Many colourants, most of the which are pigments are available for use. Certain general principles must be kept in mind. The colourants must be suitable for both processing conditions and end-use purposes.

Most of the compounds, which are transparent and which do not contain a high proportion of filler, need the addition of 1-10 phr. TiO2 to bring out the full colours. Metallic effects can be created by the addition of special pigments, usually incorporating Aluminium powder.

There are two types oof colorants used in plastics’ colouring namely Dyes and Pigments. The essential difference between dyes and pigments is one of solubility. The dyes are soluble in plastic compositions but the pigments are insoluble and are dispersed throughout the plastic mass. It is very important to select proper dye or pigment for the solubility of the colourants in the mixture. Colour stability is another important factor. Other important factors include strength , electrical properties, specific gravity, clarity and resistance to migration in the finished.

Speciality of dyes on plastics is that they hold their colour much better against fading than the pigmented plastics. Whereas in case of textiles the reverse true.

Dyes have a much lower thermal stability than the pigments, particularly inorganic pigments. Some of the inorganic pigments are even translucent which give a glossy effect with brilliant lue.

There are many world wide manufacturers of pigments and dyes and they generally give copious data on their products but correlation of one range with chemical constitution. However, a simple compounding example is given below :

PVC resin e.g. ‘corvic’ H65/33                     100 parts
Plasticizer e.g. DOP, ‘Reomol’ D 79 P          50 parts
Basic lead carbonate paste                          8-10 parts
TiO2 and colour                                            as indicated.

Though more recently the combination of barium/cadmium soap plus epoxidised oil may be used as stabilising/system.