Page 165 - Handbook of Properties of Textile and Technical Fibres

142 Handbook of Properties of Textile and Technical Fibres

5.1.4 Silk variability

Silk is light as well as strong but a review of the literature shows that it is difficult to
determine characteristic properties because of the huge variability of the material.
Fig. 5.1(a) compares representative stressestrain plots of artificial advanced fibers
with those of a spider silk, Nephila madagascariensis, and of B. mori (Type II, see
further). Compared on a weight basisdthe specific gravity of silk (1.3) is close to
that of polyamide and polyethylene terephthalatedthe properties of silk fibers
compete well with those of many advanced fibers and often show greater strain to
failure (Colomban, 2013). However, as a biomaterial, the first characteristic to be
underlined for silk fibers is their huge variability; variability from the sources (species,
individual), variability in their compositions (amino acid variety and sequences),
variability in geometry (section dimension and shape, see Fig. 5.2), and variability
due to processing (degumming, weaving, dyeing, spinning conditions). As, for
example, is the case for fine wine and wool production, the details of the silk producers
(genetics, farming, etc.) and the conditions of production (care, feed, etc.) are impor-
tant criteria for quality. The calculation of failure stress, in engineering terms, requires
the knowledge of the fiber section at the failure point; a difficult problem that is
discussed elsewhere. The range of variation appears much larger for this natural
product than for artificial fibers; however, although this variability has been occasion-
ally pointed out in the literature, the effects of the variation of fiber structure along the
fiber length have not often being considered up to recently (Colomban et al., 2012b;
Wojcieszak et al., 2014).
Traditional empirically silk conditioning techniques have evolved over the cen-
turies, and these chemical treatments, namely degumming and dyeing result in a fall
in mechanical properties. However, the removal of the soluble sericin is mandatory
so as to achieve controllable dyeing of silk cloth (Khan et al., 2010). For that reason
wild and undegummed silk fibers remain primarily used for woven textile applications,
the dyeing being performed as one of the final processes employed. It has, however,
been demonstrated that the sericin sheath has a positive effect on the mechanical
properties of the yarn (Jauzein and Bunsell, 2012). In this chapter, we will try to sum-
marize the state of art of the knowledge concerning production, processing, mechan-
ical properties, and nanostructure of representative silk fibers. Recent reviews could
complete the present overview (Heslot, 1998; Olsztynska-Janus et al., 2011; Miserez
and Guerette, 2012; Colomban, 2013; Wang et al., 2014; Su and Buehler, 2016; Lee
et al., 2016; Laity and Holland, 2016), but many questions remain incompletely
answered: what is the relationship between the different silk “structures” and their
mechanical properties, what is the reason for the better properties obtained in high
strain rate tests; in brief what determines the behavior of silk fibers? One important
interest of silk fibers is that they can be considered as the best model compound for
understanding the properties of fibrous protein materials such as ligaments, arteries
parishes, etc. (Smith and Scheibel, 2013; Camerlingo et al., 2014; Fleissner et al.,
2016).
The variability of mechanical properties and the need for a statistical approach for
their analysis are well established (Marcellan et al., 2003; Colomban et al., 2006,