| After a long period of development,domestic carbon fiber has made great progress.Taking high-strength intermediate modulus carbon fiber as an example,the domestic carbon fiber has reached or even exceeded the international similar product level in terms of mechanical properties.However,in terms of specific applications,domestic carbon fiber encounters many problems in practical applications,especially in the performance conversion rate of carbon fiber composite materials and there is a large gap with foreign products.The interface performance between the surface structure of the carbon fiber and the composite matrix is the key factor that determines the conversion rate of the performance of the carbon fiber composite.The surface structure of carbon fiber can be divided into physical structure and chemical structure.The interaction between the two structures and the matrix resin is the basis of the interface of the composite material.Using domestic high-strength intermediate modulus wet spinning precursur and dry-jet wet precursur,under the same oxidation and carbonization conditions,designing different surface treatment conditions can obtain carbon fibers with different surface structures.By evaluating the mechanical properties of carbon fiber multifilaments,the microstructure of the carbon fiber surface and the mechanical index of composite materials in different processing forms,studying the relationship between the carbon fiber surface structure and the performance conversion rate of different composite processing forms is conducive to improving the comprehensive performance of domestic carbon fiber efficient application of carbon fiber composite materials and promoting domestic production.By evaluating the performance of wet spinning carbon fiber and dry-jet wet carbon fiber multifilament with different physically surface morphology,the influence of thesurface treatment process for mechanical properties of carbon fiber multifilament.Under the same oxidation and carbonization conditions,the surface treatment improves the carbon fiber impregnated resin capacity.With the increase of the surface treatment current density,the carbon fiber tensile strength slightly increases,but the carbon fiber elastic modulus and linear density are basically not subject to surface treatment effect of current density.Due to the excellent microstructure of the dry-jet wet precursur,the tensile strength and elongation at break of the prepared dry-jet wet carbon fiber are significantly higher than that of the wet spinning carbon fiber.When the carbon fiber tow has poor collimation,it will result in poor processability and increased fuzz.The surface structure of pure carbon fiber was characterized by SEM combined with PS-matlab software analysis,Raman spectroscopy and X-ray photoelectron spectroscopy,and the wet spinning carbon fiber and dry-jet wet carbon fiber surface structure was studied.Through the SEM study,the surface of the wet-spun carbon fiber was found to have grooves,and the surface of the dry-jet wet carbon fiber was smooth.As the current density of surface treatment increases,the surface of the wet spinning carbon fiber and dry-jet wet carbon fiber has no significant changes.Through the combination of PS-matlab software and SEM,the RS and RP values of the carbon fiber cross-section were obtained.As the current density of the surface treatment increased,the carbon fiber cross-section roundness improved slightly.The surface area of carbon fibers with surface grooves is estimated to be about 4%higher than that of carbon fibers with smooth surfaces.Raman spectroscopy was used to evaluate the carbon structure on the surface of the carbon fiber.It was found that the surface morphology and current density had no significant effect on the G peak position and half-height width.As the current density increases,oxidation reduces the value of graphitization on the carbon fiber surface.Using XPS to study the structure of functional groups on the surface of carbon fibers,it was found that with the increase of the surface treatment current density,the three types of carbon-containing oxygen groups(-C-OH,-C=O,O=C-O)have a clear upward trend.Since carbon fibers with surface grooves have a larger surface area than carbon fibers with smooth surfaces,carbon fibers with surface grooves are more likely to introduce active groups on their surfaces.The dynamic contact angle tester was used to study the surface free energy of carbon fiber,and the contact angle of carbon fiber tow filament and resin.It was found that with the increase of current density,the active functional groups on the surface of the carbon fiber increased,and the free energy of the carbon fiber surface with a grooved surface or a smooth surface showed an increasing trend;The groove shape makes the free energy of the carbon fiber surface greater at the same current density.The wettability of the carbon fiber tow is closely related to the surface structure of the carbon fiber.The better the surface free energy,the better the wettability of the carbon fiber and the resin.At the same time,the better the collimation of the carbon fiber,the better the infiltration of the carbon fiber tow.In this paper,surface free energy is used as a quantitative evaluation method for the surface structure of carbon fiber,which can take into account the physical structure and chemical structure of the surface of carbon fiber,and evaluate the surface characteristics of carbon fiber more comprehensively.In the field of composite prepreg applications,the interlayer shear strength of carbon fiber increases with the improvement of the surface structure of carbon fiber.The conversion rate of the 0°tensile strength of the carbon fiber composite material can be optimized by adjusting the surface structure of the carbon fiber.When the free energy of the carbon fiber surface is 31mJ/m2,the conversion rate of carbon fiber in the domestic high-strength intermediate modulus wet spinning carbon fiber(groove surface)can reach the highest value(about 80%).When the free energy of the carbon fiber surface is 32mJ/m2,the conversion rate of carbon fiber in the domestic high-strength intermediate modulus dry-jet wet carbon fiber(smooth surface)can reach the highest value(about 73%).In the field of composite winding applications,the interlayer shear strength of carbon fiber increases with the improvement of the surface structure of carbon fiber.However,the NOL strength conversion rate of carbon fiber composites is less affected by the chemical structure of the carbon fiber surface,and is more affected by the physical structure and stress concentration defects in the NOL ring.The smooth surface structure and high elongation of the dry-jet wet carbon fiber make the NOL strength conversion rate 10%higher than the wet spinning carbon fiber,which is more conducive to composite winding applications.In the process of assembling carbon fiber into carbon fiber tow and then converting them into carbon fiber composite materials,optimizing the surface structure of carbon fibers in response to the needs of different composite material processing forms is essential to improve the conversion rate of composite performance.The high-strength intermediate modulus carbon fiber with smooth surface physical structure obtained by the dry-wet spinning process has better universality of composite material processing technology. |
