Transport of sulphur dioxide in polymers

Abstract

Studies were made of the permeation, solution, and diffusion of sulphur dioxide in polyethylene, polyamide (Nylon 11), and polycarbonate at 25 °C and at pressures up to one atmosphere. The steady-state permeability coefficients of sulphur dioxide in all three polymers were pressure-dependent, and the effect of pressure on the solution and diffusion processes, which together govern the overall permeation process, account for the observed relations. Solution and diffusion in the two glassy polymers, polyamide and polycarbonate, were strongly pressure-dependent whereas in polyethylene diffusion was pressure-dependent but solution obeyed Henrys law. The vapour pressure isotherms of the glassy polymers obeyed a modified Langmuir expression suggesting that two concurrent sorption mechanisms were operative, the filling of microvoids within the polymer structures and ordinary Henrys law solution. Kinetic sorption-desorption studies showed that diffusion was “Fickian” in polyethylene and polyamide over the entire pressure range. It was also “Fickian” in polycarbonate at pressures up to approximately 25 cm Hg, but “non-Fickian” at higher levels. For each polymer, diffusion coefficients were determined by steady-state permeation and transient sorption-desorption methods, and the separate methods gave similar values.