| In the production process of PAN based carbon fibers, there are some surface treatment processes such as finish process before collapsing, finish process before heat setting, electrochemical surface modification. Study on surface treatment process helps us to enable the production process to be smooth and improve the performance of the final carbon fibers. This paper focused on finish process in PAN based carbon fibers precursor production and electrochemical surface modification process in carbon fibers reprocessing, and the major contents include:(1) Through the influence of finish oil on fiber structure and performance in different stages of the production of carbon fibers, the influence of the types of finish oil and oil rate on collapsing and oxidative stabilization process were studied.(2) The effect of electrochemical surface modification process on the surface properties of carbon fibers, and carbon fiber composite material interface properties. The electrochemical modification mechanism was also studied in this paper.Three kinds of silicone oil were used as finish oil respectively before collapsing and heat setting. Control oil rate by changing the oil concentration, and the effect of finish oil on the PAN based carbon fibers production process were studied. It was found that finish oil rate ought directly proportional to finish oil concentration. During finish process before collapsing, it was relatively small that finish oil rate increased with the increase of the oil concentration, and the used oil concentration was high. During finish process before heat setting, it was relatively obvious that finish oil rate increased with the increase of oil concentration, and the used oil concentration was low. The influence of finish oil on PAN fibers after collapsing and the precursor was mainly the change of the apparent characteristics. When the finish oil rate was appropriate, it could effectively avoid adhesion and fusion between filaments, reduce broken filaments and improve the antistatic performance of fibers and integrity, etc. Finish oil rate not only affected the apparent characteristics of stabilized fibers, but also the mechanical properties and crystal structure. Finish oil could improve the degree of aromatization, decrease the mechanical performance of the stabilized fibers. Finish oil could hinder the pyrolysis reaction and reduce heat flow during oxidative stabilization process. As the finish oil rate increased, the block effect were higher. The use of finish oil would increase the mechanical properties of carbon fibers, but high finish oil rate would have the opposite effect. Compared to carbon fibers made from unfinished precursor, after using#1,#2and#3finish oils, when the finish oil rate was1%, the tensile strength of the carbon fibers respectively increased by4.5%,5.4%and8.2%.#3finish oil was use to produc high-performance carbon fibers. The finish oil rate of PAN fibers after collapsing was0.4%, and the finish oil rate of precursor was1%.NH4H2PO4agueous solution was used as electrolyte, and carbon fibers was electrochemical modified with different treatment intensity. Surface morphology, surface chemical properties, surface color, surface microstructure structure, tensile strength of carbon fibers and ILSS of CFRP were investigated. It was found that the carbon fibers surface groove deepened and the surface became uneven. With the increase of treatment intensity, thin skin layer of carbon fibers was etched into flake, then the exposed layer continued to be etched. When the treatment intensity was higher, the carbon fibers surface appeared deep cracks. Carbon fibers surface appeared longitudinal crack parallel to the fiber axis, bamboo-like crack perpendicular to the fiber axis or spiral crack with a certain angle to fiber axis after excessive oxidation. The regular crack was associated with the carbon fibers surface and internal structure and defect. After electrochemical surface treatment, the carbon content of the carbon fibers surface decreased, oxygen and oxygenated functional group increased. With the increase of electricity passing through the fiber bundle, the content of carbon, oxygen, and oxygenated functional group changed rapidly at first and then remained almost constant. With the increase of treatment intensity, the color of the carbon fiber surface changed, which was related with unsaturated oxygenated functional group generated in electrochemical surface modification process. These unsaturated functional groups worked together to form multiple conjugated chromophores. With the change of treatment intensity, the content of each surface functional groups changed and the absorbed wavelength of light changed, so the appeared color of the carbon fibers would change. D band and G band separated to a certain extent in the first-order Raman spectra after electrochemical modification treatment, which indicated that the surface microstructure changed. The edge of microcrystalline graphite flake layer on the carbon fibers surface was etched. Big graphite crystallite was etched into small graphite crystallite. Crystallite size decreased and disorder graphite lattice increased. Aliphatic structure, olefins structure and crystallite structure edge carbon were oxidized to generate oxygenated functional group. With the increase of treatment intensity, R increased firstly and then decreased. Linear density, elongation at break and tensile strength decreased. ILSS of CFRP increased at first, then decreased.When the electricity was160C/g, tensile strength of carbon fibers decreased by5%and ILSS of CFRP increased by39.6%. The weak layer and crack could reduce the tensile strength and ILSS obviously.Cyclic voltammetry was used to study the electrochemical behavior of carbon fiber under different experimental conditions in different electrolyte aqueous solution. The influence of the types of electrolyte, temperature and concentration on anodic oxidation reaction were studied. It was found that the scan range was different in different electrolyte aqueous solution, which was mainly determined by the pH of the electrolyte. As the pH value increases, the scan range moved toward the direction of the lower potential. In the smaller scan range, the water in the electrolyte did not produce reactive oxygen, so the carbon fibers surface was not oxidized. There was no oxidation reaction electricity. If only the oxygen evolution reaction of water happened, the carbon fibers surface can be oxidized. Oxygen evolution reaction of water was the precondition of the carbon fibers surface oxidation reaction. Scan rate had a positive relationship with surface oxidation reaction, hydrogen evolution and oxygen evolution reaction current intensity. CV curve was different in different electrolytes, which showed that the mechanism of carbon fibers surface oxidation reaction was different. Two oxidation limiting current could be observe in strong acid sulfate electrolyte, however there was oxidation limiting current in strong alkaline sodium hydroxide electrolyte. On the one hand, it was related to the pH of electrolyte. On the other hand it was because the carbon fibers surface could be easily oxidized in acidic electrolyte. The oxidation and reduction current were obvious, which showed that more surface oxidation products were generated. Temperature and the concentration would affect anodic oxidation reaction. Electrolyte temperature could affect the difficulty and reaction speed of the oxygen evolution reaction of water and oxygen reaction of carbon surface. The electrolyte concentration would not affect the difficulty of the oxygen evolution reaction and oxygen reaction of carbon fibers surface, however it could affect the reaction rate.Through the analysis of carbon fibers surface element content and functional groups before and after treated in different electrolytes, it was found that oxidation ability of the five electrolytes was different. The oxidation ability order of the five electrolytes was NH4H2PO4>H2SO4>(NH4)2HPO4>NH4HCO3>NaOH2Through the analysis of surface microstructure treated in different electrolyte, it was found that R could achieve high value at low current density in alkaline electrolyte, then R value decreasesd with the increase of the current intensity. R value was large at higher current density in acidic electrolyte. Through the analysis of the tensile strength of carbon fibers and ILSS of CFRP before and after treated in NH4H2PO4, NH4HC03and (NH4)2HPO4, the main reason of the decrease of carbon fibers tensile strength was the etching effect. Electrochemical oxidation etching mainly occurred in acidic electrolyte. Physical etching mainly occurred in alkaline electrolyte. The tensile strength of carbon fibers had a close relation with R value. When R value was larger, etching effect was stronger, and the tensile strength decrease rapidly. ILSS of CFRP had a close relation with surface chemical properties and surface roughness of carbon fibers. The optimum current density was160C/g in NH4H2PO4electrolyte. The optimum current density was80C/g in NH4HCO3and (NH4)2HPO4electrolyte. |
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