Nylon fibers belong to the category of polycondensate fibers, with the main varieties being nylon 6 and nylon 66. As a type of thermoplastic fiber, temperature variations during the manufacturing and processing stages have a significant impact on the physical properties and dyeing performance of nylon. The unique fiber properties and processing procedures both influence the final level dyeing effect.

Factors Affecting the Level Dyeing Property of Nylon

1. Influence of Fabric and Fiber Structure

From the perspective of the entire dyeing and finishing process, the most direct factors leading to uneven dyeing of nylon are the uneven warp and weft density of the fabric, variations in yarn count, and differences in the production batches of nylon fibers used in the fabric. These factors can all affect the dyeing performance of nylon, resulting in uneven dyeing. From the fiber aspect, the chemical structure of nylon fiber is polyamide, and there is a certain amount of amino groups at the ends of the fiber molecular chains. The dyeing characteristics of nylon are affected by the chemical and physical differences of the fiber. These differences may arise during the fiber manufacturing or processing process, and they directly impact the dyeing performance of the dye.

Chemical differences stem from variations in the amino group content of the fiber. Such differences may occur during the spinning process, heat drawing process, or filament combining process. Differences in amino group content lead to variations in the dyeing rate, especially the final adsorption capacity of anionic dyes. These differences can only be minimized through careful control during the production process. According to New Dyeing & Printing tips, the physical structure differences in yarns mainly include two aspects: on one hand, they come from the overall physical differences of the yarns, including variations in yarn count, the number of fibers in the yarn, or the fineness of the fibers, as well as differences between the end crimps of single fibers or multiple fibers in the yarn; on the other hand, they result from the inhomogeneity of the supermolecular structure of nylon fibers generated during processing, such as differences in crystallinity, orientation, or uneven skin-core structure within the fibers. When fabrics made of nylon fibers with chemical or physical differences are dyed, a characteristic weft-wise color stripe is likely to occur.

2. Influence of Pretreatment Conditions

Inappropriate selection of conditions during the pretreatment of nylon fabrics is also an important factor causing uneven dyeing. Improper pretreatment of fabrics mainly includes: uneven treatment of the fabric, which leads to significant differences in the adsorption performance of the fibers and results in dyeing defects like mottling; rapid temperature rise during the pretreatment process using a jigger dyeing machine, which causes sudden shrinkage of nylon filaments; or uneven temperature during heat setting, both of which can lead to structural differences in nylon fibers, resulting in uneven dyeing during the dyeing process.

3. Influence of Dyeing Conditions

(1) Influence of Temperature

Nylon is a thermoplastic fiber. Therefore, the dyeing rate of the fiber is closely related to temperature, and the dyeing temperature must be higher than the glass transition temperature (35-50℃) of the fiber. Nylon fibers start to adsorb dyes at 40℃. As the temperature rises, the dyeing rate accelerates, and the dyeing process is basically completed at 100℃. Although dyeing can be basically finished at 100℃, continuing to increase the temperature is beneficial for the migration of dyes, thereby improving the level dyeing property. Some researchers believe that the dye-uptake rate of acid dyes on nylon fibers is related to temperature. When the temperature is higher than the glass transition temperature, the mobility of the macromolecular chains in the fiber increases, the fiber expands, allowing the dye to penetrate into the fiber and react with the cationic amino groups at the end. However, if the heating rate is not properly controlled, uneven dyeing is likely to occur.

(2) Influence of pH Value

During the dyeing of nylon fibers, when the pH value of the dye bath is relatively high, very little dye is absorbed onto the fiber. When the pH value of the dye bath drops to a certain level, the dye begins to be absorbed and quickly reaches saturation. Continuing to lower the pH value of the dye bath does not significantly increase the dye-uptake. However, when the pH value further drops to 3, the dye-uptake increases sharply, resulting in super-equivalent adsorption. When nylon fibers are dyed under conditions of very low pH value, they may undergo hydrolysis. Especially after super-equivalent adsorption occurs, the pH value inside the fiber is lower than that in the solution, which accelerates hydrolysis. After hydrolysis, more amino groups are generated, the accessibility of the fiber increases, and it can adsorb more dyes, making uneven dyeing more likely to occur. Therefore, according to actual conditions, appropriately increasing the pH value can reduce the occurrence of dyeing defects like mottling.

(3) Influence of Auxiliaries

Nylon fibers are mostly dyed with acid dyes, and the levelling agents for acid dyes mainly include three types: anionic, cationic, and non-ionic. Anionic levelling agents have a certain affinity for fibers in an acidic medium and compete with dye anions for dyeing sites on the fiber. Their affinity is lower than that of dye anions, but their diffusion rate is faster than that of dye anions. At the beginning of dyeing, anionic levelling agents compete with dyes for dyeing seats. Due to their fast diffusion rate, they dye the fiber before the dye anions and combine with the -NH₂ groups on the fiber. Subsequently, they are gradually replaced by dye anions with higher affinity for the fiber. This reduces the initial dyeing rate of the dye and achieves the levelling effect. According to New Dyeing & Printing tips, cationic and non-ionic levelling agents exert a levelling effect by combining with dye anions to reduce the effective concentration of dye anions and gradually releasing the dye anions during the dyeing process. In addition, factors such as dyeing time, dye compatibility, dye bath concentration, liquor ratio, and dye liquor circulation rate also affect the level dyeing effect of nylon during dyeing.

Approaches to Improve the Level Dyeing Property of Nylon

1. Selection of Dyes

Firstly, the correct selection of dyes is a key factor in improving the level dyeing property of nylon. There is a wide range of dyes that can be used to dye nylon fibers. Among the many types of dyes suitable for nylon fiber dyeing, disperse dyes, anionic dyes such as acid dyes, 1:2 metal complex dyes, and selected direct dyes are the most important.

Disperse dyes with a simple molecular structure, due to their non-ionic nature and low molecular weight, can cover the physical and chemical differences in nylon fibers, but these dyes can only achieve moderate color fastness. Disperse dyes with a more complex structure can provide better wet fastness when dyeing nylon fibers, but due to their high molecular weight, they can only cover chemical differences and not physical differences. Selected direct dyes bring excellent wet processing fastness to nylon dyeing and are economical for producing dark-colored products. Compared with disperse dyes, dyeing with the brightest direct dyes can obtain relatively brighter dyed products (with better shade than disperse dyes). According to New Dyeing & Printing tips, acid dyes are often used for nylon dyeing. Their greatest advantage is easy dyeing, enabling the achievement of deeper shades. The dyeing procedures and requirements are relatively simple, and the wet fastness of the dyed products is good, but their level dyeing property is worse than that of disperse dyes.

2. Selection of Presetting Process

To improve the level dyeing property, strict requirements must be imposed on the processing processes of nylon fibers or fabrics, such as pretreatment, dyeing, and heat setting. In actual production, tentering and presetting can be carried out before pretreatment, generally controlled at 140-150℃ for 30 seconds, with an appropriate amount of overfeed. In particular, nylon filament fabrics are prone to creasing during high-temperature processing such as dyeing, and once creases are formed, they are difficult to eliminate, leading to uneven dyeing and affecting the appearance. Therefore, setting the fabric before dyeing can fix the fabric shape, stabilize the size, and prevent defects during the entire processing process.

3. Selection of Dyeing Conditions

(1) Selection of pH Value

Proper control of the pH value during dyeing is also crucial for improving the level dyeing property. As mentioned earlier, the dyeing effect of nylon fibers is closely related to the pH value of the dye bath. Although a lower pH value is beneficial for dye adsorption, the faster the dyeing rate, the corresponding decrease in level dyeing property. Therefore, it is essential to properly control the pH value of the dye bath. Generally, when dyeing nylon with acid dyes, the pH value of the dye bath can be strongly acidic, weakly acidic, or neutral.

(2) Selection of Auxiliaries

The selection of dyeing auxiliaries is often crucial to the success of dyeing. According to research by dyeing and finishing experts, when dyeing nylon with acid dyes, anionic auxiliaries have the best levelling effect. Their function is to block the terminal amino groups, thereby controlling the initial dyeing.

(3) Selection of Temperature

The temperature of the dye bath is proportional to the dyeing rate of acid dyes. The higher the temperature, the faster the dyeing rate. Especially when the temperature reaches above 80℃, the dyeing rate of the dye accelerates significantly, making it highly prone to dyeing defects like mottling. To achieve the most uniform dyeing effect as much as possible, some experts suggest adopting a stepwise temperature-rising dyeing process when dyeing nylon. Dyeing starts at 40℃, then the temperature is gradually raised to boiling (approximately 45 minutes), followed by continued dyeing for 45 minutes. The dyeing rate is relatively slow at low temperatures, and when the temperature exceeds 80℃, the dyeing rate increases rapidly, so the temperature should be raised slowly. In addition, proper control of factors such as the dyeing liquor ratio, dye liquor circulation rate, and dye liquor concentration is also beneficial for improving the level dyeing effect of nylon.

(4) Selection of Other Conditions

Selection of the order of adding materials before dyeing. Since nylon can be dyed at 40℃ (close to room temperature), the temperature of the dye liquor should be as low as possible before adding the dye. There are also certain requirements for the order of adding materials before dyeing. Generally, the order of adding materials is as follows: after putting the fabric and water into the dyeing equipment, adjust the liquor ratio, add the levelling agent and mix evenly, and finally add the dye. It is also recommended to add the dye in multiple batches. Because the dyeing rate is the fastest when the dye is just added to the dye bath, do not rush to raise the temperature after adding the dye. Instead, keep the temperature at room temperature for 15 minutes, and then raise the temperature according to the process.

Influence of color correction. If the color is incorrect after dyeing and color correction is required, it is recommended to drain the dye liquor and dye again. Because after dyeing is completed, the dye liquor still has a certain temperature (usually around 60℃), and this temperature easily promotes the adsorption of the dye. Therefore, when adding supplementary materials for color correction, dyeing defects like mottling may occur.

Conclusion

In summary, the dyeing effect of nylon fibers is affected by many factors. Therefore, in actual operation, appropriate dyes, auxiliaries, presetting processes, and optimal dyeing conditions such as pH value, temperature, and time should be selected according to specific dyeing requirements. Only by comprehensively considering these factors can a good level dyeing effect be achieved.