Pigments

Colorants for plastics fall into two categories, pigments and dyes.

Dyes dissolve into the polymer matrix where as pigments do not. This means that pigments can be used in most polymers where as dyes are only really suitable for styrenic type polymers which allow them to be incorporated into their chemical structure. A dye blended with other types of polymer may leach colour onto things it comes into contact with making it unsuitable.

Pigments fall into two categories, inorganic and organic:

Inorganic Pigments

These pigments are based on metal salts and oxides. They have an average particle size of 0.2 - 1.0 microns. Because of a tendency to agglomerate, special attention is made in manufacturing the pigments to breaking down and removing agglomerates of more than 5.0 microns. They are inexpensive but also weak in comparison to organic pigments, however, they have good light fastness and heat stability qualities. Inorganic pigments are good for making reliable transparent colours although they tend to add a cloudiness to clear formulas.

Organic Pigments

These pigments are made from chemical compounds and average about 0.05 microns in size. They tend to have between 10 to 20 times the strength of inorganic pigments and this, combined with a movement away from heavy metal pigments such as lead chromate, has meant a vast increase in their usage. As with inorganic pigments they are good for matching transparent colours and can achieve clearer effects due to their smaller particle size. Although organic pigments are more expensive, when used in masterbatch the formulations tend to be cheaper than using inorganic due to their higher strength.

Dyes

As explained earlier dyes have limitations as to the polymers they can be used in. However, when they can be used they exhibit some excellent properties. Dyes are easy to disperse, have good colour strength and allow a glass clear finish in polymers such as SAN and PET.