Chemical Modification Of Carbon Fiber Surface And Study Of Its Composite Interface Performance

Carbon fiber（CF） reinforced polymer composites have been widely used in many fields, such as aerospace, automotive and ship industry because of their low weight and high strength. However, weak interface and low efficiency of reinforcement become the obstacles to the development of domestic CF reinforced composites. In this paper, based on the bio-inspired dendritic structure and excellent properties of supercritical fluid, the dendritic small molecules hexamethylenetetramine（HMTA）/CF, branched polymer polyethylenemine（PEI）/CF, organic amine-titania nanowire（TiO₂ NWs）/CF hierarchical structure were prepared as reinforcement, the interphase properties of polymer composites were investigated, and the enhancement mechanism was also expounded and analyzed.Chemical grafting was used to introduce HMTA onto CF surface, HMTA evolved into three generations of dendrimers according to the features of HMTA structure. The physical and chemical properties of CF surface were studied, and the results showed that the polar group content, surface roughness and surface energy of CF surface increased markedly with the dendritic generation. Compared with untreated CF, the interface shear strength（IFSS） and impact strength of HMTA grafted CF（CF-G1-HMTA）/epoxy composite increased by 45.9% and 15.8%, and the strength of three generations of HMTA grafted CF（CF-G3-HMTA）/epoxy composite further increased, by 65.6% and 33.6%. The tensile strength also increased with the dendritic generation. In addition, the interface chain length had significant influence on the interfacial adhesion of CF/epoxy composites, the longer chain length of the grafted 1,6-dichlorohexane was crucial for increasing the IFSS of the CF/epoxy composites（by 67.5%）, which were attributed to the lower in space steric effect and more polar groups and larger surface roughness. Therefore, the IFSS could be controllable through changing the dendritic generation and chain length.Branched polymer PEI was grafted onto CF surface in supercritical methanol. The unique three-dimensional branched structure and numerous polar functional groups of PEI improved the surface roughness, wettability and chemical reactivity of fiber surface. Supercritical methanol with strong mass transfer and infiltration characteristics improved the reaction efficiency significantly, the reaction time was reduced from 80 h to 10 min now, and more PEI was evenly combined onto CF surface. The enhancement rate of IFSS and impact strength of PEI grafted CF/epoxy composite reached 82.6% and 44.0%. CF grafted with PEI retained the high tensilestrength, and supercritical methanol treatment did not cause serious damage on the CFs. Moreover, branched PEI was deposited successfully onto CF surface via Van der Waals force（CF-c-s-PEI）, zwitterionic interactions（CF-ad-s-PEI） and covalent bonds（CF-g-s-PEI） in supercritical methanol, the bonding type had some influence on interface properties of composite. The interface failure mode for CF-c-s-PEI and CF-ad-s-PEI composite was the adhesion failure mode, the interface was weak with higher impact strength. When PEI was grafted chemically onto CF surface, the interfacial failure transferred to cohesive dominant mode, and the stronger interfacial bonding led to a higher stress concentration at the interphase, which resulted in the slight decrease of impact resistance.TiO₂ NWs was grown onto the carboxyl CF surface in supercritical water, TiO₂ NWs/CF hierarchical reinforcement was prepared. TiO₂ NWs was rutile type crystalline structure. The surface morphology of TiO₂ NWs was controlled by changing the growth environment and time, and the relationship among process parameters, surface morphology and interfacial properties was systematically studied. Strong mechanical interlocking and good wettability were the main contributors for the enhanced interfacial properties of composites. Compared with untreated CF, the tensile strength of CF after TiO₂ NWs modification had no observed decrease. Furthermore, growing TiO₂ NWs onto CF-G3-HMTA and CF-g-s-PEI led to a high density and uniform morphology owing to the compatibility principle. TiO₂ NWs, as a transitional interface layer, increased the fiber surface roughness, and the mechanical interlocking between fibers and matrix, thus restricting the resin mobility in interphase region. Load could be transfered more evenly and more failure energy could be absorbed. The IFSS of TiO₂ NWs grown on CF-G3-HMTA（CF-HMTA-s-TiO₂ NWs）/epoxy composite and TiO₂ NWs grown on CF-g-s-PEI（CF-PEI-s-TiO₂ NWs）/epoxy composite increased by 72.7% and 88.6%, and the interface failure mode also changed to the cohesive mode from the original adhesion failure mode.