Page 300 - Basic Principles of Textile Coloration

DYEING PROPERTIES OF DIRECT DYES 289

structures. In general, the greater the extent of conjugation, the longer the
wavelength of maximum light absorption. Green dyes must have two absorption
bands in the red and violet regions of the visible spectrum. Green polyazo direct
dyes, however, tend to be dull and bluish in hue. Bright green direct dyes have
blue and yellow dye structures bonded together by a linking group that prevents
their mutual conjugation (3, in Figure 14.1).

There are some yellow and orange stilbene direct dyes obtained from
condensation reactions of 4-nitrotoluene-2-sulphonic acid. These are often of
unknown constitution but have stilbene, azo and azoxy groups. Sulphonated
copper phthalocyanines give turquoise direct dyes. These have good light fastness,
but low wet fastness and poor colour build-up. A number of blue dyes based on the
triphenodioxazine structure (4, in Figure 14.1) have good fastness to light. There
are also some pre-metallised azo copper complexes that give dyeings of very good
light fastness (2, in Figure 14.1).

Many of the older azo direct dyes based on benzidine and its derivatives, such as
Congo Red (Figure 1.3), and some made from 2-naphthylamine, are no longer
manufactured in many countries. Benzidine and 2-naphthylamine are proven
carcinogens.

Although direct dyes often have similar structures to acid dyes, they generally
have higher molecular weights and extended coplanar molecular structures. There
is, however, no clear demarcation between acid and direct dyes. Some direct dyes
dye protein and nylon fibres just as a few acid dyes will also dye cotton.

14.3 DYEING PROPERTIES OF DIRECT DYES

14.3.1 Classification according to dyeing characteristics
Classification of direct dyes according to their chemical structures is not of much
use to the dyer since dyes with similar chemical constitutions can have quite
different application and fastness properties. Direct dyes vary widely in their
dyeing behaviour giving compatibility problems. Grouping of direct dyes according
to their dyeing properties is therefore more useful. The most common
classification of direct dyes is that of the Society of Dyers and Colourists (SDC),
based on their levelling ability and their response to increase in the dyeing
temperature and to added salt during exhaust dyeing.