New Plastic Biodegrades in Seawater, Self-Heals Under Heat

Researchers at the University of Tokyo have developed a new, innovative plastic material that is both stronger and stretchier than current popular plastics. The new material can also self-heal scratches when warmed, is partially biodegradable and remembers complex shapes that can be restored when heated.

Their research is published in the journal ACS Materials Letters.

Developing alternative plastics

The environmental risks of plastic use have been known for years, yet despite extensive campaigns to curb plastic use, the usefulness of the material is a hard barrier to overcome. Its lightweight and relative strengths have made it the go-to material for everything from toys to infrastructure. But this does not erase the difficulties associated with plastic’s life cycle and disposal.

Developing plastic alternatives that last longer, can be reused and recycled more easily or that can be made from more environmentally-friendly source materials will be key to solving these issues and helping to realize several of the United Nations’ Sustainable Development Goals.

In pursuit of this goal, researchers at the University of Tokyo had been experimenting to create more sustainable plastics. Now, they report the synthesis of a plastic based on an epoxy resin vitrimer.

Vitrimers are a relatively new class of plastics, which are known for their superior strength at low temperatures, but which can also be reshaped multiple times when warmed to higher temperatures. However, this class of material also suffers from being extremely brittle – vitrimers cannot be stretched far before they break.

To overcome this, the researchers added a molecule called polyrotaxane into the plastic synthesis. The result is a new type of plastic they have named VPR, short for “vitrimer incorproated with polyrotaxane.”

A stronger, stretchier plastic

At low temperatures, the strong internal chemical bonds in VPR allow it to hold its shape rigidly. But at higher temperatures – around 150 degrees °C – those bonds can begin to recombine and allow the material to be reformed into different shapes.

Applying heat and a solvent to VPR will also readily break it down into its raw components. Submerging VPR in seawater for 30 days also resulted in 25% biodegredation, with the polyrotaxane breaking down into a potential food source for marine life.

“VPR is over five times as resistant to breaking as a typical epoxy resin vitrimer,” said Professor Shota Ando, a project research associate in the University of Tokyo Graduate School of Frontier Sciences.

“It also repairs itself 15 times as fast, can recover its original memorized shape twice as fast and can be chemically recycled 10 times as fast as the typical vitrimer. It even biodegrades safely in a marine environment, which is new for this material.”

Polyrotaxane had been gaining interest in scientific circles for its ability to enhance the toughness of certain materials. In this study, the addition of this compound improved the toughness of VPR to allow more complex shapes to be created and retained at lower temperatures – as demonstrated by the researchers’ use of a sheet of the material to create a complex origami crane, which could be flattened and reformed under the application of heat from a heat gun.

In addition to improving its physical properties, the addition of polyrotaxane to the vitrimer plastic also resulted in easier recycling and disposal.

“Although this resin is insoluble in various solvents at room temperature, it can be easily broken down to the raw material level when immersed in a specific solvent and heated. It also showed 25% biodegradation after exposure to seawater for 30 days,” Ando said. “By comparison, vitrimer without PR [polyrotaxane] did not undergo any apparent biodegradation. These characteristics make it an ideal material in today's society, which demands resource recycling.”

The next steps for alternative plastics

The researchers believe that their new material, and others like it, could have real-world practical applications in a wide array of sectors, including engineering, fashion, robotics and medicine.

“Just to give some examples, infrastructure materials for roads and bridges are often composed of epoxy resins mixed with compounds such as concrete and carbon. By using VPR, these would be easier to maintain as they would be stronger and healable using heat,” suggested Ando. “Unlike conventional epoxy resins, this new material is hard but stretchable, so it could also be expected to strongly bond materials of different hardness and elongation, such as is needed for vehicle manufacture. Also, as it has shape memory, shape editing and shape recovery capabilities, you might also someday be able to rearrange the silhouette of your favorite clothes at home with a hair dryer or steam iron.”

Ando and colleagues are now planning to work with various companies to determine the feasibility of their various ideas for VPR, as well as to continue their scientific research in the lab.

“I have always thought that existing plastics are very difficult to recover and dispose of because they are subdivided according to their uses,” said Ando. “It would be ideal if we could solve many of the world's problems with a single material like this.”

Reference: Ando S, Hirano M, Watakabe L, Yokoyama H, Ito K. Environmentally friendly sustainable thermoset vitrimer-containing polyrotaxane. ACS Materials Lett. 2023:3156-3160. doi: 10.1021/acsmaterialslett.3c00895

This article is a rework of a press release issued by the University of Tokyo. Material has been edited for length and content.