Aqueous Film-Coating Vaporization Efficiency

Abstract

A steady-state condition is achieved during most of an aqueous film-coating cycle, i.e., the rate of application of moisture to the tablet bed is equal to its rate of evaporation. A mathematical model was developed for the steady-state condition to describe the efficiency of evaporation of the aqueous solvent for any set of drying conditions (inlet air flow rate, temperature, and humidity) and spray rate of coating suspension. The model was tested utilizing a series of different coating conditions and coating compositions. A vaporization efficiency, E_s, was defined as a means to assess a reproducible coating end point. E_s values for each coating formula fell within a narrow range. The hydroxypropyl cellulose coating system had a significantly lower E_s value due to the higher tackiness that occurs with this polymer during film formation.