Surface Treatment Of Polyacrylonitrile Based Carbon Fibers

The interphase interaction between carbon fibers and matrix plays an important role on mechanical properties of carbon fiber reinforced matrix composites. The smooth and chemically inert surface of PAN based carbon fibers goes against the adhesion between carbon fibers and matrix. The mechanical properties of PAN based carbon fiber reinforced matrix composites can be improved, if the surface of carbon fibers was treated, owing to the increasing of active functional groups and surface roughness on the carbon fibers.This paper studied the effect of electrolytes, current density, conductivity, temperatures of solution on electrochemical treatment of carbon fibers, discussed relationship between quantity of maleic anhydride (MA) grafting on the carbon fibers' surface and different treatment conditions, researched surface property of untreated, electrochemically treated, electrochemical-ammonia treated carbon fibers, studied mechanical properties of different surface state carbon fibers reinforced nylon 66 resin composites, and the status of two phases.Scanning electron microcopy, atomic force microscope, raman spectra, infrared spectra and X ray photoelectron spectroscopy were used to study morphology, crystallite size, functional groups of carbon fibers' surface, the acidic functional groups and pore structures were evaluated by the quantity of methylene blue (MB⁺) absorbing on carbon fibers and acid-base titration, the interfacial adhesion among carbon fibers, epoxy resin and nylon 66 resin was assessed by interfacial shear strength or bending strength.The main results are summarized as following:(1) The conductivity of electrolyte solution was related to species of electrolyte, concentration, and temperature of solution. When concentration of different electrolytes in solution exceeds 5wt%, there was little change for the conductivity of electrolyte solution, the conductivity increased as the temperature of electrolyte solution increases. PAN based carbon fibers' surface wetting ability was improved, and grooves were deepened as the fibers were electrochemically treated. Carbon fibers absorbed more MB⁺ and NaOH, as quantity of acidic groups and pore structures formed on the surface of carbon fibers. Acidic electrolytes and higher temperature of solution could aggravate the etching of active oxygen on the surface of carbon fibers. The quantity of different functional groups could be controlled through changing conditions of surface treatment. The crystallite sizes of PAN based carbon fibers treated in H₃PO₄ solution decreased more than that in NH₄HCO₃ solution, and the single fiber strength increased after being treated in H3PO4 solution. The interfacial shear strength increased obviously between treated carbon fibers and epoxy resin.(2) Maleic anhydride could be grafted on the surface of PAN based carbon fibers by Diel-Alder reaction. The quantity of grafting MA was related to temperature and the time of reaction, and a lot of MA could be grafted to the surface of carbon fibers after reacting for one hour at 100℃. Diene structures of carbon fibers decreased at first when the fibers were electrochemically treated, and the diene structures increased little with the aggravation of oxidation degree. The interfacial shear strength increased between grafted carbon fibers and epoxy resin.(3) The surface wetting ability of PAN based carbon fibers was improved after being electrochemically and ammonia treated, and ammonia treated carbon fibers got better wetting ability. There were a lot of functional groups containing oxygen and nitrogen formed on the surface of carbon fibers, which were electrochemically treated in NH₄HCO₃ solution. Ammonia treated carbon fibers' surface formed more nitrogen-containing functional groups, but the oxygen-containing functional groups decreased a little. The surface roughness of carbon fibers increased and crystallite size decreased when carbon fibers were oxidized by active oxygen, and these characters remained after being ammonia treated. The electro active substances on the surface of carbon fibers decreased when the fibers were electrochemically and electrochemical -ammonia treated.(4) The bending strength of carbon fiber reinforced nylon 66 composites increased obviously as the composites containing more carbon fibers. The electrochemically treated carbon fibers reinforced nylon 66 composites shew the best bending strength, while the reinforced effect of ammonia treated and untreated carbon fibers were lower relatively. The length of carbon fibers in the composites was about 0.8mm. The interfacial adhesion between electrochemically treated or electrochemical-ammonia treated carbon fibers and nylon 66 was better than the untreated carbon fibers.