Preparation And Interface Modification Of Carbon Fiber Reinforced PA12T Composites

Carbon fiber reinforced thermoplastic composites have the advantages of short molding cycle,easy processing,excellent mechanical properties and recyclability,and have gradually become one of the research hotspots of carbon fiber composites.Poly(dodecamethylene terephthalamide)(PA12T)in the new high temperature resistant polyamide is expected to enter the composite material market of carbon fiber reinforced special engineering plastics,with its extremely low moisture absorption rate and molding shrinkage rate,good heat resistance,abrasion resistance and solvent resistance,excellent dimensional stability and mechanical strength.However,the carbon fiber surface is chemically inert,and the traditional carbon fiber surface is coated with a layer of epoxy sizing agent,which is easily degraded during high temperature processing,resulting in the formation of CF/PA12 T composites.The wettability is not good,and it is easy to debond at the weak interface during use,which affects the mechanical properties of the CF/PA12 T composite.Therefore,it is necessary to further improve the interface bond strength and mechanical properties of CF/PA12 T composites by surface treatment of carbon fibers.In order to prepare a sizing agent suitable for CF/PA12 T composites,this thesis selected monomers containing polar groups and monomers that can produce branching to prepare water-soluble copolyamides.First,nylon 66(PA66)salt and nylon 6T(PA6SSI)salt containing sodium sulfonate side groups were prepared by solution method,and then the copolyamide was further synthesized by high temperature and high pressure melt polycondensation.And before the reaction,undecylenic acid is added to cap the copolyamide,and the copolyamide is branched by the selfpolymerization of olefins.Finally,the chemical structure,heat resistance and water solubility of the synthesized copolyamides were systematically studied.The results showed that the synthesized copolyamide contained amide bond structure and sulfonic acid group structure,and the ratio of PA6 SSI salt segment to PA66 salt segment was0.4:1.It was found by gel chromatography that the number-average molecular weight(Mn)of the synthetic copolyamide was 20,000 to 25,000.It was found by nanometer particle size meter that the particle size of copolyamide in water was mostly concentrated between 200 and 700 nm,and the particle size distribution index was narrow.It could be homogeneously dissolved in water and could be used to configure sizing agent.Then,this thesis selected PA12 T as the thermoplastic resin matrix,and prepared CF/PA12 T composites by vacuum molding process.The effects of different molding temperatures and pressure gradients on the mechanical properties of CF/PA12 T composites were systematically studied,and the correlation mechanism between the structure and mechanical properties of CF/PA12 T composites was analyzed.The research results showed that under the process conditions of 350? and 8 MPa,PA12 T had the best impregnation effect on carbon fibers in CF/PA12 T composites.The storage modulus and loss modulus were the highest,and the loss angle was the lowest.The crystal structure of PA12 T was the most perfect,and its crystallinity reached 32.42%.The comprehensive mechanical properties of the CF/PA12 T composite laminate were the best,with a tensile strength of 914 MPa,a tensile modulus of 64 GPa,a flexural strength of 596 MPa,a flexural modulus of 60 GPa,and an interlaminar shear strength of 40 MPa.At this time,the bending fracture form of CF/PA12 T composite material was the coexistence of resin and carbon fiber debonding and resin cracking.Finally,the CF was heated and refluxed with acetone to remove the original epoxy sizing agent.The CF was surface activated with Michaelis acid to introduce carboxyl groups.Then,the copolyamide synthesized in this paper was used for sizing treatment of CF,and the modified CF reinforced PA12 T composite was prepared by vacuum molding process.The structure and properties of CF/PA12 T composites were systematically studied.The research results showed that the activation energy of Michaelis acid was better to introduce polar oxygen-containing functional groups on the surface of carbon fibers,which increased the oxygen-to-carbon ratio on the surface of carbon fibers.After the introduction of copolyamide sizing agent,the content of carbon elements on the surface of carbon fibers further decreased,the content of polar functional groups increased,and the surface roughness of carbon fibers increased,which greatly improved the surface energy of carbon fibers.The storage modulus of the sized modified CF/PA12 T composite was 56 GPa,which was 17% higher than that of the desized 48 GPa,and it also had a relatively low loss angle inscribed value.The flexural strength of the sizing-modified CF/PA12 T composite could reach 840 MPa,the flexural modulus could reach 74 GPa,and the interlaminar shear strength could reach49 MPa.At this time,the bending fracture form of the CF/PA12 T composite material was the cracking of the resin.