Discussion on Factors Affecting Dyeing of Bio-based Polyamide 5,6 Fibers Compared with Polyamide 6,6 Fibers

To replace petroleum-based nylon and broaden the applications of polyamide 5,6 (PA56) in textiles, it is necessary to investigate the factors affecting the dyeing of bio-based PA56. In this work, bio-based PA56 and polyamide 6,6 (PA66) fibers were dyed with an acid dye. A comprehensive comparison of the factors affecting the dyeing performance of the PA56 and the PA66 fibers was carried out by measurements of the end-group content, crystallinity, and free volume fraction. The free volume fractions of the fibers in the dyeing process were calculated by the Williams-Landel-Ferry equation. Compared to the PA66 fibers, the results showed that the PA56 fibers had a better dyeing performance and water absorption behavior. Under the conventional dyeing process with water, the free volume fraction of PA56 fibers was slightly larger than that of the PA66 fibers. The larger the free volume, the more space for segmental motion and the easily entering of the dye into the interior of a fiber. The crystallinity of the PA56 fibers was 26.9%, which was lower than the 35.6% of the PA66 fibers. The lower crystallinity of the PA56 fibers means less regularity of macromolecular arrangement. The end-amino groups of the PA56 fibers and the PA66 fibers were 39.6 mmol/kg and 44.7 mmol/kg, respectively. Since the -NH2 groups only existed on the end of the polymer chains and the amount of -NH2 groups was low, the effect of the -NH2 groups content on the dyeing results was very small. Taking all these factors into consideration, the lower arrangement and the better segmental mobility of the molecular chains in the amorphous areas gave rise to the better dyeing performance of the PA56 fibers.