Direct Dyes are water-soluble colorants that are often used in a hot water bath applications for coloring cellulose fibers (such as cotton), nylon, silk, wool, paper, and leather. The fabric or other object is typically dipped in the dye bath until the desired color intensity is achieved. Some applications involve the use of neutral or slightly alkaline salts, such as sodium sulfate or sodium carbonate.
Direct Dyes are easy to apply, and provide a vibrant coloring effect. For these reasons Direct Dyes are some of the most common dyes used for coloring paper. Direct Dyes are also used in many soaps, detergents, and cleaning products. Some Direct Dyes have been used as indicators and in laboratory purposes in specialized stains.
Most Direct Dyes fall into the Azo, Phthalocyanine, Stilbene, or Thiazol families of colorants. Direct Dyes attach directly to fibers through van der Waals attractions and hydrogen bonding. This association with fibers is known as substantivity. Direct Dyes are often used with dye fixatives, to enhance the adhesion of the dye to the fiber, and to inhibit fading of the dye with repeated washing.