Particle size, cohesiveness and charging effects on electrostatic and nonelectrostatic powder coating

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1 Journal of Electrostatics 65 (27) Particle size, cohesiveness and charging effects on electrostatic and nonelectrostatic powder coating Puntarika Ratanatriwong, Sheryl Barringer Department of Food Science and Technology, The Ohio State University, 215 Fyffe Road, Columbus, OH 4321, USA Received 5 January 27; accepted 2 May 27 Available online 15 June 27 Abstract Ten powders, differing in protein, carbohydrate and salt contents and ranging from 19 to 165 mm were coated by nonelectrostatic and electrostatic coating. Nonelectrostatic transfer efficiency (TE) increased to a maximum before leveling off with increasing particle size. Electrostatic TE either decreased or increased then decreased with increasing particle size. Powders became more free flowing as particle size increased. Since TE increases as powders become more free flowing, TE increased with particle size for both nonelectrostatic and electrostatic coating. For electrostatic coating, the effect of charge decreases with increasing particle size. Thus, the conflicting effects of ability to pick up charge and flowability caused an increase then decrease in the TE for powders coated electrostatically, and can also explain the exceptions. The average improvement in TE by electrostatic coating was 2%, with the improvement increasing as particle size decreased. r 27 Elsevier B.V. All rights reserved. Keywords: Powder; Size; Electrostatic; Coating 1. Introduction Powder coating is a common way to apply a variety of seasonings to snack foods. Manufacturers frequently use an excess amount of seasoning to ensure adequate coverage even though this increases cost due to waste. The excess seasonings fall off onto the conveyor belt and the resulting powder increases cleaning needs [1,2]. Electrostatic coating has been used in the metal painting industry for several decades to minimize waste. Its coating principle is to charge powder particles in an intense electric field known as a corona zone generated by high voltages [1]. Due to similar polarity, charged particles repel each other, forming a cloud across the coating area. The larger the charge, the wider the powder spread area [3]. The aerodynamic, gravitational and Coulombic forces help transport charged powders to the target. Coulombic force accelerates charged particles as they are driven toward the grounded target and evenly coat the products [1]. Adhesion Corresponding author. Tel.: ; fax: address: Barringer.11@osu.edu (S. Barringer). force between charged particles and targets due to the creation of an image charge decreases powder-coating loss as waste [4,5]. The advantages of electrostatic coating over traditional coating include higher transfer efficiency (TE), less waste, more uniform coating, less dust and less cleanup time [1,2,4,6,7]. Despite the fact that electrostatic coating is widely used in the metal painting industry, the effects of food powder properties on electrostatic and nonelectrostatic coating efficiency are still unclear. Food powder properties such as size, powder chargeability and flowability are reported to affect electrostatic coating TE. Size is known to play a major role in powder coating; however, it is unclear whether smaller or larger particle size increases electrostatic coating efficiency. Many studies report smaller particles producing higher electrostatic coating efficiency than larger particles [1,4,8 1]. However, one study reported that the increase in TE for electrostatic coating is independent of size [11]. Another study reported that larger particles accumulate greater charge, and since a large charge is required to overcome the charge on the powder forming the initial monolayer of powder, the larger the /$ - see front matter r 27 Elsevier B.V. All rights reserved. doi:1.116/j.elstat

2 P. Ratanatriwong, S. Barringer / Journal of Electrostatics 65 (27) particle, the more likely it is to deposit on the target [12]. All of these studies were conducted using different powders, each with a different particle size, instead of using various sizes from the same powder. No investigation on the effect of size using the same food powder has yet been done. Powder charge and flowability also affect electrostatic coating. Electrostatic TE increases with increasing charge to mass ratio [4] and powder charge decay [1]. The greater the powder charge, the higher the electrostatic TE [9,1]. Powder cohesiveness is also important to coating efficiency. Free flowing powders increase nonelectrostatic and electrostatic TE [11,13]. Thus, the objective of this study is to determine the effect of powder size on nonelectrostatic and electrostatic coating efficiency using different sizes of the same powder. 2. Materials and methods Powders with a wide range of carbohydrate, protein and ash content were selected in order to test the range of powders typically coated. Powders were granulated sugar, 1DE, demineralized whey, whey protein isolate, nonfat dry milk, sodium chloride, potassium chloride, sodium erythorbate, malic acid and citric acid. These powders were ground to obtain a range of particle sizes mm. Due to differences in the nature of the powders, the grinding process could not achieve the same particle size for all powders. Size reduction was accomplished using either a commercial food blender (Model 31BL92, Dynamics Co., New Hartford, CT, USA) or an ultracentrifugal mill (Retsch ZM1, GlenMills Inc., Clifton, NJ). Ground powders were then separated by a series of ASTME-11 standard testing sieves to obtain ranges of particle size. Particle size was determined by the polydisperse method using the Malvern Mastersizer X with a dry powder feeder unit (Worchestershire, UK). The volume mean diameter D (4.3) was reported. Food powders were coated onto aluminum foil samples using a belt conveyor electrostatic powder applicator (Terronics Development Corporation, Elwood, IN, USA) at and 25 kv for nonelectrostatic (NE) and electrostatic (E) coating, respectively. Coating was done at 45% relative humidity. Five grams of each powder was manually fed per run, with three replicates. Seven aluminum foil strips cm were horizontally placed in a fixed position across the coating area. Each strip represented the area a sample moving through time would cover on the belt. After powder was transported into the delivery system, it was fluidized with an air pressure of 345 kpa and pneumatically conveyed into the coating chamber. Powder then entered the system at a downward slope and passed through a corona zone generated by two electric wires. Due to corona discharge, powder particles picked up negative charge in the air and formed a cloud due to charge repulsion, before seeking to coat onto grounded targets. A vacuum was placed to the side at the end of the coating chamber to remove dust from the air. TE of each powder is the weight on the sample divided by the theoretical amount delivered. TE improvement was the TE of electrostatic minus that of nonelectrostatic coating. Angle of repose is the angle with the horizontal made by a pile of powder. It was determined by the modified fixed base method using a 4.5 cm diameter and 5 cm height powder-collecting cup. Powder was sifted by a powder sifter through a funnel with its tip 11 cm from the edge of the cup. The powder was slowly sifted so that it formed a pile within the edges of the cup. Calipers were used to measure the peak height of the powder pile from the base of the pile. The peak height was determined as the last height just before the pile collapsed. The arctangent of the peak height over the radius gives the angle of repose. Powders with angle of repose in the ranges of , 45 1 and 41 are free flowing, fairly free flowing and cohesive, respectively [14,15]. Statistical analyses were performed by JMP software version 4.3 (SAS Institute Inc., Cary, NC). Student s t-test, Tukey s test, analysis of variance and standard least square regression analysis with effect screening were used. All equations, variables and data analyses are significant at p-value o Results and discussion Food powders with a range of composition were ground to produce particle sizes ranging from 19 to 165 mm and were coated electrostatically and nonelectrostatically. Coating TE is the proportion of powder deposited onto the sample to powder delivered in the system. For any given powder, there was a significant difference in the nonelectrostatic TE based on particle size. As particle size increased, the nonelectrostatic TE increased to a maximum then remained constant for most powders (Fig. 1). For demineralized whey and malic acid powders, the nonelectrostatic TE increased then decreased with increasing particle size. Others have reported that nonelectrostatic coating increases with an increase in particle size [9,1]. As size decreases, powder flowability typically decreases [14,16]. This was observed in this study as well (Fig. 2). Electrostatic and molecular forces become more significant for smaller particles due to their high surface to volume ratios. For small particles, van der Waals forces are large because of the large surface area, surface energy per weight and short interparticle distance. Hence, small particles are more cohesive and have more difficulty in flowing [15,17]. The more free flowing the powder, the higher the TE [9,1,13]. It is easier for free flowing powders to be well dispersed [4] so that they can deposit more effectively on the target [17]. Cohesive powders easily aggregate or cake, causing variation in the flow and TE. Cohesive powders tend to form bridges between particles, adhere to the wall of the transporting tube [15,17] and choke at the outlet of

3 6 ARTICLE IN PRESS P. Ratanatriwong, S. Barringer / Journal of Electrostatics 65 (27) 4 8 Nonelectrostatic TE (%) Demineralized whey Whey protein isolate Fig. 1. The effect of particle size on nonelectrostatic coating efficiency of various powders. Angle of Repose (Degree) Whey protein isolate Demineralized whey Size (µm) Fig. 2. Powders became more cohesive, higher angle of repose, when particle size decreased. the transporting tube, decreasing the amount of input powder and resulting in lower TE [18]. Thus the TE increases as size and flowability increases. Similar to nonelectrostatic coating, size was a significant factor driving electrostatic coating efficiency. For most powders the electrostatic TE increased until it reached a maximum then decreased with increasing size (Fig. 3). Whey protein isolate and potassium chloride were not significantly different with size while sodium chloride and nonfat dry milk continued to decrease with increasing particle size, and did not reach a maximum within the size range tested. There are two conflicting forces which caused the electrostatic TE to increase then decrease with increasing size. One force is the effect of size on charge development. Smaller particles have a larger surface area compared to their mass, resulting in higher charge to mass ratio development in a corona discharge. The higher the charge on the powder, the higher the electrostatic TE [4]. Small masses are also more strongly influenced by an electric field [1,4]. Large particles have lower TE because the effect of gravitational force overcomes electrostatic force and adhesion between powders and target so that powders are not directed toward the target and those that do land do not adhere well [1]. The second force is the effect of size on cohesiveness. The effect is the same as for nonelectrostatic coating in that smaller particles are more cohesive and have lower TE. The two most cohesive powders, malic acid and sugar, also had the lowest TEs (Figs. 2 and 3). There were two exceptions to the pattern of increasing then decreasing, sodium chloride and nonfat dry milk, which only decreased with increasing size. They are also the only powders where the cohesiveness decreased instead of increasing as size decreased (Fig. 2). Thus, both size and flowability worked together to increase TE as size decreased for those two

4 P. Ratanatriwong, S. Barringer / Journal of Electrostatics 65 (27) Electrostatic TE (%) Demineralized whey Whey protein isolate Fig. 3. The effect of particle size on electrostatic coating efficiency of various powders. TE Improvement by Electrostatic (%) Demineralized whey Whey protein isolate Fig. 4. The effect of particle size on transfer efficiency improvement using electrostatics. powders. For the other powders, these forces were in opposition and produced an increase then decrease in TE. Electrostatic coating for all powders produced significantly greater TE than nonelectrostatic coating (Fig. 4). Electrostatic coating improved coating TE by an average of 2%, similar to a 15% efficiency improvement reported by [11] for a different coating system. However, the average is deceptive because the efficiency improvement ranges from % for small particles down to less than 1% for large particles. Thus the improvement in efficiency becomes greater the smaller the particle. 4. Conclusion The nonelectrostatic coating efficiency of a given powder increased to a maximum then leveled off. The increase in nonelectrostatic TE with increasing particle size was due to the influence of powder size on the cohesiveness. The larger the particle size the more free flowing the powder. The more free flowing the powder, the higher the nonelectrostatic TE. Electrostatic TE increased then decreased with size. The smaller the particle, the greater the charge and the greater the electrostatic TE. The conflicting effects of particle charge and cohesiveness created the increase then decrease in electrostatic TE. For all powders, electrostatic coating improved coating TE by an average of 2% over that of nonelectrostatic coating. The smaller the particle size, the greater the improvement in transfer coating efficiency by electrostatic coating. Acknowledgments Support by the Edison Industrial Systems Center and Nestle R&D Center, Inc. is gratefully acknowledged. This project was supported by the USDA Cooperative State Research, Education and Extension Service, special research Grant number

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