Page 131 - Handbook of Properties of Textile and Technical Fibres

Physical, chemical, and tensile properties of cashmere, mohair, alpaca 111

4.2.2.1 Cuticle cells

The outer surface of fibers consists of cuticle cells (scales), which overlap like tiles of a
roof to give the well-known range of distinctive surface structures of animal fibers
(Fig. 3.5; Dobb et al., 1961). The thickness of the cuticle ranges from 1 to 2 scales
for fine fibers such as wool, cashmere, and mohair, up to 8e10 cuticle cells for coarser
fibers such as goat guard hair. The cuticle constitutes 10%e20% of the weight of a
fiber and provides the tough protective layer for the 80%e90% bulk of the fiber, which
is composed of long, spindle-shaped cortical cells and medulla cells when these are
present.
The cuticle is the interface between the fiber and the environment, including chem-
ical processing media and the wearer of textiles. The extreme outer layer, the epicu-
ticle, is just a few nanometres in thickness. It consists of protein material of high
chemical resistance and plays a key role in all surface properties. The surface of the
epicuticle is covered by a chemically bounded extremely thin layer of fatty acid
with an unusual chemical structure (Leeder et al., 1985). This is responsible for the nat-
ural water-repellency of animal fibers. The amount of this fatty acid is different for
cashmere, mohair, and alpaca fibers (Rivett et al., 1988).
Mohair has an average cuticle scale frequency of z5/100 mm (range 3.5e7.5/
100 mm; Wildman, 1954; Teasdale, 1988; Phan et al., 1988). Cashmere has a cuticle
scale frequency of about 7 with a range from 4 to 10/100 mm(Wildman, 1954; Tucker
et al., 1988, 1989, 1990a). Vineis et al. (2011) summarizes the cuticle scale frequency
for a range of rare fibers from farmed and wild animals but appears to assume that fiber
properties are fixed for each species. Bison fiber has a higher scale frequency than most
of the other fibers, perhaps a result of relatively low rates of fiber growth, while qiviut
has a similar cuticle scale frequency to cashmere and alpaca (McGregor and Quispe,
2017). Garner (1967) found that up to 20% of the Merino fibers he examined had the
same scale frequency/unit length as cashmere, while Wildman (1954) showed the
overlap for mohair and New Zealand cross-breed 48s wool was greater. Vassis et al.
(2003) provide data on cuticle scale angles obtained from coarse wool, fine wool, su-
perfine wool, mohair, and cashmere. Vassis et al. (2003) concluded that a few prom-
inent scales occur repeatedly along the fibers.
Roberts (1973) reported that the cuticle edges were more pronounced on coarse
cashmere than fine and attributed some of the smoothness of fine cashmere to the
reduced scale protrusion compared with wool. Roberts (1973) found that Mongolian
cashmere contains one or two apparent scale edges per cuticle cells (“false” scale
edges), an observation made by Leeder (1969) on various animal fibers.
Tester (1987) found that cashmere fibers had on average a lower number of cuticle
cells around the fiber cross-section than did 16e18 mm Merino wool fibers. This was
explained in part by cashmere having fewer cuticle cells per unit length of fiber than
wool. In addition, the cuticle of the cashmere and wool fibers had fewer cuticle cell
layers on the orthocortical side of their fiber and cashmere had significantly more
orthocortical cells than the wool. On this basis, reduced fiber cuticle cell thickness
may be associated with reduced bending rigidity of cashmere fibers compared with
Merino wool.