Linking Process‐Induced Degradation to Mechanical Property Deterioration in Flax Fiber–Reinforced Bio‐Polymer Matrix Composites

ABSTRACT The drawback of biobased polymer matrix composites (PMCs) is their limited temperature stability, resulting from degradation, which restricts their processability in established composite manufacturing processes requiring elevated temperatures. These key issues not only affect the mechanical properties but ultimately limit the utilization of flax fibers as fiber reinforcement in PMCs. In this study, kinetic models for the thermal degradation of flax fibers and PA11 are derived and combined with a model for thermo‐chemical fiber degradation. Selective degradation of the fibers and mechanical testing establishes a link between degradation and the accompanying deterioration of the mechanical performance. The deterioration of flax fiber mechanical properties under concurrent thermal and thermo‐chemical degradation is primarily governed by the thermos‐chemical contribution (chain scission) up to 3% thermal degradation, beyond which the influence of thermal degradation becomes evident. Even 1% thermal degradation of flax fibers results in a pronounced reduction in their mechanical performance. In contrast, equal degradation values enhance the PMCs' strength, which may be attributed to improved fiber‐matrix interactions. Compiling results into processing maps establishes a framework for designing tailored processing of temperature‐sensitive materials, offering transfer opportunities to individual processing conditions and heat treatments, enabling broader research on bio‐based PMCs.