O o d h e a d p u b L i s h I n g L i m I t e d

9.3
The effect of wool processing on colour
The long processing chain from greasy wool to wool garment, by either the
worsted or woollen system, has many stages and several of these can affect
wool colour. In addition to photoyellowing, yellowing of wool can also
occur when it is treated under alkaline conditions or exposed to heat during
drying. Also several chemical treatments used in wool processing, in particular
shrink-resist treatments that use chlorine or the disodium salt of
dichloroisocyanuric acid (DCCA) can lead to significant yellowing of the
fibre. Wet bleaching treatments can also be used commercially to improve
the whiteness of wool. The effect of the various stages of wool processing on
its colour is summarised in the following subsections.
Scouring and mechanical processing
Efficient and effective scouring is important to obtain the brightest possible
wool colour. A study has shown that by simply rewashing commercially
scoured wool, in most cases, further improvements in both brightness (Y)
and yellowness (Y–Z) can be obtained (Thompson and Teasdale, 1986).
Effective scouring has been shown to depend on adequate refreshment of the
scouring liquor and failure to do this impacts particularly on scoured wool
brightness (Westmoreland et al., 2006). The use of hydrogen peroxide in the
final rinse of the scour before drying improves the colour of the scoured
wool but this practice can lead to unnecessary fibre damage if further oxidative
processes such as shrink-resist treatment or bleaching are performed in
subsequent downstream processing at the top or yarn stage. The practice can
also lead to colour instability.
A study on the processing of greasy Merino wools through to the top or
noil stage has shown very similar brightness and yellowness values at the
greasy and the top stage (Mahar and Osborne, 1996). In this study the scoured
wool values were significantly inferior to those of the greasy wool and top,
presumably due to somewhat inefficient scouring. The authors note that
commercial wool cleaning can be viewed as a two-stage process, with the
majority of contaminants removed during scouring and the remainder during
the mechanical carding and combing operations. A more recent study on
Australian, New Zealand and US wools has confirmed the improvements in
scoured wool colour after carding (Cameron and Stobart, 2006).
Wet processing and finishing
Thermal yellowing of wool can be a serious problem during wool processing,
in particular during extended dyeing at the boil, in setting with superheated
steam (decatising), and in drying for extended times, particularly under alkaline
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Improving the whiteness and photostability of wool
223
conditions (Maclaren and Milligan, 1981). The thermal yellowing of wet
wool is far more rapid than for dry wool, which is similar to the behaviour
observed for photoyellowing. Thermal yellowing of wool during dyeing is
influenced by pH, temperature and time, with chlorinated wools being especially
sensitive. Yellowing during dyeing can be counteracted by adding a bleaching
agent, based on sodium bisulfite or hydroxylamine sulphate, to the dyebath
(Duffield, 1996). Addition of hydrogen peroxide to the dyebath after exhaustion
of the dyestuffs can also be effective.
Commercial bleaching of wool is usually carried out either by an oxidative
process using alkaline hydrogen peroxide or by a reductive process using
stabilised sodium dithionite (‘hydros’) or thiourea dioxide (Duffield and
Lewis, 1985). The whitest wool is obtained by first carrying out an oxidative
hydrogen peroxide bleaching directly followed by a reductive treatment with
sodium dithionite, the so-called double bleaching process. Nevertheless the
ultimate whiteness achievable even after double bleaching is poor compared
with bleached cotton and synthetics, since some of the natural cream
chromophores of wool remain highly resistant to bleaching.
To further improve its whiteness, wool can be treated with a fluorescent
whitening agent (FWA) either after oxidative bleaching or by including the
FWA in the reductive bleaching bath. FWA-treated wool absorbs UV light
and emits blue fluorescence, which makes it appear much whiter than bleached
wool. Commercial FWAs for wool are usually based on a sulphonated stilbene,
distyrylbiphenyl or pyrazoline derivative. Wool fabrics that have been treated
with FWAs yellow rapidly when exposed to sunlight, especially when wet.
Chlorination of wool is carried out during commercial shrinkproofing at
the top, fabric or garment stage, and to prepare wool fabrics for printing. The
degree of wool yellowing that occurs during chlorination is dependent on
pH, temperature and the concentration of the chlorinating agent. Using DCCA,
a low pH (~3) at room temperature produces the least amount of yellowing
(Veldsman and Swanepoel, 1971). The mechanism of wool yellowing by
chlorine is unknown.
9.3.1
Wool colour compared with cotton and
synthetic fibres
Reflectance spectrophotometers capable of measuring tristimulus values also
provide a reflectance spectrum which represents the amount of light of any
given wavelength reflected from a sample. A perfect white material (such as
a ceramic MgO plate) exhibits close to 100% reflectance across the entire
wavelength range and is used to calibrate the instrument. 
Figure 9.1
 compares
the reflectance spectra of wool, cotton and polyester (PET) woven fabrics of
similar weight, and includes data after bleaching for the two natural fibres.
The fabrics that appear bright and white to the eye (i.e. bleached cotton and
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Advances in wool technology
224
PET) have a very flat spectrum with reflectance >80% that begins to tail off
only below ~460 nm. By contrast, the spectrum of natural wool is not flat,
having a lower level of reflectance across the whole wavelength range that
tails off further below ~500 nm. This low reflectance below 500 nm (equivalent
to absorption of blue and UV wavelengths) gives natural wool its characteristic
cream shade.
It is also clear from Fig. 9.1 that the improvement in whiteness for cotton
fabric after bleaching is far superior to wool. Even after double bleaching
(see above), wool’s reflectance across the wavelength range remains inferior
to unbleached cotton, and thus still appears cream-coloured rather than white.
90
80
70
60
50
40
30
360
400 440
480 520
560 600 640
680 720
Wavelength (nm)
CB
P
C
WDB
WB
W
Reflectance (%)
9.1
 Reflectance spectra of wool (W), cotton (C) and polyester (P)
fabrics, including H
2
O
2
-bleached wool (WB), double-bleached wool
(WDB) and H
2
O
2
-bleached cotton (CB).
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Improving the whiteness and photostability of wool
225
A more efficient bleaching process for wool capable of radically reducing its
yellowness would be of commercial value.
Treatment with a fluorescent whitening agent (FWA) improves the brightness
and yellowness of all fibres, as shown in Table 9.2 for typical lightweight
woven wool, cotton and polyester fabrics. It can be seen that the range of
brightness (Y) values is lowest for wool (68.8) and highest for FWA-treated
polyester (86.2) and also that this range is rather narrow since the difference
in perceived whiteness between these two samples is very significant. In the
textile industry it is more appropriate to use a whiteness index value (W) to
compare bleached and FWA-treated fibres, as this has a much wider range
than brightness values. There are a large number of different whiteness
indices available, and the CIE Ganz 82 index provides a good correlation
between the subjective visual whiteness and the measured value (Ganz,
1979).
The Ganz index is derived from the brightness value Y: