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Accidental chemistry leads to new superplastics: self-healing or nearly unbreakable polymers can be easily recycled.

With an accidental tweak to a classic chemical reaction, scientists have created the first easily recyclable forms of mighty plastics. The discovery could lower costs and reduce waste of everyday products, researchers say.

The two nitrogen-containing polymers, a superstrong plastic and a self-healing gel, represent new types of thermoset materials, which are heat resistant and highly stable. Thermosets are widely used, from automobiles to electronic devices, and are difficult or impossible to recycle. But the new polymers easily break down into their original components, ready to reassemble. "This is a completely different paradigm for recycling," says polymer chemist Jeannette Garcia of IBM Almaden Research Center in San Jose, Calif.

Recycling methods for standard plastics like those in water bottles often involve melting the polymers at very high temperatures, which tends to damage the materials. With the new plastics, Garcia says, researchers completely destroy and rebuild the material.

The new materials were created by serendipity. While testing a chemical recipe, Garcia missed a step. A hard chunk of plastic appeared in her flask. After smashing the glassware to retrieve the material, Garcia found a nearly unbreakable rock of thermoset that was stable up to around 300[degrees] to 350[degrees] Celsius.

Garcia's team reports in the May 16 Science that the chemistry is a revision of an old reaction. For years, chemists have known that combining nitrogen-containing molecules called amines with formaldehyde creates hexagon-shaped rings of carbon and nitrogen, called triazines. But by fortuitously adding a double amine--one with two nitrogens --Garcia's team created triazines that linked together into a 3-D network, forming a poisoner with unusual powers.

"The formaldehyde-amine reaction is one of the oldest used in polymer engineering," says organic chemist Bert Meijer of Eindhoven University of Technology in the Netherlands. By using it to create unique materials, the research "catches the eye of every materials chemist," he adds.

Combining the formaldehyde and double amine at around 50[degrees] C created a gel that, when cut apart and placed back together, reformed into a seamless whole. The self-healing gel was shelf stable, but when researchers washed it with a liquid of neutral pH, it disassembled.

When heated to about 200[degrees] C, the reaction formed the hard plastic that Garcia first discovered. That plastic breaks down in strong acid, at pH 2, regenerating the starting materials.

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Title Annotation:MATTER & ENERGY
Author:Mole, Beth
Publication:Science News
Geographic Code:1USA
Date:Jun 14, 2014
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