Advantages And Limitations Of Sulfur Dyes:
Sulfide dyes are made by fusing aromatic amines or phenolic compounds with sodium polysulfide or sulfur. During dyeing, they must be restored and dissolved with sulfide alkali, hence they are called sulfide dyes. Sulfide dyes have low prices, simple dyeing processes, convenient color matching, good dyeing fastness, and can be produced using roll dyeing and pad dyeing methods. However, their application is limited due to uneven chromatography, dull color, and poor dyeing fastness of local dyes.
Principles of Sulfide Dyeing:
Sulfide dyes are insoluble in water, but can be restored by sulfide bases to form sodium salt cryptomers that dissolve in aqueous solutions. This type of chromophore has strong directness to fibers in alkaline solutions, and is therefore absorbed by fibers. After oxidation, the dye transforms back into an insoluble state and deposits on the fibers. During the dyeing process, sulfide alkali is both a restoring agent and an alkaline agent. In order to promote the decomposition of sulfurized alkali, in addition to increasing the temperature, baking soda can also be added. However, if the dosage is too high, it may cause uneven dyeing and white core due to fast dyeing. In addition, 2% pure alkali can be added to the dyeing bath to adjust the pH value, which can help dissolve dyes and soften water quality. Adding an appropriate amount of baking soda can better decompose sulfurized alkali and promote its recovery effect. The amount of sulfide alkali varies with different dyes, generally ranging from 20% to 100% of the dye. The dyeing temperature can be high temperature, or even boiling dyeing, to achieve a higher penetration effect.
Sulfide Dye Roll Dyeing Process:
The dyeing of sulfide dyes is generally done using roll dyeing, which is not only beneficial for small batch and multi variety production, but also can give the finished product a deep and full color. However, roll dyeing can easily produce dyeing defects such as deep ends and edges. Therefore, when conditions permit, pad dyeing can be used for local color to improve production efficiency and improve product quality. In order to improve the dyeing fastness of sulfide dyes, post dyeing treatment should also be given attention. Especially for coffee, black green and other colors, in addition to necessary water washing, the selection of oxidants is also important. If hydrogen peroxide or sodium perborate is used, it will affect the soap washing fastness after dyeing. However, using red rock, copper sulfate, and acetic acid treatment can not only improve the water washing fastness of the product, but also improve the sun fastness. In addition, due to the presence of polysulfide bonds within the molecules of sulfide dyes, their high sulfur content, and relaxed molecular structure, under the action of humid heat and oxygen in the air, the release of free sulfur will gradually be oxidized into sulfuric acid, causing brittle damage to the fabric. Therefore, anti brittle treatment is required for fabrics dyed with sulfide dyes.
