HYDROXAMIC ACID-ESTERS AND OXIMES AS ISOCYANATE BLOCKING-AGENTS - STUDY ON THERMAL-DISSOCIATION

Blocked 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 se lected blocking agents were 2-butanone oxime (methyl ethyl ketoxime, M ECO), ethyl acetoacetate oxime (EAO) and benzyl methacrylohydroxamate (BMH), in order to compare their temperature of dissociation (T(d)). C hemical titration of -NCO groups was tested as suitable technique to f ollow 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-MD I adduct and allowed to predict the free -NCO ratio at high temperatur es. Benzyl methacrylohydroxamate can be used as blocking agent and the blocked system shows lower unblocking temperature than the usual oxim e-blocked isocyanates.