Description
Direct dyes are another class of
dyes, one of the two types of dyes that are mixed in 'all purpose'
dyes such as Rit. (The other type in the mixture is an acid dye,
which will not stay in any cellulose fiber for long.) The colours
of direct dyes are duller than those provided by fiber reactive
dyes, and the washfastness is poor - expect anything dyed with them
to 'bleed' forever. The one advantage is that direct dyes may be
more lightfast, that is, resistant to fading in the light, than
fiber reactive dyes. The "direct dye" classification in the Colour
Index system refers to various planar, highly conjugated molecular
structures that also contain one or more anionic sulfonate group.
It is because of these sulfonate groups that the molecules are
soluble in water. Though most direct dyes still can be obtained in
powder form, it is increasingly popular to receive them as liquid
concentrates. The advantage of concentrates is that they are easy
to handle and meter. The disadvantage is that the surfactants and
co-solvents needed to keep the dye concentrates stable may
interfere with retention and sizing in the case of very deeply
coloured grades.
Direct dyes are used on cellulose fibers such as cotton, rayon, and
linen. They lack the permanence of the cold water fiber reactive
dyes which most serious dyers prefer for use on cellulose fibers,
but in some cases they have advantages that make their use
worthwhile. For example, while many of the direct dyes are not very
lightfast, there are some dyes in the class that may be more
lightfast than similar shades of fiber reactive dyes. All direct
dyes perform rather poorly with respect to washfastness. Without an
appropriate after-treatment, direct dyes bleed a little with every
washing, losing their brightness and endangering other clothes
washed in the same load. However, there are special
after-treatments which may be used to solve this problem. (Vinegar
is not among them! In spite of claims you may see to the contrary,
you cannot use vinegar to set any dye on cotton or other cellulose
materials.) A product called Retayne, which is an ionic bulking
agent which essentially "glues" the dye into the fiber, works very
well to make fabric dyed with direct dyes washable without bleeding
of the dye.
The name 'direct dye' alludes to
the fact that these dyes do not require any form of 'fixing'. They
are almost always azo dyes, with some similarities to acid dyes.
They also have sulphonate functionality, but in this case, it is
only to improve solubility, as the negative charges on dye and
fibre will repel each other. Their flat shape and their length
enable them to lie along-side cellulose fibres and maximize the
Van-der-Waals, dipole and hydrogen bonds. Below is a diagram of a
typical direct dye. Note that the sulphonate groups are spread
evenly along the molecule on the opposite side to the hydrogen
bonding -OH groups, to minimize any repulsive effects.
