Page 306 - Basic Principles of Textile Coloration

DYEING PROPERTIES OF DIRECT DYES 295

of half dyeing of viscose with direct dyes varied by 2–3 orders of magnitude, when
determined for dyeings in which the final exhaustion was limited to 50% by
changing the concentration of added salt in each case (Section 11.2). Fortunately,
under practical dyeing conditions, giving economic exhaustion, dyeing rates do
not vary by more than ten-fold.

Rates of dyeing with single direct dyes are very dependent on the actual dyeing
conditions and may not be a good guide to their properties in the typical dye
mixtures used in most commercial dyeings. Ideally the dye mixture should behave
as a homogeneous dye. As dyeing proceeds, the colour of the goods will gradually
become deeper but will then always be of the same hue. For this, the dyes in the
mixture must have similar rates of exhaustion and are said to be compatible. The
compatibility of direct dyes depends on the rates of dyeing and migration, and the
salt sensitivity of the dyes. Hue differences as dyeing proceeds are much more
apparent than depth differences. Incompatible dyes also tend to give unlevel
dyeings. They have different rates of migration, different degrees of fibre
penetration because of dissimilar diffusion rates, and give a change of fabric hue
during dyeing because of differences in their overall rates of absorption.

It is preferable to select compatible dye combinations from within each SDC
classification group. Combination of some Group A and B, or Group B and C dyes,
is possible, however, because of similarities in their properties. Ideally, all the
selected dyes should have similar rates of exhaustion and often have the same
molecular ionic charge. Dyes with low rates of exhaustion but good migration
(SDC Class A dyes) are useful for shading towards the end of the dyeing process.

A dip test is useful for establishing dye compatibility. Small pieces of cotton of
equal weight are dyed in the same bath with a mixture of direct dyes. At various
intervals, a small dyed sample is removed from the bath and replaced by an
identical piece of undyed fabric. A series of dyed samples arranged in order of
increasing dyeing time will have gradually decreasing colour depth, but invariant
hue, when the dyes used are compatible. When this is so, the series of samples
that were added to the dyebath during dyeing will be of increasing colour depth,
but also with the same constant hue, when arranged in order of their dyeing times.
The results of compatibility tests for direct dyes are only valid under the given
dyeing conditions. Rates of exhaustion are very dependent upon the dyeing
temperature and salt concentration. For example, for a mixture of CI Direct Blue 1
(5, Figure 14.3) and Direct Yellow 12 (6) at 60 °C, the yellow dye dyes cotton
more rapidly than the blue dye in 0.01 M NaCl, but less rapidly in 0.10 M NaCl.

For some mixtures of two direct dyes, the presence of the second dye reduces