Nacre‐inspired construction of soft–hard double network structure to prepare strong, tough, and water‐resistant soy protein adhesive

Efficient fabrication of cellulose polymer networks via alkaline swelling strategy for wood bonding

Existing issues with bio-based adhesives, such as complex preparation processes, high energy consumption, and production costs, still need to be addressed. In our study, APTES was grafted onto microcrystalline cellulose (MCC) to generate active aminated cellulose, and then reacted with the epoxide group in glycerol triglycidyl ether (GTE) through a swelling strategy under alkaline solvent, forming a network structure via covalent cross-linking. The adhesive exhibits superior bonding performance and water-resistant property in the bonding strength test of poplar plywood, with a dry shear strength of 2.40 MPa, a wet shear strength of 2.16 MPa after soaking in 63 °C hot water, and a wet shear strength of 1.79 MPa after soaking in boiling water. In terms of cost calculation, the theoretical production cost of AC-GTE adhesive is calculated to be 5303.7 RMB per ton, which is comparable to that of phenol-formaldehyde (PF) resin and other petrochemical-based adhesives, and significantly lower than that of isocyanate-based adhesives. These research results can provide a practical example for producing high-efficiency, aldehyde-free, and low-cost bio-based adhesives.

Re-Assemblable, Recyclable, and Self-Healing Epoxy Resin Adhesive Based on Dynamic Boronic Esters

Thermosetting adhesives are commonly utilized in various applications. However, covalent cross-linked networks prevent thermosetting adhesives from being re-assembled, which necessitates higher machining precision. Additionally, the primary raw materials used in adhesive preparation are derived from non-renewable petroleum resources, which further constrain adhesive development. In this study, a recyclable adhesive was developed by incorporating dynamic boronic esters into epoxy resin derived from soybean oil. The successful synthesis of epoxidized soybean oil and boronic esters was confirmed through the analysis of proton nuclear magnetic resonance spectra and differential scanning calorimetry results. Swelling tests and tensile curves demonstrated the presence of covalently cross-linked networks. Self-healing and reprocessing experiments indicated that the cross-linked network topology could be re-assembled under mild conditions.

Bioinspired Natural Magnolol-Based Adhesive with Strong Adhesion and Antibacterial Properties for Application in Wet and Dry Environments

The development of environmentally friendly, green, and nontoxic adhesives with excellent dry and wet adhesion properties is of great attraction. In nature, barnacles and mussels exhibit strong adhesion by secreting a hydroxyl-rich dopa. Inspired by their adhesion mechanism, a simple biobased MAG-PETMP (MP) adhesive was prepared from magnolol (MAG) and pentaerythritol tetra (3-mercaptopropionate) (PETMP) by a thiol-ene click chemistry reaction. MP as an adhesive exhibits high bond strength with other substrates due to hydrogen bonds formed by the abundant hydroxyl groups at the interface and shows an inherent thermosetting network structure. Since MP has a thermosetting network, it exhibits excellent thermal stability, solvent resistance, and high mechanical strength, which make the adhesive stable in a humid environment. The cross-linking degree of MP can be easily controlled by adjusting the molar ratio of MAG and PETMP. Among the synthesized samples, the elongation at break of the MP 1 formulation is 174.27%, which makes it promising for use as a flexible adhesive. Moreover, the inherent antibacterial properties of MAG enable MP to exhibit antimicrobial properties and antibacterial adhesion to some extent. This work provides a simple biomimetic strategy that could enable the application of MAG for adhesives.

Preparation and Characterization of Soybean Protein Adhesives Modified with an Environmental-Friendly Tannin-Based Resin

Soybean protein-based adhesives are limited in their application due to their poor wet bonding strength and poor water resistance. Herein, we prepared a novel, environmentally friendly soybean protein-based adhesive by adding tannin-based resin (TR) to improve the performance of water resistance and wet bonding strength. The active sites of TR reacted with the soybean protein and its functional groups and formed strong cross-linked network structures, which improved the cross-link density of the adhesives and then improved the water resistance. The residual rate increased to 81.06% when 20 wt%TR was added, and the water resistance bonding strength reached 1.07 MPa, which fully met the Chinese national requirements for plywood (Class II, ≥0.7 MPa). SEM observations were performed on the fracture surfaces of all modified SPI adhesives after curing. The modified adhesive has a denser and smooth cross-section. Based on the TG and DTG plots, the thermal stability performance of the TR-modified SPI adhesive was improved when TR was added. The total weight loss of the adhesive decreased from 65.13% to 58.87%. This study provides a method for preparing low-cost and high-performance, environmentally friendly adhesives.

Improving Bond Performance and Reducing Cross-Linker Dosage of Soy Protein Adhesive via Hyper-Branched and Organic-Inorganic Hybrid Structures

Eco-friendly soybean protein adhesives could be an ideal substitute for replacing traditional formaldehyde-based adhesives in wood industry. However, a large number of cross-linking agents are required in soy protein adhesive formulations to obtain sufficiently performing properties. Inspired by the high performance of nacre and branched structures, a hyper-branched amine (HBPA) was synthesized and grafted to graphene oxide (GO), generating a hyper-branched amine-functionalized GO (FGO). A novel soy protein-based adhesive was developed by mixing FGO with soy protein (SPI) and a low dose polyamidoamine-epichlorohydrin (PAE). Results showed that the addition of only 0.4 wt% FGO and 0.75 wt% PAE to the SPI adhesive formulation enhanced the wet shear strength of plywood to 1.18 MPa, which was 181% higher than that of the adhesive without enhancement. The enhanced performance is attributed to the denser cross-linking structure and improved toughness of the adhesive layer. Using FGO in the adhesive formulation also greatly reduced the concentration of the additive cross-linker by up to 78.6% when compared with values reported in the literature. Thus, using a hyper-branched functionalized nano-material to form an organic-inorganic hybrid structure is an effective and efficient strategy to reinforce the composites and polymers. It significantly reduces the chemical additive levels, and is a practical way to develop a sustainable product.