Page 302 - Basic Principles of Textile Coloration

DYEING PROPERTIES OF DIRECT DYES 291

The SDC classification tests involve dyeing trials using a 30:1 liquor ratio. A
migration test and a salt sensitivity test are carried out using the dyes under
examination along with standard dyes for comparison [1,2]. The migration test
(Section 10.4.5) is conducted at the boil in the presence of 10% owf salt using a
dyeing of the test dye along with an equal weight of undyed cotton. Dyes that
exhibit good migration, comparable to that of the standard dyes, belong to Class
A. For the other dyes showing poor migration ability, a salt sensitivity test is
carried out. Dyeings are prepared at the boil in the presence of 0.6, 0.8 and 1.0%
owf salt for 30 min. The dyed samples are removed and replaced with the same
weight of undyed material, further salt is added to give a total of 20% owf NaCl
and dyeing continued for another 30 min. For Class B dyes, the initially removed
samples, dyed at low salt concentrations, are lighter or the same as the second
samples dyed in the presence of additional salt. For Class C dyes, the initial
dyeings are darker than the later ones.

An examination of the molecular structures of direct dyes from the three
groups, shows that in passing from Group A to Group C, dye substantivity
generally increases because of increased molecular weight and a lower number of
sulphonate groups per dye molecule. Class C dyes therefore generally have better
washing fastness than Class B dyes, with Class A dyes having even lower fastness.

Despite its value, the SDC classification is not readily applicable to dyeing at
low liquor ratios as in jet, jig or pad dyeing. In these cases, a strike test may be a
more useful indicator for selecting compatible dyes (Section 14.3.4). At low liquor
ratios, dye aggregation is more pronounced since the dye solution is more
concentrated for a given % dye owf. SDC Class B direct dyes may then require
temperature control as well as salt control to obtain level dyeings.

14.3.2 Introduction to dyeing methods

The selection of specific direct dyes for dyeing cellulosic fibres depends on their
dyeing properties, the particular fastness requirements, any aftertreatments used to
improve the washing fastness, and on the particular finishing processes involved.
Possible staining of other fibres present in the material by the direct dyes is
another consideration. Dyeing with direct dyes is carried out in neutral solution.
The dyebath is gradually heated to promote diffusion and levelling of the dyes,
usually with gradual salt addition to exhaust the dyebath. Figure 14.2 illustrates a
typical dyeing procedure.