Expert overviews covering the science and technology of rubber and plastics

ISSN: 0889-3144

Volume 15, Number 7, 2004

V. L. Shulman

Tyre RecyclingR

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RAPRA REVIEW REPORTS

A Rapra Review Report comprises three sections, as follows:

1. A commissioned expert review, discussing a key topic of current interest, and referring to the References and Abstracts section. Reference numbers in brackets refer to item numbers from the References and Abstracts section. Where it has been necessary for completeness to cite sources outside the scope of the Rapra Abstracts database, these are listed at the end of the review, and cited in the text as a.1, a.2, etc.

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Item 1Macromolecules33, No.6, 21st March 2000, p.2171-83EFFECT OF THERMAL HISTORY ON THE RHEOLOGICAL BEHAVIOR OF THERMOPLASTIC POLYURETHANESPil Joong Yoon; Chang Dae HanAkron,University

The effect of thermal history on the rheological behaviour of ester- and ether-based commercial thermoplastic PUs (Estane 5701, 5707 and 5714 from B.F.Goodrich) was investigated. It was found that the injection moulding temp. used for specimen preparation had a marked effect on the variations of dynamic storage and loss moduli of specimens with time observed during isothermal annealing. Analysis of FTIR spectra indicated that variations in hydrogen bonding with time during isothermal annealing very much resembled variations of dynamic storage modulus with time during isothermal annealing. Isochronal dynamic temp. sweep experiments indicated that the thermoplastic PUs exhibited a hysteresis effect in the heating and cooling processes. It was concluded that the microphase separation transition or order-disorder transition in thermoplastic PUs could not be determined from the isochronal dynamic temp. sweep experiment. The plots of log dynamic storage modulus versus log loss modulus varied with temp. over the entire range of temps. (110-190C) investigated. 57 refs.

GOODRICH B.F.USA

Accession no.771897

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Previous Titles Still AvailableVolume 1Report 1 Conductive Polymers, W.J. Feast

Report 2 Medical, Surgical and Pharmaceutical Applications of Polymers, D.F. Williams

Report 3 Advanced Composites, D.K. Thomas, RAE, Farnborough.

Report 4 Liquid Crystal Polymers, M.K. Cox, ICI, Wilton.

Report 5 CAD/CAM in the Polymer Industry, N.W. Sandland and M.J. Sebborn, Cambridge Applied Technology.

Report 8 Engineering Thermoplastics, I.T. Barrie, Consultant.

Report 10 Reinforced Reaction Injection Moulding, P.D. Armitage, P.D. Coates and A.F. Johnson

Report 11 Communications Applications of Polymers, R. Spratling, British Telecom.

Report 12 Process Control in the Plastics Industry, R.F. Evans, Engelmann & Buckham Ancillaries.

Volume 2Report 13 Injection Moulding of Engineering Thermoplastics,

A.F. Whelan, London School of Polymer Technology.

Report 14 Polymers and Their Uses in the Sports and Leisure Industries, A.L. Cox and R.P. Brown, Rapra Technology Ltd.

Report 15 Polyurethane, Materials, Processing and Applications, G. Woods, Consultant.

Report 16 Polyetheretherketone, D.J. Kemmish, ICI, Wilton.

Report 17 Extrusion, G.M. Gale, Rapra Technology Ltd.

Report 18 Agricultural and Horticultural Applications of Polymers, J.C. Garnaud, International Committee for Plastics in Agriculture.

Report 19 Recycling and Disposal of Plastics Packaging, R.C. Fox, Plas/Tech Ltd.

Report 20 Pultrusion, L. Hollaway, University of Surrey.

Report 21 Materials Handling in the Polymer Industry, H. Hardy, Chronos Richardson Ltd.

Report 22 Electronics Applications of Polymers, M.T.Goosey, Plessey Research (Caswell) Ltd.

Report 23 Offshore Applications of Polymers, J.W.Brockbank, Avon Industrial Polymers Ltd.

Report 24 Recent Developments in Materials for Food Packaging, R.A. Roberts, Pira Packaging Division.

Volume 3Report 25 Foams and Blowing Agents, J.M. Methven, Cellcom

Technology Associates.

Report 26 Polymers and Structural Composites in Civil Engineering, L. Hollaway, University of Surrey.

Report 27 Injection Moulding of Rubber, M.A. Wheelans, Consultant.

Report 28 Adhesives for Structural and Engineering Applications, C. O’Reilly, Loctite (Ireland) Ltd.

Report 29 Polymers in Marine Applications, C.F.Britton, Corrosion Monitoring Consultancy.

Report 30 Non-destructive Testing of Polymers, W.N. Reynolds, National NDT Centre, Harwell.

Report 31 Silicone Rubbers, B.R. Trego and H.W.Winnan, Dow Corning Ltd.

Report 32 Fluoroelastomers - Properties and Applications, D. Cook and M. Lynn, 3M United Kingdom Plc and 3M Belgium SA.

Report 33 Polyamides, R.S. Williams and T. Daniels, T & N Technology Ltd. and BIP Chemicals Ltd.

Report 34 Extrusion of Rubber, J.G.A. Lovegrove, Nova

Petrochemicals Inc.

Report 35 Polymers in Household Electrical Goods, D.Alvey, Hotpoint Ltd.

Report 36 Developments in Additives to Meet Health and Environmental Concerns, M.J. Forrest, Rapra Technology Ltd.

Volume 4Report 37 Polymers in Aerospace Applications, W.W. Wright,

University of Surrey.

Report 38 Epoxy Resins, K.A. Hodd

Report 39 Polymers in Chemically Resistant Applications, D. Cattell, Cattell Consultancy Services.

Report 40 Internal Mixing of Rubber, J.C. Lupton

Report 41 Failure of Plastics, S. Turner, Queen Mary College.

Report 42 Polycarbonates, R. Pakull, U. Grigo, D. Freitag, Bayer AG.

Report 43 Polymeric Materials from Renewable Resources, J.M. Methven, UMIST.

Report 44 Flammability and Flame Retardants in Plastics, J. Green, FMC Corp.

Report 45 Composites - Tooling and Component Processing, N.G. Brain, Tooltex.

Report 46 Quality Today in Polymer Processing, S.H. Coulson, J.A. Cousans, Exxon Chemical International Marketing.

Report 47 Chemical Analysis of Polymers, G. Lawson, Leicester Polytechnic.

Report 48 Plastics in Building, C.M.A. Johansson

Volume 5Report 49 Blends and Alloys of Engineering Thermoplastics, H.T.

van de Grampel, General Electric Plastics BV.

Report 50 Automotive Applications of Polymers II, A.N.A. Elliott, Consultant.

Report 51 Biomedical Applications of Polymers, C.G. Gebelein, Youngstown State University / Florida Atlantic University.

Report 52 Polymer Supported Chemical Reactions, P. Hodge, University of Manchester.

Report 53 Weathering of Polymers, S.M. Halliwell, Building Research Establishment.

Report 54 Health and Safety in the Rubber Industry, A.R. Nutt, Arnold Nutt & Co. and J. Wade.

Report 55 Computer Modelling of Polymer Processing, E. Andreassen, Å. Larsen and E.L. Hinrichsen, Senter for Industriforskning, Norway.

Report 56 Plastics in High Temperature Applications, J. Maxwell, Consultant.

Report 57 Joining of Plastics, K.W. Allen, City University.

Report 58 Physical Testing of Rubber, R.P. Brown, Rapra Technology Ltd.

Report 59 Polyimides - Materials, Processing and Applications, A.J. Kirby, Du Pont (U.K.) Ltd.

Report 60 Physical Testing of Thermoplastics, S.W. Hawley, Rapra Technology Ltd.

Volume 6Report 61 Food Contact Polymeric Materials, J.A. Sidwell,

Rapra Technology Ltd.

Report 62 Coextrusion, D. Djordjevic, Klöckner ER-WE-PA GmbH.

Report 63 Conductive Polymers II, R.H. Friend, University of Cambridge, Cavendish Laboratory.

Report 64 Designing with Plastics, P.R. Lewis, The Open University.

Report 65 Decorating and Coating of Plastics, P.J. Robinson, International Automotive Design.

Report 66 Reinforced Thermoplastics - Composition, Processing and Applications, P.G. Kelleher, New Jersey Polymer Extension Center at Stevens Institute of Technology.

Report 67 Plastics in Thermal and Acoustic Building Insulation, V.L. Kefford, MRM Engineering Consultancy.

Report 68 Cure Assessment by Physical and Chemical Techniques, B.G. Willoughby, Rapra Technology Ltd.

Report 69 Toxicity of Plastics and Rubber in Fire, P.J. Fardell, Building Research Establishment, Fire Research Station.

Report 70 Acrylonitrile-Butadiene-Styrene Polymers, M.E. Adams, D.J. Buckley, R.E. Colborn, W.P. England and D.N. Schissel, General Electric Corporate Research and Development Center.

Report 71 Rotational Moulding, R.J. Crawford, The Queen’s University of Belfast.

Report 72 Advances in Injection Moulding, C.A. Maier, Econology Ltd.

Volume 7Report 73 Reactive Processing of Polymers, M.W.R. Brown,

P.D. Coates and A.F. Johnson, IRC in Polymer Science and Technology, University of Bradford.

Report 74 Speciality Rubbers, J.A. Brydson.

Report 75 Plastics and the Environment, I. Boustead, Boustead Consulting Ltd.

Report 76 Polymeric Precursors for Ceramic Materials, R.C.P. Cubbon.

Report 77 Advances in Tyre Mechanics, R.A. Ridha, M. Theves, Goodyear Technical Center.

Report 78 PVC - Compounds, Processing and Applications, J.Leadbitter, J.A. Day, J.L. Ryan, Hydro Polymers Ltd.

Report 79 Rubber Compounding Ingredients - Need, Theory and Innovation, Part I: Vulcanising Systems, Antidegradants and Particulate Fillers for General Purpose Rubbers, C. Hepburn, University of Ulster.

Report 80 Anti-Corrosion Polymers: PEEK, PEKK and Other Polyaryls, G. Pritchard, Kingston University.

Report 81 Thermoplastic Elastomers - Properties and Applications, J.A. Brydson.

Report 82 Advances in Blow Moulding Process Optimization, Andres Garcia-Rejon,Industrial Materials Institute, National Research Council Canada.

Report 83 Molecular Weight Characterisation of Synthetic Polymers, S.R. Holding and E. Meehan, Rapra Technology Ltd. and Polymer Laboratories Ltd.

Report 84 Rheology and its Role in Plastics Processing, P. Prentice, The Nottingham Trent University.

Volume 8Report 85 Ring Opening Polymerisation, N. Spassky, Université

Pierre et Marie Curie.

Report 86 High Performance Engineering Plastics, D.J. Kemmish, Victrex Ltd.

Report 87 Rubber to Metal Bonding, B.G. Crowther, Rapra Technology Ltd.

Report 88 Plasticisers - Selection, Applications and Implications, A.S. Wilson.

Report 89 Polymer Membranes - Materials, Structures and

Separation Performance, T. deV. Naylor, The Smart Chemical Company.

Report 90 Rubber Mixing, P.R. Wood.

Report 91 Recent Developments in Epoxy Resins, I. Hamerton, University of Surrey.

Report 92 Continuous Vulcanisation of Elastomer Pro les, A. Hill, Meteor Gummiwerke.

Report 93 Advances in Thermoforming, J.L. Throne, Sherwood Technologies Inc.

Report 94 Compressive Behaviour of Composites, C. Soutis, Imperial College of Science, Technology and Medicine.

Report 95 Thermal Analysis of Polymers, M. P. Sepe, Dickten & Masch Manufacturing Co.

Report 96 Polymeric Seals and Sealing Technology, J.A. Hickman, St Clair (Polymers) Ltd.

Volume 9Report 97 Rubber Compounding Ingredients - Need, Theory

and Innovation, Part II: Processing, Bonding, Fire Retardants, C. Hepburn, University of Ulster.

Report 98 Advances in Biodegradable Polymers, G.F. Moore & S.M. Saunders, Rapra Technology Ltd.

Report 99 Recycling of Rubber, H.J. Manuel and W. Dierkes, Vredestein Rubber Recycling B.V.

Report 100 Photoinitiated Polymerisation - Theory and Applications, J.P. Fouassier, Ecole Nationale Supérieure de Chimie, Mulhouse.

Report 101 Solvent-Free Adhesives, T.E. Rolando, H.B. Fuller Company.

Report 102 Plastics in Pressure Pipes, T. Stafford, Rapra Technology Ltd.

Report 103 Gas Assisted Moulding, T.C. Pearson, Gas Injection Ltd.

Report 104 Plastics Pro le Extrusion, R.J. Kent, Tangram Technology Ltd.

Report 105 Rubber Extrusion Theory and Development,B.G. Crowther.

Report 106 Properties and Applications of Elastomeric Polysul des, T.C.P. Lee, Oxford Brookes University.

Report 107 High Performance Polymer Fibres, P.R. Lewis, The Open University.

Report 108 Chemical Characterisation of Polyurethanes,M.J. Forrest, Rapra Technology Ltd.

Volume 10Report 109 Rubber Injection Moulding - A Practical Guide,

J.A. Lindsay.

Report 110 Long-Term and Accelerated Ageing Tests on Rubbers, R.P. Brown, M.J. Forrest and G. Soulagnet, Rapra Technology Ltd.

Report 111 Polymer Product Failure, P.R. Lewis, The Open University.

Report 112 Polystyrene - Synthesis, Production and Applications, J.R. Wünsch, BASF AG.

Report 113 Rubber-Modi ed Thermoplastics, H. Keskkula, University of Texas at Austin.

Report 114 Developments in Polyacetylene - Nanopolyacetylene, V.M. Kobryanskii, Russian Academy of Sciences.

Report 115 Metallocene-Catalysed Polymerisation, W. Kaminsky, University of Hamburg.

Report 116 Compounding in Co-rotating Twin-Screw Extruders, Y. Wang, Tunghai University.

Report 117 Rapid Prototyping, Tooling and Manufacturing, R.J.M.

Report 118 Liquid Crystal Polymers - Synthesis, Properties and Applications, D. Coates, CRL Ltd.

Report 119 Rubbers in Contact with Food, M.J. Forrest and J.A. Sidwell, Rapra Technology Ltd.

Report 120 Electronics Applications of Polymers II, M.T. Goosey, Shipley Ronal.

Volume 11

Report 121 Polyamides as Engineering Thermoplastic Materials, I.B. Page, BIP Ltd.

Report 122 Flexible Packaging - Adhesives, Coatings and Processes, T.E. Rolando, H.B. Fuller Company.

Report 123 Polymer Blends, L.A. Utracki, National Research Council Canada.

Report 124 Sorting of Waste Plastics for Recycling, R.D. Pascoe, University of Exeter.

Report 125 Structural Studies of Polymers by Solution NMR, H.N. Cheng, Hercules Incorporated.

Report 126 Composites for Automotive Applications, C.D. Rudd,University of Nottingham.

Report 127 Polymers in Medical Applications, B.J. Lambert and F.-W. Tang, Guidant Corp., and W.J. Rogers, Consultant.

Report 128 Solid State NMR of Polymers, P.A. Mirau, Lucent Technologies.

Report 129 Failure of Polymer Products Due to Photo-oxidation, D.C. Wright.

Report 130 Failure of Polymer Products Due to Chemical Attack, D.C. Wright.

Report 131 Failure of Polymer Products Due to Thermo-oxidation, D.C. Wright.

Report 132 Stabilisers for Polyole ns, C. Kröhnke and F. Werner, Clariant Huningue SA.

Volume 12Report 133 Advances in Automation for Plastics Injection

Moulding, J. Mallon, Yushin Inc.

Report 134 Infrared and Raman Spectroscopy of Polymers, J.L. Koenig, Case Western Reserve University.

Report 135 Polymers in Sport and Leisure, R.P. Brown.

Report 136 Radiation Curing, R.S. Davidson, DavRad Services.

Report 137 Silicone Elastomers, P. Jerschow, Wacker-Chemie GmbH.

Report 138 Health and Safety in the Rubber Industry, N. Chaiear, Khon Kaen University.

Report 139 Rubber Analysis - Polymers, Compounds and Products, M.J. Forrest, Rapra Technology Ltd.

Report 140 Tyre Compounding for Improved Performance, M.S. Evans, Kumho European Technical Centre.

Report 141 Particulate Fillers for Polymers, Professor R.N. Rothon, Rothon Consultants and Manchester Metropolitan University.

Report 142 Blowing Agents for Polyurethane Foams, S.N. Singh, Huntsman Polyurethanes.

Report 143 Adhesion and Bonding to Polyole ns, D.M. Brewis and I. Mathieson, Institute of Surface Science & Technology, Loughborough University.

Report 144 Rubber Curing Systems, R.N. Datta, Flexsys BV.

Volume 13Report 145 Multi-Material Injection Moulding, V. Goodship and

J.C. Love, The University of Warwick.

Report 146 In-Mould Decoration of Plastics, J.C. Love and V. Goodship, The University of Warwick.

Report 147 Rubber Product Failure, Roger P. Brown.

Report 148 Plastics Waste – Feedstock Recycling, Chemical Recycling and Incineration, A. Tukker, TNO.

Report 149 Analysis of Plastics, Martin J. Forrest, Rapra Technology Ltd.

Report 150 Mould Sticking, Fouling and Cleaning, D.E. Packham, Materials Research Centre, University of Bath.

Report 151 Rigid Plastics Packaging - Materials, Processes and Applications, F. Hannay, Nampak Group Research & Development.

Report 152 Natural and Wood Fibre Reinforcement in Polymers, A.K. Bledzki, V.E. Sperber and O. Faruk, University of Kassel.

Report 153 Polymers in Telecommunication Devices, G.H. Cross, University of Durham.

Report 154 Polymers in Building and Construction, S.M. Halliwell, BRE.

Report 155 Styrenic Copolymers, Andreas Chrisochoou and Daniel Dufour, Bayer AG.

Report 156 Life Cycle Assessment and Environmental Impact of Polymeric Products, T.J. O’Neill, Polymeron Consultancy Network.

Volume 14Report 157 Developments in Colorants for Plastics,

Ian N. Christensen.

Report 158 Geosynthetics, David I. Cook.

Report 159 Biopolymers, R.M. Johnson, L.Y. Mwaikambo and N. Tucker, Warwick Manufacturing Group.

Report 160 Emulsion Polymerisation and Applications of Latex, Christopher D. Anderson and Eric S. Daniels, Emulsion Polymers Institute.

Report 161 Emissions from Plastics, C. Henneuse-Boxus and T. Pacary, Certech.

Report 162 Analysis of Thermoset Materials, Precursors and Products, Martin J. Forrest, Rapra Technology Ltd.

Report 163 Polymer/Layered Silicate Nanocomposites, Masami Okamoto, Toyota Technological Institute.

Report 164 Cure Monitoring for Composites and Adhesives, David R. Mulligan, NPL.

Report 165 Polymer Enhancement of Technical Textiles, Roy W. Buckley.

Report 166 Developments in Thermoplastic Elastomers, K.E. Kear

Report 167 Polyole n Foams, N.J. Mills, Metallurgy and Materials, University of Birmingham.

Report 168 Plastic Flame Retardants: Technology and Current Developments, J. Innes and A. Innes, Flame Retardants Associates Inc.

Volume 15Report 169 Engineering and Structural Adhesives, David J. Dunn,

FLD Enterprises Inc.

Report 170 Polymers in Agriculture and Horticulture, Roger P. Brown.

Report 171 PVC Compounds and Processing, Stuart Patrick.

Report 172 Troubleshooting Injection Moulding, Vanessa Goodship, Warwick Manufacturing Group.

Report 173 Regulation of Food Packaging in Europe and the USA, Derek J. Knight and Lesley A. Creighton, Safepharm Laboratories Ltd.

Report 174 Pharmaceutical Applications of Polymers for Drug Delivery, David Jones, Queen's University, Belfast.

ISBN 1-85957-489-0

Tyre Recycling

Valerie L. Shulman(European Tyre Recycling Association (ETRA))

Tyre Recycling

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Contents

1 Scope ..........................................................................................................................................................3

2 Introduction ..............................................................................................................................................3

2.1 Sustainable Development: The Context for Recycling ....................................................................3

2.2 The Size of the Problem ...................................................................................................................5

3 The Tyre: The Raw Material for Recycling ...........................................................................................6

3.1 The Structure of the Tyre ..................................................................................................................6

3.2 Tyre Composition .............................................................................................................................7

3.3 Tyre Wear and Use ...........................................................................................................................8

4 Material Valorisation of Post-Consumer Tyres .....................................................................................9

4.1 Preparation for Recycling ...............................................................................................................10

4.2 Recycling Treatments and Technologies ........................................................................................10

4.2.1 Level 1 Treatments: Destruction of the Structure of the Tyre ............................................11 4.2.2 Level 2 Treatments: Liberation and Separation of the Elements of the Tyre .....................12 4.2.3 Level 3 Treatments: Multi-Treatment Technologies ..........................................................13

4.3 Material Outputs .............................................................................................................................14

5 Traditional and Evolving Markets ........................................................................................................16

5.1 Material Production ........................................................................................................................16

5.2 Applications and Products ..............................................................................................................18

5.2.1 Whole Tyres .......................................................................................................................19 5.2.2 Shred and Chips .................................................................................................................20 5.2.3 Granulate ............................................................................................................................21 5.2.4. Powders and Speciality Powders .......................................................................................23

5.3 Energy Recovery ............................................................................................................................25

5.3.1 Use in Cement Kilns ..........................................................................................................25 5.3.2 Use for Electricity and Steam Generation .........................................................................26

6 The Future ..............................................................................................................................................26

Additional References ....................................................................................................................................30

Abbreviations and Acronyms ........................................................................................................................31

Abstracts from the Polymer Library Database ...........................................................................................33

Subject Index ................................................................................................................................................119

Company Index ............................................................................................................................................129

Tyre Recycling

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The views and opinions expressed by authors in Rapra Review Reports do not necessarily re ect those of Rapra Technology Limited or the editor. The series is published on the basis that no responsibility or liability of any nature shall attach to Rapra Technology Limited arising out of or in connection with any utilisation in any form of any material contained therein.

Tyre Recycling

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1 Scope

This review summarises current tyre recycling practices and the factors that have contributed to their growth and ef cacy as viable, economically and environmentally sound alternatives for treating post-consumer tyres. While it relies on the European model, it draws upon experiences and expertise from around the world, which have often precipitated action in the European Union. The introduction will summarise the current context for recycling, the extent of the post-consumer tyre problem, and the characteristics and composition of the tyre that is the raw material for recycling. The report will review the progress and current status of:

A. Recycling treatments and some of the advances that have facilitated the development of more diversi ed and ef cient treatments and processes.

B. Ways in which the traditional markets for post-consumer tyre materials have expanded and multiplied.

C. Industry initiatives that have contributed to the evolution of a more ‘level playing eld’ for post-consumer tyre materials.

D. Issues and actions for the future.

Material recycling appears to be one of the most signi cant future routes for sustainable development in the tyre related industries. The treatments, technologies, materials and applications presented in this Review are not exhaustive, but provide a snapshot of how the industry has evolved to date. The nal section will explore some of the issues that remain to be addressed and resolved in future.

2 Introduction

Recycling is not a new concept. Prior to World War II, recycling was a relatively common industrial practice for a variety of materials and products - including tyres. However, once synthetic rubber became readily available, recycling was, for the most part abandoned (a.1).

More than half a century later, recycling is again becoming an accepted industrial activity. However, as it is interpreted today, the concept of recycling is inextricably linked to the production and management of waste and by extension, to its prevention and

minimisation. Recycling has evolved into one of the four pillars which support improved resource management through the prevention of waste and the reuse, recycling and recovery of the wastes that do occur in order to achieve sustainable development goals.

2.1 Sustainable Development: The Context for Recycling

During the nal years of the 20th Century, it became apparent that the unbridled economic growth of the past could not be sustained in future without irreparable damage to the environment. Discussions initiated during the 1960s culminated in a proposal for change, at the global level.

The Stockholm meeting of the United Nations Conference on Environment and Development (UNCED) in 1972 is often marked as the turning point in the move towards more sustainable growth practices (a.2, a.3). It signalled a break from the past and the beginning of a new era.

The goals of the conference were limited. They were rst, to introduce the concepts and practices inherent in

sustainability and second, to provoke suf cient concern and interest for world leaders to make a commitment to de-link economic growth from negative environmental impacts.

Simply stated, sustainability requires policies and actions which foster economic and social growth, which meet current needs without detriment to the environment. The aim is to not compromise the ability of future generations to meet their own needs. ‘Environment’ was de ned in the broadest sense to include all of the conditions, circumstances and influences affecting development. The speci c issue was the improved management of natural resources, concentrating on the prevention and control of pollution and waste.

Delegates adopted the principle and accepted the challenge of implementing the sustainable model of development for the 21st Century. For the next twenty years they undertook an exhaustive awareness campaign to draw the support of national and local governments, non-government organisations (NGOs), industry and the public at large.

The global economic and social nature of the plan led to the involvement of other organisations within the United Nations infrastructure. Described in Figure 1, these bodies provide the international framework within

Tyre Recycling

4

which intra-and-inter-national trade occur, including the movement of wastes.

The Basel Convention was created in 1989 under UNEP to ll the gap between existing mandates which facilitate and monitor world trade on the one hand, and those which are concerned with sound environmental practices, on the other.

The mission of the Basel Convention is to monitor the trans-boundary movements and management of wastes to ensure their environmentally sound treatment and disposal and, to provide support to governments by assisting them to carry out national sustainable objectives (a.4, a.5).

By the 1992 UNCED meeting in Rio de Janeiro, much of the groundwork had been completed. The goal of the conference was to propose alternative strategies and actions that could be undertaken in the short, medium and long-term in order to ensure that consideration and respect for the environment would be integrated into every aspect of the development process.

The Basel Convention provided the common framework for the classi cation, management and treatment of waste. Brie y, waste was de ned as:

‘..substances or objects which are disposed of or intended to be disposed of or are required to be disposed of by the provisions under national law.’

Both the Basel Convention and the OECD independently prepared catalogues of the substances, objects, materials,

etc., that are de ned as waste and separated out those de ned as hazardous or dangerous. A nal list contains those wastes that are not perceived to pose a risk to the environment or human health. However, it is important to note that the lists are not mutually exclusive and that under certain conditions, a ‘waste’ can and often does appear on more than one list. Virtually every conceivable material, product or residue is listed - those that are not speci cally named fall under the rubric ‘other’. Tyres, tyre related wastes and other rubber wastes were identi ed as:

• B3140 Waste pneumatic tyres, excluding those destined for Annex IVA operations (recovery)

• B3080 Waste parings and scrap of rubber.

The definition and annexes served as a guide for transboundary movements of waste, principally for environmentally sound management. Examples of recovery and disposal operations were appended. Environmentally sound management was broadly de ned as:

'..taking all practicable steps to ensure that waste is managed in a manner that will protect human health and the environment against adverse effects which may result from such waste.'

Within the context of the de nitions of waste and its environmentally sound recovery and disposal, the OECD laid down the provisions for its transboundary movement and acceptance, within and outside of the member countries. Each country was invited to prepare

Figure 1 International bodies concerned with waste

United Nations Conference on Environment and Development (UNCED) formulates strategies and actions to stop and reverse the effects of environmental degradation and promote sustainable, environmentally sound development in all countries.

United Nations Conference on Trade and Development (UNCTAD) promotes trade between countries with different social and economic systems and provides a centre for harmonising the trade and development policies of governments and economic groupings.

Organisation for Economic Cooperation and Development (OECD) is a permanent body under the UNCTAD. It was created to assist in removing restrictions and facilitating trade between and among member and non-member countries, ensuring that the substances, materials and products, etc., involved do not pose a threat to the environment or humanity in the receiving country.

Basel Convention, under the UN Environment Programme (UNEP), is speci cally concerned with the control of trans-boundary movements of hazardous and other wastes and their disposal, from OECD countries to non-OECD countries. Further, it is concerned with the identi cation of those products and materials which could cause damage to the receiving country(ies).

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a list of those wastes that it would no longer accept for either recovery or disposal, due to lack of appropriate treatment facilities or risks to human health, among other reasons. Thus, procedures were also set out for the non-acceptance of wastes and their return, should they be delivered in error.

Once the framework was established, various tools were examined to assess their capacity for targeting potential environmental impacts. Life-cycle analysis (LCA) was selected as the most appropriate and effective tool for determining the points at which the greatest environmental impacts occur, thus making possible the suggestion and selection of less damaging options (61). For example, the approach permitted the evaluation of industrial outputs from the production or extraction of raw materials through the design and manufacture of materials and products, as well as during product use.

The de nitions, annexes and provisions were accepted by the delegates and also adapted by many countries to comply with national policy and priorities.

The most hazardous wastes and the most prevalent sources of pollution were targeted for immediate attention. Five priority waste streams were also distinguished. In addition to the more general category of 'household waste', post-consumer tyres (at present, subsequent to discussion and debate, post-consumer tyres are not de ned as hazardous waste and do not appear on any list as a dangerous or hazardous waste), demolition waste, used cars, halogenated solvents and hospital waste were earmarked for action.

2.2 The Size of the Problem

The priority waste streams were not necessarily hazardous or large. However, each did pose some degree of dif culty related to its management. The basic

reasons included: the inability to accurately calculate the quantity of arisings, the lack of effectiveness of the treatment or disposal practices at that point in time, and/or, the potential threat to human health.

Post-consumer tyres were not classi ed as hazardous or dangerous. However, accurate data on annual accumulations were not readily available. At the time, the principal methods of managing them were by domestic reuse, retreading, and the use of limited quantities for material recycling or as a secondary fuel. The preponderance was sent to land lls. A large percentage of those that were not landfilled were stored in warehouses or derelict buildings, on farms, or scattered around the countryside, in rivers and streams. In addition to being unsightly, they were found to be a breeding-ground for vermin and insects. Large quantities were also exported to developing countries with less well-de ned environmental regulations or the means to deal with them.

The overall market for either the raw material or the nished product is not particularly large compared to

other wastes. Table 1 illustrates the relative consumption of key material streams in the EU. It is evident that the overall quantity of rubber used is relatively small, however, the production units are comparatively large, and the units of waste conspicuous and unattractive.

World production of natural and synthetic rubber is estimated to be approximately 20,000,000 tonnes per year. About 20%, or approximately 4,000,000 tonnes are consumed in the European Union each year. Indications are that an additional 1,000,000 tonnes are imported annually from outside of the EU as nished goods, including tyres (a.6). Comparable amounts are utilised in North America and a growing percentage is consumed in Asia.

About 75% of the combined rubber resources worldwide, are used in various sectors of the automotive

Table 1 Examples of other waste streams in tonnesProduct Consumption SubsetPaper ±79,000,000Plastics ±37,000,000

Packaging ±19,980,000Glass ±15,000,000Aluminium ±8,860,000

Automotive/construction ±5,316,000Packaging ±1,594,800

Rubber ±5,000,000Tyres ±3,000,000

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industry. The bulk, close to 60%, is consumed in the production of tyres for two principal markets, passenger cars (including utility vehicles) and trucks, as well as for smaller diverse categories grouped as ‘other’ (e.g., agricultural, aeroplane, bicycle, motorcycle, civil engineering, industrial, mining). Hundreds of non-tyre automotive products, i.e., appearance items, belts, hoses, housings, mouldings, rings, and seals, among others, utilise the other 15% of the rubber.

The remaining 25% of natural and synthetic rubbers are consumed by a broad cross-section of other industrial sectors to manufacture thousands of general rubber products. More than 20 categories are represented including such diverse products as footwear, bladders, residential and commercial construction supplies, marine products, ooring and roo ng components, non-automotive equipment, consumer products such as pads and tool handles, seals and expansion joints, civil engineering and road materials, etc.

Since the 1990s world tyre production has been reported to be approximately 1,000,000,000 units per year. In units sold, which are somewhat less than those produced, passenger car tyres account for slightly more than 90%, while truck and ‘other’ categories, together constitute about 10% (a.7, a.8, a.9).

Once a tyre in any category is permanently removed from a vehicle without the possibility of being re-mounted for continued on-road-use, it is de ned as waste. It is generally accepted that for each tyre sold, whether newly manufactured, retreaded or part-worn, one tyre has become waste. In the 15 Member States of the EU alone, post-consumer tyres amounted to more than 2,600,000 tonnes of waste in 2003. Projections for 2004 indicate that the expanded Union of 25 states will account for annual arisings of approximately 2,850,000 tonnes.

3 The Tyre: The Raw Material for Recycling

A pneumatic tyre is often described as an engineering marvel. It is basically a large, round, black, hollow shell lled with compressed air that can support more than

50 times its own weight. It is meticulously constructed of over thirty different component parts to meet diverse performance standards in order to provide maximum comfort and safety on dry or wet, slippery or rutted surfaces, at high or low speeds. It is often pushed beyond its limits or abused by careless behaviour.

During its on-road life it is ignored until it needs repair or is grudgingly replaced. Once a tyre is permanently removed from a vehicle that it has diligently served over thousands of kilometres, it is a waste and follows another route.

The external features of the tyre have not changed perceptibly since the radial tyre was introduced more than fty years ago. However, internally, changes have been made and others are planned which will continue to improve performance and durability as well as environmental quality. Some of these changes will also impact upon the ways in which the tyres are valorised at the end of their on-road life.

3.1 The Structure of the Tyre

Figure 2 illustrates seven critical parts of the tyre structure, each of which serves a speci c function and impacts upon potential recycling actions. The letters in the gure correspond to the component descriptions which follow. The tread and sidewalls are observable exterior elements, while the belts, casing, beads, apex, and inner liner are interior components.

A. The tread is the part of the tyre that comes directly in contact with the road to maintain traction when the vehicle moves forward, back, turns or stops, in wet or dry weather. Rubber compounds with a high concentration of natural rubber and llers, which vary according to tyre category and local conditions, are moulded into a design in which the solid parts of the tread clear away the water while the channels allow the water to ow outwards enabling the tread to maintain contact with the surface.

Figure 2 Tyre cross-section

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B. The belts provide structural support to the tread helping to maintain the tyre shape. Made from layers of rubber sheets containing brass coated high carbon steel wires, they are placed on the bias at alternate angles under the tread. This helps to control road contact, provide a smoother ride and can reduce uneven wear. Some manufacturers have introduced aramid ‘bandages’ to replace steel belting (215).

C. The sidewalls on either side are attached to the casing. This contributes to structural integrity by reinforcing the interface between the tyre and the wheel rim and setting the inner dimension. Side-walls are designed to ex up and down over road irregularities while staying relatively rigid horizontally to respond to driving actions such as steering, braking, etc. Because they are exposed to abrasion damage as well as to UV and ozone degradation, the compounds used in these parts contain many ingredients to counteract these actions, e.g., anti-oxidants, as well as the newer anti-ozonants.

D. The casing provides the shape and internal structure of the tyre, and bears stress. It is traditionally made of twisted metal, natural rayon, nylon or polyester cords that are then coated with a natural rubber substance. As a rule, truck tyres contain a proportionately greater ratio of metals to textiles than do passenger tyres. Since the 1980s, a family of special aramids has been introduced into some products, primarily to reduce tyre weight.

E. The beads are structural components that frame the edge of the casing to anchor the tyre to the metal wheel rim so that it does not shift or become free during driving actions. They are made from coils of zinc or bronze coated single lament high strength steel wire that are coated with rubber and add appreciably to the weight of the tyre. Innovative non-metal materials are being introduced to reduce tyre weight.

F. The apex, at the end of the bead, is used to gradually shape the tyre making the transition from the almost in exible bead to the mid-point of the more pliable sidewalls. It is moulded from ller and reinforcing resins.

G. The inner liner is an integral part of the tyre, providing a lining for the casing in order to contain the air and maintain consistent pressure, which contributes to improved rolling resistance and energy savings. Like the inner tube that it has

replaced in a majority of tyres, it is most often produced from butyl rubber.

While the external features of the tyre have changed little during the past fty years, the ingredients and production processes have changed considerably. Many of the newer ingredients which have improved longevity, resistance to abrasion, etc. also contribute to the durability and effectiveness of post-consumer tyre materials downstream, when they are recycled and used in products and applications.

3.2 Tyre Composition

The material composition of a tyre varies by category, i.e., passenger car, utility vehicle, truck, other, etc. However, all categories of tyres include four fundamental groups of materials: rubbers, carbon blacks/silicas, reinforcing materials and facilitators. All but the reinforcing materials are ingredients in the tyre compounds.

Table 2 provides a generic profile of the material composition of pneumatic car and truck tyres produced for the European market.

The rst group of materials which account for ±40-45% by weight are natural and synthetic rubbers, the former tapped from the Hevea tree, the latter generally derived from petroleum based products. The ratio of natural to synthetic rubber is approximately two to one in truck tyres, four to three in car tyres. Different polymers and additives are used in each part of the tyre.

The second most prevalent materials are carbon blacks and/or silica, which amount to ±23-27% of the tyre

Table 2 Composition by weight of car and truck tyres

Material Car/utility %

Truck/Lorry %

Rubber/Elastomersa ±43% ±45Carbon black and silicab ±27% ±20Metals ±11% ±22Textiles ±5% ±1Vulcanisation aidsc ±3 ±3Additives ±3 ±3Aromatic oils ±8 ±8

a Rubber content: truck ±30% naturalb Different varieties of carbon black are used for

different purposes and may appear in other categories of material

c Sulfur, stearic acid, zinc oxide

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weight. A range of carbon blacks of varying shapes, sizes and structures are used in different parts of the tyre. Larger sizes can be used in the inner liner, while smaller particles can be used in the casing or tread. During the past twenty years, attempts have been made to replace some carbon blacks with silicas in selected applications, such as the Green tyre. More recently, several modi ed carbon blacks have come onto the market for use in tyres.

The third group are reinforcing materials, comprised primarily of metals or textiles. Metals in the beads, belts and casing can add ±25% to the weight of a truck tyre while in car tyres, which utilise a larger portion of textiles in the casing, metals are ±11-13% of the total weight. Natural rayon, nylon and polyester used in the casing cords amount to ±5% of the weight of a car and ±1% of a truck tyre. Manufacturers have experimented with a variety of materials to partially replace the metal content in order to reduce the weight of tyres and have had limited success with a class of aramids, which would increase the bre and reduce the metal content.

The fourth group of materials are used as facilitators during the various stages of tyre production. Small amounts of extender oils, waxes, anti-oxidants, the newer anti-ozonants and other ingredients are added to the tyre compound to enhance performance, or to facilitate curing and manufacturing ef ciency. Several varieties of carbon black; titanium dioxide; zinc oxide and sulfur are used to facilitate the vulcanisation process and are evenly distributed throughout the polymer matrix. Calcium and aluminium are used in small amounts as are trace amounts of magnesium, phosphorous, potassium, sodium, chloride and silica.

The compounding process modifies the hardness, strength and/or toughness of the rubber and increases its resistance to abrasion, oil, oxygen, chemical solvents and heat. Different ingredients are used to produce speci c qualities.

At times ten different rubber compounds are used in a tyre, each of which is a mix which contains a number

of ingredients that modify and improve the physical properties of the rubber. However, while each tyre manufacturer has its own special formula to provide unique characteristics, tyre compounds in general share many similarities and contain all of the ingredients necessary to provide quality on-road service.

Once all of the ingredients have been compounded and the structure has been assembled, the tyre is vulcanised. Vulcanisation is a curing process which transforms the rubber into a strong, elastic and rubbery hard state. Heat causes the vulcanisation agents to combine with the rubber to create chemical links between the rubber molecules. The crosslinking between the molecules makes the rubber stronger and more durable and contributes to improved wear and durability. At the same time, the sulfur also creates a bond between the rubber and the copper that is in the brass coating of the wires. The nal structure is an integrated whole.

Vulcanisation is generally considered irreversible. In other words, after it has been altered, the once long, convoluted rubber molecule cannot return to its original form.

Table 3 presents the average weights of three categories of new tyres produced for the European market.

3.3 Tyre Wear and Use

The average on-road life of a tyre varies by category. Truck tyre life is extended in some countries by re-grooving, i.e., re-cutting the tread grooves, or by retreading. In recent years, truck tyre manufacturers have begun to offer multiple retreadings as part of the tyre sale. The package is sold as a strategic maintenance programme which ensures tyre safety and access to replacement tyres. Similar packages are not economically viable for passenger car tyres as they can only be retreaded once. Further, the availability of budget tyres has removed the nancial incentive.

Table 3 Weight ranges of new tyresCategory Range Av.wt Units/tonnePassenger car tyres ±7-9 kg 8 kg 125Vans and light utility vehicles ±8-11 kg 9 kg 111Trucks1 (load index of 121) 40-75 kg 56 kg 18Other (bicycle, motor cycle, agriculture, air plane, construction, mining)

0.5-2.5-1000 kg NA2

1 the average of the category ‘trucks’ re ects the preponderance of smaller truck tyres2 NA: Information not available

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Longevity is also affected by driver care as well as by maintenance and driving conditions, e.g., climatic extremes, speed and road surfaces. Table 4 approximates the on-road life of different categories of tyres produced for the European market. Estimates are that the average for the North American market can be up to ±50,000 miles (±80,000 kilometres) due to differences in road surface, speed limits and climate conditions.

While a tyre can reach the end of its on-road life at almost any point after construction, the most common reasons for replacing them are accident or wear. Tyre wear is most evident on the tread, although sidewall damage is also common primarily due to driver behaviour or road conditions. It is generally accepted that tyres lose approximately 20% of their weight, principally from the tread, during their on-road life.

The required tread depth for on-road use in OECD countries is a minimum of 1.6 mm in the most worn groove. However, many non-OECD countries, particularly in Asia, Africa, parts of Latin America, among others, do not have the same restrictions. Table 5

shows the approximate tread depth for three categories of new tyres in the EU. To ensure tyre safety, a number of manufacturers mould tread wear indicators into the tread as bands or cushions which become apparent as the tread wears to the de ned legal limit for continued use.

Once a tyre has been permanently removed from a vehicle without the possibility of being returned to the road, it is waste (126, 258). Figure 3 brie y explains the four Rs of sustainable waste management, which through their use, minimise waste and reduce reliance on natural resources.

Material recycling and energy recovery offer alternative and complementary means of gaining the greatest sustainable bene t from natural resources and their wastes and thereby reducing the consumption of virgin resources. Material recycling is emerging as a commercially, technologically and economically viable option for the future.

4 Material Valorisation of Post-Consumer Tyres

The recent life-cycle assessment of ‘an average passenger car tyre’ conducted by BLIC, the European tyre manufacturers’ association confirmed the importance and ef cacy of tyre recycling as a principal means of valorising post-consumer tyres in Europe. In comparison with reuse, retreading and incineration for energy recovery and in cement kilns, material recycling is the only one which resulted in a positive impact on the environment.

The BLIC report concludes that the net environmental effect from processing post-consumer tyres is negative.

Table 4 New tyre wear expectationsTyre category Estimated kilometresPassenger car ±35,000 - 45,000 kmUtility vehicle/light truck ±60,000 - 70,000 kmLong haul truck ±180,000 - 200,000 km

Figure 3 Sustainable waste management options

Reuse: Includes the sale of part-worn tyres for domestic on-road and other uses as well as for export to countries with less restrictive road-use requirements.

Retreading: re-manufactures a tyre using as the core a carefully selected, undamaged casing, which reduces production energy as well as virgin resources.

Recycling: transforms a waste into a raw material that can be reintegrated into the economic stream as a resource to substitute the use of virgin resources.

Recovery: transforms a waste into energy or fuel, which can be reintegrated into the economic stream as a resource to substitute the use of other energy sources.

Table 5 Examples of new tyre tread depth in the EU

Tyre category Average tread depthPassenger car 7-8 mmUtility vehicle/light truck 10-12 mmLong haul truck 18-21 mm

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In other words, the bene ts accrued from recycling outweigh the environmental impacts that result from processing. One contributing factor appears to be the production of useful products from the recycled materials. Conversely, both forms of incineration for energy recovery presented produce a balanced impact - neither negative nor positive effects on the environment (a.10).

Although in considerably greater detail than earlier analyses, it corroborates the findings from studies undertaken in Germany (52), the USA (225), the UK (126), and Russia (356). It also draws parallel conclusions to those reported in the recent mass balance study in the UK (a.11), which examined the economics of recycling and projected several scenarios for the future.

Together, these studies re ect the vast improvements that have occurred within the industry during the past fteen years in terms of the effectiveness of the treatments, the quality and consistency of the material outputs and the range of applications and products in which the materials are used. Improved overall ef ciency has also had the effect of lowering production costs and increasing competivity.

4.1 Preparation for Recycling

The starting point for material recycling is the same as other industries - the sourcing of a continuous ow of raw material. In most EU States post-consumer tyres destined for recycling are collected by specialised tyre collection companies. Tyres are usually collected under commercial contract from regular sources which include garages, retail outlets, depots and vehicle dismantlers, among others. In some regions, they are also removed from long-standing stockpiles or clean-up sites.

The tyres are sorted by category, i.e., car, utility, truck, and then by size, often prior to delivery to the treatment facility. Road-worthy part-worn tyres are removed for domestic reuse or export under OECD regulations. Retreadable casings are diverted to appropriate facilities. Recently, there is even an active competition between recycling and energy recovery facilities for the available tyres.

Post-consumer tyres are a waste and, therefore, must be shipped in compliance with Basel Convention and OECD regulations. However, as they are non-hazardous wastes destined for recovery, the documentation and information required is limited to general information.

Once delivered to the treatment facility, the tyres come under the jurisdiction of the local authority(ies) which regulate the quantity and location where the tyres may be stored on the premises. Appropriate zoning and land-use permission, as well as the necessary permits and licenses must be obtained. Under certain circumstances, waste management permits may also be required before the tyres can be processed or the materials used (59).

Prior to processing, on site or at a facility, the tyres must be cleaned of debris such as glass, stones, or miscellaneous items as well as from partially burned tyre fragments. Tyres acquired from stockpiles or other long-term storage areas are often washed prior to use.

4.2 Recycling Treatments and Technologies

Under the European Commission’s waste legislation, tyre recycling is a recovery operation that encompasses two distinct but interrelated functions:

• transformation of post-consumer tyres with the use of diverse treatments, e.g., size reduction, pyrolysis and technologies (physical, chemical, thermal or biological) in order to produce a broad range of materials which will be reintegrated into the economic stream as a resource to substitute the use of virgin resources, and

• use of the materials in myriad consumer and industrial products as well as construction and civil engineering applications.

A distinction is often made between treatments and technologies. The terms are de ned as follows:

• A treatment is a specialised method of processing a material or substance to achieve a speci ed result, for example, size reduction is a treatment designed to reduce a tyre into smaller pieces or particles for which one of several different technologies can be used.

• A technology is the specialised tool, i.e., a type of equipment or process which is used to achieve a treatment's end result. Thus an ambient granulator is a speci c type of equipment that can be used to reduce a tyre into granulate or powder.

The treatments used to recycle post-consumer tyres range from the simplest mechanical cutting devices to sophisticated and complex multiphase chemical, mechanochemical and/or thermal processes. They

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appear to have overcome many of the principal obstacles inherent in the recycling of thermoset rubbers (413). Speci cally, the treatments and technologies that have evolved do not attempt to dissolve or melt the rubber into the virgin compound. Rather, they attempt to exploit and enhance the properties of the tyre compound.

There are four basic levels of treatment. Each can be described in terms of its functions, which become increasingly complex as they progress through successive levels. The capabilities can be expanded by linking two or more different technologies to operate in tandem in order to produce the desired result.

Level 1 Destruction of the structure of the tyre - Primarily simple mechanical means which destroy one or more of the physical attributes of the tyre, e.g., shape, weight bearing capacity, rigidity, among others. The most common methods include bead removal, compression or cutting.

Level 2 Liberation and separation of the elements of the tyre - Treatments which process the tyre to segregate the principal components, e.g., the rubber, metals, textiles. The most common are ambient and cryogenic size reduction technologies. Level 1 outputs are often used as feedstock.

Level 3 Multiphase treatments and technologies - Rubber materials liberated during Level 2 provide

feedstock for treatments and technologies which modify one or more characteristics of the material. Devulcanisation, reclaim, surface modi cation and pyrolysis, are among the most prominent.

Level 4 Material upgrading treatments - Materials modi ed during Level 3 provide the feedstock for treatments that further refine and upgrade them. Technologies are used that enhance selected properties or characteristics. The preparation of thermoplastic elastomers, upgraded carbon products and improved reclaim are the most representative examples.

Figure 4 illustrates the progressive, often inclusive nature of the treatments from the ' rst cut', by mechanical means, through to 'higher level' specialised treatments and technologies which add distinctive characteristics or properties to the material outputs.

4.2.1 Level 1 Treatments: Destruction of the Structure of the Tyre

This is de ned as simple mechanical means which destroy one or more of the physical attributes of the tyre, e.g., shape, weight bearing capacity, rigidity, among others. The most common means include bead removal, compression or cutting. The majority of the outputs are used directly in civil engineering or construction applications. The remainder are used as feedstock for further treatment.

Figure 4 Schematic of the four levels of treatment

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Bead removal is used on car and truck tyres as a pre-treatment for later recycling treatments. It is a mechanical procedure that removes the rubber coated steel coil wires by force (pulling) or by cutting or tearing the connecting points that anchor the bead to the casing. The carcass can be directly used for civil engineering applications or as feedstock for later recycling treatments.

Sidewall removal is used primarily on car tyres as a pre-treatment for later recycling treatments or the sidewall can be used directly in civil engineering applications. It is a mechanical cutting procedure that destroys the structure of the tyre by removing the support on either side.

Tread removal is used on car or truck tyres as a pre-treatment for later recycling treatments or the tread can be used directly in civil engineering applications. It is a mechanical cutting procedure which frees the strips of tread from the tyre carcass. The tread can be used directly to produce simple products or as a feedstock for later recycling treatments.

Compression is used on car or truck tyres. It is a mechanical procedure that destroys the structure of a tyre by placing it under the force of controlled pressure to permanently deform it. The number of tyres and pressure used is dependent upon the desired nal result. The units can be directly used for a number of civil engineering applications.

Baling is used on car and truck tyres. It is a mechanical procedure that destroys the structure of a speci ed number of tyres by placing them in a chamber under high pressure (±65 tonnes) to permanently deform them into a cube or rectangular solid, which is then secured with a stipulated number of straps at speci ed points. The number of tyres used is dependent upon the desired nal dimensions of the unit. The units can be used directly for a number of civil engineering applications.

Cutting is used on car and truck tyres as a pre-treatment for later recycling treatments or for direct use in civil engineering applications. It is a mechanical procedure that guillotines, scissors or slices the tyre through the middle of the tread producing two equal halves with the sidewall attached, or into halves or quarters across the diameter of the tyre.

Equipment can be stationary or mobile, dependent upon how the material will ultimately be used. Generally, neither the bead wires nor the belts are removed prior to or during processing. The material can be used directly

in civil engineering applications, as a secondary fuel or as feedstock for further processing.

4.2.2 Level 2 Treatments: Liberation and Separation of the Elements of the Tyre

Level 2 treatments separate out the principle components of the material, e.g., the rubber, metals, textiles. The most common technologies are ambient and cryogenic size reduction as well as some new technologies which are designed to reduce the material to pieces of ±0 to ±15 mm. Whole tyres and Level 1 outputs are generally used as feedstock. The outputs can be used directly in applications or products, or as feedstock for Level 3 processing.

Shredding and chipping are used on whole car or truck tyres. Shredding is a treatment that uses different technologies to fragment the tyre. Usually, a set of knives is used to produce material ±50 mm to ±300 mm that is irregularly shaped or equidimensional. Neither the bead wires nor belts are removed prior to, during or after processing unless it is accomplished as the rst step in size reduction processing.

Chipping is generally a second processing of shred which results in material ±10 mm to ±50 mm that is either irregularly shaped or equidimensional.

Ambient grinding uses whole or pre-treated car or truck tyres in the form of shred or chips, or sidewalls or treads. Ambient grinding is a multistep technology. Processing takes place at or above normal room temperature. The rubbers, metals and textiles are sequentially separated out. First, the material is sheared with a system of knives. If the reinforcing and bead wires are not removed prior to processing, the metals are magnetically separated out during the granulation process. The material may continue through one or more sequential granulators to further reduce it in size. The material passes through a series of screens and sifting stations to remove the nal vestiges of impurities and ensure consistency of size (126). During the nal phase, the textile residues are removed by air separators.

Cryogenic processing generally uses pre-treated car or truck tyres as feedstock, most often in the form of chips or ambiently produced granulate. Processing takes place at very low temperature using liquid nitrogen or commercial refrigerants to embrittle the rubber. It can be a four-phase system which includes initial size reduction, cooling, separation, and milling. The material enters a freezing chamber where liquid nitrogen is used to cool it from –80 to –120 °C, below the point where

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rubber ceases to behave as a exible material. The cooling process embrittles the rubber and allows it to be fractured to the desired size resulting in a smooth and regular shape (398, 419, 448). Because of its brittle state, bres and metal are easily separated out in a hammer mill. The granulate then passes through a series of magnetic screens and sifting stations to remove the last vestiges of impurities.

Both ambient and cryogenic processing can be repeated to produce ner particles. Increasingly, the two with their attendant technologies, are combined into one continuous system in order to bene t from the advantages and characteristics of each and to reduce overall costs. The ambient system is generally used for the initial size reduction phases. The cryogenic system is used to further reduce the material in size and then to remove the metals and textiles. The outputs from either or both systems can be used directly or as feedstock for further processing.

4.2.3 Level 3 Treatments: Multi-Treatment Technologies

Level 3 treatments and technologies further process the material to modify one or more characteristics by means of mechanical, thermal, chemical, mechanochemical or multitreatment procedures. The outputs of Level 2 are most often used as feedstock. Reclaim, surface activation, devulcanisation and pyrolysis are representative examples of the range of treatments used. The outputs can be used directly in applications or products, or as feedstock for Level 4 processing.

Rubber reclaim uses ambiently size reduced granulate as feedstock. It is a two-phase thermomechanical shearing process (120, 387) that changes the characteristics of the input material. During the rst phase the granulate is plasticised. During the second phase the plasticised material is processed by thermal and mechanical treatments that break down the vulcanised structure in order to restore some of the original characteristics of virgin rubber, i.e., reducing some of the crosslinks in the granulate. The resulting material has a maximum particle diameter of 0.425 mm and an average diameter of 0.360 mm. Other reclaiming processes utilise chemical treatments in order to transform the elastomeric properties (212, 375).

Surface modification/activation uses buffings or peelings from retreading, or ambiently size reduced granulate produced from truck tyre treads as the

principal feedstock. Surface modi cation is a three-phase treatment. In the rst phase, the feedstock is ambiently reduced to a ne powder of >0.04 mm from which all metals and textiles are removed. In the second phase, the powder is activated by coating it with high molecular weight unsaturated polymers in aqueous dispersion with an appropriate curing system. The third phase occurs during vulcanisation on a double belt press that rolls the material at very high pressure. During the curing process (vulcanisation), bonds are formed between the polymer chains of the coating and the polymer to which the post-consumer tyre powders are added. As a result, the rubber particles activated by the coating are bound to the new three-dimensional network. The original rubber properties are retained (330, 427).

Devulcanisation uses ambiently or cryogenically size-reduced granulate or powder as a feedstock. It is a two-phase treatment in which the rst phase is size reduction, which generally takes place in a different facility. The second phase ‘reactivates’ the material by reducing the number of crosslinks between the rubber molecules that occurred during vulcanisation so that the resulting material can be revulcanised (177). The feedstock is mechanically (85), chemically (178) or thermally broken down to restore some of the original characteristics of the rubber. Chemical activation agents can change some of the physical and/or chemical properties of the resulting material while mechanical technologies retain the same properties of the feedstock. Recent research has led to partial ‘devulcanisation’ systems, which utilise ultrasound or even microbes (84, 216, 330). These devulcanisates can be revulcanised by using traditional methods without further additives.

Pyrolysis uses pre-treated car or truck tyre chips as the principal feedstock. It is a two-phase treatment which uses thermal decomposition to heat the rubber in the absence of oxygen to break it into its constituent parts, e.g., oil, gas and carbon (71, 97). Cracking and post-cracking take place progressively as the material is heated to 450-500 °C and above. De-polymerisation and oil and gas production take place progressively, the balance of product shifting to gas as temperatures increase. A clean oil free char can be produced. Pyrolytic char is a coarse powder with a particle size ranging from 0.4 m to over 1,000 m in diameter. The char can be used in low value production processes as a colorant or ller. The output can continue on for further processing during Level 4 to produce economically more interesting carbon products, which can act as replacements for certain types of carbon black used in rubber compounding.

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4.2.4 Level 4 Treatments: Material Upgrading

Level 4 treatments re ne, upgrade, modify or generate specific characteristics or properties in materials produced by Level 3 treatment, which most often provide the feedstock. Upgraded reclaim, reactivated/surface modi ed/devulcanised materials, upgraded char (carbon products) and new compounds such as TPEs are among the most representative.

Post-treatments of pyrolytic char issued from pyrolysis are mechanical separation, physical or chemical treatments that can upgrade the char by reducing it in size and separating out impurities. Post-treatments generate materials that have similar characteristics to a variety of different grades of carbon black currently utilised for the production of a broad range of commercial products and are valuable for technical rubber goods. Resonance disintegration is an innovative example of particle fragmentation, using resonance forces to vibrate particles apart (a.12). Resonance disintegration can take pyrolytic char from a maximum particle size of 600 m to below 30 m with 50% below 1 m after a single processing with a surface area, structure and dispersion in rubber compound very similar to standard carbon blacks.

Thermoplastic elastomer production uses granulate produced from car or truck tyres as the feedstock. The treatment requires two feedstocks, i.e., granulate and a thermoplastic (polypropylene). It is a two-phase reactivation and mixing process, which combines the material qualities of the former with the processing behaviour of the latter. The reactivation and mixing processes occur under high shear forces in a conventional internal mixer. The granulate acts as an elastomeric compound during the rubber phase in crosslinked thermoplastics creating a new compound (not a blend). Chemical or mechanochemical activation of both the dispersed elastomeric domain and the matrix phases result in a linkage between granulate particles, acting as a dispersed elastomeric domain within the thermoplastic matrix (54, 93). Special crosslinking systems, compatibilisers and additives allow the properties of the new material to be varied in function for the intended use.

Table 6 shows the increasing percentage of the total weight of the tyre that is lost during processing. The principal loss is due to the removal of the metals. Generally, smaller materials retain fewer impurities from metal or textile. As speciality treatments most often use smaller granulate or powders as feedstock, there is usually no further loss during processing. One of the few exceptions could be the upgrading of pyrolytic char.

For many years, all metals and textiles were disposed of in land lls. As recycling has become increasingly ef cient, the waste materials have become cleaner. Subsequently, arrangements have been made with metal recycling facilities for further treatment (307). New uses for textile residues are also being developed. Several companies have devised equipment which forms the uff into briquettes which can be used for energy

recovery. Thus, the largest source of recycling residues is also being handled in an environmentally sound manner, reducing the overall impact of post-consumer tyres on the environment.

4.3 Material Outputs

The materials outputs from the four levels of treatments are broadly classi ed into six categories: cuts, shred, chips, granulate, powders and ne powders. Figure 5 illustrates the continuum of material that result from recycling treatments.

Each category is comprised of one or more subcategories with different parameters, creating a continuum from less than ±500 m to >300 mm. The apparent overlap between the larger granulate and the smaller chips is a function of processing speci cations. Speci cally, granulate is characterised by multistep processing in which metals and textiles are removed, while chips are characterised by processing which merely fragments the tyre leaving the metals and textiles intact.

The six basic categories of materials are described as follows.

Table 6 Percentage of processed material per tonneOutput ± % product ± % lossShred and chips (unseparated) 95% ±5%Large granulate ±7 to 12 mm 70% ±30%Granulate or powder truck 60% ±40%Granulate or powder car 40% ±60%

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• Cuts: irregularly shaped fragments >300 mm;

• Shred: irregularly shaped fragments of ±50 mm to ±300 mm in any dimension;

• Chips: irregularly shaped fragments of ±10 mm to ±50 mm

• Granulate: finely dispersed particles between ±1 mm to ±10 mm. Subcategories could include ranges of, for example, ±0.5-2 mm, ±2-7 mm, and ±7-15 mm, with variations according to purchaser specifications. Ambiently produced granulate is characterised by irregularly shaped surfaces. Cryogenically produced granulate is characterised by smooth regular surfaces.

• Powder: finely dispersed particles of less than 1 mm. Subcategories could include ranges of, for example, ±0-0.5, ±0-1.5, ±0.750-1.6 mm. Surface characteristics depend upon the treatment or technology. Ambiently produced powder is characterised by irregularly shaped surfaces. Cryogenically produced powder is characterised by smooth regular surfaces. Specialised powders: nely dispersed powders that exhibit unique characteristics dependent upon the treatment or technology used.

• Devulcanisates are powders characterised by reduced crosslinks.

• Reclaim particles are characterised by reduced crosslinks with a diameter of 0.300-0.420 mm.

• Surface modi ed powders are characterised by activated surfaces with high crosslink density with nely dispersed particles of less than 1 mm.

• Thermoplastic elastomers are powders which constitute a new compound and which exhibit the same shore hardness and polymer base of traditional thermoplastics.

• Fine powder: finely dispersed particles of

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particles. As an example, a sample of 1 mm powder contains particles ranging from less than 0.30 mm to more than 1 mm. The distribution in the sample should show that more than 90% of the particles are within the 0.30 to 1 mm range.

Particle distribution is determined with a series of screens. Dependent upon the size of the material, a set number of screens is placed in descending order. The material ows through the tier within a speci ed time. The material that remains on each screen is weighed and expressed as a percentage of the sample.

While the size and particle distribution are often critical to the selection of a material, the chemical and physical properties provide information about the content of the material and how it will react physically in response to certain conditions (81).

Table 7 provides a comparison of the different material outputs in terms of:

1. Size: each material is described as a range of particle sizes, e.g., granulate can be described as a range from ±0.5 mm to ±15 mm, while devulcanisates are described as less than 1 mm. Whole tyres are discussed as a unit, and bales by the number of tyres required to produce a unit.

2. Key characteristics: describe some of the principal characteristics of the particular material which can distinguish it from other materials of the same size, e.g., granulate produced ambiently and cryogenically present very different characteristics.

3. Traditional materials: lists some of the traditional materials for which particular post-consumer tyre materials could be used as substitute for virgin resources.

5 Traditional and Evolving Markets

It is evident that during the past fteen years tyre recycling has made great strides towards meeting its rst goal, i.e., the production of materials that can be used to substitute for virgin resources. Signi cant quantities of material are produced annually with indications that capacity will continue to grow, at least within the near future (64). Treatments and technologies have evolved to new levels of sophistication and ef ciency which allow the production of ner and cleaner materials, at more competitive prices.

However, the issues surrounding the second part of the equation, i.e., the use of the materials in applications and products, could become more dif cult to address as material production continues to increase. Three traditional large markets coupled with smaller niche markets, consistently consume almost 75% of the material produced. Newer niche markets, many of which have the capacity to use increasingly sophisticated materials, are beginning to evolve, albeit very slowly. Until now, production and use have maintained a relatively even pace. In general, there has been some expansion into new realms, and there are strong indications that this pattern could continue (72).

5.1 Material Production

In 2003, European recyclers processed slightly more than 650,000 tonnes of post-consumer tyres, 25% of total annual arisings in the 15 Member States. This represents a six-fold increase over 1992 when only ±5% of tyres was recycled. Figure 6 illustrates the steady expansion of recycling capability in the EU during the past decade (the data are collected for the ETRA Annual Report to the Waste Topic Centre from each of the EU Member States).

Figure 7 illustrates the percentage of the total treated for each category of material. Whole tyres accounted for slightly more than 65,000 tonnes. The quantity of whole tyres used for recycling has increased nominally each of the past four years, with a total increase for the period of almost 2% (a.6). It is important to note that in the EU, whole tyres, shred or chips destined for energy recovery are not included in calculations for recycling as they are most frequently delivered directly to the recovery facility and treated on site.

Approximately 78,000 tonnes of the total were used to produce shred and chips, of which 74,000 tonnes were available for use in a variety of applications. The quantity of shred and chips produced has increased marginally during each of the past four years, with a total increase for the period of almost 2%.

By far, the largest quantities of tyres are used for the production of granulate. The quantity has remained relatively stable during the past 4 years, although the ratio to other materials has changed. The ±410,000 tonnes of tyres input resulted in ±200,000 tonnes of clean material containing less than 5% of impurities, textiles or metals.

Powders, including speciality powders, used ±84,500 tonnes of tyres of which somewhat over 40,000 tonnes of

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Table 7 Summary of material characteristicsMaterial Size Key characteristics Substitutes for traditional

materialsWhole tyres unit Lightweight, low compacted density, high

void ratio, good compressibility, water permeability, thermal insulation

Concrete block, clay, quarried aggregate, gravel lled drums

Construction bale

100-125 tyres

Lightweight, low compacted density, good thermal insulation, limited de ection, exceeds speci cations for speci c gravity, compressibility, deformation, creep, hydraulic conductivity

Construction block, stone riprap, gravel, packed earth, crushed rock in wire cages or other containers

Shred

Chips

±50-±300 mm±10-50 mm

Lightweight, low compacted density ±0.5 tonne/m3, high void ratio with good water permeability between 10-1 and 10-3 m/s. Good thermal insulation and compressibility, low earth pressure and high friction road characteristics

Crushed rock or gravel, large grain sand, lightweight clay or light expanded clay aggregate, y ash from coal burning

Size reduced Dependent upon useAmbient ±0.50-

15 mmCrosslinked macro structure retains same characteristics as the tyre; irregular shape; some thermal degradation temperature stressed. May exhibit a nominal degree of reduced crosslinking

Product: Virgin rubbers, EPDM, clay, sand, gravel, y ash from coal burning polyurethane

Cryogenic ±0.5-50 mm

Clean surfaces, regular particle size and shape, glossy smooth surface, no surface decomposition or thermal stress

Powder

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clean material