Growing for gold - Natural fibres aid sporting success
While no one can deny the talent of the gold medal-winning athletes at the London 2012 Olympic Games, much of the various athletes’ success can be attributed to the equipment that helped get them to the medal podium – from Bradley Wiggins’ bicycle to Andy Murray’s tennis racket. Today, sporting success is heavily influenced by equipment performance, in addition to the skill of the sports men and women who use them, meaning materials science in the field has never been more fast-paced or innovative.
The most common type of fibre reinforcement used in composites is fibreglass, which is made by melting silica and extruding it into filaments. This process requires large amounts of energy, but the resulting fibres have good strength and stiffness, making them suitable for reinforcing a vast array of composite products.
With sustainability a hot topic in materials science, interest in natural fibre composites is growing, including their use in sports equipment. The market for natural fibre composites has seen healthy growth, particularly in Europe (270,000t), North America (1.6Mt), Japan (100,000t) and China (200,000t). The market grew by 15% between 2005–2010 from US$1,086,000 to US$2,171,000, and a further 10% year-on-year growth is projected by 2016 to US$3,085,000.
Natural fibres are renowned for their low weight and desirable properties, which include a high stiffness–weight ratio, vibration damping and thermal insulation. An added benefit that gives natural composites a commercial as well as a competitive edge is their low impact on the environment. Recyclable, biodegradable and requiring little energy to produce, most natural fibres are carbon neutral – some are even carbon positive, consuming more CO2 than they produce.
Automotive is currently the largest sector for nonwood fibres, which are widely used in car components such as door panels, door inserts, shelves, wheel pan covers, headliners, bumpers and protection trim.
Several natural plant fibres offer high mechanical properties – one example is Linum usitatissimum, more commonly known as flax. Widely available in Europe and noted for its high strength and stiffness, flax fibres are already used in many commercial materials, including those for sports. Flax is used in everything from snowboards and bike frames to canoes, surfboards and skis. Where strength and weight reduction is key, the possibilities are endless.
Bundles of fibres are present in the outer regions of the flax stem, where nature uses their strength to prevent the plant collapsing in the wind and rain. Individual fibres are extracted through a combination of microbial and mechanical processes called retting, scutching and hackling.
Round the twist
Flax fibres can be used to reinforce thermosetting resins, such as polyester, epoxy and bioresins, or thermoplastic polymers including polypropylene (PP), polyethylene (PE) and biopolymers such as polylactic acid (PLA). Flax can be used as short fibres or converted into yarn and textile reinforcement products.
A twistless technology developed by sustainable materials supplier Composites Evolution, based in Derbyshire, UK, produces long, straight, aligned flax fibres to produce materials that are both strong and lightweight. While materials in injection-moulded or press-moulded parts comprise short and randomly oriented fibres, the alignment of fibres in twistless yarns makes them 50% more effective than those in conventional twisted yarns.
Twistless yarns have the added benefit of being easier to impregnate. In conventional yarns the fibres are twisted tightly together, resulting in a closed structure that is difficult to impregnate with resin. Twistless yarns are more open so the resin can easily flow into the yarn and encapsulate each individual fibre, offering improved fibre-matrix interaction and, therefore, material performance.
But creating the twistless yarn is only half the story. For most applications the yarn has to be converted into a fabric, and the design and engineering of this fabric also influences its processability and performance. Fabrics made from such twistless yarns offer good drapability (forming easily into complex shapes) and low crimp – ie, the yarns remain as straight as possible rather than being wavy.
Vibration damping is key in equipment such as bicycle frames, tennis rackets, baseball bats and hockey sticks, where large forces and impacts on the equipment cause vibrations that travel down through the athlete’s body, impeding performance. Could the likes of Andy Murray and Novak Djokovic soon be playing with flax rackets? French racket sports manufacturer Artengo has developed a flax-carbon tennis racket made up of 25% flax material that takes advantage of flax’s natural vibration damping properties, which are almost double that of glass fibre and Kevlar.
Belgian company Museeuw has developed a flax-carbon composite bicycle frame that offers 20% better shock and vibration absorption compared to carbon alone. Similarly, UK-based RAW Bamboo Bikes has developed a stiff, lightweight, sustainable bamboo-flax frame made from bamboo tubes and flax fibre composite joints, which aid in vibration dampening.
Swedish company Oxeon manufactures flax spread tow fabrics using Composites Evolution’s highly aligned, thin flax tapes. Spread tow fabrics are very lightweight and have extremely low crimp at the weave intersections, which significantly increases performance. Spread tow fabrics have several favourable mechanical properties. Flax/PP composites made from spread tow fabric have 30–40% higher stiffness than the equivalent yarn-based laminates, and carbon-fibre variants are already widely used – in racing cars, ice hockey sticks and skis to name just a few.
Snowboarders could benefit from a biocomposite snowboard by Canadian company Magine, which comprises flax twill fabric and eco-epoxy resin over a wood laminate core. The snowboard (pictured below is a sustainable alternative to conventional fibreglass and basalt composite snowboards. The bio-fibre board was extensively tested by the company’s pro riders, responding, flexing and edging well, and in grinds and aerial maneouvres it performed comparably with freestyle snowboards.
Given a ribbing
Taking advantage of the limited compressibility of natural fibre yarns, a technique developed by natural fibre specialist Bcomp, in Switzerland, places meso-scale natural-fibre yarns as a top layer to reinforce thin-walled structures. The technology has been shown to increase the flexural strength of tubes by up to 250%, while weight increases by just 5–20%. Price–performance is near par to that of aluminium alloys or glass fibre composites, giving it potential for use in sports equipment such as ski and hiking pole shafts, bicycle seat posts and paddle shafts.
Another material by Bcomp matches the performance of traditional wooden cores currently used in equipment such as skis and snowboards, but is five times lighter. The lightweight foam comprises low-density flax fibre composite layers with wide shear webs that form rough, fibrous edges for optimum core–face adhesion. This results in a strong interface over a large surface area for efficient core–face stress transfer. The resulting core increases shear properties by a factor of 5–10 and compression properties by 2–3 when compared to their wooden counterparts. Several manufacturers have launched skis containing Bcomp cores this winter season.
These examples demonstrate how far and fast the industry for natural fibres for sports equipment is growing, although further R&D is needed to fully optimise the production and performance of these naturally clever materials.
For more information contact Dr Weager, email@example.com