Polyethylene crosslinking using the epoxy‐anhydride reaction I: A strategy for a curing process with high thermal sensitivity

The initiated epoxy‐anhydride reaction was examined as a crosslink motif for effecting cure in functionalized polyethylene for electrical cable insulation applications. A specific challenge for this application is that little to no crosslinking can occur during the processing steps (~140°C), but crosslinking must be rapid and complete within a few minutes during the curing process (~200°C). To achieve this, we coupled the kinetics of the formation of an initiator for the epoxy‐anhydride reaction to the crosslinking step to achieve a phenomenological temperature sensitivity or “latency” that would be difficult to access via a single simple reaction. Evaluations of different imidazolium and phosphonium salts as initiator precursors were conducted with model compounds in solution, and specific salts were chosen for polymer studies based on the ratio of the phenomenological rates measured for the model reaction at ~140°C and ~200°C. In polymer studies using epoxide‐functional poly(ethylene), high crosslinking rates were observed at ~200°C while crosslinking was minimal at ~140°C. However, inclusion of the anhydride‐functional polyethylene in the formulation led to loss of initiator latency, and significant crosslinking was observed at ~140°C. A reaction between the anhydride or an impurity and initiator precursor is postulated to produce a kinetically‐competent initiator at lower temperatures that destroys latency. Computational studies illuminate the basis for the preferred alternating nature of the epoxy‐anhydride reaction and its faster rate relative to epoxide homopolymerization. Exploration of the coupled kinetics of these reactions involving different latent initiators provided a framework through which polymer crosslinking could be tuned to deliver a phenomenological temperature response not attainable by a single simple chemical crosslinking reaction.