Page 108 - Handbook of Properties of Textile and Technical Fibres

Properties of wool 89

et al., 1998; Smith et al., 2005); however, degradation is only slowed down, not
stopped.
The effect of dyeing is complicated, with some dyes absorbing UV radiation and
thereby reducing the rate of degradation, whereas others can exacerbate the loss of
strength (Millington, 2006a).

3.6 Applications and examples

In service, wool is subjected to a range of deformations and mostly fails through wear
by abrasion or fatigue. This is particularly true of apparel and carpets, and the lack of
failure by straight tensile failure suggests that wool’s strength is more than adequate.
Fibers do fail during early stage processing (carding, combing, spinning, and weaving
or knitting), however, leading to a reduction in processing efficiency at significant cost
to the industry. Furthermore, many of the later stage processes (dyeing, finishing,
shrinkproofing, etc.) required to turn wool fiber into an acceptable end product result
in a reduction in strength. Several strategies employed to try to minimize this damage
are reported here. Wool garments can also be deemed to have failed on aesthetic
grounds. So, for instance, curtains or garments that have faded or yellowed may no
longer be acceptable to the consumer. Also, particularly in knitwear, pilling is an un-
desirable property which results in a product that is no longer acceptable. Fading and
yellowing involve chemical changes and are outside of the scope of this chapter; how-
ever, pilling is known to be related to fiber strength and hence will be discussed briefly.

3.6.1 Shrinkproofing

The presence of cuticle cells on the surface of wool fibers means that the friction in the
direction of the scales is lower than in the direction against the scales. This differential
friction causes fibers to migrate preferentially in one direction and, with sufficient
movement, results in felting of the wool fabric. Felting is exacerbated at elevated tem-
peratures and relative humidities; hence the need for dry cleaning of many wool gar-
ments. Although felting is desirable in the manufacture of hats, papermaking felts, and
billiard cloths, it is a major source of angst for the consumer of apparel. Early treat-
ments to shrinkproof wool involved treatment with a variety of oxidizing agents,
which had the effect of damaging or even stripping off the scales. In the process
considerable damage was also done to the fiber as a whole. The use of oxidizing agents
in organic solvents (Freney and Lipson, 1940) or saturated salt solutions (McPhee,
1959, 1960a,b) restricted damage to the fiber, presumably by restricting swelling of
the fiber and thereby confining the oxidation to the surface of the fiber. The salt/
KMnO 4 process is used commercially on a small scale in India; however, environ-
mental concerns have prevented its widespread use. None of the other processes
became commercial and nowadays the most common method, developed by the
CSIRO and the International Wool Secretariat in the early 1970s, is the chlorine/Her-
cosett treatment of tops (Rippon, 2008; Rippon and Evans, 2012; Kettlewell et al.,