Abstract: Soy-based adhesive (S) was prepared by use of defatted soy flour as feedstock, and new phenol formaldehyde resins were synthesized by using common formaldehyde and high concentration formaldehyde (PF₁, PF₂) as crosslinker agent.S and PF₁, or PF₂ were then directly blended to form the modified S.The structure and the performance of these adhesives were subsequently characterized by differential scanning calorimetry (DSC), infrared spectrum (FT-IR), dynamic thermal mechanical properties (DMA) and nuclear magnetic resonance (¹³C NMR), respectively.It was found that with the same molar ratio, plywood of S crosslined by PF₂ showed better bonding performance and strength stability.Their dry and wet strengths increased by nearly 4.3% and 11.6% compared with those crosslined by PF₁, respectively.Dynamic DSC analysis showed that PF₂ was easy to react with S by decreasing the activation energy of adhesives system.¹³C NMR showed that the hydroxymethyl content PF₂ system was 88.73%, which was significantly higher than the 80.91% of PF₁.FT-IR analysis confirmed that the phenol formaldehyde resin reacted with amino of S, and PF₂ showed much more efficient than PF₁.The DMA results showed that PF₂ could improve the mechanical performance and thermal stability of the final product, reduce the onset curing temperature and curing temperature, and accelerate the curing reaction rate of modified S.

Key words: high concentration formaldehyde, phenol formaldehyde resin, soy-based adhesive, curing performance