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

Figure 2. A: Model of transverse stem section zooming to single fibers, secondary cell
wall and finally the cell wall lamella structure. B: Model of the microfibril orientation
throughout the secondary cell wall.
Architecture of the hemp stem and the single fibers
SEM observations of the cortex showed polygonal-shaped primary and secondary single
fibers with 4 to 6 sides (Figure 1A) arranged in fiber bundles of 10-40 fibers. The
primary single fibers nearest the stem surface were largest (cell wall thickness = 7-13
μm) and were formed at the early growth stage (Mcdougall et al., 1993). The secondary
single fibers near the cambium layer were smaller (cell wall thickness = 3-6 μm) and
only present in the thick parts of the stem at the late growth stage (Figure 1B). The area
of the transverse fiber section including cell lumen (Af,, Equation 1) was 780 μm 2 ± 300
μm 2 .The single fibers had small lumina (lumen width = 0.5-5 μm) so cell wall synthesis
appears to continue until nearly all the cytoplasm space has been used. The lumen
fraction in the fibers (L) was 9 % ± 7 %. Therefore the load carrying part of the single
fibers is high (1-L = 91 %) presumably resulting in high tensile strength compared to
wood fibers with larger lumina.
Figure 3. Single fibers partially decayed by fungi (transverse sections). A: P. mutabilis
decay of the thick layers in S2. B: P. radiata (wild) decay of the cell wall from the lumen
side. C: P. radiata Cel 26 decay of the thin layers between the thick concentric S2 layers.
96 Risø-PhD-11