Study On Preparation And Interface Modification Of Carbon Fiber Reinforced Polyamide 6 Composites

Carbon fiber reinforced thermoplastic resin matrix composites have the advantages of excellent mechanical properties,short molding cycle,recyclable,corrosion resistance and impact resistance,which have broad application prospects in aerospace,automobile transportation and sporting devices.As a semi-crystalline thermoplastic resin,polyamide 6(PA6)is expected to achieve large-scale industrial application in the field of civil carbon fiber reinforced thermoplastic resin matrix composites due to its excellent comprehensive performance and low-cost nature.However,since the surface of carbon fiber is chemically inert,the interface bonding ability of carbon fiber composite with PA6 resin matrix is weak,and it is easy to produce debonding phenomenon at the weak interface during use,which limits carbon fiber reinforced polyamide 6(CF/PA6)composites play of properties.Therefore,improving the interface properties of CF/PA6 composites is of great significance to improve their mechanical properties.First of all,CF/PA6 composites were prepared by a vacuum molding process.The effects of molding temperature and molding time on the structure and properties of CF/PA6 composites were systematically studied during the preparation process,and then the CF/PA6 composites were subjected to isothermal crystallization treatment,the optimum technological conditions for vacuum molding were explored and preliminarily determined.The test results showed that the CF/PA6composites achieved the optimal mechanical performances under the conditions of 2.5MPa molding pressure,the molding temperature is 280?,the molding time is 20 min,and then the isothermal crystallization process is performed at 200?for 60 min.Currently,the tensile strength,bending strength,and interlaminar shear strength of the composites were 867.6 MPa,635.2 MPa and 35.7MPa,respectively.Subsequently,this thesis selected copolyamide(COPA)powder as raw material,and prepared copolyamide water-based carbon fiber sizing agent(COPA-SA)by phase inversion method.The surface treatment of carbon fibers after removing sizing agent and activated by Michaelis acid was carried out,and the effects of different concentrations of COPA-SA on the structure and properties of CF/PA6 composites prepared by vacuum molding were systematically studied.Infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)test results exhibited that a large number of amide groups were introduced on the surface of carbon fibers after COPA-SA treatment,which effectively improved the surface chemical activity of carbon fibers.Scanning electron microscope(SEM)observation showed that the surface of carbon fiber after COPA-SA sizing treatment was obviously attached with copolyamide.The mechanical test results displayed that when the concentration of sizing agent was 0.5%,it has the best effect on the interface enhancement of CF/PA6 composites.Compared with the treated CF/PA6 composite,the improvement was 64.4%and 97.8%,respectively.Using SEM to observe the bending section morphology of CF/PA6composites,it was found that the carbon fibers at the cross-section of CF/PA6 composites were covered with PA6 resin,indicating that COPA-SA treatment of carbon fibers can effectively improve the interfacial bonding ability between carbon fibers and PA6 resin.Finally,three copolymers of PA6/66/612,PA6/66/610/612 and PA6/56/610/612 were synthesized by melt polycondensation using caprolactam,PA56,PA66,PA610 and PA612 as raw materials.amide for subsequent preparation of water-based carbon fiber sizing agent.The chemical structures of different copolyamides were analyzed by hydrogen nuclear magnetic resonance spectroscopy(~1H-NMR)and FTIR.Through differential scanning calorimetry(DSC),it was found that the randomness of the molecular chain of the copolyamide has a great influence on its melting point,and the melting point of the quaternary copolyamide was lower than that of the ternary;At the same time,the greater the randomness of the molecular chain,the worse the crystallization ability of the copolyamide and the lower the melting point;and the addition of PA56 in PA6/56/610/612 weakens the ability to form intermolecular hydrogen bonds,so crystallization ability also weakened.Through thermogravimetric analysis(TGA),it was found that the proportion of long carbon chain dibasic acid in the copolyamide molecule increased,and its heat resistance improved,the temperature of the three copolyamides with a mass loss of 5%was greater than 300°C,which was subsequently used as the raw material and prepared into a copolyamide water-based carbon fiber sizing agent,the requirements for the processing temperature in the molding process of the CF/PA6 composite material are met.