Enhanced properties of a soybean adhesive by modification with a cycloaliphatic epoxy resin

Soybean-based wood adhesives have good application prospects, but their low water resistance limits their application in plywood manufacturing. In this study, cycloaliphatic epoxy resin 4,5-epoxycyclohexane-1,2-dicarboxylic acid diglycidyl ester (TDE-85) was used as a modifier to prepare a highly water-resistant soybean adhesive. When TDE-85 was added in an amount of only 2.5 wt%, the wet shear strength of three-layer plywood pressed by the soybean adhesive increased from 0.32 MPa to 0.79 MPa, reaching the national standard for class II plywood GB/T 9846–2015. A denser fracture surface was observed with the modified adhesive than without the adhesive using scanning electron microscopy (SEM). This result was thought to be due to the participation of glycidyl ester epoxy groups and alicyclic epoxy groups in the curing reaction of soybean adhesive, as suggested by Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC). By further comparing the modification effects of TDE-85 and trimethylolpropane triglycidyl ether (THPTG), it was deduced that the special epoxy group and alicyclic structure in TDE-85 may be another important factor associated with its better modification effect, thus offering new suggestions for soybean-based wood adhesives modification.