Recycling flax fibre composites by using a chemical solvolysis process in near- and supercritical conditions: linking fibre integrity to micro-mechanical performance

The growing demand for bio-based composites has intensified the need for efficient recycling strategies that preserve fibre integrity and reduce environmental impact. In this study, unidirectional flax-reinforced composites with epoxy and poly-amide 11 (PA 11) matrices are manufactured and subsequently recycled using a solvolysis-recycling based process under near- and supercritical conditions in a semi-continuous flow reactor. A water/ethanol mixture (50/50 in mol) is employed as solvent at 250°C and 300°C under 25 MPa. The recovered flax fibres are characterised using scanning electron microscopy (SEM), nanoindentation (NI) and atomic force microscopy (AFM). Mechanical tests on the original composites demonstrate tensile properties consistent with the best values reported in the literature, supported by low porosity levels (∼2%). After solvolysis, flax fibres recovered from PA 11 composites exhibit clean surfaces, preserved cell wall organisation, and nano-mechanical properties close to virgin fibres. In contrast, fibres from epoxy composites experience substantial degradation, especially at 300°C, with pronounced ultrastructural damage and a 65.8% reduction in indentation modulus. AFM confirms localised degradation patterns and lumen-driven damage pathways. These results highlight the superior recyclability of Flax – PA 11 systems and the intrinsic limitations of Flax – Epoxy composites for fibre-preserving solvolysis. Overall, the study provides composite manufacturers and recyclers with new insights into fibre-scale degradation mechanisms and supports the development of more circular, eco-efficient bio-composite architectures.