March 2, 2018
by Caleb Parker
Coating specification can be a complicated component of the building materials process. This is largely due to the high variability of exposure conditions and the accompanying deteriorative effects. Ultraviolet (UV) light degradation, heat, and humidity are among the leading causes of corrosion on structural and decorative steel and can result in unsightly staining from rust as well as a reduced service life. Rehabilitating steel after corrosion occurs can be expensive, adding downtime, labor, and material costs to a project’s bottom line.
Architectural coatings are specifically formulated to protect steel from the effects of UV light, humidity, and moisture intrusion, with the added benefits of ultra-durable color and gloss retention, user-friendly application, and low volatile organic compound (VOC) content.
These were some of the considerations that led to the architectural coating specifications for three glass-enclosed, sphere-shaped centerpieces of a multi-billion-dollar office complex in downtown Seattle. Designed by NBBJ, the site is a component of the headquarters for Amazon—the world’s largest online retailer.
Spanning three blocks in the city’s Denny Regrade neighborhood, the spheres are situated in the heart of the 306,580-m2 (3.3 million-sf) project and flanked by three 37-story high-rise towers and two mid-rise office buildings. In a space previously occupied by surface parking lots, the complex includes ground-level retail and restaurants, a dog park, sheltered pedestrian arcades, accessibility for bicycle commuters, and public art. (For more information, read “Amazon in the Regrade” at www.nbbj.com/work/amazon.)
The spheres serve as a workplace for Amazon employees, with the 6040-m2 (65,000-sf) interior designed to put its occupants at ease and help them think more creatively. The space houses a waterfall, tens of thousands of plants and trees, and a conference room enclosed by greenery.
The original coating specification for the complex structural domes called for a standard urethane topcoat, but this was later revised to a longer-lasting fluoropolymer coating system. This change was, in part, due to the tropical-like climate necessary for the hundreds of plant species growing inside
the spheres. The project required a coating system capable of withstanding a particularly humid environment.
“What [the designers] are doing inside is unbelievable,” says Seattle-based coating consultant Scott McConnell. “As soon as the building is closed at night, the humidity is increased to nearly 85 percent, then it drops down to 60 percent during business hours when the building is exposed to full sunlight. That is a major reason why we recommended a coating system with long-lasting corrosion protection.”
Architects at NBBJ worked with McConnell and coating consultants from Front Inc., to issue a revised coating specification for the steel Catalans. The new specification consisted of a shop-applied zinc-rich aromatic urethane primer and two field-applied coatings—a water-based epoxy and a fluoropolymer finish coat.
In addition to its low VOC, fast-curing, and rapid-handling properties, the epoxy intermediate coat was selected for the project because of the substance’s durability and corrosion resistance on architectural and industrial structural steel. Meanwhile, the fluoropolymer finish coating was specified for its durability, desirable appearance, and user-friendly application properties. For continuity, the same coating system was applied to the exterior metal of Amazon’s adjacent office building and to sculptures in the surrounding plaza.
“The change in coating systems moved the project from the short-term life cycle of a traditional aliphatic urethane coating to a fluoropolymer finish that included corrosion benefits, color and gloss longevity, and low VOC content,” McConnell says.
Zinc-rich primers have a history of success in providing corrosion protection to bridges, potable water storage tanks, and architecturally exposed structural steel—even those subjected to harsh environmental and corrosive elements. This is because when a zinc-rich primer cures and hardens, it creates a reactive barrier, protecting the steel substrate from weathering. As the degradation rate of zinc is many times less than that of steel, the zinc will sacrifice itself, thus protecting the underlying steel. (Find this information in K. Nanan’s 2017 Corrosionpedia article, “How Metallic Coatings Protect Metals from Corrosion.”)
The primer used in the spheres project contains 83 percent zinc pigment by weight in the cured film and provides sacrificial (or galvanic) protection should the structural steel become scratched or damaged during construction. The performance of this primer has been demonstrated by the coating manufacturer—both in the field and within the laboratory—using accelerated testing in accordance with ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus.
Salt spray testing
The coatings manufacturer used a standard salt spray (or fog) test, where sample panels were coated with the zinc-rich primer and placed in a controlled chamber. Intervals of hot, de-ionized water were released into the enclosed space to create humid conditions. Inside the salt spray (fog) apparatus, the specimens were exposed to a neutral (pH 6.5 to 7.2) salt fog solution, containing five percent sodium chloride, at 35 C (95 F). This acted as an accelerant to induce corrosion. (To read more, refer to “Testing Remains Relevant for Coating Selection” by M. Thomas in the July 2011 edition of The Construction Specifier. The definition is derived from Corrosionpedia, “What does ASTM B117 mean?”)
Next, to simulate what occurs when a topcoat is scratched or damaged at a construction site, testers cut a scribe through the coating film and exposed the metal substrates to the chloride solution. This component of the test had to be conducted because undercutting can result in the coating lifting and peeling away from the steel substrate, opening the surface to severe corrosion.
After 50,000 hours of exposure to the salt fog conditions in the controlled chamber, specimens coated with the primer showed no blistering, cracking, or delamination, little undercutting (or creepage) at the scribe, and no more than one percent rusting. For comparison, metal samples protected solely by two coats of epoxy were subjected to the same test and lasted only 6700 hours in the controlled chamber, while a micaceous iron oxide (MIO)/zinc prime coat exposed to 10,000 hours revealed 0.4-mm (0.016-inch) creepage and three percent rusting at the scribe (Figure 1).
The proposed fluoropolymer finish coating also underwent accelerated weathering testing to verify its durability and aesthetic performance. Under testing conducted by the coating manufacturer in accordance with ASTM D4587, Standard Practice for Fluorescent UV-Condensation Exposures of Paint and Related Coatings, the chemistry found in this fluoropolymer technology showed almost no color or gloss change after 10,000 hours of accelerated UV exposure. (ASTM D4587, Standard Practice for Fluorescent UV-Condensation Exposures of Paint and Related Coatings, is available at www.astm.org/DATABASE.CART/HISTORICAL/D4587-05.htm.)
The finish coat was also subjected to a test known as the equatorial mount with mirrors for acceleration with water (EMMAQUA)—a particularly harsh outdoor accelerated weathering test in accordance with ASTM D4141, Standard Practice for Conducting Black Box and Solar Concentrating Exposures of Coatings, designed to simulate wet climates. Metallic specimens coated with the fluoropolymer finish were exposed to UV energy, which was measured in megajoules per square meter (MJ/m2), to determine levels of gloss, color, and any physical degradation. When exposed to 3500 MJ/m2, the tested fluoropolymer coating showed no blistering, cracking or chalking, slightly less color change, and significantly better gloss retention than a standard acrylic polyurethane.
Both accelerated weather tests were consistent with the aesthetic performance of fluoropolymer coating systems observed from weathering tests conducted in South Florida. (This is based on proprietary testing conducted by the coatings manufacturer.)The aesthetic reliability of these coatings has also been supported by the research of trained coating specialists who have observed ambient-cured fluoropolymer coatings on both new and maintenance landmark architectural projects over the past 15-plus years under a wide range of exposure conditions, which include high-humidity coastal environments, desert climates, and areas with frequent freeze-thaw cycles. (This is based on anecdotal observations of manufacturer coating consultants and research and development specialists.)
The zinc-rich aromatic urethane primer was pre-applied by the steel fabricator, then the frames were shipped to the site and assembled. Once onsite, the weld seams were cleaned and touched up with a low-VOC, zinc-rich urethane. Applicators completed the coating system with an intermediate layer of a hybrid water-based epoxy before adding the fluoropolymer finish coat to ensure outstanding color and gloss retention. The field-applied intermediate and finish coatings are compliant with the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) program. Each component also meets the VOC content limits of South Coast Air Quality Management District (SCAQMD) 1113, Architectural Coatings, which helps the project earn credit toward LEED Gold certification. (For more information, read “Low-emitting Materials” in LEED BD+C: New Construction v4.)
In addition to its durability and its aesthetic properties, the fluoropolymer finish coating was formulated with fluoroethylene vinyl ether (FEVE) resins, which are ambient air-cured for user-friendly application with brush, roller, or spray equipment. As the FEVE-based fluoropolymer is ambient-cured, it can be applied in the field after erection and welding, unlike other force-cured fluoropolymer technologies. Additional advantages can include versatility, with specifiers able to select from a wide range of colors, and resistance to graffiti.
“The coatings were brush and roller-applied in the field, which helped the applicator to complete the job with an army of construction workers and tradesmen on the jobsite,” McConnell says. “From shop priming to field finishes, the coating system was flawless.”
With its highly unusual exposure conditions vital to sustaining thousands of interior plants and trees, the spherical centerpiece of Seattle’s brand-new Amazon headquarters required durable, corrosion-resistant, structural steel coatings with the added benefits of color stability and sustainability. Consultation with coatings experts led the project’s design team to a long-lasting coating system that achieved the required performance criteria, including resistance to UV light and humidity, color and gloss retention, ease of application, and low VOC content.
Caleb Parker is an inside sales manager for Tnemec Company, where he provides coating assistance to various customers and independent sales representatives in the western region of the United States, as well as various international locations. Parker is a NACE coating inspector (Level III certified) and has associations with various organizations in the industry, including the American Water Works Association (AWWA), Water Environment Federation (WEF), and Society for Protective Coatings (SSPC). He has nearly a decade of experience in assisting with the specification and application of high-performance coatings and linings. Parker can be reached via e-mail at email@example.com.
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