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

Figure 5. Microfibril angles (MFA) in fungal colonized and delignified single fibers
shown with black lines and white arrows: A: Crossing cavities parallel with two different
MFA in S2. B: A third MFA identified as dark and winding cavities in S1. C: P. radiata
Cel 26 treated fiber with the S1 layer partly stripped off revealing the underlying S2
layer.
Histochemical composition of the structure on hemp stem and single fiber level
Lignin was determined with the Wiesner - and Mäule reactions (Figure 6). The Wiesner
reaction stains lignin red by reaction of phloroglucinol with hydroxycinnamyl aldehydes
(Strivastava, 1966). The Mäule reaction stains lignin red by oxidation of syringyl lignin
with KMnO4-HCl due to the produced 3-methoxy catechol (Wu et al., 1992). The inner
part of the single fiber secondary wall was lignin poor as shown by negative Wiesner –
and Mäule reactions (Table 1). The outer part of the cell wall was lignified according to
positive Wiesner – and Mäule reactions. Lignin was present at highest concentration in
the compound middle lamellae. No lignin was present in the cambium while the
epidermis contained some lignin (Figure 6). According to the similar intensities found by
the Wiesner - and Mäule reactions, the lignin composition appears similar in the xylem
and single fibers.
Hemp belongs to the Angiosperm phylum since it has vessel elements. It is
eudicotyledon like hardwoods, numerous bushes and herbs, since it has two cotyledons
(germinating leaves). Thus it contains hardwood lignin of coniferyl alcohol, sinapyl
alcohol and a minor content of p-coumaryl alcohol. The first lignification step occurs in
the compound middle lamellae, which has the same high lignin content in thick-walled
and thin walled fibers (Figure 6). The second lignification step occurs during the
synthesis of the outer part of the S2 layer. The lignin synthesis appears reduced in later
stages of fiber development due to the low lignin content in the inner part of the S2 layer.
98 Risø-PhD-11