Comparison of methyl-DOPO and mppp as flame retardant for flexible polyurethane foam

The chemical nature of the flexible polyurethane foams, the low density, the high air permeability and the open cell structure cause this material to be highly flammable. The new phosphorus flame retardant (FR) methyl-DOPO 9,10-dihydro-9-oxa-methylphosphaphenthren-10-oxid is known to show an excellent flame retarding behavior in flexible polyurethane foam by acting mainly in the gas phase. I this study, the FR working mechanism of methyl-DOPO and its ring-opened analogue MPPP methylphenoxyphenylphosphinate is investigated by TGA, TG-MS, FMVSS 302 and Cone Calorimeter measurements. Under TG-MS conditions comparable concentrations of low molecular weight species like HPO, CH3PO or PO2 are released. These species are able to scavenge the H- and OH-radicals in the radical chain reactions of the flame leading to a significant increase of the CO/CO2 ratio and the smoke density during cone calorimeter experiments. Finally, the flame retardancy of MPPP is determined to be sligh tly less efficient in flexible polyurethane foam because of the higher vapor pressure compared to methyl-DOPO. Here, the vaporization of methyl-DOPO occurs in the same temperature region as the depolymerization of the urethane and the bisubstituted urea groups during pyrolysis of the foam leading to an optimal interaction.