Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
Alternatives to Solvent-Borne Coatings
The majority of conventional coatings are solvent borne, traditionally containing about 25% solids and a relatively high organic solvent content. These materials generally have been applied with conventional air spray, which uses compressed air at high pressures to atomize paint, a technique known as low-volume/high-pressure (LVHP). LVHP and other application techniques are discussed in chapter 7. This chapter covers the composition of conventional metal coatings, low-to-no solvent alternatives, and lower toxicity alternatives. The US EPA's Coatings Alternative Guide (CAGE) (available via the Internet at http://cage.rti.org) is a helpful tool for assistance providers to use in identifying specific alternative coatings for facilities.
The major components of paints and coatings are solvents, binders, pigments, and additives. In paint, the combination of the binder and solvent is referred to as the paint "vehicle." Pigment and additives are dispersed within the vehicle (IHWRIC, p. 2). The amount of each constituent varies with the particular paint, but solvents traditionally make up about 60% of the total formulation. Binders account for 30%, pigments for 7 to 8%, and additives for 2 to 3% (KSBEAP, p. 4).
Solvents are a major source of environmental concern because at normal temperatures and pressures they can volatilize (i.e., the liquid solvent becomes a vapor). Exposure to these solvent vapors is dangerous for a number of reasons. In the workplace, solvent vapors can result in a number of human health risks. Table 18 presents information on the health effects of solvents used in paint formulations. Solvent vapors also can pose fire/explosion hazards, necessitating careful storage and handling procedures.
a These solvents are nonhalogenated hydrocarbons; that is, they do not contain chlorine or related elements. Nonhalogenated hydrocarbon solvents are often used in paint formulations as well as in surface preparation and equipment cleaning. Halogenated hydrocarbons are hydrocarbon solvents that contain one or more of the halogens (i.e., fluorine, chlorine, bromine, iodine and astatine). Examples include trichloroethylene (TCE), perchloroethylene (PERC), 1,1,1-trichloroethane (TCA), carbon tetrachloride, methylene chloride (METH) and CFC-113. The halogenated hydrocarbon solvents are preferred for vapor degreasing operations because their flashpoints are in a higher range than those of the nonhalogenated solvents; therefore, they are usually not ignitable. However, halogenated solvents, in general, are more toxic to humans and capable of causing greater environmental damage (IWRC, p. 13-14).
When solvent vapors are released, they emit volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) into the atmosphere. VOCs combine with nitrogen oxides in the presence of sunlight to form ground-level ozone. Ground-level ozone is a precursor to smog, a major pollutant in urban and industrial areas. Smog poses a number of human health risks to respiratory function, particularly among persons with asthma or allergies.
Many pigments still contain lead, chromium, cadmium, or other heavy metals. These paints cannot be disposed of in a landfill and must be handled as a hazardous waste because the heavy metals can leach out of landfills and contaminate groundwater. Production of paints containing these heavy metals is being phased out due to their toxicity (KSBEAP, p.5). EPA banned the production of certain paints containing lead and mercury several years ago. However, some facilities may still have these paints in use if they purchased the paints prior to the phaseout.
Many manufacturers are finding that they can eliminate unnecessary paints and coatings that are used only for appearance. Not only does this reduce capital, operating and maintenance costs, it also reduces potential liability from toxic chemical use (EPA, p. 159). The use of injection-molded plastic sheets in place of painted metal cabinets in the electronics industry is one example of this trend (Freeman, p. 485). Manufacturers that are considering product redesign to eliminate unnecessary coatings must consider the substrate and its characteristics without a coating. If the coating is needed to provide an engineering function, such as improved corrosion resistance, one option may be to change to a base material that does not require a coating (EPA, p. 160). Currently available materials that are free of surface coats include plastics, aluminum, titanium and other metals. Other materials that are under development for a wide range of industries include: cement-bonded particle boards, pultruded products from fiberglass-reinforced plastic, uncoated metals, weathering steel and polymer film coatings (TURI, p. 2).
The primary advantage of conventional solvent-borne paints is their versatility. However, due to the low solids content of conventional solvent-borne paints, a high volume of paint is required to supply a small amount of coverage. In addition, because the paint solvent is highly atomized along with the paint solids in LVHP application, VOC emissions are high (MnTAP, p. 3-4). See figure 3 for more information.
Vendors have developed a number of alternative coating technologies. Environmental compliance remains the principal driver for the development of new technologies (Tilton). These new technologies include:
These coating alternatives can reduce emissions of VOCs and, in so doing, reduce the generation of hazardous wastes and decrease worker exposure to toxic air emissions (EPAd, p. 15). Each of the alternatives is discussed on the following pages. Generally, the P2 alternatives are not one-to-one substitutions. In some cases, an alternative requires a process change using a specific application and/or curing method (e.g., powder coating). Alternatives also can raise other issues (e.g., less solvent in the coating generally requires more thorough surface preparation). For an overview of alternatives to solvent-borne coatings, see table 19. Firms should consult with coatings suppliers for more detailed information on product offerings, as a number of hybrid technologies and different chemistries have recently been introduced (Tilton).
a High-solids formulations do reduce solvent in the coating when compared with low-solids formulations. In many cases, however, high-solids coatings represent the baseline for regulatory limits, and low-solids no longer comply. For this reason, high-solids are considered the baseline for solvent content, and consequently do not have a reduced solvent content even though they qualify as a cleaner technology compared to traditional low-solids coatings.
The relationship between emissions and VOC content, though obviously direct, is not linear; in other words, the transfer efficiency of the application method also can have a significant impact on the amount of VOCs emitted. This issue is explored in chapter 7 (Falcone, p. 35).
High-solids coatings have a higher percentage of paint solids and a lower percentage of solvent carriers than conventional solvent-borne coatings (MnTAP, p. 4). EPA defines high-solids paints as systems with volatile organic contents of less than 2.8 pounds per gallon. Paints with more than 85% solids content by weight are also generally referred to in the coatings industry as high-solids paints. In practice, paints with a solids content of 60 to 80% can be called high-solids paints per EPA's definition, especially if the equivalent solvent-borne paint contains more than 50% solvent (EPA, p. 162).
To achieve solids contents exceeding 70%, the binder in a high-solids paint must be chemically modified so that it has a much lower intrinsic viscosity than binders of conventional solvent-borne paints. To overcome performance limitations, additives often are used to increase crosslinking during curing (EPAd, p. 15). The binders in high-solids paints include alkyd resins, polyester resins, polyurethanes, acrylic resins, epoxy resins and polyvinyl chloride plastisols. Nondrying alkyd resins crosslinked with melamine during heat curing are often used for industrial coatings (EPA, p. 162-163).1
1 For more information see, "High Solids, Low VOC, Solvent-based Coatings," by Ron Joseph, part of the Metal Finishing Special: Organics Finishing Guidebook and Directory that provides detailed information on the advantages/disadvantages of specific resin types.
Because high-solids coatings contain less solvent than traditional formulations, VOC and HAP emissions are reduced in this process (e.g., up to 50%, in some cases) (VT DEC). High-solids paints also provide higher layer thicknesses per application cycle than conventional coatings, resulting in a savings in time. Despite past issues with viscosity, today's high-solids paints can be applied with conventional spray equipment (EPA, p. 162-163). However, surface preparation of the substrate remains a critical issue. This is because a smaller amount of solvent in the coating mixture means a smaller amount will be available to clean the substrate (TURIb, p. 9).
High-solids coatings fit into 3 general categories: air/force dry, baking and two-component.
Air/force dry coatings cure by exposure to moisture or oxygen at temperatures less than 194°F. Alkyd resins are most common in air-dry coatings. Air-dry alkyds are often termed oxidizing or auto-oxidizing because they cure in air without baking or the addition of a catalyst. However, low-temperature ovens can be used to speed cure. The recent development of new acrylic resins has resulted in a range of fast-drying high-solids paints suitable for general metal finishing applications, both indoor and outdoor. These coatings are inexpensive, offer excellent flow and drying properties, good hardness, durability, color and gloss stability, and do not suffer from air entrapment or sagging (EPAd, p. 16).
Bake coatings predominately use acrylic and polyester resins, although some alkyds and modified alkyds are also used. These resin systems cure in an oven at high temperatures (350 to 400°F) to form a crosslinked film. Crosslinking agents, such as melamine-formaldehyde (MF) or blocked isocyanates, are commonly used. MF coatings are usually one-pack systems, catalyzed by a strong acid, such a p-toluenesulfonic acid. Latent or blocked catalysts are used for fast cure and good pot life. Blocked isocyanates, such as aliphatic polyisocyanates, are recommended for coatings requiring superior weathering properties and resistance to yellowing (EPAd, p. 16-17).
In a two-component reactive liquid coating system, two low-viscosity liquids are mixed just before application. One liquid contains reactive resins, and the other contains an activator or catalyst that promotes polymerization of the resins (NCP2P, p. 4). However, once the two components are brought together, curing starts; therefore, these coatings have very short pot lives after mixing. Short pot life can be overcome by using a twin-headed sprayer that is fed from two different pots. This spray head can proportion the flow of each component to achieve the desired ratio of liquids. Thus, the two components mix both on the way to the workpiece and on the workpiece itself (VT DEC).
Two-component coatings cure at low temperatures, and do not require heating in ovens (MnTAP, p. 4). Epoxies and polyurethanes are the most common two-component systems. Epoxies are the oldest form of high-solids coatings, producing thick films for specialty applications. Two-component polyurethane coatings are suitable for use in the automotive and machine tool industries because of their excellent resistance to solvents, lubricants, cutting oils and other chemicals. However, polyurethane coatings do pose some health and safety concerns. For example, polyisocyanates used as crosslinking agents in polyurethane coatings can impair the respiratory function, causing sensitization and in some cases, permanent lung damage (EPAd, p. 17-18).
High-solids coatings usually are applied by conventional spray guns. Traditionally, the high viscosity of high-solids coatings have made them difficult to atomize, making it difficult to achieve a uniform film thickness. Today, emerging formulations are tending toward lower viscosities and, therefore, easier spraying. These new formulations might be based on new resin systems, or additives that modify viscosity and rheology for easier spraying (EPAd, p. 18-19). In addition, the use of a heated spraying system can also reduce viscosity (VT DEC).
If the viscosity of the paint needs adjustment before it can be sprayed, companies generally thin the coating with solvents. Using solvents for thinning increases air emissions and requires the purchase of additional materials. An alternative method for reducing viscosity is to use heat. The benefits from the purchase of paint heaters can include lower solvent use, lower solvent emissions, more consistent viscosities and faster curing rates (MnTAP, p. 3).
Most heaters are stainless steel and are placed between the pump and spray gun. The heaters work best on recirculating systems that return heated material to the container when operators are not spraying. These systems keep the temperature and viscosity constant and avoid cooking the material when spraying stops (MnTAPc, p. 6).
With the exception of two-component liquid coatings, which are widely used for auto and appliance painting (IWRCb, p. 26), high-solids paints have not made the inroads that other systems (such as powder coatings) have in replacing conventional coatings in product coatings applications. Particular problems have included high viscosity, viscosity changes due to temperature variation, and storage stability, as well as the control of film thickness and the drying characteristics of the film (EPAd, p. 15). A variety of new formulations, however, could mean increased growth in a wider variety of markets.
100% Solids Coatings. Because these materials are basically solids, their most distinguishing feature is their viscosity. The 100% solids coatings have a viscosity at room temperature that is approximately 10 times greater than other paint coatings. These materials are not formulated with heavy metals, HAPs or added solvents. Furthermore, once cured, they can be disposed of as nonhazardous solid waste. Because 100% solids coatings have very high viscosities, conventional handling and application methods are ineffective. Instead, mechanical agitation is needed to reduce the viscosity of the coating, making it easy to apply. Increasing the temperature can also reduce viscosity as these coatings are extremely heat sensitive (e.g., an additional 20°F can reduce viscosity by 50%). Application techniques include electrostatic spraying, airless application, roller application and dip tanks. Using these methods the material can be applied in thin layers, providing excellent coverage of the painted object (APC, p. 1.12).
The high-solids coatings that are currently available are generally similar to low-solids coatings in their application, curing and final film properties, and the capital cost for application equipment is approximately the same. The high-solids coatings themselves are slightly more expensive; however, pollution control costs may be lower. In addition, a paint heater might be required (EPAd, p. 15-21). In many cases, high-solids coatings represent the baseline for regulatory limits, and conventional solvent-based, low-solids coatings no longer comply (EPAd, p. 12).
The term waterborne refers to coating systems that primarily use water as the solvent to disperse the resin (IHWRIC, p. iv). Usually, they contain up to 80% water with small amounts of other solvents, such as glycol ethers (TURI, p. 1). Most regulations require waterborne coatings to have a VOC content of less than 3.5 pounds per gallon less water (EPAd, p. 47).
In addition to reducing VOC emissions during application, waterborne coatings reduce risk of fire, are easier to clean up (creating less hazardous residues) and result in reduced worker exposure to organic vapors (EPAd, p. 46-52). However, special equipment might be required for application, as water in the formulation can cause corrosion. For instance, water-based paints can rust plain steel or attack aluminum; therefore, application equipment must be constructed of a corrosion-resistant material such as 316 stainless steel. Humidity must also be controlled to achieve the best film formation; a microprocessor-controlled water-spray system is one method for doing so (EPAd, p. 52). For more information on other advantages and disadvantages of waterborne coatings, see the box at the end of this section.
Almost all types of resins are available in a waterborne version, including vinyls, two-component acrylics, epoxies, polyesters, styrene-butadiene, amine-solubilized, carboxyl-terminated alkyd and urethanes (EPAd, p. 47-48). Waterborne coatings are classified based on how the resin is fluidized (KSBEAP, p. 6). The three main types are: water-soluble/water-reducible (solutions), water-dispersible/colloidal (dispersions) and emulsions (latex) paints (the most commonly used form) (TURI). Within each category, physical properties and performance depend on which resins are used (KSBEAP, p. 6).2
2 For more information on the advantages and disadvantages of each resin type see, "High Solids, Low VOC, Solvent-based Coatings," by Ron Joseph, part of the Metal Finishing Special: Organics Finishing Guidebook and Directory.
Water-soluble paints are paints whose individual molecules of water-soluble resins dissolve completely in water. Water-soluble resins are generally produced via polycondensation or polymerization reactions in an organic medium. As a result, they generally contain organic co-solvents like alcohols, glycol ethers or other oxygen-containing solvents that are soluble or miscible with water (organic content less than 10 to 15%). Because of viscosity anomalies, waterborne paints made with water-soluble binders have only about 30 to 40% solids content by weight. Resins include alkyds, polyesters, polyacrylates, epoxies and epoxy esters. Despite their sensitivity to water, water-soluble paints have a high gloss and a high level of corrosion protection, along with good pigment, wetting and stabilization (EPA, p. 160).
Water-dispersible paints, or colloidal coatings, are paints that have small clusters of insoluble resin particles that are suspended in water. Mechanical agitation is sufficient to suspend the clusters (KSBEAP, p. 7). Small amounts of organic solvents (usually less than 5% by weight) are used as coalescing agents that evaporate on drying. Resins used in dispersion paints include vinyl acetate copolymers, vinyl propionate copolymers, acrylate-methacrylate copolymers, and styrene-butadiene copolymers and polymers (EPA, p. 161). Colloidal dispersions are used mainly to coat porous materials such as paper or leather (EPAd, p. 49).
Emulsions, or as they are more commonly known, latex paints, are similar to water-dispersibles. However, resin clusters in emulsions tend to be larger, and an emulsifier is required to keep the clusters in suspension (KSBEAP, p. 7). Emulsion paints are manufactured using a variety of resins including styrene-butadiene copolymers, polyvinyl acetate (the most common), acrylics, alkyds and polystyrene. Emulsion paints are widely used in the architectural market segment (IHWRIC, p. 6). The increased permeability of latex paints allows these coatings to "breathe," reducing the chances for blistering or peeling (EPA, p. 161).
Water-based alkyds may take longer to dry than solvent-borne coatins, but the resulting coatings have similar gloss, flow and leveling properties. These coatings are extremely versatile because they are thinned with water to almost any viscosity. They can be applied with spray or dip applications and are among the least expensive VOC compliant coatings (CAGE).
Application technology for waterborne coatings is comparable to that of conventional solvent-borne coatings. If a facility is using a water wash booth, overspray is easily recovered and reused if colors are appropriately segregated. Uncured waterborne coatings can be cleaned from equipment with water (TURI, p. 1).
Electrostatic spray can be used if the electrically conductive waterborne paint is isolated from the electrostatic system. Three methods can be used to avoid grounding out the electrostatics in a waterborne system. The facility can (1) isolate the entire paint system from electrical grounds; (2) isolate a small part of the wetted system with a voltage blocking device; and (3) indirectly charge the paint particles away from any wetted equipment. Each method has its own advantages and disadvantages and should be evaluated for the specific application. The use of a voltage blocking device at each atomizer is often the most cost-effective method (EPAd, p. 51) (VT DEC).
Waterborne coatings can also be applied by electrodeposition for corrosion resistance and coating of hard-to-reach areas (TURI, p. 1). However, some formulations or substrates might require special pumps and piping to prevent corrosion from water in the formulation. In addition, for product finishing, coatings need to dry or cure at elevated temperatures to ensure complete cure in a reasonable period of time. Therefore ovens are required with this process (EPAd, p. 52).
Waterborne coatings have quickly taken hold in some product-coating market segments; for more than two decades, copiers, fax machines, typewriters, printers and computers have been painted with various combinations of waterborne emulsion and other coatings (McBree et al., p. 35). However, waterborne coatings have been less accepted in market sectors with requirements that are exceptionally high for appearance and engineering. In recent years, however, the automotive OEM sector has increased its use of water-based paints and coatings in all but the heaviest coat applications. An estimated 20% of this sector now uses water-based paints, and that percentage is growing each year. With improved water-based paint technology, manufacturers have been able to change from solvent-borne paint systems and meet emissions regulations while maintaining their ultrahigh finish standards (EPA, p. 162).
Waterborne Two-Component Technology. With this new technology, coatings manufacturers can formulate high-performance coatings without cosolvents and achieve the same appearance, properties and ease of use that manufacturers have with the solvent-borne analogs. For example, an epoxy curing agent for water-based epoxy coating formulations has been designed for use with solid epoxy dispersions. This epoxy curing agent provides corrosion resistance when used as a primer in general metal applications (Iceman, p. 27).
Waterborne coatings are more expensive than conventional coatings per unit of reactive
resins. In addition, the capital costs for application equipment tends to be greater
(e.g., stainless steel is required to protect against corrosion in storage tanks and
transfer piping). However, water-based coatings generally use less organic solvents,
reducing environmental and human health risks (EPAd, p. 58-60). Technical assistance
providers should remember that, despite the use of water in waterborne formulations,
discharge of wastes from coatings must still be in compliance with federal and state
wastewater discharge regulations. Paint manufacturers, however, are developing methods for
recycling waterborne paints collected from communities and industry (EPA, p. 162).
Powder coating uses 100% resin in a dry, powdered form (MnTAP, p. 4). Powder coating works on the principle that opposite charges attract. The powder is pneumatically fed from a reservoir through a spray gun where the powder gains a low amperage, high-voltage positive charge. Parts to be painted are electrically grounded so that the positively charged powder particles are strongly attracted to the parts' surfaces. The powder-coated part is then pulled through an oven where the powder melts and fuses into a smooth coating (IHWRICe). Substrates must generally be able to withstand temperatures of 260°F or higher (EPAd, p. 33).
Powder-coating materials can provide a high-quality, durable, corrosion-resistant coating. Powder coatings do not produce hazardous overspray wastes or wastewater sludges, and most do not release VOCs when cured (some powder coatings will release VOCs, such as caprolactam, a former HAP). With powder coating, users can collect the powder overspray and reuse it, resulting in transfer efficiencies of up to 99% (MnTAP, p. 4). However, powder coating systems require the complete conversion of a coating line, which can be costly. For more information on other advantages and disadvantages of powder coating, see table 23 at the end of this section.
Product manufacturers can specify the properties required in a finish (such as resistance to ultraviolet light, high durability, corrosion resistance and color) to a powder coating manufacturer who then formulates the appropriate powder (IHWRICe). Coating powders are frequently separated into decorative and functional grades; decorative grades generally have a finer particle size than functional grades. Powders are also divided between thermoset and thermoplastic resins (EPA, p. 163-164).
Thermoset resins crosslink to form a permanent film that withstands heat and cannot be remelted. They are used for decorative and protective coatings for architectural structures, on appliances and furniture, and elsewhere. Thermosetting resins are characterized by their excellent adhesion to metal; they are one-coat systems and do not require a primer (Farrell, p. 81). The five basic families of thermoset resins are epoxies, hybrids, urethane polyesters, acrylics and triglycidyl isocyanurate (TGIC) polyesters as described below:
Thermoplastic resins form a coating, but do not undergo a change in molecular structure. These resins can be remelted after they have been applied. Thermoplastic powder coatings melt and flow when heat is applied, but retain the same chemical composition when they are cool and solidified (KSBEAP, p. 10). Although some thermoplastic materials provide adhesion to metal, most require a primer (Farrell, p. 81). Thermoplastic resins are mainly used in functional coatings, such as thick, protective coatings on dishwasher trays. Examples of thermoplastic resins used in powder coating are polyethylene, polypropylene, nylon, polyvinyl chloride (PVC), and thermoplastic polyester. These examples are described below:
See table 24 for a summary of powder coating resin properties.
There are five powder coating processes: electrostatic spraying, fluidized bed, electrostatic fluidized bed, flame spray, and tribocharge.
The main method in use today for the application of powder coatings is the electrostatic process. In the electrostatic process, electrostatic spray guns impart an electrostatic charge to the powder being sprayed via a charging electrode that is located at the front of the spray gun. This technique is called "corona charging," and these guns generate a high-voltage, low-amperage electrostatic field between the electrode and the product being coated. The charge on the electrode can be controlled by the operator. Powder particles become charged as they pass through the ionized electrostatic field, which controls the deposition rate and the powder's location on the part. The field can be adjusted to direct the powder's flow, control pattern size, shape, and powder density as it is released from the gun (KSBEAP, p. 14). The particles are attracted and held to the grounded substrate through electrostatic forces. The substrate subsequently is heated in an oven, or through chemical activation (e.g., by infrared), to fuse the particles to the substrate and to each other to create a continuous film (EPA, p. 164). This method has made it possible to apply thin layers of coatings for higher quality decorative finishes, and has allowed powders to be used on parts that should not be dipped in a fluidized bed.
Powder is supplied to the electrostatic spray gun by the powder delivery system. This system consists of a powder storage container, or feed hopper, and a pumping device that transports a stream of powder into hoses or feed tubes. Compressed air is often used as a pump because it aids in separating the powder into individual particles for easier transport. The powder delivery system is usually capable of supplying powder to one or several guns. Delivery systems are used in many different sizes, depending on the application, number of guns to be supplied, and volume of powder to be sprayed in a given time period. Recent improvements in powder delivery systems, coupled with better powder chemistries that reduce clumping, have made delivery of a consistent flow of particles to the spray gun possible. Agitating or fluidizing the powder in the feed hopper also helps prevent clogging or clumping of the powder before it enters the transport lines (KSBEAP, p. 9). Innovations in powder delivery systems also allow the powder supply reservoir to be switched easily to another color when necessary. Systems are also available for segregating colors so that several colors can be applied in the same booth (EPAd, p. 36).
Initially, powder was applied using a fluidized bed process in which heated parts were dipped into a vat with the suspended coating powders. As these particles came in contact with heated parts they softened and began to "flow" into other particles to create a coating. The coatings were thick, usually vinyl or epoxy, and demonstrated functional rather than decorative qualities (KSBEAP, p. 9). However, several methods for powder coating exist now, which makes powder coating a more versatile option, however fluidized bed is still used in certain operations.
In a fluidized bed, powder particles are kept in suspension by an air stream. A preheated workpiece is placed in the fluidized bed where the particles coming in contact with the workpiece melt and adhere to its surface. Coating thickness depends on the temperature and heat capacity of the workpiece, and its residence time in the bed. Postheating is generally not required when applying thermoplastic powder coatings. However, postheating is required to cure thermoset powder coatings completely (NEFSC).
Electrostatic Fluidized Bed
An electrostatic fluidized bed is similar in design to conventional fluidized beds, but its air stream is electrically charged as it enters the bed. The ionized air charges the particles as they move upward in the bed, forming a cloud of charged particles. The grounded workpiece is covered by the charged particles as it enters the chamber. No preheating of the workpiece is required. However, curing of the coating is necessary. This technology is most suitable for coating small objects with simple geometries (NEFSC).
Flame spray was recently developed for application of thermoplastic powder coatings. The thermoplastic powder is fluidized by compressed air and fed into a flame gun where it is injected through a flame of propane, which melts the powder. The molten coating particles are deposited on the workpiece and form a film upon solidification. Because no direct heating of the workpiece is required, this technique is suitable for applying coatings to most substrates. Metal, wood, rubber and masonry can be coated successfully using this technique. This technology is also suitable for coating large or permanently fixed objects (NEFSC).
Tribocharging relies on friction between the powder and the spray gun. The action of the powder flowing through the barrel of the gun generates a frictional charge on the powder. The charged powder is carried by the air stream to the substrate, where it adheres due to electrostatic attraction. Because no high-voltage system is used, the electric field is substantially smaller and the powder tends to follow air currents rather than field lines. The smaller electric field results in a much reduced Faraday cage effect3. Consequently, tribo guns produce smoother finishes, allow deposition of thicker films, and provide better coverage of intricately-shaped objects (EPAd, p. 31).
3 The Faraday cage effect occurs when the electrostatic-field force limits the entry of paint particles in recessed areas. To achieve coating in the recessed area, overpainting of the nonrecessed area or manual touchup often is required. In this situation, real transfer efficiency is less than the quoted transfer efficiency.
Currently, 85% of the total market for powder coatings is represented by four industrial areas: metal finishing (53%), appliances (21%), lawn and garden (8%), and architectural applications (3%) (EPA, p. 164).
Since 1986, all or most automotive manufacturers have powder coated engine blocks, the largest volume job in the history of the powder industry. Now powder has come out from "under the hood" and is being used on a wide range of trim and accent parts. Polyester and acrylic powders are used in these coatings. For example, the "metallic look" powders are delivering luster to aluminum wheels. However, there remains great potential for more powder use in the automotive industry; its use as a primer surface and anti-chip coating on body panels is becoming more common. Powder coatings have undergone extensive testing both as a primer surfacer and antichip coating and have met OEM standards for chip resistance, adhesion, durability, and heat and humidity exposure.
In addition, clear powders over liquid base coats are currently being tested for exterior auto body finishing. The advent of clearcoat finishes for base coats in the mid-1980s made it more economically feasible to use powder as an automotive topcoat. Using specially formulated acrylic and polyester powders, manufacturers are working to meet the automotive industry standard for clearcoats of absolute smoothness, clarity, perfection and performance (Bocchi, p. 21).
Can Coating. Development of powder coatings for the coating of can interiors, tops, ends and lids is well underway. In addition, application equipment is now available to apply, recover and recycle the very small particle size powders required to maintain thin films and run at line speeds common in this industry. Food and Drug Administration approval is still pending.
Lower-Temperature Cures. Powder coatings with very high reactivity have been developed to cure at temperatures as low as 121°C (250°F). Such low-curing powders will allow more types of products to be coated with powder, including plastics and preassembled products that contain heat-sensitive fluids or gaskets. In addition, manufacturers can run higher line speeds with the lower-cure powders, thereby increasing production capacity.
Weathering Capabilities. Significant advances have been made in the development of polyester and acrylic resin systems with excellent long-term weatherability, which is needed to meet the extended warranties being offered by manufacturers. Also under development are fluorocarbon-based powders that will match or exceed the weatherability of liquid fluorocarbons, with application costs similar to or lower than conventional powder coatings.
Thinner Films. Powder manufacturers are continually working to develop powders that can form films that are thinner than those previously attainable, resulting in a savings of material and money. Based on epoxy-polyester hybrids, these powder coatings provide applications in the range of 1 to 1.2 mils for colors with good hiding powder. These thin coatings are currently suitable only for indoor applications (Moore, p. 66 and Bocchi, p. 32-34).
Powder coating emits no VOCs and offers several performance advantages. However, to introduce powder coating to an existing paint line, a capital investment in special equipment must be made. Pretreatment of the part to be coated also needs to be quite thorough, which can add to the overall cost (EPAd, p. 35). For entirely new lines, however, investment in powder application equipment is comparable to that of equipment for liquid coatings (VT DEC). In addition, the cost of producing a finished coating is typically lower with powder coating than conventional coating because maintenance and operating costs are less, particularly for operations that use a single color (EPAd, p. 42).
Radiation curing uses ultraviolet (UV) and electron beam (EB) electromagnetic radiation to polymerize specially formulated coatings directly on a substrate. Called photopolymerization, the UV-curing process is a photochemical reaction. Specially formulated coatings mixed with a small amount of materials called photoinitiators are exposed to a UV-light source, initiating crosslinking. The rate of polymerization depends on the intensity of the radiation used (Radtech, p. 40). EB curing crosslinks coatings by exposing them to low-energy electrons; however, because of the high cost associated with EB generators, this method of radiation curing accounts for only about 10 to 15% of the total radiation curing market (Lucas, p. 29).
Radiation curing produces high-performance protective and decorative finishes. Radiation-curable coatings can be 100% reactive liquids, completely eliminating the use of solvents. However, some of the resins in these coatings can volatilize, resulting in VOCs. Although emissions are usually low, the amount of VOCs emitted from radiation curing depends entirely upon the coating formulation (EPAd, p. 68). In addition, the shape of the part will affect the curing; flat surfaces are easiest to cure. Capital investments for UV-curing systems are usually lower than investments for conventional ovens and use considerably less space. The cost of the coating is generally higher on a per pound basis, but not always on a coverage basis (RadTech). For more information on other advantages and disadvantages of radiation curing, see table 25 at the end of this section.
A complete formulation for a radiation-curable coating consists of a blend or mixture of oligomers (low molecular weight polymers), monomers, additives, pigments, and photoinitiators. The oligomer used in the formulation plays an important role in determining the final properties of the finish (Radtech, p. 40). Resins used in conventional solvent-based coatings can be chemically modified for use in radiation-cured systems by introducing acrylate functionality. The general physical and chemical characteristics of the resins are retained after modification (EPAd, p. 71 ). The oligomers most commonly found in today's radiation-curable formulations are acrylated urethanes, epoxies, polyesters and silicones (Radtech, p. 40). Coatings that use acrylated resins cure by free radical polymerization and comprise 85% of the total radiation-curable coatings market (Lucas, p. 28).
Coatings can also cure by cationic curing, the polymerization of cycloaliphatic epoxies or vinyl ethers. Cationic curing is an attractive option because it withstands pasteurization and promotes adhesion to metals, even during postforming operations (Lucas, p. 28-29).
UV-cured coatings can be applied using traditional spray methods, but roll-coating is often used on flat stock (KSBEAP, p. 11). Varnishes on two-piece cans are applied using an offset process, while curtain coating is used in some specialty applications (RadTech).
The use of UV-inks and overprint coatings on two-piece metal cans has been commercially successful for more than 10 years. Coating of three-piece composite and metal-can ends has been a commercial reality since the 1970s (RadTech, p. 14). UV-cured coatings are widely used to provide corrosion resistance to galvanized metal tubing. It is also used on metallized plastics. In addition, UV-cured coatings have been formulated for coil coating, in which outstanding resistance and flexibility have been achieved.
Significant growth in other metal markets could occur in the next decade as environmental and productivity requirements increase. The use of UV curing is growing rapidly for wood finishes, medical appliances, consumer products, automotive head lamp assemblies, optical fibers and electronics. Growth will be further enhanced with the development of cationic-cured epoxies, which provide improved adhesion to, and protection of, metal substrates (MPC, p. 29-36).4
4 For more information on the use of radiation curing in can manufacturing, refer to the EPA document Project Summary: Evaluation of Barriers to the Use of Radiation-Cured Coatings in Can Manufacturing.
Water-Reducible, UV-/EB-Curable Formulations. These formulations have been developed for a number of coatings and products, including flexo and gravure inks, clear coatings for wood furniture, and dip-coated or spray-coated plastics.
Water dilution of a compatible resin system provides lower viscosity, thinner films, improved flow and leveling, lower applied costs and lower amounts of monomers and solvents. The use of water as a viscosity reducer can minimize or eliminate the use of lower molecular weight diluents, which tend to be skin irritants. Some research has indicated that small amounts of water (1% of water) can reduce the viscosity of oligomers substantially, and larger amounts of water can be used as a formulation tool to vary gloss and reduce web temperatures in critical applications. Disadvantages include the increased time and energy required to remove any added water, as well as the negative effects of water on the drying and curing system and the substrate to which it is applied. If the material is cured before the water is fully evaporated, then the film properties will be reduced (Lawson, p. 16).
The UV-radiation source most commonly used in industry is the medium-pressure mercury-electrode arc lamp. These lamps can be retrofitted easily to existing production lines, but they require an extraction system to remove excess heat and ozone that is generated by UV action on oxygen in the air (EPAd, p. 72). The cost of an electrode arc system is approximately $6,400 for a 10-inch lamp, shields to contain the UV light waves that are harmful to skin and eyes, reflectors, shutters, a high-voltage power supply, and an air cooling fan (EPRI). An alternative UV system produces UV radiation through microwave excitation of the mercury vapor (EPAd, p. 72). A microwave-powered UV-curing system costs approximately $7,500. This system includes a standard-length lamp, a power supply, an air cooling system, a cable, and a detector to ensure that microwave radiation leaks do not occur (EPRI).
EB generators are expensive, complex and large. In addition, oxygen has an inhibiting effect on crosslinking initiated by EB; therefore, companies must establish an inert atmosphere of nitrogen, with oxygen concentrations of less than 100 parts per million (ppm) if adequate curing is to be achieved (EPAd, p. 72).
This section presents coating systems that have only recently become commercially available. Knowledge of other technologies that are still under research and development is important for technical assistance providers. However, presenting information on experimental systems is not within the scope of this manual. Technologies covered in this section include vapor permeation of injection-cured coatings, supercritical carbon dioxide and unicoat paint. For more information on coating research and development, consult the trade journals listed in appendix A.
Vapor Permeation of Injection-Cured Coatings (VIC). After a reactive resin is applied as a liquid, curing is induced by exposing the liquid to a vapor-containing compound that initiates polymerization. Examples are polyol-isocyanate coatings that cure by tertiary amine vapor injection (NCP2P, p. 4). The amine vapor is made by an amine generator in a predetermined concentration and is dispersed in an air stream channel in the spray gun. The generator uses dried and filtered air at 90 to 120 psi. The coating material and catalyst are mixed as they leave the spray gun.
This technology is a high-solids coating system because the coating still uses solvent in the formulation. However, its ease of use and rapid cure times can improve production efficiency (EPAd, p. 80).
VIC can produce a variety of finishes with good performance characteristics including chemical, solvent, and stain resistance; high humidity and water resistance; high mar and abrasion resistance; and color and gloss retention. These coatings can be used on a broad range of substrates including plastic, steel, aluminum, wood and castings. Heat-sensitive parts such as thermoplastics and thermosets are ideally suited to the low-temperature cure used with VIC (EPAd, p. 80). For other advantages and disadvantages of VIC, see table 26.
VIC is compatible with LVHP, HVLP, electrostatic and airless spray systems. However, electrostatic equipment might need to be modified to accommodate the amine generator. In addition, some types of spray guns might have rubber or plastic seals that degrade when exposed to the amine. Capacity is limited to two spray guns (EPAd, p. 80).
UNICOAT Paint Technology: The UNICOAT technology is a one-coat painting system for aircraft that replaces the combination of a coat primer system and a top coat system. Since only one coat is applied instead of two coats, VOC emissions and waste generated from cleanup operations can be reduced by 50 to 70%. This technology, developed by the Naval Warfare Center (NAWC), consists of a self-priming topcoat for aircraft and other industrial parts. It is applied directly to the metal substrate without priming (NFESC).
UNICOAT, which is formulated without lead or chrome, replaces the two-coat system with a blend of organic and inorganic zinc compounds that are non-toxic. UNICOAT contains polyurethane as do traditional coatings; however, corrosion inhibitors and adhesion promoters have been added to UNICOAT.
UNICOAT has performed at levels equivalent to, and superior to, the performance levels for conventional paints (in applications by the U.S. Navy and U.S. Air Force). To avoid adverse reactions, freshly painted wet surfaces must not come in contact with alcohols, amines, water or acids. Costs for the UNICOAT system varies depending on the specific application (NFESC).