Page 197 - Basic Principles of Textile Coloration

186 AN INTRODUCTION TO DYES AND DYEING

and in the distribution of residual impurities, as well as differences in temperature
and flow rate of the solution in contact with the fibres.

The strike of a dye is evaluated by a simple test. After a few minutes of dyeing a
sample of fabric, a second undyed sample is added and the two samples are then
dyed together for several minutes, removed and rinsed. For dyes of high strike, the
first sample is much deeper in colour than the second. Dyeing of the two samples
may be continued to establish how long it takes for them to have the same final
colour. Alternatively, a series of samples may be dyed in succession for a constant
short period in the same bath. As each dyed sample is removed, it is replaced by
an undyed one, and so on. After the test, the coloured samples are placed side by
side in the order they were dyed. Rapid dyeing dyes give a short series of heavily
dyed samples, the later samples being much paler. For slow dyeing dyes, the colour
is less deep but the depth is much more uniform over the entire range of samples.
This type of test may be carried out with a constant or variable bath temperature.

10.4.5 Migration and levelling

Unlevel dyeings can arise for a number of different reasons, some of which are
beyond the control of the dyer. Types of unlevel dyeing include stripiness in either
material direction or at random, end or edge differences, light and dark patches,
and skitteriness (closely spaced light and dark regions). Unlevel dyeing can usually
be traced to poor preparation, to other faults in the goods, or to problems in the
operation of the machine or in the dyeing procedure. Specific dyeing machines
give certain types of faults, such as crease marks in winch dyeing (Section 12.4.1),
or non-uniform dyeing from uneven liquor flow in a package machine (Section
12.3.2). Material faults that cause unlevelness include uneven distribution of
residual chemicals, variations of the fibres, and uneven treatment with chemicals
or by mechanical or thermal processes. Inadequate control of the rate of dye
absorption will obviously give unlevel dyeings unless the dye can subsequently
migrate from deeply dyed to lightly dyed regions of the goods.

The ability of a dye to migrate and produce a level colour, under the given
dyeing conditions, is obviously an important characteristic. It can overcome any
initial unlevelness resulting from a rapid strike. Migration of the dye demonstrates
that the dye can be desorbed from more heavily dyed fibres and re-absorbed on
more lightly dyed ones. This is important in package dyeing where uniform colour
of the yarn throughout the bobbin is essential. While migration is important for
level dyeing, it has two major drawbacks. Firstly, the dye’s ability to desorb from