Page 425 - Basic Principles of Textile Coloration

414 UNION DYEING

of end-uses, in weights ranging from light shirting to heavy industrial fabric. A
variety of different batch and continuous dyeing processes are used for these. The
dyeing of blends of polyester with other cellulosic fibres such as viscose and linen
follows much the same principles outlined here, although there are some slight
differences in the actual details.

The dyeing properties of polyester and cotton fibres are quite different and most
of the dyeing methods for their unions involve separate steps for the colouring of
the two fibres. The polyester component is invariably dyed first with a mixture of
disperse dyes. For the cotton, there is a choice of dyes, the actual selection
depending on the desired colour, the type of finishing required, the demanded
fastness properties, the costs and the type of machinery available. The cotton is
usually dyed with reactive, direct, sulphur, vat, or azoic dyes. The use of the latter
three types is decreasing because of increased costs and environmental problems.
The reactive/disperse dye combination for dyeing cotton/polyester clothing fabrics
is very popular. If the cotton is dyed first, subsequent pressure dyeing of the
polyester at 120–130 °C can change the shade of the dyed cotton because the
cotton dyes are less stable at high temperatures. Dyeing the polyester before the
cotton allows an intermediate reduction clearing of any disperse dye on the
polyester surface or remaining in the cotton (Section 15.7.4). An alkaline solution
of hydros destroys several types of cotton dyes so that reduction clearing is
precluded after dyeing the cotton. In addition, once the polyester is dyed any
detrimental effects of required cotton dyeing assistants such as alkali and salt on
the disperse dyes are avoided.

Cross-staining of the cotton by disperse dyes can be a problem. The stained
cotton has poor light and washing fastness and the weakly held disperse dyes will
transfer colour to other synthetic fibres such as nylon during washing. For deep
shades, alkaline scouring, or preferably reduction clearing with alkaline hydros and
a non-ionic detergent, removes the disperse dyes staining the cotton. Because
cotton crosslinking agents used in the finishing of cotton/polyester fabrics require
the use of relatively high curing temperatures, the disperse dyes used for the
polyester must be of the types that have higher sublimation fastness. Therefore,
high-energy disperse dyes are preferred (Section 15.7.6). For fabrics with more
than 60% polyester, the cotton component may not even be dyed if the required
shade is only pale and the two fibres are well blended in the yarn.

As always, careful fabric preparation is essential for production of high quality
dyeings. The aim is to produce a fabric that is easily wetted and absorbs dyes as
uniformly as possible. It is imperative that singeing does not produce small masses