It’s good to recycle. There’s a certain sense of accomplishment that comes from properly sorting soda bottles, plastic bags, and yogurt cups from the rest of the trash. The more plastic you put in that blue bin, the more you stay out of landfills and oceans, right?
Mal. No matter how thorough you are when cleaning and separating plastics, most end up in the trash.
Bring 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 wrappers are too fragile, prone to tangling with other materials on a conveyor belt. Polypropylene from yogurt cups and other items is also not usually recycled; recycling a polypropylene blend produces a dark, smelly plastic that few manufacturers will use.
In the United States, only two types of plastic are typically recycled: the type of plastic soda bottles, polyethylene terephthalate, or PET; and the plastic found in milk jugs and detergent packaging: high-density polyethylene or HDPE. Together, those plastics only account for about a quarter of the world’s plastic waste, researchers reported in 2017 in Science Advances. And when those plastics are recycled, they’re not good for much. Melting plastic to recycle changes its consistency, so the PET in the bottles has to be mixed with new plastic to make a durable end product. Recycling a blend of multicolored HDPE parts creates a good dark plastic just for making products like park benches and trash cans, in which properties like color don’t matter much.
The difficulty of recycling plastic into anything manufacturers want to use is one of the main reasons the world is full of so much plastic waste, says Eric Beckman, a chemical engineer at the University of Pittsburgh. In 2018 alone, the United States dumped 27 million tons of plastic and recycled just 3 million, according to the U.S. Environmental Protection Agency. Low recycling rates are not just a problem in the United States. Of the 6.3 billion tons of plastic that was discarded worldwide, only about 9 percent was recycled. Another 12 percent burned and nearly 80 percent accumulated on land or in waterways.
With the collection of plastic everywhere from the top of Mount Everest to the bottom of the Mariana Trench, it is urgent to reduce the amount of plastic that is dumped (SN: 16/1/21, p. 5). Some people propose replacing plastics with biodegradable materials, but they are usually not as strong or cheap to make as plastics (SN: 22/06/19, p. 18). Since, realistically, plastic won’t go away any time soon, chemists who understand the details of all this annoying plastic are working to make it easier to recycle and become a higher quality material that is useful for more things.
“There’s not going to be a single technology that’s the answer,” says Ed Daniels, senior project manager at the REMADE Institute in West Henrietta, New York, which funds research into new recycling techniques. Some projects are on the verge of entering the industry; others remain promising laboratory experiments. But everyone is 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.
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One of the biggest hurdles in plastic recycling is that each material has to be 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 jug of polyethylene detergent and its polypropylene lid. “If you melt those things, and I make a bottle of it and squeeze it, it would basically crack the side,” Coates says. “He’s crazy brittle. Totally useless. "
That is why the first destination of recyclable plastics is a material recovery facility, where people and machines do the sorting. Separate plastics can be washed, crushed, melted and remolded. The system works well for simple items like soda bottles and milk jugs. But not for items like deodorant packaging, where the bottle, crank and cap could be made of different types of plastic. Food packaging films containing several different layers of plastic are particularly difficult to disassemble. 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.
Saul Loeb / AFP via Getty Images
To solve this problem, Huber and colleagues devised a strategy to deal with complex mixtures of plastics. The process uses a series of liquid solvents to dissolve individual plastic components of a product. The trick is to choose the right solvents to dissolve just one type of plastic at a time, Huber says.
The team tested the technique on a packaging film containing polyethylene and PET, as well as a plastic oxygen barrier made of ethylene vinyl alcohol or EVOH, which keeps food fresh.
By removing the film in a toluene solvent the polyethylene layer was first dissolved. Dunking the remaining EVOH-PET film in a solvent called DMSO removed the EVOH. Investigators ripped off the remaining PET film and recovered the other two plastics from their separate solvents by mixing "antisolvent" chemicals. Those chemicals caused the plastic molecules that were dispersed in the liquids to clump together into solid clusters that could be fished.
This process has recovered virtually all of the plastic from the original film, researchers reported last November in Science Advances. When tested on a lot of polyethylene, PET and EVOH beads, solvent washes recovered more than 95 percent of each material, suggesting that these solvents could be used to remove the plastic components of bulky products than packaging films. . Thus, in theory, recovery facilities could use this technique to disassemble multiplastic deodorant containers and other products of various shapes and sizes.
Huber and colleagues then plan to look for solvents to dissolve more types of plastic, such as polystyrene. But it will take much more work to make this strategy efficient in selecting all the complex plastic combinations in real-world recyclables.
Making plastic mixture
There may also be chemical shortcuts that allow you to recycle multilayer films and other mixtures of plastics as they are. Additives called compatibilizers help to mix different melted plastics so that unclassified materials can be treated as one. But there is no universal compatibilizer that allows you to mix all kinds of plastic. And existing compatibilizers are not used much because they are not very powerful, and adding a lot of compatibilizer to a plastic mixture is expensive.
To increase 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 compatibilizing molecule contains two segments of polyethylene, interspersed with two segments of polypropylene. These alternating segments bind to plastic molecules of the same type in a mixture, joining polyethylene and polypropylene. It’s as if the polyethylene was Legos and the polypropylene was Duplos, and the researchers made a special building block with connectors that fit the two types of blocks.
Having two polyethylene connectors and two polypropylene for each compatibilizer molecule, instead of one, made this compatibilizer stronger than previous versions, as reported by Coates and colleagues in 2017 in Science. The first test of the new compatibilizer consisted of welding strips of polyethylene and polypropylene. Usually, both materials peel off easily. But with a layer of compatibilizer between them, the plastic strips broke, instead of the compatibilizer seal, when they separated.
In a second test, the researchers mixed the compatibilizer into a molten mixture of polyethylene and polypropylene. It only took 1 percent compatibilizer to create a new durable plastic.
“These are crazy potent additives,” Coates says. Other compatibilizers had to be added at concentrations of up to 10 percent to hold these two plastics together. The new compatibilizer is now the basis for Coates ’launch, Intermix Performance Materials, based in Ithaca, N.Y.
Good as new
Although every piece of plastic trash could be easily recycled, that still wouldn’t solve the world’s plastic problem. There are a couple of major issues about the current operation of recycling that severely limit the usability of recycled materials.
On the one hand, recycled plastics inherit all the dyes, flame retardants and other additives that have given each original piece of plastic its distinctive look. “The plastic you really recover at the end of it all is really a very complex mix,” says chemist Susannah Scott of the University of California, Santa Barbara. Few manufacturers can use plastic with a random mix of properties to make something new.
In addition, recycling breaks some of the chemical bonds in plastic molecules, affecting the strength and consistency of the material. Melting and reshaping the plastic is like reheating the pizza in the microwave; you basically get what you put in, but not so good. This limits the number of times plastic can be recycled before it has to be dumped.
The solution to both problems could lie in a new type of recycling process, called chemical recycling, that promises to make plastic pure and endless at times. Chemical recycling involves separating plastics at 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 the polymers into monomers, separate those building blocks from dyes and other contaminants, and join the monomers into a new plastic.
“Chemical recycling has really started to emerge as a force, I would say, in the last three or four years,” says Beckman of the University of Pittsburgh. But most chemical recycling techniques are too expensive or energy consuming for commercial use. “It’s not ready for peak audience hours,” he says.
Different plastics require different chemical recycling processes, and some break down more easily than others. “What’s further is PET,” Beckman says. "That polymer is easy to disassemble." 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 of these enzymes last April in Nature. Microbes typically use the enzyme, called compost cutinase from leaf branches, to decompose the waxy coating of plant leaves. But cutinase is also good for breaking down PET into its monomers: ethylene glycol and terephthalic acid.
“The enzyme is like a molecular scissors,” says Alain Marty, scientific director of Carbios. But because it has evolved to decompose plant matter, not plastic, it is not perfect. To make the enzyme better by separating the PET, “we redesigned what we call the active site of the enzyme,” says Marty. This involved exchanging some of the amino acids along that PET coupling site for others.
When the researchers tested their mutant enzyme on colored plastic flakes from PET bottles, applying 3 milligrams of the enzyme per gram of PET, about 90 percent of the plastic broke in about 10 hours. The original enzyme had reached a maximum of 50 percent. Using the terephthalic acid monomers produced in that process, the researchers made new plastic bottles as strong as the originals.
Carbios is building a plant near Lyon, France, to begin chemically recycling PET later this year.
But other plastics, such as polyethylene and polypropylene, are much harder to break through by recycling chemicals. Disassembling polyethylene molecules, for example, requires temperatures above 400 ° C. At such high heat, the chemistry is chaotic. Plastic molecules decompose at random, generating a complex mixture of compounds that can be burned as fuel but not used to make new materials.
Scott, the chemist at UC Santa Barbara, proposes to partially break down these resistant plastics in a more controlled way, under milder conditions, to make other types of useful molecules. She and her colleagues recently found a way to turn polyethylene into alkyl aromatic compounds, which can be used as biodegradable ingredients in shampoos, detergents and other products. The process involves placing the polyethylene inside a reaction chamber set at 280 ° C, with a catalyst powder containing platinum nanoparticles.
Polyethylene is a long molecule, in which hydrogen atoms are attached to a carbon backbone that can have thousands of carbon atoms. Platinum is good for breaking carbon-hydrogen bonds, Scott says. "When you do that, you generate hydrogen in the reactor and the platinum catalyst can use the hydrogen to break the carbon-carbon bonds (in the backbone of the molecule). Then it actually cuts the chain into smaller pieces."
Since this reaction takes place at a relatively mild 280 ° C, it occurs in an orderly fashion, fitting long polyethylene molecules into shorter chains that have a length of about 30 carbons. These fragments are then arranged in the six-sided annular structures characteristic of alkylaromatic compounds.
After 24 hours in the reaction chamber, “most products are liquids and most liquids are alkylomatic,” Scott says. In experiments, about 69 percent of the plastic in a low-density polyethylene bag became liquid. About 55 percent of a high-density polyethylene bottle cap was transformed. The process also produces hydrocarbon gases, which could be used to generate heat to carry out the reaction at a recycling plant, Scott says.
For now, this is just a lab demonstration and, like many new recycling strategies, is still a long way from marketing. And no upgrade to the recycling pipeline will rid the world of its growing mountains of plastic waste. “We will need a set of technologies to meet this challenge,” says Daniels, of the REMADE Institute. But each new technology, whether focused on making plastics easier to recycle or turning them into more useful materials, could help.