Preparation Of Aminated Dialdehyde Starch Comb-like Polymer And Enhancement Mechanism Of Soy-based Adhesives

Soy protein adhesive is the most promising adhesives to replace"aldehydes"resin in the preparation of wood-based panels because of its wide source,non-toxic,harmless,green and renewable advantages.At present,soy protein adhesives are modified by crosslinking structure design and chemical modification of protein,which can effectively provide water resistance of adhesives.However,the shortcomings of crosslinking modified soy protein adhesive,such as poor initial viscosity and brittle adhesive layer after curing leading to poor bonding stability,which restrict its large-scale industrial application.In this study,soy flour（SF）was used as the main raw material,and the covalent cross-linking structure and hydrogen bond network synergistic strengthening system were constructed by using strong adhesion and flexible chain segments of comb-like polymer to enhance the coating performance,toughness and bonding stability of adhesive.At the same time,by designing and regulating the functional groups at the end of comb-like polymer,the bonding properties of the adhesive can be regulated.To study the synthesis and characterization of amino dialdehyde starch comb-like polymer and its influence on adhesive strength,adhesive toughness,microstructure,coating performance and mildew resistance of soy protein adhesive.To analyze the synthesis regulation mechanism of comb-like polymer and reveal the action mechanism of comb-like polymer to enhance soy protein adhesive.It provides technical and theoretical support for the development of soy protein adhesives with high water resistance and durability.The main conclusions are as follows:（1）A comb-like polymer was designed as follows:a long-chain polyamide polyamine compound（PPC）was synthesized and grafted onto dialdehyde starch via a Schiff-based reaction to generate a water-soluble comb-like aminated dialdehyde starch（A-DAS）and the A-DAS and SPI were then mixed combined with epoxy crosslinking agent to fabricate a soy protein adhesives.The covalent cross-linking structure as the first network and the hydrogen bond as the second network in the adhesive system were constructed to realize the toughening and strengthening of soy protein adhesive.The dry/wet bonding strength of SPI/ADAS adhesive is increased to 2.77 MPa/1.36 MPa,respectively,which is 174.3%and119.4%higher than that of SPI adhesive.A large number of hydrogen bonds were formed between the end rich amino groups of comb-like polymer and the wood surface to improve the initial adhesion of the adhesive.The coating performance of the adhesive is significantly improved and can be evenly dispersed on the wood surface.The presence of Schiff-base bond in comb-like polymer endows the adhesive with mildew resistance and prolongated the storage life of the adhesive.No mildew occurred in SPI/ADAS adhesive after 24 days in the airtight condition.SPI-based composite was prepared by ADAS as a model to study its toughening effect.Compared with SPI/G composite,the tensile strength of the SPI/ADAS composite dramatically increased by 1072.2%to 25.09 MPa.The existence of the imine group of Schiff-base compounds endows the composite with excellent UV resistance,indicating that the improved anti-aging performance of the prepared composite.（2）The comb-like polymer ADAS was epoxidized by using epoxy groups to prepare the comb-like polymer with epoxy terminal group（ADEH）.ADEH was used as a crosslinking agent to react with protein molecules and form a stable crosslinking structure.By utilizing flexible chain segment and branched chain structure of comb-like polymer,the internal stress during curing of adhesive layer is dispersed to achieve toughening and strengthening of adhesive.The adhesion structure of the comb-like polymer forms a firm bond with the substrate through strong hydrogen bond,thus improving the coating performance of the adhesive and preloading performance of the plywood.Compared with SF adhesive,the solid content of SF/ADEH adhesive increased by 27.27%,the dry bonding strength and wet bonding strength increased to 1.85 MPa and 1.02 MPa,the preloading strength increased from 0.57 MPa to 1.16 MPa,and the preloading performance of the adhesive was significantly improved.（3）Inspired by the strong adhesion composition of mussel protein and the adhesion structure of gecko toes,catecheol group was introduced into the terminal of comb-like polymer ADAS to construct the comb-like polymer ADD with biomimetic catecheol terminal group to provide a strong hydrogen bond binding site.The structural adhesion of comb-like polymer could further improve the adhesion performance of adhesive.A soy flour adhesive with high strength,high toughness,uniform coating and mildew resistance was prepared by mixing ADD with epoxy compound GTE and soy flour Compared with SF adhesive,the solid content of modified SF/GTE/ADD-0.5%adhesive increased to 30.96%,the residue rate increased to 68.5%,and the hygroscopicity decreased by 34.46%to 27%.The coating performance of the prepared adhesive was significantly improved,the adhesive was evenly distributed and the residual amount of cots was significantly reduced after coating.When 0.5%ADD and 1.5%GTE were added to the modified SF adhesive,the dry and wet bonding strength of the prepared plywood increased from 1.45 MPa/0.4 MPa to 2.27 MPa and 1.26 MPa,respectively,increased by 56.6%and 215%.In addition,the antibacterial performance of Schiff-base structure in ADD made the mildew resistance of the modified adhesive reach 96 h in the exposure condition,which is 4 times higher than that of SF adhesive.（4）Prompted by the high strength derived from the biomineralized structure of oysters,Ca₃（PO₄）₂was sedimented on soy protein and combined with the comb-like polymer to construct a biomineralized structure,which endows the resultant adhesive with enhanced coating performance and flame retardancy while maintaining high bond strength and toughness.The dry and wet shear strengths of the bonded plywood by the resultant adhesive increased by 118.8%and 750%relative to those of the unmodified adhesive,to 2.21 MPa and 1.02 MPa,meeting the type II plywood standard.The initial adhesion of modified adhesive significantly improved by the adhesion performance of comb-like polymer structure and a large number of strong hydrogen bond binding sites provided by the end group:the veneer adhesive line in the surface was closely continuous,the coating was evenly distributed,and the cots have no glue residue.The comb-like polymer and organic-inorganic hybrid structure significantly improved the toughness of adhesive layer,and the work of adhesion increased from 0.31 J to 4.05 J,increased by1206.5%.Meanwhile,the organic-inorganic hybrid structure formed heat-insulating carbon layer through the synergistic effect of nitrogen and phosphorus groups,so that the flame retardant performance of the modified adhesive reached LOI standard B₁（GB 8624-2012）.Compared with commercial adhesives,the total VOCs emission of adhesives decreased by 75%.Due to the antibacterial properties of Schiff-base bond in the adhesive system,the mildew resistance of the adhesive in the exposure condition was increased to 7 days,which significantly improved the application period of the adhesive.