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High-solids, low-VOC, solvent-based coatings
coating materials HIGH-SOLIDS, LOW-VOC, SOLVENT-BASED COATINGS BY RON JOSEPH RON JOSEPH & ASSOCIATES INC., SAN JOSE, CALIF.
This article focuses on the most common high-solids, solvent-based coating technologies. Each of the resin systems discussed is available to meet the Reasonable Available Control Technology (RACT) limits of most state volatile organic compounds (VOCs) regulations. Most systems are also available to satisfY the more stringent regulations of California and the Best Available Control Technology (BACT) requirements of other states. The discussions and tables presented in this article provide sufficient detail to enable a company that is out of compliance with state regulations to make an initial selection of potential coatings that may satisfY the company's performance and production requirements; however, it will still be necessary to ask vendors for samples and to conduct comprehensive production-type tests before implementing the technology of choice.
AIR AND FORCE-DRIED ALKYDS These alkyds are air- or force-dried at temperatures less than 90°C (194°F). Alkyd resins are essentially oil-modified polyesters and are made by reacting an alcohol with an acid. [Eq. (I)} Acid + Alcohol = Oil-free polyester (1) Typical acids used are phthalic anhydride and maleic anhydride. Typical alcohols used are pentaerythratol, glycerine, ethylene glycol, trimethylol ethane and trimethylol propane. These acids and alcohols can be combined in various combinations under very precise and controlled conditions. They form a wide range of alkyd resins, each of which, either alone or in combination, has its own distinctive chemical and physical properties. The coating formulator chooses the appropriate resin, or combination of resins according to customer requirements (see Table I). Alkyds can also be modified with other resins to change or improve their final properties, such as hardness, gloss retention, color retention and sunlight resistance. [Eq. (2)) Table I. '!Yplcal End.Uses of Solvent.Borne Modified Alkyds Dark industrial air-drying enamels Maintenance enamel~ Automotive .refinishing top C()8ls Indu.trial air·dry primen and lOp coats Chemical reslManl couting. Metal primers Indus.rial maintenance enamels 11>y enamels Implement enamels
Exterior metal finishes Exterillf maintenance enamelK where color and gloss relenlion are desired 78
Acid + Resin, phenol formaldehyde, vinyl toluene, styrene, acrylic, or silicone = Modified alkyd (2) Typical modifying resins include styrene, vinyl toluene, acrylics, silicone and others. The modified resins are more commonly known as modified alkyds. Alkyd and modified alkyd enamels are available in VOC-compliant formulations as high-solids coatings with VOCs of340-420 gjL (2.8-3.51bjgal). The most important advantages of high-solids alkyds and modified alkyd [air- or force-dried, with VOCs less than 420 gjL (3.51bjgal)] are the following: (1) They are single-component coatings with performance properties similar to those of conventional solids alkyds. (2) They are available at VOC levels of 3.5 Ibjgal, with a few at 2.81bjgal. (3) They can be formulated as primers and top coats and can be air-dried at ambient (room) temperature, although they should preferably be force-dried below 90°C (194°P). (4) They can be spray applied with conventional air-atomizing spray, airless, air-assisted airless and HVLP and the full range of electrostatic spray guns. (5) They are available in a wide range of colors and all gloss levels and are easy to self-touch-up. (6) They can be applied to most substrates, although they are not recommended for application directly to zinc or zinc-coated surfaces; a nonalkyd primer should be used instead. (7) They are less sensitive to the surface cleanliness of substrates than most other coatings. (8) They are the preferred choice of coating for many low-to-medium cost items or for large machinery that cannot be subjected to high-temperature ovens. Disadvantages are the following: (1) High-solids formulations generally have long ambient air-drying times (approximately 6 to 8 hours). (2) It is often difficult to maintain film thicknesses less than 1.5 mils. This is particularly evident on complex geometries, such as weldments and assemblies. Therefore, by default more coating is applied than is actually required. (3) They tend to exhibit higher viscosities than high-solids polyurethanes of similar VOC content. (4) Some formulations require the coatings to be heated during spray application to adequately lower the viscosity for application. (5) Gloss and color variations can occur from one surface to the next, owing to uneven film thicknesses. (6) Long recoating times, sometimes several hours or overnight, are not uncommon. This is aggravated if the film thickness is too high. (7) Some modified alkyds have a "critical" recoating period. The coating cannot be recoated during a certain window, sometimes 2-10 hours. (8) They are not generally used for texture finishing. (9) They tend to be relatively soft coatings initially. Hardness improves over a period of days to a final pencil hardness value of approximately HB (compare this with a pencil hardness of 3H-6H for epoxies and polyurethanes). (10) They tend to have limited resistance to longterm ultraviolet (UV) (sunlight) exposure; chalking and color fading are prevalent. (11) They exhibit poor resistance to alkalinity, chemicals, solvents, and immersion in water. Alkyds and modified alkyds are commonly used as general-purpose shop primers for steel and other substrates; however, they are not recommended for direct application to zinc or zinc-coated substrates. Because alkyd resins can be modified in so many ways, they are still among the most popular systems recommended for general-purpose topcoats. When high-performance properties are required, such as resistance to strong chem79
icals or solvents or color and gloss retention when the coating is exposed to sunlight over the long term, other resin systems are usually more appropriate. Alkyds and modified alkyds are also among the least expensive of the VOC-compliant coating systems.
BAKED ALKYDS Baked alkyds include melamine formaldehyde, urea formaldehyde or phenolic modifications as well as polyester, oil-free and acrylic coatings. The primary difference between air- and force-dried alkyds and baked alkyds is that they do not dry at ambient (room) temperatures but must be cured at elevated temperarures, usually in the rangellO-176°C (230-350 P) for 45-10 minutes, respectively. Although the coatings may feel touch dry after air-drying, they can only achieve their optimum chemical and physical resistance properties after they have been fully cured at their specified baking schedules. Non-air-dried (baked) alkyds are cross-linked with stabilized "aminoplast" resins because the cross-linking is initiated when the high temperarures are attained. The most frequently used aminoplast resins are urea formaldehyde and melamine formaldehyde. In the white baking enamels that are used for metal shelving, metal furnirure, computer cabinets, etc., urea formaldehyde provides excellent initial color, color retention and resistance to heat, soap,water and fatty acids. Alkyd baking 0
Polyester + . _ Polyhydric alcohol Coconut ot! - Alkyd
(3)
+
Polybasic acid Alkyd +
Urea formaldehyde or = Modified alkyd melamine formaldehyde
(4)
enamels based on melamine formaldehyde are harder, more chemically resistant and faster drying. They are used to coat refrigerators, washing machines, highquality fluorescent light fixtures, and automotive components. A typical sequence of reactions for these types of coatings is shown in Eqs. (3) and (4): These coatings are available at 360 giL (3.0 lb/gal), as required in most states. Some formulations are as low as 275 giL (2.3Ib/gal). The advantages of the baked alkyds are the following: (1) They are available at VOC levels [275 giL (2.31b/gal)) to meet most regulations and offer excellent high performance properties. (2) They are single-component coatings, available in wide range of colors and gloss levels, and can be applied directly to metal substrates. (3) They are excellent for appliances, such as washing machines, driers, dishwashers, refrigerators, metal shelving and cabinets and lighting fixrures. (4) With proper controls, they can achieve uniform, thin film thickness of approximately 1 mil. (5) They have excellent pencil hardness greater than 2H. (6) In many cases, they do not need special application equipment and have good adaptability to high-speed lines. (7) They offer film properties better than the air- and force-dried alkyds. (8) Some energy savings is possible because oflower solvent concentrations. 80
Disadvantages are the following: (1) High-energy usage. They must be baked at elevated temperatures with schedules such as 45 minutes at 110°C (230°F) or 10 minutes at 176°C (350°F). (2) Some formulations remain tacky at ambient temperatures and leave walls and floors of spray booths tacky. (3) High viscosities of some compliant formulations require special spray application equipment. Alternatively, apply at fluid temperatures of 100-110°F. (4) They are not for plastic or other heat-sensitive substrates because of the high baking temperatures. (5) Stains caused by the spray washer cleaning process are often "photographed" through the coating finish. (6) As with many high-solids coatings, smooth finishes, free of orange peel, may be difficult to achieve. (7) They may require close application controls. (8) An operator learning curve required. (9) Applied costs are greater than for conventional-solids baked enamels.
EPOXY ESTERS Epoxy esters are coatings that in many ways resemble alkyds in that they are single component and require no mixing of multi components prior to application; however, they are harder and more chemically resistant. In addition, they are available in solvent- or waterborne formulations. Epoxy esters are air- or forcedried at temperatures less than 90°C (194°F). The similarity between epoxy esters and alkyds lies in the fact that they are the products of reactions between moderate equivalent weight (800-1,500) epoxy resins and fatty acids. The properties of the resulting epoxy ester polymer resins are related to the actual equivalent weight of the original epoxy resin and the type of fatty acid with which it was esterified. Consequently, some epoxy esters are softer, more flexible and slower drying than other formulations that may be harder and faster drying. They also tend to have better chemical resistance and are harder than alkyds. These resins require metallic driers, as do alkyds, to start and maintain the drying sequence. These coatings are used in situations where alkyds would normally be selected but where a harder and more chemically resistant finish is required. The advantages of epoxy esters are the following: (1) Coatings are single-component materials and therefore maintain a constant viscosity, provided that temperature remains constant. (2) They are available in high-solids formulations. (3) They can be formulated into VOC-compliant water-reducible formulations at very low VOC contents. (4) Storage stability is excellent for the solvent-solution types, with long-term stability for water-thinnable systems. (5) They can be easily pigmented with normally available mixing equipment. (6) The solvent-borne types are very similar to medium- oil-length alkyds in most characteristics. (7) The water-reducible coatings resemble their alkyd counterparts. (8) They can be applied using the full spectrum of available spray equipment. (9) Some are FDA approved and are used for applications in which such approval is important. Disadvantages of epoxy esters are the following: (1) The major disadvantage for nearly all epoxy derivatives is their very poor resistance to chalking on exterior exposure. They chalk so heavily and so soon after exposure that they have poor color retention. (2) For exterior service, they can be successfully used only as primers and must be top-coated as soon as possible after being applied on an exterior exposed surface. (3) Because of their poor exterior durability they should only be used as top coats for interior exposure. (4) Yellowing can be a problem depending on the epoxy and fatty acid from which the epoxy es81
Component A
C
ComponentB
0
/'\,. /\ CH 2-CH-R~CH-CH 2
+
I
N~-R"-NH OH I
2
1
OH RII I
I
-..... -(CH 2 -CH-R ' -CH-N)Fig. 1. Reaction for typical2-component epoxy. R' = bisphenol Aderivative; R" = polyamide chain. Component A =epoxy resin, which is the product of reaction of epichlorhydrin and bisphenol A; component B = solution of multifunctional polyamide.
ter polymer was reacted.
CATALYZED EPOXY Epoxy resins are the reaction products that result when epichlorohydrin is reacted with bisphenol A. For the coating to form a cured, useful film, the epoxy resin must be further reacted with yet another resin. The unique features of an epoxy resin are due to the epoxy groups in the molecule as well as reactive hydroxyl (OH) groups. A typical reaction is shown in Fig. l.
Typical Properties In general, epoxy coatings are known for their toughness, flexibility and excellent adhesion to a wide range of substrates, including most metals, plastics, wood, ceramics, masonry, and glass. It is understandable therefore that epoxies are a popular choice as primers. They are commonly used where resistance to many chemicals, solvents and alkalies, such as soaps and detergents, is required. In addition, they have excellent resistance to fresh water, salt (sea) water, and hot water. For these reasons, they are a popular choice for protecting structures, such as offshore drilling platforms, ships, and bridges, where resistance to marine environments is critical. They are also used to coat industrial and potable water tanks and pipelines. One of the most notable weaknesses of epoxy coatings is their relatively poor resistance to UV light. For instance, when exposed to sunlight, many epoxy coatings tend to chalk readily, causing them to lose gloss and color. Although chalking takes place primarily at the surface of the film, it does not significantly affect the chemical properties of the coating. When a decorative, corrosion-resistant or chemically resistant coating system is desired, such as on bridges, in chemical refineries, or on offshore drilling equipment, it is customary to use epoxy coatings as the primer and undercoat and then apply a more UV-resistant top coat, such as an acrylic or polyurethane. Epoxies should not be applied at low ambient temperatures, usually less than 50-60°F (lO-lS°C), because they will not cure properly. The common air- or force-dried, two-component epoxies that are used in the general metals, plas82
tics ,and industrial maintenance industries comprise two separate packages, of which component A consists of the epoxy resin and component B can be a polyamine (for example, diethylene triamine, triethylene tetramine, tetraethylene pentamine), polyamide, polysulfide, or some other resin. In the case of baked epoxy coatings that cure during a high-temperature bake, usually above 60-204 C (140-400 F), the two resins are preblended by the coating manufacturer and are supplied as a single-component package. Examples include blends of epoxy resin with amine, urea formaldehyde, or melamine formaldehyde resins. Only when the applied coating attains an elevated temperature do the two resin systems react to form the cured finish. When clear coatings are required, neither component A nor component B is colored; however, for colored finishes, component A will usually contain the pigments and other additives, and component B will be clear. 0
0
Industrial Maintenance Coatings If maximum chemical resistance is required, such as in industrial maintenance coatings that are used in chemical plants and refineries, component B is usually a polyfunctional amine. Unfortunately, these coatings tend to be very hard and sacrifice flexibility. If the painting operators do not wear proper protective clothing and appropriate respirators during the mixing and application of the coating, and if the unreacted amine comes into contact with their skin or is inhaled, the operators can experience severe dermatitis; therefore, stringent safety procedures must be followed. Epoxy-polyamine coatings have a relatively short pot life and must be used within a short time after the two components have been mixed. The manufacturers' technical data sheets will provide further details.
General-Purpose Industrial Epoxy Coatings Improved toughness and flexibility are obtained when epoxy resins are reacted with polyamide resins. Unlike the polyamines, they do not cause severe dermatitis, and their pot life tends to be longer.
Availability as VOC-Compliant Coatings For the general metals and plastics industries, several coating vendors supply VOCcompliant primers and top coats. Depending on the application, VOC contents are available as 168-420 giL (1.4-3.5Ib/gal). Compliant epoxies are available that meet military specifications, such as MIL-P-23377 (primer), MIL-P-53022 (primer), MILC-22750 (top coat), and MIL- P-24441 (primer and top coat systems). High-solids epoxies, with very low VOC contents, often well below the regulated limits, are also available for the industrial maintenance industry. The advantages of high solids, solvent-based catalyzed epoxies, with VOCs less than 420 giL (3.5 Ib/gal) , are the following: (1) They are used primarily as a primer because of excellent adhesion properties to metals, plastics, composites, wood, masonry, ceramics, glass, paper and other substrates. (2) They are available to meet many military primer and top coat specifications (MIL-P-23377, MIL-P53022, MIL-C22750, MIL-P-24441). (3) They can be formulated into a wide range of colors and gloss levels. (4) Depending on the choice of curing agent (component B) they can achieve excellent hardness and chemical resistance, particularly alkali resistance. (5) They exhibit excellent resistance to many solvents, 83
fresh water, sea water, and hot water. (6) Some formulations are more flexible than others, depending on the choice of curing agent. (7) They can be air-dried at ambient temperatures within 3 to 5 hours and force-dried at 150 F within 30 minutes. (8) They are primarily used in military, marine, offshore, and chemical plant applications. (9) Some high-build formulations allow for thick films in excess of 5 mils in one application. Disadvantages are the following: (1) Usually they are two-component systems comprising component A (clear or colored epoxy resin) and component B (curing agent); therefore, they must be accurately mixed. (2) Any unused, mixed coating must be disposed of as hazardous waste. (3) They offer poor resistance when exposed directly to UV light (sunlight). (4) High-solids materials are difficult to apply to achieve dry films less than 1.5 mils, particularly when coating complex shapes. (5) They are generally not available in small quantities of custom colors. (6) Some formulations require an induction period of20-30 minutes after the two components have been mixed before coating can be applied. (7) Pot-life limitations of 4 to 6 hours or less at ambient temperatures are common. (8) Application equipment must be cleaned before coating starts to set. (9) They are sensitive to cleanliness of the substrate. (10) It is difficult to strip coating from damaged, coated parts. (11) Some formulations, particularly those based on the 0
Component A: Polyester (clear or pigm~nted)
.
+ Co~ponent B: = Cured coating: polYlsocyanate
Polyurethane
Component A: . Ac lic + Co~ponent B: = Cured coatmg: ry. polYlsocyanate Polyurethane (clear or pigmented)
(5) (6)
more chemically resistant polyamine resins, can cause severe dermatitis and other health effects. They must be used with caution.
CATALYZED POLYURETHANES Polyurethanes are a type of coating formed by the reaction of a polyisocyanate with a polymer that contains hydroxyl functionality. Two-component polyurethanes are supplied in two separate containers, of which the first is usually labeled component A and the second component B. Component A can either be clear or pigmented, offering a wide range of colors and gloss levels. The primary resin (polyol) is usually an acrylic, polyester or polyether, each of which contains more than one hydroxyl group. The second container, component B, contains a multifunctional, prepolymerized isocyanate. When components A and B are mixed according to the manufacturers' prescribed ratios, the polymers react to form a highly cross-linked polyurethane. Figure 2 shows the simplified chemistry of the two components; Eqs. (5) and (6) depict the results of mixing the two components: Typically, polyfunctional polyisocyanates used in two-component polyurethanes are homopolymers or copolymers of toluene diisocyanate (TO I), hexamethylene diisocyanate (HOI), isophorone diisocyanate (IPOI), dicyclomethane diisocyanate (HMDI). Components A and B can be batch mixed by manually mixing immediately before the coating is applied. Alternatively, special proportioning equipment can 84
Component A ComponentB
R' I
OH
+
R" - C= N= 0 R I
HO I I
R"-N-C II
o
Aliphatic Polyurethane
Fig. 2. Reaction for typical2-component polyester-polyurethane, R' =aliphatic hydrocarbon chain; R" = aliphatic polyester chain. Component A = polyester polyol (dear or pigmented); component B = aliphatic polyisocyanate (dear).
be set to automatically measure out each component according to its prescribed ratio. Each component is then pumped separately to a manifold, where they both come together in the fluid hose leading to the spray gun. Downstream of the manifold is a short static mixer that comprises a short plastic or stainless steel tube located in the fluid hose only a few inches or feet from the spray gun. Small baffles in the tube cause the components to be thoroughly mixed immediately before entering the spray gun. In-line mixing allows components A and B to be mixed on a continuous basis. The primary advantage is that the viscosity of the coating remains constant throughout the day and is used before it can outlive its pot life. The most important advantages of two-component, high-solids polyurethanes [polyester urethanes with VOCs less than 340 g/L (2.81b/gal); acrylic urethanes with VOCs less than 420 g/L (3.51b/gal)] are the following: (1) They offer excellent physical film performance - abrasion resistance, toughness, hardness up to pencil hardness of 6H. (2) They have excellent resistance to most solvents and chemicals and excellent outdoor durability (primarily the aliphatic polyurethanes). (3) They can cure at ambient (room) temperatures, elevated temperatures or even under subzero conditions. (4) They can be used for application to steel, aluminum, plastics, composites, wood, and masonry. (5) They can be spray applied with standard equipment-conventional air atomizing, airiess, air-assisted airiess, HVLP, electrostatic. (6) They are available in a wide range of solid and metallic colors, with quick turnaround. (7) On-site intermixing of colors, predominantly automotive, is available from several manufacturers. (8) They are available in a complete range of gloss and texture levels, and self-rouch-up is possible. (9) They 85
meet performance standards for top-of-the-line products, such as computers, business machines, aircraft and truck cabs, and meet various military top-coat and camouflage specifications. (10) For polyester urethanes, formulations are available with VOCs lower than 2.81b/gal. Coatings often have better chemical resistance; therefore, they are more commonly used on laboratory instruments, machine tools, computers, business machines, aircraft (where resistance to hydraulic fluids is important), industrial maintenance coatings for chemical plants, refineries, etc. (11) For acrylic urethanes, formulations are available with VOCs under 3.5lb/gal and in some cases, under 2.81b/gal. Coatings are often reported to exhibit better exterior UV (sunlight) resistance; therefore, they are usually used on transportation equipment, such as automobiles, trucks, buses, and some private and commercial aircraft. Disadvantages are the following: (1) The two-component system requires mixing in prescribed proportions. (2) They have a limited pot life, sometimes less than four hours, particularly in high-solids formulations. (3) Like many high-solids coatings, it can be difficult to achieve a uniform film thickness on complexshaped parts. (4) Equipment must be cleaned before coating begins to set. (5) They are relatively expensive (usually $30/gal). Aliphatic polyutethanes for exterior exposure are more expensive than aromatic polyurethanes for interior exposure. (6) They must be handled with care, and paint operators must use appropriate respirators. (Consult coating vendor for appropriate recommendation). (7) Polyurethanes can have allergic sensitization in some people, particularly if they do not wear appropriate respirators. (8) They may need to be applied over epoxy primer, and like most other coatings, they must be applied over clean, pretreated surfaces. (9) At the present time, low-VOC, high-gloss acrylic polyurethanes are not readily available in small quantities of "automotive" colors, but are available in "fleet" colors. New formulation polyurethane technologies are emerging. They will have very low VOC contents, while retaining manageable viscosities.
MOISTURE-CURED POLYURETHANES In the previous section on two-component polyurethanes, we discussed the reaction of the polyhydroxy resin, such as the polyester, acrylic or polyether, with the polyisocyanate. It was explained that the chemical reaction commences as soon as the two components, A and B, are mixed; however, if the polyhydroxy resin is prereacted with a polyisocyanate but the reaction is not taken to completion, leaving some unreacted isocyanate groups, the coating then cures in the presence of moisture from the air. Such materials are called moisture cured polyurethanes. The coating is supplied in one package (the second component being atmospheric moisture) [Eq. (7)]: Prereacted polyhydroxylisocyanate (clear or pigmented) + Atmospheric moisture = Polyurethane (7) In preparing moisture-cured polyurethanes it is critical that all of the coating pigments and solvents are totally free of water, because traces of moisture in the package can cause the coating to cure before application. In addition, the fluid hose leading to the spray gun and the head space above the coating in the pressure pot or reservoir must be free of moisture. A nitrogen blanket or a desiccant can be used to keep the head space dry. 86
'Thble II. Comparison Between Methyl and Phenyl Groups Methyl Groups
Phenyl Grtlups
Hardne~s while hilt Flexibility Water repellency
Heat and oxidati()n resistance Better .helf life Less therm<>plilstic
Low temperature propenies
Chemical resl.tance Rate ()f cure Thermal .hock Heat and oxidation resistance
The advantages of high-solids polyurethane, moisture-cured industrial coatings with less than 420 gjL (3.5Ibjgal) VOCs are the following: (1) This is a single-component system, hence no pot-life limitations. (2) It has all of the advantages of two-component polyurethane coatings. (3) It achieves chemically resistant properties more quickly than some two-component polyurethanes. (4) It is readily available in military camouflage coatings complying with MIL-C-53039. Disadvantages are the following: (1) This is a technology with less field history than two-component polyurethanes. (2) Currently only a few companies supply military-specification-approved camouflage coatings. (3) They are not yet available in wide range of colors or in small quantities and may have a limited shelf life (less than 1 year). (4) They are very sensitive to moisture contamination, therefore requiring a special effort to keep moist air from the packaged or stored coating. (5) Drying time is affected by moisture in air. In very dry climates, the drying time may be longer than usual. (6) Currently they may be more expensive than two-component polyurethanes, with an average cost greater than $25jgal. (7) They have other disadvantages of two-component polyurethanes. Typical polyurethane end uses are the following: Aircraft skins Missiles and other aerospace products Over-the-road trucks Buses Rail cars Automotive refinishing Automotive (original equipment manufacturer): newly introduced high-gloss clear coat over metallic base coat; chip-resistant primer surfacers; flexible coatings for plastic facias bumpers Military ground support equipment, such as tanks personnel carriers,vehicles, with resistance to live chemical agents (chemical agents-resistant coatings) Architectural and Maintenance Structures and vessels in chemical plants Offshore drilling rigs Roofs Antigraffiti Pipelines Product finishing Machine tools 87
Garden lawn mowers, snow blowers, tractors Plastic housings, keyboards, computer monitors, etc. Computer and business machines Medical and laboratory equipment
SILICONE COATINGS Silicone resins may be considered hybrids of glass and organic resins. Because of their inertness they can be used beneficially in conjunction with many other organic resins. Silicone resins with methyl and phenyl groups have been found to confer the most desirable properties; Table II lists their most important advantages. Most silicone resins used in the coatings industry comprise combinations of methyl and phenyl groups, and the resulting properties are dependent on the ratio of one to the other.
High-Temperature Coatings High-temperature-resistant coatings cure only when exposed to high temperatures, such as 260°C (SOOOP), but formulations are available in which coatings can cure at lower temperatures. In addition, inorganic pigments are commonly used. Colored high-temperature coatings are available, but to retain their color, the exposure temperature should not exceed 204°C (400°F).
Weatherability Although temperature resistance is one of the major benefits of silicone resins, they also exhibit excellent weatherability. This implies that they have improved color and gloss retention and exhibit less chalking. They may be incorporated into coatings, such as alkyds, phenolics, epoxies, polyesters, and other oleo resinous materials containing hydroxyl groups. Generally, the organic resins that are modified with silicone resins provide better properties than if the unmodified resins were to be exposed to the same environments.
Typictil Applications Unmodified silicone resin coatings are used on steel and aluminum substrates that will be subjected to high temperatures. These include such items as exhaust manifolds, mufflers, heat stacks, furnaces, boilers, ovens, heat exchangers, and aerospace and engine components. Colored silicone coatings generally withstand lower temperatures but are used on items such as space heaters, clothes driers, stoves, and similar applications. Copolymers, such as silicone-alkyds, are popular as industrial maintenance coatings where excellent exterior durability and chemical resistance are required. Applications include outdoor storage tanks, highway guard rails, railway tank cars, bridges, and aluminum siding. Unlike the unmodified silicone coatings, the silicone-organic copolymers, such as silicone-alkyds, -acrylics, -phenolics, -melamines, can be cured at ambient temperatures or typical baking temperatures. They are also commonly used as coil coatings. The silicone-modified alkyds and other organic copolymers are available in lowVOC formulations, typically with VOC contents of approximately 420 giL (3.S lb/gal). 88
Advantages of silicone-based coatings are as follows: (1) They are generally available as single-component coatings. (2) They can be air-dried or baked, depending on formulation. (3) Depending on the formulation and pigmentation, silicone resins can withstand temperatures 260-360°C (500-600°F) up to 540-650°C (1,000-1,200° F). Although their temperature resistance is excellent, color stability will depend on the choice of pigments and their resistance to high temperatures. (4) Copolymers, such as silicone-alkyds exhibit excellent resistance to exterior weathering .(5) Nonchalking coatings are available. (6) They have excellent resistance to thermal shock and corrosion and excellent water repellency. (Clear, thin coatings are commonly used as water sealers on the exterior of buildings.) (7) They can be formulated as copolymers with organic resins to obtain the advantages of each resin. (8) They have excellent electrical insulating properties; therefore they are used in resistors, capacitors, rectifiers, etc. (9) They have good adhesion, but require excellent surface preparation of substrate. Disadvantages are as follows: (1) High-temperature-resistant coatings must usually be cured at temperatures above 260°C (500°F). This can be a problem when coating large furnaces, stacks, etc. (2) Ifhigh-temperature-resistant coatings are not fully cured before being placed in service, corrosion of the substrate may take place. (3) Silicone resin-based coatings are sensitive to surface preparation and must be applied to properly prepared substrates. (4) They exhibit generally poor hardness and poor abrasion resistance. (5) Low-VOC, high-temperature coatings are not readily available.
89
This article focuses on the most common high-solids, solvent-based coating technologies. Each of the resin systems discussed is available to meet the Reasonable Available Control Technology (RACT) limits of most state volatile organic compounds (VOCs) regulations. Most systems are also available to satisfY the more stringent regulations of California and the Best Available Control Technology (BACT) requirements of other states. The discussions and tables presented in this article provide sufficient detail to enable a company that is out of compliance with state regulations to make an initial selection of potential coatings that may satisfY the company's performance and production requirements; however, it will still be necessary to ask vendors for samples and to conduct comprehensive production-type tests before implementing the technology of choice.
AIR AND FORCE-DRIED ALKYDS These alkyds are air- or force-dried at temperatures less than 90°C (194°F). Alkyd resins are essentially oil-modified polyesters and are made by reacting an alcohol with an acid. [Eq. (I)} Acid + Alcohol = Oil-free polyester (1) Typical acids used are phthalic anhydride and maleic anhydride. Typical alcohols used are pentaerythratol, glycerine, ethylene glycol, trimethylol ethane and trimethylol propane. These acids and alcohols can be combined in various combinations under very precise and controlled conditions. They form a wide range of alkyd resins, each of which, either alone or in combination, has its own distinctive chemical and physical properties. The coating formulator chooses the appropriate resin, or combination of resins according to customer requirements (see Table I). Alkyds can also be modified with other resins to change or improve their final properties, such as hardness, gloss retention, color retention and sunlight resistance. [Eq. (2)) Table I. '!Yplcal End.Uses of Solvent.Borne Modified Alkyds Dark industrial air-drying enamels Maintenance enamel~ Automotive .refinishing top C()8ls Indu.trial air·dry primen and lOp coats Chemical reslManl couting. Metal primers Indus.rial maintenance enamels 11>y enamels Implement enamels
Exterior metal finishes Exterillf maintenance enamelK where color and gloss relenlion are desired 78
Acid + Resin, phenol formaldehyde, vinyl toluene, styrene, acrylic, or silicone = Modified alkyd (2) Typical modifying resins include styrene, vinyl toluene, acrylics, silicone and others. The modified resins are more commonly known as modified alkyds. Alkyd and modified alkyd enamels are available in VOC-compliant formulations as high-solids coatings with VOCs of340-420 gjL (2.8-3.51bjgal). The most important advantages of high-solids alkyds and modified alkyd [air- or force-dried, with VOCs less than 420 gjL (3.51bjgal)] are the following: (1) They are single-component coatings with performance properties similar to those of conventional solids alkyds. (2) They are available at VOC levels of 3.5 Ibjgal, with a few at 2.81bjgal. (3) They can be formulated as primers and top coats and can be air-dried at ambient (room) temperature, although they should preferably be force-dried below 90°C (194°P). (4) They can be spray applied with conventional air-atomizing spray, airless, air-assisted airless and HVLP and the full range of electrostatic spray guns. (5) They are available in a wide range of colors and all gloss levels and are easy to self-touch-up. (6) They can be applied to most substrates, although they are not recommended for application directly to zinc or zinc-coated surfaces; a nonalkyd primer should be used instead. (7) They are less sensitive to the surface cleanliness of substrates than most other coatings. (8) They are the preferred choice of coating for many low-to-medium cost items or for large machinery that cannot be subjected to high-temperature ovens. Disadvantages are the following: (1) High-solids formulations generally have long ambient air-drying times (approximately 6 to 8 hours). (2) It is often difficult to maintain film thicknesses less than 1.5 mils. This is particularly evident on complex geometries, such as weldments and assemblies. Therefore, by default more coating is applied than is actually required. (3) They tend to exhibit higher viscosities than high-solids polyurethanes of similar VOC content. (4) Some formulations require the coatings to be heated during spray application to adequately lower the viscosity for application. (5) Gloss and color variations can occur from one surface to the next, owing to uneven film thicknesses. (6) Long recoating times, sometimes several hours or overnight, are not uncommon. This is aggravated if the film thickness is too high. (7) Some modified alkyds have a "critical" recoating period. The coating cannot be recoated during a certain window, sometimes 2-10 hours. (8) They are not generally used for texture finishing. (9) They tend to be relatively soft coatings initially. Hardness improves over a period of days to a final pencil hardness value of approximately HB (compare this with a pencil hardness of 3H-6H for epoxies and polyurethanes). (10) They tend to have limited resistance to longterm ultraviolet (UV) (sunlight) exposure; chalking and color fading are prevalent. (11) They exhibit poor resistance to alkalinity, chemicals, solvents, and immersion in water. Alkyds and modified alkyds are commonly used as general-purpose shop primers for steel and other substrates; however, they are not recommended for direct application to zinc or zinc-coated substrates. Because alkyd resins can be modified in so many ways, they are still among the most popular systems recommended for general-purpose topcoats. When high-performance properties are required, such as resistance to strong chem79
icals or solvents or color and gloss retention when the coating is exposed to sunlight over the long term, other resin systems are usually more appropriate. Alkyds and modified alkyds are also among the least expensive of the VOC-compliant coating systems.
BAKED ALKYDS Baked alkyds include melamine formaldehyde, urea formaldehyde or phenolic modifications as well as polyester, oil-free and acrylic coatings. The primary difference between air- and force-dried alkyds and baked alkyds is that they do not dry at ambient (room) temperatures but must be cured at elevated temperarures, usually in the rangellO-176°C (230-350 P) for 45-10 minutes, respectively. Although the coatings may feel touch dry after air-drying, they can only achieve their optimum chemical and physical resistance properties after they have been fully cured at their specified baking schedules. Non-air-dried (baked) alkyds are cross-linked with stabilized "aminoplast" resins because the cross-linking is initiated when the high temperarures are attained. The most frequently used aminoplast resins are urea formaldehyde and melamine formaldehyde. In the white baking enamels that are used for metal shelving, metal furnirure, computer cabinets, etc., urea formaldehyde provides excellent initial color, color retention and resistance to heat, soap,water and fatty acids. Alkyd baking 0
Polyester + . _ Polyhydric alcohol Coconut ot! - Alkyd
(3)
+
Polybasic acid Alkyd +
Urea formaldehyde or = Modified alkyd melamine formaldehyde
(4)
enamels based on melamine formaldehyde are harder, more chemically resistant and faster drying. They are used to coat refrigerators, washing machines, highquality fluorescent light fixtures, and automotive components. A typical sequence of reactions for these types of coatings is shown in Eqs. (3) and (4): These coatings are available at 360 giL (3.0 lb/gal), as required in most states. Some formulations are as low as 275 giL (2.3Ib/gal). The advantages of the baked alkyds are the following: (1) They are available at VOC levels [275 giL (2.31b/gal)) to meet most regulations and offer excellent high performance properties. (2) They are single-component coatings, available in wide range of colors and gloss levels, and can be applied directly to metal substrates. (3) They are excellent for appliances, such as washing machines, driers, dishwashers, refrigerators, metal shelving and cabinets and lighting fixrures. (4) With proper controls, they can achieve uniform, thin film thickness of approximately 1 mil. (5) They have excellent pencil hardness greater than 2H. (6) In many cases, they do not need special application equipment and have good adaptability to high-speed lines. (7) They offer film properties better than the air- and force-dried alkyds. (8) Some energy savings is possible because oflower solvent concentrations. 80
Disadvantages are the following: (1) High-energy usage. They must be baked at elevated temperatures with schedules such as 45 minutes at 110°C (230°F) or 10 minutes at 176°C (350°F). (2) Some formulations remain tacky at ambient temperatures and leave walls and floors of spray booths tacky. (3) High viscosities of some compliant formulations require special spray application equipment. Alternatively, apply at fluid temperatures of 100-110°F. (4) They are not for plastic or other heat-sensitive substrates because of the high baking temperatures. (5) Stains caused by the spray washer cleaning process are often "photographed" through the coating finish. (6) As with many high-solids coatings, smooth finishes, free of orange peel, may be difficult to achieve. (7) They may require close application controls. (8) An operator learning curve required. (9) Applied costs are greater than for conventional-solids baked enamels.
EPOXY ESTERS Epoxy esters are coatings that in many ways resemble alkyds in that they are single component and require no mixing of multi components prior to application; however, they are harder and more chemically resistant. In addition, they are available in solvent- or waterborne formulations. Epoxy esters are air- or forcedried at temperatures less than 90°C (194°F). The similarity between epoxy esters and alkyds lies in the fact that they are the products of reactions between moderate equivalent weight (800-1,500) epoxy resins and fatty acids. The properties of the resulting epoxy ester polymer resins are related to the actual equivalent weight of the original epoxy resin and the type of fatty acid with which it was esterified. Consequently, some epoxy esters are softer, more flexible and slower drying than other formulations that may be harder and faster drying. They also tend to have better chemical resistance and are harder than alkyds. These resins require metallic driers, as do alkyds, to start and maintain the drying sequence. These coatings are used in situations where alkyds would normally be selected but where a harder and more chemically resistant finish is required. The advantages of epoxy esters are the following: (1) Coatings are single-component materials and therefore maintain a constant viscosity, provided that temperature remains constant. (2) They are available in high-solids formulations. (3) They can be formulated into VOC-compliant water-reducible formulations at very low VOC contents. (4) Storage stability is excellent for the solvent-solution types, with long-term stability for water-thinnable systems. (5) They can be easily pigmented with normally available mixing equipment. (6) The solvent-borne types are very similar to medium- oil-length alkyds in most characteristics. (7) The water-reducible coatings resemble their alkyd counterparts. (8) They can be applied using the full spectrum of available spray equipment. (9) Some are FDA approved and are used for applications in which such approval is important. Disadvantages of epoxy esters are the following: (1) The major disadvantage for nearly all epoxy derivatives is their very poor resistance to chalking on exterior exposure. They chalk so heavily and so soon after exposure that they have poor color retention. (2) For exterior service, they can be successfully used only as primers and must be top-coated as soon as possible after being applied on an exterior exposed surface. (3) Because of their poor exterior durability they should only be used as top coats for interior exposure. (4) Yellowing can be a problem depending on the epoxy and fatty acid from which the epoxy es81
Component A
C
ComponentB
0
/'\,. /\ CH 2-CH-R~CH-CH 2
+
I
N~-R"-NH OH I
2
1
OH RII I
I
-..... -(CH 2 -CH-R ' -CH-N)Fig. 1. Reaction for typical2-component epoxy. R' = bisphenol Aderivative; R" = polyamide chain. Component A =epoxy resin, which is the product of reaction of epichlorhydrin and bisphenol A; component B = solution of multifunctional polyamide.
ter polymer was reacted.
CATALYZED EPOXY Epoxy resins are the reaction products that result when epichlorohydrin is reacted with bisphenol A. For the coating to form a cured, useful film, the epoxy resin must be further reacted with yet another resin. The unique features of an epoxy resin are due to the epoxy groups in the molecule as well as reactive hydroxyl (OH) groups. A typical reaction is shown in Fig. l.
Typical Properties In general, epoxy coatings are known for their toughness, flexibility and excellent adhesion to a wide range of substrates, including most metals, plastics, wood, ceramics, masonry, and glass. It is understandable therefore that epoxies are a popular choice as primers. They are commonly used where resistance to many chemicals, solvents and alkalies, such as soaps and detergents, is required. In addition, they have excellent resistance to fresh water, salt (sea) water, and hot water. For these reasons, they are a popular choice for protecting structures, such as offshore drilling platforms, ships, and bridges, where resistance to marine environments is critical. They are also used to coat industrial and potable water tanks and pipelines. One of the most notable weaknesses of epoxy coatings is their relatively poor resistance to UV light. For instance, when exposed to sunlight, many epoxy coatings tend to chalk readily, causing them to lose gloss and color. Although chalking takes place primarily at the surface of the film, it does not significantly affect the chemical properties of the coating. When a decorative, corrosion-resistant or chemically resistant coating system is desired, such as on bridges, in chemical refineries, or on offshore drilling equipment, it is customary to use epoxy coatings as the primer and undercoat and then apply a more UV-resistant top coat, such as an acrylic or polyurethane. Epoxies should not be applied at low ambient temperatures, usually less than 50-60°F (lO-lS°C), because they will not cure properly. The common air- or force-dried, two-component epoxies that are used in the general metals, plas82
tics ,and industrial maintenance industries comprise two separate packages, of which component A consists of the epoxy resin and component B can be a polyamine (for example, diethylene triamine, triethylene tetramine, tetraethylene pentamine), polyamide, polysulfide, or some other resin. In the case of baked epoxy coatings that cure during a high-temperature bake, usually above 60-204 C (140-400 F), the two resins are preblended by the coating manufacturer and are supplied as a single-component package. Examples include blends of epoxy resin with amine, urea formaldehyde, or melamine formaldehyde resins. Only when the applied coating attains an elevated temperature do the two resin systems react to form the cured finish. When clear coatings are required, neither component A nor component B is colored; however, for colored finishes, component A will usually contain the pigments and other additives, and component B will be clear. 0
0
Industrial Maintenance Coatings If maximum chemical resistance is required, such as in industrial maintenance coatings that are used in chemical plants and refineries, component B is usually a polyfunctional amine. Unfortunately, these coatings tend to be very hard and sacrifice flexibility. If the painting operators do not wear proper protective clothing and appropriate respirators during the mixing and application of the coating, and if the unreacted amine comes into contact with their skin or is inhaled, the operators can experience severe dermatitis; therefore, stringent safety procedures must be followed. Epoxy-polyamine coatings have a relatively short pot life and must be used within a short time after the two components have been mixed. The manufacturers' technical data sheets will provide further details.
General-Purpose Industrial Epoxy Coatings Improved toughness and flexibility are obtained when epoxy resins are reacted with polyamide resins. Unlike the polyamines, they do not cause severe dermatitis, and their pot life tends to be longer.
Availability as VOC-Compliant Coatings For the general metals and plastics industries, several coating vendors supply VOCcompliant primers and top coats. Depending on the application, VOC contents are available as 168-420 giL (1.4-3.5Ib/gal). Compliant epoxies are available that meet military specifications, such as MIL-P-23377 (primer), MIL-P-53022 (primer), MILC-22750 (top coat), and MIL- P-24441 (primer and top coat systems). High-solids epoxies, with very low VOC contents, often well below the regulated limits, are also available for the industrial maintenance industry. The advantages of high solids, solvent-based catalyzed epoxies, with VOCs less than 420 giL (3.5 Ib/gal) , are the following: (1) They are used primarily as a primer because of excellent adhesion properties to metals, plastics, composites, wood, masonry, ceramics, glass, paper and other substrates. (2) They are available to meet many military primer and top coat specifications (MIL-P-23377, MIL-P53022, MIL-C22750, MIL-P-24441). (3) They can be formulated into a wide range of colors and gloss levels. (4) Depending on the choice of curing agent (component B) they can achieve excellent hardness and chemical resistance, particularly alkali resistance. (5) They exhibit excellent resistance to many solvents, 83
fresh water, sea water, and hot water. (6) Some formulations are more flexible than others, depending on the choice of curing agent. (7) They can be air-dried at ambient temperatures within 3 to 5 hours and force-dried at 150 F within 30 minutes. (8) They are primarily used in military, marine, offshore, and chemical plant applications. (9) Some high-build formulations allow for thick films in excess of 5 mils in one application. Disadvantages are the following: (1) Usually they are two-component systems comprising component A (clear or colored epoxy resin) and component B (curing agent); therefore, they must be accurately mixed. (2) Any unused, mixed coating must be disposed of as hazardous waste. (3) They offer poor resistance when exposed directly to UV light (sunlight). (4) High-solids materials are difficult to apply to achieve dry films less than 1.5 mils, particularly when coating complex shapes. (5) They are generally not available in small quantities of custom colors. (6) Some formulations require an induction period of20-30 minutes after the two components have been mixed before coating can be applied. (7) Pot-life limitations of 4 to 6 hours or less at ambient temperatures are common. (8) Application equipment must be cleaned before coating starts to set. (9) They are sensitive to cleanliness of the substrate. (10) It is difficult to strip coating from damaged, coated parts. (11) Some formulations, particularly those based on the 0
Component A: Polyester (clear or pigm~nted)
.
+ Co~ponent B: = Cured coating: polYlsocyanate
Polyurethane
Component A: . Ac lic + Co~ponent B: = Cured coatmg: ry. polYlsocyanate Polyurethane (clear or pigmented)
(5) (6)
more chemically resistant polyamine resins, can cause severe dermatitis and other health effects. They must be used with caution.
CATALYZED POLYURETHANES Polyurethanes are a type of coating formed by the reaction of a polyisocyanate with a polymer that contains hydroxyl functionality. Two-component polyurethanes are supplied in two separate containers, of which the first is usually labeled component A and the second component B. Component A can either be clear or pigmented, offering a wide range of colors and gloss levels. The primary resin (polyol) is usually an acrylic, polyester or polyether, each of which contains more than one hydroxyl group. The second container, component B, contains a multifunctional, prepolymerized isocyanate. When components A and B are mixed according to the manufacturers' prescribed ratios, the polymers react to form a highly cross-linked polyurethane. Figure 2 shows the simplified chemistry of the two components; Eqs. (5) and (6) depict the results of mixing the two components: Typically, polyfunctional polyisocyanates used in two-component polyurethanes are homopolymers or copolymers of toluene diisocyanate (TO I), hexamethylene diisocyanate (HOI), isophorone diisocyanate (IPOI), dicyclomethane diisocyanate (HMDI). Components A and B can be batch mixed by manually mixing immediately before the coating is applied. Alternatively, special proportioning equipment can 84
Component A ComponentB
R' I
OH
+
R" - C= N= 0 R I
HO I I
R"-N-C II
o
Aliphatic Polyurethane
Fig. 2. Reaction for typical2-component polyester-polyurethane, R' =aliphatic hydrocarbon chain; R" = aliphatic polyester chain. Component A = polyester polyol (dear or pigmented); component B = aliphatic polyisocyanate (dear).
be set to automatically measure out each component according to its prescribed ratio. Each component is then pumped separately to a manifold, where they both come together in the fluid hose leading to the spray gun. Downstream of the manifold is a short static mixer that comprises a short plastic or stainless steel tube located in the fluid hose only a few inches or feet from the spray gun. Small baffles in the tube cause the components to be thoroughly mixed immediately before entering the spray gun. In-line mixing allows components A and B to be mixed on a continuous basis. The primary advantage is that the viscosity of the coating remains constant throughout the day and is used before it can outlive its pot life. The most important advantages of two-component, high-solids polyurethanes [polyester urethanes with VOCs less than 340 g/L (2.81b/gal); acrylic urethanes with VOCs less than 420 g/L (3.51b/gal)] are the following: (1) They offer excellent physical film performance - abrasion resistance, toughness, hardness up to pencil hardness of 6H. (2) They have excellent resistance to most solvents and chemicals and excellent outdoor durability (primarily the aliphatic polyurethanes). (3) They can cure at ambient (room) temperatures, elevated temperatures or even under subzero conditions. (4) They can be used for application to steel, aluminum, plastics, composites, wood, and masonry. (5) They can be spray applied with standard equipment-conventional air atomizing, airiess, air-assisted airiess, HVLP, electrostatic. (6) They are available in a wide range of solid and metallic colors, with quick turnaround. (7) On-site intermixing of colors, predominantly automotive, is available from several manufacturers. (8) They are available in a complete range of gloss and texture levels, and self-rouch-up is possible. (9) They 85
meet performance standards for top-of-the-line products, such as computers, business machines, aircraft and truck cabs, and meet various military top-coat and camouflage specifications. (10) For polyester urethanes, formulations are available with VOCs lower than 2.81b/gal. Coatings often have better chemical resistance; therefore, they are more commonly used on laboratory instruments, machine tools, computers, business machines, aircraft (where resistance to hydraulic fluids is important), industrial maintenance coatings for chemical plants, refineries, etc. (11) For acrylic urethanes, formulations are available with VOCs under 3.5lb/gal and in some cases, under 2.81b/gal. Coatings are often reported to exhibit better exterior UV (sunlight) resistance; therefore, they are usually used on transportation equipment, such as automobiles, trucks, buses, and some private and commercial aircraft. Disadvantages are the following: (1) The two-component system requires mixing in prescribed proportions. (2) They have a limited pot life, sometimes less than four hours, particularly in high-solids formulations. (3) Like many high-solids coatings, it can be difficult to achieve a uniform film thickness on complexshaped parts. (4) Equipment must be cleaned before coating begins to set. (5) They are relatively expensive (usually $30/gal). Aliphatic polyutethanes for exterior exposure are more expensive than aromatic polyurethanes for interior exposure. (6) They must be handled with care, and paint operators must use appropriate respirators. (Consult coating vendor for appropriate recommendation). (7) Polyurethanes can have allergic sensitization in some people, particularly if they do not wear appropriate respirators. (8) They may need to be applied over epoxy primer, and like most other coatings, they must be applied over clean, pretreated surfaces. (9) At the present time, low-VOC, high-gloss acrylic polyurethanes are not readily available in small quantities of "automotive" colors, but are available in "fleet" colors. New formulation polyurethane technologies are emerging. They will have very low VOC contents, while retaining manageable viscosities.
MOISTURE-CURED POLYURETHANES In the previous section on two-component polyurethanes, we discussed the reaction of the polyhydroxy resin, such as the polyester, acrylic or polyether, with the polyisocyanate. It was explained that the chemical reaction commences as soon as the two components, A and B, are mixed; however, if the polyhydroxy resin is prereacted with a polyisocyanate but the reaction is not taken to completion, leaving some unreacted isocyanate groups, the coating then cures in the presence of moisture from the air. Such materials are called moisture cured polyurethanes. The coating is supplied in one package (the second component being atmospheric moisture) [Eq. (7)]: Prereacted polyhydroxylisocyanate (clear or pigmented) + Atmospheric moisture = Polyurethane (7) In preparing moisture-cured polyurethanes it is critical that all of the coating pigments and solvents are totally free of water, because traces of moisture in the package can cause the coating to cure before application. In addition, the fluid hose leading to the spray gun and the head space above the coating in the pressure pot or reservoir must be free of moisture. A nitrogen blanket or a desiccant can be used to keep the head space dry. 86
'Thble II. Comparison Between Methyl and Phenyl Groups Methyl Groups
Phenyl Grtlups
Hardne~s while hilt Flexibility Water repellency
Heat and oxidati()n resistance Better .helf life Less therm<>plilstic
Low temperature propenies
Chemical resl.tance Rate ()f cure Thermal .hock Heat and oxidation resistance
The advantages of high-solids polyurethane, moisture-cured industrial coatings with less than 420 gjL (3.5Ibjgal) VOCs are the following: (1) This is a single-component system, hence no pot-life limitations. (2) It has all of the advantages of two-component polyurethane coatings. (3) It achieves chemically resistant properties more quickly than some two-component polyurethanes. (4) It is readily available in military camouflage coatings complying with MIL-C-53039. Disadvantages are the following: (1) This is a technology with less field history than two-component polyurethanes. (2) Currently only a few companies supply military-specification-approved camouflage coatings. (3) They are not yet available in wide range of colors or in small quantities and may have a limited shelf life (less than 1 year). (4) They are very sensitive to moisture contamination, therefore requiring a special effort to keep moist air from the packaged or stored coating. (5) Drying time is affected by moisture in air. In very dry climates, the drying time may be longer than usual. (6) Currently they may be more expensive than two-component polyurethanes, with an average cost greater than $25jgal. (7) They have other disadvantages of two-component polyurethanes. Typical polyurethane end uses are the following: Aircraft skins Missiles and other aerospace products Over-the-road trucks Buses Rail cars Automotive refinishing Automotive (original equipment manufacturer): newly introduced high-gloss clear coat over metallic base coat; chip-resistant primer surfacers; flexible coatings for plastic facias bumpers Military ground support equipment, such as tanks personnel carriers,vehicles, with resistance to live chemical agents (chemical agents-resistant coatings) Architectural and Maintenance Structures and vessels in chemical plants Offshore drilling rigs Roofs Antigraffiti Pipelines Product finishing Machine tools 87
Garden lawn mowers, snow blowers, tractors Plastic housings, keyboards, computer monitors, etc. Computer and business machines Medical and laboratory equipment
SILICONE COATINGS Silicone resins may be considered hybrids of glass and organic resins. Because of their inertness they can be used beneficially in conjunction with many other organic resins. Silicone resins with methyl and phenyl groups have been found to confer the most desirable properties; Table II lists their most important advantages. Most silicone resins used in the coatings industry comprise combinations of methyl and phenyl groups, and the resulting properties are dependent on the ratio of one to the other.
High-Temperature Coatings High-temperature-resistant coatings cure only when exposed to high temperatures, such as 260°C (SOOOP), but formulations are available in which coatings can cure at lower temperatures. In addition, inorganic pigments are commonly used. Colored high-temperature coatings are available, but to retain their color, the exposure temperature should not exceed 204°C (400°F).
Weatherability Although temperature resistance is one of the major benefits of silicone resins, they also exhibit excellent weatherability. This implies that they have improved color and gloss retention and exhibit less chalking. They may be incorporated into coatings, such as alkyds, phenolics, epoxies, polyesters, and other oleo resinous materials containing hydroxyl groups. Generally, the organic resins that are modified with silicone resins provide better properties than if the unmodified resins were to be exposed to the same environments.
Typictil Applications Unmodified silicone resin coatings are used on steel and aluminum substrates that will be subjected to high temperatures. These include such items as exhaust manifolds, mufflers, heat stacks, furnaces, boilers, ovens, heat exchangers, and aerospace and engine components. Colored silicone coatings generally withstand lower temperatures but are used on items such as space heaters, clothes driers, stoves, and similar applications. Copolymers, such as silicone-alkyds, are popular as industrial maintenance coatings where excellent exterior durability and chemical resistance are required. Applications include outdoor storage tanks, highway guard rails, railway tank cars, bridges, and aluminum siding. Unlike the unmodified silicone coatings, the silicone-organic copolymers, such as silicone-alkyds, -acrylics, -phenolics, -melamines, can be cured at ambient temperatures or typical baking temperatures. They are also commonly used as coil coatings. The silicone-modified alkyds and other organic copolymers are available in lowVOC formulations, typically with VOC contents of approximately 420 giL (3.S lb/gal). 88
Advantages of silicone-based coatings are as follows: (1) They are generally available as single-component coatings. (2) They can be air-dried or baked, depending on formulation. (3) Depending on the formulation and pigmentation, silicone resins can withstand temperatures 260-360°C (500-600°F) up to 540-650°C (1,000-1,200° F). Although their temperature resistance is excellent, color stability will depend on the choice of pigments and their resistance to high temperatures. (4) Copolymers, such as silicone-alkyds exhibit excellent resistance to exterior weathering .(5) Nonchalking coatings are available. (6) They have excellent resistance to thermal shock and corrosion and excellent water repellency. (Clear, thin coatings are commonly used as water sealers on the exterior of buildings.) (7) They can be formulated as copolymers with organic resins to obtain the advantages of each resin. (8) They have excellent electrical insulating properties; therefore they are used in resistors, capacitors, rectifiers, etc. (9) They have good adhesion, but require excellent surface preparation of substrate. Disadvantages are as follows: (1) High-temperature-resistant coatings must usually be cured at temperatures above 260°C (500°F). This can be a problem when coating large furnaces, stacks, etc. (2) Ifhigh-temperature-resistant coatings are not fully cured before being placed in service, corrosion of the substrate may take place. (3) Silicone resin-based coatings are sensitive to surface preparation and must be applied to properly prepared substrates. (4) They exhibit generally poor hardness and poor abrasion resistance. (5) Low-VOC, high-temperature coatings are not readily available.
89