Functionalized lignin for halogen-free flame retardant rigid polyurethane foam: preparation, thermal stability, fire performance and mechanical properties

Halogen-free flame retardant rigid polyurethane foams (RPUF) were synthesized by substituting a part of polyther polyol with polyol-modified lignin and filled with phenolic encapsulated ammonium polyphosphate (PFAPP). Lignin was chemically modified with flame retardant elements, phosphorous, via a three-step reaction. The foams maintained a regular cell structure and had even smaller average cell size than pure foam. The effects of lignin polyol and PFAPP on the apparent density, compressive strength were discussed. The thermal stability, flame retardant properties and mechanical properties of the foams are improved. Compared with pure foam (0.2 % char residue at 700 °C), modified foam with lignin and PFAPP exhibits much higher char formation (42.7 % char residue at 700 °C). The FR properties of these modified foams were evaluated by cone calorimeter and limiting oxygen index (LOI). The modified lignin in foam system further reduces the heat release rate and shows the combustion process. The addition of 15 % PFAPP and 10 % modified lignin into RPUF led to a considerable increase of the compressive strength with respect to the neat RPUF.