Thermal characterization of drug/polymer and excipient/polymer interactions in some film coating formulation

Some probable consequences of the dissolution/migration of a major solid dosage component in or into an applied film coating during or after a film coating operation have been investigated using free films of hydroxypropyl methylcellulose (HPMC) and polyvinyl alcohol (PVA) incorporating small amounts of either lactose (a diluent) or ephedrine hydrochloride (a drug). Intrinsic features of the films such as softening, glass transition, crystallinity and melting were examined by differential scanning calorimetry and thermomechanical analysis. Generally, the results indicate that ephedrine hydrochloride exhibited plasticizing activity in both HPMC and PVA films. On the other hand, incorporation of lactose produced an opposite effect (stiffening) in PVA films as demonstrated by increased glass transition temperature and crystallinity. On the basis of these findings, it was proposed that the undesired presence of a component of a solid dosage core in the applied film coating could significantly alter its end-use properties such as diffusivity and the incidence of film coating defects. It was also shown that the application of the relationship of Moelter & Schweizer (1949) in the evaluation of the plasticizer efficiency of non-homologous additives could pose problems of interpretation.