Purpose.This study investigates the extent of tablet core relaxation by measuring the dimensional and hardness changes over time on different tablet compositions. Results from the work are to support industrialisation of the continuous film coater, where tablet relaxation time between compression and coating will be significantly shorter (minutes) compared to batch coating (hours).
Methods.Based on literature, materials with different elasticity: sodium chloride crystals, pregelatinized starch, microcrystalline cellulose (MCC) and a placebo blend which contained MCC, anhydrous lactose, croscarmellose sodium and magnesium stearate (64:32:3:1%w/w) were selected. The materials were compressed into 10mm round flat faced tablet cores using the EHS compaction simulator. In-die tablet thicknesses were obtained directly from the displacement between punches during compression. Tablet core weight, thickness, diameter and hardness were measured immediately after ejection, and at various time points (up to 24hours).
Results.There are differences in the extent of tablet relaxation amongst various tablet compositions. Change in tablet thickness was essentially complete immediately out of die for all, except auxetic material e.g. starch. Tablet hardness for sodium chloride and starch increased over time. The hardness of sodium chloride nearly double in half an hour after compression. This may be due to increased bonding between the adjacent crystals. As the porosity of a tablet core is reduced, the hardness approaches a maximum value corresponding to the intrinsic strength of the solid material. Placebo and MCC conversely showed slight decrease in hardness over time, though the tablets are still mechanically strong to withstand further handling. The hygroscopic nature of MCC tends to take up moisture, leading to reduction in surface energy which decreases tablet hardness.
Conclusions.This study highlights the importance of tablet core relaxation post compression, and the extent of relaxation can be quantified by tablet thickness and hardness. It is important to monitor the impact of APIs on mechanical properties of tablets over time in order to determine the required buffer time for continuous film coating.
Jiyi Khoo, PhD
GlaxoSmithKline R&D, New Frontiers Science Park South Site, Harlow, UK
Gurjit Bajwa
GlaxoSmithKline R&D, New Frontiers Science Park South Site, Harlow, UK