plastics

vaporizing-plastics-recycles-them-into-nothing-but-gas

Vaporizing plastics recycles them into nothing but gas

chemistry, plastics, recycling, Science, Sustainability / /

Breakdown —

Polypropylene and polyethylene can be broken down simultaneously.

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A man stands next to piles of compressed plastic bottles.

Our planet is choking on plastics. Some of the worst offenders, which can take decades to degrade in landfills, are polypropylene—which is used for things such as food packaging and bumpers—and polyethylene, found in plastic bags, bottles, toys, and even mulch.

Polypropylene and polyethylene can be recycled, but the process can be difficult and often produces large quantities of the greenhouse gas methane. They are both polyolefins, which are the products of polymerizing ethylene and propylene, raw materials that are mainly derived from fossil fuels. The bonds of polyolefins are also notoriously hard to break.

Now, researchers at the University of California, Berkeley have come up with a method of recycling these polymers that uses catalysts that easily break their bonds, converting them into propylene and isobutylene, which are gasses at room temperature. Those gasses can then be recycled into new plastics.

“Because polypropylene and polyethylene are among the most difficult and expensive plastics to separate from each other in a mixed waste stream, it is crucial that [a recycling] process apply to both polyolefins,” the research team said in a study recently published in Science.

Breaking it down

The recycling process the team used is known as isomerizing ethenolysis, which relies on a catalyst to break down olefin polymer chains into their small molecules. Polyethylene and polypropylene bonds are highly resistant to chemical reactions because both of these polyolefins have long chains of single carbon-carbon bonds. Most polymers have at least one carbon-carbon double bond, which is much easier to break.

While isomerizing ethenolysis had been tried by the same researchers before, the previous catalysts were expensive metals that did not remain pure long enough to convert all of the plastic into gas. Using sodium on alumina followed by tungsten oxide on silica proved much more economical and effective, even though the high temperatures required for the reaction added a bit to the cost

In both plastics, exposure to sodium on alumina broke each polymer chain into shorter polymer chains and created breakable carbon-carbon double bonds at the ends. The chains continued to break over and over. Both then underwent a second process known as olefin metathesis. They were exposed to a stream of ethylene gas flowing into a reaction chamber while being introduced to tungsten oxide on silica, which resulted in the breakage of the carbon-carbon bonds.

The reaction breaks all the carbon-carbon bonds in polyethylene and polypropylene, with the carbon atoms released during the breaking of these bonds ending up attached to molecules of ethylene.“The ethylene is critical to this reaction, as it is a co-reactant,” researcher R.J. Conk, one of the authors of the study, told Ars Technica. “The broken links then react with ethylene, which removes the links from the chain. Without ethylene, the reaction cannot occur.”

The entire chain is catalyzed until polyethylene is fully converted to propylene, and polypropylene is converted to a mixture of propylene and isobutylene.

This method has high selectivity—meaning it produces a large amount of the desired product. That means propylene derived from polyethylene, and both propylene and isobutylene derived from polypropylene. Both of these chemicals are in high demand, since propylene is an important raw material for the chemical industry, while isobutylene is a frequently used monomer in many different polymers, including synthetic rubber and a gasoline additive.

Mixing it up

Because plastics are often mixed at recycling centers, the researchers wanted to see what would happen if polypropylene and polyethylene underwent isomerizing ethenolysis together. The reaction was successful, converting the mixture into propylene and isobutylene, with slightly more propylene than isobutylene.

Mixtures also typically include contaminants in the form of additional plastics. So the team also wanted to see whether the reaction would still work if there were contaminants. So they experimented with plastic objects that would otherwise be thrown away, including a centrifuge and a bread bag, both of which contained traces of other polymers besides polypropylene and polyethylene. The reaction yielded only slightly less propylene and isobutylene than it did with unadulterated versions of the polyolefins.

Another test involved introducing different plastics, such as PET and PVC, to polypropylene and polyethylene to see if that would make a difference. These did lower the yield significantly. If this approach is going to be successful, then all but the slightest traces of contaminants will have to be removed from polypropylene and polyethylene products before they are recycled.

While this recycling method sounds like it could prevent tons upon tons of waste, it will need to be scaled up enormously for this to happen. When the research team increased the scale of the experiment, it produced the same yield, which looks promising for the future. Still, we’ll need to build considerable infrastructure before this could make a dent in our plastic waste.

“We hope that the work described…will lead to practical methods for…[producing] new polymers,” the researchers said in the same study. “By doing so, the demand for production of these essential commodity chemicals starting from fossil carbon sources and the associated greenhouse gas emissions could be greatly reduced.”

Science, 2024. DOI: 10.1126/science.adq731

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These household brands want to redefine what counts as “recyclable”

plastics, Policy, recycling, syndication / /
These household brands want to redefine what counts as “recyclable”

Olga Pankova/Moment via Getty Images

This story was originally published by ProPublica, a Pulitzer Prize-winning investigative newsroom. Sign up for The Big Story newsletter to receive stories like this one in your inbox.

Most of the products in the typical kitchen use plastics that are virtually impossible to recycle.

The film that acts as a lid on Dole Sunshine fruit bowls, the rings securing jars of McCormick dried herbs, the straws attached to Juicy Juice boxes, the bags that hold Cheez-Its and Cheerios—they’re all destined for the dumpster.

Now a trade group representing those brands and hundreds more is pressuring regulators to make plastic appear more environmentally friendly, a proposal experts say could worsen a crisis that is flooding the planet and our bodies with the toxic material.

The Consumer Brands Association believes companies should be able to stamp “recyclable” on products that are technically “capable” of being recycled, even if they’re all but guaranteed to end up in a landfill. As ProPublica previously reported, the group argued for a looser definition of “recyclable” in written comments to the Federal Trade Commission as the agency revises the Green Guides—guidelines for advertising products with sustainable attributes.

The association’s board of directors includes officials from some of the world’s richest companies, such as PepsiCo, Procter & Gamble, Coca-Cola, Land O’Lakes, Keurig Dr Pepper, Hormel Foods Corporation, Molson Coors Beverage Company, Campbell Soup, Kellanova, Mondelez International, Conagra Brands, J.M. Smucker, and Clorox.

Some of the companies own brands that project health, wellness, and sustainability. That includes General Mills, owner of Annie’s macaroni and cheese; The Honest Co., whose soaps and baby wipes line the shelves at Whole Foods; and Colgate-Palmolive, which owns the natural deodorant Tom’s of Maine.

ProPublica contacted the 51 companies on the association’s board of directors to ask if they agreed with the trade group’s definition of “recyclable.” Most did not respond. None said they disagreed with the definition. Nine companies referred ProPublica back to the association.

“The makers of America’s household brands are committed to creating a more circular economy which is why the industry has set sustainability goals and invested in consumer education tools” with “detailed recycling instructions,” Joseph Aquilina, the association’s vice president and deputy general counsel, wrote in an email.

The Green Guides are meant to increase consumer trust in sustainable products. Though these guidelines are not laws, they serve as a national reference for companies and other government agencies for how to define terms like “compostable,” “nontoxic” and “recyclable.” The Federal Trade Commission is revising the guides for the first time since 2012.

Most of the plastic we encounter is functionally not recyclable. It’s too expensive or technically difficult to deal with the health risks posed by the dyes and flame retardants found in many products. Collecting, sorting, storing and shipping the plastic for reprocessing often costs much more than plowing it into a landfill. Though some newer technologies have pushed the boundaries of what’s possible, these plastic-recycling techniques are inefficient and exist in such limited quantities that experts say they can’t be relied upon. The reality is: Only 5 percent of Americans’ discarded plastic gets recycled. And while soda bottles and milk jugs can be turned into new products, other common forms of plastic, like flimsy candy wrappers and chip bags, are destined for trash heaps and oceans, where they can linger for centuries without breaking down.

The current Green Guides allow companies to label products and packaging as “recyclable” if at least 60 percent of Americans have access to facilities that will take the material. As written, the guidelines don’t specify whether it’s enough for the facilities to simply collect and sort the items or if there needs to be a reasonable expectation that the material will be made into something new.

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Researchers make a plastic that includes bacteria that can digest it

bacteria, Biology, chemistry, evolution, materials science, plastics, Science / /

It’s alive! —

Bacterial spores strengthen the plastic, then revive to digest it in landfills.

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Image of two containers of dirt, one with a degraded piece of plastic in it.

Han Sol Kim

One reason plastic waste persists in the environment is because there’s not much that can eat it. The chemical structure of most polymers is stable and different enough from existing food sources that bacteria didn’t have enzymes that could digest them. Evolution has started to change that situation, though, and a number of strains have been identified that can digest some common plastics.

An international team of researchers has decided to take advantage of those strains and bundle plastic-eating bacteria into the plastic. To keep them from eating it while it’s in use, the bacteria is mixed in as inactive spores that should (mostly—more on this below) only start digesting the plastic once it’s released into the environment. To get this to work, the researchers had to evolve a bacterial strain that could tolerate the manufacturing process. It turns out that the evolved bacteria made the plastic even stronger.

Bacteria meet plastics

Plastics are formed of polymers, long chains of identical molecules linked together by chemical bonds. While they can be broken down chemically, the process is often energy-intensive and doesn’t leave useful chemicals behind. One alternative is to get bacteria to do it for us. If they’ve got an enzyme that breaks the chemical bonds of a polymer, they can often use the resulting small molecules as an energy source.

The problem has been that the chemical linkages in the polymers are often distinct from the chemicals that living things have come across in the past, so enzymes that break down polymers have been rare. But, with dozens of years of exposure to plastics, that’s starting to change, and a number of plastic-eating bacterial strains have been discovered recently.

This breakdown process still requires that the bacteria and plastics find each other in the environment, though. So a team of researchers decided to put the bacteria in the plastic itself.

The plastic they worked with is called thermoplastic polyurethane (TPU), something you can find everywhere from bicycle inner tubes to the coating on your ethernet cables. Conveniently, there are already bacteria that have been identified that can break down TPU, including a species called Bacillus subtilis, a harmless soil bacterium that has also colonized our digestive tracts. B. subtilis also has a feature that makes it very useful for this work: It forms spores.

This feature handles one of the biggest problems with incorporating bacteria into materials: The materials often don’t provide an environment where living things can thrive. Spores, on the other hand, are used by bacteria to wait out otherwise intolerable conditions, and then return to normal growth when things improve. The idea behind the new work is that B. subtilis spores remain in suspended animation while the TPU is in use and then re-activate and digest it once it’s disposed of.

In practical terms, this works because spores only reactivate once nutritional conditions are sufficiently promising. An Ethernet cable or the inside of a bike tire is unlikely to see conditions that will wake the bacteria. But if that same TPU ends up in a landfill or even the side of the road, nutrients in the soil could trigger the spores to get to work digesting it.

The researchers’ initial problem was that the manufacturing of TPU products usually involves extruding the plastic at high temperatures, which are normally used to kill bacteria. In this case, they found that a typical manufacturing temperature (130° C) killed over 90 percent of the B. subtilis spores in just one minute.

So, they started out by exposing B. subtilis spores to lower temperatures and short periods of heat that were enough to kill most of the bacteria. The survivors were grown up, made to sporulate, and then exposed to a slightly longer period of heat or even higher temperatures. Over time, B. subtilis evolved the ability to tolerate a half hour of temperatures that would kill most of the original strain. The resulting strain was then incorporated into TPU, which was then formed into plastics through a normal extrusion process.

You might expect that putting a bunch of biological material into a plastic would weaken it. But the opposite turned out to be true, as various measures of its tensile strength showed that the spore-containing plastic was stronger than pure plastic. It turns out that the spores have a water-repelling surface that interacts strongly with the polymer strands in the plastic. The heat-resistant strain of bacteria repelled water even more strongly, and plastics made with these spores was tougher still.

To simulate landfilling or litter with the plastic, the researchers placed them in compost. Even without any bacteria, there were organisms present that could degrade it; by five months in the compost, plain TPU lost nearly half its mass. But with B. subtilis spores incorporated, the plastic lost 93 percent of its mass over the same time period.

This doesn’t mean our plastics problem is solved. Obviously, TPU breaks down relatively easily. There are lots of plastics that don’t break down significantly, and may not be compatible with incorporating bacterial spores. In addition, it’s possible that some TPU uses would expose the plastic to environments that would activate the spores—something like food handling or buried cabling. Still, it’s possible this new breakdown process can provide a solution in some cases, making it worth exploring further.

Nature Communications, 2024. DOI: 10.1038/s41467-024-47132-8  (About DOIs).

Listing image by Han Sol Kim

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