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Performance Index and Determination Method of Reactive Dyes


Release time:

2023-07-28

High quality reactive dyes should have good water solubility. The solubility and the concentration of the prepared dye solution are related to the selected bath ratio, the amount of electrolyte added, the dyeing temperature and the amount of urea. Reactive dyes used in printing or pad dyeing should be selected with a solubility of about 100g/L. The dyes are required to be completely dissolved, not turbid, and not colored. Hot water can accelerate dissolution, urea has a solubilizing effect, salt, sodium sulfate and other electrolytes will reduce the solubility of dyes. When the reactive dye is dissolved, the alkali agent should not be added at the same time to prevent the dye from hydrolyzing.

1. Solubility

High-quality reactive dye products should have good water solubility. The solubility and the concentration of the prepared dye solution are related to the selected bath ratio, the amount of electrolyte added, the dyeing temperature and the amount of urea. Reactive dyes used in printing or pad dyeing should be selected with a solubility of about 100g/L. The dyes are required to be completely dissolved, not turbid, and not colored. Hot water can accelerate dissolution, urea has a solubilizing effect, salt, sodium sulfate and other electrolytes will reduce the solubility of dyes. When the reactive dye is dissolved, the alkali agent should not be added at the same time to prevent the dye from hydrolyzing.

The solubility of reactive dyes is measured by vacuum filtration, spectrophotometry and filter paper spot. The filter paper spot method is easy to operate and suitable for practical use in factories. During the measurement, a series of dye solutions with different concentrations were prepared and stirred at room temperature (20°C) for 10 minutes to fully dissolve the dye. Use a 1 ml graduated pipette to extend into the middle of the test solution, and suck and release it three times while stirring. Then suck 0.5 ml of test solution and drop it vertically on the filter paper flat on the beaker mouth, and repeat it once. After drying in the sun, the concentration of the former level without obvious spots on the filter paper is taken as the solubility of the dye, expressed in grams/liter. Some reactive dye solution, after cooling, a cloudy colloidal solution, drops on the filter paper can be evenly infiltrated, no spots precipitated, does not hinder the normal use.

2. directness

directness refers to the ability of reactive dyes to be absorbed by fibers in the dye liquor. Reactive dyes with high solubility tend to have low directness, and continuous pad dyeing and printing should choose varieties with low directness. Dyeing equipment with large bath ratio, such as rope-shaped paddle dyeing and skein dyeing, should preferably use dyes with high directness. In the rolling (cold rolling pile) dyeing method, the dye solution is transferred to the fiber through padding, and the dye with slightly lower directness is easy to be evenly dyed, with less color difference before and after, and the hydrolyzed dye is easy to wash. The direct size of

reactive dyes is expressed by the Rf value of the balanced dye percentage (I. e. color rate) or color layer analysis.

Measurement method (1): 2 grams of bleached mercerized 40x 40 cotton poplin was used for the fiber material. The concentration of dye solution is 0.2g/l, the bath ratio is 1:20, and the dyeing temperature is divided into two stages: 30 ℃ and 80 ℃. During the determination, 2g of fabric cut into pieces is put into a three-neck bottle that has reached the specified dyeing temperature (to avoid water evaporation). At regular intervals, 2 ml of dye solution is sucked (2 ml of water is added at the same time) during stirring to determine the optical density of the dye solution. With the extension of dyeing time, the adsorption reached equilibrium, and the optical density of the dye solution no longer changed. The dyeing percentage at this time indicates the magnitude of the directness of the dye.

Determination method (2): paper chromatography (Xinhua
filter paper) to observe the different rising heights of each dye spot, I .e., different Rf values. A larger Rf value indicates less directness of the dye to the cellulosic material; a smaller Rf value indicates greater directness. Prepare 0.2g/L dye solution, spot the sample on the filter paper with capillary tube, hang it in a closed chromatography cylinder containing distilled water after drying for 30 minutes, then make one end of the filter paper spot contact with water to start chromatography, and calculate the Rf value of the dye spot when the front of the developing agent rises to 20cm. Paper chromatography is a simple method for determining the substantivity of dyes, but the Rf values are not entirely consistent with the actual properties of dyes.


3. Diffusivity


Diffusivity refers to the ability of dyes to move into the fiber. Heating up is conducive to the diffusion of dye molecules. Dyes with large diffusion coefficient have high reaction rate and fixation efficiency, and good levelness and penetration. The diffusion performance depends on the structure and size of the dye, and the larger the molecule, the more difficult it is to diffuse. Dyes with high affinity for fibers have strong adsorption effect on fibers, and diffusion is difficult. Dye diffusion is usually accelerated by increasing temperature. When electrolyte is added to the dye solution, the diffusion coefficient of the dye decreases.

The diffusion properties of dyes are usually determined by the thin film method. The viscose film (cellophane) was immersed in distilled water with a thickness of 2.4 silk before immersion and 4.5 silk after immersion for 24 hours. During the measurement, the film is laminated to a certain thickness as required, and the bubbles are removed by pressing on the glass plate. Then it is sandwiched between two splints with rubber washers in the middle, one of which has a round hole in the middle. The dye solution can only diffuse into the film layer through this hole. Immerse the splint film in the dye solution at 20 ℃ for 1 hour, then take out and rinse with water, and observe the number of layers of the dye solution through the dye film and the color of each layer. There is a certain correlation between the number of diffusion layers and the half dyeing time, the half dyeing time is short, and the number of diffusion layers is more.


4. Reactivity


Reactivity of reactive dyes usually refers to the strength of the reactivity of dyes with cellulose hydroxyl groups. Dyes with strong reactivity can be fixed at room temperature and under weak alkali conditions, but the stability of dyes in this reaction is relatively poor, and they are easy to be hydrolyzed to lose their dyeing ability. Weak reactive dyes need to be bonded to the fiber under higher temperature conditions, or a strong alkali agent is used to activate the hydroxyl groups of the fiber yarn to promote the dye reaction and fixation on the fiber.


Reactive dyes of the same model have roughly the same reactivity. The strength of the reactivity depends on the chemical structure of the active group of the dye, followed by the linking group between the dye body and the active group, which also has a certain influence on the reactivity of the dye. In addition, it is also affected by the pH value. Generally, when the pH value increases, the reaction rate will increase. As for temperature, it is also a factor that affects the reaction speed. As the temperature increases, the reaction speed will be faster. The reaction rate can be increased by 2-3 times for every 10 ℃ increase in temperature. Therefore, after printing, after drying or steaming, the dye can react with the fiber.


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