Page 42 - Basic Principles of Textile Coloration

PREPARATION FOR DYEING 31

quality white goods are being produced, or if the goods will be dyed with pale
bright colours. The bleaching process also eliminates any traces of other impurities
remaining from the previous preparation steps and improves the absorbency of the
material for dyeing and printing. Bleaching agents are usually oxidising agents.
Today, the most common bleaching agent is a stabilised alkaline solution of
hydrogen peroxide (H2O2). This is a powerful oxidising agent that destroys the
natural colouring matters present in cotton without undue oxidative damage to
the fibres. Any residue of starch in the cotton is rapidly oxidised by the hydrogen
peroxide used in bleaching so that the peroxide is less effective in destroying
undesirable coloured impurities. In addition, residual starch can also reduce some
dyes during dyeing, particularly under alkaline conditions, resulting in decreased
colour depth.

Hydrogen peroxide has largely replaced solutions of sodium hypochlorite
(NaOCl) as a bleaching agent, since the latter chemical requires careful pH and
temperature control during bleaching to avoid oxidising the cotton. Hydrogen
peroxide also offers other advantages: alkali-boiling and bleaching can be
combined into one process; continuous operation is relatively easy; and thorough
washing of the bleached fabric is less critical since traces of residual peroxide are
less damaging than those of chlorine from sodium hypochlorite. Again, both
alkali-boiling and bleaching remove unwanted contaminants from cotton material,
which results in waste liquors and large volumes of washing water producing a
dilute effluent with an appreciable biological oxygen demand.

Bleaching, in the sense described above, must be distinguished from the
destruction of the colour of a dyed fabric, which may be necessary if it must be re-
dyed. Such colour stripping usually involves solutions of reducing agents such as
sodium hydrosulphite (Na2S2O4).

Materials manufactured from artificially made fibres are much easier to prepare
than those from natural fibres. Artificially made fibres are relatively clean and any
spin-finish or other chemicals such as size are, by design, removable by simple
washing with a detergent solution. Bleaching of artificially made fibres is not
usually necessary. It is most common when a fabric made of synthetic fibres has a
yellow cast from over-heating, or has picked up dirt during manufacture. The
process is much milder than for natural fibres.

Since many fabrics requiring bleaching tend to be somewhat yellow, they have a
stronger absorption of blue and violet light in the 400–500 nm wavelength range
(Table 1.3). Certain colourless compounds absorb non-visible, near ultraviolet
light at wavelengths between 300 and 400 nm and transform some of the