Page 166 - Handbook of Properties of Textile and Technical Fibres

Silk: fibers, films, and compositesdtypes, processing, structure, and mechanics 143

2012a; Dinh et al., 2008; Dinh, 2010). This is true for many materials but particularly
true for silk; however, the data to be found in the literature are incomplete and in
many case the variations in properties are not quantified. The Weibull parameter, a
parameter that measures the dispersion of measured characteristics such as Young’s
modulus, ultimate tensile strength, and strain, typically ranges between 2 and 6,
although that value can be greater than 20 or more when measured for synthetic
materials (Colomban et al., 2012a). Fig. 5.2 compares the photomicrographs of
some silk fibers and reveals the difficulty in determining cross-sectional areas. The
fibers shown are, from left to right: a set of B. mori fibers associated to form a yarn,
note that the silkworm spins a double fiberdthe “bave”dboth coated by sericin, a
waxy compound that protects and sticks the fibers together to form the cocoon. The
section of the yarn shows clearly the pair of fibers (see especially top left and bottom
right) and the variety of their forms; section variability is still much larger for wild
moths (Colomban et al., 2008a). In contrast, the shape of Nephila spider silk appears
very regular and is free of any coating. Depending on the creature and the procedure of
(forced or not) spinning the dragline can be constituted of a single fiber
(N. madagascariensis (Colomban et al., 2012a,b)) or of two (Nephila edulis (Ene
et al., 2010)). However, different diameters are observed in silk fibers produced by
the same spider. Spider silk is finer than most of the silkworm silks, typically being
in the range 2e15 mm, whereas from the B. mori “mean” diameters of the fibers range
from w10 to 20 mm. Smaller diameter (and higher length) can be obtained by forced
spinning.
Variability of spider silk arises from the number of specialized glands and associ-
ated spinnerets and the different amino acid compositions found in them (Gosline
et al., 2002; Ko, 2004). For instance, Fig. 5.2(d) shows the optical photomicrograph
of a spider dragline consisting of a main fiber with a helicoidal smaller fiber wrapped
around it (Colomban and Gouadec, 2009).
Fig. 5.3 compares the different stressestrain curve types obtained with a variety of
B. mori silk measured as raw materials (fresh and aged cocoons, flotte, degummed
fibers, dyed yarns, etc.) (Colomban et al., 2012a,b). Similar results have been obtained
for Antheraea (Tussah) and for some extent for Gonometa (Colomban et al., 2008a,b)
and Nephila (Colomban et al., 2012a) silk fibers. If the degumming procedure has a
strong influence on the silk type as shown by Dinh and Colomban (Dinh et al.,
2008; Colomban et al., 2012a) and confirmed by Perea et al. (2016), note that different
types of silk structure can be observed along the length of a single fiber (Dinh, 2010;
Colomban et al., 2012b).
Four different types of stressestrain curves include the following:
Type I: elastic behavior up to w2% of strain and then a, more or less flat, quasi
plateau; this behavior is very rare for processed fibers/yarns but frequent for fibers
extracted from fresh cocoons and dominant for just hand-spun fibers from a silk
moth extracted from its cocoon (Dinh et al., 2008; Dinh, 2010; Colomban et al.,
2012a).
Type II: elastic behavior up to w2%, then linear up to 5%e6%, and a third rather
flat plateau up to the breaking point; this behavior is common for dry, processed fibers/
yarns.