Surface microstructure of powder layers influenced by the forces ofdeposition and adhesion in electrostatic coating process

In the electrostatic powder coating process, several factors affect the deposition of charged polymer paint particles and the adhesion of the deposited particles on the grounded substrate being coated. In this paper, the roles and relative magnitudes of these forces are discussed. A model on the deposition of a charged particle approaching the surface of the substrate is presented. The electrostatic fields that are considered here to be responsible for particle deposition are the following: (1) corona field between gun tip and grounded plane; (2) image field between a particle and its image charge; (3) field due to the space charge of charged particles; and (4) net repulsive field between a charged particle and the powder layer. Once the particle deposits on the surface and the high voltage is turned off, the particle experiences the following forces: (1) the force due to the image charge of the underlying powder layer; (2) the force due to the image charge of the particle; and (3) the repulsive force between the charged particle and the charged powder layer. The model shows criteria on whether the particles will deposit on the substrate and, if deposited, whether they will remain on the surface of the powder layer once the corona field is turned off. These relative forces influence the microstructure of the powder layer and may affect the ultimate appearance or the texture of paint film after curing     

Adhesion of charged powders to a metal surface in the powdercoating process

Substrate-particle adhesion of electrostatically charged, nonconducting particles deposited on electrically grounded substrates is discussed. Glass microspheres of diameters ranging from 25.5-74.1 μm, charged by corona and tribe-charging, were deposited in a monolayer on conducting stannic-oxide coated surfaces of glass plates (NESA). The total force of adhesion due to electrostatic, van der Waals, and gravitational forces was measured by observing the removal of particles by applying a known electric field between the particle coated surface and a clean surface of a second NESA glass, placed parallel to it at a distance of 0.013 m. The adhesive force was measured as a function of particle size and charge. The net average charge on the particles was measured using a Faraday cup. The experimental values agree well with the calculated force of adhesion for a single layer deposition. The charge decay of the particles was studied using a noncontact electrostatic voltmeter. The charge relaxation time of the deposited powder was found to increase with time. A physical model of the adhesion of charged powder paints deposited on a grounded metal substrate is presented. The role of the forces acting on a spherical polymer particle deposited on the surface of a uniform coating of powder paint is investigated as a function of particle diameter and charge. The particles are assumed to be unipolarly charged and deposited uniformly on the substrate. The relative magnitudes of the electrostatic attractive and repulsive forces are analyzed as functions of powder film thickness and particle size     

Charge to Mass Distributions in Electrostatic Sprays

The dependence of the charge to mass ratio or specific charge of the spray stream on fluid and automization parameters in electrostatic spraying of various solvents and paints has been investigated. In addition, atomized paint particle sizes have been analyzed. It has been found that an optimum value of fluid conductivity exists at 2 ?ho/cm where the specific charge of the spray stream exhibits a maximum. From the particle size distribution and from the dependence of the specific charge of the spray stream on conductivity it has been concluded that the basic spray droplet charging mechanism is due to the breakup of a charged liquid surface, while corona discharge does not appear to play a significant role in the charging of the particles.     

Charge limits in corona charging of distorted liquid droplets

The efficiency of electrostatic painting depends strongly on the amount of charge on a single paint droplet. The saturation corona charging for a spherical droplet has been theoretically estimated by Pauthenier. However, in many industrial and agricultural applications, the droplets being charged become substantially distorted. The results of numerical estimations of the charge limit, due to a DC corona discharge, on conducting droplets distorted by an electric field are presented. The finite-element method (FEM) and the separation-of-variables method for the Laplace equation have been used as modeling tools. In general it is found that a distorted droplet can exceed the Pauthenier charge limit     

A Theoretical Model for Electric Field Distribution and Enhancement in Air Atomized Electrostatic Spray Painting

A theoretical model predicting the electric field distribution during air atomized electrostatic spray painting has been developed. A space charge pattern in the form of a rectangular pyramid is considered. The model was also applied to the case of two adjacent and inward tilted spray nozzles. The attainment of high intensity and broad range electric field at the vicinity of the substrate in conjunction with less possibility of opposite charge formation resulting from splashing shows that this configuration can offer significant deposition efficiency advantages for high-voltage charged organic paints or inductively charged water-based paints.     

Reducing Water-borne Paint Contamination in Electrostatic Rotary Atomizers

The electrostatic rotary atomizer has the highest paint efficiency in all kind of atomizers. The usage of an electrostatic rotary atomizer, therefore, has contributed to reduce the waste of paint and volatile organic compounds (VOCs). Concurrently, water-borne painting processes which use water-soluble paint have been introduced to reduce the amount of VOCs emissions. With the water-borne painting, the atomizer body is easily contaminated by the paint mists. The contamination has been an important problem to be solved in factories. In this report, the causes of water-borne paint contamination was considered and the experimental results of the contamination proof system were presented. The atomizer is surrounded by the repelling film that is charged and repels the incoming paint droplets which otherwise contaminate the atomizer. From the experimental results, following electrical properties are found to be important for the repelling film such as low capacitance and high insulation property to keep high surface potential. These features were also checked by the space charge measurements inside the repelling film. In addition, safety against incendiary sparking was investigated. The ground electrode of sphere, needle or blush-shape was approached to contact to the charged film. Energy of the discharge was calculated from the measured discharge current and the surface potential of the film. The discharge energy of all measured electrodes was below the minimum ignition energy of 0.24 mJ used in the painting industries. At the meantime, the results indicate that shape of the electrode is an important parameter to affect the discharge energy.     

Surface Charge Buildup During Electrostatic Spraying

In industrial electrostatic spray coating operations, liquid paint is applied to substrates of varying electrical characteristics. In many cases a metal substrate is chemically pretreated and/or primed prior to electrostatic spraying. A previously rejected part may have already received a number of thoroughly cured topcoats prior to the final spray application. Typical primers and topcoats have large surface resistivities (108 to 1010 ohm.cm-2) and exhibit slow surface potential decay rates. A surface potential measurement system has been developed to determine the time dependent electrical properties of various substrates when coated with commercial water base and organic paint systems. It has been observed that during some electrostatic recoat operations, the charged paint particles appear to be repelled from the surface to be coated. It is found that this phenomenon can be predicted when conditions are such that adequate charge leakage paths to ground are not available due to high surface and bulk resistivities of the film resulting in a repelling charge and potential buildup on the surface. It has been found that the charge removal process cannot be described as a simple exponential decay, but as a superposition of three exponential decay processes each with its own decay constant ?.     

Mathematical modeling of powder paint particle trajectories inelectrostatic painting

The trajectories of charged powder particles in an electrostatic powder coating system were modeled considering electrical and fluid forces. The mathematical model employed an iterative technique wherein the charge simulation method was used to compute the electric field strength and the method of characteristics was used to compute the charge density in the gun-to-target region. The fluid flow between the electrostatic gun and the target was modeled using interpolated experimental data assuming stagnation point flow. Particle trajectories were simulated for size range 10-40 μm and charge-to-mass ratios of -0.1 to -1 μC/g. The simulation results showed good agreement with experimental data (charge and mass measurements) at several collection points on the painting target and provided valuable information concerning particle deposition     

Charging of insulating spheres on the surface of an electrodeaffected by monopolar ions

The charge acquired by an insulating sphere in a uniform monoionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are on the surface of an electrode. Under these circumstances, Pauthenier's formula is no longer valid, because the field is nonuniform. This paper addresses this problem from both a computational and an experimental point of view. A numerical method of field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, on the surface of a plate electrode. The numerically computed values of the saturation charge in this situation were always greater than those given by Pauthenier's formula. The experiments were carried out on laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (3 mm diameter) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (150 mm diameter) connected to the ground. The experimental data were in good agreement with the theoretical predictions; the saturation charge increases linearly with the applied voltage, up to a threshold at which the self discharge of the particle occurs. The efficiency of ionic charging was shown to depend on the type of corona electrode that is employed     

Charging and discharging of insulating particles on the surface of a grounded electrode

The aim of the present paper is to analyze the corona charging of millimeter-size insulating disks, as well as their discharging when they are no longer exposed to the action of an external electric field. The experiments were carried out on a roll-type electrostatic laboratory separator, equipped with a wire-type corona electrode, simulating the actual charging/discharging conditions in an industrial unit. Disks of various sizes were charged on the surface of the roll electrode, then the high voltage supplied to the corona electrode was turned off and the particles were collected in a Faraday pail, connected to an electrometer. The charge measurements were performed at various time intervals from high-voltage turn-off. In this way, the charge decay could be recorded and the discharge process fully characterized. The measured data show that the discharge process depends on the nature, size, and shape of the particles, as well as on the contact conditions between the particles and the grounded roll electrode. These data could guide the design of the electrostatic separation experiments that precede any new industrial application of this technology.     

Electrostatic painting of insulating surfaces

The purpose of this work was to develop methods for painting insulating surfaces by the conventional electrostatic painting technique used to coat conducting samples. This technique is not applicable to insulators since the charges carried by the paint accumulate on insulating targets, thereby preventing a proper coating. A detailed investigation of the decay of charges deposited on insulators by a corona discharge simulating the charged paint has confirmed that a grounded counterelectrode (CE) on the back face of the sample speeds up the decay and that the better the contact, the faster the decay. Because a material CE with proper contacts is inconvenient for painting in production configuration, it was suggested that ionized air might behave as a satisfactory CE. Several ionizers have been tested, and good coatings have been obtained, provided that the flows of paint and ionized air are well separated. The possibility of self-creation of ionized air by the voltage of the early paint deposit has also been successfully tested     

Numerical Simulation of the 2-D Gas Flow Modified by the Action of Charged Fine Particles in a Single-Wire ESP

A numerical model for simulating precipitation of submicrometer particles in a singlewire electrostatic precipitator is discussed in this paper. It includes all important phenomena affecting the process: electric field, space charge density, gas flow, including the secondary electrohydrodynamic flow caused by the corona discharge and charged particles, and particle transport. A simplified corona model assumes just one ionic species and neglects the ionization zone. The fully coupled model for the secondary EHD flow, considering the ion convection, has been implemented. The dust particles are charged by ionic bombardment and diffusion. The gas flow pattern is significantly modified by the secondary EHD flow, which depends on the particle concentration. As for fine particles the drift velocity is small and particles practically follow the gas streamlines, the particle concentration has a very strong effect on the precipitation efficiency.     

Electrostatic powder coating of insulating surfaces using analternating polarity internal corona gun

An internal-corona (IC) gun operated at 1-Hz alternating polarity (AP) was used in an attempt to powder coat insulating plastic plates without using conductive primers, metal backings or ion “backings.” The IC gun was effective in charging the powder particles and emitted very few free ions, but it required frequent cleaning. The Q/M ratios obtained using DC voltage were very good, with negative polarity better than positive. The use of charge separation electrodes verified that the 1-Hz AP gun emitted “waves” of oppositely charged particles. The charged powder average velocity between the gun 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 and the measured transit time. The measured saturation surface charge densities (σ_s) were used together with the total Q/M ratios to calculate the saturation surface mass densities, which were in good agreement with the measured values (0.3-0.5 g/m²)     

Distribution of electric field strength around a large-scalecharged particle cloud

To investigate an electrical discharge occurring from or in a space-charge cloud, a large-scale charged particle cloud was formed by using a cloud generator consisting of a blower and corona charger. The distribution of the electric field strength around a charged particle cloud has been investigated to determine the behavior of charged particles. The soil-conditioning particles were charged by corona charging and blown by high-speed air flow in a test room, 5 m wide, 10 m long, and 3 m high. The average charge-to-mass ratio of the particles blown by this method was 170 μC/kg. The space-charge density of the cloud was calculated at the order of 10 μC/m³ from the electric field strength outside of the cloud. While the electric field strength at the outside of the cloud increased up to 52 kV/m within 2 m downstream from the cloud generator, it decreased below 25 kV/m farther than 2 m away from the cloud generator due to dispersion of charged particles. The change in the electric field strength due to dispersion of charged particles can be qualitatively explained by a simple cloud model. The velocity of charged particles transported by air flow and mobility of charged particles are found to be effective factors increasing the electric field strength around the large-scale charged particle cloud     

Electrical characterization of bell-type electrostatic paintingsystems

A numerical model that determines the electrical characteristics of a bell-type electrostatic painting system is presented. The model is capable of handling the complex geometries of practical sprayers and accounts for the space charge effect of charged paint droplets. The influence of aerodynamic forces is also considered in tracing individual droplet trajectories. As a test of tile simulation, the restricted case of a sprayer emitting ions, rather than charged paint droplets, was modeled. The simulation results were compared with experimental data and showed good agreement. For the more general case, maps of equipotential contours and the paint droplet trajectories are generated for a bell-type sprayer. It was found that the space charge tends to increase the deposition field, although not as strongly as increased bell potential or decreased bell-to-target spacing. However, the space charge also caused the spray pattern to expand, possibly leading to increased overspray     

High-voltage electrode position: a key factor of electrostatic separation efficiency

Industry application of electrostatic separation technologies still faces a major difficulty: good results can be obtained only by adequate control of a multitude of operating parameters. The aim of the present paper is to analyze a key factor of electrostatic separation efficiency: the position of the high-voltage electrodes. Experiments were performed with two types of granular materials: chopped electric wire wastes and foundry sands. The electrostatic separator employed for the tests was provided with a wire-type corona electrode, associated - in some experiments - with a tubular-type electrostatic electrode, at various angular and radial positions, with respect to a rotating roll electrode connected to the ground. The experimental data are discussed in relation to the results of the numerical analysis of the electric field, carried out with a charge simulation program. They show that the outcome of the separation process (i.e., the weight percentage of the conductor and nonconductor fractions, as well as the purity of the recovered materials) depends on the configuration of the electrode system. The position of the electrodes affects both the particles charging conditions on the surface of the roll electrode, and the trajectories of the charged particles in the high-intensity electric field of the separator. Some recommendations could be formulated for the industrial application of the electrostatic separation technology.     

Optimization of corona electrode position in roll-type electrostatic separators

The position of the corona electrode(s) is known to be a key factor of electrostatic separation efficiency, as it influences both the charging conditions of the granular materials on the surface of the roll electrode connected to the ground, and the magnitude of the electric forces exerted on the particles. Response surface methodology was employed for the design of the experiments performed on a laboratory roll-type corona-electrostatic separators, with samples of chopped electric wire wastes typically processed by such techniques. The results of the electrostatic separation tests are discussed in relation to the data obtained from two other experiments, in which corona current and particle charge measurements were performed for various positions of the standard wire-type electrode. The conclusions of this study enabled the formulation of several recommendations for the improvement of the outcome of industrial separation processes (i.e., maximize the weight percentage as well as the purity of the recovered materials).     

Charge Separation During Splashing of Liquid Droplets in the Presence of an Electric Field

When conductive liquid droplets splash on solid or liquid surfaces, fragments are ejected from the surface that in the presence of an electric field can be inductively charged to the opposite polarity. This phenomenon can reduce the deposition efficiency of electrostatically applied conductive coatings such as water base paints. The charge separation process was investigated by collecting and measuring the size and charge of splashed fragments ejected from surfaces in the presence of an electric field.     

空间介质颗粒对交流导线电晕特性影响规律

为了研究空间介质颗粒对导线交流电晕特性的影响,基于小电晕笼,应用Fi Lin?6紫外成像仪记录存在介质颗粒时模拟导线交流电晕放电发生、发展的过程,实验结果表明:空间介质颗粒会对导线交流电晕发生、发展过程产生影响,球径越大、距离导线越近则导线的起晕电压越低;存在多球时,由于颗粒间的相互影响,导线的起晕电压和放电过程会受到影响,其中垂直排列的颗粒之间对放电发展过程和起晕电压影响相对较大。将偏心偶极子模型应用到存在介质颗粒时电场计算中,结合模拟电荷法对导线周围存在颗粒时空间电场分布进行分析,仿真结果表明:导线表面的最大场强以及导线和颗粒间场强的均值随着颗粒尺寸的增大、颗粒导线距离的降低和颗粒相对介电常数的增大而增大;空间存在多个颗粒时,由于球间的相互影响使导线表面场强进一步增大,其中球间连线和电场方向一致时的影响更大。     

Dielectric charged drop break-up at sub-Rayleigh limit conditions

The maximum charge a drop may hold, for an electrically isolated, electrically conducting drop, in vacuum, is defined by the Rayleigh Limit. For spray plumes of electrically charged drops this condition is clearly not met due to the space charge field. We would like to simulate such spray plumes and to simulate drop break up within them, using stochastic methods. Since many simulated particles are required a dynamic drop stability analysis is clearly not computationally feasible. Based upon a static analysis, and a thorough review of the previous experimental data on charged drop stability, it is shown that for dielectric drops in the presence of significant electric fields, and particularly those within spray plumes, the maximum charge a drop may hold is less than the Rayleigh Limit. Typical values of stable drop charge of 70-80% of the conducting drop Rayleigh Limit are predicted, and this is supported by a majority of recent experimental work. We present an explanation of the sub-Rayleigh Limit drop fission within charged spray plumes for dielectric drops, based upon a static, rather than a dynamic analysis. This permits sub-Rayleigh Limit drop fission to be incorporated into stochastic particle simulations.