ethyl silicate binders for high performance coatings

Progress in Organic Coatings 42 2001 1–14Review Ethyl silicate binders for high performance coatings of the same have also been developed.. On the other hand, in the zinc-rich ‘inorganic

Trang 1

Progress in Organic Coatings 42 (2001) 1–14

Review Ethyl silicate binders for high performance coatings

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.

Keywords: Inorganic coatings; Silicate binders; Ethyl silicate coatings; Zinc silicate coatings; Heat resistant coatings; Anticorrosive coatings

1 Introduction

Painting is one of the most important techniques used

for the protection of metals from corrosion Effectiveness

of protection of metals against corrosion mainly depends on

the factors such as quality of the coating, characteristics of

the metal, properties of the coating/metal interface, and the

corrosiveness of the environment Typical corrosion

resis-tant coatings protect the metallic surfaces primarily by the

following two mechanisms [1]

1 By acting mainly as a physical barrier to isolate the

substrate from corrosive environment

2 By containing reactive materials (usually pigments)

which react with a component of the vehicle to form

such compounds that, in fact, inhibit corrosion Some

∗ Corresponding author Tel.: +91-512-583-507; fax: +91-512-545-312.

E-mail address: vkj@hbti.ernet.in (P Kumar).

pigments having limited solubility can give rise toinhibitive ions, such as chromates

Undoubtedly, steel is one of the most important metalsused in the modern society However, one of its main draw-backs is its tendency to corrode (rust), i.e to revert to itsoriginal state, and become useless Hence, the protection ofsteel from corrosion, i.e to keep the steel in its usable form,has always been a matter of great concern for all those whouse 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 ofsteel in the form of electroplated layers or by the applica-tion of paints containing a high percentage of zinc particlesdispersed in an organic or an inorganic binder Zinc, either

in the form of electroplated film or in the form of films ofzinc-rich coatings, protects the steel substrate by sacrificialcathodic (galvanic) protection mechanism For the cathodicprotection of steel, the direct electrical contact between the

Trang 2

Trang 3

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

14

adjacent zinc particles, and between the zinc particles in the

film and the steel substrate is required [3]

In the case of zinc-rich ‘organic’ coating films, zinc

par-ticles can be encapsulated by the organic binder, and hence

the zinc particles have restricted electrical contact

Conse-quently, the zinc particles can provide only a small amount

of galvanic protection limited to the amount of free zinc in

the coating formulation [4]

On the other hand, in the zinc-rich ‘inorganic’ coatings

(commonly referred to as zinc silicate coatings), the binders

(inorganic) used are alkali silicates and alkyl silicates,

which can chemically react with the zinc particles in the

coating film to form a zinc silicate matrix around the zinc

particles [5] This zinc silicate matrix is electrically

conductive and chemically inert [2] In addition, the silicate

based binders can chemically react with the steel substrate

also to result in an excellent adhesion and abrasion

resistance of the dried/ cured film [6]

Inorganic zinc silicate coatings are included in the

cat-egory of high performance coatings [7], as these are the

most weather resistant coatings available today [5] They

can provide an unmatched protection against corrosion for

steel structures exposed to temperatures up to 400◦C [2]

2 Silicate binders for inorganic paint coatings

Inorganic paint coatings based on silicate binders can be

classified [6] as shown in Fig 1

2.1 Alkali metal silicate binders

For the manufacture of coatings based on alkali metalsilicates, the silicates based on alkali metals such assodium, potassium and lithium, along with the quarternaryammonium 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 toalkali metal oxide of different silicates [8], which can

be used as binder systems in paints, have been given inTable 1

The ratio of silica to alkali metal oxide, in addition to thetype 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 coatingcharacteristics 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 arewater-borne due to solubility of the used alkali metal oxide

in water These coatings are generally sub-classified intobaked, post-cured and self-cured coatings

2.1.1 Baked coatings

These are the coatings which require heating to convertthe coating films into water insoluble form These coatingsare characterised by their extreme hardness and suitabilityfor application over an acid-descaled surface Bakedcoatings still have limited use today

Trang 4

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

Fig 1 Classification of inorganic paint coatings based on silicate binders.

Table 1

Ratios of silica to alkali metal oxide in alkali silicates [8]

alkali metal oxide

Trang 5

14

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

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

Higher the solubility in water Higher the pH value of the solution Higher the susceptibility to water Higher the adhesion and binding power

Trang 6

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

further higher ratio alkali

metal silicates have

become available Of the

cheaper types, potassium

silicate is preferred

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

formulation of

solvent-borne coatings However,one of the commercialforms of ethyl silicate(popularly known as ethylsilicate-40) as solution inorganic solvent(s) is mostcommonly employed

Alkyl silicates, as such,

do not have any bindingability but when theiralcoholic solutions are

calculated amount ofwater in the presence ofacid or alkali catalyst,they acquire sufficientbinding ability On thebasis of the type ofcatalyst used for thehydrolysis, these coatingscan be sub-classified asfollows

2.2.1 Alkali catalysed coatings

For the hydrolysis ofethyl silicate, bases likeammonia, ammonium

hydroxide and someamines are generallyused as catalysts [2]

One of the greatestdrawbacks of this system

is related to the fact that

in basic

conditions, even a smallamount of water willcause the silicate to gel

To avoid this problem,remedial steps musttherefore be taken toexclude all water at themanufactur- ing stage,and from the applicationequipment If water isexcluded, the liquidcomponent can remainstable for an indefiniteperiod of time Thesecoatings are available inthe market as single-packand two-pack systems In

amines, which providehydroxyl ion in the formwhich is non-reactivewith organic polysilicateuntil they are exposed tomoisture, are used [8]

2 2 2 A c i d c a t a l y s e d

c o a t i n g s

In these type ofcoatings, rapid curingmay be achieved under

3

H y d r o l y s i s

o f

e t h y l s i l i c a t

Trang 7

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

6

e

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

hydrolysis procedures are

commonly employed for

the production of paint

media Several

Trang 8

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

2 1368 parts (by weight) 138 parts (by weight) 0.16 parts (by weight) 12 N HCl 1517 parts ethanol (by weight)

4 45 parts (by weight) 53 parts (by weight) 0.1 part (by weight) 37% aqueous HCl 49.6 parts ethanol (by weight)

Trang 9

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 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

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 Ifethyl silicate is treated simultaneously with a glycolmonoether for alcoholysis and water for hydrolysis, ahydrolysate with a long shelf life is obtained [48] Thishydrolysate can be successfully used as a paint medium.Generally 80–90% hydrolysis of the ethyl silicate is carriedout for the binder preparation [2]

3.1 Factors governing the formulation of ethyl silicate binders

There are some important factors, which can affect thehydrolysis of ethyl silicate and the formulation of ethyl sili-cate binders These factors are discussed hereunder one byone

3.1.1 Effect of quantity of water

Quantity of water and the quantity of acid catalyst usedfor 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 subtractingthe quantity of water (if any) going into the paint formula-tion from the extender pigments and the solvents used inthe formulation Excessive water in the formulation canlead to gelling of the binder system in the cans or very poorapplica- tion properties and gelling of mixed paints in theapplication equipment Less than optimum quantities ofwater can result in an uncured film lacking hardness andfilm integrity [49]

3.1.2 Effect of quantity of acid

Less than optimum quantity of acid can result in silicaprecipitation, thus making less silica available for bindingthan required Excessive quantity of acid will result inaccel-

erated condensation of silanol with silanol (≡SiOH) groups

or with alkoxy groups (≡SiOR) resulting in reduced shelflife 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

Trang 10

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)

Trang 11

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 withzinc and form a zinc silanol heterobridge

(5)This hetero bridge then undergoes further chemicalreactions to form a zinc silicate polymer

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)

Định dạng
Số trang	22
Dung lượng	697 KB