THE EFFECT OF TENSILE-STRESS ON PERMEABILITY OF FREE FILMS OF ETHYL CELLULOSE CONTAINING HYDROXYPROPYL METHYLCELLULOSE

The effect of tensile stress on potassium chloride permeability in composite films containing ethyl cellulose (EC) and hydroxypropyl methylcellulose (HPMC) has been measured in a novel pressurized cell. Such a tensile stress is expected to develop in a film-coated drug formulation as a result of osmotic water imbibition. Free composite films of EC with HPMC contents ranging from 0 to 30 wt% were prepared by spraying a polymer solution on a rotating cylinder. It was found that films with 24% HPMC or less became permeable to potassium chloride only under the influence of an applied tensile stress. Increasing the film HPMC content lowered the magnitude of the applied tensile stress required to induce permeability. This tensile stress correlated with the breaking stress of the film. Additionally, provided that the tensile stress remained below the breaking stress of the film, the permeability properties were found to be reversible with respect to changes in the applied tensile stress. It is believed that the application of a tensile stress causes small alterations in film structure which affect transport and mechanical properties of the films. © 1990.