Toward recycling polyolefin wastes via single‐step solid‐state friction extrusion

Abstract Plastic recycling facilities deal with single‐ and mixed‐stream plastic waste. Clean single‐stream waste is sold as clean flake, or the flake is converted to pellets for material handling. The flake conversion process uses a conventional melt‐phase extruder to convert extruded filaments that are cooled and then pelletized. Friction extrusion (FE), a solid phase processing technique that has successfully extruded metal and metal matrix composites with desired properties, has never been utilized to address its ability to process plastic wastes. In this study, FE was performed on single‐stream low‐density polyethylene (LDPE) and single‐stream polypropylene (PP). Consolidated filaments of 2.5 mm diameter were extruded from different polymers. The energy efficiency of FE was estimated based on extrusion rates. A potentially 50%–80% reduction in energy cost was calculated for FE compared to the conventional melt extrusion process. Thermal properties of pellets and extruded filaments were measured to evaluate the effects of FE process on the molecular and morphological structures of LDPE and PP. The thermal properties of PP before and after FE were not significantly changed. For LDPE, the melting and crystallization temperatures increased by 3 and 7°C while the degree of crystallinity decreased by 3%, probably associated with side chain realignment under shearing. Highlights Direct extruded polyolefin into filaments without melting via friction extrusion. Estimated over 50% energy reduction compared melt extrusion process. This novel mechanical process does not degrade polyolefin material.