The water vapour transmission properties of selected hydrophilic, lipophilic, and mixed hydrophilic‐lipophilic polymer systems, cast as free films, have been examined as a function of time, film thickness, plasticiser concentration, and film formulation. An inverse linear relationship was established between the logarithm of the water vapour transmission rate (Rwvt) and the logarithm of film thickness in the presence of the various plasticiser concentrations for the three systems studied. The lipophilic n‐butyl methacrylate films were found to be less permeable to moisture than either the hydrophilic hydroxypropyl cellulose or mixed methyl hydroxypropyl cellulose‐ethyl cellulose films. The butyl methacrylate films closely follow Fickian diffusion, whereas the permeability constants of the other two films increased linearly with film thickness over the range of thicknesses studied. These phenomena are discussed with respect to the relative attractions these films have for water vapour.

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The rates of moisture uptake by film-coated tablet matrices containing calcium chloride have been determined as a function of polymer coat formulation, film coat thickness and calcium chloride content of the matrix. The more hydrophilic polymer film formulations were less effective in reducing the rate of moisture uptake by the tablet matrices. A linear relation was observed between rate of moisture uptake and film thickness, in contrast to the log-log relationship obtained in previous work using the same systems cast as free films. Decreasing the calcium chloride content of the matrix caused an overall lowering of the moisture uptake rate. The results indicate that the water vapour transmission characteristics of these polymer film systems are not the same when assessed using the rate of moisture uptake of tablets coated with these films as when using the rate of water vapour transmission through the free films. It is suggested that films should not be accepted or rejected solely on the basis of transmission rates across free films but that due consideration should be given to the more meaningful moisture uptake rates of the film-coated dosage form.