Unlikely bed fellows
The production routes for flax and carbon fibres couldn’t be more dissimilar, yet increasingly both are now finding favour in composite parts for the automotive industry – often in nonwoven constructions.
After having been almost completely replaced by synthetic counterparts in the 1960s and 1970s, natural bast fibres – hemp, jute, kenaf, sisal and especially flax – have been regaining ground in automotive applications since the 1990s.
Some 150,000 metric tons of natural fibre composites are now employed in new European cars and trucks each year and as such, this is a much bigger market than the global automotive industry’s use of carbon fibres to date, yet attracts far less attention.
The advantages of natural fibre composites include their light weight, crash behaviour, deformation resistance, the ability to be laminated, and their price. On the downside, they allow for limited shape and design forming and also lead to some waste in their production.
The composites are compression-moulded nonwovens and resins based on technology that has been developed mainly in Germany.
France, Belgium and The Netherlands account for 75% of global flax growth and the best soils for growing it are mainly to be found in these three countries. In addition to the 8,000 farms that grow textile flax in the EU, the industry also directly employs 15,000 workers and a further 10,000 people indirectly.
An example of a recent success story for flax nonwovens is to be found in the door trims of the latest Mercedes E Class. These are based on a material called FibriPlast Hybrid made by EcoTechnilin, a company based in the UK with a modern nonwoven production facility in the heart of the Normandy flax fields of France.
“FibriPlast Hybrid combines our regular thermo-compression process with localised injection moulding,” explains the company’s marketing director William Anthony. “Once the tool has closed with the heated flax nonwoven and polypropylene pre-form inside, small amounts of polypropylene are injected to form the reinforcements and fixings. As a result, it combines the cost-effectiveness of thermo-compression with the localised intricacy of injection moulding – all in a quick, one-shot process. It’s very neat and cost effective – much lighter than an injected part and the tool costs six times less.”
An entirely new category of nonwoven based on flax has even been developed in France and a leading car manufacturer is on target to use a million square metres of it in 2017.
Lineo, based in Belgium, calls its product Flaxtape, but to all intents and purposes it’s a nonwoven, consisting of unidirectional (UD) flax fibres which, in a patented process, are stretched and then sprayed with water and dried, so that the natural pectin in the flax is all that’s required to bind the material together.
The process was developed in a research project involving Peugeot and its Tier 1 supplier Faurecia, with Lineo’s Flaxtape being the starting point for sandwich panel composites for automotive interior parts, including load floors, door panels, roof trim, instrument panels and seat backs.
Carbon fibre, meanwhile, is a material consisting of extremely thin fibres of around 5-10 µm in diameter and composed mostly of carbon atoms. The atoms are bonded together in microscopic crystals that are more or less aligned parallel to the longitudinal axis of the fibre. This crystalline alignment makes the fibre very strong for its size.
Carbon fibre has been defined as containing at least 90% carbon, obtained by controlled pyrolysis – once the fibres are stabilized, they are heated to a temperature of between 1,000 and 3,000°C.
Carbon fibre composites consist of fibres or fabrics in combination with plastic resins to provide a high strength-to-weight ratio material. Its specific strength is considerably higher than aramids, aluminium alloys or steel.
The density of carbon fibre is also considerably lower than that of steel, making it ideal for applications requiring low weight, and this has ensured its continuing success in aerospace applications.
However, it is very expensive to make, and its cost of around US$10,000 per ton has so far prohibited it from entering the consumer automotive market to any great extent. Carbon fibre is also an isotropic anisotropic material, which means that all fibres have to point in the same direction as the force-lines through the material to be effective. It is for this reason that unidirectional carbon fibre reinforcements, rather than nonwovens, are favoured as the basis for high-performance composites.
Precisely because of its high cost however, there is a large and growing market for recycled carbon fibres, and the most effective option for converting them into a range of second life products is via drylaid or wetlaid nonwoven technologies. A number of companies such as ELG Carbon Fibre and Technical Fibre Products, both located in the UK, have recently invested in new technology to convert recycled carbon fibre into nonwovens as the basis of composites which are already beginning to appear as new components in the automotive industry.
Another vehicle lightweighting research project, called Carbio and involving Jaguar LandRover, has even explored the benefits of combining carbon and flax. A prototype of a composite roof module has been developed for the car company, consisting of alternating layers of carbon and flax materials.
“There are issues with noise, vibration and harshness (NVH) when employing composites in vehicles, which flax fibres can go a certain way to solving,” explains Gareth Davies of Composites Evolution, which led the Carbio project. “Flax and carbon may seem like strange bed fellows, but with the addition of layers of flax materials between the carbon we can considerably increase the damping properties. This has resulted in a part that reduces costs and weight, with the same bending stiffness but hugely improved damping.”
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