Further processing is as follows:
1. unhairing and degreasing:
The purpose of this process is to remove the oil in skin cells and the hair on the original skin surface, exposing the natural fine line surface. If degreasing is not complete, the skin will harden after drying.
2. Dawn:
Lime soaking, also known as alkali expansion, refers to adding alkali (usually Ca(OH)2) to make the skin expand to a certain thickness, which is beneficial to stratification. Generally, the first floor is used to put high-grade clothes, shoes and so on. The second floor is used for low-grade leather goods, and the price on the first floor is much higher than that on the second floor.
3. Desliming and pickling:
This procedure is to remove excess alkali in the previous liming process; In fact, it is also a process of acid-base neutralization. Adding proper acid to neutralize excess alkali to reach the PH value required for tanning.
4. tanning:
Tanning refers to the use of trivalent chromium ions to connect the collagen groups of leather fibers, giving leather a full feel and enhancing the tensile and tear strength of leather. This is the most important process in leather manufacturing technology, and it is also the most critical step to determine the quality of leather. Generally, the PH value is about 3.8-4.2, the temperature is about 40℃, and the time is 8- 12h. There are also many tanning agents used in this sequence, which can be selected according to the purpose of leather. If it is heavy leather (soles of hands, holsters, etc.). ), commonly used vegetable tanning agent, with strong filling performance, high hardness and thick skin. If it is light leather, you can choose according to the color. If it is light-colored leather (such as white), you can choose aluminum tanning agent, which is beneficial to dyeing. For dark leather, chrome tanning agent is generally used, because this tanning agent has higher binding rate and is more washable.
5, retanning:
It occupies a very important position in modern tanning technology, and is even known as "golden touch". For light leather, retanning is an essential process. Because this process can supplement tanning production and improve tannin content. Because of the different tightness of fiber weaving in different parts of hides, there is a position difference, which is difficult to eliminate even if chrome tanning. At this time, if retanning agent with strong filling function is used, the position difference can be effectively reduced and the soft parts such as abdomen can be well filled.
The physical properties of the whole leather are more uniform, which can improve the cutting rate of leather. Generally, the temperature of this process is about 40℃ and the PH value is about 5.0, and the time can be adjusted according to the choice of retanning agent, which is about 30 minutes.
6, dyeing:
Its purpose is to give leather a certain color. The key is to improve the combination rate of dyes and leather fibers, improve the combination strength of dyes and leather fibers, and be washable. At this time, the temperature, PH value and time should depend on the choice of different dyes. For example, reactive dyes X (dyeing at 20℃-40℃) and KN (dyeing at 40℃-60℃) generally have a PH of about 5.0, a temperature of 40℃-60℃ and a time of 1-2h.
7. Fatliquoring:
Also known as oiling, its essence is to properly and reasonably fatliquore hides, so that they can absorb appropriate fatliquoring substances, restore their original softness and elasticity, prevent leather from being stiff and cracked, and endow leather with certain use performance. In the previous degreasing process, we removed the grease in the cells, and here the grease we added is filled in the middle of leather fibers, which can reduce the friction of leather fibers and increase the fluidity between fibers, thus becoming soft and full of hand. However, fatliquoring will also reduce porosity, thus reducing air permeability. Generally, the pH is about 5.0, the temperature is about 60℃ and the time is1.5h..
8. Drying:
After this process, the leather will enter the finishing stage, so it is very important. If it is naturally dried, the shrinkage rate of the skin will be small, but the time will be too long. This method is basically not used at present. Generally, gelatinization drying is used, but this will make the leather shrink greatly and affect the leather yield, so there are the following procedures.
9. Softening:
After drying, it is also important for the skin to shrink, harden and soften. This process adopts mechanical methods, such as softening. You can also pull the leather back and forth on the stretching plate, so that the leather can be stretched as much as possible and the leather retention rate can be improved.
10, finishing:
In the process of manufacturing light leather, the appearance quality before and after plays a decisive role in the manufacture of leather products and the choice of consumers. Although the inherent quality and feel of leather can meet the requirements, the final coating is unsightly or easy to fall off, which will affect the use value of leather products.