Environmental Engineering Reference
In-Depth Information
silk dyeing, can be applied in both batch and continuous processes; continu-
ous applications are preferred because of the lower volume of dye required.
In general, sulfur blacks are the most commercially important colors and are
used where good color fastness is more important than shade brightness [39].
Vat Dyes,
which are water-insoluble dyes, are mainly applied to cellulosic
fibers as soluble leuco salts after reduction in an alkaline bath, usually with
sodium hydrogensulfite. Following exhaustion onto the fiber, the leuco
forms are re-oxidized to the insoluble keto forms and after treated, usu-
ally by soaping, to redevelop the crystal structure. The principal chemical
classes of vat dyes are known as anthraquinone and indigoid.
Cationic (Basic) Dyes,
which are water-soluble and present as colored cat-
ions in solution, and thus frequently referred to as cationic dyes, are applied
to paper, polyacrylonitrile (e.g., Dralon), modified nylons, and modified
polyesters. Their original use was for silk, wool, and tannin-mordanted
cotton when brightness of shade was more important than fastness to
light and washing. 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.
Solvent Dyes
, which are water-insoluble but solvent-soluble, are devoid of
polar solubilizing groups such as sulfonic acid, carboxylic acid, or quater-
nary ammonium. The dyes, which are used for colored plastics, gasoline,
oils, and waxes, are predominantly azo and anthraquinone, and also phtha-
locyanine and triarylmethane dyes.
Reactive Dyes
form a covalent bond with the fiber, usually cotton, although
they are used to a small extent on wool and nylon. This class of dyes, first intro-
duced commercially in 1956 by Imperial Chemical Industries (ICI), made
it possible to achieve extremely high wash-fastness properties by relatively
simple dyeing methods. A marked advantage of reactive dyes over direct dyes
is that their chemical structures are much simpler, their absorption spectra
show narrower absorption bands, and the dyeings are brighter. The principal
chemical classes of reactive dyes are azo (including metallized azo), triphen-
dioxazine, phthalocyanine, formazan, and anthraquinone. High-purity reac-
tive dyes are used in the inkjet printing of textiles, especially cotton.
1.3
Technologies for Color Removal
Most probably, the progressive accumulation of many more organic com-
pounds in natural waters has resulted in the development and growth of
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