Robert A. Sims

Influence of powder properties on the performance of electrostatic coating process

Abstract Properties of powder, e.g. particle size distribution, chemical composition, triboand corona charging characteristics, electrical resistivity, hygroscopicity, fluidity and shape distribution play significant roles on the performance of powder coating such as transfer efficiency, film thickness, adhesion and appearance. Our focus in this paper is on particle size distribution, fluidity, charging characteristics and their effects on the first pass transfer efficiency. Even a minor difference in powder properties showed differences in the applications process illustrating the importance of characterizing physical parameters and control techniques to achieve desirable performance.

Triboelectric charging of polymer powders in fluidization and transport processes

Steady flow of powder at a desired rate is a necessity for controlling thickness and uniformity of the deposited powder layer in electrostatic spray painting. In most powder coating applications, the polymer powder is fluidized to transport the powder to the spray gun using a powder pump. The powder delivery tube is often long; sometimes in excess of 10 m. During fluidization and transport, the powder particles acquire electrostatic charge due to triboelectrification. This tribocharging of powder can present some problems: (1) agglomeration of powder due to bipolar charging, (2) deposition of charged powder on the inner wall of the transport tube, and (3) biasing of the final charge-to-mass ratio (q/m) distribution of powder exiting the corona gun. In this study, we investigate the role of tribocharging in fluidization, flowability, and q/m distribution as functions of particle size distribution (PSD), fluidization time and transport tubes of different materials. A particle size distribution shift due to the polarity of the bipolarly charged particles is shown. A brief discussion is presented on minimizing the adverse effects due to powder tribocharging, therefore improving spray uniformity.

Electrostatic Effects on Dispersion, Transport, and Deposition of Fine Pharmaceutical Powders: Development of an Experimental Method for Quantitative Analysis

Currently, there is no standard method for testing the electrostatic properties of pharmaceutical powders. The objective of this study was to develop a method of characterizing the dispersion, charging, and transport properties of fine powder flowing through tubes of different materials. Powders of known composition and size distribution were dispersed pneumatically and transported through a short section of tubing containing spiral baffle inserts of the same material to simulate powder flow in long sections of horizontal and vertical tubes with bends. The test powder was dispersed using ring jet suction and passed through the baffled tube to a sampling chamber, from which the powder cloud was sampled for particle size and electrostatic charge distribution measurement using an Electrical Single Particle Aerodynamic Relaxation Time (E-SPART) analyzer. Experimental data on the tribocharging and transport properties of different powders are presented along with an explanation of the charging mechanisms. Analyses of particle size and electrostatic charge distributions in real time and on a single particle basis using the E-SPART analyzer coupled with surface structure analyses with XPS and UPS showed that: (1) most powders are charged bipolarly with relatively high charge-to-mass ratio (Q/M) values that would have a strong effect on transport and deposition of powders; and (2) surface structures, particularly adsorbates, influence the work function and tribocharging of powder. Different methods, including plasma treatment, with minimal changes or contamination of the bulk properties of the powders are also suggested. pharmaceutical powders tribocharging dispersion work function charge distributions charge decay plasma treatment

Effect of ambient relative humidity and surface modification on the charge decay properties of polymer powders in powder coating

Back corona on a powder layer deposited via the electrostatic powder-coating process using corona guns has a strong influence on the corrosion resistance and appearance of cured powder films. The presence of the back corona is often evidenced by orange peel, micro-dents, and pinholes on the film surface. The surface resistivity of the sprayed powder governs the charge decay process and, hence, the onset of back corona. The polymer powders used in powder coating are highly resistive, with surface resistivity often greater than 10/sup 15/ /spl Omega///spl square/. Surface resistivity can be altered by the adsorption of moisture on the surfaces of the powder particles. The objectives of this research were: (1) investigate the effect of decreasing surface resistivity on the appearance of the powder-coated film and (2) enhance hydrophilic properties of polymer powder by plasma treatment. By changing the relative humidity (RH) of powder during the spraying process, it was observed that the surface resistivity could be lowered by orders of magnitude. For example, the surface resistivity for an acrylic powder decreased from 2.96/spl times/10/sup 13/ /spl Omega///spl square/ at 25% RH to 9.6/spl times/10/sup 11/ /spl Omega///spl square/ at 70% RH. The plasma treatment of this powder further improved its charge decay properties. The effects of variation of RH on the appearance of powder-coated panels surface layer are presented for an acrylic polymer powder. The film texture has been characterized by microstructural surface analysis using an optical scanning instrument. Methods of plasma and corona treatments of the powder for increasing moisture adsorption on the surface and decreasing surface resistivity are discussed along with analysis of surface morphology using the atomic force microscope.

Effect of ambient relative humidity on charge decay properties of polymer powder and on the occurrence of back corona in powder coating

Back corona on a highly resistive polymer powder layer deposited via the electrostatic powder coating process using corona guns has a strong influence on the texture of the cured powder film often evidenced by orange peel, micro-dents and pinholes. The surface resistivity of the sprayed polymer powder governs the charge decay process, and therefore the onset of back corona as the thickness of the powder layer increases. The polymer powders generally used in powder coating are highly insulative, with surface resistivity often greater than 10/sup 11/ /spl Omega//. Surface resistivity can be altered by the adsorption of moisture on the surfaces of the powder particles. By changing the relative humidity (RH) of the ambient air during the powder spraying process, it was observed that the surface resistivity could be varied by orders of magnitude. For example, the surface resistivity for an acrylic powder decreased from 1.77/spl times/10/sup 15/ /spl Omega// at 25% RH to 5.7/spl times/10/sup 13/ /spl Omega// at 70% RH. The effects of variation of REI on the texture of the powder surface layer and on the cured film are presented for an acrylic polymer powder. The film texture was characterized by microstructural surface analysis using an optical scanning instrument. Methods for decreasing surface resistivity by using hygroscopic surfactants or by plasma treatment are briefly discussed.

Portable Free-Fall Electrostatic Separator for Beneficiation of Charged Particulate Materials

A portable free-fall electrostatic separator capable of analyzing gram quantities of charged powders is presented. Unlike a Faraday pail, in which only the net average charge-to-mass (Q/M) ratio of the particles sampled by the instrument is measured, an electrostatic separator is capable of separately measuring the charge-to-mass ratios of positively and negatively charged sampled powders. Thus, with an electrostatic separator it is possible to measure the mass fractions of powders that are charged with different polarities and the respective charge-to-mass ratios, along with the mass fraction of particles that are uncharged or charged below a threshold level. We describe a method of measuring the total charge of the collected particles in real time by incorporating an electrometer to integrate the current flowing through the collecting electrode to the high voltage power supply. In this manner, both the total charge and total mass of powder deposited on the two electrodes are measured in near real time, providing information on charge-to-mass ratio of the aerosol cloud sampled. Such real time measurements are often needed to analyze the electrostatic charging properties of small quantities of dispersed powder, particularly in such applications where the charge characteristics are of high importance.

Effects of Powder Velocity and Contact Materials on Tribocharging of Polymer Powders for Powder Coating Applications

Electrostatic powder deposition using corona charging is widely used in a plethora of industrial applications. Disadvantages of this technique are back corona onset and the Faraday penetration limitation. Another method to charge powders is to use tribochargers. Tribocharging depends upon the work function difference between the contacting materials and generates bipolarly charged particles. In this study, acrylic and epoxy powders were fluidized and charged by passing through stainless steel, copper, aluminum, and polycarbonate static mixers, respectively. The particle velocity and powder flow rate were varied to determine their effect on the net charge-to-mass ratio (Q/M) acquired by the powders. The Q/M increased rapidly with velocities between 1.5 to 2.5 m/s and stabilized for higher velocities but decreased with increasing powder flow rate at a constant velocity. The net positive or negative charge on each powder was determined to be dependant on the charger material. The use of an aluminum charger (net negative charge) in combination with a PTFE finger nozzle (net positive charge) resulted in a net powder Q/M of − 0.05 μC/g. The generation of an ion-free powder cloud with high bipolar charge but overall charge density of almost zero is anticipated to provide a better coverage of recessed areas.

POWDER COATING PROCESS PARAMETERS FOR A TRANSFER EFFICIENCY MODEL

ABSTRACT The trajectories of charged powder particles in a powder coating system are governed by the electrostatic, gravitational and aerodynamic forces acting on the particles. A mathematical model of particle trajectories inside a powder coating booth must consider (1) the aerodynamic flow field, (2) particle size and charge distributions, (3) the electrostatic field distribution, and (4)the geometry of the target. Our approach is to employ a grid generation and flow solver to examine the air flow pattern and an iterative technique where the Charge Simulation Method can be used to compute the electric field strength and the Method of Characteristics can be used to compute the charge density in the gun-to-target region. The electrostatic forces due to the deposited powder layer and image charge are to be taken into account to determine if the particle will deposit on the substrate or not. The model is applied to the geometry of a high-voltage electrode consisting of a long thin rod with a hemispherical e...

Measurement of Surface Topography, Packing Density, and Surface Charge Distribution of an Electrostatically Deposited Powder Layer by Image Analysis of Fluorescent Latex Spheres

A novel method of nonintrusive measurement of surface profile, packing density, and surface charge distributions of a powder layer deposited on a substrate is reported. The method employs the deposition of electrostatically charged monodispersed fluorescent latex spheres (FLS), approximately 2 m in diameter, on the surface of: (1) the substrate before deposition, (2) the powder layer after deposition, and (3) the film formed by curing the powder layer. The surface topography in all cases was mapped using an epi-fluorescent microscope with a vertical resolution of - 2 m in the z axis and - 10 m in the x and y axes. An area of 1 cm 2 1 cm is scanned in 1 mm segments, providing approximately 100 data points per cm 2 for the surface topography. For each measurement of surface topography, the substrate was positioned on the microscope stage in a manner such that the reference points (x, y, and z) remained the same for all measurements of the substrate. The surface profiles, with respect to the same reference points, were plotted using Origin 6.0 software for 3D presentation of the topography. The method was also applied to map the surface charge density distribution of electrostatically charged surfaces. The FLS imaging method provides a new tool for examination of surface profiles, packing density, and charge distribution of powder layers on a microscopic scale not provided by optical or atomic force/electrostatic force microscopy (AFM/EFM). While AFM and EFM are very effective in providing similar information with nanometer resolution, they cannot be directly applied on a larger macroscopic scale to study powder layers and for a larger surface area (up to 1 cm 2 or greater) involving deposited particles in the range of 1-50 m in diameter. For AFM, the range in the z-axis is limited to - 3 m and the x-y scan area is limited to 100 m 2 100 m. The FLS method has a much wider range but it is operated manually; an automated scanning process is required for rapid measurement. A comparison of the FLS and EFM techniques as they apply to analyzing charge distribution on coal surfaces is presented.

The use of fluorescent particle image analysis to measure the surface profile and charge distribution of electrostatically deposited powder

A novel method for mapping the surface profile of electrostatically deposited powder is reported. The method employs the deposition of electrostatically charged, monodispersed fluorescent latex spheres (FLS) with 2.26 /spl mu/m diameter on the surface before and after powder deposition. The surface of the substrates and the deposited powder layer is mapped by imaging the FLS with an epifluorescent microscope with a resolution of /spl plusmn/2 /spl mu/m in the z-axis and /spl plusmn/5 /spl mu/m in the x- and y-axes. Typically, a 1 cm/spl times/1 cm area is scanned at 1 mm intervals. The surface profiles are plotted using Origin 6 software for 3D presentation. The method is also applied to map surface charge distribution of the deposited powder layer. Application of this method to study surface charge distribution of individual particles is also discussed. The method provides an examination of surface profile and surface charge distribution in a microscopic scale not provided by atomic or electrostatic force microscopes, which are effective in the nanometer range.