The Permeability of Different Rubbers to Gases and Its Relation to Diffusivity and Solubility

Abstract

As the permeability of rubber-like substances to gases stands in relation to the solubility and rate of diffusion of the gases in those materials, these individual values should be known. The permeability of a membrane was measured manometrically and the diffusivity was derived from the time-lag of the permeation. The solubility of the gas was computed from the permeability and the diffusivity, in addition to which the solubility was also found by direct measurement. In this way eight different gases were tested with nine elastomers at different temperatures. It appeared that the permeability of a membrane to a given gas is not affected by the presence of a second gas. The differences in permeability of different elastomers to a given gas are caused mainly by differences in rate of diffusion and only in a very minor degree to differences in solubility. The differences in permeability of the same elastomer to different gases are caused not only by differences in rate of diffusion but also by differences in solubility. A linear relationship is found between the logarithm of the solubilities of different gases in natural rubber and their critical temperatures, so the higher the critical temperature of a gas, the better does it dissolve. The presence of polar groups in an elastomer reduces the solubility of non-polar gases and increases the solubility of polar gases in the elastomer. The various rubbers behave towards gases like organic liquids. The activation energy of the diffusion and the heat of solution were calculated from the temperature function of the diffusivity and the solubility. As the diameter of the molecule of the gas increases, the rate of diffusion decreases, while the activation energy of the diffusion becomes greater. The presence of polar groups and methyl groups in elastomers causes low rate of diffusion, which involves a great activation energy of diffusion. It is presumed that the activation energy of the diffusion is required to separate the rubber molecules for the displacement of the gas molecules. The attempt to elucidate the constant D0 in the equation D=D0 exp (−E/RT)—which proves to be a function of the activation energy of the diffusion E— by reference to one of the formulas published in the literature failed. An empirical formula was drawn up, by which D0 is directly related to the activation energy E.