Fabrication and characterization of sustainable polyester biocomposites using Stevia rebaudiana stem fiber and bio-lignin from Annonamuricata fruit peel

This study investigates the fabrication and characterization of sustainable polyester biocomposites reinforced with Stevia rebaudiana stem microfibers and bio-lignin particles extracted from Annonamuricata fruit peel. The composites were fabricated through a hand lay-up process with varying lignin content (1 and 3 vol.%) and surface modification using the silane coupling agent 3-APTMS to improve interfacial adhesion. Mechanical, thermal, and environmental properties of the composites were evaluated according to ASTM standards. The results demonstrate significant improvement in mechanical performance with fiber and lignin incorporation. The silane-treated composite (M21) exhibited the highest tensile strength of 121 MPa, representing a ≈142% improvement compared with the neat polyester matrix (50 MPa). Similarly, flexural load capacity increased from ≈70 N to ≈170 N (≈143% improvement). Impact toughness improved by ≈72%, reaching 5.5 kJ/m2, while Shore-D hardness increased from 74 to 82 (≈10.8% enhancement). Thermal conductivity decreased from 0.29 W/m·K to 0.20 W/m·K (≈31% reduction), indicating improved insulation behaviour. Swelling behaviour also decreased significantly, with the silane-treated composite showing a ≈37% reduction compared with the neat matrix. Although fiber incorporation increased biodegradation slightly, silane treatment effectively reduced moisture sensitivity and degradation rate. SEM analysis confirmed improved fiber–matrix interfacial bonding in treated composites. Overall, the developed biocomposites demonstrate enhanced mechanical performance, improved thermal insulation, and controlled environmental degradation, making them promising materials for lightweight structural and insulating applications.