Page 304 - Basic Principles of Textile Coloration

DYEING PROPERTIES OF DIRECT DYES 293

are surfactants, or mixtures of surfactants, based on non-ionic polyethylene oxides
and may also contain cationic agents. These chemicals form complexes with many
types of anionic dyes, including direct dyes. Complex formation at relatively low
temperatures reduces the concentration of the free dye in the bath so that the rate
of dyeing is reduced. The dye–auxiliary complex molecule is too large to penetrate
into the fibres’ pores. The surfactants in these products disperse the dye–auxiliary
complex and prevent its precipitation. As the dyeing temperature increases, the
dye–auxiliary complex gradually breaks up, liberating more free dye, which is then
available for dyeing. In this way, the rate of temperature increase controls the rate
of exhaustion, giving dyeing with improved levelness.

High dyeing temperatures for cellulosic fibres give much better dye migration,
smaller risk of unlevelness and better fibre penetration. The SDC Class B and C
dyes migrate better at 120 °C, although the dyebath exhaustion is usually lower at
this temperature. Decreasing the dyeing temperature towards the end of the
process, once the dyeing is level, will then improve the exhaustion. At 120 °C,
however, there is a greater risk of dye decomposition or azo dye reduction by the
cellulose. Dyes must therefore be selected with care.

Scouring and bleaching of cotton consume large amounts of steam and hot
water. Because of increasing energy costs, many dyehouses dye grey cotton and
cotton/polyester fabric in a combined scouring and dyeing process, provided this
gives satisfactory results. This is possible for dull and deep shades on some knitted
materials, where the major contaminant to be removed is the lubricating oil used
in knitting. For a brighter hue, it is usual to simultaneously scour and dye the
goods, and then to add hydrogen peroxide and alkali to the exhausted dyebath and
bleach at 80–90 °C. This is called post-bleaching. In this way, the bath is heated
only once, minimising steam consumption. Dye suppliers recommend dyes for
these types of combined processes that are stable to alkali and resist fading during
post-bleaching.

Problems with tailing arise in padding with solutions of direct dyes because of
the inherent substantivity of these dyes for cellulose (Section 10.5.2). To minimise
this, the selected dyes should have low, comparable strike rates. Padding may be
carried out at higher temperatures in the range 50–90 °C, with dye solution
containing a minimum of salt, to reduce the substantivity. SDC Class A direct
dyes tend to migrate during drying and steaming and Class C dyes have rapid
strike and give most tailing. Class B dyes are therefore often the best choice for
continuous dyeing. Dyes of widely differing substantivity obviously cannot be
applied together by padding.