CoatingsPro Magazine

JUL 2015

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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COATINGSPRO JULY 2015 41 Science Behind It Why Does It Stay White? By Kim Bistromowitz, Marketing Communications Specialist for Quest Construction Products, and Steve Heinje, Technical Service Manager for Quest Construction Products I nstead of using an acrylic, silicone, or polyurethane roof coating, the architect, facility manager, and contractor for the Superdome recoat chose a fuoropoly- mer to keep their landmark bright white. W hy? W hat on earth is a fuoropolymer? Fluoropolymers have been around for three-quarters of a century, although most people still aren't familiar with the term. Fluoropolymers are best known for their use as a non-stick cookware coating and a component in weather-resis- tant apparel, but they have dozens of other major applications in industries, ranging from electrical wire coating to protective automotive paints. Teir unique properties include excellent chemical and heat resistance and a great ability to withstand ultraviolet (UV)-induced degradation and corrosion, even in the most challenging environments. Fluoropolymers were a " happy accident," discovered in 1938 by Dr. Roy Plunkett. He found them to be extremely heat-resistant, chemically inert, and amazingly slippery — nothing would stick to the surface. (Tink Tefon!) Te coating that was used on the Superdome project is a fuoropolymer-based roof coating based on a polyvinylidene fuoride resin (PVDF), a fuorocarbon-based polymer with multi- ple strong carbon-fuorine (C-F) bonds. Tese bonds are the secret weapon in the Superdome's never-ending fght against UV-induced degradation, dirt, mold, and mildew. Fluorine is the most electronegative element in the periodic table. When bound to carbon, it forms the strongest bonds in organic chemistry. Bond…C-F Bond For a roof coating to stay white over time, it fghts battles on several fronts. Plasticizers in polyvinyl chloride (PVC) membranes and asphalt in shingles and asphaltic membranes attempt to migrate to the surface, causing white coatings to discolor over time. Other materials do this, too; unreacted resins in insulation and coatings, plasticizers, and fre retardants bleed, or bloom, to the surface. Tis is a universal problem with asphalt, commonly called asphalt bleeding. Te strong C-F bond resists this migration, acting as an efective blocker for staining. Simultaneously, on the surface, the coating endures extended exposure to water, humidity, a wide range of temperatures, UV rays, oxygen, and atmospheric pollutants. Due to the C-F bond, PVDF coatings have a relatively inert surface that resists dirt pick-up. Te surface friction created by this bond is low, enabling the coating to repel dirt and contamination, so the dirt doesn't "stick " to it. Additionally, moisture glides over the smooth, enamel-like fnish, aiding the coating in a self-cleaning when it rains. No Mildew Here! Te biggest challenge the Superdome faced with keeping the surface white wasn't necessarily the discoloration of the existing coating; it was the dirt, grime, and mildew that accumulated on the surface. Te morning sun dries the eastern half of the roof, leaving the opposite side in shadow until midday. Tis shadow y half stays moist longer, provid- ing the perfect breeding ground for mildew growth. Before this recoat, every year, workers had to pressure wash the dark mildew of, with each cleaning costing fve- to six-fg- ure amounts. Fluoropolymer coatings resist dirt and grime accumulation, and they resist biological growth since mold and mildew have difculty gaining a foothold on PVDF coatings even in New Orleans' harsh, humid conditions. Coatings based on PVDF technology also have excel- lent water repellency (also expressed as low surface energy) and prevent water from penetrating the surface. Even after 200 hours of immersion, a coating based on this technology shows minimal water pick-up: 6.5 times less percent water by weight versus a more traditional coating technology. Te result of low water pick-up is maintenance of the excellent adhesion of the coating to the surface and reduced repair cost related to delamination and water damage. Bright, Bright White So how can white roofs be guaranteed to stay bright white? Two key properties control the brilliance, or whiteness, of a coating: gloss and color fade. Gloss is a subjective term used to describe the amount and nature of mirror-like refection. In laymen's terms: How shiny is it? In the coatings industry, percent gloss retention is used to compare gloss ratings over time. Te reduction in gloss indicates the breakdown of the backbone of the coating. Not only do fuoropolymers retain this surface gloss longer, the fnish contributes to the dirt pick-up resistance and self-cleaning properties. Color fade is the change of color of pigmented paint due to efects of light, heat, time, temperature, and chemicals. Like gloss loss, fading color results from the breakdown of the polymer in a coating as well as the pigments used in that coating. Fluoropolymers ultimately are not afected by UV; the result is that a white surface will stay white, retaining high TSR (total solar refectance) values that remain that way over time. Who would've imagined that the technology behind nonstick cookware would be used to keep this icon bright white? CP

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