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Most people enjoy an icy drink on a hot day without thinking about the physics happening in the glass. But the phase-change reaction — in this case, as the ice absorbs heat from the drink and melts — is a powerful one. A coating NASA created to take advantage of that same reaction could turn up on passenger planes or be the key to an ice-free cooler.

With phase changes, according to Marshall Space Flight Center materials scientist Raj Kaul, the temperature holds steady at exactly the point of transition. “If you freeze a thermometer in a block of ice and start heating it, the temperature will stay at 32 °F, even in the already melted part. Only once the ice is completely melted will the water start raising its temperature.”

For water, the melting/freezing point is 32 °F but NASA developed a suite of materials with different melting points. Initially, the plan was to use these materials inside spacesuits, which can get hot as body heat accumulates. In the early 2000s, however, Kaul began researching a way to use phase-change materials on the outside of the space shuttle. The solid rocket boosters were protected by a material called MCC-1 that was “basically a mixture of cork and epoxy,” Kaul said. “The cork will burn and that burning process also removes the heat. Plus, it ablates — the layer will just come off, so new material will be exposed and then go through that again.”

Because the coating was designed to burn off, however, there remained a danger that the process would cause damage. Kaul began working on a protective coating that would not ablate and still maintain a safe temperature underneath. He turned to NASA's work with phase-change materials and used them in this new context. He incorporated the material into a coating that could be applied like paint, which required determining the right ratio of phase-change material to coating to get the best results.

After licensing the patent for the phase-change coating from Marshall, entrepreneur Chris Bilec founded PrimeBilec and is working on a number of products including an iceless cooler.

Kaul developed a material that “is encapsulated in a plastic. When it sees heat, it absorbs that heat to go through the solid-to-liquid phase. As a result, the heat cannot go through to the substrate — it all goes through this phase-transformation process.” So, the solid rocket booster wouldn't melt from the extreme heat of launch. And thanks to the epoxy binding the coating to itself and the rocket, the phase-change material wouldn't drip off when it melted. After testing by NASA, the material was approved for use on spacecraft but by that time, the shuttle program had been discontinued.

NASA advertised the material's patent for licensing and it was snapped up by Chris Bilec, an entrepreneur and Army veteran. Bilec started Austin, TX-based PrimeBilec and Marshall's Tech Transfer Office connected him with Kaul to answer questions about the material. Kaul suggested the coating had potential for exterior house paint or as a roof coating, “so that in the daytime, it will absorb heat and in the nighttime, it will release that heat and go through the cycle.”

Bilec used the material to coat the back end of an airplane to prevent heat damage from the jet engines. “For aviation, we're setting the phase-change material at 24° C,” or approximately room temperature — the temperature the airline maintains in the cabin. By using that temperature, the coating will absorb heat that would raise the internal temperature.

He has seen interest from a company that makes heat traps for Veterans Affairs hospitals. “They monitor the heat coming off the pipes in the hospital,” Bilec said. “When these pipes get too hot, they overheat and burn their sensors. Our products will extend the lifespan of their heat traps.”

Another idea is to create an “iceless cooler” along with smaller containers for lunches. For the containers, the phase-chase material is embedded in food-grade polyester to coat the container. With the coating, the food — or beer — inside the container will stay colder for about two to four hours longer, depending on the temperature outside.

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Tech Briefs Magazine

This article first appeared in the November, 2019 issue of Tech Briefs Magazine.

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