Cure chemistry of wood/polymeric isocyanate (PMDI) bonds: Effect of wood species

Abstract The wood-species dependent performance of polymeric isocyanate resin (PMDI) was investigated by fracture analysis and solid-state NMR. The fracture performance of two different woods (yellow-poplar and southern yellow pine) bonded with PMDI was evaluated using mode-I cleavage. The fracture toughness of pine samples was significantly greater than the corresponding yellow-poplar samples. Separately, the cure chemistry of wood/PMDI bondlines was investigated by solid-state NMR with nitrogen-labeled (N15) resin, and by employing spectral decomposition of the complex resonance arising from protonated nuclei. A small but statistically significant species effect was found in both the cure chemistry and in the proton rotating-frame relaxation for samples cured at lower temperatures. These differences were not found at higher cure temperatures. This suggested that a species effect might arise in the core of commercial wood-based composite panels, where lower cure temperatures are found. Wood acetylation altered the wood-PMDI cure chemistry by reducing the relative signal intensity in the urethane spectral region by 35%. However acetylated wood specimens still exhibited significant N15 signal in the urethane spectral region. Since little or no urethane was expected from acetylated samples, an unequivocal urethane detection remains elusive, and if it is detected its occurrence was grossly overestimated by the spectral decomposition employed in this work.