ethyl silicate binders.pdf

Progress in Organic Coatings 42 (2001) 114

ReviewEthyl silicate binders for high performance coatings

Geeta Parashar a, Deepak Srivastava b, Pramod Kumar a,a Department of Oil and Paint Technology, H.B. Technical Institute, Kanpur 208 002, India

b Department of Plastic Technology, H.B. Technical Institute, Kanpur 208 002, IndiaReceived 2 October 2000; accepted 15 January 2001

Abstract

Surface coatings based on ethyl silicate binders are categorised as inorganic coatings, whereas the conventional surface coatings whichare mainly based on organic binders are referred to as organic coatings. Zinc-rich inorganic coatings based on ethyl silicate are quitesuccessful for the protection of steel against corrosion under severe exposing conditions such as underground, marine atmosphere, indus-trial atmosphere, nuclear power plants, etc. These coatings provide unmatched corrosion protection to steel substrates exposed to hightemperatures. Because of the formation of conductive matrix out of the binder after film curing, zinc-rich coatings based on ethyl silicatebinder offer outstanding cathodic protection to steel structures. These coatings are mostly solvent-borne, but recently water-borne versionsof the same have also been developed. However, the commercial success of water-borne systems is not yet well established.

In the present article, the processes of hydrolysis of ethyl silicate in the presence of acidic and alkaline catalysts have been elaborated toproduce ethyl silicate hydrolysates of desired degree of hydrolysis. Effect of various factors such as amount of catalysts, amount of water,type and amount of solvent, reaction temperature and reaction time has been discussed. Calculations to find out the amount of water andsolvent required to yield the product of desired degree of hydrolysis have also been illustrated. Typical recipes useful for the preparationof ethyl silicate hydrolysates suitable for use as coating binders have also been presented. The chemistry and mechanism involved inthe preparation of binder and the curing of film has also been discussed. This article also summarises the effect of various factors, viz.particle size and shape of zinc pigment, presence of extenders in the formulations, and the application technique on film performance. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Inorganic coatings; Silicate binders; Ethyl silicate coatings; Zinc silicate coatings; Heat resistant coatings; Anticorrosive coatings

1. Introduction

Painting is one of the most important techniques usedfor the protection of metals from corrosion. Effectivenessof protection of metals against corrosion mainly depends onthe factors such as quality of the coating, characteristics ofthe metal, properties of the coating/metal interface, and thecorrosiveness of the environment. Typical corrosion resis-tant coatings protect the metallic surfaces primarily by thefollowing two mechanisms [1].1. By acting mainly as a physical barrier to isolate the

substrate from corrosive environment.2. By containing reactive materials (usually pigments)

which react with a component of the vehicle to formsuch compounds that, in fact, inhibit corrosion. Some

Corresponding author. Tel.: +91-512-583-507; fax: +91-512-545-312.E-mail address: [email protected] (P. Kumar).

pigments having limited solubility can give rise toinhibitive ions, such as chromates.

Undoubtedly, steel is one of the most important metalsused in the modern society. However, one of its main draw-backs is its tendency to corrode (rust), i.e. to revert to itsoriginal state, and become useless. Hence, the protection ofsteel from corrosion, i.e. to keep the steel in its usable form,has always been a matter of great concern for all those whouse it in one form or the other.

For the protection of steel, various materials can be used,out of which zinc has been found to be the most success-ful [2]. Zinc can prevent or at least retard the corrosion ofsteel in the form of electroplated layers or by the applica-tion of paints containing a high percentage of zinc particlesdispersed in an organic or an inorganic binder. Zinc, eitherin the form of electroplated film or in the form of films ofzinc-rich coatings, protects the steel substrate by sacrificialcathodic (galvanic) protection mechanism. For the cathodicprotection of steel, the direct electrical contact between the

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2 G. Parashar et al. / Progress in Organic Coatings 42 (2001) 114

adjacent zinc particles, and between the zinc particles in thefilm and the steel substrate is required [3].

In the case of zinc-rich organic coating films, zinc par-ticles can be encapsulated by the organic binder, and hencethe zinc particles have restricted electrical contact. Conse-quently, the zinc particles can provide only a small amountof galvanic protection limited to the amount of free zinc inthe coating formulation [4].

On the other hand, in the zinc-rich inorganic coatings(commonly referred to as zinc silicate coatings), the binders(inorganic) used are alkali silicates and alkyl silicates, whichcan chemically react with the zinc particles in the coatingfilm to form a zinc silicate matrix around the zinc particles[5]. This zinc silicate matrix is electrically conductive andchemically inert [2]. In addition, the silicate based binderscan chemically react with the steel substrate also to result inan excellent adhesion and abrasion resistance of the dried/cured film [6].

Inorganic zinc silicate coatings are included in the cat-egory of high performance coatings [7], as these are themost weather resistant coatings available today [5]. Theycan provide an unmatched protection against corrosion forsteel structures exposed to temperatures up to 400C [2].

2. Silicate binders for inorganic paint coatings

Inorganic paint coatings based on silicate binders can beclassified [6] as shown in Fig. 1.

Fig. 1. Classification of inorganic paint coatings based on silicate binders.

Table 1Ratios of silica to alkali metal oxide in alkali silicates [8]S. No. Silicate Chemical composition Ratio of silica to

alkali metal oxide

1 Sodium silicate SiO2:Na2O 2.44.5:12 Potassium silicate SiO2:K2O 2.15.3:13 Lithium silicate SiO2:Li2O 2.18.5:1

2.1. Alkali metal silicate binders

For the manufacture of coatings based on alkali metalsilicates, the silicates based on alkali metals such assodium, potassium and lithium, along with the quarternaryammonium silicates have been reported to be suitable[8]. Alkali metal silicates are relatively simple chemi-cals, which can be water soluble depending on the ratioof silica to alkali metal oxide. The ratios of silica toalkali metal oxide of different silicates [8], which canbe used as binder systems in paints, have been given inTable 1.

The ratio of silica to alkali metal oxide, in addition to thetype of alkali metal, has a remarkable effect on curing char-acteristics and properties of the dried films [9]. The effect ofratio of silica to alkali metal oxide on coating characteristicshas been shown in Table 2.

The coatings based on alkali metal silicates having sili-ca to alkali metal oxide varying from 2.1:1 to 8.5:1 arewater-borne due to solubility of the used alkali metal oxidein water. These coatings are generally sub-classified intobaked, post-cured and self-cured coatings.

2.1.1. Baked coatingsThese are the coatings which require heating to convert the

coating films into water insoluble form. These coatings arecharacterised by their extreme hardness and suitability forapplication over an acid-descaled surface. Baked coatingsstill have limited use today.

G. Parashar et al. / Progress in Organic Coatings 42 (2001) 114 3

Table 2Effects of ratio of silica to alkali metal oxide on coating characteristics

S. No. Ratio of silica to alkali metal oxide Effect on coating characteristics

1 Higher Higher the viscosity of the solutionHigher the drying speed of the filmHigher the curing speed of the filmHigher the susceptibility to low temperatureHigher the chemical resistance of the coating films

2 Lower Higher the specific weight of the solutionHigher the solubility in waterHigher the pH value of the solutionHigher the susceptibility to waterHigher the adhesion and binding power

2.1.2. Post-cured coatingsThese are the coatings which are cured by the application

of chemicals such as an acid wash just after application ofthe film to convert the film into a water insoluble condition.These coatings are formulated mainly on sodium silicatehaving higher ratio of silica to sodium oxide. This develop-ment has led to the use of inorganic zinc coatings on largefield structures.

2.1.3. Self-cured coatingsWith further advances in silicate technology, further

higher ratio alkali metal silicates have become available. Ofthe cheaper types, potassium silicate is preferred. Reliableself-curing coatings are available today, based on highratio potassium silicates with potassium oxide to silica ra-tio ranging from 1:2 to 1:5.3. If further higher ratios arerequired, and instability is to be avoided, it is necessary touse lithium silicate with lithium oxide to silica ratio as 1:2to 1:8.5. Lithium silicate based coatings are preferred foruse in food areas. Excellent curing rates can be achievedwith some lithium silicates, but their higher cost tends torestrict their use at the present time.

2.2. Alkyl silicate binders

Alkyl silicates such as ethyl silicate, methyl silicate etc.can be used as binders for the formulation of solvent-bornecoatings. However, one of the commercial forms of ethylsilicate (popularly known as ethyl silicate-40) as solution inorganic solvent(s) is most commonly employed. Alkyl sili-cates, as such, do not have any binding ability but when theiralcoholic solutions are hydrolysed with calculated amount ofwater in the presence of acid or alkali catalyst, they acquiresufficient binding ability. On the basis of the type of catalystused for the hydrolysis, these coatings can be sub-classifiedas follows.

2.2.1. Alkali catalysed coatingsFor the hydrolysis of ethyl silicate, bases like ammonia,

ammonium hydroxide, sodium hydroxide and some aminesare generally used as catalysts [2]. One of the greatestdrawbacks of this system is related to the fact that in basic

conditions, even a small amount of water will cause thesilicate to gel. To avoid this problem, remedial steps musttherefore be taken to exclude all water at the manufactur-ing stage, and from the application equipment. If water isexcluded, the liquid component can remain stable for anindefinite period of time. These coatings are available in themarket as single-pack and two-pack systems. In single-packsystem, amines, which provide hydroxyl ion in the formwhich is non-reactive with organic polysilicate until theyare exposed to moisture, are used [8].

2.2.2. Acid catalysed coatingsIn these type of coatings, rapid curing may be achieved

under most conditions. However, the period over which thepartially hydrolysed silicate remains stable is limited, andthe product thus has a finite shelf life. Coatings based onacid catalysed binder are mainly two-component systems,and the liquid component of these coatings gel in a periodof 612 months. The problem associated with one-packsystem of this type is that zinc chemically reacts with theacid catalyst present in the binder system, due to which pHof the system increases, which causes gelling in the con-tainer. Hydrochloric acid [1027], sulphuric acid [28,29],phosphoric acid [30], formic acid [31], etc., are the acidswhich are used as catalysts.

3. Hydrolysis of ethyl silicate

Ethyl silicate, by itself, has no binding ability [32]. Tointroduce binding ability, it is necessary to hydrolyse ethylsilicate by treating it with water, so that a gel can form fromthe resulting ethyl silicate hydrolysate. The actual bindingagent is this gel [33].

Usually, the hydrolysis of ethyl silicate is carried outunder alkaline or acidic conditions. Acids or alkalis areused to catalyse the hydrolysis reaction. Hydrolysis underalkaline conditions normally results in fairly rapid gelation.Alkali catalysed hydrolysis procedures are generally pre-ferred when ethyl silicate is to be used for the productionof refractories. Acid hydrolysis procedures are commonlyemployed for the production of paint media. Several


Table 3Typical compositions for single stage procedures for the hydrolysis of ethyl silicate

S. No. Quantity of ethyl silicate-40 Quantity of water Quantity of acid Quantity of solvent1 6 l 2 l 50 ml concentrated HCl 4 l ethanol2 1368 parts (by weight) 138 parts (by weight) 0.16 parts (by weight) 12 N HCl 1517 parts ethanol (by weight)3 1.6 l 100 ml 6 ml 0.1 N HCl 840 ml 640 p industrial methylated sprit4 45 parts (by weight) 53 parts (by weight) 0.1 part (by weight) 37% aqueous HCl 49.6 parts ethanol (by weight)

procedures for the acid hydrolysis of ethyl silicate areavailable [3436].

Hydrolysis procedures in which a specified quantityof ethyl silicate is added at the start of the reaction aretermed as single stage procedures, while those in whichethyl silicate is added usually after a specified temperaturerise or time interval are termed as two-stage procedures.Some two-stage procedures require two types of organicsilicates. Typical compositions for the single stage [3740]and two-stage procedures [37,41,42] taken from the patentliterature have been given in Tables 3 and 4, respectively.Out of many possible ethyl silicate hydrolysis procedures,one can be considered on its merits.

Mcleod [43] prepared silicate binder system by hydro-lysing ethyl silicate-40 in butyl cellosolve in the presenceof acid catalyst with 5% (part basis) water at 140C. Someother workers [4446] also prepared binder systems by usingpure ethyl silicate or ethyl silicate-40 of different properties.Some special procedures include the use of silica aquasoland the use of titanic acid ester in a two-stage process. Iflarge amount of phosphoric acid is used in the hydrolysisof ethyl silicate, hydrolysates which gel rapidly can be ob-tained. Conditions for the hydrolysis of ethyl silicate withoutuse of an acid or a base catalyst to obtain binding solutionshave also been established [47].

Acid hydrolysates of ethyl silicate eventually set to a gelon standing. The relatively short shelf life of some acidhydrolysed ethyl silicate solutions can cause difficulties intheir use. As a result of the development of methods forpreparing ethyl silicate hydrolysates having a long stor-age life, hydrolysed ethyl silicate solutions have becomeavailable commercially. These solutions, often referred toas prehydrolysed ethyl silicate solutions, are of particularinterest as paint media.

Ethyl silicate hydrolysates having a long storage life canbe obtained by careful choice of the proportions of ethyl

Table 4Typical compositions for two-stage procedures for the hydrolysis of ethyl silicate

S. No. Quantity of ethylsilicate-40 (first lot)

Quantity of water Quantity of acid Quantity of solvent Quantity of alkyl silicate(second lot)

1 14 parts 2.15 parts (by volume) 18 parts concentrated HCl(specific gravity 1.18)

50 parts 160 p industrialmethylated spirit

11 parts ethyl silicate-40

2 6000 parts 2000 parts (by volume) 50 parts concentrated HCl(specific gravity 1.18)

8000 parts isopropanol 2000 parts methyl silicate(50% SiO2)

3 340 parts Nil 40 parts 0.1 N HCl 140 parts isopropanol/water azeotrope

130 parts isopropyl silicate(38% SiO2)

silicate, solvent, acid and water for their preparation. If ethylsilicate is treated simultaneously with a glycol monoetherfor alcoholysis and water for hydrolysis, a hydrolysate witha long shelf life is obtained [48]. This hydrolysate can besuccessfully used as a paint medium. Generally 8090%hydrolysis of the ethyl silicate is carried out for the binderpreparation [2].

3.1. Factors governing the formulation of ethyl silicatebinders

There are some important factors, which can affect thehydrolysis of ethyl silicate and the formulation of ethyl sili-cate binders. These factors are discussed hereunder one byone.

3.1.1. Effect of quantity of waterQuantity of water and the quantity of acid catalyst used

for partial hydrolysis are the most important factors for for-mulating acid catalysed ethyl silicate binder systems. Waterto be used in hydrolysis must be calculated after subtractingthe quantity of water (if any) going into the paint formula-tion from the extender pigments and the solvents used in theformulation. Excessive water in the formulation can lead togelling of the binder system in the cans or very poor applica-tion properties and gelling of mixed paints in the applicationequipment. Less than optimum quantities of water can resultin an uncured film lacking hardness and film integrity [49].

3.1.2. Effect of quantity of acidLess than optimum quantity of acid can result in silica

precipitation, thus making less silica available for bindingthan required. Excessive quantity of acid will result in accel-erated condensation of silanol with silanol (SiOH) groupsor with alkoxy groups (SiOR) resulting in reduced shelflife of the binder system [49].


3.1.3. Effect of size of alkyl groupThe rate of hydrolysis reaction is greatly affected by the

size of alkyl group of the organic silicates. The larger alkylgroups can act as a steric barrier to hydrolytic attack. Thus,bulkier alkyl groups protect the ester much better than thesmaller groups like methyl or ethyl. N-hexyl silicates, e.g.,are difficult to hydrolyse, whereas methyl silicate hydrolysesreadily. A second effect of the size of alkyl group involvesthe volatility of the alcohol formed during hydrolysis. If thealcohol is highly volatile, reversible reaction will be forcedin the direction of the hydrolysis. This is particularly true foracid catalysed hydrolysis where the presence of the alcoholmaintains an equilibrium. With proper selection of the alkylgroup, curing properties of alkyl silicate coatings can betailored [50].

3.2. Chemistry of ethyl silicate binders

Prepared ethyl silicate contains some silanols and alkoxygroups. These silanol groups are responsible for chemi-cal reactions in these types of coatings [2]. Some of theirreactions are as follows.

3.2.1. Acid catalysed reactionsFirst, oxygen of the silanol group is protonated, and an

intermediate species is formed, as shown in Eq. (1).

(1)This intermediate species then reacts with the silanol,

which results into the formation of siloxane bond [49].

(2)

3.2.2. Effect of pH on stabilityWhen pH of the system is low, then the hydrolysed alkyl

silicate has long pot life due to the repulsion of O+H groupwith O+H group.

(3)When pH of the system is high, the rate of formation of

water is high and due to fast dehydration, pot life of thesystem is short.

(4)

3.2.3. Reaction with zinc pigmentsThe silanol groups of hydrolysed ethyl silicate react with

zinc and form a zinc silanol heterobridge.

(5)

This hetero bridge then undergoes further chemicalreactions to form a zinc silicate polymer.

(6)

3.3. Stoichiometry of binder preparation

The overall stoichiometry of hydrolysis is given in thefollowing equations. Total hydrolysis of pure ethyl silicate[2] can be given as shown in Eq. (7).

(7)

Ethyl silicate hydrolysed to x degree can be shown bythe following equation:

(8)


The empirical equation for ethyl silicate hydrolysed to xdegree of hydrolysis, SiO2x(OC2H5)4(1x), can be used toderive the equivalent weight of the commercial ethyl polysil-icate and its exact degree of hydrolysis. This allows calcu-lation of the amount of water necessary to give a binder ofany desired percentage hydrolysis. Equivalent weight canbe obtained by substituting atomic weights in the empiricalformula.

Equivalent weight= SiO2x(OC2H5)4(1x) = 28+ 16(2x)+ 45(4 4x)= 28+ 32x + 180 180x = 208 148x

or

Equivalent weight = 208 1.48 H (H = %hydrolysis)(9)

The concentration of SiO2 in the ethyl polysilicate is equalto

Molecular weight of SiO2 100Equivalent weight of ethyl polysilicateor

% SiO2 = 60 100208 1.48 H (10)

Calculation for the amount of water to be added to oneequivalent weight of ethyl polysilicate to prepare a binderof any desired degree of hydrolysis is given as

Weight of water= 0.36(% hydrolysis desired% hydrolysis in ethyl polysilicate) (11)

The amount of solvent to be added to achieve the desiredsilica content of the binder is determined from the followingequation:

Weight of solvent to be added

= 6000% SiO2 desired

weight of ethyl polysilicateweight of water added (12)

For example, to prepare 85% hydrolysed binder contain-ing 18% SiO2 from commercial ethyl silicate containing41% SiO2, calculate the % hydrolysis in the ethyl polysili-cate from Eq. (10), as below:

41 = 6000208 1.48(H)

H = 41.66This allows the calculation of the equivalent weight of the

ethyl polysilicate using Eq. (9).

Equivalent weight of ethyl polysilicate= 208 1.48(41.66) = 146.34

In order to prepare a binder that is 85% hydrolysed, theweight of water to be added can be calculated by Eq. (11).Weight of water = 0.36(85 41.66) = 15.6 kg

The amount of solvent that must be added to give a finalsilica content of 18% is calculated from Eq. (12).= ( 600018 ) 146.34 15.6 = 171.4 kg

The solvents that can be used are ethanol, isopropanol,ethoxyethanol, ethoxy ethyl acetate or mixture of these. Thesolvent and ethyl silicate are combined and agitated. Watercontaining some acid catalyst is added and the mixture isthen agitated until the exotherm subsides. The binder is readyfor use after 24 h of preparation.

In general, curing of ethyl silicate involves hydrolyticpolycondensation occurring in two steps. The first isreversible as shown in Eq. (13).nSi(OC2H5)4 + 4nH2O nSi(OH)4 + 4nC2H5OH (13)

In the absence of alcohol, the silicic acid formed under-goes polycondensation as given in Eq. (14):nSi(OH)4 SiO2 + 2nH2O (14)

Because Eq. (14) contributes 2 mol of water for each moleof ethyl silicate, only 2 mol of water are needed for 100%hydrolysis of the reactants. Thus according to Eqs. (13) and(14), the total water necessary for 100% hydrolysis will rep-resent 17.36% by weight of the ethyl silicate used. If ethylsilicate-40 is used as the raw material, then for 100% hydrol-ysis, 14.5% water by weight of ethyl silicate-40 is required.

3.4. Paint compositions based on ethyl silicate binder

For the formulation of paints based on hydrolysed ethylsilicate binder, care should be taken for the selection ofpigments, because with this binder system, only those pig-ments are suitable which are chemically inert, non-basic andnot very reactive. Thus lead chromate, strontium chromate,mica, talc and zinc dust are some of the pigments which canbe suitable to formulate ethyl silicate based coatings. Partic-ularly good protection against high temperature and rust canbe obtained if zinc dust is used as the pigment. Some typicalformulations of these paint systems are given hereunder:

Formulation 1 [51]

S. No. Ingredient Amount (%)1 Ethyl silicate (partially hydrolysed) 20.02 Anti-settling agent (Bentone 38) 1.43 Talc 4.04 Toluene 5.35 Isopropanol 5.36 Cellosolve 4.07 Zinc dust 60.0

100.0


Formulation 2 [56]

S. No. Ingredient Amount (%)1 40% ethyl silicate liquid 26.02 30% ethyl silicate liquid 4.83 Zinc powder 39.14 Zinc flakes 6.55 Ferro phosphate 19.56 Crystalline silica 3.27 Amorphous silica 0.48 Wetting agent 0.5

100.0

Formulation 3 [52]

S. No. Ingredient Amount (%)1 Bindera 19.62 Powdered zinc (spherical particles) 32.93 Titanium dioxide (rutile) 13.34 Ilmenite 17.95 Aluminium 17.3

100.0a Binder can be prepared [52] by using 50 parts ethyl

silicate-40, 43.2 parts isopropyl alcohol, 5 parts water, onepart 5% HCl, and by stirring the contents for 5 h at 40C.

Specifications of the zinc dust commonly used in the ethylsilicate based paint formulations are given hereunder [4].

Specifications of zinc dust

(i) CompositionTotal zinc 9899.2%Metallic zinc 9497%Zinc oxide 36%Lead 0.2% maximumCadmium as (CdO) 0.7% maximumVolatile 0.1% maximumMoisture and volatile 0.1% maximumIron 0.04% maximum

(ii) Coarse particlesRetention on 100 mesh NilRetention on 200 mesh NilRetention on 325 mesh 4% maximum

(iii) Particle size distribution (Coulter counter)Medium particle size 610 micronsSpecific surface 0.17 m2/gSpherical particles, specific gravity 7.0 g/cm3

(iv) DispersibilityShould disperse satisfactorily in a high speed disperser

4. Chemistry of hydrolysis reaction of alkyl silicates

Hydrolysis of alkyl silicates is influenced by variousfactors [53] such as,1. Nature of the alkyl group.2. Nature of the solvent used.3. Concentration of each species in the solution or reaction

mixture.4. Molar ratio of water to alkoxide.5. Reaction temperature.

In addition to these influencing factors, pH of the solu-tion is also an important factor which governs the rate ofhydrolysis reaction and condensation of the hydrolysedproduct. In acidic condition, hydrolysis reaction takes placethrough electrophilic substitution and in basic condition, thehydrolysis proceeds through nucleophilic reaction. WhenpH of the solution is 2.5, alkoxy groups remain unaf-fected because silicate particles are not charged at this pH.Above or below this pH, they can be attacked by water.Rate of hydrolysis increases with increase in pH of thesolution. At pH below 2.5, silicate particles are negativelycharged and at pH above 2.5, they are positively charged.At lower pH, hydrolysis takes place through SE2 mecha-nism and at higher pH, this reaction corresponds to SN2mechanism.

In case of alkyl silicates, nucleophilic attack is sensitiveto electron density around the central silicon atom. Thiselectron density increases due to the size of substituentgroups. Susceptibility to nucleophilic attack increases withdecrease in bulky and basic alkoxy groups around the cen-tral silicon atom. However, reactivity of the tetrahedrontowards electrophilic attack is enhanced by an increase inelectron density around silicon. Initial hydrolysis of sili-con ester monomer produces silanol groups, whereas fullhydrolysis can lead to silicic acid monomer. This acid isnot stable and condensation of silanol groups occur lead-ing to polymer formation before all alkoxy groups aresubstituted by silanol groups. Condensation polymerisa-tion reactions proceed with an increase in viscosity of thealkoxide solution until an alcogel is produced. In gen-eral, acid catalysed reactions yield alcogels, whereas basecatalysed hydrolysis reaction precipitates hydrated silicapowders.

4.1. Mechanism of the hydrolysis reaction

Alkyl silicates are not water soluble in nature, becauseof which a mutual solvent is needed to hydrolyse it. Thus,hydrolysis is carried out in the form of solution, and ethylalcohol and isopropyl alcohol are generally used as themutual solvent.

When pH of the aqueous solution is 2.5, the silicate par-ticles are not electrically charged. However, when pH ofan aqueous solution is quite acidic and the silicate particlesget negatively charged, the relatively high concentration of


protons catalyses the hydrolysis reaction. The mechanismthen corresponds to an electrophilic substitution in whichan (H3O)+ hydronium ion attacks the oxygen of one of thealkyl groups.

In the intermediary complex of this mechanism, thecoordination number of Si increases. The rate of reactiondepends as much on the concentration of H3O+ as on theone of the alkoxides. The mechanism is consequently anSE2, and steric strain is also an important factor. The rate ofhydrolysis decreases as the length of alkyl group increases.The reaction mechanism is as given below:

(15)

In alkaline conditions, silicate particles are positivelycharged and OH anion attacks the alkoxide through anSN2 mechanism in order to form the silanol group. Since(OR)complex < (OR)alcohol, at least one OR or ORligand must leave the intermediary complex formed by sili-con. The anion then recombines with a proton so as to forman alcohol molecule. The mechanism of the reaction hasbeen shown below:

(16)

For this reaction, another more complex mechanism isalso proposed which involves two intermediary complexes.

Since Lewis bases are strong nucleophiles, they candeprotonate the OH ligands of cations, which form acidicoxides, thus creating oxo ligands. Lewis base such as

sodium hydroxide, ammonium hydroxide, etc. can effectthis type of reaction.

The silanol group (SiOH) resulting from the hydrolysisof silicon alkoxide can be converted to oxo ligand. For thisreaction, base is a necessary catalyser, and the reaction canbe as given hereunder:

(17)

Traces of water vapour can also hydrolyse metal alkoxidesthus transforming them into oxi-alkoxides. Such a hydrolysisfollows a reaction of the following type:

(18)

4.2. Condensation of alkyl silicates

In acidic conditions, silicon alkoxide condenses througha two step mechanism which corresponds to SN2 type ofmechanism. In first step, silanol groups are protonated whichincreases the electrophilic character of the surroundingsilicon atoms.

As a consequence, this protonated silanol combines toanother silanol group while liberating a (H3O)+ ion. Thetwo silicon atoms of the resulting polymer are then linkedthrough an oxo bridge called, in this specific case, as silox-ane bond. It can be noted that the Si of the intermediary com-plex of this mechanism is either tetra or penta coordinated.Mechanism of condensation reaction is as given below:

(19)

(20)

Rate of condensation reaction depends on the second stepof the mechanism and is proportional to the concentration ofthe protons. Hence condensation is a slower transformation


than hydrolysis. Silanols are protonated more easily whenthey are present at the end of the polymer chain.

In basic conditions, they build siloxane bridge by anotherSN2 mechanism. This mechanism involves two interme-diary complexes with penta coordinated silicons. In basicconditions, condensation rate is not only proportional tothe concentration of OH anions but also superior to thatof hydrolysis. Furthermore, since the reticulation inside thesilicon polymers is more developed than when conditionsfor acidic catalysis are used, hence the denser solids areobtained.

(21)

Overall basic catalysts, including Lewis bases, accel-erate condensation and alcohol molecules are better leav-ing groups than water. Efficient Lewis bases include, forinstance, DMAP (dimethyl aminopyridine), n-Bu4NF andNaF.

5. Mechanism of film curing of inorganic zinc silicatecoatings

Hydrolysed ethyl silicate based zinc-rich coatings areself-curing in nature. These coatings cure differently thanthat of the alkali silicate based inorganic zinc silicatecoatings. A simple distinction is that the water-borne al-kali silicate coatings lose water during the initial curingstages, whereas the solvent-borne alkyl silicate coatingsabsorb water with subsequent release of ethyl alcoholinitially [6].

As discussed previously that the principal raw materialsused for the preparation of vehicle of inorganic zinc coatingsare potassium silicate, lithium silicate, colloidal silica solu-tions and ethyl silicate. Even with all these different startingmaterials, quite similar ultimate reactions occur within thecoating and on steel surface during film curing [2].

In general, the curing of ethyl silicate involves hydrolyticpolycondensation reaction, which occurs in two steps. Thefirst reaction is reversible which has already been given asEq. (15). The product of this reaction, in the absence ofalcohol, undergoes polycondensation reaction as shown inEq. (22).

During the curing process, first of all, most of the solventis lost by the evaporation which leads to the concentrationof the zinc ethyl silicate mixture. At this point, coating isuncured and sensitive to moisture or water.

(22)

The moisture and carbon dioxide in the air react with eachother to form carbonic acid, as shown below:

H2O+ CO2 H2CO3 (23)

This carbonic acid causes ionisation of some zinc on thesurface of zinc particles. The slightly acidic water helpsto hydrolyse the prehydrolysed binder completely to yieldsilicic acid as given hereunder:

(24)

The ionic zinc then reacts with silanol groups on the sili-cate molecules in the silicate gel structure. This insolubilisesthe coating and provides its initial properties. This reactionis as follows.


(25)At this time, some reaction between poly silicic acid and

the iron surface also takes place to form a chemical bond.This bonding prevents the creepage of moisture and liftingof paint film seen in organic coatings. From this point on, thereactions will be those that take place over a long period oftime and depends on the characteristics of the environment inwhich zinc coatings are placed. Humidity and carbon dioxidecreate a very mild acidic condition that results in continuedhydrolysis of the vehicle and ionisation of the zinc. Zincions diffuse deeper and deeper into the gel structure untilthere is a zinc silicate cement matrix formed around eachof the zinc particles binding the coating together and to thesteel surface.

(26)

Ethyl silicate based binders can be cured by IR radia-tion [54], alkali metal salts of thio acids, barbutaric acids,and/or, 1,3-dicarbonyl compounds [55], and also by treatingthe substrate with an aqueous solution of a base over whichthey are applied [56].

6. Film performance of ethyl silicate based zinc-richcoatings

Uncured films of zinc-rich coatings are rough and irregu-lar while fully cured zinc-rich paint films are grey in colourand textured in nature [57], as in cured films, round glob-ules of zinc are present. These cured films have metal likehardness and these films remain unaffected by radiation in-cluding X-rays, neutron bombardment and other forms ofradioactivity [58]. Some other advantages of these systemsare given hereunder:1. They can be applied by conventional spray equipment or

by brush [2].2. They have quick drying properties.3. These systems are applicable in relative humidities

between 20 and 95% and tolerate slight surface moisture[58].

4. They have good chemical resistance and they remainunaffected by organic solvents [5].

5. Inorganic zinc-rich paints offer excellent adhesionbecause the binder chemically reacts with the underlyingsteel surface [2,8]. Such an excellent adhesion preventsunder cutting of coating by corrosion even after 10 yearsof exposure. As a matter of fact, these are the mostcorrosion resistant coatings available today [2].

6. These coatings offer excellent corrosion resistance dueto the involvement of conductive matrix in the protectionmechanism.

7. These coatings have excellent weather resistance. Theycan withstand rain just after half an hour of the application[2].

8. These films are weldable at a low dry film thickness anddo not have adverse effect on welding and gas cutting[49].

9. They will protect steel under insulation in the criticaltemperature range 066C.

10. Coatings can withstand temperature up to 400C.Along with these advantages, they have some limitations

also such as:

1. They have poor resistance for acidic or alkaline condi-tions outside the pH range 510.

2. These coatings generally exhibit more pinholing andbubbling upon top coating as compared to organic zinccoatings.

3. They are not recommended for immersion service in freshor salt water.

4. In wet condition, they are not recommended beyond 60Cdue to rapid depletion of zinc.


5. Coatings are not flexible.6. They are higher in cost as compared to the conventional

coatings.7. The major problem with this system is that the cure rate

of alkyl silicates is dependent upon relative humidity. Indry climate, cure rate may be reduced greatly, especiallyat temperature below 10C and where the films of highthickness are involved [59].

8. However, alkyl silicate primers have somewhat better tol-erance for slightly poorer surface preparation than thealkali silicate based paints, but a properly cleaned (sandblasted) surface is a must for these coatings.

Under cathodic protection, organic binder based zinc-richprimers have tendency to degrade, and also to cause blis-tering of the subsequent coats. In this respect, inorganiczinc-rich primers have superlative record. Another reasonfor the popularity of zinc silicate primers is their capacityto offer longer anticorrosive protection at lower dry filmthickness and at lower zinc loading levels [2].

These systems form coherent adhesive coating of silicawhich results due to hydrolysis and gelation of the ethylsilicate binder. Because of inertness and refractoriness ofsilica, these systems are heat stable and durable.

7. Factors influencing film performance

There are various factors, which affect performance of theapplied ethyl silicate zinc-rich coatings. These factors arediscussed hereunder one by one.

7.1. Particle shape and size of zinc pigment

Zinc is most commonly used as zinc dust in ethyl sili-cate based zinc-rich coatings. Zinc particles are generallyspherical in shape. Studies have been carried out by Hare[59] using zinc flakes in organic zinc-rich primers and ethylsilicate zinc-rich primers. It was theorised that a flat platezinc particle can be utilised advantageously in several ways.Theoretically, zinc dust particles having a particle diame-ter of about 10 times the thickness of a zinc flake plateletwould require much more minimum primer film thicknessfor a given degree of protection than would the flake do.In a 25 micron film thickness, as many as 20 zinc flakeplatelets might be superimposed as compared to approx-imately three rows of spheres of zinc dust. The lamellarnature of the flake would ensure a significantly enhancedelectrical contact area. In fact, reactivity of zinc flake in saltfog environments was found to be too great to provide thesort of long-term performance profile required. Apparently,zinc flake produced far more current than was necessary toprotect the steel cathode, and was soon exhausted. Hence re-duction of zinc reactivity by the addition of small quantitiesof inhibitors such as potassium chromate along with micaextender significantly improved performance effectiveness.

Performance comparisons between zinc dust primers andzinc flake primers have shown that chromated zinc flakesystems outperform zinc dust primers (of same vehicle type)in both salt fog and bullet hole studies.

7.2. Extender pigments

The metallic zinc content in the dry film is a very im-portant parameter to be emphasised in the technical specifi-cations of zinc-rich paints. According to the most technicalspecifications, minimum content of metallic zinc in the dryfilm required is 75% (by weight) for zinc-rich paints basedon ethyl silicate. For the same metallic zinc content in dryfilm the solids balance can be made using only the binderand zinc dust or partial substitution of binder with auxiliarypigments. It is observed by Land quest that metallic zinccontent in the dry film is not only a factor determining theperformance of this kind of paints while Fragata et al. [60],Del and Giudice [61] and Pereira et al. [62] verified that thechemical nature of the binder and the zinc particle size arealso very important.

In order to obtain contrast between sand blasted steelsubstrate and the paint, some manufacturers use colour-ing pigments such as chromium oxide and iron oxide, andbecause of technical reasons some other manufacturers useextender pigments such as barytes, mica, talc etc.

Experimental studies have been carried out by Fragataet al. [63], on ethyl silicate based paints having a metalliczinc content of 75 and 60% (Table 5). Panels coated withthese paints were subjected to salt spray, field exposure andelectrochemical tests. The results showed that addition offillers agalmatolite (A) and barytes (B) to the paints with60% metallic zinc in the dry film improves their behaviour.Salt spray results for 75% zinc content up to 2060 h ofexposure did not show any influence of fillers.

In the paints which contain fillers, for the same metalliczinc content in the dry film, the PVC/CPVC ratio is higher,which leads more porous and permeable films due to whichthe electrical contact between zinc particles and steel sub-strate improves. These factors contribute to the improve-ment of paint performance from the galvanic point of view.It is important to mention that effectiveness of the zinc-richpaint does not depend solely on electrochemical factors.Some other factors such as mechanical properties viz. cohe-sion, flexibility, etc. are also important. So the addition ofauxiliary pigments should be controlled carefully in orderto not impair the physical and chemical characteristics ofthe films.

In inorganic zinc silicate coatings, water, ground mus-covite mica is also used widely. On the basis of experimentalstudies, Hare [64] reported that upgraded corrosion resis-tance and reduced cost of the system can be obtained byusing flake zinc in combination with mica and zinc potas-sium chromate. It is also observed in the mica modifiedformulations that they produce reduced amount of zinc cor-rosion product, which indicates the general reduction in zinc


Table 5Salt spray results of ethyl silicate coatings pigmented with zinc dust and fillers

Paint designation Metallic zinc con-tent in the dry film

Main componentsof dry film

Time (h) necessary for appearance ofred corrosion in scratch (ASTM B-117)

Zn60 60.0 Ethyl silicate 460Zinc dust

ZnA60 60.0 Ethyl silicate 740Zinc dustAgalmatolite

ZnB60 60.0 Ethyl silicate 660Zinc dustBarytes

Zn75 75.0 Ethyl silicate 2060Zinc dust

ZnA75 75.0 Ethyl silicate 2060Zinc dustAgalmatolite

ZnB75 75.0 Ethyl silicate 2060Zinc dustBarytes

corrosion. This effect is thought to be related to the controlof current transfer that such non-conductive extenders mightallow. Electrical conductivity is reduced in this case not onlyby the resistance of the vehicle cover but also by the micalaminate.

Besides these, various other conductive extenders havebeen used such as cadmium, aluminium, magnesium, ironand carbon along with zinc dust. Of these, only cadmiumwith zinc and inhibitors gave results comparable to normalzinc-rich primers. Others have proved to be inferior. Prob-lems of toxic fumes during welding, however, precludesthe use of cadmium in these coatings. Out of various exten-ders used in ethyl silicate based zinc-rich paints, the bestresults have been obtained from di-iron phosphide (Fe2P),which is a refractory conductive compound. In ethyl sili-cate zinc-rich coatings evaluation of this extender has beencarried out by Filire et al. [65]. Results of the test carriedout by them show that it is possible to replace up to 25% ofzinc with minimal decrease in the ability of the coating toprovide cathodic protection to the steel substrate. Composi-tions of some ethyl silicate vehicles formulated with higherconcentration of Fe2P lead to abnormally high zinc corro-sion products. Ethyl silicate zinc-rich coatings with Fe2Padditions tend to act as porous electrodes probably becausea majority of the metal and conductive extender particlesmaintain electrical contact between each other and with thesteel surface. This explains the greater ability of silicatecoatings to provide cathodic protection to the steel substrate.Further, the inclusion of Fe2P extender does not disturbthe marked capability of ethyl silicate zinc-rich paints todevelop barrier coats. The weldability of primers is alsoimproved by the use of Fe2P. Zinc appears to be consumedmore efficiently in the presence of Fe2P with the result thatimproved corrosion protection is obtained with lower initial

zinc content while a greater fraction of the zinc initiallypresent remained unoxidised after a given period of time.

7.3. Application techniques

Application techniques and relative humidity also haveinfluence on the curing of inorganic zinc ethyl silicate basedprimers [57]. The experimental results also revealed thatcuring is affected by incorrect mix ratio of base to filler,inadequate mixing and/or settling out of the zinc portion,and this will be dictated by spray equipment and technique,and also by spray parameters such as air pressure, nozzlesizes, distance from the surface, etc. It was also reportedthat spray coating methods yielded results which were notreadily reproducible and gave both poor and good curingresults, while flow coating methods yielded reproducible re-sults conforming to manufacturers data sheets under theconditions tested.

8. Areas of applications for zinc-rich inorganic silicatepaints

Because of the excellent corrosion protection offered bythese coatings to steel, these coatings find applications invarious critical fields [66]. Some of their application areasare given hereunder:

1. Harbour structures. The corrosion conditions encoun-tered by off-shore petroleum production platforms are themost severe. Many hundreds of drilling and productionstructures have been coated with inorganic zinc silicatecoatings, located in the highly humid tropical areas ofIndonesia, Singapore and the Persian Gulf to the United


States Gulf coast and extending into the Arctic areas ofAlaska and the North Sea. The inorganic coatings basedon hydrolysed ethyl silicate, applied alone or overcoatedfor additional protection and for safety colouration, areproviding outstanding protection to these essential piecesof equipment.

2. Bridges. Bridges, like off-shore structures, are extremelyvulnerable to corrosion, perhaps so since many bridgestructures are formed from structural steel shapes, withall the corners, edges, crevices and surface defects in-herent in such shapes. One of the very early bridgescoated is a Drawbridge across a Tidal river in Florida.This bridge was coated in 1956 with the open grill workbeing the most difficult part of the structure to fullyprotect it. It is still well protected by the original singlecoat of inorganic zinc silicate coating. Other bridgessuch as Baleman bridge in Tasmania which was coatedprior to installation, the golden gate bridge on the orig-inal Morgan Whylla pipeline, are some examples of fullprotection provided by inorganic zinc silicate paints overmany years of continuous exposure.

3. Nuclear power facilities. One interesting application ofinorganic zinc silicate paints is the protection of nuclearpower plants. The steel surface within the reactor buildingrequires coating with a 40-year expected life. In fact, it ishoped that such surfaces will never have to be painted af-ter the plant goes into operation. Alkyl silicate inorganiczinc-rich primers are used in nuclear applications formany reasons. These primers are applied at 3.0 mil min-imum thickness, mainly at the steel plate manufacturersfactory before shipping to the job site. These coatingsare unaffected by -rays or neutron bombardment.

4. Tank coatings. One of the major uses of inorganic zinccoatings has been in the lining of ship tankers, primarilyfor transporting refined fuel. One of the oldest docu-mented applications of inorganic zinc coatings is theNo. 1 centre tank in Utah standard. This was applied in1954 to a previously corroded tank surface. This tankwas inspected in 1966, after 11 years approximately, andwith the exception of holidays or missed areas in origi-nal application, there was no further rust or loss of metalin the tank. Inorganic zinc-rich coatings are suitable ingeneral for tank interiors carrying petroleum products,crude oils, lubricants, edible oils and solvents like ketoneesters, chlorinated hydrocarbons, etc. [66]. However, un-pigmented hydrolysed ethyl silicate binder is also usedfor various purposes such as stone preservation, for thesurface treatment of concrete to reduce dusting, etc. [33].

9. Conclusion

Surface coatings based on inorganic binders can besuccessfully used as primers for the effective protection ofsteel against corrosion. For the formulation of inorganiccoatings, alkali metal silicates such as sodium, potassium

and lithium silicates and alkyl silicates such as ethyl sili-cate are commonly employed as inorganic binders. Ethylsilicate based binders have proved to be superior to alkalimetal silicates in overall performance, despite the fact thatformer ones produce solvent-borne compositions, whereasalkali metal silicate based coatings are water-borne. Ethylsilicate based coating films are self-curable at room tem-perature in the presence of adequate atmospheric moisture.The final (cured) films of ethyl silicate based coatings arecomposed mainly of silica, or silica and zinc, if zinc is usedas a pigment. Therefore, cured films of ethyl silicate based(inorganic) coatings are considered better, in view of envi-ronmental aspects, than organic coatings which invariablyproduce films composed of organic polymers. The films ofthese coatings, being silica based, are resistant to temper-ature up to 400C, where most organic coating films fail.Further, films of zinc-rich ethyl silicate based coatings pro-tect the substrate (steel) by providing much more effectivecathodic protection than that provided by zinc-rich organiccoating films. In addition, ethyl silicate based binders reactwith the iron (substrate) chemically, and hence provide un-matched adhesion to restrict corrosion creepage, if any kindof corrosion at all starts on the substrate. The films, beingrock-like hard and quite rough, provide excellent inter-coatadhesion to the subsequent coat.

On account of these attractive features, ethyl silicate basedcoatings can be successfully used for high performanceapplications in critical areas such as harbour structures, nu-clear power plants, etc. As on today, no organic coating isavailable which can match these inorganic coatings in termsof long-term corrosion protection performance clubbedwith their high temperature resistance. It can, therefore, beexpected that ethyl silicate based coatings will find wider andwider application in further more challenging areas in future.

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