Effects of Particle Size and Density on Separation of Mixtures During Nonelectrostatic and Electrostatic Powder Coating

EFFECTS OF PARTICLE SIZE AND DENSITY ON SEPARATION OF MIXTURES DURING NONELECTROSTATIC AND ELECTROSTATIC POWDER COATING

A Thesis

Presented in Partial Fulfillment of the Requirements for

The Degree Master of Science in the

Graduate School of The Ohio State University

Chanun Somboonvechakarn, B.S.

*****

The Ohio State University

2009

Master’s Examination Committee:

Dr. Sheryl A. Barringer, Advisor Approved by

Dr. V.M. Balasubramaniam ______

Dr. John Litchfield Advisor

Graduate Program in Food Science and Technology Copyright by Chanun Somboonvechakarn 2009 ABSTRACT

Powders used during coating usually consist of a mixture with different physical properties, such as particle size and density. Two mixtures with the same composition but different sizes: 44 and 256 µm NaCl, and 64 and 191 µm starch; and two mixtures with sizes close to one another but different densities: 44 µm NaCl and

64 µm starch, and 197 µm NaCl and 191 µm starch, were nonelectrostatically and electrostatically coated on grounded targets. Separation of mixtures was measured by comparing the percentages of each powder on the target to determine whether size caused separation. Targeting loss and adhesion loss, when coated individually and in mixtures, were determined. During nonelectrostatic coating, mixtures of NaCl and starch separated by size due to the difference in targeting losses between small and large powders, both individually and in mixtures. Adhesion losses had a small effect.

Interactions occurred due to being in a mixture reduced separation for NaCl, but increased separation in starch. Separation occurred because there was more large powder on most or all locations. During electrostatic coating, separation by size occurred due mainly to differences in adhesion loss in mixtures. For both NaCl and starch mixtures, there were more small powders closer to the nozzle due to electrostatic charge, while more large powders landed further away from the nozzle, causing separation, however, electrostatics did not affect separation compared to nonelectrostatic coating.

ii Differences in density caused separation during nonelectrostatic coating of small mixtures. The separation was due mainly to differences between individual targeting losses, while the difference in adhesion losses was a less significant factor.

There was more starch than NaCl on all targets, due to the differences in targeting losses. The interactions due to mixture increased separation. No separation was observed in the mixtures of large particles with different densities. During electrostatic coating, the differences in the individual targeting losses were large, but became insignificant when in the mixture. The differences in the individual adhesion losses were also large, but became smaller when in a mixture. In the small mixtures, there were more starch particles closer to the nozzle during electrostatic coating.

Overall, nonelectrostatic coating causes high targeting loss for all powders, therefore coating systems need to be designed to minimize the problem. Electrostatics coating increased the differences in adhesion loss, therefore methods such as adding oil to the surface of the targets need to be applied in order to reduce adhesion loss difference.

iii ACKNOWLEDGEMENTS

I wish to thank my advisor, Dr. Sheryl A. Barringer, for her guidance and encouragement, which made this thesis possible, and for her patience in correcting my errors. It has truly been a privilege for me to be under her guidance and supervision for the past two years.

I thank Yichi Xu and Yang Huang, my fellow colleagues, for discussing with me various aspects of this thesis, and for helping me with various equipment problems.

I am grateful to Inggrayani Herlambang for helping me to handle various statistical problems I encountered during the data analysis, and her moral support.

I also wish to thank my parents, Sittichai and Ladda Somboonvechakarn, for their encouragement and moral support throughout my graduate school years.

iv VITA

January 9, 1984…………………………………………….. Born- Bangkok, Thailand

2007…………………………………... B.S. Food Science, The Ohio State University

2007-present…………………………………………… Graduate Research Associate,

The Ohio State University

FIELDS OF STUDY

Major Field: Food Science and Technology

v TABLE OF CONTENTS

ABSTRACT...... ii ACKNOWLEDGEMENTS...... iv VITA...... v LIST OF TABLES ...... viii LIST OF FIGURES...... ix

Chapters:

1 LITERATURE REVIEW: SEPARATION IN MIXTURES DURING NONELECTROSTATIC AND ELECTROSTATIC POWDER COATING ...... 1 1.1 Introduction ...... 1 1.2 Transfer efficiencies...... 3 1.2.1 Effects of particle size on transfer efficiency ...... 3 1.2.2 Effects of density on transfer efficiency...... 5 1.2.3 Effects of resistivity on transfer efficiency...... 6 1.2.4 Losses contributing to decreased transfer efficiency ...... 7 1.3 Adhesion...... 8 1.3.1 Effects of particle size on adhesion...... 8 1.3.2 Effects of resistivity on adhesion...... 9 1.4 Factors affecting separation of mixtures during nonelectrostatic and electrostatic coating ...... 10 1.4.1 Effects of transfer efficiencies of individual components on separation .. 12 1.5 Trajectories of powders in a polydisperse system ...... 13 1.5.1 Trajectories of powders in mixtures during nonelectrostatic coating...... 13 1.5.1.1 Particle size profile of mixtures during nonelectrostatic coating...... 13 1.5.1.2 Particle velocity profile of mixtures during nonelectrostatic coating . 15 1.5.2 Trajectories of powders in mixtures during electrostatic coating...... 16 1.5.2.1 Particle size distribution and trajectories of particles in a polydisperse system during electrostatic coating...... 17 1.5.3 Particle velocity profile of mixtures during electrostatic coating...... 20 1.6 Conclusion...... 20

2 EFFECTS OF PARTICLE SIZE ON SEPARATION OF MIXTURES DURING NONELECTROSTATIC AND ELECTROSTATIC POWDER COATING ...... 22 2.1 Abstract ...... 22 2.2 Introduction ...... 23 2.3 Materials and Methods...... 25 2.3.1 Determination of separation ...... 25 2.3.2 Targeting loss and adhesion loss determination ...... 26 2.3.3 Determination of interactions between particles in mixtures of different sizes ...... 27 2.3.4 Statistical analysis...... 28 vi 2.4 Results and discussion...... 28 2.4.1 Differences in particle size caused separation in NaCl and starch mixtures during both nonelectrostatic and electrostatic coating...... 28 2.4.2 Individual targeting losses...... 30 2.4.3 Targeting losses of powders in a mixture...... 32 2.4.4 Individual adhesion loss ...... 34 2.4.5 Adhesion losses in mixtures ...... 36 2.4.6 Separation...... 37 2.5 Conclusion...... 39 2.6 References ...... 41 2.7 Tables and Figures ...... 44 2.7.1 Tables ...... 44 2.7.2 Figures...... 46

3 EFFECT OF PARTICLE DENSITY ON SEPARATION OF MIXTURES DURING NONELECTROSTATIC AND ELECTROSTATIC POWDER COATING ...... 50 3.1 Abstract ...... 50 3.2 Introduction ...... 51 3.3 Materials and methods ...... 52 3.3.1 Determination of separation ...... 53 3.3.2 Targeting and adhesion loss...... 54 3.3.3 Statistical analysis...... 55 3.4 Results and discussion...... 55 3.4.1 Separation during nonelectrostatic and electrostatic coating ...... 55 3.4.2 Individual targeting losses...... 58 3.4.3 Targeting losses in mixtures ...... 60 3.4.4 Individual adhesion losses...... 61 3.4.5 Adhesion losses in mixtures ...... 63 3.4.6 Separation...... 65 3.5 Conclusion...... 66 3.6 References ...... 68 3.7 Tables and Figures ...... 71 3.7.1 Tables ...... 71 3.7.2 Figures...... 73

BIBLIOGRAPHY ...... 77

APPENDIX A: STANDARD CURVES...... 80

vii LIST OF TABLES

Chapter 2

Table 1. Actual and predicted percentage of the smaller powder by location, after nonelectrostatically and electrostatically applying a 50:50 mixture of 44 and 256 µm NaCl or 61 and 191 µm starch ...... 44 Table 2. Adhesion losses (%) of powders, with and without the other size of powder coated on the unoiled target ...... 44 Table 3. Average mass of powder landed on each location on all locations, that is 44 µm and 256 µm NaCl after nonelectrostatic coating of 3.0000 g mixture vs. the average mass of powder on each location that is 44 µm and 256 µm NaCl after electrostatic coating of 3000mg mixture...... 45 Table 4. Average mass of powder on each location on all locations, that is 64 µm and 191 µm starch after nonelectrostatic coating of 3.0000 g mixtures vs. the average mass of powder on each location that is 64 µm and 191 µm starch after electrostatic coating of 3000mg mixture...... 45

Chaper 3

Table 5. Actual percentage and predicted percentage of starch powder on each location after nonelectrostatically applying a 50:50 mixture of 44 µm NaCl and 64 µm starch, or 197 µm NaCl and 191 µm starch...... 71 Table 6. Average mass of powder landed on each location on all locations, that is 44 µm NaCl and 64 µm starch after nonelectrostatic coating of 3000 mg mixture vs. the average mass of powder on each location that is 44 µm NaCl and 64 µm starch after electrostatic coating of 3000 mg mixture...... 72 Table 7. Average mass of powder on each location on all locations, that is 197 µm NaCl and 191 µm starch after nonelectrostatic coating of 3000 mg mixtures vs. the average mass of powder on each location that is 197 µm NaCl and 191 µm starch after electrostatic coating of 3000 mg mixture...... 72

viii LIST OF FIGURES

Chapter 2

Figure 1. Target setup reference...... 46 Figure 2. Targeting and adhesion loss for all powders when coated individually and when in a mixture ...... 46 Figure 3. Difference in losses in each mixture with two powders of different sizes .. 47 Figure 4. Average mass of powder landed on each location on all locations, that is 44 µm and 256 µm NaCl after nonelectrostatic coating of 3.0000 g mixture ...... 48 Figure 5. Average mass of powder landed on each location on all locations, that is 64 µm and 191 µm starch after nonelectrostatic coating of 3.0000 g mixture ...... 48 Figure 6. Average mass of powder landed on each location on all locations, that is 44 µm and 256 µm NaCl after electrostatic coating of 3.0000 g mixture ...... 49 Figure 7. Average mass of powder landed on each location on all locations, that is 64 µm and 191 µm starch after electrostatic coating of 3.0000 g mixture ...... 49

Chapter 3

Figure 8. Target setup reference...... 73 Figure 9. Targeting and adhesion loss for all powders when coated individually and when in a mixture ...... 73 Figure 10. Difference in losses in each mixture with two powders with different density...... 74 Figure 11. Average mass of powder landed on each location on all locations, that is 44 µm NaCl and 64 µm starch after nonelectrostatic coating of 3.0000 g mixture . 75 Figure 12. Average mass of powder landed on each location on all locations, that is 197 µm NaCl and 191 µm starch after nonelectrostatic coating of 3.0000 g mixture ...... 75 Figure 13. Average mass of powder landed on each location on all locations, that is 44 µm NaCl and 64 µm starch after electrostatic coating of 3.0000 g mixture...... 76 Figure 14. Average mass of powder landed on each location on all locations, that is 197 µm NaCl and 191 µm starch after electrostatic coating of 3.0000 g mixture ...... 76

Appendix A: Standard Curves

Figure 15. Standard curve of percent of 256 um NaCl in mixture of 44 and 256 um NaCl...... 81 Figure 16. Standard curve of percent of 191 um starch in the mixture of 64 and 191 um starch against particle size...... 81 Figure 17. Standard curve of percent of 44 um NaCl in the mixture of 44 and 64 um starch against conductivity...... 82

ix Figure 18. Standard curve of percent of 197 um NaCl in the mixture of 197 and 191 um starch against conductivity...... 82

x CHAPTER 1

1 LITERATURE REVIEW: SEPARATION IN MIXTURES DURING NONELECTROSTATIC AND ELECTROSTATIC POWDER COATING

1.1 Introduction

Coating foods is a value-added process, especially for snack foods, because it adds flavor and variety (Ricks and others 2002). Powders such as seasoning are often coated on snack foods to improve flavor and visual appearance after they are fried, baked, or processed. There are many coating methods for coating food powders such as tumble drum and conveyor systems. The objective of coating is to apply seasonings, usually as a mixture, in a uniform and consistent manner with the same appearance on both sides and equal amount on every individual piece as a percentage by weight (Clark 1995; Hanify 2001). Most manufacturers over-apply seasoning powders in order to get an adequate amount on the snack, therefore producing more powder loss (Pannell 1980).

The electrostatic coating concept was originated in the USA in the 1950s for industrial applications (Bailey 1998). Corona charging is one widely used application.

During corona charging, the corona gun charges the particles as they pass through a corona discharge in the gun exit region. Mono-polar negative ions are created during ion attachment in which collision of neutral atoms and electrons from the corona discharge occur when powder particles pass through an intense voltage gradient.

These ions move rapidly outwards from a source, such as the region around a sharp point, due to the strong Coulombic force of the intense electric field. Particles that 1 pass through this ion-rich region perturb the field, increasing the intensity of the field at the particle surface, and thus particles are charged when ions are attached to the particles (Bailey 1998). According to Coulombs’ Law, because of their charges, the powder particles seek a grounded surface (Hughes 1997). Aerodynamics and gravity guides the charged powders particles to the nearest grounded surface, and a good design ensures that the nearest grounded surface is the object to be coated. The evenly dispersed cloud of particles deposit themselves onto the nearest grounded target, creating a uniform layer. As opposed to nonelectrostatic coating where no charge is used, the powder particles seek a grounded surface rather than staying suspended in the air, reducing powder loss (Bailey 1998).

Powder coatings usually consist of a mixture of powders with different physical properties (Seighman 2001). These properties, such as particle size, density, charge, and resistivity have a significant effect on coating transfer efficiency (Yousuf and Barringer 2007). Several studies have been done to determine the effect of size, density, charge and resistivity on nonelectrostatic and electrostatic transfer efficiency

(Biehl and Barringer 2003; Mayr and Barringer 2006; Miller and Barringer 2002;

Yousuf and Barringer 2007). However, few studies have been done to determine the effect of those powder properties on separation and distribution of mixtures.

Studies have also been conducted to determine the effect of particle size on the trajectories of the particles in a monodisperse and polydisperse system, but with different results (Adamiak 1997; Wang and others 2005; Ye and Domnick 2003).

Trajectories of particles are good indicators of separation. Apart from particle size, many factors may affect separation of mixtures. These factors include density, resistivities of the powders, operating conditions and machine settings. Separation occurs when the proportion of one powder in the mixture is significantly different 2 from the proportion of the other powder in the mixture on the target. Separation is caused by unequal distribution of different powders on the target due to different properties, such as density, and charge (Yousuf and Barringer 2007). Separation of powders in mixtures results in uneven distribution of flavors and appearances. The main purpose of this review is to introduce the different physical properties of powders, and how they affect transfer efficiencies and trajectories of powders.

1.2 Transfer efficiencies

Transfer efficiency is a measurement of the percent mass of the powder deposited on the target relative to the mass of powder entering the coating system

(Yousuf and Barringer 2007).

TE = m f/m i X 100%

Where TE is the transfer efficiency, m f is the mass of powder deposited on the target and m i is the mass of powder entering the coating system.

1.2.1 Effects of particle size on transfer efficiency

During nonelectrostatic coating, transfer efficiency increases when particle size increases (Ricks and others 2002; Miller and Barringer 2002; Yousuf and

Barringer 2007). Ricks and others’ (2002) experiment used a particle size range of 7.6 to 130 µm, 75 to 300 µm for Miller and Barringer’s (2002), and 25 to 340 µm for

Yousuf and Barringer’s (2007). Gravity and aerodynamics facilitate the delivery of the particles to the target, however the air force and air turbulence blow the particles away (Mayr and Barringer 2006). As the particle size increases, the mass increases.

Particles with higher mass are more affected by the gravitational force relative to the effect of air velocity. Large particles fall onto the target rather than remaining in the 3 air that is passed out of the coating system, while small particles tend to remain in the air as dust and are not readily transported to the target compared to large particles

(Ricks and others 2002). Both experiments and simulation have produced the same results, in which transfer efficiency increases with particle size (Yousuf and Barringer

2007).

During electrostatic coating, transfer efficiency increases with particle size, until it reaches a certain particle size where electrostatic force is overcome by gravitational force. Mixed results have been observed. Transfer efficiency increases with particle size range of 4.9 to 70 µm (Mazumder and others 1997). The transfer efficiency of sucrose increases with increasing particle size (13 to 80 µm) but starts to level off from 80 to 139 µm (Mayr and Barringer 2006). For sucrose, malic acid, KCl, citric acid, and maltodextrin, the electrostatic transfer efficiency increases until it levels off at 60 µm for sucrose; decreases at 100 µm for malic acid; levels off at 90

µm for KCl; levels off at 50 µm for citric acid; and levels off at 50 µm for maltodextrin (Ratanatriwong and Barringer 2007). The transfer efficiency of NaCl decreases when particle size increases from 30 to 120 µm (Ratanatriwong and

Barringer 2007). The forces of gravity, aerodynamics, and electrostatics act on the particles to deposit the powders onto the target during electrostatic coating (Horinka

1995). Different sizes cause change in the magnitude of each force. Two conflicting forces cause the electrostatic transfer efficiency to stop increasing at a certain size: the effect of size on charge development from the corona discharge, and the effect of gravitational force (Ratanatriwong and Barringer 2007). Smaller particles have higher charge to mass ratio than larger particles so they are more strongly influenced by an electric field, making them more attracted to the grounded target, and are not as influenced by gravity as the large particles (Ratanatriwong and Barringer 2007). 4 However, for the large particles the gravitational force overcomes electrostatic force, so large particles are not as strongly directed towards the target by charge as the small particles. Electrostatic force has more influence on small than large particles, therefore as particle size increases, the transfer efficiency levels off or decreases

(Ratanatriwong and Barringer 2007).

As particle size increases, gravitational force increases while electrostatic force declines, therefore the electrostatic transfer efficiency becomes closer to the nonelectrostatic transfer efficiency (Mayr and Barringer 2006). The transfer efficiencies of sucrose during nonelectrostatic and electrostatic coating differ by 15% at particle size of 15 µm, however at particle size of 140 µm, the difference is only

5.0% (Mayr and Barringer 2006). Transfer efficiency improvement due to electrostatics is greater for small particles than larger particles (Mayr and Barringer

2006; Ratanatriwong and Barringer 2007; Ricks and others 2002).

1.2.2 Effects of density on transfer efficiency

Particle density is a significant factor in determining nonelectrostatic coating transfer efficiency, but not for electrostatic coating transfer efficiency (Biehl and

Barringer 2003). During nonelectrostatic coating as particle density increases from

1300 to 2200 kg/m 3, transfer efficiency increases from 27 to 46% (Biehl and

Barringer 2003, Biehl and Barringer 2004). Particles with lower densities have lower masses and inertia compared to the particle with the same size but with higher density, therefore are more susceptible to remain in the air as dust (Yousuf and

Barringer 2007). Simulation shows that as density increases from 1200 to 2200 kg/m 3, transfer efficiency increases by 10% (Yousuf and Barringer 2007). However, Ricks

5 and others (2002) found that the effect of density on transfer efficiency is not significant.

During electrostatic coating, density is not a reliable factor in determining transfer efficiency (Biehl and Barringer 2003; Biehl and Barringer 2004). As density increases from 1600 to 2200 kg/m 3, transfer efficiency increases only by 4% (Biehl and Barringer 2003). Electrostatic forces dominate over gravitational forces, making density insignificant as charge plays a more important role (Biehl and Barringer

2004). A different result was observed in Yousuf and Barringer’s (2007) experiments and simulations, where an increase in particle density from 1200 to 2200 kg/m 3 results in higher transfer efficiency. Yousuf and Barringer’s (2007) method only used

NaCl and starch with voltage of -25 kV during coating in a conveyor belt system.

However, Biehl and Barringer’s (2004) method used sugar, corn starch, NaCl, cellulose powder, maltodextrin, and soy flour with +25 kV during coating in a tumble drum system.

1.2.3 Effects of resistivity on transfer efficiency

Powder resistivity plays an important role in the performance of the system.

There is no ideal resistivity for optimum performance in a system, but it can be adjusted to maximize charge deposition on the powder particles (Hughes 1997). At the point where the particles are charged, the resistivity of a powder should be as low as possible to maximize charge deposition, while the resistivity should be as high as possible after particle charging to ensure slow charge relaxation rate and good powder adhesion to the target surface (Hughes 1997). High resistivity powder (more than 10 14

Ω) is recommended for good adhesion properties, while powder with resistivity of

10 11 to 10 12 Ω is a good recommended range for both good charging and adhesion 6 properties (Hughes 1997). Below 10 11 Ω value, the charging properties are good, but the adhesion is poor (Hughes 1997).

Resistivity is not a good indicator of the transfer efficiency of all the food powders tested. NaCl and maltodextrin powders are conductive powders (resistivity less than 10 10 Ω), while powders such as soy flour and sugar, whey and cocoa powders are in the intermediate range (in between 10 11 – 10 13 Ω) (Halim and

Barringer 2007). The transfer efficiencies of these powders during both nonelectrostatic and electrostatic coating did not show a pattern of correlation between resistivity and transfer efficiencies (Sumawi and Barringer 2005). NaCl (14

µm), maltodextrin (29 µm), Soy protein (25 µm), and sucrose (22 µm) have nonelectrostatic transfer efficiencies of 26%, 48%, 40%, and 32% respectively ,and have electrostatic efficiencies of 59%, 84%, 59%, and 76% respectively (Sumawi and

Barringer 2005).

1.2.4 Losses contributing to decreased transfer efficiency

There are three main types of powder loss characterized by Xu and Barringer

(2008): targeting loss, adhesion loss, and transportation loss. Targeting loss is the loss that occurs only after the powder enters the coating system, but before landing on the target. Engineering design, aerodynamic forces, electrostatics forces, and powder physical properties such as size and mass are the main factors affecting the targeting loss. Adhesion loss occurs when the powder lands on the target, but does not adhere to the target surface due to either surface roughness or lack of adhesive medium such as oil, and is immediately lost. Aerodynamics, target surface, powder cohesiveness, and particle size of the powder affect the adhesion loss. Transportation loss occurs

7 during the transportation of the coated targets, which is affected by target surface roughness, powder adhesiveness, and also human error.

1.3 Adhesion

1.3.1 Effects of particle size on adhesion

Particle adhesion is the outcome of forces that exist between particles and a solid surface in contact, and is affected by many factors (Podczeck 1998). The effect of particle size on adhesion is complex. Adhesion can be proportional or inversely proportional to the particle size. These differences can be explained by the influence of surface roughness on the area of contact between the particles and the target surface. If the scale of surface roughness is approximately equal to the scale of the particle size, so that the particle fits inside the asperities and adhesion will increase

(Podczeck 1998).One of the most important mechanisms that affect the adhesion is the geometry of the contact between the particles and the surface. The effects of particle size on the adhesion force cannot be described specifically, as it depends on the surface roughness, particle shape, and the force involved in the adhesion process

(Podczeck 1998).

Adhesion is quantified as the work done from separating two surfaces that are in contact with each other (Mittal 1997). Adhesion varies with particle size in many different ways: it maybe directly proportional, or inversely proportional to the average diameter, or even independent of particle size (Zimon 1969). There are many studies about the effect of particle size on adhesion during nonelectrostatic coating, with different results. Enggalhardjo and Narsimhan (2005) found that the total adhesion force between seasoning particles and the chip surface increases when particle size

8 increases from 32 to 300 µm. The adhesion between sucrose particles and crackers also increased with particle size of 20 to 140 µm (Mayr and Barringer 2006).

Adhesion increases with particle size as there are more contact areas between the large particles and the substrate (Mayr and Barringer 2006). However, Halim and

Barringer (2007) found that adhesion of sucrose on crackers decreased as particle size increased up to 200 µm. Increasing particle size decreases the particle surface contact area per volume ratio, which decreases the attraction force (Halim and Barringer

2007).

During electrostatic coating, as particle size increases adhesion decreases. As particle size increases from 20 to 140 µm, the total adhesion decreases significantly

(Mayr and Barringer 2006). In tests of coating sucrose on crackers, adhesion also decreased with increasing particle size from 20 to 250 µm. Charge to mass ratio decreases with increasing particle size, thus a smaller charge to mass ratio and lower electrostatic force is expected with larger particles (Halim and Barringer 2007).

Coulomb force is inversely proportional to the separation distance squared, so as the particle size increases, the separation distance between the charge and its image charge within the target increases, thus Coulombic force decreases with increasing size and adhesion is lower (Halim and Barringer 2007).

1.3.2 Effects of resistivity on adhesion

The powder resistivity has a great influence on the electrostatic adhesion of powders (Grosvenor and Staniforth 1996). Powder should be resistive to remain charged for a long time, and the target surface should be conductive to remove the charge from the particles so more powder can deposit onto the surface (Grosvenor and

Stanifoth 1996). 9 Bailey (1998) classified the ranges of powder resistivities. Three ranges were distinguished: greater than 10 13 Ω, less than 10 10 Ω, and in between 10 11

– 10 13 Ω, high resistivity powders (greater than 10 13 Ω) are insulators with a charge relaxation time of minutes to hours, and have good electrostatic adhesion, however, most food powders do not fall into these categories (Bailey 1998; Halim and

Barringer 2007). Powders with intermediate range resistivity (in between 10 11 – 10 13

Ω) have a very fast charge decay time, thus poor adhesion, resulting in lower adhesion to the target after landing. Powders with low resistivity (less than 10 10 Ω) are conductors which charge more effectively than insulators, but tend to lose charge immediately after being in contact with the target, therefore have low adhesion

(Bailey 1998; Halim and Barringer 2007).

Halim and Barringer (2007) investigated the effect of 11 powders with different resistivity on adhesion. NaCl and maltodextrin did not show good electrostatic adhesion. They are conductive powders (less than 10 10 Ω) which can transfer charges between particles and to surfaces easily. Cellulose, whey protein, soy flour, nonfat dry milk, sucrose, and cocoa powders have intermediate range resistivity

(in between 10 11 – 10 13 Ω) and showed significant electrostatic adhesion.

1.4 Factors affecting separation of mixtures during nonelectrostatic and electrostatic coating

A difference in density causes mixture to separate on the target (Yousuf and

Barringer 2007). During mixing of powders, differences in density cause mixtures to separate. Two equal-sized fine granular materials with different densities exhibit spontaneous separation under vertical vibration in the presence of air (Biswas and others 2003). However, few studies have been about the separation of mixtures due 10 to density during coating of powders. To determine the effect of density on separation of mixtures, an equal amount of NaCl powder with density of 2200 kg/m 3, and a starch powder with density of 1491 kg/m 3, with sizes close to each other (234 and 195

µm respectively) were used to nonelectrostatically coat the target. After each coating process, the powder on each location was transferred into deionized water and conductivity of the solution was measured and compared to the standard curve to determine the amount of NaCl and starch on each target (Yousuf and Barringer 2007).

The proportion of NaCl was then compared to the proportion of starch. Separation is defined as when the proportion of a powder in the mixture is significantly different from the proportion of the other powder in the mixture (Yousuf and Barringer 2007).

During nonelectrostatic coating, more than half the targets produced significantly different amounts between the NaCl powder and the starch powder, with the percent of NaCl on the targets ranging from 32% to 62% (Yousuf and Barringer 2007). The distribution appeared to be random without patterns. The NaCl and the starch powders had transfer efficiency of 71% and 73% respectively in the mixture, so differences in the masses on the targets were not due to one powder being lost significantly more than the other powder (Yousuf and Barringer 2007).

Differences in particle surface charge cause separation of the mixture (Yousuf and Barringer 2007). To determine the effect of particle charge on separation of mixtures during electrostatic coating, the method previously mentioned from the density section was used, except electrostatic coating at -25kV was used, and the proportion of NaCl was compared with the proportion of the starch on the same target

(Yousuf and Barringer 2007). During electrostatic coating, more than half the targets produced significantly different amounts between the NaCl powder and the starch powder. The proportion of NaCl was significantly greater than the proportion of 11 starch on the targets furthest from the air flow, while four out of six targets closer to the airflow showed significantly less NaCl than starch (Yousuf and Barringer 2007).

Although NaCl particle has higher total charge on particle and surface charge density than a starch particle with the same size, the slightly smaller starch particle has higher charge to mass ratio than a NaCl particle in this experiment (Yousuf and Barringer

2007). The higher charge to mass ratio of the smaller starch particle causes the starch powder to land closer to the airflow. The NaCl and starch particles have particle size of 234 and 195 µm respectively. The higher charge to mass ratio of the starch particles allows the starch particles to be attracted to the nearest grounded target faster than a NaCl particle, therefore more starch particles are found closer to the air flow than NaCl particles. Both the NaCl and starch powders had transfer efficiencies of

81%, so the separation was not caused by one powder being lost. The effects of size on the separation made this study invalid. Therefore either particles of the same size or particles with a closer size range are needed to investigate the effects of density on separation of mixtures.

1.4.1 Effects of transfer efficiencies of individual components on separation

Powders with different sizes and densities have different transfer efficiencies

(Biehl and Barringer 2003; Ricks and others 2002). Assuming that there is no interaction between particles in mixtures during either nonelectrostatic or electrostatic coating, the transfer efficiency of each individual component would remain the same.

If a mixture consists of two powders with different transfer efficiencies, separation will occur due to one powder being lost during coating more than the other powder.

12 1.5 Trajectories of powders in a polydisperse system

There were several studies and models about the trajectories of particles with different sizes and charges in a polydisperse system (Adamiak 1997; Adamiak 2001;

Wang and others 2005; Ye and Domnick 2003). However, the experimental conditions were different, thus producing different results. The effects of density and resistivity on particle trajectories have yet to be studied.

1.5.1 Trajectories of powders in mixtures during nonelectrostatic coating

1.5.1.1 Particle size profile of mixtures during nonelectrostatic coating

Particle trajectories were studied using a light scattering technique. Wang and others (2005) conducted experimental studies of particle trajectory in an isolated

Plexiglass coating booth. The spray rate was maintained at 35g/min, while no charge was used during nonelectrostatic coating. Particle trajectory was measured using a multi-Particle Dynamic Analyzer (PDA), which was based on the light scattering from spherical particles. Non-spherical black polyester paint powder with an average diameter of 35 µm was used.

When the powder is nonelectrostatically coated onto a vertical plane target, the mean diameter is larger at the bottom, and smaller at the top (Wang and others

2005). The total amount of small particles distributed near the top is significantly higher than near the bottom. Wang and others (2005) concluded that the difference in proportion of small and large particles at the bottom region is due to the gravitation force, as larger particles are more susceptible to gravity and fall towards the bottom region of the target than the smaller particles. Wang and others (2005) conclusion about the effects of gravity on large particles mirrors several studies where more large

13 particles tend to fall to the target due to gravity than small particles (Mayr and

Barringer 2006; Ricks and others 2002). Large particles fall onto the target rather than remaining in the air that passed out of the coating system (Ricks and others 2002).

Gravity facilitates the large powder particles to the bottom region of the target, while the smaller particles which follow the air flow are more likely to be on the top region of the target. Since particle mass increases with size, the force of gravity acting on each particle increases relative to the effect of air turbulence, therefore more large powders deposited onto the bottom of the vertical target than small particles (Wang and others 2005). In horizontal targets, the larger particles are more likely to land on the targets, while the majority of the smaller particles remains in the air or gets carried away by the airflow (Mayr and Barringer 2006).

In contradiction to Wang and others’ (2005) results, Yousuf and Barringer

(2007) found that mixtures do not separate due to differences in particle size. To determine the effects of particle size on separation of powders, equal amounts of

NaCl powders of 28 and 342 µm were used to coat nine 15 cm long and 10 cm wide aluminum sheets on a horizontal plane nonelectrostatically (Yousuf and Barringer

2007). The mean diameter of the powders on each target location was measured, and compared to the mean diameter of the original mixture to determine whether they were significantly different (Yousuf and Barringer 2007). Results showed that mixtures do not separate by size. In both simulation and experiments, the percentage of each powder is not significantly different from the other powder in all locations.

The main differences in Yousuf and Barringer’s (2007) and Wang and others’ (2005) methods are: horizontal distance between the gun and the target, vertical distance between the gun and the target, and particle size range. The horizontal distances between the gun and the target were 0 mm, and 260 mm for Yousuf and Barringer 14 (2007) and Wang and others (2005) respectively. The vertical distances between the gun and the target were 13 cm, and 0 cm for Yousuf and Barringer (2007) and Wang and others (2005) respectively. Yousuf and Barringer (2007) used equal amounts of

NaCl powders of 28 and 342 µm, while Wang and others (2005) used polyester paint powder with size ranges from 2.5 to 180 µm. These settings differences result in the contradiction in the results. Although the distances between the gun and the target in vertical and horizontal directions do not change the powder trajectory, they change the position that the powders on targets are tested or collected for results. In vertical targets, the results change greatly, since adhesion of the powder to the target on a vertical plane is lower than the adhesion of the powder to the target on a horizontal plane, therefore the results are different.

1.5.1.2 Particle velocity profile of mixtures during nonelectrostatic coating

Particles with different sizes have different velocities during the nonelectrostatic coating of mixtures (Wang and others 2005). In Wang and others’

(2005) experiment, particles smaller than 2.5 µm follow the airflow field in terms of velocity (1.6 m/s), thus the particle velocity of the particles smaller than 2.5 µm represents the airflow’s velocity. There are three size ranges of powders that were studied: 0-2.5 µm, 10-20 µm, and 160-180 µm. Large particles (160-180 µm) have only slightly higher velocity than the velocity of the particles with size 10-20 µm in the horizontal direction (1.5 m/s for large particles and 1.4 m/s for small particles 20 mm from the target in the horizontal direction), but the velocity difference is the most prominent in the vertical (gravitational) direction (0.05 m/s for large particles and

0.02 m/s for small particles 20 mm from the target). Larger particles have higher mass and inertia, therefore the effect of gravitational force is higher for large particles than

15 smaller particles thus they travel faster in the vertical direction due to gravity. The mean velocity is averaged over all sizes of particles in the system, and the velocity profiles of all particles in the vertical direction exhibit that the highest velocities occur closest to the gun, and lower velocities further away from the gun. The mean velocity increases at the bottom half of the powder cloud, as that area consists of more larger particles, which fall faster due to gravitational force. The velocity profiles in the vertical direction show more of the larger particles at the bottom of the target than the smaller particles, as larger particles move faster, thus the average speed of all particles at the bottom is higher than the average speed at the top.

The main finding in this experiment is that particles with different sizes have different trajectories during nonelectrostatic coating due to different velocities (Wang and others 2005). Larger particles have higher mass, so more gravitational force acts upon them, facilitating them to fall at higher velocity in vertical directions. Smaller particles tend to follow the airflow, so in vertical targets higher proportions of smaller particles are found at the top. However, when horizontal targets are used, the small particles will be suspended in the air as dust, lowering transfer efficiency and also making them land further away from the targets, while more large particles fall onto the target due to gravity and more larger particles will fall closer to the gun.

1.5.2 Trajectories of powders in mixtures during electrostatic coating

Several models and experiments have been conducted to study the effect of particle size on trajectories of mixtures during electrostatic coating (Adamiak 1997;

Adamiak 2001; Wang and others 2005; Ye and Domnick 2003).

16 1.5.2.1 Particle size distribution and trajectories of particles in a polydisperse system during electrostatic coating

Studies with different settings have been conducted to determine the particle trajectories and distribution in mixtures (Adamiak 1997; Ye and Domnick 2003;

Yousuf and Barringer 2007). Adamiak (1997) developed a mathematical model using tribocharging in his coating systems. The process is simulated assuming a conductive grounded target, and spherical particles with the same initial velocity for all particles.

Particle trajectories result from the balance of inertia, air drag, electrical, and gravitational forces. In Adamiak’s (1997) model, the simulated system has parameters as follows: 1.0 m/s particle initial velocity, 1500 kg/m 3 density, 40, 60, 80, and100

µm powder diameter, 0.3 m distance to the target, 0.3 µC/g, and 0.6 g/s powder flow rate. The model has a low initial particle velocity (1.0 m/s) compared to Yousuf and

Barringer (2007) and Ye and Domnick (2003), which have 3.3 m/s and 18 m/s initial particle velocity respectively. Adamiak’s (2001) other model is similar to the first model (1997), but only the transport part of the coating process is investigated.

Adamiak’s (2001) model shows that substantial inertia and electric forces drives all particles toward the target along linear trajectories, and a dense cloud forms next to the gun nozzle which deflects the trajectories in a radial direction before turning to the target. Small particles are more affected than larger particles by air drag, so they are more deflected at the gun nozzle, while the larger particles are less deflected at the gun nozzle. The smaller particles are more deflected at the gun nozzle, as charge per mass ratio increases due to larger surface area to volume ratio, and the radial electric field is magnified, causing powder dispersion. The larger particles have lower charge per mass ratio, therefore less powder dispersion occurs (Adamiak 2001). The final result shows that the distribution of particles on the target is identical to that in the 17 spray gun (Adamiak 2001). Although smaller particles are more deflected by the cloud formed next to the nozzle, they are more attracted to the grounded target than larger particles. This results in smaller particles covering the same area as the larger particles, eliminating the separation effect.

Adamiak (1997, 2001) and Yousuf and Barringer (2007) found that separation did not occur due to size, but Ye and Domnick (2003) found separation. Adamiak

(2001) and Yousuf and Barringer (2007) used a much lower air velocity (1 to 3.3 m/s) than Ye and Domnick (18 m/s) (2003). The low air velocity is more likely to carry the small particles to the target, rather than getting blown away as dust, therefore separation does not occur when low velocity is used.

Higher charge to mass ratio changes the trajectories of powders so particles land closer to the gun (Adamiak 1997). In a constant electric field, as a particle increases in size, the charge to mass ratio decreases (Mayr and Barringer 2006;

Yousuf and Barringer 2007). Powder deposition is almost uniform, and no separation occurs when all powders are charged at 0.3 µC/g (Adamiak 1997). Adamiak’s (1997) simulation uses the same particle size (100 µm) but different charge to mass ratio

(0.2, 0.3, 0.4, and 0.5 µC/g). The particles have different trajectories. Therefore, it is not the particle size, but the charge to mass ratio of the particle that controls the trajectories. As the charge per mass ratio increases, the particles are more deflected away from the gun (Adamiak 1997). Adamiak’s (2001) other simulations also show identical results where no separation occurs when powder with different sizes are charged at the same level at 0.8 µC/g, as his previous model (1997). The flaw of these simulations (Adamiak 1997; Adamiak 2001) is that the particle sizes used were not very different (40, 60, 80 and 100 µm). As very large particles have more

18 gravitational force acting upon them than small particles, trajectories of large and small particles will differ.

Size does not cause separation of mixtures in the air during electrostatic coating near the nozzle, but separation occurs at the bottom of a vertical target as the distance between the nozzle and the target increases (Wang and others 2005). The particle size distribution changes significantly when particles are used to electrostatically coat the target at 50kV compared to when nonelectrostatic coating is used (Wang and others 2005). However, no particle separation occurs in the air 10mm from the target. The particle size profiles during electrostatic coating were similar across the entire spray in the air. However, as the distance from the spray gun increases, the more the larger particles were observed at the bottom of the target, and separation occurs between the large powders (160-180 µm) and the small powders (1-

20 µm). There are differences in separations at different points because when the particles are close to the gun, the electrostatic force is dominant and can overcome gravity. However, moving away from gun, gravitational force becomes stronger and electrostatic force becomes weaker, thus larger particles and smaller particles separate

(Wang and others 2005).

Higher air velocity causes mixtures with different particle sizes to separate

(Ye and Domnick 2003). Ye and Domnick’s (2003) model resembles that of

Adamiak’s (2001), but a corona spray gun was used instead of a tribocharging gun, while charge per mass remains the same (0.8 µC/g). The particle trajectory equation takes air drag force, gravity force, particle velocity, air velocity, and electric force into account (Ye and Domnick 2003). Ye and Domnick’s (2003) model shows the trajectories of particles with different particle sizes ranging from 1 to 120 µm with air velocity of 18 m/s. The particles separate within the spray cone by size (Ye and 19 Domnick 2003). Small particles follow the airflow. Large particles are observed to fall down quickly due to gravity. Aerodynamics, and electrostatic force are taken into account. Compared to Adamiak’s (1997, 2001) models, this model has an airflow 18 times higher (18 m/s compared to 1 m/s). With higher airflow, the effect of aerodynamics becomes dominant, making electrostatic force on the particles weaker compared to Adamiak’s (2001) model in which separation does not occur.

1.5.3 Particle velocity profile of mixtures during electrostatic coating

Velocities of large particles (160-180 µm) deviate significantly from the air velocity in the vertical direction, and the influence of size on the velocity is greater during electrostatic coating than nonelectrostatic coating (Wang and others 2005).

Bigger particles, with higher inertia, accumulate more total charge (Wang and others

2005). Charged particles also experience the strongest electrostatic forces in a region close to the grounded target, changing trajectories while depositing.

1.6 Conclusion

Separation of mixtures during nonelectrostatic and electrostatic coating is undesirable in the snacks food industry. Size has an effect on transfer efficiency and separation during both nonelectrostatic and electrostatic coating. Generally, transfer efficiency increases with particle size. Electrostatics has larger effects on small particles than large particles, and greater improvement in transfer efficiency due to electrostatics was observed in small particles than large particles. Mixtures of different sizes separate when the air velocity is high during electrostatic coating. At low velocity, mixtures do not separate during either nonelectrostatic or electrostatic

20 coating. The effects of density and resistivity on transfer efficiency have been studied, but not the effects on separation.

21 CHAPTER 2

2 EFFECTS OF PARTICLE SIZE ON SEPARATION OF MIXTURES DURING NONELECTROSTATIC AND ELECTROSTATIC POWDER COATING

2.1 Abstract

Two mixtures: 44 and 256 µm NaCl, and 64 and 191 µm starch, were nonelectrostatically and electrostatically coated on grounded targets. Separation of mixtures was measured by comparing the percentages of each powder on the target to determine whether size caused separation. Targeting loss and adhesion loss, when coated individually and in mixtures, were determined. During nonelectrostatic coating, mixtures of NaCl and starch separated during coating mainly due to the difference in targeting losses between small and large powders, both individually and in mixtures. Adhesion losses had a small effect. The interactions reduced the targeting loss difference between the small and large powders in NaCl, but increased the difference between the small and large starch. On the target, separation occurred because there was more large powder on most or all locations. During electrostatic coating, differences in targeting losses were only a small effect, because electrostatics decreased targeting loss. Overall, separation occurred due to the increase of adhesion loss of the large particles when in a mixture. For both mixtures, there were more small powders closer to the nozzle due to electrostatic charge, while more large powders landed further away from the nozzle, causing separation. Using particles with the same size reduces separation, as size caused separation of mixtures. Electrostatics did not increase or decrease separation in either mixture. 22 2.2 Introduction

Powders used during coating usually consist of a mixture of powders with different physical properties (Seighman 2001). One of the important properties is particle size of the powders. During nonelectrostatic coating, transfer efficiency increases when particle size increases (Ricks and others 2002; Miller and Barringer

2002; Yousuf and Barringer 2007). As the particle size increases, the mass increases.

Particles with higher mass are more affected by gravitational force relative to the effect of air velocity. Large particles fall onto the target rather than remaining in the air that is passed out of the coating system, while small particles tend to remain in the air as dust and are not readily transported to the target compared to large particles

(Ricks and others 2002). Therefore small particles have higher targeting loss than large particles, causing separation as the amounts of the two powders are different on the targets.

Unlike nonelectrostatic coating, the principle of electrostatic coating is the attraction between negatively charged powder and the grounded target. The charged particles are attracted to the grounded target, decreasing total coating loss as particles are less likely to be lost in the air (Miller and Barringer 2002). During coating, aerodynamics, electrostatics, and gravity guide the powder particles to the nearest surface, and a good design ensures that the nearest surface is the object to be coated.

The dispersed cloud of particles deposit themselves onto the nearest grounded target, creating a uniform layer. As opposed to nonelectrostatic coating where no charge is used, the powder particles seek a grounded surface rather than staying suspended in the air, reducing powder loss, and increasing transfer efficiency (Bailey 1998).

During electrostatic coating, transfer efficiency increases with particle size, until it

23 reaches a certain particle size where electrostatic force is overcome by gravitational force (Mayr and Barringer 2006; Ratanatriwong and Barringer 2007). However, few studies have been done to determine the effect of size on separation of mixtures.

(Yousuf and Barringer 2007). Uneven appearance and distribution of flavors is caused by the separation of the powder mixture.

Studies have been conducted to determine the effect of particle size on the trajectories of the particles in a polydisperse system, but with different results

(Adamiak 1997; Wang and others 2005; Ye and Domnick 2003; Yousuf and

Barringer 2007). Trajectories of particles are good indicators of separation because if particles with different sizes have different trajectories, mixtures will separate.

Adamiak (1997) and Yousuf and Barringer (2007) found that differences in particle size did not caused separation in mixtures during either nonelectrostatic or electrostatic coating. Others have found that differences in particle size did cause separation of mixtures during coating (Wang and others 2005; Ye and Domnick

2003).

The objectives of this study were to determine whether separation occurs when mixtures of powder with different sizes were coated nonelectrostatically and electrostatically, to quantify the amount the losses of powders of difference sizes, both when in a mixture and individually, and to determine the major losses that caused separation.

24 2.3 Materials and Methods

All experiments were carried out at 25±2ºC and 35±5% relative humidity. The food powders used were 44 µm NaCl (325 Extra Fine Salt, Morton International, Inc.,

Chicago, IL), 256 µm NaCl (Alberger Flake Salt, Cargill, Inc., Minneapolis, MN), and 191 µm starch (Pregelatinized Starch, National Starch & Chemical (Thailand)

Ltd., Amphur Muang, Thailand). Milling was done using an ultracentrifugal mill

(Glenmill Inc., Clifton, NJ) 5 times at 18000 rpm to reduce the particle size of the starch to 64 µm. Mean diameters were measured using the Malvern Mastersizer X with a powder dispenser (Malvern Instruments Ltd., Worcestershire, UK). The volume mean diameter D (4,3) was reported. Three replicates were done for all experiments.

An aerodynamic powder coating machine (Terronics Development Corp.,

Elwood, IN) was used to coat the food powders on nine 15 cm x 10 cm aluminum sheets. No voltage was applied for nonelectrostatic coating and -25kV was used for electrostatic coating. All experiments were carried out at air velocity 3.3 m/s. The positions of the targets with reference to the nozzle are as shown in Figure 1.

2.3.1 Determination of separation

The percentages of the two powders on each location were measured and compared to each other to determine whether they were significantly different.

Separation occurs when there is a significant difference between the percentages of the two powders on each location.

25 2.3.2 Targeting loss and adhesion loss determination

To calculate the targeting loss and adhesion loss of the powders, 3.0000±0.2 g of individual powders and mixtures of powders were used for each coating experiment nonelectrostatically and electrostatically. Percent targeting loss was measured as:

100x (mass entering the chamber – mass collected on oiled targets )

mass entering the chamber

Approximately 1 g vegetable oil (Crisco Pure Vegetable Oil, The J.M. Smucker

Company, Orrville, OH) was applied on each target using a brush. Percent adhesion loss was measured as:

100x (mass entering the chamber- mass collected on unoiled targets) -% targeting loss

mass entering the chamber

Targeting loss in a mixture was measured by determining the average particle size of the mixture collected that was not on the oiled target after coating of 30 g of the mixture and comparing to the standard curve. The standard curves were constructed using mixtures containing different percentages of the small vs. large powders (Figure

15 and 16). The standard curves convert the measured average diameter into the percentage of large powder in a mixture. The mixture that was not on the target was collected by using a special tray (area 51cm x 50 cm, with 10 cm height at the end of the tray to prevent powder escaping) with a large area that captured most of the powder not on the target. Average size of the powder collected on the tray but not on the targets was determined, and mass of each powder that was lost was determined by the standard curves. The mass of the powder loss over the total mass of the same powder in the mixture before entering the chamber was the targeting loss in the

26 mixture. Adhesion loss in a mixture was calculated by total loss of powder in a mixture on unoiled targets minus the targeting loss in a mixture. Adhesion loss of 256

µm NaCl on the targets coated with 44 µm NaCl was also measured. The aerodynamic powder coating machine was used to coat 44 µm NaCl followed by 256

µm NaCl. The same experiments were conducted for 44 µm NaCl on 256 µm NaCl,

64 µm starch on 191 µm starch, and 191 µm starch on 64 µm starch.

2.3.3 Determination of interactions between particles in mixtures of different sizes

To observe whether there was any interaction between smaller and larger particles in a mixture, 44µm and 256 µm NaCl powder, and 64 µm and 191 µm starch powders were used in the experiment. The targets were coated with 30.00 g of 50:50 mixtures. No oil was coated on the targets in this experiment.

If there are no interactions, powders in a mixture will distribute themselves across the target (actual percentage) in a similar pattern to the individual powder

(called the predicted percentage). The actual percentage was measured by applying a

50:50 mixture, weighing the amount of powder on each location and converting the weight to percent of the total powder on all the targets. The percentage of small vs. large powder on each location was calculated by measuring the mean diameter.

Standard curves were constructed using mixtures containing different percentages of the small vs. large powder (Figure 15 and 16, in Appendix A). The percentage of each powder on the target was then calculated from the standard curve. Figure 15 shows the standard curve for percent of 256 µm NaCl in a 50:50 mixture of 256 µm NaCl and 44 µm NaCl powders. Figure 16 shows the standard curve for percent 191 µm starch in a 50:50 mixture of 191 µm starch and 64 µm starch powders. 27 The predicted percentage of powder A in the 50:50 mixture of A and B in each location was calculated as:

Percentage of the powder A = Mass of A/ (mass of A+ mass of B) X 100%, where mass of A and B were the mass measured on each location after coating of the powder individually.

2.3.4 Statistical analysis

For the statistical analysis, one-way ANOVA with LSD analysis for means was performed for all the losses. To determine the separation on the targets, independent two tailed, two-sample T-test with unequal variance was performed. A p- value of 0.01 or lower indicated significant difference between the two groups.

2.4 Results and discussion

2.4.1 Differences in particle size caused separation in NaCl and starch mixtures during both nonelectrostatic and electrostatic coating

Powders of the same composition but different sizes were mixed in equal proportions, and coated on the targets. The amount of one size powder was then measured and compared with the amount of the other size powder on the target to determine if differences in size cause separation. Separation is defined as when there is a significant difference between the amounts of large and small powders on the target. During nonelectrostatic coating, seven out of nine targets showed a significant difference between percentages of 44 and 256 µm NaCl, and of 64 µm and 191 µm starch (Table 1). During electrostatic coating particles distributed themselves across the target so that eight out of nine targets showed a significant difference between

28 percentages of 44 µm NaCl and 256 µm NaCl and six out of nine targets showed a significant difference between the percentages of 64 µm starch and 191 µm starch that landed on each location (Table 1). Thus, all the mixtures separated due to size.

Adamiak’s model (2001) predicts that during electrostatic coating, the trajectories of a powder in a mixture of different powder sizes will only be slightly different from the monodisperse system assuming all the particles are charged at the same level. However, separation by size was observed in another model (Ye and

Domnick 2003). The contradictory results may come from differences in air velocity:

1.0 m/s for Adamiak (2001), 18 m/s for Ye and Domnick (2003), and 3.3 m/s in this experiment. Higher air velocity increases separation. During both nonelectrostatic and electrostatic coating, air velocity increases the magnitude of losses, and the differences in losses due to size become larger (Yousuf and Barringer 2007).

Separation occurred in both mixtures during electrostatic coating (Table 1) which was similar to Ye and Domnick’s (2003) model rather than that of Adamiak’s (2001) due to the higher velocity.

There are two types of losses that affect the separation. Targeting loss is a loss in which the powder is lost in the air before landing on the target, and adhesion loss is due to poor adhesion in which powder is lost after landing and does not remaining on the target (Xu and Barringer 2008). Transportation loss occurs during the transportation of the coated targets (Xu and Barringer 2008). The transportation loss in this experiment was assumed to be 0%, as measurements were directly taken immediately after coating. Separation in mixtures occurs due to differences in four losses in the two powders: individual targeting loss, targeting loss when in mixtures, individual adhesion loss, and adhesion loss when in mixtures. Two powders with different sizes separate due to the differences in the amount of powder loss that occurs 29 for each powder. If one powder in a mixture loses more during coating than the other, the total amount of each powder that lands on the targets will be different, resulting in separation. In a mixture, collisions and scattering occur, which causes the loss in the mixtures to be different from the loss when coated individually.

The magnitude of the separation in all mixtures was large, as all mixtures showed an average difference of at least 15% from the 50:50 proportion for each target (Table 1). Electrostatics did not affect separation. There was no significant difference between the average difference from the 50:50 proportion for each target during nonelectrostatic, and electrostatic coating for both mixtures (Table 1).

2.4.2 Individual targeting losses

Differences in individual targeting losses caused a significant separation in both NaCl and starch mixtures during nonelectrostatic coating. It was the largest for

NaCl, and the second largest loss that caused separation in the starch mixture. The targeting loss difference in the NaCl mixture was 44% (70% and 26% in small and large respectively), and 17% in the starch mixture (46% and 29% in small and large respectively) (Figure 2).

As particle size increases, targeting loss decreases during nonelectrostatic coating (Xu and Barringer 2008). Large particles fall onto the target rather than remaining in the air due to gravitational force, while small powders are more likely to be carried away by the airflow, resulting in more coating loss (Ricks and others 2002;

Mayr and Barringer 2006; Xu and Barringer 2008). The higher targeting loss of small particles caused separation in both the NaCl and starch mixtures, because 44% more large NaCl particles and 17% more large starch particles landed on the targets than the small particles. For NaCl, almost three times more of the small powder was 30 carried away by the air than large NaCl. The difference between the targeting losses of the small and large powders was smaller in starch. However, individual targeting loss was still a major loss in the starch mixture.

The targeting losses of the large powders were not significantly different (26% for NaCl, and 29% for starch) (Figure 2), but the targeting losses of the small powders were higher in NaCl than in starch (70% for NaCl, and 46% in starch). Slight size and composition differences were not important in determining the targeting loss of large particles, as gravitational force dominated. Since particle mass increases with particle size, the force of gravity overcomes aerodynamics (Mayr and Barringer 2006).

However, the effect of slight size differences was more evident for small particles.

For NaCl, there is a 20% decrease in coating loss when particle size increases from 25 to 75 µm, (Yousuf and Barringer 2007). Therefore, the targeting loss was lower in starch because in this experiment, the particle size of the starch was 20 µm larger than the particle size of NaCl.

During electrostatic coating, the difference in individual targeting losses was a small but significant cause of separation for NaCl, and not significant for the starch mixture. There was a significant difference between the targeting loss of small and large NaCl (21% and 15% respectively), while the targeting losses in small and large starch were not significantly different (5% and 7% respectively) (Figure 2).

Electrostatics greatly reduced targeting loss. There was a greater decrease in targeting losses for small particles than large during electrostatic coating (Figure 2).

Targeting loss decreased by 41%, for small NaCl, and 11%, for large NaCl (Figure 2).

The targeting loss of small and large starch also decreased, by 41% and 22% respectively, compared to nonelectrostatic coating. Charge to mass ratio is the total charge on the particle during electrostatic coating divided by the mass of the particle. 31 Charge to mass ratio is inversely proportional to the size of the particle

(Ratanatriwong and Barringer 2007; Yousuf and Barringer 2007). With higher charge to mass ratio, the small particles are more attracted to the grounded targets than large particles (Ricks and others 2002). As a result, the smaller the particle size, the more effective electrostatic coating is at reducing targeting losses compared to nonelectrostatic coating (Figure 2). During electrostatic coating, less powder of all sizes was lost due to targeting, but the improvement was more for small than large.

Therefore, while there was a large different in individual losses during nonelectrostatic coating, electrostatics decreased the small powder loss so that there was no difference in the losses in the starch mixture, and a small difference in the

NaCl mixture.

2.4.3 Targeting losses of powders in a mixture

If there are significant differences between the targeting losses when powder is coated individually and the targeting loss of that component in a mixture, this indicates that powder interactions are occurring. Interactions between particles in the mixtures during targeting did not cause separation in the NaCl mixture, but was the largest cause of separation in the starch mixture (Figure 3).

The targeting loss changes when powders are mixed together because of differences in particle velocities that affect collisions between small and large particles. Large particles have a higher velocity than small particles during nonelectrostatic coating (Wang and others 2005). The midair collisions between high velocity large particles and low velocity small particles result in changes in particle trajectories for both sizes of powders. The large particles’ velocity decreases, more large particles land on the targets and targeting loss is reduced. The velocity of small 32 starch particles increases, because of the interactions with the large particles, therefore targeting loss increases due to more particles being carried away from the targets by the air. Starch behaved as expected. The targeting loss of the small starch increased by 22% compared to the individual targeting loss, while the targeting loss of the large starch decreased 14% when in a mixture (Figure 2). For the NaCl mixtures, there was no significant difference between the targeting losses of small NaCl in a mixture and when coated individually which could be due to the fact that 70% of the powder was already lost, so there is little additional loss that could occur. For the large NaCl there was a significant increase (12%) in targeting loss in a mixture than when coated individually (Figure 2). The effect that caused the increase in targeting loss of NaCl was unknown, however the same result was observed during electrostatic coating

(Figure 2).

During electrostatic coating, the targeting loss of small (11% in mixture, and

5% when coated individually) and large (4% in mixture, and 7% when coated individually) starch was not significantly different in the mixture than when coated individually (Figure 2). The collisions observed during nonelectrostatic coating which increased the difference in losses between the small and large starch were not observed during electrostatic coating. Electrostatic force attracts the particles to the grounded targets, thus reducing the effects of collisions (Miller and Barringer 2002).

Unlike the starch, targeting losses of both small and large NaCl increased significantly in the mixture compared to the individual powders (Table 2). The targeting loss of small NaCl increased by 10% while the targeting loss of large NaCl increased 8% when in a mixture (Table 2). During electrostatic coating, the area between the gun and target becomes filled with free ions, which together with charged powder particles, form the space charge which alters the electric field (Guskov 2002). 33 The space charge will vary when a mixture of powders with different sizes is used during electrostatic coating, therefore causing complex interactions which are difficult to predict (Zhao and others 2002). The space charge causes repulsions between powder particles during electrostatic coating, therefore interactions could increase during electrostatic coating compared to nonelectrostatic coating (Zhao and others

2002). NaCl picks up more charge than starch assuming the powders are the same size and charged under the same conditions (Yousuf and Barringer 2007). As a result,

NaCl has a higher space charge, and more repulsion occur in the NaCl mixture compared to starch, resulting in more interactions. For NaCl, there was more loss in mixtures compared to the individual coated powders, but because of the charge, there was no separation due to the powders being in the mixture, and also less loss was observed compared to during nonelectrostatic coating.

2.4.4 Individual adhesion loss

Differences in individual adhesion losses were a small cause of separation in

NaCl mixtures, and did not cause separation in starch mixtures during nonelectrostatic coating. When NaCl powders were coated individually, adhesion loss accounted for

5% and 12% for 44 µm NaCl and 256 µm NaCl respectively (Figure 2). In starch mixtures, there was no significant difference between the adhesion loss of the small and large powders. Adhesion loss was 6% for 64 µm starch and 2% for 191 µm starch

(Figure 2).

Adhesion increased with decreasing size for NaCl. As particle size decreases, adhesion increases, because the van Der Waal’s forces dominate over the gravitational forces as particle size decreases (Halim and Barringer 2007). The adhesion of starch is high with very little loss, so that size does not affect the adhesion 34 loss. Starch has a higher adhesion compared to NaCl powders (Halim and Barringer

2007; Mayr and Barringer 2006).

During electrostatic coating, differences in adhesion losses between small and large particles had a small effect on separation for both NaCl and starch mixtures

(Figure 3). However, the adhesion loss did not change from during nonelectrostatic coating. Therefore electrostatics had no additional effect on separation due to adhesion. There were significant differences between the individual adhesion losses of small and large powders in both NaCl and starch. The adhesion loss in small and large NaCl were 26% and 16% respectively, while the adhesion loss in small and large starch were 0% and 5% respectively (Figure 2).

Electrostatic coating generally improves adhesion, because an electric field is created between charged powder particles and the targets (Halim and Barringer 2007).

As the distance between the particles and targets decreases, the attractive force increases, resulting in adhesion (Halim and Barringer 2007). The change in adhesion of each component relative to nonelectrostatic coating was expected for starch, but not for NaCl. The adhesion loss of the small starch decreased significantly, from 6 to

0%, compared to during nonelectrostatic coating, while the electrostatic adhesion loss of the large starch remained the same. There was no significant change in the adhesion loss of the large NaCl compared to nonelectrostatic coating, but the adhesion loss unexpectedly increased in the small NaCl (from 5 to 26%) (Figure 2). Initially this seems counter-intuitive since electrostatics generally decreases adhesion loss.

However, during electrostatic coating, the targeting losses were reduced significantly for small (by 49%) NaCl powders (Figure 2). Therefore, adhesion loss increased as more amount of powder was available due to a significant decrease in targeting loss.

35 It was also found that NaCl can have a higher electrostatic adhesion loss compared to during nonelectrostatic coating (Buck and Barringer 2007).

2.4.5 Adhesion losses in mixtures

Adhesion loss in mixtures had no effect on separation for NaCl and starch mixtures during nonelectrostatic coating compared to when coated individually

(Figure 3). There was no significant difference between the adhesion losses when coated individually and when in a mixture of the small (5%) or large NaCl (12%) or small (<6%) and large starch (<5%) when coated individually and when in a mixture

(Figure 2). Thus no interactions occurred on the targets.

During electrostatic coating, adhesion loss in mixtures was the major cause of separation in both NaCl and starch mixtures. The adhesion loss in mixtures was higher during electrostatic coating (0 and 37% for small and large NaCl; 0 and 11% for small and large starch respectively) compared to nonelectrostatic coating (6 and

13% for small and large NaCl; 2 and 4% for small and large starch respectively).

Adhesion of large powders decreased due to interactions between the powders on the target surface (Table 2). The small particles in the mixture acted as a lubricant on the target preventing the large particles from adhering to the target, resulting in higher adhesion loss of the large NaCl particles. The adhesion loss of the large NaCl increased by 16% (Table 2), when small NaCl was coated to the target prior to the coating of large NaCl. However, the effect was not observed when the sequence was reversed (Table 2). The lubrication effect is only caused by the small NaCl. The adhesion loss in mixture increased from 12% during nonelectrostatic coating to 37% during electrostatic coating (Figure 2). This was because more of the small NaCl

36 landed on the targets during electrostatic coating, therefore adhesion loss of the large

NaCl increased due to the higher lubrication effect of the small NaCl.

During electrostatic coating of mixtures, the differences in the adhesion losses between the small and large powders were larger than when they were coated individually (Figure 3). The mixture increased separation in NaCl. The adhesion loss of the small NaCl decreased from 26% when coated individually to 0% when in a mixture, while the adhesion loss of the large NaCl increased from 16 to 37% when in a mixture (Figure 2). In starch, there was no difference between the individual and adhesion losses of the small starch; however, the adhesion loss of the large starch increased by 6% when in a mixture (Figure 2). In both NaCl and starch mixtures during electrostatic coating, mixtures increased separation by increasing the difference in adhesion loss.

2.4.6 Separation

During nonelectrostatic coating of NaCl and starch mixtures, there were more of the large particles on most (NaCl) or all (starch) of the target locations (Figure 4, and 5). The small particles had higher targeting loss in the mixture compared to the large particles, causing the separation (Figure 2). For NaCl, majority of the large particles landed further away from the nozzle (15-45 cm), with small amount landed near the targets (0-15 cm) (Figure 4). The interactions that occurred due to mixing the different sized powders reduced separation, as shown by the fact that seven out of nine targets produced a closer proportion to 50:50 than what was predicted from the individual targeting and adhesion losses (Table 1). Mixtures decreased separation in

NaCl, as the difference between the small and large losses in NaCl mixtures were reduced compared to the difference between the individual losses. Unlike NaCl, 37 creating a mixture of sizes with the starch increased separation by increasing the difference in targeting losses between the small and large powders (Figure 3). The interactions that occurred due to coating in a mixture increased separation, as shown by the fact that seven out of nine targets produced a proportion closer to 50:50 than what was predicted from the individual targeting and adhesion losses (Table 1).

During electrostatic coating of the mixtures, three out of three target locations close to the nozzle (0-15 cm) had more of the small particles than large particles

(Figure 6, and 7). More small particles landed closer to the targets compared to during nonelectrostatic coating. Small particles have higher charge per mass ratio compared to large particles, therefore are more strongly attracted to the grounded target during electrostatic coating (Mayr and Barringer 2006; Yousuf and Barringer 2007). The interactions due to creating a mixture reduced separation by reducing the targeting loss difference between small and large NaCl (Figure 3), and five out of nine target areas produced a proportion closer to 50:50 than what was predicted from individual targeting and adhesion losses (Table 1). Electrostatics changed the distribution in the

NaCl mixture. During nonelectrostatic coating, there were more of the large NaCl particles overall, and more of the large NaCl further from the nozzle (Table 1,3 and

4). Electrostatics caused more small NaCl to land on the targets, as it reduced targeting loss by a large amount, while increased the adhesion loss of the large NaCl.

Unlike NaCl, separation in the starch mixture increased, as only two out of nine targets produced a proportion closer to 50:50 than what was predicted from individual targeting and adhesion losses (Table 1). The separation increased in the starch mixture as the differences of adhesion loss between the small and large powders increased when in a mixture (Figure 3).

38 2.5 Conclusion

Mixtures separated due to differences in size during both nonelectrostatic and electrostatic coating. During nonelectrostatic coating of NaCl and starch mixtures, differences between both the individual targeting losses, and the targeting losses in mixtures of small and large powders caused separation. In NaCl, interactions between large and small powders during targeting reduced the separation, while the opposite was observed in starch where interactions during targeting increased the separation.

During electrostatic coating, differences in mixture adhesion losses between small and large particles were the main cause of separation in both NaCl and starch.

However, the difference was much lower in starch than in NaCl.

Electrostatics reduced the targeting losses of all particles, both when coated individually and when in a mixture, by a large margin. Targeting loss was the largest source of loss for all powders during nonelectrostatic coating, therefore electrostatic coating greatly reduced the targeting loss. Electrostatics reduced separation in the starch mixture, but not in the NaCl mixture because of increased adhesion loss.

Food coatings usually consist of mixtures of powders with different sizes. It is desirable to minimize the separation of powders during coating to improve appearance and quality, and increase consumer acceptability. Mixtures separate by size during both nonelectrostatic and electrostatic coating. During nonelectrostatic coating, the differences in targeting losses is the biggest cause of separation, so coating systems need to be designed to minimize the differences in targeting losses to eliminate separation. Electrostatic coating reduced targeting loss by a large amount, therefore it is a method to reduce separation caused by targeting loss. However, electrostatic coating increased the adhesion loss, so electrostatic coating systems need

39 to be designed to minimize the adhesion loss to eliminate separation. One method to reduce adhesion loss is to add oil on the target surface. Since differences in size cause separation in mixtures, it is recommended that powders in the mixture should have a similar size range in order to reduce separation during coating.

40 2.6 References

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Adamiak K. 2001. Numerical investigation of powder trajectories and deposition in tribocharge powder coating. IEEE Transactions on Industrial Applications. 37(6): 1603-1609.

Bailey AG. 1998. The science and technology of electrostatic powder spraying transport and coating. J Electrostat 45: 85-120.

Biehl HL, Barringer SA. 2003. Physical properties important to electrostatic and nonelectrostatic powder transfer efficiency in a tumble drum. J Fd Sci 68(8): 2512-2515.

Biehl HL, Barringer SA. 2004. Comparison of the effect of powder properties on coating transfer efficiency and dustiness in two nonelectrostatic and electrostatic systems. Innovative Food Sci and Emerg Tech 5(2): 191-198.

Biswas P, Sanchez P, Swift MR, King PJ. 2003. Numerical simulations of air-driven granular separation. Phys Rev E 68 (5): 050301 – 050304.

Buck VE, Barringer SA. 2007. Factors dominating adhesion of NaCl on potato chips. J Food Sci. 72(8): 435-441.

Clark G. 1995. Processing: electrostatic coating technology for savoury snacks. Food Tech Eur 2(3): 90-92.

Enggalhardjo M and Narsimhan G. 2005. Adhesion of dry seasoning particles onto tortilla chips. J. Food Sci. 70 (3): 215-222.

Grosvenor MP, Staniforth JN. 1996. The influence of water on electrostatic charge retention and dissipation in pharmaceutical compacts for powder coating. Pharmaceut. Res. 13(11);1725-1728.

Guskov S. Electrostatic phenomena in powder coating. Nordson Corporation. 1996 [cited on 2009 January 13]. Available from: http://www.nordson.com/NR/rdonlyres/57C6C120-1F05-40B0-9DC9- B81B9B2270DE/0/PWR1164.pdf.

Halim F, Barringer SA. 2007. Electrostatic adhesion in food. J Electrostat 65(7): 168- 173.

Hanify DE. 2001. Snack seasoning application. In: Lucas EW, Rooney LW, editors. Snack foods processing. Lancaster, Pa: Rooney Technomic Publishing Co. p 517- 527. 41 Horinka PR. 1995. Powder particle size: Its role in electrostatic coating. Powder coating 4: 51-58.

Hughes JF. 1997. Electrostatic particle charging: industrial and healthcare applications. Taunton, Somerset, England: Research Studies Press, Ltd. 170p.

Mayr MB, Barringer SA. 2006. Corona compared with triboelectric charging for electrostatic powder coating. J Fd Sci. 71(4): 171-177.

Mazumder MK, Wankum DL, Sims RA, Mountain JR, Chen H, Pettit P, Chaser T. 1997. Influence of powder properties on the performance of electrostatic coating process. J Electrostat 40/41: 369-374.

Miller MJ, Barringer SA. 2002. Effect of sodium chloride particle size and shape on nonelectrostatic coating and electrostatic coating popcorn. J Fd Sci 67(1): 198- 201.

Mittal KL. 1997. Role of interface in adhesion phenomena. Polym Eng Sci 17(7): 467-473.

Pannell RJH. 1980. Electrostatic coating of crisp and snack foods. Confection Manuf Mrktg 17(6): 7-8.

Podczeck F. 1998. Particle-particle adhesion in pharmaceutical powder handling. London, UK: Imperial College Press. 231p.

Ratanatriwong P, Barringer SA. 2007. Particle size, cohesiveness and charging effects on electrostatic and nonelectrostatic coating. J Electrostat. 65(10-11): 704-708.

Ratanatriwong P, Barringer SA, Delwiche J. 2003. Sensory preference, coating evenness, dustiness, and transfer efficiency of electrostatically coated potato chips. J Fd Sci 68(4): 1542-1547.

Ricks NP, Barringer SA, Fitzpatrick JJ. 2002. Food powder characteristics important to nonelectrostatic and electrostatic coating and dustiness. J Fd Sci 67(6): 2256-2263.

Seighman J. 2001. Snack food seasonings. In: Lusas EW, Rooney LW, editors. Snack foods processing. Lancaster, Pa: Rooney Technomic Publishing Co. p 495- 527.

Sumawi H, Barringer SA. 2005. Positive vs. negative electrostatic coating using food powders. J Electrostat 63:815-821.

Wang F, Martinuzzi R, Zhu J. 2005. Experimental study of particle trajectory in electrostatics powder coating process. Powder Technology 150 (12): 20-29.

Xu Y, Barringer SA. 2008. The effect of relative humidity on coating efficiency in nonelectrostatic and electrostatic coating. J Fd Sci 73(6): E297-E303. 42

Ye Q, Domnick J. 2003. On the simulation of space charge in electrostatic powder coating with a corona spray gun. Powder Tech 135-136(10): 250-260.

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43 2.7 Tables and Figures

2.7.1 Tables

Table 1. Actual and predicted percentage of the smaller powder by location, after nonelectrostatically and electrostatically applying a 50:50 mixture of 44 and 256 µm NaCl or 61 and 191 µm starch

Target 44 µm NaCl 64 µm starch Nonelectrostatic Electrostatic Nonelectrostatic Electrostatic

Actual Predicted Actual Predicted Actual Predicted Actual Predicted

BR 22.4# 30.7* 43.6 40.9 39.4 49.1 10.4# 21.2 BC 19.6# 18.0 29.8# 29.5 11.1# 44.7* 7.1# 20.5 BL 30.2# 24.3 62.0# 15.7* 37.4# 51.3 41.8# 40.7 MR 48.6 45.4 78.7# 50.2* 36.6# 48.4* 57.4 55.6 MC 28.7# 16.0 34.7# 28.2 17.1# 36.0* 13.4# 31.5 ML 35.9# 26.9 76.1# 48.5* 25.3# 33.2 53.1 55.3 FR 62.1# 69.9 86.3# 73.7 44.1# 28.3 80.6# 72.6 FC 53.6 64.3* 75.2# 39.3* 31.5 24.1 83.0# 64.1* FL 41.2# 49.7* 81.3# 87.4 0.9# 9.1* 61.5 58.1 AVG. 15.5 19.2 22.4 17.7 23.0 14.3 23.7 15.8

# A significant difference between actual percentages of large and small powders landed on the target * A significant difference between actual and predicted percentage of applied powder landed on the target AVG. Average difference of a powder on all targets compared to the 50:50 distribution Table 2. Adhesion losses (%) of powders, with and without the other size of powder coated on the unoiled target Adhesion loss (%) 256 µm NaCl 44 µm NaCl 191 µm starch 64 µm starch Control Target Control Target Control Target Contro Target coated coated coated l coated with 44 with with 64 with µm 256 µm µm 191 µm NaCl NaCl starch starch Nonelectrostatic 11.9 27.3* 5.1 9.3 1.3 0.4 5.7 1.4* Electrostatic 16.1 32.4* 25.9 21.4 4.3 0.9 0.0 0.6

*indicates a significant difference between adhesion loss of NaCl or starch individually and NaCl or starch on targets coated with other powder

44 Table 3. Average mass of powder landed on each location on all locations, that is 44 µm and 256 µm NaCl after nonelectrostatic coating of 3.0000 g mixture vs. the average mass of powder on each location that is 44 µm and 256 µm NaCl after electrostatic coating of 3000mg mixture Plate 44 µm NaCl 256 µm NaCl Mass after Mass after Mass after Mass after nonelectrostatic electrostatic nonelectrostatic electrostatic coating (mg) coating (mg) coating (g) coating (mg) BR 32.6 30.9 113.0 39.9* BC 23.8 38.7 97.6 91.1 BL 54.4 85.7* 125.8 52.5* MR 108.1 276.3* 114.3 74.8* MC 64.1 106.2* 159.3 199.9* ML 51.0 192.9* 91.0 60.6* FR 24.1 154.8* 14.7 24.6 FC 29.4 120.4* 25.4 39.7* FL 10.1 97.3* 14.4 22.4 *indicates significant difference between mass landed after nonelectrostatic and electrostatic coating of mixture

Table 4. Average mass of powder on each location on all locations, that is 64 µm and 191 µm starch after nonelectrostatic coating of 3.0000 g mixtures vs. the average mass of powder on each location that is 64 µm and 191 µm starch after electrostatic coating of 3000mg mixture Plate 64 µm starch 191 µm starch Mass after Mass after Mass after Mass after nonelectrostatic electrostatic nonelectrostatic electrostatic coating (mg) coating (mg) coating (g) coating (mg) BR 84.0 8.6* 129.0 74.0* BC 9.3 3.2 74.9 41.5 BL 106.2 95.7 177.7 133.2 MR 142.0 44.76* 246.1 332.2* MC 62.4 50.7 302.2 328.2 ML 50.6 293.0* 150.6 258.6* FR 18.7 236.7* 23.9 57.1 FC 29.2 254.8* 63.7 52.0 FL 0.1 21.0 14.2 13.2 *indicates significant difference between mass landed after nonelectrostatic and electrostatic coating of mixture

45 2.7.2 Figures

Column 1 Column 2 Column 3 BR MR FR Row 1

BC MC FC Nozzle Row 2

BL ML FL Row 3

Figure 1. Target setup reference

* significant difference between nonelectrostatic and electrostatic losses in mixtures

# significant difference between small and large losses

! significant difference between individual losses and losses in mixtures

Figure 2. Targeting and adhesion loss for all powders when coated individually and when in a mixture

individual targeting loss targeting loss in mixture 50 individual adhesion loss adhesion loss in mixture

30 Difference (%) in losses 20

10 @ @ @

0 44 and 256 um NaCl 64 and 191 um starch 44 and 256 um NaCl 64 and 191 um starch mixture nonelectrostatic mixture nonelectrostatic mixture electrostatic mixture electrostatic @ no significant difference between the losses

Figure 3. Difference in losses in each mixture with two powders of different sizes

Figure 4. Average mass of powder landed on each location on all locations, that is 44 µm and 256 µm NaCl after nonelectrostatic coating of 3.0000 g mixture

Figure 5. Average mass of powder landed on each location on all locations, that is 64 µm and 191 µm starch after nonelectrostatic coating of 3.0000 g mixture

Figure 6. Average mass of powder landed on each location on all locations, that is 44 µm and 256 µm NaCl after electrostatic coating of 3.0000 g mixture

Figure 7. Average mass of powder landed on each location on all locations, that is 64 µm and 191 µm starch after electrostatic coating of 3.0000 g mixture

49 CHAPTER 3

3 EFFECT OF PARTICLE DENSITY ON SEPARATION OF MIXTURES DURING NONELECTROSTATIC AND ELECTROSTATIC POWDER COATING

3.1 Abstract

Two mixtures: 44 µm NaCl and 64 µm starch, and 197 µm NaCl and 191 µm starch were nonelectrostatically and electrostatically coated on grounded targets.

Separation of mixtures was measured by comparing the percentages of each powder on the target to determine whether density caused separation. Targeting loss and adhesion loss, both when coated individually, and in mixtures were determined.

During nonelectrostatic coating, mixtures of small particles separated due mainly to the differences between individual targeting losses, and differences between adhesion losses in the mixture. There was more starch than NaCl on all targets due to differences in individual targeting losses. The interactions in the mixture increased separation, as it increased the adhesion loss difference between the NaCl and starch powders. No separation was observed in the mixtures of large particles, as the differences in individual targeting and adhesion losses became insignificant when powders were combined in a mixture. The interactions due to being in a mixture reduced separation. During electrostatic coating, separation occurred due to the differences in adhesion losses, both individually and in mixtures. In the small mixtures, there were more starch particles closer to the nozzle during electrostatic

50 coating. Nonelectrostatic coating of large powders with different densities showed the least separation.

3.2 Introduction

Powders used during coating usually consist of a mixture of powders with different physical properties (Seighman 2001). One of the important properties is particle density of the powders. Particle density is a significant factor in determining nonelectrostatic coating transfer efficiency (Biehl and Barringer 2003). Particles with lower densities have lower masses and inertia compared to particles with the same size but with higher density, therefore are more susceptible to remain in the air as dust

(Yousuf and Barringer 2007).

During electrostatic coating, density is not a reliable factor in determining transfer efficiency (Biehl and Barringer 2003; Biehl and Barringer 2004). Electrostatic forces dominate over gravitational forces, making density insignificant as charge plays a more important role (Biehl and Barringer 2004). 51 Although several studies investigate the effect of density on transfer efficiency and adhesion, few studies have been done to determine the effect of powder density on separation and distribution of mixtures (Biehl and Barringer 2003; Buck and

Barringer 2007; Halim and Barringer 2007; Mayr and Barringer 2006). NaCl has lower adhesion compared to starch during both the nonelectrostatic and electrostatic coating on saltine crackers (Halim and Barringer 2007; Mayr and Barringer 2006).

There was no significant difference between the adhesion during nonelectrostatic and electrostatic coating of NaCl powders (Halim and Barringer 2007). One study found that density caused mixtures to separate during both nonelectrostatic and electrostatic coating (Yousuf and Barringer 2007).

Separation of the powder in a mixture during coating is undesirable in the food industry. Separation occurs when the proportion of one powder in the mixture is significantly different from the proportion of the other powder in the mixture on the target. Separation is caused by unequal distribution of different powders on the target due to different properties (Yousuf and Barringer 2007). Separation of powders in mixtures results in uneven distribution of flavors and appearances.

The objectives of this study were to determine if separation occurs when mixtures of powder with different densities were coated nonelectrostatically and electrostatically, to identify the key losses of powders of difference densities, both when coated individually and when in a mixture, and to determine the major losses that caused separation.

3.3 Materials and methods

All experiments were carried out at 25±2ºC and 35±5% relative humidity. The food powders used were 44 µm NaCl (325 Extra Fine Salt, Morton International, Inc., 52 Chicago, IL), 197 µm NaCl (Alberger Fine Flake Salt, Cargill, Inc., Minneapolis,

MN), and 191 µm starch (Pregelatinized Starch, National Starch & Chemical

(Thailand) Ltd., Amphur Muang, Thailand). Milling was done using an ultracentrifugal mill (Glenmill Inc., Clifton, NJ) 5 times at 18000 rpm to reduce the particle size of the starch to 64 µm. Mean diameters were measured using the

Malvern Mastersizer X with a powder dispenser (Malvern Instruments Ltd.,

Worcestershire, UK). The volume mean diameter D (4,3) was reported. Three replicates were done for all experiments.

An aerodynamic powder coating machine (Terronics Development Corp.,

3.3.1 Determination of separation

The percentages of the two powders on each location were measured and compared to each other to determine whether they were significantly different.

Separation occurs when there is a significant difference between the percentages of the two powders on each location. Two sets of mixtures with similar sizes but different densities were used: 44 µm NaCl and 64 µm starch, and 197 µm NaCl and

191 µm starch. Similar sizes were used in the experiment to eliminate the effects of particle size on separation. The targets were coated with 3.0000 g ± 0.2000g of 50:50 mixtures by mass. No oil was coated on the targets in this experiment. After the completion of each coating process, the mass of the powders on each location was measured. The powder was then transferred to 100 ml volumetric flasks and deionized 53 water was added to make a 100 ml solution. The conductivity of the solution was measured using a digital conductivity meter (E C Meter 19101-00, Cole-Parmer

Instrument Co. Vernon Hills, IL), and compared to a standard curve (Figure 17 and

18, in Appendix A). Since starch contributed negligible amount of solution conductivity, the standard curve was constructed using different amounts of NaCl.

The percent of NaCl on each location was determined from the conductivity and total powder weight.

3.3.2 Targeting and adhesion loss

(mass entering the chamber – mass collected on oiled targets ) x 100

mass entering the chamber

Approximately 1 g vegetable oil (Crisco Pure Vegetable Oil, The J.M. Smucker

Company, Orrville, OH) was applied on each target using a brush. Percent adhesion loss was measured as:

100x (mass entering the chamber- mass collected on unoiled targets) -% targeting loss

mass entering the chamber

Targeting loss in a mixture was measured by determining the conductivity of the mixture collected that was not on the oiled target after coating of 3 g of the mixture and comparing to the standard curve. The standard curves were constructed using mixtures containing different percentages of the NaCl vs. starch powders (Figure 17 and 18, in Appendix A). The standard curves convert the measured conductivity into 54 the percentage of NaCl powder in a mixture. The mixture that was not on the target was collected by using a special tray (area 51cm x 50 cm, with 10 cm height at the end of the tray to prevent powders escaping) with a large area that captured most of the powder not on the target. Conductivity of the powder collected on the tray but not on the targets was determined, and mass of each powder that was lost was determined by the standard curves. The mass of the powder loss over the total mass of the same powder in the mixture before entering the chamber was the targeting loss in the mixture. Adhesion loss in a mixture was calculated by total loss of powder in a mixture on unoiled targets minus the targeting loss in a mixture.

3.3.3 Statistical analysis

For the statistical analysis, one-way ANOVA with LSD analysis for means was performed for all the losses. Independent two tailed, two-sampled T-test with unequal variance was performed to determine significant differences between the proportions of powders on the targets, and also to compare the means for the actual proportion and the predicted proportion of the powders on the targets. A p-value of

0.01 or lower indicated significant difference between two groups.

3.4 Results and discussion

3.4.1 Separation during nonelectrostatic and electrostatic coating

To determine if density has an effect on the separation in mixtures, powders of the same size but different densities were mixed in equal proportions, and coated on the targets. As a difference in particle size causes separation of powders, NaCl (2200 kg/m 3) powder and starch (1491 kg/m 3) powder with sizes close to each other (±20

55 µm) were used to determine the effects of density on separation. It was assumed that density is the only difference in this experiment, but there are other differences such as composition, shape, and cohesiveness. It is usually assumed that all particles are spherical, since the effects of shape are unlikely to be important during coating

(Bailey 1998). Size and mass of the particles are the major factors affecting aerodynamic trajectory (Wang and others 2005). Therefore, composition and cohesiveness are not likely to affect the particle trajectory as the effect of size and mass of the particles is more significant. In this experiment, two small powders with different densities, and two large powders with different densities were used.

Equal proportions of 44 µm NaCl and 64 µm starch were used for the experiment during nonelectrostatic coating. All nine targets showed a significant difference. A larger size was also used in order to determine whether the difference in size between the two mixtures would show a difference in separation. The same densities of powders were used but with larger particles to investigate the difference in total separation between small particles with different densities and large particles with different densities. A mixture of large particles with different densities was nonelectrostatically coated on grounded targets: 191 µm starch and 197 µm NaCl.

There were six out nine targets in which there were significant differences between the proportions of large starch and NaCl (Table 5). However, there were only three out of nine locations where the percentages of the two powders differed by more than

7% (Table 5). There was less separation for the mixture of large powders than for the mixture of small powders, as the percentages of the two powders on the different target locations were closer in the large particle mixture than the small particle mixture. During electrostatic coating, density also caused separation during the coating of both small and large NaCl and starch mixtures as there were significant 56 differences between the percentages of NaCl and starch on seven out of nine target locations for the small mixture, and six out of nine target locations for the large mixture (Table 5).

There are three main losses that occur during coating: targeting loss, adhesion loss, and transportation loss. Targeting loss is a loss in which the powder is lost in the air before landing on the target, and adhesion loss is due to poor adhesion in which powder is lost after landing and does not remaining on the target (Xu and Barringer

2008). Transportation loss occurs during the transportation of the coated targets (Xu and Barringer 2008). The transportation loss was assumed to be 0%, as the measurements were directly taken immediately after coating. Targeting loss and adhesion loss were measured to determine the cause of separation.

Separation in mixtures occurred due to differences in four losses in the two powders: individual targeting loss, targeting loss when in mixtures, individual adhesion loss, and adhesion loss when in mixtures. Two powders with different sizes separate due to the differences in the amount of powder loss that occurs for each powder. If one powder in a mixture loses more during coating than the other, the total amount of each powder that lands on the targets will be different, resulting in separation. In a mixture, collisions and scattering occur, which causes the loss in the mixtures to be different from the loss when coated individually.

During nonelectrostatic coating, the magnitude of the separation in the small mixture was large, as the mixture showed an average difference of 25% from the

50:50 proportion for each target (Table 5). However, a smaller magnitude of separation was observed for the large mixture, as the mixture only showed an average difference of 8% from the 50:50 proportion for each target (Table 5). During electrostatic coating, the magnitude of separation in both large and small mixtures 57 were small, as the mixture showed an average difference of 12% and 10% from the

50:50 proportion for each target (Table 5). Electrostatics reduced separation in the small mixture, but had no effect on separation in the large mixture.

3.4.2 Individual targeting losses

Differences in individual targeting losses between NaCl and starch were significant for both small and large mixtures during both nonelectrostatic and electrostatic coating. Thus density affects targeting loss and increases separation.

Differences in individual targeting losses were the major cause of separation of the small mixtures. The difference in the individual targeting losses of the small NaCl and starch powders was 24% (Figure 9). For the small powders, more of the NaCl particles were lost in the air during targeting than starch, causing separation. The small NaCl particle was 20 µm larger than the small starch particle. Size has a larger effect on small particles than on large particles. As the particle size increases from 25 to 75 µm, the losses decrease by 15%, but only a 5% decrease is observed when the particle size increases from 175 to 225µm (Yousuf and Barringer 2007).

For the large particles, the individual targeting loss of NaCl was lower than that of starch by 6% (Figure 9). NaCl has a higher density (2200 kg/m 3) than starch

(1491 kg/m 3). The higher the particle density, the lower the coating loss (Biehl and

Barringer 2003; Biehl and Barringer 2004; Yousuf and Barringer 2007). Particles with lower density are more prone to get carried away by the air rather than being carried onto the targets because they have lower mass compared to particles with similar size but higher density (Yousuf and Barringer 2007).

During electrostatic coating of the small and large powder mixtures, the differences in individual targeting loss were moderate in both the NaCl and starch 58 mixtures. The differences in individual targeting losses of the small NaCl and starch were 16% (Figure 9). The differences in individual targeting losses of the large NaCl and starch were 10% (Figure 9). When the composition of the powders is the same, the powder with higher density should have a lower total loss than the powder with lower density, however, density is not a reliable factor in predicting the total losses of individual powders when the composition of the powders is different (Biehl and

Barringer 2003; Biehl and Barringer 2004; Yousuf and Barringer 2007). Therefore, the results were not as expected, as both small and large NaCl powders had higher individual targeting loss compared to starch (Figure 9).

Electrostatics reduced targeting loss compared to nonelectrostatic coating.

According to Coulombs’ Law, during electrostatic coating, because of their surface charge, the powder particles seek a grounded surface (Hughes 1997). As opposed to nonelectrostatic coating where no charge is used, the powder particles seek a grounded surface rather than staying suspended in the air, reducing targeting loss of the powder (Bailey 1998). For small particles, size has a large effect on targeting loss.

Therefore, the targeting loss of NaCl (49% decrease) decreased more due to electrostatics than starch (41%) (Figure 9). However, the individual targeting loss of

NaCl (21%) was still higher than that of starch (5%). There were two reasons that the targeting loss of NaCl was higher than starch: size difference between the NaCl and starch, and the ability of NaCl to lose charge to grounded targets and moisture. The particle size was smaller and the nonelectrostatic individual targeting loss was already high (70%) (Figure 9). More NaCl powder was lost during targeting than starch powder. Electrostatics also reduced targeting loss for the large particles, but electrostatics had a smaller effect on the large particles compared to the small particles. Charge is a more important factor as particle size decreases. NaCl has a 59 higher charge than starch at the same size (Yousuf and Barringer 2007; Biehl and

Barringer 2003). NaCl power is conductive (resistivity less than 10 10 Ωm), which transfer charges between the particles and surfaces easily, but tend to lose charge easily (Bailey 1998). The relative humidity in the experiment was 35%, therefore

NaCl particles lost charge to the moisture in the air. Therefore, the individual targeting loss of the large starch was higher than that of NaCl (Figure 9).

3.4.3 Targeting losses in mixtures

During nonelectrostatic coating, targeting losses due to interactions that occurred in the mixtures did not cause separation in either the small or large particle mixtures. There was no significant difference in the targeting losses in mixtures from when powders were coated individually for either small NaCl (66%) or starch (41%)

(Figure 9). For the large mixture, there was no significant difference between the targeting loss in mixtures of the large NaCl (27%) and starch (30%) powders compared to individual targeting losses (Figure 9). Since all targeting losses in mixtures were the same as the individual targeting losses, there were no significant interactions in the mixtures.

During electrostatic coating, interactions in the mixture reduced the large differences in individual targeting loss so that there was no difference in targeting losses in mixtures of either small or large particles. For small particles, the interactions during targeting reduced the separation in the mixture by increasing the starch targeting loss by 10%, while the targeting loss of the small NaCl remained the same, reducing the total difference of the targeting loss in mixture, compared to the individual (Figure 10). The interactions during targeting also reduced separation in the large mixture, as targeting loss of NaCl decreased by 8% while the targeting loss 60 of the large starch powder did not change from individual to when in a mixture

(Figure 9). NaCl and starch particles of the same size have different charge (Yousuf and Barringer 2007). During electrostatic coating, the particles are charged, and interactions between particles result in charge exchange even though the targets are identical (Bailey 1998). Repulsions occurred during targeting. As the particles changed charge due to particle-particle interactions, the magnitude of repulsion was different for different particles. For the small mixtures, the charge of starch increased due to the interactions, thus more repulsion occurred and the targeting loss of the starch increased. For the large mixtures, the interactions caused the large NaCl to lose charge, therefore less repulsion occurred, and the targeting loss decreased. In both mixtures, the repulsions caused the difference in targeting losses between NaCl and starch to decrease. Mixtures had a large effect on the targeting losses, however the effect was to decrease separation.

3.4.4 Individual adhesion losses

There was no significant difference between the individual adhesion loss of small NaCl and starch powders during nonelectrostatic coating. However, individual adhesion losses caused separation in the large mixture during nonelectrostatic coating.

The adhesion losses were 5%, and 6% for small NaCl and starch respectively (Figure

9). As particle size decreases, adhesion loss decreases when sugar is coated on the saltines crackers (Halim and Barringer 2007). It is size, and not density, that determines the adhesion for the small mixtures. In this experiment, the particle size of the two powders in the mixture was kept similar, so here was no significant adhesion loss difference between the small NaCl and starch. There was a significant difference between the individual adhesion loss of the large NaCl and starch powders (Figure 9). 61 The adhesion loss of NaCl was 38%, while the adhesion loss of starch was only 1%.

There was more NaCl powder lost compared to starch during adhesion, resulting in separation. The adhesion of NaCl is significantly lower than that of starch (Halim and

Barringer 2007; Mayr and Barringer 2006). The separation during nonelectrostatic coating of the large mixture was caused by the composition of the powder, not the density.

During electrostatic coating, the difference in individual adhesion losses between the NaCl and starch powders in both small and large mixtures caused separation. There was a significant difference between the individual adhesion loss of both small and large NaCl and starch. The adhesion losses of the small NaCl and starch were 26% and 0% respectively, while the adhesion losses of the large NaCl and starch were 35% and 4% respectively (Figure 9). More NaCl than starch was lost after the powder landed on the targets. The adhesion of NaCl was lower than that of starch during electrostatic coating of small powders (Halim and Barringer 2007). There was no change in the individual adhesion losses for both large powders from during nonelectrostatic coating (Figure 9). For the large mixture, electrostatics had no effect on the adhesion losses, therefore the existing differences from nonelectrostatic coating remained. The individual adhesion loss of the small NaCl increased by 21% due to electrostatics, because there were more powders landing on targets due to decrease in targeting loss, and because NaCl loses charge immediately when in contact with the targets. During electrostatic coating, the targeting loss of small NaCl was greatly reduced compared to during nonelectrostatic coating, therefore more small NaCl landed on the targets, thus adhesion loss increased during electrostatic coating, due to more powder that did not land on the targets during nonelectrostatic coating landed on the targets due to electrostatics. Therefore, more small NaCl particles were available 62 to be loss due to adhesion. NaCl is a conductive powder, which tends to lose charge immediately after adhering on the targets resulting in repulsion away from the targets, therefore it will have higher adhesion loss than starch, an intermediate resistivity powder, which does not lose charge as quickly as the conductive powder (Bailey

1998; Sumawi and Barringer 2007).

3.4.5 Adhesion losses in mixtures

During nonelectrostatic coating, being in a mixture increased the total difference in adhesion losses between the two small powders, therefore separation increased. For the large particles, mixtures decreased the total difference in adhesion losses, therefore separation decreased. The total difference between the adhesion losses of small NaCl and starch powders increased by 12% in mixtures compared to the individual adhesion losses (Figure 10). The adhesion loss in the small mixture increased by 9% for the NaCl compared to when coated individually, while the adhesion loss in the mixture for starch did not change significantly (Figure 9).

The total differences between the adhesion losses of large NaCl and starch powders decreased by 34% in mixtures compared to the individual adhesion losses

(Figure 10). Due to powders being in a mixture, there was no significant difference between the adhesion loss of NaCl and starch in a mixture. The adhesion loss of large

NaCl and starch in mixture were 6 and 9% respectively (Figure 9). The adhesion loss in mixture of the NaCl decreased by 32%, while the adhesion loss in mixture of starch increased 7% compared to when coated individually (Figure 9).

Starch has a stronger adhesion than NaCl during nonelectrostatic coating

(Halim and Barringer 2007; Mayr and Barringer 2006). Therefore, the adhesion loss 63 of starch did not change compared to the individual adhesion loss (Figure 9). Both the

NaCl powders had larger changes compared to starch. The adhesion loss increased by

9% for small NaCl, and decreased by 32% for the large NaCl in mixtures due to the change in roughness of the target surface (Figure 9). The directions of the change in mixtures were different for small and large NaCl. During coating of the powders, targets were coated with the mixtures of powder, creating a new layer on the surface of the targets. Adhesion can be proportional or inversely proportional to the surface roughness (Podczeck 1998). In the small mixtures, the NaCl and starch powders created a smooth layer on the targets, which resulted in higher adhesion loss for the

NaCl as the particles could not adhere to the targets well. However, the larger particle size in the large mixtures resulted in a rougher surface layers, where the large NaCl adhered to the surface better, therefore the adhesion loss decreased.

During electrostatic coating of the mixture of NaCl and starch particles, the interactions on the target decreased separation in both small and large mixtures. In small mixtures, the adhesion loss of NaCl in a mixture decreased by 12% to 14% compared to the individual adhesion loss, while the adhesion loss of starch stayed the same in mixture (0%). For the large mixture, the adhesion loss of NaCl decreased by

18% to 17% when in a mixture compared to the individual adhesion loss, while the adhesion loss of starch stayed the same in a mixture (5%) More NaCl was lost after landing on the target compared to starch in a mixture, decreasing separation. During electrostatic coating, the particles are charged through high voltage, and particle- particle interactions result in charge exchange even though the targets are identical

(Bailey 1998). NaCl has a higher charge than starch with the same particle size

(Yousuf and Barringer 2007). Therefore, NaCl loses charge, while starch gains charge through the charge exchange. When the NaCl is coated individually, more repulsion 64 occurred due to the high charge of the NaCl particles. However, the loss in charge of

NaCl due to interactions caused the adhesion loss to decrease in mixtures for both small and large NaCl compared to the individual adhesion losses, as less repulsion occurred due to the high charge. Although the starch gained charge, the individual adhesion losses were already very low (0 and 5% for small and large starch respectively), therefore no additional adhesion loss occurred.

3.4.6 Separation

During nonelectrostatic coating of a mixture of small NaCl and starch, there was more of the small starch then the small NaCl on all target locations (Figure 11, and Figure 12). Individual targeting loss in the mixture was 20% lower in small starch than small NaCl (Figure 9). The difference in individual targeting losses resulted in the separation. The mixture of small NaCl and starch increased separation, as seven out of nine target locations showed distribution further from 50:50 compared to the distribution predicted from individual losses (Table 5).

Unlike the mixture of small particles, the mixture consisting of large NaCl and large starch showed a more even distribution. 7 out of 9 target locations were within

3% of a 50:50 distribution (Table 5).

During electrostatic coating of the mixture of small particles, more starch landed on the targets closer to the nozzle compared to NaCl, as 3 out of 3 targets within 0-15 cm of the nozzle had more starch than NaCl (Figure 13, and Figure 14).

For the small mixture, moving away from the nozzle, the mixture was evenly spread, as half of the 6 target locations had higher NaCl than starch, and vice versa (Table 5 and 6). The mixture reduced separation, as 8 out of 9 target locations showed a closer distribution to 50:50 compared to the distribution predicted from individual losses 65 (Table 5). For the large mixture, moving away from the nozzle (15-45 cm), there were more NaCl than starch in 4 out of 6 target locations (Table 5 and 7). The large mixture also reduced separation, as 8 out of 9 target locations showed a closer distribution to 50:50 compared to the distribution predicted from individual losses

(Table 5). Electrostatics changed the distribution in the small mixture, and also decreased separation. During nonelectrostatic coating, there were more of the starch particles on all targets due to the higher targeting loss of NaCl. Electrostatic forces caused more NaCl to land on the targets compared to during nonelectrostatic coating.

The magnitude of the separation was lower during electrostatic coating, as the small mixture showed an average difference of 12% compared to the 50:50 distribution during electrostatic coating, compared to 25% during nonelectrostatic coating (Table

5). For the large mixtures, electrostatics did not change the magnitude of separation, as there was no significant difference between the average difference on the targets to the 50:50 distribution during nonelectrostatic and electrostatic coating (Table 5).

3.5 Conclusion

During nonelectrostatic coating, the mixture of small particles separated due mainly to the difference in individual targeting losses between NaCl and starch powders, while adhesion losses in the mixture caused a small separation. The interactions due to being in a mixture increased separation, as it increased the adhesion loss difference between the NaCl and starch powders. Mixtures of large particles had very little separation. The difference between the individual adhesion losses of NaCl and starch was high, but interactions decreased the difference, reducing separation.

66 During electrostatic coating of a mixture with small or large particles, differences in adhesion losses, both individual and in mixtures, caused separation of mixtures. Differences between the individual targeting losses of NaCl and starch were large, but interactions reduced the differences, thus differences in individual losses were not an effect.

Electrostatic coating decreased all the targeting losses, both individually and in mixtures of both NaCl and starch. However, there was no clear pattern in correlating types of powder (NaCl and starch), size, and systems (individual and mixtures) with adhesion losses. The least separation was observed when a mixture of large NaCl and starch was nonelectrostatically coated.

Food coatings usually consist of mixtures of powders with different densities.

It is desirable to minimize the separation of powders during coating to improve appearance and quality, and increase consumer acceptability. During nonelectrostatic coating, targeting loss is the biggest cause of separation for the small mixtures.

Therefore, the coating systems need to be designed to minimize the targeting loss difference between the powders to eliminate separation. Electrostatic coating reduced targeting loss by a large amount, but had high adhesion loss, therefore the coating systems need to be designed to minimize adhesion loss in order to reduce separation.

The large mixtures have very little separation, as the differences in both targeting losses and adhesion losses are eliminated due to being in mixtures. Separation is reduced when large particles are used during nonelectrostatic coating. There is no difference in the magnitude of separation between the small and large mixtures.

Differences in both size and density cause separation in mixtures during nonelectrostatic and electrostatic coating. During nonelectrostatic coating, differences in individual targeting losses cause separation in mixtures with different sizes, and 67 small mixtures with different densities. For the large mixtures with different densities, being in a mixture reduces differences in targeting and adhesion losses of the two powders, thus reducing separation. Nonelectrostatic coating also causes high targeting loss for all powders. Therefore, nonelectrostatic coating systems need to be designed to minimize the targeting loss of all powders in order to reduce separation and also powder waste. Electrostatic coating does not have an effect on separation in the mixture with different sizes and large mixtures with different densities, but reduces separation in the small mixture with different densities. The least separation occurs in the large mixture with different densities. Differences in adhesion loss cause separation for both mixtures with different size and density. It is difficult to design coating systems which minimize adhesion loss. However, the surface of the targets can be manipulated. One way to reduce adhesion loss is to introduce adhesive agents such as oil on the targets. During nonelectrostatic coating, differences in size between the powders in mixtures cause higher differences in targeting loss compared to differences in density between powders in mixtures. During electrostatic coating, both differences in size and density between the powders in mixtures cause difference in adhesion losses, and are the main problems in creating separation.

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