coating materials and application methods

ELECTROSTATIC SPRAY PROCESSES

BY JOEL RUPP, ERIC GUFFEY, AND GARY JACOBSEN

TW RANSBURG ELECTROSTATIC SYSTEMS, TOLEDO, OHIO

PRINCIPLES OF ELECTROSTATICS

Electrostatic Theory

Electrostatic finishing got its start in the early 1950s. Coatings engineers needed anapplication method that would significantly increase transfer efficiency and reducefinishing costs. They reasoned that particles and objects with like charges repel eachother, and objects with unlike charges attract each other. The same would apply tocharged spray coatings and a part to be painted. They discovered that by negativelycharging the atomized paint particles and positively charging the workpiece to becoated (or making it a neutral ground), an electrostatic field would be created thatwould pull paint particles to the workpiece. (See Fig.1.)With a typical electrostatic spray gun, a charging electrode is located at the tip ofthe atomizer. The electrode receives an electrical charge from a power supply. Thepaint is atomized as it exits past the electrode, and the paint particles become ionized(pick up additional electrons to become negatively charged)An electrostatic field is created between the electrode and the grounded workpiece.The negatively charged paint particles are attracted to the neutral ground. As the particlesdeposit on the work piece, the charge dissipates and returns to the power supplythrough the ground, thus completing the electrical circuit. This process accounts forthe high transfer efficiency. Most of the atomized coating will end up on the part.The degree to which electrostatic force influences the path of paint particlesdepends on how big they are, how fast they move, and other forces within the spraybooth such as gravity and air currents. Large particles sprayed at high speeds havegreat momentum, reducing the influence of the electrostatic force. A particle’sdirectional force inertia can be greater than the electrostatic field. Increased particlemomentum can be advantageous when painting a complicated surface, because themomentum can overcome the Faraday cage effect — the tendency for charged paintparticles to deposit only around the entrance of a cavity. (See Fig. 2.)On the other hand, small paint particles sprayed at low velocities have low momentum,allowing the electrostatic force to take over and attract the paint onto the workpiece.This condition is acceptable for simple surfaces but is highly susceptible toFaraday cage problems. An electrostatic system should balance paint particle velocityand electrostatic voltage to optimize coating transfer efficiency.

Electrostatic Advantages

The main benefit offered by an electrostatic painting system is transfer efficiency. Incertain applications electrostatic bells can achieve a high transfer efficiency exceeding90%. This high efficiency translates into significant cost savings due to reducedoverspray. A phenomenon of electrostatic finishing known as “wrap” causes somepaint particles that go past this workpiece to be attracted to the back of the piece,further increasing transfer efficiency.Increased transfer efficiency also reduces VOC emissions and lowers hazardouswaste disposal costs. Spray booth cleanup and maintenance are reduced.

Coating Application

Any material that can be atomized can accept an electrostatic charge. Low-, medium- and high-solids solvent borne coatings, enamels,lacquers,and two-component coatingscan be applied electrostatically.The various types of electrostatic systems can apply coatings regardless of their conductivity.Waterborne and metallic coatings can be highly conductive. Solvent-bornecoatings tend to be nonconductive. Any metallic coatings can contain conductive metalparticles. These metallic coatings mustbe kept in circulation to prevent a shortcircuit in the feed line. As high voltageis introduced into the system, the metalparticles can line up to form a conductivepath.System modifications may berequired because of coating conductivityto prevent the charge from shorting toground. (See Fig. 3.)

Operating Electrostatics Safely

Electrostatic finishing is safe if theequipment is maintained properlyand safety procedures are followed.All items in the work area must begrounded, including the spray booth,conveyor, parts hangers, applicationequipment (unless using conductive/waterborne coatings), and the sprayoperator.As electrical charges come in contact with ungrounded components, the chargescan be absorbed and stored. This is known as acapacitive charge buildup. Eventually,enough charge is built up so that when the ungrounded item comes within sparkingdistance of a ground, it cand is charge as a spark. Such a spark may have enoughenergy to ignite the flammable vapors and mists that are present in the spray area.An ungrounded worker will not know that the capacitive charge has been absorbeduntil it is too late. Workers should never wear rubber- or cork-soled shoes, which canturn then into ungrounded capacitors. Special shoe-grounding devices are available.If workers are using hand-held guns, they should grasp them with bare hands orwith gloves with cut-outs for fingertips and palms that allow adequate skin contact.Proper grounding of all equipment that is not used for the high-voltage processis essential. Grounding straps should be attached to equipment and connected to aknown ground. A quick inspection of all equipment, including conveyors and parthangers, can reveal improper grounding.Good housekeeping can pay dividends. Removing paint buildup from parts hangerscan help ensure that workpieces are grounded. Ungrounded objects, such as toolsand containers, should be removed from the finishing area.

PAINT PARTICLE CHARGING

Electrostatic charging of paint particle got its start back in the early 1950s. Engineerswere looking for methods to reduce the cost of finishing products. Harold Ransburg,the inventor of the electrostatic process, reasoned that since unlike electrical chargesare attracted to each other, the same idea would apply to charged paint particles anda part to be painted.Everyone’s heard the saying that “opposites attract, and likes repel.” This is truewith both a magnetic field and with the electrostatic process of charging paintparticles. The electrostaticprocess is almost identicalto the way a commonmagnet works. Bycreating an electrostaticfield between a negativelycharged paintparticle and a positivegrounded workpiece,the paint particles areattracted and depositthemselves onto theworkpiece. The basicbuilding block ofelectrical energy is thecharged particle. Allmatter is made fromelectrically chargedparticles. These particlesare either neutral, negative, or positive.Back in the early days of particle charging, a process referred to as the Number OneProcess was developed by Harold Ransburg to charge paint particles. Paint particleswere sprayed into an electrostatic field by conventional air spray guns. Two wire gridswere aligned parallel to each other at a certain distance, then the parts were conveyedthrough these grids. At one end of the grids, atomized paint particles were sprayedinto the electrostatic field. The paint particles would become negatively charged andwould be attracted to the positively grounded parts.These wire grids are now the wire electrode in an electrostatic spray gun. Thethree most common ways of charging paint particles are the electrostatic spray gun,a rotary bell, or a rotary disk.All three of these methods work by the same common principle of the electrostaticfield between the atomizer and the workpiece then introduce atomized paintparticles into the field and they will be attracted to and deposit themselves on thepositive grounded workpiece.With an air spray or an HVLP electrostatic spray gun, a high voltage DC charge issupplied to the applicator’s nozzle electrode, creating an electrostatic field betweenthe gun and the grounded target object. (See Fig. 4.) The coating materials arecharged at the point of atomization. The charged paint particles are attracted to anddeposited on the grounded target object. This electrostatic charge allows a more efficient,uniform application of the coating material to the front, edges, sides, and backof the product. The electrostatic forces allow for a high percentage of the chargedpaint particles to be deposited on the workpiece.The electrostatic process can also be used to charge paint particles using airlessand air-assisted airless electrostatic spray guns.The only difference is the coatingmaterial is atomized by different methods. An air spray or HVLP electrostatic gunutilizes much lower air pressure to atomize the coating material, the airless and airassistedairless methods use a much higher pressure. Coating material is delivered athigh pressure to the atomizer. There, the material is atomized by passing through avery small orifice under high pressure. The resulting spray mist particles then becomeelectrostatically charged and are attracted to the workpiece in the same manner aselectrostatic air spray or electrostatic HVLP.Today, rotary bells are generally about 1 to 3 in. in diameter and rotary disks areabout 6 to 12 in. These atomizers operate on the same principle except they are posi- tioned differently to the workpiece.Bells are positioned with their axis horizontal tothe part, and disks are positioned vertically.A rotating disk or bell distributes a thin, even coating to the edge of the atomizer.There the coating is atomized either by the electrostatic force or centrifugal force. Alow speed rotary atomizer utilizes almost all electrostatic forces, a high speed rotaryatomizer relies on the centrifugal force of the atomizer to atomize the coating material.A DC high voltage charge is then supplied to the rotating atomizer, creating anelectrostaticfield between it and the grounded target object. The negatively chargedpaint particles are attracted to and deposited on the positive grounded workpiece.The forces between the charged particles and the grounded target are sufficientto turn normal overspray around and deposit it on the back surface of the target;therefore, a very high percentage of the paint particles are deposited on the part.Paint resistivity, often referred to as conductivity, is critical when spraying materialselectrostatically. Waterborne materials are very conductive; therefore, measuressuch as voltage blocking devices,external charging probes, or completely isolating thefluid supply and fluid lines must be taken or the paint particles will not be able tomaintain the electrostatic charge. Due to the low resistance of waterborne materials,all of the electrostatic voltage will drain off to ground and short out the system. Ifone of the three previous methods mentioned are not used, the paint particles cannotbe charged electrostatically. Solvent-borne materials paint resistivity will vary from one material to another.When spraying solvent-borne coatings with electrostatics,it is critical to measure andmonitor the resistivity of the paint being sprayed. Materials that are too conductive,(very low resistance,often referred as “hot”) will also drain some or all of the electrostaticvoltage off to ground. This will greatly reduce the electrostatic effects on thepaint particle. On the other hand, when using materials with a very high resistance,often referred to as “dead,” the paint particles will not readily accept the electrostaticcharge and the transfer efficiency will be very poor.Coating suppliers can easily formulate their solvent-borne materials tobe withina specific resistivity range. The optimum resistivity may differ depending on the toolused for application. For example, with an electrostatic disk or bell, the optimumresistivity range is between 0.05 and 1 megohms on a (Ransburg) paint resistivitymeter. An electrostatic spray gun however, can effectively spray coating materialsbetween 0.1 to 00 megohms of resistance.Another example is the No. 2 Process on-site electrostatic spray gun.This gunrequires a more precise paint resistivity because it relies solely on the electrostaticcharge to atomize the coating materials. The paint used with this gun must readbetween 0.1 to 1 megohms on the (Ransburg) paint test meter to work properly.Another key element in theelectrostatic process or charging of paint particlesis particle size. Large particles sprayed at high speed have greater momentum andreduce the influence of the electrostatic force.Increased particle size and momentumcan be an advantage when coating complicated surfaces because the momentum canovercome the Faraday cage areas (where paint particles are attracted to the edges ofa work piece while avoiding inside corners and recessed areas).On the other hand, small paint particles sprayed at low velocities have lowmomentum, thus allowing the electrostatic force to take over and attract thecoating material to the target object. This condition is acceptable for simplesurfaces but is highly susceptible to Faraday cage problems.

ELECTROSTATIC PROCESSES/EQUIPMENT

The electrostatic application of atomized materials was developed to enhance finishquality and improve transfer efficiency. (See Fig.5.)Presently, there are seven types of electrostatic processes for spray application:

Electrostatic air spray atomization

Electrostatic high-volume, low-pressure (HVLP) atomization

Electrostatic airless atomization

Electrostatic air-assisted airless atomization

Electrostatic electrical atomization

Electrostatic rotary-type bell atomization

Electrostatic rotary-type disk atomization

Regardless of the electrostatic finishing systems, each has its advantages andlimitations. What may be suitable for one situation may not be suitable in another.(See Table I.)

Electrostatic Air Spray Atomization

Electrostatic air spray uses an air cap with small precision openings that allowscompressed air to be directed into the paint for optimum atomization. Electrostaticair spray is the most widely used type of atomization in the industry today due to itscontrol and versatility. Electrostatic air spray provides very high transfer efficiencyby utilizing the electrostatic charge to wrap paint around edges and capture overspraythat would have been unusable waste. Standard electrostatic air spray providestransfer efficiencies in the 40 to 90% range depending on the type of material andapplication.

Electrostatic HVLP Spray Atomization

Electrostatic HVLP spray utilizes the same atomization characteristics as electrostaticair spray technology with slight modifications. When using air HVLP, the pressureof the compressed air at the air cap must be reduced to a range of 0.1 to 10 psi.Transfer efficiency is greater when using HVLP spray to lower the particle velocityand atomize the material thus causing less waste and blow-by of material. Someelectrostatic equipment can be easily converted or transformed between air sprayand HVLP spray technology by simply changing four parts. HVLP spray technologyhelps meet stringent EPA codes requiring reduced VOCs and waste. ElectrostaticHVLP spray provides transfer efficiencies in the 60 to 90% range depending on thetype of material and application.

Electrostatic Airless Spray Atomization

Electrostatic airless spray technology utilizes the principle of fluid at high pressures(500-5,000 psi) atomizing through a very small fluid nozzle orifice. Size and shapeof the spray pattern along with fluid quality is controlled by the nozzle orifice.Airless spray technology evolved after air spray to aid in faster application ratesusing higher delivery and heavier viscosities on larger parts.

Electrostatic Air-Assisted Airless Atomization

Electrostatic air-assisted airless spray technology uses the airless spray principleto atomize the fluid at reduced fluid pressure with assisted atomizing air to aid inreducing pattern tailing and affect pattern shape. Air-assisted airless spray technologyoffers some of the desirable characteristics of both airless spray and air spray. The, desirable characteristics being medium to high delivery rates, ability to spray heavyviscosities at low velocities, and high transfer efficiency.

Electrostatic Electrical Atomization

Electrostatic electrical atomization is accomplished by using a rotary bell on the endof a gun to evenly dispense paint to the edge of the bell. Once the coating materialreaches the edge of the bell it is introduced to an electrical charge. The electricalcharge at the sharp edge (approximately 100 kV) causes paint of a medium electricalresistance range (0.1 to 1 megohms) to disperse onto the product. The pure electricalapplication is a slightly slower process than an air spray or air-assisted airless technologyand requires a rotational type spray paint technique, due to the bells spraypattern, but is the most transfer efficient spray gun process in the industry today.The ultrasoft forward velocity of the spray pattern achieves transfer efficiencies ofnearly 100% on most products. This high transfer efficiency spawned the industryof painting and refurbishing machinery and furniture in place.

Electrostatic Rotary-Bell-Type Atomization

An electrostatic bell atomizer is a high-speed rotary bell that uses centrifugal forceas well as electrical atomization to atomize material and efficiently transfer materialfrom the bell edge to the target being painted. (See Fig. 6.) The bell is used ona turbine motor where the pattern is carefully directed by the use of compressedair,introduced to the pattern at the edge of the bell cup. The compressed air gives thematerial forward velocity to aid in penetrating recessed areas. The bells are usuallymounted stationary or reciprocated to coat products on straight line conveyors. Thebells may also be positioned on both sides of the conveyor. Rotary-bell-type atomizationprovides transfer efficiencies in the 70 to 95% range.

Electrostatic Rotary-Disk-Type Atomization

An electrostatic rotary-disk atomizer is a high-speed flat rotary atomizer that usescentrifugal force along with electrical atomization to atomize coating material andefficiently transfer the material from the disk edge to the target being painted. Thedisk is used in an enclosed omega shape loop (see Fig. 7) to coat the product. Disksmay be mounted stationary and tilted (up to 45°) to coat small parts of 12 in. or less,or mounted on reciprocating arms to coat parts up to 40 ft. in height but generally nowider than 4 ft.in width. The disk produces transfer efficiencies in the 80 to 95% range.

WATERBORNE ELECTROSTATICS

Over the last several years, government regulations on VOC emissions coming frompaint application facilities, have fueled the need for coating manufacturers to reducethe amount of VOC from their coating materials. Waterborne coatings have beenaround for many years,but due to tougher government regulations they are rapidlygaining more and more momentum in today’s finishing industry. Many of currentusers of solvent borne coatings will be forced to make the switch to a more compliantcoating in the future. And many of these manufacturers, in an effort to utilizeas muchof their existing finishing equipment possible, will make the move towaterborne coatings.Although the application of these waterborne coatings is basically the same aswith solvent borne coatings, many factors must be taken into consideration. Aremy system’s componentscompatible with waterborne materials? Many alloys andmetals will rust and corrode over time when coming in contact with waterbornematerials; therefore, you must ensure that all components such as pumps, valves,piping and the atomizer itself are constructed of materials compatible with waterbornecoatings such as 316 stainless steel or Teflon.A decision must be made as to how the system will be isolated from high voltagegrounding out back through the to waterborne fluid supply. Water is a good conductorof electricity, and all components that come in contact with the waterborne materialwill be at high voltage. This includes all atomizers, fluid supply hoses, pumps,regulators, valves,and the fluid supply itself.In today’s finishing environment waterborne materials must be safely isolated.This is accomplished by: (1) complete system isolation; (2) voltage blocking device;or (3) indirect charging of the coating material.

Complete System Isolation

Complete system isolation is the most commonly used method of isolating highvoltage from the waterborne fluid supply. This low-tech approach has been aroundfor decades. (See Fig. 3.)In an isolated system, any components that come in contact with the waterbornematerial must be kept isolated from any possible grounds.The fluid supply must beenclosed in a caged area with the supply bucket, drum, or tote on an isolation stand.The gates to these cages must be equipped with safety interlocks. When an operatoropens the gate to enter the cage, a pneumatically operated ground rod must shortthe systems’ high voltage to ground. This ensures that the operator will not comein contact with a charged waterborne fluid supply. In addition, one of the isolationstand’s legs should have a 1,050 megohm bleed resistor installed inside it andattached to earth ground so that when the high voltage is turned off the voltage canbleed off to ground in a timely manner.Despite the fact that these properly confirmed waterborne systems may havesafety interlocks and bleed resistors, never assume that all of the high voltage has beendischarged to ground. Before approaching any of the wetted systems components,always take a secondary ground wire and touch it to all system components to makesure that the system is fully discharged. Failure to do so could result in a painfulshock to the operator.Failure to keep the entire system properly isolated from ground can result in a shorting condition. This can potentially short some or all of the high voltage toground. This can greatly reduce the electrostatic affect which can lead to poor transferefficiency. Example: A fluid supply hose, of a fluid supply container too close toground, can short the system out completely or create a high load (high microampreading) on the power supply which in turn lowers the actual voltage at your applicator.This can significantly reduce transfer efficiency.In addition to keeping all the equipment isolated, the cages (fluid supply) mustbe kept relatively close to the application equipment.This can result in a significantamount of lost floor space. In many occasions, the amount of floor space it takes toenclose the fluid supply may not be available. In many installations, floor space isextremely valuable and cannot be afforded when lost.

External Charging (Indirect Charging of Material)

External charging of waterborne coatings allows the fluid supply to remain grounded.The fluid supply area can remain the same as it was configured for a solvent based coating.Since the paint particles are charged externally, or as some say “indirect,” the highvoltage does not follow the conductive path through the fluid lines back to ground.The indirect charge of the material is accomplished by placing a probe, whichis at high voltage, a few inches away from the gun electrode.This probe creates theelectrostatic field to charge the paint particles without coming in direct contact withthe waterborne material. Thus, thehigh voltage does not follow the conductive path back through the fluid lines.

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Fig. 7. Typical bell-type installation

With automatic applicators such a rotary atomizers, a ring of probes (6-8) isplaced around the applicator a few inches back and away from the rotary bell. Thisconfiguration is often referred to as a “Copes” ring. Many U.S. automotive assemblyplants have switched to waterborne basecoats and the Copes bells have become widelyaccepted in the automotive market. Utilizing Copes technology, color changes in theten-second range can still be achieved.Unfortunately, of the three common methods of spraying waterbornes electrostatically,the external or indirect charging method is the least efficient. Voltageblocks and isolated systems have been proven to provide higher transfer efficiencies.

Voltage Blocks

In recent years, the application of waterborne coatings has become simpler and saferwith the development of voltage blocking devices.Voltage blocking devices isolate the spray applicators from the grounded fluidsupply. This prevents the high voltage from following the conductive path throughthe fluid lines back to the ground fluid supply and grounding (shorting) out thesystem high voltage.These devices can be used to feed both manual and automatic spray applicators.In a handgun situation, only one applicator can be fed from a single voltage blockingdevice. Where as with an automatic applicator the voltage blocking device canfeed multiple applications.This is due to the fact that any and all applicators willbe charged back through their fluid lines when connected to one blocking device.Voltage blocking devices eliminate the need for safety cages and interlocks andprotect the operator from coming in contact with a charged fluid supply. Thiseliminates the need for isolation stands and the isolation of the fluid supply fromground. It is now a grounded fluid supply. This can lead to a significant amount ofsavings in floor space.

Summary

Of the three methods discussed for spraying waterbornes electrostatically all havetheir advantages and disadvantages. The end user must decide as to which methodis best suited for their application. Voltage blocks are the simplest and can be usedwith any type of fluid supply, but up front cash can sometimes be a factor in themind of the decision maker.Isolated systems can be cheaper on most occasions, but can also take up a lot ofvaluable floor space. Isolated systems are also the least safe and may be impracticalwhen your fluid supply is a remote paint kitchen.Although indirect charging may be the least efficient of the three methods discussed,it may be the most practical in some applications. For example, in automotiveassembly plants where a large paint kitchen is involved or extremely fast colorchanges are necessary. Voltage blocks and isolated systems have been proven toprovide higher transfer efficiencies.

ELECTROSTATIC PROCESS FOR PLASTICS

& OTHER NONCONDUCTIVE SUBSTRATES

The ideal application for the use of electrostatics is metal because the only thing thatneeds to be done to spray electrostatically is to connect a ground wire to the product;however, when you try to electrostatically spray a nonconductive substrate, such asplastics, it must be made conductive. There are several ways of making the part beingcoated or the application conductive. The most common of these being:

1. Build a bracket of grounded metal and place the nonconductive part betweenthe applicator and the conductive fixture.(The charged particles will see the groundand be drawn to the part being coated. Examples for utilizing this method of technologywould be the coating of fabrics, paper or other thin structures.)

2. Certain materials become conductive with heat. Materials such as glass, rubberproducts, and some plastics may be heated until they are conductive and electrostaticallysprayed while warm.

3. All nonconductors, such as wood, rubber, plastic and glass, may also be treatedwith chemical sensitizers. These are generally hydroscopic chemicals that attractmoisture onto the surface of the product to create conductivity. Controlled concentratesof the sensitizer may be applied by dipping, wiping, spraying or a mist chamber.After treatment, the part becomes conductive when exposed to adequate humiditysuch as a humidity chamber or high ambient humidity (70% relative humidity).Sensitizers are non film-forming liquids.

4. Another method of making the part conductive is by using a conductive primer.The conductive primer can be applied to the substrate by conventional means, thusallowing the top coat to be applied electrostatically. Conductive primers may besprayed, dip coated, flow coated, or molded in.The reason for making nonconductive parts more acceptable to an electrostaticcharge is to utilize the most efficient process with the highest quality finish at themost minimal cost. By utilizing the electrostatic process, you will achieve each ofthese benefits.

COST SAVINGS

Transfer Efficiency/Paint Savings

The cost savings associated with the use of electrostatic equipment can be realized inmany different areas. The most obvious savings is in paint usage. With the increase of high-solids,plural-component, and base/clear finishes, it is not unrealistic to pay$100 per gallon for these coatings. Considering this cost, it is crucial that the coatingis applied to the product as efficiently as possible. With a conventional air spray gun,roughly 15 to 40% of the paint sprayed from the gun is applied on the part. This isknown as transfer efficiency. The remaining 60 to 85% is lost in the filters or left asoverspray on the floor and walls. Conventional HVLP guns are more efficient thanconventional air spray guns. HVLP guns will typically yield transfer efficiencies of30-60%.Electrostatic guns can obtain even greater transfer efficiency. An electrostatic airspray gun is normally in the 40 to 80% transfer efficiency range. This means you cancoat twice as many parts with an electrostatic air spray gun, compared to a nonelectrostaticair spray gun given the same quantity of paint. As with nonelectrostaticguns, HVLP technology shows significant improvement in transfer efficiency.Thesame holds true with electrostatic HVLP technology as well. In some cases, electrostaticHVLP has obtained efficiencies as high as 90%.Typically, cost justification is obtained from paint cost savings alone. Its typicallyenough to cost justify the purchase of the electrostatic applicator. Table II displaysthe dollar figure in paint savings that can be achieved by slightly increasing transferefficiency.

VOC Reduction

Another savings area is emission reduction. With federal and local regulations becomingtougher by the day, VOC (volatile organic compound) emissions has become amajor issue. We are constantly trying o reduce the amount of VOCs emitted into theatmosphere. By increasing transfer efficiency you lower VOC emission. (See Fig. 8.)This is a result of more paint being applied on the part and less paint being deposited into the booth filters or atmosphere.