Effect of Solvents on De-Cross-Linking of Cross-Linked Polyethylene under Subcritical and Supercritical Conditions

Abstract

The effect of various solvents on the de-cross-linking reaction of cross-linked polyethylene under subcritical and supercritical conditions was studied. The gel content of the de-cross-linked polyethylene decreased from 60 to 0.8–2.5% at 380 °C within 5 min in all of the solvents tested in this study. When water, ethanol, and 2-propanol were used, the M w of the de-cross-linked polyethylenes decreased from 349,000 to 200,000–227,000 g/mol, and much lower M w values of 70,000–90,000 g/mol were observed when acetone and diisopropyl ether were used. The de-cross-linked polyethylenes treated in the different solvents had a very similar chemical structure to raw polyethylene. According to a kinetic study using a first-order reaction model, the de-cross-linking rate in each solvent increased in the order 2-propanol < ethanol < n-hexane < water < methanol ≪ diisopropyl ether ≪ acetone at 350 °C.

Continuous supercritical decrosslinking extrusion process for recycling of crosslinked polyethylene waste

“…The observation indicates that XLPE is basically decrosslinked because of thermal dissociation of the C–C bonding and the supercritical methanol conditions accelerate the decrosslinking reaction. These results are well in agreement with those reported from the batch reaction . However, the continuous supercritical extrusion process resulted in lower degree of decrosslinking effect than that of the batch type reactor…”

“…Goto et al reported that silane‐XLPE could be depolymerized through selective decomposition of the siloxane bonds in supercritical methanol or water . In our previous studies, we also observed that the irradiation‐crosslinked, the silane‐crosslinked, and the peroxide‐crosslinked PE could successively be decrosslinked into the thermoplastic PE using supercritical methanol . The decrosslinking reaction rate was observed as linearly proportional to the gel content and exponentially related to the reaction temperature.…”

“…Recently, several research groups reported that supercritical fluids can be used as a reaction medium for the depolymerization reaction of several condensation polymers and decrosslinking reaction of XLPE . Polyethylene terephthalate has been depolymerized via hydrolysis or solvolysis reactions in supercritical water and methanol medium .…”

“…In this work, we demonstrated that an extruder can be used as a reactor for the supercritical methanol decrosslinking reaction of peroxide‐crosslinked PE, XLPE. The C–C crosslinking bond of peroxide‐crosslinked PE has much better thermal stability than the silane‐crosslinking bond in silane‐crosslinked PE . The multi‐stage single screw extruder specially designed for peroxide crosslinked PE provides XLPE with the reliable supercritical conditions of methanol during continuous extrusion.…”

Ultrasonic decrosslinking of crosslinked high-density polyethylene: effect of degree of crosslinking

“…Since the value of M n is not available, the weight average molecular weight, M w of HDPE is used in calculation of eqn (2). The M w of HDPE is 5.2 Â 10 4 g mol À1 .…”