Valorization of Velvet Bean Pod Agricultural Residues in Recycled HDPE Composites: The Role of Recycling Cycles and Maleic Anhydride

Abstract

The growing demand for plastics, combined with low recycling rates and poor waste management, has led to severe plastic accumulation. Concurrently, rising agricultural residues motivate efforts to valorize both waste streams. This study investigates recycled high-density polyethylene (rHDPE) reinforced with 20 wt % velvet bean pod (VBP) powder, with and without 1 phr maleic anhydride (MA) as a compatibilizer, across one, three, and five successive injection molding cycles. Mechanical, thermal, morphological, rheological, spectroscopic (FTIR), and flammability properties were examined. Without MA, repeated recycling induced chain scission and voids, reducing toughness and density while slightly increasing stiffness due to filler fracture. FTIR spectra of MA-modified composites showed stronger, shifted carbonyl peaks (1718–1740 cm^–1), confirming ester bond formation between anhydride groups of MA and hydroxyl groups of VBP and indicating improved interfacial adhesion and mechanical integrity. MA addition stabilized the melt flow index (1.3–1.5 g/10 min), preserved tensile strength (∼22 MPa), increased modulus (up to 558 MPa) and impact toughness (10.6 kJ/m²), reduced water absorption (0.39 to 0.17%), and maintained high crystallinity (∼81%). MA-modified composites also exhibited higher onset degradation temperature and achieved a UL-94 HB flame rating with lower burn rate (∼29 mm/min). The rHDPE5/VBP/MA composite demonstrated the best stiffness–toughness balance, confirming that agricultural residues can be upcycled with recycled plastics into dimensionally stable, reprocessable composites. This approach advances circular-economy goals, aligns with UN SDGs 12 and 13, and offers scalable potential for automotive, landscaping, and construction uses.