Unhairing
Unharing and Liming are often linked because the traditional process of hair dissolved and alkaline hydrolysis combine the process step in one.
Unhairing is the method of removing hair from the pelt. It is traditionally one of the direct aspects of leather processing.
Unhairing is the method of removing hair from the pelt. It is traditionally one of the direct aspects of leather processing.
Mechanism of Unhairing
There are two groups of Methods of hair removing .
1. By destroying or modifying the epidermis tissue surrounding the hair bulb, so that the hair is loosening and mechanically removed.
2. By attacking the hair itself, reducing it to pulp. this more drastic method is in practice connected with the use of alkali and sulphide. this group of methods includes 'liming' in the solutions or lime liquor and painting which consist in spreading a layer of paste over the flesh side of the skins. As a result, only the roots and lower parts of the hair slip are destroyed. Thus preserve most of the hair.
Hair can be destroyed by the action of alkali only this action is too slow to be of any use in depletion by hair - destructive method, unless the concentration of alkali or the temperature is so high that serious damage to the hide fibres would occur. Destruction of hair is brought about much more rapidly by adding moderate amounts of accelerators or sharpeners. In practice the alkali used is calcium hydroxide and the sharper is a sulphide, generally sodium sulphide.
Liming and painting are the oldest and still remain as classical method of unhairing they are based in principle on action of hydroxylic groups,in the presents of cation, on soaked hides/skins. The reaction of metal hydroxylic and the functional group of the protein can be expresssed by the formula
1. By destroying or modifying the epidermis tissue surrounding the hair bulb, so that the hair is loosening and mechanically removed.
2. By attacking the hair itself, reducing it to pulp. this more drastic method is in practice connected with the use of alkali and sulphide. this group of methods includes 'liming' in the solutions or lime liquor and painting which consist in spreading a layer of paste over the flesh side of the skins. As a result, only the roots and lower parts of the hair slip are destroyed. Thus preserve most of the hair.
Hair can be destroyed by the action of alkali only this action is too slow to be of any use in depletion by hair - destructive method, unless the concentration of alkali or the temperature is so high that serious damage to the hide fibres would occur. Destruction of hair is brought about much more rapidly by adding moderate amounts of accelerators or sharpeners. In practice the alkali used is calcium hydroxide and the sharper is a sulphide, generally sodium sulphide.
Liming and painting are the oldest and still remain as classical method of unhairing they are based in principle on action of hydroxylic groups,in the presents of cation, on soaked hides/skins. The reaction of metal hydroxylic and the functional group of the protein can be expresssed by the formula
COO-
+ MeOH → COO⁻Me⁺NH₂ + H₂O
NH3+
This is the important part of the process. Change in the charge distribution in the protein molecules causes changes in the system of hydrozen bonds due to connection of carboxylics by ionic bond. Cations are surrounding by the solvation water molecules which is term causes swelling and increase of plumping of hide fibres.
Immunization of keratin and its solution
When hides/ skins are immersed into alkaline solution which does not contain any sharpening agent, a peculiar phenomenon will take place. The alkali will disintegrate only 50% of keratin and the remaining 50% keratin will become so resistant that they are immune to attack by sulphide and other reducing agents, their removal become a problem. Thus phenomena so technically called "Immunization of keratin".
Most theories of immunization are based on the premise that the first reaction of keratin with alkali is a hydrolytic splitting of the disulphide bridges, yielding a sulphonic acid and a thiol group.
R—CH2—S—S—CH2—R + H2O = R—CH2—SOH + HS—CH2—R
The mechanism of the formation of new cross-links from these fragments has been variously postulated. According to Speakman, the sulphonic acid group is supposed to react directly with the side chain amino group of a neighbouring lysine residue.
Most theories of immunization are based on the premise that the first reaction of keratin with alkali is a hydrolytic splitting of the disulphide bridges, yielding a sulphonic acid and a thiol group.
R—CH2—S—S—CH2—R + H2O = R—CH2—SOH + HS—CH2—R
The mechanism of the formation of new cross-links from these fragments has been variously postulated. According to Speakman, the sulphonic acid group is supposed to react directly with the side chain amino group of a neighbouring lysine residue.
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