Screw extrusion technology in waste plastic recycling

Screw extrusion stands as a cornerstone technology in the mechanical recycling of waste plastics, a critical endeavor in addressing the global plastic pollution crisis. This review provides a comprehensive analysis of the multifaceted role of screw extrusion in this domain. It begins by elucidating the fundamental principles of extruder design and operation, specifically comparing single-screw and twin-screw systems and their respective suitabilities for processing diverse plastic recyclates. A significant portion is dedicated to polymer-specific degradation phenomena - thermal, thermo-mechanical, and thermo-oxidative - that occur during extrusion, detailing their impact on common polymers such as polyethylene terephthalate (PET), polyolefins (polyethylene and polypropylene), polyvinyl chloride (PVC), and polystyrene (PS, HIPS). Subsequently, the review explores a wide array of quality improvement strategies essential for enhancing the properties and value of extruded recyclates. These include pre-extrusion feedstock preparation, in-process melt filtration and devolatilization, and post-extrusion modifications through additive incorporation (antioxidants, acid scavengers, light stabilizers) and reactive extrusion (chain extension, compatibilization, filler addition). The report also critically examines the industrial challenges inherent in the screw extrusion of waste plastics, including feedstock variability and contamination, process control intricacies, economic viability concerns, and energy efficiency. Finally, an outlook on future trends is presented, covering advancements in extrusion technology, the importance of Design for Recycling, the evolving interface between mechanical and chemical recycling, and innovations in additives for upcycling. The review synthesizes current knowledge to provide an expert-level understanding, aiming to inform research, industrial practices, and policy development toward more effective and sustainable plastic recycling.