Investigation On Radiation Effects And The Related Rheology Behaviors Of Polyolefin

Nuclear cable is regarded as one of the most important parts in the nuclear power plant?NPP?system.The nuclear cables must be qualified to be functional against both low-dose irradiation in service NPP condition and high-dose irradiation of?-ray and?-ray during design basis event?DBE?.Furthermore,the requirement of?-ray irradiation under low dose rate has been proposed according to the third generation NPP technology?e.g.AP1000?.Hence,the investigation on radiation resistance of polyolefin,which is one of the most utilized base materials in insulation industry,is imperative in consideration of the nuclear safety.While the radiation effect of polyolefin has been studied previously,the structural changes of irradiated polymers under high-energy ray,especially under low dose rate?-ray?tested using electron beam?are far from clarified.It was reported that rheological measurement exhibited higher sensitivity in distinguishing topologic changes of polymer chains than other characterization techniques.Therefore,in this work,a systematic study is made on the radiation effect of traditional polyolefin,i.e.Low-Density Polyethylene?LDPE?and Polypropylene?PP?,by rheology measurements.The effects of absorbed dose,ray-species,dose rate,atmosphere and temperature have been investigated systematically,in order to clarify the radiation mechanism of LDPE and PP under true NPP condition.Detailed contents and conclusions are listed as follows:?1?The influence of absorbed dose?ranging from 10 kGy,25 kGy,50 kGy,100kGy,200 kGy,300 kGy to 400 kGy?on the structure and property of irradiated LDPE and PP was studied using electron-beam?EB?accelerator?1.2MeV,2.3×10⁴ kGy/h?.All samples were irradiated at room temperature?RT?in air.Soxhlet extraction,FT-IR,ESR,tensile tests,DSC,TGA and rheology measurements have been performed to investigate the changes of gel content,molecular structure,radical concentration,mechanical property,crystallization behavior,thermos-stability and viscoelasticity of LDPE and PP under EB with different doses,respectively.The results are summarized as follows:For LDPE,crosslinking and degradation reactions coexisted under EB irradiations.It was indicated that LDPE predominantly undergone crosslinking reaction at lower dose level?10 kGy-200kGy?while radio-oxidation turned dominant at higher doses?300 kGy-400 k Gy?.For PP,the oxidative degradation predominated at lower doses?10 kGy-300 kGy?while branching reaction played as the leading role when the absorbed dose was 400 kGy.Besides,it was suggested that the rheology method was more sensitive in characterizing topologic changes?linear,crosslinked and long-chain branched structure?.?2?The influence of ray-species and dose rate??-ray,EB@high dose rate?EB-hdr?and EB-ldr?was investigated using rheology measurements.All specimens were irradiated at RT in air.Amplitude sweeping,small amplitude oscillatory shear?SAOS?,stress relaxation and creep-recovery tests were carried out in assessing the viscoelastic differences of irradiated specimen.It was inferred that when absorbed dose was identical,the radio-induced reaction showed strong dependence on ray-species?i.e.permeability and processing time?.For LDPE,the crosslinking density?v_d?was EB-hdr?EB-ldr??-ray;For PP,the degree of degradation obeyed EB-hdr>EB-ldr>?-ray.?3?The influence of atmosphere?radiated in N₂,air or O₂ at RT?on linear viscoelasticity of EB-hdr irradiated LDPE and PP was studied through SAOS tests and analyzed by the weighted relaxation spectrum and criteria.The absorbed doses were ranged from 10 kGy,25 kGy,50 kGy,to 100 kGy.Rheology results showed that when the absorbed dose was identical,LDPE's v_d values showed negative correlation with oxygen content?N₂?Air?O₂?while the degradation degree of PP showed positive correlation with that?O₂>Air>N₂?.?4?The influence of temperature?radiated at RT or 80?in air?on rheology behaviors of EB irradiated LDPE and PP were studied by SAOS test.The representative criteria such as vGp plot,Cole-Cole plot and the plot of|?*??,??|vs.|G*??,??|were used for investigation.It was concluded that when the absorbed dose were identical,both LDPE's v_d value?under EB-hdr?and PP'degradation degree?against EB-ldr?showed positive correlation with the temperature:80??25?.?5?Different parament influenced above were reorganized for comparison.It was showed the atmosphere-factor seemed to be the dominating factors than the others.