Synthesis And Properties Of Polyimide Sizing Agent Containing Ether And Ketone Structure

Carbon fiber(CF)-reinforced poly(ether ether ketone)(PEEK)composites have been widely used in aerospace,rail transportation,automobiles,sports goods,and other areas due to their high fracture toughness,excellent mechanical properties,and processing repeatability.In the carbon fiber reinforced composites system,the interface plays a crucial role in the stress transfer between the fiber and the surrounding matrix,and its structure and performance directly affect the comprehensive properties of the composites.However,traditional epoxy sizing is not appropriate for PEEK matrix as the sizing layer would be degraded at typical processing temperatures(up to 370C),which would result in weak interfacial adhesion between the CFs and the PEEK matrix.To solve this problem,a suitable sizing agent for PEEK system was synthesized to bond CF and PEEK together as a bridge and enhance the interfacial properties of the composites.For this purpose,the sizing agent should have good wettability with CF surface and good compatibility with PEEK matrix,resulting load transfer effectively.The main research contents and conclusion were as follow:(1)To exclude the effects of epoxy sizing from the interfacial shear strength(IFSS)in the system,the commercial CFs were first refluxed by acetone for 24 h before being used.SEM and XPS results informed that the expected desizing effect was achieved.To explore the wettability between PEEK resin and CF,the IFSS of bare CF-reinforced PEEK with different melt flow index(MFI)was evaluated by microdroplet test.The values of IFSS were almost no changed,indicating that the molecular weight has little effect on the interfacial adhesion between fiber and matrix,and the interfacial adhesion between CF and PEEK was weak.Hence,several high-temperature thermoplastics matrix was selected to evaluate the interfacial wettability.Versus the other thermoplastics,PEI exhibited the highest IFSS with the value of 97.9MPa,indicating that the imide group have a strong interfacial adhesion with the bare CF surface.(2)The diamine monomer with the same as the repeating unit of PEEK,was used to synthesize the polyamic acid(PAA)with different imide content.Furthermore,PAA was used as sizing agent to prepare polyimide(PI)-modified CF.To evaluate the mechanical properties of short carbon fiber(SCF)-reinforced PEEK composites,standard tensile and flexural test bars were obtained by extrusion and injection molding techniques.Versus the untreated CF/PEEK composites,the mechanical properties was enhanced due to the introduction of PI layer,and the tensile/flexural strength were increased with the content of imine group in the PI sizing.Morphological analysis of tensile fracture surfaces indicated that the interfacial adhesion between SCFs and PEEK has been effectively enhanced by PI sizing treatment on the SCF surfaces.The interfacial shear strength(IFSS)was carried out by microdroplet test,and the results shown that the PI sizing treatment can effectively enhanced the interfacial adhesion between CF and PEEK.Moreover,APBP-PMDA PI-modified CF present the highest IFSS values.SEM images of microdroplet specimens after debonding shown that many resin residue adhered on the surface of CFs coated with PI,revealing a relatively strong interfacial bonding at the interface.(3)The PAA was synthesized by APBP and PMDA,and the untreated CF were immersed into the PAA solution varying from 0 to 2.0 wt%,followed by thermal imidization to obtain APBP-PMDA PI-modified SCF.The tensile and flexural strength of SCF/PEEK composites increase with increasing PI sizing concentrations,and when the PI sizing concentrations was 1.0 wt%,the tensile strength and flexural strength of the corresponding SCF/PEEK composite was significantly increased by 16.8% and 8.2%respectively in comparison with the untreated case.The SEM images of tensile fracture surfaces was elucidated that PI coating on the CF could effectively improve the bonding and compatibility between the CFs and the PEEK matrix.DMA curves demonstrated that storage modulus of SCF/PEEK composites were enhanced with the increasing concentration of PI sizing,resulting the higher stiffness and the lower damping behavior of the composites.The single fiber pull-out testing was also measured,with the increased IFSS of CF/PEEK composites by 24.8% from 70.6 to 88.1 MPa for the CF modified with 1.0 wt% PI sizing.(4)The CF tows would be bonded together under the appropriate concentration of PI sizing,which affects the application of industrial applications.Hence,in order to explore the influence of the molecular weight of the sizing agent on the interfacial performance,PA-terminated was used to prepared imide oligomers with different degree of polymerization.The interfacial properties of CF/PEEK increased initially with the increase of molecular weight of sizing agent,and reaches the maximum when the degree of polymerization is 16,and the corresponding tensile strength,flexural strength and IFSS values were 173.3MPa 、 270.6MPa and 93.8MPa,respectively.Morphological analysis of fracture surfaces indicated that interfacial bonding has been improved by the introduction of low molecular weight of PI sizing.Consequently,reducing the molecular weight of the polyimide sizing agent can maintain the interfacial performance while facilitating commercial production.