Study On The Construction Of Multiple Network Structure Of Soybean Protein Adhesive And Strengthening Mechanism

Traditional formaldehyde-based adhesives have problems such as release of formaldehyde,environmental pollution,endangering human health,and dependence on petrochemical resources.Soybean protein adhesives are the most promising alternative to formaldehyde-based adhesives because of their green,non-toxic and sustainable features.However,the disadvantages of soy protein adhesives,such as poor water resistance,poor toughness,and easy mildew,hinder their industrial applications.Therefore,it is an important challenge for the wood adhesive industry to develop a soybean protein adhesive with high water-resistant bonding strength,toughness,mildew resistance and flame retardancy to replace formaldehyde-based adhesives.In this study,based on the theory of multiple network structure enhancement,the water resistance,toughness,mildew resistance and flame retardancy of soybean protein adhesives were improved by constructing multiple network structures such as hydrogen bond,covalent bond,and ionic bond in soybean protein adhesives.The influence of the construction of multiple network structures on the performance of soybean protein adhesives and the enhancement mechanism were studied,and the effect of multiple network structures on the properties of soybean protein adhesives was clarified,which provided a theoretical basis for the enhancement and modification of soybean protein adhesive.The main conclusions are as follows:（1）The covalent-hydrogen bond double network structure was constructed in the soybean protein adhesive,and the self-synthesized crosslinking agent triglycidylamine（TGA）reacted with soybean protein（SPI）to form a covalent bond network,which improved the strength and water resistance of the adhesive;polyvinyl alcohol（PVA）interacted with the active groups on the soybean protein to form a hydrogen bond network,which absorbed energy as a sacrificial bond during the fracture process,thereby improving the toughness of the adhesive.Compared with the SPI adhesive,the dry/wet shear strength of the SPI/TGA/PVA adhesive was increased by 54.4%and 71.4%,respectively,and the residual rate was increased by 19.3%.To characterize the toughness of the adhesive,soybean protein-based films were prepared.Compared with the SPI/TGA film,the tensile strength of the SPI/PVA/TGA film was increased by 90.5%to 10.21 MPa,the elongation at break was increased by 163%to 421%.（2）Inspired by the structure of nacre,a soft-hard dual network structure was constructed in soybean protein adhesive.Cross-linking agent（TGA）cross-linked with soybean protein to form a hard-phase network structure as a skeleton structure,providing strength and water resistance for adhesives;epoxidized rubber（ER）reacted with soybean protein to form a soft-phase cross-linked network structure,which improved the toughness of the adhesive by dissipating energy when the adhesive broke.Compared with the SPI adhesive,the residual rate of SPI/4%TGA/3%ER adhesive was increased by 7.9%,and the moisture absorption rate was decreased by 16.7%,indicating that the water resistance of the adhesive was improved;the dry/wet shear strengths of plywood prepared with SPI/4%TGA/3%ER adhesive increased by 58.9%and 160.4%to 1.86 MPa and 1.38 MPa,respectively,indicating that the adhesive performance was improved;elongation at break increased by 55.2%to 1.86 MPa and 45%to 1.38 MPa,respectively;the toughness increased by 233%,and the surface cracks of the adhesive disappeared,indicating that the toughness of the adhesive was improved.（3）The triple network structure of covalent bond,hydrogen bond and ionic bond was constructed in soybean protein adhesive.The cross-linking agent PTGE reacted with the active groups in the enzymatically hydrolyzed soybean protein peptides to recombine the soybean protein molecules to form a covalent bond network.The covalent bond network provided the water resistance of the adhesive as a backbone structure.The phenolic hydroxyl groups on the tannic acid interacted with the active groups of soybean protein to form a hydrogen bond network;in addition,Zn²⁺chelated with the catechol/pyrogallol in the tannic acid to form an ionic bond network,thereby forming a dynamic network,the dynamic network consumed energy during the fracture process,which improved the toughness of the adhesive.Compared with the SPI adhesive,the viscosity of the triple network adhesive was reduced by 94.3%to9952 m Pa·s,and the dry and wet shear strengths were increased by 33.9%（2.57 m Pa）and 116%（1.36m Pa）,respectively;the fracture strain of the sample increased by 36.4%;the mildew resistance of the adhesive increased from 1 d to more than 15 d.（4）Inspired by the cuticle of arthropods,this study used epoxide cross-linked soybean protein isolate to form a covalent bond network,and synthetic dopamine-functionalized waste paper fibers（WPF-PDA）as catechol group donor for hydrogen bonding network formation and physical enhancement,copper hydroxide as an inorganic material chelated with the catechol group of WPF-PDA to form an ionic bond network,thereby forming an organic-inorganic hybrid multiple bonding structure in soybean protein adhesives.Compared with SPI adhesive,the dry/wet shear strength of synthetic adhesive was increased by 46%and 144%,reaching 2.0 MPa and 1.22 MPa,respectively;the toughness was increased by 145%;the residual rate of the adhesive increased by 18.6%,indicating that the adhesive strength,toughness and water resistance were improved.In addition,the mildew resistance time of the adhesive increased from 1d to more than 40 d;the maximum heat release rate and total heat release decreased by 32.5%and 5.8%,respectively,indicating that the mildew resistance and flame retardancy of the modified adhesive were also improved.