Study On The Key Factors Of Interfacial Bonding Strength Of Carbon Fiber Reinforced Resin Composites

The interface between carbon fibers and matrix is a main component of the composite. Good adhesion is a precondition for stress transfer in carbon fiber reinforced resin composites and is helpful to effectively improve the mechanical properties of composites. On the investigation of interface, it is the key to enhance interfacial bonding strength. Therefore, it is important to comprehensively understand the effect of various factors on the interfacial bonding strength of composites. In this paper several technologies, such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Laser Raman spectroscopy (LRS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and some mechanical properties tests were used to investigate the effect of different preparation technologies on the properties and structures of carbon fibers, discuss the process of electrochemical treatment of wet-spinning PAN-based carbon fibers and the evolution of surface property and structure during electrochemical treatment systemically, grope reasonable surface desizing technology of carbon fibers, study the effect of electrochemical treatment of carbon fibers, surface sizing agent and matrix modification on the interfacial bonding strength of carbon fiber reinforced resin composites, put forward electrochemical treatment mechanism of carbon fibers and matrix modification mechanism.Analysis of properties and structures of carbon fibers with different preparation technologies show that compared with carbon fibers prepared by wet-spinning process, the compactness of carbon fibers prepared by dry-jet wet spinning process is higher and the axial pore size is smaller, while the content of non-carbon elements on the surface of carbon fibers is lower. From X-ray diffraction, the structural parameters of both carbon fibers vary. For carbon fiber prepared by dry-jet wet-spinning process, its interlayer spacing d₀₀₂ is smaller than that prepared by wet-spinning process. The crystallite size Lc and La are larger and stacked layers of graphite lamella increases. The results show that carbon fiber prepared by dry-jet wet-spinning process has a higher degree of graphite and more integrated structure. The surface micro-structure of both carbon fibers is almost the same, while there is a big difference in surface morphology. The surface of carbon fibers prepared by wet-spinning process is rough and possesses many axial grooves, which are beneficial to increase the mechanical hinge effect between carbon fibers and resin matrix and improve the interfacial bonding strength of composites, while the surface of carbon fibers prepared by dry-jet wet-spinning process is smooth. In addition, clean production environment and fine processing equipment are beneficial to the reduction of carbon fiber surface defects.The surface properties of wet-spinning PAN-based carbon fibers was investigated with the treatment technologies by XPS during electrochemical treatment. The results indicate that the effect is obvious at early stage. However, the trend of surface treatment effect become gently forward. Higher electrolyte concentration, higher electrolyte temperature, longer treatment time and larger current density are helpful to the electrochemical treatment. Experimental determination is that NH4H2PO4 is the most suitable electrolyte for electrochemical treatment of carbon fibers.LRS was used to study on the surface structure change of wet-spinning PAN-based carbon fibers during electrochemical treatment. And the characteristics of first-order Raman spectra of carbon fibers with different treatment time were investigated. The results indicate that the peak separation between D band and G band increases in the first-order Raman spectra after electrochemical treatment, so does R value and ratio of D3 band and G band, while the ratio of D2 band and G band decreases. R value gradually increases with the treatment time, while the variation trends of the ratio of D2 band and G band and the ratio of D3 band and G band are not obvious. The ratio of all the disordered structure and G band increases continuously, and its change trend is consistent with that of R value, which can be used to comprehensively explain the variation of the surface disordered structure of carbon fibers. So the change rules of the structure of carbon fibers can be investigated by laser Raman spectroscopy during electrochemical treatment.The carbon fibers electrochemically treated in 1mol/L NH4H2PO4, for 4 mininutes, and high current density.were investigated. The results show that after electrochemical treatment specific surface area enlarges, surface roughness increases while oxygen content and oxygen-containing functional groups increase. These factors can improve the surface polarity and increase wettability of resin matrix. More oxygen-containing functional groups are helpful to enhance chemical action between carbon fibers and resin matrix to improve interfacial bonding strength of composites, in which the content increase of carbon-oxygen bond plays a main part in improving interfacial bonding strength of composites. The surface micro-structure of carbon fibers is destroyed after electrochemical treatment, the surface disordered degree increases and surface grain size becomes larger.The structures of T700 carbon fibers and graphite fibers after excessive electrochemical treatment were analyzed. The results indicate that electrochemical treatment is a kind of effective method, which has effect on the surface structure and does not destroy bulk structure. Under the same treatment condition, surface striations disappear and surface of T700 become rough along axial direction of carbon fibers. The surface of graphite fibers is almost unchanged. Modification mechanism is analyzed in the way of internal structure of carbon fibers. The existence of surface non-carbon elements is the main reason to destroy the structure of graphite layers, and the region of poor bonding force is easily destroyed to expose more axial grooves on the surface of carbon fibers.Carbon fibers were treated by different sizing agents, the effect of sizing agents on carbon fibers and interfacial bonding strength of composites were investigated. The results show that after sizing the broken filaments decrease. Sizing agents uniformly attach to the surface of carbon fibers. The dispersion property of sized carbon fibers is good to effectively avoid adhesion between carbon fibers. But different sizing agents result in different interfacial bonding strength of composites. Most sizing agents made in Japan can improve the surface wettability of carbon fibers, which is beneficial to improve the interfacial bonding strength of composites. Compared with imported sizing agent, the domestic is poor in improving interfacial bonding of composites, whose effect is close to ordinary epoxy resin.The structures and properties of carbon fibers treated by different desizing technology were investigated by FESEM, XPS, LRS and XRD. The results indicate that the immersion technology in acetone has little action to remove surface sizing agent. The instant high-temperature treatment can melt surface sizing agent, but it also damages the surface micro-structure of carbon fibers. The high-temperature treatment in N2 can effectively remove the surface sizing agent, eliminate the residue of the pyrolytic products of sizing agent and prevent the damage of the carbon fibers surface at high temperature under reasonable technology parameters. In summary, the high-temperature treatment in N2 is most effective to remove the surface sizing agent.Phenolic resin modified by hydroxyl silicone oil was used as matrix to prepare carbon fibers reinforced resin composites. It is found that when the content of hydroxyl silicone oil increases, the interfacial bonding strength of composites firstly increases and then decreases, flexural strength gradually increases and the variation trend of tensile strength is basically identical with that of interfacial bonding strength. Unsuitable addition of hydroxyl silicone oil leads to more defects in matrix and poor interfacial bonding of composites. When the weight percent of hydroxyl silicone oil to phenolic resin is about 2.5%, the interfacial bonding strength of composite is highest, meanwhile tensile strength and flexural strength enhance. But the effect of matrix modification on interfacial bonding strength of composites is inferior to that of carbon fiber treatment. In addition, suitable proportion design of matrix and proper operation in the preparation of carbon fiber composites are key factors to avoid structure defects and improve interfacial bonding strength of composites.