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

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Figure 7. A: Model of transverse hemp stem section zooming to single fibres, secondary
cell wall and finally the cell wall lamella. B: Model of the microfibril orientation
throughout the secondary cell wall (Paper II).The S3 layer was not found in hemp fibres.
5.2.2 Structure on the fibre bundle level (1-100 μM)
The cortex part of the hemp stems in Felina contains bundles of 100-300 polygonalshaped
primary and secondary single fibres with 4 to 6 sides (Figure 7; Figure 9; Paper
II). The single fibres are long (5-55 mm) compared with the fibres in the xylem (0.2-0.6
mm; Table 2; Vignon et al., 1995). The primary fibres nearest the stem surface are large
(cell wall thickness = 7-13 μm; length = 20 mm; Sankari, 2000). These fibres are formed
at the early growth stage during the phase of rapid stem elongation and contribute in
Fedora, Felina and Futura to 92-95% of the bast fibres located in the cortex (Mcdougall
et al., 1993; Sankari, 2000). The secondary fibres near the cambium layer are smaller
(cell wall thickness = 3-6 μm; length = 2 mm; Sankari, 2000) and only present in the
thick part of the stem (Mcdougall et al., 1993; Figure 9b).
The average area of the transverse fibre section including cell lumen was calculated to
780 μm 2 ± 300 μm 2 and the lumen fraction in the fibres to 9% ± 7% (Paper II). The
variation in area is due to actual variation in fibre size and not due to inaccurate
measurement. Therefore the load carrying part of the single fibers is high (91%)
compared with wood and straw fibres with larger lumens. Jute fibres and flax fibres have
also small lumens (Cichocki and Thomason, 2002; Henriksson et al., 1997).
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