Solubility and Diffusivity of Propylene, Ethylene, and Propylene-Ethylene Mixtures in Polypropylene

The solubility of propylene, ethylene, and propylene-ethylene mixtures in isotactic polypropylene (iPP) is measured with a high-pressure sorption balance at different temperatures and pressures. The obtained sorption isotherms of both binary systems (propylene-iPP and ethylene-iPP) are fitted with the Sanchez-Lacombe equation of state. It is found that both binary interaction parameters for ethylene and propylene in iPP decrease with temperature. At temperatures above 70 degrees C, the overall solubility of ethylene plus propylene mixtures can be modeled using purely binary systems. However, at temperatures under 70 degrees C it is necessary to use a ternary solubility model that accounts for interactions between the penetrants and the polymer. Furthermore, at lower temperatures, the total solubility of the gas mixture is higher than the sum of gas solubilities of individual components because the anti-solvent effect of ethylene (on propylene) is weaker than the co-solvent effect of propylene on ethylene. Moreover, the molar ratio of ethylene to propylene dissolved in iPP increases with temperature and pressure. Desorption studies show that the effective diffusion coefficient increases with pressure and temperature and that the effective diffusivity of gaseous ethylene-propylene mixtures is lower than that of either of the individual gases.