Synchronized Falling-Film\nDevolatilization and Viscosity\nEnhancement for Continuous Production of High-Strength PA 6 Industrial\nYarn

Direct spinning of polyamide 6 (PA 6) fibers, traditionally\nimpeded\nby the presence of approximately 10% residual monomer and oligomers\nin the polymerized form, has been made feasible through our innovative\napproach. Current extraction methods predominantly employ hot water\nto extract monomer and oligomers. However, this approach is characterized\nby time-consuming procedures, high energy consumption, and substantial\nwater usage. Therefore, in this study, a straightforward falling-film\ndevolatilization method is proposed to synchronize the liquid-phase\nviscosity enhancement and oligomer removal of PA 6. PA 6 industrial\nyarns were successfully produced using devolatilized PA 6 (D-PA 6)\nobtained by this method. The viscosities of D-PA 6 samples were increased\nfrom 2.5 and 2.8 to 2.8 and 3.4, their monomer contents were reduced\nto 0.26%, and their dimer and residual oligomer contents were reduced\nto 0.54 and 2.54%, respectively, with a lower removal of dimer and\ntrimer. The process led to a wider molecular weight distribution,\nconsistent with that typical of PA 6 spinning. The obtained PA 6 industrial\nyarn exhibited a high degree of crystallinity, and its orientation,\nthermal properties, and mechanical properties were comparable to those\nof conventional yarn. Although its dyeing ability was slightly compromised,\nthis did not hinder the practical application of the PA 6 industrial\nyarn. The falling-film devolatilization process enables simultaneous\nviscosity enhancement and devolatilization without the need for hot-water\nextraction (24 h) and drying (24 h), drastically shortening the production\ntime from more than 48 h to less than 10 h, thereby greatly reducing\nenergy consumption and wastewater discharge. This study provides technical\nguidance for the sustainable industrial production of PA 6 industrial\nyarn by direct melt-spinning, which significantly reduces energy and\nenvironmental impacts while improving production efficiency.