Page 310 - Basic Principles of Textile Coloration

THE EFFECTS OF VARIATIONS IN DYEING CONDITIONS 299

increasing temperature decreasing the dyebath exhaustion because dyeing is
exothermic, and its enhancement of the dyeing rate particularly at lower
temperatures. Increasing temperature also promotes dye de-aggregation in the
dyeing solution liberating more individual dye molecules to enter the fibre.

Manufacturers often cite a temperature of maximum exhaustion and provide
optimum dyeing temperature profiles. The dyes CI Direct Yellow 12, Direct Red
81 and Direct Yellow 28 have maximum exhaustion at 30 °C, 60 °C and 100 °C,
respectively, corresponding approximately to the behaviour of dyes (a), (b) and (c)
in Figure 14.4. Despite this, the actual dyeing temperature may often depend on
machinery limitations. For example, open jig dyeing machines cannot achieve
dyeing temperatures much above 85 °C.

14.4.3 The influence of dyeing pH
Dyeing with direct dyes is usually carried out in neutral solution. Under alkaline
conditions, cellulose fibres have an even greater negative potential, partly because
of increasing dissociation of a number of cellulose hydroxyl groups, and exhaustion
is lower. Oxycellulose, present in cotton that has been over-bleached, has a higher
proportion of carboxyl groups, and is dyed much paler than undamaged cellulose
because carboxylate ions repel the dye anions of like charge. Dyeing in the
presence of formic acid suppresses ionisation of the carboxyl groups and the
oxycellulose is then dyed to about the same depth of shade as regular cellulose.
Under alkaline conditions, reducing end groups, particularly prominent in low
molecular weight celluloses such as viscose, reduce some azo direct dyes,
decreasing the colour yield. In general, however, if dyeing of cellulosic fibres is in
acidic solution, only weak acids are used and the residual acid in the goods is
thoroughly rinsed out before drying. Strongly acidic dyeing conditions favour the
acid-catalysed hydrolysis of cellulose. Any traces of residual mineral acid dried into
the material can cause considerable damage.

14.4.4 The effect of liquor ratio
As we saw in Section 10.4.3, dyebath exhaustion should increase with decrease in
the dyeing liquor ratio. This is certainly the case for direct dyes. This means that
dyeing at low liquor ratio decreases the amount of waste dye in the effluent. It also
consumes less water and steam, and allows a given salt concentration with less
added salt. There has been a strong trend towards dyeing at as low a liquor ratio as
is practicable. It should always be remembered, however, that the required amount