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

Figure 8. Lignin (a), pectin (b) and wax (c) stained in transverse sections of hemp stem.
Lignin stained red by the Mäule reaction (a) (arrows = strongly lignified part of the cell
wall). Pectin stained by Ruthenium Red (b) (arrows = pectin rich compound middle
lamellae). Wax stained by Sudan IV (c) (arrows = wax rich cuticle layer).
The microstructure of the fibre wall in the hemp fibres and the decay patterns caused by
fungal cultivation on hemp were investigated by transmission electron microscopy. The
fibre middle lamellae (ML) and primary wall (P) were found to have thickness of 30-50
nm and 70-110 nm, respectively (Figure 10A, B). The secondary cell wall was composed
of a 100-130 nm thick S1 layer and a 3-13 μm thick S2 layer (Paper II). Both SEM and
TEM observations showed that the S2 layer had a laminate structure of 1 to 4 concentric
layers of 1-5 μm in thickness (Figure 9C-D; Figure 10B-C). These layers were
constructed of 100 nm thick lamellae. Thin layers of 200-240 nm in thickness were
located in between the concentric layers (Figure 10A). These thin layers seems to lack
cellulose, since they were selectively degraded by the mutated white rot fungus P.
radiata Cel 26, which does not degrade cellulose at significant degree (Nyhlen and
Nilsson, 1987; Figure 10C).
Inter-laminar cracks were also observed in the fibre wall after fibre bundle test showing
that the interface between the wall-layers is weak (Paper II). The cracks might have a
negative effect on the overall fibre strength due to lack of stress transfer through the cell
wall.
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