Researchers at Japan’s RIKEN Center for Emergent Matter Science (CEMS) have developed a durable plastic that breaks down in seawater and can supply the soil with phosphorous and nitrogen, similar to a fertilizer. The scientists say the new material won’t contribute to microplastics pollution in our oceans.
Around the world, a million plastic bottles are purchased every minute. Up to five trillion plastic bags are used every year. Making, using, and disposing of conventional fossil-fuel based plastics is forecast to be the source of 19% of the global carbon budget by 2040. The oceans already hold up to 199 million tons of plastic, much of it microplastics.
According to one study, plastic pollution results in a 1-5% loss in marine ecosystem services costing about $500 billion to $2.5 trillion per year. That’s about $33,000 per metric ton of plastic pollution.
One problem researchers have had coming up with alternatives is that many are water insoluble. They break down into microplastics small enough to enter the bodies of aquatic life and find their way into the food chain, including our own bodies.
The RIKEN center solved this problem by creating polymers with structures held together by reversible interactions. The new plastics combined two ionic monomers–a common food additive called sodium hexametaphosphate and a guanidinium (a colorless solid that dissolves in polar solvents)ion-based monomer. Both monomers can be metabolized by bacteria, so once the plastic is dissolved into its components it’s biodegradable.
“While the reversible nature of the bonds in supramolecular plastics have been thought to make them weak and unstable, our new materials are just the opposite,” said says chemist Tazuko Aida who led the research. In the new material, the salt bridges structure that makes the substance strong is only reversible when exposed to electrolytes like those found in seawater. The key discovery was how to create these selectively irreversible cross links.
When scientists “desalted” the material it made it usable; but “resalting” the plastic by placing it in salt water caused the interactions to reverse and the plastic’s structure destabilized in a matter of hours. Thus, having created a strong and durable plastic that can still be dissolved under certain conditions, the researchers next tested the plastic’s quality.
Scientists said the new plastics are non-toxic and non-flammable—meaning no CO2 emissions—and can be reshaped at temperatures above 120°C like other thermoplastics. By testing different types of guanidinium sulfates, the team was able to generate plastics that had varying hardnesses and tensile strengths, all comparable or better than conventional plastics. This means that the new type of plastic can be customized 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 fertilizer.
Researchers said that the new material is as strong as conventional plastics and biodegradable. The experimental findings were published Nov 22 in Science.
“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.
