JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014 - 26

biocomposites
at low temperature are left at
the fibre surface, and that the
fibres are damaged as little as
possible during the processing
cycle. There are now plant fibre
reinforced parts that comply
with auto builder specifications
on odours (Baley, 2013).

Bottleneck 8: optimizing
plant fibre presentation

Microstructure of hemp and flax fibres

individualization with the idea
of reinforcing a polymer using
elementary fibres (single plant
cells) to increase the bonding
surface area; cleaning fibre
surfaces; promoting wetting
and adhesion; minimizing the
tendency for the wall to absorb
water; improving the heat
stability; minimizing odours;
changing the colour; improving the fire behaviour; including a fungicide; or facilitating
processing. So there is a broad
range of potential treatments,
and some of these can have significant environmental impacts.
It is also important to consider
a plant fibre as a stack of plies
that is reinforced by cellulose
fibrils. The notion of interface
can't be summed up as just the
link between fibre and matrix,
i.e. primary wall/polymer. There
are multiple interfaces (e.g. between the primary and secondary walls, between the secondary wall's sublayers, between the
macrofibrils, and between the
cellulose microfibrils) (Baley
et al., 2014) and there can be a
problem recognizing which is
the relevant one. Note that treatments and processing cycles can
affect these interfaces (Baley
et al., 2012), (Le Duigou et al.,
2013).

Bottleneck 7: adapting
the converting process to
preserve wall properties
Temperature affects the plant fibre properties (Bourmaud et al.,
2010), (Baley et al., 2012). The
fibres' heat resistance (about
200-230°C) places limitations
not only on their structural
use, but also on the processing
temperatures, especially with
thermoplastic matrices. However, temperature is not the only
important parameter, it is time
and temperature that count.
When the fibres are processed at
uncontrolled high temperature,
the water contained in the fibres
evaporates (if the fibres have not
been oven-dried or if the process is not perfectly controlled),
changing the fibre mechanical
properties (loss of resistance)
and leading to the risk of gas
emission, which creates porosity. For injected compounds,
the extrusion phase can damage
the fibres, but it also enables
individualization (Ausias et al.,
2013), (Bourmaud et al., 2013),
(Beaugrand et al., 2012).
In a number of applications,
e.g. automotive interior trim
panels, it is also important that
the parts not give off much
odour. This means ensuring
that no elements which degrade

This bottleneck concerns the
presentation of fibres in forms
like mats, fabrics, unidirectionals and biaxials. As an example,
the fibres in a flax plant are in
the form of bundles of discontinuous fibres. An elementary
flax fibre ranges from about 15
to 60 mm in length. Retting
alters the intercellular layers,
facilitating their later extraction.
Scutching enables the recovery of fibre bundles along the
length of the plant, and hackling
(combing) finishes separating
the bundles into line fibre that
can be used in spinning. During
spinning operations, the objective is to individualize and twist
the fibres into a yarn with good
tensile strength (load transfer
between the fibres in a yarn that
is under tensile stress takes place
through friction).

The yarn is then woven.
In polymer reinforcement, other
load transfer mechanisms come
into play through the interfaces.
Plant fibres are highly anisotropic, and to make the best possible
use of them in a ply, you have
to take that into account. Using a fabric that consists of an
assembly of twisted yarns does
not give the benefit of their mechanical performance, and leads
to a non-uniform distribution of
the reinforcement throughout
the matrix. Several ongoing
research projects to create a tow
that is handleable are working
along different lines: reducing
yarn twist, applying a binder to
create unidirectional tows, or
using stitch assembly (CELC,
2012). The permeability of
these reinforcements is also
being delved into (Ouagne et
al., 2009). Various types of ply
(e.g. mat, biaxial, unidirectional,
fabric) are under development
to take the specific features of
this type of reinforcement into
account. The price of some of
the semi-finished products is
holding back their more widespread use, and this parameter
needs to be taken into account
in the choice of a technology.

Bottleneck 9:
understanding and
controlling the evolution
of properties over time

Electron microscope view of a flax
yarn for textile applications (note
the individualization of fibres )

26 jec composites magazine / No92 October - November 2014

There has not yet been much
research on how the properties
of a biocomposite part evolve
in a hostile environment (Le
Duigou et al., 2009), (Celino et
al., 2013), (Liang et al., 2013).
The tools to study, analyse and
simulate the ageing of structures made of synthetic fibre
reinforced composites currently
exist (Davies et al., 2014), (Harris, 2000). For these new materials, one problem is that there

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

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014