The Effect Of Plasma Grafting Bismaleimide On The Surfacial Properties Of Carbon Fiber

Carbon fiber is widely used in resin-based composites as reinforcement.There is a two-phase interface layer in the combination of carbon fiber and resin,which affect the interface and mechanical properties of composite materials.Carbon fiber is a kind of inorganic material with strong surfacial inertia since it was undergone high temperature treatment.As a result,it is not easy to react with resin matrix chemically.Therefore,the bonding between carbon fiber and resin matrix is poor.In order to improve the interfacial bonding performance,it is very important to modify the surface of carbon fiber.In this thesis,bismaleimide was grafted onto the surface of carbon fiber by thermal initiation and plasma methods respectively to improve the interfacial bonding between carbon fiber and bismaleimide resin.Compared with the thermal initiation grafting method,the plasma method can better utilize the energy of plasma to graft bismaleimide effectively on the surface of carbon fiber.The grafting process and mechanism of plasma treatment on carbon fiber surface were investigated.The effect of plasma grafting on the surface structure of carbon fiber was also studied by controlling the conditions such as plasma grafting time,the concentration of bismaleimide monomer and the distance from the plasma torch nozzle to the carbon fiber surface.Finally,the effect of plasma grafting bismaleimide on the surface properties of carbon fiber bismaleimide resin system was evaluated.The indication of the results is as following.(1)Bismaleimide could be grafted onto the surface of untreated and plasma-treated carbon fiber by thermal initiation.According to the analysis of the surfacial structure and properties of carbon fiber,it was found that the increase of reactive functional groups on the carbon fiber surface,which was induced by plasma treatment,can promote the grafting of bismaleimide on the surface of carbon fiber.(2)Since air plasma is a kind of ionized gas group with high potential energy and high kinetic energy,it could be used to treat carbon fiber and bismaleimide simultaneously.The results indicated that it is practicable to use plasma to graft bismaleimide onto the surface of carbon fiber.The reaction mechanism of grafting carbon fiber by plasma treatment is deduced from the analysis of X-ray photoelectron spectroscopy,electron paramagnetic resonance spectroscopy and optical emission spectroscopy.In the process of plasma grafting,the plasma will provide energy to initialize the formation of free radicals of bismaleimide and generate hydroxyl groups on the surface of carbon fiber.These two kinds of free radicals will be joined together by the hydrogen capture reaction under the action of plasma as well as the free radical polymerization of bismaleimide.Thus the grafting of bismaleimide was introduced to the surface of carbon fiber.(3)The carbon fiber treated under different plasma grafting conditions were analyzed by scanning electron microscope,X-ray photoelectron spectroscopy and thermogravimetric analysis.It was found that the extending of plasma grafting time could increase the reaction extent of the grafting of bismaleimide on the carbon fiber surface.However,too long grafting time would result in the peeling of original formed graft layers.When increase the concentration of bismaleimide monomer,the grafting of bismaleimide on the carbon fiber surface could make more effective applying of the plasma energy.When the concentration exceeded a certain amount,the‘cage effect’for radicals would hinder the grafting process.The distance from the plasma torch nozzle to the surface of carbon fiber will affect the energy provided to the grafting reaction.The energy is too high to cause the agglomeration of grafts,and too low to complete the grafting reaction.(4)After the plasma grafting of bismaleimide on the carbon fiber surface,the wettability of carbon fiber was improved.The interfacial shear strength with bismaleimide resin will show a trend of increasing at first and then decreasing with the further increase of grafting time.The same situation was found by increasing the concentration of bismaleimide and the distance from the plasma torch nozzle to the carbon fiber.In particular,when the grafting time was 12 s,the concentration of bismaleimide was 0.05 mol/L and the distance from the plasma torch nozzle to the carbon fiber surface was 1.5 cm,the surface energy of carbon fiber increased from 19.2 m J/m~2to 37.2 m J/m~2compared to untreated sample.Meanwhile,the interfacial shear strength of treated carbon fiber with bismaleimide resin increased from 30.1 MPa to 68.2MPa with an improvement of 127%.The interfacial properties between carbon fiber and bismaleimide matrix was greatly improved.