N-15 CP MAS NMR-STUDY OF THE ISOCYANATE/WOOD ADHESIVE BONDLINE - EFFECTS OF STRUCTURAL ISOMERISM/

Structurally isomeric N-15-labeled polymeric methylenebis(phenylisocya nate), pMDI, adhesives were synthesized. One resin had a high content of 4,4'-MDI, and another was prepared with a high content of 2,4'-MDI. Both resins were cured with wood (Liriodendron tulipifera) for variou s times and temperatures and then analyzed using N-15 CP/MAS NMR. It w as found that resin polymerization occurs via the reaction of isocyana te with wood moisture to form polyurea. Biuret formation and isocyanat e dimerization were detected. Urethane formation probably also occurs; however, signal overlap of urea and urethane signals prevents a defin itive conclusion. These findings are similar to previous ones; however , subtle differences are noted. The structurally isomeric resins displ ayed similar chemistries. Of the two resins, the resin prepared with a high content of 2,4'-MDI cured more slowly, and resulted in a network that was more mobile in the midkilohertz frequency range. This leads to the prediction that resins high in 2,4'-MDI may have a superior per formance in impact loading.