MONITORING POSTCURE REACTION CHEMISTRY OF RESIDUAL ISOCYANATE IN 4,4'-METHYLENEBIS(PHENYL ISOCYANATE) BASED ISOCYANURATE RESINS BY N-15 AND C-13 CP MAS NMR

The fate of residual isocyanate groups in three cured, N-15-enriched isocyanurate-rich resins based on 4,4'-methylenebis(phenyl isocyanate) (MDI) was monitored by both N-15 and C-13 CP/MAS NMR. Data obtained on the samples immediately after curing are compared with results obtained after 7-month exposure to air. The comsumption of isocyanate and the subsequent formation of urea linkages during this 7-month period are clearly identified from changes in the C-13 spectra. N-15 CP/MAS NMR results readily identify the consumption of isocyanate to form the amine as the product of isocyanate hydrolysis. In addition, the formation of urea condensation products from the reaction of amine and isocyanate groups is clearly shown. No significant reaction of urea linkages with isocyanate groups to form biuret linkages is observed in the N-15 spectra of air-exposed samples. Deconvolutions and integrations of the N-15 spectra yield estimates for the extent of isocyanate postcure reaction chemistry for each MDI-polyisocyanate resin. N-15 spin-lattice relaxation times obtained on the samples exposed to air provide insight into the molecular motion of the various nitrogen environments. N-15 CP/MAS NMR is seen to be especially suited for monitoring the solid-state reaction chemistry of the isocyanate group.