The influence of particle size on the surface roughness of pharmaceutical excipient compacts

The effect of initial powder particle size on the surface roughness of compacted pharmaceutical materials was studied using non-contact optical profilometry. Various particle size grades of two commonly used pharmaceutical excipients, microcrystalline cellulose (MCC) and lactose monohydrate (LMH), were compacted to a solid fraction of 0.85. The compression stress values CSm (maximum) and CS0 (equilibrium), were recorded during uniaxial compression and triaxial decompression profiles for assessing the extent and mechanism of deformation. MCC samples were compressed beyond the reported yield pressure (Yp) values for these materials, whereas LMH samples were compressed only up to 55% of the reported Yp. As a result, two distinct compression behaviors could be classified for these excipients where CS0 > Yp (MCC) or CS0 < Yp (LMH). The brittle–ductile transition diameter ( D crit) values for these materials were important in predicting the deformation mechanism for each powder. The surface roughness parameter, Sq (root mean square roughness), was correlated with particle size and compression stress. Sq values increased with particle size for both excipient categories; LMH samples were found to have a stronger dependence on particle size than MCC since most LMH powders experienced incomplete fracture of particles during compression, whereas all MCC powders showed evidence of complete plastic deformation. The tensile strength (TS), an important mechanical property of the compacts, decreased with increasing particle size for both material types, and showed strong correlations with surface roughness. Smoother compacts exhibited higher values of TS, possibly due to the more intricate bonding between smaller particles. Hence, surface roughness parameters can be used as a non-destructive method for predicting the mechanical properties of materials. Profilometry was found to be a useful tool for understanding the relationship between initial particle size and the surface roughness of compacts from either brittle or ductile materials.