JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022 - 53

New bio-based composite successfully used
Fig. 3: Tensile and stress-strain bending diagrams of the Green Moulding Compound.
The blue line is the resin bending test without flax fibre reinforcement
Fig. 2: The Hedera vertical garden
created by the Hikari start-up
Overcoming complexity
However, replacing glass fibres with fibres
of plant origin is not an easily achievable
goal. The initial assumption mainly
concerned the mechanical properties of
natural fibres, and several issues had to
be addressed. First, the average rigidity
and strength values of natural fibres are
lower than those of common glass fibres,
and their standard deviation is greater
than that of synthetic fibres (see Table
1). The second issue is particularly complex:
lower mechanical properties can
be addressed by optimizing the part design,
provided that a minimum threshold
is reached. For dispersion, however, it is
something that can be considered a peculiar
feature of materials of natural origin,
and the only viable solution is an oversized
design to ensure safety.
An additional issue related to the use of
natural fibres is the fibre-matrix interface.
Fibres of plant origin are very hydrophilic
since their main structural component is
cellulose, which is a polysaccharide. Polymer
matrices, on the other hand, are often
hydrophobic, or in any case not as hydrophilic
as cellulose. Therefore, appropriate
coupling agents may be required to " build
a bridge " between the two components
and thus achieve efficient load transfer
between the matrix and the fibres.
The fibres' hydrophilia involves another
important obstacle. Polyesters, which
are often used as composite matrices, are
very sensitive to hydrolytic degradation.
Therefore, careful drying of the fibres is
necessary before impregnation, otherwise
the water vapour developing from
the undried fibres would degrade the polyester,
further lowering the composite's
mechanical properties.
Another major challenge comes from the
production process of GMC®, which is the
subject of two patents. Due to their hydrophilic
nature, flax fibres tend to agglomerate
together and form bundles, which is an
issue from many points of view. First, the
Tab.1: Comparison between natural fibres and traditional reinforcing fibres
Strength [MPa]
Fibre
Min.
Cotton
Jute
Sisal
Flax
E-glass
T300 graphite
200
400
300
600
3000
3500
Max.
800
900
900
2000
Min.
5.5
10
9
12
76
230
Max.
13
55
38
85
Stiffness [GPa]
resulting material would become very disomogenius
with areas reach of fibers and
almost complete devoid of them. This inevitably
leads to stress concentration points
while the parts are mechanically loaded in
service, which in turn can cause excessive
failure or deformation. Reducing the fibre
length would certainly help from this point
of view, but the strengthening effect of short
fibres is much lower than that of long fibres;
therefore, a compromise between mechanical
properties and de-bundling of fibres
must be sought. To meet such a challenge,
the resin flow inside the mould during
compression moulding must be carefully
planned so that the bundles can be broken
due to the viscous action of the liquid resin
flowing in the cavity.
A final processing issue is the thermal
degradation of the fibres. Vegetable fibres
consist of cellulose, hemicellulose and
lignin. Although cellulose, the structural
component, degrades at around 250°C, it
is strongly recommended not to exceed
the degradation temperature of hemicellulose
(approximately 200°C) during processing.
While hardening temperatures
are generally lower than these values,
hardening times can be long enough to
still allow thermal degradation of the fibres,
drastically reducing the mechanical
properties of the resulting material, and
giving rise to unsightly burns.
With the help of the Engineering Department
of the University of Ferrara, many
of these issues were addressed and solved.
Today, GMC® is a reality with the commercialization
of its first market application:
the Hedera vertical garden system.
The tensile and bending properties of
GMC® are illustrated in Figure 3.
N°146 June - July 2022 / jec composites magazine
53
Feature Sustainability
JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022
