The manufacturing and handling of polyethylene (PE) powders are associated with undesired charging, resulting in agglomeration of charged particles, wall sheeting and eventually leading to plugging of reactors/conveyors. Although some triboelectric theories were developed; these are mostly specialized on one type of charging and are rarely useful for the quantitative prediction of charging. We aim to quantitatively describe the charging of PE powders by charging them in the shaking apparatus, in which we modify the frequency of particle-particle and particle-wall collisions. The key part of our research is the ability to measure the charge distribution, because the particle-particle charging exhibits broadening of charge distribution among the particles of charged sample of PE. To measure charge distribution of our PE samples, we utilized the free-fall electrostatic separator that separated the particles according to their charge densities into several fractions. Afterwards, we measured the charge of each such fraction in the Faradayâs pail connected to an electrometer. Some particles of the PE sample reached significantly larger charges of opposite polarities and therefore they formed agglomeration centres. Such bipolarity in charging due to particle-particle collisions, as well as the formation of the agglomeration centres was also predicted and quantified using our particulate mathematical model based on discrete element method. Apart from agglomeration, fouling can play role in industrial systems, as PE particles of any charge polarity are attracted towards the metal walls of reactors and conveyors due to the effect of electrostatic induction. In our experiments, we observed significant build-up of charge of only one sign in cases where particle-wall collisions were frequent relative to the particle-particle collisions. Particle-wall charging thus doesnât necessarily leads to the formation of agglomerates, it may, however, lead to the fouling. Our experiments regarding the effect of temperature on charging showed that the saturation charge of PE particles increases with temperature of particles; however, no increase in charging was observed when only the aluminium slide (serving as a counterpart used for charging of PE particles) was heated. This indicates that the increase in charging at elevated temperatures is associated with the properties of PE particles, rather than with properties of the metal. A possible explanation could be the softening of PE particles at elevated temperatures. As for the studies of impact velocity effect on charging, we observed that the saturation charge of PE particles increased with the maximum impact velocity. Such behaviour could be explained by the structure of PE particles that effectively enables the particles to be elastic and also by the surface roughness of the material. Specifically, if force is applied on a PE particle, the asperities on the particle surface bend inside the particle, thus effectively increasing the total particle area accessible for contact with a wall and thus also for charging. This work can be valuable for further work on the understanding of the charging mechanism, as well as it can provide some guidance for the industry.
Konopka, L. and J. Kosek, Discrete element modeling of electrostatic charging of polyethylene powder particles. Journal of Electrostatics, 2017. 87(Supplement C): p. 150-157.
JantaÄ, S., Konopka, L. and J. Kosek Experimental study of triboelectric charging of polyethylene powders: effect of humidity, impact velocity and temperature. Advanced Powder Technology (Submitted)