Direct dyes chemical constitution

It is important to recognise that although a dye has a C.I. number, the purity and precise chemical constitution will vary depending upon the name. Thus, the dye C.I. Direct Blue 99 from company A will not be identical to C.I. Direct Blue 99 from Company B. 

With the increasing demands of the dye plants, the purity 6f the raw materiab has steadily improved, and today many of these products may be called chemically pure. Modem methods have permitted the direct manufacture of pure compounds by fractional distillation and fractional crystallization. These improved techniques of the tar industry have resulted from extensive work and they constitute one of the foundations for the manufacture of intermediates for the dye industry. 

Dyes may be classified according to their constitution, such as azo dyes, phthaleins, alizarines, etc., but from the standpoint of the dyer the more important classification is that which is based on the behavior of the dyes with fibers. Dyes are grouped in this way as substantive or direct, and adjective or mordant. Dyes are also classed as acidic and basic. In addition to the dyes belonging to these classes, there are a few, such as indigo and other Vat dyes, aniline black, and the so-called ingrain colors, which are produced by chemical reactions within the fiber. 

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