Basic Concept of Wool: Production to Application

Basic Concept of Wool: Production to Application
Md. Nazmul Huda Sohag
Dept. of Textile Engineering
TMSS Engineering College (Afflicted by Rajshahi University)
Email: sohagnazmul124@gmail.com


Wool:
Wool is a natural fiber which we obtain from the fleece of domesticated sheep. It is a natural, protein, multicellular, staple fiber. Wool is a quality fiber and so it is unique and valuable in its properties. In a sheep under the layer of coarse hair fibers, there is an undercoat of finer hairs, much more delicate. This inner insulating layer has given us the textile fiber known as wool.

Wool fiber
Fig: Wool fiber
Wool producing countries:
Wool is generally produces in cool condition. The following are the wool producing country:
  1. Spain, 
  2. New Zealand 
  3. France
  4. Australia 
  5. Argentina 
  6. Brazil
  7. U.S.A 
  8. South Africa 
  9. India
  10. U.K 
  11. Uruguay 
  12. Turkey etc.
Composition of wool:
  • Keratin -------------------------- 33%
  • Dirt ------------------------------ 26%
  • Suint ---------------------------- 28%
  • Fat ------------------------------- 12%
  • Mineral materials -------------- 1%
Composition of Keratin:
  • C ------------------- 50%
  • O ------------------- 22-25%
  • N ------------------- 16-17%
  • H ------------------- 7%
  • S ------------------- 3-4%
Classification of wool:

Classification by sheep quality:
On the basis of sheep there are four types of wool. These classifications depending on fineness and length. These are:
  1. Class I, Merino Wool,
  2. Class II Wool,
  3. Class III Wool,
  4. Class IV Wool.
1) Class I, Merino Wool:
It is mainly produce in Australia, New Zealand, South Africa, South America, Europe (Germany, France, Spain)

The staple length of:


Features:
  • Fiber is fine and strong.
  • Highly elastic
  • Good working properties,
  • Greatest amount of crimp,
  • The scale of unit length is very high,
  • Maximum no. of scales about 8000 to the inch.
2) Class II Wool:
This class of wool mainly produces in England, Scotland, and Ireland. It qualities are almost same to Class-I Wool but length is little but higher.

Staple length = 2// - 8//

Features:
  • This quality is not good as merino wool,
  • It has large no. of scale per inch.
  • Uses mainly in hosiery and knitting; suitable for coating and apparel
3) Class-III Wool:
Origin of this wool is U.K.

Staple length = 4// - 8//

Features:
  • Coarse and less elastic,
  • Have fewer scale,
  • Have less crimp then merino and Class-II Wools.
  • Used for clothing.
4) Class-IV Wool: (Cross-breed Wool)
This class is actually referred as cross breed wool.

Staple length = 1// - 16//

Features:
  • Coarse and hair like.
  • Have relatively fewer scales and very little crimp.
  • Smoother and have more lusture.
  • Used for carpet, rugs and low grades fabrics.
Classification by fleece:
These are of seven types:
  1. Lambs wool, 
  2. Cotty wool,
  3. Hogget wool, 
  4. Taglocks.
  5. Wether wool,
  6. Pulled wool,
  7. Dead wool,
1) Lambs wool:
Refers to the first fleece sheared from a lamb about 6 to 8 months old.

2) Hogget wool:
Refers to the first fleece come from a sheep about 12 to 14 months old.

3) Wether wool:
Any fleece clipped after the first shearing is called wether wool.

4) Pulled wool:
When the sheep are slaughtered for meal, their wool is pull from the pelt by the use of chemicals.

5) Dead wool:
Wool has been covered from the sheep that have died on the range or have been accidently killed.

6) Cotty wool:
Sheep that are exposed to severe weather conditions or lack of nourishment yield a wool is called cotty wool.

7) Taglocks:
The torn, ragged or discoloured parts of a fleece are known as taglocks.

Classification by quality:
According to quality there are five types of wool.

These are:
  1. Fine wool,
  2. Medium wool,
  3. Long wool,
  4. Cross breed,
  5. Carpet or mixed wool.
1) Fine wool:
  • Length = 6// - 7//
  • Source = only the merino sheep produces this kind of fleece.
  • Producing sheep = the principle merino families of today are the Spanish, French, Australian, American, South African.
2) Medium wool:
  • Length = 4// - 6//
  • Source = It is proced from the firbst breed of sheep. Great England produce the largest percentage of this type wool.
  • Producing sheep = South down, Hampshire, Dorest, Succex.
3) Long wool:
  • Length = 3//- 4//
  • Source = Breeds of largest sheep.
  • Producing sheep = Lincoin, Cots wold, Leicester, Romney marsh.
4) Cross breed wool:
  • Length = 4// - 6//
  • Source = Medium wool
  • Producing sheep = Columbia, Panama, Romledale, Polwarth & Targhee.
5) Carpet or mixed wool:
  • Length = 2//- 4//
  • Source = Primitive sheep (lower quality)
  • Producing = Mainly in Asiatic countries, America, Australian.
The manufacturing process of wool:

Shearing:
Wool is a natural, protein, multicellular, staple fiber which may be sheared form the living animal, domesticated sheep or pulled from the hide after the animal has been slaughtered for its meat. The sheared wool is called fleece or cup wool. Sheep are sheared once or twice a year. This shearing process is done by an expert shearer, who can clip as many as 100 to 200 sheep a day.

The sheep is sheared by early spring and the fleece removed in one piece by expert shares.

Sorting:
Wool sorting is done by skilled workers who sorts according to fineness, length and some times strength of fibers.

Scouring:
Following sorting, the wool is scouring. This involves washing in warm soapy water several times. It moves the natural grease in the fiber and saint dirt and dust.

Oiling:
The wool fiber is treated by various oil including animal, vegetable and mineral because of unmanageable.

Garneting:
The shredding process of recycled wool fiber is called garneting.

Carbonizing:
After garneting, the wool fibers are put through a dilute solution of HCl or H2SO4 which destroys any vegetable fibers. This process is known as carbonizing

A morphological diagram of a wool fiber:

morphological diagram of a wool fiber
Fig: Morphological diagram of a wool fiber
The micro structure of wool consists of three main components:
  1. The cuticle
  2. Cortex
  3. Fibrils.
1) The cuticle:
The cuticle is the layer of overlapping epithelial cells surrounding the wool fiber. It consists of the epicuticle, exocuticle and endocuticle.

The epicuticle is the outermost layer which covers the wool fiber. It is only few molecules thick and composed of a water repellent, wax-like substance.

The overlapping epithelial cells form the exocuticle. An epithelial cell is about 1 long and 36 wide. The epithelial cells are largely responsible for the felting shrinkage of untreated wool textile materials.

The endocuticle is an intermediate cementing layer bonding the epitheial cells to the cortex of the wool fiber.

2) Cortex:
The cortex of wool fiber forms about 90% of the fiber volume. It consists of countless long, spindle-shaped cells. If a specially selected dye is applied to the fiber and the fiber cross-section examined, the ortho and para cortex become apparent. The ortho cortex absorbs more dye than para cortex. The cortex of the wool fiber is composed of two distinct sections.

a) Ortho-cortex, b) Para-cortex.

The ortho and para cortex spiral around one another, along the length of the wool fibre.

3) Fibril:
The cortical cells of the wool fiber consists of a number of macro fibrils each about 100-200 nm in diameter. The macro fibrils are held together by a protein matrix. Each macro fibrils consists of hundreds of micro fibrils, each about 5nm in diameter. Each micro fibril consists of eleven photos fibrils about 500nm in length and 2nm in diameter. Finally, each photo fibril consists of three wool polymers, which also spiral around each other.

Why ortho-cortex absorbs more dye than the para-cortex?
The ortho-cortex absorbs more dye than the para-cortex. The reason for this different staining is the different composition of the para-cortex and the ortho-cortex. The chemical composition of the para-cortical cells shows a higher cystine (cystine is a sulpher containing amino acid, capable of forming disulphide cross-links) content than the ortho-cortical cells.

Since there is a greater amount of cystine in the para cortical cells, a greater number of disulphide cross links exist in the para-cortex. This increased cross-linking tends towards greater chemical stability resulting in less dye absorption.

Why wool is fine to wear?
Due to helical configuration of ortho and para cortex, wool fiber has a smoothness, flexibility, elasticity and more durability. So we can say that wool fiber has higher resiliency properties. That is why wool fiber is fine to wear.

General structure of protein fiber:
The wool polymer is linear, keratin polymer, with some very short side groups and it normally has a helical configuration. The repeating unit of the wool polymer is the amino acid which has the following general formula.


(The steps in the formation of the wool polymer are not known. So the amino acid is considered the repeating unit of wool. As a result, it is not possible to determine the extent or degree of polymerization for wool.)

Amino acids are linked to each other by the peptide bond (i.e. --CO--NH--) to the wool polymer. The peptide bond is identical with the amide bond of the nylon polymer. The wool polymer is composed of twenty amino acids, but only a general formula can be given for the wool polymer.


It is known that, in general, the amino acids arginine, crystine and glutamic acid constitute at least one-third of the wool polymer. Wool is composed in 20 number of amino acid and silk is composed in 16 number of amino acid.

Polymer system:
The wool polymer is linear keratin polymer with some very short side groups and it normally has a helical configuration. A wool polymer is about 140nm long and about 1nm thick 25-30% crystalline. It has peptide bond (-CONH-), H-bond, cystine linkage (disulphide bond).

Keratin:
Keratin is an amphoteric substance i.e. reacts as both acids and bases. Keratin is made up of five chemical elements-carbon, hydrogen, oxygen, nitrogen and sulpher.

α-Keratin:
When wool polymer is in its relaxed state then it is spiral/helical. This spiral or helical configuration of wool polymer is called -Keratin.

β-Keratin:
When wool polymer is stretched, then it unfolds. The unfolded configuration of the wool polymer is called -Keratin.

Why wool fiber is easy to dye?
Wool is a protein fiber which has more amorphous region than crystalline region. So dye molecules can easily enter to the amorphous region of the fiber. Moreover wool is more absorbent in nature. So, wool is easy to dye.

Difference between ortho and para cortex/ common properties of ortho & para cortex:


Ortho Cortex
Para Cortex
1. Lower density.
1. Higher density.
2. More absorbent.
2. Lower absorbent.
3. Tends to be in the outer sides.
3. Tends to be in the insides.
4. Contains lower cystine content.
4. Contains higher cystine content.
5. Elastic & Flexible.
5. Stable and rigid and tends to tighten the spiral.
Macro structure of wool:
The wool fiber is a crimp, fine to thick, regular fiber. As the diameter of wool fiber increases, the number of crimps per unit length decreases. A single wool fiber is rod like and tapers from the root end to its tip.

1. Length: 5-35cm

2. Diameter:
  • Fine 14
  • Coarse 45
3. Length width ration:
  • Fine short- 2500:1
  • Long coarse- 75:1
4. Color:
Off white, light cream.

5. Crimp: 

10 per centimeter

Physical Properties of Wool:

01. Tenacity:
8.8-15 CN/Tex (1.0-1.7gm/den) in dry state and 7-14 CN/Tex (0.8-1.6gm/den) in wet.

02. Elongation:
25-35% under standard conditions and 25-50% when wet.

03. Elastic properties:
It has an elastic recovery of 99% at 2% extension and 63% at 20% extension.

04. Specific Gravity of wool:
1.32 and so fabrics feel lighter than cellulose.

05. Resiliency:
Higher and so resist wrinkling.

06. Hygroscopisity:
Higher

07. Cross section:
Oval to roughly circular.

08. Appearance and colour:
Appearance depends on colour, long and smooth fiber characterized by two features. Sometimes microscopically shows dark in the middle. This kemp, which are hair like character. By selective breeding kemp can be minimized.

Chemical properties of Wool:

01. Effect of moisture:
Wool absorbs moisture to a greater extent than any other fiber and yield up readily to the atmosphere. Under ordinary conditions wool will hold 16-18% of it weight of moisture. Wool loses about 40% of its strength and silk loses about 15% in wet condition.

02. Effect of acids:
Wool is attacked by hot concentrated sulphuric acid and decomposes completely. It is in general resistant to other mineral acids of all strength. Even at high temperature, though nitric acid tends to cause damage by oxidation. Dilute acids are used for removing cotton from the mixture of two fibers.

03. Effect of alkalis:
The chemical nature of wool keratin is such that it is particularly sensitive to alkaline substances. Wool will dissolve in caustic soda solutions that would have little effect on cotton. The scouring and processing of wool is carried out under conditions low alkalinity (NaOH, NaCO3). Ammonium Carbonate, borax and sodium phosphate are mild alkalis that have a minimum effect on wool.

04. Effect of organic solvents:
Wool has a good resistance to dry cleaning and other common agents.

05. Effect of bleaches:
Wool fibrion is attacked by oxidizing agents or bleaches such as H2O2, NaOCl, calcium hypochlorite Ca(OCl)2 , KMnO4, K2Cr2O7, O3, NaCl. Wool becomes yellowish in sodium hypochloride (NaOCl) and dissolve. It is less harmed by reducing agents or bleaches such as ZnO, SnCl2, SO2, H2S and FeSO4.

06. Effect of sunlight:
The keratin of wool decomposes under the action of sunlight. The sulphur in wool is converted into sulphuric acid so the fiber becomes discolored and develops a harsh feel. It losses its strength and the dyeing properties are affected. Tends to yellow white or dull color or surface polymer degraded by ultraviolet radiation.

Thermal properties of Wool:
Wool becomes weak and losses its softness when heated at the temperature of boiling water for long periods of time. At 1300C, it decomposes and turns to yellow and it damages at 3000C. Wool doesn’t continue to burn when it is removed from a flame. Do not burns readily is self extinguishing, have odor of burning hair and have a black crushable ash.

Biological properties of Wool:
Wool is attacked by moth-grubs and by other insects. Wool has a poor resistance to mildews and bacteria and it is not advisable to leave for too long in a damp condition.

End uses of wool:

  1. Knitted appeals,
  2. Suiting, over coat, sweater,
  3. Carpet, lining fabric,
  4. Lustrous dress,
  5. Designs for kurtain,
  6. Blanket,
  7. Hosiery fabric,
  8. Home uses furnishing fabric
You can also like:
  1. Physical and Chemical Properties of Wool Fiber
  2. Dyeing of Wool with Onion Skin
  3. Wool Sorting Process
  4. What are the Impurities Present in Wool? How They can be Removed?
  5. An Overview of Wool Fiber
  6. Production Processes for Wool Yarns
  7. Carbonizing Process of Wool | Techniques of Wool Carbonizing
  8. Effects of Plasma Treatment on Wool
  9. What is Woollen and Worsted Yarns?
  10. Wool Glazing Machine | Process of Wool Glazing
  11. Wool Finishing Processes
  12. Characteristics of Wool Fabrics | Properties of Wool Fabrics
  13. Difference between Woolens and Worsted Yarn
  14. Manufacturing of Worsted and Woolen Yarns
 
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Editor-in-Chief:

Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.


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