Neutralization:
Neutralization is the removal of protein bound acid from the surface as well as interior of leather up to certain depth depending on penetration of re-tanning material used in re-tanning system.
pH range 4.3 to 5.5.
During neutralization the free & protein bound acids are efficiently removed either by repeated washing with water or by chemical treatment.
Neutralization is necessary to stop acid hydrolysis
& to remove salt spue.
Objectives of neutralization:
The objective of neutralization is not to remove totally all acidity present in the hide or skin but only to remove harmful acidity. By harmful acidity means the pH of the leather is below 3.3. So, it is necessary to take the pH of the leather a little over this value so that even if a little or more of acid if produced after some time no harmful effect will be observed.
Parameters of Neutralization:
1. Temperature:
Carried out in a range of 30-40°c. If use sodium-bi-carbonate it will be <38°c.
2. Duration:
About 30 minutes to 2 hours depending on the thickness of leather & texture, type of neutralizing agent & type of leather to be produced.
3. Intensity:
Depends on the type of leather to be produced. For soft leathers the pH range is 5.0-6.0, for firm upper leathers the pH range is 4.2-5.0. The outer zones are neutralised in the higher pH range, the inner zones are slightly neutralised.
Controls of Neutralization:
Cut off a small piece of leather & then let an 0.1% indicator solution of bromocresol green (BCG) onto cross section. The change of color indicates the progressive depth of penetration of neutralizing agents & the pH value on the leather. Transition interval of BCG:
Yellow = pH 3.4 & below.
Yellow green = pH 4.0
Green = pH 4.5
Blue green = pH 5.0
Effects of Neutralization:
Acid or direct dye, both these dye are sodium or potassium salts of certain colored acid when dissolved in water.
(-) charged acid fraction x- react with the (+) charged group of protein & thus color is imparted.
* Result of no neutralization is uneven dyeing.
* Result of over neutralization is impossible dyeing.
Neutralizing chemicals/agents:
1. Strong products:
Products of this category are seldom used, in combination of mild products. Used in small quantities neutralizing action is superficial. Larger amounts give good penetration, however over neutralization of the grain & simultaneous de-tanning effect. Example: Sodium carbonate (Soda).
2. Mild products:
- Sodium-bi-carbonate:
Frequently used product. Over-neutralization is possible if large amounts are used.
- Ammonium-bi-carbonate:
Rapidly penetrating product. Very suitable for leathers of dense texture (pig leathers, reptiles)
- Sodium formate:
Over-neutralization is not possible. Mostly used in conjunction with sodium-bi-carbonate.
- Calcium formate:
No risk of over-neutralization but formation of calcium sulphate may produce stain.
Beside sodium acetate, sodium sulphite, sodium thiosulphate, sodium aluminium silicate, polyphosphate etc are used.
Ideal condition of Neutralization/'N':
Is to make the leather surfaces electrically neutral & the acidity should increase as the middle layer is approached.
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The leather surface will be electrically neutral if the pH of the leather surfaces are kept in range i.e pH=5-6 & at middle layer pH=4.1 to 5.0. The leather will be properly neutralised if the pH value reaches its ISO-electric point.