STRUCTURE OF WOOL FIBREVIS-À-VIS FIBRE PROPERTIES

Wool has a highly complex chemical and physical structure, which is responsible for superior natural fibre properties. Wool represents one of the most fascinating materials, a masterpiece of design which could never be duplicated in a factory.

Wool fibre exhibits A typical core-shell structure consisting of an inner protein core, the cortex (which is covered by overlapping cuticle cells with scale edges pointing in the direction of the fibre).

The fibre is surrounded by cuticle cells which over lap in one direction and which consist of at least four layers, the epicuticle, exocuticle and endocuticle.

The cortex is built up of spindle-shaped inter-digited cells, consisting mainly of ortho and denser para cells, which divide the stem of a fine fibre into two halves. Cuticle cells and curticle cells are separated by the so called cell membranes complex comprising of internal lipids and proteins. The cell membrane complex is the component between the cells that guarantees strong intercellular bonding via protein generally called as Demasones.

This bilateral asymmetry results in a natural crimp of the fibre being jointly responsible for crease resistance and due to permitting enmeshed air, for insulation against loss of heat or protection from heat.

The cortex contains macrofibrils formed by fibril-structured α-helical keratin proteins embedded in a cystine-rich protein matrix.The α-helical material is responsible for the fibre resilience.A diamino acid, cystine, cross-links the protein chains and thus stabilizes the wool fibre towards environmental influences.Cystine is also responsible for high wet strength, moderate swelling and insolubility of the fibre.Wool is hygroscopic and the amount of water taken up corresponds to the relative humidity and temperature of the surrounding air.

However, only the interior of wool fibre is able to absorb water vapour, whereas the fibre surface is water repellent due to hydrophobicity of the outer surface of the cuticle.

This apparently contradictory behavior results in an intelligent moisture management system being responsible for well-known wearing comfort of wool.

The hydrophobic nature of the fibre surface and the high cross-linking density of the protein layer immediately below the lipid layer act as a natural diffusion barrier.

COMPOSITION OF RAW WOOL

This is a natural coat of sheep. It contains wool wax, suint soil and vegetable matter and water as impurities. Suint is the inorganic salts produced due to perspiration of sheep.

Extent of vegetable matter is more on belly and leg and on the tip of the fibre.

Wool from shoulder and sides are considered best, wool from leg is of poor quality.

Sorting is done as per fineness, coarseness and fibre length.

• The fibre consists of :a) Epidermis or Cuticle (b) Cortex © Medula

Cuticle : It is the outer surface of the fibre, made up of irregular horney scales which over lap with the protecting edges pointing towards the fibre tip. This serrated pattern is similar to fish scales or like that of tiles of a roof.This scaly epidermis serves as a protection for the main part of the fibre and also gives it rigidity.The cuticle cell width is around 36 micron, the thickness is 0.5 – 1 micron and visible length is around 16 micron. The size and density of the scales is vary along the fibre and fibre to fibre.In finer wool one scale is enough to go all the way around the fibre so that it looks like a series of funnels set inside each otherWhen diameter increases, number of scales also increases. The scaly layer consists of three separate fractions i.e Epicuticle (outermost), Exocuticle and Endocuticle which constitutes the scales.The epicuticle is extremely thin layered, reported to have a thickness of 50 – 200 Au.

Cortex : The cortex constitutes main portion of the body ofwool fibre. It consists of long slightly flattered and twistedspindle shaped cells. The cells are approximately 80 – 110 micron long and 1.2 & 2.8 micron for major and mine diameters respectively.

The cross section of the fibre may be circular but more often they are elipticle.

The cortex is divided into its cells by treatment of enzymeswhich consists of fibrils that are oriented along the length of cell.The cells appear to have an enclosing membrines and joined together by cementing tissue. The cortex of the fibre is mainly responsible for its strength, elasticity and dyeing behaviour of wool.

Medula : In medium coarser wool, there is a centre core or medula that runs length wise through the fibre. This medula arises from the growing root and its loosely filled with many super imposed cells of various shapes, often polygonal.

The size of the medula varies greatly and volume of wool fibres occupied by the medula may run from 10 – 80%. It is probably through the medula, that solutions of wet processing treatment and dyes penetrate the fibre.

Chemical Composition : The composition of raw wool will vary greatly because of many complicated factors. The important constituent is known as Keratin. It defers from others on account of its high sulphur content.

The composition of raw wool is as follows :Keratin – 45 – 75%Grease - 5 – 15%Moisture – 10 – 12%Suint - 2 – 12 %Sand and dust – 4 – 30%Vegetable matter – 0-5%

Grease : It is an impurity in raw wool, insoluble in water but can be partially emulsified by it.

It is soluble inorganic solvents. Wool grease is an ester of high molecular weight fatty acids and monohydric alcohol in cholestrol and Iso cholesterol. This is not a fat but wax.

Wool was has power of absorbing large quantities of water.

Suint : It is soluble in water and can be isolated from raw wool by aqueous extraction. It consists of potassium salts of fatty acids and organic amino acids and is a complex mixture.

Sand and Dust : The fibre is in natural state contains a consiserable amount of dirt. This is held by adhesion action of grease and fall away when the grease is removed by scouring.Wool keratin is composed of five elements Carbon, Hydrogen, Nitrogen, Oxygen and Sulphur.

The compositions are :Carbon – 50 – 53%Hydrogen – 6 – 7 %Nitrogen – 16 – 18 %Oxygen - 21 – 25 %Sulphur - 0.7 – 5.00%The elementary composition of Keratin is nearly same for different types of wool but sulphur content varies within great limits. Most of the stability and instability of wool depends upon functional nitrogren and sulphur atoms.

Chemical Structure of wool :The Keratin molucule is supposed to be polypeptide naturally composed of 15 – 20 amino acids.The amino acids with one or two exceptions have common featues. The molecule consists of carbon atoms joined to four different groups, three of which are common to all amino acids.Those are 1) H Atom 2) An amino group- ie basic in nature 3) An acid group – COOH4) The fourth group represented by R is one which distinquishes are amino acids from one

another.5) All amino acids are condensed with each other by means of one NH2 group with COOH

group of another. The condensation.would join groups of molecules together by ploypeptide chain.

The Keratin molecule is long polypeptide chain (NH-CH-CO) with 15-20 different side chains of R. About 50% of the wt of the wool fibre is due to side chain.:

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Sr.no. SUBSTITUENT R AMINO ACID FORMULA % PRESENCE

1. Hydrocarbon Glycine H 10.00

2. Hydrocarbon Alanine CH3 5.00

3. Hydrocarbon Valine CH3 CH 4.2

4. Hydrocarbon Leucine (CH3 ) 2 CH CH2 8.7

5. Hydrocarbon Isoleucine (CH3 ) CH2

CH CH3

--

6. Hydrocarbon Phenyl Alamine C6 H5- CH2 2.3

7. Hydroxyl Serine CH2 OH 9.1

8. Hydroxyl Thereonine CH3CHOH 5.5

Amino Acid Composition : The Amino Acid composition of wool is as Follows

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Sr.no SUBSTITUENT R AMINO ACID FORMULA % PRESENCE

9. Hydroxye Tyrosine C6 H4CH2CH 2.7

10. Acidic Aspartic Acid CH2 CO OH 7.3

11.. Acidic Glutamic Acid CH2 CH2 CO OH 15.3

12. Basic Arginine NH2 C (NH) NH (EN2) 3

8.00

13. Basic Lycine NH2 (CH2 ) 4

CH CH3

3.00

14. Basic Histidine C3 H3N2CH2 0.5

15.. Basic Tryptophane C8 H5NH CH2 0.6

16.. Sulphur Methioning CH3SCH2 CH2 0.5

17. Sulphur Cystine CH2 SS CH2

CH (NH2) COOH

11.1

18. Proline Proline Proline 5.9

The structure of the Wool protein can be summarized as follow :1. Amino acids are joint together to form a long chain.2. Side groups of amino acids project from the main chain.3. Some side groups are polar i.e may be acidic, basic or contains

hydroxyl as amino groups.4. Some side as per nonpaln like Hydrocarbon groups.5. Adjacent chains may be attracted to each other by Hydrogen

bonds.6. Dicarboxylic and diamino acids inside chain can form salt

linkages which connect to adjacent chains by electro static forces.

7. Cystine form a covalent linkage between two adjacent chains.

The above discussion shows that wool is a very complex network of polypeptide chain link together by covalent and electrovalent bonds of which system linkage is most important.Main polypeptide bond, water repellent bond between non polar side chain of valine, leucine and phynyl alanine residues.