Characterization of moisture interactions in some aqueous-based tablet film coating formulations

Three methods--radiotracer, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)--have been employed to evaluate moisture interactions in aqueous-based hydroxypropyl methylcellulose films containing polyethylene glycol 400, polyethylene glycol 1000 or polyvinyl alcohol. The radiotracer employed was tritiated water. Using a proposed classification, the water present in the films was placed in three categories: tightly bound, moderately bound and free. The tightly bound water in hydroxypropyl methylcellulose and hydroxypropyl methylcellulose/polyvinyl alcohol films was related to the crystallinity of the films whilst tightly bound water was practically non-existent in the plasticized systems thus indicating the absence of crystallinity. Free (or freezable) water could not be detected in the films using DSC. The moisture content data obtained by TGA were considered to represent moderately bound water. The moderately bound water content of the film systems was in the rank order of the hydrophilicity of the polymer additives: polyethylene glycol 400 greater than polyethylene glycol 1000 greater than polyvinyl alcohol. This order was reversed for the calculated free water content.