The influence of relative humidity in the pan during coating on polymer deposition and film formation was investigated. Four tablet substrates, differing in hydrophobicity, porosity, and surface roughness, were prepared and coated with Eudragit® RS/RL 30 D (8:2 ratio). The spray rate and atomization air pressure were varied to create two distinct micro-environmental conditions in the coating pan. PyroButton data logging devices placed directly in the pan were found to more accurately reflect the relative humidity to which tablets were exposed in comparison to measurements taken at the exhaust. Polymer deposition was shown to be influenced by the properties of the substrate, rather than the processing conditions used during coating, with higher polymer weight gains observed for the more porous tablets. Differences in the film-tablet interface and in the release performance of the coated products, however, were attributed to both the relative humidity in the pan and tablet porosity. Overall, this study demonstrated that a more humid coating process (86% vs 67%) promoted surface dissolution and physical mixing of the tablet ingredients with the forming film and the extent of this phenomenon was dependent on the tablet porosity.

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