Closing the loop on plastic recycling?
Conventional plastics recycling is really just “downcycling” that is, reusing the material in ever-lower-quality products before ultimately sending it to the landfill. But scientists inspired by nature are developing ways to remake plastic from the ground up, with almost no waste.
The word “recycle” suggests movement in a circle. But when it comes to plastics, that vision doesn't quite match reality.
Since the 1950s, humanity has generated some 6 billion metric tons of plastic waste. Just 9 percent of that waste has been recycled, 12 percent was incinerated, and the remaining 79 percent ended up in landfills or as litter.
But even when plastic does make it to a recycling plant, there are limitations to how much recycling can happen. Current modes of recycling usually result in some form of downgraded product. Researchers are searching for solutions in plastic’s very chemistry.
“The forward-looking goal is to have a truly closed-loop relationship with plastic,” says Jeannette Garcia, a polymer chemist at IBM Research – Almaden in San Jose, Calif.
From the recycling bin, most discarded plastic is processed in mechanical recycling facilities, where plastic is cleaned, sorted, broken or melted down, and then remolded. But that process can erode some valuable properties, such as flexibility or clarity.
As a result, recycled plastics are often “downcycled,” such as when plastic water bottles are turned into carpeting. And plastic can be downcycled only so many times before ending up in a landfill.
But scientists realized that there might be another way to return products to their original uses – or even better ones.
At the chemical level, plastics are made up of long-chain molecules called polymers. The idea is to break those polymers down into individual links, or monomers. Then scientists could rebuild the same plastic products from the ground up, without chemical distortions.
Dr. Garcia and other scientists are also working on ways to “upcycle” plastics, breaking them down into new types of monomers. That way they could take something like the plastic that is used for soda bottles (polyethylene terephthalate, or PET), and turn it into the plastic used for high-performance products, like airplane parts.
Is 100 percent attainable?
To make chemical recycling a widespread reality, chemists like Garcia and her colleagues must first clear some major hurdles.
“In mechanical recycling, you’re treating everything the same,” Garcia says. “But in chemical recycling, you’re actually treating each plastic differently, because each plastic is structurally different” on the chemical level.
Researchers have to figure out a specific catalyst for each type of plastic, some of which have already been identified. For some others, inspiration has come from nature in the form of plastic-eating caterpillars, mealworms, wax worms, and fungi.
In 2016, researchers discovered a plastic-eating bacteria in a Japanese plastic recycling plant. Since then, an international team has been researching how those organisms may have evolved to take advantage of this new food source in hopes of learning how they might bioengineer an organism to break down PET into the desired monomers. And in April, they announced that they had figured it out.
The catch with these chemical and biochemical recycling innovations is that they’re expensive, consume a lot of energy, and aren’t ready for an industry-level scale, says Gregg Beckham, a member of that team and a chemical engineer at the National Renewable Energy Laboratory in Golden, Colo.
“But that’s the promise of research,” he says. “We are constantly trying to improve every step of the process that will one day make this cost-effective to do.”
These recycling techniques likely wouldn’t be deployed alone, says Susan Selke, director of the School of Packaging at Michigan State University. Ideally, she says, plastic would be put through mechanical recycling as many times as possible before it is too degraded for another round. Only then would it be chemically recycled or burned for fuel.
Still, it’s unlikely that we could get to a point where all plastic is recycled, Dr. Selke says. “To get to 100 percent, you have to collect 100 percent of everything and not have any waste in the processing. And that just doesn’t happen in the real world. So can we get way higher than we are now? Absolutely. But 100 percent? I don’t think so.”
There’s also the question of should we, Selke adds. If you look at the whole system, she says, it might not always make sense environmentally to recycle plastic. For example, if a plastic bottle would need to be shipped hundreds to thousands of miles to get to the right recycling facility, a lot of fuel would be consumed just to get it there. If there was an incinerator nearby, at least some energy could be extracted from the plastic without expending too much more.
Even if all plastic were to be recycled or upcycled, more virgin plastic would likely still need to be created, to accommodate economic growth. So some scientists are rethinking the other end of the lifecycle, focusing on developing plastics from more readily recyclable materials.
Some plant-based “bioplastics” already exist. Packing peanuts, for example, are sometimes made with starch instead of styrofoam. But almost all of the current bioplastics are made from sugars, which offers just one set of chemical building blocks.
Beckham and his colleagues are looking to lignin, a durable polymer that makes trees and grasses stand tall, and could be used to make a whole suite of other plastics and useful materials. In June the team reported in a paper published in the journal Nature Communications that they found some enzymes to break down lignin.
Not all plastics made from biological sources are biodegradable. If bioplastics are chemically identical to petroleum-based plastics, they will still take centuries to degrade. So Beckham and others are innovating.
“The great thing about using plant-based feedstocks and using a combination of biology and chemistry to convert them,” says Beckham, “is we don’t necessarily have to be bound by the set of molecules that we make from petroleum today.”