Effect of moisture content on the isocyanate/wood adhesive bondline by15N CP/MAS NMR

A 99% 15N-labeled polymeric diphenylmethane diisocyanate (pMDI) resin was used to make a series of wood/15N-pMDI composites at various wood precure moisture contents. Cure chemistry and bondline morphology was monitored using 15N CP/MAS (15N cross-polarization, magic-angle spinning) NMR (nuclear magnetic resonance). Biuret-type structures were identified as the predominant chemical moieties when dry wood flakes were bonded together, while urea linkages were predominant at higher precure moisture. Urea and urethane signal overlap make the unambiguous identification of urethane difficult. Relaxation studies using variable contact times were complicated by excessively long cross-polarization rates for nonprotonated nitrogens. Experiments using variable spin lock periods prior to a fixed contact time indicate that the cured resin in these composites is homogeneous.