Abstract
AbstractBlocked isocyanates were prepared and their thermal dissociation into isocyanates and blocking agents was studied. Three blocked versions of 4,4′‐diphenylmethane diisocyanate (4,4′‐MDI) were synthesized. The selected blocking agents were 2‐butanone oxime (methyl ethyl ketoxime, MECO), ethyl acetoacetate oxime (EAO) and benzyl methacrylohydroxamate (BMH), in order to compare their temperature of dissociation (Td). Chemical titration of —NCO groups was tested as suitable technique to follow the unblocking with the temperature. Confirmation of the results by infrared spectroscopy was successful with the hydroxamic ester as blocking agent. Thermodynamic parameters were evaluated for the BMH‐MDI adduct and allowed to predict the free —NCO ratio at high temperatures.Benzyl methacrylohydroxamate can be used as blocking agent and the blocked system shows lower unblocking temperature than the usual oxime‐blocked isocyanates.
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