ethyl silicate binders for high performance coatings

Progress in Organic Coatings 42 (2001) 1–14 Review Ethyl 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, India Received 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 which are mainly based on organic binders are referred to as organic coatings. Zinc-rich inorganic coatings based on ethyl silicate are quite successful 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 high temperatures. Because of the formation of conductive matrix out of the binder after film curing, zinc-rich coatings based on ethyl silicate binder offer outstanding cathodic protection to steel structures. These coatings are mostly solvent-borne, but recently water-borne versions of 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 to produce ethyl silicate hydrolysates of desired degree of hydrolysis. Effect of various factors such as amount of catalysts, amount of w ater , type and amount of solvent, reaction temperature and reaction time has been discussed. Calculations to find out the amount of water and solvent required to yield the product of desired degree of hydrolysis have also been illustrated. Typical recipes useful for the preparation of ethyl silicate hydrolysates suitable for use as coating binders have also been presented. The chemistry and mechanism involved in the 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 used for the protection of metals from corrosion. Effectiveness of protection of metals against corrosion mainly depends on the factors such as quality of the coating, characteristics of the metal, properties of the coating/metal interface, and the corrosiveness of the environment. Typical corrosion resis- tant coatings protect the metallic surfaces primarily by the following 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 form such compounds that, in fact, inhibit corrosion. Some ∗ Corresponding author. Tel.: +91-512-583-507; fax: +91-512-545-312. E-mail address: vkj@hbti.ernet.in (P. Kumar). pigments having limited solubility can give rise to inhibitive ions, such as chromates. Undoubtedly, steel is one of the most important metals used in the modern society. However, one of its main draw- backs is its tendency to corrode (rust), i.e. to revert to its original state, and become useless. Hence, the protection of steel from corrosion, i.e. to keep the steel in its usable form, has always been a matter of great concern for all those who use 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 of steel in the form of electroplated layers or by the applica- tion of paints containing a high percentage of zinc particles dispersed in an organic or an inorganic binder. Zinc, either in the form of electroplated film or in the form of films of zinc-rich coatings, protects the steel substrate by sacrificial cathodic (galvanic) protection mechanism. For the cathodic protection of steel, the direct electrical contact between the 0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 )0012 8 - X 2 G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14 adjacent zinc particles, and between the zinc particles in the film 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 hence the zinc particles have restricted electrical contact. Conse- quently, the zinc particles can provide only a small amount of galvanic protection limited to the amount of free zinc in the 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, which can chemically react with the zinc particles in the coating film to form a zinc silicate matrix around the zinc particles [5]. This zinc silicate matrix is electrically conductive and chemically inert [2]. In addition, the silicate based binders can chemically react with the steel substrate also to result in an 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 the most weather resistant coatings available today [5]. They can provide an unmatched protection against corrosion for steel structures exposed to temperatures up to 400 ◦ C [2]. 2. Silicate binders for inorganic paint coatings Inorganic paint coatings based on silicate binders can be classified [6] as shown in Fig. 1. 2.1. Alkali metal silicate binders For the manufacture of coatings based on alkali metal silicates, the silicates based on alkali metals such as sodium, potassium and lithium, along with the quarternary ammonium silicates have been reported to be suitable [8]. Alkali metal silicates are relatively simple chemi- cals, which can be water soluble depending on the ratio of silica to alkali metal oxide. The ratios of silica to alkali metal oxide of different silicates [8], which can be used as binder systems in paints, have been given in Table 1. The ratio of silica to alkali metal oxide, in addition to the type of alkali metal, has a remarkable effect on curing char- acteristics and properties of the dried films [9]. The effect of ratio of silica to alkali metal oxide on coating characteristics has 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 are water-borne due to solubility of the used alkali metal oxide in water. These coatings are generally sub-classified into baked, post-cured and self-cured coatings. 2.1.1. Baked coatings These are the coatings which require heating to convert the coating films into water insoluble form. These coatings are characterised by their extreme hardness and suitability for application over an acid-descaled surface. Baked coatings still have limited use today. Fig. 1. Classification of inorganic paint coatings based on silicate binders. Table 1 Ratios 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 SiO 2 :Na 2 O 2.4–4.5:1 2 Potassium silicate SiO 2 :K 2 O 2.1–5.3:1 3 Lithium silicate SiO 2 :Li 2 O 2.1–8.5:1 Table 2 Effects 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 solution Higher the drying speed of the film Higher the curing speed of the film Higher the susceptibility to low temperature Higher the chemical resistance of the coating films 2 Lower Higher the specific weight of the solution Higher the solubility in water Higher the pH value of the solution Higher the susceptibility to water Higher the adhesion and binding power 2.1.2. Post-cured coatings These are the coatings which are cured by the application of chemicals such as an acid wash just after application of the film to convert the film into a water insoluble condition. These coatings are formulated mainly on sodium silicate having higher ratio of silica to sodium oxide. This develop- ment has led to the use of inorganic zinc coatings on large field structures. 2.1.3. Self-cured coatings With further advances in silicate technology, further higher ratio alkali metal silicates have become available. Of the cheaper types, potassium silicate is preferred. Reliable self-curing coatings are available today, based on high ratio potassium silicates with potassium oxide to silica ra- tio ranging from 1:2 to 1:5.3. If further higher ratios are required, and instability is to be avoided, it is necessary to use lithium silicate with lithium oxide to silica ratio as 1:2 to 1:8.5. Lithium silicate based coatings are preferred for use in food areas. Excellent curing rates can be achieved with some lithium silicates, but their higher cost tends to restrict 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-borne coatings. However, one of the commercial forms of ethyl silicate (popularly known as ethyl silicate-40) as solution in organic solvent(s) is most commonly employed. Alkyl sili- cates, as such, do not have any binding ability but when their alcoholic solutions are hydrolysed with calculated amount of water in the presence of acid or alkali catalyst, they acquire sufficient binding ability. On the basis of the type of catalyst used for the hydrolysis, these coatings can be sub-classified as follows. 2.2.1. Alkali catalysed coatings For the hydrolysis of ethyl silicate, bases like ammonia, ammonium hydroxide, sodium hydroxide and some amines are generally used as catalysts [2]. One of the greatest drawbacks of this system is related to the fact that in basic conditions, even a small amount of water will cause the silicate to gel. To avoid this problem, remedial steps must therefore be taken to exclude all water at the manufactur- ing stage, and from the application equipment. If water is excluded, the liquid component can remain stable for an indefinite period of time. These coatings are available in the market as single-pack and two-pack systems. In single-pack system, amines, which provide hydroxyl ion in the form which is non-reactive with organic polysilicate until they are exposed to moisture, are used [8]. 2.2.2. Acid catalysed coatings In these type of coatings, rapid curing may be achieved under most conditions. However, the period over which the partially hydrolysed silicate remains stable is limited, and the product thus has a finite shelf life. Coatings based on acid catalysed binder are mainly two-component systems, and the liquid component of these coatings gel in a period of 6–12 months. The problem associated with one-pack system of this type is that zinc chemically reacts with the acid catalyst present in the binder system, due to which pH of the system increases, which causes gelling in the con- tainer. Hydrochloric acid [10–27], sulphuric acid [28,29], phosphoric acid [30], formic acid [31], etc., are the acids which are used as catalysts. 3. Hydrolysis of ethyl silicate Ethyl silicate, by itself, has no binding ability [32]. To introduce binding ability, it is necessary to hydrolyse ethyl silicate by treating it with water, so that a gel can form from the resulting ethyl silicate hydrolysate. The actual binding agent is this gel [33]. Usually, the hydrolysis of ethyl silicate is carried out under alkaline or acidic conditions. Acids or alkalis are used to catalyse the hydrolysis reaction. Hydrolysis under alkaline conditions normally results in fairly rapid gelation. Alkali catalysed hydrolysis procedures are generally pre- ferred when ethyl silicate is to be used for the production of refractories. Acid hydrolysis procedures are commonly employed for the production of paint media. Several 4 G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14 Table 3 Typical 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 solvent 1 6 l 2 l 50 ml concentrated HCl 4 l ethanol 2 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 sprit 4 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 are available [34–36]. Hydrolysis procedures in which a specified quantity of ethyl silicate is added at the start of the reaction are termed as ‘single stage’ procedures, while those in which ethyl silicate is added usually after a specified temperature rise or time interval are termed as ‘two-stage’ procedures. Some two-stage procedures require two types of organic silicates. Typical compositions for the single stage [37–40] and two-stage procedures [37,41,42] taken from the patent literature 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 presence of acid catalyst with 5% (part basis) water at 140 ◦ C. Some other workers [44–46] also prepared binder systems by using pure ethyl silicate or ethyl silicate-40 of different properties. Some special procedures include the use of silica aquasol and the use of titanic acid ester in a two- stage process. If large amount of phosphoric acid is used in the hydrolysis of ethyl silicate, hydrolysates which gel rapidly can be ob- tained. Conditions for the hydrolysis of ethyl silicate without use of an acid or a base catalyst to obtain binding solutions have also been established [47]. Acid hydrolysates of ethyl silicate eventually set to a gel on standing. The relatively short shelf life of some acid hydrolysed ethyl silicate solutions can cause difficulties in their use. As a result of the development of methods for preparing ethyl silicate hydrolysates having a long stor- age life, hydrolysed ethyl silicate solutions have become available commercially. These solutions, often referred to as prehydrolysed ethyl silicate solutions, are of particular interest as paint media. Ethyl silicate hydrolysates having a long storage life can be obtained by careful choice of the proportions of ethyl silicate, solvent, acid and water for their preparation. If ethyl silicate is treated simultaneously with a glycol monoether for alcoholysis and water for hydrolysis, a hydrolysate with a long shelf life is obtained [48]. This hydrolysate can be successfully used as a paint medium. Generally 80–90% hydrolysis of the ethyl silicate is carried out for the binder preparation [2]. 3.1. Factors governing the formulation of ethyl silicate binders There are some important factors, which can affect the hydrolysis of ethyl silicate and the formulation of ethyl sili- cate binders. These factors are discussed hereunder one by one. 3.1.1. Effect of quantity of water Quantity 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. Water to be used in hydrolysis must be calculated after subtracting the quantity of water (if any) going into the paint formula- tion from the extender pigments and the solvents used in the formulation. Excessive water in the formulation can lead to gelling of the binder system in the cans or very poor applica- tion properties and gelling of mixed paints in the application equipment. Less than optimum quantities of water can result in an uncured film lacking hardness and film integrity [49]. 3.1.2. Effect of quantity of acid Less than optimum quantity of acid can result in silica precipitation, thus making less silica available for binding than required. Excessive quantity of acid will result in accel- erated condensation of silanol with silanol ( ≡ SiOH) groups or with alkoxy groups ( ≡ SiOR) resulting in reduced shelf life of the binder system [49]. Table 4 Typical compositions for two-stage procedures for the hydrolysis of ethyl silicate S. No. Quantity of ethyl silicate-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 50 parts 160 p industrial 11 parts ethyl silicate-40 (specific gravity 1.18) methylated spirit 2 6000 parts 2000 parts (by volume) 50 parts concentrated HCl 8000 parts isopropanol 2000 parts methyl silicate G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14 5 3 340 parts Nil (specific gravity 1.18) 40 parts 0.1 N HCl 140 parts isopropanol/ (50% SiO 2 ) 130 parts isopropyl silicate water azeotrope (38% SiO 2 ) 3.1.3. Effect of size of alkyl group The rate of hydrolysis reaction is greatly affected by the size of alkyl group of the organic silicates. The larger alkyl groups can act as a steric barrier to hydrolytic attack. Thus, bulkier alkyl groups protect the ester much better than the smaller groups like methyl or ethyl. N-hexyl silicates, e.g., 3.2.3. Reaction with zinc pigments (4) are difficult to hydrolyse, whereas methyl silicate hydrolyses readily. A second effect of the size of alkyl group involves the volatility of the alcohol formed during hydrolysis. If the alcohol is highly volatile, reversible reaction will be forced in the direction of the hydrolysis. This is particularly true for acid catalysed hydrolysis where the presence of the alcohol maintains an equilibrium. With proper selection of the alkyl group, curing properties of alkyl silicate coatings can be tailored [50]. 3.2. Chemistry of ethyl silicate binders Prepared ethyl silicate contains some silanols and alkoxy groups. These silanol groups are responsible for chemi- cal reactions in these types of coatings [2]. Some of their reactions are as follows. 3.2.1. Acid catalysed reactions First, 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]. The silanol groups of hydrolysed ethyl silicate react with zinc and form a zinc silanol heterobridge. (5) This hetero bridge then undergoes further chemical reactions to form a zinc silicate polymer. (6) 3.3. Stoichiometry of binder preparation 3.2.2. Effect of pH on stability (2) The overall stoichiometry of hydrolysis is given in the following equations. Total hydrolysis of pure ethyl silicate [2] can be given as shown in Eq. (7). When pH of the system is low, then the hydrolysed alkyl silicate has long pot life due to the repulsion of –O + H group with 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 the system is short. (7) Ethyl silicate hydrolysed to ‘x’ degree can be shown by the following equation: (8) 6 G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14 H = 41 . 66 This allows the calculation of the equivalent weig 4 Toluene 5.3 t of the 5 Isopropanol 5.3 ethyl polysilicate using Eq. (9). 6 Cellosolve 4.0 7 Zinc dust 60.0 Equivalent weight of ethyl polysilicate 2 The empirical equation for ethyl silicate hydrolysed to x degree of hydrolysis, SiO 2x ( OC 2 H 5 ) 4(1 − x) , can be used to derive 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 of any desired percentage hydrolysis. Equivalent weight can be obtained by substituting atomic weights in the empirical formula. Equivalent weight = SiO 2x ( OC 2 H 5 ) 4(1 − x) = 28 + 16(2x) + 45(4 − 4 x) = 28 + 32x + 180 − 180x = 208 − 148 x or Equivalent weight = 208 − 1.48 H (H = %hydrolysis) (9) The concentration of SiO 2 in the ethyl polysilicate is equal to Molecular weight of SiO 2 × 100 Equivalent weight of ethyl polysilicate or 60 × 100 In order to prepare a binder that is 85% hydrolysed, the weight 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 final silica content of 18% is calculated from Eq. (12). 6000 = ( 18 ) − 146.34 − 15.6 = 171.4 kg The solvents that can be used are ethanol, isopropanol, ethoxyethanol, ethoxy ethyl acetate or mixture of these. The solvent and ethyl silicate are combined and agitated. Water containing some acid catalyst is added and the mixture is then agitated until the exotherm subsides. The binder is ready for use after 24 h of preparation. In general, curing of ethyl silicate involves hydrolytic polycondensation occurring in two steps. The first is reversible as shown in Eq. (13). n Si ( OC 2 H 5 ) 4 + 4 n H 2 O → nSi(OH) 4 + 4 n C 2 H 5 OH (13) In the absence of alcohol, the silicic acid formed under- goes polycondensation as given in Eq. (14): nSi(OH) 4 → SiO 2 + 2 n H 2 O (14) Because Eq. (14) contributes 2 mol of water for each mole of ethyl silicate, only 2 mol of water are needed for 100% % SiO 2 = 208 (10) − 1.48 H hydrolysis of the reactants. Thus according to Eqs. (13) and (14), the total water necessary for 100% hydrolysis will rep- Calculation for the amount of water to be added to one equivalent weight of ethyl polysilicate to prepare a binder of 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 desired silica content of the binder is determined from the following equation: Weight of solvent to be added 6000 = % SiO desired − weight of ethyl polysilicate −weight of water added (12) For example, to prepare 85% hydrolysed binder contain- ing 18% SiO 2 from commercial ethyl silicate containing resent 17.36% by weight of the ethyl silicate used. If ethyl silicate-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 ethyl silicate binder, care should be taken for the selection of pigments, because with this binder system, only those pig- ments are suitable which are chemically inert, non-basic and not very reactive. Thus lead chromate, strontium chromate, mica, talc and zinc dust are some of the pigments which can be suitable to formulate ethyl silicate based coatings. Partic- ularly good protection against high temperature and rust can be obtained if zinc dust is used as the pigment. Some typical formulations of these paint systems are given hereunder: Formulation 1 [51] 41% SiO 2 , calculate the % hydrolysis in the ethyl polysili- cate from Eq. (10), as below: 6000 S. No. Ingredient Amount (%) 1 Ethyl silicate (partially hydrolysed) 20.0 41 = 208 − 1 . 48 ( H ) 2 Anti-settling agent (Bentone 38) 1.4 3 Talc 4.0 = 208 − 1.48(41.66) = 146.34 100.0 G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14 7 Formulation 2 [56] S. No. Ingredient Amount (%) 1 40% ethyl silicate liquid 26.0 2 30% ethyl silicate liquid 4.8 3 Zinc powder 39.1 4 Zinc flakes 6.5 5 Ferro phosphate 19.5 6 Crystalline silica 3.2 7 Amorphous silica 0.4 8 Wetting agent 0.5 100.0 Formulation 3 [52] S. No. Ingredient Amount (%) 1 Binder a 19.6 2 Powdered zinc (spherical particles) 32.9 3 Titanium dioxide (rutile) 13.3 4 Ilmenite 17.9 5 Aluminium 17.3 100.0 a Binder can be prepared [52] by using 50 parts ethyl silicate-40, 43.2 parts isopropyl alcohol, 5 parts water, one part 5% HCl, and by stirring the contents for 5 h at 40 ◦ C. Specifications of the zinc dust commonly used in the ethyl silicate based paint formulations are given hereunder [4]. Specifications of zinc dust (i) Composition Total zinc 98–99.2% Metallic zinc 94–97% Zinc oxide 3–6% Lead 0.2% maximum Cadmium as (CdO) 0.7% maximum Volatile 0.1% maximum Moisture and volatile 0.1% maximum Iron 0.04% maximum (ii) Coarse particles Retention on 100 mesh Nil Retention on 200 mesh Nil Retention on 325 mesh 4% maximum (iii) Particle size distribution (Coulter counter) Medium particle size 6–10 microns Specific surface ≤ 0.17 m 2 /g Spherical particles, specific gravity 7.0 g/cm 3 (iv) Dispersibility Should disperse satisfactorily in a high speed disperser 4. Chemistry of hydrolysis reaction of alkyl silicates Hydrolysis of alkyl silicates is influenced by various factors [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 of hydrolysis reaction and condensation of the hydrolysed product. In acidic condition, hydrolysis reaction takes place through electrophilic substitution and in basic condition, the hydrolysis proceeds through nucleophilic reaction. When pH 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 the solution. At pH below 2.5, silicate particles are negatively charged and at pH above 2.5, they are positively charged. At lower pH, hydrolysis takes place through SE 2 mecha- nism and at higher pH, this reaction corresponds to SN 2 mechanism. In case of alkyl silicates, nucleophilic attack is sensitive to electron density around the central silicon atom. This electron density increases due to the size of substituent groups. Susceptibility to nucleophilic attack increases with decrease in bulky and basic alkoxy groups around the cen- tral silicon atom. However, reactivity of the tetrahedron towards electrophilic attack is enhanced by an increase in electron density around silicon. Initial hydrolysis of sili- con ester monomer produces silanol groups, whereas full hydrolysis can lead to silicic acid monomer. This acid is not stable and condensation of silanol groups occur lead- ing to polymer formation before all alkoxy groups are substituted by silanol groups. Condensation polymerisa- tion reactions proceed with an increase in viscosity of the alkoxide solution until an alcogel is produced. In gen- eral, acid catalysed reactions yield alcogels, whereas base catalysed hydrolysis reaction precipitates hydrated silica powders. 4.1. Mechanism of the hydrolysis reaction Alkyl silicates are not water soluble in nature, because of which a mutual solvent is needed to hydrolyse it. Thus, hydrolysis is carried out in the form of solution, and ethyl alcohol and isopropyl alcohol are generally used as the mutual solvent. When pH of the aqueous solution is 2.5, the silicate par- ticles are not electrically charged. However, when pH of an aqueous solution is quite acidic and the silicate particles get negatively charged, the relatively high concentration of 8 G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14 protons catalyses the hydrolysis reaction. The mechanism then corresponds to an electrophilic substitution in which an (H 3 O) + hydronium ion attacks the oxygen of one of the alkyl groups. In the intermediary complex of this mechanism, the coordination number of Si increases. The rate of reaction depends as much on the concentration of H 3 O + as on the one of the alkoxides. The mechanism is consequently an SE 2 , and steric strain is also an important factor. The rate of hydrolysis decreases as the length of alkyl group increases. The reaction mechanism is as given below: (15) In alkaline conditions, silicate particles are positively charged and OH − anion attacks the alkoxide through an SN 2 mechanism in order to form the silanol group. Since δ(OR) complex < δ ( OR ) alcohol , at least one OR or OR − ligand must leave the intermediary complex formed by sili- con. The anion then recombines with a proton so as to form an alcohol molecule. The mechanism of the reaction has been shown below: (16) For this reaction, another more complex mechanism is also proposed which involves two intermediary complexes. Since Lewis bases are strong nucleophiles, they can deprotonate the OH ligands of cations, which form acidic oxides, thus creating oxo ligands. Lewis base such as sodium hydroxide, ammonium hydroxide, etc. can effect this type of reaction. The silanol group ( ≡ SiOH) resulting from the hydrolysis of silicon alkoxide can be converted to oxo ligand. For this reaction, base is a necessary catalyser, and the reaction can be as given hereunder: (17) Traces of water vapour can also hydrolyse metal alkoxides thus transforming them into oxi-alkoxides. Such a hydrolysis follows a reaction of the following type: (18) 4.2. Condensation of alkyl silicates In acidic conditions, silicon alkoxide condenses through a two step mechanism which corresponds to SN 2 type of mechanism. In first step, silanol groups are protonated which increases the electrophilic character of the surrounding silicon atoms. As a consequence, this protonated silanol combines to another silanol group while liberating a (H 3 O) + ion. The two silicon atoms of the resulting polymer are then linked through 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 step of the mechanism and is proportional to the concentration of the protons. Hence condensation is a slower transformation [...]... inorganic binders can be successfully used as primers for the effective protection of steel against corrosion For the formulation of inorganic coatings, alkali metal silicates such as sodium, potassium 13 and lithium silicates and alkyl silicates such as ethyl silicate are commonly employed as inorganic binders Ethyl silicate based binders have proved to be superior to alkali metal silicates in overall performance, ... Coatings 42 (2001) 1– 14 12 Table 5 Salt spray results of ethyl silicate coatings pigmented with zinc dust and fillers Paint designation Metallic zinc content in the dry film Main components of dry film Zn60 60.0 Ethyl silicate Zinc dust 460 ZnA60 60.0 Ethyl silicate Zinc dust Agalmatolite 740 ZnB60 60.0 Ethyl silicate Zinc dust Barytes 660 Zn75 75.0 Ethyl silicate Zinc dust 2060 ZnA75 75.0 Ethyl silicate. .. Lewis bases include, for instance, DMAP (dimethyl aminopyridine), n-Bu4 NF and NaF 5 Mechanism of film curing of inorganic zinc silicate coatings Hydrolysed ethyl silicate based zinc-rich coatings are self-curing in nature These coatings cure differently than that of the alkali silicate based inorganic zinc silicate coatings A simple distinction is that the water-borne alkali silicate coatings lose water... can be successfully used for high performance applications in critical areas such as harbour structures, nu- clear power plants, etc As on today, no organic coating is available which can match these inorganic coatings in terms of long-term corrosion protection performance clubbed with their high temperature resistance It can, therefore, be expected that ethyl silicate based coatings will find wider... that former ones produce solvent-borne compositions, whereas alkali metal silicate based coatings are water-borne Ethyl silicate based coating films are self-curable at room temperature in the presence of adequate atmospheric moisture The final (cured) films of ethyl silicate based coatings are composed mainly of silica, or silica and zinc, if zinc is used as a pigment Therefore, cured films of ethyl silicate. .. which affect performance of the applied ethyl silicate zinc-rich coatings These factors are discussed hereunder one by one 7.1 Particle shape and size of zinc pigment Zinc is most commonly used as zinc dust in ethyl silicate based zinc-rich coatings Zinc particles are generally spherical in shape Studies have been carried out by Hare [59] using zinc flakes in organic zinc-rich primers and ethyl silicate. .. some ethyl silicate vehicles formulated with higher concentration of Fe2 P lead to abnormally high zinc corrosion products Ethyl silicate zinc-rich coatings with Fe2 P additions tend to act as porous electrodes probably because a majority of the metal and conductive extender particles maintain electrical contact between each other and with the steel surface This explains the greater ability of silicate. .. the initial curing stages, whereas the solvent-borne alkyl silicate coatings absorb water with subsequent release of ethyl alcohol initially [6] As discussed previously that the principal raw materials used for the preparation of vehicle of inorganic zinc coatings are potassium silicate, lithium silicate, colloidal silica solu- tions and ethyl silicate Even with all these different starting materials,... Organic Coatings 42 (2001) 1– 14 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 In dry climate, cure rate may be reduced greatly, especially ◦ at temperature below 10 C and where the films of high thickness are involved [59] 8 However, alkyl silicate. .. an aqueous solution of a base over which they are applied [56] 6 Film performance of ethyl silicate based zinc-rich coatings (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 lifting of paint film seen in organic coatings From this point on, the reactions will be those that take place . their higher cost tends to restrict 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. 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; . Progress in Organic Coatings 42 (2001) 1–14 Review Ethyl silicate binders for high performance coatings Geeta Parashar a , Deepak Srivastava b , Pramod