A soybean adhesive with excellent hygrothermal resistance and enhanced mildew resistance via optimal synthesis of polyamidoamine–epichlorohydrin resin

Despite the fact that soybean-based adhesives offer huge advantages such as sustainability and being eco-friendly, their wider application in wood composites has been severely restricted by the poor resistance towards wet environments and mildew. In this study, a soybean-based adhesive with excellent hygrothermal and enhanced mildew resistance was developed by optimizing the synthesis of a polyamidoamine–epichlorohydrin (PAE) resin with varying epichlorohydrin (ECH) content and applying formic acid as a terminating agent. FTIR, NMR analyses and plywood evaluation indicated that the ECH content had a significant effect on the hygrothermal resistance of the PAE-modified soybean adhesive. This is attributed to the greater number of azetidine groups resulting from higher ECH content, which effectively crosslinked the soybean meal to form compact networks. However, owing to the more powerful mildew inhibition of formic acid than sulfuric acid and ECH-derived organochlorine, higher ECH content also led to a higher amount of organochlorine in the PAE resin but required less formic acid to terminate PAE polymerization, causing reduced mildew resistance of the soybean adhesive. The PAE resin synthesized with 50% ECH was optimum for formulating a soybean adhesive for wood composites, which showed superior hygrothermal and mildew resistance compared to commercial adhesives.