Evaluation of the plug formation process of silicified microcrystalline cellulose

To investigate the powder plug formation process of silicified microcrystalline cellulose (SMCC) under compression forces consistent with automatic capsule-filling machines, a single-ended saw-tooth wave was used to make powder plugs with different heights (6, 8, 12 mm), at two different punch speeds (1 and 50 mm/s) on a tablet compaction simulator. SMCC was compared to Starch 1500, anhydrous lactose (direct tableting grade), and microcrystalline cellulose. Heckel analysis showed that 'apparent mean yield pressures' (AMYP) of all tested materials increased with an increase in the plug height and punch speed. AMYP appeared to depend on the material type and punch speed. Not all materials fit the Shaxby-Evans relationship at such low compression forces (less than 250 N). Only SMCC 90, SMCC HD90 and anhydrous lactose data fit the equation at both punch speeds. Due to poor axial load transmission, the R values of all tested materials decreased with an increase in the plug height. The experimental data fit the Kawakita equation quite well. Overall, Kawakita's b values were inversely related to AMYP values. The maximum breaking force (MBF) of a 12 mm plug formed at a punch speed of 50 mm/s correlated well with the work of compaction, except for SMCC HD90 and SMCC X, which exhibited very high MBF values. This research demonstrated that several grades of SMCC produced plugs having higher MBF than anhydrous lactose and Starch 1500 under similar compression conditions. The apparently higher compactability of these materials at low plug formation forces may be beneficial in developing direct fill formulations for automatic capsule filling machines.