Page 96 - Basic Principles of Textile Coloration

COTTON PROCESSING 85

preparation operations, and particularly those where stages are combined,
although often giving acceptable results, are less effective and give a white of
lower quality. Semi-continuous pad–batch methods, with cold or hot storage, are
also used. Section 21.6.6 discusses evaluation of the degree of whiteness of a
fabric.

Cotton may also be effectively bleached with weakly acidic solutions of sodium
chlorite (NaClO2), more widely used in bleaching wood pulp. The acid liberates
the toxic gas ClO2, which is the real bleaching agent. This, however, promotes
corrosion of steel and a corrosion inhibitor such as sodium nitrate is needed in the
solution.

5.4.5 Fluorescent whitening agents [10]

One remarkable way of improving the apparent whiteness of a cotton fabric is to
treat it with a fluorescent whitening agent (FWA), sometimes also called an
optical whitener or optical bleach. These chemicals are colourless or very pale
yellow. They do not absorb visible light significantly, but do absorb ultraviolet light
with wavelengths around 340–380 nm. Some of the absorbed ultraviolet energy is
re-emitted as blue fluorescence in the 430–450 nm wavelength range. The
absorbed ultraviolet light has greater energy than the re-emitted fluorescence, the
remainder being converted into heat. This is illustrated in Scheme 5.3. In this,
FWA and FWA* represent the normal and excited electronic states of the
fluorescent chemical, respectively, n and l are the frequency and wavelength of
the absorbed or emitted light, h is Planck’s constant, and c the speed of light.

FWA + hυ (360nm) FWA*

FWA* FWA + hυ (440nm)+ heat

Energy of light = hυ = hc
λ
Scheme 5.3

A perfectly white material will reflect all wavelengths of white light to the same
high degree. Most textile fabrics requiring bleaching, however, have a slight cream
or yellowish appearance because they preferentially absorb blue light and therefore
have lower reflectance in the 400–500 nm region of the spectrum. The blue