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DIRECT DYES
Dyeing is normally carried out in a neutral
or slightly alkaline dyebath, at or near the boil, with the
addition of either sodium chloride (NaCl) or sodium sulphate
(Na2SO4). Also known as hot-water dye, this type can be used
with hot tap water and requires no leveling or exhausting
agents. It is convenient but lacking in light-fastness and
wash-fastness. Direct dyes are used on cotton, paper, leather,
wool, silk and nylon. They are also used as pH indicators
and as biological stains.
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
colors 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 Color 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 colored 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 are 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
maximise 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 minimise any repulsive
effects.
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