Direct Dyes. These watersoluble anionic
dyes, when dyed from aqueous solution in the presence of
electrolytes, are substantive to, i.e., have high affinity for,
cellulosic fibers. Their principal use is the dyeing of cotton and
regenerated cellulose, paper, leather, and, to a lesser extent,
nylon. Most of the dyes in this class are polyazo
compounds, along with some stilbenes,
phthalocyanines, and oxazines. Aftertreatments,
frequently applied to the dyed material to improve washfastness
properties, include chelation with salts of metals (usually copper
or chromium), and treatment with formaldehyde or a
cationic dyes complexing
resin.
Vat Dyes. These
waterinsoluble dyes are applied mainly to cellulosic fibers as
soluble leuco salts after reduction in an alkaline bath, usually
with sodium hydro- gensulfite. Following exhaustion onto the fiber,
the leuco forms are reoxidized to the insoluble keto forms and
aftertreated, usually by soaping, to redevelop the crystal
structure. The principal chemical classes
of vat dyes are anthraquinone and
indigoid.
Cationic (Basic)Dyes. These watersoluble cationic
dyes are applied to paper, polyacrylonitrile (e.g.
Dralon), modified nylons, and modified polyesters. Their original
use was for silk, wool, and tanninmordanted cotton when brightness
of shade was more important than fastness
to light and washing. Basic dyes are watersoluble and yield colored cations in solution.
For this reason they are frequently referred to as cationic dyes.
The principal chemical classes are diazahemicyanine,
triarylmethane, cyanine, hemicyanine, thiazine,
oxazine,and acridine. Some basic dyes show biological
activity and are used in medicine as antiseptics.
Acid
Dyes. These watersoluble
anionic dyes are applied to nylon, wool, silk, and modified
acrylics. They are also used to some extent for paper, leather,
ink-jet printing, food, and cosmetics.
Solvent
Dyes. These
water-insoluble but solvent-soluble dyes are devoid of polar
solubilizing groups such as sulfonic acid, carboxylic acid, or
quaternary ammonium. They are used for coloring plastics, gasoline,
oils, and waxes. The dyes are predominantly azo and
anthraquinone, but phthalocyanine and
triarylmethane dyes are also used.
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