Page 103 - Handbook of Properties of Textile and Technical Fibres

84 Handbook of Properties of Textile and Technical Fibres

et al., 2002) or any evidence of peptide bond breakdown after 20 days at 110 C in air
(Dhingra et al., 1989).
Several authors (Feughelman, 1963, 1997; Cook and Fleischfresser, 1990) have
shown that treatments, which cause a decrease in the disulfide content, result in fibers
and yarns with reduced modulus, stress at 15% strain, and stress at break along with an
increase in breaking strain.
This is in contrast to fibers with a natural variation in sulfur where the picture is less
clear. Whiteley and McMahon (1965) showed that within flock, a decrease in sulfur
content brought about a decrease in intrinsic strength of wool fibers; however, no
such relationship was found between breeds. Wool that has a reduced tensile strength
due to an inadequate supply of copper (Purser, 1979; Gillespie, 1983), as well as “ten-
der” wools (Orwin et al., 1980; Huson and Turner, 2001), have been shown to have
less ultrahigh-sulfur proteins or half-cystine. In contrast, Feughelman and Reis
(1967) found no significant differences in the tensile properties of wool fibers where
the sulfur content was increased from 3.1% to 4.2% by the abomasal infusion of
methionine.
Setting is also known to be implicated in strength losses associated with dyeing.

During dyeing, at temperatures above 70 C, chemical stress relaxation via the thiole
disulfide interchange reaction results in strained wool fibers being set in a new config-
uration. Thus fibers in a curved configuration as a result of twisting in a yarn, or weave
crimp, become permanently set in this curved state during dyeing.
Gullbrandson (1958) has suggested that all of the reduction in fiber tenacity result-
ing from dyeing is due to setting of bends (i.e., curvature) in the fibers. However, in a
more detailed study, Huson (1992) showed a 10% decrease in strength for straight fi-
bers and a 20% decrease in strength for curved fibers (Table 3.2). A mechanism was
proposed to explain this additional strength loss in terms of the distribution of covalent
bonds in the wool and the homogenous transferral of stresses onto the molecular chains
(Fig. 3.25).
SEM lends support to this theory, showing the fracture initiating at the inside edge
of a permanently set helix (Fig. 3.26(a)). The fracture lines propagate radially out from
the point of origin, converge slightly, and then terminate in a rougher region in the
opposite half of the fiber. This rougher region shows the distinctive V- or U-shaped
ramps characteristic of tear fracture (Engel et al., 1981). The open end of the V-
shaped ramps points in the direction of the fracture propagation. A close-up of the
origin of the fracture (Fig. 3.26(b)) shows that the fracture initiates at the edge of
the cortex rather than in the cuticle. The absence of any fibrils greater than 1 mmin
length indicates an essentially brittle failure. The change in mode of fracture probably
occurs as a result of the decrease in the rate of propagation of the fracture as stresses are
relieved.
Modulus values were shown to be relatively unaffected by setting operations but
highly sensitive to the degree of curvature in the fiber (Table 3.2). The decreased stiff-
ness of the curved fibers is attributed to the disruption of a stable network of secondary
bonds between the polypeptide chains and suggests that secondary bonds play a domi-
nant role at low strain levels (<1.5%).