The resins and other paint components that have been discussed in the previous chapter are used to provide properties to coatings for specific applications. These properties are determined by the substrate to be covered, the end use for the product,and the environment in which the coating will be applied.
There are a tremendous number of industrial and trade sales paints available.Paints are manufactured for industrial applications, building maintenance, house painting, architectural steel, tanks, and bridges. This text is focused on industrial coatings applied to manufactured durable goods.
3.1 COATINGS SELECTION
Virtually any desired property can be built into the coating if the need exists. Ultraviolet light resistance, chemical resistance, impact resistance, glossy appearance, smoothness, and many more characteristics can be part of the formula. It is important to carefully define the needs of the end product to be sure that the coating will perform properly. Keep in mind that any special characteristics will probably raise the cost of the paint.
The primary factors to consider in choosing a coating are the task to be performed,(appearance or performance) and the environment the product will be used in.
• Gloss level—high, medium, low
• Finish appearance—smooth, slight texture or orange peel, heavily textured or spatter coated
• UV resistance
• Corrosion resistance
• Solvent/chemical attack
• Impact resistance
3.1.3 OTHER FACTORS
• Manufacturing abuses (forming, machining, or welding after coating)
• Substrate to be coated: plastic, wood, steel, aluminum and castings
• Value of the product and cost per square foot applied
• Exterior or interior use
• Product life expectancy
3.2 PAINT CLASSIFICATIONS
There are different ways to classify paint materials based on their formulations and use. The specific task they perform, the physical type of paint they are known as, and other factors may be used to identify a particular product.
3.2.1 PAINTS CLASSIFIED BY TASK
Paints are often identified by the task they perform, such as primers, sealers, surfacers, and topcoats. Each type of paint has a different function.
A primer is the first coat applied to a surface. Its usual job is to fight corrosion and enhance the adhesion of subsequent coatings applied over the primed surface. Most industrial primers are cross-linking or thermosetting type. The binder used in the primer must be compatible with the substrate and the second coat. Thus, different primers are required for wood, stone, metal, and plastic surfaces. Primers must be able to tolerate the differences in thermal expansion between substrate and paint.
When primers are used for protection against corrosion, they are usually highly pigmented. They usually have relatively low gloss, because the primer particles extend up to the surface of the film and give it a rough, pebbled texture. The best corrosion resistance is usually achieved at a pigment level near the critical pigment volume concentration (CPVC). Primer film thickness is typically 1.0–3.0 mil.
Primers for metal are frequently alkyd systems that cure by a solvent reaction mechanism. A typical industrial primer might have the following approximate
composition, as sprayed:
• Alkyd resin: 20%—binder
• Pigment: 15%—for corrosion protection
• Solvent: 60%—to liquefy
• Additives: 5%—viscosity control, antifoam, and so forth
In recent years, there has been a trend toward using epoxy-modified alkyd binders for primers. These resins have better adhesion and corrosion resistance than conventional alkyds.
188.8.131.52 Sealers and Surfacers
Sealers are paint films applied between a primer and a topcoat to improve adhesion. This is often necessary if the primer and topcoat have different adhesive, Liquid Organic Coatings thermal expansion, or impact resistance properties. When the interface between these films is stressed, the layers sometimes separate, and the topcoat can then chip, crack, or break off.
Sealers are also sometimes used to provide a barrier coat between a paint film and the solvent in a subsequent coat. This may be necessary if the pigment in the lower coat has a tendency to bleed into the solvent of the upper coat.
Sealers have special binders that stick well to both the primer and the topcoat. In addition, they are slightly elastic, acting as flexible glue to hold the primer and the topcoat together during periods of mechanical or thermal stress. Sealers are usually low in pigment. Typically, sealers are applied as thin films less than 0.5 mil thick when dry.
A surfacer is a coating used to fill irregularities in the primer. Since primers are often heavily loaded with pigment, they sometimes bake out with rough surfaces that are difficult to sand.Asurfacer is designed to be sanded and provide a smooth surface before topcoat application. The typical film thickness for a surfacer is 1.0–3.5 mil.
The final layer of paint, the topcoat, is the one that provides the necessary appearance and possibly improves performance. Topcoat may be applied as a single coat, or it may be applied as more than one thin coat to achieve the final film build. When several coats are applied, it is not always necessary to completely dry each coat. The first coat is allowed to flash off before receiving the next coat. This is referred to as wet-on-wet application.
The topcoat provides the surface with the desired color, gloss, abrasion resistance, and weatherability. Usually, the corrosion resistance of the topcoat is minimal.The highly pigmented primer is expected to provide most of the paint’s corrosion resistance.
Many binders are used for topcoats. The final choice of a topcoat usually requires a trade-off among several properties (Table 3.1). In the automotive industry, for example, it may be necessary to sacrifice some impact resistance to achieve the desired gloss.
3.2.2 PAINTS CLASSIFIED BY PHYSICAL TYPE
Paints are also grouped by physical type such as high-solids, waterborne, or powder coating. The following is a description and comparison of the most common types of industrial liquid coatings, high-solids, and waterborne coatings.
184.108.40.206 Waterborne Coatings
Waterborne coatings are materials in which water is the major solvent or dispersant. The coating may be a true solution, a colloidal dispersion, or an emulsion. The characteristics of these formulations are shown in Table 3.2.
Waterborne paints contain 5–40% organic cosolvent to aid in wetting, viscosity control, and pigment dispersion. Some systems, especially emulsions, may also contain a polymeric thickener.
Numerous resins are used for waterborne systems. Modified epoxies, polyesters,acrylics, alkyds, vinyl acetate-acrylics, and styrene-acrylics are available. Both thermoplastic and thermosetting (cross-linking) formulations are used. Some resin systems cure when heated while others require the addition of a cross-linking agent.
Advantages of Waterborne Coatings
• Reduced fire hazard (some are flammable because of the organic cosolvent)
• Reduced solvent emission
• Lower toxicity
• Use equipment similar to solvent spray
Limitations of Waterborne Coatings
• Stainless steel or plastic pipe and fittings are necessary
• Some formulations must be protected from freezing
• Better control of booth temperature and humidity is required
• Longer flash-off times are necessary
• Requires better pretreatment
Waterborne systems are available for most substrates although some plastics may be difficult to wet and have poor adhesion. Color, impact resistance, gloss, weatherability, corrosion resistance, and repairability characteristics of waterbornes are similar to conventional coatings.
There are certain differences from conventional coatings that will affect the plumbing, tanks, booths, and pumps used with waterborne products. These differences are important enough to influence the installation cost and ease of application of waterborne coatings.
Pumps, tanks, and piping must be of noncorrosive material. Black iron systems will corrode, so plastic or stainless steel is recommended due to the composition of the paint. Screens and filters are an important part of the paint distribution system, especially when emulsions are used, and they should be generously sized and regularly serviced. Centrifugal pumps can be used, but positive displacement pumps give better control of pressure and flow rate.
Provisions should be made to keep waterborne coatings from freezing and to avoid storage above 90◦F (32◦C). The viscosity of many waterborne coatings is temperature sensitive, so paint heaters are often used to ensure good control of this important variable.
Spray booth air velocities for waterborne coatings can be lower than those used for solvent paints, because they contain little toxic, flammable, or bad-smelling solvent. This can provide savings in capital and operating cost. In some cases, booths may need to be longer than usual because of the slow evaporation rate of water. Line speeds may have to be reduced for the same reason.
Keeping the part surface and the spray equipment clean is particularly important with waterborne coatings, because they are less tolerant to oil and dirt. This is because water does not “wet” or “pick-up” these materials as readily as solvents. Even small amounts of oil or grease, which might find its way into tanks, pumps, or lines will not mix with the paint and can cause surface defects on the parts.
Dip systems and air spray systems are commonly used for the application of waterborne coatings. Electrostatic systems are less common. Since waterborne formulations are somewhat electrically conductive, it is necessary to isolate electrostatic systems from ground to avoid loss of current through the supply line.
Temperature and humidity control are more critical for waterborne coatings than for solvent paints, because water evaporates more slowly than many organic solvents.
The slow evaporation rate of water affects several areas of the system:
1. Longer flash-off times are needed
2. Sagging and popping are more likely
3. Wet-on-wet coating may not be possible
In areas where the humidity is very high for an extended period, it may be necessary to air-condition the spray booth to avoid excessive sagging or popping. Air pressure, wet-film build, and gun-to-part distance may also require extra-close attention.
The cure of waterborne coatings is similar to that of solvent paints. The liquids in the film must be driven off in a reasonable time and, in the case of thermosetting enamels, cross-linking is accomplished in an oven. Waterborne paints must have somewhat longer flash time than solvent-based paints. Sometimes, it is also necessary to use a slower heat-up rate in the oven.
If temperature is applied to the wet film too quickly, it may cause popping. Since water from the paint may be corrosive, it is necessary to use rust resistant panels in the oven and exhaust system.
Once cured, waterborne films are much like other coatings. Overbaking leads to cold-cracking and poor adhesion of subsequent coats. Underbaking means reduced durability and lowered corrosion resistance. The repairability of the finished film is related to the resin and additives in the paint rather than the fact that it is a waterborne formula.
220.127.116.11 High-Solids Coatings
High-solids coatings are solvent-based paints that contain greater amounts of pigment and binder than traditional solvent-based paints contain. At least 65% and as high as 85% of the formula is solids content. Ultra-high solids paints may have an even higher percentage of solids. The much lower solvent content of high-solids paints offers some advantages over traditional materials.
Advantages of High-Solids Coatings
• Less paint must be shipped, stored, pumped, and sprayed compared to traditional solvent-based paint.
• Lower oven-air volumes are required.
• Less paint must be sprayed to provide a given film build.
• Booths may sometimes be shorter.
• Less oven exhaust volume is necessary.
• Less solvent is emitted to the atmosphere. This means less difficulty in
meeting regulatory requirements.
Limitations of High-Solids Coatings
• High viscosity. Paints must often be heated to around 100–120◦F to achieve
• High viscosity does not perform well in dip tanks or flow-coaters.
• Difficulty in pumping and atomizing, especially when cold.
• Cleaning and phosphating quality may be more important than for conventional
paints, because there is less solvent present to “clean as it coats”.
• Sticky over-spray which is messy to clean up, because it remains in the
High-solids paints are baking enamels. Alkyds are the most common resins used for high solids, but there are also polyester and acrylic products. Once the paint has been applied and cured, film properties are essentially the same as for other solvent formulations based on the same resin.
Because of the high viscosities involved, high-solids paint lines are usually equipped with paint heaters. Heaters are usually located near the guns and are set to keep the paint in the range of 110–150◦F (43–95◦C) with tolerance of a few degrees.
The elevated temperature drops the paint viscosity enough so that good atomization is possible.
Most spray guns can handle the additional air pressure required for breaking up high-solids paints. However, the capacity of lines, pumps, and filters may have to be increased to ensure trouble-free performance. Pressure-feed pumps are usually required. Paint agitators should be of low-speed, high-torque design rather than the other way around.
Since high-solids paints are heavy (up to 360 kg or 800 lb per barrel), it may be necessary to increase hoist and forklift capacities.
Cure oven exhaust volumes can be lower than conventional solvent-based coatings, because there are less volatile gases in the oven. Filters and floor papers may need to be changed more often and they are difficult to handle due to the fact that the material may not air-dry.
High-solids coatings are applied with air, airless, conventional, or electrostatic spray. Electrostatic disks and bells are also commonly used. Atomization with these devices is usually excellent at speeds of at least 25,000 rpm.
Film buildup of less than 12.5 μm(1/2 mil or 0.0005′′) are possible with 15–30 cm (6–12′′) discs. Film builds up about twice as rapidly with high-solids paints as it does with conventional products, so there may be an initial tendency to apply too much paint.
Overspray can be a real nuisance, since it contains about twice as much solids as conventional paint.
Like all liquid coatings, it is important to control the application process and use the right equipment to reduce overspray. Fluid and atomizing pressures, fluid tips, and air caps should be carefully chosen to maximize application efficiency.
Some equipment modifications and operator retraining will be required when converting to higher solids coatings. Larger diameter lines and more powerful pumps are the most common changes required.
The cure of high-solids coatings is similar to conventional enamels. Thirty minutes at 300◦F (150◦C) is typical although cure schedules can vary considerably. No special ovens are required for high-solids coatings.
Before the paint can be applied, it must be prepared and set up for delivery to the spray system. Good paint preparation and system set up can help to avoid application problems.
When the paint container is first opened, the paint should be checked visually for kick-out. Kick-out refers to the binder coming out of solution as small lumps of soft or hard material. Sometimes, the process can be reversed by agitation, but in many cases the paint is no good. Kick-out results when the binder and solvent are no longer compatible.
The paint should be thoroughly mixed so that there is no separation of the ingredients. Normally, over the course of time, the paint solids will tend to settle to the bottom while the solvents will rise to the top. If applied unmixed, an uneven film will result with differences in film properties. In small batches, the paint can be stirred and in larger operations it is mixed with a powered agitator. To avoid application problems and surface defects, it must be blended to consistent, uniform viscosity.
Proper paint viscosity is critical for obtaining a quality finish. Viscosity is the resistance to flow exhibited by fluids, a result of the internal friction of molecules against each other during movement. In conjunction with air and fluid pressure, viscosity controls the size of the paint droplets when the paint is spray-applied.
Excessive viscosity can cause orange peel while a low viscosity can create a film that is too “wet” and creates runs.Ageneral rule of thumb for application viscosity is 20–25 s on a #2 Zahn cup for conventional air spray and 25–40 s for airless application. However, the product data sheet should always be referred to before application. For
high-solids paints, the spray viscosity is usually between 13–20 s on a #3 Zahn cup. Materials and applications vary widely, so viscosity adjustments should be made according to the manufacturers’ recommendations.
The viscosity of the paint should always be checked before and during application.
An important factor in checking viscosity is the temperature of the paint. Significant changes in viscosity will occur with relatively small changes in temperature. For example, a coating that has a viscosity of 40 s on #2 Zahn at 70◦F could drop to 25 s at 90◦F. Viscosities should always be tested at 77◦F (25◦C).
Viscosity is measured with a viscosity cup and a stopwatch. Viscosity is expressed in terms of seconds of time it takes to flow through the cup that has been filled to a specified level. To take this measurement, the cup is immersed in the paint, filled, and then withdrawn. When the cup is withdrawn from the liquid level of the paint,the stopwatch is started. Paint will flow out of the orifice in the bottom of the cup.
The watch is stopped when a definite break in paint flow is seen.Viscosity is sometimes lowered by reduction with solvent. In some cases, a compliant coating may be delivered ready to spray. Reduction of compliant coatings will reduce the solids content as sprayed and may constitute a violation of an air quality permit. In some cases, it is best to have the coating supplier adjust the material
viscosity to make sure that it is not out of compliance.
The relationship of viscosity to temperature can be problem if the spray area does not have climate controls. The plant temperature and humidity will change with the seasons and time of day. This can affect paint viscosity and paint cure characteristics.
Different solvent blends may be needed to provide the proper cure during different seasons. Temperature control of the paint can provide the paint consistency needed to maintain proper application viscosity throughout the day. This temperature relationship is illustrated in Figure 3.1.
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