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 F-19 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 F-19 resonance contributed to the product vs. that of alpha,alpha,alpha-trifluorotoluene as an internal standard in the (FNMR)-F-19 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 F-19 NMR method and the standard ASTM method. It was found that the F-19 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. (C) 2013 Elsevier Ltd. All rights reserved.