Abstract
The opacity of tablet film coatings consisting of hydroxypropyl methylcellulose containing titanium dioxide has been analyzed quantitatively with particular emphasis on the theoretical and practical significance of the various methods for evaluating the relative opacity of different film formulations. By taking reflectance measurements with both black and white backing tiles at various film thicknesses it is possible, by the application of the Fell relationship, to estimate the approximate conditions (e.g. pigment concentration and film thickness) required to produce opaque films. However, if a quantitative assessment of opacity is required, the application of the Kubelka-Munk equation provides data not only of a more fundamental nature but also more precise. The scattering coefficients calculated for the various film coating formulations compare favourably with those for the same pigment in paint formulations.
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