Page 350 - Basic Principles of Textile Coloration

BATCH DYEING OF COTTON WITH REACTIVE DYES 339

light fastness, allow relatively simple and diverse dyeing methods and are available
in a range of bright colours. They have contributed significantly to the decline of
direct cotton dyes. Their fastness properties, though generally good, do not match
those of pigmented cotton dyed with vat dyes. In particular, the fastness to
bleaching by chlorine, and to a lesser extent by peroxides present in modern
household detergents, is often only moderate.

16.2.5 Evidence for covalent bond formation with cellulose
The good fastness to washing of dyeings with reactive dyes on cellulosic fibres is a
consequence of the stable covalent bond formed between the dye’s reactive group
and the cellulose polymer. There is considerable evidence to support the formation
of this dye–fibre bond. Dyeings are resistant to colour stripping with hot aqueous
pyridine, a solvent that effectively removes direct dyes from cotton. Dyeings of
cotton obtained with bifunctional dyes often exhibit reduced swelling and
decreased solubility in cuprammonium solution. A dye molecule with two reactive
groups crosslinking two different cellulose chains would explain this. If the colour
of a dyeing obtained with an azo reactive dye is destroyed by chemical reduction
with alkaline sodium hydrosulphite, which cleaves the azo dye into two primary
aromatic amines, the amine remaining attached to the cellulose can be diazotised
and coupled with an appropriate phenol to reform a coloured fibre. Finally,
bacterial degradation of reactive dyed cotton that depolymerises the cellulose, but
avoids breaking the dye–fibre bond, gives coloured products containing the
original dye still bonded to glucose.

16.3 BATCH DYEING OF COTTON WITH REACTIVE DYES

16.3.1 Preparation for dyeing
Level well-penetrated dyeings require careful preparation of the material. All
sizing compounds capable of reacting with the dye, such as starch or polyvinyl
alcohol, must be removed from the material and any traces of residual alkali must
be uniformly neutralised. Good alkali boiling to remove wax is essential for goods
to be dyed with cold-dyeing reactive dyes because penetration of the dyes into the
fibres is much more difficult at lower dyeing temperatures. Reactive dyes often
give such bright colours that bleaching may not be necessary. Once size has been
removed, grey cotton goods can sometimes be simultaneously scoured and dyed
using hot-dyeing reactive dyes and an efficient detergent. Because of the