Study On Toughening Design And Reversible Self-Healing Property Of PU-PDA/EP Composite Materials

Since the 21st century,the construction of marine infrastructure has been continuously advancing.However,due to the complexity of the marine environment,it faces many factors such as wave erosion,chloride ion erosion in seawater,and dry wet alternation,resulting in a huge test of the durability of marine engineering.At present,the main way to extend the service life of marine engineering is through coating protection.Epoxy resin(EP),as one of the most widely used materials in marine protective coatings,has excellent mechanical properties,corrosion resistance,and easy processing advantages.However,after curing with EP,the crosslinking density is high,the matrix appears brittle,and the impact toughness is poor.In practical engineering,EP coatings often exhibit cracking,pulverization,and other phenomena.Therefore,this article chooses polyurethane(PU)prepolymer as the toughening agent for EP.By combining PU with EP,the toughening effect is achieved without affecting the tensile strength of EP.At the same time,reversible hydrogen bonds are formed between polydopamine(PDA)nanoparticles and EP molecular chains.It is expected to obtain a PU-PDA/EP composite material with both toughening and self-healing effects,and the composite material is coated on the surface of cement-based materials as a coating,Conduct research on the relevant mechanical properties of modified coating cement-based materials.PU/EP composite materials with different ratios were prepared through different process formulas.Orthogonal experiments were conducted using the tensile strength,elongation at break,and impact toughness of the composite materials as evaluation indicators.The optimal preparation formula for PU/EP composite materials was determined by combining single index method and response surface analysis method as follows:The R value(ratio of PU prepolymer raw material n(OH~-)to n(NCO~-))is1,the PU content is 15wt%(percentage of PU in the mass of PU/EP composite material),and the preparation temperature is 80℃.The chemical structures of EP and PU/EP were tested and analyzed using scanning electron microscopy(SEM)and Fourier transform infrared spectroscopy(FTIR),indicating that the reaction between PU and EP was complete.The maximum tensile strength of PU/EP composite material is 32.35MPa,the elongation at break is 26.21%,and the impact toughness is 30.40 J/m2,which is 37.61%,52.21%,and 47.07%higher than pure EP,respectively.Thermogravimetric testing showed that PU/EP composite materials have good thermal stability,and the hardness,modulus,stiffness,etc.at different depths were tested using the continuous stiffness method of nanoindentation.PDA/EP composites were prepared,and their tensile mechanical properties and self-healing properties were studied.The results showed that the mechanical strength of 0.6%PDA/EP composites was higher than that of EP.The pre-damaged composites were tested after being repaired in the atmosphere and 3.5wt%Na Cl solution for 3 days,and the self-healing ability of the composites was verified by comparative analysis.When the content of PDA is 0.6%(the percentage of PDA in the mass of EP matrix),the strength recovery rate of the composite is 104.46%.In order to study the toughening effect and self-healing mechanism of PU and PDA on EP matrix,PU-PDA/EP composites were prepared.The mechanical test results show that the tensile strength of PU/EP composites is slightly higher than that of PU/EP composites,and it has self-healing performance.This is because the surface of PDA carries a large number of functional groups(amine,imine,catechol)that can react with epoxy groups,which makes the composites form hydrogen bonds.Digital speckle correlation method was used to characterize the deformation behavior of composite materials before and after repair under load,and the variation laws of stress-strain characteristic value(Stc)and X-direction strain characteristic value(Sts)of composite materials before and after repair under different environments were obtained,which further confirmed the self-repair performance of PU-PDA/EP composite materials.In order to verify whether the prepared composite meets the practical engineering application requirements of the surface coating of cement-based materials,four kinds of epoxy composite materials,namely pure EP,15wt%PU/EP,0.6%PDA/EP and15wt%PU-0.6%PDA/EP,were coated on the surface of cement-based materials,and used as the surface coating of cement-based materials,and the adhesion,compressive strength and three-point bending strength of the coated cement-based materials were tested.The results show that the adhesion of 15wt%PU/EP coating is as high as5.53MPa,and the compressive strength of cement-based materials coated with15wt%PU-0.6%PDA/EP is the highest,which is 33.49MPa.Three-point bending test and broken photos of specimens also prove that the cement-based materials coated with15wt%PU-0.6%PDA/EP have the highest bending performance.The deformation behavior of cement matrix with different coatings under load was characterized by digital speckle correlation method.The analysis results show that the 15wt%PU-0.6%PDA/EP coating can effectively enhance the mechanical strength of cement,and the mechanical properties of cement matrix in water environment are effectively improved under the sealing protection of the coating.