Bubbles and collapses: Fire phenomena of flame-retarded flexible polyurethane foams

Flexible polyurethane foams (FPUF) are easy to ignite and exhibit rapid flame spread. In this paper, the fire phenomena of two standard foam formulations containing tris(1,3-dichloro-2-propyl) phosphate (FR-2) and a halogen-freepoly (ethyl ethylene phosphate) (PNX), respectively, as flame retardants are compared. A multi-methodological approach is proposed which combines standard fire tests as well as new investigatory approaches. The thermophysical properties of the foams were determined by thermogravimetric analysis (TG), reaction to small flames was studied by means of the limiting oxygen index (LOI) and UL 94 HBF test, and the burning behavior was investigated with the cone calorimeter. Further, temperature development in burning cone calorimeter samples was monitored using thermocouples, and rheological measurements were performed on pyrolyzed material, delivering insight into the dripping behavior of the foams. This paper gives comprehensive insight into the fire phenomena of flame-retarded FPUFs that are driven by the two-step decomposition behavior of the foams. LOI and UL 94 HBF tests showed a reduced flammability and reduced tendency to drip for the flame-retarded foams. TG and cone calorimeter measurements revealed that the two-step decomposition behavior causes two stages during combustion, namely structural collapse and pool fire. The flame-retardant mode of action was identified to take place primarily during the foam collapse and be based mainly on flame inhibition. However, some condensed-phase action was been measured, leading to significantly increased melt viscosity and improved dripping behavior for foams containing PNX.