WOOL
Wool Fiber
Wool fiber is the natural hair grown on sheep and is composed of protein substance called as keratin. Wool is composed of carbon, hydrogen, nitrogen and this is the only animal fiber, which contains sulfur in addition. The wool fibers have crimps or curls, which create pockets and give the wool a spongy feel and create insulation for the wearer. The outside surface of the fiber consists of a series of serrated scales, which overlap each other much like the scales of a fish. Wool is the only fiber with such serration’s which make it possible for the fibers to cling together and produce felt.
Properties of Wool Fiber
The characteristics of Wool fiber or protein fibers are as follows:
*They are composed of amino acids.
*They have excellent absorbency.
*Moisture regain is high.
*They tend to be warmer than others.
*They have poor resistance to alkalis but good resistance to acids.
*They have good elasticity and resiliency.
Flow chart of wool production:
Shearing
↓
Sorting and grading
↓
Scouring
(this involves washing in warm soapy water several times. Followed by through rinsing and drying. It removes the natural grease in fiber dirt and dust.)
↓
Oiling
(the fiber is usually treated with various oils including animal, vegetable and mineral.)
↓
Yarn formation
Classification of Wool
The quality of wool fibers produced is based on the breeding conditions, the weather, food, general care etc. For example, excessive moisture dries out natural grease. Similarly the cold weather produces harder and heavier fibers.
The wool could be classified in two different ways:
*Classification by sheep
*Classification by fleece
1. Classification by sheep
* Merino wool:
The staple is relatively short, ranging from 1 to 5 inches, but the fiber is strong, fine and elastic and has good working properties. Growing area → Australia, New Zealand, South America, South Africa and Spain.
* Class two wool:
It is 2 to 8 inches in length, elastic, fine, strong and has good working properties. Growing area → England, Scotland, Ireland.
* Class three wool:
This class of sheep originated in the U.K. The fibers are about 4 to 8 inches long are coarser and have fewer scales and crimp than Merino.
* Class four wool:
The fibers are from 1 to 16 inches long are coarse and hair like. Some time refereed to as half breeds.
2. Classification by fleece
* Lambs wool:
The first fleece sheared from a lamb about six to eight months old is known as lambs wool.
* Hogged wool:
It comes from sheep twelve to fourteen month old that not been previously shorn.
* Weather wool:
Any fleece clipped after the first shearing is called weather wool.
* Pulled wool:
When sheep are slaughtered for meat, their wool is pulled from the plot by the use of a chemical.
* Dead wool:
The wool that was been recovered from the sheep that have died on the range or have been accidentally killed.
* Catty wool:
Sheep that are imposed to severe weather condition or lack of nourishment yield wool that is matted or felted.
* Tag locks:
The torn, ragged or discolor parts of a fleece are known as tag locks.
Chemical composition of wool
Keratin → 33%
Dust → 26%
Suint (Grease) → 28%
Fat → 12%
Mineral matter → 1%
The micro structure of wool
Cuticle
On the outside of the wool fibre is a protective layer of scales called cuticle cells. They overlap like tiles on a roof. The exposed edges of the cells face away from the root end so there’s more friction when you rub the fibre in one direction than the other. This helps wool expel dirt and gives it the ability to felt. Wool felts when fibres are aligned in opposite directions and they become entangled.
The scales have a waxy coating chemically bound to the surface. This stops water penetrating the fibre but allows absorption of water vapour. This makes wool water-repellent and resistant to water-based stains.
It consists of the
* Epicuticle → The epicuticle is the outer most layer or sheath which covers the wool fiber.
* Exocuticle → The overlapping epithelial cells from the exocuticle.
* Endocuticle → The endocuticle is an intermediate cementing layer bonding the epithelial cells to
the cortex of the wool fiber.
Cortex
The cortex – the internal cells - make up 90% of the fibre. There are 2 main types of cortical cells – ortho-cortical and para-cortical. Each has a different chemical composition. In finer fibres, these two types of cells form in two distinct halves. The cells expand differently when they absorb moisture, making the fibre bend - this creates the crimp in wool. In coarser fibres, the para-cortical and ortho-cortical cells form more randomly so there’s less crimp.
Fibre crimp makes wool feel springy and provides insulation by trapping air.
Cortical cell
The cortical cells are surrounded and held together by a cell membrane complex, acting similarly to mortar holding bricks together in a wall.
The cell membrane complex contains proteins and waxy lipids and runs through the whole fibre. The molecules in this region have fairly weak inter-molecular bonds, which can break down when exposed to continued abrasion and strong chemicals.
The cell membrane complex allows easy uptake of dye molecules.
Macrofibril
Inside the cortical cells are long filaments called macrofibrils. These are made up of bundles of even finer filaments called microfibrils, which are surrounded by a matrix region.
Matrix
The matrix consists of high sulphur proteins. This makes wool absorbent because sulphur atoms attract water molecules. Wool can absorb up to 30% of its weight in water and can also absorb and retain large amounts of dye. This region is also responsible for wool’s fire-resistance and anti-static properties.
Microfibril
Within the matrix area, there are embedded smaller units called microfibrils. The microfibrils in the matrix are rather like the steel rods embedded in reinforced concrete to give strength and flexibility. The microfibrils contain pairs of twisted molecular chains.
Twisted molecular chain and helical coil
Within the twisted molecular chains are protein chains that are coiled in a helical shape much like a spring. This structure is stiffened by hydrogen bonds and disulphide bonds within the protein chain. They link each coil of the helix, helping to prevent it stretching. The helical coil – the smallest part of the fibre – gives wool its flexibility, elasticity and resilience, which helps wool fabric keep its shape and remain wrinkle-free in use.
The wool fiber is a crimped, fine to thick, regular fiber. As the diameter of wool fibers increases the number of crimps per unit length decrease.
Length → (1.5 to 15 inches) (3.8 to 38 cm)
Diameter → Fine 14 μm and coarse 45 μm
Color → Wool fibers vary from off white
Crimp → Fine wool 10 crimp/cm
Coarse wool 4 crimp/cm
Wool grading
* In America the wool sorter grades wool by comparison with pure blood Merino. But English is more specific in several instances.
Comparative grading system:
| US | British |
| Fine | 80’s,70’s |
| Half blood | 62’s,60’s |
| Three eight blood | 56’s |
| Quarter blood | 50’s,48’s |
| Low quarter blood | 46’s |
| Common | 44’s |
| Braid | 40’s,36’s |
Physical properties of wool fibers
* Specific gravity→ 1.31
* Moisture regain→ 13 to 16%
* Strength→ Tenacity→ Dry→ 1.35 g/d
* Elasticity→ Breaking extension→ 42.5%
* Feel or Hand→ Soft
* Resiliency→ Excellent (due to crimp)
* Abrasion resistance→ Good
* Dimensional stability→ Bad (for tendency of felting)
Chemical Properties of Wool Fiber
Effects of:
Acids: Fairly stable, acid hydrolyze the peptide bond so weakened in acid concentrated HNO2 and H2SO4 destroy the fiber.
Alkali: Destroy all bonds and dissolves.
Heat: Poor conductivity.
Bleach: Treating the wool textile material with reducing bleach such as acidifies sodium sulphite; sodium dithionitre converts the discoloration on the fiber surface to colorless compound.
Sunlight: Tends to yellow white or dull color or surface polymer degraded by ultraviolet radiation.
Dyeability: Easy to dye acid, mordant, premetallized and reactive dyes.
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