Reactive dyes have a very good solubility in water. Reactive dyes mainly rely on the sulfonic acid group on the dye molecule to dissolve in water. For meso-temperature reactive dyes containing vinylsulfone groups, in addition to the sulfonic acid group, the β -Ethylsulfonyl sulfate is also a very good dissolving group.

In the aqueous solution, the sodium ions on the sulfonic acid group and the -ethylsulfone sulfate group undergo hydration reaction to make the dye form anion and dissolve in the water. The dyeing of the reactive dye depends on the anion of the dye to be dyed to the fiber.

The solubility of reactive dyes is more than 100 g/L, most of the dyes have a solubility of 200-400 g/L, and some dyes can even reach 450 g/L. However, during the dyeing process, the solubility of the dye will decrease due to various reasons (or even completely insoluble). When the solubility of the dye decreases, part of the dye will change from a single free anion to particles, due to the large charge repulsion between the particles. Decrease, particles and particles will attract each other to produce agglomeration. This kind of agglomeration firstly gathers dye particles into agglomerates, then turns into agglomerates, and finally turns into flocs. Although the flocs are a kind of loose assembly, because of their The surrounding electric double layer formed by positive and negative charges is generally difficult to decompose by the shear force when the dye liquor circulates, and the flocs are easy to precipitate on the fabric, resulting in surface dyeing or staining.

Once the dye has such agglomeration, the color fastness will be significantly reduced, and at the same time it will cause different degrees of stains, stains, and stains. For some dyes, the flocculation will further accelerate the assembly under the shear force of the dye solution, causing dehydration and salting out. Once salting out occurs, the dyed color will become extremely light, or even not dyed, even if it is dyed, it will be serious color stains and stains.

Causes of dye aggregation

The main reason is the electrolyte. In the dyeing process, the main electrolyte is the dye accelerant (sodium salt and salt). The dye accelerant contains sodium ions, and the equivalent of sodium ions in the dye molecule is much lower than that of the dye accelerant. The equivalent number of sodium ions, the normal concentration of the dye accelerator in the normal dyeing process will not have much influence on the solubility of the dye in the dye bath.

However, when the amount of dye accelerant increases, the concentration of sodium ions in the solution increases accordingly. Excess sodium ions will inhibit the ionization of sodium ions on the dissolving group of the dye molecule, thereby reducing the solubility of the dye. After more than 200 g/L, most of the dyes will have different degrees of aggregation. When the concentration of the dye accelerator exceeds 250 g/L, the degree of aggregation will be intensified, first forming agglomerates, and then in the dye solution. Agglomerates and floccules are formed quickly, and some dyes with low solubility are partially salted out or even dehydrated. Dyes with different molecular structures have different anti-agglomeration and salt-out resistance properties. The lower the solubility, the anti-agglomeration and salt-tolerant properties. The worse the analytical performance.

The solubility of the dye is mainly determined by the number of sulfonic acid groups in the dye molecule and the number of β-ethylsulfone sulfates. At the same time, the greater the hydrophilicity of the dye molecule, the higher the solubility and the lower the hydrophilicity. The lower the solubility. (For example, dyes of azo structure are more hydrophilic than dyes of heterocyclic structure.) In addition, the larger the molecular structure of the dye, the lower the solubility, and the smaller the molecular structure, the higher the solubility.

Solubility of reactive dyes
It can be roughly divided into four categories:

Class A, dyes containing diethylsulfone sulfate (ie vinyl sulfone) and three reactive groups (monochloros-triazine + divinyl sulfone) have the highest solubility, such as Yuan Qing B, Navy GG, Navy RGB, Golden: RNL And all reactive blacks made by mixing Yuanqing B, three-reactive group dyes such as ED type, Ciba s type, etc. The solubility of these dyes is mostly around 400 g/L.

Class B, dyes containing heterobireactive groups (monochloros-triazine+vinylsulfone), such as yellow 3RS, red 3BS, red 6B, red GWF, RR three primary colors, RGB three primary colors, etc. Their solubility is based on 200～300 grams The solubility of meta-ester is higher than that of para-ester.

Type C: Navy blue that is also a heterobireactive group: BF, Navy blue 3GF, dark blue 2GFN, red RBN, red F2B, etc., due to fewer sulfonic acid groups or larger molecular weight, its solubility is also low, only 100-200 g/ Rise. Class D: Dyes with monovinylsulfone group and heterocyclic structure, with the lowest solubility, such as Brilliant Blue KN-R, Turquoise Blue G, Bright Yellow 4GL, Violet 5R, Blue BRF, Brilliant Orange F2R, Brilliant Red F2G, etc. The solubility of this type of dye is only about 100 g/L. This type of dye is particularly sensitive to electrolytes. Once this type of dye has agglomerated, it does not even need to go through the process of flocculation, directly salting out.

In the normal dyeing process, the maximum amount of dye accelerator is 80 g/L. Only dark colors require such a high concentration of dye accelerator. When the dye concentration in the dyeing bath is less than 10 g/L, most reactive dyes still have good solubility at this concentration and will not aggregate. But the problem lies in the vat. According to the normal dyeing process, the dye is added first, and after the dye is fully diluted in the dye bath to uniformity, the dye accelerant is added. The dye accelerant basically completes the dissolution process in the vat.

Operate according to the following process

Assumption: dyeing concentration is 5%, liquor ratio is 1:10, cloth weight is 350Kg (double pipe liquid flow), water level is 3.5T, sodium sulfate is 60 g/liter, the total amount of sodium sulfate is 200Kg (50Kg/package total 4 packages) ) (The capacity of the material tank is generally about 450 liters). In the process of dissolving sodium sulfate, the reflux liquid of the dye vat is often used. The reflux liquid contains the previously added dye. Generally, 300L reflux liquid is first put into the material vat, and then two packets of sodium sulfate (100 kg) are poured.

The problem is here, most dyes will agglomerate to varying degrees at this concentration of sodium sulfate. Among them, the C type will have serious agglomeration, and the D dye will not only be agglomerated, but even salt out. Although the general operator will follow the procedure to slowly replenish the sodium sulfate solution in the material vat into the dye vat through the main circulation pump. But the dye in the 300 liters of sodium sulfate solution has formed flocs and even salted out.

When all the solution in the material vat is filled into the dyeing vat, it is severely visible that there is a layer of greasy dye particles on the vat wall and the bottom of the vat. If these dye particles are scraped off and put into clean water, it is generally difficult. Dissolve again. In fact, the 300 liters of solution entering the dye vat are all like this.

Remember that there are also two packs of Yuanming Powder that will also be dissolved and refilled into the dye vat in this way. After this happens, stains, stains, and stains are bound to occur, and the color fastness is seriously reduced due to surface dyeing, even if there is no obvious flocculation or salting out. For Class A and Class B with higher solubility, dye aggregation will also occur. Although these dyes have not yet formed flocculations, at least part of the dyes have already formed agglomerates.

These aggregates are difficult to penetrate in the fiber. Because the amorphous area of ​​cotton fiber only allows the penetration and diffusion of mono-ion dyes. No aggregates can enter the amorphous zone of the fiber. It can only be adsorbed on the surface of the fiber. The color fastness will also be significantly reduced, and color stains and stains will also occur in serious cases.

The solution degree of reactive dyes is related to alkaline agents

When the alkali agent is added, the β-ethylsulfone sulfate of the reactive dye will undergo an elimination reaction to form its real vinyl sulfone, which is very soluble in genes. Since the elimination reaction requires very few alkali agents, (often only accounting for less than 1/10 of the process dosage), the more alkali dosage is added, the more dyes that eliminate the reaction. Once the elimination reaction occurs, the solubility of the dye will also decrease.

The same alkali agent is also a strong electrolyte and contains sodium ions. Therefore, excessive alkali agent concentration will also cause the dye that has formed vinyl sulfone to agglomerate or even salt out. The same problem occurs in the material tank. When the alkali agent is dissolved (take soda ash as an example), if the reflux solution is used. At this time, the reflux liquid already contains the dye accelerating agent and dye in the normal process concentration. Although part of the dye may have been exhausted by the fiber, at least more than 40% of the remaining dye is in the dye liquor. Suppose a pack of soda ash is poured during operation, and the concentration of soda ash in the tank exceeds 80 g/L. Even if the dye accelerator in the reflux liquid is 80 g/L at this time, the dye in the tank will also condense. C and D dyes may even salt out, especially for D dyes, even if the concentration of soda ash drops to 20 g/l, local salting out will occur. Among them, Brilliant Blue KN.R, Turquoise Blue G, and Supervisor BRF are the most sensitive.

Dye agglomeration or even salting out does not mean that the dye has been completely hydrolyzed. If it is agglomeration or salting out caused by a dye accelerator, it can still be dyed as long as it can be re-dissolved. But to make it re-dissolve, it is necessary to add a sufficient amount of dye assistant (such as urea 20 g/l or more), and the temperature should be raised to 90°C or more with sufficient stirring. Obviously it is very difficult in the actual process operation.
In order to prevent the dyes from agglomerating or salting out in the vat, the transfer dyeing process must be used when making deep and concentrated colors for the C and D dyes with low solubility, as well as the A and B dyes.

Process operation and analysis

1. Use the dye vat to return the dye accelerant and heat it in the vat to dissolve it (60～80℃). Since there is no dye in the fresh water, the dye accelerator has no affinity for the fabric. The dissolved dye accelerator can be filled into the dyeing vat as quickly as possible.

2. After the brine solution is circulated for 5 minutes, the dye accelerant is basically fully uniform, and then the dye solution that has been dissolved in advance is added. The dye solution needs to be diluted with the reflux solution, because the concentration of the dye accelerant in the reflux solution is only 80 grams /L, the dye will not agglomerate. At the same time, because the dye will not be affected by the (relatively low concentration) dye accelerator, the problem of dyeing will occur. At this time, the dye solution does not need to be controlled by time to fill the dyeing vat, and it is usually completed in 10-15 minutes.

3. Alkali agents should be hydrated as much as possible, especially for C and D dyes. Because this type of dye is very sensitive to alkaline agents in the presence of dye-promoting agents, the solubility of alkaline agents is relatively high (the solubility of soda ash at 60°C is 450 g/L). The clean water needed to dissolve the alkali agent does not need to be too much, but the speed of adding the alkali solution needs to be in accordance with the process requirements, and it is generally better to add it in an incremental method.

4. For the divinyl sulfone dyes in category A, the reaction rate is relatively high because they are particularly sensitive to alkaline agents at 60°C. In order to prevent instant color fixation and uneven color, you can pre-add 1/4 of the alkali agent at low temperature.

In the transfer dyeing process, it is only the alkali agent that needs to control the feeding rate. The transfer dyeing process is not only applicable to the heating method, but also applicable to the constant temperature method. The constant temperature method can increase the solubility of the dye and accelerate the diffusion and penetration of the dye. The swelling rate of the amorphous area of ​​the fiber at 60°C is about twice as high as that at 30°C. Therefore, the constant temperature process is more suitable for cheese, hank. Warp beams include dyeing methods with low liquor ratios, such as jig dyeing, which require high penetration and diffusion or relatively high dye concentration.

Note that the sodium sulfate currently available on the market is sometimes relatively alkaline, and its PH value can reach 9-10. This is very dangerous. If you compare pure sodium sulfate with pure salt, salt has a higher effect on dye aggregation than sodium sulfate. This is because the equivalent of sodium ions in table salt is higher than that in sodium sulfate at the same weight.

The aggregation of dyes is quite related to water quality. Generally, calcium and magnesium ions below 150ppm will not have much impact on the aggregation of dyes. However, heavy metal ions in water, such as ferric ions and aluminum ions, including some algae microorganisms, will accelerate dye aggregation. For example, if the concentration of ferric ions in the water exceeds 20 ppm, the anti-cohesion ability of the dye can be significantly reduced, and the influence of algae is more serious.

Attached with dye anti-agglomeration and salting-out resistance test:

Determination 1: Weigh 0.5 g of dye, 25 g of sodium sulfate or salt, and dissolve it in 100 ml of purified water at 25°C for about 5 minutes. Use a drip tube to suck the solution and drop 2 drops continuously at the same position on the filter paper.

Determination 2: Weigh 0.5 g of dye, 8 g of sodium sulfate or salt and 8 g of soda ash, and dissolve it in 100 ml of purified water at about 25°C for about 5 minutes. Use a dropper to suck the solution on the filter paper continuously. 2 drops.

The above method can be used to simply judge the anti-agglomeration and salting-out ability of the dye, and basically can judge which dyeing process should be used.