PAINTS

Paint is mainly composed of two ingredients, viz., a base which is a solid material and a liquid called a vehicle. The vehicle carries the solid matter, distributes it evenly and enables the solid matter to adhere to the surface.

12.2.1 Functions of Paints

Irrespective of the type of paint, in general, the functions of paints are as follows:

1. It has to protect the surface from the weathering effects of atmosphere such as sun, wind, other liquids, fumes, etc.
2. It has to prevent corrosion of metal bases and decay of wooden surfaces.
3. It has to provide a smooth surface for easy cleaning.
4. It should provide the surface good aesthetic appearance and a hygienically sound surface.

12.2.2 Constituents of an Oil Paint

Metals and wooden surfaces are generally painted by oil paints. Sometimes walls are also painted by oil paints. Thus, it is worth discussing the constituents of an oil paint. An oil paint is made up of six main constituents, viz., base, inert filler, colouring pigment, vehicle, thinner and drier.

1. Base

The base forms the chief ingredient of a paint. The most important purpose of adding a base to the paint is to make an opaque coating so as to hide the surface to be painted and to reduce shrinkage cracks. The bases that are commonly used for the paints are white lead, red lead, oxide of zinc, oxide of iron, titanium white, etc.

2. Inert Filler

It is a cheap pigment added to a paint to reduce its cost. Further, it modifies the weight of the paint and makes it more durable. Commonly used inert fillers, also called as extenders, are barium sulphate, lithophone, silicate of magnesia or alumina, gypsum, etc.

3. Colouring Pigments

It is a white or coloured pigment added to the paint to get the desired colour of the paint.

4. Vehicle

Vehicle acts as a binder for various pigments, viz., bases, extender and colouring pigment. Vehicles are required for two purposes, mainly:

1. To make it possible to spread the paint as a thin layer on the surface evenly and uniformly.
2. To provide a binder for the ingredients of a paint such that the paint sticks to or adheres to the surface.

Vehicles that are commonly used are: linseed oil, tung oil, poppy oil, nut oil, etc.

5. Thinner

It is also known as solvent. It is a liquid which thins the consistency of the paint and evaporates after the paint film has been applied. It increases the workability of the paint. The commonly used solvents are: turpentine, pure oils, petroleum spirit and highly solvent naphtha.

6. Drier

It is a material used in small quantities for accelerating the drying of paint film. It is a metallic compound that acts as a catalyst for the oxidation and polymerization of the vehicle used in the paint. The commonly used driers are: litharge, lead acetate, manganese dioxide and cobalt. In an oil paint only 10% (by volume) of the drier should be used. The use of excess drier leads to flaking.