overview
The Evolution of Tanning The tanning process used originally was a many months’
long process using vegetable tannins such as roots or bark from trees such as oak; “Tannenbaum” means oak and fi r in old High German, from the Latin “tanna”, hence the Eng- lish words tanning and tanner. At the beginning of the 20th century a quick process was developed using chrome salts (Chrome III sulfate). In the last 15 years another process has been developed based on the use of aldehydes, which has similarities with vegetable tan- ning, and does not use metals such as chrome. This was initially utilized in the automo- tive fi eld by Audi and BMW, to make many components chromium-free and recyclable. Today, chrome tanning and aldehyde tanning coexist, especially in the automotive
Cangilones tannery drums from Spain are used in the liming and tanning processes.
industry; however traditional vegetable tanning is still used for some products such as bags, saddles and sole leather. From a production engineering point of view, the chal- lenge for the tanner is to produce from a variable raw material such as a cattle hide, leather with consistent qualities that meet the required standards for each application. For automotive leather specifi cally, the fi nished product has to meet strict tensile, fl ex, wear and abrasion standards, as well as light fastness and resistance to weathering as leather in a vehicle can be subject both to severe cold as well as extreme heat from sun shining through the windows.
The Process Begins The fi rst steps of leather production occur at a slaughter- house, which in the US is mostly also a meat-packing com- pany. Approximately 33 million head of cattle are slaughtered in the US each year. The hides of about 35% are tanned at the meat packing houses, producing what in the leather industry is called “wet blue” (chrome tanned) or “wet white” (aldehyde tanned) hides. The hide is removed from the animal immediately after slaughter with “hide pullers”. Then, in the US, the hide usually goes through concrete brine-fi lled raceways, and afterwards
through fl eshing machines, which have spiral blades that remove most of the fl esh from the underside of the hide. Brine curing or salting of hides is done to conserve them, especially if they next have to travel to a distant location. Next the hides are processed in large wooden drums manufactured of special tropical hardwood. Traditionally these drums used “pegs” or “pins” in their interior, these being similar in shape to those used in a bowling alley. Nowadays to decrease the water usage, increase the loading capacity, lower the power consumption and increase the chemicals’ penetration, a patented type of drum with wide shelves of the Cangilones brand from the Olcina Group (Lorca, Spain) is being used. Cangilones means water wheel in Spanish. In the drums the hides go through a washing/soaking
process, then a liming process to remove the hair, a deliming, bating and a pickling process to reach the suitable pH. Then while still in the drums, comes the tanning process itself where the hides reach the “wet blue” or “wet white” stage. These drums typically can have a diameter of up to 4.2 m and a width of 4.5 m, capable of handling loads of up to 20 to 24 tons.
Going Through the Wringer The next process is wringing with either a machine with cylinders covered with felt sleeves or in the more modern process with through-feed machines with felt conveyors, and then the splitting process, which consists of a rotating band- knife to separate the “grain” side of the hide from the split (underside). The hides can also be split after liming. If tanned in a meat-packing plant the hides then are trans- ported for further processing to other plants. The next process after wetting back, if necessary, is “shaving”, where machines with spiral blades on a cylinder placed against a rubber roll take shavings of the underside of the hides, to equalize the thickness to a desired value. Next chemical processes are used including retanning, fatliquoring and dyeing. These pro- cesses also are done in drums, but of a smaller size. Fatliquor- ing reintroduces fats into the leather to replace those which
24 — Motorized Vehicle Manufacturing 2015
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