Construction of an efficient ternary flame retardant system for rigid polyurethane foam based on bi-phase flame retardant effect

The flame-retardant rigid polyurethane foams (RPUF) were prepared by using modified ammonium polyphosphate (MAPP) combined with expandable graphite (EG) and dimethyl methylphosphonate (DMMP). The thermal stability, flame retardant property, and mechanical property of flame-retardant RPUF were evaluated based on thermogravimetric analysis (TGA), limiting oxygen index (LOI) test, cone calorimetry tests, scanning electron microscopy (SEM) and compressive strength tests. The results showed that an efficient ternary flame retardant system, DMMP/EG/MAPP, for rigid polyurethane foam was constructed. When the ratio of DMMP/EG/MAPP was 4/12/4, the cell size of RPUF composite was the most homogeneous, and the LOI value of RPUF composite reached 31.9%. The peak heat release and total smoke release value decreased to 97 kW/m(2)and 347 m(2)/m(2), respectively. The RPUF composites with 4%DMMP/12%EG/4%MAPP exhibited the best flame retardant property because DMMP/EG/MAPP played great free radical quenching effect in the gas phase by releasing the PO center dot and PO2 center dot free radicals during the whole combustion process. Meanwhile, the compact char layer with "connected worm-like" structure exerted barrier effect in the condensed phase.