Study On Interface Strengthening Mechanism Of Carbon Fiber/Polyamide 6 Composites

Carbon fiber(CF)reinforced polyamide 6(PA6)composite is a kind of thermoplastic composite with great application potential.However,due to the limitation of low interfacial strength,the mechanical properties of composite still have a large space for improvement.In this paper,a series of CF surface modification techniques have been carried out to improve the interfacial properties of CF reinforced PA6 composites.Firstly,the surface activity of CF was improved by desized and activation treatment.PA6 and polydopamine(PDA)coatings were then synthesized in situ on the CF surface,respectively,so that the composite interface was improved by hydrogen.Subsequently,on the basis of PDA modified CF surface technology,δphase manganese dioxide(δ-MnO2)nanosheets were grown in situ on the CF surface,which further improved the interface bonding properties of the composite by increasing the mechanical linkage between fiber and resin.Finally,the modified silane coupling agent was grafted on the CF surface to generate chemical grafting between CF and PA6 resin,thus the interfacial mechanical properties of the composite are improved even more.The commercial CF was desized,and then activated by NaOH solution and hydrogen peroxide.In this case,the surface activity of CF was improved to a certain extent,and hydrogen peroxide had a stronger activation effect on CF.NaOH solution and hydrogen peroxide solution etched and oxidized the CF surface,which is the main reason for the increase in the CF surface activity.However,the treatment method also caused mechanical damage to CF,which resulted in the decrease of ILSS properties of composites by 0.26%and 2.30%,respectively.Therefore,to strengthen the force damage flaws of CF without lowering its surface activity,additional surface treatment of activated CF is required.In order to solve the above problems,PA6 coating was synthesized in situ on the surface of NaOH-activated CF(CF-NaOH)by CPL anionic ring-opening polymerization.By adjusting the concentration of the precursor solution,the amount of PA6 on the CF surface was effectively controlled.The modified CF with complete coating and non-adhesion can be obtained by treating CF with a 20 wt%precursor solution.The introduction of PA6 coating improved the CF surface activity,strengthened the monofilament tensile strength,reduced the CF surface roughness,and improved the ILSS and IFSS properties of the composite by 49.92%and 31.94%compared with desized CF composites,respectively.The mechanical interlock between PA6 coating and CF surface grooves enhanced the stress transfer ability of the composite interface,which is the main reason for the improvement of the composite interfacial properties.In order to solve the problem of poor stability of the CPL in situ polymerization method,PDA coating was synthesized on the surface of H2O2-activated CF(CF-H2O2)to modify the CF.In the process of modification,the ultrasound-assisted method was introduced.Because of the ultrasonic cavitation,water molecules produce H2O2,which can play the oxidation role to promote the synthesis of PDA.Therefore,the use of ultrasonic reduced the dependence on oxygen in the synthesis process of PDA,greatly improved the coating efficiency of PDA on the CF surface.At the same time,the difference in PDA coating between the inner and outer layers of CF bundles was reduced,and the stability of CF properties was increased.PDA coating improved the CF surface hydrophilicity and enhanced the tensile strength of the monofilament.The ILSS properties of the CF-reinforced PA6 composite are 11.11%higher than those of the desized CF composite.PDA coating contains a large number of phenolic hydroxyl and amino groups,which can produce a large number of hydrogen bonding with CF and resin matrix,which is the key factor to improve the interface performance of the composite.Cracks were easy to propagate along the PDA coating when the composite was damaged by force.To solve this problem,δ-MnO2 nanosheets were grown in situ on the surface of PDAmodified CF(UPCF-1)by hydrothermal method with potassium permanganate(KMnO4)solution,which increased the tearing resistance of PDA coating.The δ-MnO2 nanosheets were grown on the surface of desized CF and UPCF-1,and the CF properties before and after growth were analyzed.It was found that the PD A coating can protect the CF and improve the formation rate of δ-MnO2 nanosheets in the hydrothermal reaction.The upright growth of δ-MnO2 nanosheets on CF was promoted by the redox reaction between KMnO4 and the hydroxyl and amino groups of PDA.The effects of KMnO4 solution at different concentrations on the CF treatment effect were compared.It was clear that the composite interface properties could be improved most by using 0.03 wt%KMnO4 solution.The ILSS of composites increased by 18.25%compared with that of desized CF composite.The nanosheets embedded within both the PDA coating and the resin matrix can effectively prevent the propagation of cracks along the PDA layer,thus improving the interlayer properties of the composite.In order to generate chemical covalent bonding between CF and PA6 resin matrix,and further improve the composite interface properties,the CF surface was grafted with the isocyanate group by the coupling agent method.To enable the hydrolysis of 3isocyanatopropyltriethoxysilane(IPTS),the isocyanate group was blocked by CPL.By adjusting the hydrolysis conditions and concentration of the modified coupling agent solution,the CPL-blocked IPTS was covalently grafted to the surface of CF-H2O2 and MnO2-modified CF(UCF-M3).After treating CF with 1.0 wt%coupling agent solution,the ILSS of the composite reached 52.53 MPa,which was 21.82%greater than that of the desized CFreinforced PA6 composites.High-temperature composite molding caused CPL-blocked IPTS to unblock,releasing highly active isocyanate thermally.This isocyanate then reacted with the terminal amino group of PA6 to form an amide bond,and the composites established a highstrength interface through covalent grafting,which is the main reason for the improvement in ILSS.After UCF-M3 was treated with 1.0 wt%coupling agent solution,the coupling agent accumulated at the top of the nanosheet,which hindered the penetration of resin into the δMnO2 nanosheet layer,resulting in a decrease in ILSS.Therefore,when the CF surface is modified at multiple and multi-scale,not only effective modification methods should be adopted,but also reasonable coordination of various methods should be carried out.Only in this way,the interlaminar properties of the composite can be improved to the maximum extent.