Page 181 - Handbook of Properties of Textile and Technical Fibres

158 Handbook of Properties of Textile and Technical Fibres

1200
Nephila madasgascarensis
Antharea AAA
1000
Ultimate strength / MPa 800 Bmd Bmd Bombyx mori
Bombyx mori degummed O
Bombyx mori degummed (cocoon)
600

400
AAA
200
BmC
0
6 8 10 12 14 16 18 20 22 24 26
Fibre diameter / μm
Figure 5.11 Mean dependence of ultimate stress measured for silk with various mean
diameters: Bombyx mori (from different batches: Bm, BmO, BmO*; in the later silk a spider gen
has been substituted) (Dinh, 2010), Antharea/Tussah (Anth) and Nephila madagascariensis
(Neph). BmdO are degummed fibers of BmO, Bmd is very old degummed fiber of Bm, AAA is
extracted fiber from textile yarn of Antharea silk, BmC is extracted fiber from a cocoon.
After Colomban P, Tournié A, Dinh HM, Jauzein V: Origin of the variability of the mechanical
properties of silk fibers: 3, order and macromolecule orientation in Bombyx mori bave,
hand-stretched strings and Nephila madagascariensis spider fibres, J Raman Spectrosc
43:1042e1048, 2012b.

Mechanical tests can be carried out under different conditions or atmospheres.
Notably, some testshavebeenconductedinair,aswellasimmersedinwater,
ethanol, acetone, or isopropanol. We can see that the different organic solvents
have the same effects on the fibers: they increase the stiffness and reduce the elon-
gation at break. Water has the opposite effect as observed for keratin fiber (Paquin
and Colomban, 2007). Normally, water affects weak hydrogen bond interactions
and solvents the Van der Waals bonds. However, they act in opposing ways. The
explanation could be that silk fibers are slightly structured by weak hydrogen bonds,
and water is able to disrupt them leading to plasticization. However, the hydrogen
bonds calculated from the wave numbereH bond distance relationship (Novak,
1974; Gruger et al., 1995)are very weak (d X H.Y > 0.293 nm), the corresponding
potential well-being less than kT at room temperature (Colomban and Dinh, 2012).
The effect of the H-bonding in the solid state is thus not significant. Takahashi et al.
(1999) identifies 5 H-bonds longer than 0.305 nm plus one shorter (0.276 nm).
Organic solvents have only a role in desiccation of fibers (Pérez-Rigueiro et al.,
2000a,b).
Finally, regeneration has to be improved to achieve fiber characteristics closer to
those of natural silk fibers. Indeed, the failure stress decreases to a third of its original
value during silk regeneration, independent on the solvents used (Holland et al.,
2007).