Chemists develop novel method to recycle mixed plastics

Recycling plastic is an inefficient business. According to an August 2022 Department of Energy report, only 5 percent of the estimated 44 million metric tons of plastic waste processed for recycling in 2019 was actually recycled. The rest went to a landfill or was incinerated.

Plastic is difficult to recycle because not all the different types are chemically compatible with one another. Some plastics are formed by covalent bonds, in which the individual atoms equally share electrons. Other plastics are formed by noncovalent bonds, in which the individual atoms do not share electrons equally. For example, combining a sandwich bag composed of covalently bonded low-density polyethylene (LDPE) with a salad dressing bottle made up of noncovalently bonded polyethylene terephthalate (PET) results in a weak mixed plastic with limited applications.

A novel chemical process designed by researchers at Colorado State and Columbia universities might make recycling mixed plastic waste more feasible. Using crosslinkers, small molecules that form bridges between chemicals, the scientists were able to make chemically different plastics compatible. Their work was published April 26 in Nature.

While laboratory tests demonstrated the crosslinker technology’s remarkable effectiveness, real-world application is still far off. The biggest question is whether the technology is cost-effective in comparison to production of new plastic.

The researchers heated and processed mixed plastics with the crosslinker bis-diazirine. Scanning electron microscopy images demonstrated the superior structure of the resulting material. In comparison to the blob-like, irregular surface of virgin mixed plastics, the crosslinked plastics had a smooth, homogenous surface. Lead author Tomislav Rovis, professor of chemistry at Columbia University, compared the crosslinkers to Velcro. They act like hook-and-loop fasteners in stitching together plastics of different polymer compositions.

“The [crosslinked] material behaves as well as the unique individual virgin polymers, in terms of it doesn’t degrade over time and all these other physical tests,” said Rovis. He emphasized that stress tests, which measure a polymer’s ability to resist deformation, showed similar strength profiles for the crosslinked mixed plastics and individual polymers.

The scientists successfully created blends of the incompatible plastics LDPE and poly L-lactic acid (PLLA), then blended those two plastics with isotactic polypropylene (iPP). Lead author Sanat Kumar, professor of chemical engineering at Columbia University, said they chose to focus on those three plastics because they represented both traditional plastic polymers (LDPE and iPP) and a new class of bio-based polymers (PLLA).

Kumar and Rovis explained how their chemistry could transform plastic recycling. The 5 percent of plastic waste that is successfully recycled using traditional methods is downgraded into lower quality products. High-density polyethylene (HDPE) products, such as milk containers, are converted into items like picnic tables and plastic lumber. PET products, including soda bottles, are converted into items such as fleece jackets or carpeting. But the new material formed using the crosslinkers can be used to transform degraded products into higher quality materials.

“The advantage of this technology is you can get the degraded water bottle, or the degraded tire, put it back into the melt, and remake something without those deficiencies because these crosslinkers are in it,” said Rovis.

Gregory Rummo, a lecturer in chemistry at Palm Beach Atlantic University and an adjunct scholar at the Cornwall Alliance for the Stewardship of Creation, thinks the polymer crosslinkers have potential. But Rummo cautioned that their use must be economically feasible. He noted that the new technique would likely require recycling facilities to carve out space to perform the crosslinker process. He questioned whether this process, once scaled up, would cost more than producing new plastic.

Rovis said the only byproduct of the crosslinking process is nitrogen gas, which is already abundant in Earth’s atmosphere. And while his team didn’t conduct an environmental assessment, Rovis doesn’t think the crosslinkers are toxic.