Page 105 - Handbook of Properties of Textile and Technical Fibres

86 Handbook of Properties of Textile and Technical Fibres

(a) (b)

Figure 3.26 Scanning electron micrograph of the fracture surface of a wool fiber permanently
set into a helical configuration: (a) part of the helix showing the point of origin of the fracture
occurring at the inside edge of the helix; (b) close-up of the fracture origin.
Adapted from Huson MG: The mechanism by which oxidizing-agents minimize strength losses
in wool dyeing, Text Res J 62:9e14, 1992.

the fiber is strained up to about 30% strain no damage is done and the strength of the
fiber increases (Tsobkallo et al., 2012). Beyond this point damage is effected, however,
because many weak fibers break during the process and are removed as noil, the over-
all change in fiber strength can be complicated. This was confirmed by Wang and
Wang (1998, 2000) who monitored the diameter and strength distributions of Merino
wool in early stage processing (from scouring to combing). They showed a decrease in
breaking force after carding and then a steady recovery through gilling, combing, and
top making as weak fibers were removed (Wang and Wang, 2000). Stana et al. (2015)
showed a 7% drop in fiber strength as a consequence of spinning and then a further
2%e20% on weaving, dependant on the type of weave. The decrease was more pro-
nounced in the warp direction and in plain weave fabric. Halfaoui and Chemani (2013)
subjected yarns to cyclic strain in order to simulate the strain imposed on yarns (partic-
ularly warp yarns) during weaving. They showed a decrease in the breaking force of
yarns with an increase in the number and amplitude of the extensions. After 3500 cy-
cles at an amplitude of 1.9% the breaking force had decreased by about 10%.
Increasing the speed of the deformation had a small positive effect on breaking force.

3.5.7 Effect of UV light
When wool or other keratin is exposed to sunlight for extended periods it is prone to
loss of lipid (Habe et al., 2011), yellowing (Millington, 2006a,b), and eventually loss
of strength or tendering (Holt and Milligan, 1984; Millington, 2006a). The yellowing
of wool in particular is a serious commercial shortcoming compared with cotton and
synthetic fibers, particularly when photostable brilliant whites and bright pastel shades