Purpose: Particulate rearrangement is the very first step of a tablet compaction process. Tablet feed rearrangement within the die is highly dependent on flowability and this activity can be gauged by the rearrangement energy component derived from analysis of the force-displacement curve. Understanding the factors in powder rearrangement could provide a better understanding of the energy utilization during tablet compaction and in turn, the quality of tablets produced. This study aims to investigate the influence of particle surface roughness on the in-die flow and its consequence to tablet properties.
Methods: Rough lactose granules (RC) were produced by roller compaction. Further surface alteration was carried out by spraying water on RC in a fluid bed till two end points of 50 % (RC50W) and 100 % (RC100W) of the liquid weight equivalent to the total load weight were reached. Smoother surface crystalline lactose (S100) particles were used as the control. Samples were all fractionated to 63 – 180 µm using an air jet sieve and evaluated for their flow properties and mean arithmetic roughness (Ra). Samples were then compacted into 300 mg tablets using flat faced punches and deep concave punches while modifying the compression profile using a compaction simulator. Tablet properties and compaction energy profiles were analyzed in relation to their Ra.
Results: Particle surface roughness was varied with RC achieving the highest Ra, followed in declining order by RC50W, RC100W and S100. Increasing the particle roughness resulted in an increase in rearrangement energy of the particles during tableting, which was also observed during compressibility test of the FT4 powder rheometer where the particles with smoother surface exhibited superior compressibility under a fixed force. The work done by the impeller blade of the FT4 rheometer also increased with Ra, suggesting an increase in interparticulate frictional forces with increased roughness, which in turn increased the energy requirement for rearrangement. Good positive correlation could be obtained between basic flowability energy, specific energy, powder compressibility with rearrangement energy required during tableting. Tablets produced using the deep concave punches were found to experience greater rearrangement effort, limiting the amount of energy available for bond formation, thus forming weaker tablets. Rearrangement energy increased with increasing Ra, indicating higher energy requirement for particulate movements within the die for rougher particles. Plastic energy was found to decrease linearly with an increase in the Ra values of the samples. Plastic energy utilization only provided a rough gauge on the amount of energy available for bond formation process although it was not necessary representative of the strength of the bonds formed. Tablets formed with roller compacted lactose were stronger than those prepared using crystalline lactose, possibly due to the lack of fragmentation which is energy consuming, for roller compacted lactose. Longer dwell time was found to increase the dissipation of destructive elastic energy through stress relaxation also increased bond strength.
Conclusion: Flow measurements such as basic flowability energy, specific energy and powder compressibility could accurately predict the powder rearrangement requirements in the die cavity. High rearrangement energy decreased the energy available for bond formation, which translates to weaker tablets produced. Higher interparticulate frictional forces associated with rougher surfaces increased the rearrangement energy of roller compacted particles.
Berlinda Wen Ting Kok– National University of Singapore
Celine Valeria Liew– Senior Lecturer, National University of Singapore
Paul Wan Sia Heng– Associate Professor, National University of Singapore