Studies of Catalysed Isocyanate Reactions by High Performance Liquid Chromatography

An analytical method based on high performance liquid chromatography (HPLC) has been developed to investigate the competing isocyanate reactions under the influence of a catalyst. A model reaction between phenyl isocyanate and n-butanol/water was chosen to simulate the water-blown foam production. The kinetics of this model reaction were studied in acetonitrile at 50°C. Effects of tertiary amine catalysts with various structures on the reaction rate and the formation of reaction products were investigated. The reactions were followed by determining the NCO disappearance using the standard di-n-butylamine back titration method as well as measuring the formation of various reaction products using the HPLC method. The relative specificity of a catalyst in isocyanate reactions can thus be determined from the profile of the model reaction which depends upon the structure of the catalyst employed.

The gelling (urethane formation) and blowing (urea formation) activities of these catalysts can be determined from the amount of carbamate and urea in the reaction solution. The amine catalysts containing a hydroxyethyl group catalyse trimerisation reaction, but those with a hydroxypropyl group do not. The nitrogen active centre (N) is essential for the catalytic activity of the amine catalysts. The amines are urea catalysts when their active centres (N or O) are separated with ethylene linkages; whereas they are urethane catalysts when their active centres are separated with propylene linkages.