Investigation of film curing stages by dielectric analysis and physical characterization.

A dielectric analysis (DEA) method was developed to monitor the curing of Eudragit RS30D plasticized with 20% acetyltributyl citrate and to investigate the effects of over curing on the barrier properties of the film. Permittivity (epsilon') obtained from DEA is related to the degree of mobility of permanent dipoles present in the sample. Changes in the epsilon' values were observed in various film sample types cured at 60 degrees C. Four distinct regions of curing were identified from the epsilon' versus time curves; namely, under-cured, optimally-cured, over-cured, and super-cured. The epsilon' values were high in under-cured films, and decreased to a minimum when the films became optimally-cured. Continued curing beyond the optimum stage caused an increase in the epsilon' values, corresponding to an over-cured stage, whereas in the super-cured region, the values decreased again. The changes in dielectric properties manifested the changes in the structure of the film, which correlated with the dissolution rates of the active drug from the film-coated beads as well as with the changes on their surface morphology at the same stage of cure. A mechanism of film curing was proposed based not only on the dielectric properties and dissolution data but also on the amount of plasticizer remaining in the film-coated beads, the changes in film thickness, and the surface electron micrographs of the cured film-coated beads.