Preparation of De-crosslinked Polyethylene from Waste Crosslinked High-Density Polyethylene Using Supercritical Twin-Screw Extrusion

This study examined a method for de-crosslinking high-density polyethylene (HDPE) for use in heating pipes using supercritical fluids and recycling them into polyethylene. Waste crosslinked HDPE is mostly incinerated because it is a thermosetting plastic and cannot be recycled. Therefore, there is an urgent need to develop new recycling technologies for crosslinked HDPE to prevent environmental pollution. Many experiments have been conducted under various subcritical and supercritical conditions using methanol as the supercritical solvent to recycle crosslinked HDPE. Consequently, PE can be prepared via a de-crosslinking reaction. To recycle crosslinked high-density polyethylene, we conducted experiments at three rotational speeds (80, 250, and 400 rpm) of the screw under the conditions of various temperatures (300–360 ℃) by using methanol, ethanol, acetone, and water. As a result, we could prepare polyethylene with a linear structure at temperatures above 300 ℃ through the de-crosslinking reaction. This study evaluated the characteristics of recycled polyethylene based on the reaction conditions using Fourier transform infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and tensile strength analyses. In this study, samples with the same chemical and crystal structures were prepared under all conditions.