Page 386 - Basic Principles of Textile Coloration

PRE-PIGMENTATION DYEING METHODS 375

dyeing. In some cases, an intermediate drying of the padded fabric may be
included to minimise colour bleeding into the development bath.

17.7.3 The fully continuous pad–steam process

The pad–steam process for dyeing cellulosic materials with vat dyes is a
particularly important process. It involves five stages:
(1) padding the fabric with the fine vat pigment dispersion containing a wetting

and an anti-migration agent;
(2) drying of the padded fabric and cooling;
(3) padding of the dried, pigmented fabric with hydros and caustic soda solution;
(4) steaming in air-free steam at about 102–105 °C;
(5) rinsing, oxidation and soaping using a series of wash boxes.

Liquid forms of vat dyes are preferred for continuous dyeing as they give less
migration on drying and are more convenient for preparing the large volumes of
dye liquor required. The drying of the fabric after padding ensures a high pick-up
of alkaline hydros solution from the chemical pad. Some dye invariably bleeds from
the fabric into the alkaline hydros solution. This is less when padding a dried
fabric, and an initial addition of some dye to the chemical pad solution minimises
colour tailing. Drying of the initially padded fabric is usually carried out in two
steps involving a gentle pre-drying stage before the complete drying process. Pre-
drying is to control migration of the pigment particles to the yarn surfaces where
water is evaporating. The heating should be as uniform as possible. It is also
common practice to use an anti-migrant such as sodium alginate in the first
padding solution. The anti-migrant causes flocculation and aggregation of pigment
particles during drying thus limiting their movement within the fabric. Some anti-
migrant agents may also increase the solution viscosity. The ease of migration
depends on the fibres and fabric, the solution pick-up and the pigment particle size
(Section 10.5.3).

After the chemical pad, the fabric is steamed for 30–60 s in saturated, slightly
superheated, air-free steam (Section 12.6.3). It is essential to avoid condensation
falling onto the fabric in the steamer as this causes a spotty colour (Section
12.6.3). The steamer has a water seal at its exit with water flowing through the
trough to maintain a temperature of not more than 40 °C. After steaming the
fabric is rinsed by spraying to remove alkali and hydros. This is followed by rinsing,
oxidation, soaping, rinsing and neutralising in a series of wash boxes.