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Pollution

Chemistry may have solutions to our plastic trash problem

Chemists explore ways to convert plastics into valuable products and to develop intrinsically recyclable polymers

by Sam Lemonick
June 15, 2018 | APPEARED IN VOLUME 96, ISSUE 25

 

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Credit: Jesse Lenz

The world’s mounting plastic trash crisis is hard to solve because it has many dimensions: social, technical, and economic. But because chemistry brought the problem into the world, it doesn’t seem unreasonable to look to chemistry for a solution.

Such a solution will require that today’s chemists figure out how to undo the hard work of their predecessors. The polymers we use as plastics were designed to be durable and stable. They’re difficult to break down on purpose.

Now, as the need for finding better ways to handle plastic waste grows, some researchers are finding ways to take plastics apart. Several companies have started up in the past decade to capitalize on these processes. Some methods return plastics to their monomeric form in the hope that the reclaimed building blocks might replace fossil fuels as the feedstock for new materials. Other processes yield fuels or additives for other products.

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Credit: Adapted from Periodic Graphics/Andy Brunning for C&EN
U.S. recycling status
While existing methods can recycle some polymers, like poly(ethylene terephthalate), at relatively high rates, new technology is needed for other types. Note: These data, from EPA, are for the U.S. in 2012 and were published in 2014.

Developing new recycling methods is especially important as the kinds of polymers we use have started to change. A growing number of products and applications, such as cars and wind turbines, are relying on the strength of composite materials made with fiberglass and carbon fiber. These materials use polymer resins that cannot simply be melted and re-formed like other plastics, and chemists are just starting to develop methods for recycling them in research labs.

But other researchers are thinking about recycling as they develop new materials that might not be as difficult to deal with as today’s plastics. These projects could yield resins and plastics that are intrinsically easy to recycle.

With such developments, it’s conceivable that, one day, chemists might deliver a plastic bottle that can be reincarnated infinitely.

Trash to treasure

All plastics are not equal when it comes to recycling. Polyethylene polymers are the easy-to-handle favorites. Poly(ethylene terephthalate) (PET) and high-density polyethylene are the most commonly recycled plastics in the U.S. Products made with these plastics are stamped with a recycling symbol encircling the numbers “1” and “2,” respectively. When emblazoned with these numbers, called resin identification codes (RICs), plastics can be shredded, cleaned, and remade into new bottles or lower-quality materials like carpet fiber.

On the other end of the scale, many curbside recycling programs don’t even accept polystyrene (RIC 6), used in food packaging, packing peanuts, and disposable cutlery. Like polyethylene plastics, polystyrene waste can be processed and reused to make new products. It just doesn’t happen often.

“Plastic foam is troublesome for most material recovery facilities in the country,” says Chris Faulkner, vice president of technology and project management at Agilyx, which has developed a chemical process to recycle polystyrene.

A big challenge in recycling polystyrene is contamination. Actually, it’s a problem for all plastics recycling; if oily molecules, water, and other contaminants make it into recycled materials, the substances can disrupt and weaken the polymers. Polystyrene clamshell containers and coffee cups are especially likely to be dirty, adding to the cost of processing them for recycling.

Agilyx uses pyrolysis to break down polystyrene at its Tigard, Ore., facility, heating it in an oxygen-deprived environment so the plastics don’t burn. The company can revert polystyrene back to monomeric styrene, toluene, and ethylbenzene, which is a precursor to styrene. Faulkner wouldn’t share the exact details of Agilyx’s process but says it can select for different products by tuning the time, temperature, and pressure of the pyrolysis process.

Pyrolysis can address the polystyrene contaminant problem. Faulkner says that because the desired products vaporize as the polymer breaks down, the valuable compounds can be separated from some of the dyes, processing aids, pesticides, and food contaminants during the reaction. Later separation steps are still needed, though, he says.

Agilyx started out with a more generalized process for turning plastic waste into valuable products. In 2004, then called Plas2Fuel, the company built reactors to convert mixed plastic waste into a mixed hydrocarbon product called Agilyx Synthetic Crude Oil. This mixture can be refined like natural crude oil. Agilyx had built and sold several of these systems to waste management companies across the country before the price of oil dropped in 2015, making the crude alternative less competitive with what comes out of the ground. That’s when the company started getting its polystyrene recycling plant on-line. But Faulkner says with oil prices climbing again and China limiting how much plastic waste it will accept from the U.S. and other countries, Agilyx is seeing renewed interest in its oil production process.

Other companies are hoping to turn waste plastics into valuable chemical products or feedstocks. A lot of the attention has focused on polyethylene.

GreenMantra Technologies in Brantford, Ontario, employs a thermocatalytic process to turn plastic into waxes for asphalt roads and roofs, as well as additives for plastics, adhesives, and coatings. Domenic Di Mondo, vice president of technology and business development, says those products haven’t typically come from recycled materials. “We’re driving a circular economy and in most cases creating products with even higher value than the virgin starting material,” he says.

When the company started in 2010, its first target was polyethylene. GreenMantra uses a heterogeneous thermocatalytic process to turn the plastic into different specialty chemical products. Di Mondo says the process requires lower temperatures than pyrolysis and gives the company a high degree of control over what gets produced. Di Mondo wouldn’t specify what catalysts GreenMantra uses, but patent documents related to the polyethylene process describe it as using an iron- and copper-based catalyst. Di Mondo says that because the method uses a solid catalyst and no solvents and has a small physical footprint, the process can be easily scaled up.

The company now processes polyethylene and polypropylene at its Brantford facility, and it plans to open a polystyrene pilot plant in 2019. But it hasn’t targeted all types of plastic. Chlorine in poly(vinyl chloride) poses too many health and environmental risks for most recyclers, even the ones using mechanical recycling, and Di Mondo says GreenMantra has not focused on PET because existing recycling processes are sufficient to keep large amounts of it out of the landfill.

The trouble with thermosets

Another class of polymers called thermosets presents a unique set of recycling challenges. Unlike thermoplastics, such as polyethylene, polystyrene, and polypropylene that can be melted and molded into new forms, thermoset polymers harden irreversibly thanks to covalent cross-linkers that bridge polymer strands. These polymers are increasingly used as resins and combined with carbon fiber or other materials to achieve tensile strength and elasticity that the polymers alone don’t have.

The makers of cars, planes, and wind turbine blades rely on these composite materials because of their high strength-to-weight ratio. Unlike plastic bottles, these products don’t get thrown away every day. But when their lifetimes do end, a lot of thermoset-polymer-based composites get sent to landfills.

“We really have no way of dealing with those polymers,” says Megan Robertson, a chemical engineer at the University of Houston.

Jinwen Zhang, a polymer scientist at Washington State University, is one person trying to change that. He’s developed mild catalytic processes to break down ester linkages in amine-cured epoxy resins, a type of thermoset that is common in composite materials. Zhang showed he could dissolve resin in carbon-fiber scraps from a major aircraft maker using a ZnCl2-ethanol catalyst at 250 ºC. His group recovered carbon fibers and un-cross-linked oligomers from the resin. A scanning electron microscope showed that the fibers were still smooth, indicating that they were mostly undamaged by the process (Green Chem. 2017, DOI: 10.1039/c7gc01737e).

Although Zhang says he has licensed some of his technology to a Chinese company and has had interest from others, these methods are a long way from commercial applications. As a result, Zhang and others are working on a possibly easier path to recycling thermosets: designing new polymers with recycling in mind. Such research could save the chemists a lot of trouble trying to break down the materials when they’re thrown away.

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Polymers like the vitrimer shown could be used as recyclable resins in composite materials. When heated at 12 atm with ethanol and zinc catalyst left over from the polymerization process, this particular vitrimer breaks down at its ester linkages, making it recycling ready.

To create these new recycling-ready materials, Zhang has zeroed in on the source of thermoset stability—the cross-linkers. The materials he’s making are known as vitrimers, a subset of thermosets with cross-linking bonds that form and break depending on temperature. In a way, they act like a glass, malleable at high temperatures and hardening when they cool. If manufacturers used such vitrimers as resins in composite materials, the resulting products could be recycled through mechanical processes similar to those used for thermoplastics.

Zhang developed a vitrimer based on eugenol, a renewable phenylpropene found in nutmeg, cinnamon, and other plants (Macromolecules 2017, DOI: 10.1021/acs.macromol.7b01889). When heated with ethanol and some zinc catalyst left over from the polymerization process, the vitrimer breaks down at its ester linkages. Zhang says he’s also experimented with lignin, which can be extracted from plants, as a vitrimer.

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University of Houston’s Robertson has also developed recyclable thermosets by finding renewable chemicals to replace some or all of bisphenol A (BPA), which is used as an epoxy precursor in many resins. The BPA alternatives she’s identified include epoxidized soybean oil, salicylic acid, and other plant derivatives. Robertson says key features she looks for in these biobased molecules are convenient functional groups for conversion to epoxides and aromatic rings to provide strength, mimicking the chemical structure of BPA (ACS Sustainable Chem. Eng. 2016, DOI: 10.1021/acssuschemeng.6b01343). Because many of these molecules contain esters, Robertson says, chemical recycling methods under development for polyesters could be applied to her thermoset polymers.

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Boroxine rings can break and easily re-form throughout this thermoset, enabling it to be reshaped (shown). With boiling water, the thermoset can even revert to its monomers.

A research group at the University of California, Irvine, led by Zhibin Guan has turned to boroxine rings to produce recyclable thermosets (J. Am. Chem. Soc. 2018, DOI: 10.1021/jacs.8b03257). These rings form through reversible reactions between boronic acid groups on the monomers, a property that also allows the thermoset to be reshaped and re-formed. In boiling water, the polymer breaks down to its monomers.

Researchers at IBM are also interested in chemically recyclable thermoset polymers because computers use the materials in many ways, including as insulators for electronics and in the cases that house them. Jeannette Garcia, a chemist at IBM, created poly(hexahydrotriazine), which can be converted back into its monomer with an acid catalyst that selectively hydrolyzes the hexahydrotriazine linkers (Science 2014, DOI: 10.1126/science.1251484).

Rethinking thermoplastics

Thermoset polymers aren’t the only materials that chemists are designing to be easily recycled. Some researchers have turned to the field of self-destructive, or so-called self-immolative, polymers to develop new thermoplastics. Pioneered by chemist Doron Shabat of Tel Aviv University, self-immolative polymers are inherently unstable but have an endcap that prevents their depolymerization. When triggered by light or a specific chemical, the endcap releases and triggers depolymerization. “Self-immolative polymers embody that ideal notion of how we could potentially recycle plastics,” says Elizabeth Gillies, a chemist at the University of Western Ontario.

Gillies developed an ethyl glyoxylate polymer that reverts to its monomer, ethyl glyoxylate, when the polymer’s endcap is removed via light, hydrogen peroxide, or mild acid (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja504727u). In theory, that’s an ideal system for a recyclable polymer. In reality, she says, the current versions of these polymers can’t compete with the properties or cost of PET and other commercial plastics.

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Two-electron reactions can degrade self-immolative polymers linearly when a trigger removes the endcap.

Scott Phillips, a chemist at Boise State University, has designed self-immolative polymers with responsive endcaps, including phenoxides and alkoxides (Green Chem. 2015, DOI: 10.1039/c5gc01090j). When these molecules get cleaved off the polymer, they liberate two electrons that then cascade down the polymer, selectively breaking off monomers.

But not all recyclable thermoplastics need to rely on self-immolation. Eugene Y.-X. Chen, a chemist at Colorado State University, recently described a fully recyclable polymer with properties on par with plastics on the market right now (Science 2018, DOI: 10.1126/science.aar5498).

Chen has long focused on ring-opening reactions to synthesize polymers. “Ring-opening polymerization is a highly effective way of making high-molecular-weight polymers in a short period of time,” he says. He recently reported polymerization of a strained, two-ring monomer called 3,4-T6GBL into either a linear or cyclic polymer, depending on the metal catalyst used. Using heat and a ZnCl2 catalyst, Chen and his team can return the polymer to 3,4-T6GBL, a process they think can be repeated infinitely.

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Eugene Y.-X. Chen of Colorado State University calls this polymer, in its linear (right) or cyclical (left) form, infinitely recyclable, capable of repeatedly breaking down to its monomer (center).

“The design of the monomer is the key for developing chemically recyclable polymers with high depolymerization selectivity and useful materials properties,” he says. Ensuring that its core structure can be chemically recycled is the first step, according to Chen. Then adding functional groups can achieve desirable physical properties.

One of Chen’s former postdocs, Miao Hong, now developing her own recyclable polymers at the Shanghai Institute of Organic Chemistry, says chemically recyclable polymers are the best solution to the problem of plastic trash. Such materials could not only make possible the infinitely recyclable plastic bottle but also avoid issues of quality loss, seen with mechanical recycling, and the inability to recover valuable products from biodegradable polymers.

Still, intrinsically recyclable plastics are a long way from commercial reality. Besides technical hurdles, there are also economic ones. For manufacturers, it’s usually easier to use a tried-and-true material than a brand-new one, Garcia says. Although the barrier to adoption is high, it isn’t insurmountable. So she and other chemists continue to work on chemical solutions to the rapidly growing problem of plastic trash.

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