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Soda bottles and sour cream containers—these plastics typically arrive at recycling plants mixed together in the same bin. But because they are made of different monomers (单体), 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. To seek new solutions to old recycling problems, his team had to solve a fundamental chemical difficulty: when different plastics are melted together, their various monomers tend to separate from one another. They have 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.

The new process solves this problem by adding chemicals called universal dynamic cross-linkers (交联剂) to the mix. Just as soap brings together oil and water, which creates a bridge, allowing the oil and water to mix together and form a stable liquid, these cross-linkers(when applied under heat)form covalent molecular bonds (共价键) that attach the diverse monomers together. These materials can then be melted and remade again and again because the cross-linkers can break and re-form their bonds.

The researchers hope the technique could eventually help handle more plastic waste—but it’s not yet cheap enough to be widely used at existing recycling facilities. Still, Kumar wishes it would be one way to help recyclers address the plastic waste crisis. “The plastics problem is huge,” he says, “and you’re going to have to look for multiple solutions.”