J. of Supercritical Fluids 35 (2005) 175–181 Phase behavior of crosslinked polyisoprene rubber and supercritical carbon dioxide Masaaki Kojima a,∗,1 , Masatoshi Tosaka a , Eita Funami b , Kouei Nitta b , Masahiro Ohshima b , Shinzo Kohjiya a a Kyoto University, Institute for Chemical Research, Uji, Kyoto 611-0011, Japan b Kyoto University, Department of Chemical Engineering, Kyoto 615-8510, Japan Received 4 August 2004; accepted 15 February 2005 Abstract The phase behavior of crosslinked polyisoprene rubber (PIR) in CO 2 at temperatures ranging from 293 to 453 K and pressures ranging from 0.1 to 20 MPa were investigated. Visual swelling measurements based on the sample dimensions were performed using a high-pressure view cell. The change of the degree of swelling with time provided a diffusion coefficient of CO 2 in crosslinked PIR. The solubility measurements were carried out by using a magnetic suspension balance (MSB) method. The weight change of the sample in CO 2 under elevated pressure and temperature was converted to solubility taking the buoyancy effect and the degree of swelling into account; the degree of swelling was derived both from the visual swelling measurement and from theoretical estimation on the basis of the Sanchez–Lacombe equation of state. The solubility from the two methods was in good agreement with each other in the range from 8 to 13 MPa. © 2005 Elsevier B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Polyisoprene rubber; Swelling; Solubility; Diffusion coefficient 1. Introduction Application of supercritical carbon dioxide (scCO 2 ) as a reaction medium has been extensively studied. Its ability to permeate into polymer materials to facilitate transport of low molar mass reagents into the polymer bulk is well doc- umented and as a result, more effective progress of the re- action under study can be expected [1]. On the basis of this idea, the authors are developing a devulcanization process for crosslinked rubber in scCO 2 [2,3], which is targeted towards chemical recycling of pneumatic tires. In previous reports [2,3], it was interesting to find that more than 60% of diphenyl disulfide (DD), which was used as a devulcanizing reagent, in the reaction vessel was ab- sorbed into the crosslinked rubber during the devulcanizing process, though the rubber sample occupied only 1.6% vol- ∗ Corresponding author. Tel.: +81 72 622 6891; fax: +81 72 625 7703. E-mail address: mkojima@toyo-rubber.co.jp (M. Kojima). 1 On leave from Toyo Tire & Rubber Co., Ltd. ume of the vessel. Such absorption of DD did not occur when using a common organic solvents (e.g., toluene). By understanding how the absorption of DD occurs in scCO 2 , we hope to develop a practical devulcanizing process. How- ever, to discuss the absorption phenomena, phase equilib- rium of the ternary system composed of the rubber, DD and scCO 2 must be known. As one step in process devel- opment, the phase equilibrium of the CO 2 /rubber system is studied. The rubber sample, synthetic poly(cis-1,4-isoprene) rub- ber (PIR) is used for the experiment. PIR is the main con- stituent of natural rubber (NR); the main differences between NR and PIR (namely, synthetic NR) are the degree of stere- oregularity and the existence of minor constituents [4]. PIR contains a few percent of trans-1,4-isoprene units which are randomly positioned along the main chains, while NR con- tains the trans units only at the both ends of the chains. Also, PIR is free from proteins or fatty acids that are intrinsically contained in NR. For elucidating physicochemical properties, PIR is preferred in this study. 0896-8446/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2005.02.004