Design Of Polyurethane Structure And Study On The Mechanism Of Strengthened And Toughened Soy Protein Adhesive

As the important part of wood processing industry,the development of high-performance aldehyde-free adhesives has gained a growing interest under the sustainable and eco-friendly concept.Among the family of“green”wood adhesives,the typical soy protein adhesives present great application potential.To improve the adhesion performances of soy protein adhesives,crosslinking modification,molecular modification,and biomimetic structure construction strategies have been employed to optimize the protein crosslinking structure.Nevertheless,despite the improvements in mechanical bonding performance,the highly rigid crosslinking characteristics of soy protein structure increases the stiff and brittle characterization of the adhesive layer thus leading to low impact resistance,which restricts its large-scale application in the wood-based panel industry.Taking advantages of the adjustable structural properties and unique microphase-separated structure,waterborne polyurethane(WPU)elastomer can effectively solve the defect of poor energy dissipation in rigid crosslinking structures,and presents significant improved effects for strengthening and toughening polymer materials.Based on the above background,this study employees WPU as the reinforcer to improve soy protein adhesives,in which the reactivity and structure of WPU was designed to induce protein molecules to construct strong yet tough multiple crosslinking systems thus achieving the significant improvement of comprehensive adhesion properties.The effect of the modified WPU enhancer on the crosslinked structure,bonding performance,microstructure,water resistance and toughness of the soy protein adhesive was studied,and the structure-activity relationship between the polyurethane elastomer structure and soy protein adhesives performances was also established.Meanwhile,its modification mechanism was explored and revealed on the basis of the above research.The main conclusions of the paper are as follows:(1)Mussel-inspired WPU crosslinker was synthesized by using dopamine as the end-capping agent,and employed to modify soy protein adhesives.It is found that the grafted dopamine unit not only improves the intermolecular interactions between polyurethane and protein molecules,but also optimizes the polyurethane-induced microphase-separated structure within protein matrix.Meanwhile,it serves as the water-resistant barrier to reduce the erosion effect of water on the crosslinking structure of the protein.As a result,the dry bonding strength,wet bonding strength,and adhesive layer toughness of the modified soy protein adhesive reach 2.08 MPa,1.31 MPa,and 12.04 MJ/m~3,which presents the increment of 70.5%,133.9%,and 141.8%compared to the unmodified protein adhesive,respectively.(2)The sodium bisulfite,as the blocking agent of isocyanate groups,cooperated with dopamine unit to prepare thermally-driven dual-functionality WPU crosslinker,aiming to optimize the reactivity of dopamine-functionalized WPU thus for further improving the protein crosslinking network.Under the curing condition of soy protein adhesive,the sodium bisulfite blocking agent is deblocked from the polyurethane molecule to expose the isocyanate groups,which serves as chemical crosslinking unit together with dopamine physical crosslinking unit to construct dual-crosslinking protein network.In this way,the crosslinking structure and microphase-separated structure of the soy protein adhesive provides an obvious optimization.And,there exhibits the increment of 124.1%,50.8%,171.9%and 166.4%in the residual rate,dry bonding strength,wet bonding strength,and work of adhesion of the soy protein adhesive,reaching 48.65%,3.03 MPa,1.74 MPa,and 5.94 J,respectively.(3)Polydopamine(PDA)-modified attapulgite nanofiller was anchored onto polyurethane molecule chains by means of in-situ polymerization process to endow WPU with dopamine chemistry property and rigid reinforcement effect.The intermolecular multiple interactions of physical and covalent bonding between WPU nanohybrid and protein are constructed,and the polyurethane cooperates with modified nanofiller to achieve a significant improvement in cohesive strength of the protein system.Compared to the control soy protein composite,the tensile strength and fracture toughness are increased by 145.6%and 118.3%,reaching 10.29 MPa and 15.5 MJ/m~3,respectively.(4)Based on the PDA chemical reaction platform,siloxane-functionalized polyurethane was“one-step”co-deposited onto the cellulose nanocrystals(CNC)surface for a core-shell polyurethane nanohybrid containing PDA adhesion effect and polysiloxane hydrophobic properties.The multiple-scale interfacial crosslinking system has been constructed between core-shell nanohybrid and protein molecules,and the robust and water-resistant protein crosslinking structure is formed within soy protein adhesives.As a result,the dry bonding strength,wet bonding strength,and work of adhesion of the modified soybean protein adhesives reach 4.48 MPa,2.10 MPa,and 8.37 J,which are 102.7%,311.8%,and 147.6%higher than the control adhesives,respectively.(5)Using tannic acid as polyol to prepare hyperbranched isocyanate-terminated polyurethane,the pre-crosslinking active polyurethane hybrid(PMEU)was subsequently prepared by in situ polymerizing polyurethane molecules onto polydopamine-modified microfibrillated cellulose with the epoxy monomer as the pre-crosslinking agent.The results show that PMEU polyurethane,as an independent modifier,successfully induces the soy protein molecules to construct an epoxy chemical crosslinking-dominated multi-level protein crosslinking system.Given on this effect,the crosslinked structure,bonding performance,fracture toughness,and water resistance of soy protein composites exhibit obvious improvement.With the introduction of PMEU,the tensile strength,fracture toughness,and contact angle of the modified protein composite are increased by 146.7%,102.1%,and 49.1%,respectively,in comparison with unmodified protein composite.Meanwhile,the wet bonding strength of the PMEU-modified soy protein adhesives-bonded plywood reaches 1.35 MPa,which satisfies the Type II indoor plywood strength requirements of GB/T 17657-2013(?0.7 MPa).