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Studies have been made of the flammability of thermoplastic polyurethane incorporating a halogen-free additive flame-retardant system. Clear-cut evidence of synergism has been shown for the interaction between the additives. Thermoanalytical studies have shown that the crucial reaction for synergism is the enhanced production of amnonia and that this only occurs when the two phosphorus- and nitrogen-containing additives are present. The overall pattern of thermal decomposition of the polymer-additives system shows slow but steady thermal activity over a wide range of temperatures; this is indicative of synergism in a condensed-phase mechanism. The most important flame-retardant effect is thus the formation of a charry intumescent foam, but there is also some physical and chemical gas-phase activity. The resulting thermoplastic polyurethanes can be produced with an LO1 of over 40 and a UL94 ranking of VO, in the total absence of halogen.