The influence of additives on the spreading coefficient and adhesion of a film coating formulation to a model tablet surface.

The surface energies of film coating formulations based on hydroxypropyl methylcellulose and containing microcrystalline cellulose, lactose and Tween 20, respectively, have been assessed. The approach taken allowed the components of the surface energy, in terms of the Lifshitz-van der Waals and the acid-base contributions, to be determined. Spreading coefficients of these coating formulations were determined on a model tablet surface whose surface energy had been similarly characterised. The determined spreading coefficients were high and positive indicating that spreading and wetting would not be a controlling factor in the formation of an adequate film coat. The adhesion of the coats to the core was measured and showed that the inclusion of additives influenced the adhesion of the film. Maximum adhesion was obtained when microcrystalline cellulose was included in the coating formulation that presumably allowed a strong interaction with the same component in the tablet core. Adhesion was enhanced when the tablet cores were made at a higher compaction force. Atomising air pressure had little influence on the adhesion.