The thermal decomposition of biurea

Abstract

Biurea decomposes in the temperature range 230–260° to yield gas, a white sublimate, and a viscous liquid residue. The gas is mainly ammonia (ca. 71%), but some carbon dioxide (ca. 17%) and nitrogen (ca. 12%) are also present. The main constituent of the sublimate is urea and that of the residue urazole. The products are accounted for by four reactions involving hydrogen atom transfer, together with two subsequent reactions of isocyanic acid. The decomposition takes place in three stages, an initial small evolution of gas, an induction period, and a final stage during which most of the decomposition occurs and which is autocatalytic. Recrystallisation largely eliminates the first stage. Lowering the external gas pressure lengthens the induction period. The final stage is more reproducible than the other two. It remains acceleratory almost to the very end of the reaction and has an activation energy of 114 kJ mol^–1. The kinetics of the decomposition are interpreted in terms of a nucleation and growth mechanism.