Preparation Of Carbon Nanotubes/Lyocell Composite Fibers And Their Application

Lyocell fiber is a new kind of regenerated cellulose fiber and is expected to replace the rayon fiber in the future due to its simple, advanced and environmentally friendly process. Lyocell fiber has lots of excellent properties such as high dry strength and wet strength, good hand-feeling and water-absorbing properties, and so on. Therefore, it is widely used in the textile field. It can expectedly be used as precursor of carbon fiber, tire cord, electrical conductive fiber and anti-bacterium fiber etc. after being modified and will have wide potential application prospects in such fields as industry, spaceflight and medical treatment etc..To the date, there are a few reports on modified Lyocell fiber and no work has been reported on Lyocell fiber modified with carbon nanotubes (CNTs). In this thesis, multiwalled carbon nanotubes (MWNTs) were used as an additive of Lyocell process. The purification and functionalization of MWNTs, as well as their dispersion stability in the NMMO aqueous solution were studied. Moreover, the effect of MWNTs on the rheological behaviors and spinnability of the spinning dope, the structure and properties of MWNTs/Lyocell composite fiber were also investigated. Based on these works, the MWNTs/Lyocell-based carbon fibers were prepared by using MWNTs/Lyocell composite fibers as the precursor. In addition, the effects of the other additives such as nano-scale carbon black and single-walled carbon nanotubes (SWNTs) on the structure and properties of Lyocell fiber were further studied.In this thesis, the purification and functionalization of MWNTs, and their dispersion stability in the NMMO aqueous solution were studied firstly. The results from TEM, TGA, FTIR and Raman showed that the purity of MWNTs was increased and the carboxylic and hydroxyl groups existed on the surface of MWNTs after being treated with the nitric acid. The results by sedimentation test and using microscope further showed the MWNTs modified with appropriate amount of sodium dodecylbenzene sulfonate (SDBS) could be dispersed uniformly in the NMMO aqueous solution and cellulose/NMMO·H2O solution.The effect of MWNTs on the rheological behavior and spinnability of the cellulose/NMMO·H2O solution were also studied by HAAKE rheometer and two types of spinning devices. The results showed that the cellulose/NMMO·H2O solution filled with MWNTs belongs to the typical shear thinning fluid. The apparent relative molecular weight of cellulose, viscosity, flow activation energy, structural viscosity index and elasticity of the solution were increased firstly with the addition of MWNTs and reached the highest value with 1% MWNTs, and then decreased with the further addition of MWNTs. In the range of our experiments, it was found that the uniformity and spinnability of cellulose/NMMO·H2O spinning dope were also affected by the MWNTs content, and the spinning process was run smoothly if the content of MWNTs was not higher than 1%.Based on the above works, the structures and properties of MWNTs/Lyocell composite fibers were further investigated by using tensile meter, SEM, FESEM, WAXD, TGA, two dimentional X-ray diffraction and low current-high resistance meter etc.. It was found that by the addition of MWNTs the crystal structure and crystallinity of Lyocell fiber were hardly affected, while its thermal stability was increased in some extent. An appropriate amount of MWNTs could be dispersed uniformly in the Lyocell matrix and that could improve the mechanical properties of the resultant composite fiber. Compared with the pure Lyocell fiber, the modulus and tenacity of the composite fiber containing 1% MWNTs were increased by 41.9% and 22.2%, respectively. Moreover, the mechanical properties of MWNTs/Lyocell composite fibers could be also improved by increasing the jet stretch ratio appropriately. In addition, MWNTs in the fiber almost aligned along the axis of the fiber and the orientation of MWNTs increased with the increasing jet stretch ratio. In our case it was found that more MWNTs content and lower jet stretch ratio could improve the electrical conductivity of the composite fiber. The volume conductivity of the 5% MWNTs/Lyocell composite fiber prepared under a weaker spinneret draft was 5 orders higher than that of pure Lyocell fiber.The MWNTs/Lyocell based carbon fiber was prepared by using the MWNTs/Lyocell composite fiber as precursor and compared with the pure Lyocell based carbon fiber. The results showed that both MWNTs/Lyocell composite fiber and pure Lyocell fiber had smooth surface and round cross-section. Compared with pure Lyocell fiber, the composite fiber with appropriate amount MWNTs was much more suitable for using as carbon fiber precursor due to its higher tenacity and more compact structure. The Lyocell fiber and MWNTs/Lyocell composite fiber were treated under different pre-oxidation and carbonization temperatures. It was found that the crystallinities of the two kinds of fibers were gradually decreased with the increasing of the treatment temperature. After being treated at 250℃, the crystal structures of MWNTs/Lyocell composite fiber and Lyocell fiber were seriously damaged and the monoclinic crystal system of their original celluloseⅡdisappeared. The monoclinic crystal system of cellulose II of precursor was converted to a two-dimensional turbostratic graphite structure of carbon fiber after carbonization but the crystal sturcture of MWNTs was always kept in the fiber during the treatment process. The resultant carbon fiber had low crystallinity. It was further fount that the structure of carbon fiber was affected by the spinning process of precursor, its fineness and the content of MWNTs in the precursor and so on. The diameter of precursor made from spinning process B was finer and more uniform than that from the spinning process A. Therefore, the surface of resultant carbon fiber was more smooth and no obvious hole flaw existed in the carbon fiber. Compared with Lyocell-based carbon fiber, the tenacity and modulus of 1% MWNTs/Lyocell-based carbon fiber were increased by 70% and 116%, respectively.At the same time, the effects of the other additives such as nano-scale carbon black and SWNTs on the structure and properties of Lyocell fiber were also investigated. It was found that by the addition of carbon black the mechanical properties of the composite fiber were decreased, but its conductivity was improved. Compared with the MWNTs/Lyocell composite fiber, the conductivity of MWNTs /carbon black/Lyocell composite fiber was improved in some extent. Furthermore, because SWNTs had more perfect structure and better electricity property than MWNTs, the mechanical properties, volume conductivity and thermal stability of SWNTs/Lyocell composite fiber were better than those of MWNTs/Lyocell composite fiber in the case of the same carbon nanotubes'content. In addition, the variation of the morphological structure of composite fibers with the addition of SWNTs was similar to that of the MWNTs/Lyocell composite fibers.