Dye

A dye is a colored substance used to impart color to other materials, typically by dissolving or dispersing in a medium and forming a chemical or physical association with the substrate. Dyes differ from pigments in that dyes are usually soluble and bond with fibers or surfaces, whereas pigments are insoluble particles that require a binder.

Historically, dyes were derived from natural sources such as plants (for example, indigo and madder), insects (such as cochineal), and minerals. The development of synthetic dyes began in the mid-19th century with William Perkin’s discovery of mauveine, which spurred rapid expansion of the dye industry and the diversification of dye chemistry.

Dyes are classified in several ways, including source (natural vs synthetic) and chemical structure or application. Common classes include reactive dyes (which form covalent bonds with cellulose fibers), direct dyes, acid dyes, basic dyes, disperse dyes (for synthetic fibers like polyester), vat dyes (formed as insoluble compounds and reduced in the bath), sulfur dyes, and mordant dyes (requiring metal salts to fix to substrates).

Application and fixation depend on the substrate and the dye type. Binding can be covalent, ionic, or through adsorption, often aided by mordants, auxiliaries, or specific bath conditions. Fastness properties, such as resistance to washing and light, are key performance measures.

Environmental and safety considerations address dyeing effluents and resource use. Modern practices emphasize wastewater treatment, reduced water consumption, and the development of more sustainable dyes, with regulatory oversight in many regions to control dye composition and discharge.