Page 99 - Basic Principles of Textile Coloration

88 NATURAL CELLULOSIC FIBRES

cationic and non-ionic dyes. The types for cotton behave as colourless anionic
direct dyes whereas the non-ionic types are similar to disperse dyes. Use of these
compounds at low concentrations of 0.1 g l–1 or less is usual, with application
procedures closely resembling the analogous dyeing processes. Many of these
compounds are stable in alkaline solutions of hydrogen peroxide commonly used in
beaching and their presence in the bleaching bath allows milder bleaching
conditions. They often have poor fastness to washing and light but this is not a
problem since most synthetic washing detergents contain FWAs and reproduce
the whitening effect after each wash.

5.4.6 Mercerisation

Mercerisation is a process named after its discoverer, John Mercer (1850). He
treated cotton cloth with cold 20% aqueous NaOH solution, without any applied
tension, and then washed out the alkali. This caused considerable fibre swelling
and shrinkage of the fabric, but increased its strength, elasticity and ability to
absorb cotton dyes. Later, around 1890, Horace Lowe showed that if the material
was held under tension during the treatment, and until most of the NaOH had
been washed out, no shrinkage occurred and the fibres developed an attractive,
subdued lustre. The development of this lustre is still one of the purposes of
modern mercerisation.

During the process, absorption of NaOH by the cotton fibres causes acidic
dissociation of a significant number of hydroxyl groups. The concentration of ionic
sites within the cotton fibres is high in comparison to the ion concentration in the
external solution. A considerable pressure develops in the fibres as water is
absorbed by osmosis and the fibre swells, loosing the characteristic convolutions.
After washing out of the alkali, the fibres have a much rounder cross-section and
therefore higher lustre. Today mercerisation is also appreciated because mercerised
cotton absorbs larger amounts of dyes and gives a higher rate of dyeing. The more
uniform swollen fibres absorb more and scatter less light so that a given amount of
dye produces a deeper shade than on regular cotton. This is significant when the
dyes are expensive. In addition, the swelling of immature fibres, with thin
secondary walls, during mercerisation results in improved dye absorption and a
depth of colour close to that of normal fibres, so that the paler dyed neps are less
obvious.

The effects on the dyeing properties and the changes in mechanical properties
are a consequence of the fibre swelling and changes in the crystalline structure of