Structure Regulation And Mechanical Reinforcement Of Carbon Fiber/polyetherketoneketone Composite Interface

Poly（aryletherketone）s（PAEK）have excellent properties such as good impact,chemical corrosion,anti-ultraviolet and high-temperature performance.As the matrix of carbon fiber（CF）reinforced composites,they can be widely used in aerospace,rail transit,medical repair and other advanced fields.In recent years,polyetherketoneketone（PEKK）,a member of the Poly（aryletherketone）s family,has attracted increasing interest due to its outstanding mechanical properties and low processing temperature.However,the high viscosity of the thermoplastic resin melt and poor affinity to the surface of the CFs leads to high porosity of the composites and poor interfacial bonding between the fibres and resin,which greatly reduces its practical application value.In addition,the high viscosity of PEKK melt makes it difficult to measure the interfacial bond strength of CF/PEKK directly by microdroplet test.Fiber surface modification or adjustment of the processing parameters are widely used to improve the structure and interfacial adhesion of the composites,but some of them could damage the fibers or compromise the properties of resin.Herein,the interfacial bond strength of CF/PEKK composites was evaluated through appropriate impregnation techniques and testing methods.The interfacial structure was analysed in terms of microscopic morphology and resin crystallisation,then the moulding parameters were regulated to further achieve interfacial reinforcement and maximise the advantages of both carbon fibre and polyetherketoneketone.The first part systematically analysed the current status of researches on the interface of CF /PEKK and other similar composites,then expounded the factors affecting the bond strength of CF and PEKK from the aspects of composite process conditions,surface properties and modification methods,matrix condensed structure and so on.The formation of a strong interface relies on good wetting,physical adsorption and chemical chain linkage.By using solution-based method,the wetting effect of the resin on the fibres can be significantly improved and close contact between the two materials can be achieved.Thus,the CF/PEKK composite with uniform structure and excellent interface bonding is expected to be prepared.In the second part,the type and viscosity of the impregnation solutions were investigated,and it was found that Trifluoroacetic acid（TFA）and 4-chlorophenol（4CP）could completely dissolve PEKK.By adding 1,2-dichloroethane（DCE）,the polarity of PEKK molecule could be enhanced to further promote PEKK dispersion in the solutions and improve the infiltration effect.The contact angle of PEKK microdroplets formed by brush PEKK solution alone a single fiber was about 20～23°,which is smaller than that of the two epoxy resins composite systems,CF/Epoxy（AG-80）and CF/Epoxy（JC-02A）.By heat treatment of CF/PEKK microdroplets,the resin coated the carbon fiber more tightly,and the interface interaction between the two phases can be strengthened.When the treatment temperature is 340 ℃,the effect is best.The average interfacial shear strength（IFSS）of CF/PEKK can reach 96.0 MPa（the maximum value can reach 107.8 MPa）,which is 119.6% higher than that of the samples treated at 240 ℃.CF/PEKK exhibits higher IFSS values than CF/Epoxy due to the strong polar carbonyl group in the molecular chain and the better affinity of the PEKK solution for carbon fibres.In the third part,CF/PEKK laminates were prepared by hot compression molding on the basis of solution impregnation.Comparing the composite laminates formed by PEKK/TFA/DCE and PEKK/4CP/DCE impregnation solutions,it was found that rapidly solvent evaporation would cause serious uneven distribution of the resin in the outer and inner layer of the CF/PEKK impregnated tapes,resulting in significantly poorer interfacial mechanical properties of CF/PEKK-F than CF/PEKK-Cl.The interfacial mechanical properties of CF/PEKK-F are obviously worse than those of CF/PEKK-Cl.The interlaminar shear strength（ILSS）,bending strength and bending modulus of CF/PEKK-F are only64.24%,70.17% and 75.13% of those of CF/PEKK-Cl.By adjusting the processing temperature,the fluidity of PEKK melt can be improved,so that the excess resin on the surface of the prepreg can be dispersed into the bundle,and the the structural homogeneity of the composite can be improved accordingly.It can be observed that the composite has the best interfacial performance at the processing temperature of 360 ℃.In the fourth part,it is considered that the strong interfacial bonding is also attributed to the configuration of a large number of amorphous phases in the matrix by analyzing the crystallization of the resin in the composites,which closely cover around the surface of CF and promote uniformly transfer of the load.CF/PEKK and CF/Epoxy（AG-80）laminates exhibit two completely different interface failure modes.PEKK has high toughness and absorbs more energy through plastic deformation,while CF/Epoxy（AG-80）fracture surfaces exhibit obvious brittle characteristics.