Use of acid scavengers to improve durability of acid- catalyzed adhesive wood bonds

Abstract

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 acidcatalyzed degradation of wood polymer adjacent to the bondline. With a phenolic adhesive with p-toluenesulfonic 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 hydrox­ ide, magnesium oxide, soda lime glass, a kraft pine lignin, a carbamated walnut shell flour, and a magnesium hydroxide-impregnated lignocellulosic ex­ tender. Primary criteria for utility were lack of in­ terference 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 achieve­ ment 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 con­ trol. The better materials allowed no significant strength losses during 60 days of accelerated 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 acidcatalyzed systems. However, these and other scavenger candidates would require much more extensive evalua­ tion for any particular application than was carried out in this study. The use of acidic catalysts offers distinct advan­