Structurally isomeric 15N-labeled polymeric methylenebis(phenylisocyanate), 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 various times and temperatures and then analyzed using 15N CP/MAS NMR. It was found that resin polymerization occurs via the reaction of isocyanate with wood moisture to form polyurea. Biuret formation and isocyanate dimerization were detected. Urethane formation probably also occurs; however, signal overlap of urea and urethane signals prevents a definitive conclusion. These findings are similar to previous ones; however, subtle differences are noted. The structurally isomeric resins displayed 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 performance in impact loading.
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Ni et al. (1998) studied this question.
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