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

· + FWA
–
·
  
Proton transfer

→


Wool· ·FWAH 
  
Combination

→
  Wool — FWAH
(15)
covalent wool–FWA bonding
FWA
–
· + H
+
  
                          

→

FWAH· 
→ FWAH
2
(16)
Reduced FWA products
FWA
–
· + O
2
  
                          

→

FWA + 
  
O
2
–
·
(17)
  
O
2
–
· + H
+
  
                          

→

·OOH
(18)
  
O
2
–
· + H
+
 + ·OOH
  
                          

→

H
2
O
2
 + O
2
Dismutation of superoxide (19)
H
2
O
2
 + M
n
+
  
                          

→

M
(
n
+1)+
 + OH· + OH
–
(20)
M
(
n
+1)+
 + 
  
O
2
–
·
  
                          

→

M
n
+
 + O
2
(21)
H
2
O
2
  
  
          
           
h
ν

→


2 ·OH
Generation of ·OH
(22)
FWA + 
n ·OH
  
                          

→

mono and dihydroxylated FWAs
(23)
Dihydroxylated FWA
  
         – 2H         

→


yellow stilbene quinones
(24)
Yellow FWA photoproducts
Wool + ·OH
  
                          

→

yellow wool oxidation products
(25)
Wool
+
· + 
  
O
2
–
·
  
                          

→

yellow wool oxidation products
(26)
Yellow wool photoproducts
M
n
+
, M
(
n
+1)+
 = catalytic metal ions
9.9
 Photoreduction mechanism of FWA on wool and superoxide
generation via photoinduced electron transfer.
© 2009 Woodhead Publishing Limited

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Advances in wool technology
238
by H atom abstraction by ·OH), to form the stilbene quinone as shown in
Fig. 9.10.
It is not possible from mass spectrometry to determine whether the stilbene
quinone adopts an ortho- or para-geometry or a mixture of isomers, but both
9.10
 Formation of a coloured stilbene quinone from an aqueous
DSBP FWA irradiated in the presence of H
2
O
2
.
NaO
3
S
SO
3
–
NaO
3
S
NaO
3
S
NaO
3
S
NaO
3
S
NaO
3
S
[O]
DSBP Uvitex NFW
OH
[O]
OH
and/or
HO
HO
HO
–2H
O
SO
3
–
SO
3
–
SO
3
–
SO
3
–
SO
3
–
O
O
O
and/or
ortho- and para-stilbene quinones
© 2009 Woodhead Publishing Limited

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Improving the whiteness and photostability of wool
239
forms would be highly coloured. It is interesting that ortho-quinones, in
particular dopaquinone, are involved in the formation of melanin.
9.5
Methods for improving the whiteness and
photostability of wool
There are currently a number of effective ways in which wool whiteness can
be improved on the farm. As discussed in 
Section 9.2.1,
 heritability data
show that selection of sheep for high clean wool brightness and low yellowness
would result in improved wool colour characteristics. In addition, the rugging
or shedding of sheep, or the annual application of UV absorbers to the backs
of sheep would lead to reduced weathering damage and improved colour.
Several treatments have been described in the literature that can improve
the photostability of wool, but so far only one has been found suitable for
commercialisation, and mainly as a treatment against phototendering rather
than photoyellowing. A UV-absorber with a sulphonated 2-
hydroxyphenylbenzotriazole structure was developed by CSIRO in Australia
that offers wool good protection against the reduced tear strength, abrasion
resistance and dye fading usually experienced following prolonged sunlight
exposure, with some reduction in photoyellowing for undyed wools (Mosimann
et al., 1990). Intramolecular hydrogen-bonded 2-hydroxyphenylbenzotriazoles
are commonly used as UV-absorbers in other polymers, and they dissipate
the absorbed UV energy as heat through conversion to a tautomeric form in
the excited state (Fig. 9.11).
Hydroxyphenylbenzotriazoles used commercially for stabilisation of other
polymers are highly hydrophobic materials that have no affinity for wool.
Introduction of the sulphonate group imparts both water solubility and
substantiveness for application to wool under slightly acid conditions, similar
to acid dyes. However, for wool the key to development of an effective
additive was in the nature of the R group. A bulky hydrophobic substituent
is required to shield the internal hydrogen bond from interaction with polar
groups in wool (Mosimann et al., 1990). This chemistry was commercialised
in 1990 for use as Cibafast W (Ciba), mainly for use on upholstery and
curtaining fabrics. It cannot be used on FWA-treated wool since it absorbs
the UV wavelengths necessary to excite the FWA.
H
O
N
N
N
R
SO
3
Na
H
O
N
N
N
R
SO
3
Na
+
h
ν
9.11
 Tautomerisation of a 2-hydroxyphenylbenzotriazole UV absorber.
© 2009 Woodhead Publishing Limited

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Advances in wool technology
240
Improved bleaching processes are an obvious way to improve wool colour.
Working on crossbred wools for use in wool carpets, New Zealand workers
at Canesis found that performing an acid extraction during scouring produces
a much brighter fibre (i.e. a higher Y tristimulus value) (McKinnon et al.,
1999). It is claimed that the acid extraction, performed at pH 2–3 using
sulphuric acid and ethylenediaminetetraacetic acid (EDTA), removes absorbed
iron from the wool. This agrees with previous work by Simpson who detected
iron (10 mg/kg) and copper ions (1 mg/kg) after extraction of scoured wool
fabric with molar sulphuric acid and with phosphate buffer at pH 7 (Simpson,
1987). Although acid extraction improves the brightness of treated wool, its
perceived yellowness (Y–Z) also increases since the Z tristimulus value is
not significantly improved. The new scouring process, marketed as Glacial
®
wool, therefore includes a peroxide bleaching stage and an optional reductive
bleach. Incorporating acid extraction and an oxidative and reductive bleaching
stage into the scouring process produces wool at least 7Y tristimulus units
brighter than conventionally scoured wool. The Z value is also improved by
~8 units, making the wool appear significantly less yellow.
Another recent development in wool bleaching has been the development
by Australian workers at CSIRO of a new reductive bleaching technology
for wool, ColorClear™ WB (Australian Wool Innovation, 2006). This method
utilises the reaction between sodium borohydride and sodium bisulphite to
produce the active bleaching species, sodium dithionite, in situ. This technique
was already in use by the textile industry to strip excess dyestuffs from
synthetic fibres. Large-scale commercial trials on wool and wool blends
have confirmed that the developed procedures are technically robust and
give significant benefits over the use of conventional ‘hydros’ (sodium
dithionite) and ‘thiox’ (thiourea dioxide) reductive bleachings in terms of
improved whiteness. This is illustrated in 
Figure 9.12
 for natural and machine-
washable (chlorine/Hercosett-treated) wool fabric.
ColorClear™ WB can also be used for the whitening of wool blends. A
polyester disperse FWA is first applied to a wool/polyester blend, followed
by a sequential double bleaching procedure using H
2
O
2
 followed by reductive
bleaching with a mixture of sodium borohydride and sodium bisulphite. In
this process, the ColorClear™ WB carries out a dual role. It not only acts as
a reductive bleach to whiten the wool component, but also strips any disperse
FWA from the wool component. This is necessary since any FWA remaining
on the wool component would lead to rapid photoyellowing of the wool on
exposure to sunlight. ColorClear™ WB is produced and marketed by Rohm
and Haas.
On the basis of the metal-catalysed oxygen free radical yellowing mechanism
discussed in 
Section 9.4.4,
 a combination of a water-soluble antioxidant and
a metal chelator was expected to be a more effective treatment for preventing
the rapid photoyellowing of FWA-treated wool than an antioxidant alone.
© 2009 Woodhead Publishing Limited

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Improving the whiteness and photostability of wool
241
9.12
 Plot of the CIE Ganz 82 whiteness index of wool fabric after
various bleaching treatments. Ox = H
2
O
2
 bleaching, Thiox = thiourea
dioxide, Hydros = sodium dithionite.
Rinsing FWA-treated wool with low concentrations of an antioxidant such
as N-acetylcysteine (NAC) was highly effective against photoyellowing when
combined with a metal chelator such as oxalic acid, and provided much
better photoprotection than the antioxidant alone, particularly under wet
conditions (Millington, 2006a). Unfortunately the benefits of the rinse treatment
are lost on laundering, and efforts to identify a substantive antioxidant/metal
chelator combination that could be applied during wet finishing and provide
effective protection against photoyellowing under both wet and dry conditions
have so far been unsuccessful.
Improving the photostability of wool to photobleaching can also be important
for certain applications. When wool carpets are exposed to sunlight through
window glass they undergo photobleaching soon after the carpet is laid
during a period when consumers are particularly sensitive to product
performance. The effect is particularly apparent when the yellowness of the
wool is fairly high and the carpet is dyed to a pale shade. New Zealand
workers at WRONZ developed a dyebath additive that photoyellows at the
same rate as photobleaching occurs, so that the overall colour is unchanged
throughout exposure. This is now marketed by Clariant as Lanalbin APB™.