Effects Of CNT Dispersion And Contents On The Electrical And Mechanical Properties Of The Epoxy Matrix Composites

The mechanical and electrical properties of fiber reinforced composites are extremely anisotropic, due to the traditional one dimensional reinforcement. Carbon nanotubes(CNTs) exhibit excellent physical and chemical properties, which could be used as a kind of nano-sized reinforcement in the composites. Because of its high specific surface area, high aspect ratio and extended π-electron system, CNTs possess outstanding mechanical and electrical properties. This paper studied the effect of surfactants, CNT contents and chemical groups on the dispersion of CNTs in the epoxy resin and the conductivity of CNT/epoxy composites. T700 carbon fiber, and amino-functionalized multi-walled carbon nanotubes(MWCNTs-NH2) or buckypapers made of MWCNTs, co-reinforced epoxy resin composites were also fabricated. Both the mechanical and electrical properties of the above composites were investigated, with respect to CNT dispersion and contents.Firstly, the CNT dispersion in the epoxy resin was assessed by optical microscope images. Experimental results showed that a more homogenous dispersion of CNTs was obtained via high speed shearing, ultrasonication, surfactants treatment and chemical functionalization of CNTs. The dispersion of CNTs, treated by nonionic surfactants like Triton X-100, was more homogenous than that treated by anionic surfactants like sodium dodecyl sulfate(SDS) or without surfactants. Dispersion quality of CNTs grafted with different functionalization was also characterized, and the amino-functionalized CNTs achieved the best dispersion among the four kinds of CNTs.Then, epoxy composites reinforced by different CNT contents were prepared by casting method. The electrical property and the relationship between microstructure and property of the composites were studied by megger and SEM. The experimental results showed that the conductivity of the composites added with SDS treated CNTs was better than that added with Triton X-100 treated CNTs or pristine CNTs. The results also indicated that an effective electron transport channels formed in the composites with CNTs, and that the electrical conductivity of the epoxy composites was increased by seven orders of magnitude with 2.0 wt% MWCNTs-NH2.Based on the above results, the mechanical and electrical properties of CNTs and unidirectional T700 carbon fibers(CF) co-reinforced epoxy composites were further investigated. Results showed that the mechanical and electrical properties of CNT-CF/epoxy composites were increased with the increasing contents of CNT addition. Compared to the CF/epoxy composites, 7.3 % improvement in inter-laminate shear strength of CNT-CF/epoxy composites with 1.5 wt% CNTs was observed. An increase of 21.2 % and 15.9 % for the flexural strength and modulus of the composites with 2.0 wt% of MWCNTs-NH2 was obtained, respectively,, and meanwhile an improvement factor as high as five and ten for the longitudinal and transverse electrical conductivities respectively, was obtained.However, due to its large aspect ratio and high surface energy, CNTs are easy to agglomerate, and low CNT contents in the matrix so far was kept quite low. Relatively high contents of CNTs might be incorporated into the composites in the form of buckypaper. Unidirectional T700 CF and buckypaper co-reinforced epoxy composites were fabricated. SEM images of the fracture surfaces of buckypaper-CF/epoxy composites indicated that abundant well-dispersed CNTs existed at the fracture surfaces and the structure of buckypaper in the composites remained quite intact. The mechanical and electrical properties of buckypaper-CF/epoxy composites were observed to be even more highly improved. An increase of 6.9 % for the inter-laminate shear strength of the composites was obtained for the composites with six CF prepregs and five buckypaper interleaves. Especially, the transverse electrical conductivity of buckypaper-CF/epoxy composites was increased by two orders of magnitude approximately.Finally, mechanisms of the reinforcements on the electrical property of the composites were discussed. There were two kinds of conducting network in the composites: one is formed by the contacts between carbon fibers; the other is formed by the contacts between CNTs. With the addition of CNTs, a uniform CNT network was formed, and electron transfer paths between CF were improved by the brige of CNTs, resulting in the improvement of the electrical conductivity of CF/epoxy composites.