Decomposition of Silane-Crosslinked Polyethylene with Supercritical Alcohol

Abstract

For the purpose of the development of a new recycling technique of thermosetting silane-crosslinked polyethylene (silane-XLPE) turned into thermoplastic polyethylene, the optimum conditions are determined to decompose crosslinking points consisting of siloxane bonds, while keeping the main chain consisting of carbon-carbon bonds in silane-XLPE using supercritical alcohols. As a result, more than 90% of the siloxane bonds are broken selectively and rapidly without the destruction of the main chain and silane-XLPE is converted to thermoplastic polyethylene using supercritical primary alcohols such as methanol at 320–360°C, 10 MPa in 20 min. On the other hand, in the case of supercritical secondary alcohol such as iso-propanol, around two thirds of the siloxane bonds did not decompose even at the high temperature region above 350°C. This is because it was difficult for bulky secondary alcohol to approach the crosslinking points inside the polymer. The reaction rates are measured for the decomposition of the crosslinking points with supercritical methanol and n-propanol. They are expressed by the first-order as well as the second-order reaction models well, because the alcohol concentration is much higher than that of the crosslinking points and can be considered to be constant during the reaction. Arrhenius plots of the second-order rate constants lay well on straight lines for both alcohols and the activation energies are 21.7 kJ/mol for supercritical methanol and 66.5 kJ/mol for supercritical n-propanol.