Analysis Of Anodic Oxidation Efficiency And Effect Of Electrolyte On Surface Structure Of Pan-based Carbon Fiber

PAN-based carbon fiber as an important reinforcing material has attracted more and more attention for its excellent properties such as light weight,high specific strength and modulus.As the preparation of PAN-based carbon fiber undergoes high temperature carbonization or even graphitization,with the escape of non-carbon elements and the enrichment of carbon element,the carbon content of carbon fiber achieves more than 90%and the surface appears to be inert.When such carbon fiber is made into composite materials with other materials,especially with resin,the interfacial adhesion is weak,which leads to the poor performance of the whole composite material.Therefore,it is necessary to modify the surface of carbon fiber.There are many surface modification methods and they are mainly used to increase the surface active functional groups and roughness,and then improve the interfacial adhesion between carbon fiber and matrix.Currently anodic oxidation is the most widely used and practical continuous on-line surface treatment method.The principle is that carbon fiber is electrolyzed as anode in aqueous electrolyte solution,and the generated activated oxygen can oxidize the surface of carbon fiber.In the anodic oxidation of carbon fiber,the main function of electrolyte is to improve the conductivity and the electrolytic efficiency of water.However,previous studies showed that different electrolytes had a great influence on the surface structure of carbon fiber and its composites'interfacial properties but there is lack of rational explanation.Moreover,potential distribution on carbon fiber bundles and oxidation efficiency have not been reported.The process conditions of voltage and current given in different studies are an average value.In the actual anodic oxidation experiment,the distribution of potential and current along the fiber bundle is different due to the large resistance of carbon fiber in different experimental devices,which will inevitably lead to different oxidation results under the same process conditions.Therefore,the reference of previous research results is poor.Based on these problems,the effects of device parameters of anodic oxidation experiment on the attenuation of potential and current along the direction of fiber bundle,power consumption of different parts and efficiency during the experiment are studied firstly by means of calculation and simulation,which provides theoretical guidance for further research on anodic oxidation surface treatment of carbon fiber and optimization of experimental device.On this bases,under a series of same process conditions in the same anodization experimental device,sulfuric acid,sodium hydroxide and ammonium bicarbonate were used as electrolytes to further study the effect of different electrolytes on the surface and interface structure of carbon fiber and the corresponding mechanism.The details are as follows.?1?We set up a model and got a series of formulas capable of simulating the distributions of potential and current along a carbon fiber tow,and power consumption under different cell parameters such as fiber gauge and electrode distance.The results show that the potential and current are in exponential attenuation along the direction of carbon fiber in the anodic oxidation experiment.Carbon fiber resistance consumes the most in different parts of the power consumption,and accounts for at least 50%of the total consumption under different device parameters.Through analysis we found that under a certain voltage between the fiber roller and the graphite cathode,the longer the fiber length soaked in the bath,the greater the attenuation of potential and current along fiber tow,and the more the useless power is consumed.It is probably considered that the distance between carbon fiber tow and graphite cathode should be as close as possible,however,when the treatment length of fiber is determined,such as 1.00 m,when the distance is too close,less than 0.01 m,the potential decays much sharply along the direction of fiber bundle and the power consumption of carbon fiber resistance is very high,resulting in low efficiency.When the distance is 0.01 m,the efficiency is only 7%.The model or the formulas can help find the optimum choice of cell parameters.?2?Effects of different electrolytes on the surface structure and interfacial property with epoxy resin of PAN-based carbon fiber are evaluated,which is used to find out the key factor that affects the improvement of the interfacial adhesion.It is found that the carbon fiber treated in H₂SO₄ electrolyte owns the most abundant acidic groups,which optimum value is more than twice that of other electrolytes,but bonds relatively weak with epoxy resin by the maximum interlaminar shear strength of 60 MPa,an increase of 14%compared with that of untreated.The carbon fiber oxidized in NaOH electrolyte and then restored in HCl solution owns less oxidative groups but shows a strongest bonding with epoxy resin by the maximum interlaminar shear strength of 69 MPa.We conclude this phenomenon is mainly ascribed to the graphene layers covering on the fiber surface that are formed during carbonization and connect weakly with the substrate of carbon fiber,thus called weak layers.The weak layers can be peeled off effectively in NaOH electrolyte but not in H₂SO₄ electrolyte.After they are removed,oxidative groups grow on the exposed edge sides of embedded graphitic layers.Therefore it is more important for a high interfacial bonding that acidic groups grow on somewhere solid rather than grow more.An attempt for modification is tried by removing the weak layers in NaOH electrolyte and adding more oxygen groups in H₂SO₄ electrolyte.The interlaminar shear strength of composites is not increased as expected,however,it shows oxidation in H₂SO₄ electrolyte doesn't generate weak layers.?3?This study is aimed to further investigate the stripping mechanism of graphene layers off PAN-based carbon fiber during anodic oxidation in NaOH electrolyte.Microstructures of the treated carbon fibers were characterized by atomic force microscope,X-ray photoelectron spectroscope and Raman spectroscopy.While the exfoliated fragments in the electrolyte were investigated with UV spectroscopy,scanning electron microscope and transmission electron microscopy.The stripping is assumed to be caused by intercalation of OH^-ions,which was detected by electric capacity of the carbon fiber as positive electrode.The results show that the capacitance at initial circle is about twenty times higher in NaOH electrolyte than in H₂SO₄,which proves intercalation happens and induces the carbon sheets to be stripped.Moreover,the stripping occurs on the carbon fiber surface in both crystalline and amorphous regions,while in H₂SO₄electrolyte only etching takes place,and preferentially in amorphous region.The difference can be caused by the ion structure and size difference between OH^-and SO₄^2-.The linear structure and much smaller size of OH^-ions permit them to enter into the interlayers on the carbon fiber surface,while the tetrahedral structure and much bigger size of SO₄^2-ions can only permit them to react on the fiber surface or somewhere loose in amorphous region.It is hoped that this paper will provide a theoretical basis for the optimization of the experimental device and better application of carbon fiber anodic oxidation surface treatment.