Preparation And Mechanical Properties Of Self-healing Polyurethane Modified-epoxy Resin Based On Diels-Alder Reaction

Epoxy resin(EP)exhibits good physical and chemical properties.EP has many advantages such as high adhesive strength,good corrosion resistance,good formability and thermal stability,etc.It has been widely used in adhesives,building materials,electronics,aerospace and other fields.However,due to the brittleness of the cured epoxy resin,it is prone to stress cracking under the impact force.This limits its application in the case of high impact resistance and fracture resistance.Polyurethane(PU)is widely used as foam plastics,elastomers,coatings,adhesives,sealants etc because of its excellent wear resistance,oil resistance,and low temperature resistance.But the high temperature performance of polyurethane is poor.Interpenetrating network polymer(IPN)technology can combine both the epoxy resin and polyurethane,and it is an effective method to modify polyurethane and epoxy resin.IPN is a kind of unique polymer blend,and which is formed by the continuous interpenetration of the networks of cross-linked polymer I and cross-linked polymer II.The interpenetration between networks makes the polymer achieve the effect of“forced compatibilization”and“synergistic effect”.IPN is widely used in toughening resin,reinforcing rubber and damping material.Micro-cracks may produce in polymers due to the effect of factors as thermal,mechanical and chemical during the process of processing,forming and using.Consequently,the mechanical properties of polymers can be reduced and the service life can be shortened.Inspired by the fact that animals and plants can heal themselves after being damaged,researchers have proposed a variety of methods to prepare self-healing polymer materials to solve problems above.It is one of the most effective method of introducing the thermally reversible Diels alder(DA)covalent into polymers.DA reaction is a thermo-reversible dynamic reaction,and this cycloaddition reversible reaction can be multiple cycled.At the same time,DA reaction has the advantages of economic and environmental-friendly,mild reaction conditions,less side reactions.And meanwhile,no catalyst is needed.Therefore,it provides an effective way for self-repairing materials.Moreover,materials containing DA bonds can be repaired and recycled for many times.In this thesis,two systems of polyurethane-epoxy resin(PU-DA-EP)based on thermo-reversible DA bonds were designed and synthesized.Firstly,E51 epoxy-modified self-healing polyurethane system was prepared.The effect of mechanical properties of the self-healing polyurethane reinforced by epoxy resin was investigated.Then the self-healing polyurethane-modified furyl glycidyl ether(FGE)epoxy resin based on DA bonds was prepared.The effect of different contents of self-repairing polyurethane on the toughening effect of epoxy resin was examined.Meanwhile,how to improve the repair efficiency of epoxy resin,and self-repairing mechanism were discussed in detail.The main research and conclusions are as follows:(1)Preparation and properties of E51 epoxy resin-modified self-healing polyurethane based on Diels-Alder reaction:A furan-capped polyurethane prepolymer(MPF)was designed and synthesized.The self-healing polyurethane(PU-DA)was synthesized by DA reaction between the resultant MPF and bismaleimide(BMI)at 60 ~oC.Then E51-modified polyurethane(PU-DA-E51)was prepared by using E51 epoxy resin to modified PU-DA.The chemical structure and thermal properties of PU-DA-E51 were characterized by FT-IR and DSC.The results showed that DA bonds have been introdued into PU-DA-E51 successfully,and results also confirmed that DA bonds in PU-DA-E51 exhibit excellent thermal reversibility.At the same time,the tensile test results showed that the introduction of E51epoxy resin enhanced the tensile strength of polyurethane materials.(2)Repairing behavior and self-healing mechanism of E51 epoxy resin-modified self-healing polyurethane:The self-healing behavior of PU-DA-E51 was investigated by polarizing microscope and quantitative analysis of tensile test.The results showed that PU-DA-E51 with DA bonds exhibited good repairing ability.The cracks disappeared completely when the PU-DA-E51 with a scratch depth of 0.5 mm was heat-treated for 15 min at 130 ~oC,and the repair efficiency could reach 79.8%.It is the studies indicated that PU-DA-E51 was repaired by a mechanism conbined with two self-heaing methods.I.e.,the repair was realized by both the reversible DA reaction and the thermal movements of molecular chains.The repair behavior of PU-DA-E51 with different contents of E51 was further investigaed by polarizing microscope and tensile test.It was further confirmed that the repair of PU-DA-E51 was realized by dual repair mechanism.(3)Preparation and properties of FGE epoxy resin modified by self-healing polyurethane(FGE-DA-PU)based on Diels-Alder reaction:A furan-terminated polyurethane prepolymer(MPF)and FGE epoxy monomer were synthesized by molecular design.Then the self-healing FGE-DA-PU was prepared by DA reaction between bismaleimide(BMI)and the mixture of MPF and FGE at 85 ~oC.The chemical structure of FGE-DA-PU was characterized by FT-IR,and the cross-linking of MPF and FGE was determined by DA bond.The thermal properties of FGE-DA-PU were tested by DSC,and the heat treatment condition was determined to be 85 ~oC 24 h.Its rDA reaction temperature is 130~oC.Meanwhile,through the impact and three-point bending test,it is proved that when the polyurethane content is 20%,the impact strength and three-point bending strength of FGE-DA-PU are the best.Observation of the surface structure of the impact fracture sample by scanning electron microscope proved that with the increase of polyurethane content,it first appeared as a"island structure"and finally showed a two-phase separation structure.(4)Repairing behavior and mechanism of self-healing polyurethane-modified self-healing epoxy resin:The repairing behavior of FGE-DA-PU with different contents of self repairing polyurethanes was examined by observing with digital camera.It was found that the crack healing effect of FGE-DA-PU became better and better with the increase of self-repairing polyurethane content.Meanwhile,results showed that the self-healing efficiency of FGE-DA-PU enhanced with the increasing of polyurethane contents.Moreove,the multiple self-healing efficiency had been increased.The repair efficiency of FGE-DA-PU can still reach 65%after three times of damage-repairing,and which proved to that the FGE-DA-PU have had good multiple repair performance.It was proved that the double repair mechanism played a role in self-healing epoxy resin-modified by polyurethane.