12
Direct dyes
N. SEKAR, Institute of Chemical Technology Mumbai, India
Abstract: Structural characteristics and classification of direct dyes
are discussed. The chemistry of direct dyes belonging to azo, azine,
copperphthalocyanine and other classes are discussed. Chemistry of
benzidine-free direct dyes and after-treatment dyes is also discussed.
Key words: azo dyes, phthalocyanines, triphenodioxazines, substantivity,
cellulosic substrates
12.1 Introduction
Direct dyes are inherently substantive to cotton and other cellulosic sub-
strates. Their aqueous solutions dye cotton generally in the presence of an
electrolyte such as sodium chloride or sodium sulphate. Historically they
replaced naturally occurring mordant dyes. Direct dyes are the most impor-
tant single class for cotton in the sense that they are simple to use. Optimum
conditions vary, but dyeing is usually carried out at or near boiling point in
the presence of an electrolyte such as sodium chloride or sodium sulphate.
Direct dyes are relatively inexpensive. They are available in a full range of
hues but are not of high colour brilliance. They show poor to moderate fastness
to washing. They are therefore used on materials where good fastness is not of
prime importance.The light fastness of dyeing with direct dyes on cellulosic fibres
varies from poor to fairly good, although some copper complex direct dyes have
very good light fastness. The light fastness of metallisable dyes can be increased
by post-treatment with copper salts, and wash fastness can often be improved by
resin treatment. Shades are generally affected by metal salts but less so by resins
imparting crease resistance. As usual the deeper the dyeing colour, the lower the
fastness is to wet treatments, and the higher the fastness to light.
Substantivity of direct dyes was initially attributed to secondary valence
bonding between the dye and the fibre. The observation that coplanar mole-
cules are always more substantive than nonplanar ones led to the coplanar-
ity theory with the assumption that the coplanar dyes are in contact with the
cellulose molecules along their entire length. The affinity was then explained
as an outcome of the presence of hydrogen bonds (Boulton, 1951); how-
ever, such bonds are probably prevented by a water layer between fibre and
dye (Rattee and Breuer, 1974). A very convincing explanation is as follows
(Bach et al., 1963). Single dye molecules are absorbed by the intermicellary
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426 Handbook of textile and industrial dyeing
cavities of the cellulosic fibres and they form aggregates in these cavities.
Because direct dyes become aggregated in aqueous solutions at normal
temperatures, substantivity is initiated only at higher temperatures since the
diffusion is possible only at higher temperatures. Direct dyes are therefore
characterised by their tendency to aggregate and therefore they need to be
coplanar.
The affinity or substantivity of direct dyes for cellulose fibres depend on
the type of chromophore and can also be influenced by the choice of dyeing
conditions. They invariably contain solubilising sulphonic acid groups that are
ionised in aqueous solution. The dye molecule exists in the dyebath as the
anion, and thus an electrolyte must be added to the dyebath because cellulose
fibres have a negative charge on the surface in water. The cations of the elec-
trolyte neutralise the negative charge and help the aggregation of dye anions
on the fibre which is called salting out effect. With the help of Freundlich
or Langmuir adsorption isotherms exothermic adsorption can be described
(Porter, 1993). After adsorption, dye molecules diffuse from the surface into
the amorphous areas of the cellulose fibre. Again, the rate of dyeing can be
controlled by the dyeing parameter. The diffusion process is in general under-
stood in terms of Fick’s law (Brady, 1992). The dye–fibre interaction can be
described by hydrogen bonds or van der Waals forces (Lewis, 1998). The
strength of these bonds is low, so that the dyes can be washed out of the fibre
again. At dyeing equilibrium, the rate of absorption is equal to the rate of des-
orption. In addition treatment improves the longevity of the dye on the fibre.
Direct dyes usually cannot meet today’s more stringent wet fastness
requirements for apparel and linens. Recently their share in the market
has gradually declined in favour of the reactive dyes, which have often very
good washing fastness and have bright hues. Only in individual cases do
direct dyes achieve the brilliance of reactive dyes. The light fastness of direct
dyes covers the entire scale from one to eight, meeting even the highest
requirements. With increasing depth of shade the wet fastness decreases to
such an extent that dyeing must generally be after-treated. The majority of
direct dyes are characterised by excellent affinity to the fibre.
12.2 Classification of direct dyes according to dyeing
characteristics
Direct dyes with similar chemical constitutions can have quite different
application and fastness properties (SDC, 1982). They also vary widely in
their dyeing behaviour creating problems of compatibility. Grouping of
direct dyes according to their dyeing properties is therefore more practical.
The classification by the Society of Dyers and Colourists (SDC) is based on
their levelling ability and their response to increase the dyeing temperature
and to added salt during exhaust dyeing.
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