Preparation and characterization of cationic dyeable polycaprolactam modified with 5-sulfoisophthalic acid sodium and poly(ethylene glycol)

Unmodified nylon is dyeable to a single color only, and is almost exclusively dyed with acid dyes that are absorbed with amine groups of nylon molecules. Two types of polycaprolactam (PA6) copolyamide were successfully prepared with 5-sulfoisophthalic acid monosodium salt and poly(ethylene glycol) (PEG) units named cationic dyeable polyamide (CD-PA6) and easy cationic dyeable polyamide (ECD-PA6). The chemical and crystalline structures of CD-PA6 and ECD-PA6 were characterized by Fourier transform infrared spectroscopy and wide angle x-ray diffraction, and their thermal properties were tested by differential scanning calorimetry and thermogravimetric analysis, respectively. In addition, the rheological behavior and mechanical properties of copolyamide are presented in this paper. The influence of chemical modification in polyamide 6 fibers on the dyeing properties was investigated using cationic dye (methylene blue). The incorporation of PEG not only destroyed the regularity of the molecular chain arrangement and created more amorphous regions of ECD-PA6 samples, but also led to nylon 6 changing from the α-form to the γ-form. In addition, the crystallinities and degradation temperatures of samples which corresponded to different mass losses of CD-PA 6 and ECD-PA6 declined as the sulfonic group content increased, since large –SO ₃ Na side groups in the copolymers prevented the chain molecules from tightly coagulating and obstructed the formation of larger crystals. Based on the analysis of the dyeing, a distinct improvement in tinctorial affinity and wash fastness for modified fibers compared with unmodified fibers was revealed.