In this thesis the film coating of tablets is reviewed with reference to the coating materials and processes which are available. The effects of formulation and processing variables are discussed together with techniques for the evaluation of film properties. The discrepancies observed in the literature between the properties of free films and those applied to tablets in commercial coating equipment formed the basis for one section of the work, in which a more realistic model coating system was developed. The coating parameters used successfully in a 24" Accela-Cota for the aqueous coating of tablets were isolated and then reproduced on a model system. The water vapour permeability of coatings produced on the model system and in the Accela-Cota were not significantly different. The viscosity and thermal behaviour of aqueous solutions of film coating polymers were investigated to compare the film formers and to gain an understanding of the process of film formation on tablets; in particular to evaluate the possibility that thermal gelation of the polymer may play a role in the transition from a sol to a gel state during film formation on the tablets. A mass spectrometry permeability technique was developed and evaluated in order to follow the low level transfer of gas across a small area of film coating. Techniques were also developed to measure the tensile properties and water vapour permeability of films and to follow the changes in surface roughness of a tablet during deposition of a coating on the model system. The effects of film preparation technique, substrate and titanium dioxide content on the oxygen permeability, water vapour permeability and mechanical properties of hydroxypropylmethyl cellulose (HPMC) films were investigated. The changes in oxygen permeability of HPMC films with increasing relative humidity were also demonstrated.
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