The manufacture of leather follows the same general steps for a great variety of leathers (figure below). The largest category of hides tanned is cattle hides . Of the cattle hides chrome tanning of unhaired hides is by far the dominant system used throughout the world. The tanning of other types of hides and skins requires variations in the systems used for cattle hides.
Fig: Process flow diagram for the production of chrome-tanned cattle hides leather
The temporary preservation of hides or skin is known as curing. Curing is not tanning in that the hides or skin not stabilized to a bio-refractive state, cured hides immediately begin to decay if there is a change in conditions that removes the curing agent or permits bacterial actions.Salt curing is generally accepted curing method for cattle and other large animal hides. Curing can be done using solid salt or brine. The solid salt method is the more general and is used worldwide.
Salt cured cattle hides, when recovered at the tannery are individually bundled to prevent excessive moisture loss. The bundles of hides may be sorted for different weight or quality classification at this point. In the modern large tannery, the size/quality classification is not necessary because the hides arrive in car load quantities under specification as to size, type, and month of slaughter.
Soaking: The hides are weighed and counted in to production batches. For pre-fleshed and trimmed cattle hides .The batches are about 3-5 ton when the processing is in drums. If the tannery has hide processors the batches may be up to 10 ton. Water is added to cover the hides and allow free movement of the load. The drum is turned intermittently during the normal 8 -16 hours soaking period.The concentration of the salt solution is about 3-5g/100ml. At this concentration some of the soluble proteins disperse. The soak water removes the salt, some proteins, some loose fat, blood, dirt, and manure. If the hides were not fleshed before curing, the soaked hides are usually fleshed and trimmed at this time. If the hides are not to be trimmed or fleshed, they are drained and washed to decrease the salt concentration, drained, and the drum refilled with cold water.
Unhairing: Unhairing can be done either by a hair save or a hair pulp system. The hair pulp system is preferred by most tanners for its speed and labor efficiency. In the hair pulp system the hides are treated with sodium sulfide (sulfhydrate) and lime (calcium hydroxide). The hair is quickly destroyed by the strong alkaline reducing conditions.
Trimming and fleshing: the limed hides not fleshed before curing are usually trimmed and fleshed at this point in the production. The trimming is done by hand to remove any portion of the hide that could interfere with the subsequent machine processes, e.g.: the shanks, ears, and snout.
The fleshing is done on a multi roller machine that pulls the hide over a rotating blade, similar to rolling machine that cuts off the flesh from the inside of the hide. The machine includes a rubber roller that holds the hide near the rotating fleshing blades.
Splitting:- In splitting the hides are cut to the desired thickness with a horizontal belt knife. The hides are fed in to the machine grain up. The clearance between the grain and the blade is maintained by a series of narrow rollers supported by a rubber roller underneath the spacing rollers. The grain layer is then cut to the thickness desired to an accuracy of about 0.1mm.
The grain layer is most valuable part of the hide and serves as the outside of the shoe. Splitting allows the grain layer to spread to the maximum area yield and also allows an efficient use of the valuable tanning chemicals.
Deliming and Bating:-The limed hides have a PH around 12. Because chrome tanning is done at PH 2-4 , the lime must be removed for PH adjustment. In addition to the undesirable materials in the hide, i.e. both natural and the degradation products from the unhairing must be removed.
For deliming, ammonium salts and acids are used. The proportion of ammonium salts to acids and the type of acids employed is a matter of tanners’ choice. The acid neutralizes the lime, Ca (OH) 2, there by adjusting the PH. The ammonium salts have two functions: to buffer the solution to a PH required for bating and to form calcium ammonium complexes. The acidity and the complexes’ formation solublize the calcium and serve to bring the hide to the desired PH.
Bating is a part of the deliming step in cattle hide leather production. The hides contain some elastins proteins which are very inert to the action of acids and bases and react to tanning chemicals in a limited manner if the elastins are not broken down sufficiently. The leather may be firm and stiff for the desired use, and the grain may not be as smooth as desired. The bating enzymes and the PH adjustment disperse much of the degradation products from the unhairing. The resulting hides are clean and flaccid.
Pickling:-Pickling is the term used for acidification of the hides. For chrome tanning, the desired PH is about 2.0, thus the hides are placed in a solution of salt and acid.
Once pickled, the hides can be drained and stored indefinitely. The possibility of bacterial or mold damage is always present, but under proper pickling and storage the hides or skins can be traded on the world market.
Chrome tanning is the most widely used tannery system worldwide. It has the advantages of light color, speed of processing, low costs and great stability of the resulting leather. Chrome tanned leather is so stable that exposure to boiling water for short (<2-3 minutes) periods of time usually shows no adverse effect.
Tanning refers to a specific reaction of the tanning chemicals combining with the hide or skin to stabilize the protein and make it resistant to bacterial degradation. Tanned leather is so biorefractive that leathers even centuries old found under adverse natural storage conditions are often in almost usable condition.
Chrome tanning is done in a drum similar to that used for deliming. The salt solution from the pickle is present and the solution is at about PH 2.0. The chrome tanning material is usually a basic chromium sulfate. The general formula for the most common commercial chrome tanning product is 2Cr(OH)SO4,Na2 SO4.After the addition of the chrome tanning salts, the PH of the solution rises to about 2.5. At this PH the chrome salt is taken up by the hide and the tannage begins.
At a PH > 3.0 the reactivity of the hide to the chromium complex is greatly increased. The PH is therefore raised gradually to the desired point by addition of a mild alkali, usually sodium bicarbonate. The chrome tanned leather is removed from the drum and wrung to remove the absorbed tanning solution. The leather is then inspected for quality of the grain and other characteristics of importance for the leather being made. In large tanneries where very uniform hides are worked ,the leather may be trimmed and split to the desired thickness. Hides split before tanning need no splitting at this point.
Retanning, coloring and fatiliquoring
Chrome-tanned leather is light blue in color. The fibers are only stabilized against microbial action and do not have feel of leather. If the leather were dried at this point only a stiff unattractive product would result. The characteristics of desired leather result from the retanning, coloring, and fatiliquoring. Fatiliquoring is term applied to the oiling of leather.
The retanned leather is stretched to increase the area for the best yield and to produce a flat leather surface. The leather is the then dried by hanging in a dry loft.
Staking is a mild flexing of the leather to bring the leather to the desired final softness. Modern tanneries use a staking machine employing a number of vertically opposing oscillating rounded rods.
The buffing step consists of a light sand papering of the grain. If the grain surface of the leather is free of blemishes, it may be good enough for full grain where the full beauty of the natural hide surface is visible.
The application of finish results in a protective and/or decorative coat. Leather finishes must minimally be abrasion resistant and flexible, and must adhere to the leather. Formation of tough water –resistant film is also desirable.
Grading, measuring and shipping
Grading is done on the basis of hide defects, shape of the skin, manufacturing defects, or any other factors of importance to the specifications of the sale. The leather in the grade to be shipped is measured for area. In modern tanneries an electronic measuring machine is used.
Adapted from: Shreves, 1984, Chemical Industrial processes, McGraw Hill Publishing
Adapted from: Shreves, 1984, Chemical Industrial processes, McGraw Hill Publishing
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