UNHAIRING and LIMING
Loosening (depilation) or unhairing may be considered as an extension of soaking. Its purpose is to separate the two structural proteins keratin and collagen.The aim of unhairing (depilation) and liming is to remove the hair, epidermis and to some degree the inter-fibrillary proteins, and to prepare the hide for removal of loose flesh and fat by the fleshing process.
Unhairing:
Methods of hair removing can be divided into two groups:
1) methods based on destruction or modification of the epidermis tissue surrounding the hair, so that it can be loosened and removed
2) methods in which hair itself is attacked and its structure is destroyed (use of alkali Ca(OH)2 or NaOH and Na2S).
-Sweating: The earliest method was “sweating” and may still be used if the wool is of much greater value than the skins. Soaked skins are hung up in dark humid rooms (22-27 0C), bacteria attack keratin cells of hair and epidermis, until wool is loose. The wool is then “pulled” and sorted and the skins are rinsed and thrown in lime liquor to stop further putrefaction. The pulled wool is hydroextracted to remove water and dried to 16 % moisture in which condition it is marketed.
Cathepsins contained in the lysosomes of the cells of the skin are also participating in this process. For autolytic unhairing an optimal pH is about 4. In this ‘lysosomal’ unhairing probably proteins and gycosaminoglycans are equally attacked. In the products separated from the skin, hydroxyproline is found in some amounts, equivalent to 0.3% of skin collagen. This may be result of attacking of collagen-containing ‘lining of the hair pocket.
A development of sweating process is enzyme unhairing.
-Enzyme unhairing: sweating is an uncontrolled enzyme process. Enzyme unhairing is acontrolled process. Enzyme preparations that preferentially attack the keratin cells at base of hair roots or epidermis are available. Enzymes are specific in their action and are active within a narrow range of temperature and pH. Since optimum temperature for unhairing enzymes are close to those for bacterial growth, it is essential to use some disinfectant to prevent bacterial putrefaction.
Enzymatic processes may give some difficulties in a tannery. Enzyme attack may be veryvigorous, too much protein may be removed from the skin and collagen may be partly decomposed. The resultant hides are then thin and stiff. If the enzyme action is too weak, insufficient amount of protein will be digested, and additional operations necessary, like eg. Alkali swelling of hides before pickling(after liming). The enzymatically unhaired hides as a rule have to be tanned and dyed in a different way than hides processed by other techniques.
According to Felicjaniak, who invested in detail the unhairing activity of pancreatic enzymes, there is a distinct difference in unhairing and proteolytic action of those enzymes. To increase unhairing activity, the pelt has to be prepared by applying an inorganic chemical before enzyme is used. The compounds giving optimal results are ammonium chloride, thiocyanate, sodium thiosulphate and some others. These substances increase the unhairing activity when used in 1% per pelt weight. Sodiumthiosulphate increases proteolytic(not unhairing) activity of the enzyme.Optimal unhairing activity was reached at pH 8-9 (somewhat higher than optimal proteolytic activity pHopt=7.5).
Soaked skins are paddled or drained in a water float 28-300C with 1-2 % of specific enzyme preparation at pH 8-9 for ~4 hours. Bacterial contamination may be prevented by adding 0.2 % sodium chlorite.
-Paint unhairing: The washed or soaked skins are piled to drain off surplus water and then painted, or sprayed on the flesh side with a “paint” which may be made from approximately 50 parts hydrated lime, 50 parts water and 5-2- parts sodium sulphide (fused). The sodium sulphide and lime dissolves in the water and penetrate through the corium and dissolve the keratin cells which enclose the hair roots. The process may take 5-12 hours depending on thickness of the skin, tightness of fiber structure, and amount of fat and flesh left on the skin. Green fleshing before painting can be good.
Green fleshing is a method of giving some mechanical action. May be done by hand by scraping the flesh with a curved knife on a wooden beam or by a fleshing machine. Apart from the squeezing action loose fat, flesh or muscle tissue is removed, aiding entry of water from the flesh side. It also flattens and stretches the skin and has a cleaning action.
Advantages of the process:
-strong alkali (lime) and sodium sulphide prevent putrefaction therefore give better skins than sweating.
-with reasonable control, hair loosening is reliable.
-unhairing is quicker and owing to shortage of water in painted skins, the strong alkali cannot cause undue swelling, buckling and distortion of the skin.
-the amount of paint can be varied over the area of the skin, giving more to thicker backbone and less to thinner, loose flanks and bellies.
Disadvantages of the process:
-requires more labour
-wool yield is less (owing to the disintigration of wool roots)
Lime and sodium sulphide damage hair and wool, causing a harsness to the touch or weakening of strength and eventually complete disintigration. This action is a function of sulphide concentration.
Variations of the process:
Sodium sulphide + water à caustic soda + sodium hydrosulphide too much sodium sulphide may weaken the skin as it produces caustic soda in water. Adding wetting agents does not improve penetration unless used in large quantities (excessive spread down the wool shaft, with loss of wool or quality). On greasy skin, excessive soluble lime produce lime soaps which give water resistance to the skin and may result in poor tan.
The painted skins are usually given a modified liming after unhairing, to remove unwanted protein and prepare them for fleshing or splitting.
Alternative materials:
Sodium hydrosulphide when used instead of sodium sulphide gives no alkaline swelling, does not weaken the skin, causes less damage to the wool. The penetration power is not as good as sodium sulphide, especially on greasy skins.
Calcium hydrosulphide is even milder and gives very good wool and skins.
Arsenic sulphide was popular(before the above chemicals were introduced), poisonous.
If sharpened lime is used, pH is higher than in the absence of alkali, but the swelling is greater, despite the fact that the pH is higher than that of maximum swelling due to
( i) an increased breakdown of structural restrains
( ii)replacement of divalent calcium ions by monovalent sodium ions.
When Na2S is used for depilation the swelling effect is the same as that for NaOH, since H2S is a very weak acid.
When calciumhydrosulphide is used, the depilatory process can take place without any additional swelling. The tanner can vary independently depilatory and swelling effects.
Liming:
The major chemical modification of collagen during liming is the hydrolysis of some of the amido groups attached to aspartic and glutamic acid residues.
-CONH2 + HOH -> -COOH + NH3 (pH decrease)
as the carboxyl group can ionize:
-COOH -> COO- + H+
more potential negatively charged centers are introduced so that the isoelectric pH of collagen is invariably reduced by liming.
A small portion of arginine residues is also converted to ornithine and urea.
-CH2CH2CH2NH-C=NH -> -CH2CH2CH2NH2 + CO(NH2)2
I
……………………..NH2
Apart from these reactions, some modifications of covalent cross-links may occur, especially of ester type, which are thought to join chains together.
The total effect is that liming produces a pelt which swells more at all pH values than does native, unlimed skin. Forces of swelling lead to a general loosening of the fiber- network layer and to the splitting of larger collagen fibers.
In normal tannery liming, apart from breakage of a few peptide main chains by hydrolysis, the fibrils swell and not show any marked changes in their general appearance. However they are cleaner, as interfibrillary material (mostly globular proteins and mucopolysaccharides) is removed.Thus liming helps to prepare a clean system of fibrils.
Liming may be enhanced by duration process, raising temperature,and raising pH of liquor.
Unhairing and liming may be carried out at the same time by immersing the hides and skins completely in lime and water mixtures, often with the addition of other chemicals called sharpeners.
Liming process may be carried out without lime in certain cases, e.g., with greasy skins the lime is replaced by other alkali such as caustic soda.
Straight lime liquors:
Water dissolves a relatively small amount of lime. And lime is unusual because less dissolves with increasing temperature. Approximately 1/8 parts lime per 100 parts water gives a clear solution.
The alkali solution causes the collagen fiber of the corium to swell by absorbing more water. The hair and epidermis swell to a lesser extent, and the interfibrillary proteins become more soluble and are loosened from the structure.
These effects occur with all soluble alkalis and the stronger the alkalinity, the greater the effect. As lime has a limited solubility compared with other alkalis, it is considered safe for hides and skins.
Under very alkali conditions, some of the young keratin decomposes to produce sulphur compounds, these, in conjuction with lime, accelerate the break-down of further keratin. Thus, the lime causes unhairing and the more keratin break-down impurities it contains, the more rapidly it unhairs. This is why old lime liquors are more effective for unhairing. All these reactions areaccelerated by increase in time, temperature and high alkalinity.
The alkali also modifies and breaks down the collagen fibre of the skin, but much more slowly than the keratin. Therefore, if skins are limed too long they suffer from thinness, looseness and weakness.
For skins that have already been unhaired: straight lime liquors of 2 parts lime per 100 parts water are used. The skins are immersed in about 5-6 times their weight of this liquor (in paddles or slowly revolving drums-agitated), for 12-60 hours at 13-18 0C.
This quantity of lime is in excess of that needed to get the necessary alkalinity (pH 12-13) plus the lime chemically fixed to the hide. The surplus may be spent by adsorption on grease, loose protein or by carbonation. It also makes handling the slippery hides easier. However this excess lime often requires expensive effluent treatment.
One Pit Liming system:
Hides are laid or suspended in a pit in a similar lime solution. The undissolved lime tends to settle to the bottom of the pit and therefore not available for dissolving to maintain the 1/8 % solution. It may be agitated by moving the frame the hides are suspended from or by rocking a scraper arm moving along the pit bottom.
After 1-2 days in this liquor to allow the lime to start swelling the hides uniformly and gradually, 1/8 – ¼ % sodium sulphide may be added. This is a sharpener that speeds up the process:
– by producing sodium hydrosulphide, which very quickly attacks the keratin, giving hair loosening;
– by producing caustic soda, which increases the alkalinity and therefore the rate of swelling.
On the third or fourth day, in the case of ox hides, a further similar addition of sulphide may be made to finish the process. If too much sulphide is added too quickly, however, rapid unhairing results, but this is accompanied by excessive swelling of the surfaces of the hide, while the interior is unswollen.
The used liquor may be drained away but as it has become stronger in unhairing power and less alkaline, it has good properties for starting the liming of the next pack of hides.
According to old technologists ‘art of tanning’ was the knowledge how to mix the fresh and used lime liquors. In fresh liquor swelling is stronger and opening up weaker, old liquors (often infected by microorganisms) are better hair looseners. A similar effect may be obtained by adding methyl amine to lime liquor.
The “modified one pit” or the “three pit” system may be used.
Modified One Pit System:
About half of the previously used lime liquor is left in the pit, which is then topped up with water. 1 % lime is added and the goods limed in this as before, except that the time and the amount of sodium sulphide added may be reduced.
Three Pit system:
This system is more thorough development of the same idea, the pack of hides being placed succesively in three pits for 2-3 days each.
A) The first pit contains a twice used lime liquor.
B) The second pit contains a once-used lime liquor with an addition of ½ % lime and ¼ % sodium sulphide.
C) The third pit contains a fresh liquor of 2% lime and ¼ % sodium sulphide.
All these percentages are based on the volume of the water. This is called a “counter current” system, the hides moving from pit to pit one way, whilst the oldest liquor A is run away and replaced with a fresh liquor C.
Disadvantage: laborious
One liquor system more common (can controlled more accurately).
Drum Unhairing:
When the hair is of little value and the hides are of a quality which will not suffer from the process, they may be drummed in a relatively strong sodium sulphide solution, for example: 300 % water on hide wt. At 16 C, 2-5 % sodium sulphide (fused), 12 % salt.
After 6 hours the hair and epidermis are reduced to a pulp (lapa), which can be washed off, and the hides are well swollen. The salts limit the swelling, which may also be reduced by replacing part of sodium sulphide with sodium hydrosulphide.
1 % lime is often added to the sulphide solution .
NO LIME TREATMENTS are favored for thinner skins(hair, sheep,goat). Advantages are absence of lime soap formation which may cause uneven tanning and dyeing.
Hair or wool may be previously removed by a “lime free” sulphide paint. Usually sulphide or hydrosulphide is used adjusted to pH 12.6-13 at a liquor concentration of about 0.2 % caustic soda. As the temperature increases to 28-30 C less swelling occurs giving a finer flatter grain and although hydrolysis of the skin occurs this is slower and can be controlled by time(6-8 hours).
Sharpening Agents:
Sodium sulphide- speeds up unhairing and alkaline swelling
Sodium hydrosulphide and arsenic sulphide- speeds up unhairing only(not alkaline swelling)
Caustic soda-sodium hydroxide- causes increased swelling only.
Sodium carbonate- mild alkali, reacts with lime to give caustic soda.
Ammonia– has a gentle swelling action on the skin, particular in early stages. It is formed naturally in old lime liquor.
Amines- dimethyl amines are added to lime liquors, having a gentle swelling action and helping hair removal.
Salt: at low concentration (below 2 % ) it increases the swelling or plumping action of the lime liquors, but at high concentration it tends to decrease the swelling or plumping and to give a softer, spongier type of leather.
Temperature: temperature control is important. An increase from 16-27 C in the liming temperature will halve the time required for loosening the hair; more significantly, it will double the rate of solution of collagen. Normally temperatures 10-16 are used in Europe but some modern processes go higher.
Limed skin should never be unduly exposed to the air, as carbondioxide in the air may convert the lime to hard, insoluble calcium carbonate.
Use of amines in lime liquors:
In recycling lime liquors, aliphatic amines were found to be in increasing amounts with time and enhance unhairing. Perhaps the unhairing effect is due to their properties as reductors. They bleach pelts which may be explained by a reduction process. Unhairing by dimethylamine sulfate (DMAS) in alkali (NaOH) was found to satisfactory and two significant remarks were done:
1) DMAS may be used for unhairing as main component (suitable for purposes where hair saving is required).
2) NaOH was successfully used.
3) Further observations of sulfideless unhairing have led(Somerville) to the conclusions that it is possible to apply short liquors (1:2)in unhairing without sulfides and a (1:1) liquor in case of liquors containing 3% NaOH, 1% Na2SO4, 0.5% NaSH, 1%DMAS calculated on hide weight.
Operation time 24 hours.
Lyotropic agents act by structure making of the solution, they contribute to the dissolving of the protein. Urea and NaCl affect hide in the same way. Unhairing affect is a property of compounds dissolving protein molecule through transferring H-bonds stabilizing protein molecules on the solute molecules thus forming protein-solute bonds.
Thermal unhairing (scalding): used for pigskins. Epidermis is more sensitive to thermal decomposition than other layers and short time scalding doesn’t influence collagen. Pigskin is passed very quickly between rollers, one of them heated to 160-200 C, contact time is 0.05-1.0 s. In this process the grain proteins are denatured, however denaturation is a very shallow, and nubuck skin is obtained without buffing. Denaturated layer dissolves very easily or may be seaparated spontaneously.
Oxidative unhairing (Rosenbusch):
5 NaClO2 + 4 HCl -> 4 ClO2 + 5 NaCl + 2 H2O
4 keratin-S-S-keratin + 10 ClO2 + 4 H2O -> 8 keratin-SO3H + Cl2
Chlorine dioxide reacts with keratin and splits disulfide bonds. Glycolic acid is used to maintain pH 3.0-3.5, t<40 C, time:24 hours.
When the process is finished excess ClO2 decomposed by use of sodiumthiosulfate, inorder to prevent oxidizing of chrome tanning agents(later).
This process replacing all the beamhose operations: degreasing,pickling and alumpretanning by binding of chlorine to collagen, has some disadvantages, therefore not used in industry.
1) Has to be carried in acidic medium. All the materials used in construction of machines in the beamhouse need to be replaced by acid resistant ones.
2) Expensive
3) Toxic gases are formed
4) Leathers obtained are stiff and spready.
Measurements used in control processes:
In many processing the quantity of chemicals to be used are based on the percentages on skin weight. It is always necessary to check whether this is dry skin weight or wet skin weight. It is often specified “limed weight”, “shaved weight”, “crust dry weight”, where these refer to the weight of the leather passing through these processes, which may have caused it to absorb more water, or cut away some of its thickness, or reduced its weight by drying.
Leather is normally sold on the basis of a price per unit weight or per unit area.
