Descriptive notes on paints

Paints are used to protect metals, timber or plastered surfaces from the corrosive effects of weather, heat, moisture or gases etc. and also to improve their appearance.

Functions of Paint:
Paint performs following functions :
(i) It protects wood from decaying.
(ii) It prevents corrosion of metals.
(iii) It renders surface hygienically safe and clean.
(iv) It gives decorative and attractive appearance to the surface.
(v) It also protects the surface from harmful effects of atmospheric agencies.

Characteristics of an Ideal Paint:
An ideal paint should have the following characteristics :
(i) The paint should be cheap.
(ii) It should be easy and harmless to the user.
(iii) It should retain its original colour for a long time.
(iv) It should be able to cover maximum area of the surface with minimum quantities.
(v) The painted surface should dry neither too slowly nor too rapidly.
(vi) When applied, the paint should form a thin uniform film on painted surface.
(vii) The paint should form a hard and durable coat on the painted surface.
(viii) The paint should not peel off from painted surface.
(ix) It should be good fire and moisture resistant.
(x) The painted surface should not show any cracks.
(xi) The painted surface should possess attractive and decorative pleasing appearance.
(xii) Atmospheric agencies should not be able to affect the painted surface.

Constituents of an Oil Paint or Oil Borne Paints:
An oil paint essentially consists of the following ingredients :
(i) A base
(ii) An inert extender or filter
(iii) A vehicle or carrier
(iv) A drier
(v) A solvent or thinner
(vi) A colouring pigment

It provides body to the paint and on it depends upon the nature of paints to a
great extent. A base in general should possess the following properties :
(i) It is the solid matter forming the main body of the paint.
(ii) It makes the paint film harder and more resistant to abrasion.
(iii) It reduces shrinkage cracks formed on drying.
(iv) It forms an opaque layer to obscure the surface of material to be painted.
White lead, Red lead, Zinc oxide, Iron oxide, and metallic powders such as Aluminium, Copper and Bronze etc. are the commonly used bases.

An Inert Extender or Filter:
They are added in the paint to reduce the cost and also to modify some of the properties of the paints. They reduce weight of the paint and render paint more durable. They should not be used in excess amount as otherwise paint may loose its original character and may become weak. Barium sulphate silica, lithophone, whiting, charcoal, gypsum silicate of magnesia or alumina etc. are the usual inert extenders.

Vehicles or Carriers.
They are liquid substances which hold solid ingredients of the paint. In general vehicles or carriers possess the following properties :
(i) It is an oily liquid in which the base and pigment are soluble.
(ii) It facilitates the paint to be conveniently spread evenly over the surface by means of a brush.
(iii) It acts as a binder for the base and causes it to stick to the surface.
(iv) On drying, it forms a tough and an elastic film. Oils most commonly used as vehicles are linseed oil, poppy oil, nut oil and tung oil.