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Performance index and determination method of reactive dyes
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1、 Solubility

High quality reactive dyes should have good water solubility. The solubility and concentration of the dye solution are related to the bath ratio, the amount of electrolyte added, dyeing temperature and the amount of urea. The reactive dyes used in printing or pad dyeing should be of the solubility of about 100g / L. the dyes should be completely dissolved without turbidity and color spots. Hot water can accelerate the dissolution, urea can enhance the solubility, salt, sodium sulfate and other electrolytes will reduce the solubility of dyes. When the reactive dyes are dissolved, alkali should not be added at the same time to prevent the dye from hydrolysis.

There are vacuum filtration method, spectrophotometry method and filter paper spot method to determine the solubility of reactive dyes. The filter paper spot method is easy to operate and suitable for practical use in factories. In the determination, a series of dye solutions of different concentrations were prepared and stirred at room temperature (20 ℃) for 10 minutes to fully dissolve the dyes. Use a 1 ml graduated pipette into the middle of the test solution, stir while sucking for three times. Then draw 0.5 ml of test solution, drop it vertically on the filter paper on the beaker mouth, repeat once. After drying, the liquid permeating ring is tested by eye. The concentration in the filter paper without obvious spots is taken as the solubility of the dye, expressed in g / L. Some reactive dye solution, after cooling, presents a turbid colloidal solution, drops on the filter paper can be evenly permeated, no spot precipitation, does not hinder the normal use.

2、 Directness

Directness refers to the ability of reactive dyes to be absorbed by fibers in dyeing solution. Reactive dyes with high solubility tend to have low directness, so low directness should be selected for continuous pad dyeing and printing. For dyeing equipment with large bath ratio, such as rope matching dyeing and hank dyeing, high direct dyes should be preferred. In the rolling (cold pad batch) dyeing process, the dye liquor is transferred to the fiber by dip rolling, and the dye with a little lower directness is easy to be leveled, and the color difference between the front and the back is small, and the hydrolyzed dye is easy to clean.

The directness of reactive dyes is expressed by equilibrium dye uptake (i.e. dye uptake) or RF value of chromatography analysis.

Determination method (1): 2 g of bleached mercerized 40x40 cotton poplin for fiber material. 2 g / L, bath ratio 1: 20, dyeing temperature 30 ℃ and 80 ℃. During the determination, put 2G fabric into three necked bottle which has reached the specified dyeing temperature (to avoid water evaporation). At regular intervals, 2 ml of dye solution (2 ml of water is added at the same time) in the mixing process to determine the optical density of the dye solution. With the prolongation of dyeing time, the adsorption reached equilibrium, and the optical density of dye solution did not change. The dye uptake at this time represents the directness of the dye.

Determination method (2): paper chromatography (Xinhua 3 filter paper), observe the rising height of each dye spot, that is, RF value is different. The larger the RF value is, the less direct the dye and cellulose material is; the smaller the RF value is, the greater the directness is. 0.2 g / L dye solution is prepared. The sample is sampled on the filter paper by capillary tube. After drying, it is hung in a closed chromatographic cylinder containing distilled water for 30 minutes. Then, one end of the filter paper is contacted with water to start chromatography. When the front edge of developing agent rises to 20 cm, the RF value of dye spot is calculated. Paper chromatography is a simple and convenient method to determine the directness of dyes, but the RF value is not completely consistent with the actual properties of dyes.

3、 Diffusivity

Diffusivity refers to the ability of dye to move to the interior of the fiber. Temperature rise is conducive to the diffusion of dye molecules. The dye with high diffusion coefficient has high reaction rate and fixation efficiency, and good levelness and penetration. The diffusion performance depends on the structure and size of the dye. The larger the molecule, the more difficult the diffusion. The dye with high affinity to the fiber has strong adsorption force by the fiber, so it is difficult to diffuse. Generally, the dye diffusion is accelerated by increasing the temperature. When electrolytes are added to the dye solution, the diffusion coefficient of the dye decreases.

The film method is usually used to determine the diffusion properties of dyes. The viscose film (cellophane) was immersed in distilled water with a thickness of 2.4 wires before immersion and 4.5 wires after 24 hours of immersion. During the measurement, the film is folded into a certain thickness according to the need and pressed under the glass plate to remove bubbles. Then they are sandwiched in two splints with rubber gasket in the middle. One of them has a round hole in the middle. The dye can only diffuse into the film layer through this hole. Immerse the splint film in the dye solution at 20 ℃ for 1 hour, and then take it out and wash it with water. Observe the number of layers and the color of each layer of dye. There is a certain correlation between the number of diffusion layers and the half dyeing time.

4、 Reactivity

The reactivity of reactive dyes usually refers to the ability of reactive dyes to react with cellulose hydroxyl groups. The reactive dyes can be immobilized at room temperature and weak alkali conditions. However, the stability of reactive dyes in this reaction is relatively poor, which is easy to be hydrolyzed and lose dyeing ability. The weak reactive dyes need to be bonded with the fiber at higher temperature, or the hydroxyl group of the fiber yarn should be activated by strong alkali agent to make the dye react and fix on the fiber.

The reactivity of the same type of reactive dyes is roughly the same. The degree of reactivity depends on the chemical structure of the reactive group of the dye, followed by the linker between the dye body and the reactive group, which has a certain influence on the reactivity of the dye. In addition, the reaction rate will increase with the increase of pH value. As for the temperature, it is also a factor that affects the reaction rate. If the temperature increases, the reaction speed will be faster. When the temperature increases by 10 ℃, the reaction rate can be increased by 2-3 times. Therefore, the reaction can occur after steaming with the dye.


Updated:2020-08-24 | Return
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