A new kind of plastic can break down into the same building blocks from which it was made. Like a child’s plastic Lego bricks, the molecular blocks can link and detach again and again, a new study finds. And the material is more durable than earlier plastics designed to be recycled more than once.
Maybe you’ve learned to throw plastic bottles and containers into a recycling bin rather than the trash. News flash: Today, only about 10 percent of plastic ever gets recycled. That’s according to a 2017 study in Science Advances. Most of the rest lingers in landfills or ends up floating in the ocean. But plastic is so cheap and useful that hundreds of millions of tons more of it are made each year.
Even plastic that does get recycled will likely reach the landfill before long. That’s because most plastics today don’t turn back into their starting material. Instead, they break down into molecules that can’t be reused right away. Rather than pulling a Lego creation apart into its starting blocks, it might be more like smashing the structure into bits.
Transforming those bits, now, into something useable can take many chemical reactions. All that extra processing uses time, water and energy. Those steps make recycling not very efficient. And few recycled plastics are remade into the same type of object they were before. Instead, most get turned into items such as carpet, car parts and park benches — things that themselves are hard to recycle. But a plastic that could easily break down into its building blocks and be reused over and over? That could greatly help cut the global plastic-waste problem.
Michael Shaver is a chemist at the University of Edinburgh in Scotland. He studies plastics but wasn’t part of the team that made the new material. Designing a reusable plastic is a balancing act, he says. That’s because plastics are a type of polymer. Polymers are long chains of small molecules called monomers (MAH-nuh-merz). Those small starter molecules link together like beads on a string.
To be good for making polymers, the monomer building blocks should be fairly easy to connect. Monomers that need extreme temperatures or too much chemical coaxing to link up might not be practical. What’s more, the end polymers can’t be brittle or fall apart too easily, Shaver notes. They need to be stable at a high enough temperature to be used. Pouring a hot drink into a plastic cup? You don’t want it to weaken the chains and melt the cup into a sticky puddle.
Rigid and reversible
Polymer chemist Jianbo Zhu and his colleagues set out to solve this challenge. They work at Colorado State University in Fort Collins. The team previously made a polymer that could be broken down into its starting molecules. But the resulting plastics weren’t usable. They were too soft and temperature-sensitive.
This time, Zhu and his colleagues started with one of their previous creations. The monomer’s atoms were arranged in a ring shape. Then they added a second ring of atoms in a way that braced the molecule into a rigid form. That rigidity helped the monomers link together. These quickly assembled into polymer chains at room temperature. And the chains were stable even when heated up. The team reported its results April 26 in Science.
Especially important, the process was reversible. Certain mild chemicals or really high heat broke the polymers apart. And they broke down into the same initial monomers. The researchers were able to repeat this cycle several times. Link, snap, link, snap. In theory, they say, the polymer could be infinitely recyclable.
The same chemical recipe can make either of two monomer shapes. Mixing the two monomers together created an even stronger plastic, reports Zhu.
“This is probably the best system out there,” Shaver says.
Still, it’s not perfect yet. Zhu and his colleagues plan to keep tinkering with the monomer’s design. They want the resulting plastic to be a bit less brittle. Someday, they hope to have a product that can be sold.