Flax, Linen

8-15 usd/kg
Circularity potential
High
Strength
Very high
Production energy
Low
Stiffness
Medium
Embodied CO2
Medium
Density
Medium

Flax is a bast fibre, like hemp and jute, which is extracted from the stalk of the blue-flowered flax plant grown for its seeds (linseed) and fibre. As a natural material, its properties will vary widely, depending on the plant, growing conditions and where in the stalk the fibre came from. Even so, flax and the other bast fibres have impressive qualities. As a technical fibre it competes alongside manmade fibres like glass (GF) and aramid (AF) in terms of strength to weight. And as linen fabric – thought to be one of the oldest textiles – used for clothing and interiors, flax is prized for its high strength, soft hand and high lustre. In addition, it gets stronger when wet, and can soak up around 20% moisture before feeling damp.

Flax is widely cultivated and grows without much need for fertiliser, pesticide or herbicide. Once harvested, the stems require retting to release the fibres from their bundles. This is typically carried out by lying them in water or in the field where they were grown. Microbes breakdown the hemicellulose-pectin matrix that locks the fibres in.

Yarn production involves decorticating and combing the fibres (this is also known as scutching and hackling when carried out by hand) to produce a sliver free from contamination; bleaching; drying; spinning; and winding. Of course, for natural colour fibres, some of these processes are avoided.

Nothing is wasted in production. The seeds are used for food (linseed oil comes from another type of flax plant), the woody core of the stem goes into particleboard or mycelium materials (fungi), and the short fibres are converted into paper or board.


Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Low circularity potential
Microplastics


Flax is strong, lightweight and sustainable. But its properties are variable and, as a result, a little unpredictable for structural applications. One major advantage it offers as fibre reinforcement in composites is vibration dampening (dissipation of mechanical energy). Therefore, it is often used in combination with glass (GF) or carbon fibre (CF) to enhance the overall performance of the composite. In this way, it has found application in a wide range of demanding applications, such as automotive seats structures and door panels, wind turbine blades and sports equipment.

On it own, it is used to reinforce both thermosetting plastic systems, as well as thermoplastics, such as polypropylene (PP), and polylactic acid (PLA) for a fully biodegradable composite. It is used in the production of furniture, automotive interiors and even musical instruments. As a natural fibre, it has more bulk than synthetics like GF and CF, which tend to lie flat. This means the best results are achieved with processes that apply positive pressure during moulding, such as compression moulding, pressing and resin transfer moulding (RTM). It is also compatible with processes like resin infusion, used in wind turbines and boat building for example, but less well-suited to dry processes that use vacuum consolidation alone.

Its high moisture absorption properties, which are very useful as a textile fibre, can cause problems in a composite. Moisture leads to micro-cracks in the matrix-fibre bond, and this weakens the structure. Therefore, in structure critical applications, the fibre must be completely sealed in with resin – for example, cut edges must be treated to avoid exposed fibre.


Design properties
Cost usd/kg
3-7
Embodied energy MJ/kg
71.5-81.5
Carbon footprint kgCO2e/kg
2.2-3.1
Density kg/m3
1040
Tensile modulus GPa
6-7
Tensile strength MPa
70-90
Flexural strength MPa
75
Hardness Mohs
1
Thermal conductivity W/mK
0.12
Temperature min-max °C
-10 to 100
Thermal
insulator
Electrical
insulator