The non-uniformity of microcrystalline cellulose bilayer tablets

The material response of the constrained microcrystalline cellulose particles during the manufacture of bilayer tablets has been investigated. This work exploits the ability of a non-contact optical profilometer to elucidate both the intricate roughness detail and the inherent form of a compacted sample surface within one measurement. Thus the effects of the imposed non-uniform stress pattern that develops during the two sequential compaction cycles have been determined. The indirect measurement procedure has been able to infer the degree of particle deformation and hence the relative local porosity can be determined. The application of two compaction cycles has shown to exasperate the inhomogeneity of the localised regions of stored elastic strain energy and this is highlighted by the non-uniform and complex ejected tablet geometry. The localised stored energy release by volume expansion in the radial direction is thought to be responsible for the fracture of tablets during the ejection phase of the manufacturing process. When the magnitude of the final layer compaction stress is greater than the initial layer compaction stress the radial volume expansion can even result in the rupture of junctions between particles in the adjacent layers at the periphery of the interfacial zone. Thus the energy dissipation by volume expansion is a possible explanation for the stress concentrating crack commonly present at interfacial boundary zone within the bilayer formulations.