Development of boiling water resistance starch-based wood adhesive via Schiff base crosslinking and air oxidation strategy

Starch adhesives are promising biomass adhesives. However, their polyhydroxy structure leads to poor water resistance. In this study, DAS-HBP adhesive with a densely cross-linked structure was prepared by reacting dialdehyde starch (DAS) with hyperbranched polyamide (HBP) via the Schiff base. In order to increase the water resistance, glucose was introduced for secondary cross-linking to obtain DAS-HBP-G adhesive with a double cross-linked structure. The cross-linking mechanism of glucose and DAS-HBP was investigated by FTIR, XPS, LC-MS, and 13C NMR. Glucose was oxidized to various aldehyde structures under heating conditions to provide sufficient reactive groups for secondary cross-linking. The introduction of glucose not only reduced the curing temperature of the adhesive and enhanced its thermal stability but, most importantly, substantially enhanced its water resistance. The wet strength of DAS-HBP was 0.72 and 0.67 MPa, respectively, while DAS-HBP-G reached 1.26 and 1.15 MPa. The excellent water resistance is attributed to the synergistic effects of the internal hyperbranched structure of the adhesive, hydrogen bonding, imide, and amide bonding. This double cross-linking strategy provides a potential approach for the development of water-resistant starch adhesives.