2.5.6 Influence of Rinsing Temperature
The dilution effects so far considered have been appropriate
for non-substantive chemicals and for dyes already desorbed from the fibre
surface into the liquor.
However, temperature can influence dye diffusion even during
the phase 1 dilution step. It has been shown that the rate and amount of
unfixed dye removal during phase 1 increase with temperature from 50C to 70C.
There is a similar increase in the quantity of dye removed between 50C and
60C in phase 3 (after 'soaping') but there is no improvement above 60C. The
increasing rate of dye diffusion is assisted by the swollen fibres at the
higher temperature, particularly under alkaline pH conditions. These effects
are illustrated in figure 3
Figure 3: Influence of Rinsing
Temperature
Following the drain and initial rinsing/dilution phase, the
diffusion-out of the unfixed hydrolysed reactive dye in phase 2 can be achieved
either by:
- - continuing the Smart Rinsing up to the required
temperature (ideally greater than 90C),
- running at this temperature for a defined period,
- cooling back at a pre-determined rate by Combined Cooling and Rinsing, and
- continuing with Smart Rinsing at 60 - 70C or
- - closing off the low level overflow weir,
- filling to the nominal running level and carrying out a conventional
boiling 'soap',
- cooling back at a pre-determined rate by Combined Cooling and Rinsing, and
- continuing with Smart Rinsing at 60 - 70C
The choice will depend upon the particular machine set up
and the ability to continue feeding in hot water at the required rate and
temperature. The relative water consumption will also determine the design of
the rinsing sequence. Option ii) will normally consume less water and has the
added advantage that any required chemicals (e.g., sequestrants) can be dosed
into the 'soaping' bath and thus used more effectively.
Phase 3, is a continuation of smart rinsing to dilute the
diffused out dyestuff to an acceptable level. As with phase one this is
accomplished most efficiently with hot water, 60 - 70C (the colour content of
cold rinse liquors give no indication of the extent of unfixed dye left on the
fibre because the dye becomes 'frozen' into the constricted fibres). The
rinsing time will again depend on depth of shade and will normally take 10 - 30
minutes. The rinsing can be continued down to 40C where after-treatments may
be exhausted from a new discrete bath, if required. Whichever of the above
options is used the entire controlled rinsing sequence should take only 45 - 90
minutes depending on depth of shade and flow rates. This represents a saving of
more than 2 hours compared with conventional rinsing of dark shades.
