Predicting the pressure distribution during roll compaction from uniaxial compaction measurements

The accuracy of predicting the pressure distribution in a roll compaction process using confined uniaxial compaction was assessed using microcrystalline cellulose (MCC, Avicel-PH102) as a model pharmaceutical powder. This involved an instrumented roll press and a range of roll gaps and roll speeds. The measured pressure-displacement data were described using the Johanson equation and a comparison was made with data obtained from uniaxial compaction. In addition, an existing uniaxial compression relationship was adapted to describe the data with the advantage that it can be applied to the whole pressure range especially at the lower pressure range. It has been shown that the predicted maximum pressure values using uniaxial compaction data are generally less than those measured. This was attributed to the more complex powder flow field in roll compaction compared to uniaxial compaction, which leads to (1) a non-uniform pressure distribution across the roll width with the pressure in the centre of the roll width being greater than the mean value and (2) a shear component of the flow field, and even the formation of shear band when the roll gap is small. Despite the differences between the measured and predicted pressure distributions, it is believed that uniaxial compaction is a useful method for screening different formulations in terms of ranking their expected performance in roll compaction.