In recent times, wood as a construction material has experienced a renaissance in Europe. However, for medium and tall buildings – which are typical of urban areas – wood is still rarely used. One reason for this is the insufficient fire safety offered by wood. Although new construction methods and fire-protection concepts have expanded the application possibilities of wood in recent years, architects and engineers continue to be faced with challenges. Through fire-retardant treatments, the fire behavior of wood and wood-based materials can be significantly improved.

In collaboration with the VTT Technical Research Centre of Finland Ltd, the Norwegian Institute of Wood Technology, and Teknos Oy R&D, researchers at the Fraunhofer WKI have developed fire-retardant coatings for indoor and outdoor applications on the basis of functionalized MFC. These are weather-resistant when used for exterior applications and are beneficial for the room climate when used indoors. As the starting material for their new development, the project partners chose the natural substance cellulose in the form of tiny fibers, known as microfibrils.

“For interior applications, we have designed a multi-layer macrostructure similar to that of Canary pine bark. The MFC-based multilayer structure enables high water-vapor permeability. This is a positive factor for indoor air quality. For exterior applications, we have used phosphorylated MFC, both as an additive in wood coatings and chemically bonded directly to the binder during the polymerization process. The bonding can counteract the leaching of flame retardants in outdoor applications,” explained Dr. Claudia Schirp, Project Manager at the Fraunhofer WKI.

The researchers have succeeded in functionalizing MFC with phosphorus compounds. It was possible to increase the solids content of the MFC to a maximum of 25 percent. A combination of functionalized and unmodified MFC improved the adhesion to the wood substrate. The fire-protection properties were improved in comparison to the uncoated reference wood. In flammability tests, the coated wood exhibited self-extinguishing and delayed-ignition properties. For indoor application of the coatings, the best results were achieved in combination with inorganic additives in a multilayer structure.

For exterior coatings, the MFC was applied in the binder. The researchers achieved success with an approach using the mini-emulsion-polymerization technique and, in this way, were able to incorporate a very high quantity of 15 percent modified MFC into the binder. In the formulation, MFC and modified MFC were used both as an additive and with chemical bonding to the binder polymer, and coatings were produced. For comparison and to improve the flame-retardant performance, a conventional intumescent mixture was added to selected formulations. The application quantity of 15 percent MFC in the binder significantly reduced the heat-release rate. The flame-retardant performance remained unchanged, even after three months of artificial weathering. Natural weathering is currently being continued, as even after 17 months, no failure in the form of crack formation has occurred.

The fire-protection coatings developed in the project offer the construction industry the possibility of utilizing wood to a larger extent for medium and tall buildings. Timber construction in itself already offers advantages for the indoor climate. With a fire-protection coating that is favorable for the indoor climate, wooden construction will become even more attractive. In order to further expand the range of applications for wood in the construction sector, researchers at the Fraunhofer WKI will continue to work on environmentally-friendly biogenic flame retardants for wood coatings.