FACTORS AFFECTING DIFFERENCES IN FILM THICKNESS OF BEADS COATED IN FLUIDIZED-BED UNITS

Previous work (Wesdyk et al., Int. J. Pharm., 65 (1990) 69-76) demonst rated that various size beads, dispersed in a single bed, display diff erences in film thickness when coated in a fluidized bed apparatus equ ipped with a Wurster column. The differences in film thickness were at tributed to differences in fluidization patterns and velocities of the various size beads. The present study examines the effect of spray mo de and process parameters on the variation of film thickness for a fix ed particle size range and distribution. Beads with a size distributio n in the no. 14-no. 20 mesh range were coated with an aqueous polymeri c dispersion using top, bottom, and tangential spray fluidized bed uni ts. Film thickness of the coated beads was measured using a scanning e lectron 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 re sults are consistent with the differences observed in the surface area normalized release rates of the various size beads coated in the thre e spray modes. The results also indicated that for the bottom spray mo de, a reduction in the atomizing air pressure, while substantially red ucing differences in fluidization patterns, had only a marginal effect in reducing the differences in film thickness.