Selective decomposition of the siloxane bond constituting the crosslinking element of silane-crosslinked polyethylene by supercritical alcohol

Material recycling of silane-crosslinked polyethylene (silane-XLPE) was investigated to make thermoplastic polyethylene (PE). To make thermoplastic PE from silane-XLPE, a crosslinking element that consists of a siloxane bond must selectively be decomposed. Supercritical alcohol and water were adopted in this study. An autoclave was applied to expose silane-XLPE to the supercritical fluid. The structure of the products was analyzed with ²⁹Si-NMR, Fourier transform infrared, gel fraction, and molecular weight measurements. The results showed that the siloxane bond was successfully decomposed selectively by supercritical alcohol. It was expected that the structure of the recycled PE would be close to that of the silane-grafted PE. To confirm this expectation, the recycled PE was cured in saturated water vapor to investigate the crosslinking ability. The recycled PE, silane-grafted PE, and silane-grafted PE with a catalyst for condensation were compared. The kinetics and activation energy were calculated from the data of the temperature dependence of the increment of the gel fraction. The results showed that the recycled PE still had the ability to undergo a crosslinking reaction. The kinetics and activation energy of the recycled PE were closer to those of silane-grafted PE than to those of silane-grafted PE with a catalyst. The activity of the catalyst must have been lost by supercritical alcohol. These data support the expectation of the structure of the recycled PE.