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02 | 2008

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Natural Fibres Natural

Natural Fibres Natural Fibres in Automotive Applications Article contributed by Andrzej K. Bledzki, Adam Jaszkiewicz, Markus Murr and Volker E. Sperber Institut für Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Germany Omar Faruk, Department of Forestry, Michigan State University Michigan, USA Automotive exterior components made from flax fibre reinforced composites (Photo: DBU) Automotive components made with materials from renewable resources have moved successfully from pipe dreams long ago to the state-of-the-art applications of today: Natural fibres are enjoying a comeback in high-tech development. The related research has experienced an explosion of interest, particularly with regard to natural fibre‘s comparable properties to glass fibres within composites materials. Above all, the automotive industry is interested because cars have been required to be partially decomposable or recyclable since 2006. The main area of increased usage is in interior applications, because the need is the greatest here. A DEFRA report from 2002 projected the growth rates for bio-fibres in automotive components at 54% per year. In the last decade bio-fibre reinforced polymer composites have been embraced by European car makers for door panels, seat backs, headliners, package trays, dashboards, and trunk liners. Recently the trend has reached North America where bio-fibre composites are gaining widespread acceptance. Nowadays, in the USA more than 1.5 million vehicles include applications for bio-fibres such as kenaf, jute, flax, hemp and sisal in combination with thermoplastic polymers such as polypropylene and polyester. Bio-fibres have benefited from being eco-friendly, and even more by providing enhanced stiffness and sound damping at lower cost and density than glass fibres and mineral fillers. Furthermore, those composites contribute to the automotive manufacturer‘s final goal by delivering a 30% weight reduction and a cost reduction of 20%. Lightweight and recyclable Increased social awareness of environmental problems posed by the non-degradable, non-recyclable contents of salvaged automobiles is forcing automotive manufacturers to enhance the biodegradable content by switching to bio-fibres. To accelerate this process legislators in the USA and Europe have issued a specific directive on end-oflife vehicles that promotes the use of environmentally safe products and reduces landfill. The directive, which came into effect at the turn of this century, predetermined the 18 bioplastics MAGAZINE [02/08] Vol. 3

Under-floor protection trim of a Mercedes „A“ class made from banana fibre reinforced composites (Photo: Rieter Automotive) Natural Fibres (Photo: Daimler AG) deposition fraction of a vehicle to 15% for the year 2005, and then gradually reduces it to 5% for the year 2015. Bio-fibre composites in the automotive industry both reduce material waste and increase fuel efficiency. On the one hand a major problem is the waste disposal of glass fibre from composite materials after their life cycle. This leads to a clear advantage for bio-fibre composites. On the other hand, the second environmental benefit is the reduction of fuel emissions. Europe is committed to the Kyoto protocol. If lighter materials are used in the automotive industry, fuel efficiencies rise, making it easier to meet the goals. In the current situation, it is estimated that no more than 50 kg of natural fibres can be used in a car. This corresponds to a reduction of about 10 kg if glass fibre composites are replaced with bio-fibre composites in an automobile. If the weight of a car can be reduced by 10 to 20 kg, the effect on the environment will be significant. Processing bio-fibre reinforced composites In principle, the production techniques for natural fibre composites can be similar to those with glass fibres. Exceptions to this are techniques where continuous fibres are used, such as pultrusion (a yarn has to be made first), or where fibres are chopped up, as in spray-up or SMCprepreg preparation. The most important technology is undoubtedly compression moulding. Different variations of this process (details depending on the company adopting the process) are suitable for the processing of plant fibres. Such compression moulding of resinated natural fibre mats, natural fibre/PP hybrid mats and NMT (natural fibre mat reinforced thermoplastics), was developed by BASF in Germany about 15 years ago. A specific improvement of NMT made it closely equivalent to the performance of GMT (glass fibre mat reinforced thermoplastics). In general, the differences lie in the way that fibres and binding polymers are combined and brought into the mould. Some processes use a pre-melted polymer (e.g. the EXPRESS technology), some use a fibrous polymer that is combined with the plant fibres into hybrid mats before compression moulding, and others use polymer powder that is introduced into the fibre mats before compression moulding. As almost all processes use the fibres in the form of mats, decortication of the fibres and the processing of the mats are key issues for the technology. The extrusion press processing (express-processing) was developed for the production of flax fibre reinforced polypropylene at the research centre of DaimlerChrysler, Ulm, Germany. A short overview of the current main technologies is given in the following: FIBRIT Process The FIBRIT process is the first process based on renewable raw materials. Chips of pine are washed, ground to wood-like fibres and silted with the addition of water, phenolic resin (as a binder) and cellulose, thereby giving a wood fibre pulp. This is applied as slurry to a contoured part of the interior door panel, eliminating any water surplus. Through compression moulding at 230 to 250°C, a blank is obtained with only 50% of the former thickness. Following this the blank undergoes vacuum forming in a pre-heated mould. Once specific holes have been stamped out, the finished support is obtained. Wood Fibre/Phenolic Resin (Lignotock Process) This process uses wood chips (preferentially red pine) as the base material. The chips are shredded in two stages and mixed into a fibre mass under the action of steam. After mixing in phenolic resin (as binder) hot thermoplastic and an adhesive cross-linking agent, the basic compound thus obtained is processed on a needle-punching unit into mat nearly 6 mm thick. From this the finished support is finally formed in a heated mould. Polywood Process In this process a compound made up of 50% wood flour and 50% polypropylene is extruded into a Polywood sheet that is the support material of the trim panel. The sheet is heated in an infrared oven, making it easy to form. The finished trim panel is then obtained through compression moulding. bioplastics MAGAZINE [02/08] Vol. 3 19

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