Wood-polymer composite material features low flammability

Fraunhofer scientists and partners have developed a wood-polymer composite material for furniture that is resistant to humidity and has low flammability.

Researchers at the Fraunhofer Institute for Wood Research in Braunschweig, Germany are collaborating with industrial partners in Belgium, Spain, France and Germany on the development of materials suitable for pressing into moisture-resistant wood-polymer composites (WPC) boards for indoor furniture manufacturing.

These boards are composed of around 60% wood particles and 40% thermoplastic material, either 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% recyclable. The pressed WPC boards are formaldehyde-free, the researchers said. 

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.

The team 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%, provided the wood particles they contained were also flame-proofed. By comparison, the limiting oxygen index of a standard wood particle board is 27%, and that of an untreated WPC board is 19%. 

Another feature of the new WPC material is that it absorbs very little water and can be used in bathrooms and kitchens. Even after being immersed in boiling water for five hours, the material emerges intact, whereas conventional wood particle board was completely destroyed by this test. 

Wood-polymer composites can be produced in many ways. The most commonly used processes are injection molding and extrusion, in which the various components – wood fibers, thermoplastic materials, and additives – are melted under high pressure at a high temperature and formed in a continuous mold. The team has placed its focus on press technology, because it is the best way to produce boards for use in furniture construction.  But there are many other applications for wood-polymer composites, including exterior weatherboarding of buildings, the construction of trade-show booths and interior fittings for houses and ships.