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

INTRODUCTION
This study presents a structural and chemical investigation of hemp fibers (Cannabis
sativa L.), which have been tested as reinforcement agents in biocomposites in recent
years (Madsen & Lilholt, 2003; Mwaikambo & Ansell, 2003). Hemp fibers are
advantageous compared to glass fibers with respect to lower density (1.56 g cm -3 ),
similar stiffness (50-70 GPa) and environmental sustainability. Hemp fibers are 1-3 cm
long with 60-70 % cellulose whereas wood fibers are 1-3 mm long with 40-50 %
cellulose. The part of the hemp plant, which is useful as reinforcement agents in
biocomposites, are the primary- and secondary single fibers (bast fibers) present in the
bark (cortex) of the plant (Garcia-Jaldon et al., 1998). Parenchyma cells separate bundles
of single fibers from one another. Between the single fibers, there are mainly
polysaccharides and lignin, which must be selectively removed to obtain fibers for
composites. Thus, both improvement of defibration methods and better understanding of
single fibers composition are required to develop stronger composites.
Transmission electron microscopy (TEM) has been used previously for observations on
mercerized hemp (18 % NaOH) (Purz et al., 1998). However it is difficult to prepare 100
nm thick transverse sections for TEM, so only limited studies have been performed on
untreated hemp. The microfibril angle to the fiber axis (MFA) is a major parameter that
affects the mechanical properties of plant fibers (Page & Elhosseiny, 1983). The MFA
has been determined for hemp fibers as 4° (Fink et al., 1999) and other non woody fibers
as 6-25° (Mukherjee & Satyanarayana, 1986) using X-ray diffraction providing an
average value across the cell wall.
This study concerns a thorough investigation of hemp fibers. The hemp stems were
defibrated by the mutated white rot fungus Phlebia radiata Cel 26, which is
characteristic due to its lack of cellulases. Histochemical investigations were made to
visualize the distribution of lignin, pectin and waxes in the hemp stem – and at the single
fiber level. Observations with scanning electron microscopy provided novel and detailed
information on the microfibril angle. Cell wall structure and MFA were further
characterized by partial decay using the soft rot fungus Phialophora mutabilis, which
develops cavities orientated along the cellulose microfibrils in plant fibers and wood
fibers (Khalili et al., 2000, 2001). Further structural knowledge was obtained by TEM
observations of partial cell wall decay patterns of P. radiata wild and P. radiata Cel 26.
Tensile tests of hemp fiber bundles followed by SEM microscopic investigation of the
fractured fiber ends revealed details on the microstructure and mechanical properties of
the cell wall.
MATERIAL STUDIED
Hemp (Cannabis sativa L.) variety Felina was sown May 7 th 2001 with 32 kg seed ha -1
and harvested October 17 th 2001. The site is located at 55°20’N, 11°10’E and 0-100 m
above sea level. The hemp stems were cut 10 cm above the soil surface and dried at 25-
30°C for six days. From the dried stems, the middle section 40-140 cm above the root
was investigated. Some stem pieces were treated using the fungi and others were not
treated before analysis.
92 Risø-PhD-11