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No one can deny the advantages of natural fibres, and yet they are rarely used in composite materials. The Lineo Company is now offering prepregs reinforced with natural flax fibres.
(Published on December 2007 – JEC Magazine #37)
FRANCOIS VANFLETEREN, PRESIDENT LINEO
People around the world have been using flax fibre as a textile fibre for thousands of years. Nowadays, composites constitute a new outlet for flax – one that is equally as interesting as it is unknown to most people. Flax fibres have been used in composites for a number of years to improve materials by their insulating properties and low weight, but this has involved very little added value. In such applications, short flax fibres must compete in the market with other fibres such as kenaf, jute and sisal. But there is another potential outlet that could be sustainable for long flax fibres, in parallel with the textile industry, by using them as reinforcement in composite materials, just like glass or carbon fibres. This utilization would be based on the intrinsic physical properties of flax fibre, and therefore would exclude any competition with other natural fibres.
Lineo: the fruit of partnership
Lineo was created by four companies:
The joint project consisted in developing a process that would make it possible to use natural flax fibres as reinforcement in composite structures. Following research by IPA and Stanislas Cock, a patent was filed to protect the flax-fibre sizing process and allow its use in high-performance composites. Lineo offers an innovative product in a market that is dominated by glass and carbon fibres.
The mechanical properties of flax fibre are similar to those of standard glass fibre. Flax has other advantages, however. It is lighter than carbon and has a density of 1.45 (Table I).
Like natural fibres in general, it also has vibration-absorbing properties that are due to the links between cellulose and hemicellulose chains – low-energy bonds that are known as the Van der Waals force. During vibration, these contact bonds break easily, allowing the molecules to move slightly. Thanks to this potential for movement, the fibre is able to absorb the vibrations. There are many similarities between flax and carbon fibres (such as a coefficient of expansion that is practically zero and an elongation at break of about 1.5%), so it is possible to make high-performance hybrid composite parts that are reinforced with both flax and carbon (Figure 1). Water has a density of 1, corresponding to the weight in kilograms of a litre of water. Flax fibre is lighter than steel, and lighter even than carbon and glass fibre.
A patented process
One of the founders of Lineo developed and patented a process to impregnate natural flax fibres with thermosetting resins. The company now has exclusive licence status under the patent to manufacture and market flax-fibre-reinforced prepreg materials. The process appears to solve all the problems confronting manufacturers who have tried to introduce flax into composites.
Two of the more significant problems have been 1) the poor adhesion between the flax fibre and the resin, and 2) the tendency of the flax fibre in the composite material to absorb moisture. The process limits water uptake to 2% (equivalent to glass and carbon), thereby preventing the flax fibre from degrading within the composite material. The fibre-matrix bond created also helps to transfer the mechanical properties of the flax fibre to the composite part.
A specific treatment results in a strong bond between the flax fibres and the epoxy resin, practically eliminating water uptake in the flax fibres.
Five different prepregs are available so far (Figure 2):
the experimental data are well predicted by the model lines.
A half-flax-/half-carbon-reinforced racing bicycle
Former world cycling champion Johann Museeuw decided to develop a bicycle that would be 50%-50% flax/carbon reinforced. The resulting bicycle has definite advantages: the carbon gives stiffness and the flax absorbs the vibrations from flaws in the road. The bicycle has been tested by a number of cycling professionals, unanimous in their approval (Figure 3). The Museeuw Bikes Company is currently working to develop flax-reinforced wheel rims. As we have seen, flax fibre has its own specific mechanical properties and characteristics. It can be used as reinforcement in composite materials. It can even be used as reinforcement in high-performance composite materials, where the fibres are used for their strength and mechanical properties in structural parts. This is a potential outlet for the high-value-added long fibres, one that could open up to the entire flax industry.
JEC Composites Magazine: What makes natural fibres so attractive for composite materials today?
FRANÇOIS VANFLETEREN: The main attraction of natural fibres is their low weight. Some bast fibres like flax and hemp also have mechanical properties that are on a par with glass fibre. So natural fibres can be used for high-performance composite structures. There is also the environmental aspect, since these fibres are renewable and non-polluting, and have a very small carbon imprint.
JCM: Why choose flax over other natural fibres?
FV: Flax is the natural fibre with the best mechanical strength properties. Furthermore, in addition to these qualities, there already exists a complete industry in Europe for flax, from crops all the way to the manufacturing of fabrics. This means that we can develop a range of customized flax reinforcements relatively easily.
JCM: Which application sectors are you targeting as a priority, and in which geographical areas?
FV: The automotive sports sector is very interested in hybrid flax/carbon composites. These are both stiff and vibration-free, thanks to the flax properties – an aspect that also has a broader appeal in the sports and leisure industry and for machine builders. We also note that the transportation sector is interested, as a way to replace glass-fibre reinforcement while retaining the same mechanical properties and making the materials lighter. Because flax has roughly the same coefficient of expansion as carbon, Axson, our distributor, is helping us develop a range of tooling products to replace carbon moulds with flax-based ones, which will enable us to save a lot of money.We are established mainly in France and Europe.