Page 432 - Basic Principles of Textile Coloration

DYEING COTTON/POLYESTER BLENDS 421

cotton by the disperse dyes is heavy because a reduction clearing can be carried
out before the vat dyes are added to the bath.

20.3.6 Disperse/sulphur dye combinations
Because of the low price of sulphur dyes, this combination is useful for dyeing deep
shades where the fastness requirements are less demanding. A two-bath dyeing
procedure is common, initially dyeing the polyester with disperse dyes under
pressure. The cotton is dyed in the second bath with the leuco sulphur dye
maintained in solution with some sodium hydrosulphide rather than sodium
sulphide, which can attack the polyester fibre. The reducing conditions help in
clearing disperse dye from the cotton. Finally, the dyeing is rinsed, the leuco dye
oxidised and the fabric well soaped. The process takes a long time but is useful for
relatively inexpensive dull, deep shades. The reversed procedure, in which the
cotton is first dyed with the sulphur dye, is not recommended because the alkaline
solution must be completely neutralised before application of the disperse dye.

20.3.7 Semi-continuous dyeing methods
This type of process offers high productivity for batches of material that are larger
than is usual for exhaust dyeing but too small to justify fully continuous operation.
There are a number of possible variants. In the case where the polyester is dyed by
padding and thermofixation, the dyeing of the cotton may be carried out in a
batch process. This process may involve:
(1) cold pad–batch application of reactive dyes;
(2) batch treatment of the fabric in any suitable dyeing machine with a solution

of salt and alkali if the reactive dyes were padded simultaneously with the
disperse dyes;
(3) a separate dyeing process for the cotton in a jet or beam dyeing machine.

Alternatively, the polyester may be pressure dyed and the cotton dyed with
reactive dyes by the cold pad–batch process. The reverse of this, cold pad–batch
dyeing of the cotton followed by batch dyeing of the polyester, is particularly
economic because the padded cotton can be batched directly onto the perforated
beams for a beam dyeing machine and no intermediate drying is required. It is less
successful in deep shades because reduction clearing of disperse dye from the