Reaction of Isocyanates with Urea Groups

Similarly to the allophanate formation, the -N-H groups of urea react with isocyanates, to generate a biuret [1, 3, 6-18, 21, 23-25]  

Also similarly to the allophanate formation, the reaction between urea and isocyanates is an equilibrium reaction and needs higher temperatures too ( 110 °C). 

Formation of allophanates and biurets in polyurethane chemistry, especially when an excess of isocyanate is used, is in fact a supplementary source of crosslinking. 

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