The effect of mechanical strain on properties of lubricated tablets compacted at different pressures

TL;DR

This study investigates how mechanical shear strain during powder blending affects the compaction behavior, porosity, and tensile strength of pharmaceutical tablets, revealing that shear strain reduces bonding efficiency and tensile strength.

Contribution

It provides new insights into the impact of shear strain on tablet properties, highlighting the importance of considering shear effects in tablet formulation and manufacturing.

Findings

Shear strain decreases initial and minimum porosity of tablets.

Shear strain reduces tensile strength by 60% at zero porosity.

Post-compaction relaxation compensates for increased in-die compaction with shear strain.

Abstract

A full factorial design of experiments was used to study the effect of blend shear strain on the compaction process, relative density and strength of pharmaceutical tablets. The powder blends were subjected to different shear strain levels (integral of shear rate with respect to time) using an ad hoc Couette shear cell. Tablets were compressed at different compaction forces using an instrumented compactor simulator, and compaction curves showing the force-displacement profiles during compaction were obtained. Although the die-fill blend porosity (initial porosity) and the minimum in-die tablet porosity (at maximum compaction) decreased significantly with shear strain, the final tablet porosity was surprisingly independent of shear strain. The increase in the in-die maximum compaction with shear strain was, in fact, compensated during post-compaction relaxation of the tables, which also increased significantly with shear strain. Therefore, tablet porosity alone was not sufficient to predict tablet tensile strength. A decrease in the 'work of compaction' as a function of shear strain, and an increase in the recovered elastic work was observed, which suggested weaker particle-particle bonding as the shear strain increased. For each shear strain level, the Ryskewitch Duckworth equation was a good fit to the tensile strength as a function of tablet porosity, and the obtained asymptotic tensile strength at zero porosity exhibited a 60% reduction as a function of shear strain. This was consistent with a reduced bonding efficiency as the shear strain increased.