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

7 Defibration methods
Defibration methods for production of fibres from the hemp plant can be divided into
combined physical and chemical methods and biotechnological methods. The combined
physical and chemical methods combine chemical reactions and physical disruption
obtained at high pressure and high temperature. Procedures of this type include wet
oxidation and steam explosion (Thomsen et al., 2005; Thygesen et al., 2002; Paper I).
The biotechnological methods include water retting (Meijer et al., 1995; Paper IV),
enzymatic defibration (Paper I) and fungal defibration (Henriksson et al., 1997; Paper
IV).
7.1 Combined physical and chemical treatments
Both wet oxidation and steam explosion of hemp fibres increased the fraction of
cellulose and decreased the fraction of lignin and non cell-wall materials like pectin and
water extractives in the hemp samples (Figure 24). The amount of lost dry matter in a
sample was an indication of the amount of lost cellulose, which in general was 12-17%
w/w higher than the cellulose loss (Table 5).
In wet oxidation, the losses of dry matter were smaller (15-29 %) than in steam
explosion (17-73 %). Correspondingly losses of cellulose were smaller (0-12 %) than in
steam explosion experiments (0-69 %). Base addition resulted in large removal of
cellulose so it might have a too strong effect on opening the structure of the plant
constituents. The losses were in general higher when raw hemp fibres were treated (a)
than when water-retted hemp fibres were treated, since some pectin and water extractives
are extracted during the retting step (Paper IV).
The content of cellulose in the fibres increased by wet oxidation or steam explosion in
retted hemp fibres from 73% to 85-90% and in raw hemp fibres from 60-64% to 73-75%
(Figure 24). The most lignin was removed when oxidative conditions were applied since
it is decomposed to low molecular phenolic compounds and oxidized to carboxylic acids
(Klinke et al., 2002; Figure 25).
Impregnation with base before steam explosion reduced the hemicellulose extraction and
increased the cellulose extraction (Thomsen et al., 2005). It has been shown that
degradation of hemicellulose monomers to furans and carboxylic acids is reduced by
base addition (Klinke et al., 2002).
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