Solid state NMR studies of polyurethane in wood

Solid state nuclear magnetic resonance (NMR) experiments were conducted on samples of engineered wood composites. Several NMR techniques including 13C variable contact time (VCT) cross polarization magic-angle spinning (CPMAS), 13C inversion recovery cross polarization (IRCP), 15N VCT-CPMAS and 15N-1H Heteronuclear correlation (HETCOR) NMR experiments were employed to study the reaction of 15N enriched diphenylmethane diisocyanate adhesive (MDI) in aspen and pine wood samples. The use of 15N enriched MDI permitted detection of resonances from MDI-derived species without interference from wood resonances. In VCT/CPMAS experiments, the influence of temperature, moisture content and the type of wood on reaction were investigated. No evidence for reaction of MDI with hydroxyl of lignin or cellulose could be found. When the temperature is higher than 140°C, reaction of MDI produces cross linking biuret and/or trimer structures. The influence of moisture is more apparent in pine than in aspen when the moisture content is in the range from 7% to 14%. In HETCOR experiments, resonances due to both urea and biuret structures were evident in the spectra of the wood/MDI composites. Two peaks in the range of 104--106 ppm were attributed to the different chemical environments of urea structures, and not to the urea/urethane structure. Both intramolecular hydrogen bonding of poly-MDI (PMDI) and intermolecular hydrogen bonding between PMDl and wood molecules were observed.