OVERVIEW

The distinction between pigments and dyes, which is based on the differences in their solubility characteristics, has been discussed in detail in Chapter 2. A pigment is a finely divided solid colouring material, which is essentially insoluble in its application medium. Pigments are used mostly in the coloration of paints, printing inks, and plastics although they are applied to a certain extent in a much wider range of substrates, including paper, textiles, rubber, glass, ceramics, cosmetics, crayons, and building materials such as cement and concrete. In most cases, the application of pigments involves their incorporation into a liquid medium, for example a wet paint or ink or a molten thermoplastic material, by a dispersion process in which clusters or agglomerates of pigment particles are broken down into primary particles and small aggregates. The pigmented medium is then allowed to solidify, either by solvent evaporation, physical solidification or by polymerisation, and the individual pigment particles become fixed mechanically in the solid polymeric matrix. In contrast to textile dyes where the individual dye molecules are strongly attracted to the individual polymer molecules of the fibres to which they are applied, pigments are considered to have only a weak affinity for their application medium, and only at the surface where the pigment particle is in contact with the medium.

Pigments are incorporated to modify the optical properties of a substrate, the most obvious effect being the provision of colour. However, this is not the only optical function of a pigment. The pigment...