Effect of functionalization and adding method of carbon nanotube on interlaminar property of carbon fiber/epoxy composite

Using carbon nanotubes (CNTs) to toughen polymer and its fiber reinforced composites has been extensively studied, but the selection of nanotube and preparation process optimization still lack adequate theoretical support and data basis. Meanwhile, most researches adopted CNTs to modify polymer prepared in laboratory, rather than ones with high performances in practical application. This paper provided a research on an aerospace grade carbon fibers/epoxy composite system which has been used in practice, and proved the feasibility of toughening composite by CNTs. CNTs modified epoxy resin was prepared by directly adding non-functionalized CNTs, carboxyl-CNTs and amino-CNTs into resin, respectively. The resin was used to prepare carbon fiber prepreg, and the laminates were cured by autoclave process. Adding carboxyl-CNTs increases Mode II interlaminar fracture toughness of composite laminate evidently, but all kinds of CNTs show no significant effect on Mode I interlaminar fracture toughness, which is related to the distribution and orientation of CNTs. Furthermore, the influences of CNTs on resin properties and the interface between carbon fiber and resin were evaluated. The results show that adding CNTs increases the resin viscosity, and decreases the degree of curing reaction. The carboxyl-CNTs greatly enhance the interfacial bonding strength due to chemical reaction with sizing agent of carbon fiber. Moreover, spraying the carboxyl-CNTs on the surface of prepreg was applied to prepare composite laminates, and larger improving degree on interlaminar property than that with directly dispersing the CNTs in resin is found. It is attributed to the high utilization degree of CNTs in the spraying method.