Investigation of Degradation Mechanism from Shear Deformation and the Relationship with Mechanical Properties, Lamellar Size, and Morphology of High-Density Polyethylene

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.