Plastic waste is a global problem. Most plastic waste is incinerated or landfilled, which has obvious drawbacks. Incineration releases CO₂ and landfill pollutes the environment. Ideally, plastic waste would be broken down into monomers and turned into new plastics. Unfortunately, depolymerising bulk plastics is rather difficult to achieve, because the reaction is hard to control. Catalysts can improve selectivity, but can suffer from performance degradation.

Now, a group of US researchers from universities including the University of Maryland, Princeton University and the University of Wisconsin-Madison have presented an alternative method for depolymerising polypropylene (PP) and polyethylene terephthalate (PET) in Nature. They use a catalyst-free, electrified spatial temporal heating (STH) method.

The researchers built a layered system with two layers of porous carbon felt and a reservoir of plastic reactant. They heat the top carbon layer with electrical pulses, while the bottom carbon layer acts as a reactor. This creates a temperature gradient in the reactor layer. It allows the plastic material to melt, capillary move up the reactor, vaporize and finally decompose into free monomers. The gaseous products are discharged above the heating layer. Using this method, they were able to selectively depolymerise PP and PET into their monomers with yields of 36 and 43%, respectively.

The researchers also tested the scalability of the process by enlarging the double layer and increasing the polypropylene input tenfold from 0.1 to 1.0 grams. The yield remained the same. According to the researchers, this shows that the process has potential for practical application. They hope that it will help to solve the global problem of plastic waste.

Dong et al, (2023), Nature, https://doi.org/10.1038/s41586-023-05845-8