coating materials and application methods

CONVERTING TO WATERBORNES

BY RONALD KONIECZYNSKI

NORDSON CORP., AMHERST, OHIO

Just as government regulations concerning gasoline mileage in the ‘70s helpedspur today’s improved automobile engines, so government regulations concerningVOC emissions from coating lines have spurred the development of someremarkable waterborne and water-based coating materials. Now manufacturersneed to know how to convert their operations to use these new materialsefficiently. This means that they need to know what application equipment issuitable and how best to use it to apply waterbornes.A likely first question is “What is the best way to apply a waterborne material?”The answer is “There is no best way.” The process that works best in a particularapplicationwith a solvent-based coating will usually continue to be the best afterconverting the application to a waterborne coating. The dynamics of gettingmaterial from the applicator to the part are similar whether the material containsmostly solvents or mostly water.

ATOMIZER SELECTION

Most coating material applicators break the material up into fine droplets orparticles, which are then carried through the air to the part being coated. Theprocess of breaking up the material is called atomization, and the equipmentthat does the atomization is called an atomizer. Typical atomizers are air sprayguns, rotary atomizers, and disks. None of these is “best” for all waterborneapplications. Instead, the shape of the part being coated, the coverage required,and the production rate determine the best atomizer for a particular applicatiousing waterbornes, just as they do with solvent-based materials.To illustrate the importance of part configuration on atomizer selection,consider a simple box-like part, open on one end, requiring paint on both theinside and outside surfaces. The outside of the box might best be paintedwith asoft spray using electrostatics to get good part coverage, high transfer efficiency(TE), and good wrap. A rotary atomizer with electrostatics would be a goodchoice for the outside of the box.The rotary atomizer and electrostatics would be a poor choice, however, for

the inside of the same box because the Faraday cage effect caused by the electrostaticsand the walls of the box would actually keep most of the paint out of thebox. A better choice for the inside of the box would be an airless spray atomizer.Airless spray uses the momentum of the paint particles to get the paint to thepart, rather than electrostatic attraction.The point is that a manufacturer who has spent a lot of time perfecting thecoating application process for solvent-based material should stick with thatprocess when converting to waterbornes, if the latest technology and goodequipment are already in place and good TE is being obtained.Sometimes a particular waterborne coating formulation may need to bemodified slightly to accommodate the atomizer. For example, an emulsionmay tend to separate when subjected to severe centrifugal force on a spinningrotary atomizer cup.Does this mean that you don’t have to change anything in order to convertto waterbornes? No, it doesn’t. Even though the basic application process maynot change, some of the specific pieces of equipment used for that processmay not be suitable for waterborne materials. Metal parts may corrode. Sealsmay swell or leak.

CONSTRUCTION MATERIALS

Waterbornes rust plain steel and in some cases attack aluminum. Even stainlesssteel parts can be damaged by some formulations. For example, 400 seriesstainless steel can dissolve over time in contact with a highly acidic formulation.On the other hand, parts made from 316 stainless steel hold up well withmost waterbornes.This means that at least some of the application equipment will need to bereplaced when a system is converted to waterbornes. Piston pumps made of plainor alloy steel have to be replaced with pumps made of stainless steel. Pipes anddistribution systems need to be made of corrosion-resistant materials such asstainless steel. Atomizers should contain only stainless steel or plastic wettedparts. Parts made from aluminum will perform satisfactorily for some waterbornematerials, but will corrode quickly in contact with others. Some waterborneformulations can even become “explosive” in contact with aluminum.Seals in atomizers and pumps need to be changed if they are not compatiblewith the waterborne material. There is no single best seal material for waterbornesbecause the formulations vary so much. In some cases, the seals in equipmentused for solvent-based coating materials are also suitable for waterbornes.For example, Buna-N is suitable for some solvent-based paints and is also a goodchoice for many waterbornes.One caution about reusing equipment from a solvent-based coating operationfor waterbornes, a surprising amount of “dirt” from the old coating material canturn up in the new coating material after the conversion to waterbornes. A fewfilters in the fluid lines can prevent a lot of downtime due to plugged nozzlesand orifices.As with the coating application process and most of the equipment, the physicalplant does not necessarily need to change in order to convert to waterbornes.Often the formulation of the waterborne material can be tweaked a little toaccommodate the facility. For example, the drying time for a waterborne primermay need to be adjusted for the time available before the color coat is applied.The TE can drop after converting to waterbornes even though the applicationprocess is the same and much of the equipment is unchanged. This is especiallytrue if the application process includes atomizing the material.

ELECTROSTATICS

All spray guns and centrifugal applicators, like rotaries and disks, atomize the coatingmaterials and propel the atomized particles toward the part being coated. Withthese devices, all the particles that are not aimed directly at the part will miss it andbe wasted. The waste can be minimized and TE improved if the atomized coatingmaterial is given an electrical charge that is opposite in polarity to the charge onthe part being coated. Opposite electrical charges attract and some of the materialthat would miss the part entirely instead gets drawn to it by the electrical forces.The technique is called electrostatics and has been used for years by painters andcoating applicators to improve the TE of their operations.Until recently, most coating materials were solvent based and did not conductelectricity readily. This made it easy to use electrostatics with these materials bysimply placing a high-voltage electrode in the coating material at the atomizernozzle. The coating material picks up a static charge of electricity as it is atomized.Waterborne coating materials conduct electricity much more readily thansolvent-based materials, however, and the electrical charge drains off downthe paint hoses. None of the charge gets into the particles of atomized coatingmaterial, so there is no electrostatic attraction between the particles and thepart being coated. Without electrostatics, TE drops to unacceptably low levels.The challenge in converting to waterbornes is getting comparable TE withthese conductive materials as with the nonconductive solvent-based materialsthey replace. This means finding a way to get the electrostatic voltage into theatomized particles rather than letting it drain away through hoses and equipmentmade conductive by the waterborne coating material. Any equipment thatcontacts an electrical ground, such as a pipe or a damp concrete floor, providesan electrical pathway that drains off the electrostatics. This means that the electrostaticswon’t work in a waterborne system unless all the wetted equipment isisolated from potential grounds.

SYSTEM ISOLATION

Waterborne systems are commonly isolated in one of three ways. (1) Completeisolation of all equipment that contacts wet coating material. (2) Isolation ofthe charging electrode from the wet coating material. (3) Isolation of only theatomizer and its feed hose by using a voltage blocking device. Each method hasadvantages and disadvantages.

Complete Isolation

The advantage of completely isolating the entire application system is that thecoating material can be directly charged with electrostatic voltage. If isolation issuccessful, the resulting TE will be the highest possible for the specific application.The exact TE that can be achieved in a specific application depends on thepart geometry, line speed, application equipment, and other factors, the sameas it would with a solvent-based coating material.To isolate a complete waterborne system, every pump, tank, pipe, atomizer, orother piece of equipment that sees wet coating material must be set on a plastictable or hung from a plastic rod or stuffed in a plastic pipe sleeve. Suitable commonand inexpensive plastic materials for this purpose include polyvinyl chloride(PVC), polyethylene, and polypropylene. Teflon, some nylons, and Delrinare also good isolation materials for high voltage, but are relatively expensive.Dry air is one of the best isolation materials. A 12-in. air gap will isolate equipmentcharged with electrostatic voltage, except in cases of extreme humidity. Airhas some advantages over plastic as an isolator for an electrostatic system. A paintspill down a plastic table leg can make it conductive. Humidity in a thick coating ofdust on a plastic pipe can make it conductive. The disadvantage of air as an isolatoris that it is an easily penetrated barrier between a charged part and a groundedpart — too easily penetrated by personnel or by loose hoses or other equipment.Unfortunately, it’s almost impossible to design an isolated system that won’taccidentally ground out, and a system that grounds out is less efficient. In onecase, for example, an 18-in. long, hollow PVC table leg provided a direct path toground because it was set on a concrete floor and humidity from the concretemade the inside of the leg conductive. That particular short took a full week oftroubleshooting to find and correct.Besides being inefficient, isolated systems can be dangerous because theycan store too much electrical energy. All the equipment that gets wetted withelectrically charged coating material stores electrical energy, much like a giantcapacitor. All that stored energy gets discharged if the system gets shorted out. Ifthe system is big enough, and stores enough electrical energy, an operator can getinjured by shorting it out accidentally by touching a charged hose or atomizer.It is impossible to draw a definite line that says, “A system this small is safe,and a system that big is dangerous.” Trying to define a safe electrical shock islike trying to define a safe height from which to fall. For example, a shock itselfmight only be annoying, but the victim might be so startled by it that it resultsin a bump on the head or an injury in some other way. Although a safe system,with regard to storage of electrical energy, may be a contradiction in terms,some guidance regarding the size of a “probably unsafe” system would be useful.Unfortunately, no regulations directly applicable to electrically chargedwaterborne systems are available.By making some assumptions about the meager data that is published,extrapolating to the 70,000\100,000 V range used for electrostatics, and pluggingthe resulting voltage and capacitance values into the standard equation forstoring electrical energy in a capacitor, the following can be developed:Maximum\Energy = 3.5 Joules = CV2where C=Capacitance (farads)Rearranging: CMAX =7/V2where the voltage is the maximum available from the power pack.This equation can be used as an indicator of the potential for a given isolatedsystem to pose a serious shockhazard. The capacitance of the system, as measuredwith a capacitance bridge or a suitable capacitance meter, must be lessthan the value of CMAX if there is a possibility of accidental human contact, whichcould result in an electrical shock. For example, if a 100,000-V electrostaticpaint system has a capacitance G700 picofarads, caging and interlocks shouldbe considered for operator protection.For comparison purposes, a single 55-gal. drum and 200 ft of 3/8-in. innerdiameter hose, all set 12 in. above a ground, can have between 450 and 900 picofaradsof capacitance. This means that a typical paint system, which has muchmore hardware, would almost certainly exceed the maximum capacitance valueand could store potentially dangerous levels of electrical energy.The storage of electrical energy can be reduced by lowering the electrostatic voltage.The voltage term is squared in the equation for energy storage in a capacitor.This means that a given system at 100,000 V will store four times the energy that itwould at 50,000 V. At the lower voltage, not only will the system be safer, but gunsand cables will last much longer before breaking down electrically. Perhaps an evenmore compelling reason for lowering the voltage is to maximize TE. The maximumTE for most waterborne coating materials occurs between 40,000 and 60,000 V. Bycomparison, the maximum TE for a less conductive solvent-based material can be90,000 V or more. Handguns present a special problem when a coating applicationsystem is converted from solvent-based materials to waterbornes.An isolated electrostatic system for waterbornes can have multiple automaticatomizers or it can have a single handgun. It cannot have both, nor canit have more than one handgun. National Fire Prevention Association (NFPA)regulations dictate that the electrostatic voltage to any handgun must turn offwhen the trigger is released. Since all the atomizers in a waterborne system areconnected electrically by their fluid hoses, the voltage remains “on” to an idlehandgun as long as it is “on” to any atomizer in the system. This means that ahandgun cannot be used with electrostatics if there are other atomizers in thesystem, and without electrostatics it is impossible to achieve the maximum TE.To summarize, completely isolated systems have the potential to allow themaximum TE for a given application because they allow the coating materialto be directly charged with electrostatics. In practice, that potential is rarelyachieved unless the application system is very small because it is difficult to keepthe electrostatic charge isolated.Fully isolated systems can store too much electrical energy and become dangerous.To prevent operator injury, such systems need to be caged and equippedwith interlocks to prevent access while the system is operating. Unfortunately,this means that even minor maintenance to the equipment is impossible whileany part of the system is operating at high voltage because all the equipment iselectrically connected by the fluid hoses. This is also why only one handgun canbe permitted in a completely isolated system.

Indirect Charging

Indirect charging avoids many of the problems of completely isolated systems,but at a price. Indirect charging systems charge the coating material between thenozzle of the atomizer and the part, rather than at the atomizer. This is done byplacing the high-voltage electrode in the air stream near the nozzle but not indirect contact.The coating material particles pick up a charge after they leave theatomizer.Because the high voltage never directly contacts the application equipment,there is no opportunity for the charge to drain away down paint hoses. On theother hand, any charge inadvertently imparted to the application system drainsaway harmlessly to ground because the system is not isolated from ground. Infact these systems can, and should, be intentionally grounded to prevent storageof electrical energy.There is little capacitive storage of electrical energy in an indirect chargedelectrostatic system so any electrical hazard is greatly reduced. This means thatsafety caging and interlocks can be less intrusive, or eliminated completely. Withhandguns no longer connected electrically by their hoses, there is no need to limitthe number of handguns in a particular application system.Several coating application equipment manufacturers offer atomizers speciallydesigned for indirect charging. These devices position the electrostatic electrodeaway from the coating material stream so that there is no direct electrical contactbetween the application equipment and components charged with high voltage.Some conventional atomizers can also be retrofit with indirect charging apparatus,making the conversion to waterbornes easy and relatively inexpensive.The downside of indirect charging is lower than optimum TE. Indirect chargingdoes improve TE over comparable nonelectrostatic systems. Unfortunately, testsprove that the TE with indirect charging is less than the TE that can be achieved bydirect charging in any given application — that means using the same applicators,coating material, part shape, etc.The difference can be considerable, up to 40% improvement in TE in extremecases. Even the best indirect charged systems rarely achieve TEs within 10 percentagepoints of what is possible for the same application but using direct chargingfor the electrostatics.To maximize TE, the coating material should be directly charged with electrostaticvoltage, but to minimize shock hazards and operational problems, the sizeof the charged parts of the system should be minimized. This can be achievedby using a voltage block at each atomizer. Each atomizer then becomes a miniisolatedsystem with no electrical connection to any other atomizer in the system.

Voltage Blocks

Voltage blocks are devices that allow coating material to pass through to theatomizer but prevent voltage from leaking back the other way. They allow coat-ing material to flow from the grounded pumps or kitchen to charged atomizers,yet block voltage from leaking back from the atomizers to the pumps or kitchen.This means that the hardware in the pump house and distribution system can bevirtually the same as for a conventional solvent-based system, or for an indirectcharged waterborne system.Since the primary advantage of voltage block technology is that it limits theamount of hardware at high voltage, it is important to install these devices as closeto the atomizer as possible. The connecting hoses between the voltage block andthe atomizer are at high voltage, so keeping them short minimizes both the capacitanceand the opportunity for accidental grounding. A voltage block for one atomizeris compact, requiring about as much space as a small electrical control box,so it can be mounted inside the spray or ventilation booth close to the atomizer.The mini-isolated systems created by voltage blocks do not have the problemsfound in large isolated systems because less hardware is charged with electricalenergy. Capacitance is greatly reduced, making the system inherently safer. Safersystems mean easier access to the inside of the spray booths. Often a simpleguard rail and warning sign can replace elaborate caging and interlocks. Voltageleakage problems are minimized, since only the atomizer and a short hose arecharged, making it easy to keep the TEs up to a high level. By isolating atomizersfrom each other, mini-isolated systems have some unexpected advantages. First,the NFPA limitation concerning handguns no longer applies. Each handgun isindependently isolated from every other handgun so the voltage to idled gunscan be turned “off.”In fact, spraying waterbornes with ahandgun and voltage block can be easierthan spraying the old solvent-based material with the handgun. Solvent-basedmaterial is charged at the gun barrel so a high-voltage cable to the gun is required.Since waterborne coating material conducts electricity, however, it can bedirectlycharged at the voltage block and the cable to the gun can be eliminated. With thecable gone, the gun feels lighter and the hose bundle flexes more easily.Even automatic atomizers, such as rotaries or disks, require less maintenanceif the coating material is charged at the voltage block rather than at the atomizer,as it was when spraying solvent-based material. This is because the high-voltagecables last longer when they don’t get flexed over and over by the motion of thegun mover or robot.A second unexpected advantage of making each atomizer into a mini-isolatedsystem is that every atomizer can operate at a different voltage, or at zero voltage,as desired. For example, in-plant experience might show that the rotary atomizersin a paint line run best at 60,000 V, but the handguns perform better at45,000 V. With voltage blocks, the handguns and the rotary atomizers can runat different voltages, yet all can be supplied from a common paint distributionsystem.Finally, in an application system for waterbornes and using voltage blocks,any atomizer in the system can be shut down and repaired or changed out, eventhough the other atomizers are operating at high voltage. The ease of access toproduction application equipment is comparable between a voltage-blockedwaterborne system and the solvent-based coating material system it replaces.

CONCLUSION

An often unstated goal when converting a coating application system to waterbornesis to disrupt “the way it’s done now” as little as possible, particularly ifthe existing system has good equipment and is performing well. That goal is notout of reach because the existing process, and much of the existing equipment,an often be used for waterbornes. Usually only the atomizers will need to bemodified for waterbornes, or replaced with atomizers specifically designed tohandle waterborne materials. The remaining equipment and distribution systemcan be reused unless made of materials that will corrode in waterbornes or bedamaged by exposure to them.Well-engineered conversions from solvent-based coating materials to waterbornesresult in the highest possible operating efficiency at low cost and withmaximum operator safety. The operating cost, in terms of TE, should be aboutthe same as that of a good solvent-based paint system. To get high TE, electrostaticsmust operate at peak efficiency. This means directly charging the materialwith electrostatic voltage, but limiting the hardware that gets charged.Voltage-blocking devices confine high electrostatic voltage to only the atomizerand hoses to the atomizer. This means that the rest of the coating materialapplication system can be the same or similar to the system before the conversionis made. The electrostatics will still operate at high efficiency because thecoating material can be directly charged so the TE will be comparable before andafter conversion. Because system capacitance, or the capacity to store electricalenergy, can be controlled to “safe”’ levels, safety issues with theconverted systemsare not prohibitive. In other words, a voltage-blocked waterborne system isas close as possible to the solvent-based material system it replaces with a coatingmaterial that conducts electricity.To summarize, here is how to convert an application system from solvent-basedcoating materials to waterbornes: (1) Reuse the existing process and hardware ifit is up to date and performing well for the existing solvent-based system. Changecomponents where materials are not compatible with waterbornes. (2) Turn eachatomizer into a mini-isolated system by installing a voltage block in the coatingmaterial hose, as close to the atomizer as possible. Directly charge the materialfor maximum TE. (3) Lower the voltage to maximize TE, extend equipment life,and reduce shock hazard. (4) Take advantage of the fact that waterbornes conductelectricity. Remove the electrostatic cables from the atomizers and charge at the voltage blocks. Cables will last longer and the guns will move easier.