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.

Plastic mechanical recycling is an attractive method for reducing the amounts of waste plastics. However, the alterations in the mechanical properties (degradation) in recycled plastics is a limitation to the material’s mechanical recycling. In this study, the mechanical recycling was enhanced by the addition of a “molten resin reservoir” unit at the end of the twin-screw extruder. Recycled polypropylene (RPP) obtained from a household was re-extruded with this developed extrusion unit. The tensile properties, type of crystalline, and conformation of polypropylene polymorphs were evaluated and compared for virgin polypropylene (VPP), recycled polypropylene (RPP) without extrusion (RPP-original), and RPP with extrusion by using a new type of extruder (RPP-extrusion). It could be found that the tensile properties of RPP-extrusion were improved, so as to be similar to those of VPP. In addition, the conformation of RPP-extrusion was similar to that of VPP by increasing the ratio between the helix and parallel band. This study succeeded in regenerating the tensile properties and inner structures in recycled PP, which could prolong the used lifetime and decrease the amount of waste from single-use plastic.

Mechanical recycling is the most efficient way to reduce plastic pollution due to its ability to maintain the intrinsic properties of plastics as well as provide economic benefits involved in other types of recycling. On the other hand, molecular dynamics (MD) simulations provide key insights into structural deformation, lamellar crystalline axis (c-axis) orientations, and reorganization, which are essential for understanding plastic behavior during structural deformations. To simulate the influence of structural deformations in high-density polyethylene (HDPE) during mechanical recycling while paying attention to obtaining an alternate lamellar orientation, the authors examine a specific way of preparing stacked lamella-oriented HDPE united atom (UA) models, starting from a single 1000 UA (C₁₀₀₀) chain of crystalline conformations and then packing such chain conformations into 2-chain, 10-chain, 15-chain, and 20-chain semi-crystalline models. The 2-chain, 10-chain, and 15-chain models yielded HDPE microstructures with the desired alternating lamellar orientations and entangled amorphous segments. On the other hand, the 20-chain model displayed multi-nucleus crystal growth instead of the lamellar-stack orientation. Structural characterization using a one-dimensional density profile and local order parameter {P₂(r)} analyses demonstrated lamellar-stack orientation formation. All semi-crystalline models displayed the total density (ρ) and degree of crystallinity (χ) range of 0.90–0.94 g/cm⁻³ and ≥42–45%, respectively. A notable stress yield (σ_yield) ≈ 100–120 MPa and a superior elongation at break (ε_break) ~250% was observed under uniaxial strain deformation along the lamellar-stack orientation. Similarly, during the MD simulations, the microstructure phase change represented the average number of entanglements per chain (<Z>). From the present study, it can be recommended that the 10-chain alternate lamellar-stack orientation model is the most reliable miniature model for HDPE that can mimic industrially relevant plastic behavior in various conditions.

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.