Sustainable Composites From Almond Shell Powder and Recycled Polyolefin Blends: Effect of Compatibilizer on Interfacial Adhesion and Properties

ABSTRACT This study reports injection‐molded sustainable composites developed from a hybrid recycled polymer matrix reinforced with agricultural food waste as a biofiller. A 50:50 (w/w) blend of automotive recycled polypropylene and recycled polyolefins derived from industrial pipe‐scrap waste was combined with almond shell powder (ASP) and processed via melt extrusion at 190°C followed by injection molding. Maleic anhydride‐grafted polypropylene was incorporated to enhance polymer–filler interfacial adhesion. Increasing ASP content up to 30 wt.% significantly improved tensile and flexural moduli, heat deflection temperature (HDT), and decreased the linear thermal expansion coefficient (CLTE), reflecting the inherent stiffness of ASP. Rheological analysis showed increased complex viscosity at low frequencies, indicating enhanced polymer–filler interactions. Compatibilization further enhanced tensile strength (~16%), flexural strength (~24%), flexural modulus (~10%), and HDT (~10%), while reducing CLTE (~7.4%) relative to uncompatibilized systems. The composites retained high notched Izod impact strength (> 50 J m ⁻¹ ), indicating a favorable balance between stiffness and toughness. Scanning electron microscopy confirmed improved filler dispersion and interfacial bonding. A quantitative correlation between void fraction and stiffness efficiency factor was established, yielding a maximum efficiency of 3.87 with a minimum void content of 0.06%. The sustainable composites maintained low density (< 1 g cm ⁻³ ) and high notched Izod impact strength (> 50 J m ⁻¹ ), indicating their suitability for lightweight automotive interior applications.