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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 29 Si-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.

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UV/visible absorption spectroscopy was applied to measure the microscopic solvent dipolarity/ polarizability, π*, and hydrogen-bond-acceptor basicity, , parameters of supercritical CO 2 /nalcohol mixtures. Experiments were conducted at 45°C by varying the pressure between 86 and 230 bar, the type of n-alcohol from methanol to n-hexanol, and the n-alcohol mole fraction, x 2 , from 0 to 0.05. The effect of our experimental conditions on maximum ∆π* changes can be represented in the following order: system pressure (∆π* ≈ 0.4) > n-alcohol mole fraction (∆π* ≈ 0.3) > type of n-alcohol (∆π* ≈ 0.1). However, the corresponding order for ∆ changes was as follows: n-alcohol mole fraction (∆ ≈ 0.6) > type of n-alcohol (∆ ≈ 0.3) > system pressure (∆ ≈ -0.1). Under our experimental conditions, the π* parameters of the mixtures were below the corresponding value for liquid cyclohexane, whereas parameters at x 2 ) 0.05 approached the values for liquid alcohols under ambient conditions. Because the solvent strength for CO 2 / methanol mixtures can be varied over the widest range by changing the methanol mole fraction and system pressure, methanol is the best cosolvent among the n-alcohols studied here.