If recycling plastics isn’t making sense, remake the plastics

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Picture of a forklift surrounded by plastic bottles.
Zoom in / Workers sort plastic waste as a forklift that transports plastic waste at the Yongin Recycling Center in Yongin, South Korea.

A few years ago, it looked like plastic recycling would become an essential part of a sustainable future. After that, the price of fossil fuels fell, making it cheaper to manufacture new plastics. Then China basically stopped importing recycled plastics for use in manufacturing. The bottom of the recycle plastic, however, has leaked, and the best thing you can say to most plastics is that they have isolated the carbon they were made of.

However, the lack of a market for recycled plastics has also inspired researchers to consider other ways to use them. Two papers this week examined processes that allow for “recycling,” or the conversion of plastic into materials that can be more valuable than newly manufactured plastic.

Make me some nanotubes

The first paper, made through international cooperation, actually got the plastics they tested from a supermarket chain, so we know it’s working on the related materials. The recycling it describes also has the advantage of working with very cheap iron-based catalysts. Usually, to break down the plastic, the catalysts and the plastic are heated together. But in this case, the researchers simply mixed the catalyst, grinded the plastic, and heated the iron using microwave ovens.

Like water, iron absorbs microwave radiation and turns it into heat. This causes the heat to be focused on the site where the catalytic activities take place, rather than spreading out evenly throughout the reaction.

The difference is amazing. Compared to conventional heating, microwave heating released more than 10 times the amount of hydrogen from the plastic, leaving very little other than pure carbon and some iron carbide. Even better, the carbon was almost entirely in the form of carbon nanotubes, which is a very valuable product. And all of this happened so quickly, that the hydrogen was released after less than a minute of using the microwaves. The process was completed in less than two minutes.

Although some of the iron ended up binding to the carbon, this did not disrupt the catalyst. The researchers found that they could mix more ground plastic and start the process again, and repeat that up to 10 times in their tests, even though hydrogen production was significantly reduced by the tenth cycle. On the plus side, the later courses produced nearly pure hydrogen, as pollutants such as oxygen and water were removed in the earlier cycles. At the end of 10 cycles, the carbon-rich material was 92 percent of the nanotubes by weight.

The only thing missing in the work is an indication of how easily iron can be reformed into iron oxide, which is the catalytic form of the substance.

We’ll take this hydrogen

If you’re ever worried about what to do with this hydrogen, a US-based group has a potential answer. The group was also concerned about the problems other researchers saw when they simply heated up a catalyst and plastic together: the results were a complex mixture of chemicals, rather than the two clean products seen on rapid heating with microwaves. But this team looked in biology for possible solutions.

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