Modification of Carbon Fiber Surface by Triazine and Study of Its Composite Performance

Author:Zhao Min

Supervisor:meng ling hui


Degree Year:2019





Carbon fibers(CFs)have been widely used as the reinforcement of polymer composites due to the excellent mechanical properties.However,the interfacial bonding between CFs and resin matrix is poor,which further affects the overall performance of the composites due to the strong inertness surface of CFs.Therefore,the interface should be optimized for preparing high-performance CF composites.In this thesis,in order to improve the interfacial properties of carbon fiber/epoxy resin composites,the interface of the composites was designed combined with the structural characteristics of triazine molecules.Melamine,triazine-based dendrimers and carbon nanotubes(CNTs)were respectively chemically grafted onto the surface of carbon fiber,and then the interfacial strengthening mechanism of CFs and their composites were investigated.Melamine was first chemically grafted onto CFs surface in subcritical water and supercritical methanol,respectively,and the properties of CFs and their composites treated for different reaction time were investigated.The results showed that with the reaction time increasing from 5 min to 35 min,the amino content,wettability and surface energy of CFs were significantly improved.The interfacial shear strength(IFSS)and impact strength of carbon fiber composites were also enhanced.Compared with those of untreated CF composite,the IFSS and impact strength of the composites increased by 35.1%and 24.4%,respectively after treatment for 35 min.When the supercritical methanol was used as the medium,the mechanical properties of the CFs/epoxy resin composite were the optinal at 25 min,while the impact strength was still highest at 35 min.In addition,the relatively mild reaction conditions of supercritical methanol were more conducive to the reaction and have less damage to the strength of carbon fiber,thus the properties of carbon fiber and its composites were improved significantly.The triazine-based dendrimers derived from triazine molecules were grafted onto the surface of CFs.Initially,the first generation dendrimer was formed on CFs surface with the cyanuric chloride as the active center,meanwhile ethylenediamine(EDA)and p-phenylenediamine(PPD)was used as the amino donors,respectively.The bonding ability between CFs and resin has been enhanced for the strong rigidity of PPD,which is more conducive to the improvement of composite properties.On this basis,three generations of triazine dendrimers containing terminal amino groups were grafted.With the increase of generation,the rigid branched structure of dendrimers and a large number of polar groups caused obvious changes in fiber surface activity and roughness,thus enhancing the chemical and physical interaction at the interface.The gradually increasing molecular chain was helpful to the load transferring uniformly at the interface and enhanced the restricting of the resin mobility in the interphase,which effectively prevent the interface cracking.After grafting the third generation of dendrimer,the IFSS of CFs/epoxy resin composite was increased by61.8%.Meanwhile,it also showed the best impact resistance.Chemical grafting was used to immobilize functional carbon nanotubes(CNTs)onto CFs surface and the CNTs/CFs hierarchical reinforcement was prepared.The results showed that functional CNTs can improve the wettability of CF and increase the bonding strength between CFs and resin.Furthermore,CNTs can also increase the surface area and surface roughness of fibers,which could improve the interlocking between CFs and resin and make the load the transfer more uniformly in the interphase.Melamine and functionalized CNTs were alternately grafted onto the surface of CFs by the LBL and chemical grafting.The interfacial compatibility among the fibers and the resin was improved due to the multiscale structure and higher polar groups on CFs after grafting.In addition,a higher density and uniform distribution of CNTs on the CFs surface was realized on account of the"support"effect of melamine between CNTs layer.Moreover,CNTs can restrict the movement of epoxy molecular on fiber surface,which give rise to the enhancement of interfacial properties.As a result,the IFSS of the CF-MC-L3 composite was enhanced by 104.4%and the interfacial failure mode changed into cohesive failure of the matrix.