Synthesis,Characterization And Properties Of Spiroacetal-Containing Epoxy Resins

Epoxy resin has excellent mechanical properties,insulation properties,corrosion resistance,adhesion properties and low shrinkage properties.It is widely used in adhesives,anticorrosive coatings,electrical insulation materials,composite materials and other applications.However,there are certain limitations in the performance of epoxy resins.For example,the internal molecular stress of aromatic epoxy resins is large,resulting in brittle materials and easy cracking.In extreme environments,brittle fractures are easy to occur,which is difficult to achieve Standards for the use of engineering plastics and aerospace composite materials.The spiroacetal polymer containing the thioether structure has good flexibility,and introducing it into the epoxy resin crosslinking network can solve the above problems.In addition,traditional epoxy resins form insoluble and infusible cured products through curing reactions,which are neither difficult to degrade nor recoverable.Under acidic conditions,the epoxidized spiroacetal resin and its cured product can be degraded into small molecules of aldehyde or alcohol.In this paper,two spiroacetal-structured epoxy resins synthesized by click chemistry,which molecular structures were characterized by nuclear magnetic resonance?NMR?,infrared spectroscopy?FTIR?,gel permeation chromatography?GPC?,and mass spectrometry?MS?,and differential scanning calorimetry?DSC?,universal sample machine,small angle X-ray scattering?SAXS?,contact angle measuring instrument,ultraviolet-visible spectroscopy?UV?and other means was used to carry out curing reaction kinetics,fracture toughness,surface properties,degradation kinetics and other research.Some achievements are as follows:1.Using benzoin dimethyl ether?DMPA?as an initiator,and 3,9-vinyl-2,4,8,10-tetraoxaspiro[5.5]undecane?U?and ethanedithiol?EDT?as the raw materials,two linear mercapto-terminated spiroacetal oligomers?EU?with different numbers of structural units were synthesized through mercapto-ene click chemistry.By controlling the monomer feeding ratio,temperature and time,the molecular weight distribution index?PDI?of EU is within 1.5².0.2.Through the method of mercapto-epoxy click chemistry,under the conditions of 75 ^oC and triethylamine catalysis,two different structural unit numbers of EU were reacted with bisphenol A epoxy resin?DGEBA?to obtain epoxy terminal spiroacetal polymer l-P?EU-DGEBA?.For l-P?EU-DGEBA?with the same molecular weight and different EU structural units,the glass transition temperature?T_g?decreases with the increase in the degree of polymerization of EU,and when the T_g of l-P?EU-DGEBA?is 35.9 ^oC,bisphenol A epoxy thermosetting resin has the best toughening effect.l-P?EU-DGEBA?is 15wt%of DGEBA,and its composition ratio EDT:U:DGEBA is 4:3:1,the critical stress field strength factor(K_Ic)of l-P?EU-DGEBA?/DGEBA reaches the maximum value?3.89?and 2.3 times of the blank sample;Its flexural strength and flexural modulus are 231.3 MPa and5.2 GPa,respectively,which are in compliance with the flexural strength and flexural modulus of epoxy resin cured materials in aviation composite materials that should reach 100.0 MPa and 4.0 GPa,respectively.At the same time,the cured product shows good damping characteristics and high transparency,and has very good application prospects.3.Using allyl glycidyl ether?AGE?as the EU end-capping agent,spiroacetal resins?EUE?with two different structural unit numbers were synthesized by a mercapto-ene click reaction.The tensile strength of the cured products of EUE with two degrees of polymerization is above 1 MPa,and the elongation at break is between 26%and 32%.In the EUE cured surface contact angle test,the surface tension??_L?of solvents dioxane and water were 36.9 and 72.7 mN/m,respectively,and the?_Lof formic acid and hydrochloric acid were 23.5 and 75.4 mN/m,and organic acid has good wettability to the cured product.Under the condition of 60 ^oC and 2 h,when the mass ratio of EUE cured product to formic acid is 1:20,the cured product is completely degraded,and the degradation rate of EUE cured product with a small number of structural units is fast.The products of EUE cured products after acid degradation are small molecules of aldehydes and alcohols.