Environmental durability of biochar-reinforced recycled polypropylene composites: Moisture transport, mechanical retention and microstructural analysis

The environmental durability under 720 h water immersion of recycled polymer composites is a key requirement for their application in humid service environments. This study investigates the influence of biochar on the moisture transport behaviour and mechanical durability of recycled polypropylene (rPP) composites fabricated by melt compounding followed by injection molding. Biochar derived from lignocellulosic biomass was incorporated at filler loadings of 0–8 wt.% to evaluate its effects on density, void content, water absorption, mechanical performance before and after water immersion, and fracture morphology. The composite density increased slightly with increasing biochar content, while the void content remained below 1%, indicating effective filler dispersion and good structural integrity. All composites exhibited exceptionally low water absorption (<0.25%) throughout the immersion period, demonstrating highly restricted moisture transport due to the hydrophobic nature of the rPP matrix, low porosity, and the diffusion barrier effect of biochar. The composite containing 4 wt.% biochar exhibited the optimum mechanical performance, achieving tensile and flexural strengths of 23.053 ± 0.31 MPa and 35.52 ± 0.37 MPa, respectively. Following water immersion, all composites retained more than 95% of their original mechanical properties, with strength reductions limited to approximately 2–5%, confirming excellent resistance to moisture-induced degradation. SEM analysis revealed uniform filler dispersion and strong interfacial adhesion at intermediate biochar loadings, whereas minor particle agglomeration was observed at higher filler contents. The findings establish a clear relationship between void content, moisture transport behaviour, and mechanical property retention, demonstrating that biochar serves as both an effective reinforcing filler and a moisture diffusion barrier for enhancing the environmental durability of recycled polypropylene composites for engineering applications under humid service conditions.