Recycling different types of plastics jumbled together is quite the pickle. Separation takes work, but recycled mixed plastic is weak and often unsuitable for reuse. To get the plastics to mix better, scientists led by Eugene Y.-X. Chen from Colorado State University and Sanat Kumar and Tomislav Rovis from Columbia University incorporated cross-linker molecules as part of the recycling process (Nature 2023, 10.1038/s41586-023-05858-3). The result is stronger recycled plastic from common polymers, including low-density polyethylene and high-density polyethylene. When combined for recycling, different types of plastic tend to form blobs instead of mixing smoothly, Chen says. But adding 5% by weight of bis(diazirine)-based cross-linkers evens out the blobs. “That [cross-linker] molecule can literally activate those dead polymer chains and link them together, to form this living, multiblock copolymer,” Chen says. The ends of the linker molecule snag the polymer chains, while a reversible chemical reaction at the center uses a thioester, a disulfide, or an acid anhydride to bind the polymer chains together. This strategy worked with multiple types of plastics, including zip-top plastic bags and colored cups.
“One issue with recycling postconsumer plastic waste is the cost of separation, which can be prohibitive relative to the value of the recycled plastic,” says polymer chemist Susannah Scott from the University of California, Santa Barbara. Adding cross-linkers could reduce some of this need to separate plastics before recycling, which may cut costs, she says in an email. However, there still might be issues with long-term stability when scientists combine biodegradable polymers with nonbiodegradable ones.