How does a Wool Fiber work - Structure and Properties of Wool
Sheep’s wool is a protein fiber and is one of the natural fibers and renewable raw materials. Worldwide, 1.5 million tons of sheep’s wool are sheared and processed each year. This corresponds to more than 96 percent of all animal hair produced.
The remaining four percent is made up of mohair, angora, cashmere, llama and camel hair.
The term new wool is intended to express that the wool was sheared from the living sheep and is not a recycled product. Wool is the oldest fiber material in the world.
Fine Cashmere Wool with a wool fiber diameter of only 12 microns
Structure and Properties of Wool Fiber
The Alpha Helix
Hair (Wool is the Hair of the Sheep), fingernails and feathers are made of keratin, an almost insoluble protein with high resistance.
The polypeptides of the wool fiber form a so-called αlpha-helix. This means that the molecule winds around its axis like a corkscrew.
The αlpha-helix is right-handed. The helix is mainly formed by relatively weak “hydrogen bonds” which, however, are present in large numbers.
Since these attractive forces act within a molecule, this case is also referred to as an intramolecular interaction. →
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The Alpha Helical Structure of Wool Keratin - Creative Commons CC BY-NC-ND
Hydrogen Bonds – Roland.chem, CC0, via Wikimedia Commons
The Hydrogen Bonds
The “Hydrogen Bonds” are mainly responsible for the flexibility of the wool fibers.
The polar Carbonyl group and the polar Amino group can bind water molecules since water is a dipole itself.
Polypeptide chains are therefore hygroscopic. A wool sweater weighing 1kg can absorb up to 0.3 liters of humidity and sweat without feeling damp or clammy.
It also does not lose its warming effect. The heat-insulating property and water-absorbing capacity of wool is due in particular to the cavities inside the fiber.2
When water molecules accumulate around these functional groups, the intramolecular interactions are no longer effective so that wet wool, unlike other fibers, can be stretched to twice its length.
When wet Wool is stretched, the helical structure changes to a protein folded leaflet structure – breaking the intramolecular hydrogen bonds.
In this process, intermolecular Hydrogen Bridges are formed between adjacent polypeptide chains:
Protein Folding - Leaflet Structure
In the leaflet folded protein structure, amino acid residues with the same charge lie opposite each other: the leaflet structure of the wool is not stable. After stretching, the wool reverts to the lower-energy helical structure on its own.
Depending on the nature of the amino acid residues, additional bonds such as ionic bonds “salt bridges” or covalent bonds “sulfur bridges” can provide stability to the helix.
Significant, non-polar amino acid residues also play a role here with their van der Waals forces.3
Icebreaker - Merino Wool Shirt
Icebreaker - Merino Wool Shirt
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The Structure of a Wool Fiber
Detailed Model of a Merino Wool Fiber – Licence Creative-Commons – Science Image, Credit to CSIRO
The wool fiber has a cuticle that resembles the trunk of a palm tree and is impermeable to liquid water. A thin skin covers the scales, the epicuticle. Beneath the scales lies the cortex, or bark layer, which is the main component of the fiber.
Chemically, wool consists of proteins; the chemical composition can be about 50 percent carbon, 25 percent oxygen, and 15 percent nitrogen, plus hydrogen and sulfur.
Natural fibers are air-permeable, breathable and support a healthy body climate. In addition, the skin-friendly properties are retained if the fibers are processed naturally, and no chemical finishes are used.
When a wool fiber is stretched, the scales slide apart. When the fiber contracts again, the scales slide into each other![efn_note]The Chemical & Physical Structure of Merino Wool[/efn_note]
The Excellent Properties of Wool
Fineness of Wool
The fineness of wool is an essential property and determines its value and usability.
The term fineness generally refers to the average diameter of fibers that occur in a fiber composite. The diameter of wool fibers is measured in microns!
Stretchability and elasticity
Elasticity refers to the ability of the wool fiber to return to its original shape after being stressed.
Finer fibers are more elastic than coarser fibers. Because of its elasticity, wool keeps its shape very well; it has the best crease resistance of all natural fibers.
When exposed to tensile stress, wool stretches up to 30% without tearing. It then contracts back to its original length. However, wool is virtually wrinkle-resistant, supple and soft.
When wool fibers are stretched, there is a transition from the helical structure to the energetically less favorable folded sheet structure. In this process, the covalent bonds of the “sulfur bridges” counteract the tearing.
The scales of the cuticle slide apart when stretched. When the fiber contracts again, the scales slide into each other, and the polypeptide chains revert to the energetically more favorable helix structure.
Damp wool is heavier than light wool because a considerable amount of water is bound by the formation of hydrogen bonds.
However, the pre-existing hydrogen bonds between the carbonyl and the amino groups are also broken in the process, which leads to wear-out.
Icebreaker - Merino Wool Shirt
Icebreaker - Merino Wool Shirt
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Color and sheen
The refraction of light causes a silky sheen in healthy wool, which only really comes into its own after washing.
This luster is important for the subsequent brilliance of the colors.
Absorption and release of moisture
Wool fibers can absorb and release moisture without chemically combining with it – they are hygroscopic. Wool does not feel damp even at 30 percent water absorption.
The polar carbonyl and amino groups can bind water molecules via hydrogen bonds store them without this water reaching the skin. Since the water molecules are only held by the “hydrogen bonds,” these water molecules can also be released again when heated.
Water repellent
The so-called mantle or cuticle of a wool fiber consists of three layers, whereby the outermost layer consists of lipids and is thus hydrophobic.
Water droplets, therefore, roll-off. Water vapor, on the other hand, can pass through the cuticle.
Discovering Wool
No electrostatic charge
Wool textiles do not become electrically charged under normal atmospheric conditions.
Thus, the attraction of dust particles is absent, so frequent washing of textiles is not necessary.
Odor Neutral and Antibacterial
Wool fibers are antibacterial: the fiber can kill bacteria or slow their growth.
The fibers are technically hair and therefore have keratin. Keratin has anti-fungal, and anti-microbial, features. Antibacterial properties help to reduce odor in garments and require less washing.4
Washability
To wash the wool should use a mild detergent and do not exceed the temperature of 30 ° C. During the first wash, relaxation shrinkage occurs, which can account for a change in length of up to 7 percent.
This shrinkage is reversible by pulling on the wet or dried garment. Today, wool textiles can be washed in the washing machine without any problems, using a special wool program.
Spinning does not damage the fibers: due to the centrifugal force, the wool particles are pressed against the outer wall of the drum and remain still there during spinning.
During drying, the wool remains in its most comfortable position and, to prevent deformation, should be dried lying down.
Flame resistant
Due to the high nitrogen and moisture content, virgin wool does not melt and ignites only at a temperature of 560°C.
Wool is Cool – The Properties of Wool
Queenfur Merino
Smartwool - Merino
Merino BeddingsMore Wool Knowledge
The term wool is used primarily to refer to sheep’s Wool. The term “wool” merely indicates that the article is made of pure Wool – however, it says nothing about the quality of the Wool.
Only Wool from healthy, living sheep may be called pure new Wool. It is particularly breathable and temperature balancing.
Recycled Wool is Wool reprocessed from waste and rags. Compared to virgin Wool, it has shorter fiber lengths and is of inferior quality.
Iceland Wool: As protection against the harsh Nordic climate, Icelandic sheep develop a particularly dense wool coat called Iceland Wool. It is very warm, robust, strong and water repellent. The undercoat is characterized by a very soft, fluffy quality.
Lambswool is the name given to Wool from young lambs that are no more than six months old and have not been shorn up to that point. Lambswool is very fine and exceptionally soft.
Sheep breeds are divided according to the character of the Wool. The fine merino wool, the medium-fine to strong cheviot wool and the crossbred Wool. Crossbred sheep are a cross between merino sheep and Cheviot Sheep.
Merino wool is the finest quality of sheep wool due to its fineness and softness. It is characterized by particular uniformity, elasticity and lightness.
Shetland wool is the name given to wool types that come from sheep living on the Shetland Islands. Often the surface is slightly milled.
Angora wool has its animal origin in the angora rabbit of the same name. The also known Angora goat, on the other hand, does not provide Angora wool but Mohair wool.
Cashmere Wool is derived from the cashmere goat and is one of the most valuable and expensive natural fibers. Therefore Cashmere is often offered mixed with merino wool or other sheep wool.
The selling price depends on the quality of the Cashmere. The hair should be as fine, long, curly and light as possible.
Alpaca is the name given to the wool of the domesticated alpaca, originally native only to the Andes. The hair is very fine, soft and silky shiny. It is only slightly curly but still produces fairly durable wool.
There are two types of the Alpaca, the Huacaya and the Suri Alpaca. They both have a different structure of their fiber: the Huacaya alpaca has a fine, evenly curled fiber and some guard hairs.
The Suri alpaca, has no crimps in the fiber, and the hair forms curly, straight strands. The wool must be sorted by over 20 natural colors.
If you want to know more about wool, read the very informative article about the History of Wool.
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Thank You for Reading
Marco Heitner
- Gruen, L. C., & Woods, E. F. (1983). Structural studies on the microfibrillar proteins of wool. Interaction between alpha-helical segments and reassembly of a four-chain structure. The Biochemical journal, 209(3), 587–595. https://doi.org/10.1042/bj2090587
- 1. Farnworth AJ. A Hydrogen Bonding Mechanism for the Permanent Setting of Wool Fibers. Textile Research Journal. 1957;27(8):632-640. doi:10.1177/004051755702700807
- Church JS, Corino GL, Woodhead AL. The analysis of Merino wool cuticle and cortical cells by Fourier transform Raman spectroscopy. Biopolymers. 1997;42(1):7-17. doi: 10.1002/(SICI)1097-0282(199707)42:1<7::AID-BIP2>3.0.CO;2-S. PMID: 9209155.
- Wool: Stucture, Properties and Processing - John A. Rippon,John R. Christoe,Ronald J. Denning,David J. Evans,Mickey G. Huson,Peter R. Lamb,Keith R. Millington,Anthony P. Pierlot
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