The effect of acid soap detergent on improving the color fastness of fabrics in the washing process after printing and dyeing is closely related to its chemical properties, washing mechanism and interaction with dyes. This type of detergent significantly improves the color fastness performance without damaging the fabric fibers through multiple ways such as adjusting the pH of the washing environment, enhancing the stability of dye fixation, and removing floating colors, becoming a key auxiliary agent in the post-printing and dyeing treatment process.
The pH value regulation of acid soap detergent is the basic mechanism for improving color fastness. During the printing and dyeing process, some dyes (such as reactive dyes and acid dyes) need to complete the fixation reaction under a specific acid-base environment, and the acidic conditions in the washing process can inhibit the occurrence of the reverse reaction of dye hydrolysis. For example, reactive dyes covalently bond with fibers under alkaline conditions, but if the pH value is too high during washing, the fixed dyes may break due to alkaline hydrolysis, resulting in fading. Acid soap detergent reduces the pH value of the washing liquid by releasing hydrogen ions, so that the system is maintained in the weak acid range (usually pH 4~6), thereby stabilizing the covalent bond or ionic bond between the dye and the fiber and reducing the problem of color fastness degradation caused by chemical environment fluctuations.
The surfactant components it contains play a dual role in improving color fastness. On the one hand, anionic or nonionic surfactants reduce the surface tension of water, enhance the penetration of detergent into fabric fibers, and promote the effective dispersion and emulsification of floating color particles adsorbed on the fiber surface. If these floating colors are not completely removed, they are prone to migration or shedding during subsequent friction, washing, etc., resulting in reduced color fastness. On the other hand, the dye-philic groups of surfactant molecules can form weak hydrogen bonds or van der Waals forces with dye molecules, stably encapsulating the dye particles detached from the fiber in micelles, preventing them from reattaching to the fabric surface to form color stains, thereby reducing the negative impact of floating colors on color fastness.
The chelation effect of acid soap detergent on metal ions is also an important factor in improving color fastness. Printing and dyeing water often contains metal ions such as calcium, magnesium, and iron. These ions may combine with dye molecules to form precipitation, or catalyze the redox reaction of dyes, resulting in color changes or fading. Chelating agents (such as aminocarboxylic acids and hydroxycarboxylic acid compounds) in detergents can form stable complexes with metal ions, reduce their free concentration in the washing liquid, and avoid the damage of metal ions to the dye structure. For example, iron ions are easily fixed by chelating agents under acidic conditions, thereby inhibiting their catalytic decomposition of the azo structure of reactive dyes and maintaining the stability of fabric color.
In terms of removing hydrolyzed dyes and unfixed dyes, acid soap detergent shows unique advantages. During the printing and dyeing process, some dyes exist in a hydrolyzed state or a physically adsorbed state because they fail to fully react with the fiber. These dyes have weak binding force with the fiber, which is the main reason for poor color fastness. The acidic environment can fully dissociate the water-soluble groups (such as sulfonic acid groups and carboxyl groups) of the hydrolyzed dyes, enhance their solubility in the washing liquid, and the dispersing effect of the surfactant further washes these dye particles out of the fiber pores. Through repeated washing and soap boiling, acid soap detergent can effectively reduce the residual hydrolyzed dyes and unfixed dyes on the fabric, reduce the risk of dye shedding during subsequent use, and thus improve the color fastness to washing and rubbing.
The adjustment of the surface charge state of the fiber under acidic conditions also indirectly helps to improve color fastness. Most textile fibers (such as cotton, wool, and silk) have negative charges on the surface under neutral or alkaline conditions, and dye molecules (especially anionic dyes) also have negative charges. The electrostatic repulsion between the two may cause the dye to be adsorbed loosely. In an acidic environment, the negative charge density on the fiber surface decreases, and the electrostatic repulsion between the dye molecules is weakened. At the same time, the fiber molecular segments expand to a certain extent due to protonation, making it easier for the dye molecules to embed into the fiber and form a stable bond. This adjustment of the charge state is not only conducive to the uniform dyeing of the dye during printing and dyeing, but also reduces the phenomenon of dye shedding due to electrostatic repulsion during washing, further consolidating the color fastness.
In practical applications, the process parameters of acid soap detergent are crucial to the effect of improving color fastness. Washing temperature, time, detergent concentration and bath ratio and other factors need to be comprehensively regulated: under low temperature (such as 40~50℃), the surface activity of detergent is low and the ability to remove floating color is limited; while too high temperature (such as over 90℃) may cause excessive fiber expansion, and the fixed dye molecules will be separated from the fiber due to the intensified thermal motion. It is usually recommended to perform soap boiling in the range of 60~80℃, at which time the activity of detergent and the expansion degree of fiber are balanced, which can effectively remove floating color without damaging the combination of dye and fiber. In addition, the detergent concentration needs to be adjusted according to the dyeing depth of the fabric and the type of dye. If the concentration is too low, it cannot fully exert the decontamination and chelation effects, and if the concentration is too high, it may cause surfactant residues, affecting the feel of the fabric and subsequent processing.
While improving color fastness, acid soap detergent also needs to take into account the protection of fabric performance. Excessive acidic conditions may damage protein fibers (such as wool and silk), causing yellowing and decreased strength of the fibers. Therefore, in practical applications, the acidity of the washing liquid needs to be controlled within a safe range through a pH buffer system, and sufficient water washing should be carried out after washing to remove residual detergent. In addition, for sensitive colors (such as bright red and blue) or special fibers (such as regenerated cellulose fibers), small sample tests are required in advance to optimize detergent formulas and process parameters to ensure a balance between color fastness improvement and fabric quality.
In short, acid soap detergent systematically improves the color fastness of printed and dyed fabrics through multiple mechanisms such as pH regulation, surfactant action, metal ion chelation, floating color removal, and fiber charge adjustment. Its scientific application in the washing process can not only meet the quality requirements of color stability of textiles, but also achieve dual optimization of environmental protection and economic efficiency by reducing dye waste and washing times, becoming an indispensable key technical link in modern textile printing and dyeing processing.
