Factors affecting differences in film thickness of beads coated in fluidized bed units

Abstract

Previous work (Wesdyk et al., Int. J. Pharm., 65 (1990) 69–76) demonstrated that various size beads, dispersed in a single bed, display differences in film thickness when coated in a fluidized bed apparatus equipped with a Wurster column. The differences in film thickness were attributed to differences in fluidization patterns and velocities of the various size beads. The present study examines the effect of spray mode and process parameters on the variation of film thickness for a fixed particle size range and distribution. Beads with a size distribution in the no. 14-no. 20 mesh range were coated with an aqueous polymeric dispersion using top, bottom, and tangential spray fluidized bed units. Film thickness of the coated beads was measured using a scanning electron microscopic method. It was observed that unlike batches coated in the bottom spray (Wurster) mode, the batches coated in the top and tangential spray modes exhibited no trend in film thickness. These results are consistent with the differences observed in the surface area normalized release rates of the various size beads coated in the three spray modes. The results also indicated that for the bottom spray mode, a reduction in the atomizing air pressure, while substantially reducing differences in fluidization patterns, had only a marginal effect in reducing the differences in film thickness.