LEATHER SCIENCE: Post Tanning Operations

Crust leather obtained after post tanning operations is a higher added value skin and hides already tanned ready for the finishing stage. The semi-tanned wet-blue leather undergo final tanning to produce crust leather. The distinct feature of this product from the wet-blue leather is that it has visible physical properties such as size, thickness, fullness, looseness of grain and grain damages. It is rather a product of better tensile and tearing strength, chrome and fat content etc. than wet-blue leather.

“Crust is also known as unfinished leather” and It is used by the leather finishing industries. The different methods of making Crust are given below:

A. Chrome-Retan Process

B. Semi Chrome Process

Post tanning Oprations for Chrome-Retan Crust Leather

PREPARATION FOR POST TANNING 

After the completion of Chrome tanning, the leathers have to pass through few operations to prepare them for Post tanning operations further. Following are the preparatory stages:

1. Wet Blue Sorting

2. Samming

3. Splitting

4. Shaving

5. Weighing

1. Wet-Blue Sorting

The Wet-blue leather is considered as raw material for Crusting further and therefore sorting of Wet-blue stock is done as per the following criteria:

a. Area

b. Thickness

c. Defective area percentage

d. Grain Structure 

e. Shade

After sorting the wet blue leather may be trimmed properly so as to get a regular shape.

2. Samming

In this operation the excess water content of the leather is reduced so as to bring it to a suitable condition for subsequent Splitting & Shaving operations. It is carried out by mechanical squeezing using Samming machine.

3. Splitting

After proper samming the thick /heavy leathers could be splitted into two layers The Grain split & Flesh split using Band knife splitting machine. Thus it is possible to utilize the flesh split by processing it further for conversion into a valuable by-product to reduce the manufacturing cost. Excessive splitting could lower the tensile strength of the grain split. Splitting is carried out at 0.2 mm above the thickness of the shaved leather.

4. Shaving

The objective of shaving is to make the thickness uniform all over the hide/skin area. The appearance and the value of the finished leather depend upon efficient shaving which require high skill.

Two types of shaving machines are available in the market:

1) Single width (older) and

2) Double width.

The thickness of the shaved leather should be kept about 0.2 mm above the required thickness of the final leather. The allowance also depends upon the type & method of Retanning.

5. Weighing

After shaving the shaved weight is taken and for all further post- tanning operations the percentage of chemicals will be based on this weight.

POST TANNING OPERATION

Following are the Post tanning opertions for making Chrome-Retan Crust:

1. Washing

2. Neutralisation

3. Dyeing

4. Re-Tanning

5. Fatliquoring

6. Setting out

7. Drying

8. Conditioning

9. Staking

10. Toggling

11. Crust Sorting

12. Buffing/Snuffing

1. Washing 

Leather is properly washed after shaving so as to clean the H/S for effective dyeing and other post tanning operations. The washing is carried out in drum with soft water for approximately 30 minutes. The bath is then exhausted and then Neutralization is carried out in new bath.

2. Neutralization

The process of neutralization signifies the removal of neutral salts, uncombined chromium salts & strong acids from the chrome tanned leather. The main objective is to

reduce the positive charges to a level (p H) suitable for dyeing, fat liquoring etc. This operation is of great importance and must be carefully controlled to avoid difficulties during subsequent operations affecting the physical characteristics of the final leather.

Objective:

l Removal of Acids and Salts

l Bringing pH to neutral ranges.

l Preparing chrome tanned leather for effective Dyeing, Retanning & Fat liquoring.

Process Control:

l p H - 4.5-5.5

l Float Temp. - 25-30*C

Process Equipment: Drum.

Chemicals used:

Neutralizing agents should be mild so as to avoid drastic change in p H on the surface of the leather, causing rough leather surface and poor grain break of the finished leather. The agents are:

l Sodium Bicarbonate

l Sodium Sulphite

l Ammonium Bicarbonate

l Sodium Format

l Calcium format

l Neutralising Syntans

l Borax etc.

The neutralizing Syntans can be very effective for leveling effect and also for the extra filling effect.

Process & Duration:

The Process and the duration of neutralization depend upon the acidity level of the incoming wet blue. Normally the process is carried out with water (100%) and neutralizing agents (1.5- 2.0%) for 30-40 minutes in total adding the agents in installments of 10 minutes. The indicator is used to check the p H of the cross section of leather. The optimum p H level of the central layer should be less (4.5-5.0) than the surface p H (5.5) for better penetration of dyes, fat liquors etc. subsequently.

3. Dyeing 

The Objective of this process is to give color and appearance to the leather by which the value can be improved. The dyeing is carried out after neutralization and is a continuous process.

Process Equipment: Drum

Process Controls:

l pH: 5.5- 6.0

l Float Temp. : 60 *C

l R.P.M. of Drum: 15-20

Dyeing Agents:

The selection of the dyeing agents depends upon various factors:

l Fiber-structure (The Origin of animal)

l End use of leather

l Types of tannage /Re tannage / fat liquoring

The Dyes can be broadly classified as:

l Anionic

l Cationic

l Reactive 

Anionic Dyes: These dyestuffs can be further classified into three sub-groups as: i) Acid dyes, ii) Direct dyes, iii) Metal-complex dyes

i) Acid dyes:

The acid dyes are the compounds possessing negative charges having good affinity towards Chrome tanned Leather.

ii) Direct dyes:

These are also having negative charges but bigger particle size compared to the acid dyes. Hence these dyes are having less penetrating power and good covering power. These dyes are also fast to light.

iii) Metal-complex dyes:

These dyes possess metal radical coordinately bonded with the complex. The metals are Cr, Co, Cu, Fe etc. There are 1:1 / 2:1 Metal complex dyes available in the market in which the dye: metal ratio determines the properties of the dyes. These dyes give shade levelness and very good fastness characteristics to leather.

Cationic Dyes: 

These dyes are also known as Basic dyes. They have got very good affinity with the vegetable tanned leather but poor affinity for chrome leathers. The cationic dyes are mainly used for top dyeing (in case of black ) so as to impart brilliancy of colour on chrome leather. These dyes give poor light fastness and dry-rub fastness.

Reactive Dyes: 

These dyes are having good covalence characteristics with substrate in question. It gives good wash fastness, very good leveling and exhaustion characteristics.

Dyeing Auxiliaries:

Neutral syntans capable to reduce the cationic charges of chrome leather for dyeing with anionic dyes are the dyeing auxiliaries. The dyeing auxiliaries assist in:

a) Better penetration of dyes

b) Good leveling effect.

Dyeing Method:

The method of dyeing depends upon the quality of final product required. There are basically three methods for leather dyeing:

l Drum Dyeing

l Brush Dyeing

l Spray dyeing

Drum dyeing: For through & through dyeing the drum dyeing method is carried out during other post tanning operations.

l Float - 100 % (Temp. 60-65*C)

l For black color - 1.0 %

l Direct dye for Other Color - 0.75-1.0 %

Acid dye Run for 60 minutes, check penetration, if through, Fix with Formic Acid - 1.0 % (Diluted with 10% Water) Add in 2 installments, Total running 15-20 minutes. In case of black color, top dyeing is preferred with basic dyes (1.5%) to give richness of color on the surface. The total dyeing process can take 90-120 minutes. For better exhaustion of dyes the process can be carried out along with Fat liquoring / Retanning.

Brush Dyeing: 

Application of dye by means of brush is carried out in this method. This method is used during the Finishing operation so as to give leveling of color and making a suitable base for the season application. The dye-solution (solvent based) is used for this purpose.

Spray Dyeing: 

Spray dyeing method is used for dyeing the leathers meant for aniline look. This method may cause color bleeding if not finished or fixed properly. Generally basic dyes are used for this purpose. The other means of dyeing can be:

l By Curtain Coater

l By Roller Coater

l By Screen Printing etc

Factors affecting dyeing:

The dyeing process is very critical regarding the final quality related to uniformity of shade as well as the dye penetration. There are various factors influencing the final quality of leather:

l Pre-tanning & Tanning operations

l Retanning & Fat liquoring

l The type of Dye & its quantity

l Dyeing auxiliaries

l Duration of process

l Dye-fixation to prevent bleeding

Precautions for effective Dyeing:

For better quality of dyeing, some precautionary measures must be taken:

l Wet-blue Selection

l Proper washing before neutralization

l Neutralization

l Proper fixation of dyes

l Proper cleaning of drum especially for light colors

l Proper piling after drumming (Covered well in case of light colors).

4. Retanning

The main objectives of this important post tanning operation are:

l To improve the quality by filling up the looser & softer parts.

l To give tightness of grain (esp. in shanks & belly areas).

l To enhance the strength properties.

Retanning Agents: The Retanning agents can be:

l Vegetable tannins eg. Quebracho, Wattle ext., G.S.Powder etc.

l Synthetic tanning agents (Syntans) eg. Resin syntan, Chome syntan etc.

l Other tanning agents eg. Alum, Aldehyde etc.

These retanning agents are used in combination as per the final properties of leather required. Also the type of retannage determines the percentage of different tanning agents used in combination.

Process Control:

l pH - 5.0-6.0

l Float Temp. - 55-60* C (If used with fat liquors) - 25-30* C (If only retanning is done)

The retanning process is generally preferred to be carried out with low float (For better penetration and uniform distribution of retanning agents and also the risk of D/G* is minimized). Sometimes the re- chroming of the wet-blues is also required before actual retanning, in case the chrome content of the wet-blue stock is found to be less than 8%. But, this Rechroming is essential for the Semi-chrome leather manufacturing. The total retannage for upper leather should be 10-15%, depending upon the type of final leather and also upon the quality of W/B, E/I as an input material. Retanning process is generally carried out for 3-4 hours but if the percentage of retannage is required more, as per specific quality requirements (Thicker leather, shrunken grain leather etc.), the duration can be more accordingly. The fixation of retanning agents is carried out using the same formic acid (0.5-1.0%) added in installments.

Types of Syntans (Synthetic tanning agents):

There are variety types of synthetic tanning agents available in the market according to the requirements of the tannery people and also research & development activities are going on to get better results at lesser cost . Broadly the syntans are classified into:

Replacement Syntans:Replaces veg. Tannins, Bigger Particle-size, good filling effect

Exchange Syntans:Neutralizes the positive charges of chrome for better penetration of vegetable tannins inside

Resin Syntans: They have also got very good tightening effect

Other Combination Syntans: Combination of various mineral tanning agents after the drumming operations are over, unloading of drum is carried out and the stock is piled grain to grain and left overnight for ageing. Next day on wards, some mechanical operations are carried out:

5. Fat-liquoring 

Fat liquoring is the process by which the oils & fats are incorporated into leather to make the leather soft, stretchy and waterproof. In this process, some amount of selected oils (2-6% on the dry leather weight) is provided to the leather in place of the natural fats removed during pre-tanning operations. The purpose is to provide lubrication for fibres, so that they can slip over each other more easily when the leather is flexed and also to avoid crankiness when the leather is dry. Fat liquoring greatly affects the physical properties of leather such as:

l Break peppiness

l Tensile Strength

l Comfort characteristics etc.

Factors affecting fat liquoring:

Fat liquoring process is influenced by a number of factors such as:

l Selection of oils/fat liquors

l Tanning / Retanning operations

l Drying system (Rapid drying cause’s patches on the surface).

l Float concentration

l Temperature of float

l Running time

l pH of the substrate as well as the liquor

Types of Fat liquors:

Fat liquor is a solution consists of an emulsifier and a water insoluble oil or fatty matter (fatty acids etc.). Useful oil should be of:

l High molecular weight

l Non Volatile

l Fairly linear structure

l Reasonably viscous fluid or paste.

The types of Oils/Fats commonly used for making fat liquors are:

l Vegetable Oils : Castor Oil, Olive Oil, Coconut Oil

l Marine Oils : Cod-Liver Oil / Fish Oil

l Animal Oil : Neat's Foot Oil, Lanoline

l Mineral Oil : Petroleum products act as solvent

l Synthetic Oil

The emulsifiers are means of dispersing the oils finely in water to enable it to penetrate into leather at 60- 65*C float temperature, eg. Soap, Sulphated oils, Polyamines etc.

Classification of Fat-liquors:

The classification of fat liquors used for leather manufacturing can be carried out on the basis of:

a) Ionic charge:

There are three types of fat-liquors categorized as per ionic charge:

i) Anionic, ii) Cationic iii) Non-ionic

i) Anionic fat liquors: These possess negative charged particles and they are having good penetration into chrome leather.

ii) Cationic fat liquors: They are having positively charged particles and used for surface-oiling effect in case of chrome leathers. For vegetable leathers these fat liquors show good penetration.

iii) The Non-ionic fat liquors: These do not combine with the leather cross-links and serve as free oil inside the leather.

b) Chemical Properties:

i) Sulphonated fat liquor:This fat liquor is obtained by the treatment of vegetable/mineral/animal oils with sulphuric acid in controlled conditions. The sulphur-oxygen-carbon linkage forms in this case. 1.0-1.5% combined sulphate is better for penetration of fat liquor inside the leather. Sulphonated or sulphated fat liquors contain other constituents as neutral oils, emulsifying agents, & filling agents.

ii) Sulphited fat liquor: Treatment of fish oil or cod oil (oxidized) with sodium bisulphate gives this type of fat liquor. Very soft leathers can be produced using this fat liquor. It is best suited for garment and gloving leathers.

iii) Synthetic fat liquor: The Sulpho-chlorinated fat liquors are the category of the synthetic fat liquors. These do not contain free fatty acids (hence chances of mould growth are minimum). Partial replacement of sulphonated fat liquors & oils gives improved lubrication effect inside the leather.

iv) Solvent fat liquors: Oils in high boiling solvents come under this category. These fat liquors can provide softness with minor staking in case of upper leather manufacturing. This type is recommended for the leather used for vulcanized shoes.

Fat liquoring process:

Fat liquoring is carried out with an emulsion in water at 60-65*C. The Float-Temperature is required to be maintained throughout the process so as to avoid the surface oiliness. Anionic Fat liquor - 3.0-5.0% (in 3-4 installments) (in combination) Run for approx. 45-60 minutes for every installment of adding fat liquor. Fixation is carried out with Formic acid (0.751.0%), added in 2-3 installments. When crosssectional p H is found to be 3.0-3.5 cationic fat liquor (0.5-1.0%) can be added (run for another 15-20 minutes) in case of making soft leathers. The total fat liquoring process can be completed in 5-6 hours.

Fatty Spues:

The Spue formation on leather may occur when the leathers contain high amount of free fatty acids (Palmitic or Stearic acids), they are more liable to occur on chrome tanned leather than vegetable tanned leathers. At higher temperature of Plating or Glazing operation, the free fatty acids are liable to migrate to the surface of the leather and they solidify on cooling. The other reasons may be the hydrolysis of these fatty acids by moulds. It is recommended to use 0.2- 0.3% fungicides during the post tanning operations. The addition of mineral oils/Chlorinated paraffin’s also helps in this case.

6. Setting out

The objective of Setting out is to make the leather flat (removal of wrinkles & fold marks) and to make smooth grain. This operation is carried out using Setting M/c, having helical cylindrical blunt knives with other pressure rollers. The Pressure adjustment can be done for light & heavy leathers.

7. Drying

After Setting, the leather is dried for subsequent mechanical processes. The moisture content should be 15-20% after drying. The methods of drying could be:

Normal Air Drying: 

i) Ordinary drying: The stock is hanged to dry in normal environmental condition.

ii) Nailed drying: The leather are nailed on a flat surface in stretched condition & dried under natural air. The dried stock will be flatter. Direct Sunlight is not recommended for this purpose.

Forced Drying:

i) Hot Air-Blow: For this, the stock is hooked in stretched condition with the wire-frame & the stock can be dried inside the drying chamber with the arrangement of blowing hot air.

ii) Vacuum Drying: 

This is the latest method of drying leather & the drying time is also very less. The stock is spread in between hot metal plates and the moisture is removed under vacuum. It is required to hang the leather in normal condition for some time after removal from the Vacuum dryer.

8. Conditioning

Condition is the process of dampening back of the dried lot for quality staking subsequently. This process is carried out by applying the damp saw-dust on the flesh side of the stock or by light-spray of water. The stock should be kept under such condition overnight. The moisture content after proper conditioning should be 25-30%.

9. Staking

The objective of Staking is to provide proper softness to the leather by separating the opening the fiber- structure mechanically. There are two types of Staking machines used for the purpose:

Molissa type Staking: 

This machine gives the mild effect by pin-vibration method. Suitable for Upper leather.

Slocomb type Staking: This type of machine can give additional softness (as desired) and suitable for garment/gloving leathers. The mechanism is based on two combs acting together.

10. Toggling

By this method the leather is dried finally and the area yield is also observed to be approx. 10%. Nowadays the machine for toggling came in practice which reduces the drying time and also the stretch of leather can be done automatically during the process.

11. Crust Sorting

After toggle drying the Crust is ready for finishing operation further. The crust is finally sorted out as per the buyer’s specifications of Area, Thickness, Colour, Grain structure,Nap etc. The Snuffing or Buffing will be carried out as subsequent processes after Edge Trimming.

12. Edge-Trimming

The Stock should be trimmed well round about the area for efficient Buffing and Snuffing subsequently and also it prevents the damage of leather. This objective of this process is to give proper shape to the leather also.

13. Snuffing / Buffing 

The process of removal of a very thin layer from the grain surface is known as Snuffing, whereas if the same process is carried out on the flesh surface of leather, it is called as Buffing. The removal of the layer is carried out by means of rubbing with emery paper roller (using pressure) provided with the Buffing / Snuffing machine.Normally, 80-120 Grit paper is suggested for Buffing whereas. 600-800 Grit Emery paper can be used for Snuffing operation. Dust removal is also very essential operation before the leather is sent for finishing.

Post tanning Operations for Semi-Chrome Crust Leather: 

The stages of making Chrome - Retain crust are given below:

a) Re-Chroming

b) Neautralization

c) Dyeing, Retanning, Fat-liquoring

d) Setting out

e) Drying

f) Conditioning

g) Staking

h) Toggling

i) Crust Sorting

Since in case of Semi-Chrome leather, Vegetable tanning is carried out first and then chrome tanning is done, in the Crusting process Re-chroming is carried out in Drum to incorporate the required chrome content in the leather. Before Re-chroming, Stripping and Bleaching process may be carried out for making light colour Semi-Chrome Crust. Other processes will be followed after Neutralization same as in case of Chrome - Retain Crust leather.