Thermal decomposition and flammability of rigid PU foams containing some DOPO derivatives and other phosphorus compounds

A series of 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) based derivatives were synthesized and incorporated as flame retardant additives in rigid polyurethane foam (RPUF). The flame retardant performance of DOPO derivatives in RPUF was investigated and compared with traditional flame retardant tris(1-chloro-2-propyl) phosphate (TCPP) and reactive flame retardant (diol) based on oligomeric ethyl ethylene phosphate (PLF140). The flame retardant performance of rigid foams was evaluated by UL 94 HB test, limiting oxygen index (LOI) and cone calorimetry (CONE). Thermal stability of RPUF samples was analyzed by thermogravimetric analysis (TGA) and pyrolysis combustion flow calorimeter (PCFC). The thermal decomposition mechanism of the flame retardant RPUF and the mode of action of the flame retardants were investigated via pyrolysis-gas chromatography-mass spectrometry (Py-GC–MS) and thermogravimetric analyzer coupled with FTIR and MS (TG-FTIR-MS). Scanning electron microscope-energy dispersive X-ray (SEM-EDX) was used to analyze the char residue of the RPUF. It was observed that, addition of 2wt% phosphorus in RPUF helps achieve HF1 rating in UL 94 HB test and reduces the peak of heat release rate by 23%–42% in cone calorimeter measurements. Additionally, in cone calorimeter measurements, compared to RPUF containing TCPP and PLF140, RPUF containing DOPO derivatives exhibit lower smoke and toxicant production and increased char residue. For RPUF containing 6-(2-(4,6-diamino-1,3,5-triazin-2-yl)ethyl) dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DTE-DOPO), production of decomposition products such as –NCO containing compounds, HCN, hydrocarbons, amines and cyanic acid were significantly decreased compared to virgin RPUF. RPUF/DTE-DOPO formulation resulted in a compact and tough char structure which proves its strong barrier effect in the condensed phase. The detailed flame retardant mechanisms of TCPP and DTE-DOPO in RPUF are further elaborated in this work.