Sunday, June 26, 2022

New clothes cool you when it’s hot, warm you when it’s cold

Imagine if the same jacket that warmed you on cold days would cool you down on hot ones too. Fabrics with “phase-change” properties can do this. And a research team from China now shows that 3-D printing technology can yield a sturdier phase-change fabric – one that hides a few more tricks up its sleeve.

This new fabric not only helps regulate temperature, but also conducts electricity. It also resists the radio waves used in Wi-Fi.

Metamorpho was a 1960s comic-book hero whose superpower was changing his state of matter—like a phase-change material.Graham Nolan/Grand Comics Database™/(CC BY-SA 4.0)

“The combination of those qualities is very interesting,” says Sergio Granados-Fosile. He did not participate in the development of the fabric. But this polymer chemist at Clark University in Worcester, Mass., is familiar with phase-change materials.

To understand the phase-change properties of fabric, consider DC Comics’ Metamorpho. This superhero could do it – poof! – Suddenly change its solid body to gas. Or, when it needs it, become liquid.

The transformation is imaginary. But the phase-change is very real. Water undergoes a phase-change when it freezes or evaporates. In each case, it is the same molecule – just in different chemical phases. The change between phases occurs by removing or adding heat. Imagine that ice cubes are melting in a glass of water. Ice absorbs the heat of the water. It heats the ice, but cools the water.

The phase-change cloth also serves to move the heat around. The Chinese team trapped the phase changing polymer inside the cloth. It alternates between a crystal and a non-crystal form. Crystal formation absorbs – or removes – body heat. This will give you coolness. But this summer is not over. Cloth stores it. And the polymer will release that heat when it turns back to non-crystal. It warms you now.

The new fabric consists of a new blend of materials. In developing this, says Yongyi Zhang, “for the first time we demonstrate a scalable and controllable 3-D printing strategy.” Zhang studies nanotechnology at the Chinese Academy of Sciences in Suzhou.

His team described its innovative fabric on January 31 ACS Applied Materials and Interfaces,

A composite image of four photographs showing a black gloved hand holding a black piece of cloth of various shapes;  folded in half, rolled into a cylinder, folded along a different axis, and turned into a paper airplane
This 3-D printed phase-change fabric was folded, crumpled and stretched. Yet even after being folded 2,000 times, it retained its unique qualities.Reprinted with permission (adapted): Jade Yang et al. 3D-printed flexible phase-change nonwoven fabrics to multi-functional fabrics. ACS Applied Materials and Interfaces. 4(5), 7283-7291, 2022. Copyright 2022 American Chemical Society

Shape is the key to polymer’s magic

The polymer of the new fabric will change its shape at different temperatures. That’s the phase-change part. In the non-crystal phase, “the polymer chains can spin around each other,” noted Emily Pantzer, who did not participate in the new work. Polymer scientists show that this polymer re-crystallizes when it cools again. Pantzer works at Texas A&M University in College Station.

A diagram of the polymer PEG and how it changes from crystalline to non-crystalline forms
The polymer PEG (short for polyethylene glycol) was used to make the new phase-change fabric. This illustration shows how the shape of the PEG changes when the phase changes from non-crystal (left) to crystalline (right).Reprinted with permission (adapted): Jade Yang et al. 3D-printed flexible phase-change nonwoven fabrics to multi-functional fabrics. ACS Applied Materials and Interfaces. 4(5), 7283-7291, 2022. Copyright 2022 American Chemical Society

The Chinese team 3-D printed their fabric using “ink” made from a mixture of the new polymer and carbon nanotubes. As their name implies, each of those tubes was only a few billionths of a meter in length. X-ray diffraction – an imaging tool that reveals the arrangement of individual molecules – shows that polymers change from their dirty non-crystal phase at hot temperatures to an ordered crystal phase when cooled. This phase change occurs between 40º and 55º Celsius (104º and 131º Fahrenheit). Changing the chemical recipe of the polymer may allow it to change phases at different temperatures.

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The new fabric performed well, Zhang says — better than his team expected. Even after being folded 2,000 times, it worked as it was designed to do.

But Clark University’s Granados-Fossil points out that the team never did a side-by-side comparison of the durability of the fabric with and without the phase-change component. He would like to see such a comparison.

Why add carbon nanotubes?

The Chinese team added nanotubes to the fabric to help conduct electricity. Zhang explains that the nanotubes also accelerate the fabric’s ability to move heat around. An added benefit, he noted: The shorter tubes add “radiation resistance.”

Here, Granados-Focil explains, “they’re talking about radio waves … Wi-Fi connections, that kind of radiation.” He says that if they are used on a smartphone, others “can’t access the information.” But this feature also confuses him. Considering the electrical conductivity of the fabric, he wonders: “In what scenario would you need to combine the two of them?”

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“But being able to absorb all that radiation” And Don’t feel hotter or colder than your surroundings,” he notes—”which may give you some camouflage property.” This characteristic can add appeal to military or defense-type applications.

Byron Jones believes that ordinary fashion designers would have little need or interest in such applications. A mechanical engineer, Jones works at Kansas State University in Manhattan. But years ago, he worked with companies that had actually tried to develop phase-change fabrics for everyday use. “My personal opinion,” he says, “is more about marketing hype than about the useful effect of phase-change materials in everyday clothing.”

Here’s the problem, Jones says. Your phase-change jacket absorbs heat when it’s hot, then releases it when it cools. You hope this puts you at ease. But it can’t absorb all your heat. Still, absorbing even 10 percent will give you moderate comfort. And if you walk around in that jacket for about 10 minutes, 10 percent of the heat you give off in that time will be about 72,000 joules. (Joule is a unit of energy.) For comparison, a regular light bulb emits about 60 joules per second.

An infrared photo of a man wearing a jacket lined with the new material
This infrared image shows a volunteer wearing a jacket with a phase-change cloth. Light colors indicate warm areas. Dark colors are cool.Reprinted with permission (adapted): Jade Yang et al. 3D-printed flexible phase-change nonwoven fabrics to multi-functional fabrics. ACS Applied Materials and Interfaces. 4(5), 7283-7291, 2022. Copyright 2022 American Chemical Society

According to team data, each gram of their fabric absorbs 65 joules. Not 65 joules per second. It is once 65 Joules during the phase-change. Afterwards, the polymer ceases to store heat. So to absorb 72,000 joules, Jones explains, your jacket must contain 1,100 grams — or 2.4 pounds — of phase-change fabric. And it’s like carrying about two and a half cans of thick soup. He concluded, “No matter how you propagate it, you can’t get around these physics.” So it can work for jackets, but hardly for t-shirts.

However, Jones sees some potential uses. This can prove useful in a situation where you only need “a few minutes” of heating or cooling capacity. “Then you go back to an environment where the phase-change material is recharged” – back to its heat-absorbing or heat-shedding state.

Granados-Fossil agrees. “This electrical conductivity business — it’s interesting,” he says. For example, in gloves, it must mean that you can operate the touchscreen. And it can let you touch something really hot or cold for 30 seconds without the need for clunky gloves.

This technology “is not going to replace all our sweaters tomorrow,” concluded Granados-Fossil. But for niche-type applications, he says, “it could be interesting.”

This was made possible by the generous support of the Lemelson Foundation, one of a series of news stories on technology and innovation.

Nation World News Desk
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