| Abstract: | In fires, flexible polyurethane foams can decompose to give smokes which are subsequently degraded with the generation of hydrogen cyanide. In order to understand the general nature of this hazard it is first necessary to obtain information about the decomposition mechanism of the polyurethane foams and the structures of the intermediate smokes. The complexity of this problem has required the use of suitable model compounds. The preparation is described of (i) the four bis-carbamates based on pure tolylene 2, 4-diisocyanate and pure tolylene 2,6-di-isocyanate, each in combination with 2-ethoxyethanol and with triethylene glycol monomethyl ether, and (ii) a urethane-carbodi-imide-urethane and a urethane-urea-urethane based on pure tolylene 2,6-di-isocyanate and 2-ethoxyethanol. All were decomposed by heating under nitrogen at temperatures near 300°C. The two di-2-ethoxyethyl bis-carbamates ( la and IIa ) gave volatile monoisocyanate-monourethanes and 2-ethoxyethanol. The two di-2-[2-(2-methoxyethoxy)ethoxy] ethyl bis-carbamates ( Ib and IIb ) gave corresponding products and residue which contained carbodi-imide functions. The urethane-carbodi-imide-urethane ( Va ) gave a volatile urethane-carbodi-imide-isocyanate ( Via ) and 2-ethoxyethanol. The urethane-urea-urethane ( VIIIa ) decomposed through isocyanate- and carbodi-imide-containing materials to a volatile polymeric urea, which was obtained as a smoke and appeared to be virtually identical with the smoke obtained in earlier work from a commercial polyurethane. These results suggest that poly-urethanes based on tolylene di-isocyanate and polyether polyols decompose, when heated under nitrogen at 300°C, by cleavage of urethane groups to regenerate isocyanate groups and liberate alcohols. The isocyanato groups may react either with other isocyanato groups to give carbodi-imides, which are then rapidly hydrated to ureas, or with water to give amines, which react with other isocyanato groups again to give ureas. Whether these processes occur separately or together, the ultimate product at 300°C, i.e. the smoke, will be a polymeric urea. |
