Unit - Chemistry of Garments: Leather

While an animal is alive, its skin is soft, flexible, very tough and hard wearing and essentially semipermeable: meaning that although water vapour can travel out, it is not able to penetrate and come in. This changes when the animal dies. If the skin is then kept moist it deteriorates by rotting, and if it is dried it goes hard and brittle.
The purpose of the tanning process is stabilise its structure so that the skin (from animals including goats and kangaroos, but more often cattle hide) can retain it's natural properties. After chemically processing the skin, it will no longer be subject to putrefecation.

Tanning is essentially the reaction of collagen fibers in the hide with tannins, chromium, alum, or other chemical agents. The most common tanning agents used in the U.S. are trivalent chromium and vegetable tannins extracted from specific tree barks. Alum, syntans (man-made chemicals), formaldehyde, glutaraldehyde, and heavy oils are other tanning agents.

There are approximately 111 leather tanning facilities in the United States. However, not every facility may perform the entire tanning or finishing process. Leather tanning and finishing facilities are most prevalent in the northeast and midwest states; Pennsylvania, Massachusetts, New York, and Wisconsin account for almost half of the facilities. The number of tanneries in the United States has significantly decreased in the last 40 years due to the development of synthetic substitutes for leather, increased leather imports, and environmental regulations.

Traditionally, tanning used tannins, bitter plant polyphenolic compounds that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids.

The term tannin (from tanna, an Old High German word for oak or fir tree, as in Tannenbaum) refers to the use of wood tannins from oak in tanning animal hides into leather; hence the words "tan" and "tanning" for the treatment of leather. However, the term "tannin" by extension is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with proteins and other macromolecules.
tannic acid
The structure of some (gallo)tannic acids

Leather is a durable and flexible material and can be produced through manufacturing processes that range from cottage to heavy industries.
The processing can be divided into three sub-processes: the preparatory stage, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating may be added into the sequence. The list of operations that leathers undergo varies with the type of leather required, but can be roughly described by the following eight steps:

Step 1 - Unhairing
The animal skins are steeped in an alkali solution that breaks down the structure of the hair at its weakest point (the root) and so removes the hair.
Step 2 - Liming
The hairless skin is immersed in a solution of alkali and sulfide to complete the removal of the hair and to alter the properties of the skin protein (collagen). The collagen becomes chemically modified and swells, leaving a more open structure.
Step 3 - Deliming and Bating
The skin structure is then opened further by treatment with enzymes, and further unwanted material is removed.
Step 4 - Pickling
The skins are then treated with acid to preserve them for up to two years
Step 5 - Tanning
This is the most chemically complex step. During tanning, the skin structure is stabilised in its open form by replacing some of the collagen with complex ions of chromium. Depending on the compounds used, the colour and texture of the leather changes. When leather has been tanned it is able to 'breathe' and to withstand 100 °C boiling water, as well as being much more flexible than an untreated dead skin.
Step 6 - Neutralising, Dyeing and Fat Liquoring
The leather is then treated with alkali to neutralise it and so prevent deterioration, and then dyed. This involves fixing a variety of compounds onto the chromium, as that is the most reactive site present. Once the leather is dyed, it is treated with reactive oils that attach themselves to the fibrous structure, improving suppleness and flexibility.
Step 7 - Drying
Water is removed from the leather, and its chemical properties stabilised.
Step 8 - Finishing
A surface coating is applied to ensure an even colour and texture, and to improve its ability to wear. Suede leather is also buffed at this point to give it its distinctive finish.

Heavy leathers and sole leathers are produced by the vegetable tanning process, the oldest of any process in use in the leather tanning industry.

In the vegetable tanning process, the concentration of the tanning materials starts out low and is gradually increased as the tannage proceeds. It usually takes 3 weeks for the tanning material to penetrate to the center of the hide. The skins or hides are then wrung and may be cropped or split; heavy hides may be retanned and scrubbed. For sole leather, the hides are commonly dipped in vats or drums containing sodium bicarbonate or sulfuric acid for bleaching and removal of surface tannins. Materials such as lignosulfate, corn sugar, oils, and specialty chemicals may be added to the leather. The leather is then set out to smooth and dry and may then undergo further finishing steps. However, a high percentage of vegetable-tanned leathers do not undergo retanning, coloring, fatliquoring, or finishing.

Leather may be dried by any of five common methods. Air drying is the simplest method. The leather is hung or placed on racks and dried by the natural circulation of air around it. A toggling unit consists of a number of screens placed in a dryer that has controlled temperature and humidity. In a pasting unit, leathers are pasted on large sheets of plate glass, porcelain, or metal and sent through a tunnel dryer with several controlled temperature and humidity zones. In vacuum drying, the leather is spread out, grain down, on a smooth surface to which heat is applied. A vacuum hood is placed over the surface, and a vacuum is applied to aid in drying the leather. High-frequency drying involves the use of a high frequency electromagnetic field to dry the leather.

Chrome-tanned leather tends to be softer and more pliable than vegetable-tanned leather, has higher thermal stability, is very stable in water, and takes less time to produce than vegetable-tanned leather. Almost all leather made from lighter-weight cattle hides and from the skin of sheep, lambs, goats, and pigs is chrome tanned. The first steps of the process (soaking, fleshing, liming/dehairing, deliming, bating, and pickling) and the drying/finishing steps are essentially the same as in vegetable tanning. However, in chrome tanning, the additional processes of retanning, dyeing, and fatliquoring are usually performed to produce usable leathers and a preliminary degreasing step may be necessary when using animal skins, such as sheepskin.

Leather that is not subject to scuffs and scratches need only be surface dyed. For other types of leather (e.g., shoe leather) the dye must penetrate further into the leather. Typical dyestuffs are aniline-based compounds that combine with the skin to form an insoluble compound.

Leather Finishing
Leathers may be finished in a variety of ways: buffed with fine abrasives to produce a suede finish; waxed, shellacked, or treated with pigments, dyes, and resins to achieve a smooth, polished surface and the desired colour; or lacquered with urethane for a glossy patent leather. Water-based or solvent-based finishes may then be applied to the leather. Plating is used to smooth the surface of the coating materials and bond them to the grain. Hides may be embossed.

Environmental Issues
1 metric ton of raw hide yields 250-300 kg of leather but also leaves 600 kg of solid waste, including sludge.

The presence of Cr(VI) in leather and leather products is a cause of concern. Cr(VI) is bioaccumulating, highly toxic, mutagenic and carcinogenic to humans. Cr(VI) is not used at any stage in the manufacturing process, only Cr(III) which is not carcinogenic. Where does the oxidation occur and how it can be avoided are critical questions for chemists working in the leather industry.

See the YouTube video for how Kiwi shoe polish is made.

Prior to 1906, shoe polish was not well known as a commercial product, nor was it particularly sophisticated. While sales were not especially high, a few brands existed, like Nugget, during the 19th century. The practice of shining shoes gradually caught on and soon shoeshine boys in city streets were offering shoe shines using a basic form of shoe polish along with a polishing cloth.

Shoe polish (or boot polish), is a waxy paste, cream, or liquid used to polish, shine, and waterproof leather shoes or boots to extend the footwear's life, and restore, maintain and improve their appearance. Various substances have been used as shoe polish for hundreds of years, starting with natural substances such as wax and tallow. Modern polish formulas were introduced early in the 20th century and many of those original formulations are still in use today. Today, shoe polish is usually made from a mix of natural and synthetic materials, including naphtha, turpentine, dyes, and gum arabic, using straightforward chemical engineering processes. Shoe polish is usually flammable, can be toxic, and, if misused, can stain skin.

The first shoe polish to resemble the modern varieties (aimed primarily at inducing shine) was Kiwi. Scottish expatriates William Ramsay and Hamilton McKellan began making "boot polish" in a small factory in 1904 in Melbourne, Australia. Their formula was a major improvement on previous brands. It preserved shoe leather, made it shine, and restored color. By the time Kiwi Dark Tan was released in 1908, it incorporated agents that added suppleness and water resistance. Australian-made boot polish was then considered the world's best. Black and a range of colors became available, and exports to Britain, continental Europe, and New Zealand began.

Shoe polish consists of a waxy colloidal emulsion, a substance composed of a number of partially immiscible liquids and solids mixed together. It is usually made from ingredients including some or all of naphtha, lanolin, turpentine, wax (often Carnauba wax), gum arabic, ethylene glycol, and if required a colourant, such as carbon black or an azo dye (such as aniline yellow). It typically has a specific gravity of 0.8, is negligibly soluble in water, and is made of between 65 and 77% volatile substances - usually naphtha.

Carnauba wax consists mostly of aliphatic esters (40 wt%), diesters of 4-hydroxycinnamic acid (a.k.a. p-coumaric acid 21.0 wt%), ω-hydroxycarboxylic acids (13.0 wt%) and fatty acid alcohols (12 wt%). The high amount of volatile substances means that the shoe polish will dry out and harden after application, while retaining its shine.

New Zealand Institute of Chemistry (NZIC) article on Leather processing
FAO report
EPA report on Leather Tanning

Much of the information in these course notes has been sourced from Wikipedia under the Creative Commons License. Students taking this course will be expected to contribute to Wikipedia as a part of their course assignments.

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Created February 2013. Links checked and/or last modified 12th October 2016.
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