| Epoxy resins have been widely used for coatings,adhesives,electronic materials and matrices for fiber reinforced composites because of their excellent mechanical properties,perfect adhesive properties,good heat resistance,desirable corrosion resistance,high electrical resistance and lower curing shrinkage.However,the intrinsic drawback of low fracture toughness of epoxy resins limits their practical applications in advanced technical field such as aerospace industry,so it has been the research hotspot to undertake toughness to epoxy resins.Nowadays,a great deal of research work has been carried out to modify epoxy matrices.The addition of secondary phase such as thermoplastics,elastomers and block copolymers modification of epoxies could support an improvement in fracture toughness,but undergo sacrifice in strength,elastic modulus and heat resistance.Research work is still ongoing to further toughen the epoxies avoiding much diminution in favorable properties.Based on an extensive literature review of previous investigations,a concept used in the toughening of epoxies is the inclusion of polyurethane prepolymer in epoxy E-51 in this paper.Curing behavior of polyurethane modified epoxy cured with 4,4-diaminodiphenyl sulfone?DDS?was investigated under non-isothermal DSC at different heating rates,then an optimum curing procedure was obtained for this curing system.The curing kinetic parameters such as apparent activation energy E?of polyurethane toughened epoxy were obtained utilizing Kissinger,Ozawa and Crane methods.Additionally,the autocatalytic-type kinetic model of graft-interpenetrating polymer networks of epoxy with polyurethane was investigated by Friedman isoconversional method,and the stuructures of polymers were characterized using FTIR spectrometer.Mechanical properties change with the inclusion of varying polyurethane concentrations were analyzed.In comparison to the unmodified epoxy,the elongation at break and fracture energy of 10phr polyurethane modified epoxy showed 45.22%and 75.13%increase respectively,meanwhile the tensile and flexural strength was respectively increased by 23.70%and 24.50%.In contrast,the elastic modulus remained stable and vickers hardness decreased slightly with increasing loading of polyurethane.Fracture surface analysis of the polyurethane modified epoxy via SEM identified the toughening mechanism as the formation of interpenetrating polymer networks?IPN?.The surface morphological changes were clearly indicative of the significant amount of plastic deformation which occured in the material prior to its ductile failure.Dynamic mechanical properties and thermal stability of DDS cured polyurethane modified epoxy were comparatively investigated with DMA and TG,respectively.Results showed that the initial decomposition temperatures?Td5?of neat epoxy,PU5-EP95,PU10-EP90,PU15-EP85 and PU20-EP80 curing system were 339.5?,359.66?,352.52?,338.15? and324.16?,respectively.Additionally,the temperatures for maximum decomposition rate(Tmax)and char yield?Yc?at 800? of polyurethane modified epoxy were slightly higher than the neat epoxy sample.It was found that little sacrifice in glass transition temperature?Tg?of modified epoxy with increasing polyurethane content up to 10phr as a result of reduced cross-link density.Moreover,the IPN also exhibited slightly improved storage modulus as compared with the neat epoxy in glassy state,indicating that the inclusion of polyurethane didn't sacrifice storage modulus.In general,polyurethane modified epoxy curing system showed a fairly good processability,as indicated by isothermal rheological analysis with low viscosity?<800 mPa·s,120??within 95min.Overall,this paper identifies the curing procedure of polyurethane-modified epoxy,and obtains the effects of polyurethane content on the mechanical and thermal properties of epoxy resin.It provides a theoretical basis for further expanding the application of epoxy resin,and also presents reference for the study of polyurethane toughening modified epoxy. |
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