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Understand The Four Characteristics Of Reactive Dyes, The Ultimate Success Rate Of Dyeing!
Jan 19, 2019

Reactive dyes, also known as reactive dyes. A type of dye that chemically reacts with fibers during dyeing. Such dye molecules contain a group capable of chemically reacting with the fiber, and the dye reacts with the fiber during dyeing, forming a covalent bond between the two to form a whole, which improves the washing and rubbing fastness. Reactive dyes are a new class of dyes. In 1956, the UK first produced Procion grade reactive dyes. The reactive dye molecules include two main components of the parent dye and the active group, and the groups reactive with the fiber are referred to as active groups.

The properties of reactive dyes are as follows:

First, solubility

Quality reactive dyes are applied with good water solubility. The solubility and the concentration of the dye solution are related to the selected bath ratio, the amount of electrolyte added, the dyeing temperature and the amount of urea used. The solubility of reactive dyes varies widely, see the monographs, and the solubility listed refers to the range allowed for the application of the dye. For reactive dyes applied to printing or padding, a variety of solubility of about 100 g / liter should be used. It is required that the dye is completely dissolved, not turbid, and does not produce color points. The hot water can accelerate the dissolution, the urea has a solubilizing effect, and the electrolytes such as salt and Yuanming powder can reduce the solubility of the dye. When the reactive dye is dissolved, the alkali agent should not be added at the same time to prevent the dye from being hydrolyzed.

The method for measuring the solubility of the reactive dye includes a vacuum filtration method, a spectrophotometry method, and a filter paper spot method. The filter paper spot method is easy to operate and is suitable for practical use in the factory. For the determination, a series of dye solutions of 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 aspirate it three times while stirring. Then, pipette 0.5 ml of the test solution and place it vertically on the filter paper placed on the beaker mouth, repeating once. After the drying, the test solution is oozing, and the concentration of the dye in the filter paper is the solubility of the dye in grams per liter. Some solutions of reactive dyes, after cooling, show a turbid colloidal solution, which can be evenly oozing on the filter paper without speckle precipitation, which does not hinder normal use.


Second, diffusivity

Diffusivity refers to the ability of a dye to move inside a fiber, and the temperature is diffused by the dye molecules. Dyes with large diffusion coefficients have high reaction rate and fixation efficiency, and the degree of leveling and permeation is also good. The quality of the diffusion 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 are strongly adsorbed by fibers, and diffusion is difficult. It is usually accelerated by temperature to accelerate dye diffusion. The electrolyte is added to the dye solution and the diffusion coefficient of the dye is lowered.

The film method is usually determined by measuring the diffusion properties of the dye. The adhesive film (cellophane) was immersed in distilled water, and the thickness was 2.4 filaments before immersion, and the thickness was 4.5 filaments after immersion for 24 hours. During the measurement, the film was laminated to a certain thickness as needed, and pressed under a glass plate to remove air bubbles. Then, it is sandwiched between two splints with rubber washers in the middle, and one of them has a round hole in the middle of the splint. The dye solution can only diffuse into the film layer through the hole, and the splint film is immersed in the dye solution at 20 ° C for 1 hour. Then, it was taken out and rinsed with water, and the number of layers of the dye-transparent film and the dye color of each layer were observed. 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 large.


Third, directness

Directness refers to the ability of a reactive dye to be absorbed by a fiber in a dye liquor. Reactive dyes with high solubility tend to be less direct, and continuous padding and printing should use low direct varieties. For dyeing equipment with large baths such as rope-like dyeing and skein dyeing, direct dyes should be preferred. In the coiling (cold-rolled pile) dyeing method, the dyeing liquid is transferred to the fiber by padding, and is also easily dyed by a dye having a slightly lower directness, and the color difference is small before and after, and the hydrolyzed dye is easily washed.

The direct size of the reactive dye is expressed as a percentage of equilibrium dyeing (i.e., coloring rate) or Rf value of the chromatographic analysis.

Measuring method (1): The fiber material was bleached with a mercerized 40X40 cotton poplin product 2 g. The dyeing solution has a concentration of 0.2 g/L, a bath ratio of 20:1, and a dyeing temperature of 30 ° C and 80 ° C. 2 g of the fabric which will be cut into pieces will be put into the three-necked flask which has reached the specified dyeing temperature (to avoid evaporation of water), and at a certain time, 2 ml of the dye solution is taken up while stirring (while adding 2 ml of the mixture) Water), determine the optical density of the dye solution. As the dyeing time is prolonged, the adsorption reaches equilibrium and the optical density of the dye liquor no longer changes. The percentage of dyeing at this time indicates the directness of the dye.

Measurement method (2): On-paper chromatography (Xinhua #3 filter paper), it was observed that the height of each dye spot was different, that is, the Rf value was different. The larger the Rf value, the smaller the directness of the dye and the cellulosic material; the smaller the Rf value, the greater the directness. Prepare a 0.2 g/L dye solution, spot it on a filter paper with a capillary tube, dry it, and hang it in a closed chromatography cylinder containing distilled water for 30 minutes. Then, the end of the filter paper is contacted with water to start chromatography. The Rf value of the dye spot was calculated when the front of the developer was raised to 20 cm. Paper chromatography is a convenient method for determining the directness of a dye, but the Rf value is not completely consistent with the actual performance of the dye.


Fourth, the reaction is lively

The reactivity of reactive dyes usually refers to the ability of the dye to react with cellulose hydroxyl groups. The reactive dyes can be fixed at room temperature and under weak conditions, but the dye stability in this reaction is relatively poor and easy. It is hydrolyzed to lose dyeing ability. A weakly reactive dye needs to be bonded to the cellulose under relatively temperature conditions, or a strong alkali agent is used to activate the hydroxyl group of the fiber yarn to cause the dye reaction to adhere to the fiber.

The reactivity of the same type of reactive dye is about the same. The reactivity depends on the chemical structure of the dye active group, followed by the link between the dye and the reactive group, and has a certain influence on the reactivity of the dye. In addition, due to the influence of pH, the general pH value increases and the reaction rate increases. As for the temperature, it is also a factor affecting the reaction speed. When the temperature is increased, the reaction speed is fast. For every 10 °C increase in temperature, the reaction rate can be increased by 2 to 3 times. Therefore, after printing, drying or steaming can promote the reaction of the dye with the fiber.