Wet blue production of Cow Hides

Production of Wet Blue Cow Hides

By Samina Tannery,Hazaribag,Dhaka,Bangladesh

USER (U) -

Leather: Leather is a durable and flexible material created via tanning from putrescible raw hides and skins, primarily from cattle hide.Tanning is the process of controlled denaturation of protein of the raw hides and skins.

Flow Chart of Wet Blue Production

Selected raw stock

Trimming

Weighing

Soaking

Liming

Fleshing

Selection(if required)

Weighing

Chemical Wash

De liming

Bating

Pickling

Chrome tanning(with or without pre tanning)

Basification

Drain out the chrome liquor and pile up for 4-7 days for completion of fixation and polymerization reactions

Selection,Trimming and Weighing

At first select raw hides/skins and put aside the putrefied stock to treat separately

Next we trim the unnecessary part of the hides or skins

After that measure weight, because we use chemical till fleshing based on this weight.

SoakingIt is a process of rehydration of preserved raw hides or skins

Quality Control

pH Check-9.0 to 9.5(if pH is 10,no problem)

Cross Section Check-Take cross section, if the entire part feel wet uniformly then soaking has done.

Duration-22to24 hours in industry. Agitate the drum for 10 minutes interval of every hour

Different checks-1.Two hour after soaking, touch the hides with hand,if it feels softy then its ok.

2.Drop the soaked hide from high place,if it falls down in a place uniformly then its ok.

3.If flesh side become slippery then soaking has done

We use NaCl to remove Hyaloglunic acid

Beume check- Conc.check of salt.Its measured during soaking and pickling(sometime in liming).For soaking the value is 3-4(in several tannery they keep 2-3)

If the pH goes over 11,it is called immunization of soaking.In this case hair gets fixed in its root and cause problem in liming process.

Soaking

Chemicals

Soda Ash or Na2CO3- It increases the pH & works as bleaching agent. It helps to keep pH within 6.5 to 7.0 of raw h/s

Caustic Soda or NaOH- It works as Soda ash

Wetting agent-It wets back wet or dry salted leather during soaking. Some wetting agents are:- 1.LD600(buckman) 2.Lissapol EF(ACI) 3.Eusapon FB(BASF) 4.Sinteral BD(Alpha) 5.Metapol WA 6.7794

Bactericide- It prevents bacteria's growth in raw h/s

Soaking Enzyme- It works as catalyst to return the wet condition completely in h/s just like green condition and also remove loose plunk which grows during mechanical agitation of soaking.It also assists to control the pH in optimum level.

Liming-Liming is the treatment of soaked h/s with milk of lime(CaO) with or without addition of Na2S to remove the hair from the grain surface of the h/s.

Objectives

Remove the hair from h/s

Swelling & plumping the collagen fiber

Split up the fiber structure at the fibril bundle layer

Remove the non collagenous substances

Remove inter fibrillary cementing materials

Emulsify fats and greases

Opening up more reactive site for tanning

Hydrolysis the peptide bonds, amide side chains, guanine side chains

Removal of dermatan sulfate

Controls

pH-12 to12.5

Cross section test-Drip phenolphthalein in the cross section of limed pelt, if deep pink color appears, liming is confirmed.

Layer- If double layer appears in the cross section of limed pelt, then liming has done.

Swelling & Plumping- If the pelt uniformly distended then thats plumping, it needs 8000lb pressure to bring the water from inside. If the pelt inflate in different place thats called swelling,it requires 48000lb pressure to bring out the water.

Duration- 24 hours in industries,but it varies in laboratory.

Hair Slip test-Before soaking we should test hair slipness, if hair slips then the raw stock is putrefied.

Chemicals & Use condition

Chemicals

Sodium Sulfide (Na2S)

Calcium Oxide (CaO)

Conditions

If the hide has great thickness and area

Sodium Sulfide (Na2S)-3.5%

Calcium Oxide (CaO) -3.25%

If the hide has mediocre thickness and area

Sodium Sulfide (Na2S)-3.5%

Calcium Oxide (CaO)-3.5%

If the hide is thin and small in size

Sodium Sulfide (Na2S)-3.5% to 3.75%

Calcium Oxide (CaO)-4.0%

In case of goat skin(Any size and thickness)

Sodium Sulfide (Na2S)-5.0%

Calcium Oxide (CaO)-7.0% to 7.5%

Different facts of liming

CaO inflates the hides and skins

Normally after 3 months of Qurbani, the h/s got Wrinkle, after that h/s got Jam in belly portion

In liming we have to add water in afternoon

If the stock got Wrinkle we have to take water less than the stock equal to two joint segment of finger, if the stock got Jam then we have to add water more than three joint segments of finger.

We should add reactive chemicals in three steps in 30 minutes interval or applicable time interval

Piled up limed pelt

Durrani Tannery

Deliming: It is an intermediate process between liming and pickling

Objects

Lowering the pH from 12-12.5 to 8.5-9.0

Remove the lime

Remove swelling and plumping

Depleting the pelt

Quality Control

pH- 8.2 to 8.8

Cross section test-Drip phenolphthalein in the cross-section, if light pink color appears or the pelt become colorless deliming has done

After deliming we press the pelt by hand, if it feels soft, then it does mean the protein inside pelt has prepared for fixation reaction

Deliming Chemicals

We can use anything which has ability to reduce pH, but the popular chemical is NH4Cl

Water

Hydrochloric acid

Sulfuric acid

Boric acid

Lactic acid

Formic acid

Acetic acid

Ammonium sulfate

Carbon di oxide

Condition

If the pelt is thick and large

(NH4)2SO4- 2.75% to 3.0% (duration-75 mins)

Medium thick and medium size pelt

(NH4)2SO4- 2.5% (duration-75 mins)

Less thick and small pelt

(NH4)2SO4- 2.0% to 2.25% (duration-50 min-60min)

For all types and sizes goat and sheep skin

(NH4)2SO4- 1.5% (duration- 45 min)

If bath falls down, upload the pelts in the drum, add 80% water and 0.25% (NH4)2SO4 then run the drum for 10 minutes

Bating : Depending on the use of leather we treat h/s with enzymes to soften them, this process is done after deliming is called bating.

Purpose: Peptizing the fiber structure and eliminate the alkali swelling with the aid of specific enzyme.

Tests

pH check- 7.5 to 8.0

Cross section test- Drip phenolphthalein in the cross section of the pelt and if it turns into completely colorless, then bating has finished.

Thumb test- Press the bated pelt with thumb, if the left a print their that means bating has done

Bubble test- It basically does for goat & sheep skins as those have more compact weaved structure so it requires both acid and basic bate. Well wrap the bated skin if bubble comes out then its ok

Bating-things to keep in mind

Hides bating depends on the final stage of the deliming

If light pink color exist after deliming and the pelt remain light soft and thick duration of bating 60 minutes

If the pelt turn into colorless and become too soft then the duration of bating is 45-50 minutes

In case of sheep and goat skin, alkali bating is done prior to acid bate. Add 0.2% HCOOH in the alkali bate and revolve 10 minutes then start the acid bate and the duration will be 90 minutes

Enzymes

Theoretically there are three types bating enzymes. They are:-

Weak bating enzymes- 500 to 1000 LVU

Mid strong bating enzymes- 1000 to 1500 LVU

Strong bating enzymes- 1500 to 2000 LVU

Practically the available enzymes are mixture of the upper classified enzymes. There are two types available commercially, they are:-

Alkali bate

Acid bate

Available enzymes are-

1.Oropon K (TFL, Alkali bate)

2.Buzyme 146 (Buckman, Alkali bate)

3.Nissabate EF (Bantage, Alkali bate)

4.Tripsol D (Alkali bate)

5.Oropon BRS (TFL, Acid bate)

** LVU- Lohlein Volhard Unit

PicklingThe treatment of delimed or bated pelt with a solution of acid and salt is known as pickling. Recently its been strongly supported that by proper adjusting of salt & acid in this operation a large amount of hydrolysable tannin can be easily reduced.

Objects:

Preserve the pelt for further treatment

Acidify the pelt, the pH bring down from 7.8 to 2.8

Oxidize hair root and remove if hair remains after liming

Remove all swelling and plumping

Produce softer, thinner white pelt

Reduce astringency of chrome tanned leather

Tests and Controls

pH- 2.6 to 2.9

Cross section test- CS area+BCG =yellow color (if yellow color appear pickling done)

Beume Check- In pickling the range for beume will be 7-8

Emergencies in pickling

If light green color shows in the pelt we have to add Sulfuric acid from 0.1M to 0.6M and run 30 minutes

If cow hides, the volume of H2SO4- 1-1.2%

If goat skins, the volume of H2SO4- 0.8%

Run 3-4 hours after adding H2SO4 in cow hides and run 1 hour for sheep or goat skins

If pickling bath falls down, load the stock in drum+ 80% water+ 7.5% salt+ 0.5% HCOOH and run additional 10 minutes

Salts & Acids

Salts

Common salt NaCl

Sodium Sulfate NaSO4

Sodium Formate - HCOONa

Sodium Acetate CH3COONa

Acids

Sulfuric acid- H2SO4

Formic acid- HCOOH

Pickled pelt

Pretannage

Which pelts are normally white those are chrome tanned and pre tanned simultaneously after pickling. The pre tanning chemicals are prepared from replacement synthetic tans. In this step the pH value is 3.4-3.6

Chrome Tanning

Control

pH check- 3.7 to 4.0

CS test- CS area+BCG= If Reddish yellow turns into green, then tanning has ended

Penetration check- Take CS of wet blue leather if color is uniform through all the parts then the Cr(OH)SO4 has penetrated into the pelt.

Surface tanning is known as case hardening, its a defect. If pickling is defective then it happens

Chrome tanning process

These are added to pickle bath

4.0% Cr(OH)SO4 (run for 30 min)

4.0% Cr(OH)SO4

1.0% HCOONa

0.5% Carbonyl sulfide (OCS) as chrome stable fat

Piled up pelts for fixation reaction

Wet blue leather

Basification

Its done to fix up the Cr salt in the hide protein

In industry MgO is used as basification agent as it reacts slowly than NaHCO3 thus basicity of the bath get increased gradually. Thats why chromium molecules get fixed up inside hide protein properly and uniformly

Advantage of Basification

Increase strength properties

Tensile strength- 200 kg/cm2

Stitch tear strength- 120-180 kg/cm2

Ball burst strength- 20 kg/cm2

Increase soften property

Miscellaneous

Soaking & liming is done in same drum

Deliming and bating is done in same bath

Chrome tan is accomplished in other drum

One day prior to soaking Cao & Na2S keep mixed

Enzyme prevents hides from forming loose plunk and reduce the spots sculptured during mechanical action

Water level measurement for industry is

Equal to hides level= 100% water

1 segment of finger joint less= 80% water

1 segment of finger joint up = 200% water

2 segment of finger joint up = 300% water

Cross section test is the major and confirmatory test for every steps except goat or sheep skin require to perform bubble test in bating

Soda Ash is known as in industries

NaHCO3 is known as in industries

References:

Syed Rubel Hossain

Tanning chemistry: The science of leather

A VERY SPECIAL THANKS TO

ABDULLAH AL MAHMUD SOHEL SIR

Thanks to All