Valorization of walnut shell biochar in hemp fibre reinforced isosorbide epoxy/eugenol benzoxazine copolymer composites: physical, water absorption, mechanical, and thermal properties

Growing eco-awareness drives research into natural filler-reinforced composites, but hydrophobicity and incompatibility of fillers with matrices are significant challenges. This study investigates the effects of walnut shell biochar (WSBC) as an additive filler in silane-treated hemp fibre (SHF) composites produced with a bio-based epoxy/benzoxazine copolymer via an isothermal compression molding technique. The WSBC particles having 43.66 μm average particle size were produced in a N 2 atmosphere pyrolysis at 400 °C. Produced composites were tested for mechanical, thermal, and thermomechanical properties. The water absorption was significantly reduced as the loading of WSBC share was enhanced. The optimal mechanical properties were recorded on 12 wt% WSBC particle reinforcement, including tensile strength (87.47 ± 3.98 MPa), Young’s modulus (5.66 ± 0.16 GPa), flexural strength (167.47 ± 4.89 MPa), flexural modulus (6.51 ± 0.24 GPa), and impact strength (18.51 ± 0.54 kJ/m 2 ). The highest storage modulus and the lowest damping factor values were observed for the WSBC15 specimen. Moreover, thermal stability was gradually enhanced, and the highest char yield (53.46 ± 0.15%) was recorded for the WSBC15 specimen. Results confirm that these lightweight and eco-friendly composites are promising for structural applications, sporting goods, and internal parts of automotive and aerospace industries.