Home Search Collections Journals About Contact us My IOPscience The characteristics of particle charging and deposition during powder coating processes with ultrafine powder This content has been downloaded from IOPscience. Please scroll down to see the full text. 2009 J. Phys. D: Appl. Phys. 42 065201 (http://iopscience.iop.org/0022-3727/42/6/065201) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 132.203.227.62 This content was downloaded on 14/06/2017 at 09:15 Please note that terms and conditions apply. You may also be interested in: The characteristics of particle charging and deposition during powder coating processes with coarse powder Xiangbo Meng, Hui Zhang and Jingxu (Jesse) Zhu Electrophoretic deposition characteristics of an EDMed micro-tool for micro-hole polishing in quartz Cheng-Kuang Yang, Jung-Chou Hung, Chih-Ping Cheng et al. 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Phys. 42 (2009) 065201 (12pp) doi:10.1088/0022-3727/42/6/065201 The characteristics of particle charging and deposition during powder coating processes with ultrafine powder Xiangbo Meng, Jingxu (Jesse) Zhu and Hui Zhang Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, N6A 5B8, Canada Received 18 August 2008, in final form 21 January 2009 Published 25 February 2009 Online at stacks.iop.org/JPhysD/42/065201 Abstract In a preceding work, the mechanisms of particle charging and deposition during powder coating processes were explored with coarse polyurethane powder. In this paper, the developed mechanisms were further examined with ultrafine polyurethane powder in order to meet the growing needs for ultrafine powder in finishing industries. This study first verified the previous findings in particle deposition, which account for a cone-shaped pattern formed by deposited particles on the substrate and a rise in particle accumulation in the fringe region. It was further demonstrated with ultrafine powder that, as disclosed by using coarse powder, the primary charging of in-flight particles competes with back corona in particle deposition processes, and the highest deposition efficiency is a compromise by balancing their effects. In comparison with coarse powder, ultrafine powder presents a faster reduction in the deposition rate with extended spraying duration, but shows some superiority in the uniformity of the deposited layer. In the case of charging characteristics of the deposited particles, it was further substantiated with ultrafine powder that the secondary charging mechanism takes predominance in determining the distribution of local charge-to-mass ratios. It was also disclosed that ultrafine powder shows a decreasing charge-to-mass ratio with increased charging voltage in the deposited layer, opposite to the increasing tendency of coarse powder. However, it was commonly demonstrated by both coarse and ultrafine powders that the charge-to-mass ratio of the deposited particles decreases with the extended spraying durations. In comparison, ultrafine powder is more likely to produce uniform charge-to-mass ratio distributions in the deposited layer, which contrast sharply with the ones associated with the coarse powder. In conclusion, it is believed that this study supplements the preceding study and is of great help in providing a comprehensive understanding of the mechanisms of corona charging processes of different powders. (Some figures in this article are in colour only in the electronic version) the finishing industries emerged in the USA in the 1950s as electrostatic powder coating [10]. Since then, powder coating has promptly occupied an important place in the finishing industries in place of conventional liquid coatings, due to its overwhelming advantages in saving energy and cost, resisting corrosion and not releasing any volatile organic compounds (VOCs) [10]. A typical powder coating system consists of a spray gun and a metal substrate in a point-to-plane geometry, and usually negative high voltages are supplied to the gun tip in order 1. Introduction Since the studies on point-to-plane corona discharge started from the end of the nineteenth century, it is now involved in a number of commercial and industrial applications [1–10]. Of these applications, corona discharge is often used to provide ions for charging materials, and corona charging performs more reliably and controllably than induction charging and tribo-charging [6–10]. After its first successful practice with an electrostatic precipitator in 1907 [5], corona charging in 0022-3727/09/065201+12$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al Q/M of the overall deposited particles, but they overlooked the characteristics of the non-uniform inter-electrode electric field, which may produce some influence on the charging behaviour of the local deposited particles. In the case of deposited particles, the accumulated charge in the deposited layer often incurs an abnormal discharge, named back corona. Masuda and Mizuno [27–29] defined the initiation of back corona by using the following correlation: to initiate coronas. During the coating processes, powder particles of high resistivity are entrained in transportation gas streams and thereby sprayed out from the gun. Usually particles bypass a deflector at the gun outlet to form a dispersed powder cloud in the inter-electrode space. Wu [11] pointed out that the charging of in-flight particles is mainly within 50 mm of the corona electrode, named primary charging. Earlier studies disclosed that the charge obtained by in-flight particles only accounts for less than 10% of the total corona current while free ions take 90% [8, 10, 11]. Due to the convergence of free ions on the substrate, the deposited particles may receive extra charge by the so-called secondary charging. During the trip to the substrate, charged in-flight particles are subjected to several kinds of forces (gravitational, aerodynamic and electrostatic forces), and their precise trajectories mainly depend on the balance between electrostatic and aerodynamic forces [10]. Adamiak [12, 13] numerically predicted that the powder cloud exhibits a dispersing tendency, and both the charge and sizes of in-flight particles are important in determining their trajectories. However, aerodynamic forces are dominant in the region close to the gun and electrostatic forces become increasingly important as charged particles approach the substrate [10]. In particular, the motions of in-flight particles are only dominated by electrostatic forces in the vicinity of the substrate of about 10 mm [7], and the deposited particles mainly depend on their image forces to adhere onto the substrate [10, 11]. Thus, a powder coating process in essence is both a particle charging and a deposition process [8]. In the case of particle charging, the Pautheniner limit was widely referred to in the literature to interpret the maximal (saturation) charge, which is proportional to the electric field and the square of the particle radius [11]. Furthermore, the time to reach the saturation charge is proportional to the electric field but inversely proportional to the corona current [7, 10]. However, in order to evaluate the charging efficiency of a particle, its charge-to-mass ratio (Q/M) is usually employed, which is proportional to the electric field but inversely proportional to the particle radius [11]: Qmax /M ∝ E/r. Ed = ρd J Eb , (2) where ρd is the resistivity of the dielectric and Ed and Eb the electrical field across the layer and the breakdown field of the layer, respectively. Because back corona produces ions of opposite polarity with respect to the fore corona at the gun, the charge of the deposited and the in-flight particles is possibly reduced by neutralization. Tachibana [30] observed that arriving particles changed their paths inversely in the vicinity of the substrate and were charged oppositely under the onset of back corona. Thus, back corona is commonly regarded as the main cause of reduced deposition efficiency and makes particle deposition a self-limiting process [7, 10, 27–29]. In particular, as reviewed by Bailey [10], Basset et al illustrated that free ions play a very important role in limiting the thickness of the deposited layer and concluded that the self-limiting process during powder coating is caused by back corona rather than the repulsion of the already deposited charged particles to arriving particles. In practice, due to the difficulties in directly detecting free ions of both polarities, an increase in the current density often serves as evidence of back corona [4, 31]. With reference to particle deposition during powder coating processes, some earlier efforts [26, 32, 33] simply inspected the first-pass-transfer-efficiency (FPTE), a mass ratio of the overall deposited particles to the overall sprayed particles. Although Ye et al [34, 35] noticed cone-shaped particle accumulation on a substrate, no further explanations were provided. Based on the above knowledge, it is clear that, partially due to its complexity, few studies correlated the particle charging and the deposition process to provide a deep insight into the powder coating process. Motivated to explore the underlying mechanisms in powder coating processes, recently the authors conducted a series of investigations using two powder systems (coarse and ultrafine, as defined by Zhu and Zhang [36], the former larger than 30 µm and the latter smaller than 25 µm in their mean particle sizes). In a preceding work [37], the authors demonstrated that ultrafine powder would produce a stronger suppression on the corona current in comparison with the coarse powder, but both powders resulted in distorted electric fields between the electrodes. Thus, the properties of the applied powders influence the characteristics of the powder coating processes. In another preceding work [38], the characteristics of particle charging and deposition with the coarse powder were disclosed. In this paper, the characteristics of particle charging and deposition with ultrafine powder are revealed, and the differences in the two powders are discussed. More importantly, in the current state of art powder coating is suffering from its extensively applied coarse powder, whose mean particle size is typically in the range 30–40 µm, because many aesthetic problems (such as thick film, orange (1) Obviously, charging behaviour of particles is closely related to the characteristics of the inter-electrode electric field. For point-to-plane corona discharges, several empirical formulae were proposed to describe the current–voltage relation [2, 14, 15], and Warburg’s law was extensively applied to predict the current density distribution [16–18]. However, the particle charging process is even more complex in the powder coating processes, for charged in-flight particles not only distort the inter-electrode electric field but also incur corona quenching [19], a drop in the corona current. In earlier studies, some works [20–22] were dedicated to investigating the charging behaviour of in-flight particles and a suction-type Faraday pail with an insulator pipe attached to its inlet [20] was found to be more reliable in providing precise measurements. Unfortunately, those studies provided no clues to correlate the primary charging of particles with their deposition. Some other works [23–26] were conducted to examine the average 2 J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al Booth Substrate Corona Gun Electrometer Current Analog Signal Screw Feeder Negative Voltage Digital A/D Board Data Powder Supply LabVIEW Computer On/Off Signal Pulse Signal Venturi Pump Feeding Gas Gun Control Unit (a) G On/Off Signal A P System Controller A/D T PC (b) V Figure 1. Experimental setup: (a) schematic diagram of the experimental system; (b) the nominal circuit followed by the corona current: V—voltage supply, G—corona gun, P—corona electrode, T—substrate, A—electrometer, A/D—A/D board and PC—computer. in a Nordson® Model 902 booth. An electrometer (Model 6514 Keithley® ) was used to receive currents, which were then transferred to a computer for storage via an A/D board (NI Lab-PC-1200). The nominal circuit of the current is illustrated in figure 1(b). The corona gun equipped with a coneshaped deflector (as illustrated in figure 2(a)) was mounted on a support stand 300 mm away from the substrate. The configuration of the gun tip was introduced previously [14], and the tip was supplied with high voltages to induce corona discharges. In this study, three charging voltages (30, 60 and 90 kV) and spraying durations (5, 10 and 20 s) were applied. In addition, the air relative humidity was controlled at 50 ± 2% and room temperature of approximately 23 ◦ C. peel, pinholes and craters) are incurred on the coated films [10, 36, 39]. These problems are preventing powder coating from widening into the core of liquid coatings. For instance, all the applications of powder coating in automotive industries are limited to underbody, trim components, steel and aluminium car wheels [39]. However, ultrafine powder was successfully demonstrated in the literature of being capable of improving the film appearance and decreasing the film thickness [36, 39– 41]. Therefore, ultrafine powder is attracting more and more attention from the finishing industries due to its superior properties and is considered as the next generation of powder coating [8, 36, 39, 42, 43]. In this case, this study is important not only for understanding the related physical mechanisms but is also informative for practical applications. This paper is a strong supplement for gaining comprehensive knowledge of powder coating processes. 2.2. Measurement techniques and materials As shown in figure 2(b), the substrate (i.e. the collecting electrode) was a 300 mm diameter blank printed circuit board covered with a 0.2 mm thick layer of copper. It was divided into ten 15 mm wide annular regions by nine marked borderlines in order to facilitate the study of the local charging and deposition characteristics of the deposited particles. The regions were named as A1, A2, . . . , A10 in sequence outwards. Nine tiny protruding points (as shown in figure 2(b) by dark dots) made of glue help discriminate different regions of the substrate after a coating cycle. In the investigation of the charging and deposition characteristics of the deposited particles, a suction-type Faraday pail, as illustrated in figure 3, was used to collect particles deposited in each region. The inner pail was connected with a polyethelyne (HDPE) insulator suction pipe, 2. Experimental 2.1. Apparatus The experimental setup in this study is the same as that used in [38] and illustrated in figure 1(a). A system controller synchronized the powder feeding, voltage supply and current data collecting by sending three signals simultaneously to the gun control unit, the screw feeder and the A/D board, respectively. The powder was accurately fed into a Venturi pump by a SCHENCK AccuRate® screw feeder at 1.0 g s−1 and then pneumatically transported to a Nordson Surecoat™ negative corona gun by a feeding gas of 1.5 bar. The powder particles were charged and then coated on a metallic substrate 3 J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al On the other hand, the charging behaviour of in-flight particles was investigated selectively. The method is illustrated in figure 4(b): the extension pipe of a Faraday pail passed through the centre of the substrate from its backside and sucked in inflight particles into the pail at different locations along the centreline. In this case, the pail was free of the outer bass pipe used in figure 4(a), and the extension pipe was 500 mm in length. In this study in-flight particles were collected in the vicinities (within 50 mm) of the gun tip and the substrate, respectively, and their charges were compared. Furthermore, another substrate was applied to measure the local current densities associated with the coating processes. Its dimensions were the same as the ones in figure 2(b), but annular regions were physically insulated by 0.6 mm air gaps, instead of marked borderlines. The powder used in this study was black polyurethane paint with a mean particle diameter (D50 ) of around 12 µm and supplied by Links Coatings (London, Ontario). Its representative particle size distribution (PSD) determined by Malvern Mastersizer® was compared with the coarse powder used in the preceding work [38] and is shown in figure 5. D10 , D50 and D90 represent the volume percentages of powder with the diameter less than the stated diameter, e.g. D50 is the equivalent volume diameter where 50% of the particles in volume have a diameter smaller than the stated diameter. Gun Tube Gun Tip (a) Deflector A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 3. Results and discussion 3.1. Characteristics of the particle deposition At a spraying duration of 20 s and various charging voltages (30, 60 and 90 kV), the profiles of the local particle mass-tosurface ratio (M/S) are illustrated in figure 6, in which particle deposition was assumed centrosymmetric. It is obvious that the deposited particles of the ultrafine powder present a coneshaped pattern in the internal regions (A1–A9) and a M/S rise in the fringe region (A10) in all the cases, both of which are identical to the previous observation of the coarse powder in [38]. In the preceding work [38], it was believed that the coneshaped distribution of deposited particles was mainly attributed to inhomogeneous concentrations of in-flight charged particles in the powder cloud whereas the M/S rise in the fringe region was due to the edge effect. It is well known that electrostatic forces are the main reasons for the adherence of the employed particles to the substrate in powder coating processes. So, the primary charging of in-flight particles becomes the premise of particle deposition. On the other hand, the particles form a powder cloud initially in the vicinity of the corona electrode while receiving their primary charges. Thereafter the cloud evolves more dispersed in travelling towards the substrate, for the newly incoming charged particles suffer retardation from the foregoing space charge (consisting of charged particles and free ions) and repulsion in the radial direction from the surrounding space charge. Earlier studies [10, 12, 13] disclosed that the trajectories of in-flight particles strongly depended on the balance between aerodynamic and electrostatic forces, which was particle size dependent. In (b) Figure 2. (a) Schematic view of the gun tip and deflector; (b) the configuration of the substrate. which was 6.5 mm in inner diameter (I.D.), 9.5 mm in outer diameter (O.D.) and 300 mm in length. The outer pail was attached with a brass pipe (10 mm I.D. and 11 mm O.D.) used to level the inner insulator pipe and electrically shield the device. The sampling method of the deposited particles is shown in figure 4(a): the corona gun was removed promptly after a coating cycle and replaced with a compass; the holes on the compass’s arm helped direct the suction pipe of the pail in a certain region of the substrate for each hole corresponding to one annular region, and the deposited particles were sucked into the pail by vacuum in the rotating processes of the compass’s arm. No particles remained in the pipe during samplings. The collected particles were then determined with their charge and weight by a Model 6514 Keithley electrometer and a digital balance (with an accuracy of 0.001 in grams), respectively. In this case, the mass-to-surface ratio (M/S: g m−2 ) and the charge-to-mass ratio (Q/M: µC g−1 ) of a certain region would be known and used to characterize the particle deposition and the charging, respectively. To alleviate the influence of the charge decay during the samplings, only one annular region was sampled right after a coating cycle and the procedures were repeated at least twice for each region. 4 J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al Outer Metallic Faraday Pail Inner Metallic Faraday Pail Metallic Extension Pipe Insulator Suction Pipe Vacuum Suction Porous Thimble Filter BNC Connector to Electrometer Figure 3. Schematic configuration of the suction-type Faraday pail with an extension pipe. Earthed Plane 7 Ultrafine Powder D10 = 2.65 µm 6 Extension Pipe D50 = 12.36 µm Vacuum Volume (%) Faraday Pail 5 D90 = 33.72 µm 4 Coarse Powder D10 = 11.24 µm 3 D50 = 35.13 µm D90 = 90.78 µm 2 (a) Compass 1 Earthed Plane 0 1 10 Particle Size (µm) 100 Figure 5. The PSDs of the applied ultrafine and coarse powder. Corona Gun Extension Pipe 140 Spraying Duration 20 s: 60 kV; 30 kV; 90 kV Vacuum Faraday Pail 120 (M/S)i (g/m2) 100 (b) Figure 4. Schematic diagrams of the samplings: (a) samplings of deposited particles; (b) samplings of in-flight particles. 80 60 40 20 particular, it was demonstrated in the preceding work [38] that small particles had stronger propensities to move towards the peripheries of the powder cloud, due to their higher charging capability and less inertia (both evaluated by their mass). In this study, the characteristics of the powder cloud were examined and shown in figures 7(a) and (b). First, by applying the method shown in figure 4(b), the in-flight particles were sampled in the vicinities of the gun tip and the substrate for a spraying duration of 20 s. The results in figure 7(a) imply that the powder cloud evolved more dispersed while approaching the substrate, for fewer particles were collected in the vicinity of the substrate and higher charging voltages promoted the dispersing progress. Furthermore, the deposited particles in each region were collected by applying the method illustrated in figure 4(a) and their particle sizes are shown in figure 7(b). 0 10 8 6 4 2 0 2 4 6 8 10 Annular Region 10 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 10 Figure 6. The distributions of the local mass-to-surface ratio ((M/S)i ) for deposited particles. 5 J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al 50 2.0 20 s: 50 mm to the Gun Tip; 50 mm to the Substrate 45 Spraying Duration 5 s: 30 kV 60 kV 90 kV 40 35 (M/S)i (g/m2) M (g) 1.5 1.0 0.5 30 25 20 15 10 5 0.0 0 30 60 Charging Voltage (kV) (a) 90 10 8 6 4 (a) 2 0 2 4 6 8 10 Annular Region 70 24 20 s: Original Ultrafine Powder 30 kV; 60 kV; Spraying Duration 10 s: 30 kV; 60 kV; 90 kV 60 90 kV 22 50 (M/S)i (g/m2) D50 (µm) 20 18 16 40 30 20 14 10 12 0 10 A1-A4 (b) A5-A7 A8-A10 (b) 10 8 6 4 2 0 2 4 6 8 10 Annular Region Annular Region Figure 8. The effects of charging voltage on the local mass-to-surface ratio ((M/S)i ) distributions: (a) in a spraying duration of 5 s; (b) in a spraying duration of 10 s. Figure 7. (a) The number (in mass: M) of in-flight particles collected in a spraying duration of 20 s in the vicinity (50 mm) of the gun tip and in the vicinity (50 mm) of the substrate; (b) the size evolutions of deposited particles in local regions. in figures 6, 8(a) and (b), it is easy to learn that there are more particles to deposit on the substrate with an extended spraying duration, and the effects of the charging voltage account for an improvement in the deposition efficiency with the voltage increased from 30 to 60 kV but a comparable efficiency between 60 and 90 kV. To clarify the characteristics of particle deposition due to different powders, the deposition rates of the ultrafine powder were illustrated in figure 9(a) in comparison with those induced by the coarse powder of the preceding work [38]. It is evident that the rates with ultrafine powder show a faster decrease with time, except for a slight increase in the segment between 10 and 20 s for the voltage of 30 kV. It can be further observed that there exists an optimal voltage to achieve the highest deposition efficiency, which is around 60 kV for both powders. Exceeding this optimal voltage, the deposition efficiency exhibits a comparable value for the ultrafine powder but a decrease for the coarse powder. Furthermore, the uniformity of the deposited layers was evaluated by using It is obvious that the deposited particles present a sizedecreasing tendency along the radial direction, which accounts for the variations in the trajectories of different particles. Thus, it is easy to understand that particle concentrations are locally inhomogeneous in the powder cloud. As a result, the discrepancy in local particle fluxes is responsible for the coneshaped pattern formed by the deposited particles. However, the exception in the M/S of the fringe region is thought to be mainly due to a much stronger electric field between the fringe and the corona electrode, which compensates the correspondingly lower particle concentration by capturing particles with a higher efficiency and thereby contributes to the M/S rise in the fringe region. This is called the edge effect and will be further demonstrated later in this paper. Similarly, cone-shaped distributions of the internal M/S and the edge effects of the fringe M/S were observed at spraying durations of 5 s and 10 s, which are illustrated in figures 8(a) and (b) respectively. Combining the M/S results 6 J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al 0.6 4.0 Ultrafine Powder Coarse Powder 30 kV 30 kV 60 kV 60 kV 90 kV 90 kV 0.3 2.0 1.5 1.0 0.0 0.5 0.0 5 10 15 20 35 30 Ultrafine Powder 30 kV 60 kV 90 kV 15 10 5 10 15 90 increases with the charging voltage for both powders, for higher voltages produce higher electric fields and higher corona currents. However, the ultrafine powder shows a lower Q/M with respect to the coarse powder, which may be partly due to a shorter exposure of the ultrafine powder to the electric field of the gun tip but more possibly due to a weaker electric field and much more severe corona quenching induced by the ultrafine powder. As for corona quenching effects of both powders, more details were discussed in a preceding work [37] and the ultrafine powder showed a stronger propensity in weakening the electric field and in reducing the corona current. More importantly, the Q/M of in-flight particles becomes lower in the vicinity of the substrate for both powders. In particular, the higher the charging voltage is, the more severe the reduction in Q/M is induced. The reason is that, as disclosed by equation (2), the back corona has a propensity to intensify with the charging voltage, due to higher current densities produced by higher voltages. In addition, it can be seen from figure 10 that in the vicinity of the substrate the Q/M values with the voltages of 60 and 90 kV are at a comparable level for both powders. On the other hand, it was demonstrated in [38] that the back corona showed an intensifying tendency with time, which was evaluated by the increased part of the corona current and is verified in figure 11 in this study. The initial drops in the current profiles are due to the effect of corona quenching, and thereafter the increasing currents account for the intensifying back corona. In addition, it was also observed in this study that some particles could be ejected from the deposited layer with some severe occurrences of back corona and left pinholes or craters in the deposited layer. In figure 12, the appearance of the coated film is exemplified in the case of 90 kV in an elongated spraying duration of 60 s in order to get clear information on pinholes. If the charging voltage of 30 kV was applied at the same conditions, the film appearance became fluffy and loosened without evident pinholes. As a result, it was demonstrated by the results of figures 10–12 that increasing the charging voltage improves 20 5 60 Figure 10. Comparisons of the charge-to-mass ratio (Q/M) of in-flight particles in the vicinity (50 mm) of the gun tip with those in the vicinity of the substrate (50 mm) for different powders. Coarse Powder 30 kV 60 kV 90 kV 25 0 30 Charging Voltage (kV) Spraying Duration (s) (a) Standard Deviation σ 2.5 0.2 0.1 (b) 50 mm to the Gun Tip 50 mm to the Substrate Ultrafine Powder Ultrafine Powder Coarse Powder Coarse Powder 3.0 0.4 Q/M (µC/g) Deposition Rate (g/s) 0.5 3.5 20 Charging Voltage (kV) Figure 9. (a) Comparisons of deposition rates due to different powders; (b) Comparisons of standard deviations (σ ) of the local mass-to-surface ratio ((M/S)i ) distributions due to different powders. standard deviations of local M/S distributions, as shown in figure 9(b). Remarkably, the ultrafine powder shows a more uniform deposited layer under the same conditions, and the uniformity of the deposited layer is opposite to the deposition efficiency. With reference to the characteristics disclosed in figure 9(a), the preceding work in [38] attributed them to the competing result between the primary charging of in-flight particles and the occurrences of back corona in the deposited layer. As the underlying mechanisms, the primary charging and the back corona exert opposite influences on the ultimate charges of in-flight particles and thereby combine to determine their electrostatic forces in the vicinity of the substrate. To demonstrate the so-called competition between the primary charging and the back corona, the charge of the in-flight particles was investigated in both the vicinities of the gun tip and the substrate by using the method described in figure 4(b) in a spraying duration of 20 s. As shown in figure 10, in the vicinity of the gun tip the Q/M of in-flight particles 7 350 300 250 200 150 100 50 6 5 4 3 2 1 0 X Meng et al 2.0 Spraying Duration 20 s: 30 kV; 60 kV; 90 kV 1.5 90 kV (Q/M)i (µC/g) JOverall (µA/m2) J. Phys. D: Appl. Phys. 42 (2009) 065201 60 kV 30 kV 1.0 0.5 0 5 10 Spraying Duration (s) 15 20 0.0 Figure 11. The dependence of overall current densities on the charging voltage. 10 8 6 4 2 2 0 Annular Region 4 6 8 10 Figure 13. The distributions of local charge-to-mass ratio ((Q/M)i ) for deposited particles. 90 kV additional charges by the secondary charging mechanism due to the convergence of ions on the substrate. In the meantime, the back corona induced by the accumulated charge of the deposited layer may discharge the deposited particles with an intensifying tendency. Therefore, back corona and secondary charging also compete in determining the particle charging characteristics of the deposited layer. In the preceding work [38] it was demonstrated with the coarse powder that the secondary charging of deposited particles was predominant in determining the characteristics of local Q/M distributions, and the back corona reduced the Q/M of the deposited layer with the extended spraying durations. In the preceding work [38] secondary charging was interpreted and evaluated by employing the concept of the current-to-mass ratio (A/M). In this study, the local Q/M distribution profiles of the deposited particles are first illustrated for ultrafine powder in figure 13 under a spraying duration of 20 s and various charging voltages (30, 60 and 90 kV). It is clear that the Q/M distributions of the ultrafine powder are fairly uniform, despite a rising tendency with 90 kV in some external regions (A8–A10). In order to explore the effects of secondary charging on the Q/M distributions of the ultrafine powder, the profiles of local current densities are shown in figure 14 under a spraying duration of 20 s and various voltages, whose characteristics have been discussed in [37]. It is evident that there is a remarkable rise in the current density of the fringe region, which is called the edge effect and is thought to be responsible for the M/S rise in the fringe region (as shown in figures 6, 8(a) and (b)). By further dividing the local current density (Ji : µA m−2 , see figure 14) by the corresponding M/S (g m−2 , see figure 6) of a certain region, the profiles of local A/M for a spraying duration of 20 s are obtained and shown in figure 15. Comparing the Q/M profiles in figure 13 with the A/M profiles in figure 15, it is noticeable that the uniformity of the A/M profiles underlies the uniform Q/M distributions, despite some variance between them existing in the fringe region at the voltages of 60 and 90 kV. The variance possibly implies that the local current of the fringe region concentrates more at the edge of the fringe region rather than distributes Figure 12. The surface defects incurred by back corona. the primary charging efficiency of the in-flight particles in the vicinity of the gun tip, but will also induce a severe back corona in the deposited layer at the same time. Thus, the characteristics of the ultrafine powder in the deposition efficiency can be interpreted as follows: the improved deposition efficiency from 30 to 60 kV was due to the improved primary charging efficiency of the in-flight particles, which was dominant in competing with the occurrences of back corona; but a further increase to 90 kV induced a much severe back corona, which resulted in a more remarkable reduction in the charge of in-flight particles and thereby suppressed further improvement in the deposition efficiency. On the other hand, the intensifying back corona was responsible for the decreasing deposition rate in powder coating processes. In particular, the ultrafine powder showed some inferiority in its primary charging efficiency with respect to the coarse powder, which might make it more sensitive to the charge loss incurred by back corona, and thereby results in a faster decrease in the deposition rate. 3.2. Characteristics of particle charging In the above section, it is clearly demonstrated that, as the premise of particle deposition, the primary charging of in-flight particles competes with back corona during the powder coating processes. However, the deposited particles may accept 8 J. Phys. D: Appl. Phys. 42 (2009) 065201 Spraying Duration 20 s: X Meng et al 30 kV; 1000 60 kV; 4.0 90 kV 3.5 100 Q/M(µC/g) Ji (µA/m 2 ) 3.0 10 2.5 2.0 1.5 1.0 0.5 1 8 10 6 4 2 2 0 Annular Region 4 6 8 0.0 10 Spraying Duration 5 s: 10 Spraying Duration 20 s: 60 kV; 2 Spraying Duration 5 s: 0 2 4 6 90 kV 8 10 30 kV; 60 kV; 90 kV 10 90 kV 1 1 0.1 0.1 10 (b) 0.01 4 60 kV; Annular Region (A/M)i (µA/g) (A/M)i (µA/g) 30 kV; 6 (a) Figure 14. The dependences of local current density (Ji ) distributions on charging voltages during powder coating processes. 10 8 30 kV; 10 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 10 Annular Region Figure 16. Comparisons between the local charge-to-mass ratio ((Q/M)i ) and the local current-to-mass ratio ((A/M)i ) at a spraying duration of 5 s: (a) local charge-to-mass ratio distributions; (b) local current-to-mass ratio distributions. 10 Annular Region Figure 15. The dependences of local current-to-mass ratio ((A/M)i ) distributions on charging voltages during powder coating processes. for the ultrafine powder while an increasing tendency for the coarse powder. The characteristics of the average Q/M for both powders were supported by the behaviour of their average A/M, as shown in figure 18(b). Obviously, the different Q/M tendencies of the two powders are simply due to the differences in their secondary charging efficiencies with the increasing voltages. The different secondary charging efficiencies lie in the differences in their particle sizes of the two powders, which lead to the differences in their particle number and specific surface. Further, different powders incur variations in the suppression of corona currents (as disclosed in [37]), in their primary charging efficiencies (as shown in figure 10) and in the current density distributions (as disclosed in [37]). As a result, the deposition efficiency and the secondary charging efficiency vary with different powders. On the other hand, the average Q/M of both powders in figure 18(a) decreases with the extended spraying duration. This should be attributed to the intensifying back corona in powder coating processes, which produces more positive ions and thereby neutralizes the negative charge of the deposited particles more severely with the extended spraying duration. The deposited particles in the evenly in the whole fringe region. More importantly, it can be further observed from figures 13 and 15 that the Q/M and A/M profiles nearly change synchronously with the charging voltage. Thus, the dominance of secondary charging on the Q/M characteristics of deposited particles is witnessed again with the ultrafine powder. Furthermore, the dominance of secondary charging on the characteristics of local Q/M distribution is supported at the spraying durations of 5 s and 10 s by comparing the local Q/M profiles with the local A/M profiles, as shown in figures 16(a) and (b) and figures 17(a) and (b) respectively. In addition, the results in figures 13, 16(a) and 17(a) commonly disclosed that the Q/M roughly shows a decreasing tendency in most of the local regions with the increase in voltage. To contrast the charging characteristics of the deposited layers due to the two powders, the average Q/M of the deposited particles in the internal regions of A1–A9 was illustrated in figure 18(a). It is obvious that the average Q/M presents a decreasing tendency with the increase in voltage 9 J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al 2.5 10 Spraying Duration 10 s: 30 kV; 60 kV; 90 kV Ultrafine Powder 5s 10 s 20 s (Q/M)AV. (µC/g) (Q/M)i (µC/g) 2.0 Coarse Powder 5s 10 s 20 s 1.5 1.0 1 0.5 0.0 30 10 8 6 4 (a) 2 0 2 4 6 8 Annular Region 30 kV; 60 kV; Ultrafine Powder Coarse Powder 5s 5s 10 s 10 s 20 s 20 s 90 kV 10 (A/M)i (µA/g) 90 Charging Voltage (kV) 10 (A/M)AV. (µA/g) Spraying Duration 10 s: 60 (a) 10 1 1 0.1 0.1 30 10 (b) 8 6 4 2 0 2 4 6 8 60 (b) 10 Annular Region 90 Charging Voltage (kV) Figure 18. (a) The dependences of the average charge-to-mass ratio ((Q/M)AV ) on charging voltages and spraying durations; (b) the dependences of the average current-to-mass ratio ((A/M)AV ) on charging voltages and spraying durations. Figure 17. Comparisons between the local charge-to-mass ratio ((Q/M)i ) and the local current-to-mass ratio ((A/M)i ) at a spraying duration of 10 s: (a) local charge-to-mass ratio distributions; (b) local current-to-mass ratio distributions. 1.4 fringe region (A10) were not taken into account in figures 18(a) and (b), due to the difficulty in interpreting the part of the local current of the fringe region working on its local deposited particles. Nevertheless, the Q/M distributions of deposited particles with both powders were compared by evaluating their standard deviations, as shown in figure 19. It is implied by standard deviations that the Q/M distributions with the ultrafine powder are more uniform than those with the coarse powder, except for the case of 30 kV under the spraying durations of 5 and 10 s. In addition, the results of standard deviations indicate that the Q/M uniformity with the coarse powder gets much worse with increasing voltage while the one with the ultrafine powder suffers a small influence from the changing voltage and mostly has a less deviation. The results in figure 19 still imply that the extended spraying duration can improve the Q/M uniformity of both powders, but there is an exception in the case of 90 kV of the ultrafine powder. Ultrafine Powder 5s 10 s 20 s Standard Deviation σ 1.2 Coarse Powder 5s 10 s 20 s 1.0 0.8 0.6 0.4 0.2 0.0 30 60 90 Charging Voltage (kV) Figure 19. Comparisons of standard deviations (σ ) of the local charge-to-mass ratio ((Q/M)i ) distributions due to different powders. 10 J. Phys. D: Appl. Phys. 42 (2009) 065201 X Meng et al The authors are grateful to Links Coatings (London, Ontario) for supplying the paint powder. 4. Conclusion The understanding of powder coating processes is not yet fully clear, due to the many physical mechanisms involved. As an essential supplement to the studies in exploring the mechanisms of particle charging and deposition during powder coating processes, this study employed an ultrafine powder with respect to the coarse powder used in the preceding work [38]. By investigating the charge-to-mass ratio and the massto-surface ratio of the deposited particles, the characteristics of particle charging and deposition with ultrafine powder were revealed for the first time in this paper. It was disclosed that the ultrafine powder behaves similarly in many ways as the coarse powder studied in the preceding work. First, a cone-shaped pattern of deposited particles across the substrate and a rise in particle accumulation in the fringe region were observed with both powders. It was demonstrated again in this study that the inhomogeneous concentrations of charged in-flight particles contribute to the former, and the edge effect is responsible for the latter. In addition, it was verified by the ultrafine powder that the particle deposition efficiency has a strong dependence on the primary charging of the in-flight particles but suffers a severe influence from the back corona. Thus, the highest efficiency is a competing result between the primary charging of in-flight particles and the back corona in the deposited layer and can be realized by compromising the effects from primary charging and back corona. It was indicated in this study that the optimal voltage for the highest efficiency is around 60 kV for both powders. However, the ultrafine powder presents a faster reduction in the deposition rate and has some superiority in improving the uniformity of the deposited layer. As for the charging characteristics of the deposited particles, it was demonstrated again with the ultrafine powder that secondary charging dominates the characteristics of the local charge-to-mass ratio distribution across the substrate. The ultrafine powder exhibits a decreasing Q/M of the deposited particles with increasing voltage, which is opposite to the increasing tendency of the coarse powder. In particular, the ultrafine powder is more likely to produce more uniform Q/M distributions in the deposited layer with respect to the coarse powder. However, the charge-to-mass ratio of the ultrafine powder decreases with extended spraying durations due to the intensifying back corona, as is also the case for the coarse powder. Significantly, the characteristics of particle charging and deposition with ultrafine powder are disclosed in this study. By combining the findings from both coarse and ultrafine powders together, comprehensive knowledge on powder coating processes is possible. It is believed that the outcome will be of great help in understanding the mechanisms of corona charging processes of powder coating as well as improving the applications of powder coating in finishing industries. References [1] Meek J M and Craggs J D 1978 Electrical Breakdown of Gases (New York: Wiley) [2] Leal Ferreira G F, Oliveira O N and Giacometti J A 1986 Point-to-plane corona: corona-voltage characteristics for positive and negative polarity with evidence of an electronic component J. Appl. Phys. 59 3045–9 [3] Abdel-Salam M and Singer H 1991 Onset voltages of fore and back coronas in point–plane gaps J. Phys. D: Appl. Phys. 24 2000–7 [4] Lawless P A and Sparks L E 1980 A mathematical model for calculating effects of back corona in wire-duct electrostatic precipitators J. Appl. Phys. 51 242–56 [5] McLean K J 1988 Electrostatic precipitators IEE Proc. 135 347–61 [6] Chang J S, Lawless P A and Yamamoto T 1991 Corona discharge processes IEEE Trans.Plasma Sci. 19 1152–66 [7] Cross J A 1987 Electrostatics: Principles, Problems and Applications (Bristol: Hilger) [8] Mazumder M K, Sims R A, Biris A S, Srirama P K, Saini D, Yurteri C U, Trigwell S, De S and Sharma R 2006 Twenty-first century research needs in electrostatic processes applied to industry and medicine Chem. Eng. Sci. 61 2192–211 [9] Dascalescu L, Morar R, Iuga A, Samuila A, Neamtu V and Suarasan I 1994 Charging of particulates in the corona field of roll-type electroseparators J. Phys. D: Appl. Phys. 27 1242–51 [10] Bailey A G 1998 The science and technology of electrostatic powder spraying, transport and coating J. Electrostat. 45 85–120 [11] Wu S 1976 Electrostatic charging and deposition of powder coatings Polym.-Plast. Technol. Eng. 7 119–220 [12] Adamiak K 1997 Numerical modeling of tribo-charge powder coating systems J. Electrostat. 40–41 395–400 [13] Adamiak K 2001 Numerical investigation of powder trajectories and deposition in tribocharge powder coating IEEE Trans. Indust. Appl. 37 1603–9 [14] Meng X, Zhang H and Zhu J 2008 A general empirical formula of current–voltage characteristics for point-to-plane geometry corona discharges J. Phys. D: Appl. Phys. 41 065209 [15] Henson B L 1981 A space-charge region model for microscopic steady coronas from points J. Appl. Phys. 52 709–15 [16] Boutlendj M and Allen N L 1993 Current-density distribution on a plane cathode in dc glow and streamer corona regimes in air IEEE Trans. Electr. Insul. 28 86–92 [17] Jones J E 1997 A theoretical explanation of the laws of Warburg and Sigmond Proc. R. Soc. A: Math., Phys. Eng. Sci. 453 1033–52 [18] Adamiak K and Atten P 2004 Simulation of corona discharge in point–plane configuration J. Electrostat. 61 85–98 [19] Awad M B and Castle G S P 1975 Efficiency of electrostatic precipitators under conditions of corona quenching J. Air Pollut. Control Assoc. 25 172–6 [20] Masui N and Murata Y 1982 Method for measuring the powder charge in the electrostatic powder-coating process Rev. Sci. Instrum. 53 532–3 [21] Inculet I I and Castle G S P 1992 Faraday pail with self regulating ion repulsion IAS Annual Meeting (IEEE Industry Application Society) (Houston, TX) vol 2 pp 1502–5 [22] Inculet I I and Castle G S P 1995 Q/M distribution in pulsed corona powder coating spray cone IAS Annual Meeting Acknowledgments The authors thank Mr Xianzhong Zhu and Mr Jianzhang Wen for their help and advice on the design of experimental setup. 11 J. Phys. D: Appl. Phys. 42 (2009) 065201 [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] X Meng et al [33] Shah U, Zhang C and Zhu J 2006 Comparison of electrostatic fine powder coating and coarse powder coating by numerical simulations J. Electrostat. 64 345–54 [34] Ye Q, Steigleder T, Scheibe A and Domnick J 2002 Numerical simulation of the electrostatic powder coating process with a corona spray gun J. Electrostat. 54 189–205 [35] Ye Q and Domnick J 2003 On the simulation of space charge in electrostatic powder coating with a corona spray gun Powder Technol. 135–136 250–60 [36] Zhu J and Zhang H 2005 Ultrafine powder coatings: an innovation Powder Coat. 16 39–47 [37] Meng X, Zhu J and Zhang H 2008 The characteristics of current density distribution during corona charging processes of different particulates J. Phys. D: Appl. Phys. 41 172007 [38] Meng X, Zhang H and Zhu J 2008 The characteristics of particle charging and deposition during powder coating processes with coarse powder J. Phys. D: Appl. Phys. 41 195207 [39] Misev T A and van der Linde R 1998 Powder coatings technology: new developments at the turn of the century Prog. Org. Coat. 34 160–8 [40] Satoh H, Harada Y and Libke S 1998 Spherical particles for automotive powder coatings Prog. Org. Coat. 34 193–99 [41] Biris A S, Mazumder M K, Yurteri C U, Sims R A, Snodgrass J and De S 2001 Gloss and texture control of powder coated films Particulate Sci. Technol. 19 199–217 [42] Biris A S, Mazumder M K, Yurteri C U, Sims R A, Snodgrass J and De S 2001 Gloss and texture control of powder coated films Particulate Sci. Technol. 19 199–217 [43] Meschievistz T, Rahangdale Y and Pearson R 1995 US council for automotive research (USCAR) low-emission paint consortium: a unique approach to powder painting technology development Metal Finish. 26–31 (IEEE Industry Application Society) (Orlando, FL) vol 2 pp 1289–94 Dastoori K, Makin B, Telford J 2001 Measurement of thickness and adhesive properties of electrostatic powder coatings for standard and modified powder coating guns J. Electrostat. 51–52 545–51 Sharma R, Biris A S, Sims R A and Mazumder M K 2001 Effect of ambient relative humidity on charge decay properties of polymer powder and on the occurrence of back corona in powder coating IAS Annual Meeting (IEEE Industry Application Society) (Chicago, IL) vol 3 pp 1961–5 Sims R A, Mazumder M K, Biris A S, Sharma R and Kumar D 2000 Effect of electrical resistivity on the adhesion and thickness of electrostatically deposited powders layers IAS Annual Meeting (IEEE Industry Applications Society) (Rome, Italy) vol 2 pp 820–3 Sims R A, Mazumder M K, Liu X, Chok W, Mountain J R, Wankum D L, Pettit P and Chasser T 2001 Electrostatic effects on first pass transfer efficiency in the application of powder coatings IEEE Trans. Indust. Appl. 37 1610–7 Masuda S and Mizuno A 1977/1978 Initiation condition and mode of back discharge J. Electrostat. 4 35–52 Masuda S and Mizuno A 1978 Flashover measurements of back discharge J. Electrostat. 4 215–31 Masuda S and Mizuno A 1976/1977 Light measurements of back discharge J. Electrostat. 2 375–96 Tachibana N 1989 Back discharge and intermittent energization in electrostatic precipitation of fly ash J. Electrostat. 22 257–72 Cross J A 1985 An analysis of the current in a point-to-plane corona discharge and the effect of a back-ionising layer on the plane J. Phys. D: Appl. Phys. 18 2463–71 Li Z, Zhu J and Zhang C 2005 Numerical simulations of ultrafine powder coating systems Powder Technol. 150 155–67 12