Page 95 - Basic Principles of Textile Coloration

84 NATURAL CELLULOSIC FIBRES

The active species in bleaching with hydrogen peroxide is probably the
hydroperoxyl anion (HO2–). Hydrogen peroxide is quite unstable in alkaline
solution, particularly on heterogeneous surfaces and in the presence of many
transition metal ions. The bleaching liquor normally contains a silicate,
polyphosphate or protein stabiliser. These probably function by sequestering
(complexing) metal ions in the water, such as Fe3+, and by coating rough surfaces.
Thus the stabiliser suppresses catalysis of the decomposition of the peroxide.

Hydrogen peroxide bleaching of cotton offers a number of advantages:
(1) a range of continuous, semi-continuous and batch bleaching processes are

possible;
(2) the complete removal of proteins from the cotton is not essential since

chloramine formation is not possible with peroxide;
(3) for some materials, the combining of alkali boiling and bleaching, or even of

desizing, alkali boiling and bleaching, into a single operation is possible;
(4) there is a reduced risk of over-bleaching and oxycellulose formation.

On the negative side, careful control of the bleaching pH is necessary and heavy
metal ions such as those of copper and iron, which catalyse rapid peroxide
decomposition, must be complexed with an appropriate sequestering agent such as
a stabiliser or ethylenediamine tetra-acetic acid (EDTA) (Section 8.3.3). The use
of buffers and stabilisers is rather critical. The presence of magnesium ions in the
water is essential for effective peroxide stabilisation by silicates. Silicate stabilisers
tend to deposit on the yarns or fabric and form silica particles on drying. These can
give a rough handle to a fabric and considerable friction of yarns with their guides
during knitting. Protein stabilisers are preferred over silicates for a softer, smooth
handle.

In the preparation of cotton yarn packages for knitting, a one-step process is
common (combined mild alkali boiling and bleaching). Pressurised alkali boiling
promotes swelling of the yarns and can create excessive pressure in the package
interior.

Much of the success of hydrogen peroxide bleaching is a consequence of the
development of rapid processes combining some or all of the three preparation
operations, allowing considerable economies in labour, time, steam and hot rinsing
water. Good water absorbency only requires rupture of the cotton fibre cuticle wall
and not necessarily complete removal of the cuticle so vigorous alkali boiling, as in
a kier, is not always essential. On the other hand, the traditional enzyme desizing,
kier boiling and hypochlorite bleaching give the whitest cotton. Continuous