Mechanical performance and failure mechanisms of biochar-reinforced recycled polypropylene composites

Mechanical degradation during recycling limits the reuse of recycled polypropylene (rPP) in load-bearing applications. The incorporation of sustainable carbonaceous fillers such as biochar offers a promising strategy for improving the mechanical performance of rPP while maintaining environmental sustainability. In this study, biochar derived from mixed hardwood residues was incorporated into rPP at loadings of 0–8 wt.% to investigate its influence on the multi-mode mechanical behaviour. Composites were fabricated through injection-moulded process and mechanical performance was evaluated using tensile, flexural, Izod impact, short-beam shear (SBS), and punch shear tests. The results revealed a non-linear reinforcement response, with optimum performance achieved at 4 wt.% biochar loading. At this composition, the tensile, flexural, SBS, and punch shear strengths increased by ca. 12%, ca. 12%, ca. 2 3%, and ca. 11%, respectively, compared with neat rPP. These improvements were attributed to enhanced stress transfer and stronger filler–matrix interactions resulting from the uniform dispersion of biochar within the polymer matrix. In contrast, impact strength decreased progressively with increasing biochar content, indicating a stiffness–toughness trade-off typical of particulate-reinforced thermoplastics. At higher filler loadings (6–8 wt.%), the mechanical performance declined because of reduced filler dispersion, increased filler–filler interactions, and greater microstructural heterogeneity within the composites. Scanning electron microscopy of the optimum 4 wt.% biochar composite revealed a rougher fracture surface, improved filler–matrix adhesion, and evidence of crack-deflection mechanisms compared with neat rPP. The findings demonstrate that controlled incorporation of biomass-derived biochar can effectively upgrade recycled polypropylene into mechanically enhanced composite materials for lightweight engineering applications where moderate mechanical performance is required.