Phase behavior of rubber and supercritical CO2: Operating parameters and swelling characteristics

Abstract This study aims to compare the difference in the swelling process between styrene butadiene rubber (SBR) and natural rubber (NR) in ScCO 2 . A high‐pressure visual stainless‐steel autoclave was used to carry out swelling measurements based on the sample dimensions. The swelling rates for both rubbers followed a first‐order kinetic model. The diffusion coefficient for CO 2 was determined from the change in the swelling degree with time. The shrinking process for the expanded rubber obeyed a second‐order kinetic model. A 2‐level full factorial experimental design was employed to discuss the influence of variables on rubber swelling and CO 2 diffusion. Temperature and pressure were found to have a great influence on the equilibrium swelling degree, swelling rate and diffusion coefficient. For both SBR and NR, the maximum equilibrium swelling degrees were observed at 120°C and 20 MPa, yielding values of 1.18 and 1.16, respectively. The maximum diffusion coefficient reached approximately 3 × 10 −9 m 2 s −1 for both types of rubber at 120°C and 8 MPa. The crosslink density failed to affect these three responses, but the volume of expansion and the shrinkage rate were found to significantly decrease and increase, respectively, with increasing crosslink density. The motion of polymer segments forced by ScCO 2 led to rearrangement of the material structure, which governed the nature of swelling, expansion, and shrinkage. Due to a lower number of entanglements and a weak electrostatic interaction exists between CO 2 and the π‐system, SBR can absorb more CO 2 and easily restructure than NR under the same conditions, resulting in higher equilibrium swelling degree, larger expansion and faster shrinkage than NR. Highlights Swelling of nature rubber and styrene‐butadiene rubber in ScCO 2 is compared. Swelling degree is positively correlated with temperature and pressure. Diffusion coefficient increases with temperature but decreases with pressure. Swelling degree varies with rubber type, while CO 2 diffusion dose not. Crosslink density significantly affects expansion and shrinkage of rubber.