Aqueous ethyl cellulose dispersion containing plasticizers of different water solubility and hydroxypropyl methyl-cellulose as coating material for diffusion pellets II: properties of sprayed films.

This study investigates the properties of sprayed films prepared from aqueous ethyl cellulose dispersions (ECD) containing hydroxypropyl methylcellulose (HPMC) and plasticizers of different water solubility in order to clarify the drug release mechanisms of pellets coated with the respective material. It is of special interest to measure the migration of the water soluble components as well as the physical properties of the swollen ethyl cellulose film. Swelling experiments with sprayed films in 0.1 N-HCl at 37 degrees C show that fairly water soluble plasticizers and the pore forming agent (HPMC) migrated rapidly and almost completely out of the films. The water insoluble plasticizers remain predominantly in the film and the migration rate of HPMC is reduced in a release medium of high ionic strength. The glass transition temperature (T(g)) and the softening temperature (T(s)) of these films after swelling are dependent on the water solubility of the plasticizer. The T(g) of ECD films plasticized with triethyl citrate is above the swelling temperature of 37 degrees C after migration of the plasticizer, transforming the polymer in the glassy state. In contrast, dibutyl phthalate-containing ECD films demonstrate a T(g) below the swelling temperature, leaving the polymer in the rubbery state. The mechanical properties of dry and wet films are studied as a function of the state of curing of the films and of the swelling temperature. On contact with water, a pronounced shrinkage of ECD/HPMC films plasticized with water insoluble plasticizers is observed. All these results are used to explain the different drug release mechanisms of the coated pellets and to enable the prediction and optimization of drug release-rates from coated pellets.