Analysis of Auxiliaries for Synthetic Fiber Dyeing and Printing

In the dyeing and printing of synthetic fibers, surfactants are mainly used as leveling agents, retarding agents and dispersants.

The molecular structure of polyester fiber is very dense, so auxiliaries are needed to relax and expand the space between fiber molecules, so as to facilitate the diffusion of dyes. Generally, non-ionic surfactants have good migration properties for disperse dyes, but poor dispersibility, which easily causes agglomeration of dye particles. Anionic surfactants have good dispersibility, but poor migration properties. Therefore, most of the rapid leveling agents used in practice are blends of non-ionic and anionic surfactants.

The leveling effect involves the uniform distribution of dyes between fibers and within fibers. The role of surfactants can be explained by the following two mechanisms, whose basic principle is to delay the adsorption of dyes and slow down the dyeing rate: 1) Competition between dyes and surfactants for dyeing sites on the fiber surface. 2) Interaction between dyes and surfactants in the dye bath.

Usually, surfactants as dispersants are fully mixed with dyes during the dye processing, or dispersants can be added to the dye bath. Dispersants play two main special roles in dyeing: one is the deflocculation effect of breaking up agglomerated particles; the other is the ability to maintain the stability of dispersed particles.

In the high-temperature dyeing of polyester fibers with disperse dyes, adding anionic surfactants as dispersants can increase the solubility of dyes by several times due to the solubilization effect. Dispersants can not only prevent the growth of dye particles (inhibit crystal growth) and maintain the dispersibility of disperse dyes under high temperature and pressure, but also help dyes diffuse into polyester fibers and prevent dye coking. In addition, dispersants have the characteristics of no inhibition on final dyeing, little dye discoloration, low foaming, as well as de facto dye retarding and leveling properties, and do not affect the fastness of dyeing. When dyeing polyester fibers with disperse dyes, surfactants need to be added to improve the stability of disperse dyes in the dye bath. When dyeing at 130℃, anionic surfactants with polycyclic structures are superior to those with fewer rings in terms of affinity for disperse dyes. However, excessive use of surfactants will inhibit dyeing and even decompose azo disperse dyes. Non-ionic surfactants can improve the solubility of disperse dyes in water, facilitate dye migration, and enhance the leveling and permeability of dyeing. For hardly dyeable dyes with high fastness and large relative molecular mass, they can increase the dyeing rate. However, in high-temperature dyeing above the cloud point, non-ionic surfactants synergize with dyes to deposit on the fiber, easily causing pollution such as spots and tar-like impurities. This phenomenon can be overcome by adding anionic surfactants to increase the cloud point, or by using modified non-ionic surfactants without cloud point and non-ionic surfactants with a large number of ethylene oxide addition molecules as leveling agents.

The main anionic surfactants used as dispersants include naphthalene sulfonate formaldehyde condensates, lignin sulfonates, cresol sulfonate formaldehyde condensates, alkylphenol ethylene oxide-propylene oxide block polyethers, nonylphenol polyoxyethylene ethers, N-methyl-N-(β-hydroxyethyl) laurylamide, etc. Surfactants can be used alone or in combination as dispersants.