Many polymers cannot be recycled easily but that could change for thermoset plastics thanks to work at the Massachusetts Institute of Technology. [Shieh, P., et al. Nature, 2020; DOI: 10.1038/s41586-020-2495-2]

Thermoset polymers including epoxy resins, polyurethanes, and synthetic rubber are durable and resistant to heat making them widely useful in vehicles, electrical appliances, and many other areas. Unfortunately, aside from separation issues, they cannot be broken down for recycling at end-of-life. This is in stark contrast to several thermoplastics, such as polyethylene and polypropylene.

The MIT have found a way to circumvent this problem by using a silyl linker that does not compromise mechanical strength nor other useful properties of thermoset plastics, but means that they can be recycled. The team has demonstrated proof of principle with the synthesis of a degradable modification of polydicyclopentadiene. At end of life, the polymer can be broken down into a powder and used to remake the polymer. Their theoretical analysis of the process suggests that the same approach would work for other thermoset plastics.

"This work unveils a fundamental design principle that we believe is general to any kind of thermoset with this basic architecture," explains team leader Jeremiah Johnson. Given that thermoset plastics cannot be returned to a liquid state after fabrication, unlike thermoplastics, there were until now completely limited in their re-use or recycling.

The silyl linker in the team's modified pDCPD allows them to retrieve a soluble form of the polymer using fluoride. The new material made from this recycled plastic powered is almost indistinguishable from virgin polymer and in some ways improves on the original thermoset plastic material's properties. "Showing that you can take the degradation products and remake the same thermoset again using the same process is exciting," Johnson enthuses.

Given how much plastic waste we are discarding in the environment the time is ripe for materials scientists and chemists to turn their attention to simple technology that allows us to retrieve even unrecyclable plastics for fabrication into new products. If researchers can find appropriate degradable monomers for other types of modified polymers, we could ultimately have recyclable acrylics, epoxies, and silicones, perhaps even degradable vulcanized rubber.

The new approach could allow car manufacturers and others to address sustainability concerns as well as providing the "raw materials" for recycling plants that are missing out on a vast waste stream when it comes to thermoset plastics.