Leather is natural material and is still the most suitable material to be used as upper
making because of its physical properties, elasticity, strength, plasticity, flexibility, ease
of working, availability in varieties of color and finish, thermal conductivity etc. For
making footwear, leather is being replaced nowadays by varieties of synthetic materials
available in the market due to the cost factor. But leather is still the most suitable
material for making footwear and is superior to synthetic leather or any other leather
substitutes.
Leather is derived from raw hide and skin, which is a byproduct of meat industry. The outer covering of large animals is called hide whereas the outer covering of smaller animals is called skin. Raw hide/skin (H/S) is highly susceptible to bacterial attack. To make this non-putrescible material and usable they are converted into a permanently preserved form. This permanently preserved form of raw hide/side is called leather. Raw hide/skin is first converted to wet blue, which is in turn converted to crust and finally converted into finished leather by finishing. This can be explained by the following diagram.
Raw hide/skin – –> Wet blue – –>Crust – –> Leather
The leather is fibrous in nature and due to this it can perspire giving extra comfort the feet. Also, the leather has got poor thermal conductivity, keeping the wearer cool in summer hot and warm when it is cold. The elasticity & plasticity of leather allows it to adjust to individual foot shape. The tensile strength, bursting strength, tear strength, flexing endurance, shrinkage temperature etc. are also observed to be considerably high compared to synthetic materials.
Additionally, the leather is repairable & eco-friendly justifying its suitability as the best material. Now the structure of hides & skins and the tanning methods involved determine the actual properties of the given leather. Special tanning methods and finishes prepare the leather for product manufacturing depending on what its final use is to be. Shoe soles for example require other attributes as would leather for shoe upper, furniture, garments, gloves, book bindings or small leather goods.
Structure of Hides & Skin
Raw hides & skins of domestic animals are commonly used for making leather. They consist of cattle hides (cow, ox, and bull), calf skins, goat skins and sheep skins. Hides and skins differ in their structure depending upon the breed and origin of the animal, its mode of life, its food, its environmental conditions, age, sex etc. These factors also influence the leather manufacturing process.
Furthermore, there are differences in quality between different parts of the same hide or skin. In case of cattle hide the fibres are heavier in the back (butt) areas than in the belly and the hair is longer.
From anatomical structure point of view, in case of calf skins there is firmness of grain, smaller collagen fibre-bundles, smaller and closer hair follicles found as compared to cattle hide. As a result, calf skins have a very fine structure and therefore useful for finest of leather. Goat skins as compared to sheep skins have very tight, firm fibre structure but have less fat (grain is also very compact) and so are used in the manufacture of shoe upper leather for premium market segment. The wool sheep are more porous whereas the hair sheep is tighter.
The pig skin cross section shows the hair follicles going completely through & through the skin, the grain surface is rough having grouping of follicles. The horse hides have a structure resembling hair sheep or goat. It has also got a strong & tight fibre structure. The alligator (reptiles) has the characteristic woven network of hide fibres with no hair follicles or fat glands.
ANATOMICAL STRUCTURE OF HIDES / SKINS
The hide or skin mainly consist of three layers :
1. Epidermis
2. Dermis or true skin
3. Hypodermis or flesh layer
1. Epidermis : This layer forms the upper boundary of the skin. It constitute only 1% of the total thickness of the H/S and serves to protect the layer beneath known as corium. This layer is made up of hard wearing layer of keratinous cells. The dead cell of keratin can be identified in the form of dandruff. The epidermis are removed from H/S during pretanning operations (liming & unhairing).
2.Dermis : This layer consists of two layers :
a) Grain layer and
b) Corium layer.
a) Grain layer: Grain layer is also known as corium minor. It is the top of corium constituting about one fifth of the total thickness in case of cattle hide. This layer has a characteristic grain pattern due to the presence of hair follicles (responsible for the perspiration) in this layer. The grain pattern thus depends upon the density and structure of the hair follicles.
b) Corium layer: Corium or reticular layer is the main part constituting leather and appearing like net like fibres of connective tissues. The entire corium is an inter-woven structure consisting of several fibres grasped together. Again each fibre consist of several fibrils bundled together. The unique properties of leather are largely dependent upon the interwoven fibre structure which is more or less preserved as it was in the original skin. The fibrous elements of skin float in or surrounded by a soft mucous substance called “Cementing substance” or “Interfibrillary tissue”. By the partial or complete removal of the cementing substance and by the permeation of tannins into the fibres and even into fibrils, the fibres are prevented from being cemented together when the hides or skin are dried. They are thus free to move over each other and the leather becomes flexible & pliable.
c) Hypodermis or Flesh Layer: This layer is found below Dermis adjacent to the flesh of the animal therefore known as flesh layer. It consists chiefly of fat cells, with a few scattered fibres. There is also some muscular tissue. All these structures are useless for leather manufacture and must be removed during Pre-Tanning (Soaking and Liming) operations.
c) CHEMICAL STRUCTURE OF HIDES/SKINS: The chemical constituents of hides & skins can be divided into following groups:
a. Water: 60 - 70%
b. Proteins: 19.2 - 32.75%
c. Fats: 1.5 - 10.5%
d. Minerals, Pigments etc.: 0.36 - 1.5%
a) Water: Two forms of water are present in H/S: 1. Free water 2. Bound water The free water can be removed easily by squeezing or mechanical pressure. The bound water can be partly removed by chemical treatment and partly by constant drying.
b) Proteins: Proteins are structural units of a living thing containing carbon, hydrogen, nitrogen, oxygen, sulphur etc.
i) Structural or fibrous proteins: There are four types of structural proteins found in hides & skins namely- Keratin, Collagen, Elastin & Reticulin.
ii) Non Structural or non fibrous proteins: Three types of non fibrous proteins known as Albumins, Globulins & various protein Mucins are also the constituents of hides & skins removed during Pre-Tanning operations. In leather manufacturing, Collagen is of interest to the tanner. Collagen, a fibrous protein is composed of rope like bundle of smaller fibrils (made of molecules consisting very fine & long thread like structure). Collagen constitutes 90% of the total hide or skin protein. Proteins other than Collagen are removed during Pre- tanning operations.
Protein Synthesis: Protein synthesis can be understood by the following chain:
Amino Acid →Simpler Peptide → Polypeptides → Peptone → Proteose → Protein
Amino acids: Amino acids are derivatives of the carboxylic acids (say Acetic acid) in which the hydrogen atom in the carbon chain has been replaced by an amino (-NH2) group.
CH3-COOH → NH2.CH2.COOH
Amino acids have both acidic & basic properties. The result is that, in aqueous solution, Glycine is neutral to litmus but forms salts with acids as well as bases. Amino acids have properties like high melting point, low solubility in organic solvents and high solubility in water.
Zwitter ion: In aqueous solution the amino acids lose a proton and the amino group accepts that proton. Hence the resultant molecule exists largely as an internal salt known as a Dipolar ion or a Zwitter ion.
Simpler Peptide: When two amino acids form amide-linkge together eliminating water,
it is known as Peptide-linkage.
Polypeptide: Polypeptides are the chain of peptides constituting protein molecule.
Structure of proteins: There are Primary, Secondary and Tertiary levels of the protein structures. The Primary structure gives a complete description of covalent linkage in a protein molecule. The Secondary structure is geometrical configuration of polypeptide chains (Spatial relationships of near neighbours). The Tertiary structure refers to as the arrangement of several polypeptide chains (Spiral, Stretched or both) in one molecule. It also determines the shape & size of protein molecule (Triple Helix). The bonds which are of importance could be listed as : Electrostatic, Covalent, Hydrogen, Sulphide etc.
c) Fats: The fats found in hides and skins are of two types:
1. Glycerides of fatty acids
2. Non glycerides
The glycerides of fatty acids found in corium and flesh layers, whereas the non - glycerides are found in the fat glands of the grain layer. The fat content in hides and skins may vary widely and the maximum is found in sheep skins. Both these fats are removed during Pre-Tanning operations (Liming, Bating and Degreasing).
d) Minerals, Pigments etc.: Raw hides and skins contain mineral salts in small quantity in the form of chlorides, sulphates, carbonates, phosphates of sodium, potassium, calcium, and magnesium. Hides and skins also contain small amount of carbohydrates, pigments (colouring matter) enzymes etc. All these minerals are removed before Tanning operation.
Leather is derived from raw hide and skin, which is a byproduct of meat industry. The outer covering of large animals is called hide whereas the outer covering of smaller animals is called skin. Raw hide/skin (H/S) is highly susceptible to bacterial attack. To make this non-putrescible material and usable they are converted into a permanently preserved form. This permanently preserved form of raw hide/side is called leather. Raw hide/skin is first converted to wet blue, which is in turn converted to crust and finally converted into finished leather by finishing. This can be explained by the following diagram.
Raw hide/skin – –> Wet blue – –>Crust – –> Leather
The leather is fibrous in nature and due to this it can perspire giving extra comfort the feet. Also, the leather has got poor thermal conductivity, keeping the wearer cool in summer hot and warm when it is cold. The elasticity & plasticity of leather allows it to adjust to individual foot shape. The tensile strength, bursting strength, tear strength, flexing endurance, shrinkage temperature etc. are also observed to be considerably high compared to synthetic materials.
Additionally, the leather is repairable & eco-friendly justifying its suitability as the best material. Now the structure of hides & skins and the tanning methods involved determine the actual properties of the given leather. Special tanning methods and finishes prepare the leather for product manufacturing depending on what its final use is to be. Shoe soles for example require other attributes as would leather for shoe upper, furniture, garments, gloves, book bindings or small leather goods.
Structure of Hides & Skin
Raw hides & skins of domestic animals are commonly used for making leather. They consist of cattle hides (cow, ox, and bull), calf skins, goat skins and sheep skins. Hides and skins differ in their structure depending upon the breed and origin of the animal, its mode of life, its food, its environmental conditions, age, sex etc. These factors also influence the leather manufacturing process.
Furthermore, there are differences in quality between different parts of the same hide or skin. In case of cattle hide the fibres are heavier in the back (butt) areas than in the belly and the hair is longer.
From anatomical structure point of view, in case of calf skins there is firmness of grain, smaller collagen fibre-bundles, smaller and closer hair follicles found as compared to cattle hide. As a result, calf skins have a very fine structure and therefore useful for finest of leather. Goat skins as compared to sheep skins have very tight, firm fibre structure but have less fat (grain is also very compact) and so are used in the manufacture of shoe upper leather for premium market segment. The wool sheep are more porous whereas the hair sheep is tighter.
The pig skin cross section shows the hair follicles going completely through & through the skin, the grain surface is rough having grouping of follicles. The horse hides have a structure resembling hair sheep or goat. It has also got a strong & tight fibre structure. The alligator (reptiles) has the characteristic woven network of hide fibres with no hair follicles or fat glands.
ANATOMICAL STRUCTURE OF HIDES / SKINS
The hide or skin mainly consist of three layers :
1. Epidermis
2. Dermis or true skin
3. Hypodermis or flesh layer
1. Epidermis : This layer forms the upper boundary of the skin. It constitute only 1% of the total thickness of the H/S and serves to protect the layer beneath known as corium. This layer is made up of hard wearing layer of keratinous cells. The dead cell of keratin can be identified in the form of dandruff. The epidermis are removed from H/S during pretanning operations (liming & unhairing).
2.Dermis : This layer consists of two layers :
a) Grain layer and
b) Corium layer.
a) Grain layer: Grain layer is also known as corium minor. It is the top of corium constituting about one fifth of the total thickness in case of cattle hide. This layer has a characteristic grain pattern due to the presence of hair follicles (responsible for the perspiration) in this layer. The grain pattern thus depends upon the density and structure of the hair follicles.
b) Corium layer: Corium or reticular layer is the main part constituting leather and appearing like net like fibres of connective tissues. The entire corium is an inter-woven structure consisting of several fibres grasped together. Again each fibre consist of several fibrils bundled together. The unique properties of leather are largely dependent upon the interwoven fibre structure which is more or less preserved as it was in the original skin. The fibrous elements of skin float in or surrounded by a soft mucous substance called “Cementing substance” or “Interfibrillary tissue”. By the partial or complete removal of the cementing substance and by the permeation of tannins into the fibres and even into fibrils, the fibres are prevented from being cemented together when the hides or skin are dried. They are thus free to move over each other and the leather becomes flexible & pliable.
c) Hypodermis or Flesh Layer: This layer is found below Dermis adjacent to the flesh of the animal therefore known as flesh layer. It consists chiefly of fat cells, with a few scattered fibres. There is also some muscular tissue. All these structures are useless for leather manufacture and must be removed during Pre-Tanning (Soaking and Liming) operations.
c) CHEMICAL STRUCTURE OF HIDES/SKINS: The chemical constituents of hides & skins can be divided into following groups:
a. Water: 60 - 70%
b. Proteins: 19.2 - 32.75%
c. Fats: 1.5 - 10.5%
d. Minerals, Pigments etc.: 0.36 - 1.5%
a) Water: Two forms of water are present in H/S: 1. Free water 2. Bound water The free water can be removed easily by squeezing or mechanical pressure. The bound water can be partly removed by chemical treatment and partly by constant drying.
b) Proteins: Proteins are structural units of a living thing containing carbon, hydrogen, nitrogen, oxygen, sulphur etc.
i) Structural or fibrous proteins: There are four types of structural proteins found in hides & skins namely- Keratin, Collagen, Elastin & Reticulin.
ii) Non Structural or non fibrous proteins: Three types of non fibrous proteins known as Albumins, Globulins & various protein Mucins are also the constituents of hides & skins removed during Pre-Tanning operations. In leather manufacturing, Collagen is of interest to the tanner. Collagen, a fibrous protein is composed of rope like bundle of smaller fibrils (made of molecules consisting very fine & long thread like structure). Collagen constitutes 90% of the total hide or skin protein. Proteins other than Collagen are removed during Pre- tanning operations.
Protein Synthesis: Protein synthesis can be understood by the following chain:
Amino Acid →Simpler Peptide → Polypeptides → Peptone → Proteose → Protein
Amino acids: Amino acids are derivatives of the carboxylic acids (say Acetic acid) in which the hydrogen atom in the carbon chain has been replaced by an amino (-NH2) group.
CH3-COOH → NH2.CH2.COOH
Amino acids have both acidic & basic properties. The result is that, in aqueous solution, Glycine is neutral to litmus but forms salts with acids as well as bases. Amino acids have properties like high melting point, low solubility in organic solvents and high solubility in water.
Zwitter ion: In aqueous solution the amino acids lose a proton and the amino group accepts that proton. Hence the resultant molecule exists largely as an internal salt known as a Dipolar ion or a Zwitter ion.
Polypeptide: Polypeptides are the chain of peptides constituting protein molecule.
Structure of proteins: There are Primary, Secondary and Tertiary levels of the protein structures. The Primary structure gives a complete description of covalent linkage in a protein molecule. The Secondary structure is geometrical configuration of polypeptide chains (Spatial relationships of near neighbours). The Tertiary structure refers to as the arrangement of several polypeptide chains (Spiral, Stretched or both) in one molecule. It also determines the shape & size of protein molecule (Triple Helix). The bonds which are of importance could be listed as : Electrostatic, Covalent, Hydrogen, Sulphide etc.
c) Fats: The fats found in hides and skins are of two types:
1. Glycerides of fatty acids
2. Non glycerides
The glycerides of fatty acids found in corium and flesh layers, whereas the non - glycerides are found in the fat glands of the grain layer. The fat content in hides and skins may vary widely and the maximum is found in sheep skins. Both these fats are removed during Pre-Tanning operations (Liming, Bating and Degreasing).
d) Minerals, Pigments etc.: Raw hides and skins contain mineral salts in small quantity in the form of chlorides, sulphates, carbonates, phosphates of sodium, potassium, calcium, and magnesium. Hides and skins also contain small amount of carbohydrates, pigments (colouring matter) enzymes etc. All these minerals are removed before Tanning operation.
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