In NCCA Tool Kit # 26, “Factors Influencing the Long-Term Performance of Prepainted Metal Building,” an emphasis was justifiably placed on the selection of materials. That selection process starts with a substrate that must provide the corrosion-resistant properties for the environment in which the prepainted product will be used, probably for decades. Whether the specifier is considering steel or aluminum, the mechanical properties of the material are of paramount importance. After all, all prepainted metal is post-formed, so the substrate, as well as the paint system, must be able to withstand the rigors of the fabrication process. With a steel substrate, it’s the thickness of the metallic coating layer; with hot-dipped galvanized steel, it’s the zinc; and with Galvalume®, it’s the Zn-Al metallic blend that needs to have an adequate thickness to provide the necessary lifetime and level sacrificial of galvanic properties. There are many parameters to consider.
After the proper substrate for the job is determined, all prepainted metal must be chemically treated. This pretreatment process does two things: It adds further to the corrosion protection that the substrate already provides, and it creates a surface that enhances the adhesion of the organic coatings to the substrate. This may not sound like much, but the thin pretreatment layer can make or break the long-term performance of the prepainted metal system.
In the coil coating process, once the metal is cleaned and pretreated, primer is applied. Depending on the application, the back side of the strip may—or may not be—primed. Like everything else having to do with prepainted metal, the choice of the primer—both its chemistry and its thickness—plays an important role in the lifetime of the coated metal. All primers incorporate a corrosion-fighting pigment, but the chemistry of the resin system is chosen specifically for the fabrication properties to which the prepainted metal will be subjected. Epoxy, polyester, and urethane primers are commonly used.
Finally, we get to the topcoat—the coating the consumer sees. They probably believe it’s all about the topcoat aesthetics, since that’s what they see, but the real foundation of long-term performance is in what they don’t see. Indeed, the topcoat needs to be formulated to resist environmental staining and weathering to maintain the aesthetics of the metal building. Various resin systems are available, but the three most common are based on polyester, silicone-modified polyester, and polyvinylidene difluoride (PVDF) resin chemistry, with the latter providing unparalleled long-term performance. But resin chemistry is just one aspect of a topcoat’s performance.
Prepainted metal is available in myriad colors, and the pigments chosen to create these colors are as important as the resin chemistry in providing excellent performance over years of exposure. Today’s topcoat colors can range from earth-tone tans and browns to saturated reds and greens, and can sometimes have pearlescent- and color-shifting effects. Most topcoat colors are available with the ability to reflect infrared radiation, which helps to keep the roof cool and to minimize cooling costs.
So although consumers may only see the topcoat and only expect it to provide an acceptable level of aesthetics over the long run, they would benefit from a better understanding of how the additional layers under the topcoat also contribute to the long-term performance of prepainted metal.
The National Coil Coating Association (NCCA), with the support of its member companies, is pleased to offer a free seminar for architects and specifiers with the opportunity to earn up to 3 CEU credits.
Attendees will learn about the long-term performance of prepainted metal and how it can meet modern design and environmental requirements while offering cost-effective solutions for builders and building owners. The seminar will be held Monday, April 30, 2018, from 1:30 p.m. – 5:30 p.m. at the Eilan Hotel in San Antonio, Texas. To learn more about the seminar and how to register, click here for full details.