SAMPE Journal - May/June 2016 - 27

Feature Article

Figure 4. Typical stress-strain profiles of the various
cocoon reinforced polyurethane foams. All the syntactic
foams exhibited a yield point in the strain range of 4-5%.

reinforced foams to be substitutes to unreinforced
foams, the specific compressive properties of the
former would have to exceed that of the latter's.
The volumetric composition of the various foams
is also described in Figure 3. A high volume fraction
of macro-sphere fillers is attractive as it drastically
reduces resin consumption. In the cocoon reinforced
foams, the cocoons had replaced between 40-70%
by volume (60-90% by weight) of polyurethane
resin. Foams reinforced with B. mori cocoons had a
consistent fibrous cocoon volume fraction vf of 2.7-
3.1 %. This figure was much higher at n¦ = 7.8% for
G. postica cocoon reinforced foams. The larger volume

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

of G. postica cocoons also resulted in a larger void
volume fraction of nn = 61.6 %, yielding a total nonmatrix volume fraction (n¦ + nn) of just under 70%. In
comparison, foams reinforced with B. mori cocoons
with closed ends had a non-matrix volume fraction
of 50-55%. Pierced ends in B. mori cocoons were a
potential entry point for the expanding foam and
consequently their foams comprised of lower nonmatrix volume fraction of about 40%. The orientation
of the non-spherical cocoons was also observed to
marginally affect the volumetric composition with
foams incorporating longitudinal-oriented cocoons
having 5% higher total non-matrix component than
foams with transverse- and randomly-oriented
cocoons.
Our results demonstrate that silkworm cocoon
macro-fillers, which are prolate spheroids, can
yield high packing fractions ranging between 40 to
70% in their uncompressed state. This compares
to geometrically-permissible, maximum achievable
packing fractions of spherical microspheres π/6 ≈
52% (simple cubic packing), π√3/8 ≈ 68% (centered
cubic packing) and π/3√2 ≈ 74% (hexagonal close
cubic packing). Sherwood14 has also shown that
prolate spheroids represent a local maxima (at 41%)
while spheres represent a local minimum (at 38%)
for asymptotic packing densities close to the jamming
limit. It is noteworthy however that if the particles
were to be compressed, spheres allow more scope
for an increase in the packing fraction than prolate
spheroids14. Therefore, silkworm cocoon macro-fillers
are more suitable for syntactic foams manufactured
via casting rather than press moulding.
Cocoons Impart Structural Properties to
Foams
The
compressive
mechanical
properties of the unreinforced and
cocoon reinforced polyurethane foam
were evaluated and are presented as
representative stress-strain curves
in Figure 4 and a comparative radar
plot in Figure 5. The general trends in
compressive properties of the cocoon
reinforced foams were well-matched to
the trends in compressive properties of
the cocoons8.
Figure 5. Radar plot comparing the
mechanical properties of the various cocoon
reinforced polyurethane foams.

Table of Contents for the Digital Edition of SAMPE Journal - May/June 2016