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Wool fibre properties

Wool’s range of desirable properties make it a valuable material for many different purposes, from high-end fashion to fire-resistant products and heavy-duty carpet. Discover how wool’s unique structure creates its many desirable properties.

Wool’s unique cellular structure gives it a number of desirable properties. Most properties are typical of all wool, but there are some differences in wool from different sheep breeds. The differences affect the appearance and feel of the wool and what it’s used for.

Wool’s surface structure

Wool fibres have a unique surface structure of overlapping scales called cuticle cells. The cuticle cells anchor the fibre in the sheep’s skin. Wool’s surface is very different to typical synthetic fibres, which have a very smooth surface.

Cuticle cells give ability to felt

The exposed edges of the cuticle cells point towards the tip of the fibre, creating a jagged edge. This allows fibres to slip over one another easily in one direction but not the other, giving wool the ability to felt.

Felt is created when wool fibres are agitated in water – they slip over one another and the scales interlock, preventing the fibre from returning to its original shape. The process can be controlled to create very dense fabrics such as felt and wool blanket and jacket fabric, but can also be caused unintentionally during laundering and ruin a garment.

Fibre surface is water repellent

The cuticle cells provide a tough exterior, protecting the fibre from damage. The cells have a waxy coating, making wool water repellent, but still allowing absorption of water vapour. The water-repellent surface makes wool garments naturally shower-proof and also reduces staining because spills don’t soak in easily.

Wool’s interior structure

The interior of the wool fibre, called the cortex, is made up of long tapering cells that overlap and are surrounded by the cell membrane complex (CMC). The CMC runs throughout the fibre and is made up of proteins and waxy lipids. The molecules in this part of the fibre have fairly weak intermolecular bonds, so with prolonged wear and abrasion, they tend to break down. The weak bonds also make this area susceptible to chemical attack, such as strong alkaline conditions, which is why washing with a pH-neutral detergent is recommended.

Interior structure creates flexibility and absorbency

The cortical cells also have a complex interior structure. The smallest component within these cells is a spring-like structure, which gives wool its flexibility, elasticity, resilience and wrinkle recovery properties.

This spring-like structure is surrounded by a matrix, which contains high sulphur proteins that readily attract and absorb water molecules. Wool can absorb up to 30% of its weight in water without feeling wet. It also absorbs and retains dyestuffs very well, helps remove sweat and absorbs odours.

The matrix also creates wool’s fire-resistant and antistatic properties.

Absorbency creates comfort

When wool absorbs moisture, it produces heat, so if you go from a warm room into a cold, damp night wearing a wool jersey, the wool picks up water vapour from the air, keeping you warm. The reverse occurs when you go back into the warm room – the moisture in your jersey passes into the atmosphere, cooling you down. Tiny pores in the cuticle cells allow water vapour to pass through the wool fibre. This makes wool comfortable to wear in both warm and cool conditions.

Arrangement of interior cells creates fibre crimp

There are 2 main types of cell in the cortex – orthocortical and paracortical – and each has a slightly different chemical composition. In finer fibres, these cells are arranged in 2 distinct halves. In coarser fibres, the arrangement is less distinct.

These cells create the crimp in wool. The 2 types of cell expand differently when they absorb moisture, causing the fibre to bend. When the cells are arranged in 2 halves, there is more crimp, and the more random arrangement in coarser fibres creates less crimp, so crimp relates directly to fibre diameter.

Fibre crimp affects wool’s properties

The crimp in wool fibres makes it soft and springy to touch. It also adds bulk and traps a large volume of air between the fibres, giving it good insulation properties. Finer fibres with more crimp such as Merino create fabrics that drape better than coarser fibres.

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