The Fundamental Research On The Functionalized Carbon Nanotube/Carbon Fiber Multi-Scale Reinforced Epoxy Composites

As the weakest and first damaged component, resin matrix is the main factor to determine the total performance of the resulting fiber reinforced composites. At present, the use of functionalized carbon nanotubes (CNTs) modified epoxy resin to prepare multi-scale carbon fiber reinforced composites has been seen as one of effectively technicals to solve these issues, such as ply delamination, low impact strength and interfacial interaction between carbon fiber and resin matrix, as well as poor mechanical properties out of the plain. However, for the practical application, there are many factors, including the dispersion of CNTs in the resin matrix, the physical and chemical interactions between the functionalized CNTs and epoxy/curing agent, cure kinetics of the matrix material, the processing technology. Furthermore, each factor always influences one another, resulting in the unstable and unsatisfactory enhancing effects of nanoparticles in resin matrix and the resulting carbon fiber reinforced composites. To address these issues above, this dissertation carried out a series of research work, which would provide theoretical and experimental basis for preparation of advanced fiber reinforced composites.(1) MWCNTs-NH2 with surface covalently linked ethylenediamine (EDA) were prepared through the amidation reaction process from commercial MWCNTs-COOH. The potential chemical reaction between functionalized MWCNTs and epoxy/aromatic amine curing agents and its effect on the dispersion stability of MWCNTs during the mixing and curing process, and the mechanical, glass transition temperature (Tg) as well as interfacial properties of resin matrix and the resulting carbon fiber reinforced composites were investigated. The results showed that MWCNTs-COOH tended to re-aggregate during the fabrication process of resin, whereas MWCNTs-NH2 can maintain the dispersion stability due to the presence of amine curing agents, which would ensure MWCNTs-NH2 more efficiently participate in the cross-linking reaction of epoxy resin and the interface strengthening between carbon fiber and resin matrix, and then resulting in the improved mechanical, thermal-mechanical and interfacial performances compared to the carbon fiber composites based on the neat epoxy and MWCNTs-COOH reinforced ones.(2) The interfacial interaction between MWCNTs-NH2 and TDE-85/AFG-90 epoxy molecule and its relation with the dispersion of MWCNTs-NH2 in the resin matrix, the curing kinetics, and crosslinking network as well as the mechanical properties of the final nanocomposites were investigated by using molecular simulation and experiments. The results showed that higher interaction between MWCNTs-NH2 and epoxy molecular was beneficial to improve the dispersion of MWCNTs-NH2 and tensile toughness reinforcement in the resulting cured resin matrix. Moreover, the results of non-isothermal DSC and FT-IR showed that MWCNTs-NH2 can regulate the original curing reaction and the crosslinking network of epoxy-amine system, characterised by the changes of etherification reaction between hydroxyl and epoxy groups. When composites were under compression stress, compared with neat epoxy, the crosslinked network with increased relative aliphatic ether bond can give full play to the synergistic enhancement effect of MWCNTs as evidence of the improved compressive strength of the composites.(3) The MWCNTs-NH2 were converted into liquid-like MWCNT reinforcements through the reaction with butyl glycidyl ether. Then the liquid-like MWCNT reinforcements were added into the epoxy matrix for domestic T300 carbon fiber filament wound composites. The effects of liquid-like MWCNT reinforcements on the processing, mechanical and interfacial properties of carbon fiber filament wound composites were investigated. The results confirmed that some critical issues, such as the dramatically increased viscosity, dispersion and variation of processing with addition of the nano-reinforcements can be effectively solved by the liquid-like MWCNT reinforcements. The liquid-like MWCNT reinforcements with reasonable material composition can be uniformly dispersed in the epoxy matrix and participate in the cross-linking reaction with epoxy groups. In addition, the liquid-like MWCNT reinforcements can efficiently enhance the interfacial properties of carbon fiber filament wound composites, improving the stress transfer from resin matrix to carbon fibers. Compared with neat epoxy matrix, an enhancement of 8 ℃ in Tg and 8% in interfacial laminar shear stress (ILSS) as well as 17% in tensile strength of T300 carbon fiber NOL-rings were achieved by adding the liquid-like MWCNT reinforcements.(4) A kind of semi-solid MWCNT reinforcements was prepared via regulating the solid-liquid ratio of the liquid-like MWCNT reinforcements above. Different contents of the semi-solid MWCNT reinforcements were dispersed in the epoxy resin, and unidirectional T700 carbon fiber prepreg with containing MWCNTs was prepared by using hot-melt method. Subsequently, the resulting composite laminates were prepared via hot-pressing process and the effects of semi-solid MWCNT reinforcements on the interlaminar shear, impactive and compressive performance of T700 carbon fiber composites were investigated. The results showed that the semi-solid MWCNT reinforcements improved the impregnation of MWCNTs with epoxy and effectively solved the dispersion challenge of MWCNTs in the high viscosity resin system. Moreover, spontaneous formation of crosslinking network of MWCNTs between semi-solid MWCNT reinforcements and epoxy was observed, resulting in the improvement of the dispersion stability of MWCNTs in the epoxy (without curing agents). Compared with neat epoxy, the addition of semi-solid MWCNT reinforcements can effectively improve the interfacial properties between carbon fiber and resin matrix of composites, and therefore the synergistic effects of reinforcing and toughening of MWCNTs have been realized, leading to the enhancement of ILSS, impactive and compressive performance of laminated composites. Compared to composites based on the neat resin system, an enhancement of 14% in ILSS and 27% in compression-after-impact (CAI), as well as an enhancement of 11% in 0° compressive strength and 11% in 90° compressive strength was achieved with addition of 2 wt% semi-solid MWCNT reinforcements, which can better satisfy the requirements, i.e. high compressive and impact performance of the carbon fiber composites for certain practical application.