Vehicles are the liquid substances which hold the ingredients of paint in liquid suspension (Table 10.2). They serve mainly two purposes:
- To make it possible to spread the paint evenly and uniformly on the surface in the form of a thin layer.
- To provide a binder for the ingredients of a paint so that they may stick or adhere to the surface.
10.2.3 Driers
These act as catalysts. These are substances that are used with a view of accelerating the rate of drying of the paint film. A drier absorbs oxygen from the air and transfers it to the vehicle, which in turn gets hardened. Driers may be either in the form of soluble driers or paste driers. Soluble driers are compounds of cobalt, lead, manganese, etc. dissolved in linseed oil or some other volatile liquid. Paste driers are compounds of the same metal.
Table 10.2 Materials Commonly Used as Vehicles and Their Properties
| Number | Name | Description |
|---|---|---|
| 1. | Linseed oil | This is the most common material used as a vehicle of paint. It is extracted from flax seeds. It is used in various grades. |
| a. Raw linseed oil | This oil is thin and pale. It requires more time for drying and is used for interior work of inferior nature. | |
| b. Boiled linseed oil | This oil is thicker and darkly coloured than raw oil. It dries quickly. It is used for exterior surfaces. | |
| c. Pale boiled linseed oil | This is similar to boiled linseed oil except that it does not possess a dark colour. It is more suitable for painting plastered surfaces. | |
| d. Double boiled linseed oil | This oil dries very quickly and is suitable for external work. It however requires a thinning agent like turpentine. | |
| e. Stand oil | This oil dries slowly and provides a durable, clear and shining finish. | |
| f. Tung oil | This oil is far superior to linseed oil and is used for preparing paints of superior quality. | |
| 2. | Poppy oil | This oil is prepared from poppy seeds. It dries slowly but its colour lasts long. It is used for making paints of delicate colours. |
| 3. | Nut oil | This oil is extracted from ordinary walnuts. It is nearly colourless and dries rapidly. It is used for ordinary work as it is cheap. |
Litharge, red lead and sulphate of manganese can also be used as driers. Litharge is the most commonly used drier. Red lead is less effective than litharge. The following precautions should be taken while using driers:
- A drier should not be added until the paint is about to be used.
- More than one drier should not be used in a mixture.
- The driers need not be used with pigments that dry well.
- The driers should not be used unnecessarily nor in excess especially in the finishing coat as they may destroy the elasticity of the paint and injure its colour.