| Electron beam radiation modification technology is to use high-energy electron beam to irradiate polymer,which can make ionization and excitation of polymer macromolecules,generate secondary ions,molecules or free radicals,initiate polymer degradation,crosslinking or graft reaction with other monomers.Electron beam radiation modification technology is widely used in the fields of polymer grafting,rubber crosslinking,polymer degradation and so on.In the field of PAN based carbon fibers production,radiation treatment can effectively reduce the pre-oxidation time of PAN fiber,improve the production efficiency of carbon fibers and reduce energy consumption.At present,the effect of radiation modification on the chemical reaction and physical structure change of PAN precursor fiber during pre-oxidation is unclear.There are many chemical reactions in the process of radiation treatment and carbon fibers production.How to combine the two processes to complete the practical application is also an urgent problem.In view of the above problems,in this paper self-shielded electron accelerator was used as radiation source,which was connected with pre-oxidation and carbonization equipment through fiber draft device,realized a continuous treatment of radiation,pre-oxidation and carbonization in our lab for the first time.On this basis,through the analysis of the influence of radiation process on each temperature zone stage of pre-oxidation,and the preoxidation process in line with the radiation modified PAN fibers was formulated,and the feasibility of the scheme was analyzed by analyzing the performance indexes of preoxidation fibers and carbon fibers.Finally,the effect of radiation modification on the microstructure of PAN fibers during pre-oxidation process was analyzed in detail by wide angle X-ray diffraction(WAXD)of synchrotron radiation Facility.The details are as follows:(1)PAN fibers were treated by electron beam irradiation at room temperature with selfshielded electron accelerator.The effects of irradiation on the molecular composition,mechanical properties and microstructure of PAN fibers were studied by gel content,Fourier-transform infrared spectroscopy(FTIR),X ray diffraction(XRD),differential scanning calorimetry(DSC)and thermogravimetric analysis(TGA).The results showed that,after electron beam radiation,PAN fibers mainly underwent cross-linking reaction,which could be accelerated by increasing the radiation dose rate.When the absorbed dose of PAN fibers was < 200 k Gy,the effect of radiation treatment on the molecular structure of PAN fibers was insignificant,but when the dose reached 400 k Gy,there would be carbon-carbon double bond formation in the fibers.The oxygen atoms were introduced into the PAN macromolecular chain.The oxygen content increased with the increase of absorbed dose and dose rate.At the same time,the surface morphology of the PAN fibers was rougher.After radiation treatment,the specific heat capacity of PAN fibers decreased obviously,and the increase of absorbed dose also led to a further decrease of the specific heat capacity.The crystallinity of PAN fibers decreased with the increase of absorbed dose,and the crystal surface spacing increased the dose.The cyclization reaction of PAN fibers could occur at lower temperature after radiation treatment,which could reduce the small molecules produced by thermal cracking and effectively improved the carbon yield of carbon fiber production.(2)The types,structures and properties of the free radicals in PAN fibers induced by irradiation are closely related to the reactions of cross-linking,dehydrogenation and degradation of PAN fibers.At the same time,thermal treatment induced radicals were also produced in the process of pre-oxidation.In this paper,electron paramagnetic resonance(ESR)was used to monitor the types,structures and changes of free radicals induced by electron beam irradiation and heat treatment.The kinds and concentrations of free radicals induced by irradiation in PAN fibers varied with the dose rate and absorbed dose.When the dose rate and absorbed dose were low,the free radicals induced by irradiation were mainly alkyl radicals.With the increase of dose rate and absorbed dose,the alkyl radicals gradually decreased,and the polyenyl radicals gradually increased.When the dose rate reached 13 k Gy/s and the absorbed dose reached 300 k Gy,the free radicals were mainly polyenyl radicals.At the dose rate of 13.0k Gy/s,the concentration of free radicals first increased and then decreased with the increase of absorbed dose.At the dose of 200 k Gy,the concentration of free radicals reached the maximum.The surface temperature of PAN fibers increased with the increase of dose rate and irradiation time.When the dose rate of 13 k Gy/s was used to irradiate PAN fibers to 400 k Gy,the surface temperature of PAN fibers could rise to 135-140 ?.The alkyl radicals in radiation modified PAN fibers would rearrange and recombine rapidly in the early stage of pre-oxidation,forming carbon-carbon double bond and then decay rapidly;the dissociation energy of allyl hydrogen near carboncarbon double bond was far less than that of alkyl radicals,so the dehydrogenation reaction of PAN fibers modified by irradiation in the early stage of pre-oxidation was obviously faster than that of PAN fibers without irradiation.The production of polyenyl radicals could continue to rearrange and recombine to produce more carbon-carbon double bonds and allylhydrogen.Therefore,there was a mutual promotion between polyenyl radicals and allylhydrogen.The dehydrogenation and cyclization of PAN fibers were accelerated obviously under the action of allyl hydrogen and polyene radicals.In this paper,a parameter(IDI)was set to represent the growth rate of the concentration of polyenyl radicals in each temperature zone of pre-oxidation.By calculating the concentration change of polyenyl radicals,the effect of radiation treatment on each temperature zone in the preoxidation process could be analyzed.According to the data of IDI,the effect of radiation treatment on the initial stage of pre-oxidation was very significant.With the increase of pre-oxidation temperature,the effect of radiation treatment gradually decreased,but in the later stage of pre-oxidation,the effect was also very obvious.(3)In this paper,the pre-oxidation process of irradiated PAN fibers was set up by analyzing the change of free radical concentration at each temperature point in the process of preoxidation,and the pre-oxidation test was carried out.The retention time of PAN fibers with absorbed dose of 400 k Gy in the early stage of pre-oxidation was reduced to one tenth,the retention time in the middle stage was reduced to one quarter,the retention time in the later stage was reduced to one half,and the total retention time was reduced to one quarter.By comparing the free radical concentration,cyclization degree,reaction degree,oxygen content,pre-oxidation index,density and other parameters,the optimization effect of preoxidation process was analyzed.It was found from the analysis data that the preoxidized PAN fibers can completely meet the industrial requirements of carbon fibers production.(4)In order to accurately analyze the influence of radiation treatment on the crystal structure of PAN fibers in the process of pre-oxidation,the evolution of crystal structure and orientation structure of radiation modified PAN fibers in the process of pre-oxidation was studied by using WAXD at bl16b1 beamline station of Shanghai Light Source(SSRF).The results showed that irradiation treatment could inhibit the formation of quasicrystalline structure at the early stage of pre-oxidation(200-220 ?),reduced the grain size,crystallinity and orientation of the fibers during the pre-oxidation process,and the residual grains in the pre-oxidized fibers could produce defects during the carbonization process,which would seriously affect the performance of carbon fibers,Therefore,the radiation modification could improve the performance of the carbon fibers by eliminating the residual grains of the pre-oxidation wire. |
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