PLA coating effect on water aging of basalt and basalt/flax fiber composites

This study investigates the effectiveness of a biodegradable polylactic acid (PLA) coating in mitigating water-induced ageing of basalt fibre and hybrid basalt/flax fibre epoxy composites, addressing the limited literature on sustainable protective coatings for mineral–natural fibre hybrid systems exposed to marine and humid environments. Composite laminates, with and without PLA coating, were immersed in distilled water and salt water for 30 and 120 days at room temperature. Water absorption behaviour, flexural modulus degradation, and thermal–chemical stability of the coating were analysed through gravimetric measurements, three-point bending tests, DSC and FTIR analyses. Water immersion tests showed that the PLA coating significantly reduced moisture uptake in both composite systems. After 120 days, the flexural modulus reduction of coated basalt composites was limited to 4% in distilled water, compared to 8–9% for uncoated samples, while in hybrid composites exposed to salt water the modulus reduction decreased from 20% (uncoated) to 12% (PLA-coated). The protective effect of PLA was particularly evident during the early ageing stage (30 days), where coated samples retained flexural modulus values comparable to unaged laminates. Thermal and spectroscopic analyses revealed environment-dependent degradation mechanisms of the PLA coating, with enhanced degradation in salt water for basalt composites and a recrystallisation increase of about 10% in hybrid systems. Long-term drying tests on hybrid composites highlighted a more pronounced flexural stiffening in uncoated laminates due to hornification of flax fibres, whereas the PLA coating moderated this effect by limiting moisture ingress. These results demonstrate that PLA represents a viable, eco-friendly alternative to conventional synthetic coatings for improving the durability of fibre-reinforced composites in marine, automotive and sustainable structural applications.