Inhibition of cyclic oligomer formation via rare earth coordination and long-chain amine end capping during PA6 synthesis process

Abstract

In the industrial production of polyamide 6 (PA6), hot water extraction is commonly used to remove monomers (caprolactam, C1) and cyclic oligomers. This cumbersome process hinders the realization of direct melt spinning and results in considerable energy waste. Herein, the occurrence of the cyclization reaction was effectively decreased using long-chain ammonia as a capping agent and reducing the ring-opening temperature. At the same time, the coordination between rare earths and carbonyl oxygen significantly increased the free energy barrier that needs to be overcome for the formation of cyclic dimers (C2) and formed steric hindrance that hinders the attack of terminal amino groups on amide bonds. Under the synergistic effect, the cyclic oligomer and cyclic dimer contents in PA6 were 1.75 wt% and 0.19 wt%, respectively, which were 48% and 76% lower than those in terminal aggregation PA6 (PA6-C, 3.36 wt% and 0.79 wt%), respectively. In order to meet the standard of the total oligomer content in spinning grade PA6, C1 and the partial cyclic oligomer were rapidly removed under high temperature and vacuum. The content of the oligomer decreased to 1.47 wt%, which was less than 2.0 wt% of the spinning requirement, and the relative viscosity increased to 2.53. This is similar to the oligomer content of industrial PA6 with a relative viscosity of 2.47, extracted by hot water (1.53 wt%). In addition, the appearance of the γ-crystal form was conducive to the subsequent processing of PA6 and the saving of energy consumption. The significant increase in the content of terminal amino groups is beneficial for improving dyeing performance. This study provides a new solution for direct melt spinning of PA6 fibers, reducing the entire polymerization process time from 55 h to 10 h and increasing production efficiency by nearly 450%. Eliminating complex hot water extraction steps greatly saved water and energy, with a quantifiable electric energy savings of approximately 32.13% compared to hot water extraction methods.