JEC COMPOSITES MAGAZINE - Issue #92 - October/November 2014 - 25

The edge of a flax field

and plastics processors. The
agricultural sector is used to
fluctuating prices (speculative
markets), while the industrial
sector seeks stability for longterm commitments. Development is also complicated
by the fact that the price of
semi-finished products depends
on the volumes consumed, and
vice-versa.

Bottleneck 4: acquiring
a broader scientific and
technical culture around
plant fibres
While there are many types of
fibre plants (Franck, 2005),
(Mussig, 2010), not all can be
used as polymer reinforcement.
The most useful fibres are the
ones that play a structural role
in the plant as supporting tissue,
and so we need to take an interest in plants and agronomy (Roland, 2002), (Reis et al., 2006).
Agricultural land is not just a
growing medium, it is a living
environment. A seed has a gene
pool, and there are a number of
parameters (e.g. weather, soil
type, distance between plants,
fertilization) that influence the
plant development. The plants
will then be harvested and
undergo various treatments (e.g.
retting for flax, scutching, hack-

ling) to facilitate fibre extraction and bundle separation. All
these steps will affect the wall
properties (Martin et al., 2013),
and as such, they need to be well
understood.
The "naturalness" of plant fibres
is often presented as a handicap,
as it leads to non-uniform properties. There is no point in denying the non-uniformity - but it
is possible, through inspection
and homogenizing of batches, to
obtain reproducible properties.
For textile applications, this has
been done for many years. Note
that glass (Coroller et al., 2013)
and carbon fibres also display
non-uniform tensile strength.
Few people are able to control
the mechanical properties of
synthetic fibres. The reproducibility of flax fibre mechanical
properties from one year to
the next has been studied by
Lefeuvre et al. (2013). The
mechanical properties of the
Marylin variety that is grown in
a given region (the Neubourg
Plateau in northwestern France)
change very little whatever the
meteorological conditions.
This kind of research needs to
be done for other varieties and
other types of plant. We must
not forget the "homogenizing"
effect of a composite ply.

Bottleneck 5: acquiring a
better understanding of
supporting tissues
Even though plant fibres have
been used for many years
(Franck, 2005), (Mussig,
2010), there are still a number
of fields of study on cell wall
composition, organization
and microstructure. Such data
are needed to understand the
origin of the properties and to
analyse wall behaviour. For example, cell wall microstructure
is usually described as a stack

(Evert, 2006), (Glimn-Lacy
et al., 2006), (Cutler, 2007),
(Beck, 2010), (Nultsch, 1998)
composed of a primary wall and
a secondary wall that consists of
three layers, called S1, S2, and
S3. For flax fibres, transmission
electronic microscopy (TEM)
has been used to examine this
microstructure (Roland et al.,
2005), (Morvan et al., 2003).
The technique enables observation of the transition zones, i.e.
changes in the orientation of the
cellulose microfibrils between
the layers. For the woody tissue,
a number of studies describe
an identical microstructure,
with an additional layer called
G in the case of reaction wood
(Pilate et al., 2004). This
organization is not always
identical for all of a plant's fibres
and for all plant families. For
hemp fibres, Thygesen (2006)
proposes another description:
a primary wall and a secondary
wall that consists of two sublayers, S1 and S2; the S2 sublayer
itself is composed of 1-4 layers.
Understanding microstructures
requires studying how these
supporting tissues develop
during plant growth. A recent
article (Gorshkova et al., 2012)
presents both the latest level of
knowledge about fibre formation and some open questions.
In addition, the stiffening
mechanisms in play when an
elementary fibre is subjected
to tensile stress are not per-

fectly understood. Is it mainly a
mechanism whereby the cellulose microfibrils shift direction
as is described for wood (Fratzl
et al., 2007) and flax (Baley,
2002), or is it a phenomenon
of stress-induced crystallization
as proposed for hemp (Placet
et al., 2014)? Of course, there
is no single answer, given the
differences in fibre composition, organization and technical
itinerary.

Bottleneck 6: choosing
constituent materials
The choice of constituent
materials (polymer, plant fibre
and treatment) is very similar
to bottleneck 1. If the goal is to
reduce environmental impacts,
it is important to limit transport
and use local resources like flax
or hemp (Ernst and Young,
2005). For the choice of matrix,
it is possible to use different criteria, e.g. origin for a biobased
polymer, end of life (EOL) by
biodegradation for a compostable polymer, or possibility of
recycling for a thermoplastic
polymer.
Review articles exist on the
treatments (e.g. chemical, heat,
physical, mechanical, enzymatic) applied to the fibres (Bledzki
et al., 1999), (George et al.,
2001), (John et al.,2008), (Xie
et al., 2010), (Zafeiropoulos,
2011), (Faruk et al., 2013). The
treatments can target different
objectives, e.g. promoting fibre

Rupture of a flax/PLA unidirectional ply in liquid nitrogen [Baley 2014]

No92 October - November 2014 /

jec composites magazine 25

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

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