Each year, thousands of tons of nylon end up in landfills. But small-scale experiments may offer big hope for efficient recycling of some types of the material.
Nylon-6, an artificial polymer used in carpets, clothing, and car parts, is made by chemically linking large numbers of molecules derived from a petroleum product called caprolactam. Current processes to break apart, or depolymerize, nylon-6 typically must take place at high temperatures and high pressures. The processes are also relatively inefficient, says Akio Kamimura, an organic chemist at Yamaguchi University in Ube, Japan.
On the other hand, incinerating the polymers in mixed trash can create prodigious amounts of toxic compounds (SN: 1/29/00, p. 70). That’s why nylon-6 usually ends up in landfills. Each year in the United States alone, carpets containing about 500,000 metric tons of nylon-6 end up at the dump.
Now, Kamimura and his colleague Shigehiro Yamamoto have developed a process that depolymerizes nylon-6 and regenerates caprolactam. The researchers describe their bench-scale experiments, which use common laboratory equipment, in the June 21 Organic Letters.
Kamimura and Yamamoto placed chips of nylon-6 and small amounts of a catalyst in various ionic liquids, which consist solely of positively and negatively charged ions (SN: 9/8/01, p. 156). At a temperature of 270°C, the depolymerization reaction was inefficient, and the team recovered only 7 percent of the caprolactam contained in the nylon chips, says Kamimura. At temperatures above 330°C, the reaction was more efficient, but only 55 percent of the caprolactam was recovered because some of the substance decomposed in the heat.
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At the intermediate temperature of 300°C—low by industrial standards—the yield of caprolactam approached 86 percent, says Kamimura. More important, he notes, at that temperature the ionic liquid didn’t become tainted with by-products of the reaction. The researchers were able to reuse their ionic liquid five times without significant drops in caprolactam yield.
The team’s approach is novel because it uses ionic liquids under conditions less harsh than those needed for other solvents, says Michael P. Harold, a chemical engineer at the University of Houston. He suggests, however, that several issues may stand in the way of making the process economically feasible. For instance, because ionic liquids are typically quite costly, expanding the process to an industrial scale would require the solvent to endure hundreds of depolymerization cycles.
“Ultimately, the economics [of the process] will dictate the success,” says Harold. “If the ionic liquid is very expensive and not sufficiently durable, the concept will not be viable.”
John D. Muzzy, a chemical engineer at the Georgia Institute of Technology in Atlanta, and his colleagues are developing a different sort of chemical reaction to unzip nylon-6. In the lab, they’ve used a liquid catalyst to melt the nylon and cleave its long molecules. The researchers haven’t yet published their findings, but Muzzy and his team estimate that a single facility using this process to recycle nylon-6 would be able to recover about 90 percent of its caprolactam. It could generate more than 4,600 metric tons of an impure solution of caprolactam each year at a cost of about half the current market price.