Research On Surface Modification Of UHMWPE Fibers And Preliminary Investigation On The Composite Interface

Ultra-high molecular weight polyethylene(UHMWPE)fibers are one of most important reinforcing fibers in the composite material due to their high specific strength and specific modulus.However,due to their smooth surface and chemically inertness,the bonding properties of the fibers and the resin matrix are poor,which greatly limits fibers' application in the field of composite materials.Therefore,surface modification of UHMWPE fibers is necessary.In the research and applications of fiber reinforced composite materials,interfacial properties have a crucial impact on the overall performance of the composites.This thesis focuses on exploring the surface modification of UHMWPE fibers via chromic acid oxidation treatment and comparing with the dielectric barrier discharge(DBD)plasma modification.Besides,the micro-debonding method and Raman spectroscopy are used to study the interfacial micro force transmission in UHMWPE fiber reinforced composite materials.Firstly,UHMWPE fibers are oxidized modified using chromic acid oxidation solution under the specific concentration.The changes in the surface functional groups,morphology,crystalline properties and mechanical properties of the fibers before and after treatment were analyzed by using Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD),scanning electron microscopy(SEM)and mechanical test.The results show that,after chromic acid oxidation processing under appropriate conditions,the oxygen-based functional groups are produced on fibers' surface and the surface roughness is increased,fibers'surface wetting properties and fiber-epoxy interfacial bonding performance was are improved,and crystallinity and crystal sizes in fiber's surface layer are obviously decreased,the crystal orientation is also decreased,while the decreases in fiber's core layer are relatively lower.When chromic acid treatment temperature is 55 ? and the processing time is 5min,fibers' IFSS reaches the maximum value with no obvious damage to fibers' mechanical properties.Secondly,UHMWPE fiber/resin composites are made by embedding fibers with the droplet of epoxy resin.The stress distribution along the fiber in droplets is explored via Raman spectroscopy when the fiber is subjected to different stress.The results show that,for the untreated fiber/epoxy resin composite material droplets,the stress distribution along the UHMWPE fiber in droplet renders the "M" shape which is caused by the solidification of epoxy without stress.As the stress increases,the UHMWPE fibers stress distribution in droplet renders the "U" shape,which indicates that the fiber stress inside the droplets has been partly transferred to the resin.Comparing the UHMWPE fiber stress distribution of treated and untreated fiber,the stress transferring effect of treated fiber is better.The interfacial shear stress also changes along the fiber,it reaches the maximum on the end of the droplet and the least shear stress occurred at the droplet center.Thirdly,the modification effects of UHMWPE fibers with different concentrations of chromic acid(K2Cr2O7:H2O:H2SO4=1:4:10-1:4:40)are discussed.The results show that the fiber-resin interfacial shear strength increases gradually with increasing the concentration of chromic acid solution,and the IFSS is increased by 75.5%at the chromic acid concentration of 1:4:40.As the chromic acid concentration increases,UHMWPE fiber's surface irregularities phenomenon becomes increasingly apparent,convex area increases,and its roughness and nicks are increasing depth.Considering the mechanical properties of the treated fibers,the optimum chromic acid oxidation processing condition is:chromic acid concentration is 1:4:30(or 1:4:20),treating temperature is 55 ?,and treating time is 5min.Lastly,UHMWPE fibers aretreated with the DBD plasma discharge under pure argon atmosphere,the modification effects are discussed by changing the plasma discharge treating time and Ar flow,and the optimal treating condition is got.In order to seek the optimum condition of improving the fiber-matrix bonding properties,the modificaiton effects of DBD plasma treatment and chromic acid treatmentare compared with detailed measurements on fibers' surface structure and properties.The results show that the optimum DBD plasma treatment condition is processing 30s at the Ar flow of 3L/min.After treated under the optimal plasma treatment conditions,UHMWPE fiber's IFSS increasing rate and the contact angle decreasing rate are both lower compared with those of chromic acid treated fibers.The changes in the surface functional groups and surface morphology are analyzed by XPS and AFM.It is found that the plasma modification mainly works by introducing polar functional groups on the fiber's surface,while,chromic acid modification mainly works by etching the fiber's surface.