AbstractThe thermal decomposition of a number of TDI‐ and MDI‐based biscarbamates (model compounds for polyurethane foams) between 200°C and 1000°C showed that the urethane linkage undergoes anO‐acyl fission at about 300°C to generate the free isocyanate and alcohol. In the case of the flexible foam analogues, the newly generated TDI reacts further to generate volatile polyureas, termed ‘yellow smoke’. The MDI residues generated in the decomposition of a rigid foams react to yield non‐volatile polycarbodiimides. Both the yellow smokes and the polycarbodiimides decompose above 600°C to give a mixture of nitriles (including HCN) as well as a number of olefinic and aromatic compounds. The use of13C labeling indicated that HCN and all the other nitriles generated during the high temperature decompositions originate in the thermal fission of the aromatic ring, the nitrile carbon being the 2‐, 4‐
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