Polyester and nylon fiber dyeing technology
After a series of pretreatments, fabrics typically undergo dyeing. The purpose of dyeing is to apply the desired colors to the natural fabric.
This process involves many factors, but the most basic principle is that specific dyes and auxiliaries, using water as a medium, under specific conditions (such as a certain temperature and pH value) for a certain period of time, allow the dye to migrate into the fiber, penetrate, and ultimately bind. The following example illustrates the entire dyeing process in detail.
Polyester (Polyester Fiber)
Polyester fiber dyeing uses disperse dyes. Because polyester fiber itself lacks reactive groups, the dyeing conditions are relatively demanding. High temperatures cause the polyester fiber molecular chains to move, increasing the internal space of the fiber, allowing tiny dye particles to penetrate and bind covalently.
This process requires sufficient energy to cause the polyester fiber molecular chains to move; the temperature needs to reach 130℃. Simultaneously, glacial acetic acid, softeners, leveling agents, and dispersing agents need to be added to the dye bath to coordinate the entire process.
1. 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.
2. The function of the softener in the dye bath is to soften the fabric during the dyeing process, preventing fabric defects such as creases and "chicken claw" marks.
3. 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.
4. 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.
In dyeing polyester fibers, the sublimation fastness of disperse dyes must be considered. Sublimation is the process of a solid directly turning into a gas under heat. Since polyester fibers and dyes do not have reactive groups fixed, the dye will sublimate during the setting process due to high temperatures, resulting in a lighter color. Therefore, the sublimation fastness of disperse dyes must be considered when dyeing polyester. Generally, the dye is selected according to the final requirements.
Because dyeing occurs at high temperatures, and the movement of polyester fiber molecules requires significant energy, the wash fastness of dyed polyester is quite high and not a major concern. However, for dark colors like extra black, due to the large amount of dye used, it cannot completely penetrate and fix into the fiber's interior. Excess dye will adhere to the fiber surface after dyeing. If not thoroughly removed, this will affect the wash fastness. Therefore, soaping and washing are necessary after dyeing.
Soaping involves using detergent and soda ash at a specific temperature to wash away the dye adsorbed on the fiber surface, followed by thorough washing to ensure the fabric's wash fastness.
Nylon Fiber:
Nylon fiber dyeing primarily uses acid dyes and some disperse dyes. Disperse dyes are mainly used for lighter colors, while acid dyes are the most commonly used. Acid dyes are categorized into leveling acid dyes (strong acid dyes), semi-leveling acid dyes (weak acid dyes), and neutral dyes.
The selection of dyes is based on the depth of color:
1. Light colors: disperse dyes and leveling acid dyes;
2. Medium to dark colors: semi-leveling acid dyes;
3. Dark colors: neutral dyes; for colors requiring particularly high wash fastness, metal complex dyes are used (rarely applied). The principle of acid dyes for nylon is that the dye and fiber form covalent bonds and van der Waals forces in an acidic bath, and after fixation, the dye is fixed onto the fiber. Dyeing conditions require specific temperature, pH value, and leveling agent.
The following describes the process for different dyes:
1. Leveling acid dyes: also known as strongly acidic dyes, these require dyeing under strongly acidic conditions, with glacial acetic acid added to promote dyeing. They are characterized by excellent leveling properties but poor wash fastness. Therefore, fixation treatment is necessary to achieve a certain level of fastness.
2. Semi-leveling acid dyes: also known as weakly acidic dyes, these require dyeing under weakly acidic conditions. Because these dyes have relatively good fastness after bonding with fibers, the amount of glacial acetic acid used needs to be carefully controlled. The addition of a leveling agent is also crucial to prevent excessively rapid dyeing, which can lead to color differences and uneven coloring.
3. Neutral dyes: Dyeing requires near-neutral conditions. Since these dyes are not easily re-detached after bonding with fibers, the amount of glacial acetic acid used is minimal or even unnecessary, and the addition of a leveling agent is particularly important. The leveling agent acts as a retarder.
Except for disperse dyes, which generally do not require fixing treatment, acid dyes used to dye nylon fibers usually require fixing with a fixing agent to improve wash fastness. In general, the dyeing of nylon fibers requires consideration of the combination of acid dyes and their wash fastness. Process control varies depending on the dye used.
For acid dyed nylon fibers, a significant issue is the influence of water quality, especially heavy metal ions such as Fe and Mn, which can cause changes in color hue—darkening.
Therefore, the water used for dyeing nylon with acid dyes needs to undergo ion exchange, which is the process of Na ions replacing some heavy metal ions such as Fe and Mn after passing through an ion exchanger.
Since the dyeability of nylon fibers is greatly affected by heat, the unevenness of heating during pretreatment will affect the uniformity of dyeing. This is particularly important in the dyeing of nylon fibers. The dyeing process described above all occurs in a dyeing vat; therefore, it is necessary to provide a general explanation of the structure and operation of the dyeing vat, mainly using a high-temperature, high-pressure overflow dyeing machine as an example.