![ Chromium(III) sulfate ([Cr(H2O)6]2(SO4)3) has long been regarded as the most
efficient and effective tanning agent. Chromium(III) compounds of the sort
used in tanning are significantly less toxic than hexavalent chromium.
Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation,
[Cr(H2O)6]3+, which at higher pH undergoes processes called olationto give
polychromium(III) compounds that are active in tanning, being the cross-
linking of the collagen subunits.
The chemistry of [Cr(H2O)6]3+ is more complex in the tanning bath rather
than in water due to the presence of a variety of ligands. Some ligands
include the sulfate anion, the collagen's carboxyl groups, amine groups from
the side chains of the amino acids and masking agents.
Masking agents are carboxylic acids, such as acetic acid, used to suppress
formation of polychromium(III) chains. Masking agents allow the tanner to
further increase the pH to increase collagen's reactivity without inhibiting the
penetration of the chromium(III) complexes.
Collagen is characterized by a high content of glycine proline, and
hydroxyproline usually in the repeat -gly-pro-hypro-gly-.[These residues
give rise to collagen's helical structure. Collagen's high content of
hydroxyproline allows for significant cross-linking by hydrogen bonding
within the helical structure.
Ionized carboxyl groups (RCO2
−) are formed by hydrolysis of the collagen by
the action of hydroxide. This conversion occurs during the liming process,
before introduction of the tanning agent (chromium salts). The ionized
carboxyl groups coordinate as ligands to the chromium(III) centers of the
oxo-hydroxide clusters.
Tanning increases the spacing between protein chains in collagen from 10 to
17 Å.]The difference is consistent with cross-linking by polychromium
species, of the sort arising from olation and oxolation.](https://image.slidesharecdn.com/unit2-190128094741/85/Unit-2-Materials-and-methods-of-Tanning-4-320.jpg)
Unit 2-Materials and methods of Tanning
- 1. Vegetable tans The vegetable tans are water extracted from plant barks ,leaves etc. It consists of large polyphenol molecules with some acidic groups and high secondary valency of potential. catechol tans The catechol process is a increases in molecular size when boiled in an acid solution. pyrogallol tans The pyrogallol tans are usually more yellow brown catehols.its sugar content often leads to acid fermentation during long tannage. vegetable tanning process and applications of vegetable tanned leather Vegetable tanning is one of the oldest methods known. Primitive people have developed vegetable tanning system based on meterials available locally. Application of vegetable tanned leather The process is used in order to stabilise the skin and be able to process the resulting leather.
- 2. Chrome tanning There are two process of chrome tanning i) One bath process ii) Two bath process. Chemicals used-method type of leather The chemical name is sodium sulphate, sodium hydrosulphide, causdic soda soda ash ammonium sulphate, ammonium chloride ,fomic acid, sulphuric acid Leather uses Leather isoften used to bind or finish books,foot wear fashionable foot wear is one of the most common uses for leather,including boots,shoes,sillpers, gloves, watches, cases,acessories. Artificial leather It is a material intended to substitute for leather in fields such as clothing, footwear, and fabrics Synthetic tans It is used in tanneries as replacement syntans ,to compliment the tanning action made by the natural tanning. Aldehyde tannage This type of tannage is done by Formalin solution contanning 40%, formaldehyde 8-10%, methanol.It is used for tanning white, washable leather.
- 3. Vegetable tanning uses tannins(a class of polyphenol astringent chemicals), which occur naturally in the bark and leaves of many plants. Tannins bind to the collagen proteins in the hide and coat them, causing them to become less water-soluble and more resistant to bacterial attack The process also causes the hide to become more flexible. The primary barks processed in bark mill and used in modern times are chestnut oak redoul, tanoak, hemlock, quebracho mangrove, wattle (acacia; see catecholand myrobalans from Terminalia spp., such as Terminalia chebula. Hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. Vegetable-tanned hide is not very flexible and is used for luggage, furniture, footwear, belts, and other clothing accessories.
- 4. Chromium(III) sulfate ([Cr(H2O)6]2(SO4)3) has long been regarded as the most efficient and effective tanning agent. Chromium(III) compounds of the sort used in tanning are significantly less toxic than hexavalent chromium. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, [Cr(H2O)6]3+, which at higher pH undergoes processes called olationto give polychromium(III) compounds that are active in tanning, being the cross- linking of the collagen subunits. The chemistry of [Cr(H2O)6]3+ is more complex in the tanning bath rather than in water due to the presence of a variety of ligands. Some ligands include the sulfate anion, the collagen's carboxyl groups, amine groups from the side chains of the amino acids and masking agents. Masking agents are carboxylic acids, such as acetic acid, used to suppress formation of polychromium(III) chains. Masking agents allow the tanner to further increase the pH to increase collagen's reactivity without inhibiting the penetration of the chromium(III) complexes. Collagen is characterized by a high content of glycine proline, and hydroxyproline usually in the repeat -gly-pro-hypro-gly-.[These residues give rise to collagen's helical structure. Collagen's high content of hydroxyproline allows for significant cross-linking by hydrogen bonding within the helical structure. Ionized carboxyl groups (RCO2 −) are formed by hydrolysis of the collagen by the action of hydroxide. This conversion occurs during the liming process, before introduction of the tanning agent (chromium salts). The ionized carboxyl groups coordinate as ligands to the chromium(III) centers of the oxo-hydroxide clusters. Tanning increases the spacing between protein chains in collagen from 10 to 17 Å.]The difference is consistent with cross-linking by polychromium species, of the sort arising from olation and oxolation.
- 5. Vegetable-tanned leather is tanned using tannins extracted from vegetable mattersuch as tree barkprepared in bark mills. It is the oldest known method. It is supple and brown in color, with the exact shade depending on the mix of materials and the color of the skin. The color tan derives its name from the appearance of undyed vegetable- tanned leather. Vegetable-tanned leather is not stable in water; it tends to discolor, and if left to soak and then dry, it shrinks and becomes harder. This is a feature of oak-bark-tanned leather that is exploited in traditional shoemaking. In hot water, it shrinks drastically and partly congeals, becoming rigid and eventually brittle. Boiled leather is an example of this, where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances. Historically, it was occasionally used as armorafter hardening, and it has also been used for book binding. Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other chromium salts . It is also known as "wet blue" for the pale blue color of the undyed leather. The chrome tanning method usually takes approximately one day to complete, making it best suited for large-scale industrial use. This is the most common method in modern use. It is more supple and pliable than vegetable-tanned leather and does not discolor or lose shape as drastically in water as vegetable-tanned. However, there are environmental concerns with this tanning method, as chromium is a heavy metal.
- 6. Aldehyde-tanned leather is tanned using glutaraldehyde or oxazolidine compounds. It is referred to as "wet white" due to its pale cream color. It is the main type of "chrome-free" leather, often seen in shoes for infants and automobiles. Formaldehyde has been used for tanning in the past; it is being phased out due to danger to workers and sensitivity of many people to formaldehyde. This type of tannage is done by formalin solution containing 40% foramaldehyde 8-10% methanol. It is used for tanning white washable leather. The skin are prepared toapH of 4.0 or 5.0 in any suitable manner heavily picked skin requires de pickling to attain the correct pH’ A suitable de picking bath is prepared by sodium acetate.salt and water. The skin are well in the closed drum overnight and the ashed up.in this process ½-1 ½% of soda ash is dissolved in 50- 100% water and added to the drum until the pH of the skin is not less than 8.
- 8. Artificial leathers are often used in clothing fabrics, furniture upholstery, and automotive uses.One disadvantage of plastic- coated artificial leather is that it is not porous and does not allow air to pass through; thus, sweat can accumulate if it is used for clothing, car seat coverings, etc. One of its primary advantages, especially in cars, is that it requires little maintenance in comparison to leather, and does not crack or fade easily. Synthetic tan They are products made by the condensation of formaldehyde with the sulphonic acids of phenol and naphthalene. Recently, Silvateam has invested significant resources developing products with a low environmental impact by reducing the content of phenol and free formaldehyde. Synthetic tannins, Blancotan, are used in tanneries as replacement syntans, to compliment the tanning action made by the natural tanning extracts. They are used as auxiliaries for pretanning, retanning and combined tanning and as dispersing agents for vegetable tannins and dyestuffs.
- 9. Leather is often used to bind or finish books. . Clothing It is often used to make clothing, including pants, skirt, raincoats, and jackets. 3. Cuir Bouilli Cuir bouilli, French for boiled leather, was historically used to create body armor and flasks.
- 10. Couches, chairs, recliners, and even automotive interiors are made from leather. . Gloves From fashionable winter gloves to durable work gloves, leather is often used.
- 11. Leather is commonly used to make arrow quivers, knife sheaths, and gun holsters. 10. Bags Satchels, backpacks, wallets, and purses are all made from leather.