Cocuring of Epoxidized Soybean Oil-Based Wood Adhesives and the Enhanced Bonding Performance by Plasma Treatment of Wood Surfaces

Application of epoxidized soybean oil (ESO) in the wood industry as an adhesive has been limited due to the difficulty in fast curing ESO. Due to the advantages of the urea-formaldehyde (UF) resin, like low cost, high molecular weight, rich functional groups, as well as fast self-curing property, fast cocuring ESO-UF adhesives were prepared by mixing UF with ESO in different ratios (15–50%). Bonding performance tests indicated that the ESO-UF adhesives well balanced bonding strengths and the formaldehyde emission level; especially, the adhesives provided satisfactory water resistance. Structural characterization of cured ESO-UF adhesives showed that a copolymerization reaction occurred between epoxy groups of ESO and the hydroxyl groups of UF. Combined with gliding arc discharge (GAD) plasma treatment of the veneer surface, better bonding performances were achieved. X-ray photoelectron spectroscopy (XPS) analysis on the wood surfaces showed that the GAD treatment significantly increased the content of oxygen-containing groups like −OH and −COOH through oxidation and degradation of the chemical components on the wood surface. The generated reactive groups on the wood surface participated in the curing reaction of ESO-UF adhesives and led to improved bonding performances. Thus, the ESO-UF-activated wood surface (ESO-UF-AWS) system was developed in this study. The results of this study have demonstrated that the combination of seeking an efficient cocuring agent along with activating the wood surface is a valid strategy for realizing applications of ESO in the wood product industry.