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The four characteristics of reactive dyes are fundamental to the success rate of dyeing!

Views: 1     Author: Site Editor     Publish Time: 2022-05-03      Origin: Site Inquire

Reactive dyes, also known as reactive dyes. A class of dyes that chemically react with fibers during dyeing. This type of dye molecule contains groups that can chemically react with fibers. During dyeing, the dye reacts with the fibers, forming a covalent bond between the two to form a whole, which improves the fastness to washing and rubbing. Reactive dyes are a new class of dyes. In 1956, the United Kingdom first produced the reactive dyes of the Procion brand. Reactive dye molecules include two main components, parent dyes and reactive groups, and the groups that can react with fibers are called reactive groups.


The properties of reactive dyes are as follows:


1. Solubility


Good quality reactive dyes should be used commercially with good water solubility. The solubility and the concentration of the prepared dye solution are related to the selected liquor ratio, the amount of electrolyte added, the dyeing temperature and the amount of urea. The solubility of reactive dyes varies greatly, please refer to the monograph, and the listed solubility refers to the allowable range for the application of the dye. Reactive dyes used in printing or pad dyeing should be selected with a solubility of about 100 grams per liter. Hot water can accelerate the dissolution, urea has a solubilizing effect, and electrolytes such as salt and yuanming powder will reduce the solubility of dyes. When the reactive dye is dissolved, alkali agent should not be added at the same time to prevent the dye from being hydrolyzed.


The determination methods of the solubility of reactive dyes include vacuum filtration, spectrophotometry and filter paper spot method. The filter paper spot method is easy to operate and suitable for practical use in factories. When measuring, first prepare a series of dye solutions with different concentrations, and stir at room temperature (20°C) for 10 minutes to fully dissolve the dye. Insert a pipette with a 1 ml scale into the middle of the test solution, and suck it three times while stirring. Then draw 0.5 ml of the test solution, drop it vertically on the filter paper on the beaker mouth, and repeat once. After drying in the sun, visually test the liquid permeability circle, and the previous concentration without obvious spots in the filter paper is used as the solubility of the dye, expressed in grams per liter. Some reactive dye solutions are turbid colloidal solutions after cooling, which can be evenly infiltrated on the filter paper without spot precipitation, which does not hinder normal use.


2. Diffusion


Diffusivity refers to the ability of the dye to move into the fiber, and the temperature has the effect of diffusing the dye molecules. A dye with a large diffusion coefficient has a high reaction rate and fixing efficiency, as well as a good level dyeing and dyeing degree. The quality of the diffusion performance depends on the structure and size of the dye, the larger the molecule, the more difficult it is to diffuse. Dyes with high affinity to fibers have a strong adsorption force by fibers, so it is difficult to diffuse. Usually, the dye diffusion is accelerated by increasing the temperature. When electrolyte is added to the dye liquor, the diffusion coefficient of the dye decreases.


Determination of the diffusion properties of dyes is usually performed by the thin film method. Take the viscose film (cellophane) and immerse it in distilled water, the thickness before immersion is 2.4 silk, and the thickness after immersion for 24 hours is 4.5 silk. During the measurement, the film is stacked to a certain thickness as required, and pressed under the glass plate to remove air bubbles. Then sandwiched between two splints with rubber gaskets in the middle, one of the splints has a round hole in the middle, the dye solution can only diffuse into the film layer through this hole, and the splint film is immersed in the dye solution at 20 °C for 1 hour. , and then take out and rinse with water, observe the number of layers of the dye liquor permeable film and the color of each layer of dye. There is a certain correlation between the number of diffusion layers and the half-dye time. The short half-dye time and the more diffusion layers.


3. Directness


Immediateness 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. For dyeing equipment with large liquor ratio, such as rope piece dyeing and hank dyeing, dyes with high directness should be preferred. Pad-coil (cold pad stack) dyeing method, the dye liquor is transferred to the fiber by padding, and it is easy to get level dyeing with dyes with slightly lower directness, with less color difference before and after, and the hydrolyzed dyes are easy to wash.


The directness of reactive dyes is expressed by the percentage of equilibrium dyeing (that is, the dyeing rate) or the Rf value of chromatographic analysis.


Determination method (1): 2 grams of bleached mercerized 40X40 cotton poplin is used as the fiber material. The concentration of dye solution is 0.2 g/L, the liquor ratio is 20:1, and the dyeing temperature is divided into two grades: 30°C and 80°C. When measuring, put 2 grams of fabric cut into pieces into a three-necked bottle that has reached the specified dyeing temperature (to avoid evaporation of water), and absorb 2 ml of dye liquor during stirring at regular intervals (add 2 ml of dye at the same time). water) to measure the optical density of the dye solution. With the prolongation 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 directness of the dye.


Determination method (2): Chromatography on paper (Xinhua #3 filter paper), and observe that the height of each dye spot is different, that is, the Rf value is different. The larger the Rf value, the smaller the directness between the dye and the cellulose material; the smaller the Rf value, the greater the directness. Prepare 0.2 g/L dye solution, spot the sample on the filter paper with a capillary tube, dry it in the sun, hang it in a closed chromatography tank filled with distilled water and saturate it for 30 minutes, then make the spotting end of the filter paper contact with water to start the chromatography. When the front of the developer reaches 20 cm, the Rf value of the dye spot is calculated. Chromatography on paper is a convenient method for determining the directivity of dyes, but the Rf values are not completely consistent with the actual properties of dyes.


4. Reactivity


The reactivity of reactive dyes usually refers to the strength of the ability of the dye to react with cellulose hydroxyl groups. The dyes with strong reactivity can be fixed at room temperature and under weak alkali conditions, but the stability of the dyes in this reaction is relatively poor and easy to use. It loses its dyeing ability by hydrolysis. Dyes with weak reactivity need to bond with cellulose under relatively high temperature conditions, or use a strong alkaline agent to activate the hydroxyl groups of fiber yarns to promote the reaction of dyes and fix them on the fibers.


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


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