Study On Microwave Heating Of Carbon Fiber/epoxy Resin Prepreg And Its Curing Behavio

Carbon fiber reinforced polymer（CFRP）is a kind of lightweight and high strength composite material,which is widely utilized in aerospace,energy equipment,transportation,sports and leisure fields,etc.Although carbon fiber composites have ideal properties,the curing process of carbon fiber composites is energy-consuming and time-consuming,which leads to the high price of CFRP and restricts the use of CFRP.Microwave heating technology for the curing carbon fiber composites has the advantages of rapid heating,high energy conversion efficiency,energy saving and consumption reduction.However,after years of research,this technology has not been successfully applied to industrial production.The main reason is that microwave heating for the curing CFRP has technical problems such as uneven heating and multidirectional laminates are difficult to be heated by microwaves.In this dissertation,aiming at the problem of uneven heating of carbon fiber composites by microwave heating,a prototype of microwave heating equipment for carbon fiber composites was designed and manufactured,and a special microwave curing process was developed based on the equipment.The specific research contents and results are as follows:The carbon fiber/epoxy prepreg was analyzed by two-round dynamic differential scanning calorimetry（DSC）scans,and the glass transition temperatures of uncured prepreg and fully cured prepreg were obtained to be 7.5°C and 106.2°C respectively.Based on the results of dynamic DSC scans,Kissinger,Ozawa and Friedman methods were used to analyze the activation energy of the curing kinetic,which gave the results of 77.49,80.24and 70.30 k J/mol,respectively.Moreover,the isothermal DSC analyses of the prepreg was carried out at 100,110 and 120°C,respectively.It was observed that the ultimate curing degree of the prepreg could reach 0.92,0.96 and 1,respectively at the three above-mentioned isothermal curing temperatures.Combined the results of isothermal DSC scans and isothermal rheological measurements,the curing degree at the gelation point of the prepreg was determined to be 0.41.The variation of the axial dielectric properties of unidirectional carbon fiber prepreg during three isothermal curing processes under 100,110 and 120°C respectively were tested by technique of cylindrical resonant cavity perturbation.Combined with rheological and dynamic mechanical analysis（DMA）tests,it was found that the isothermal curing temperature and the evolution of curing process had little influence on the axial dielectric constant,which ranged from 122.1 to 131.9 during all three curing processes.However,the evolution of curing process had significant effect on the axial dielectric loss factor.When the resin matrix underwent gel transition,the formation of macromolecular network structure would limit the free movement of macromolecules and restrict the polarization of dipoles in the alternating electric field,resulting in a significant reduction in the axial dielectric loss from 9.7 to 0.5.The subsequent glass transition resulted in the slightly increase of axial dielectric loss factor from 0.5 to 1.4.In order to realize the uniform heating of carbon fiber composite materials in the microwave field,through the design of microwave resonant cavity,microwave transmission device and the number of microwave feed sources,Comsol software was used to simulate the characteristics of electromagnetic field distribution in six microwave heating cavities under no-load condition.Finally,it was determined that the large-size cylindrical multimode cavity can obtain relatively uniform electromagnetic field distribution.Based on this,a set of cylindrical microwave heating equipment（L2800mm×Φ1600 mm）was manufactured,which matched six magnetrons as microwave sources and transmitted microwaves by BJ-26 waveguide.The output power was continuously adjustable between 0.1 k W and 9 k W,equipped with a fluorescent fiber multi-point temperature measurement device,and innovatively designed a three-dimensional motion support for the material.By adding multi-dimensional motion to the material,the difference in the cumulative absorption of microwave energy in each region of the material was reduced to achieve uniform heating.Based on the newly developed microwave heating equipment,the microwave heating process of carbon fiber unidirectional laminate(300×300 mm,[0°]₁₀)was explored.The effects of laminate placement position,three-dimensional motion mode,magnetron opening and closing combination and other factors on heating uniformity were studied.The dynamic thermal equilibrium control method was innovatively introduced to further improve the uniformity of microwave heating by regulating the heat dissipation rate of each region of the laminate to the surrounding environment.Under the optimal conditions,the temperature difference between the six temperature measurement points on the laminate could be controlled within 8.7°C.On the basis of the optimal control process,combined with the rheological test results,a series of two-stage microwave heating curing processes were proposed.The optimal microwave curing process of isothermal curing at 110°C for10 min,and then isothermal curing at 140°C for 30 min was obtained by techniques of FTIR,DMA,porosity,tensile and compressive properties.The 0°tensile strength of the laminate prepared under optimal microwave curing process was 2039.91 MPa,which was12.1%higher than the tensile strength of T300 carbon fiber reinforced epoxy composite provided by Toray group（1820 MPa）.This dissertation took the unidirectional carbon fiber/epoxy prepreg as research object.In order to improve the heating uniformity of carbon fiber composite in the microwave field,a prototype of microwave heating device equipped with three-dimensional motion system was innovatively designed and manufactured.Based on the new equipment,a series of exclusive microwave curing processes were developed,and carbon fiber unidirectional laminates with good comprehensive performance were successfully prepared.These works were of great significance to promote the application of microwave heating technology in the curing process of carbon fiber composites.