A solution-state NMR approach to elucidating pMDI-wood bonding mechanisms in loblolly pine

Solution-state nuclear magnetic resonance spectroscopy (NMR) is a powerful tool for unambiguously determining the existence or absence of covalent chemical bonds between wood components and adhesives. Finely ground wood cell wall material dissolves in a solvent system containing dimethylsulfoxide- d6 and N-methylimidazole-d6, keeping wood component polymers intact and in a near-native state. Two-dimensional NMR experiments, using 13--C-1H one-bond Heteronuclear Single Quantum Coherence (HSQC) on non-derivatized cell wall material from loblolly pine reveal details about the major cell wall polymers. This technique can determine covalent bond formation between cell wall polymers and wood adhesives. Monofunctional model compounds of polymeric methylene diphenyl diisocyanate (pMDI) were reacted with loblolly pine under moisture-controlled conditions to derivatize the wood cell wall polymer hydroxyls. The reacted loblolly pine was ball-milled, dissolved, and characterized via 2D NMR experiments. NMR chemical shift data from carbamylated wood model compounds were then used to assign carbamate signals in the HSQC spectra from carbamylated wood polymers. The data obtained allows us to determine whether covalent bonds form between loblolly pine and pMDI at different adhesive concentrations and moisture levels. Results show that high concentrations of the pMDI model react with the loblolly pine to form quantifiable carbamate linkages essentially only with lignin sidechain units under dry conditions. No covalent bond formation was detected after using a hot-press on loblolly pine with pMD1 models at 5% and 14% moisture content.