DYE

coloured substance synthesized from certain chemical compounds called benzenoid hydrocarbons, obtained from either coal tar or petroleum. Its most important use is in dyeing textiles. Wool or cotton fabrics may have dyes applied at any stage of manufacture from fibre to finished garment, depending on the nature of the textile, its subsequent processing, and the requirements in use. Dyeing, as the term is generally understood, occurs only when the dye is in solution, usually in an aqueous medium. Some dyes whose solubility is small are called disperse dyes, because they are dispersed rather than dissolved in the water. Some colorants, called pigments, are completely insoluble; they are melted together with synthetic resins to impart colour before the synthetic fibres are extruded. Synthetic textiles such as nylon are coloured in this way. (The term colorant is used to describe both dyes and pigments.) At the present time, dyeing from nonaqueous solvents, called simply solvent dyeing, shows considerable promise as a low-cost process that conserves water and reduces pollution. Very few dyes of natural (animal or vegetable) origin are presently used by the commercial dyer; but before the mid-19th century they were the only colorants available. Their use over the centuries, especially from the Middle Ages on, laid the foundations of the art, and later the science, of the modern dyeing industry, which is of considerable complexity. More than 7,000 synthetic organic colorants are presently in commercial use, differing in fastness and other properties and requiring different methods of application. Most of the dyestuffs are used by the textile industry, but the leather, paper, food, and cosmetic industries are also important users. Synthetic organic pigments are used in the manufacture of printing inks, paints, and automobile finishes and for incorporation in plastics and rubber. Since each industry requires different properties in the dyes and pigments it employs, the large dye-making concerns usually provide technical service and advice to their customer industries on the selection and use of products. Despite the fact that colorants form only a small proportion of the cost of a finished article (less than 1.5 percent of the cost of a woman's dress and less than one percent of that of a woollen overcoat), the dye-making industry remains of considerable economic importance. any of a group of complex organic compounds that are intensely coloured and are utilized to colour other materials. Dyes are used to colour textiles, paper, leather, and many other substances. During the process, dye molecules are deposited from solution onto the material in such a way that they cannot be subsequently removed by the solvent in which they were dissolved. Dyes are generally prepared synthetically from coal tar and petrochemicals. The craft of dyeing is several thousand years old. The major dyes known to the ancients were those obtained from the madder and indigo plants, together with Tyrian purple (chemically, an indigo derivative), which was obtained from mollusks. Many of the dyeing processes discovered during antiquity are still used today. An example is the application of an inorganic chemical called a mordant. This substance is applied to a material before the dye to help the dye adhere to the material by precipitating in it as an insoluble metal salt. The first synthetic dye, mauveine, was discovered in 1856 by the British chemist William Henry Perkin. It was an unexpected product of a reaction with a coal-tar derivative. As a result, a large-scale coal-tar-dye industry arose during the latter part of the 19th century. The basic raw materials were organic substances containing so-called aromatic hydrocarbons in which carbon atoms are linked in six-membered rings. Aromatics are characteristic of most dyestuffs today, although the raw materials come from petroleum as well as from coal tar. In addition to the aromatics, many dyestuffs contain a chemical subunit called a chromophore. The chromophore acts to produce strong absorption of radiation in the visible spectrum. Various categories of atomic groupings called auxochromes are also used to shift the absorption to make the colour more intense. The chemical structure of dye molecules has proved to be relatively easy to modify, and a wide range of coal-tar dyes have become available. Developments in chemical synthesis have led to the production of many new dyes that adhere strongly to many different types of substances. Fibres absorb dyes particularly well because they are porous and exert chemical forces on dye molecules. Dyes are held onto fibres by ionic forces, hydrogen bonding, or other attractive forces. One of the major advances in dye chemistry has been the development of fibre-reactive dyes. Here, the dye molecule forms a covalent chemical bond with the fibre that it colours. Such a bond is the strongest means of attaching a dye molecule to a fibre. With natural materials such as cotton, wool, and silk, it is necessary to synthesize dyes that are suited to the specific chemical structures of those materials. On the other hand, it is possible to modify the chemical structure of synthetic fibres to alter their dyeing characteristics. Additional reading General works Encyclopaedic coverage of every aspect of the chemical industry is provided by Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (197884), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991; Ullmann's Encyclopedia of Industrial Chemistry, 5th, completely rev. ed., edited by Wolfgang Gerhartz et al. (1985 ); and Thorpe's Dictionary of Applied Chemistry, 4th ed., 12 vol. (193756). History Authoritative articles on dyes and dyeing from classical antiquity to the present may be found in the journal Ciba Reviews (1937 ). The commemorative volume, Perkin Centenary, London: 100 Years of Synthetic Dyestuffs (1958), contains a chapter on the life and work of Sir William Perkin by J. Read. The article by C. Paine surveys the development of the synthetic-dye-making industry.E.N. Abrahart, Dyes and Their Intermediates (1968); and H.E. Fierz-David and L. Blangey, Fundamental Processes of Dye Chemistry (1949, Eng. trans. from the 5th Austrian ed. of 1943), are textbooks written from the industrial viewpoint. Edward Noah Abrahart The Editors of the Encyclopdia Britannica