Soy protein has shown potential as a renewable and environmentally friendly adhesive because of its superior performance and affordability compared with urea formaldehyde-based adhesives, but poor water resistance has limited its application as a high-performance wood adhesive comparable to phenol formaldehyde. This work focused on developing and characterizing undecylenic acid (UA)-modified soy proteins to improve their water resistance. The reaction between amine groups from protein and carboxyl groups from UA was proposed to be the main chemical pathway for grafting, which was proven by Fourier transform infrared spectroscopy (FTIR) and a ninhydrin test. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showed that UA modification led to reduced thermal stability because of protein unfolding and decreased protein-protein cross-linkages. The increased attractive force between carbon chains of UA and protein residues resulted in higher viscosity and dynamic modulus values. Atomic force microscopy (AFM) images showed changes in protein particle size and surface properties. The wet strengths of modified soy protein adhesives were significantly improved by 35–62% compared with the control. UA with hydrophobic carbon chains and reactive carboxyl groups is an ideal bio-based modifier for soy proteins.
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