Wool Fibre
Engr. Md. Ahshanul Hoq
Lecturer(Textile)
Sheikh Rehana Textile Engineering College(SRTEC), Gopalgonj.
Introduction
Wool is the natural highly crimped textile fibre obtained from different variety of
sheep. Wool is possibly the oldest fibre know to the humans. It is the first fibres to
be spun into yarn and into fabric. Wool is composed of very complicated protein
known as keritain along with many active sides groups. The finess and properties of
the wool will depend on the variety of sheep from which it was derived. Major
varieties of wool come from merino, alpacas, camels, goats,and other breeds of
sheep. Wool fibres of 5cm to 12 cm in length are preferred for wool textile
manufacturing because of their most versitle and economical yarn manufacturing. It
varies greatly in its diameter ranging from about 14micron(fine) to 45 micron
(coarse) wools. The chemical structure of the wool fibre ( more functional groups
than cotton) makes it possible to form chemical bonds with a great number of
chemical and dyes. Highly natural stable crimp is in wool fibre. An important
characteristic of wool is its fineness i.e. the smaller the diameter the softer and more
comfortable the fabric specially when worn directly next to the skin.
Wool producing countries:
The entire range of protein fibers accounts for only about 6% of world fiber
consumption, where wool fiber contributes 90% of protein fiber consumption. The
major wool producing countries are- Argentina, Australia, Britain, England, Holland,
Spain, Germany, New Zealand, South Africa, France, Canada, U.S.A., Soviet Russia,
Pakistan, Iran etc
Wool and Hair:
Wool and hair both are protein fiber comes from fibrous covering of sheep, goat and camel .But wool
fibers have some special characteristics that are absent in hair. Wool are short, fine, curly, elastic, but
hair are long, coarse, less elastic, no scale and stiffer than wool.
Manufacturing process of Wool:
Wool is a natural; multicellular; staple fibre which may be sheared from the living animal;
domesticated sheep or pulled from the hide after the animal has been slaughtered for its sweet meet.
The sheep is sheared by early spring and the fleece removed in one piece by expert shares.
Sorting: Wool sorting is done by good skilled workers who sorts according to fineness; length and
sometimes strength of fibres.
Scouring: It involves washing in warm soapy water several times to remove the natural grease or gum
from the fiber and the saint dirt and dust.
Oiling: The wool fibre is treated by various oil including animal, vegetable and mineral because
unmanageable.
Garneting: The picking and shredding process of recycled and unused wool fibre is called garneting.
Carbonizing: After garneting, the wool fibres are put through a dilute solution of HCl or H2SO4 which
destroys any vegetable fibres is called carbonizing.
Felting of wool: When a fabrics made from wool fibres is wet and is subjected to a suitable effective
mechanical treatment of some sorts such as rubbing; pressing; squeezing. Twisting or pounding.
Wool felts because of their serrated surface structure or over-lapping epithelial cells but they remove
in one direction only to root ward due to loss friction.
Extraction of Wool from Sheep Step by Step
The steps involved in wool production are as follows.
•Shearing – The process of removal of the woollen coat or fleece from the animal is called shearing. This is done
without harming the animal by using shearing tools such as scissors, hand blades and electric shears. Shearing is
usually done during the hot season. This allows them to grow back hair by the time winter arrives. The amount of
wool produced by one sheep varies from 1 to 3 kg.
•Scouring – Wool taken directly from the sheep is called raw or grease wool. The raw sheared wool is washed with
detergent and alkali in tanks to remove grease, dust and dirt. This is called scouring. Nowadays it is done by
machine.
•Carbonising- Carbonising is a continuous process which combines scouring to remove the wool grease and a
chemical process which removes vegetable matter such as seeds, burs and grass. Carbonising occurs if the
greasy wool contains a high percentage of vegetable matter. Dilute Sulphuric acid (H2SO4) is used in
carbonising process.
•Sorting and grading – After scouring the damaged or inferior wool is removed. This process is called sorting. The
process of sorting the wool according to the length, colour and texture of fibres is called grading.
•Carding – Before wool can be used for making fabric it is disentangled and cleaned. The intermixed fibres are
separated to form continuous fibres. This process is called carding, the wood fibres are passed through a series of
metal teeth to straighten the fibres.
•Making yarn – Carded wool is twisted into a rope called silver. The silver is stretched and twisted into a thin yarn.
Spinning for woollen yarns is typically done on a mule spinning machine.
•Washing and finishing – Woollen yarn is woven or knitted into fabric which is then used to make finished
products such as clothes, table cloths and bags.
Morphology
Macro-structure of wool
The wool fiber is a crimped, fine to thick, regular fiber. Fine wools have as many as
10 crimps per centimeter, while coarse wool has less than 4 crimps per 10
centimeters. As the diameter of wool fibres increases, the number of crimps per
unit length decreases. The number of crimps per unit length may be taken as an
indication of wool fiber diameter or wool fiber fineness. As the diameter of
the wool fiber increases the crimp per unit length decreases.
The crimped configuration prevents wool fibres from aligning themselves too
closely when being spun into yarn. As a result it is possible to have wool textile
materials with air spaces occupying about two-thirds of the volume. The warmth of
wool fabrics is due more to the air spaces in material than to the fibres.
•Length of the fiber ranges from 5cm for finest to 35cm for the coarsest wools.
•Diameter for finer 14μm, coarse 45μm,
•Length width ratio ranges from 2500:1 for the fine and shorter, 7500:1 for coarse
and longer
•Colors vary from off white to light cream.
Color of wool varies due to the di-sulfide bonds, which acts as chromophores. As a
result the incident light may be modified to cause the reflected light to have a tinge
of yellow, giving the wool fibres their off white appearance. When the fibre is
cream to dark cream in color, this is due more to the polymer degradation on the
surface of the fibre, as wool polymer is very sensitive to atmospheric oxygen and
air pollutants.
Microscopic appearance of wool
1) Longitudinal microscopic appearance of wool is the overlapping surface cell
structure. These surface cells, known as epithelial cells and commonly known
as scales, which point towards the tip of the fiber.
Microscopic appearance of wool
1) Longitudinal microscopic appearance of wool is the overlapping surface
cell structure. These surface cells, known as epithelial cells and commonly
known as scales, which point towards the tip of the fiber.
Fig 1: Untreated wool, magnified over 25000 times and the surface
cells or scales which overlap toward the tip of the fiber. [1]
Fig: Morphological Structure of Wool Fibre
2) The cross section of wool fibre is usually oval in shape.
3) Felting: Felting of wool is the irreversible shrinkage of the length, breadth and
thickness of the material. Wool felts because of the serrated surface of its fibres which is
formed by the overlapping epithelial cells or scales. Because of this serrated structure,
less friction will result if the fiber moves in a rootward direction than if it moves in a
tipward direction. This difference in surface friction between the two directions is known
as the directional frictional effect (DFE). Felting of fiber is enhanced by heat, acid or
alkali.
4) The micro structure of wool fiber consists of three main components, the cuticle,
cortex and medulla.
•Cuticle : The cuticle is the layer of overlapping epithelial cell's surrounding the wool fiber.
There are three cuticle.
§ Epi Cuticle: The epicuticle is the outermost layer covers of the wool fiber.
• Exocuticle : The overlapping epithelial cell forms the exocuticle.
• Endocuticle: The endocuticle is the intermediate connecting layer bonding the
epithelial cell of the cortex of the wool fiber.
•The Cortex: The cortex or core, of the fiber forms about 90% of the fiber volume. It consists of
countless, long, spindle shaped cells or cortical cells. It is composed of two regions known as ortho
and para cortex. The ortho cortex absorbing more dye than para cortex. The ortho and para cortex
spiral around one another. Fine wool fibers have about 20 such cells, whereas coarse wool fibers
have about 50 cortical cells across diameter of their cross-section.
•Medulla: Coarser fibers have a hollow space running lengthwise through the center. This is
medulla.
Fig 2 : Exploded view of the various structural units of the wool
fiber [3]
Ortho-cortex absorb more dye than the para-cortex
Different staining is due to the different composition of the para-cortex and the ortho-cortex. The
chemical composition of the para-cortical cells shows a higher cystine content than ortho-cortical
cells. Cystine is a sulphur containing amino acid, capable of forming di-sulphide cross-links. This
increased cross-linking tends towards greater chemical stability resulting in less dye absorption of
para-cortical cells.
The cortical cells of the wool fibre consist of a number of macro-fibrils. These macro-fibrils held
together by a protein matrix. Each macro-fibril consists of micro-fibrils of indeterminate length.
And each micro-fibril composed of eleven proto-fibrils these protofibrilsspiral about each other.
Finally, each proto-fibril consist of three wool polymers (alpha keratin plymers), which also spiral
around each other. It is the fibrillar and spiralling structure, within the cortical cells, which
contributes towards the flexibility, elasticity and durability of the wool fibre.
Wool polymer is a linear, alpha-keratin polymer which has a helical configuration. Steps in
the formation of wool polymer are not known. So the repeating unit of wool polymer is amino
acid which is linked to each other by the peptide bond (-CO-NH-). As a result, it is not
possible to determine the extent or degree of polymerization for wool. It consists of a long
polypeptide chain constructed from 18 amino acids.
•wool polymer is about 140 nm and about 1nm thick
•in its normal relaxed state , the wool polymer has alpha keratin structure
•stretching of the wool fiber tend to stretch, straighten with unfolded configuration called
called beta-keratin. A beta-keratin wool polymer always tends to return to its relaxed alpha
keratin structure.
•Amorphous : Wool polymer system is extremely amorphous, as it is about 25 to 30%
crystalline. The spiraling of the proto-fibrils, micro-fibrils and macro-fibrils does not imply a
well aligned polymer system.
Complex structure of wool
The complexity of the wool polymer is due to important chemical groupings it contains and
the inter-polymer forces of attraction.
I. Polar peptide groups: The oxygen of the carbonyl groups (-CO-) is slightly negatively
charged and as a result will form hydrogen bonds with the slightly positively charged
hydrogen of the amino groups (-NH-) of another peptide group.
II. Salt linkages or ionic bonds: carboxyl radicals (-COOH) and (-NH2) as side groups of
amino acids which are basically the acidic and basic groups, salt linkages or ionic bond will
forms.
III. Covalent bonds: cystine, the sulphur containing amino acid which is present in wool,
makes the wool polymer system the only one with cystine linkages, also known as di-sulphide
bonds. Cystine bonds are covalent bonds, they occur within and between wool polymers.
IV. Van der Waals forces
Felting properties of wool:
The tendency of wool to felt is a distinctive property that is not found in many other textile fibers. The scaly layer of
wool fiber is responsible for this property.
Washing of woolen articles causes irreversible shrinkage and felting.
Mechanical compression and relaxation of the fibers in a woolen fabric during washing allow the edges of wool fibers
to migrate only in the direction of the root end. The migrated fibers, owing to its scale structure are interlocked with
each other preventing the fiber from returning to its original position. This irreversible shrinkage is called felting.
It closes up the fabric structure, making it much more compact and have increased rigidity.
Woolen and worsted yarns:
Woolen yarns:
Woolen yarns are thick and full; the fibers arc held loosely and subjected to only a limited twist during spinning.
These yarns, made usually from short staple wool, are woven into thick, full-bodied materials such as tweeds or
blankets and used for knitting. Often, woolen yarns are spun from a mixture of new wool with reclaimed wool, or
with rayon, cotton or other fibers.
Worsted yarns:
Worsted yarns are finer, smoother and firmer than woolen yarns. In worsted yarn, the fibers are lying more parallel
and arc more tightly twisted, producing a thinner yarn with smoother surface. Worsted yarns are spun commonly
from fibers 5-38 cm (2-15 inch) long. These yarns are woven into fine dress materials and suiting. Worsted spun yarns
with less twist arc used for knit fabrics.
Physical properties:
1) Tenacity:
When wool absorbs moisture, the water molecules gradually force sufficient polymers apart to cause a
significant number of hydrogen bonds to break. Water molecules hydrolyze salt linkages in the
amorphous regions of the wool fiber. Breakage of these inters –polymer forces of attraction are
apparent as swelling of the fiber and results in a loss in tenacity of the wet wool textile material.
Wool is comparatively weak fiber
Wool is composed principally of proteins which are polycondensation products in which the different
amino acids are linked together to form a polypeptide chain:
They possess a large number of highly polar peptide linkages which can give rise to inter-
and intra-molecular hydrogen bonding. While these bonds contribute much toward
increasing the strength of the fiber, such close spacing of these groups along the
molecular chain would be detrimental to other desirable fiber properties. They contain
relatively large side chains (R groups in the scheme of the polypeptide chain) which
prevent close packing of the protein molecules and thus decrease the extent to which
hydrogen bonding can occur. The low tensile strength of wool is due to the relatively few
hydrogen bonds that are formed.
2) Elasticity: wool has very good elastic recovery and excellent resiliency. The ability of wool fibres to recover from
being compressed is due to:
a) crimped configuration of wool fiber
b) alpha–keratin configuration of the wool polymer
The ability of the polymers to return to their alpha-keratin configuration is due to inter-polymer di-sulphide bonds,
salt-linkages and hydrogen bonds.
3) Hygroscopic nature: absorbent nature of wool is due to the polarity of the peptide groups, salt linkages and
amorphous nature of its polymer system. The peptide groups and salt linkages attract water molecules which readily
enter the amorphous polymer system of the wool fiber.
Polymer system of wool easily takes dye molecules
It is due to the polarity of its polymers and its amorphous nature. The polarity will readily attract any polar dye
molecules and draw them into the polymer system. The inter-polymer spaces in the crystalline regions of the
polymer system are too small and prevent the relatively large and bulky dye molecules from entering. Therefore the
dye molecules can enter the amorphous regions of the polymer system of wool.
Stress strain curve for wool
Commonly, the stress-strain curve of a single wool fiber which provides tensile characteristics is
depicted by 3 different deformation regions, namely, initial Hookean region from 0 to 2% strain
corresponding to the reversible deformations of mainly bond angles and lengths, a yield region
ranging from 2% to 25-30% in which the α- helices of micro-fibrils unfold and are replaced by β-
pleated sheets, and a post yield region beyond 30% in which some irreversible degradation
process occur. In one of the interesting studies of mechanical behavior of wool fibers as a function
of temperature it was reported that with increasing temperature, tensile properties and durability
of the wool fibers decreased considerably. A great decrease on tensile properties was seen at
temperatures higher than ~200°C.
Physical properties:
Length and fineness: Merino wool fibers are very fine (17-25 m) bill not very long (60-100mm) whereas Lincoln wool
is coarser (around 40m) diameter but much on r. 75- 250 mm).
Crimp: The crimp of wool fibers is most pronounced in the fine wool fibers. The best merino wools, for example, will
have 30 waves per inch. The elasticity of wool fiber is due to its crimp. Due to its crimp, wool yarns trap air and when
used in garments, providing an insulating barrier to loss of body heat.
Luster: Wool fibers have natural luster, which varies in its characteristics. Depending on the type of wool, Luster
depends very largely on the nature of the fiber surface.
Tenacity: Wool has a tenacity of 8.8-15 cN/tex (1.0-1.7g/den) in dry state and 7-14 cN/tex(0.8-1.6g/den) in wet state.
Elongation: Wool has an elongation at break of 25-35% under standard condition and 25-50% when wet.
Elastic property: Wool fibers are highly elastic and resilient. The elastic recovery of wool fibers is 65% for 20%
extension and almost 100% for short extensions.
Specific gravity: Wool is a light-weight fiber of specific gravity 1.32
Effect of moisture and water: Wool fibers are hygroscopic and the most hydrophilic of textile fibers. Under ordinary
atmospheric conditions, wool will hold 16-18% of its weight of moisture.
Chemical properties:
Effect of acid: Wool is attacked by hot concentrated H2SO4 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.
Effect of alkalis: The chemical nature of wool keratin is such that it is particularly sensitive to alkaline s-substances. Wool will dissolve in
caustic soda solutions that would have little effect of cotton fibers. Ammonium carbonate, borax and sodium phosphate arc mild alkalis
that have minimum effect on wool.
Effect of bleaching:
Oxidizing agents: Oxidizing agents cause considerable change in composition and properties of wool attacking preferentially in cystaline
linkage. For bleaching purpose, hydrogen peroxide is commonly used as oxidizing agent.
Reducing agents: Wool is most strongly attacked by reducing agents in an alkaline solution they attack preferably on cystaline linkage.
Effect of Organic solvents: Wool has a good resistance to dry-cleaning and other common solvents.
Insects: Wool is attacked by moth-grubs and by other insects.
Micro organism: Wool has a poor resistance to mildews and bacteria and it is not advisable to leave wool for too long in a damp
condition.
Dye ability: Easy to dye. Acid ,mordant ,premetalized, reactive dye is suitable
Effect of heat: Wool becomes weak and loses its softness when heated at the temperature of boiling water for long periods of time. At
1300C,it is decomposes and turns yellow, and it chars at 3000C. As it decomposes, wool gives off a characteristic smell, similar to that from
burning feathers.
Wool does not continue to burn when it is removed from a flame.
Action of sunlight: The keratin of wool decomposes under the action of sunlight, a process which begins before the wool has been
removed from the sheep. The sulphur in wool is converted into sulphuric acid; the fiber becomes discoloured and develops a harsh feel. It
losses strength and the dyeing properties are affected.
Identification Of wool fibre :
Thanks
