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: vkj@hbti.ernet.in (P. Kumar).
pigments having limited solubility can give rise to
inhibitive ions, such as chromates.
Undoubtedly, steel is one of the most important metals
used in the modern society. However, one of its main 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
2
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
adjacent zinc particles, and between the zinc particles in the
film and the steel substrate is required [3].
In the case of zinc-rich ‘organic’ coating films, zinc 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.
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
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
3
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 development 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 ratio ranging from 1:2 to 1:5.3. If further higher ratios are
required, and instability is to be avoided, it is necessary to
use lithium silicate with lithium oxide to silica ratio as 1:2
to 1:8.5. Lithium silicate based coatings are preferred for
use in food areas. Excellent curing rates can be achieved
with some lithium silicates, but their higher cost tends to
restrict their use at the present time.
2.2. Alkyl silicate binders
Alkyl silicates such as ethyl silicate, methyl silicate etc.
can be used as binders for the formulation of solvent-borne
coatings. However, one of the commercial forms of ethyl
silicate (popularly known as ethyl silicate-40) as solution in
organic solvent(s) is most commonly employed. Alkyl silicates, as such, do not have any binding ability but when their
alcoholic solutions are hydrolysed with calculated amount of
water in the presence of acid or alkali catalyst, they acquire
sufficient binding ability. On the basis of the type of catalyst
used for the hydrolysis, these coatings can be sub-classified
as follows.
2.2.1. Alkali catalysed coatings
For the hydrolysis of ethyl silicate, bases like ammonia,
ammonium hydroxide, sodium hydroxide and some amines
are generally used as catalysts [2]. One of the greatest
drawbacks of this system is related to the fact that in basic
conditions, even a small amount of water will cause the
silicate to gel. To avoid this problem, remedial steps must
therefore be taken to exclude all water at the manufacturing stage, and from the application equipment. If water is
excluded, the liquid component can remain stable for an
indefinite period of time. These coatings are available in the
market as single-pack and two-pack systems. In single-pack
system, amines, which provide hydroxyl ion in the form
which is non-reactive with organic polysilicate until they
are exposed to moisture, are used [8].
2.2.2. Acid catalysed coatings
In these type of coatings, rapid curing may be achieved
under most conditions. However, the period over which the
partially hydrolysed silicate remains stable is limited, and
the product thus has a finite shelf life. Coatings based on
acid catalysed binder are mainly two-component systems,
and the liquid component of these coatings gel in a period
of 6–12 months. The problem associated with one-pack
system of this type is that zinc chemically reacts with the
acid catalyst present in the binder system, due to which pH
of the system increases, which causes gelling in the container. Hydrochloric acid [10–27], sulphuric acid [28,29],
phosphoric acid [30], formic acid [31], etc., are the acids
which are used as catalysts.
3. Hydrolysis of ethyl silicate
Ethyl silicate, by itself, has no binding ability [32]. To
introduce binding ability, it is necessary to hydrolyse ethyl
silicate by treating it with water, so that a gel can form from
the resulting ethyl silicate hydrolysate. The actual binding
agent is this gel [33].
Usually, the hydrolysis of ethyl silicate is carried out
under alkaline or acidic conditions. Acids or alkalis are
used to catalyse the hydrolysis reaction. Hydrolysis under
alkaline conditions normally results in fairly rapid gelation.
Alkali catalysed hydrolysis procedures are generally preferred when ethyl silicate is to be used for the production
of refractories. Acid hydrolysis procedures are commonly
employed for the production of paint media. Several
4
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
Table 3
Typical compositions for single stage procedures for the hydrolysis of ethyl silicate
S. No.
Quantity of ethyl silicate-40
Quantity of water
Quantity of acid
Quantity of solvent
1
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)
procedures for the acid hydrolysis of ethyl silicate are
available [34–36].
Hydrolysis procedures in which a specified quantity
of ethyl silicate is added at the start of the reaction are
termed as ‘single stage’ procedures, while those in which
ethyl silicate is added usually after a specified temperature
rise or time interval are termed as ‘two-stage’ procedures.
Some two-stage procedures require two types of organic
silicates. Typical compositions for the single stage [37–40]
and two-stage procedures [37,41,42] taken from the patent
literature have been given in Tables 3 and 4, respectively.
Out of many possible ethyl silicate hydrolysis procedures,
one can be considered on its merits.
Mcleod [43] prepared silicate binder system by 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 two-stage process. If
large amount of phosphoric acid is used in the hydrolysis
of ethyl silicate, hydrolysates which gel rapidly can be obtained. 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
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 application 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 )
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
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.,
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].
5
(4)
3.2.3. Reaction with zinc pigments
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)
6
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
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 polysilicate and its exact degree of hydrolysis. This allows calculation 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
In order to prepare a binder that is 85% hydrolysed, the
weight of water to be added can be calculated by Eq. (11).
Weight of water = 0.36(85 − 41.66) = 15.6 kg
The amount of solvent that must be added to give a final
silica content of 18% is calculated from Eq. (12).
= ( 6000
18 ) − 146.34 − 15.6 = 171.4 kg
The solvents that can be used are ethanol, isopropanol,
ethoxyethanol, ethoxy ethyl acetate or mixture of these. The
solvent and ethyl silicate are combined and agitated. Water
containing some acid catalyst is added and the mixture is
then agitated until the exotherm subsides. The binder is ready
for use after 24 h of preparation.
In general, curing of ethyl silicate involves hydrolytic
polycondensation occurring in two steps. The first is
reversible as shown in Eq. (13).
nSi(OC2 H5 )4 + 4nH2 O → nSi(OH)4 + 4nC2 H5 OH
In the absence of alcohol, the silicic acid formed undergoes polycondensation as given in Eq. (14):
nSi(OH)4 → SiO2 + 2nH2 O
or
(13)
(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
Because Eq. (14) contributes 2 mol of water for each mole
of ethyl silicate, only 2 mol of water are needed for 100%
hydrolysis of the reactants. Thus according to Eqs. (13) and
(14), the total water necessary for 100% hydrolysis will represent 17.36% by weight of the ethyl silicate used. If ethyl
silicate-40 is used as the raw material, then for 100% hydrolysis, 14.5% water by weight of ethyl silicate-40 is required.
Weight of water = 0.36(% hydrolysis desired
3.4. Paint compositions based on ethyl silicate binder
60 × 100
% SiO2 =
208 − 1.48 H
(10)
−% 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 weight of the
ethyl polysilicate using Eq. (9).
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. Particularly 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
Equivalent weight of ethyl polysilicate
= 208 − 1.48(41.66) = 146.34
100.0
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
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
1
2
3
4
5
Bindera
Powdered zinc (spherical particles)
Titanium dioxide (rutile)
Ilmenite
Aluminium
Amount (%)
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
Medium particle size
Specific surface
Spherical particles, specific gravity
7
counter)
6–10 microns
≤0.17 m2 /g
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
8
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
protons catalyses the hydrolysis reaction. The mechanism
then corresponds to an electrophilic substitution in which
an (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 complex 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
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
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 intermediary 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.
9
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)
(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.
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:
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 solutions 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 absence of
alcohol, undergoes polycondensation reaction as shown in
Eq. (22).
(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.
10
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–14
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 zinc-rich
coatings
(25)
At this time, some reaction between poly silicic acid and
the iron surface also takes place to form a chemical bond.
This bonding prevents the creepage of moisture and lifting
of paint film seen in organic coatings. From this point on, the
reactions will be those that take place 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 irregular while fully cured zinc-rich paint films are grey in colour
and textured in nature [57], as in cured films, round globules of zinc are present. These cured films have metal like
hardness and these films remain unaffected by radiation including 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, they have some limitations
also such as:
(26)
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 tolerance 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 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.
In a 25 micron film thickness, as many as 20 zinc flake
platelets might be superimposed as compared to approximately 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 reduction 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 resistance and reduced cost of the system can be obtained by
using flake zinc in combination with mica and zinc potassium chromate. It is also observed in the mica modified
formulations that they produce reduced amount of zinc corrosion 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
Metallic zinc content in the dry film
Main components
of dry film
Zn60
60.0
Ethyl silicate
Zinc dust
460
ZnA60
60.0
Ethyl silicate
Zinc dust
Agalmatolite
740
ZnB60
60.0
Ethyl silicate
Zinc dust
Barytes
660
Zn75
75.0
Ethyl silicate
Zinc dust
2060
ZnA75
75.0
Ethyl silicate
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
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 results 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 after 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 minimum 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, unpigmented 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].
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 temperature 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, nuclear 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.
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