Sustainable Production of Polyamide 6 Fibers: Direct Melt Spinning and Efficient Reuse of Residual Oligomers during Polymerization

Direct melt spinning of PA6 from a polymerization reactor is currently unachievable in the industrial production of polyamide 6 (PA6) fibers because the hydrolysis polymerization of its monomer (caprolactam) is a reversible reaction which requires a boiling water extraction of oligomers from the polymer before melt spinning. Here, we proposed a polymerization method where the content of the cyclic dimer is controlled by reducing the temperature at the monomer ring-opening stage and the content of oligomers is monitored by rapid devolatilization through the polycondensation stage. As a result, the content of oligomers in PA6 is less than 1.5 wt % and the cyclic dimer is at 0.1–0.3 wt %, which are 80 and 48% lower than the values for the conventional PA6 production process. This allows for direct melt spinning of PA6 fibers at the industrial scale (spinning speed: 4000 m/min and a continuous production line of 10 kg/h), which reduces the production time from over 40 h to within 10 h, and the resource consumption reduces by over 30%. The resulting PA6 fibers show comparable mechanical and dyeing properties to those spun with the conventional multistep spinning method. In addition, the unreacted oligomers extracted during the polymerization stage can be directly reused in the polymerization, with no further treatment. This work provides an effective route to the sustainable production of PA6 fibers at the industrial scale, which significantly decreases the consumption of resources and the generation of chemical wastes while improving the production efficiency.