Newly developed plastic is recyclable and ocean-degradable

Researchers from Japan’s RIKEN Center for Emergent Matter Science (CEMS) have developed a new plastic material that is as strong as conventional plastics but also breaks down in sea water, meaning it should help reduce harmful microplastic pollution that accumulates in our oceans and soil and eventually enters the food chain. The team’s experimental findings have been published in the journal Science.

Scientists have been trying for some time to develop safe and sustainable materials that can replace traditional plastics, which are non-sustainable and harm the environment. But while some recyclable and biodegradable plastics exist, one big problem remains: current biodegradable plastics like PLA often find their way into the ocean, where they cannot be degraded as they are water insoluble. As a result, microplastics — plastic bits smaller than 5 mm — are harming aquatic life and finding their way into the food chain, including our own bodies.

The RIKEN team focused on solving this problem with supramolecular plastics — polymers with structures held together by reversible interactions. These new plastics were made by combining two ionic monomers that form cross-linked salt bridges, which provide strength and flexibility. In the initial tests, one of the monomers was a common food additive called sodium hexametaphosphate and the other was any of several guanidinium ion-based monomers. Both monomers can be metabolised by bacteria, ensuring biodegradability once the plastic is dissolved into its components.

“While the reversible nature of the bonds in supramolecular plastics had been thought to make them weak and unstable, our new materials are just the opposite,” said research leader Takuzo Aida. This is because the salt bridges structure is irreversible unless exposed to electrolytes like those found in sea water. The key discovery was how to create these selectively irreversible cross links.

As with oil with water, after mixing the two monomers together in water, the researchers observed two separated liquids. One was thick and viscous and contained the important structural cross-linked salt bridges, while the other was watery and contained salt ions. For example, when sodium hexametaphosphate and alkyl diguanidinium sulfate were used, sodium sulfate salt was expelled into the watery layer. The final plastic, alkyl SP2, was made by drying what remained in the thick viscous liquid layer.

The ‘desalting’ turned out to be the critical step; without it, the resulting dried material was a brittle crystal, unfit for use. Resalting the plastic by placing it in salt water caused the interactions to reverse and the plastic’s structure destabilised in a matter of hours.

A thin square of the glassy new plastic.

The new plastics are non-toxic and non-flammable — meaning they generate no CO₂ emissions — and can be reshaped at temperatures above 120°C, just like other thermoplastics. By testing different types of guanidinium sulfates, the team was able to generate plastics with varying levels of hardness and tensile strength, all comparable or better than conventional plastics. This means that the new type of plastic can be customised for need; hard scratch-resistant plastics, rubber silicone-like plastics, strong weight-bearing plastics, or low tensile flexible plastics are all possible. The researchers also created ocean-degradable plastics using polysaccharides that form cross-linked salt bridges with guanidinium monomers. Plastics like these can be used in 3D printing as well as medical or health-related applications.

Lastly, the researchers investigated the new plastic’s recyclability and biodegradability. After dissolving the initial new plastic in salt water, they were able to recover 91% of the hexametaphosphate and 82% of the guanidinium as powders, indicating that recycling is easy and efficient. In soil, sheets of the new plastic degraded completely over the course of 10 days, supplying the soil with phosphorous and nitrogen similar to a fertiliser.

“With this new material, we have created a new family of plastics that are strong, stable, recyclable, can serve multiple functions and, importantly, do not generate microplastics,” Aida said.

Top image shows an artistic rendering of the new plastic. Cross-linked salt bridges visible in the plastic outside the sea water give it its structure and strength. In sea water (and in soil), resalting destroys the bridges, making it water soluble and thus preventing microplastic formation.