Influence of drying on the mechanical behaviour of flax fibres and their unidirectional composites

Influence of drying on the mechanical behaviour of flax fibres
and their unidirectional composites
C. Baley a
, A. Le Duigou a,⇑
, A. Bourmaud a
, P. Davies b
a
LIMATB Laboratoire d’Ingénierie des Matériaux de Bretagne, Equipe Polymères et Composites, Rue de Saint Maudé, BP 92116, 56321 Lorient Cedex, France
b
IFREMER, Materials and Structures Group, Centre de Brest, BP 70, 29280 Plouzané, France
a r t i c l e i n f o
Article history:
Received 3 January 2012
Received in revised form 5 March 2012
Accepted 11 March 2012
Available online 19 March 2012
Keywords:
A. Flax fibres
C. Mechanical properties
B. Water absorption
a b s t r a c t
The microstructure of flax fibres can be considered as a laminate with layers reinforced by cellulose
fibrils. During a single fibre tensile test the S2 layer is subjected to shear. At room temperature, natural
fibres contain water absorbed in the cell-walls. This paper examines the influence of this water at two
scales: on the tensile behaviour of the flax fibres and on unidirectional plies of flax reinforced epoxy. Dry-
ing (24 h at 105 °C) is shown to reduce both failure stress and failure strain significantly. Analysis of nor-
mal stresses at the accomodation threshold provides an estimation of the shear strength of secondary cell
walls as 45 MPa for fibres containing 6.4% by weight of water and only 9 MPa for dried fibres. Results
from tensile tests on unidirectional flax/epoxy composites, reinforced by as-received and dried fibres,
confirm the influence of drying on strength properties.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Plant fibres are made up of composite layers reinforced by cel-
lulose fibrils [1,2], (Fig. 1) whose axial modulus is in the range from
134 to 160 GPa [3,4]. According to the variety cellulose represents
between 65% and 75% of the total weight of flax fibres [5].
The hierarchical architecture of flax is composed of an outer
layer with a primary wall (0.1–0.5 lm thick) which is flexible
enough to allow wall expansion during growth [6]. The primary
layer is coated with a polymer matrix that ensures the intercellular
cohesion in a fibre bundle. A secondary wall represents around 80%
of the fibre cross-section and is composed of 3 layers, S1 (0.5–2 lm
thick), S2 (5–10 lm) and S3 (0.5–1 lm) and provides the reinforce-
ment of the plant structure. The S2 layer is reinforced by cellulose
fibrils with a helix angle of 10° to the fibre axis [7]. In the centre of
the fibre there may be a cavity called a lumen.
The vocabulary used to describe these reinforcements is often
rather confusing. With a diameter between 2 and 4 nm, Burgert
et al. [8], Altaner and Jarvis [5] and Peterlin and Ingram [7] talk
of cellulose micro-fibrils whereas Fratzl et al. [3] simply uses the
term fibrils. Macrofibrils of cellulose have diameters between
100 and 200 nm according to Altaner and Jarvis [5] and Peterlin
and Ingram [7]. However, Bos et al. [9,10] in their work on tensile
and compression behaviour use the term mesofibrils (assemblies
of microfibrils) for cellulose elements whose diameter is around
200 nm. In this paper we will use the latter definition. In general,
while the names differ there is agreement on the hierarchical
structure of plant fibres: cellulose microfibrils are grouped in bun-
dles to form mesofibrils.
In flax fibres, the cellulose mesofibrils are embedded in an
amorphous polysaccharide matrix composed mainly of pectins
and hemicellulose. The hemicellulose (glucomananes) not only
acts as a matrix but also, thanks to a branched structure similar
to that of cellulose [11], may act as a coupling agent [2,12]; its nat-
ure varies according to whether it is in the primary layer of fibres
(xyloglucanes or arabinoxylanes) or in the secondary layer (xyl-
anes and glucomananes) [6,13,14].
Two types of pectins are present in flax fibres [2]: those which
are found in the adjoining and the primary layers, which form large
macromolecules and ensure cohesion between cells, and those in
the secondary layers which are part of the matrix covering the
cellulose mesofibrils [15]; these are assemblies of elementary
sugars (oses). The most common pectins in flax fibres are homogal-
acturonanes and rhamnogalacturonanes [2,16–18].
Plant fibres are thus composite nano-structures, which are
renewable, durable and biodegradable. Their hierarchical
architecture results in complex mechanical behaviour [19]. The
tensile properties of flax fibres have been widely studied and their
stiffness is comparable to that of glass fibres [12,20,21]. The com-
pression strength of single fibres, measured by Bos [22] is also
high, with values up to 1200 MPa. The composite structure of these
plant fibres is highly anisotropic [13,23] with a transverse modulus
for flax of around 7 GPa [13].
At room temperature, flax fibres contain (depending on the
variety, the retting and growth conditions) between 6% and 10%
1359-835X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.compositesa.2012.03.005
⇑ Corresponding author. Tel.: +33 2 97 87 45 07; fax: +33 2 97 87 45 88.
E-mail address: Antoine.le-duigou@univ-ubs.fr (A. Le Duigou).
Composites: Part A 43 (2012) 1226–1233
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of water by weight of fibres which acts as a cell wall plasticizer
[24].
The presence of absorbed water is frequently invoked as a con-
cern when plant fibres are being considered for polymer reinforce-
ment. Most thermoplastic manufacturing processes involve steps
at high temperatures, and the amount of absorbed water in the fi-
bres will evaporate according to the temperature and duration of
exposure. Manufacturers are therefore concerned that this water
will affect both fibre/matrix interface properties and initiate de-
fects such as porosity. However, removing the water from flax
fibres by increasing storage temperature results in a drop in their
tensile strength [25]. A drying step before composite manufactur-
ing may therefore be detrimental as the plant layers are sensitive
to thermal exposure. Thus drying at 120 °C for tens of minutes
leads to complete water removal but also the loss of surface waxes
[26]. Exposure to higher temperatures can alter the layer structure
and degrade both pectins (above 180 °C) and the cellulose (above
230 °C). High temperatures have an effect on mechanical proper-
ties; as shown by Baley et al. [24] or Stamboulis et al. [27], a high
temperature exposure induces a decrease in the fibre strength and
the appearance of defects inside the cell walls. Cichocki and Tho-
mason [28] showed a decrease of the transverse and shear modu-
lus of jute fibres with the increase in temperature. Gassan and
Bledzki [29] showed that the mechanical properties of jute or flax
are affected by the temperature of around 170 °C; in another study,
Placet [30], by using DMA on hemp bundles, showed a decrease in
the storage modulus and an increase in the loss factor during a
heating ramp between 20 and 200 °C. Ho and Ngo [31] performed
tensile tests on coir and hemp bundles between 23 and 80 °C; they
also showed a large drop in the Young’s modulus, the strength and
the strain at break with the increase of the temperature. Manufac-
ture of flax reinforced thermoplastics at 150 °C should not modify
the fibre structure but will affect the absorbed water content,
depending on the temperature and exposure time, and can modify
the interfaces between the constituents. The potential of fibres is
generally not realised in natural fibre composites [32,33].
Various authors have studied the relationships between plant fi-
bre tensile properties and unidirectional (UD) ply behaviour. For
example, Charlet et al. [34] examined the influence of the height at
which fibres were taken from the plant stem (near the root, in the
middle and near the top) on the longitudinal tensile properties of
UD flax/epoxy. The fibre properties depended on the position in
the stem, as did the resulting composite stiffness (but not the
strength). Other authors have studied other UD plies, such as
hemp/PP, hemp/epoxy [35], flax/epoxy [13] and flax/starch [36].
Many types of flax fibre have been characterised previously,
including both textile flax [12,14,34,37] and oleaginous flax [38].
The differences in properties observed for the different varieties
can be explained by the biochemical composition of the cell walls
which depends on the species, the climate conditions during growth,
and the retting conditions. Work on linseed [39] has revealed signif-
icant differences in both the cellulose content and the cell wall struc-
ture.Inordertorelate theseparametersto themechanicalproperties
of single fibres it is important to understand the behaviour of the cell
walls,so it is essential to understand the role of water,as this is one of
their main constituents. The aim of this paper is to examine the influ-
ence of a drying operation on the tensile behaviour of single flax fi-
bres and to estimate the shear strength of the S2 layer. The
behaviour of a UD flax/epoxy composite, subjected to the same dry-
ing operation, will also be examined. The fibres used in the compos-
ite are from the same batch as those studied individually.
2. Materials and methods
2.1. Material
Flax plants, variety Ariane, were cultivated in Normandy
(France) and retted in the field (dew retting). After harvest, the
stems were scutched and the fibres hackled. The batch used is sim-
ilar to those studied previously by Baley [12].
2.2. Thermogravimetric analysis
In order to quantify the influence of the drying on the flax fibre,
TGA analyses were carried out, using Mettler Toledo TGA/DSC 1
equipment. The analyses were performed according to the follow-
ing protocol:
Ramp from 25 °C to 105 °C at 20 °C/min.
Isothermal at 105 °C for 14 h.
Around 100 mg of fibres were used. Weight changes versus time
were recorded. A temperature of 105 °C was chosen as this has
been used previously in order to measure the amount of water in
the cell walls of wood, by drying until a constant weight was ob-
tained [40].
2.3. Tensile tests on single fibres
Tensile tests were performed on individual fibres. As the plant
fibres are quite short (about 30–80 mm) and as there are variations
S3
S2
S1
θ
S3
S2
S1
P
Secondary wall
Primary wall
Amorphous polymers
Cellulose microfibrils
(70%) crystalline
Fig. 1. Schematic representation of flax fibre showing helix arrangement of cellulose fibrils. Diagram on right adapted from Hearle [1]. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
C. Baley et al. / Composites: Part A 43 (2012) 1226–1233 1227

in cross-section along the fibre length, a gauge length of 10 mm
was chosen. The fibre was clamped on a universal MTS type tensile
testing machine equipped with a 2 N capacity load cell and loaded
at a constant crosshead displacement rate of 1 mm/min up to fail-
ure. The determination of the mechanical properties was
performed in accordance with the NFT 25-704 standard, which
takes into account the compliance of the loading frame. For each
kind of fibre, at least 50 fibres were tested. Before each tensile test,
the fibre diameter was measured with an optical microscope. The
diameter used to calculate stress is an average value from three
points obtained along each fibre. Fibres with a large section varia-
tion (above 5%) were excluded.
Storage and testing for raw fibres were performed in a temper-
ature and relative humidity controlled laboratory (23 ± 0.5 °C,
48 ± 2% RH).
For dried fibres the following protocol was used: the fibres were
fixed to a support, inspected under a microscope, dried for 14 h at
105 °C. The oven was close to the test machine, in the temperature
and humidity controlled laboratory. The tensile test lasted less
than 2 min including the time needed to place the sample on the
machine. The temperature drops very quickly (testing at room
temperature) but the fibre will pick up a small amount of moisture
during this period. This is similar to the industrial process. During
manufacture of compounds (PP/flax or PLLA/flax), the fibres are
first dried, then compounding involves a period in which the fibres
are in contact with the air for about 2 min [21]. A previous study of
the water uptake during this period [24] showed that for dried fi-
bres (24 h at 105 °C), the amount of absorbed water by weight was
around 1.5% in 2 min, compared to 6.4% in the un-dried fibres.
Moreover dried fibres do not recover their initial properties due
to modification of element involved in load transfer inside cell-
wall. The diameter is not measured in the dry state, so the stress
measured will be probably underestimated.
2.4. Unidirectional composite specimens
Unidirectional composite specimens were prepared by com-
pression moulding after wet impregnation of the fibres (to control
the fibre content the specimen thickness was calibrated). The ma-
trix was an epoxy resin (Axson 2020). Samples were made directly
in a mould to avoid subsequent machining. After hardening at
room temperature they were post-cured for 12 h at 65 °C to obtain
complete cure. Both dried and as-received flax fibres were used for
impregnation. The specimen cross section was 4 Â 1 mm2
, end tabs
were bonded to improve load introduction.
2.5. Tensile tests on UD composite specimens
The static tensile tests were carried out in an environmentally
controlled laboratory, according to NF T 57-101 ISO 527 on an
MTS Synergie RT1000 (MTS, Eden Prairie, MN, USA) testing ma-
chine. The loading speed was 1 mm minÀ1
. An MTS extensometer
was used to measure strain, with a nominal length of 25 mm. At
least ten specimens were tested for each condition, and the results
were averaged arithmetically.
3. Results and interpretation
3.1. TGA study
Fig. 2 shows the weight loss of the Ariane fibre during two iso-
therms, the first one of 14 h at 105 °C and the second one of 5 h at
150 °C. Most of the weight loss occurs during the first few minutes,
Time (h)
0 5 10 15 20
Weight(mg)
56
58
60
62
64
66
68
70
72
74
Temperature(°C)
0
20
40
60
80
100
120
140
160
Time (s)
0 100 200 300 400 500 600
Weight(mg)
68
69
70
71
72
73
74
Temperature(°C)
20
40
60
80
100
120
Fig. 2. Thermo-gravimetric analysis of flax fibres held at 105 °C for 14 h and 150 °C for 5 h. The insert shows the initial part of the curve at higher magnification. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
0
200
400
600
800
1000
1200
1400
1600
0 0,5 1 1,5 2 2,5
Stress(Mpa)
Strain (%)
Raw fibres
Dried fibres
Fig. 3. Typical stress–strain plots for flax fibres (black curve). The dashed line is
simply drawn to indicate the non-linearity of the curve). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
1228 C. Baley et al. / Composites: Part A 43 (2012) 1226–1233

during the temperature ramp. This 5.6% weight loss can be attrib-
uted to water removal. The weight loss continued throughout the
first isotherm, and after 14 h the total loss was 6.2%.
The water content in flax fibres evaluated by TGA dynamic heat-
ing is generally in the range from 7% to 10% [26,37]. In our case, by
using an isothermal heating at 105 °C during 14 h, this value was
lower, indicating that all the water contained into the cell walls
was not removed. In the flax fibre the water is mainly linked to
the pectins [41] located at the fibre surface or inside the polymeric
matrix between the cellulose mesofibrils [2,42]. Heating at 105 °C
for 14 h does not result in complete extraction of the strongly
linked water located in the polysaccharide matrix, whereas this
extraction is complete when a dynamic thermal ramp is applied
[26,37] with complete water release around 150 °C.
In order to estimate the exact water quantity contains into our
fibres, a second isotherm of 150 °C for 5 h has been carried out. The
final weight loss was 7.1%. This value is well correlated with liter-
ature values [26,37] and shows that the linked water needs high
temperature to be extracted.
The work of Velde and Baetens [26] has shown that prolonged
heating at 120 °C results in degradation of surface waxes, which
may be occurring here. This can change the behaviour of the poly-
mers and modify the load transfer between fibrils [24].
3.2. Influence of the loss of water on tensile mechanical behaviour of
fibres
Fig. 3 shows a typical tensile stress–strain plot for as-received
flax fibres.
Table 1 summarizes the mechanical properties. Since the
Young’s modulus and failure stress depend on the fibre diameter
[12,43], the values are given for two mean diameter ranges, from
20 to 22.5 lm and from 22.5 to 25 lm.
Although there is scatter in the results, for the two types of fi-
bres drying does not appear to result in a significant change in
Young’s modulus. This modulus depends on the volume fraction
of reinforcement (cellulose), which is not modified by drying
unless temperatures above 230 °C are reached [26].
Fig. 4 shows how the slope of the stress–strain curve changes
with strain for the as-received and dried fibres. A particular form
of curve was noted, with changes in slope as strain increases. Sim-
ilar published curves can be found for regenerated cellulose [44]
and synthetic fibres [45].
Although the initial values of Young’s modulus were not af-
fected by drying there were significant differences in the way the
slope of the stress–strain curve changed with tensile strain during
the tests on as-received and dried fibres. In both cases stiffness in-
creased with strain, due to reorientation of cellulose fibrils. Various
authors have noted the re-alignment of micro fibrils and reduction
in the microfibril angle during tensile tests, such as Keckes et al.
[46] on wood using a synchrotron, or Baley [12] and Wild et al.
[47] by cyclic loading, on flax and wood fibres, respectively. Astley
and Donald [48] showed (by X-ray scattering) that, during a tensile
test the strain can induce crystallization of the amorphous part of
the micro-fibrils. According to Hearle [1], the sequence of events is
the following: first, an increase in length of fibrils and
noncrystalline regions (polysaccharide matrix), then a spiral spring
extension with flexure and torsion of the mesofibrils accompanied
by a reduction in volume of mesofibrils and matrix. The non-crys-
talline regions are then sheared to conform to the new fibrillar
Table 1
Influence of drying on the tensile properties of flax fibres.
Designation Diameter range (lm) Number of fibres tested Diameter (lm) Young’s modulus (GPa) Failure strain (%) Failure stress (MPa)
As-received 20–22.5 21 21.57 (±0.95) 64.10 (±13.65) 2.93 (±0.74) 1499 (±346)
22.5–25 23 23.86 (±0.68) 51.28 (±12.02) 3.34 (±0.71) 1317 (±529)
Dried 20–22.5 23 20.94 (±0.76) 59.24 (±19.36) 2.07 (±0.30) 870 (±266)
22.5–25 18 23.77 (±0.72) 58.66 (±15.86) 1.74 (±0.37) 711 (±251)
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2
0
10000
20000
30000
40000
50000
60000
Strain (%)
Variationinslope(MPa)
BA
0 0,5 1 1,5 2 2,5 3 3,5 4
0
10000
20000
30000
40000
50000
60000
Strain (%)
Variationinslope(MPa)
Fig. 4. Variation in slope versus strain derived from stress–strain curve for as-received fibre (A) and dry fibre (B). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Mesofibrils aligned with the
fibre axis after the tensile test
Fig. 5. Failure area of a flax fibre after a tensile test. The secondary wall shows a
structure of unidirectional composite, micro fibrils are aligned with the fibre axis
[11]. (For interpretation of the references to colour in this figure legend, the reader
is referred to the web version of this article.)

architecture. Shear of the polysaccharide network (xyloglucane
and galactanes) beyond a shear stress threshold leads to flow
(breaking of hydrogen bonds). A stick–slip (velcro) mechanism, de-
scribed in detail for wood [3,46], allows a rearrangement of the
deformable network, so that loads can be transferred between
the polysaccharide matrix and the mesofibrils [5]. This original
mechanism is the result of the special conformation of the xyloglu-
canes and galactanes linked to the cellulose mesofibrils, which are
able to entangle and disentangle with the pectin matrix.
For the as-received fibres (containing water), the increase in
stiffness with strain, corresponding to an accommodation phe-
nomenon [30], followed two distinct slopes, the change in slope
may be considered as the end of microfibrils reorientation. For
the dried fibres, the change in slope was accentuated (Fig. 4) and
a pronounced knee was noted at low strains. A clear decrease of
the slope is noticed at this step. Beyond this point, there is a new
improvement of the Young’s modulus with strain before the final
break.
Fig. 5 [12] shows an example of the damage undergone by flax
fibres during tensile testing, note the unidirectional structure of
the S2 wall and the longitudinal orientation of the meso-fibrils.
In order to analyse the loss of linearity shown in Fig. 4, the nor-
mal stresses in the fibres at the damage threshold (knee) have been
calculated. It was assumed that these axial stresses caused the lo-
cal shear stresses which initiated relative slipping of the cellulose
mesofibrils. If we consider the fibre to consist of a symmetrical se-
quence of plies oriented at ±10° to the fibre axis (and assuming
that the strain at the knee is too low to cause significant changes
to the microfibril angle), it is possible to estimate the shear stress
in the fibre at this elastic limit with the following expression [49]:
sLT ¼
F sinð2hÞ
2S
ð1Þ
with F: the applied load, S: the cross-section and h the microfibrillar
angle (10° for flax). In this case, shear stress distribution in the ra-
dial direction is not taken into account.
Table 2 shows the mean values of the axial and estimated shear
stresses for both as-received and dried flax fibres.
The results in Table 2 shows that the axial stress at the knees is
around 20% of the failure stress of the as-received fibres. The
corresponding calculated shear stress is of similar magnitude to
those of common composites such as glass/polyester or glass/
epoxy [49]. The dried fibres show much lower values, and the axial
stress at the knee is only 7% of the break stress of these fibres.
The loss of certain mechanical properties of thermally treated
wood has been explained by the breakage of glucosidic bonds be-
tween glucose monomers and inter and intra molecular hydrogen
bonds [50]. This brittleness is accentuated by the presence of sugars
such as galactose, mannose and xylose, within the wood fibres [51].
In order to improve the behaviour of plant cell walls some heat
treatments (at temperatures above 150 °C) have been proposed to
limit the sensitivity of flax fibres [27] and wood [52] to humidity.
For example, the Duralin treatment consists of a steam or water-
heating step of the rippled straw-flax at temperatures above
160 °C for approximately 30 min in an autoclave. This first step is
followed by a drying step and a heating (curing) step above
150 °C for approximately 2 h. During this treatment, the hemicellu-
lose and lignin are depolymerised into lower molecular weight
aldehyde and phenolic groups, which are combined by the subse-
quent curing reaction into a water resistant resin, which keeps
the cellulose microfibrils together [27]. Heat treatments are also
used for wood, various thermo-transformation procedures are
used to stabilise wood through a progressive increase in tempera-
ture in the absence of oxygen (the inert atmosphere slows down
the thermo-chemical reactions and makes cycle control easier).
This treatment is a controlled pyrolysis in the range of tempera-
tures between 150 and 280 °C, an intermediate between drying
and carbonisation. The resulting chemical modifications, in partic-
ular to the intermolecular bonds (chemical links between wood
constituents, cross-linking of ligno-cellulosic matter), improve
dimensional stability [52] and durability [53] but reduce
mechanical properties, particularly the toughness [54].
Table 2
Analysis of the influence of drying on the mechanical properties of flax fibres in tension and shear.
Designation Diameter range (lm) Failure stress (MPa) Normal stress at knee (MPa) Estimated shear stress at knee (MPa)
As-received 20–22.5 1499 (±346) 292 (±82) 50 (±14)
22.5–25 1317 (±529) 225 (±109) 39 (±20)
Dried 20–22.5 870 (±266) 61 (±29) 10 (±5)
22.5–25 711 (±251) 48 (±27) 8 (±4)
0
50
100
150
200
250
300
350
400
Stress(MPa)
Strain (%)
0
50
100
150
200
250
300
350
400
0 0,5 1 1,5 20 0,5 1 1,5 2
Stress(MPa)
Strain (%)
A B
Fig. 6. Examples of stress–strain plots for UD composite reinforced by as-received (A) and dried flax fibres (B). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
Table 3
Tensile properties of plies reinforced by unidirectional flax fibres.
Material Vf (%) EL (GPa) rUD L (MPa) e UD L (%)
Flax/epoxy 40.4 ± 1.2 22.50 ± 1.51 328 ± 18 1.6 ± 0.2
Dry flax/epoxy 40.0 ± 0.9 22.94 ± 2.65 210 ± 25 1.2 ± 0.1

In order to evaluate the influence of drying fibres on composite
behaviour a series of tensile tests was performed on flax reinforced
epoxy composites.
3.3. Influence of drying of fibres on the tensile behaviour of
unidirectional composites
The flax fibres were either dried or not before impregnation
with epoxy resin, post-cure was performed after resin hardening
at room temperature to limit loss of water from the undried fibres.
The stress/strain plots for the two composites are shown in
Fig. 6. Their form is similar, with an initial non-linear region, sim-
ilar also to those of the single flax fibres (Fig. 3).
Table 3 presents the mean mechanical properties in the fibre
direction from the two series of specimens for the same volume fi-
bre fraction (40%). The axial modulus was not affected by fibre
drying whereas the stress and strain at failure were reduced signif-
icantly by the drying operation. These results confirm those mea-
sured on the fibres alone. The average loss in strength of the UD
is 36%, a little lower than those measured on the fibres (42% for
diameters between 20 and 22.5 lm, 46% for diameters between
22.5 and 25 lm). However, the tensile strength of a composite
reinforced by discontinuous unidirectional fibres (fibre length be-
tween 30 and 80 mm) is complex and not directly proportional
to the strength of the fibres, since cumulative damage needs to
be taken into account.
The average value of Young’s modulus for this batch of flax fi-
bres is 58.6 GPa [12] and the mean failure stress is 1496 MPa
(mean values obtained taking into account diameter variations
and the influence of diameter on properties). The longitudinal
modulus estimated from a simple law of mixtures approach
(EL % Vf Â EfL) for a fibre volume fraction (Vf) of 0.4 is 23.45 GPa,
close to the measured values shown in Table 3.
4. Discussion
At room temperature plant fibres contain moisture (6.4% by
weight for the flax fibres studied here). This water is located both
on the surface and within the fibres [41] and absorption on the sur-
face is assisted by the high polarity due to the large quantity of hy-
droxyl groups in the pectins. However, the pectin content can be
controlled by the retting conditions [37]. Drying (and thus the
amount of absorbed water in the cell walls) influences both the
tensile behaviour of flax fibres (failure properties, appearance of
a knee on the stress–strain plot and the reorientation of fibrils de-
scribed above) and the failure properties of UD composites
manufactured with these fibres. Different phenomena are involved
at different scales:
4.1. At the scale of the fibre constituents
Removal of moisture will result in reduced interactions be-
tween fibrils [55]. Dehydration of flax fibres results in the forma-
tion of hydrogen bonds between the cellulosic surfaces (fibrils)
causing a reduction in the space between cellulose mesofibrils
(from 7.7 to 5.8 nm) [5,56]. The water also plays the role of plasti-
cizer [24], so its removal embrittles the constituents, and espe-
cially the hydrated gel network formed by the polysaccharide
matrix [16]. These phenomena will affect the interactions between
mesofibrils and pectin matrix. The exact scale at which these occur
(fibrils, micro fibrils, mesofibrils) is not well understood yet and
requires further study. Moreover, the diameter is not measured
in the dry state, so the stress measured will be probably
underestimated.
4.2. At the fibre scale
Removal of moisture during drying can cause mechanical
stresses to develop within the fibre, due to differences in behaviour
of the cellulose, pectins, and hemicellulose as well as physico-
chemical changes [26] resulting in a reduction in mechanical
properties of the fibres. The physico-chemical changes can begin
as soon as drying starts (see TGA curve) with the extraction of
low molecular weight components.
4.3. At the UD composite scale
The tensile stress–strain plot for the unidirectional flax fibre
reinforced composite is non-linear (similar to that of the fibres,
even though the composite behaviour does not only depend on
the fibres). Drying the fibres before impregnation causes a reduc-
tion in their mechanical properties at failure, which is transferred
to the composite.
However, with flax fibres, which have a complex layered struc-
ture, there is an additional damage mechanism, as cracks can also
develop within the fibres themselves [13] which can influence the
global behaviour of the composite. Fig. 7 shows an example of the
shear failure of the fibre along the microfibril direction.
Fig. 8 shows a fracture surface of a flax fibre after UD composite
failure (dried fibres). Debonding within a fibre is observed, appar-
ently between the primary and secondary cell walls.
Furthermore, geometrical singularities (kink bands) on the
natural fibre can result in stress concentrations in both fibre and
matrix [35]. In addition some flax fibres will be aggregated in bun-
dles, which have not been completely separated during fibre pro-
duction, and cracks can develop in the adjoining cell walls
between fibres (each fibre is then not covered by a layer of matrix)
[13]. The quality of the fibre/matrix interface will also affect the
Shear failure
Fig. 7. Damage within a flax fibre in a UD composite after tensile failure. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
Fibre peeling
Fig. 8. Fracture surface of flax fibre within UD composite (fibre dried before resin
impregnation). (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)

behaviour of the composite, though the interfacial shear strength
of flax in the epoxy matrix studied here has been measured by
micro-droplet tests to be similar to that of E-glass/epoxy with a
standard multi-compatible fibre sizing [57]. An important addi-
tional factor which will affect this transfer is the influence of dry-
ing on the fibre/matrix interface properties. This is currently being
studied, and should allow a global appreciation of the influence of
drying on composite performance.
5. Conclusion
The use of composites reinforced by natural fibres is expanding.
Flax fibres provide significant reinforcement of polymers, can re-
duce environmental impacts [58] and result in recyclable and com-
postable materials (with an appropriate matrix). The moisture in
fibres is generally considered to be detrimental to composite per-
formance, as manufacturers are concerned that it will reduce fi-
bre/matrix bonding quality and cause porosities. Plant fibres are
therefore usually dried before mixing with polymers.
This paper clearly shows that drying fibres (14 h at 105 °C) re-
sults in significant loss of strength (À44% on average) and failure
strain (À39%), together with a change in the stress–strain plot
slope. If we treat the flax fibre structure as a layered composite
reinforced by helical cellulose fibrils, when this is loaded in tension
the microfibril angle to the fibre axis decreases and stiffness in-
creases. Assuming that the knee on the tensile modulus-strain plot
corresponds to the onset of mesofibril sliding a damage threshold
can be defined in terms of the local shear stress in the S2 layer
(secondary cell wall). This shear stress drops after drying from
45 MPa to 9 MPa.
Longitudinal tensile tests on unidirectional composites rein-
forced with dried and as-received flax fibres show that drying does
not affect axial stiffness but causes a large drop (À36%) in compos-
ite strength, in a similar way to the drop in fibre strengths.
If the use of natural fibres as composite reinforcement is to be
optimised therefore the application of fibre drying cycles must be
reconsidered. If these are really necessary they must be carefully
defined with respect to the particular fibre used, the matrix poly-
mer and the manufacturing process. This may require modifica-
tions to the process or the development of special technology to
protect the cell wall properties.
The use of natural fibres to reinforce thermoplastic polymers re-
quires high temperature cycles. This provides additional parame-
ters and the temperature and time must also be optimised to
allow complete impregnation without fibre degradation.
Finally, the removal of moisture will also influence the proper-
ties of the fibre surfaces and hence the interfacial bonding. This
requires more detailed study.
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