Use of Acid Scavengers to Improve Durability of Acid-Catalyzed Adhesive Wood Bonds

Acid catalysis of adhesive cure can provide benefits of shorter press times and/or lower press temperatures, plus increasing the potential of using some adhesives from renewable resources. Those benefits can be outweighed, however, by a loss of durability that results, in the case of phenolic or furan adhesives, from acid-catalyzed degradation of wood polymer adjacent to the bondline. With a phenolic adhesive with ptoluenesulfonic acid and yellow birch veneer as a model system, individual additives and combinations of additives were examined for their effectiveness in increasing the durability of acid-catalyzed joints via neutralization of the acid at the proper rate. These additives were all water-insoluble powders: aluminum, aluminum hydroxide, magnesium oxide, soda lime glass, a kraft pine lignin, a carbamated walnut shell flour, and a magnesium hydroxide-impregnated lignocellulosic extender. Primary criteria. for utility were lack of interference with adhesive cure at 100?C for 3 minutes or at ambient for 24 hours and ability to reduce rate of joint strength loss during aging at 60?C/67 percent relative humidity and 90?C/dry. Some additives interfered with cure at small particle size but not at larger; some interfered at ambient/24 hours but not at 100?C/3 minutes. Those nine materials that allowed the achievement of good bond strength at 100?C/3 minutes were tested for durability enhancement. Of these, all but aluminum hydroxide provided some improvement in durability relative to a nonscavenger-containing control. The better materials allowed no significant strength losses during 60 days of acclerated aging. These results confirm the viability of the acid scavenger concept and indicate that concerns about durability need not necessarily rule out the use of acid-catalyzed systems. However, these and other scavenger candidates would require much more extensive evaluation for any particular application than was carried out in this study.

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