Bridging the wood gap

A polymer-wood product said to offer the benefits of wood without the disadvantages of rotting or warping is being used for rail sleepers for the first time in Europe, on Vienna’s Zollamt Bridge.

Austria has laid the first rail sleepers in Europe to use a Japanese-designed composite that is claimed to offer all the benefits and none of the drawbacks of that most venerable of all materials – wood.

New sleepers on the Zollamt bridge in Vienna are made from a composite called Esion Neo Lumber FFU – also known as polyurethane wood.

The material is designed for applications in which natural timber – with its versatility and ease of use  – would normally be first choice but cannot be used because of its tendency to rot or warp.

Polymer-wood substitutes tend to be used in less demanding applications, but its developer claimed the material used in Vienna is robust enough to withstand the unique strains of rail tracks.

The polyurethane wood is produced using materials and long glass fibres from Sumika Bayer Urethane, a subsidiary of materials technology and chemicals giant Bayer.

It has been extensively used as a construction material in Asia for some years, forming the sleepers on the Japanese Shinkansen high-speed lines, for example. Other common applications for the material in Asia include fish ponds, farming, pedestrian walkways and soil-based anchors as an alternative to concrete.

The Zollamt bridge renovation project is the first time Eslon Neo Lumber FFU has been used in Europe.

According to its developers, the wood substitute is a highly dimensionally stable composite material. Eslon Neo Lumber FFU (fibre-reinforced foamed urethane) can be processed in the same way as natural wood, said Bayer.

Components made from the material can be screwed together, nailed and sawed using conventional woodworking tools. The material is also said to bond more effectively than timber. The closed cell structure of the polyurethane glass fibre compound ensures minimal water absorption and high compressive and tensile strength.

Railway sleepers are a classic application for polyurethane wood, said Bayer, because its durability reduces the need for expensive and disruptive track upgrades.

Rail tracks, particularly those running across bridges and through tunnels, are often at the mercy of a high level of permanent air moisture and changes in temperature, all of which spell trouble for natural wood – even when resistance-treated.

The plastic wood is also claimed to offer good electrical insulation properties, assisting smooth running of trains in the winter when points need to be heated to prevent them from freezing, and is resistant to frost and de-icing salt.

Concrete sleepers are the conventional alternative used where wood is unsuitable.

However, these are heavy, more brittle and difficult to handle on-site. For example, a separate mould is needed for each individual sleeper length, causing problems when various sizes are needed for the construction of points. Its developers claim the polyurethane wood substitute is lighter and easier to machine at the point of use than concrete.

The sleepers can be made to any length using the pultrusion process, a common production technique for composite components. It involves pulling fibre reinforcement through resin in a continuous process to form bundles of fibre/resin composites with a constant cross-sectional area.

The composites are then heat-set into a permanent reinforced shape at the required dimensions.

Pultrusion is considered to be extremely efficient by the composites industry because it produces high quality materials with little waste.

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