The effect of some formulation and process variables on the surface roughness of film‐coated tablets

The effect of some formulation and process variables on the surface appearance of film‐coated tablets has been examined by measuring the arithmetic mean roughness, Ra, values across the faces of tablets before and after they were coated with hydroxypropyl methyl‐cellulose. For all tablet cores except those that were very porous, film coating resulted in an increasing surface roughness; for very porous cores a decrease was found. Tablets with rough surfaces were produced by coating with low molecular weight grades of the polymer; increaseing the polymer molecular weight resulted in a smoother finish with a minimum roughness at intermediate molecular weight grades. Increasing the polymer concentration above 2% w/v caused an increase in roughness as did increasing film thickness to 140 μm. There was a minimum in roughness at film thicknesses of 20 μm. The addition of pigment in low concentrations (0–25% v/v) caused a marginal increase in surface roughness but at concentrations above the critical pigment volume concentration, the surfaces were very rough. The results illustrate the potential of the method in the optimization of film formulations and process conditions during product development.