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

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Key issues in applying organic paint coatings 5

An inorganic zinc silicate coating is entirely different from an organic

zinc-rich coating. Here also, zinc dust is added in an organic resin, but in

this case the organic resin is an active one which bonds strongly with the

zinc atom, which in turn forms a chemical bond with the substrate [1].

Hence inorganic zinc coatings are applied only on very clean surfaces

resin used is ethyl silicate to which more than 80% of good quality zinc is

added. Figure 1.2 shows how an ethyl zinc silicate coating forms a strong

bond with the substrate.

An inorganic zinc silicate (IOZ) coating, when applied along with two

coats of epoxy and followed by one polyurethane top coat, can give a life

of more than 5–6 years in an aggressive environment, while the same

coating without IOZ will not have a life of more than 2–3 years. An IOZ

coating, if applied alone, can also last for six months to one year in a rela-

tively moderate environment, compared to an organic zinc-rich coating

which may survive for as little as a few hundred hours. Furthermore, while

an organic zinc-rich coating cannot sustain a temperature of more than

150°C, an IOZ coating can withstand a temperature of more than 450°C.

OZn

ZnO

SiZnO

OZn

ZnO

OZn

ZnO

OZn

ZnO

OZn

Zinc particle

Zinc atom

1.2 Structure of an inorganic zinc silicate coating.

having a surface finish equivalent to or better than Sa 2-. The active organic

6 High-performance organic coatings

IOZ is the most common coating applied on heat exchanger components

in power plants.

Shop primers are special kinds of primer that have a longer life than

the normal organic zinc-rich primers. A shop primer such as red oxide in

epoxy is a well-known primer used in the ship industry and other heavy

machinery construction. Along with the advantage of having a longer life

than the conventional primers, shop primers can be painted on as a single

coating together with a top coat without surface preparation or another

coat of the primer.

1.3 Intermediate and top coats

The intermediate and top coats are generally organic paint coatings. Based

upon the kind of environment, application technique and climatic condi-

tions, different types of paint coatings can be chosen. The range of coatings

Conventional coatings

• Alkyd

• Epoxy

• Polyurethane

• Chlorinated rubber

• Coal-tar

• Vinyl

• Polyester

Advanced coatings

• Solventless coatings

• Fluoroelastomeric coatings

• Polysiloxane coatings

• Polyurea

Conventional coatings are basically solvent-based coatings and can be of

one or two components. A one-component coating is simply a solution of

a resin in a solvent. These are also called lacquers. They provide a coating

which remains on the surface after the evaporation of the solvent. These

coatings do not give very strong corrosion protection and also do not have

good rigidity, strength, etc. In a two-component system, the coating dries by

reaction between the resin and the hardener which cures with time and

becomes hard. The coating becomes harder by strong crosslinking between

the resin and the hardener. The following sections deal with different types

of coating and also look at ways of improving coating properties.

can be classified as follows:

• Glassflake-modified coatings

Key issues in applying organic paint coatings 7

1.4 Curing mechanisms

Coatings dry by several mechanisms. The simplest is by evaporation of the

solvent. Whether the paint is one-component or two-components, in

solvent-based coatings solvent evaporates, leaving the resin on the sub-

strate. In a two-component system, crosslinking of the coating also proceeds

simultaneously.

The second most common mechanism is reaction with oxygen. Oil-

based coatings such as alkyd coatings dry by the reaction of oils with

oxygen. Because this process is very slow, it takes several days to completely

cure an alkyd coating. The third main mechanism of curing is the chemical

reaction between resin and hardener in two-component systems. As the

resin and the hardener or catalyst are mixed, crosslinking starts and

with time the hardness of the mixture increases till it becomes the complete

solid.

The fourth mechanism is radiation curing, where the paint is dried

the paints are dried by heating. This process is called stoving. In coil

coatings, where the strips are coated online, the immediate coated strip

The coated coil travels through this furnace. It is important that the

coating temperature does not rise above 150°C, above which many coatings

disintegrate.

1.5 Pigments

Pigment is an essential constituent of a coating. It is a common perception

that the main role of pigment in a coating is to provide color and opacity.

However, pigments can also be added for several other important reasons.

The main functions of pigments are summarized below:

• Providing color and opacity

• Strengthening of paint resin

• Providing barrier properties

• Enhancing corrosion resistance

protection.

variation in the properties of these two groups. Inorganic pigments are heat

resistant with long durability and are often costly. Organic pigments have

varying properties with respect to durability, heat resistance and cost. In the

by exposing it to microwave radiation or UV radiation. In the fifth case,

• Adding functional properties such as heat resistance and fire

It is difficult to classify pigments meaningfully [4]. The traditional classi-

fication is in terms of organic and inorganic pigments. There is significant

last few years, the use of nanopigments has also become significant. Very

is passed through a long furnace, maintained at a fixed temperature.

8 High-performance organic coatings

common pigments such as ZnO, SiO

, TiO

, Al

, etc., when added as

ings [5]. It has been found that only 0.03% of ZnO added as a nanopowder

in a waterborne coating improved the mechanical, corrosion and UV prop-

erties several times [6]. In the same way addition of 0.3% of nano-alumina

improved abrasion and scratch resistance of an alkyd-based waterborne

coating by almost 10% [7].

1.5.1 Color and opacity

While inorganic pigments normally give high opacity and are often dull in

color, organic pigments are usually bright and normally transparent. Table

1.1 lists some of the organic and inorganic pigments that are commonly

used for giving different colors to paint coatings.

The most important pigment used in many coatings is titanium dioxide,

commonly known as universal white pigment. Its high refractive index gives

high opacity and a pure white color to the coating. More than 70% of

organic pigments belong to the azo group, especially in the yellow, orange

membered ring in their chemical structure. Phthalocyanine and quinacri-

done are also included in this category [4].

Table 1.1 A list of a few selected organic and inorganic pigments

Inorganic pigments Organic pigments

Carbonates White

– colorless

Isoindolinone Yellow, orange

Oxides White, green,

yellow, red,

brown

Quinophthalone Yellow

Sulfates Colorless Anthrapyrimidine Yellow

Ferrocyanides Blue Flavanthrone Yellow

Chromates,

molybdates

Yellow, orange,

red

Perinone Orange, red

Silicates White, colorless,

blue

Carbazole Violet

Carbon Black Perylene Red, maroon

Metallic Aluminum

bronze

Benzimidazolone Yellow, orange,

red, brown

Sulﬁ des,

sulfoselenides

White, yellow

orange, red

β-naphthol Red, orange

nanopowders, have significantly improved the performance of paint coat-

and red colors. Many of the non-azo pigments are classified as polycyclic

pigments. They are organic compounds with more than one five- or six-

Key issues in applying organic paint coatings 9

1.5.2 Strengthening of paint resin

The principle of strengthening paint resin using a pigment is to form a com-

there is enhancement of the permeability resistance of the coating.

coating and inhibits moisture permeation, thereby enhancing the life of the

an epoxy matrix. If we now consider, for example, an ideal distribution of

a permeating pollutant, for example moisture, chloride, etc., will follow will

tion. This results in the enhancement of coating life.

size up to a concentration of 15% [8].

1.5.3 Providing barrier properties

Micaceous iron oxide, commonly used in Europe and to a lesser degree in the

United States, has lamellar (platelike) particles. Any relatively large-sized

pigment particle, even if it is not plate-shaped, will increase the permeating

path of moisture and will result in a barrier-type protection. Mechanism of

barrier protection by micaceous iron oxide is shown schematically in Fig. 1.4.

The moisture which can penetrate the organic coating is resisted by a lamellar

micaceous iron oxide internal layer next to the substrate.

1.5.4 Enhancing corrosion resistance

An organic coating applied on a steel substrate is called a simple barrier

coating, as the organic coating just separates the steel substrate from the

Substrate

Moisture penetration

1.3 The tortuous path being followed through a coating cross-section

posite matrix by adding some inorganic pigment material as particles, fibers

in imparting the optimum benefit to the coating properties. An ideal com-

Glassflakes

with an ideal distribution of glassflakes.

or flakes. Depending upon the size, thickness and distribution of the parti-

cles, fibers or flakes, in addition to the increment in the strength of the matrix,

As an example, adding glassflake as a pigment provides strength to the

coating. This is characterized typically by a random distribution of flakes in

flakes in cross-section as shown in Fig. 1.3, it can be seen that the path which

be several times longer than the path followed without glassflake incorpora-

The addition of glassflakes, their size and distribution are very important

position has been found to be achieved with glassflakes about 0.04 mm in

10 High-performance organic coatings

environment. In the case of any damage to the coating, the substrate

surface would be exposed to the environment, resulting in its corrosion,

because there is no additional protection to the steel surface. By the

addition of certain pigments, this corrosion can be reduced. Depending

upon the type and concentration of the pigment, the coating will become

sence of zinc pigment in a small concentration of zinc (pure dust, zinc

chromate or phosphate) can provide protective properties to the paint

as shown in Fig. 1.5. The coating under these conditions has better

properties than a pure barrier coating with no inhibitor at all. As the

concentration of the pigment increases, the barrier properties of the

centration of the pigment may result in an organic coating which can give

a complete cathodic protection to the steel surface even if the organic

coating is damaged. This happens when more than 70% of zinc dust is

added into the organic resin. Such a coating is called an organic zinc-rich

coating. The inhibitive pigments are usually water soluble. Whenever

Main coating

MIO layer

Substrate

1.4 Mechanism of barrier protection by lamellar micaceous iron oxide

layer.

(R)

(S)

(R)

CPVC

Steel substrate

Pigment

Binder sheet

around paint

Interstitial binder

Air-filled voids

Barrier coating

Inhibitive coating

Inorganic zinc-rich coating

Organic zinc-rich coatings

1.5 The change in properties of a coating by changing the pigment

concentration.

slightly modified as a result of passivity on the surface, while a large

concentration of the pigment may fully protect it sacrificially. The pre-

coating change to inhibitive properties. A sufficient increase in the con-

Key issues in applying organic paint coatings 11

paint coatings are exposed to a moist environment, with the permeation of

some moisture from the environment to the coating, the pigment dissolves

and reaches the surface of the steel. It passivates the steel and so inhibits

corrosion [9].

The role of these active pigments is thus quite different from that of

barrier pigments. Active pigments result in an electrochemical reaction to

form a passive layer, while barrier pigments provide a physical barrier to

the movement of corrosive species. Table 1.2 differentiates the role of active

corrosion pigments from those of barrier pigments.

1.5.5 Functional properties such as heat resistance and

There are several pigments which, when added in paint, enhance its heat

chemicals, which on burning cause thick char formation, prevent a steel

structure from losing its mechanical properties and hence give additional

time for safe evacuation of people and valuables. Pigments such as tetra-

ping it from spreading. Phosphorus-containing compounds act in the solid

phase of burning materials. When heated, the phosphorus reacts to give a

polymeric form of phosphoric acid (PO

), causing the material to char.

polyphosphate, red phosphorus metal and organics. Ammonium polyphos-

phate is preferred for coating applications.

Active pigments Barrier pigments

Principal functions:

• Anti-corrosion effect on

steel by electro-chemical

mechanism

• Inhibiting anodic or

cathodic areas

Principal functions:

• Anti-corrosion effect on steel by

barrier mechanism

• Ultraviolet (UV) radiation absorption

Examples:

• Zinc metal pigments

• Zinc chromate pigments

• Zinc phosphate pigments

• Red lead pigments

Examples:

• Micaceous iron oxide (MIO) pigments

• Aluminum pigments

• Talc pigment extender

• Mica pigment extender

Table 1.2 Classification of various pigments

fire protection

resistance properties or make it fireproof. The addition of intumescent

• Glassflake pigments

bromobisphenol-A hexabromocyclododecane are flame retardants which

remove hydrogen and hydroxyl radicals in the gas flame phase, thus stop-

Examples of phosphorus-containing flame retardants include ammonium

providing film protection

12 High-performance organic coatings

1.6 Additives

Additives are important constituents of paint systems which modify a spe-

two manufacturers, having the same resin, hardener and pigment, may not

be the same because of the use of different additives to improve various

other properties of the paint coatings such as wettability, surface tension,

various additives can be given as:

• Wetting and dispersing additives

• Surface additives

• Defoamers

• Anti-skin additives

• Preservatives

• Ultraviolet stabilizers.

Wetting additives reduce interfacial tension between pigment particles

and resin. They also reduce sagging and settling of paint. Silicones and

acrylic copolymers are well-known surface additive chemicals. They reduce

different chemicals are added to avoid foaming of paints (for example,

mineral oils or polysiloxane). Other chemicals protect paint from attack by

fungi and bacteria. For this purpose, various biocides are added [9].

One of the biggest problems of paints is their stability in UV light. White

paints usually go yellowish, and dark-colored paints become faded due to

exposure to UV light. Chemicals such as phenyl salicylate and 2-hydroxy

4-methyl benzotriazole act as UV absorbers and hence prevent the paint

from deteriorating.

1.7 Alkyds

Organic paint resins can be either natural or synthetic. Examples of natural

resins are:

• Rosins, from sap exuded from pine trees

• Shellac, a secretion of a coccid insect

• Copals, derived from vegetable remains.

Use of these resins in paint applications is limited. Synthetic resins are used

improved properties such as better resistance to moisture, chemical and UV

attack, compared to the natural resins, they are used in a variety of different

service environments as corrosion protective coatings [9].

cific property of the paint coating. The performance of paint systems from

reaction to UV light, dispersion, etc. Each specific property is taken care

of by a different additive and its concentration. A broad classification of

as film formers for both protective and decorative coatings. Since they have

surface tension, improve slip and control surface flaws. In the same way,

Key issues in applying organic paint coatings 13

An alkyd resin is synthesized by reacting an oil with an acid and alcohol.

The principal raw materials are oils (castor oil, soya bean oil, etc.), fatty

acids, polyhydric alcohols (glycerol) and dibasic acids (phthalic acid). Figure

1.6 illustrates the formation of an alkyd resin.

Alkyd coatings possess properties derived from the drying oils used in the

manufacture of the alkyd resins. Properties such as drying time, hardness,

color and moisture sensitivity all depend on the drying oil, its type and the

COOH

-CH

C-O-CH

Polyol

Anhydride

Alkyd resin

Fatty acid

+ 7 Water

1.6 Formation of an alkyd resin from alcohol, fatty acid and a

dibasic acid.

14 High-performance organic coatings

degree of unsaturation. Soya bean oil has been shown to give good drying

rates and good color retention. However, linseed oils generally dry faster

but darken on exposure to light. Castor and coconut oils have good color-

retentive properties and are used as plasticizing resins because of their non-

oxidizing characteristics. The extent of the reaction of the oil with the resin

oil resins (2 l/kg) have poor moisture and chemical resistance and longer

drying times. Low-oil alkyds (0.80 l/kg) are fast-drying coatings and have

good moisture and chemical resistance but are relatively hard and brittle [9].

The drying mechanism of an alkyd coating initially involves a solvent

evaporation mechanism followed by curing by auto-oxidative crosslinking

of the oil constituents. Alkyd coatings do not have strong chemical resis-

tance and hence cannot be used in highly aggressive environments and are

generally not preferred for any immersion application. They are, therefore,

used in environments which are not vulnerable to pollution. The biggest

advantage of these coatings is their low cost and their ability to adhere

nicely to poorly prepared, rough, dirty or chalked surfaces.

It is possible to enhance the properties of alkyd coatings. Several proper-

ties such as corrosion resistance, weathering, and mechanical properties

other resins, is given below. Phenolic resin improves gloss retention, water

and alkali resistance. Phenolic alkyd resins have performed satisfactorily in

water immersion conditions, where ordinary alkyd resins fail.

resistance. These coatings are used frequently as shop primers or as tie coats

between different generic coatings, for example over inorganic, zinc-rich

tions such as incorporation of epoxy resin also provide alkyd coatings with

improved chemical and moisture resistance. Though relatively costly, these

are used by equipment manufacturers for appliances, machinery, pumps and

valves.

silicon. A silicone intermediate is added to the alkyd resin in quantities up

to 30% to provide polymers with greatly improved durability, gloss reten-

tion and heat resistance. Moisture resistance is also greatly improved by the

and maintenance paint. Further improvements in drying time of the coating

and enhanced resistance to chemicals, moisture, weathering and abrasion is

provided when alkyd resins are reacted with isocyanates to form so-called

uralkyd or urethane oil coatings.

such as hardness and abrasion, can be modified by addition of other resins.

An example is the modification of alkyd resin with the incorporation of

Alkyd resins with vinyl modification are commonly formulated as uni-

versal primers. The alkyd constituent improves adhesion, film build-up and

thermal resistance. The vinyl modification enhances chemical and moisture

primers or between alkyd primers and epoxy topcoats [9]. Other modifica-

The most widely used modification of alkyd coatings is the addition of

silicone modification, and this type of paint is used extensively as marine

influences the protective ability of the applied alkyd coating. High-solubility