Despite the advantages offered by natural fibre-based thermoplastic composites in terms of environmental impact and cost, their mechanical performance is generally lower than that of synthetic counterparts. Hybridisation with mineral fibres (basalt) can broaden the industrial applications of natural fibre reinforced composites. The present study focused on the performance of injection-moulded short basalt fibre, hemp fibre and hemp/basalt fibre hybrid high density polyethylene (HDPE) composites. Effects of a maleated coupling agent on the thermal and mechanical properties of the resulting composites were evaluated as a function of the fibre mass fraction. Hybridisation of hemp fibres with basalt fibres was found to significantly increase the mechanical properties and the crystallinity of hemp-fibre reinforced composites thus suggesting that short hemp/basalt fibre hybrid HDPE composites are promising candidates for semi-structural applications. Additionally, a sizing removal procedure mimicking the conditions experienced in an end-of-life composite thermal recycling process was defined and discussed in terms of residual mechanical properties of basalt/HDPE composites.

The Achilles heel of thermoplastic natural fibre composites is their limited durability. The environmental degradation of the mechanical properties of hemp and hemp/basalt hybrid-reinforced high-density polyethylene (HDPE) composites has been investigated with a special focus on the effects of water ageing and accelerated ageing, including hygrothermal and UV radiation. Modification of the matrix was carried out using a maleic anhydride high-density polyethylene copolymer (MAPE) as a compatibilizer. Hybridization of hemp fibres with basalt fibres and the incorporation of MAPE were found to significantly decrease the water uptake (up to 75%) and increase the retention of mechanical properties after accelerated ageing. Secondary crystallization phenomena occurring in the composites, as confirmed by differential scanning calorimetry (DSC) analysis, were able to counteract the severe combined effects of hygrothermal stress and UV radiation, with the exception of hemp-fibre composites where permanent damage to the fibres occurred, with 2% and 20% reduction in tensile strength and modulus, respectively, for a 30 wt % hemp fibre-reinforced HDPE.

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