JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022 - 31

Sustainable, multifunctional hemp-based structural composites
a
b
c
d
e
Fig. 1: Properties of hemp versus flax. Top (a,b): tensile properties of fibres determined using impregnated fibre bundle tests (IFBT);
bottom (c,d,e): tensile properties of GreenPoxy composites reinforced with the quasi-unidirectional woven hemp fabrics developed in
the SSUCHY project and the commercial unidirectional Flaxtape (Ecotechnilin); fibre volume fraction 50%
Fig. 2: Production of hemp woven fabrics in the SSUCHY project
second one consists in breaking and carding
(combing) randomly aligned straw. In both
cases, low-twisted rovings suitable for weaving
were produced.
The properties of the fibres obtained with the
two routes are equivalent to those of hackled
flax (combed long fibres) used in commercial
fabrics (Figure 1, top).
Using these rovings, a variety of 100% hemp
reinforcements suitable for composite applications
has been produced (Figure 2), including
balanced and unbalanced fabrics, quasi
unidirectional fabrics and braided preforms.
These primary and secondary processing
steps have been demonstrated at lab scale, but
also using industrial machinery at the Linificio
e Canapificio Nationale company. At the
composite's scale, results showed that hemp
can achieve properties comparable to high
quality long flax fibres (Figure 1, bottom).
Improving the moisture
durability of plant fibre composites
Perhaps, the main potential weak point of
plant fibre composites is the substantial
amounts of water that the fibres can absorb.
Water absorption lowers fibre and composite
stiffness, but also leads to fibre swelling in
humid conditions, and shrinking in dry states.
Swelling and shrinking of natural fibres cause
damage inside the composite material over
time and reduce its durability. In the SSUCHY
project, the material durability was monitored
and analysed under various loading conditions
(hygrothermal ageing, hygrothermal
creep and fatigue) and various strategies were
investigated to mitigate the problem. The
results showed that, when fully dried before
production and subsequently equilibrated
under high humidity (80% RH or more),
bio-fibres develop a substantial amount
of micro-damage due to fibre swelling. At
the start of the project, this issue made the
SSUCHY team hypothesise that if the fibres
are processed in a non-dried condition, this
should reduce damage during hygrothermal
ageing and thus improve mechanical property
retention. This hypothesis was proven during
the project. The resin can tolerate water in
the fibres when they are not dried. Within
SSUCHY, this issue was solved by Nouryon
in the case of polyester resins, by furthering
the development of water-tolerant cobalt-free
accelerators. several iron and copper-based
catalysts were launched commercially as a
result of this work.
Exploiting the damping
properties of plant fibre composites
Bio-based composites can strongly contribute
to solving a technical challenge related to vibration
and acoustic performance. Indeed, the
damping capability of bio-based composites is
generally much higher than that of synthetic
fibre composites: this is measured by the loss
factor, which typically ranges between 0.7%
and 14% for bio-based composites, compared
to 0.24-2.5% for synthetic fibre composites.
Hence, this category of materials offers a
novel compromise between damping and
stiffness, which is of first interest for various
applications where lower vibration or acoustic
levels are expected. The development of
specific microscale techniques during the
project provided a new understanding of how
dissipation mechanisms are distributed in
the matrix, the fibres and the interface. The
results were exploited by the Wilson Benesch
company to develop the Precision and A.C.T.
series loudspeaker demonstrators (Figure 3).
The modular approach used for the A.C.T.
loudspeaker was based on a monocoque
component using hemp reinforcements and
vacuum-RTM technology.
Lightweight structural solutions
for the transportation sector
Over 95% of the plant fibre composites produced
for commercial applications in Europe
are used in non-structural components. In
this type of products, plant fibres are used
in the form of short fibres or nonwoven
reinforcements. Considering their high
mechanical performance, the objective of the
SSUCHY project was also to demonstrate
that continuous hemp-based reinforcements
can replace traditional reinforcements (glass
or carbon) in structural composite applications.
For
the ground transportation sector, the
NPSP company developed and produced a
new hemp-based body (a large monocoque
structure) for an electric scooter using
balloon blowing techniques (see Figure 4).
The advantages of hemp-based composites
include a low environmental impact, specific
Fig. 3: High-end " green " loudspeakers made of hemp-based
composites produced by Wilson Benesch. Left: Prototype of a
Precision speaker; right: oblique views of all the loudspeaker
systems in the Fibonacci Series
N°146 June - July 2022 / jec composites magazine
31
Feature Sustainability

JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022

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