acid base cements

Although acid-base cements have been known since the mid 19thcentury, and have a wide variety of applications, there has been a failureto recognize them as constituting a single, well-defined class of material.This book remedies the situation by unifying the subject and treating thisrange of materials as a single class.

These cements are defined as materials that are formed by mixing abasic powder with an acidic liquid, and offer an alternative to polymer-ization as a method for forming solid substances. They are quick-settingmaterials, with unusual properties, which find diverse applications asbiomaterials and in industry.

Chemistry of Solid State Materials

Acid-base cementsTheir biomedical and industrial applications

Chemistry of Solid State Materials

Series EditorsA. R. West, Department of Chemistry, University of AberdeenH. Baxter, formerly at the Laboratory of the Government Chemist,London

1 Segal: Chemical synthesis of advanced ceramic materials2 Colomban: Proton conductors3 Wilson & Nicholson: Acid-base cements

Acid-base cementsTheir biomedical and industrialapplications

Alan D. Wilsonformerly Head, Materials Technology, Laboratory of the Government ChemistSenior Research Fellow, Eastman Dental Hospital

John W. NicholsonHead, Materials Research, Laboratory of the Government Chemist

m0

CAMBRIDGEUNIVERSITY PRESS

CAMBRIDGE UNIVERSITY PRESSCambridge, New York, Melbourne, Madrid, Cape Town, Singapore, Sao Paulo

Cambridge University Press

The Edinburgh Building, Cambridge CB2 2RU, UK

Published in the United States of America by Cambridge University Press, New York

www.cambridge.org

Information on this title: www.cambridge.org/9780521372220

© Cambridge University Press 1993

This book is in copyright. Subject to statutory exceptionand to the provisions of relevant collective licensing agreements,no reproduction of any part may take place withoutthe written permission of Cambridge University Press.First published 1993

This digitally printed first paperback version 2005

A catalogue record for this publication is available from the British Library

Library of Congress Cataloguing in Publication dataWilson, Alan D.

Acid—base cements: their biomedical and industrial applications /Alan D.Wilson, John W. Nicholson

p. cm. - (Chemistry of solid state materials; 3)Includes bibliographical references and index.ISBN 0-521-37222-41. Adhesives. 2. Dental cements. I. Nicholson, John W.

II. Title. III. Series.TP968.W54 1993620.1'35-dc20 91-38946 CIP

ISBN-13 978-0-521-37222-0 hardbackISBN-10 0-521-37222-4 hardback

ISBN-13 978-0-521-67549-9 paperbackISBN-10 0-521-67549-9 paperback

Dedicated to the past and present members of theMaterials Technology Group at the Laboratory of theGovernment Chemist

Contents

Preface xviiAcknowledgements xix

1 Introduction 1References 4

2 Theory of acid-base cements 52.1 General 52.2 The formation of cements 7

2.2.1 Classification 72.2.2 Requirements for cementitious bonding 82.2.3 Gelation 10

2.3 Acid-base concepts 122.3.1 General 122.3.2 History of acid-base concepts 122.3.3 Acid-base concepts in AB cement chemistry 142.3.4 Relevance of acid-base theories to AB cements 192.3.5 Acid-base strength 202.3.6 Acid-base classification 222.3.7 Hard and soft acids and bases (HSAB) 24

References 26

3 Water and acid-base cements 303.1 Introduction 30

3.1.1 Water as a solvent 303.1.2 Water as a component 30

3.2 Water 313.2.1 Constitution 313.2.2 Water compared with other hydrides 33

3.3 The structure of water 343.3.1 The concept of structure in the liquid state 343.3.2 The structures of ice 353.3.3 Liquid water 36

3.4 Water as a solvent 40

IX

Contents

3.4.1 Hydrophobic interactions 403.4.2 Dissolution of salts 413.4.3 Ion-ion interactions in water 443.4.4 Dissolution of polymers 45

3.5 Hydration in the solid state 473.5.1 Coordination of water to ions 47

3.6 The role of water in acid-base cements 483.6.1 Water as a solvent in AB cements 483.6.2 Water as a component of AB cements 483.6.3 Water as plasticizer 51

References 52

4 Polyelectrolytes, ion binding and gelation 564.1 Polyelectrolytes 56

4.1.1 General 564.1.2 Polyion conformation 58

4.2 Ion binding 594.2.1 Counterion binding 594.2.2 The distribution of counterions 594.2.3 Counterion condensation 634.2.4 Effect of valence and size on counterion binding 654.2.5 Site binding - general considerations 674.2.6 Effect of complex formation 694.2.7 Effect of the polymer characteristics on ion binding 704.2.8 Solvation (hydration) effects 724.2.9 Hydration of the polyion 734.2.10 Hydration and ion binding 764.2.11 Desolvation and precipitation 774.2.12 Conformational changes in polyions 794.2.13 Interactions between polyions 824.2.14 Polyion extensions, interactions and precipitation 82

4.3 Gelation 83References 85

5 Polyalkenoate cements 905.1 Introduction 905.2 Adhesion 92

5.2.1 New attitudes 925.2.2 The need for adhesive materials 925.2.3 Acid-etching 935.2.4 Obstacles to adhesion 935.2.5 The nature of the adhesion of polyalkenoates to tooth

material 945.3 Preparation of poly(alkenoic acid)s 975.4 Setting reactions 98

Contents

5.5 Molecular structures5.6 Metal oxide poly electrolyte cements5.7 Zinc polycarboxylate cement

5.7.1 Historical5.7.2 Composition5.7.3 Setting and structure5.7.4 Properties5.7.5 Modified materials5.7.6 Conclusions

5.8 Mineral ionomer cements5.9 Glass polyalkenoate (glass-ionomer) cement

5.9.1 Introduction5.9.2 Glasses5.9.3 Poly(alkenoic acid)s5.9.4 Reaction-controlling additives5.9.5 Setting5.9.6 Structure5.9.7 General characteristics5.9.8 Physical properties5.9.9 Adhesion5.9.10 Erosion, ion release and water absorption5.9.11 Biocompatibility5.9.12 Modified and improved materials5.9.13 Applications

5.10 Resin glass polyalkenoate cements5.10.1 General5.10.2 Class I hybrids5.10.3 Class II hybrids5.10.4 Properties

References

Phosphate bonded cements6.1 General

6.1.1 Orthophosphoric acid solutions6.1.2 Cations in phosphoric acid solutions6.1.3 Reactions between oxides and phosphoric acid

solutions6.1.4 Effect of cations in phosphoric acid solutions6.1.5 Important cement-formers

6.2 Zinc phosphate cement6.2.1 General6.2.2 History6.2.3 Composition6.2.4 Cement-forming reaction6.2.5 Structure

99101103103103104106112113113116116117131133134143146147152156159162166169169170171173175

197197197198

201203204204204204205207212

XI

Contents

6.2.6 Properties 2146.2.7 Factors affecting properties 2186.2.8 Biological effects 2196.2.9 Modified zinc phosphate cements 2196.2.10 Hydrophosphate cements 220

6.3 Transition-metal phosphate cements 2206.4 Magnesium phosphate cements 222

6.4.1 General 2226.4.2 Composition 2226.4.3 Types 2226.4.4 Cement formation and properties 2236.4.5 Cement formation with phosphoric acid 2236.4.6 Cement formation with ammonium dihydrogen

phosphate 2236.4.7 Cement formation with diammonium hydrogen

phosphate 2316.4.8 Cement formation with ammonium polyphosphate 2326.4.9 Cement formation with aluminium acid phosphate 2326.4.10 Cements formed from magnesium titanates 235

6.5 Dental silicate cement 2356.5.1 Historical 2356.5.2 Glasses 2376.5.3 Liquid 2416.5.4 Cement-forming reaction 2436.5.5 Structure 2496.5.6 Physical properties 2536.5.7 Dissolution and ion release 2556.5.8 Biological aspects 2606.5.9 Conclusions 2616.5.10 Modified materials 262

6.6 Silicophosphate cement 2636.7 Mineral phosphate cements 265References 265

Oxysalt bonded cements 2837.1 Introduction 283

7.1.1 Components of oxysalt bonded cements 2847.1.2 Setting of oxysalt bonded cements 284

7.2 Zinc oxychloride cements 2857.2.1 History 2857.2.2 Recent studies 286

7.3 Magnesium oxy chloride cements 2907.3.1 Uses 2907.3.2 Calcination of oxide 2907.3.3 Setting chemistry 291

xn

Contents

7.3.4 Kinetics of cementation 2937.3.5 Phase relationships in the MgO-MgCl2-H2O system 2947.3.6 Consequences for practical magnesium oxychloride

cements 2957.3.7 Impregnation with sulphur 297

7.4 Magnesium oxy sulphate cements 2997.4.1 Setting chemistry 2997.4.2 Phase relationships in the MgO-MgSO4-H2O system 3007.4.3 Mechanical properties of magnesium oxysulphate

cements 3027.5 Other oxy salt bonded cements 304References 305

8 Miscellaneous aqueous cements 3078.1 General 3078.2 Miscellaneous aluminosilicate glass cements 3078.3 Phytic acid cements 3098.4 Poly(vinylphosphonic acid) cements 310

8.4.1 Metal oxide polyphosphonate cements 3118.4.2 Glass polyphosphonate cements 314

8.5 Miscellaneous copper oxide and cobalt hydroxidecements 315

References 316

9 Non-aqueous cements 318318320320321321322323331333334334335335336336336337337337339

Xlll

9.19.2

9.2.19.2.29.2.39.2.49.2.59.2.69.2.79.2.89.2.99.2.109.2.11

9.39.3.19.3.2

9.49.4.19.4.29.4.3

GeneralZinc oxide eugenol (ZOE) cementsIntroduction and historyEugenolZinc oxideCement formationSettingStructurePhysical propertiesBiological propertiesModified cementsImpression pastesConclusionsImproved ZOE cementsGeneralReinforced cements2-ethoxybenzoic acid eugenol (EBA) cementsGeneralDevelopmentSetting and structure

Contents

9.4.4 Properties 3409.5 EBA-methoxyhydroxybenzoate cements 342

9.5.1 EBA-vanillate and EBA-syringate cements 3429.5.2 EBA-divanillate and polymerized vanillate cements 3449.5.3 EBA-HV polymer cements 3459.5.4 Conclusions 3469.5.5 Other zinc oxide cements 347

9.6 Calcium hydroxide chelate cements 3479.6.1 Introduction 3479.6.2 Composition 3489.6.3 Setting 3489.6.4 Physical properties 3509.6.5 Biological properties 3509.6.6 The calcium hydroxide dimer cement 351

References 352

10 Experimental techniques for the study of acid-basecements 35910.1 Introduction 35910.2 Chemical methods 360

10.2.1 Studies of cement formation 36010.2.2 Degradative studies 361

10.3 Infrared spectroscopic analysis 36110.3.1 Basic principles 36110.3.2 Applications to AB cements 36210.3.3 Fourier transform infrared spectroscopy 364

10.4 Nuclear magnetic resonance spectroscopy 36410.4.1 Basic principles 36410.4.2 Applications to AB cements 365

10.5 Electrical methods 36610.6 X-ray diffraction 367

10.6.1 Basic principles 36710.6.2 Applications to AB cements 368

10.7 Electron probe microanalysis 36910.7.1 Basic principles 36910.7.2 Applications to dental silicate cements 36910.7.3 Applications to glass-ionomer cements 369

10.8 Measurement of mechanical properties 37010.8.1 Compressive strength 37110.8.2 Diametral compressive strength 37210.8.3 Flexural strength 37210.8.4 Fracture toughness 373

10.9 Setting and rheological properties 37410.9.1 Problems of measurement 37510.9.2 Methods of measurement 375

xiv

Contents

10.10 Erosion and leaching 37810.10.1 Importance in dentistry 37810.10.2 Studies of erosion 379

10.11 Optical properties 37910.11.1 Importance in dentistry 37910.11.2 Measurement of opacity 380

10.12 Temperature measurement 38010.13 Other test methods 381References 382

Index 386

xv

Preface

The senior author first became interested in acid-base cements in 1964when he undertook to examine the deficiencies of the dental silicate cementwith a view to improving performance. At that time there was muchconcern by both dental surgeon and patient at the failure of this aestheticmaterial which was used to restore front teeth. Indeed, at the time, onecorrespondent commenting on this problem to a newspaper remarked thatalthough mankind had solved the problem of nuclear energy the samecould not be said of the restoration of front teeth. At the time it wassupposed that the dental silicate cement was, as its name implied, a silicatecement which set by the formation of silica gel. Structural studies at theLaboratory of the Government Chemist (LGC) soon proved that this viewwas incorrect and that the cement set by formation of an amorphousaluminium phosphate salt. Thus we became aware of and intrigued by aclass of materials that set by an acid-base reaction. It appeared that therewas endless scope for the formulation of novel materials based on thisconcept. And so it proved.

Over the years, from 1964 to date, a team at the LGC, with its expertisein Materials Chemistry, has studied many of the materials described in thisbook, elucidating structures, setting reactions and behaviour. Thisexperience has formed a strong experimental background against whichthe book was written. In addition we have maintained contact with leadersin this field throughout the world. We should mention Professor DennisSmith of Toronto University, who amongst his many achievementsinvented the adhesive zinc polycarboxylate cement (Chapter 5); Dr G. M.Brauer, who was for many years at the Institute for Materials Research,National Bureau of Standards, Washington, D.C., and is the acknowl-edged authority on cements formed by the reaction between zinc oxideand phenolic bodies (Chapter 9); and Dr J. H. Sharp of the University ofSheffield, who has developed magnesium phosphate cements (Chapter 6).

xvu

Preface

In particular we thank Dr J. H. Sharp for supplying original photographsfor use in the section on magnesium phosphate cements and for criticallyreading the draft manuscript and making constructive suggestions. Onclinical matters we have benefited from a 20-year collaboration withDr J. W. McLean OBE.

Our own research at the LGC, while not confined to, has centred on,cements formed by the reactions between acid-decomposable glasses andvarious cement-forming acids (Chapters 5, 6, 8, 9). One of these materialsinvented at the LGC, the glass polyalkenoate or glass-ionomer cement,has proved of immense importance. Indeed, so successful has this materialbeen in general dentistry, that the Materials Technology Group earned theQueen's Award for Technology in 1988. This material illustrates the usefulcombination of properties that can be found in the acid-base cements, forit has the aesthetic appearance of porcelain, the ability to adhere to teeth,and also the ability to release fluoride with its beneficial effect of reducingcaries.

We hope that this work will encourage, stimulate and assist otherschoosing to work in this interesting field.

Alan D. WilsonJohn W. Nicholson

xvin

Acknowledgements

We make a particular acknowledgement to the late Dr John LongwellCBE, Deputy Government Chemist in 1964, who encouraged the Labor-atory to enter the field, and to the line of Government Chemists whosupported the work over the long years; the late Dr David Lewis CB, thelate Dr Harold Egan, Dr Ron Coleman CB (who became Chief Scientistof the Department of Trade and Industry), Mr Alex Williams CB andDr Richard Worswick.

We note the particular contributions of Brian Kent, present Head of theMaterials Technology Group, as co-inventor of the glass polyalkenoatecement way back in 1968, and of Dr John McLean OBE in developingclinical applications. It was Surgeon Rear Admiral Holgate CB, OBE,Chief Dental Officer at the Ministry of Health in 1964, who introduced DrMcLean to the Laboratory of the Government Chemist (LGC) to initiatea collaboration that proved so fruitful. Since then there has been constantsupport from the Department of Health and its various officers and alsofrom the British Technology Group, particularly from G. M. Blunt andR. A. Lane.

Most importantly we acknowledge the contribution of those whoworked at that essential place, the laboratory bench, on which everythingdepends.

Our colleagues in the Materials Technology Group (formerly the DentalMaterials Group) who have worked with one or other of us since 1964 are:R. F. Batchelor, B. G. Lewis, Mrs B. G. Scott, J. M. Paddon, G. Abel,Dr S. Crisp, A. J. Ferner, Dr H. J. Prosser, M. A. Jennings, Mrs S. A.Merson, M. Ambersley, D. M. Groffman, S. M. Jerome, D. R. Powis,Mrs P. J. Brookman (nee Brant), R. P. Scott, J. C. Skinner, Dr R. G. Hill,G. S. Sayers, Dr C. P. Warrens, Miss A. M. Jackson, Dr J. Ellis,Miss E. A. Wasson, Miss H. M. Anstice, Dr J. H. Braybrook, Miss S. J.Hawkins and A. D. Akinmade.

xix

Acknowledgements

In addition we have received support from members of other divisionsat the LGC: Dr R. J. Mesley, M. A. Priguer, D. Wardleworth, Dr I. K.O'Neill, B. Stuart, R. A. Gilhooley, Dr C. P. Richards, Dr O. M. Lacyand Dr S. L. R. Ellison.

Guest workers to the Materials Technology Group who have con-tributed include Professor P. Hotz (Klinik fur Zahnerhaltung der Uni-versitat, Bern), Ms T. Folleras (NIOM, Scandinavian Institute of DentalMaterials).

Workers in other Government Research Stations and the Universitieswho have collaborated with us are: R. P. Miller, D. Clinton, Dr T. I.Barry, Dr I. Seed (National Physical Laboratory); K. E. Fletcher (Build-ings Research Station); Miss D. Poynter (Warren Spring Laboratory);Professor L. Holliday, Dr J.H.Elliott, Dr P. R. Hornsby, Dr K. A.Hodd, Dr A. L. Reader (Brunei University); R. Manston, Dr B. F.Sanson, Dr W. M. Allen, P. J. Gleed (Institute for Research on AnimalDiseases); Professor Braden (London Hospital); A. C. Shorthall (Bir-mingham University), I. M. Brook (University of Sheffield); andR. Billington (Institute of Dental Surgery, London).

We thank Dr L. J. Pluim of the Rijksuniversiteit te Groningen fordrawing our attention to the early and neglected work of E. van Dalen onzinc phosphate cements.

We thank Mrs Margaret Wilson for her help in checking the proofs andthe indexing.

We acknowledge the stoic forbearance of our wives in putting up withthe disturbances and neglect of domestic routines and duties occasionedby the writing of a book.

Alan D. WilsonJohn W. Nicholson

xx

1 Introduction

Acid-base (AB) cements have been known since the mid 19th century.They are formed by the interaction of an acid and a base, a reaction whichyields a cementitious salt hydrogel (Wilson, 1978) and offers an alter-native route to that of polymerization for the formation of macro-molecular materials. They are quick-setting materials, some of which haveunusual properties for cements, such as adhesion and translucency.They find diverse applications, ranging from the biomedical to theindustrial.

Despite all this there has been a failure to recognize AB cements asconstituting a single, well-defined class of material. Compared with organicpolymers, Portland cement and metal alloys, they have been neglected and,except in specialized fields, awareness of them is minimal. In this book weattempt to remedy the situation by unifying the subject and treating thisrange of materials as a single class.

Human interest in materials stretches back into palaeolithic times whenmaterials taken from nature, such as wood and stone, were fashioned intotools, weapons and other artifacts. Carving or grinding of a material is aslow and time-consuming process so the discovery of pottery, which doesaway with the need for these laborious processes, was of the greatestsignificance. Here, a soft plastic body, potter's clay, is moulded into thedesired shape before being converted into a rigid substance by firing.Pottery is but one of a group of materials which are formed by the physicalor chemical conversion of a liquid or plastic body, which can be easilyshaped by casting or moulding, into a solid substance. Other examples ofthis common method of fabrication are the casting of metals and theinjection moulding of plastics.

Into this category come the water-based plasters, mortars, cementsand concretes which set at room temperature as the result of a chemicalreaction between water and a powder. Some of these have been known

1

Introduction

since antiquity. The AB cements are related to these materials except thatwater is replaced by an acidic liquid.

The first AB cement, the zinc oxychloride cement, was reported by Sorelin 1855. It was prepared by mixing zinc oxide powder with a concentratedsolution of zinc chloride. Its use in dentistry was recommended byFeichtinger in 1858 but it did not prove to be a success (Mellor, 1929).However, other AB cements have proved to be of the utmost value todentistry, and their subsequent development has been closely associatedwith this art (Wilson, 1978). The AB cements, developed against thebackcloth of the severe demands of dentistry, have interesting properties.Some possess aesthetic appeal and the ability to adhere to base metals andother reactive substrates. Most have superior properties to plasters,mortars, and Portland cements, being quick-setting, stronger and moreresistant to erosion. These advantageous properties make them strongcandidates for other applications. In fact, one of these cements, themagnesium oxychloride cement of Sorel (1867), is still used to surface wallsand floors on account of its marble-like appearance (Chapter 7).

In the 1870s more effective liquid cement-formers were found: ortho-phosphoric acid and eugenol (Wilson, 1978). It was also found that analuminosilicate glass could replace zinc oxide, a discovery which led to thefirst translucent cement. Thereafter the subject stagnated until the late1960s when the polyelectrolyte cements were discovered by Smith (1968)and Wilson & Kent (1971).

In recent years Sharp and his colleagues have developed the magnesiumphosphate cements - Sharp prefers the term magnesia phosphate cement- as a material for the rapid repair of concrete runways and motorways(Chapter 6). These applications exploit the rapid development of strengthin AB cements. This cement can also be used for flooring in refrigeratedstores where Portland cements do not set. Interestingly, this materialappears to have started life as an investment for the casting of dental alloys.

The glass polyalkenoate, a polyelectrolyte cement, of Wilson & Kent(Chapter 5), was originally developed as a dental material but has sincefound other applications. First it was used as a splint bandage materialpossessing early high-strength and resistance to water. Currently, it isbeing used, as a biocompatible bone cement, with a low exothermicity onsetting and the ability to adhere to bone, for the cementation of prostheses(Jonck, Grobbelaar & Strating, 1989).

Outside the field of biomaterials it has been patented for use as a cementfor underwater pipelines, as a foundry sand and as a substitute for plaster

Introduction

in the slip casting of pottery. Quite often it appears as a substitute forplaster of Paris, for it is stronger, less brittle and more resistant to water.There are other possibilities. Its translucent nature suggests that it could beused for the production of porcelain-like ceramics at room temperature.

Phosphate and polyelectrolyte AB cements are resistant to attack byboiling water, steam and mild acids and this suggests that they could beemployed in technologies where these properties are important.

The ability of the polyelectrolyte-based AB cements (Chapter 5) to bondto a variety of substrates, combined with their rapid development ofstrength - they can become load-bearing within minutes of preparation -suggests that they have applications as rapid-repair and handymanmaterials.

A current area of interest is the use of AB cements as devices for thecontrolled release of biologically active species (Allen et aL, 1984). ABcements can be formulated to be degradable and to release bioactiveelements when placed in appropriate environments. These elements can beincorporated into the cement matrix as either the cation or the anioncement former. Special copper/cobalt phosphates/selenates have beenprepared which, when placed as boluses in the rumens of cattle and sheep,have the ability to decompose and release the essential trace elementscopper, cobalt and selenium in a sustained fashion over many months(Chapter 6). Although practical examples are confined to phosphatecements, others are known which are based on a variety of anions:polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) anda variety of organic chelating anions (Chapter 9). The number of cementsavailable for this purpose is very great.

A recent development has been the incorporation of a bioactive organiccomponent into the AB cement during preparation. Since AB cements areprepared at room temperature, this can be done without causingdegradation of the organic compound. In this case, the AB cement maymerely act as a carrier for the sustained release of the added bioactivecompound.

Another development has been the advent of the dual-cure resin cements.These are hybrids of glass polyalkenoate cements and methacrylates thatset both by an acid-base cementation reaction and by vinyl polymerization(which may be initiated by light-curing). In these materials, the solvent isnot water but a mixture of water and hydroxyethylmethacrylate which iscapable of taking dimethacrylates and poly(acrylic acid)-containing vinylgroups into solution. In the absence of light these materials set slowly and

Introduction

have extended working times, but they set in seconds when illuminatedwith an intense beam of visible light. These hybrids are in their infancy buthave created great interest.

From this account we are to expect diversification of these AB cementsboth for biomedical and for industrial usages. There should be furtherdevelopments of the glass polyalkenoate cements both as bone substitutesand as bioadhesives. We also expect more types of AB cements to beformulated as devices for the sustained release of bioactive species. Thesematerials would have applications in agriculture, horticulture, animalhusbandry and human health care. In industrial fields we expect that therewill be continued interest in developing AB cements as materials for therapid repair of constructural concrete, as materials for the surfacing offloors and walls, and as adhesives and lutes for cementation in aqueousenvironments. The hybrid light-cured cements also appear to be apromising new line of development which may give us entirely novel classesof materials.

ReferencesAllen, W. M., Sansom, B. F., Wilson, A. D., Prosser, H. J. & Groffman, D. M.

(1984). Release cements. British Patent GB 2,123,693 B.Jonck, L. M., Grobbelaar, C. J. & Strating, H. (1989). The biocompatibility of

glass-ionomer cement in joint replacement: bulk testing. Clinical Materials, 4,85-107.

Mellor, J. W. (1929). A Comprehensive Treatise on Inorganic and TheoreticalChemistry, vol. IV, p. 546. London: Longman.

Sorel, S. (1855). Procede pour la formation d'un ciment tres-solide par 1'actiond'un chlorure sur l'oxyde de zinc. Comptes rendus hebdomadaires des seancesde T Academie des sciences, 41, 784-5.

Sorel, S. (1867). On a new magnesium cement. Comptes rendus hebdomadairesdes seances de VAcademie des sciences, 65, 102—4.

Wilson, A. D. (1978). The chemistry of dental cements. Chemical SocietyReview, 7, 265-96.

2 Theory of acid-base cements

2.1 General

From the chemical point of view AB cements occupy a place in the vastrange of acid-base phenomena which occur throughout both inorganicand organic chemistry. Like Portland cement they are prepared by mixinga powder with a liquid. However, this liquid is not water but an acid, whilethe powder, a metal oxide or silicate, is a base. Not surprisingly, thecement-forming reaction between them is extremely rapid and a hardenedmass is formed within minutes of mixing.

AB cements may be represented by the defining equation

Base + Acid = Salt + Water(powder) (liquid) (cement matrix)

The product of the reaction, the binding agent, is a complex salt, andpowder in excess of that required for the reaction acts as the filler. Eachcement system is a particular combination of acid and base. The number ofpotential cement systems is considerable since it is a permutation of allpossible combinations of suitable acids and bases.

Cement-forming liquids are strongly hydrogen-bonded and viscous.According to Wilson (1968), they must (1) have sufficient acidity todecompose the basic powder and liberate cement-forming cations, (2)contain an acid anion which forms stable complexes with these cations and(3) act as a medium for the reaction and (4) solvate the reaction products.

Generally, cement-forming liquids are aqueous solutions of inorganic ororganic acids. These acids include phosphoric acid, multifunctionalcarboxylic acids, phenolic bodies and certain metal halides and sulphates(Table 2.1). There are also non-aqueous cement-forming liquids which aremultidentate acids with the ability to form complexes.

Potential cement-forming bases are oxides and hydroxides of di- and

Theory of acid—base cements

Table 2.1. Examples of acids used for cement formation

Protonic acids Aprotic acids(used in aqueous solution) (used in aqueous solution)

Phosphoric acid Magnesium chloridePoly(acrylic acid) Zinc chlorideMalic acid Copper(II) chlorideTricarballylic acid Cobalt(II) chloridePyruvic acid Magnesium sulphateTartaric acid Zinc sulphateMellitic acid Copper(II) sulphateGallic acid Cobalt(II) sulphateTannic acid Magnesium selenate

Zinc selenateProtonic acids Copper(II) selenate(liquid non-aqueous) Cobalt(II) selenate

Eugenol2-ethoxybenzoic acid

Table 2.2. Examples of bases used for cement formation

Copper(II) oxideZinc(II) oxideMagnesium oxideCobalt(II) hydroxideCobalt(II) carbonateCalcium aluminosilicate glassesGelatinizing minerals

trivalent metals, silicate minerals and aluminosilicate glasses (Table 2.2).All cement-forming bases must be capable of releasing cations into acidsolution. The best oxides for cement formation are amphoteric (Kingery,1950a,b) and the most versatile cement former is zinc oxide, which canreact with a wide range of aqueous solutions of acids, both inorganic andorganic, and liquid organic chelating agents. Gelatinizing minerals, that isminerals that are decomposed by acids, can act as cement formers, as canthe acid-decomposable aluminosilicate glasses.

In this chapter the nature of the cementitious bond and the acid-basereaction will be discussed.

The formation of cements

2.2 The formation of cements2.2.1 Classification

Before proceeding further it is well to consider the term cement, for itsdefinition can be the source of some confusion. Both the Oxford EnglishDictionary and Webster give two alternative definitions. One defines acement as a paste, prepared by mixing a powder with water, that sets to ahard mass. In the other a cement is described as a bonding agent. These twodefinitions are quite different. The first leads to a classification of cementsin terms of the setting process, while the second lays emphasis on aproperty. In this book the term cement follows the sense of the first of thesedefinitions.

Cements can be classified into three broad categories:

(1) Hydraulic cements. These cements are formed from two con-stituents one of which is water. Setting comprises a hydration andprecipitation process. Into this category fall Portland cement andplaster of Paris.

(2) Condensation cements. Here, cement formation involves a loss ofwater and the condensation of two hydroxyl groups to form abridging oxygen:

R-OH + HO-R = R-O-R + H2O

One example is silicate cement where orthosilicic acid, chemicallygenerated in solution, condenses to form a silicic acid gel. Anotheris refractory cement where a cementitious product is formed bythe heat treatment of an acid orthophosphate, a process whichagain involves condensation to form a polyphosphate.

(3) Acid-base cements. Cement formation involves both acid-baseand hydration reactions (Wilson, Paddon & Crisp, 1979). Thesecements form the subject of this book.

This classification differs from that given by Wygant (1958), whosubdivides cements into hydraulic, precipitation and reaction cements. Theadvantage of the present classification is that it clearly differentiatesphosphate cements formed by condensation from those formed by anacid-base reaction (Kingery, 1950a). Wygant includes these in the samecategory, which can be confusing. Moreover, he puts silicate cements andthe heat-treated acid phosphate cements into separate categories, althoughboth are condensation cements.

Theory of acid—base cements

2.2.2 Requirements for cementitious bonding

The essential property of a cementitious material is that it is cohesive.Cohesion is characteristic of a continuous structure, which in the case of acement implies an isotropic three-dimensional network. Moreover, thenetwork bonds must be attributed to attractions on the molecular level.Increasingly, recent research tends to show that cements are not bonded byinterlocking crystallites and that the formation of crystallites is incidental(Steinke et al., 1988; Crisp et al., 1978). The reason is that it is difficult toform rapidly a mass which is both cohesive and highly ordered.

Cement formation requires a continuous structure to be formed in situfrom a large number of nuclei. Moreover, this structure must be maintaineddespite changes in the character of the bonds. These criteria are, obviously,more easily satisfied by a flexible random structure than by one which ishighly-ordered and rigid. Crystallinity implies well-satisfied and rigidly-directed chemical bonds, exact stoichiometry and a highly orderedstructure. So unless crystal growth is very slow a continuous molecularstructure cannot be formed.

In random structures, stoichiometry need not be exact and adventitiousions can be incorporated without causing disruption. Bonds are not highlydirected, and neighbouring regions of precipitation, formed arounddifferent nuclei, can be accommodated within the structure. Continuousnetworks can be formed rapidly. Thus, random structures are conducive tocement formation and, in fact, most AB cements are essentially amorph-ous. Indeed, it often appears that the development of crystallinity isdetrimental to cement formation.

The matrices of AB cements are gel-like, but these gels differ from thetobermorite gel of Portland cement. In AB cements, setting is the result ofgelation by salt formation, and the cations, which cause gelation, areextracted from an oxide or silicate by a polyacid solution. The conversionof the sol to a gel is rapid and the cements set in 3 to 5 minutes. Two basicprocesses are involved in cement formation: the release of cations from theoxide or silicate and their interaction with polyacid. This interactioninvolves ion binding and changes in the hydration state which areassociated with gelation and structure formation (Section 4.3). Thus, thereare two reaction rates to be considered: the rate of release of cations andthe rate of structure formation. These two reaction rates must be balanced.If the rate of release of cations is too fast a non-coherent precipitate ofcrystallites is formed. If too slow the gel formed will lack strength.

The formation of cements

During cement formation, domains are formed about numerous nucleiand there must be bonding between the domains as well as within them. InAB cements bonding within the domains is mainly ionic, with a degree ofcovalency. The attractive forces between domains are those of a colloidaltype. In random structures, residual force fields exist which act in a similarfashion to polar forces and serve to bond domains. These forces mustinclude hydrogen bonds, for the addition of fluoride ions always enhancescement strength and the fluoride-hydrogen bond is a strong one.

The structures of cement gels bear some relationship to the structure ofglasses. Spatially, the O2~ ion is dominant. The matrices are based on acoordinated polyhedron of oxygen ions about a central glass-formingcation (Pauling, 1945). In effect, these are anionic complexes where thecations are small, highly charged, and capable of coordinating with oxygenor hydroxyl ions. Examples of these polyhedra are [SiOJ, [POJ and [A1OJ.Thus, we find that there are silicate, phosphate and aluminosilicate glassesand gels.

There are, however, differences which are best illustrated by reference tothe simple example of silica glass and silica gel. In silica glass, Si4+ is four-coordinate and the polymeric links are of the bridging type:

-«>Si—O—Si<^-

In aqueous solution, coordination increases to 6, Si-OH links are possibleas well as Si-O-Si, and H2O is a possible ligand. In silica cements thecondensation of silicic acid, Si(OH)4, to SiO2 is only partial. Silica geltherefore contains both bridging oxygen and non-bridging hydroxyllinkages. Again, in contrast to the situation in glasses the possibility ofhydrogen bond formation will also exist.

In AB cements the gel-forming cations are frequently Zn2+, Mg2+, Ca2+

or Al3+. As Kingery (1950b) has pointed out, it is the amphoteric cations,for example Zn2+ and Al3+, that possess the most favourable cement-forming properties. Their oxides are capable of glass formation, not bythemselves, but in conjunction with other glass formers. Kingery alsoindicated that weakly basic cations, for example Mg2+, are less effective,and more strongly basic cations, for example Ca2+, even less effective.

The nature of the association between cement-forming cation and anionis important. As we shall see from theoretical considerations of the natureof acids and bases in section 2.3, these bonds are not completely ionic incharacter. Also while cement-forming cations are predominantly a-

Theory of acid-base cements

acceptors and the anions cr-donors, both have weak ^-capabilities also.This topic is treated in more detail in the next section. Complex formationis clearly important and this view is supported by the anomaly that B2O3forms cements with acids, not as a result of salt formation, but because ofcomplex formation (Chapters 5 and 8).

A final point needs to be made. Theory has indicated that AB cementsshould be amorphous. However, a degree of crystallization does sometimesoccur, its extent varying from cement to cement, and this often misled earlyworkers in the field who used X-ray diffraction as a principal method ofstudy. Although this technique readily identifies crystalline phases, itcannot by its nature detect amorphous material, which may form the bulkof the matrix. Thus, in early work too much emphasis was given tocrystalline structures and too little to amorphous ones. As we shall see, theformation of crystallites, far from being evidence of cement formation, isoften the reverse, complete crystallinity being associated with a non-cementitious product of an acid-base reaction.

2.2.3 Gelation

The formation of AB cements is an example of gelation, and the matricesmay be regarded as salt-like hydrogels. They are rigid and glass-like. A gelhas been defined by Bungenberg de Jong (1949) as 'a system of solidcharacter, in which the colloidal particles somehow constitute a coherentstructure'. A more exact definition is not possible, for gels are easier torecognize than define; they include a diversity of substances. Coherence ofstructure appears, however, to be a universal criterion for gels.

Flory (1974) classified gels into four types on the basis of theirstructures:

(1) Well-ordered lamellar structures. The lamellae are arranged inparallel, giving rise to long-range order. Examples are soaps,phospholipids and clays.

(2) Covalent polymeric networks which are completely disordered.Continuity of structure is provided by an irregular three-dimensional network of covalent links, some of which arecrosslinks. The network is uninterrupted and has an infinitemolecular weight. Examples are vulcanized rubbers, condensationpolymers, vinyl-divinyl copolymers, alkyd and phenolic resins.

10

The formation of cemen ts

(3) Polymer networks formed through physical aggregation; theseare predominantly disordered, but have regions of local order.Linear structures of finite length are connected by multiple-stranded helices, which may be crystalline. Examples are gelatinand sodium alginate gels.

(4) Particulate, disordered structures. These include flocculent pre-cipitates where particles generally consist of fibres in brush-heapdisarray or connected in irregular networks.

Since the matrices of AB cements bear some similarity to alginate gelsthey most probably fall into type 3.

The classical theory of gelation, due to Flory (1953, 1974), sees gelationas the result of the formation of an infinite three-dimensional network.According to Flory, the theory can be applied without ambiguity to thetype 2 (covalent) gels and is also applicable to type 3 gels. The conditionsfor the formation of such an infinite network are critical. Flory conceivesthe growth of a random network as a sequential condensation processbetween difunctional and multifunctional units involving a branchingprocess. During growth, the probability of branching (a) at each potentialbranching point has to reach a critical value (ac) for an infinite network tobe formed. In the case of condensation between di- and trifunctionalgroups, the probability has to be more than 50 % for an infinite network tobe formed. If it is 50 % or less then an infinite network is not formed. Thistheory explains why gelation occurs suddenly.

In general, the critical value for a, ac, is given by the expression

where / i s the degree of functionality of the multifunctional group.The most investigated examples are to be found in the precipitation of

polyelectrolytes by metal ions. Here, networks are formed by the randomcrosslinking of linear polymer chains, and the theory requires somemodification. The condition for the formation of an infinite network isthat, on average, there must be more than two crosslinks per chain. Thus,the greater the length of a polymer chain the fewer crosslinks in the systemas a whole are required.

11


2.3 Acid-base concepts2.3.1 General

The cement-forming reaction is a special case of an acid-base reaction sothat concepts of acid, base and salt are central to the topic. In AB cementtheory, we are concerned with the nature of the acid-base reaction andhow the acidity and basicity of the reactants are affected by theirconstitution. Thus, it is appropriate at this stage to discuss the variousdefinitions and theories available.

Although acids and bases have been recognized since antiquity, ourconcepts of them are still the subject of debate and development (Walden,1929; Hall, 1940; Bell, 1947, 1973; Luder, 1948; Kolthoff, 1944; Bjerrum,1951; Day & Selbin, 1969; Jensen, 1978; Finston & Rychtman, 1982). Thehistory of these concepts is a long one and can be seen as a prolonged andcontinuous refinement of inexact and commonsense notions into precisescientific theories. It has been a long and difficult journey and one that is byno means ended.

There are various definitions of acids and bases, and in discussing themit should be emphasized that the question is not one of validity but one ofutility. Indeed, the problem of validity does not arise because of thefundamental nature of a definition. The problem is entirely one of choosinga definition which is of greatest use in the study of a particular field ofacid-base chemistry. One point that needs to be borne in mind is that aconcept of acids and bases is required that is neither too general nor toorestrictive for the particular field of study.

2.3.2 History of acid-base concepts

From early times acids were recognized by their properties, such assourness and ability to dissolve substances, often with effervescence. Thestory of Cleopatra's draught of a pearl dissolved in vinegar illustrates thispoint (Pattison Muir, 1883). Vinegar, known to the Greeks and Romans,was associated with the concept of acidity and gives its name to the termacid which comes from the Latin acetum. Boyle (1661) observed that acidsdissolve many substances, precipitate sulphur from alkaline solution,change blue plant dyes to red and lose these properties on contact withalkalis.

It also has been known since antiquity that aqueous extracts of the ash

12

Acid-base concepts

of certain plants have distinctive properties: slipperiness, cleansing powerin the removal of fats, oils and dirt from fabrics, and the ability to affectplant colours (Day & Selbin, 1969; Pattison Muir, 1883). These substanceswere called alkalis, a name which comes from the Arabic for plant ash, alhalja (Finston & Rychtman, 1982) or algali. The term alkali applies onlyto the hydroxides and carbonates of sodium and potassium, and it wasRouelle in 1744 who extended the concept to include the alkaline earthanalogues and used the term base to categorize them (Walden, 1929; Day& Selbin, 1969).

Salt formation as a criterion for an acid-base interaction has a longhistory (Walden, 1929). Rudolph Glauber in 1648 stated that acids andalkalis were opposed to each other and that salts were composed of thesetwo components. Otto Tachenius in 1666 considered that all salts could bebroken into an acid and an alkali. Boyle (1661) and the founder of thephlogistic theory, Stahl, observed that when an acid reacts with an alkalithe properties of both disappear and a new substance, a salt, is producedwith a new set of properties. Rouelle in 1744 and 1754 and William Lewisin 1746 clearly defined a salt as a substance that is formed by the union ofan acid and a base.

It can be seen that these definitions are derived from experimentalobservation and are no more than classifications based on a set ofproperties shared by a group of substances. They are scientificallyinadequate for the interpretation of results, which requires a definitionbased on concepts. Historically, the attempt to provide a model rather thana classification comes in the form of a search for underlying universalprinciples. It seems that the alchemists recognized vague principles ofacidity and alkalinity, and in the 17th century the iatrochemists made thesethe basis of chemical medicine. Disease was attributed to a predominanceof one or other of these principles (Pattison Muir, 1883).

Boyle (1661) attempted to provide a more definite concept and attributedthe sour taste of acids to sharp-edged acid particles. Lemery, anothersupporter of the corpuscular theory of chemistry, had similar views andconsidered that acid-base reactions were the result of the penetration ofsharp acid particles into porous bases (Walden, 1929; Finston &Rychtman, 1982). However, the first widely accepted theory was that ofLavoisier who in 1777 pronounced that oxygen was the universal acidifyingprinciple (Crosland, 1973; Walden, 1929; Day & Selbin, 1969; Finston &Rychtman, 1982). An acid was defined as a compound of oxygen with anon-metal.

13


After this theory was disproved, other acidifying principles wereproposed. The most significant was the recognition, by Davy & Dulongearly in the 19th century, of hydrogen as the acidifying principle (Walden,1929; Finston & Rychtman, 1982). During this period no such search wasmade for a basic principle. Bases were merely regarded as a motleycollection of antiacids with little in common apart from the ability to reactwith acids.

The first substantial constitutive concept of acid and bases came only in1887 when Arrhenius applied the theory of electrolytic dissociation toacids and bases. An acid was defined as a substance that dissociated tohydrogen ions and anions in water (Day & Selbin, 1969). For the first time,a base was defined in terms other than that of an antiacid and was regardedas a substance that dissociated in water into hydroxyl ions and cations. Thereaction between an acid and a base was simply the combination ofhydrogen and hydroxyl ions to form water.

This theory was a milestone in the development of acid-base concepts:it was the first to define acids and bases in terms other than that of areaction between them and the first to give quantitative descriptions.However, the theory of Arrhenius is far more narrow than both itspredecessors and its successors and, indeed, it is the most restrictive of allacid-base theories.

Since Arrhenius, definitions have extended the scope of what we meanby acids and bases. These theories include the proton transfer definition ofBronsted-Lowry (Bronsted, 1923; Lowry, 1923a,b), the solvent systemconcept (Day & Selbin, 1969), the Lux-Flood theory for oxide melts, theelectron pair donor and acceptor definition of Lewis (1923, 1938) and thebroad theory of Usanovich (1939). These theories are described in moredetail below.

2.3.3 Acid—base concepts in AB cement chemistry

We now review the various concepts of acids and bases in order to see howappropriate and useful they are in the field of AB cements.

The definition of ArrheniusThis definition of acids and bases is of restricted application. The reactionbetween acids and bases is seen as the combination of hydrogen andhydroxyl ions in aqueous solution to form water.

14

Acid-base concepts

An acid is defined as a species that dissociates in aqueous solution to givehydrogen ions and onions, and a base as a species that dissociates in aqueoussolution to give hydroxyl ions and cations.

Thus, acids and bases are defined as aqueous solutions of substances andnot as the substances themselves. It follows that ionization is a necessarycharacteristic of Arrhenius acids and bases. Another restriction of thisdefinition is that acid-base behaviour is not recognized in non-aqueoussolution.

The Arrhenius definition is not suitable for AB cements for severalreasons. It cannot be applied to zinc oxide eugenol cements, for these arenon-aqueous, nor to the metal oxychloride and oxysulphate cements,where the acid component is not a protonic acid. Indeed, the theory is,strictly speaking, not applicable at all to AB cements where the base is nota water-soluble hydroxide but either an insoluble oxide or a silicate.

The protonic Bronsted-Lowry theoryThe theory of Bronsted (1923) and Lowry (1923a, b) is of more generalapplicability to AB cements. Their definition of an acid as' a substance thatgives up a proton' differs little from that of Arrhenius. However, the sameis not true of their definition of a base as' a substance capable of acceptingprotons' which is far wider than that of Arrhenius, which is limited tohydroxides yielding hydroxide ions in aqueous solution. These concepts ofBronsted and Lowry can be defined by the simple equation (Finston &Rychtman, 1982):

Acid = Base + H+ [2.2]

Thus, the relationship between acid and base is a reciprocal one and anacid-base reaction involves the transfer of a proton. This concept is notrestricted to aqueous solutions and it discards Arrhenius' prerequisite ofionization.

This concept covers most situations in the theory of AB cements.Cements based on aqueous solutions of phosphoric acid and poly(acrylicacid), and non-aqueous cements based on eugenol, alike fall within thisdefinition. However, the theory does not, unfortunately, recognize saltformation as a criterion of an acid-base reaction, and the matrices of ABcements are conveniently described as salts. It is also uncertain whether itcovers the metal oxide/metal halide or sulphate cements. Bare cations arenot recognized as acids in the Bronsted-Lowry theory, but hydrated

15


cations are. Thus, in the case of the group III elements, the octahedral[M(H2O)6]3+ aquo ions are quite acidic (Cotton & Wilkinson, 1966):

[M(H2O)6]3+ = [M(H2O)5 (OH)]2+ + H+ [2.3]

However, although both zinc and magnesium ions, the cations of the oxy-cements, are hydrated as [M(H2O)6]2+ ions, these hydrated ions hydrolyseonly slightly (Baes & Mesmer, 1976). Thus, in magnesium chloridesolutions the aquo ions, in contrast to beryllium aquo ions, are notperceptibly acidic. So there must be some doubt as to whether thesehydrated ions can be regarded as protonic acids. But for this, theBronsted-Lowry theory would almost exactly define AB cements.

Aluminosilicate glasses are used in certain AB cement formulations,and the acid-base balance in them is important. The Bronsted-Lowrytheory cannot be applied to these aluminosilicate glasses; it does notrecognize silica as an acid, because silica is an aprotic acid. However, formost purposes the Bronsted-Lowry theory is a suitable conceptualframework although not of universal application in AB cement theory.

The solvent system theoryAlthough the protonic theory is not confined to aqueous solutions, it doesnot cover aprotic solvents. The solvent system theory predates that ofBronsted-Lowry and represents an extension of the Arrhenius theory tosolvents other than water. It may be represented by the defining equation:

Acid + Base = Salt + Solvent [2.4]

This theory is associated in its early protonic form with Franklin (1905,1924). Later it was extended by Germann (1925a,b) and then by Cady &Elsey (1922,1928) to a more general form to include aprotic solvents. Cady& Elsey describe an acid as a solute that, either by direct dissociation or byreaction with an ionizing solvent, increases the concentration of the solventcation. In a similar fashion, a base increases the concentration of thesolvent anion. Cady & Elsey, in order to emphasize the importance of thesolvent, modified the above defining equation to:

Acidic solution + Basic solution = Salt + Solvent [2.5]

Thus, acids and bases do not react directly but as solvent cations andanions. Since emphasis is placed upon ionization interactions, inherentacidity and basicity is neglected, as are interactions in the non-ionic state.The theory is a simple extension of the Arrhenius theory and suffers from

16

Acid-base concepts

the same drawbacks. The definition cannot be applied directly to thereaction between a basic solid and acidic liquid characteristic of ABcements.

The Lux-Flood theoryThe Lux-Flood theory relates to oxide melts. Geologists have often usedacid-base concepts for the empirical classification of igneous silicate rocks(Read, 1948). Silica is implicitly assumed to be responsible for acidity, andthe silica content of a rock is used as a measure of its acid-base balance:

Rock typeAcidIntermediateBasicUltra-basic

Silica content (SiO2) %> 6 652-6645-52< 4 5

Lux (1939) developed an acid-base theory for oxide melts where the oxideion plays an analogous but opposite role to that of the hydrogen ion in theBronsted theory. A base is an oxide donor and an acid is an oxide acceptor(Lux, 1939; Flood & Forland, 1947a,b; Flood, Forland & Roald, 1947):

Base = Acid+ O2~ [2.6]

Thus an acid-base reaction involves the transfer of an oxide ion (comparedwith the transfer of a proton in the Bronsted theory) and the theory isparticularly applicable in considering acid-base relationships in oxide,silicate and aluminosilicate glasses. However, we shall find that it issubsumed within the Lewis definition.

The Lewis theoryThis theory was advanced by G. N. Lewis (1916, 1923, 1938) as a moregeneral concept. In his classic monograph of 1923 he considered andrejected both the protonic and solvent system theories as too restrictive. Anacid-base reaction in the Lewis sense means the completion of the stableelectronic configuration of the acceptor atom of the acid by an electronpair from the base. Thus:

A base has the ability to donate a pair of electrons and an acid the ability toaccept a pair of electrons to form a covalent bond. The product of a Lewisacid—base reaction may be called an adduct, a coordination compound or acoordination complex (Vander Werf 1961). Neither salt nor conjugateacid—base formation is a requirement.

17


Although Lewis and Bronsted bases comprise the same species, the same isnot true of their acids. Lewis acids include bare metal cations, whileBronsted-Lowry acids do not. Also, Bell (1973) and Day & Selbin (1969)have pointed out that Bronsted or protonic acids fit awkwardly into theLewis definition. Protonic acids cannot accept an electron pair as isrequired in the Lewis definition, and a typical Lewis protonic acid appearsto be an adduct between a base and the acid H+ (Luder, 1940; Kolthoff,1944). Thus, a protonic acid can only be regarded as a Lewis acid in thesense that its reaction with a base involves the transient formation of anunstable hydrogen bond adduct. For this reason, advocates of the Lewistheory have sometimes termed protonic acids secondary acids (Bell, 1973).This is an unfortunate term for the traditional acids.

Lewis (1938) was not content with a purely conceptual view of acids andbases, for he also listed certain phenomenological criteria for an acid-basereaction. The process of neutralization is a rapid one, an acid or basedisplaces a weaker acid or base from its compounds, acids and bases maybe titrated against each other using coloured indicators, and both acids andbases have catalytic effects.

The Lewis definition covers all AB cements, including the metaloxide/metal oxysalt systems, because the theory recognizes bare cations asaprotic acids. It is also particularly appropriate to the chelate cements,where it is more natural to regard the product of the reaction as acoordination complex rather than a salt. Its disadvantages are that thedefinition is really too broad and that despite this it accommodatesprotonic acids only with difficulty.

The Usanovich theoryThe Usanovich theory is the most general of all acid-base theories.According to Usanovich (1939) any process leading to the formation of asalt is an acid-base reaction. The so-called' positive-negative' definition ofUsanovich runs as follows.

An acid is a species capable of yielding cations, combining with onions orelectrons, or neutralizing a base. Likewise a base is a species capable ofyielding anions or electrons, combining with cations, or neutralizing an acid.

When developed, this theory proved to be more general than the theory ofLewis, for it includes all the above acid-base definitions and also includesoxidation-reduction reactions.

18

Acid—base concepts

It is better than the Lewis theory for describing acid-base cements, forit avoids the awkwardness that the Lewis definition has with protonicacids. However, as Day & Selbin (1969) have observed, the generality ofthe theory is such that it includes nearly all chemical reactions, so thatacid-base reactions could simply be termed 'chemical reactions'.

2.3.4 Relevance of acid-base theories to AB cements

The various acid-base definitions are summarized in the Venn diagram(Fig. 2.1). From this it can be seen that the Usanovich definition subsumesthe Lewis definition, which in turn subsumes all other definitions (i.e.Arrhenius, Bronsted-Lowry, Germann-Cady-Elsey, Lux-Flood).

Also shown is how the topic of AB cements relates to these definitions.An ideal definition for a subject should be one that exactly fits it. It shouldcover all aspects of the subject while excluding all extraneous topics. Thus,a theory should be neither too restrictive nor too general. The Arrheniusand Germann-Cady-Elsey definitions do not relate to the subject at all as

USANOVICH

LEWISElectron-pair acceptor

BR0NSTEDproton-donorany solvent

ARRHENIUSproton-donor

in water

GERMANNsol vent-cation

donor

Figure 2.1 Venn diagram showing the relationship between the various definitions of acidsand bases.

19


the basic component of an AB cement is a powdered solid. TheBronsted-Lowry definition is not broad enough to include all AB cementsand excludes the concept of salt, which is unfortunate since the matrices ofAB cements are salts. Both the Lewis and Usanovich definitions cover allaspects of AB cement theory at the cost of including topics not relevant tothis subject.

From this discussion it can be seen that there is no ideal acid-base theoryfor AB cements and a pragmatic approach has to be adopted. Since thematrix is a salt, an AB cement can be defined quite simply as the productof the reaction of a powder and liquid component to yield a salt-like gel.The Bronsted-Lowry theory suffices to define all the bases and theprotonic acids, and the Lewis theory to define the aprotic acids. The subjectof acid-base balance in aluminosilicate glasses is covered by the Lux-Floodtheory.

2.3.5 Acid-base strength

Ever since the formulation of the Bronsted-Lowry theory, efforts havebeen made to develop a general approach to acid-base strength. Theinfluence of ionic charge and size of the central atom on acidity andbasicity is important. In 1926, Bronsted found that an increase in aciditycorresponded to an increase in positive charge or a decrease in negativecharge on an ion. Cartledge (1928a,b), against the background of theprotonic theory, proposed to correlate acidity or basicity with a functionhe called ionic potential, by considering acids and bases to be hydroxides ofnon-metals and metals, respectively. He defined ionic potential, (/>, as

</> = Z/r (2.1)

where Z is the charge on the central atom and r its ionic radius. Cartledge(1928b) then used values of ^05 to define acidity and basicity of a species.

f5 value>3-22-2-3-2<2-2

Acid-base statusacidicamphotericbasic

Thus, highly charged smaller cations are highly acidic. This point isillustrated for the series Na+, Mg2+, Al3+, Si4+, P5+, S6+ and Cl7+ in Table2.3a.

Note, however, that Zn(OH)2 is not classified as amphoteric as it should

20

Acid-base concepts

Table 2.3a. Effect of cation on acidity-basicity (Cartledge, 1982a,b)

Cation

Na+

Ca2+

Zn2+

Mg2+

Al3+

Si4+

p5+S 6 +

Cl7+

Ionic potential \

1021-421-641-762-453133-834-555-20

Species

NaOHCa(OH)2

Zn(OH)2

Mg(OH)2

A1(OH)3

Si(OH)4

H3PO4

H2SO4

HC1O4

Acidity-basicity

Strong baseWeak baseWeak baseWeak baseAmphotericWeak acidIntermediate acidStrong acidStrongest acid

Table 2.3b. Effect of cation on acidity-basicity

Cation Ionization potential In Species Acidity-basicity

Na+

Ca2+

Mg2+

Cd2+

Zn2+

Cu2+

Bi3+

Al3+

Si4+

p5+

s6+

5-1411-87150316-8417-9620-2025-4228-4545-14650288-05

NaOHCa(OH)2

Mg(OH)2

Cd(OH)2

Zn(OH)2

Cu(OH)2

Bi(OH)3

A1(OH)3

Si(OH)4

H3PO4

H2SO4

Strong baseWeak baseWeak baseAmphotericAmphotericAmphotericAmphotericAmphotericWeak acidIntermediate acidStrong acid

In is the nth ionization potential.

be. Clearly, ionic potential alone is not a sufficient criterion for classi-fication. As will be shown, unlike other cations in Table 2.3a which areclassified as hard acids, Zn2+ is an intermediate because of the presence ofd orbital electrons. The effect of d electrons in increasing the polarizingpower of the cations, because of ineffective screening, has been demon-strated by Hodd & Reader (1976). They found that Cd2+ was a moreeffective cement-former than Ca2+, because although both have a similarionic radius, Ca2+ has no d electrons. For these reasons, ionizationpotential is a better criterion than ionic potential. As Table 2.3b shows,Zn2+ is ranked correctly by this criterion and can be classified as

21


amphoteric. Inspection of this table throws some light on the requirementsfor cement formation. If judged by strength and hydrolytic stability ofcements formed with orthophosphoric acid, poly(acrylic acid) and poly-(vinylphosphonic acid), the common cement-forming cations can beranked in the following order of decreasing effectiveness.

Al3+> Cu2+ > Zn2+ > Mg2+ > Ca2+

The first three form amphoteric oxides and are distinctly superior, ascement-formers, to the latter two which form weakly basic oxides. Datafrom Table 2.3b indicate that optimum cement formation occurs withcations that have In values lying between 18 and 29.

2.3.6 Acid-base classification

The strength of a Lewis acid or base depends on the particular reaction,and for this reason there is no absolute scale for the strengths of Lewisacids and bases. However, certain qualitative features have been observed.Ahrland, Chatt & Davies (1958) divided metal ions (which are Lewisacids), on the basis of the stability of their complexes, into what theytermed class (a) and class (b) acceptors (Table 2.4). They stated that class(a) acceptors form their most stable complexes with ligands of the lightestmember of a non-metal group. By contrast, class (b) acceptors form theirmost stable complexes with heavier members of each group. Thus, complexstability can be ranked according to the ligand as follows. For class (a)acceptors O P S and for class (b) acceptors O < S. Class (a) metal ions aresmall and non-polarizable, whereas class (b) metal ions are large andpolarizable. The class of a given element is not constant and depends onoxidation state; class (a) character increases with increase in the positivecharge. Chatt (1958) considers that the important feature of class (b) acidsis the presence of loosely held outer d orbital electrons which can form n-bonds to certain ligands. These ligands would contain empty d orbitals onthe basic atom; examples are P and As.

In the context of AB cements, Al3+, Mg2+, Ca2+ and Zn2+ are in class (a)while Cu2+ is in the border region. Zn2+ contains a completed 3d shell andforms stronger complexes with O than with S ligands, as do other class (a)cations.

22

Table 2.4. Classification of acceptor atoms in their normal valent states (Ahrland, Chatt & Davies, 1958)

H

Li

Na

K

Rb

Cs

Be

Mp

Ca I

Sr ^

Ba 1

>c

ft

a

Class (a)

Ti V

Zr Nb

Hf Ta

Cr

Mo

W

Mn

Tc

Re

a/b border

Fe

Ru

Os

Co

Rh

Ir

Ni

Pd

Pt

a/b border

Cu

Ag

Au

B

Al

Zn Ga

Cd

Hg

In

TI

C

Si

Ge

Sn

Pb

N

P

As

Sb

Bi

Class

0

s

Se

Te

Po

(a)

F

Cl

Br

I

At

Class (a) a/b border Class (b) a/b border


2.3.7 Hard and soft acids and bases (HSAB)

This concept of Chatt and his coworkers was developed further by Pearson(1963, 1966, 1968a,b) in his theory of hard and soft acids and bases. Hardacids correspond with class (a) acceptors and soft acids with class (b)acceptors.

Hard acids prefer to react with hard bases and soft acids prefer to reactwith soft bases.

Hard acids are characterized by small size, high positive charge andabsence of outer electrons which are easily excited to higher states; they arethus of low polarizability. In this class are the common protonic acids, HA,the hydrogen-bonding molecules in the Lewis scheme and Mg2+, which areall acids of relevance to AB cements. The soft acids have low or zeropositive charge, large size and several easily excited outer electrons (oftend orbital electrons). These properties lead to high polarizability. Thedivision between these two classes is not sharp; amongst the intermediateclass are Zn2+ and Cu2+.

Pearson (1966) defines a soft base as 'one in which the donor atom is ofhigh polarizability and low electronegativity and is easily oxidized orassociated with empty, low-lying orbitals'. A hard base has oppositeproperties. 'The donor atom is of low polarizability and high electro-negativity, is hard to reduce, and is associated with empty orbitals of highenergy.'

The underlying theory for hard-hard and soft-soft preferences isobscure and no one factor is responsible (Pearson, 1966). Pearson (1963,1968b) advanced several explanations. He stated that the ionic-covalenttheory provides the most obvious explanation. Hard acids are assumed tobind bases primarily by ionic forces and soft acids by covalent bonds. Highpositive charge and small size favour strong ionic bonding, and bases oflarge negative charge and small size would be most strongly held. Soft acidsbind to bases by covalent bonding, and the atoms should be of similar sizeand electronegativity for good bonding.

The classification of Lewis acids and bases relevant to AB cements isshown below.

Hard acids: HA, H + , Ca2+ , Mg2 + , Al3+, Si4+

Borderline acids: Zn2+, Cu2+

Hard bases: H2O, O H , F", POJ", SO2", RCOO"

24

Acid-base concepts

Table 2.5. YatsimirskiVs hardness indices {Yatsimirskii, 1970)

Base Indices Acid Indices

OH-F-HPOJ-CH3COO-

sorH2O

6-31-71-70-80-5zero

H+

In3 +

Cu2 +

Zn2 +

La3 +

9-01-2100-201

Extension of HSAB theoryYatsimirskii (1970) attempted to quantify HSAB theory and producedhardness indices (S) for acids and bases. These indices were obtained byplotting the logarithms of the equilibrium constants for the reactions ofbases with the proton (the hardest acid) against similar values for thereactions with CH3Hg+ (one of the softest acids). For acids, the hydroxylion (the hardest base) and the chloride ion (a soft base) were chosen.

These S indices for cations and anions relevant to AB cements are shownin Table 2.5. Bases which add on through F or O and do not form Tr-bondshave similar hardness values; they are hard bases. Soft bases form dative7r-bonds with many cations. They have high-energy-level occupied orbitalswith unshared electron pairs.

Yatsimirskii considered that the hard and soft classification was toogeneral and proposed instead a more detailed approach. He classifiedLewis acids and bases into six groups, based on the nature of the adductbonding.

Group (1) Cations and anions which are incapable of donor-acceptorinteractions. These are the large univalent ions. Bonding is purelyby Coulomb and Madelung electrostatic interactions. From theLewis point of view these are not acids or bases. They have nocement-forming potential.

Group (2) Strong a-acceptor acids and donor bases. Included here areprotonic acids, which are relevant to AB cements. Their adductscan only contain one coordinate bond.

Group (3) G- and n-acceptor acids and donor bases with o-interactionspredominating. In this group acceptors are capable of adding onelectron pairs of donors in both types of interactions. Includescations with stable closed electron shells: Al3+, Mg2+, Ca2+ and

25


Zn2+. Donors are ligands coordinated through oxygen atoms orfluoride ions: RCOO", PO*~, OH", F" and H2O. These acceptorsand donors are of relevance to AB cements.

Group (4) Strong a- and n-acceptor acids and donor bases. Bi3+, In3+

and Sn2+ are of some relevance to AB cements.Group (5) Acids that are o-acceptors but capable of n-donation in

backbonding. This group includes cations with mobile d electronse.g. Cuw+, Cow+, Few+.

Group (6) Bases that are a-donors but n-acceptors.

According to Yatsimirskii, group (2) and (3) species are equivalent toPearson's hard acids and bases, and group (4), (5) and (6) speciescorrespond to Pearson's soft acids and bases. This classification is of morevalue than HSAB theory to our subject. It can be seen that all cement-forming anions come from group (3) and cations from groups (3), (4) and(5). Thus, the bonding in cement matrices formed from cation-anioncombinations is not purely a but contains some n character.

References

Ahrland, S., Chatt, J. & Davies, N. R. (1958). The relative affinities of ligandatoms for acceptor molecules and ions. Quarterly Reviews, 12, 265-76.

Baes, C. F. & Mesmer, R. E. (1976). The Hydrolysis of Cations. New York:John Wiley.

Bell, R. P. (1947). The use of the terms 'acid' and 'base'. Quarterly Reviews, 1,113-25.

Bell, R. P. (1973). The Proton in Chemistry. Ithaca, New York: CornellUniversity Press.

Bjerrum, J. (1951). Die Entwickhmgsgeschichte des Saure-Basenbegriffes undiiber die ZweckmaBigkeit der Einfuhrung eines besonderen Antibasenbegriffesneben dem Saurebegriff. Naturwissenschaften, 38, 461-4.

Boyle, R. (1661). The Sceptical Chymist. Everyman Library Edition, 1911.Brensted, J. N. (1923). Einige Bemerkungen iiber den Begriff der Sauren und

Basen. Recueil des Travaux chimiques des Pays-Bas et de la Belgique, 42,718-28.

Bronsted, J. N. (1926). The acid-base function of molecules and its dependencyon the electronic charge type. Journal of Physical Chemistry, 30, 777-90.

Bungenberg de Jong, H. G. (1949). In Kruyt, H. R. (ed.) Colloid Science II, p. 2.Amsterdam: Elsevier Publishing Co. Inc.

Cady, H. P. & Elsey, H. M. (1922). A general conception of acids, bases andsalts. Science, 56, 27 (Lecture abstract).

Cady, H. P. & Elsey, H. M. (1928). A general definition of acids, bases andsalts. Journal of Chemical Education, 5, 1425-8.

26

References

Cartledge, G. H. (1928a). Studies on the periodic system. I. The ionic potentialas a periodic function. Journal of the American Chemical Society, 50, 2855-63.

Cartledge, G. H. (1928b). Studies on the periodic system. II. The ionic potentialand related properties. Journal of the American Chemical Society, 50, 2863-72.

Chatt, J. (1958). The stabilisation of low valent states of the transition metals.Journal of Inorganic & Nuclear Chemistry, 8, 515-31.

Cotton, F. A. & Wilkinson, G. (1966). Advanced Inorganic Chemistry, 2nd edn.New York, London & Sydney: Wiley Inter science.

Crisp, S., O'Neill, I. K., Prosser, H. J., Stuart, B. & Wilson, A. D. (1978).Infrared spectroscopic studies on the development of crystallinity in dentalzinc phosphate cements. Journal of Dental Research, 57, 245-54.

Crosland, M. P. (1962). Historical Studies in the Language of Chemistry.London: Heinemann.

Crosland, M. (1973). Lavoisier's theory of acidity. Isis, 64, 306-25.Day, M. C. & Selbin, J. (1969). Theoretical Inorganic Chemistry. New York:

Reinhold.Finston, H. L. & Rychtman, A. C. (1982). A New View of Current Acid-Base

Theories. New York: John Wiley & Sons.Flood, H. & Forland, T. (1947a). The acidic and basic properties of oxides.

Ada Chemica Scandinavica, 1, 592—604.Flood, H. & Forland, T. (1947b). The acidic and basic properties of oxides. II.

The thermal decomposition of pyrosulphates. Acta Chemica Scandinavica, 1,781-9.

Flood, H., Forland, T. & Roald, B. (1947). The acidic and basic properties ofoxides. III. Relative acid-base strengths of some polyacids. Acta ChemicaScandinavica, 1, 790-8.

Flory, P. J. (1953). Principles of Polymer Chemistry, Chapter 11. Ithaca, NewYork: Cornell University Press.

Flory, P. J. (1974). Introductory lecture. In Gels and Gelling Processes. FaradayDiscussions of the Chemical Society, No. 57, pp. 7-18.

Franklin, E. C. (1905). Reactions in liquid ammonia. Journal of the AmericanChemical Society, 27, 820-51.

Franklin, E. C. (1924). Systems of acids, bases and salts. Journal of theAmerican Chemical Society, 46, 2137-51.

Germann, A. F. O. (1925a). What is an acid? Science, 61, 71.Germann, A. F. O. (1925b). A general theory of solvent systems. Journal of the

American Chemical Society, 47, 2461-8.Hall, N. F. (1940). Systems of acids and bases. Journal of Chemical Education,

17, 124^8.Hodd, K. A. & Reader, A. L. (1976). The formation and hydrolytic stability of

metal ion-polyacid gels. British Polymer Journal, 8, 131-9.Jensen, W. B. (1978). The Lewis acid-base definitions: a status report. Chemical

Reviews, 78, 1-22.Kingery, W. D. (1950a). Fundamental study of phosphate bonding in

refractories. I. Literature review. Journal of the American Ceramic Society, 33,239-41.

27


Kingery, W. D. (1950b). Fundamental study of phosphate bonding inrefractories. II. Cold setting properties. Journal of the American CeramicSociety, 33, 242-7.

Kolthoff, I. M. (1944). The Lewis and Bronsted-Lowry definitions of acids andbases. Journal of Physical Chemistry, 48, 51-7.

Lewis, G. N. (1916). The atom and the molecule. Journal of the AmericanChemical Society, 38, 762-85.

Lewis, G. N. (1923). Valence and the Structure of Atoms and Molecules. NewYork: Chemical Catalog Co.

Lewis, G. N. (1938). Acids and bases. Journal of the Franklin Institute, 226,293-337.

Lowry, T. M. (1923a). The uniqueness of hydrogen. Chemistry & Industry, 42,43.

Lowry, T. M. (1923b). Co-ordination and acidity. Chemistry & Industry, 42,1048-52.

Luder, W. F. (1940). The electronic theory of acids and bases. ChemicalReviews, 27, 547-83.

Luder, W. F. (1948). Contemporary acid-base theory. Journal of ChemicalEducation, 25, 555-8.

Lux, H. (1939). 'Sauren' und 'Basen' im Schelzfluss: Die Bestimmung derSauerstoffionen-Konzentration. Zeitschrift fur Elektrochemie, 45, 303-9.

Pattison Muir, M. M. (1883). Heroes of Science-Chemists, Chapter IV, pp.171-89. London: Society for Promoting Christian Knowledge.

Pauling, L. (1945). The Nature of the Chemical Bond. Ithaca, New York:Cornell University Press.

Pearson, R. G. (1963). Hard and soft acids and bases. Journal of the AmericanChemical Society, 85, 3533-9.

Pearson, R. G. (1966). Acids and bases. Science, 151, 172-7.Pearson, R. G. (1968a). Hard and soft acids and bases, HSAB. Part I.

Fundamental principles. Journal of Chemical Education, 45, 581-7.Pearson, R. G. (1968b). Hard and soft acids and bases, HSAB. Part II.

Underlying theories. Journal of Chemical Education, 45, 643-8.Read, H. H. (1948). Rutle/s Elements of Mineralogy, 24th edn. London:

Thomas Murby & Co.Smith, D. C. (1968). A new dental cement. British Dental Journal, 125, 381-4.Steinke, R., Newcomer, P., Komarneni, S. & Roy, R. (1988). Dental cements:

investigation of chemical bonding. Materials Research Bulletin, 23, 13-22.Usanovich, M. I. (1939). On acids and bases. Journal of General Chemistry

(USSR), 9, 182-92.Vander Werf, A. (1961). Acids, Bases, and the Chemistry of the Covalent Bond.

New York: Reinhold.Walden, P. (1929). Salts, Acids and Bases: Electrolytes, Stereochemistry. New

York: McGraw-Hill.Wilson, A. D. (1968). Dental silicate cements: VII. Alternative liquid cement

formers. Journal of Dental Research, 47, 1133-6.Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement: a new

28

References

translucent cement for dentistry. Journal of Applied Chemistry andBiotechnology, 21, 313.

Wilson, A. D., Paddon, J. M. & Crisp, S. (1979). The hydration of dentalcements. Journal of Dental Research, 58, 1065-71.

Wygant, J. F. (1958). Cementitious bonding in ceramic fabrication. In Kingery,W. D. (ed.) Ceramic Fabrication Processes, pp. 171-88. New York: JohnWiley & Sons.

Yatsimirskii, K. (1970). Acid-base and donor-acceptor properties of ions andmolecules. Theoretical and Experimental Chemistry (USSR), 6, 376-80.

29

3 Water and acid-base cements

3.1 Introduction

The setting reaction for the great majority of acid-base cements takes placein water. (The exceptions based on o-phenols are described in Chapter 9.)This reaction does not usually proceed with formation of a precipitate butrather yields a substance which entrains all of the water used to prepare theoriginal cement paste. Water thus acts as both solvent and component inthe formation of these cements. It is also one of the reaction products,being formed in the acid-base reaction as the cements set.

3.1.1 Water as a solvent

It is widely recognized that the solvent in which any chemical reactiontakes place is not merely a passive medium in which relevant moleculesperform: the solvent itself makes an essential contribution to the reaction.The character of the solvent will determine which chemical species aresoluble enough to enter solution and hence to react, and which species areinsoluble, and thus precipitate out of solution, thereby being preventedfrom undergoing further chemical change. In the case of water, as will beseen, polar and ionic species are the ones that most readily dissolve. Buteven so, mere polarity or ionic character is not sufficient to ensuresolubility. Solubility depends on a number of subtle energetic factors, andthe possible interactions between water and silver chloride, for example, donot fulfil the requirements despite the ionic nature of the silver salt. Hencesilver chloride is almost completely insoluble in water.

3.1.2 Water as a component

In AB cements water does not merely act as solvent for the setting reaction.It also acts as an important component of the set cement. For example,

30

Water

glass-ionomer dental cements as generally formulated include at least15% by mass of water, all of which becomes incorporated into thecomplete cement (Wilson & McLean, 1988). Indeed, great importance isattached to the retention of water by these cements, since if they areallowed to dry out by storage under conditions of low humidity, theyshrink significantly, and develop cracks and crazes.

Another class of AB cement, the oxychloride cements of zinc andmagnesium, are also formulated in aqueous solution and retain substantialamounts of water on setting (Sorrell & Armstrong, 1976; Sorrell, 1977).

Water may have a number of roles in the set versions of these cements.It is capable of solvating the cement-forming ions, such as Ca2+ or Zn2+,depending on the cement. It also contributes a sheath of solvatingmolecules around polyelectrolytes such as poly(acrylic acid) in glass-ionomer and zinc polycarboxylate cements. Significant amounts of waterare known to be retained by metal polyacrylate salts at equilibrium andthis water contributes to reducing the glass transition temperature of suchmaterials by acting as a plasticizer (Yokoyama & Hiraoko, 1979).

These various aspects of water in AB cements are covered in the presentchapter. Its solvent character, structure and hydration behaviour aredescribed, and the chapter concludes with a more thorough considerationof the precise role of water in the various AB cements.

3.2 Water3.2.1 Constitution

Water has a deceptively simple chemical constitution, consisting as it doesof molecules containing two atoms of hydrogen and one of oxygen. It wasviewed by the ancients as one of the four 'elements', following Aristotle'sclassification, the others being air, fire and earth. The modern view that itis a compound composed of hydrogen and oxygen was first established in1789 by two amateur chemists, Adriaan Paets van Troostwijk (1752-1837),a merchant, and Jan Rudolph Deiman (1743-1808), a pharmacist (Hall,1985). They were able to show by synthesis which elements combine tomake water, forming it from reaction of hydrogen gas with oxygen. Theirwork was important historically for the part it played in undermining thephlogiston theory of combustion. It was left to the great Swedish chemistJ. J. Berzelius (1779-1848) to determine that the ratio of hydrogen tooxygen is 2:1.

31

Water and acid-base cements

Table 3.1. Molecular dimensions of normal and isotopic water inthe vapour phase {Benedict, Gailar & Plyler, 1956)

Bond length, Bond angle,Molecule pm degrees

D2O 95-75 104-474H2O 95-718 104-523HDO 95-71 104-529

As a compound water is remarkable. It is the only inorganic liquid tooccur naturally on earth, and it is the only substance found in nature in allthree physical states, solid, liquid and vapour (Franks, 1983). It is the mostreadily available solvent and plays a vital role in the continuation of life onearth. Water circulates continuously in the environment by evaporationfrom the hydrosphere and subsequent precipitation from the atmosphere.This overall process is known as the hydrologic cycle. Reports estimatethat the atmosphere contains about 6 x 1015 litres of water, and this iscycled some 37 times a year to give an annual total precipitation of224 x 1015 litres (Franks, 1983; Nicholson, 1985).

The bond lengths and bond angle for the water molecule are knownvery precisely following studies of the rotation-vibration spectra of watervapour, and also the vapour of the deuterated analogues of water, D2 Oand HDO (Eisenberg & Kauzmann, 1969). The data for these compoundsare shown in Table 3.1. The nuclei of the water molecule, regardless of theisotopes involved, form an isosceles triangle having a slightly obtuse angleat the oxygen atom. All of the data in Table 3.1 refer to the equilibriumstate of the water molecules, which is formally acceptable, but is actuallya hypothetical state, since it assumes neither rotation nor vibration in themolecule.

The equilibrium bond lengths and bond angles can be seen to differ littlebetween the different isotopic molecules. Such a finding agrees with thepredictions of the Born-Oppenheimer approximation, that the electronicstructure of a molecule is independent of the mass of its nuclei, it being theelectronic structure of a molecule alone which determines the geometry.

The bond angle in water is slightly less than the ideal tetrahedral angleof 109-5°. This is attributed to the presence of lone pairs of electrons on theoxygen atom which repel more strongly than the bonding pairs of electronsbetween the oxygen and hydrogen nuclei (Speakman, 1975). The valence-

32

Water

Table 3.2. Properties of hydrides of first row elements (Weast, 1985-6)

Compound

CH4

NH3

H2OHF

Relativemolar mass

16171819

Melting point,°C

-182-0-11-1

00- 8 3 4

Boiling point,°C

- 1 6 4 0-33-4100019-5

Gasphasedipolemoment,debyes

0001-471-851-82

shell electron-pair repulsion concepts of Gillespie & Nyholm (1957) showthat such increased repulsion by lone pairs closes the angle between thebonding pairs slightly but significantly for the water molecule.

The O-H bond energy of water is taken as half the energy of formationof the molecule, since water has two such bonds. This gives a value of458-55 kJ mol"1 at 0 K (Eisenberg & Kauzmann, 1969). Related to thebond energy is the dissociation energy, i.e. the energy required to break thebond at 0 K. Neither of the O-H bonds in water has a dissociation energyequal to the O-H bond energy. Instead, the first O-H dissociation energyhas been found experimentally to be 424-27 kJ mol"1. From conservationof energy considerations which lead to the requirement that the sum of thetwo dissociation energies must equal the energy of formation, it is foundthat the second O-H dissociation energy has to take a value of492-83 kJ mol"1. This has been explained (Pauling, 1960) by postulating anelectronic rearrangement on the oxygen atom of the O-H fragment leftbehind after scission of the first O-H bond, and that breaking the bondbetween oxygen in this new electronic configuration and the remaininghydrogen requires greater energy.

3.2.2 Water compared with other hydrides

Water shows properties that are interestingly different compared withhydrides of the neighbouring elements of the first row of the periodic table.Some of these properties are given in Table 3.2. From this table, water canbe seen to have a very high melting point and a very high boiling point forits relative molar mass. Indeed, it is the only one of the hydrides of the

33


elements from this portion of the periodic table to be liquid at roomtemperature and atmospheric pressure. In the gas phase it has a dipolemoment that, while only slightly greater than that of hydrogen fluoride, isthe highest for this group of hydrides. All of these properties point to waterhaving a structure in which its constituent molecules are more highlyassociated and interact more strongly than the molecules of the closelyrelated hydrides.

3.3 The structure of water

At first sight the concept of a 'structure' for liquid water appears strange.In the solid state atoms are relatively fixed in space, albeit with somevibrational motion about equilibrium positions, and no difficulty isassociated with the idea of locating these equilibrium positions by someappropriate physical technique, and thereby assigning a structure to thesolid.

3.3.1 The concept of structure in the liquid state

With water or any other liquid, molecules do not occupy even reasonablyfixed locations but have considerably more freedom for movement than inthe solid state. What then do we mean by the term structure applied to aliquid?

To answer this question we need to consider the kind of physicaltechniques that are used to study the solid state. The main ones are basedon diffraction, which may be of electrons, neutrons or X-rays (Moore,1972; Franks, 1983). In all cases exposure of a crystalline solid to a beamof the particular type gives rise to a well-defined diffraction pattern, whichby appropriate mathematical techniques can be interpreted to giveinformation about the structure of the solid. When a liquid such as wateris exposed to X-rays, electrons or neutrons, diffraction patterns areproduced, though they have much less regularity and detail; it is also moredifficult to interpret them than for solids. Such results are taken to showthat liquids do, in fact, have some kind of long-range order which canjustifiably be referred to as a 'structure'.

In considering the structure of a liquid, two possible conceptualapproaches exist. One is to begin from an understanding of the gaseous

34

The structure of water

state, characterized as it is by gross translational movement of theconstituent molecules and substantial disorder. The liquid is then viewed asa gas that has been condensed and in which translational motion hasbecome constrained. Alternatively, consideration can start from the solidstate, with its well-characterized structure, having little or no translationalmotion, but some vibrational motion of the constituent atoms ormolecules. The liquid state is then viewed as a solid in which some degreeof translational motion has become allowed, but with a structure stillrecognizable as being derived from that existing in the solid state (Franks,1983). With the growth in application of the techniques of X-ray andneutron diffraction to the study of the liquid state, the latter approach hasbecome increasingly favoured in recent years.

In this section, rather than give a detailed account of theories of theliquid state, a more qualitative approach is adopted. What follows includesfirst a description of the structure of ice; then from that starting-point,ideas concerning the structure of liquid water are explained.

3.3.2 The structures of ice

Water is capable of solidifying into a number of different structural statesor polymorphs depending, for example, on the external pressure appliedduring solidification. The simplest and most common of these polymorphsis known as ice I, whose structure was first determined by W. H. Bragg(1922). In this structure, every oxygen atom occupies the centre of atetrahedron formed by four oxygen atoms, each about 0-276 nm away. Thewater molecules are connected together by hydrogen bonds, each moleculebeing bonded to its four nearest neighbours. The O-H bonds of a givenmolecule are oriented towards the lone pairs on two of these neighbouringmolecules, and in turn, each of its lone pairs is directed towards an O-Hbond of one of the other neighbours. This arrangement gives an openlattice in which intermolecular cohesion is large.

The arrangement of oxygen atoms in ice I is isomorphous with thewurtzite form of zinc sulphide, and also with the silicon atoms in thetridymite form of silicon dioxide. Hence, ice I is sometimes referred to asthe wurtzite or tridymite form of ice (Eisenberg & Kauzmann, 1969).

Location of the hydrogen atoms in ice I has caused more problems. Thisis because hydrogen is less effective at scattering X-rays or electrons thanoxygen. For a long time, arguments about the position of hydrogen werebased on indirect evidence, such as vibrational spectra or estimates of

35


residual entropy at 0 K (Eisenberg & Kauzmann, 1969). Since the adventof neutron diffraction the positions of the hydrogen atoms have becomeclearer. These studies have shown that the water molecules have verysimilar dimensions in ice I to those in the isolated molecule: the O-H bondlength is 0-101 nm and the bond angle 104*5°.

Ice I is one of at least nine polymorphic forms of ice. Ices II to VIIare crystalline modifications of various types, formed at high pressures;ice VIII is a low-temperature modification of ice VII. Many of thesepolymorphs exist metastably at liquid nitrogen temperature and atmos-pheric pressure, and hence it has been possible to study their structureswithout undue difficulty. In addition to these crystalline polymorphs, so-called vitreous ice has been found within the low-temperature field of ice I.It is not a polymorph, however, since it is a glass, i.e. a highly supercooledliquid. It is formed when water vapour condenses on surfaces cooled tobelow -160°C.

It is not appropriate in this chapter to give a detailed review of the solid-state behaviour of water in its various crystalline modifications. However,there are some general structures which are relevant and worth high-lighting. Firstly, water molecules in these various solids have dimensionsand bond angles which do not differ much from those of an isolated watermolecule. Secondly, the number of nearest neighbours to which eachindividual molecule is hydrogen-bonded remains four, regardless of the icepolymorph.

The differences in structure between the polymorphs, particularly thehigh-pressure ones, lie in (a) the distances between the non-hydrogenbonded molecules, and hence the amount of' free volume' in the structure,(b) the angles of the hydrogen bonds, which may differ markedly from the180° of ice I, and (c) the distance between nearest neighbouring oxygenatoms, which may fall to well below the 0-276 nm value in ice I. All of theseare consistent with closer packing of the water molecules, and a closing upof the cage structure by comparison with that found for ice I.

3.3.3 Liquid water

Before considering the details of the structure of liquid water, it isimportant to define precisely what is meant by the term structure as appliedto this liquid. If we start from ice I, in which molecules are vibrating aboutmean positions in a lattice, and apply heat, the molecules vibrate withgreater energy. Gradually they become free to move from their original

36


lattice sites and acquire significant translational energy. However, trans-lational energy is not confined to molecules in the liquid state. There is afinite possibility of any molecule in ice I moving from its lattice site, thusacquiring translational energy. In principle, a given molecule can movethrough the solid structure in a process that is essentially diffusion.

From this model of ice I we derive three meanings of the term structurefor the solid. We may refer to the positions of the molecules at an instantof time. We may allow some averaging of the positions, i.e. we may havea vibrationally averaged structure, considered over a short time-period,during which molecules have time to undergo only minor vibrationalreorientations. Finally we may have a diffusionally averaged structure,considered over longer time-periods, in which the minor translationalmotion has been allowed to proceed to such an extent as to be significant.These three possible structures, the instantaneous, the vibrationallyaveraged and the diffusionally averaged, are referred to as I-, V- and D-structures respectively.

Let us now turn our attention to liquid water. Just as in ice I, molecularmotions may be divided into rapid vibrations and slower diffusionalmotions. In the liquid, however, vibrations are not centred on essentiallyfixed lattice sites, but around temporary equilibrium positions that arethemselves subject to movement. Water at any instant may thus beconsidered to have an I-structure. An instant later, this I-structure will bemodified as a result of vibrations, but not by any additional displacementsof the molecules. This, together with the first I-structure, is one of thestructures that may be averaged to allow for vibration, thereby con-tributing to the V-structure. Lastly, if we consider the structure around anindividual water molecule over a long time-period, and realize that there isalways some order in the arrangement of adjacent molecules in a liquideven over a reasonable duration, then we have the diffusionally averagedD-structure.

No experimental technique exists for determining I-structures in eitherthe liquid or the solid state. Techniques do exist for obtaining informationon both the V- and D-structures of liquid water; the results of applyingthese techniques are considered next.

Spectroscopic studies have established that for liquid water, the V-structure has the following features.

(a) Considerable local variation between the environments of theindividual water molecules, compared with the relatively uniform

37


molecular environments in a crystal of ice I. The frequency spansof the uncoupled O-H and O-D spectral bands indicate that somenearest neighbours are as close as 0-275 nm, while others areseparated by 0.310 nm or more. The most probable equilibriumseparation is about 0.285 nm (Eisenberg & Kauzmann, 1969).

(b) The differences between the various molecular environments arecontinuous. In other words, the V-structure does not containdiscrete types of molecular environment.

(c) The frequency of the stretching band indicates that hydrogenbonds in the V-structure are weaker than those in ice I, though stilldistinctly present.

Ideas about the D-structure have come mainly from two sources, namelya consideration of the underlying reasons for the values of certain physicalproperties, such as heat capacity or compressibility, and a study of radialdistribution functions that arise from X-ray diffraction work on liquidwater. The D-structure represents the average arrangement of moleculesaround an arbitrary central water molecule. This average is either the'space average' for several central molecules in different V-structures, orthe 'time average' for a single molecule over very long periods of time.

Near the freezing point, the D-structure is found to have relatively highconcentrations of neighbours at distances 0-29, 0-50 and 0*70 nm from thecentral water molecule. This suggests that a substantial hydrogen-bondednetwork is discernible, even in the liquid state. As the temperature is raised,so the distinct concentrations at 0-50 and 0-70 nm disappear. Thermalagitation thus distorts or destroys the hydrogen-bonded networks, and theamount of observable long-range order decreases significantly.

Structural studies on liquid water reveal that the majority of moleculesare effectively tetrahedral, since the O-H bonds and the lone pairs are usedin hydrogen-bonding. Questions remain about the nature of thesehydrogen-bonds (Symons, 1989). Specifically: on average, how many suchhydrogen bonds are formed per molecule, how strong and how linear arethey, and what is their lifetime? One recent approach has been to considerthe possibility that, because of their weakness, some of the hydrogen bondsin liquid water will break. This then gives concentrations of free O-Hbonds, OHfree, and free lone pairs, LPfree, on certain molecules which arebonded to only three others (Symons, 1989). Symons (1989) also suggeststhat the chemical properties of liquid water depend on the relativeconcentrations of these species. Fully hydrogen-bonded water can be

38


considered as inert; reactions requiring attack by O-H depend on theconcentration of OHfree molecules; and those requiring nucleophilic attackby lone pairs depend on the concentration of LPfree molecules. Evidencefor OHfree and LPfree molecules has been obtained spectroscopically usingmonomeric deuterated water, HOD, in inert solvents such as dimethylsulphoxide, though debate continues over interpretation of the resultsobtained in such studies.

An important phenomenon when considering the differences betweenice I and liquid water is that water achieves its maximum density not in thesolid state, but at 4 °C, i.e. in the liquid state. The reasons for this were firstdiscussed by Bernal & Fowler (1933). They noted that the separation ofmolecules in ice I is about 0-28 nm, corresponding to an effective molecularradius of 014 nm. Close packing of molecules of such radius would yielda substance of density 1*84 g cm"3. To account for the observed density of10 g cm"3, it was necessary to postulate that the arrangement of moleculeswas very open compared with the disordered, close-packed structures ofsimple liquids such as argon and neon.

The increase in density on melting is assumed to arise from twocompeting effects that occur as water is heated. First, increasing trans-lational freedom for the water molecules weakens the hydrogen-bondednetwork that exists in ice I. This network thus collapses, and reduces thevolume. Second, increased vibrational energy for the molecules causes aneffective increase in the volume occupied by any one molecule, thusenlarging the overall volume of the liquid. The first effect is considered topredominate below 4 °C, the second above 4 °C.

Overall, the main conclusions that are to be drawn concerning thestructure of liquid water are as follows.

(a) Water has a degree of long-range order that is appropriatelydescribed as structure; it is possible to measure detailed para-meters for either a vibrationally averaged or a diffusionallyaveraged structure.

(b) The force between molecules that sustains this order in the liquidstate is the hydrogen bond.

(c) The bond lengths and angles of individual water molecules arealmost independent of whether they occur in ice I or liquid water.

39


3.4 Water as a solvent

The general criterion for solubility is the rule that 'like dissolves like'. Inother words polar solvents dissolve polar and ionic solutes, non-polarsolvents dissolve non-polar solutes. In the case of water, this means thationic compounds such as sodium chloride and polar compounds such assucrose are soluble, but non-polar compounds such as paraffin wax arenot.

In general, solubility depends on the relative magnitudes of three pairsof interactions, namely solute-solute, solvent-solvent and solute-solvent(Robb, 1983). For a substance to be soluble in a given liquid, thesolute-solvent interactions must be greater than or equal to the other twointeractions.

Insolubility does not only result from the kind of energetic consider-ations outlined above. It can also be the result of essentially kineticbarriers. For example, the naturally occurring macromolecule cellulose isnot soluble in water, yet its monomer, D( + )-glucose is extremely water-soluble (Morrison & Boyd, 1973). This is because cellulose adopts a well-ordered structure, in which individual hydroxyl groups are aligned viahydrogen bonds; the overall structure simply has too great an integrity toallow water molecules to enter and hydrate the individual molecules inorder to carry them off into solution.

3.4.1 Hydrophobic interactions

The qualitative discussion of solubility has focussed so far on the attractiveforces in solute-solvent interactions. However, where water is concerned,it is also important to consider the forces of repulsion due to the so-called'hydrophobic' interactions that may arise in certain cases (Franks,1975). These hydrophobic interactions may be explained in terms ofthermodynamic concepts.

Measuring enthalpy changes for the dissolution of hydrocarbons, suchas alkanes, in water shows that heat is evolved, i.e., AH is negative andenergetically water and alkanes attract each other. However, suchattraction does not make alkanes soluble in water to any appreciableextent. This is because the free energy change AGsolution opposes theprocess and is positive.

From the Gibbs equation,

Absolution — Absolution 1 Absolution

40

Water as a solvent

it follows that the ^ASsolution term (and hence A*Ssolution itself) must benegative. This means that the proposed solution has lower entropy and ismore ordered than pure water, which is a striking conclusion, since entropyis usually increased by mixing. It occurs because the relatively orderedstructure of the liquid water, based as it is on a hydrogen-bonded array ofwater molecules, actually becomes more ordered when alkane moleculesenter it. This result is attributed to the formation of a 'cage' structure ofwater molecules around the non-polar alkane molecule, in which water hasless vibrational and translational freedom than in the pure liquid (Franks,1983).

In cases where the solvation energies are large, as for example when ioniccompounds dissolve in water, these hydrophobic effects, based on adversechanges in entropy, are swamped. Dissolving such compounds can bereadily accomplished due to the very large energies released when the ionsbecome hydrated.

3.4.2 Dissolution of salts

Salts dissolve in water with dissociation of the constituent ions, thisconcept having been proposed originally by S. Arrhenius in 1887. His firstidea was that all salts, including those of what would now be regarded asweak acids or bases, are completely dissociated at extreme dilution (Hall,1985). It was eventually realized that substances such as NaCl, KC1, etc,are effectively completely dissociated at all concentrations.

Dissolution of an ionic salt is essentially a separation process carried outby the interaction of the salt with water molecules. The separation isrelatively easy in water because of its high dielectric constant. Comparisonof the energies needed to separate ions of NaCl from 0-2 nm to infinityshows that it takes 692-89 kJ mol"1 in vacuum, but only 8-82 kJ mol"1 inaqueous solution (Moore, 1972). Similar arguments have been used to tryto estimate solvation energies of ions in aqueous solution, but there aredifficulties caused by the variations in dielectric constant in the immediatevicinity of individual ions.

In order to dissolve ionic solutes so readily, water molecules mustsolvate the ions as they enter solution. Consequently, water molecules losetheir translational degrees of freedom as a result of their association withspecific ions. It is possible to estimate the number of water molecules inclusters of the type M+ (H2O)W using mass spectrometry (Kebarle, 1977).

41


The number of water molecules in such a cluster, the hydration number,varies with ionic size; it is four for Li+, three for Na+, but only one for Rb+.

Mass spectrometry has been used to study the energetics of solvationand has shown that the enthalpies of attachment of successive watermolecules to either alkali metal or halide ions become less exothermic asthe number of water molecules increases (Kebarle, 1977). The Gibbs freeenergies of attachment for water molecules have also been found to benegative.

The different hydration numbers can have important effects on thesolution behaviour of ions. For example, the sodium ion in ionic crystalshas a mean radius of 0-095 nm, whereas the potassium ion has a meanradius of 0133 nm. In aqueous solution, these relative sizes are reversed,since the three water molecules clustered around the Na+ ion give it aradius of 0-24 nm, while the two water molecules around K+ give it a radiusof only 0-17 nm (Moore, 1972). The presence of ions dissolved in wateralters the translational freedom of certain molecules and has the effect ofconsiderably modifying both the properties and structure of water in thesesolutions (Robinson & Stokes, 1955).

The precise orientation of water molecules around cations is not clear,though two models have been proposed for the possible structures thatoccur (Vaslow, 1963). In one, the water molecules are arranged so that thedipole moments are aligned with the centres of the ions. In the other, watermolecules are arranged so that interaction between the lone-pair orbitalson the oxygen atoms and orbitals on the cation is maximized. This lattermodel is supported by molecular dynamics calculations (Heinzinger &Palinkas, 1987; Heinje, Luck & Heinzinger, 1987).

Less uncertainty surrounds the structure of hydrated anions: thehydrogen atoms are almost collinear with the oxygen atoms and thecentres of the ions (Briant & Burton, 1976). Monte-Carlo calculations haveshown that F~ is surrounded by four hydrogen atoms each 017 nm away(Watts, Clementi & Fromm, 1974).

Neutron scattering has been used for studying the state of solvation ofions in aqueous solution (Enderby et ai, 1987; Salmon, Neilson &Enderby, 1988). These studies have shown that a distinct shell of watermolecules of characteristic size surrounds each ion in solution. Thisimmediate hydration shell was called zone A by Frank & Wen (1957); theyalso postulated the existence of a zone B, an outer sphere of molecules, lessfirmly attached, but forming part of the hydration layer around a givenion. The evidence for the existence of zone B lies in the thermodynamics of

42

Water as a solvent

the hydration process, and may be appreciated by considering theisoelectronic species KCl and two moles of argon.

The standard enthalpy of hydration is significantly more exothermic forKCl than for the two moles of argon; however, the corresponding entropyof hydration is less for KCl than for argon. The results for the values ofenthalpy can be readily understood in terms of the greater intensity of theinteractions between water molecules and the ions of KCl than betweenthose of water molecules and the uncharged argon atoms. At first sight thegreater loss of freedom in the water molecules involved in hydrating theions of KCl would be expected to reduce disorder in such solutions. Inother words the entropy of hydration for KCl ought to be greater than theentropy of hydration for two moles of argon. To explain the fact that theopposite is found experimentally, Frank & Evans (1945) suggested thatthere is a compensating gain in entropy which can be attributed todisruptions in the water-water interactions within zone B.

As a result of these electrostatic effects aqueous solutions of electrolytesbehave in a way that is non-ideal. This non-ideality has been accounted forsuccessfully in dilute solutions by application of the Debye-Huckel theory,which introduces the concept of ionic activity. The Debye-Huckel limitinglaw states that the mean ionic activity coefficient y± can be related to thecharges on the ions, z+ and z_, by the equation

Iog10y±=-0-509z+z_

Ionic activity essentially represents the concentration of a particulartype of ion in aqueous solution and is important in the accurateformulation of thermodynamic equations relating to aqueous solutions ofelectrolytes (Barrow, 1979). It replaces concentration because a given iontends not to behave as a discrete entity but to gather a diffuse group ofoppositely charged ions around it, a so-called ionic atmosphere. Thismeans that the effective concentration of the original ion is less than itsactual concentration, a fact which is reflected in the magnitude of the ionicactivity coefficient.

Debye-Hiickel theory assumes complete dissociation of electrolytes intosolvated ions, and attributes ionic atmosphere formation to long-rangephysical forces of electrostatic attraction. The theory is adequate fordescribing the behaviour of strong 1:1 electrolytes in dilute aqueoussolution but breaks down at higher concentrations. This is due to achemical effect, namely that short-range electrostatic attraction occurs

43


either between ion-pairs or between solvent-separated ion-pairs (Russo &Hanania, 1989); this effect becomes important in concentrated aqueoussolutions of the type used to form AB cements.

3.4.3 Ion-ion interactions in water

Two ions are particularly important in the chemistry of water, namely H+

and OH~ (Clever, 1963). Hydrogen ions do not exist as discrete entities.This is because ionization of the hydrogen atom leaves behind a proton,which is very small compared with a typical ion. Thus the local chargedensity developed around the proton is very high. The polarizing effect ofsuch a high charge density is such that the resulting system is simply toounstable to form to any detectable extent in aqueous solutions.

When water undergoes self-ionization, a range of cationic species areformed, the simplest of which is the hydronium ion, H3O+ (Clever, 1963).This ion has been detected experimentally by a range of techniquesincluding mass spectrometry (Cunningham, Payzant & Kebarle, 1972), ashave ions of the type H+ (H2O)W with values of n up to 8. Monte-Carlocalculations show that H3O+ ions exist in hydrated clusters surrounded bythree or four water molecules in the hydration shell (Kochanski, 1985).These ions have only a short lifetime, since the proton is highly mobile andmay be readily transferred from one water molecule to another. The timetaken for such a transfer is typically of the order of 10~14 s provided thatthe receiving molecule of water is correctly oriented.

Several other discrete species have been found to arise from the self-ionization of water. These include H5Og (Kearley, Pressman & Slade,1986), H4O2+ (Bollinger et al., 1987), H9O+ (Robinson, Thistlethwaite &Lee, 1986) and hydrated electrons (Hart & Anbar, 1970).

Intense ion-ion interactions which are characteristic of salt solutionsoccur in the concentrated aqueous solutions from which AB cements areprepared. As we have seen, in such solutions the simple Debye-Huckellimiting law that describes the strength goes up so the repulsive forcebetween the ions becomes increasingly important. This is taken account ofin the full Debye-Hiickel equation by the inclusion of a parameter relatedto ionic size and hence distance of closest approach (Marcus, 1988).

For concentrated solutions, there are approaches that are moresophisticated than that of Debye & Hiickel. A particularly successfulmethod of describing such solutions is that due to McMillan & Mayer(1945) which has subsequently been developed by Ramanathan &

44

Water as a solvent

Friedman (1971). This approach is described as the hypernetted chainprocedure and in it ion-ion pair potentials are expressed as the sum of fourterms. These are:

COULU, the charge-charge interactions between two ions, / andj , as a function of their separation,CORU, a repulsive core potential term,CAVU, arising from the dielectric cavity effect, andGURU, the so-called Gurney potential (Gurney, 1953), whichdescribes the effect of co-sphere overlap.

Using this approach, calculations can be made of volumetric, entropicand energy parameters taking account of the effect of overlapping co-spheres. Some indication of the organization in the solution is alsopossible. The properties of a number of concentrated salt solutions havebeen analysed by this procedure, including simple 1:1 salts, alkaline earthsalts and alkylammonium salts.

A number of other attempts have been made to account for theproperties of concentrated aqueous solutions of ionic compounds byprocedures that represent further improvements on the simple Debye-Hiickel approach. However, they lie outside the scope of the presentchapter. The important point to emphasize is that the concentratedaqueous solutions that are generally employed in the preparation of ABcements tend to exhibit significant ion-ion interactions; such interactionslead to significant deviations from ideality which may be accounted for bysubstantial extension of the ideas of simple dilute solution theory.

3.4.4 Dissolution of polymers

AB cements are not only formulated from relatively small ions with welldefined hydration numbers. They may also be prepared from macro-molecules which dissolve in water to give multiply charged species knownas polyelectrolytes. Cements which fall into this category are the zincpolycarboxylates and the glass-ionomers, the polyelectrolytes beingpoly(acrylic acid) or acrylic acid copolymers. The interaction of suchpolymers is a complicated topic, and one which is of wide importance to anumber of scientific disciplines. Molyneux (1975) has highlighted the factthat these substances form the focal point of 'three complex andcontentious territories of science', namely aqueous systems, ionic systemsand polymeric systems.

45


In polyelectrolytes, the ionic charges are carried by groups which arethemselves attached covalently to the macromolecular backbone. When allof the groups are negatively charged, as with polyacrylate, the poly-electrolyte that results is referred to as a polyanion. Polyelectrolytes are ofhigh solubility in water, especially when compared to most organicmacromolecules. This increased solubility may be attributed not only tothe more favourable interactions between the charged groups and thewater molecules, but also to the fact that entropy strongly favoursdissolution and dissociation of these molecules (Molyneux, 1975).

The conformations adopted by polyelectrolytes under different condi-tions in aqueous solution have been the subject of much study. It is known,for example, that at low charge densities or at high ionic strengthspolyelectrolytes have more or less randomly coiled conformations. Asneutralization proceeds, with concomitant increase in charge density, sothe polyelectrolyte chain uncoils due to electrostatic repulsion. Eventuallyat full neutralization such molecules have conformations that areessentially rod-like (Kitano et ai, 1980). This rod-like conformation forpoly(acrylic acid) neutralized with sodium hydroxide in aqueous solutionis not due to an increase in stiffness of the polymer, but to an increase in theso-called excluded volume, i.e. that region around an individual polymermolecule that cannot be entered by another molecule. The excludedvolume itself increases due to an increase in electrostatic charge density(Kitano et al.9 1980).

In a study of the transition in conformation from random coil to stiff rodby poly(acrylic acid), it was found that the point of transition depended ona number of factors, including the nature of the solvent, the temperature,the particular counterion used and the degree of dissociation (Klooster,van der Trouw & Mandel, 1984).

Methanol was used in this study, though in terms of Flory-Hugginssolution theory it is not a good solvent for poly(acrylic acid) at roomtemperature. In other words, the polymer adopts a tightly coiledconformation that excludes solvent, thereby approaching the point ofprecipitation. This phenomenon may be responsible for the observationthat the addition of methanol to poly(acrylic acid) solutions intended foruse in glass-ionomer cements prevents gelation (Wilson & Crisp, 1974).This was originally attributed to methylation of the polymer, leading to areduction in the stereoregularity of the poly(acrylic acid) and hence to alessening of the readiness with which stable hydrogen-bonded links couldbe formed. However, there is the alternative possibility that the presence of

46

Hydration in the solid state

methanol altered the conformation of the polymer and that this con-formational change prevented the development of the hydrogen-bondednetwork necessary for gelation to occur.

5.5 Hydration in the solid state

Many ionic compounds contain what used to be referred to as 'water ofcrystallization'. For example, magnesium chloride can exist as a fullyhydrated salt which was formerly written MgCl2. 6H2O, but is moreappropriately written Mg(OH2)6Cl2, since the water molecules occupycoordination sites around the magnesium ions. This is typical. In mostcompounds that contain water of crystallization, the water molecules arebound to the cation in an aquo complex in the manner originally proposedby Alfred Werner (1866-1919) in 1893 (Kauffman, 1981). Such anarrangement has been confirmed in numerous cases by X-ray diffractiontechniques.

3.5.1 Coordination of water to ions

The ions that tend to be involved in AB cements include such species asAl3+, Mg2+, Ca2+ and Zn2+. These are all capable of developing acoordination number of six, and hexaquo cations are known to be formedby each of these metal ions (Hiickel, 1950). The typical requirements for anion to develop such coordination characteristics are that the ion shouldexist in the + 2 or +3 oxidation state, and in this state should be of smallionic radius (Greenwood & Earnshaw, 1984).

Another feature of the metal ions that are typically involved incementitious bonding in AB cements is that most of them fall into thecategory of hard in Pearson's Hard and Soft Acids and Bases scheme(Pearson, 1963). The underlying principle of this classification is that basesmay be divided into two categories, namely those that are polarizable orsoft, and those that are non-polarizable or hard. Lewis acids too may beessentially divided into hard and soft, depending on polarizability. Fromthese classifications emerges the useful generalization that hard acidsprefer to associate with hajd bases and soft acids prefer to associate withsoft bases (see Section 2.3.7).

Of the ions most often implicated in cementitious bonding in ABcements, Ca2+, Mg2+ and Al3+ are classified as clearly hard; Zn2+ bycontrast falls into the category that Pearson designated 'borderline', as

47


does Cu2+ (Pearson, 1963). This means that most of these ions formparticularly stable complexes with hard bases, i.e. those which are notreadily polarizable. This requirement demands that the bases be those offirst row elements, such as oxygen or nitrogen. Water is thus a hard base,and the complex that it forms with these ions involves coordination by theoxygen atoms. As predicted by the Hard and Soft Acids and Bases concept,the aquo complexes of the cement-forming metal ions are extremely stableand do not readily lose their coordinated water. Hence, one of thefunctions of water in fully set AB cements is coordination to the metal ions.

3.6 The role of water in acid-base cements

Water has three possible roles in acid-base cements. First, it acts as themedium for the setting reaction of these materials, and second, it is one ofthe components of the set cement, actually becoming incorporated into thecement as it hardens. Third, water may act as plasticizer in these cements.All of these roles are reviewed here.

3.6.1 Water as solvent in AB cements

Water as the solvent is essential for the acid-base setting reaction to occur.Indeed, as was shown in Chapter 2, our very understanding of the terms' acid' and' base', at least as established by the Bronsted-Lowry definition,requires that water be the medium of reaction. Water is needed so that theacids may dissociate, in principle to yield protons, thereby enabling theproperty of acidity to be manifested. The polarity of water enables thevarious metal ions to enter the liquid phase and thus react. The solubilityand extent of hydration of the various species change as the reactionproceeds, and these changes contribute to the setting of the cement.

3.6.2 Water as a component of AB cements

Water is also a component of set AB cements. In glass-ionomer cements,for example, it may serve to coordinate to certain sites around the metalions. It also hydrates the siliceous hydrogel that is formed from the glassafter acid attack has liberated the various metal ions (Wilson & McLean,1988). Such reactions continue long after the initial hardening of thecement is complete, and for this reason water must be retained as far aspossible during the first hours and days after formation of the cement. Ifwater is lost from the cement and desiccation occurs, these post-hardening

48

The role of water in acid-base cements

reactions stop, and the cement will not achieve maximum possible strength.Moreover, if desiccation is excessive, the material will also shrink andcrack.

Water occurs in glass-ionomer and related cements in at least twodifferent states (Wilson & McLean, 1988; Prosser & Wilson, 1979).These states have been classified as evaporable and non-evaporable,depending on whether the water can be removed by vacuum desiccationover silica gel or whether it remains firmly bound in the cement whensubjected to such treatment (Wilson & Crisp, 1975). The alternativedescriptions 'loosely bound' and 'tightly bound' have also been applied tothese different states of water combination. In the glass-poly(acrylic acid)system the evaporable water is up to 5 % by weight of the total cement,while the bound water is 18-28 % (Prosser & Wilson, 1979). This amountof tightly bound water is equivalent to five or six molecules of water foreach acid group and associated metal cation. Hence at least ten moleculesof water are involved in the hydration of each coordinated metal ion at acarboxylate site.

It has been suggested by Ikegami (1968) that the carboxylate groups ofa polyacrylate chain are each surrounded by a primary local sphere oforiented water molecules, and that the polyacrylate chain itself issurrounded by a secondary sheath of water molecules. This secondarysheath is maintained as a result of the cooperative action of the chargedfunctional groups on the backbone of the molecule. The monovalent ionsLi+, Na+ and K+ are able to penetrate only this secondary hydrationsheath, and thereby form a solvent-separated ion-pair, rather than acontact ion-pair. Divalent ions, such as Mg2+ or Ba2+, cause a much greaterdisruption to the secondary hydration sheath.

The effectiveness with which divalent ions cause gelation of poly(acrylicacid) has been found to follow the order Ba2+ > Sr2+ > Ca2+ (Wall &Drenan, 1951) and this has been attributed to the formation of salt-likecrosslinks. Gelation has been assumed to arise in part from dehydration ofthe ion-pairs (Ikegami & Imai, 1962), and certainly correlates withprecipitation in fairly dilute systems. Indeed, the term precipitation hassometimes been applied to the setting of AB cements derived frompoly(acrylic acid) as they undergo the transition from soft manipulablepaste to hard brittle solid.

At the molecular level, a number of features are associated with thephenomenon of gelation or precipitation. In particular the disruption ofthe secondary hydration sheaths around the polyacrylate chains appears

49


important (Prosser & Wilson, 1979). In glass-ionomer cements the boundwater has been assumed to be associated with the intrinsic water spheresaround the carboxylate anion-metal cation units, while evaporable wateris associated with the secondary hydration sheath around the polyacrylatechain. As these cements age, the ratio of tightly bound to loosely boundwater increases. This is accompanied by an increase in strength andmodulus of the cement and by a decrease in plasticity (Paddon & Wilson,1976; Wilson, Crisp & Paddon, 1981).

The loosely bound water in glass-ionomer cements is labile, and is easilylost or gained. Indeed, such cements are stable only in an atmosphere of80% relative humidity (Hornsby, 1980). In higher humidities the cementsabsorb water and the resulting hydroscopic expansion can exceed theshrinkage that usually accompanies setting, which is a distinct clinicaladvantage for the use of these cements in dentistry. By contrast, the cementcan lose water under drying conditions leading to shrinking, crazing andfailure to develop full strength.

Glass-ionomer cements become less susceptible to desiccation as theyage, because a greater proportion of the water in older cements has become'tightly bound'. Early contact with moisture is also damaging, and thisproblem is overcome clinically to some extent by using some sort ofprotection such as clear nail varnish to seal the cement during its early life(Wilson & McLean, 1988). However, this does not give perfect results, andas yet there is no ideal barrier material for this purpose (Earl, Hume &Mount, 1985).

The role of water in dental silicate cements was studied by Wilson et al.(1970), and they found that the properties of these materials includingsetting time, compressive strength and resistance to attack by water andacids were markedly affected by the amount of water in the original cementpaste. Water in these materials was also found to fall into the twocategories of evaporable and non-evaporable. In this case non-evaporablewater was defined as that water remaining in the cement after heating at105 °C for 24 hours. With increasing acid concentration (i.e. decreasingamounts of water in the initial cement paste), the amount of non-evaporable water went down, until at 75 % phosphoric acid concentrationnearly all of the water in the cement was found to be evaporable. Moreoverthe cements containing almost no non-evaporable water were found to beextremely weak. Hence the non-evaporable (bound) water could beequated with bonding water. Infrared analysis had previously shown thatnon-bonding water was associated with the water-soluble hydrated salt

50

The role of water in acid-base cements

sodium dihydrogen phosphate, NaH2PO4.H2O (Wilson & Mesley, 1968).The presence of this compound was known to have a deleterious effect oncement properties and the water associated with it was known to be readilyremoved. Hence in cements prepared from aqueous solutions having highconcentrations of phosphoric acid this salt was assumed to be present inquantity and to be responsible for the relatively high levels of evaporablewater (Wilson et aL, 1970).

A series of AB cements can be prepared from aqueous solutions ofoxides and halides (or sulphates) of magnesium or zinc. These cements aredescribed in detail in Chapter 7. For the moment we will confine ourdiscussion to a consideration of the role of water in these cements.

In the cements of this type a number of phases are known to be present.For example, in the zinc oxychloride cement two discrete phases,corresponding to the composition ZnO. ZnCl2. H2O in the ratios 4:1:5and 1:1:2 respectively, are known to occur (Sorrell, 1977). Similarly,in the magnesium oxychloride cement, phases corresponding toMg(OH)2. MgCl2. H2O in the ratios 5:1:8 and 3:1:8 have been shown toexist and have been studied by X-ray diffractometry (Sorrell & Armstrong,1976).

The precise structural role played by the water molecules in thesecements is not clear. In the zinc oxychloride cement, water is known to bethermally labile. The 1:1:2 phase will lose half of its constituent water atabout 230 °C, and the 4:1:5 phase will lose water at approximately 160 °Cto yield a mixture of zinc oxide and the 1:1:2 phase. Water clearly occursin these cements as discrete molecules, which presumably coordinate to themetal ions in the cements in the way described previously. However, thepossible complexities of structure for these systems, which may includechlorine atoms in bridging positions between pairs of metal atoms, makeit impossible to suggest with any degree of confidence which chemicalspecies or what structural units are likely to be present in such cements.One is left with the rather inadequate chemical descriptions of the phasesused in even the relatively recent original literature on these materials, fromwhich no clear information on the role of water can be deduced.

3.6.3 Water as plasticizer

An additional possible role for water in AB cements is plasticization.Water is known to act as plasticizer for a number of polymeric materials,whether synthetic or natural, and whether or not they are predominantly

51


polar. Thus water has been found to affect the properties of poly(methylmethacrylate) (Turner, 1982) and of alkyd resins used in surface coatings(Mayne & Mills, 1982). As plasticizer, the principal effect of water in thesesystems is to reduce the glass transition temperature, Tg, and this in turnaffects a number of other properties of the materials, including rigidity,dimensional stability and diffusion coefficients within the bulk. Given thepolar nature of the components of AB cements, the known water contentof set cements, and the fact that water has been shown to act as plasticizerin pure metal poly(acrylate) salts (Yokoyama & Hiraoko, 1979) it seemsprobable that one of the roles of water in the solid state of AB cements isthat of plasticizer.

ReferencesBarrow, G. M. (1979). Physical Chemistry, 4th edn. Tokyo: McGraw-Hill

Kogakusha Ltd.Benedict, W. S., Gailar, N. & Plyler, E. K. (1956). Rotation-vibration spectra of

deuterated water vapour. Journal of Chemical Physics, 24, 1139-65.Bernal, J. D. & Fowler, R. H. (1933). A theory of water and ionic solutions with

particular reference to hydrogen and hydroxyl ions. Journal of ChemicalPhysics, 1, 515-48.

Bollinger, J. C, Faure, R., Yvernault, T. & Stahl, D. (1987). On the existence ofthe protonated dication H4 O2+ in sulfolane solution. Chemical Physics Letters,140, 579-81.

Bragg, W. H. (1922). The crystal structure of ice. Proceedings of the PhysicalSociety of London, 34, 98-103.

Briant, C. L. & Burton, J. J. (1976). Molecular dynamics study of the effects ofions on water microclusters. Journal of Chemical Physics, 64, 2888-95.

Clever, H. L. (1963). The hydrated hydronium ion. Journal of ChemicalEducation, 40, 637^41.

Cunningham, A. J. C, Payzant, J. D. & Kebarle, P. (1972). A kinetic study ofthe proton hydrate H+(H2O) and equilibria in the gas phase. Journal of theAmerican Chemical Society, 94, 7627-32.

Earl, M. S. A., Hume, W. R. & Mount, G. J. (1985). Effect of varnishes andother surface treatments on water movement across the glass-ionomer cementsurface. Australian Dental Journal, 30, 298-301.

Eisenberg, D. & Kauzmann, W. (1969). The Structure and Properties of Water.Oxford: Oxford University Press.

Enderby, J. E., Cummings, S., Herdman, G. H., Neilson, G. W., Salmon, P. S.& Skipper, N. (1987). Diffraction and study of aqua ions. Journal of PhysicalChemistry, 91, 5851-8.

Frank, H. S. & Evans, M. W. (1945). Entropy in binary liquid mixtures; partialmolal entropy in dilute solutions; structure and thermodynamics in aqueouselectrolytes. Journal of Chemical Physics, 13, 507-32.

52

References

Frank, H. S. & Wen, W-Y. (1957). Structural aspects of ion-solvent interactionsin aqueous solutions-water structure. Discussions of the Faraday Society, 24,133^0.

Franks, F. (1975). The hydrophobic interaction. In Franks, F. Water. AComprehensive Treatise, vol. 4, Chapter 1. London and New York: PlenumPress.

Franks, F. (1983). Water. London: Royal Society of Chemistry.Gillespie, R. J. & Nyholm, R. S. (1957). Inorganic stereochemistry. Quarterly

Reviews of the Chemical Society, 11, 339-80.Greenwood, N. N. & Earnshaw, A. (1984). The Chemistry of the Elements.

Oxford: Pergamon Press.Gurney, R. W. (1953). Ionic Processes in Solution. New York: McGraw-Hill.Hall, V. M. D. (1985). In Russell, C. A. (ed.) Recent Developments in the History

of Chemistry. London: Royal Society of Chemistry.Hart, E. J. & Anbar, M. (1970). The HydratedElectron. New York: Wiley.Heinzinger, K. & Palinkas, G. (1987). In Kleeberg, H. (ed.) Interactions of

Water in Non-ionic Hydrates. Berlin: Springer-Verlag.Heinje, G., Luck, W. A. P. & Heinzinger, K. (1987). Molecular dynamics

simulation of an aqueous sodium perchlorate solution. Journal of PhysicalChemistry, 91, 331-8.

Hornsby, P. R. (1980). Dimensional stability of glass-ionomer cements. Journalof Chemical Technology and Biotechnology, 30, 595-601.

Hiickel, W. (1950). Structural Chemistry of Inorganic Compounds, vol. 1. NewYork: Elsevier.

Ikegami, A. (1968). Hydration of polyacids. Biopolymers, 6, 431-40.Ikegami, A. & Imai, N. (1962). Precipitation of polyelectrolytes by salts. Journal

of Polymer Science, 56, 133-52.Kauffman, G. B. (1981). Coordination Chemistry. New York: Hey den.Kearley, G. J., Pressman, H. A. & Slade, R. C. T. (1986). The geometry of the

H5OJ ion in dodecatungstophosphoric acid hexahydrate, (H5O£)3 (PW^O^),studied by inelastic neutron scattering vibrational spectroscopy. Journal of theChemical Society Chemical Communications, 1801-2.

Kebarle, P. (1977). Ion thermochemistry and solvation from gas phase ionequilibria. Annual Reviews in Physical Chemistry, 28, 445-76.

Kitano, T., Taguchi, A., Noda, I. & Nagasawa, M. (1980). Conformation ofpolyelectrolytes in aqueous solution. Macromolecules, 13, 57-63.

Klooster, N. Th. M., van der Trouw, F. & Mandel, M. (1984). Solvent effects inpoly electrolyte solutions. 3. Spectropho tome trie results with (partially)neutralised poly(acrylic acid) in methanol and general conclusions regardingthese systems. Macromolecules, 17, 2087-93.

Kochanski, E. (1985). Theoretical studies of the system H3O+(H2O)n forn = 1—9. Journal of the American Chemical Society, 107, 7869-73.

Marcus, Y. (1988). Ionic radii in aqueous solution. Chemical Reviews, 88,1475-98.

Mayne, J. E. O. & Mills, D. J. (1982). Structural changes in polymer films.Part 1. The influence of the transition temperature on the electrolytic resistance

53


and water uptake. Journal of the Oil and Colour Chemists' Association, 65,138^2.

McMillan, W. G. & Mayer, J. E. (1945). The statistical thermodynamics ofmulticomponent systems. Journal of Chemical Physics, 13, 276-305.

Molyneux, P. (1975). Synthetic polymers. In Franks, F. Water. A ComprehensiveTreatise, vol. 4, Chapter 7. London and New York: Plenum Press.

Moore, W. J. (1972). Physical Chemistry, 5th edn. London: Longman GroupLtd.

Morrison, R. T. & Boyd, R. T. (1973). Organic Chemistry, 3rd edn. New York:Allyn and Bacon.

Neilson, G. W., Schioberg, D. & Luck, W. A. P. (1985). The structure aroundthe perchlorate ion in concentrated aqueous solutions. Chemical PhysicsLetters, 111, 475-9.

Nicholson, J. W. (1985). Waterborne Coatings. OCCA Monograph No. 3.London: Oil and Colour Chemists' Association.

Paddon, J. M. & Wilson, A. D. (1976). Stress relaxation studies on dentalmaterials. 1. Dental cements. Journal of Dentistry, 4, 183-9.

Pauling, L. (1960). The Nature of the Chemical Bond, 3rd edn. Ithaca, NewYork: Cornell University Press.

Pearson, R. G. (1963). Hard and soft acids and bases. Journal of the AmericanChemical Society, 85, 3533-9.

Prosser, H. J. & Wilson, A. D. (1979). Litho-ionomer cements and theirtechnological applications. Journal of Chemical Technology and Biotechnology,29, 69-87.

Ramanathan, P. S. & Friedman, H. L. (1971). Refined model for aqueous 1-1electrolytes. Journal of Chemical Physics, 54, 1086-99.

Robinson, R. A. & Stokes, R. H. (1955). Electrolyte Solutions. London:Butterworth Scientific Publications.

Robinson, G. W., Thistlethwaite, P. J. & Lee, J. (1986). Molecular aspects ofionic hydration reactions. Journal of Physical Chemistry, 90, 4224-33.

Robb, I. D. (1983). Polymer-small molecule interactions. In Finch, C. A. (ed.)Chemistry and Technology of Water Soluble Polymers. New York: PlenumPress.

Russo, S. O. & Hanania, G. I. H. (1989). Ion association solubilities andreduction potentials in aqueous solution. Journal of Chemical Education, 66,148-53.

Salmon, P. S., Neilson, G. W. & Enderby, J. E. (1988). The structure of Cu2+

aqueous solutions. Journal of Physics C, 21, 1335-49.Sorrell, C. A. & Armstrong, C. R. (1976). Reactions and equilibria in

magnesium oxychloride cements. Journal of the American Ceramic Society, 59,51-4.

Sorrell, C. A. (1977). Suggested chemistry of zinc oxychloride cements. Journalof the American Ceramic Society, 60, 217-20.

Speakman, J. C. (1975). The Hydrogen Bond. London: Chemical Society.Symons, M. C. R. (1989). Liquid water- the story unfolds. Chemistry in Britain,

25, 491-4.

54

References

Turner, D. T. (1982). Poly(methyl methacrylate) plus water. Sorption kineticsand volumetric changes. Polymer, 23, 197-202.

Vaslow, F. (1963). The orientation of water molecules in the field of an alkaliion. Journal of Physical Chemistry, 67, 2773-6.

Wall, F. T. & Drenan, J. W. (1951). Gelation of polyacrylic acid by divalentions. Journal of Polymer Science, 1, 83-8.

Watts, R. O., Clementi, E. & Fromm, J. (1974). Theoretical study of the lithiumfluoride molecule in water. Journal of Chemical Physics, 61, 2550-5.

Weast, R. C. (ed.) (1985-6). Handbook of Physics and Chemistry. Ohio:Chemical Rubber Company.

Wilson, A. D. & Crisp, S. (1974). Unpublished data cited in Wilson & McLean,1988.

Wilson, A. D. & Crisp, S. (1975). Ionomer cements. British Polymer Journal, 1,279-96.

Wilson, A. D., Crisp, S. & Paddon, J. M. (1981). The hydration of aglass-ionomer (ASPA) cement. British Polymer Journal, 13, 66-70.

Wilson, A. D., Kent, B. E., Batchelor, R. F., Scott, B. G. & Lewis, B. G. (1970).Dental silicate cements. XII. The role of water. Journal of Dental Research,49, 307-14.

Wilson, A. D. & McLean, J. W. (1988). Glass-ionomer Cement. Chicago,London, etc.: Quintessence Publishers.

Wilson, A. D. & Mesley, R. J. (1968). Dental silicate cements. VI. Infraredstudies. Journal of Dental Research, 47, 644—52.

Yokoyama, T. & Hiraoko, K. (1979). Hydration and thermal transition ofpoly(acrylic acid) salts. Polymer Preprints of the American Chemical Society,Division of Polymer Chemistry, 20, 511-13.

55

4 Polyelectrolytes, ion bindingand gelation

4.1 Polyelectrolytes4.1.1 General

The setting of AB cements is an example of gelation, and gelation is relatedto ion binding. A theoretical examination of the various phenomenaassociated with ion binding and gelation finds its clearest exposition in thefield of polyelectrolytes. Moreover, this field may be wider than it seems atfirst.

Polyelectrolytes form the basis of those modern cements which aredistinguished by their ability to adhere to reactive surfaces. At present themain use of such cements lies in the medical field, principally in dentalsurgery. They adhere permanently to biological surfaces where they haveto withstand adverse conditions of wetness, chemical attack, the stress ofbiological activity, and chemical and biological changes within thesubstrate. Nevertheless, adhesive bonds are maintained.

Polyelectrolytes are polymers having a multiplicity of ionizable groups.In solution, they dissociate into polyions (or macroions) and small ionsof the opposite charge, known as counterions. The polyelectrolytes ofinterest in this book are those where the polyion is an anion and thecounterions are cations. Some typical anionic polyelectrolytes are depictedin Figure 4.1. Of principal interest are the homopolymers of acrylic acidand its copolymers with e.g. itaconic and maleic acids. These are used in thezinc polycarboxylate cement of Smith (1968) and the glass-ionomercement of Wilson & Kent (1971). More recently, Wilson & Ellis (1989) andEllis & Wilson (1990) have described cements based on polyphosphonicacids.

There is also the question of whether there are inorganic polyelectrolyteswithin the field of AB cements. A number of cements are based onorthophosphoric acid, and in the two most important ones aluminium is

56

Polyelectrolytes

known to be essential for cement formation. Aluminium forms complexaluminophosphoric acids with orthophosphoric acid. The solutions ofthese complexes are markedly viscous and there is some NMR spec-troscopic evidence that these aluminophosphoric acids form linearpolymers based on the Al-O-P linkage (Sveshnikova & Zaitseva, 1964;Akitt, Greenwood & Lester, 1971; O'Neill et al., 1982). Callis, Van Wazer& Arvan (1954), Salmon & Wall (1958) and Wilson et al. (1972) considerthat aluminophosphate polymers are formed in which [POJ tetrahedra arelinked by aluminium atoms.

Polyanion chains containing many linked charged groups exert aconsiderable electrostatic effect on the orientation of dipolar solventmolecules and on the counterions. The counterions are constrained toremain in the neighbourhood of the charged polymer chains, a phenom-enon known as ion binding. This phenomenon is supported by a wealth ofexperimental evidence (Morawetz, 1975; Wilson & Crisp, 1977; Rymden& Stilbs, 1985a, b) and an early illustration of it is found in the work ofHuizenga, Grieger & Wall (1950a, b) who observed that, in an electric field,cations were sometimes transported with the polyanion.

rCH—COOH

T2CH—COOH

Poly (acrylic acid)

CH2

1CH—SO2OH1CH2

CH—SO2OH

CH2

CH— COO"1

CH2

CH— COO~

1Polyacrylate

1CH21CH—PO(OH)2

T2CH—PO(OH)2

Poly (vinyl sulphonic acid) Poly (vinyl phosphonic acid)

Figure 4.1 Some typical anionic polyelectrolytes.

57

Polyelectrolytes, ion binding and gelation

4.1.2 Polyion conformation

The shape, configuration or morphology of a polyion is usually known asits conformation. There are very many possible conformations available toa polyion because of the flexibility of the main chain due to the freerotation of bonds. The particular conformation adopted will be the onewith the lowest free energy. This free energy has two components, onearising from chain flexibility and the other from electrical interactions. Theintrinsic free energy of rotation is a function of the relative position ofneighbouring bonds. There are energy minima, at the trans position, whichcorresponds to the stretched form of the chain, and at the two gauchepositions, corresponding to the contracted form. The difference in energybetween the trans and gauche positions is one important factor determiningthe flexibility of the chain. The other component of free energy arises frominteractions between charged groups on the polyion, counterions andsolvent molecules.

There are two broad kinds of polyion conformation; the random coiland the ordered helix. In a helix there are regularly repeated structuresalong the coil; there are none in the case of a random coil. In this book weare concerned with the latter where there are often several conformationswith approximately equal free energies and, thus, conformational changesoccur readily.

Random coil conformations can range from the spherical contractedstate to the fully extended cylindrical or rod-like form. The conformationadopted depends on the charge on the polyion and the effect of thecounterions. When the charge is low the conformation is that of acontracted random coil. As the charge increases the chains extend underthe influence of mutually repulsive forces to a rod-like form (Jacobsen,1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformationchanges from that of a compact random coil to an extended chain. Forexample poly(acrylic acid), degree of polymerization 1000, adopts aspherical form with a radius of 20 nm at low pH. As neutralizationproceeds the polyion first extends spherically and then becomes rod-likewith a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry ofpolyelectrolyte cements.

The situation is more complex in the reactions found in AB cementsbecause neutralization is accompanied by ion binding. Although a polyionchain extends as the number of ionized groups increases, the binding of

58

Ion binding

counterions has the reverse effect because intrachain repulsive forces aredecreased. An increase in the concentration of polyions in solution has thesame effect, for an increase in interchain repulsion will inhibit theunwinding of polymer chains. Thus, the effects predicted by dilute solutiontheory will be much less in the concentrated conditions found in ABcements. From this it can be seen that the effect of ion binding onconformation change is complex.

Conversely, conformation affects the binding of counterions to polyions(Jacobsen, 1962). In the compact spherical conformation some ionizedgroups on polymer chains will be inaccessible for ion binding.

4.2 Ion binding

4.2.1 Counterion binding

Oppositely charged ions are attracted to each other by electrostatic forcesand so will not be distributed uniformly in solution. Around each ion orpolyion there is a predominance of ions of the opposite charge, thecounterions. This cloud of counterions is the ionic atmosphere of thepolyion. In a dynamic situation, the distribution of counterions dependson competition between the electrostatic binding forces and the opposing,disruptive effects of thermal agitation.

The phenomenon has been studied by a number of techniques: titration(Gregor & Frederick, 1957; Kagawa & Gregor, 1957); viscosity andelectrical conductance measurements (Gregor, Gold & Frederick, 1957;Bratko et al., 1983); determination of counterion activity (Kagawa &Katsuura, 1955); measurement of transference (Ferry & Gill, 1962);dilatometry (Strauss & Leung, 1965; Begala & Strauss, 1972); and NMRspectroscopy (Rymden & Stilbs, 1985a, b).

Ion binding is affected by the size and charge of the counterion, thecharge and conformation of the polyion, and states of hydration. We willexamine these effects in some detail.

4.2.2 The distribution of counterions

The potential distribution around the polyion is important to anydiscussion of counterion binding and hydration effects. Oosawa (1971) hasdistinguished four regions of potential about a polyion (Figure 4.2): (1) alocalized potential hole around each charged group, (2) a cylindricalpotential valley or tube along the polyion chain, (3) a spherical trough in

59

Poly electrolytes, ion binding and gelation

the apparent volume occupied by the whole of the coiled chain, and (4) theregion outside the polyion. Counterions are distributed between these fourpotential regions and may be classified as free, bound but mobile(atmospheric) and localized {site-bound). Free ions remain outside thevolume of the polyion (in region 4); the remaining ions are bound to thepolyion. Of the bound ions, the mobile atmospheric ions occupy thepotential trough or valley around each polyion (regions 2 and 3). Localizedbinding occurs in the potential holes at the sites of the individual chargedgroups of the polyion, and ion-pairs are formed.

Oosawa (1971) used a simple calculation to illustrate the effect of ahighly charged polyion on the binding counterions. The distributionbetween free ions and bound ions depends on the ratio between potentialenergy and kinetic energy. In the case of a random coil, containing nionized groups of charge — e0, and of spherical conformation, radius/?, thepotential drop, Si//, for a counterion of charge + e0 at the edge of thepolyion is given by

neo/eop (4.1)

\\

rRegion offree counterions

Figure 4.2 The four regions of potential about a polyion. Based on Oosawa (1971).

60

Ion binding

where the dielectric constant of the solvent is equal to e0. Hence thepotential energy is

nel/eop (4.2)

The ratio of the potential energy to the kinetic energy, kT, is

ne*/eopkT (4.3)

This ratio is related linearly to the degree of polymerization n. In the caseof a poly(acrylic acid) where n = 1000 and p = 20 nm, this ratio works outat 35. Thus, many of the counterions must enter the region of the polyion.Even when 90 % of the counterions are within the polyion this ratio is stillhigh with a value of 3-5. A similar calculation for the rod-like random coilgives an energy ratio of 26 and similar arguments apply.

Oosawa (1971) developed a simple mathematical model, using anapproximate treatment, to describe the distribution of counterions. Weshall use it here as it offers a clear qualitative description of thephenomenon, uncluttered by heavy mathematics associated with thePoisson-Boltzmann equation. Oosawa assumed that there were twophases, one occupied by the polyions, and the other external to them. Healso assumed that each contained a uniform distribution of counterions.This is an approximation to the situation where distribution is governed bythe Poisson distribution (Atkins, 1978). If the proportion of site-boundions is negligible, the distribution of counterions between these phases isthen given by the Boltzmann distribution, which relates the populationratio of two groups of atoms or ions to the energy difference between them.Thus, for monovalent counterions

nJK = (nJV^xpi-e^/kT) (4.4)

where Sy/ is the average potential difference between the two phases, nh isthe number of bound ions contained in a total volume Vh9 nt is the numberof free ions contained in a total volume Vt and n is the total number ofcounterions in a total volume V.

This case can be rewritten

In {(1-/?)/# = ln{0/(l-0)}-eo<ty/£r (4.5)

where p is the apparent dissociation constant, i.e. the ratio of free to totalcounterions, nt: n, and 6 is the volume concentration of the polyion, Vh: V.

For a rod-like or cylindrical polyion, the potential difference Sy/ between

61


the inside and outside of a cylindrical polyion of length / and radius r, withan average distance between the polyions of 2R, is given by

dy, = -2(nie0/s0)\n(R/r) (4.6)

If v is the mean effective volume occupied by a single polyion andN is the number of polyions then v = nrH and V/N = nR2l; thusr*/R2 = Nv/V=0 and

Hence equation (4.5) becomes

= ln{0/(l-0)}-/fein0

(4.7)

(4.8)where Q is the charge density along the polyion and equals nel/sokTl.

The equation for a spherical polyion conformation of radius a is similar:

ln{(l -fi)/fi = In{0/(1 -0)}-/?P(l -01/3) (4.9)

where P is the charge density along the polyion and equals nel/skTa.Thus /?, and hence the extent of the ion binding, depends both on the

volume concentration of the polyion, 9, and the charge density, Q. Theconsequences of these equations, are not easy to see, because they cannot

1.0

0.5•

1

0 = 1

Q=2•

Q=3

•

I I0.1 0.2 0.3

Figure 4.3 Variation of the apparent degree of dissociation, /?, with v and Q. Based onOosawa (1971).

62

Ion binding

be solved. However, when 9 is sufficiently small the equations can besimplified and rendered easy to discuss. Although the conditions in verydilute solutions are far removed from practical reality, the simplifiedsituation can be used to illustrate certain basic points. Thus, for a rod-likeconfiguration where 9^0, equation (4.8) reduces to

In {(1-/?)//?} = ( l - / ? 0 In 0 (4.10)or

(\-P)/P=8a-pQ) (4.11)

The apparent degree of dissociation, /?, varies in a complex way with 9and depends on the value of Q (Figure 4.3). Apart from the case Q = 1,where /? decreases with 9, fi shows little variation with 9. It slowly decreaseswhen Q = 2, and when Q = 3 or 4 it increases slightly to a plateau.Consequently, in practical cases, /? is unaffected by increases in the chargeon the polyion associated with ionization. This conclusion is supported bythe results of Nagasawa & Kagawa (1957).

4.2.3 Counterion condensation

The theory of counterion condensation is implicit in Oosawa (1957) but theterm was coined later (Imai, 1961). The phenomenon was demonstrated byIkegami (1964), using refractive index measurements of the interactionbetween sodium and polyacrylate ions. It has since been confirmed formany mono-, di- and trivalent counterions and polyionic species(Manning, 1979).

Manning (1969) suggested that there is a critical charge density abovewhich counterions condense on the surface of the polyion. This phenom-enon is most clearly illustrated by the simple case of infinite dilution. As9^0 in equation (4.11), the graph of j$ against Q falls into two partsabout the critical point Q = 1:

P^\ andy->l for Q 1 (4.12)£-> \/Q and y-> \/Q for Q ^ 1 (4.13)

where y is the activity coefficient and equals /?/(l — 0).The consequences of these solutions are shown in Figure 4.4. The

abscissa is n, the total number of counterions or charged groups on thepolyion, and is proportional to Q. Along the ordinate are the numbers ofcounterions bound, nh, and free, nt, equal to n(\—fi) and n^respectively.

The increase of counterion binding with the charge on the polyion has

63


been termed counterion condensation as it is analogous to the con-densation of a vapour. This point is illustrated by Figure 4.4. As the chargeon the polyion increases from zero there is a proportional increase in thenumber of counterions. At first, all the counterions are free and noneare bound. This continues until a plateau is reached at a critical value ofQ = 1. Above this point, all additional counterions are bound to thepolyion and the number of free ions remains constant. Thus, PQ remainsconstant and /? decreases as Q increases.

The simple situation depicted in Figure 4.4 is a limiting one, and thediscontinuity does not occur when 6 > 0; however, for large values of g,PQ increases only slowly. Nor does the discontinuity appear in the case ofthe spherical conformation, but again, for large values of P, PP increasesonly logarithmically. Thus, the situation is similar to that for the rod-likeconfiguration although there is no specific critical value for P.

The above treatment is based on the assumption that 9 is small.However, as Figure 4.3 shows, /? does not greatly change with con-centration so that counterion condensation is probably insensitive toconcentration. The delayed binding of counterions is of some importanceto the onset of gelation.

a

H

C

r—t

a

c

free //

counterions, /nf /

//

//

//

// :/ •

/ ;

bound /counterions, /

nb 7

:Q = i /

Total number of counterions, n[Q]Figure 4.4 The abscissa is n, the total number of counterions or charged groups on thepolyion, and is proportional to Q. Along the ordinate are the number of counterions bound,nh, and free, «f, equal to n(\ —f$) and «/? respectively. Based on Oosawa (1971).

64

Ion binding

Theories of counterion condensation have been reviewed by Manning(1979, 1981); and Satoh, Komiyama & Iijima (1984) have extended thetheory.

4.2.4 Effect of valence and size on counterion binding

Cations of small ionic radius and high charge are more firmly bound thanmonovalent ions of large ionic radius (Ikegami & Imai, 1962; Strauss &Leung, 1965; Begala & Strauss, 1972). Divalent ions are more stronglybound than monovalent ions and the interaction is often localized. Thiscan be examined theoretically by applying Oosawa's two-phase model tocounterions with a valence z and charge of + e0, and a polyion with a totalcharge — ne.

Equations (4.5) and (4.8), which were developed for univalent ions, canbe rewritten, thus:

In {(1-/?)//?} = \n{e/(\-e)}-zeQ5¥/kT (4.14)(4.15)

The critical value for Q is 1/z. There is a proportional increase in thenumber of free counterions, njz, as Q increases from zero, reaching aplateau when Q = 1/z. Also, below this value the degree of dissociation, /?,increases as the concentration decreases, and tends to unity as v tends tozero. When Q > 1/z, f5 decreases with 9 and tends to 1/zQ as 6 tends tozero. The number of free ions cannot exceed n/z2Q. Note that this numberis inversely proportional to the square of the valence. The condensation ofions is thus very sensitive to valence; for multivalent counterions it takesplace at a lower value of Q and the number of free ions is much smaller(l/*a).

Imai (1961) has observed that multivalent counterions are more stronglybound than are monovalent ones. This phenomenon can be demonstratedtheoretically by considering equilibrium conditions for two counterionswith valencies zx and z2 (z2 > zx) and degrees of dissociation fix and /?2.For a cylindrical model the equilibrium equations are

^ - ^ / . A ) ^ (4.16)

fflwJi+fMW (4.17)

where fx and/2 represent the proportions of the total charge carried by thecounterions, i.e./i+/2 = 1.

65


As 9^0 then the solutions to these equations fall into four groupsdepending on the value of Q.

A->1, &"• ! forg<l /z 2 (4.18)& ^ 1 , A^l//2z2 2- / i / / 2

f o r l/z2 < Q < l/fiz2 (4.19)Px-> 1, p2->0 for I/ft z2 ^ Q I/ftzx (4.20)P1->l/f1z1Q, 02^O for I / f t z ^ e (4.21)

These equations are represented graphically in Figure 4.5. Increase in thebinding of counterions as Q increases is reflected as a decrease in ft values.No binding occurs until Q reaches l/z2, when the binding of the higher-valence ions begins. This process is complete when Q attains a value ofl/ftz2. There is no further binding of counterions until Q reaches l / / 1z1

when the binding of the lower-valence ions commences. Figure 4.5 showsthat when there is a mixture of counterions then those of the higher valenceare preferentially bound. Lower-valence ions can completely suppress thedissociation of those of higher valence.

Figure 4.5 The effect of Q on the dissociation {fix /?2) of ions of two valencies. Note thesuppression of the dissociation ( /y of ions of higher valence zx by those of lower valence z2.Based on Oosawa (1957).

66

Ion binding

The selective binding of cations is not as sensitive to size as to valence.The value of Q for the condensation of counterions of the same valence isunaffected. In the case of monovalent cations, the dissociation of allcounterions is complete at infinite dilution, when Q ^ 1. When Q ^ 1 thedissociation of the smaller counterion is always greater than that of thelarger one and increases relatively as Q increases.

A number of workers have observed that the strength of binding ofmonovalent counterions depends on ionic radius. However, the effect ofionic radius is somewhat obscure as it depends on hydration phenomenaand whether the size of the bare ion or that of the hydrated ion is thesignificant parameter (Wilson & Crisp, 1977).

4.2.5 Site binding - general considerations

Not all ions are mobile within the ionic atmosphere of the polyion. Aproportion are localized and site-bound-a concept apparently firstsuggested by Harris & Rice (1954). Localized ion binding is equivalent tothe formation of an ion-pair in simple electrolytes. Experimental evidencecomes mainly from studies on monovalent counterions.

This concept is due to Bjerrum, who in 1926 suggested that in simpleelectrolytes ions of the opposite charge could associate to form ion-pairs(Szwarc, 1965; Robinson & Stokes, 1959). This concept of Bjerrum arosefrom problems with the Debye-Hiickel theory, when the assumption thatthe electrostatic interaction was small compared with kT was not justified.

Bjerrum considered the case of spherical ions in a solvent of dielectricconstant e. The probability of finding two ions of opposite charge at adistance A from each other is calculated from the number of ionssurrounding a central ion of opposite charge in a spherical shell ofthickness dA and radius A. This probability, W{A), is given by

W(A) dA = (4nA2 dA/v) exp (e2/sAkT) (4.22)for monovalent ions. This distribution has a minimum, Am, at

Am = e'/2ekT (4.23)For a cation of charge z+ and anion of charge z_, this minimum becomes

Am = z+z_e2/2ekT (4.24)When A > Am the ions are free and the Debye-Hxickel theory applies.When A < Am the two ions tend to approach each other and form an ion-pair, and there is no contribution to the electrostatic energy from theinteraction between an ion and its atmosphere.

67

Poly'electrolytes, ion binding and gelation

The high dielectric constant of water normally militates against theformation of ion-pairs for simple salts because a high dielectric constantreduces the strength of the electrostatic forces. The phenomenon is morereadily observed in solvents of low dielectric constant; for a typical mono-monovalent salt, ion-pair formation takes place only when the dielectricconstant is less than 41 (Fuoss & Kraus, 1933).

The fraction of all ions forming ion-pairs is

J2cW(A)dA (4.25)

where a is the radius of the central ion.This distribution has some inconsistencies - for example it diverges

when R is large - and was modified by Fuoss (1934); see Figure 4.6.These arguments for simple electrolytes can be extended to the

relationship between the two types of bound counterion in poly-electrolytes: the bound but mobile (atmospheric) and the localized(site-bound). Under equilibrium conditions, the relationship between site-bound and atmospheric ions is

(4.26)

2I

Con tactdistance

Ion-pairrange

Inter-ion distanceFigure 4.6 The Fuoss (1934) distribution function.

68

Ion binding

where ns is the number of site-bound ions, «a is the number of atmosphericions and K is the equilibrium constant. For monovalent cations in dilutesolution (0-1 M) the degree of localized ion binding is negligible; in moreconcentrated solutions some site binding does occur. In general, localizedion binding can be expected only with multivalent cations.

When site binding occurs, the equations which relate the numbers of freeand bound ions require some modification. The relationship is thenbetween free ions and those bound ions that are mobile. The equations aresimilar to equations (4-8), (4-9), (4-14), and (4-15), but the number of site-bound ions has to be discounted in all calculations for P, Q, /? etc.

4.2.6 Effect of complex formation

In a discussion of papers by Rice & Harris (1954) and Harris & Rice (1954),Van Wazer (1954) suggested that there could be covalent binding as well aselectrostatic interaction and that cations could be held at specific sites bycomplex formation.

This is a reasonable inference, because site binding is significant onlywith multivalent cations and strong electrostatic interactions. Under theseconditions ion polarization occurs and bonds have some covalent character(Cotton & Wilkinson, 1966). This is illustrated by the data of Gregor,Luttinger & Loebl (1955a,b). They measured the complexation constantsof poly(acrylic acid), 0-06 N in aqueous solution, with various divalentmetals, which, as it so happens, are of interest to AB cements (Table 4.1).The order of stability was found to be

Mg < Ca < Co < Zn < Mn < CuMandel & Leyte (1964) found a similar order for the complexes ofpoly(methacrylic acid):

Mg < Co < Ni < Zn < Cd < Cu

Some of these divalent cations form part of the Irving-Williams series:Mn, Fe, Co, Ni, Cu and Zn. Irving & Williams (1953) examined thestability constants of complexes of a number of divalent ions and foundthat the order

Mn < Fe < Co < Ni < Cu > Zn

held for the stability of most complexes irrespective of the nature of thecoordinated ligand. The stability constants of metal-poly(alkenoic acid)

69


Table 4.1. Metal PAA complexes (Gregor, Luttinger & Loebl,195 5 a,b)

Metal ion Crystal ionic radius A Complexation constant

6-0 xlO3

2-3 x 103

2-1 x 103

4-0 x 102

1-OxlO2

6-0 x 101

complexes, for the most part, follow the Irving-Williams series as do thestabilities and strengths of poly(alkenoic acid) cements.

The complexation constant for copper(II) is particularly high and Wall& Gill (1954) have suggested that chelate formation takes place with twocarboxyl groups:

Cu2+

Mn2+

Zn2+

Co2+

Ca2+

Mg2+

0-720-800-800-720-990-66

From all of this discussion it is apparent that, as Manning (1979) said, thebinding between counterion and polyion can range from atmospheric tocovalent site binding.

4.2.7 Effect of the polymer characteristics on ion binding

The extent of ion binding depends on a number of characteristics of thepolyion: degree of dissociation, acid strength, conformation, distributionof ionizable groups and cooperative action between these groups (Wilson& Crisp, 1977; Oosawa, 1971; Harris & Rice, 1954, 1957). The hydrationstate of the macromolecule, which is in turn dependent on conformation,also affects ion binding (Begala, 1971).

70

Ion binding

There are differences in ion binding between different polyacids. Thus,alkali metal ions are bound more strongly to poly(acrylic acid) than to theweaker poly(methacrylic acid) (Wilson & Crisp, 1977). Again, the rankingorder for the binding strength of alkali metal ions depends on the nature ofthe polyanion, and the order is different for poly(acrylic acid) than forpoly(maleic acid) or poly(itaconic acid). Thus, for poly(acrylic acid) thebinding strength increases in descending order of the ionic radius of thebare cation:

K+ < Na+ < Li+

For poly(maleic acid) and poly(itaconic acid), the binding strengthincreases in descending order of the size of the hydrated metal ion, whichis the reverse of that for the bare ion (Muto, Komatsu & Nakagawa, 1973;Muto, 1974). This observation has been explained by postulating theformation of a stable ring structure with a hydrogen bridge betweenionized and non-ionized carboxylate groups.

The strength of ion binding is enhanced when the arrangements of thefunctional groups permit chelate formation (Begala & Strauss, 1972).Thus, magnesium is more firmly bound to poly(vinyl methyl ether-maleicacid) than to either poly(acrylic acid) or poly(ethylene maleic acid).

The charge or number of dissociated groups on a poly acid chain dependson the degree of neutralization and is reflected by the pH of the solution.Behaviour is determined by the site binding of hydrogen ions; in the caseof a weak polyacid the number of free hydrogen ions may be neglected.It follows that decrease of site binding of hydrogen ions is directlyproportional to the amount of added alkali. In the case of poly(acrylicacid) its polymer chain can be regarded as a copolymer containing pendantCOOH and COO" groups, the relative amounts of each depending on thedegree of neutralization.

When the degree of neutralization is small the charge on the polyionand the number of counterions will also be small and the majority ofcounterions will be free. As the degree of neutralization, a, increases, thepolyion charge, Q, will increase. This observation follows from thefollowing equations:

Q = nel/eokTl (4.27)

where n = number of ionized groups on the polyion. It follows that

Q = anoel/sokTl (4.28)

71


where n0 = the potential number of ionizable groups on the polyion. WhenQ is low, most of the counterions are free, but as neutralization increasesa point is reached at which the counterions condense; above this point,additional counterions are bound. This follows from the discussion inSection 4.2.3.

4.2.8 Solvation (hydration) effects

The solvation (hydration) and desolvation of ions is important to thegelation process in AB cement chemistry. The large dipole moment of ion-pairs causes them to interact with polar molecules, including those of thesolvent. This interaction can be appreciable. Much depends on whether thesolvent molecule or molecules can intrude themselves between the two ionsof the ion-pair. Thus, hydration states can affect the magnitude of theinteraction. The process leading to separation of ions by solvent moleculeswas perceived by Winstein et al. (1954) and Grunwald (1954).

Consider two ions in contact. As they are pulled apart the potentialenergy of the two ions increases. At some critical point the separationbecomes sufficient for a polar solvent molecule to occupy the spacebetween them, which reduces the energy of the system. Further separationincreases the energy of the system again. These changes demonstrate thattwo types of ion-pair exist: contact and solvent-separated.

This distinction is meaningful if the resultant distribution function is ofthe type shown in Figure 4.7 (Szwarc, 1965). This figure shows that thereis a high probability that the cation and anion are either in contact,separated by a solvent molecule or far apart (Szwarc, 1965). Intermediatepositions are improbable. The structure of solvated ion-pairs has beenstudied by Grunwald (1979) using dipole measurements.

Winstein & Robinson (1958) used this concept to account for thekinetics of the salt effects on solvolysis reactions. They considered thatcarbonium ions (cations) and carbanions could exist as contact ion-pairs,solvated ion-pairs and as free ions and that all these forms participated inthe reactions and were in equilibrium with each other. These equilibria canbe represented, thus:

X:Y= X+Y" = X+SY~ = X+

contact solvent-separated free ionsion-pair ion-pair

72

Ion binding

where X is a carbonium radical and X+ the carbonium ion, Y~ thecarbanion and Y the associated radical. S represents a solvent molecule.

Eigen & Tamm (1962a,b) and Atkinson & Kor (1965, 1967) envisage amore complex situation and consider that there are two kinds of solvent-separated ion-pairs: those with one intervening molecule of solvent andothers where the ion-pair is fully solvated (Wilson & Crisp, 1977).

4.2.9 Hydration of the polyion

The electric potential around a polyion can aflfect the structure of water.There are three regions of potential about a polyion to consider: thepotential holes at the site of the individual charged groups, cylindricalregions along the polymer chain, and the outlying region. In the outlyingregion the potential is small and the water molecules have a normalstructure. In the other two regions there are strong electric fields, and watermolecules are oriented and have special structures. Oriented water isdenser and has a higher refractive index than normal water (Begala &Strauss, 1972; Ikegami, 1964, 1968).

The structure of this water can be affected by ion binding. If thecounterions are tightly bound at the sites of individual charged groups, the

Solvention-pair

Inter-ion distance

Figure 4.7 Distribution function for contact and solvent-separated ion-pairs.

73


structure of the water around them will be profoundly modified. If thecounterions are not localized but mobile, the influence on water structuremay be small. Thus, the state of the binding of a counterion will be reflectedin changes in water structure which in turn can be measured by changes inrefractive index or density.

The effect has been studied experimentally by Ikegami (1964, 1968) whomeasured changes in refractive index, Asai (1961) employing an ultrasonicmethod, Begala & Strauss (1972) who measured changes in molar volume,and Grunwald (1979) using dipole moment measurements. When an acidis neutralized by a base the refractive index of the salt solution formed isless than the weighted mean of the refractive indices of the acid and basesolutions from which it is formed. Likewise, the density increases. By thesemeans, the progress of neutralization may be followed.

At low degrees of neutralization, the average distance between ionizedgroups is great, so that the rearrangement of neighbouring water moleculesinduced by the ionization of a carboxyl group is solely due to the charge onthat individual group. Individual hydration spheres of oriented water,intrinsic water, are formed at each charged site. In the case of poly(acrylicacid) when the degree of neutralization, a, is 0-3 the radius of these spheresis 031 nm (Ikegami, 1964).

As a increases, the average distance between ionized groups decreases sothat these neighbouring groups begin to have an effect. When a exceeds 0-3,individual water spheres begin to overlap and eventually coalesce into acylindrical form. With further increases in a, a second outer cylindricalsheath of water appears in which water molecules are oriented by thecooperative effect of two or more carboxyl groups.

When neutralization is complete, the inner layer of intrinsic waterassumes a cylindrical form along the length of the polyion with a diameterof 0-5-0-7 nm (Ikegami, 1964). The outer second cylindrical hydrationregion has a diameter of 0-9-1-3 nm (Figure 4.8).

The explanation for this volume increase is as follows. For a cylindricalmodel of uniform charge density the electric field around the cylinder is

2cm0e0/e0rl (4.29)

where a is the degree of neutralization, r the radius of the cylinder and /itslength. When the magnitude of the electric field exceeds a certain value,water molecules are reoriented; the above expression shows that theradius, r, of the cylinder increases as the degree of neutralization, a,increases.

74

Ion binding

According to Ikegami (1968) the presence of hydrophobic groups, forexample the methyl group in poly(methacrylic acid), can induce anadditional hydration region around neighbouring charged groups.

The arrangement of carboxyl groups on the polyacid is also important.Thus, poly(ethylene maleic acid), PEMA, which is an 'isomer' ofpoly(acrylic acid), PAA, has a different hydration structure. Whereas inPAA the COOH groups are pendant on alternate chain C atoms, those inPEMA are paired on adjacent chain C atoms. These structural differencesaffect hydration (Begala, 1971). The separation of the hydrophilic carboxylgroups by a pair of hydrophobic chain C atoms effectively prevents thecooperative effect between ionizable groups. Thus, by contrast with PAA,as the degree of ionization increases, the hydration regions around PEMAnever coalesce to form a cylindrical sheath. In the fully ionized state thereis a spherical region of intrinsic water around each carboxyl group and anouter spherical region of water which encloses each pair of carboxyls.

The formation of a stable hydrogen-bonded ring structure as inpoly(itaconic acid) and in poly(maleic acid) has also been shown to affecthydration states (Muto, Komatsu & Nakagawa, 1973; Muto, 1974).

0.9-1.3nm

0.5-0.7 nm

a < 0 . 3 a=0.3 a=1.0Figure 4.8 Cylindrical and spherical hydration regions around poly(acrylic acid) at variousdegrees of neutralization (or charge densities). Based on Ikegami (1964).

75

Poly electrolytes^ ion binding and gelation

4.2.10 Hydration and ion binding

Counterions can affect the structure of hydration regions, and converselyhydration regions can affect ion binding. We have already touched on thissubject in discussing contact and solvent-separated ion pairs in Section4.2.8.

Large bound monovalent cations, e.g. tetrabutylammonium ions, aretoo large to penetrate any of the hydration regions. However, the smallerlithium, sodium and potassium ions are able to penetrate the outermosthydration region of the neutralized polyacid and this is accompanied byvolume increases (Figure 4.9). These cations are probably not site-boundbut are mobile in the outer cylindrical region of hydration (Figure 4.10).

Divalent cations cause a much greater disruption of the hydration

0 . 5 1 . 0

Figure 4.9 Volume increases associated with the binding of various counterions topoly(acrylic acid). Based on Ikegami (1964).

76

Ion binding

regions. These ions completely penetrate the outer hydration region andpartly penetrate the inner one (Figure 4.10). Such effects manifestthemselves in much greater changes in molar volume than are the case formonovalent ions (Figure 4.9). Divalent ions may be considered to be partlymobile and partly site-bound.

Even greater disruption is encountered in the case of trivalent cations(Figures 4.9, 4.10). They completely penetrate both hydration regions anddestroy the structure of water around the polyion. This amounts tocomplete desolvation. The same is true of bound hydrogen ions which arelocalized.

4.2.77 Desolvation and precipitation

Divalent and trivalent ions can precipitate PAA, and this phenomenon isrelated to the loss of a hydration region. Such precipitation is to bedistinguished from salting-out effects which occur with high concentrationsof monovalent ions.

Figure 4.10 The effect of monovalent, divalent and trivalent counterions on the hydrationstate of neutralized poly (acrylic acid). Based on Ikegami (1964).

77


Ikegami & Imai (1962) made a study of precipitation and hydrationusing turbidity, conductance, refractive index and viscosity measurements.The following account is based on their description.

Although all divalent ions precipitate PAA when the degree ofdissociation, a, approaches 1-0, there are differences when a = 0-25 (Figure4.11). Small amounts of barium and calcium ions precipitate PAA at thislow a value, whereas magnesium ions do not. These differences are not tobe attributed to differences in the amounts of counterions bound, forcondensation theory (Section 4.2.3) predicts that all divalent counterionsare bound to polyanions to the same extent (Imai, 1961). Therefore,differences must arise from differences in solubility between the variouspolyacrylates. At low degrees of neutralization barium polyacrylate haslow solubility, while magnesium polyacrylate is very soluble. This is relatedto the extent of disruption of hydration regions as cations are bound topolyions.

Ikegami & Imai (1962) explained their results by assuming that divalentions can be bound to PAA in two forms, which they represented as

(I) COO-Me+ + COO"(II) COO-Me-OOC

1.2 -

0 . 9 -

0 . 6 •

0 . 3

0.25 0.50 0.75

Figure 4.11 The effect of a on the precipitation of PAA by divalent ions. Cs is the critical saltconcentration. Based on Ikegami & Imai (1962).

78

Ion binding

According to these workers the formation of COO-Me+ causes a smalldegree of dehydration, while that of COO-Me-OOC is accompanied byconsiderable dehydration. The experimental results showed that, whena = 1*0, divalent ions are bound as COO-Me-OOC, a form which favoursprecipitation. However, as a decreases, the COO-Me+ form becomes moreapparent.

The ratio of the two forms depends on the cation as well as on a. Ba2+

has a greater tendency to make linkages of the COO-Me-OOC type thanMg2+ and this difference is accentuated when the density of COO~ inthe polyanion is low. Thus, at a = 0*25 more Ba2+ ions are in theCOO-Ba-OOC form than in the COO-Ba+ form, while the reverse is truefor Mg2+ ions. Moreover, the structure COO-Mg+ is more stable andsoluble than COO-Ba+ because Mg2+ is more hydrophilic than Ba2+. Forthese reasons, Ba2+ is precipitated at a = 0-25 while Mg2+ is not. Thisinterpretation is supported by titration experiments in the presence ofdivalent cations (Jacobsen, 1962). Magnesium forms very stable hydratesand would be expected to be more difficult to desolvate.

It is, perhaps, more in line with other thinking to represent form (I) as asolvent-separated ion-pair: COO" H2O Me2+ H2O OOC, and form (II) asa contact ion-pair: COO~ Me2+ OOC. Thus, precipitation occurs when asolvent-separated ion-pair is desolvated.

4.2.12 Conformational changes in polyions

The conformation of macro- or polyions has been defined and discussedbriefly in Section 4.1.1. The conformation of a polyion is determined by abalance between contractile forces, which depend on conformation freeenergy, and extension forces, which arise from electrical free energy. Theextent of conformational change is determined by several factors. Changesare facilitated by the degree of flexibility of the polyion, and conform-ational change is greatest at low concentration of polyions.

Conformation depends on the degree of ionization and concentration ofthe polyion, the type and concentration of the counterion and theinteraction between counterion and polyion. Extension is favoured by lowconcentrations of counterion and polyion. Conformational change is alsoaffected by the extent of the charge on the polyion. As the charge on apolyion increases, the chain uncoils and expands under the influence ofrepulsive forces. Thus, the neutralization of a polyacid is accompanied by

79


chain expansion as carboxyl groups ionize. The distribution of ionizedgroups is labile and depends on conformation; an extended polyion has alarger number of ionized groups than a contracted one.

Ionization of the carboxyl groups is accompanied by binding of thecations. But if counterions are site-bound the charge on the carboxylgroups is neutralized and chain contraction results. A special case is that ofthe polyacid which adopts a contracted form because the close associationof hydrogen ions with carboxyl groups results in a neutral chain.

Extensive forces arise from the electrical interaction between counterionsand polyions. There are two repulsive forces which act to extend a polyion.One results from coulombic repulsion between the charged groups on thepolyion and the other from osmotic pressure of the counterions within,which seek to increase the space in which they can move.

The coulombic force is proportional to the square of the effective chargeon the polyion, i.e. n\. (The effective charge is equivalent to the number offree counterions, nv) When the charge along the polyion, Q, is small theextensive forces involved are those of purely coulombic repulsion.

The most important factor determining the sensitivity of the con-formation to the concentration of polyions is the change in ion activity orosmotic pressure with conformation. If the activity coefficient of thecounterions is sensitive to conformation then conformational changeresulting from concentration changes of polyions becomes large.

Osmotic pressure results from the difference in concentration betweenthe bound but mobile counterions within the polyion and the freecounterions outside it. The concentration of counterions is greater withinthe polyion so that solvent molecules tend to enter this region. The osmoticforce is proportional to the difference n — nt, where n equals the totalnumber of counterions or the number of ionizable groups on the polyion.

The predominant force is that of osmotic pressure, unless both thecharge density and the concentration are low. These statements may besubstantiated by a simple and approximate mathematical treatment whichapplies for very dilute solutions.

Coulombic forces oc n\ oc (nflf oc (/?g)2

where ji is the degree of dissociation;

Osmotic force oc (n-nt) oc n{\~P) oc Q(\-p)

These relationships apply for both forces along and perpendicular to thechain, although the proportionality constants differ. These simple

80

Ion binding

expressions are represented graphically in Figure 4.12. As the figureshows, when Q is low the extensive force depends solely on coulombicrepulsion, thus when Q ^ 1, p = 1

Coulombic force oc Q2, Osmotic force = 0

At higher Q values, when Q>l,PQ=l

Osmotic force oc 1 — 1 /Q

and the Coulombic force is constant.At high Q values the contribution from osmotic pressure predominates.

This is shown by the ratio of the two forces which is given by

Coulombic force: Osmotic force oc /?2/(l —/?)

These considerations apply to dilute solutions. In concentrated solutionsthe extensive forces will be diminished. Also if the bound counterionsbecome site-bound then both extensive forces are diminished. These areimportant factors to consider in the theory of acid-base gelation in ABcements, where solutions are concentrated and many counterions are site-bound.

Osmoticforce

Figure 4.12 The effect of Q on the extensive forces, coulombic and osmotic, acting on acylindrical and a coiled polyion. Based on Oosawa (1971).

81


4.2.13 Interactions between poly ions

Repulsive coulombic forces exist between charged polyions. These areattenuated by the bound counterions; conversely they are stronger forpolyions having a higher concentration of free counterions. When thecharge along the polyion, Q, is small the forces involved are purelycoulombic repulsion forces. However, when Q exceeds a certain value,counterions condense on the polyions and reduce the repulsive forces.

Attractive forces arise from dipole interaction, a result of the fluctuationsin the cloud of counterions. Although the mean distribution of counterionsis uniform along the length of the polyion, there are fluctuations in thecloud of counterions which induce transient dipoles. When two polyionsapproach each other counterion fluctuations become coupled and enhancethe attractive force. Since polyions have a high polarizability theseattractive forces can be considerable.

The repulsive force between polyions, calculated for the meanequilibrium distribution of the counterions, is

ell/eQz* (4.30)

where / is the length of the polyion. The attractive forces are

kTl/D2 (4.31)

where D is the average distance between polyions. If

D<z2e20/s0kT (4.32)

then the attractive force predominates over the repulsive one. This occursfor monovalent ions when D is 0*7 nm and for divalent ions when D is2-8 nm. This means that attraction before direct contact between two rod-like macroions will occur only in the case of multivalent counter ions. Theattractive force is important at high charge densities because it continues toincrease with charge density whereas the repulsive forces become constant.

4.2.14 Polyion extensions, interactions and precipitation

The precipitation of polyelectrolytes by the addition of multivalentcounterions may be explained in these terms. When there are nomultivalent ions in solution there is a strong repulsive force betweenpolyions and the osmotic pressure is large. The solubility of polyions is aresult of these repulsive forces.

82

Gelation

The binding of multivalent counterions decreases the repulsion andcauses attraction between polyions. This attraction is the result of thefluctuation of the counterion distribution and is equivalent to a multivalentcounterion bridge between polyions.

4.3 Gelation

The theory of gelation (Flory, 1953,1974) has been summarized in Section2.2.3. This theory regards gelation as the consequence of the randomcrosslinking of linear polymer chains to form an infinite three-dimensionalnetwork. The phenomenon is, of course, well illustrated by examplesdrawn from the gelation of polycarboxylic acids by metal ions.

Since chemical gelation occurs only when the cations have a valencygreater than one, an early view was that it resulted from the formation ofionic crosslinks, a concept which is useful when applying Flory's theory ofgelation. Thus, in early studies, the gelation of alginates and pectinates byCa2+ ions was attributed to the crosslinking of COO~ groups by Ca2+

bridge formation. Wall & Drenan (1951) had a similar view in their studyof the gelation of poly(acrylic acid) with various divalent alkaline earthions. However, they noted that the concentration of cations required toproduce gelation differed widely between cations and so concluded that thephenomenon could not be explained in terms of simple ionic equilibrium.Implicitly their mechanism assumes that chain extension occurs during ionbinding.

The concept of ionic crosslinking is in accord with the idea that a gelmust possess a coherent structure. However, although crosslinking may beessential to gel formation it does not necessarily have to be a simple ionicsalt bridge.

Michaeli (1960) opposed these views. He concluded that whatever theexact mechanism was, the binding of divalent cations caused contractionand coiling of the polyelectrolyte as was the case with acids. He disagreedwith the concept of ionic crosslinking. The phenomenon of precipitationcould be explained simply in terms of reduced solubility. From this heconcluded that precipitation took place in an already coiled molecule andthe matrix consisted of spherical macromolecules containing embeddedcations.

These early views are, perhaps, too simplistic to explain in full therheological changes that occur in polyelectrolyte cement pastes before andat gelation. There are several physicochemical processes that underlie

83

Poly electrolytes^ ion binding and gelation

such changes: ionization, ion binding, desolvation of the ion-pair,conformational changes in the polymer chain and interpolyion attraction.The extent and rate of interaction between hydrated counterion andpolyanion depends on polymer structure and conformation, acid strength,degree of dissociation, and distribution and density of ionic charge on thepolymer chain.

The underlying physicochemical process leading to gelation in ABcements may be summarized as follows. The interaction between metaloxide or silicate and the polyacid solution involves a neutralizationprocess. As neutralization proceeds and the charge on the polymer chaingrows, the polymer chain, which is initially in random coil form, unwinds,a process which causes the cement paste to thicken. The forces which causethis unwinding are osmotic pressure and coulombic repulsion between thecharged groups on the polymer chain.

The cations released become bound by electrostatic forces to thepolyanionic chain. These counterions can be either mobile (atmospheric)or site-bound at specific centres.

Ion binding reduces the repulsive forces between the charged groups onthe polyanion but, unless the counterions are site-bound, the repulsiveosmotic forces are not affected. At full neutralization the coulombic forcesalong the polymer chain become zero. However, the polymer does notcontract, because the osmotic forces remain; unless, of course, all thecations become site-bound. (Of course, in the case of a free weak acid theconcentration of mobile hydrogen ions is very small and the polymeradopts a compact form.)

Ion binding by reduction of repulsive forces also causes the attractiveforces between polyions to increase, and the cement paste thickens. Thisinteraction between polyions may be regarded as a kind of bridge formedby multivalent ions located between the polyions. At this stage the cementpaste has the characteristic of a lyophilic sol - high viscosity.

It is well known that lyophilic sols are coagulated by the removal of astabilizing hydration region. In this case, conversion of a sol to a gel occurswhen bound cations destroy the hydration regions about the polyanion,and solvated ion-pairs are converted into contact ion-pairs. Desolvationdepends on the degree of ionization, a, of the polyacid, and the nature ofthe cation. Ba2+ ions form contact ion-pairs and precipitate PAA when ais low (0-25), whereas the strongly hydrated Mg2+ ion disrupts thehydration region only when a > 0*60.

More than one type of site binding is possible. There is the simple

84

References

Bjerrum ion-pair formation based on purely coulombic attraction. There isalso complex formation and, if the ligand is bidentate, chelate formationenhances this effect, as in the case of Cu2+ (Wall & Gill, 1954). In cementgels Crisp, Prosser & Wilson (1976) found that the binding of Na+, Mg2+

and Ca2+ was purely ionic, whereas Al3+ binding had some covalentcharacter. There was a suggestion, too, that the binding of Zn2+ might notbe purely ionic. Nicholson et al. (1988a,b) found positive evidence that thebinding of Zn2+ and Al3+ involved some covalent character.

It is difficult to avoid the view, which is consistent with gelation theory,that crosslinking is involved in gelation. Simple ionic bridges fit in with thisview, but there are alternatives. Networks of type 3 (see Section 2.2.3) canbe formed by crosslinks consisting of bundles of chains or multistrandedhelices (Flory, 1974). In gelatin, triple helices are involved, i.e. three chainsare joined at a point (Peniche-Covas et al., 1974). In alginates, gelation isbelieved to result from the formation of a junction zone where there is localchain dimerization with cavities formed capable of holding calcium ions(Reid, 1983). This complicated junction of chain association and ionbinding is known as the 'egg-box' model.

Although this account of gelation is made with reference to organicpolyelectrolytes, it is of wider application and may be applied tophosphoric acid cements. Orthophosphoric acid solutions used in thesecements contain aluminium, and soluble aluminophosphate complexes areformed. Some appear to be multinuclear and there is evidence for polymersbased on the bridging Al-O-P unit. These could be termed polyelectrolytes(Akitt, Greenwood & Lester, 1971; Wilson et al, 1972; O'Neill et al.,1982).

References

Akitt, J. W., Greenwood, N. N. & Lester, G. D. (1971). Nuclear magneticresonance and Raman studies of aluminium complexes formed in aqueoussolutions of aluminium salts containing phosphoric acid and fluoride ions.Journal of the Chemical Society, A, 2450-7.

Asai, H. (1961). Study of the hydration-dehydration in polyelectrolyte solutionsby the ultrasonic technique. Journal of the Physical Society of Japan, 16,761-6.

Atkins, P. W. (1978). Physical Chemistry, p. 338. Oxford: Oxford UniversityPress.

Atkinson, G. & Kor, S. K. (1965). The kinetics of ion association in manganesesulphate solutions. I. Results in water, dioxane-water mixtures, andmethanol-water mixtures at 25 °C. Journal of Physical Chemistry, 69, 128-33.

85


Atkinson, G. & Kor, S. K. (1967). The kinetics of ion association in manganesesulphate solutions. II. Thermodynamics of stepwise association in water.Journal of Physical Chemistry, 71, 673-7.

Begala, A. J. (1971). Interactions of cations with polycarboxylic acids. PhDDissertation. Rutgers University, The State University of New Jersey.

Begala, A. J. & Strauss, U. P. (1972). Dilatometric studies of counterion bindingby polycarboxylates. Journal of Physical Chemistry, 76, 254-60.

Bratko, D., Dolar, D., Godec, A. & Span, J. (1983). Electric transport inpoly(styrenesulfonate) solutions. Makromolekulare Chemie RapidCommunications, 4, 697-701.

Bungenberg de Jong, H. G. (1949). In Kruyt, H. R. (ed.) Colloid Science II, p. 2.Amsterdam: Elsevier Publishing Co. Inc.

Callis, C. F., Van Wazer, J. R. & Arvan, P. G. (1954). The inorganic phosphatesas polyelectrolytes. Chemical Reviews, 54, 777-96.

Cotton, F. A. & Wilkinson, G. (1966). Advanced Inorganic Chemistry, 2nd edn,Chapter 2. New York: Wiley Inter science.

Crisp, S., Prosser, H. J. & Wilson, A. D. (1976). An infra-red spectroscopicstudy of cement formation between metal oxides and aqueous solutions ofpoly (acrylic acid). Journal of Materials Science, 11, 36-48.

Eigen, M. & Tamm, K. (1962a). Schallabsorption in Elektrolytlosungen alsFolge chemischer Relaxation. 1. Relaxationtheorie der mehrstufigenDissoziation. Zeitschrift fur Elektrochemie, 66, 93-107.

Eigen, M. & Tamm, K. (1962b). Schallabsorption in Elektrolytlosungen alsFolge chemischer Relaxation. 2. Messergebnisse und Relaxationmechanismenfur 2-2-wertige Elektrolyte. Zeitschrift fur Elektrochemie, 66, 107-21.

Ellis, J. & Wilson, A. D. (1990). Polyphosphonate cements: a new class ofdental materials. Journal of Materials Science Letters, 9, 1058-60.

Ferry, G. V. & Gill, S. (1962). Transference studies of sodium polyacrylateunder steady state electrolysis. Journal of Physical Chemistry, 66, 999-1003.

Flory, P. J. (1953). Principles of Polymer Chemistry. Ithaca, New York: CornellUniversity Press.


Fuoss, R. M. (1934). Distribution of ions in electrolyte solutions. Transactionsof the Faraday Society, 30, 967-80.

Fuoss, R. M. & Kraus, C. A. (1933). Properties of electrolytic solutions. IV. Theconductance minimum and the formation of triple ions due to the action ofCoulomb forces. Journal of the American Chemical Society, 55, 2387-99.

Gregor, H. P. & Frederick, M. (1957). Titration studies of polyacrylic acid andpolymethacrylic acids with alkali metals and quaternary ammonium bases.Journal of Polymer Science, 23, 451-65.

Gregor, H. P., Gold, D. H. & Frederick, M. (1957). Viscometric andconductometric titrations of polymethacrylic acids with alkali metals andquaternary ammonium bases. Journal of Polymer Science, 23, 467-75.

Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955a). Metal-polyelectrolyte

86

References

complexes. I. The polyacrylic acid-copper complex. Journal of PhysicalChemistry, 59, 34-9.

Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955b). Metal-polyelectrolytecomplexes. IV. Complexes of polyacrylic acid with magnesium, calcium,cobalt and zinc. Journal of Physical Chemistry, 59, 990-1.

Grunwald, E. (1954). Interpretation of data obtained in nonaqueous media.Analytical Chemistry, 26, 1696-701.

Grunwald, E. (1979). Structure of solvated ion pairs from electric dipolemoments. Journal of Pure and Applied Chemistry, 51, 53-61.

Harris, F. E. & Rice, S. A. (1954). A chain model for polyelectrolytes. I. Journalof Physical Chemistry, 58, 725-32.

Harris, F. E. & Rice, S. A. (1957). A model for ion binding and exchange inpolyelectrolyte solutions and gels. Journal of Physical Chemistry, 58, 725-32.

Huizenga, J. R., Grieger, P. F. & Wall, F. T. (1950a). Electrolytic properties ofaqueous solutions of polyacrylic acid and sodium hydroxide. I. Transferenceexperiments using radioactive sodium. Journal of the American ChemicalSociety, 72, 2636-42.

Huizenga, J. R., Grieger, P. F. & Wall, F. T. (1950b). Electrolytic properties ofaqueous solutions of polyacrylic acid and sodium hydroxide. II. Diffusionexperiments using radioactive sodium. Journal of the American ChemicalSociety, 72, 4228-32.

Ikegami, A. (1964). Hydration and ion binding of polyelectrolytes. Journal ofthe Polymer Society, A2, 907-21.

Ikegami, A. (1968). Hydration of polyacids. Biopolymers, 6, 431-40.Ikegami, A. & Imai, N. (1962). Precipitation of polyelectrolytes by salts. Journal

of Polymer Science, 56, 133-52.Imai, N. (1961). Interaction between polyions and low molecular weight ions.

Journal of the Physical Society of Japan, 16, 746-60.Irving, H. & Williams, R. J. P. (1953). The stability of transition-metal

complexes. Journal of the Chemical Society, 3192-210.Jacobsen, A. (1962). Configurational effects of binding of magnesium to

polyacrylic acids. Journal of Polymer Science, 57, 321-36.Kagawa, I. & Gregor, H. P. (1957). Theory of the effect of counter ion size

upon titration behavior of polycarboxylie acids. Journal of Polymer Science,23, 477-84.

Kagawa, I. & Katsuura, K. (1955). Activity of counterions in polyelectrolytesolutions. Journal of Polymer Science, 17, 365-74.

Mandel, M. & Leyte, J. C. (1964). Interactions of poly(methacrylic acid) andbivalent counterions. Journal of Polymer Science, A2, 2883-99.

Manning, G. S. (1969). Limiting laws and counterion condensation inpolyelectrolyte solutions. 1. Colligative properties. Journal of ChemicalPhysics, 51, 924-33.

Manning, G. S. (1979). Counterion binding in polyelectrolyte theory. Accountsof Chemical Research, 12, 443-9.

Manning, G. S. (1981). Limiting laws and counterion condensation in

87


polyelectrolyte solutions. 6. Theory of the titration curve. Journal of PhysicalChemistry, 85, 870-7.

Michaeli, I. (1960). Ion-binding and the formation of insoluble polymethacrylicsalts. Journal of Polymer Science, 48, 291-9.

Morawetz, H. (1975). Macromolecules in Solution, 2nd edn, Chapter 7. NewYork: Wiley.

Muto, N., Komatsu, T. & Nakagawa, T. (1973). Counterion effect on thetitration behaviour of poly(maleic acid). Bulletin of the Chemical Society ofJapan, 46, 2711-15.

Muto, N. (1974). Counterion effect on the titration behaviour of poly(itaconicacid). Bulletin of the Chemical Society of Japan, 47, 1122-8.

Nagasawa, M. & Kagawa, I. (1957). Colligative properties of polyelectrolytesolutions. IV. Activity coefficient of sodium ion. Journal of Polymer Science,25, 61-76.

Nicholson, J. W., Brookman, P. J., Lacy, O. M., Sayers, G. S. & Wilson, A. D.(1988a). A study of the nature and formation of zinc polyacrylate cementusing Fourier transform infrared spectroscopy. Journal of BiomedicalMaterials Research, 22, 623-31.

Nicholson, J. W., Brookman, P. J., Lacy, O. M. & Wilson, A. D. (1988b).Fourier transform infrared spectroscopic study of the role of tartaric acid inglass-ionomer cements. Journal of Dental Research, 67, 1450-4.

O'Neill, I. K., Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). NMRspectroscopy of dental materials. 1.31P studies on phosphate-bonded cementliquids. Journal of Biomedical Materials Research, 16, 39-49.

Oosawa, F. (1957). A simple theory of thermodynamic properties ofpolyelectrolyte solutions. Journal of Polymer Science, 23, 421-30.

Oosawa, F. (1971). Poly electrolytes. New York: Marcel Dekker.Peniche-Covas, C. A. L., Dev, S. B., Gordon, M., Judd, M. & Kajiwara, K.

(1974). The critically branched state in covalent synthetic systems and thereversible gelation of gelatin. In Gels and Gelling Processes. FaradayDiscussions of the Chemical Society, No. 57, pp. 165-80.

Reid, D. S. (1983). Ionic polysaccharides. In Wilson, A. D. & Prosser, H. J.(eds.) Developments in Ionic Polymers-1, Chapter 6. London and New York:Applied Science Publishers.

Rice, S. A. & Harris, F. E. (1954). A chain model for polyelectrolytes. II.Journal of Physical Chemistry, 58, 733-9.

Robinson, R. A. & Stokes, R. H. (1959). Electrolyte Solutions, 2nd edn, Chapter14. London: Butterworths.

Rymden, R. & Stilbs, P. (1985a). Counterion self-diffusion in aqueous solutionsof poly (aery lie acid) and poly (methacry lie acid). Journal of PhysicalChemistry, 89, 2425-8.

Rymden, R. & Stilbs, P. (1985b). Concentration and molecular weightdependence of counterion self-diffusion in aqueous poly(acrylic acid)solutions. Journal of Physical Chemistry, 89, 3502-5.

Salmon, J. E. & Wall, J. G. L. (1958). Aluminium phosphates. Part II. Ion-

References

exchange and pH-titration studies of aluminium phosphate complexes insolution. Journal of the Chemical Society, 1128-34.

Satoh, M., Komiyama, J. & Iijima, T. (1984). Counterion condensation inpolyelectrolyte solutions: a theoretical prediction of the dependences on theionic strength and degree of polymerization. Macromolecules, 18, 1195-2000.

Szwarc, M. (1965). Ions, ion-pairs, and their agglomerates. DieMakromolekulare Chemie, 89, 44-80 (in English).

Smith, D. C. (1968). A new dental cement. British Dental Journal, 125, 381-4.Strauss, U. P. & Leung, Y. P. (1965). Volume changes as a criterion for site

binding of counterions by poly electrolytes. Journal of the American ChemicalSociety, 87, 1476-80.

Sveshnikova, V. N. & Zaitseva, S. N. (1964). Aluminophosphates aspoly electrolytes. Russian Journal of Inorganic Chemistry, 9, 672-5.

Van Wazer, J. R. (1954). In Rice, S. A. & Harris, F. E. (1954). A chain modelfor poly electrolytes. II. Journal of Physical Chemistry, 58, 739.

Wall, F. T. & Drenan, J. W. (1951). Gelation of polyacrylic acids by divalentcations. Journal of Polymer Science, 7, 83-8.

Wall, F. T. & Gill, S. J. (1954). Interaction of cupric ions with polyacrylic acid.Journal of Physical Chemistry, 58, 1128-30.

Wilson, A. D. & Crisp, S. (1977). Organolithic Macromolecular Materials,Chapters 2 & 4. London: Applied Science Publishers.

Wilson, A. D. & Ellis, J. (1989). Poly-vinylphosphonic acid and metal oxide orcermet or glass-ionomer cements. British Patent Application 2, 219, 289A.

Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement: a newtranslucent cement for dentistry. Journal of Applied Chemistry andBiotechnology, 21, 313.

Wilson, A. D., Kent, B. E., Clinton, D. & Miller, R. P. (1972). The formationand microstructure of dental silicate cement. Journal of Materials Science, 1,220-38.

Winstein, S., Clippinger, E., Fainberg, A. H. & Robinson, G. C. (1954). Salteffects and ion-pairs in solvolysis. Journal of the American Chemical Society,76, 2597-8.

Winstein, S. & Robinson, G. C. (1958). Salt effects and ion-pairs in solvolysisand related reactions. IX. The //zre0-3-/>-anisyl-2-butyl system. Journal of theAmerican Chemical Society, 80, 169-81.

89

5 Polyalkenoate cements

5.1 Introduction

Poly(acrylic acid) and its salts have been known to have useful bindingproperties for some thirty years; they have been used for soil consolidation(Lambe & Michaels, 1954; Hopkins, 1955; Wilson & Crisp, 1977) and asa flocculant (Woodberry, 1961). The most interesting of these applicationsis the in situ polymerization of calcium acrylate added to soil (de Mello,Hauser & Lambe, 1953). But here we are concerned with cements formedfrom these poly acids.

The polyelectrolyte cements are modern materials that have adhesiveproperties and are formed by the cement-forming reaction between apoly(alkenoic acid), typically poly(acrylic acid), PAA, in concentratedaqueous solution, and a cation-releasing base. The base may be a metaloxide, in particular zinc oxide, a silicate mineral or an aluminosilicateglass. The presence of a polyacid in these cements gives them the valuableproperty of adhesion. The structures of some poly(alkenoic acid)s areshown in Figure 5.1.

The polyelectrolyte cements may be classified by the type of basicpowder used to form the cement.

(1) The metal oxide cements (Section 5.6)(2) The zinc polycarboxylate cement (Section 5.7)(3) The mineral ionomer cements (Section 5.8)(4) The glass-ionomer or glass polyalkenoate cement (Section 5.9)

Only two of these materials are of practical importance: the zincpolycarboxylate cement of Smith (1968) and the glass-ionomer cement ofWilson & Kent (1971). Both are used in dental applications and both havebeen used as bone cements. The glass-ionomer cement is, perhaps, themost versatile of all AB cements. It has many applications in dentistry: a

90

Introduction

filling material for the restoration of anterior (front) teeth, a cementingagent for the attachment of crowns and bridges, a cavity liner and a baseunder amalgams and composite resins, and a general repair material.Outside dentistry it is marketed as a splint bandage material and as a bonecement. It has also been considered as an underwater cement for North Seapipelines, as a replacement for plaster of Paris in slip casting, and as amodel material.

The invention and development of the zinc polycarboxylate andglass-ionomer cements was brought about by a change in basic attitudes inmaterials science in dentistry. This largely revolved around the necessity ofinventing materials which would adhere to tooth enamel and dentine.

I Acrylic acid unitCH—COOH

CH2

.COOH Itaconic acid unit-CH2COOH

TCH—COOH

CH—COOH

Maleic acid unit

7CH9

.COOH

XH—COOH

CH2COOH

3-Butene 1,2,3-tricarboxylicacid unit

Figure 5.1 The structure of poly(alkenoic acid)s containing acrylic, itaconic, maleic and 3-butene 1,2,3-tricarboxylic acid units.

91

Polyalkenoate cements

5.2 Adhesion5.2.1 New attitudes

Up to the 1950s the quality of a dental material was judged entirely by itsphysical and mechanical properties (Wilson, 1991). This proved to be aconcept which hampered development. We may take the amalgam asrepresenting the traditional dental restorative material with all itsadvantages and disadvantages. The amalgam is strong and resistant toabrasion, but it is essentially a foreign body in the tooth, an unattractiveblack mass of metal that does not bond to tooth structure. In order toensure its mechanical retention, cavities have to be cut which are wastefulof sound tooth material. It does nothing for the tooth and, despite itsexcellent mechanical properties, is little more than a mechanical plug.

In the late 1940s a reaction against this idea of a dental material tookplace. Increasing attention was paid to problems of compatibility betweenthe restoration and the tooth. We now believe that a restorative should beat one with the tooth material in all respects. It should possess identicalproperties. Its thermal characteristics should be the same as those of thetooth and its appearance should match that of the enamel. It shouldprovide some therapeutic action. In fact, a restorative material should nolonger be regarded as a 'filling' but as an 'enamel or dentine substitute'.

5.2.2 The need for adhesive materials

To achieve such compatibility the primary requisite is that the restorativeadheres to tooth material. This concept of adhesion is hardly to be foundin the literature of the 1920s and 1930s. For that reason we find no attemptat developing tooth adhesives in that period. Adhesion was, apparently,only recognized as a desirable property in the 1950s. It seems for somereason to be associated with the introduction of simple resins as dentalrestorative materials. Although they were not a great success, attemptswere made to bond them to tooth material.

The Conference on Adhesive Restorative Dental Materials held inIndianapolis in 1961 (Phillips & Ryge, 1961) may be considered as usheringin the era when dental adhesives were actively sought. Buonocore (1961)summed up the new thinking.

The lack of adhesion of available filling materials to tooth structure isconsidered as one of their shortcomings. A solution to this problemwould indeed represent a milestone in dentistry.

92

Adhesion

Thus, thought became directed towards developing adhesive dentalmaterials, an approach that has led to considerable successes and hasrevolutionized restorative dentistry.

5.2.3 Acid-etching

The first experimental study on adhesion appears in a paper by Kramer &McLean (1952). They reported on the use of glycerol phosphoric aciddimethacrylate as a dentine bonding agent. They achieved some successbut, unfortunately, the bond deteriorated with time (Buonocore, 1961).More significant was the finding reported by Buonocore in 1955. This washis innovative technique for the acid-etching of enamel for the micro-mechanical attachment of dental resins. Resins are capable of penetratingan etched surface and, when polymerized, are bonded to the enamel byresin tags. Buonocore's significant innovation proved to be far ahead of itstime because the simple restorative resins then available were not a clinicalsuccess. Buonocore's invention remained unnoticed until the arrival of thecomposite resin some ten years later. This technique has ensured the lastingsuccess of the composite resin. The dental surgeon now has the means toachieve the aesthetic restoration of damaged incisal edges on anteriorteeth. Formerly such damaged teeth would have had to be crowned.

Although the importance of Buonocore's discovery cannot be over-emphasized, micromechanical attachment cannot be regarded as trueadhesion. True adhesion must be on the molecular level and must involvechemical or physicochemical bonds.

5.2.4 Obstacles to adhesion

There are many obstacles to permanent adhesion under oral conditions.The substrate is a biological tissue and subject to change, and the presenceof moisture represents the worst kind of situation for adhesion. Water isthe great barrier to adhesion. It competes for the polar surface of toothmaterial against any potential polymer adhesive. It also tends to hydrolyseany adhesive bond formed. These twin obstacles gave rise to considerabledoubt as to whether materials adhesive to tooth material could bedeveloped at all (Cornell, 1961).

Nevertheless adhesive materials were developed, for in 1968 DennisSmith announced the zinc polycarboxylate cement (Smith, 1968,1969) and

93


this material was followed by the glass-ionomer cement of Wilson andKent in 1969 (Wilson & Kent, 1971, 1972, 1973, 1974). These inventionsdemonstrated that materials based on poly(acrylic acid) or similarpoly acids are effective dental adhesives. Even today, these materials are theonly ones known with certainty to form a permanent bond to toothmaterial - that is a bond that does not deteriorate with time; if anythingthe bond strength increases (Tyas et al., 1988).

5.2.5 The nature of the adhesion of polyalkenoates to tooth material

Dentine forms the bulk of a tooth and is covered by a harder material,enamel. Enamel is almost entirely inorganic, with only 0-25-0-45 % proteinand 0-60% lipids (Hess, 1961). Enamel is composed of rod-shapedstructural units known as enamel prisms c. 5 jam in diameter (Silverstone,1982). It is generally accepted that the mineral is hydroxyapatite (Posner,1961; Silverstone, 1982), although the evidence is not entirely conclusive.Dentine is composed of 70 % inorganic material, 20 % organic materialand 10 % water (Ten Cate & Torneck, 1982). The mineral portion is largelya hydroxyapatite-like mineral and the organic portion is largely the proteincollagen.

The precise nature of the adhesion of the polyelectrolyte cements tountreated dental enamel and dentine has yet to be established. The earliesttheory was due to Smith (1968) who speculated that the polyacrylatechains of the cement formed a chelate with calcium ions contained in thehydroxyapatite-like mineral in enamel and dentine. Beech (1973) con-sidered this unlikely since it involved the formation of an eight-memberedring. Beech studied the interaction between PAA and hydroxyapatite,identified the formation of polyacrylate and so considered that adsorptionwas due to ionic attraction.

Wilson (1974) emphasized the importance of wetting the substratesurface. Later, as the reaction proceeded, these hydrogen bonds would bereplaced by ionic salt bridges. Wilson stressed the importance of thepolymeric nature of these cements in adhesion. Their polymeric natureallowed interfacial gaps between cement and substrate to be bridged andalso provided a multiplicity of bonds. Under oral conditions, where thesubstrate is subject to change, adhesive bonds will be broken, but if thereare a multiplicity of these, attachment of the cement to the substrate willendure and allow broken bonds to be re-established. It is significant that

94

Adhesion

the related phosphate cements based on monomeric [POJ units do nothave this adhesive property.

Wilson, Prosser & Powis (1983) studied the adsorption of polyacrylateon hydroxyapatite using infrared and chemical methods. They observed anexchange of ions and concluded that polyacrylate displaced surfacephosphate and calcium, and entered the hydroxyapatite structure itself(Figure 5.2). They postulated that an intermediate layer of calcium andaluminium phosphates and polyacrylates must be formed at the cement-

0" 0 0 " o-

Hydroxy apatiteSurface

0"

Ca2+

O 0"

Ca2+

O

Ca2+

COO"

POf

o- oo- oCa2+ Ca2+

Figure 5.2 The adsorption of polyacrylate on hydroxyapatite.

95


hydroxyapatite interface. This layer has actually been observed by Mount(1990) who observed debonding at the interface between this intermediatelayer and the body of the cement when the cement was dehydrated.

Adhesion in vivo appears to be dynamic. Bonding to bone was observedto be disrupted as extensive bone remodelling took place, and then re-established once damage had been repaired (Brook, Craig & Lamb,1991b).

Adsorption studiesIn order to elucidate the mechanism of adhesion of ionomer-carboxylatecements, Wilson and his coworkers have carried out several studies on theadsorption of carboxylates - aliphatic, aromatic and polymeric-onhydroxyapatite (Skinner et ai, 1986; Scott, Jackson & Wilson, 1990; Elliset al9 1990).

Aliphatic monocarboxylates are not adsorbed at all (Skinner et al,1986). The extent of the adsorption of aliphatic dicarboxylates depends onthe spacing between the carboxyl groups, and is greatest when the numberof carbon atoms in the molecule is three or four (malonate and succinate).Adipate (six carbon atoms) is not adsorbed at all. Adsorption is, therefore,dependent on a cooperative effect between pairs of carboxyl groups.Adsorption cannot occur at OH sites in hydroxyapatite for these are 0-69or 0-94 nm apart and, remembering that the length of the C-C bond liesbetween 014 to 016 nm, could only be bridged by the pairs of carboxyls inadipate, which is not adsorbed. However, attachment to H2PC>4 sites ispossible via hydrogen bonds.

Similar results were found in a study of aromatic carboxylates with oneto six carboxyl groups (Scott, Jackson & Wilson, 1990). Adsorptionincreased with the number of carboxyl groups and was also dependent onthe spacing between the carboxyl groups. With the benzene dicarboxylates,maximum permanent adsorption was obtained with the 1,3-dicarboxylate,while the 1,4-dicarboxylates was not adsorbed at all. This is again evidenceof the cooperative effect between carboxyl groups.

Polymeric aliphatic carboxylates, the poly(alkenoic acid)s, were verymuch more strongly adsorbed than the difunctional carboxylates (Ellis etal., 1990). Results showed that adsorption depended on the conformationof the polyanion. When extended, as in dilute solutions, a polyanion isadsorbed onto a relatively large number of sites and further adsorption ishindered. Thus, increases in acidity (and concentration) were found toresult in greater adsorption because the polyanion adopted a more compact

96

Preparation of poly(alkenoic acid)s

conformation. Above a certain concentration, further adsorption had areversible rather than an irreversible (permanent) character. High levels ofadsorption were achieved under conditions of high chain entanglement,that is with polyanions of high molecular weight.

In all studies it was noted that calcium and phosphate ions weredisplaced, the amount generally increasing with the degree of carboxylateadsorption. It would appear that negatively charged carboxylate groupsdisrupt the hydroxyapatite surface, upsetting the equilibrium betweenphosphate in solid phase and solution phase thus allowing phosphate to beexchanged by carboxylate.

5.3 Preparation of poly(alkenoic acid)s

The most common poly(alkenoic acid) used in polyalkenoate, ionomeror polycarboxylate cements is poly(acrylic acid), PAA. In addition, co-polymers of acrylic acid with other alkenoic acids - maleic and itaconicand 3-butene 1,2,3-tricarboxylic acid - may be employed (Crisp & Wilson,1974c, 1977; Crisp et al, 1980). These polyacids are prepared by free-radical polymerization in aqueous solution using ammonium persulphateas the initiator and propan-2-ol (isopropyl alcohol) as the chain transferagent (Smith, 1969). The concentration of poly(alkenoic acid) is keptbelow 25 % to avoid the danger of explosion. After polymerization thesolution is concentrated to 40-50% for use.

Poly(alkenoic acid)s may be prepared as follows. 200 cm3 of a solutioncontaining between 0-5 and 2-5 g of ammonium persulphate contained in aflask is heated to a controlled temperature, lying between 80 and 95 °C,while purging with nitrogen to displace dissolved oxygen. Two solutions,Solution (I) and Solution (II) are added, in the ratio 3-4:1-0, to the flaskcharge, with continuous stirring, over a period of two hours. Thesesolutions are:

Solution (I) 100 g redistilled inhibitor-free alkenoic acid, 20 g propan-2-ol in 100 cm3 water

Solution (II) 0-5-2-5 g ammonium persulphate in 60 cm3 water.

After the addition is completed the contents of the flask are heated fora further two hours. The reaction mixture is then concentrated by vacuumdistillation at 40-45 °C until the desired concentration is attained.

97


The molecular mass of the poly acid obtained lies between 10000 and55000. Increasing the temperature of polymerization and the concen-tration of ammonium persulphate serves to decrease the molecular mass ofthe poly(alkenoic acid).

Poly(acrylic acid) is very soluble in water as are its copolymers withmaleic and itaconic acids. Solutions of 50% by mass are easily obtained.The 'isomer' of PAA, poly(ethylene maleic acid), is not so soluble.However, solutions of PAA tend over a period of time to gel when theirconcentration in water approaches 50 % by mass (Crisp, Lewis & Wilson,1975); this is attributed to a slow increase in the number of intermolecularhydrogen bonds. Copolymers of acrylic acid and itaconic acid are morestable in solution and their use has been advocated by Crisp et al. (1975,1980).

5.4 Setting reactions

The cement-forming reaction of the polyelectrolyte cements may beconsidered to take place in a number of overlapping stages. These are theattack by the acid on the oxide or glass, the migration of the liberated ionsfrom the oxide or glass into the aqueous phase, the ionization of thepolyacid with consequent unwinding of the polymer chain, the interactionbetween the charged chains and oxide or glass cations leading to ionbinding and gelation, and lastly the hardening phase represented by thecontinuation of ion binding.

Setting results from the gelation of the poly(alkenoic acid) by metal ionsliberated from the metal oxide or silicate by acid attack. The gelation ofpolysalts, which has been discussed in Sections 2.2.3 and 4.3, occurs as thepH of the cement increases. As pointed out in those chapters there areseveral physicochemical processes that underlie these rheological changes.Amongst these are conformational changes in the polymer chain, bindingof the cations to the polymer chains, and hydration changes.

As reaction proceeds, the polymer chain (which is in random coil form)unwinds as the charge on it grows as a result of neutralization andionization. This contributes to thickening of the cement paste. Cationsreleased become bound to the polymer chain. Countercations can either bebound to a polyanionic chain by general electrostatic forces or be site-bound at specific centres. More than one type of site binding is possible.Complex formation and, if the ligand is bidentate, chelate formationenhance the effect.

98

Molecular structures

The extent and rate of interaction between hydrated counterion andpolyanion depend on polymer structure, acid strength, conformation,degree of dissociation, and distribution and density of ionic charge on thepolymer chain. This interaction between the cations - the counterions -and the polyanion chain disrupts the hydration regions surrounding both.Desolvation of the ion-pair, which depends on the nature of the cation andthe degree of neutralization, results in gelation. Gelation itself occurssuddenly when the critical condition for the formation of an infiniterandom network is met, that is when there are more than two crosslinks perpolymer chain (Flory, 1953, 1974).

5.5 Molecular structures

The molecular structure of the polyelectrolyte cements has been examinedby a number of workers using infrared spectroscopy (Crisp et ah, 1914;Crisp, Prosser & Wilson, 1976; Wilson, 1982; Nicholson et al., 1988a,b).The asymmetrical COO" stretching modes in particular can be used to

0 0 \ ^

C « 0 Z n 2 + 0 i C ZrC C , Zn

0 0^—•-" P I ' c ———

(a) IONIC \ \ / / (c ) CHELATING BIDENTATE

(b) BRIDGING BIDENTATE

\CH-

\^ CH, Zn

CH C ^(d) ASYMMETRIC UNIDENTATE / \

0(e) CHELATE BIDENTATE

8-membered ring

Figure 5.3 Metal polyacrylate molecular structures: (a) purely ionic, (b) bridging bidentate,(c) chelating bidentate, (d) asymmetric unidentate, (e) chelate bidentate.

99


obtain structural information; if the metal-carboxylate bond is not purelyionic and coordination complexes are formed then there are frequencyshifts. The types of structure are given in Figure 5.3. These structures are(a) purely ionic, (b) bridging bidentate, (c) chelating bidentate, (d)asymmetric unidentate, (e) chelate bidentate (Nakamoto, 1963; Mehrotra,Bohra & Gauer, 1978; Mehrotra & Bohra, 1983). Infrared spectroscopy isnot able to distinguish between all these structures but asymmetricstretching bands can be used to distinguish between COOH (c. 1700 cm"1),ionic COO" (c. 1540 cm"1) and certain coordination complexes. Infraredspectroscopy shows complexes with asymmetric bands both above andbelow the ionic COO" band.

The bonds between PAA and Na+, Mg2+, Ca2+ are purely ionic, but theabsorption bands of other cation-PAA interactions - Zn2+ (1540-1560 cm"1), Cu2+ (1605 cm-1) and Al3+ (1600 cm"1)-show evidence ofcomplex formation. The strength and stability of cements parallel the

ICH2

|

H20 H20

/

CH2

I o \ /CH—C — 0"—Ca2—"0—C CH

I / \ oCH2 H20 H20 CH2

CH2 H20 H20

| 0 \ /CH — C — 0 Ca A

CH2 H20 H20

CH2

CH2 A' H20 CH2

I 8 \ / ICH—C—0'—Al3— 0 — C — CH

I / \ 5 ICH2

IH20 H20 CH2

CH—C H200 " |

CH2 Ca2

CH — C H20

I °CH2

/ H20

Figure 5.4 Hypothetical molecular structures in polyalkenoate cements, where A representsOH" or F-.

100

Molecular structures

magnitude of the complexation constants of the cations and are in theorder

Al3+ > Cu2+ > Zn2+ > Ca2+ > Mg2+.

The detailed molecular structure of the polyelectrolyte cements remainsa subject for conjecture. The structure is determined basically by the chargeand coordination number of the cation. Firstly, we must consider thequestion of coordination and examine it in respect of the three mostimportant cations in these cements: Zn2+, Ca2+ and Al3+. Of the divalentcations, Zn2+ can assume a coordination number of 4, 5 or 6 and Ca2+ of6 or 8. If we assume a coordination number of 6, then an electrically neutralcoordination complex would have to contain two ligands with a singlenegative charge and four neutral ligands. The coordination of Al3+ inaqueous solution is 6 and for an electrically neutral complex this requiresthat there should be three single-charged ligands and three neutral ligands.In AB cements the ligands available are COO~, F", OH~ and H2 O; thereis a possibility of chelate formation and there are a number of possiblecomplexes. Chelate formation and bridging between chains cannot beexcluded.

Cations can be seen as acting as ionic crosslinks between polyanionchains. Although this may appear a naive concept, crosslinking can be seenas equivalent to attractions between polyions resulting from the fluctuationof the counterion distribution (Section 4.2.13). Moreover, it relates to theclassical theory of gelation associated with Flory (1953). Divalent cations(Zn2+ and Ca2+) have the potential to link two polyanion chains. Of course,unlike covalent crosslinks, ionic links are easily broken and re-formed;under stress there could therefore be chain slipping and this may explainthe plastic nature of zinc polycarboxylate cement.

A trivalent cation, for example Al3+, has the potential to link threechains. Sterically, this is improbable; Mehrotra & Bohra (1983) assert thatsimple aluminium tricarboxylates are not known in solution. Neverthelesswe consider it probable that a small proportion of Al3+ ions link threechains, in which all three charged ligands are COO". More probablemolecular structures would contain one or two F~ ions; with a single F",an [A1F(H2O)3]2+ unit could bridge two polyanion chains, while an[A1F2(H2O)2]+ would have no crosslinking ability.

Some possible molecular structures are depicted in Figure 5.4.

101


Table 5.1. Compressive strength of metal oxide-poly{acrylic acid) cements{Elliott, Holliday & Hornsby, 1975; Hornsby, 1977)

Oxide

ZnOCuOHgOPbOMgOBi2O3

liquid,gem"3

1-4201020102-0

Wet compressive

Strength,MPa

768329265832

Modulus,GPa

1-401020-660-610-450-97

Strain atfailure,%

5-4814.44-3

12-93-3

Trup

porosity,%

18242526—12

5.6 Metal oxide poly electrolyte cements

Many divalent and trivalent oxides form cements with PAA (Crisp,Prosser & Wilson, 1976; Hodd & Reader, 1976; Hornsby, 1977). Cementformation was observed using infrared spectroscopy and physical andchemical tests. Of these cements that of ZnO (Smith, 1968) was the first andremains by far the most important; it is given detailed treatment in Section5.7.

Certain oxides of divalent metals, those of ZnO, CuO, SnO, HgO, andPbO, form cements that are hydrolytically stable; in addition MgO, CaO,BaO and SrO form cements that are softened when exposed to water.Compressive strengths of these materials range from 26 to 83 MPa, thestrongest being the copper(II) and zinc polyacrylate cements (Table 5.1).Crisp, Prosser & Wilson (1976) found that for divalent oxides the rate ofreaction increased in the order

CuO < ZnO < CaO < MgO.This is in descending order of the stability constants of the cations.

Trivalent oxides A12O3, La2O3, Bi2O3 and Y2O3 are also capable ofcement formation but the reaction is only partial (Hornsby, 1977).Hornsby also made the interesting observation that B2O3 forms a cementwith poly(acrylic acid), but, since B2O3 is acidic, an acid-base reactiondoes not take place. Although the cement is hydrolytically unstable it is oftheoretical interest; it is to be presumed that cement formation takes placeby the formation of hydrogen-bonded complexes rather than by saltformation.

102

Zinc polycarboxylate cement

The nature of the poly(alkenoic acid) can affect the hydrolyticalstability of metal oxide cements (Hodd & Reader, 1976). For example theB2O3-poly(ethylene maleic acid) cement, unlike its poly(acrylic acid)counterpart, is not hydrolytically stable.

5.7 Zinc poly carboxy late cement

5.7.1 Historical

The zinc polycarboxylate cement was the first of a new generation of dentalcements. It is based on the gelation of concentrated solutions of apoly(alkenoic acid) by zinc ions provided by a zinc oxide powder (Wilson,1975a,b, 1978a). It was invented as a result of a search by Smith (1968,1969) for a luting cement that would, unlike the traditional zinc phosphatedental cement, adhere to tooth material. It was the first adhesive dentalcement discovered and represented a considerable advance in dentalcement technology. It combined the strength of the zinc phosphate cementwith the bland qualities of the zinc oxide eugenol cement.

It is used for luting, lining and as a periodontal pack. Indeed, it can beused to replace the zinc phosphate dental cement in all applications withthe possible exception of post crowns (crowns which are placed on a metalpost placed in the tooth root) and cantilever bridges (Smith, 1982a).

There are a number of brands on the market and, as far as can beascertained, development of this cement has virtually ceased since the mid1970s.

5.7.2 Composition

In their original form these cements came as a zinc oxide powder and aconcentrated solution of poly(acrylic acid) (Wilson, 1975b). Since thenthey have been subject to a number of chemical modifications.

The liquid is usually a 30-43 % solution of a poly(alkenoic acid) whichis a homopolymer of acrylic acid or a copolymer with itaconic acid, maleicacid, or 3-butene 1,2,3-tricarboxylic acid (Smith, 1969; Bertenshaw &Combe, 1972a; Jurecic, 1973; ESPE, 1975; Wilson, 1975b; Suzaki, 1976;Crisp, Lewis & Wilson, 1976a; Crisp & Wilson, 1974c, 1977; Crisp et aL,1980). The method of preparation has already been given in Section 5.3,and the structures of these alkenoic acid units are shown in Figure 5.1.

The molecular mass of these polyacids varies from 22000 to 49000

103


Table 5.2. Composition of zinc polycarboxylate cements(Bertenshaw & Combe, 1972a, b, 1976)

Zinc oxide powder: 85-2-96-8% ZnO; 4-73-10-06% MgOPoly(acrylic acid) solution: 32-4-42-9 %Molecular weight: 15000-50000

Copolymers of acrylic acid with maleic or itaconic acid are sometimessubstituted for poly (acrylic acid).

(Smith, 1969; Bertenshaw & Combe, 1976). There is an optimum molecularmass. A high molecular mass gives high-strength cements but leads todifficulties in manipulation of the cement paste.

Two methods are available for the preparation of the powder (Smith,1969). In one, zinc oxide is ignited at 900 to 1000 °C for 12 to 24 hours untilactivity is reduced to the desired level. This oxide powder is yellow,presumably because zinc is in excess of that required for stoichiometry.Alternatively, a blend of zinc oxide and magnesium oxide in the ratio of9:1 is heated for 8 to 12 hours to form a sintered mass. This mass is groundand reheated for another 8 to 12 hours. The powder is white. Altogetherthe powder is similar to that used in zinc phosphate cements.

Commercial powders are composed chiefly of a deactivated zinc oxidecontaining up to 10% magnesium oxide (Bertenshaw & Combe, 1972b;Kohmura & Ida, 1979). In addition they may contain silica, alumina orbismuth salts. The most important additive is stannous fluoride (4-5 %),which strengthens the cement although it was originally added as a fluoriderelease agent (Foster & Dovey, 1974).

In some brands the polyacid is in dry form and blended with the zincoxide powder (Baumann & Gerhard, 1970; Jurecic, 1973; Bertenshaw &Combe, 1972a). The cement is formed by mixing this powder blend withwater. In early examples, sodium dihydrogen phosphate was added tothe liquid (Bertenshaw & Combe, 1972a); as a result the viscosity of thecement paste was lowered and setting was retarded, possibly because of theslow dissolution of the solid polyacid (Bertenshaw, Combe & Grant, 1979).

Typical compositions of zinc polycarboxylate cements are given in Table5.2.

104


5.7.3 Setting and structure

The cement sets as the result of an acid-base reaction between a zinc oxidedental powder and a poly(alkenoic acid). The pH increases and aninsoluble amorphous salt is formed which acts as the cement matrix. Ageneral account of the gelation processes is given in Section 5.4.

Wilson (1982) studied the setting reaction of a stoichiometric cementand a cement containing 100% excess of zinc oxide, using infraredspectroscopy and physical and chemical methods. The setting of thecement, measured by an oscillating rheometer, was paralleled by the loss ofbands associated with COOH groups (1700 cm"1 asymmetric C-O stretch)and the appearance and growth of carboxylate bands at 1540-1560 cm"1

(asymmetric C-O stretch). Using deuterated cements, two bands wereobserved in the young cement paste (1550 and 1560 cm"1), but only one inthe set cement (1550 cm"1). Nicholson et al (1988a) clarified this pictureusing Fourier transform infrared spectroscopy. An initial fast ionicreaction, associated with a band at 1562 cm"1, was attributed to a purelyionic structure (Figure 5.3a). Later, as the cement matured, bands at 1554and 1548 cm"1 became predominant; these were tentatively assigned tochelated structure (Figure 5.3c). Finally, when the cement had set there wasone band at 1537 cm"1 (Figure 5.3d). This was attributed to a change inbond type during setting and hardening. Of course, assignment of bands tobond type is rendered more difficult by hydration and dehydrationprocesses. Wilson, Paddon & Crisp (1979) and Wilson, Crisp & Paddon(1981) noted that as the cement matured the proportion of bound water tototal water increased.

X-ray diffraction shows that both the cement matrix and the salt areamorphous (Wilson, 1982; Smith, 1971; Steinke et al, 1988). On the basisof chemical analysis, Wilson (1982) assigned the following empiricalformula to the zinc polyacrylate salt:

Zn0.98H0.004(CH2. CH. COO)2 ,{H,O\ 61

He compared the infrared spectra of cements with that of zinc polyacrylatesalt and found differences. Inspection of his data shows that, unlike thecements, the salt was purely ionic, so that it seems here that cementformation is associated with the formation of coordination complexes.There are no ligand field stabilization effects with the Zn2+ ion because ithas a completed d shell (Cotton & Wilkinson, 1966). For this reason the

105


stereochemistry of Zn2+ compounds is determined solely by size, elec-trostatic forces and covalent bonding forces. Zinc can be four-, five- or six-coordinate. Most commonly it is four-coordinate, although six-coordinatecompounds are known. Five-coordination is rare.

Scanning electron microscopy shows the cement to consist of zinc oxideparticles embedded in an amorphous matrix (Smith, 1982a). As with thezinc phosphate cement, a separate globular water phase exists since thecement becomes uniformly porous on dehydration. Porosity diminishes asthe water content is decreased. Wilson, Paddon & Crisp (1979) distinguishbetween two types of water in dental cements: non-evaporable (tightlybound) and evaporable (loosely bound). They found, in the example theyexamined, that the ratio of tightly bound to loosely bound water was0-22:1-0, the lowest for all dental cements. They considered that looselybound water acted as a plasticizer and weakened the cement.

Most practical cements contain Mg2+ which is less strongly bound to thepolyacrylate than Zn2+ (Gregor, Luttinger & Loebl, 1955a). Magnesiumoxide forms a paste with PAA which sets to a plastic mass; this is nothydrolytically stable, for when placed in water it swells and softens(Hornsby, 1977; Smith, 1982a). Moreover, if ZnO powder contains morethan 10% MgO, the resultant cement deteriorates under oral conditions.

Evidence for the firm binding of Zn2+ comes from studies using labelledzinc polyacrylate containing 65Zn and 14C. Only small amounts of theseions were lost to a saline solution over a three-month period, even in thepresence of calcium (Peters et al., 1972; Peters, Jackson & Smith, 1974).There is some evidence, from leaching studies, that Zn2+ is more firmlybound to a copolymer of acrylic and itaconic acids than to poly(acrylicacid), and less firmly bound to a copolymer of maleic and acrylic acids.

5.7.4 PropertiesSetting

The zinc polycarboxylate cement sets within a few minutes of mixing andhardens rapidly. Strength is substantially developed within an hour.However, even when fully hardened the cement exhibits marked plasticbehaviour. Its most important property is its ability to bond permanentlyto untreated dentine and enamel.

The early zinc polycarboxylate cement did not possess the ease of mixingcharacteristic of the zinc phosphate and zinc eugenolate cements. Itsuffered because it was expected to mix exactly as a traditional zinc

106


Table 5.3. Properties of zinc polycarboxylate cements (Jendresen& Trowbridge, 1972; Plant, Jones & Wilson, 1972; Paddon &Wilson, 1976; Powers, Johnson & Craig, 1974; Powers, Farah &Craig, 1976; Chamberlain & Powers, 1976; Levine, Beech &Garton, 1977; 0ilo & Espevik, 1978; Bertenshaw, Combe &Grant, 1979; Peddy, 1981; Hinoura, Moore & Phillips, 1986)

Working time, 23 °C 2-5 minutesSetting time, 37 °C 3-12 minutesCompressive strength (wet), 24 h 48-80 MPaa

Tensile strength (wet), 24 h 4-8-15-5 MPab

Compressive modulus (wet), 24 h 3-2-6-2 GPaAdhesion to enamel, 24 h (tensile) 4-1-6-9 MPaAdhesion to dentine, 24 h (tensile) 2-2-5-1 MPa

a Omitting atypical high and low values of 10 and 100 MPa6 Measured by the diametral compression method, omitting anatypical low value of 1-5 MPa

phosphate cement and the viscous nature of the polyacid liquid coulddeceive the operator as to the actual fluidity of the cement paste (McLean,1972). Frequently, the cement was mixed too thinly in a misguided attemptto make it appear as fluid as a zinc phosphate cement paste. This led topoor properties. In fact, the fluidity of the cement is greater than theapparent consistency of the cement paste would suggest, because it ispseudo-plastic. Thus, it exhibits shear thinning when a restoration is seatedon it, and it flows as readily as a zinc phosphate cement (Mortimer &Tranter, 1969; McLean, 1972).

An unfortunate characteristic of early zinc polycarboxylate cements wasthe early development of elastomeric characteristics - 'cobwebbing' - inthe cement pastes as they aged, thus shortening working time (McLean,1972). Improvements in cement formulation, the addition of stannousfluoride to the oxide powder (Foster & Dovey, 1974, 1976) and modi-fications in the polyacid have eliminated this defect. However, the cementshave to be mixed at quite a low powder/liquid ratio, 1*5:1-0 by mass, whenused for luting.

The properties of these cements - the fluidity of the mix, the workingand setting times of the cement paste, and the strength of the set cement- are affected by a number of factors. These include the composition of thepowder, the concentration, molecular mass and type of the polyacid, the

107


powder/liquid ratio and the presence or absence of metal fluorides (Smith,1971; Foster & Dovey, 1974, 1976).

Working time varies from 2 to 5 minutes (at 23 °C) and setting time from3 to 12 minutes (at 37 °C) (Plant, Jones & Wilson, 1972; Jendresen &Trowbridge, 1972; Chamberlain & Powers, 1976; Powers, Johnson &Craig, 1974) (Table 5.3). These ranges are suitable, at the lower end, for thecementation of single crowns and, at the upper end, for bridges. As withother cements, working time can be prolonged by refrigerating the mixingslab (McLean, 1972; Chamberlain & Powers, 1976).

Bertenshaw, Combe & Grant (1979) found the film thickness of cementsto vary widely from 20 |am to 110 |im, but this property depends on theplasticity of the paste which changes rapidly with time. Thus, 0ilo & Eyje(1986) found for one cement that film thickness increased from 15 \mi at1 min, to 25 jim at 3*5 min, and to 60 \xm at 5 min. In practice, film thick-nesses lower than 25 jim, the specification upper limit for luting agents,can be obtained (Jendresen & Trowbridge, 1972; 0ilo & Evje, 1986).

There is a hardening stage after set when the cement rapidly becomesstronger and less plastic (Plant & Wilson, 1970; Bertenshaw, Combe &Grant, 1979; Paddon & Wilson, 1976; Wilson, Paddon & Crisp, 1979).

Mechanical propertiesAll properties are time-dependent. Smith (1982a) reported one examplethat developed 80% of its ultimate tensile strength in one hour andmaximum strength in 24 hours. Watts, Combe & Greener (1979) notedlittle change in strength in seven days while Smith (1971) reported a slightdecline which he attributed to water sorption. Small increases in strengthhave been recorded after 30 days (Osborne et al.9 1978; Smith, 1971) and228 days (Smith, 1977). Paddon & Wilson (1976) found little increase ineither strength or modulus after 24 hours.

When cements are mixed to a luting consistency, compressive strengthvaries, typically, from 48 to 80 MPa, compressive modulus from 3-2 to6-2 GPa and tensile strength from 4-8 to 15-5 MPa (Table 5.3), allmeasurements being made on 24-hour-old cements. These figures are notabsolute as they depend on test conditions. Thus, 0ilo & Espevik (1978)found a temperature dependency; an increase in temperature from theusual 23 °C to 37 °C reducing the compressive strength of a cement from48 MPa to 36 MPa and modulus from 3-2 GPa to 1-9 GPa.

The cement shows marked viscoelastic properties. Thus, measuredstrength is affected by the crosshead speed of the testing machine and this

108


effect is particularly noticeable if the crosshead speed is less thanO^mmmnT1 (0ilo & Espevik, 1978; Wilson & Lewis, 1980). Wilson &Lewis (1980) recorded a compressive strength of 65 MPa with crossheadspeed of 0-05 mm min"1 which increased to 100 MPa when the crossheadspeed was increased to 20 mm min"1.

Unlike other aqueous dental cements, the zinc polycarboxylate retainsplastic characteristics even when aged and shows significant stressrelaxation after four weeks (Paddon & Wilson, 1976). It creeps under staticload. Wilson & Lewis (1980) found that the 24-hour creep value for onecement, under a load of 4-6 MPa, was 0-7 % in 24 hours, which was morethan that of a zinc phosphate cement (0-13 %) and a glass-ionomer cement(0-32%), but far less than that of the zinc oxide eugenol cement (2-2%).

Plastic deformation is observed when the freshly set cement is subjectedto a slowly increasing load at 37 °C (Plant & Wilson, 1970; Hertet et al.,1975; Paddon & Wilson, 1976; 0ilo & Espevik, 1978; Wilson, Paddon &Crisp, 1979). 0ilo & Espevik (1978) recorded strain at failure of 1-7%, at23 °C, and 4-3%, at 37 °C, values which are greater than that of a zincphosphate cement and far less than that of ZOE and EBA cements(Chapter 9).

The plastic strain at fracture decreases markedly with time as the cementages; also the elastic modulus increases (Wilson, Paddon & Crisp, 1979;Barton et al., 1975). There is an increase in dynamic modulus with time(Barton et al., 1975).

Properties are affected by temperature. Compressive strength is reducedfrom 48 MPa at 23 °C to 36 MPa at 37 °C. Strain at failure increases from1-7% at 23 °C to 4-3% at 37 °C. But these are nothing like the massivechanges encountered with the ZOE and EBA cements. Although theseappear to be about as strong as the zinc polycarboxylate cement whenmeasurements are made at 23 °C, they are far weaker when tested at 37 °C,the temperature of the mouth.

ErosionErosion depends on the solubility of the powder (the filler) and the matrixin the aqueous medium. Here, acidity and complexing power of thesolution for metal ions compared with the stability of the metal PAAcomplexes are important.

The solubility of these cements in water (when aged from one to 24hours) is small and ranges from 0-1 to 0-6% (Gourley & Rose, 1972;Bertenshaw, Combe & Grant, 1979; Crisp, Lewis & Wilson, 1976a; Smith,

109


1971; Chamberlain & Powers, 1976; Jendresen & Trowbridge, 1972). Theaddition of stannous fluoride to the cement increases dissolution, but thisis an advantage rather than a disadvantage, for the fluoride released istaken up by neighbouring enamel (Bitner & Weir, 1973).

Once the cement has aged, dissolution occurs mainly at the site of theoxide particles rather than at the matrix (Crisp, Lewis & Wilson, 1976a;Anzai et al., 1977). The addition of high levels of magnesium oxide to zincoxide (for the purpose of densification) is undesirable. Early commercialexamples, containing 10% or more MgO added to the ZnO powder,absorbed water, swelled and showed high dissolution. Crisp, Lewis &Wilson (1976a) found that both zinc and magnesium are steadily elutedfrom these cements with magnesium predominating. This observation ledthem to recommend the omission of magnesium oxide from cementformulations. The nature of the poly(alkenoic acid) was found to affect therate of elution of zinc. This elution was highest for cements based on acopolymer of maleic and acrylic acids and lowest for those based on acopolymer of acrylic and itaconic acids. Values for cements based onpoly(acrylic acid) lay between these two extremes. Water absorptionvaried, according to brand, from 1-2 to 3-4 % and appeared to increase withthe ratio of COO to total C in the poly(alkenoic acid). It was also affectedby powder/liquid ratio.

More important is the behaviour of these cements in solutionsapproximating to conditions in the mouth. Calcium does not affect thestability, but phosphate, also a constituent of saliva, increases dissolution(Peters et al, 1972; Peters, Jackson & Smith, 1974).

Acidic conditions greatly increase the erosion of the cement, to an extentdepending on the nature of the acid. Using the impinging jet method withlactic acid/lactate solutions, Wilson et al. (1986b) found, for one cement,erosion rates of 0-4% per hour at pH = 5-0, 5-3 % at pH = 4-0 and 16-2%at pH = 2-7. For a group of different cements, Wilson et al. (1986a) founderosion rates in solutions of pH = 2-7 in the range 8-5 to 19-8 %, and in oneexceptional case 0-1 %. These workers found that the zinc polycarboxylatecement was markedly less resistant to acid erosion than the aluminosilicateglass cements, the glass-ionomer and dental silicate cements. They alsofound that, with one exception, zinc polycarboxylate cements weresomewhat less resistant to acid erosion than the zinc phosphate cement.These results have been confirmed by Smink & Arends (1980) and Beech &Bandyopadhyay (1983) using a similar method.

In vivo studies show that zinc polycarboxylate cements are much less

110


resistant to erosion than aluminosilicate cements, but there is no consensuson their durability vis-a-vis the zinc phosphate cement (Ritcher & Ueno,1975; Mitchem & Gronas, 1978, 1981; Osborne et al9 1978; Pluim &Arends, 1981, 1987; Mesu & Reedijk, 1983; Theuniers, 1984; Pluim et al.,1984; Pluim, Arends & Havinga, 1985).

AdhesionAn important property of the zinc polycarboxylate cement is its ability tobond to untreated dentine and enamel. It also adheres to bone. Thisadhesion can be observed visually (Mizrahi & Smith, 1969b; Smith, 1975;Abramovich et al., 1977) and there is, of course, mechanical and chemicalevidence as well. After fracture, areas of cement remained attached to thesubstrate (Smith, 1975; Eick et al., 1972). The principles and mechanism ofadhesion have already been discussed in Section 5-2. The bond strength totooth material develops rapidly in a matter of hours (Mizrahi & Smith,1969a) and is maintained over many months (Mizrahi & Smith, 1969b,1971). The tensile bond strength of the zinc polycarboxylate cement tountreated dentine is 2-2 to 5-1 MPa (Bertenshaw, Combe & Grant, 1979;Peddy, 1981; Levine, Beech & Garton, 1977; 0ilo, 1981; Hinoura, Moore& Phillips, 1986). The bond strength to enamel is somewhat higher at 4-1to 6-4 MPa (Peddy, 1981; Levine, Beech & Garton, 1977). Jemt, Stalblad& 0ilo (1986) find that in vivo bond strengths are much lower and reportedtensile values as low as 1-7 and 2-8 MPa. The lower bond strength todentine is significant and emphasizes that the bonding is to apatite. Thus,demineralization of tooth material by acids reduces bond strength (Smith,1975).

Both fluoride and calcium ions lead to an increase in bond strength.Fluoride-containing cements bond more strongly to dentine, with a shearbond strength of 7-0 MPa against 5-2 MPa for others (Causton, 1982);bond strength can be increased by increasing the calcium concentrations atthe dentine surface (Beech, 1973). Such observations led to successfulattempts to improve bond strength by pre-treating dentine with speciallyformulated solutions. Calcifying solutions were developed to pre-treat thetooth surface and so improve bond strength. Levine, Beech & Garton(1977) used a solution containing calcium hydrogen phosphate, sodiumfluoride and disodium hydrogen phosphate to pre-treat dentine and soraised tensile bond strength from 2-4 to 5-5 MPa. Similarly, Causton &Johnson (1982) used their so-called IT-S solution, a calcifying isotonicsolution buffered with carbonate and phosphates to pH of 7-4, and

111


improved the shear bond strength of two examples of cement from 5-2 and7-0 MPa to 7-2 and 12-9 MPa respectively.

The cement also bonds to metals. Saito et al. (1976) found that the bondstrength of zinc polycarboxylate cement to alloys decreased in the followingorder of substrate: copper alloy, nickel-chromium alloy, silver-palladiumalloy, type III gold alloy. The bond strength to stainless steel has beenreported as varying from 6 to 9 MPa (Moser, Brown & Greener, 1974;Jendresen & Trowbridge, 1972; Mizrahi & Smith, 1969a,b) with similarvalues for the cobalt-chromium alloy (Moser, Brown & Greener, 1974).The cement does not bond to the inert surfaces of porcelains.

Biological propertiesThe biocompatibility of these materials is in general excellent (Beagrie,Main & Smith, 1972; Peters et al, 1972; Peters, Jackson & Smith, 1974;Beagrie et al, 1974; Lawrence, Beagrie & Smith, 1975; Eames, Hendrix &Mohler, 1979; Main et al., 1975). Results from in vitro experiments areconflicting, but it would appear that cytotoxic effects (inhibition of cellgrowth or cell death) are low unless test conditions permit the release oftoxic concentrations of zinc ions, fluoride ions and free poly(acrylic acid)(Peters et al., 1972; Leirskar & Helgeland, 1977; Spangberg, Rodrigues &Langeland, 1974; Welker & Neupert, 1974). It is probable that thesecements do not cause cytotoxic effects in use. However, the possible releaseof zinc rules them out as bone cements.

The effects of zinc polycarboxylate cements on calcified tissue appearminimal and they are as bland as the zinc oxide eugenol cement is towardsdental pulp (Smith, 1969; Plant, 1970; von Klotzer et al., 1970; Barnes &Turner, 1971; Beagrie, Main & Smith, 1972; Jendresen & Trowbridge,1972); the traditional zinc oxide eugenol is recognized as exceptional in thisrespect. Reactions were mild in the teeth of monkeys even in deep cavities(El-Kafrawy et al., 1974). In clinical practice these cements give little painand there are no long-term adverse effects (McLean, 1972). They are lessirritating than the zinc oxide eugenol cement when used in implants in softtissue and bone (Beagrie, Main & Smith, 1972; Lawrence, Beagrie &Smith, 1975).

There are several reasons why these cements are bland. Acid irritation isprobably minimal. Poly(acrylic acid) is a weak acid and, in addition,because of its high molecular weight will not readily diffuse along dentinaltubules and is also immobilized by phosphatic material in these tubules.Moreover, once set these cements rapidly become neutral.

112


Another cause of inflammation is leakage of bacteria from the mouth atthe interface between the cement and tooth material. Adhesion at theinterface reduces this effect.

Full accounts of the biological responses of these cements are to befound in reviews by Smith (1982b), Helgeland (1982) and Granath (1982).

5.7.5 Modified materials

The most important modification of these materials was the discovery ofthe effect of adding stannous fluoride (Foster & Dovey, 1974, 1976).Originally added to provide fluoride release, it was found to improve themixing qualities of the cement and to increase strength by about 50 %. Thisis reflected also in improved adhesion to enamel and dentine (Section5.7.4).

Attempts have been made to improve the mechanical properties of thesecements by adding reinforcing fillers (Lawrence & Smith, 1973; Brown &Combe, 1973; Barton et al, 1975). Lawrence & Smith (1973) examinedalumina, stainless steel fibre, zinc silicate and zinc phosphate. The mosteffective filler was found to be alumina powder. When added to zinc oxidepowder in a 3:2 ratio, compressive strength was increased by 80% andtensile strength by 100 % (cements were mixed at a powder/liquid ratio of2:1). Because of the dilution of the zinc oxide, setting time (at 37 °C) wasincreased by about 100 %. As far as is known, this invention has not beenexploited commercially.

5.7.6 Conclusions

The cement is to be regarded as a replacement for the zinc phosphatecement. Its advantages over that traditional cement are its adhesion totooth substance and a more bland reaction towards living tissues. Itsdisadvantages are that it is less user-friendly than the zinc phosphatecement and requires greater care in preparation. Unlike other aqueous-based dental cements, it retains distinct plastoelastic properties even afterageing for months. Thus, it is less rigid and more liable to creep. Whetherthis constitutes an advantage or disadvantage is a matter of opinion. Thecurrent view is that cement properties should match those of dentine, inwhich case it is too plastic. On the other hand, the zinc phosphate cementis too rigid.

113


5.8 Mineral ionomer cements

Cements can be formed by reacting silicate minerals with solutions of apoly(alkenoic acid) but they are weaker than zinc polycarboxylate andglass polyalkenoate cements and so far have found little practical use. Theyare, however, of theoretical interest for they give some insight into thesilicate structures required for cement formation. Broadly, silicates that arecapable of cement formation fall into the class of naturally occurringsilicates known as gelatinizing minerals. These are minerals decomposedby acids with the formation of silica gel; this behaviour is related to silicatestructure. Larsen & Berman (1934) recorded the effect of acids on a largenumber of minerals and their work has been used by others to classifygelatinizing minerals (Murata, 1943; Mase, 1961; Petzold, 1966).

It is helpful in the discussion to describe silicate structures using the Qn

nomenclature, where Q represents [SiOJ tetrahedra and the superscript nthe number of Q units in the second coordination sphere. Thus, isolated[SiO4]4~ are represented as Q° and those fully connected to other Q units asQ4. In general, minerals based on Q°, Q1, and Q2 units are decomposed byacids. Such minerals are: those containing isolated silicate ions, theorthosilicates, SiO4^ (Q°); the pyrosilicates, Si2C>7~ (Q1); ring and chainsilicates, (SiO3)2w~ (Q2). Certain sheet and three-dimensional silicates canalso yield gels with acids if they contain sites vulnerable to acid attack. Thisoccurs with aluminosilicates provided the Al/Si ratio is at least 2:3 whenattack occurs at Al sites, with scission of the network (Murata, 1943).

Mase (1961) explains the reactivity of silicate minerals towards acids interms of the polarizing ability of the cation and the bond energy of themineral. Clearly, too, the reactivity of minerals towards acids is connectedwith their basicity, and one notes that the orthosilicates are basic minerals.According to the ideas of Flood & Forland (1947a,b) (Section 2.3.3)basicity is related to the residual polarizability of the oxygen atom. If thisis large, as will be the case if the associated cation has little polarizingpower, then the oxygen atom is basic. Thus, sodium silicates are the mostbasic of the silicates, and silica itself is acidic and so resistant to acid attack.Basicity is also related to the silicate structure: orthosilicates are morebasic than pyrosilicates which, in turn, are more basic than ring and chainsilicates.

Using this information, Crisp et al. (1977, 1979) and Hornsby et al.(1982) selected candidate minerals for cement formation with poly (acrylicacid) and found a number of minerals that formed cements (Table 5.4).

114

Table 5.4. Properties of mineral ionomer cements {Crisp et al, 1979; Hornsby et aL, 1982)

Powder:liquid,gem"3

2-02-1

1-010

1-0

1-5

0-5

10

1-0

Settingtime,min

26140

486

14

29

20

13

59

Compressivestrength(24 h), MPa

Humid

1940

19

18

28

160

89

134

Water

2140

612

3

35

30

37

2

Sohibil

0-35002

2-40-4

3-4

10

1-35

2-4

3-6

OrthosilicatesWillemiteGadolinite

PyrosilicatesGehleniteHardystonite

Chain silicateWollastonite

Sheet silicateThuringite

ZeoliteScolecite

UltramarineHackmanite

FeldsparLabradorite

Zn2[SiOJBe3Fe(YO)2[SiOJ2

Ca2Al[AlSiO7]Ca2Zn[Si2O7]

Ca3[(SiO3)3]

(Fe(II), Fe(III), Mg, Al)12[(Si, Al)8O20](O, OH, F) r

Ca[Al2Si3O10]3H2O

Na8[Al6Si6O2J(Cl2,S)

(Ca,Na)[Al1_2Si2_3O8]


While some formed hard, rigid cements that were stable in water, othersyielded rubbery or plastic masses that were hydrolytically unstable.Minerals with cement-forming capability were found in the followingclasses:

(1) Island silicates containing discrete ions. Orthosilicates,SiO4~ (Q°); pyrosilicates, Si2O*~ (Q1); and ring silicates, Si3OJ-,S&.O"- (Q2).

Most orthosilicates reacted completely with poly(acrylic acid)solution; an exception was andradite, Ca3Fe2 [SiO4]3. Even so, thecements of gehlenite and hardystonite were very weak and affectedby water. Only gadolinite and willemite formed cements of somestrength which were unaffected by water, probably because onecontained beryllium and iron and the other zinc.

(2) Chain silicates, consisting of connected metasilicate units,(SiO3)2w~ (Q2), and of an open structure. Wollastonite Ca(SiO3)reacted completely with poly(acrylic acid), but the cement wasmuch affected by water.

(3) Sheet silicates (Q3) with significant isomorphic replacement of Si4+

by Al3+ or Fe3+. These were decomposed by poly (aery lie acid) tosilica gel. The chlorite, thuringite, formed a strong cement but wasmuch affected by water.

(4) Aluminosilicates with a three-dimensional network (Q3 and Q4)where the Al/Si ratio was 2:3. These reacted with poly(acrylicacid), but none reacted completely.

The zeolite, scolecite, the feldspar, labradorite, and the ultra-marine, hackmanite, gave high-strength cements but all weremuch affected by water - the strength of the labradorite cementdisappeared almost entirely - possibly because of the presence offree acid.

5.9 Glass polyalkenoate (glass-ionomer) cement

5.9.1 Introduction

The glass polyalkenoate cement, formerly known as the glass-ionomercement, was invented by Wilson and Kent in 1969 (Wilson & Kent, 1973)and is now well established as a material that has an important role inclinical dentistry. It has proved to have considerable development potentialand has been subjected to continuous development, improvement and

116

Glass polyalkenoate (glass-ionomer) cement

diversification. It is the most versatile of all dental cements and currentlyaccounts for most of the research and development on them. There areother applications of the cement as a biomaterial. It is used as a splintbandage material and as a bone cement.

Glass polyalkenoate cement has a unique combination of properties. Itadheres to tooth material and base metals. It releases fluoride over a longperiod and is a cariostat. In addition it is translucent and so can be colour-matched to enamel. New clinical techniques have been devised to exploitthe unique characteristics of the material.

The material originated from the general dissatisfaction with the clinicalperformance of the dental silicate cement. Wilson and his coworkers madeextensive studies on the dental silicate cement (Section 6.5) and drew theconclusion that this cement could not be further improved. Wilson (1968)examined several alternatives to orthophosphoric acid, including organicchelating agents, as a liquid cement-former, but none of these weresuccessful. Finally, after considerable research, the glass polyalkenoatecement was developed (Wilson & Kent, 1971, 1972, 1973). The cement isformed by mixing an ion-leachable glass powder with an aqueous solutionof a poly(alkenoic acid). The glass is generally a fluoride-containingcalcium aluminosilicate but calcium may be replaced by strontium orlanthanum.

The cement was originally known as ASPA, an acronym of Alumino-silicate Polyacrylic Acid. The term ASPA is now applied to materialsdeveloped by the Laboratory of the Government Chemist in the UK, andwas once also the brand name of an early commercial material. For manyyears it was known as the glass-ionomer cement - indeed, that is still theterm in common use-but the International Standards Organizationofficially adopted the name glass polyalkenoate cement. The termglass-ionomer cement is now used as a generic term to cover these cementsand the new glass polyphosphonate cements invented by Ellis and Wilsonin 1987-9 (Ellis & Wilson, 1990).

5.9.2 GlassesGeneral

The powders used in glass polyalkenoate cement formulations are preparedfrom glasses and not opaque sintered masses. In this they resemble thetraditional dental silicate cement from which they are descended. The glassplays several roles in the chemistry and physics of the glass polyalkenoate

117


cement. It acts as a source of ions for the cement-forming reaction, controlsthe setting rate and strength of the cement and imparts the property,unusual in a cement, of translucency.

Chemically these are special aluminosilicate glasses. Until quite recently,all were calcium aluminosilicates, but now calcium is sometimes wholly orpartly replaced by strontium and lanthanum. Most glasses also containfluorides which, besides lowering the temperature of glass fusion, play arole in cement formation and affect cement properties. Provided the Al/Siratio is high enough, these glasses are decomposed by acids to releasecement-forming ions (Wilson & Kent, 1973, 1974; Crisp & Wilson,1978a,b, 1979; Kent, Lewis & Wilson, 1979; Wilson et al., 1980; Hill &Wilson, 1988a). They are similar to the glasses used for dental silicatecements, although the Al/Si ratio is higher.

Types of glassThere are a great number of potential glasses and some can be extremelycomplex. All contain silica and alumina and an alkaline earth or rare earthoxide or fluoride. The two essential glass types are SiO2-Al2O3-CaO andSiO2-Al2O3-CaF2, from which all others are derived.

Oxide glasses have been reported by Crisp & Wilson (1978a,b, 1979),Wilson et al. (1980), and Hill & Wilson (1988a). The fusion mixturescontain silica, alumina and calcium carbonate to which sodium carbonateor calcium orthophosphate may be added. They may be represented thus,with fusion temperature given in parentheses:

SiO2-Al2O3-CaO (1350-1550 °C)SiO2-Al2O3-CaO-P2O5 (1370-1450 °C)SiO2-Al2O3-CaO-Na2O (1200-1350 °C)

In fluoride glasses, calcium fluoride is an essential constituent, but generallycryolite, Na3AlF6, is also added as a flux to lower the temperature offusion. Aluminium orthophosphate is also generally added to the fusionmixture for various reasons. Of course, the various elements may be addedin different ways. Thus, calcium orthophosphate, aluminium fluoride andsodium carbonate are often used in the preparation of fluoride glasses.

Apart from lowering the temperature of glass fusion, fluoride improvesthe handling qualities of the cement paste, increases cement strength andtranslucency, and has a therapeutic quality when used as a dental fillingmaterial. In fluoride glasses the ratio of alumina to silica controls thesetting time of the cement; fluoride tends to slow setting while aluminium

118


orthophosphate improves the mixing of the paste. Sodium in the glassimproves the translucency of the cement but can affect its hydrolyticstability. In addition, glasses have been reported where calcium is replacedby strontium or lanthanum (Akahane, Tosaki & Hirota, 1988) whichimpart radio-opacity to the cement.

Fluoride glasses are difficult to classify because the various constituentscan be added to the fusion mixture in several ways. However, glasses of theLaboratory of the Government Chemist (Wilson & Kent, 1973; Kent,Lewis & Wilson, 1979; Wilson et ai, 1980; Hill & Wilson, 1988a), whichform the basis of many commercial cements, can be represented as

SiO2-Al2O3-CaF2 (1150-1350 °C)SiO2-Al2O3-CaO-CaF2 (1320-1450 °C)SiO2-Al2O3-CaF2-AlPO4 (1150-1300 °C)SiO2-Al2O3-CaF2-AlPO4-Na3AlF6-AlF3 (1100-1300 °C)

where again the temperature of fusion is given in parentheses.After fusion the molten glass is shock-cooled by pouring it onto a metal

plate and then into water. The glass fragments are then finely ground topass either a 45-|im sieve for a filling material, or a 15-jim sieve for a fine-grained luting agent. The glass powders may be annealed after preparationby heating at 400 to 600 °C; in general, the effect is to slow down the settingreaction. Sometimes the powder is acid-washed to improve the mixingqualities of the cement.

Structure of aluminosilicate glassesThe formation of a cement is dependent on the ability of the glass to releasecations to acid solutions. It is not sufficient for network-modifying cationsto be exchanged for hydrogen ions as this would restrict the attack to theglass surface only. It is required that the glass structure itself be completelydecomposed if all the glass ions are to be available for release.Aluminosilicate glasses have this property. To discuss why this is so it isuseful to have an appropriate conceptual framework, and one can bedeveloped from the Random Network model of Zachariasen (1932).

Zachariasen (1932) conceived of a glass structure as a random assemblyof oxygen polyhedra, these polyhedra consisting of a central glass-formingcation surrounded by a small number of oxygen atoms, e.g. [SiOJtetrahedra. These polyhedra were considered to be linked at corners only,via 2-coordinate oxygen atoms. This concept amounts to regarding a glassas a type of highly crosslinked polymer based on -Si-O-Si- linkages. This

119


idea, although much criticized (Rawson, 1967), has proved to be a fruitfulone. Zachariasen (1932) added another criterion, namely that the randomnetwork was three-dimensional, and, therefore, in modern terminology,composed only of Q4 and Q3 units. Hagg (1935) considered that thisrequirement was not always necessary and a glass might contain largeirregular anionic groups. The work of Trap & Stevals (1959) supported thisview, for they prepared so-called invert glasses containing only Q2 and Q1

units, that is glasses with no crosslinking. In these glasses at least half theoxygen atoms are non-bridging -O" groups, so the -Si-O-Si- chains areanionic and are held together by network-modifying cations (these do notform part of the glass structure). Today, following Ray (1975, 1983) wewould call these ionic polymers.

We are now in a position to discuss requirements for ionomer glassesfurther. Consider the case of the simple silica (SiO2) glass where we canrepresent the network diagrammatically thus:

O O O O

I I I I— Si—O—Si—O—Si—O—Si — O

I I I IThis infinite three-dimensional network is electrically neutral and im-pervious to acid attack. If so-called network-modifying cations areintroduced then this network must acquire a negative charge leading to thebreaking of an Si-O-Si bridge to form non-bridging oxygens:

\ / Ca2+ \ „ /— S i — O — S i — • — S i — Or Ca2+ "O — S i —/ \ / \

This is a type of ionic polymer where the negative charge on the networkis balanced by the positively charged network modifier. Statistically alltypes of [SiOJ tetrahedra, Q1, Q2, Q3 and Q4, will be present in varyingproportions, depending on the ratio of bridging to non-bridging oxygens.

Aluminosilicates are more complex as aluminium can be either anetwork modifier in sixfold coordination or a network former in fourfoldcoordination. In the latter case, Al3+ is able to replace Si4+ in the glassnetwork because it has a similar ionic radius, but the network then acquiresa negative charge. If this charge becomes sufficiently high then the networkbecomes susceptible to acid attack. Again this charge on the network hasto be balanced by positively charged network-modifying cations. Thus, we

120


can regard an aluminosilicate glass structure as consisting of linked [SiOJand [A1OJ~ tetrahedra. There are restrictions on the replacement of Si4+ byAl3+. The Al/Si ratio apparently cannot exceed 1:1 (Lowenstein, 1954).Nor can all the aluminium go into the network if there are insufficientnetwork-modifying cations to balance the network charge. Under suchconditions aluminium adopts a sixfold coordination.

We must note that recently Ellison & Warrens (1987), using 27A1NMRspectroscopy, have found evidence for the existence of aluminium inpentacoordination in asymmetric or distorted sites using previouslyestablished assignments (Kirkpatrick et al. 1986; Risbud et al., 1987;Cruikshank et al., 1986).

A negatively charged network of non-bridging oxygens and aluminiumsites renders these glasses susceptible to acid attack. Overall the in-troduction of network-modifying cations and aluminium ions increases thepolarizability of oxygen ions and, therefore, vulnerability to acid attack.The mode of acid decomposition of an aluminosilicate glass is depicted inFigure 5.5. It can be seen that attack by hydrogen ions involves exchangeof network-modifying cations (Ca2+, Na+) and rupture of the alumino-silicate network at aluminium sites to yield silicic acid and aluminium ions(Wilson, 1978b; Prosser & Wilson, 1979). Glasses used in glass poly-alkenoate cements have been observed to release cations, fluoride if presentand silicic acid (Crisp & Wilson, 1974a; Wasson & Nicholson, 1990,1991).Similar observations have been made for the related dental silicate cement

polymerizessilica gel

Figure 5.5 The mode of acid decomposition of an aluminosilicate glass.

121


Table 5.5. Glass compositions and acid extracts {Crisp & Wilson,1974a; Wasson & Nicholson, 1990)

Mole ratio

Si:AlCa:AlNa:AlF:A1

G-200

Glass

0-980-890142-46

Cementextract

1-170-250-97

G-338

Glass

0-670-260-441-67

Cementextract

0-630-360-65

Table 5.6. Composition of oxide glasses used in studies on polyalkenoatecements, parts by mass {Wilson et al., 1980; Crisp, Merson & Wilson,1980)

SiO2A12O3CaOCa3(PO4)2

AppearanceCrystallites

PropertiesPowder .liquid, g cm"3

Setting time (37 °C), minStrength (24 h), MPa

G-273

120102168—

clear—

202-75

95

G-275

240102112—

clear—

3040

35

G-287

18010256—

clear—

308-25

29

G-255

12010256—

opaqueAn

3040

56

G-247

160100—

140opal—

—5-25

72

An = Anorthite

(Wilson & Kent, 1970). The release of each glass species is roughlygoverned by the amount contained in the glass and so varies with glasstype (Table 5.5).

The reactivity of a glass towards acids depends on its acid-baseproperties and both the Bronsted-Lowry and Lewis theories have beenapplied to oxide glasses (Volf, 1984). Basic components of a glass are themetal oxides, and acidic ones are silicon, boron or phosphorus oxides. Theimportant factor is the state of the oxygen atoms. In purely oxide glassesthe basicity of a glass depends on the ability of the oxygen atoms to give up

122


electrons. This is greatest when the oxygen atoms are associated withcations of low electrostatic field strength, for example Na+ and Ca2+, andleast when the cations have a high electrostatic field strength, for examplethe highly charged, small Si4+ ion.

Lux (1939) introduced the symbol pO (note it is not an exponent like pH)to quantify the acid-base balance in a glass, and various attempts havebeen made to obtain values for this parameter. All are based on theelectronegativity of the cation or a related characteristic, such aselectrostatic field strength (Volf, 1984).

SiO2-Al2O3-CaO glassesIn these glasses (Table 5.6) the coordination state of aluminium depends onits chemical environment and can only be entirely fourfold when the Ca/Al

SiO

1:3

* Non-settingD Slow settingA Moderate settingo Fast setting9 Fast setting

crystalline mass• Ultra-fast setting

1:2

Al /Si mole ratio

CaO

C9S

C,S

A12O3

Figure 5.6 Triangular composition diagram for SiO2-Al2 O3-CaO glasses, showing thatglasses with cement-forming ability fall within the gehlenite and anorthite compositionregion, and that only glasses with less than 61 to 62 % by mass of silica have the potential toform a cement (Hill & Wilson, 1988a).

123


Table 5.7. Properties of cements formed from glassescorresponding to the generic formula xSi02.Al203.Ca0

Cement properties

moles(x) mass %setting time,minutes

strength,MPa

10204060

21-935-952-862-7

3-52-2540non-setting

1047435

zero

SiO

1:3

1:2

* Zero strengthD Unworkable• Weako Low strength• Moderate strength

Al /Si mole ratio

CaO

C2S

C,S

A12O3

Figure 5.7 Triangular composition diagram for SiO2-Al2 O3-CaO glasses. Glasses in thegehlenite region yield stronger cements (95 to 104 MPa in compression) than those in theanorthite region (29 to 56 MPa) (Hill & Wilson, 1988a).

124


Table 5.8. Composition of fluoride glasses used in studies on polyalkenoatecements, parts by mass (Kent, Lewis & Wilson, 1979; Wilson et al., 1980)

SiO2A12O3CaF2Na3AlF6A1F3A1PO4SiO2:Al2O3bymassAppearanceCrystallites

PropertiesPowder: liquid, g cm"3

Setting time (37 °C), minStrength (24 h), MPa

G-200

175100207303260

1-75opaqueFl

305-2

185

G-307

133100100———

1-33clear

303-5

107

G-309

67100100———

0-67opaqueFl,Co

306-5

166

G-235

175100166——60

1-75opalFl

3-53-0

149

G-237

175100117——60

1-75opaqueCo

3030

199

G-338

17510090

13532

1701-75

opalAp

1-83-75

149

Fl = Fluorite, Co = Corundum, Ap = Apatite

ratio > 1:2 and Al/Si ratio < 1:1 (Isard, 1959; Lowenstein, 1954). Thus,aluminium is in fourfold coordination in anorthite glass (Ca: Al = 1:2,Al:Si = 1:1); the glass is composed of Q4 and Q3 units, i.e. it is three-dimensional. Gehlenite glass must contain some aluminium in sixfoldcoordination (Ca: Al > 1:2, Al:Si > 1:1) and is composed of paired Q1

units, i.e. [AlSiO7].A study by Ellison & Warrens (1987) on two of these glasses, using 27A1

and 29Si NMR, produced results not too dissimilar from theoreticalpredictions. In glass G-273, Ca3Al2Si2O9 (Table 5.6), aluminium wasfound to be mainly in tetrahedral coordination with a minor amountin octahedral coordination. Similar results were found for glass G-275,Ca2Al2Si4O13 (Table 5.6), but, in addition, some aluminium was foundto be pentacoordinate. Possible structural units were considered to beQ3 (1A1) and Q2 (0A1) with some Q4 (3A1). The number of Al replacingSi in the second coordination sphere is given in parentheses.

Glasses that have cement-forming ability fall within the gehlenite andanorthite composition regions of this system, and only glasses with lessthan 61 to 62 % by mass of silica have potential to form a cement (Figure5.6). Cements are not formed if the Si/Al mole ratio exceeds 3:1. When the

125


ratio is less than 2:1 fast-setting cements are obtained with setting time of2 to 10 minutes. There is only a very small region for slow-setting cementsand as Table 5.7 shows there is a critical region between setting and non-setting. Glasses in the gehlenite region yield stronger cements (95 to104 MPa in compression) than those in the anorthite region (29 to 56 MPa)(Figure 5.7).

SiO2-Al2O3-CaF2 glassesThese (Table 5.8) are the basic type from which most biomedical glasspolyalkenoate cements are derived. Although the fluoride content is high,many of these shock-cooled glasses are clear. Clear glasses are confined toa narrow central compositional range at the centre of the phase diagramwhere the Al2O3/CaF2 ratio is around 1:1 by mass and the SiO2/Al2O3ratio exceeds 1-33:1 by mass (Figure 5.8). Outside this region fluorite, andsometimes corundum, phase-separate. Even the clear glasses can be

SiO2

1:3

CaF2/Si02 ratio by mass

• Clear, non-setting• Opal, non-settingA Clear, slow setting, low strength• Opal, slow setting, low strengtho Clear, fast setting, high strength• Opal, fast setting, high strength

ratio by mass

CaF2

3:1

AI2O3ratio by mass

Figure 5.8 Clear glasses are confined to a narrow central compositional range at the centreof the phase diagram where the Al2O3/CaF2 ratio lies in the region 1:1 by mass and theSiO2/Al2O3 ratio exceeds 1-33:1 (Hill & Wilson, 1988a).

126


induced to phase-separate when heated to 450 °C, and this reduces theirreactivity.

The ability of a glass to form a cement is governed by the SiO2/Al2O3

ratio which represents the acid-base balance in these glasses. If this ratio is3-0 or more by mass then the glass will not form a cement. If it is below 2-0then the cements formed are rapid-setting (2-5 to 5-0 minutes). Glasses ina very narrow band around a ratio of about 2-0 are slower-setting (6-5 to18 minutes). The critical ratio for non-setting lies somewhere between 2-0and 3-0. The effect of SiO2/Al2O3 ratio on setting time and compressivestrength is shown in Figure 5.9. Note that compressive strength increasessteadily as the SiO2/Al2O3 ratio decreases. Setting time decreases as theSiO2/Al2O3 ratio decreases until a point is reached when phase separation

10.0i

8.0

| 6 . 0

Z 4.0I/)

2.0

200 - | 1 -

Opal glasses I Clear glasses

G-309I

G-308I

|G-307J

G-379

0.8

150

100

50

o>c<b^3if)

%

essi

Q . "£ou

0.6

oo

••5a0

0.4

0.2

0.5 1.0 1.5 2.0SiO2 • AI2O3 by mass

Figure 5.9 The effect of SiO2/Al2O3 mass ratio on setting time, compressive strength andopacity (Hill & Wilson, 1988a).

127


occurs in the glass. Phase separation has the effect of deactivating the glass.In these glasses the main phase is depleted in calcium and fluoride, whichreduces its reactivity. Acid attack occurs selectively at the phase-separateddroplets which are rich in calcium and fluoride. This selective attack isshown in Figure 5.10.

Phase-separated glasses produce stronger cements than clear glasses.The strongest cements produced from a clear glass have a compressivestrength of 130 MPa and a flexural strength of 20 MPa, whereas phase-separated glasses produce cements with compressive strength exceeding200 MPa and flexural strengths exceeding 35 MPa. Note that fluorideglasses produce much stronger cements than oxide glasses. The strongestcement produced from an oxide glass has a compressive strength of only104 MPa.

Ellison & Warrens (1987) have reported NMR results on an atypicalphase-separated glass of extreme composition G-309 (Table 5.8) finding

Figure 5.10 In these glasses, the main phase is depleted in calcium and fluoride, which reducesits reactivity. Acid attack occurs selectively at the phase-separated droplets which are rich incalcium and fluoride (Hill & Wilson, 1988a).

128

01201021120

0-2012010210115-6

0-331201029326

0-51201028439

101201025678

1-512010228117

1-812010211-2140


Table 5.9. Composition (%) of oxide/fluoride glasses used in studies onpolyalkenoate cements, parts by mass. Generic composition:2SiO2.Al2O3. (2-x)CaO.xCaF2 (Kent, Lewis & Wilson, 1979;Wood & Hill, 1991a)

G-241 G-278 G-276 G-279 G-280 G-281 G-282

SiO2A12O3CaOCaF2Tg n d 745 n d 717 642 n d 636Appearance below clear clear clear clear clear clear opaque

Crystallites below — — — — — — Fl

PropertiesPowder: liquid, 2-5 2-5 20 20 2-0 20 2-5

gcnr 3

Setting time (37 °C), 2-25 2-25 2-5 u/w u/w u/w 3-0min

Strength (24 h), 74 125 120 u/w u/w u/w 165MPa

Fl = Fluorite, nd = not determinedu/w = unworkable pastes that set during mixing

that the Al is mainly in sixfold coordination, not surprising in view of thehigh alumina content. The structural units are mainly Q4 (3A1).

The role of fluoride is a matter of speculation and debate (Kumar, Ward& Williams, 1961). According to Weyl and Marboe (1962), in additionto [SiOJ and [A1OJ, such glasses contain tetrahedra such as [SiO3F],[A1O3F] etc. The replacement of O2~ by F~ reduces the screening of thecentral cation and so strengthens the remaining cation-oxygen bonds, butfluoride is non-bridging and so structure-breaking. This role of fluoridemay be represented, thus

Q4 Q3

Another view of the role of fluoride is that metal fluorides occupy holes inthe major glass network (Rabinovich, 1983). There is experimental

129


evidence to support both views. However, these views are not necessarilymutually exclusive.

SiO2-Al2O3-CaO-CaF2 glassesThese glasses (Table 5.9) with generic composition 2SiO2.Al2O3.(2-x)CaO.xCaF2, first described by Wilson et al (1980) and recentlystudied by Wood & Hill (1991a), are of theoretical interest. In this series,starting with the oxide glass G-241, O atoms are progressively replaced byF atoms (Table 5.9). Fluoride is often considered to be a structure-breakerand this is reflected in the reduction of the glass transition temperature, Tg,as x increases (Wood & Hill, 1991a). Another indication of structurebreaking is shown by a decrease in setting time of cements as x increases(Wilson et al, 1980). When x reaches 0-5 (G-279) the reaction betweenglass powder and polyacid liquid becomes very vigorous and cement pastesset during mixing. Glasses with x values of 1-0 and 1-5 behave similarly.However, when x reaches 1-8 (G-282) phase-separation of fluorite (CaF2)occurs, fluoride is removed from the main phase, and the reactivity of theglass is reduced. The cement of G-282 has the longest setting time of theseries.

Wood & Hill (1991b) induced phase-separation in the clear glasses byheating them at temperatures above their transition temperatures. Theyfound evidence for amorphous phase-separation (APS) prior to theformation of crystallites. Below the first exotherm, APS appeared to takeplace by spinodal decomposition so that the glass had an interconnectedstructure (Cahn, 1961). At higher temperatures the microstructure con-sisted of distinct droplets in a matrix phase.

When x was 1-0, fluorite and anorthite crystallites were formed. Withglasses of lower fluoride content (x < 1-0) gehlenite crystallites were alsofound. As x decreased, increasingly more gehlenite was formed at theexpense of anorthite and fluorite. In connection with this observation itshould be noted that the chemical composition of the glass correspondsto gehlenite when x = 0 and to a mixture of anorthite and fluorite whenx=\.

Wood and Hill consider that the role of fluoride in these glasses isuncertain. Phase-separation studies suggest that the structure of the glassmight relate to the crystalline species formed, in which case a micro-crystallite glass model is appropriate. But other evidence cited above on thestructure-breaking role of fluoride is compatible with a random networkmodel.

130


Table 5.10. Examples of practical glasses used in glass polyalkenoatecement {Crisp, Abel & Wilson, 1979; Wilson & McLean, 1988; Brook,Craig & Lamb, 1991)

SiO2A12O3A1F3CaF2CaONaFNa2OA1PO4SiO2: A12O3 by mass

ASPA IVG-200

30119-92-6

34-5

3-7

1001 51

ASPAXG-338

24-914-211012-8

12-8

24-21-75

GCFuji

41-928-6

1-615-7

9-3

3-8146

ESPEKetac

34-92018-8

20-8

3-6

11-81-74

PilkingtonMP4

30-838-5

28-6

21

0-80

G-200 and G-338 are LGC formulations used in certain commercial materialsGC Fuji and ESPE Ketac are dental cementsMP4 is used in splint bandage materials

SiO2-Al2Oz-CaF2-AlPO^ glassesNot much needs to be said on these glasses, except that one, G-235 (Table5.8) has been subjected to solid state NMR (Ellison & Warrens, 1987). Itwould appear that aluminium is mainly in tetrahedral coordination withsome octahedral and pentahedral coordination and that the structuralunits are mainly Q3 (1A1) and Q4 (2A1).

glassesThese are practical glasses and some examples are given in Table 5.10. Oneof the most important of these is G-200, an unusual glass in that it formsa practical cement without the need for ( + )-tartaric acid. It was used inearly commercial materials and in fundamental studies on setting andcement structure. The glass must now be regarded as atypical as it is veryhigh in fluoride and low in sodium. Barry, Clinton & Wilson (1979)examined the glass structure. They found that it was heavily opal andphase-separated with droplets rich in calcium and fluoride of complexmorphology. These droplets were of average size 1-7 \im and volumefraction 20%. There were also massive inclusions of fluorite. When theglass was fused at a higher temperature, 1300 °C as against 1150 °C,

131


Table 5.11. Effect of ( + )-tartaric acid on glass polyalkenoate cementproperties

Tartaric acida

Working time (23 °C),minutes

Setting time (37 °C),minutes

Setting rate, arbitraryunits

Compressive strength,MPa

G-237

Absent

10

4-3

50

121

Present

10

2-7

165

140

G-309

Absent

1-2

3-5

65

125

Present

1-7

2-8

100

136

G-200

Absent

1-7

60

20

100

Present

2-7

3-8

55

120

Cement-forming liquid: 45 % poly(acrylic acid)Powder: liquid ratio of 3:1a Added in 5 % concentration

fluoride was lost, the morphology of the glass changed to one with smallerdroplets, and it became more reactive towards poly(acrylic acid).

5.9.3 Poly(alkenoic acid)s

The poly(alkenoic acid)s used in glass polyalkenoate cement are generallysimilar to those used in zinc polycarboxylate cements. They are homo-polymers of acrylic acid and its copolymers with itaconic acid, maleic acidand other monomers e.g. 3-butene 1,2,3-tricarboxylic acid. They havealready been described in Section 5.3. The poly (aery lie acid) is not alwayscontained in the liquid. Sometimes the dry acid is blended with glasspowder and the cement is activated by mixing with water or an aqueoussolution of tartaric acid (McLean, Wilson & Prosser, 1984; Prosser et al.,1984).

Increase in concentration of the polyacid increases solution viscosity,quite sharply above 45% by mass (Crisp, Lewis & Wilson, 1977). Thestrength of glass polyalkenoate cements also increases, almost linearly,with polyacid concentration. This is achieved at the cost of producing over-thick cement pastes and loss of working time.

The molecular weight of the polyacid affects the properties of glasspolyalkenoate cements. Strength, fracture toughness, resistance to erosionand wear are all improved as the molecular weight of the polyacid is

132


Table 5.12. Effect of the various tartaric acids on glass polyalkenoatecement properties {Crisp, Lewis & Wilson, 1979)

None(+ )-tartaric acid( —)-tartaric acidMeso-tartaric acidRacemic tartaric acid

Setting time,minutes (37 °C)

8-5504-75

10-255-25

Compressive strength,MPa (24 hours)

10411212072

109

Powder: G-200Liquid: 47-4% poly(acrylic acid), 5-3% tartaric acid

increased (Wilson, Crisp & Abel, 1977; Hill, Wilson & Warrens, 1989;Wilson et al.91989). Setting is accelerated and working time is lost, and thisplaces a restriction on improving cement properties by this route. Themaximum molecular weight of the polyacid that can be used would appearto be 75000.

5.9.4 Reaction-controlling additives

The glass polyalkenoate cement system was not viable until Wilson andCrisp discovered the action of ( + )-tartaric acid as a reaction-controllingadditive (Wilson & Crisp, 1975,1976, 1980; Wilson, Crisp & Ferner, 1976;Crisp & Wilson, 1976; Crisp, Lewis & Wilson, 1979). It may be regardedas an essential constituent and is invariably included in glass polyalkenoatecements as a reaction-controlling additive.

It affects the nature of the setting reaction profoundly and this subject isdiscussed in Section 5.9.5. It sharpens set and increases the hardening rate,without decreasing, and even sometimes increasing, working time. Strengthis also increased (Table 5.11). Crisp, Merson & Wilson (1980) foundmoreover that the addition of ( + )-tartaric acid conferred the settingproperty on a glass (G-288) that otherwise did not form a cement.

No other additive has the same effect although many alternatives wereexamined by Wilson, Crisp & Ferner (1976) and Prosser, Jerome & Wilson(1982). Other multifunctional carboxylic acids, including citric acid, hadlittle effect, apart from a slight tendency to shorten working time andincrease the setting rate. That the effect is a subtle one is shown by the fact

133


Table 5.13. Effect of fluorides on glass polyalkenoate cementcompressive strength, MPa {Crisp, Merson & Wilson, 1980)

Fluoride

NoneA1F3

MgF2

SnF2

ZnF.

(+ )-tartaric acid

Absent

7291

101128111

Present

112158145128

Powder: G-247Liquid: 50 % acrylic-itaconic acid copolymer, 5 % tartaric acid

that raeso-tartaric acid does not have the effect of sharpening the set(Table 5.12).

Ethanolamines and polyphosphates slow the reaction down as a whole.Both tetrahydrofurantetracarboxylic acid and polyphosphates are some-times to be found in commercial examples.

Crisp, Merson & Wilson (1980) found that the addition of metalfluorides to formulations had the effect of accelerating cement formationand increasing the strength of set cements; the effect was enhanced by thepresence of ( + )-tartaric acid (Table 5.13). Strength of cements formedfrom an SiO2-Al2O3-Ca3 (PO4)2 glass, G-247, can be almost doubled bythis technique.

5.9.5 Setting

General

The setting and hardening reactions are first outlined in general terms.They can be considered to take place in a number of overlapping stages.

(1) On mixing the cement paste, the calcium aluminosilicate glass isattacked by hydrogen ions from the poly(alkenoic acid) anddecomposes with liberation of metal ions (aluminium and cal-cium), fluoride (if present) and silicic acid (which later condensesto form a silica gel).

(2) As the p H of the aqueous phase rises, the poly(alkenoic acid)ionizes and most probably creates an electrostatic field which aidsthe migration of liberated cations into the aqueous phase.

134


(3) As the poly(alkenoic acid) ionizes, polymer chains unwind as thenegative charge on them increases, and the viscosity of the cementpaste increases. The concentration of cations increases until theycondense on the polyacid chain. Desolvation occurs and insolublesalts precipitate, first as a sol which then converts to a gel. Thisrepresents the initial set.

(4) After gelation or initial set, the cement continues to harden ascations are increasingly bound to the polyanion chain andhydration reactions continue. Recent evidence suggests that asiliceous hydrogel may be formed in the matrix.

In considering the setting reaction in more detail, cognizance must betaken of the nature of the glass and the presence of reaction-controllingadditives. These affect both the nature of the cement-forming reaction andsetting characteristics. These effects stem from complex formation; the twomost important complexing agents are fluoride, derived from the glass, and(4- )-tartaric acid, by far the most important of the reaction-controllingadditives. We distinguish four possible systems: (1) oxide glasses, (2) oxideglasses with added (+ )-tartaric acid, (3) fluoride glasses and (4) fluorideglasses with added (H-)-tartaric acid.

Cement formation with oxide glassesLittle more can be said concerning the cement-forming reactions betweenoxide glasses and poly(alkenoic acid)s because they have not been studied.It is almost impossible to prepare cements from oxide glasses and apolyacid, because the paste formed on mixing is intractable. The evidencepoints toward the premature bonding of aluminium ions as the cause (Ellis& Wilson, 1987). This does not occur in the related dental silicate cement,which is formed using a concentrated solution of orthophosphoric acid;here aluminium forms complexes with orthophosphoric acid, which delaysprecipitation. Practical cements can be formed from oxide glasses if( + )-tartaric acid is added to the system. Since aluminium forms solublecomplexes with ( + )-tartaric acid it is reasonable to suppose that theformation of these complexes prevents the premature ion-binding ofaluminium to poly(alkenoic acid) and so allows workable cement pastes tobe formed. This view is supported by the solution studies of Ellis & Wilson(1987).

135


Table 5.14. Infrared spectroscopic bands of reference carboxylates{Nicholson et aL, 1988b)

C-O stretch of salt, cm"1

Ca-PAAAl-PAACa-tartrateAl-tartrate

1550159915951670

Salt asymmetric symmetric

1410146013851410

Cement formation with fluoride glasses - no tartaric acidCements can be formed using fluoride-containing glasses in the absence of( + )-tartaric acid, but high-fluoride glasses have to be used to obtaincements with sufficient working time (Crisp & Wilson, 1976; Kent, Lewis& Wilson, 1979). Only very few glasses, those very high in fluoride, can beused in practical cements. Fluoride clearly has a considerable effect on thereaction, probably because it forms strong soluble complexes withaluminium such as A1F2+ and A1F+; these have been reported by Connick& Poulsen (1957), O'Reilly (1960), and Akitt, Greenwood & Lester (1971).These complexes probably prevent the premature gelation of the poly-anions by aluminium ions.

In this system there is a useful cooperative effect between aluminium,fluoride and calcium, which has been demonstrated by the solution studiesof Ellis & Wilson (1987). In the absence of aluminium, calcium precipitatesas the fluoride at all pHs. Aluminium has the effect of preventing theprecipitation of calcium as fluoride, again because it forms strong solublecomplexes with fluoride.

Detailed studies of the cement-forming reaction using fluoride-con-taining glasses and aqueous solutions of poly(acrylic acid) have beencarried out; mainly by Wilson and his coworkers (Crisp & Wilson,1974a,b, 1976; Crisp et aL, 1974; Barry, Clinton & Wilson, 1979; Prosser,Richards & Wilson, 1982; Hill & Wilson, 1988b; Nicholson et aL, 1988b),but the work of Cook (1982, 1983a) should also be noted.

The following account is based mainly on the early studies of Crisp,Wilson and coworkers (Crisp & Wilson, 1974a,b, 1976; Crisp et aL, 1974)who used glass G-200 and 50 % solutions of poly(acrylic acid), and issupported by the later studies of Nicholson et aL (1988b) using glass G-309. The compositions of materials used are shown in Table 5.8.

136


These workers applied chemical and infrared spectroscopic methods tostudy cement-formation. Infrared spectroscopy exploits the fact thatcalcium polyacrylate and aluminium polyacrylate give rise to differentcarboxylate bands (Table 5.14). On mixing the powder and liquid,hydrogen ions from the poly(acrylic acid) solution rapidly attack the glassparticles, which are decomposed to silicic acid, and Al3+, Ca2+, Na+ and F~ions are released (Crisp & Wilson, 1974a,b; Cook, 1983c; Crisp, Lewis &Wilson, 1976d; Wasson & Nicholson, 1990). Originally, it was supposedthat this attack occurred only at the surface layer of the glass particles(Barry, Clinton & Wilson, 1979) but later observations using 29Si NMR byEllison & Warrens (1987) suggest that attack occurs throughout the glassparticles. In the case of glass G-235 pentacoordinated aluminium dis-appeared and Q4 (2A1) units were converted to Q3 (1A1) units, i.e.aluminium was lost from the glass structure.

Since hydrogen ions are six to twelve times more mobile than othercations, there will be a delay between loss of hydrogen ions from solutionand migration of glass cations into the aqueous phase. Presumably, thiselectrical imbalance results in an electric field which acts as a driving forcefor the migration of cations. Aluminium and fluoride are almost certainlytransported as cationic aluminofluoride complexes, A1F2+ and A1FJ,mentioned above.

Figure 5.11 (Crisp & Wilson, 1974b) shows the time-dependent variationof the concentration of soluble ions in setting and hardening cements. Notethat the concentrations of aluminium, calcium and fluoride rise to maximaas they are released from the glass. After the maximum is reached theconcentration of soluble ions decreases as they are precipitated. Note thatthis process is much more rapid for calcium than for aluminium and thesharp decline in soluble calcium corresponds to gelation. This indication issupported by information from infrared spectroscopy which showed thatgelation (initial set) was caused by the precipitation of calcium poly-acrylate. This finding was later confirmed by Nicholson et al. (1988b) who,using Fourier transform infrared spectroscopy (FTIR), found that calciumpolyacrylate could be detected in the cement paste within one minute ofmixing the cement. There was no evidence for the formation of anyaluminium polyacrylate within nine minutes and substantial amounts arenot formed for about one hour (Crisp et al., 1974).

Crisp & Wilson (1974b, 1976) attributed the slowness of binding in thecase of aluminium to several effects: preferential leaching of calcium ions,lack of mobility of the hydrated or multinuclear aluminium species

137


(Aveston, 1965; Akitt, Greenwood & Lester, 1971; Waters & Henty, 1977)and steric problems because of the triple charge on the ion. However, whenit is remembered that the Al3+ ion is responsible for the premature gelationof the oxide glass system, it may be that the true explanation lies in thestability of the fluoride complexes or the formation of large multinuclearaluminium complexes. Such complexes have been reported by a numberof workers - Aveston (1965), Akitt, Greenwood & Lester (1971) andWaters & Henty (1977) - and may be represented by the generic formula[Alx(OH,F)y(H2O)z]{3xy). The presence of such complexes may explainthe slow reactivity of the Al3+ ion with EDTA in slightly acid solutions.Of course, this argument does not apply to the premature binding of Al3+

ions in the dental glass system when the pH is low.Gelation involves an extended structure and some type of linking

between chains. The concept of salt-like crosslinks has already beendescribed (Section 5.5). Other possibilities may be considered. Hill, Wilson& Warrens (1989) examined the possibility that chain entanglements mightaccount for the strength of polyelectrolyte cements. They used in particular

6 -

5 -

\ 3

-0-t

1A. 1 y••••F *'

P A1 1 1 1 1

vMil 1 1 1 1 1 1 1 1 1

• • • • • • • • • • . . .

1 1 1 1 1 1 1 1

•

* •

J

2 -

1 -

10 100 1000time, min

Figure 5.11 The time-dependent variation, in setting and hardening cements, of theconcentration of soluble ions Al3+, Ca2+, F~ and VO\~ (expressed as P2 O5). These ions arereleased from the glass powder into the cement matrix (Crisp & Wilson, 1974b).

138


the reptation (chain pull-out) model described by de Gennes (1979) andEdwards (1969). Here a polymer chain is considered to be trapped by atube of entanglements formed by neighbouring chains, and strength isrelated to the forces required to pull out the trapped chain. They found,however, that this model did not account for the effect of polyacidmolecular mass on cement strength (Nicholson, 1992).

After initial set the cement continues to harden and strengthen (Crisp,Lewis & Wilson, 1976b) as cations are increasingly bound to the polyanionchain (Cook, 1983c) and hydration processes continue (Wilson, Paddon &Crisp, 1979; Wilson, Crisp & Paddon, 1981). There are still free COOHgroups present after 24 hours of reaction (Nicholson et al., 1988b). Cook(1983c) observed that the transfer of aluminium and calcium from the glassto the matrix continued for at least five weeks, during which time bothstrength and modulus increased (Paddon & Wilson, 1976). The increase instrength after set is illustrated by the results of Elliot, HoUiday & Hornsby(1975) presented in Figure 5.12. The reaction probably never ceasesentirely, for Crisp, Lewis & Wilson (1976b) have observed that strengthcontinues to increase, logarithmically, for at least a year if specimens arestored in kerosene. This slow increase in strength has been attributed tohydration processes and the slow diffusion of cement-forming cations,especially aluminium, seeking anionic sites.

There remains the question of the role of silica. This aspect of setting

200 r

o

\ 150 -

oI«» 100

aEo50

1.0 100 100010Time, hours

Figure 5.12 The time-dependent increase in strength after set (Elliott, HoUiday & Hornsby,1975).

139

73-392-073-312-59

600-550-241-270-73

14400100030-490-73


chemistry has been neglected and there are some ambiguities in theliterature. Recently, Wasson & Nicholson (1990, 1991) have revivedinterest in this topic. The release of orthosilicic acid, which accompaniesthe release of glass ions, has been observed and is implicit in the theory ofacid decomposition of the glass (Section 5.9.2). During the setting reactionorthosilicic acid is converted to silica gel (Crisp et aL, 1974). Crisp, Lewisand Wilson (1976d) found that the reduction in the amount of silicic acideluted as a cement aged was matched by similar reductions in amounts ofcations and anions eluted.

Age of cement, minutesSilica (SiO2) eluted mg/g cementAluminium (A12O3) eluted mg/g cementSodium (Na2O) eluted mg/g cementFluoride (F) eluted mg/g cement

The insolubilization of cations and anions during the setting and hardeningprocess is thus paralleled by that of silica. Under acid conditionsorthosilicic acid condenses first to form polymeric silicic acid and thensilica gel (Her, 1979; Andersson, Dent Glasser & Smith, 1982). Theseprocesses are discussed more fully in Section 6.5.4. Gelation of silica, likethe formation of salt gels, is enhanced by a reduction in the acidity ofsolutions.

Much of the siliceous hydrogel formed is found enveloping the glassparticles, and the attacked glass particles maintain their original mor-phology (Barry, Clinton & Wilson, 1979). The mechanism of this processis obscure and it is uncertain whether it results from an ion depletionprocess or deposition of a siliceous gel on the site of the decomposed glassnetwork. Recently, Wasson & Nicholson (1990, 1991) have suggested thata hydrated silicate is formed in the matrix and may contribute to thehardening process. The composition of this species is at present unknown.It could be a silicate gel, similar to tobermorite gel found in Portlandcement (Taylor, 1966) which could account for the long term increases instrength of glass-ionomer cement. Alternatively, it could be a type of silicagel. Silica gel cements are known: they are weak with a compressivestrength of 14 MPa, but are acid-resistant (Trautschold, 1940). In thisconnection, it may be noted that glass-ionomer cements are much moreresistant to acid attack than are the related zinc polycarboxylates. Clearly,further studies are required into this aspect of glass-ionomer cementchemistry.

140


Cement formation with fluoride glasses - ( + )-tartaric acidThe presence of ( + )-tartaric acid in a cement formulation exerts aprofound effect on the cement-forming reaction. The nature of theunderlying chemical reaction is changed and this is reflected in time-dependent changes in viscosity.

Both Cook (1983a,b) and Hill & Wilson (1988b) have studied the effectof (+ )-tartaric acid on the development of viscosity. Hill & Wilson usedtwo glasses in their study, an oxide glass, G-287 (Table 5.6), and a fluorideglass, G-307 (Table 5.8). They noted that in the absence of ( + )-tartaricacid there was an almost exponential increase in apparent viscosity withtime and that this effect was exaggerated when small amounts of ( + )-tartaric acid, 0-3 %, were added to the liquid (Figure 5.13). However, whenadded in larger amounts ( + )-tartaric acid reduced the apparent viscosityof the pastes and also delayed the increase in apparent viscosity for aperiod of time which depended on the amount of ( + )-tartaric acid added;this plateau is very evident in the curves of apparent viscosity against time.Clearly, there are competing reactions taking place.

The early reaction leading to gelation has been studied by Prosser,Richards & Wilson (1982) who used 13C NMR to examine the interactionbetween glass G-200 and poly(acrylic acid) solutions, and by Nicholson et

Viscosity (+) tartaric acid

no (+) tartaric acid

Time 1Figure 5.13 Effect of tartaric acid on viscosity development (Hill & Wilson, 1988b).

141


al (1988b) who used FTIR and glass G-309 (Table 5.8). The addition of( + )-tartaric acid totally changes the chemistry of the cement-formingreaction. It reacts preferentially with the glass because it is the stronger acidand its complexes are fully formed at pH = 3-4 while those of poly (aery lieacid) only appear at higher pH. The formation of calcium polyacrylate issuppressed, the extent of this suppression being dependent on the amountof ( + )-tartaric acid present. Instead, within the first minute, calciumtartrate makes a transient appearance most probably causing gelation. Itdisappears within nine minutes as aluminium polyacrylate appears.

From these experiments, it appears that ( + )-tartaric acid prolongsworking time by preventing the premature formation of calcium poly-acrylate, and later sharpens set and accelerates hardening by enhancing therate at which aluminium polyacrylate is formed. By contrast, while meso-tartaric acid delays the premature formation of calcium polyacrylate itdoes not enhance the rate of formation of aluminium polyacrylate. Thus,it prolongs working time without accelerating hardening. The effects of

Figure 5.14 The microstructure of the set cement is clearly revealed by Nomarski reflectanceoptical microscopy. Glass particles are distinguished from the matrix by the presence ofetched circular areas at the site of the phase-separated droplets (Barry, Clinton & Wilson,1979).

142


(H-)-tartaric acid and raeso-tartaric acid on the physical properties ofglass-ionomer cements described in Section 5.9.4 are explained in terms ofthe underlying chemistry.

5.9.6 Structure

Barry, Clinton & Wilson (1979) examined the structure of cementsprepared from a glass powder from which very fine particles had beenremoved to improve resolution. The microstructure of the set cement isclearly revealed by Nomarski reflectance optical microscopy (Figure 5.14).Glass particles are distinguished from the matrix by the presence of etchedcircular areas at the site of the phase-separated droplets. The micrograph

Figure 5.15 More detail than seen in Fig. 5.14 is obtained in a scanning electron image. Thereacted glass particles are covered by a distinct reaction layer of silica gel (Barry, Clinton &Wilson, 1979).

143


is similar to that of the dental silicate cement (Section 6.5.5). More detailcan be seen in a scanning electron image (Figure 5.15). The reacted glassparticles are covered by a distinct, gel-like reaction layer which is identifiedas silica gel. This layer has detached itself from the glass core, which,

Figure 5.16 (a) An electron image of a glass-ionomer cement; (b) SL diagrammaticrepresentation of this image: unshaded areas are glass particles, lightly shaded areas arecement matrix, and deeply shaded areas are voids or cracks. The remaining micrographsshow the Ka characteristic radiation of (c) Ca, (d) Si, (e) Al, and ( / ) Si: Al (Barry, Clinton &Wilson, 1979).

144

Glass polyalkenoate {glass-ionomer) cement

according to Brune & Smith (1982), is consistent with weak hydrogen-bonding between layer and core. Fluorite particles appear as brighthighlights because any calcium present gives rise to enhanced electronscattering.

Barry, Clinton & Wilson (1979), using a scanning electron microscopefitted with an X-ray analyser, obtained an element distribution map for Ca,Si and Al (Figure 5A6c,d,e). They found that whereas the glass particlescontained major amounts of all three elements, the matrix contained onlyaluminium and calcium in major amounts, with more aluminium thancalcium; there were only minor amounts of silicon. Interestingly, elementdistributions showed that the glass particles appeared slightly larger whenjudged by silicon radiation than by aluminium radiation. A map of theSi/Al ratio confirmed this observation. An electron image of a glass-ionomer cement specimen is shown in Figure 5.16a together with an Si: Aldistribution map, Figure 5.16/. At the time, this effect was attributed to thedepletion of ions from the outer layer of the glass particles, but now we aremore inclined to believe that this layer is really due to an outgrowing ofsilica gel, which, of course, has the capacity to absorb metal ions (Her,1979; Hazel, Shock & Gordon, 1949). Certainly the siliceous gel layer inFigure 5.15 does have the appearance of a reaction zone rather than thatof a relict. Moreover, an aluminosilicate glass matrix is decomposed tosilicic acid (Wasson & Nicholson, 1990, 1991), which is detected insignificant amounts in young cements (Crisp, Lewis & Wilson, 1976d).

The latter interpretation of data is more in accord with the recent 27A1and 29Si NMR findings of Ellison & Warrens (1987), who found that thestructure of an appreciable fraction of the glass changed under acid attackwith some loss of aluminium including all in fivefold coordination (seeSection 5.9.2). Thus, acid attack was not entirely confined to the surfacelayer of a glass particle. If this is so then silicic acid as well as ions mustmigrate from the body of the particle and it is reasonable to suppose thatsilicic acid deposits as siliceous gel at the particle-matrix interface.

The picture of cement microstructure that now emerges is of particles ofpartially degraded glass embedded in a matrix of calcium and aluminiumpolyalkenoates and sheathed in a layer of siliceous gel probably formedjust outside the particle boundary. This structure (shown in Figure 5.17)was first proposed by Wilson & Prosser (1982, 1984) and has since beenconfirmed by recent electron microscopic studies by Swift & Dogan (1990)and Hatton & Brook (1992). The latter used transmission electronmicroscopy with high resolution to confirm this model without ambiguity.

145


The form of silica in the matrix is at present unknown. In the freshlyprepared cement there are appreciable amounts of silicic acid presentwhich decline as the cement ages (Crisp, Lewis & Wilson, 1976d). In the setcement silica could be present as a polymeric silicic acid, a siliceous gel oreven a hydrated silicate gel, such as the tobermorite gel present in Portlandcements (Taylor, 1966).

The molecular structure is mainly a matter of conjecture but thecoordination state of aluminium in the matrix is known to be six (Ellison& Warrens, 1987). Therefore, there must be three monovalent anions,COO", F" or OH", and three neutral H2O. This matter has been discussedin detail in Section 5.5. As noted there, Al3+ has the potential to link threechains but this is sterically unlikely. Also, as Mehrotra & Bohra (1983)have pointed out, tricarboxylates tend to hydrolyse into complexstructures that involve Al-O-Al bridges. Nevertheless we must notethat glass polyalkenoate cement loses its original plastic behaviour andbecomes increasingly rigid as it ages over the weeks following set. Suchtime-dependent changes are not found with other cements and it istempting to speculate that they may arise from the slow transformationin the nature of the Al bonding.

Rolysalt matrix

Figure 5.17 Diagrammatic representation of a glass-ionomer cement (Laboratory of theGovernment Chemist: Crown copyright reserved).

146


5.9.7 General characteristics

The glass polyalkenoate cement uniquely combines translucency with theability to bond to untreated tooth material and bone. Indeed, the onlyother cement to possess translucency is the dental silicate cement, while thezinc polycarboxylate cement is the only other adhesive cement. It is also anagent for the sustained release of fluoride. For these reasons the glasspolyalkenoate cement has many applications in dentistry as well as beinga candidate bone cement. Its translucency makes it a favoured materialboth for the restoration of front teeth and to cement translucent porcelainteeth and veneers. Its adhesive quality reduces and sometimes eliminatesthe need for the use of the dental drill. The release of fluoride from thiscement protects neighbouring tooth material from the ravages of dentaldecay. New clinical techniques have been devised to exploit the uniquecharacteristics of the material (McLean & Wilson, 1977a,b,c; Wilson &McLean, 1988; Mount, 1990).

This cement also has a low setting exotherm, lower than any otheraqueous dental cement (Crisp, Jennings & Wilson, 1978), which meansthat it can be mixed swiftly as there is no need to dissipate heat. Thisproperty also gives it an advantage over bone cements based on modifiedpoly (methyl methacrylate) which have high exo therms.

5.9.8 Physical properties

The glass polyalkenoate cement sets rapidly within a few minutes to forma translucent body, which when young behaves like a thermoplasticmaterial. Setting time (37 °C) recorded for cements mixed very thickly forrestorative work varied from 2-75 to 4-7 minutes, and for the more thinlymixed luting agents from 4-5 to 6-25 minutes. Properties are summarized inTable 5.15.

Strength develops rapidly and after 24 hours in water (37 °C) can reach225 MPa (compressive) and 39 MPa (flexural) (Williams & Billington,1989; Pearson & Atkinson, 1991; Pearson, 1991). Compressive modulusreaches 9 to 18 GPa after 24 hours (Paddon & Wilson, 1976; Wilson,Paddon & Crisp, 1979).

Crisp, Lewis & Wilson (1976a) found that for two early types of glass-ionomer cement (ASPAII and ASPAIV) compressive strength continuedto increase for at least a year. Recently, Williams & Billington (1989) havefound that this behaviour does not hold for all modern commercial

147


Table 5.15. Mechanical properties of glass polyalkenoate cement (Prosseret al. 1984; 0ilo, 1988; Paddon & Wilson, 1976; Wilson, Paddon &Crisp, 1979; Pearson & Atkinson, 1991; Williams & Billington, 1989)

Powder .liquid, g cm 3

ConventionalWater hardening

Consistency disc diameter, mmMaximum particle size, urnFilm thickness (2 min), urnWorking time (23 °C), minutesSetting time (37 °C), minutesWet compressive strength (24 h), MPaWet compressive modulus (24 h), GPaWet flexural strength (24 h), MPaWet tensile strength, MPaCreep (24-48 h), %Opacity, C07 (1 mm)Water leachables (7 min), %Water leachables (1 h), %

Fillingmaterials

20-3-46-7-7-218-33——1-3-3-82-75-4-7140-2259-188-9-3919-0-19-3019-0-330-44-0-840-29-2-120-13-0-70

Lutingagents

1-67-1-833-3-3-621-3120-4024-402-3-5-754-5-6-2582-162—4-1-15-5

5-3-10-90-32-1-370-67-0-880-9-3-20-3-1-0

Load: 2-5 Kgf for filling materials, 220 gf for luting agents, applied after2 minutes

materials. Thus, whereas the compressive strength of Opusfil W increasedsteadily from 225 MPa (after 24 hours) to 258 MPa (after 115 days), thatof Ketac Fil increased from 171 MPa (after 24 hours) to a peak of 262 MPa(after 50 days) and, thereafter, declined to 167 MPa (after 115 days).

The young glass polyalkenoate cement is similar to zinc polycarboxylatecement in that it is not as rigid as dental silicate cement and shows markedstress relaxation characteristics (Paddon & Wilson, 1976). However, as theglass polyalkenoate cement ages its properties progressively depart fromthose of the zinc polycarboxylate and approach those of the dental silicatecement. It loses its viscoelastic properties and becomes increasingly rigid.This is shown by a significant increase in modulus and decline in stressrelaxation (Figure 5.18).

148


Fracture toughnessFlexural strength and fracture toughness are clinically more significantthan compressive strength. The flexural strength of a glass-ionomercement can reach 39 MPa after 24 hours (Pearson & Atkinson, 1991) whichis a much higher value than that attained by any dental silicate cement.

N/mm2

ExiO3 a

Cement AgeFigure 5.18 This figure shows how the properties of a glass polyalkenoate cement change asit ages. S is the compressive strength, E the modulus, a* a stress-relaxation function, and e*a strain-conversion function from elastic to plastic strain (Paddon & Wilson, 1976).

149


Table 5.16. Strength and fracture toughness of glass polyalkenoate fillingmaterials (Seed & Wilson, 1980; Prosser et ai, 1984; Lloyd & Mitchell,1984; Goldman, 1985; Prosser, Powis & Wilson, 1986; Lloyd &Adamson, 1987)

Glass polyalkenoate cementPhase-dispersed glass polyalkenoate

cementCarbon fibre reinforced glass

polyalkenoate cementSilver-glass polyalkenoate cementDental silicate cementAnterior composite resinsPosterior composite resinsDental amalgam

Flexuralstrengtha, MPa

9-3038

53

362529-4974^12976

Fracturetoughness# l c ,MNm3 / 2

0-45-0-55—

—

0-350-12-0-300-63-1-840-95-2-00-74

However, these values are less than those recorded for composite resinsused in dentistry. Goldman (1985) reports values of 29 to 49 MPa foranterior composite resins and Lloyd & Adamson (1987) values of 76 to125 MPa for posterior composite resins. A typical amalgam has a flexuralstrength of 6 MPa (Lloyd & Adamson, 1987) (Table 5.16). However, theflexural strengths of some glass-ionomer cements increase with time andvalues as high as 59 MPa (after 3 months) and 70 MPa (after 7 days) havebeen reported (Pearson & Atkinson, 1991).

Fracture toughness values for glass polyalkenoate cement vary from0-25 to 0-55 MN m3/2 (Lloyd & Mitchell, 1984; Goldman, 1985; Lloyd &Adamson, 1987). The values are generally higher than those found for thetraditional dental silicate cement but lower than those found for anteriorcomposite resins (Lloyd & Mitchell, 1984; Goldman, 1985) and muchlower than those for posterior composite resins and dental amalgams(Lloyd & Adamson, 1987).

These low values for flexural strength and fracture toughness comparedwith the values for composite resins and dental amalgams make theglass-ionomer cement less suitable than these materials in high-stresssituations.

150


TranslucencyA restorative material can be used for the aesthetic restoration of the front(anterior) teeth only if it is as translucent as tooth enamel. This is becausecolour matching depends on translucency as well as hue and chroma.

The glass polyalkenoate cement is translucent and so can be colour-matched to enamel (Kent, Lewis & Wilson, 1973; McLean & Wilson,1977a,b,c; Crisp, Abel & Wilson, 1979; Wilson & McLean, 1988); seeFigure 5.19. Early glass polyalkenoate cements were significantly deficientin this quality because high-fluoride glasses had to be used before othermeans of controlling the cement-forming reaction were discovered. Theseglasses were heavily phase-separated and almost opaque. This has beenremedied in recent formulations by employing clear or slightly opalescentglasses in combination with reaction-controlling additives.

Another barrier to achieving translucency is mismatch between therefractive indices of the glass and the matrix; the refractive index of theglass is greater than that of the matrix, which causes light-scattering. Thedental silicate cement tends to be naturally more translucent than the glass

Figure 5.19 The translucent appearance of glass polyalkenoate cements when placed on ablack and white striped background.

151


polyalkenoate cement because the refractive index of a phosphate matrixis greater than that of a polyacrylate one. However, by incorporating largeamounts of leachable phosphate into a polyalkenoate cement glass it hasbeen possible to increase matrix refractive index and to produce fullyaesthetic glass polyalkenoate cements (Crisp, Abel & Wilson, 1979).

The translucency of dental materials is normally represented as theinverse of opacity, although the scattering coefficient is a more fundamentalproperty (Section 10.11). Opacity is equivalent to contrast ratio, which isthe ratio of the light reflected from a disc of cement (1 mm thick) placed ona black background to that when it is placed on a white background. Thereflectivity of this background used in the dental context is 70 % andopacity is reported as C0.7 values (Paffenbarger, 1937). The C0.7 values forthe enamel of incisor teeth vary from 0-31 to 0*67 (Paffenbarger,Schoonover & Souder, 1938) and it is generally accepted that an aestheticfilling material should have a C0.7 between 0-35 and 0-50.

The first glass polyalkenoate cement had a C0.7 of 0-76, which was far toohigh, but improved modern materials are more acceptable and a value aslow as 0-52 has been reported for one of these (Crisp, Abel & Wilson, 1979).Knibbs, Plant & Pearson (1986b) have found that most glass polyalkenoatecements have a good optical match with tooth enamel.

5.9.9 Adhesion

Bonding to tooth material and metalsThe glass polyalkenoate cement has the important property of adhering tountreated enamel and dentine as many workers have shown (Wilson &McLean, 1988; Lacefield, Reindl & Retief, 1985). It also appears to adhereto bone and base metals (Hotz et al., 1977).

The bond strength to enamel (2-6 to 9-9 MPa) is greater than that todentine (1-5 to 4-5 MPa) (Wilson & McLean, 1988). Bond strength developsrapidly and is complete within 15 minutes according to van Zeghbroeck(1989). The cement must penetrate the acquired pellicle (a thin mucousdeposit adherent to all surfaces of the tooth) and also bond to debris ofcalciferous tooth and the smear layer present after drilling. Whatever theexact mode of bonding to tooth structure, the adhesion is permanent. Theprinciples and mechanism of adhesion have already been discussed inSection 5.2.

Various attempts have been made to improve bonding by pre-

152


conditioning the tooth surfaces with surface conditioners. The earliestconditioner, citric acid solution, was proposed by Hotz et al. (1977). Itetches the surface of enamel, revealing its prismatic structure, and removescalciferous debris (Figure 5.20a,&). It removes the smear layer from dentineand opens up dentinal tubules (Figure 5.20c,d). Doubt was cast on itsefficacy and since then more effective surface conditioners have beenfound. These include poly(acrylic acid), which has a less drastic effect ontooth material than citric acid, and tannic acid, which forms reaction layerson both dentine and enamel (Powis et al., 1982). Surface cleansing agentsand the fluoride ion also improve adhesion. Prati, Nucci & Montanari

Figure 5.20 The effect of a citric acid solution on tooth structure: (a) enamel surface beforeapplication, (b) enamel surface after application showing etching, (c) dentine surface beforeapplication, (d) dentine surface after application showing the opening-up of the dentaltubules (Powis et al, 1982).

153


(1989) have reviewed recent studies on surface conditioners using modernglass polyalkenoate cements. They found that bond strength depended onboth the cement and the surface conditioner used. The present consensusfavours treatment with a solution of poly(acrylic acid).

Permanent adhesion is an important attribute in a restorative materialfor it demands only minimal cavity preparation (i.e. drilling etc.) as thereis no need to provide a retentive undercut. In cervical erosion lesions (smallcavities at the gum line) it is especially important not to enlarge the lesionby drilling and in this situation the glass polyalkenoate cement is thematerial of choice. In addition, the glass polyalkenoate cement provides anexcellent seal because it is adhesive and shows little or no microleakagecompared with composite resins (Hembree & Andrews, 1978; Welsh &Hembree, 1985; Powis, Prosser & Wilson, 1988). This quality accounts forthe biocompatibility of glass polyalkenoate cement because a good sealeliminates bacterial invasion at the interface between cavity wall andrestoration. The biological consequences of this are described in Section5.9.11.

Molecular attachment

Acid etch attachment

Enamel

Figure 5.21 The laminate restoration, showing the glass polyalkenoate cement as a dentinesubstitute and a composite resin as an enamel substitute.

154


Bonding to composite resinsAn idea with important clinical implications to dentistry was advanced byMcLean & Wilson (1977b). This was to use a laminate restorationcomposed of a glass polyalkenoate cement and a composite resin. In thistechnique the glass polyalkenoate cement and the composite resin are usedtogether in a laminate restoration - the glass polyalkenoate cement as adentine substitute and the composite resin as an enamel substitute (Figure5.21). The glass polyalkenoate cement provides adhesion to dentine andfluoride release. The overlying composite resin provides excellent aestheticsand wear resistance and is bonded both to the enamel and the glasspolyalkenoate cement by acid etching. The advantages of both materialsare obtained with the laminate restoration.

In recent years this concept has been revived and combined with the ideaof bonding the composite resin to the glass polyalkenoate cement by usingan acid-etch technique to provide micromechanical attachment (McLeanet ah, 1985; Wilson & McLean, 1988). In this technique the surface of a setglass polyalkenoate cement is etched using a solution of orthophosphoricacid, and a thin coat of mobile resin is allowed to flow into the etchedsurface and is then polymerized by light activation. The composite resin isthen bonded to this surface. The appearance of the etched cement surfaceis shown in Figure 5.22. The bond strength appears to be about the sameas the cohesive strength of the cement; McLean et al. (1985) reported avalue of 10 MPa.

There is, however, a considerable variation in the strength of the union,and the combination of glass polyalkenoate cement and composite resinhas to be selected with considerable care in order to achieve good results(Hinoura, Moore & Phillips, 1987; Mount, 1989; Prati, Nucci &Montanari, 1989). The strength of the union depends on several factors,including the strength of the cement, its rate of development and the abilityof the bonding resin to wet the cement (Mount, 1989). There are problemsof adaptation with heavily filled composite resins, and excessive poly-merization contraction can destroy the bond. So far in these laminatesleakage between the glass polyalkenoate cement and the dentine has notbeen completely eliminated, but the use of surface pre-treatments hassignificantly reduced its extent (Prati, Nucci & Montanari, 1989).

155


5.9.10 Erosion, ion release and water absorption

Although the glass polyalkenoate cement is the most durable of all dentalcements it is susceptible to attack by aqueous fluids under certainconditions. There are three related phenomena to consider: erosion, ionrelease and water absorption.

Ion release and water absorptionWhen fully hardened, the cement is resistant to erosion provided thesolution has a pH above 4. However, the glass polyalkenoate cement issusceptible to erosion immediately after set because some of the matrix-forming cations and anions are still in soluble form. In fact, the hardeningprocess is one where these cations and anions continue to precipitate. Forthis reason these cements have to be protected, temporarily, by a varnish.

When immature glass polyalkenoate cements are exposed to neutralsolutions, such as normal saliva, they release ions and absorb water. The

Figure 5.22 Effect of acid-etching on the surface of a glass polyalkenoate cement (McLean etaL, 1985).

156


matrix-forming cation Al3+, but not Ca2+, can be lost and then the cementis permanently damaged. Other ions lost are sodium and fluoride, togetherwith silicic acid, but loss of these species does not seem to be significant asfar as erosion is concerned. Water is also rapidly absorbed: 3-2% in 24hours and 3-8 % in 7 days in one example cited by Crisp, Lewis & Wilson(1980).

As the cement ages, absorption of water and loss of aluminium ionsceases (after 7 days). Other species - sodium and fluoride ions and silicicacid - continue to be eluted. The release of fluoride is important, for theglass polyalkenoate cement can be seen as a device for its sustained release.

Fluoride releaseA number of workers have observed the sustained release of fluoride fromthe glass polyalkenoate cement (Crisp, Lewis & Wilson, 1976d; Forsten,1977; Maldonado, Swartz & Phillips, 1978; Causton, 1981; Cranfield,Kuhn & Winter, 1982; Swartz, Phillips & Clark, 1984; Tay & Braden,1988; Wilson, Groffman & Kuhn, 1985). The last-named found thatrelease continued for at least 18 months. Release can be fitted to a log-logplot (Cranfield, Kuhn & Winter, 1982) although it is difficult to give aphysical meaning to this expression. Wilson, Groffman & Kuhn (1985)fitted accumulated release of fluoride, sodium and silica to an equation ofthe following form:

Total amount released = C+At*+Bt (5.1)

The three terms correspond respectively to initial washout, diffusionand erosion. Unfortunately, although the mathematical fit was good(c. 99-9 %), C and B proved to be negative, making it difficult to assigna physical meaning to the equation.

Wilson, Groffman & Kuhn (1985) calculated that only about 4-5% ofthe total fluoride is available for release, and Meryon & Smith (1984) foundthat the amount released was not related to the fluoride content of thecement. Wilson, Groffman & Kuhn (1985) observed that release of fluoridewas accompanied by the release of sodium, necessary to maintainelectroneutrality, and silica (as silicic acid). The release of these speciescould also be fitted to equation (5.1).

Crisp, Lewis & Wilson (1980) found that these same three species werereleased, in greater amounts but in roughly the same proportions, underacid attack. The association of silica with fluoride suggests, perhaps, thatit is principally the glass particles that are attacked rather than the matrix

157


phase. Cranfield, Kuhn & Winter (1982) also found that the rate of releaseof fluoride is greater in acid than in neutral solution. These results suggestthat release of these species may be an erosive rather than a diffusiveprocess, which may explain the perplexity of Cranfield, Kuhn & Winter(1982) in attempting to elucidate the mechanism of fluoride release in termsof a diffusive process. Kuhn & Jones (1982) suggested that a porousgranular monolith as described by Kydonieus (1980) was appropriate todescribe fluoride release. More recently, Tay & Braden (1988), in aprolonged study over 2\ years, suggested that there were two processesinvolved, a rapid surface elution and a slower bulk diffusion. But we mustconclude, at present, that the mechanism of fluoride release is still far fromunderstood.

The release of fluoride is biologically important because it is taken up byadjacent tooth material (Retief et al, 1984; Shimoke, Komatsu & Matsui,1987), presumably by the ion exchange of F~ for OH" in hydroxyapatite.This fluoride uptake has the effect of improving the resistance of the toothmaterial to acid attack (Maldonado, Swartz & Phillips, 1978; Wesenberg& Hals, 1980; Kidd, 1978). Maldonado, Swartz & Phillips (1978) havefound that the solubility of enamel in acid was reduced by 53 % when incontact with a glass polyalkenoate cement. Also, fluoride adsorptionreduces surface energy (Glanz, 1969) making adhesion of caries-promotingplaque more difficult (Rolla, 1977). It also decreases demineralization andincreases remineralization of teeth (Wei, 1985) and reduces the fer-mentation of carbohydrates and the growth of plaque bacteria (Hamilton,1977; Tanzer, 1989). Of course, it has been known since the early 1940sthat fluoride inhibits dental decay (Horowitz, 1973).

Acid erosion and clinical durabilityThe glass polyalkenoate cement is the most durable of all dental cements.In a very early study, Kent, Lewis & Wilson (1973) found that the surfaceof the glass polyalkenoate cement was much less affected by acids andstained much less than traditional dental silicate cement. This earlyobservation has since been substantiated by both in vivo studies (Mitchem& Gronas, 1978, 1981; Ibbetson, Setchell & Amy, 1985; Mesu & Reedijk,1983; Pluim et al, 1984; Pluim, Arends & Havinga, 1985) and laboratorytests (Mesu, 1982; Beech & Bandyopadhyay, 1983; Sidler & Strub, 1983;Kuhn, Setchell & Teo, 1984; Theuniers, 1984; Pluim et al.9 1984; Pluim,Arends & Havinga, 1985; Walls, McCabe & Murray, 1985; Wilson et ai,1986a,b; Gulabivala, Setchell & Davies, 1987).

158


Clinical durability largely depends on resistance to acid erosion, for acidconditions occur in stagnation regions of the mouth where dental plaqueaccumulates. Plaque contains streptococci and lactobacilli which degradeplaque polysaccharides and sucrose to lactic acid (Tanzer, 1989; Jenkins,1965), and lactic acid is the most potent driving force in causingdemineralization of teeth. The lower extreme of acidity found in the mouthis pH = 40 (Stephan, 1940; Kleinburg, 1961). Glass polyalkenoatecements begin to erode only at this pH (Crisp, Lewis & Wilson, 1980);Walls, McCabe & Murray (1988) and Wilson et al. (1986b) found that onebrand did not erode at all at this pH. Susceptibility to acid erosion is loweven when the pH is 2-7.

Crisp, Lewis & Wilson (1980) made a chemical study of the erosion ofa glass polyalkenoate cement under acid attack. They found that the chiefspecies eluted were sodium and fluoride ions and silicic acid suggesting thatattack occurred mainly on the glass particles rather than on the matrix.

Resistance to acid erosion depends on brand and varies from 0-04 to0-54% per hour (Setchell, Teo & Kuhn, 1985; Wilson et al., 1986a; Walls,McCabe & Murray, 1988). It would appear that cements based oncopolymers of acrylic and maleic acids are less durable than those based onpoly (aery lie acid). The extent of erosion varies inversely with the timeallowed for the cement to cure prior to exposure (Walls, McCabe &Murray, 1988).

McKinney, Antonucci & Rupp (1987) found that the clinical wear of theglass polyalkenoate cement compared favourably with that of thecomposite resin, but they noted that it was prone to brittle fracture andchemical erosion.

Clinical experience shows that these cements are durable. For example,a failure rate as low as 2 % has been reported by Mount (1984) in a clinicaltrial lasting seven years, and Wilson & McLean (1988) have cited a numberof clinical trials attesting to the durability of this cement.

5.9.11 Biocompatibility

Dental materialThe biocompatibility of the glass polyalkenoate cement is good (Wilson &McLean, 1988; Nicholson, Braybrook & Wasson, 1991) and its capacity torelease fluoride in a sustained fashion makes it cariostatic (Hicks, Flaitz &Silverstone, 1986; Kidd, 1978). Its ability to provide an excellent seal(Section 5.9.9) is an important attribute because in recent years it has

159


become generally accepted that pulpal inflammation is caused not so muchby chemical toxicity as by the percolation of harmful bacteria betweencavity wall and the restorative (Brannstrom & Nyborg, 1969; Paterson,1976; Browne et al, 1983). The seepage of harmful bacteria beneath arestoration can be the cause of secondary caries (Bergenholtz et al, 1982)and is the cause of much of the failure of dental amalgams (Mjor, 1985).

Early studies showed that glass polyalkenoate cement has less of anadverse effect on the dental pulp than has silicate cement (Kawahara,Imanishi & Oshima, 1979; Pameijer, Segal & Richardson, 1981). Ap-parently, the glass polyalkenoate cement causes only mild pulpal inflam-mation which reaches a maximum 14 days after placement and thenprogressively diminishes (Kawahara, Imanishi & Oshima, 1979). The effectis greatly diminished by a layer of 0-5 mm of dentine. More recentmaterials appear to cause less inflammation than earlier ones (Plant et ai,1984; Yoshii et al.9 1987). A recent careful study of biocompatibility byPameijer & Stanley (1988) on primates, using a standard methodologyrecommended by the American National Standards Institute and theAmerican Dental Association, showed a modern cement to be bland.

The glass-ionomer cement is well-tolerated by living cells (Kawahara,Imanishi & Oshima, 1979; Kasten et al, 1989). An important distinctionis to be made between the freshly mixed and fully set glass-ionomercement. The glass-ionomer cement exhibits an antibacterial effect whenfreshly mixed which diminishes with time (Tobias, Browne & Wilson,1985). It exhibits some cytotoxicity when freshly mixed, but none whenfully set (Tyas, 1977; Kawahara, Imanishi & Oshima, 1979; Meryon &Browne, 1984; Hume & Mount, 1988; Brook, Craig & Lamb, 1991a;Hetem, Jowett & Ferguson, 1989). Both the antibacterial and cytotoxiceffects appear to be associated with the leachate from the cements. Theprecise cause of these effects remains unclear. Some workers have suggestedthat it is the combination of fluoride and low pH in the young cement(Leirskar & Helgeland, 1987; McComb & Ericson, 1987), while othersimplicate the release of metal ions and free poly(acrylic acid)s (Kawahara,Imanishi & Oshima, 1979; Nakamura et al., 1983). Recent studies byBrook, Craig & Lamb (1991a,b) on bone substitutes have shed some lighton this problem and are described later.

No problems arise when the glass-ionomer cement is used to restoreabrasion/erosion lesions in primary teeth and as a lining material inshallow cavity preparations (Tobias et ah, 1978, 1987). In deeper

160


preparations, when the dentine layer is very thin it is advisable to use anantibacterial calcium hydroxide liner.

Post-operative sensitivity has occasionally been reported when theglass-ionomer cement has been used as a luting agent. This observation ismore than anecdotal, but the reason for it is unknown. It is not connectedwith pulpal irritation but may be related to hydraulic pressures (Pameijer& Stanley, 1988). The indication is that sensitivity is related to clinicaltechnique and is exacerbated if certain slow-setting glass-ionomer cementsare used, especially if they are mixed too thinly.

Bone cement and bone substituteNot surprisingly, this cement has been studied as a possible replacementfor the poly(methyl methacrylate), PMMA, bone cement, a material whichis not entirely satisfactory (Jonck, Grobbelaar & Strating, 1989a,b; Jonck& Grobbelaar, 1990). Its use has been linked with the pathogenesis offailure; there is tissue sensitivity and it is not suitable in reconstructivesurgery following cancer or replacement procedures following radiationtreatment of diseased bone.

Jonck, Grobbelaar & Strating (1989a,b) and Jonck & Grobbelaar (1990)have made biological evaluations, using baboons, of glass polyalkenoatecement for use in joint replacement surgery. It was found to be stablewithin the bone environment and there were no signs of surface dissolutionover a period of three years. It was also observed to bond to bone and actas a sealant. The promotion of new bone formation and calcification on thesurface of the cement was observed. This reparative process was tentativelyattributed to the ion-exchange properties of the cement and, in particular,to the release of fluoride. The glass polyalkenoate cement was found tohave no inhibitory effect on bone tissue development. Rather, it appearedto promote the formation of osteoblasts (bone-forming cells) and thenormal differentiation of haemopoietic (blood-forming) tissue.

The glass-ionomer cement was found to be non-toxic. There were nosigns of inflammation or irritation with any of the glass polyalkenoatecement implants even after several months. By contrast a proportion ofPMMA cement implants caused swelling and bone reactions. There werealso signs of possible hyperaemia (blood congestion) and infarcts (areasdeprived of blood supply) and dead tissue.

Thus, the glass polyalkenoate cement has distinct biological advantagesstemming from its dynamic surface chemistry, which is favourable to bone

161


mineral precipitation. This process, in turn, promotes normal tissueresponses essential for the processes of bone formation.

Brook, Craig & Lamb (1991a,b), who studied the use of the cement as analveolar bone substitute, have confirmed these findings. They find that thecement forms an intimate bioactive bond with bone cells and their workhas highlighted the complex role of fluoride. Their in vitro studies showedthat of the cement pastes only the one derived from a non-fluoride-containing glass (MP4) did not inhibit cell growth. But this cement did notintegrate with bone as effectively as did cements based on fluoride-containing glasses.

Jonck, Grobbelaar & Strating (1989a,b) suggested that the slow releaseof fluoride had a beneficial in vivo effect on osteogenesis (the formation ofbone) similar to that of osteoid formation stimulated by fluoride therapy inthe treatment of osteoporosis (abnormal porosity of the bone) (Frost,1981). There appeared to be an optimum level of fluoride for thestimulation of bone-forming cells (Turner et al, 1989) since mild toxiceffects are encountered in the closed in vitro situation (Kawahara, Imanishi& Oshima, 1979; Nakamura et ai, 1983; Hetem, Jowett & Ferguson, 1989;Kasten et al., 1989; Jonck, Grobbelaar & Strating, 1989a,b; Brook, Craig& Lamb, 1991a,b). Brook, Craig & Lamb (1991b) have suggested that inthe dynamic in vivo situation the leaching of fluoride may stimulateintegration with the bone (osseointegration) thus accounting for thesuperiority of cements based on fluoride-containing glasses in this respect.

Brook, Craig & Lamb (1991a) found that set glass-ionomer cementimplants compared favourably with those of other materials. More bonewas formed around glass-ionomer cement implants than those ofhydroxyapatite. Direct bonding of glass polyalkenoate cement to mineral-ized collagenous extracellular bone matrix was found without an inter-vening layer. Behaviour was similar to that of bioglass and glass ceramics.To sum up, glass-ionomer cement forms an intimate bond to living bone,a process which is enhanced by the release of fluoride.

5.9.12 Modified and improved materials

Various attempts have been made to improve the glass polyalkenoatecement. We have already described (Section 5.9.5) the most importantinnovation, the use of (+ )-tartaric acid to improve setting characteristics(Wilson & Crisp, 1976). Before its use was discovered, only one glass,

162


G-200, could be used to form practical cements; afterwards a whole rangeof glasses became available, including clear ones. The original glass poly-alkenoate cement based on G-200 had poor translucency because this high-fluoride glass was heavily opal. The use of the new clear glasses yielded anew generation of cements with good translucency. Radio-opacity has alsobeen introduced into glasses by replacing calcium with strontium orlanthanum (Section 5.9.2).

All additives, ( + )-tartaric acid and metal fluorides, which improvesetting also increase cement strength (Section 5.9.4). The same point can bemade about the technique of acid-washing glasses (Schmidt et al., 1981a)which, by removing calcium ions from the surface of glass particles,enhances the mixing and setting qualities of cements. All of whichillustrates the general point that strength is related to the working andsetting qualities of cements.

The realization that cement strength could be increased by increasing themolecular weight of the poly(alkenoic acid) was important (Section 5.9.3).Unfortunately, this also increased the stiffness of the cement mix,necessitating a reduction in powder/liquid ratio. Thus, the benefits gainedon the one hand were lost on the other. Fortunately, there was a way roundthis problem. By drying the poly(alkenoic acid) and blending it with theglass powder cement, water could be used to initiate cement formation,with the benefit of lowering the stiffness of the mix (Prosser et al., 1984;McLean, Wilson & Prosser, 1984). This technique permitted the use ofpoly(alkenoic acid) of higher molecular weight.

A combination of all these technical improvements has led to asignificant improvement in the quality of the glass polyalkenoate cement.

Reinforced glass polyalkenoate cementsMore direct attempts at improving cement strength have been made.Wilson et al (1980) and later Prosser, Powis & Wilson (1986) observed thatdisperse-phase glasses yielded stronger cements. The strongest cementyielded by a clear glass had a flexural strength of 21 MPa, whereas a glasscontaining corundum and fluorite as disperse phase gave a cement with aflexural strength of 33 MPa.

The use of reinforcing fillers was examined by Seed & Wilson (1980). Analumina-fibre cement had a flexural strength of 44 MPa, while onereinforced by carbon fibre had a flexural strength of 53 MPa. Metalreinforcement has also been examined. Seed & Wilson (1980) found that acement reinforced with silver-tin alloy had a flexural strength of 40 MPa.

163


However, restorations made from this material could not be polished andwere aesthetically very poor. Simmonds (1983) has pursued this idea, anda material has been placed on the market. But according to Moore, Swartz& Phillips (1985) such cements have less resistance to abrasion than asimple glass polyalkenoate cement.

Recently, Oldfield & Ellis (1991) have examined the reinforcement ofglass-ionomer cement with alumina (Safil) and carbon fibres. Theintroduction of only small amounts of carbon fibres (5% to 7*5% byvolume) into cements based on MP4 and G-338 glasses was found toincrease considerably both the elastic modulus and flexural strength. Therewas an increase in work of fracture attributable to fibre pull-out. Amodulus as high as 12-5 GPa has been attained with the addition of 12%by volume of fibre into MP4 glass (Bailey et ai, 1991). Results usingalumina fibre were less promising as there was no fibre pull-out because ofthe brittle nature of alumina fibres which fractured under load.

McLean & Gasser (1985) had the idea of fusing silver with ionomer glassto produce a fused metal-glass (cermet) powder which replaced the glasspowder in the conventional cement. The idea was to improve strength,toughness and abrasion resistance. Resistance to abrasion is improved(Moore, Swartz & Phillips, 1985; Swift, 1988b), probably by a lubricatingeffect since the surface of the restoration can take a polish (McLean &Gasser, 1985; McKinney, Antonucci & Rupp, 1985, 1987). However,Lloyd & Adamson (1987) found that a cermet polyalkenoate cement wasno stronger and no tougher than a conventional material. In an in vitrostudy, Thornton, Retief & Bradley (1986) showed that the bond strength toenamel and dentine was lower, and consequently microleakage has beenfound to be higher (Robbins & Cooley, 1988). Fluoride release was foundto be lower than that of conventional materials (Thornton, Retief &Bradley, 1986). The material is radio-opaque, which is an advantage, for itspresence in living tissues can then be detected by X-rays. Obviously, itcannot be used for aesthetic restorations and finds chief use in preparing acore to receive porcelain and gold crowns. One example of this material isavailable on the market.

Very recently, Williams, Billington & Pearson (1992) have examined theeffect of reinforcement by silver or silver-tin alloy on the mechanicalproperties of three glass-ionomer cements. Measurements of compressive,flexural, tensile (measured by the diametral compressive procedure) andshell strength are given in Table 5.17. These results show that the effect ofreinforcement varies from cement to cement but, in general, increases it.

164


Table 5.17. Effect of reinforcement on various strength properties of threeglass polyalkenoate cements (Williams, Billing ton & Pearson, 1992)

Cement Standard Reinforced

Powder: liquid ratio, g cm"

Wet compressive strength (24 h), MPa

Wet flexural strength (24 h), MPa

Wet tensile strength, MPa

Wet shell strength (24 h), MPa

ABCABCABCABCABC

3-82-47-0

14613616221-324-725118-613-316-235-732-346-9

3-25111013713822129036-367012-814-325144-639-675-8

The effect on cement C is particularly dramatic and the flexural strength ofcement C is exceptionally high. In part, this is to be attributed to the highpowder/liquid ratio. These results are to be compared with the flexuralstrengths of early polyalkenoate cements which were c. 10 MPa.

Aluminoborate glassesBrief mention may be made of the aluminoborate glasses developed byBertenshaw et al. (1979). These glasses are prepared by fusing a mixture ofboric oxide (replacing silica), alumina and a metal oxide, usually zincoxide. The fusion temperature is much lower than for the aluminosilicateglasses. The compositional region for glass formation is restricted andglasses are only obtained when the alumina content lies between 1 and13 mole. Boric oxide content ranges from 35 to 73 mole boric acid andmetal oxide content from 27 to 59 mole.

The cement-forming reaction will be similar to glass polyalkenoatecement. The cement matrix will consist of metal polyacrylates, but boricacid will be produced instead of silica gel. Since boric acid has a watersolubility of 2-7 % compared with the near insolubility of silica gel, it would

165


be expected that these cements would be less durable than the conventionalglass polyalkenoate cements.

Compressive strengths of these cements were found by Bertenshaw et al.(1979) to range from 20 to 50 MPa and tensile strengths from 5 to 9 MPa.These values are inferior to those of the conventional glass polyalkenoatecements but similar to those of the zinc polycarboxylate cements. They arereported to have a good translucency and have a low solubility in water.These materials do not appear to be manufactured commercially.

Recently, Wilson & Combe (1991) have studied the reactivity ofmagnesium, zinc, calcium and strontium boroaluminate glasses towardspoly (aery lie acid) solutions. The controlling factor would seem to be thealumina content of these glasses which serves to moderate the setting rateof the cements.

5.9.13 Applications

The glass polyalkenoate cement is a versatile material and finds use indentistry and more generally as a biomaterial. There have also beenapplications outside these fields.

DentalThe glass-ionomer cement is the most versatile of all the dental cementsand has been developed for a variety of applications (McLean & Wilson,1974, 1977a,b,c; Swift, 1988b; van de Voorde, 1988; Wilson & McLean,1988; Mount, 1990). Many of its applications depend on its adhesivequality which means that, unlike the non-adhesive traditional fillingmaterials, it does not require the preparation of mechanical undercuts forretention and the consequent loss of sound tooth material.

The glass polyalkenoate cement was originally intended as a substitutefor dental silicate cements for the aesthetic restoration of front (anterior)teeth (Wilson & Kent, 1972; Knibbs, Plant & Pearson, 1986a; Osborne &Berry, 1986; Wilson & McLean, 1988). It is suitable for restoring anteriorcavities in low-stress situations, that is when the restoration is completelysupported by surrounding tooth material. These cavities occur on theadjacent surfaces of neighbouring teeth (class III cavities) and at the gumline (class V cavities).

Quite early, McLean & Wilson (1977b) found that the glass poly-alkenoate cement was particularly effective for restoring cervical lesions -

166


small cavities that occur at the gum line. These are often found in middle-aged people and are caused by abrasion or erosion rather than by dentaldecay (caries). These lesions are so small that it is unwise to enlarge themto provide mechanical retention (Wilson & McLean, 1988). For this reasonthe adhesive glass polyalkenoate cement is the material of choice for therestoration of cervical lesions (Tyas & Beech, 1985).

The cement is commonly used to restore primary (children's) teeth sincethe trauma of drilling may be minimized or avoided altogether (Wilson &McLean, 1988; Walls, Murray & McCabe, 1988).

Some recent ingenious applications are of particular interest as theyexploit the adhesive properties of the glass-ionomer cement to the full.Clinicians have now developed the concept of minimal cavity preparation(McLean, 1980, 1986; Hunt, 1984; Knight, 1984; Wilson & McLean,1988). The idea is that since caries is mainly a disease of the dentine, thenonly a minimal amount of enamel need be removed, just sufficient to allowfor the excavation of the carious dentine. A small channel is drilled throughthe enamel and the carious dentine is removed through it. Sound enamel isthus preserved. The excavated region is then filled with glass polyalkenoatecement, which by virtue of its adhesive nature holds the enamel shelltogether.

Another use of the glass polyalkenoate cement is as a base materialwhich can be placed under dental amalgam or composite resin in therestoration of posterior (molar and semi-molar) teeth (Smith, Ruse &Zuccolin, 1988; Wilson & McLean, 1988). Its role is to adhere to dentine,provide a protective seal against bacteria and release fluoride: functionswhich prevent caries occurring under the restoration. In the laminaterestoration, fully described in Section 5.9.9, it is used, in effect, as a dentinesubstitute. Base cements used under other restoratives are frequently maderadio-opaque so that they can be distinguished from carious dentine. Thisis achieved by adding zinc oxide, using silver-glass cermets in place of theglass or using glasses in which the calcium is replaced by lanthanum orstrontium. Base cements are generally quick-setting.

The glass polyalkenoate cement is also used in the fitting of crowns. It isused to build up a substructure, known as a core, if there is insufficienttooth material to take a crown (Wilson & McLean, 1988). Core build-upmaterials are generally made radio-opaque and the silver cermet is oftenused. A fine-grained version was developed to be used as a luting agent forthe cementation of cement crowns and veneers; this is based on a fine-grained glass (Wilson et aL, 1977; Wilson & McLean, 1988).

167


Another important use for the glass polyalkenoate cement is inpreventive dentistry where it can be used to fill and seal naturally occurringpits and fissures in molar teeth which are sites for the initiation of caries(McLean & Wilson, 1974, 1977b; Komatsu, 1981; Wilson & McLean,1988). Its adhesive quality and ability to act as a long-term fluoride-releasing gel make it particularly suitable for this purpose. Specialformulations for this application have been placed on the market.

Splint bandageThe glass polyalkenoate cement forms the basis of a novel splint bandagethat was developed in the early 1970s (Parker, 1974; Potter et ai, 1977,1979; Hall, 1977) and marketed by Smith & Nephew. In this application,a powder blend of glass and poly(acrylic acid) is applied to a bandage.When required for use the bandage is immersed in water, a techniqueidentical to that used for the conventional plaster bandage. The addition ofwater activates the cement-forming reaction and the bandage sets hard,but remains more flexible than the plaster bandage. It has otheradvantages. It is stronger and attains strength more rapidly than theconventional bandage. It is also impervious to water once set. Theserepresent significant advantages for the patient. It can be applied over anormal plaster bandage to protect the latter from the softening effect ofwater.

Bone cementExisting bone cements for orthopaedic use are based on a modifiedpoly(methyl methacrylate) resin. It has disadvantages. The formation ofthis polymer in situ is accompanied by the marked evolution of heat whichcan damage tissues (Feith, 1975). The presence of unreacted monomer,which can leach out, also damages tissues (Petty 1980; Pople & Phillips,1988). The glass polyalkenoate cement has an exceptionally low exothermand good biocompatibility, which together with its ability to bond to bonegive it potential as an improved bone cement.

A biological evaluation of the glass polyalkenoate cement as a bonecement has been carried out by Jonck, Grobbelaar & Strating (1989a,b;Jonck & Grobbelaar, 1990) on baboons and is reported in Section 5.9.11;initial findings are promising.

168

Resin glass polyalkenoate cements

Alveolar bone substituteIn the UK about 20 % of the population over 16 are without natural teeth(Brook, Craig & Lamb, 1991b). Dentures are supported by the alveolarridge which over the years is subjected to progressive resorption. Thereduction of the alveolar ridge gives rise to functional problems. Brook,Craig & Lamb (1991b) have used the glass-ionomer cement successfully torestore this ridge showing that it successfully integrates with bone (seeSection 5.9.11).

Slip casting mouldThe glass polyalkenoate cement can also be used to replace plaster as amould in the slip process for pottery. It possesses the same property asplaster of Paris, of causing material to deposit on its surface from slipsuspensions. So far this property has not been exploited in the manufactureof pottery.

5.10 Resin glass polyalkenoate cements

5.10.1 General

One of the most interesting recent developments has been the advent of theresin glass polyalkenoate cements (Antonucci, McKinney & Stansbury,1988; Mitra, 1989; Wilson, 1989, 1990; Mathis & Ferracane, 1989;Minnesota Mining & Manufacturing Company, 1989; Albers, 1990). Theyare dual-cure hybrids that set by a combination of acid-base andpolymerization reactions, and there are several types. Polymerization iseffected by either chemical or light initiation.

At its most basic, the resin glass polyalkenoate cement can be seen as aglass polyalkenoate cement in which the water component is replaced by awater-HEMA mixture. HEMA is hydroxymethyl methacrylate, itshydroxy group making it water-soluble:

CH2 = C — C O — O — CH2-CH2-OH

CH3

Also, included in these formulations are water-soluble initiators/activatorsfor the polymerization of HEMA.

The resin glass polyalkenoate cements are mixed in the same way asconventional materials. In the case of the light-activated systems they

169


remain workable for 10 or more minutes unless exposed to light. Whenlight is shone on them they are activated and set hard in 30 seconds. Thereis a dual setting reaction: the normal glass polyalkenoate cement acid-basereaction and, additionally, a free-radical or photochemical polymerizationprocess, similar to that occurring in composite resins. These may berepresented as:

(1) Acid—base reaction'.

Calcium aluminosilicate Polyacrylic Calcium and aluminiumglass (base) acid polysalt hydrogel

(2) Polymerization reaction:

HEM A + photochemical initiator/activator ^PolyHEMA matrix

Two matrices are formed: a metal polyacrylate salt and a polymer. Thereis a lack of water in the system because some of it has been replaced byHEM A, and lack of water in glass polyalkenoate cements is known to slowdown the ionomer acid-base reaction (Hornsby, 1977). Thus, the initial setof these materials results from the polymerization of HEM A and not thecharacteristic acid-base reaction of glass-ionomer cements. The laterreaction serves only to harden and strengthen the already formed matrix.

The two matrix-forming reactions are shown in more detail in Figure5.23.

5.10.2 Class I hybrids

In more complex forms of this resin hybrid, other dimethacrylates may bepresent, such as the ethylene glycol dimethacrylates, and bis-GMA, whenHEMA acts as a co-solvent for water and bis-GMA (Antonucci,McKinney & Stansbury, 1988). The general composition of these materials,which we term class I hybrids, is summarized below:

Powder component'.Glass (Chemfil II) + poly(acrylic acid) + tartaric acid

Liquid component {replaces water):

Water/HEMAOther difunctional hydroxy dimethacrylates, e.g. the ethyleneglycol dimethacrylates

170


Bis-GMAInitiator/activator

In chemically-cured materials, one example of an initiator/activatorsystem is: hydrogen peroxide as initiator, ascorbic acid as activator andcupric sulphate as co-activator. In light-cured materials, camphorquinoneis used as a visible-light photochemical initiator, sodium /?-toluene-sulphinate as activator and ethyl 4-dimethylaminobenzoate as photo-accelerator.

If there is too little water in a composition then the acid-base reaction

DUAL CURE 1ACID — BASE REACTION

ICH2

CH-COOH

' CalciumI 2 aluminosilicateI •CH-COOH

Poly (acrylic acid)

I ICH2 F CH2

CH-COO — A l 3 + -= 00C— CH

CH-, CH,

CH-COO"— Ca2 + — 00C—CHI I

Ca, Al polysalt hydrogel

HEMA

C=CH2

C=0

0

ICH2

CH2I

DUAL CURE 2

HEMAPOLYMERIZATION

Photo or chemicalinitiator / activator

- C - C H 2 -

c=o

0

CH2

CH2

OH

poly HEMA

Figure 5.23 The two matrix-forming reactions in class I resin-based glass polyalkenoatecements.

171


will be completely inhibited and only the polymerization reaction will takeplace, in which case the material is not strictly speaking a glasspolyalkenoate cement.

5.10.3 Class II hybrids

The two matrices in these cements are of a different nature: an ionomer salthydrogel and polyHEMA. For thermodynamic reasons, they do notinterpenetrate but phase-separate as they are formed. In order to preventphase separation, another version of resin glass polyalkenoate cement hasbeen formulated by Mitra (1989). This is marketed as VitraBond, which weterm a class II material. In these materials poly(acrylic acid), PAA, isreplaced by modified PAAs. In these modified PAAs a small fraction of thependant -COOH groups are converted to unsaturated groups by con-densation reaction with a methacrylate containing a reactive terminalgroup. These methacrylates can be represented by the formula:

T — R — C = C H 2

CH3

where T is a terminal group, for example:

HO— H2N— OCN— CH2 — CH —

The condensation reaction can be represented thus:

— COOH + H O — R — C = C H 2 • — C O — O — R — C = C H 2

CH3 CH3

Modified PAAs can be represented by the generic formula:

—CH2—CH—CH2—CH—CH2—CH — CH2—CH—CH

CO CO CO CO

OH OH O OHR—C=CH 2

CH3

The liquids used for class II hybrids contain 2 5 ^ 5 % modified PAA and21-41 % HEM A. The initiator system for light activation contains

172


camphorquinone and diphenyliodonium chloride (Mitra, 1989). The glasspowder has the following percentage composition:

SiO2 27-24; A12O3 0-81; P2O50-95; NH4F3-37; A1F3 20-97;Na3AlF6 10-81; ZnO 20-97; MgO215; SrO 12-74.

Formation of matricesWhen a resin glass polyalkenoate cement, containing a modified PAA andHEMA, is mixed, a paste is formed which sets only slowly in the absenceof light. When activated by light the paste sets in 30 s. Several types ofpolymerization can then take place. Both HEMA and the modified PAA,because it contains unsaturated groups, will polymerize. PolyHEMA anda crosslinked PAA of high molecular weight will be formed. In addition,the modified PAA may copolymerize with HEMA; thus, polyHEMA ischemically linked to the polyacrylate matrix and so cannot phase-separate.The matrix of such a cement contains both ionic and covalent crosslinks(Figure 5.24). Thus, the cement matrix is reminiscent of an ion-exchangeresin.

5.10.4 Properties

These resin-modified glass polyalkenoate cements have both advantagesand disadvantages over conventional glass polyalkenoate cements. How-ever, because of their poor translucency they are recommended only asliners or bases.

They have improved setting characteristics. They have a long workingtime because HEMA slows the acid-base reaction, yet set sharply once thepolymerization reaction is initiated by light. They are also resistant to earlycontamination by water because of the formation of an organic matrix,and so do not require protection by varnish. This combination of propertiesis bound to appeal to the clinician.

The freshly set class II (Vitrabond) resin glass polyalkenoate cementappears to have rubbery characteristics and there is some debate as to

CH2 CH2 CH2 CH2

CH-COO" M2+ "OOC-CH CH-COO-R-CH-CH 2 -CH 2 -CH-R-OOC-CH

CH2 CH2 CH2 CH3 CH3 CH2

I II IFigure 5.24 The matrix of a class II resin-based glass polyalkenoate cement, showing ionicand covalent crosslinks.

173


Table 5.18. Strength of resin glass polyalkenoate cements (Antonucci,McKinney & Stansbury, 1988; Wilson & McLean, 1988; Mathis &Ferracane, 1989; Mitra, 1989; Minnesota Mining & ManufacturingCompany, 1989; Alters, 1990)

Wet strength,MPa (24 hours)

CompressiveFlexuralTensileAdhesion

(dentine)

Resin

Class I

94-139—

16-4^33-91-6

Class II

53-9625-511-2-17-49-8-11-3

Conventional

Liner/base

56-795-2-10-33-4-9-13-4^3-9

Filling

140-1958-9-30-39-0-19-31-7-6-8

whether this is advantageous or not. The reason for this rubberiness is thatthe polymer is only lightly crosslinked, and at set the acid-base reactionhas not proceeded very far. Most probably, these rubbery characteristicswill disappear as the cement ages and the acid-base reaction is completed.But this may take a very long time.

Some data have been published on the mechanical properties of thesecements (Antonucci, McKinney & Stansbury, 1988; Mathis & Ferracane,1989; Mitra, 1989; Minnesota Mining & Manufacturing Company, 1989;Albers, 1990), but much of it comes from patents and company reports soit would be unwise to draw firm conclusions from these figures alone.

Although both class I and II resin hybrids are stronger than conventionalliners and bases, class II materials are not as strong as conventional fillingmaterials (Table 5.18). According to Mathis & Ferracane (1989) a class Imaterial developed 82 % of its 24-hour ultimate compressive strength inone hour, which compares favourably with a figure of about 52 % for aconventional glass polyalkenoate cement. Rapid development of strengthis to be expected because of the polymerization process.

Both class I and class II resin glass polyalkenoate cements are claimed tobond to dentine. This can be accepted. But that the bond is stronger anddevelops more rapidly than that of the conventional glass polyalkenoatecement, as is claimed for class II materials (Minnesota Mining &Manufacturing Company, 1989) requires confirmation.

It may be, because of the slowness of the acid-base reaction in resin glasspolyalkenoate cements, that free poly(acrylic acid) is available for alonger period than in conventional glass polyalkenoate cements, for the

174

References

formation of a stronger adhesive bond. However, it seems doubtful thatthe adhesive bond will be developed more rapidly than in the conventionalglass polyalkenoate for, according to van Zeghbroeck (1989), the adhesivebond of a conventional glass polyalkenoate cement develops its maximumstrength rapidly (within five minutes). The resin glass polyalkenoatecement has the undoubted advantage of bonding directly to compositeresins and this makes it ideal for use in the glass polyalkenoatecement/composite resin laminates.

There is bound to be one problem with resin glass polyalkenoate cement.Because the matrix is a mixture of hydrogel salt and polymer, light-scattering is bound to be greater than in the conventional material.Moreover, the zinc oxide-containing glass of class II materials is bound tobe opaque. This makes it difficult to formulate a translucent material andis the reason why their use is restricted to that of a liner or base. However,the class II material cited will be radio-opaque because it uses strontiumand zinc, rather than calcium, in the glass.

A fundamental criticism of the resin-modified glass polyalkenoatecements is that, to some extent, they go against the philosophy of the glasspolyalkenoate cement: namely, that the freshly mixed material shouldcontain no monomer. Monomers are toxic, and HEMA is no exception.This disadvantage of composite resins is avoided in the glass polyalkenoatecement as the polyacid is pre-polymerized during manufacture, but thesame cannot be said of these new materials. For this reason they may lackthe biocompatibility of conventional glass polyalkenoate cements. Thesematerials also absorb excessive amounts of water because of the hydro-philic nature of polyHEMA (Nicholson, Anstice & McLean, 1992).

It is far too soon to make a judgement on these materials, which are ofconsiderable interest and only in the very early days of their development.Most probably, much development will take place in this area.

References

Abramovich, A., Kaluza, J. J., Macchi, R. L. & Ribas, L. (1977). Enamelsurface treated with zinc polyacrylate dentine cements. Journal of DentalResearch, 56, 471-3.

Akahane, S., Tosaki, S. & Hirota, K. (1988). Fluoroaluminosilicate glasspowder for dental ionomeric cements. German Patent DE 3,804,469.

Akitt, J. W., Greenwood, N. N. & Lester, G. D. (1971). Nuclear magneticresonance and Raman studies of aluminium complexes formed in aqueoussolutions of aluminium salts containing phosphoric acids and fluoride ions.Journal of the Chemical Society, A, 2450-7.

175


Albers, H. F. (ed.) (1990). Light-cured fluoride releasing liners. The AdeptReport. 1990; 1, No. 1, l^k

Andersson, K. R., Dent Glasser, L. S. & Smith, D. (1982). Polymerization andcolloid formation in silicate solutions. In Falcone, J. G. (ed.) Soluble Silicates.ACS Symposium Series No. 194, Chapter 8. Washington, DC: AmericanChemical Society.

Antonucci, J. M., McKinney, J. E. & Stansbury, J. W. (1988). Resin-modifiedglass-ionomer cement. US Patent Application 160,856.

Anzai, M., Hirose, H., Kikuchi, H., Goto, J., Azuma, F. & Higasaki, S. (1977).Studies on soluble elements and solubility of dental cements. I. Solubilities ofzinc phosphate cement, carboxylate cement and silicate cement in distilledwater. Journal of the Nihon University School of Dentistry, 19, 26-39.

Aveston, J. (1965). The hydrolysis of the aluminium ion: ultracentrifugal andacidity measurements. Journal of the Chemical Society, 4438-43.

Bailey, J. E., Ellis, B., Howarth, L. G. & Oldfield, C. W. B. (1991). The fractureof glass-ionomer cements. Conference on the Fractography of Glasses andCeramics II, New York State College of Ceramics, Alfred University, NewYork, July 1990. American Ceramic Society.

Barnes, D. S. & Turner, E. P. (1971). Initial response of the human pulp to zincpolycarboxylate cement. Journal of the Canadian Dental Association, 37,265-6.

Barry, T. I., Clinton, D. J. & Wilson, A. D. (1979). The structure of aglass-ionomer cement and its relationship to the setting process. Journal ofDental Research, 58, 1072-9.

Barton, J. R., Brauer, G. M., Antonucci, J. M. & Raney, M. J. (1975).Reinforced polycarboxylate cements. Journal of Dental Research, 54, 310-23.

Baumann, E. & Gerhard, V. (1970). Vormischung, Verfahren zu ihrerHerstellung und Verwendung. German Patent 1,903,807.

Beagrie, G. S., Main, J. H. P. & Smith, D. C. (1972). Inflammatory reactionevoked by zinc polyacrylate and zinc eugenate cements: a comparison. BritishDental Journal, 132, 351-7.

Beagrie, G. S., Main, J. H. P., Smith, D. C. & Walshaw, P. R. (1974).Polycarboxylate cements as a pulp capping agent. Journal of the CanadianDental Association, 40, 378-83.

Beech, D. R. (1972). A spectroscopic study of the interaction between humantooth enamel and polyacrylic acid (polycarboxylate cement). Archives of OralBiology, 17, 907-11.

Beech, D. R. (1973). Improvement in the adhesion of polycarboxylate cementsto human dentine. British Dental Journal, 135, 442-5.

Beech, D. R. & Bandyopadhyay, S. (1983). A new laboratory method forevaluating the relative solubility and erosion of dental cements. Journal ofOral Rehabilitation, 10, 57-63.

Beech, D. R., Solomon, A. & Bernier, R. (1985). Bond strength ofpolycarboxylic acid cements to treated dentine. Dental Materials, 1, 154-7.

Bergenholtz, G., Cox, C. F., Loesche, W. J. & Syed, S. A. (1982). Bacterialleakage around dental restorations: its effect on dental pulp. Journal of OralPathology, 11, 439-50.

176

References

Bertenshaw, B. W. & Combe, E. C. (1972a). Studies on polycarboxylates andrelated cements. I. Analysis of cement liquids. Journal of Dental Research, 1,13-16.

Bertenshaw, B. W. & Combe, E. C. (1972b). Studies on polycarboxylates andrelated cements. II. Analysis of cement powders. Journal of Dental Research,1, 65-8.

Bertenshaw, B. W. & Combe, E. C. (1976). Studies on polycarboxylates andrelated cements. III. Molecular weight determinations. Journal of DentalResearch, 4, 87-90.

Bertenshaw, B. W., Combe, E. C. & Grant, A. A. (1979). Studies onpolycarboxylates and related cements. 4. Properties of cements. Journal ofDentistry,!, 117-25.

Bertenshaw, B. W., Combe, E. C, Tidy, P. C. & Laycock, J. N. C. (1979).Surgical cement composition containing aluminoborate glass and a polymerglass and a polymer containing recurring carboxylic or carboxylate groups.US Patent 4,174,334.

Bitner, T. J. & Weir, S. H. (1973). Fluoride uptake and acid solubility of enamelexposed to carboxylate cement containing MFP. Journal of Dental Research,52, 157-62.

Bitter, N. C. (1986). Glass ionomer-microfil technique for restoring cervicallesions. Journal of Prosthetic Dentistry, 56, 661-2.

Bovis, S. C, Harrington, E. & Wilson, H. J. (1971). Setting characteristics ofcomposite filling materials. British Dental Journal, 131, 352-6.

Brannstrdm, M. & Nyborg, H. (1969). Comparison of pulpal effect of twoliners. Ada Odontologica Scandinavica, 27, 433-51.

Brook, I. M., Craig, G. T. & Lamb, D. J. (1991a). In vitro interaction betweenprimary bone organ cultures, glass-ionomer cements andhydroxyapatite/tricalcium phosphate ceramics. Biomaterials, 12, 179-86.

Brook, I. M., Craig, G. T. & Lamb, D. J. (1991b). Initial in-vitro evaluation ofglass-ionomer cements for use as alveolar bone substitutes. Clinical Materials,7, 295-300.

Brown, D. & Combe, E. C. (1973). Effects of stainless steel filler on theproperties of polycarboxylate cement. Journal of Dental Research, 52, 388.

Browne, R. M., Tobias, R. S., Crombie, I. K. & Plant, G. C. (1983). Bacterialmicroleakage and pulpal inflammation in experimental cavities. InternationalEndodontics Journal, 16, 147-55.

Brune, D. & Smith, D. (1982). Microstructure and strength properties of silicateand glass-ionomer cements. Acta Odontologica Scandinavica, 40, 389-96.

Buonocore, M. G. (1955). A simple method of increasing the adhesion of acrylicmaterials to enamel surfaces. Journal of Dental Research, 34, 849-53.

Buonocore, M. G. (1961). Test of an adhesive containing glycerophosphoricacid dimethacrylate. In Phillips, R. W. & Ryge, G. (eds.) Adhesive RestorativeDental Materials, pp. 172-6. Spencer, Indiana: Owen Litho Service.

Cahn, J. W. (1961). On spinodal decomposition. Acta Metallurgica, 9, 795-801.Causton, B. E. (1981). The physio-mechanical consequences of exposing glass

ionomer cements to water during setting. Biomaterials, 2, 112-15.

177


Causton, B. E. (1982). Primers and mineralizing solutions. In Smith, D. C. &Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II.Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter7. Boca Raton, Florida: CRC Press Inc.

Causton, B. E. & Johnson, N. W. (1982). Improvement of polycarboxylateadhesion to dentine by the use of a new calcifying solution. British DentalJournal, 152, 9-11.

Chamberlain, B. B. & Powers, J. N. (1976). Physical and mechanical propertiesof three zinc polycarboxylate cements. Journal of the Michigan DentalAssociation, 58, 494-500.

Connick, R. E. & Poulsen, R. E. (1957). Nuclear magnetic resonance studies ofaluminium fluoride complexes. Journal of the American Chemical Society, 79,5153-7.

Cook, W. D. (1982). Dental polyelectrolyte cements. I. Chemistry of the earlystages of the setting reaction. Biomaterials, 3, 232-6.

Cook, W. D. (1983a). Dental polyelectrolyte cements. II. Effect ofpowder/liquid ratio on their rheology. Biomaterials, 4, 21-4.

Cook, W. D. (1983b). Dental polyelectrolyte cements. III. Effect of additives ontheir rheology. Biomaterials, 4, 85-8.

Cook, W. D. (1983c). Degradative analysis of glass-ionomer polyelectrolytecements. Journal of Biomedical Materials Research, 17, 1015-27.

Cornell, J. (1961). Adhesion to natural tooth. In Phillips, R. W. & Ryge, G.(eds.) Adhesive Restorative Dental Materials, p. 159. Spencer, Indiana: OwenLitho Service.

Cotton, F. A. & Wilkinson, G. (1966). Advanced Inorganic Chemistry. NewYork: Wiley Interscience.

Cranfield, M., Kuhn, A. T. & Winter, G. (1982). Factors relating to the rate offluoride-ion release from glass-ionomer cement. Journal of Dentistry, 10,333^1.

Crisp, S., Abel, G. & Wilson, A. D. (1979). The quantitative measurement ofthe opacity of aesthetic dental materials. Journal of Dental Research, 58,1585-96.

Crisp, S., Ferner, A. J., Lewis, B. G. & Wilson, A. D. (1975). Properties ofimproved glass-ionomer cement formulations. Journal of Dentistry, 3, 125-30.

Crisp, S., Jennings, M. A. & Wilson, A. D. (1978). A study of temperaturechanges occurring in setting dental cements. Journal of Oral Rehabilitation, 5,139^4.

Crisp, S., Kent, B. E., Lewis, B. G., Ferner, A. J. & Wilson, A. D. (1980).Glass-ionomer cement formulations. II. The synthesis of novel polycarboxylieacids. Journal of Dental Research, 59, 1055-63.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1975). Gelation of polyacrylic acidaqueous solutions and the measurement of viscosity. Journal of DentalResearch, 54, 1173-5.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1976a). Zinc polycarboxylate cements.A chemical study of erosion and its relationship to molecular structure.Journal of Dental Research, 55, 299-308.

178

References

Crisp, S., Lewis, B. G. & Wilson, A. D. (1976b). Characterization ofglass-ionomer cements. 1. Long term hardness and compressive strength.Journal of Dentistry, 4, 162-6.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1976c). Characterization ofglass-ionomer cements. 2. Effect of powder: liquid ratio on the physicalproperties. Journal of Dentistry, 4, 287-90.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1976d). Glass-ionomer cements:chemistry of erosion. Journal of Dental Research, 55, 1032-41.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1977). Characterization ofglass-ionomer cements. 3. Effect of poly acid concentration on the physicalproperties. Journal of Dentistry, 5, 51-6.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1979). Characterization ofglass-ionomer cements. 5. Effect of tartaric acid concentration in the liquidcomponent. Journal of Dentistry, 7, 304-5.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1980). Characterization ofglass-ionomer cements. 6. A study of erosion and water absorption in bothneutral and acidic media. Journal of Dentistry, 8, 68-74.

Crisp, S., Merson, S. A. & Wilson, A. D. (1980). Modification of ionomercements by the addition of simple metal salts. Industrial & EngineeringChemistry, Product Research & Development, 19, 403-8.

Crisp, S., Merson, S., Wilson, A. D., Elliott, J. H. & Hornsby, P. R. (1979). Theformation and properties of mineral—poly acid cements. Part 1. Ortho- andpyro-silicates. Journal of Materials Science, 14, 2941-58.

Crisp, S., Pringuer, M. A., Wardleworth, D. & Wilson, A. D. (1974). Reactionsin glass—ionomer cements. II. An infrared spectroscopic study. Journal ofDental Research, 53, 1414-19.

Crisp, S., Prosser, H. J. & Wilson, A. D. (1976). An infra-red spectroscopicstudy of cement formation between metal oxides and aqueous solutions ofpoly(acrylic acid). Journal of Materials Science, 11, 36-48.

Crisp, S. & Wilson, A. D. (1974a). Reactions in glass ionomer cements. I.Decomposition of the powder. Journal of Dental Research, 53, 1408-14.

Crisp, S. & Wilson, A. D. (1974b). Reactions in glass ionomer cements. III. Theprecipitation reaction. Journal of Dental Research, 53, 1420-4.

Crisp, S. & Wilson, A. D. (1974c). Poly(carboxylate) cements. British Patent1,484,454.

Crisp, S. & Wilson, A. D. (1976). Reactions in glass ionomer cements. V. Effectof incorporating tartaric acid in the cement liquid. Journal of Dental Research,55, 1023-31.

Crisp, S. & Wilson, A. D. (1977). Cements comprising acrylic and itaconic acidcopolymers and fluoroaluminosilicate glass powder. US Patent 4,016,124.

Crisp, S. & Wilson, A. D. (1978a). Poly(carboxylate) cements. British Patent1,532,954.

Crisp, S. & Wilson, A. D. (1978b). Fluoroaluminosilicate glasses. British Patent1,532,955.

Crisp, S. & Wilson, A. D. (1979). Cements. US Patent 4,143,018.Crisp, S., Wilson, A. D., Elliott, J. H. & Hornsby, P. R. (1977). Cement-forming

179


ability of silicate minerals and polyacid solution. Journal of AppliedChemistry, 27, 369-74.

Cruikshank, M. C , Dent Glasser, L. S., Barri, S. A. I. & Pollett, J. F. (1986).Penta-co-ordinated aluminium: a solid-state 27A1 N.M.R. study. Journal ofthe Chemical Society, Chemical Communications, 23—40.

Eames, W. B., Hendrix, K. & Mohler, H. C. (1979). Pulpal responses in rhesusmonkeys to cementation agents and cleaners. Journal of the American DentalAssociation, 98, 40-5.

Edwards, S. F. (1969). Theory of crosslinked polymerized material. Proceedingsof the Physical Society of London. Solid State Physics, 2 (2), 1-13.

Eick, J. D., Johnson, L. N., Fromer, J. R., Good, R. J. & Neuman, A. W.(1972). Surface topography: its influence on wetting and adhesion in a dentaladhesive system. Journal of Dental Research, 51, 780-8.

El-Kafrawy, A. H., Dickey, D. M., Mitchell, D. F. & Phillips, R. W. (1974).Pulp reaction to a polycarboxylate cement in monkeys. Journal of DentalResearch, S3, 15-19.

Elliott, J., Holliday, L. & Hornsby, P. R. (1975). Physical and mechanicalproperties of glass-ionomer cement. British Polymer Journal, 7, 297-306.

Ellis, J., Jackson, A. M., Scott, R. P. & Wilson, A. D. (1990). Adhesion ofcarboxylate cements to hydroxyapatite. III. Adsorption of poly(alkenoicacid)s. Biomaterials, 11, 379-84.

Ellis, J. & Wilson, A. D. (1987). Report. Laboratory of the Government Chemist.Ellis, J. & Wilson, A. D. (1990). Polyphosphonate cements: a new class of

dental materials. Journal of Materials Science Letters, 9, 1058-60.Ellison, S. & Warrens, C. (1987). Solid-state nmr study of aluminosilicate

glasses and derived dental cements. Report of the Laboratory of theGovernment Chemist.

ESPE. (1975). Mixing liquid for dental filling materials. British Patent 1,382,882.Feith, R. (1975). Side effect of acrylic cement implanted into bone. Acta

Orthopaedica Scandinavica Supplementum, 161, 1-36.Fields, J. E. & Nielsen, L. E. (1968). Dynamic mechanical properties of some

polymeric zinc salts. Journal of Applied Building Science, Yl, 1041-51.Flood, H. & Forland, T. (1947a). The acidic and basic properties of oxides.

Acta Chemica Scandinavica, 1, 592-604.Flood, H. & Forland, T. (1947b). The acidic and basic properties of oxides. II.

The thermal decomposition of pyrosulphates. Acta Chemica Scandinavica, 1,781-9.

Flory, P. J. (1953). Principles of Polymer Chemistry, Chapter 11. Ithaca, NewYork: Cornell University Press.


Forsten, L. (1977). Fluoride release from a glass ionomer cement. ScandinavianJournal of Dental Research, 85, 503-4.

Foster, J. F. & Dovey, E. H. (1974). Surgical cements of improved compressivestrength containing stannous fluoride and poly aery lie acid. US Patent3,856,737.

180

References

Foster, J. F. & Dovey, E. H. (1976). Improvements in and relating to surgicalcements. British Patent 1,423,133.

Frost, H. M. (1981). Coherence treatment of osteoporosis. Orthopedic Clinics ofNorth America, 12, 649-69.

de Gennes, P. (1979). Scaling Concepts in Polymer Physics. Ithaca: CornellUniversity Press.

Glanz, P-O. (1969). On wettability and adhesiveness. Odontologisk Revy, 20,Supplement 20.

Goldman, M. (1985). Fracture properties of composite and glass ionomer dentalrestorative materials. Journal of Biomedical Materials Research, 19, 771-83.

Gourley, J. M. & Rose, D. E. (1972). Cavity bases under liners. Journal of theCanadian Dental Association, 38, 246.

Granath, L. E. (1982). Pulp capping materials. In Smith, D. C. & Williams,D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibilityof Preventive Dental Materials and Bonding Agents, Chapter 11. Boca Raton:CRC Press Inc.

Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955a). Metal polyelectrolytecomplexes. I. The polyacrylic acid-copper complex. Journal of PhysicalChemistry, 59, 34^9.

Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955b). Metal polyelectrolytecomplexes. IV. Complexes of polyacrylic acid with magnesium, calcium,manganese, cobalt and zinc. Journal of Physical Chemistry, 59, 990-1.

Gulabivala, K., Setchell, D. J. & Davies, E. H. (1987). An application of the jettest method for the evaluation of disintegration of dental luting cements inmarginal gaps analogous to those of crowns and bridges. Clinical Materials,2, 221-30.

Hagg, G. (1935). The vitreous state. Journal of Chemical Physics, 3, 42-9.Hall, J. I. (1977). Orthopedic bandage. US Patent 4,044,761.Hamilton, I. R. (1977). The effects of fluoride on enzymatic regulation of

bacterial carbohydrate metabolism. Caries Research, 11 (Supplement 1),321-7.

Hatton, P. V. & Brook, I. M. (1992). Characterization of the ultrastructure ofglass-ionomer (glass polyalkenoate) cements. British Dental Journal, 173,275-7.

Hazel, F. M., Shock, R. U. & Gordon, M. (1949). Interactions of ferric ionswith silicic acid. Journal of the American Chemical Society, 71, 2256-7.

Helgeland, K. (1982). In vitro testing of dental cements. In Smith, D. C. &Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II.Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter9. Boca Raton, Florida: CRC Press Inc.

Hembree, J. H. & Andrews, J. T. (1978). Microleakage of several class Vanterior restorative materials: a laboratory study. Journal of the AmericanDental Association, 91, 179-83.

Hertet, R. S., Huget, E. F., de Simon, L. & Cosgrave, J. H. (1975). Short-termstress relaxation behaviour of non-metallic restoratives. Journal of DentalResearch, 54, 1149-53.

181


Hess, W. C. (1961). Organic components of enamel and dentine. In Phillips,R. W. & Ryge, G. (eds.) Adhesive Restorative Dental Materials, pp. 10-14.Spencer, Indiana: Owen Litho Service.

Hetem, S., Jowett, A. K. & Ferguson, M. W. (1989). Biocompatibility of aposterior composite and dental cements using a new organ culture model.Journal of Dentistry, 17, 155-61.

Hicks, M. J., Flaitz, C. M. & Silverstone, L. M. (1986). Secondary cariesformation in vitro around glass ionomer restorations. QuintessenceInternational, 17, 527-32.

Hill, R. G. & Wilson, A. D. (1988a). Some structural aspects of glasses used inionomer cements. Glass Technology, 29, 150-88.

Hill, R. G. & Wilson, A. D. (1988b). A rheological study of the role of additiveson the setting of glass ionomer cements. Journal of Dental Research, 68, 89-94.

Hill, R. G., Wilson, A. D. & Warrens, C. P. (1989). The influence of poly(acrylicacid) molecular weight on the fracture toughness of glass-ionomer cements.Journal of Materials Science, 24, 363-71.

Hinoura, K., Moore, B. K. & Phillips, R. W. (1986). Influence of dentin surfacetreatments on the bond strength of dentin-lining cements. Operative Dentistry,11, 147-54.

Hinoura, K., Moore, B. K. & Phillips, R. W. (1987). Bonding agent influence onglass ionomer-composite resin. Journal of the American Dental Association,114, 167-72.

Hodd, K. A. & Reader, A. L. (1976). The formation and hydrolytic stability ofmetal ion-polyacid gels. British Polymer Journal, 8, 131-9.

Hopkins, R. P. (1955). Acrylate salts of divalent metals. Industrial & EngineeringChemistry, 47, 2258-65.

Hornsby, P. R. (1977). A study of the formation and properties of ionic polymercements. Thesis for PhD, Brunei University, Middlesex, England.

Hornsby, P. R., Merson, S., Prosser, H. J. & Wilson, A. D. (1982). Theformation and properties of mineral-poly acid cements. Part 2. Chain, sheetand three-dimensional silicates. Journal of Materials Science, 17, 3575-92.

Horowitz, H. S. (1973). A review of systematic and topical fluorides for theprevention of dental caries. Community Dentistry Oral Epidemiology, 1, 104-14.

Hotz, P., McLean, J. W., Seed, I. & Wilson, A. D. (1977). The bonding ofglass-ionomer cements to metal and tooth substrates. British Dental Journal,142, 41-7.

Hume, W. R. & Mount, G. J. (1988). In vitro studies on the potential for pulpalcytotoxicity of glass-ionomer cements. Journal of Dental Research, 67,915-18.

Hunt, P. R. (1984). A modified class II cavity preparation for glass ionomerrestorative materials. Quintessence International, 15, 1011-18.

Ibbetson, R. J., Setchell, D. J. & Amy, D. J. (1985). An alternative method forthe clinical evaluation of the disintegration of dental cements. Journal ofDental Research, 64, 672 Abstract No. 89.

Her, R. K. (1979). The polymerization of silica. The Chemistry of Silica,Chapters 3 & 6. New York: Wiley-Inter science.

182

References

Isard, J. O. (1959). Electrical conduction in aluminosilicate glasses. Journal ofthe Society of Glass Technology, 43, 113-23.

Jemt, T., Stalblad, P. A. & 0ilo, G. (1986). Adhesion of polycarboxylate-baseddental cements to enamel: an in vivo study. Journal of Dental Research, 65,885-7.

Jendresen, M. D. & Trowbridge, H. D. (1972). Biological and physicalproperties of a zinc polycarboxylate cement. Journal of Prosthetic Dentistry,28, 264^71.

Jenkins, G. N. (1965). The equilibrium between plaque and enamel in relation todental caries. In Wolstenholme, G. E. W. & O'Connor, M. (eds.) CariesResistant Teeth, pp. 192-210. London: Churchill.

Jonck, L. M. & Grobbelaar, C. J. (1990). Ionos bone cement (glass-ionomer):an experimental and clinical evaluation in joint replacement. ClinicalMaterials, 6, 323-59.

Jonck, L. M., Grobbelaar, C. J. & Strating, H. (1989a). The biocompatibility ofglass-ionomer cement in joint replacement: bulk testing. Clinical Materials, 4,85-107.

Jonck, L. M., Grobbelaar, C. J. & Strating, H. (1989b). Biological evaluation ofglass-ionomer cement (Ketac-0) as an interface material in total jointreplacement. A screening test. Clinical Materials, 4, 201-24.

Jurecic, A. (1973). Water-soluble acrylic copolymer dental cements. BritishPatent 1,304,987; US Patent 3,741,926.

Kasten, F. H., Pineda, L. F., Schneider, P. E., Rawls, H. R. & Foster, T. A.(1989). Biocompatibility testing of an experimental fluoride releasing resinusing human gingival epithelial cells in vitro. In Vitro Cellular DevelopmentBiology, 25, 57-62.

Kawahara, H., Imanishi, Y. & Oshima, H. (1979). Biological evaluation onglass-ionomer cement. Journal of Dental Research, 58, 1080-6.

Kent, B. E., Lewis, B. G. & Wilson, A. D. (1973). The properties of aglass-ionomer cement. British Dental Journal, 135, 322-6.

Kent, B. E., Lewis, B. G. & Wilson, A. D. (1979). Glass ionomer cementformulations. I. The preparation of novel fluoridealuminosilicate glasses highin fluorine. Journal of Dental Research, 58, 1607-19.

Kidd, E. A. M. (1978). Cavity sealing ability of composite and glass ionomerrestoration; an assessment in vitro. British Dental Journal, 144, 139-42.

Kirkpatrick, R. J., Oestrike, R., Weis, jr, C. A., Smith, K. & Oldfield, E.(1986). Higher resolution aluminium-27 and silicon-29 NMR spectroscopyof glasses and crystals along the join calcium magnesium silicate-calciumaluminium silicate (CaMgSi2 O6-CaAl2 SiO6). American Mineralogist, 71,705-11.

Kleinburg, I. (1961). Studies on dental plaque. I. The effect of differentconcentrations of glucose on the pH of dental plaque in vivo. Journal ofDental Research, 40, 1087-110.

von Klotzer, W. T., Tronstad, L., Dowden, W. E. & Langeland, K. (1970).Polycarboxylatzemente im physikalischen und biologischen Test. DeutscheZahndrztliche Zeitschrift, 25, 877-86.

183


Knibbs, P. J., Plant, C. G. & Pearson, G. J. (1986a). The use of a glass-ionomercement to restore class III cavities. Restorative Dentistry, 2, 42-8.

Knibbs, P. J., Plant, C. G. & Pearson, G. J. (1986b). A clinical assessment of ananhydrous glass-ionomer cement. British Dental Journal, 161, 99-103.

Knight, G. M. (1984). The use of adhesive materials in the conservativerestoration of selected posterior teeth. Australian Dental Journal, 29, 324-31.

Kohmura, T. T. & Ida, K. (1979). A new type of hydraulic cement. Journal ofDental Research, 58, 1461.

Komatsu, H. (1981). Glass ionomer cement for caries prevention. Journal ofConservative Dentistry, 24, 32-45.

Kramer, I. R. H. & McLean, J. W. (1952). Alterations in the staining reactionsof dentine resulting from a constituent of a new self-polymerizing resin.British Dental Journal, 150, 150-3.

Kuhn, A. T. & Jones, M. P. (1982). A model for the dissolution and fluoriderelease from dental cements. Biomaterials, Medical Devices and ArtificialOrgans, 10, 281-93.

Kuhn, A. T., Setchell, D. J. & Teo, C. K. (1984). An assessment of the jetmethod for solubility measurements of dental cements. Biomaterials, 5, 161-8.

Kuhn, A. T. & Wilson, A. D. (1985). The dissolution mechanisms of silicate andglass-ionomer dental cements. Biomaterials, 6, 378-82.

Kumar, D., Ward, R. G. & Williams, D. J. (1961). Effects of fluorides onsilicates and phosphates. Discussions of the Faraday Society, 32, 147-54.

Kydonieus, A, F. (1980). Controlled Release Technologies. West Palm Beach,Florida: CRC Press.

Lacefield, W. R., Reindl, M. C. & Retief, D. H. (1985). Tensile bond strength ofa glass-ionomer cement. Journal of Prosthetic Dentistry, 53, 194-8.

Lambe, T. W. (1951). Stabilization of soils with calcium acrylate. Journal of theBoston Society of Civil Engineers, 38, 127-54.

Lambe, T. W. & Michaels, A. S. (1954). Altering soil properties with chemicals.Chemical Engineering News, 32, 488-92.

Larsen, E. S. & Berman, H. (1934). Microscopic determination of thenonopaque minerals. US Geological Survey Bulletin, No. 834. Washington.

Lawrence, L. G. (1979). Cervical glass-ionomer cement restorations: a clinicalstudy. Journal of the Canadian Dental Association, 45, 58-60.

Lawrence, L. G., Beagrie, G. S. & Smith, D. C. (1975). Zinc polyacrylatecements as alloplastic bone implant. Journal of the Canadian DentalAssociation, 41, 456-61.

Lawrence, L. G. & Smith, D. C. (1973). Strength modification ofpolycarboxylate cements with fillers. Journal of the Canadian DentalAssociation, 39, 405-9.

Lee, H. & Orlowski, J. (1973). Handbook of Dental Composite Restoratives, p.3107. South El Monte, California: Lee Pharmaceuticals.

LeGeros, R. Z. & LeGeros, P. (1984). Phosphate minerals in human tissues. InNriagu, J. O. & Moore, P. B. (eds.) Phosphate Minerals, Chapter 12. Berlin:Springer-Verlag.

184

References

Leirskar, J. & Helgeland, K. (1977). Toxicity of some dental cements in cellculture systems. Scandinavian Journal of Dental Research, 85, 471-9.

Leirskar, J. & Helgeland, K. (1987). Mechanism of an in vitro toxicity ofrestorative materials: pH, fluoride and zinc. International Endodontic Journal,20, 246-7.

Levine, R. S., Beech, D. R. & Garton, B. (1977). Improving the bond strengthof poly aery late cements to dentine. A rapid technique. British Dental Journal,143, 275-7.

Lloyd, C. H. & Adamson, M. (1987). The development of fracture toughnessand fracture strength in posterior restorative materials. Dental Materials, 3,225-31.

Lloyd, C. H. & Mitchell, L. (1984). The fracture toughness of tooth colouredrestorative materials. Journal of Oral Rehabilitation, 11, 257-72.

Low, T. (1981). The treatment of a hypersensitive cervical abrasion cavity usingASPA cement. Journal of Oral Rehabilitation, 8, 81-9.

Lowenstein, W. (1954). The distribution of aluminum in the tetrahedra ofsilicates and aluminates. American Mineralogist, 39, 92-6.

Lux, H. (1939). 'Sauren' und 'Basen' in Schmelzfluss: Die Bestimmung derSauerstoffionen-Konzentration. Zeitschrift fur Elektrochemie, 45, 303-9.

McComb, D. & Ericson, D. (1987). Antimicrobial action of new, proprietarylining cements. Journal of Dental Research, 66, 1025-8.

McKinney, J. E., Antonucci, J. M. & Rupp, N. W. (1985). Wear and micro-hardness of a metal-filled ionomer cement. Journal of Dental Research, 64,Special Issue 344 (IADR Abs 1577).

McKinney, J. E., Antonucci, J. M. & Rupp, N. W. (1987). Wear andmicrohardness of glass-ionomer cement. Journal of Dental Research, 66,134^9.

McLean, J. W. (1972). Polycarboxylate cements. Five years experience ingeneral practice. British Dental Journal, 132, 9-15.

McLean, J. W. (1980). Aesthetics in restorative dentistry: the challenge for thefuture. British Dental Journal, 149, 368-72.

McLean, J. W. (1986). New concepts in cosmetic dentistry using glass-ionomercements and composites. Journal of the Californian Dental Association, April1986, 20-7.

McLean, J. W. & Gasser, O. (1985). Glass-cermet cements. Quintessence, 16,333-43.

McLean, J. W., Powis, D. R., Prosser, H. J. & Wilson, A. D. (1985). The use ofthe glass-ionomer cement in bonding composite resins to dentine. BritishDental Journal, 158, 410-14.

McLean, J. W. & Wilson, A. D. (1974). Fissure sealing and filling with aglass-ionomer cement. British Dental Journal, 136, 269-76.

McLean, J. W. & Wilson, A. D. (1977a). The clinical development of theglass-ionomer cements. I. Formulations and properties. Australian DentalJournal, 22, 31-6.

McLean, J. W. & Wilson, A. D. (1977b). The clinical development of the

185


glass-ionomer cements. II. Some clinical applications. Australian DentalJournal, 22, 120-7.

McLean, J. W. & Wilson, A. D. (1977c). The clinical development of theglass—ionomer cements. III. The erosion lesion. Australian Dental Journal, 22,190-5.

McLean, J. W., Wilson, A. D. & Prosser, H. J. (1984). Development and use ofwater-hardening glass-ionomer luting cements. Journal of ProstheticDentistry, 52, 175-81.

Main, J. H. P., Mock, D., Beagrie, G. S. & Smith, D. C. (1975). Investigationsof possible oncogenic action of zinc polycarboxylate cement. Journal ofBiomedical Materials Research, 9, 69-78.

Maldonado, A., Swartz, M. L. & Phillips, R. W. (1978). An in vitro study ofcertain properties of a glass-ionomer cement. Journal of the American DentalAssociation, 96, 785-91.

Mase, H. (1961). The reactivity of various silicate minerals toward acids.Bulletin of the Chemical Society of Japan, 34, 214-25.

Mathis, R. S. & Ferracane, J. L. (1989). Properties of a glass-ionomer cementresin-composite hybrid material. Dental Materials, 5, 355-8.

Mehrotra, R. C. & Bohra, R. (1983). Metal Carboxylates. London & NewYork: Academic Press.

Mehrotra, R. C , Bohra, R. & Gauer, D. P. (1978). Metal Diketonates and AlliedDerivatives. London & New York: Academic Press.

de Mello, V. H. F., Hauser, E. A. & Lambe, T. W. (1953). Stabilization of soils.US Patent 2,65\,6\9.

Meryon, S. D. & Smith, A. J. (1984). A comparison of fluoride release fromthree glass ionomer cements and a polycarboxylate cement. InternationalEndodontic Journal, 17, 16-24.

Meryon, S. D. & Browne, R. M. (1984). In vitro cytotoxicity of a newgeneration. Cell Biochemistry and Function, 2, 43-8.

Mesu, F. P. (1982). Degradation of luting cements measured in vitro. Journal ofDental Research, 61, 665-72.

Mesu, F. P. & Reedijk, T. (1983). Degradation of luting cements measured invitro and in vivo. Journal of Dental Research, 62, 1236-40.

Michaeli, I. (1960). Ion-binding and the formation of insoluble polymethacrylicsalts. Journal of Polymer Science, 48, 291-9.

Minnesota Mining & Manufacturing Company. (1989). Vitrabond Light CureGlass Ionomer Cement Liner/Base. Report of Dental Products, 3M HealthCare.

Mitchem, J. C. & Gronas, D. G. (1978). Clinical evaluation of cement solubility.Journal of Prosthetic Dentistry, 40, 453-6.

Mitchem, J. C. & Gronas, D. G. (1981). Continued evaluation of theclinical solubility of luting cements. Journal of Prosthetic Dentistry, 45,289-91.

Mitra, S. B. (1989). Photocurable ionomer cement systems. European PatentApplication 0 323 120 A2.

Mizrahi, E. & Smith, D. C. (1969a). Direct cementation of orthodontic brackets

186

References

to dental enamel. An investigation using zinc polycarboxylate cement. BritishDental Journal, 127, 371-410.

Mizrahi, E. & Smith, D. C. (1969b). The bond strengths of a zincpolycarboxylate cement. British Dental Journal, 127, 410-14.

Mizrahi, E. & Smith, D. C. (1971). Direct attachment of orthodontic brackets todental enamel. British Dental Journal, 130, 392-6.

Mjor, I. A. (1985). Frequency of secondary caries at various anatomicallocations. Operative Dentistry, 10, 88-92.

Moore, B. K., Swartz, M. L. & Phillips, R. W. (1985). Abrasion resistance ofmetal reinforced glass ionomer cement. Journal of Dental Research, 64,Special Issue 371 (Abstract 1766).

Morawetz, H. (1965). Macromolecules in Solution. New York: IntersciencePublishers.

Mortimer, K. V. & Tranter, T. C. (1969). A preliminary laboratory evaluationof polycarboxylate cements. British Dental Journal, 111, 365-9.

Moser, J. B., Brown, D. B. & Greener, E. H. (1974). Short-term bond strengthsbetween adhesive cements and dental alloys. Journal of Dental Research, 53,1377-80.

Mount, G. J. (1984). Glass ionomer cements: clinical considerations. In Clarke,J. W. (ed.) Clinical Dentistry, Chapter 20A. Philadelphia: Harper & Row.

Mount, G. J. (1986). Longevity of glass-ionomer cements. Journal of ProstheticDentistry, 55, 682-5.

Mount, G. J. (1988). The tensile strength of the union between various glassionomer cements and various composite resins. Australian Dental Journal, 34,136-46.

Mount, G. J. (1989). The wettability of bonding resins used in the compositeresin/glass—ionomer 'sandwich' technique. Australian Dental Journal, 34,32-5.

Mount, G. J. (1990). An Atlas of Glass-ionomer Cements. London: MartinDunitz.

Murata, K. J. (1943). Internal structure of silicate minerals that gelatinize withacid. American Mineralogist, 28, 545-62.

Murata, K. J. (1946). The significance of internal structure of gelatinizingsilicate minerals. Contributions to Geochemistry, 1942-5. In Geological SurveyBulletin, No. 950, 25-33.

Nakamoto, K. (1963). Infrared Spectra of Inorganic and CoordinationCompounds. New York: John Wiley.

Nakamura, M., Kawahara, H., Imia, K., Tomoda, S., Kawata, Y. & Hikari, S.(1983). Long-term biocompatibility test of composite resins andglass-ionomer cement (in vitro). Dental Materials Journal, 1, 100-12.

Nicholson, J. W. (1992). The application of the reptation hypothesis topolyelectrolyte biomaterials. Journal of Materials Science, Materials inMedicine, 3, 157-9.

Nicholson, J. W., Anstice, H. M. & McLean, J. W. (1992). A preliminary reporton the effect of storage in water on the properties of commercial light-curedglass-ionomer cements. British Dental Journal, 173, 98-101.

187


Nicholson, J. W., Braybrook, J. H. & Wasson, E. A. (1991). Thebiocompatibility of glass-poly(alkenoate) (glass-ionomer): a review. Journalof Biomedical Science, Polymer Edition, 2, 277-85.

Nicholson, J. W., Brookman, P. J., Lacy, O. M., Sayers, G. S. & Wilson, A. D.(1988a). A study of the nature and formation of zinc polyacrylate cementusing Fourier transform infrared spectroscopy. Journal of BiomedicalMaterials Research, 22, 623-31.

Nicholson, J. W., Brookman, P. J., Lacy, O. M. & Wilson, A. D. (1988b).Fourier transform infrared spectroscopic study of the role of tartaric acid inglass-ionomer cements. Journal of Dental Research, 67, 1450—4.

0ilo, G. (1981). Bond strength of new ionomer cements to dentin. ScandinavianJournal of Dental Research, 89, 344-7.

0ilo, G. (1988). Characterization of glass-ionomer filling materials. DentalMaterials, 4, 129-33.

0ilo, G. & Espevik, S. (1978). Stress/strain behaviour of some dental lutingcements. Acta Odontologica Scandinavica, 36, 45-9.

0ilo, G. & Evje, D. M. (1986). Film thickness of dental luting cements. DentalMaterials, 2, 85-9.

Oldfield, C. W. B. & Ellis, B. (1991). Fibrous reinforcement of glass-ionomercements. Clinical Materials, 7, 313-24.

Oosawa, F. (1971). Poly electrolytes. New York: Marcel Dekker.O'Reilly, D. E. (1960). NMR chemical shifts of aluminum: experimental data

and variational calculation. Journal of Chemical Physics, yi, 1007-12.Osborne, J. W. & Berry, T. G. (1986). Clinical assessment of glass ionomer

cements as Class III restorations: a one-year report. Dental Materials, 2,147-50.

Osborne, J. W., Swartz, M. L., Goodacre, C. J., Phillips, R. W. & Gale, E. M.(1978). A method for assessing the clinical solubility and disintegration ofluting cements. Journal of Prosthetic Dentistry, 40, 413-17.


Paffenbarger, G. C. (1937). Dental silicate cements. In Judd, D. B. Opticalspecifications of light scattering materials. Report No. 1026. Journal of theNational Bureau of Standards, 19, 314-16.

Paffenbarger, G. C , Schoonover, I. C. & Souder, W. (1938). Dental silicatecements: Physical and chemical properties and a specification. Journal of theAmerican Dental Association, 25, 52-87.

Pameijer, C. H., Segal, E. & Richardson, J. (1981). Pulpal response to aglass ionomer cement in primates. Journal of Prosthetic Dentistry, 46, 36-40.

Pameijer, C. H. & Stanley, H. R. (1988). Biocompatibility of a glass ionomerluting agent in primates. Part I. American Journal of Dentistry, 1, 71-6.

Parker, R. S. R. (1974). Hardenable sheet materials. British Patent 1,504,972.Paterson, R. C. (1976). Bacterial contamination and the exposed pulp. British

Dental Journal, 140, 231-6.Pearson, G. J. (1991). Physical properties of glass-ionomer cements influencing

clinical behaviour. Clinical Materials, 1, 325-32.

188

References

Pearson, G. J. & Atkinson, A. S. (1991). Long-term flexural strength ofglass-ionomer cements. Biomaterials, 12, 658-60.

Peddy, M. (1981). The bond strength of polycarboxylic acid cements to dentine:effect of surface modification and time after extraction. Australian DentalJournal 26, 178-80.

Peters, W. J., Jackson, R. W., Iwako, K. & Smith, D. C. (1972). The biologicalresponse to zinc polyacrylate carboxylate. Clinical Orthopaedics and RelatedResearch, 88, 228-33.

Peters, W. J., Jackson, R. W. & Smith, D. C. (1974). Studies of the stability andtoxicity of zinc polyacrylate cements (PA2). Journal of Biomedical MaterialsResearch, 8, 53-60.

Petty, W. (1980). Methylmethacrylate concentrations in tissues adjacent to bonecement. Journal of Biomedical Materials Research, 14, 88-95.

Petzold, A. (1966). Die Reaktionfahigkeit einiger complexer Hydrosilikatergegeniiber dem Angriff von Saure und ihre strukturelle und atomistischeDeutung. Wissenschaftliche Zeitschrift der Hochschule fur Architektur undBauwesen Weimar, 113, 343-50.

Phillips, R. W., Hamilton, A. J., Jendresen, M. D., McHarris, W. H. &Schallhorn, R. G. (1986). Report of the Committee on Scientific Investigationof the American Academy of Restorative Dentistry. Journal of ProstheticDentistry, 55, 736-72.

Phillips, R. W. & Ryge, G. (1961). Adhesive Restorative Dental Materials.Spencer, Indiana: Owen Litho Service.

Plant, C. G. (1970). The effect of polycarboxylate containing stannous fluorideon the pulp. British Dental Journal, 135, 317-21.

Plant, C. G., Browne, R. M., Knibbs, P. J., Britton, A. S. & Sorhahan, T.(1984). Pulpal effects of glass ionomer cements. International EndodonticJournal, 17, 51-9.

Plant, C. G., Jones, I. H. & Wilson, H. J. (1972). Setting characteristics of liningand cementing materials. British Dental Journal, 133, 21-4.

Plant, C. G. & Wilson, H. J. (1970). Early strengths of lining materials. BritishDental Journal, 129, 269-74.

Pluim, L. J. & Arends, J. (1981). In vivo solubility of dental cements. Journal ofDental Research, 60, 1213, Abstract No. 61.

Pluim, L. J. & Arends, J. (1987). The relationship between salivaryproperties and in vivo solubility of dental cements. Dental Materials, 3,13-18.

Pluim, L. J., Arends, J. & Havinga, P. (1985). Comparison of in vivo and in vitrosolubility of 6 luting cements. Journal of Dental Research, 64, 714, AbstractNo. 98.

Pluim, L. J., Arends, J., Havinga, P., Jongebloed, W. J. & Stokroos, I. (1984).Quantitative cement solubility experiments in vivo. Journal of OralRehabilitation, 11, 171-9.

Pople, I. K. & Phillips, H. (1988). Bone cement and the liver. A dose-relatedeffect? Journal of Bone and Joint Surgery, 70B, 364-6.

Posner, A. S. (1961). The structure of the mineral portion of teeth. In Phillips,

189


R. W. & Ryge, G. (eds.) Adhesive Restorative Dental Materials, pp. 15-27.Spencer, Indiana: Owen Litho Service.

Potter, W. D., Barclay, A. C, Dunning, R. J. & Parry, R. R. (1977). Curablecompositions. US Patent 4,034,327.

Potter, W. D., Barclay, A. C, Dunning, R. J. & Parry, R. R. (1979). Calciumfluoroaluminosilicate glass. British Patent 1,554,555.

Powers, J. M., Farah, J. W. & Craig, R. G. (1976). Modulus of elasticity andstrength properties of dental cements. Journal of the American DentalAssociation, 92, 588-91.

Powers, J. M., Johnson, Z. G. & Craig, R. G. (1974). Physical and mechanicalproperties of zinc polyacrylate dental cements. Journal of the American DentalAssociation, 88, 380-3.

Powis, D. R., Folleras, T., Merson, S. A. & Wilson, A. D. (1982). Improvedadhesion of a glass ionomer cement to dentin and enamel. Journal of DentalResearch, 61, 1416-22.

Powis, D. R., Prosser, H. J. & Wilson, A. D. (1988). Long-term monitoring ofmicroleakage of dental cements by radiochemical diffusion. Journal ofProsthetic Dentistry, 59, 651-7.

Prati, C, Nucci, C. & Montanari, G. (1989). Effects of acid and cleansingagents on shear bond strength and marginal microleakage of glass-ionomercements. Dental Materials, 5, 260-5.

Prosser, H. J., Jerome, S. M. & Wilson, A. D. (1982). The effect of additives onthe setting properties of a glass-ionomer cement. Journal of Dental Research,61, 1195-8.

Prosser, H. J., Powis, D. R., Brant, P. & Wilson, A. D. (1984). Characterizationof glass-ionomer cements. 7. The physical properties of current materials.Journal of Dentistry, 12, 231^40.

Prosser, H. J., Powis, D. R. & Wilson, A. D. (1986). Glass-ionomercements of improved flexural strength. Journal of Dental Research, 65,146-8.

Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). NMR spectroscopy ofdental cements. II. The role of tartaric acid in glass-ionomer cements. Journalof Biomedical Materials Research, 16, 431-45.

Prosser, H. J. & Wilson, A. D. (1979). Litho-ionomer cements and theirtechnological applications. Journal of Chemical Technology and Biotechnology,29, 69-87.

Rabinovich, E. M. (1983). On the structural role of fluorine in silicate glasses.Journal of Physical Chemistry, 24, 54-6.

Rawson, H. (1967). Inorganic Glass-Forming Systems, Chapter 2. London &New York: Academic Press.

Ray, N. H. (1975). Inorganic glasses as ionic polymers. In Holliday, L. (ed.)Ionic Polymers, Chapter 8. London: Applied Science Publishers.

Ray, N. H. (1983). Oxide glasses as ionic polymers. In Wilson, A. D. & Prosser,H. J. (eds.) Developments in Ionic Polymers- 1, Chapter 2. London and NewYork: Applied Science Publishers.

Retief, D. H., Bradley, E. L., Denton, J. C. & Switzer, P. (1984). Enamel and

190

References

cementum fluoride uptake from a glass ionomer cement. Caries Research, 18,250-7.

Richter, W. A. & Ueno, H. (1975). Clinical evaluation and dental clinicaldurability. Journal of Prosthetic Dentistry, 33, 294-9.

Risbud, S. H., Kirkpatrick, R. J., Taglialavore, A. P. & Montez, B. (1987).Solid-state NMR evidence of 4-, 5- and 6-fold aluminium sites in roller-quenched SiO2-Al2 O3 glasses. Journal of the American Ceramic Society, 70,C10-12.

Robbins, J. W. & Cooley, R. L. (1988). Microleakage of Ketac-Silver in thetunnel preparation. Operative Dentistry, 13, 8-11.

Rolla, G. (1977). Effects of fluoride on initiation of plaque formation. CariesResearch, 11, 243-61.

Saito, C, Sakai, Y., Node, H. & Fusayama, T. (1976). Adhesion ofpolycarboxylate cements to dental casting alloys. Journal of ProstheticDentistry, 35, 543-8.

Seed, I. & Wilson, A. D. (1980). Poly(carboxylic acid) hardenable compositions.British Patent Application GB 2,028,855A.

Schmidt, W., Purrmann, R., Jochum, P. & Glasser, O. (1981a). Calciumaluminosilicate glass powder and its use. European Patent Application 23,013.

Schmidt, W., Purrmann, R., Jochum, P. & Glasser, O. (1981b). Mixingcompounds for glass-ionomer cements and use of a copolymer for preparingthe mixing components. European Patent Application 24,056.

Scott, R. P., Jackson, A. M. & Wilson, A. D. (1990). Adhesion of carboxylatecements to hydroxyapatite. II. Adsorption of aromatic carboxylates.Biomaterials, 11, 341-4.

Setchell, D. J., Teo, C. K. & Kuhn, A. T. (1985). The relative solubilities of fourmodern glass-ionomer cements. British Dental Journal, 158, 220-2.

Shimoke, H., Komatsu, H. & Matsui, I. (1987). Fluoride content in humanenamel after removal of the applied glass ionomer cement. Journal of DentalResearch, 66, Special Issue 131, Abstract No. 196.

Sidler, P. & Strub, J. R. (1983). In-vivo Untersuchung der Loslichkeit und desAbdichtungsvermogens von drei Befestigungszementen. DeutscheZahndrztliche Zeitschrift, 38, 564-71.

Silverstone, L. M. (1982). The structure and characteristics of human dentalenamel. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of DentalMaterials. Volume I. Characteristics of Dental Tissues and their Response toDental Materials, Chapter 2. Boca Raton: CRC Press Inc.

Simmonds, J. J. (1983). The miracle mixture glass-ionomer and alloy powder.Texas Dentist, October 6-12.

Skinner, J. C, Prosser, H. J., Scott, R. P. & Wilson, A. D. (1986). Adhesion ofcarboxylate cements to hydroxy apatite. I. The effect of the structure ofaliphatic carboxylates on their uptake by hydroxy apatite. Biomaterials, 7,438^0.

Smink, L. A. & Arends, J. (1980). Oplosbaarheid en disintegratie vanTandhelkundige cementen in vitro. Nederlands Tijdschrift voor Tandheelkunde,87, 389-93.

191


Smith, D. C. (1968). A new dental cement. British Dental Journal, 125, 381^4.Smith, D. C. (1969). Improvements relating to surgical cements. British Patent

1,139,430.Smith, D. C. (1971). A review of the zinc polycarboxylate cements. Journal of

the Canadian Dental Association, 37, 22-9.Smith, D. C. (1975). Approaches to adhesion to tooth structure. In Silverstone,

L. M. & Dogson, I. L. (eds.) The Acid Etch Technique, pp. 119-38. St Paul,Minnesota: North Central Publishing Company.

Smith, D. C. (1977). Past, present and future of dental cements. In Craig, R. G.(ed.) Dental Materials Review. Ann Arbor: University of Michigan Press.

Smith, D. C. (1982a). Composition and characteristics of dental cements. InSmith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials.Volume II. Biocompatibility of Preventive Dental Materials and BondingAgents, Chapter 8. Boca Raton: CRC Press Inc.

Smith, D. C. (1982b). Tissue reactions to cements. In Smith, D. C. & Williams,D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibilityof Preventive Dental Materials and Bonding Agents, Chapter 10. Boca Raton:CRC Press Inc.

Smith, D. C. & Cartz, L. (1973). Crystalline interface formed by polyacrylic acidand tooth enamel. Journal of Dental Research, 52, 1155.

Smith, D. C , Ruse, D. N. & Zuccolin, D. (1988). Some characteristics of glassionomer cement lining materials. Canadian Dental Journal, 54, 903-8.

Sneed, W. D. & Looper, S. W. (1985). Shear bond strength of a composite resinto an etched glass-ionomer. Dental Materials, 1, 127-8.

Spangberg, L., Rodrigues, H. & Langeland, K. (1974). Biologic effects of dentalmaterials. 4. Effect of polycarboxylate cements on HeLa cells in vitro. OralSurgery, Oral Medicine, Oral Pathology, 37, 113-17.

Steinke, R., Newcomer, P., Komarneni, S. & Roy, R. (1988). Dental cements:investigation of chemical bonding. Materials Research Bulletin, 23, 13-22.

Stephan, R. M. (1940). Changes in the hydrogen-ion concentration on toothsurfaces and in carious lesions. Journal of the American Dental Association,27, 718-23.

Suzaki, M. (1976). Dental cement composition. US Patent 3,962,267.Swartz, M. L., Phillips, R. W. & Clark, H. E. (1984). Long-term F release from

glass ionomer cements. Journal of Dental Research, 63, 158-60.Swift, E. J. (1988a). Silver-glass ionomers. A status report for the American

Journal of Dentistry. American Journal of Dentistry, 1, 81-3.Swift, E. J. (1988b). An update on glass ionomer cements. Operative Dentistry,

19, 125-30.Swift, E. J. & Dogan, A. U. (1990). Analysis of glass-ionomer cement with

use of scanning electron microscopy. Journal of Prosthetic Dentistry, 64,167-74.

Tanzer, J. M. (1989). On changing the cariogenic chemistry of coronal plaque.Journal of Dental Research, 68, 1576-87.

Tay, W. M. & Braden, M. (1988). Fluoride ion diffusion from polyalkenoate(glass-ionomer) cements. Biomaterials, 9, 454-6.

192

References

Taylor, H. F. W. (1966). The Chemistry of Cements. Lecture Series 1966, No. 2.London: Royal Institute of Chemistry.

Ten Cate, A. R. & Torneck, C. (1982). The dentin-pulp complex: development,structure and repair. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibilityof Dental Materials. Volume I. Characteristics of Dental Tissues and theirResponse to Dental Materials, Chapter 3. Boca Raton: CRC Press Inc.

Theuniers, G. (1984). Een onderzock naar een duurzame afdichting door kroon-en brugcementen. Thesis. Leuvan.

Thornton, J. B., Retief, D. H. & Bradley, E. L. (1986). Fluoride release fromand tensile bond strength of Ketac-Fil and Ketac-Silver to enamel anddentine. Dental Materials, 2, 241-5.

Tobias, R. S., Browne, R. M., Plant, C. G. & Ingram, D. V. (1978). Pulpalresponse to a glass ionomer cement. British Dental Journal, 144, 345-50.

Tobias, R. S., Browne, R. M. & Wilson, C. A. (1985). Antibacterial activity ofdental restorative materials. International Dental Research, 18, 161-71.

Tobias, R. S., Plant, C. G., Browne, R. M., Knibbs, P. J. & Britton, A. (1987).Pulpal response to an anhydrous glass-ionomer luting cement. Journal ofDental Research, 66, 836. Abstract 12.

Trap, H. J. L. & Stevals, J. M. (1959). Physical properties of invert glasses.Glastechnische Berichte, 32K, VI/31-52.

Trautschold, R. (1940). Non-corrosive, non-metallic, materials. Rayon TextileMonthly, 21, 373-4.

Turner, R. T., Francis, R., Brown, D., Garand, J., Hannon, K. S. & Bell, N. H.(1989). The effects of fluoride on bone implant histomorphology in growingrats. Journal of Bone Mineralogy Research, 4, 477-84.

Tyas, M. J. (1977). A method for the in vitro toxicity testing of dentinerestorative materials. Journal of Dental Research, 56, 1285.

Tyas, M. J. & Beech, D. R. (1985). Clinical performance of three restorativematerials for cervical abrasion lesions. Australian Dental Journal, 30, 260-4.

Tyas, M. J., Alexander, S. B., Beech, D. R., Brockenhurst, P. J. & Cook, W. D.(1988). Bonding - retrospect and prospect. Australian Dental Journal, 33,364-74.

Volf, M. B. (1984). Chemical Approach to Glass, Chapter 2. Amsterdam, etc.:Elsevier.

van de Voorde, A. (1988). Clinical uses of glass ionomer cement: a literaturereview. Quintessence International, 19, 53-60.

Wall, F. T. & Drenan, J. W. (1951). Gelation of polyacrylic acids by divalentcations. Journal of Polymer Science, 7, 83-8.

Walls, A. W. G. (1986). Glass polyalkenoate (glass-ionomer) cements: a review.Journal of Dentistry, 14, 231-46.

Walls, A. W. G., McCabe, J. R. & Murray, J. J. (1985). An erosion test fordental cements. Journal of Dental Research, 64, 1100-4.

Walls, A. W. G., McCabe, J. F. & Murray, J. J. (1988). The effect of variationof glass polyalkenoate (ionomer) cements. British Dental Journal, 164, 141-4.

Walls, A. W. G., Murray, J. J. & McCabe, J. F. (1988). The use of glasspolyalkenoate (ionomer) cements in deciduous dentition. British DentalJournal, 165, 13-17.

193


Wasson, E. A. & Nicholson, J. W. (1990). A study of the relationship betweensetting chemistry and properties of modified glass-poly(alkenoate) cements.British Polymer Journal 23, 179-83.

Wasson, E. A. & Nicholson, J. W. (1991). Studies on the setting chemistry ofglass-ionomer cements. Clinical Materials, 7, 289-93.

Waters, D. N. & Henty, M. S. (1977). Raman spectra of aqueous solutions ofhydrolysed aluminium(III) salts. Journal of the Chemical Society: DaltonTransactions, 243-5.

Watts, D. C , Combe, E. C. & Greener, E. H. (1979). Effect of storageconditions on the mechanical properties of poly electrolyte cements. Journal ofDental Research, 58, Special Issue C, Abstract No. 18.

Wei, S. H. Y. (1985). Clinical Uses of Fluoride. Philadelphia: Lea & Febiger.Welker, D. & Neupert, G. (1974). Vergleichender biologischer Test von

Polyakrylate- und Phosphatzement an Monolayer-Kulturen. Stomatologie(DDR), 24, 602-10.

Welsh, E. L. & Hembree, J. H. (1985). Microleakage of the gingival wall withfour class V anterior restorative materials. Journal of Prosthetic Dentistry, 54,370-2.

Wesenberg, G. & Hals, E. (1980). The in vitro effect of a glass ionomer cementon dentine and enamel wall. Journal of Oral Rehabilitation, 7, 35-42.

Weyl, W. A. & Marboe, E. C. (1962). The Constitution of Glasses: a DynamicInterpretation. Volume 1. Fundamentals of the Structure of Inorganic Liquidsand Solids. New York: Interscience Publishers.

Williams, J. & Billington, R. W. (1989). Increase in compressive strength ofglass-ionomer cements with respect to time: a guide to their use in theposterior deciduous dentition. Journal of Oral Rehabilitation, 16,475-9.

Williams, J. & Billington, R. W. (1991). Changes in compressive strength ofglass ionomer restorative materials with respect to time periods of 24 h to 4months. Journal of Oral Rehabilitation, 18, 163-8.

Williams, J. A., Billington, R. W. & Pearson, G. J. (1992). The comparativestrengths of commercial glass-ionomer cements with and without metaladditions. British Dental Journal, 111, 279-82.

Wilson, A. D. (1968). Dental silicate cements. VII. Alternative liquid acidformers. Journal of Dental Research, 47, 1133-6.

Wilson, A. D. (1974). Alumino-silicate poly aery lie acid and related cements.British Polymer Journal, 6, 165-79.

Wilson, A. D. (1975a). Dental cements - general. In von Fraunhofer, J. A. (ed.)Scientific Aspects of Dental Materials, Chapter 4. London and Boston:Butterworths.

Wilson, A. D. (1975b). Zinc oxide dental cements. In von Fraunhofer, J. A. (ed.)Scientific Aspects of Dental Materials, Chapter 5. London and Boston:Butterworths.

Wilson, A. D. (1975c). Dental cements based on ion-leachable glasses. In vonFraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 6.London and Boston: Butterworths.

194

References

Wilson, A. D. (1978a). The chemistry of dental cements. Chemical SocietyReviews, 7, 265-96.

Wilson, A. D. (1978b). Glass-ionomer cements - ceramic polymers. In Young,J. F. (ed.) Chemical Research Progress. Columbus, Ohio: American CeramicSociety.

Wilson, A. D. (1982). The nature of the zinc polycarboxylate cement matrix.Journal of Biomedical Materials Research, 16, 549-57.

Wilson, A. D. (1989). Developments in glass-ionomer cements. InternationalJournal of Prosthodontics, 2, 438-46.

Wilson, A. D. (1990). Resin modified glass-ionomer cements. InternationalJournal of Prosthodontics, 3, 425—46.

Wilson, A. D. (1991). Glass-ionomer cement - origins, development and future.Clinical Materials, 7, 275-82.

Wilson, A. D. & Crisp, S. (1975). Ionomer cements. British Polymer Journal, 1,279-96.

Wilson, A. D. & Crisp, S. (1976). Poly(carboxylate) cements. British Patent1,422,337.

Wilson, A. D. & Crisp, S. (1977). Polymer-clay compounds and soil treatment.In Organolithic Macromolecular Materials, Chapter 5. London: AppliedScience Publishers.

Wilson, A. D. & Crisp, S. (1980). Dental cement containing poly(carboxylicacid), chelating agent and glass ionomer powder. US Patent 4,209,434.

Wilson, A. D., Crisp, S. & Abel, G. (1977). Characterization of glass-ionomercements. 4. Effect of molecular weight on physical properties. Journal ofDentistry, 5, 117-20.

Wilson, A. D., Crisp, S. & Ferner, A. J. (1976). Reactions in glass ionomercements: IV. Effect of chelating comonomers on setting behavior. Journal ofDental Research, 55, 489-95.

Wilson, A. D., Crisp, S., Lewis, B. G. & McLean, J. W. (1977). Experimentalluting agents based on the glass-ionomer cements. British Dental Journal, 142,117-22.

Wilson, A. D., Crisp, S. & Paddon, J. M. (1981). The hydration of aglass-ionomer (ASPA) cement. British Polymer Journal, 13, 66-70.

Wilson, A. D., Crisp, S., Prosser, H. J., Lewis, B. G. & Merson, S. A. (1980).Aluminosilicate glasses for polyelectrolyte cements. Industrial & EngineeringChemistry Product Research & Development, 19, 263-70.

Wilson, A. D. & Ellis, J. (1989). Poly-vinylphosphonic acid glass ionomercement. British Patent Application 8,924,129.3.

Wilson, A. D., Groffman, D. M. & Kuhn, A. T. (1985). The release of fluorideand other chemical species from a glass-ionomer cement. Biomaterials, 6, 431-3.

Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986a). Anevaluation of the significance of the impinging jet method for measuring theacid erosion of dental cements. Biomaterials, 7, 55-60.

Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986b). A studyof variables affecting the impinging jet method for measuring the erosion ofdental cements. Biomaterials, 7, 217-20.

195


Wilson, A. D., Hill, R. G., Warrens, C. P. & Lewis, B. G. (1989). The influenceof poly(acrylic acid) molecular weight on some properties of glass-ionomercement. Journal of Dental Research, 68, 89-94.

Wilson, A. D. & Kent, B. E. (1970). Dental silicate cements. IX. Decompositionof the powder. Journal of Dental Research, 49, 7-13.

Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement, a newtranslucent cement for dentistry. Journal of Applied Chemistry andBiotechnology, 21, 313.

Wilson, A. D. & Kent, B. E. (1972). A new translucent cement for dentistry.British Dental Journal, 132, 133-5.

Wilson, A. D. & Kent, B. E. (1973). Surgical cements. British Patent 1,316,129.Wilson, A. D. & Kent, B. E. (1974). Poly(carboxylic acid)-fluoroaluminosilicate

glasses surgical cement. US Patent 3,814,717.Wilson, A. D. & Lewis, B. G. (1980). The flow properties of dental cements.

Journal of Biomedical Materials Research, 14, 383-91.Wilson, A. D. & McLean, J. W. (1988). Glass-ionomer Cement. Chicago,

London, etc.: Quintessence Publishing Company Inc.Wilson, A. D., Paddon, J. M. & Crisp, S. (1979). The hydration of dental

cements. Journal of Dental Research, 58, 1065-71.Wilson, A. D. & Prosser, H. J. (1982). Biocompatibility of the glass ionomer

cement. Journal of the Dental Association of South Africa, 37, 872-9.Wilson, A. D. & Prosser, H. J. (1984). A survey of inorganic and polyelectrolyte

cements. British Dental Journal, 157, 449-54.Wilson, A. D., Prosser, H. J. & Powis, D. M. (1983). Mechanism of adhesion of

polyelectrolyte cement to hydroxyapatite. Journal of Dental Research, 62,590-2.

Wilson, M. A. & Combe, E. C. (1991). Effects of glass composition andpretreatment on the reactivity of a novel glass polyalkenoate (glass ionomer)dental cement. Clinical Materials, 7, 15-21.

Wood, D. & Hill, R. (1991a). Structure-property relationships in ionomerglasses. Clinical Materials, 1, 301-12.

Wood, D. & Hill, R. (1991b). Glass ceramic approach to controlling theproperties of a glass-ionomer cement. Biomaterials, 12, 164-70.

Woodberry, N. T. (1961). A new, anionic, polyacrylamide flocculant. Tappi, 44,156A-60A.


Yoshii, E., Homma, T., Hirota, K. & Tomioka, K. (1987). Cytotoxic evaluationof the improved glass-ionomer cement. Journal of Dental Research, 66,Special Issue 133, Abstract No. 215.

Zachariasen, W. H. (1932). The atomic arrangement in glass. Journal of theAmerican Chemical Society, 54, 3841-51.

van Zeghbroeck, L. (1989). Bond capacity of adhesive luting cements. Thesis forDoctor in de Tandheelkunde, Leuvan University, Belgium.

196

6 Phosphate bonded cements

6.1 General

The phosphate bonded cements described in this chapter are the productsof the simple acid-base reaction between an aqueous solution oforthophosphoric acid and a basic oxide or silicate. Such reactions takeplace at room temperature. Excluded from this chapter are the cemen-titious substances that are formed by the heat treatment of aqueoussolutions of acid metal phosphates.

The most important of these are the refractory cements formed by theheat treatment of aluminium acid phosphate solutions. This subject hasbeen well reviewed by Kingery (1950a), Morris et al. (1977), Cassidy (1977)and O'Hara, Duga & Sheets (1972). The chemistry of these binders isextremely complex as the action of heat on acid phosphates gives rise topolymeric phosphates, with P-O-P linkages, and these are very complexsystems (Ray, 1979).

Here we are concerned with the cement-forming reaction betweenorthophosphoric acid solutions and basic oxides and silicates where thereaction is much simpler. Polymeric phosphates are not involved, there areno P-O-P bonds, and the structural unit is the simple [POJ tetrahedron.

6.1.1 Orthophosphoric acid solutions

Concentrated solutions of orthophosphoric acid, often containing metalsalts, are used to form cements with metal oxides and aluminosilicateglasses. Orthophosphoric acid, often referred to simply as phosphoric acid,is a white crystalline solid (m.p. 42-35 °C) and there is a crystallinehemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasicand in aqueous solution has three ionization constants (pKa): 2-15, 7-1 and12-4.

197

Phosphate bonded cements

X-ray diffraction (XRD) studies have shown that the crystalline acid andits hydrate contain tetrahedral [POJ groups (Van Wazer, 1958). In theanhydrous acid, three of the oxygen atoms are bonded to hydrogen atomsand the P-O bonds are 0-157 nm or 0-158 nm in length. The P-O bond ofthe fourth O has more n character than the others and is shorter (0-152 nm).The [POJ tetrahedra are interconnected by hydrogen bonds and there arealso internal hydrogen bonds between pairs of O atoms within the [POJtetrahedra. Radial distribution curves from XRD studies indicate that theintramolecular hydrogen bonds persist in 86 % phosphoric acid solution.Mostly, a single hydrogen bond connects any two [POJ groups, but doubleand triple hydrogen bonding occurs to a lesser extent. In more dilutesolution (54 %) the [POJ groups are linked to the water lattice rather thanto other [POJ groups. Raman spectroscopy supports these structuralviews. In 75 % solution not all [POJ groups are hydrogen-bonded to watermolecules (Wilson & Mesley, 1968).

Results from infrared spectroscopy indicate that the only species presentin 50 % phosphoric acid are H3PO4, H2PO4 and their oligomers (Wilson &Mesley, 1968). There is evidence that H6P2O8, the phosphoric acid dimer,and H5P2O~, the triple ion HgPO^. H+. H2PO~, are also present (Elmore,Mason & Christensen, 1946; Selvaratnam & Spiro, 1965). Akitt, Green-wood & Lester (1971), on the basis of 31P NMR studies, suggest furtherthat there are oligomers of the type (H3P4O)W.

6.1.2 Cations in phosphoric acid solutions

Cement-forming phosphoric acid liquids nearly always contain cations.The most important of these are aluminium and zinc, but other metals maybe used. Manly et al. (1951) have laid down criteria for the choice ofmodifying metals. They must be moderately soluble in oxide solution, notform coloured sulphides and be non-toxic. To these criteria, followingKingery (1950a,b), may be added another, that the modifying cations mustbe capable of remaining in vitreous form in the cement gel. This criterionis satisfied only by cations having a low coordination number, multiplecharge and small ionic radius, that is having a high ionic potential. Theseare ions of amphoteric or weakly basic metals such as aluminium, zinc,beryllium and magnesium. More basic metals, for example calcium,barium and thorium, weaken the cement.

As noted above, however, aluminium and zinc are the most importantand are often found in combination in the liquids used for the zinc

198

General

phosphate and dental silicate cement. The presence of aluminium inphosphoric acid solution serves to retard the setting reaction in zincphosphate cements and accelerate it in dental silicate cements.

Both these metals are soluble to a limited extent in phosphoric acidsolutions. Phase diagrams have been constructed for the systemsZnO-P2O5-H2O (Figure 6.1) and A12O3-P2O5-H2O (Figure 6.2). Stablezinc phosphate phases that have been found to exist are Zn3(PO4)2. 4H2O,ZnHPO4. 3H2O, ZnHPO4. H2O, Zn(H2PO4)2. 2H2O andZn(H2PO4)2.2H3PO4 (Eberly, Gross & Crowell, 1920; Salmon & Terrey,1950). All are apparently crystalline. Stable aluminium phosphate phasesthat have been found to exist are 2A1PO4.7H2O, 2A1PO4.4H2O,2A1PO4.2H3PO46H2O, 2A1PO4.2H3PO4.3H2O; with the possible excep-tion of the first all are crystalline (Jameson & Salmon, 1954). Althoughthese phase diagrams have been used to deduce the course of cementformation, they are of limited use because it is doubtful whether conditionsof thermodynamic equilibrium are reached; moreover, many cements aremainly amorphous.

H2O

rangeforcements

ZnO

Figure 6.1 The system ZnO-P2O5-H2O (Salmon & Terrey, 1950).

P2O5

199


The actions of zinc and aluminium differ. In general, metal ions such aszinc merely serve to neutralize the acid and are present in solution as simpleions (Holroyd & Salmon, 1956; O'Neill et ai, 1982). But aluminium has aspecial effect: in contrast to zinc, it prevents the formation of crystallitesduring the cement-forming reaction in zinc phosphate cements.

Aluminium has long been known to form complexes with phosphoricacid (Bjerrum & Dahm, 1931; Jameson & Salmon, 1954; Genge et aL,1955; Holroyd & Salmon, 1956; Salmon & Wall, 1958; Van Wazer, 1958;Van Wazer & Callis, 1958; Genge & Salmon, 1959). When it is dissolved inphosphoric acid solution the viscosity of the solution increases sharply as,according to Sveshnikova & Zaitseva (1964), aluminophosphoric acids areformed which behave as polyelectrolytes. A number of workers using 31PNMR spectroscopy have found evidence for the formation of alumino-phosphoric acid complexes (Akitt, Greenwood, & Lester, 1971; O'Neill etal., 1982). Combining these NMR observations with those from infraredspectroscopy (Wilson & Mesley, 1968) the species present in 50%

rangeforcements

A12O3 P2O5

Figure 6.2 The system A12O3-P2O5-H2O (Jameson & Salmon, 1954).

200

General

Table 6.1. Cement-forming oxides

BeOBe(OH)2

ZnOCuOCu2OMgOCaOBi2O3

CdOSnOPb3O4

Co(OH)3

Aluminosilicate glass

Conditionof oxide

CalcinedCalcined

CalcinedCalcined

Modificationof liquid

Al salt

Al or NH4 saltAl salt

phosphoric acid solution containing aluminium appear to be H 3PO 4 ,H 6P 2O 8 , H 2 PO", H 5 P 2 O-, A l H 3 P O r , A1H2POJ+, A1(H2PO4)+ andA1(H3PO4)W, where n ^ 2, of unknown protonation. Binuclear aluminiumphosphate complexes are also present and Salmon & Wall (1958) considerthat bridge structures exist. All this points to the formation of analuminophosphate polymer in the solution based on -P -O-Al -bond ing .

6.1.3 Reactions between oxides and phosphoric acid solutions

In a classic study, Kingery (1950b) examined a large number of oxides forcement formation with orthophosphoric acid. He observed three types ofreaction: no reaction, violent reaction with crystallization, and controlledreaction with cement formation.

There was no reaction with oxides of an acidic or inert nature, forexample SiO2, A12O3, ZrO2 and Co2O3. There was violent reaction withreactive oxides, yielding non-cementitious products, which were crys-talline, porous and friable. This type of reaction tended to occur when theoxides were alkaline, although it could be affected by calcining the oxide.Examples cited were calcined CaO, SrO and BaO, and uncalcined MgOand La2O3. However, calcined CaO did form cements when ortho-phosphoric acid was partly neutralized by CaO.

201


Lastly, there was the cementitous reaction which Kingery (1959b)reported with BeO, Be(OH)2, CuO, Cu2O, CdO, SnO and Pb3O4. Inaddition, calcined ZnO and MgO formed cements.

These observations require some comment and amendment. Theformation of cements with ZnO, CuO and Bi2O3 has been known for manyyears and they are found in dental cements (Wilson, 1975a,b; 1978). Also,although Co2O3 does not form a cement, Co(OH)3 does (Prosser et al.,1986). MgO is a doubtful cement-former with orthophosphoric acid, butforms useful cements with ammonium acid phosphate and aluminium-containing orthophosphoric acid solutions (Finch & Sharp, 1989). There isalso cement formation with calcium aluminosilicate glasses (Wilson 1975c,1978). This material, the dental silicate cement, is unusual in beingtranslucent.

Table 6.1 summarizes known basic cement-formers based on theobservations of the workers cited above.

Because of its importance and the breadth of the investigation, the workof Kingery (1950b) requires critical examination. He considered that theessential feature of phosphate-bonded cements was an acid phosphatematrix. Extended hydrogen bridges between acid phosphate groups canthen be cited as the matrix-forming bond (Wygant, 1958). However, weconsider that Kingery was mistaken, for his work has its limitations.

Kingery based his conclusions on a literature survey and someexperimental studies. The literature survey indicated that in many cases thereaction product of a metal oxide with orthophosphoric acid was an acidphosphate. However, these studies did not relate to cements, where themetal oxide is always present in excess. For example, he cites the phasediagram of Eberly, Gross & Crowell (1920) as showing ZnHPO4. 3H2O asthe reaction product. But in the presence of excess ZnO the phase diagramshows Zn3(PO4)2. 4H2O as the stable species. Kingery also neglected toconsider the effect of phosphoric acid concentration on the nature of thereaction product.

Kingery himself used XRD, but except in one case failed to positivelyidentify crystalline hydrogen phosphates in any of the cements heexamined. He found either the neutral orthophosphate or unidentifiedcrystalline species and then the lines were weak.

Although an acid phosphate matrix cannot be excluded it is not essentialfor cement formation. In fact, it must be remembered that when thesecements are prepared the oxide or silicate powder is normally in excess ofthat required for the reaction. Under these conditions most oxides (MgO

202

General

is a notable exception) form neutral orthophosphates. An acid phosphatematrix is to be expected only when there is excess acid in the cement mix;the only practical cements where these conditions obtain are those used forcontrolled release which are designed to be hydrolytically unstable (Prosseret al., 1986). Hydrated neutral orthophosphates are the normal reactionproducts in zinc phosphate and dental silicate cements, both of which havebeen studied in detail. The reaction between magnesium oxide andphosphoric acid is an exception. An acid phosphate is formed, but it issoluble in water (Finch & Sharp, 1989) and, in fact, MgO forms usefulcements only with ammonium dihydrogen phosphate. Hydrogen bondscan still be considered as playing an important role, even in the case of aneutral orthophosphate, but they would act via water of hydration.

As research progresses over the years it is becoming apparent that themajority of these cements are essentially amorphous, and that crystaUinityis secondary and sometimes very slight. Kingery's arguments based solelyon XRD data are, perhaps, not very relevant.

6.1.4 Effect of cations in phosphoric acid solutions

As we have already shown, the presence of cations in orthophosphoric acidsolution can have a decisive effect on cement formation. As noted above,Kingery (1950b) found it necessary to modify orthophosphoric acid, by theaddition of calcium, to obtain cement formation with calcium oxide. Also,Finch and Sharp (1989) had to modify orthophosphoric acid, with eitherammonium or aluminium, to achieve cement formation with magnesiumoxide.

Even when modifiers are not necessary for cement formation, they canlead to improved cement properties. Kingery (1950b) also examined thiseffect. He found that optimum bonding was achieved with cations that hadsmall ionic radii and were amphoteric or weakly basic, such as beryllium,aluminium, magnesium and iron. By contrast, cations that were highlybasic and had large ionic radii, for example calcium, thorium and barium,had a detrimental effect on bonding.

We have noted earlier that aluminium is unusual in forming alumino-phosphate complexes in phosphoric acid solution which may be of apolymeric nature. Bearing in mind the analogies between aluminiumphosphate and silica structures, it may well be that during cementformation an aluminium phosphate hydrogel is formed. Its character maybe analogous to that of silica gel, where a structure is built up by the

203


condensation of pairs of hydroxyl groups to form oxygen bridges. Thus,the structure may consist of P in 4- and Al in 6-coordination, linked byoxygen bridges with H2O and OH~ as other ligands.

6.1.5 Important cement-formers

The most important of the phosphate bonded cements are the zincphosphate, dental silicate and magnesium ammonium phosphate cements.The first two are used in dentistry and the last as a building material.Copper(II) oxide forms a good cement, but it is of minor practical value.In addition, certain phosphate cements have been suggested for use ascontrolled release agents. The various phosphate cements are described inmore detail in the remainder of this chapter.

6.2 Zinc phosphate cement6.2.1 General

Zinc phosphate cement, as its name implies, is composed principally of zincand phosphate. It is formed by mixing a powder, which is mainly zincoxide, with a solution based on phosphoric acid. However, it is not assimple chemically as it appears because satisfactory cements cannot beformed by simply mixing zinc oxide with phosphoric acid solution.

The zinc phosphate cement finds use only in dentistry. Here it is usedmainly as a 'luting agent' for the attachment of inlays, crowns, bridges,posts and orthodontic bands (Wilson, 1975a,b; Smith, 1982). It is used alsoas a cavity liner in crowns and bridges (dental prosthesis). Although newtypes of cement have been introduced in dentistry in the 1970s and 1980s,this traditional cement continues to hold its own, particularly on thecontinent of Europe.

6.2.2 History

The early history of the material is obscure. According to Palmer (1891) itgoes back to 1832, but this statement has never been corroborated.Rostaing (1878) patented a series of pyrophosphate cements which couldinclude Zn, Mg, Cd, Ba and Ca. Rollins (1879) described a cement formedfrom zinc oxide and syrupy phosphoric acid. In the same paper hementions zinc phosphate cements recently introduced by Fletcher andWeston. Similar information is given in a discussion of the Pennsylvania

204

Zinc phosphate cement

Association of Dental Surgeons (1879), where Peirce describes a cementsimilar to that of Rollins. Many brands were on the market by 1881(Miller, 1881).

The earliest formulations, as reported by Rollins (1879), Gaylord (1889),Ames (1893), Hinkins & Acree (1901) and Fleck (1902), were variouslybased on syrupy orthophosphoric acid or unstable mixtures of meta-phosphoric acid and sodium metaphosphate in solution. Some used solidpyrophosphoric acid. Many were grossly inferior cements which werehydrolytically unstable.

Later, better cements appeared based on c. 50 % solutions of ortho-phosphoric acid. But even these were far from satisfactory. As always withdental cements, the problems revolved around the control of the settingreaction: the reaction between zinc oxide and orthophosphoric acid wasfound to be far too fierce. By the time of Fleck's 1902 paper these problemshad been solved. The importance of densifying and deactivating the zincoxide powder to moderate the cement reaction had been recognized. Ofequal importance was the realization that satisfactory cements could beproduced only if aluminium was incorporated into the orthophosphoricacid solution. The basic science underlying this empirical finding waselucidated only in the 1970s.

Comparison of the chemical composition of brands available in the1960s and 1970s (Axelsson, 1965; Wilson, Abel & Lewis, 1974) shows littlevariation from those of the 1930s (Paffenbarger, Sweeney & Isaacs, 1933)and it is doubtful whether the composition has changed in essence since thebeginning of the century (Table 6.2).

6.2.3 CompositionPowder

The powder is principally composed of zinc oxide (Table 6.2). Magnesiumoxide is found in all current commercial brands in amounts that range from3 to 10%. Alumina and silica are sometimes to be found. Present daycompositions show less variation than formerly when bismuth, calciumand barium oxides, or sometimes no additives, were to be found incommercial examples (Paffenbarger, Sweeney & Isaacs, 1933).

The chief problem with these cements, as with many AB cements, is tomoderate the cement-forming reaction. If the reaction is over-vigorousthen a crystalline mass rather than a cement is formed (Komrska & Satava,1970; Crisp et al, 1978). Therefore, the zinc oxide used in these cements

205


Table 6.2. Chemical composition of commercial zinc phosphate cements(Axelsson, 1965; Wilson, Abel & Lewis, 1974)

Powders

Species

ZnOMgOA12O3SiO2

% by mass

89-1-92-73-2-9-70-0-6-80-0-2-1

Liquids

Species

H3PO4AlZn

% by mass

45-3-63-21-0-3-10-0-9-9

Based on the results of nine examples.

has to be deactivated and densified by sintering at temperatures whichrange from 1000 to 1350 °C to moderate its reaction with aqueousphosphoric acid solutions. The sintering of a non-stoichiometric solid,such as zinc oxide, is affected by its initial physical condition and thesurrounding atmosphere. Several processes are involved. Sintering reducesspecific surface area and densities zinc oxide. It also reduces surface energy.According to Dollimore & Spooner (1971) freshly prepared zinc oxide hasa high surface energy because of its preparation in an oxygen-richatmosphere. They suggested that during sintering the initial excess ofoxygen at the surface is reduced by the diffusion of zinc ions from the bulkto the surface.

Magnesium oxide is always blended with the zinc oxide prior to ignition.Magnesium oxide promotes densification of the zinc oxide, preserves itswhiteness and renders the sintered powder easier to pulverize (Crowell,1929). The sintered mixed oxide has been shown to contain zinc oxide anda solid solution of zinc oxide in magnesium oxide (Zhuravlev, Volfson &Sheveleva, 1950). Specific surface area is reduced compared with that ofpure zinc oxide and cements prepared from the mixed oxides are stronger(Crowell, 1929; Zhuravlev, Volfson & Sheveleva, 1950).

In the presence of silica, zinc silicate is formed, the sintering process isimproved and the increase in grain size is enhanced (Zhuravlev, Volfson, &Sheveleva, 1950). Mineralizers, such as fluorite, cryolite and borax, have asimilar effect (Zhuravlev, Volfson & Sheveleva, 1950). These mineralizersenhance sintering and promote growth in grain size. As a result thesintering temperature can be reduced from 1350 °C to 1150-1200 °C.

206


LiquidThe liquid is an aqueous solution of phosphoric acid, always containing 1to 3 % of aluminium, which is essential to the cement-forming reaction(Table 6.2). Zinc is often found in amounts that range from 0 to 10% tomoderate the reaction. Whereas zinc is present as simple ions, aluminiumforms a series of complexes with phosphoric acid (Section 6.1.1). This hasimportant consequences, as we shall see, in the cement-forming reaction.

6.2.4 Cement-forming reaction

The cement sets rapidly within a few minutes of preparation. The reactionis strongly exothermic and is greater than with any other dental cement(Crisp, Jennings & Wilson, 1978). The excessive heat generated in thereaction has to be dissipated by progressively incorporating the powderinto the liquid. Strength develops rapidly. About half the ultimate strengthis attained within ten minutes of preparation and 80 % after one hour(Plant & Wilson, 1970; Williams & Smith, 1971).

The setting reaction is an acid-base one and the course of the reaction isshown by pH changes in the cement. Two minutes after mixing the pH isas low as 1-6, after 60 minutes it increases to about 4 and reaches between6 and 7 after 24 hours (Plant & Tyas, 1970).

The nature of the setting reaction and the set cement remainedimperfectly understood for many years. This is not surprising, for theproducts of the reaction depend on a number of factors, including thephosphoric acid concentration and the presence or absence of aluminiumin the solution. These complexities have caused considerable confusion inthe literature.

Early workers, and some later ones, ignored the fact that aluminium isalways found in the orthophosphoric acid liquid of the practical cement;its presence profoundly affects the course of the cement-forming reaction.It affects crystallinity and phase composition, and renders deductionsbased on phase diagrams inappropriate. Nevertheless we first describe thesimple reaction between zinc oxide and pure orthophosphoric acidsolution, which was the system studied by the earliest workers.

In the earliest attempt to explain the reaction, Crowell (1929) used, inpart, arguments based on the phase diagrams of the ZnO-P2O5-H2Osystem constructed by Eberly, Gross & Crowell (1920). Later, Darvell(1984) advanced similar arguments using the phase diagrams of Salmon &

207


Terrey (1950). In the case of the simple zinc oxide-orthophosphoric acidsystem, but only for this simple system, these phase-diagram arguments arevalid.

In the presence of excess zinc oxide, the final product of reaction withorthophosphoric acid solution is always hopeite, Zn3(PO4)2. 4H2O, and asearly as 1933 Halla & Kutzeilnigg (1933) found that zinc phosphatecements in service in the mouth contain hopeite. But this substance is notresponsible for initial set in the absence of aluminium. Crowell (1929)attributed setting to the formation of ZnHPO4. 3H2O, which he foundslowly converted over the weeks to hopeite, Zn3(PO4)2. 4H2O. Vieira & DeArujo (1963) confirmed this result. The most definitive study in this fieldwas that of Komrska & Satava (1970) who used XRD analysis to identifythe crystalline products formed. They found that a cement prepared fromZnO and 82-5% H3PO4 set as the result of the formation ofZn(H2PO4)2. 2H2O crystallites. Under humid conditions at 37 °C thefollowing conversions occurred:

Zn(H2PO4)2. 2H2O - ZnHPO4. H2O -> ZnHPO4. 3H2O ->Zn3(PO4)2.4H2O

After 7 days all four species were found in the cement. These conversionswere speeded up when the cement was placed in water, so that after 7 daysonly Zn3(PO4)2. 4H2O was found. With 65 % H3PO4, setting was found toresult from the formation of ZnHPO4.H2O crystallites. On ageing, thesame conversions occurred as with the 82*5% H3PO4 cement. All thesechanges are in accordance with phase-diagram predictions.

Unfortunately, Komrska & Satava (1970) did not examine the reactionwith orthophosphoric acid solutions having concentrations in the range45 to 63% H3PO4 which, according to Wilson (1975b), is the rangeencountered in practical materials. As Darvell (1984) has deduced, whenZnO is added to orthophosphoric acid solutions within this compositionalrange, ZnHPO4. 3H2O starts to precipitate. As precipitation continues, theacid concentration declines until it reaches the isoelectric point (22-5 %) atwhich ZnHPO4. 3H2O becomes unstable with respect to the liquid andstarts to convert to Zn3(PO4)2. 4H2O, the final reaction product. Darvell'sdeductions for higher initial concentrations of acid are in accordance withthe experimental findings of Komrska & Satava (1970).

These simple zinc oxide-orthophosphoric acid cements are very weak;indeed, it may be that they are just weakly-bonded aggregates of

208


crystallites. Nevertheless, they have been used to advance theories ofcementation. Kingery (1950b) in his XRD investigations of the reactionreported a crystalline matrix that was not hopeite but was taken to be anacid phosphate. He was apparently unaware of the slow conversion of thematrix to this species. Unfortunately, both he and Wygant (1958) went onto use this and similar observations to construct a theory that attributedcementation to hydrogen bonding between acid phosphate units. This ideacan now be seen as dubious. The role of hydrogen bonding in the cementmatrix is to be envisaged between particles of colloidal dimensions ratherthan between molecular units.

For many years the picture remained of a cement consisting of zinc oxideparticles bonded by a crystalline matrix of hopeite (Dobrowsky, 1942). Butthese ideas were erroneous for several reasons. Phase-diagram argumentsapplied only to systems in thermodynamic equilibrium and, clearly, theseconditions are not obtained in rapidly setting cements. XRD analysis canbe misleading as it ignores amorphous phases. Finally, these early ideasignored the role of aluminium, although the necessity of having it in theliquid for the production of satisfactory cements had long been known(Section 6.2.2).

Komrska & Satava (1970) showed that these accounts apply only to thereaction between pure zinc oxide and phosphoric acid. They found that thesetting reaction was profoundly modified by the presence of aluminiumions. Crystallite formation was inhibited and the cement set to anamorphous mass. Only later (7 to 14 days) did XRD analysis reveal thatthe mass had crystallized directly to hopeite. Servais & Cartz (1971) andCartz, Servais & Rossi (1972) confirmed the importance of aluminium. Inits absence they found that the reaction produced a mass of hopeitecrystallites with little mechanical strength. In its presence an amorphousmatrix was formed. The amorphous matrix was stable, it did not crystallizein the bulk and hopeite crystals only grew from its surface under moistconditions. Thus, the picture grew of a surface matrix with some tendencyfor surface crystallization.

Surprisingly, the effect of aluminium on the reaction had beenanticipated by van Dalen many years before in his thesis of 1933, but hadnot made its way into the scientific literature. The authors are indebted toDr L. J. Pluim of the Rijksuniversiteit te Groningen for this information.Van Dalen (1933) was convinced that aluminium had an important rolein cement formation and that Crowell had been wrong to ignore it. VanDalen found that the reaction between zinc oxide and phosphoric acid was

209


greatly moderated by the presence of aluminium in the liquid. He attributedthis to the formation of a gelatinous coating of aluminium phosphatearound each zinc oxide particle. This observation has never been repeated,but it appears entirely reasonable. Van Dalen also found that aluminiuminhibited the formation of crystallites and that this inhibition increasedwith increase in the aluminium content of the liquid.

Crisp et al. (1978) were able to follow the course of the cement-formingreaction using infrared spectroscopy and to confirm previous observations.They found that the technique could be used to distinguish betweencrystalline and amorphous phases of the cement. Hopeite shows a numberof bands between 1105 and 1000 cm"1; this multiplicity has been explainedby postulating a distortion of the tetrahedral orthophosphate anion. (Two-thirds of the zinc ions are tetrahedrally coordinated to four phosphate ions,and the remainder are octahedrally coordinated to two phosphate and fourwater ligands.)

Crisp et al. (1978) were able to detect the formation of crystallites bothon the surface and in the bulk of the reaction product. In the absence ofaluminium the reaction between zinc oxide and phosphoric acid was veryrapid and the cement set in less than two minutes. Hopeite was formed,within minutes, both at the surface and in the bulk of the reaction mass. Itwas doubted whether this mass constituted a true cement.

The addition of aluminium to the liquid slowed down the reaction. Anamorphous cement was formed and there was no crystallization in the bulkof the cement. However, after some time crystallites were formed at thesurface. Thus, the presence of aluminium exerts a decisive influence on thecourse of the cement-forming reaction. This effect is to be attributed to theformation of aluminophosphate complexes (see Sections 6.1.2 and 4.1.1).These complexes may delay the precipitation of zinc from solution and alsointroduce an element of disorder into the structure, thus inhibitingcrystallization. It is significant that zinc, which does not form complexes,has little effect on the nature or speed of the reaction.

Crisp and coworkers found that the development of surface crystallinitywas related to the speed of set. The faster the reaction, the shorter was theinhibition period before surface crystallization took place. When thesetting time of a cement was between two and three minutes, surfacecrystallinity developed in a few minutes. When it was seven minutes,surface crystallinity was delayed by three hours. The reaction rate wasaffected by the chemical composition and physical state of the cementcomponents. Well-ignited zinc oxide, the presence of magnesium in the

210


oxide powder, high phosphoric acid concentration, a low powder/liquidratio and the presence of aluminium in the liquid all served to retard thereaction. It is notable that these workers, like Servais & Cartz, found noevidence for the formation of crystalline acid phosphates in the cements.

More recently, Steinke et al. (1988) examined four commercial cementsand found that the matrices were mainly amorphous; indeed, they foundhopeite in only one of them. This indicates that manufacturers today areable to formulate to prevent crystallization.

Wilson, Paddon & Crisp (1979) have shown that the water present in thecement can be divided, somewhat arbitrarily, into bound water ofhydration (non-evaporable) and loosely held (evaporable) water. Theamount of tightly bound water increases as the cement ages and in oneexample reached 42 % of the total water.

It is interesting that this cement has been known for over 100 years andyet certain features of its chemistry remain obscure. The exact nature of thematrix is still a matter for conjecture. It is known that the principal phaseis amorphous, as a result of the presence of aluminium in the liquid. It isalso known that after a lapse of time, crystallites sometimes form on thesurface of the cement. A cement gel may be likened to a glass and thisprocess of crystallization could be likened to the devitrification of a glass.Therefore, it is reasonable to suppose that the gel matrix is a zincaluminophosphate and that entry of aluminium into the zinc phosphatematrix causes disorder and prevents crystallization. It is not so easy toaccept the alternative explanation that there are two amorphous phases,one of aluminium phosphate and the other of zinc phosphate. This isbecause it is difficult to see how aluminium could act in this case to preventzinc phosphate from crystallizing.

Summary of experimental evidenceA summary of this evidence may be attempted to give a probable reactionmechanism. After mixing, the zinc oxide powder is attacked by the acidsolution, water acting as the reaction medium. Zinc ions are extracted andthe pH at the powder-liquid interface rises, causing aluminium phosphateor more probably a zinc aluminophosphate to precipitate as a gel at theparticle surface. This gel coating moderates the reaction. Zinc ions diffusethrough this layer and, as the pH rises, precipitate as an amorphous gel,probably a zinc aluminium phosphate. This reaction mechanism thuspostulates both a topochemical and a through-solution reaction. Asreaction proceeds, the cement matrix becomes increasingly hydrated.

211


Increase in concentration of aluminium and phosphoric acid in the liquidserves to slow the reaction. This observation is in line with the abovereaction scheme. Increase in the aluminium content will serve to increasethe thickness of the coating formed around zinc oxide particles. Increasein phosphoric acid content implies a decrease in water content and animpairment of the hydration reaction.

The phenomenon of surface crystallization could be represented by theequation.

H2O

Zinc aluminophosphate -> Zn3PO4. 4H2O + A1PO4. «H2Oamorphous hydrogel hopeite amorphous gel

6.2.5 Structure

The set cement is completely opaque and can be regarded as a compositeof unconsumed zinc oxide particles, possibly coated with an aluminiumorthophosphate gel, bonded together by an amorphous neutral zincorthophosphate gel. There is some tendency for hopeite crystallites to growfrom the surface (Servais & Cartz, 1971; Cartz, Servais & Rossi, 1972;Crisp et al, 1978). Growth is related to the speed of set and the presence ofmoisture. Under dry conditions the surface is stable and undulating withno sign of crystallites (Figure 6.3a). When the environment is maintainedat 100% relative humidity, crystal growth is observed (Figure 63b) andmay be compared with the devitrification of a glass. Servais & Cartz (1971)observed that under certain conditions the layer could be 8 |am thick. It isonly loosely attached to the body of the cement matrix. The presence ofthis layer of crystallites may explain why the cement lacks adhesion.

The bulk of the cement is extremely porous as the fractured surface of aspecimen shows (Figure 6.3c). The pores are 0-5 \im in diameter and moreabundant in the depth of the cement. The porosity arises from excessunbound water which separates out as globules in the cement and istrapped by the rapid setting. Subsequent diffusion of these globules leavesthe cement porous. This makes the cement permeable to dyes (Wisth,1972).

As mentioned previously, the cement contains both tightly bound andloosely bound water. The set cement can both lose and gain waterdepending on its environment. Under drying conditions (say 50 % relativehumidity) it loses water and shrinks. When placed in water there is an

212


(b)Figure 6.3 The effect of environmental conditions on the surface of a zinc phosphate cement:(a) stable and undulating surface with no sign of crystallites observed under dry conditions,(b) crystal growth observed in an atmosphere of 100 % relative humidity, (c) extreme porosityobserved in the bulk of the cement; pores are 0-5 urn in diameter (Servais & Cartz, 1971).

213


Table 6.3. Specification properties of commercial zinc phosphate cements{Wilson 1975b)

Powder: liquid6

Setting time (37 °C), minFilm thickness, urnCompressivec

strength (24 h), MPaSolubility &disintegration (24 h), %

Value

20-303-9-7-524-4070-131

004-3-3

Specification limits"

—5-925 maximum70 minimum

0-2 maximum

aBS 3364: 1961 Specification for Dental Zinc Phosphate Cement.b For a consistency spread of 30 mm diameter for 0-5 cm3 of cement paste undera load of 1-96 N (200 g weight) applied after 3 minutes at 23 °C.c After storage for 24 hours in water at 37 °C, based on the results of nineexamples.

equilibrium water uptake: 9 % for two zinc phosphate cements examinedby Eichner, Lautenschlager & von Radnoth (1968).

6.2.6 Properties

Zinc phosphate cement is used as a luting agent for the cementing ofcrowns and bridges in dentistry. It is not an ideal material. It does notpossess the adhesion of modern polyelectrolyte cements, but its clinicalperformance has been good enough to ensure it a place in dentistry forabout 100 years. Despite the challenge of modern polyelectrolyte cementsit still holds its own. Undoubtedly this is because it is easy to mix, the fluidpaste is easy to manipulate, its working time is long and the paste setssharply to a hard mass. These are important properties for a dentalmaterial. In practice it means that these cements are easier to use than thepolyelectrolyte cements and this alone explains their continued popularitywith the dentist.

Preparation and settingZinc phosphate cement is prepared by introducing small incrementalamounts of powder into the liquid and mixing the paste over a large areaon a glass slab in order to dissipate heat because of the excessive exotherm

214


Table 6.4. Mechanical properties of commercial zinc phosphatecements (Housten & Miller, 1968; Wilson, 1975b; Wilson &Lewis, 1980; Powers, Far ah & Craig, 1976; 0ilo & Espevik,1978)

Property" Value

Compressive strengthCompressive modulusTensile strengthStrainCreep (over 24 hours)

70-131 MPa11-9—13-5 GPa4-3-8-3 MPa

0-2 %b

0-13 %b

a After storage for 24 hours in water at 37 °C.b Only one example.

of this cement (Crisp, Jennings & Wilson, 1978). To achieve a consistencysuitable for cementing crowns this cement is normally mixed with apowder/liquid ratio that ranges from 2-3 to 2-7 g cm"3 (Table 6.3) (Wilson,1975b). The working time (at 23 °C) then varies from 3 to 6 minutes andsetting time from 5 to 14 minutes (Plant, Jones & Wilson, 1972; Jendresen,1973; Wilson, 1975b; Myers, Drake&Brantley, 1978; E a r n e s t a l , 1977).The linear contraction of the cement on setting (0-5 %) can give rise to slitsat the cement-tooth and cement-restoration interfaces (0ilo, 1978).

Zinc phosphate cement mixes to a paste which is thin and mobile. Underpressure it flows readily to give a film 24 to 40 jim thick (Table 6.3). Thisfilm thickness is adequate to seat restorations, especially as McLean & vonFraunhofer (1971) and Dimashkieh, Davies & von Fraunhofer (1974) haveshown that in practice the gap between tooth and restoration can be asmuch as 100 |iim or more.

Mechanical propertiesFully hardened cements have brittle characteristics (Williams & Smith,1971; Skibell & Shannon, 1973) and show little creep under load (Wilson& Lewis, 1980). When mixed to a luting (cementation) consistency, theircompressive strength reaches 70 to 131 MPa after 24 hours (Wilson,1975b) depending on brand (Table 6.4). There is little subsequent increasein strength (Paffenbarger, Sweeney & Isaacs, 1933; Smith, 1977).

The tensile strength of these cements lies between 4-3 and 8-3 MPa andis thus lower than their compressive strength (Table 6.4) (Williams &

215


Smith, 1971; Hannah & Smith, 1971; Powers, Farah & Craig, 1976). Themodulus of elasticity in compression is 12 to 13 GPa (Table 6.4) (Powers,Farah & Craig, 1976; Wilson, Paddon & Crisp, 1979).

ErosionGood cements show little dissolution in water, less than 0-1 % using thestandard test (see Chapter 10); the amount can be much greater for poorerexamples. Dissolution represents the amount of material eluted from aone-hour-old cement as it ages in water for a further 24 hours. As thecement ages further the rate of dissolution falls although it always remainssignificant (Wilson, Kent & Lewis, 1970; Wilson, 1976; de Freitas, 1973).

Wilson, Abel & Lewis (1974), in a detailed chemical study of erosion inaqueous solution, found that in the first 24 hours of the cement's life theions eluted were Zn2+, Mg2+, HPO2" and H2PC>4. Far more Mg2+ ions wereeluted than Zn2+ ions, despite zinc being the major metal constituent of thezinc phosphate cement. These workers deduced that magnesium is far lessfirmly bound to phosphate than is zinc and that, consequently, its presencein the oxide is a source of weakness. These results were later confirmed byAnzai et al (1977).

Wilson, Kent & Lewis (1970) in a long-term study found that most ofthe soluble phosphate was eluted from the cement in the first 24 hours. Bycontrast the rate of elution of zinc remained constant for 160 days, thelength of the study. They concluded that long-term erosion took place atthe surface of the oxide particles rather than in the matrix.

Of more significance, however, were their results for the effect of pH.These showed that the cement is at its most stable in neutral solution andthat dissolution increases sharply with increasing acidity (Figure 6.4). Thisis of clinical significance, for pHs as low as 4 can occur in the stagnationregions of the mouth (Stephan, 1940; Kleinburg, 1961), and it is nowgenerally believed that the life of a dental cement is determined by itsresistance to acid conditions. The dissolution in lactic acid and especiallycitric acid is much higher (Norman, Swartz & Phillips, 1957). This effectmust be attributed to the complexing effect of these acids.

The laboratory impinging jet test for evaluating the acid erosion ofdental cements is described in Chapter 10. Using this method with lacticacid-lactate solutions, Wilson et al. (1986b) found, for one cement, that theerosion rate was virtually zero at pH = 5-0, 0-38 % at pH = 4-0 and 5-7 %at pH = 2-7. For a range of cements Wilson et al. (1986a) found erosionrates varying from 3-0 to 5-7 % in lactic acid solutions of pH = 2-7. The

216


zinc phosphate cement is markedly less resistant to acid erosion than thealuminosilicate glass cements, glass-ionomer cements and dental silicatecements. They also found that, with one exception, zinc phosphate cementswere somewhat more resistant to acid erosion than zinc polycarboxylatecement. These results have been confirmed by other workers using similarmethods (Beech & Bandyopadhyay, 1983; Kuhn, Setchell & Teo, 1984;Gulabivala, Setchell & Davies, 1987; Mesu, 1982).

In vivo studies have indicated that zinc phosphate cements erode underoral conditions. Also, cements based on zinc oxide, including the zincphosphate cement, are less durable in the mouth than those based onaluminosilicate glasses, the dental silicate and glass-ionomer (Norman etal., 1969; Ritcher & Ueno, 1975; Mitchem & Gronas, 1978,1981; Osborneet al, 1978; Pluim & Arends, 1981, 1987; Sidler & Strub, 1983; Mesu &Reedijk, 1983; Theuniers, 1984; Pluim et al, 1984, Arends & Havinga,1985). However, there is some disagreement on whether the zinc phosphatecement is more durable than the zinc polycarboxylate cement.

Dissolution of the cement has been associated with increased marginal

2",

\

silicatecement

P/L >4 2q/ml

-

I

P/L26q/ml

\ \

VJI > ~r

pH

Figure 6.4 Effect of pH on the elution of phosphate from a zinc phosphate cement mixed attwo different consistencies (Wilson, Kent & Lewis, 1970).

217


leakage (Andrews & Hembree, 1976) and the penetration of bacteria(Brannstrom & Nyborg, 1974).

6.2.7 Factors affecting properties

Cement properties are affected by a number of factors. Some aredetermined by the manufacturer, for example the chemical composition ofthe cement components. Others are under the clinician's control. Theseinclude the powder/liquid ratio of the cement mix and the temperature ofthe surgery. Increase in either of these variables accelerates the reactionand affects properties.

Properties are also affected by the composition of the phosphoric acidliquid. We have already pointed out that the liquid has to containaluminium for satisfactory cement formation. In addition the H3PO4/H2Oratio exerts a considerable effect on cement properties. This was investi-gated by Worner & Docking (1958). If the cement liquid is exposed to air,water can be readily gained or lost, depending on humidity, and this clearlyaffects the H3PO4/H2O ratio and therefore the cement properties (Figure6.5). If water is lost, then setting time is prolonged, strength increased and

3O°h water in the liquid

4O

Figure 6.5 Effect of water content of the liquid (H2O:H3PO4) on the properties of a zincphosphate cement (Worner & Docking, 1958).

218


resistance to aqueous attack decreased. The converse also occurs, althoughthe reduction in setting time is slight.

Both setting time and working time are reduced if the mixingtemperature is increased. Longer mixing time gives slower set (Paffen-barger, Sweeney & Isaacs, 1933). As long ago as 1892, Evans advocated theuse of a cool slab for cement preparation. Low-temperature preparationextends working time and enables more powder to be incorporated into thecement; this is advantageous for it increases strength and resistance todissolution, although this varies from brand to brand. This effect has sincebeen confirmed by many workers: Paffenbarger, Sweeney & Isaacs (1933);Henschel (1943); Jendresen (1973); Myers, Drake & Brantley (1978);Windeler (1978); Kendzior, Leinfelder & Hershey (1976); Tuenge, Sugel &Izutsu (1978); Williams et al. (1979). However, there is a danger with thistechnique, that if the atmosphere is excessively moist, water may condenseon the slab and the cement be weakened (Tuenge, Sugel & Izutsu, 1978;Norman et al., 1970).

The powder/liquid ratio used in the cement mix affects a number ofproperties. As it is increased, setting time and working time are reduced.Compressive strength increases almost linearly with powder/liquid ratio(Savignac, Fairhurst & Ryge, 1965).

Film thickness is controlled by a number of factors. The grain size of thepowder imposes a lower limit on its value and rheological characteristics ofthe cement affect flow (Jorgensen & Peterson, 1963). An increase in thepowder/liquid ratio or a delay in seating a restoration leads to an increasein film thickness. The geometry of the surfaces to be cemented also affectsflow and hence film thickness (Windeler, 1979).

6.2.8 Biological effects

The zinc phosphate cement is not bland towards living tissues. Whenfreshly mixed it is highly acidic with a pH as low as 1-6 (Plant & Tyas,1970). Even after it has aged one hour the pH may be lower than 4. This cangive rise to pulpal irritation and pain. Prolonged pulpal irritation in deepcavities cannot be allowed and some form of pulpal protection is needed inthese cases.

There are other causes of pain and pulpal irritation. Hydraulic pressuredeveloped during the seating of a restoration can lead to pulpal damage(Hoard et al., 1978). The movement of fluid under osmotic pressure hasbeen cited as a cause of pain (Brannstrom & Astrom, 1972).

219


If the cement is mixed too thinly it may etch the tooth enamel because ofits excess acidity (Docking et al., 1953; Abramovich, Macchi & Ribas,1976). Of course, etching can promote mechanical attachment to the tooth(Ware, 1971).

6.2.9 Modified zinc phosphate cements

Fluoride is found in some zinc phosphate cements, generally as stannousfluoride. The cements are weaker and have less resistance to dissolutionthan normal zinc phosphate cements (Myers, Drake & Brantley, 1978;Williams et al., 1979). They release fluoride over a long period (de Freitas,1973) and this is taken up by enamel (Wei & Sierk, 1971). This results inreduced enamel solubility (Gursin 1965; Skibell & Shannon, 1973) andincreased hardness (Yamano, 1968). Fluoride-releasing cements shouldreduce the incidence of enamel decalcification under orthodontic bandsbut this effect has not been recorded.

Although fluoride is added as the tin salt, fluoride release is accompaniedby the release of aluminium and not tin (de Freitas, 1973). There is littleleaching out of tin apart from an initial wash-out. Of course, aluminium isnot released from the normal cement (Wilson, 1976; Wilson, Abel & Lewis,1974).

These cements are of very minor interest in dentistry.

6.2.10 Hydrophosphate cements

Interesting attempts have been made to formulate water-setting cementsby blending solid acid phosphates with the zinc oxide powder. The cementis then prepared by mixing this powder blend with water. These attemptsmay be considered to have failed.

Nakazawa et al. (1965) used calcium or magnesium dihydrogenphosphate as the acid phosphate. The powders were hygroscopic. Themagnesium salt yielded cements which showed excessive contraction onsetting and were weak and prone to aqueous attack. The calcium salt wassomewhat better.

A more successful approach was that of Higashi et al. (1969a,b 1972).They blended solid acid phosphate salts with zinc oxide powder. One acidsalt used was a precipitated hydrate of ZnH2PO4. The cement was formedby mixing this powder blend with water. Work progressed to the pointwhere three commercial brands of these so-called 'hydrophosphate'cements appeared on the market. None met the specification requirements

220

Transition-metal phosphate cements

(Laswell et ai, 1971; Arato, 1974). All were prone to excessive dissolutionand only one had adequate strength and film thickness. Their workingcharacteristics were found to be unduly sensitive to changes in temperatureand humidity (Simmons, D'Anton & Hudson, 1968). All were inferior toconventional zinc phosphate cements. No further development of thesecements has taken place, nor is it likely that interest in them will be revived.The modern water-activated glass-ionomer cement has filled this nicheand has vastly superior properties including adhesion to tooth material.

6.3 Transition-metal phosphate cements

Copper oxides are good cement-formers but copper phosphate cementsfind little practical use. They have a very minor use in dentistry, the lastdescription of their chemistry appearing in 1940 (Worner, 1940). Theyhave also been proposed for the controlled release of copper as a traceelement supplement. They were introduced into dentistry by Ames in 1892and came into extensive use between 1914 and 1916; Poetschke describedthem in 1916. The rationale for their use was based on the germicidalaction of copper. There appears to be some basis for this belief (Worner,1940; Babin, Hurst & Feary, 1978). They are, however, excessively acidicand cause pulpal irritation (Worner, 1940; Ware, 1971). At one time theywere used for the filling of pits and fissures of deciduous (children's) teeth,lining, restoring badly decayed teeth, luting and the placement oforthodontic bands. By 1971 their use was confined to the last twoapplications (Ware, 1971).

There is virtually no knowledge of the setting and structure of copperphosphate cements. Mostly, they are complex materials. The simplest wasbased on a powder containing 91-5% CuO and 8-4% Co3O4. Otherscontained respectively 62-2 % CuO and 29-8 % ZnO, and 23-9 % Cu2O and66-7 % ZnO, with other metal oxides. The strength of these cements isabout the same as the zinc phosphate cement (Ware, 1971). There are alsopseudo-copper cements, which are zinc phosphate cements coloured byminor amounts of copper(II) oxide.

Vashkevitch & Sychev (1982) have identified the main reaction productof the cement-forming reaction between copper(II) oxide and phosphoricacid as Cu3(PO4)2. 3H2O. The addition of polymers - poly(vinyl acetate)and latex - was found to inhibit the reaction and to reduce the compressivestrength of these cements. However, impact strength and water resistancewere improved.

221


More recently copper phosphate cements have been suggested for use ascontrolled-release agents for supplying trace amounts of copper to cattleand sheep over an extended period (Allen et al, 1984; Manston et al., 1985;Prosser et al., 1986). The cements were prepared with a Cu/P ratio of 1:1to ensure that the matrix was an acid phosphate and so subject todissolution in aqueous solutions. They released copper at a constant ratefor 90 days.

Here we might note that cobalt(II) hydroxide, but not the oxide, alsoforms cements (Allen et al., 1984; Manston & Gleed, 1985; Prosser et al.,1986). It also is used in controlled-release devices for supplying traceelements to cattle and sheep. Nothing is known of its structure.

6.4 Magnesium phosphate cements6.4.1 General

Cements based on the reaction between magnesium oxide and phosphateshave long been known as investment materials for the casting of alloys(Prosen, 1939, 1941; Earnshaw, 1960a,b). More recently versions havebeen used as materials for the emergency repair of roads, runways andindustrial floors (El-Jazairi, 1982). This is because of their fast-settingproperties and ability to set at low temperatures. Setting reactions haveonly been elucidated in the last decade or so. The important point to noteis that although a cementitious mass is formed when magnesium oxide isreacted with phosphoric acid, it is soluble in water (Finch & Sharp, 1989).Practical cements are formed only when ammonium or aluminiumphosphates are incorporated into the cement liquid. Interestingly, theyappear to occur as a natural cementitious binder in kidney stones(Abdelrazig & Sharp, 1988). Unlike other phosphate-bonded cements thathave been fully studied, these appear to have a mainly crystalline matrix.

6.4.2 Composition

Magnesium (or magnesia) phosphate cements are based on the reactionbetween ignited magnesium oxide and acid phosphates, which are generallymodified by the addition of ammonium and aluminium salts. Thephosphates may be either in solution or blended in solid form with themagnesium oxide. In the latter form the cement is formed by mixing thepowder blend with water.

222

Magnesium phosphate cements

Table 6.5. Crystalline phosphate species found in magnesiumphosphate cements

Crystalline species Formula

Dittmarite MgNH4PO4. H2O' Hayesite' MgHPO4. 2H2O or MgHPO4. H2O ?Newberyite MgHPO4. 3H2OPhosphorresslerite MgHPO4. 7H2OSchertelite Mg(NH4)2(HPO4)2. 4H2OStercorite NaNH4HPO4. 4H2OStruvite MgNH4PO4. 6H2O

The most important characteristic of the magnesium oxide powder usedin these cements is its reactivity (Glasson, 1963). Magnesium oxide needsto be calcined to reduce this, otherwise the cement pastes are too reactiveto allow for placement. Surface area and crystal size are important andrelate to the calcination temperature (Eubank, 1951; Harper, 1967; Sorrell& Armstrong, 1976; Matkovic et ai, 1977). The lower reactivity ofcalcined magnesium oxide relates to a lower surface area and a largercrystallite size.

6.4.3 Types

There are several types of cement and mortar (cement plus filler):

(1) Those based on the reaction between magnesium oxide powderand orthophosphoric acid solution.

(2) Those based on the reaction between magnesium oxide andammonium dihydrogen phosphate (ADP), often in the presenceof sodium tripolyphosphate (STPP), which is mixed with water(El-Jazairi, 1982; Abdelrazig et aL, 1984; Abdelrazig, Sharp & E1-Jazairi, 1988, 1989).

(3) Those based on the reaction between magnesium oxide anddiammonium hydrogen phosphate (DAP) (Sugama & Kukacka,1983a).

(4) Those based on the reaction between magnesium oxide andammonium polyphosphate (APP) (Sugama & Kukacka, 1983b).

223


(5) Those based on the reaction between magnesium oxide andaluminium hydrogen phosphate solutions (Finch & Sharp, 1989).

6.4.4 Cement formation and properties

Cement formation between MgO and various acid phosphates involvesboth acid-base and hydration reactions. The reaction products can beeither crystalline or amorphous; some crystalline species are shown inTable 6.5. The presence of ammonium or aluminium ions exerts a decisiveinfluence on the course of the cement-forming reaction.

6.4.5 Cement formation with phosphoric acid

The reaction between magnesium oxide and 85-3 % phosphoric acid hasbeen studied by Finch & Sharp (1989). The reaction products were foundto be two highly crystalline phases, one unidentified, and an amorphousphase. One crystalline phase was most probably Mg(H2PO4)2. 2H2O.The other was believed to be another dihydrogen phosphate,Mg(H2PO4)2.4H2O. These findings are in accordance with the phasediagrams of Belopolsky, Shpunt & Shulgina (1950) who reported boththese species.

These cements are soluble in water and so of no practical significance.

6.4.6 Cement formation with ammonium dihydrogen phosphate

The reaction between MgO and ammonium dihydrogen phosphate (ADP)in aqueous solution yields struvite, MgNH4PO4. 6H2O, and schertelite,Mg(NH4)2(HPO4)2. 4H2O, as the main reaction products. Both may beregarded as cementing species. Only minor amounts of MgO are consumedduring these reactions as it is present in excess. The reaction is exothermic.Sodium tripolyphosphate (STPP) or borax may be added to retard thereaction. The main course of the reactions may be represented thus:

MgO + 2NH£ + 2H2PO4 + 3H2O = Mg(NH4)2(HPO4)2. 4H2O [6.2]Mg(NH4)2(HPO4)2. 4H2O + MgO + 7H2O = 2MgNH4PO4. 6H2O [6.3]

The reaction will not go to completion unless there is sufficient water.Mortars of this system are prepared by blending ignited magnesium

oxide, ADP and STPP with a filler, normally quartz sand. On mixing withwater a cementitious mass is formed. The reaction has been studied by anumber of workers: Kato et al (1976), Takeda et al. (1979), Neiman &

224


Sarma (1980), Abdelrazig et al (1984), Popovics, Rajendran & Penko(1987), Abdelrazig, Sharp & El-Jazairi (1988, 1989). The systems useddiffered in the proportions of MgO: ADP: H2O which did affect someaspects of the reaction. Nevertheless there are common features to allcompositions and some broad conclusions can be drawn.

On mixing, an exothermic reaction takes place with some loss ofammonia. As the reaction proceeds crystalline phases are formed. Allworkers are in agreement that these are the tetrahydrate schertelite, or thehexahydrate struvite, or a mixture of both. They also agree that scherteliteis first formed, which is then hydrated further, if water is available, to formstruvite. The relative amounts thus depend on the water available forhydration. Sometimes dittmarite, MgNH4PO4. H2O, and stercorite,NaNH4HPO4. 4H2O, are found in minor amounts.

The most extensive studies on these materials are those of Abdelrazigand coworkers, who used XRD, differential thermal analysis, thermo-gravimetry and scanning electron microscopy to characterize the reactionproducts. The reaction has been described by Abdelrazig et al. (1984), withsubsequent revision (Abdelrazig, Sharp & El-Jazairi, 1988, 1989). Onadding water to ADP a solution is immediately formed that containsammonium and various complex phosphate ions. This solution reacts withthe suspended magnesium oxide particles. Hydration products are formedprobably by a through-solution mechanism, an idea originally advancedby Neiman & Sarma (1980), rather than by a topochemical reaction.Initially, when the concentration of phosphate is high, the tetrahydrateschertelite (MgO:PO4 = 1:2) is formed. Schertelite can be seen as areaction intermediate which reacts further with MgO. As reactionproceeds, the phosphate content of the solution drops and the formation ofstruvite or dittmarite (MgO:PO4 = 1:1) is favoured. The reaction can befrozen by lack of either water or phosphate. If there is an abundance ofwater then the formation of the hexahydrate struvite is favoured. If,however, water is lacking then some monohydrate dittmarite is formed,and moreover the reaction does not proceed. Schertelite remains in the setmaterial and some ADP remains undissolved. Thus, the reaction is not oneof progressive hydration from monohydrate to tetrahydrate to hexa-hydrate.

The presence of colloidal species has been a subject of some debate.Neiman & Sarma (1980) found with their material that crystallinity did notdevelop for at least the first two hours of setting. However, Abdelraziget al (1984) and Abdelrazig, Sharp & El-Jazairi (1988, 1989) reported that

225


abundant amounts of crystallites are formed in only a few minutes. Thesedifferences probably relate to the use of different molar ratios of reactants,although this cannot be confirmed because Neiman & Sarma did not givethe molar ratios that they used.

(3) 100 ym (b) 100 ]im

(c) 10 pm

For legend see opposite

226

(d) 100


Abdelrazig, Sharp & El-Jazairi (1988, 1989) prepared a series ofmortars based on a powder blend of MgO and ADP with a quartzsand filler. They were hydrated by mixing with water. A mortar I(MgO: ADP: silica: water = 17-1:12-9:70-0:12-5), with a water/solid ratioof 1:8, formed a workable paste which set in 7 minutes with evolution ofammonia. The main hydration product, struvite, was formed in ap-preciable amounts within 5 minutes and continued to increase. Schertelitealso appeared, but only in minor amounts, within the first 5 minutes andpersisted only during the first hour of the reaction. Dittmarite appeared inminor amounts after 15 minutes, and persisted.

Microstructural changes were observed during hydration. After the firsthour the microstructure appeared to be predominantly crystalline,although the crystallinity was poor. The crystallites appeared to growbetween silica grains (Figure 6.6a). After seven hours the crystallinitybecame well defined (Figure 6.6b); the platy and tabular morphologiesobserved were tentatively assigned to dittmarite and struvite respectively(Figure 6.6c). After 24 hours there was an abundance of struvite crystalliteswith a rod-like appearance (Figure 6.6d,e).

The compressive strength of the mortar reached 14-9 MPa in 24 hours.

V(e) 10 \im

Figure 6.6 Scanning electron micrographs of magnesium ammonium phosphate mortar I(Abdelrazig, Sharp & El-Jazairi, 1989) after hydration at 22 °C: (a) after 1 hour, lowmagnification, (b) after 7 hours, low magnification, (c) after 7 hours, high magnificationshowing platy and tabular morphologies, (d) after 24 hours, low magnification, (e) after 24hours, high magnification showing rod-like crystallites of struvite.

227


Total pore (3-75 nm to 7-5 (am) volume was 73-1 mm3 g"1 and coarse pore(1 |im to 7-5 |im) volume was 60-1 mm3 g"1.

The addition of STPP (1-7%) acted as a retarder and increasedcompressive strength (mortar II). Less heat and ammonia were evolvedand the cement set more slowly in 10 minutes. The paste hardened in 30 to60 minutes. Traces of ADP persisted for 30 minutes but no STPP wasdetected in the reaction products. Struvite, the main hydration product,schertelite and dittmarite all appeared within 5 minutes. Struvite continuedto increase in amount as the cement aged; schertelite disappeared after 3hours and dittmarite after a week. Stercorite was found only during thefirst 7 hours.

The presence of STPP affected the morphology of the cement. After 24hours struvite with ellipsoidal morphology was observed (Figure 6.1a).Later (7 and 28 days) there was a morphological change with the formationof well-crystalline hexagonal plates (Figure 6.1b). This morphology is quitedifferent from that of mortar I.

The addition of STPP improved the compressive strength of the mortarwhich reached 19-5 MPa in 24 hours. The total pore volume was reducedto 70-4 mm3 g"1 and the coarse pore volume to 55-4 mm3 g"1.

•J

10 10 ym

Figure 6.7 Scanning electron micrographs of magnesium ammonium phosphate mortar II(Abdelrazig, Sharp & El-Jazairi, 1989): (a) after 24 hours, high magnification showingstruvite of ellipsoidal morphology, (b) after 28 days, high magnification showing well-crystalline hexagonal plates of struvite.

228


When a mortar was prepared with insufficient water (mortar III,water: solid = 1:16) some ADP remained even after 3 weeks and themortar took longer to set (12 minutes). Schertelite was the major reactionproduct for the first hour; thereafter struvite became the major product,but schertelite remained a significant constituent. The microstructurediffered from that of the other two mortars. After one hour there was adense crystalline microstructure with needle-like and cuboid crystallitesgrowing between silica grains (Figure 6.&a,b). These crystallites weretentatively assigned to struvite and schertelite respectively. These featuresdid not change with time. The compressive strength of the mortar was27-4 MPa, the total pore volume was reduced considerably to 20-6 mm3 g"1

and the coarse pore volume to 5-3 mm3 g"1. This incompletely hydratedmortar was stronger than the other two fully hydrated mortars. In this,these materials obey the general rule that strength of a cement is increasedas the water content is reduced, irrespective of the exact microstructure ofthe matrix.

Abdelrazig et al. (1984) studied the commercial FEB SET-45 cementsand mortars (Set Products Inc., Master Builders Division, Martin MariettaCorporation). Their hydration behaviour is similar to those describedabove. The mortars normally set in 15 minutes and hardened in 30 to 60

100 \m 10 ymFigure 6.8 Scanning electron micrographs of magnesium ammonium phosphate mortar III(Abdelrazig, Sharp & El-Jazairi, 1989): (a) after 1 hour, low magnification, (b) after 1 hour,high magnification showing needle-like and cuboid crystallites.

229


minutes. The compressive strength of an FEB SET-45 mortar (water: solid= 1:16) increases with time. Compressive strength reached 24 MPa in the

first hour and 47 MPa after 24 hours. Thereafter, compressive strengthincreased more slowly to 54 MPa after one week and 56 MPa after one

4U

Figure 6.9 The morphology of a commercial mortar, showing well-developed needle-likecrystallites. Micrograph span (a) 75 \im, (b) 30 (im (Abdelrazig et aL, 1984).

230


month. Strength depends on the amount of water in the system, andcompressive strength was drastically reduced when the water/ADP ratiowas increased to 1:8.

Microstructure is also affected by water content. The 1:16 mortarcontained hexagonal plates, while the 1:8 mortar had an ellipsoidalmorphology. The 1:5 mortar when prepared at — 5 °C developed striking,well-formed, needle-like crystals (Figure 6.9).

The action of heat on these cements is complex (Abdelrazig & Sharp,1988). The principal sequence based on XRD and thermal analysis isshown in Figure 6.10.

6.4.7 Cement formation with diammonium hydrogen phosphate

Sugama & Kukacka (1983a) described cements based on magnesium oxideand a 40 % solution of diammonium hydrogen phosphate (DAP) liquid.The powder was a fine magnesium oxide that had been calcined above1500 °C and had a surface area of c. 1 m2 g"1. These cements set within 3minutes and developed an early strength of 5-7 MPa after 30 minutes and19-3 MPa in 15 hours. Sugama & Kukacka, using XRD, considered thatMg3(PO4)2. 4H2O was the main reaction product. They also reported thepresence of other hydrates, Mg(OH)2 and struvite. However, Abdelrazig &Sharp (1985) discounted the presence of Mg3(PO4)2. 4H2O and Mg(OH)2in this system, and we are inclined to agree that it is unlikely that such

MgNH4PO4.6H2O

Temperajtureincreasing

MgNH4PO4.H2O

lMgNH4PO4.H2O (amorphous phase)

\

MgHPO4 (amorphous phase)

Mg2P2O7

Mg(NH4)2(HP04)2.4H20

[6.4]

[6.5]

[6.6]

[6.7]

1 + MgO

Mg3(PO4)2 [6.8]Figure 6.10 Action of heat on ammonium magnesium phosphate cements (Abdelrazig &Sharp, 1988).

231


species are formed in the presence of ammonium ions. Most likely, thereaction products are the hydrates of magnesium ammonium phosphate:schertelite, struvite and dittmarite.

6.4.8 Cement formation with ammonium polyphosphate

Sugama & Kukacka (1983b) described cements based on magnesium oxideand a 56 % aqueous solution of ammonium polyphosphate (APP). Thepowder was a fine magnesium oxide that had been calcined above 1300 °Cand had a surface area of 1 to 5 m2 g"1. The reaction was stronglyexothermic; the cements set within 3 minutes and developed an earlystrength of 13*8 MPa after 1 hour and over 20 MPa after 5 hours.

Micrographs taken after 30 minutes reaction showed the matrix to beamorphous (Figure 6.11). After 1-5 hours, crystallites were observed. XRDanalysis showed that the most abundant phase was struvite. Sugama &Kukacka also considered that Mg3(PO4)2. 4H2O was another majorreaction product and that other hydrates, Mg(OH)2 and newberyite,MgHPO4.3H2O, were also present. Again, Abdelrazig & Sharp (1985)

Figure 6.11 Scanning electron micrographs showing the microstructure of a cement formedfrom magnesium oxide and ammonium hydrogenphosphate solutions (Sugama & Kukacka,1983b).

232


argued that Mg3(PO4)2. 4H2O, MgHPO4. 3H2O and Mg(OH)2 wereunlikely to be present in this system. Thus, it would appear that thereaction products of these cements are struvite and other phases yet to beidentified. During the course of the reaction it would appear that P-O-Pbridges hydrolyse into orthophosphates.

6.4.9 Cement formation with aluminium acid phosphate

Ando, Shinada & Hiraoka (1974) examined cements formed by thereaction between magnesium oxide and concentrated aqueous solutions ofaluminium dihydrogen phosphate. Later, Finch & Sharp (1989) made adetailed examination of the cement-forming reaction and reported that thereaction yielded cements of moderate strength.

They considered that cement formation was the result of an acid-basereaction leading to the formation of hydrates by a through-solutionmechanism, by nucleation and precipitation from pore fluids. Two phaseswere found in the matrix, one amorphous and the other crystalline. Thecrystalline phase was newberyite. Finch & Sharp concluded that theamorphous phase was a hydrated form of aluminium orthophosphate,A1PO4, which almost certainly contained magnesium. They ruled out apure A1PO4.«H2O, for they considered that the reaction could not berepresented by the equation

2MgO + A1(H2PO4)3 A 2MgHPO4. 3H2O + A1PO4. «H2O [6.9]

because cement formation was poor when the MgO/Al(H2PO4)3 moleratio was 2:1. It followed that the amorphous phase must be a compositionwithin the Al2O3-MgO-P2O5-H2O system. This conclusion was in linewith results of an energy-dispersive X-ray microanalysis of the fracturesurface using a scanning electron microscope.

As we have seen in Section 6.2, there is some evidence for supposing thatzinc phosphate cements contain an amorphous aluminium phosphate orzinc aluminophosphate phase. Also, as we shall see in Section 6.5,amorphous aluminium phosphate is the binding matrix of dental silicatecement.

Scanning electron micrographs of fracture surfaces revealed thepresence of both amorphous and crystalline phases which corresponded toresults from XRD analysis (Figure 6.12). What is of interest is that thecrystalline phase is MgHPO4. 3H2O and not Mg(H2PO4)2. 2H2O or

233


Mg(H2PO4)2. 4H2O as was the case in the reaction between MgO andsimple phosphoric acid solutions. Inspection of the diagrams ofBelopolsky, Shpunt & Shulgina (1950) shows that newberyite is the stablephase at lower concentrations of phosphate. Presumably, in the presentcase, aluminium locks up some phosphate and so reduces the phosphateavailable for the magnesium phosphate phase.

Finch & Sharp (1989) found the mole ratio of MgO to A1(H2PO4)3 to bean important parameter that affected both the reaction rate and the natureof the reaction products. The critical mole ratio was 2:1. When the ratiowas less than 2:1 cements were not formed at all, and when it was exactly2:1 the paste set slowly and always remained tacky. Further increases inthe ratio caused cements to set faster with greater evolution of heat. Finch& Sharp (1989) also found that this ratio affected the proportion ofcrystalline phase to amorphous phase in the cement matrix. The proportionof newberyite in the matrix reached a maximum when theMgO/A1(H2PO4)3 ratio was 4:1 and decreased to a low level when the ratiowas 8:1.

When Finch & Sharp (1989) used solutions of lower water content theyfound an unknown XRD pattern that was distinct from that ofMgHPO4.3H2O. This unidentified phase they dubbed hayesite andspeculated that it might be a lower hydrate, either MgHPO4.2H2O orMgHPO4.H2O. Infrared spectroscopy showed that hayesite was less well

Figure 6.12 Microstructure of MgO-aluminium hydrogenphosphate cement (Finch &Sharp, 1989).

234

Dental silicate cement

crystalline than newberyite, but did contain bands at c. 3400 cm"1 and1640 cm"1, showing it to be hydrated.

A higher hydrate, phosphorresslerite, MgHPO4. 7H2O, was formedwhen these cements were exposed to water and dried in air. On ageing, thishydrate readily transformed back to newberyite.

6.4.10 Cements formed from magnesium titanates

Cements have been prepared from magnesium titanates (Mg2TiO4,MgTiO3 and Mg2Ti2O5) and phosphoric acid (Sychev et ai, 1982; Sudakas,Turkina & Chernikova, 1982). The reaction product, MgHPO4. 3H2O, iscrystalline when Mg2TiO4 is used and amorphous with the othermagnesium titanates. The amorphous product gives the stronger cements.

6.5 Dental silicate cement6.5.1 Historical

Dental silicate cement was once the most favoured of all anterior (front)tooth filling materials. Indeed, it was the only material available for theimportant task of aesthetic restoration from the early 1900s to the mid1950s, when the not very successful simple acrylic resins made theirappearance (Phillips, 1975). In the mid sixties there were some 40 brandsavailable (Wilson, 1969) and Wilson et al. (1972) examined some 17 ofthese. Since that time the use of the cement has declined sharply. It is rarelyused and today only two or three major brands are on the market. Thereason for this dramatic decline after some 50 years of dominance is closelylinked with the coming of modern aesthetic materials: the composite resinfrom the mid 1960s onwards (Bowen, 1962), and the glass-ionomer cement(Wilson & Kent, 1971) from the mid 1970s.

Dental silicate cement was also variously known in the past as atranslucent, porcelain or vitreous cement. The present name is to someextent a misnomer, probably attached to the cement in the mistaken beliefthat it was a silicate cement, whereas we now know that it is a phosphate-bonded cement. It is formed by mixing an aluminosilicate glass with anaqueous solution of orthophosphoric acid. After preparation the cementpaste sets within a few minutes in the mouth. It is, perhaps, the strongestof the purely inorganic cements when prepared by conventional methods,with a compressive strength that can reach 300 MPa after 24 hours(Wilson et al., 1972).

235


The early history of the cement is obscure. Dreschfeld (1907) andSanderson (1908) attributed its invention to Fletcher. Fletcher (1878,1879)certainly described cements formed from concentrated orthophosphoricacid solutions and sintered mixtures of oxides which included SiO2, A12O3,CaO and ZnO. One was reported by Fletcher (1879) as being slightlytranslucent. These cements were not successful in clinical use.

Voelker (1916a) reported three early dental silicate cements whichappeared in 1895, 1897 and 1902; all proved inadequate. The firstsuccessful material was developed by Steenbock (1903,1904) who explicitlysought and formulated a translucent cement (Voelker, 1916a,b). It wasmarketed by Ascher in 1904 as New Enamel Richters; Harvadid cementfollowed in the same year. Thereafter development was rapid and eightvarieties were reported by Morgenstern in 1905. However, from theirchemical composition we doubt whether they were sufficiently translucent.

The liquids used in these early formulations were 50% solutions oforthophosphoric acid, often containing aluminium and zinc. Chemicalanalyses were published between 1904 and 1972 (Voelker, 1916a; Greve,1913; Watts, 1915; Paffenbarger, Schoonover & Souder, 1938; Axelsson,1964; Wi l sons al, 1972).

The glasses used by Steenbock in his original compositions were mixturesof calcium aluminosilicates and beryllium silicates; but, as Dreschfeldreported in 1907, subsequent developments moved away from the use ofberyllium compounds. Published chemical analyses in the period to 1916(Voelker, 1916a; Greve, 1913; Watts, 1915) confirmed Dreschfeld'sstatement. In the following we shall refer to these as oxide glasses.

In 1908 Schoenbeck made the most important of all compositionalinnovations when he introduced the use of fluoride-fluxed glasses intodental silicate cement. In this discussion we shall refer to these as fluorideglasses, although they are, in fact, mixed fluoride and oxide glasses. Overthe years fluoride glasses have progressively replaced the purely oxide ones.Although materials based on fluoride-containing aluminosilicate glasseswere rare before 1920 (Watts, 1915; Greve, 1913), and Wright (1919)ignored them in his studies, by 1938, Paffenbarger, Schoonover & Souder(1938) reported that most dental silicate cements were of the Schoenbecktype. This development is not surprising because, besides lowering thetemperature of fusion, fluoride confers greater translucency and strengthon the cement and has beneficial therapeutic effects. There have been nomajor compositional innovations since.

Dental silicate cement is used exclusively for the aesthetic restoration of

236


anterior (front) teeth. In this situation, unlike cements used for cemen-tation, the cement is fully exposed to erosive attack by oral fluids. Thus,considerable interest was shown in the physical, chemical and biologicalproperties of these cements in the years following their introduction. Earlyworkers in the field were Morgenstern (1905), Kulka (1907), Rawitzer(1908, 1909), Proell (1913) and Poetschke (1916). These workers reporteda number of faults, finding the cements to be porous, prone to staining,attacked by mouth acids, shrinking under drying conditions and lacking inadhesion. These observations have been confirmed during the course oftime. In certain mouths, dental silicate cement stains, erodes and evenwashes away. For this reason it has never been fully satisfactory and,consequently, now that alternatives have become available, it has fallenout of general use.

The first period of development ended with the research of Wright(1919) who published the results of an extensive survey of cementsprepared from experimental SiO2-Al2O3-CaO glasses and orthophos-phoric acid solutions containing aluminium phosphate. By this time themain cement formulations had been established. Between 1919 and 1950only minor improvements were attempted; these were of a technologicalnature and unsuccessful.

After 1950 some serious attempts were made to improve dental silicatecement. Notable were those of Manly et al. (1951) and Rockett (1968);also, Pendry reported an acid-resistant cement containing indium (Pendry& Cook, 1972; Pendry, 1973). None of these experimental materials wentinto production. These attempts came too late in view of progress made inother directions, and the picture of dental silicate cement remains one of anessentially traditional material which has changed little since the originaldevelopments made mainly by German chemists prior to 1914.

The main line of development now lies with its successor, theglass-ionomer cement, which uses a similar glass, but in which phosphoricacid is replaced by poly(acrylic acid); this cement is more resistant to aciderosion and staining and has the great advantage of adhesion to toothmaterial.

6.5.2 Glasses

The powders used in dental silicate cement are unusual in being groundopal glasses rather than crushed crystalline clinker. The glassy nature ofthe powder gives the set cement the unusual property of translucency

237


which it shares with one other cement, its successor, the glass-ionomercement. These glasses are calcium aluminosilicates which, in materialsavailable since the 1940s, always contain fluoride. Essentially they arebased on the SiO2-Al2O3-CaF2 system.

The aluminosilicate glass has a dual role: it acts as a filler and is also thesource of ions required to gel phosphoric acid solutions. These glasses actas a source of ions because they are decomposed by acids. This property isdependent on the Al/Si ratio being sufficiently high, approaching 1:1. Thereasons and criteria for the decomposition of aluminosilicates areexamined fully in Section 5.9.2.

The glasses are similar to those used in glass-ionomer cements but theirreactivity towards acids has to be less, as orthophosphoric acid is astronger acid than the poly(alkenoic acid)s. The consequence is that theAl/Si ratio, which determines reactivity, is lower than in the glass-ionomercement glasses.

Typical glass compositions are given in Table 6.6. The preparation ofthese glasses is given in Section 5.9.2. The essential components are groundsilica (quartz), alumina and fluorite (which also acts as a flux). Cryolite isadded as an additional flux and minor amounts of aluminium phosphateare present, which apparently improve the mixing qualities of the cement.The ratio of alumina to silica controls the setting time of the cement.Fluoride tends to slow setting while aluminium phosphate improves themixing of the paste.

The temperature of fusion is 1050 to 1300 °C, a somewhat lower rangethan that for the glass-ionomer cement glasses. The melts are shock-cooledand ground. The median particle size ranges from 8-6 to 11-5 |im. Theglasses are slightly opal in appearance and evidence from electronmicroscopy shows that this arises from phase separation (Wilson et al.,1972). The separated phase appears as droplets although it may bespinodal. An example of one glass (Super Syntrex) showed the segregationof larger droplets of uniform size c. 400 nm in diameter and smallerdroplets of 20 to 30 nm (Figure 6.13). Etching with phosphoric acidshowed that the droplets were selectively attacked by acids.

Effect of glass composition on cement propertiesAs we have indicated previously, two types of glass have been used indental silicate cements: the obsolete oxide glass and the modern fluorideglass. Only four studies on glass composition and its relationship to cementproperty have been published (Wright, 1919; Crepaz, 1951; Manly et al.,

238


Table 6.6. Chemical composition of commercial dental silicatecements {Wilson et al., 1972)

Powders

Species

SiO2A12O3CaONa2OFP AZnOMgOSrOH2O

% by mass

31-5-41-6 (35-9)27-2-29-1 (290)

7-7-9-0(6-1)7-7-11-2(14-5)

13-3-22-0(15-2)3-0-5-3 (4-4)0-1-2-9 (0-3)0-0-0-1 (00)0-0-0-2 (00)1-6-2-2

Liquids

Species

H3PO4AlZnMgBe

% by mass

48-8-55-5 (65-9)1-6-2-5 (00)4-2-9-1 (0-0)

0(1-6)0 (0-65)

Based on four typical examples and one atypical example.Composition of the atypical example is in parentheses.

o • O:

Figure 6.13 Electron micrograph of a single-stage replica of a dental silicate cement glass,showing phase-separated droplets rich in calcium and fluoride: large droplets 400 nm indiameter and small droplets 20 to 30 nm in diameter (Wilson et al., 1972).

239


1951; Rockett, 1968) and these are concerned almost entirely with non-fluoride glasses of the SiO2-Al2O3-CaO and SiO2-Al2O3-CaO-P2O5 types.

In their studies on SiO2-Al2O3-CaO glasses Manly et al. (1951) foundthat the SiO2/CaO ratio controlled the setting rate of the cement pastes. Ifthis ratio was greater than 2-08 by mass then the glass powder-liquid pastedid not set; when it was less than 1-74 pastes set rapidly to form cements.Between these extremes there were narrow compositional bands corres-ponding to slow-setting cements (ratio = 1-95) and moderate-settingcements (ratio = 1-77-1-89).

Since the 1940s all commercial dental silicate cements have used fluorideglasses. Fluoride glasses yield more translucent cements than oxide glassesbecause they have lower refractive indices. It is also probable, by analogywith glass-ionomer cements, that they yield stronger cements (Section5.9.4). Unfortunately, in the four studies cited above, the workers wereconcerned almost entirely with non-fluoride glasses and made no sys-tematic studies on fluoride glasses. Only Manly et al. (1951) made even acursory examination of these glasses, which are the basis of practicalcements. Consequently, none of these workers were able to improve on oreven equal the performance of the commercial examples. Nor were theyable to shed much light on fundamental compositional factors controllingthe setting of cements based on fluoride glasses.

It was left to Kent & Wilson (1968), in unpublished observations, todiscover that in glasses based on SiO2-Al2O3-CaF2 compositions theAl/Si ratio controlled the rate at which the cement paste set. Theseobservations laid the foundation for the development of the glass-ionomercement, during which most of the work on fluoride glasses was done. Thistopic is covered in detail in Section 5.9.2.

The degree of subdivision of the powder has considerable effect on theproperties of cements (Swanson, 1936; Charbeneau, 1961; Kent & Wilson,1971). Kent & Wilson (1971) showed that the cements prepared from afine-grain powder when mixed at the same powder/liquid ratio as normal-grain powder had greater strength, but set too fast for practical use andlacked translucency (Table 6.7). Moreover, cements prepared fromnormal-grain powder could be mixed to a higher powder/liquid ratio,resulting in a notable increase in strength combined with an optimumsetting time.

240


Table 6.7. Effect of particle size of powder on cement properties (Kent &Wilson, 1971)

Surface area, um *Powder: liquid ratio, g cm"3


Wet compressive"strength (24 h), MPa

Wet tensile"strength (24 h), MPa

Powder

Fine

1-6231252-5

289

14-7

Normal

0-87312590

176

6-2

Normal

0-874004-0

217

13-6

a After storage for 24 hours in water at 37 °C.

6.5.3 Liquid

Dental silicate cement liquids are concentrated aqueous solutions oforthophosphoric acid generally containing aluminium and zinc (Wilson,Kent & Batchelor, 1968; Kent, Lewis & Wilson, 1971a,b; Wilson et al,1972). The optimum orthophosphoric acid concentration is 48 to 55 % bymass (Wilson et al., 1970a), although higher concentrations are en-countered. Aluminium is present as phosphate complexes and zinc as asimple ion (see Section 6.1.2). Examples are given in Table 6.6.

Effect of liquid composition on cement propertiesThe properties of dental silicate cements are affected both by theconcentration of phosphoric acid and by the presence of metal salts.

The effect of the concentration of orthophosphoric acid on cementproperties has long been known (Poetschke, 1916; Ray, 1934; Worner &Docking, 1958; Wilson et al., 1970a). The setting time of a cement pasteincreases as the orthophosphoric acid concentration increases; this effect isparticularly marked above 65% H3PO4 (Figure 6.14). There are severalreasons for this sharp change. Water is required to act as a reactionmedium and also to hydrate reaction products; a deficiency could thereforeretard or even arrest the reaction. There is also the possibility of a changein structure of the orthophosphoric acid solution as its concentrationincreases (see Section 6.1.1).

241


Cement strength and resistance to aqueous attack are also criticallydependent on phosphoric acid concentration, and there is an optimumconcentration for the development of maximum strength and resistance toaqueous attack (Wilson et ai, 1970a; Wilson, Paddon & Crisp, 1979). Theeffect is particularly critical when the phosphoric acid liquid containsaluminium and zinc.

This sensitivity to water has practical implications. A cement liquid isstable in an atmosphere of 70 % relative humidity. It will gain water inmore humid atmospheres and lose it to drier ones, and this will adverselyaffect cement properties (Paffenbarger, Schoonover & Souder, 1938;Worner & Docking, 1958; Wilson et al., 1970a).

All commercial examples of phosphoric acid solutions used in thesecements contain metal ions, whose role has been discussed in Section 6.1.2.In the case of the dental silicate cement, aluminium and zinc are the metalsadded to liquids of normal commercial cements and have a significanteffect on cement properties (Table 6.8) (Wilson, Kent & Batchelor, 1968;Kent, Lewis & Wilson, 1971a,b). Aluminium accelerates setting for itforms phosphate complexes and is the principal cation of the phosphaticmatrix. Zinc retards setting for it serves to neutralize the acidic liquid - it

3OO

4O 5O 6O 7Oliquid composition °/ow/w H3PC

8O

Figure 6.14 Effect of liquid composition on the setting time and strength of dental silicatecements (Wilson et al, 1970a).

242


Table 6.8. Effect of metals contained in the phosphoric acid liquid oncement phosphate properties (Wilson, Kent & Batchelor, 1968)

Al, Zn, Powder: liquid, Setting time, Compressive strength% % g cm"3 minutes (wet, 24 hours), MPa

00002-51-25

00130006-5

3-9403-940

3-54-53-03-5

169267269291

forms simple salts - and thus reduces the rate of extraction of cations fromthe aluminosilicate powder. When both metals are present these opposingeffects tend to cancel out. Both metals, alone or in combination in theliquid, serve to improve the strength of cements, a combination being mosteffective.

6.5.4 Cement-forming reaction

The setting reaction of dental silicate cement was not understood until1970. An early opinion, that of Steenbock (quoted by Voelker, 1916a,b),was that setting was due to the formation of calcium and aluminiumphosphates. Later, Ray (1934) attributed setting to the gelation of silicicacid, and this became the received opinion (Skinner & Phillips, 1960).Wilson & Batchelor (1968) disagreed and concluded from a study of theacid solubility that the dental silicate cement matrix could not be composedof silica gel but instead could be a silico-phosphate gel. However, infraredspectroscopy failed to detect the presence of P-O-Si and P-O-P bonds(Wilson & Mesley, 1968).

The nature of the setting reaction was finally elucidated by Wilson et al.(1970a), who established that formation of an aluminium phosphate gelwas responsible; although siliceous gel was also formed it merely coatedthe partly reacted glass particles.

The following account is based mainly on the studies of Wilson andcoworkers, with some re-interpretation of experimental data. The com-position of the cement used is given in Table 6.9. In brief, the reaction takesplace in several overlapping stages: extraction of ions from the glass,migration of cations into the aqueous phase, precipitation of insolublesalts as pH increases, leading to formation of an aluminium phosphate gel.

243


Table 6.9. Chemical composition of the dental silicate cementused in reaction and structural studies

Powders

Species

SiO2A12O3CaONa2OFP 2 O 5ZnOH2OLess O for 2F

Total

% by mass

41-628-28-87-7

13-33-30-32-2

-5-6

99-8

Liquids

Species

H3PO4AlZn

% by mass

48-81-661

In the subsequent hardening phase, precipitation and hydration continue.The set cement consists, essentially, of partly-reacted glass particlesembedded in an aluminium phosphate gel. The morphology of the fillerparticles is one where a glass core is sheathed by silica gel.

The cement-forming reactions may be described as follows. On mixingthe powder and liquid, hydrogen ions from the phosphoric acid solutionattack the glass particles, which are decomposed to silicic acid (Wilson &Batchelor, 1967a; Wilson & Mesley, 1968). Al3+, Ca2+, Na+, and F~ ionsare released (Wilson & Kent, 1970a), the pH of the aqueous phase increases(Kent & Wilson, 1969) and, as infrared spectroscopy shows, H3PO4 ionizesto H2PO4 (Wilson & Mesley, 1968). An electrical imbalance results andunder the influence of an electrostatic field cations migrate into theaqueous phase where they accumulate. Most probably, aluminium andfluoride form cationic A1F2+ and A1F2 complexes, whose existence hasbeen reported by Connick & Poulsen (1957), O'Reilly (1960) and Yamazaki& Takeuchi (1967).

The most important cation is aluminium. In the absence of fluoride,aluminium is present in solution as the Al(H2O)g+ complex whichhydrolyses to form complex multinuclear species such as [A12(OH)2]4+ and[A113O4(OH)24(H2O)12]7+ (Aveston, 1965; Waters & Henty, 1977). Thereare also two kinds of phosphate complexes (Akitt, Greenwood & Lester,1971; O'Neill et al, 1982): those based on the H3PO4 ligand, A1(H3PO4)3+

244


and complexes of unknown protonation, A1(H3PO4)W, where n ^ 2; andthose where the ligand is H2PO4, A1(H2PO4)2+ and A1(H2PO4)J.

As pH increases and the ionization of H3PO4 to H2PO4 continues (Kent& Wilson, 1969), the formation of those aluminophosphate complexesbased on the H2PO4 ligand is favoured. Further increases in theconcentration of aluminium and H2PO4 give rise to the formation ofbinuclear complexes in which aluminium and phosphate are linked byoxygen bridges (Akitt, Greenwood & Lester, 1971; O'Neill et ai, 1982).Most probably, this process continues with the formation of multinuclearcomplexes and networks based on Al-O-P linkages, which leads togelation.

The presence of fluoride complicates this picture but does not change itsessence. Using 31P, 19F and XH NMR Akitt, Greenwood & Lester (1971)found numerous complexes in such solutions. There were 19F NMR peaksassociated with A1F2+ and A1F2 complexes and, if the fluoride content washigh, a peak corresponding to exchanging AlFjf"w)+, F", HF and HF2.In addition there were peaks corresponding to fluorine-containingaluminophosphate complexes which were similar to the aluminophosphatecomplexes noted above but with the addition of one, two or three fluorideligands.

The insolubilization of ions during setting and hardening was followedexperimentally by Wilson & Kent (1970b) and is shown in Figure 6.15.This figure shows the time-dependent variation of the concentration ofsoluble ions during setting and hardening. There are two competingprocesses which govern the concentration of ions: extraction from theglass and removal by precipitation. The extractive process is illustrated bythe [Na]/time curve, as Na+ is not precipitated during cement formation.Judging by the concentration of soluble sodium ions extraction is halfcomplete during mixing of the paste and over within 10 minutes, when20 % of the glass powder is decomposed.

The progress of precipitation is revealed by the concentration/timecurves for zinc and phosphate, since both these species are present initiallyin solution. There should be maxima for the soluble aluminium, calciumand fluoride which are extracted from the glass, but because of the earlyonset of precipitation these are not observed. Precipitation is accompaniedby an increase in pH; when it reaches 1-8, at which juncture 50% of bothzinc and phosphate have been precipitated, the cement paste gels (5minutes after preparation).

Hardening continues after gelation, rapidly reaching 65 % of its final

245


value within 30 minutes, and ceasing after about 72 hours. The pHcontinues to rise, reaching 5-2 after 48 hours. Precipitation of aluminiumand calcium ions appears to be complete within an hour but zinc continuesto precipitate. Sodium and fluoride ions do not completely precipitate.

- IOO

30CEMENT AGE (min)

Figure 6.15 The time-dependent variation of the concentration of soluble ions during thesetting and hardening of a dental silicate cement (Wilson & Kent, 1970b).

246


Evidence from electrical conductivity experiments (Wilson & Kent,1968) indicates that, even after hardening is apparently complete, thereaction continues for at least 7 weeks; indeed it is known from the workof Paffenbarger, Schoonover & Souder (1938) that the cement continues tostrengthen for at least a year.

The correlation of phosphate precipitation with decrease of conductivity(Wilson & Kent, 1968), increase in pH (Kent & Wilson, 1969) and hardness(Wilson et al., 1972) is shown in Figure 6.16. These results demonstrate therelationship between the development of physical properties and theunderlying chemical changes, but there are no sharp changes at the gelpoint. Evidence from infrared spectroscopy (Wilson & Mesley, 1968) andelectron probe microanalysis (Kent, Fletcher & Wilson, 1970; Wilson etal., 1972) indicates that the main reaction product is an amorphousaluminophosphate. Also formed in the matrix were fluorite (CaF2) andsodium acid phosphates.

The fate of silicic acid is of some interest. Silicic acid polymerizes, bycondensation, and finally a silica gel is formed (Wilson & Mesley, 1968).The insolubilization of silicic acid has been observed to parallel closely theprecipitation of phosphate (Wilson & Batchelor, 1967b) and is related toan increase of pH within the cement (Kent & Wilson, 1969). A lowconcentration of silicic acid must remain in the matrix. All this is in accordwith the known aqueous chemistry of silica.

Orthosilicic acid at concentrations above 100 ppm in solution condensesto a dimer. At higher concentrations cyclic and polymeric species areformed (Her, 1979; Andersson, Dent Glasser & Smith, 1982). Theseprocesses are ones where silanol groups condense to form siloxanelinkages:

—OH + HO—Si^- = -^Si—O—Si^- + H2O

According to Vysotskii et al. (1974) gelation is the result of a complexprocess, and polymerization does not necessarily ensure gelation. Theformation of polymeric particles is followed by their growth. Theseparticles are then linked by siloxane bonds to form branched chains.Networks extend throughout the liquid phase, and finally gelation occurs,provided the pH is below 7. Gelation is most rapid for pH = 5 to 6 andminimal for pH = 1-5 to 3 (Her, 1979). The observations of Wilson &Batchelor (1967b) are in accord with this theory.

The role of water is important, for it acts as a reaction medium and

247


100r

Conductivity

500 50

40

30

20

10

20 40 OOmin 4h 24hFigure 6.16 The relationship between the development of hardness and the underlyingphysicochemical process: decrease in phosphate concentration, increase in pH, and decreasein electrical conductivity (Wilson et al., 1972).

248


serves to hydrate reaction products. During setting and hardening, waterbecomes progressively bound to the matrix and in the fully hardenedcement the ratio of water of hydration (non-evaporable water) to looselybound (evaporable) water reaches 1:1 (Wilson, Paddon & Crisp, 1979).

A deficiency of water can inhibit the reaction. Kent & Wilson (1969)noted that storage conditions affected the rate of reaction as measured bythe increase in pH over 18 hours at 37 °C. When water was allowed neitherto enter nor to leave the cement the pH rose to 5-15; when cured in waterthe reaction was enhanced and the pH rose to 5-40; but when stored underdesiccating conditions the reaction was retarded and the pH only rose to4-30.

A deficiency of water in the cement liquid has the same effect and thisoccurs when the H3PO4 content exceeds 60%. Wilson & Mesley (1968)noted that in a cement formed from a solution of 65 % H3PO4 there wasevidence of incomplete reaction even after 6 hours. We have noted inSection 6.5.3 that there is a sharp decline in the rate of reaction when theorthophosphoric acid concentration exceeds 65% H3PO4 (Figure 6.14).The avidity of cements to absorb water from humid surroundings alsoincreases sharply when the phosphoric acid in the cement-forming liquidexceeds 60%. It is difficult to avoid the conclusion that these twophenomena are related and that a deficiency of water retards the cement-forming reaction.

6.5.5 Structure

Information on the microstructure and molecular composition of the setcement comes almost entirely from the optical, electron microscopic andelectron probe microscopic analysis of Kent, Fletcher & Wilson (1970),Wilson et al. (1970a, 1972) and Brune & Smith (1982). There are somedifferences which require resolution. Although under optical microscopythe set cement appears as an irregular mosaic of angular, and seeminglyunattacked, glass particles connected by an apparently structurelessmatrix, its appearance under the reflectance optical microscope is morerevealing (Figure 6.17). Here the glass particles are shown to be pitted withhemispherical craters and to vary in size from 1 to 100 urn. The matrix isseen as particulate. A stereoscan of a fractured surface reveals more detailand shows an angular glass particle which has debonded from a particulatematrix. The surface of the glass particle is spotted where selective acid

249


attack has occurred at the site of phase-separated droplets (Figure 6.18).Apart from this etching the glass particles appear unattacked, but this isnot the case.

A combination of infrared spectroscopy and electron probe micro-analysis shows that the matrix of the set cement is amorphous aluminiumphosphate (Wilson et al., 1970a). Silica gel formed appears to sheathe thepartly reacted glass particles. Two crystalline phases have been detected inthe matrix: fluorite, CaF2, and augelite, A12(OH)3PO4 (Wilson et al., 1972).However, the XRD pattern of the matrix did not quite correspond to thatof augelite in published powder diffraction data, and the possibility existsthat F replaces OH to give A12F3PO4; this would be in accordance with theformation of fluorine-containing aluminophosphate complexes notedpreviously.

The element distributions of Si, P, F, and Na within a dental silicatecement were recorded by Kent, Fletcher & Wilson (1970) and Wilson et al.(1972) who used cements from which fine particles had been removed, and

Figure 6.17 Phase contrast micrograph of a polished section of a dental silicate cement,showing angular glass particles, size 1 to 100 urn, embedded in a featureless matrix (Wilsonet al, 1972).

250


by Brune & Smith (1982) who used a normal powder. The elementdistribution maps for Si, Al and P are shown in Figure 6.19, where whitehighlights indicate the positive presence of an element. An opticalmicrograph of the area of study shows the presence of a distinct outer layeraround each reacted glass particle (Figure 6.19a). Silicon is present only inthe particles and its concentration is enhanced at the boundary (Figure6.196). This implies the existence of a layer of silica gel surrounding eachglass particle. The gel-like nature of the silica sheath can be clearly seen inFigure 6.19a where it has detached from the glass core. Phosphorus isfound substantially only in the matrix (Figure 6.19c). Aluminium is foundboth in the glass particles and in the matrix, showing that it has migrated.A prominent feature is the depletion of Al at the boundary (Figure 6A9d).Calcium, sodium and fluorine were also shown to have migrated from theglass particles to the matrix.

These findings support the view that during the reaction ions areextracted from the surface of the glass particles, migrate to the aqueousphase where they form the matrix, and leave a silica gel relict. This explainswhy the glass particles appear to be unattacked when examined under themicroscope. The presence of both Al and P in the cementing matrix and the

•

Figure 6.18 A stereoscan of a fracture surface of a dental silicate cement. The debonded glassparticle is to be identified by its pitted surface, the result of selective acid attack. Note theparticulate nature of the matrix (Wilson et al., 1972).

251


knowledge that infrared spectroscopy had identified the presence of analuminium phosphate led to the conclusion that the matrix was analuminium phosphate hydrogel (50 % of the water is bound to the matrix).

Some of these conclusions may require revision, since recent findings ofEllison & Warrens (1987) on the related glass-ionomer cement (Section5.9.6) suggest that acid attack occurs throughout the body of the glassparticle and not just at the surface layer. In that case the silica gel layer isnot a relict but a zone of gelation. This view is more in accord with ideason the decomposition of aluminosilicate glasses.

Figure 6.19 Element distribution maps for a scanning electron micrograph of a dental silicatecement: (a) optical micrograph of area of study, (b) Si distribution, (c) P distribution, (d) Aldistribution (Wilson et al, 1972).

252


A close examination of Figure 6.19&, especially the two closely adjacentparticles formed by the splitting of one particle, reveals that the Si regionsappear to extend slightly beyond the boundaries of the glass particles. Thusit is possible that the whole of the glass structure is slightly acid-degraded,although remaining essentially glassy, and that silicic acid and ions arereleased. These species migrate outwards, the ions into the aqueous phase,while the majority of the silicic acid condenses to a shell of silica gel justbeyond the particle-matrix boundary.

This alternative hypothesis may explain the observational differencesbetween different workers. Brune & Smith (1982), unlike Wilson et al(1972), found Si distributed throughout the cement but were uncertainwhether it was due to Si in the matrix or the degradation of fine particlesto silica gel. But Brune & Smith (1982) used a normal glass powder whileWilson et al. (1972) removed fine particles to improve resolution. Thesediffering observations are reconciled if the silicic acid which is formedmigrates slightly before condensing to silica gel.

SummaryOverall, the formation of dental silicate cement can be represented inoutline as in Figure 6.20.


Dental silicate cement is used for the aesthetic restoration of anterior(front) teeth because it is translucent and so can be made to colour-matchtooth enamel. It is prepared by introducing powder into the liquidgradually in order to dissipate heat, although the exotherm is not so great

Calcium aluminosilicate glass+

Phosphoric acid solution

Aluminophosphatecomplexes

Calciumfluoride

Sodiumacid phosphates

(soluble)

Silicagel

Al phosphatepolynuclear complexes- Aluminium phosphate gel

Figure 6.20 Formation of dental silicate cement.

253


Table 6.10. Properties of commercial dental silicate cements

Powder: liquid*, g cm"3



Compressivec

strength (24 h), MPaCompressivec

modulus (24 h), GPaFlexuralc

strength (24 h), MPaTensilec

strength (24 h), MPaFracture toughness0

(24 h), MN m-fSolubility &disintegration (24 h), %

Opacity, C07

Refs.

[1][2]

[1,2]

[1,2,3]

[4,5]

[2]

[6]

[7]

[1]

[1]

Value

2-70-4-023-6

3-25-7-0

68-5-255

18-0-21-0

24-5

13-6

012-0-30

0-34-3-8

0-42-0-71

Specification"limits

nn

3-8

166 minimum

n

n

n

n

1-0 maximum

0-35-0-55

aBS 3365/1: 1969 Specification for Dental Silicate Cement and DentalSilicophosphate Cement. Part 1 Dental Silicate Cement.6 For a consistency spread of 25 mm diameter for 0-5 cm3 of cement paste undera load of 1-5 Kgf applied after 2 minutes at 23 °C.c After storage for 24 hours in water at 37 °C.n no specification test.[1] Wilson et aL, (1972); [2] 0ilo (1988); [3] Wilson (1975c); [4] Paddon &Wilson (1976); [5] Wilson, Paddon & Crisp (1979); [6] Kent & Wilson (1971);[7] Lloyd & Mitchell (1984).

as that of zinc phosphate cement (Crisp, Jennings & Wilson, 1978). Thecement is mixed very thickly and the powder/liquid ratio can be as high as4 g cm"3 for good examples of this cement (Wilson et aL, 1972). At thesethick consistencies the working time is good; an isolated observationindicates that it is 3-6 minutes at 23 °C (0ilo, 1988). Setting time (37 °C)varies from 3-25 to 7-0 minutes (Wilson et aL, 1972; 0ilo, 1988). Propertiesare summarized in Table 6.10.

254


The dental silicate cements have brittle characteristics which areimmediately evident after set (Paddon & Wilson, 1976). The best examplesdevelop a compressive strength of about 250 MPa after 24 hours (Wilsonet al., 1972; Wilson, 1975c; 0ilo, 1988), which is higher than that recordedfor any other acid-base cement, including the glass polyalkenoate cement(Table 6.10). Strength continues to increase for at least a year (Paffen-barger, Sweeney & Isaacs, 1933). Compressive modulus is 18 to 21 GPaafter 24 hours (Paddon & Wilson, 1976; Wilson, Paddon & Crisp, 1979).Modulus changes with time and for one example increased from 18-0 GPaafter 24 hours to 30-6 GPa after 30 days.

Little information is available on other tests of strength. Isolatedmeasurements give a flexural strength of 24-5 MPa (0ilo, 1988) and atensile strength of 13-6 MPa (Kent & Wilson, 1971). These values lie withinthe range of those recorded for glass polyalkenoate cement. Translucencyis easily achieved as values for the inverse property of opacity show (Table6.10).

Most of the properties of a dental silicate cement are affected bypreparative variables, particularly the powder/liquid ratio (Jorgensen,1963; Wilson & Batchelor, 1967b). Increase in the powder/liquid ratioaccelerates set and increases strength and resistance to erosion (Figure6.21). Temperature and, to a lesser extent, humidity during mixing havesome effect, but chiefly they affect setting time.

Although these cements have high compressive strength, their lowflexural and tensile strengths coupled with brittleness and lack of toughnessmakes them suitable only for low-stress anterior (front teeth) restorations.

6.5.7 Dissolution and ion release

The dissolution and ion release from dental silicate cement have been themost investigated characteristics; with good reason, for they are central toits clinical performance. Erosion limits its life but release of fluoride hasimportant clinical consequences.

Ion releaseIn neutral solution when fully hardened, dental silicate cements areresistant to aqueous attack. Before they have fully hardened, set cementscontain soluble reaction intermediates - soluble sodium salts, acid phos-phates and fluorides - which render them vulnerable to attack even byneutral solutions including saliva (Wilson, 1976).

255


Irretrievable loss of matrix-forming cations and anions can result inpermanent damage to the cement surface. This is visible as milky or chalkypatches or even raised blisters. For this reason it is customary to protect,temporarily, the freshly placed cement by varnish. Once hardened, attackby neutral solutions causes failure only when a cement has been poorlyformulated and contains excessive amounts of soluble reaction products.In this case osmotic effects can cause blistering or even disintegrationunder the action of internal forces, as Figure 6.22 illustrates (Wilson &Batchelor, 1967a).

The composition of the leachates does not correspond to the com-position of the cement at all (Wilson & Batchelor, 1967a,b). Thepredominant species eluted are the soluble sodium salts of phosphate andfluorides, although sodium is only a minor constituent of the cement. Forone example of cement examined, the leachate contained 0-28 % sodiumand 0-20 % phosphate (expressed as a percentage of the amount of thespecies contained in the cement). For the major constituents of the glass thefigures were 0-07 % fluoride, 0-02 % A12O3, 0-01 % SiO2 and 0-003 % CaO.

3.5 4.0Powder/Liquid Ratio (g/ml)

Figure 6.21 The effect of powder/liquid ratio on setting time and compressive strength of adental silicate cement (Wilson & Batchelor, 1967b).

256


The rate of elution declines sharply with time and the pattern of elutionchanges. The acid phosphate ions, HgPO^ and HPO|~, are removed byfurther reaction or by elution, and the release of phosphate changes frompredominant to minor. Thereafter, the rate of loss of phosphate isgoverned by the phosphate concentration of the solution; indeed if thephosphate concentration of the solution is sufficiently high the process isreversed and the cement takes up phosphate. So, clearly, an ion exchangephenomenon is involved (Kuhn & Wilson, 1985; Kuhn, Winter & Tan,1982).

Elution of ions is accompanied by absorption of water and this canamount to as much as 20% by mass in five days (Kuhn et al., 1982). Theextent of water uptake is affected by the ionic concentration of the solution.

Fluoride releaseAs the cements age, sodium, fluoride and silica become the major specieseluted, although the amounts involved are small. Fluoride is released in asustained fashion over a prolonged period (Wilson & Batchelor, 1967a; de

Figure 6.22 The effect of water on a poor example of a dental silicate cement. An osmoticforce causes blistering and disintegration (Wilson & Batchelor, 1967a).

257


Freitas, 1968; Cranfield, Kuhn & Winter, 1982; Kuhn & Jones, 1982). Thisis a biologically advantageous property. The release of fluoride is governedby the following expression:

Rate of release of fluoride = Af~* + B

The A term relates to a diffusion-controlled process and the B zero-orderterm to an erosive process.

Kuhn & Jones (1982) examined various models for fluoride release andshowed that release did not fit the membrane and homogenous monolithmodel. Instead, they concluded that the cement behaved as a porousgranular monolith, as described by Kydonieus (1980). The release offluoride appears to be an ion exchange phenomenon, as dental silicatecement takes up rather than releases fluoride from solution if it is presentin sufficient concentration (Kuhn, Lesan & Setchell, 1983).

Fluoride release is biologically important. Since the early 1940s, fluoridehas been known to inhibit dental decay (Dean, Arnold & Elvove, 1942),but the effect is not fully understood and several mechanisms have beensuggested (Levine, 1976). Tooth enamel, dentine and bone all possess amineral phase that has a hydroxyapatite-like substance which takes upfluoride by replacement of the hydroxyl group (Hallsworth & Weatherall,1969). In fact fluoride released by dental silicate cement is taken up byadjacent tooth enamel (Halse & Hals, 1976). This apparently increases theresistance of enamel to dissolution (McLundie & Murray, 1972) and, if theacidogenic theory of caries is accepted (Levine, 1976) must have acariostatic effect. Maldonado, Swartz & Phillips (1978) have found that thesolubility of enamel in acid was reduced by 39 % when it was in contactwith a dental silicate cement. Moreover, the surface energy of fluorapatiteis lower than that of hydroxyapatite (Glanz, 1969) making the adhesion ofunwanted cariogenic substance, such as dental plaque, more difficult(Rolla, 1977). For these reasons secondary caries is rarely observed underdental silicate cement restorations (Bock, 1971; Hals, 1975) and in thisrespect the cement is superior to composite resins and dental amalgams(Updegraff, Change & Joos, 1971).

Aluminium ions released from the dental silicate cement are alsoabsorbed by hydroxyapatite and have a similar beneficial effect to that offluoride (Halse & Hals, 1976; Putt & Kleber, 1985). Thus, the dentalsilicate cement confers protection against caries (dental decay) onsurrounding tooth material.

258


Acid erosionDental silicate cement has always been regarded as suspect in service andhas a variable life-span (Paffenbarger, 1940; Wilson, 1969). It was earlysuspected that this was associated with a susceptibility to acid attack(Kulka, 1907; Rawitzer, 1909; Voelker, 1916b; Poetschke, 1916), a viewwhich has been confirmed by subsequent in vitro and in vivo work(Norman, Swartz & Phillips, 1957, 1959; Jorgensen, 1963; Wilson &Batchelor, 1968; Norman et al., 1969). Thus, although the cement is stablein neutral media, such as normal saliva, it becomes progressively moreeroded under acid conditions (Wilson & Batchelor, 1968). This effect isshown in Figure 6.23. Such conditions occur in stagnation regions of themouth. In these regions, dental plaque containing streptococci and

Figure 6.23 The effect of pH on the removal of ions from a dental silicate cement (Wilson &Batchelor, 1968).

259


lactobacilli degrades sugars, including plaque polysaccharides, to lacticacid (Jenkins, 1965) and pH values as low as 4-0 have been recorded(Stephan, 1940; Kleinburg, 1961). In fact preferential breakdown of dentalsilicate cement has been observed in these regions (Henschel, 1949;McLean & Short, 1969).

Tay et al. (1974, 1979) have studied the mechanism of erosion of thedental silicate cement in service, finding that grooving occurs at the marginbetween the restoration and the tooth. Erosion exposes the cavity andprovides sites for the accumulation of food debris and bacteria which cancause inflammation of the gingiva (Larato, 1971). It also leads to stainingof the restoration (Bock, 1971; Kent, Lewis & Wilson, 1973).

The extent of acid erosion depends on the nature of the acid; acids withstrong complexing function, such as citric acid, are particularly erosive(Wilson & Batchelor, 1968; Stralfors & Eriksson, 1969). These acids arefound in citrus drinks.

Despite the failing of the dental silicate cement under acid conditions itis more resistant to acid attack than all other dental cements with thenotable exception of the glass polyalkenoate cement (Norman, Swartz &Phillips, 1959; Walls, McCabe & Murray, 1985; Beech & Bandyopadhyay,1983; Kuhn, Setchell & Teo, 1984; Wilson et al., 1986a). These studieshave been confirmed by in vivo observations (Norman et al., 1969). Aclinical study carried out by Robinson (1971) over many years showed thatwhen carefully prepared and placed, the dental silicate cement was capableof giving good performance. Many of the failures of this material must beattributed to faulty preparation.

6.5.8 Biological aspects

The adverse pathological effects of dental silicate cement have been knownsince Kulka (1911a,b). Since then many workers have observed that thiscement causes significant pulpal inflammation (McComb, 1982).

Manley (1936, 1943) reported that major histological changes occurredin the pulp 24 hours after placing a silicate restoration, a finding confirmedby other workers (Zander, 1946; Brannstrom & Nyborg, 1960; Stanley,Swerdlow & Buonocore, 1967; Qvist, 1975). The silicate cement alsoinflames the gingiva (gum tissues) (Larato, 1971; Trivedi & Talim, 1973)and demineralizes both dentine and enamel (Grieve, 1974).

At one time, irritation of the pulp was entirely attributed to the acidity

260


of the cement which is most marked when it is freshly mixed (Manley,1936; Harvey, Le Brocq & Rakowski, 1944; Roydhouse, 1961; Svare &Meyer, 1965; Matsui et al., 1967). This theory is supported by observationsthat tissue reactions become less marked with time (Svare & Meyer, 1965).Moreover, it was observed that phosphoric acid penetrated dentine to aconsiderable depth (Svare & Meyer, 1965; Swartz et al., 1968).

Despite this evidence, some workers have doubted this theory(Roydhouse, 1961; Antonioli, 1969; Johnson et al., 1970). In particular,Brannstrom and coworkers have strongly advocated an alternative theory,that bacterial contamination causes pulpal damage (Brannstrom &Nyborg, 1971; Bergvall & Brannstrom, 1971; Brannstrom & Vojinovic,1976). They considered that virtually all the damage to pulp under a silicaterestoration was caused by bacterial infestation. This idea has been amplyconfirmed by subsequent observations (Qvist, 1975; Brannstrom &Nyborg, 1971;Mjor, 1977).

Watts (1979), while agreeing that bacterial contamination plays animportant role in causing irritation to tissues, showed that a silicate cementeven under germ-free conditions produced tissue damage. Of course, theacidic dental silicate cement does not possess the antiseptic action of thealkaline cements.

Cell culture tests show that dental silicate cement is strongly cytotoxic- that is it severely damages cells - even after set (Spangberg et al., 1973).This effect has been attributed to the hydrogen and fluoride ions present(Helgeland & Leirskar, 1972, 1973; Tyas, 1979).

Another biological disadvantage is that dental silicate cement does notbond to tooth material, and harmful substances and bacteria can percolatebetween it and the tooth, giving rise to secondary caries and pulpalirritation (Going, Massler & Dute, 1960). These effects are magnified whendissolution of the cement occurs.

One advantageous biological property possessed by dental silicatecement is the sustained release of fluoride; this has been discussed inSection 6.5.7.

6.5.9 Conclusions

Dental silicate cement is solely used for restoring anterior (front) teeth. Itis probably the strongest purely inorganic cement and develops its strengthrapidly. Although satisfactory in areas of the mouth washed by saliva it is

261


not quite up to the demands of more severe oral conditions. Its greatadvantage is that it acts as a fluoride release agent and protects adjacenttooth enamel.

6.5.10 Modified materials

A number of innovations made in the 1920s and 1930s may be noted.Several attempts were made to reduce the dissolution of these cements inoral fluids and their adverse effect on the pulp by inclusion of oils andgreases (Simon, 1929, 1932; Eberly, 1934). None have been consideredbeneficial (Paffenbarger, Schoonover & Souder, 1938), a not surprisingresult because the inclusion of hydrophobic substances is bound tointerfere in the setting of an aqueous cement.

Poetschke (1925) patented a dental silicate powder prepared by fusingzinc silicate with calcium fluoride. This is a kind of silicophosphate cement(Section 6.6). Thomsen (1931) attempted to formulate a water-settingdental cement. Heynemann (1931) included lithium salts in the flux andBrill (1935) included them in the liquid.

During this period a number of attempts at reinforcing these cementswere made. Fillers described include carborundum (Salzmann, 1930),cellulose fibres (Schonbeck & Czapp, 1936) and even diamonds (Salzmann,1930). None of these innovations found their way into commercialmaterials (Paffenbarger, Schoonover & Souder, 1938).

More recently, Stanicioiu, Chinta & Hartner (1959) attempted toreinforce the cement with glass fibres, but this was not successful. The mostserious study on the reinforcement of dental silicate cement was made byJ. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibresand alumina powder were introduced into the cement mix. Unfortunately,the glass powder/liquid ratio had to be reduced, and the strength gained byreinforcement was thereby lost. It is clear that dental silicate cement cannotbe strengthened by fibre or particulate reinforcement.

Systematic attempts to formulate improved materials have met with nosuccess (Manly et al., 1951; Rockett, 1968). The last and, in some ways,most promising attempt at improving the dental silicate cement was madeby Pendry (Pendry & Cook, 1972; Pendry, 1973) who improved itsresistance to acid by adding indium to both powder (5*8 %) and liquid(5-65 %). The cement, however, lacked sufficient translucency, and by thistime the glass-ionomer cement had arrived with its advantages oftranslucency and resistance to staining and acid attack.

262

Silicophosphate cement

Table 6.11. Specification properties of commercial luting silicophosphatecements {Anderson & Pajfenbarger, 1962)

Powder: liquid,6 g cm 3


Film thickness(2 min), urn

Compressivec


Value

2-00-2-706-14

42-53

101-171

0-7-2-3


n

50

135 minimum

1-0 maximum

aBS 3365/2: 1971 Specification for Dental Silicate Cement and DentalSilicophosphate Cement. Part 2. Dental Silicophosphate Cement.b For a consistency spread of 25 mm diameter for 0-5 cm3 of cement paste undera load of 220 gf applied after 2 minutes at 23 °C.c After storage for 24 hours in water at 37 °C.n no specification test.

6.6 Silicophosphate cement

The silicophosphate cement has always been a minor and somewhatobscure material. There have been no investigations into its settingreaction and structure. Its chief uses are as a cementing agent and as atemporary posterior filling material in dentistry. It can be regarded as ahybrid of the dental silicate and zinc phosphate cements since the powderis a physical mixture of an aluminosilicate glass and zinc oxide, the amountof zinc oxide varying from 9-3 to 17*9% (Wilson, Crisp & Lewis,1982).

The silicophosphate cement originated with the dental silicate cement,for there is no doubt that early investigators experimented with mixtures ofaluminosilicate glass and zinc oxide (Fletcher, 1878,1879; Eberly, 1928). Itappears to have no particular advantages. As is often the case with hybrids,it can combine the worst features as well as the best of the parents, andoften properties have intermediate values. Nevertheless, it continues tohave a small but persistent usage arising from its one advantage over the

263


Table 6.12. Specification properties of commercial filling silicophosphatecements {Wilson, 1975c)

Powder: liquid,6 g cm 3


Compressivec


Value

3-3-4-13-5-5-75

179-209

0-3-0-7


n3-6

170 minimum

0-7 maximum

aBS 3365/2: 1971 Specification for Dental Silicate Cement and DentalSilicophosphate Cement. Part 2. Dental Silicophosphate Cement.b For a consistency spread of 23 mm diameter for 0-5 cm3 of cement paste undera load of 1-5 Kgf applied after 2 minutes at 23 °C.c After storage for 24 hours in water at 37 °C.n no specification test.

zinc phosphate cement: it releases fluoride and in certain clinical situationsthis protection against dental caries is invaluable.

The flow properties are not as good as those of zinc phosphate cement(Eames et ai, 1978; Hembree, George & Hembree, 1978) and filmthickness is greater (Table 6.11). Moreover, it does not have thetranslucency of dental silicate cement (Wilson, 1975c).

The strength of silicophosphate cement lies between that of dentalsilicate cement and zinc phosphate cement. Anderson & Paffenbarger(1962) have reported properties of luting cements (Table 6.11) and Wilson(1975c) those of filling materials (Table 6.12). Cameron, Charbeneau, &Craig (1963) have confirmed these results. Housten & Miller (1968)reported the properties for silicophosphate cements used for cementingorthodontic brackets, where the consistency of the mix, and consequentlycement properties, lie between those of luting agents and filling materials.

Silicophosphate cement acts as an agent for the sustained release offluoride, although different cements behave very differently (Wilson, Crisp& Lewis, 1982). Silicophosphate cement has a durability in the mouthsimilar to that of dental silicate cement. It is less resistant to oral fluids thanglass polyalkenoate cement, but more resistant than all other dentalcements, as is shown by both in vivo studies (Norman et aL, 1969; Ritcher& Ueno, 1975; Clark, Phillips & Norman, 1977; Mitchem & Gronas, 1978;

264

References

Osborne et al., 1978) and the laboratory impinging jet method (Beech &Bandyopadhyay, 1983; Wilson et ai, 1986a).

It is superior to the zinc phosphate cement for bonding orthodonticbands to teeth (Clark, Phillips & Norman, 1977). It has greater durabilityand there is less decalcification in adjacent tooth enamel. This latterbeneficial effect must arise from the release of fluoride which is absorbed bythe enamel, so protecting it in a clinical situation where caries-producingdebris and plaque accumulate.

6.7 Mineral phosphate cements

Semler (1976) reported cements formed by reacting ground wollastonite(CaSiO3) with phosphoric acid solution containing aluminium and zinc.The setting times of these cements varied from 4 to 60 minutes. Best resultswere obtained using a liquid of composition 69% H3PO4, 6-0% Zn,2-0 % Al with a selected grade of wollastonite. The cement set in 4 minutesand developed a compressive strength of 73 MPa in 24 hours comparedwith a value of 20 MPa obtained with a fast-setting Portland cement. Thematrix of the wollastonite phosphate cement was observed to containsilica, calcium and phosphate.

Serpentinite, Mg6Si4O10(OH)8, phosphate cements have been reportedby Ter-Grigorian et al (1982, 1984) and Zenaishvili, Bakradze & Chelidze(1984). The reaction products are MgHPO4.3H2O and Mg(H2PO4)2,which are transformed on heating to Mg2P2O7 and Mg3(PO4). Setting time,strength and resistance to aqueous attack are all affected by theconcentration of phosphate used to prepare the cements. These materialsshould be compared with the magnesium phosphate cements described inSection 6.4.

Naturally occurring phosphate cements are also known (Krajewski,1984).

ReferencesAbdelrazig, B. E. I. & Sharp, J. H. (1985). A discussion of the papers on

magnesia-phosphate cement by T. Sugama and L. E. Kukacka. Cement &Concrete Research, 15, 921-2.

Abdelrazig, B. E. I. & Sharp, J. H. (1988). Phase changes on heating ammoniummagnesium phosphate hydrates. Thermochimica Acta, 129, 197-215.

Abdelrazig, B. E. L, Sharp, J. H. & El-Jazairi, B. (1988). The chemicalcomposition of mortars made from magnesia-phosphate cement. Cement &Concrete Research, 18, 415-25.

265


Abdelrazig, B. E. L, Sharp, J. H. & El-Jazairi, B. (1989). Microstructure andmechanical properties of mortars made from magnesia-phosphate cement.Cement & Concrete Research, 19, 247-58.

Abdelrazig, B. E. I., Sharp, J. H., Siddy, P. A. & El-Jazairi, B. (1984). Chemicalreactions in magnesia-phosphate cements. Proceedings of the British CeramicSociety, 35, 141-54.

Abraham. (1913). Pulpentod unter Silikatfullungen. Deutsche Monatsschrift fiirZahnheilkunde, 33, 532-6.

Abramovich, A., Macchi, R. L. & Ribas, L. M. (1976). Enamel corrosionproduced by zinc phosphate dental cement. Journal of Dental Research, 55,107-10.

Akitt, J. W., Greenwood, N. N. & Lester, G. D. (1971). Nuclear magneticresonance and Raman studies of aluminium complexes formed in aqueoussolutions of aluminium salts containing phosphoric acids and fluoride ions.Journal of the Chemical Society, A, 2450-7.

Allen, W. M., Sansom, B. F., Wilson, A. D., Prosser, H. J. & Groffman, D. M.(1984). Release cements. British Patent GB 2, 123, 693 B.

Ames, W. B. (1892). Discussion - American Dental Association. Dental Cosmos,34, 926-7.

Ames, W. B. (1893). Oxyphosphates. Dental Cosmos, 35, 869-75.Anderson, J. N. & Paffenbarger, G. C. (1962). Properties of silicophosphate

cements. Dental Progress, 2, 72-5.Andersson, K. R., Dent Glasser, L. S. & Smith, D. (1982). Polymerization and

colloid formation in silicate solutions. In Falcone, J. G. (ed.) Soluble Silicates.ACS Symposium Series No. 194, Chapter 8. Washington, DC: AmericanChemical Society.

Andrews, J. T. & Hembree, J. H. (1976). In vivo evaluation of the marginalleakage of four inlay cements. Journal of Prosthetic Dentistry, 35, 532-7.

Ando, J., Shinada, T. & Hiraoka, G. (1974). Reactions of monoaluminiumphosphate with alumina and magnesia. Yogyo-Kyokai-Shi, 82, 644-9.

Antonioli, C. A. (1969). Etude sur la penetration des ions H+ de differentsciments de scellement a travers la dentine. Schweizische Monatsschrift furZahnheilkunde, 79, 533-7.

Anzai, M., Hirose, H., Kikuchi, H., Goto, J., Azuma, F. & Higasaki, S. (1977).Studies on soluble elements and solubility of dental cements. I. Solubilities ofzinc phosphate cement, carboxylate cement and silicate cement in distilledwater. Journal of the Nihon University School of Dentistry, 19, 26-39.

Arato, A. (1974). Physikalische Eigenschaften von verschiedenen Zementen furkieferothopadische Zwecke. Schweizische Monatsschrift fiir Zahnheilkunde,54, 535-58.

Asch, W. & Asch, D. (1913). Silicates in Industry and Commerce, pp. 199-336.London: Constable.

Aveston, J. (1965). The hydrolysis of the aluminium ion: ultracentrifugal andacidity measurements. Journal of the Chemical Society, 4438-43.

Axelsson, B. (1964). Kemisk analys av dentala silikatcement och deras losligakomponenter. Odontologisk Revy, 15, 150-68.

266

References

Axelsson, B. (1965). Kemisk analys av dentala zinkfosfatcement. OdontologiskRevy, 16, 126-30.

Babin, J. B., Hurst, R. V. V. & Feary, T. (1978). Antibacterial activity of dentalcements. Journal of Dental Research, 37, Special Issue B, Abstract No. 214.

Beech, D. R. & Bandyopadhyay, S. (1983). A new laboratory method forevaluating the relative solubility and erosion of dental cements. Journal ofOral Rehabilitation, 10, 57-63.

Belopolsky, A. P., Shpunt, S. Ya. & Shulgina, M. N. (1950). Physicochemicalresearches in the field of magnesium phosphates (the system MgO-P2O5-H2Oat 80 °C). Journal of Applied Science (USSR), 23, 873-84.

Bergvall, O. & Brannstrom, M. (1971). Measurement of the space betweencomposite resin fillings and cavity walls. Swedish Dental Journal, 64,217-26.

Bjerrum, N. & Dahm, C. R. (1931). Studies on aluminium phosphate. I.Complex formation in acid solution. Zeitschrift fiir physikalische Chemie,Bodenstein Festband 627-37 {Chemical Abstracts, 26, 666).

Bock, B. (1971). Klinische Nachuntersuchungen von Silikatzementfullungen.Deutsche Zahndrtzliche Zeitschrift, 26, 665-71.

Bowen, R. L. (1962). Dental filling material comprising vinyl silane treated fusedsilica and a binder consisting of a reaction product of bisphenol and glycidylacrylate. US Patent 3,066,112.

Brannstrom, M. & Astrdm, A. (1972). The hydrodynamics of the dentine: itspossible relationship to dentinal pain. International Dental Journal, 22,219-27.

Brannstrom, M. & Nyborg, H. (1960). Dentinal and pulpal responses.Odontologisk Revy, 1, 37-50.

Brannstrom, M. & Nyborg, H. (1971). The presence of bacteria in cavities filledwith silicate cement and composite resin materials. Swedish Dental Journal,64, 149-55.

Brannstrom, M. & Nyborg, H. (1974). Bacterial growth and pulpal changes andinlays cemented with zinc phosphate cement and epoxylite CBA 9080. Journalof Prosthetic Dentistry, 31, 556-65.

Brannstrom, M. & Vojinovic, O. (1976). Responses of the dental pulp toinvasion of bacteria around three filling materials. Journal of Dentistry forChildren, 43, 83-9.

Brill, E. (1935). Improvements in or relating to dental cement. British Patent430,624.

Brune, D. & Smith, D. (1982). Micro structure and strength properties of silicateand glass-ionomer cements. Acta Odontologica Scandinavica, 40, 389-96.

Cameron, J. C, Charbeneau, G. T. & Craig, R. G. (1963). Some properties ofdental cements of specific importance in the cementation of orthodonticbands. Angle Orthodontics, 33, 233-45.

Cartz, L., Servais, G. E. & Rossi, F. (1972). Surface structure of zinc phosphatedental cements. Journal of Dental Research, 51, 1668-71.

Cassidy, J. E. (1977). Phosphate bonding then and now. American CeramicSociety Bulletin, 56, 640-3.

267


Charbeneau, G. T. (1961). Influence of specific surface on some properties ofsilicate cement. Journal of Dental Research, 40, 763.

Clark, R. J., Phillips, R. W. & Norman, R. D. (1977). An evaluation ofsilicophosphate cement. American Journal of Orthodontics, 71, 190-6.

Connick, R. E. & Poulsen, R. E. (1957). Nuclear magnetic resonance studies ofaluminium fluoride complexes. Journal of the American Chemical Society, 79,5153-7.

Cranfield, M., Kuhn, A. T. & Winter, G. B. (1982). Factors relating to the rateof fluoride-ion release from glass-ionomer cement. Journal of Dentistry, 10,333-41.

Crepaz, E. (1951). Constitution and reactivity against phosphoric acid of dentalcements. Chimica e Industria (Milan), 33, 137—40 (Chemical Abstracts, 45:63211).


Crisp, S., O'Neill, I. K., Prosser, H. J., Stuart, B. & Wilson, A. D. (1978).Infrared spectroscopic studies on the development of crystallinity indental zinc phosphate cements. Journal of Dental Research, 57,245-54.

Crowell, W. S. (1929). Physical chemistry of dental cements. Journal of theAmerican Dental Association, 14, 1030-48.

van Dalen, E. (1933). Orienteerende onderzoekingen over tandcementen. Thesis.Delft.

Darvell, B. W. (1984). Aspects of the chemistry of zinc phosphate cements.Australian Dental Journal, 29, 2M-A.

Dean, H. T., Arnold, F. A. & Elvove, E. (1942). Domestic water and dentalcaries. V. Public Health Reports (USA), 57, 1155-81.

Dimashkieh, M. R., Davies, E. H. & von Fraunhofer, J. A. (1974). Ameasurement of the cement film thickness beneath full crown restorations.British Dental Journal, 137, 281-4.

Dobrowsky, A. (1942). Uber die Herstellung eines neuen Zinkphosphatzementeshochster Volumkonstanz. Die Chemische Technik, 15, 159-61.

Docking, A. R., Donnison, J. A., Newbury, C. R. & Storey, E. (1953). Theeffect of orthodontic cement on tooth enamel. Australian Journal of Dentistry,57, 139^9.

Dollimore, D. & Spooner, P. (1971). Sintering studies on zinc oxide.Transactions of the Faraday Society, 67, 2750-9.

Dreschfeld, H. T. (1907). Chemistry of translucent cements. British DentalJournal, 28, 1061-9.

Eames, W. B., Monroe, S. D., Roan, J. D. & O'Neal, S. J. (1977). Proportioningand mixing of cements: a comparison of working times. Operative Dentistry,2, 97-104.

Eames, W. B., O'Neal, S. J., Monteiro, J., Miller, C, Roan, R. D. & Cohen,K. S. (1978). Techniques to improve the seating of castings. Journal of theAmerican Dental Association, 96, 432-7.

268

References

Earnshaw, R. (1960a). Investments for casting cobalt-chromium alloys. Part I.British Dental Journal 108, 389-96.

Earnshaw, R. (1960b). Investments for casting cobalt-chromium alloys. Part II.British Dental Journal 108, 429-40.

Eberly, J. A. (1934). Development of silicate cement tending to eliminate pulpirritation. Dental Cosmos, 76, 419-24.

Eberly, N. E. (1928). Dental Cement, US Patent 1,671,104.Eberly, N. E., Gross, C. V. & Crowell, W. S. (1920). The system zinc oxide,

phosphorus and water at 25° and 37°. Journal of the American ChemicalSociety, 42, 1433-9.

Eichner, K., Lautenschlager, E. P. & von Radnoth, M. (1968). Investigationconcerning the solubility of dental cements. Journal of Dental Research, 47,280-5.

El-Jazairi, B. (1982). Rapid repair of concrete pavings. Concrete, 12-15.Ellison, S. & Warrens, C. (1987). Solid-state nmr study of aluminosilicate

glasses and derived dental cements. Report of the Laboratory of theGovernment Chemist.

Elmore, K. L., Mason, C. M. & Christensen, J. H. (1946). Activity oforthophosphoric acid in aqueous solutions at 25 °C from vapour pressuremeasurements. Journal of the American Chemical Society, 68, 2528-32.

Eubank, W. R. (1951). Calcination studies of magnesium oxides. Journal of theAmerican Ceramic Society, 34, 225-9.

Evans, G. (1892). Discussion - American Dental Association. Dental Cosmos,34, 927.

Finch, T. & Sharp, J. H. (1989). Chemical reactions between magnesia andaluminium orthophosphate to form magnesia-phosphate cements. Journal ofMaterials Science, 24, 4379-86.

Fleck, H. (1902). The chemistry of oxyphosphates. Dental Items of Interest,906-35.

Fletcher, T. (1878). Compound for filling decayed teeth etc. British Patent 3028.Fletcher, T. (1879). New filling. British Journal of Dental Science, 22, 74.Frankel, E. (1913). Uber Silikatzement und Pulpatod. Deutsche Monatsschrift

fur Zahnheilkunde, 33, 529-32.de Freitas, J. R. (1973). The long term solubility of a stannous fluoride-zinc

phosphate cement. Australian Dental Journal, 18, 167-73.Gaylord, E. S. (1889). Oxyphosphates of zinc. Archives of Dentistry, 33, 364^80.Genge, J. A. R., Holroyd, A., Salmon, J. E. & Wall, J. G. L. (1955). Phosphoric

acid as a complexing agent in ion-exchange chromatography. Chemistry inIndustry, 357-8.

Genge, J. A. R. & Salmon, J. E. (1959). Aluminium phosphates. Part III.Complex formation between tervalent metals and orthophosphoric acid.Journal of the Chemical Society, 1459-63.

Glanz, P.-O. (1969). On wettability and adhesiveness. Odontologisk Revy, 20,Supplement 20.

Glasson, P. R. (1963). Reactivity of lime and related oxides. VIII. Production ofactivated lime and magnesia. Journal of Applied Chemistry, 13, 111-19.

269


Going, R. E., Massler, M. & Dute, H. L. (1960). Marginal penetration of dentalrestorations as studied by crystal violet dye and 131I. Journal of the AmericanDental Association, 61, 285-300.

Greve, H. Chr. (1913). Theoretische und practische Studien iiber Zahnzemente.Deutsche Monatsschrift fur Zahnheilkunde, 33, 346-58.

Grieve, A. R. (1974). The effect of silicate cement on enamel and dentine.Scandinavian Journal of Dental Research, 82, 510-16.

Griffith, J. R. & Cannon, R. W. S. (1974). Cementation - materials andtechniques. Australian Dental Journal, 19, 93-9.

Gulabivala, K., Setchell, D. J. & Davies, E. H. (1987). An application of the jettest method for the evaluation of disintegration of dental luting cements inmarginal gaps analogous to those of crowns and bridges. Clinical Materials,2, 221-30.

Gursin, A. V. (1965). A study of the effect of stannous fluoride incorporated ina dental cement. Journal of Oral Therapeutics and Pharmacology, 1, 630-6.

Halla, F. & Kutzeilnigg, A. (1933). Zur Kenntnis des Zinkphosphatzements.Zeitschrift fur Stomatologie, 31, 177-81.

Hallsworth, A. S. & Weatherall, J. A. (1969). The microdistribution, uptake andloss of fluoride in human enamel. Caries Research, 3, 109-18.

Hals, E. (1975). Histology of natural secondary caries associated with silicatecement restorations. Archives of Oral Biology, 20, 291-6.

Halse, A. & Hals, E. (1976). Electron probe microanalysis of secondary cariouslesions adjacent to silicate fillings. Calciferous Tissues Research, 21, 183-93.

Hannah, C. M. & Smith, D. C. (1971). Tensile strengths of selected dentalrestorative materials. Journal of Prosthetic Dentistry, 26, 314-23.

Harper, F. C. (1967). Effect of calcination temperature on the properties ofmagnesium oxides for use in magnesium oxychloride cements. Journal ofApplied Chemistry, 17, 5-10.

Harvey, W., Le Brocq, L. F. & Rakowski, L. (1944). The acidity of dentalcements. British Dental Journal, 11, 61-9.

Helgeland, K. & Leirskar, J. (1972). A further testing of the effect of dentalmaterials on growth and adhesion of animal cells in vitro. ScandinavianJournal of Dental Research, 80, 206-12.

Helgeland, K. & Leirskar, J. (1973). Silicate cement in a cell culture system.Scandinavian Journal of Dental Research, 81, 251-9.

Hembree, J. H., George, T. A. & Hembree, M. E. (1978). Film thickness ofcements beneath complete crowns. Journal of Prosthetic Dentistry, 39, 533-5.

Henschel, C. J. (1943). The effect of mixing surface temperatures on dentalcementation. Journal of the American Dental Association, 30, 1582-9.

Henschel, C. J. (1949). Observations concerning the in vivo disintegration ofsilicate cement restorations. Journal of Dental Research, 28, 528.

Heynemann, H. (1931). Verfahren zur Herstellung von Zahncementen. GermanPatent 520,138.

Higashi, S., Nakazawa, S., Yamanaka, A., Jyoshin, K. & Yamaga, R. (1963).On the properties of the hydraulic zinc phosphate cement during and after thesetting process. Journal of the Osaka University Dental School, 3, 77-88.

270

References

Higashi, S., Morimoto, K., Satomura, A., Horie, K., Anzai, M., Kimura, K.,Takeda, K. & Miyazima, T. (1969a). Studies on water-settable cements. Part9. Examination of the physical properties of water-settable tertiary zincphosphate. Journal of the Nikon University School of Dentistry, 11, 60-4.

Higashi, S., Morimoto, K., Satomura, A., Horie, K., Anzai, M., Oitate, M.,Hirata, Y. & Haga, H. (1969b). Studies on water-settable cements. Part 10.Manipulation of the newly made A type zinc phosphate cement and itscompressive strength. Journal of the Nihon University School of Dentistry, 11,105-8.

Higashi, S., Yamaga, R., Satomura, A., Anzai, M., Nishijima, K., Kubo, M. &Kawakami, R. (1972). Studies on water-settable cements. Part 11. Preparatoryexperiments on water-settable phosphate cement and properties of water-settable primary calcium phosphate. Journal of the Nihon University School ofDentistry, 14, 58-63.

Hinkins, J. E. & Acree, S. F. (1901). The disintegration of cement fillings. DentalCosmos, 43, 581-97.

Hoard, R. J., Caputo, A. A., Contino, R. M. & Koenig, M. E. (1978).Intracoronal pressure during crown cementation. Journal of ProstheticDentistry, 40, 520-30.

Holroyd, A. & Salmon, J. E. (1956). Ion-exchange studies of phosphates. Part I.Ion-exchange and pH-titration for detection of complex formation. Journal ofthe Chemical Society 269-72.

Housten, W. J. B. & Miller, M. W. (1968). Cements for orthodontic use. TheDental Practitioner, 19, 104-9.

Her, R. K. (1979). The polymerization of silica. The Chemistry of Silica. NewYork: Wiley-Inter science Chapters 3 and 6.

Jameson, R. F. & Salmon, J. E. (1954). Aluminium phosphates: Phase-diagramand ion-exchange studies of the system aluminium oxide-phosphoricoxide-water at 25 °C. Journal of the Chemical Society, 4013-17.

Jendresen, M. D. (1973). New dental cements and fixed prosthodontics. Journalof Prosthetic Dentistry, 30, 684-8.

Jenkins, G. N. (1965). The equilibrium between plaque and enamel in relation todental caries. In Wolstenholme, G. E. W. & O'Connor, M. (eds.) CariesResistant Teeth, pp. 192-210. London: Churchill.

Johnson, R. H., Christensen, G. J., Stigers, R. W. & Laswell, H. R. (1970).Pulpal irritation due to the phosphoric acid component of silicate cement.Oral Surgery, 29, 447-54.

Jorgensen, K. D. (1960). Factors affecting the film thickness of zinc phosphatecements. Ada Odontologica Scandinavica, 18, 479-90.

Jorgensen, K. D. (1963). On the solubility of silicate cements. Acta OdontologicaScandinavica, 21, 141-58.

Jorgensen, K. D. & Peterson, G. F. (1963). The grain size of zinc phosphatecements. Acta Odontologica Scandinavica, 21, 255-70.

Kato, K., Shiba, M., Nakamura, M. & Ariyoshi, T. (1976). Report of theInstitute of Medical and Dental Engineering (Tokyo Medical and DentalUniversity), 10,45-61.

271


Kendzior, G. M., Leinfelder, K. F. & Hershey, H. G. (1976). The effect of coldtemperature mixing on the properties of a zinc phosphate cement. AngleOrthodontics, 46, 345-50.

Kent, B. E., Fletcher, K. E. & Wilson, A. D. (1970). Dental silicate cements: XI.Electron probe studies. Journal of Dental Research, 49, 86-92.

Kent, B. E., Lewis, B. G. & Wilson, A. D. (1971a). Dental silicate cements:XIII. The crazing and dulling of the surface. Journal of Dental Research, 50,393-9.

Kent, B. E., Lewis, B. G. & Wilson, A. D. (1971b). Dental silicate cements:XIV. Crazing, cement properties and liquid composition. Journal of DentalResearch, 50, 400-4.

Kent, B. E., Lewis, B. G. & Wilson, A. D. (1973). The properties of aglass-ionomer cement. British Dental Journal, 135, 322-6.

Kent, B. E. & Wilson, A. D. (1968). Unpublished observations.Kent, B. E. & Wilson, A. D. (1969). Dental silicate cements: VIII. Acid-base

aspect. Journal of Dental Research, 48, 412-18.Kent, B. E. & Wilson, A. D. (1971). Dental silicate cements: XV. Effect of the

particle size of the powder. Journal of Dental Research, 50, 1616-20.Kingery, W. D. (1950a). Fundamental study of phosphate bonding in

refractories. I. Literature review. Journal of the American Ceramic Society, 33,239-41.

Kingery, W. D. (1950b). Fundamental study of phosphate bonding inrefractories. II. Cold setting properties. Journal of the American CeramicSociety, 33, 242-7.

Kleinburg, I. (1961). Studies on dental plaque. I. The effect of differentconcentrations of glucose on the pH of dental plaque in vivo. Journal ofDental Research, 40, 1087-110.

Komrska, J. & Satava, V. (1970). Die chemischen Prozesse bei der Abbindungvon Zinkphosphatzementen. Deutsche Zahndrtzliche Zeitschrift, 25, 914-21.

Krajewski, K. P. (1984). Early diagenetic phosphate cements in the albiancondensed glauconitic limestone of the Tatra mountains, WesternCarpathians. Chemical Abstracts, 10, 114382.

Kuhn, A. T. & Jones, M. P. (1982). A model for the dissolution and fluoriderelease from dental cements. Biomaterials, Medical Devices and ArtificialOrgans, 10, 281-93.

Kuhn, A. T., Lesan, W. & Setchell, D. (1983). Some further factors affecting thesolubility of silicate cements. Journal of Materials Science Letters, 2, 191-4.

Kuhn, A. T., Setchell, D. J. & Teo, C. K. (1984). An assessment of the jetmethod for solubility measurements of dental cements. Biomaterials, 5, 161-8.

Kuhn, A. T., Tan, W. K., Davies, E. H. & Winter, G. B. (1982). Water uptakeand loss in silicate cements. Journal of Materials Science Letters, 17, 3263-7.

Kuhn, A. T. & Wilson, A. D. (1985). The dissolution mechanisms of silicate andglass-ionomer dental cements. Biomaterials, 6, 378-82.

Kuhn, A. T., Winter, G. B. & Tan, W. K. (1982). Dissolution rates of silicatecements. Biomaterials, 3, 136-44.

Kulka, M. (1907). Ueber die wichtigsten mechanischen and einige chemische

272

References

Eigenschaften der Silikat- und Zinkphosphatzemente. O ester reichisch-Ungarische Vierteljahrschrift fiir Zahnheilkunde, 23, 568-602.

Kulka, M. (1911a). The possible chemical or pathological effects of cementfilling. Dental Items of Interest, 33, 360-74.

Kulka, M. (1911b). Ueber die Moglichkeit chemischer bzw. pathologischerWirkungen von Zementfiillungen. Oesterreichisch-UngarischeVierteljahrschrift fiir Zahnheilkunde, 27, 43-60.

Kydonieus, A. F. (1980). Controlled Release Technologies. West Palm Beach,Florida: CRC Press.

Larato, D. C. (1971). Influence of silicate cements on gingiva. Journal ofProsthetic Dentistry, 26, 186-8.

Laswell, H. R., Welk, D. A., Dilts, W. E. & Thompson, J. A. (1971). Evaluationof a water activated zinc phosphate dental cement. Journal of the TennesseeDental Association, 51, 14—17.

Levine, R. S. (1976). The action of fluoride in caries prevention. A review ofcurrent concepts. British Dental Journal, 140, 9-14.

Lloyd, C. H. & Mitchell, L. (1984). The fracture toughness of tooth colouredrestorative materials. Journal of Oral Rehabilitation, 11, 257-72.

McComb, D. (1982). Tissue reactions to silicate, silicophosphate, glass ionomercements and restorative materials. In Smith, D. C. & Williams, D. F. (eds.)Biocompatibility of Dental Materials. Volume III. Biocompatibility of DentalRestorative Materials, Chapter 4. Boca Raton: CRC Press Inc.

McLean, J. W. & von Fraunhofer, J. A. (1971). The estimation of cement filmthickness by an in vivo technique. British Dental Journal, 131, 107-11.

McLean, J. W. & Short, I. G. (1969). Composite anterior filling materials. Aclinical and physical approach. British Dental Journal, 111, 9-18.

McLundie, A. C. & Murray, F. D. (1972). Silicate cements and composite resins- a scanning electron microscope study. Journal of Prosthetic Dentistry, 27,544-51.

Maldonado, A., Swartz, M. L. & Phillips, R. W. (1978). An in vitro study ofcertain properties of a glass-ionomer cement. Journal of the American DentalAssociation, 96, 785-91.

Manley, E. B. (1936). A preliminary investigation into the reaction of the pulpto various filling materials. British Dental Journal, 50, 321-31.

Manley, E. B. (1943). Pulp reaction to dental cements. Proceedings of the RoyalSociety of Medicine, 36, 488-99.

Manly, R. S., Baker, C. F., Miller, P. N. & Welch, F. E. (1951). The effect of thecomposition of liquid and powder on the physical properties of silicatecements. Journal of Dental Research, 30, 145-56.

Manly, R. S. & Harrington, D. P. (1959). Solution rate of tooth enamel in anacetate buffer. Journal of Dental Research, 38, 910-19.

Manston, R. & Gleed, P. T. (1985). Reaction cements as materials for thesustained release of trace elements into the digestive tract of cattle and sheep.II. Release of cobalt and selenium. Journal of Veterinary PharmacologyTherapeutics, 8, 374-81.

Manston, R., Sansom, B. F., Allen, W. M., Prosser, H. J., Groffman, D. M.,

273


Brant, P. J. & Wilson, A. D. (1985). Reaction cements as materials for thesustained release of trace elements into the digestive tract of cattle and sheep.I. Copper release. Journal of Veterinary Pharmacology Therapeutics, 8,368-73.

Matkovic, B., Popvic, S., Rogic, V., Zunic, T. & Young, J. F. (1977). Reactionproducts in magnesium oxychloride cement pastes. SystemMgO-MgCl2-H2O. Journal of the American Ceramic Society, 60, 504-7.

Matsui, A., Buonocore, M. G., Sayegh, F. & Yamaki, M. (1967). Reactions toimplants of conventional and new dental restorative materials. Journal ofDentistry for Children, 34, 316-22.

Mesu, F. P. (1982). Degradation of luting cements measured in vitro. Journal ofDental Research, 61, 665-72.

Mesu, F. P. & Reedijk, T. (1983). Degradation of luting cements measured invitro and in vivo. Journal of Dental Research, 62, 1236-40.

Miller, W. D. (1881). Concerning oxyphosphate cements. Dental Cosmos, 33,14-22.

Mitchem, J. C. & Gronas, D. G. (1978). Clinical evaluation of cement solubility.Journal of Prosthetic Dentistry, 40, 453-6.

Mitchem, J. C. & Gronas, D. G. (1981). Continued evaluation of the clinicalsolubility of luting cements. Journal of Prosthetic Dentistry, 45, 289-91.

Mjor, I. A. (1977). Histological demonstration of bacteria subjacent to dentalrestorations. Scandinavian Journal of Dental Research, 85, 169-74.

Morgenstern, M. (1905). Untersuchungen der Silikat- undZinkphosphatzemente unter besonderer Berucksichtigung ihrer physikalischenEigenschaften. Oesterreichisch-Ungarische Vierteljahrschrift filr Zahnheilkunde,21, 514-36.

Morris, J. H., Perkins, P. G., Rose, A. E. A. & Smith, W. E. (1977). Thechemistry and binding properties of aluminium phosphates. Chemical SocietyReviews, 6, 173-94.

Myers, C. L., Drake, J. T. & Brantley, W. A. (1978). A comparison ofproperties for zinc phosphate cements mixed on room temperature and frozenslabs. Journal of Prosthetic Dentistry, 40, 409-12.

Nakazawa, S., Jyoshin, K., Tsutsumi, S. & Yamaga, R. (1965). Studies on thehydraulic zinc phosphate cement containing Mg(H2PO4)2 • 2H2O. Journal ofthe Osaka University Dental School, 5, 61-71.

Neiman, R. & Sarma, A. C. (1980). Setting and thermal reactions of phosphateinvestments. Journal of Dental Research, 59, 1478-85.

Norman, R. D., Swartz, M. L. & Phillips, R. W. (1957). Studies on thesolubility of certain dental materials. Journal of Dental Research, 36,977-85.

Norman, R. D., Swartz, M. L. & Phillips, R. W. (1959). Additional studies onthe solubility of certain dental materials. Journal of Dental Research, 38,1025-37.

Norman, R. D., Swartz, M. L., Phillips, R. W. & Sears, R. (1970). Properties ofcements mixed from liquids with altered water content. Journal of ProstheticDentistry, 24, 410-17.

274

References

Norman, R. D., Swartz, M. L., Phillips, R. W. & Virmani, R. (1969). Acomparison of the intraoral disintegration of three dental cements. Journal ofthe American Dental Association, 78, 777-82.

O'Hara, M. J., Duga, J. J. & Sheets, H. D. (1972). Studies in phosphatebonding. American Ceramic Society Bulletin, 51, 590-5.

0ilo, G. (1978). Extent of slits at the interface between luting cements andenamel dentin and amalgam. Acta Odontologica Scandinavica, 36, 257-61.

0ilo, G. (1988). Characterization of glass-ionomer filling materials. DentalMaterials, 4, 129-33.


O'Neill, I. K., Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). NMRspectroscopy of dental materials. I. 31P studies on phosphate-bonded cementliquids. Journal of Biomedical Materials Research, 16, 39-49.

O'Reilly, D. E. (1960). NMR chemical shifts of aluminium: experimental dataand variational calculation. Journal of Chemical Physics, 32, 1007-12.



Paffenbarger, G. C. (1940). Silicate cements: an investigation by a group ofpractising dentists under the direction of the ADA Research Fellowship at theNational Bureau of Standards. Journal of the American Dental Association,27, 1611-22.

Paffenbarger, G. C, Schoonover, I. C. & Souder, W. (1938). Dental silicatecements: Physical and chemical properties and a specification. Journal of theAmerican Dental Association, 25, 32-87.

Paffenbarger, G. C, Sweeney, S. J. & Isaacs, A. (1933). A preliminary report onzinc phosphate cements. Journal of the American Dental Association, 20,1960-82.

Palmer, S. B. (1891). Zinc phosphates. Dental Cosmos, 33, 364-80.Peirce, C. B. (1879). Filling materials of oxide of zinc and glacial phosphoric

acid. Dental Cosmos, 21, 696-7.Pendry, N. R. (1973). Siliceous dental cement. British Patent 1,314,090.Pendry, N. R. & Cook, D. J. (1972). Improvements in or relating to dental

cement. British Patent 1,270,195.Pennsylvania Association of Dental Surgeons. (1879). Dental Cosmos, 21,

695-7.Phillips, R. W. (1975). Polymeric material for anterior restorations. In von

Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 7.London and Boston: Butterworths.

Plant, C. G., Jones, I. H. & Wilson, H. J. (1972). Setting characteristics of liningand cementing materials. British Dental Journal, 133, 21-4.

Plant, C. G. & Tyas, M. (1970). Lining materials with special reference toDropsin. British Dental Journal, 128, 486-91.

275


Plant, C. G. & Wilson, H. J. (1970). Early strength of lining materials. BritishDental Journal 129, 269-74.

Pluim, L. J. & Arends, J. (1981). In vivo solubility of dental cements. Journal ofDental Research, 60, 1213, Abstract No. 61.

Pluim, L. J. & Arends, J. (1987). The relationship between salivary propertiesand in vivo solubility of dental cements. Dental Materials, 3, 13-18.

Pluim, L. J., Arends, J. & Havinga, P. (1985). Comparison of in vivo and in vitrosolubility of 6 luting cements. Journal of Dental Research, 64, 714, AbstractNo. 98.

Pluim, L. J., Arends, J., Havinga, P., Jongebloed, W. J. & Stokroos, I. (1984).Quantitative cement solubility experiment in vivo. Journal of OralRehabilitation, 11, 171-9.

Poetschke, P. (1916). Physical properties of dental cements. Journal of Industrial& Engineering Chemistry, 8, 302-9.

Poetschke, P. (1925). Dental cement. US Patent 1,552,341.Popovics, S., Rajendran, N. & Penko, M. (1987). ACI Materials Journal, 84,

64-73.Powers, J. M., Farah, J. W. & Craig, R. G. (1976). Modulus of elasticity and

strength properties of dental cements. Journal of the American DentalAssociation, 92, 588-91.

Powis, D. R., Prosser, H. J., Shortall, A. C. & Wilson, A. D. (1988). Long-termmonitoring of microleakage of composites. Part I: radiochemical diffusiontechnique. Journal of Prosthetic Dentistry, 60, 304-7.

Powis, D. R., Prosser, H. J. & Wilson, A. D. (1988). Long-term monitoring ofmicroleakage of dental cements by radiochemical diffusion. Journal ofProsthetic Dentistry, 59, 651-7.

Proell. (1913). Experimentelle Untersuchungen iiber die Ursache desZahnpulpatodes unter Silikaphosphatzementen nebst theoretisch-praktischenStudien iiber Zemente und andere Fullmaterialen. Deutsche Monatsschrift filrZahnheilkunde, 33, 81-122 (English abstract: Experimental investigationsregarding the cause of death of the dental pulp under silicate cements withtheoretical and practical studies on cements and other filling materials. TheDental Cosmos, 750).

Prosen, E. M. (1939). Refractory material for use in making dental casting. USPatent 2,152,152.

Prosen, E. M. (1941). Refractory material suitable for use in casting dentalinvestments, models, etc. US Patent 2,209,404.

Prosser, H. J., Wilson, A. D., Groffman, D. M., Brookman, P. J., Allen, W. M.,Gleed, P. T., Manston, R. & Sansom, B. F. (1986). The development ofacid-base reaction cements as formulations for the controlled release of traceelements. Biomaterials, 7, 109-12.

Putt, M. S. & Kleber, C. J. (1985). Dissolution studies of human enamel treatedwith aluminium solutions. Journal of Dental Research, 64, 437-40.

Qvist, V. (1975). Pulp reactions in human teeth to tooth coloured fillingmaterials. Scandinavian Journal of Dental Research, 83, 54-66.

Rawitzer, J. (1908). Wie miissen Silicatzemente zusammengezetzt sein damit ein

276

References

Absterben der Pulpa unter ihnen vollkommen ausgeschlossen ist?Correspondenz-Blatt fur Zahndrzte, 37, 340-5.

Rawitzer, J. (1909). Chemie der Silikatphosphatzemente. Deutsche Monatsschriftfur Zahnheilkunde, 27, 269-81.

Ray, K. W. (1934). The behavior of siliceous cements. Journal of the AmericanDental Association, 21, 237-51.

Ray, N. H. (1979). The structure and properties of inorganic polymericphosphates. British Polymer Journal, 11, 163-77.

Richter, W. A. & Ueno, H. (1975). Clinical evaluation and dental clinicaldurability. Journal of Prosthetic Dentistry, 33, 294-7.

Robinson, A. D. (1971). The life of a filling. British Dental Journal, 130, 206-8.Rockett, T. J. (1968). Experimental silicate cements. International Association of

Dental Research, 46th General Meeting, Abstract No. 433.Rolla, G. (1977). Effects of fluoride on initiation of plaque formation. Caries

Research, 11, 243-61.Rollins, W. H. (1879). A contribution to the knowledge of cements. Dental

Cosmos, 21, 574-6.Rostaing di Rostagni, C. S. (1878). Verfahrung zur Darstellung von Kitten fur

zahnarztliche und ahnliche Zwecke, bestehend von Gemischen vonPyrophosphaten des Calciums oder Bariums mit den Pyrophosphaten desZinks oder Magnesiums. German Patent 6015 (Berlin). Also in (1881)Correspondenz-Blatt fur Zahndrzte, 10, 67-9.

Roydhouse, R. H. (1961). Silicate cements and acid production. Journal ofDental Research, 40, 258-63.

Salmon, J. E. & Terrey, J. H. (1950). The system zinc oxide-phosphoricacid-water at temperatures between 25° and 100°. Journal of the ChemicalSociety, 2813-24.

Salmon, J. E. & Wall, J. G. L. (1958). Aluminium phosphates. Part II. Ion-exchange and pH-titration studies of aluminium phosphate complexes insolution. Journal of the Chemical Society, 1128-34.

Salzmann, W. (1930). Verbesserung von Zahnzementen. German Patent 553,425.Sanderson, R. S. (1908). Silicate cements. The Dental Record, 28, 74-6.Savignac, J. R., Fairhurst, C. W. & Ryge, G. (1965). Strength, solubility and

disintegration of zinc phosphate cement with clinically determinedpowder/liquid ratio. Angle Orthodontics, 35, 126-30.

Schoenbeck, F. (1908). Process for the production of tooth cement. US Patent897,160.

Schonbeck, F. & Czapp, E. (1936). Verfahren zur Herstellung vonZahnzementen. German Patent 635,310.

Selvaratnam, M. & Spiro, M. (1965). Transference numbers of orthophosphoricacid and the limiting equivalent conductance of the H2PO~ ion in water at25 °C. Transactions of the Faraday Society, 61, 360-73.

Semler, C. E. (1976). A quick-setting wollastonite phosphate cement. AmericanCeramic Society Bulletin, 55, 983-8.

Servais, G. E. & Cartz, L. (1971). Structure of zinc phosphate dental cement.Journal of Dental Research, 50, 613-20.

277


Sidler, P. & Strub, J. R. (1983). In-vivo Untersuchung der Loslichkeit und desAbdichtungsvermogens von drei Befestigungszementen. DeutscheZahndrztliche Zeitschrift, 38, 564-71.

Simmons, F. F., D'Anton, E. W. & Hudson, D. C. (1968). Property studies of ahydrophosphate cement. Journal of the American Dental Association, 76,337-9.

Simon, O. (1929). Improvements in and relating to dental cements. BritishPatent 333,325.

Simon, O. (1932). Process of producing cements for teeth. US Patent 1,886,982.Skibell, R. B. & Shannon, I. L. (1973). Addition of stannous fluoride to

orthodontic cement. International Journal of Orthodontics, 11, 131-5.Skinner, E. W. & Phillips, R. W. (1960). Silicate cement. In The Science of

Dental Materials, Chapter 15. Philadelphia and London: W. B. SaundersCompany.

Smith, D. C. (1977). Past, present and future of dental cements. In Craig, R. G.(ed.) Dental Materials Review. Ann Arbor: University of Michigan Press.

Smith, D. C. (1982). Composition and characteristics of dental cements. InSmith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials.Volume II. Biocompatibility of Preventive Dental Materials and BondingAgents, Chapter 8. Boca Raton: CRC Press Inc.

Sorrell, C. A. & Armstrong, C. R. (1976). Reactions and equilibria inmagnesium oxychloride cements. Journal of the American Ceramic Society, 59,51-4.

Spangberg, L., Rodrigues, H., Langeland, L. & Langeland, K. (1973). Biologicaleffects of dental materials. 2. Toxicity of anterior tooth restorative materialson HeLa cells in vitro. Oral Surgery, Oral Medicine, Oral Pathology, 36,713-24.

Stanicioiu, Gh., Chinta, Gh. & Hartner, Al. (1959). Possibilities of improvingsilicate cements. Stomatologia, 6, 369-73 {Chemical Abstracts, 54, 5983g).

Stanley, H. R., Swerdlow, H. & Buonocore, M. G. (1967). Pulp reactions toanterior restorative materials. Journal of the American Dental Association, 75,132-41.

Steenbock, P. (1903). Verfahren zur Herstellung einer durch Behandlung mitPhosphorsauren und deren Salzen erhartenden Masse. Kaiserliches Patent174,558.

Steenbock, P. (1904). Improvements in and relating to the manufacture of amaterial designed for the production of cement. British Patent 15,181.


Stralfors, A. & Eriksson, S. E. (1969). The rate of dissolution of dental silicatecement. Odontologisk Tidskrift, 11, 185-210.

Stephan, R. M. (1940). Changes in the hydrogen-ion concentration on toothsurfaces and in carious lesions. Journal of the American Dental Association,27, 718-23.

Sudakas, G. L., Turkina, L. I. & Chernikova, A. A. (1982). Properties ofphosphate binders. Chemical Abstracts, 96, 202,472.

278

References

Sugama, T. & Kukacka, L. E. (1983a). Magnesium monophosphate cementsderived from diammonium phosphate solutions. Cement & Concrete Research,13, 407-16.

Sugama, T. & Kukacka, L. E. (1983b). Characteristics of magnesiumpolyphosphate cements derived from ammonium polyphosphate solutions.Cement & Concrete Research, 13, 499-506.

Svare, C. W. & Meyer, M. W. (1965). Available acidity of silicate cements.Journal of the American Dental Association, 70, 354-61.

Sveshnikova, V. N. & Zaitseva, S. N. (1964). Aluminophosphates aspolyelectrolytes. Russian Journal of Inorganic Chemistry, 9, 672-5.

Swanson, E. W. (1936). Effect of particle size on the physical properties ofsilicate cements. Journal of the American Dental Association, 23, 1620-31.

Swartz, M. L., Niblack, B. F., Alter, E. A., Norman, R. D. & Phillips, R. W.(1968). In vivo studies on the penetration of dentin by constituents of silicatecement. Journal of the American Dental Association, 76, 573-8.

Sychev, M. M., Medvedeva, I. N., Biokov, V. A. & Krylov, O. S. (1982). Effectof reaction kinetics and morphology of neoformation on the properties ofphosphate cements based on magnesium titanates. Chemical Abstracts, 96,222252e.

Takeda, S. et al. (1979). Thermal changes of binder in phosphate-bondedinvestment. Shika Igaku, 42, 429-36.

Tay, W. M., Cooper, I. R., Morrant, G. A., Borlace, H. R. & Bultitude, F. W.(1979). An assessment of anterior restorations in vivo using the scanningelectron microscope. Results after three years. British Dental Journal, 146,71-6.

Tay, W. M , Morrant, G. A., Borlace, H. R. & Bultitude, F. W. (1974). Anassessment of anterior restorations in vivo using the scanning electronmicroscope. Results after one year. British Dental Journal, 137, 463-71.

Ter-Grigorian, M. S., Beriya, V. V., Zedginidze, E. N. & Sychev, M. M. (1984).Problem of the setting of serpentinite-phosphate cement. Chemical Abstracts,101, 156469.

Ter-Grigorian, M. S., Zedginidze, E. N., Sychev, M. M., Papuashvili, S. M.,Teideishvili, L. K. & Dateshidze, R. I. (1982). Study of serpentinite-phosphatecement during heat treatment at 110-1200 °C. Chemical Abstracts, 96, 23920.

Theuniers, G. (1984). Een onderzock naar een duurzame afdichting door kroon-en brugcementen. Thesis. Leuvan.

Thomsen, J. C. (1931). Dental cement and process making the same. US Patent1,792,200.

Trivedi, S. C. & Talim, S. T. (1973). The response of human gingiva torestorative materials. Journal of Prosthetic Dentistry, 29, 73-80.

Tuenge, R., Sugel, I. A. & Izutsu, K. T. (1978). Physical properties of a zincphosphate cement prepared on a frozen slab. Journal of Dental Research, 57,593-6.

Tyas, M. J. (1979). The effects of silicate cement on mitochondria and lyosomesof cultured cells assessed by quantitative enzyme histochemistry. Journal ofOral Rehabilitation, 6, 55-60.

279


Updegraff, D. M., Change, R. W. H. & Joos, R. W. (1971). Antibacterialactivity of dental restorative materials. Journal of Dental Research, 50, 382-7.

Van Wazer, J. R. (1958). Properties and Chemistry of Phosphorus and itsCompounds, pp. 486-91. New York: Interscience Publishers Inc.

Van Wazer, J. R. & Callis, C. F. (1958). Metal complexing by phosphates.Chemical Reviews, 58, 1011^6.

Vashkevich, N. K. & Sychev, M. M. (1982). Setting of copper phosphatecements in the presence of organic polymers. Chemical Abstracts, 97,23921.

Vieira, D. F. & De Arujo, P. A. (1963). Estudo a cristizacao de cemento defosfato de zinco. Revista da Faculdade Odontologia da Universidade de SaoPaolo, 1, 127-31.

Voelker, C. C. (1916a). The place of silicates in dentistry. The Dental Summary,36, 177-200.

Voelker, C. C. (1916b). Dental silicate cements in theory and practice. TheDental Cosmos, 36, 1098-111.

Vysotskii, Z. Z., Galinskaya, V. I., Kolychev, V. I., Strelko, V. V. & Strazhesko,D. N. (1974). The role of polymerization and depolymerization reactions ofsilicic acid. In Strazhesko, D. N. (ed.) Adsorption and Adsorbents, vol. 1, p. 75.New York: Wiley.

Walls, A. W. G., McCabe, J. F. & Murray, J. J. (1985). An erosion test fordental cements. Journal of Dental Research, 64, 1100-4.

Ware, A. L. (1971). Properties of cements for orthodontic bonding. AustralianOrthodontics Journal, 2, 254-61.

Waters, D. N. & Henty, M. S. (1977). Raman spectra of aqueous solutions ofhydrolysed aluminium(III) salts. Journal of the Chemical Society: DaltonTransactions, 243-5.

Watts, A. (1979). Bacteria contamination and the toxicity of silicate and zincphosphate cements. British Dental Journal, 146, 7-13.

Watts, A. S. (1915). Dental porcelains. Transactions of the American CeramicSociety, 17, 190-9.

Wege. (1908). Zur Frage betr. die Ursache des Absterbens der Pulpa unterSilikatzementen, sowie einige Worte iiber Phenakit. Deutsche ZahndrztlicheWochenschrift 346-58.

Wei, S. H. Y. & Sierk, D. L. (1971). Fluoride uptake by enamel from zincphosphate cement containing stannous fluoride. Journal of the AmericanDental Association, 83, 621-4.

Williams, J. I., Gates, G. L., Hembree, J. H. & MacKnight, J. P. (1979). Thefrozen-aluminium-slab mixing technique: its effect on zinc phosphate cements.Journal of Dentistry for Children, 46, 398-403.

Williams, P. D. & Smith, D. C. (1971). Measurement of the tensile strength ofdental restorative materials by use of a diametral compressive strength test.Journal of Dental Research, 50, 436-42.

Wilson, A. D. (1969). A survey of dental practice in the use of silicate cements.Ministry of Technology Report. British Dental Journal, 127, 7 (abstract).

Wilson, A. D. (1975a). Dental cements - general. In von Fraunhofer, J. A. (ed.)

280

References

Scientific Aspects of Dental Materials, Chapter 4. London and Boston:Butterworths.

Wilson, A. D. (1975b). Zinc oxide dental cements. In von Fraunhofer, J. A. (ed.)Scientific Aspects of Dental Materials, Chapter 5. London and Boston:Butterworths.

Wilson, A. D. (1975c). Dental cements based on ion-leachable glasses. In vonFraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 6.London and Boston: Butterworths.

Wilson, A. D. (1976). Specification test for the solubility and disintegration ofdental cements: a critical evaluation of its meaning. Journal of DentalResearch, 55, 721-9.

Wilson, A. D. (1978). The chemistry of dental cements. Chemical SocietyReviews, 7, 265-96.

Wilson, A. D., Abel, G. & Lewis, B. G. (1974). The solubility and disintegrationtest for zinc phosphate dental cements. British Dental Journal, 137, 313-17.

Wilson, A. D., Abel, G. & Lewis, B. G. (1976). The 'solubility anddisintegration' test for zinc phosphate dental cements: the use of smallspecimens. Journal of Dentistry, 4, 28-32.

Wilson, A. D. & Batchelor, R. F. (1967a). Dental silicate cements. I. Thechemistry of erosion. Journal of Dental Research, 46, 1075-85.

Wilson, A. D. & Batchelor, R. F. (1967b). Dental silicate cements. II.Preparation and durability. Journal of Dental Research, 46, 1425-32.

Wilson, A. D. & Batchelor, R. F. (1968). Dental silicate cements. III.Environment and durability. Journal of Dental Research, 47, 115-20.

Wilson, A. D., Crisp, S. & Lewis, B. G. (1982). The aqueous erosion ofsilicophosphate cements. Journal of Dentistry, 10, 187-97.

Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986a). Anevaluation of the significance of the impinging jet method for measuring theacid erosion of dental cements. Biomaterials, 1, 55-60.

Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986b). A studyof variables affecting the impinging jet method for measuring the erosion ofdental cements. Biomaterials, 1, 217-20.

Wilson, A. D. & Kent, B. E. (1968). Dental silicate cements. V. Electricalconductivity. Journal of Dental Research, 44, 463-70.

Wilson, A. D. & Kent, B. E. (1970a). Dental silicate cements. IX.Decomposition of the powder. Journal of Dental Research, 49, 7-13.

Wilson, A. D. & Kent, B. E. (1970b). Dental silicate cements. X. Theprecipitation reaction. Journal of Dental Research, 49, 21-6.

Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement, a newtranslucent cement for dentistry. Journal of Applied Chemistry andBiotechnology, 21, 313.

Wilson, A. D., Kent, B. E. & Batchelor, R. F. (1968). Dental silicate cements.IV. Phosphoric acid modifiers. Journal of Dental Research, 47, 233-43.

Wilson, A. D., Kent, B. E., Batchelor, R. F., Scott, B: G. & Lewis, B. G.(1970a). Dental silicate cements. XII. The role of water. Journal of DentalResearch, 49, 307-14.

281


Wilson, A. D., Kent, B. E., Clinton, D. & Miller, R. P. (1972). The formationand microstructure of the dental silicate cement. Journal of Materials Science,7, 220-38.

Wilson, A. D., Kent, B. E. & Lewis, B. G. (1970). Zinc phosphate cements:chemical study of in vitro durability. Journal of Dental Research, 49,1049-54.

Wilson, A. D., Kent, B. E., Mesley, R. F., Miller, R. P. Clinton, D. & Fletcher,K. E. (1970b). Formation of dental silicate cement. Nature, 225, 272-3.

Wilson, A. D. & Lewis, B. G. (1980). The flow properties of dental cements.Journal of Biomedical Materials Research, 14, 383-91.

Wilson, A. D. & Mesley, R. F. (1968). Dental silicate cements. VI. Infraredstudies. Journal of Dental Research, 47, 644-52.

Wilson, A. D., Paddon, J. M. & Crisp, S. (1979). The hydration of dentalcements. Journal of Dental Research, 58, 1065-71.

Windeler, A. S. (1978). The use of film thickness to measure working time ofzinc phosphate cements. Journal of Dental Research, 57, 697-701.

Windeler, A. S. (1979). Powder enrichment effects on film thickness of zincphosphate. Journal of Prosthetic Dentistry, 42, 299-303.

Wisth, P. J. (1972). The ability of zinc phosphate and hydrophosphate cementsto seal space bands. Angle Orthodontics, 42, 395-8.

Worner, H. K. (1940). The properties of commercial zinc phosphate cements.Australian Journal of Dentistry, 44, 123-41.

Worner, H. K. & Docking, A. R. (1958). Dental materials in the tropics.Australian Dental Journal, 3, 215-29.

Wright, J. W. (1919). A study of some dental cements. Journal of DentalResearch, 1, 35-60.


Yamano, C. (1968). Effect of NaF-phosphate cement on enamel of humantooth. Journal of the Osaka University Dental School, 13, 123-37.

Yamazaki, T. & Takeuchi, M. (1967). The Chemical Society of Japan, IndustrialChemistry Section, 10, 656.

Zander, H. A. (1946). The reaction of dental pulps to silicate cements. Journal ofthe American Dental Association, 33, 1233^43.

Zenaishvili, N. V., Bakradze, E. G. & Chelidze, D. V. (1984). Serpentinite-phosphate mortar containing iron and boron. Chemical Abstracts, 101,156519r.

Zhuravlev, V. F., Volfson, S. L. & Sheveleva, B. I. (1950). The processes thattake place in the roasting of zinc-phosphate dental cement. Journal of AppliedChemistry (USSR), 23, 121-8.

282

7 Oxysalt bonded cements

7.1 Introduction

Oxysalt bonded cements trace their origin to studies by Sorel in the thirdquarter of the nineteenth century. The first of these cements which hestudied was zinc oxychloride (Sorel, 1855). Later he described a series ofmagnesia-based cements which included both the magnesium oxychlorideand magnesium oxysulphate types (Sorel, 1867). Of this second group, themagnesium oxychlorides in their hydrated form have been shown to havelarger values of modulus of elasticity, microhardness and compressivestrength than does Portland cement for a wide range of porosites (Beaudoin& Ramachandran, 1975). The magnesium oxysulphate cements haveproperties that have led to their being considered for nuclear applications,since they have good fire resistance, low thermal conductivity and aboveall, in marked contrast to the related oxychloride cements, no potential toinitiate corrosion of the reinforcing steel (Beaudoin & Ramachandran,1978). These cements are also employed as binders in lightweight panels, ininsulating materials and in architectural applications (Urwongse & Sorrell,1980b).

Oxysalt bonded cements are formed by acid-base reactions between ametal oxide in powdered solid form and aqueous solutions of metalchloride or sulphate. These reactions typically give rise to non-homo-geneous materials containing a number of phases, some of which arecrystalline and have been well-characterized by the technique of X-raydiffraction. The structures of the components of these cements and thephase relationships which exist between them are complex. However, aswill be described in the succeeding parts of this chapter, in many cases thereis enough knowledge about these cements to enable their properties andlimitations to be generally understood.

283

Oxysalt bonded cements

7.1.1 Components of oxysalt bonded cements

The three major types of oxysalt bonded AB cement are the zincoxychloride, the magnesium chloride and the magnesium oxysulphatecements. The bases employed, therefore, are either zinc oxide or mag-nesium oxide, both of which readily undergo hydration in aqueoussolution, behaving as M(OH)2 species and acting as a source of hydroxylions. They are thus both clearly bases in the Bronsted-Lowry sense.

By contrast, the acidity of the metal salts used in these cements has a lessclear origin. All of the salts dissolve quite readily in water and give rise tofree ions, of which the metal ions are acids in the Lewis sense. These ionsform donor-acceptor complexes with a variety of other molecules,including water, so that the species which exists in aqueous solution is awell-characterized hexaquo ion, either Mg(OH2)g+ or Zn(OH2)g+. How-ever, zinc chloride at least has a ternary rather than binary relationshipwith water and quite readily forms mixtures of ZnO-HCl-H2O (Sorrell,1977). Hence it is quite probable that in aqueous solution the metal saltsinvolved in forming oxysalt cements dissolve to generate a certain amountof mineral acid, which means that these aqueous solutions function asacids in the Bronsted-Lowry sense.

7.1.2 Setting of oxysalt bonded cements

The nature of the solidification process in these cements has received littleattention. Rather, attention has focussed on the crystalline componentsthat form in cements which have been allowed to equilibrate for someconsiderable time; the nature of such phases is now quite well understood.Gelation is reasonably rapid for these cements and occurs within asignificantly shorter time than does development of crystalline phases. Theconclusion may be drawn that initial cementition is not the same ascrystallization, but must occur with the development of an essentiallyamorphous phase. Reactions can continue in the amorphous gelled phase,but are presumably limited in speed by the low diffusion rates possiblethrough such a structure. However, reactions are able to proceedsubstantially to completion, since in many cases X-ray diffraction hasdemonstrated almost quantitative conversion of the parent compoundsto complex crystalline mixed salts, though several days or weeks ofequilibration are required to bring this about.

284

Zinc oxychloride cements

7.2 Zinc oxychloride cements7.2.1 History

These cements were the earliest of the oxysalt bonded cements to beprepared (Sorel, 1855) and their chemistry has been the subject ofnumerous investigations over the years. There are considerable difficultiesassociated with such investigations. Not only does the cement contain acomplex mixture of different crystalline precipitates but it is unaffected byboiling water and dissolves only slowly in strong acids. Consequentlyseparation or analysis of any of the phases which may be present is difficult.Nonetheless, as early as 1925 at least 17 crystalline compounds wereclaimed to occur in the zinc oxychloride cement (Mellor, 1925).

Of the early studies, two are still of value in providing an outline of thepossible phase compositions and equilibria existing in this material. Thefirst, by Droit (1910), concentrated on measuring the solubility of zincoxide in zinc chloride solutions at 18 °C, while the second, by Holland(1930), was based on the analysis of saturated solutions and moist residuesequilibrated at 25 °C and 50 °C. Droit assigned the compositions4ZnO. ZnCl2. 6H2O (4:1:6) and 2ZnO. 2ZnCl2. 3H2O (2:2:3 or 1:1:1-5)to solids which he found in equilibrium. The former phase he described asamorphous and reported that five of the six water molecules were lost at200 °C. The last remaining molecule of water was not lost until a muchhigher temperature, and then both HC1 and ZnCl2 were lost as well. Droitdescribed the 1:1:1-5 phase as microcrystalline and reported that it lostone of its water molecules at 230 °C while the remaining water, togetherwith HC1, was lost at a higher temperature.

Holland, by contrast, reported the existence of three well-defined phasesin this system, corresponding to ZnO:ZnCl2:H2O ratios of 5:1:8, 1:1:1and 1:1:2 respectively (Holland, 1930). More recently it was pointed outthat these claims lack any unequivocal support such as X-ray charac-terization of the phases, so that their validity must remain in doubt(Sorrell, 1977).

A number of other workers have reported the existence of an additionalphase, corresponding to 4:1:5 (Feitknecht, 1930; Hayek, 1932; Aspelund,1933), which may or may not be associated with a 1:1:1 phase. Feitknecht(1933) also carried out a detailed study of a phase described as 4:1:4 usingX-ray diffraction and concluded that the material had a layer structurewith interspersed water molecules. Such a structure would permit theaddition and removal of water molecules without altering the interlayer

285


distance. Hence all of the phases based on a 4:1 ratio of ZnO to ZnCl2 maybe closely related and readily interconverted, depending on the preciseconditions of cementition.

Droit's original 4:1:5 phase has been studied by X-ray diffraction(Nowacki & Silverman, 1961, 1962) and found to have a rhombohedrallayer structure. The 1:1:1 phase was also found to have a layer structure,which consisted of pseudohexagonal layers of zinc atoms separated byordered layers comprising oxygen and chlorine atoms (Feitknecht, Ostwald& Forsberg, 1959). This fundamental structure was apparently found forboth of the crystalline modifications in which this phase has been found tooccur, namely the monoclinic and the orthorhombic (Sorrell, 1977).

7.2.2 Recent studies

Sorrell (1977) further elucidated the structure and phase relationships inzinc oxychloride cements and produced a phase diagram for the system(Figure 7.1). Sorrell prepared his solutions of aqueous zinc chloride in oneof two ways: either by dissolving reagent-grade ZnCl2 in distilled water, orby dissolving zinc metal cut from an ingot in aqueous hydrochloric acidfollowed by boiling to low volume to remove the excess acid. Cementsamples were prepared by reacting either aqueous HC1 or aqueous ZnCl2solutions with zinc oxide powder and sealing them in polyethylenecontainers to equilibrate for at least four days before examining them.Cements were analysed by X-ray diffractometry using Cu Ka radiation.Powdered cement samples were also examined by quantitative differentialthermal gravimetry (DTG) from 25 °C to 715 °C at a heating rate of 10 °Cper minute.

X-ray analysis of the various samples that were produced indicated thatthe system ZnO-ZnCl2-H2O includes four crystalline phases, two ofwhich, ZnO and ZnCl2.1-5H2O, are essentially the starting materials.Sorrell also found the 4:1:5 phase, reported by Droit, with an identical X-ray powder diffraction pattern to that reported by Nowacki & Silverman(1961, 1962), and a 1:1:2 phase. Since neither the 1:1:2 nor the 4:1:5phase lost or gained weight on exposure to air at about 50% relativehumidity and 22 °C and no changes developed in the X-ray diffractionpattern following this exposure, he concluded that the previously reported1:1:1 phase cannot be formulated from mixtures of ZnO and aqueousZnCl2.

286


The cementition reaction between zinc oxide powder and aqueous zincchloride was found to be both rapid and extremely exothermic. Althoughat least four days equilibration was allowed before examining any of thecements in detail, Sorrell found evidence that reaction was complete within20 to 30 minutes and occurred without observable development ofintermediate phases. He also found that, as the concentration of reactantswas increased, so reaction rate increased until, at sufficiently highconcentrations, reaction occurred too quickly to allow proper mixing ofthe reactants. Preheating the zinc oxide at 900 °C for 16 hours was foundto slow the reaction down, but only slightly.

Sorrell showed that the two discrete phases could be readily andreversibly interconverted. For example, the 1:1:2 phase was found to react

ZnO

90,

O ZnO ond ZnClg Solution*

• ZnO and MCI Solution!

H2O10 20 30 60 90 ZnCU40 50 60

Wt%ZnCI2

Figure 7.1 Phase relationships in the ZnO-ZnQ2-H2O system at room temperature (Sorrell,1977).

287


with water to generate the 4:1:5 phase so rapidly that it was not possibleto record an X-ray diffraction pattern for the mixture. Samples whosecomposition fell into the triangle bounded by the phases 4:1:5,1:1:2 andpure water dried in air to give a solid mixture of the two discrete phases. Bycontrast, samples containing less ZnO did not dry out. Thus all changescaused by variation in composition proved to be predictable from thephase diagram and at no time were there any departures from this, such asformation of Zn(OH)2.

Thermogravimetric analysis was carried out on both the 4:1:5 and the1:1:2 phases (Sorrell, 1977). The latter was found to undergo a two-stepdissociation beginning at about 230 °C. Above 275 °C, the melting point ofZnCl2, weight loss corresponding to removal of half of the constituentwater occurred. Above c. 680 °C weight loss ceased, having reached a levelcorresponding to complete loss of ZnCl2 and water. Attempts were madeto complement this thermogravimetric study by X-ray analysis on thequenched samples but these were unsuccessful due to the deliquescentnature of the dissociation products. Above 275 °C, however, the sampleexisted as a solid mixed with a liquid and this crystallized on cooling as itabsorbed water from the atmosphere. The X-ray patterns which were madeshortly after quenching showed the presence of zinc oxide, and repeatedscanning indicated a rapid reaction to regenerate the 1:1:2 phase.

The 4:1:5 phase was shown by thermogravimetric analysis to dissociateat about 160 °C to zinc oxide and the 1:1:2 phase, a process which wasverified using X-ray diffraction (Sorrell, 1977). Once the 1:1:2 phase wasformed it underwent characteristic dissociation at temperatures above160 °C.

The equilibrium relationships found by Sorrell (1977) were valid only forroom temperature (22 + 2 °C) and, because samples were allowed to cure insealed containers, for equilibrium water vapour pressures determined bythe assembly of phases present. The phases which exist under suchconditions were quite unequivocally found to be 4:1:5 and 1:1:2. HoweverSorrell pointed out that it is entirely possible that lower hydration states ofeither phase could be stable at higher temperatures or lower humidities. Inparticular the 4:1:4 phase (Feitknecht, 1933) may well be such a phase,particularly as one of the five waters of hydration is known to be held onlyloosely in the structure. Indeed, Sorrell reported that he observed a slightshoulder on the larger dehydration peak of the DTG curve of the 4:1:5phase that might be assigned to the loss of this first water molecule. He didnot, however, succeed in isolating or characterizing a 4:1:4 phase.

288


Similarly Sorrell (1977) had no success in attempts to isolate the 1:1:1phase reported by Forsberg & Nowacki (1959), though he conceded that it,too, might exist as a lower hydration state of his well-defined 1:1:2phase. Unfortunately Forsberg and Nowacki did not provide details ofthe alleged 1:1:1 phase, so comparison of their results with SorrelPs wasnot possible.

To summarize these findings, the thermal decomposition of the twoternary phases was found to be complex (Sorrell, 1977). On the basis ofthermogravimetric data alone, the loss of water appeared to occur in sucha way that both a 4:1:4 and a 1:1:1 phase might result. However, whatactually occurred was that a mixture of ZnCl2 and ZnO was obtained as aresidue; such a result is typical of what is found for aqueous solutions ofdivalent metal chlorides that have been evaporated to dryness. Anadditional complexity was the possible reoxidation of zinc as the mixturescontaining ZnCl2 were evaporated, thus leading to larger amounts of ZnOin the residue than were originally present in the original mixture. Overall,the thermogravimetric studies gave results that were extremely difficult tointerpret, largely because of the highly complicated set of relationships thatcan exist in this system.

In very dilute HC1 solutions, specifically those with a pH above 5-48, the4:1:5 phase was found to be insoluble. By contrast, addition ofconcentrated HC1 to the 4:1:5 phase was shown to lead to formation of the1:1:2 phase (Sorrell, 1977). Below 35wt% HC1, the 4:1:5 phase wasfound to dissolve congruently. Since the 1:1:2 phase was also found todissolve congruently in hydrochloric acid solutions with concentrationsabove 23 wt %, it follows that there is a range of concentrations over whichboth phases are soluble in aqueous HC1. This behaviour explains why thezinc oxychlorides have proved to be unsatisfactory in attempts to use themas dental cements. The preparation of such cements from concentratedaqueous solutions of ZnCl2 results in the formation either of the 1:1:2phase alone or of mixtures of the 4:1:5 and 1:1:2 phases, neither of whichis stable in the presence of water. Preparing dental cements from lessconcentrated solutions also results in the formation of mixed phases,unless the bulk composition has excessive amounts of ZnO present. Inthese latter cases the cement stability is acceptable but it lacks both aworkable consistency and a reasonable working time.

SorrelFs study of zinc oxychloride cement (1977), in addition to makingan important contribution to our understanding of the nature of thismaterial, also highlighted a more general feature of the chemistry of zinc

289


chloride. Since the 4:1:5 phase separated from dilute solutions, theZnCl2-H2O system was shown not to be binary. Instead, it represents asection through the ZnO-HCl-H2O ternary system. Moreover, zincchloride is known to be one of the most soluble of all substances and is verydifficult to prepare in a completely anhydrous condition (Greenwood &Earnshaw, 1984). This feature also presumably derives from the nature ofthe phase relationships that develop in the ZnO-HCl-H2O system.

7.3 Magnesium oxychloride cements7.3.1 Uses

Magnesium oxychloride cements are widely used for the fabrication offloors. They find application for this purpose because of their attractiveappearance, which resembles marble, and also because of their acousticand elastic properties and their resistance to the accumulation of staticcharge. They have also been used for plastering walls, both interior andexterior; for exterior walls the cement often includes embedded stoneaggregate (Sorrell & Armstrong, 1976). However, there have beenproblems with this latter application, since the base cement has been foundto be dimensionally unstable and, in certain circumstances, to releasecorrosive solutions and show poor weather resistance.

7.3.2 Calcination of oxide

The quality of magnesium oxychloride cements is highly dependent on thereactivity of the magnesium oxide used in their preparation. Typically,such oxides are prepared by calcination of the basic carbonate (Eubank,1951; Harper, 1967), but their reactivity varies according to the conditionsunder which such calcination is carried out. As the reactivity alters so doesthe amount of oxide that can be incorporated into a cement relative to theamount of aqueous MgCl2 (Harper, 1967).

A detailed study of the effect of calcination conditions on properties ofthe resulting oxide was carried out by Harper (1967), with the resultsshown in Table 7.1 Oxides prepared from basic carbonate at 600 °C and700 °C give weak cements because the reactivity of the oxide is so high thatreaction occurs during mixing and the cement is broken in the process. Thelow strengths of these cements contrasted with the much higher strengthsobtained from oxides that had been calcined at higher temperatures. For

290

Magnesium oxychloride cements

Table 7.1. Compressive strengths of magnesium oxy chloride cements madefrom basic carbonate (Harper, 1967)

Calcinationtemperature,°C

600

700

800

900

1000

Age,days

71428

71428

71428

71428

71428

Compressive strength, MPaMgO/MgCl2, mo

6

7-67-48-3

15-916-317-349-655-957-547-748-655-660-565-983-2

7

55-363160-058-268-668-871-785-790-4

1

8

57-463-550-067-569165-877-887-580-6

9

73-871-767-873-687-088-2

10

78-169066-680-068-488-5

example, oxides prepared at 1000 °C were much less reactive, whichallowed time for adequate mixing with the aqueous magnesium chlorideand thus allowed the strength to develop to a greater extent. Such cementswent on increasing their strength for at least 28 days after mixing. Theselower-reactivity oxides could also be added in greater quantities to theMgCl2 solution, thus allowing for greater variation in the composition ofthe cement.

7.3.3 Setting chemistry

There have been a number of studies aimed at understanding the chemistryof the curing and setting of magnesium oxychloride cements and atidentifying the phases that are present in the final material. Investigationsin the first half of the twentieth century revealed that cement formation inthe MgO-MgCl2-H2O system involves gel formation and crystallization of

291


ternary oxychloride phases of uncertain composition (Robinson &Waggaman, 1909; Bury & Davies, 1932). In the years around 1950, thesystem was studied by Walter-Levy and coworkers; they succeeded inidentifying crystalline phases and determining the structure of one of them(de Wolff & Walter-Levy, 1953). Walter-Levy had previously recognizedan oxychlorocarbonate phase in the cement which forms by reaction withcarbon dioxide in the atmosphere (Walter-Levy, 1937, 1938). A veryextensive study of this system was carried out by Cole and coworkers(Demediuk, Cole & Hueber, 1955; Cole & Demediuk, 1955), who wereable to define the temperature ranges over which the various phases arestable, though they confined their studies to cements of low MgO content,e.g. 1-5 g MgO in 75 cm3 MgCl2 solution. Below 100 °C two ternary phaseswere found, one with an Mg(OH)2: MgCl2: H2O composition of 5:1:8, theother with a compositon of 3:1:8. These phases have been referred to asthe 5-form and the 3-form respectively. The 3-form was the reactionproduct of MgO with solutions of higher MgCl2 concentration than thosewhich tended to yield the 5-form (Cole & Demediuk, 1955). Both formswere found to change gradually with time via slow reaction withatmospheric CO2, so that after long periods a basic magnesium carbonate,corresponding to a composition of Mg(OH)2. 2MgCO3. MgCl2. 6H2O,had formed (Cole & Demediuk, 1955). Hence the results previouslyreported by Walter-Levy (1937, 1938) were confirmed. This basic car-bonate was found to be much less soluble in water than either of the twooxychloride phases.

Above 100 °C, a different set of phases was stable in the simplemagnesium oxychloride system (Demediuk, Cole & Hueber, 1955; Cole& Demediuk, 1955); a 2-form 2Mg(OH)2. MgCl2. 4H2O and a 9-form9Mg(OH)2. 5H2O were found to occur. Apart from identifying thesephases, these workers were not able to give details on the structures.

The phase relationships prevailing in this system have received moreattention at temperatures below 100 °C, and there is greater understandingof the equilibria which occur in this temperature range. These equilibriainvolve a range of solids, namely Mg(OH)2, the 5:1:8 and 3:1:8 materialsand MgCl2. 6H2O, depending on the range of concentrations of MgCl2

used in aqueous solution and the ratio of solution to MgO powderemployed in fabricating the cement.

Of all the studies on this system, that by Sorrell & Armstrong (1976) hasprovided the most useful information, both on the phase relationships andon the kinetics of interconversion. They used three different grades of

292


MgO, produced by different but well-defined routes and having differentreactivities towards aqueous MgCl2; in this way, it was possible to studythe cementation reactions in some detail and to ensure a reasonably closeapproach to equilibrium.

To study reaction kinetics, cement batches of total mass 300 g wereprepared using ingredients measured to the nearest 0-1 g. Mixing wascarried out for 10 minutes using a kitchen blender, after which specimenswere cast in slabs 10 x 10 x 1-2-1-5 cm in polyethylene moulds. When thesetting reaction had proceeded to a sufficient extent and viscosity had risento give a reasonably stiff paste, a small portion was removed, placed on aglass microscope slide and immediately examined by X-ray diffraction.The remainder of the sample was allowed to set.

7.3.4 Kinetics of cementation

Sorrell & Armstrong formulated cements in proportions corresponding tothe 5:1:8 and 3:1:8 compositions. The initial mixtures were thick slurrieswith no observable tendency to separate provided a sufficiently reactiveoxide was used. They tended to set within about 90 minutes, at which timesamples were prepared for X-ray determination. Initially, although thepreliminary hardening process was apparently complete, the only crys-talline phase that could be found was MgO; moreover, this material wasfound in amounts that approximated to the quantity in the initial mixture.

After some two hours, the X-ray diffraction pattern corresponded toeither the 5:1:8 or the 3:1:8 phase; warming of the sample had alsooccurred. Growth of the crystalline oxychloride phases continued rapidlyup to about 15 hours, and more slowly thereafter, until after four daysthere was no trace of MgO in the diffraction pattern of the cement.

The fact that the initial setting process for magnesium oxychloridecements takes place without observable formation of either the 5:1:8 orthe 3:1:8 phase is important. It indicates that formation of an amorphousgel structure occurs as the first step, and that crystallization is a secondaryevent which takes place from what is effectively a supersaturated solution(Urwongse & Sorrell, 1980a). This implies that crystallization is likely to beextremely dependent upon the precise conditions of cementition, includingtemperature, MgO reactivity, heat build-up during reaction and purity ofthe components in the original cement mixture.

The reactivity of the magnesium oxide in particular was shown to becrucial. A relatively unreactive batch gave thin slurries which showed some

293


tendency to settle and took at least 20 hours to set. Reaction was notcomplete after 14 days and in the mixture corresponding to the 5:1:8composition it was the 3:1:8 phase which actually crystallized out.

7.3.5 Phase relationships in the MgO-MgCl2-H2O system

Sorrell & Armstrong (1976) prepared cement slabs in proportionscorresponding to the 5:1:8 phase from each of the three magnesium oxidesamples that were available. The samples were allowed to set for at least30 days exposed to air, after which time the phase gradient was determinedby X-ray diffraction of surfaces exposed by incremental grinding. Theseexperiments revealed that the most reactive sample of MgO had yielded amaterial consisting essentially of the 5:1:8 phase uniformly distributedthroughout the bulk and containing no more than 2 % residual MgO. Theless reactive samples of oxide, by contrast, gave much less homogeneouscements in which the surface layer consisted mainly of unreacted MgOtogether with MgCl2 solution. The amount of crystalline 5:1:8 phaseincreased with depth into the sample, thus demonstrating the developmentof phase gradients for these materials.

For studies of the phase equilibria, 20-gram batches of cement wereprepared by mixing appropriate solutions of MgCl2 with the most reactiveof the three samples of MgO that was available (Sorrell & Armstrong,1976). Mixing was carried out by stirring with a glass rod and cements weresealed in polyethylene containers for at least four days to allow sufficienttime for equilibration. The use of relatively small samples avoided theproblems of large temperature increases as the cementition reactionoccurred. The studies of phase equilibria resulted in compilation of thephase diagram Figure 7.2.

The portion of the phase diagram representing the MgCl2-rich compo-sitions received little attention in the work of Sorrell & Armstrong (1976),but was the subject of a separate study reported a few years later(Urwongse & Sorrell, 1980a). Since it had previously been shown that theMgO-MgCl2-H2O system is a portion of the MgO-HCl-H2O system(Robinson & Waggaman, 1909; Bury & Davies, 1932), and the equilibriumassemblages will be the same whichever reagents are used, Urwongse &Sorrell (1980a) decided to use magnesium oxide and aqueous hydrochloricacid for their work. This approach had the advantage that magnesiumoxide is much more soluble in hydrochloric acid solutions than inmagnesium chloride solutions, and hence measurements were easier and

294


more reliable. Having obtained data on solubilities for the MgO-HCl-H2Osystem, these were recalculated to give the compositions in theMgO-MgCl2-H2O phase diagram.

This work was of value in constructing that part of the phase diagraminvolving solutions in water. Of greater value in understanding cementformulations were the results obtained in the earlier study (Sorrell &Armstrong, 1976) for the MgO-rich portion of the phase diagram.

7.3.6 Consequences for practical magnesium oxychloride cements

The results of Sorrell & Armstrong (1976) show clearly that care is neededin selecting the magnesium oxide when practical oxychloride cements arebeing prepared. In particular, the powder must be of good reactivity withsmall uniform crystallites and minimum agglomeration. Such an oxidesample is capable of being rapidly dissolved by the aqueous magnesiumchloride, thereby forming a thixotropic suspension. This suspension then

MgO

90

Mgcl2.6H2o

Phase assemblages

A Mgo-Mg(OH)2-5:1:8

B MgO-5:l:8-3:l:8

C MgO-3:1:8-MgCl2.6H2O

D 3:1:8-MgCl2.6H2O-gel

E 3:l:8-5:l:8-gel

F 5:1:8-Mg(OH)2-gel

G Mg(OH)2-gel

H 5:l:8-gel

I 3:l:8-gel

J MgCl2.6H2O-gel

K gel

L gel-liquid

Wt% Mgci2

Figure 7.2 Phase relationships in part of the MgO-MgCl2-H2O system at room temperature(Sorrell & Armtrong, 1976).

295


develops firstly into a homogeneous gel and finally into a crystallinematerial consisting of a dense aggregate of the equilibrium ternary oxidephase.

Relatively unreactive powders lead to a different sequence of events.First, long periods of time are required for dissolution into the MgCl2

solution, during which water is lost from the liquid surface and acompositional gradient is established. Second, under such conditions,MgCl2 is able to migrate to the surface where it may either precipitate orremain as a deliquescent layer. The combined effects of the compositionalgradient and the presence of unreacted MgCl2 and MgO are almost certaindimensional instability, leaching of corrosive salts, and poor weatherresistance.

The amount of variation in reactivity which may be tolerated is small,since a reasonable balance has to be struck between rapid and uniformreaction on the one hand and practical working times on the other. Sorrell& Armstrong (1976) found that the mean crystallite diameter could bedetermined adequately by X-ray diffraction, using line-broadening as anindication of crystallite size, and also by electron microscopy. Thesetechniques were able to distinguish between suitable and unsuitable oxidepowders.

Both the 5:1:8 and 3:1:8 phases were shown to be unstable in water,dissociating to give Mg(OH)2 and MgCl2 solution (Sorrell & Armstrong1976). This clearly has consequences when magnesium oxychloridecements are employed as exterior stuccos on buildings. In fact, thesecements do have good weather resistance, but not because of any inherentstability of the parent oxychloride cement. Rather, the slow conversion ofthe oxychloride to the much less soluble basic magnesium carbonate as aresult of reaction with atmospheric carbon dioxide creates a material ofinherently good weather resistance.

Studies of samples of magnesium oxychloride cements used as exteriorstructures have been carried out on specimens between one month and 50years in age (Sorrell & Armstrong, 1976). From these it has been possibleto gain a general understanding of the mechanism of the reaction withcarbon dioxide and hence the way in which weather resistance develops. Ifthe cement has initially reacted completely to give the 5:1:8 phase, as it willif the magnesium oxide used is sufficiently reactive, then the weatherresistance develops due to formation of the chlorocarbonateMg(OH)2.2MgCO3.MgCl2.6H2O. This product arises following inter-action with atmospheric carbon dioxide, as first reported by Cole &

296


Demediuk (1955). A surface coating of this carbonate protects theunderlying cement from attack by water. The longer-term stability,however, depends on slow leaching of the chloride from this newsurface of the cement, resulting in its conversion to hydromagnesite,5MgO • 4CO2. 5H2O. The precise sequence of these reactions remains to beelucidated.

73.7 Impregnation with sulphur

One method of overcoming some of the instability and loss in strength ofoxychloride cements when exposed to water has been to modify them byimpregnation with sulphur (Beaudoin, Ramachandran & Feldman, 1977).The resulting material appears to be a composite in which the respectivecomponents complement each other. The magnesium oxychloride part hasrelatively poor resistance to water as initially formed, whereas the sulphuris difficult to wet and is completely insoluble in water.

Beaudoin, Ramachandran & Feldman (1977) studied this system indetail, examining both the mechanical properties and water resistance ofsulphur-filled cements. Mechanical properties evaluated included porosity,modulus of elasticity and microhardness. Thermal characteristics weredetermined by DTG, and for certain fractured specimens microstructurewas determined by examining fracture surfaces on the scanning electronmicroscope. Specimens were prepared as 5-1-cm cubes and cured at 50%r.h. for 13 months, after which thin discs were cut from them to enablemechanical properties to be evaluated. Two series of samples wereprepared, which were respectively (1) immersed in water for 88 days,impregnated with sulphur, then exposed again to water, and (2) impreg-nated first, immersed in water for 88 days, followed by further im-pregnation, then water exposure. Impregnation was carried out in a bathat 128 °C, at which temperature the sulphur was molten and would diffusethoroughly into all the pores of the native cement. Samples were removedand, after cooling to allow solidification of the sulphur, the excess sulphurwas removed by washing in kerosene.

For the native cement exposed first to water, there was a dramatic andrapid drop in microhardness, 30^0% in the first hour, and 55-60% ateight hours. Compressive strength was assumed to have undergone asimilar decrease, since it is linearly related to microhardness for cemen-titious materials (Beaudoin & Feldman, 1975). Scanning electron micro-scopy revealed clearly the differences that occurred on soaking in water.

297


These include a change from a non-distinct and amorphous appearanceto a more porous structure with larger platy crystals. The reason for thiswas not completely clear, though it was suggested that some degree ofrecrystallization might have occurred during immersion in water. It mayalso be that in the sample that had not been soaked in water the finelydivided portion obscured the large platy crystals.

The response of magnesium oxychloride cements to water was found tovary according to whether or not they were impregnated with sulphur. Formicrohardness, the impregnated samples gave initially higher values thanthe unimpregnated samples. Microhardness decreased after 88 days'immersion in water, but the decline was greater in the unimpregnatedsamples. Unimpregnated samples had extremely low values of micro-hardness, 10-20 MPa compared with 400-600 MPa for different formu-lations prepared by impregnation with sulphur (Beaudoin, Ramachandran& Feldman, 1977).

Modulus of elasticity showed similar behaviour. Impregnated sampleswere found to have initially a higher modulus value than unimpregnatedones; the decrease in modulus was less for the impregnated cementsfollowing the 88 days' immersion in water.

The mechanism by which sulphur has these observed effects is as follows.Immersion of native magnesium oxychloride cement in water brings abouta slow dissolution which creates pores. When those pores are filled withsulphur, sites of possible stress concentration at points of contact betweenparticles are modified. Similar effects occur when sulphur is used toimpregnate hydraulic cements based on Portland cement and silica(Beaudoin, Ramachandran & Feldman, 1977).

There were, however, important differences between the water-resistanceproperties of sulphur-impregnated Portland cement and sulphur-impregnated magnesium oxychloride cement. The former showed poorresistance, in one case breaking into small pieces after about two hours'exposure to water vapour. The latter, by contrast, remained completelyintact after exposure to water for 88 days, and remained in good conditionfollowing reimpregnation with sulphur and a further 28 days of exposureto water. Beaudoin, Ramachandran & Feldman (1977) attributed thisresult in part to the difference in pore size and distribution in the twocements, which leads to the magnesium oxychloride cement having asmaller surface area exposed to water than Portland cement.

Overall, these studies showed that sulphur could be used to impregnatemagnesium oxychloride cements thereby yielding materials of superior

298

Magnesium oxysulphate cements

properties in terms of both mechanical characteristics and water resistance.The preferred method of preparation was to soak the cement in water priorto impregnation, since of the two impregnation regimes employed this gavecements of marginally superior properties.

7.4 Magnesium oxysulphate cements

7.4.1 Setting chemistry

The magnesium oxysulphate cements are formed by reactions betweenhigh-reactivity magnesium oxide powder and aqueous solutions ofmagnesium sulphate. They have a number of applications in architecture,including use as binders in lightweight panels and as insulating materials.There have been problems in fully characterizing these cements, becauseoften it has not been appreciated that they have not been allowed toequilibrate, and as a result their composition may be uncertain. To rectifythis situation, Urwongse & Sorrell (1980b) carried out a study of thesematerials under conditions which allowed equilibrium to be attained. Inthis way they were able to gain an insight into the phase relationships thatoccur in these cements.

Previous studies had been carried out on mixtures rich in aqueousmagnesium sulphate and maintained at various temperatures between30 °C and 120 °C (Demediuk & Cole, 1957). These revealed the existenceof three crystalline phases at lower temperatures, namely magnesiumhydroxide, hydrated magnesium sulphate, MgSO4.7H2O, and a com-plex crystalline salt whose composition corresponded to anMg(OH)2: MgSO4: H2O ratio of 3:1 :8. At higher temperatures, a numberof other well-defined crystalline phases formed, including 5:1:3,1:1:5 and1:2:3. Their relationships to each other and to the starting materials wereestablished. For example, at lower concentrations of magnesium sulphatesolution the 5:1:3 phase was found to be stable at temperatures greaterthan 40 °C in equilibrium with Mg(OH)2 and liquid. At high concentra-tions the equilibrium altered, so that the 5:1:3 phase occurred inassociation with the 1:1:5 phase rather than with Mg(OH)2. Above100 °C, the 1:2:3 phase was stable in equilibrium with the 5:1:3 phase andliquid at lower concentrations, and with the solid MgSO4. 7H2O and liquidat higher concentrations (Demediuk & Cole, 1957).

Other crystalline phases have been reported for the magnesiumoxysulphate system. The 2:3:5 phase has been claimed to be formed from

299


magnesium sulphate solutions with concentrations exceeding 17-5 wt%(Adomavichiute, Yanitskii & Vektaris, 1962). Unfortunately, no analyticaldata accompanied this report and so it is not certain how reliable the claimis.

Acid salts as well as basic salts have been claimed to occur in this system.In particular, the complex salt MgSO4. H2SO4. 3H2O was found at 12-6 °C(Montemartini & Losana, 1929) arising as the product of reaction betweenMgSO4 and aqueous H2SO4. This acid salt was found to coexist withMgSO4.7H2O, MgSO4.H2O and liquid.

7.4.2 Phase relationships in the MgO-MgSO\-H2O system

A detailed study of the phase relationships in the magnesium oxysulphatecement was carried out by Urwongse & Sorrell (1980b). They used X-rayanalysis to examine the phases present in the cement, and established thecomposition of the invariant liquids after equilibration by measuringspecific gravity with the aid of a pycnometer. Specific gravities were relatedto concentration by means of a calibration exercise in which 30 stocksolutions of sulphuric acid at concentrations between 0 and 79-5 wt % wereprepared with distilled water.

A number of species were found at equilibrium in the solid state in thissystem, including MgSO4.7H2O, MgO, MgSO4.6H2O and MgSO4.H2O.Only one complex salt, the 3:1:8 phase, was found under the conditionsstudied, with both H2SO4 and H2O occurring as discrete entities in certainof the phases. Non-equilibrium phases were also apparent, including the1:1:5 phase in certain samples. The non-equilibrium phase MgSO4. 7H2Owas also found in numerous samples, particularly those prepared fromsulphuric acid solutions with concentrations above 20wt%. The oc-currence of these non-equilibrium phases, and also uncertainty about thepossible existence of the phase MgSO4. H2SO4. 3H2O, led to problems inconstructing the complete phase diagram for the MgO-H2SO4-H2Oternary system. In particular there were difficulties in establishing phaserelationships on the H2SO4-rich side of the diagram.

The X-ray diffraction lines corresponding to Mg(OH)2 were distinctive.The basal line (001) was very broad, the prism line (110) was very sharp andthe other lines (hkl) were intermediate in breadth. This was interpreted asimplying that Mg(OH)2 crystallites adopt an exaggerated sheet-likemorphology in this system (Urwongse & Sorrell, 1980b). The (001) line

300


became sharper with time for a series of samples of Mg(OH)2 crystals leftin contact with the liquid in sealed containers. By contrast, the prism line(110) remained sharp and unchanged for 64 days. These findings indicatethat not only were the initially formed crystallites in the form of very thinsheets, but that crystal growth continued slowly with time in the cdirection.

The phase diagram constructed by Urwongse & Sorrell (1980b) for thechemical equilibria in the MgO-H2SO4-H2O system applies also to theMgO-MgSO4-H2O system, and is thus relevant for gaining an insight intothe phase relationships which occur in the magnesium oxysulphate cements(Figure 7.3). There is, though, the possibility of non-equilibrium phasesappearing. For example the phases 1:1:5 and MgSO4. 4H2O which havebeen found experimentally in this system have been assumed to arisebecause of localized temperature increases on mixing. These phases, which

O PREPARED SAMPLES

• REPORTED COMPOUNDS

80 © 1-1-3 NON-EQUILIBRIUM

MgS04"4H20 NON-EQUILIBRIUM/ \ _

Mg(OH);

SOLUBILITY MEASUREMENTS

40 50WEIGHT PERCENT

9 0 H2S04

Figure 7.3 Phase relationships in the MgO-H2SO4-H2O system at room temperature(Urwongse & Sorrell, 1980b).

301


are known to be stable above room temperature, are then presumablystranded as the mixture cools. Further work over longer periods of timewould be necessary to confirm this.

The phase diagram of Urwongse & Sorrell (1980b) has importantconsequences for the formulation of magnesium oxysulphate cements. Inparticular, it indicates the impossibility of preparing cements at 23 °Cwhich have more than 50 wt % of the 3:1:8 phase in them if the startingmaterials are MgO and aqueous MgSO4. Attempting to obtain more of thephase by the alternative tactic of reacting the oxide with sulphuric acidsolutions is not practical because of the rapid formation of MgSO4. 7H2Ofrom these starting materials. From these and other observations of thephase relationships, the bonding phase in commercial oxysulphate cementsis concluded to be 5:1:3. In addition, varying amounts of MgSO4. 7H2Oare likely to occur in the finished product, though this phase is not desirableand minimizing its formation must be part of the aim of formulatingpractical cements with optimum properties.

7.43 Mechanical properties of magnesium oxysulphate cements

The use of magnesium oxysulphate cement as a binder in building materialshas been recognized since the earliest report of it by Sorel (1867). A studyof the development of strength in these cements cured under pressure wascarried out (Beaudoin & Ramachandran, 1978) in order to determinewhether strength could be improved by this means. Other cementitiousmaterials that usually produce weak specimens can undergo significantand technically useful increases in strength when cured under pressure, andthe study aimed to discover whether magnesium oxysulphate also fell intothis category.

For this study a series of magnesium oxysulphate cements was preparedhaving different initial compositions from those prepared by Urwongse &Sorrell (1980b) in their study of the phase equilibria. Beaudoin &Ramachandran (1978) formulated their cements from solutions of MgOand MgSO4. 7H2O, the latter being dissolved in distilled water to give asaturated solution.

Having prepared the cements, a number of physical techniques wereemployed to study them. Porosity was determined by measuring theapparent volume and the solid volume, the former by straightforwardlinear measurement, the latter using a helium pycnometer. This techniquewas used to avoid the problems of dissolution that arise when water is used

302


in a displacement technique to evaluate porosity. However, because of thewater-sensitivity of these cements, care was taken to condition them fullyat 11 % r.h. prior to taking any measurements.

In addition to porosity, modulus of elasticity and microhardness weredetermined. Differential thermograms were recorded, at a heating rate of20 ° min"1.

Compaction pressure and resulting porosity were found to be related ina logarithmic way, such that plotting log(compaction pressure) againstporosity gave a straight line (Beaudoin & Ramachandran, 1978). This istypical for cementitious materials, though what was not typical was theabrupt change of slope at approximately 7-5 % porosity. Below this valuethe slope of the graph was found to be much steeper than at higherporosity. This result was held to arise from the porous nature of theparticles bound together in the cement matrix. Above the critical value ofporosity (7-5 %) compaction occurs when particles slide past each other asthe matrix deforms. Below this value, compaction is possible only when theparticles themselves are heavily deformed or fractured; this difference inmechanism results in the observed change in relationship betweencompaction pressure and porosity.

The microhardness and modulus of elasticity were found to alter oncompaction but in opposite directions. For microhardness, compactedspecimens gave higher values than uncompacted, whereas for modulus ofelasticity compacted specimens gave lower values. The reason for this wasnot clear, though a tentative explanation was advanced by Beaudoin &Ramachandran (1978). They suggested that modulus of elasticity isdetermined predominantly by the bonding between particles, whilemicrohardness is largely dependent on the strength of the solid component.In the case of interparticle bonding, compaction causes bond fracture andparticle slippage, and once the compacting force is removed only a fractionof the interparticle bonds will remain unaffected. Hence the property thatdepends on the strength of these interparticle forces, i.e. modulus ofelasticity, will decrease. By contrast, microhardness depends on the extentto which the measurement of hardness reflects the value for the crystallinecomponent, rather than the matrix phase, and this would favourcompacted specimens. These explanations are somewhat speculative andqualitative, and it is not clear just how helpful they are in understandingthe effect of compaction on these cements.

One important finding which did emerge from the work of Beaudoin &Ramachandran (1978) was that, contrary to previous assumptions,

303


magnesium oxysulphate cements are not significantly weaker than mag-nesium oxychloride cements, provided equal porosities are considered.Previous strength measurements had been done on samples at varyingporosities, and since the oxysulphate cement more readily develops aporous structure, such a comparison always showed it up unfavourably.However, when specimens of equal porosity were examined, the oxy-chloride cements proved to be only marginally stronger than theoxysulphate cements.

Overall, the major conclusion from this study was that the magnesiumoxysulphate cement system responds differently from the Portland cementsystem to being formed under compaction. The principal difference is thatthe oxysulphate cements are weakened by compaction. This appears to bebecause compaction, particularly at higher porosities, alters the mor-phology of the particulate phase, changes the pore size distribution andreduces the extent of bonding between particles. Nevertheless, from thepractical point of view, pressing oxysulphate cements in order to formthem is capable of producing specimens of adequate strength for use inarchitectural applications.

7.5 Other oxysalt bonded cements

The three major oxysalt bonded cements that have already been describedin detail in this chapter are not the only ones that have been prepared,though they are the ones that have been the most thoroughly studied. Forexample, Demediuk, Cole & Hueber (1955) gave some details about thecalcium analogues of the magnesium oxychloride cements. Like themagnesium cements, they were fabricated by reaction of powdered metaloxide with aqueous solutions of metal chloride. The resulting calciumoxychloride cements were similar to the magnesium oxychloride cements.However, unlike the latter materials, they have found few or noarchitectural or similar applications, and as a result there has been hardlyany interest in developing an understanding of their setting chemistry orstructure.

In a similar way there has been a passing reference to a cobaltoxychloride cement (Prosser et ai, 1986). No explicit details of thefabrication or chemical behaviour of this material were provided, but theingredients were listed among series of acids and bases for forming cementsas agents for the sustained release of trace elements to grazing animals. Theimplication of this paper was that cobalt oxide would function as the base

304

References

and aqueous cobalt chloride as the acid, and that these two compoundswould form cements that were chemically and structurally similar to thezinc and magnesium oxychloride materials.

ReferencesAdomavichiute, O. B., Yanitskii, I. V. & Vektaris, B. I. (1962). On the

hardening of magnesium cement. Zhurnal Priklandnoi Khimii, 35, 2551-4.Aspelund, H. (1933). Basic salts of bivalent metals: II. Acta Academiae

Aboensis, 7 (6), 1-25.Beaudoin, J. J. & Feldman, R. F. (1975). Mechanical properties of autoclaved

calcium silicate systems. Cement and Concrete Research, 5 (2), 103-18.Beaudoin, J. J. & Ramachandran, V. S. (1975). Strength development in

magnesium oxychloride and other cements. Cement and Concrete, 5 (6),617-30.

Beaudoin, J. J. & Ramachandran, V. S. (1978). Strength development inmagnesium oxysulphate cement. Cement and Concrete, 8 (1), 103-12.

Beaudoin, J. J., Ramachandran, V. S. & Feldman, R. F. (1977). Impregnationof magnesium oxychloride cement with sulphur. Ceramic Bulletin, 56, 424-7.

Bury, C. R. & Davies, E. R. H. (1932). System magnesium oxide-magnesiumchloride-water. Journal of the Chemical Society, 2008-15.

Cole, W. F. & Demediuk, T. (1955). X-ray, thermal and dehydration studieson magnesium oxychlorides. Australian Journal of Chemistry, 8, 234-51.

Demediuk, T. & Cole, W. F. (1957). A study of magnesium oxysulphates.Australian Journal of Chemistry, 10, 287-94.

Demediuk, T., Cole, W. F. & Hueber, H. V. (1955). Studies on magnesium andcalcium oxychlorides. Australian Journal of Chemistry, 8, 215-33.

Droit, A. (1910). Oxychlorides of zinc. Comptes rendus hebdomadaires desseances de V Academie des Sciences, 150, 1426-8.

Eubank, W. R. (1951). Calcination studies of magnesium oxides. Journal of theAmerican Ceramic Society, 34, 225-9.

Feitknecht, W. (1930). Reaction of solid substances in liquids: 1. HelveticaChimica Acta, 13, 22-43.

Feitknecht, W. (1933). Structure of the basic salts of bivalent metals. HelveticaChimica Acta, 16, 427-54.

Feitknecht, W., Ostwald, H. R. & Forsberg, H. E. (1959). Uber die Struktur derHydroxidchloride MeOHCl. Chimia, 13 (4), 113.

Forsberg, H. E. & Nowacki, W. (1959). Crystal structure of ZnOHCl. ActaChemica Scandinavica, 13 (5), 1049-50.

Greenwood, N. N. & Earnshaw, A. (1984). The Chemistry of the Elements.Oxford: Pergamon Press.

Harper, F. C. (1967). Effect of calcination temperature on the properties ofmagnesium oxides for use in magnesium oxychloride cements. Journal ofApplied Chemistry, 17, 5-10.

Hayek, E. (1932). Basic salts: I. Zeitschrift fur anorganische undallgemeineChemie, 207, 41-5.

305


Holland, H. C. (1930). The ternary system zinc oxide-zinc chloride-water.Journal of the Chemical Society, 643-8.

Mellor, J. W. (1925). A Comprehensive Treatise on Inorganic and TheoreticalChemistry, vol. IV, pp. 535-46. London: Longmans Green.

Montemartini, C. & Losana, L. (1929). Equilibria between double sulfates andaqueous solutions of sulfuric acids of various concentrations. IndustriaChimica (Rome), 4, 199-205.

Nowacki, W. & Silverman, J. N. (1961). Crystal structure of zinchydroxychloride II, Zn5(OH)8Cl2.1H2O. Zeitschriftfur Kristallographie,Kristallgeometrie, Kristallphysik, Kristallchemie, 115, 21-51.

Nowacki, W. & Silverman, J. N. (1962). Appendix to the paper 'Crystalstructure of zinc hydroxychloride II, Zn5(OH)8Cl2.1H2O\ Zeitschrift furKristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie, 117, 238-40.

Prosser, H. J., Wilson, A. D., Groffman, D. M., Brookman, P. J., Allen, W. M.,Gleed, P. T., Manston, R. & Sansom, B. F. (1986). The development ofacid-base cements as formulations for the controlled release of trace elements.Biomaterials, 7, 109-12.

Robinson, W. O. & Waggaman, W. H. (1909). Basic magnesium chlorides.Journal of Physical Chemistry, 13, 673-8.

Sorel, S. (1855). Procedure for the formation of a very solid cement by theaction of a chloride on the oxide of zinc. Comptes rendus hebdomadaires desseances de P Academie des sciences, 41, 784-5.

Sorel, S. (1867). On a new magnesium cement. Comptes rendus hebdomadairesdes seances de TAcademie des sciences, 65, 102^.

Sorrell, C. A. (1977). Suggested chemistry of zinc oxychloride cements. Journalof the American Ceramic Society, 60, 217-20.

Sorrell, C. A. & Armstrong, C. R. (1976). Reactions and equilibria inmagnesium oxychloride cements. Journal of the American Ceramic Society, 59,51-4.

Urwongse, L. & Sorrell, C. A. (1980a). The system MgO-MgCl2-H2O. Journalof the American Ceramic Society, 63, 501—4.

Urwongse, L. & Sorrell, C. A. (1980b). Phase relationships in magnesiumoxysulfate cements. Journal of the American Ceramic Society, 63, 523-6.

Walter-Levy, L. (1937). Neutral chlorocarbonate of magnesium. Comptes rendushebdomadaires des seances de VAcademie des Sciences, 204, 1943-6.

Walter-Levy, L. (1938). Contribution to the study of halogenocarbonates ofmagnesium. Comptes rendus hebdomadaires des seances de VAcademie dessciences, 205, 1405—7.

de Wolff, P. M. & Walter-Levy, L. (1953). Crystal structure of Mg2(OH)3(Cl,Br), 4H2O. Acta Crystallographica, 6, 40-4.

306

8 Miscellaneous aqueous cements

8.1 General

This chapter is devoted to a miscellaneous group of aqueous acid-basecements that do not fit into other categories. There are numerous cementsin this group. Although many are of little practical interest, some are oftheoretical interest, while others have considerable potential as sustained-release devices and biomedical materials. Deserving of special mention asbiomedical materials of the future are the recently invented polyelectrolytecements based on poly(vinylphosphonic acids), which are related both tothe orthophosphoric acid and poly(alkenoic acid) cements.

8.2 Miscellaneous aluminosilicate glass cements

In 1968 Wilson published an account of his early search for alternatives toorthophosphoric acid as a cement-former with aluminosilicate glasses.Aluminosilicate glasses of the type used in dental silicate cements were usedin the study and were reacted with concentrated solutions of variousorganic and inorganic acids. Wilson (1968) made certain general observa-tions on the nature of cement formation which apply to all cements basedon aluminosilicate glasses.

(1) Silica gel is formed in the reaction but is not associated withcement formation.

(2) Water is essential to the reaction and cements are not formedwhen the acid is present in an organic solvent rather than inaqueous solution. Water acts as an effective reaction medium andprobably hydrates reaction products.

(3) Cements are formed only with acids that are capable of formingcomplexes with calcium and aluminium. Thus, neither hydro-chloric nor acetic acid forms cements.

307

Miscellaneous aqueous cements

Table 8.1. Properties of aluminosilicate glass cements prepared withvarious acids in aqueous solution {Wilson, 1968)

Acidaqueous solution,% by mass

Tannic acid, 50 %Tartaric acid, 50 %Citric acid, 50 %Pyruvic acid, 50 %Malic acid, 50%Fluoboric acid, 42 %Glycerol phosphoric

acid, 35%Conventional silicate

cement phosphoricacid liquid

Powder:liquid,gem"3

40404040403-540

40

Settingtime,minutes

8-04-56-55-5

1502-23-2

3-5

Compressivestrength(24 hour),MPa

7775423635

10638

272

Water-leachablematerial,"% mass

1115CD.CD.CD.6-33-3

01

a On 24-hour-old cements.CD. Complete disintegration.

(4) Cements set rapidly.(5) Cement formation with carboxylic acids is associated with

carboxylate (salt) formation.(6) The ability of a cement to resist aqueous attack depends on the

nature of the acid anion.

The properties of these cements are given in Table 8.1.All the cement-forming organic acids were multifunctional and capable

of forming strong chelates with aluminium and weaker chelates withcalcium (Perrin, 1964; Martell & Calvin, 1952). The cements of citric,pyruvic and malic acids were found to disintegrate completely when placedin water, in accordance with the soluble nature of their metal complexes.The disintegration of the tartrate cement, though substantial, wasincomplete - the aluminium salt is soluble but that of calcium is not. Thetannic acid cement alone had reasonable hydrolytic stability, but thentannic acid forms an insoluble salt with aluminium (Welcher, 1947). Thiscement was the strongest, with a compressive strength of 77 MPa.

Fluoboric acid was also found to form a cement of some strength(106 MPa) but of poor hydrolytic stability.

308

Phytic acid cements

From the practical point of view these results were disappointing, but itwas from this unpromising start that the successful glass polyalkenoatecement was eventually developed.

8.3 Phytic acid cements

Phytic acid, myo-inositol hexakis(dihydrogen phosphate), is an abundantconstituent of plants (Graf, 1986). Its structure is that of a symmetrical six-membered ring carrying six dihydrogen phosphate groups (Figure 8.1).This structure suggests that it should form strong complexes, and this is so,with the strength of the complex increasing with the valency of the cation(Graf, 1986). Most of the complexes with polyvalent cations are insoluble,which makes phytic acid a candidate for cement formation. Indeed, it hasbeen found to form cements with zinc oxide and aluminosilicate glasses(Lion Corporation, 1980; Prosser et ai, 1983; Prosser & Wilson, 1986) andthese may be compared with phosphate-bonded cements.

Cements based on phytic acid set more quickly than their glasspolyalkenoate or dental silicate cement counterparts, but have similarmechanical properties (Table 8.2). They are unique among acid-basecements in being impervious to acid attack at pH = 2-7. Unfortunately,they share with the dental silicate cement the disadvantage of not adheringto dentine. They do bond to enamel but this is by micromechanicalattachment - the cement etches enamel - and not by molecular bonding.Lack of adhesive property is a grave weakness in a modern dental or bone

Figure 8.1 The structure of phytic acid.

309


Table 8.2. Properties of cements based on phytic acid (Prosser et al.,1983)

Powder: liquid, g cm 3



Compressive strength"(24 hour), MPa

Tensile strength"(24 hour), MPa

Opacity, C07

Water-leacnable material7 minute cure, % mass

Water-leachable material1 hour cure, % mass

Acid erosion pitting depth,urn/hour

G-200

Phyticacid, 40%

401-5

2-7

201

12-6

0-780-88

0-40

0

PAA50%

3050

4-3

166

140

0-8521

0-45

11-7

G-5

Phyticacid, 40 %

303-2

3-8

168

5-7

0-880-74

0-46

0

Cementliquid6

3-5—

50

183

13-0

0-551-25

0-70

12-3

a Specimens stored for 24 hours in water (37 °C).b Cement Liquid: 48-3% H3PO4; 2-3% Al; 4-9% Zn.G-200: 29-0% SiO2; 16-6% A12O3; 34-3% CaF2; 3-0% NaF; 2-0% A1F3;9-9% A1PO4.G-5: 43-7% SiO2; 23-0% A12O3; 12-9% CaF2; 10-5% NaF; 7% A1F3;2-9% A1PO4.

cement. Nor do phytic acid cements have the translucency of the dentalsilicate cement. Unless progress is made in remedying these two dis-advantages it is doubtful whether they will find any use despite theirexcellent resistance to erosion. More promising are the poly(vinyl-phosphonic acid) cements that are described next.

8.4 Poly(vinylphosphonic acid) cements

Ellis and Wilson studied cements formed from concentrated solutions ofpoly(vinylphosphonic acid) (PVPA) and oxides and silicate glasses, whichthey termed metal oxide and glass polyphosphonate cements (Wilson &

310

Poly{vinylphosphonic acid) cements

Ellis, 1989; Ellis, 1989; Ellis & Wilson, 1990, 1991, 1992; Ellis, Anstice &Wilson, 1991). They are poly electrolyte cements related to the poly-alkenoate cements and represent an attempt to improve on them. PVPA(Figure 8.2) has a structure similar to that of PAA (Figure 5.2).

PVPA was prepared by the free-radical homopolymerization of vinyl-phosphonyl dichloride using azobisisobutyronitrile as initiator in achlorinated solvent. The poly(vinylphosphonyl chloride) formed was thenhydrolysed to PVPA (Ellis, 1989). No values are available for the apparentpATas of PVPA, but unpolymerized dibasic phosphonic acids have pKal andpKa2 values similar to those of orthophosphoric acid, i.e. 2 and 8 (VanWazer, 1958). They are thus stronger acids than acrylic acid, which as apKa of 4-25, and it is to be expected that PVPA will be a stronger and morereactive acid than poly (aery lie acid).

8.4.1 Metal oxide polyphosphonate cements

Ellis & Wilson (1991, 1992) examined cement formation between a largenumber of metal oxides and PVPA solutions. They concluded that settingbehaviour was to be explained mainly in terms of basicity and reactivity,noting that cements were formed by reactive basic or amphoteric oxidesand not by inert or acidic ones (Table 8.3). Using infrared spectroscopythey found that, with one exception, cement formation was associated withsalt formation; the phosphonic acid band at 990 cm"1 diminished as thephosphonate band at 1060 cm"1 developed. The anomalous result was thatthe acidic boric oxide formed a cement which, however, was soluble inwater. This was the result, not of an acid-base reaction, but of complexformation. Infrared spectroscopy showed a shift in the P=O band from1160 cm"1 to 1130 cm"1, indicative of an interaction of the type

Setting times and hydrolytic stability of these cements are given in Table8.3. In some cases the speed of reaction was very high, and practicalcements could not be formed from ZnO or CaO even when these oxideswere deactivated by heating. All the faster-setting cements exhibited goodhydrolytic stability. The stability of the complexes between divalentcations and PVPA was found by a titrametric procedure to follow theorder Mg ~ Ca < Cu ~ Zn (Ellis & Wilson, 1991). This result was

311


Table 8.3. Properties of metal oxide phosphonic acid cements {Ellis, 1989;Ellis & Wilson, 1991)

Oxide

ZnO, MgO, CaO, Co(OH)2,HgO, CdO

Bi2O3, PbOB ACuO, Pb3O4, Y2O3, La2O3

Cu2OCoO, SnO, MoO3

Fe3O4, MnO2

In2O3, Cr2O3, CrO3

A12O3, SiO2, ZrO2, WO3

Setting time

< 1 minute

1-5 minutes1-5 minutes5-20 minutes

20-60 minutes1-24 hours1-24 hours1-24 hours

non-setting

Hydrolytic stability

Stable

stablecomplete disintegrationstablestablestablesoftened in watercomplete disintegration—

expected, since Cu2+ and Zn2+ with filled d orbitals tend to form bonds ofa covalent character. However, Mg2+ and Ca2+ do form stable cementswith PVPA, though not with PAA. This is to be attributed to the higherdegree of site binding of cations to PVPA compared with PAA (Begala &Strauss, 1972; Strauss & Leung, 1965).

ICH 2

ICH —

CH2

CH —

OH

OH

OH

OHFigure 8.2 The structure of poly(vinylphosphonic acid).

IP-O — MI

0I

M

= P - 0

0— M o'l\>I o iM I M

M MFigure 8.3 Possible structures of metal-polyphosphonate complexes.

312

Poly(vinylphosphonic acid) cements

Table 8.4. Properties of MgO, CuO, Cu2O, Bi2Oz, La2O z phosphoric acidcements (Ellis, 1989; Ellis & Wilson, 1990)

Oxide MgO CuO Cu2O Bi2O3 La2O3

Powder:liquid, gcm"3 10 50 50 50 50Setting time (37 °C), minutes 5-7 7-3 — 2-0 —Compressive strength 7 day (water), 56-6 54-5 31*5 18-2 11-9

MPaFlexural strength 7 day (water) 4-5 120 — 5-1 —

24 hour, MPaWater-leachable material 1 hour 0-24 00 — 005 —

cure, % massShrinkage, linear % 6-8 1-7 1-3 3-0 31

Table 8.5. Adhesive bond strength of the magnesium phosphonatecement to dentine and enamel (Ellis, 1989)

Cement

MgO-PVPAZinc polycarboxylate [1]Glass polyalkenoate [2]

Bond strength, MPa

Dentine

3-43-2-5-51-9-6-8

Enamel

4-94-1-6-93-2-9-9

[1] Walls (1986); [2] Wilson & McLean (1988b).

The exact nature of metal-poly(phosphonic acid) interaction is un-known (Ellis, 1989) although a number of structures can be drawn (Figure8.3).

The properties of the most promising cements - those of MgO, CuO,and Bi2O3-are given in Table 8.4. All had a short and sharp set.Compressive and flexural strengths, although moderate, compare fairlywell with those of commercial zinc polycarboxylate cements. All cementsshrank when exposed to an atmosphere of 50 % relative humidity. Themost promising is the cement based on MgO which adheres to both dentineand enamel with about the same bond strength as the glass polyalkenoateand zinc polycarboxylate cements (Table 8.5).

313


8.4.2 Glass polyphosphonate cements

Ellis and Wilson also examined cement formation from aluminosilicateglasses and concentrated solutions of PVPA (Wilson & Ellis, 1989; Ellis,1989; Ellis & Wilson, 1990). These cements, like the glass polyalkenoatecements, are a type of glass-ionomer cement.

Disadvantages of the glass polyalkenoate cements are the susceptibilityof the young cement to aqueous attack and problems in achieving sufficienttranslucency to match that of tooth enamel. PVPA has a higher refractiveindex than PAA, and should lessen the refractive index mismatch betweenglass particle and cement matrix which is the cause of light-scattering.PVPA is also a much stronger acid than PAA and the pendant groups arebifunctional. For this reason it should form stronger associations withcations and produce cements that set more rapidly.

The reactivity of the PVPA solutions with SiO2-Al2O3-CaO andSiO2-Al2O3-CaF2 glasses did, as expected, prove greater than that of PAAsolutions. Its cements had shorter setting times, as is shown by Figure 8.4which depicts the setting of cements formed from glasses having the genericcomposition (mole ratios) zSiO2, 1-00 A12O3, 1-30 CaF2 (Ellis, Anstice &Wilson, 1991). Note that the glass with Si/Al mole ratio of 2-55 formsa cement with PVPA but not with PAA. Decreasing the Si/Al mole ratioaccelerates cement formation, and the setting time of cements is corre-

1000 i -

0.5 1.0

Si / Al mole ratio

Clearglasses

2.0 2.5

Figure 8.4 The effect of Si/Al ratio in the glass on the setting time of glass polyphosphonateand glass polyalkenoate cements (Ellis, 1989).

314

Copper oxide and cobalt hydroxide cements

spondingly reduced. This result, which is similar to that found for glasspolyalkenoate cements, is to be expected as replacement of Si by Al mustweaken the glass network for reasons discussed in Section 5.9.2. When theSi/Al mole ratio reaches 1-1, glass PVPA cements set impossibly fast.Below a mole ratio of 0-57 the effect is reversed as phase separation offluorite (CaF2) and corundum (A12O3) occurs (glasses with an Si/Al ratioat or above 0-80 are clear). Phase separation reduces the reactivity of themain glass phase as the Si/Al ratio is increased and fluoride is withdrawn.

Heat treatment of the glasses at 600 °C for 6 hours reduced theirreactivity, by promoting phase separation, and prolonged the setting timeof cements. Thus, the setting time of the cement formed from one glass(Si:Al = 1-7) was increased from 2-0 minutes to 5-3 minutes and that ofanother glass (Si: Al = 2-0) from 3*6 minutes to 25 minutes.

These glass polyphosphonate cements are still in an early stage ofdevelopment. A recent paper by Ellis, Anstice & Wilson (1991) reports amaximum compressive strength of 90 MPa and a maximum flexuralstrength of 10 MPa. Although more recent data indicate that a compressivestrength of 150 MPa is possible these values are still much lower than thoserecorded for the best dental silicate and glass polyalkenoate cements -systems which are, of course, fully developed. The translucency of the glasspolyphosphonate cement is good, with an opacity value sufficiently low(C07 = 0-55) to match that of tooth enamel.

Its resistance to early contamination by water is very good and muchsuperior to that of the glass polyalkenoate cement. The solubility of aseven-minute-old cement is 0*5 % which compares favourably with valuesof 1-0-2-1 % reported for glass polyalkenoate cement (Wilson & McLean,1988a). The solubility of one-hour-old cements is infinitesimal (< 0-05%)and very much lower than that of the glass polyalkenoate cement(0-17-0-33%).

8.5 Miscellaneous copper oxide and cobalt hydroxide cements

Copper(II) oxide and cobalt(II) hydroxide form cements with solutions ofmany multifunctional organic acids: propanetricarboxylic acid, tartaricacid, malic acid, pyruvic acid, mellitic acid, gallic acid, tannic acid andphytic acid (Allen et al., 1984; Prosser et al., 1986). These have been usedmainly in cement devices for the sustained release of copper and cobalt(Manston et al., 1985; Manston & Gleed, 1985). Little is known about

315


their structure and mechanical properties. Most devices that have beenused in animal husbandry have been based on acid phosphates and havebeen dealt with in Section 6.3.

ReferencesAllen, W. M., Sansom, B. F., Wilson, A. D., Prosser, H. J. & Groffman, D. M.

(1984). Release cements. British Patent GB 2,123,693 B.Begala, A. J. & Strauss, U. P. (1972). Dilatometric studies of counterion binding

by polycarboxylates. Journal of Physical Chemistry, 76, 254-60.Ellis, J. (1989). Materials based on polyelectrolytes. PhD. Thesis (Council for

National Academic Awards): Thames Polytechnic and Laboratory of theGovernment Chemist, London.

Ellis, J., Anstice, M. & Wilson, A. D. (1991). The glass polyphosphonatecement: a novel glass-ionomer cement based on poly(vinylphosphonic acid).Clinical Materials, 7, 341-6.

Ellis, J. & Wilson, A. D. (1990). Polyphosphonate cements: a new class ofdental materials. Journal of Materials Science Letters, 9, 1058-60.

Ellis, J. & Wilson, A. D. (1991). A study of cements formed between metaloxides and polyvinylphosphonic acid. Polymer International, 24, 221-8.

Ellis, J. & Wilson, A. D. (1992). The formation and properties of metal oxidepoly(vinylphosphonic acid) cements. Dental Materials, 8, 79-84.

Graf, E. (1986). Chemistry and applications of phytic acid: an overview. InGraf, E. (ed.) Phytic Acid, Chapter 1. Minneapolis: Pilatus Press.

Lion Corporation. (1980). Dental cements. Nihon Kokai Tokkyo Koho80,139,311. Chemical Abstracts, 94, 903,488b.

Manston, R. & Gleed, P. T. (1985). Reaction cements as materials for thesustained release of trace elements into the digestive tract of cattle and sheep.II. Release of cobalt and selenium. Journal of Veterinary PharmacologyTherapeutics, 8, 374-81.

Manston, R., Sansom, B. F., Allen, W. M., Prosser, H. J., Groffman, D. M.,Brant, P. J. & Wilson, A. D. (1985). Reaction cements as materials for thesustained release of trace elements into the digestive tract of cattle and sheep.I. Copper release. Journal of Veterinary Pharmacology Therapeutics, 8,368-73.

Martell, A. E. & Calvin, M. (1952). Chemistry of the Chelate Compounds, p. 415.New York: Prentice-Hall.

Perrin, D. D. (1964). Organic Complexing Reagents, p. 269. New York:Interscience Publishers.

Prosser, H. J., Brant, P. J., Scott, R. P. & Wilson, A. D. (1983). The cement-forming properties of phytic acid. Journal of Dental Research, 62, 598-600.

Prosser, H. J. & Wilson, A. D. (1986). The cement-forming properties of phyticacid. In Graf, E. (ed.) Phytic Acid, Chapter 17. Minneapolis, Minnesota:Pilatus Press.

Prosser, H. J., Wilson, A. D., Groffman, D. M., Brookman, P. J.? Allen, W. M.,Gleed, P. T., Manston, R. & Sansom, B. F. (1986). The development of

316

References

acid-base reaction cements as formulations for the controlled release of traceelements. Biomaterials, 1, 109-12.

Strauss, U. P. & Leung, Y. P. (1965). Volume changes as a criterion for sitebinding of counterions by polyelectrolytes. Journal of the American ChemicalSociety, 87, 1476-80.

Van Wazer, J. R. (1958). Phosphorus and its Compounds, pp. 486-91. NewYork: Interscience Publishers Inc.

Walls, A. W. G. (1986). Glass polyalkenoate (glass-ionomer) cements: a review.Journal of Dentistry, 14, 231-46.

Welcher, F. J. (1947). Organic Analytical Reagents, vol. 2, pp. 142-3. NewYork: Van Nostrand.

Wilson, A. D. (1968). Dental Silicate Cements: VII. Alternative liquid cementformers. Journal of Dental Research, 47, 1133-6.

Wilson, A. D. & Ellis, J. (1989). Poly-vinylphosphonic acid glass ionomercement. British Patent Application 2,219,289A.

Wilson, A. D. & McLean, J. W. (1988a). Glass-ionomer Cement, Chapter 4.Chicago, London and Berlin: Quintessence Publishing Co.

Wilson, A. D. & McLean, J. W. (1988b). Glass-ionomer Cement, Chapter 6.Chicago, London and Berlin: Quintessence Publishing Co.

317

9 Non-aqueous cements

9.1 General

The non-aqueous or chelate cements are an exceptionally diverse group ofmaterials (Wilson, 1975a,b, 1978; Smith, 1982b). The term chelate cementis not strictly speaking correct, as a minority of them do not form chelates,and some aqueous AB cements do. However, the term is a convenient one.They are of interest in that the reaction media for the acid-base reactionare non-aqueous, although sometimes water may play a role in cementformation. In these cements water is replaced by an organic acid that isliquid at room temperature and generally has chelating ability.

The low permittivity of these liquids compared with water inhibitsdissociation of the acids so that cement formation demands much morereactive basic oxides. Oxides and hydroxides that are capable of cementformation are ZnO, CuO, MgO, CaO, Ca(OH)2, BaO, CdO, HgO, PbO andBi2O3 (Brauer, White & Moshonas, 1958; Nielsen, 1963). In practice theseare confined to two: calcium hydroxide and special reactive forms of zincoxide.

Examples of liquid organic acids suitable for cement formation are:

(1) Alkoxyphenols, for example the 2-methoxyphenols, which includeeugenol, guaiacol and vanillates. Also, 2,5-dimethoxyphenol.

(2) /?-diketones, e.g. acetylacetone.(3) /?-keto esters, e.g. jS-keto-ethylate.(4) Keto acids, e.g. acetyl acetic acid.(5) Other difunctional aliphatic carboxylic acids, e.g. lactic acid,

pyruvic acid, ethoxyacetic acid.(6) Aldehydic aromatic acids, e.g. salicylaldehyde.(7) Alkoxy aromatic acids, e.g. 2-ethoxybenzoic acid.

318

General

These examples are drawn from the work of Brauer, White & Moshonas(1958), Nielsen (1963), Brauer, Argentar & Durany (1964), Stansbury,Argentar & Brauer (1981), and Brauer, Argentar & Stansbury (1982).

All of these organic liquids are capable of forming chelates. They allcontain an acidic group (COOH, phenolic OH or enol OH) and a secondfunctional group, such as an ester or an ether, containing an oxygencapable of donating an electron pair. The structural requirement is that afive- or six-membered chelate ring is formed. Typical examples are shownin Figure 9.1.

That chelate formation is involved in cement formation is demonstratedby the behaviour of the methoxyphenols. The only methoxyphenols whichare capable of cement formation, the 2-substituted, are those which areable to form chelates (Brauer, Argentar & Durany, 1964). Further,Douglas (1978a,b) has shown that magnesium, calcium and zinc formchelates with potassium guaiacol-4-sulphonate, that the zinc chelate is thestrongest, and that at pH ~ 6 hydroxy complex formation does notseriously compete with chelate formation. In addition, chelate formationhas been reported between /?-diketones and a number of metals includingzinc and copper (Graddon, 1968). Some of these complexes are multi-nuclear. In zinc complexes the preferred coordination number is fivealthough four- and six-coordination are also observed.

Related to these cements are the long chain aliphatic acids and aryl-substituted butyric acid (Skinner, Molnar & Suarez, 1964). These materialsare on the market as non-eugenol cementing agents but they are unduly

(a) (b)

Figure 9.1 Chelating agents capable of forming cements, (a) 2-methoxyphenol type.Guaiacol: R, = R2 = H. Eugenol: Rx =-CH2-CH=CH2, R2 = H. (b) ^-diketone type.

319

Non-aqueous cements

weak and can only be used for the temporary cementation of crowns(Powers, Farah & Craig, 1976).

Although there are many potential chelate cements only three groupsare of any significance: the zinc oxide eugenol (ZOE) cement, the2-ethoxybenzoic acid (EBA) cements and the calcium hydroxide alkyl-salicylate cements. Only cements based on these three groups have beencommercially exploited. Their mechanical properties are poor, but all arenoted for having therapeutic effects and this is perhaps their mostimportant asset in biological applications. In effect, they are also devicesfor the sustained release of biologically active agents.

They are extensively used for temporary cementation of crowns and astemporary filling materials because of their therapeutic action. It is claimedthat when strengthened some can be used for permanent cementation or asan intermediate restoration, that is a restoration that has a life somewhatgreater than a temporary filling material.

However, it must be pointed out that these claims have been made on thebasis of strength measurements made at room temperature rather than37 °C (body temperature), and so may be misleading. This is especially trueof these materials. When measurements have been made at 37 °C (0ilo &Espevik, 1978), these cements, in contrast to the water-based ones, show amarked decline in all mechanical properties. Flow under load increasessharply and strength decreases by almost an order of magnitude. Thispoint must be borne in mind when reading the following section of thischapter where the reported strength measurements have been made atroom temperature.

9.2 Zinc oxide eugenol {ZOE) cements9.2.1 Introduction and history

The zinc oxide eugenol (ZOE) cement is formed by mixing a reactive formof zinc oxide with eugenol. It is widely used in dentistry for the temporarycementation of crowns, for the temporary filling of teeth, as a root canalsealer, as a sedative cavity liner, as a base in deep cavities and in soft tissuepacks for use in oral surgery (Brauer, 1965; Wilson, 1975b; Smith, 1982a).Suitably modified with diluents and fillers it can also be used as animpression paste (Phillips, 1982a,b).

The ZOE cement has a long history. Eugenol is the essential constituentof oil of cloves, which has been used medically since the fourth century

320

Zinc oxide eugenol (ZOE) cements

(Molnar, 1942). Its use specifically to relieve toothache was recorded byVigo in the sixteenth century and reactions with metal oxides were reportedby Bonastre (1827a,b). The earliest zinc oxide chelate cements usedcreosote (King, 1872) and later this was mixed with oil of cloves (Chisholm,1873). Then oil of cloves was used by itself (Flagg, 1875) and finally itsessential constituent, eugenol (Wessler, 1894).

This cement has retained its popularity to this day, despite its poormechanical properties, because it is easy to use - it makes no demands ontechnique - is tolerant towards living tissues and is a palliative or anodyne.It is unlikely to be replaced, because, fortuitously, it is formed from twomedicaments and, again fortuitously, is a sustained release agent foreugenol.

Cement formation is the result of an acid-base reaction between zincoxide and eugenol, leading to the formation of a zinc eugenolate chelate.Water plays a vital role in the reaction.

Important reviews on these materials have appeared from time to time:Brauer (1965), Wilson (1975a,b, 1978), Smith (1982a).

9.2.2 Eugenol

Eugenol, 4 allyl-2-methoxy phenol, is capable of forming cements withZnO, CuO, MgO, CaO, CdO, PbO and HgO (Brauer, White & Moshonas,1958; Nielsen, 1963). Other 2-methoxy phenols are also capable of formingcements with metal oxides, provided the allyl group is not in a 3- or6-position where it sterically hinders the reaction (Brauer, Argentar &Durany, 1964). These include guaiacol, 2-methoxyphenol, and the allyland propylene 2-methoxy phenols.

Eugenol is a very weak acid with a pK of 10-4 (Brauer, Argentar &Durany, 1964) and occurs as a hydrogen-bonded dimer (Gerner et al.,1966; Wilson & Mesley, 1972). The dimer contains both intra- andintermolecular hydrogen bonds (structure I in Figure 9.2a). The presenceof an intramolecular hydrogen bond indicates that eugenol is in the cisform.

9.2.3 Zinc oxide

Active zinc oxide is capable of forming chelate cements with a number ofliquid organic chelates. These include the /?-diketones, ketoacids andketoesters as well as the 2-methoxy phenols (Nielsen, 1963).

321

Non-aqueous cements

The zinc oxide used in ZOE cements differs entirely from that used inzinc phosphate cements. Whereas the latter has to be ignited to a very hightemperature to deactivate it, the opposite is true of the zinc oxides used inthe ZOE cement, which are of an activated variety. They are normallyprepared by the thermal decomposition of zinc salts at 350 °C to 450 °C;such oxides are yellow. Zinc oxides prepared by oxidizing zinc in oxygenmay also be used; these are white.

Further discussion of zinc oxide is deferred until the setting reaction isconsidered (Section 9.2.5).

9.2.4 Cement formation

The earliest ZOE cements were prepared simply by mixing zinc oxide witheugenol. These cements set under the moist conditions of the mouth andthen only slowly. Unlike other dental cements, the cement-formingreaction of the ZOE cement requires acceleration rather than retardation(Smith, 1958; Wilson & Batchelor, 1970; Crisp, Ambersley & Wilson,1980). Although it is possible to make cements from zinc oxide and plaineugenol by using a very reactive zinc oxide (Smith, 1960), the settingbehaviour of these cements is sensitive to variations in humidity and thesource of materials (Smith, 1982a). Commercial cements always containaccelerators.

The most common accelerators are acetic acid (0-1 % to 2%) dissolvedin the eugenol, and zinc acetate or other zinc alkanoate (0-1 % to 8%)blended with the zinc oxide powder (Wilson & Batchelor, 1970; Wilson1975b). Zinc propionate or succinate is also effective (Phillips, 1982b).Molnar & Skinner (1942) found that many salts acted as accelerators. Themost effective acetates were those of silver, sodium and zinc. Chlorides andnitrates were even more effective. Rosin (abietic acid) also produced anaccelerating effect. In practice, nearly all commercial materials used eitheracetic acid in the liquid, a zinc alkanoate, or even zinc eugenolate (20 %)itself. Zinc chloride has been found dissolved in massive amounts ineugenol (10 %); the resulting cement is a cross between a zinc oxychlorideand eugenolate cement. The question of 2-ethoxybenzoic acid (EBA) as anaccelerator is deferred until the EBA-eugenol cements are discussed(Section 9.4). The setting reaction of all cements is accelerated by increasein temperature.

322


9.2.5 Setting

Zinc oxide eugenol cements set and harden as the result of ionic reactions,and physical changes are related to these underlying chemical ones. Thesetting reaction has been studied systematically first by Copeland et al.(1955) and by a number of workers since. It is still not fully understood.The reaction is essentially an acid-base one with eugenol providinghydrogen ions. Copeland et al. (1955) established that the product ofreaction had the empirical molecular formula Zn(C10H11O2)2 and that itsXRD pattern corresponded closely to that of zinc eugenolate salt. Wilson& Mesley (1972) confirmed this finding.

The overall reaction can be represented by the following equation, whereHE represents eugenol and E eugenolate.

ZnO + 2HE = ZnE2 + H2O [9.1]

In fact, the reaction is an ionic one involving zinc and eugenolate ions.A number of infrared spectroscopic studies have been made which have

thrown light on the cement-forming reaction (Copeland et al., 1955;Gerner et al., 1966; Wilson & Mesley, 1972). Wilson & Mesley (1972) usedATR spectroscopy to follow the course of the reaction and showed thatmajor spectral changes were almost entirely associated with loss of the

CH2 = CH CH2 v CH2 = CH CH,

,0 - CH3

CH2CH = CH,

\

II I CH2 CH = CH2

Matrix Region Water eluted

Figure 9.2a Hydrolysis of the zinc eugenolate bis chelate to the hydrogen bonded eugenoldimer and zinc hydroxide. After Wilson & Mesley (1974).

323

Non-aqueous cements

O-H group. In the cement-forming reaction, phenolic hydrogens (O-Hstretching bands at 3520 and 3460 cm"1) are replaced by zinc ions, and aweak chelate is formed. In effect, the hydrogen bonds of the eugenol dimerare replaced by stronger Zn2+ bridges. Wilson & Mesley (1972) found littleor no free eugenol after 30 minutes, when the reaction was deemed to becomplete.

The bisligand chelate structure which is formed differs little from that ofthe parent eugenol dimer (Structure II in Figure 92a). The molecule is anelectrically neutral chelate where two eugenolate molecules are attached toa central zinc atom in square planar or tetrahedral configuration(Figure 92b).

The CH3O-Zn coordinate bond in the zinc eugenolate chelate is veryweak (Gerner et ai, 1966) and the chelate has poor stability thus, thecement-forming reaction [9.1] can be reversed. This occurs when thecement is placed in water, when the matrix is easily hydrolysed to eugenoland zinc hydroxide (Figure 9.2a) (Wilson, 1978; Wilson & Batchelor,

H2O

CH2=CH.CH

CH2=CH.CH

CH2=CH.CH

CH2.CH=CHo

CH2.CH=CH2

CH CH2.CH=CH2

H2O

Figure 9.2b The bisligand chelate structure of zinc eugenolate, showing bridging watermolecules. After Wilson & Mesley (1974).

324


1970). Eugenol can also be extracted from the cement matrix by methanol(Molnar, 1967); this is further evidence of the weakness of the chelate,which is decomposed during the extraction.

Water was not found in the set cement by Wilson & Mesley (1972), whoreasoned that it had entered the matrix. They speculated that zinc was inoctahedral coordination with two eugenolate molecules in planar positionsand two shared water molecules occupying diametrically opposed sites.These water molecules acted as bridges in the individual chelates.

This hypothesis received support from the electrical studies of Braden &Clarke (1974) and Crisp, Ambersley & Wilson (1980), who attributedmaxima in curves of permittivity and conductivity against time to theliberation of water and its subsequent reabsorption into the matrix (Figure93a,b). Crisp, Ambersley & Wilson (1980) also considered that thesemaxima were due to generation of both water and ionic zinc species.Subsequently, as the reaction proceeds the zinc ions are fixed as insolublezinc eugenolate.

160

I 120

80

40

20 40 120 140 160 180 200 220 240 260 280~TIME(minutM)

Figure 9.3a Permittivity/time curves (s) of cements, showing maxima. Prepared from ZnOignited at 600 °C with: A - - - A dry eugenol; # . . . • eugenol+1% water; O - . - Oeugenol + 1 % chloracetic acid; • • • • • eugenol + 1 % acetic acid; • . . . • eugenol + 1 %acetic a c i d + 1 % water. Cement powder/liquid ratio = 2-5 g cm"3 (Crisp, Ambersley &Wilson, 1980).

325

Non-aqueous cements

Eugenol is a very weak acid (pK= 10-4) and will not react with zincoxide in the absence of promoters. These reaction promoters includewater, acetic acid and zinc acetate.

The role of water in settingThe importance of water as an initiator and catalyst for the reactionbetween zinc oxide and plain eugenol has been demonstrated by a numberof studies (Smith, 1958; Crisp, Ambersley & Wilson, 1980; Batchelor &Wilson, 1969; Prosser & Wilson, 1982). In particular, the reaction isaccelerated by the humidity of the atmosphere during mixing (Batchelor &Wilson, 1969; Crisp, Ambersley & Wilson, 1980).

As we shall see later, the catalytic effect of water is connected with the

120 140 1M I NTim* (minute)

Figure 93b Conductivity/time curves (a) of cements, showing maxima. Prepared from ZnOignited at 600 °C with: A . . . A dry eugenol; * . . . • eugenol+1% water; O - . - Oeugenol + 1 % chloracetic acid; • • • • • eugenol + 1 % acetic acid; • . . . • eugenol + 1 %acetic a c i d + 1 % water. Cement powder/liquid ratio = 2-5 g cm"3 (Crisp, Ambersley &Wilson, 1980).

326


presence of loosely absorbed water on the surface of zinc oxide particlesand not with water contained in eugenol (Prosser & Wilson, 1982). Theaddition of water to eugenol has relatively little effect on the setting rate(Crisp, Ambersley & Wilson, 1980). On the other hand zinc oxide powdersthat do not have water absorbed on the surface react very slowly with plaineugenol (Prosser & Wilson, 1982). Of course, a reaction will take place evenif there is only a trace of water present, for water is generated in thereaction.

One might suppose that, since water is essential, the first step in thereaction would be the formation of zinc hydroxide. This is not so, forProsser & Wilson (1982) found that zinc hydroxide did not react witheugenol. They therefore agreed with the earlier suggestion of Crisp,Ambersley & Wilson (1980) that the reaction was activated by theformation of ZnOH+ on the surface of zinc oxide powders by water.Douglas (1978a,b) has also suggested that the reaction takes place in thewater layer or at the water-eugenol interface; this suggestion is based onmeasurement of zinc chelate formation constraints and the reflectancespectrum of the admittedly water-soluble guaiacol-4-sulphonate.

The reaction may be represented as follows. First, the generation ofions:

ZnO + H2O ± ZnOH+ + OH"ZnOH+ — Zn2+ + OH"

HE^±H+ + E"

bination of ions:

Zn2+ + 2E—ZnE 2

H+ + OH "^±H2OZnE2 + H2O ZnE2. H2O

[9.2][9.3][9.4]

[9.5][9.6][9.7]

The role of additives in settingThis scheme applies only to the reaction in simple zinc oxide and eugenolsystems. The presence of an accelerator such as zinc acetate profoundlymodifies it. The addition of Zn2+ ions or acetic acid (HAc) to the systemeliminates the need for water to initiate the reaction. The reactions can thenbe represented by the following series of equations.

ZnO + 2H+^=HE^±

Zn2+ + 2E"^±

Zn2+ + H2OH+ + E"ZnE,

[9.8][9.9]

[9.10]

327

Non-aqueous cements

Table 9.1. Effect of zinc oxide type on setting time ofZOE cements{Prosser & Wilson, 1982)

Cement Type

Indirect process ZnOThermally decomposed ZnO (500 °C)

Dry

15 days3 hours

Eugenol

Activated"

5-5 min90 min

a 1 % by mass of acetic acid added to eugenol. A powder:liquid 20 g cm 3 wasused for cement preparation.

In this case acetic acid and acetate are reaction promoters. The reactionis greatly accelerated and the setting time of cements is shortened (Table9.1).

The role of zinc oxide in settingThe physical and chemical characteristics of zinc oxide powders are knownto affect cement formation (Smith, 1958; Norman et ai, 1964; Crisp,Ambersley & Wilson, 1980; Prosser & Wilson, 1982). The rate of reactiondepends on the source, preparation, particle size and surface moisture ofthe powder. Crystallinity and lattice strain have also been suggested asfactors that may change the reactivity of zinc oxide powders towardseugenol (Smith, 1958).

Zinc oxide is made either by the oxidation of the metal in oxygen (theindirect, IP, or French process), by the direct decomposition of zinc ores inair (the direct or American process) or by the thermal decomposition ofzinc salts (TD zinc oxide). IP zinc oxides differ from TD zinc oxides in thattheir surfaces do not contain absorbed water. Also, whereas TD zinc oxidereacts with plain eugenol, IP zinc oxide hardly reacts unless activated by anacetic acid or zinc acetate accelerator (Table 9.2).

Particle size is the rate-controlling factor in the case of cements formedusing IP zinc oxide (Smith, 1958; Norman et ai, 1964). Setting time appearsto be proportional to the median particle size (Prosser & Wilson, 1982). Bycontrast, the setting times of cements prepared from TD zinc oxide do notappear to relate to particle size.

The heat treatment of zinc oxide powders reduces their reactivitytowards eugenol, because of an increase in particle size or a decrease inabsorbed water. In the case of zinc oxide powders prepared by the thermal

328


Table 9.2. Effect ofZnO ignition temperature on cement settingtime (Prosser & Wilson, 1982)

Ignition Setting Absorbed Particletemperature time water size

400500600800

°c°c°c°c

18 min3 hours

18 hours120 hours

1-26%—0-33%0-11%

0-33 jam0-65 urn—0-84 jam

Zinc oxide prepared by ignition of zinc carbonate. A powder: liquid2-0 g cm"3 was used for cement preparation.

decomposition of carbonate or oxalate, increasing the temperature of heattreatment causes them to react more slowly with eugenol (Smith, 1958;Crisp, Ambersley & Wilson, 1980; Prosser & Wilson, 1982). This isparalleled by a decrease in amount of water physically held on the surfaceof the powder and an increase in the particle size (Prosser & Wilson, 1982).These effects are shown in Table 9.2.

The effect of heating zinc oxide powders in various atmospheres hasbeen studied by several workers (Blackman, 1962, 1963; Lee andParravano, 1959; Marshall, Enrigh & Weyl, 1952; Dollimore & Spooner,1971; Prosser & Wilson, 1982). Heating causes sintering, a process ofcoalescence and densification, which finally leads to the formation of anon-porous body. For example, in air, freshly prepared zinc oxide spheressinter at 700 °C (Lee & Parravano, 1959). This sintering is shown clearly inelectronmicrographs of powders before and after heating at 800 °C(Prosser & Wilson, 1982; Figure 9Aa,b,c,d). Heating results in the loss ofoxygen which creates anion vacancies (F). The structure of such zincoxides may be represented as

Thus, there is excess of zinc over that required by stoichiometry. Sinteringwas attributed by Lee & Parravano (1959) to the diffusion of Zn2+ ionsconsequent on the difference in concentration of excess Zn2+ ions betweenthe surface and the bulk.

The presence of water on the oxide surface can enhance the sintering ofzinc oxide particles (Dollimore & Spooner, 1971). The amount of waterreversibly absorbed on zinc oxide surfaces is affected by heat treatment

329

Figure 9.4 The effect of sintering temperature on the morphology of zinc oxide particles. Zinc oxide from zinc oxalate: (a)400 °C, (b) 800 °C. Carmox zinc oxide (c) 400 °C, (d) 800 °C (Prosser & Wilson, 1982).


because of the reduction in specific surface area. Nagoe & Morimoto(1969) found that adsorption of water on zinc oxide particles reached amaximum after heating the oxide to 450 °C in vacuo and concluded thatboth chemisorption and physicosorption occurred. At this temperature,surface hydroxyls are lost from the surface:

-Zn-OH + HO-Zn- > -Zn-O-Zn- + H2O [9.11]

However, these are rapidly regenerated on exposure to water vapour. Thisability is lost on ignition to higher temperatures and must relate to theconversion of surface hydroxyls to oxygen bridges that are resistant torehydroxylation.

9.2.6 Structure

The set cement consists of zinc oxide particles bonded together by a loosematrix of zinc eugenolate (Wilson, Clinton & Miller, 1973). Electron-micrographs show that the zinc oxide particles are covered by zinceugenolate (Figure 9.5a).

Different workers have reported different results on the nature of thezinc eugenolate matrix. Earlier work and some recent work has indicated

Figure 9.5a Electronmicrograph of a ZOE cement matrix, showing zinc oxide particlescovered by zinc eugenolate (Wilson, Clinton & Miller, 1973).

331

Non-aqueous cements

that crystallites are present (Copeland et al., 1955; Wilson, Clinton &Miller, 1973; Bayne et aL, 1986). By contrast, Steinke et al (1988) foundthat the matrix was entirely amorphous.

El-Tahawi & Craig (1971), using thermal analysis, came to the followingconclusions. The setting of ZOE mixes, containing 0 to 1 % of zinc acetate,does not result in the formation of more than trace amounts of zinceugenolate crystallites, so that setting has nothing to do with the formationof a crystalline phase. Zinc eugenolate crystallites are formed in appreciableamounts only when the cement formulation contains large amounts of zincacetate. It is unlikely, therefore, that the classical explanation of Copelandet al (1955), that coherence is due to the interlocking of crystals, is correct.Recently, Bayne & Greener (1985) have cast doubt on El-Tahawi & Craig'sinterpretation, so the exact nature of the matrix remains to be resolved.

On exposure to water the matrix decomposes, with release of eugenol(Figure 9.2a). Wilson, Clinton & Miller (1973) found that the zinceugenolate matrix was degraded to a weak zinc hydroxide matrix and thezinc oxide particles were washed clean of zinc eugenolate (Figure 9.56).

Figure 9.56 Electronmicrograph of a ZOE cement matrix after aqueous attack. The zincoxide particles are washed clean of zinc eugenolate and the matrix is degraded to zinchydroxide (Wilson, Clinton & Miller, 1973).

332



The ZOE cement is easy to mix and a greater amount of powder can beincorporated into this cement (5:1 by mass) than any other, where even 4:1by mass is unusual. Because the ZOE cement is sensitive to moisture it canbe formulated to have a long working time under normal room conditions(23 °C, relative humidity 50 %) and a rapid set once placed in the warm andmoist conditions of the mouth. This is a considerable clinical advantage,making it convenient to use. The cement can be used in a war pack for useon the battlefield. Nevertheless, sensitiveness to humidity can give rise toproblems in use under tropical conditions.

The working time of commercial materials varies from 4 to 14 minutes(Plant, Jones & Wilson, 1972; Wilson, 1975a). The International Standard(ISO, 1988) requires that these cements when used for temporarycementation or as a cavity liner should set in 4 to 10 minutes and when usedas a base or for temporary restoration should set in 3 to 10 minutes. Thesetting of commercial ZOE bases can be rapid and so sufficient strengthcan be developed for an amalgam to be placed over them after 10 minutes(Plant & Wilson, 1970). Linear shrinkage during setting is high, 0-86 % dryand 0-32 % wet (Civjan & Brauer, 1964).

Compressive strength is much lower than that of the water-based dentalcements and ranges from 13 MPa to 38 MPa (24 hours) for unreinforcedmaterials (Brauer, 1972; Wilson, 1975a). ZOE cements are suitable for useas liners and temporary cements. Gilson & Myers (1970) pointed out thatZOE cements for temporary cementation should be strong enough toensure retention of devices yet weak enough to allow for ease of removal.They concluded from clinical studies that materials with compressivestrengths ranging from 2 to 24 MPa were required. The InternationalStandard (ISO, 1988) requires that for temporary cementation compressivestrength should not exceed 35 MPa and for use as liners must be greaterthan 5 MPa. Tensile strength is much lower, 1-2 to 2-8 MPa (Civjan &Brauer, 1964; Hannah & Smith, 1971).

These cements have marked creep characteristics and flow underpressure even when fully set. In this they contrast markedly with the rigidphosphate cements (Wilson & Lewis, 1980). This plastic behaviour explainswhy such cements provide a good seal despite a high setting shrinkage andthermal expansion of 35 x 10"6 X"1 (Civjan & Brauer, 1964).

The hydrolytic instability of ZOE cements, arising from the weakness ofthe zinc eugenolate chelate, has been discussed in Section 9.2.5. For this

333

Non-aqueous cements

reason these cements easily decompose under oral conditions. They cansurvive, however, when used as a liner where they are not exposed toaqueous conditions. Otherwise they are strictly temporary materials.Indeed, the weakening of the cement may prove an advantage when theyare used for the temporary cementation of crowns.

One serious fault of these materials is that the presence of an electron-rich phenolic hydroxyl group inhibits free-radical polymerization. Thus,composite resins placed over them do not polymerize completely.

9.2.8 Biological properties

Pulp reaction is minimal: there is a slight reduction in odontoblasts (cellsresponsible for dentine formation) but these recover in a few weeks(Wilson, 1975b). The sealing ability and bactericidal action appear tofacilitate pulpal healing (Beagrie, Main & Smith, 1972). The release ofeugenol by hydrolysis of the zinc eugenolate matrix relieves pain in thepulp in deep cavities (Smith, 1982a). There are, however, disadvantagesassociated with the release of eugenol. Eugenol (and zinc also) is cytotoxic,and causes toxic cell reactions (Coleman, 1962; Roydhouse & Weiss,1964). Thus, eugenol is an irritant and can cause inflammatory responsesin soft tissues (Beagrie, Main & Smith, 1972) with haemolysis and delayedhealing (Smith, 1982b). It is also a potential allergen (Smith, 1982a). Forthese reasons direct contact with connective tissues must be avoided.

For full accounts of biological responses the reader is referred to reviewsby Brauer (1965), Helgeland (1982) and Smith (1982b).

9.2.9 Modified cements

The ZOE cements are susceptible to modification. Modification by theaddition of accelerators has already been discussed in Sections 9.2.4 and9.2.5.

One of the oldest additives is rosin (abietic acid) which improvesworking, hardening rate and strength (Wallace & Hansen, 1939; Molnar &Skinner, 1942). El-Tahawi & Craig (1971) report that hydrogenated rosininhibits the formation of crystallites.

Some materials intended for temporary cementation and cavity liningare formulated as two pastes. One paste is formed by blending the zincoxide powder with a mineral or vegetable oil and the other by mixingan inert filler into the liquid. These cements are much weaker, with

334


compressive strengths between 1-7 and 7 MPa (Gilson & Myers, 1970).This strength is adequate for the temporary cementation of somerestorations. Mechanical properties are reduced by water immersionalthough the effect is much less than with simple cements. Mechanicalretention is less than for zinc phosphate cements (Grieve, 1969; Richter,Mitchem & Brown, 1970) but an 83*5% success rate has been reported(Silvey & Myers, 1976).

Sometimes antimicrobial agents such as thymol or 8-hydroxyquinolinemay be present (Wilson, 1975b). The latter is also capable of forming acementitious chelate with zinc.

9.2.10 Impression pastes

The ZOE impression paste is used in taking impressions of the mouth priorto constructing a denture. It is used as a corrective impression material aftera preliminary impression has been taken (Phillips, 1982a). A preliminaryimpression lacks detail so it is necessary to take a secondary or correctiveimpression which is placed on a tray that has the contours of thepreliminary impression.

The ZOE impression paste is essentially a two-paste ZOE cement. Onepaste is formed by plasticizing the zinc oxide powder with 13 % of mineralor vegetable oil. The other paste consists of 12 % eugenol or oil of cloves,50% polymerized rosin, 20% silica filler, 10% resinous balsam (toimprove flow) and 5 % calcium chloride (accelerator).

9.2.11 Conclusions

The ZOE cement has been in use for over 100 years and is still popularbecause it is easy to prepare and handle and has palliative and antisepticproperties. Its plastic nature ensures that it adapts well and even itsweakness is an advantage in its applications as a temporary filling materialand cementing agent. In these applications a cement must be sufficientlystrong for the restoration to be held in place for a limited period, but notso strong as to make removal difficult. Although not by intention, thecement happens to be prepared from two medicaments, eugenol and zincoxide, and these impart anodyne and antibacterial properties. Moreover,the cement is a sustained-release device for eugenol and zinc. Theseaccidental benefits make it a difficult material to supplant.

Surprisingly, although the material has been in use for over 100 years,the cement structure has yet to be fully elucidated.

335

Non-aqueous cements

9.3 Improved ZOE cements9.3.1 General

The main disadvantage of the ZOE cements is their low strength. Thefollowing approaches have been made to remedy this disadvantage.

(1) Incorporation of reinforcing fillers into the powder(2) Addition of reinforcing polymers to the liquid(3) Replacement of eugenol by other chelating agents(4) Replacement of zinc oxide by other oxides

9.3.2 Reinforced cements

Several attempts have been made to improve the strength of ZOE cementseither by adding fillers to the powder or by dissolving resin in the liquid(Wilson, 1975b; Smith, 1982a). Examples of fillers used include rosin,hydrogenated rosins, poly(methyl methacrylate), polystyrene, polycar-bonate, fused silica and dicalcium hydrogen phosphate.

Messing (1961) found that he could improve ZOE cements by adding10 % polystyrene to the liquid. Strength developed more rapidly than in anunmodified but accelerated cement and after 24 hours reached 42 MPacompared with 36 MPa.

The most important approach was to use poly(methyl methacrylate),PMMA, in formulations, either as a particulate filler or as a coating on zincoxide particles (Jendresen & Phillips, 1969; Jendresen et ai, 1969; Civjanet al, 1972). It is claimed that such materials can be used for permanent aswell as temporary cementation.

ZOE cements intended for permanent cementation are required by theInternational Standard (ISO, 1988) to set in 4 to 10 minutes with a filmthickness of 25 um.

The compressive strength of these reinforced ZOE cements ranges from36 to 52 MPa (Table 9.3). This compares with a strength of 13 to 38 MPafor unreinforced cements. ZOE cements are required by the InternationalStandard (ISO, 1988) to have a minimum compressive strength of 35 MPaif they are to be used for permanent cementation and 25 MPa if they areintended for use as a base or for temporary restoration.

The tensile strength of these materials is very much lower and liesbetween 2-6 and 4-2 MPa (Table 9.3) and their modulus of elasticity variesfrom 2-1 to 3-0 GPa (Powers, Farah & Craig, 1976). All these strength

336

2-ethoxybenzoic acid eugenol (EBA) cements

Table 9.3. Strength (MPa) ofZOE cements

ZOE simpleZOE acceleratedZOE reinforcedZOE pastesEBA reinforced

Compressive

13-38 [1,2]36-52 [1-4]1-7-7 [5]40-70 [4, 8]

Tensile

1-2-2-8 [1,6]1-2-2-1 [6]2-6- -2 [4, 6, 7]

—5-5-7-0 [4, 6, 7]

[1] Brauer (1972); [2] Wilson (1975b); [3] Jendresen et al (1969); [4]Powers, Farah & Craig (1976); [5] Gilson & Myers (1970); [6] Hannah& Smith (1971); [7] Williams & Smith (1971); [8] 0ilo & Espevik(1978).

figures have to be accepted with some reservations, for they representmeasurements made at room temperature and it is certain that thesecements are very much weaker at oral temperatures (0ilo & Espevik,1978).

9.4 2-ethoxybenzoic acid eugenol (EBA) cements9.4.1 General

The term EBA cement is not quite exact, but is convenient to use and hasbeen generally accepted. Originally this cement was a variant of the ZOEcement - its most important variant - and was based on a liquid which wasa mixture of 2-ethoxybenzoic acid (EBA) and eugenol. More recently,eugenol has been replaced by other compounds of similar structure. Allthese cements contain EBA as a major constituent.

The structure of EBA has been examined using infrared spectroscopy(Bagby & Greener, 1985) and there are apparently three conformations,two hydrogen-bonded (Figure 9.6).

9.4.2 Development

In Section 9.1 it was noted that this cement was the only important zincoxide cement other than the ZOE cement. Its invention and developmentis largely associated with Brauer who, with coworkers, has been carryingout a programme of research and development from the late 1950s to thepresent day.

In 1958, in an attempt to improve on the ZOE cement, Brauer, White &

337

Non-aqueous cements

Table 9.4. Composition of the EBA cement (Brauer, McLaughlin& Huget, 1968)

Liquid: 62-5 % 2-ethoxybenzoic acid37-5 % eugenol

Powder: 64% zinc oxide30 % tabular alumina (particle size from < 1 urn to 20 urn,

with few particles > 20 urn)6 % hydrogenated rosin

Moshonas (1958) investigated the reactions between zinc oxide and a largenumber of chelating agents. Of these, EBA proved to be the mostpromising. They then examined cement formation between EBA andvarious metal oxides. Cement formation was found with MgO, CaO, BaO,ZnO, CdO, HgO and PbO.

Finding that pure EBA cements tended to be unduly water soluble, theseworkers went on to study cement formation between zinc oxide andmixtures of EBA and eugenol. Small amounts of EBA added to eugenolproduced a marked acceleration in cement formation and reduced thesetting time from 2 hours to 3 minutes. Setting time remained short in therange 25 to 75 % EBA. From this basis, Brauer went on to develop what hetermed the EBA cement. Later studies showed that the optimum liquid hadthe composition 62-5 % EBA and 37*5 % eugenol, the EBA-eugenol liquid.This liquid is important for it was used in most of the later experimentalstudies and in commercial examples (Table 9.4).

The weakness of this cement was its tendency to dissolve in water. Thiswas prevented by including rosin (mainly abietic acid) or hydrogenatedrosin in the formulation (Brauer, Simon & Sangermano, 1962). Rosin andfused quartz or calcium hydrogen phosphate monohydrate were added to

,0 H — 0

0 — H-

C2H5 C 2H 5

Figure 9.6 The structure of 2-ethoxybenzoic acid, showing the two hydrogen-bondedconfigurations (Bagby & Greener, 1985).

338


the zinc oxide powder as strengthening fillers. The highest compressivestrengths were obtained with the EBA-eugenol liquid and two powdersthat contained respectively 10% rosin (71 MPa), and 6% rosin and 10 to30% quartz (74 to 81 MPa). These are stronger than the strongest of thereinforced ZOE cements (55 MPa).

In a further attempt to improve properties, Brauer, McLaughlin &Huget (1968) examined the use of alumina as a reinforcing filler. Aluminais considerably more rigid than fused quartz. They achieved a considerableincrease in strength. The preferred composition was the powder defined inTable 9.4, which had a compressive strength of 91 MPa. This zinc oxidebased powder was the one most commonly used in subsequent studies byBrauer and coworkers. We shall refer to it as the EBA powder for it is theone used in commercial formulations and in a number of experimentalstudies.

Although these materials had a high early (10-minute) compressivestrength of 46 MPa, their brittleness limited their use for the temporaryrestoration of multiple surfaces subject to heavy masticatory forces (Ciyjan& Brauer, 1964). Stress bearing was improved by incorporating powderedpolymethacrylate polymers of low elastic moduli into the zinc oxidepowder (Brauer, Huget & Termini, 1970). One such cement used a powdercontaining 58-2% zinc oxide, 27*3% alumina, 5-4% rosin and 9*1%methylmethacrylate copolymer. The compressive strength was 65 MPaand the tensile strength high at 11 MPa.

Despite all these endeavours, as Brauer himself admits, the rapiddisintegration under oral conditions prevents their use in permanentrestorations (Brauer, Stansbury & Argentar, 1983) and subsequentdevelopment has taken place in a quite different direction.

9.4.3 Setting and structure

Little is known of the setting reaction and structure of EBA cement. Theabsence of an infrared band at 1750 cm"1 in the set cement indicates that nounreacted COOH is present (Brauer, 1972). So far, it is not certain whetherzinc forms a six-membered chelate or merely a simple salt with EBA.Neither infrared spectroscopy nor solution studies are able to distinguishbetween these two forms. Eugenol is much less readily extracted and somore firmly bound in the complex than is EBA. The suspicion is that theEBA cement is fundamentally more prone to hydrolysis than the ZOEcement.

339

Non-aqueous cements

The only other studies are contained in one brief paper by Wilson &Mesley (1974). The infrared spectrum of the pure EBA cement containingexcess zinc oxide was found to correspond to a salt of EBA. The bindingmatrix was amorphous save for a trace of unidentified crystalline material.Infrared spectroscopy indicated that cements prepared from EBA-eugenolmixtures were not a simple mixture of the two parent cements. The mixedcements contained three crystalline phases: zinc eugenolate and twounidentified phases. Probably, one of these unidentified phases is a zinc 2-ethoxybenzoate and the other a zinc eugenolate-2-ethoxybenzoate chelatesalt. Different results were obtained when zinc oxide was present only instoichiometric amounts. Then the evidence was that only two crystallinephases were present: a zinc eugenolate and a zinc 2-ethoxybenzoate. Theindications are that there is an equilibrium:

ZnO

eugenolate + 2-ethoxybenzoate • eugenolate-2-ethoxybenzoate

Thus, if zinc 2-ethoxybenzoate is a weak chelate it will be preferentiallyextracted and the reaction will move to the left. Eventually the matrix willcontain only eugenol, as, indeed, Brauer (1972) found.

9.4.4 Properties

These cements have unusual rheological properties (Wilson, 1975b). Theycan be mixed to higher powder/liquid ratios (6:1 by mass, or more) thanany other dental cements and are very fluid. Whereas pastes of othercements behave as plastic bodies, the EBA cement has the characteristicsof a very viscous Newtonian liquid and flows under its own weight, evenwhen mixed very thickly (Wilson & Batchelor, 1971). High powder/liquidratios are required for optimum properties: 3-5 g cm"3 for luting and 5 to6 g cm"3 for linings and bases.

The linear setting shrinkage is 0-24 to 0-52% (dry) and 0-12 to 0-38%(wet), which compares with figures of 0-85 % (wet) and 0-31 % (dry) foundfor a ZOE cement by Civjan & Brauer (1964). The working and settingtimes of EBA cements range from 7 to 13 minutes (Smith, 1982a) and aredependent on both humidity and temperature. Film thickness ranges from40 to 70 |im which is greater than the 25 |im required by the InternationalStandard (ISO, 1988). Hembree, George & Hembree (1978) found thatunder simulated clinical conditions an alumina EBA cement always gave agreater film thickness (c. 50 Jim) than the 20 jim of the unfilled material.

340


Table 9.5. Strength (MPa) of reinforced EBA cements

ZOEEBAEBA-HVEBA-di-HVEBA-poly-HVGlass-EBA-poly-HVEBA polymer cement

Compressive

36-52 [1-4]40-70 [4, 8]42-60 [9]48-70 [10]67-70 [10]64-73 [10]73-112 [11]

Tensile

2-6-4-2 [4, 6, 7]5-5-7-0 [4, 6, 7]5-0-6-3 [9]6-2-7-5 [10]5-8-6-8 [10]9-9-11-2 [10]

101-15-8 [11]

[I] Brauer (1972); [2] Wilson (1975b); [3] Jendresen et al (1969);[4] Powers, Farah & Craig (1976); [6] Hannah & Smith (1971);[7] Williams & Smith (1971); [8] 0ilo & Espevik (1978); [9] Brauer,Stansbury & Argentar (1983); [10] Stansbury & Brauer (1985);[II] Brauer & Stansbury (1984b).

EBA cements have marked viscoelastic characteristics. They creep underload to an even greater extent than the ZOE cements (Wilson & Lewis,1980). When subject to a slowly increasing load they exhibit marked strainat fracture and low strength (0ilo & Espevik, 1978). These characteristicsmay be the reason why retention values for crowns and orthodontic bands,although better than other ZOE cements, are inferior to those of the zincphosphate cements (Williams, Swartz & Phillips, 1965; Grieve, 1969;Richter, Mitchem & Brown, 1970). Thermal expansion is 60 to90 x 10~6 °C-\ which is higher than that of the ZOE cement (Civjan &Brauer, 1964).

Compressive strength depends on powder/liquid ratio and test con-ditions. At the thin cementing consistency, the compressive strength ofthese materials ranges from 40 to 70 MPa (Powers, Farah & Craig, 1976;0ilo & Espevik, 1978). Tensile strength is much lower: 4-7 to 7-1 MPa(Williams & Smith, 1971; Powers, Farah & Craig, 1976; Hannah & Smith,1971). The modulus of elasticity is 5400 MPa (Powers, Farah & Craig,1976).

In vitro studies by Wilson et al. (1986) using the impinging jet methodshow the EBA cement to be the least resistant of all the cement types toerosion in neutral solution. Clinical studies confirm this result and showthat there is greater dissolution in the mouth than for other dental cements(Mitchem & Gronas, 1978; Osborne et al, 1978; Andrews and Hembree,1976; Shilling, 1977). Despite this, a clinical survey by Silvey and Myers

341

Non-aqueous cements

(1976,1977) indicated that the performance of an alumina-reinforced EBAcement over 3 years was only slightly worse than that of zinc phosphateand polycarboxylate cements.

Results for cement strengths are summarized in Table 9.5.

9.5 EBA-methoxyhydroxybenzoate cements9.5.1 EBA-vanillate and EBA-syringate cements

All cements that contain eugenol inhibit the polymerization of acrylates,and those of EBA-eugenol are no exception. In order to remedy this andother defects, Brauer and his coworkers examined alternatives to eugenol(Figure 9.7). These included the esters of vanillic acid (3-methoxy-4-hydroxybenzoic acid, HV) and syringic acid (3,5-dimethoxy-4-hydroxy-benzoic acid). Both are 3-methoxy-4-hydroxy compounds and are thuschemically related to eugenol and guaiacol. Both are solids and have to bedissolved in EBA where they form satisfactory cements with EBA zincoxide powder. The vanillate (EBA-HV) cements are the more important.

EBA-vanillate cementsUsing the EBA powder (Table 9.4) and liquids containing 12-5 to 18-3 % ofa vanillate ester (either n-hexyl, n-heptyl or n-decyl) Brauer, Stansbury &Argentar (1983) obtained cements that set in 4-5 to 5-5 minutes withcompressive strengths from 42 to 60 MPa and tensile strengths from 5-0to 6-3 MPa (Table 9.5). The preferred liquid was one containing 87-5%eugenol and 12-5% HV. Brauer & Stansbury (1984a) found also that theEBA-HV cements bonded much more strongly to composite resins(5-5 MPa), stainless steel (4 MPa), nickel-chrome (5 MPa) and porcelain(4-1 to 5-5 MPa) than the ZOE cement (Table 9-6).

COOR COOR

OCHq H3CO

(a)Figure 9.7 Structure of (a) vanillates and (b) syringates.

342

OCH3

EBA-methoxyhydroxybenzoate cements

Table 9.6. Bond strength (MPa) to substrates

ZOEEBA-HVEBA-poly HVSilica EBA-poly HVEBA polymer cement

Composite resin

0-3 [12]5-5 [12]3-8-6-9 [10]60-7-9 [10]4-1-10-3 [11]

Stainless steel

0-6 [12]4-1 [12]5-9-7-9 [10]7-1-9-4 [10]9-8-15-6 [11]

[10] Stansbury & Brauer (1985); [11] Brauer & Stansbury (1984b);[12] Brauer & Stansbury (1984a).

EBA-syringate cementsWith a liquid containing 7 to 14% n-hexyl syringate, Brauer & Stansbury(1984a) obtained cements that set in 4 minutes with a compressive strengthfrom 54 to 62 MPa and a tensile strength of 5-5 MPa, but they were brittle(Table 9.5). Replacement of n-hexyl syringate by 2-ethylhexyl syringateyielded cements that, depending on powder/liquid ratio, set in 6 to 9-5minutes with compressive strengths of 40 to 50 MPa and tensile strengthsof 5-2 to 5-7 MPa. These cements were less brittle than those of n-hexylsyringate.

The best formulation proved to be one based on a liquid containing 88 %EBA, 5% n-ethylhexyl syringate and 7% n-hexyl vanillate. Cementsprepared from these liquids set in 5-5 to 6-5 minutes with a compressivestrength of 66 MPa and tensile strength of 6 to 7 MPa.

All these vanillate and syringate cements are about as strong as those ofEBA-eugenol.

SettingThere is little information available on their setting and structure. Bagby &Greener (1985) used Fourier transform infrared spectroscopy (FTIR) toexamine the cement-forming reaction between zinc oxide and a mixture ofEBA and n-hexyl vanillate. Although they found evidence for reactionbetween zinc oxide and EBA, they were unable to find any for reactionbetween zinc oxide and n-hexyl vanillate because of peak overlaps, theminor concentration of n-hexyl vanillate and the subtle nature of thespectral changes.

343

Non-aqueous cements

AdvantagesBrauer (1988) considers that EBA-HV cements possess a number ofadvantages over eugenol cements. They bond much more strongly tocomposite resins and stainless steel than does the ZOE cement. They arecompatible with composite resins for, unlike eugenol, vanillates do notinhibit the polymerization of acrylate polymers, because the phenolichydroxyl is electron-poor. Brauer, Stansbury & Argentar (1983) speculatethat they are probably less toxic, as n-hexyl vanillate has been consideredas a food preservative. Vanillates should yield cements with bactericidalproperties. But all these supposed biological advantages need to besubstantiated. One recent evaluation by Keller et al. (1988) has shown thatthis cement has an acceptable performance when compared with theclinically acceptable zinc oxide eugenol and zinc phosphate cements.

Syringate cements possess similar advantages to the vanillate cements.In addition, syringic acid possesses cariostatic properties, so syringatesmay inhibit the development of caries (dental decay). Again theseadvantages need to be confirmed.

ModificationsBrauer, Stansbury & Flowers (1986) modified these cements in severalways. The addition of various acids - acetic, propionic, benzoic etc. -accelerated the set. The use of zinc oxide powders coated with propionicacid improved mixing, accelerated set, reduced brittleness and increasedcompressive strength from 63 to a maximum of 72 MPa. The addition ofplasticizing agents such as zinc undecenylate yielded flexible materials.Incorporation of metal powders had a deleterious effect and greatlyincreased the brittleness of these cements. The addition of fluorides was notvery successful, for fluoride release was not sustained.

9.5.2 EBA-divanillate and polymerized vanillate cements

Stansbury & Brauer (1985) investigated the use of divanillates in EBAcements (Figure 9.7). Divanillates consist of two vanillate groups linked byesterifying the COOH groups with diols. Cements based on EBAcontaining 10 to 11% of divanillates set in 4-5 to 5 minutes withcompressive strengths of 48 to 70 MPa and tensile strengths of 6-2 to7-5 MPa using the EBA zinc oxide powder (Table 9.5).

In addition, Stansbury & Brauer (1985), taking advantage of the factthat vanillates do not inhibit free-radical polymerization, incorporated

344

EBA-methoxyhydroxybenzoate cements

polymerizable vanillates in cement formulations.They used the methyl-acryloylethyl, -CH2. OOC. C(CH3)=CH2, ester of vanillic acid (Figure9.7), and added 3 to 10 % to the EBA liquid. In this system the zinc oxidepowder contained 1 % benzoyl peroxide as a polymerization initiator andthe accelerator, 7V,7V-dihydroxyethyl-/?-toluidine, was added to the liquid.These cements set in 6-5 to 8-5 minutes with compressive strengths of 67 to70 MPa and tensile strengths of 5-8 to 6-8 MPa (Table 9.6). Thus, thesecements are no stronger than other vanillate cements and they are brittle.They possess adhesive properties: bond strengths of 5*9 to 7-9 MPa tostainless steel and 3-8 to 6-9 MPa to composite resins were recorded (Table9.6).

Some improvement in physical properties was obtained by adding a fillerof silanized glass (325 mesh) to the EBA zinc oxide powder (1:1). Althoughthe compressive strengths of these reinforced cements were no greater thanthose of the unreinforced cement (64 to 73 MPa) there were significantimprovements in tensile strengths with values of 9-9 to 11-2 MPa beingobtained (Table 9.5). There was improved adhesion, with bond strengthsof 7-1 to 9-4 MPa to stainless steel and 6-0 to 7-9 MPa to composite resins(Table 9.6).

9.5.3 EBA-HVpolymer cements

The last stage in the development of the EBA cement is represented by thepolymer cements. Brauer & Stansbury (1984b), taking advantage of thefact that the EBA-HV liquid does not inhibit vinyl polymerization,included methacrylates into the cement composition. The object was toproduce a material that set after mixing, both by polymerization and bysalt or chelate formation.

The liquids used were 1:1 mixtures of EBA-HV and liquid methacrylatewhich also contained dihydroxyethyl-/?-toluidine as the accelerator. Bothmono- and di-methacrylates were used. The benzoyl peroxide initiator wasincluded in the EBA zinc oxide/silanized (1:1) glass powder. Thesepolymer cements set 5 to 10 minutes after mixing. Since there is asubstantial amount of monomer in the liquid (50%) the contribution ofthe polymer to the strength of the cement must be considerable. Brauer &Stansbury (1984b) suggested that the two matrices, the polymer matrix andthe salt matrix, may be interpenetrating; but separation of the two phasesis likely.

Very high strengths were obtained compared with other ZOE and EBA

345

Non-aqueous cements

cements. The most important monomethacrylates studied were methyl-,cyclohexyl- and dicyclophenyl- and mixtures of them. Cements containingthese monomers had compressive strengths ranging from 73 to 112 MPaand tensile strengths from 10-1 to 15-8 MPa (Table 9.5).

Good bonding was obtained to several substrates under aqueousconditions. Values obtained were 4-1 to 10-3 MPa to composite resins, and9-8 to 15-6 MPa to stainless steel (Table 9.6). They were also reported asadhering to porcelain. No adhesion was obtained to untreated dentine orenamel. The cements could be bonded to enamel etched with acid (3-5 MPa)and to dentine conditioned with poly(acrylic acid) (1-0 MPa).

The mechanism of adhesion to various substrates has not been fullyexplained. Brauer & Stansbury (1984b) consider that bonding to compositeresins occurs by the diffusion of methacrylate polymer chains into theresin. Bonding to base metals is, perhaps, by salt or chelate bridges. Hereit is significant that ZOE cements do not bond, so perhaps bonding is dueto the action of free EBA on the substrate. The adhesion to porcelain issurprising. Porcelain is inert so that the attachment can hardly be chemical.Also, it would be expected that if a cement adheres to porcelain then itshould adhere to untreated enamel and dentine, but this is not so.

The use of dimethacrylates led to even greater cement strengths.Compressive strengths ranged from 50 to 199 MPa, although the higherstrengths were obtained with pastes that were hardly workable and132 MPa represents the practical limit of compressive strength. Tensilestrength ranged from 8-0 to 15-9 MPa. Unfortunately, the EBA-dimethacrylate liquids were unstable and partial polymerization occurredwithin hours.

The biological properties of these materials are unknown, but thepresence of methacrylate monomers must adversely affect their bio-compatibility.

Brauer & Stansbury (1984b) claim that these materials can be used in'intermediate' restorations which are used in holding-type situationswhere an extensive cleaning-up regime is required over many weeks priorto the placement of a permanent restorative.

9.5.4 Conclusions

We have seen how Brauer and coworkers have developed EBA cementssince 1958, during which time they have become increasingly morecomplex. They are somewhat stronger than reinforced ZOE cements. EBA

346

Calcium hydroxide chelate cements

cannot be used alone but always requires the addition of a 2-methoxy-phenol. Chemical differences between the different 2-methoxyphenols havelittle effect on cement strength. Non-eugenol EBA cements have thedistinct advantage, however, in not inhibiting vinyl polymerization and arethus compatible with composite resins. This is an important attributebecause these cements are used as bases under composite resins. It alsomeans that methacrylate monomers can be mixed with EBA to givepolymer cements. These materials are considerably stronger than plaincements.

Unfortunately, although EBA cements have been subjected to aconsiderable amount of development, this work has not been matched byfundamental studies. Thus, the setting reactions, microstructures andmolecular structures of these EBA cements are still largely unknown. Inaddition, the mechanism of adhesion to various substrates has yet to beexplained. Such knowledge is a necessary basis for future developments.

9.5.5 Other zinc oxide cements

Skinner, Molnar & Suarez (1964) studied the cement-forming potential of28 liquid aromatic carboxylic acids with zinc oxide. Twelve yieldedcohesive products of some merit. Of particular interest were cementsformed with hydrocinnamic, cyclohexane carboxylic, ^-tertiary butyl-benzoic, thiobenzoic and cyclohexane butyric acids. One of these cementsis on the market as a non-eugenol cement. It is very weak with acompressive strength of 40 MPa, a tensile strength of 11 MPa and amodulus of 177 MPa, and is only suitable as a temporary material (Powers,Farah & Craig, 1976).

9.6 Calcium hydroxide chelate cements9.6.1 Introduction

Pastes of calcium hydroxide with water have been used as pulp-cappingmaterials for many years and it is the material of choice for this application(Granath, 1982). Its favourable tissue responses have been known formany years (Zander, 1939). It has a healing effect, for it induces theformation of hard tissues of reparative dentine when pulp has beenexposed (Eidelman, Finn & Koulourides, 1965). This action seems to beassociated with its high alkalinity (pH ~ 12-5) and consequent bactericidaland proteinlysing effect (Fisher, 1977).

347

Non-aqueous cements

Table 9.7. Composition of a calcium hydroxide chelate cement {AmericanDental Association, 1977)

Basic paste: 51 % calcium hydroxide9-23 % zinc hydroxide0-29 % zinc stearate

in JV-ethyl toluene sulphonamide

A cid paste: 13-8% titanium dioxide31*4% calcium sulphate15-2% calcium tungstate

in 1-methyl trimethylene disalicylate butane-1-3-diol ester

The manipulation of calcium hydroxide paste is not easy, however, andDougherty (1962) introduced the calcium hydroxide salicylate cements.These are based on the reaction between calcium hydroxide and salicylateesters and come in two-paste packs which are easy to mix in the dentalsurgery.

9.6.2 Composition

All commercial materials are based on calcium hydroxide and liquid alkylsalicylates (Prosser, Groffman & Wilson, 1982) and are supplied as a two-paste pack. Zinc oxide is sometimes added to the calcium hydroxide, as areneutral fillers. A paste is formed from this powder by the addition of aplasticizer; examples include Af-ethyl toluenesulphonamide (p- or/?-) andparaffin oil, with sometimes minor additions of polypropylene glycol. Theother paste is based on an alkyl salicylate as the active constituentcontaining an inorganic filler such as titanium dioxide, calciumsulphate, calcium tungstate or barium sulphate. Alkyl salicylates usedinclude methyl salicylate, isobutyl salicylate, and 1-methyl trimethylenedisalicylate. An example of one commercial material, Dycal, is given inTable 9.7, but its composition has been subjected to change over the years.

9.6.3 Setting

Prosser, Stuart & Wilson (1979) and Prosser, Groffman & Wilson (1982)examined the setting of a number of these cements using infraredspectroscopy. The infrared spectrum of the alkyl salicylates showed anO-H stretch band at 3190 cm"1 and a C-O stretch band at 1675-95 cm"1,

348


which were displaced from the normal frequencies of these groups becauseof very strong intramolecular hydrogen-bonding conjugate chelation. Thisconjugate chelation arises from resonance between the ester and itsenolized form, and is shown for an alkylsalicylate in Figure 9.8.

The essential chemical reaction is an acid-base one between calciumhydroxide and the phenolic group of an alkylsalicylate. Cement formationoccurs as calcium replaces phenolic hydrogen. During setting, the O-Hstretch band diminished as a carboxylate band appeared at 1540-60 cm"1

(asymmetric stretch). The ester band at 1675-95 cm"1 diminished, showinga conversion of C=O to C~O". The molecular structure can be representedas a chelate containing two bidendate ligands (Figure 9.9). In this case,unlike that of the zinc oxide eugenol cement, infrared spectroscopy canprovide proof of a chelate structure, although the setting reaction appearsto be very similar. The cement structure consists of an amorphousdisalicylate complex filled by the unreacted calcium hydroxide and otherinorganic fillers.

The case of the disalicylate, 1-methyl trimethylene disalicylate, isinteresting. Because of steric hindrance it is unlikely that the two salicylateligands can chelate to one calcium atom. In theory the disalicylate

Figure 9.8 Alkylsalicylate structure.

Figure 9.9 Chelate of calcium and an alkylsalicylate.

349

Non-aqueous cements

structure could bridge calcium ions and so form an ionically linked chain,but this too is unlikely. All the calcium hydroxide cements are weak andfriable, indicating that the chelates are bound together by only weaksecondary forces. The coordination number of calcium is usually six, andsince two water molecules are generated for every calcium ion during thecourse of the reaction, it is possible that these two water molecules areattached to the central calcium ion to form an octahedral complex. Thisstructure would be similar to that proposed for the zinc eugenolate cement,and water bridges may play a similar structural role.


These materials are prepared by mixing two strips of paste of equal length,and when mixed form fluid pastes. Some set rapidly, but setting depends onthe availability of water (Plant & Wilson, 1970; Bryant & Wing, 1976a,b).In contact with water one example set in 2 minutes, and in the absence ofwater another did not set at all (Bryant & Wing, 1976b). Normally they setin 3-5 to 8 minutes (Plant, Jones & Wilson, 1972).

They show marked plastic deformation under compressive load andalthough this decreases as they harden they still deform rather thanfracture when 25 minutes old (Plant & Wilson, 1970). This probablyaccounts for the observation that there is little incidence of fracture ordisplacement when amalgams are placed on these cements (Fisher, 1977).The cements are very weak and after 24 hours their compressive strengthsonly range from 11 to 14 MPa (Bryant & Wing, 1976b).

These materials are hydrolytically unstable and weaken when stored inwater for a week (Bryant & Wing, 1976b). Prosser, Groffman & Wilson(1982) found that calcium and hydroxide ions and salicylates were releasedand that the rate of release was controlled by the plasticizer used in thecement formulation. Hydrophilic sulphonamide plasticizers allowed readyingress of water and promoted decomposition, whereas the hydrophobichydrocarbon plasticizer repelled water and retarded hydrolytic decompo-sition.

9.6.5 Biological properties

Hydrolytic decomposition of these cements is clinically advantageous.Free calcium hydroxide is present in excess so that large amounts ofcalcium are released which, together with high alkalinity, promotes

350


sterilization and calcification of carious dentine (McWalter, El-Kafrawy &Mitchell, 1976). There is formation of dentine bridges when they are usedfor pulp capping.

Fisher (1977) found that the bactericidal effects varied from brand tobrand and Fisher & McCabe (1978) related this to chemical composition.Only cements which give rise to high alkalinity (pH = 11) are effective.These are the cements which are readily decomposed by water, and thisrelates to the plasticizer used. Hydrophilic plasticizers are required if thesecements are to be clinically effective.

Hydrolytic decomposition brings disadvantages. There is continuedleakage at the margins where complete dissolution can occur (Gourley &Rose, 1972) and, indeed, these bases have been observed to disappearentirely (Akester, 1979; Barnes & Kidd, 1979).

These cements are the materials of choice for pulp capping (a wounddressing for covering an exposed or surgically treated pulp). They aresuperior to zinc oxide eugenol cements for this purpose (Mjor, 1963;Paterson, 1976).

These materials also protect the pulp against invasion by acids fromoverlying dental cements of the phosphate or polyacrylate type and act asa barrier to the penetration of harmful chemicals such as the unpolymerizedmethacrylates (Smith, 1982a).

9.6.6 The calcium hydroxide dimer cement

Cowan & Teeter (1944) reported a new class of resinous substances basedon the zinc salts of dimerized unsaturated fatty acids such as linoleic andoleic acid. The latter is referred to as dimer acid. Later, Pellico (1974)described a dental composition based on the reaction between zinc oxideand either dimer or trimer acid. In an attempt to formulate calciumhydroxide cements which would be hydrolytically stable, Wilson et al.(1981) examined cement formation between calcium hydroxide and dimeracid. They found it necessary to incorporate an accelerator, aluminiumacetate hydrate, A12(OH)2(CH3COO)4. 3H2O, into the cement powder.

These cements set in 3-5 to 56 minutes (at 37 °C). Infrared spectroscopyshowed that as the cement set there was loss of acid carbonyl groups andOH groups associated with calcium hydroxide, and simultaneouslyformation of ionic carboxylate groups and hydrogen-bonded OH groups.

Although these cements were capable of setting under water and wereimpervious to aqueous attack they were not a success. They failed to

351

Non-aqueous cements

release calcium hydroxide into solution; consequently, there was noalkaline reaction and hence no favourable biological responses.

References

Andrews, J. T. & Hembree, J. H. (1976). In vivo evaluation of the marginalleakage of four inlay cements. Journal of Prosthetic Dentistry, 35, 532-7.

American Dental Association. (1977). Accepted Dental Therapeutics, 36th edn.,p. 235.

Akester, J. (1979). Disappearing Dycal. British Dental Journal, 146, 369.Bagby, M. & Greener, E. H. (1985). Infrared spectral analysis of EBA-hexyl

vanillate-ZnO cement. Dental Materials, 1, 86-8.Barnes, I. E. & Kidd, E. A. M. (1979). Disappearing Dycal. British Dental

Journal, 141, 111.Batchelor, R. F. & Wilson, A. D. (1969). Zinc oxide eugenol cements. I. The

effect of atmospheric conditions on rheological properties. Journal of DentalResearch, 48, 883-7.

Bayne, S. C. & Greener, E. H. (1985). ZnO cements: phase identification bythermal analysis. Dental Materials, 1, 165-9.

Bayne, S. C , Greener, E. H., Lautenschlager, E. P., Marshall, S. J. & Marshall,jr, G. W. (1986). Zinc eugenolate crystals: SEM detection andcharacterization. Dental Materials, 2, 1-5.

Beagrie, G. S., Main, J. H. P. & Smith, D. C. (1972). Inflammatory reactionevoked by zinc polyacrylate and zinc eugenate cements: a comparison. BritishDental Journal, 132, 351-7.

Blackman, L. C. F. (1962). Lattice defects and the sintering of oxides. IndustrialChemist, 38, 620-6.

Blackman, L. C. F. (1963). Lattice defects and the sintering of oxides. IndustrialChemist, 39, 23-6.

Bonastre, J. F. (1827a). De la combinaison des huiles volatiles de girofle et depimet de la Jamai'que, avec des alcalis et autres bases salifiables. Journal dePharmacie, 13, 464-76.

Bonastre, J. F. (1827b). De la combinaison des huiles volatiles de girofle et depiment de la Jamai'que avec des alcalis. 2. Combinaison d'huiles volatiles degirofle avec les oxides metalliques. Journal de Pharmacie, 13, 513-21.

Braden, M. & Clarke, R. L. (1974). Dielectric properties of zinc oxide-eugenoltype cements. Journal of Dental Research, 53, 1263-7.

Brauer, G. M. (1965). A review of zinc oxide-eugenol type filling materials andcements. Revue Beige de Medecine Dentaire, 20, 323—64.

Brauer, G. M. (1972). Cements containing 2-ethoxybenzoic acid (EBA).National Bureau of Standards Special Publication 354, pp. 101—11.

Brauer, G. M. (1988). Vanillate or syringate cements. Trends & Techniques, 5,No. 1, 6.

Brauer, G. M., Argentar, H. & Durany, G. (1964). Ionization constants andreactivity of isomers of eugenol. Journal of Research of the National Bureau ofStandards, 68A, 619-24.

352

References

Brauer, G. M., Argentar, H. & Stansbury, J. W. (1982). Cementitious dentalcompositions which do not inhibit polymerization. US Patent 4,362,510,December 7, 1982.

Brauer, G. M., Huget, E. F. & Termini, D. J. (1970). Plastic modifiedo-ethoxybenzoic acid cements as temporary restorative materials. Journalof Dental Research, 49, Supplement, 1487-94.

Brauer, G. M., McLaughlin, R. & Huget, E. F. (1968). Aluminum oxide as areinforcing agent for zinc oxide eugenol-o-ethoxybenzoic acid cements.Journal of Dental Research, 47, 622-8.

Brauer, G. M., Simon, L. & Sangermano, L. (1962). Improved zincoxide-eugenol type cements. Journal of Dental Research, 41, 1096-102.

Brauer, G. M. & Stansbury, J. W. (1984a). Cements containing syringic acidester-o-ethoxybenzoic acid and zinc oxide. Journal of Dental Research, 63,137^0.

Brauer, G. M. & Stansbury, J. W. (1984b). Intermediate restorative fromn-hexyl vanillate-EBA-ZnO-glass composites. Journal of Dental Research,63, 1315-20.

Brauer, G. M., Stansbury, J. W. & Argentar, H. (1983). Development of high-strength, acrylic resin-compatible adhesive cements. Journal of DentalResearch, 62, 366-70.

Brauer, G. M., Stansbury, J. W. & Flowers, D. (1986). Modification of cementscontaining vanillate or syringate esters. Dental Materials, 2, 21-7.

Brauer, G. M., White, jr, E. E. & Moshonas, M. G. (1958). The reaction ofmetal oxides with o-ethoxybenzoic acid and other chelating agents. Journal ofDental Research, 37, 547-60.

Bryant, R. W. & Wing, G. (1976a). The rate of development of strength in baseforming materials for amalgam restorations. Australian Dental Journal, 21,153-9.

Bryant, R. W. & Wing, G. (1976b). The effects of manipulative variables onbase forming materials for amalgam restorations. Australian Dental Journal,21,211-16.

Chisholm, E. S. (1873). Discussion. Dental Register, 27, 517.Civjan, S. & Brauer, G. M. (1964). Physical properties of cements based on zinc

oxide, hydrogenated resin, o-ethoxybenzoic acid and eugenol. Journal ofDental Research, 43, 281-99.

Civjan, S., Huget, E. F., Wolfhard, G. & Waddell, L. S. (1972). Characteristicsof zinc oxide eugenol cements reinforced with acrylic resin. Journal of DentalResearch, 51, 107-14.

Coleman, G. (1962). A study of some antimicrobial agents used in oral surgery.British Dental Journal, 113, 22-8.

Copeland, H. I., Brauer, G. M., Sweeney, W. T. & Forziati, A. F. (1955).Setting reaction of zinc oxide and eugenol. Journal of Research of the NationalBureau of Standards, 55, 133-8.

Cowan, J. H. & Teeter, H. M. (1944). Salts of residual dimerized fat acids: anew class of resinous substance. Industrial and Engineering Chemistry, 36,148-52.

353

Non-aqueous cements

Crisp, S., Ambersley, M. & Wilson, A. D. (1980). Zinc oxide eugenol cements.V. Instrumental studies of the catalysis and acceleration of the settingreaction. Journal of Dental Research, 59, 44-54.

Dollimore, D. & Spooner, P. (1971). Sintering studies on zinc oxide.Transactions of the Faraday Society, 67, 2750-9.

Dougherty, E. W. (1962). Dental cement material. US Patent 3,047,408.Douglas, W. H. (1978a). The metal oxide/eugenol cements. I. The chelating

power of eugenol type molecules. Journal of Dental Research, 57, 800-4.Douglas, W. H. (1978b). The metal oxide/eugenol cements. II. A diffuse

reflectance spectrophotometric study of the setting of zinc oxide andmagnesium oxide cements. Journal of Dental Research, 57, 805-9.

Eidelman, E., Finn, S. B. & Koulourides, T. (1965). Remineralization of cariousdentin treated with calcium hydroxide. Journal of Dentistry for Children, 32,218-25.

El-Tahawi, H. M. & Craig, R. G. (1971). Thermal analysis of zincoxide-eugenol cement. Journal of Dental Research, 50, 430-5.

Fisher, F. J. (1977). The effect of three proprietary lining materials on micro-organisms in carious dentine. An in vivo investigation. British Dental Journal,143, 231-5.

Fisher, F. J. & McCabe, J. F. (1978). Calcium hydroxide base materials: aninvestigation into the relationship between chemical structures andantibacterial properties. British Dental Journal, 144, 341-4.

Flagg, J. F. (1875). Dental pathology and therapeutics. Dental Cosmos, 27,465-9.

Gerner, M. M., Zadorozhnyi, B. A., Ryabina, L. V. & Batovskii, V. N. (1966).Infrared spectra of eugenol and zinc eugenolate. Russian Journal of PhysicalChemistry, 40, 122-3 (translation).

Gilson, T. D. & Myers, G. E. (1970). Clinical studies of dental cements. III.Seven zinc oxide eugenol cements used for temporarily cementing completedrestorations. Journal of Dental Research, 49, 14-20.

Gourley, J. M. & Rose, D. E. (1972). Cavity bases under liners. Journal of theCanadian Dental Association, 38, 246.

Graddon, D. P. (1968). Divalent transition metal /Mtetone-enolate complexes asLewis acids. Coordination Chemistry Reviews, 4, 1-28.

Granath, L. E. (1982). Pulp capping materials. In Smith, D. C. & Williams,D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibilityof Preventive Dental Materials and Bonding Agents, Chapter 11. Boca Raton:CRC Press Inc.

Grieve, A. R. (1969). A study of dental cements. British Dental Journal, 127,405-10.

Hannah, C. M. & Smith, D. C. (1971). Tensile strengths of selected dentalrestorative materials. Journal of Prosthetic Dentistry, 26, 314-23.

Helgeland, K. (1982). In vitro testing of dental cements. In Smith, D. C. &Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II.Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter9. Boca Raton: CRC Press Inc.

354

References

Hembree, J. H., George, T. A. & Hembree, M. E. (1978). Film thickness ofcements beneath complete crowns. Journal of Prosthetic Dentistry, 39, 533-5.

ISO. (1988). International Standard, ISO 3107. Dental zinc oxide/eugenolcements and zinc oxide non-eugenol cements.

Jendresen, M. D. & Phillips, R. W. (1969). A comparative study of four zincoxide eugenol formulations as restorative materials. Part II. Journal ofProsthetic Dentistry, 21, 300-9.

Jendresen, M. D., Phillips, R. W., Swartz, M. L. & Norman, R. D. (1969). Acomparative study of four zinc oxide eugenol formulations as restorativematerials. Part I. Journal of Prosthetic Dentistry, 21, 176-83.

Keller, J. C , Hammond, B. D., Kowlay, K. K. & Brauer, G. M. (1988).Biological evaluation of zinc hexyl vanillate cement using two in vivo testmethods. Dental Materials, 4, 341-50.

King, J. (1872). Treatment of exposed pulps. Dental Cosmos, 14, 193-4.Lee, V. J. & Parravano, G. (1959). Sintering reactions on zinc oxide. Journal of

Applied Physics, 30, 1735-^0.McWalter, G. K., El-Kafrawy, A. H. & Mitchell, D. F. (1976). Long term study

of pulp capping in monkeys with three agents. Journal of the American DentalAssociation, 93, 105-111.

Marshall, P. A., Enrigh, D. P. & Weyl, W. A. (1952). On the mechanism ofsintering and recrystallization of oxides. In The Proceedings of theInternational Symposium on the Reactivity of Solids, pp. 273-84. Gothenburg.

Messing, J. J. (1961). A polystyrene-fortified zinc oxide/eugenol cement.Investigation into its properties. British Dental Journal, 110, 95-100.

Mitcham, J. C. & Gronas, D. G. (1978). Clinical evaluation of cement solubility.Journal of Prosthetic Dentistry, 40, 453-6.

Mjor, I. A. (1963). The effects of calcium hydroxide zinc oxide/eugenol andamalgam on pulp. Odontologisk Tidsskrift, 71, 94-105.

Molnar, E. J. (1942). Cloves, oil of cloves and eugenol. Their medico-dentalhistory. Dental Items of Interest, 64, 521-8.

Molnar, E. J. (1967). Residual eugenol from zinc oxide-eugenol compounds.Journal of Dental Research, 46, 645-9.

Molnar, E. J. & Skinner, E. W. (1942). A study of zinc oxide-rosin cements. I.Some variables which affect hardening time. Journal of the American DentalAssociation, 29, 744-51.

Nagoe, M. & Morimoto, T. (1969). Differential heat of adsorption and entropyof water absorbed on zinc oxide surface. Journal of Physical Chemistry, 73,3809-14.

Nielsen, T. H. (1963). The ability of 39 chelating agents to form cements withmetal oxides, respecting their usability as root-filling materials. AdaOdontologica Scandinavica, 21, 159-74.

Norman, R. D., Phillips, R. W., Swartz, M. L. & Frankiewicz, T. (1964). Theeffect of particle size on the physical properties of zinc oxide-eugenolmixtures. Journal of Dental Research, 43, 252-62.


355

Non-aqueous cements


Paterson, R. C. (1976). The reaction of the rat molar pulp to various materials.British Dental Journal, 140, 93-6.

Pellico, H. A. (1974). Settable dental compositions. US Patent 3,837,865.Phillips, R. W. (1982a). Skinner's Science of Dental Materials, Chapter 7.

Philadelphia: W. B. Saunders.Phillips, R. W. (1982b). Skinner's Science of Dental Materials, Chapter 29.

Philadelphia: W. B. Saunders.Plant, C. G., Jones, I. H. & Wilson, H. J. (1972). Setting characteristics of lining

and cementing materials. British Dental Journal, 133, 21-74.Plant, C. G. & Wilson, H. J. (1970). Early strengths of lining materials. British

Dental Journal, 129, 269-74.Powers, J. M., Farah, J. W. & Craig, R. G. (1976). Modulus of elasticity and

strength properties of dental cements. Journal of the American DentalAssociation, 92, 588-91.

Prosser, H. J., Groffman, D. M. & Wilson, A. D. (1982). The effect ofcomposition on the erosion properties of calcium hydroxide cements. Journalof Dental Research, 61, 1431-5.

Prosser, H. J., Stuart, B. & Wilson, A. D. (1979). An infra-red spectroscopicstudy of the setting reaction of a calcium hydroxide dental cement. Journal ofMaterials Science, 14, 2894-900.

Prosser, H. J. & Wilson, A. D. (1982). Zinc oxide eugenol cements. VI. Effect ofzinc oxide type on the setting reactions. Journal of Biomedical MaterialsResearch, 16, 585-98.

Richter, W. A., Mitchem, J. C. & Brown, D. (1970). The predictability ofretentive value of dental cements. Journal of Prosthetic Dentistry, 24, 298-303.

Roydhouse, R. H. & Weiss, M. E. (1964). Tissue reactions in restorativematerials. Journal of Dental Research, 43, 807.

Shilling, G. (1977). The permanency of EBA cements. Journal of the AmericanDental Association, 95, 187-9.

Silvey, R. G. & Myers, G. E. (1977). Clinical studies of dental cements. V.Recall evaluation of restorations cemented with a zinc oxide-eugenol cementand a zinc phosphate. Journal of Dental Research, 55, 289-91.

Silvey, R. G. & Myers, G. E. (1976). Clinical studies of dental cements. VI. Astudy of zinc phosphate EBA-reinforced zinc oxide eugenol and polyacrylicacid cements as luting agents in fixed prostheses. Journal of Dental Research,56, 1215-18.

Skinner, E. W., Molnar, E. J. & Suarez, G. (1964). Reactions of zinc oxide withcarboxylic acids - physical properties. Journal of Dental Research, 43, 915.

Smith, D. C. (1958). The setting of zinc oxide/eugenol mixtures. British DentalJournal, 105, 313-21.

Smith, D. C. (1960). A quick-setting zinc oxide/eugenol mixture. British DentalJournal, 108, 232.

Smith, D. C. (1982a). Composition and characteristics of dental cements. In

356

References

Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials.Volume II. Biocompatibility of Preventive Dental Materials and BondingAgents, Chapter 8. Boca Raton: CRC Press Inc.

Smith, D. C. (1982b). Tissue reactions to cements. In Smith, D. C. & Williams,D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibilityof Preventive Dental Materials and Bonding Agents, Chapter 10. Boca Raton:CRC Press Inc.

Stansbury, J. W., Argentar, H. & Brauer, G. M. (1981). Cements from 2,5-dimethyloxyphenol and zinc oxide. Journal of Dental Research, 60, 373.

Stansbury, J. W. & Brauer, G. M. (1985). Divanillates and polymerizablevanillates as ingredients of dental cements. Journal of Biomedical MaterialsResearch, 19, 715-25.


Wallace, D. A. & Hansen, H. L. (1939). Zinc oxide eugenol cements. Journal ofthe American Dental Association, 26, 1536-40.

Wessler, J. (1894). Pulpol, ein neues medicamentoses Cement. DeutscheMonatsschrift fur Zahnheilkunde, 12, 478-84.

Williams, J. D., Swartz, M. L. & Phillips, R. W. (1965). Retention oforthodontic bands as influenced by the cementing media. Angle Orthodontics,4, 276-85.

Williams, P. D. & Smith, D. C. (1971). Measurement of the tensile strength ofdental restorative materials by use of a diametral compressive strength test.Journal of Dental Research, 50, 436—42.

Wilson, A. D. (1975a). Dental cements - general. In von Fraunhofer, J. A. (ed.)Scientific Aspects of Dental Materials, Chapter 4. London: Butterworths.

Wilson, A. D. (1975b). Zinc oxide dental cements. In von Fraunhofer, J. A. (ed.)Scientific Aspects of Dental Materials, Chapter 5. London: Butterworths.

Wilson, A. D. (1976). Examination of the test for compressive strength appliedto zinc oxide eugenol cements. Journal of Dental Research, 55, 142-7.


Wilson, A. D. & Batchelor, R. F. (1970). Zinc oxide eugenol cements. II. Studyof erosion & disintegration. Journal of Dental Research, 49, 593-8.

Wilson, A. D. & Batchelor, R. F. (1971). The consistency of dental cements. Thespecification test for filling materials. British Dental Journal, 130, 437-41.

Wilson, A. D., Clinton, D. J. & Miller, R. P. (1973). Zinc oxide eugenolcements. IV. Microstructure and hydrolysis. Journal of Dental Research, 52,253-60.

Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986). Anevaluation of the significance of the impinging jet method for measuring theacid erosion of dental cements. Biomaterials, 7, 55-60.

Wilson, A. D. & Lewis, B. G. (1980). The flow properties of dental cements.Journal of Biomedical Materials Research, 14, 383-91.

Wilson, A. D., Prosser, H. J., Paddon, J. M. & Gilhooley, R. A. (1981). Calciumhydroxide dimer (Cal-mer) cements. British Dental Journal, 150, 351-3.

357

Non-aqueous cements

Wilson, A. D. & Mesley, R. J. (1972). Zinc oxide eugenol cements. III. Infra-redspectroscopic studies. Journal of Dental Research, 51, 1581-8.

Wilson, A. D. & Mesley, R. J. (1974). Chemical nature of cementing matrices ofcements formed from zinc oxide and 2-ethoxybenzoic acid-eugenol liquids.Journal of Dental Research, 53, 146.

Zander, H. A. (1939). Reaction of the pulp to calcium hydroxide. Journal ofDental Research, 18, 373-9.

358

10 Experimental techniques for thestudy of acid-base cements

10.1 Introduction

The chief problem in studying the chemical nature of AB cements is thatmany are essentially amorphous, so that the powerful tool of X-raydiffraction (XRD) analysis cannot be used. Some AB cements do exhibit adegree of crystallinity, but rarely in significant amounts; indeed, completecrystallinity is usually a sign that the reaction product is not cementitious.The literature contains numerous examples of workers being misled bythe results of XRD analysis into neglecting the presence and significance ofthe amorphous phase.

A number of techniques have been employed that are capable of givinginformation about amorphous phases. These include infrared spectro-scopy, especially the use of the attenuated total reflection (ATR) or Fouriertransform (FT) techniques. They also include electron probe micro-analysis, scanning electron microscopy, and nuclear magnetic resonance(NMR) spectroscopy. Nor are wet chemical methods to be neglected forthey, too, form part of the armoury of methods that have been used toelucidate the chemistry and microstructure of these materials.

In addition to spectrosopic studies of the setting chemistry of ABcements, numerous mechanical tests have been used to measure propertiesof the set materials. This latter group has included determination ofcompressive and flexural strengths, translucency, electrical conductivityand permittivity. The present chapter describes each of these techniques inoutline, and shows how they have been applied. Results obtained usingthese techniques are described in earlier chapters which deal morethoroughly with each individual type of AB cement.

359

Experimental techniques

10.2 Chemical methods

Broadly speaking, two groups of purely chemical methods have beenemployed in the study of AB cements. These have been (1) studies ofcement formation and (2) degradative studies on set cements. The firstgroup has generally used the simple approach of attempting to react thecandidate acid, usually as an aqueous solution, with the candidate base,which is generally a powder that is only sparingly soluble in water. Thesuccess or otherwise of the attempted cementition has then been assessedby a simple criterion, such as stability of the product in water.

10.2.1 Studies of cement formation

A number of studies have been carried out using the criterion of waterstability to assess the success of cementation. For example, Hodd &Reader (1976) studied a range of metal oxide-poly acid cements by thistechnique in order to determine the influence of the metal cation and thepolymer structure on stability. A number of polymeric carboxylic acidswere used in this study, namely poly(acrylic acid), poly(ethylene-maleicacid), poly(methacrylic acid) and poly(vinyl methyl ether-maleic acid).Poly(ethylene sulphonic acid) was also used, but it proved to be a poorcement former and did not form stable cements with many of the cationsexamined. A large number of metal oxides of both main group andtransition metals were examined, and a number of them were found toform water-stable cements with most of the poly carboxylic acids. In rarecases, metal oxides formed stable cements with a few of the acids, butyielded only unstable mixtures with others. For example, bismuth oxide,Bi2O3, gave a stable cement with poly(acrylic acid) that showed nodisintegration after 16 hours immersion in water at room temperature, butwith ethylene-maleic acid gave a mixture which disintegrated completely.More common, however, was the behaviour of zinc oxide, which gavestable cements with all of the polycarboxylic acids.

An equally simple chemical study was carried out on phytic acid-aluminosilicate cements (Prosser et al, 1983). Phytic acid, myo-inositolhexakis(dihydrogen phosphate), is a naturally occurring substance foundin seeds, and it is a stronger acid than phosphoric acid. Cements wereprepared using aqueous solutions of phytic acid, concentrated to 50 wt%,and with 5 wt% zinc dissolved in the acid to moderate the rate of reactionwith the glass powder. Discs of cement were prepared and these were

360

Infrared spectroscopic analysis

placed in distilled water exactly seven minutes after the start of mixing ofthe aqueous acid with powder. Soluble material was determined gravi-metrically by evaporating the eluates to dryness. This approach demon-strated that these cements had excellent resistance to early attack by water.Other properties of these cements were promising for dental applicationand a patent protecting this use has been sought (Lion Corporation, 1980).

10.2.2 Degradative studies

Degradative methods have been employed on cements that have beenallowed to set. In a typical degradative study, Cook (1983) treatedglass-ionomer cements with 3-3-molar potassium hydroxide solution. Theresulting solution was analysed for release of ions using atomic absorptionspectroscopy. This overall degradation technique allowed the measure-ment of time-dependent concentrations of Al3+, Ca2+ and Na+ ions whichhad entered the matrix from the glass. Cook concluded that bothaluminium and calcium ions were involved in the initial setting reaction ofthese cements, although this has been disputed by other workers(Nicholson et ai, 1988b; Wilson & McLean, 1988). Cook also concludedthat aluminium ions were the least easily extracted of all the cationsremoved from the glass particles, which is not surprising given that thealuminium is present in the glass initially as part of the aluminosilicatenetwork structure, and is anionic in character (Hill & Wilson, 1988).

Extraction studies have also been carried out by grinding the ageingcements and extracting the soluble ions with water (Wilson & Kent, 1970;Crisp & Wilson, 1974). Ion content was determined using atomicabsorption spectroscopy. The experiments give different, but complemen-tary, results to those of Cook (1983), since what is extracted are those ionsthat have been released from the glass powder but not yet insolubilized byreaction with the polyacid.

10.3 Infrared spectroscopic analysis10.3.1 Basic principles

The infrared region of the electromagnetic spectrum lies between thewavelengths 1000 and 15000 nm (Kemp & Vellaccio, 1980). Absorption ofradiation in this region by organic compounds has been known since 1866,when Tyndall first conducted experiments on the interaction of radiationwith compounds such as chloroform, methyl and ethyl iodides, benzene,

361


ethyl acetate and so on (Tyndall, 1866). More systematic work in the 30 orso years from 1881 by a number of workers, most notably W. Coblentz,showed that the interaction was attributable to individual groups withinthe molecule (Cesaro & Torracca, 1988). From this deduction, and thesubsequent painstaking acquisition of empirical data, the modern ap-proach to infrared spectroscopy has emerged, in which individualfunctional groups can readily be identified and structural conclusions canbe drawn on the basis of a rapidly prepared infrared absorption spectrum.

In the most usual modern form of infrared spectroscopy, absorption isplotted against reciprocal wavelength or wavenumber in cm"1 (Williams &Fleming, 1973). The usual range of such a spectrum is 4000 cm"1 at thehigh-frequency end to 625 cm"1 at the low-frequency end. Functionalgroups in organic molecules absorb infrared radiation at well-defined partsof this spectrum. Although this actually occurs due to absorption by thewhole of a complex organic molecule, such an absorption can be consideredto a reasonable approximation, for a number of spectral bands, to belocalized at individual functional groups. These localized absorptions giverise to vibrations of the particular chemical bonds, which may includestretching, bending, rocking, twisting or wagging (Williams & Fleming,1973).

The specific requirement for a vibration to give rise to an absorption inthe infrared spectrum is that there should be a change in the dipole momentas that vibration occurs. In practice, this means that vibrations which arenot centrosymmetric are the ones of interest, and since the symmetryproperties of a molecule in the solid state may be different from those of thesame molecule in solution, the presence of bands may depend on thephysical state of the specimen. This may be an important phenomenon inapplying infrared spectroscopy to the study of AB cements.

10.3.2 Applications to AB cements

It is apparent from the preceding discussion that the kinds of moleculesgenerally studied by infrared spectroscopy are organic. This means that theAB cements which have been particularly studied by this technique arethose containing organic functional groups, most typically carboxylic acidand carboxylate groups. In particular, extensive studies have been carriedout on the setting reactions and final structures of glass-ionomer cements(Wilson & McLean, 1988), zinc polycarboxylates (Wilson, 1982) and othermetal oxide-poly(acrylic acid) cements (Crisp, Prosser & Wilson, 1976).

362

Infrared spectroscopic analysis

The range of acids for which infrared spectroscopy can be used islimited. Despite this, the amount of detailed information which can beobtained using the technique is very high. This is because of the extent andcomprehensiveness of the data available concerning the effect of subtlechanges in the bonding of carboxylate groups on the position of thecorresponding infrared absorption bands. Many detailed structures ofsimple metal carboxylates have been established by complementarytechniques, such as X-ray crystallography, and these findings have beenused to establish correlations between band position in the infraredspectrum and structure (Mehrotra & Bohra, 1983). There are limits to theextent to which band position and structure can be correlated, however,since there are exceptions to many of the apparently established empiricalrules. Moreover, some literature data have been shown to be plainly wrongsince they have ignored, for example, possible spectral changes caused bycation and halide exchange due to mounting the sample between alkali-metal halide discs (Deacon & Phillips, 1982). Nonetheless, some valuableconclusions have been drawn using general correlations of carboxylateband position with the nature of the bonding in set or setting cements.

Broadly speaking, in the study of AB cements derived from poly-carboxylic acids, the band of interest falls in the region 1550-1620 cm"1

(Mehrota & Bohra, 1983; Bellamy, 1975). This band is the asymmetricstretch of the carboxylate group and its exact position depends on both thenature of the bonding involved (i.e. whether purely ionic or partiallycovalent), and the nature of any chelation by the carboxylate group(Bellamy, 1975).

The four possible modes of carboxylate bonding which have beenidentified are purely ionic, unidentate, bridging bidentate and chelatingbidentate, as illustrated in Figure 5.3.

The ionic band falls at 1570-1575 cm"1, as it does in sodium and lithiumpoly(acrylates) (Nicholson & Wilson, 1987) as well as in the simplemonomeric carboxylates (Mehrotra & Bohra, 1983). The unidentatecovalent binding gives a band close to 1550 cm"1, as does the chelatingbidentate, while the bridging bidentate generally gives a band in the region1600-1620 cm"1. The unidentate mode of binding has been found to berelatively rare in monomeric carboxylate compounds (Mehrotra & Bohra,1983), and on these grounds was rejected as a probable structure inpolycarboxylate materials by Nicholson, Wasson & Wilson (1988).

363


10.3.3 Fourier transform infrared spectroscopy

In recent years, infrared spectroscopy has been enhanced by the possibilityof applying Fourier transform techniques to it. This improved spectro-scopic technique, known as Fourier transform infrared spectroscopy(FTIR), is of much greater sensitivity than conventional dispersive IRspectroscopy (Skoog & West, 1980). Moreover, use of the Fouriertransform technique enables spectra to be recorded extremely rapidly, withscan times of only 02 s. Thus it is possible to record spectra of AB cementsas they set. By comparison, conventional dispersive IR spectroscopyrequires long scan times for each spectrum, and hence is essentiallyrestricted to examining fully-set cements.

FTIR has been applied to both zinc polycarboxylate cements (Nicholsonet al., 1988a) and glass-ionomer cements (Nicholson et al, 1988b), in bothcases yielding significant findings. The zinc polycarboxylate was shown forthe first time to become partially covalent with time after setting, while therole of (+ )-tartaric acid in glass-ionomer cements was shown to be tosuppress early formation of calcium polyacrylate acid and to enhance laterformation of aluminium polyacrylate. These results are discussed in moredetail in Chapter 5.

10.4 Nuclear magnetic resonance spectroscopy10.4.1 Basic principles

The NMR spectrum can be recorded for compounds containing thoseelements whose nuclei have spin values of \ (Williams & Fleming, 1973).A large number of such nuclei exist, including 1H, 13C, 19F, 27A1 and 31P.Unfortunately, many other nuclei of importance in chemistry, such as 12Cand 16O, have nuclear spin values of 0 and hence do not give nuclearresonance signals in a magnetic field.

To study NMR spectra of compounds, apparatus is required thatconsists of three sets of components. These are a radio-frequencytransmitter, a homogeneous magnetic field and a radio-frequency receiver.In addition to these, the apparatus includes a unit to sweep the magneticfield over a small range, a mere few parts per million.

The earliest NMR technique to gain importance in chemistry was that ofproton NMR. Spectra could be obtained for compounds containing the *Hnucleus by continuously sweeping the field at constant frequency. This

364

Nuclear magnetic resonance spectroscopy

approach results in so-called continuous wave spectra, which suffer fromthe general disadvantage that only a small portion of the spectrum isexcited at any one time. Such spectra have unduly low signal-to-noiseratios, which is particularly undesirable when studying nuclei of lowabundance and/or low sensitivity, such as 13C. As a result continuous waveis not used for 13C NMR spectra but instead pulsing techniques are usedtogether with Fourier transformation of the data thus obtained.

For 13C NMR, the radio frequency is applied as a short, powerful pulsewhich acts like a spread of frequencies. All the NMR-active nuclei areexcited by this pulse and then decay back to their equilibrium states. Thesedecays result in a series of complex sine waves which diminish exponentiallywith time. By the application of Fourier transform techniques, such decaycurves can be converted into spectra. By making use of the capability ofstoring many such pulsed spectra on a computer and adding the signalstogether prior to applying the Fourier transform, a significant improve-ment in the signal-to-noise ratio can be obtained.

The nuclei studied by NMR spectroscopy are affected by the precisenature of their electronic environments. This means, for example, thatprotons will resonate at different frequencies according to their position ina molecule. In this way, it is possible to distinguish between protons inmethyl groups and methylene groups, in hydroxyl groups or as part of thebenzene ring. In addition, protons interact with each other, leading to whatis known as spin-spin coupling and giving rise to well-characterizedsplitting patterns in their NMR spectrum. All of these features may be usedto give structural information about molecules containing XH nuclei.

Carbon-13 nuclei, due to their low natural abundance, do not interactwith each other in a molecule, though they are affected by adjacentprotons. In practice, such couplings are removed by irradiation of thewhole spectrum as it is recorded, in a technique known as proton noisedecoupling. This means that practical 13C NMR spectra exhibit one unsplitsignal for each type of carbon atom present in the sample.


NMR spectroscopy of various nuclei has been used in the study of ABcements derived from various acids, including phosphoric acid andpoly(acrylic acid). For example, 31P NMR has been used in studies ofdental silicate cement, i.e. the AB cement made from aqueous phosphoricacid and powdered aluminosilicate glass (Wilson, 1978). In this cement, the

365


setting reaction is controlled by dissolving small amounts of aluminiumand/or zinc metal in the phosphoric acid. Using 31P NMR, O'Neill et al.(1982) were able to distinguish between the various complexes formed byions of these metals in the presence of phosphoric acid.

The chemistry of polyelectrolyte cement liquids has been studied using13C NMR. Watts (1979) used this technique to distinguish between thehomopolymer of acrylic acid and its copolymer with itaconic acid invarious commercial polyelectrolyte dental cements. This was readilyachieved because of the ability of 13C NMR to differentiate betweencarbon atoms in chemical environments that are only slightly different.

Prosser, Richards & Wilson (1982) used 13C NMR spectroscopy to studythe role of ( + )-tartaric acid in modifying the setting behaviour ofglass-ionomer dental cements. For this, a model system was used, with alower ratio of glass powder to poly (acrylic acid) liquid than in conventionalcements; this slowed the reaction, enabling the spectra to be recorded onacceptable time scales. This study showed for the first time that ( + )-tartaric acid was an effective additive for controlling the setting character-istics of these cements because it reacts preferentially with Ca2+ ionsreleased from the glass. Hence, (+ )-tartaric acid acts to extend the workingtime of these cements. The work also showed that there was no differencebetween the reactivity of the acrylic acid and the itaconic acid segmentswhen the copolymer was used as the acidic component.

10.5 Electrical methods

Changes in electrical conductivity have occasionally been used to study thesetting chemistry of AB cements. Conductivity has been particularly usedin the study of dental cements, notably the dental silicate (Wilson & Kent,1968), the zinc polycarboxylate (Cook, 1982), the glass-ionomer cement(Cook, 1982) and the ZOE cement (Crisp, Ambersley & Wilson, 1980).

In a typical study of conductivity, Cook (1982) used a cell consisting oftwo platinum disc electrodes, 12 mm in diameter and 1-5 mm apart. Thesetting AB cement was examined in this cell which had been calibratedusing a standard solution of 0-02 M potassium chloride. Plots were recordedof specific conductance against time for each of the setting cements. Forzinc polycarboxylate there was found to be a rapid drop in specificconductance about 10 minutes after the start of mixing. This behaviourwas consistent with the replacement of relatively mobile protons bysignificantly less mobile zinc ions in the polycarboxylate chain. Con-

366

X-ray diffraction

ductivity was found to drop rapidly by a factor of 200 which was held tobe consistent with quantitative neutralization of the poly(acrylic acid) andwith a very low diffusion coefficient for zinc ions in the set cement. Bycontrast, the specific conductance of glass-ionomer cements was found todecrease much more gradually, indicating that the setting reaction in thesecements is much slower than for the zinc polycarboxylates. Moreover,setting was still not complete even 1000 minutes after the start of mixing.

An alternative electrical method that has been used in the study ofglass-ionomer cements has been the measurement of dielectric properties.Tay & Braden (1981, 1984) measured the resistance and capacitance ofsetting cements at various times from mixing. From the results obtained,relative permittivity and resistivity were calculated. In general, as thesecements set, their resistivity was found to fall rapidly, then to rise again.Both these results and the results of relative permittivity measurementswere consistent with the cements comprising highly ionic and polarstructures.

10.6 X-ray diffraction

10.6.1 Basic principles

X-ray diffraction is the most accurate and powerful method of bothidentifying solids and determining their structure. This is essentiallybecause the regular array of atoms or ions in a crystalline solid is spaced atdimensions corresponding to the wavelength of X-rays, and such arraysare consequently able to act as an X-ray diffraction grating. The resultingdiffraction pattern is recorded either on photographic film or by anelectronic detector.

The underlying principle of X-ray diffraction is as follows. When a beamof X-rays passes through a crystalline solid it meet various sets of parallelplanes of atoms. The diffracted beams cancel out unless they happen to bein phase, the condition for which is described in the Bragg relationship:

nk = 2dsin9

where X is the wavelength of the X-rays, d is the distance between theplanes, and 6 is the angle of incidence of the X-rays on the planes.

In practice, values of 6 can be measured and, since the wavelength of theX-rays is known in any given experiment, values of d can be calculated.These lvalues are related to the unit cell symmetry and dimensions. If the

367


intensities of the diffracted beam in each direction are also measured, thecomplete structure of the solid can be determined (Mackay & Mackay,1972).

Each atom in the lattice acts as a scattering centre, which means that thetotal intensity of the diffracted beam in a given direction depends on theextent to which contributions from individual atoms are in phase. Relatingthe underlying structure to the observed diffraction pattern is notstraightforward, but is essentially a trial-and-error search involvingextensive computer-based calculations.

For materials which are available not in the form of substantialindividual crystals but as powders, the technique pioneered by Debye andScherrer is employed (Moore, 1972). The powder is placed into a thin-walled glass capillary or deposited as a thin film, and the sample is placedin the X-ray beam. Within the powder there are a very large number ofsmall crystals of the substance under examination, and therefore allpossible crystal orientations occur at random. Hence for each value of dsome of the crystallites are correctly oriented to fulfil the Bragg condition.The reflections are recorded as lines by means of a film or detector fromtheir positions, the lvalues are obtained (Mackay & Mackay, 1972).


X-ray diffraction has been applied to certain AB cements. For example,Crisp et al. (1979), in a study of silicate mineral-poly(acrylic acid) cements,used the technique both to assess the purity of the powdered mineralsemployed and to monitor mineral decomposition in mixtures withpoly(acrylic acid), in order to indicate whether or not cement formationhad taken place. They employed Cu Ka radiation passed through a nickelfilter; for most of the samples, a seven-hour exposure time was found to beadequate for the development of a discernible diffraction pattern. Sampleswere identified by reference to published powder diffraction data.

A number of other studies of AB cements have used X-ray diffraction.For example, Sorrell (1977) and Sorrell & Armstrong (1976) employed thetechnique in the study of oxychloride cements formed in aqueous solutionby interaction of oxides and chlorides of either zinc or magnesium.Individual phases were identified, again using Cu Ka radiation, this timecomparing results with those previously obtained for pure compounds.Results from these two studies are described in detail in Sections 7.2 and7.3 respectively.

368

Electron probe microanalysis

10.7 Electron probe microanalysis10.7.1 Basic principles

This technique can be applied to samples prepared for study by scanningelectron microscopy (SEM). When subject to impact by electrons, atomsemit characteristic X-ray line spectra, which are almost completelyindependent of the physical or chemical state of the specimen (Reed, 1973).To analyse samples, they are prepared as required for SEM, that is they aremounted on an appropriate holder, sputter coated to provide an electricallyconductive surface, generally using gold, and then examined under highvacuum. The electron beam is focussed to impinge upon a selected spot onthe surface of the specimen and the resulting X-ray spectrum is analysed.

Analysis is most frequently done qualitatively since there are problemsin quantification (Reed, 1973). Although intensity is approximatelyproportional to mass concentration of a given element there are significantdeviations, depending on which other elements are present.

There is also a minimum atomic number which can usually be detected,since the practical maximum X-ray wavelength that is used is fixed at0-12 nm. This in turn fixes sodium (atomic number 11) as the lightestelement for which this technique is valid. Special techniques are availableto overcome this limitation, but they are not in general use, and have notbeen applied to AB cements.

10.7.2 Applications to dental silicate cements

In a study of dental silicate cements, Kent, Fletcher & Wilson (1970) usedelectron probe analysis to study the fully set material. Their method ofsample preparation varied slightly from the general one described above,in that they embedded their set cement in epoxy resin, polished the surfaceto flatness, and then coated it with a 2-nm carbon layer to provide electricalconductivity. They analysed the various areas of the cement for calcium,silicon, aluminium and phosphorus, and found that the cement compriseda matrix containing phosphorus, aluminium and calcium, but not silicon.The aluminosilicate glass was assumed to develop into a gel which wasrelatively depleted in calcium.

10.7.3 Applications to glass-ionomer cements

A similar study was carried out on glass-ionomer cements (Barry, Clinton& Wilson, 1979), which showed some interesting similarities to the dental

369


silicate cements, as well as some differences. Firstly, aluminium andcalcium were found to be removed from the glass particles and to reside inthe continuous phase. However, by contrast with the dental silicatecement, some silicon was also found in the continuous phase. Attack by thepoly(acrylic acid) had apparently occurred preferentially at the calcium-rich sites of the glass, a finding that was significant in formulating thetheory of the setting chemistry of these materials.

10.8 Measurement of mechanical properties

Strength has been widely measured for AB cements. It is formally definedas the force experienced by a material at the point where fracture occurs(Gillam, 1969). The study of strength is complicated in that fracture is apoint of discontinuity, so cannot be readily interpreted in terms of eventsleading up to it.

Strength can be measured in compression, in tension, in shear andtransversely (flexural strength). However, if we exclude plastic flow as ameans of failure, then materials can only fracture in one of two ways: (1)by the pulling apart of planes of atoms, i.e. tensile failure, or (2) by theslippage of planes of atoms, i.e. shear failure. Strength is essentially ameasure of fracture stress, which is the point of catastrophic anduncontrolled failure because the initiation of a crack takes place atexcessive stress values.

AB cements tend to be essentially brittle materials. This means thatwhen subjected to mechanical loading, they tend to rupture suddenly withminimal deformation. There are a number of different types of strengthwhich have been identified and have been determined for AB cements.These include compressive, tensile and flexural strengths. Which one isdetermined depends on the direction in which the fracturing force isapplied. For full characterization, it is necessary to evaluate all of theseparameters for a given material; no one of them can be regarded as the solecriterion of strength.

Generally, strength is determined by applying forces uniaxially using anapparatus consisting of a pair of jaws which move either together or apartin a controlled manner. A chart recorder is employed to give a permanentrecord of results obtained, so that the force at fracture can be determined.Whether such an apparatus measures tensile, compressive or flexuralstrength depends on how the sample is oriented between the jaws and onthe direction that the jaws are set to travel relative to each other. Such tests

370

Measurement of mechanical properties

need to be repeated on several samples in order to provide sufficient datafor statistical analysis and to allow calculation of both the mean and thescatter of the results.

10.8.1 Compressive strength

The most common mechanical property of cements that has been measuredroutinely is compressive strength (Polakowski & Ripling, 1966). Measure-ment is easy to carry out but there are several reasons to consider that theresults from the technique are unsatisfactory. Interpretation of results isuncertain because of the complexities in the mode of failure. Minorimperfections in the material lead to localized stress concentrations whichaffect the magnitude of the result.

Failure is complex because both the mode and plane of failure arevariable. Failure under compressive load can occur by plastic yielding,cone failure (secondary shear forces) or axial splitting (secondary tensileforces) (Kendall, 1978). The mode of failure depends on the size andgeometry of the specimen, the nature of the material tested and the rate ofloading. The studies of Selenrath & Gramberg (1958) are of interest inshowing the effect of the nature of the material on fracture. They foundthat when cylindrical specimens were placed unrestrained in the testingmachine, glass and lithographic limestone exhibited simple vertical fractureor axial splitting, evidence of tensile failure. By contrast, coarse-grainedmarble and both fine-grained and coarse-grained sandstone yielded conesat the ends of the specimen with typical shearing fracture planes. However,when the ends of the lithographic limestone were clamped they exhibitedboth types of fracture.

The variation in the mode of failure makes comparison of different typesof cement quite impossible. As Darvell (1990) has pointed out, compressivestrength is not a material property under any condition, but can only beused to compare materials of a very similar nature.

The compressive strength of AB cements used in dentistry has beenwidely studied (Wilson & McLean, 1988). It is the method, for example,specified in the British Standard on dental cements. However, there isconcern that the result is less clinically relevant than the evaluation offlexural strength. Moreover, the latter is more discriminating (Prosser etal., 1984). Despite this, compressive strength has been used to indicateclinical acceptability; phosphate-bonded cements with low compressivestrength tend to be unsatisfactory in other respects such as durability, and

371


hence there is value in using compressive strength as a criterion of generalmaterial quality (Wilson & McLean, 1988).

10.8.2 Diametral compressive strength

The diametral compressive strength has been used to estimate the tensilestrength of certain AB cements (Smith, 1968). In this test, the load isapplied diametrically across a cylinder of cement. Theoretical consider-ation of the test geometry shows that for a perfectly brittle material thefailure that occurs is tensile in character. The difficulty in applying this testto AB cements is that they are not sufficiently brittle for this to hold true.In particular, the zinc polycarboxylate and glass-ionomer cements showsufficient plastic character to make the relationship between diametralcompressive and tensile strength vary between AB cements of differenttypes; like the compressive strength test, this test is valid only as a meansof comparison between similar materials (Darvell, 1990).

For glass-ionomer cements, there have been several studies on thefactors affecting strength. For example Crisp, Lewis & Wilson (1977)showed that both compressive and tensile strengths increased linearly withconcentration of polyacid in the liquid component, though neitherextrapolated to zero at zero acid concentration. Ageing was also shown toinfluence compressive strength of these materials, older cements beingstronger than younger ones. However, the exact development of strengthwas found to depend on storage conditions (Crisp, Lewis & Wilson, 1976).

10.8.3 Flexural strength

Flexural strength is determined using beam-shaped specimens that aresupported longways between two rollers. The load is then applied by eitherone or two rollers. These variants are called the three-point bend test andthe four-point bend test, respectively. The stresses set up in the beam arecomplex and include compressive, shear and tensile forces. However, at theconvex surface of the beam, where maximum tension exists, the material isin a state of pure tension (Berenbaum & Brodie, 1959). The disadvantageof the method appears to be one of sensitivity to the condition of thesurface, which is not surprising since the maximum tensile forces occur inthe convex surface layer.

Of all the methods of determining strength, the flexural test appears tobe the most satisfactory. While not ideal, it does have the advantage of

372

Measurement of mechanical properties

measuring a clearly defined parameter. However, very few studies of ABcements involving this test have been carried out. Prosser, Powis & Wilson(1986) studied the influence of various formulation changes on the flexuralstrength of glass-ionomer cements. They found that the flexural strengthof glass-ionomer cements was dependent on the glass and the polyacidused to prepare them. Opaque and opal glasses containing crystallites werefound to yield cements of higher flexural strength than those prepared fromclear glasses; increasing the relative molar mass of the polyacid was alsofound to improve flexural strength.

10.8.4 Fracture toughness

The use of fracture stress as a measure of resistance to fracture is suspect.In all these tests, whether compressive, tensile or flexural, failure iscatastrophic because there is no suitable flaw for crack propagation. Ahigh force is needed to start a crack and as a result the subsequentpropagation takes place under too great a stress. Then there is the curiousfinding that compressive strength values are 10 times those for tensilestrength although, in principle, both are measures of cohesion.

For these reasons, attention has been paid to energy criteria as a measureof toughness. The following points need to be noted. Materials do notreach their theoretical strength (that is of their primary chemical bonds),because of the presence of minute flaws. Stress is concentrated at theseflaws and so is enhanced. In effect this amounts to a weakening of thematerial. Under load, cracks propagate from these flaws and lead tofailure.

Propagation of cracks requires energy to create new surfaces but alsoreleases stored energy. Unstable propagation of cracks occurs when thestrain energy released exceeds that required to create new surfaces, andoccurs when the crack reaches a certain length. This is because strainenergy released is proportional to (crack length)2, and the energy to createa new surface is proportional to the crack length.

Fracture toughness is the resistance to propagation of cracks through amaterial and is usually quantified by the stress intensity factor, Kx, definedas

Kx = GF(na)*

where a is the flaw size and aF the fracture stress.There are a number of methods of determining fracture toughness, but

the one used so far for AB cements is the double torsion method introduced

373

Experimen tal techn iques

by Outwater et al. (1974), which appears to have some advantages overother methods.

Double torsion test specimens take the form of rectangular plates witha sharp groove cut down the centre to eliminate crack shape corrections.An initiating notch is cut into one end of each specimen (Hill & Wilson,1988) and the specimen is then tested on two parallel rollers. A load isapplied at a constant rate across the slot by two small balls. In essence thetest piece is subjected to a four-point bend test and the crack is propagatedalong the groove. The crack front is found to be curved.

The double torsion test specimen has many advantages over otherfracture toughness specimen geometries. Since it is a linear compliance testpiece, the crack length is not required in the calculation. The crackpropagates at constant velocity which is determined by the crossheaddisplacement rate. Several readings of the critical load required for crackpropagation can be made on each specimen.

When the load has reached a critical plateau value, the crack continuesto propagate at constant load. Crack propagation can be stopped byremoving the load, with the implication that several readings can be madeon one test specimen. Crack velocity is determined by the crosshead speed,modulus of the material and specimen dimensions.

To calculate fracture toughness using the double torsion test piece, thefollowing equation is used:

where Kx = fracture toughness, Pc = critical load for crack propagation,Wm = specimen width, t = specimen thickness, tc = crack depth, Wc =distance between the supports and v = Poisson's ratio.

10.9 Setting and rheological properties

The rheological properties of AB cements are important where thosecements have been used in dentistry. It is not sufficient simply to have acement which eventually sets to give a resistant, strong material. Thecement must also remain fluid for a sufficient time to allow placement, andideally must develop a good degree of hardness very rapidly followingplacement. Thus the setting chemistry must be such that the reaction whichoccurs immediately the acid and the base are mixed does not lead toorapidly to the development of a viscous paste that is difficult to manipulate.Ideally, viscosity should be low enough to allow manipulation but highenough so that the fluid cement does not flow appreciably once in place.

374

Setting and rheological properties

Thus, the rheological requirements of materials for this application aredemanding, and their evaluation has been important in the study of thisgroup of AB cements.

The rheological characteristics of AB cements are complex. Mostly, theunset cement paste behaves as a plastic or plastoelastic body, rather thanas a Newtonian or viscoelastic substance. In other words, it does not flowunless the applied stress exceeds a certain value known as the yield point.Below the yield point a plastoelastic body behaves as an elastic solid andabove the yield point it behaves as a viscoelastic one (Andrade, 1947). Thismakes a mathematical treatment complicated, and although the theories ofviscoelasticity are well developed, as are those of an ideal plastic (Binghambody), plastoelasticity has received much less attention. In many ABcements, yield stress appears to be more important than viscosity indetermining the stiffness of a paste.

10.9.1 Problems of measurement

Consistency, working time, setting time and hardening of an AB cementcan be assessed only imperfectly in the laboratory. These properties areimportant to the clinician but are very difficult to define in terms oflaboratory tests. The consistency or workability of a cement paste relatesto internal forces of cohesion, represented by the yield stress, rather thanto viscosity, since cements behave as plastic bodies and not as Newtonianliquids. The optimum stiffness or consistency required of a cement pastedepends upon its application.

Three useful tests have been used to evaluate the working and settingproperties of experimental cements. These are the parallel plate plas-tometer, the penetrometer and the oscillating rheometer. They aredescribed in the following sections of this chapter.

10.9.2 Methods of measurement

Initially, the test that was used to determine setting time was one based onresponse of the newly mixed AB cements to the application of a weightedneedle of known dimensions. This test was originally devised by Gillmore(1864) for his studies on the setting of hydraulic cements and mortars, andthe weighted device, known as a Gillmore needle, had a mass of 454 g (1 lb)and a tip diameter of 105 mm (Crisp, Merson & Wilson, 1980).

The drawback with the use of the Gillmore needle is that it is not a testof any well-defined rheological property, but of resistance to indentation.

375


This property does not necessarily correlate with the changes in rheologicalcharacteristics undergone by a cement as it sets. A more satisfactory test,developed in the early 1970s, is oscillating rheometry, first described byBovis, Harrington & Wilson (1971) and subsequently refined slightly byWilson, Crisp & Ferner (1976). A theoretical treatment of the results ofoscillating rheometry was provided by Cook & Brockhurst (1980).

The apparatus used for oscillating rheometry consists of a pair ofgrooved metal plates clamped together, between which the sample ofcement is placed. The bottom plate is connected via a spring to a motor,which causes reciprocating motion at the end of the spring furthest fromthe plate. Initially, motion of the motor causes corresponding motion inthe bottom plate. As the cement sets, and the force required to move theplate increases, so the motion in the lower plate diminishes. A trace isproduced on a chart recorder which shows the changes in oscillation withtime as the cement sets, beginning from the large initial amplitude anddeclining to negligible, or in some cases, zero oscillation when the cementis fully set.

The original definition of working time using this apparatus was the timetaken from the start of mixing to reach an oscillation 95 % of the originalvalue (Bovis, Harrington & Wilson, 1971). An alternative method ofestimating working time was suggested by Wilson, Crisp & Ferner (1976),which consisted of drawing lines as extensions to the initial straight portionof the rheogram and extending the tangent at the maximum setting rateback to cross these lines. The point of intersection is then taken as theworking time. In practice very little difference is observed in the workingtimes from oscillating rheometry obtained by either of these constructionmethods.

Since its development as a technique, oscillating rheometry has beenwidely applied to the study of AB cements for use in dentistry, mostnotably to the glass-ionomer cement. For example, it was used to study theeffect of chelating comonomers on the setting of glass-ionomers (Wilson,Crisp & Ferner, 1976). This study examined a range of compoundsincluding hydroxybenzoic acids, diketones, and most significantly,hydroxyacids. The technique was useful in identifying the particularadvantages which are associated with the use of 5% ( + )-tartaric acid inglass-ionomer cements, namely delayed onset of gelation and eventualincreased rate of set. These improvements in handling properties have beencrucial in making glass-ionomer cements fully acceptable for use in clinicaldentistry (Wilson & McLean, 1988).

376

Setting and Theological properties

In another study, oscillating rheometry was used to examine the effect ofadding various simple metal salts to glass-ionomer cements (Crisp, Merson& Wilson, 1980). It was found that cement formation for certain glasseswhich react only slowly with poly(acrylic acid) could be acceleratedsignificantly by certain metal salts, mainly fluorides such as stannousfluoride and zinc fluoride. Some non-reactive glasses could be induced toset by the addition of such compounds.

As a further example of the use of the technique of oscillating rheometry,the work of Crisp et al. (1979) can be cited. This study was of the formation

Figure 10.1 Parallel-plate plastometer for determination of consistency.

377


of AB cements from poly(acrylic acid) and basic minerals. The mineralsexamined were ortho- and pyro-silicates, which had been ground into finepowders of sufficiently small particle size to pass through a 38-//m testsieve. With a number of such minerals, including willemite, gehlenite andhardystonite, oscillating rheometry demonstrated that there was a reason-ably rapid setting reaction at 23 °C. Infrared spectroscopy was used toconfirm reaction in fully hardened cements and mechanical propertymeasurements were carried out, though it was concluded that the cementswere too weak and porous to be practically useful.

Oscillating rheometry continues to be useful in the study of AB cements,and has recently been used to give further insight into the role of ( + )-tartaric acid in glass-ionomer cements (Hill & Wilson, 1988). Furtherexamples of its use are described in earlier chapters of this book.

Consistency is tested on a measured volume of freshly mixed cement inthe form of a cylinder. This specimen is placed between two horizontalplates using the apparatus illustrated in Figure 10.1 and subjected to avertically applied load. The cement then flows out rapidly to form a disc.This radial flow ceases almost instantaneously because the applied stressdecreases as the disc expands and rapidly reaches the yield stress, at whichpoint outward flow ceases. This is the behaviour expected for a plasticbody.

The diameter of the disc is measured and this gives an indication of theshear strength of the paste. It is not a measure of viscosity because flow hasceased at this point. The shear strength of the paste can be calculated fromthe following formula, which was derived by Wilson & Batchelor (1971).

Shear strength = 4SPV/TZ2D5

where P = applied load, V = volume of the cement paste, and D =diameter of the disc.

The consistency depends on the powder/liquid ratio used to mix thecement, and the parallel plate plastometer can be used to determine theoptimum ratio for a particular cement system.

10.10 Erosion and leaching10.10.1 Importance in dentistry

For the various AB cements used in clinical dentistry, erosion and/orleaching of components have been considered important in assessingdurability (Wilson & McLean, 1988). In fact, the two aspects are not

378

Optical properties

necessarily related to durability. Loss of soluble species from the set cementaffects durability only if the species concerned are matrix-formers. If not,such loss has no effect and indeed may be beneficial. The fluoride release byglass-ionomer cements is regarded as clinically advantageous, andapparently takes place by ion exchange, there being no discernible loss ofmaterial as the process occurs (Wilson & McLean, 1988).

10.10.2 Studies of erosion

Erosion is the result of both chemical attack and mechanical wear. Indentistry the chemical attack comes from acids either generated in themouth by dental plaque or present in foods and beverages (Pluim &Arends, 1987; Wilson & Batchelor, 1968). To mimic this attack inlaboratory testing, a static solubility test was originally carried out, whichemployed appropriate solutions of eroding acids (Kent, Lewis & Wilson,1973). More recently, the mechanical aspect has been introduced by usinga test in which jets of aqueous acid impinge on a sample of cement (Wilsonet al.,1986); this test gives results for erosion that agree with clinical studiesof durability (Setchell, Teo & Kuhn, 1985). In particular, using dilute lacticacid as the eroding agent in an impinging jet test, it has been shown (Wilsonet al., 1986) that extent of erosion increased in the order

glass-ionomer < silicate < zinc phosphate < zinc polycarboxylate

Setchell, Teo & Kuhn (1985) observed that glass-ionomer cementsprepared from poly(acrylic acid) were more resistant to erosion than suchcements prepared from maleic acid copolymers. This has been confirmedby Wilson et al. (1986) and by Billington (1986), even when, as in the lattercase, the same glass was used in both cements. The method has beenreviewed recently by Billington, Williams & Pearson (1992).

10.11 Optical properties10.11.1 Importance in dentistry

For certain AB cements, used in dentistry, optical properties are importantfor their overall acceptability as materials. The two particular properties ofinterest have been colour and translucency, both of which need to matchnatural tooth material as closely as possible if good aesthetics are to bedeveloped (Wilson & McLean, 1988). Of the AB cements currently used indentistry, the glass-ionomer cement has the best aesthetics, since it has a

379

Experimen tal techniques

degree of translucency. This translucency arises because the filler is a glass,and unlike zinc oxide used in the zinc polycarboxylate cements, is notopaque.

Evaluation of these optical properties may be done by simple ob-servation; this approach is useful clinically (Knibbs, Plant & Pearson,1986), since acceptability of the colour match to the surrounding toothmaterial can be readily seen without the need for instrumental measure-ment. On the other hand, for quantitative evaluation of optical properties,some kind of instrumental measurement is necessary, and the propertyusually evaluated is opacity.

10.11.2 Measurement of opacity

The main technique that has been used for the measurement of opacity hasbeen to prepare a standard disc of AB cement 1-0 mm thick and aged for24 hours at 37 °C. This disc, contained in a small trough of water to preventdesiccation, is placed in a reflectometer on a black background. It is thenilluminated with diffuse light and the amount of light reflected from it, Ro,is measured. The disc is then placed on a white background of 70%reflectivity, and the new amount of reflected light, R07, measured. Thecontrast ratio R0/R07 is defined as the C0.7 opacity (Crisp et al., 1979).

Using this technique, it has been shown that the opacity of glass-ionomercements decreases as they age; in other words, their translucency increasesover this time. This change has been found to be rapid in the first hour aftermixing, but becomes much slower after this time (Wilson & McLean,1988).

Colour and opacity have been found to be connected for glass-ionomercements (Crisp et al., 1979; Asmussen, 1983), with darker shades givingincreased opacity. However, this is merely a consequence of the underlyingphysical relationships, and is not thought to be a clinical problem (Wilson& McLean, 1988), mainly because the stained tooth material for which thedarker shades are necessary for colour match is itself of reducedtranslucency.

10.12 Temperature measurement

When AB cements harden there may be a considerably exothermicreaction. For those cements used in dentistry, this may have clinicalsignificance, since excessive temperature rise can cause damage to the

380

Other test methods

dental pulp. In a survey of a wide range of AB dental cements, Crisp,Jennings & Wilson (1978) made use of a fairly simple miniaturecalorimeter. This consisted of a block of expanded polystyrene, into whicha hole 6 mm in diameter and 6 mm deep was drilled. This hole could becovered with a lid also made of polystyrene. Cements were mixed andplaced in this calorimeter and the temperature rise on setting wasmonitored using a sealed NiCr-NiAl thermocouple.

Of the cements examined, the zinc phosphate cement showed thegreatest exotherm on setting, a maximum temperature rise of 22-1 °C beingobserved. By contrast, glass-ionomer cement gave the smallest exotherm,only 3-9 °C. Dental silicate and silicophosphate cements also showedsignificant exotherms, at about 8 °C; should such an amount of heat begenerated in clinical use, this would be considered potentially harmful todental pulp (Paffenbarger et al, 1949). Of the remaining cements, the zincoxide-eugenol and the zinc polycarboxylate gave smaller exotherms ofabout 5 °C.

Difficulties have been found in relating laboratory measurements ofexotherm behaviour in these AB cements to what happens when they areused in clinical practice. As a consequence there have been few otherstudies of temperature rise in the setting of such materials.

10.13 Other test methods

This chapter has covered the more important test methods used in thestudy of AB cements and their underlying principles, but it is notexhaustive. A number of experimental methods have been used to assessparticular properties of certain cements for particular applications. Forexample, for AB dental cements, and especially glass-ionomer cements,adhesion to tooth material has been studied (Wilson & McLean, 1988).However, this is not a property of general interest for AB cements, norhave satisfactory standard methods been developed. Consequently,adhesion to tooth material has not been covered in the present chapter, buthas been left to chapters covering cements for which it is appropriate.Other properties of glass-ionomer cements, such as fluoride-ion releaseand effect of relative molar mass of the poly(acrylic acid) on strength havebeen studied, but using techniques that are of interest only in the contextof these particular cements. Once again, these tests are discussed in thechapter devoted to the AB cements in question.

381


References

Andrade, E. N. da C. (1947). Introduction to Rheology. London: Oil & ColourChemists' Association.

Asmussen, E. (1983). Opacity of glass-ionomer cements. Acta OdontologicaScandinavica, 41, 151-7.

Barry, T. I., Clinton, D. J. & Wilson, A. D. (1979). The structure of aglass-ionomer cement and its relationship to the setting process. Journal ofDental Research, 58, 1072-9.

Bellamy, L. J. (1975). The Infrared Spectra of Complex Molecules. London:Chapman and Hall.

Berenbaum, R. & Brodie, I. (1959). The tensile strength of coal. Journal of theInstitute of Fuel, 32, 320-7.

Billington, R. W. (1986). Personal communication, cited in Wilson & McLean(1988).

Billington, R. W., Williams, J. A. & Pearson, G. J. (1992). In vitro erosion of20 commercial glass ionomer cements measured using the lactic acid jet test.Biomaterials, 13, 343-7.

Bovis, S. C, Harrington, E. & Wilson, H. J. (1971). Setting characteristics ofcomposite filling materials. British Dental Journal, 131, 352-6.

Cesaro, S. N. & Torracca, E. (1988). Early applications of infrared spectroscopyto chemistry. Ambix, 35, 39—46.

Cook, W. D. (1982). Dental polyelectrolyte cements. I. Chemistry of the earlystages of the setting reaction. Biomaterials, 3, 232-6.

Cook, W. D. (1983). Degradative analysis of glass-ionomer polyelectrolytecements. Journal of Biomedical Materials Research, 17, 1015-27.

Cook, W. D. & Brockhurst, P. (1980). The oscillating rheometer-what does itmeasure? Journal of Dental Research, 59, 795-9.

Crisp, S., Abel, G. & Wilson, A. D. (1979). The quantitative measurement ofthe opacity of aesthetic dental filling materials. Journal of Dental Research, 58,1585-96.

Crisp, S., Ambersley, M. & Wilson, A. D. (1980). Zinc oxide eugenol cements.V. Instrumental studies of the catalysis and acceleration of the settingreaction. Journal of Dental Research, 59, 44-54.


Crisp, S., Lewis, B. G. & Wilson, A. D. (1976). Characterisation ofglass-ionomer cements. 1. Long term hardness and compressive strength.Journal of Dentistry, 4, 162-6.

Crisp, S., Lewis, B. G. & Wilson, A. D. (1977). Characterisation ofglass-ionomer cements. 3. Effect of polyacid concentration on the physicalproperties. Journal of Dentistry, 5, 51-6.

Crisp, S., Merson, S. A. & Wilson, A. D. (1980). Modification of ionomercements by the addition of simple metal salts. Industrial and EngineeringChemistry, Product Research and Development, 19, 403-8.

382

References

Crisp, S., Merson, S., Wilson, A. D., Elliott, J. H. & Hornsby, P. R. (1979). Theformation and properties of mineral-poly acid cements. Part 1. Ortho- andpyro-silicates. Journal of Materials Science, 14, 2941-58.

Crisp, S., Prosser, H. J. & Wilson, A. D. (1976). An infrared spectroscopic studyof cement formation between metal oxides and aqueous solutions ofpoly(acrylic acid). Journal of Materials Science, 11, 36-48.

Crisp, S. & Wilson, A. D. (1974). Reactions in glass-ionomer cements: I.Decomposition of the powder. Journal of Dental Research, 53, 1408-13.

Darvell, B. W. (1990). Uniaxial compression tests and the validity of indirecttensile strength. Journal of Materials Science, 25, 757-80.

Deacon, G. B. & Phillips, R. (1982). Relationship between the carbon-oxygenstretching frequency of carboxylate complexes and the type of carboxylatecoordination. Coordination Chemistry Reviews, 33, 227-50.

Gillam, E. (1969). Materials under Stress. London: Newnes-Butterworth.Gillmore, Q. A. (1864). Practical Treatise on Limes, Hydraulic Cements and

Mortars. New York.Hill, R. G. & Wilson, A. D. (1988). Some structural aspects of glasses used in

ionomer cements. Glass Technology, 29, 150-8.Hill, R. G., Wilson, A. D. & Warrens, C. P. (1989). The influence of poly(acrylic

acid) molecular weight on the fracture toughness of glass-ionomer cements.Journal of Materials Science, 24, 363-71.

Hodd, K. A. & Reader, A. L. (1976). The formation and hydrolytic stability ofmetal ion-polyacid gels. British Polymer Journal, 8, 131-9.

Hondras, G. (1959). The evaluation of Poisson's ratio and the modulus ofmaterials of a low tensile resistance by the Brazilian (indirect tensile) test withparticular reference to concrete. Australian Journal of Applied Science, 10,245-68.

Kemp, D. S. & Vellaccio, F. (1980). Organic Chemistry. New York: Worth.Kendall, K. (1978). Complexities of compression failure. Proceedings of the

Royal Society of London, A 361, 245-63.Kent, B. E., Fletcher, K. E. & Wilson, A. D. (1970). Dental silicate cements. XL

Electron probe studies. Journal of Dental Research, 49, 86-92.Kent, B. E., Lewis, B. G. & Wilson, A. D. (1973). The properties of a

glass-ionomer cement. British Dental Journal, 135, 322-6.Knibbs, P. J., Plant, C. G. & Pearson, G. J. (1986). A clinical assessment of an

anhydrous glass-ionomer cement. British Dental Journal, 161, 99-103.Lion Corporation. (1980). Dental cements. Nihon Kokai Tokkyo Koho

80,139,311. Chemical Abstracts, 94: 903488, 1981.Mackay, K. M. & Mackay, R. A. (1972). Introduction to Modern Inorganic

Chemistry. London: Intertext Books.Mehrotra, R. C. & Bohra, R. (1983). Metal Carboxylates. London and New

York: Academic Press.Moore, W. J. (1972). Physical Chemistry, 5th edn. London: Longman Group

Ltd.Nicholson, J. W., Brookman, P. J., Lacy, O. M., Sayers, G. S. & Wilson, A. D.

(1988a). A study of the nature and formation of zinc polycarboxylate cement

383


using Fourier transform infrared spectroscopy. Journal of BiomedicalMaterials Research, 22, 623-31.

Nicholson, J. W., Brookman, P. J., Lacy, O. M. & Wilson, A. D. (1988b).Fourier transform infrared spectroscopic study of the role of tartaric acid inglass-ionomer dental cements. Journal of Dental Research, 67, 145-4.

Nicholson, J. W., Wasson, E. A. & Wilson, A. D. (1988). Thermal behaviour offilms of partially neutralised poly(acrylic acid). 3. Effect of calcium andmagnesium ions. British Polymer Journal, 20, 97-101.

Nicholson, J. W. & Wilson, A. D. (1987). Thermal behaviour of films ofpartially neutralised poly (aery lie acid). 1. Effect of different neutralising ions.British Polymer Journal, 19, 67-72.

O'Neill, I. K., Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). Nuclearmagnetic resonance spectroscopy of dental materials. 1.31P studies onphosphate-bonded cement liquids. Journal of Biomedical Materials Research,16, 39-49.

Outwater, J. O., Murphy, M. C, Kumble, R. G. & Berry, J. T. (1974). Doubletorsion techniques as a universal fracture toughness test method. FractureToughness and Slow-Stable Cracking, ASTM Special Technical Publication559, pp. 127-37, American Society for Testing and Materials.

Paffenberger, G. C, Swaney, A. C, Schoonover, I. C, Dickson, G. & Glasson,G. F. (1949). An investigation of Diafil, a dental silicate cement. Journal ofthe American Dental Association, 39, 283.

Pluim, L. J. & Arends, J. (1987). The relationship between salivary propertiesand in vivo solubility of dental cements. Dental Materials, 3, 13-18.

Polakowski, N. H. & Ripling, E. J. (1966). The Strength and Structure ofEngineering Materials, Chapter 10. Englewood Cliffs, New Jersey: Prentice-Hall Inc.

Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). NMR spectroscopy ofdental materials. II. The role of tartaric acid in glass-ionomer cements.Journal of Biomedical Materials Research, 16, 431-45.

Prosser, H. J., Brant, P. J., Scott, R. P. & Wilson, A. D. (1983). The cement-forming properties of phytic acid. Journal of Dental Research, 62, 598-600.

Prosser, H. J., Powis, D. R., Brant, P. & Wilson, A. D. (1984). Characterizationof glass-ionomer cements. 7. The physical properties of current materials.Journal of Dentistry, 12, 231-40.

Prosser, H. J., Powis, D. R. & Wilson, A. D. (1986). Glass-ionomer cements ofimproved flexural strength. Journal of Dental Research, 65, 146-8.

Reed, S. J. B. (1973). In Anderson, C. A. (ed.) Microprobe Analysis. New York:Wiley-Inter science.

Selenrath, Th. R. & Gramberg, J. (1958). Stress-strain relations and breakagesof rocks. In Walton, W. H. (ed.) Mechanical Properties of Non-metallic BrittleMaterials, Chapter 6, pp. 79-105. London: Butterworths.

Setchell, D. J., Teo, C. K. & Kuhn, A. T. (1985). The relative solubilities of fourmodern glass-ionomer cements. British Dental Journal, 158, 220-2.

Skoog, D. A. & West, D. M. (1980). Principles of Instrumental Analysis, 2ndedn, Chapter 8. Tokyo: Holt-Saunders Japan Ltd.

384

References

Smith, D. C. (1968). A new dental cement. British Dental Journal, 125, 3 8 1 ^ .Sorrell, C. A. (1977). Suggested chemistry of zinc oxychloride cements. Journal

of the American Ceramic Society, 60, 217-20.Sorrell, C. A. & Armstrong, C. R. (1976). Reactions and equilibria in

magnesium oxychloride cements. Journal of the American Ceramic Society, 59,51-4.

Tay, W. M. & Braden, M. (1981). Dielectric properties of glass-ionomercements. Journal of Dental Research, 60, 1311-14.

Tay, W. M. & Braden M. (1984). Dielectric properties of glass-ionomercements-further studies. Journal of Dental Research, 63, 74-5.

Tyndall, J. (1866). On calorescence. Philosophical Magazine, Series 4, 31,386-96; 435-50.

Watts, D. C. (1979). C-13 NMR spectroscopic analysis of polyelectrolyte cementliquids. Journal of Biomedical Materials Research, 13, 423-35.

Williams, D. H. & Fleming, I. (1973). Spectroscopic Methods in OrganicChemistry, 2nd edn. London: McGraw-Hill.


Wilson, A. D. (1982). The nature of the zinc polycarboxylate cement matrix.Journal of Biomedical Materials Research, 16, 549—57.

Wilson, A. D. & Batchelor, R. F. (1968). Dental silicate cements: III.Environment and durability. Journal of Dental Research, 41, 115-20.

Wilson, A. D. & Batchelor, R. F. (1971). The consistency of dental cements. Thespecification test for filling materials. British Dental Journal, 130, 437-41.

Wilson, A. D., Crisp, S. & Ferner, A. J. (1976). Reactions in glass-ionomercements: IV. Effect of chelating comonomers on setting behavior. Journal ofDental Research, 55, 489-95.

Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986). A study ofvariables affecting the impinging jet method for measuring the erosion ofdental cements. Biomaterials, 7, 217-20.

Wilson, A. D., & Kent, B. E. (1968). Dental silicate cements. V. Electricalconductivity. Journal of Dental Research, 47, 463-70.

Wilson, A. D. & Kent, B. E. (1970). Dental silicate cements. IX. Decompositionof the powder. Journal of Dental Research, 49, 7-13.

Wilson, A. D. & McLean, J. W. (1988). Glass-ionomer Cement. Chicago,London, etc.: Quintessence Publishers.

385

Index

abietic acid (rosin) 322, 334, 338-9acid-base balance

in glasses 123-5in silicate rocks 17

acid-base (AB) cementscrystallinity in 8-10, 205-13formation 7-11, 307-8theory 1-5, 5-26, 307-8

acid-base concepts 12-26aprotic acids 6, 17-20Arrhenius theory 14-5, 19Bronsted-Lowry theory 15-6, 19-20,

48, 284Cartledge theory 20-1classification 22-3hard acids and bases see HSAB theoryhistory 1 2 ^HSAB theory 24-6ionic potential 20-1ionization potential 21-2Lewis theory 17-20Lux-Flood theory 17, 19-20relevance 19-20soft acids and bases see HSAB theorysolvent system theory 16-7, 19strength 20-2Usanovich theory 18-20

acid-decomposable glasses 6, see alsoaluminosilicate glasses,aluminoborate glasses

acid-etching of enamel 93acids for cement-formation see cement-

forming acidsacrylic acid

copolymers see poly(alkenoic acid)shomopolymer see poly(acrylic acid),

poly(alkenoic acid)sadhesion 1, 4, 56, 92-7, 107, 152-4, 381

to bone 94-6, 111to dentine and enamel (tooth) 92-6of glass polyalkenoate cements 152-4to hydroxyapatite 95-6

obstacles to 93-4of polyalkenoates 94-6of zinc polycarboxylates 107

adsorption of carboxylates (alkenoates)96-7

agriculture 4A12O3 cements 102A12O3-P2O5-H2O 199-201aldehydic aromatic acid cements 318,

321alkanoate adsorption on hydroxyapatite

96-7alkanoate bonding modes 363alkanoic acids 5-6, 308, 315, 318,

320-1, 337, 348, 351alkoxy aromatic acids 318, 320, 337citric acid 308dimer (dimerized fatty) acids 3512-ethoxybenzoic acid 6, 318, 320-1,

337, see also EBA cementmalic acid 6, 308, 315mellitic acid (benzenehexacarboxylic

acid) 6, 3151,2,3-propanetricarboxylic acid 6,

315pyruvic acid 6, 308, 315salicylates 318, 348tartaricacid 6, 308, 315tricarballylic acid 6

alkenoic acid polymers see poly(akenoicacid)s

alkoxy aromatic acid cements 318, 320,337

allyl-2-methoxyphenol cements 318, 321aluminium coordination 101-2, 120-1,

123, 125, 129, 131,137-8alumino complexes 244

fluoride 135-8,244phosphate 57, 85, 200-1, 210, 244-5

aluminoborate glasses 165-6aluminophosphoric acids 57, 200-1aluminosilicate gels 91

386

Index

aluminosilicate glass cements 307-9, seealso glass polyalkenoate cement,dental silicate cements

aluminosilicate glasses 2, 6, 9, 90,117-32,236-40,310,314-5

acid-base balance in 123acid-decomposition of 119-23, 127-8acid-washing of 163Al2O3:SiO2 123-7aluminium coordination 120-1, 123-5,

128-9, 130, 137anorthite in 122, 130apatite in 125beryllium-containing 236Ca:Al 123-5calcium-containing 6, 118-20,

123-33,236-7,310,314-5composition and properties 118-9,

122-9, 238^1coordination polyhedra 119-21corundum in 125-6electron micrographs 128, 238-9fluoride role 118-9, 129^30, 236fluoride type 117-9, 125-132, 135,

140, 236-40fluorite in 125-6, 129-30indium-containing 237lanthanum-containing 117-9NMR studies 121, 125, 128, 131oxide type 117, 122-6, 135, 236-40phase-separation 126-8, 130-1, 238-9SiO2-Al2O3-CaF2 119, 126-9, 238SiO2-Al2O3-CaF2-AlPO4 119,131SiO2-Al2O3-CaF2-AlPO4-Na3AlF6-

A1F3 119,131-2SiO2-Al2O3-CaO 118, 123-6, 237^0SiO2-Al2O3-CaO-CaF2 119, 130SiO2-Al2O3^CaO-Na2O 118SiOa-AlaO8-CaO-PaO6 118,239-40spinodal decomposition 130strontium-containing 117-9structure 118-131,238-9transition temperatures 130types 118-9,235-7

animal husbandry 4applications

agriculture 4animal husbandry 4architecture 283battlefield dental material 333bone cements 2, 90-1, 117, 147,

161-2, 168bone substitute 4, 161-2, 169controlled release devices see sustained

release devicesdental cements 2, 90-1, 103, 116-7,

147, 166-8, 204, 214, 235-6, 320-1,333

fire resistant materials 283floor fabrication 2, 290floor repair 222foundry sands 2handyman materials 3horticulture 4human health care 4impression material 335insulating materials 283investment materials 222nuclear 283plasters 290pulp capping 347road repairs 222runway repairs 222slip casting 2-3,91, 169soil consolidation 90splint bandage 2, 91, 117, 168surfacing 4sustained release devices 3, 4, 157-8,

222, 304underwater cements 2, 91

architectural applications 283aromatic carboxylic acid 96-7, 347attenuated total reflectance (infrared)

spectroscopy (ATR) 359; see alsoinfrared spectroscopy

B2O3 cements 102, 312BaO cements 204, 318, 338bactericides 335bases for cement formation see cement-

forming liquidsbattlefield dental material 333BeO and Be(OH)2 cements 201-2Bi2O3 cements 102, 201-2, 312-3,

318bioactive materials 3bioadhesion see adhesionbis-GMA 170-1Bjerrum ion-pairs 67Boltzmann distribution 61bone 2

adhesion to 94-6, 111bone cements 2, 90-1, 117, 147,

161-2, 168bone substitute 4, 161-2, 169Born-Oppenheimer approximation 32Bragg equation 367Bronsted-Lowry theory 15-6, 19-20,

48, 2843-butene 1,2,3-tricarboxylic acid/acrylic

acid copolymer 91, 103^ , 131-2,see also poly(alkenoic acid)s

387

Index

CaO cements 204, 318, 321, 338Ca(OH)2 cements see calcium hydroxide

chelate cements, calcium hydroxidedimer (dimerized) acid cements

calcium aluminosilicate glasses seealuminosilicate glasses

calcium hydroxide chelate cements 318,347-50

applications 347composition 348properties 350-1setting 348-9

calcium hydroxide dimer (dimerized) acidcements 351

carboxylate adsorption onhydroxyapatite 95-7

carboxylate bonding modes 363carboxylic acids see alkanoic acids,

poly(alkenoic acid)scaries, effect of fluoride on 258CdO cements 201-2, 204, 312, 318, 321,

338cement classification 7cement-forming acids 3, 5-6, 308

aldehydic aromatic acids 318, 321alkoxy aromatic acids 318, 321, 337allyl-2-methoxyphenol 318-9, 321aromatic carboxylic acids 347citric acid 308cobalt chloride, selenate, sulphate 6copper chloride, selenate, sulphate 6creosote 321yS-diketones 318-9,321dimer (dimerized fatty) acids 351-22,5-dimethoxyphenol 3182-ethoxybenzoic acid 318, 320-1, 337,

see also EBA cementseugenol 318, 320, see also zinc oxide

eugenol cementsfluoboric acid 308gallic acid 6, 315glycerol phosphoric acid 308guaiacol (2-methoxyphenol) 318-9,

321ketoacids 318, 321ketoesters 318, 321magnesium chloride 6, 2 8 3 ^ , 290,

292-6magnesium selenate 6magnesium sulphate 6, 283-4,

299-302malic acid 6mellitic acid (benzene hexacarboxylic

acid) 6, 315methoxyhydroxybenzoic acids 342-62-methoxyphenols 318-9, 321

oil of cloves 320-1orthophosphoric acid see phosphoric

acidphosphoric acid 6, 22, 56, 85,

197-201phytic acid 3, 5, 309-10poly(acrylic acid) 6, 22, 56-8, 69,

70-1, 74^5, 78-9, 90-4, 97-8, 1 0 3 ^poly(alkenoic acid)s 56-8, 69-71,

74-5, 78-9, 90-1, 97-8, 103-5, 360poly(phosphonic acid)s 310-1poly(vinyl phosphonic acid) see

poly(phosphonic acid)spropylene-2-methoxyphenol 321pyruvic acid 6salicylates 348syringic acid see

methoxyhydroxybenzoic acidstannic acid 6,308,315tartaric acid 6, 308, 315tricarballylic acid 6vanillic acid see

methoxyhydroxybenzoic acidszinc chloride 6, 283-9zinc selenate 6zinc sulphate 6

cement-forming cations 9, 19-22,198-9, 201-4, 244

cement-forming liquids see cement-forming acids

cement-forming metal oxides 5-6, 102,201-2, 312-6, 318, 321

A12O3 102B2O3 102, 312BaO 204,318,338BeO and Be(OH)2 201-2Bi2O3 102, 201-2, 312-3, 318CaO 312,318Ca(OH)2 204, 311-2, 318, 321, 338CdO 201-2, 312, 318, 321, 338CoO 312Co(OH)2 6, 202, 222, 312, 315-6Cr2O3, CrO3 312CuO 6, 102, 201-2, 204, 311-2,

315-6, 318, 321, see also CuOcements

Cu2O 201-2, 311-2, see also Cu2Ocements

Fe2O3 312HgO 102,312,318,321,338In2O3 312La2O3 102, 312MgO 6, 102, 201-2, 204, 311-3, 321,

338, see also magnesium phosphatecements

MoOo 312

388

Index

MnO2 312PbO 102, 312, 318, 321, 338Pb3O4 201-2, 312SnO 201-2,312WO3 312Y2O3 102, 312ZnO 6, 102, 201-2, 204, 311-3, 318,

see also zinc polyalkenoate cement,zinc phosphate cement, zinc oxideeugenol cements, EBA cements

cement gels 8-10cementitious bonding 7-11, 307-8cementitious substances

mortars 1plaster of Paris 1, 7Portland cement 1, 2, 5, 7refractory cements 197silicate/silica gel cements 140

chelate agents 6chelate cements 318-52chelate formation 71citric acid cement 308cloves, oil of 321CoO cements 312Co(OH)2 cements 202, 222, 312, 315-6composite resin 154—6, 235compressive strength 359, 370-2, see

also experimental techniquesconcrete 1condensation see counterion

condensationcondensation cements 7configuration see polymer conformationconformation see polymer conformationconsistency 375, 378controlled release devices see sustained

release devicescopper phosphate cements 201-2, 221-2coulombic forces 80-2counterion

binding 7-8, 59-83, 106, see also ionbinding

distribution 59-63, 82condensation 63-7, 78

creosote cement 321Cr2O3, CrO3 cements 312crystallinity in cements 8-10, 205-13CuO cements 102, 201-2, 221-2, 231,

311-2, 315-6, 318,321chelate 231miscellaneous 315-6phosphate 201-2, 221-2polyalkenoate 101-2polyphosphonate 311-2

Cu2O cements 6, 201-2, 220-1,311-2phosphate 201-2, 220-1

polyalkenoate 101-2polyphosphonate 311—2

Debye-Hiickel theory 43-5, 67degradative studies (chemical) 105,

136-9, 244-7, 339, 360-1dehydration

and gelation 72, 84and precipitation 77-9

dental cements 90-1, 103, 106-13,116-7, 146-7, 166-8, 204, 214,235-6, 320-1, 333

dental impression material 335dental materials 2dental plaque 379dental silicate cement 235-63, 366, 369,

381applications 236-7, 249composition: glasses 238-9, see also

aluminosilicate glasses; liquids 218,241-3, see also phosphoric acid

history 235-7modified materials 237-8: acid-

resistant 237; indium-containing237

properties 253-65: acid erosion259-60; bacterial contamination261; biological 260-1; erosion255-8; fluoride release 255-6,257-8; physical 253-7;powder: liquid effect 256;translucency 255; water absorption256-7

setting 243-9: electrical conductance247, 366-7; exotherm 381;hydration 247, 249; ion release25-7; infrared spectra 243; NMRspectra 245,252,365-6;permittivity 367; pH changes249; precipitation of ions 243-8;salt formation 244-8; silicaformation 247, 250-1; waterdeficiency effect 249

structure 249-53: electronmicroscopy 250-1; electron probeanalysis 250-2, 369; elementdistribution 250-3; opticalmicroscopy 250

dentine 91bonding to 92-5, 111, 152-4composition 94treatment 152-^

desolvationand gelation 72, 84and precipitation 77-9

diffusion in water 37

389

Index

/?-diketone cements 318-9, 321dilatometry 59dimer acid (dimerized fatty acid) cements

351-22,5-dimethoxyphenol cements 318dipole interactions 82dissolution

of polymers in water 45-7of salts in water 41

distribution functions for ion-pairs67-8, 72-3

double-torsion test of fracture toughness374

dual-cure resin cement see resin glasspolyalkenoate cement

EBA 6,318,320,337EBA cements 320, 337-47EBA divanillate cements 344-5EBA eugenol cement 337-42

composition 338-9physical properties 340-2setting 339-40structure 339-40

EBA-methoxyhydroxybenzoate cements342-4

composition 343-3properties 3 4 2 ^setting 343-4

EBA polymer cement 344-6EBA syringate cement see EBA-

methoxyhydroxybenzoate cementsEBA vanillate cement see EBA-

methoxyhydroxybenzoate cements'egg-box' model for gelation 85electrical conductance 247, 325-6,

359, 366-7electron diffraction 33, 35electron probe microanalysis 105,

144-5, 233, 247, 250, 369-70electrons, hydrated 44enamel (tooth)

acid-etching 93, 153—4aluminium uptake 258bonding to 92-6, 111-2, 152^composition 94conditioning 15 3—4fluoride uptake 158, 258opacity 152translucency 152

erosion measurement 378-92-ethoxybenzoic acid see EBAethylene glycol dimethacrylate 170eugenol 2, 6, 318, 321, see also zinc

oxide eugenol cementsexotherm measurements 147, 308-1

experimental techniques 359-381adsorption 95-7compressive strength 359, 370-2degradative studies (chemical) 105,

136-9, 244-7, 339, 360-1diametral compressive strength 372dilatometry 59dipole interactions 82double torsion test of fracture

toughness 374electrical conductance 247, 325-6,

359, 366-7electron diffraction 33, 35electron probe analysis 105, 144—5,

233, 247, 250erosion 378-9exotherm measurements 147, 380-1flexural strength 359, 370, 372-3four-point bend test 374Fourier transform infrared

spectroscopy (FTIR) 105, 359, 364,see also infrared spectroscopy

fracture toughness 373-4impinging jet 158-9, 216-8, 341, 379infrared spectroscopy 99-101, 105-6,

137, 142, 198, 210, 243-4, 247,250-2, 311, 323-5, 337, 339-40, 343,348-9, 351, 359, 361-4

leaching studies 106, 378-80mass spectrometry 42mechanical properties 370-4neutron diffraction 33, 35-6, 42NMR spectroscopy 59, 141, 198,

200-1, 245, 359, 364-66NMR spectroscopy solid state 121,

125, 131, 145-6, 252opacity 127, 148, 151-2, 379-80, see

also translucencyoptical microscopy 143, 249optical properties 127, 146-8, 151-2,

166, 379-80parallel plate plastometer 377permittivity 325-6, 359, 367Raman spectroscopy 198rheometry 141,374-8scanning electron microscopy 106,

128, 226-30, 232-3, 329, 331setting measurements 374-8tensile strength 370titrimetric methods 311transition temperatures 130translucency 147, 151-2, 166, 359,

379, see also opacitytransmission electron microscopy 145viscosity measurements 141water analysis 105-6

390

Index

X-ray diffraction (XRD) spectroscopy9-10, 33, 35, 47, 51, 105, 125-6, 130,198, 202-3, 208-9, 224^31, 250,283-6, 293, 323, 359, 367-8

Fe2O3 cement 312fire-resistant materials 283flexural strength 359, 370, 372-3floor fabrication 290floor repair 222fluoboric acid cement 308fluoride

additive to glass polyalkenoate cement133

alumino complexes 135-8, 244bone remineralization 161-2and caries 258cement reaction 106-8glass polyalkenoate cement reactions

133-41release from cements 117, 147, 157-8,

255-8, 379fluoride glasses 136-46foundry sand 2four-point bend test 374Fourier transform infrared spectroscopy

(FTIR) 105, 359, 364, see alsoinfrared spectroscopy

fracture toughness 373-4freezing point of water, D-structure 38Fuoss distribution 68-9

gallic acid cement 6gelatinising minerals 6, 114-6gels and gelation 8-11, 49, 56, 64, 83-5,

138cations, gel-forming 9* egg-box' model 8 5hydration 72, 83-4ion binding 84-5models 10-1,85neutralization 84polyion interaction 84polymer conformation 77, 84structures 10-1,85

Gibbs equation 40Gillmore needle 375glass-ionomer cement see glass

polyalkenoate cement, glasspolyphosphonate cement

glass polyalkenoate cement 2-4, 56,90-1, 116-175, 235

applications 117, 147, 160-2, 166-9composition 123-46, 162-3: additives

133-4, 376; fluoride glasses136-46; glass effect on properties

123-31; glasses see aluminosilicateglasses; liquids see poly(alkenoicacid)s; metal fluoride additives134, 163; oxide glasses 135;poly(alkenoic acid), molecular masseffect 163; poly(alkenoic acid)type, effect on properties 132;tartaric acid effect 133-4, 376

history 116—7light-cured see resin glass

polyalkenoate cementproperties 94-7, 117, 146-165, 174:

acid erosion 158-9; acid-etching of155-6; adhesion 94-7, 117, 147,152-6, 164, 174-5; biological159-61; bonding to composite resins154-6; bonding to tooth material152-4; bone remineralization161-2; consistency of pastes 148;creep 148; erosion 148, 156-9,165;exotherm 147; fluoride release157-8, 379; glass composition, effect123-31; modulus 149, 164;opacity, see translucency; plasticity147-8; poly(alkenoic acid),molecular mass effect 163;poly(alkenoic acid) type, effect 132;setting behaviour 122-8, 132-3,165; strength 122, 125, 127, 132-4,138-9, 147-50, 163-6, 372-3; stressrelaxation 148-9; tartaric acid,effect 133-4; translucency 127,147-8, 151-2, 166, 380;viscoelasticity 148-9; waterabsorption 156-7; wear 159

resin hybrids see resin glasspolyalkenoate cement

setting reaction 98-9, 134-43:aluminofluoride complexes in 137-8;coordination changes 145-6;desolvation 135; exotherm 147;fluoride, effect of 13^41, 134,163; Fourier transform infraredspectra 364; gelation 134-5,137-8;glass ions release 361; hydration139; infrared spectra 137, 362,364; ion binding during setting137-9; NMR spectra 145-6, 366;pH changes 134, 136, 138; polymerconfiguration 135; precipitation ofions 137-8; release of ions from134, 137-8; rheometry 376; saltformation 135; silicic acid andsilica gel formation 134, 139-40,145-6; solvation 139; strengthincreases 139, 148-9; tartaric acid,

391

Index

setting reaction (cont.)effect 131-5, 141-3, 162-3;viscosity increases 135, 141

structure 138, 142-6: electronmicroscopy 143-145; electronprobe analysis 145-6, 369-70;element distribution 144-5;molecular structure 99-101; opticalmicroscopy 142-3; reptationmodel 139; water states 31,49,146

glass polyphosphonate cement 117,314-5

glass transition temperatures 52glasses see aluminoborate glasses,

aluminosilicate glassesglycerol phosphoric acid cement 308guaiacol cement 318, 321Gurney potential 45

handyman materials 3hard acids see HSAB theoryhard and soft acids and bases see HSAB

theoryhard bases see HSAB theoryHEMA see hydroxyethyl methacrylatehexaquo cations 16, 47, 284HgO cements 102, 312, 318, 321horticulture 4HSAB theory 2^6 ,47-8human health care 4hybrid light-cured cements see resin glass

polyalkenoate cementshydrated electrons 44hydration 74-9, 139, 247, 249, 307

and gelation 49-50, 72, 77-9, 84and ion binding 76-7and ionization 74—7of ions 31,41-4,47-8of polyions 31, 73-5and precipitation 77-9in solid state 47

hydration number 42hydration regions see hydration shellshydration shells 42-3, 49-50, 72-7hydration states 31, 49-50, 59hydration zones see hydration shellshydraulic cements 7hydrides 33hydrogel 1hydrogen bonding

in cements 9, 203in 2-ethoxybenzoic acid 338in phenols 321-2in phosphoric acid 198of water 38

hydrologic cycle 32hydrophobic interactions 40-1hydrosphere 32hydroxyapatite 95-7

adsorption on 95-7aluminium uptake 258carboxylate uptake 95-7fluoride uptake 258

hydroxydimethyl acrylates 170hydroxyethyl methacrylate (HEMA) 3,

169-173

ice structures 35-6impinging jet test 158-9, 216-8, 341,

379In2O3 cement 312infrared spectroscopy 359, 361-4

attenuated total reflectance (ATR)359

calcium hydroxide chelate cements348-9

calcium hydroxide dimer cements 351dental silicate cement 243, 247, 250-1EBA (2-ethoxybenzoic acid) cements

339^0EBA-methoxyhydroxybenzoate

cements 3432-ethoxybenzoic acid (EBA) 337eugenol 323-^Fourier transform (FTIR) 105, 359,

364glass polyalkenoate cement 136-7,

142metal oxide polyphosphonate cements

311molecular structure of polyalkenoate

(polyelectrolyte) cements 99-101phosphate bonded cements 198, 210,

244, 247, 250-2polyalkenoate cements 104-5, 136-7,

142zinc phosphate cement 210zinc polycarboxylate cement 104-5ZOE cements 323-6

insulating materials 283investment materials 222ion binding 7-8, 59-83, 106

cation effect 65-67cements 106complex formation effect 69-70density changes 73-4dipole changes 7electric conductance changes 59hydration effects 72-9molar volume changes 74polymer type effect 70-2

392

Index

refractive index changes 63, 73-5turbidity 79ultrasonic changes 74viscosity changes 78

ion coordination 47-8, 69, 99-101,117

ion-ion interactions 44-5, see also ionpairs

ion-pairs 49, 72-3, 79contact 72-3, 79distribution functions 67-8, 72-3hydration (solvation) of 72-3solvent separated 72-3, 79types 72-3

Irving-Williams series 69-70itaconic acid/acrylic acid copolymer 56,

91, 97-8, 103-4, 132-3, see alsopoly(alkenoic acid)s

jet test see impinging jet test

ketoacid cements 318, 321ketoester cements 318, 321

La2O3 cements 102, 312leaching studies 106Lewis acids 6, 18, 22-4, 47, 284Lewis theory 17-20light-cured cements 3-4, see also resin

glass polyalkenoate cementliquids for cement formation see cement-

forming acidsLux-Flood theory 17-20

magnesia cements 283, see alsomagnesium oxychloride cement,magnesium oxysulphate cement,magnesium phosphate cements

magnesia phosphate cements seemagnesium phosphate cements

magnesium chloride solution 6, 283-4,290, 292-6

magnesium oxide 103-4, 290-1cements 283, see also magnesium

oxychloride cement, magnesiumoxysulphate cement, magnesiumphosphate cements

deactivation 290-1magnesium oxychloride cement 2,31,

51, 283, 290-9applications 290components 290phases 294: MgO-MgCl2-H2O

294-5setting chemistry 291-3setting kinetics 293—4

magnesium oxysulphate cement299-304

phases 300-2: MgO-H2SO4-H2O301; MgO-MgSO4-H2O 300-2

porosity 303properties 302-304setting chemistry 299-300

magnesium phosphate cements 2, 102,204, 222-35

aluminum acid phosphate type 233-5ammonium dihydrogen phosphate type

224-31ammonium polyphosphate type 232composition 222-3diammonium phosphate type 231-2phosphoric acid type 224types 223-4

magnesium polyalkenoate cement 235magnesium polyphosphonate cement

312-3magnesium titanate phosphate cement

235maleic acid/acrylic acid copolymer 56,

91, 97-8, 103-4, 132-3, see alsopoly(alkenoic acid)s

malic acid cements 6mass spectrometry 42mellitic acid cement 6, 315metal oxide cements

eugenol cements 321; see also zincoxide eugenol cements

oxysalt bonded cements 2-3, 5-6,304-5, see also magnesiumoxychloride cement, magnesiumoxysulphate cement, zincoxychloride cements

phosphate cements 201-2, 221-22,see also CuO cements, Cu2Ocements, dental silicate cement,magnesium phosphate cements, zincphosphate cement

polyalkenoate cements 90, 102-3, seealso glass polyalkenoate cement, zincpolycarboxylate cement

polyelectrolyte cements seepolyalkenoate cements,polyphosphonate cements

polyphosphonate cements 311-3metal oxides for cement formation see

cement-forming metal oxidesmetal poly(acrylic acid) complexes

69-70methods see experimental techniquesmethoxyhydroxybenzoate cements see

EBA-methoxyhydroxybenzoatecements

393

Index

methoxyhydroxybenzoic acids 342-62-methoxyphenol cements 318-9, 321,

342MgO cements 6, 102, 201-2, 204, 311-3,

318, 321, see also magnesiumphosphate cements

micromechanical attachment 93, 154-5mineral cements

ionomer 90, 114-6phosphate 265polyalkenoate 90, 114-6

MnO2 polyphosphonate cement 312MoO3 polyphosphonate cement 312model materials 91molecular dynamics 42mortars 1-2mouth fluids 216

neutron diffraction 34-6, 42NMR spectroscopy 59, 141, 198, 200-1,

245, 359, 364-66NMR spectroscopy solid state 121, 125,

131, 145-6,252non-aqueous cements 318-52nuclear applications 283

oil of cloves 320-1opacity see optical propertiesoptical microscopy 143, 249optical properties 127, 146-8, 150-2,

165, 379-80opacity 127, 148, 151-2, 379-80, see

also translucencytranslucency 1, 3, 147, 151-2, 166,

see also opacityorganic polymers 1orthophosphoric acid see phosphoric

acidorthosilicic acid 7, 121, 134, 139-40,

243-4, 247osmotic forces 80-1osteogenesis 162oxychloride cements 3, 5-6

calcium 304cobalt 305magnesium 2, 31, 51, 283, 290-9zinc 2, 31, 51, 285-90

oxygen polyhedra 9, 120, 123, 125,128-9

oxysalt bonded cements 2-3, 5-6, 31, 51,283-305

components 6, 284setting 284

oxyselenate cements 3, 6oxysulphate cements 3, 5-6

magnesium 283-4

parallel plate plastometer 377PbO cements 102, 312, 318, 321, 338Pb3O4 cements 201-2,312permittivity 325-6, 359, 367phenolic cement-formers 5-6, 308,

318-21allyl 2-methoxyphenol see eugenol2,5-dimethoxyphenol 318eugenol 318, 321, see also zinc oxide

eugenol cementsgallic acid 6, 315guaiacol (2-methoxyphenol) 318, 3212-methoxyphenols 318-9, 321propylene-2-methoxyphenol 321tannicacid 6, 308, 315

phlogiston theory 31phosphate-bonded cements 3, 7,

197-265liquids for 218, 241-3see also copper phosphate cements,

dental silicate cement, magnesiumphosphate cements, metal oxidephosphate cements, zinc phosphatecement

phosphoric acid 2, 5-6, 22, 56, 85,197-204, 241-3

aluminium complexes 198-200cations in 198-201, 203-4cement forming liquids 218, 241-3concentration effect on cement

properties 218, 241-3dimers H6P2O8 198hydrogen bonding 198infrared spectra 198NMR spectra 198, 200phase diagrams 199-200:

A12O3-H2O-P2O5 199-200;ZnO-H

pK 197properties 197-8structure 198triple ion H5P2Og 198see also dental silicate cement,

magnesium phosphate cements,mineral phosphate cements, zincphosphate cement

phytic acid [myo-inositolhexakis(dihydrogen phosphate)]cement 3, 5, 309-10

cements 309-10, 360pipelines 2, 91plaster of Paris 1,91plasters 290Poisson distribution 61poly(acrylic acid) 6, 22, 31, 46, 49, 56-9,

69-71, 90-1, 97-8, 103-4, 132-3,

394

Index

237, 360, 366, see also poly(alkenoicacid)s

poly(acrylic acid), modified vinylcontaining 3, 170-2

polyalkenoate cements 2, 3, 90-175molecular structure 99-101setting 98-9see also glass polyalkenoate cement,

mineral polyalkenoate cements, zincpolycarboxylate cement

polyalkenoatesadhesion 94-6adsorption 96-7complexes 69-70

poly(alkenoic acid)s 56-8, 69-71, 74-5,90-1, 97-8, 103-5, 132-3, 360

acrylic acid homo and copolymers56-7, 70-1, 74-5, 91, 97-8, 103—4,132-3, 360

binding properties 903-butene 1,2,3-tricarboxylic acid

copolymers 91-2, 103-4, 132and cement properties 132,162-3concentration effect on cement

properties 132, 162conformation 46, 58-9dissolution in water 45-7ethylene-maleic acid homopolymer

71, 75, 98, 360gelation 83-5in glass polyalkenoate cements 132-3hydration 74-7ion binding 77-9itaconic acid polymers 56, 71, 75,

91-2, 103-4, 132-3maleic acid polymers 56, 71, 75, 91-2,

103^4, 132-3methacrylic acid polymer 70-1, 75,

360modified vinyl containing 3, 170-3molecular mass 98, 103-4polymer preparation 97-8preparation 97-8solid form use of 163vinyl methyl ether maleic acid polymer

71, 360zinc polycarboxylate cements 103-4

polycarboxylates see polyalkenoatespolyelectrolyte cements 2-3, 90-169,

310-1, see also polyalkenoatecements and polyphosphonatecements

polyelectrolytes 31, 45-6, 56-85, 91-8,200-1, 310-1, see also poly(alkenoicacid)s, polyions, poly(phosphonicacid)

poly(ethylene maleic acid) 71, 75, 98,360

polyHEMA see poly(hydroxyethylmethacrylate)

poly(hydroxyethyl methacrylate) 169-73polyion-polyion interactions 82-3polyions

attraction 82-3conformation 58-9extension 82-3hydration 73-5interaction 82-3ion binding to 56-7, 59-64repulsion 82-3

poly(itaconic acid) 71, 75poly(maleic acid) 71, 75polymer configuration see polymer

conformationpolymer conformation 58-9, 79-81

coulombic attraction effects 80-1, 84factors affecting 79-81gelation and 81, 83^4neutralization effects 84osmotic pressure, effect on 80-1, 84

polymer morphology see polymerconformation

polymer shape see polymer conformationpolymerization 1, 3poly(methacrylic acid) 70-1, 75, 360polyphosphonate cements 117,310-5

glass (aluminosilicate) 314-5metal oxide 311-4

poly(phosphonic acid)s 56, 311poly(vinyl methyl ether-maleic acid) 71,

360poly(vinyl phosphonate) cements see

polyphosphonate cementspoly(vinyl phosphonic acid) 56-7, 311Portland cements 1, 2, 5, 298pottery 1precipitation cements 7propylene-2-methoxyphenol cement 321protonic acids 6, 14—6pulp capping 347pyruvic acid cement 6

Q nomenclature for silicates 114, l ib,120, 125, 129, 131, 137

Raman spectroscopy 198rapid repair materials 3, 4reaction cements 7refractive index 63, 74—5refractory cements 197resin glass polyalkenoate cement 3, 4,

16^-175

395

Index

composition 170-3light activation 171properties 173setting reaction 170-2structure 172-3

rheometry 141, 374-8road repairs 222runway repairs 222

salicylate cements 348salts, dissolution in water 41scanning electron microscopy 106,

128, 226-30, 233, 329, 331setting measurements 374-8silica gel 7, 9, 121, 139^0, 247, 307silicate cements 7silicate glass 9silicate minerals 6, 90, 114—6, 265silicic acid 7, 9, 121, 140, 244, 247silicophosphate cement 263-5slip casting 3,91, 169SnF2 113SnO cements 201-2, 312soft acids see HSAB theorysoft bases see HSAB theorysoil consolidation 90solvation see hydrationSorel's cements see magnesium

oxychloride cement, zinc oxychloridecement

splint bandage 2, 91, 117, 168SrO polyacrylate cement 102sulphur impregnation of cements 297sustained release devices 3, 4, 157-8,

222, 304syringate cements see


tannic acid cements 6, 308, 315tartaric acid cements 6, 308, 315tartaric acid in glass polyalkenoate

cement 132—5temperature measurements 147, 380-1tensile strength 370titrimetric methods 59, 311tobermorite gel 140tooth material see dentine, enameltrace element release 3, 4, 156-8, 222,

304transition temperatures 130translucency see optical propertiestransmission electron microscopy 145tricarballylic acid (1,2,3-

propanetricarboxylic acid) cements6,315

turbidity measurements 78

ultrasonic methods 74underwater cement 2, 91Usanovich theory 18-20

V-structure of water 37-40vanillate cements see


vinyl polymerization 3viscoelasticity 148-9, 341, 375viscosity measurements 59, 141

water 30-55in AB cements 30-1: hydration 31,

74-9, 139, 247, 249, 307; in cementformation 7, 249; component of30, 48-51; ligands 101, 137-8;' loosely bound' (evaporable)49-50; as plasticizer 31, 51-2; asreaction medium 247, 307; assolvent 30, 48, 249, 307; states49-50; ' tightly bound' (non-evaporable) 49-50

as a base 49coordination to ions 47-8, 101, 137-8and gelation 49-50, 72, 77-9, 83-4hydration shells 50, 74-7and hydrides 33-4hydrosphere 32ion binding effect 73-4structure 34—40: anomalous density

39; bond angles 32; bond lengths32; constitution 31-3; D-structure37-40; density and 7 3 ^ ; hydrogenbonding 38; hydronium ions 44;I-structure 3 7 ^ 0 ; intrinsic 74;nucleophilic attack 39; O-H bondenergy 33; orientated 73;refractive index 73-4; translationalmotion 35; translational energy37; V-structure 37-8; see also ice

as a solvent 30, 40-8: dissolution ofsalts 41; dissolution of polymers45; hydrophobic interactions 40-1;solvation energies 41;thermodynamics 40-1

working time 375

X-ray diffraction (XRD) spectroscopy9-10, 33, 35, 47, 51, 105, 125-6, 130,198, 202-3, 208-9, 224-31, 250,283-6, 293, 323, 359, 367-8

Y2O3 cements 102, 312

396

Index

zinc chloride solutions 2, 6, 283-9zinc oxide 103^ , 205-6, 321-2, 329-31

active forms 321-2, 328-31deactivation 205-6defect structures 329densification 103^ , 205-6ignition 103-4, 205-6, 329magnesium oxide in 205-6mineralizers 206preparation by thermal decomposition

salts 328-9preparation by zinc oxidation 328sintering 206, 329water in 328-9,331

zinc oxide chelate cements 318-20, seealso EBA cements, EBA divanillatecements,EBA-methoxyhydroxybenzoatecements, zinc oxide eugenol cements

zinc oxide eugenol (ZOE) cements321-37, 381

applications 320composition 321-2, 336: abietic acid

(rosin) 322, 334; accelerators322-3; antimicrobial agents in 322,335; impression paste 335;modified materials 336; reinforced336-7

history 320-1, 335properties 333-7: biological 334-5;

creep 333; hydrolysis 332—4;mechanical 333, 336-7; settingcontraction 333; setting time328-9; zinc oxide type, effect328-31

setting reaction 322-31: electricalconductance 325-6; permittivity325-6; water role 324-9, 331; zincoxide type, effect 328-31

structure 331-2zinc oxychloride cements 2, 31, 51, 283,

285-90history 2,283,285-6phases 286-90: ZnO-HCl-H2O

290; ZnO-ZnCl2-H2O 51,286-90

setting 287-8structure: thermogravimetry 288—9;

XRD 286-7zinc oxysulphate cements 6zinc phosphate cement 204-21, 381

applications 204, 214composition 205-6: aluminium role

205, 207, 209-12; fluoride-containing220; modified 220; liquid 207,see also phosphoric acid; MgO in

205-6; powder (ZnO) 205-6, seealso zinc oxide

history 204-5phases: ZnO-P2O5-H2O 199, 207-9properties 214: biological 219;

erosion 216-7; exotherm 207;factors affecting 218-9; filmthickness 215; fluidity 215;mechanical 215-6; phosphoricacid:water effect 218;powder: liquid effect 219;preparation 214-5; settingcontraction 215; setting time214-5, 219; strength 215; workingtime 215

setting reaction 205, 207-12:aluminium role 205, 207, 209-12;crystallite formation 205, 208-12;exotherm 207; hydration 211-2;infrared spectroscopy 210; pHchanges 207; phosphoric acidconcentration effect 207; XRD209-10

structure 99-101,212-3:microstructure 212-3; molecular99-101

zinc phosphates 199-200zinc polyacrylate cement see zinc

polycarboxylate cementzinc polyalkenoate cement see zinc

polycarboxylate cementzinc polycarboxylate cement 31, 45, 56,

90-1, 93, 103-116, 362, 366, 372,380-1

applications 103composition 103—4: liquids 103—4,

see also poly(alkenoic acid)s;modified materials 113; poly-(alkenoic acid) liquid 103-4, seealso poly(alkenoic acid); powder(ZnO) 103-4, see also zinc oxide;reinforcing fillers 113; stannousfluoride 107

history 103properties 94-7, 106-18: adhesion

94-7, 111-3; biological 112;erosion 109-11; setting 106-8;mechanical 107-9; metal fluorideeffect 108

setting reaction 98-9, 105-6:electrical conductivity andpermittivity 366-7; infraredspectroscopy 105, 361, 365

structure 105-6: hydration states105-6; microstructure 105-6;molecular structure 99-101, 105

397

Index

zinc selenate solution 6 cements, zinc phosphate cement, zinczinc sulphate solution 6 polycarboxylate cementZnO cements 102, 201-2, 204, 311-2, ZnO-P2O5-H2O phases 199, 207-9

see also EBA cements, zinc oxide ZOE cements see zinc oxide eugenolchelate cements, zinc oxide eugenol cements

398

acid base cements

Documents