SAMPE Journal - May/June 2016 - 23

Feature Article
Silk for Light-Weight Syntactic Foams
and Tough Textile Composites
D. U. Shah1, 2, F. Vollrath1
University of Oxford, Dept. of Zoology, Oxford, UK
2
Present Address: University of Cambridge, Dept. of Architecture, Cambridge, UK
1

Abstract
Growing interest in bio-based materials has powered the recent industrial uptake of plant fibre reinforced
plastics. In contrast, nature's wonder-fibre silk has had no commercial applications and only limited scientific
investigations as a composite reinforcement. In addressing the question 'are silks suitable as polymer
reinforcements?', we explored two routes to silk composites: i) syntactic foams, where silk cocoons were employed
as natural, macroballoon, particulate reinforcements in a bio-based polyurethane foam, and ii) laminate
composites, where nonwoven mats and woven textiles of silk were fibre reinforcements in an epoxy matrix.
In the syntactic foams, cocoons were effective volume-occupying, structural fillers; the cocoons replaced 60-
90 wt% (40-70 v%) of the polymer foam, and yet a marked increase in compressive properties was observed.
Notably, the cocoon reinforced foams were useful hybrids between honeycomb structures (that are anisotropic,
but difficult to form into complex shapes) and foams (easy to form, but are isotropic).
As a fibre reinforcement, silk was a superior alternative to flax, and a potential sustainable option against glass,
in appropriate applications, viz. i) light-weight, impact-critical components, such as high-performance helmets
and drones, and ii) light-weight, flexural stiffness- or strength-critical components, such as construction beams,
automotive load-floors, and sporting equipment.
Introduction
With the growing interest in material technologies
incorporating bio-based constituents, substitutes to
both conventional, petrochemical-derived polymers
and their synthetic reinforcements are being sought.
For example, biocomposites reinforced with plant
fibres such as flax, jute and hemp have been widely
investigated as potential eco-friendly alternatives
to glass fibre reinforced composites1-3. These plantbased biocomposites have penetrated various
markets, including the automotive industry (for
interior panels), building and construction industry
(for decking), and sporting equipment industry (e.g.
for surfboards and bicycles)4. Today, plant-based
biocomposites have captured around 10-15% (by
volume) of the EU fibre-reinforced composites market,
making them several (five to eight) times larger than
the carbon fibre composites market1,5. Strikingly, silk,
the only natural fibre to exist as a continuous filament,
has had no commercial applications, and only limited
scientific investigations, as a reinforcement for
engineering composites.
Silk has a long, illustrious history as well as a
strong following in today's society. While the lustrous
silk threads from the cocoons of the domesticated
mulberry silkworm,
have for millennia formed
the basis of the sericulture industry and a hugely
lucrative textile market, the technical properties of
silks, especially spider silks, have attracted extensive
SAMPE Journal, Volume 52, No. 3, May/June 2016

research and development. Yet, today's silks are still
almost exclusively used in garments and textiles,
barring some niche technical applications (e.g. in
high-end, road-racing tubular tires).
The question arises: is there a case for silks as
suitable polymer reinforcements? More specifically,
what advantages do silks and their composites offer
in comparison to conventional materials? To address
these questions, in this two-part paper we fabricate
and characterise two forms of silk composites:
i) novel syntactic foams, where silk cocoons are
employed as volume-occupying, structural particulate
reinforcements in polymer foams, and ii) laminate
composites, where silk fibre nonwoven mats and
woven textiles are employed as fibre reinforcements
in polymer resins. In light of the achieved properties,
the potential applications of these new silk composite
material technologies are briefly explored.

Silk Cocoon Reinforced Bioforms
The Structure and Properties of Silkworm Cocoons
The cocoons of silkworms, from which we unravel silk
filaments, have evolved over millions of years to protect
the larvae from predators as they metamorphose into
moths. Broadly speaking, the cocoon shells themselves
are remarkable hierarchical, non-woven composite
laminate structures (Figure 1). Our previous studies
demonstrate the impact resistance and damage
tolerance of such cocoon shells6,7.

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

Contents
SAMPE Journal - May/June 2016 - Cover1
SAMPE Journal - May/June 2016 - Cover2
SAMPE Journal - May/June 2016 - Contents
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