Problems and Improvement Methods of Dyeing Color Fastness for Nylon Fabrics
September 10, 2025
The common problems in dyeing with acid dyes mainly include uneven dyeing, color change, yellowish shade, and insufficient levelness. The causes of these problems are not only poor dye quality but also improper process operation or inadequate handling of details. This article mainly takes the dyeing of nylon with acid dyes as an example to discuss the dyeing color fastness problem of acid dyes.
Influencing Factors of Nylon Dyeing Color Fastness
Nylon fiber is a type of synthetic fiber, and its molecular ends contain amino groups or carboxyl groups. Theoretically, it can be dyed with disperse dyes, reactive dyes, and acid dyes. However, reactive dyes have a low dye-uptake rate on nylon, making it difficult to obtain dark-colored varieties. Disperse dyes also have problems such as poor deep dyeing property, insufficient color brightness, and difficulty in controlling the shade. Therefore, in the actual dyeing and finishing process, acid dyes are usually used for dyeing nylon knitted elastic fabrics.
For acid dyes used in dyeing nylon fibers, due to the presence of anionic polar groups such as sulfonic acid groups and carboxylic acid groups in their molecular structures, they usually exist as water-soluble sodium salts in water. During dyeing, they ionize to form R-SO₃⁻ and R-COO⁻. The macromolecular chains of nylon fibers contain a large number of imino groups and amino groups, which form -NH₂⁺- and -NH₃⁺- under acidic conditions. These cationic groups in the fiber molecules can combine with the anionic groups of the dye molecules through ionic bonds and fix on the fabric, thus achieving high dyeing fastness.
At the same time, the affinity between the dye and the fiber macromolecules is relatively strong, and they can combine through intermolecular forces and other forces, which greatly increases the adsorption capacity of the dye, improves the color yield, and reduces the residual amount of the dye in the dye bath. However, nylon textiles often need to be washed during use, and household washing is generally carried out under alkaline conditions. The dye will fall off from the fabric due to ionization in the alkaline aqueous solution, and under the action of surfactants such as soap or washing powder, it will move away from the fabric and disperse in water. At the same time, it may also stain other fabrics, resulting in a decrease in various wet fastness properties.
Acid dyes refer to dyes that contain acidic groups in their structural formulas and can dye polyamide fibers and protein fibers in acidic, weakly acidic, and neutral dye baths. Most of their acidic groups are sulfonic acid groups, and a few are carboxylic acid groups. They are easily soluble in water and ionize in water to form dye anions. The molecular structure of acid dyes is relatively simple; most of them are monoazo dyes, and a few are disazo dyes. The dye molecules have fewer long conjugated double bond structures, and the coplanarity and linear characteristics of the molecular aromatic rings are not obvious. Their affinity for cellulose fibers is small, so the wet processing fastness and light fastness vary greatly depending on the variety. Among them, dyes with a relatively simple structure and a large number of sulfonic acid groups have relatively poor wet fastness. Therefore, after dyeing, fixation treatment is usually required.
Compared with the dyeing of cotton with direct dyes, the dyeing of nylon with weakly acidic dyes has an additional ionic bond combination. Since the number of terminal amino groups provided by nylon fibers that can react with weakly acidic dyes is relatively small, the dyeing of weakly acidic dyes relies on ionic bonds, hydrogen bonds, and van der Waals forces. Therefore, the color fastness of weakly acidic dyes should be better than that of direct dyes. However, the number of terminal amino groups in nylon fiber molecules is limited (only 1/12 of that of wool), resulting in poor color fastness for medium and dark colors. Moreover, the more water-soluble groups the dye has, the worse the color fastness. In order to improve the color fastness of weakly acidic dyes on nylon fibers, especially the wet rubbing fastness, fixation treatment is necessary, which is a particularly important process step for medium and dark color dyeing of nylon.
Influence of Dyeing and Finishing Process on Color Fastness
- After nylon is processed into fabrics, it has the defects of poor hygroscopicity and difficulty in draining sweat when worn. Therefore, chemical auxiliaries are used for post-finishing of nylon fabrics. Adding moisture-wicking agents, hydrophilic silicone oil emulsions, etc., can make the fabric have a certain moisture-wicking and softening effect. However, the used auxiliaries will have a certain impact on the fixation effect of the fixing agent, so more attention should be paid to the fixation treatment process of nylon.
- When nylon fabrics undergo pre-setting and final setting, changes in molecular structure and the content of fiber terminal amino groups will occur under different thermal conditions. This is the reason why the deep dyeing effect of nylon fabrics on dyes increases after pre-setting. After the wet heat setting process such as water-setting or pre-treatment oil removal, the structure of nylon fibers becomes loose, the dye adsorption rate increases, and the color yield also improves. These changes may affect the excessive adsorption of dyes and have an impact on color fastness.
- The temperature, heat preservation time, adjustment of dye bath pH, and addition of dyeing auxiliaries during the dyeing process can effectively control the dye-uptake rate and levelness of dyeing. However, changes in external conditions will also have a certain impact on the color fastness of nylon.
- For the selection of dyes, when choosing weakly acidic dyes, the first consideration is the dye manufacturer and brand. Even for the same type of weakly acidic dyes, their own dyeing fastness is different, which is determined by the production and processing costs. The quality of the dye itself is also a major factor directly affecting the color fastness of textiles after dyeing. If the problem lies in the dye itself, no matter how good the fixing agent is selected in the subsequent process, the color fastness cannot be improved.
Fixation of Nylon Fabric Dyeing
In terms of improving the color fastness of weakly acidic dyes on nylon, efforts are basically focused on the improvement of dye structure and acid fixing agents. There is little change in the structure of weakly acidic dyes at present, so the current work mainly lies in the improvement of fixing agents and the coordination of the process in printing and dyeing factories. However, due to differences in the location, water source, equipment, and process of printing and dyeing factories, the fixation effects they achieve vary, and it cannot be generalized. Choosing a fixing agent suitable for the use conditions of the enterprise and designing a corresponding process are one of the details that printing and dyeing factories need to pay attention to.
Like all other textile materials, the color fastness treatment after dyeing basically follows two approaches:
- Remove the dye that is not fixed on the fiber;
- Block the channels through which the dye on the fiber can be redissolved. Removing the unfixed dye is the most practical method, but in general, this method cannot remove the dye completely, so a suitable fixing agent must be selected for fixation.
Mechanism of Action of Acid Fixing Agents
Acid fixing agents are generally high-molecular-weight aromatic sulfonic acid condensates. After fixation, they can improve water immersion fastness, seawater fastness, washing fastness, and perspiration fastness. The high-molecular-weight aromatic sulfonic acid condensates coat the fiber surface. The negative charge of the sulfonic acid groups on the surface has a repulsive force on the acid dyes inside the fiber, preventing the dye from leaching out. The coating effect also reduces the swelling of the fiber, reducing the chance of combination between the dye and water molecules, thereby improving the fastness.
Selection of Fixing Agents
After nylon is dyed, it is necessary to carry out a water washing step to remove surface floating dye before fixation. A relatively mild condition can be used for treatment to prevent color fading. In the conventional processing of dyeing and finishing enterprises, the nylon textiles that need to use acid fixing agents include swimsuit fabrics, underwear fabrics, lace fabrics, mesh fabrics, webbings, buckles, and cheese yarns. According to different processing methods, swimsuit and underwear fabrics are divided into piece-dyed fabrics, yarn-dyed fabrics, and printed fabrics. Lace fabrics are divided into warp-knitted fabrics and embroidered water-soluble fabrics according to different processing methods.
Advantageous Products of Acid Fixing Agents
Guangdong Qiantai Chemical Co., Ltd. has years of technical accumulation in high-end nylon chemical fiber dyeing and finishing technology. It has sufficient technical experience in dealing with dyeing/printing defects of high-quality nylon/polyester chemical fiber elastic fabrics, color fastness (including water immersion fastness, color bleeding fastness, etc.), and finishing of sports fabrics and functional fabrics.