Thermal and mechanical characterization of cellulose acetate phthalate films for pharmaceutical tablet coating: effect of humidity during measurements.

Films from a polymer used in pharmaceutical coating (cellulose acetate phthalate) were analyzed by thermomechanical techniques including dynamic mechanical thermal analysis (DMTA) and tensile tests. Emphasis was placed on relative humidity (RH) at the measurement site (as opposed to storage or conditioning RH). The films were plasticized with either triethyl citrate or diethyl phthalate. The results show that the films respond rapidly to changes in the environmental humidity. This in turn influences the data obtained from DMTA and tensile testing; thus, good control of the humidity is essential. Absorption isotherms have been obtained for the two types of films, and the results were interpreted in terms of the equilibrium moisture content which is determined by the polar nature of the plasticizer. This factor must be considered when formulating a film composition because moisture can, apart from its influence on mechanical properties, also speed the chemical degradation processes. When the stability of a film composition during storage is studied, the actual measurements on the films should preferably be done at 0% RH, irrespective of the storage RH. At 0% RH, the mechanical testing results will reflect the chemical stability correctly, without interference from plasticizing effect of absorbed water. A functionality test for films has been suggested: DMTA under isothermal conditions using either step or continuous humidity scans. Isothermal dynamic humidity scans on the DMTA were performed for the first time, and the utility of these measurements is discussed.