The main reasons for the phenomenon of "black skin" when reducing olive R in dyeing cotton yarn threads are as follows.
① reduction olive R if the dye particle size is coarse, because the dye redox potential negative value is high, the dye specific surface area is relatively small, in the reduction due to incomplete dissolution of the dye, resulting in insufficient reduction, part of the unreduced dye precipitation adsorbed in the spinning line to form the surface of the "black skin".
② dry cylinder restore, caustic soda and insurance powder concentration is too high, restore when the water is too little, the sodium salt concentration of the hidden color body is too high, can prevent the normal dissolution of dyes. Or coloring bath ratio is too small, high concentration, will also make the dye dissolve poorly to form a precipitate, staining the surface of the spinning yarn, resulting in "black skin".
③ reduction olive R structure contains acylamino, in the concentrated alkali or insurance powder is not enough, easy to produce local oxidative hydrolysis, so that part of the dye precipitation, once adsorbed on the surface of the spinning yarn to form a "black skin". And no matter when dry cylinder reduction, or in the process of coloring, the temperature is too high will produce different degrees of hydrolysis, the generation of dark color and dark amino compounds, resulting in "black skin".
④ If the dye with alkali oxidation, also easy to produce "black skin", the reason is because the spinning yarn surface insurance powder quickly decomposition, so that the ratio of its and caustic soda out of balance.
⑤ with cowhide gum for slow dyeing agent, if the amount is too much, reducing the rate of combination of dyestuff and fiber, resulting in surface coloring, serious when caused by the "black skin". How to prevent "black skin", first of all, the commercial dyes require fine particles, high force, dry tank conditions, caustic soda, insurance powder dosage should be appropriate.
To have produced "black skin" defects, the introduction of two removal methods.
① coloring after spinning yarn first oxidation with water washing, remove part of the alkali, and then with 97.7% (66 ° B) sulfuric acid 0.8 ~ 1.0ml / L, at room temperature and neutralize the alkali, and then wash with water to neutral, soap cooking bath with soda ash 1g / L.
② with 85% insurance powder 0.1 ~ o.2g / L solution to gradually clean, and soap cooking with the bath, but the amount of insurance powder needs to be doubled.
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