Page 106 - Handbook of Properties of Textile and Technical Fibres

Properties of wool 87

are required. For this reason considerable effort has gone into understanding the factors
that affect photoyellowing, including oxidative bleaching, trace metals, fluorescent
whitening, and the presence of moisture (Maclaren and Milligan, 1981a; Simpson,
1999; Millington, 2006a,b; King and Millington, 2013; Grosvenor et al., 2016).
On the sheep’s back the tip is more prone to weathering than the root end of the
fiber, leading to effects such as increased swelling and tippy dyeing (Maclaren and
Milligan, 1981a). Dunn and Weatherall (1992) compared the tensile properties of
the tip and root halves of fibers taken from the midback region of sheep reared out-
doors. The tip halves were shown to have lower modulus but no difference in failure
properties was detected. Haly et al. (1957) showed that exposing wool to irradiation for
2 h by wavelengths above 290 nm resulted in the loss of approximately 25% of the
cystine content; however, no change could be detected in the loadeextension curves
of single fibers in water. Smith et al. (2005) tested fabric strips and reported an initial
increase in breaking strength of almost 20% after 500 h of exposure to simulated sun-
light. The strength then declined with further exposure, eventually reaching a value
10% lower than the initial strength after 2500 h. In contrast, many researchers (Lee
and Finkner, 1967; Waters and Evans, 1983; Holt and Milligan, 1984; Evans et al.,
1986a,b; Dhingra et al., 1989; Schmidt and Wortmann, 1994; Riedel and H€ ocker,
1996; Zimmermann and H€ ocker, 1996; Jones et al., 1998) have used fiber bundle tests
and fabric tensile, tear, and abrasion tests to show that exposure of wool to a range of
wavelengths results in considerable loss of mechanical integrity. Although UV-B
(280e320 nm) is more damaging than the lower energy UV-A (320e400 nm), it is
more strongly absorbed at the surface of the fiber and hence less likely to penetrate
into the cortical cells, suggesting that UV-A is the primary cause of the decrease in
strength of photoirradiated wool (Millington, 2006a). Schmidt and Wortmann
(1994) showed increased damage for UV-A radiation when compare to UV-B radia-
tion; however, it should be noted that the intensity of the UV-A radiation was much
greater than that of the UV-B. In contrast, Nogueira et al. (2004) found no loss of
strength when the UV-B radiation was filtered out and concluded that changes in me-
chanical properties are mainly due to the UV-B range of the solar spectrum. Increased
humidity and temperature during the UV exposure significantly accelerated the degra-
dation (Lee and Finkner, 1967; Holt and Milligan, 1984; Schmidt and Wortmann,
1994); however, temperature on its own appeared to have very little effect (Dhingra
et al., 1989).
The influence of temperature is of particular importance in automotive upholstery
where temperatures of 95 C are not uncommon in hot climates (Dhingra et al.,

1989). In the case of vehicles and also for curtains and carpets the radiation is often
filtered through glass. This can have a significant protective effect on the wool (Evans
et al., 1986a)by filtering out wavelengths below about 300 nm but can be offset by
higher service temperatures as in vehicles. Figs. 3.27 and 3.28 are typical examples
(Holt and Milligan, 1984) of the damage caused by photodegradation. They show
the effect of time and temperature of exposure for fabric exposed 215 mm from a
500 W Phillips ML G/74 mercury vapor tungsten phosphor lamp B. Wool can be pro-
tected to some extent against photodegradation by the application of UV absorbers
(Waters and Evans, 1983; Evans et al., 1986a,b; Riedel and H€ ocker, 1996; Jones