Stronger, stretchier, self-healing plastic | The National Tribune

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Video of opaque VPR plastic changing into the shape of an origami crane as it is heated on a hot plate.

Good memory. The complex shape of an origami crane that was restored using heat after being flattened.©2023, Shota Ando

An innovative plastic, stronger and more stretchable than the current standard type and that can be heat-cured, retains its shape and is partially biodegradable, has been developed by researchers at the University of Tokyo. They created it by adding the molecule polyrotaxane to an epoxy resin vitrimer, a type of plastic. The material, called VPR, can retain its shape and has strong internal chemical bonds at low temperatures. However, at temperatures above 150 degrees Celsius, these bonds recombine and the material can be reformed into different shapes. By applying heat and a solvent, VPR is broken down into its raw components. Immersion in seawater for 30 days also resulted in a 25% biodegradation, breaking down the polyrotaxane into a food source for marine life. This new material could have far-reaching applications for a more circular economy to recirculate resources and reduce waste, from engineering and manufacturing to medicine and sustainable fashion.

Despite global campaigns to reduce plastic use and waste, it is difficult to avoid the ubiquitous material. From toys and clothing, household goods and electronics to vehicles and infrastructure, these days it may seem like it’s in almost everything we use. Although useful, there are many problems associated with the life cycle and disposal of plastic. Developing alternatives that last longer, are more easily reused and recycled, or are made from environmentally friendly sources is critical to helping solve these problems and achieving some of the United Nations Sustainable Development Goals.

With this in mind, researchers at the University of Tokyo have created a more durable plastic based on an epoxy resin vitrimer. Vitrimers are a relatively new class of plastics, which are firm and strong at lower temperatures (such as thermoset plastics, used to make heat-resistant tableware), but can also be deformed several times at higher temperatures (such as thermoplastics, used for plastic bottles). However, they are typically brittle and cannot be stretched far before breaking. By adding a molecule called polyrotaxane, the team was able to create a dramatically improved version that they called VPR (vitrimer incorporated with polyrotaxane (PR)).

Two small arcs, one made with VPR and one without, in a race to flatten back to their original shape when heated. Two small arcs, one made with VPR and one without, in a race to flatten back to their original shape when heated.

Flatten the curve. Shape recovery test of vitrimer plastic with polyrotaxane, left, and without heating to 120 degrees Celsius on a hot plate.©2023, Shota Ando.

“VPR is more than five times more resistant to breakage than a typical epoxy resin vitrimer,” says project assistant Professor Shota Ando from the Graduate School of Frontier Sciences. “It also repairs itself 15 times faster, can recover its original stored form twice as fast, and can be chemically recycled 10 times faster than typical vitrimer. It even biodegrades safely in a marine environment, which is new for this material.”

Polyrotaxane is gaining interest in science and industry due to its ability to improve the toughness of various materials. In this study, VPR’s improved toughness meant that more complex shapes could be created and maintained even at low temperatures (such as the origami crane in the video included with this release). Disposal or recycling was also easier than for vitrimers without polyrotaxane, Ando explains: “Although this resin is insoluble in various solvents at room temperature, it can easily be degraded to the level of the raw material when immersed in a specific solvent and heated. It also showed a biodegradation of 25% after exposure to seawater for 30 days. In comparison, vitrimer without PR did not undergo any apparent biodegradation. These characteristics make it an ideal material in today’s society, which requires resource recycling.”

A grid of black scratches on a gray-white surface fades into a smooth surface.A grid of black scratches on a gray-white surface fades into a smooth surface.

Just a scratch. A VPR surface scratched with a scalpel to a depth of 0.1 millimeters was able to heal completely within 60 seconds after being heated to 150 degrees Celsius with a heat gun.©2023, Shota Ando

From technology to fashion, from robotics to medicine, the team envisions both practical and playful applications for VPR. “To name just a few examples, infrastructure materials for roads and bridges often consist of epoxy resins mixed with compounds such as concrete and carbon. Using VPR would make these easier to maintain as they would be stronger and curable with the help of heat,” Ando said. “Unlike conventional epoxy resins, this new material is hard but stretchable, so you might expect it to also strongly bond materials of varying hardness and stretch, as required for vehicle production. And because it has shape memory, shape editing and shape recovery, you might also one day be able to rearrange the silhouette of your favorite clothes at home with a hair dryer or a steam iron.”

The team’s next step will be to work with companies to determine the feasibility of the various ideas for VPR, and to continue the research in the laboratory. “I have always thought that existing plastics are very difficult to reclaim and dispose of because they are divided according to their use,” says Ando. “It would be ideal if we could solve many of the world’s problems with a single material like this.”

/Public publication. Watch the full thing here.

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