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Effect of particle size on powder compaction and tablet strength using limestone

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The compaction of powders at high loads poses a challenge due to the simultaneous occurrence of particle rearrangement, compression, and breakage. Although tableting is a pivotal process in pharmaceutical technology, there is still a need for a more comprehensive understanding of the connection between macroscopic powder behavior and its micro-mechanical properties. This study focuses on both the powder compaction behavior and the quality properties of final tablets using a compaction simulator. Tableting was conducted over a broad size range of limestone powders, ranging from 10 to 400 MPa, to investigate and compare the powder compaction behavior under low and high confining stresses. The compactibility of limestone, specifically the relationship between porosity and stress, was evaluated using both the traditional (logarithmic) Heckel model and the newly proposed (double logarithmic) Wünsch model. The results confirmed the superiority of the latter in describing porosity changes during compaction and demonstrated the model's robustness for non-pharmaceutical powders. The qualitative impact of particle size and cohesion on bulk density during high-pressure compaction was observed to be similar to the low-pressure regime. However, the geometric interlocking effect of large-sized powders, identified in a previous study, became negligible at these high pressures. For particles with a median size (d50) less than 10μm, the tablet tensile strength exhibited almost no sensitivity to size variation. Conversely, for coarser grades, the tensile strength decreased with increasing d50 at all compaction stresses. Furthermore, the tablet tensile strength displayed a non-monotonic trend with median particle size.
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