The flame retardancy of wood-based materials such as OSB, particle board or fiberboard can be achieved in various ways: either on the surface, by coating the wood-based materials, or by mass protection, i.e. the integration of flame retardants into the wood-based materials. A surface protection must be applied retrospectively and leads to optical alterations. Mass protection is therefore attractive for manufacturers and processors.
Inorganic substances such as ammonium polyphosphate, magnesium hydroxide or aluminum hydroxide are often used as flame retardants in the mass of wood-based materials. These flame retardants offer good environmental compatibility and are relatively inexpensive. In order to achieve sufficient flame retardancy, large application quantities are, however, required, as a result of which the mechanical properties of wood-based materials generally deteriorate.
The vegetable raw material lignin could solve this problem, as it has inherent advantageous properties and is available in large quantities. Lignin is one of the three main constituents of wood and a hitherto unused by-product from the paper-production industry. It is known that lignin can be used as both a bio-based flame retardant and - in modified form - as an adhesive. Until now, however, its flame-retardant effect has only been investigated in plastics, not in wood-based materials.
The flame-retardant properties of lignin are attributable to the fact that it chars when exposed to intense heat. In combination with phosphorus and a gas generator, such as a nitrogen-containing molecule, it can therefore form an intumescent system. Intumescent systems expand in the event of a fire and insulate the wood, as a result of which the fire is considerably decelerated or extinguished. In our project, we therefore initially investigate combinations of lignin, phosphorus and gas-formers and test them regarding their flame-retardant effect.
We subsequently modify the lignin, enabling it to be used as an adhesive in the production of wood-based materials. Advantage: The adhesive which bonds the fibers and/or particles together - which is necessary anyway - provides the wood-based material with the desired flame-retardant effect. There is no need to add a supplementary flame retardant. As a result, the wood-based material retains its high proportion of fibers and/or particles. A flame-retardant on the basis of lignin can therefore be integrated without impairing the mechanical properties of the wood-based materials.
Simultaneously, we are creating a possibility for the high-quality utilization of lignin, which is currently being burned in pulp mills in order to generate energy.
With this project, we are making a contribution towards an increased use of bio-based construction materials and a more efficient utilization of plant-based raw resources such as wood. In doing so, we are supporting the development of a bio-based recycling economy (bioeconomy).
Fraunhofer Institute for Wood Research 
