(276a) Segregation in Powders: An Interplay between Particle Properties and Cohesion

Most of the powders used in industrial processes are polydisperse materials, i.e. the particle size is not unique but distributed over a range of sizes and usually described by the particle size distribution. However, when such polydisperse materials are submitted to flow or vibrations, segregation mechanisms come into play. The segregation will modify the size distribution across the material and as a consequence induce changes in the powder properties.

In lots of industries, powder batches have to meet strong requirements to guarantee good processability and the quality of the final product. The production of pharmaceutical products is a good example in regards to this, as the powder properties must comply with strict regulations to ensure the consistency of the production. The evaluation of the powder characteristics is usually done in the lab after the formulation. However, if a change in powder properties occurs when the powder goes through the different steps of the production line (feeding, mixing/blending, coating, tableting or capsule filling, ...) the processability will be affected leading to the final products being out of specification. Segregation can be one of the mechanisms at the origin of this drift in powder processing and therefore have to be properly understood to give better prediction of the actual processability of the batches. Moreover, continuous processing commonly used in production and currently under development in pharma industries is expected to highlight the problems associated with segregation.

The size distribution is one of the most important particle properties influencing segregation and probably the most studied. However, other properties such as shape and interparticle friction can lead to different segregating behaviors. The cohesive interactions lying between the particles, mainly due to the presence of capillary bridges and electrostatic interactions, play also an important role. Usually, the presence of cohesive forces will prevent an easy reorganization of the particles and thus will oppose the segregation mechanisms. However, an inhomogeneous distribution of the cohesive interactions across the material can lead to cohesion-driven segregation mechanisms. Despite the omnipresence of cohesive polydisperse powders in industrial applications, most of the studies have focused on dry non-cohesive materials.

In this study, the effect of cohesion on segregation has been investigated over a wide range of different materials. The cohesion of the powder samples is controlled via pre-conditioning at different relative humidities. The resulting change in cohesiveness is then evaluated in a quasi-static regime with a tap density analysis (GranuPack, Granutools, Belgium), and in a dynamic regime with a rotating drum method (GranuDrum, Granutools, Belgium). The segregation is investigated via two methods. Using the ability of the GranuPack instrument to measure the bulk density evolution after each tap allows us to investigate the effect of segregation on the packing dynamics. In addition, the rotating drum method used in this study allows for performing hysteresis analysis. The measurement is done by following an increasing rotating speeds sequence then following the same sequence in a decreasing order. As segregation will lead to temporal modifications of the flowability during the measurement, that will be seen as an hysteretic behavior between the two speeds sequence.