The manufacture and mechanical testing of thermosetting natural fibre composites

High volume fraction hemp and flax fibre composites were manufactured using low viscosity epoxy and phenolic resins. Using 80% volume fraction of flax fibres in epoxy resin, composites with a mean stiffness of 26 GPa and a mean strength of 378 MPa were produced. By reducing processing damage of the plant fibres mechanical properties could be increased by 40%. Strips of retted fibre tissue were found to be just as effective for reinforcement as fibre bundles and individual fibres. Phenolic resin and decorticated flax fibres produced very poor composites. Using 40% volume fraction of fibres the mean stiffness was 3.7 GPa and the mean strength was 27 MPa. Two fibre pre-treatments were devised to improve adhesion with resins. The first, 6 M urea was used only in natural fibre-epoxy composites where it increased the stiffness but not the strength. The second pre-treatment was a 50% PVA solution, which was cured prior to the addition of space filling resin. The PVA treatment improved the stiffness and strength of both natural fibre-epoxy composites and natural fibre-phenolic composites.