Electrostatic powder coating of insulating surfaces using an alternating polarity internal corona gun

An internal-corona (IC) gun operated at l-Hz alternating polarity (AP) was used in an attempt to powder coat insulating plastic plates without using c onductive primers, metal backings, or ion "backings." The IC gun was effect ive in charging the powder particles and emitted very few free ions, but it required frequent cleaning. The Q/M ratios obtained using de voltage were very good, with negative polarity better than positive. The use of charge s eparation electrodes verified that the l-Hz AP gun emitted "waves" of oppos itely charged particles. The charged powder average velocity between the gu n and the target (3.8 m/s) was calculated from the measured transit time (0 .13 s), and was dominated by the airflow. The current waveform produced by the charged particle motion yielded results consistent with the Q/M data an d the measured transit time. The measured saturation surface charge densiti es (sigma(s)) were used together with the total Q/M ratios to calculate the saturation surface mass densities, which were in good agreement with the m easured values (0.3-0.5 g/m(2)). The low free ion emission of the IC gun al lowed charged powder to deposit (until target saturation) instead of just t he higher mobility free ions. The sigma(s) increased at higher powder feed rates, most likely due to an increase in the powder space-charge field. Fie ld charging theory for an insulating plane target was derived and used to c alculate the target charging rate and sigma(s). The theory was also used to obtain the applied electric field strength near the target surface from th e measured cr,, The very low de transfer efficiencies (TE's) of 1%-2% obtai ned using the IC gun to coat insulating targets were only improved to 9% by using AP voltage. The calculated optimum AP frequency was 2 Hz, which was in excellent agreement with the measured value. However, the AP TE's were b elow those expected, most likely because of powder adhesion problems result ing from the use of AP, Microphotographs of the deposited powder layer show ed significant agglomeration and some tower formation, which suggests that the oppositely charged particles formed dipole pairs and were attracted mor e to each other than to the target surface. Poor adhesion could have also r esulted from layer neutralization by free ions or ejection of particles fro m title layer by the reversed field. Due to the adhesion problem, AP may be better suited for use with preheated insulating targets or with liquid pai nts.