Processability of powders in high load compaction constitutes a challenge due to particle rearrangement, compression and breakage occurring simultaneously. Although tableting is a central operation in pharmaceutical technology, a better understanding of the link between the macroscopic powder behaviour and its micro-mechanical properties is still required. In the present study, a dual focus on the powder compaction behaviour and the quality properties of final tablets using a compaction simulator is presented. Tableting has been performed for a wide size range of limestone powders from 10 to 400 MPa, in order to understand and compare the powder compaction behaviour at both low and high confining stresses. Compactibility of limestone, the relation between porosity and stress, has been assessed with both the classical (logarithmic) Heckel model and the newly proposed (double logarithmic) Wünsch model, confirming the improvement of the latter to enhance the description of the porosity change during compaction, as well as the model robustness towards non-pharmaceutical powders. The qualitative effect of particle size and thus cohesion on the bulk density at high pressure compaction is found to be very similar to the low pressure regime. However, the geometrical interlocking influence of large size powders found in a previous study becomes irrelevant at such high pressures. For d₅₀<10 μm, the tablet tensile strength remains almost insensitive to the size variation. However, for the coarsest grades, the tensile strength decreases with increasing d₅₀ at all compaction stresses. In addition, the tablet tensile strength is found to follow a non-monotonic trend with median particle size.

The bulk properties of powders depend on material characteristics and size of the primary particles. During storage and transportation processes in the powder processing industry, the material undergoes various modes of deformation and stress conditions, e.g., due to compression or shear. In many applications, it is important to know when powders are yielding, i.e. when they start to flow under shear; in other cases it is necessary to know how much stress is needed to keep them flowing. The measurement of powder yield and flow properties is still a challenge and will be addressed in this study. In the framework of the collaborative project T-MAPPP, a large set of shear experiments using different shear devices, namely the Jenike shear tester, the ELE direct shear tester, the Schulze ring shear tester and the FT4 powder rheometer, have been carried out on eight chemically-identical limestone powders of different particle sizes in a wide range of confining stresses. These experiments serve two goals: i) to test the reproducibility/ consistency among different shear devices and testing protocols; ii) to relate the bulk behaviour to microscopic particle properties, focusing on the effect of particle size and thus inter-particle cohesion. The experiments show high repeatability for all shear devices, though some of them show more fluctuations than others. All devices provide consistent results, where the FT4 powder rheometer gives lower yield/steady state stress values, due to a different pre-shearing protocol. As expected, the bulk cohesion decreases with increasing particle size (up to 150 μm), due to the decrease of inter-particle cohesion. The bulk friction, characterized in different ways, is following a similar decreasing trend, whereas the bulk density increases with particle size in this range. Interestingly, for samples with particle sizes larger than 150 μm, the bulk cohesion increases slightly, while the bulk friction increases considerably—presumably due to particle interlocking effects—up to magnitudes comparable to those of the finest powders. Furthermore, removing the fines from the coarse powder samples reduces the bulk cohesion and bulk density, but has a negligible effect on the bulk friction. In addition to providing useful insights into the role of microscopically attractive, van der Waals, gravitational and/or compressive forces for the macroscopic bulk powder flow behaviour, the experimental data provide a robust database of cohesive and frictional fine powders for industrially relevant designs such as silos, as well as for calibration and validation of models and computer simulations.