Aqueous ethylcellulose dispersion of ethylcellulose. I. Evaluation of coating process variables.

Formulation and process variables play an important role in the film-forming properties of coating polymers. Three selected independent coating process variables, namely, percent solids content in the coating polymeric dispersion, inlet-air temperature, and spray rate of the polymeric dispersion, were investigated in this study to determine their effect on the performance characteristics of tablets coated with a plasticized aqueous ethylcellulose dispersion (Sure-lease) in a fluid-bed equipment. Response surface methodology (RSM) was utilized to study the complex relationship between these process variables and selected response variables. Three response variables were considered, namely, rate of drug release from the "untreated" coated tablets and the "thermal-treated" coated tablets and microindentation hardness of the untreated coated tablets. A 12-point factorial experimental design was utilized, and three-dimensional (3-D) response surface plots were generated using a second-order polynomial model. The model provided information needed to predict optimal process conditions. Drug release from the coated tablets followed zero-order kinetics. Inlet-air temperature was found to be the most critical process variable for all the three response variables studied. A correlation was observed between the drug release rate and the microindentation hardness of the applied polymeric coat in the case of untreated coated tablets. The 3-D response surface plots indicated that lower rates of drug release from the coated tablets may be obtained by using high inlet-air temperature and low spray rate of the polymeric dispersion during coating.