The¹⁵N CP/MAS NMR Characterization of the Isocyanate Adhesive Bondline for Cellulosic Substrates

A 99% 15 5N-labeled polymeric diphenylmethane diisocyanate (pMDI) resin with desirable adhesive properties was synthesized. This adhesive was used to make a series of cellulose/15N-labeled pMDI composites at various cellulose precure moisture contents. Cure chemistry and bondline morphology were monitored using 15N CP/MAS NMR. Biuret-type structures were identified as the predominant chemical moieties at low precure moisture conditions, while urea linkages were predominant at higher precure moisture. Significant amounts of urethane were not detected, however low amounts could be obscured by signal overlap. Relaxation studies using variable contact times were complicated by excessively long cross-polarization rates for nonprotonated nitrogens. Experiments using variable spin lock times prior to fixed contact periods suggest that the cured resin is homogeneous. The utility of 15N CP/MAS for elucidating fine structural and morphological information from complex isocyanate-cured composites is clearly demonstrated.