Studies On Toughening Characteristics And Mechanism Of Cryogenic Epoxy Resin

When epoxy resin matrix composites are applied in polar environments,embrittlement of materials is a key issue that needs to be addressed.Due to the high crosslink density,epoxy resin has the problem of high brittleness.When used in low temperature environment,the insufficient toughness of epoxy resin will become more prominent,which restricts the application of epoxy resin and composite materials in polar environment.In this paper,two kinds of cryogenic epoxy resin system E54/EE41/DDS and E54/PEKC/DDS were researched.The bisphenol A epoxy resin E54 was matrix,and cryogenic epoxy resin system were designed and prepared by changing the type of ring toughening agent,including physical modification and active reaction modification.The curing characteristic temperature was determined by non-isothermal DSC testing and extrapolation.The apparent activation energy and reaction order of the two epoxy resin curing systems were calculated by Kissinger and Ozawa method.The autocatalytic model kinetic equations of two epoxy resin systems were established by nonlinear fitting method.The validated model equations can be well matched with the experimental results.Through non-model fitting method,it was found that the activation energy of both systems first decreases and then increases with the increase of curing degree.Both systems were autocatalytic reactions.It was found that the curing reaction mechanism of the two epoxy resin systems was the autocatalytic reaction of amines by infrared spectroscopyFor the E54/EE41/DDS epoxy resin system,when m(EE41):m(DGEBA)was 3:1 at room temperature,the tensile strength and elongation of the epoxy resin were the largest compared to untoughened epoxy resin,which increased by 53.3% and 131.4%.At low temperature,when m(EE41):m(DGEBA)was 3:1,the tensile strength,elongation at break and flexural strength of E54/EE41/DDS epoxy system increased by 13.7%,44.3% and 8.5% compared with untoughened epoxy resin.For the E54/PEK-C/DDS epoxy resin system,at room temperature,when the content of PEK-C was 8wt.%,the tensile strength,elongation at break and flexural strength of the epoxy resin were the largest compared to untoughened epoxy,which increased by 18.8%,10.4% and 5.7% respectively.At low temperatures,when the PEK-C content was 8wt.%,the tensile strength,elongation at break and flexural strength of the E54/PEK-C/DDS system increased by 10.7%,22.8% and 4.1%,respectively,compared to untoughened epoxy resin.It was indicated that EE41 showed better toughening effect than PEK-C.The molecular structure characteristics of two cryogenic epoxy resin systems were studied by dynamic mechanical analysis.It was found that the mechanical loss of epoxy resin increased with the addition of EE41 increasing.And the regularity reduced,as well as the molecular chain's mobility enhanced,thus the flexibility of the resin improved.However,the glass transition temperature decreased.When m(EE41):m(DGEBA)was 3:1,the Tg of the epoxy resin was reduced by 41.3% compared with the pure epoxy resin.The addition of PEK-C causes a slight phase separation in the epoxy ring,as well as the peak width of the mechanical loss peak became larger,and the regularity of the system decreased.When the content of PEK-C was 8wt.%,the mechanical loss peak of epoxy resin was a single peak,and the system compatibility was better,meanwhile,the Tg was improved by 6.95% compared with untoughened epoxy resin.The fracture morphology of the epoxy resin system of different toughening systems was observed by SEM at normal temperature and low temperature.It was found that,at room temperature,EE41 toughened epoxy resin by microcrack and yield deformation,and PEK-C toughened epoxy resin by bridging and shear yielding;At low temperature,EE41 mainly toughened by forming microcracks Epoxy resin,while PEK-C toughen epoxy resin by shear yielding.