Properties of hemp fibre polymer composites - An optimisation of fibre properties using novel defibration methods and fibre characterisation

Characterization of hemp fibres was carried out with fibres obtained with low handling damage and defibration damage to get an indication of how strong cellulose based fibres that can be produced from hemp. Comparison was made with hemp yarn produced under traditional conditions where damage is unavoidable. The mild defibration was performed by degradation of the pectin and lignin rich middle lamellae around the fibres by cultivation of the mutated white rot fungus Phlebia radiata Cel 26. Fibres with a cellulose content of 78% w/w could thereby be produced which is similar to the cellulose content obtained by steam explosion of hemp fibres prior defibrated with pectin degrading enzymes. The S2 layer in the fibre wall of the hemp fibres consisted of 1-4 cellulose rich and lignin poor concentric layers constructed of ca. 100 nm thick lamellae. The microfibril angle showed values in the range 0-10° for the main part of the S2-layer and 70-90° for the S1-layer. The microfibrils that are mainly parallel with the fibre axis explain the high fibre stiffness, which in defibrated hemp fibres reached 94 GPa. The defibrated hemp fibres had higher fibre stiffness (88-94 GPa) than hemp yarn (60 GPa), which the fibre twisting in hemp yarn explains. The hemp fibre stiffness appeared to increase linearly with cellulose content and crystallinity and to decrease with cellulose twisting angle. Pure crystalline cellulose had an estimated stiffness of 125 GPa. The defibration with P. radiata Cel 26 resulted in fibre strength of 643 MPa, which is similar to the strength of traditionally produced hemp yarn (677 MPa) even though mild processing was applied. The plant fibre strength seemed therefore to be linearly dependent on the cellulose content and not clearly dependent on the introduced physical damage during handling and defibration. Pure cellulose appeared to have effective strength of 850 MPa that is about 10% of the strength on the molecular level.