Please note that the colours on this document are
for demonstration purposes only as colours vary
with individual monitors and therefore cannot be
guaranteed as accurate.
Dyeing is normally
done 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 wash fastness is poor
- except 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 do 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 named
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.
|