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

solution (pH 4) for 4 h at 85°C. Lignin was oxidized and extracted in chlorite solution
for 1 h at 75°C (300 ml water+10 g NaClO2+1 ml 100% acetic acid). Hemicellulose was
extracted in 100 ml 12% NaOH+2% H3BO3 solution for 90 minutes at 25°C. After each
step the suspension was filtrated on pre-weighed glass filters and the filter cake washed
in water. The contents were determined from the dry matter loss in the extraction steps
by drying overnight at 60°C and weighing after each step. The mineral content was
determined by incineration of 1 g raw sample at 550°C for 3 h.
Tensile tests on fibre bundles (15 mm long lf) were done with a 3 mm test span lspan at a
strain rate of 1.7×10 -3 s -1 using an Instron 5566 with pressley clamps [Type: Stelometer
654 from Zellweger Uster] (Thygesen et al., 2005a). Following fracture the fibre pieces
of 15 mm were weighed wf. The number of repetitions was at least 13 for each sample.
The failure stress σfu was calculated based on the failure force Ffu and the fibre density
ρf:
σ
F l × ρ
fu
f f
fu = = Ffu
×
Equation 1
Af
w f
The fibre density for the investigated hemp fibres and hemp yarn is 1.58 g/cm 3 (Madsen
2004; Thygesen 2006). The maximum density for pure and fully crystalline cellulose is
in the range 1.61 to 1.64 g/cm 3 , based on crystallographic information (Nishiyama et al.,
2002; Thygesen et al., 2005b).
Preparation of composites
The fibre bundles of 6 – 25 cm length were wetted in water and aligned by carting with a
comb. In cases of short fibre pieces (6 cm), the aligned fibres were placed on top of each
other in three layers with different fibre-end positions to allow stress transfer between
the fibres and reduce movement of the fibres in the epoxy flow when the lay-up was
being compressed. The hemp yarn composites were made both by hand carting and
filament winding (Madsen & Lilholt, 2003). The fibre content in the composites was
controlled by the displacement rate during winding. The frame was displaced at a rate
that allowed 11 rotations/cm. The epoxy resin (SPX 6872) and hardener (SPX 6873)
were mixed in a ratio of 100 g resin to 36 g hardener and poured over the fibres and left
for 5 – 10 minutes to allow impregnation. Spacers of 1.5 mm thickness were used to
control the laminate thickness. Tape was placed beside the fibres to avoid fibre
movement. Finally, a steel plate was placed on the mould and excess resin with air
bubbles was squeezed out with clamps. The lay-up was pre-cured at 40°C for 16 hours
and thereafter cured at 120°C for 6 hours.
Analysis of the composites
Standard dumb-bell-shaped tensile test specimens (180 × 20 mm with gauge section of
25 × 15 mm) were cut from the laminates. Tensile tests were performed at room
temperature on an Instron machine with crosshead speed of 1 mm/min. The strain was
measured with two extensometers placed on each side of the test specimens.
Composite structure and fibre bundle size were investigated by SEM-microscopy.
Composite pieces were polished on the side perpendicular to the fibre direction using
wetted silicon carbide paper [Struers, Denmark] in a series of decreasing roughness
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