Blocked isocyanate. Reaction and thermal behaviour of the toluene 2,4‐diisocyanate dimer

Due to the difference in reactivity between free isocyanates and uretdione rings, diisocyanate dimers can be used as blocked isocyanates. They offer the potential advantage that no volatile by‐products are formed. The toluene 2,4‐diisocyanate dimer contains one uretdione ring, formed by dimerization of two isocyanate groups, and two free isocyanate groups. The latter react rapidly with the hydroxy groups of a polycaprolactone diol at temperatures above 80°C. The pure dimer thermally dissociates above 150°C; dissociation is reversible. In the presence of OH groups, the regenerated isocyanate groups can then react to form urethanes. Our experiments have also revealed the formation of allophanate groups as reaction products of an uretdione ring with a hydroxy group. The influence of initial molar ratio of reactants and of temperature on the mechanisms have been discussed. Allophanates are not stable at high temperature in presence of remaining hydroxy groups, their dissociation leads to urethanes. Application of branching theory has shown that the system can gel due to allophanate formation in a wide range of initial molar ratios of NCO to OH groups.