|Author:||Jiang, Xiuying Debbie|
|Title:||Tensile drawing behavior of rotor spun yarn|
Hong Kong Polytechnic University -- Dissertations
|Department:||Institute of Textiles and Clothing|
|Pages:||xxi, 224, 52 leaves : ill. (some col.) ; 30 cm|
|Abstract:||Rotor spun yarn, despite of its growing popularity, suffers the disadvantages of lower tenacity, limited count range, and especially harsh hand of its end fabrics. This project is thus conducted with an aim to probe the possibility of amending these drawbacks by a tensile drawing process. Yams produced with different specifications from cotton and Tencel fibers are used. The results are encouraging since it is confirmed that staple yarn treated this way can still serve its proper function. Especially, it is found that the tensile drawing process can improve rotor spun yarn quality in terms of structural/mechanical evenness and processability. Other features of modified rotor spun yam include increased packing density, reduced yam diameter, enhanced mechanical rigidity, and even a potential of tenacity growth. However, the major breakthrough from this process might be the significant changes in the fabrics produced from drawn rotor spun yarns, viz. both subjective evaluation and rational experimental results indicate a phenomenal improvement in their comfort property. Besides, an appreciable improvement is also found in most of the fabric mechanical properties. What is more important, the dimensional stability and pilling performance are not seriously influenced. Other benefit include more attractive appearance as reflected by the whiter appearance and better luster. These changes from tensile drawing process would no doubt indicate the possibility of gaining an attractive return on investment. A specially designed device is also prepared to improve the drawing effect in terms of tenacity enhancement and breaking elongation loss moderation. The device comprises a two-roller drawing system, with an air jet nozzle in between. In the two-roller drawing system, the feed roller is always at a speed lower than the delivery roller, and thus the rotor spun yam would undergo tensile stretching when passing through these two rollers. The adoption of the air jet nozzle can subject rotor spun yarn to the action of false twisting when it travels along a path predetermined by two rollers so as to improve tensile drawing effect via yam restructuring. Experimental results demonstrate that the designed device is a success as reflected by the rise in yam tenacity and even some improvements in breaking elongation. This device enhances the potential of commercial applicability of the proposed tensile drawing process. Although millions of yarn models must have been developed, the feasibility of these models to rotor spun yams is still doubtful in consideration of the peculiar structure of rotor spun yam. A new tensile model specially for rotor spun yam is thus necessary, which may contribute to an in-depth understanding of the tensile drawing behavior of rotor spun yarn so as to optimize the proposed tensile drawing process. The study is carried out on the basis of a coaxial-helix structural model, taking account of non-uniform yarn packing density. Especially, a changing-pitch system is for the first time introduced in yam modeling. A pitch function has thus been determined by the aid of non-linear regression method and curve-fitting approach on the basis of fiber lengthwise images using tracer fiber technique. Fundamental approaches adopted in modeling include discrete-fiber-modeling principle, an energy method, and a "shortest-path" hypothesis. A significant feature of the proposed yam model is that for the first time in yarn mechanics research history, pitch is no longer considered as a constant but a variant. Therefore, the need for precise and concise information about pitch becomes apparent for the success of the present model and this project thus also involves a method to determine the pitch distribution function of a rotor spun yarn, which is discerned to be a cubic function of correlated radial positions. It is confirmed that the model using the identified pitch distribution function presents substantial advantage to constant pitch model. The proposed model also considers non-uniform packing density, and thus the method of determining packing density function is also given. Detailed sampling procedures are shown, together with a concrete elaboration of the general algorithm for program writing. It is found that yarn cross sectional packing density function is also a cubic function of related radial position, with the maximum around yam axis or one quarter to one third from yarn axis. Evaluation of the present model is given against experimental data and constant-pitch model. It is confirmed that the present model, due to the introduction of changing-pitch system, is very successful in explaining the experimental observations. Also presented is a theoretical analysis of fiber strain distribution and the induced fiber strain curves provide a good explanation for the discrepancy between the present model and traditional constant pitch model. Particularly, additional information about the influence of pitch function on stress-strain curves is also provided to investigate the significance of pitch on yam mechanical behavior. The result not only indicates the necessity of introducing changing-pitch function in yam mechanics study but also confinns the accuracy of obtained pitch distribution function.|
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