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Fiber reinforced PU used as ties or sleepers to support railroad tracks |
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Wood and concrete, typically used asmaterials to make ties or sleepers that are used to support railroad tracks are being increasingly replaced by plastics. The demands on railroad ties are very high as they must be able to withstand high mechanical loads and must also be dimensionally stable and weather-resistant over a long period to comply with the conditions for safe rail operation and low maintenance costs. They also get affected by frequent temperature changes, UV radiation and permanent atmospheric moisture after a relatively shorter time. Repairing the track involves considerable amount of material, organization and personnel, as well as heavy internal and external costs.
As a replacement for the wooden ties that are generally considered not very environmentally friendly, railroad builders are turning increasingly to a polyurethane composite marketed under the name Eslon Neo Lumber FFU (Fiber reinforced Foamed Urethane) from Sekisui Chemical Co. Ltd. In use in Japan for over two decades, the material is noted among other things for the far greater durability of the ties and the correspondingly lower lifecycle costs. In the production of the ties, for which Sekisui Chmical 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. It is possible to make special customized models because of the high design freedom offered by the material. Due to the material's fiber reinforcement, its high compressive and tensile strength place it among the current top high-tech construction materials. It loses none of its favorable mechanical properties even after long-term service in the open air.
Test results have shown that the flexural strength of the polyurethane ties is very much higher than that of wood, even after 15 years. Because of these favorable mechanical properties, the material is also suitable for the construction of high-speed tracks. The PU 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. Compared with concrete, the PU material weighs much less and offer reproducible evenness that is particularly important with turnouts. Their lifetime is estimated to be more than three times that of a conventional track construction. The ties are manufactured by Sekisui in virtually any desired length and cross-section up to a current maximum of 9.60 meters.
Because of its closed-cell structure, 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 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 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 PU ties are currently being laid every year, with over 1.3 million units in service there.
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 to improve the railroad links between the production site and the German Railways network, supplied by Sekisui Chemical, while the polyurethane system based on long-fiber reinforced Baydur� 60 integral skin foam comes from Sumika Bayer Urethane Co., Ltd.
The most common applications of the composite material are pools for fish farming, silos, walkways, soil anchors (in this instance as a substitute for concrete) and above all railway sleepers. Sekisui offers a service life of at least 50 years, resulting in considerably longer maintenance cycles and associated cost benefits for the railroad operator. Moreover, individual ties can be replaced quickly and accurately, helping to lower construction costs even further. The ecological compatibility of PU is a great advantage. PU sleepers in railroad increase durability, safety and decrease cost. |
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