Page 219 - Basic Principles of Textile Coloration

Exhaustion/%208 DYEING THEORY

the equilibrium position (Figure 10.2). Exhaustion curves at different temperatures
show how the dyeing rate increases with increasing temperature, and, in the usual
case of exothermic dyeing, the suppression of the equilibrium exhaustion at higher
temperatures (Figure 10.5).

The information in an exhaustion curve is often summarised in terms of the
time of half dyeing and the equilibrium exhaustion. The time of half dyeing is the
time required for the exhaustion to reach 50% of its equilibrium value. The rate of
isothermal dyeing constantly decreases over time, becoming zero at equilibrium,
and the half dyeing time gives a convenient measure of the dyeing rate. Thus, a
dye with a short half dyeing time absorbs more rapidly than one with a long half
dyeing time. This approach, however, has several limitations. A major criticism is
that two dyes with different final exhaustions can have the same time of half
dyeing, but do not necessarily have the same dyeing rate (Figure 11.6). Therefore,
compatible dyes do not necessarily have the same half dyeing times. In addition,
for pale shades, a dye often initially exhausts more rapidly and gives higher final
exhaustion than when dyeing a deeper shade. The time of half dyeing therefore
depends on the actual amount of dye being used.

100

(c)
80 (b)

60 (a)

40
time of half dyeing = 16 min (a)

20
time of half dyeing = 8 min ((b) and (c))

0
0 20 40 60 80 100
Time/min

Figure 11.6 Graphs of exhaustion versus time to show variations in the time of half dyeing