Chemists have spent the past century trying to make plastics that will break down in seawater. As it is, most plastics appear to take centuries to fully degrade in the ocean. That’s one reason plastics make up 80 percent of ocean trash. But that may change.
Scientists have just designed a new plastic that can break down in seawater within weeks, not decades or more.Back in the 1930s, scientists created a now-popular plastic out of corn and potato starch. It’s known as polylactide, or PLA. It’s a polymer, which is a molecule made by linking many building blocks — called monomers — into a long string. Scientists had hoped PLA would quickly break down in the environment. And in some places, like compost pits, it does. But not in seawater. Even after three years in ocean water, PLA remains largely unchanged.
Timo Rheinberger is a PhD student at the University of Twente in the Netherlands. His work on polymers has focused on boosting PLA’s breakdown. As part of that work, he became part of a team that just added some biology-inspired breaking points to PLA. They put those breaking points to places where monomers in the PLA molecules are linked.
They weakened the links that joined up to 15 percent of a PLA’s monomers. Then, they soaked their samples in artificial seawater and measured how fast these tweaked versions of PLA broke down. The expected final product of PLA’s breakdown was a small molecule called lactic acid. So they tested for that too.
Polylactide, or PLA, is a plastic made from starch, usually from corn or potatoes. Disposable cutlery, such as the fork shown in the picture, is often made of PLA.
Borrowing from genetics
Think of a PLA molecule as a necklace of pearls, where each pearl is a monomer. Normally, those pearls stay firmly linked. That’s why normal PLA resists breaking down. But with the right chemical reaction, these pearls can separate.
Other chemists tried to rip apart PLA by swapping out some PLA monomers for other, weaker types. Chemists also tried blending pure PLA with other substances — materials that do degrade in the environment. Examples included plant-based fibers such as flax or hemp.
At first, that seemed to work. “The fibers get degraded,” Rheinberger says. “And the structure gets weak and falls apart.” But the PLA itself was not degrading, he adds. Its polymers remained intact.
That’s when his group decided to alter the links between monomers. Their inspiration for how to do this came from genetics.
All living things contain DNA and RNA. Together, these molecules help make and operate living things. DNA stores the instructions. RNA uses those instructions to make things. Like PLA, both are polymers. But they are made of different monomers.
As the ultimate instruction manual, DNA is made to last. RNA is not. Its molecules “have a short life,” says Rheinberger. Each RNA molecule is made for one job. When it’s done, the RNA breaks down, freeing its monomers for re-use.
Environment: The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
Plastic: Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
Polymer: A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).
Theory: (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
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