Progress in Organic Coatings 42 (2001) 1–14Review 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, industrial 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 water, 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 resistant 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 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 drawbacks 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 successful [2]. Zinc can prevent or at least retard the corrosion of steel in the form of electroplated layers or by the application 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 ∗ Corresponding author. Tel.: +91-512-583-507; fax: +91-512-545-312. E-mail address:
[email protected] (P. Kumar). 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 ) 0 0 1 2 8 - X 2.1.1–8. On the other hand.1:1 to 8. which can be water soluble depending on the ratio of silica to alkali metal oxide. potassium and lithium.4–4. Baked coatings still have limited use today. has a remarkable effect on curing characteristics and properties of the dried films [9]. which can chemically react with the zinc particles in the coating film to form a zinc silicate matrix around the zinc particles [5]. 2.3:1 2. as these are the most weather resistant coatings available today [5]. 1. The coatings based on alkali metal silicates having silica to alkali metal oxide varying from 2. Parashar et al.5:1 .5:1 2. Fig. Alkali metal silicate binders For the manufacture of coatings based on alkali metal silicates. In the case of zinc-rich ‘organic’ coating films. 1 2 3 Silicate Sodium silicate Potassium silicate Lithium silicate Chemical composition SiO2 :Na2 O SiO2 :K2 O SiO2 :Li2 O Ratio of silica to alkali metal oxide 2. which can be used as binder systems in paints.1. / Progress in Organic Coatings 42 (2001) 1–14 adjacent zinc particles. and hence the zinc particles have restricted electrical contact. 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]. Classification of inorganic paint coatings based on silicate binders.1. The ratios of silica to alkali metal oxide of different silicates [8]. Consequently. the binders (inorganic) used are alkali silicates and alkyl silicates.1–5. along with the quarternary ammonium silicates have been reported to be suitable [8]. 1. Table 1 Ratios of silica to alkali metal oxide in alkali silicates [8] S. These coatings are generally sub-classified into baked. 2. The ratio of silica to alkali metal oxide. the zinc particles can provide only a small amount of galvanic protection limited to the amount of free zinc in the coating formulation [4]. The effect of ratio of silica to alkali metal oxide on coating characteristics has been shown in Table 2.5:1 are water-borne due to solubility of the used alkali metal oxide in water. zinc particles can be encapsulated by the organic binder. the silicates based on alkali metals such as sodium. in the zinc-rich ‘inorganic’ coatings (commonly referred to as zinc silicate coatings). In addition. Baked coatings These are the coatings which require heating to convert the coating films into water insoluble form. have been given in Table 1. They can provide an unmatched protection against corrosion for steel structures exposed to temperatures up to 400◦ C [2]. Silicate binders for inorganic paint coatings Inorganic paint coatings based on silicate binders can be classified [6] as shown in Fig.2 G. No. Alkali metal silicates are relatively simple chemicals. and between the zinc particles in the film and the steel substrate is required [3]. in addition to the type of alkali metal. This zinc silicate matrix is electrically conductive and chemically inert [2]. post-cured and self-cured coatings. These coatings are characterised by their extreme hardness and suitability for application over an acid-descaled surface. Inorganic zinc silicate coatings are included in the category of high performance coatings [7]. bases like ammonia. Acid catalysed coatings In these type of coatings.2. To introduce binding ability. even a small amount of water will cause the silicate to gel. so that a gel can form from the resulting ethyl silicate hydrolysate.5. Alkyl silicate binders Alkyl silicates such as ethyl silicate. methyl silicate etc. but their higher cost tends to restrict their use at the present time.1. This development has led to the use of inorganic zinc coatings on large field structures. they acquire sufficient binding ability. These coatings are formulated mainly on sodium silicate having higher ratio of silica to sodium oxide. Acid hydrolysis procedures are commonly employed for the production of paint media. Of the cheaper types. The actual binding agent is this gel [33]. 2. 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.3. Alkali catalysed coatings For the hydrolysis of ethyl silicate. one of the commercial forms of ethyl silicate (popularly known as ethyl silicate-40) as solution in organic solvent(s) is most commonly employed.2. are the acids which are used as catalysts. Excellent curing rates can be achieved with some lithium silicates. sulphuric acid [28. On the basis of the type of catalyst used for the hydrolysis. Coatings based on acid catalysed binder are mainly two-component systems. further higher ratio alkali metal silicates have become available. the liquid component can remain stable for an indefinite period of time. and the product thus has a finite shelf life. rapid curing may be achieved under most conditions.G. it is necessary to use lithium silicate with lithium oxide to silica ratio as 1:2 to 1:8. No. and the liquid component of these coatings gel in a period of 6–12 months. Self-cured coatings With further advances in silicate technology.29]. these coatings can be sub-classified as follows. One of the greatest drawbacks of this system is related to the fact that in basic conditions.2. 1 Ratio of silica to alkali metal oxide Higher Effect on coating characteristics Higher Higher Higher Higher Higher Higher Higher Higher Higher Higher the the the the the the the the the the 3 viscosity of the solution drying speed of the film curing speed of the film susceptibility to low temperature chemical resistance of the coating films specific weight of the solution solubility in water pH value of the solution susceptibility to water adhesion and binding power 2 Lower 2. Hydrolysis under alkaline conditions normally results in fairly rapid gelation. Hydrolysis of ethyl silicate Ethyl silicate. ammonium hydroxide. These coatings are available in the market as single-pack and two-pack systems. In single-pack system. 2. can be used as binders for the formulation of solvent-borne coatings. However. 2.3. the period over which the partially hydrolysed silicate remains stable is limited. Several . / Progress in Organic Coatings 42 (2001) 1–14 Table 2 Effects of ratio of silica to alkali metal oxide on coating characteristics S. Acids or alkalis are used to catalyse the hydrolysis reaction. 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. sodium hydroxide and some amines are generally used as catalysts [2]. and instability is to be avoided. Alkali catalysed hydrolysis procedures are generally preferred when ethyl silicate is to be used for the production of refractories. the hydrolysis of ethyl silicate is carried out under alkaline or acidic conditions.1. 2. has no binding ability [32].2. are used [8]. phosphoric acid [30]. The problem associated with one-pack system of this type is that zinc chemically reacts with the acid catalyst present in the binder system. by itself. which provide hydroxyl ion in the form which is non-reactive with organic polysilicate until they are exposed to moisture. and from the application equipment. it is necessary to hydrolyse ethyl silicate by treating it with water. remedial steps must therefore be taken to exclude all water at the manufacturing stage. formic acid [31]. amines. based on high ratio potassium silicates with potassium oxide to silica ratio ranging from 1:2 to 1:5. potassium silicate is preferred. etc. If water is excluded. due to which pH of the system increases. Hydrochloric acid [10–27]. Alkyl silicates.2. Lithium silicate based coatings are preferred for use in food areas. However. which causes gelling in the container.. 3. Usually. If further higher ratios are required. Parashar et al. Reliable self-curing coatings are available today.1. as such. To avoid this problem. 3. The relatively short shelf life of some acid hydrolysed ethyl silicate solutions can cause difficulties in their use. which can affect the hydrolysis of ethyl silicate and the formulation of ethyl silicate binders.2. 3.16 parts (by weight) 12 N HCl 6 ml 0.15 parts (by volume) 2000 parts (by volume) Nil Quantity of acid 18 parts concentrated HCl (specific gravity 1. 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].1. These factors are discussed hereunder one by one. hydrolysed ethyl silicate solutions have become available commercially.6 parts ethanol (by weight) procedures for the acid hydrolysis of ethyl silicate are available [34–36].42] taken from the patent literature have been given in Tables 3 and 4.18) 50 parts concentrated HCl (specific gravity 1.1 N HCl Quantity of solvent 50 parts 160 p industrial methylated spirit 8000 parts isopropanol 140 parts isopropanol/ water azeotrope Quantity of alkyl silicate (second lot) 11 parts ethyl silicate-40 2000 parts methyl silicate (50% SiO2 ) 130 parts isopropyl silicate (38% SiO2 ) . often referred to as prehydrolysed ethyl silicate solutions. 1 2 3 Quantity of ethyl silicate-40 (first lot) 14 parts 6000 parts 340 parts Quantity of water 2. Table 4 Typical compositions for two-stage procedures for the hydrolysis of ethyl silicate S. Out of many possible ethyl silicate hydrolysis procedures. while those in which ethyl silicate is added usually after a specified temperature rise or time interval are termed as ‘two-stage’ procedures. 3. Effect of quantity of water Quantity of water and the quantity of acid catalyst used for partial hydrolysis are the most important factors for formulating acid catalysed ethyl silicate binder systems. Factors governing the formulation of ethyl silicate binders There are some important factors. These solutions. thus making less silica available for binding than required. 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. Excessive quantity of acid will result in accelerated condensation of silanol with silanol (≡SiOH) groups or with alkoxy groups (≡SiOR) resulting in reduced shelf life of the binder system [49]. are of particular interest as paint media. Water to be used in hydrolysis must be calculated after subtracting the quantity of water (if any) going into the paint formulation from the extender pigments and the solvents used in the formulation. solvent.1. If ethyl silicate is treated simultaneously with a glycol monoether for alcoholysis and water for hydrolysis. hydrolysates which gel rapidly can be obtained. 1 2 3 4 Quantity of ethyl silicate-40 6l 1368 parts (by weight) 1. Effect of quantity of acid Less than optimum quantity of acid can result in silica precipitation. acid and water for their preparation. Less than optimum quantities of water can result in an uncured film lacking hardness and film integrity [49]. Ethyl silicate hydrolysates having a long storage life can be obtained by careful choice of the proportions of ethyl silicate.41.4 G. one can be considered on its merits. Excessive water in the formulation can lead to gelling of the binder system in the cans or very poor application properties and gelling of mixed paints in the application equipment. Some other workers [44–46] also prepared binder systems by using pure ethyl silicate or ethyl silicate-40 of different properties. / Progress in Organic Coatings 42 (2001) 1–14 Table 3 Typical compositions for single stage procedures for the hydrolysis of ethyl silicate S. Typical compositions for the single stage [37–40] and two-stage procedures [37. No. Some two-stage procedures require two types of organic silicates. Mcleod [43] prepared silicate binder system by hydrolysing ethyl silicate-40 in butyl cellosolve in the presence of acid catalyst with 5% (part basis) water at 140◦ C.1.1 N HCl 0. No. Hydrolysis procedures in which a specified quantity of ethyl silicate is added at the start of the reaction are termed as ‘single stage’ procedures. Acid hydrolysates of ethyl silicate eventually set to a gel on standing. As a result of the development of methods for preparing ethyl silicate hydrolysates having a long storage life.6 l 45 parts (by weight) Quantity of water 2l 138 parts (by weight) 100 ml 53 parts (by weight) Quantity of acid 50 ml concentrated HCl 0.1. Parashar et al.1 part (by weight) 37% aqueous HCl Quantity of solvent 4 l ethanol 1517 parts ethanol (by weight) 840 ml 640 p industrial methylated sprit 49. a hydrolysate with a long shelf life is obtained [48]. This hydrolysate can be successfully used as a paint medium. respectively. Generally 80–90% hydrolysis of the ethyl silicate is carried out for the binder preparation [2].18) 40 parts 0. This is particularly true for acid catalysed hydrolysis where the presence of the alcohol maintains an equilibrium. Stoichiometry of binder preparation The overall stoichiometry of hydrolysis is given in the following equations. (7). Acid catalysed reactions First. whereas methyl silicate hydrolyses readily. curing properties of alkyl silicate coatings can be tailored [50]. (1) This intermediate species then reacts with the silanol. (6) 3. If the alcohol is highly volatile. (8) .1. Effect of size of alkyl group The rate of hydrolysis reaction is greatly affected by the size of alkyl group of the organic silicates.G. reversible reaction will be forced in the direction of the hydrolysis.2. oxygen of the silanol group is protonated. as shown in Eq. Total hydrolysis of pure ethyl silicate [2] can be given as shown in Eq. N-hexyl silicates. Parashar et al. 3. Chemistry of ethyl silicate binders Prepared ethyl silicate contains some silanols and alkoxy groups. Reaction with zinc pigments The silanol groups of hydrolysed ethyl silicate react with zinc and form a zinc silanol heterobridge.3.2.g. (4) 3.2. With proper selection of the alkyl group.3. Some of their reactions are as follows. 3. pot life of the system is short.3. The larger alkyl groups can act as a steric barrier to hydrolytic attack. Effect of pH on stability When pH of the system is low. e. the rate of formation of water is high and due to fast dehydration. A second effect of the size of alkyl group involves the volatility of the alcohol formed during hydrolysis. (7) Ethyl silicate hydrolysed to ‘x’ degree can be shown by the following equation: (3) When pH of the system is high. and an intermediate species is formed. Thus. bulkier alkyl groups protect the ester much better than the smaller groups like methyl or ethyl. (2) 3. These silanol groups are responsible for chemical reactions in these types of coatings [2]. are difficult to hydrolyse.2. / Progress in Organic Coatings 42 (2001) 1–14 5 3.2. (5) This hetero bridge then undergoes further chemical reactions to form a zinc silicate polymer. which results into the formation of siloxane bond [49]. (1). then the hydrolysed alkyl silicate has long pot life due to the repulsion of –O+ H group with O+ H group..1. 5% water by weight of ethyl silicate-40 is required. The solvent and ethyl silicate are combined and agitated.48(41.66) = 15. care should be taken for the selection of pigments. nSi(OC2 H5 )4 + 4nH2 O → nSi(OH)4 + 4nC2 H5 OH (13) In the absence of alcohol. talc and zinc dust are some of the pigments which can be suitable to formulate ethyl silicate based coatings. (14) contributes 2 mol of water for each mole of ethyl silicate. non-basic and not very reactive. 14.36(85 − 41.6 G. (12). (9).4 4. In general.0 5.6 kg The amount of solvent that must be added to give a final silica content of 18% is calculated from Eq. only 2 mol of water are needed for 100% hydrolysis of the reactants. Parashar et al.4. Equivalent weight = SiO2x (OC2 H5 )4(1−x) = 28 + 16(2x) + 45(4 − 4x) = 28 + 32x + 180 − 180x = 208 − 148x or Equivalent weight = 208 − 1. (14): nSi(OH)4 → SiO2 + 2nH2 O (14) 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. 3. then for 100% hydrolysis.34 − 15. / Progress in Organic Coatings 42 (2001) 1–14 The empirical equation for ethyl silicate hydrolysed to x degree of hydrolysis. mica. Thus according to Eqs. calculate the % hydrolysis in the ethyl polysilicate from Eq. to prepare 85% hydrolysed binder containing 18% SiO2 from commercial ethyl silicate containing 41% SiO2 . the silicic acid formed undergoes polycondensation as given in Eq. Water containing some acid catalyst is added and the mixture is then agitated until the exotherm subsides.48 H (H = %hydrolysis) (9) The concentration of SiO2 in the ethyl polysilicate is equal to Molecular weight of SiO2 × 100 Equivalent weight of ethyl polysilicate or 60 × 100 % SiO2 = 208 − 1. (10).0 (12) For example. (13).6 = 171. If ethyl silicate-40 is used as the raw material. the weight of water to be added can be calculated by Eq.36% by weight of the ethyl silicate used.0 100.3 4. No. (11). SiO2x (OC2 H5 )4(1−x) . The first is reversible as shown in Eq.48 H (10) In order to prepare a binder that is 85% hydrolysed. curing of ethyl silicate involves hydrolytic polycondensation occurring in two steps. Paint compositions based on ethyl silicate binder For the formulation of paints based on hydrolysed ethyl silicate binder.34 . ethoxyethanol. Equivalent weight of ethyl polysilicate = 208 − 1. Weight of water = 0.0 1.3 5. ethoxy ethyl acetate or mixture of these. as below: 41 = 6000 208 − 1. This allows calculation of the amount of water necessary to give a binder of any desired percentage hydrolysis. can be used to derive the equivalent weight of the commercial ethyl polysilicate and its exact degree of hydrolysis. Some typical formulations of these paint systems are given hereunder: Formulation 1 [51] S. isopropanol. The binder is ready for use after 24 h of preparation. (13) and (14). strontium chromate.48(H) H = 41. the total water necessary for 100% hydrolysis will represent 17. only those pigments are suitable which are chemically inert. Ingredient 1 2 3 4 5 6 7 Ethyl silicate (partially hydrolysed) Anti-settling agent (Bentone 38) Talc Toluene Isopropanol Cellosolve Zinc dust Amount (%) 20.0 60. Equivalent weight can be obtained by substituting atomic weights in the empirical formula.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 = − weight of ethyl polysilicate % SiO2 desired −weight of water added Because Eq.66 This allows the calculation of the equivalent weight of the ethyl polysilicate using Eq.4 kg 18 The solvents that can be used are ethanol. because with this binder system.66) = 146. Thus lead chromate. Particularly good protection against high temperature and rust can be obtained if zinc dust is used as the pigment. = ( 6000 ) − 146. Initial hydrolysis of silicon ester monomer produces silanol groups. the relatively high concentration of 98–99.0 4.9 17. Molar ratio of water to alkoxide. whereas full hydrolysis can lead to silicic acid monomer. 5 parts water. Mechanism of the hydrolysis reaction Alkyl silicates are not water soluble in nature. 1 2 3 4 5 6 7 8 Ingredient 40% ethyl silicate liquid 30% ethyl silicate liquid Zinc powder Zinc flakes Ferro phosphate Crystalline silica Amorphous silica Wetting agent Amount (%) 26.2% 94–97% 3–6% 0. Thus. hydrolysis reaction takes place through electrophilic substitution and in basic condition. this reaction corresponds to SN2 mechanism. reactivity of the tetrahedron towards electrophilic attack is enhanced by an increase in electron density around silicon. one part 5% HCl.7% maximum 0. 3.1. 1 2 3 4 5 Ingredient Bindera Powdered zinc (spherical particles) Titanium dioxide (rutile) Ilmenite Aluminium Amount (%) 19.5.5. pH of the solution is also an important factor which governs the rate of hydrolysis reaction and condensation of the hydrolysed product. When pH of the aqueous solution is 2. Nature of the alkyl group. Chemistry of hydrolysis reaction of alkyl silicates Hydrolysis of alkyl silicates is influenced by various factors [53] such as. However. hydrolysis is carried out in the form of solution. At pH below 2.5 19.1% maximum 0. No. whereas base catalysed hydrolysis reaction precipitates hydrated silica powders.8 39. This electron density increases due to the size of substituent groups. At lower pH. No.04% maximum Nil Nil 4% maximum counter) 6–10 microns ≤0. silicate particles are negatively charged and at pH above 2. acid catalysed reactions yield alcogels.1% maximum 0. Concentration of each species in the solution or reaction mixture.0 Binder can be prepared [52] by using 50 parts ethyl silicate-40. 5. the silicate particles are not electrically charged. specific gravity a 4.1 6. Specifications of the zinc dust commonly used in the ethyl silicate based paint formulations are given hereunder [4].0 g/cm3 (iv) Dispersibility Should disperse satisfactorily in a high speed disperser .5 3. and by stirring the contents for 5 h at 40◦ C. the hydrolysis proceeds through nucleophilic reaction. 2. Specifications of zinc dust (i) Composition Total zinc Metallic zinc Zinc oxide Lead Cadmium as (CdO) Volatile Moisture and volatile Iron (ii) Coarse particles Retention on 100 mesh Retention on 200 mesh Retention on 325 mesh (iii) Particle size distribution (Coulter Medium particle size Specific surface Spherical particles.3 100. This acid is not stable and condensation of silanol groups occur leading to polymer formation before all alkoxy groups are substituted by silanol groups. Reaction temperature. In general. Above or below this pH. In case of alkyl silicates.17 m2 /g 7.9 13. 4. In addition to these influencing factors. 43.2 0.5 100.G.6 32. In acidic condition. Rate of hydrolysis increases with increase in pH of the solution. because of which a mutual solvent is needed to hydrolyse it. 4. Susceptibility to nucleophilic attack increases with decrease in bulky and basic alkoxy groups around the central silicon atom. alkoxy groups remain unaffected because silicate particles are not charged at this pH. they can be attacked by water. when pH of an aqueous solution is quite acidic and the silicate particles get negatively charged.4 0. hydrolysis takes place through SE2 mechanism and at higher pH.3 17. 1. they are positively charged.2% maximum 0. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14 7 Formulation 2 [56] S. When pH of the solution is ≈2. However. and ethyl alcohol and isopropyl alcohol are generally used as the mutual solvent. Condensation polymerisation reactions proceed with an increase in viscosity of the alkoxide solution until an alcogel is produced. Nature of the solvent used.2 parts isopropyl alcohol.5.0 Formulation 3 [52] S. nucleophilic attack is sensitive to electron density around the central silicon atom.5. The mechanism then corresponds to an electrophilic substitution in which an (H3 O)+ hydronium ion attacks the oxygen of one of the alkyl groups. thus creating oxo ligands. Condensation of alkyl silicates In acidic conditions. The anion then recombines with a proton so as to form an alcohol molecule. Lewis base such as (20) Rate of condensation reaction depends on the second step of the mechanism and is proportional to the concentration of the protons. ammonium hydroxide. they can deprotonate the OH ligands of cations. base is a necessary catalyser. in this specific case. the coordination number of Si increases.2. It can be noted that the Si of the intermediary complex of this mechanism is either tetra or penta coordinated. The silanol group (≡SiOH) resulting from the hydrolysis of silicon alkoxide can be converted to oxo ligand. / Progress in Organic Coatings 42 (2001) 1–14 protons catalyses the hydrolysis reaction. silanol groups are protonated which increases the electrophilic character of the surrounding silicon atoms. etc. Hence condensation is a slower transformation .8 G. The rate of hydrolysis decreases as the length of alkyl group increases. this protonated silanol combines to another silanol group while liberating a (H3 O)+ ion. Mechanism of condensation reaction is as given below: (15) In alkaline conditions. In first step. which form acidic oxides. as siloxane bond. and the reaction can be as given hereunder: (17) Traces of water vapour can also hydrolyse metal alkoxides thus transforming them into oxi-alkoxides. The two silicon atoms of the resulting polymer are then linked through an oxo bridge called. at least one OR or OR− ligand must leave the intermediary complex formed by silicon. another more complex mechanism is also proposed which involves two intermediary complexes. Such a hydrolysis follows a reaction of the following type: (18) 4. For this reaction. As a consequence. and steric strain is also an important factor. can effect this type of reaction. Since δ(OR)complex < δ(OR)alcohol . Parashar et al. The rate of reaction depends as much on the concentration of H3 O+ as on the one of the alkoxides. silicate particles are positively charged and OH− anion attacks the alkoxide through an SN2 mechanism in order to form the silanol group. The mechanism is consequently an SE2 . The reaction mechanism is as given below: sodium hydroxide. The mechanism of the reaction has been shown below: (19) (16) For this reaction. Since Lewis bases are strong nucleophiles. silicon alkoxide condenses through a two step mechanism which corresponds to SN2 type of mechanism. In the intermediary complex of this mechanism. (24) The ionic zinc then reacts with silanol groups on the silicate molecules in the silicate gel structure. n-Bu4 NF and NaF. As discussed previously that the principal raw materials used for the preparation of vehicle of inorganic zinc coatings are potassium silicate. These coatings cure differently than that of the alkali silicate based inorganic zinc silicate coatings. Silanols are protonated more easily when they are present at the end of the polymer chain. for instance.G. A simple distinction is that the water-borne alkali silicate coatings lose water during the initial curing stages. colloidal silica solutions and ethyl silicate. / Progress in Organic Coatings 42 (2001) 1–14 9 than hydrolysis. At this point. (22). In basic conditions. most of the solvent is lost by the evaporation which leads to the concentration of the zinc ethyl silicate mixture. This carbonic acid causes ionisation of some zinc on the surface of zinc particles. In general. This insolubilises the coating and provides its initial properties. coating is uncured and sensitive to moisture or water. the curing of ethyl silicate involves hydrolytic polycondensation reaction. DMAP (dimethyl aminopyridine). hence the denser solids are obtained. undergoes polycondensation reaction as shown in Eq. as shown below: H2 O + CO2 → H2 CO3 (23) (21) Overall basic catalysts. This mechanism involves two intermediary complexes with penta coordinated silicons. (15). (22) The moisture and carbon dioxide in the air react with each other to form carbonic acid. first of all. lithium silicate. accelerate condensation and alcohol molecules are better leaving groups than water. which occurs in two steps. The product of this reaction. During the curing process. including Lewis bases. quite similar ultimate reactions occur within the coating and on steel surface during film curing [2]. condensation rate is not only proportional to the concentration of OH− anions but also superior to that of hydrolysis. Even with all these different starting materials. in the absence of alcohol. This reaction is as follows. . The first reaction is reversible which has already been given as Eq. Furthermore. The slightly acidic water helps to hydrolyse the prehydrolysed binder completely to yield silicic acid as given hereunder: 5. whereas the solvent-borne alkyl silicate coatings absorb water with subsequent release of ethyl alcohol initially [6]. In basic conditions. Parashar et al. Mechanism of film curing of inorganic zinc silicate coatings Hydrolysed ethyl silicate based zinc-rich coatings are self-curing in nature. they build siloxane bridge by another SN2 mechanism. since the reticulation inside the silicon polymers is more developed than when conditions for acidic catalysis are used. Efficient Lewis bases include. 3. 6. 2. Coatings can withstand temperature up to 400◦ C. 4. neutron bombardment and other forms of radioactivity [58]. 5. barbutaric acids. These systems are applicable in relative humidities between 20 and 95% and tolerate slight surface moisture [58]. Along with these advantages. 4. They can be applied by conventional spray equipment or by brush [2]. the reactions will be those that take place over a long period of time and depends on the characteristics of the environment in which zinc coatings are placed. Humidity and carbon dioxide create a very mild acidic condition that results in continued hydrolysis of the vehicle and ionisation of the zinc. These coatings offer excellent corrosion resistance due to the involvement of conductive matrix in the protection mechanism. These coatings generally exhibit more pinholing and bubbling upon top coating as compared to organic zinc coatings. and also by treating the substrate with an aqueous solution of a base over which they are applied [56]. 1. and/or. In wet condition. They are not recommended for immersion service in fresh or salt water. They have good chemical resistance and they remain unaffected by organic solvents [5]. 3. 8. as in cured films. These coatings have excellent weather resistance. 9. Some other advantages of these systems are given hereunder: 1. This bonding prevents the creepage of moisture and lifting of paint film seen in organic coatings.8]. These cured films have metal like hardness and these films remain unaffected by radiation including X-rays. They have poor resistance for acidic or alkaline conditions outside the pH range 5–10. they have some limitations also such as: 1. 6. They can withstand rain just after half an hour of the application [2]. Zinc ions diffuse deeper and deeper into the gel structure until there is a zinc silicate cement matrix formed around each of the zinc particles binding the coating together and to the steel surface. Inorganic zinc-rich paints offer excellent adhesion because the binder chemically reacts with the underlying steel surface [2. they are not recommended beyond 60◦ C due to rapid depletion of zinc. alkali metal salts of thio acids.10 G. some reaction between poly silicic acid and the iron surface also takes place to form a chemical bond. Film performance of ethyl silicate based zinc-rich coatings Uncured films of zinc-rich coatings are rough and irregular while fully cured zinc-rich paint films are grey in colour and textured in nature [57]. / Progress in Organic Coatings 42 (2001) 1–14 Ethyl silicate based binders can be cured by IR radiation [54]. Such an excellent adhesion prevents under cutting of coating by corrosion even after 10 years of exposure. Parashar et al. They have quick drying properties. As a matter of fact.3-dicarbonyl compounds [55]. They will protect steel under insulation in the critical temperature range 0–66◦ C. round globules of zinc are present. From this point on. 7. (25) At this time. 2. (26) . these are the most corrosion resistant coatings available today [2]. These films are weldable at a low dry film thickness and do not have adverse effect on welding and gas cutting [49]. 10. However. Hare [64] reported that upgraded corrosion resistance and reduced cost of the system can be obtained by using flake zinc in combination with mica and zinc potassium chromate. on ethyl silicate based paints having a metallic zinc content of 75 and 60% (Table 5). Extender pigments The metallic zinc content in the dry film is a very important parameter to be emphasised in the technical specifications of zinc-rich paints. cure rate may be reduced greatly. ground muscovite mica is also used widely. especially at temperature below 10◦ C and where the films of high thickness are involved [59]. 7. Theoretically. Because of inertness and refractoriness of silica. In order to obtain contrast between sand blasted steel substrate and the paint. They are higher in cost as compared to the conventional coatings.1. zinc flake produced far more current than was necessary to protect the steel cathode. etc.G. which affect performance of the applied ethyl silicate zinc-rich coatings. The lamellar nature of the flake would ensure a significantly enhanced electrical contact area.2. . these systems are heat stable and durable. flexibility. talc etc. for the same metallic zinc content in the dry film. which leads more porous and permeable films due to which the electrical contact between zinc particles and steel substrate improves. These factors are discussed hereunder one by one. In a 25 micron film thickness. In inorganic zinc silicate coatings. Factors influencing film performance There are various factors. [60]. 8. According to the most technical specifications. Under cathodic protection. 6. / Progress in Organic Coatings 42 (2001) 1–14 11 5. inorganic zinc-rich primers have superlative record. It is also observed in the mica modified formulations that they produce reduced amount of zinc corrosion product. mica. Coatings are not flexible. Hence reduction of zinc reactivity by the addition of small quantities of inhibitors such as potassium chromate along with mica extender significantly improved performance effectiveness. Particle shape and size of zinc pigment Zinc is most commonly used as zinc dust in ethyl silicate based zinc-rich coatings. These factors contribute to the improvement of paint performance from the galvanic point of view. [62] verified that the chemical nature of the binder and the zinc particle size are also very important. Del and Giudice [61] and Pereira et al. So the addition of auxiliary pigments should be controlled carefully in order to not impair the physical and chemical characteristics of the films. some manufacturers use colouring pigments such as chromium oxide and iron oxide. Some other factors such as mechanical properties viz. reactivity of zinc flake in salt fog environments was found to be too great to provide the sort of long-term performance profile required. Experimental studies have been carried out by Fragata et al. zinc dust particles having a particle diameter of about 10 times the thickness of a zinc flake platelet would require much more minimum primer film thickness for a given degree of protection than would the flake do. The results showed that addition of fillers agalmatolite (A) and barytes (B) to the paints with 60% metallic zinc in the dry film improves their behaviour. field exposure and electrochemical tests. are also important. It is important to mention that effectiveness of the zinc-rich paint does not depend solely on electrochemical factors. minimum content of metallic zinc in the dry film required is 75% (by weight) for zinc-rich paints based on ethyl silicate. The major problem with this system is that the cure rate of alkyl silicates is dependent upon relative humidity. It was theorised that a flat plate zinc particle can be utilised advantageously in several ways. which indicates the general reduction in zinc 7. and also to cause blistering of the subsequent coats. [63]. Studies have been carried out by Hare [59] using zinc flakes in organic zinc-rich primers and ethyl silicate zinc-rich primers. In the paints which contain fillers. Panels coated with these paints were subjected to salt spray. On the basis of experimental studies. the PVC/CPVC ratio is higher. as many as 20 zinc flake platelets might be superimposed as compared to approximately three rows of spheres of zinc dust. 7. and was soon exhausted. Apparently. water. In fact. 7. Performance comparisons between zinc dust primers and zinc flake primers have shown that chromated zinc flake systems outperform zinc dust primers (of same vehicle type) in both salt fog and bullet hole studies. Another reason for the popularity of zinc silicate primers is their capacity to offer longer anticorrosive protection at lower dry film thickness and at lower zinc loading levels [2]. In this respect. It is observed by Land quest that metallic zinc content in the dry film is not only a factor determining the performance of this kind of paints while Fragata et al. and because of technical reasons some other manufacturers use extender pigments such as barytes. Salt spray results for 75% zinc content up to 2060 h of exposure did not show any influence of fillers. For the same metallic zinc content in dry film the solids balance can be made using only the binder and zinc dust or partial substitution of binder with auxiliary pigments. Parashar et al. but a properly cleaned (sand blasted) surface is a must for these coatings. cohesion. In dry climate. organic binder based zinc-rich primers have tendency to degrade. alkyl silicate primers have somewhat better tolerance for slightly poorer surface preparation than the alkali silicate based paints. These systems form coherent adhesive coating of silica which results due to hydrolysis and gelation of the ethyl silicate binder. Zinc particles are generally spherical in shape. 0 660 Zn75 ZnA75 75. 7. Harbour structures. magnesium. It was also reported that spray coating methods yielded results which were not readily reproducible and gave both poor and good curing results. aluminium. which is a refractory conductive compound. while flow coating methods yielded reproducible results conforming to manufacturers’ data sheets under the conditions tested. only cadmium with zinc and inhibitors gave results comparable to normal zinc-rich primers. [65]. In ethyl silicate zinc-rich coatings evaluation of this extender has been carried out by Filire et al. nozzle sizes. located in the highly humid tropical areas of Indonesia. these coatings find applications in various critical fields [66].0 Main components of dry film Ethyl silicate Zinc dust Ethyl silicate Zinc dust Agalmatolite Ethyl silicate Zinc dust Barytes Ethyl silicate Zinc dust Ethyl silicate Zinc dust Agalmatolite Ethyl silicate Zinc dust Barytes Time (h) necessary for appearance of red corrosion in scratch (ASTM B-117) 460 740 ZnB60 60. Results of the test carried out by them show that it is possible to replace up to 25% of zinc with minimal decrease in the ability of the coating to provide cathodic protection to the steel substrate. Out of various extenders used in ethyl silicate based zinc-rich paints. Parashar et al. Zinc appears to be consumed more efficiently in the presence of Fe2 P with the result that improved corrosion protection is obtained with lower initial zinc content while a greater fraction of the zinc initially present remained unoxidised after a given period of time. inadequate mixing and/or settling out of the zinc portion. and this will be dictated by spray equipment and technique.12 G. The weldability of primers is also improved by the use of Fe2 P. Others have proved to be inferior. various other conductive extenders have been used such as cadmium.0 2060 corrosion. the best results have been obtained from di-iron phosphide (Fe2 P). Many hundreds of drilling and production structures have been coated with inorganic zinc silicate coatings. Problems of toxic fumes during welding. 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. 8. Areas of applications for zinc-rich inorganic silicate paints Because of the excellent corrosion protection offered by these coatings to steel. The corrosion conditions encountered by off-shore petroleum production platforms are the most severe. Some of their application areas are given hereunder: 1. Electrical conductivity is reduced in this case not only by the resistance of the vehicle cover but also by the mica laminate.0 60. however. Compositions of some ethyl silicate vehicles formulated with higher concentration of Fe2 P lead to abnormally high zinc corrosion products. / Progress in Organic Coatings 42 (2001) 1–14 Table 5 Salt spray results of ethyl silicate coatings pigmented with zinc dust and fillers Paint designation Zn60 ZnA60 Metallic zinc content in the dry film 60. the inclusion of Fe2 P extender does not disturb the marked capability of ethyl silicate zinc-rich paints to develop barrier coats. Besides these. Singapore and the Persian Gulf to the United . Application techniques Application techniques and relative humidity also have influence on the curing of inorganic zinc ethyl silicate based primers [57]. Of these. and also by spray parameters such as air pressure. This effect is thought to be related to the control of current transfer that such non-conductive extenders might allow.3. Further.0 2060 2060 ZnB75 75. The experimental results also revealed that curing is affected by incorrect mix ratio of base to filler. precludes the use of cadmium in these coatings. etc. distance from the surface. This explains the greater ability of silicate coatings to provide cathodic protection to the steel substrate.0 75. iron and carbon along with zinc dust. 1 centre tank in Utah standard. C. B. Surface Coatings. Chem. etc. The films of these coatings. Japanese Patent No. and hence provide unmatched adhesion to restrict corrosion creepage. Alkyl silicate inorganic zinc-rich primers are used in nuclear applications for many reasons. 9. 7. 23 (1993) 185. As on today. N.H. mainly at the steel plate manufacturer’s factory before shipping to the job site. 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 is still well protected by the original single coat of inorganic zinc silicate coating. 90 (1979) 123285n. Parashar et al. 81 (12) (1998) 596. be expected that ethyl silicate based coatings will find wider and wider application in further more challenging areas in future. [33]. These primers are applied at 3. Cavalcanti. unpigmented hydrolysed ethyl silicate binder is also used for various purposes such as stone preservation. Bridges. Paintindia 33 (3) (1988) 19. One of the major uses of inorganic zinc coatings has been in the lining of ship tankers. Encyclopedia of Chemical Technology. D. etc. Coat. Rieber. 471. In addition. S. Gettwert. 1984. M. E. T. Abstr. O. Kirk-Othmer. than organic coatings which invariably produce films composed of organic polymers. Vol.0 mil minimum thickness. after 11 years approximately. Coat. and with the exception of holidays or missed areas in original application. Minoru. Rani. J.H. are some examples of full protection provided by inorganic zinc silicate paints over many years of continuous exposure. Surf.S. Laurin. Rotenberg. Jagannath. Kusuhara. US Patent No.939. Tanaka. the golden gate bridge on the original Morgan Whylla pipeline.L. / Progress in Organic Coatings 42 (2001) 1–14 13 States Gulf coast and extending into the Arctic areas of Alaska and the North Sea. Abstr. O. This bridge was coated in 1956 with the open grill work being the most difficult part of the structure to fully protect it. For the formulation of inorganic coatings. 2. in view of environmental aspects. 2. One interesting application of inorganic zinc silicate paints is the protection of nuclear power plants. 4. These coatings are unaffected by -rays or neutron bombardment. W. Y. 91 (1979) 58881h. Takahashi. New York. etc. Tanaka.439 (1979). 91 (1979) 22611f. Therefore. This tank was inspected in 1966. Prog. therefore. 1984. Cuffe. p. like off-shore structures. Nuclear power facilities. One of the oldest documented applications of inorganic zinc coatings is the No. The final (cured) films of ethyl silicate based coatings are composed mainly of silica. 4. Ferraz. or silica and zinc.173. ethyl silicate based coatings can be successfully used for high performance applications in critical areas such as harbour structures. Y. Naito. edges. The steel surface within the reactor building requires coating with a 40-year expected life. M. Paintindia 49 (7) (1999) 31. crude oils. where most organic coating films fail. Nirvan. there was no further rust or loss of metal in the tank. S. Chapman & Hall.K. despite the fact that former ones produce solvent-borne compositions.R. Chem. nuclear power plants. G. if zinc is used as a pigment.H. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] B. R. 3. Paintindia 48 (4) (1998) 47. cured films of ethyl silicate based (inorganic) coatings are considered better. In fact.P. Ethyl silicate based coating films are self-curable at room temperature in the presence of adequate atmospheric moisture. lubricants. whereas alkali metal silicate based coatings are water-borne. P. Bonarius. perhaps so since many bridge structures are formed from structural steel shapes. edible oils and solvents like ketone esters. Abstr. Chem. Conclusion [11] Surface coatings based on inorganic binders can be successfully used as primers for the effective protection of steel against corrosion. OCCA Australia. 92 (1980) 24414p. p. provide excellent inter-coat adhesion to the subsequent coat. Mukherjee. alkali metal silicates such as sodium. J. and lithium silicates and alkyl silicates such as ethyl silicate are commonly employed as inorganic binders. The inorganic coatings based on hydrolysed ethyl silicate. Marphatia. are extremely vulnerable to corrosion. Other bridges such as Baleman bridge in Tasmania which was coated prior to installation. Imahigashi. applied alone or overcoated for additional protection and for safety colouration. 78. being silica based.140. The films. Japanese Patent No. Ramirez. Org.M. 6. Int. for the surface treatment of concrete to reduce dusting.340 (1979). P. Japanese Patent No. Hare. However. S. On account of these attractive features.490 (1979). 484. Paintindia 31 (7) (1981) 3. being rock-like hard and quite rough. Ishii. Paintindia 38 (8) (1988) 31. [66]. Vol. Tank coatings. This was applied in 1954 to a previously corroded tank surface. films of zinc-rich ethyl silicate based coatings protect the substrate (steel) by providing much more effective cathodic protection than that provided by zinc-rich organic coating films. with all the corners. primarily for transporting refined fuel. 7. Ethyl silicate based binders have proved to be superior to alkali metal silicates in overall performance. Further. crevices and surface defects inherent in such shapes.332 (1979). 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