Page 200 - Basic Principles of Textile Coloration

TERMS USED IN DIRECT EXHAUST DYEING 189

molecules, however, are able to diffuse into the fibres. A rapid equilibrium exists
between single molecules and aggregates in solution. Aggregation is slightly
exothermic. As the dyeing temperature increases, the aggregates gradually break
up (heat absorbing) so that more individual molecules are available for penetration
into the fibres. The exhaustion thus increases with increasing temperature.
Eventually, if the dyeing temperature increases further, and aggregation is no
longer significant, the final exhaustion will begin to decrease with increasing
temperature. In the dyeing of synthetic fibres with disperse dyes, the dyebath
exhaustion often apparently increases with increasing temperature at temperatures
around the dyeing transition temperature. This is more of a kinetic effect related
to the rate of dye diffusion, controlled by polymer chain segment mobility.

Practical dyeing rarely reaches equilibrium. A rapidly dyeing dye may soon
reach equilibrium and dyeing may only require a relatively short time ((a), in
Figure 10.5). For such a dye, dyeing at lower temperatures gives the best
exhaustion ((a), at temperature T2), assuming that dyeing is still reasonably rapid.
For a slowly dyeing dye ((b) in Figure 10.5), the best exhaustion normally occurs at
higher dyeing temperatures ((b), at temperature T1) because dyeing is far from
equilibrium and the exhaustion depends on the rate of dyeing and the time.

10.4.7 Dye compatibility

Exhaustion curves characterise the dyeing properties of a dye and are useful for
selecting compatible dyes. One of the dyer’s major objectives is to produce goods
having the exact shade desired by the client. This usually requires the use of a
mixture of dyes, often a mixture of red, blue and yellow. These should all be dyes
with about the same rate of dyeing. They should have very similar rates of
exhaustion when used in mixtures under the given dyeing conditions. During
dyeing, the colour of the goods will then gradually become deeper, but, since the
dyes are absorbed in the same proportions throughout the process, the hue does
not change and the goods will always be on shade. The dyer can then stop the
process as soon as the colour is deep enough. If the dyes do not have
approximately equal dyeing rates, the hue will be constantly changing from that of
the more rapidly absorbed dyes towards that of the more slowly absorbed ones that
remain in the dyebath longer. The dyer then has to contend with both the gradual
increase in depth of shade and the shifting hue. It will be lucky indeed if the
dyeing has the correct colour depth at the moment when it has the correct hue.
Mixtures of dyes that build up on shade, with no change in hue, are said to be