Growing back to nature

Dr Neil Calder looks at the facts on flax fibres to discover whether one of nature’s forgotten crop sources is ready to challenge the domination of synthetic materials.

The need to establish more sustainable manufacturing routes is driving a return to a very traditional natural fibre from a renewable crop source. The manufacture of goods from flax fibres has been a part of human existence for thousands of years. Biofibre sources are being developed to challenge the position of long glass fibres in the middle cost, middle performance range as their low density means that specific material properties compare favourably. Compared with other composite fibres, there is a dramatic reduction in the energy required for their production: under 10MJ/kg for flax, which is five times lower than for glass fibres and 25 times lower than for carbon fibre. After a century or so of synthetic materials taking centre stage in the manufacture of industrial and consumer goods, we are starting to see a swing back in favour of more sustainable natural materials like flax, hemp and bamboo in the places where glass fibre composites have previously been the materials of choice. This provides proof that the tide of technological progress can ebb as well as flood. Lest the high-performance, no compromise world of aerospace composites forgets its roots, it is worth remembering that some of the earliest aerospace composites applications used flax fibres: in 1940, Aero Research - the company that was eventually to become Hexcel in Duxford - used Gordon Aerolite’s flax fibres in a phenolic resin for the manufacture of a Spitfire fuselage. This provided a serious alternative to aluminium even in those days, although motivated by reasons of material scarcity during a period of global conflict rather than the sustainability drivers of today. Flax is one of our oldest fibre crops, with its cultivation and use dating back to Neolithic times in Northern Europe, but it appears to have been rediscovered by those seeking sustainability in production. The early days of the composites sector has been described figuratively as a cottage industry: flax production really was one, as it was locally grown, processed and used throughout the western world. Planting the seed The centre of gravity for European flax production is particularly around Northern France and the Low Countries. As with any agricultural process, production has to follow the seasons. As this piece is being written in mid-April, the flax seeds in northern Europe are just going into the ground. Harvesting takes place in August and subsequent manufacturing operations follow that. The flax which is grown in Northern Europe takes longer to mature than that from warmer climates where agricultural productivity may be greater, but this produces a plant with better mechanical properties. These materials have been used for some years as short fibres in sheet moulding compound for semi-structural applications, and as unwovens in applications such as acoustic absorbent linings for automobiles. The real sea change is in the provision of materials technology and commercial availability in the use of these materials as structured, orientated fibres in engineered components. Although these are termed semi-structural (for automotive interior panels etc.) there is a real notion that more major applications are just around the corner. Lucintel predicts 10% annual growth in the use of these in the next five years to around $3.8billion, with the largest growth potential in the automotive sector. Some of the most significant challenges are in the natural variations which occur in organic, farmed materials. Tensile strength is usually quoted in the range 70-150MPa which can be misleading, as the cross-section is irregular and continuously variable. This means that calculated mechanical property values are always going to have a significant margin of error or uncertainty. This irregularity does have some positive benefits, however, with considerable improvements over synthetic fibres in inherent ‘wetability’ with typical resin systems. Natural fibre composites (which some are calling NFCs) face a double whammy of intrinsic variations in properties and a lack of compatibility with analysis and design methods created for much-closer-to-idealised synthetic fibres like glass and carbon. This requires a change in mindset from having materials (fibres, resins etc.) created to a technical specification, to the NFC situation where you have to work with what you’re given. The property tweaking has to happen downstream in the engineering value chain instead of upstream in an interesting collision of agricultural and industrial production techniques. A 1,400MPa tensile strength limit is theoretically possible with careful processing of the fibres, i.e. deviating from the traditional methods of handling these materials for non-structural applications. Fibre damage, breaks and dislocations degrade this, as does fibre misalignment. A new set of engineering compromises between the ease of separating the fibres from other parts of the flax stems and the residual mechanical properties after this have to be negotiated before optimised solutions are available. The twist in yarns is necessary to hold fibres together, however this reduces the strength of subsequent composite materials as the angles of the fibres in the yarn takes load carrying capability away from the material. To realise structural applications for NFCs requires a new portfolio of capabilities, relying on many technologies such as appropriate resin systems, processing, fibre conditioning, handling, design, etc. The methods of extracting the fibres from the plant and processing these to produce textiles products will have to change to suit these new structural requirements. Traditional scutching and hackling processes, designed to break the flax stem down into its constituent fibres and fibre bundles, can introduce damage and defects that degrade the fibre. Whilst in the plant, the fibres are aligned to a high degree of consistency providing the strength required for the plant to survive and grow. The production of aligned fibre semiproducts is opening up a range of applications for this new/old material. The early producers of unidirectional prepreg in these materials are starting to put their offerings onto the market and UD prepreg material is now available off the shelf which pioneers are experimenting within applications. The fibre revolution Composites Evolution is exploring less twisted, less crimped Biotex textiles to make the numbers work for long fibre structural applications. These quasi unidirectional materials utilising the Oxeon spread tow method provide 0/90° layup capability for engineered products. Whilst energy and materials have been in plentiful supply, there has been no reason to turn back to the engineering of these more traditional materials. As the lifetime cost of materials is becoming more important relative to the production cost, it’s clear that NFCs are going to become increasingly attractive. Whilst the hype from participants in this growing sector may still be somewhat on the optimistic side, there is inevitability in the march of progress back towards more traditional materials and processes. www.flaxland.co.uk www.compositesevolution.com