The problem of reactive wet rubbing fastness
The dyeing mechanism of reactive dyes on cellulose fibers is different from other dyes. As a result, the dyes on the fibers do not rely on hydrogen bonds. Ionic bonds or van der Waals forces physically dye the fiber, but chemically combine 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.
Causes of poor wet rubbing fastness
How to improve dyeing Does the wet rubbing fastness of the product meet the 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 rating it with a standard gray staining sample card according to the degree of staining of the white cloth.
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 rubbing process.
2. There are microscopic particles of colored fibers on the surface of the dyed cloth, which transfer 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 particles 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, it is necessary to study the selection of dyes, formulation of processes and other aspects, and implement corresponding technical measures in a targeted manner.
Selection of dyes
Reactive dyes with different structures, their dyeing Performance varies. In terms of dyeing with deep and deep colors, the key conditions that must be considered when choosing dyes should be:
1. The solubility should be high because the amount of dye used for deep and deep colors is large (usually >5% o.w.f). If the dye is poorly dissolved, it will not only affect the leveling and dyeing effects, but also cause 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 better the dyeing depth. 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 worse the washability of the dye. The floating dye that remains in the physical state inside and outside the fabric (or fiber) after dyeing is more difficult to wash away. Therefore, the dye’s Directness 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 worse the smoothness of the fabric surface, the larger 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. Therefore, The friction fastness is 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 crystalline area of natural cotton fiber accounts for 70% and is inaccessible to dyes. 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. Therefore, good mercerization effect plays a decisive role in improving various wet fastnesses, especially wet rubbing fastness. In actual production, there are two negative problems, which will seriously harm the mercerizing effect.
First, in recent years, most printing and dyeing companies have adopted alkali-oxygen-one bath or enzyme-oxygen one-bath cold padding short-process pre-treatment process. This process has outstanding advantages in “energy saving and emission reduction”. However, the bleaching effect often suffers from poor water absorption (low wool efficiency) (especially thick and tight fabrics). Therefore, when mercerizing, due to the water absorption capacity of the refining and bleaching semi-finished products,The washing temperature can effectively improve the water solubility of floating dyes and reduce the adhesion of dyes, so that the floating dyes on the fibers can dissolve faster and more, improving the soaping effect.
When soaping at high temperature (95°C) under neutral conditions, there is no need to worry about the dye on the fiber falling off too much. 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. Unfixed dyes and hydrolyzed dyes should be washed away 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.
Perform color-fixing treatment if necessary
Production practice shows that active Even if the dyed goods are washed and soaped, there will still be some residual floating dye on the fiber, especially when dyeing dark colors such as scarlet, maroon, sauce red, and green orchid. The wet rubbing fastness and soaping fastness , often very poor. At this time, fixative treatment is usually required.
Commonly used fixing agents or cross-linking agents, after entering the fiber, on the one hand, combine with the dyes on the fibers (fixed dyes and unfixed dyes), and the dyes The water-soluble group is blocked, making it less water-soluble. 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. </p
