Soda bottles, sour cream containers and disposable cutlery—these plastics (and many others) typically arrive at recycling plants mixed together in the same bin. But because they are made of different molecular building blocks, called monomers, they must be sorted into different streams before they can be melted to make new products.
“Until about a year ago, everybody thought the only thing you could do is take a plastic, break it back down to a monomer and then re-form it,” says Sanat Kumar, a chemical engineer at Columbia University. “Now we've come up with a different way of doing it.” His team has developed a process that allows different kinds of plastic to be recycled together. Their findings, reported recently in Nature, could give new life to many items that end up in landfills.
A disturbingly small portion of our plastic waste is recycled, and production of new plastic—made from fossil fuels—continues to increase. The worsening situation has prompted scientists to seek new solutions to old recycling problems, including the difficulty of recycling mixed plastics. But they have faced a fundamental chemical hurdle: when different plastics are melted together, their various monomers tend to separate from one another like oil and water.
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The new process solves this problem by adding chemicals called universal dynamic cross-linkers to the mix. Just as soap brings together oil and water, these cross-linkers (when applied under heat) form covalent molecular bonds that tether the diverse monomers together. This process creates polymers that can retain certain useful properties of each constituent plastic, such as keeping oxygen or ultraviolet light from passing through. These materials can then be melted and remade again and again because the cross-linkers can break and re-form their bonds.
“This chemistry is a really important step,” says University of Washington chemical engineer Julie Rorrer, who works on plastics recycling and was not a part of the study. “They're demonstrating that this was industrially processable, which is very exciting.”
The researchers hope the technique could eventually help repurpose more plastic waste, and Kumar says the process consumes less energy than breaking plastics down into their original monomers—but it's not yet cheap enough to be widely used at existing recycling facilities. Still, showing that it works could lead to lucrative markets for less expensive recycled plastics, which Kumar says would be one way to help recyclers eventually address the plastic waste crisis. “The plastics problem is huge,” he says, “and you're going to have to look for multiple solutions.”