Page 426 - Basic Principles of Textile Coloration

DYEING COTTON/POLYESTER BLENDS 415

of melted polyester on the fibre ends since these tend to dye more deeply than the
fibres resulting in a fabric with a spotty appearance. Heat setting the polyester
before dyeing, to stabilise the fabric dimensions, must be uniform since it
influences both the rate and extent of dyeing with disperse dyes. Heat setting after
dyeing helps to remove creases in the fabric and to set its final dimensions. It can,
however, cause thermal migration of the disperse dyes in the polyester fibres. In
the case of poorly penetrated fibres, disperse dyes may migrate further into the
polyester fibre interior, thus deepening the shade. Alternatively, if any
hydrophobic chemical such as non-ionic surfactant or fabric softener remains on
the polyester fibre surfaces, disperse dyes may migrate into this layer, in which they
are very soluble. This results in greater ease of colour removal by washing or
rubbing during use, and is undesirable.

In many cases of union dyeing, the dyes and dyeing conditions for colouring one
of the fibres influence the other fibre and its dyes. The major areas of concern are:
(1) the degree of cross-staining. Although disperse dyes will stain cotton, the

anionic cotton dyes usually completely reserve the polyester. The disperse
dyes selected should be those that give minimal cotton staining;
(2) interactions between dyes and auxiliaries when present in the same bath. For
example, many disperse dyes are not stable under the alkaline reducing
conditions used in leuco vat dyeing. The salt and alkali required for dyeing
cotton with reactive dyes often have a deleterious effect on the dispersing
agent for disperse dyes and cause particle aggregation;
(3) the conditions for fixation or aftertreatment of one type of dye on the other
types often limits the processing methods that are possible. Thus, if direct
dyes are used for continuous dyeing in combination with disperse dyes they
must be stable to the high temperatures used in the Thermosol process. This
factor influences when reduction clearing can be used. It is not possible to
clear disperse dyes staining the cotton with an alkaline hydros solution if the
cotton is already dyed. All the types of dyes used to colour cotton will be
reduced under these conditions and the colour destroyed.

Even when staple polyester is the predominant fibre in intimate blends with
cotton, the cotton fibres have a tendency to migrate to the yarn surfaces and have
higher visibility. The colour and the fastness properties of the dyed cotton are
therefore important, even though it is the minor component.

One of the major problems in dyeing cotton/polyester fabrics is that of colour
control. This is true for cross-dyeings and for solid shades. To examine the colours