Wool Fibre

Amit Chakrabortty
Assistant Professor , DTE, AUST.
 Introduction
 Wool is the natural highly crimped textile fibre obtained from different varieties of sheep.

 Major varieties of wool come from merino, alpaca, camel, goats, and other breeds of sheep.

 Wool is composed of very complicated protein known as keratin along with many active side groups.

 Fine wool (<25 𝜇m) fibre has been used as a raw material for apparel textiles, and medium (25–35 𝜇m) and coarse
(>35 𝜇m) wool have been utilized for different technical textiles such as carpets, blankets. Wool fibres of 5-12 cm in
length are preferred for wool textile manufacture.

 The world produces around 1.2 million tons of clean wool fibre annually, and it is mainly utilized for high quality
apparel. Fine wool is mainly contributed by Australia and exported to other countries.

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 Major varieties of wool
Merino wool (Merino sheep) Alpaca Angora Rabbit Mohair wool (Angora Goat)

 It is mainly produced in Australia, New  It is mainly produced in  It is mainly produced in  It is mainly produced in
Zealand, South Africa and South China and South Africa South Africa, Australia
Peru and Argentina
America, Europe (Germany, France, and Argentina
and Spain). Fibre length: 62-100 mm,
fineness: 15-23 µm, very fine, highly
crimped fibre.

Cashmere wool
(Cashmere goat)
Vicuna Yak

 It is mainly  It is mainly  It is mainly produced in

produced in China, produced in China and Mongolia.
Mongolia and Chile, Bolivia,
Kashmir. and Peru.

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 Processes involved in wool fibre production
1. Shearing
 The process of removing the wool fleece from the adult sheep is called
shearing.
 All sheep require shearing of hairs at regular intervals as a key part of caring
for sheep, which is a simple process like a haircut and not painful to the sheep.
 Shearing can be carried out using either a scissor or shearing instrument.

 A process by which fleece or skin wool is divided up into

2. Sorting various qualities is called sorting of wool. Fibre fineness
and fibre length can be used to sort the wool fleece.
 Based on the portion of wool fleece, wool growers sort
the fleece from 1 to 14 grade, in which 1 is from the
shoulder portion with long, fine and even wool fibre,
while 14 is from the shank (legs) with rough and hard
wool. In general, the fleece is shorter around the face,
legs and belly , but longer and softer across the back and
sides of the body.
 Sorted wool is usually packed by a mechanical press with a minimum weight of
3. Baling 120 kg (260 lb.), tagged with proper level, and graded based on the region.

Fleece: It is the main wool covering the sheep’s body. The fleece is obtained by shearing
process.
Greasy wool/Raw wool: It refers to the state of wool that has been shorn from the sheep but
has undergone no further processing. It may also be called as raw wool, which is harvested
in the form of fleece containing various contaminants such as wool grease, suint, dirt and
vegetable matter.
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 Chemical processing of Wool fibre
 Wool fiber when sheared from the skin of sheep contains 25%–70% impurities by mass. Such impurities
accumulate on the surface of the wool fiber. Impurities present in the wool fibres can be classified into
1) Natural impurities (wax, suint, grease) 2) Added impurities/Vegetable matter (dried grass, burrs and
straws). All impurities are removed by chemical processing.

Scouring of wool
 Scouring is the process of removal of impurities from raw wool . Grease,
suint and dirt materials of the raw wool fibres can be removed by treating it
with a sequence of soda ash (Sodium carbonate) solutions in the presence of a
detergent at 40°C-60°C for 30-60 minutes. During scouring, fatty acids
present in the fibres react with sodium cation and are emulsified by detergent
in the presence of water.
Carbonization of wool
 Vegetable matter that adheres to the wool fleece can be removed by carbonization
process. Wool are treated with a 60% sulfuric acid solution at 60°C-80°C and
degrade the glucosidic linkages of vegetables matter , form carbon matter and can
be dusted.
Vegetable Matter Convert into
(Burrs, seed, grass) Heating carbon
(60°C-80°C) 5
 Morphological structure of Wool fibre
 Morphologically, the fibres are composed of a cuticle (epicuticle, exocuticle, and endocuticle) and cortex (cortical
cells and the cell membrane complex, orthocortex, paracortex, macrofibrils, and microfibrils) . Moreover, in coarse
fibers, a central medulla is made up of air-filled cell walls, which can be continuous or discontinuous.

Medulla

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 1. Cuticle
(1) The cuticle cells, or scales, constitute the outermost surface of the wool fibre (2) The cells overlap
like tiles on a roof. (3) Cuticle consists of several layers such as epicuticle, exocuticle and endocuticle.
Cuticle cells are covered with a hydrophobic outer layer called epi-cuticle. The epicuticle is however,
permitted by many microscopic pores, and through which water vapor may penetrate into the interior of
the fibre.

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 2. Cortex
 The cortex contains cortical cells and the cell membrane complex. The cuticle and cortical cells in wool fibres are
separated by the cell membrane complex. The cortical cells are made up of macrofibrils.
 The cortex may be formed from up to two types of cells i) Ortho-cortex ii) Para-cortex. The ortho- and para-cortex
cells twist, forming a crimp. The outer part of the curl shows the ortho-cortex, while the inner part reveals the para-
cortex. The cortex consists of microfibrils and keratin-associated proteins, forming the intermicrofibrillar matrix.
 The matrix consists of high-Sulphur proteins. This makes wool absorbent because Sulphur atoms attract water
molecules.
 The microfibrils in the matrix are similar to steel rods in reinforced concrete, providing strength and flexibility. The
protein chains that form the helical coil (Alpha helix) are the smallest parts of the wool fibre. They give wool its
flexibility and elasticity.

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 3. Medulla
 A central stream of cells along the length of wool fibre is termed as medulla. It may be a) Fragmental
b) Interrupted c) Continuous medulla

The presence of medulla contributes to thermal insulation and

increasing the light scattering properties of fibres, particularly for blue
light (The shorter the wavelength, the stronger the light scattered).
This makes medullated fibres appear whiter than those of unmedullated
wools.

Heat, Q1 Heat, Q2

Figure: a) Fragmental b) Interrupted c) Continuous medulla

• Medullated fibre suffers from lower bundle of strength.

• They causes lower spinning properties.
• They produce lighter shade than the true wool fibres.

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 Polymeric structure and different bonds in wool fibre
 Wool fiber is a cross-linked protein called keratin. Keratin consists of carbon, hydrogen, oxygen,
nitrogen, and sulfur. These combine to form over 17 different amino acids. The flexible molecular
chains of wool are held together by natural cross links—cystine (or sulfur) linkages and salt
bridges—that connect adjacent molecules.

 The individual peptide

chains in wool are held
together by various types
of covalent crosslink and
noncovalent interaction.

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 Different bonds in wool fibre
1. ‘Ionic bonds’ or ‘salt linkages (Noncovalent Bonds) : Strong electrostatic interactions occur between the ionized
terminal groups of lysine and aspartic acid, is shown in following figure. These ionic interactions have also been
referred to as ‘ionic bonds’ or ‘salt linkages’. It contributes to Strength, react with acid and dyes.

2. Hydrogen Bonds (Noncovalent Bonds) : The –CO and –NH groups in the peptide chains and the amino and
carboxyl groups in the side chains can interact through hydrogen bonds. It contributes to strength and react with
moisture.

3. Disulphide crosslink (Covalent Crosslinks): The disulphide bonds of cystine form crosslinks, either between
different protein chains (the interchain bonds) or between different parts of the same protein chain (the intrachain
bonds). The disulfide bridges help wool to be more stable and less soluble.

Intrachain crosslink in wool Interchain bonds 11

 Special properties of wool
1. Acid & Basic nature
 The specific characteristic of wool is its amphoteric behavior (the ability to appear as a function of the pH value
as an acid or as a base). Basic amino and acidic carboxyl groups are responsible for the amphoteric nature of
the fibre . The reason for this amphoteric nature lies in its composition that it contains many cationic and anionic
groups at the same time.

 In acidic solution, the carboxylate ions combine with protons to form neutral carboxylic acid groups and the
ammonia cation make the fibre cationic.
 Conversely, in alkaline solution, reaction with hydroxide ions convert ammonia cation to amino groups and make
the fibre becomes anionic.

 At neutralized condition, both types of group are fully ionized , and the net electrical charge carried by the fibre is
zero. This condition is known as the isoelectric point/ state.

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 2. Felting behavior of wool fibre
 The surface of wool fibers has many scales, which is responsible for the high shrinkage of wool
apparel during laundering. When the wool fabric is subjected to mechanical action in the presence of
moisture, the individual wool fibers start to move. Because of the surface scales, the fibers can move
only in one direction. During washing or drying, under mechanical agitation and friction, the scale
edge of one fiber locks into the inter-scale gap of another fiber like a ‘ratchet’ mechanism as shown in
Fig. The fibers interlock and cannot return to their original positions. This irreversible shrinkage is
called felting.
There are some reasons behind this such as
a) The scaly layer of wool
b) Crimpiness

Fig: Ratchet mechanism

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 Crimp
 Wool fibers have a special wavy structure that enables the fibers to hold together when twisted into
yarn, and it is called crimp. This curliness comes from two core sections, ortho and para cortex, twisting
together like a spiral.
 The crimp provides unique elasticity and thermal insulation due to its springy structure.
 Crimpiness is expressed as the number of waves per inch or per centimeter.
 In general, the more crimps per inch, the finer the fibre. As for example, the number of crimps per inch
in very fine grades wool is 22-30, whereas in fine wool it is 14-22.

Figure: Crimp of wool fibre

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 Properties of Wool
Aesthetics: Wool has a matte appearance. Drape, luster, texture, and hand can be varied by choice
of yarn structure, fabric structure, and finish.
Durability: Wool fabrics are durable. Their moderate abrasion resistance stems from the fiber’s scale
structure and excellent flexibility. Wool fibers can be bent back on themselves 20,000 times without
breaking, compared to 3,000 times for cotton and 75 times for rayon. Wool fibers have a low tenacity,
1.5 g/d dry and 1.0 g/d wet. The durability of wool fibers relates to their excellent elongation (25
percent) and elastic recovery (99 percent).
Comfort: Wool is more hygroscopic than any other fiber, with a moisture regain of 13 to 18 percent
under standard conditions. Wool is a poor conductor of heat, so warmth from the body is not dissipated
readily. Wool has a medium specific gravity (1.32). One way to compare fiber densities is to think of
blankets. A winter blanket of wool is heavy and warm. An equally thick blanket of cotton would be even
heavier (cotton has a higher density), but not as warm.
Appearance Retention: Wool is a very resilient fiber. Wool maintains its shape fairly well during normal
use. When wool fabrics are dry-cleaned, they retain their size and shape well. When wool items are
hand-washed, they need to be handled carefully to avoid shrinkage. Wool has an excellent elastic
recovery—99 percent at 2 percent elongation.
Care: Dry cleaning is the recommended care method for most wool items. Dry cleaning minimizes
potential problems that may occur during hand or machine washing.

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Effect of alkalis: The chemical nature of wool keratin is such that it is particularly sensitive to alkaline
substance. Wool will dissolve in caustic soda solution that would have little effect on cotton. In fact,
3% solution of sodium hydroxide will completely dissolve wool at boil.

Effect of acids: Wool is more resistant to acid. This is because they hydrolyze the peptide groups but
leave the disulfide bonds intact, which cross link the polymer. Although this weakens the polymer system, it
doesn’t dissolve the fibre. Hot-dilute or cold concentrated acid hardly influence the strength characteristics
of wool fibre.

Effect of bleach: Bleaches that contain chlorine compounds will damage wool. Products with hypochlorite
will cause wool to become yellow and dissolve it room temperature. Various forms of chlorine are used
to make unshrinkable wool by destroying the scales. This wool is weaker, less elastic and has no felting
property.
Wool can absorb moisture but repels water
Wool is a hygroscopic fibre because it absorbs water vapor. Wool fibers naturally pull moisture away from the body
and it can absorb up to 30% of its weight. Wool can absorb moisture but repels water. The waxy outer layer of wool
repels water because of its lower surface energy in comparison to that of water. The pores are so small that the water
droplets cannot pass through the fibre surface, but evaporated or molecular water (sweat) can pass through the surface
pores. Chemical treatments, such as oxidation and reduction, are commonly used methods in the industry. These techniques
were applied to increase the hydrophilicity of wool fibers.

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Performance of Wool in Apparel

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 Identification of Wool Fibre
Burning test: The smell of burnt hair or feathers.

Microscopic View:

Chemical test:

Wool fibre dissolves in concentrated sodium hydroxide and Sodium Hydrochlorite and
slowly dissolves in Nitric acid (70%).

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 Mechanical processing of Wool Fibre
 Worsted and woolen (woolen or woollen) spinning can be used in wool yarn
production, and it depends on the fibre length and fibre fineness of a fleece.

Woolen yarn Worsted yarn

1. Fibres are short (2 in. length) 1. Fibres are long (2–8 in.)

2. Woolen yarns are only carded, less 2. Worsted yarns are carded and

twisted and hence weak in strength combed, highly twisted and stronger.

3. Fabrics made from woolen yarn are soft, 3. Fabrics made from worsted yarn are

fuzzy, thick and warm but not durable flat, rough but more durable.

4. Less expensive than worsted 4. Expensive than woolens

5. Uses: Sweater, Carpets 5. Uses: Suits, Dresses

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Performance Analysis of Various Natural Fibers in Apparel
Properties Cotton Flax Silk Wool
Aesthetics Attractive Excellent Variable Variable
1. Luster Matte, pleasant High Beautiful and soft Matte
2. Drape Soft to stiff --- ------ -----
3. Texture Pleasant Thick-and-thin ------- -------
4. Hand Smooth to rough Stiff --- ---
Durability Good Good High High
1. Abrasion resistance Good Good Moderate Moderate
2. Tenacity Good Good High for natural fibers Poor
3. Elongation Poor Poor Moderate High
Comfort Excellent High High High
1. Absorbency Excellent High High High
2. Thermal retention Poor Good Good High

Appearance retention Moderate Poor Moderate High

1. Resiliency Poor Poor Moderate High
2. Dimensional stability Moderate Moderate High Poor
3. Elastic recovery Moderate Poor Moderate Excellent
Machine-wash Dry-clean or
Recommended Care and dry machine-wash Dry-clean (apparel) Dry-clean (apparel)
(apparel) (apparel)

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 Comparison of Wool and Silk

Price per Kilogram of Various Natural Fibres

Fibre Name Cost/kg ($)

Wool 8-12
Raw silk 35-50
Flax 5-7
Cotton 2.74-4.80
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Thank you

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