SAMPE Journal - May/June 2016 - 24

Feature Article

Figure 1. The Bombyx mori cocoon seen in (a) is a hierarchical polymer composite laminate structure with multiple nonwoven laminas of 2D randomly-oriented, semi-crystalline protein fibres that are bonded by an amorphous protein sericin
binder (b), where the continuous fibres (upto 1500 m in length when unravelled) exist as dual-strands (c). The cocoon
acts as a protective barrier for the pupating silkworm (a). The Gonometa postica cocoon seen in (d) is also a non-woven
composite and like many wild silkworm cocoons its outer surface is extensively covered with reinforcing particulate
minerals (<5 wt%) (e) that are columnar in dimensions (f). JCM-5000 NeoScope (JEOL) scanning electron microscope
(SEM) was used for the SEM images. Samples, sputter coated with Au/Pd alloy, were viewed at an acceleration voltage of
10 kV.

Syntactic foams are a class of particulate composites
comprising of typically hollow particles (referred to
as 'microballons' if particle diameters are between
1-1000 μm, and 'macroballoons' if diameters are
between 1-100 mm) that are systematically dispersed
in a matrix phase. These reinforced foams are
commonly used in engineering applications where a
combination of ultralight-weight (including buoyancy
potential), and high compressive and energy
absorption properties are required. We hypothesised
that cocoon shells, such as those of domesticated
Bombyx mori and wild Gonometa postica silkworms
(Figure 1), may be interesting natural macroballoon
fillers in polymer foams. As per our knowledge, biobased naturally-constructed, or even pre-constructed,
macroballoon fillers have not been explored in
literature so far.
Table 1 compares properties of silkworm cocoon
shells with conventional man-made fillers (i.e. glass
and polymer microballoons) and typical unreinforced
polyurethane foam. It is evident that cocoon shells,
which are natural polymer macroballoons, share many
similarities with man-made polymer microballons,
including low apparent density (and therefore lightweighting potential), comparable glass transition
temperature (and therefore processing temperatures)
and similar compressive strength. However, the lower
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economic and environmental cost of cocoon shells in
comparison to synthetic polymer microballons makes
the former particularly appealing. In comparison
to glass microballons, the advantage of polymer
microballons and cocoon shells is that the latter are
not only significantly lighter, but also less susceptible
to breakage during processing as they flatten under
pressure, unlike brittle glass microballons which
burst under pressure.
By providing large volume displacements of the
polymeric foam, the renewably-sourced and lowercost natural cocoon shells may aid in reducing
the consumption of the higher-cost and more
environmentally-polluting polymeric matrix, as well
as improving the absolute and specific mechanical
properties of the foam (Table 1). We envisage that
the laminate structure of cocoon shells and their
behaviour under compressive loads7 may introduce
more gradual deformation mechanisms into the
syntactic foams.
Experimental Methodology
For this study, a rigid, closed-cell polyurethane
(Envirofoam Sustain 16.235; IFS Chemicals Ltd, UK)
was used as the polymer foam, and dried cocoons of
the B. mori and the G. postica silkworms were used as
particulate reinforcements. Notably, the Envirofoam
SAMPE Journal, Volume 52, No. 3, May/June 2016

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