Mechanical, Water Absorption, and Thermal Characterisation of Jute–Sisal Hybrid Natural Fibre Reinforced Epoxy Composites

Natural fibre reinforced polymer (NFRP) composites have gained accelerating research attention as sustainable, lightweight alternatives to glass and carbon fibre reinforced plastics in semi-structural applications including automotive interior panels, agricultural equipment housings, and low-cost construction components, driven by growing regulatory and consumer pressure to reduce lifecycle environmental impact in the manufacturing sector. Jute (Corchorus capsularis) and sisal (Agave sisalana) are two of the most abundantly available bast and leaf natural fibres in India, with annual production of approximately 1.8 million tonnes and 0.3 million tonnes respectively, and their epoxy-matrix composites have individually attracted substantial research interest. However, systematic comparative data on hybrid laminates combining both fibre types under varying fibre volume fractions and surface chemical treatments, evaluated across the full spectrum of structural (tensile, flexural, impact, interlaminar shear), environmental (water absorption, thermal stability), and microstructural (FTIR, SEM fracture surface) properties, remains limited — particularly under the temperature and humidity conditions representative of central and northern Indian operating environments. This study fabricates and characterises hand-layup jute/epoxy, sisal/epoxy, and hybrid jute-sisal/epoxy laminated composites at fibre volume fractions of 5, 10, 15, 20, 25, and 30 vol%. Alkali (NaOH) treatment at 5% and 10% concentrations, silane coupling agent treatment, and benzoyl peroxide chemical treatment are evaluated for their effect on fibre-matrix interfacial adhesion quantified through interlaminar shear strength (ILSS) and compared against untreated controls. Water absorption kinetics, thermogravimetric analysis up to 600°C, and FTIR spectroscopy on treated and untreated fibres complete the characterisation matrix. Results confirm that the hybrid 20 vol% composite with 10% NaOH-treated fibres achieves peak tensile strength of 86 MPa (105% improvement over neat epoxy), flexural strength of 118 MPa (103% improvement), and Charpy impact energy of 14.6 J — the highest values among all compositions tested. Alkali treatment at 10% NaOH concentration removes hemicellulose and surface wax, confirmed by FTIR, improving ILSS by 75% relative to untreated hybrid laminate.