What's the dyeing principle for sulfur dyes?

Date:Oct 29, 2020

The dyeing of cotton textiles with sulfur dyes is indeed very economical, and the color fastness is also good, but it often produces erythema and red streaks, which need to be reworked, which affects the output and cost. To solve this problem, we must pay attention to the following points:


1.Use alkali sulfide (Na2S)

Alkali sulfide is an indispensable main auxiliary for dyeing sulfur dyes. Sulfur dyes must be dyed with different proportions of alkali sulfide (50%) according to the specific product. Generally, it is 1:1 with dyes (100%), and it can also be 1:1.5 like sulphur blue. Its main function is to promote the dissolution and reduction of dyes. Sometimes the alkali sulfide used is not reddish brown, but becomes off-white powder, which is also added to the dye bath. In fact, alkali sulfide is in the form of off-white powder and has been weathered. Adding it to the dye bath will not only not dissolve or reduce the dye, but will also cause the dye to be prematurely oxidized into lakes. And other staining defects.


2.Control the dyeing process conditions

Practice has proved that sulfur dyes cannot be dyed at 80-85°C in theory, especially sulfur blue and sulfur black. After adding the dye and alkali sulfide, gradually increase the temperature to 98℃, keep the dyeing for 15-20min, then stop heating; while dyeing, let the temperature in the tank drop to 80-85℃, continue dyeing for about 20min, and then drain the liquid Clean and oxidize while entering cold water. Regardless of removing the residual liquid (hot) and adding cold water, the speed should be slow, not too fast. This dyeing method of heating first and then warming not only facilitates level dyeing, but also facilitates the full dissolution and reduction of dyes, and can overcome dyeing defects.


3.Dye should be washed slowly

As mentioned above, the dyed fabric cannot be washed in a cold water bath at once. Especially like sulphur blue, after dyeing, it should transition from the dye bath to mild, and even wash it in water containing a certain reducing agent. The term is called "foot tank". This is not only conducive to the full cleaning of the alkali and reducing agent on the fabric, but also conducive to the full color development (oxidation) of the dye on the fabric. Of course, for some hard-to-oxidize dyes, such as Sulfur Blue CV/3G, Red Brown GN, Dark Brown, etc., 1-2g/L sodium perborate and hydrogen peroxide should be added to the bath in the 2-3 tank to make the dye full Oxidation hair color.


4.Pay attention to the stability of the dye bath, and solve the problem before the vat

Sulfur dyes are dyes with high sulfur content, especially for black, which have many unstable factors in application. For example, the leuco body in the dye bath is dark green or turmeric when dyeing starts (the leuco body is different depending on the type of dye), before the end of dyeing, the leuco body in the dye bath has changed and is different from the original Up. Then, you can't get out of the tank. Generally speaking, add 1/2 or 1/3 of the original amount of alkali sulfide, and then continue dyeing for 10-15 minutes at about 85°C. The leuco body of the visual dye bath is similar to the beginning. Sip again at that time. Otherwise, problems will occur after the cylinder is released, leading to rework. Practice has proved that when dyeing, if 2-3g/L baking soda, soda ash, glucose (industrial grade) or 0.5-1g/L sodium hydroxide and 1-2g/L urea are added, it can promote the dissolution of the dye and make the dye bath stable , Significantly reduce the chance of erythema and red streaks.


5.Repair of erythema and red streaks

Generally, use a dilute solution of alkali sulfide (about 5-10g/L) at 85-90°C for 20 minutes, and the defects can be eliminated. When the defect is serious, the amount of alkali sulfide should be increased accordingly. When the amount of alkali sulfide reaches 10g/L or more, add an appropriate amount of dye to the treatment bath, otherwise although the erythema and other defects have been repaired, the color may become lighter. The specific amount and method of repair depends on the actual situation.