OUTER PLANET 46 PETplanet Insider Vol. 26 No. 11/25 www.petpla.net OUTER PLANET Recyclable and ocean-degradable plastics Researchers led by Takuzo Aida at the RIKEN Center for Emergent Matter Science (CEMS), part of Japan’s largest comprehensive research institution, have developed a durable plastic that breaks down in seawater. Scientists have been working to develop safe and sustainable alternatives to conventional plastics, which are typically derived from non-renewable resources and persist in the environment. Although biodegradable plastics have been introduced, challenges remain. For instance, many biodegradable materials such as PLA are waterinsoluble and therefore do not readily degrade in marine environments but break down to microplastics. In their new study, polymer chemist Takuzo Aida and his team focused on developing a water-soluble form of supramolecular plastics - polymers with structures held together by reversible interactions. The 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 have been thought to make them weak and unstable,” says Aida, “our new materials are just the opposite.” In the new material, the salt bridges structure is irreversible unless exposed to electrolytes like those found in seawater. 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 sulphate were used, sodium sulphate 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. Thus, having created a strong and durable plastic that can still be dissolved under certain conditions, the researchers next tested the plastic’s properties. The new plastics are claimed to be 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 sulphates, 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 customised as needed; 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 healthrelated 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 ten days, supplying the soil with phosphorus and nitrogen, similar to a fertiliser. The experimental findings were published in Science. www.riken.jp Artistic rendering of the new plastic: cross-linked salt bridges visible in the plastic outside the seawater give it its structure and strength. In seawater (and in soil, not depicted), resalting destroys the bridges, making it water-soluble, thus preventing microplastic formation and allowing the plastic to become biodegradable. A thin square of the glassy new plastic
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