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

fibre mats, which are made by air-drying and needle punching (Figure 17-left) (Dobel,
2002; Eriksen and Pallesen, 2002) with uncontrolled and thereby random fibre
orientation. Production of aligned fibre assemblies can also be done at laboratory scale
by combing the fibres in water, stretching them and air-drying them (Paper IV).
Figure 17. Retting and defibration processes, used to make fibre mats and aligned fibre
assemblies of fibres from hemp stems.
6.1.1 Defibration
During water retting, the hemp bast is separated into large fibre bundles (Paper IV).
Additional treatment is required to defibrate the fibre bundles into single fibres and small
fibre bundles useful for production of hemp yarn. It is done by degradation or disruption
of the middle lamellae between the single fibres. Enzyme treatment (Paper I; Brühlmann
et al., 2000; Madsen et al., 2003), wet oxidation (Thomsen et al., 1999; Paper I) and
NaOH treatment (Mwaikambo and Ansell, 1999; Wang et al., 2003) can degrade pectin
and lignin in the middle lamellae between the single fibres.
Physical defibration methods include steam explosion (Paper I; Garcia-Jaldon et al.,
1998; Madsen et al., 2003; Vignon et al., 1996) and ultrasound treatment (Zimmer and
Kloss, 1995). It is suspected that these treatments introduce damage to the fibres. Fungal
treatment of hemp stems has the advantage of degrading lignin, so single fibres can be
obtained without a defibration step (Paper II). This procedure has been used for
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