Page 356 - Basic Principles of Textile Coloration

BATCH DYEING OF COTTON WITH REACTIVE DYES 345

but this consumes much more water. For winch and jig dyeing machines operating
at atmospheric pressure, it is not possible to carry out soaping at temperatures
above 90 °C, even when they are closed. For any given dyeing machine, each
rinsing and washing stage should be evaluated and a general satisfactory washing
protocol established. One point requires particular attention. After each stage in
the washing cycle, dye transfers from one bath to the next, in the solution retained
by the fabric. This transfer must be as low as possible. This means that machines
should be completely drained between stages, and wash boxes in continuous
washing must have effective mangles to squeeze out as much solution as possible
from the fabric leaving the box.

16.3.4 Dyeing compatibility of reactive dyes
Ideally, reactive dyes in a mixture should all exhaust and react with the fibre at
about the same rate so that the shade builds up on tone. Dyes from different
ranges, with different reactive groups, can rarely be used together because of their
different dyeing characteristics and reactivities. It is therefore usual to mix dyes
with the same type of reactive group having about the same substantivity. Since
there is often a great deal of uncertainty about the particular type of reactive group
in a given reactive dye, dye selection must often be from one particular
manufacturer’s dye range and based on his recommendations.

Compatible dyeing behaviour is a function of all the process variables and
requires careful control of the dyeing temperature, salt and alkali concentrations,
the dyeing time and the liquor ratio. Once the dye has reacted with the cellulose,
it is completely immobilised and cannot migrate. Control of the process variables
determines whether a given shade will be reproducible from batch to batch. When
dyeing with mixtures of reactive dyes, shading is usually possible by addition of low
substantivity dyes to the alkaline bath. The dyebath may be partially drained and
re-filled with cold water, the solution of shading dyes added and the bath then re-
heated if necessary. Further additions of salt or alkali are often not required.

16.3.5 Problems in dyeing with reactive dyes
One of the major problems in exhaust dyeing with many reactive dyes is the rather
low level of fixation, particularly when dyeing using a high liquor ratio. Often less
than 70% of the original dye reacts with the fibre. This results in appreciable dye
concentrations in the dyehouse effluent. This environmental problem is