(225a) Combined Experimental and Modeling Approach to Triboelectric Charging of Polyethylene Powders

The charging of objects caused by their mutual friction is called triboelectric charging or triboelectrification. Triboelectrification accompanies not only our daily activities like brushing hair or walking in shoes, but also several industrial processes. And because the charging is a surface-related phenomenon, it is most apparent in systems with high specific surface like powders (polymers, pigments, flour, drugs). For example, in fluidized-bed polymerization reactor, the polymer particles collide with other particles and with the device wall, thereby generating the excess electrostatic charge on their surfaces. Charged particles, in general, are attracted to the metal walls of devices due to electrostatic induction. Moreover, oppositely charged particles attract each other. Such behavior contributes to the unwanted particle agglomeration and fouling, which can impair the production and manufacturing of polymers. Although triboelectric charging has been documented for centuries, its mechanism hasnâ??t been fully explained yet. Thus we combined the cascade method apparatus (that is a slide followed by the Faradayâ??s pale) with Discrete Element Method (DEM) based model of colliding particles, in order to study the charging of polyethylene (PE) powder particles. Using the apparatus, we measured not only the saturation charge of PE particles, but also the charging dynamics as a function of industrially relevant parameters (temperature, air humidity, material of the slide, presence of sorbent). The charging was less pronounced at low humidities and low temperatures. Moreover, we observed significant charging even for the contact of â??identicalâ?? materials (PE particle with PE slide). Our DEM model of particle-wall contact charging was complemented by the capacitor model of charging. Another model was used for the prediction of particle-particle contact charging, in which Surface States theory was implemented. The model predicted fast charging for systems of a wide particle size distribution and/or spatial segregation of â??smallâ?? and â??bigâ?? particles. Such a result correlates well with the observations of charging in natural and industrial granular systems. Moreover, the model predicted more pronounced charging at high temperatures, which correlates well with our measurements. Combining particle charging and elasticity measurements with modeling, we further address the question of whether the charging depends only on the contact area of colliding objects (which correlates with the spontaneous release of electrons during collision) or also on the amount of dissipated mechanical energy during the contact (which correlates with the excitation energy necessary for the electron transfer). Our study thus contributes to a better understanding of triboelectric charging fundaments and, at the same time, may provide valuable information for polyolefin industry