Aug 19 2008
In Europe, wood or concrete has traditionally been the material of choice for manufacturing the ties or sleepers that are used to support railroad tracks. Now, especially as a replacement for the wooden ties that are generally regarded as not very environmentally friendly, railroad builders are turning increasingly to a polyurethane composite marketed under the name Eslon Neo Lumber FFU from SEKISUI CHEMICAL CO. LTD., Tokyo.
The material is noted among other things for the far greater durability of the ties and the correspondingly lower lifecycle costs. In the company's home country, plastic ties have been in service for more than a quarter of a century. They have been laid, for example, under the track for the Shinkansen high-speed train.
Following several successful applications in Europe, the FFU polyurethane ties have now been premiered in Germany. In the Leverkusen CHEMPARK, a turnout system was installed recently that should improve the railroad links between the production site and the German Railways network. The 74-meter-long component was produced by voestalpine BWG GmbH & Co. KG at its Butzbach plant. The supplier of the 136 ties is SEKISUI CHEMICAL, while the polyurethane system based on long-fiber reinforced Baydur® 60 integral skin foam comes from Sumika Bayer Urethane Co., Ltd., the Japanese polyurethane systems house in Bayer MaterialScience's global BaySystems® network.
The demands made on railroad ties are very high, regardless of whether fast passenger trains or heavy goods trains roll over the tracks. They must be able to withstand high mechanical loads and must also be dimensionally stable and weather-resistant over a long period in order to comply with the conditions for safe rail operation and low maintenance costs. "Frequent temperature changes, UV radiation and permanent atmospheric moisture start to affect wooden ties after a relatively short time. Repairing the track involves not only a considerable amount of material, organization and personnel, it also involves heavy internal and external costs," explains Bodo Blume, Sales Manager at SEKISUI CHEMICAL GmbH, Düsseldorf. The company gives the polyurethane ties a service life of at least 50 years. "This means considerably longer maintenance cycles and associated cost benefits for the railroad operator," continues Blume. Moreover, individual ties can be replaced quickly and accurately, helping to lower construction costs even further.
The flexural strength of the polyurethane ties is very much higher than that of wood, even after 15 years. This has been demonstrated in cyclical flexural and long-term load tests. Whereas several natural wood ties failed at a pressure of 40 megapascals after around 50,000 cycles, all the tested polyurethane ties managed 1 million cycles at more than twice that load, namely 94 megapascals. Because of these favorable mechanical properties, the material is also suitable for the construction of high-speed tracks, which, in the case of the Shinkansen, have been proving their worth for more than 20 years. "The polyurethane material looks like wood and combines all the positive properties of the natural product with those of a modern composite product. The polyurethane ties can be sawn, milled, screwed, nailed and bonded with the same tools that are used for timber processing - and even more durably than wood," says Thorsten Eschmeier, head of Diversified Industries Marketing at BaySystems in the Europe, Middle East and Africa Region (EMEA). BaySystems® is the umbrella brand for the global polyurethane systems business of Bayer MaterialScience and stands for the extensive know-how in the field of polyurethanes that the company has gathered over many decades, and for an intensive cooperation with customers.
"Compared with concrete, the polyurethane material weighs much less and above all boasts the reproducible evenness that is particularly important with turnouts," says Heinz Holtschke, railroad manager at the CHEMPARK Leverkusen. "With a lifetime estimated to be more than three times that of a conventional track construction, these turnout systems and railroad ties are also of interest for operators who are not part of the German Federal Railways system. Because of their light weight, the ties are also very suitable for bridges," adds Holtschke. Over the entire length of the turnout in Leverkusen, a total of 136 ties have been used, with individual lengths of between 2.20 and 4.50 meters. This is another advantage of the polyurethane composite, because the ties are manufactured by Sekisui in virtually any desired length and cross-section up to a current maximum of 9.60 meters. With concrete ties, on the other hand, a separate mold is needed for every length.
Because of the closed-cell structure of Eslon Neo Lumber FFU (FFU stands for Fiber reinforced Foamed Urethane), the material absorbs very little water even in heavy rainfall. The favorable electrical insulating properties are therefore not impaired. This is especially important in the winter in case the turnout has to be heated. The material is also resistant to hydrolysis, grease, oil and deicing salt.
The ecological compatibility of the polyurethane ties is also an advantage.
They can be produced without the impregnation that is needed for wooden ties. As a rule, ties that have already been in service can be reused, and, if they are not needed any more, can be recycled in the same way as the production scrap.
Because of all these advantages, polyurethane ties are particularly recommended for turnouts, in tunnels, on bridges and for through-tracks. In Japan, more than 90,000 polyurethane ties are currently being laid every year, and there are now more than 1.3 million units in service there.
In the production of the ties, for which SEKISUI CHEMICAL uses the pultrusion process, the reinforcing fibers are impregnated with the polyurethane system and the composite is then cured at elevated temperature. The whole process is kept going by a pultrusion mold, which pulls the finished profile from the curing tool, allowing economical production and consistent quality. Thanks to the high design freedom offered by the material, special customized models can also be made.
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