They are used for the direct dyeing of cotton and regenerated
cellulose, paper, and leather. They are also used to dye union goods (mixed
cotton, and wool or silk) and to a lesser extent nylon fiber. Most of the dyes
in this class are polyazo compounds, along with some stilbenes, phthalocyanines,
and oxazines. The solubility of the dye in the dye bath is often
reduced by adding common salt or Glauber's salt. The presence of excess
sodium ions favors establishment of equilibrium with a minimum of dye
remaining in the dye bath.
Direct dyes are water-soluble anionic dyes, but are not classified
as acid dyes because the acid groups are not the means of attachment
to the fiber.
A group of dyes that are
mostly azo-compounds derived from benzidene
or benzidene derivatives. They are
used to dye cotton, viscose rayon, and
other cellulose fibers directly, using a neutral
bath containing sodium chloride or
sodium sulfate as a mordant.
