Page 424 - Basic Principles of Textile Coloration

DYEING COTTON/POLYESTER BLENDS 413

Union dyeing often involves colouring each fibre component separately using
appropriate dyes in two different dyebaths. This will be the longest type of union
dyeing process. Ideally, the dyeing of each type of fibre is so selective that the dyes
for colouring one fibre completely reserve the other. This is rarely the case. Some
of the dyes selected for colouring one component of the blend often stain the
other type of fibre. Such cross-staining must be distinguished from cross-dyeing.
Staining of a fibre implies that the dye is not well fixed and the colour will have
poor fastness properties. In most union dyeing, therefore, minimising the degree of
cross-staining is essential.

It is obviously desirable that union dyeing takes less time than that required for
dyeing each fibre in separate steps. Direct dyeing processes, in which both fibres
are dyed simultaneously with a mixture of different or similar types of dyes in a
single bath, are more economical. A number of factors, however, must be
considered. These include:
(1) the colour effects and fastness properties required;
(2) the compatibility of different dyes in the dyebath;
(3) the degree of anticipated cross-staining of each fibre;
(4) the influence of the dyeing conditions on the different fibres present;
(5) the stability of the dyeings to finishing processes;
(6) the type and availability of dyeing equipment;
(7) the process costs.

One-bath union dyeing to give a solid shade is difficult to achieve and there are
few union dyeing processes of this type. For example, there are several important
limitations in the dyeing of unions containing wool or cotton because of their
different sensitivity to alkalis and acids. Thus, the cotton in cotton/wool blends
cannot be dyed with vat dyes because the high alkalinity required for the leuco dye
solution causes extensive wool degradation. Large amounts of cotton/polyester
blends are still dyed in two separate processes, one for application of disperse dyes
to the polyester, followed by a second dyeing of the cotton with reactive or direct
dyes.

20.3 DYEING COTTON/POLYESTER BLENDS

20.3.1 Introduction
There is a major market for 65/35 and 50/50 polyester/cotton fabrics for a variety