Upcycling of Diaper and Toothpaste Plastic Waste into Sustainable Biocarbon-Reinforced Composites for Automotive Applications

This study investigates the development of sustainable lightweight composites for automotive applications by combining recycled polyolefins and wheat straw-derived biocarbon. Recycled polypropylene (rPP) from disposable diapers and recycled high-density polyethylene–polypropylene blends (rHDPE-PP) from toothpaste tubes were melt compounded with wheat straw biocarbon produced via pyrolysis at 600 °C and 950 °C. The effects of biocarbon pyrolysis temperature and compatibilization using maleic-anhydride-grafted polyolefins on the resulting composite performance were evaluated. Among the base polymers, rHDPE–PP exhibited the highest tensile strength, which may be attributed to the typically higher crystallinity and closer chain packing of HDPE, whereas rPP demonstrated superior flexural strength, consistent with its more rigid molecular backbone. Blending rPP with rHDPE–PP yielded intermediate flexural, impact, and heat deflection properties while retaining a tensile strength similar to that of rHDPE–PP. Incorporation of biocarbon into the rPP/rHDPE–PP blend increased both tensile and flexural stiffness, with the 950 °C biocarbon producing the greatest enhancement as a result of its higher carbon content and structural rigidity, despite its correspondingly higher structural disorder as indicated by Raman analysis. However, these stiffness gains were accompanied by a reduction in impact strength, attributed to the brittle nature of biocarbon and local stress-concentration effects. Thermal stability improved upon biocarbon incorporation. The composite containing 950 °C biocarbon reduced the coefficient of linear thermal expansion (CLTE) in the flow direction by up to 25% and increased the heat deflection temperature from 74.1 ± 6.6 °C to 77.3 ± 1.0 °C, indicating improved dimensional stability under thermal loads. Maleic anhydride-grafted polypropylene (MA-g-PP) and polyethylene (MA-g-PE) compatibilizers were incorporated to enhance filler–matrix adhesion. Compatibilization with 3 phr MA-g-PP enhanced tensile strength by 7% and flexural strength by approximately 12% compared to the uncompatibilized composite. The integration of recycled polymers and renewable biocarbon produced composites with balanced mechanical and thermal performance, offering a viable route to valorize post-consumer waste streams into functional materials for automotive and other engineering applications.