Influence of Coupling Agents and a Carbodiimide-Based Hydrolysis Stabilizer on the (Thermo)Mechanical Properties and Water Degradation of Poly(L-Lactide)- and Polyamide 11-Based Flax Fiber-Reinforced Composites

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Abstract This study examines the thermal and (thermo)mechanical properties of flax twill-reinforced composites (FFRCs) with a fiber volume content of approx. 51 vol%. The bio-based thermoplastics poly(L-lactide) (PLLA) and polyamide 11 (PA11), as well as a PLLA/PA11 blend were used as matrices. Their influence on the performance of the FFRCs was investigated after fabrication (pristine) and water absorption (1000 h immersion). Young’s moduli of approx. 16 GPa (PLLA) and 13 GPa (PA11) were reached. The specific Young’s moduli are similar for bending criterions due to PA11’s lower density. Water absorption proceeded faster for PLLA-based FFRCs, which could be attributed to PLLA’s tendency for crack formation which facilitates water penetration. In addition, commercially available glycidyl methacrylate- and maleic anhydride-based coupling agents and a carbodiimide polymer-based hydrolysis stabilizer were blended into the matrices to improve the fiber-matrix bond and reduce delamination during moisture absorption-related fiber swelling. This might cut channels for water penetration and decrease the deterioration of the mechanical properties. The mechanism proved effective in FFRCs with the more brittle PLLA matrix, where water absorption was delayed by the coupling agents and the decline in strength was slightly reduced at saturation. The hydrolysis stabilizer showed by far the most significant effect probably due to its strong adhesion-promoting and ductility-enhancing properties. Water absorption was delayed, reduced over the long term, and the tensile (0°, 45°) and flexural strength were improved both before and after water exposure. However, the additives caused a strong reduction of PLLA‘s heat deflection temperature.

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