Kenaf Fibre-Reinforced Polyolefin Composites

The study was undertaken to compare the mechanical properties and moisture absorption of ground kenaf fibre thermoplastic composites with conventional wood flour thermoplastic composites. A 40 wt% reinforcement loading was used. Both high density polyethylene (HDPE) and polypropylene (PP) were used as the matrix materials. The composites were compounded by extrusion and then hot pressed into sheet. Composites were made with and without maleic anhydride (MA) coupling agent. A 50 wt% untreated kenaf fibre/polypropylene matrix composite made from hot pressed comingled mat was also examined.
The kenaf fibre composites were found to have generally superior mechanical properties to their wood fibre counterparts with higher tensile and flexural moduli as well as higher impact strength. However there was no significant improvement in either the tensile strength or the flexural strength. The polypropylene composites were stiffer and stronger than the polyethylene composites but the impact strength was substantially lower. The addition of MAPE and MAPP coupling agents substantially increased the tensile modulus and strength. There was a more modest improvement in flexural strength but no significant improvement in flexural modulus. Impact strength was unaffected by the coupling agents in the PP composites but substantially reduced in the HDPE composites. The effects of coupling agent addition were more pronounced for the kenaf fibre composites.
Both the kenaf fibre and wood flour composites absorbed 8-9% moisture when moisturized to equilibrium. However, moisture uptake occurred more slowly in the kenaf fibre composites. Moisturization caused approximately 40% degradation in the mechanical properties of the composites with the effect being slightly worse in the wood flour composites than in their kenaf fibre counterparts. The effect of moisturization was also more pronounced for the PE composites than the PP composites while the composites made with coupling agents were more affected than those made without coupling agent.
The kenaf composites fabricated from comingled mat had similar tensile properties to the other kenaf fibre composites when the difference in fibre fraction was taken into account, but had greatly improved impact strength.
Figure 1. SEM micrographs showing fracture surface of kenaf fibre reinforced PP composite.
N. Soatthiyanon, S. Deng, A. Crosky and A. Beehag
This research was undertaken as part of the research program of the Cooperative Research Centre for Advanced Composite Structures Ltd.