Surface effects in film coating Michael SUMMARY This chapter will explain the significance of the stages of impingement, wetting, spreading and penetration of atomized droplets at the surface of tablet or multiparticulate cores. It will explain some of the fundamental aspects of solid-liquid interfaces which are important to the process of film coating. This chapter will emphasize the importance of controlling the ‘wetting power’ of the spray and the ‘wettability’ of the substrate, and will explain how this can be achieved by changes in formulation and process parameters. Both surface tension and contact angle are important properties in influencing. | Page 118 5 Surface effects in film coating Michael SUMMARY This chapter will explain the significance of the stages of impingement wetting spreading and penetration of atomized droplets at the surface of tablet or multiparticulate cores. It will explain some of the fundamental aspects of solid-liquid interfaces which are important to the process of film coating. This chapter will emphasize the importance of controlling the wetting power of the spray and the wettability of the substrate and will explain how this can be achieved by changes in formulation and process parameters. Both surface tension and contact angle are important properties in influencing the wetting of a substrate surface whether this be tablets granules or spheronized pellets by the coating formulation. These properties have been evaluated in coating polymer systems because of their possible relationship with wetting spreading and subsequent adhesion. These aspects are discussed in detail in this chapter. The chapter also contains a discussion on the adhesion properties of the final dried film coats and some data are presented to illustrate the factors influencing the magnitude of these adhesive forces. INTRODUCTION In our deliberations on the process of film coating of pharmaceutical solid dosage forms one cannot escape a consideration of surface aspects relating to the wetting of granule pellet or tablet cores by the coating solution and the subsequent adhesion of the dried films. Page 119 This chapter will consider some fundamental aspects of these stages and explain the mechanisms involved in the spreading and wetting of droplets once they hit the substrate. While it is not always necessary to have a firm grasp of these concepts to produce a satisfactory film coat in practice an awareness and understanding of some of these theories will help to produce much more efficient and elegant films. Film coatings are invariably applied in the pharmaceutical industry by spraying a coating .
