Continuous flame-retardant actions of two phosphate esters with expandable graphite in rigid polyurethane foams

The flame-retardant rigid polyurethane foams (RPUFs) with dimethyl methylphosphonate (DMMP)/[bis(2-hydroxyethyl)amino]-methyl-phosphonic acid dimethyl ester (BH)/expandable graphite (EG) were prepared by box foaming. The DMMP/BH/EG flame-retardant system with certain components ratio increased the LOI value, decreased the peak value of heat release rate, sustained the effective heat of combustion and total heat release in low level, and promote the formation of phosphorus-rich compact char layer comparing with DMMP/EG and BH/EG systems. The results imply that DMMP/BH/EG possessed the trinary synergistic flame-retardant effect. The results from thermogravimetric analysis (TGA) and TGA-gas chromatography-mass spectrometer (TGA-GC-MS) all confirmed that the trinary synergistic effect of DMMP/BH/EG was caused by the continuous release of DMMP/EG and BH/EG flame-retardant actions with increasing temperature. DMMP/EG and BH/EG in sequence worked in inhibiting flame and forming phosphorus-rich char layer, thus forming stable flame-retardant action on matrix and endowing RPUFs with the increased flame-retardant performance.