Decrosslinking of AOTEMPO-derived polyethylene thermosets

Abstract

Recycling thermosets poses a significant challenge due to their three-dimensional network of covalent bonds. Among these, crosslinked high-density polyethylene (XHDPE) is valued for its superior thermal and chemical properties, though crosslinking hinders reprocessing by conventional methods. This research investigates the reprocessibility of XHDPE produced using cure formulations comprised of 4-acryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl (AOTEMPO) and dicumyl peroxide (DCP), whose chemical composition may be susceptible to degradation by external stimuli such as heat and pH. Decrosslinking by the hydrolysis and thermolysis of XHDPE thermosets was assessed by measurements of gel content and dynamic storage modulus (G’). Base-catalyzed hydrolysis was more effective than acid-catalyzed hydrolysis and alcoholysis, reducing gel content from 85% to 55% after three days at 90°C and two days at 140°C. However, thermolysis at 230°C achieved the greatest reduction of XHDPE crosslink density, decreasing gel content from 80% to 14% through disproportionation of alkoxyamine crosslink functionality. Moreover, decrosslinked XHDPE could be cured once again with DCP to yield crosslink densities comparable to those achieved with unmodified HDPE. Based on the storage modulus and gel content of these materials, the efficient recycling of XHDPE and sustainable reuse appears to be possible.