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direct dye[də¦rekt ′dī]
(also substantive dye, commercial dye), a synthetic dye that does not require mordanting before application, because the dye has an affinity to cellulosic fibers. It is believed that direct dyes are retained in the fibers owing to the presence of hydrogen bonds and dispersed van der Waals forces. Hydrogen bonds may form hydroxyl groups of cellulose and OH groups, NH2 groups, and CONH groups of dyes.
Most direct dyes are classified as azo dyes by chemical structure, but some are placed in the dioxazine and phthalocyanine groups. Direct dyes usually contain sulfo groups and are readily soluble in water. A typical example of a direct dye is direct light blue:
Direct dyes are available in all colors. They are inferior to reactive dyes in brightness, and many have low photosensitivity. To increase stability to water, perspiration, and the mechanical effects of laundering, fabrics colored with direct dyes are treated with fixatives. These agents are special substances that form high-molecular protective films on fiber or interact with the dyes to produce water-insoluble salts, which are retained in the pores of the fiber.
Direct dyes are used for cotton fibers, viscose, leather, and paper; to a lesser degree, they are used for natural silk, wool, and polyamide fibers. Their widespread use is the result of their low cost and easy application.
REFERENCESChekalin, M. A., B. V. Passet and B. A. loffe. Teknologiia organicheskikh krasitelei i promezhutochnykh produktov. Leningrad, 1972.
Emel’ianov, A. G. Priamye krasiteli i ikh primenenie v tekstil’noi promyshlennost’. Moscow, 1963.
M. A. CHEKALIN