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Powder electric charge buildup: an important lever to improve processability
This article explains how the Granucharge can be used to improve the processability of the powders
Powder electric charge buildup: an important lever to improve processability
Powders are complex materials involving numerous physical mechanisms. The macroscopic behavior of a powder is usually quantified in terms of flowability, packing ability, mixing, or blending depending on the industrial process involved. The collective behavior observed is a consequence of multiple interactions acting at the scale of the particles that can be classified into contact interactions, driven by friction, and cohesive interactions. The cohesive nature of the powder is a consequence of the cohesive interactions that reduce the freedom of displacement of the particles. Cohesive interactions are mainly due to capillary bridges lying between the particles, Van der Walls forces, electrostatic forces, and possibly hydrodynamic forces due to a surrounding fluid.
Electrostatic forces arise from electric charges lying at the surface of the particles. Indeed, when a powder flows, the contacts between the particles and with the conveying material (pipes, blades, machine parts) create electric charges due to the triboelectric effect. This leads to an electric charge buildup that strengthens the electrostatic cohesive interactions. Depending on the powder material properties, these electric charges may be able to dissipate when the powder is let at rest. Especially, the presence of capillary bridges usually helps to dissipate these electric charges by the setting of a conductive network. Unfortunately, the mechanisms involved in the creation of these electric charges in powders are complex and still not well understood.
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Powder electric charge buildup is commonly accompanied by a decrease in processability. Indeed, the decrease of flowability as a consequence of stronger electrostatic cohesive interactions leads to problems in powder conveying, silo discharge, or more generally flow through the different parts of the process. As an example, good flowability is critical to ensure mass feeding consistency in the die filling during the production of pharmaceutical tablets. In additive manufacturing, powder cohesiveness has to be minimized to produce homogeneous powder layers during the recoating. Moreover, fine powders submitted to electric charge buildup tends to exhibit a stickier behavior, which can lead to production stop due to clogging of the pipes as well as higher machine cleaning time. In addition, electric charge buildup can trigger more drastic phenomena like dust explosions with important consequences on safety.
Although powder electric charge buildup needs to be avoided to preserve good powder behavior, some applications take benefit of it. For example, in dry powder inhaling applications, the API particles are bonded to a lactose carrier to carry it to the lung. Electric charges mainly contribute to this bonding and are thus an essential part of the process. A good understanding of the electrostatic properties of both materials is critical to ensure proper carrying of the API and prevent early release before the material has reached the targeted release spot.
Assessing and controlling the electrostatic properties of powders is thus an interesting lever to improve their processability. However, because of the complexity and the lack of knowledge on the mechanisms involved in electric charge buildup and its effect on the macroscopic behavior of the powder, it has only been investigated recently. Thanks to state-of-the-art characterization method, we can now have a deeper understanding of how the different properties influence powder electric charge buildup ability. The GranuCharge instrument by GranuTools has been specifically designed for this purpose by allowing to quantify the variation of the charge density in the powder due to its flow. Therefore, the important information gathered then allows to identify and select the relevant parameters to improve the electrostatic properties of new high-end products.