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02 | 2008

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Natural Fibres Go, Trabi

Natural Fibres Go, Trabi Go! „Back to the future?“ Article contributed by Rosemarie Karner It started about 50 years ago The Zwickau motor factory (AWZ) produced its first pilot series of small cars in 1957. In total 50 of the so-called P50‘s were built. Because the spirit of the times was still one of optimism and a bright future, a simple name like the P50 for such a ‘modern example of ingenious socialist engineering’ was hardly in keeping with the mood. It was therefore fortuitous that in October 1957 the Soviet Union launched the first „Sputnik“ earth satellite. This led to the adoption of the name „Trabant“, which, like „Sputnik“, means „travelling companion“. In 1964 the 601 model was introduced, with one of the main features being new bodywork. The car no longer looked quite so rotund, and there were small changes to the rear „fins“ that were popular at the time, especially in the USA. After 1964 nothing on the Trabant really changed. Between 1957 and 1991 a total of 3,051,385 Trabants were built (source: www.sueddeutsche.de) An old Trabant is also shown on this issue’s cover photo and you can see the original 601 commercial at http://www.teamburg.de/bioplastics/misc/trabi.php The Trabant - it still to be seen on German roads (and not only German roads). A mass-produced car from the former DDR (German Democratic Republic). Fans of the Trabant call it the ‘Trabi’, others call it ‘Rennpappe’ (‘cardboard racer’). But as well as being a nostalgic icon the Trabant has some interesting features, particularly with regard to the material used for the bodywork. Let’s go back to the 1950‘s, where we can take a closer look. History At that time the automotive engineers at the VEB Sachsenring car factory in Zwickau were faced with a serious problem. The situation regarding automobile mass production was far from good. There was a serious lack of machines and materials. During the Cold War there was an embargo on sheet steel, and a regular supply from the USSR was also subject to limitations. Something had to be done. A solution had to be found, and the East Germans started experimenting with different kinds of materials. After initial trials with a fibre mixture based on PVC with wood chip and cotton fibres the search was on to find a better solution. The need to find a material that could withstand substantial temperature variations, both heat and cold, with better mechanical properties, and that would be reasonably easy to process led researchers down various routes until they finally developed a thermosetting material whose properties they were able to improve as time went by and the manufacturing process was perfected. Cotton fibres and phenolic resin In the early trials a carding machine was used to produce a fibrous mat from cotton, which was then wound onto a drum. As it was wound onto the drum the cotton mat was impregnated, by a sprinkler device, with a synthetic resin powder until the resin content reached 52%. The impregnated mat was then compressed by passing it between calendar rollers. The next step was to produce two more resin-impregnated mats with modified resin levels. The aim of the three layer process was to achieve a particularly weatherproof outer surface with a high level of Photo: Th. Kraft / Wikipedia 16 bioplastics MAGAZINE [02/08] Vol. 3

Natural Fibres (Photo: Daimler) rigidity, and to avoid delamination of the pressed parts if they became distorted or warped. For the series production of what was finally a 5-layer resin mat the following process was developed. Five carding machines processed the loose cotton fibre flock on a conveyor belt, converting it into a cohesive mat. Before the continuous mat was laid in the press it was sprinkled with a specially produced synthetic resin. Because each machine fed a slightly different thickness and mixture of material onto the conveyor the final layered mat was given the desired characteristics. The compressed cotton and phenolic resin mat finally consisted of five layers with different resin contents and different fibre orientation. The weatherproof outer layer had to be made only of fibres that were totally free of components which could swell. The fibres were chemically pre-treated by the so-called bucking process, designed to remove fats and waxes. Criss-cross layers to improve performance With the other layers care was taken to place them on top of each other with the fibres in a criss-cross pattern. One reason was the fact that shrinkage across the fibres was greater than the linear shrinkage, so that placing the layers at 90 degrees to each other compensated for the difference in shrinkage, thus largely avoiding any warpage. In addition the oriented fibres made a significant contribution to the rigidity of the finished sheet. Using randomly scattered fibres would have produced no more than conventional moulded mass properties. In a subsequent step the multilayer resin mat was cut into rectangular pieces. The dimensions of the individual pieces could be adjusted, and later the mat could even be cut, and the sizes altered, as it passed down the production line. The scrap material that was trimmed off during cutting was reclaimed, separating the cotton and the resin using a beating opener and fed back into the middle layers at the material preparation stage. The parts taken out of the press-tool still had loosely compacted edges which were not trimmed off during pressing but were removed later by various methods. To finish the edges they were partially die-cut and then milled smooth in a special fixture. At times a band saw was used. Made in this way the production of a roof panel took only a few seconds. The curved cut-outs in the front wing panels were done by hand on the finished pressing using electric rotary cutters. Final assembly looked just like sheet metal The process for manufacture and assembly of the Trabant thermoset parts had to be appropriate to the material being used (draft angles). The final bodywork design even fooled experts in sheet metal bodywork, who assumed that it was made of metal-clad parts. Fixing the parts to the steel framework was done by baking on a heat-hardening resin adhesive, using screws and rivets, and crimping. The picture above shows the basic skeleton with the pressed parts that are to be mounted onto it. When repairs were required the individually mounted parts could be removed without damaging the framework. The material research carried out included investigating, for example, flexural strength, elasticity, heat resistance and moisture absorption. Material testing was carried out not only in Germany but in Hainan in the Chinese tropics, in African deserts and in the Arctic Circle. Ongoing tests and modifications were necessary for various reasons, including the variations in the price, quality and availability of cotton and resin, with different fibres resulting in different levels of shrinkage. The pressings themselves, even over an extended period of time and with no painting, proved to be almost indestructible. The Trabant is „history on the move“, at least as long as a few examples keep running. On the other hand, cotton fibres, are enjoying a new lease of life in modern automobile manufacture, with applications in moulded parts. Is this a case of „back to the future“? Source: Sonntag W./Barthel W.: „Kunststoffe für Karosserieverkleidungen“ (Plastics for bodywork panels), 2002, 4th International Wood and Natural Fibre Composites Symposium bioplastics MAGAZINE [02/08] Vol. 3 17

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