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

Photosynthesis:
Solar energy + CO2+H2O Hemp plant Fibres
Defibration +
Decortication
Shives
Composite
Hydrolysis +
Fermentation
Burning materials
Car
Ethanol
Transport
Figure 1. The carbon cycle of the application of the hemp plant for both material- and
ethanol production.
The current application of plant fibres in composites is for non-structural components
with random fibre orientation. These components are used by the automotive industry
and the building industry (Broge, 2000; Clemons, 2000; Karus et al., 2002; Parikh et al.,
2002). For example, flax fibres are used instead of synthetic fibres as reinforcement of
polypropylene in inner panels for cars. This type of usage is primarily driven by price
and demand of ecological awareness, and to a lower extent by the reinforcing effect of
the fibres (Bledzki et al. 2002; Kandachar, 2002). The next step is to attract industrial
interest to use plant fibres in load bearing materials in for example cars.
Bast fibres from hemp possess by nature a high variability in structure. Fundamental
structural characterization is therefore needed before hemp fibres can be used as
reinforcement in composite materials. The steps the hemp stems have to go through to
get fibres for reinforcement of composites have to be improved too. Traditionally field
retting is used for defibration to degrade the binding between the bast fibres on the stem
surface and the shives (Meijer et al., 1995), resulting in fibres of variable quality
dependent on weather conditions and the attacking microorganisms. The retted stems are
mechanically decorticated to isolate the fibres. The decortication equipment has not been
developed sufficiently, so the production rate and fibre quality are not as good as
required for production of fibres for advanced purposes. Thus, milder and more effective
defibration and decortication processes must be developed before hemp fibres can
compete with synthetic fibres.
4.1 The aim of the thesis
The aim of the thesis was to study the morphology and the chemical composition of
hemp fibres and relate these properties to the composite properties obtained with hemp
fibres as reinforcement. The developed methods for determination of the fibre properties
were used to investigate the tested pre-treatment methods. For the fibre production to be
feasible it is essential to use pre-treatment conditions, which maximise the yield of fibres
and introduce as low damage as possible to the fibres. Biological pre-treatment was used
since it has the advantage of being performed at low temperature and thereby cause low
thermal and physical damage to the fibres. The traditional bio-treatment of the hemp
stems, water retting (Meijer et al., 1995; Rosember, 1965) was compared with cultivation
of the mutated white rot fungus P. radiata Cel 26, which does not degrade cellulose to a
significant degree. The treated and untreated hemp stems were hand peeled, since it is a
mild decortication procedure for production of fibres compared to mechanical methods.
12 Risø-PhD-11