Engineered wood-based products including plywood, fiberboard, and particleboard have been largely used in interior furnishing and construction fields, which promotes the rapidly increased consumption of wood adhesives. Formaldehyde-based adhesives such as urea formaldehyde and phenol formaldehyde are the most widely used wood adhesives because of their low cost, excellent bonding performance, and water resistance. Many biomass materials including cellulose, tannin, and lignin have been utilized as main components for developing biobased adhesives, but they mostly show high cost, week bonding strength, and poor water resistance. Plant oils are composed of triglycerides that are intrinsically water resistance, and thus are perfect materials for preparing water-resistant adhesives. As one of the main commercially available plant oil derivatives, Acrylated Epoxidized Soybean oil (AESO) is derived from soybean oil via an epoxidization and acrylation process. AESO possesses the advantages of abundance, water resistance, and environmental friendliness. The development of AESO-based wood adhesives is a novel approach to reduce the consumption of aldehyde-based adhesives. AESO contains carbon-carbon double bonds and hydroxyl groups as well as epoxy groups. Thus, Hexamethylene Diisocyanate (HDI) was introduced as a modifier and tert-butyl Peroxybenzoate (TBPB) was used as a free-radical initiator to prepare the formaldehyde-free AESO wood adhesives and their poplar plywoods. The chemical structure, rheological and curing behaviors, water-resistance, and bonding mechanism of the modified AESO adhesives and their effects on the physical and mechanical properties of the plywoods were investigated. FTIR and NMR analyses confirmed that HDI could react with both the hydroxyl and epoxy groups of AESO to form carbamate structure, hence leading to improved crosslinking density of the adhesives and interfacial bonding between wood and the adhesives. Due to the improved cohesion and bonding force of the adhesives, and the water-resistant bonding strength of poplar plywood was up to 1.18 MPa, which meets the requirement of Chinese standard. The addition of HDI significantly reduced the viscosity and the maximum curing temperature of AESO resins, and improved the processibility of the adhesives. The HDI modification significantly increased the physical and mechanical properties of the plywoods (P<0.05). The bonding strength, flexural strength, elastic modulus, and water resistance of the plywoods significantly increased with the increased mass ratio of HDI to AESO from 0.5:9.5 to 1:9, and then leveled off as the further increased HDI usage. The optimum mass ratio of HDI to AESO was considered as 1:9, in which the transverse flexural strength and elastic modulus of the plywood reached 57.6 MPa and 5 920 MPa, respectively, and the plywood did not crack at 120 °C for 2 h. SEM images indicated that the HDI modification significantly increased the interfacial adhesion between wood and adhesives due to the formation of covalent bonds. The work provides a feasible way to prepare formaldehyde-free adhesives by using biomass soybean oil as raw materials, which is beneficial for the green and sustainable development of wood industry. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.
