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SYNOPSISThis study of the pyrolysis of a flexible polyurethane foam showed that the composition of the products depends on the conditions of the pyrolysis. If the volatiles are removed rapidly from the system, they will contain tolylene diisocyanate (TDI). However, under confined conditions, the TDI will be replaced by diamino toluene (DAT).These results can be explained by assuming two decomposition routes to be operative; one leading to the regeneration of the two source materials, TDI and the polyol; the other to DAT and a double-bond-terminated polyether (the polyol with its terminal hydroxy groups replaced by double bonds). The TDI route is the faster, however if the volatile TDI is confined to the pyrolysis zone, an equilibrium will be established between the urethane group and the TDI plus polyol. Such conditions will favor the slower, though irreversible, route leading to the formation of DAT.On pyrolysis, the urea groups in the foam dissociates into TDI and DAT. These will recombine in the vapor phase to form an aerosol of polyurea. It is proposed that this aerosol is the "yellow smoke" reported in the literature. 12:27 c, 1997 John Wiley & Sons, Inc. is completed by about 1700C.7 On pyrolysis, both types of materials are stated to regenerate their precursors, isocyanate and urea, in the case of biuret, isocyanate, and urethane from allophanate.z>]s Next in thermal stability are the urethanes, closely followed by ureas, with the isocyanurate group being the most stable of these nitrogen-containing deriv-

ABSTRACT:A flexible polyurethane foam fire-retarded with 7.8% tris(1,3-dichloro-2-propyl)phosphate (TDCPP) was found to lose by volatilization 80% of this material at 200ЊC, a temperature at which there is as yet little decomposition of the foam. It is therefore to be expected that when this foam burns, most of its TDCPP will vaporize into the flame. The latter would therefore be expected to be the main site of activity of this retardant. However, when TDCPP was injected directly into the flame of the burning unretarded foam, no inhibiting effect was apparent. This, in contrast to the above conclusion, points to the condensed phase as the main site of the retarding activity. Although there is some additional evidence that supports the latter interpretation, it is difficult to reconcile it with the small amount of residual TDCPP available for reaction in the condensed phase. Flames of unretarded foams were also unaffected by the injection of other halogen containing materials, such as trichloropropane, HCl, and HBr. It is suggested that the temperatures of these polyurethane flames may not be sufficiently high to initiate the radical-trapping-based flame-poisoning mechanism classically attributed to the halogenated fire retardants. This could be the reason why the latter are less effective as fire retardants for polyurethanes than they are for many other substrates.

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Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) is used commercially as a fire retardant for flexible polyurethane foams. It was found to act both in the condensed (pyrolysis zone) and the vapor (flame) phases. The extent of its activity in the individual phases depends on the way in which the specimen is ignited. Under conditions of candle-like, top-down burning, retardation seems to occurs mainly in the condensed phase by a mechanism apparently based largely on the barrier properties of a phosphorus-containing carbonaceous layer that builds up on top of the liquid pyrolyzing layer beneath the flame. As the formation of this barrier requires time, extinguishment in this mode is relatively slow. Extinguishment is much faster in bottom-up burning, where the flame appears to be the main site of the retardation. In this mode, because of the orientation of the specimen relative to the flame, a disproportionately large amount of TDCPP enters the flame. It is proposed that TDCPP decomposition products, HCl and other low-fuel-value materials dilute the fuel vapors sufficiently to reduce their flame propagation velocity to below that at which they stream out of the pyrolysis zone. This pushes the flame away from the pyrolysis zone, uncoupling the thermal feedback mechanism that produces the fuel.