By Eman Abdallah Kamel
Eman is a writer and engineer. She received her bachelor’s degree in textile sciences from the Faculty of Applied Arts in Egypt.
Wool is a natural fibre that provides warmth, insulation, and comfort. This article covers the physical and chemical properties of wool fibre and the benefits of fabrics.
Wool
Wool is a natural protein-based fibre that grows on the bodies of sheep and other animals, such as camels, Angora rabbits, goats, and muskox. To learn more about different types of wool fabrics, visit Types of Wool Fabrics and the Properties of Each Type.
Wool is a natural, renewable resource. Fibres are curly and flexible, and they grow in staples or clusters. Lanolin and grease hold the staples of wool together.
Wool’s internal structure and chemical composition are intricate. The main proportion of the fibre material is α-keratin, a biological polymer consisting of polypeptide chains made up of amino acids such as lysine, cystine, arginine, and glutamine.
The molecules in the polypeptide chain are linked together by a covalent bond formed between the carboxyl group COOH of one molecule and the amino group NH₂ of the other, resulting in a functional group CONH, which is called a peptide bond.
Wool fibres are used in men’s suits, women’s suits and dresses, coats, shirts, scarves, hats, carpets, blankets, and upholstery.
Properties
1. Physical Properties
- Crimps: They are the ripples in wool or hair. Crimps range from small ridges to longer waves. The size of the crimp and the quantity of crimps per unit length are crucial crimp parameters. Crimps are responsible for:
- Breaking stress,
- Stretchability,
- Internal modulus of elasticity,
- Lustre of the final product.
Did You Know?
The crimps in wool trap air and insulate the warmth of body heat.
Fibres with high crimp, 40-60 per mm, are finer and can be spun to a finer yarn. They perform well for woollen spinning. On the other hand, a fibre with a high crimp is less suitable for worsted spinning as it is difficult to compact the fibres enough to give the drape associated with worsted spun yarns.
- Elastic recovery refers to a textile fibre’s ability to regain its original length after being stretched.
Elasticity plays a major role in wool’s mechanical properties and felting. Alpha-helical cells give wool its elastic quality, which allows it to act as a spring up to 2.5 percent of its length.
- Stress: Wool exhibits three distinct zones called the Hookean, yield, and post-yield zones when stretched longitudinally. The load in the Hookean region is held by cortical cells with elongation of up to 2.5%. In the post-yield zone, the wool fibres continuously bear the full load until they break, with an elongation range of 25% to 35%. Wool fibre breakage occurs due to complete stretching and subsequent disulfide bond breakage.
Coarse wool has a tensile strength of 12 to 14 g/tex, whereas fine wool has a tensile strength of 8 to 10 g/tex. Wool’s stress-strain behaviour changes depending on temperature, relative humidity, and surface modification.
- Resilience: Wool contains orthocortical and paracortical cells and tends to elongate during loading or compression without changing the dimension to the desired limit. It retains its shape after the load is removed. The proportions of orthocortical and paracortical cells can determine the elasticity property of wool fibres, which are low in fine wool fibres and high in carpet wool.
- Moisture: Wool has multiple functional groups and amorphous regions, so it can easily absorb water molecules and retain 13% to 18% over the weight of dry fibre at 65% relative humidity. It can reach 40% at 100% relative humidity.
Did You Know?
Wool is a moisture-absorbent material, so it easily absorbs water vapour, helping to resist moisture buildup inside clothing.
- Water Repellent: Wool fibres’ surfaces are naturally water-resistant. Small water droplets do not easily seep into the wool fabric but remain on the surface in droplets that can be easily shaken off.
- Flame Resistant: High concentrations of sulphur and nitrogen in wool make it difficult to ignite wool because they are natural fire retardants.
When wool is extinguished, it forms a soft ash residue that dissipates. Various chemicals have been developed to improve the fire-resistant performance of wool fibers. These include zirconium, phosphonates, and halogenated compounds, which are generally applied by exhaust in the manner of conventional mordant dyeing.
- Lustre: Wool fibres with shorter interdigitations and more scales per unit cross-section have a high luster. The natural lustre can be improved by increasing the alignment of the fibres in the yarn through worsted spinning.
- Durability: The interwoven protein molecules in wool fibres can stretch and return to their original shape, resulting in a durable, long-lasting fabric.
- Colour: Wool is creamy white, but in some cases, it is coloured due to the presence of melanin produced through the oxidation of tyrosine, followed by its polymerization during growth. Eumelanin and pheomelanin are responsible for the black-to-brown and yellow-to-red colour, respectively.
- Anti-static: Wool does not tend to accumulate static electricity because it naturally absorbs moisture from the air. Wool clothing is also less likely to “spark” or stick to the body. Because of this characteristic, wool does not draw in dust from the air.
2. Chemical Properties
- Dyeability: The scales on the surface of wool fibres diffuse light, reducing reflection and giving a softer color. The proteins in the core of the fibre absorb and combine with a variety of dyes, allowing the wool to retain its colour.
- Effect of Alkalies: Wool is susceptible to any alkaline (a chemical that can dissolve in water and combine with acids to form salts) solution in heat since it easily breaks the polypeptide linkages into respective amino acids. In a controlled mild condition, cystine bonds split into hydrogen sulphide with a reduction in the amount of cystine content partially and form a new lanthionine bond. Also, cross-linking occurs between an integral aminoacrylic acid and the free amino groups of the lysine residue to form a lysinoalanine bond.
In general, alkali treatment in the presence of heat affects the mechanical properties and handle of wool with additional yellowing due to the hydrolysis of aspartic and glutamic residues to aspartic acid and glutamic acid.
- Effect of Acids: Wool fibres are more resistant to acids than alkalis. Under mild conditions, acid cation (A+) can combine with basic functional groups (-NH₂) of wool fibre to form protonated functional groups (-NH₃+) and form chemical bonds with anionic dye and final chemicals.
- Effect of Oxidizing Agents: Wool fibres are sensitive to oxidizing agents in acidic or alkaline conditions. The oxidizing agent breaks the covalent bonds and salt bonds of the cystine bond and converts it into two cysteic acids; the oxidized cystine bonds also form a new bond with the adjacent protonated amino groups.
- Effect of Reducing Agents: The reduction process can change the dimensions of wool fibres by selecting a reducing agent. Reduction occurs on the cysteine sulphate bond, and other amide bonds cannot react. Tributyl phosphine, sulphate, mercaptoethanol, and thioglycolic acid can be used to reduce wool fibres.
- Effect of Insects: Clothes moths or carpet beetle larvae feed on the protein found in wool fibers. Wool is treated with chemicals to keep insects away, but of course, most chemicals are toxic and harmful to the environment. Some companies are using more sustainable methods, such as wool protection ionic. Wool is passed through a machine that generates an energy-rich plasma field. The electrons and ions interact with the wool and change its molecular structure.
Benefits of Woolen Fabrics
- Wool is comfortable to wear because of its elasticity and ability to absorb moisture.
- Wool does not attract dust from the air due to its high moisture absorption capacity and therefore low static electricity buildup.
- Wool produces heat and keeps the body warm instead of cold and damp if it gets wet because it is hydrophobic by nature. Although it absorbs moisture, it repels liquids. This is one of the unique qualities of wool.
- Wool fabrics burn slowly and have limited flammability.
- Wool is used as an insulating material to reduce noise.
Sources
- Crimp: How important is it in modern wool?
- Wool Fiber
- Flame-resistant wool and wool blends… pdf
- The wool fiber and its applications
- Physical and chemical properties of wool fibers…pdf
- Wool protection against pests
©Eman Abdallah Kamel, 2024
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