Plastics are unfortunately so cheap useful that they’ve ended up everywhere. They’re filling our landfills, polluting our rivers, and even infiltrating our food chain as microplastics. As much as we think of plastic as recyclable, too, that’s often not the case—while some plastics like PET (polyethylene terephthalate) are easily reused, others just aren’t.
Indeed, the world currently produces an immense amount of polyethylene and polypropylene waste. These materials are used for everything from plastic bags to milk jugs and for microwavable containers—and it’s all really hard to recycle. However, a team at UC Berkeley might have just figured out how to deal with this problem.
Catalytic
Here’s the thing—polyethylene and polypropylene are not readily biodegradable at present. That means that waste tends to pile up. They’re actually quite tough to deal with in a chemical sense, too. It’s because these polymers have strong carbon-carbon bonds that are simply quite difficult to break. That means it’s very hard to turn them back into their component molecules for reforming. In an ideal world, you can sometimes capture a very clean waste stream of a single type of these plastics and melt and reform them, but generally, the quality of material you get out of this practice is poor. It’s why so many waste plastics get munched up and turned into unglamorous things like benches and rubbish bins.
At Berkley, researchers were hoping to achieve a better result, turning these plastics back into precursor chemicals that could then be used to make fresh new material. The subject of a new paper in Science is a new catalytic process that essentially vaporizes these common plastics, breaking them down into their hydrocarbon building blocks. Basically, they’re turning old plastic back into the raw materials needed to make new plastic. This has the potential to be more than a nifty lab trick—the hope is that it could make it easy to deal with a whole host of difficult-to-recycle waste products.
Combining the plastics in a high-pressure reactor with ethylene gas and the catalyst materials breaks the polymer chains up into component molecules that can be used to make new plastics. Credit: UC Berkeley
The team employed a pair of solid catalysts, which help push along the desired chemical reactions without being consumed in the process. The first catalyst, which consists of sodium on alumina, tackles the tough job of breaking the strong carbon-carbon bonds in the plastic polymers. These materials consists of long chains of molecules, and this catalyst effectively chops them up. This typically leaves a broken link on one of the polymer chain fragments in the form of a reactive carbon-carbon double bond. The second catalyst, tungsten oxide on silica helps that reactive carbon atom pair up with ethylene gas which is streamed through the reaction chamber, producing propylene molecules as a result. As that carbon atom is stripped away, the process routinely leaves behind another double bond on the broken chain ready to react again, until the whole polymer chain has been converted. Depending on the feed plastic, whether it’s polyethylene, polypropylene, or a mixture, the same reaction process will generate propylene and isobutylene as a result. These gases can then be separated out and used as the starting points for making new plastics.
Before and after—the plastic has been converted to gas, leaving the catalytic material behind. Credit: UC Berkeley
What’s particularly impressive is that this method works on both polyethylene and polypropylene—the two heavy hitters in plastic waste—and even on mixtures of the two. Traditional recycling processes struggle with mixed plastics, often requiring tedious and costly sorting. By efficiently handling blends, this new approach sidesteps one of the major hurdles in plastic recycling.
To achieve this conversion in practice is relatively simple. Chunks of waste plastic are sealed in a high-pressure reaction vessel with the catalyst materials and a feed of ethylene gas, with the combination then heated and stirred. The materials react, and the gas left behind is the useful precursor gases for making fresh plastic.
In lab tests, the catalysts converted a near-equal mix of polyethylene and polypropylene into useful gases with an efficiency of almost 90%. That’s a significant leap forward compared to current methods, which often result in lower-value products or require pure streams of a single type of plastic. The process also showed resilience against common impurities and should be able to work with post-consumer materials—i.e. stuff people have thrown away. Additives and small amounts of other plastics didn’t significantly hamper the efficiency, though larger amounts of PET and PVC did pose a problem. However, since recycling facilities already separate out different types of plastics, this isn’t a deal-breaker.
The process can even run efficiently with a mixture of polypropylene and polyethylene. Note that propene is just another word for propylene. Credit: UC Berkeley
One of the most promising aspects of this development is the practicality of scaling it up. The catalysts used are cheaper and more robust than those in previous methods, which relied on expensive, sensitive metals dissolved in liquids. Solid catalysts are more amenable to industrial processes, particularly continuous flow systems that can handle large volumes of material.
Of course, moving from the lab bench to a full-scale industrial process will require further research and investment. The team needs to demonstrate that the process is economically viable and environmentally friendly on a large scale. But the potential benefits are enormous. It could actually make it worthwhile to recycle a whole lot more single-use plastic items, and reduce our need to replace or eliminate them entirely. Anything that cuts plastic waste streams into the environment is a boon, too. Ultimately, there’s still a ways to go, but it’s promising that solutions for these difficult-to-recycle plastics are finally coming on stream.
This articles is written by : Nermeen Nabil Khear Abdelmalak
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