Measuring Electrostatic Behavior of Nonconductive Powders
Often powders are stressed by pneumatic transport, aerosolization or other mechanical movements which can cause electrostatic charging of the powder particles. These processes can be emulated in the powder flow cell from Anton Paar. This report describes the electrostatic charging and discharging behavior of powders and granules using the model sample of glass beads. The dependency of electrostatic charging on the flow rate during fluidization and on the fluidization duration was investigated. In addition, how the wall material of different measuring tube materials affects the charging and discharging was also explored. Furthermore, methods to discharge the powders were explored.
Many powders show electrostatic charging during movement and flow. This is known as the “triboelectric effect” and is caused by the rubbing together of the particles. The electrostatic charge can impact the powder behavior and cause problems in transport and manufacturing processes. This can also result in a higher tendency to agglomerate or to start caking.
For pneumatic transport systems, the material(s) used is an essential economic element. In this context, knowledge about the electrostatic behavior of powders and granules is important. A conductive pipeline material can reduce electrostatic charging, while nonconductive material will produce more electrostatic charging. These phenomena in turn can change the flow behavior of a non-conductive material and influence various process constants.
Another example where electrostatic charging is necessary but can cause problems, is electrostatically sprayed powder coatings. The electrostatic charging impacts atomization, as well as the adhesion of the powder coatings onto the substrate. Too much electrostatic charging can cause issues during spraying by clogging the nozzle.
Electrostatic charging can be found in many different kinds of materials, like polymers (PE, PTFE, PVC, PS), cotton, paper, wool and glass.
Two measurement methods were used in this application report – pressure drop and cohesion strength.
The pressure drop method is used to study fluidization properties of powders, e.g. which air flow is necessary for incipient and full fluidization.
The cohesion strength method is a fast and reproducible measurement method with a high sensitivity to changes in powders. Cohesion strength is a measure of flowability of powders, and depends on the bonding forces between the particles. Therefore this method is very sensitive to electrostatic charging of particles, as electrostatic charging of powders and granules causes a decrease in flowability, or as shown in this report, an increase in cohesion strength.
More information about the two methods can be found in the application report “An Introduction to Powder Rheology”.
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