Effects of wood dyestuff on the bonding properties of soybean-based adhesive for commercial plywood production

The two main components of commercial soybean-based adhesives (SA), polyamidoamine-epichlorohydrin (PAE) resin and soybean protein, are extremely ion-sensitive. When SA is used to bond dyed wood substance and/or mix with anion dyes for labelling wood composites during commercial manufacturing, the regularly used anion dyes in the wood industry may produce significant differences in the bonding capabilities of SA. In the current study, three common tricolor anion dyes in the wood industry were chosen to evaluate their impacts on the structures and properties of soybean protein, PAE resin, and PAE-modified SA, respectively. Analytical data from FTIR, XRD, TGA, NMR, sol-gel test, and plywood evaluation indicated that dye ions had varying effects on the bonding properties of SAs. The addition of chosen red and yellow dyes to SAs enhanced plywood cycling wet strengths by 41.7–42.8%, whereas the blue dye had little impact. The promoter interactions of red- and yellow-dye ions on the unfolding of globular structures of soybean protein, the self-crosslinking of PAE resin, and the sufficient co-crosslinking of soybean protein by PAE resin all contributed to the formation of compact three-dimensional networks of cured SAs. As a result, this work illuminates a novel method for conveniently improving the bonding capabilities of SAs by adding appropriate dyes.