Wool fiber – Basics, Characteristics, & Properties

flock of sheep

Of the major apparel fibres, wool is the most reusable and recyclable fibre on the planet. The eco-credentials of wool are enhanced by its long service life and suitability to be recycled to new textiles for clothing, resilient upholstery or products that call on its natural resistance to fire and temperature extremes. Aside from premium next-to-skin apparel, wool can be used in industrial applications such as thermal and acoustic insulation or in pads to soak up oil spills.

At the disposal stage, natural fibres such as wool reduce the impact of the textile industry on pollution and landfill build-up. In warm, moist conditions such as in soil, wool biodegrades rapidly through the action of fungi and bacteria to essential elements (i.e. Nitrogen and Sulphur) for the growth of organisms as part of natural carbon and nutrient cycles.

Growth

Wool is possibly the oldest fibre known to humans. It was one of the first fibres to be spun into yarn and woven into a fabric. Wool mostly comes from sheep but also from alpacas, camels, and goats. Australia, Eastern Europe, New Zealand, and China are major wool producers. The American woollen industry began in the Massachusetts settlements in 1630, where each household was required by law to produce wool cloth.

Harvesting

wool fiber

Then, they need a haircut. The process is called sheering. A sheering specialist can sheer 200 sheep in a day. A ewe, or female sheep, can produce up to 15 pounds of wool. A ram, or male sheep, can 20 pounds of wool. The sheared wool is called raw wool and since sheep don’t take showers, it must first be cleaned.

Next, the wool is carded – that means brushing the wool to straighten the fibres. Once done by hand, these days a carding machine passes the wool through a series of rollers covered with wire bristles. The carded fibres are gently scraped into strands called roving. The roving is spun into yarn that is then woven into cloth. In the past, the task of spinning usually was the job of unmarried females – they became spinsters.  Fleece is a ball of wool taken from a single animal in a shearing. But not all wool is equal – even when it comes from the same animal. The highest quality wool comes from the sides, shoulders, and back. The lowest quality comes from the lower legs.

Grading of Wool Fibers

Wool is graded for fineness and length. The length varies from place to place on the animal, but it mostly varies amongst sheep breeds. Australian Merino wool is 3-5 inches long. Breeds found in Texas and California produce fibres 2.5 inches long. Wool from other breeds and other animals may be as long as 15 inches.

Properties of Wool Fibers

This microscopic view shows us why wool is special. The surface is a series of overlapping scales of protein, pointing toward the tip. On the animal, this enables the foreign matter to work its way out of the fleece. In a strand of yarn, it enables the fibres to lock with one another. This is the key to wool’s strength. Wool’s surface repels water. Since moisture does not remain on the surface, woollen fabrics tend to feel dry and comfortable even in damp weather. The inner core does absorb moisture – so much so that wool can absorb almost double its own weight in water and still feel reasonably dry. This absorbency also gives wool its natural resistance to wrinkles. The absorbed moisture also holds down static electricity. And because of the inner moisture, wool is naturally flame resistant.

Moisture in wool

The amount of water absorbed by wool is usually referred to as ‘regain’. Regain is the ratio of the mass of water to dry wool expressed as a percentage. Another term also used by the textile industry, predominately for cotton, is ‘moisture content’. Moisture content is the ratio of the mass of water to the mass of water plus wool expressed as a percentage.  Wool, along with cotton and to a lesser extent nylon, is a fibre that absorbs moisture from the surrounding air to reach an equilibrium, which depends on the relative humidity of the environment. At ambient humidity, wool will contain 10 to 15% by weight of water and up to 35% water at high relative humidity, which is more than most other fibres. This water is incorporated in the internal structure of the fibre and, therefore, is hardly noticed by the wearer. Wool garments do not feel damp or clammy. This property enables wool to act as a buffer against sudden environmental changes, for example, excessive perspiration during exercise or changes in climatic conditions such as going outside from an air-conditioned room.

Although the wool fibre can readily absorb water vapour from the air, a garment made from wool will be water repellent to some extent. This is because the surface of the fibre has a very thin, waxy, lipid coating chemically bonded to the surface that cannot be easily removed. Scouring, washing or processing will not remove this layer. The only way to
remove this layer, if required, is by chemical treatment.

Shrinkage and felting

wool felt

There are two major sources of shrinkage in wool-containing fabrics, which may be encountered during laundering: relaxation and felting shrinkage.  Relaxation shrinkage is an irreversible change in fabric dimensions (expansion or shrinkage) that occurs only once when a fabric is first immersed in water (without agitation) and then allowed to dry. Relaxation shrinkage is caused by the release of temporarily set strains imposed on the fabric in dry finishing and can be minimised by suitable finishing routes.

Felting shrinkage is a form of shrinkage unique to wool and wool-rich fabrics and is caused by the scales on the surface of wool fibres that have a ratchet-like action allowing preferential movement of the fibre in the direction of the fibre root. This results in the entanglement of the fibres as they move under mechanical action in water (as occurs in
machine washing).

Physical properties

Chemical properties

Wool is resistant to acid, whereas cotton and cellulose are severely damaged if exposed to acid. This difference is utilised in carbonising wool to remove excessive cellulosic impurities, such as burr and vegetable matter. Wool is treated with a solution of sulfuric acid and is then baked to destroy the impurities with only minimal damage to the wool.  The differences in the chemical structure of the various fibres mean that different classes of dyestuffs are required to cover the range of fibre types. Polypropylene and polyethylene do not absorb the dyes used to colour wool so any wool pack contamination from these fibres in the final product will appear obvious. The move to nylon woolpacks does not
necessarily prevent contamination; however, as the chemical structure of nylon is similar to wool, it absorbs wool dyes and this makes any contamination less obvious.

Application of Wool Fibers

Wool today is prized for its beauty and durability. It is still the prime choice for high-quality business suits, warm sweaters, and premium carpets. The majority of wool (72.8%) is used in apparel, home furnishing account for 15-45%, industrial uses 6 to 7% and exports 5%, wool accounts for 3.3% of all fibres for apparel. The most important use of wool is for apparel coats, jackets, suits, dresses, skirts, slacks made from woven fabrics of varying weights and knitted fabrics’.

In the home furnishing area, the major use of wool is in carpets and rugs where wool is used more, cover to the carpets and warm in the rugs. Blends of different synthetic fibres with wool for suiting materials are increasingly important. This result in fabrics that are more appropriate in warmer conditions. Polyester is the most important fibre used in blending with wool.

Characteristics of Wool Fibers and Products