adhesion of food powders with nonelectrostatic and electrostatic coating

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ADHESION OF FOOD POWDERS WITH NONELECTROSTATICAND ELECTROSTATIC COATING

YANG HUANG and SHERYL A. BARRINGER1

Department of Food Science and TechnologyThe Ohio State University

2015 Fyffe Road, Columbus, OH 43210

Accepted for Publication December 18, 2009

ABSTRACT

This study investigated the effects of powder resistivity, coating voltage,relative humidity (RH) and coating density on adhesion. Cocoa powder with ahigh resistivity (1.15 ¥ 1013 Wm) showed a stronger electrostatic adhesionthan starch powder with a medium resistivity (2.56 ¥ 1010 Wm) and NaClpowder with a low resistivity (7.31 ¥ 105 Wm). The adhesion of starch andcocoa powders coated at 0, 40 and 95 kV increased with increasing voltage.The adhesion at 0 kV should be dominated by the van der Waals force, at 40and 95 kV by the electrostatic image force and at high RH by capillary force.Theoretical calculations were in the correct range, but the assumptions inthose calculations make them unreliable. For nonelectrostatic coating (0 kV),there was no significant change in adhesion when RH increased from 30to 60%, while adhesion increased when RH increased from 60 to 80%. Forelectrostatic coating, the adhesion decreased when RH increased from 30 to60%, but the adhesion at 80% RH was larger than the adhesion at 30 and 60%RH. For both nonelectrostatic and electrostatic coating, the adhesion forcedecreased as coating density increased to 1.0 mg/cm2, but there was nosignificant change from 1.0 mg/cm2 to 2.0 mg/cm2.

PRACTICAL APPLICATIONS

The adhesion of seasoning powders on snack foods plays an importantrole in the product quality, but insufficient adhesion between food powders andthe surface of snacks results in the seasonings falling off the snacks, leadingto a poor distribution and waste of seasonings. This study suggests that theadhesion between food powders and targets can be effectively improved

1 Corresponding author. TEL: 614-688-3462; FAX: 614-292-0218; EMAIL: [email protected]

Journal of Food Process Engineering •• (2011) ••–••. All Rights Reserved.© 2011 Wiley Periodicals, Inc.DOI: 10.1111/j.1745-4530.2010.00587.x

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by controlling the properties of the powders, as well as optimizing coatingconditions. For example, using food powders with a high resistivity and highvoltage during electrostatic coating and increasing relative humidity willincrease adhesion.

INTRODUCTION

Snack foods such as potato chips, pretzels, popcorn and tortilla chips areoften coated with seasonings or flavorings to enhance flavor and taste beforethey are packaged. Even distribution of seasoning toppings is important to theappearance and taste of snacks and it is essential to maintain a consistentquality of the product. However, insufficient adhesion between seasonings andthe surface of snacks results in the seasonings falling off the snacks, leading toa poor distribution of flavor on the product. Thus, it is common for traditionalsnack manufacturers to put 30% and, in some cases, as much as 50% moreseasonings than actually needed into the coating process to ensure sufficientcoating (Mitchell 1992). This powder waste increases product cost becausethe seasonings are expensive.

Electrostatic coating is one solution to the insufficient adhesion problemduring food powder coating. Electrostatic powder coating applies charge ontofood powders, and the charged particles repel each other because they allpossess a similar charge, resulting in improved coating evenness. Once thecharged particles deposit onto the grounded food product, they adhere to theproduct via electrostatic image forces (Bailey 1998). Based on Coulombic’slaw, the electrostatic image force will be large as the distance is now small.Therefore, the adhesion will be increased when charge is present (Hughes1997). The resistivity of the food powder determines how long the powder canretain the charge. Powders with higher resistivity can keep the charge longerthan powders with lower resistivity; thus, the adhesion between a powder withhigher resistivity and the target is stronger than that between a powder withlower resistivity and the target.

Adhesion is the interaction of particles with a solid surface (Zimon1969). It has been quantified as the work done in separating two surfacesthat are in contact with each other (Michalski et al. 1997). Adhesion iscaused by a variety of forces. Forces between particles and a surface, bring-ing about adhesion, include long-range attractive interactions such as van derWaals forces and electrostatic forces (Bowling 1988). Electrostatic forcesinclude both electrostatic image force and electrical double-layer forces.Adhesion also occurs because of the establishment of liquid and solidbridges between particles and a surface, and the consequent capillary force(Bowling 1988). Under different conditions, different individual forces of

2 Y. HUANG and S.A. BARRINGER

adhesion will prevail. For dry, uncharged particles on a dry, unchargedsurface, only van der Waals forces and electrical double layer forces areimportant (Bowling 1988). Charged particles add an additional electrostaticimage force to the total adhesion. Different forces may dominate adhesionunder different conditions. For instance, van der Waals force was the domi-nant force for uncharged toner particles at a relative humidity (RH) below30%, while electrostatic force was the dominant force for corona-chargedtoner particles (Takeuchi 2006). Capillary forces appear in wet systems andcan be predominant over other forces for small particles (Bowling 1988). Anincrease of RH progressively modified the surface topology of zanamivirparticles on lactose monohydrate and increased the adhesion force (Bérardet al. 2002).

The objective of this study is to investigate how the powder resistivity,coating voltage, RH and coating density affect adhesion during electrostaticand nonelectrostatic coating, and if the theoretical adhesion forces can becalculated.

MATERIALS AND METHODS

Food Powders

Cocoa powder, starch and NaCl were applied in this study. Powderparticle size was measured using a Malvern Mastersizer X with a dry powderfeeder (Malvern Instruments Ltd, Worcestershire, UK). The volume meandiameter D (4, 3) of each powder was measured three times and the averagewas reported as the particle size for each powder. The cocoa powder (TheGreat American Spice Co., Fort Wayne, IN) had a particle size of 34 mm. TheNaCl powder (Extra-Fine 325, Morton International Inc. Chicago, IL) had aparticle size of 24 mm. The starch powder was modified starch (NationalStarch & Chemical [Thailand] Ltd., Amphur Muang, Thailand). The originalparticle size of the starch was 195 mm. It was ground in an ultracentrifugal mill(Glenmill Inc., Clifton, NJ) at 18,000 rpm for six times to produce 59 mmparticles.

Resistivity was measured using a resistivity cell (powder resistivity testcell, Electrostatic Solutions Ltd., Southampton, Hampshire, UK) connected toan electrometer (614 Electrometer, Keithley Instruments Inc, Cleveland, OH)and voltage supply (DC supply, Kepco Inc., Flushing, NY). Food powder(5 cm3) was filled into the cell and tapped 30 s to remove air. A voltmeter (LCDAuto Range Digital Multimeter, Model 22-163, Radio Shack, Tandy Corpo-ration, Fort Worth, TX) measured the voltage (175 � 5 volts) from the voltagesupply and the current was read from the electrometer. The resistivity of thepowder was calculated using the equation r = (KV)/I, where r = resistivity

ADHESION OF FOOD POWDERS 3

(Wm), K = the cell constant (0.014), V = the voltage applied (V) andI = current (A).

The charge-to-mass ratio produced by tribocharging was determinedusing an aluminum foil-covered piece of cardboard, which was first groundthen insulated from contact with the environment. The food powder wascoated onto the aluminum foil by a corona charging system (Sure Coat ManualPowder Spray Gun, Part 302123D, Nordson Corporation, Amherst, OH). Thecoating voltage was set at 0 kV. An electrometer (Model 610c, Keithley Instru-ments, Inc.) connected to the aluminum foil measured the charge captured bythe aluminum foil. The mass of powder deposited on the aluminum foil wasmeasured. The charge-to-mass ratio was determined from the charge of thepowder divided by the mass of the powder.

Coating System

All experiments were conducted in an environmentally controlled room,where the RH was kept constant at 35 � 5% using humidifiers (Environizer,Koz Inc., Southborough, MA; MoistAir, Emerson Electric, Hatfield, PA) ordehumidifiers (Maytag, Maytag Inc., Effingham, IL; Hampton Bay, FeddersCorporation, Liberty Corner, NJ) except for the study of effect of RH onadhesion. The temperature was controlled at 25 � 2C. The corona chargingsystem (Sure Coat Manual Powder Spray Gun, Part 302123D, NordsonCorporation) was set at 40 kV or 95 kV for electrostatic coating. The voltagewas adjusted to 0 kV for nonelectrostatic coating. For both electrostatic andnonelectrostatic coating with the corona gun, an air compressor (5.0 hp, 15.0gallon tank, Model WL650800AJ, Campbell Hausfeld, Harrison, OH) wasconnected to the corona charging system to supply airflow. The flow rate airpressure was set at 6.9 ¥ 104 Pa (10.0 psi) and the atomizing air pressure wasset at 3.5 ¥ 104 Pa (5.0 psi).

The powders were sprayed into a cardboard booth (47 ¥ 38 ¥ 40 cm)through an opening to contain the excess powder. The powder is charged as itexits the gun. The bottom of the booth was ground foil. A rigid foam insulationsheet (Owens Coring Foam Insulation, LCC, Toledo, OH) was cut to producethe coating targets (11.6 ¥ 3.5 cm). A coating zone was drawn on the foil,which ensured the vertical position of the gun tip was 27 cm above the foil, andthe horizontal distance of the gun tip to the coating zone was 15 cm. Thetargets were cleaned and dried at room temperature. Both the targets and thepowders were put in the coating room at least 2 h before coating for tempering.The coating density was adjusted by controlling the amount of powder fedto the gun. The coating density of salt on commercial potato chips wasdetermined to be 0.5 to 1.0 mg/cm2. Thus, all the tests were performed at thiscoating density range unless otherwise specified.


Adhesion Force Measurements

The adhesion was determined by applying detachment forces to removethe powder from the surface of the target. Centrifugal force and gravity werethe two types of detachment forces employed. Targets (12 pieces) were put intwo rows on the ground foil for coating. After coating with the corona gun,three targets with similar coating density were selected as replicates foreach run. They were gently inverted three times after coating to remove loosepowders by gravity. Then they were vertically placed into 250-mL centrifugalbottles (Fisher Scientific Company L.L.C., Morris Plains, NJ). The bottleswere placed into the rotor of a Sorvall RC5C plus (Sorvall Company,Asheville, NC), with the powder-covered surfaces of the targets facing outsidethe rotor. The targets were centrifuged at 100, 500, 1,000, 1,500 and 2,000 rpmfor 1 min, and the powder mass before and after applying each centrifugalforce was recorded by weighing. The force due to gravity (Fg) was calculatedwith the equation:

F mg g=

where m is the mass of a single particle, which equals the density of the powder(1,491, 2,200 and 1,450 kg/m3 for starch, NaCl and cocoa powder, respec-tively) times the volume of a single particle. The volume of a single particlewas calculated by assuming all particles were perfect spheres and had adiameter equal to the mean particle size measured. The gravitational accelera-tion (g) is 9.8 m/s2. The centrifugal force at each speed was calculated withthe equation:

F mc x= ω2

where Fc is the centrifugal force, w is the angular velocity of the rotation, whichequals to 2 pn, n is the spinning speed in revolutions per second and x (8.63 cm)is the distance of the target surface from the axis of rotation. The detachmentforce was assumed to be the sum of gravity and centrifugal force at differentspeeds, and the adhesion force F50 was determined as the sum of gravity andcentrifugal force required to remove 50% of the powders on the target.

Calculation of The Individual Forces

Capillary Force. The interaction between a sphere and plate surfaceassociated with capillary force takes the following form:

F rk = 4πσ θ αcos


where s is the surface tension (water at 25C is 0.072 N/m), r is the radius ofthe particle, q is the contact angle of water to the target surface, which isassumed to be 10° and a is the roughness coefficient of the surface, whichwas determined to be 118.61 for tortilla chips (Enggalhardjo and Narsimhan2005).

Electrostatic Image Force. The charged food particles induce equal andopposite charges on the target’s surface. According to Coulombic law, theelectrostatic image force for a single particle (Fim) is theoretically calculatedwith the equation (Cross 1987)

F q him r r= +( )⎡⎣ ⎤⎦2

0216πε ε

where e0 is the vacuum permittivity constant, 8.85 ¥ 10-12 C2/J ¥ m, and er

is the relative permittivity constant. As the medium between the particle andsubstrate is air, the value of relative permittivity is close to one, h is thedistance between the particle and target, which is assumed to be 0.05 mm,and q is the charge on a single particle, which was estimated to be thesaturation charge qmax due to the Pauthenier limit. qmax is given by (Cross1987)

q pEmax = 4 02πε r

where p = 3e/(e + 2), which varies between three for a conducting particle andone for an insulating particle, the permittivity of the particle is e, and E is theelectric field in Vm-1.

Van Der Waals Forces. The van der Waals force (Fvd) between aspherical food particle and a plate substrate at the distance of h is given by

F Avd r h= ( )6 2

where A is the van der Waals constant, which is assumed to be 4.7 ¥ 10-19 Jbased on sodium borate spheres (Zimon 1969).

Statistical Analysis. Two-way analysis of variance with post hoctests was performed to analyze the difference between different coatingvoltage, RH and particle size. Tukey’s honestly significant difference wasused to determine the significant factors. A P value less than 0.05 was usedto indicate significant difference.


RESULTS AND DISCUSSION

Powder Resistivity

Resistivity is a property of a material which indicates how stronglythe material opposes the flow of electric charge. Powder resistivity deter-mines the ability of a powder to hold charge and is critical to electrostaticadhesion. In terms of the magnitude of resistivity, particles with resistivitygreater than 1013 Wm are insulators, and less than 1010 Wm are conductors(Bailey 1998). NaCl (7.31 ¥ 105 Wm), starch (2.56 ¥ 1010 Wm) and cocoa(1.15 ¥ 1013 Wm) powders were chosen because they have low, mediumand high resistivity, respectively, and represent the range of resistivitiespresent in food products. In order to minimize the size effect, powders withsimilar particle size were chosen to investigate the effect of resistivity onadhesion.

For nonelectrostatic coating, the percent of powder removed for differentamounts of force for starch and NaCl powders were close, while cocoa powdershowed a greater loss (lower adhesion) (Fig. 1). Although no charge wasintentionally added in nonelectrostatic coating, tribocharging always occursbecause of friction during the handling of the powders. The net charge accu-mulated because of tribocharging can be large, and tribocharging is greatest on

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FIG. 1. ADHESION OF DIFFERENT POWDERS AT 30% RELATIVE HUMIDITY, 0 kV


powders and targets with high resistivity (Bailey 1998). The charge to massratio produced on cocoa powder (+62.0 nC/g) was significantly higher thanthat of NaCl (+48.8 nC/g) and starch (+30.0 nC/g). If the plastic foam targetswere tribocharged positively during handling, then the powders would berepelled. NaCl and starch have lower resistivity so they lose their chargequickly, while both cocoa powder and plastic foam have a high resistivity.Therefore, the repulsion between cocoa powder and the targets may have beenenough to lower the adhesion force.

As capillary force is not significant at low RH and electrostatic imageforce is not dominant for low charge to mass values, van der Waals force is thekey force in nonelectrostatic coating. The measured adhesion forces at 0 kV,30% RH for cocoa powder (F50 = 0.76 nN), NaCl (F50 = 4.6 nN) and starch(F50 = 14.0 nN) were somewhat similar to the theoretically calculated van derWaals forces for cocoa (0.53 nN), NaCl (0.38 nN) and starch (0.92 nN);however, they were not that close. There are critical assumptions that must bemade in calculating van der Waals force, including the distance betweenpowder and the target, and the value of the van der Waals constant for thatpowder, which cannot be measured. Because they must be assumed, someauthors adjust them until they match the experimental results. Without doingthat, it appears that a poor correlation will result, because of the inability todirectly measure the necessary variables.

After electrostatic coating, on the other hand, cocoa powder showed thehighest adhesion, followed by starch then NaCl powder (Fig. 2). Particles withlower resistivity are charged more efficiently than those with higher resistivitywhen passing through a corona discharge, but when in contact with the target,they lose charge more quickly (Bailey 1998). Charge decay is exponential witha time constant. Cocoa powder is an insulating powder with a high resistivity,which has a slow charge decay rate and can retain charge longer, resulting inhigher electrostatic adhesion. Starch powder has an intermediate resistivityand the electrostatic adhesion is between cocoa powder and NaCl powder.NaCl powder has a low resistivity and is a conductor. The charge applied on itdecays very fast and does not show significant electrostatic adhesion. Halimand Barringer (2007) also found that electrostatically coated cocoa powder hadhigher adhesion than NaCl.

Coating Voltage

Starch powder coated onto the targets at 95 kV showed the highest adhe-sion force, followed by 50 kV, then 0 kV (Fig. 3). Because of its higherresistivity, the increase in adhesion force at high voltage for cocoa powder waseven larger than for starch (Fig. 4), while NaCl showed no difference (Figs. 1and 2). In the remainder of the paper, the results for starch powder are


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FIG. 2. ADHESION OF DIFFERENT POWDERS AT 30% RELATIVE HUMIDITY, 95 kV

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FIG. 3. EFFECT OF COATING VOLTAGE ON ADHESION FOR STARCH POWDER AT 30%RELATIVE HUMIDITY


presented because it has an intermediate resistivity and the majority of foodpowders have a resistivity in the intermediate range (Halim and Barringer2007). Electrostatic coating has a greater effect on cocoa powder and a lesseffect on NaCl, compared with starch.

Electrostatic image force is an important contributor to adhesion forcewhen the powders are charged. The electrostatic image force increases as thevoltage increases because higher voltage creates higher charge density in thecorona field, allowing the powders to pick up more net charge when passingthrough the corona area, thus achieving a higher electrostatic image force. Themeasured adhesion force (F50 = 1.4 nN) for cocoa powder at 40 kV, 30% RHwas similar to the theoretically calculated electrostatic image force (2.6 nN),and larger than the calculated van der Waals force (0.53 nN). At 95 kV, themeasured adhesion force (F50 = 115 nN) for cocoa powder was higher than thecalculated electrostatic image force (10.6 nN). The theoretically calculatedelectrostatic image force was determined by assuming that the food powderswere saturated with charge when they passed through the electrical field, hencehave the maximum charge levels. However, the actual charge the powdercaptures is frequently lower than the saturation level, thus lowering the actualelectrostatic image force.

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FIG. 4. EFFECT OF COATING VOLTAGE ON ADHESION FOR COCOA POWDER AT 30%RELATIVE HUMIDITY


RH

For nonelectrostatic coating (0 kV), there was no significant change inadhesion between starch powder and the targets when RH increased from 30to 60%, while adhesion increased when RH increased from 60 to 80% (Fig. 5).There is little capillary force at 30 and 60% RH, while capillary force becomessignificant at 80% RH. RH greatly influences adhesion by forming liquidbridges resulting in strong capillary forces. Capillary forces are the primaryadhesion force at high RH, which exceed all other adhesive components.Liquid bridges form from the condensation of atmospheric moisture at a RHabove 65% (Zimon 1969). Above 70% RH, capillary force predominates overother forces in adhesion (Busnaina and Elsawy 2001). The measured adhesionforce at 80% RH (F50 = 277 nN) was similar to the theoretically calculatedcapillary force (226 nN).

In electrostatic coating, the adhesion force at 30% RH, 95 kV was stron-ger than at 60% RH, 95 kV (Fig. 6). As RH increases, powder absorbs waterfrom the air, decreasing the powder’s resistivity and increasing the chargedecay rate (Halim and Barringer 2007), therefore decreasing electrostaticimage force. A further increase of RH to 80% leads to a lower resistivity andfaster charge decay rate, which makes the electrostatic image force negligible.However, water bridges also form at high RH; thus, capillary force appears and

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FIG. 5. EFFECT OF RELATIVE HUMIDITY (RH) ON ADHESION FOR STARCH AT0 kV COATING


becomes the dominant force at 80% RH. Because capillary force is muchstronger than other individual forces, the adhesion force at 80% RH forelectrostatic coating is larger than that at 30 and 60% RH.

Coating Density

For both electrostatic and nonelectrostatic coating, as coating densityincreased to 1.0 mg/cm2, the percentage of starch powder removed at 153 nN(1,000 rpm) detachment force increased (Fig. 7). However, when the coatingdensity increased from 1.0 mg/cm2 to 2.0 mg/cm2, there was no significantchange in the percentage removed. The fact that adhesion did not changeindicated that multilayers of powder were formed when coating densityincreased above 1.0 mg/cm2. The theoretical monolayer value, based on theassumptions that all the particles are perfect spheres with the same size andthat there is no space between particles, is 2.9 mg/cm2. This indicates that thestarch powders are not perfect spheres and/or multilayers form while there isstill space between particles. The strength of multiple layers depends not onlyon their adhesion to the target surface but also on the autohesion between theparticles (Zimon 1969). The adhesion force becomes smaller as layers increasebecause gravity dominates over autohesion, causing the particles to beremoved from the targets more easily. Across the range of coating densities

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FIG. 6. EFFECT OF RELATIVE HUMIDITY (RH) ON ADHESION FOR STARCH AT95 kV COATING


tested, the percentage of powder removed for 95 kV coating was lower than for40 kV and 0 kV, indicating that high voltage improves both monolayer andmultilayer coating.

Electrostatic coating, especially high voltage (95 kV) coating, showed alower coating density than nonelectrostatic coating when the same mass ofpowder was applied to the gun. The charged powder evenly spread across thetarget, ground and surrounding area due to the repelling force between thecharged powder particles instead of unevenly dropping where gravity directedit onto the target, which occurred during nonelectrostatic coating. To achievesimilar coating density for both nonelectrostatic and electrostatic coating, thefeeding mass for electrostatic coating has to be increased. A feeding mass of1 g for 0 kV, 3 g for 40 kV and 5 g for 95 kV coating was needed to create asimilar coating density range of 0.5~1.0 mg/cm2.

CONCLUSION

Powders with higher resistivity can retain charge better, thus have higherelectrostatic adhesion force than powders with lower resistivity. Adhesionforce becomes stronger with increasing coating voltage, especially frommedium voltage to high voltage. Adhesion force increased greatly when RH isabove 60% due to the formation of liquid bridges resulting in strong capillary

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FIG. 7. EFFECT OF COATING DENSITY ON ADHESION FOR STARCH AT 35%RELATIVE HUMDITY


force. For electrostatic coating, water in the air at moderate RH (60%)decreases the resistivity of powder and causes faster charge decay, decreasingthe effectiveness of electrostatic coating compared with lower RH. Adhesionforce decreased as the coating density decreased for both nonelectrostatic andelectrostatic coating. While adhesion force can be theoretically calculated, thenumber of assumptions that must be made keeps the calculations from beingvery accurate.

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BOWLING, R.A. 1988. A theoretical review of particle adhesion. In Particleson Surfaces, Vol. 1 (K.L. Mittal, ed.) pp. 129–142, Plenum Press, NewYork, NY.

BUSNAINA, A.A. and ELSAWY, T. 2001. The effect of relative humidity onparticle adhesion and removal. In Particle Adhesion: Applications andAdvances (D.J. Quesnel, D.S. Rimai and L.H. Sharpe, eds.) pp. 391–410,Plenum Press, New York, NY.

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MICHALSKI, M.C., DESOBRY, S. and HARDY, J. 1997. Food materialsadhesion: A review. Crit. Rev. Food Sci. Nutr. 37, 591–619.

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adhesion of food powders with nonelectrostatic and electrostatic coating

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