Study Unit 4 Natural Protein Fibres: Wool Flashcards
History
Sheepskin, with the hair, was used as a covering long before human beings realised that the fibres could be spun into yarn or felted into fabric. The oldest fragments of woollen fabric, dating back to about 4000 to 3500 BC, have been found in Egypt. There is evidence that woollen fabrics were first made in Mesopotamia. The earliest example of woollen fabric found in Europe dates back to about 1500 BC and was unearthed in Germany.
Fibre production
The best quality wool is obtained from the Merino sheep. Its fibre is strong, fine and elastic with the greatest degree of crimp of all wool fibres, and a maximum number of scales. Australia, South Africa and New Zealand are the world’s largest producers of Merino wool. Worldwide the demand for wool fibres has declined steadily, for example, from 6% in 1977 to 3% in 1997.
Wool can be sheared from the living sheep or pulled from the hide after the animal has been slaughtered. The first fleece sheared from a lamb aged about 6–8 months is known as lamb’s wool. It is wool of very fine quality and the fibres are tapered because the ends have never been clipped. Lamb’s wool is not as strong as that of a mature sheep.
After shearing, wool will be classified, sorted and baled for sale. Before the fibre is ready for spinning into yarn form, the wool must be cleaned in a process called scouring. Impurities such as natural wool grease and suint, as well as sand and seeds are removed during scouring.
Fibre properties
Wool can be irritating to the skin. The scale may scratch the skin and the chemical composition of wool can cause an allergic reaction. Wool is an expensive fibre because of the economics of sheep farming and the amount of processing that the fibre must go through before being in fabric form. However, wool has many good properties that make it desirable for a number of end-uses. For example, wool has a low flammability which improved on applying a flame retardant finish. Wool is a matte fibre.
Physical structure
Wool fibres vary in length from about 12 to 381 mm. The surface of the fibre is covered with overlapping scales (Refer to figure 3.1). Fine wools can have as many as 790 scales per cm and course wools as few as 275 per cm (figure 4.1). The tips of the scales point towards the tip of the fibre. In cross-section, wool fibres are roughly cylindrical and they have a multi-cellular structure. Typical diameters range from 10–60 micrometres. Fine wools have an average diameter of about 15–17 micrometres, while course wools have an average diameter of 40 micrometres. Wool has natural crimp. This crimp gives wool its elasticity and helps in yarn manufacture.
Properties relating to durability
- Abrasion resistance
- Strength
- Flexibility
- Elongation
- Abrasion resistance
Wool fibres have moderate resistance to abrasion due to the scale structure of the fibre and its flexibility.
- Strength
Wool fibres have a low tenacity. They have a tenacity of about 8–13 cN/tex when dry, and about 7–12 cN/tex when wet. Wool is a weak fibre but it can produce a strong fabric because of its resiliency and elasticity.
- Flexibility
The flexibility of wool is excellent. Wool fibres can be bent back on themselves 20000 times compared to 3000 times for cotton. Atmospheric moisture helps wool retain its flexibility.
- Elongation
Wool has excellent elongation (25% elongation at break).
Properties relating to comfort
- Moisture absorption
2. Heat conductivity
- Moisture absorption
Wool is more hygroscopic than any other fibre. It has a moisture regain of 13–18% under standard conditions. Wool is comfortable to wear in cold and humid conditions. Wool accepts colour readily because of its ability to absorb moisture.
- Heat conductivity
Wool is a poor conductor of heat. Air is also a poor conductor of heat and the crimp of wool fibres traps air in between the fibres. Therefore, wool is warm to wear.
Properties relating to appearance retention
- Resiliency
- Dimensional stability
- Elasticity
- Resiliency
The resiliency of wool is exceptionally good. It will readily spring back into shape after crushing. This property is needed in a carpet fibre. The unique structure of the wool fibre also allows a durable crease or pleat to be pressed into the fabric with the use of heat, moisture and pressure. This is of importance in tailoring, as the fabric is permanently shaped during the tailoring process. Wrinkling may occur if the fabric is wet.
- Dimensional stability
Wool fibres are not dimensionally stable. Wool is susceptible to relaxation shrinkage. During the spinning of a yarn or weaving and finishing of a fabric, the product is often stretched. In the presence of moisture, the product will then relax and return to its original shape, which may mean it will shrink. In wool, relaxation shrinkage can be controlled by steaming the fabric. This is usually done only once before the fabric is cut for garment manufacture.
Wool is also subject to felting shrinkage. This shrinkage cannot be reversed. When wool fibres are subjected to heat, moisture and agitation, the scales on the fibre surfaces interlock and the fibres become entangled and matted. This property is used when manufacturing wool felts, but for untreated wool garments it can mean shrinkage of the item in the wash. Treatments can be applied to wool to prevent this type of shrinkage.
