Influence of PA11 and PP thermoplastic polymers on recycling stability of unidirectional flax fibre reinforced biocomposites

In this study, we explore the interest of recycling PA11 and PP unidirectional flax fibre composites. After describing the mechanical performance of the various components of the composite, its tensile properties are investigated before and after recycling. The results indicate a good stability of the modulus and strength of the composites during the multiple recycling steps, which also applies to the fibre properties, as highlighted by nanoindentation tests throughout the recycling process. However, interestingly, the elongation at break of the PA11 composites shows a significant increase with the number of injection cycles, ranging from 5.2 to 14.7%. Further analyses show that the PA11 matrix has highly stable thermal, rheological and mechanical properties, so the matrix cannot be involved in this increased elongation at break. Flax fibers display a steady decrease in length with recycling, especially in the case of PA11 matrix compared to PP matrix. This drastic decline is explained by the high viscosity of the matrix, but above all by the presence of defects (kink bands) in the flax fibers, spaced at intervals of 60 μm, which is consistent with the residual lengths measured at the end of recycling. PA11 and PP flax UD composites prove to be very efficient after recycling, while PA11 biocomposites exhibit an increase of elongation after recycling that makes it a material of first choice in the biocomposites industry.