Page 102 - Handbook of Properties of Textile and Technical Fibres

Properties of wool 83

suggesting that the inside of the crimp is under greater tension than the outside. Barach
and Rainard (1950), as well as Ross (1971), crimped wool artificially and obtained
large reductions in tensile strength, modulus, and elongation at break. They ascribed
the weaknesses to stress concentration effects at bends in the fibers. However, Huson
and Turner (2001) showed no significant correlation between fiber strength and crimp
or curvature in a study involving wool from 11 different bloodlines. A decrease in
bundle tenacity with curvature was ascribed to the increased difficulty of aligning
the more highly curved fibers in the bundle. Huson (1992) also showed that the reduc-
tion in strength of artificially crimped wool was due to the redistribution of disulfide
bonds during setting and that bends per se do not reduce tensile failure properties.

3.5.5 Effect of chemical processing
During chemical processing, wool is subjected to a variety of chemical and enzymatic
reagents, the intended or side effects of which often include the removal of the outer
lipid layer (Huson et al., 2008; Habe et al., 2011) and cleavage of internal protein and
disulfide bonds (Sweetman, 1967a,b). The cleavage of internal bonds leads to a reduc-
tion in the mechanical strength and durability of wool fabrics. Common processes
include dyeing (Lewis, 1992), bleaching (Cai et al., 2008; Montazer and Ramin,
2010; Raja and Thilagavathi, 2010), shrinkproofing (Zhang and Zaisheng, 2009; Mon-
tazer and Ramin, 2010; Raja and Thilagavathi, 2010; Zhang et al., 2010; Kettlewell
et al., 2015; Shahid et al., 2016), and anti-pilling treatments (Montazer and Ramin,
2010; Wan and Yu, 2012), and there are several strategies to try to either minimize
the strength loss (Cai et al., 2008; Lewis, 1989) or repair the damage caused to the fiber
during processing. The most common strategy employed to reduce damage in dyeing
is to use a reactive dye that cross-links the protein during dyeing or to reduce the tem-
perature of dyeing (Lewis, 1989). Typical repair strategies include treating the wool
with a cross-linking agent such as transglutaminase (Cortez et al., 2004; Ge et al.,
2009; Zhang and Zaisheng, 2009; Montazer and Ramin, 2010; Zhang et al., 2010;
Motaghi et al., 2014), formaldehyde (Lewis, 1989), or glutaraldehyde (Lin et al.,
2014) or applying a coating such as chitosan (Ghosh et al., 2013).
The most common chemical treatment of wool is to dye it. This usually involves
holding the wool at the boil for prolonged periods (up to 2 h). These conditions almost
always result in a reduction in strength of the wool, particularly if the pH of the dye-
bath is outside the isoelectric region of the wool (c. pH 4e5; Harrigan and Rippon,
1988; Lewis, 1990; Huson, 1992; Long et al., 2008; Zhang et al., 2014). Xue and
Jin-Xin (2011) showed that even as little as 4 min in a 700 W microwave oven resulted
in more than a 10% loss in strength of wet wool fibers, almost certainly due to rapid
heating of the water in the fibers. The loss of strength has been ascribed to extraction of
soluble proteins from the CMC (Baumann, 1979), a breakdown of cystine linkages to
form thiol groups (Peryman, 1954; R€ omer, 1979; R€ omer et al., 1980; Maclaren and
Milligan, 1981b; Cook and Fleischfresser, 1990) and, under severe conditions, hydro-
lysis of peptides to form amino groups (R€ omer, 1979; R€ omer et al., 1980; Maclaren
and Milligan, 1981b). By contrast, heating wool dry yielded no apparent change in ten-

sile properties of fabrics after treatment for 4 h at 160 C in air or nitrogen (Schmidt