Hygrothermal ageing and mechanical degradation behaviour of alkali-treated sunn hemp fibre/bio-epoxy composites

The long-term durability of natural fibre-reinforced bio-composites under hygrothermal environments remains a major barrier to their deployment in building and construction. This study provides a methodical evaluation of the mechanical, viscoelastic, and chemical degradation mechanisms of bio-epoxy composites reinforced with alkali-treated sunn hemp fibres (SHFs) under accelerated hygrothermal ageing (60 °C, 98% RH, up to 3,000 h). Fibre modification was performed using sodium hydroxide (NaOH) concentrations of 6%, 8%, and 10% for 2 h to investigate the influence of treatment severity on durability. Moisture uptake followed Fickian diffusion behaviour, with the 8% NaOH condition exhibiting the lowest equilibrium moisture content (13.5%) due to controlled removal of amorphous constituents and improved fibre–matrix compatibility. While tensile and interlaminar shear strength (ILSS) decreased with exposure time, a treatment-dependent trade-off was identified: 8% NaOH maximised initial mechanical performance, whereas 10% NaOH provided comparatively superior long-term strength retention. Dynamic mechanical analysis (DMA) demonstrated sustained viscoelastic stability, with the 8% system retaining a storage modulus of 7 GPa at ~ 25 °C (glassy region) and a glass transition temperature of 83 °C after prolonged exposure. Fourier-transform infrared (FTIR) analysis revealed moderated carbonyl development in optimally treated systems, linking chemical stability to mechanical retention. These results establish a conceptual framework describing the balance between interfacial strengthening and ageing resistance in SHF-reinforced bio-composites, offering predictive insights for service-life design in low-carbon construction applications.