Impact of compression pressure on tablet appearance.

The color of model tablets deepened and tablet gloss increased with increase in compression pressure regardless of colorant type. The possible reasons for these phenomena were examined. These observations could not be explained either by polymorphic transition of the colorant or by condensation of the colorant during compression. Instead, a change in surface roughness was assumed to be the most likely reason. The increase in compression pressure made the tablet surface smoother. With the increase in the surface smoothness, diffuse reflection decreased while specular reflection increased, indicating the opposite relation between diffuse and specular reflection as a function of surface roughness. A decrease in diffuse reflection results in a deepening of color. In contrast, an increase in specular reflection causes an increase in glossiness. Thus, the surface roughness seemed to govern gloss and color. The compression pressure seems to be one of the most important factors to control tablet appearance.