The ecological advantages of flax fibre reinforced thermoplastic composites make them an attractive alternative to glass fibre reinforced polymer composites with comparable specific properties.

1. Institute of Polymer Engineering Klosterzelgstrasse 2 T +41 56 202 73 87 info.ikt.technik@fhnw.ch
CH-5210 Windisch F +41 56 202 75 74 www.fhnw.ch/technik/ikt
The ecological advantages of flax fibre reinforced thermoplastic composites make them an
attractive alternative to glass fibre reinforced polymer composites with comparable specific
properties. However, one must overcome the relatively high viscosities of engineering
polymers whilst respecting the low degradation temperature and inherent variation of natural
fibres. We demonstrate that this may be achieved, and complex geometries can be produced.
Flax Fibre Thermoplastic Composites: Processing
Fig. 1 Thermogravimetric analysis of flax fibres
Thermoplastic matrices allow for part welding.
Thus, integral parts may be manufactured in a
single processing step or by direct injection
moulding.
At the end of service, the material may be
recycled via reprocessing, down-cycling, or
energy recovery, proving its sustainability in
mobility.
Acknowledging the Swiss Commission for Technology and Innovation Grant No. 15091.1 PFIW-IW and Connova AG, EMS-CHEMIE AG for their support
The production of flax fibres requires much less
energy than the production of their synthetic
counterparts (flax fibres: ~6.5 MJ/kg, carbon
fibres: 289 MJ/kg and glass fibres: 48 MJ/kg).
However, a major challenge when working with
natural fibres is their variability (both stochastic
and seasonal) and their low degradation
temperature (Fig. 1).
Polypropylene (PP) as a matrix is attractive due
to its low cost and ease of processing; however,
it is less suited for high performance natural
fibre composites due to low mechanical
performance. Polymers such as co-
polyoxymethylene (coPOM), cellulose derived
thermoplastic (Cell_TP) and poly-L-lactide
(PLLA) can offer comparatively high stiffness,
strength, toughness and low creep. These
polymers have inherently high viscosities
(Fig. 2) and unfavourable wetting characteristics
compared to epoxy (EP).
Flax fibre thermoplastic layers with varying fibre
orientation may be processed into organo-
sheets. These sheets may be used to form
complex parts via a stamping process (Fig. 4),
for example a bicycle saddle shape (Fig. 5) in a
cost effective and fast process.
Surface functionalisation of the fibre further aids
impregnation of the polymer and fibre-matrix
adhesion. This enhances the mechanical
properties of the composite.
When exposing the flax fibre to a plasma and
impregnating them via a novel process, one may
achieve flax fibre composites with fibre volume
fractions of 55 % and void contents of less than
2 % (Fig. 3).
0.1 1 10 100 1000
0.1
1
10
100
1000
10000
EP
PP
PLLA
Cell_TP
coPOM
Viscosity(Pa.s)
Angular frequency (Hz)
Fig. 2 Viscosity as a function of frequency of the studied
matrix materials at 190 °C
Fig. 4 Schematic of thermoplastic composite process line
0 10 20 30 40 50 60 70 80 90
92
93
94
95
96
220 °C
215 °C
210 °C
205 °C
200 °C
190 °C
Weight(%)
Time (min)
180 °C
Fig. 3 Optical microscopy of flax fibre PLLA composite
By characterising the surface energies of
polymer melts and the flax natural fibres, one
may understand the physics of wetting and
spreading of the polymer during impregnation. Fig. 5 Flax fibre thermoplastic composite saddle shell

2. Institute of Polymer Engineering Klosterzelgstrasse 2 T +41 56 202 73 87 info.ikt.technik@fhnw.ch
CH-5210 Windisch F +41 56 202 75 74 www.fhnw.ch/technik/ikt
The ecological advantages of flax fibre reinforced thermoplastic composites make them an
attractive alternative to glass fibre reinforced polymer composites whilst having comparable
specific properties. In order for these materials to be adopted, we aim to develop natural fibre
composites with supreme toughness and impact resistance.
Flax Fibre Thermoplastic Composites: Performance
The high cellulose content (65-75 wt%) of flax
fibres provides high stiffness (up to 70 GPa) and
strength (700 MPa). In addition, thermoplastic
matrices are tough and damage tolerant (and in
some cases crack blunting or self healing) and
offer the possibility of rapid processing.
Within this study, several application specific
polymers were identified (stiffness, strength,
strain or impact performance driven):
polypropylene (PP), poly-L-lactide (PLLA),
cellulose derived thermoplastic (Cell_TP), co-
polyoxymethylene (coPOM) and epoxy (EP) as
a reference.
The impact performance of the polymers was
evaluated using the ISO 179 Charpy test
(Fig. 2). By tuning the polymer (Cell_TP) we
may develop impact resistance and lower notch
sensitivity.
Flax fibre composites also suppress vibration
efficiently due to their hierarchical microstructure
and discontinuous make up. For example, our
studies show that the characteristic loss factor is
up to 130 % higher than a carbon fibre
composite at room temperature. The creep
behaviour and environmental effects of the
developed natural fibre thermoplastic
composites are currently being studied.
0
50
100
150
200
Charpyimpactstrength
(unnotched)(kJ/m
2
)
Unnotched
Notched
EP
PP
PLLA
Cell_TP
coPOM
0
5
10
15
Charpyimpactstrength
(notched)(kJ/m
2
)
0
25
50
75
100
125
150
coPOM
Cell_TP
PLLA
PP
max
(MPa)
max
E
EP
0.0
2.5
5.0
E(GPa)
Fig. 1 Flexural properties of the studied matrices
Fig. 2 Charpy impact properties of the studied matrices
Fig. 5 Specific density, 50% volume fraction normalised flexural
transverse properties of the FF and GF composites
The developed composite materials were
characterised using longitudinal (Fig. 4) and
transverse (Fig. 5) flexure tests. The results are
highly dependent on quality of impregnation but
show that comparable properties to glass fibre
composites may be achieved.
0
100
200
300
FF/coPOM
FF/Cell_TP
FF/PLLA
FF/PP
FF/EP
GF/PP
maxlong.
(MPa)
max long.
EFlex long.
0
10
20
30
EFlexlong.
(GPa)
0
10
20
30
40
FF/coPOM
FF/Cell_TP
FF/PLLA
FF/PP
FF/EP
GF/PP
maxtrans.
(MPa)
max trans.
EFlex trans.
0
1
2
3
4
EFlextrans.
(GPa)
Fig. 4 Specific density, 50% volume fraction normalised flexural
longitudinal properties of the flax fibre (FF) and glass
fibre (GF) composites
Acknowledging the Swiss Commission for Technology and Innovation Grant No. 15091.1 PFIW-IW and Connova AG, EMS-CHEMIE AG for their support
Fig. 3 From flax plants and cellulose based polymers to natural fibre composite parts