Although various shoe products with bleached canvas uppers have clear texture and pure white color, they are prone to yellowing. This is because it is cellulose fiber, contains 6%-10% natural impurities, and is subjected to 1.96×105Pa (2kg/cm2) pressure, 130℃, and vulcanization baking for 1 hour in a vulcanization cylinder containing a large number of chemical reagents. .
Ways to improve the yellowing of bleached cotton canvas caused by baking are:
1. Scouring conditions: Boiling cloth pot scouring is usually at 1.96×105Pa
(2kg/cm2) pressure for 4-5 hours. Although most impurities have been removed at 100°C, increasing the temperature to 116-140°C is beneficial to the removal of wax. The caustic soda content in refining should be 7-8g/L, and scouring agents such as water glass, sodium bisulfite, and surfactants need to be added. The use of synthetic detergents will help improve wool efficiency.
2. Rolling alkali and stacking detergent: Directly scouring the fabric, the wool effect only reaches about 6cm, while after rolling alkali and stacking detergent, the wool effect is greatly increased. Experience shows that when caustic soda is 10-12g/L and detergent is 2g/L, the liquid temperature is 85°C and the semi-finished product is stacked for 20 hours, the whiteness of the semi-finished product is high and the degree of yellowing is very small after baking. Moreover, the above-mentioned cooking can be omitted.
3. “Acid-bleach-acid” bleaching process: After scouring the fabric, the “acid-bleach-acid” process is used, which is beneficial to removing pigments and improving whiteness. Pickling after scouring can remove acid-soluble impurities and reduce the pressure of bleaching, thereby significantly improving the whiteness. Add oxalic acid g/L during the second acid rolling, because oxalic acid has a complexing effect and can more effectively remove impurities and improve whiteness.
4. Water washing: Water washing will not cause serious yellowing. Measures to improve the washing effect and save water include: First, the number of liquid quenching rollers should be larger and the pressure should be higher to facilitate the exchange of impurities and lotion and reduce the liquid carrying rate on the fabric. Secondly, the water supply method should be reverse flow, and the route of the lotion flow should be appropriate, rather than overflowing in a short flow.
5. Reasonable bluing and whitening: The fabric has been scoured and bleached, and the pigment has been basically removed. It has a considerable whiteness, but still has a slight brown color. Through fluorescent whitening and bluing at the same time, the fabric can be made to have an obvious white feeling without leaving traces of yellow light.
6. pH value: For bleached cotton canvas for shoes, the pH value should be 6.5-9.5. Otherwise, the whitening effect of the fluorescent whitening agent will be quickly lost once baked at high temperature. This is because the structure of VBL determines its resistance to acids and bases. sensitivity.
7. Fluorescent whitening agent dosage: Generally, the dosage of brightener in pad dyeing whitening solution should be 1.3-1.8g/L, and should not exceed 2g/L. Excessive dosage will not only have no whitening effect, but will cause yellowing on the contrary.
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