Earth

Chemists are reimagining recycling to keep plastics out of landfills

Too much of today’s plastic is impossible to recycle

It feels good to recycle. There’s a certain sense of accomplishment that comes from dutifully sorting soda bottles, plastic bags and yogurt cups from the rest of the garbage. The more plastic you put in that blue bin, the more you’re keeping out of landfills and the oceans, right?

Wrong. No matter how meticulous you are in cleaning and separating your plastics, most end up in the trash heap anyway.

Take flexible food packages. Those films contain several layers of different plastics. Because each plastic has to be recycled separately, those films are not recyclable. Grocery bags and shrink wrap are too flimsy, prone to getting tangled up with other materials on a conveyor belt. The polypropylene in yogurt cups and other items doesn’t usually get recycled either; recycling a hodgepodge of polypropylene produces a dark, smelly plastic that few manufacturers will use.

Only two kinds of plastic are commonly recycled in the United States: the kind in plastic soda bottles, polyethylene terephthalate, or PET; and the plastic found in milk jugs and detergent containers — high-density polyethylene, or HDPE. Together, those plastics make up only about a quarter of the world’s plastic trash, researchers reported in 2017 in Science Advances. And when those plastics are recycled, they aren’t good for much. Melting plastic down to recycle changes its consistency, so PET from bottles has to be mixed with brand-new plastic to make a sturdy final product. Recycling a mix of multicolored HDPE pieces creates a dark plastic good only for making products like park benches and waste bins, in which properties like color don’t matter much.

The difficulties of recycling plastic into anything manufacturers want to use is a big reason why the world is littered with so much plastic waste, says Eric Beckman, a chemical engineer at the University of Pittsburgh. In 2018 alone, the United States landfilled 27 million tons of plastic and recycled a mere 3 million, according to the U.S. Environmental Protection Agency. Low recycling rates aren’t just a problem in the United States. Of the 6.3 billion tons of plastic that have been discarded around the world, only about 9 percent has gotten recycled. Another 12 percent has been burned, and almost 80 percent has piled up on land or in waterways.
Good news/bad news

The amount of plastic recycled in the United States has increased over the last few decades — but those levels still pale in comparison with the amount of plastic that goes into landfills.

With plastic collecting everywhere from the top of Mount Everest to the bottom of the Mariana Trench, there’s an urgent need to reduce the amount of plastic that gets thrown away (SN: 1/16/21, p. 5). Some people propose replacing plastics with biodegradable materials, but those replacements are generally not as strong or cheap to make as plastics (SN: 6/22/19, p. 18). Since, realistically, plastic is not going away any time soon, chemists who understand the ins and outs of all this pesky plastic are working to make it easier to recycle and turn into higher-quality material that’s useful for more things.

“There’s not going to be a single technology that’s going to be the answer,” says Ed Daniels, senior project manager at the REMADE Institute in West Henrietta, N.Y., which funds research into new recycling techniques. Some projects are on the brink of breaking into industry; others are still just promising lab experiments. But all are focused on designing a future where any plastic that ends up in the recycling bin can have a second and third life in a new product.

One of the biggest bottlenecks in plastic recycling is that every material has to get processed separately. “Most plastics are like oil and water,” says chemist Geoffrey Coates of Cornell University. They just don’t mix. Take, for example, a polyethylene detergent jug and its polypropylene cap. “If you melt those down, and I make a bottle out of that, and I squeeze it, it would basically crack down the side,” Coates says. “It’s crazy brittle. Totally worthless.”

That’s why the first destination for plastic recyclables is a material recovery facility, where people and machines do the sorting. Separated plastics can then be washed, shredded, melted and remolded. The system works well for simple items like soda bottles and milk jugs. But not for items like deodorant containers — where the bottle, crank and cap could all be made of different kinds of plastic. Food packaging films that contain several layers of different plastic are particularly tricky to take apart. Every year, 100 million tons of these multilayer films are produced worldwide. When thrown away, those plastics go to landfills, says chemical engineer George Huber of the University of Wisconsin–Madison.
workers sorting at a waste management facility

To tackle that problem, Huber and colleagues devised a strategy for dealing with complex mixtures of plastics. The process uses a series of liquid solvents to dissolve individual plastic components off a product. The trick is choosing the right solvents to dissolve only one kind of plastic at a time, Huber says.

The team tested the technique on a packaging film that contained polyethylene and PET, as well as a plastic oxygen barrier made of ethylene vinyl alcohol, or EVOH, that keeps food fresh.

Stirring the film into a toluene solvent first dissolved the polyethylene layer. Dunking the remaining EVOH-PET film in a solvent called DMSO stripped off the EVOH. The researchers then plucked out the remaining PET film and recovered the other two plastics from their separate solvents by mixing in “antisolvent” chemicals. Those chemicals caused the plastic molecules that were dispersed in the liquids to bunch together into solid clumps that could be fished out.

This process recovered practically all of the plastic from the original film, the researchers reported last November in Science Advances. When tested on a jumble of polyethylene, PET and EVOH beads, the solvent washes recovered more than 95 percent of each material — hinting that these solvents could be used to strip plastic components off bulkier items than packaging films. So in theory, recovery facilities could use this technique to disassemble multiplastic deodorant containers and other products of various shapes and sizes.

There may also be chemical shortcuts that allow multilayer films and other mixtures of plastics to be recycled as they are. Additives called compatibilizers help different melted-down plastics blend, so that unsorted materials can be treated as one. But there is no universal compatibilizer that allows every kind of plastic to be mixed together. And existing compatibilizers are not widely used because they are not very potent — and adding a lot of compatibilizer to a plastic blend gets expensive.

To boost viability, Coates and colleagues created a highly potent compatibilizer for polyethylene and polypropylene. Together, those two plastics make up more than half of the world’s plastic. The new compatibilizer molecule contains two segments of polyethylene, interspersed with two segments of polypropylene. Those alternating segments latch onto plastic molecules of the same kind in a mixture, bringing polyethylene and polypropylene together. It’s as if polyethylene were made of Legos, and polypropylene were made of Duplos, and the researchers made a special building block with connectors that fit both types of blocks.

Having two polyethylene and two polypropylene connectors for each compatibilizer molecule, rather than one, made this compatibilizer stronger than previous versions, Coates and colleagues reported in 2017 in Science. The first test of the new compatibilizer involved welding together strips of polyethylene and polypropylene. Ordinarily, the two materials easily peel apart. But with a layer of compatibilizer between them, the plastic strips broke, rather than the compatibilizer seal, when pulled apart.

In a second test, the researchers mixed the compatibilizer into a melted blend of polyethylene and polypropylene. It took only 1 percent compatibilizer to create a tough new plastic.

“These are crazy potent additives,” Coates says. Other compatibilizers had to be added at concentrations up to 10 percent to hold these two plastics together. The new compatibilizer is now the basis for Coates’ start-up, Intermix Performance Materials, based in Ithaca, N.Y.

Even if every piece of plastic trash could easily be recycled, that still wouldn’t solve the world’s plastic problem. There are a couple major issues with how recycling currently works that severely limit the usability of recycled materials.

For one thing, recycled plastics inherit all the dyes, flame retardants and other additives that gave each original plastic piece its distinctive look and feel. “The plastic that you actually recover at the end of all this is really a very complex mixture,” says chemist Susannah Scott of the University of California, Santa Barbara. Few manufacturers can use plastic with a random mishmash of properties to make something new.

Plus, recycling breaks some of the chemical bonds in plastic molecules, affecting the strength and consistency of the material. Melting down and remolding plastic is sort of like reheating pizza in the microwave — you get out basically what you put in, just not as good. That limits the number of times plastic can be recycled before it has to be landfilled.

The solution to both problems could lie in a new kind of recycling process, called chemical recycling, which promises to make pure new plastic an infinite number of times. Chemical recycling involves taking plastics apart on the molecular level.

The molecules that make up plastics are called polymers, which are made of smaller monomers. Using heat and chemicals, it is possible to disassemble polymers into monomers, separate those building blocks from dyes and other contaminants, and piece the monomers back together into good-as-new plastic.

“Chemical recycling has really started to emerge as a force, I would say, within the last three or four years,” says University of Pittsburgh’s Beckman. But most chemical recycling techniques are too expensive or energy intensive for commercial use. “It’s not ready for prime time,” he says.

Different plastics require different chemical recycling processes, and some break down more easily than others. “The one that’s farthest along is PET,” Beckman says. “That polymer happens to be easy to take apart.” Several companies are developing methods to chemically recycle PET, including the French company Carbios.

Carbios is testing enzymes produced by microorganisms to break down PET. Researchers at the company described their work on one such enzyme last April in Nature. Microbes normally use the enzyme, called leaf-branch compost cutinase, to decompose the waxy coating on plant leaves. But the cutinase is also good at breaking PET down into its monomers: ethylene glycol and terephthalic acid.