Appendix C

REVIEW OF ORGANIC COATING TECHNOLOGY

Military equipment, vehicles, ships, and aircraft generally require coatings that are both hard and flexible, with high requirements for outdoor durability, corrosion protection, adhesion, and resistance to chemicals. This combination of properties is best achieved if there is a chemical reaction during and/or after film formation to “cure” the coatings. This chemical reaction provides connections between the polymer molecules, leading to formation of cross-links in the binder. This class of coatings is generally referred to as thermoset, as compared to thermoplastic coatings, which are not cross-linked. Thermoplastic paints such as architectural latexes, industrial plastisols, and nitrocellulose lacquers are used in some industries; the Navy uses modified rosin gum-based antifoulant paints. Several good references are available on the complex and sophisticated chemistry of organic coatings and on their applications.1,2,3

Thermoset coatings are used in several demanding civilian applications and have military uses as well. In the civilian economy, such coatings are generally classified as special-purpose or original equipment manufacture (OEM) coatings. Thermoplastic marine antifoulant paints are an exception.

Special-purpose coatings include paints for automotive refinishing and for aircraft and marine applications; these meet technical requirements broadly similar to the requirements for coatings for military needs. Other significant specialty coatings are for industrial maintenance, roof coatings, and traffic striping. A salient feature of specialty coatings is that they are cured at or close to ambient temperature, as are most depot and field-applied military coatings.

Most coatings applied during original manufacturing are cured at elevated temperatures. Application in factories requires fast throughput, which can be best achieved with coatings cured at temperatures higher than 110 °C (230 °F) for metal substrates and around 82 °C (180 °F) for plastics and wood. Important OEM markets include automobiles, appliances, furniture, containers, machinery, small implements, and flat stock. Chemical coating sales in the United States in 2000 were $17 billion at a volume of 5.24 billion liters (1 billion gallons).4 The market mix is shown in Table C-1.

Table C-1 Market Share of Coating Sales in the United States in 2000

 

Percentage of Dollar Value (2000 total: $17 billion)

Percentage of Gallon Volume (2000 total: 1.3 billion gallons)

Original equipment

40

36

Special purpose

22

14

Architectural

38

50

 

Source: Chemical Week. November 7, 2001. Available at <http://www.chemweek.com>. Accessed January 2002.

Five of the largest paint suppliers in the United States are Sherwin Williams, PPG, DuPont, BASF, and Akzo Nobel. These highly diversified global companies have significant sales in the automotive and refinish markets. Other major paint suppliers include Valspar and Benjamin Moore. The industry is consolidating, but about 450 paint manufacturers remain active in the United States and more than 17,000 remain active globally.

1  

Wicks, Z.W., Jr., F.N. Jones, and S.P. Pappas. 1999. Organic Coatings. 2nd edition. New York: Wiley-Interscience.

2  

National Research Council. 1994. Coatings, subsection in Polymer Science and Engineering: The Shifting Research Frontier. Washington, D.C.: National Academy Press, pp. 98-100.

3  

Koleske, J.V. 2000. A century of paint. Paint & Coatings Industry Magazine 1:54-114.

4  

Chemical Week. November 7, 2001. Available at <http://www.chemweek.com>. Accessed January 2002.



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Transfer of Pollution Prevention Technologies Appendix C REVIEW OF ORGANIC COATING TECHNOLOGY Military equipment, vehicles, ships, and aircraft generally require coatings that are both hard and flexible, with high requirements for outdoor durability, corrosion protection, adhesion, and resistance to chemicals. This combination of properties is best achieved if there is a chemical reaction during and/or after film formation to “cure” the coatings. This chemical reaction provides connections between the polymer molecules, leading to formation of cross-links in the binder. This class of coatings is generally referred to as thermoset, as compared to thermoplastic coatings, which are not cross-linked. Thermoplastic paints such as architectural latexes, industrial plastisols, and nitrocellulose lacquers are used in some industries; the Navy uses modified rosin gum-based antifoulant paints. Several good references are available on the complex and sophisticated chemistry of organic coatings and on their applications.1,2,3 Thermoset coatings are used in several demanding civilian applications and have military uses as well. In the civilian economy, such coatings are generally classified as special-purpose or original equipment manufacture (OEM) coatings. Thermoplastic marine antifoulant paints are an exception. Special-purpose coatings include paints for automotive refinishing and for aircraft and marine applications; these meet technical requirements broadly similar to the requirements for coatings for military needs. Other significant specialty coatings are for industrial maintenance, roof coatings, and traffic striping. A salient feature of specialty coatings is that they are cured at or close to ambient temperature, as are most depot and field-applied military coatings. Most coatings applied during original manufacturing are cured at elevated temperatures. Application in factories requires fast throughput, which can be best achieved with coatings cured at temperatures higher than 110 °C (230 °F) for metal substrates and around 82 °C (180 °F) for plastics and wood. Important OEM markets include automobiles, appliances, furniture, containers, machinery, small implements, and flat stock. Chemical coating sales in the United States in 2000 were $17 billion at a volume of 5.24 billion liters (1 billion gallons).4 The market mix is shown in Table C-1. Table C-1 Market Share of Coating Sales in the United States in 2000   Percentage of Dollar Value (2000 total: $17 billion) Percentage of Gallon Volume (2000 total: 1.3 billion gallons) Original equipment 40 36 Special purpose 22 14 Architectural 38 50   Source: Chemical Week. November 7, 2001. Available at <http://www.chemweek.com>. Accessed January 2002. Five of the largest paint suppliers in the United States are Sherwin Williams, PPG, DuPont, BASF, and Akzo Nobel. These highly diversified global companies have significant sales in the automotive and refinish markets. Other major paint suppliers include Valspar and Benjamin Moore. The industry is consolidating, but about 450 paint manufacturers remain active in the United States and more than 17,000 remain active globally. 1   Wicks, Z.W., Jr., F.N. Jones, and S.P. Pappas. 1999. Organic Coatings. 2nd edition. New York: Wiley-Interscience. 2   National Research Council. 1994. Coatings, subsection in Polymer Science and Engineering: The Shifting Research Frontier. Washington, D.C.: National Academy Press, pp. 98-100. 3   Koleske, J.V. 2000. A century of paint. Paint & Coatings Industry Magazine 1:54-114. 4   Chemical Week. November 7, 2001. Available at <http://www.chemweek.com>. Accessed January 2002.

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Transfer of Pollution Prevention Technologies The major driving force for technology change in the past 30 years has been reduction of emission of volatile organic compounds (VOCs) in compliance with the Clean Air Act of 1970 and the successive federal and state regulations. Compliance was accomplished primarily by reducing the solvent content in paints and secondarily by improving the deposition efficiency. Elimination of toxic ingredients is another major issue for the paint industry. Today’s paints contain no heavy-metal-based pigments, with the exception of lead in some electrocoats and chromates. Lead is gradually being eliminated from electrocoat. Strontium chromate is still a key ingredient for improving the corrosion resistance of primers for aircraft and coil coatings, and no satisfactory replacement has yet been found. Chromates are also part of many inorganic conversion coatings for aluminum, steel, and galvanized steel. Such conversion coatings are applied before metal substrates are painted with organic coatings. New chromate-free conversion coatings are now being market tested. Formulation of paints without hazardous air pollutants (HAPs) is a high priority. Toxic aromatic solvents and ethylene-oxide-based oxygenated solvents are being increasingly replaced by more benign ingredients. There is also much research to reduce formaldehyde emissions from heat-cured coatings that contain amino resin crosslinkers. Amino resins are condensation products of formaldehyde with certain nitrogenous compounds. In recent years, the Environmental Protection Agency has exempted a few solvents, including acetone, methyl acetate, and 4-(trifluoromethyl)chlorobenzene from VOC regulations on the grounds that their potential for generating ozone is very low. When used in coatings, these exempt solvents do not count as VOCs. While exempt solvents are expensive and/or very volatile, they can be blended with other solvents to enable formulators to meet stringent VOC limits. HIGH-TEMPERATURE-CURE COATINGS The cross-linking reaction in a thermoset material is heat induced in most industrially applied coatings. Many cars, trucks, small implements, appliances, metal containers, metal furniture, and other industrial products are covered by heat-cured paints when originally manufactured. Most often, acrylic and polyester resins are used in topcoats. The topcoat resins contain pendant hydroxyl (-OH) groups, which are reactive toward cross-linkers. The most widely used crosslinkers are the amino resins. Other cross-linkers such as blocked isocyanates also find considerable use. The design of resins and cross-linkers is continuously improved. Such binder systems provide excellent performance at relatively low cost. The long durability and high esthetic value of modern automotive paints illustrate this benefit. Also, modern washing machines, dryers, and dishwashers break down mechanically long before their paint fails. Special property requirements for automotive topcoats such as acid rain resistance require various functional resins and cross-linkers. These include cross-linking of hydroxyl functional resins with blocked isocyanates, carbamate functional resins with amino resins, and carboxyl functional resins with epoxies, to name a few examples. Corrosion protection is an essential requirement for most coating systems for metal. Topcoats provide modest protection and can be used without primer for some indoor applications, such as metal file cabinets and shelving. In applications requiring rigorous corrosion protection, primers, usually based at least partly on epoxy resins, are used. Where corrosion protection in recesses and crevices is important, as for autos, trucks, farm implements, or appliances, electrodeposited coatings based on modified epoxy resins and cross-linked with blocked isocyanates are almost universal. Such primers require high baking temperatures, generally in the range of 160 to 190 °C (320 to 374 °F). Heat-induced cross-linking reactions often involve the evolution of volatile by-products. This happens even in the cure of solventless powder coatings, which consequently cannot be designated as “zero VOC.” Ambient-temperature-cure coatings, generally referred to as special-purpose coatings, are particularly important for the refinishing of large objects such as airplanes, naval vessels, and cars and trucks. Also, because heat curing of large objects is not practical, airplanes, naval vessels, combat vehicles, large implements, bridges, and industrial machinery are painted with ambient-temperature-curing paints both in original manufacture and in refinishing. The most commonly used high-performance ambient-curing paints are two-component urethanes for topcoats and two-component epoxies for primers. Most urethane binders are hydroxyl functional acrylics or polyesters cross-linked with trifunctional isocyanates. Amine functional compounds are used

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Transfer of Pollution Prevention Technologies for cross-linking epoxy resins. It is one of the major achievements of modern coating technology that ambient-curing automotive refinishing systems based on two-component urethane topcoats and two-component epoxy primers now provide properties matching those of heat-cured coating systems. Fundamentally similar two-component coatings are used for painting aircraft, both in original manufacturing and in refinishing, and for above-water-line paints for commercial ships. Alternate two-component technologies for topcoats have been developed and have the advantage that they do not involve the use of isocyanates, which are highly allergenic. They are important in a few markets. Such systems include the cross-linking of glycidyl, carbonyl, or activated methylene or acryloyl functional binders with polyamines generated by the reaction of ketimines with moisture. A special two-component solventless system is a blend of unsaturated polyester and styrene. Such gel coats are cured by incorporation of peroxides. Boat and yacht coatings, particularly for fiberglass hulls, and some “wet look” wood furniture coatings are based on gel coats. A single-component and ambient-temperature cross-linking is achieved by air oxidation of alkyd resin binders. Though alkyd paints are generally inferior to isocyanate-cured paints in outdoor durability and chemical resistance, they are used extensively in high-gloss architectural paints. Modification with silicones, acrylics, or urethanes improves performance, though not enough to match that of two-component systems. The Navy aims to avoid the use of isocyanates. Instead of two-component urethanes, the Navy uses silicone-modified alkyds for topcoats. These are applied over two-component epoxy primers in above-water-line paints. ORGANIC COATINGS WITH REDUCED SOLVENT CONTENTS Before the 1970s most industrial and specialty paints were applied from organic solvents at relatively low solids contents, causing significant emission of ozone-depleting chemicals. In the past 30 years, the solvent content of paints has been drastically reduced by the means outlined below. High-Solids Coatings High-solids paints are one of two dominant nonpolluting paint technologies in the United States. New binders were developed that allowed convenient paint application at 50 to 80 weight percent solids content, or 1000 to 250 grams of solvent per kilogram of paint solids (1 to 0.25 pounds of solvent per pound of paint solids). In contrast, a typical old-style solvent-borne paint contained about 20 to 40 weight percent solids, or 4000 to 1500 grams of solvent per kilogram of paint solids (4 to 1.5 pounds of solvent per pound of paint solids). According to these examples, the change to high-solids paints often reduced solvent emissions 4-fold, and in some instances even 16-fold. The current high-solids technology for heat-cured coatings has been evolving since about 1970. Of the many technical expedients involved, the most critical is the reduction in the molecular weight of the binder resins without compromising the physical properties of the final coating film. This was accomplished using modern polymer chemistry so that the new high-solids paints generally perform better than their low-solids counterparts. High-solids automotive and appliance coatings have been particularly successful in the past 25 years. The Clean Air Act of 1970 initially regulated only high-temperature-curing paints. The requirements to reduce the VOC content of ambient-temperature-curing paints originated in the early 1980s. High-solids, two-component urethane topcoats and epoxy primers are now routinely used in car refinishes, aircraft manufacture and refinish coatings, primers for above- and below-waterline marine paints, heavy-duty industrial maintenance paints, and many other applications. The alkyd, silicone-modified alkyd, and thermoplastic antifoulant technologies have also been adapted for high-solids coatings. High-solids ambient-curing organic coatings dominate military painting operations. Recently, the formulation of high-solids coatings has been greatly facilitated by the use of the exempt solvents listed above. While these solvents are unsuitable to serve as the total solvent of most paints, they can be blended with nonexempt solvents to reduce the total VOC content. Waterborne Coatings Waterborne coatings are the second dominant nonpolluting paint technology. Two waterborne organic coatings were widely used even before environmental concerns became important. These are

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Transfer of Pollution Prevention Technologies architectural latex paints and electrocoat. Latexes provide the best available binder technology for low-to medium-gloss architectural paints. Electrocoat is a high-temperature-cured system. The articles are dipped into the coating bath and electric current deposits the coating. The main merit of electrocoat is that it provides the best corrosion protection for articles with complex shapes by uniform coating deposition onto exposed surfaces and into crevices. VOC emission by electrocoat processes is negligibly small. Aqueous binder resins may be grouped into three classes: latexes, polyurethane dispersions, and water reducibles. For specialty and OEM coatings acrylic latexes are used; vinyl-acetate-based latexes are limited to architectural coatings. Latexes and polyurethane dispersions have high molecular weights and form good films without cross-linking. However, cross-linking improves their toughness and chemical resistance. Polyurethane dispersions provide particularly high quality coatings but are expensive. Water-reducible resins have relatively low molecular weights and can be considered as water-dispersed analogues of high-solids resins. For good film formation such resins must be cross-linked. Electrocoat technology is based on water-reducible resins. Waterborne organic coatings contain some solvents for enhancing substrate wetting and film formation. However, the solvent content of aqueous coatings per pound of solids is often, but not always, lower than that for high-solids paints. In many applications aqueous coatings provide an option for reducing VOC emissions beyond what can be achieved by high-solids coatings. High-solids coatings formulated partly with exempt solvents can, in some circumstances, provide equal or lower VOC emissions than those typical for waterborne coatings. Spraying is the most common method of applying paint. High-temperature-cure aqueous spray paints are widely used for automotive base coats, can coatings, business machines, furniture, shelving, and many other applications. In the past 5 years the technology of two-component, aqueous urethanes and epoxies has been greatly improved; such paints match the performance of their high-solids counterparts in several, though not all, applications. These novel paints and the more traditional latexes cross-linked through carbonyl or carboxyl groups are now widely used for painting plastics, composition boards, and wood furniture. Powder Coatings Powder coatings are almost completely solventless. The powder is applied by electrostatic means. The object with its layer of powder is then heated; the powder liquefies and flows out and then cross-links. In addition to providing very low VOC emissions, powder coatings can, in favorable circumstances, offer advantages such as low energy consumption and excellent film mechanical properties. However, they are best suited to high-volume applications of a single color, or a limited range of colors. High-solids, waterborne, and powder coatings are competitive high-temperature-cure technologies. Radiation-Cured Coatings Radiation cure provides a method for applying solventless paints and curing them at ambient temperature. The precursor of the binder is in a liquid state and polymerizes under ultraviolet or electron beam radiation. Major end uses include can varnishes and coatings for sports equipment, wood furniture and paneling, optical fibers, plastics, and wheels. Radiation curing is now also used for cross-linking solid binders. Aqueous or powder paints are allowed to form continuous uncross-linked films first and are subsequently cross-linked by UV irradiation. Radiation cure provides coatings with excellent chemical resistance. In favorable circumstances, the relatively high raw material costs are offset by savings due to very fast throughput without the use of expensive heat-curing ovens. APPLYING ORGANIC COATINGS Reduced solvent content in paints is complemented by novel technologies allowing improved efficiency of paint application. In conventional spray applications 30 to 80 percent of the paint does not hit the target surface. This overspray is generally wasted, but its solvent content contributes to VOC emissions. The percentage of paint hitting the surface, known as the transfer efficiency, is improved by the technologies described below.

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Transfer of Pollution Prevention Technologies Electrostatic spaying of liquid paints greatly improves the wrap of paint particles around the targets, and therefore the transfer efficiency. This technology is very sophisticated and includes air-assisted and airless guns, as well as rotating disks and bells used to atomize paint. Novel equipment design even allows electrostatic application of aqueous paints. Supercritical carbon dioxide partly replaces organic solvents in some high-solids paints. It is particularly effective in improving atomization so that particle size distribution in the paint aerosol becomes uniform and the efficiency of electrostatic spray is enhanced. Robotic applications are now widespread in the automotive industry. The robots follow the shape of the target very closely and thus minimize overspray. New low-pressure/high-volume guns improve the transfer efficiency of liquid paints even without robotics or electrostatics. Overspray is recycled in the application of powder coatings, leading to better than 99 percent effective paint utilization. There are now experimental systems available for recycling the overspray originating from liquid coatings. Electrocoat provides an application method that also allows complete paint utilization. Application of paints by rollers over flat surfaces for coil coating or radiation-curable coatings is accomplished with almost 100 percent transfer efficiency. Coil coating provides prefinished steel or aluminum sheets, which are later stamped and formed to shape by the OEM end user. This technology is widely used for building siding, roof coatings, and appliances. Much development activity in the automotive industry is aimed at manufacturing automotive components from prefinished steel to eliminate electrocoat primers, as well as at powder- or high-solids-based primer-surfacer coatings. Reducing the solvent content of paints, reusing emitted solvents, and improving transfer efficiency not only yield environmental benefits but also often provide considerable cost savings. Reducing the costs of waste disposal provides additional economies.