JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014 - 48

reinforcement
devices.

Tab. 1: Silks in comparison to plant and glass fibres [8]

Properties

Silk fibresa

Annual global production
of fibres [kt]

(150)

Cost of commercial
raw fibre [£/kg]

(2.0-30.0)

0.5-1.5

1.3-20.0

Chemical nature

protein-based

cellulose-based

silica-based

Fibre length

continuous

discrete

continuous

15-600 (15-50)

5-25

1.35-1.55

2.40-2.70

Fibre diameter [μm]
Density [gcm ]
-3

Moisture absorption [%]
Softening temperature [°C]

1-15

31,000

(8-15)

1.25-1.35
5-35

Plant fibresb

(20-35)

170-220

7-25

(600)

(7-10)

Glass fibresc
4,000

0-1

190-230

700-1,100

30-80 (50-80)

As part of projects funded by the European Research
Council (ERC) and the Air Force Office of Scientific
Research (AFOSR), the Oxford Silk Group is also
investigating the potential of silkworm silks as sustainable alternative reinforcements for engineering
(i.e. non-biomedical) composites. This timely work
is in the context of the growing research interest and
industrial acceptance of natural fibre composites as
sustainable material solutions to help industry meet
environmental legislations in the short term, and to
help develop a circular, green economy in the long
term.

70-85

Table 1 and Figure 2 compare some technical
properties of natural silks with natural plant fibres
(the more widely studied natural reinforcement)
Specific tensile stiffness
4-20
(4-12)
20-60 (30-60)
27-34
and synthetic glass fibres (the more commonly used
[GPa/gcm-3]
reinforcement). In general, the primary disadvan0.2-1.8 (0.3-0.6)
0.4-1.5 (0.5-0.9)
2.0-3.7
Tensile strength [GPa]
tages of silks in comparison to plant and glass fibres
Specific tensile strength
are: i) higher cost (of processed fabrics), ii) higher
0.1-1.5 (0.3-0.7) 0.3-1.1 (0.3-0.7)
0.7-1.5
[GPa/gcm-3]
moisture absorption (than even plant fibres), iii)
lower softening (and therefore processing) tempera15-60
(15-25)
2-30
(2-4)
2.5-5.3
Tensile failure strain [%]
tures, and iv) poor stiffness. However, silks possess
-3
25-250
(70)
5-35
(7-14)
40-50
Toughness [MJm ]
i) lower density (than even plant fibres), ii) natural
Specific toughness
flame resistance, iii) moderate strength, iv) unpa20-185
(50-55)
3-26
(4-10)
16-19
[MJm-3/gcm-3]
ralleled toughness, and iv) a favourable environa
Includes silks from various spiders and silkworms. As most of the commercial silk is cultivated from
mental profile of the raw material. As the silk textile
the Bombyx mori silkmoth, typical properties of this variety of silk are presented in brackets.
industry is well-established, common technical
b
Includes bast, leaf and seed fibres. Typical properties of flax fibre are presented in brackets.
c
Includes E- and S-glass fibres. Properties for E-glass are in the lower range.
textile precursors for composites are not difficult to
produce or source. In addition, the Oxford Silk Group
has recently shown that the higher compactibility of silk
preforms in comparison to both plant and glass fibre textiles
provides a unique opportunity to produce high fibre content
natural fibre composites using conventional manufacturing
processes [7].
Tensile stiffness [GPa]

5-25

(5-15)

As part of its research, the group first conducted a comprehensive literature review on silk composites and simultaneously
worked on the "design of silk fibre composites" noting the
apparent differences between the traditionally used brittle-stiff
reinforcing fibres and ductile-compliant silk fibres. This paper
presents some results from a subsequent cross-comparative
study evaluating the properties of nonwoven and woven silk
composites against similar flax and glass composites.
Fig. 2: Ashby plot comparing natural silks with other natural fibres (gold
balloons) and their building blocks (pink), and synthetic fibres (green) -
Adapted from [9]

and bone repair [3-6]. Indeed, the companies spun out of
the Oxford Silk Group (see www.oxfordbiomaterials.com)
are developing many such silk-based biocomposite medical

Composite manufacture
All composite laminates were manufactured using vacuum
infusion in a rigid aluminium mould. Silk composites were
fabricated using specially produced nonwoven mat and biaxial
woven fabric reinforcements at fibre volume fractions of 36%
and 45%, respectively (Figure 3) [8]. The nonwoven silk mat
is unique as the low-cost fabrication process employed (des-

48 jec composites magazine / No92 October - November 2014
http://www.oxfordbiomaterials.com
JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014

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