Because the amino groups and carboxyl groups at both ends of the macromolecules of nylon fibers are sensitive to light, heat and oxygen, especially the amino groups, the number of groups will decrease during the process of oxidation and thermal cracking. Nylon fibers also turn yellow at the same time. In addition, the various oils used in the fiber spinning and weaving process of nylon fabrics may also change their properties under the action of light and oxidation, resulting in oil stains that are difficult to remove on the surface of the fabric, which will also cause "white stains" , leukoplakia" phenomenon. Therefore, it is required that nylon fabrics should be packaged with opaque packaging materials after weaving.
Dyes that can be used for nylon dyeing include acid (weak acid) dyes, disperse dyes, reactive dyes, direct dyes, and the like. Although reactive dyes have the advantages of bright color, good light fastness and no heavy metal ions, their dye depth and cover dyeing properties are poor. Disperse dyes can only dye light colors, so the dyes currently used for nylon dyeing are still mainly acid or weak acid dyes.
The isoelectric point of nylon fiber is pH 5-6. When the pH value of the solution is lower than the isoelectric point, the nylon fiber is positively charged, and the dye is combined with the fiber in the form of ionic bonds, and the dye is mainly fixed on the terminal amino group of the fiber; When the pH value of the solution is higher than the isoelectric point, the nylon fiber is negatively charged, and the dye can only dye the nylon fiber by the combination of intermolecular attractive force such as van der Waals force or hydrogen bond.
In the dyeing of nylon fabrics, problems such as "willows" and "crossbars" are often prone to occur. The following is an analysis of various influencing factors in the dyeing process.
1. The effect of pH value of dye solution on dyeing
The industry has always believed that the control of the pH value of nylon dyeing is the key to success or failure. After years of production accumulation, we found that if the use of buffer system auxiliaries and the selection of good leveling agents in dyeing, it will play a decisive role in the success or failure of nylon dyeing. .
When acid (or weakly acidic) dyes are dyed, the pH value of the dye solution tends to increase as the dyeing proceeds. For dyes with low affinity and sensitive to the pH value of the dye bath, the pH value of dyeing should be controlled at about 5, so that the use of buffer system auxiliaries can achieve a multiplier effect on the pH value control of nylon dyeing. Stabilizing the pH of the dye solution is one of the key factors to achieve uniform dyeing.