Specific oxidants improve the wood bonding strength of soy and other plant flours

To meet newer environmental standards, modified plant proteins have been studied as no-added formaldehyde wood adhesives for interior applications. Many methods have been developed to increase the wet strength of wood products bonded with soy adhesives. These methods involve modifying the soy in separate steps prior to formulating the adhesive or adding a polymerizable co-reactant to the soy. We show that adding periodate, permanganate, or iodate to soy flour improved the strength of soy adhesive bonds in small-scale testing and in plywood shear, especially when tested under wet conditions. Periodate improved the bond strength of other plant materials (lupine, canola, and cottonseed) but none of these produced as high of a wet strength as the soy flour. We investigated other oxidants with plant proteins. Permanganate was quite effective and iodate was somewhat effective, whereas nitric acid, chlorate, perchlorate, and bromate were not effective in increasing wet strength. The available data are consistent with oxidation of the carbohydrate-protein mixture in plant flours to provide adhesives with increased wet strength in wood bonds. This mechanism was also supported by the improved wet strength with the addition of dialdehydes (glyoxal and glutaraldehyde). The purified soy protein also gave strength improvement with periodate. Published 2019. This article is a U.S. Government work and is in the public domain in the USA. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1017-1023