What spray painters are doing to lessen VOCs and improve transfer efficiency...
The traditional air-atomize spray painting gun is not about to disappear from the industrial scene. There are still more of them out there than any other kind of device for spraying liquid coatings.
But one supplier predicts that this staple of our industry will be essentially eliminated in the next ten years, to be replaced by spray guns that offer higher transfer efficiency (TE). That may be wishful thinking, since he sells equipment that would be one possible replacement. Nevertheless, the status quo is not firmly in place. Not at all. Under pressure from environmental authorities to lessen smog-causing VOC emissions and from upper management to lower costs and improve efficiency, spray painters are clearly changing their equipment-buying habits.
Last October the editors of Products Finishing surveyed subscribers, asking about their spray-painting equipment. We selected survey recipients from our database of plants that perform spray painting. You may have received a survey. If you returned it, thanks for the help!
We mailed 711 surveys to subscribers randomly selected from plants all across the United States. A total of 240 finishers involved in spray painting filled in and returned the surveys. That's 34 pct, an outstanding rate of return for any mail survey, and one that provides a high level of confidence in the results.
Here's what you told us:
Type % of
Manual, non-electrostatic air-atomize guns 64 Manual, non-electrostatic airless guns 22 Manual, non-electrostatic air-assisted airless guns 17 Manual, non-electrostatic HVLP Guns 37 Electrostatic Equipment 42 Plural-Component Equipment 11 Liquefied CO2 Application 2.5 *Total of percentages is not 100 because many plants have more than one type of spray equipment
The fact that such substantial percentages of the plants have airless, air-assisted airless, HVLP and electrostatic guns shows that finishers are intent upon improving transfer efficiency and lessening VOCs. But conventional manual non-electrostatic air-atomize guns are still the most versatile, used to some degree in 64 pct of the plants, including many that have more advanced equipment.
Important? % of
Extremely 58 Moderately 27 Not 10 *Percentages do not total 100 because some respondents did not answer the question.
The overwhelming majority of these paint-line managers are very concerned about discharge of volatile organic compounds. "No kidding," you say. "Who ISN'T concerned about VOCs?" As the survey indicates, most finishers certainly recognize the VOC problem. But it's a matter of degree; if you are in a rural area fifty miles from nowhere, you may be getting less heat from EPA and local authorities; or if you paint only a few castings daily, maybe no one cares. In any case, the survey confirms what we would have predicted: control of VOCs is a key factor in selecting spray-painting equipment.
Important? % of
Extremely 50 Moderately 35 Not 8 *Percentages do not total 100 because some respondents did not answer this question.
Transfer efficiency rates the ability of spray-painting equipment to put paint on the parts being painted rather than allowing it to escape as overspray or in other forms of paint waste. If you spray 10 pounds of paint on uncoated parts and those parts gain two pounds as the result of being coated, you've achieved 20 pct TE, i.e., 20 pct of the paint ended up on the part and the rest landed on spray booth filters, booth walls, hangers, etc.
In selecting new spray-painting equipment, 85 pct of those who responded rated transfer efficiency as extremely or moderately important. Most survey respondents said cost is the reason they consider TE so important, but in some areas of the country air-pollution-control is an equal driving force. In these areas transfer efficiency percentages are mandated by air-pollution-control authorities intent upon using improved transfer efficiency to lessen VOCs. In 1989, authorities in southern California required finishers to begin using application equipment that met transfer-efficiency standards deemed possible with the more advanced spray equipment.
If conventional air-atomize spray is used, it is possible that only 20 pct of the paint becomes a coating on the surfaces being sprayed, and 80 pct is wasted. And the paint isn't the only cost associated with poor transfer efficiency. Spray-booth-cleanup costs often equal the cost of the paint. Plus the fact that you may end up with waste that is either difficult or very expensive to dispose of.
Installed % of
HVLP 38 Electrostatic 29 Paint Heaters 15 Voltage-Block 3 CO2 2 *Percentages do not total 100 because some plants have installed more than one type of equipment while others have installed no new equipment.
If you're like most paint-line managers you've already made significant changes that improve transfer efficiency and/or lessen VOC discharge as shown above. You've bought new equipment, changed to coatings that contain less VOC, or both.
Answer % of
Yes 26 Plan to in next 2 yrs 15
As the survey shows, replacement of some or all of the surveyed plants' spray-painting equipment with powder-coating equipment has been taking an ever-increasing share of the market for equipment that improves TE and lessens VOCs. Taking into account the fact that we purposely excluded from this survey all plants that apply powder coatings and not liquid coatings, this is an impressive change. The number of plants that have already installed powder coating equipment would be even higher if those that now apply powder only had been included.
Type of Coating % of
Solvent-Thinned Coatings 54 Powder Coatings 23 Waterborne Coatings 20 UV Cured Coatings 1 *Does not total 100.
Some recipients did not reply to this question.
Despite all the pressure to eliminate VOCs, more than half the spray painters who responded to this survey still use coatings thinned with organic solvents. Undoubtedly some of these are higher-solids coatings, though we did not attempt to quantify this. But it is quite evident that many plants have requirements that make it difficult or too expensive to change from their "tried and true" conventional-solvent-thinned coatings. For some it is a matter of meeting customer specifications or their own specs. Others are hesitant to make changes that might compromise product quality or appearance, while still others paint configurations that may be difficult to coat with certain types of equipment that lessen VOCs and improve transfer efficiency.
Nevertheless, as environmental regulations tighten and finishers continue to search for ways to cut costs, the trend is clearly toward alternative coatings. Indeed, 43 pct of the finishers surveyed said most of their spray coating is done with powder coatings or waterborne coatings.
Many applicators of liquid coatings who do not now use waterborne coating materials may be forced to do so in the future, as authorities require lower VOCs.
Low-VOC waterbornes offer code compliance but are more sensitive to shearing, air entrapment and settling than are solvent-thinned coatings. These characteristics can lead to degradation and inconsistency of viscosity and variations in flow rates. Finish quality suffers as a result.
With these facts in mind, you should buy equipment that helps to avoid the aforementioned problems, even if you are presently continuing to use solvent-thinned coatings. By purchasing waterborne-compatible equipment, with wetted surfaces made of stainless steel or other corrosion-resistant materials, you build in flexibility for the future.
Type of Equipment % of
Manual, Non-Electrostatic** Guns 34 HVLP Guns 25 Electrostatic Equipment 22 *Does not total 100 because some plants do not plan to buy while others have plans to buy more than one type. **Includes air-atomize, airless and air-assisted airless guns.
How many spray booths are in use in your plant?
Number of Booths % of
1 27 2-5 49 6-10 15 11 or more 7 *Does not total 100 because some respondents did not answer this question.
We asked this question to give you an idea of the sizes of the plants surveyed. A total of 117 respondents work in installations with between two and five spray booths. That's 49 pct of those who responded. Many of these finishers 64 pct to be exact are in plants that have between two and ten spray booths. Seven pct are very-high-production installations, with 11 or more spray booths.
57% of the plants have conveyors. This is another question related to the sizes of installations.
Most of the alternatives to conventional air spray guns would never have been invented had it not been for the relatively poor transfer efficiency of air-atomize spray. Two of the most common replacements electrostatic application and airless spray were commercialized long before the advent of the Environmental Protection Authority (EPA). The driving force was to reduce overspray (and cost of paint). There was certainly room for improvement, since conventional air-spray guns can waste 60-80 pct of the paint purchased.
The major types of spray equipment in use today are as follows:
These are "conventional" because they have been around since the early part of this century. They were widely used before any of the others. Basically they emit a stream of paint from a small opening in a fluid nozzle. The paint is broken up into tiny droplets by compressed air emerging from jets adjacent to the fluid nozzle. Relatively high air pressures at low volumes will quickly atomize large amounts of paint.
Despite their tendency to spray more paint on booth filters than onto parts, they remain an important tool in most painting facilities. Why? Because they are so versatile. They can spray a class A finish at high speeds on almost any surface that needs to be painted. So even if you have one of the more efficient types of spray apparatus, you probably still need a few conventional air-atomize spray guns to do what the more advanced equipment will not.
Very poor transfer efficiency. Wastes paint, increases cleanup costs, emits more VOCs.
When you think of airless spray, think of a garden hose. It sprays water under high pressure through a nozzle. When the water emerges from the nozzle its velocity causes the stream to disintegrate into droplets as it encounters resistance from the atmosphere. The airless paint gun is similar in that it pressurizes paint to 900-1200 psi (or higher) and forces it through a nozzle. Unlike conventional air spray, there are no jets of atomizing air to break up the paint and propel it to the surface. Atomization is dependent upon high fluid pressure.
A hybrid of airless spray and conventional air-atomize spray, this kind of gun uses fluid pressures higher than those used in conventional air-atomize guns but lower than those employed in normal airless spray. Unlike normal airless guns, these guns do have compressed air jets that supply atomizing air, but the air pressure is far lower than that used in conventional air-atomize guns. The result is that the coarse spray provided by the airless atomization is further broken up into a finer spray by the compressed air.
In operation, air-assisted airless guns provide atomization much better than is normal with airless spray. Some suppliers of this type of equipment claim that finish quality and productivity equal that produced by air-atomize spray. Danger of airless injection is lessened, as is wear of fluid nozzles.
The main reason for considering use of air-assisted airless spray, however, is its much better transfer efficiency. The softer spray also makes it easier to spray into recesses.
Both air-assisted and pure airless spray operate at high fluid pressures and thus can use smaller-diameter fluid lines. This translates into paint and solvent savings. The reason is that it takes less solvent to flush smaller-diameter lines. One manufacturer reports a study showing that by replacing a 25-ft, 1/4-inch-diameter hose with a 20-ft 3/16-inch-diameter hose, the reduction in paint and solvent waste was 100 gallons per year.
High-Volume Low-Pressure (HVLP) Spray
HVLP is a variation of conventional air-atomize spray. The difference is that these guns are designed to atomize paint using a high volume of air delivered at low pressure. The lower pressure results in far less overspray and "bounce-back."
Some problems in achieving proper atomization with HVLP may be caused by "starving" the spray gun for air. Causes of this problem include use of air hoses that are too long or too small in diameter; use of too many "quick-disconnect" fittings; and use of low-performance air compressors and air regulators. Any one of these factors may result in too little air being delivered to the air cap, causing poor atomization from the gun.
Some of these guns use air compressors to deliver the atomizing air, while others use a turbine. The turbine is a series of fans mounted inside a housing, designed to produce pressurized air for one or more guns. In the process of forcing the air through the turbine the fans create friction and warm the air. This helps to heat the paint and in turn lowers its viscosity, thus making it easier to atomize.
Electrostatic painting begins with a spray gun or other device (discs or bells) to atomize paint. The atomizing principle could be any of those previously discussed conventional air-atomize, HVLP, or airless.
The difference is that an electrostatic application device is equipped with a means of electrically charging the particles of paint. A common method is to build in an electrode near the point where paint is atomized. This electrode electrostatically charges the particles negatively. Parts are grounded, usually by hanging them on a conveyor securely connected to a ground. The grounded parts attract the negatively charged paint particles (opposites attract).
The result is that fewer of the paint particles are propelled into space as overspray and more are electrostatically guided to the surfaces of the parts being painted. Sprayed particles will even turn the corner and be attracted to the back side of a part if the velocity of the particles causes them to initially travel past the parts being painted. This is called "wraparound."
Transfer efficiency is greatly improved. The amount of improvement depends upon the parts being painted, the booth design, etc. Electrostatic spray is particularly beneficial in improving TE when parts with lots of open areas lacy patterns, for example are being sprayed.
Rotary atomizers utilize centrifugal force rather than compressed air or fluid pressure to atomize paint.
Discs. Imagine a spinning flat round disc with a hole in the center. Feed paint through a hose so that it overflows through the hole and onto the spinning disc. Centrifugal force propels paint over the surface of the spinning disc until it flies off the edge. The paint atomizes as it is propelled through the atmosphere. Add electrostatics and one has an electrostatic spray-painting device.
Parts on hangers travel around the periphery of the disc in an "omega loop," housed by a circular spray booth. The disc spins and simultaneously moves up and down, on a floor- or ceiling-mounted reciprocator. As the parts travel around this loop, paint particles are being propelled toward the parts and electrostatically attracted to them. Since the disc moves up and down, the entire length of a long part or racks of parts is painted.
Bells. Electrostatic bells are similar in principle, except that in this case the paint is fed through a hole at the closed end of the spinning bell-shaped atomizer. Centrifugal force propels paint from the edges of the bell. Bells may be mounted on reciprocators or on hand-held guns.
Higher-Speed Rotational Devices. The latest bells and discs utilize higher rotational speeds, producing finer atomization, the ability to apply higher-solids and waterborne coatings, and high transfer efficiency. These devices are often mounted on reciprocators in very-high-production installations. Less operator time is required in the disc application itself, so labor is conserved.
But high-speed discs and bells also may have problems in reaching into deep recesses (Faraday-Cage areas). Thus some of the labor conserved by their use may be required to hand-spray reinforce the areas of parts not properly covered by the automatic spray.
Waterborne coatings are widely used to lessen the VOC content of coating materials. To oversimplify, if water replaces some or all of the organic solvents used in paints, the resultant coating material contains less VOC.
Electrostatically applying waterborne coatings can be a problem, in that their water content increases their electrical conductivity. Unless special precautions are taken, the waterborne coating provides a conductive path from the electrostatic applicator to the grounded paint supply.
To circumvent this, manufacturers have developed increasingly sophisticated "voltage-block" systems. These electrically isolate the spray applicator and prevent high voltage from following the conductive path through the paint-supply line to the waterborne coating supply.
Switching to waterbornes is now easier because of the new technologies available in voltage-isolating systems. Waterbornes can be electrostatically applied safely, economically and in minimum floor space (newer voltage-block systems eliminate the need for isolation cages to deal with the conductivity of waterbornes).
One supplier urges finishers to look carefully at the latest technologies for electrostatic application of waterbornes. "There are several emerging technologies that make this switch more plausible and less costly," he says.
Another supplier claims that some HVLP guns have advantages in waterborne application because they supply heated atomizing air, which provides faster set-up times and improved drying of waterborne coatings.
Some coatings, principally urethanes, are supplied as two components. After being mixed, the components chemically react with one another to form a solid coating. They are often referred to as "catalyzed" since the "catalyst" causes a reaction that leads to curing of the coating. An advantage is that low temperatures are sufficient to cure the coating and thus plastic parts that cannot tolerate high temperatures can be coated. The coatings also exhibit unusual durability in certain applications and require less solvent for thinning, thus improving VOC control.
If the two components are mixed before entering a paint pump or pressure pot, the mixed material must be sprayed promptly or the reaction of the two components increases viscosity to the point where the coating is no longer sprayable. It is said to have limited "pot life."
For this reason, spray guns have been developed that bring each of the two components into the spray gun through separate feed lines. The components mix just prior to application. This remedies the "pot-life" problem, since mixing occurs only at the moment before application.
Two-component application equipment is used in some very high-production applications, but as the survey shows, it is not used to the extent of the more conventional technologies. The reason is obvious the equipment is more costly, as are the coating materials.
The most recent development in spray application equipment is built to spray coatings formulated with heated, compressed carbon dioxide. In this form carbon dioxide is a liquid and can be used as a paint thinner. When the coating is sprayed the liquefied carbon dioxide is no longer compressed and reverts to a gas. Since carbon dioxide is naturally present in the atmosphere and is not considered hazardous, it is interesting to finishers searching for alternatives that lessen VOCs. The system was developed and patented by Union Carbide under the trade name "Unicarb."
Thus far CO2 spray has been used in a few commercial applications, as the survey indicates. But this is a relatively recent development, and its use may proliferate in coming years.
When you consider combinations of the technologies for spray painting, there are additional possibilities. One can add electrostatics to almost any of the basic spray-application technologies, for example.
Such technologies as HVLP and air-assisted airless, already more efficient in raising TE, can be equipped with electrostatics to further improve TE and lessen VOCs. But you have to test any system that appears to meet your requirements in YOUR plant, while painting YOUR mix of parts.
Heat reduces the viscosity of paint. Heating paint before it is atomized makes it possible to spray more viscous paint. In some cases it is possible to use paint containing less solvent, since not as much is required to lower paint viscosity. Obviously less solvent in the coating material equals lower VOC content. Thus paint heaters are a well-established, viable means of lowering VOCs. They are probably under-utilized. If the coating formulation permits use of a paint heater, it will keep viscosity more constant, improve TE, lower wear of equipment and improve finish quality and consistency.
Manufacturers of spray equipment are constantly innovating to produce guns, bells and discs that are better able to apply waterborne and higher solids coatings. The latter require higher air and fluid pressures for proper atomization, but a careful balance must be struck to keep from defeating the purposes better transfer efficiency and lower VOCs. Hand-held guns that are lighter in weight and easier to trigger are increasingly important in preventing work-related injury.
You face an almost bewildering array of equipment and materials offered by suppliers to improve transfer efficiency and lessen VOCs. To measure TE, count the number of parts or square feet painted at a given film thickness, per gallon of paint from a pressure pot; or use a flow meter to determine how many ounces flow from your pumping device. Do that for every type of spray equipment being evaluated.
You also have to look at your specifications for film thickness, coating composition, appearance and corrosion resistance. Consider needs for color change, spraying odd shapes, speed of production, operating and materials costs, and capital-equipment budgets. Rate ergonomics, so that your spray painters are comfortable with the guns they operate and not at risk of electrical shock, fire hazard or repetitive-motion injury. And after you've done all that, you still may be forced by local or federal regulations to make some changes just to comply with your permit.
The survey results reported here give you a good idea of what others have been doing to improve TE and lessen VOCs. They've been busy switching to:
Convertible Air-Spray Guns. As the survey indicates, many of you continue to use conventional air-atomize spray guns, and you plan to replace these guns as they age. If you're concerned about EPA regulations tightening, however, you might consider conventional air-spray guns that are made to be converted to HVLP. By inexpensively changing a few components of these guns, one can move to HVLP without buying a whole new gun.
Training operators of spray equipment has always been important, but it becomes even more so when one considers how operators' techniques can influence TE. Operator training and annual re-training may be the best investment you can make. Knowledgeable operators with good spray techniques can measurably improve TE without changing equipment.
Pressures. Operators sometimes perceive that they can increase air and fluid pressures to improve speed or reduce orange peel. Had they used the right air caps and fluid tips they might have applied a smoother finish at lower fluid and air pressures. As much as 20 pct of the paint you apply can be wasted by too-high fluid and air pressures. Use the lowest pressures consistent with finish quality and productivity. It's worth some experimentation to find the optimum combination of fluid tips, air caps and pressures. Begin lowering pressures and see how far down you can go while maintaining finish quality.
Exhaust Rates. In trying to improve transfer efficiency plant engineers must set air-exhaust rates in spray booths to the lowest consistent with operator safety and comfort. Too-high rates can pull more paint onto air-exhaust filters than is being applied on parts being painted. OSHA regulations specify minimum flow rates and you should aim to be close to these rates.
Gun Handling. A well-trained operator holds the gun perpendicular to the surface being painted, at a distance of six to eight inches. He avoids "arcing" (holding the gun at less than a 90-degree angle to the surface). Holding the gun at a 45-degree angle to the surface, for example, wastes 65 pct of the paint.
Good operators move the spray gun toward the surface to be painted before triggering. Just as they reach they edge of the surface, they trigger to begin applying paint. Then as they reach the end of the surface being painted, they release the trigger to stop paint flow. Lots of paint can be wasted by sloppy triggering.
Manufacturers of equipment have been paying much more attention to ergonomic design, particularly low trigger-pull force and ease of holding and moving guns. Companies using ergonomically designed guns report not only greater worker satisfaction and higher-quality finishes, but actual improvements in TE, resulting from proper triggering and gun motion.
A competent operator learns how to overlap his strokes to apply a smooth finish without excessive thickness. The objective is to apply an even coating of the specified thickness. Too-thick coatings waste paint and increase VOCs.
Your equipment and coating suppliers can help you with operator training and tell you where to find training courses for your operators.
Products Finishing acknowledges information provided by the following sources:
- "Industrial Painting: Principles and Practices," by Norman R. Roobol, available from Hanser Gardner Publications, 6915 Valley Ave., Cincinnati, OH 45244-3029, for $49.95, postpaid.
- Carl Izzo, consultant and Products Finishing columnist, 2245 Manordale Dr., Export, PA 15632.
- Accuspray, 23350 Mercantile Rd., Cleveland, OH 44122-5939.
- Binks Manufacturing Co., 6915 Belmont Ave., Franklin Park, IL 60131-2887.
- Can-Am Engineered Products, Inc., 30850 Industrial Rd., Livonia, MI 48150.
- Graco, Inc., PO Box 1441, Minneapolis, MN 55440-1444.
- ITW DeVilbiss, 1724 Indian Woods Circle, Maumee, OH 43537.
- ITW Ransburg Electrostatic Systems, 320 Phillips Ave., Toledo, OH 43612-1493.
- Kremlin, Inc., 201 S. Lombard Rd., Addison IL 60101.
- Lex-Aire Spray Systems, 34 Hutchinson Rd., Arlington MA 02174.
- Nordson Corp., Liquid Systems Group, 555 Jackson St., Amherst, OH 44001.