Determination of NCO content of the urethane prepolymers by 19F NMR spectroscopy

A novel, reliable and efficient test method for quantification of isocyanate functional groups is developed using 19F NMR spectroscopy. The procedure involves an in situ derivatization of isocyanate with 1,1,1, 3, 3,3,3-hexafluoro isopropyl alcohol (HFIP) in an NMR tube, followed by integration of the unique 19F resonance contributed to the product vs. that of α,α,α-trifluorotoluene as an internal standard in the 19FNMR spectrum. The spectrum of the reaction mixture appears in a manner such that an explicit and accurate resolution can be achieved. The method is applicable to simple, polymeric polyisocyanates and also urethane prepolymers. Accuracy of this method was verified using a known diisocyanate such as pure MDI as a model sample. A desirable correlation was observed between the results obtained by the current 19F NMR method and the standard ASTM method. It was found that the 19F NMR approach has distinct advantages over the titration method, such as having high sensitivity and entailing a convenient procedure within a short period of time.