For high-profile architecturally exposed structural steel (AESS), water tanks, bridges, and public art, the long-term color and gloss properties of protective coatings are essential to the image of the project. As such, the basis of design should be specified to provide the longest service life, lowest life-cycle cost, and most sustainable solution. This means avoiding costly repaints or repairs down the road due to corrosion and cosmetic problems.

As with any performance-based specification, a balance of applicable case histories, real-world testing, and accelerated testing is the best way to give specifiers and owners confidence that the coating system is up to the task.

These three ASTM tests should be considered when specifying a high-performance topcoat, as they run the gamut of laboratory accelerated weathering with real-life exterior exposure to form a well-rounded battery of testing.

UV (lab/accelerated)

ASTM D4587 (Standard Practice for Florescent UV – Condensation, Exposures of Paint, and Related Coatings), also known as QUV, is an accelerated test performed in a lab. Widely used because it gives fast and reproducible results, this test mimics two main causes of topcoat degradation—UV radiation and moisture. Coated test panels are exposed to prescribed cycles of UV light and condensation.

To compare test results, first look at the number of hours tested, then the reported color change and gloss loss. Keep in mind that this test allows for a couple of variations that can significantly affect the color and gloss numbers. First is the type of lightbulb used. UVB-313 bulbs include the shortest wavelengths found in sunlight. A significant portion of this type of solar radiation does not reach the Earth's surface and is typically used for quickly screening different resins. UVB-340 bulbs use a wider spectrum that includes wavelengths not emitted by the sun.

Variations in the durations of the UV-condensation cycle can impact the results. Review testing information carefully to determine which cycle was utilized.

EMMAQUA (natural + accelerated)

ASTM D4141 (Standard Practice for Conducting Black Box and Solar Concentrating Exposures of Coatings), generally referred to by its trade name EMMAQUA, is an accelerated outdoor test. Using a rack mounted with 10 mirrors that follow the sun, the test concentrates the solar radiation onto the coating and then periodically sprays high-purity water to further degrade the film. Instead of hours, EMMAQUA results are reported in units of total energy exposure by the coating, as megajoules per square meter (MJ/m2). The accumulated irradiance, combined with the color and gloss results, makes EMMAQUA a very useful test when comparing the performance capabilities of various topcoats.

South Florida (natural—not accelerated)

South Florida marine exposure provides the most correlative real-world results, with the downside being the time it takes to get those results. Test panels are placed in a rack, located along Florida's southern coast, at a 45-degree angle and then evaluated for corrosion, film defects, color change, and gloss reduction. The combination of high UV levels, humidity, rain, and salt spray from the ocean stresses and degrades the coating film over time.

South Florida results are typically measured in years. Due to the natural exposure, some coatings can take 5, 10, or more years to produce meaningful results. Therefore, newer coating technologies may not yet have data.

If you are looking for a high-performance topcoat that best suits your region and assets, reach out to your local Tnemec representative to learn more about Tnemec’s coatings. At Tnemec we believe that testing matters, formulation matters, and above all else, performance matters.