Glass paint could keep metal roofs and other structures cool even on sunny days

August 16, 2015, American Chemical Society

Sunlight can be brutal. It wears down even the strongest structures, including rooftops and naval ships, and it heats up metal slides and bleachers until they're too hot to use. To fend off damage and heat from the sun's harsh rays, scientists have developed a new, environmentally friendly paint out of glass that bounces sunlight off metal surfaces—keeping them cool and durable.

The researchers present their work today at the 250th National Meeting & Exposition of the American Chemical Society (ACS).

"Most paints you use on your car or house are based on polymers, which degrade in the ultraviolet light rays of the sun," says Jason J. Benkoski, Ph.D. "So over time you'll have chalking and yellowing. Polymers also tend to give off , which can harm the environment. That's why I wanted to move away from traditional polymer coatings to inorganic glass ones."

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

To address that aspect in a new coating, Benkoski, who is at the Johns Hopkins University Applied Physics Lab, started with silica, one of the most abundant materials in the earth's crust. He modified one version of it, potassium silicate, that normally dissolves in water. His tweaks transformed the compound so that when it's sprayed onto a surface and 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 its counterparts that contain organic compounds. His paint is also designed to expand and contract with to prevent cracking.

Mixing pigments with the silicate gives the coating an additional property: the ability to reflect all and passively radiate . Since it doesn't absorb sunlight, any surface coated with the paint will remain at air temperature, or even slightly cooler. That's key to protecting structures from the .

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

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

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

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

Explore further: Self-healing paint could halt rust on military vehicles

More information: Passive cooling with UV-resistant siloxane coatings in direct sunlight, the 250th National Meeting & Exposition of the American Chemical Society.

Solar exposure is a leading cause of material degradation in outdoor use. Polymers and other organic materials photo-oxidize due to ultraviolet (UV) exposure. Even in metals, solar heating can cause unwanted property changes through precipitation and Ostwald ripening. In more complex systems, cyclic temperature changes cause fatigue failure wherever thermal expansion mismatch occurs. Most protective coatings designed to prevent these effects inevitably succumb to the same phenomena because of their polymeric matrix. In contrast, siloxane coatings have the potential provide indefinite solar protection because they do not undergo photo-oxidation. This study therefore demonstrates UV-reflective siloxane coatings with low solar absorptance and high thermal emissivity that prevent any increase in temperature above ambient conditions in direct sunlight. Mathematical modeling suggests that even sub-ambient cooling is possible for ZnO-filled potassium silicate. Preventing widespread adoption of potassium silicates until now has been their tendency to crack at large thicknesses, dissolve in water, and delaminate from untreated surfaces. This investigation has successfully addressed these limitations by formulating potassium silicates to behave more like a flexible siloxane polymer than a brittle inorganic glass. The addition of plasticizers (potassium, glycerol), gelling agents (polyethylenimine), and water-insoluble precipitates (zinc silicates, cerium silicates, organosilanes) make it possible to form thick, water resistant coatings that exhibit excellent adhesion even to untreated aluminum surfaces.

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5 / 5 (3) Aug 16, 2015
Reflects ALL sunlight. And this is physics website. You need to get some physics trained journalists fast
not rated yet Aug 17, 2015
This reminds me of an recent announce from Stanford about a cooling mirror (http://news.stanf...4.html).

If you can cover a building with a reflective coating and also radiate the internal heat to space, how cold exactly can it get??
Lex Talonis
not rated yet Aug 17, 2015
I did the research on lime wash paint - and it actually reflects and emits MORE energy than it absorbs.

The slowly "cured" (kept wet during carbonation) surface, is both WHITE, there is no polymer bonding to keep the emissions within the paint structure, and the calcium carbonate crystals refract the light energy back out.

The total emissivity is around 105%.

In the hot summer sun, the surface temperature of the spray painted brick (lime wash) is barely above ambient temperature - despite 40*C+ temperatures for days on end, while the uncoated brickwork and cement pathways, are hot enough to fry eggs on / burn bare skin.

Lex Talonis
not rated yet Aug 17, 2015
So for HOT climates, and porous surfaces, where there is no abrasion or people washing it with garden hoses etc., lime wash is an excellent heat emitting coating.

But it does not stick very well to very smooth surfaces, metals, or where people / animals / plants rub against it.

But for the hot sun lit sides of houses and other buildings - absolutely fabulous.

Application method - preferable cool wet / foggy day, with minimal drying winds - wet down brick / stone / wood / rough cement surfaces.

Make a thin watery milk like solution of builders lime and water, filter the crap from it, so it can be sprayed, and apply thin coats over the walls, until an opaque single coating is in place.

Keep surface "slightly wet" by spraying surface over with water - to allow the calcium carbonate to form bicarbonate crystals - over a day or two.

Bingo! Ultra white bright heat emitting paint.

1 / 5 (1) Aug 18, 2015
"Sta-Kool 770 Ultra White Elastomeric Roof Coating is a tough, highly flexible, rubberized roof coating. This high-performance, 7-year warranted elastomeric roof coating resists cracking and peeling. The coating also provides a highly protective barrier which reflects the sun's heat and destructive UV rays."
not rated yet Aug 18, 2015
OK, but what to do during winter, when you actually need that heat to enter the house?
You start scraping it down and repeat the spraying next summer?
(I speak about temperate climate regions.)
not rated yet Aug 19, 2015
Why will it take 2 years for the lab to start field testing the material? That's a very long time...

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