BLEACHING OF LIGHT LEATHERS

 

Bleaching does not mean the complete destruction of colour. A substance is considered to be bleached if its colour becomes pale due to some treatment.

CLASSIFICATION OF BLEACHING

At present there are many ways of bleaching.

  1. Sun or photo bleaching

  2. Bleaching by oxidation

  3. Bleaching by reduction

  4. Bleaching by solublization and superimposition

  5. Bleaching by change of pH value

  6. Optical bleaching

1) Sun or photo bleaching: Sun rays possess bleaching property. Many coloured materials get bleached by sun rays in the presence of moisture. According to modern electronic theory of colour production, certain electrons in the dye or pigment remain in the state of tension between chromophores and auxochromes with certain level of energy in the system. When such material absorbs light of certain wavelength, The electrons under tension begin to vibrate due to increase of energy level in the system and produce different shade to the material.

2) Bleaching by oxidation: By oxidation certain coloured materials so the materials containing those colours can be bleached my the method of oxidation. It is nascent oxygen which causes bleaching.

Oxidative bleaching is not suitable for bleaching chrome or vegetable tanned leathers. Due to oxidation the basic chromium compound in the chrome-tanned leathers is converted into chromate compound which has no tanning property. This type of bleaching on the other hand darkens the colour of veg-tanned leather.

  1. Chlorine bleach: Chlorine cannot bleach in the dry state. The chlorine first combines with water to form hypochlorous acid which readily decomposes into hydrochloric acid and nascent oxygen.

    2. Cl2 + H2O = HCl + HOCl

    2HOCl = 2HCl + [O]

    Chlorine when allowed to remain in the cloth or the fiber it tends to rot the cloth or fiber so the chlorine has to be removed by anti-chlor. Sodium thiosulphate or

    2Na2S2O3 + Cl2 = Na2S4O6

    Cl2 + SO2 + 2 H2O = 2HCl + H2SO4

    b) Hydrogen peroxide bleach:

    H2O2 = H2O + [O]

  2. Potassium permanganate bleach:

2KmNO4 = K2O + MnO2 + 5O

3) Bleaching by reduction: Like nascent oxygen nascent hydrogen can also bleach many coloured substances. Vegetable tannins and tanned leathers are generally bleached by this method of reduction.

4) Bleaching by solublization and superimposition: Though in true sense this is not a method of chemical bleaching but the most widely accepted way of bleaching in the leather industry.

5) Bleaching by change of pH values: The colour of vegetable tannins becomes both light and bright when its pH value is lowered by the addition of acid. The maximum brightening effect is noticed within the pH range 4.5 to 3.0 in most of the cases.

6) Optical bleaching: Optical bleaching or brightning agents are generally colourless fluorescent water-soluble materials but they absorb appreciable amount of invisible ultraviolet radiation and emit visible blue rays. When used , these optical bleaching agents, therefore, not only give normal bluing effect but increase the intensity of reflected light.