Valorization of coal and hybrid bio-composites as a reinforcement for HDPE composites: Synergistic-property relationships and performance analysis

This study explores the development of sustainable HDPE-based hybrid bio-composites reinforced with bean-pod (BP) and soybean fibre (SF) agricultural wastes, coal mineral waste (CO), and maleic anhydride (MAH) as a compatibilizer. The objective was to evaluate the combined effects of CO and MAH on the physical, mechanical, and structural properties of the composites. Hybrid composites containing 70 wt.% HDPE, 10 wt.% BP, and 10 wt.% SF were fabricated with varying CO (3–10 wt.%) and MAH (1–7 wt.%) contents using compression moulding. The results showed that HDPE/10BP/10SF/8CO/2MAH (PTS5) achieved the highest density (1.35,251 g/cm 3 ), while HDPE/10BP/10SF/3CO/7MAH (PTS7) exhibited the lowest water absorption, indicating improved moisture resistance. The highest stiffness values were recorded for HDEP/10SF/10CO (PTS1) (89.4 MPa) and HDPE/10BP/10SF/9CO/1MAH (PTS4) (82.4 MPa), demonstrating effective stress transfer and enhanced fibre–matrix bonding. MAH improved stiffness at optimal concentrations, although excessive MAH reduced performance. All composites showed increased crystallinity, confirming the role of the fillers in promoting crystalline structure formation within the HDPE matrix. Overall, the synergistic use of coal waste, agricultural fibre residues, and optimized MAH content significantly enhanced composite performance, highlighting their potential as sustainable, eco-friendly materials for applications in construction, automotive, packaging, agriculture, electrical components, and consumer products.