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 A new thermoset plastic does not decompose when heated. (Photo: The American Chemical Society) |
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For Tough Recyclables, a Self-Mending Plastic by Henry Fountain, NYTimes Most of the plastics that are recyclable today — water bottles and grocery bags, for instance — are what are called thermoplastics. They are polymers that can be melted down and molded into something else. But there is another category of plastics, thermoset resins, that can’t be easily recycled. These polymers — the stuff of circuit boards, electrical insulation and epoxy glue, among other things — have strong cross-links and when heated tend to decompose. Most products made from these plastics end up as waste. But chemists at the University of Groningen in the Netherlands have devised a thermoset plastic that, rather than decomposing, heals itself when heated. Writing in the journal Macromolecules, the researchers, Youchun Zhang, Antonius A. Broekhuis and Francesco Picchioni, say the material has the potential to be recycled and reused many times. The building blocks of the polymer are polyketones, and they are cross-linked using another organic compound, bismaleimide. One secret to the material’s success is that the cross-linking reaction is reversible: when heated to about 300 degrees Fahrenheit, the material becomes unlinked, but as it cools the links re-form, creating a rigid polymer once again. The researchers demonstrated that the material can be shredded, melted and remolded at least seven times with no loss of mechanical properties. Their discovery, they say, adds to scientific understanding of the nature of self-healing materials, and with more research may eventually lead to the full development of recyclable thermoset plastics. |
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