Page 86 - Handbook of Properties of Textile and Technical Fibres

Properties of wool 67

Surrounding and protecting the assembly of cortical cells are the overlapping
cuticle cells. They have a higher level of cystine and hence are more heavily cross-
linked. They are generally believed to not contain any microfibrils or other crystalline
material (Rippon, 1992); however, recently Stani c et al. (2015) provided evidence that
the cuticle of hair fibers contains b-keratin sheets. Cuticle cells have a laminar sub-
structure (Fig. 3.5) with different layers containing different levels of cysteine; epicu-
ticle (12%), exocuticle-A (35%), exocuticle-B (15%), and endocuticle (3%). On the
very outer surface of the fiber there is a unique covalently bound lipid, 18-
methyleicosanoic acid, which forms a hydrophobic barrier (Rippon et al., 2016;
Okamoto et al., 2011). Huson et al. (2008) suggested that the surface lipid is not
present as a discrete outer layer on the fiber, but rather that the lipid is intimately asso-
ciated with the surface proteins, which allows for changes in lipid concentration at the
surface in response to changes in environmental conditions. The notion of a dynamic
surface is challenged by Rankin and Carr (2013), who give a good summary of work in
this area. In fine Merino wool, the cuticle is normally 1 cell thick (approximately
20 30 0.5 mm) (Rippon et al., 2016) and is partially wrapped around the fiber.
In contrast, in human hair the cuticle is from 5 to 10 scales thick (Robbins, 1994).
The cuticle cells are believed to be weakly attached to the cortex by the CMC and
therefore not to contribute to the tensile properties (Feughelman, 1982); however, there
is no definitive proof. Swift (2000) has suggested that in hair fibers, with multiple
layers of cuticle, the cuticle could play a significant role in bending stiffness. Fine Me-
rino fibers have a higher proportion of cuticle, therefore if it was not contributing to the
strength of the fiber, one might expect these fibers to appear weaker by virtue of a
greater overestimation of their effective cross-sectional area. The reality is that the
intrinsic strength has been shown to increase as fibers become finer (Thompson,
1998; Huson and Turner, 2001).
Feughelman and Haly (1960b) showed decreased stress at 15% strain on abrading
the outer layers of Lincoln wool fibers. They attributed the result to the removal of the
stiffer paracortical cells compared with orthocortical cells, but in their experiment they
also removed the cuticle and the results could equally be explained by the removal of
the stiffer cuticle. Further work is needed in this area.
Scanning probe microscopy (SPM) studies investigating scale height changes pro-
posed increased swelling of the cuticle relative to the fiber as a whole (Phillips et al.,
1995), in agreement with torsional measurements on human hair, which also suggest
that the cuticle takes up more moisture than the whole fiber and has a lower wet
torsional modulus (Wolfram and Albrecht, 1985; Feughelman, 1997). More recent
studies using SPM suggest that the exocuticle may be as much as five times as stiff
as the cortex in air (Parbhu et al., 1999). Another SPM study (Maxwell and Huson,
2005) estimated the exocuticle stiffness to be about 2 GPa, similar to values for the
whole fiber determined via tensile and transverse compression tests (Kawabata
et al., 1995). This study also showed the viscoelastic nature of the cuticle. Force
curves, in which the silicon probe is pushed into the sample, left indents in the fiber
cuticle which closed slowly over a few minutes, leaving narrow slits (Fig. 3.9).