SAMPE Journal - May/June 2016 - 32

Feature Article
fibres. The resulting composites also showed a similar
trend. As reducing weight is a top priority in many
applications, considering the specific properties
becomes of interest. Figure 7 presents flexural stressstrain curves normalised for the material density.
We found that while the stress-strain profiles of silk,
flax and glass composites lay almost super-imposed,
silk composites were relatively superior to both flax
and glass composites in terms of specific flexural
properties, particularly noting the high failure strain
of woven silk composites.
Alongside specific flexural properties, various other
measured properties are compared in Figure 8. Poor
impact strength and low interfacial strength have
often been described as the principal Achilles heels
of plant fibre composites1,19-21. We found that silk
composites not only outperformed flax composites in
those departments, but also gave a strong competition
to glass fibre composites. The high interfacial bonding
between the silk fibres and the polar epoxy matrix
achieved without any active surface modification was
revealing as a similar interfacial bonding for glass
composites is only obtained due to the silane-based
surface treatment of composite-grade glass fibres. The
high impact strength of the woven silk composites, on
the other hand, was not surprising given the enviable
balance of strength and ductility of silk fibres, and the
high failure strain capacities of their composites. Silk
composite panels, unlike glass fibre composite panels,
did not show any delamination upon bullet-impact,
corroborating their ductile, tough performance.

Afterwords
We found that silk composites are an excellent
alternative to plant fibre composites, and even a
potential sustainable option against glass composites,
in appropriate applications; for instance, in i) lightweight, tough components, such as high-performance
helmets (Figure 9) and aerial surveying drones, and
ii) light-weight, flexural stiffness- or strength-critical
components, such as composite construction beams,
automotive load floors, and sporting equipment.
To demonstrate the material technologies, we have
fabricated a prototype outer-shell of a helicopter pilot
crash helmet with the silk textile composite, finding it
to be over 30% lighter than a similar glass composite
helmet.
Certainly, depending on the application, other factors
such as materials cost and materials environmental
aging properties will require attention. Nevertheless,
considering the increasing renewed interest in
engineering materials of natural origin, silks seem to
be a strong natural fibre candidate for reinforcements
in polymer composites.
In summary, we have demonstrated the potential of
silk to be used as particulate and fibre reinforcement
in polymer composites. To take this work further, we
are currently investigating novel silk-based sandwichstructured composites employing both the described
material technologies: silk textile laminate for skins
and a core based on cocoon reinforced biofoams.

Figure 8. Comparison of various mechanical properties of nonwoven and woven silk, flax and E-glass composites.

SAMPE Journal, Volume 52, No. 3, May/June 2016

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