Dyes

colored organic compounds used to color fabrics, leather, furs, paper, plastic, cured rubber, wood, and other materials. Dyes also include colorless compounds from which a colored substance is formed after it is applied to the material (for example, dyes for cold dyeing and optical bleaches).

Since ancient times natural dyes, such as alizarin and indigo, have been obtained from plants and, less frequently, from animals. The first synthetic dyes were produced independently in 1856 by the Polish chemist J. Natanson (fuchsin) and the English chemist W. H. Perkin (mauve); the commercial production of mauve began in 1857. In 1869 alizarin was synthesized by the German chemists K. Graebe and K. T. Liebermann, and soon a large number of other synthetic dyes were produced that were superior in quality to natural dyes. By the beginning of the 20th century, synthetic dyes had almost completely replaced natural dyes. The synthesis of dyes became possible after the discovery by N. N. Zinin of the general method of producing aromatic amines (the Zinin reaction). In the early 1970’s more than 9, 000 dyes were produced industrially, and the number is growing each year. The world production of dyes exceeds 600, 000 tons per year.

Dyes are divided according to chemical structure into the following groups: nitro dyes, nitroso dyes, azo dyes, arylmethane dyes, quinone imine dyes, sulfur dyes, indigoid dyes, anthraqui-none dyes, polycyclic dyes, phthalocyanine dyes, polymethine dyes, and azomethine dyes. According to their region and method of use, dyes are divided into acid, direct, vat, sulfur, mordant, basic, cation, reactive, oxidizing, and disperse dyes; pigments and lacquers; fat-, alcohol-, and acetone-soluble dyes; dyes for cold (ice) dyeing; and dyes for leather, aluminum, fur, and wood.

The color index of dyes—that is, their ability to selectively absorb visible light rays—is associated with their chemical structure—the presence of a sufficiently extensive system of coupled double bonds, often including heteroatoms. The dye adheres to the dyed material (substrate) because of the formation of chemical bonds with it (covalent bonds in the case of active dyes or ionic bonds in the case of acid dyes), as well as by the forces of adsorption and by hydrogen bonds (direct dyes); many dyes form water-insoluble particles (vat dyes, sulfide dyes, and dyes for cold dyeing), which “stick” in the pores of the substrate. Adhesives are also used to keep the dye on the material (if the dye is part of lacquers, enamels, or paints), as are polymer films. During use of the material its dye must not change substantially under the influence of light, weak acids and bases, washing, rubbing, or pressing. The stability of a dye depends on many factors, including chemical structure, the type of bond with the substrate, and the nature of the substrate. For example, basic dyes are unstable on wool but are sufficiently durable for use on polyacrylonitrile fiber. The stability of dyes under various influences is measured on a five-point scale, except for light stability, which is measured on an eight-point scale.

Dyes are used not only to color various materials but also in color and black-and-white cinematography and photography, in analytical chemistry, in medicine (as diagnostic materials), in biochemical research, in liquid lasers, and as photoconductive components of various physics instruments.

The raw materials for dye production include benzene, naphthalene, anthracene, pyrene, and other aromatic and heterocyclic compounds, as well as various acids, bases, salts, and alcohols. The first step is the production of intermediate products, which are further treated to produce dyes by the processes of condensation, diazotization, azo compounding, and oxidation. The production of dyes is often a complex process, consisting of ten or more stages.