Repairing Behavior Of Thermo-reversible Self-healing Polyurethanes Modified With SiO₂ Nanoparticles

As a large-scale application and production of nanomaterials,SiO₂ nanoparticls have been widely used in various production and life.Depend on their small size,high surface energy,and large specific surface area,and so on,nanoparticles can effectively locate the damage area and diffuse to the area of micro-crack to repair the damage site when micro-cracks and damage generated in the matrix material.Consequently,the matrix polymer material can be repaired and reinforceed.Polyurethanes have been widely used as coatings,foamed plastics,and adhesives because of their excellent performance of oil resistance,solvent resistance,wear resistance,and high elasticity.However,micro-cracks may be generated in polyurethanes when suffered from heat,mechanical attack,and chemical erosion during the production and application process,etc.By incorporating Diels-Alder?DA?covalent bond,which has thermal reversible property,polyurethane?PU-DA?with self-healing property can be achieved by utilizing the transformation of reversible covalent bonds inside the material.In this dissertation,the synthesis and surface modification of nano-SiO₂ have been studied firstly.Second,the self-healing polyurethanes with thermo-reversible DA bonds have been prepared,and the structure,thermal property,and self healing capability of the resultant polyurethances have been investigated.Finally,the surface-modified nano-SiO₂?SiO₂-NH₂?is introduced into the self-healing polyurethane?PU-DA?to synthesize the-nano-SiO₂-modified thermo-reversible self-healing polyurethanes?PU-DA-SiO₂?.The repair behavior,strength recovery rate,and repair mechanism of PU-DA-SiO₂ are explored.The main contents and conclusion are as follows:?1?Preparation and surface modification of nano-SiO₂:Monodispersed SiO₂nanoparticles are successfully prepared according to the classic St?ber method.The surface of nano-SiO₂ is modified by silane coupling agent of KH550.The FT-IR test proves that the surface of nano-SiO₂ is modified successfully.SEM test results show that the agglomeration of nano-SiO₂ is serious and the dispersion effect is poor.However,the surface modification of nano-SiO₂ is in favor of dispersion.Consequently,the modified nano-SiO₂ has better dispersion.The particle size and distribution are characterized by DLS.The results show that the size of nano-SiO₂ is at 114-120 nm.?2?Preparation and properties of self-healing polyurethane based on Diels-Alder reaction:The conventional thermoplastic polyurethane?PU-BDO?is prepared through the extension of-NCO terminated polyurethane precursor?MPF?.The self-healing polyurethane?PU-DA?based on Diels-Alder reaction is synthesized by the reaction between polyurethane prepolymer and furan ring-terminated bismaleimide?BMI?.Using FT-IR and DSC tests,it is proved that DA bonds are introduced successfully.Meanwhile,the introduction of DA bond gives PU-DA with excellent thermal reversibility and good reprocessing property.POM observation for the crack healing test shows that PU-DA samples are partially healed with heat treatment at 60 ^oC for 6 h and 120 ^oC for 10 min,and which indicates that the realization of repair is shared double repair mechanism of molecular migration of thermoplastic resin and reto Diels-Alder reaction.The tensile test results show that the mechanical strength and self-healing ability of PU-DA are greatly improved because of the introduction of the rigid group of DA bond,and the recovery rate of the strength is up to 78.2%.With the increase of the number of damage-repair in the same part,the intensity recovery rate of PU-DA decreases gradually.Still,it can reach 44.3%at the third cycles.?3?Nano-SiO₂ modified thermo-reversible self-healing polyurethane:the nano-SiO₂ is successfully introduced into the self-healing polyurethane to obtain nano-SiO₂ modified polyurethane with thermo-reversible self-repairing property?PU-DA-SiO₂?.The results of FT-IR and DSC analysis show that the introduction of nano-SiO₂ has no significant influence on the structure and thermal reversible properties of PU-DA.The crack healing of the specimen is observed by POM,and results show that the crack in the sample narrows gradually when the heat treatment is at 120 ^oC.When the heat treatment time reaches 15 min,the crack disappears completely.Tensile test shows that the highest repair efficiency of PU-DA-SiO₂ can reach 87.6%.As compared to that of the self-healing polyurethane without nano-SiO₂,the repair efficiency of self-healing polyurethane with 4 wt.%nano-SiO₂ is increased by about 15%.Results confirm that the self-healing property of PU-DA-SiO₂ results from thermal reversible DA reaction with thermal movement of molecular chains and nano-SiO₂.Nano-SiO₂ modified self-healing polyurethane can also achieve multiple damage-repair of the same part.Even after three times of damage and heat treatment,the repair efficiency can still reach 43.6%.