Page 360 - Basic Principles of Textile Coloration

CONTINUOUS DYEING PROCESSES FOR COTTON 349

hue. These effects can be minimised by adjusting the initial concentration
and composition of the feed solution of dyes;
(2) when using pre-prepared alkaline solutions of reactive dyes, reasonable
stability of the dye/alkali mixture is essential. Any premature hydrolysis of the
dye’s reactive group in the reservoir or pad bath will result in a loss of fixed
colour. This can be controlled using a dye solution with the lowest possible
pH. When the pre-mixed dye and alkali solution is not sufficiently stable, and
it cannot be stored in a reservoir without an unacceptable degree of
hydrolysis, metering pumps deliver known flow rates of separate neutral dye
and alkali solutions, which are mixed just before entering the pad bath. A
constant padding temperature must be maintained for an invariant colour
yield. Having two separate padding stages solves the problem of dyes that are
very susceptible to hydrolysis. In this case, a neutral solution of the reactive
dye is first padded onto the goods and, after drying, the alkali solution is
padded on just before the fixation stage. In some cases, a wet-on-wet two-
stage padding is possible;
(3) when dyeing very deep shades, concentrated dye solutions are required. Dye
solutions containing more than 50 g l–1 of dye may be needed if the solution
is to be applied at relatively low wet pick-up to avoid migration problems
during drying and excessive drying costs. For example, a 4.2% owf dyeing
requires a dye solution of 70 g l–1 at 60% solution pick-up. The desired
concentration may exceed the dye solubility, particularly if an appreciable
amount of electrolyte is present. Low dye solubility can be improved by
addition of relatively large amounts of urea (100 g l–1). Urea helps to break
up dye aggregates by hydrogen bonding to the dye molecules. It is sprinkled
onto the cooled dye solution before the salt and stirred in to dissolve it;
(4) after padding with a neutral solution of reactive dyes, the fabric will often be
dried. This must be done under conditions that minimise migration of
unfixed dye to the fibre and yarn surfaces. Heating of the wet material must
be as uniform as possible to avoid side-to-centre and back-to-face colour
differences due to migration. The padding solution will usually contain salt as
an anti-migrant and also a thickening agent such as sodium alginate
(1 g l–1);
(5) in fully continuous operations, the final rinsing and soaping of goods dyed
with reactive dyes are also carried out continuously. These processes take
place at high fabric speeds and the goods are only in contact with the washing
solutions for short periods of time. Therefore, washing-off is usually less