Long-term closed-loop recycling of high-density polyethylene/flax composites

This work investigates the loss of performance and the recyclability of natural fibre composites for a long-term closed-loop process. Composites based on flax fibres and high-density polyethylene are subjected up to 50 extrusion cycles under constant processing conditions with or without maleic anhydride grafted polyethylene as a coupling agent. The results show that the addition of fibre increases the rigidity but decreases the elongation properties. The initial processing cycle leads to an important decrease of the fibre length and modification of the molecular weight distributions, thus indicating that the addition of fibre enhances chain scission and that fibre breakup mainly happens during the initial processing. The effect of recycling is much less significant, except for the mechanical properties. Negligible variations are observed for density, differential scanning calorimetry, thermogravimetric analysis, gel permeation chromatography and impact results. On the contrary, the mechanical properties are strongly affected by recycling as most of them increase with recycling. The addition of a coupling agent improves the composite properties, but this effect disappears with recycling. These trends are associated to a balance between fibre breakup and macromolecular chain scission compared to more homogeneous materials (better fibre distribution) taking place in the materials during recycling. The results show that long-term recycling of composites is possible as their overall performances remain acceptable.