Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
Various application methods are available to coat metal, the most common being spray painting and electrodeposition (EPAb, p. 20). Coatings also can be applied by dipping parts into tanks filled with paint and then allowing the excess paint to drain off, or by direct application methods such as roller coating and flow coating. This chapter provides information on: conventional air-spray guns; high-volume/low-pressure spray guns; airless spray guns; electrostatic spray guns; electrodeposition; roll coating; flow coating; and plural component systems. Which paint application process is chosen depends on the type of substrate to be coated, the type of coating, and the size and shape of the surface (IHWRIC, p. 35).
Paints and coatings can be applied to surfaces in a number of ways. Industrial coatings often are applied on a production line using spray application techniques. Curing is done usually by an accelerated curing operation involving heat, surface catalysts or radiation (EPA, p. 155-156).
In general, spray methods use specially designed guns to atomize paint into a fine spray. For industrial applications, the paint is typically contained in a pressure vessel and fed to the spray gun using compressed air. Traditionally, hand-held or automated guns (mounted on a mechanical control arm) have been used to apply liquid paints to metal substrates.
Although spray systems are easy to operate and have low equipment costs, they have a certain amount of overspray and rebound from the sprayed surface and, therefore, are unable to transfer a substantial portion of the paint to the part (Freeman, p. 710). Spray booths with an open front and exhaust at the rear are generally used to remove the overspray as it is generated (EPA, p. 155).
During conventional spray painting, some of the paint is deposited on the surface being painted; while much of it, in the form of overspray, is sprayed into the air. As the paint dries, the solvent evaporates into the air in the form of VOCs. Often exhaust from paint booths is run through dry filters to capture the particulates. Though it can be run through a water scrubber that separates the paint from the air, scrubber water is normally recycled, and paint solids are concentrated in the scrubber sump. When the sump fills with paint sludge, it is removed and put in drums for disposal. Paint sludge that fails the TCLP test must be disposed of as a hazardous waste (Higgins, p. 118).
Emissions of VOCs from coatings application can be significantly reduced by substituting a paint with a lower solvent content (e.g., high-solids, waterborne or powder), and by increasing transfer efficiency. The type of coating and the application method selected can have a significant effect on transfer efficiency (MnTAP, p. 2). For more information on alternatives to solvent-borne coating formulations, see chapter 6.
Whatever type of paint and application method is chosen, the best environmental solution may be to redesign the product to eliminate unnecessary coating. This is a P2 option known as surface-free coating. Many of the resins used in alternative paints are made from regulated chemicals, and surface-free coating can eliminate the use of these substances (EPA, p. 165).
A number of other P2 techniques in coating applications also are available. For coating operations that involve manual spray application, for example, training operators to practice proper spray techniques is a cost-effective method for reducing VOC emissions and other wastes. Wastes generated during the application of paints and coatings (as well as during surface preparation and equipment cleaning) can also be reduced by adopting improved housekeeping, maintenance and operating practices. Additional P2 options include: installing a paint heater to reduce the need for paint thinning with solvents, and setting application standards to avoid unnecessary coating. Each of these options is discussed below.
Improvements in transfer efficiency can lead to less paint waste and lower emissions of VOCs. Transfer efficiency depends on a large number of parameters. Some of these parameters are under the control of the operator, while others are not. Important parameters that should be considered when optimizing spray gun application include:
By definition, transfer efficiency is the amount of paint solids deposited on an object, divided by the amount of paint solids sprayed at the object, multiplied by 100%. The definition of transfer efficiency does omit some related factors for optimum material use. Minimizing waste is not necessarily achieved by simply using the application technique that has the highest rated transfer efficiency. "Real" transfer efficiency depends on a number of other factors including:
In summary, real transfer efficiency depends on the particular coating situation. Replacing a system (manual or automatic) will not reduce VOC emissions by improving transfer efficiency alone, hence another step must be taken to use less paint. This may require changing the flow rates, triggering times, and/or spray tip sizes. For instance, electrostatic can be added to increase transfer efficiency, but if nothing else is changed, VOC emissions will stay the same and paint thickness on the part will increase. A study by the Research Triangle Institute found that real transfer efficiency depends heavily on solids content, wet film thickness, application equipment and operator experience. Therefore, if a firm is considering a change in paint application methods to improve transfer efficiency, careful testing should be done to ensure that paint and solvent waste are truly being minimized. When comparing application techniques for possible use in a particular plant, spray efficiency and the above factors should all be considered (VT DEC).
Following are methods that facilities can use to increase their transfer efficiencies:
Before deciding whether an operation can improve transfer efficiency, determine the current transfer efficiency rates. Appendix G provides information on how to estimate current transfer efficiency (EPAq, p. 74-76).
Table 28 provides an overview of the relative costs and benefits of the different spray application methods relative to conventional air spray guns.
NOTE: Capital cost refers to the cost of the system in comparison to conventional air spray. The higher the process complexity, the higher the associated costs (i.e., training for employees and maintenance)
The monitoring of applied film thickness is critical to ensure that a uniform and consistent coating of paint is being applied. Too thin a coat will result in premature failure in the field, while too thick a coat represents excess cost and waste. Other standards that should be established include the levels of crosshatch adhesion, film hardness and solvent resistance. Specification of and adherence to standards can do much to minimize the level of rejects and ease troubleshooting when problems arise (Freeman, p. 487). Different tests have been used over the years for liquid and cured paints. A consistent system should be used for evaluating coating properties. The American Society for Testing Materials (ASTM) standards has developed many useful standards; see appendix E for more information (KSBEAP, p. 25).
Untrained and hurried workers using poorly maintained equipment can contribute to the need to rework products and to clean up and dispose of wasted coatings, thereby increasing costs. A well-trained operator is far more important than the type of gun used. By training operators on proper equipment setup, application techniques and maintenance, companies can reduce the use of materials by 20 to 40% (Callahan). These savings will depend on the parts coated, material sprayed, and operator technique and experience level (MnTAPd, p. 6). The fundamentals of effective spray technique that operators can follow are:
Whenever helping companies adjust the spray technique of operators, technical assistance providers should keep in mind that, over a period of time, the firm may have selected a coating and application equipment to conform to an incorrect technique. Equipment settings and materials might need to be changed to conform to an improved technique (De Vilbiss).
Improving operating practices is another cost-effective pollution prevention method for reducing the amount of wastes generated. The following methods require minimal capital outlays, and can be very effective (KSBEAP, p. 21):
The following sections provide more detailed information on specific application equipment and on methods to optimize their performance.
Conventional air spray technology, which has been the standard for the past 40 years, uses a specially designed gun and air at high pressures (i.e., 40 to 90 psi) to atomize a liquid stream of paint into a fine spray. This technology is known as low-volume/high-pressure (LVHP) but is commonly referred to as conventional air spray. Air is usually supplied to the LVHP gun by an air compressor, and paint is supplied via a pressure feed system (siphon and gravity systems are also used). A typical picture of an air spray gun features clouds of overspray around the part.
Conventional air spray produces a smooth finish, and can be used on many surfaces. It offers the best control of spray pattern and the best degree of atomization. This system produces the finest atomization and, therefore, the finest finishes. It also sprays the widest range of coating materials (CAGE). However, this technology produces a great deal of overspray, resulting in low transfer efficiencies (i.e., 30 to 60%) and uses large amounts of compressed air (7 to 35 cfm at 100 psi). In addition, because the solvent in the paint is highly atomized along with the paint solids, transfer efficiency is low and VOC emissions are high (MnTAP, p. 3).
The essential components of an air atomizing system are gun body, fluid inlet, fluid nozzle, fluid needle assembly, fluid control assembly, air inlet, air nozzle, air valve, fan control and trigger. Other parts of the spray coating system may include a compressed air supply, fluid supply and paint heater. Recirculation booths are often used with these systems. These booths are designed to reduce process exhaust volumes while maintaining minimum ventilation flow rates in order to lower operating costs for both emission control systems and the facility in general (e.g., heating, ventilation and air conditioning). These systems have built-in safety limits that are based on the concentration of hazardous constituents present in the recirculated stream.
The main advantages of conventional air spray systems are the high level of control that the operator has of the gun and the versatility of the systems. Disadvantages of this system include high air emissions, low transfer efficiencies and high compressed air use. However, using proper training and setting the gun at low pressure (20 psi), transfer rates similar to HVLP can be achieved (Eck).
The capital investment for a new conventional air spray system that includes spray gun, two-gallon pressure pot, hoses and fittings can range from $500 to $1,500.
Painters are required to wear respirators to prevent inhalation of overspray, hazardous vapors and toxic fumes. Depending on the noise level in the spray booth, ear protection may also be required.
There are a number of alternative spray gun systems, including high-volume/low-pressure (HVLP) air spray, airless spray, and electrostatic spray. There are also variations on each of these techniques. Of the many available methods, electrostatic air-assisted airless spray is considered to have the best transfer efficiency (IWRCb, p. 39). Other available paint application methods include electrodeposition, and dip, roll and flow coating.
As the name suggests, this technology uses a high-volume of air at low pressures (i.e., 0.1 to 10 psi) to atomize paint. This technology reduces overspray and improves transfer efficiency. HVLP guns have nozzles with larger diameter openings than LVHP guns for atomizing air. They can be bleeder (i.e., controls only the fluid flow to the gun) or non-bleeder (i.e., controls air flow and fluid flow to the gun by use of a trigger) types, and may require airflows of 10 to 30 cubic feet per minute. Air can be supplied to the sprayer by turbine air blowers or conventional shop compressors (KSBEAP, 13). Typical transfer efficiencies with HVLP systems are 65 to 75%. Figure 5 shows a typical configuration for a HVLP system.
An HVLP gun is portable and easy to clean, and has a lower risk of blowback to the
worker. In many cases, HVLP guns are mandated to comply with state air regulations
(KSBEAP, p. 14). However, the atomization of HVLP guns might not be good enough for fine
finishes, and production rates might not be as high as with conventional LVHP spray.
Generally, fluid delivery rates of up to 10 ounces per minute with low viscosity paints
work best with HVLP guns (MnTAP, p. 3). For more information on other advantages and
disadvantages of HVLP, see table 29.
Several different configurations of HVLP systems are available. The specific air supply (i.e., turbine or compressor) and fluid delivery system (described below) will affect the efficiency, ease of use, cost and versatility of the particular system (KSBEAP, p. 13).
In a siphon-fed system, air pressure to the sprayer is used to pull paint from a cup located below the gun, producing a fully atomized pattern for even surface coverage. The simple design of siphon-fed guns has made it possible to buy conversion kits for conventional siphon sprayers, making HVLP technology very affordable for small shop owners (KSBEAP, p. 13).
Gravity-fed systems are well adapted to high viscosity paints such as clears, water-based paints, high-solids paints and epoxy primers because of the design of the system. The cup, located on top of the gun, allows paint to completely drain, minimizing paint waste (KSBEAP, p. 13).
The pressure assist cup system uses a cup that is mounted beneath the gun with a separately regulated air line to feed paint to the gun. This design increases transfer efficiency and makes it possible for the operator to spray evenly while the gun is inverted, offering maximum flexibility in application techniques (KSBEAP, p. 13-14).
Although covering every aspect of equipment selection is not possible in this manual, see appendix D for a list of some of the more important points to consider when evaluating HVLP spray equipment.
HVLP paint spray systems can be used in a variety of painting applications. The finer atomization of HVLP systems produce smoother finishes. There are many paint gun models with a variety of tip sizes to accommodate most coatings including solvent-based paints, water-based coatings, fine finish metallic, high-solids polyurethane, contact adhesives, varnish, top coats, lacquer, enamel primer, latex primer, epoxy and vinyl fluids. The efficiency of these systems is greatly reduced if the painting is done in an exposed area.
LVHP systems can be easily converted to HVLP by retrofitting the air gun and installing the appropriate diameter air hoses (5/16 in. I.D.); however, the air supply system must be able to deliver 10 to 30 cubic feet per minute of airflow at 10 psi or lower. If a firm has a large investment in high-pressure air compressors, conversion air systems (CAS) can be used. The CAS reduces high-pressure compressed air in two ways: 1) by using an air-restricted HVLP gun that is specially equipped to restrict air pressure within the gun body, and 2) by using a small air conversion unit that takes in high-pressure compressed air and restricts its flow, delivering low-pressure air to the HVLP gun (CC and Binksd). Costs can vary depending on specific applications, painting/coating type, paint volume, workpiece specifications and technique. Generally, costs for HVLP paint-spray system equipment range from $500 to $1,500 for a gun, hose and paint pot.
Painters are required to wear respirators to prevent inhalation of overspray, hazardous vapors and toxic fumes when using HVLP equipment. Depending on the noise level in the spray booth, ear protection may also be required.
Low-pressure/low-volume paint spraying, which is similar to air-assisted airless, is a relatively new development. Paint and air separately exit through the spray nozzle into a secondary fluid tip assembly. The exiting paint stream is of low pressure (less than 100 psig), flattened by the spray nozzle, but unatomized. Atomization occurs by impinging low amounts of compressed air (5-35 psig) from two small holes in the fluid tip assembly into the flattened paint stream. Table 30 presents an overview of the advantages and disadvantages of LPLV Systems.
Airless spray does not use compressed air. Instead, paint is pumped at increased fluid pressures (500 to 6,500 psi) through a small opening at the tip of the spray gun to achieve atomization. Pressure is generally supplied to the gun by an air-driven reciprocating fluid pump (KSBEAP, p. 16). When the pressurized paint enters the low pressure region in front of the gun, the sudden drop in pressure causes the paint to atomize. Airless systems are most widely used by painting contractors and maintenance painters (Binksc).
Airless spraying has several distinct advantages over air spray methods. This method is more efficient than the air spray because the airless spray is softer and less turbulent, thus less paint is lost in bounce back. The droplets formed are generally larger than conventional spray guns and produce a heavier paint coat in a single pass. This system is also more portable. Production rates are nearly double, and transfer efficiencies are usually greater (65 to 70%). Other advantages include the ability to utilize high-viscosity coatings (without thinning with solvents) and its ability to have good penetration in recessed areas of a workpiece.
The major disadvantage of the airless spray is that the quality of the applied coating is not as good as conventional coatings, unless a thicker coating is required. Airless spray is limited to painting large areas and requires a different nozzle on the spray gun to change spray patterns. In addition, the nozzle tends to clog and can be dangerous to use or clean because of the high pressures involved (IHWRICb). For more information on other advantages and disadvantages of airless spray, see table 31.
Small fluid nozzle orifices limit the coating materials that can be sprayed with airless systems to those that are finely ground. This rules out fiber-filled and heavily pigmented materials. In addition, airless spraying lacks the feather capability that air guns have. This can result in flooding of the surface and sags or runs if gun movement is too slow. The high pressures used with airless spray deliver a high rate of paint flow through the nozzle, tending to enlarge the orifice, increase flow rates and change spray pattern characteristics. This is especially true at very high pressures and with paints containing high amounts of pigments or abrasive pigments. Strict maintenance is required for this system. Foreign objects in the fluid that are larger than the nozzle tips can block or shut off the system. Equipment maintenance on pumps is high because of the high pressures used (CAGE).
The capital investment required for a new airless spray system consisting of an airless spray gun, carted mount pump, hoses, and fittings, can range from $3,500 to $7,500.
The high velocity of the fluid stream and spray pattern as it exits the gun and hose is a potential hazard. Operators should never allow any part of their body to come into contact with this high-pressure material. Failure to keep several inches away from the coating as it exits the gun will result in serious injury. As with other spray systems, respirators are required, and hearing protection may be required as well.
Air-assisted airless systems are a variation of airless spraying. These systems use supplemental air jets to guide the paint spray and to boost the level of atomization. Approximately 150 to 800 psi of fluid pressure and 5 to 30 psi of air pressure are used. Air-assisted airless spray systems atomize paint well, although not as well as air spray methods. The use of air-assisted airless systems improves the quality of the finish, presumably because finer paint particles are formed. The transfer efficiency of the airless, air-assisted spray gun is greater in comparison to airless, and with proper operator training, the manufacturer can obtain finishes comparable to conventional guns (Batelle, p. III-5). This system has the same dangers as airless spraying, but it also requires more maintenance and operator training and has a higher capital cost (IHWRICb).
The major difference in gun construction between an air-assisted airless gun and an air-atomized gun is found in the atomizing tip. The air-atomized tip incorporates a fluid nozzle and an air nozzle. The fluid orifice in the center of the tip is surrounded by a concentric atomizing ring of air. The air-assisted tip delivers a flat fan spray of partially atomized paint. Jets of atomizing air, exiting from ports in small projections on each side of the tip impacts at a 90 degree angle into the spray. The air jets break up the large droplets and complete the atomization, assisting the airless spray process.
The capital investment required for a new air-assisted airless spray system, including an air-assisted airless spray gun, 10:1 ratio carted mount pump, hoses and fittings, can range from $2,500 to $5,000.
Table 31 (above) presents an overview of the advantages and disadvantages of airless spray systems.
Air and air-assisted electrostatic spray guns resemble nonelectrostatic guns. An electrostatic gun has a wire charging electrode positioned in front to ionize the air. The ionized air passes its charge to the paint particles exiting the gun. Some guns have no external electrode. Instead, an internal electrode located inside the gun barrel is used to charge the paint. In another variation, a metal electrode is situated in the paint tank, and the paint is delivered to the gun already charged.
LVHP systems cannot be converted to airless systems. Therefore, the capital cost for implementing airless spray is usually high. However, this cost might be offset by the number of advantages that airless spray provides.
This spray method is based on the principle that negatively charged objects are attracted to positively charged objects. Atomized paint droplets are charged at the tip of the spray gun by a charged eletrode; the electrode runs 30 to 140 kV through the paint at 0 to 225 microamperes (CAGE). Paint can be atomized using conventional air, airless, or rotary systems. The electrical force needed to guide paint particles to the workpiece is 8,000 to 10,000 volts per inch of air between the gun and its workpiece. The part to be painted, which is attached to a grounded conveyor, is electrically neutral, and the charged paint droplets are attracted to that part. If the charge difference is strong enough, the paint particles normally fly past the part and reverse direction, coating the edges and back of the part. This effect is called "wraparound" and increases transfer efficiency (KSBEAP, p. 15). Electrostatic spray is used by most appliance manufacturers (Binksc).
The major advantage of using electrostatic spraying is that it saves in material costs
and labor. The labor savings is often associated with a changeover to automated lines,
although labor savings for cleanup is significantly reduced in either automated or manual
lines. Another benefit of electrostatic is its ability to completely cover an object with
a uniform thickness, including areas that are normally inaccessible (Batelle, p. III-10).
Rotary atomization is a variation of electrostatic spraying that uses centrifugal force generated by discs or bells to atomize paint, which drives it from the nozzle. The atomization of this method is excellent as is the transfer efficiency. This method also can be used with paints of different viscosity. However, the equipment needed for this type of application is very specialized and usually requires a major conversion of a painting line (IHWRICb). Typical costs for a new rotary atomization system consisting of a rotary atomizer, 2-gallon pressure-pot, and hoses and fittings may range from $5,000 to $7,500 .
An LVHP air spray system can be converted to an electrostatic system. In most cases, however, airless, air-assisted airless, or rotary atomization is used with electrostatic spray. This is because LVHP air-atomized electrostatic spray has a transfer efficiency of only 60 to 70%. Airless, however, runs from 70 to 95%, and rotary runs from 80 to 90% (IHWRICb).
Part and gun cleanliness are essential for efficient electrostatic operation. Dirt or oversprayed paint can form on a conductive track on the plastic gun tip and short out the system. For top efficiency, the part to be coated should be the closest grounded object to the charging needle on the spray gun. The charged paint particles are attracted to the nearest electrically grounded item; the larger the item, the greater the attraction.
Ungrounded objects in the vicinity of the charged gun electrode can pick up a considerable electrical charge. The charge buildup can arc over or spark if a grounded object is brought near. The intense heat of the arc may be sufficient to ignite the solvent-laden atmosphere typically found in a paint booth.
Paint buildup on hooks or hangers can act as an insulator and block the flow of electric current in the electrostatic circuit. Hangers and hooks should be regularly stripped or otherwise cleaned of paint buildup to maintain good grounding contact between the parts and the conveyor.
Because of high transfer efficiencies, air velocity in spray booths may be reduced from 100 to 60 feet/minute. This results in a 40% reduction in make-up air costs and reduces emissions.
In 1995, the National Fire Protection Association (NFPA) rewrote the NFPA 33 Standard to require fast-acting flame detectors for all automatic electrostatic liquid painting applications. These are also required for automatic electrostatic powder coating applications. All electrically conductive materials near the spray area such as material supply, containers and spray equipment should be grounded as well.
The capital investment for a new liquid electrostatic spray system consisting of an electrostatic spray gun, 2-gallon pressure pot, and hoses and fittings can range from $4,900 to $7,500. The capital investment required for a new electrostatic powder coating spray system, including powder application equipment, powder booth, cleaning system and bake oven, may range from $75,000 to $1,000,000. (CAGE).
This section presents brief descriptions of a variety of other paint application methods, including electrodeposition, various dip processes, and direct application methods such as roller and flow coating.
Electrodeposition/Electrocoating (E-coat). This process applies paint in a method that is similar to electroplating. In the E-coat process, a paint film from a waterborne solution is electrically deposited onto a part. Parts are usually made primarily of steel. An E-coat bath contains resin, pigment (unless it is a clearcoat), solvent (water and a cosolvent) and additives. The most commonly used resins in this process are epoxies and acrylics. These systems have no or low VOC emissions and produce little toxic waste.
The liquid is a very dilute emulsion of waterborne paint. Reactions between the paint particles and certain bath components cause the resin to be ionic. The electric current causes the paint particles to migrate to the metal surface. As more and more particles collect, water is squeezed out and cross linking of the resin particles occurs. The transfer efficiency of electrodeposition is greater than 90%. High production rates are possible, and production can be automated. However, this method is costly and requires a lot of energy. Also, employees need a high level of training to use this system (IHWRICc).
E-coat is extremely efficient, depositing a mostly uniform coating on all surfaces that can be reached by electricity. Waterborne electrocoating systems may be used to apply uniform, pinhole-free coatings. For films that require high appearance standards, E-coat uses acrylic resins. Electrocoatings are resistant to attack by UV light and have good weatherability. Typical applications include truck beds, engine blocks, water coolers, microwave ovens, dryer drums, compressors, furnace parts, housings for the automotive industry, shelving, washers, air conditioners, file cabinets, switch boxes, refrigerators, transmission housings, lighting fixtures, farm machinery, and fasteners.
One drawback to the electrocoating system is that it is limited to one color at a time. Each color requires its own tank.
Autodeposition. Autodeposition is a process used to deposit organic paint films onto iron-, steel-, zinc- and zinc alloy-plated substrates. Autodeposition is typically an 6-step process, including alkaline cleaning, rinsing with plant water and deionized water, autodeposition (immersion), immersion sealing rinse and curing. The part is immersed into a solution containing paint compounds, usually a vinyl emulsion, hydrofluoric acid and hydrogen peroxide. When the part is submersed, the paint compound precipitates out of the solution and coats the part. The part is then removed from the tank, rinsed and cured (KSBEAP, p. 20).
Autodeposition is an effective method for achieving corrosion resistance and coverage of objects. Autodeposited films also provide extremely uniform thicknesses, typically 13 to 30 micrometers (0.6 to 1.2 mils). These resins also have excellent hardness, formability and adhesion characteristics. Two other advantages of autodeposition are that organic solvents are not needed, and little or no VOCs are emitted. Autodeposited films have high transfer efficiencies (approximately 95%), further reducing environmental impacts. This system also does not have fire hazards. However, autodeposition produces a dull or low gloss finish and has few available colors (IHWRICc). The largest application for autodeposition coatings have been for nonappearance and under-hood parts in cars and trucks due to their excellent anticorrosion properties. It is also used on drawer slides for office furniture, replacing zinc-plating.
Dip Coating. With this process, parts are dipped (usually by conveyor) into a tank of paint. Dip coating allows for a high production rate and high transfer efficiency and requires relatively little labor. The effectiveness of dip coating depends greatly on the viscosity of the paint, which thickens with exposure to air unless it is carefully managed. The viscosity of the paint in a dip tank must remain practically constant if the deposited film quality is to remain high. To maintain viscosity, solvent must be routinely added as makeup. This results in higher VOC per gallon ratios.
Dip coating is not suitable for objects with hollows or cavities, and generally the finish is of lower quality (IHWRICc). Color change is slow and not feasible for most dip operations. This process is usually used to apply primers and to coat items whose appearance is not vitally important. Top coats are not commonly applied by dipping. Coatings applied by dipping have only a poor to fair appearance unless parts are rotated during drippage. Dipping is well suited for automation with conveyerized paint lines.
Capital investment required for dip coating is minimal. All that is required is a tank for the coating. The parts may be dipped manually, or automatically with a conveyor. Given the large surface area of the dip tank, adequate ventilation must be provided to prevent buildup of fumes and vapors. An efficient fire-extinguishing system must be installed as a safety measure if flammable paints are used (CAGE).
Flow Coating. In a flow coat system, 10 to 80 separate streams of paint coat all surfaces of the parts as they are carried through the flow coater on a conveyor. This system has the advantages of dip coating along with low installation costs and low maintenance requirements. The quality of the finish is also comparable to dip coating (IHWRICc).
Flow coating is usually used for large or oddly shaped parts that are difficult or impossible to dip coat. Coatings applied by flow coating have only a poor to fair appearance unless the parts are rotated during drippage. Flow coating is fast and easy, requires little space, involves relatively low installation cost, requires low maintenance, and has a low labor requirement. Required operator skill is also low. Flow coating achieves a high paint transfer efficiency, often 90% and higher (CAGE).
Principal control of dry-film thickness depends on the paint viscosity. If viscosity is too low, insufficient paint will be applied. If the paint viscosity rises, extra paint will be applied. This can increase paint costs and also plug small holes in the part (CAGE).
Curtain Coating. Instead of the multiple streams of paint found in flow coating, curtain coating uses a waterfall flow of paint to coat parts on a conveyor belt. The paint flows at a controlled rate from a reservoir through a wide variable slot. Curtain coating has a high transfer efficiency and covers parts uniformly, but is suitable only for flat work. The quality of the finish depends on the viscosity of the paint (IHWRICc).
Roll Coating. Roll coating is the process of applying a coating to a flat substrate by passing it between rollers. Paint is applied by one or more auxiliary rolls onto an application roll, which rolls across the conveyed flat work. After curing, the coated substrate is then shaped or formed into the final shape without damaging the coating. The paint-covered rollers have large surface areas that contribute to heavy solvent evaporation. This can pose a fire hazard from flammable solvents in solvent-borne formulations.
Roll coating is divided into two types: direct and reverse roll coating. In direct roll coating, the applicator roll rotates in the same direction as the substrate moves. In reverse roll coating, metal feed stock is fed between the rolls as a continuous coil. The applicator roll rotates in the opposite direction of the substrate.
Roll coating is limited to flatwork and is extremely viscosity dependent. Coating properties should be checked often to ensure proper results. These tests should include adhesion, impact resistance, flexibility and hardness. A well-known application of roll coating is coil coating, in which coiled metal strip is uncoiled, pretreated, roller coated with paint, cured and then recoiled (IHWRIC, p. 36).
Roll coaters are typically custom made for each application. Roll coaters can be made-to-order to accommodate widths ranging from 14 to 100 inches.
Plural Component Proportioning System for Epoxy Paints. Plural component proportioning systems are self-contained epoxy paint measuring and mixing systems. These systems accurately mix the epoxy paint components, produce the precise amount of paint required by an application, and consequently minimize waste.
Epoxy paint mixtures are prepared by premixing a base and a catalyst and then combining them in appropriate proportions in a separate container. After mixing and waiting the specified time, application of the paint to the workpiece may proceed. Once mixed, epoxy paints have a limited pot-life that cannot be exceeded without affecting the characteristics of the paint. If the pot life is exceeded, the mixture must be disposed of, and the application equipment must be cleaned. Under conventional methods, these mixtures are prepared by hand, a process that frequently leads to the generation of excess paint. The solvents used to cleanup and dispose of excess paint generates hazardous waste consisting of spent solvents and waste paint.
Plural component proportioning systems are used in conjunction with application devices. A typical proportioning and application system layout includes the following components: proportioning pump module, mix manifold, mixer, application device, materials supply module, and purge or flush module. These systems optimize painting operations by maximizing efficiency and minimizing waste generation.
The plural component proportion system for epoxy paints provides for total control of materials from container(s) to application. The system is accurate and can provide more consistent material quality than hand mixing. These systems can also keep pace with higher production requirements. The systems mix the coating on demand (i.e., as the gun is triggered). This does not result in significant quantities of waste materials because no excess paint is mixed. Material cleanup requires less labor and maintenance, and generates less waste because the mixed material can be purged with solvent from the mix manifold, mixer, hose, and applicator before it cures. The plural component system is a closed system and, as a result, there are fewer spills, less contamination or waste to cleanup, and less exposure of toxic materials to personnel. In addition, the proportioning system makes bulk purchase of material practical.
If an epoxy paint requires significant induction time (i.e., 15 minutes or longer), the plural component system can still be used, provided that the mixed paint is allowed to stand in a separate container prior to application.
Capital costs for plural component proportioning systems can range from $6,000 to $7,500 for basic units that mix two materials, up to $50,000 to $70,000 for systems that mix multiple materials. Application systems are an additional component, and their capital costs can range from $500 to $5,000. Each application needs to be evaluated on a case-by-case basis with respect to material and labor costs and savings.
Supercritical Carbon Dioxide (CO2). Supercritical fluid spray application allows substitution of supercritical carbon dioxide for up to two-thirds of conventional solvents concentration in spray-applied coatings, reducing VOC emissions by 30 to 70%. The proportioning and supply system from Union Carbide (UNICARB) mixes supercritical CO2 solvent with coating concentrate and supplies the material to a specially designed spray gun (i.e., internal mixing). The CO2 solvent is compatible with high molecular weight resins and existing painting facilities and procedures; therefore, this compatibility enables the use of solvent-borne formulations with substantial VOC reductions.
In the supercritical CO2 spray process, the solvent-like properties of supercritical CO2 are exploited to replace a portion of the solvent in the conventional solvent-borne coating formulation. The addition of supercritical CO2 acts as a diluent solvent to thin the viscous coating just before application, so that the coating can be atomized and applied with a modified spray gun (EPAl). Supercritical fluid spray application can be used to coat metal and plastics. The applied coating has a higher viscosity that allows thicker coatings without runs or sags. However, care is required in working with high-pressure gas at high operating temperatures (100 to 150°F) (TURI, p. 2).
This system requires investment in new equipment for paint mixing, handling and spraying. In 1991, five coating formulators were licensed to develop, manufacture and market UNICARB systems, including Akzo (automotive components, furniture), BASF (automotive), Guardsman (furniture), Lilly (furniture, plastics, heavy equipment) and PPG Industries (automotive, heavy equipment) (EPAd, p. 82).
Table 41. Transfer Efficiencies of Various Application Technologies (IHWRIC, p. 37, KSBEAP, p. 23 and CC)
A paint booth is an enclosure that directs overspray and solvent emissions from painting operations away from the painter and toward an entrainment device. Spray booths are designed to capture particulate matter that is released into the air during coating operations. They are not abatement devices for VOCs. A spray booth's primary function is to protect the painter and other employees from exposure to potentially toxic vapors and particulates. Another function of the booth is to prevent fires within a facility by venting high concentrations of flammable solvent vapors out of the building (EPAq, p.149).
Discharges from paint booths consist of particulate matter and organic solvent vapors. Particulates result from solids in the paint that are not transferred to the part. Organic solvent vapors are from the solvent, diluent or thinner that is used with the coating to reduce the viscosity of the paint. Much of the particulate matter is captured by a dry, water-wash or baffle filter (these are discussed below). Solvent vapors are controlled or recovered by the application of control technologies such as condensation, compression, absorption, adsorption or combustion. Solvent vapors can be minimized by using more efficient equipment, and low or no VOC materials. Increasing the transfer efficiency of the painting operation can result in both reduced particulate and solvent emissions (EPAq, p. 149).
There are two basic types of enclosures that are used in most painting applications: dry booths and wet booths. The key difference between the two is that a dry booth depends on a filter of paper, fiberglass or polystyrene to collect overspray, while the wet booth uses water with chemical additives to collect overspray. The type of booth selected can affect the volume and type of paint waste. A third type of booth is used exclusively in powder coating operations.
Although a spray booth is generally thought of as an enclosed painting area, this is not always the case. For instance, facilities that paint very large pieces may have a booth that only has one side, consisting of an exhaust plenum that draws solvent and particulates away from the operator. It is also not uncommon to see two spray booths opposite one another. This set-up allows for very large workpieces to be transported in between the booths either by a conveyor or a forklift truck that runs between the booths. Often neither booth has a ceiling, and they draw air from the surrounding factory (EPAq, p. 149).
Regardless of the size or design of the booth, they consist of one of three basic designs for directing air flow.
Decisions about equipment should be made based on the type and volume of painting done and the volume of waste generated.
Choosing between a dry filter, water-wash or baffle spray booth encompasses many different issues. The following section provides information on these three systems. Analysts estimate that 80% or more of the spray booths in use today are of the dry filter type (EPAq, p. 151). In recent years, however, many facilities have switched to water-wash booths because of their lower maintenance and hazardous waste costs. However, there are other concerns with these booths. The following section provides more detail on dry filter and water-wash booths.
There are many types of dry filter systems, however, they all operate on the same principle: particulate-laden air flowing toward the filter medium is forced to change directions rapidly. The particulate, having more inertia than the surrounding air, impacts the filter medium and is removed from the air flow. The scrubbed air is then vented to the atmosphere.
There are four general types of filters currently used: fiberglass cartridges, multilayer honeycombed paper rolls or pads, accordion-pleated paper sheets, and cloth rolls or pads. Each type of filter has different characteristics for particulate capacity, removal efficiency, cost and replacement time. Filter performance is characterized by three basic parameters: particulate capacity, resistance to air flow and particulate removal efficiency. Filter replacement is required when the filter becomes heavily laden with captured particles, resulting in a reduction in removal efficiency and an increase in the pressure differential across the filter face. The primary waste stream generated by dry booths is spent filters. When using lead or zinc chromate paints, the dry filter will eliminate 50 to 90% of the hazardous waste generated by water-curtain paint booths.
Generally, small-volume painting operations find that the lower cost of a dry-filter booth meets their requirements. This equipment requires a low capital investment relative to wet-booths and are simple in design. The filters act to remove paint in airborne particles by capturing them as they are forced through the filter. Ease of replacing a relatively low number of filters produced by small operations makes such an approach attractive. As paint volume increases, though, filter replacements must be made more often. This may increase costs for labor and materials significantly (Mitchell, p.10).
Dry filters effectively remove up to 95 to 99% of particulates. These systems are also versatile. They can be used in booths of all designs (small, large, cross-draft, down-draft and semidown- draft). These booths can also be operated for a variety of coating technologies, including polyurethanes, epoxies and alkyds. However, they cannot be used for nitrocellulose paints and some waterborne coatings (proper filter selection is critical in these cases). They are inexpensive to purchase, and depending on the nature of the paint (i.e., pass or fail TCLP test), they are also inexpensive to operate.
A disadvantage of dry filter booths is that they are generally not appropriate for facilities with high coating use (i.e., greater than 5 gallons per square foot of filter areas per day). They also have problems with VOC emissions, since they do not remove VOCs.
Regarding safety, dry filters are a potential fire hazard, especially if dry overspray is allowed to build up. Typically, the majority of this waste is the filter media, which can be contaminated by a relatively small amount of paint. Reusable filters may decrease waste volume and reduce disposal cost. In some applications, such as powder coatings, overspray can be reused.
Choosing the proper type of dry filter is important for a facility's operations. Dry filter characteristics that should be considered include:
Filters made from expanded polystyrene are also available. Facilities can reuse these types of filters after carefully brushing the overspray off the surface with a bristle brush. Hence, the same filters can be used several times until they break or become unusable. Manufacturers have promoted the practice of dissolving the filter in a drum of solvent and paint waste when a facility is ready to scrap the filter. The solvents dissolve the filter into the waste, which must then be treated as a hazardous waste. Some facilities have argued that this is counterproductive due to disposal costs of liquids versus solids. Others argue that this qualifies as treatment of a hazardous waste and therefore is a violation of RCRA regulations1 (EPAq, p. 151).
1For more information, see the May 1995 issue of Metal Finishing.
Water-wash booths capture overspray paint by using positive air pressure to force the particles into a cascading curtain of water. As a result of being captured in the water curtain, uncured particles of paint accumulate in a wash-water pit, located either beneath a grating that the painters stand on or above ground behind the booth itself.
When overspray enters the water, it remains sticky and can plug up holes, nozzles, pipes and pumps. In addition, it can form a deposit on the water curtain, slowly building up a layer that eventually impedes the smooth water flow down the water curtain's face. With time, the water becomes contaminated with bacteria and requires disposal. To prevent this from occurring, the water needs to be treated with chemicals designed to de-tack overspray particles (EPAq, p. 152).
If overall painting volume can justify the investment, a water-wash booth has substantial advantages. 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. However, the primary disadvantage of this technology is the resulting generation of large quantities of wastewater and paint sludge. Typically, spent wastewater and sludge requires offsite treatment, and the paint sludge is disposed of as a hazardous waste. Depending on the amount of coating used, this option could use more energy, require more maintenance time, add to chemical use for water treatment, and/or result in additional cost to dispose of "wet," low BTU value, heavy paint sludges than a dry filter booth. These units are also more expensive to install and to operate than dry filter booths.
The water-wash booth design faces substantial challenges and more restrictive landfill regulations than they have in the past. Prior to 1993, some liquid nonhazardous special wastes could be disposed of in a landfill with little or no treatment. EPA's decision to redefine liquid wastes and ban certain materials from landfill disposal pertains to sludge generated from water-wash booths.2 This material still can be disposed of, however, the material must be processed prior to disposal, resulting in a significant increase in waste treatment costs (Mitchell, p.10).
2EPA's definition of "liquid wastes" is: any material that will exude droplets of liquid through a standard conical paint filter within a prescribed period of time.
A baffle spray booth is an uncommon alternative to both dry filter and water-wash booths. In a baffle spray booth, the face of the booth has steel baffles that run the height of the booth and are several inches wide. The baffles usually overlap each other, forcing the air that passes through the booth to change direction in order to reach the back of the booth. When the air does reach the entrainment section in the back, the paint particulates that the air is carrying fall into the trough for reuse. These booths are used less frequently because unless the facility is reclaiming paint, this type of booth offers no advantages.
In most powder coating operations, the coating is reclaimed and reused in the process, optimizing material use. Powder coating booths have smooth sides with steep, hopper-like sloping bottoms that empty into collectors and an exhaust system that removes powder suspended in the air. The powder is drawn into a cylindrical chamber that has a centrifugal blower to force the powder to the outside walls where the powder collects and then falls through an opening in the cone-shaped bottom. The air flows through a filter at the top to remove any fine suspended powder particles. The reclaimed powder can then be blended with fresh material.
There are a number of steps that a company can take to minimize the defects that result in rejected work. Most of the defects require painters to perform rework or, in some cases, completely reject a part. Higher reject rates result in increased waste generation and reduced profits. The most common coating defects that relate to paint booths include: