Solution properties and atomization in film coating Michael and Andrew SUMMARY A little-considered stage of the film-coating process is the atomization of the coating solution by the spray gun. This chapter will show how formulation and process factors can cause marked changes in the characteristics of the spray, which may have important consequences for film formation and film properties. The chapter begins by describing how film-coating solution or suspension properties, such as density, surface tension and viscosity, alter with changing formulation and then continues by presenting predictions of how these properties could influence spray droplet size. The chapter. | Page 64 4 Solution properties and atomization in film coating Michael and Andrew SUMMARY A little-considered stage of the film-coating process is the atomization of the coating solution by the spray gun. This chapter will show how formulation and process factors can cause marked changes in the characteristics of the spray which may have important consequences for film formation and film properties. The chapter begins by describing how film-coating solution or suspension properties such as density surface tension and viscosity alter with changing formulation and then continues by presenting predictions of how these properties could influence spray droplet size. The chapter then discusses various techniques for measuring and representing mean droplet size and size distribution. The influence of formulation and atomization conditions on spray characteristics is discussed and data are presented for aqueous HPMC droplets produced under a wide range of conditions. Parameters examined include concentration of polymer in solution atomizing air pressure liquid flow rate spray rate gun-to-substrate distance spray-gun design spray shape liquid nozzle diameter and atomizing air velocity. INTRODUCTION The overall process of film coating comprises a number of important stages Solution or suspension preparation. Droplet generation. Page 65 Droplet travel from the spray gun to the substrate bed. The substrate in question will usually be either a tumbling bed of tablets or a fluidized bed of multiparticulates . beadlets or pellets. Impingement wetting spreading and coalescence of the droplets at the surface of the tablet or multiparticulate. Subsequent drying gelation and adhesion of the film. All the above are important stages that need to be understood and where possible controlled. These stages are outlined schematically in Fig. . Atomization has been found to be a particularly important stage in the overall process of film coating. This chapter .
