Characterization of the film formation of the dry coating process

As the film formation of the dry coating process differs completely from conventional coating methods it is of certain interest to define a parameter like the minimum film formation temperature (MFT) used for aqueous dispersion based processes in order to describe an efficient film formation. Film formation occurs mainly during the curing step following the coating phase. Therefore, the film formation of the dry coating process was analyzed with regard to the two process parameters influencing film formation, namely curing temperature and time.Theophylline pellets were coated using the enteric polymer HPMCAS and TEC/Myvacet® as plasticizer composition. The polymer and the plasticizer were applied to the pellets simultaneously, except for the beginning of the coating step when the plasticizer has been sprayed 30 s before powder feeding was started. The coated pellets were cured for five different time periods at eight different temperatures. Drug release was determined in 0.1 N HCl. Their surface and cross-sectional morphologies were examined by scanning electron microscopy. The glass transition temperature of the obtained films as well as of the polymer plasticizer mixture obtained by casting a film using an organic solution of the coating components was determined by thermomechanical analysis.At higher curing temperature and/or extended curing time an enhancement of acid resistance is observed. The glass transition temperature of the coating mixture was determined to be 51.7 ± 3.3 °C. It is close to the temperature needed for film formation. Enteric resistant pellets are obtained after curing for 0.75 h at 55 °C. However, enteric resistance was achieved as well slightly below the glass transition temperature at 45 °C applying long curing periods of at least 12 h. This may be caused by the plasticizer gradient along the coat which is caused by spraying the plasticizer 30 s before starting powder feeding. This results in a higher plasticizer concentration of the inner layers of the coat relatively to the outer ones. Due to this film formation starts at the inner layers even below the glass transition temperature. As the plasticizer diffuses to the outer layers during curing according to the plasticizer gradient, film formation proceeds.