The Research On Preparation Of High Strength And Modulus Lyocell Fiber

Lyocell fiber is a new regenerated cellulose fiber, which use N-methylmorpholine-N-oxide monohydrate (NMMO·H2O) as the solvent to dissolve the cellulose and spin the Lyocell fiberby a special dry-jet wet spinning process. Compared with viscose fibers, it has higher strength and modulus and the Lyocell production process is simpler and more environmental-friendly. Therefore, Lyocell fiber is expected toreplace the viscose fiber to be a new material of tire cord.The heat resistance, croop property and dimensional stability of Lyocell fiber is better than the traditional tire cord material such as polyester and the adhesion between Lyocell fiber and rubber is excellent. At present, the strength and modulus of conventional Lyocell fiber is unable to meet the requirements of high performance tire cord. Thus the mechanical properties of Lyocell fiber should be improved to prepare high strength and modulus Lyocell fiber.In thisthesis, the rheological properties of various spinning dopes were investigated firstly. Then the effects of various factors such as the addition of polyvinyl alcohol and ammoniumchloride, the spinning condition and the heat treatment on the structure and properties of the resultant Lyocell fiber were also investigated. Base on the above studies, some feasible ways for preparing high strength and modulus Lyocell fiber have been found.In this thesis, the effect of the addition of the polyvinyl alcohol on the structure and properties of the resultant Lyocell fiber was investigated. The rheological properties of cellulose solutions with different polyvinyl alcohol contents were studied by HAAKE rheometer. The results showed that the apparent viscosity, zero-shear viscosity and structural viscosity index of cellulose solution decreased firstly and then increased with the increase of the polyvinyl alcohol, and the solution showed the minimum apparent viscosity when the content of polyvinyl alcohol was3%wt. Besides, the dissolution homogeneity of cellulose decreased with the addition of polyvinyl alcohol. The result of mechanical properties test showed that the tensile strength and initial modulus of Lyocell fiber increased obviously with the addition of polyvinyl alcohol. XRD result showed that the modified Lyocell fiber still had the cellulose II crystal and the crystallinity increased firstly and then decreased. The addition of polyvinyl alcohol improved the thermostability and fibrillation resistance of Lyocell fiber.In order to enhance the mechanical properties of Lyocell fiber, the feasibility of preparing high strength and modulus Lyocell fiber by adding polyvinyl alcohol and ammoniumchloride as composite modifier in the dope was further investigated. The rheological properties of cellulose solutions with different composite modifier contents showed that polyvinyl alcohol and ammoniumchloride would decrease and increase the apparent viscosity, zero-shear viscosity and structural viscosity index of cellulose solution respectively and the dissolution homogeneity of cellulose decreased. It was also found that the mechanical properties of Lyocell were improved by adding the composite modifier and the modification effect of composite modifier was better than simplex modifier. The mechanical properties reached the maxinum when the mass ratio between polyvinyl alcohol and ammoniumchloride was3:7.XRD result showed that the modified Lyocell fiber still had the cellulose II crystal and the crystallinity could be increased with adding modifier. In addition, the simplex modifierand the composite modifier could both improve the fibrillation resistance of Lyocell fiber.The effect of spinning condition and the heat treatment on the structure and properties of the resultant Lyocell fiber were also investigated. The results showed that the tensile strength and initial modulus increased with the increase of spinneret diameter, but the oversize diameter could lead to the decrease of the crystallinity and orientation of the fiber and decrease the mechanical properties; the increase of spinneret length/diameter ratio or spinning speed could also increase the crystallinity and orientation of Lyocell fiber and result in the increase of tensile strength and initial modulus of the fiber;after the heat treatment, the mechanical properties of Lyocell fiber were improved significantly, compared with Lyocell fiber without heat treatment, the tensile strength and initial modulus were increased by21.6%and67.3%respectively.