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

DISCUSSION
The defibration of hemp fibres by cultivation of P. radiata resulted in fibres with higher
cellulose content than water retted hemp and pectinase treated hemp (Thygesen et al.,
2002) due to increased degradation of pectin and lignin. The cellulose crystallinity
tended to decrease with the cellulose content obtained by the fungal treatments (Figure
3) showing that during processing part of the crystalline cellulose is converted into its
amorphous form. A similar chemical composition to P. radiata defibrated hemp has been
obtained by wet oxidation (Thygesen et al., 2002; Thomsen et al., 2005), which is also an
oxidative process resulting in decreased cellulose crystallinity (Thygesen, 2006).
The machine winding and hand lay-up techniques resulted in roughly the same
composite strength (Figure 6a), showing that the hand lay-up technique results in the
same optimal fibre alignment generally obtained by machine (Madsen & Lilholt, 2003).
The fibre length was an important parameter for both calculation of the fibre strength and
stiffness due to the high risk of composite failure near the fibre ends, when these are
located in the strained specimen section. Therefore, the calculated fibre strength
decreased by 1.23 times for composites based on the short fibre length (Figure 6a) and
further reduction might occur if the fibre ends are not at random positions.
The fibre strength and stiffness were in general compared based on back calculation
from composite data since this involves identical test specimens, accurate measurement
of the force and accurate measurement of the strain. However this will cause inaccuracy
due to the effect of fibre length and fibre-matrix interface. The advantage with the fibre
bundle tensile test is that the fibre strength is measured in a more direct manner.
However since the test samples are small and of variable size and geometry it was
decided to rely on the composite data.
The composite strength for the laminate with 35% v/v hemp yarn (230 MPa; Figure 6a),
was slightly lower than for flax yarn based composites (251 MPa; Vf = 43% v/v)
(Madsen & Lilholt, 2003). However the calculated fibre strength was lowest in the flax
yarn (575 MPa). The lower strength in composites with P. radiata defibrated hemp
fibres (174 MPa, Figure 6a) was due to the lower cellulose content, the lower fibre
content (32 % v/v) and the lower failure strain of the fibres resulting in lower matrix
stress of 25 MPa compared with 39 – 43 MPa (Table 4). Investigations reported by
Hepworth et al. (2000b) with raw hemp bast and water retted hemp in epoxy-composites
have shown much lower composite strength (80 – 90 MPa) using 20% v/v fibres leading
to a fibre strength of 290 – 340 MPa, which can be due to incomplete fibre alignment.
The composite stiffness has been found to 27 GPa in composites containing 43 % v/v
flax fibres (Madsen & Lilholt, 2003). This corresponds to a fibre stiffness of 58 GPa,
which is close to the stiffness for hemp yarn that was calculated in this study (60 GPa).
Even though the hemp fibres were handled with mild methods like hand peeling and
fungal defibration, a higher calculated fibre strength than 643 MPa was not obtained,
which is similar to the strength of traditionally produced hemp yarn (677 MPa). The
similar results are surprising and indicate that the final fibre strength is not very
dependent on the defibration method applied (Table 4).
Risø-PhD-11 137