Leather production processes

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The leather manufacturing process is divided into 3 fundamental sub-processes: preparatory stages, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating can be added into the leather process sequence but not all leathers receive surface treatment. It's difficult to create a list of operations that all leathers must undergo, as there are so many types of leather.

Contents

[edit] Production management

The leather making process is in general restricted to batch processing, but if the surface coating sub-process is added then some continuous processing can be included. The operation flow has to follow the preparatory → tanning → crusting → surface coating sub-process order without deviation, but some of the sub-processes can be omitted to make certain leathers (or partially tanned/ untanned products). It is not uncommon for tanneries to

[edit] Preparatory stages

The preparatory stages are when the hide/skin is prepared for tanning[1]. During the preparatory stages many of the unwanted raw skin components are removed. Many options for pretreatment of the skin exist. Not all of the options may be performed. Preparatory stages may include[2]:

  • preservation- the hide/skin is treated with a method which renders it temporarily unputrescible.
  • soaking - water for purposes of washing or rehydration is reintroduced.
  • liming - unwanted proteins and "opening up" is achieved.
  • unhairing - the majority of hair is removed.
  • fleshing - subcutaneous material is removed.
  • splitting - the hide/skin is cut into two or more horizontal layers.
  • reliming - the hide/skin is further treated to achieve more "opening up" or more protein removal.
  • deliming - liming and unhairing chemicals are removed from the pelt.
  • bating - proteolytic proteins are introduced to the skin to remove further proteins and to assist with softening of the pelt[3].
  • degreasing - natural fats/oils are stripped or as much as is possible from the hide/skin[4]..
  • frizing - physical removal of the fat layer inside the skin[5]. Also similar to Slicking.
  • bleaching - chemical modification of dark pigments to yield a lighter coloured pelt.
  • pickling - lowering of the pH value to the acidic region. Must be done in the presence of salts. Pickling is normally done to help with the penetration of certain tanning agents, e.g., chromium (and other metals), aldehydic and some polymeric tanning agents
  • depickling - raising of the pH out of the acidic region to assist with penetration of certain tanning agents

[edit] Tanning

Tanning is the process converts the protein of the raw hide or skin into a stable material which will not putrefy and is suitable for a wide variety of end applications. The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible material that when re-wetted (or wetted back) putrefy, whilst tanned material dries out to a flexible form that does not become putrid when wetted back. There is a large number of different tanning methods and materials that can be used, the choice is ultimately dependent on the end application of the leather. The most commonly used tanning material is chromium, which leaves the leather once tanned a pale blue colour (due to the chromium), this product is commonly called “wet blue”. The hides once they have finished pickling will typically be between pH of 2.8-3.2. At this point the hides would be loaded in a drum and immersed in a float containing the tanning liquor. The hides are allowed to soak (while the drum slowly rotates about its axle) and the tanning liquor slowly penetrates through the full substance of the hide. Regular checks will be made to see the penetration by cutting the cross section of a hide and observing the degree of penetration. Once a good even degree of penetration exists, the pH of the float is slowly raised in a process called basification. This basification process fixes the tanning material to the leather and the more tanning material fixed the higher the hydrothermal stability and increased shrinkage temperature resistance of the leather. The pH of the leather when chrome tanned would typically finish somewhere between 3.8-4.2[6].

[edit] Crusting

Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations:

  • wetting back - semi-processed leather is rehydrated.
  • sammying - 45-55%(m/m) water is squeezed out the leather.
  • splitting - the leather is split into one or more horizontal layers.
  • shaving - the leather is thinned using a machine which cuts leather fibres off.
  • rechroming - more chrome is fixed to the leather.
  • neutralisation - the pH of the leather is adjusted to a value between 4.5 and 6.5.
  • retanning - additional tanning agents are added to impart properties.
  • dyeing - the leather is coloured.
  • fatliquoring - fats/oils and waxes are fixed to the leather fibres.
  • filling - heavy/dense chemicals that make the leather harder and heavier are added.
  • stuffing - fats/oils and waxes are added between the fibres.
  • stripping - superficially fixed tannins are removed.
  • whitening - the colour of the leather is lightened.
  • fixation - all unbound chemicals are chemically bonded/trapped or removed from the leather
  • setting - area, grain flatness are imparted and excess water removed.
  • drying - the leather is dried to various moisture levels (commonly 14-25%).
  • conditioning - water is added to the leather to a level of 18-28%.
  • milling
  • staking
  • buffing

[edit] Surface coating

For some leathers a surface coating is applied. Tanners refer to this as finishing. Finishing operations may include:

  • oiling
  • brushing
  • padding
  • impregnation
  • buffing
  • spraying
  • roller coating
  • curtain coating
  • polishing
  • plating
  • embossing
  • ironing
  • ironing/combing (Hair-on)
  • glazing

[edit] References

  1. ^ Sharphouse, J.H., Leather Technician's Handbook, Leather Producer's Association, p. 37, ISBN 0950228516 
  2. ^ Leafe, M.K., Leather Technnologist's Pocket Book, Society of Leather Technologists and Chemists, p. 23 
  3. ^ Heidemann, E., Fundamentals of Leather Manufacture, Eduard Roether KG, p. 211, ISBN 3792902060 
  4. ^ Bienkiewiecz, K., Physical Chemistry of Leather Making, Robert E. Krieger, p. 298, ISBN 0898743044 
  5. ^ Sharphouse, J.H., Leather Technician's Handbook, Leather Producer's Association, p. 212, ISBN 0950228516 
  6. ^ Heidemann, E., Fundamentals of Leather Manufacture, Eduard Roether KG, p. 296, ISBN 3792902060