Permeation, diffusion, and solution of cyclopropane in silicone rubber

Permeability, solubility, and diffusion coefficients have been determined for cyclopropane (c‐C3H6) in silicone rubber at temperatures between −8 and 70°C at relative pressures from 0.04 to 0.30. The permeability coefficients, $&\[\bar P\]$, are of the order of 10−6 cm3 (STP) · cm/(s · cm2 · cmHg). $&\[\bar P\]$ increases slightly with increasing penetrant pressure and decreases with increasing temperature, the energy of activation for permeation being −1.27 kcal/gmol at zero pressure. The solubility of cyclopropane in silicone rubber can be represented over the experimental concentration range by the Flory‐Huggins equation. The solubility decreases with increasing temperature and the partial molar heat of solution is −4.95 kcal/gmol. The solubility coefficient in the Henry's law limit, S(0), for cyclopropane and many other gases and vapors can be correlated with (Tc/T)2, where T and Tc are the experimental and critical temperatures, respectively. The mutual diffusion coefficients, D, increase with increasing concentration and temperature, the energy of activation for diffusion being 3.68 kcal/gmol. The pressure dependence of &\[\bar P\] is described satisfactorily by a free‐volume model proposed by Fujita and extended by Stern, Frisch, and coworkers. The permeability, diffusion, and solubility behavior of cyclopropane in silicone rubber is similar to that of propane (C3H8).