Reactive dyes have a very good solubility in water. Reactive dyes mainly rely on the sulfonic acid groups on the dye molecules to dissolve in water. For medium-temperature reactive dyes containing vinyl sulfone groups, in addition to the sulfonic acid groups, their β -Ethyl sulfonyl sulfate is also a very good solubilizing group.
In an aqueous solution, the sodium ions on the sulfonic acid group and the -ethylsulfonyl sulfate group undergo a hydration reaction, causing the dye to form negative ions and dissolve in water. Reactive dye dyeing relies on the negative ions of the dye to be dyed onto the fiber. The solubility of reactive dyes exceeds 100 g/L. The solubility of most dyes is 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 (or even become completely insoluble) due to various reasons.
When the dye solubility decreases, some dyes will transform from single free negative ions into particles, because the charge repulsion between particles is greatly reduced.
Particles will attract each other to produce agglomeration. In this agglomeration, the dye particles first assemble into agglomerates, then transform into aggregates, and finally transform into flocs. Although the flocs are a relaxed collection, due to the double electric layer formed by positive and negative charges around them, it is difficult to decompose them due to the shear force during the circulation of the dye liquor, and the flocs are easy to form on the fabric. Precipitate on the surface, causing surface staining or staining.
Once the dyes produce such agglomeration, the dye fastness will be significantly reduced, and at the same time, varying degrees of color flowers, stains, and stains will be caused. For some dyes, their flocs will further accelerate their aggregation under the shear force of the dye liquor, causing dehydration and salting out. Once salting out occurs, the dyed color will become very light or even uncolored. Even if it is dyed, it will cause serious stains and stains.
The main reason for the agglomeration of dyes is caused by electrolytes. In the dyeing process, the main electrolytes are dye accelerators (yuanming powder and salt). The dye accelerators contain sodium ions, but the equivalent of sodium ions in the dye molecules is much lower. Due to the sodium ion equivalent number of the dye accelerator, the normal dye accelerator concentration will not have much impact on the dye solubility in the dye bath during normal dyeing.
However, when the amount of dye promoter increases, the concentration of sodium ions in the solution also increases accordingly. Excess sodium ions will inhibit the ionization of sodium ions on the dissolving groups of the dye molecules, thereby reducing the solubility of the dye. When the dye accelerator concentration exceeds 200 g/L, most dyes will agglomerate to varying degrees. When the concentration of the dye accelerator exceeds 250 g/L, the degree of aggregation will intensify. Aggregates will be formed first, and then aggregates and floccules will quickly form under the shear force of the dye liquor. For some dyes with low solubility, Part of the salt precipitates out or even becomes dehydrated.
Dyes with different molecular structures also have different anti-aggregation and salting-out resistance. The lower the solubility, the worse the anti-aggregation and salting-out resistance. The solubility of dyes is mainly determined by the number of sulfonic acid-containing groups and the number of β-ethyl sulfonyl sulfate in the dye molecules.
At the same time, the greater the hydrophilicity of the dye molecule, the higher the solubility, and the smaller the hydrophilicity, the lower the solubility. (For example, dyes with azo structures are more hydrophilic than dyes with heterocyclic structures.) In addition, the larger the molecular structure of the dye, the lower the solubility, and the smaller the molecular structure, the higher the solubility.
The solubility of reactive dyes can be roughly divided into four categories:
Category A, dyes containing bisethyl sulfone sulfate (i.e. vinyl sulfone) and three reactive groups (monochloro-s-triazine + bis vinyl sulfone) have the highest solubility, such as Yuanqing B, Navy Blue GG, Navy Blue RGB, Golden Yellow: RNL As well as all reactive blacks made by mixing Yuanqing B, three-reactive base dyes such as ED type, Ciba S type, etc. The solubility of this type of dye is mostly around 400 g/L.
Category B, dyes containing heterobi-reactive groups (monochloro-s-triazine + vinyl sulfone), such as yellow 3RS, red 3BS, red 6B, red GWF, RR type three primary colors, RGB three primary colors, etc. Their solubility is based on 200 to 300 grams /L or so, among which the solubility of meta-ester is higher than that of para-ester.
Category C: Navy blue with heterobivalent reactive groups: BF, navy blue 3GF, dark blue 2GFN, red RBN, red F2B, etc. Due to fewer sulfonic acid groups or larger molecular weight, their solubility is also low, only 100 to 200 g/ Lift. Category D: dyes containing monovinyl sulfone groups and heterocyclic structures, with the lowest solubility, such as brilliant blue KN-R, emerald blue G, bright yellow 4GL, purple 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 agglomerates, it does not even need to go through the flocculation process and will directly salt out.
In the normal dyeing process, the maximum dosage of dye accelerator is 80 grams/liter. Only dark colors require such a high concentration of dye accelerator. When the dye concentration in the dyeing bath is below 10 g/L, most reactive dyes still have good solubility at this concentration and will not agglomerate. But the problem lies in the material vat. According to the normal dyeing process, the dye is added first. After the dye is fully diluted to uniformity in the dye bath, the dye accelerator is then added. The dye accelerator basically completes the dissolution process in the material vat.
Operate according to the following process, assuming: dyeing concentration is 5%, liquor ratio is 1:10, cloth weight is 350Kg (double-tube liquid flow), water level is 3.5T, Yuanming powder is 60g/L, and the total amount of Yuanming powder is 200Kg (50Kg /pack, 4 packs in total) (the capacity of the material tank is generally about 450 liters).
When operating and dissolving Yuanming powder, the reflux fluid from the dye vat is often used to return it., Yuanming powder currently available on the market is sometimes highly alkaline, and its pH value can reach 9 to 10. This is very dangerous. If we compare pure Yuanming powder with pure salt, the effect of salt on dye aggregation is higher than that of Yuanming powder. This is because the sodium ion equivalent in table salt is higher than the sodium ion equivalent in Yuanming powder under the same weight.
The aggregation of dyes is closely related to the water quality. Generally, calcium and magnesium ions below 150 ppm will not have much impact on the aggregation of dyes. However, heavy metal ions in the water, such as ferric ions and aluminum ions, including some algae and microorganisms, will accelerate the aggregation of dyes. If the concentration of ferric ions in water exceeds 20ppm, the anti-aggregation ability of the dye can be significantly reduced, and the impact of algae is even more serious.
Test of dye resistance to aggregation and salting out resistance
Determination 1: Weigh 0.5 grams of dye, 25 grams of Yuanming powder or salt, and dissolve them in 100 ml of purified water at about 25°C for about 5 minutes. Use a dropper to absorb the solution and drop 2 drops continuously on the same position on the filter paper.
Determination 2: Weigh 0.5 grams of dye, 8 grams of sodium chloride powder or salt and 8 grams of soda ash, dissolve it in 100 ml of purified water at about 25°C, wait for about 5 minutes, use a dropper to draw the solution continuously at the same position on the filter paper Add 2 drops.
The above method can be used to simply judge the anti-aggregation and salting-out resistance of the dye, and basically determine which dyeing process should be used.
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