Synthesis And Properties Of The Reversible Covalent Crosslinking Toughened Epoxy Resins Via Click Chemistry

Traditional thermosetting epoxy resins are widely used in paints,electrical and electronics,building materials,machinery manufacturing,aerospace,insulation materials and advanced composite materials owning to their excellent product properties.However,irreversible crosslinked network structures make epoxy resins difficult to be dissolved in solvent or melten for secondary processing,resulting in inefficient use of resources and a certain degree of waste.Meanwhile,high cross-link density results in intensive internal stress of the epoxy resin,inherent brittle mechanical properties,poor impact resistance,and low external loading,which limit its application to a certain extent.This work takes advantage of the fast and high selectivity of click chemistry reaction,using multiple click chemical reactions to introduce the flexible epoxy resins,and the use of furan/maleimide in the Diels-Alder reaction as a reversible cross-linking functional group to toughen the epoxy resin.First,the chain-extending flexible epoxy resin Polypropylene glycol diglycidyl ether(LPPGDGE)was prepared.The degree of the reaction and structure of products were analyzed by hydrochloric acid-acetone titration,FT-IR and1H tests.Adjusting the amount between flexible epoxy resin LPGGDGE and rigid epoxy resin EPN while fixing the molar ratio of furan functional group to maleimide functional group at 1:1,the reversible cross-linked toughened epoxy resin material was prepared.The thermally reversible Diels-Alder reaction enables repetitive processing of materials.The thermal reversible nature of the cross-linked toughened epoxy resin was demonstrated using FT-IR,swelling solubility,DSC and rheological tests.The tensile tests,impact tests and TGA tests were used to investigate the mechanical properties and thermal stability of the cross-linked toughened epoxy resin.The effects of curing temperature and crosslinking density on the properties of the epoxy resin were investigated.It has been found that an epoxy resin network LPPGDGE/EPN-DA owning thermo-reversible crosslinking properties can be prepared through the method above.Adjusting the amount between flexible epoxy resin LPGGDGE and rigid epoxy resin EPN can produce epoxy resin with a variety of mechanical properties.With the increase of the crosslinking density,the tensile strength and tensile modulus of the material will increase,and breaking elongation decreases.Compared with the phenolic epoxy resin crosslinking system,the impact strength of the modified toughened epoxy resin can be increased up to 68%.The curing temperature around the DA reaction is beneficial to increase the mechanical strength of the sample,the gelation rate will increase,and the degree of swelling will decrease.The chain-extending flexible epoxy resin LPEGDGE was prepared by using the raw material Polyethylene glycol diglycidyl ether(PEGDGE).The reaction degree and structure of the epoxy-functional group were investigated by hydrochloric acid-acetone titration,FT-IR and 1H tests.By fixing the molar ratio of furan functional group to maleimide functional group at 1:1,the reversible cross-linked toughened epoxy resin was prepared by the thermo-reversible Diels-Alder reaction,achieving the repetitive processing of materials.The thermal reversible nature of the cross-linked toughened epoxy resin was demonstrated using FT-IR,swelling solubility,DSC and rheological tests.The tensile tests,impact tests and TGA tests were used to investigate the mechanical properties and thermal stability of the cross-linked toughened epoxy resin.The effects of curing temperature,crosslinking density and network structures on the properties of the epoxy resin were investigated.It has been found that an epoxy resin LPEGDGE/EPN-DA with thermo-reversible cross-linking properties can be prepared by the method above.Adjusting the amount of flexible epoxy resin LPEGDGE and rigid epoxy resin EPN can also produce a variety of mechanical properties of epoxy resin,with the increase of crosslinking density,rigid epoxy resin increased leading to the increase of tensile strength and tensile modulus.Meanwhile,the elongation at break decreases.For samples of different composition at the same crosslink density,a much denser crosslinked network has higher tensile modulus and tensile strength,and a soft chain molecule with no methyl group on the side chain has a greater elongation at break.The sample cured at 70? can achieve the best mechanical properties.Compared with the phenolic epoxy resin crosslinking system,the impact strength of the modified toughened epoxy resin can increase up to 151%.