6. Coating Principles

6.4 Binders


The binders fall into the following categories

Examples are given below:


These are formed by reacting alcohols with acids like pthalic anhydride or maleic acid; oils such as linseed, chinawood, soya oils may be used. They are commonly used for plant painting. The properties of alkyd paints are determined by the oils used.

Advantages: User friendly, surface tolerant, expands and contracts with substrate.

Disadvantages: Can contain high amounts of solvent, slow drying, tacky. Only oleoresinous phenolics can be used in severe environments.

Epoxy Esters

These are formulated with an epoxy resin and a drying oil. These are hard and have good adhesion and flexibility. They have improved water and hydrocarbon resistance. They are not very popular because of their tendency to crack.


These are formed by the reaction of isocyanates with hydrogen compounds, which contain a urethane radical. There are three types: oil modifier, moisture cured and catalyses. The oil properties are dominating and they have a high abrasion and water resistance.

Advantages: Best gloss and color retention, protects epoxy intermediate coat from UV, hard but flexible.

Disadvantages: Highly toxic (needs personnel protection), loses gloss when applied in high humidity, user unfriendly, pot life limitation, more expensive than epoxy.

Silicone Alkyds

They are produced by combining alkyds with silicone molecules. They possess a silicon oxygen structure. They can withstand high temperatures.


Unlike alkyds, the lacquers are coatings converted from a liquid material to a solid film by the evaporation of solvent. Examples of lacquers.

Chlorinated rubbers: These are rapid drying, non toxic and highly resistant to water and most chemicals. They degrade above 75oF. They are used for coatings on steel and concrete.


Bituminous materials: These are derived from coal and petroleum sources and often combine with solvents in order to form lacquer type films. There are four major groups.

1. Coaltar: It is manufactured during the production of coal tar pitch. It offers a good resistance to corrosion.

Advantages: Lower cost, improved water resistance and greater film build.

Disadvantages: Becomes brittle in sunlight, shorter pot life, toxic, high solvent content.

2. Cutbacks (cutback alphalt or coal tar): The bitumen is dissolved in a solvent to produce coating by solvent evaporation. They are often used for temporary protection of pipes.

3. Emulsions: These are obtained by emulsification of water. They are resistant to attack by hydrocarbons.

4. Varnishes: The resin part of oleoresinocil paints are partially replaced by the bitumen. They are not very popular in engineering applications.


These are of high molecular weight combined with vinyl (non-convertible class) to improve their weather resistance. They are not highly resistant to chemical attack and are water resistant. Polyacrylics are binders consisting of copolymers of acrylates and methcrylate esters. They are used widely in automotive sectors and domestic appliances.

Advantages: Excellent flexibility, ease of overcoat and repair, good gloss/color retention, user friendly.

Disadvantages: Poor immersion resistance, difficulty in bonding to smooth surfaces, can only be used if ambient temperature is above 50 oF.


Examples are coal tar epoxy and epoxy phenol. Col tar epoxies are produced by blending epoxy resin, coal tar and solvent. The epoxy phenolics are produced by cross linking epoxy and phenolic resins with a catalyst. Vinyl chloride polymer offers a good resistance to water and outstanding resistance to alkalies.

Co-reactive binders

This class type of binders are formed from low molecular weight resins. Their binders are combined just before application. They form a homogeneous film just by co-reacting, hence their name. A number of epoxy based coatings have been developed; epoxy esters (air drying), amine or polyamide (catalyses) and epoxy co-polymers (coal tar, amine, polyamide). They offer a high degree of chemical resistance and good weatherability.


These are formed by the reaction of isocynates with hydroxyl components containing a urethane radical. There are three types:

1. oil modified

2. moisture cured

3. catalysed

The catalysed aliphatic urethane is most common.

Inorganic binders

Inorganic silicates are generally dissolved in solvents or water to obtain inorganic films. There are three main categories.

1. Post-cured silicates: Post cured water based silicate coatings are obtained by reacting silicate solution with zinc dust and an acid curing solution. They are extensively used for lining of steel tanks. The pigment portion is metallic zinc.

2. Self-cured water based silicate coatings: The film formation mechanism is the same as for the post-cured silicates except that carbon dioxide from the environment reacts with the siltcake binder to form an insoluble film. The corrosion resistance is slightly inferior.

3. Solvent based silicates: The organic ethyl silicate is converted to solid  film by reacting with air. In desert areas, curing may be a problem due to lack of insufficient moisture in the air. They are suitable for higher humidities. However they lack resistance to corrosion.