Researchers led by Takuzo Aida at the RIKEN Center for Emergent Matter Science (CEMS) in Japan have one-upped themselves in their quest to solve our microplastic problem. In a recent study published in the Journal of the American Chemical Society they report a new type of plastic made from plant cellulose, the world's most abundant organic compound. The new plastic is strong, flexible, and capable of rapid decomposition in natural environments, setting it apart from other plastics marketed as biodegradable.
Microplastics are a global contaminant found in nearly every ecosystem, from the soil and the ocean to the animals and plants that live there. They have even been found in human tissue and the bloodstream where they likely have adverse effects. While biodegradable plastics and even cellulose-derived plastics are not new, most plastics labeled "biodegradable" do not degrade in marine environments or they take a very long time to degrade, leaving microplastics behind in the meantime.
Last year, Aida and his team developed a plastic that could quickly degrade in salt water within several hours, without leaving any microplastics behind. That plastic was a supramolecular plastic made from two polymers held together by reversible interactions. In the presence of salt water, the bonds holding the two polymers together came apart and the plastic decomposed. But this plastic wasn't as practical as it could be for real-world manufacture.
The new plant-based plastic is similar, except that one of the two polymers is a commercially available, FDA approved, biodegradable wood-pulp derivative called carboxymethyl cellulose. Finding a compatible second polymer took some trial and error, but eventually the team found a safe crosslinking agent made from positively charged polyethylene-imine guanidinium ions. When the cellulose and guanidinium ions were mixed in room temperature water, the negatively and positively charged molecules attracted each other like magnets and formed the critical cross-linked network that makes this kind of plastic strong. At the same time, the salt bridges holding the network together broke as expected in the presence of salt water. To avoid unintentional decomposition, the plastic can be protected with a thin coating on the surface.
So far so good. But even though the new plastic decomposed quickly, it initially suffered from being too brittle because of the cellulose. The resulting plastic was colorless, transparent, and extremely hard, but had a fragile glass-like quality. What the team needed was a good plasticizer, some small molecule they could add to the mix to make the plastic more flexible, yet remain hard. After much experimenting, they discovered that the organic salt choline chloride worked wonders. By adding varying amounts of this FDA-approved food additive to the plastic, the researchers were able to fine-tune exactly how flexible they wanted the plastic to be. Depending on the amount of choline chloride, the plastic can range from being hard and glass-like to being so elastic that it can be stretched up to 130% of its original length. It can even be made into a strong yet thin film with a thickness of only 0.07 mm. A video of a bag made from the new plant-based biodegradable plastic can be seen decomposing here: https://youtu.be/glBYYhk1STQ .
The improvements on the original design are not trivial. "While our initial study focused mostly on the conceptual," explains Aida, "this study shows that our work is now at a more practical stage." The new carboxymethyl cellulose supramolecular plastic, dubbed CMCSP, is as strong as conventional petroleum-based plastics and its mechanical properties can be adjusted as needed, without spoiling the intrinsic transparency, processability, seawater dissociability, or close-loop recyclability. By using common and inexpensive FDA-approved biodegradable ingredients, Aida and his team have ensured that their plastic will be able to move quickly to real-world, practical applications.
"Nature produces about one trillion tons of cellulose every year," says Aida. "From this abundant natural substance, we have created a flexible yet tough plastic material that safely decomposes in the ocean. This technology will help protect the Earth from plastic pollution."
