Time-dependent densification behaviour of cyclodextrins

Understanding of volume reduction mechanisms is a valuable aid in the development of robust cyclodextrin tablet formulations. The particle and powder properties of alpha-, beta-, gamma- and hydroxypropyl (HP)-beta-cyclodextrins and their behaviour under compression were examined. The cyclodextrins studied showed big differences in particle-size distribution and particle shape. The highest densification on tapping was found for cyclodextrins having the smallest particle size. Cyclodextrins were compressed using single-sided saw-tooth displacement-time profiles at rates of 3 and 300 mm s-1 with a compaction simulator. The densification of the powders was examined by Heckel treatment, using the tablet-in-die and ejected-tablet methods. The cyclodextrins were denser at the beginning of the tableting process (at low pressures) if high rather than low velocity was used. Ranking according to their tendency toward total deformation and permanent plastic deformation was: HP-beta-cyclodextrin > beta-cyclodextrin > gamma-cyclodextrin > alpha-cyclodextrin. The ranking order in strain-rate sensitivity (SRS) of total deformation was HP-beta-cyclodextrin > > gamma-cyclodextrin > or = alpha-cyclodextrin > or = beta-cyclodextrin. On the basis of yield pressure values and the Heckel plot profiles, all the cyclodextrins were highly prone to plastic deformation. Cyclodextrins showed time-dependent consolidation behaviour manifested as increased yield pressure with decreased contact time. A ratio was defined between the SRS of fast elastic recovery and total elastic recovery. The two materials with high ratios, HP-beta-cyclodextrin and beta-cyclodextrin, were especially prone to fast elastic recovery with increasing punch velocities; gamma-cyclodextrin and alpha-cyclodextrin had low values and were less prone. On the basis of this parameter it might be possible to categorize pharmaceutical materials according to capping tendency.