Study On Strengthening And Functionalization Of Soybean Adhesive Based On Branched Polyamide

Wood-based panel is a model of efficient utilization of wood.More than 90%of wood-based panel adhesives are aldehydes resin adhesives,which will release harmful gases such as formaldehyde in the process of production and application,endangering human health.Aldehyde-free soy protein adhesives are the most promising wood adhesives to replace aldehyde-free resin in large-scale preparation of wood-based panels.However,it is also a challenge to improve the toughness,coating,precompression,mildew resistance and antibacterial properties of soybean protein adhesives while ensuring their high water resistant bonding properties.In addition,the research on the flame retardant and electromagnetic shielding properties of plywood is the key content of high value-added applications of wood-based panels.In this study,branched polyamide polymers were designed and synthesized to enhance soybean protein adhesives,improve the properties of existing adhesives and realize the functionalization of plywood.Through the study on the construction and regulation of water-resistant structure of branched polyamide of soybean protein adhesive,the influence of water-resistant structure construction of branched polyamide on the performance of soybean adhesive and its strengthening mechanism were understood,so as to realize the enhancement of adhesion and toughness of soybean protein adhesive,mildew proof and antibacterial modification and preparation of flame retardant plywood.Electromagnetic shielding and other functions provide theoretical basis and technical support for enhanced modification of vegetable protein adhesives.The main research contents and conclusions are as follows:（1）Hyperbranched polyamide（HBPA）was synthesized from diethylenetriamine（DETA）and succinic anhydride（SA）by melting polycondensation process.HBPA and benzaldehyde reacted with dynamic Schiff base to synthesize HBPA-TPA prepolymer.Soybean protein adhesive was introduced and TGA was used to prepare high strength and toughness soybean protein adhesive.Due to the construction of crosslinking network structure between HBPA-TPA prepolymer TGA and SPI,the crosslinking density of adhesive is improved and the water-resistant bonding strength is increased by 71.2%,which meets the requirements of national plywood standard.In addition,a large number of terminal amino groups in HBPA-TPA prepolymer formed strong hydrogen bonds with wood,which improved the interface bonding,and the dry bonding strength of adhesive increased by 69.2%.The preparation of adhesive film was used as the model to analyze the toughening and strengthening mechanism.The construction of branched polyamide crosslinking structure reduced the crystallinity of adhesive film,improved the thermal stability,and presented ductile fracture in section shape.The tensile strength,toughness and elongation at break of hb PA-TPA prepolymer were increased to 49.07 MPa,28.36 MJ/m³and 186.67%,which were attributed to the fact that the branched structure of HBPA-TPA prepolymer contained a large number of cavities and a large number of terminal amino groups,which made the molecular chain segment soft.At the same time,the C=N double bond rearranged under heat,releasing internal stress and improving the toughness of the film（2）Using polyethylene imine（PEI）and carboxymethyl chitosan（CCTS）as raw materials,hyperbranched polyamide-carboxymethyl chitosan functionalized polyethylene imine（HPFC）was synthesized by one-pot Schiff base reaction.Instead of traditional epoxy crosslinking agent,2,3-butadione（BD）was combined with HPFC to prepare high performance soy protein adhesive based on Maillard reaction.The insolubility fraction of synthetic adhesive increased by 16.6%,and the swelling rate of water absorption decreased by 90.6%The wet bonding strength increased by 55%and 357%,respectively,and the wood breakage rate of plywood specimens increased from 0%to 70%.The coating performance of adhesive on wood surface was significantly improved,which was attributed to the cross-linking network structure formed by BD,SM and HPFC through Maillard reaction,and the interface bonding strength between adhesive and wood was improved.In addition,the synergistic effect of C=N bond and chitosan made the adhesive have excellent anti-mildew and antibacterial properties,and the storage time was extended from 24 h to 240 h.（3）Inspired by the organic-inorganic hybrid structure of oysters,polyethylene imine（PEI）was used to coat the surface of nano-silica（Si O₂）through electrostatic interaction,and then the branched polyamide was synthesized by grafting 3,4-hydroxybenzaldehyde（DBA）onto branched polyamide.Soybean protein hybrid adhesive was prepared by combining multifunctional crosslinker-Hexachlorocyclotriphosphazene（HCCP）and soybean protein isolate（SPI）.The swelling rate of soybean protein hybrid adhesive was reduced by 66.74%and the insolubility fraction was increased by 266%.The dry and wet bonding strength of the prepared plywood was increased by 110%（2.60 MPa）and 143%（1.46 MPa）,the adhesive precompression strength was increased by 304%,and the coating performance was significantly improved.This is attributed to the stable organic-inorganic hybrid crosslinking structure formed by HCCP as a crosslinking agent with SPI and DP@Si O₂.The introduction of catecheol structure improves the interfacial bonding strength between adhesive and plywood.In addition,the limit oxygen index（LOI）of adhesive is 39.4%due to the presence of HCCP and DP@Si O₂.（4）Quaternary ammonium salt hyperbranched polyamide（QHBPA）antibacterial agent was synthesized from long-chain quaternary ammonium salt 2,3-epoxy propyl trimethylammonium chloride（EPTAC）and hyperbranched polya mide（QHBPA）.Graphene nanosheets（GNSs）were compounded with QHBPA through cation-πinteraction to prepare G-co-Q hybrid.Phytic acid（PA）was introduced to replace the traditional epoxy crosslinking agent and mixed with SPI and G-co-Q hybrid to prepare branched polyamide non-covalent crosslinking hybrid adhesive.The dry and wet bonding strength of plywood is increased by 76.6%and 78.7%respectively,and the wood breaking rate of plywood was increased from 0%to 80%.The cured film smooth,crack down significantly,toughness increase significantly.Thanks to PA as crosslinking agent,the SPI with G-co-Q hybrid mixed by electrostatic interactions,strong hydrogen bond and multiple non covalent interaction built crosslinking network,improve the adhesive with wood interface interaction,forming strong mechanical interlocking,improve the adhesive bonding performance.At the same time,due to the presence of QHBPA,no mildew appeared after 144 h of the test,the antibacterial rate of E.coli,S.aureus,A.viscosis and C.albicans was up to99.99%99.93%99.77%and 99.72%,respectively.Due to the presence of PA and G-co-Q hybrid,the LOI of adhesive is 35.5%.In addition,due to the addition of GNSs,plywood has excellent electromagnetic shielding function.