Compared with the pulp-free polyester fabric or knitted fabric, the sizing polyester fabric dyeing and finishing process takes a long time, and most of the additives used in spinning and weaving have been removed. However, there may still be a small amount of auxiliaries used in the dyeing and post-treatment process remaining in the fabric and the free water it carries. In addition, although the hot washing and the first pickling have been carried out after desizing, the caustic soda used in the desizing may still remain in a small amount inside the monofilament of the polyester fiber or even inside the amorphous region.
In the same way, in the process of stacking the fabric before the mold is set, the free water can carry the residual alkali and the auxiliary agent to the reciprocating fold. At this time, the pH value of the reciprocating fold is significantly higher than that of other parts of the fabric, which is also confirmed by the universal indicator, and the concentration of the auxiliary agent is also larger than other parts. During the high-temperature setting process, the polyester macromolecular chain moves violently, and the OH-ion can be instantaneously drilled into the amorphous region to react with the disperse dye.
Disperse dyes are stable under acidic conditions (pH 5). Under alkaline conditions, there are generally two effects on disperse dyes: alkaline conditions can dissociate certain dye structures. This reaction is reversible and neutral. The undissociated structure reappears in the slightly acidic structure. The base can hydrolyze some of the disperse dyes. After hydrolysis, the dye not only changes color, but also has different affinity. Therefore, the base can permanently destroy some of the disperse dyes.
Therefore, the color change of the sizing polyester fabric at the reciprocating fold is caused by the excessive pH value, the hydrolysis or dissociation of the disperse dye, and the combined effect of the auxiliary agent on the thermal migration of the disperse dye.