Properties of heat-humidity cured cellulose acetate phthalate free films

Cellulose acetate phthalate (CAP) free films containing diethyl phthalate (DEP) or triethyl citrate (TEC) as the plasticizer were prepared by the spray method. The chemical and mechanical properties of films were compared following heat-only (50 degrees C for 24 h) and heat-humidity curing (50 degrees C/75% RH for 24 h) conditions. The surface roughness of the heat-humidity cured films decreased compared to that of the uncured and heat-only cured films. The heat-humidity curing condition suppressed evaporation of the plasticizer, resulting in higher plasticizer levels remaining in the films, as compared to the heat-only curing condition. The heat-humidity curing also significantly increased the mechanical strength and decreased the water vapor permeability of the films. When exposed to the acidic media, despite rapid leaching of plasticizer, the heat-humidity cured films retained the most mechanical strength of the films prior to exposure. The moisture content and phthalic acid content after heat-humidity curing were increased slightly, but did not reach a level that would interfere with enteric performance. TEC was less volatile and produced films with increased % elongation, and decreased tensile strength and elastic modulus compared to the films plasticized with DEP. However, the DEP plasticized films were less permeable than TEC-plasticized films following heat-humidity curing. The results indicated that a short-term exposure of the CAP films to heat and humidity during the curing process greatly improved the degree of film coalescence and mechanical strength, without causing significant chemical degradation of the polymer.