Flax fibre physical structure and its effect on composite properties: Impact strength and thermo-mechanical properties

Abstract

Earlier investigations by the authors showed that the tensile modulus of flax fibre mat polypropylene composites (NMT) could surpass the values of glass mat reinforced thermoplastic (GMT) on fibre weight basis. The tensile and flexural strength could reach values of up to 65% of the GMT strength values, however, very much dependent on the fibre physical structure. This study deals with the Charpy impact and the thermo-mechanical properties of flax NMT materials. The trend is that the Charpy impact strength decreases with increasing fibre internal bonding and enhanced fibre-matrix adhesion, which is opposite to the trend for the tensile and flexural properties. The impact strength of the NMT materials is lower than generally reported for GMT materials. Dynamic mechanical thermal analysis reveals that with increasing temperature the storage modulus of the NMT materials reduces more slowly when the fibre internal bonding and the fibre-matrix adhesion are improved. In order to approach the tensile, flexural and impact strength of GMT materials, composites should be based on the strong elementary flax fibres. The axial tensile strength of elementary fibres approaches the strength of glass fibres and the lateral strength of the elementary fibres is higher than the technical flax fibres lateral strength. The thermo-mechanical properties can probably be improved when non-cellulosic material can be removed from the flax fibre surface without damaging the fibre.