In order to turn hide into a usable raw material, hide is processed many times during kneading process. The main purpose of skin treatment can be divided into two points. First, soften leather, second, apply anti-rust treatment to leather to make it anti-corrosion. The so-called softening refers to meaning of word "tanning". If leather is dried directly without tanning, collagen fibers in leather will bind together to form a hard "raw hide". The purpose of tanning is to make gaps in collagen fibers in skin moderate, soft and flexible.
Today, given cost and purpose of tanning, most leather manufacturers use "vegetable tannins" (also "synthetic tannins") or "chromium" metal compounds as tannins in leather tanning process. The first option is called vegetable tanning. ", and latter is called "chrome tanning". Due to differences in tanning processes, leather obtained from two tanning processes will also have different characteristics. In addition, there is also a "combination tanning method" in which different tanning methods are mixed. The main method of combined tanning is to properly add tannins after chrome tanning.Based on firmness and softness of chrome tanning, after addition of vegetable tanned tannins, plasticity of leather is increased and it has a natural feel similar to natural raw materials, thus forming a leather that combines respective advantages of two tanned leathers.During leather tanning process, people can properly control subtle changes in leather texture by adjusting ratio of chromium and tannins.
Characteristics of vegetable tanned leather
①High voltage and good strength
Vegetable tanned leather can be made into tougher leather. At present, vegetable tanned leather is still main material for manufacture of wear-resistant, tough harness and technical leathers. People have mastered technology of making vegetable tanned leather. Mastered technology of oiling, calendering and vegetable tanning.
Plasticity refers to property of vegetable-tanned leather to retain its existing shape after it has been altered. According to nature of vegetable tanned leather, people can make leather products with good toughness, which is fully consistent with storage items, and can also flexibly shape clothes and shoes according to user's physical condition, allowing users to experience benefits of leather, convenience. But on other hand, since vegetable tanned leather easily stains and dents, it is also called plastic leather.
③Strong water absorption
Compared to chrome tanned leather, vegetable tanned leather has better water absorption performance and removes stains easily, which makes it not only easily damaged by moisture, but also has advantage of being easily stained.
④Can change color over time
Vegetable tanned leather has a high plasticity and changes color over time. Vegetable tanned leather can become dark brown over time, which gives vegetable tanned leather a unique charm and is widely popular with public.
⑤ Vegetable tanned leather takes a long time to tan
The tanning process for vegetable tanned hides can take months, depending on circumstances, making finished vegetable tanned leather extremely time consuming and expensive. Because of this, its output is small, and price is high.
Characteristics of chrome tanned leather
①Soft, thin and flexible
Since chrome-tanned leather is soft, light and pleasant to touch, it is suitable for making tight-fitting garments.
②Elasticity and heat resistance
Because chrome-tanned leather is elastic, it is not easy to scratch or deform, and it also has good heat resistance. Chrome-tanned leather can be used for products that require high strength.
③Easy to produce and process, low price
Chrome-tanned leather can be produced in large quantities in a short period of time, and its supply is stable and affordable.
Because chrome-tanned leather can have different colors and patterns, it is a raw material with different ductility.
⑤Contains metallic substances
At low temperatures and unsafe environments, when chrome-tanned leather burns, harmful substances are formed - hexavalent chromium, so it is dangerous. However, chrome-tanned leather produced in recent years can only burn when temperature reaches a certain level, and danger is reduced. Chromium-free tanned leather will not cause adverse reactions in people with metal allergies or children with sensitive skin, and chromium-free tanned leather attracts people's attention.
The process is pretty much same. It is generally divided into preparation process, tanning process and finishing process. The following is how each process works:
When tanning fur, raw materials must first be softened to restore state of fresh skin, and also remove components that are unnecessary during fur processing (subcutaneous tissue, connective tissue, meat residue, sarcolemma, etc.). Including following process:
Has a big impact on product quality. The working method must be strictly observed. The purpose of soaking in water is to restore raw hide to a fresh hide, remove some of soluble protein, and remove blood stains, feces, and other contaminants. The immersion temperature depends on type of raw hide, usually 15-18°C is suitable, if softening of skin is slow at temperatures below 18°C, bacteria easily multiply at temperatures above 20°C. Soaking time, generally salt or salt-dried skin soaked in running water for 5-6 hours, if dry skin or salt-dried skin has been stored for a long time, it should be softened by physical or chemical methods during soaking. . Soaking time can be 20-24 hours. It is required that skin is not exposed to water surface, wet, wet and uniform. Particular attention should be paid to temperature control and immersion time when immersed in water. Adopt rawhide swelling principle in alkaline water to shorten rawhide soaking time and inhibit breeding of bacteria. Sodium sulfide, sodium hydroxide, ammonium hydroxide, sodium sulfite, etc. are often added to soaking water. Caustic soda consumption 0.2-0 .5 g / l, sodium sulfide 0.5-1 g / l.
Soaked in water to soften fur. Lay inside up on semi-circular log and use an onion knife to scrape off fat and meat residue stuck to surface of meat. log when cleared. Cutting is closely related to next degreasing step. If there is a lot of residual fat, degreasing is not good. The effect of scraping inside with an onion knife is also that residual fat in skin is lifted to surface of skin through extrusion, which is beneficial for degreasing.
③Degreasing and washing
The quality of finished fur products depends on thoroughness of degreasing. During degreasing process, fat should be removed without damaging fur. The saponification method is relatively gentle, and its principle is to accurately use characteristics of alkali and oil to produce soap to remove oil from wool. If lye is too concentrated or a strong lye is used, keratin protein of fur can be damaged, making hair dull and brittle. Commonly used soda ash, its alkalineIt is weak and it can remove oil without damaging hair. However, concentration of alkali should also be controlled, if concentration is too low, purpose of degreasing will not be achieved, product will harden and leave original animal smell, which will also affect next step of tanning and make it difficult for leather to be hard. Not durable. Degreasing method: first prepare a degreasing liquid, 3 parts soap, 1 part sodium carbonate, 10 parts water, first cut soap, put it in water and boil until dissolved, then add sodium carbonate, dissolve and let cool for later use. Add warm water (38-40°C) 4-5 times weight of wet skin into container, add 5-10% of above degreasing solution (5% rabbit skin, sheep skin, 10% dog skin), then put cleaned fur and stir thoroughly. After 5-10 minutes, change liquid again, and then stir until specific oily smell of fur disappears, and degreasing liquid foam disappears. The degreasing solution can also be prepared from washing powder (3 g/l) and soda ash (0.5 g/l), so that ratio of treatment liquid to weight of wet skin is 10-12 times, heated to 38°C. , degreased and mixed for 40 minutes. It is better to use an enzyme-based laundry detergent on market, 3 grams of detergent per 1 liter of water, and 0.5 grams of soda ash. After degreasing fur for specified time, it should be washed immediately to remove soapy liquid, and then washed and rinsed. Let skin dry.
(Explain only alum tanning method, its source is simple and convenient). Prepare a tanning solution, 4-5 parts of alum, 3-5 parts of table salt, 100 parts of water. Dissolve alum in warm water first, then add remaining water and salt so that they are evenly mixed. Principle: after alum is dissolved in water, free sulfuric acid will be produced, which can make protein fiber in skin absorb water and swell. The purpose of adding salt is to suppress expansion, but amount of salt added depends on temperature. At low temperature, less salt can be added, and at high temperature, more salt can be added. Generally, 0.7-2 parts of salt can be added to 1 part of alum.
The ratio of solid and liquid is 4-5:1 (with weight of raw leather 1, tanning solution 4-5). Place it in a container so that fur is completely saturated with feed liquid. In order for feed liquid to penetrate bark evenly, it must be thoroughly mixed (it is best to use a drum), after overnight mixing once a day. , about 30 minutes each time, soaking for 7-10 days of tanning is finished.
When tanning is complete, you can take out impregnated leather, fold leather board outward, foldPush plush inside and press corners to squeeze out water. If folded part is white and unidentifiable, it looks like tissue paper, which proves that tanning is over. The water temperature during tanning is low, which not only prolongs tanning time, but also hardens bark. It is best to keep temperature around 30°C. After tanning, do not wash inner surface with water. Fur tanned with alum is white and soft, but not water or heat resistant.
The original fat in hide has been removed during processing. In order to form a protective layer of fat film around fibers of hide and improve softness, flexibility and strength of hide, fat must be added. Here is a relatively simple fattening formula for reference: 10 parts castor oil, 10 parts soap, 100 parts water, boil soap bars with water, and then slowly add castor oil to emulsify. Greasing method: Apply above grease solution to semi-dry inner surface of fur, after overlapping, allow to dry overnight.
After greasing and drying, skin becomes very hard. To facilitate scraping and softening, it must be properly sprinkled with water inside. This process is called "humidification". It can be applied with a brush or sprayed onto interior surface with a spray gun. For fur tanned with alum, due to its lack of water resistance, it is best to cover with a solution of alum tanning. Fold slathered fur in and out, wrap with oilcloth or polyethylene cloth, press down with a stone and leave overnight. Let it absorb water evenly, and then gently scrape it off.
Spread rehydrated fur on a semi-circular log, fur side down, and lightly scrape inside with a dull knife to lengthen fur fiber, expand area, and soften skin.
To make skin smooth after scraping, it must be shaped. Nail rough side to a wooden board, dry in shade, avoid exposure to sunlight and, after complete drying, sand surface with a pumice stone or sandpaper. dry in shade and then remove from nail board Remove trim. Then use a comb to comb your hair. If skin is torn, it should be sewn on, and that's it.