Determination of process
Production practice tells us that the practical performance of dyes determines the quality of dyeing The key factor in fastness. However, the same dyes are dyed using different processes, and their dye fastnesses are also different. Therefore, in order to improve the dyeing fastness of cotton and nylon fabrics, in addition to the correct selection of dyes, the dyeing process must also be formulated correctly.
1.1 Dyeing cotton
As we all know, reactive dyes dye cotton. The dyeing mechanism is different from other dyes. As a result of dyeing, the dye on the cotton fiber is not physically dyed on the cotton fiber by hydrogen bonds and van der Waals forces, but is chemically combined with the cotton fiber in the form of covalent bonds. After dyeing, the dye molecules become cellulose. part of a molecular chain. Therefore, theoretically, the covalent bond between dyes and cotton fibers can give dyed goods excellent dye fastness.
But in fact, during the testing, use, washing and even storage processes of dyed goods, fading, discoloration or staining often occurs, especially the wet rubbing fastness and soaping fastness when dyeing dark colors. The light fastness when dyeing light colors is often unsatisfactory.
1.1.1 Reasons for poor color fastness
Production Practice shows that when cotton is dyed with reactive dyes, poor color fastness occurs. Improper dye selection is the first factor; improper dyeing process is the second factor.
As we all know, reactive dyes exist on cotton fibers in two ways: chemical combination and physical adsorption. Dyes that are chemically bonded to cotton fibers have good dyeing fastness and are not easy to fall off. The dye (floating color) that is physically adsorbed inside and outside the cotton fiber has a weak bond with the cotton fiber. When it is rubbed, soaped or soaked in hot water, it will partially fall off, showing low wet fastness.
Practice has shown that these floating dyes (including partially hydrolyzed or completely hydrolyzed dyes and dyes that are not hydrolyzed but not bonded to cotton fibers), on cotton fibers It is not only the source of poor rubbing fastness, soaping fastness and water immersion fastness of cotton fiber, but also an important influencing factor of poor light fastness.
Studies have shown that the presence of reactive dyes on cotton fibers is different, and the performance of light fastness is also different. The light fastness of dyes that are covalently bonded to cotton fibers is Relatively good; hydrolyzed dyes that have lost their ability to react have relatively poor light fastness; dyes that are not hydrolyzed and not bonded to cotton fibers have relatively poor light fastness. However, the actual dyeing situation is that the maximum color absorption rate during dip dyeing with reactive dyes is always higher than the maximum color fixation rate. The difference between the maximum color absorption rate and the maximum color fixation rate is the floating dye adsorbed by the cotton fiber but not bonded to the cotton fiber.
Research shows that vinyl sulfone reactive dyes (KN type) and reactive dyes with mixed double reactive groups (M type, ME type, A type, B type, etc.) are commonly used for dip dyeing 1 At the %o.w.f depth, before soaping, the floating dye remaining on the cotton fiber is about 7% to 30% of the maximum color absorption capacity. For example, reactive black KN-B is about 7%, reactive brilliant blue KN-R is about 20%, and reactive emerald blue M-GB is about 30%.
It is worth noting that these floating dyes have two sides in terms of washing performance. First, they have different degrees of affinity for cotton fibers. Therefore, in actual production, it is necessary to It is not objective to completely remove the floating dye on cotton fibers through washing and soaping. Furthermore, after water flow, soaping, drying and finishing, new floating dyes will be produced due to the breakage of the dye-fiber bond. Therefore, when it comes to cotton dyeing with reactive dyes, the existence of floating dyes is objective. Phenomenon. Second, these floating dyes also have good water solubility. When the dyed goods are washed, especially under the conditions of color fastness testing, part of the floating dye will dissolve and then contaminate the test white cloth, resulting in poor fastness.
Obviously, reducing the amount of floating dyes physically adsorbed on the fibers is the key to improving the color fastness of cotton dyed with reactive dyes. There are only two ways to reduce floating dyes. One is to improve the fixation rate of dyes and reduce floating dyes as much as possible. The second is to strengthen washing and soaping to remove floating dyes as much as possible.
1.1.2 Issues that should be paid attention to in each process to improve color fastness
Production practice shows , To achieve this goal, it is not enough to just strengthen the dyeing process. Corresponding measures must be taken in every aspect of the entire processing process. For this reason, the following issues must be paid attention to when formulating the processing technology:
1.1.2.1 The gross efficiency of semi-finished products should be high
Everyone As we all know, the wool effect of dyed semi-finished products directly affects the dyeing effect. The better the dyeing is, the less dye is attached to the surface of the fiber, the greater the chance that the dye that diffuses into the fiber will be bonded and fixed to the fiber, the higher the fixation rate, and the better the dye fastness will naturally be. Therefore, in the actual production of cotton nylon fabrics, whether it is bleached by a jet stream machine, a jigger dyeing machine, or a cold rolling stack, the semi-finished products must have uniform and transparent effects. This is A principle.
1.1.2.2 The color absorption capacity of semi-finished products should be large
Practice shows that cotton fiber The greater the color absorption capacity (color absorption capacity), the higher the accessibility of the dye inside the fiber, the more complete the bonding reaction with the fiber, the less floating dye attached to the inside and outside of the fiber, and the relatively stronger the dye fastness. good.
As we all know, when dyeing, dyes can only enter the amorphous area of the fiber.�Anionic active agents commonly used in disperse dye exhaust dyeing, such as dispersants N, MF, CNF, etc., will coat disperse dye particles in water to form dispersant-dye colloidal particles. Because the outer layer of these colloidal particles is negatively charged, they repel each other and will not aggregate due to mutual collisions, which can effectively improve the dispersion stability of the dye liquor. However, the binding energy between these dispersants and dye particles is weak. As the dyeing temperature increases and the thermal movement of the dye liquor intensifies, the dispersant-dye colloid particles formed in the initial stage will gradually break, and the dye liquor will Dispersion stability gradually decreases.
Practice shows that this type of dispersant has better dispersion effect when used below 70°C and is not suitable for high-temperature exhaust dyeing. Sodium lignosulfonate dispersants such as CMN have relatively strong binding force with disperse dyes in water. The resulting dispersant-dye colloidal particles have good heat resistance and stability. Under boiling temperature conditions, the dye particles are Still has good dispersion and stabilizing effect. The main disadvantage of these anionic dispersants (diffusing agents) for disperse dye dyeing of nylon is the lack of promotion of dye migration. Nonionic surfactants, like anionic surfactants, can also coat disperse dye particles in water to form colloidal particles.
Practice shows that anionic surfactants and nonionic surfactants can exert a synergistic effect when used in a certain proportion and have good practical effects.
1.2.3 Careful fixation treatment
Disperse dyes are used to dye nylon, only light colors , so as long as the dye is properly selected, there is usually no problem with its light fastness and wet fastness. Nylon dyed with neutral dyes not only has good light fastness, but also has good wet fastness. Therefore, even if you dye medium to dark colors, there is no need to fix the color. When acidic dyes are used to dye nylon in dark colors, such as red, maroon, orange, brilliant purple, sapphire, etc., the color fastness to water immersion, soaping, wet ironing, wet rubbing, etc. is often poor. If necessary, it needs to be treated with color fixing agent or cross-linking agent, which is effective in improving wet fastness.
However, one thing should be noted: for cotton and nylon fabrics, when nylon is dyed with weak acid dyes, the process of dyeing cotton with reactive dyes first and then dyeing nylon with acid dyes is usually used. (Because the color fastness of weakly acidic dyes on nylon is poor, it cannot withstand the alkaline bath treatment and high-temperature soaping treatment when dyeing cotton).
After nylon dyeing, if the color-fixing treatment is performed, the fixing agent must be cross-linked with acid dyes and reactive dyes at the same time. The result is often:
“(1) Not only does the shade of nylon change, but the shade of cotton fiber also changes, which may even cause the overall shade of the dye to be inconsistent; (2) The overall light fastness of dyed goods often has a downward trend; (3) Dyeing Once the color and light of the object do not match, it will be difficult to modify the color of the dye. Therefore, when dyeing cotton and nylon fabrics with weak acid dyes, you must be cautious in fixing the color, especially when making export orders.
</p
