A technique being developed in Japan depolymerizes polyamides to yield caprolactam, the monomer used to create them. A team at Yamaguchi University in Ube led by Akio Kamimura of the Department of Applied Molecular Bioscience in the Graduate School of Medicine has achieved yields of 86% of the caprolactam from nylon 6 in small-scale laboratory equipment (Figure 1). The method relies on an ionic liquid as solvent and works at 300°C, which while a high temperature is far lower than that used in other depolymerization processes, notes Kamimura.
In the route, chips of nylon 6 are placed in a vessel along with N,N-dimethyl-aminopyridine catalyst and N-methyl-N-propylpiperidinium bis(trifluoro-methylsulfonyl)imide, an ionic liquid solvent, and held for about an hour at ambient pressure and under a nitrogen atmosphere, explains Kamimura. The caprolactam is recovered and purified by distillation while the ionic liquid can be reused five times without substantial impact on yield, he adds.
The researchers now are working both to fine-tune the technology for nylon 6 and to extend it to other polymers. The major challenge, says Kamimura, is economic. In my opinion, price of ionic liquids is the hardest problem at the moment. Indeed, I sometimes feel it is too expensive to carry out our research even in micro-scale, he says.