Faraday Pail with Self Regulating Ion Repulsion.

Authors: Ion I. Inculet & G.S. Peter Castle Applied Electrostatics Research Centre

The University of Western Ontario

Abstmct - Measurements of Charge to Mass ratios of paint powders applied electrostatically with corona guns are often subject to m r on account of the ions which are collected at the same time with the charged powder. Several systems have been reported which attempt to prevent the very mobile ions from reaching the inside of the Faraday pail. Some such systems involve a conductive grid placed in the front of the orifice of the Faraday pail. When the grid is c o ~ e c t e d to ground, it collects both ions and paint powder which rapidly clogs the grid. If the grid is maintained at an electric potential of the same polarity as that of the corona, many of the ions are repelled while some powder penetrates through the grid and reaches the inside of the Faraday pail. The authors present the results of tests with a Faraday pail with a teflon shield with holes mounted between the corona painting gun and the pail. The teflon shield acquires a high surface charge from the ionic bombardment. When the charge reaches a saturation level, it repels the ionic bombardment almost completely and only powder is entrained in the Faraday pail by the air flow.

I. INTRODUCTION

The phenomenal increase in electrostatic coating with powder paints has reopened the search for an accurate method of measuring the charge-to-mass ratio of the paint powders applied electrostatically. In the majority of the cases, electrostatic painting of powders is camed out with corona guns. The ionic bombardment emerging from the corona gun which strikes and charges the paint particles represents only a small fraction of the total current. The majority of the ionic current traverses the space between the corona gun and the painting target without striking the paint powder. If one attempts to take a sample of the charged aerosol in a Faraday pail to measure the charge to mass ratio of the powders, one invariably collects a substantially larger charge than the charge brought by the paint particles. As a result, the calculated charge-to-mass ratios are always larger unless some ways are devised to prevent the free irons from reaching the Faraday pail.

In the past, several attempts have been made to remove the free ionic flow. Such methods have involved:

0 open grids maintained either at ground or at a sufficiently large potential to attract or repel the oncomingions while letting the powder paint particles penetrate the grid and reach the Faraday pail. Such systems require an adjustment of the potential of the grid and it has been found that the grid becomes coated with a substantial layer of the paint powder and may be subject to back ionization.

0 washed grids consisting of a series of vertical wires between the corona gun and the painting target maintained at ground potential and continuously washed by streams of water. In this way the build up of powder on the vertical wires is eliminated thus allowing for a indefinitely long operation time.['] However, depending on the velocity of the corona wind, some of the ions do penetrate the washed vertical wires adding to the measurement errors of the charge-to-mass ratios.

0 The use of a long (up to 50 cm.) plastic tube of small diameter connected to the intake of the Faraday pail['' has demonstrated some improvement in preventing a large number of the ions to penetrate into the Faraday pail. This system also has drawbacks; once the ions are inside the small diameter tube, the higher velocity and the absence of an internal field allows a substantial number of ions to reach the inside Faraday pail. Furthermore, depending upon the powder characteristics, a significant quantity of the paint particles may deposit on the inside of the tube thus avoiding detection in the Faraday pail.

The Faraday pail with self regulating ion repulsion presented in this paper involves the addition of a teflon shield in front of the usual double Faraday pail system. The arrangement is shown in Figure 1 and consists of a cylindrical double Faraday pail with a section of cylindrical insulation (ABS tubing) of the same diameter as the pail, attached to the front. The end of the ABS cylinder is covered with a highly insulating Teflon shield with holes to allow for taking of samples.

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II. PRINCIPLE OF OPERATION

The ionic bombardment emerging from the corona gun will charge the Teflon shield to the same polarity as that of the electrostatic painting gun. Depending on the electric field intensity at the plane of the Teflon shield, the potential will reach a sufficient value to repel practically all the ions. Figure 2 (a) & (b) shows the difference in the ionic current which reaches the Faraday pail at various potentials of the corona gun when the Teflon shield is on (see Figure 2(a)) and when the Teflon shield is removed (see Figure 2(b)). It can be seen that the Teflon shield reduces the ionic current by two orders of magnitude. Figure 3 (a) and (b) shows the effect of the Teflon shield during the actual operation of the painting gun when both charged powder and ionic current are drawn into the thimble of the double Faraday pail by means of vacuum suction. The total charge which reaches the Faraday pail when the Teflon shield is mounted, is substantially smaller than the charge when the Teflon shield is removed.

By measuring the total charge accumulated in the Faraday pail when the Teflon shield is on and dividing it by the mass trapped in the inside thimble, one can calculate the charge to mass ratios of the paint particles. Note: m e tests presented in this paper were carried out with silica particles in the form of a fine dust. n e particles diameter range (0.1 pm to 50 pn).

Figure 4 (a) and (b) shows the calculated charge-to-mass ratios of the silica dust powder with and without the Teflon shield. Without the Teflon shield and without taking into consideration the free ionic current which reaches the Faraday pail, the charge-to-mass ratios show to be one order of magnitude higher than the true charge-to-mass ratio values.

III. CONCLUSIONS

A Teflon shield with holes placed between the standard Faraday pail and the corona gun in electrostatic painting provides an effective self regulating repulsion system of the ions while allowing the air entrained less mobile paint particles to be collected by the Faraday pail for charge-to- mass measurements.

For very fine paint powders one must consider that some of the powder will be repelled at the same time with the ions. As such, in an industrial application it is recommended that every type of powder paint be analyzed individually in terms of the fraction which would be repelled by the Teflon shield and thus not contribute to the measured charge-to-mass ratio. However, if such small

particles are repelled by the shield, there is an indication that such particles are very well charged - and as such, the determined charge-to-mass ratios by the Faraday pail with the self regulating ionic repulsion will be underestimated rather than overestimated as is the case without the shield.

IV. REFERENCES

[l] B.D. Moyle & J.F. Hughes, "Particle Charging and Absolute Measurement of Charge to Mass", Conf. Rs. IEEE/IAS Annual Meting 1982, pp 1191-1195.

[2] N. Masui and Y. Murata, "Method for Measuring the Powder charge in the electrostatic Powder-Coating Process, " Rev. Sci. Instum. a(4 ) 1982, pp. 532- 533.

Figure 1.

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