Construction Of High-Performance Gel System For Soy Protein Adhesive With Excellent Cold-Pressing Adhesion

Developing green formaldehyde-free adhesives is an effective way to alleviate the shortage of petrochemical resources and solve the problem of formaldehyde release from aldehyde-based adhesives.Soy protein（SP）adhesive has the advantages of low price and easy access,and is an effective substitute for aldehyde adhesives.The water resistance and bonding strength of SP adhesives can be significantly improved by adding highly active crosslinking agents,but their practical production is still limited by poor cold-pressing bonding performance.However,the chemical crosslinking agents are expensive,and the brittleness of the adhesive layer after chemical crosslinking increases.These problems hinder the practical application of SP adhesives.Adhesive hydrogels have excellent adhesion to various substrates.The introduction of gel system into SP adhesives is expected to improve the cold-pressing bonding strength of SP adhesives.Therefore,based on the gelation mechanism under the radical polymerization of vinyl monomers and the coacervation effect induced by cation-πinteraction,the in situ gelation of soy protein adhesive was successfully achieved in this study.The gel skeleton formed by the free radical polymerization of vinyl monomers effectively crosslinked SP,and the strong cation-πinteraction effectively"tackified"the adhesive system,which made the prepared SP adhesive present excellent cold-pressing adhesion performance.At the same time,based on the construction of the gel system,a new chemical cross-linking agent was developed to reduce the cost of the cross-linking agent,and fiber-reinforced system was constructed to improve the toughness of the adhesive layer.The interaction mechanism between the gel skeleton and SP was investigated,and the influence of the gel system on the chemical structure,microscopic morphology,cold-pressing bonding performance,and water-resistant bonding performance of the SP adhesive was investigated.The relationship between the structure and the peroformance of the adhesive was built to clarify the internal mechanism of the gelation modification of SP.The main conclusions of the study are as follows:（1）Based on the hydrolysis exothermic properties,the inorganic calcium sulfoaluminate（CSA）was used to promote the decomposition of persulfate to generate free radicals,further inducing the in-situ polymerization of vinyl monomer acrylic acid to realize the gelation of SP adhesive.The inorganic crystals produced by the hydrolysis of CSA not only helped to form a stable cross-linking system to achieve strong cohesion,but also improved the interfacial bonding strength between the adhesive layer and the plywood veneer through physical anchoring.The cold-pressing bonding strength of the modified SP adhesive could reach 0.54 MPa,which was significantly higher than that of the pure SP adhesive（0.02 MPa）.At the same time,the water-resistant adhesive strength of the modified SP adhesive could reach 1.21 MPa.（2）The redox reaction between tannic acid and Fe³⁺was used to promote the decomposition of ammonium persulfate to generate free radicals,triggering the free radical polymerization of acrylamide,and further realizing the gelation of SP adhesive.The dynamic cross-linking system induced by the tannic acid-Fe³⁺complex enhanced the cohesion of the adhesive;the topological entanglement of the gel skeleton in the veneer realized the tight interfacial bonding between the adhesive layer and the veneer.The cold-pressing bonding strength of the modified SP adhesive could reach 0.65 MPa,which was significantly improved compared with the pure SP adhesive.At the same time,the water-resistant bonding strength could reach 1.28 MPa.（3）The highly reactive polyamidoamine-epichlorohydrin and folic acid were added to SP to construct a“double network”system stabilized by strong cation-πinteractions,and the coacervation produced by polyamidoamine-epichlorohydrin and folic acid acted as an“internal adhesive”to adhere the components of the SP together,and the prepared adhesive had a high initial viscosity but was easy to sizing.The cold-pressing bonding strength of the modified adhesive was increased to 0.42 MPa,which was 295%higher than that of the pure SP adhesive.Inorganic calcium carbonate particles were introduced into the adhesive system to construct an organic-inorganic hybrid adhesive system to improve the water-resistant adhesive strength of the adhesive.The water-resistant bonding strength was increased from 0.63 MPa to 0.96 MPa.（4）Waterborne epoxy emulsion was prepared by grafting vinyl monomers such as methacrylic acid onto the molecular chain of epoxy resin through radical polymerization,and the prepared waterborne epoxy emulsion was then used to modify SP adhesive.The grafting monomer induced multiple non-covalent interactions with SP,which increased the viscosity of the modified SP adhesive from108.7 Pa·s to 456.3 Pa·s,significantly improving the cold-pressing adhesion performance.Benefited from the chemical cross-linking effect of epoxy groups,the prepared SP adhesive presented excellent water-resistance bonding performance,and the water-resistance bonding strength could reach 1.14 MPa.（5）Mussel-inspired surface functional modification was used to improve the surface activity of silk fibers.Then,the catechol derivative-coated silk fibers and the cross-linking agent were added to the SP.Based on the chemical crosslinking effect of the crosslinking agent and the silk fiber-enhanced“reinforced concrete”structure,the adhesive exhibited excellent water-resistant bonding strength and toughness,and the water-resistant bonding strength could reach 1.50 MPa.