Flame retardation and thermal stability of novel phosphoramide/expandable graphite in rigid polyurethane foam

A novel flame retardant named diethylene N,N',N''-tri (diethoxy)phosphoramide (DTP) was synthesized using diethyl phosphate and diethylenetriamine via Atherton-Todd reaction. Then, series of flame-retardant water-blown rigid polyurethane foams (RPUFs) with expandable graphite (EG) and DTP were prepared through box-foaming. The results of thermogravimetric analysis showed that DTP/EG changed thermal degradation process of RPUF and promoted enhancing char residues. The complex flame-retardant system (EG/DTP) endowed RPUF higher limiting oxygen index (LOI) values (29.1%-30.2%) and lower heat release rate peak (PHRR) values according to LOI and microscale combustion calorimeter tests. More importantly, the synergistic flame-retardant effect between EG and DTP in RPUF was proved by the analysis of synergistic effectivity values. Based on the analysis of cone calorimetric tests, EG/DTP revealed remarkable effects to inhibit the fire intensity and smoke release of RPUF with decreased PHRR and total smoke production due to good char-forming action. To further investigate the char-residues of the foams after combustion, scanning electron microscope and energy dispersive X-ray spectroscopy analyses were conducted. The results suggested that EG/DTP flame-retardant system promoted RPUF forming a compact, continuous and phosphorus-rich char layer as a good fire barrier in combustion. (c) 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46434.