The crocodile skin is long and narrow, with thick and hard scales on its surface, and its granules are very close and delicate.

On the organizational structure, the abdomen and the dorsal part of the body and the lateral part of the body are quite different. The abdominal scales are mostly square, relatively flat, soft and white, while the dorsal part is mostly bulge like the scales of the broad bean. There are lots of pigments on the scales, and there are strong choked bones in the scales as support.

The skin value of the skin of the crocodile is different from that of the abdomen. Therefore, the back and abdomen are often separated from the side of the body, and some special treatments are made according to different requirements.

Make

Whether it is the skin shape of the alligator skin or the characteristics of its grain organization, it decides that the technology of crocodile leather is different from other kinds of leather making technology. Aiming at the characteristics of the crocodile skin, we mainly design the technology from several key points, such as reducing crocodile skin wear, gray scale, bleaching and tanning.

Mechanical action and chemical material selection

Because crocodile skin has fine grain, with raised patterns, and a skeleton structure on the side, it is easy to abrade the skin in operation. Therefore, around this problem, we have carefully considered the choice of machinery and chemical materials.

The strength of mechanical action depends mainly on the rotation speed, rotation time and liquid ratio of rotary drum. Therefore, in the operation of soaking and immersing, we take a rotational speed of 1-2r/min, a liquid ratio of 10, and turn to 2min per 2H; in the operation of degrading softening, the rotary speed of rotary drum is 3-4r/min, the liquid ratio is 4 (with tare weight), stopping 10min at every turn of 20min, total 3H; before tanning, pre tanning, pre tanning speed 3-4r/min, initial liquid ratio of 3, rotating drum stop combination, first turn 20min, after every 0.5h turn to 5min, and stop drums for the night.

The speed of chrome tanning and retanning is 3-4r/min..

Selection of chemical materials

It is difficult to achieve the desired effect only by lowering the mechanical function.

In the course of practice, we have carefully chosen chemical materials.

在浸水工序為了加速回軟速度及回軟均勻選擇了德瑞公司(TEL)的浸水和浸水助劑，浸灰使用了德瑞公司的浸灰助劑，使灰咸對皮纖維的作用更深透、更均勻，并且使皮避免過度堿膨脹，同時選擇超細(xì)度的石灰粉并進(jìn)行過濾，盡量減少浸灰時固體顆粒對粒面的磨損;復(fù)類除使用一定的浸灰助劑外還加入一定量的增滑劑，以減輕機(jī)械磨擦對皮的傷害;軟化采用德瑞公司的OO酶制劑，進(jìn)行低溫、長時間的處理;在鞣制中，由于轉(zhuǎn)鼓轉(zhuǎn)動時間長，同時皮子因鞣制粒面變緊實;使機(jī)械擦傷機(jī)會變大;所以工藝制定更應(yīng)該慎之又慎，從浸酸開始我們就分步加入一定量的增滑劑，以減輕摩擦作用，并采取預(yù)加脂的辦法，這里我們選用了Trumpler公司的OSL加脂劑，這種加脂劑耐酸、堿、鹽性能好，與皮結(jié)合牢固，皮表面油蠟感強(qiáng)，從而避免粒面的擦傷，避免皮子的纏結(jié);選用高溶解度的鉻粉，以

Avoid excessive impurity scratching grain, alkalinity 33%, such as Bayer company's ChromsalB; in retanning, fatliquor dyeing process, is also in line with the role of mild, fast penetration, plump and soft as the principle of choice, the latter part will introduce these contents.

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Liming alkali

Soot base is the key to leather technology of crocodile skin.

The scale of the crocodile skin is thick and hard, and it is difficult to dissolve it with the usual amount of sulphide base.

But it is well known that high concentration of ash and alkali can easily cause excessive expansion of leather and a great loss of cortex, resulting in irreparable effects on the quality of leather.

Therefore, after many tests and adjustments, we have adopted the combined process of pre calcining scale and ash alkali scale.

The pre calcining scale is pre treated with sodium sulphide, which is 6% of the wet weight of the salt before the degreasing operation. The scale is loosened to a certain extent before the liming, so as to facilitate the scaling and dissolution of the scales and make the leather expand slowly.

In the alkali leaching operation, because of the large liquid ratio, in order to avoid excessive expansion of the leather caused by the use of large dosage of sodium sulfide (20% of the salt temperature tare), 5Be concentration of salt was added to inhibit the possible excessive expansion. Then, the liquid ratio was expanded step by step and lime powder was added at the same time, so as to ensure the leather to dilate and expand, then stop the drum for the night, add a certain amount of sulphide alkali to the next day and stop for the night.

After this treatment, the ash scales are thoroughly scald, with moderate expansion, soft handle and clear grain.

bleach

There is a lot of pigments on the surface of crocodile skin. If not removed, it will seriously affect the subsequent dyeing effect and ultimately affect the quality of finished leather.

There are many ways to remove pigments: the main methods are oxidation and reduction.

The chemical materials commonly used in oxidation are H2O2, KMnO4, NaCIO2 and so on.

Materials commonly used in reduction are SO2, NaH-SO3 and other substances.

Comparing these methods, we found that the bleaching effect of H2O2, KmnO4 and NaCIO2 alone is better, but the skin with heavier pigment is still not thorough.

Here we choose the bleaching process according to the color of the raw leather and the color required in the finishing process. If the pigment is lighter or the color of the finishing is deeper, we will use 1 oxidation decolorization or simply do not decolorization.

On the contrary, the combination of oxidation and reduction is adopted.

According to the material properties and its influence on the cortex, we used materials such as NaCIO2, H2O2, KmnO4 and Na2S2O5 in operation.

Tanning technology is a very important process in tanning process. The tanning process will determine the physical and chemical properties and uses of leather. On the contrary, the different varieties and uses also determine the difference of tanning process.

Because crocodile leather is mostly used for shoes, bags, belts and other leather materials, and men wear more and dark color is the main factor. In combination with these factors, we have adopted the route of pre fatliquoring, fatty aldehyde pre tanning and chrome tanning.

The OSL and fatliquoring agent of Trumpler company were selected. The fatliquoring agent has strong ability of resisting acid, alkali, salt and mineral substance. It is firmly combined with leather and has good softness. The surface wax feeling is strong, which can protect the surface of the leather very well. It is also beneficial to the uniform distribution of chrome tanning agent.

It is expected that the PF aldehyde tanning agent of Zschimmer&Schwarz company is selected. It has a mild effect on the skin surface, improves the skin fullness and softness, brightens the color, and has a strong skin reservation ability. It is very conducive to the distribution and combination of the chrome tanning agent.

The chrome powder is ChromsalB. of Bayer company. It is a low alkalinity chrome tanning agent. It has good solubility and permeability. It is evenly distributed in the skin, making the blue leather uniform and soft.

The CromnoFB of TFL company is chosen as the caustic agent. Its mild alkali extraction and temperature raising function can further improve the uniform distribution of chromium and the uniformity of surface color and chromium, so that the leather can achieve better tanning effect.

Retanning, dyeing and Fatliquoring

With the improvement of people's living standard, people pursue leather's softness, fullness and comfort.

Crocodile leather is a kind of special, high-grade leather and dry finishing. It usually needs polishing and polishing. Therefore, in combination with these 2 reasons, the purpose is to strengthen tanning with the aim of selecting mild and highly permeable materials. In process operation, PAK neutralization tannins, which are better in neutralization, neutralization, permeability and cushioning, are used in the process operation, combined with sodium formate to stop the drum for the night so as to make the whole leather evenly and uniformly, open the fibre channel, create good conditions for the subsequent retanning dyeing and fatliquoring, and finally make the leather coloring thoroughly and thoroughly, soft, plump and firm.

Retanning materials are mainly OS, R7 and LevotanC of Bayer company, ReluganGTW, BasytanANF and Rel-uganRV of BASF company, and a certain amount of wattle tannin extract is used to make leather complete, soft and firm. The fatliquoring agent is chosen by using TrumponDB and TrumaonBRD and TrumponDB of Trumpler company, so that the fullness and softness of leather can reach the requirement of soft upper leather.