Cure cycle determination of neat and nano-reinforced carbon/epoxy composites used in aerospace applications through investigation of cure kinetics by differentaial scanning calorimetry and rheometer

Official URL: https://risc01.sabanciuniv.edu/record=b2586032_(Table of contents)

In carbon/epoxy composite laminates, epoxy resin has an important role as a component that holds the carbon fibers together and provides load distribution. Particularly, the interfacial bond between carbon fiber and epoxy resin should be good against shear stresses and stresses occurring in the perpendicular direction to the fibers. In order to achieve this, it is necessary to maximize the adhesion and strength properties of the epoxy resin and to remove the voids in the structure. This depends on a good examination of the epoxy resin curing kinetics. For this purpose, in this thesis, dynamic and isothermal tests were performed under different pressure conditions using HP-DSC for two different resins and prepregs, the rheological behavior of the resin and prepregs were examined, and then rheology and HP-DSC data were combined to determine cure cycles. The proposed cure cycles were applied for the manufacturing of carbon/epoxy composite laminate in the autoclave, and the working condition of the cure cycle was evaluated with the thermal and mechanical tests. In the first part of the thesis, materials, devices, and test methods used were introduced. In the second part, the curing kinetics of epoxy resin and the effect of pressure on the curing behavior were examined in the light of the literature. Afterwards, the curing kinetic behavior was investigated with HP-DSC under different pressures, heating rates and temperatures. These tests were done for resin and twill prepregs. The results revealed that the air bubbles and the voids formed as a result of the removal of water vapor during the process in the crimp zones in the prepreg were evacuated with the application of pressure, which increased the reaction surface area and the reaction enthalpy. The application of pressure considerably increased the curing time of the resin. The effect of pressure was more pronounced at low heating rate since there was more time for molecular mobility at low temperatures and the heat distribution was more uniform. The rheological data showed that the viscosity values of the B-stage prepreg were higher than the resin. Lastly, two different cure cycles determined with 3 and 7 bar pressures were applied, and the reference characterization and mechanical test results were successfully performed. In the third part, UD prepregs were studied under similar test conditions. No effect of pressure on enthalpy was observed for UD prepregs since they have unidirectional fibers and do not have crimp zones. The differences in curing behavior between neat and nano-reinforced UD prepregs were also investigated. Small increases in reaction enthalpy were observed with the utilization of MWCNT in the reactions, and it was concluded that the curing rate increased since the use of MWCNT increased the reaction surface area. It was also observed that the viscosity increased with the incorporation of MWCNT into the structure. Finally, the cure cycle was determined for these prepregs. Two different pressures (3 and 7 bars) were used. As a result of the tests applied to the manufacturing with these cure cycles, it was seen that 3 bar did not provide the reference manufacturing values in this system, while 7 bar pressure did.