Isocyanate-Wood Adhesive Bond

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Abstract
We have used infrared spectroscopy to study the reaction between phenyl isocyanate and glucose, cellulose, lignin, and wood. In the latter instance we have investigated oven-dried wood, wood at its fiber saturation point, and wood at 19% and 7% moisture levels. Our results show that the isocyanate reacts with all these entities, but that its reaction with water is quicker than with any of the other hydroxyl-containing compounds, and when water is present the water-isocyanate reaction dominates all others. Lignin was found to react more readily than any of the sugar derivatives. In oven-dried wood, the presence of strong new absorptions at 1712 and 1694 cm−1 (indicative of a carbamate group) is evidence of a direct chemical bond between the isocyanate and the wood polymer structure. We have also studied the reaction between 4,4′-diphenylmethane diisocyanate (MDI)—a commercially used adhesive—with wood flour at various levels of moisture content (MC), and the infrared spectra again support the presence of a wood-isocyanate bond for oven-dried wood and for wood at 7% moisture content. For wood at higher moisture content levels, our infrared results suggest that the isocyanate reacts with the water in preference to the wood polymers.

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