Generally after undergoing a series of pretreatments, fabrics typically proceed to the dyeing process. The purpose of dyeing is to impart the desired colors onto the natural-colored fabric.
This process involves numerous factors, but the most fundamental principle is to use specific dyes and auxiliaries in water as a medium, under specific conditions (such as a certain temperature and PH value)
Polyester fiber
Polyester fibers are dyed using disperse dyes. Since polyester fibers themselves lack relative groups, the dyeing conditions are relatively higher demanding. Specifically, high temperatures are required to activate the polyester fiber molecular chains, increasing the internal space of fiber. This allows fine dye particles to penetrate the fiber and bond covalently, thus fixing the dye.
This process requires sufficient energy to activate the polyester fiber molecular chains; the temperature needs to reach 130℃. Simultaneously, glacial acetic acid, softeners, leveling agents, and dispersing agents must be added to the dye bath to coordinate the entire process.
- The function of glacial acetic acid is to adjust the PH value (acidity/alkalinity) of the dye bath to a slightly acidic level, thereby increasing the solubility and penetration of disperse dyes and making dyeing easier.
- The function of softener in the dye bath is to soften the fabric during the dyeing process, preventing fabric defects such as creases and "chicken claw" effects.
- The function of the leveling agent is to adjust the distribution balance between the dye and the fabric during the dyeing process to achieve a level dyeing effect.
- The function of the dispersing agent is to help disperse dyes disperse evenly in the dye bath, preventing the dispersion of disperse dyes and the formation of color spots.
When dyeing polyester fibers, the sublimation fastness of disperse dyes must be considered. Sublimation refers to the process where a substance transitions directly from a solid to a gas under heat. Since polyester fibers and dyes lack reactive groups for fixation, high temperatures during setting cause dye sublimation, resulting in color fading. Therefore, when dyeing polyester with disperse dyes, sublimation fastness must be considered. Dye selection generally depends on the final requirements.
Since dyeing occurs under high-temperature conditions and requires significant energy to induce movement in polyester fiber chains, the wash fastness of dyed polyester is exceptionally high and not a concern. However, for deep colors like ultra-black, the substantial dye quantity prevents complete penetration and fixation within the fiber interior. resulting in excess dye adsorbed on the fiber surface. If not thoroughly removed, this residual dye will compromise wash fastness. Therefore, soap washing and rinsing must be performed after dyeing.
Soap washing refers to the process of using detergent and soda ash at a specific temperature to remove dyes adsorbed onto the fiber surface, followed by thorough rinsing to ensure the fabric's wash fastness.
Nylon fiber
The dyeing of nylon fibers primarily employs acid dyes and certain disperse dyes. Dispersed dyes are mainly used for lighter shades, while acid dyes see the widest application. Acid dyes are categorized into: - Uniform-dyeing acid dyes (strong acid dyes); - Semi-uniform-dyeing acid dyes (weak acid dyes); and neutral dyes.
The selection of dyes is based on color depth:
1. Light colors: Disperse dyes and evenly-dyeing acid dyes;
2. Medium-dark colors: Semi-evenly-dyeing acid dyes;
3. Dark shades: Neutral dyes;
For applications requiring exceptionally high wash fastness, metal-complex dyes are employed (rarely used). The principle of dyeing nylon with acid dyes involves covalent bonding and van der Waals forces between the dye and fiber in an acidic bath. After fixation, the dye is anchored to the fiber to impart color. Dyeing conditions include specific temperature, pH, and leveling agents.
The following describes the processes for different dyes:
1. Equal-dyeing acid dyes: Also known as strong acid dyes, these require dyeing under strongly acidic conditions, with glacial acetic acid added to promote dyeing. They feature excellent leveling but poor wash fastness. Therefore, fixation treatment is essential to achieve adequate fastness.
2. Semi-leveling acid dyes: Also known as weak acid dyes, these require dyeing under weakly acidic conditions. Since these dyes exhibit acceptable fastness after fiber binding, acetic acid usage must be carefully controlled. The addition of leveling agents is also crucial to prevent uneven dye uptake and color streaks caused by excessive dye penetration speed.
3. Neutral dyes: These require dyeing under near-neutral conditions. Since these dyes bond firmly to fibers and are difficult to remove, acetic acid usage is minimal or unnecessary. The addition of leveling agents becomes particularly crucial, as they serve to slow the dyeing process.
Except for disperse dyes, which generally do not require fixation treatment, acid dyeing of nylon fibers typically necessitates the use of a fixing agent to enhance wash fastness. Overall, dyeing nylon fibers involves considering the combination of acid dyes and their wash fastness. Process control is achieved by employing different methods based on the specific dye used.
For acid-dyed nylon fibers, a critical issue is the impact of water quality, particularly the presence of heavy metal ions such as Fe and Mn. These ions cause color shifts-specifically, a darkening effect.
Therefore, water used for acid dyeing nylon must undergo ion exchange. This process involves passing the water through an ion exchanger where Na ions replace heavy metal ions like Fe and Mn.
Since the dyeability of nylon fibers is significantly affected by heat, uneven heating during pretreatment can compromise dyeing uniformity. This factor is particularly critical in the dyeing of these fibers. As all dyeing processes described above occur within dyeing vats, it is necessary to provide a general explanation of vat structure and operation, primarily using high-temperature, high-pressure overflow dyeing machines as an example.
