Understanding the four characteristics of reactive dyes is fundamental to the success rate of dyeing once!

Reactive dyes, also known as reactive dyes. A type of dye that undergoes a chemical reaction with fibers during dyeing. These types of dye molecules contain functional groups that can undergo chemical reactions with fibers. During dyeing, the dye reacts with the fibers, forming covalent bonds between the two and forming a whole, which improves the washing and rubbing fastness. Reactive dyes are a new type of dye. Reactive dye molecules consist of two main components: the parent dye and the reactive group. The group that can react with fibers is called the reactive group.
The properties of reactive dyes are as follows:
1、 Solubility
High quality reactive dye products have good water solubility. The solubility and concentration of the prepared dye solution are related to factors such as the selected bath ratio, the amount of electrolyte added, the dyeing temperature, and the amount of urea used. The solubility of reactive dyes varies greatly, as can be seen in various treatises. The listed solubility refers to the allowable range for the application of the dye. Reactive dyes used for printing or pad dyeing should be selected with a solubility of around 100 grams per liter, requiring complete dissolution of the dye, no turbidity, and no color spots. Hot water can accelerate dissolution, urea has a solubilizing effect, and electrolytes such as salt and sodium hydroxide can reduce the solubility of dyes. Alkaline agents should not be added simultaneously when dissolving reactive dyes to prevent dye hydrolysis.
The methods for determining 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, prepare a series of dye solutions of different concentrations and stir at room temperature (20 ℃) for 10 minutes to fully dissolve the dyes. Insert a 1ml graduated pipette into the middle of the test solution and mix while suctioning three times. Then take 0.5 milliliters of the test solution and drop it vertically onto the filter paper placed flat on the mouth of the beaker, repeating once. After drying, visually test the liquid infiltration circle, and use the previous concentration without obvious spots in the filter paper as the solubility of the dye, expressed in grams per liter. Some solutions of reactive dyes, after cooling, appear as turbid colloidal solutions that can be evenly dispersed on filter paper without spot precipitation, and do not hinder normal use.

2、 Diffusivity
Diffusivity refers to the ability of dyes to move towards the interior of fibers, with temperature dependent diffusion of dye molecules. Dyes with high diffusion coefficients have high reaction rates and fixation efficiency, as well as good uniformity 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 forces on fibers, making diffusion difficult. Therefore, it is usually necessary to increase the temperature to accelerate dye diffusion. Adding electrolyte to the dye solution reduces the diffusion coefficient of the dye.
The diffusion performance of dyes is usually measured by thin film method. Soak the adhesive film (glass paper) in distilled water, with a thickness of 2.4 strands before immersion and 4.5 strands after 24 hours of immersion. When measuring, stack this film to a certain thickness as needed and press it under a glass plate to remove bubbles. Then it is sandwiched between two clamps with rubber washers in the middle, one of which has a circular hole in the middle. The dye solution can only diffuse into the film layer through this hole. Immerse the clamp film in the dye solution at 20 ℃ for 1 hour, then take it out and rinse it with water. Observe the number of layers of dye solution penetrating the film and the dye color of each layer. There is a certain correlation between the number of diffusion layers and the half dyeing time, with shorter half dyeing time and more diffusion layers.

3、 Directness
Directness refers to the ability of reactive dyes to be absorbed by fibers in the dye solution. Reactive dyes with high solubility often have low directness, and varieties with low directness should be selected for continuous pad dyeing and printing. Dyeing equipment with high bath ratios, such as rope like matching dyeing and yarn twisting dyeing, should prioritize the use of dyes with high directness. Rolling (cold rolling pile) dyeing method, the dye solution is transferred to the fibers through immersion rolling, and it is also easy to achieve uniform dyeing with slightly less direct dyes, with less color difference before and after, and easy to clean with hydrolyzed dyes.
The directness of reactive dyes is represented by the equilibrium dye uptake percentage (i.e. coloring rate) or the Rf value of color layer analysis.
Measurement method (1): The fiber material is packaged in 2 grams of bleached silk 40X40 cotton poplin. The dye concentration is 0.2 grams per liter, the bath ratio is 20:1, and the dyeing temperature is divided into two levels: 30 ℃ and 80 ℃. When measuring, cut 2 grams of fabric into pieces and put them into a three necked bottle that has reached the specified dyeing temperature (to avoid water evaporation). At regular intervals, draw 2 milliliters of dye solution while stirring (while adding 2 milliliters of water), and measure the optical density of the dye solution. As the dyeing time prolongs, adsorption reaches equilibrium and the optical density value of the dye solution no longer changes. The percentage of dye uptake at this time indicates the degree of directness of the dye.
Measurement method (2): Paper chromatography (Xinhua # 3 filter paper), observe that the height of each dye spot rising is different, that is, the Rf value is different. The larger the Rf value, the less direct the dye has with the cellulose material; The smaller the Rf value, the greater the directness. Prepare a 0.2 g/L dye solution, sample it on filter paper using a capillary tube, dry it and hang it in a closed chromatography cylinder containing distilled water for 30 minutes. Then, let one end of the filter paper sample come into contact with water to start chromatography. When the front edge of the developing agent rises to 20 cm, calculate the Rf value of the dye spot. Paper chromatography is a simple method for determining the directness of dyes, but the Rf value is not completely consistent with the actual performance of the dye.

4、 Reactivity
The reactivity of reactive dyes usually refers to the strength of their ability to react with cellulose hydroxyl groups. Dyes with strong reactivity can be immobilized under room temperature and weak alkaline conditions, but the stability of the dyes in this reaction is relatively poor, and they are easily hydrolyzed and lose their dyeing ability. Dyes with weak reactivity need to bond with cellulose under higher temperature conditions, or activate the hydroxyl groups of fiber yarns with strong alkaline agents to promote dye reaction and fixation on fibers.
The reactivity of reactive dyes of the same model is roughly the same, and the strength of reactivity depends on the chemical structure of the dye's active group, followed by the linking group between the dye body and the active group, which also has a certain impact on the reactivity of the dye. In addition, it is also affected by the pH value. Generally, as the pH value increases, the reaction rate also increases. As for temperature, it is also a factor that affects the reaction rate. As the temperature increases, the reaction rate increases. For every 10 ℃ increase in temperature, the reaction rate can be increased by 2-3 times. Therefore, after printing, drying or steaming can promote the reaction between dye and fiber.