(735am) Evaluation of Powder Triboelectric Properties: A Must Have for Material Optimization

The charge buildup appearing during the processability of powders is a major problem in lot of industries involving the handling of powders. Indeed, during the flow of particulate materials the friction between the particles leads to the generation of electrostatic charges, a mechanism called tribocharging. As a consequence, a continuous accumulation of charges can appear inside the material, leading to numerous processability problems. Indeed, the charged particles have a high tendency to stick on the machine parts causing clogging of the pipes and time-consuming cleaning of the process line. Fine charged particles will also tend to agglomerate or stick on large grains, thus inducing a shift from the optimal particle size distribution defined at the powder formulation stage. The charge buildup will also result in the increase of the powder cohesiveness, electrostatic charges being one of the main cohesive forces, which is associated with a decrease of flowability. In some applications/materials, the presence of charges can also trigger dust explosions and thus must be avoided. Finally, with the development of continuous manufacturing, especially in the pharmaceutical industry, problems with charge buildup which where manageable for batch-to-batch processes become a major challenge as continuous manufacturing promotes the charge accumulation. Being able to accurately evaluate the sensitivity of a powder to tribocharging is thus a must have to move forward in the development of optimized processes and materials.

The tribocharging mechanisms are still not well understood, mainly due to their complexity. The charging ability of a powdered material is highly dependent on numerous properties such as the size and shape of the particles, the surface chemistry or the presence of water due to environment relative humidity. In addition, during the processing the powder interact with the machine parts (feeders, blades, pipes, ...), and the material of these pieces will also drastically change the tribocharging sensitivity. Therefore, the inherent complexity of tribocharging of powders prevent from using theoretical models for accurate prediction of the charge buildup.

But how can a powder formulator or process line designer evaluate and predict if the newly developed material will behave correctly during the production? One option is to produce a large batch and test it directly into the production line. However, this is a cost and time consuming process, especially with costly products like APIs. Fortunately, state of the art characterization methods have been developed during the last decade to assess this problematic. Especially, a powder tribocharging analyser (GranuCharge, Granutools, Belgium) allows to precisely evaluate the charge buildup during the flow. The instrument is composed of an electrometer allowing to accurately measure the net charge of the powder before and after the flow through a set of pipes. The material of the pipes can be changes to assess the influence of the process parts material on the charge buildup. The new characterization method opens up new possibilities for material optimization. In this presentation, we will show how the information gathered from this characterization method can be of great interest for tribocharging investigation. Several case studies for real applications will be presented to demonstrate the usefulness of the method.