Bi-phase flame-retardant actions of water-blown rigid polyurethane foam containing diethyl-N,N-bis(2-hydroxyethyl) phosphoramide and expandable graphite

•The complex system EG/DEPA greatly enhanced the LOI values of RPUFs from 19.5% to 30.4%.•EG and DEPA jointly acted inhibitive effects on the combustion intensity and smoke production according to cone tests.•Bi-phase flame retardant mechanism of EG/DEPA in RPUF were investigated in detail.

Flame-retardant water-blown rigid polyurethane foams (RPUFs) with a reactive-type phosphoramidate named diethyl-N,N-bis(2-hydroxyethyl) phosphoramide (DEPA) and expandable graphite (EG) were fabricated through free foaming technique. The flame retardancy and fire behaviors of RPUFs were characterized using limiting oxygen index (LOI), microscale combustion calorimeter (MCC) and cone calorimetric tests. Compared to the effects of individual flame retardant agent (DEPA or EG), the complex system (EG/DEPA) endowed RPUFs enhanced LOI values (28.8%-30.4%), higher protective char yields, lower heat release and smoke production. EG and DEPA jointly acted to remarkably inhibit the combustion intensity and smoke production through their addition or synergistic flame-retardant effects. To further validate the flame-retardant mechanism of EG/DEPA in RPUF, thermogravimetric analysis connected with Fourier transform infrared spectrometry (TG-FTIR), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) tests were conducted. These results demonstrated that DEPA decomposed potential free radical scavenger (PO) and promoted generating more H2O and CO2 in gaseous phase. Furthermore, DEPA facilitated the formation of phosphorus-rich residue, which was combined with worm-like expanded graphite to constitute protective char layers. On the basis of these results, a bi-phase flame-retardant mechanism of EG/DEPA in RPUF was proposed.