Preparation Of Lyocell Fibers Used As Tire Cord

Lyocell fiber is a new kind of regenerated cellulose fiber developed in the 1990s. Compared with traditional rayon fibers, it has higher strength and modulus. Lyocell process is a simple and environmentally friendly process. It can use the cellulose pulps with higher polymerization degree to prepare fiber directly, which is impossible for rayon process. Therefore, Lyocell fiber can expectedly be used as tire cord to replace the existing rayon tire cord.To the date, there is no report on Lyocell fiber used as tire cord. In this thesis, the dissolving characteristic of various cellulose raw materials and the rheological properties of the spinning dopes were investigated firstly. Then the effects of various factors such as the raw material properties, the spinning condition, the addition of ammonium chloride and 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 performance Lyocell fiber used as tire cord have been found.In this thesis, the dissolving process of cellulose raw materials were observed by using a polarizing microscopy with CCD and heating stage and the rheological properties of the dopes were studied by HAAKE rheometer. The results showed that the higher the relative molecular weight of cellulose, the stronger dependence of dissolving rate on temperature, the higher the activation energy and the more notable the shear-thinning. The results based on the mechanical properties of the resultant fibers also showed that the polymerization degree of cellulose raw materials played a more important role in improving the mechanical properties of fibers thanα-cellulose content of raw materials. If the polymerization degrees of raw materials were similar, the mechanical properties of Lyocell fibers can be improved by increasingα-cellulose content of raw materials. The highα-cellulose content of raw material had positive effect on heat and size stability of Lyocell fiber. Moreover, the spinning properties of dopes from the cellulose mixture formed by mixing high polymerization degree pulp with generally used pulp were studied by GPC. It was found that the mechanical properties of Lyocell fiber could be improved by mixing an appropriate amount of high polymerization degree cellulose raw material with generally used cellulose raw material. However, if the difference between the relative molecular weights for two raw materials was too big, the resultant dope would be separated during its passing through filter and therefore the mechanical properties of prepared Lyocell fiber could not be improved effectively. Base on the above studies, Lyocell fibers with the properties which could accord with those of the fiber for tire cord were prepared by using appropriate cellulose raw materials with high relative molecular weight.In this thesis, the feasibility of preparing Lyocell fiber used as tire cord by heat treatment of as-spun fiber from medium relative molecular weight cellulose raw material was also investigated. The heat treatment of never dried as-spun was carried out by using a home-made heat treatment equipment. It was found that after heat treatment with a pre-tension of 0.75cN/dtex and a heat-treatment tension of 0.60cN/dtex under 130℃, the tensile strength and initial modulus of Lyocell fiber could be up to 5.8cN/dtex and 86.9cN/dtex respectively, and its dry heat shrinkage was lower than 1.1%, which accord with the properties of the fiber for tire cord.In addition, the feasibility of preparing high performance Lyocell fiber by adding ammonium chloride in the dope was further investigated in this thesis. The results showed that the viscosity of dope was increased with increasing ammonium chloride content in dope. The mechanical properties of Lyocell fiber could be improved by adding an appropriate amount of ammonium chloride and the modified Lyocell fiber has a mixture of cellulose II crystal structure and cellulose III crystal structure. Especially, the strength and the modulus of Lyocell fiber modified by ammonium chloride combined with heat treatment can be up to 7.21cN/dtex and 92.6cN/dtex, respectively. This means that the modification by ammonium chloride is also an effective method to prepare Lyocell fiber with excellent mechanical properties.