7.3.1 Degreasing
Domestic sheepskins normally contain large amount of natural grease (natural fat
content is estimated to be between 10% and 20% on dry weight) which must be
removed by degreasing operation. Excess fat is eliminated from fatty skins to avert
the development of insoluble chrome soaps or block fat spumes formation in
subsequent stage. Excess grease on the skin prevents even penetration of dye or
tan, causing difficulties in the finishing processes and creating dark and greasy
smudge on surface of the finished leather. Three methods are usually employed for
degreasing, i.e.:
• Use of aqueous medium with organic solvent and non-ionic surfactant
• Use of aqueous medium with non-ionic surfactant
• Degreasing in solvent medium
Application of proteolytic enzyme in dehairing greatly helps in the removal and
recovery of grease/fat/oil in the skin/hide by dissolving protein matter surrounding
fatty tissues. Tanneries currently apply alkaline lipases at different steps of
processing to eliminate natural fats from skins [44]. Lipases can remove fats and
grease from skins and hides, particularly those with a moderate fat content. By
combining the effects of alkaline protease, alkaline lipase and acid active lipases in
dehairing, one may establish an effective method of degreasing prior to tanning and
also improving the brightness and uniformity in dyeing [44]. The main advantages of
using lipases are a more uniform colour and a cleaner appearance. Lipases also
improve the production of hydrophobic (waterproof) leather; makers of leather for
car upholstery have commented that ‘fogging’ is reduced [45].
7.3.2 Tanning
Tanning process introduces a tanning agent into the skins/hides, thereby stabilizing
collagen fibres in the skin such that the hide is no longer susceptible to putrefaction
or rotting. Collagen fibres are stabilized by cross-linking action of the tanning
agents. The dimensional stability, resistance to mechanical action and heat resistance
increase upon the tanning treatment. There are different tanning materials and
methods, and one’s choice depends mainly on characteristics of the desired finished
leather, cost and availability of the chemicals and the type of hides and skins. The
majority of tanning agents fall into one of the following groups:
• Vegetable tannins
• Syntans
• Aldehydes
• Mineral tannages
• Oil tannage
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W. C. Wanyonyi and F. J. Mulaa
Domestic sheepskins normally contain large amount of natural grease (natural fat
content is estimated to be between 10% and 20% on dry weight) which must be
removed by degreasing operation. Excess fat is eliminated from fatty skins to avert
the development of insoluble chrome soaps or block fat spumes formation in
subsequent stage. Excess grease on the skin prevents even penetration of dye or
tan, causing difficulties in the finishing processes and creating dark and greasy
smudge on surface of the finished leather. Three methods are usually employed for
degreasing, i.e.:
• Use of aqueous medium with organic solvent and non-ionic surfactant
• Use of aqueous medium with non-ionic surfactant
• Degreasing in solvent medium
Application of proteolytic enzyme in dehairing greatly helps in the removal and
recovery of grease/fat/oil in the skin/hide by dissolving protein matter surrounding
fatty tissues. Tanneries currently apply alkaline lipases at different steps of
processing to eliminate natural fats from skins [44]. Lipases can remove fats and
grease from skins and hides, particularly those with a moderate fat content. By
combining the effects of alkaline protease, alkaline lipase and acid active lipases in
dehairing, one may establish an effective method of degreasing prior to tanning and
also improving the brightness and uniformity in dyeing [44]. The main advantages of
using lipases are a more uniform colour and a cleaner appearance. Lipases also
improve the production of hydrophobic (waterproof) leather; makers of leather for
car upholstery have commented that ‘fogging’ is reduced [45].
7.3.2 Tanning
Tanning process introduces a tanning agent into the skins/hides, thereby stabilizing
collagen fibres in the skin such that the hide is no longer susceptible to putrefaction
or rotting. Collagen fibres are stabilized by cross-linking action of the tanning
agents. The dimensional stability, resistance to mechanical action and heat resistance
increase upon the tanning treatment. There are different tanning materials and
methods, and one’s choice depends mainly on characteristics of the desired finished
leather, cost and availability of the chemicals and the type of hides and skins. The
majority of tanning agents fall into one of the following groups:
• Vegetable tannins
• Syntans
• Aldehydes
• Mineral tannages
• Oil tannage
212
W. C. Wanyonyi and F. J. Mulaa