A Search for Nondestructive Acid Catalysts for Wood Bonding

Acid catalysts for bonding exterior grade wood products would allow use of several potential new adhesives and would increase cure speeds for phenolic adhesives. The problem is that the acid leaches out of the bondline to attack nearby wood, leading to serious decline of initially high strength. A novel approach to stop acid movement was investigated: selecting sulfonic acids that might become chemically or physically trapped within the cross-linked adhesive structure. An acid-curable phenolic resol was used as the model adhesive. The acids included four with bulky molecular structures that might be physically trapped, and four containing a phenolic structure that might react into the resin matrix. The relative catalytic effectiveness of acids was judged by calorimetric measurements. Both aqueous extractions of acid from cured resin and strength durability measurements on bonded wood specimens were used to judge how well the concept worked. Toluene sulfonic acid, used as a control catalyst, was the most effective acid, per equivalent of acid, in accelerating the resin cure. None of the acids tested was significantly retained within the resin upon extraction. This indicates that the phenolic resin is somehow porous to even the large catalyst molecules. At acid levels needed to give faster cures than alkaline catalysts, bond strengths degraded as much as 30 percent during 8 weeks of accelerated moist aging. Degradation correlated with initial acid level. No successful trapping technique is yet available for preventing acid damage to wood near highly acidic gluelines.

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