Storage stability of polyamidoamine-epichlorohydrin resin and its effect on the properties of defatted soybean flour-based adhesives

A polyamidoamine (PA) without introducing epichlorohydrin, and two epichlorohydrin-modified polyamidoamine (PAE) samples with solid contents of 12% (PAE-12) and 25% (PAE-25) were synthesized, and their short-term storage stabilities, evaluated at intervals of at least 3 months were assessed for chemical structure, viscosity, pH, thermal degradation behavior, crystalline degree and wet bond strength. The results showed that PA was stable during storage for 98 days, while PAE-12 had better storage stability than PAE-25. PAE was active due to complex side reactions, as the number of azetidinium groups within PAE significantly decreased resulting from the ring open reaction after storage for 42-56 days, leading to decreased crosslinking degree and thermal stability, and increased crystalline content of defatted soy flour (DSF)-PAE adhesives. Thus, the wet bond strength of the corresponding plywood decreased with the increasing storage time of PAE-12 and PAE-25, and finally decreased to 1.05 MPa and 0.66 MPa after storage for 98 days, respectively. In order to ensure the water resistance of DSF-PAE adhesives, the preferable application time at room temperature is 3 months for PAE-12 and 4 weeks for PAE-25.