Page 355 - Basic Principles of Textile Coloration

344 REACTIVE DYES

Viscose fibres give higher fixation and exhaustion of reactive dyes than cotton.
In fact, for identical conditions, exhaustion and fixation increase in the order:
cotton, mercerised cotton, viscose. The washing fastness of reactive dyes on
viscose is also somewhat better than on cotton. Because of the ease of swelling of
viscose, the dyeing pH and temperature for a given dye may be different than for
cotton, particularly if dye penetration may be problematic.

The so-called ‘all-in’ dyeing method has the advantage that no additions are
required during dyeing and is therefore the most rapid dyeing method. The goods
are run in the filled machine with all the added salt and alkali and the reactive dye
solution then added over 10–15 min. The rate of dye fixation is controlled by
gradual heating. The starting temperature and heating gradient must be well
controlled for good colour reproducibility. The NT reactive dyes, mixed with
disperse dyes, are useful for dyeing cotton/polyester at 130 °C under neutral
conditions.

16.3.3 Washing-off of unfixed reactive dyes

Removal of hydrolysed and unreacted dye from the goods is a vital step after
dyeing. The amount of unfixed dye remaining in a cotton fabric dyed with reactive
dyes may have to be less than 0.002% owf. Although bleeding out of such a small
amount during subsequent washing by the consumer will not significantly alter the
depth of shade of the material, it can visibly stain adjacent white goods. This is
usually unacceptable.

Both batch and continuous washing processes involve three stages. Initially, the
goods are rinsed in cold and warm water. This is a dilution stage aimed at
removing as much salt and alkali from the goods as possible. This makes the next
soaping stage much more efficient since at lower electrolyte concentrations the
substantivity of the dye is less, making its desorption easier. The final stage is again
a warm rinsing stage to dilute the final dye solution adhering to the fibres to the
point that the amount of unfixed dye carried over to the final drying is minimal.
This residual quantity of dye will be deposited on the fibre surface on evaporation
of the water during drying and will be easily removed by later washing. Obviously,
the amount must be as small as possible.

The entire washing operation involves achieving a compromise between the
effectiveness of removal of unfixed dye and the cost of the large volumes of water
used, including the heating costs. Low liquor ratio washing saves water but gives
less dilution of the washing liquors. Some dyeing machines allow overflow rinsing