Avoiding Fading Paint

Transcription

1 45 Clues to a Coating Failure Avoiding Fading Paint Uneven paint at a retail store offers lessons for specifying high-performance paint. Top-coated with a shop-applied high-solids flouropolymer paint selected specifically for severe weather exposure, the 386-footlong (118-meter-long) SkyDance Bridge in Oklahoma City is inspired by Oklahoma s State Bird, the scissor-tailed flycatcher. Photo courtesy of Tnemec Company Inc. By Jayson L. Helsel, P.E., KTA-Tator Inc. Within five years of application, areas of blue paint had faded on a store exterior in the South Atlantic United States, leading to a mottled, uneven appearance. Such premature fading was surprising, given that high-performance fluoropolymer coatings, known for their weather-resistance, were used on the store s architectural block exterior and metal awnings. To the trained eye, the faded areas appeared to be caused by UV exposure. But why would some areas fade and not others? Faded and non-faded areas were mostly equally exposed. And why should there be fading at all? Fluoropolymer coatings are known for service lives measured in decades. Fading is a Problem While the primary purpose of a coating is to protect a substrate in a particular service environment, aesthetics may also be important. The appearance of a retail or commercial building may be important to customers, more so than a warehouse, factory or other strictly utilitarian building. The coating s uneven appearance was a particularly poor reflection on the brand and could be considered a failure, even though its protective characteristics were intact.

2 46 D+D JUNE 2014 A coating s appearance may become unacceptable due to many factors. Moisture exposure during curing, such as from high relative humidity during application or a falling dew point after application, could be a cause. On concrete or masonry, efflorescence or alkaline burn from moisture intrusion may be issues. Or a coating simply may not have good resistance to sunlight, leading to premature or uneven fading. Polyurethanes have long been the workhorses of the high-performance coating world. Here, the highprofile steel supports on Philadelphia s Pennsylvania Convention Center are protected by two polyurethane finish coats over a zinc-rich primer and epoxy intermediate coat, all shop-applied. Photo courtesy of Tnemec Company Inc. Measure the Change in Color and Gloss In the case of the retail store, the color change was apparent, but it isn t always so noticeable. To determine whether there is a fading problem, you can measure color and/or gloss with portable instruments such as a spectrophotometer or gloss meter. Take measurements at areas where coatings have clearly faded or lost gloss. Compare them to measurements in shaded locations or other areas of little or no change. A project s approved samples are ideal for measuring the specified color and gloss. Instrumental measurement quantifies a color by defining three coordinates representing a specific point in a three-dimensional color system or space. One of the most common color systems is known as CIELAB. CIE stands for a French name that translates to the International Commission on Illumination, the main international organization concerned with color and color measurement. LAB refers to the three color coordinates or values that comprise the system: L*, a*, and b*. A general description of these values is that L* indicates the lightness of a color (where 0 is black and 100 is pure white), a* indicates the degree of red/green for a color (where a negative value indicates green and the positive value is red), and b* indicates the degree of blue/yellow for a color (where a negative value indicates blue and the positive value is yellow).

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4 48 D+D JUNE 2014 The numeric color difference between two sets of values, known as Delta E, is calculated using a standard equation. Theoretically, a difference between two colors should be visible when the Delta E reaches 1.0. In practice, however, a visual difference may be unnoticeable until Delta E approaches 3.0. The value at which a color difference is noticeable depends on the colors involved and how the values differ. Gloss is measured on a 0 to 100 scale, with 0 indicating a flat coating and 100 a high-gloss coating. Measurements are compared directly, with the numeric difference representing the gloss change. with the two parts resin and curing agent combined at the time of application. The cured film is hard and dense, and is typically applied at 3 to 5 mils dry film thickness (DFT). Polyurethane coatings are characterized by excellent chemical resistance, and aliphatic polyurethane formulations exhibit good resistance to weathering. generally more user-friendly than two-part coatings, can be more surface-tolerant and can be applied in a wider range of temperatures; significantly, they can tolerate much cooler application temperatures. Polyurethane coating systems usually include a primer, possibly an intermediate coat, and the finish coat. When the system is used on blast-cleaned steel substrates, a Not All High-Performance Coatings are Equal High-performance coatings such as polyurethanes, polyaspartics, polysiloxanes and fluoropolymers are the best bet for an appearance that withstands weathering and UV over time. Despite the fact that a fluoropolymer coating appeared to perform unevenly at the store, fluoropolymers are considered the best of this high-performing field. They re also the most expensive. Polysiloxanes are generally the next-highest performers. Polyurethanes have long been the workhorses of this group, balancing performance with economy. Polyaspartics, with performance similar to polyurethanes, have recently joined the group. Higher-performing 100 percent acrylic coatings are another good option where aesthetics and protective capability are important; however, acrylics typically can t match the other high-performance coatings mentioned for weather- and UVresistance. Let s examine each of these coatings. Polyurethanes: Polyurethane coatings cure by chemical reaction and are most often applied as two-component products, Although recommended for recoating after several years of weathering, the protective characteristics of this acrylic coating on a stadium in the Northeastern United States remain intact. Photo courtesy of KTA-Tator. Polyurethanes are also offered in singlecomponent products that cure by reaction with moisture in the surrounding atmosphere. These moisture-cure urethanes offer many of the same high-performance characteristics of two-component urethanes in a single-pack product. They are zinc-rich primer and epoxy intermediate coat are typically specified. When the coating is applied to galvanized sheet material, aluminum or concrete, the system includes an epoxy primer and the polyurethane finish. On galvanized or aluminum surfaces, pay special attention to the manufacturer s

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6 50 D+D JUNE 2014 This 1 million-gallon, 95-feet-tall (29-meter-tall) water tower in Brookfield, Ill., was recoated with polyurethane paint in in Aliphatic polyurethanes are known for good weather-resistance. Photo courtesy of Tnemec Company Inc. surface-preparation recommendations (e.g., chemical etching and/or specialized primers). Polyaspartics: Polyaspartic coatings are modified polyureas with application and performance properties similar to polyurethanes. One way they differ from polyurethanes is in their increased application thickness, typically 6 to 9 mils DFT. This increased thickness may allow elimination of an intermediate coat on blastcleaned steel. In this case, the system would include a zinc-rich primer and the polyaspartic finish. When the system is applied to galvanized, aluminum or concrete surfaces, the applicator will usually apply an epoxy primer along with the finish coat. As always, follow the manufacturer s surface-preparation methods and primer recommendations. Polysiloxanes: Polysiloxanes are siliconbased coatings offering a high degree of thermal stability and heat-resistance compared to typical organic compounds. These properties give polysiloxanes excellent weathering characteristics. Polysiloxanes are applied at a relatively higher application thickness than the other coatings reviewed here generally in the range of 3 to 7 mils DFT. As with polyaspartics, the increased film thickness may allow use of a two-coat system (zinc primer and finish coat) on blast-cleaned steel rather than a three-coat system. An epoxy primer is typical for galvanized, aluminum or concrete surfaces, again with the coating manufacturer s recommendations for primer and surface preparation being a priority. Fluoropolymers: Fluoropolymer coatings generally are regarded as unmatched for weathering resistance. However, they cost more and exhibit less tolerance for application error than the other types of coatings. Fluoropolymer coatings are based on polyvinylidene fluoride (PVDF) or flouroethylene vinyl ether (FEVE), which give the

7 Clues to a Coating Failure 51 coating film a high degree of mechanical hardness, abrasion resistance, chemical resistance, thermal stability and resistance to weathering. Factory finishes that require cure by baking are often based on PVDF, while field-applied coatings are typically formulated with FEVE resins. Both types were used at the store, and both exhibited an uncharacteristic fading in places. Fluoropolymer coatings are most often applied to sheet materials such as galvanized steel and aluminum, usually in factory settings. Fluoropolymer coatings are thin-film materials with a DFT in the range of 1 mil, depending on the particular coating and color. A typical fluoropolymer coating system includes a thin primer layer (0.5 mils DFT or less), and may also include a clear coat for certain colors. The coating manufacturer will specify the primer, either an epoxy or a PVDFbased coating, for use with the finish coat. When fluoropolymer coatings are applied as specified by the coating manufacturer, the coating manufacturer or metal-product installer can warrant them for 20 to 30 years. 100 Percent Acrylics: These coatings are most commonly available as latex paint. Though they lack the weather- and UV-resistance of the previously cited coatings, they are more affordable. They are popular residential coatings and, for the price, deliver good color and gloss retention. They are appropriate for mild atmospheric exposures, particularly in areas of increased volatile organic content (VOC) restrictions. SSPC, AAMA Publish Specs Although few industry standards exist for high-performance coatings, SSPC: The Society for Protective Coatings provides guidance for performance of polyurethanes in Paint Specification No. 36 for Two-Component Weatherable Aliphatic Polyurethane Topcoats. The Paint 36 standard specifies three levels of performance as measured by accelerated weathering requirements, with level 3 representing the highest-performing coatings. These performance requirements essentially address color and gloss retention as measured with specialized instruments. Although the standard is written for polyurethane coatings, with corresponding compositional requirements, specifiers can apply the performance requirements to other coating types such as polysiloxanes. The American Architectural Manufacturers Association (AAMA) publishes other industry references on the performance of these types of coatings. Of the many specifications published by AAMA, three that apply here are AAMA 2603, 2604 and 2605, all titled Voluntary Specification, Performance Requirements and Test Procedures for Pigmented Organic Coatings on Aluminum Extrusions and Panels. The specifications include performance requirements for properties such as color uniformity, gloss, dry-film hardness and adhesion, impact resistance, chemical resistance and corrosion resistance. Of these standards, AAMA 2603 is the least rigorous, while AAMA 2605 spells out the most demanding performance requirements. Another consideration in specifying color, even when selecting high-performance coatings with good weathering characteristics, is how well the desired color will perform. For example, bright reds, due to pigmentation, typically fade much faster than other colors. When there is a question as to whether particular colors will resist fading, consider performance testing for color and/or gloss retention. Knowledge Helps Solve the Mystery By following appropriate industry standards, we can increase our ability to specify and apply coatings that deliver optimum performance. Understanding how various types of high-performance coatings perform is an important first step in specifying their use. In the case of the store, that knowledge contributed to solving the mystery of the faded paint. Realizing that the original fluoropolymer coatings were unlikely to fade after five years, investigators found the faded areas were actually touch-ups done with color-matched 100 percent acrylic paints. It wasn t clear why the building needed touching up, or why a 100 percent acrylic was selected to go over a fluoropolymer. Budget may have been one reason for selection, though a short-sighted one. The recommendation, of course: Remove the faded acrylic and re-apply the fluoropolymer. About the Author Jayson Helsel is a senior coatings consultant with KTA-Tator Inc. He is a registered professional engineer, an SSPC protective coatings specialist, an SSPC-certified concrete coatings inspector and a NACE-certified coatings inspector. At KTA, Helsel manages coating projects, performs failure investigations and coating surveys, writes coating specifications and is a regular instructor for coating inspection courses. He holds a master s degree in chemical engineering from the University of Michigan. D+D