Wood is a popular material in interior design, but its water absorbency limits its use in bathrooms, where natural wood easily becomes discolored or moldy. Fraunhofer scientists and partners have developed a wood-polymer composite material for furniture that is resistant to humidity and has low flammability.
Resource-saving wood-polymer composites (WPCs) are the latest trend in materials for garden furniture and other outdoor applications, especially for terrace decking and also for weatherboarding and fencing panels. As part of the EU-sponsored LIMOWOOD project, researchers at the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut WKI in Braunschweig are now collaborating with industrial partners in Belgium, Spain, France and Germany on the development of materials suitable for pressing into moisture-resistant WPC boards for indoor furniture manufacturing.
These boards are composed of around 60 percent wood particles and 40 percent thermoplastic material, generally polypropylene or polyethylene. Both wood and plastic components can be sourced from recycling streams. The wood component in WPC boards can be replaced by other lignocellulose products derived from the fibrous part of plants such as hemp or cotton, or the husks of rice grains and sunflower seeds. All of these materials are 100-percent recyclable. Moreover, the pressed WPC boards produced by the WKI researchers are formaldehyde-free. “The controversial question of formaldehyde emissions due to the binder used in conventional pressed wood products is therefore not an issue in this case,” says WKI research scientist Dr. Arne Schirp.
By choosing appropriate additives, the researchers were able to enhance the fire-retardant properties of their WPC boards. They initially developed their formula on a laboratory scale, using commercially available, halogen-free fire retardants which were added to the wood-polymer mixture during the melt phase. The first step involved determining the limiting oxygen index of the item under test: this parameter defines the behavior of plastics or wood-filled plastic compounds when exposed to flames. It represents the minimum concentration of oxygen at which the material will continue to burn after catching fire. The higher this value, the lower the material’s flammability. Schirp and his colleagues obtained the best results with a combination of fire retardants such as red phosphorus and expanded graphite. The limiting oxygen index of WPC boards treated in this way extends up to 38 percent, provided the wood particles they contained were also flame-proofed. By comparison, the limiting oxygen index of a standard wood particle board is 27 percent, and that of an untreated WPC board is 19 percent. Even in a single-flame source test, in which a Bunsen burner is held against the test sample, the treated WPC boards demonstrated a high fire resistance. Even after 300 seconds’ exposure, the boards didn’t catch fire. By contrast, the reference samples – of a standard wood particle board and an untreated WPC board – caught fire and continued to burn.