Page 81 - Handbook of Properties of Textile and Technical Fibres

62 Handbook of Properties of Textile and Technical Fibres

30

Regain (%) 20

10

0
0 20 40 60 80 100
Relative humidity (%)
Figure 3.2 Moisture regain of a wool fiber as a function of relative humidity.
0
Data from Watt IC, D Arcy RL: Waterevapour adsorption isotherms of wool, J Text Inst 70:
298e307, 1979.

been presented to suggest that a small amount of water is absorbed in the interior of the
microfibrils (Watt, 1980).
Of the matrix amino acids, cysteine is particularly important. The thiol side groups
in adjacent protein molecules react to form disulfide cross-links, thus stabilizing the
matrix structure. Note that coupling of two cysteine amino acids results in the amino
acid cystine. Because chemical analysis of wool is generally done by acid hydrolysis,
the concentration of the reduction product, cysteine (also termed “half-cystine”), is
generally reported. Besides disulfide cross-links, lysine and either aspartic or glutamic
acid can form covalent isopeptide cross-links and a number of amino acids can form
noncovalent bonds, all of which have a significant influence on the physical properties
of the fiber (Feughelman, 1973). All the polar amino acids can form hydrogen bonds,
forming both inter- and intramolecular linkages. Carboxyl and amino groups in some
of the side chains can form strong electrostatic interactions (ionic bonds or “salt link-
ages”) when ionized. Finally, amino acids such as leucine and phenylalanine, with hy-
drophobic side chains, can form hydrophobic bonds between protein molecules. The
different types of cross-links are shown in Fig. 3.3.
The hydrophobic bonds play a significant role in the mechanical properties of wet
keratin fibers, whereas all the other secondary bonds are disrupted by water. Conse-
quently, wool is a material whose properties are highly susceptible to changes in hu-
midity. For instance, the tensile strength of dry wool is about 200e250 MPa, whereas
fully saturated fibers have a strength of approximately 150e200 MPa (Morton and
Hearle, 1993; Huson and Turner, 2001; Hillbrick and Huson, 2008). In addition, on
going from completely dry to wet, wool swells 16% in the radial direction (Warburton,

1947)(Fig. 3.4), shows a decrease of 180 C in glass transition temperature (Wortmann

et al., 1984; Kure et al., 1997), a decrease of 70 C in melting point temperature
(Haly and Snaith, 1967; Wortmann, 2007), a decrease of 60% in the initial modulus
(Feughelman and Robinson, 1967, 1971; Postle et al., 1988; Huson, 1998), and an
increase of 80% in elongation at break (Postle et al., 1988).