Method of restoring soft for air-dried wet blue leather

It is a very important operation step to make the retanning wet blue leather fully restoring soft before wet processing after tanning.  The effect of retanning, dyeing and fatliquoring will be greatly reduced due to insufficient soft and water filling. The problems such as rigid and stiff leather, split surface and color ton  are often closely related to insufficient restoring soft and water filling. In general, fresh wet blue leather with uniform chrome tanning is relatively easy to restore soft, but if wet blue leather is partially or completely air-dried due to improper storage or storing for too long time, it becomes very difficult to restore soft. Experience tells us that it is difficult to make the water content of air-dried wet blue leather reach the requirement even if the leather is soaked for several days in the restoring soft bath if we use conventional method. Therefore, in the actual production, some strengthening measures must be taken when the air-dried wet blue leather restoring soft. After summing up, the main measures to strengthen the air-dried wet blue leather restore soft are as follows:

1. Extend the soaking time of restoring soft leather, usually it is 20-60 minutes for normal leather, but for air-dried leather, we can extend the soaking time by a large margin, and even soak overnight

2. Increase the restore soft temperature. The conventional restore soft temperature is usually about 40℃, and the temperature can be raised to about 50℃for air-dried leather.

3. Use an appropriate volume of osmotic agent to help the leather filling water and restoring soft.

4. Use saline water to restore soft. The temperature can be controlled at 40-50℃, the concentration of saline water can be controlled at about 10%, the volume of water will be enough if it can inundate the leather, the restore soft time depends on the degree of restore soft

5. Use saline water to restore soft under the condition of low pH value. Adjust the pH value to about 3 by using sulfuric acid and formic acid, the temperature was controlled at 35-40℃, and the salt concentration should be controlled above 4 waves. Do not wash the blue leather before it restores soft to retain its salt and save the added salt. When it restores soft under low pH value, the liquid ratio should not be too large, otherwise it will consume a lot of salt.

6, Use waste chrome liquid to restore soft. While using the waste chromium solution to restore soft, the pH value can be reduced appropriately by using formic acid, and an appropriate volume of penetrant should be added.

7. Use modified glutaraldehyde to restore soft. First, we can restore soft by conventional methods, in which the temperature was controlled at about 45 ° C and the pH value was controlled at about 3.5. After several hours of softening, add 3-4% modified glutaraldehyde and rotate for 20-30 minutes, soak overnight.

The above measures can be used seperately or in conjunction with each other. When the air drying is not too serious, you can take measures 1 & 2 & 3, when the air drying is more serious, you should consider taking measures 5 & 6 & 7, when the pH value is low, you need to be careful about the high temperature, the dosage of degreasing agent depends on the oil content of leather

Excerpted from 500 Questions on Tanning Technology (edited by Gao Xiaozhong and Gao Lingyun)