Flame retardancy and thermal decomposition of flexible polyurethane foams: Structural influence of organophosphorus compounds

In this study the structural effect of different phosphorus compounds on the flame retardant (FR) properties of flexible polyurethane foams (FPUF) was investigated. A series of organophosphorus compounds (phosphonates, phosphates and phosphoramidates) with systematical structure variations were synthesized and incorporated into the flexible foam during its polymerization process. The flame-retarding efficiency of these phosphorus compounds was subsequently evaluated by subjecting their corresponding flame-retarded foams to standard fire tests such as limiting oxygen index (LOI), BKZ-vertical burning test and UL 94-horizontal burning test for foamed materials. It was observed that the synthesized phosphonates and analogous phosphoramidates were more effective than the corresponding phosphates. The presence of an allyl moiety within the molecular structure further improved the FR efficacy of the phosphonate and phosphoramidate compounds, whereas a benzyl group reduced the flame retardant efficiency significantly. The thermogravimetric (TGA) analysis and pyrolysis combustion flow calorimetry (PCFC) results for the FPUF indicated that the most effective FRs might act predominantly in gas phase and exhibit lower condensed phase action (i.e. catalyze the decomposition of FPUF to happen at lower temperatures). Mass spectrometric analysis of FRs indicates that phosphonate and phosphoramidate derivatives appear to support the phosphoryl radical formation more than the investigated phosphate compounds.