Pigments are defined as inorganic or organic, chromatic or achromatic coloring agents that are practically insoluble in the application medium (DIN 55 943). Dyes are coloring agents that are soluble in the application medium. See also → Pigments, Inorganic, 1. General.
In coloring , the crystalline pigment is applied in the solid state, not in dissolved form, to the medium being colored. Both the chemical and the physical properties of pigments (e.g., particle size, particle‐size distribution, special types of surface and specific surface area, crystal modification, and crystal form) are important for their industrial applications.
Pigment fractions may, however, also be soluble, depending on the composition of the system to be colored and processing conditions. Important properties of the pigmented system, especially migration, recrystallization, thermal stability, lightfastness, weather resistance, and coloristic properties, are often decisively influenced by the dissolved pigment fractions.
Many organic pigments and dyes have the same basic chemical structure. The insolubility required in pigments can be obtained by excluding solubilizing groups, by forming insoluble salts (lake formation) of carboxylic or sulfonic acids, by metal‐ complex formation in compounds without solubilizing groups, and particularly by incorporating groups that reduce solubility (e.g., amide groups).
Organic pigments can be classified in various ways, the two main criteria being chemical structure and coloristic features. A strict subdivision according to one of these criteria is not possible because of overlap. In this article, pigments are classified largely on the basis of chemical structure. They are broadly divided into azo pigments and nonazo or polycyclic pigments. Azo pigments are by far the largest group, not least because they are generally easy to produce and therefore more economical to use. Polycyclic pigments are synthesized according to many widely varying principles. Both groups of pigments can be further subdivided according to their chemical properties and uses. Characteristic properties and most important uses are given for each of the pigment classes described. Examples have been selected on the basis of technical and industrial importance.
