Answer The main reasons for the “black skin” phenomenon produced by reduced olive R when dyeing cotton yarn are as follows.
①Reduced olive R If the dye particle size is relatively coarse, due to the redox potential of the dye The negative value is high, and the specific surface area of the dye is relatively small. During the reduction, due to incomplete dissolution of the dye, the reduction is insufficient, and part of the unreduced dye lake is adsorbed on the yarn to form a “black skin” on the surface.
② During dry cylinder reduction, caustic soda and insurance powder The concentration is too high, the amount of water during reduction is too little, and the concentration of leuco sodium salt is too high, which can hinder the normal dissolution of the dye. Or the dyeing bath ratio is too small and the concentration is high, which will also cause the dye to dissolve poorly and form a lake, which will contaminate the yarn surface and produce “black skin”.
③The reduced olive R structure contains amido group. When the concentrated alkali or insurance powder is insufficient, It is prone to local oxidation and hydrolysis, causing some dyes to precipitate. Once adsorbed on the yarn surface, “black skin” will be formed. Regardless of whether it is reduced in a dry vat or during the dyeing process, excessive temperature will produce varying degrees of hydrolysis, producing deep and dark amino compounds, resulting in “black skin”.
④If the dye is oxidized with alkali, it is also easy to produce “black skin”.The reason is that the insurance powder on the surface of the yarn decomposes quickly, causing an imbalance in the ratio between the yarn and the caustic soda.
⑤When using cowhide glue as a retarder, if too much is used, the dye and the The binding speed of fibers causes surface staining, and in severe cases, “black skin”. How to prevent “black skin”, first of all, commercial dyes require fine particles, high strength, dry vat conditions, and appropriate amounts of caustic soda and insurance powder.
Two removal methods are introduced for the “black skin” defects that have occurred.
①After dyeing, the yarn is first washed and oxidized with water to remove part of the alkali, and then neutralized with 0.8-1.0ml/L 97.7% (66°B) sulfuric acid at room temperature, and then washed with clean water until neutral. properties, add 1g/L of soda ash to the soap cooking bath.
② Use 85% insurance powder 0.1~o.2g/L solution to gradually clean, you can also It is carried out in the same bath as soap cooking, but the amount of insurance powder needs to be doubled.
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