Fact Sheet, Minnesota Technical Assistance Program, University of Minnesota
Many products require some type of coating such as paint, lacquer or varnish. Because the use of coatings is widespread, there is great potential for savings by including waste reduction techniques as part of the coating application process.
The first opportunity for waste reduction comes from using less coating material per finished product. In turn, reduced use of conventional coating materials can lead to savings through reduced use of thinners. Minimizing material use also translates directly into less paint waste needing disposal. Whether it is paint sludge from a wet booth or spent filters from a dry booth, reducing this waste volume reduces the need for regulated waste disposal and the associated disposal costs.
Through waste reduction, large-volume painting operations may be able to avoid reaching the threshold level for chemical usage, which would require reporting emissions to the Minnesota Emergency Response Commission and compliance under the Minnesota Toxic Pollution Prevention Act.
In the future, the Minnesota Pollution Control Agency may adopt more-limiting air emission standards for painting operations. Painting operations that begin adopting waste reduction now will have an advantage if stricter air emission standards must be met in the future.
In order to help Minnesota businesses benefit from the application of waste reduction techniques to painting operations, the Minnesota Technical Assistance Program (MnTAP) has developed this overview on the assessment of techniques for waste reduction in processes used for spray painting and coating.
Many products require a preparation step prior to painting. This step is commonly called "pretreatment" for new products, and "paint stripping" for products that need to be reworked.
For waste reduction in pretreatment of new parts, the first step is assessing cleanliness of the parts: To what degree are the surfaces contaminated with substances such as oil from machining, dirt from the manufacturing environment, and finger oil from shop personnel? An important part of the assessment is to determine the sources of contamination.
The next step is to determine the cleanliness level or standard needed to satisfy the pretreatment process. Once the contamination sources are identified and cleanliness standards are set, determine whether some or all contamination sources can be eliminated. Then, if contamination cannot be reduced enough through process changes, cleaning methods must be assessed.
In the past, halogenated organic solvents (i.e., CFCs) were commonly used as cleaning agents. But environmental concerns and regulations limiting production of these solvents have caused many to reassess their use of this type of solvent as a cleaner. MnTAP has developed two reference lists on this topic called "Manufacturers of Aqueous Cleaning Equipment and Aqueous and Semi-Aqueous Cleaners for Metal Parts Degreasing" which can be obtained by contacting MnTAP. Additional information and case studies are available on the SAGE Web site at http//clean.rti.org/tools.htm.
Phosphatizing is another pretreatment method often used in the surface preparation of metal parts. The primary waste reduction option for phosphatizing is reduced water use. The water added to maintain the solution in the phosphatizing bath can be reduced by analyzing and controlling each solutions temperature, chemical concentration, and pH level in each step; and by recirculating solution or rinse water from one bath to others where possible. An added benefit is the potential for reduced chemical use.
When repainting a part, the old paint often must be removed prior to application of the new paint coat. The waste reduction assessment should start by examining what causes the need for repainting: inadequate initial part preparation, defects in coating application, equipment problems, or coating damage due to improper handling.
While no process is perfect, reducing the need for repainting has a direct effect on the volume of waste generated from paint removal.
Once the need for paint stripping has been reduced to a minimum, alternate paint-stripping approaches can be considered. For example, it may be more cost-effective to send paint-stripping work to a company specializing in paint removal. Advantages include reduced environmental liability; avoidance of worker exposure to paint- stripping hazards; and elimination of capital expenses for purchase, operation and maintenance of stripping equipment.
If you decide that it is more cost-effective to maintain a paint removal operation, the key concern is the type and volume of waste produced.
Chemical stripping has commonly been used in a number of applications, but alternate methods that are less toxic and less costly are available. For example, a barrel reconditioning operation was able to replace chemical stripping with mechanical stripping using metal and nylon brushes.
Paint-stripping technologies that are alternatives to chemicals include:
Key factors that must be considered when selecting a paint-stripping method are the characteristics of the substrate to be stripped; the type of paint to be removed; plus the volume and type of waste produced. Waste type and volume can have a major impact on cost-benefits associated with a change. Often, a combination of removed paint and chemical stripper requires disposal as hazardous waste.
Once parts are ready to be painted, the type of coating material and application method selected have an impact on transfer efficiency. Transfer efficiency is the amount of paint applied to the object being painted, divided by the amount of paint used. High transfer rates offer financial incentives by reducing the amount of paint wasted while minimizing solid, liquid, and air emissions. Simply stated, transfer efficiency measures how much paint makes it from the paint can on to the surface being painted.
In spray-painting applications, liquid paint is converted to an atomized spray in order to coat the object being painted. Differences in spray-painting equipment are based on how the equipment atomizes paint. As paint becomes more highly atomized, the more likely transfer efficiency is to decrease. Highly atomized paint spray can more readily drift away from the painting surface due to forces such as air currents and gravity.
To achieve the best transfer efficiency, it is advisable to study the application equipment available and actually evaluate equipment performance using each coating material considered acceptable for your application. Because each application equipment combination has its own characteristics, the advantages and disadvantages must be weighed against the coating specifications set for your product. Increased transfer efficiency can waste materials if the result is a coating film thickness greater than specified.
The viscosity of the paint may need adjustment before it can be sprayed. Most often this is accomplished by thinning with organic solvents. Using solvents for thinning requires the purchase of additional materials and increases air emissions. An alternative method of reducing the viscosity is to use heat. Benefits from the purchase of paint heaters include lower solvent usage, lower solvent emissions, more consistent viscosities, and faster curing rates.
Conventional Spray. This technology, in use for over 40 years, uses air at high pressure (40 - 70 pounds per square inch [psi]) to atomize a liquified stream of paint. The high-energy air stream that is mixed with the paint causes atomization that is generally very fine and easily applied. This yields very good finishes with high-quality visual characteristics.
A disadvantage is that along with a high degree of atomization comes a spray that is very fine and highly susceptible to overspray, resulting in more paint waste and less transfer efficiency. The solvent in the paint is also highly atomized along with the paint solids, meaning that volatile organic compound (VOC) emissions from the solvent in paint are increased.
High-Volume/Low-Pressure (HVLP). As the name suggests, a high volume of air at low pressure is used to atomize paint. The defined air-pressure limit for HVLP is 10 psi at the spray gun. It is this reduced gun spray energy level that reduces overspray and improves transfer efficiency. Generally, fluid delivery rates up to 10 ounces per minute with low viscosities will work best with the HVLP gun. At higher fluid-delivery rates and heavier materials, HVLP may not atomize well enough to achieve an acceptable finish.
Airless. This is a method of atomizing paint without the use of compressed air. The paint is pumped at high pressure through a small opening at the spray tip to achieve atomization. Adjustments in airless spraying are done through adjusting the viscosity or the system pressure. This method has higher transfer efficiencies than conventional spray. Many high-viscosity coatings can be applied without costly solvent thinning. Also, this method allows for rapid application of a heavy paint coat useful for keeping up with a fast-moving painting line.
Air-Assisted. This is a spraying system that helps or "assists" airless systems by using supplemental air jets to guide the paint spray and boost the level of atomization. Air-assisted airless technology combines the best characteristics of both air and airless spray. Benefits include substantial material savings and reduced overspray when compared to conventional air spray; and improved transfer efficiency and finishing appearance when compared to airless technology. The ability to reduce the fluid pressure from airless is the primary factor in the increased finish quality. Operator technique is also enhanced as the application rate is reduced and the operator can more easily coat the product.
Electrostatics. With this method, the paint and the part are given opposite electrical charges. The result is that transfer efficiency is increased because the paint is drawn to the part by an electric field. As a result, paint spray is less susceptible to drafts and air currents that increase overspray. Even water-based paints can be electrostatically applied with special equipment.
Rotary Atomization. This application system atomizes paint by dropping a stream of liquid on a disk or bell-shaped object spinning at high speed. Rotary atomizers utilize electrostatics to attract paint to the part. Rotary atomization is useful for high-viscosity paints. This process can create a spray without use of thinner and tends to have high transfer efficiency. However, the equipment needed for this type of application is very specialized and usually requires a major conversion of a painting line.
Spray Booths. Two basic types of enclosures are used in most painting applications: dry booths and wet booths. The key difference is that a dry booth depends on a filter of paper, fiberglass, Styrofoam® or metal to collect overspray, while the wet booth uses water with chemical additives. The type of booth selected can affect the volume and type of paint waste generated. Decisions made about this equipment should be made based on the type and volume of painting done and the volume of waste generated.
Generally, small-volume painting operations will find the lower purchase cost of a dry filter booth will meet their requirements. One disadvantage in the use of a dry-filter booth is in the disposal of the waste. Most often the majority of this waste is the filter media itself contaminated by a relatively small amount of paint. Reusable filters may decrease waste volume and reduce disposal cost. In some applications, overspray can be collected for reuse.
If overall painting volume can justify the investment, a wet booth may work to your advantage. This type of booth eliminates disposal of filter media and allows waste to be reduced in weight and volume. This is achieved by separating the paint from the water through settling, drying, or using a centrifuge or cyclone.
Organic Solvent-Based. This is the traditional type of painting material, typically containing about 40 percent solids with a relatively high organic-solvent content. While this coating material is one of the most versatile, its low solid content and high percentage of solvent carrier can cause it to have low overall (solids) transfer efficiency. To get the required coverage, more material must be sprayed compared to materials with higher solids content and lower VOC emissions.
High-Solids. This paint type has a higher percentage of paint solids and a lower percentage of solvent carrier. Overall transfer efficiency tends to be better than traditional solvent-based paint. The increased solids content means that fewer applications are needed to get the required film thickness. Air emissions from the solvent are generally less due to reduced organic solvent content. However, a paint heater may be required to reduce viscosity and the film thickness is more difficult to control.
Water-Based. These paint types typically have a high solids content, utilize water as the solvent, and have very low or no organic-solvent content. Advantages of these paint types include reduced VOC emissions, reduced fire hazard, minimized or eliminated hazardous waste disposal, and easy cleanup. However, using a water-based coating may require a cleaner surface, longer drying times, increased oven temperatures, and a temperature-controlled paint storage area. The switch to water-based materials must be done carefully. Water-based coating technology is the fastest changing in the market today.
Catalyzed or Two-Component. These coatings are created by mixing two low-viscosity liquids just before entering the application system. One liquid contains reactive resins, and the other contains a catalyst that promotes polymerization of the resins. These coatings eliminate or reduce solvents and cure at low temperatures.
However, it is important to remember that catalysts and paint components may be hazardous themselves and create a different set of emission and exposure problems than those of organic solvents. Catalyzed painting also means that more material may be used if pot life is neglected.
Powder Coating. These coatings use 100 percent resin in dry, powdered form which must cure in an oven. Powder-coating materials can provide a high-quality, durable, corrosion-resistant coating. There are no VOC emissions, hazardous overspray wastes, or wastewater sludges. With powder coating it is also possible to collect the dry coating material that does not stick to the part and reuse it. Reuse allows powder coaters to achieve very high transfer efficiencies.
Powder coating requires specialized application equipment using electrostatic charges to apply the material. Its use also means that the substrate must be able to tolerate the curing temperature of the oven (typically 300 - 450°F). However, advancements in powder coating formulations are occurring at a rapid pace, and new powder coatings are becoming available to meet special manufacturing needs.
Radiation Cured. Ultraviolet (UV), Electron Beam (EB), and Infrared (IR) coatings use electromagnetic radiation to cure. These coatings typically have lower VOC content than conventional coatings, require smaller ovens, and allow for increased production rates due to shorter curing period. The shape of the part will effect the curingflat surfaces are easiest to cure. Capital investments are usually higher than conventional ovens and the cost of the raw material coating is higher.
This partial listing of equipment and coating options is only a summary of the technology available. MnTAP can provide reference materials on these topics. Valuable information and hands-on training can also be obtained from your equipment vendors and suppliers.
The techniques that spray painters use during application have a definite effect on transfer efficiency and have waste reduction potential. When using spray equipment, maintain the proper distance and angle between the gun tip and painting surface in accordance with equipment specifications. This will help assure the proper film thickness. Maintaining proper distance is equally important for electrostatic application.
For electrostatic applications, the distance between the spray tip and the part affects the degree of charge the paint spray maintains; the position is important to achieve optimal transfer efficiency.
Gun position relative to the painting surface also affects transfer efficiency. It is important to keep the spray gun perpendicular to the painting surface, thus avoiding uneven coverage that might otherwise require more paint than necessary to produce an acceptable finish.
Triggering the gun on and off at the appropriate time will minimize overspray and improve finish quality. Also important are proper spray overlap and the speed of the stroke.
Operator training and experience will provide operators with knowledge of the various painting techniques needed to paint parts of different configurations. Different techniques are helpful when painting inside and outside corners, or slender, round, flat, large, or small parts.
When a painting process is completed, a color change is needed, or maintenance is required during regular operation, equipment cleaning is required. Equipment cleaning offers opportunities for waste reduction and for reductions in air emissions.
When assessing the cleaning process, all the typical cleaning tasks should be reviewed to learn whether cleaning is necessary. While it is assumed that spray guns, tips and lines must be cleaned for reuse, cleaning some low-cost items may not be advisable. Costs from cleaning-solvent purchases, solvent waste disposal and solvent emissions could be higher than simply replacing the item being cleaned. However, the cost of proper disposal must be taken into account.
Next, the ways in which cleaning solvents are handled should be reviewed. All solvents should be in covered containers when not in use. Leaving solvents in the open air creates unnecessary solvent waste and VOC emissions. In addition, a standard should be set in order to assure that used solvent is disposed of or recycled only when it loses its cleaning effectiveness, not just because it looks dirty.
For equipment that requires cleaning, methods that minimize solvent use and reduce evaporation should be implemented wherever practical. Using a gun washer to clean spray guns is one example. A gun washer is a piece of equipment similar to a dishwasher. It is designed to hold a number of spray guns and related equipment and cleans by circulating solvent inside a closed chamber. The result is rapid cleaning and extended solvent-cleaning life while reducing solvent waste and the emissions from evaporation. Line cleaning is another area where use of special equipment can decrease cleaning time, improve efficiency of solvent use, and decrease waste. One method used to improve line-cleaning efficiency is to introduce turbulence into the solvent going through the line during cleaning. Equipment that forces alternating pulses of solvent and compressed air is one way to accomplish this. Payback on this equipment can come from increased production output through more-rapid color changes as well as from material savings through decreased solvent use.
Solvent Reuse Alternatives
On-site recycling of used solvent is another way to reduce waste and save money. First, savings come from reducing the amount of solvent purchased. Second, savings come from reduced disposal cost by reducing the volume of spent solvent that must be sent off-site. Three common methods of solvent recycling are settling, filtering and distilling.
Settling is putting used solvent in a container and letting the particulate matter settle out. The container should be designed to allow removal of solvent without shaking up the sludge which has settled out. Filtering equipment which removes the particulate matter from solvents is also available.
Distillation is an attractive option for many organic solvent users. Equipment is available in a variety of sizes, one gallon and larger. For more information, request MnTAP reference materials on solvent recycling and selecting a still.
Due to the increased need to reduce VOC and hazardous air pollutant emissions, alternative cleaning solvents are being used. Alternatives include formulations containing: Terpenes, Dibasic Esters (DBE), N-Methyl 2-Pyrrolidone (NMP) and Dimethyl Sulfoxide (DMSO). Although toxicology information specific to these chemicals is relatively limited at this time, it is believed that the relative safety of similar chemicals means that they are a viable alternative to organic solvents in certain applications.
MnTAP provides technical assistance and services to help Minnesota businesses prevent pollution at its source and to properly manage industrial wastes.
In addition to this overview on "Waste Reduction Alternatives for Painting and Coating," further information is available from MnTAP through the following publications:
- Aaron Carlson Co.: Water-Based Substitutes for Wood Finishing Lacquers.
- Crenlo Inc.: Evaluating Methods for Reducing Solvent Emissions in Painting Operations
- Medallion Kitchens: Increasing Transfer Efficiency through Part Placement, Spray Adjustment, and Overspray Reuse.
For assistance with your painting issues or to request any of these publications, call MnTAP at 612/627-4646 or 800/247-0015.
The Minnesota Technical Assistance Program is supported with a grant from the Minnesota Office of Environmental Assistance to the School of Public Health, Division of Environmental and Occupational Health, at the University of Minnesota. The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, religion, color, sex, national origin, disability, age, veteran status, or sexual orientation.