Potential of Carbohydrates for Exterior-Type Adhesives

New exterior-quality adhesives for wood products need to be available if the cost or supply of phenol-based adhesives should become unfavorable. Carbohydrates represent a large, inexpensive, renewable resource from which to obtain polymerizable materials. Preliminary trials led to exploratory evaluation of a system in which carbohydrates and some urea replace as much as 35 percent of the phenol in a softwood plywood formulation. Carbohydrates included glucose, fructose, sucrose, methyl glucoside, corn syrup, and xylose. Acid-catalyzed dehydration drives the initial reaction. During exposure to acid at high temperature, weight loss from hexose carbohydrates is equivalent to over 4 moles of water per mole of monosaccharide, over 3 moles for a pentose. In the initial synthesis reaction, the carbohydrate is degraded and reacted with urea. The resulting product is neutralized and reacted with phenol and formaldehyde. Finally, alkali is added as the curing catalyst. Adhesive resins synthesized with various molar ratios of the ingredients were used to bond two-ply laminated veneer panels, which were tested for bond strength. Promising results were obtained. If the molar ratio of phenol-to-monosaccharide did not fall below 1:1 and that for formaldehyde-to-phenol did not fall below 2:1 then bond strengths were equal to those for phenolic resins, wet, or after 2 hours of boiling. Slightly higher pressing temperatures were needed compared to a phenolic resin. Preliminary 13C nuclear magnetic resonance analysis did not indicate expected furan ring structures in the liquid resin.

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