Adsorption charring flame retardant effect of phosphaphenanthrene derivate intercalated micro-expanded graphite composite system in rigid polyurethane foams

Phosphaphenanthrene derivate molecule (DT) was synthesized by a simple method, and exploring the flame retardant effect of DT, micro-expanded graphite (MEG) with different combination modes compounded dimethyl methylphosphonate (DMMP) in polyurethane foam rigid polyurethane foams (RPUFs). The results showed that the properties of an intercalated DT-MEG system was better than a DT blended MEG system. Specific data of DT-MEG/DMMP were as follows: the LOI value of the 6DT-8MEG/16D sample reached 30.3%, the peak heat release rate (PHRR) value decreased by 70.4%, the total smoke release value decreased by 42.4%, the total heat release (THR) value was the lowest at only 14.9 MJ/m2, and the residue yields reached 31.6%. According to the test results, the flame retardant mechanism of the DT-MEG/DMMP compounded system was also explored. DMMP of DT-MEG/DMMP can exert flame retardant effects by releasing phosphorus free radicals in the gas phase to inhibit flame intensity at the beginning of combustion. Then, when the DT-MEG structure was heated, it expanded to form a caged char layer to exert adsorption charring flame retardant effects to filter and absorb the smoke particles. Meanwhile, the intercalated DT-MEG structure produced stronger adhesion with MEG during combustion, which generated more phosphoric acids to carbonize more RPUF matrix. Therefore, DT-MEG/DMMP compound flame retardant system can endow RPUFs with excellent synergistic flame retardant effects in both the gas and condensed phases. This exerts adsorption charring flame retardant effects in the DT-MEG microscopic form, and decreases the combustion intensity and obtains a compacted char layer.