Scientific papers
Powder densification during uniaxial compaction involves concurrent processes influenced by stress gradients within the packing, as well as frictional and adhesive forces between the powder and the die walls. This leads to the development of density and stiffness anisotropy across the axial and radial directions. In this investigation, microindentation was employed to evaluate and quantify the variation of the modulus of elasticity (E_mod) across the surface of cylindrical tablets. A diverse set of deformation behaviors exhibited by pharmaceutical excipients, ranging from soft/plastic (microcrystalline cellulose) to medium (lactose) and hard/brittle (calcium phosphate), were analyzed at different compaction pressures. Results depicting the local stiffness distribution on tablet faces revealed a linear and directly proportional relationship between solid fraction and E_mod for the upper and lower faces. Additionally, significant stiffness anisotropy between the axial and radial compaction directions was observed. The most pronounced stiffness anisotropy was identified in ductile grades of microcrystalline cellulose (MCC) compared to brittle powders, attributed to the dual effects of overall elastic recovery and Poisson’s effect on relaxation kinetics. To reinforce this analysis, the evolution of specific surface area elucidated the respective densification mechanism and its implications for anisotropy. For ductile excipients, the increase in contact surface area and the reduction/closing of interstitial pores explained the decrease in surface area with rising compaction pressure. In contrast, densification for brittle powders occurred through fragmentation and subsequent void filling.
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