LEATHER SCIENCE: Pre - Tanning Operations

 PRE-TANNING OPERATIONS

1. Curing

2. Soaking

3. Liming

4. Fleashing and Splitting

5. Unhairing

6. Reliming

7. Deliming

8. Degreasing

9. Bating

10.Scudding

11.Pickling

A.Curing or Preservation of Hide/Skin:

Hides & skins after flaying from the dead animals liable to be decomposed by chemical or bacterial attacks if they are not preserved properly or processed immediately for Tanning. The smell of ammonia coming out from H/S is clear indication that the putrefaction has started.

Proper and timely preservation or curing will prevent the H/S from both bacterial decomposition & bio-mechanical changes. The curing is done to eliminate or dehydrate the moisture from H/S (up to less than 30%) to prevent the development of bacteria.

Three popular methods of curing are:

1. Wet Salting

2. Dry Salting

3. Drying

Before curing, the loose fleshes must be separated off and the dirt, the blood adhering to the hides or skins should be washed off well.

1. Wet salting:

The method involves application of a mixture of salt and preservative evenly on the flesh side by rubbing well and piling the H/S flesh to flesh. Quantity of salt used for hides (35%) is less than skins (50%) due to lesser moisture content. The percentage of salt is used based on green weight of H/S. Common salt or Khari salt (Sulphate of Magnesium & Sodium) are most commonly used for curing. After wet salting, the water content reduces to 35-40% and the H/S can be sound for one month approximately.

2. Dry salting: 

This method is applied for longer preservation (intended for long storage or export). The practice is to rub the flesh side with a solution of khari salt and then to dry the H/S in the shade. The process is repeated 3-4 times or more, if required. The final coat of salting often mixed with chalk with a view to make the flesh side white and attractive. This method is also known as “Plaster curing”.

3. Drying:

Two methods of curing by drying are: a.) Frame drying b.) Ordinary drying

a.) Frame drying: It consists of stretching the H/S lengthwise on bamboo frames and drying these in mild sun exposing the flesh side to the sun. Rapid drying under direct sun is not done to avoid formation of a hard crust on flesh and grain side.

b.) Ordinary drying: In this method the H/S are spread out on the ground and dried under sun. The H/S shrink and crump up, losing the shape. The over drying in this method can also cause Gelatinization of collagen and “Blisters”.

Dried hides are also known as “Flints”.

B. Soaking:

This is very first operation of making leather intended to restore the hides/skins to its original state.

Objective of Soaking:

l Cleaning by washing: To remove dirt, soluble proteins, blood stains, curing salts etc.

l Rehydrating: To rehydrate the stock so as to restore it as far as near to its fresh flayed condition, that is to say, about 70% water content. There should be no further gain of water.

Process Control:

· pH: 8 - 8.5

· Float temp. : 20-25* C

Processing Equipments:

Since water alone cannot make the cured hides and skins sufficiently soft, some mechanical action is required in Pit / Drum / Paddle along with addition of wetting agents as per the processing required.

Chemicals Used:

Some chemicals are added to the soak bath to accelerate and assist soaking.

Preservative prevents from bacterial decay during the operation.

· Wetting agent - 0.2%

· Surface active agents - 0.5%

(e.g. Sodium sulphide, sodium hydro sulphide, caustic soda, soda ash etc.)

Process Duration:

The soaking should be continued till all cementing together of the fibre structure resulting due to curing has been broken down. In case of dry salted & dried H/S the soaking takes from 24-36 hours (in winters 48 hours). But for green H/S (fresh stock) the soaking usually carried out by washing in 3-4 changes of water (2-3 hrs.). The wet salted H/S take 8-12 hours of soaking time. Too short soak can result in variation in the tightness of grain of upper leather within one batch. Badly soaked portion of hides and skins will not take up chemicals uniformly in subsequent operations. This can lead to an uneven grain break or to cracking of grain. On the other hand too long soak (particularly during the summer months) affects adversely on the flanks and other poorer structural parts of H/S. Hence the duration of soaking should be adjusted according to hide condition and the climate.

Soaking of green H/S:

The green hides and skins are washed in drum or paddle for 2-3 hours. In the absence of drums or paddles, the washing may be carried out in pits by 3-4 changes of water.

C. Liming: 

The most important operation after proper soaking is the process of removal of hair and the epidermal structures known as liming.

Objective of Liming:

l Unhairing/dewooling

l Removal of inter-fibrillary proteins (cementing substances) to make the final

l leather pliable, soft & porous.

l Swelling of fibre bundles into individual fibres and splitting of fibres into fibrils in

l order to expose to the subsequent action of tanning and other post tanning

l operations.

l To remove the natural fats present by saponification in order to obtain non

l patchy and uniform surface of final leather.

Process Control:

Hair destruction, degree of swelling and plumping, p H of liquor and the amount of soluble protein should be checked. The control limits are:

pH: 12 - 12.5  Float temp. : 25 - 28*C  

Process Equipment: Pit, Paddle, Drums etc.

The liming of goat skins is usually carried out in the Pit or Paddle whereas in case of

hides drum or paddle is used.

Chemicals used:

l Lime (Slaked)

l Sodium sulphide

l Sodium hydo sulphide (Less alkaline)

l Arsenic sulphide (mild reducing agent, expensive & poisonous)

l Sodium hydro sulphite (used in combination with sod. sulphide)

l Dimethylamine or Dimethylamine sulphate ( DMAS)

(Replacement of Sodium Sulphide for clean & pollution free liming)

Process Duration:

The total duration of liming depends upon the type of method involved and also upon the processing equipment.

· For Paddle or Drum: 18-24 hours

· For Pit : 36-48 hours

The drum or paddle should run 5 minutes every hour. Too much movement can impair the tightness of grain.

Liming Process: Liming process basically carried out in two steps:

a) Unhairing

b) Reliming

 

Unhairing:

It is the process of removal of keratinous material from the hides & skins. There are different basically two systems of unhairing or dewooling:

i) Hair saving system (By Painting)

ii) Hair pulping system (Hair is dissolved & reduced to pulp).

i) Hair saving system:

This system produces a softer leather with smoother grain but the quality of flanks is found poor. Drawn grain and Growth marks are less pronounced and area yield also improves in this case.

Paint Preparation

The paint is prepared with slaked lime,Sodium sulphide and sufficient water to make a solution of soupy consistency. The proportion of lime and sodium sulphide will depend upon the type of hides & skins, degree of unhairing and the duration for which the H/S are kept painted. For cow hide the usual composition of the paint is:

l Sodium sulphide : 2.5%

l Slaked Lime : 4%

l Water: 12-15% (On soaked wt.)

(Light calf and goat skins requires less concentration).

Paint application:

The paint is applied uniformly on the flesh side of hides & skins and piled for 10-12 hours (preferably overnight) keeping grain side exposed (flesh to flesh piling). The hides can be folded also down the spinal line& then piled. Unhairing is done by scrapping off with a blunt knife.

ii) Hair pulping system:

This system has a favourable effect on tightness of grain & quality of flanks The

unhairing is done with strong sodium sulphide lime liquor in a pit or in adrum or paddle.

The composition of the liquor is:

l  Water: 300-400% (In case of pit or paddle)

l 100-150% (In case of drum)

l Slaked lime: 3 - 4%

l Sodium Sulphide: 3 - 4.5%

l Surface active agent: 0.25%

Reliming: 

After unhairing process the actual swelling and plumping of H/S (depending upon the type of final leather produced) takes place during reliming operation. In other words the leather is made (laying the foundation) in this operation only. The fibre splitting, Reorientation of fibres after removal of cementing substances, fats etc. takes place during this operation. For reliming the concentration of sulphide is reduced ( upto 1%) and the lime concentration is increased (upto 10%) using caustic soda alongwith for enhanced swelling effect. For repressing swelling calcium chloride or sodium chloride can be

used.

When the desired swelling & plumping effect is obtained ( As per the final thickness of leather required) the p H of the cross section is checked to be upto the mark (12-13). Liming process is stopped at this stage.

D.Fleshing: 

After Liming, Fleshing operation (By hand or by machine) is carried out to remove the subcutis tissues, fat and flesh residues mechanically adhering to the pelt.

E. De-liming: 

De-liming is removal of mechanically deposited and chemically bound lime and of capillary lime by conversion into readily soluble salts. Delimimg also reduces swelling occurred during liming process.

Objective of De-liming:

l Removal of lime.

l Dissolving interfibrillary or cementing substances.

l Total or partial removal of Elastic fibres

l Loosening of Scud, removal of fats.

l Reduction of swelling.  

Process Control:

p H - 8 - 8.5

· Float Temp. - 28-30*C

Process Equipments: Drum, Paddle etc.

Chemicals used:

l Ammonium sulphate (commonly used) 0.75-1.0 %,

l Ammonium chloride,

l Boric acid,

l Acetic acid,

l Sodium bisulphate etc.

Deliming agents should be mild so as to avoid rapid change in p H on the surface of the leather, causing fibre structure distortion, poor grain break and loose flank in finished leather.

Process Duration: 45 minutes approx.

De-liming Process:

First the limed pelts are kept in fixed float overnight. Next morning they are washed with plain water for 15-30 minutes ( in drum/paddle). The de-liming agents should be added in new float. The p H is lowered as the lime is gradually removed from the skin by diffusion and the swelling effect on the hide during this operation are kept at a minimum. The drumming is done for 15-20 minutes. The cross sectional p H is checked to be 7-8. Then the bath is exhausted.

F. Degreasing: 

Objective:

The process of fats or grease removal from bated or pickled pelts is called Degreasing. The objective of degreasing is removal of natural fats from the hide/skin which could not be saponified during liming operation. The natural fats present in hides & skins cause trouble in tanning, dyeing & finishing operations. Also due to oxidation etc of fats during storage of leather Spue- formation is observed with objectionable odour in the surroundings.

Process:

The fat solvents like kerosene, methyl chloride, trichloroethylene, etc. are used for this purpose. These non-aqueous solvents are used alongwith emulsifiers (preferably non-ionic) 0.5- 1.5% on the pelt weight. The process takes 30-45 minutes (R.P.M. of drum should be17-18). After degreasing the pelts are washed well with 6% brine solution to remove the degreasing agents from the pelt.

G. Bating: 

It is the process of treatment hides & skins with enzymatic agents so as to prepare them for subsequent tanning which gives soft, pliable & permeable leather.

Objective of Bating:

l Further removal of lime & reduction of swelling

Loosening of dirt, short hair, grease, soap etc.

l Dissolution of some of protein degradation products present in the inter-fibrillary spaces & pores.

Process Control:

· p H - 7.5 - 8.5

· Float Temp. – 35 - 38* C

Process Equipments: Drum, Paddle etc. with minimum mechanical action.

Chemicals used:

In earlier times, fermented infusion of hen, pigeon or dog dung wear used for this purpose. Later on artificial bates found out to substitute the animal dung and are safer & more satisfactory. The artificial or synthetic bates consist of proteolytic enzymes absorbed on saw-dust, flour or china-clay, carries and contain in addition a known quantity of deliming agent. The enzymes (alkaline) are either pancreatic or bacterial in origin. After deliming upto required extent 0.25-0.75 % of bate on pelt weight is used.

Process Duration

In general, bating time for a side is 20-45 minutes. For calf skin the bating time is longer & amount of bate is the same. In case of glazed kid, a stronger bate and longer time is required.

Bating Process:

Bating process is carried out as per the requirement of the final leather. After treatment, the pelts are washed well with running water for 15-20 minutes and subsequently pickling operation can be carried out in fresh bath. The practical empirical tests for proper bating are:

l Pelt should be flaccid.

l The flesh should come off easily when scratched with finger nail

l The feel should be slippery.

l In case of glazed kid, the pelt should be porous & air must escape through folded skin when pressed.

H. Scudding:

Scud is basically the residual deposition of short hair, degraded epidermis skin pigments, soaps and soluble proteinous matter etc on the grain surface. Scudding is an essential operation of removing the scuds so as to get a very clear and smooth grain, also it imparts dye-levelling & uniform finished surface, specially for light colour leathers.

I. Pickling: 

Picking is the process by which the de-limed or bated pelt is brought to the desired p H level before tannage (Treatment of acid-salt solution). At the end of pickling operation, the hide/skin obtained is theoretically a purified network of protein.

Objective of Picking:

l p H adjustment before the tannage.

l Modification of the fibre structure.

l Adjustment of water contents for either preservation or tanning.

Process Control:

· p H - 2.4-2.6 (Final p H)

· Float Temp. – 20 - 25* C

Process Equipments: Drum

Chemicals used:

l  Acid: Sulphuric acid, Formic acid, Some aromatic acids

l Salt: Common salt, sodium sulphate, calcium formate, aluminium sulphate, potash alum etc.

The usual method of pickling is to treat the pelt with common salt and sulphuric acid, the object of salt being to prevent excessive acid swelling. The stronger salt solution gives the resulting leather flatter and loose in grain break (A good effect on grain tightness is achieved if all or half of the common salt is replaced by sodium sulphate). The use of buffer salts like calcium formate or sodium formate in sulphuric acid common salt pickle liquor, gives smoother leather with a fine break and round feel. Further, if formic acid and calcium formate are used in pickling, the drumming time can be shortened which is an advantage in the belly parts. Some tanners add weakly tanning substance such as potash alum (0.2-0.3%) and formaldehyde (0.2-0.3%) to give good results in pickle.

Process Duration:

The duration of pickling depend upon the type of pelts, method & degree of liming and the subsequent tanning operation. A short pickling leads to a better uptake of chromium and also a better grain strength and tensile strength of the leather. In summer, the pickling time is reduced to prevent the loss of hide substance as far as practicable.

Normally the duration of pickling is 2-3 hours as applicable. But, in case of pelts which have not been splitted after lining, to obtain uniform distribution of acid throughout the pelt, overnight pickling is advisable. In low float processing (30-50% float), 30-60 minute run is sufficient time for pickling.

Pickling Process:

In the pickling operation, the normal float is 60-80% on pelt weight. Salt (5%) is added to the float and run for 10-15 minutes Then sulphuric acid (1.0 %) and formic acid (0.5%), dissolved in 15% water, are added the float and the drum is run for 2-3 hours to achieve a final p H level of 2.4-2.6 of the cross section of the leather. In many tanneries, chrome tanning is carried out in the pickle bath. In this case the acid requirement for the pickle bath will be adjusted according to the degree of deliming. Such method of tanning enhances the chrome uptake and gives smoother grain, larger area and less growth marks on the final leather.