Research On Radiation Oxidation Effect Of Ultra-High Molecular Weight Polyethylene

Ultra-high molecular weight polyethylene（UHMWPE）,as a kind of special polymer with high performance,has become the ideal material for high-end manufacturing field such as medical and aerospace,depending on excellent mechanical properties and unique chemical properties.As a green and fast modification technology,ionizing radiation is widely used in radiation crosslinking of artificial joint UHMWPE,which can greatly improve the wear resistance of UHMWPE.However,the radiation processing process will lead to the slow oxidation degradation of UHMWPE,which will reduce the comprehensive performance of material such as impact resistance and fatigue resistance,and threaten the long-term performance.How to suppress the oxidative side effects caused by the radiation crosslinking of UHMWPE has always been the focus of research in this field and attracted the attention of scientific community and industry.Although it has been recognized that the oxidation of radiation-modified UHMWPE were related to radiation-induced free radicals,most of the current literature focused more on the influence of radiation-induced oxidation on the properties of materials,and rarely focused on the study of radiation-induced oxidation process itself.Thus there is a lack of systematic awareness on the radiation-induced oxidation of UHMWPE.Obviously,in-depth understanding of the radiation oxidation process is the basis for controlling radiation oxidation.So the research of UHMWPE radiation oxidation process was systematically carried out in this paper.Low density polyethylene（LDPE）and high density polyethylene（HDPE）were used for comparative study,in view of the factor that the radiation oxidation process of UHMWPE were related to crystallinity and lamellar structure.The main research contents and results of this paper were as follows:（1）Study on radiation-induced radicals of UHMWPE.UHMWPE was irradiated byγrays in argon.The radical species induced by radiation in UHMWPE and their evolution behavior under argon and different partial pressures of oxygen were studied by electron spin resonance spectroscopy（ESR）.Irradiated UHMWPE mainly formed alkyl radicals and allyl radicals in argon,and total radical radiation chemical yield G value was about 0.48.The stability of alkyl radicals was poor at room temperature,and the life was about 1 d,while the allyl radicals were relatively stable.The decay rate of irradiation-induced initial radicals increased with the increase of oxygen partial pressure in oxygen-containing atmosphere.At room temperature,when irradiated UHMWPE were stored in argon and 5×10⁵ Pa O₂,the half-life of radicals were 224.0 h and 1.8 h,respectively.Thus it could be inferred that the trapped radicals migrated from the interior of lamella to the surface at a very fast rate,in the order of hours.（2）Post-irradiation oxidation of UHMWPE.Three kinds of polyethylene（UHMWPE,HDPE and LDPE）were irradiated byγrays in argon and then exposed to air.The post-irradiation oxidation behavior was studied by Fourier Transform Infrared Spectroscopy（FTIR）and ESR.The results showed that the post-irradiation oxidation effect of UHMWPE and HDPE was significant due to the large lamellar size and high crystallinity.LDPE with small lamellae and low crystallinity had no obvious post-irradiation oxidation effect.Ketone carbonyl compounds and long-life oxidation-induced radicals（OIR）were the main oxidation products during post-irradiation oxidation of UHMWPE and HDPE,and the trans-vinylene double bonds were highly involved in the post-irradiation oxidation reaction.Further analysis showed that the formation of oxidation products lagged far behind the initial radical decay,suggesting that the oxidation products did not result from the direct oxidation of the initial radicals,but evolved from the intermediate generated by the reaction of initial radicals with oxygen.The subsequent oxidation of the intermediate produced carbonyl compounds and OIR.（3）Study on irradiation oxidation of UHMWPE.Three kinds of polyethylene（UHMWPE,HDPE and LDPE）were irradiated byγrays in air and the oxidation during irradiation was called as irradiation oxidation.And then the irradiated samples were stored in air for post-irradiation oxidation.The changes of carbonyl groups and radicals during the irradiation oxidation and post-irradiation oxidation were studied by FTIR and ESR techniques.The results showed that the all polyethylene underwent rapid irradiation oxidation and the degree of oxidation was similar.The degree of oxidation was estimated by the production of carbonyl groups.It was found that the oxidation degree of UHMWPE and HDPE after irradiation oxidation for 17 h was higher than that after post-irradiation oxidation for 52 d.The irradiation oxidation of LDPE was severe,and the irradiation oxidation products could promote its post-irradiation oxidation behavior.It was further proved that ozone produced during irradiation contributed little to irradiation oxidation process.Therefore,the irradiation oxidation process was mainly carried out by in-situ oxidation of radiation-induced active oxygen species and the oxidation of carbon radicals,in which the former was the main pathway of rapid irradiation oxidation and stopped with the cessation of irradiation.To sum up,a systematic study on the radiation-induced radicals and their evolution behavior was carried out in this paper.The influence of polyethylene structure on post-irradiation oxidation was deeply studied and the post-irradiation oxidation and irradiation oxidation behavior were further compared.A new understanding on the radical decay mechanism,irradiation oxidation and post-irradiation oxidation process was obtained.This provides new ideas for reducing the effects of radiation oxidation and developing UHMWPE radiation crosslinking technology with anti-wear and anti-oxidation integration.