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Bulk polymerization of elemental sulfur yields new flame-retardant plastics

By Mary Page Bailey |

A group of researchers from the University of Arizona (Tucson; www.arizona.edu), led by chemistry and biochemistry professor Jeff Pyun, are aiming to make use of sulfur waste streams from petroleum refineries. To meet environmental regulations, sulfur must be removed from crude oil during refining, but economic end-uses for high-volume sulfur streams are very limited.

“Typically, sulfur is very challenging to work with, as it is insoluble in most solvents and difficult to work with as a solid. Rather than trying to dissolve it, we developed a new process to melt elemental sulfur and use it as a monomer for polymerization,” explains Pyun. The group developed a process called inverse vulcanization that uses sulfur as both the monomer and the salt for the polymerization reaction, resulting in a polymer with a sulfur-based backbone. Initially, the group looked at using the polymers in battery electrodes or optics materials, but they also wanted to investigate higher-volume end products, such as polyurethanes. To do so, Pyun’s group split the polymerization process into steps, first starting with a prepolymer that is reacted with an olefin that carries an alcohol group, forming a high-sulfur-content precursor for segmented polyurethane products. “We’re taking very inexpensive materials, such as sulfur and the alcohol 1-undecanol, and using them to make conventional plastics,” adds Pyun.

These new sulfur-based polymers provided a valuable benefit over traditional polymers — flame retardancy. Typically, flame-retardant polymers involve either the addition of specialty monomers to conventional plastics, which drive up costs and inhibit large-scale production; or the addition of small-molecule flame-retardant agents, which are typically heavily halogenated, and introduce serious environmental concerns. The new sulfur-based polymers could unlock a pathway to cheaper, more sustainable flame retardants, says Pyun, noting that there are currently no similar products on the market. “I anticipate this chemistry should be scalable, as the materials and methods are inexpensive. The technology is also quite modular. We’ve chosen polyurethanes, but other polymers could certainly be made,” adds Pyun.

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