Page 180 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 157

Figure 5.10 Tensile test fracture of Bombyx mori silk fiber.

degummed and undegummed are compared, a reduction in failure stress can be seen to
have occurred (Jauzein and Bunsell, 2012). Hence, sericin is not completely passive
during loading even if its modulus seems to be very low. Studies about sericin
behavior and fibroinesericin interaction and adhesion are still to be undertaken.
Treatments, notably degumming, can induce modifications in the mechanical
behavior. Hence sericin is removed from the fibroin by different techniques, mainly

in water heated to close to 100 C. Proteases (Freddi et al., 2003) can be used like
mineralsdsodium carbonate and borate bufferdor organicdsuccinic acid and urea
solutions (Jiang et al., 2006). These processes seem to lower the properties of the
silk and increase the variability. Notably, boiling water reduces the modulus and yield
stress; failure strain and stress (Pérez-Rigueiro et al., 2002). Pretreatment by methacry-
lamide can change completely the mechanical behavior of silk. Hence, failure stress
and strain reduce but, rigidity increases much more and the stressestrain curve
changes a lot with a more drawn out plasticization (Kawahara et al., 1996).
Another way to control the mechanical properties of spun fibers is a physical
method: the forced silking. It consists of controlling the spinning rate of the fiber by
stretching the fibers directly during the spinning. Increasing the rate increases the value
of failure stress but decreases those of strain to failure (Shao and Vollrath, 2002; Du
et al., 2006) as well as the fiber diameter. But it has also been shown, as expected, that
reducing the diameter of the fiber allows improved properties to be obtained (Fig. 5.11)
(Colomban et al., 2012b). Notably, reducing the aperture of the spinneret and
increasing the draw ratio is expected to permit the fraction of b-sheet fraction to be
increased and lead to an increase in properties. Residual sericindfrom 0% to 18.9%
of initial sericindseems to increase mechanical properties by increasing crystalliza-
tion (Ki et al., 2007b).