SAMPE Journal - May/June 2016 - 28

Feature Article
The unreinforced foam was found to have
compressive stiffness and strength of 4.3 ± 0.5 MPa
and 201 ± 6 kPa, respectively. These coincided with
the datasheet properties. Reinforcing the foam
with cocoons improved the compressive strength,
but also increased the density, in all cases. Notably,
the compressive stiffness of the unreinforced foam
was only bettered by reinforced foams employing
longitudinal-oriented cocoons; transverse and
randomly oriented B. mori cocoon reinforced foams
had a comparable stiffness to the unreinforced foam
in the range of 3.6-4.3 MPa. Therefore, it could be
said that in terms of absolute properties, reinforcing
the foam with cocoons proved beneficial. However,
cellular materials are valued for their low density
and hence specific properties are more relevant
for comparison than absolute properties. Notably,
longitudinal-oriented cocoons were effective in
enhancing the specific strength and stiffness of the
unreinforced foam.
G. postica cocoon reinforced foams outperformed
all other reinforced foams with both absolute
stiffness (12.9 ± 3.7 MPa) and strength (657 ± 47
kPa) about three and two times better than that of
the unreinforced foam and longitudinal-oriented B.
mori cocoon reinforced foam, respectively. However,
the lower density of the B. mori cocoon reinforced
foams resulted in comparable specific properties to
G. postica cocoon reinforced foams, with both having
specific stiffness (0.10-0.11 MPa/(kg/m3)) and
strength (5.5-6.1 kPa/(kg/m3)) up to 15% and 35%
better than that of the unreinforced foam.
Due to the anisotropic compressive behavior of the
cocoons8, the cocoon reinforced foams also exhibit
anisotropy. Longitudinal-oriented B. mori cocoon
reinforced foams clearly outperformed the transverseand randomly-oriented B. mori cocoon reinforced
foams. The only slightly better stiffness and strength
of the transverse-oriented B. mori cocoon reinforced
foams over the randomly-oriented B. mori cocoon
reinforced foams indicated that inter-laminar shear
failure between the non-woven layers of the cocoon
may be an important damage mechanism in cocoon
reinforced foams, as it is in the cocoons6,15. While the
specific strength of the transverse- and randomlyoriented B. mori cocoon reinforced foams was only
slightly (4-14%) lower than the unreinforced foam,
the specific stiffness was significantly (30-40%)
lower. These results have direct implications on
the manufacture and applications of the cocoon
reinforced foams. Firstly, for improved absolute
and specific properties systematic longitudinal
28

orientation of the cocoons is required. Although this
would increase production times in comparison to
the use of randomly-oriented cocoons and produce
an anisotropic syntactic foam structure. Therefore,
secondly when employing them into a component, the
anisotropy should be aligned to the principal loading
direction.
Foams reinforced with pierced cocoons exhibited
higher strength and comparable stiffness to the
unreinforced foams, but significantly lower properties
in comparison to the foams incorporating un-pierced
cocoons. This demonstrated that preserving the
cocoon structure was vital for improved performance
as macro-balloon fillers.
Interestingly, the stress-strain curves (Figure 4)
displayed that the reinforced and unreinforced foams
had a yield point (i.e. sudden change in stiffness)
in the range of 4-5%. As the load-deflection curve
for cocoons did not show any yield point8, it was
most likely that the yield point was related to that
of the foam material. Indeed, the stress-strain curve
effectively plateaued for the unreinforced foam
beyond the yield point. This long collapse plateau is
typical of closed-cell foams and is associated with
the collapse (via buckling, yielding or fracture) of cell
walls and a precursor to the densification of the foam
at higher strain levels (>50%)11. It is in this constant
stress-strain range where the foam acts as an energyabsorbing material. The cocoon reinforced foams
did not exhibit this plateau region and continued
to support higher loads, albeit at a lower stiffness,
beyond the yield point. Therefore, the cocoon macrofillers were indeed reinforcing the cellular foam and
supporting the cell walls in carrying the load.
Using specific compressive strength and stiffness as
performance indicators, and comparing the various
cocoon reinforced foams evaluated in this study with
a range of conventional cellular materials, we found
that the unreinforced polyurethane foam falls in the
range of a medium-density (65-115 kg/m3) flexible
polymer foam, while the longitudinal-oriented cocoon
reinforced foam has properties of the order of a verylow-density (18-37 kg/m3) rigid polymer foam.
Interestingly, natural cork, with a density between
120-240 kg/m3, has very similar specific compressive
properties to our materials.
Summary and Conclusions
By investigating novel syntactic bio-foams based on
a bio-polyurethane matrix (with a substantial polyol
component derived from recycled vegetable oils) and
natural silkworm cocoons fillers, we have shown that
SAMPE Journal, Volume 52, No. 3, May/June 2016

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