Delineating pMDI model reactions with loblolly pine via solution-state NMR spectroscopy. Part 2. Non-catalyzed reactions with the wood cell wall

Abstract

Abstract Solution-state NMR provides a powerful tool to observe the presence or absence of covalent bonds between wood and adhesives. Finely ground wood can be dissolved in an NMR-compatible solvent system containing dimethylsulfoxide-d 6 and N-methylimidazole-d 6 , in which the wood polymers remain largely intact. High-resolution solution-state two-dimensional NMR correlation experiments, based on 13C–1H one-bond heteronuclear single quantum coherence, allow structural analysis of the major cell wall components. This technique was applied to loblolly pine that was treated with polymeric methylene diphenyl diisocyanate (pMDI) related model compounds under controlled moisture and temperature conditions. Chemical shifts of carbamates formed between the pMDI model compounds and loblolly pine were determined. The results show that: (a) under dry conditions and a high concentration of isocyanate, carbamates will form preferentially with side-chain hydroxyl groups on β-aryl ether and phenylcoumaran-linked lignin units in a swelling solvent; (b) phenyl isocyanate is more capable of derivatization in the cell wall than the bulkier 4-benzylphenyl isocyanate; (c) at 5% and 14% moisture content, detectable carbamates on lignin side-chains dramatically decrease; and (d) under typical conditions of industrial oriented strand-board production in a hot press at 5% and 14% moisture content, no carbamate formation was detected.