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

The hemp fibres and the Norway spruce had tensile strength approximately linearly
increasing versus the cellulose content showing that pure cellulose has 850 MPa in
tensile strength (Figure a). Previous investigations by Klinke et al. (2001) have shown
that the strength of pure cellulose is 600 MPa and that the fibre strength increases with
cellulose content squared. The different conclusion in this study is due to that aligned
fibres were used as reinforcement, which makes it easier to calculate the correct fibre
properties than when randomly orientated fibres are used. The determined strength of
pure cellulose was about 10% of the theoretical strength reported by Lilholt (2002).
Therefore, strength reduction seems to occur from the molecular level to the single fibre
level (8000 MPa →1200±400 MPa; Madsen et al., 2003) and from the single fibre level
to the fibre assembly level (1200±400 MPa → 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 (Thygesen et al., 2005a) and
insufficient binding strength between the reinforcing cellulose, hemicellulose, lignin and
pectin (Morvan et al., 1990). The fibre damage introduced by the fungal defibration is
mainly caused by enzymatic degradation of pectin and lignin. This may lead to
degradation of the inter microfibril bonding in the fibres (Thygesen et al., 2005a).
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 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 & Lambros, 2002). It has also been suggested
that mild handling and defibration result in high single fibre strength but with larger
scatter, counteracting the higher fibre strength in fibre assemblies (bundles) (Bos et al.,
2002). These facts explain the similar strength in the composites with traditionally
produced hemp yarn and mildly defibrated hemp fibres. This investigation shows thereby
that the effective fibre strength is 677 MPa in hemp yarn and 643 MPa in the P. radiata
defibrated hemp fibres (Table 4) which is within a narrow range, and high compared to
literature data on hemp fibres (300-800 MPa) and similar to literature data for flax fibres
(500-900 MPa) (Lilholt & Lawther, 2000).
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 8b). 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 3; Thygesen et al., 2005b). 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.
138 Risø-PhD-11