Application of a self-crosslinking strategy based on oxidization modification for the preparation of soy-based adhesive

Crosslinking modification is a key to acquiring soy protein-based adhesives with super performances, for which external crosslinkers have been adopted in most of the present studies. Carbohydrates are the important causes restricting the water resistance of soy protein-based adhesives. In this study, defatted soy flour was treated using different oxidizing agents, which mainly acted upon carbohydrates, specifically, converted these side-effect carbohydrates into components with the crosslinking action (internal crosslinking agents) to facilitate the crosslinking reaction with soy proteins, thereby preparing self-crosslinking soy protein-based adhesives. The bonding strength and water resistance of the prepared plywood were mainly analyzed, and the performances of adhesives were also analyzed via differential scanning calorimetry (DSC), Fourier transform-infrared spectrometry (FT-IR), thermogravimetry (TG) and X-ray diffraction (XRD). The results showed that: (1) The oxidizing agents presented not strong oxidizing properties, resulting by the low content of nonvolatile aldehyde and large molecular weight and steric hindrance of reactions for the oxidation products. The bonding performance of soy protein-based adhesives prepared through the oxidization of H2O2, (NH4)2S2O8 and NH4NO3 was not obviously improved. (2) Both the bonding strength and water resistance of soy protein-based adhesives prepared through the oxidization of NaIO4 and KMnO4 met requirements for Class II plywood specified in GB/T 17657–2013. (3) The oxidization of KMnO4 is a nonselective oxidization, with great variability in the degree of oxidization and degradation. The oxidization of NaIO4 was partially selective, and the products were mostly poly-aldehydes, which were more beneficial to the construction of the spatial network structure for adhesives, with the better corresponding bonding strength and water resistance.