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
Today, all food colour additives are carefully regulated by authorities to
ensure that foods are safe to eat and accurately labeled.
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.
MF: C26H15N2O4, C26H21N5Na2O19S6
CAS No.: 1064-48-8
Minimum Order Quantity: 500 Kgs.