ethyl silicate binders for high performance coatings

Progress in Organic Coatings 42 (2001) 1–14
Review
Ethyl silicate binders for high performance
coatings
Geeta
Parashar

a

, Deepak
Srivastava
b
, Pramod
Kumar
a
,
∗
a
Department of Oil and Paint Technology, H.B. Technical Institute, Kanpur 208 002,
India
b
Department of Plastic Technology, H.B. Technical Institute, Kanpur 208 002,
India
Received 2 October
2000
; accepted 15 January
2001
Abstract
Surface coatings based on ethyl silicate binders are categorised as inorganic coatings, whereas the conventional surface coatings
which
are mainly based on organic binders are referred to as organic coatings. Zinc-rich inorganic coatings based on ethyl silicate are

quite
successful for the protection of steel against corrosion under severe exposing conditions such as underground, marine atmosphere,
indus-
trial atmosphere, nuclear power plants, etc. These coatings provide unmatched corrosion protection to steel substrates exposed to
high
temperatures. Because of the formation of conductive matrix out of the binder after film curing, zinc-rich coatings based on ethyl
silicate
binder offer outstanding cathodic protection to steel structures. These coatings are mostly solvent-borne, but recently water-borne
versions
of the same have also been developed. However, the commercial success of water-borne systems is not yet well
established.
In the present article, the processes of hydrolysis of ethyl silicate in the presence of acidic and alkaline catalysts have been elaborated
to
produce ethyl silicate hydrolysates of desired degree of hydrolysis. Effect of various factors such as amount of catalysts, amount of
w
ater
,
type and amount of solvent, reaction temperature and reaction time has been discussed. Calculations to find out the amount of water
and
solvent required to yield the product of desired degree of hydrolysis have also been illustrated. Typical recipes useful for the
preparation
of ethyl silicate hydrolysates suitable for use as coating binders have also been presented. The chemistry and mechanism involved
in
the preparation of binder and the curing of film has also been discussed. This article also summarises the effect of various factors,
viz.
particle size and shape of zinc pigment, presence of extenders in the formulations, and the application technique on film
performance.
© 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Inorganic coatings; Silicate binders; Ethyl silicate coatings; Zinc silicate coatings; Heat resistant coatings; Anticorrosive coatings

1. Introduction
Painting is one of the most important techniques used
for the protection of metals from corrosion. Effectiveness
of protection of metals against corrosion mainly depends on
the factors such as quality of the coating, characteristics of
the metal, properties of the coating/metal interface, and the
corrosiveness of the environment. Typical corrosion resis-
tant coatings protect the metallic surfaces primarily by the
following two mechanisms [1].
1. By acting mainly as a physical barrier to isolate the
substrate from corrosive environment.
2. By containing reactive materials (usually pigments)
which react with a component of the vehicle to form
such compounds that, in fact, inhibit corrosion. Some
∗
Corresponding author. Tel.: +91-512-583-507; fax: +91-512-545-312.
E-mail address: (P. Kumar).
pigments having limited solubility can give rise to
inhibitive ions, such as chromates.
Undoubtedly, steel is one of the most important metals
used in the modern society. However, one of its main draw-
backs is its tendency to corrode (rust), i.e. to revert to its
original state, and become useless. Hence, the protection of
steel from corrosion, i.e. to keep the steel in its usable form,
has always been a matter of great concern for all those who
use it in one form or the other.
For the protection of steel, various materials can be used,
out of which zinc has been found to be the most success-
ful [2]. Zinc can prevent or at least retard the corrosion of
steel in the form of electroplated layers or by the applica-

tion of paints containing a high percentage of zinc particles
dispersed in an organic or an inorganic binder. Zinc, either
in the form of electroplated film or in the form of films of
zinc-rich coatings, protects the steel substrate by sacrificial
cathodic (galvanic) protection mechanism. For the cathodic
protection of steel, the direct electrical contact between the
0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII:
S 0 3 0 0 - 9 4 4 0 ( 0 1 )0012 8 - X

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G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–
14
adjacent zinc particles, and between the zinc particles in the
film and the steel substrate is required [3].
In the case of zinc-rich ‘organic’ coating films, zinc par-
ticles can be encapsulated by the organic binder, and hence
the zinc particles have restricted electrical contact. Conse-
quently, the zinc particles can provide only a small amount
of galvanic protection limited to the amount of free zinc in
the coating formulation [4].
On the other hand, in the zinc-rich ‘inorganic’ coatings
(commonly referred to as zinc silicate coatings), the binders
(inorganic) used are alkali silicates and alkyl silicates,
which can chemically react with the zinc particles in the
coating film to form a zinc silicate matrix around the zinc
particles [5]. This zinc silicate matrix is electrically
conductive and chemically inert [2]. In addition, the silicate
based binders can chemically react with the steel substrate
also to result in an excellent adhesion and abrasion

resistance of the dried/ cured film [6].
Inorganic zinc silicate coatings are included in the cat-
egory of high performance coatings [7], as these are the
most weather resistant coatings available today [5]. They
can provide an unmatched protection against corrosion for
steel structures exposed to temperatures up to
400
◦
C
[2].
2. Silicate binders for inorganic paint coatings
Inorganic paint coatings based on silicate binders can be
classified [6] as shown in Fig. 1.
2.1. Alkali metal silicate binders
For the manufacture of coatings based on alkali metal
silicates, the silicates based on alkali metals such as
sodium, potassium and lithium, along with the quarternary
ammonium silicates have been reported to be suitable
[8]. Alkali metal silicates are relatively simple chemi-
cals, which can be water soluble depending on the ratio
of silica to alkali metal oxide. The ratios of silica to
alkali metal oxide of different silicates [8], which can
be used as binder systems in paints, have been given in
Table 1.
The ratio of silica to alkali metal oxide, in addition to the
type of alkali metal, has a remarkable effect on curing char-
acteristics and properties of the dried films [9]. The effect
of ratio of silica to alkali metal oxide on coating
characteristics has been shown in Table 2.
The coatings based on alkali metal silicates having sili-

ca to alkali metal oxide varying from 2.1:1 to 8.5:1 are
water-borne due to solubility of the used alkali metal oxide
in water. These coatings are generally sub-classified into
baked, post-cured and self-cured coatings.
2.1.1. Baked coatings
These are the coatings which require heating to convert
the coating films into water insoluble form. These coatings
are characterised by their extreme hardness and suitability
for application over an acid-descaled surface. Baked
coatings still have limited use today.
Fig. 1. Classification of inorganic paint coatings based on silicate binders.
Table 1
Ratios of silica to alkali metal oxide in alkali silicates [8]
S. No. Silicate Chemical composition Ratio of silica to
alkali metal oxide
1 Sodium silicate
SiO
2

:Na
2

O 2.4–4.5:1
2 Potassium silicate
SiO
2

:K
2

O
2.1–5.3:1
3 Lithium silicate
SiO
2

:Li
2

O
2.1–8.5:1
Table 2
Effects of ratio of silica to alkali metal oxide on coating characteristics
S. No. Ratio of silica to alkali metal oxide Effect on coating characteristics
1 Higher Higher the viscosity of the solution
Higher the drying speed of the film
Higher the curing speed of the film
Higher the susceptibility to low temperature
Higher the chemical resistance of the coating films
2 Lower Higher the specific weight of the solution
Higher the solubility in water
Higher the pH value of the solution
Higher the susceptibility to
water
Higher
the adhesion and binding power
2.1.2. Post-cured coatings
These are the coatings which are cured by the application
of chemicals such as an acid wash just after application of
the film to convert the film into a water insoluble condition.

These coatings are formulated mainly on sodium silicate
having higher ratio of silica to sodium oxide. This develop-
ment has led to the use of inorganic zinc coatings on large
field structures.
2.1.3. Self-cured coatings
With further advances in silicate technology, further
higher ratio alkali metal silicates have become available. Of
the cheaper types, potassium silicate is preferred. Reliable
self-curing coatings are available today, based on high
ratio potassium silicates with potassium oxide to silica ra-
tio ranging from 1:2 to 1:5.3. If further higher ratios are
required, and instability is to be avoided, it is necessary to
use lithium silicate with lithium oxide to silica ratio as 1:2
to 1:8.5. Lithium silicate based coatings are preferred for
use in food areas. Excellent curing rates can be achieved
with some lithium silicates, but their higher cost tends to
restrict their use at the present time.
2.2. Alkyl silicate binders
Alkyl silicates such as ethyl silicate, methyl silicate etc.
can be used as binders for the formulation of solvent-borne
coatings. However, one of the commercial forms of ethyl
silicate (popularly known as ethyl silicate-40) as solution in
organic solvent(s) is most commonly employed. Alkyl sili-
cates, as such, do not have any binding ability but when
their alcoholic solutions are hydrolysed with calculated
amount of water in the presence of acid or alkali catalyst,
they acquire sufficient binding ability. On the basis of the
type of catalyst used for the hydrolysis, these coatings can
be sub-classified as follows.
2.2.1. Alkali catalysed coatings

For the hydrolysis of ethyl silicate, bases like ammonia,
ammonium hydroxide, sodium hydroxide and some amines
are generally used as catalysts [2]. One of the greatest
drawbacks of this system is related to the fact that in basic
conditions, even a small amount of water will cause the
silicate to gel. To avoid this problem, remedial steps must
therefore be taken to exclude all water at the manufactur-
ing stage, and from the application equipment. If water is
excluded, the liquid component can remain stable for an
indefinite period of time. These coatings are available in the
market as single-pack and two-pack systems. In single-pack
system, amines, which provide hydroxyl ion in the form
which is non-reactive with organic polysilicate until they
are exposed to moisture, are used [8].
2.2.2. Acid catalysed coatings
In these type of coatings, rapid curing may be achieved
under most conditions. However, the period over which the
partially hydrolysed silicate remains stable is limited, and
the product thus has a finite shelf life. Coatings based on
acid catalysed binder are mainly two-component systems,
and the liquid component of these coatings gel in a period
of 6–12 months. The problem associated with one-pack
system of this type is that zinc chemically reacts with the
acid catalyst present in the binder system, due to which pH
of the system increases, which causes gelling in the con-
tainer. Hydrochloric acid [10–27], sulphuric acid [28,29],
phosphoric acid [30], formic acid [31], etc., are the acids
which are used as catalysts.
3. Hydrolysis of ethyl silicate
Ethyl silicate, by itself, has no binding ability [32]. To

introduce binding ability, it is necessary to hydrolyse ethyl
silicate by treating it with water, so that a gel can form from
the resulting ethyl silicate hydrolysate. The actual binding
agent is this gel [33].
Usually, the hydrolysis of ethyl silicate is carried out
under alkaline or acidic conditions. Acids or alkalis are
used to catalyse the hydrolysis reaction. Hydrolysis under
alkaline conditions normally results in fairly rapid gelation.
Alkali catalysed hydrolysis procedures are generally pre-
ferred when ethyl silicate is to be used for the production
of refractories. Acid hydrolysis procedures are commonly
employed for the production of paint media. Several
4
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–
14
Table 3
Typical compositions for single stage procedures for the hydrolysis of ethyl silicate
S. No. Quantity of ethyl silicate-40 Quantity of water Quantity of acid Quantity of solvent
1 6 l 2 l 50 ml concentrated HCl 4 l ethanol
2 1368 parts (by weight) 138 parts (by weight) 0.16 parts (by weight) 12 N HCl 1517 parts ethanol (by weight)
3 1.6 l 100 ml 6 ml 0.1 N HCl 840 ml 640 p industrial methylated sprit
4 45 parts (by weight) 53 parts (by weight) 0.1 part (by weight) 37% aqueous HCl 49.6 parts ethanol (by weight)
procedures for the acid hydrolysis of ethyl silicate are
available [34–36].
Hydrolysis procedures in which a specified quantity
of ethyl silicate is added at the start of the reaction are
termed as ‘single stage’ procedures, while those in which
ethyl silicate is added usually after a specified temperature
rise or time interval are termed as ‘two-stage’ procedures.
Some two-stage procedures require two types of organic

silicates. Typical compositions for the single stage [37–40]
and two-stage procedures [37,41,42] taken from the patent
literature have been given in Tables 3 and 4, respectively.
Out of many possible ethyl silicate hydrolysis procedures,
one can be considered on its merits.
Mcleod [43] prepared silicate binder system by hydro-
lysing ethyl silicate-40 in butyl cellosolve in the presence
of acid catalyst with 5% (part basis) water at
140
◦
C.
Some
other workers [44–46] also prepared binder systems by
using pure ethyl silicate or ethyl silicate-40 of different
properties. Some special procedures include the use of
silica aquasol and the use of titanic acid ester in a two-
stage process. If large amount of phosphoric acid is used
in the hydrolysis of ethyl silicate, hydrolysates which gel
rapidly can be ob- tained. Conditions for the hydrolysis of
ethyl silicate without use of an acid or a base catalyst to
obtain binding solutions have also been established [47].
Acid hydrolysates of ethyl silicate eventually set to a gel
on standing. The relatively short shelf life of some acid
hydrolysed ethyl silicate solutions can cause difficulties in
their use. As a result of the development of methods for
preparing ethyl silicate hydrolysates having a long stor-
age life, hydrolysed ethyl silicate solutions have become
available commercially. These solutions, often referred to
as prehydrolysed ethyl silicate solutions, are of particular
interest as paint media.

Ethyl silicate hydrolysates having a long storage life can
be obtained by careful choice of the proportions of ethyl
silicate, solvent, acid and water for their preparation. If
ethyl silicate is treated simultaneously with a glycol
monoether for alcoholysis and water for hydrolysis, a
hydrolysate with a long shelf life is obtained [48]. This
hydrolysate can be successfully used as a paint medium.
Generally 80–90% hydrolysis of the ethyl silicate is carried
out for the binder preparation [2].
3.1. Factors governing the formulation of ethyl silicate
binders
There are some important factors, which can affect the
hydrolysis of ethyl silicate and the formulation of ethyl sili-
cate binders. These factors are discussed hereunder one by
one.
3.1.1. Effect of quantity of water
Quantity of water and the quantity of acid catalyst used
for partial hydrolysis are the most important factors for for-
mulating acid catalysed ethyl silicate binder systems. Water
to be used in hydrolysis must be calculated after subtracting
the quantity of water (if any) going into the paint formula-
tion from the extender pigments and the solvents used in
the formulation. Excessive water in the formulation can
lead to gelling of the binder system in the cans or very poor
applica- tion properties and gelling of mixed paints in the
application equipment. Less than optimum quantities of
water can result in an uncured film lacking hardness and
film integrity [49].
3.1.2. Effect of quantity of acid
Less than optimum quantity of acid can result in silica

precipitation, thus making less silica available for binding
than required. Excessive quantity of acid will result in
accel-
erated condensation of silanol with silanol (
≡
SiOH) groups
or with alkoxy groups (
≡
SiOR) resulting in reduced shelf
life of the binder system [49].
Table 4
Typical compositions for two-stage procedures for the hydrolysis of ethyl silicate
S. No. Quantity of ethyl
silicate-40 (first lot)
Quantity of water Quantity of acid Quantity of solvent Quantity of alkyl silicate
(second lot)
1 14 parts 2.15 parts (by volume) 18 parts concentrated HCl 50 parts 160 p industrial 11 parts ethyl silicate-40
(specific gravity 1.18) methylated spirit
2 6000 parts 2000 parts (by volume) 50 parts concentrated HCl 8000 parts isopropanol 2000 parts methyl silicate
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–
14
5
3 340 parts Nil
(specific gravity 1.18)
40 parts 0.1 N HCl 140 parts isopropanol/
(50%
SiO
2

)

130 parts isopropyl silicate
water azeotrope
(38%
SiO
2

)
3.1.3. Effect of size of alkyl group
The rate of hydrolysis reaction is greatly affected by the
size of alkyl group of the organic silicates. The larger alkyl
groups can act as a steric barrier to hydrolytic attack. Thus,
bulkier alkyl groups protect the ester much better than the
smaller groups like methyl or ethyl. N-hexyl silicates, e.g.,
3.2.3. Reaction with zinc pigments
(4)
are
difficult
to hydrolyse, whereas methyl silicate
hydrolyses
readily. A second effect of the size of alkyl group involves
the volatility of the alcohol formed during hydrolysis. If the
alcohol is highly volatile, reversible reaction will be forced
in the direction of the hydrolysis. This is particularly true
for acid catalysed hydrolysis where the presence of the
alcohol maintains an equilibrium. With proper selection of
the alkyl group, curing properties of alkyl silicate coatings
can be tailored [50].
3.2. Chemistry of ethyl silicate binders
Prepared ethyl silicate contains some silanols and alkoxy
groups. These silanol groups are responsible for chemi-

cal reactions in these types of coatings [2]. Some of their
reactions are as follows.
3.2.1. Acid catalysed reactions
First, oxygen of the silanol group is protonated, and an
intermediate species is formed, as shown in Eq. (1).
(1)
This intermediate species then reacts with the
silanol,
which results into the formation of siloxane bond [49].
The silanol groups of hydrolysed ethyl silicate react with
zinc and form a zinc silanol heterobridge.

(5)
This hetero bridge then undergoes further chemical
reactions to form a zinc silicate polymer.
(6)
3.3. Stoichiometry of binder preparation
3.2.2. Effect of pH on stability
(2)
The overall stoichiometry of hydrolysis is given in the
following equations. Total hydrolysis of pure ethyl silicate
[2] can be given as shown in Eq. (7).
When pH of the system is low, then the hydrolysed alkyl
silicate has long pot life due to the repulsion of –O
+
H group
with O
+
H group.
(3)

When pH of the system is high, the rate
of

formation
of
water is high and due to fast dehydration,
pot
life of
the
system is short.
(7)
Ethyl silicate hydrolysed to ‘x’ degree can be shown by
the following equation:
(8)
6
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–
14
H =
41
.
66
This allows the calculation of the equivalent weig
4
Toluene
5.3
t of the 5 Isopropanol
5.3
ethyl polysilicate using Eq. (9).
6
Cellosolve

4.0
7
Zinc dust
60.0
Equivalent weight of ethyl polysilicate
2
The empirical equation for ethyl silicate hydrolysed to x
degree of hydrolysis, SiO
2x
(
OC
2
H
5
)
4(1
−
x)
, can be used
to derive the equivalent weight of the commercial ethyl
polysil- icate and its exact degree of hydrolysis. This
allows calcu- lation of the amount of water necessary to
give a binder of any desired percentage hydrolysis.
Equivalent weight can be obtained by substituting atomic
weights in the empirical formula.
Equivalent weight
=
SiO
2x
(

OC
2
H
5
)
4(1
−
x)
=
28
+
16(2x)
+
45(4
−
4
x)
= 28
+
32x
+
180
−
180x = 208
−
148
x
or
Equivalent weight = 208
−

1.48 H (H = %hydrolysis)
(9)
The concentration of SiO
2
in the ethyl polysilicate is equal
to
Molecular weight of SiO
2
×
100
Equivalent weight of ethyl
polysilicate or
60
×
100
In order to prepare a binder that is 85% hydrolysed, the
weight of water to be added can be calculated by Eq. (11).
Weight of water = 0.36(85
−
41.66) = 15.6 kg
The amount of solvent that must be added to give a final
silica content of 18% is calculated from Eq. (12).
6000
=
(
18
)
−
146.34
−

15.6
=
171.4 kg
The solvents that can be used are ethanol, isopropanol,
ethoxyethanol, ethoxy ethyl acetate or mixture of these. The
solvent and ethyl silicate are combined and agitated. Water
containing some acid catalyst is added and the mixture is
then agitated until the
exotherm subsides.
The binder is
ready for use after 24 h of preparation.
In general, curing of ethyl silicate involves hydrolytic
polycondensation occurring in two steps. The first is
reversible as shown in Eq. (13).
n
Si
(
OC
2
H
5
)
4
+
4
n
H
2
O → nSi(OH)
4

+
4
n
C
2
H
5
OH (13)
In the absence of alcohol, the silicic acid formed under-
goes polycondensation as given in Eq. (14):
nSi(OH)
4
→ SiO
2
+
2
n
H
2
O (14)
Because Eq. (14) contributes 2 mol of water for each
mole
of ethyl silicate, only 2 mol of water are needed for 100%
% SiO
2
=
208
(10)
−
1.48 H

hydrolysis of the reactants. Thus according to Eqs. (13) and
(14), the total water necessary for 100% hydrolysis will rep-
Calculation for the amount of water to be added to one
equivalent weight of ethyl polysilicate to prepare a binder
of any desired degree of hydrolysis is given as
Weight of water
=
0.36(%
hydrolysis
desired
−% hydrolysis
in ethyl polysilicate) (11)
The amount of solvent to be added to achieve the desired
silica content of the binder is determined from the
following equation:
Weight of solvent to be added
6000
=
% SiO desired
−
weight of ethyl polysilicate
−weight
of water added (12)
For example, to prepare 85% hydrolysed binder contain-
ing 18% SiO
2
from commercial ethyl silicate containing
resent 17.36% by weight of the ethyl silicate used. If ethyl
silicate-40 is used as the raw material, then for 100%
hydrol- ysis, 14.5% water by weight of ethyl silicate-40 is

required.
3.4. Paint compositions based on ethyl silicate binder
For the formulation of paints based on hydrolysed ethyl
silicate binder, care should be taken for the selection of
pigments, because with this binder system, only those pig-
ments are suitable which are chemically inert, non-basic
and not very reactive. Thus lead chromate, strontium
chromate, mica, talc and zinc dust are some of the pigments
which can be suitable to formulate ethyl silicate based
coatings. Partic- ularly good protection against high
temperature and rust can be obtained if zinc dust is used as
the pigment. Some typical formulations of these paint
systems are given hereunder:
Formulation 1 [51]
41%

SiO
2
,

calculate

the

%

hydrolysis

in

the

ethyl

polysili-

cate from Eq. (10), as
below:
6000
S. No. Ingredient Amount (%)
1 Ethyl silicate (partially hydrolysed) 20.0
41
=
208
−
1
.
48
(
H
)
2 Anti-settling agent (Bentone 38) 1.4
3 Talc 4.0
= 208
−
1.48(41.66) = 146.34
100.0
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–
14
7

Formulation 2 [56]
S. No. Ingredient Amount (%)
1 40% ethyl silicate liquid 26.0
2 30% ethyl silicate liquid
4.8
3 Zinc powder 39.1
4 Zinc flakes
6.5
5 Ferro phosphate 19.5
6 Crystalline silica
3.2
7 Amorphous silica
0.4
8 Wetting agent
0.5
100.0
Formulation 3 [52]
S. No. Ingredient Amount (%)
1 Binder
a
19.6
2 Powdered zinc (spherical particles) 32.9
3 Titanium dioxide (rutile) 13.3
4 Ilmenite 17.9
5 Aluminium 17.3
100.0
a
Binder can be prepared [52] by using 50 parts ethyl
silicate-40, 43.2 parts isopropyl alcohol, 5 parts water, one
part 5% HCl, and by stirring the contents for 5 h at

40
◦
C.
Specifications
of the zinc dust commonly used in the
ethyl
silicate based paint formulations are given hereunder [4].
Specifications of zinc dust
(i) Composition
Total zinc 98–99.2%
Metallic zinc 94–97%
Zinc oxide 3–6%
Lead 0.2% maximum
Cadmium as (CdO) 0.7% maximum
Volatile 0.1% maximum
Moisture and volatile 0.1% maximum
Iron 0.04% maximum
(ii) Coarse particles
Retention on 100 mesh Nil
Retention on 200 mesh Nil
Retention on 325 mesh 4% maximum
(iii) Particle size distribution (Coulter counter)
Medium particle size
6–10
microns
Specific surface
≤
0.17
m
2

/g
Spherical particles, specific gravity 7.0 g/cm
3
(iv) Dispersibility
Should disperse satisfactorily in a high speed disperser
4. Chemistry of hydrolysis reaction of alkyl silicates
Hydrolysis of alkyl silicates is influenced by various
factors [53] such as,
1. Nature of the alkyl group.
2. Nature of the solvent used.
3. Concentration of each species in the solution or reaction
mixture.
4. Molar ratio of water to alkoxide.
5. Reaction temperature.
In addition to these influencing factors, pH of the solu-
tion is also an important factor which governs the rate of
hydrolysis reaction and condensation of the hydrolysed
product. In acidic condition, hydrolysis reaction takes place
through electrophilic substitution and in basic condition, the
hydrolysis proceeds through nucleophilic reaction. When
pH of the solution is ≈2.5, alkoxy groups remain unaf-
fected because silicate particles are not charged at this pH.
Above or below this pH, they can be attacked by water.
Rate of hydrolysis increases with increase in pH of the
solution. At pH below 2.5, silicate particles are negatively
charged and at pH above 2.5, they are positively charged.
At lower pH, hydrolysis takes place through SE
2
mecha-
nism and at higher pH, this reaction corresponds to SN

2
mechanism.
In case of alkyl silicates, nucleophilic attack is sensitive
to electron density around the central silicon atom. This
electron density increases due to the size of substituent
groups. Susceptibility to nucleophilic attack increases with
decrease in bulky and basic alkoxy groups around the cen-
tral silicon atom. However, reactivity of the tetrahedron
towards electrophilic attack is enhanced by an increase in
electron density around silicon. Initial hydrolysis of sili-
con ester monomer produces silanol groups, whereas full
hydrolysis can lead to silicic acid monomer. This acid is
not stable and condensation of silanol groups occur lead-
ing to polymer formation before all alkoxy groups are
substituted by silanol groups. Condensation polymerisa-
tion reactions proceed with an increase in viscosity of the
alkoxide solution until an alcogel is produced. In gen-
eral, acid catalysed reactions yield alcogels, whereas base
catalysed hydrolysis reaction precipitates hydrated silica
powders.
4.1. Mechanism of the hydrolysis reaction
Alkyl silicates are not water soluble in nature, because
of which a mutual solvent is needed to hydrolyse it. Thus,
hydrolysis is carried out in the form of solution, and ethyl
alcohol and isopropyl alcohol are generally used as the
mutual solvent.
When pH of the aqueous solution is 2.5, the silicate par-
ticles are not electrically charged. However, when pH of
an aqueous solution is quite acidic and the silicate particles
get negatively charged, the relatively high concentration of

8
G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–
14
protons catalyses the hydrolysis reaction. The mechanism
then corresponds to an electrophilic substitution in which
an
(H
3
O)
+
hydronium ion attacks the oxygen of one of the
alkyl groups.
In the intermediary complex of this mechanism,
the
coordination number of Si increases. The rate of
reaction
depends as much on the concentration of
H
3
O
+
as on the
one of the alkoxides. The mechanism is consequently an
SE
2
, and steric strain is also an important factor. The rate
of hydrolysis decreases as the length of alkyl group
increases. The reaction mechanism is as given below:
(15)
In alkaline conditions, silicate particles are

positively
charged and OH
−
anion attacks the alkoxide through an
SN
2
mechanism in order to form the silanol group. Since
δ(OR)
complex
<
δ
(
OR
)
alcohol

, at least one OR or OR
−
ligand must leave the intermediary complex formed by sili-
con. The anion then recombines with a proton so as to form
an alcohol molecule. The mechanism of the reaction has
been shown below:
(16)
For this reaction, another more complex mechanism
is
also proposed which involves two intermediary
complexes.
Since Lewis bases are strong nucleophiles, they
can
deprotonate the OH ligands of cations, which form

acidic
oxides, thus creating oxo ligands. Lewis base such
as
sodium hydroxide, ammonium hydroxide, etc. can effect
this type of reaction.
The silanol group (
≡
SiOH) resulting from the
hydrolysis
of silicon alkoxide can be converted to oxo ligand. For this
reaction, base is a necessary catalyser, and the reaction can
be as given hereunder:
(17)
Traces of water vapour can also hydrolyse
metal

alkoxides
thus
transforming
them into
oxi-alkoxides.
Such a
hydrolysis
follows a reaction of the following type:
(18)
4.2. Condensation of alkyl silicates
In acidic conditions, silicon alkoxide condenses through
a two step mechanism which corresponds to SN
2
type of

mechanism. In
first
step, silanol groups are protonated
which increases the electrophilic character of the
surrounding silicon atoms.
As a consequence, this protonated silanol combines to
another silanol group while liberating a
(H
3
O)
+
ion. The
two silicon atoms of the resulting polymer are then linked
through an oxo bridge called, in this specific case, as silox-
ane bond. It can be noted that the Si of the
intermediary
com- plex of this mechanism is either tetra or penta
coordinated. Mechanism of condensation reaction is as
given below:
(19)
(20)
Rate of condensation reaction depends
on

the
second
step
of the mechanism and is proportional to the concentration
of
the protons. Hence condensation is a slower transformation

G. Parashar et al. / Progress in Organic Coatings 42 (2001) 1–
14
9
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
SN
2
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, accel-
erate condensation and alcohol molecules are better leav-
ing groups than water. Efficient Lewis bases include, for
instance, DMAP (dimethyl aminopyridine),
n-Bu
4
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 al-
kali 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 absence of
alcohol, undergoes polycondensation reaction as shown in
Eq. (22).
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:
H
2

O
+
CO
2
→
H
2
CO
3
(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 sili-
cate 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
(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.
(26)
Ethyl silicate based binders can be cured by IR radia-
tion [54], alkali metal salts of thio acids, barbutaric acids,
and/or, 1,3-dicarbonyl compounds [55], and also by 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
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, they have some limitations
also such as:
1. They have poor resistance for acidic or alkaline condi-
tions 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
11
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 blis-
tering 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 sili-
cate 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.
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 im-
portant parameter to be emphasised in the technical specifi-
cations 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 colour-
ing 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 sub-

strate improves. These factors contribute to the improve-
ment of paint performance from the galvanic point of view.
It is important to mention that effectiveness of the zinc-rich
paint does not depend solely on electrochemical factors.
Some other factors such as mechanical properties viz. cohe-
sion, flexibility, etc. are also important. So the addition 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 mus-
covite 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 Metallic zinc
con-
tent in the dry
film
Main components
of dry film
Time (h) necessary for appearance of

red corrosion in scratch (ASTM B-117)
Zn60 60.0 Ethyl silicate 460
Zinc
dust
ZnA60 60.0 Ethyl silicate 740
Zinc
dust
Agalmatolite
ZnB60 60.0 Ethyl silicate 660
Zinc
dust
Barytes
Zn75 75.0 Ethyl silicate 2060
Zinc
dust
ZnA75 75.0 Ethyl silicate 2060
Zinc
dust
Agalmatolite
ZnB75 75.0 Ethyl silicate 2060
Zinc
dust
Barytes
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. Prob-
lems of toxic fumes during welding, however, precludes
the use of cadmium in these coatings. Out of various exten-
ders used in ethyl silicate based zinc-rich paints, the best
results have been obtained from di-iron phosphide
(Fe
2
P),
which is a refractory conductive compound. In ethyl sili-
cate 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. Composi-
tions of some ethyl silicate vehicles formulated with higher
concentration of
Fe
2
P lead to abnormally high zinc corro-
sion products. Ethyl silicate zinc-rich coatings with
Fe
2
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
Fe
2
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
Fe
2
P. Zinc appears to be
consumed more efficiently in the presence of
Fe
2
P with the
result that improved corrosion protection is obtained with
lower initial
zinc content while a greater fraction of the zinc initially
present remained unoxidised after a given period of time.
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 encoun-
tered 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
13
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 in-
herent 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 orig-
inal 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 docu-
mented 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 origi-
nal 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
and lithium silicates and alkyl silicates such as ethyl sili-
cate 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 tem-
perature 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 envi-
ronmental 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 pro-
tect 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 un-
matched 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.
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