Permeability of pure and mixed gases in silicone rubber at elevated pressures

The transport properties of silicone rubber are reported at 35°C for a series of pure gases (He, N2, CH4, CO2, and C2H4) and gas mixtures (CO2/CH4 and N2/CO2) for pressures up to 60 atm. The effects of pressure and concentration on the permeability of various gases have been analyzed to consider plasticization and hydrostatic compression effects. Over an extended pressure and concentration range, both compression of free volume and eventual plasticization phenomena were observed for the various penetrants. In pure component studies, plasticization effects tended to dominate hydrostatic compression effects for the more condensible penetrants (C2H4 and CO2) while the reverse was true for the low sorbing N2 and He. These issues are discussed in terms of penetrant diffusion coefficients versus pressure to clarify the interplay between the opposing effects for the penetrants of markedly different solubilities. Additional insight into the somewhat complex interplay of the plasticization and hydrostatic compression effects are given by mixed gas permeation results. It was found that the permeability of nitrogen in a 10/90 CO2/N2 and a 50/50 CO2/N2 mixture was increased by the presence of CO2 because the plasticizing nature of CO2 is able to overcome nitrogen's compression effect.