Properties of hemp fibre polymer composites -An optimisation of ...

Table 2. Tensile strength (σ) and elastic modulus (E) of hemp fibres tested in bundles
and in epoxy-composites (Thygesen et al., unpublished) and the chemical composition of
the hemp fibres with or without the P. radiata Cel 26 defibration.
Fibre source Fibre bundle tests Composite tests
Mechanical properties σ (MPa) E (GPa) σ (MPa) E (GPa)
Raw hemp 960±220 23±5 450±120 72±12
Treated hemp, liquid culture 850±240 20±3 590±90 90±17
Chemical composition (% w/w) Cellulose Hemicellulose Lignin Pectin Rest
Raw hemp 64 15 4 6 11
Treated hemp, liquid culture 77 15 2 1 5
Treated hemp, soil culture 76 15 2 3 6
The potential of hemp fibers as reinforcing agents in composite materials
The histochemical investigation showed that pectin degradation can provide separation
of the fiber bundles from the xylem, but separation of the fiber bundles into single fibers
requires lignin degradation (Table 1). Hemicellulose degradation should be avoided to
prevent disintegration of the fibers into microfibrils resulting in lower fiber bundle
strength (Morvan et al., 1990). Practically no change occurred in the hemicellulose
content during the P. radiata Cel 26 treatment (Table 2). Epidermis free and cellulose
rich (77 % w/w) hemp fiber bundles were produced on the 100 g scale by the P. radiata
Cel 26 treatment. This could not be done by enzymatic treatment with Flaxzyme
(Thygesen et al., 2002) since enzymes presumably cannot penetrate the wax-covered
epidermis.
The single fibers were completely penetrated by the low viscosity acrylic resin used as
embedding material for TEM microscopy even though the fibers were arranged in
bundles. Bundle arrangement of fibers with small lumina has also been reported for jute
(Cichocki & Thomason, 2002). Since penetration of the epoxy polymer was complete in
composites made with jute fibers, the same should be possible using hemp fibers.
The elastic modulus of 25 GPa has previously been measured in single hemp fibers
(Madsen et al., 2003). On fiber bundle level, the P. radiata Cel 26 treatment resulted in
decreases in stiffness from 23 GPa to 20 GPa and in tensile strength from 960 MPa to
850 MPa caused by extraction of most of the non-cellulosic binding materials (Table 2).
Based on the microfibril angle, the cell wall stiffness is lowest in the S1 layer and
highest in the inner 70-90 % of the S2 layer (Figure 1B; Davies and Bruce, 1997).
Cracks between the concentric layers (Figure 4A, B) observed in the fractured fibers
were presumably formed by interlaminar stress.
Composites have been made with hemp fibers from stems treated with P. radiata Cel 26.
Fiber strength and stiffness of 590 MPa and 90 GPa, respectively, were calculated based
on the composition of these composites (Thygesen et al., unpublished; Table 2). The
same properties of composites with fibers from untreated hemp were only 450 MPa and
72 GPa due to a poor binding between the hemp bast fibers and the epoxy resin caused
by the wax rich cuticle layer. The higher fiber stiffness in composites gives evidence for
polymer penetration into the fibers like that reported for flax (Hepworth et al., 2000),
which glues the cell wall aggregates together and forms a stiff structure. This
compensates for the loss of stiffness and tensile strength induced by degradation of non
cellulosic components (Table 2). The obtained fiber stiffness and strength were thereby
25-32 % higher, when the treated fibers were used.
Risø-PhD-11 101