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

9.4 Effect of cellulose structure on fibre mechanical properties
Even though the hemp fibres were handled with mild methods like hand peeling and
fungal defibration, higher fibre strength than 643 MPa was not obtained, which is similar
to the strength of traditionally produced hemp yarn (677 MPa). The similar results are
surprising and indicate that the final fibre strength is not very dependent on the
defibration method applied (Table 8).
The hemp fibres and Norway spruce had tensile strength approximately linear increasing
versus the cellulose content showing that pure cellulose has 850 MPa in tensile strength
(Figure 40a). The determined cellulose strength was about 10% of the obtainable
strength in agreement with Lilholt (2002). Therefore, strength reduction seems to occur
from the molecular level to the single fibre level (8000→1200±400 MPa; Madsen et al.,
2003) and from the single fibre level to the fibre assembly level (1200±400 → 643±111
MPa).
Single fibres are presumably weakened due to flaws like kinks inside the fibres (Bos et
al., 2002), weak interface between the fibre wall concentric lamellae (Paper II) and
insufficient binding strength between the reinforcing cellulose, hemicellulose, lignin and
pectin (Morvan et al., 1990). Fibre assemblies are weakened due to variations in single
fibre strength as explained by Weibull analysis (Lilholt, 2002) describing the fact that the
fibre bundle strength is always lower than the average strength of the same fibres
(Coleman, 1958). For flax fibres and cotton fibres, the bundle efficiency has been found
as 0.46 – 0.60 indicating that the effective strength of many fibres in for example a
composite is roughly half the single fibre strength (Bos et al., 2002; Kompella and
Lambros, 2002). It has also been suggested that mild handling and defibration result in
high single fibre strength but with larger scattering counteracting the higher fibre
strength. These facts explain the similar strength in the composites with traditionally
produced hemp yarn and mildly defibrated hemp fibres.
The fibre stiffness increased with the cellulose content in the fibres obtained by the
fungal defibration and water retting. A linear dependence on the crystalline cellulose
content could be established (Figure 40b). The hemp yarn had lower stiffness than
implied from the cellulose content, which may be due to the high twisting angle
introduced during the spinning process. It has been stated, that increasing twisting angles
decrease fibre stiffness (Page et al., 1977).
The wood fibres had lower stiffness, which can be explained by the low cellulose
crystallinity (60 – 70%) compared with the hemp fibres (90 – 100%; Figure 14).
Amorphous cellulose, hemicellulose, lignin and pectin are expected to have lower
stiffness than crystalline cellulose, which are linear molecules orientated in the test
direction resulting in high stiffness. In contrast, the plant fibre stiffness appeared to
increase linearly versus the cellulose content to 125 GPa for pure crystalline cellulose.
64 Risø-PhD-11