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 higher relative molecular weight cellulose as raw material 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 date,there is no report on Lyocell fiber used as tire cord.In this thesis,the dissolving characteristic of various celluloses 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,heat treatment and cross-link treatment on the structure and properties of the resultant Lyocell fiber were also investigated.Base on the above studies,some feasible ways for preparing Lyocell fiber used as tire cord have been found.In this thesis,the dissolving process of cellulose pulps were observed by using a polarizing microscopy with CCD and a heating stage and the rheological properties of the cellulose solutions were studied by HAAKE rheometer.The results showed that the higher the relative molecular weight of cellulose,the stronger the dependence of dissolving rate on temperature and the higher the activation energy of the cellulose solution.Also,the homogeneity of the solution became poorer and its critical shear rate was decreased.Therefore,the parameters for dissolution and spinning,such as temperature and so on,should be controlled strictly when the cellulose pulp having a higher relative molecular weight was used to prepare Lyocell fiber.The results based on the mechanical properties of the resultant fibers also showed that the relative molecular weight of cellulose played a more important role in improving the mechanical properties of fibers thanα-cellulose content.If the relative molecular weights of celluloses were similar,the mechanical properties of Lyocell fibers can be improved with the increasing ofα-ceUulose content in cellulose.The highα-cellulose content in cellulose had positive effect on heat and size stability of Lyocell fiber.Moreover,the properties of spinning dopes from the cellulose mixed by high（or super-high） relative molecular weight cellulose and medium relative molecular weight cellulose were analyzed by GPC.It was found that the mechanical properties of Lyocell fiber could be improved by mixing an appropriate amount of high relative molecular weight cellulose with medium relative molecular weight cellulose.However,if the difference between the relative molecular weights for two celluloses was too big,the resultant dope would be separated during its passing through filter of spinning components and therefore the mechanical properties of prepared Lyocell fiber could not be improved effectively.Therefore,using high relative molecular weight cellulose or its mixture with medium relative molecular weight cellulose as raw material is a feasible way for improving mechanical properties of Lyocell fiber and preparing Lyocell fiber used as tire cord.In this thesis,the feasibility of preparing high performance Lyocell fiber by adding ammonium chloride in the dope was further investigated. The results showed that the effect of adding ammonium chloride on the rheological properties of cellulose/NMMO·H₂O solution was similar to that of increasing relative molecular weight of cellulose.With the increase of ammonium chloride in the solution,the apparent viscosity,zero-shear viscosity and structural viscosity index for cellulose solution were increased, while the dissolution homogeneity of cellulose was decreased.In order to insure good spinnablity and spinning stability,the adding amount of ammonium chloride couldn’t be too high.It was also found that the mechanical properties of resultant Lyocell fiber modified by an appropriate amount of ammonium chloride were improved and the modified Lyocell fiber has a mixture structure of celluloseⅡcrystal and celluloseⅢcrystal. Furthermore,the total crystallinity and orientation could be increased with adding ammonium chloride.This means that the modification by ammonium chloride is also an effective way to prepare Lyocell fiber with excellent mechanical properties.In order to find an effective method to improve the crystalline and orientation structure of Lyocell fiber,the formation of crystal structure of Lyocell fiber was also studied in this thesis.It was found the tridimensional order of never dried as-spun Lyocell fiber was not fully formed yet.With the decreasing water content of as-spun Lyocell fiber,its paracrystal structure disappeared gradually and meanwhile the crystal structure was improved.Once the Lyocell fiber was dried completely,its crystal structure had been stabilized and was difficult to be destroyed and rearranged. Therefore,it is hopeful to improve crystalline structure and mechanical properties of Lyocell fiber by appropriate post treatment using never dried Lyocell fiber as precursor.Therefore,in this thesis the never dried as-spun Lyocell fiber was heat-treated under tension by using a home-made heat treatment equipment. And the effect of pretension,temperature and time as well tension of heat treatment on the mechanical properties of resultant Lyocell fiber was also investigated.It was found that after optimal heat treatment,the tensile strength and initial modulus of Lyocell fiber could be increased by 29.7% and 97.4%,respectively,and its dry heat shrinkage and creep ratio were decreased obviously,which could accord with the requirements of tire cord. That means the heat treatment is a feasible way to obtain the Lyocell fiber used as tire cord.Based on the above studies,the never dried Lyocell fiber, which had been modified by ammonium chloride,was heat-treated.The strength and the modulus of resultant Lyocell fiber could reach 7.21 cN/dtex and 92.6cN/dtex,respectively.And the creep ratio and dry heat shrinkage were lower than 0.5%and 0.8%,respectively.Therefore,the modification by ammonium chloride combined with heat treatment was an effective method to prepare high performance Lyocell fiber.It is generally believed that the Lyocell fiber has high fibrillation tendency,which has negative effect on its fatigue properties.In order to increase the fibrillation resistance of Lyocell fiber to improve its fatigue resistance and wear resistance,the optimal cross-link treatment was also investigated by using the never dried Lyocell fiber as precursor.The results showed that the cross-link treatment could effectively decrease the sizes of elementary fibril and elementary fibril cluster and increase their contents, which led to reduce the possibility of the fibril separating from the fiber and improve fatigue resistance of Lyocell fiber.Therefore,the wet abrasion value of Lyocell fiber after cross-link treatment was 13 times higher than that of common Lyocell fiber.These results indicated that the appropriate cross-link treatment could improve the comprehensive properties of Lyocell fiber,which were necessary for tire cords.Based on the above studies,the never dried Lyocell fiber modified by ammonium chloride was cross-link treated firstly and then heat-treated.It can be found that the mechanical properties,fatigue resistance,heat and size stability of the resultant Lyocell fiber are much better than those of HLMS polyester tire cord and high strength rayon tire cord.The above investigations showed that the Lyocell fiber used as tire cord could be prepared by（1） using high relative molecular weight cellulose or its mixture with medium relative molecular weight cellulose as raw material, （2） modification by adding ammonium chloride,and（3） the heat treatment under tension using never dried Lyocell fiber as precursor.The high performance Lyocell fiber with excellent comprehensive properties,which can be used as tire cord,could be obtained by combining the modification by ammonium chloride with the heat treatment（or further with cross-link treatment）.