The effect of product bed temperature on the microstructure of Aquacoat-based controlled-release coatings

Abstract

Diphenhydramine HCl pellets were coated at various bed temperatures in a laboratory-size fluid-bed equipment. Optimum coalescence of an Aquacoat formulation was achieved when the bed temperature was kept between 30 and 40°C. Coating applied outside this temperature range resulted in the formation of poorly coalesced films and faster release rates. The fast release rates of pellets coated at low bed temperatures were attributed primarily to drug migration into the film layer during the coating process and to incomplete film formation due to hardening of the uncoalesced polymer spheres, which, presumably, had a relatively higher minimum film formation temperature. At high bed temperatures, the rate of evaporation of water is so fast that it does not allow the migration of drug from the substrate into the film layer. The fast release rates observed were, therefore, believed to be due to the development of porous films during the coating process.