Each year, increasing amounts of plastic waste are generated, causing environmental pollution and resource loss. Recycling is a solution, but recycled plastics often have inferior mechanical properties to virgin plastics. However, studies have shown that holding polymers in the melt state before extrusion can restore the mechanical properties; thus, we propose a twin-screw extruder with a molten resin reservoir (MSR), a cavity between the screw zone and twin-screw extruder discharge, which retains molten polymer after mixing in the twin-screw zone, thus influencing the polymer properties. Re-extruded recycled polyethylene (RPE) pellets were produced, and the tensile properties and microstructure of virgin polyethylene (PE), unextruded RPE, and re-extruded RPE moldings prepared with and without the MSR were evaluated. Crucially, the elongation at break of the MSR-extruded RPE molding was seven times higher than that of the original RPE molding, and the Young’s modulus of the MSR-extruded RPE molding was comparable to that of the virgin PE molding. Both the MSR-extruded RPE and virgin PE moldings contained similar striped lamellae. Thus, MSR re-extrusion improved the mechanical performance of recycled polymers by optimizing the microstructure. The use of MSRs will facilitate the reuse of waste plastics as value-added materials having a wide range of industrial applications.

The degradation of mechanical properties is the most challenging point for the development of plastic mechanical recycling processes. Remelting and shear deformation contained in the mechanical process are a part of degradation in recycled plastics. In this study, virgin high-density polyethylene (HDPE) was simulated to be recycled by remelting and treating with shear deformation being measured at different shear treatment rates (0–100/s) using a cone-plate rheometer. The obtained shear treatment product was remolded as a thin film. The evaluation was performed comparing virgin HDPE (VPE) without any processing with shear-treated HDPE with various shear treatment rates. Tensile property, X-ray crystallography, and morphological observations were performed in order to investigate the relationship between mechanical properties, thickness of lamellar size, and the morphology of shear-treated HDPE as compared to VPE. It can be found that the elongation at break of shear-treated HDPE at a high shear treatment rate (100/s) was significantly decreased from VPE. This degradation mechanism was related to the decreased degree of crystallinity, thickness of the crystalline layer, intermediate layer, and occurrence of crystalline orientation. This study expected to explain the degradation mechanism of HDPE from shear deformation which can be further improved by the processing conditions of the mechanical plastic recycling process.