Reactivity of isocyanates with urethanes: Conditions for allophanate formation

The reactions between two polyisocyanates, 4,4′-methylenebis(phenyl isocyanate) (MDI) and trimer of isophorone diisocyanate (tIPDI) and a model aryl-alkyl diurethane were carried out at high temperature (≥170 °C) and several NCO/urethane ratios. A combination of 1H and 13C NMR and MALDI-TOF spectroscopies was used and allows the identification of reaction products. When MDI or tIPDI is reacted with a diurethane at high temperature and after cooled, allophanates are formed but not isocyanurates. 13C NMR was used to quantify the different reaction products obtained under different experimental conditions. Only a few allophanates (≤10%) are obtained after 1 h of reaction.