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

RESULTS
The effect of defibration on hemp fibres
The defibration of hemp by cultivation of P. radiata resulted in hemicellulose, pectin
and lignin degradation, and the cellulose content in the fibres increased to 78% after 14
days of cultivation (Table 2). The content of crystalline cellulose in the fibres increased
from 60 to 66 g/100 g dry matter (DM) due to the increased cellulose content after the
fungal cultivation (Table 2). However the cellulose crystallinity decreased from 94 to 84
g/100 g cellulose due to conversion of crystalline cellulose into its amorphous form. The
recovery of cellulose was 87% after 14 days using medium scale cultivation (Table 3) so
there appeared to be decay of crystalline cellulose. Using small-scale experiments for 31
days there was no further cellulose decay due to the constant fibre yield (Table 3).
Previous observations with transmission electron microscopy have also shown the fibre
wall to remain intact (Thygesen et al., 2005a). The fibre bundle strength was reduced
from 950 MPa in raw hemp bast to 820 MPa after the fungal defibration (Table 2).
The defibration of hemp fibres by cultivation of C. subvermispora resulted in cellulose
degradation as shown by the cellulose recovery of 87% after 14 days cultivation (Table
3) and by further reduction of the fibre yield after 31 days cultivation (Table 3). The
fungus formed a lot of mycelia above the water surface, while hemp stem pieces below
the surface were not colonised and the obtained cellulose content was 72% (Table 2).
The fungus grew well with low and medium scale cultivation resulting in reduction of
pH from 6 to 4.6 (Table 3). The fibre bundle strength was reduced by the cultivation to
780 MPa (Table 2). At large-scale cultivation, sterilisation was insufficient resulting in
lower fibre yield after 14 days cultivation (27 g/100 stem) and unchanged pH in the
culture broth (Table 3).
Lignin degradation was obtained during the cultivation of P. radiata on the hemp due to
peroxidase enzymes, as suggested by the presence of H2O2 in the cultivation broth (Table
3). Smaller hemp fibre bundles were also obtained by cultivation of P. radiata
(3×10 3 μm 2 ) than by water retting and by C. subvermispora cultivation (Table 2), due to
the greater degradation of the lignin rich middle lamellae between the fibres. Water
retting was a faster process than fungal defibration (Table 1) due to the lacking of
sterilisation and the higher temperature (35°C). However, a more variable quality was
obtained due to inhomogeneous colonisation by bacteria and fungi resulting in lower
fibre bundle strength (590 MPa, Table 2) and unchanged cellulose crystallinity (Figure
3).
128 Risø-PhD-11