[123doc] - Ethyl-silicate-binders-for-high-performance-coatings.doc

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, 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 ∗

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 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

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

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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 particles can be encapsulated by the organic binder, and hence the zinc particles have restricted electrical contact. Consequently, 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 category 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 chemicals, 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 characteristics 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 silica 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.

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3

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 2 3

Sodium silicate Potassium silicate Lithium silicate

SiO2 :Na2 O SiO2 :K2 O SiO2 :Li2 O

2.4–4.5:1 2.1–5.3:1 2.1–8.5:1

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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 Higher Higher Higher Higher

the the the the the

viscosity of the solution drying speed of the film curing speed of the film susceptibility to low temperature chemical resistance of the coating films

2

Lower

Higher Higher Higher Higher Higher

the the the the the

specific weight of the solution solubility in water pH value of the solution susceptibility to water 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-

G. Parashar et al.However, / Progress in borne coatings. 14 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

Organic Coatings 42 (2001) conditions, even a 1– 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 . A c i d c a t a l y s e d c o a t i n g s In these type of coatings, rapid curing may be achieved under most conditions.

5 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 twocomponent 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 . H y d r o l y s i s o f e t h y l s i l i c a t

e6 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

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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 2 3 4

6l 1368 parts (by weight) 1.6 l 45 parts (by weight)

2l 138 parts (by weight) 100 ml 53 parts (by weight)

50 ml concentrated HCl 0.16 parts (by weight) 12 N HCl 6 ml 0.1 N HCl 0.1 part (by weight) 37% aqueous HCl

4 l ethanol 1517 parts ethanol (by weight) 840 ml 640 p industrial methylated sprit 49.6 parts ethanol (by weight)

G. Parashar al. / Progress procedures for the acid hydrolysis of ethylet silicate arein 14 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 hydrolysing 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 twostage 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 storage 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

Organic Coatings 42 (2001) 1–

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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 silicate 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 formulating 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 formulation 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 accelerated 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)

6000 parts

2000 parts (by volume)

50 parts 160 p industrial methylated spirit 8000 parts isopropanol

11 parts ethyl silicate-40

2 3

340 parts

Nil

18 parts concentrated HCl (specific gravity 1.18) 50 parts concentrated HCl (specific gravity 1.18) 40 parts 0.1 N HCl

140 parts isopropanol/ water azeotrope

2000 parts methyl silicate (50% SiO2 ) 130 parts isopropyl silicate (38% SiO2 )

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.,

(4) 3.2.3. Reaction with zinc pigments

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].

The silanol groups of hydrolysed ethyl silicate react with zinc and form a zinc silanol heterobridge.

3.2. Chemistry of ethyl silicate binders Prepared ethyl silicate contains some silanols and alkoxy groups. These silanol groups are responsible for chemical reactions in these types of coatings [2]. Some of their reactions are as follows.

(5) This hetero bridge then undergoes further chemical reactions to form a zinc silicate polymer.

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]. (6) 3.3. Stoichiometry of binder preparation

(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).

3.2.2. Effect of pH on stability 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. (7) Ethyl silicate hydrolysed to ‘x’ degree can be shown by the following equation: (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. (8)

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The empirical equation for ethyl silicate hydrolysed to x degree of hydrolysis, SiO2x (OC2 H5 )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 = SiO2x (OC2 H5 )4(1−x) = 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 equal to Molecular weight of SiO2 × 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 ) − 146.34 − 15.6 = 171.4 kg 18 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). nSi(OC2 H5 )4 + 4nH2 O → nSi(OH)4 + 4nC2 H5 OH (13) In the absence of alcohol, the silicic acid formed undergoes polycondensation as given in Eq. (14): nSi(OH)4 → SiO2 + 2nH2 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%

% SiO2 =

208 − 1.48 H

G. Parashar et al. / Progress Coatings 42 (2001) 1– (10) in Organic hydrolysis of the reactants. 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 = 0.36(% 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

(12)

For example, to prepare 85% hydrolysed binder containing 18% SiO2 from commercial ethyl silicate containing

41% SiO2 , calculate the % hydrolysis in the ethyl polysilicate from Eq. (10), as below: 41 =

6000 − 208 1.48(H)

H = 41.66 This allows the calculation of the equivalent weig t of the ethyl polysilicate using Eq. (9). Equivalent weight of ethyl polysilicate = 208 − 1.48(41.66) = 146.34

Thus according to Eqs. (13) and7 (14), the total water necessary for 100% hydrolysis will represent 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 pigments 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] S. No. 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 1.4 4.0 5.3 5.3 4.0 60.0 100.0

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Formulation 2 [56] S. No.

Ingredient

1 2 3 4 5 6 7 8

40% ethyl silicate liquid 30% ethyl silicate liquid Zinc powder Zinc flakes Ferro phosphate Crystalline silica Amorphous silica Wetting agent

4. Chemistry of hydrolysis reaction of alkyl silicates Amount (%) 26.0 4.8 39.1 6.5 19.5 3.2 0.4 0.5 100.0

Formulation 3 [52] S. No. Ingredient

Amount (%)

Bindera Powdered zinc (spherical particles) Titanium dioxide (rutile) Ilmenite Aluminium

1 2 3 4 5

7

19.6 32.9 13.3 17.9 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 Metallic zinc Zinc oxide Lead Cadmium as (CdO) Volatile Moisture and volatile Iron

98–99.2% 94–97% 3–6% 0.2% maximum 0.7% maximum 0.1% maximum 0.1% maximum 0.04% maximum

(ii) Coarse particles Retention on 100 mesh Retention on 200 mesh Retention on 325 mesh

Nil Nil 4% maximum

(iii) Particle size distribution (Coulter counter) Medium particle size 6–10 microns ≤0.17 m2 /g Specific surface Spherical particles, specific gravity 7.0 g/cm3 (iv) Dispersibility Should disperse satisfactorily in a high speed disperser

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 solution 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 unaffected 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 SE2 mechanism and at higher pH, this reaction corresponds to SN2 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 central silicon atom. However, reactivity of the tetrahedron towards electrophilic attack is enhanced by an increase in electron density around silicon. Initial hydrolysis of silicon 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 leading to polymer formation before all alkoxy groups are substituted by silanol groups. Condensation polymerisation reactions proceed with an increase in viscosity of the alkoxide solution until an alcogel is produced. In general, 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 particles 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

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protons catalyses the hydrolysis reaction. The mechanism then corresponds to an electrophilic substitution in which + an (H3 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 H3 O as on the one of the alkoxides. The mechanism is consequently an SE2 , 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:

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

(15) In alkaline conditions, silicate particles are positively − charged and OH anion attacks the alkoxide through an SN2 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 silicon. The anion then recombines with a proton so as to form an alcohol molecule. The mechanism of the reaction has been shown below:

In acidic conditions, silicon alkoxide condenses through a two step mechanism which corresponds to SN2 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 (H3 O) ion. The two silicon atoms of the resulting polymer are then linked through an oxo bridge called, in this specific case, as siloxane 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)

(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

(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

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than hydrolysis. Silanols are protonated more easily when they are present at the end of the polymer chain. In basic conditions, they build siloxane bridge by another SN2 mechanism. This mechanism involves two interme- diary complexes with penta coordinated silicons. In basic conditions, condensation rate is not only proportional to − the concentration of OH anions but also superior to that of hydrolysis. Furthermore, since the reticulation inside the silicon polymers is more developed than when conditions for acidic catalysis are used, hence the denser solids are obtained.

(21 ) Overall basic catalysts, including Lewis bases, accelerate condensation and alcohol molecules are better leaving groups than water. Efficient 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 during 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, quite similar ultimate reactions occur within the coating and on steel surface during film curing [2]. In general, the curing of ethyl silicate involves hydrolytic polycondensation reaction, which occurs in two steps. The first reaction is reversible which has already been given as Eq. (15). The product of this reaction, in the

9

absence of alcohol, undergoes polycondensation reaction as shown in Eq. (22).

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

(22) The moisture and carbon dioxide in the air react with each other to form carbonic acid, as shown below: H2 O + CO2 → H2 CO3

(23)

This carbonic acid causes ionisation of some zinc on the surface of zinc particles. The slightly acidic water helps to hydrolyse the prehydrolysed binder completely to yield silicic acid as given hereunder:

(24) The ionic zinc then reacts with silanol groups on the silicate molecules in the silicate gel structure. This insolubilises the coating and provides its initial properties. This reaction is as follows.

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Ethyl silicate based binders can be cured by IR radiation [54], alkali metal salts of thio acids, barbutaric acids, and/or, 1,3-dicarbonyl compounds [55], and also by treating the substrate with an aqueous solution of a base over which they are applied [56]. 6. Film performance of ethyl silicate based zincrich 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 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. 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.

Uncured films of zinc-rich coatings are rough and irregu- lar while fully cured zinc-rich paint films are grey in colour and textured in nature [57], as in cured films, round glob- ules of zinc are present. These cured films have metal like hardness and these films remain unaffected by radiation in- cluding X-rays, neutron bombardment and other forms of radioactivity [58]. Some other advantages of these systems are 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 remain unaffected by organic solvents [5]. 5. Inorganic zinc-rich paints offer excellent adhesion because the binder chemically reacts with the underlying steel surface [2,8]. Such an excellent adhesion prevents under cutting of coating by corrosion even after 10 years of exposure. As a matter of fact, these are the most corrosion resistant coatings available today [2]. 6. These coatings offer excellent corrosion resistance due to the involvement of conductive matrix in the protection mechanism. 7. These coatings have excellent weather resistance. They can withstand rain just after half an hour of the application [2]. 8. These films are weldable at a low dry film thickness and do not have adverse effect on welding and gas cutting [49]. 9. They will protect steel◦ under insulation in the critical temperature range 0–66 C. ◦ 10. Coatings can withstand temperature up to 400 C. Along with these advantages, limitations also such as:

(26)

they

have

some

1. They have poor resistance for acidic or alkaline conditions outside the pH range 5–10. 2. These coatings generally exhibit more pinholing and bubbling upon top coating as compared to organic zinc coatings. 3. They are not recommended for immersion service in fresh or salt water. 4. In wet condition, they are not recommended beyond ◦ 60 C due to rapid depletion of zinc.

G. Parashar et al. / Progress in 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 primers have somewhat better tol- erance for slightly poorer surface preparation than the alkali silicate based paints, but a properly cleaned (sand blasted) surface is a must for these coatings. Under cathodic protection, organic binder based zinc-rich primers have tendency to degrade, and also to cause blistering of the subsequent coats. In this respect, inorganic zinc-rich primers have superlative record. 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]. These systems form coherent adhesive coating of silica which results due to hydrolysis and gelation of the ethyl silicate binder. Because of inertness and refractoriness of silica, these systems are heat stable and durable.

7. Factors influencing film performance There are various factors, 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 zinc-rich primers. It was theorised that a flat plate zinc 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 platelet would require much more minimum primer film thickness for a given degree of protection than would the flake do. In a 25 micron film thickness, as many as 20 zinc flake platelets might be superimposed as compared to approx- imately three rows of spheres of zinc dust. The lamellar nature of the flake would ensure a significantly enhanced electrical contact area. In fact, reactivity of zinc flake in salt fog environments was found to be too great to provide the sort of long-term performance profile required. Apparently, zinc flake produced far more current than was necessary to protect the steel cathode, and was soon exhausted. Hence re- duction of zinc reactivity by the addition of small quantities of inhibitors such as potassium chromate along with mica extender significantly improved performance effectiveness.

11

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. 7.2. 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. According to the most technical specifications, minimum content of metallic zinc in the dry film required is 75% (by weight) for zinc-rich paints based on ethyl silicate. 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. 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. [60], Del and Giudice [61] and Pereira et al. [62] verified that the chemical nature of the binder and the zinc particle size are also very important. In order to obtain contrast between sand blasted steel substrate and the paint, some manufacturers use colouring pigments such as chromium oxide and iron oxide, and because of technical reasons some other manufacturers use extender pigments such as barytes, mica, talc etc. Experimental studies have been carried out by Fragata et al. [63], on ethyl silicate based paints having a metallic zinc content of 75 and 60% (Table 5). Panels coated with these paints were subjected to salt spray, field exposure and electrochemical tests. 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. Salt spray results for 75% zinc content up to 2060 h of exposure did not show any influence of fillers. In the paints which contain fillers, for the same metallic zinc content in the dry film, the PVC/CPVC ratio is higher, which leads more porous and permeable films due to which the electrical contact between zinc particles and steel substrate improves. These factors contribute to the improvement of paint performance from the galvanic point of view. It is important to mention that effectiveness of the zinc-rich paint does not depend solely on electrochemical factors. Some other factors such as mechanical properties viz. cohesion, flexibility, etc. are also important. So the addition of auxiliary pigments should be controlled carefully in order to not impair the physical and chemical characteristics of the films. In inorganic zinc silicate coatings, water, ground muscovite mica is also used widely. On the basis of experimental studies, Hare [64] reported that upgraded corrosion resis- tance and reduced cost of the system can be obtained by using flake zinc in combination with mica and zinc potas- sium chromate. It is also observed in the mica modified formulations that they produce reduced amount of zinc cor- rosion product, which indicates the general reduction in zinc

12

G. Parashar et al. / 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

G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– Metallic Main components 14 zinc content in the dry film 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 Zinc dust Agalmatolite

2060

ZnB75

75.0

Ethyl silicate Zinc dust Barytes

2060

corrosion. This effect is thought to be related to the control of current transfer that such non-conductive extenders might allow. Electrical conductivity is reduced in this case not only by the resistance of the vehicle cover but also by the mica laminate. Besides these, various other conductive extenders have been used such as cadmium, aluminium, magnesium, iron and carbon along with zinc dust. Of these, only cadmium with zinc and inhibitors gave results comparable to normal zinc-rich primers. Others have proved to be inferior. Problems of toxic fumes during welding, however, precludes the use of cadmium in these coatings. Out of various extenders used in ethyl silicate based zinc-rich paints, the best results have been obtained from di-iron phosphide (Fe2 P), which is a refractory conductive compound. In ethyl silicate zinc-rich coatings evaluation of this extender has been carried out by Filire et al. [65]. 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. Compositions of 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 coatings to provide cathodic protection to the steel substrate. Further, the inclusion of Fe2 P extender does not disturb the marked capability of ethyl silicate zinc-rich paints to develop barrier coats. The weldability of primers is also improved by the use of Fe2 P. 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

13 Time (h) necessary for appearance of red corrosion in scratch (ASTM B-117)

zinc content while a greater fraction of the zinc initially present remained unoxidised after a given period of time. 7.3. Application techniques Application techniques and relative humidity also have influence on the curing of inorganic zinc ethyl silicate based primers [57]. The experimental results also revealed that curing 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, nozzle sizes, distance from the surface, etc. It was also reported that spray coating methods yielded results which were not readily reproducible and gave both poor and good curing results, while flow coating methods yielded reproducible re- sults conforming to manufacturers’ data sheets under the conditions tested.

8. Areas of applications for zinc-rich inorganic silicate paints Because of the excellent corrosion protection offered by these coatings to steel, these coatings find applications in various critical fields [66]. Some of their application areas are given hereunder: 1. Harbour structures. The corrosion conditions encountered by off-shore petroleum production platforms are the most severe. Many hundreds of drilling and production structures have been coated with inorganic zinc silicate coatings, located in the highly humid tropical areas of Indonesia, Singapore and the Persian Gulf to the United

G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14

States Gulf coast and extending into the Arctic areas of Alaska and the North Sea. The inorganic coatings based on hydrolysed ethyl silicate, applied alone or overcoated for additional protection and for safety colouration, are providing outstanding protection to these essential pieces of equipment. 2. Bridges. Bridges, like off-shore structures, are extremely vulnerable to corrosion, perhaps so since many bridge structures are formed from structural steel shapes, with all the corners, edges, crevices and surface defects inherent in such shapes. One of the very early bridges coated is a Drawbridge across a Tidal river in Florida. This bridge was coated in 1956 with the open grill work being the most difficult part of the structure to fully protect it. It is still well protected by the original single coat of inorganic zinc silicate coating. Other bridges such as Baleman bridge in Tasmania which was coated prior to installation, the golden gate bridge on the original Morgan Whylla pipeline, are some examples of full protection provided by inorganic zinc silicate paints over many years of continuous exposure. 3. Nuclear power facilities. One interesting application of inorganic zinc silicate paints is the protection of nuclear power plants. The steel surface within the reactor building requires coating with a 40-year expected life. In fact, it is hoped that such surfaces will never have to be painted af- ter the plant goes into operation. Alkyl silicate inorganic zinc-rich primers are used in nuclear applications for many reasons. These primers are applied at 3.0 mil min- imum thickness, mainly at the steel plate manufacturer’s factory before shipping to the job site. These coatings are unaffected by -rays or neutron bombardment. 4. Tank coatings. One of the major uses of inorganic zinc coatings has been in the lining of ship tankers, primarily for transporting refined fuel. One of the oldest documented applications of inorganic zinc coatings is the No. 1 centre tank in Utah standard. This was applied in 1954 to a previously corroded tank surface. This tank was inspected in 1966, after 11 years approximately, and with the exception of holidays or missed areas in original application, there was no further rust or loss of metal in the tank. Inorganic zinc-rich coatings are suitable in general for tank interiors carrying petroleum products, crude oils, lubricants, edible oils and solvents like ketone esters, chlorinated hydrocarbons, etc. [66]. However, un- pigmented hydrolysed ethyl silicate binder is also used for various purposes such as stone preservation, for the surface treatment of concrete to reduce dusting, etc. [33]. 9. Conclusion Surface coatings based on 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, despite the fact 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 based (inorganic) coatings are considered better, in view of environmental aspects, than organic coatings which invariably produce films composed of organic polymers. The films of these coatings, being silica based, are resistant to temper◦ ature up to 400 C, where most organic coating films fail. Further, 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. In addition, ethyl silicate based binders react with the iron (substrate) chemically, and hence provide unmatched adhesion to restrict corrosion creepage, if any kind of corrosion at all starts on the substrate. The films, being rock-like hard and quite rough, provide excellent inter-coat adhesion to the subsequent coat. On account of these attractive features, ethyl silicate based coatings 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 and wider application in further more challenging areas in future. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] N.

B. Rani, Paintindia 31 (7) (1981) 3. S. Mukherjee, Paintindia 49 (7) (1999) 31. E. Cavalcanti, O. Ferraz, Prog. Org. Coat. 23 (1993) 185. R.K. Marphatia, Paintindia 33 (3) (1988) 19. Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 6, 1984, p. 471. OCCA Australia, Surface Coatings, Vol. 2, Chapman & Hall, New York, 1984, p. 484. Y.P.S. Nirvan, J.H. Jagannath, Paintindia 38 (8) (1988) 31. C.H. Hare, Paintindia 48 (4) (1998) 47. G. Gettwert, W. Rieber, J. Bonarius, Surf. Coat. Int. 81 (12) (1998) 596. T. Minoru, M. Takahashi, O. Ishii, M. Naito, Y. Kusuhara,

Imahigashi, Japanese Patent No. 7,929,340 (1979); Chem. Abstr. 91 (1979) 22611f. [11] S. Tanaka, Japanese Patent No. 78,140,332 (1979); Chem. Abstr. 90 (1979) 123285n. [12] S. Tanaka, Japanese Patent No. 7,939,439 (1979); Chem. Abstr. 91 (1979) 58881h. [13] D.H. Rotenberg, P.M. Cuffe, B.L. Laurin, P.R. Ramirez, US Patent No. 4,173,490 (1979); Chem. Abstr. 92 (1980) 24414p.

14

G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1– 14

[14] Japanese Patent No. 8,124,464 (1981), to Nippon Shokubai Kagaku Kogyo Co. Ltd.; Chem. Abstr. 95 (1981) 82476h. [15] K.H. Brown, K.M. Wolma, US Patent No. 4,290,811 (1981); Chem. Abstr. 95 (1981) 205594e. [16] Y. Tanabe, N. Matsuzoe, H. Endo, K. Adachi, Japanese Patent No. 06,340,848 (1984); Chem. Abstr. 123 (1995) 12011a. [17] Japanese Patent No. 81,129,270 (1981), to Dainippon Toryo Co. Ltd.; Chem. Abstr. 96 (1982) 37048e. [18] T. Fukushima, Japanese Patent No. 61,204,282 (1986); Chem. Abstr. 106 (1987) 51838u. [19] Japanese Patent No. 57,198,767 (1982), to Nippon Oils and Fats Co. Ltd.; Chem. Abstr. 88 (1983) 217303j. [20] Japanese Patent No. 58,185,660 (1983), to Mitsui Engineering and Ship Building Co. Ltd., Nippon Paint Co. Ltd. and Mitsui Mining and Smelting Co. Ltd.; Chem. Abstr. 101 (1984) 39911b. [21] Japanese Patent No. 5,945,361 (1984), to Kansai Paint Co. Ltd.; Chem. Abstr. 101 (1984) 39973y. [22] Japanese Patent No. 5,951,951 (1984), to Hitachi Ship Building and Engineering Co. Ltd.; Chem. Abstr. 101 (1984) 74467h. [23] Japanese Patent No. 5,936,157 (1984), to Otsuka Chemical Co. Ltd.; Chem. Abstr. 101 (1984) 74454b. [24] Japanese Patent No. 59,129,268 (1984), to Matsushita Electrical Industrial Co. Ltd.; Chem. Abstr. 101 (1984) 173216g. [25] Japanese Patent No. 6,051,756 (1985), to Nippon Paint Co. Ltd., Mitsui Engineering and Shipbuilding Co. Ltd., Kawasaki Heavy Industries Ltd., Hitachi Shipbuilding and Engineering Co. Ltd. and Mitsubishi Heavy Industries Ltd.; Chem. Abstr. 103 (1985) 55530y. [26] T. Fukushima, Japanese Patent No. 61,204,283 (1986); Chem. Abstr. 106 (1987) 51839v. [27] H. Ito, M. Yokota, Japanese Patent No. 62,181,370 (1987); Chem. Abstr. 108 (1988) 57996g. [28] T. Toyo, K.K. Toryo, Japanese Patent No. 81,159,256 (1981); Chem. Abstr. 96 (1982) 105930p. [29] W. Wojnowaki, J.M. Nawak, J. Gaszkowski, M. Jaczewaki, S. Konieczny, K. Sienkiewicz, P. Rosciszewski, Polish Patent No. 122,889 (1984); Chem. Abstr. 103 (1985) 217047a. [30] Japanese Patent No. 8,124,464 (1981), to Nippon Shokubai Kagaku Koggo Co. Ltd.; Chem. Abstr. 95 (1981) 82476n. [31] S. Sono, Y. Chihara, Japanese Patent No. 61,101,566 (1986); Chem. Abstr. 106 (1987) 20106z. [32] H.F. Payne, Organic Coating Technology, Vol. 1, Wiley, New York, 1964, p. 594. [33] H.G. Emblem, Res. Ind. 23 (12) (1978) 207. [34] H.D. Cogan, C.A. Sellerstrom, Ind. Eng. Chem. 67 (1947) 1364. [35] R.K. Iler, The Colloidal Chemistry of Silica and Silicates, Cornel University Press, Ithaca, NY, 1955. [36] G.H. Emblem, Fridry Trade J. 132 (1975) 1364. [37] British Patent No. 1,075,379 (1967), to Sulzer Bros Ltd.; Chem. Abstr. 64 (1966) 17234g. [38] British Patent No. 1,126,955 (1968), to Stauffer Chemical Co.; Chem. Abstr. 66 (1967) 30082h.

[39] G.H. Allen, H.G. Emblem, R.D. Shaw, British Patent No. 1,302,462 (1972); Chem. Abstr. 77 (1972) 922522z. [40] G.D. Mcleod, US Patent No. 3,428,556 (1969); Chem. Abstr. 70 (1969) 90430n. [41] J. Aston, H.G. Emblem, R.H. Hancock, British Patent No. 770,527 (1957). [42] H.G. Emblem, E.W. Fothergill, US Patent No. 1,356,249; Chem. Abstr. 61 (1964) 5518c. [43] G.D. Mcleod, European Patent No. 64,344 (1982); Chem. Abstr. 98 (1983) 74010y. [44] M. Ando, T. Katayama, Japanese Patent No. 61,235,469 (1986); Chem. Abstr. 106 (1987) 103930q. [45] Japanese Patent No. 78,120,743 (1978), to Stauffer Chemical Co.; Chem. Abstr. 90 (1979) 56420n. [46] Japanese Patent No. 78,120,743 (1983), to Hitachi Chemical Co. Ltd.; Chem. Abstr. 100 (1984) 211800g. [47] H.G. Emblem, I.R.J. Walters, Appl. Chem. Biotechnol. 27 (1977) 618. [48] British Patent No. 1,292,938 (1972), to Anderson Development Co.; Chem. Abstr. 77 (1972) 36575v. [49] R.S. James, Mod. Paint Coat. 6 (1983) 48. [50] T. Ginsberg, I.G. Kaufman, Mod. Paint Coat. 10 (1981) 138. [51] T.A. Banfield, Protective Painting of Ships and Structural Steel, Trade and Technical Press Limited, Modern Surrey, London, 1984. [52] T. Nakano, T. Ishikawa, Japanese Patent No. 62,275,173 (1987); Chem. Abstr. 108 (1988) 133551a. [53] C.P. Alain, Introduction to Sol–Gel Processing, Kluwer Academic Publishers, Boston, 1998. [54] J. Nakajima, K. Nishimura, Y. Yamamoto, Japanese Patent No. 06,190,271 (1994); Chem. Abstr. 122 (1995) 58262r. [55] M. Yuyama, M. Futagam, Japanese Patent No. 2,487,324 (1982); Chem. Abstr. 96 (1982) 219353h. [56] Japanese Patent No. 80,108,473 (1980), to Kansai Paint Co. Ltd.; Chem. Abstr. 94 (1981) 17193d. [57] Special Report, Paintindia 49 (7) (1999) 79. [58] G.M. Charles, Structural Steel Painting Council, 1982, p. 125. [59] C.H. Hare, Coat. World 9/10 (1997) 38. [60] F.L. Fragata, M. Sebrao, C.R. Mussoi, J.E. Dopico, in: Proceedings of the Third Congress Ibero-Americano de Cor. e. Prot., Brazil, Vol. 3, 1989, p. 1205. [61] B.A. Del, C.A. Giudice, in: Proceedings of the 11th International Congress on Corrosion, Italy, Vol. 2, 1990, p. 347. [62] O. Pereira, G.J.O. Scantle, M.G.S. Ferreira, M.C. Almeido, Corros. Sci. 30 (11) (1990) 1135. [63] F.L. Fragata, C.R.S. Mussoi, C.F. Moulin, I.C.P. Margarit, O.R. Mattos, J. Coat. Technol. 1 (1993) 103. [64] C.M. Hare, Mod. Paint Coat. 4 (1982) 48. [65] S. Filire Jr., M. Marcillo, J.M. Bastidas, F. Feliu, J. Coat. Technol. 3 (1991) 67. [66] V.S. Bhakre, Paintindia 29 (3) (1979) 85.