Enhancing polyamide 6: Acid hydrolysis for functionalization and amino group quantification

In this study, our primary aim was to increase the presence of amine groups in polyamide 6 fibers. To achieve this, we employed a functionalization process that mainly depended on increasing the number of reactive sites via acid-catalyzed hydrolysis. The effects of several acids were rigorously examined, including hydrochloric acid (HCl), sulfuric acid (H2SO4), and formic acid (HCOOH). Following this comprehensive acid-based treatment, the method that demonstrated the most significant increase in amine groups was selected for further evaluation in dyeing experiments and full characterization. The quantification of amino groups was performed using potentiometry, evaluating the equilibrium state of acid removal from the fibers. To corroborate these results, the acid dye Levaset Blue 2R was employed, which measured the amount of dye bound to the fibers during the dyeing process. Characterization of the polyamide 6 fiber structural changes was conducted using multiple techniques, including tensile tests, Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC). Among the acid treatments, the 11% HCl treatment exhibited the most significant increase in amine groups, amounting to an approximately 110% increase as measured by potentiometry and an approximately 150% increase using the dyeing method. During the dyeing process, the 11% HCl-treated fibers absorbed 91% of the dye from the dye bath, indicating enhanced color intensity and K/S value, thus affirming the efficacy of the amine group increasing. Tensile testing revealed a decrease in the strength of the functionalized fiber. In the FTIR spectra, there was a notable increase in the intensity of bands associated with the fiber's crystalline regions and hydrogen bonds. DSC tests validated the FTIR results, demonstrating a 9% increase in crystallinity. Furthermore, SEM micrographs depicted surface alterations induced by the hydrolysis process. These results collectively highlight the effectiveness of hydrolysis in introducing functional groups, enhancing color intensity during dyeing with acid dyes, and reducing dye content in the bath. Additionally, the potentiometric method for quantifying amine group demonstrated practicality and accuracy, serving as a valuable contribution to the field.