Phosphorus-containing lignin intermediates as reactive bio-based flame-retardants for polyurethane and acrylic coatings for wood

Wood has limited fire-retardant properties but this can be improved by using coatings with sustainable and cost-effective flame retardants. For this purpose, lignin is a potential candidate due to its highly aromatic structure and low cost. However, its structure should be modified to achieve good fire behaviour. Thus, phosphorus was incorporated into kraft lignin and the compounds obtained were used for the synthesis of two types of wood coatings: Polyurethanes and Polyacrylates. Phosphorus nuclear magnetic resonance was used to characterize the modified lignin, together with elemental analysis. The incorporation of lignin into the polymers was confirmed by infrared spectroscopy and gel content measurements. The effectiveness of the phosphorus-containing lignin intermediates as bio-based flame retardants was measured by thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC) and cone calorimetry. According to the results, phosphorylated lignin increased the residue and decreased the total heat release of the polyurethane and acrylic coatings. In addition, slight improvements in fire resistance were observed in wood panels coated with acrylic, while no changes were observed in wood coated with polyurethane.