Effect of Alkali and Silane Surface Treatments on Mechanical, Hygroscopic and Thermomechanical Properties of Kenaf Fibre Reinforced Epoxy Composites

as sustainable alternatives to glass-fibre reinforced polymers in semi-structural applications, driven by natural fibres’ lower density (1.30–1.55 g/cm³ versus 2.54 g/cm³ for E-glass), renewable origin, CO₂ sequestration during growth, lower abrasive tool wear during machining, and end-of-life biodegradability. Kenaf (Hibiscus cannabinus L.), a bast fibre crop cultivated extensively in Maharashtra, Gujarat, and Rajasthan, produces fibres with specific tensile strength approaching 60–70% of E-glass’s at one-half the density, making kenaf/epoxy composites theoretically competitive with glass-fibre laminates on a specific property basis. However, the inherently hydrophilic nature of kenaf cellulose—arising from abundant hydroxyl groups on the fibre surface—results in poor fibre-matrix interfacial adhesion in hydrophobic epoxy systems and significant moisture uptake that degrades mechanical properties under humid service conditions. This study evaluates the effectiveness of two surface treatment protocols—sodium hydroxide (NaOH, 5 wt%, 2 h alkali treatment) and 3-aminopropyltriethoxysilane (silane, 1 wt% aqueous solution)—in improving the mechanical, hygroscopic, and thermomechanical performance of short kenaf fibre/epoxy composites at 5, 10, 15, 20, 25, and 30 wt% fibre loadings. Flexural strength, Young’s modulus, Charpy impact strength, water absorption (Fickian diffusion analysis over 28 days), TGA, and DMA are characterised for all combinations. The silane-treated 20 wt% composite achieves optimal performance: flexural strength of 89 MPa (112% improvement over pure epoxy), Young’s modulus of 5.6 GPa, impact strength of 66 kJ/m², and 28-day water absorption of 1.06%—substantially reduced from 1.41% for the untreated equivalent. FTIR and SEM analysis confirm that both treatments successfully remove hemicellulose and surface wax, increasing surface roughness and hydroxyl-group reactivity, with silane coupling producing additional covalent Si-O-C bonds at the fibre-matrix interface.