Page 138 - Handbook of Properties of Textile and Technical Fibres

Physical, chemical, and tensile properties of cashmere, mohair, alpaca 117

found increasing proportions of orthocortical cells with increasing fiber and cortical
diameter. The reversion to mainly orthocortial cells under nutritional stress indicated
that this was the basic cortical cell in wool. Single follicles produced varying propor-
tions of all cortical cell types and organized them in a variety of ways within the fiber.
Only one study investigating of the size of cortical cells in a range of rare animal
fiber is known (McGregor and Quispe, 2017). There was a large relative range in
the size of cortical cell length, diameter, and volume. Cortical cell diameter showed
less variation (mean diameters 4.5e4.9 mm) than cell length (mean lengths
42.5e57.3 mm). The relative shape of cortical cells was predictable as cortical cell
length was strongly related to cortical cell diameter. The ratio length:diameter
increased by 0.19/1 mm increase in cell length and declined by 0.03/1 mm increase
in MFD. Generally the cortical dimensions of these animal fibers were similar with
the exception that qiviut had longer cortical cells with a greater volume compared
with cashmere, mohair, bison, and vicu~ na.
Nutritional manipulation resulted in cashmere grown by goats with higher levels of
nutrition having longer cortical cells compared with cashmere grown by goats in
restricted nutrition treatments (50.8 vs. 34.5 mm) with greater diameter (4.57 vs.
3
4.14 mm), volume (274 vs. 170 mm ), and a higher length:diameter ratio (11.1 vs.
8.4). These results indicate that the internal physical structure of cashmere fibers are
not fixed and are influenced by nutritional status (McGregor and Liu, 2017).

4.2.2.3 Medullation

Fibers that have a hollow or a partially filled central canal running either as a contin-
uous or in a fragmented form along their length are known as medullated fibers, and
are present to some extent in the fleece of all animals. Some of these fibers have a
chalky white appearance and are often referred to as “kemp.” Kemp fibers show other
characteristics including being relatively shorter, coarser, more brittle and pigmented,
and have flattened portions and sharp bends compared with normal fibers (Frazer
Roberts, 1926). The medullated fibers are those in which the diameter of the medulla
is less than 60% of the diameter of the fiber. Kemp fibers are medullated fibers in
which the diameter of the medulla is 60% or more of the diameter of the fiber when
viewed in longitudinal section. Kemp fibers have a different cross-section, which is
oval or kidney shaped, compared with wool that approximates a circular cross-
section. In mohair, Lupton et al. (1991) showed that a limiting thickness of about
5 mm of solid keratin surrounds the medulla, that the limiting minimum medulla thick-
ness was 5 mm, and generally the diameter of the medulla increases with increasing
fiber diameter.
All medullated fibers are contaminants in mohair, cashmere, alpaca, llama, vicu~ na,
and yak fibers. The medullated fibers present as guard hairs and kemp can be removed
by commercial dehairing. The content of medullated fibers in raw fibers is substantial:
for combed Chinese cashmere 8%e15%; shorn cashmere fleeces 50%e75%; llama
30%e70%; alpaca 5%e90%; Dromedry camel 40% (Frank et al., 2011; McGregor,
2006a; Msahli et al., 2008). Good-quality greasy mohair has <1% medullated fiber
and after processing the level is generally <0.3% (Hunter, 1993). For high-quality