(656e) Evaluating Electrostatic Charging of Powders - the Challenges

Since ancient times, it’s been known that materials such as amber attract low mass particles after rubbing, indeed the terms electron and electricity are derived from the Ancient Greek for amber. Whilst this phenomenon is widely recognised and accepted, electrostatic charging and triboelectrification are poorly understood, especially with regards to powder and particulate handling. The propensity of some powders to electrostatically charge can have implications with regards to in-process performance, for example it can lead to adhesion onto equipment surfaces and segregation of different components due to charge differences. Furthermore, there are fire/explosion safety considerations associated with static discharge.

The complex interactions between powder handling, including flowability, and charging make effective measurement difficult, impacting the ability to design processes to control/mitigate charging problems. In most cases the propensity to charge is managed by controlling the relative humidity, earthing equipment allowing the charge to dissipate and/or in-line ionising equipment.

Many measurement methods have been devised for investigating this phenomenon, such as single particle impaction [1]; charging in gas/solids flow – either pneumatic transport/cyclones [2,3] or fluidised beds [4,5]; vibrating plates [6]; or cascades [7], however due to the inability to control the initial state of the powder and the poor reproducibility of the test method, these techniques have not been widely adopted within industry. One reason for poor repeatability is the potential to change the charge state of the powder each time it’s handled or comes in to contact with a chemically different surface. For this reason, the test conditions, including the equipment, environment and test protocols must be carefully controlled in order to minimise unwanted powder and equipment charging.

This presentation addresses the challenges of developing an industrially appropriate tool for measuring charge and its effect on powder flowability, investigating the electrostatic behaviour of both powders and contact (equipment) surfaces. This on-going study includes the following areas of investigation:

• Discharging powders and equipment
• Powder handling protocols for reducing unwanted charging
• Propensity of powders to charge – including rate of charge and maximum charge
• Interaction between different powders and equipment surfaces

A strong understanding of the fundamental principles of electrostatic behaviour and triboelectric charge will aid the development of a suitable tool for charging powders and surfaces in a controlled manner and measurement of the resultant dynamic flow properties using powder rheology, allowing process engineers to predict changes in flow behaviour due to charging as well as selecting suitable equipment and equipment settings.

The presentation will highlight the challenges associated with controlling and quantifying electrostatic charging and the steps required to counteract them, followed by discussion of the latest results from the study.

References

[1] Matsuyama, T., Yamamoto, H., 1994. Charge transfer between a polymer particle and a metal plate due to impact. IEEE Transactions on Industry Applications 30, 602–607.

[2] Masuda, H., Komatsu, T., Iinoya, K., 1976. The static electrification of particles in gas–solids pipe flow. AIChE Journal 22,558–564.

[3] Rowley, G., 2001. Quantifying electrostatic interactions in pharmaceutical solid systems. International Journal of Pharmaceutics 227, 47–55.

[4] Iuga, A., Calin, L., Neamtu, V., Mihalcioiu, A., Dascalescu, L., 2005. Tribocharging of plastics granulates in a fluidized bed device. Journal of Electrostatics 63, 937–942.

[5] Zhao, H., Castle, G.S.P., Inculet, I.I. 2002. The measurement of bipolar charge in polydisperse powders using a vertical array of Faraday pail sensors. Journal of Electrostatics 55, 261–278.

[6] Higashiyama, Y., Ujiie, Y., Asano, K., 1997. Triboelectrification of plastic particles on a vibrating feeder laminated with a plastic film. Journal of Electrostatics 42, 63–68.

[7] Oguchi, T., Tamatani, M., 1993. Contact electrification phenomena and powder surface treatments. Wear 168, 91–98