Page 363 - Basic Principles of Textile Coloration

352 REACTIVE DYES

disperse dyes to cotton/polyester materials using the Thermosol process (Section
15.7.6). Baking temperatures of 200–220 °C for 1 min are typical but may be
lower (150–170 °C) if only cotton is being dyed. When dyeing deep shades, the
degree of fixation is usually lower. The fabric tends to yellow at high baking
temperatures and this can influence the hue of pale shades. The most popular
variant of this process for dyeing cotton/polyester fabrics is the neutral thermofix
method. For this a solution of reactive and disperse dyes containing sodium
bicarbonate is padded onto the cotton/polyester using dicyanodiamide,
H2N–C(=NH)–NH–CN, in place of urea. Both fibres are dyed simultaneously by
heating at around 200 °C for 1 min. As in all reactive dyeing processes, the final
step is a thorough washing to remove unfixed dyes.

16.5.4 Pad–steam dyeing
In this process, the goods are padded with a solution containing the reactive dyes,
salt and the appropriate alkali. Again only selected dyes are suitable and the
manufacturer’s recommendations should be followed. The hot humid conditions
during steaming tend to cause excessive hydrolysis of the reactive group and thus
lower the colour yield. In dyeing terry towelling and other pile fabrics, a two-stage
wet-on-wet padding version of this process is used to avoid an intermediate drying
step. The towelling is first padded with a neutral solution of the reactive dyes, and
then with a solution of the alkali prior to steaming. In wet-on-wet padding, the
pick-up of the second solution should be sufficiently high to give good fixation,
and colour bleeding into the second pad bath must not be excessive.

16.5.5 Pad–dry–pad–steam dyeing
This is the predominant fully continuous reactive dyeing process. The fabric is
padded with a neutral solution of the reactive dyes, dried and then padded with
the alkali solution containing salt before steaming. The dye solution is quite stable,
because there is no alkali present in the dyebath. Some salt or anti-migrant agent
helps to minimise migration of the dyes to the yarn surfaces during the initial
stages of drying. The intermediate drying ensures a uniform and high pick-up of
the alkaline salt solution and minimises bleeding of dye into the chemical pad.
After drying, the fabric must be cooled on cold cylinders to avoid heating the
chemical pad solution. Since sodium carbonate is difficult to dissolve in salt
solution, sodium hydroxide is the preferred alkali. There are a number of