Separation and characterisation of elementary kenaf fibres as reinforcement in high-density polyethylene-matrix composites and tensile behaviour of flax fibres as reinforcement in vinyl ester-matrix composites

The work involved studies of two separate aspects of natural fibre composites. The first part of the work examined the use of high aspect ratio short fibres in thermoplastic matrix composites. This required that the technical fibres were first broken down into elementary fibres. Kenaf fibres were used in this part of the study with high-density polyethylene (HDPE) as the matrix. Several different treatments were examined for isolating the elementary fibres from the technical fibres, the most successful being 60% nitric acid treatment and 20% hydrogen peroxide/glacial acetic acid treatment. The hydrogen peroxide/acetic acid treatment produced full length elementaries with an average length of 2.3 mm and an average aspect ratio of 180. However, the nitric acid treatment caused fragmentation of the elementary fibres resulting in an average length of only 0.2 mm and an average aspect ratio of 15. It also caused an increase in the defect density of the fibres. Both treatments increased the cellulose crystallinity but caused some oxidation of the fibres. The elementary fibres were used to produce HDPE composites with a 40 wt% fibre fraction. The behaviour of these composites was then compared with that of composites of the same fibre fraction, prepared using chopped technical fibres with an average length of 0.7 mm and an average aspect ratio of 8. Breakup of the elementaries occurred during extrusion reducing the fibre length by as much as a factor of 10. However, the chopped technical fibres were unaffected. No improvement in modulus or strength over that of the chopped fibre composites was obtained for the nitric acid treated fibre composites, due to the very low fibre aspect ratio. However, an improvement of 20% in both modulus and strength was obtained for the hydrogen peroxide/acetic acid treated fibre composites. The second part of the work examined the suitability of using the results obtained from single fibre testing and from flat fibre bundle testing to predict the tensile properties of unidirectional composites made from the fibres. Flax fibres were used in this part of the study. Unidirectional composites were prepared with a fibre volume fraction of 25% using vinyl ester as the matrix resin.The modulus and strength obtained by backing out the fibre properties from tensile data obtained from the composites were both within 7% of those obtained from single fibre testing, once proper account was taken of the true fibre cross sectional shape and the effect of fibre length on fibre strength. The flat fibre bundle tests gave values which were only 43% of the single fibre data. However, the scatter in results was much lower indicating that this test may be useful for assessing batch to batch variation