Mechanical, thermal conductivity, dielectric, and water absorption performance of vinyl ester composites reinforced with Allium microfibre and activated macadamia nut biochar

Abstract The present study investigates the development of sustainable vinyl ester composites reinforced with Allium hollandicum stem microfibers and activated biochar derived from macadamia nut shells. Activated biochar was produced through pyrolysis at 500 °C followed by KOH activation at 700 °C, while the extracted microfibers were incorporated as natural reinforcement. Hybrid composites containing a constant 30 vol% microfiber and varying biochar contents (0–7 vol%) were fabricated using a hand lay-up process. Mechanical, thermal conductivity, dielectric, and water absorption properties were systematically evaluated. The results revealed that the composite containing 30 vol% microfiber and 3 vol% activated biochar (D3) exhibited optimum mechanical performance, achieving a tensile strength of 87 MPa, flexural load of 335 N, and impact strength of 11.3 kJ/m², corresponding to improvements of 67.3%, 76.3%, and 85.2%, respectively, compared with neat vinyl ester resin. Thermal conductivity increased progressively with biochar loading, reaching 0.56 W/m·K for D5, representing a 143.5% enhancement. Dielectric constant increased from 1.95 to 3.78 at 100 Hz, while water absorption decreased significantly from 5.2% to 1.5% after 168 h immersion. The findings demonstrate that Allium microfiber and activated macadamia biochar can effectively produce eco-friendly multifunctional composites suitable for lightweight structural, thermal management, and electrical insulation applications.