Reactive wet rubbing fastness issues
Reactive dyes dye cellulose fibers. The dyeing mechanism is different from other dyes. As a result, the dyes on the fibers are not physically dyed on the fibers by hydrogen bonds, ionic bonds or van der Waals forces. Instead, it chemically combines with the fiber in the form of covalent bonds. After dyeing, the dye molecules become part of the macromolecular bonds of cellulose. Therefore, theoretically, the covalent bond between the dye and the fiber can give the dyed goods excellent dye fastness. However, in actual production, fading, discoloration or staining often occur during testing, washing, and even storage. Especially when dyeing deep colors (such as black, navy blue, brown, soap red, etc.), the wet rubbing fastness is usually only Level 2, and often does not reach Level 2-3 or Level 3.
Reasons for poor wet rubbing fastness
How to improve the wet rubbing fastness of dyed products to meet market demand?
The so-called wet rubbing fastness of dyed cloth refers to using wet white cloth to rub the dyed cloth under specified conditions, and then according to the wet rubbing fastness of the white cloth. The degree of staining is rated using the standard gray staining sample card.
Obviously, there are two reasons for poor wet rubbing fastness:
1. The floating dye on the surface of the fabric (or fiber) transfers from the white dyed cloth to the wet white cloth during the friction process.
2. The micro-crumbs of colored fibers on the surface of the dyed cloth are transferred from the white dyed cloth to the wet white cloth during the friction process.
It can be seen that the amount of floating dye and the amount of colored fiber micro-crumbs on the surface of the fabric (or fiber) are the decisive factors that determine the wet rubbing fastness. There are many factors that affect the amount of floating dye and the amount of colored fiber microfibers. It is related to the performance of the dye and the dyeing and finishing process.
To improve the wet rubbing fastness of reactive dyes, we must start from the selection of dyes, The formulation of the process and other aspects must be studied, and corresponding technical measures must be implemented in a targeted manner.
Selection of dyes
Reactive dyes with different structures have different dyeing properties. In terms of dyeing deep and intense colors, the key conditions that must be considered when choosing dyes should be:
1. The solubility should be high
Because a large amount of dye is used to dye deep and deep colors (usually >5% o.w.f), if the dye is poorly dissolved, it will not only affect the leveling and deep dyeing effects, but also This results in an increase in floating dyes and a decrease in color fastness.
2. The fixation rate should be high
Because the higher the fixation rate of the dye , the darker the dye, the better. The lower the color floating rate (the E value of the color absorption rate – the F value of the color fixation rate) can be.
3. The directness should be moderate
This is because the greater the directness, the less The worse the washability, the more difficult it is to remove the floating dye that remains in the physical state inside and outside the fabric (or fiber) after dyeing. Therefore, the directness of the dye should not be too high. However, the directness of the dye should not be too low. Otherwise, during dip dyeing, the color absorption stage (primary color absorption) will be too slow and too little, resulting in the fixation stage (secondary color absorption) color absorption being too fast and excessive. Too much will not only affect the leveling and dyeing effect, but also increase floating dyes and reduce various wet fastnesses.
Development of process
1. To improve the quality of singeing
Due to the smoothness of the fabric surface The worse it is, the greater the actual contact area with the test white cloth, the greater the friction coefficient, and the greater the transfer of floating dye and fiber particles on the surface of the fabric (fiber) to the test white cloth, so the friction fastness is lower. Relatively worse. For example, under all conditions being equal, the wet rubbing fastness of dyed fabrics is better than plain weave fabrics and better than twill weave fabrics.
So the fabric must be singed cleanly. For this reason, the traditional pre-singing process should be changed to desizing and cooking and then singeing. Because the gray cloth is slurry before deboiling, the hairiness is inverted and sticky, making it difficult to burn. (The current slurries are mainly chemical pulps such as polyvinyl alcohol, polyacrylamide, etc., and singeing before deboiling will cause the chemical slurry to further polymerize and age, and the water solubility will decrease, making subsequent desizing difficult). For cotton fabrics, it is best to use an electric roller contact friction singeing machine to singe. The singeing effect is more uniform and smoother than that of a gas singeing machine. For orders with high requirements, if necessary, use cellulase to “polish” before dyeing. This can not only effectively improve the wet rubbing fastness, but also improve the brightness of the color.
2. To improve the quality of mercerization
The crystallization area of natural cotton fiber accounts for 70%. Dye is out of reach. Only 30% of the amorphous area can be dyed. Therefore, cotton fiber has the disadvantage of poor color absorption capacity, low color absorption capacity, and difficulty in dyeing deeply. For this reason, it must be improved by mercerizing with concentrated caustic soda before dyeing. Because mercerizing treatment can expand the amorphous area of cotton fibers and increase the voids in the microstructure, it can greatly improve the diffusion and fixation effects of dyes, thereby effectively reducing the floating rate during dyeing. So good mercerizationUnder high temperature conditions with strong acidity and alkalinity, the bonded dye may undergo hydrolysis and bond breaking and become a new floating dye, which will affect the color depth and the soaping effect.
In actual production, in order to increase output and reduce energy consumption, some water after dyeing is not clean and is soaped, forming “alkaline soaping”; some If it is directly neutralized with acid, it often forms “acid soaping”.
Production practice has proved that whether it is a s-triazine type dye (K type, X type), a vinyl sulfone type dye (KN type), or a mixed bis-triazine dye, Reactive dyes (M type, ME type, A type, B type, etc.), their dye-fiber bonds are most stable at pH=6-7, the pH value increases or decreases, the stability of the dye-fiber bond All dropped. “Alkaline soaping” is prone to hydrolytic cleavage of vinyl sulfone dye-fiber bonds, and “acidic soaping” is prone to hydrolytic cleavage of s-triazine type dye-fiber bonds, and contains two active groups: monochloros-triazine and vinyl sulfone. Reactive dyes have better stability against acids and alkalis than dyes with a single reactive group.
Fifth, it must be soaped at high temperature.
This is because during the soaping process, the floating dye remaining on the fabric is acted upon by two forces at the same time. First, it comes from the affinity between the dye and the fiber, which has a tendency to adhere. Second, it comes from the hydrophilicity of the dye’s strong water-soluble group – SO3Na, which has a tendency to dissolve. Increasing the soaping temperature can effectively increase the water solubility of the floating dye and reduce the adhesion of the dye, so that the floating dye on the fiber can dissolve faster and more, improving the soaping effect.
Soaping at high temperature (95℃) under neutral conditions, there is no need to worry about too much dye on the fiber falling off. Because the dye chemically bonded to the fiber has become part of the cellulose molecular chain, it has good bonding fastness. Some manufacturers are worried that soaping will cause more color fading and will cause changes in color and light, so they use 60-70℃ soaping. This way the dye fastness is definitely not guaranteed.
Fading after soaping is normal, because the color absorption rate of reactive dyes is always higher than the fixation rate, among which unfixed dyes and hydrolyzed dyes , should be washed as much as possible. If there is too much color fading after soaping, it must be checked from aspects such as the color absorption of the semi-finished product, the leveling and dyeing properties of the dyeing, the adequacy of dye fixation, and the acidity and alkalinity of the soaping solution. Low-temperature soaping is not allowed. to make amends.
Do color fixing if necessary
Production practice shows that even if the dyed goods with reactive dyes are washed and soaped, there will still be some residual floating dyes on the fibers, especially when dyeing dark colors such as scarlet, maroon, sauce red, and green orchid. Its wet rubbing fastness and soaping fastness are often very poor. At this time, it is usually treated with a fixing agent.
Commonly used fixing agents or cross-linking agents, after entering the fiber, on the one hand It combines with the dyes on the fiber (fixed dyes and unfixed dyes) through ionic bonds, and the water-soluble groups of the dyes are blocked, reducing their water solubility. On the other hand, it will produce cross-links between fibers and dyes. Therefore, after the dyed material is treated with the fixing agent, the dye will not easily fall off during the wet fastness test. Even if it falls into the water, it will stain the white cloth less, so the soaping fastness of the colored cloth can be significantly improved. Wet ironing fastness.
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