sodium silicate binder

Iranian Polymer Journal / Volume 10 Number 4 (2001)

1026-126512001

Sodium Silicate Glass as an Inorganic Binder in Foundry Industry

Ahmad RabbitDepartment of Adhesives and Resins, Iran Polymer Institute, P .O. Box 14965/115, Tehran, I .R. Iran

Received 14 Jan 2001 ; accepted 23 May 2001

ABSTRACT

Mixtures of sand, sodium silicate, some additives such as iron oxide, pitch or

sugar and an ester can form a temporary mould . Sodium silicate is used in

foundries to bind sand grains by means of an appropriate acid, either directlysuch as CO2, or indirectly with an organic ester, which hydrolyzes and

subsequently gels the silicate sand mass. A gel network is prepared by

acidifying concentrated sodium silicate (3.3 ratio of SiO2 :Na2O) solution.

Sodium silicate solution, under acidic conditions is polymerized to silica and

acts as an inorganic binder . In other word, lowering the pH of an alkali silicate

solution below about 10.9 causes the polymerization to occur, thus leading to

high molecular weight aggregates of hydrated silica . The produced hydrated

silica namely silica gel is responsible for giving the necessary strength to the

mould. Concentration of the sodium silicate solution is adjusted to 55%(wlw)in water. The viscosity of this solution is adjusted on 500 cp by addition of

sugar in an amount of 1 .0—2 .0% based on weight of sodium silicate . Thesugar or pitch is used as a thickening agent, to concentrate the binder

solution at a desirable limit Sodium silicate solution as an inorganic binder isof particular importance mainly because, the necessary time for curing the

silicate is low, and it is environmentally extremely beneficial.

Key Words : sodium silicate, inorganic binder, adhesive, foundry, water glass

INTRODUCTION

Casting is the formation of a solid body of predeter-mined size and shape from a melt by a process of

solidification in a mould . Foundry technology is theart and science of the processes and materials used in

manufacturing castings from metals and alloys. Itdeals with the manufacture of moulds and pattern,

melting practice, metal treatment, pouring methods

and processes for finishing castings and theequipment used.

Moulding practice deals with the production of

moulds and there are two types as follows:— Permanent moulds— Temporary moulds

Permanent moulds are reusable, and are usually

made of metal, more rarely of graphite or refractory

ceramic . These moulds are mainly used for non-

(*)To whom correspondence should be addressed . E-mail: [email protected]

229

Sodium Silicate Gloss man Inuryunic Binder in Foundry Industry

ferrous metals and iron . Important advantages ofthese moulds are the high rates of cooling and theclean casting surfaces . Temporary moulds consist ofthe basic materials, a binder system and someadditives.

The basic materials are granular substances ofwashed, classified and dried different silica sands.These basic materials (moulding sands) may beclassified into four different types as follows:– Natural moulding sand

- High silica sand– Special sand such as zircon, olivine, chamotte,

chromite, chrome-magnesite, corundum, magnesite,sillimanite and mullite.

– Bonding clays (bentonite)Binder systems may be organic or inorganic,

natural or synthetic (as in Table 1).Special sand moulding processes have been

developed to enable moulding with less effort andskill, saving time and expenses, produce better qualitymoulds and cores and effectively help in improvingproductivity [1].

Generally, these processes eliminate the needfor drying or baking of moulds and cores and rapidhardening action takes place due to chemicalreactions. Also, the casting can be produced to ahigher degree of accuracy and finish than thatpossible by conventional dry sand moulding . Specialsand moulding processes, which are used for makingtemporary mould, may be classified under threeheads:– Processes based on sodium silicate binder– Processes based on organic binders

- Other special moulding processes

Processes Based on Sodium Silicate BinderThe use of sodium silicate as a binder has consider-ably increased in recent years . It enables the pre-paration of mould and cores without any need fordrying and baking in certain cases even withoutramming the sand . The various processes based onsodium silicate are classified into four categories asfollows:– Cement moulding process– Dicalcium silicate process

Table 1 . Sand binders.

Source Natural Synthetic

_

Inorganic Clays

Bentonites

Sodium silicate

Cement

Plaster

Phosphate binders

Sulphite lye

Organic Oil binders

Core oils

Sugar

Aminoplast resins(UF)

Phenoplast resins(PF)

Furan resins

Epoxy resinsPolyester resins

Polyurethane resins

– Ferro-silicon process– Carbon dioxide process

Differences between these various processes liein the quality of hardener used, the type of catalysts orother additives, and in the nature of the chemicalreactions that cause the hardening. These processes arebeing used for production of cast iron, steel as well asnon-ferrous castings required in small, medium andheavy sizes. Patterns used with these mouldingprocesses may be made of wood, metal or plastic.

Cement Moulding ProcessPortland cement may be used as an inorganic bindingmaterial to bind sand grains together . Efficiency,however is further increased and a good combinationof strength, permeability and flowability achieved byusing cement along with sodium silicate . The sandmixture may consist of about 2% cement, 4-5%sodium silicate and I% pitch or molasses . The largesized moulds can be produced for ferrous castings bythe sand mixture, By using fuming chemicals in thesand mixture, flowable cement slurry can also beproduced.

Dicalcium Silicate ProcessDicalcium silicate has been found to be a veryeffective hardening agent when used with sodiumsilicate as a binder. Unlike the ferro-silicon process

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Rabbii A.

where hardening is due to exothermic reaction, thechemical reactions taking place in this process do notcause any evolution of heat. The rate of hardeningdepends on the grain fineness of the silicate and thetemperature of the sand . The fineness of the silicategrains should not be less than 200 mesh . To preparethe mixture, about 2-3% of dicalcium silicate and 5%sodium silicate are mixed with sand, along withsuitable foaming chemicals . The mixture can floweasily in the mould . Mixing time is from 3-5 min . Thesodium silicate used in this process should have ahigh mass ratio 1 :2 .3 to 1 :2 .8 and specific gravitybetween 1 .48 and 1 .50 . This process finds its mainapplication in medium and heavy castings, both ingrey iron and steel.

Ferro-silicon ProcessIn this process, silica sand is hardened and bondedwith the reaction product obtained by exothermicreaction between sodium silicate and ferro-siliconpowder. The method is based on the principle that ifsodium silicate and powdered ferro-silicon are mixedin a weight ratio of 2 .25 :1, foaming action takes placeand the temperature is rises simultaneously, reachinga boiling condition at about 90 'C. During thechemical reaction, steam and hydrogen are liberated.

The reactions taking place are:

Na2 O .nSiO 2 + H2O —N.-2NaOH + nSiO2 ( 1 )

2NaOH + mSi + H2 O -3.-Na2O.mSiO2 + 2H2 (2)

The reaction continues as long as silicon andwater are present and finally the products of reactionform a hard spongy mass . The sand used in themixture should be dry and should have a grain size tosuit the requirements of the metal to be cast.Normally, for ferrous castings, silica sand of 65 meshand for non-ferrous work, 100 mesh would besuitable . The sand is first mixed with about 2% of itsweight of ferro-silicon, containing 75-80% silicon,which should be powdered to 3-35 microns size.Sodium silicate of the correct grade, specific gravityand mass ratio is then added (about 5% of the weightof the sand) and the mixing continued . The specific

gravity and mass ratio suitable under moderateclimatic conditions are 1 .3-1 .35 and 1 :2 .0-1 :23,respectively, precise control of the mixing cycle andbasic additives is necessary to obtain the desiredmould characteristics . When using ferro-silicon, thesame process may be modified by adjusting the basicadditives and mixing it by a foaming agent to enablethe use of the mixture as flowable slurry in the mould.

The slurry is poured over the pattern and itoccupies all the areas uniformly.

Carbon Dioxide ProcessThe principle of working the CO2 process is based onthe fact that if CO2 gas is passed through a sandmixture containing sodium silicate as the binder,immediate hardening of sand takes place as a result ofthe chemical reaction between sodium silicate andcarbon dioxide. The bonding strength obtained by thehardening action is sufficient to eliminate the need forany drying or baking of the mould and the metal canbe immediately poured . The main chemical reactioncan be represented in simplified form as:

Na2 0 .mSiO 2 .xH2 O + CO 2- Na 2CO3 +

mSi0 2 .xH2O

(3)

The sodium silicate used is based onNa 2O.mSiO2 .xH 20 system where the ratio of totalsoda to total soluble silica, called mass ratio is avariable factor affecting the characteristics of theprocess. The common sodium silicate used for theCO2 process in foundries should have a mass ratiovarying from 1 :2 .1 to 1 :2 .5 . Adding a suitablequantity of sodium hydroxide can raise this ratio.

The specific gravity of the liquid depends onthe mass ratio and its water content, which forfoundry use may vary from 1 .55 to 10.71 . The sandused for the process must be dry and free of clay . Thissand is usually mixed in a sand mixer with about 3-5% of sodium silicate.

But since sodium silicate systems withhardeners such as ferro-silicon, portland cement anddicalcium silicate are less widely used, we pay ourattention to detailed discussion of sodium silicate

Iranian Polymer Journal / Volume 10 Number 4 (2001) 231

Solium SilioaIIe GI

an inorg nic Binder in Foundry Industry

systems with an appropriate acid such as CO2 or anorganic ester [2, 3], where sand mixture is hardenedby gasification with carbon dioxide or by heating withflame for a few seconds respectively . In both twocases of acidification, either directly or indirectly, theproduced SiO2 from the. reaction contains a certainnumber of water molecules and is represented asSiO2.xH2 O, which is called silica gel . This silica gel isresponsible for giving the necessary strength to themould .

Suitable additives may be introduced to obtainspecific properties . Additives to the mouldingmaterials improve their properties; such as: flow-ability, computability, bond improvement,prevention of sand expansion defects and breakdownproperties . Common additives are sawdust, ironoxide, sugar, invert sugar, dextrin, glycol, coalpowder, wood flour, sea coal and kaolin clay amongmany others . Dextrin, as also coal powder, wood flourand sea coal, improves collapsibility . Iron oxideprevents hot deformation of cores and produces asmooth interface between the mould and the metal,thus helping prevention of metal penetration andachieving good surface finish . Kaolin clay is added topromote mould stability and is often used in steelcasting work . Invert sugar is often mixed in thesecompounds, as it is an effective breakdown agent andan aid in improving collapsibility.

Some moulding sand additives produce lustrouscarbon, e .g . coal dust and hydrocarbons . These arevaporized at high temperatures and aresimultaneously decomposed and therefore, the sandparticles become coated with lustrous carbon and then.are not easily wetted by molten metal. The aims ofusing these kinds of additives are to reduce metalpenetration into the sand, to improve the castingsurface finish and facile separation of the castingfrom the temporary mould [4].

Finally the temporary mould is then filled withmolten metal . The production of molten metalinvolves selection of the alloy composition, rawmaterials and alloying elements. The refractory

materials that come into contact with the moltenmetals are of vital importance . Breaking down thebond of the moulding material with heavy hammers,

vibrating grates, and tumbling mills or water jetscauses the separation of the casting from the mould.The casting surfaces are then cleaned and descaled byshot or sand blasting and ground smooth.

EXPERIMENTAL

MaterialsThe sodium silicate of technical grade and pitch orsugar was obtained from a local supplier in Iran andwas used as received.

ApparatusThe viscosity of prepared solutions was measuredusing a Brookfield-viscometer model LV 1 at 25 'C.

Preparation of Sodium Silicate SolutionSodium silicate (65 g) was added to 35 mL water . Themixture was then heated to 70 'C, while stirring. Thesolution was kept under these conditions for 60 minuntil the dissolution was completed . Theconcentration of this solution was adjusted to 65%(wlw) in water. The mixture was stirred during thewhole time . Adequate granular sugar was added tothis solution in an amount of 1 .0—2 .0% weight percentwith respect to sodium silicate used to achieve aconcentrated silicate solution . The viscosity of thissolution was controlled to 500 centipose by additionof sugar . The solution was cooled to roomtemperature and its apparent viscosity was measuredat 25 'C . This solution was then ready for use as anadhesive.

RESULTS AND DISCUSSION

The soluble silicate glasses ase prepared by fusion ofglass sand with sodium or potassium carbonate.Glasses over a wide range of mass ratios may beobtained, but only those glasses containing less SiO2than the proportion of about 1Na2 O : 4SiO 2 can bedissolved in water . The soluble sodium silicates ofcommercial value include orthosilicate (NaaSiO4),metasilicate (Na2 SiO3 ), disilicate (Na2Si2 O 5) and more

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Ra6Wi A.

highly siliceous products (Na2 Si4O9 ) [5] . One of themost common types of sodium silicate, or water glass,contains the oxides in the proportion 1Na20 : 3 .3SiO2.This material as a viscous solution containing around28% SiO2 , is used as an adhesive, in waterpurification and for making silica gel. Silica isslightly soluble in water and the dissolved silicareacts with hydroxyl ion to form silicate ions . Silicadissolves in alkali when the pH exceeds about 10.5.This reaction involves depolymerization of silicathrough hydration and dissolution to Si(OH) 4,followed by addition of hydroxide ion to form silicateion . The conversion of silica to silicate ion in terms ofcoordination number of 6, for silicon ; may berepresented as follows:

SiO 2 + 2H2 O

- SiO2 .2H 20 -- Si(OH)4

(4)

Si(OH)4 + H 2O + OH -[(H20)Si(OH)5 ] - (5)

[(H 2 O)Si(OH)5 ] - +

[Si(OH) 6] -2 + H2O (6)

On the other hand, when a solution of a solublesilicate, which is always highly alkaline, isneutralized by acid to a pH below 10 .5, the silicateions decompose to silicic acid, which thenpolymerizes to silica. The neutralization of a solutionof soluble metasilicate may be represented as follows:

[Si(OH)6] -2 + H 2 O [(H20)Si(OH)5]-+

OH- (pH = 13 .6) ''Ys[Si2(OH)5 ]_2

+H2 0 (7)

[(H 2 0)Si(OH) 5 ] - ~ Si(OH)4 + OH - +

(8)H 2O (pH— 10.9)

1 /x(S iO2)x + H2O

In the above and next formulations, for theexplanation of polymerization phenomena in thesilica-water system, the coordination number ofsilicon is proposed to be 6.

Further studies [6], show that above pH 13 .6the metasilicate ion, corresponding to monomer, isstable; and between pH 13 .6 and 10 .9, the disilicateion, corresponding to dimmer is the principal anion

present in solution. The dimerization of the silicateion can be expressed by the following way;

[Si(OH)s]-2 + H20--; [(HAS i(OH)sr + OH- (9)

OH

2[(H20)Si(OH)5]-

[(HO)4 Si

Si(OH) 4 ]_2

+

OH2H2 0

(10)

High molecular weight aggregates [7] of silicaare formed when the pH of an alkali silicate solutionis lowered below about 10.9. This corresponds toSiO2/Na2O ratio of 4 .0, namely highly siliceoussodium silicate (Na2 0 :4SiO2) . These higherpolysilicate ions may be in equilibrium withmetasilicate and disilicate anions, and thispolymerization process can be express by thefollowing way:

2[S i(OH)6]-2

OH

OHOH, 1 .OH

"ii"

] -2 + 20H-

"OHOH

OH

(I1)

[ Si2(OH)to]-2 + [Si(OH)6]-2 --sp.-

0H

OH

OHOH, l OH,

OH[

;Si,

] -2 +OH'

"OW

'OH`

'OHOH

OH

,OH

20W

(12)

So higher molecular weight polymer can beformed according to the following reaction:

[Si(OH)5.[Si(OH)4 ],,.Si(OH)5] -2 + [Si(OH)6 ]-2

233Iranian Polymer Journal / Volume 10 Number 4 (200!)

Sodium Silicate Glass as an Inorpnic Binder in Foundry Industry

[Si(OH)5 .[Si(OH)4 ]„+l .Si(OH)5] -2 +20W

(13)

It is seen that each Si(OH)4 unit is coordinated withthe former polymer in a linear chain and a highmolecular weight polymer is produced.

According to these chemical reactions, thepolymerization of produced Si(OH) 4 begins in firststage, its concentration in solution is quickly reduced,and high molecular weight aggregates are formed atlow pH in aqueous solution . These correspond topolysilicate ions.

These polysilicate ions are responsible forbinding the sand particles.

In other word, when the alkali is added tosoluble silicate, the polysilicic acid is depolymerizedand by addition of acid to soluble silicate, the poly-merization is occurred . Therefore, breaking inter-particle forces holding the polymeric silicate anions insolution by adding a mineral acid, to lower the pH ofthe solution, silica can be produced . The equilibriumshifts from the monomeric to the disilicate ion whenthe pH drops below 13 .6, while transformation ofdisilicate to higher polysilicates occurs only as the pHfalls considerably lower, namely below about 10 .9.

In studying the rheology of polymer solution,the appropriate concentration variable for the particlesis their volume fraction (0), which is the ratio of thevolume of the particles to that of the entire dispersion.In prepared solutions the concentration is selected at aconstant level, so volume fraction is constant andequal to 0 = 0 .65 . According to experimental results,the suitable concentration and viscosity of the silicatesolution is 65%(w/w) and 500 cp, respectively . Sinceviscosity is directly proportional to volume fraction ofsilicate solution, viscosity increases with increasingthe concentration of solution . Viscosity studies onthese silicate solutions indicate that the high viscosityof such concentrated solution which may evenbecome syrupy, is not due to aggregation orpolymerization, but due to the fact that the moleculesare close to one another [8].

Sodium silicate solution in presence of an esteracts as a binder in a two stage reaction; hydrolysis ofthe ester followed by gelling of the silicate at thelower pH . The bond strength and reaction velocity is

dependent on the SiOzINa 7O ratio of the sodiumsilicate and the type and the amount of ester used.Heating the mixture of moulding sand, sodiumsilicate, hardener (an organic ester), and together withsuitable additives for about 1 .0 min, produce thetemporary mould. The flame causes the ester todecompose to produce acid and alcohol . Theproduced acid lowers the pH, thereby causing thesilicate to gel and binds the sand particles.

In case of using CO2 as a hardener, the bondstrength of the sand mixture depends on theNa2 0/Si02 ratio of the sodium silicate, length of timespent and pressure of gasification rate step by carbondioxide . The hardening process involving gassing ofthe sand mixture with carbon dioxide first establishedthe importance of sodium silicate as binder . Thisprocess became of particular importance for coremaking on account of its rapid turn-around time(10–25 s), and environmentally it is extremelybeneficial [4].

CONCLUSION

The concentrated 3 .3 ratio (SiO2 :Na2 O) sodiumsilicate solution, the moulding sands, and pitches orsugar with suitable additive and cross-link agent arecomposed to make a temporary mould . The 3 .3 ratiosodium silicate solution with a suitable viscosity, asan inorganic binder is responsible for binding thedifferent sand particles, in presence of an organicester (by using flame) or carbon dioxide. A viscosity

. study indicates that the high concentrated silicatesolution is used to wet the sand grains . Since asuitable viscosity is necessary for improvingwettability of sand particles and consequently forincreasing the temporary mould stability, the sugar isused as a thickening agent, to conentrate the bindersolution at a desirable limit . The suitable limit ofviscosity for sand moulding is about 500 centipoise.The produced concentrated silicate solution forms athin film around the sand particles . The so-called sandparticles, when come into contact with each other canform a rigid network rapidly due to gellation inpresence of a suitable curing agent.

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Rabbii A,

REFERENCES 4. Ullmanws Encyclopedia of Industrial Chemistry, Elvers B .,Hawkins S ., Ravenscroft M., Rouosaville I .F.and Schulz G .,

1 . Jain P .L., Principles of Foundry Technology, 2nd ed., Tata Eds., VCII, Weinheim, Al2, 1989.Megaw-Hill, New York, 1986. 5. Vail J.G., Soluble Silicate, Reinhold, New York, I and II .,

2. Nicholas K.E.L. The COrsilicate Process in Foundries,

British Cast Iron Research Association (BCIRA), Alvechurch,UK, 1972.

3. Roberts M., "Use of organic ester hardeners in selflrardening

sodium-silicate-bonded sands", Foundry Trade J., 133, 783,

2925, 1972 .

1952.

6. Iter JUL. The Colloid Chemisty of Silica and Silicates,Cornell University Press, New York, 1955.

7. lander G. and Jahr K.F., Kolloid-Beihefie, 41, 48-57, 1934.

8. Nauman R.V . and Debye P., "Light-scattering investigation ofcarefully filtered sodium silicate solutions", J. Phys. Chem .,55, 14, 1951.


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