Effects of Lysine on the Interfacial Bonding of Epoxy Resin Cross-Linked Soy-Based Wood Adhesive

Abstract

Soy protein isolate (SPI) is an attractive natural material for preparing wood adhesives that has found broad application. However, poor mechanical properties and unfavorable water resistance of wood composites with SPI adhesive bonds limit its more extensive utilization. The combination of lysine (Lys) with a small molecular structure as a curing agent for modified soy-based wood adhesive allows Lys to penetrate wood pores easily and can result in better mechanical strength of soy protein-based composites, leading to the formation of strong chemical bonds between the amino acid and wood interface. Scanning electron microscopy (SEM) results showed that the degree of penetration of the S/G/L-9% adhesive into the wood was significantly increased, the voids, such as ducts of wood at the bonding interface, were filled, and the interfacial bonding ability of the plywood was enhanced. Compared with the pure SPI adhesive, the corresponding wood breakage rate was boosted to 84%. The wet shear strength of the modified SPI adhesive was 0.64 MPa. When Lys and glycerol epoxy resin (GER) were added, the wet shear strength of plywood prepared by the S/G/L-9% adhesive reached 1.22 MPa, which increased by 29.8% compared with only GER (0.94 MPa). Furthermore, the resultant SPI adhesive displayed excellent thermostability. Water resistance of S/G/L-9% adhesive was further enhanced with respect to pure SPI and S/GER adhesives through curing with 9% Lys. In addition, this work provides a new and feasible strategy for the development and application of manufacturing low-cost, and renewable biobased adhesives with excellent mechanical properties, a promising alternative to traditional formaldehyde-free adhesives in the wood industry.

Waterborne Lignin-Based Epoxy Resin: A Green and Effective Chemical Cross-Linker to Form Soy Protein-Based Wood Adhesive with High Performance

“…When the WLEP content reached 30 wt %, the interfacial interaction between the wood and adhesive was high, many wood fibers were pulled out at the bonding interface, and 90% of the wood fractures were observed at the wood surface . These results verified that the bonding performance and water tolerance of the SPI adhesive were enhanced effectively by introducing WLEP into the system. , …”

“…61 These results verified that the bonding performance and water tolerance of the SPI adhesive were enhanced effectively by introducing WLEP into the system. 51,62 It can be explained as follows: the multiple interactions between SPI, WLEP, PA, and hydroxyl groups of the veneer led to the formation of strong covalent bonds and a complex network of hydrogen bonds (Figure 6b), which resulted in a compacted adhesive structure, thus improving their moisture resistance, cohesion strength, and interfacial connection with the wood surface. 63 Furthermore, the presence of PEG within WLEPs introduced numerous sites to form hydrogen bonds, offering both stress conduction and dissipation to enhance the mechanical properties of the adhesive.…”