TEACHING PLASTIC TO DIE
Scientists are developing a new material that will self-destruct for reuse,
Adam Feinberg had no sooner made a bright yellow thin sheet of plastic than he had to shred it into little pieces. He chose an “I”-shaped mould for the logo of the University of Illinois at Urbana-Champaign, where he is a chemist. Then, he filled it with the plastic bits and stuck it in a hot oven.
“I opened up the mould” he recalled. His new plastic passed the first test — it was mouldable with heat like regular plastic. But there was another important step left in rethinking the world of durable plastics.
Feinberg placed the “I” under a white light, and five minutes later, only half of it remained. The other half had fallen on the ground. Pieced back together, the “I” had a hole in the middle and in its place was yellow goo.
The plastic did not simply melt. Its building blocks, the synthetic polymers within, had reverted to their molecular units. “It was a phenomenal feeling,” he said of the successful experiment.
Most synthetic polymers were not designed to disintegrate or disappear. On the contrary, they were meant to last as long as possible once they began replacing metals and glass in long-lasting things such as automobiles and airplanes.
The environmental effects of plastic buildup and the declining popularity of plastics have helped to spur chemists on a quest to make new materials with two conflicting requirements: They must be durable, but degradable on command. In short, scientists are in search of polymers or plastics with a built-in self-destruct mechanism.
“It’s two diametrically opposed criteria that we’re trying to juggle,” Feinberg said. It is easier to mould a robust plastic without destroying it, he said, but at the same time, it should not last forever.
“The real trick is to make them stable when you’re using them, and unstable when you don’t want to use them,” said Marc Hillmyer, who leads the Center for Sustainable Polymers at the University of Minnesota.
While not a silver bullet for the problem of plastic waste, self-destructing plastics could also enable new applications in drug delivery, self-healing materials and even some electronics.
The starting point requires picking polymers that are inherently unstable, and often historically overlooked because of their fragility. Given a choice, their units would rather stay as small molecules. What scientists do is force those molecules to link up into long chains, and then trap the resulting polymers.
Dismantling these polymers is sometimes called unzipping them, because once the polymers encounter a trigger that removes those traps, their units fall off one after another until the polymers have completely switched back to small molecules.
“As soon as you start the process,” explained Jeffrey Moore, Feinberg’s supervisor at the University of Illinois, “they just keep going.”
Feinberg’s polymers were imprisoned in circular loops instead of being open-ended chains. By themselves, the loops were stable. For the self-destructing plastic, Feinberg mixed the polymers with a little bit of yellow, light-sensitive dye. When light shines on the plastic, the energized dye molecules rip electrons out from the polymers. The loops break, exposing the polymer ends, and the polymers unzip.
Other scientists trap their polymers by capping the ends of the long chains or linking the chains together into networks. By designing these traps to fail upon meeting certain triggers like light or acid, scientists can control exactly how and when their polymers unzip.
“We can have a big change in properties or complete degradation of the polymer just from one event,” said Elizabeth Gillies, a polymer chemist at Western University in London, Ont.
In theory, these next-generation polymers could help mitigate pollution problems associated with plastic products.
Economically, replacing the polymers used ingrocery bags, fishing nets or single-use bottles with unzipping polymers is not feasible.
“Packaging plastic is the cheapest thing ever,” Gillies said.
Instead, scientists such as Hillmyer are focusing on higher-value materials such as the poly- urethane foams commonly found in mattresses and car seats. In 2016, Hillmyer and his team made a polyurethane from unzipping polymers that was chemically recyclable. Molecular units derived from sugar linked up to make the polymers, which then cross-linked into polyurethane networks. The foam remains stable at room temperature but unzips into units if heated above 205 C (400 F).
Using chemically recyclable materials could become practical if companies begin taking responsibility for their products after their useful life, Hillmyer said.
He co-founded a startup, Valerian Materials, to commercialize the recyclable polyurethane. If car companies had to take back a used car, for instance, it might make sense to have an internal chemical recycling system to make new materials from old ones, he said.
“It is literally feedstock recovery,” said Jeannette Garcia, a polymer chemist at IBM.
“The real trick is to make them stable when you’re using them, and unstable when you don’t want to use them.” MARC HILLMYER UNIVERSITY OF MINNESOTA