Page 148 - Handbook of Properties of Textile and Technical Fibres

126 Handbook of Properties of Textile and Technical Fibres

4.3.3 Effects of weathering and influence of fleece structure

Weathering refers to the degradation of animal fibers that occurs during growth from
exposure of the keratin fibers to sunlight, water, and air. Weathering damage reduces
quantities of wool fiber that are harvested, reduces length in both raw and processed
wools, reduces spinning performance, results in discolored yellower wool, and lowers
the quality of dyeing. There are few reports on weathering damage of rare natural fibers
despite their production in often harsh environments, e.g., alpaca and vicu~ na from the
high plateaus in South America, or cashmere produced in desert regions and higher
altitudes in central Asia. A survey of 38 lots of commercial white dehaired cashmere
and tops showed the extent of weathering varied by up to six-fold (McGregor, 2016).
Increased weathering reduced bundle tenacity and bundle extension, increased the yel-
lowness, and reduced reflectance of white cashmere. Bundle tenacity of cashmere
declined as fiber diameter variability increased from 20% to 22.5%. These effects
also varied with the origin of the cashmere. Given the range in bundle tenacity and
the similar effect of weathering and fiber diameter coefficient of variation on bundle
tenacity in wool (Yang et al., 1996), it appears that variation in bundle tenacity is likely
to be an important physical attribute of commercial cashmere. With South African kid
mohair, a fleece with a similar low skin follicle density and fleece structure to that of
cashmere goats and Suri alpaca, weathering damage extends deep down the staples,
and to the staple root (Louw and van Wyk, 1958).

4.4 Examples based on textile applications

4.4.1 Dehairing and topmaking
Dehairing is a special industrial process to separate the finer valuable fibers from the
coarse guard hairs and medullated fibers present in most raw animal fibers. The me-
chanical efficiency of dehairing depends upon the number of separation points in
the processing machinery, design of specific components, and the relative humidity
or moisture regain of the product (Algaa and M€ agel, 1992). Differential removal of
coarse fibers during dehairing in cashmere processing was reported to be optimal
where the diameter of the coarse fibers is about 3.5e4 times than that of the preferred
fibers (Smith et al., 1984). At fiber diameter ratios less than 3.5, it is reported to be too
difficult to remove all the guard hairs owing to the differences in the elastic recovery
and rigidity between the finer fibers and guard hairs. Once these fibers become inti-
mately mixed and intermingled, then dehairing is more difficult (Townend et al.,
1980) with the implication that maximum separation is required during the first pas-
sage through machinery. Similar findings were reported by Couchman and Holt
(1990) for heterogeneous fleece types and for removing short fine medullated fibers
(Couchman, 1989) using laboratory scale dehairing equipment. Production rates for
dehairing are about 5%e10% of the commercial rates for carding wool, with report
dehairing rates of 2.9 kg/h/m width (Townend et al., 1980).
During dehairing using laboratory scale equipment, repeated mechanical action re-
sults in breakage of cashmere fibers (Couchman, 1989; Couchman and Holt, 1990) and