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

8.2 Effect of pre-treatment of hemp
Steam explosion used for pre-treatment of hemp fibres resulted in single fibre strength
reduced with 30% and lower standard deviation. That indicates that mainly the strong
single fibres are weakened or degraded so the fraction of weak fibres increases (Madsen
et al., 2003).
The effect of temperature during the treatment was only significant when defibration
with pectin degrading enzymes was used for defibration before the steam explosion
experiments. Thereby it was not critical if the treatment was performed at 185°C or
200°C, presumably due to low cellulose degradation at this temperature level. This
combination of defibration procedures resulted in reduced variation on the single fibre
strength (950±230 MPa; Figure 29) and more cellulose rich fibres (78%; Figure 26).
Enzyme treatment of hemp fibres resulted in increased standard deviation of the single
fibre strength but with no significant reduction in strength (1150±550 MPa). The larger
standard deviation will presumably have a negative effect on the effective fibre strength
in composites due to the bundle effect explained with the Weibull distribution.
Defibration of hemp by cultivation of P. radiata Cel 26 resulted in decrease in fibre
bundle strength from 960 MPa to 850 MPa (Figure 30) due to extraction of most of the
non-cellulosic binding materials (Figure 26b).
Single fibre strength [MPa]
2000
1500
1000
500
0
Raw hemp Mild STEX Hard STEX Enzyme E-mild STEX E-hard STEX
Only steam explosion Enzyme tr. followed by steam explosion
Figure 29. Single fibre strength of enzyme treated and steam exploded hemp fibres (mild
STEX = 2 min at 185°C; hard STEX = 2 min at 200°C (Madsen et al., 2003).
Defibration of hemp by cultivation of C. subvermispora resulted in weaker fibres (780
MPa; Paper IV) presumably due to cellulose decay. The hemp yarn had a lower strength
(Madsen, 2004) but with lower standard deviation. Barley straw had roughly the half
strength (280 MPa) of hemp fibres presumably due to the lower cellulose content (45%)
and the different fibre structure (Table 2; Liu et al., 2005).
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