Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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246 High-performance organic coatings

surface tension and go up to the top and spread there, suppressing the

material that has been on the top before. Again the diluent evaporates and

a circulation starts; this is repeated until the wet paint is no longer move-

able, due to increased viscosity. This circulation results in hexagonal cells

(under ideal conditions), called Benard cells. In coatings we can see the

results of this effect as ‘orange peeling’ or horizontal pigment separation.

the ingredients with the lowest surface tension, i.e. they migrate to the top

and spread there in a very thin layer (from a monolayer to one some

molecules thick). As there is nothing in the paint formulation that has a

lower surface tension, there will be no circulation in the paint during drying.

Of course, in reality the above-mentioned effects are always overlapping,

but depending on the paint formulation and/or the chemical structure of

the siloxane-polyether copolymer, one or the other effect dominates. Quite

often the best results are achieved by combinations of different structures

truly synergistic one, not just an addition of the single effects.

Now, after adding siloxanes (or modified siloxanes), without doubt they are

Smooth films without Benard cell-related defects will be achieved.

that influence different parameters, and quite often the resulting effect is a

247

Waterborne coatings for corrosion protection

S S PATHAK and A S KHANNA, Department of

Metallurgical Engineering & Materials Science, India

13.1 Introduction

A conventional coating consists of a resin, pigments and additives, dis-

persed in a suitable solvent. Film formation and solvent evaporation start

simultaneously after coating application. Complete evaporation of solvent

is a must for complete curing of a coating. However, evaporation of solvents

during the curing process affects the health not only of the paint applicators

but also of persons in the vicinity. The most commonly used solvents are

toluene, xylene, methylethylketone, etc. Some of the health effects caused

by these solvent vapours are cardiac arrhythmia, fatigue, sleepiness, head-

aches and nausea, and irritation of the nose, throat and eyes.

13.2 Alternatives to solventborne coatings

Due to the diverse effects on human health and the environment, stricter

legislation has been imposed on the paint and coating industries to reduce

and monitor the amount of hazardous volatile solvents. There are two pos-

sible ways to make coating formulations environmentally friendly:

• Use of solvents that are not hazardous to health and the environment

as well as use of hazardous vapour collection equipment during applica-

tion of coatings.

• Development of waterborne coatings, high solid coatings and solvent-

less coatings.

A brief introduction to these alternative coating technologies is presented

in Table 13.1. The alternative paint formulations are high build systems

where the solvent level is reduced to less than 15–20%, and solventless

systems where the solvent level is reduced to 1–5% by volume. An addi-

tional formulation is to use alternative solvents such as water, which is

available abundantly and is much cheaper. This class of coating is called

waterborne coatings.

248 High-performance organic coatings

13.3 Waterborne coatings

Waterborne means something derived from water. A coating formulation

that utilizes water as the main volatile medium (at least 80% water as the

main volatile medium) to disperse its components is called a waterborne

coating. Most regulations require waterborne coatings to have a VOC of

less than 425 g per litre of paint. The increasing importance of waterborne

coatings also lies in the environmental as well as the biological importance

of water, which is highlighted by the water-covered Earth and the essential-

ity of water to all known forms of life. Water has been referred to as the

naturally occurring universal solvent.

Many types of resins are available in a waterborne version, including vinyls,

two-component acrylics, epoxies, polyesters, styrene-butadiene, amine-

solubilized, carboxyl-terminated alkyd and urethanes. Waterborne coatings

of the polymer particles they contain (Table 13.2). The three main types

are water-soluble/water-reducible (solutions), water-dispersible/colloidal

(dispersions) and emulsions (latex) paints (the most commonly used form).

Within each category, the physical properties and performance depend on

Table 13.1 Environment-friendly alternative coating technologies

Nomenclature % Solid

content

Advantages Disadvantages

High-build

coatings

60–80% Higher thickness per

coat, low VOC

High viscosities, slight

application

Solventless

coatings

100% Very high thickness per

coat

Costly equipment;

airless spray technique

used for coating

Waterborne

coating

Same as

solvent-

borne

system

Low VOC emission; low

viscosity; reduced

toxicity; no odour and

tional application pro-

cesses; suitable for thin

Resins for waterborne

formulations are costly

compared with

conventional coatings;

tendency to form

foam; requires longer

drying times or

increased oven

temperatures

difficulty in brush

film application

13.4 Classification of waterborne coatings

the type of resin used [1]. Before discussing the specific features of water-

non-flammable; conven-

can be classified either on the basis of how the resin is fluidized or in terms

Waterborne coatings for corrosion protection 249

Water-soluble/water-

reducible (solutions)

Water-dispersible/

colloidal (dispersions)

Emulsion

Paints whose individual

molecules of water-soluble

resins dissolve completely in

water.

Paints that have small

clusters of insoluble

solid resin particles that

are suspended in water.

Emulsions are

formed from

dispersion of liquid

in polymer in

water.

Water-soluble resins are

generally produced via

polycondensation or polym-

erization reactions in an

organic medium.

Mechanical agitation is

the clusters.

On the basis of size of resin particle

True solution Dispersion Emulsion

<0.01 μm 0.01–0.1 μm 0.1–1.0 μm

borne coatings, a basic introduction to the physical properties of water

which are dramatically different from those of solvents used in coatings, is

necessary. The effects of these unique properties on the performance of

waterborne systems are outlined below.

the recipe of the coating as well as its end user properties will be discussed.

The surface tension, evaporation rate and freezing point of water are dra-

matically different from those of other solvents used in coating formula-

tions. The explanation for water’s unusual behaviour lies in its molecular

structure.

13.5.1 Polarity

Water (H

O) is a polar molecule consisting of two hydrogen atoms cova-

lently bonded to a single oxygen atom. The oxygen pulls the electron of the

hydrogen towards itself because of the higher electronegativity of oxygen

than that of the hydrogen atom. This accumulation of electrons towards the

oxygen leads to the generation of a negative charge on the oxygen and a

positive charge on the hydrogen.

Table 13.2 Classification of waterborne coatings

sufficient to suspend

On the basis of resin fluidization

13.5 Water properties influencing coating formulation

In this section, the physical and chemical nature of water that influences

250 High-performance organic coatings

13.5.2 Association of water

The three-dimensional water molecule is held together by strong hydrogen

bonding between oxygen and hydrogen (Fig. 13.1). The hydrogen-bonded

structure of water makes it more resistant to cleavage by normal molecular

vibration. Therefore, hydrogen bonding increases the heat energy require-

ment for the conversion of water to vapour as compared to other solvents

used in coating formulations. The high surface tension of water is also

caused by strong cohesion between the water molecules. Various properties

of water are compared with those of solvents widely used in coating for-

mulations in Table 13.3.

13.5.3 Freezing

The higher freezing point of water than that of organic solvents of intro-

duces the need for special storage systems in climates where low tempera-

tures are possible. The storage temperature should be at least 10°C. The

anomalous density/temperature characteristics of water should also be con-

sidered, to avoid thaw, separation and/or coagulation of the coating formu-

lation during storage.

Hydrogen bond

Covalent bond

13.1 Association of water molecules.

Waterborne coatings for corrosion protection 251

13.5.4 Evaporation rate

Complete evaporation of the solvent from the coating is necessary and is

the most important criterion for a good coating formulation. The evapora-

tion rate of the solvent depends on the boiling point and latent heat of

vaporization. As with surface tension, water has a high value for the latent

heat of vaporization as compared to solvents used in coating formulations

(Table 13.3). Therefore, waterborne coatings use up more heat energy for

the complete evaporation of water than do solventborne coatings for the

evaporation of the solvent. Also, the rate of evaporation of water, unlike

sphere. At higher relative humidity, the capacity of the atmosphere to

accept more water vapour decreases. This leads to a slower rate of evapora-

tion of water from waterborne coatings.

13.5.5 Surface tension

The surface tension of a coating formulation is related to the wetting and

adhesion of the coating to the substrate. For good wetting, the surface

Table 13.3 Properties of water in comparison to various solvents and

substrates

Properties Solvents

Water Acetone Xylene White spirit

Melting point

(°C)

0 −94 −25

Boiling point

(°C)

100 56 138–142 152–198

Flash point (°C) −17 25 36–41

capacity

73 24 30 18

Latent heat of

vaporization

(cal/g at b.p.)

540 135 94 115

Surface tension (dyne/cm)

Solvent (in

air at 20°C)

Water 72.8 Xylene 28.2 C

OH 22.7 CHCl

26.8 Benzene

28.8

Substrate

Treated

steel 40–50

Aluminium

Untreated

steel 30

Glass

70–80

Specific heat

non-flammable

Teflon 18

that of organic solvents, is influenced by the relative humidity of the atmo-

252 High-performance organic coatings

tension of the coating formulation should be lower than that of the sub-

strate on which it has to be applied. Table 13.3 shows that water has a very

high surface tension when compared to widely used solvents for coating

formulations as well as common substrates. Therefore, waterborne coatings

bility, non-toxicity and easy availability of water favour its use as a potential

solvent for coating formulations. Also, strict regulations on the use of non-

toxic materials for coating formulations have encouraged greater use of

water in industrial coatings [2].

13.6 Technology behind development of

waterborne coatings

13.6.1 Development of water soluble/reducible resin

In this section, the techniques of synthesis of water-soluble/reducible poly-

discussed. Such moieties are water loving or can be made water loving after

using one of the following techniques:

• Salt formation, or simply the incorporation of carboxylic or amino

groups onto the backbone of the polymer. These can be converted into

anions or cations by a number of acid–base reactions.

• Introduction of non-ionic groups such as polyols or polyethers into the

backbone. These polymers are of lower molecular weight and can

remain water-sensitive.

• The formation of intermediate zwitterions (groups containing both posi-

tive and negative charge).

13.6.2 Development of sol-gel derived waterborne coatings

Apart from conventional techniques of waterborne coating formulation,

the sol-gel process [3] is one of the emerging technologies for the develop-

ment of high-performance waterborne coatings. It has provided a new tool

through the in situ formation of an inorganic network by hydrolysis and

polycondensation of metal alkoxide R(MX

)

where X = OR, R, H, Cl and

M = Ti, Si, Zr as shown in Table 13.4.

Silicon alkoxides are extensively used metal alkoxides for the incorpora-

silicon alkoxides such as alkyl silanes, amino silanes, epoxy silanes and

require some modification to reduce the surface tension for easy wetting

and good adhesion to low surface energy substrates. Modification involves

mers by incorporation of specific moieties into the polymer backbone are

appropriate modification. These water-loving moieties can be incorporated

for the efficient incorporation of inorganic moieties into organic polymers

tion of inorganic moieties into organic networks. Organically modified

the addition of surfactants and wetting agents. However, the non-flamma-

Waterborne coatings for corrosion protection 253

acrylic silanes are formed by the silane crosslinking principle or by complex

formation which involves hydrolysis and condensation of liquid monomers

in solution. The result of the hydrolysis reaction is a sol (colloidal solution)

as an intermediate product which is converted to a transparent gel on sub-

sequent condensation. The solution-based nature of the sol allows the

incorporation of appropriately functionalized organic moieties into it that

can undergo the same condensation reaction as the silicon dioxide sol.

Sol is a system that allows a chemical species (macromolecules and par-

ticles) to become weightless suspended bodies in solution. It is made of

solid particles of diameter a few hundred nanometres. The hydrolysis and

condensation reaction starts between the chemical species when the sol is

applied to the substrate, resulting in gelation (three-dimensional network

formation). After gelation, a highly solvated solid forms in which the solvent

removal of the solvent by evaporation or thermal treatment as shown in

Fig. 13.2. The sol-gel coating formulation and its application process to the

substrate involves three steps, namely:

1. Generation of sol (colloidal suspension) of coating recipe

2. Application of sol onto substrate and its thermal curing

Table 13.4 Metal alkoxide for sol-gel coating

Element Alkoxide (OR) Structure of alkoxide

Aluminium

–

OCH

–

Silicon

–

OCH

–

OCH

Titanium

–

OCH

–

Zirconium

–

Cerium

–

OCH

is trapped, which further gives rise to a dense film on the substrate after

Continuous and dense film formation on solvent evaporation.

254 High-performance organic coatings

Sol-gel coatings have found use in different applications, for example

abrasion resistance, anti-soiling and anti-fogging coatings on various sub-

strates. In particular, it is found that they can provide good corrosion

resistance for metal substrates, because they blend the mechanical and

chemical characteristics of the constituent organic and inorganic networks

paint systems. The structural characteristic of various precursors and coating

formulations will be discussed later.

OCH

Organic

polymer

Organic

polymer

OH HO

Organic polymer

Interaction between hydroxyl group of silanol

and organic polymer (condensation reaction)

Where organic polymer, epoxy, alkyd,

polyester, urethane, vinyl or amine

derived

+ Curing agent

Metallic and non-metallic substrate

Room temperature drying or

thermal curing

Gelation and then

continuous sol-gel

water evaporation

Application of

waterborne sol

on substrate

Hydrolysis reaction (in aqueous medium)

Tetramethoxysilane

Silanol

application of sol to substrate.

Organically modified silane

film formation on

13.2 Sol-gel technology for organic modification of silane and

[4]. Sol-gel films are durable, scratch resistant, adherent to the metal sub-

strates, flexible, dense, and functionally compatible with organic polymer

Waterborne coatings for corrosion protection 255

13.7 Development of resins for waterborne

coating formulations

13.7.1 Cellulose ether

it is necessary to replace some of the hydroxyl group by other substituents.

Water-soluble cellulose ether can be produced by replacing the hydrogen

sodium hydroxide and then the alkali cellulose is reacted with the appropri-

ate moiety to introduce the required groups (Fig. 13.3).

13.7.2 Polyester

polyhydric alcohol with di- and polybasic acids or anhydride (Fig. 13.4).

The method of preparation of waterborne polyester is similar to that of

waterborne alkyds. It involves the use of either hydroxy acids such as

dimethylpropionic acid, or acid anhydride such as trimetallic anhydride

(Figs 13.5 and 13.6). In both cases, there is a considerable residual acid

functionality on the polyester chain. In case of trimetallic anhydride ring,

only the anhydride ring of the acid is initially involved in the synthesis

reaction. At the coating formulation stage, the residual acidity is neutral-

ized with amines to form a salt, rendering the polymer water soluble. In

OROH

where R is cellulose

R ROH + NaOH ONa

R OCH

(methyl cellulose)

(hexaethyl cellulose)

R OCH

(S)

(R)

13.3 Conversion of cellulose to water-soluble ether derivatives.

Cellulose ethers are derived from cellulose fibres which find application as

a film former or thickener in textile, paper coating and adhesive applica-

tions. Cellulose fibres are insoluble in water. To make them water soluble,

atom. For this, cellulose is first converted to alkali cellulose by reaction with

Polyester resins [5] are the reaction product of the esterification of di- or