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

9.4
The chemistry of wool colour and
photostability
9.4.1
The natural chromophores in cream wools
Chromophores are molecules in a given material that absorb particular
wavelengths of visible light, and in doing so confer colour on the material.
In wool there are several ultraviolet (UV) chromophores present in the form
of the aromatic amino acid residues tryptophan (Trp), tyrosine (Tyr) and
phenylalanine (Phe), and also cystine (Cys) which can absorb light in the
region between 250 and 320 nm. There are also visible chromophores present
that absorb from the near-UV well into the visible region of the spectrum
(350–550 nm), and the presence of these results in the cream colour of natural
undyed wool. The reflectance spectrum of wool in 
Fig. 9.1
 clearly shows
that at wavelengths below ~500 nm its reflectance decreases rapidly, even
after double bleaching. The presence of visible chromophores is also illustrated
in the UV–visible absorption spectrum of a section from a single Merino
wool fibre (Nicholls and Pailthorpe, 1976) shown in Fig. 9.2. Also shown is
the calculated absorption spectrum based on the amino acid composition of
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Absorbance
240
260
280
300
320
340
360
380
400
Wavelength, nm
0.000 20
0.000 15
0.000 10
0.000 05
0
Irradiance, W/cm
2
/nm
9.2
 The UV/visible absorption spectrum of a 6 
µ
m section of Merino
wool keratin (solid line), and absorption spectrum calculated from its
amino acid composition (dashed line) (redrawn from Nicholls and
Pailthorpe, 1976). Also shown is the relative spectral intensity of
Geelong summer sunlight.
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Improving the whiteness and photostability of wool
227
wool. The visible chromophores are important even if their concentration in
the wool fibre is low, because 
Fig. 9.2
 also shows the solar spectrum contains
significantly higher intensities of visible than UV wavelengths.
Surprisingly, very little is known regarding the identity of these natural
yellow chromophores in wool. It is likely that they are a complex mixture of
compounds that includes some wool protein oxidation products. Fundamental
knowledge of the origin, chemical nature and location within the fibre of the
natural yellow chromophores is well overdue, so that future research projects
aimed at improving wool colour using both genetic and chemical approaches
can be better targeted.
9.4.2
Influence of fibre morphology and physical
properties on wool colour
Calculating the absorption spectrum of wool from its bulk amino acid
composition assumes that the fibre is homogeneous. In reality the wool fibre
has a highly complex morphology and consists of cells, with flattened
overlapping cuticle cells forming a protective sheath around the inner cortical
cells (Fig. 9.3). In coarser fibres, a central hollow medulla may be present.
In fine wool, such as that obtained from Merino sheep, the cuticle is normally
one cell thick (approximately 20 
× 30 × 0.5 µm
3
) and usually constitutes
about 10% by mass of the total fibre. Sections of cuticle cells show an
High-S
proteins
Nuclear
remnant
High-tyr
proteins
Low-S
proteins
Right
handed
α-helix
Left handed
coiled-coil
rope
Intermediate
filament
(microfibril)
1
2
7
200
2000
20 000 nm
Matrix
Cell membrane
complex
Para-cortical
cell
Meso-cortical cell
Ortho-cortical
cell
Root end
Cortex
Epicuticle
Exocuticle
Endocuticle
Cuticle
a
9.3
 Schematic of the structure of a fine Merino wool fibre.
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Advances in wool technology
228
internal series of laminations, comprising outer sulphur-rich bands known as
the exocuticle and inner regions of lower sulphur content called the endocuticle.
On the exposed surface of cuticle cells, a membrane-like proteinaceous band
(the epicuticle) and a covalently bound lipid component form a hydrophobic
resistant barrier. Wool is composed of between 50 to 100 different proteins
representing some 14 different protein families, making it by far the most
complex fibre used for textile production.
While non-medullated wool fibres are substantially transparent to visible
wavelengths, they appear opaque in air because some of the incident light is
reflected or scattered at the interface between air and wool. When fibres are
immersed in a colourless liquid with a refractive index very close to that of
wool (
η = 1.553), such as benzyl alcohol (η = 1.540), there is no reflection
of light at the liquid–wool interface and the wool appears to be perfectly
transparent. This property has recently been used to develop a test method
for detecting the presence of dark fibre contamination in wool core samples,
and also for detecting coarse medullated fibres in fine wools (Australian
Wool Testing Authority, 2008). Unpigmented medullated fibres contain a
central core of air-filled cells that are excellent light scatterers, making them
appear whiter. This is particularly important for Arctic mammals to whiten
their winter fur and act as camouflage, in addition to increasing the insulating
properties of the fibres (Russell and Tumlinson, 1996).
Penetration of light into the wool fibre depends mainly on three factors,
the distribution and concentration of chromophores in the different
morphological regions of the wool fibre and the wavelength of the radiation.
Higher-energy UV wavelengths (< 320 nm) have very limited penetration
into the fibre and are strongly absorbed mainly by the outer region of the
cuticle cells, whereas most visible wavelengths above 400 nm that are not
scattered or absorbed by the visible chromophores are able to pass through
the fibre. Because wool has a very high absorption coefficient for high-
energy UV, a fabric exposed specifically to UVB (280–320 nm) wavelengths
is oxidised and discoloured specifically at the surface to a depth of ~1–2
µm
(Simpson, 1999).
One would expect the sulphur-rich proteins in the exocuticle to absorb
UVB wavelengths strongly. For an irradiated single fibre there will be a
gradation of damage from the exposed surface to the core and this gradation
may become more severe as new absorbing chromophores are formed at the
fibre surface due to oxidation. This suggests that lower-energy UVA
wavelengths (320–400 nm) that have greater penetration into the fibre are
mainly responsible for the loss of tensile strength observed in fibres and
fabrics exposed to sunlight, as fibre strength is determined by the cortical
cells. This is consistent with research into the phototendering of wool which
occurs following exposure to sunlight filtered by window glass. Phototendering
(or simply tendering) refers to the effects of prolonged sunlight exposure on
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Improving the whiteness and photostability of wool
229
the mechanical properties of wool fabrics, such as loss of tensile strength,
elasticity and abrasion resistance. Its effects are of commercial significance
for wool upholstery, curtaining and carpets exposed to filtered sunlight through
window glass, which absorbs light at wavelengths below 350 nm.
9.4.3
The photostability of wool: photoyellowing and
photobleaching
The colour of natural wool is far less stable to sunlight than cotton and
synthetics, and its photostability is further reduced if the fibre is oxidatively
bleached with hydrogen peroxide. Application of an FWA to wool after
bleaching results in a fabric that yellows very rapidly in sunlight.
The four principal factors which influence the observed colour change of
wool exposed to radiation are the wavelength distribution of the incident
light, the presence of oxygen, the presence of water and the processing
history of the wool, in particular whether the wool has been bleached or
whitened. There are two types of wavelength-dependent colour change that
are observed for natural cream wool. UV radiation (280–380 nm) in the
presence of atmospheric oxygen results in rapid wool photoyellowing that is
accelerated in the presence of water, whereas exposure of natural cream
wool to blue light (400–460 nm) causes photobleaching.
When natural wool is exposed to sunlight, which contains both UV and
blue light wavelengths, photoyellowing and photobleaching occur concurrently,
so that the overall effect is determined by the initial colour of the wool and
the relative intensities of the component wavelengths of the light. For example,
wool carpets photobleach because window glass attenuates the UV wavelengths
of light below 350 nm that would cause photoyellowing, but transmits blue
light wavelengths.
The intensity of UVB radiation in sunlight (290–320 nm) is variable and
dependent on factors such as time of day, season, altitude and latitude. Seasonal
variations in the levels of UVB radiation in sunlight explains why wool may
be photoyellowed by summer sunlight but photobleached during winter
(Maclaren and Milligan, 1981).
Wet photoyellowing of wool always occurs far more rapidly than under
dry conditions. 
Figure 9.4
 compares the wet photostability of natural, peroxide-
bleached and bleached/FWA-treated wool fabric to simulated sunlight exposure.
After three hours of exposure, the whiteness of bleached/FWA-treated wool
is lower than natural wool, demonstrating that its photostability is extremely
poor. The whiteness of bleached wool decreases most rapidly during the
initial 30 minutes of exposure, whereas natural wool is slightly photobleached
over the 3 hour irradiation period. A comparison of the relative rates of dry
and wet photoyellowing is also shown in 
Fig. 9.5
 (Leaver and Ramsay,
1969).
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Advances in wool technology
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Most natural materials undergo photobleaching when exposed to blue
light in the absence of UV radiation (Launer, 1968a,b). Such materials include
several of commercial significance, such as paper, wood and leather, and
other natural textile fibres such as cotton and silk. Blue light selectively
destroys the yellow chromophores that absorb in this region, converting
Natual wool
Peroxide bleached wool
Bleached/fluorescent whitened wool
0
20
40
60
80
100
120
140
160
180
200
Irradiation time (min)
80
70
60
50
40
30
20
10
0
Whiteness index (CIE Ganz 82)
9.4
 Comparison of the wet photostabilities of natural, peroxide-
bleached and bleached/FWA-treated wool fabric to simulated sunlight
exposure.
20
10
0
20
10
0
Dry
Wet
Whitened
Bleached
Natural
Whitened
Bleached
Natural
Change in yellowness index
0
2
4
Exposure time (days)
0
2
4
Exposure time (days)
9.5
 Comparison of the rates of yellowing of wet and dry treated wool
fabric (redrawn from Leaver and Ramsay, 1969).
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Improving the whiteness and photostability of wool
231
them to colourless products, resulting in increased reflection of blue light
and hence improved whiteness. The most effective wavelengths for
photobleaching of wool are 400–450 nm. The initial colour of wool affects
the observed colour changes; deeply yellowed wool bleaches rapidly in sunlight,
whereas the whitest wools may undergo little or no photobleaching (Lennox
and King, 1968). This can be seen in 
Fig. 9.6,
 which shows the change in
yellowness index for five wools covering a range of natural yellowness on
exposure to sunlight and artificial light sources. Sources which include UV
and visible radiation (Xenotest and sunlight) bleach the yellower wools during
the early stages of exposure, whereas whiter wools are yellowed. When
exposed to UV radiation only (Sunlamp) all wools undergo photoyellowing,
whereas blue light produces only photobleaching.
The photobleaching of wool with blue light has been studied extensively
although it is not used commercially. Launer used a small high-pressure
mercury arc fitted with a UV filter to photobleach dry wool fabric in around
15 seconds (Launer, 1971). Other workers passed scoured wool through a
conveyor system fitted with blue light fluorescent tubes overhead, but this
process was not adopted by industry due to slow throughput and the need for
a great many fluorescent tubes (Garrow et al., 1971). Simpson (1992) found
that photobleaching with similar low-intensity blue fluorescent tubes could
be speeded up significantly by irradiating the wool in the presence of alkaline
H
2
O
2
 at pH 10–11.5. Using a parallel array of low-power blue fluorescent
tubes, irradiation times of 20 to 30 minutes were necessary to obtain a good
bleaching effect using peroxide-padded wool, compared with over 24 hours for
dry wool. However the irradiation times were still too long for a commercially
viable continuous treatment. Recently a process for continuous photobleaching
of wool fabric has been described that uses a modified UV-curing source
(Millington, 2005). Doped medium-pressure mercury arcs developed for the
rapid curing of polymer films offer very high intensities of blue light. Filtering
the UV light at wavelengths below 350 nm from such a source and exposing
wool fabric padded with either H
2
O
2
/oxalic acid or zinc formaldehyde
sulphoxylate allowed continuous photobleaching at a rate of 2 m min
–1
.
9.4.4
The mechanism of wool photoyellowing
Most natural and synthetic polymers undergo UV-induced discoloration, which
results in an increase in yellowness on exposure. For synthetic polymers the
photoyellowing mechanisms have been well studied, and in all cases the
mechanism proceeds via a free radical route (Gugumus, 1993).
Photodegradation of cellulosic materials such as cotton, paper and wood
also proceeds via a free radical mechanism, with lignin playing the major
role in both the absorption of UV light and the generation of yellow
chromophores (Muller et al., 2003).
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Advances in wool technology
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A number of different mechanisms have been described in the literature to
account for the photoyellowing of wool, and these have recently been critically
reviewed (Millington, 2006b). In common with other less complex polymers,
most of the yellowing mechanisms proposed for wool involve free radical
Sunlight irradiation
Xenotest irradiation
Sunlamp irradiation
Bluelamp irradiation
20
40
60
80
100 120 140 160 180 200
220
Hours irradiation
40
36
32
28
24
22
40
36
32
28
24
44
40
36
32
28
24
38
34
30
24
22
18
Y
ellowness index
9.6
 Exposure of scoured wools from five sheep, covering a range of
initial yellowness indices, to sunlight and three artificial sources of
radiation (redrawn from Lennox and King, 1968).
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Improving the whiteness and photostability of wool
233
oxidation processes. Electron spin resonance (ESR) spectroscopy has shown
that both carbon- and sulphur-centred free radicals are formed directly in
wool after irradiation in the absence of atmospheric oxygen (Shatkay and
Michaeli, 1970). In the presence of air or oxygen these signals are to some
extent quenched, indicating that the radicals are reacting with oxygen.
It has been shown by studies where wet fibres have been irradiated with
UVA or blue light in the presence of a highly specific fluorescent probe (the
terephthalate anion), that wool, silk cotton, nylon and polyester all produce
hydroxyl (·OH) radicals (Millington and Kirschenbaum, 2002). Wool produces
higher concentrations of hydroxyl radicals than other fibres, and the attack
of these highly oxidising species on the aromatic amino acid residues present
in the protein leads to yellowing. Trace metal ions are involved in ·OH
generation since [·OH] was significantly reduced in the presence of the
metal chelator deferoxamine. Analysis of hydrolysed Merino wool fabric
has reported ~15 mg/kg iron, ~5 mg/kg copper and ~2 mg/kg manganese
(Millington and Kirschenbaum, 2002).
In common with synthetic polymers and cellulosics, it has recently been
shown that wool and other fibrous proteins emit light when heated above
40
°C due to chemiluminescence (CL) (Millington et al., 2007). In a non-
isothermal CL experiment in an inert nitrogen atmosphere, fibrous proteins
show a CL peak near 130
°C similar to that observed in many synthetic
polymers and characteristic of polymer hydroperoxides (POOH in Fig. 9.7)
Initiation
l