Page 187 - Handbook of Properties of Textile and Technical Fibres

164 Handbook of Properties of Textile and Technical Fibres

yarn a composite behavior. In silkworm silk, sericin has a role of cohesion, protection,
and environmental control; sericin is hydrophilic, the opposite of fibroin, which leads
to weak adhesion between them.

5.3.3.1 Reinforced composites
Chopped silk and commercial silk fabric have been used to reinforce biodegradable or
synthetic polymers such as poly(butylene succinate) (Lee et al., 2005; Kim et al.,
2007), poly(3-caprolactone) (Li et al., 2008; Wang et al., 2009), poly(lactic acid)
(Cheung et al., 2008), epoxy (Craven et al., 2000), or even polychloroprene rubber
(Setua and Dutta, 1984) and gelatine (Shubhra et al., 2011). Chopped silk loses the
interest of continuous fiber reinforcement, and woven fabric suffers from the wetting
problem of twisted fibroin threads in the matrix, leading to poor performances. Micro-
fiber reinforcement (10e600 mm) has also been tested (Li et al., 2016). Unidirectional
composites containing hundreds of fibers embedded in a matrix of regenerated silk
fibroin (filtered) were manufactured using different processing techniques (Yuan
et al., 2010; Wojcieszak et al., 2013, 2017). The standard deviation for the porosity
was quite large (up to 23%), and the approximate fiber volume varied from 35% to
73% of the total composite volume (porosity excluded). Fig. 5.14 shows characteristic
tensile behaviors. The Young’s moduli range between 2 and 3 GPa. The tensile curves
can be divided into several regimes: (1) first a linear elastic regime imposed by the
fibers; (2) a second nonlinear regime due to the fracturing of the (interface/)matrix
(resulting in a decrease of the tangent modulus); (3) a plateau corresponding to the
sliding of the fibers in the matrix; and (4) a failure, which can be clean (brittle) or
take place in successive stages (dissipative).
Nonlinearity indicates cracking in the matrix, or at the fiberematrix interface as
observed (Fig. 5.14 (inset)). The fiber volume fraction ensures the maximum strength

125
1
2
100
Stress / MPa 75

50

25

0
0 5 10 15 20
Strain / %
Figure 5.14 Representative stressestrain curve of silk fiber reinforced-regenerated silk
matrix composites with different fiber volumes and corresponding example of fibrous fracture
(see inset).