Date of Award
2008
Degree Type
Thesis
Degree Name
Doctor of Philosophy
Program
Chemical and Biochemical Engineering
Supervisor
Dr. Jingxu (Jesse) Zhu
Second Advisor
Dr. Hui Zhang
Third Advisor
Dr. Sohrad Rohani
Abstract
A series of experiments was conducted in exploring the fundamentals of negative point- to-plane geometry corona discharge and the underlying mechanisms of corona charging of powder coating processes. In exploring the characteristics of corona discharge, it was found that relative humidity influences the evolution and magnitude of corona currents by changing charge mobility and the suppression effect of space charge, and that the increasing relative humidity produces a concentrating effect on the current density distribution. In particular, edge effects were demonstrated by the evident rise in the corona current density of the fringe region, and such edge effects intensify with the increased inter-electrode distance and the reduced plane size. Further characterization of corona discharge led to a new general formula in characterizing the current-voltage relation. The formula was defined as: 1 = K(V-Vo)" and n in the range of 1.5-2.0. Under the examinations of the results from earlier studies, it was revealed that an optimal exponent n always exists in the designed range, which can best represent the current-voltage characteristics. It was disclosed that the exponent n has the strongest dependence on the point radius but not so sensitive to the change of ambient conditions and corona polarities. The superiority of this formula was exhibited in explaining the inconsistencies of previous formulae and in being applicable to point-to- ring geometries as well. Upon the understanding of corona discharge, more efforts were given to investigate corona charging process of different particulates in electrostatic powder coating. First, the distorted electric field between inter-electrodes was revealed by examining current density distribution due to two particulate (coarse and ultrafine) systems. It was demonstrated that both particulates cause strong corona quenching in corona currents and deform the current density distribution. By comparison, the coarse particulate system is iii prone to produce weaker quenching effect on its current density but severer deformation on its current density distribution with the increasing charging voltage. The characteristics of particle charging and deposition are of great importance in powder coating processes, for corona charging is both a charging and deposition process. It was discovered that charged powder particles are liable to deposit on the substrate with coneshaped distributions, except for the fringe region where the edge effect of the electric field contributes more particles to accumulate. In particular, the charging efficiency of inflight particles and back corona in the deposited layer are competing in determining the deposition efficiency, and the highest deposition efficiency is a compromise between the two. Even though the amount of overall deposited particles may increase with an extended spraying duration, the deposition rate decreases. For the first time, it was disclosed that the secondary charging dominates the characteristics of the charge-to-mass ratio of locally deposited particles. In comparison, the ultrafine particulate is prone to exhibiting a fairly uniform distribution of local charge-to-mass ratio in the deposited layer, while the coarse particulate shows a concave distribution. The underlying mechanisms are attributed to their differences in corona quenching and current density distributions. For the overall deposited particles, the average charge-to-mass ratio of the ultrafine powder decreases with the increased charging voltage while that of the coarse powder increases. However, for both particulates the average charge-to-mass ratio of the overall deposited particles decreases with an extended spraying duration, due to the intensified back corona. The particle size evolution in the deposited layer was further investigated with the two particulates. The study demonstrated that the powder coating process is a size selective process, in which deposited particles show some size discrepancies between different regions. However, the ultrafine particulate system can greatly alleviate the size discrepancy. It was also revealed that small particles preferentially adhere to the substrate for the coarse system while large particles preferentially deposit for the ultrafine system. With the extended spraying duration, the deposited particles show a decreasing tendency iv in size for the two particulate systems, which may be attributed to the intensified back corona. The discoveries in this thesis contribute to the understanding corona discharge and corona charging processes of electrostatic powder coating.
Recommended Citation
Meng, Xiangbo, "STUDY ON CORONA DISCHARGE AND CORONA CHARGING IN ELECTROSTATIC POWDER COATING PROCESS" (2008). Digitized Theses. 4520.
https://ir.lib.uwo.ca/digitizedtheses/4520