Enhancing the Cross-Linking Efficiency of Polyamidoamine-Epichlorohydrin Resin for Soybean-Based Adhesives via Proportioning Its Raw Materials

Although soybean-based adhesives modified using polyamidoamine-epichlorohydrin (PAE) are promising alternatives to formaldehyde-based synthetic resins in wood composites, their cross-linking efficiency and bonding properties require further improvements. In this study, the molecular structures of a PAE resin and its precursor were designed to enhance the cross-linking efficiency of the PAE resin with soybean flour. The results indicated that both adipic acid/diethylenetriamine and epichlorohydrin/polyamide ratios greatly affected the azetidinium content, branched structure, and molecular weight of the resulting PAE resin. A higher azetidinium content facilitated the formation of a more compact cross-linked network, which provided the resulting soybean-based adhesive with better water resistance. The branched structure and optimal molecular weight promoted the diffusion and reaction between PAE and soybean protein molecules, which helped enhance the curing rate. As a result, soybean-based adhesive SP/0.95–45 modified with the optimal PAE resin had a soaked wet strength of 1.36 MPa and a faster curing rate, as demonstrated by its 10 °C lower hot-pressing temperature compared to that of the resin modified with commercial PAE. Additionally, the optimal PAE resin remained stable over a 120-day storage period. This research offers insights into enhancing the cross-linking efficiency and bonding properties of soybean-based adhesives.