Devulcanization of Sulfur-Cured Isoprene Rubber in Supercritical Carbon Dioxide

A new devulcanization process that utilizes supercritical CO2 (scCO2) along with devulcanizing reagents was studied. Unfilled polyisoprene rubber samples (vulcanizates) with different crosslink distributions were prepared by controlling the cure time and the curatives. Each of the vulcanizates was subjected to the Soxhlet extraction using azeotropic acetone/chloroform to remove residual curatives. The devulcanization was performed at various temperatures (140–200 °C) in the presence of scCO2 for 60 min. The product was fractionated into sol and gel components, and molecular weight of the sol component and the crosslink density of the gel component were determined. Thiol-amine reagent was found to be effective among several devulcanizing reagents; the molecular weight of the resulted sol component was about tens of thousands and the crosslink density of gel component decreased substantially from the initial ones. Yield of the sol component increased with the increase in the CO2 pressure. In the supercritical fluid state of CO2, the vulcanizate was more efficiently devulcanized than in an ordinary gaseous state of CO2. The sol fraction depended considerably on the crosslink distribution in the vulcanizate. These results suggest that the devulcanizing reagents penetrate and diffuse into the vulcanizate in the presence of scCO2.