Permeation and sorption of water vapour in varnish films

An experimental investigation on the moisture permeability and sorption of detached varnish and polystyrene films is described. Details for preparation of samples and methods of test, under various conditions of vapour pressure and temperature, are given. Both the permeability and sorption of the films have been determined at constant moisture concentrations, that is, by using streams of moist air, the amounts permeating and the amounts sorbed being directly weighed. In addition to moisture permeability, some measurements of that of hydrogen and carbon dioxide are reported. Consideration of the experimental results shows that they can be interpreted by assuming that the diffusion process is analogous to that of substances in solution. There is insufficient evidence to show the effect of vapour pressure and temperature on the moisture permeability, but the results for carbon dioxide and hydrogen indicate that the diffusion process is dependent on the microphysical properties of the film. The effect of structure on diffusion is discussed, and the relations of the results to the problems of electrical insulation are considered.