Page 428 - Basic Principles of Textile Coloration

DYEING COTTON/POLYESTER BLENDS 417

To minimise staining of the cotton and the need for clearing, dye selection is
important. The use of jet dyeing machines allows low liquor ratios, reduced
chemical and energy requirements and good colour reproducibility. A liquor-to-
goods ratio of say 10:1 for a 65/35 polyester/cotton fabric represents a liquor ratio
of 1000/35 or 28:1 with respect to the cotton. This high ratio for the cotton clearly
has an influence on the exhaustion of the cotton dyes, particularly reactive dyes
that may have low substantivity for cotton.

20.3.3 Disperse/reactive dye combinations

For bright shades of superior washing fastness, reactive dyes are the best choice for
colouring the cotton in a blend with polyester, despite their greater cost for deep
dyeings.

There are four major types of process used for the batch dyeing of cotton/
polyester materials using a combination of reactive and disperse dyes. There are
also a number of variants designed to take advantage of specific dye products.
Most procedures are quite lengthy, the major problem being the long times needed
for complete removal of the hydrolysed reactive dyes from the cotton.

The most conservative method uses four separate steps:
(1) dye the polyester with the disperse dyes at 130 °C;
(2) reduction clear to remove any disperse dyes staining the cotton;
(3) apply the reactive dyes to the cotton;
(4) rinse and scour to remove hydrolysed reactive dyes from the cotton.

The overall process can take as long as 10–12 h. The required time is less if the
reactive dyes are added to the cooled exhausted disperse dye bath without
draining, re-filling and reduction clearing. Some clearing of loosely held disperse
dye particles from the fabric occurs during scouring to remove unfixed reactive
dyes from the cotton.

In the reversed form of this two-bath process, the reactive dyes are applied first,
the fabric rinsed with warm water to remove salt and alkali from the cotton, and
then the polyester is dyed with the disperse dyes under pressure. The high
temperature dyeing of the polyester helps to eliminate much of the residual
hydrolysed reactive dye from the cotton. Pressure dyeing the polyester may require
a bath pH of 6.5 to minimise reactive dye–fibre bond hydrolysis during dyeing at
130 °C. It is essential that the disperse dyes do not significantly cross-stain the
cotton since the usual hydros reduction clearing is not possible without some