Morphological modification of the technical flax fibre bundles to improve the longitudinal tensile properties of flax fibre reinforced epoxy composites

Résumé : As far as the tensile properties of natural fibres as reinforcements for composites are concerned, flax fibres will stay at the top-end. However, an efficient conversion of fibre properties into their corresponding composite properties has been a challenge, due to the fibre damages done through the conventional textile methods utilised to process flax. These techniques impart disadvantageous features onto fibres at both micro, and mesolevel, which degrade the mechanical performances of flax fibre reinforced composites, FFRC. Undulation of fibre is one of those detrimental features that occur during traditional fibre extraction and fabric manufacturing routes. The undulation or waviness causes micro compressive defects or kink bands in elementary flax fibres, which significantly undermines the performance of FFRC. Manufacturing flax fabric with minimal undulation could diminish the micro compressive defects up to a substantial extent. In this research, nonwoven flax tapes of highly aligned flax fibres, blended with a small proportion of PLA, Polylactic Acid have been manufactured deploying a novel technique. Composites reinforced from those nonwoven tapes have been compared with composites reinforced with woven Hopsack fabrics and warp knitted unidirectional,UD fabrics from flax that are comprised of undulating fibres. The composites reinforced with the highly aligned tape have shown 49 percent higher fibre bundle strength, and 100 percent higher fibre bundle stiffness in comparison with that of the Hopsack fabric reinforced composites. The results have been discussed in the light of fibre undulation, elementary fibre individualisation, homogeneity of fibre distribution, extent of resin rich areas, and impregnation of the fibre lumens.