Polymer Promises to Pare Sulfur Surplus

Simple process converts waste sulfur into lightweight plastic.

By Chemical Processing Staff

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Many petroleum refineries must contend with a glut of sulfur generated from hydrodesulfurization processes. That sulfur may become a feedstock for making plastics, hope a worldwide team of researchers. They have developed a simple process that directly copolymerizes sulfur with vinylic monomers. The team is using the new plastic to make lithium-sulfur (Li-S) batteries.

 

"We are aiming to exploit the electrochemical properties of these sulfur-based plastics as electrode materials for next-generation batteries. If the technology for electric vehicles becomes [more prevalent] these materials are envisioned worthwhile for large scale production," says Jeffrey Pyun, lead researcher and associate professor of chemistry and biochemistry at the University of Arizona, Tucson, Ariz.

The new plastic has great promise as something that can be produced easily and inexpensively on an industrial scale, he adds. "This process is already amenable to kilogram-scale production, so we envision that pilot-scale production could be readily implemented."

Sulfur doesn't easily form polymers, and out of more than 20 chemicals Pyun and his team identified as most likely to polymerize sulfur only one chemical worked. The process, dubbed "inverse vulcanization," creates a copolymer made mostly of sulfur with a small amount of vinylic monomers. The method enabled the sulfur to convert to processable copolymer forms with tunable thermocmechanical properties. More details appear in a recent article in Nature Chemistry.

The researchers report the new plastic is comparable to the elemental sulfur currently used in Li-S batteries. It boasts high specific capacity (823 mAh/g at 100 cycles) and enhanced capacity retention. In comparison, batteries with elemental sulfur cathodes can be recharged only a limited number of times before they fail. The team envisions high levels of sulfur, around 50–95 wt% compositions, to be optimal for applications in energy and sustainability.

"Elemental sulfur is inherently a brittle, intractable solid. Our inverse vulcanization process enables the preparation of sulfur copolymers, which are largely amorphous glasses with [glass transition temperatures] ranging from around -13° to 30°C. Furthermore, these copolymer materials are now amenable to melting or solution processing, where elemental sulfur is not," adds Pyun.

The team now is comparing the plastic's properties to those of existing polymers and exploring other practical applications such as photonics.

"This work is still in the early stages of development. There remain numerous basic science and engineering problems to tackle with this chemistry. We are focusing on developing new comonomers and copolymers with enhanced functionality," notes Pyun.
Several companies have expressed interest in the new plastic and battery, and the researchers have filed international patents for both.

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