Glass paint could keep metal roofs andother structures cool even on sunny days16 August 2015

Sunlight can be brutal. It wears down even thestrongest structures, including rooftops and navalships, and it heats up metal slides and bleachersuntil they're too hot to use. To fend off damage andheat from the sun's harsh rays, scientists havedeveloped a new, environmentally friendly paintout of glass that bounces sunlight off metalsurfaces—keeping them cool and durable.

The researchers present their work today at the250th National Meeting & Exposition of theAmerican Chemical Society (ACS).

"Most paints you use on your car or house arebased on polymers, which degrade in theultraviolet light rays of the sun," says Jason J.Benkoski, Ph.D. "So over time you'll have chalkingand yellowing. Polymers also tend to give off volatile organic compounds, which can harm theenvironment. That's why I wanted to move awayfrom traditional polymer coatings to inorganic glassones."

Glass, which is made out of silica, would be anideal coating. It's hard, durable and has the rightoptical properties. But it's very brittle.

To address that aspect in a new coating, Benkoski,who is at the Johns Hopkins University AppliedPhysics Lab, started with silica, one of the mostabundant materials in the earth's crust. He modifiedone version of it, potassium silicate, that normallydissolves in water. His tweaks transformed thecompound so that when it's sprayed onto a surfaceand dries, it becomes water resistant.

Unlike acrylic, polyurethane or epoxy paints,Benkoski's paint is almost completely inorganic,which should make it last far longer than itscounterparts that contain organic compounds. Hispaint is also designed to expand and contract with metal surfaces to prevent cracking.

Mixing pigments with the silicate gives the coatingan additional property: the ability to reflect all sunlight and passively radiate heat. Since it doesn'tabsorb sunlight, any surface coated with the paintwill remain at air temperature, or even slightlycooler. That's key to protecting structures from the sun.

"When you raise the temperature of any material,any device, it almost always by definition agesmuch more quickly than it normally would,"Benkoski says. "It's not uncommon for aluminum indirect sunlight to heat 70 degrees Fahrenheit aboveambient temperature. If you make a paint that cankeep an outdoor surface close to air temperature,then you can slow down corrosion and other typesof degradation."

In fact, the paint Benkoski's lab is developing isintended for use on naval ships. But it has manyother potential commercial applications.

"You might want to paint something like this on yourroof to keep heat out and lower your airconditioning bill in the summer," he says. It couldeven go on metal playground slides or bleachers.And it would be affordable. The materials needed tomake the coating are abundant and inexpensive.

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Benkoski says he expects his lab will start fieldtesting the material in about two years.

More information: Passive cooling with UV-resistant siloxane coatings in direct sunlight, the250th National Meeting & Exposition of theAmerican Chemical Society.

AbstractSolar exposure is a leading cause of materialdegradation in outdoor use. Polymers and otherorganic materials photo-oxidize due to ultraviolet(UV) exposure. Even in metals, solar heating cancause unwanted property changes throughprecipitation and Ostwald ripening. In morecomplex systems, cyclic temperature changescause fatigue failure wherever thermal expansionmismatch occurs. Most protective coatingsdesigned to prevent these effects inevitablysuccumb to the same phenomena because of theirpolymeric matrix. In contrast, siloxane coatingshave the potential provide indefinite solar protectionbecause they do not undergo photo-oxidation. Thisstudy therefore demonstrates UV-reflectivesiloxane coatings with low solar absorptance andhigh thermal emissivity that prevent any increase intemperature above ambient conditions in directsunlight. Mathematical modeling suggests thateven sub-ambient cooling is possible for ZnO-filledpotassium silicate. Preventing widespread adoptionof potassium silicates until now has been theirtendency to crack at large thicknesses, dissolve inwater, and delaminate from untreated surfaces.This investigation has successfully addressedthese limitations by formulating potassium silicatesto behave more like a flexible siloxane polymerthan a brittle inorganic glass. The addition ofplasticizers (potassium, glycerol), gelling agents(polyethylenimine), and water-insoluble precipitates(zinc silicates, cerium silicates, organosilanes)make it possible to form thick, water resistantcoatings that exhibit excellent adhesion even tountreated aluminum surfaces.

Provided by American Chemical SocietyAPA citation: Glass paint could keep metal roofs and other structures cool even on sunny days (2015,August 16) retrieved 16 August 2015 from http://phys.org/news/2015-08-glass-metal-roofs-cool-sunny.html

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