We know that the practical performance of dyes is the key factor that determines the color fastness. However, the same dyes dyed using different processes will have different color fastnesses. 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 Dyeing cotton
As we all know, the dyeing mechanism of reactive dyes for cotton dyeing 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. Reasons for poor color fastness
Production practice shows that reactive dyes dye cotton , the phenomenon of poor color fastness occurs, improper selection of dyes 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 the presence of these floating dyes (including dyes that are partially or completely hydrolyzed and dyes that are not hydrolyzed but not bonded to cotton fibers) on cotton fibers is not only the The root cause of poor rubbing fastness, soaping fastness and water immersion fastness is also an important factor affecting poor light fastness.
Research shows that the presence of reactive dyes on cotton fibers is different, and the performance of light fastness is also different. Dyes that are covalently bonded with cotton fibers have relatively better light fastness; they lose reaction. The light fastness of hydrolyzed dyes with low capacity is relatively poor; while the light fastness of dyes that are not hydrolyzed and not bonded to cotton fibers is relatively poor. 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 when the commonly used vinyl sulfone reactive dyes (KN type) and mixed dual reactive group reactive dyes (M type, ME type, A type, B type, etc.) are dipped to a depth of 1% o.w.f. Before washing, 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 remove the floating dyes on cotton fibers. It is not objective to completely remove color dye 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 fiber 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.
2. Issues that should be paid attention to in each process to improve color fastness
Production practice shows that, 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:
2.1 The gross efficiency of semi-finished products should be high
Everyone knows that 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.
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 relative dye fastness. The better.
As we all know, during dyeing, dyes can only enter the amorphous region of the fiber, and dyes in the crystalline region are inaccessible. The amorphous area of natural cotton fiber is generally only about 30%. Obviously, the color absorption capacity of cotton fiber for dyes is very low.Under high temperature conditions, it still has a good dispersion and stabilizing effect on dye particles. 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 has shown that anionic surfactants and nonionic surfactants can exert a synergistic effect when used in a certain proportion and have good practical effects.
3. Be careful with the color fixation process
When dyeing nylon with disperse dyes, it only dyes light colors. Therefore, 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 using weak acid dyes to dye nylon, the process of dyeing cotton with reactive dyes first and then dyeing nylon with acid dyes is usually used (because weak acid dyes are used in The color fastness of nylon is poor and cannot withstand the alkaline bath treatment and high temperature soaping treatment when dyeing cotton).
After nylon dyeing, if the color fixation is performed, the fixing agent must be cross-linked with acid dyes and reactive dyes at the same time. The results are often:
1. Not only the nylon color light has The color and light of the cotton fiber will also undergo some changes, which may even cause the overall color and light of the dyed product to be inconsistent
2. The overall light fastness of the dyed product often has a downward trend;
3. Once the color of the dyed material does not match, it will be difficult to correct the color of the dye. Therefore, when dyeing cotton and nylon fabrics with weakly acidic dyes, care must be taken in fixing the color, especially when making export orders. </p
