Prof. Chuan-Yu Wu
Department of Chemical and Process Engineering, University of Surrey 

Air entrapment is regarded as one of the factors responsible for tablet fracture during the manufacturing process, especially for formulations with low air permeability. However, there is a little information on entrapped air induced fracture during tablet manufacturing in the literature. In the current study, a combined experimental and numerical investigation was performed to model tablet fracture induced by an entrapped air pocket. For the experimental study, microcrystalline cellulose (MCC) and mixtures of MCC and paracetamol at various mass fractions were considered. To simulate the deformation behaviour of an entrapped air pocket, a rubber ball was placed in the powder bed at a specified position (i.e. in the centre of ¼ of the initial powder height away from the bottom punch). The powder bed is then compressed at various maximum compression loads.  The produced tablets were examined using X-ray Micro-Computed Tomography (XMCT), in order to identify the occurrence and patterns of internal cracks and fracture surfaces. Finite element modelling was also performed to provide a mechanistic analysis of the compaction behaviour of powders with an entrapped air pocket.  It was found that the entrapped air pockets can rupture compressed tablets and the fracture patterns observed experimentally are consistent with the localized effective stress during unloading. It was also noticed that the fracture tendency increases as the maximum compression pressure decreases and as the apparent tensile strength decreases. Furthermore, the fracture tendency dramatically increases when the entrapped air pocket is located closer to the tablet surface.