Torsional behavior of short-staple Torque-Balanced Singles ring spun yarns and spirality of resultant knitted fabrics

Abstract

Spirality, the distortion in the wale lines, is one of the major quality problems of single jersey knitted fabrics. It influences not only the fabric aesthetics, but also decreases fabric utilization yield during the cutting process. Many factors cause the Spirality, some associated with the yarn, and some with the knitting and other processes. Among all these factors, yarn residual torque or twist liveliness is the fundamental one. Spirality occurs when the residual torque is released. A new spinning technique had previously been developed in the laboratory to make a new Torque Balanced Singles (TBS) ring spun yarn. The TBS technique takes the advantages of the low twist yarn of a low residual torque and a higher productivity, and the high twist yarn of increased fiber migration and higher strength in one step by using a specially designed device which is similar with a pin false twister. In this study, the spinning mechanism was closely examined and the effect of reduced triangle was investigated. An extremely high twist was introduced for increasing fiber migration and obtaining a compact yarn structure. Then the yarn was de-twisted and a low twist yarn was finally produced. The spinning triangle was also examined by using a transparent top front roller and colored rovings. The structure of the TBS yarn was examined and compared with corresponding conventional ring spun yarns. Firstly, yarn surface was studied experimentally. Yarn diameter variation was investigated by calculating yarn diameters and observing Blackboard images; yarn surface features were demonstrated by analyzing SEM photos; and yarn surface hairiness was evaluated by using a Zweigle G566 hairiness tester. The results of these experiments indicate that the TBS yarn exhibits a greater variation of yarn diameter: in some parts of the yarn, fibers are tightly wound on the yarn body, showing a compact structure, and in some parts, the yarn demonstrates a slightly looser structure. Yarn cross-sectional structures were also studied by using the microscopy. The TBS yarn has a radial packing density pattern of a compact core which differs from its corresponding conventional ring spun yarns. This may contribute to the reduction of yarn residual torque and reinforce yarn strength. Fiber path within the TBS yarn was examined by using the tracer fiber technique. The 2-D images of the colored fibers show that the TBS yarn has more frequent fiber migrations which differ from conventional ring spun yarns. 3-D reconstruction of the fiber path demonstrates that the fiber basically follows a helical path with many migrations. Further analysis shows that a certain proportion of fiber segments have a rotational tendency whose direction is opposite to the yarn twist direction. This probably explains the reduction of yarn residual torque in another way. A mathematical model was established based on the results of the investigation of yarn structure and the assumption that the yarns are elastic and no inter-fiber friction and viscosity. An energy method and the discrete fiber modelling principle were adopted. A series of sinuous functions was used for simulating the fiber migration, and a changeable radial packing density was adopted for representing the fiber distribution in the yarn cross-section. It was demonstrated to be possible to further reduce yarn torque by introducing an appropriate fiber migration pattern. Several optimization experiments of the yarn modification system were conducted by using the response surface methodology. A two-step scheme was adopted. The yarn twist factor and the speed ratio were identified as the two key factors. Response surface experiments for 16Ne, 20Ne and 30Ne yarns were carried out, and optimal conditions were determined. A comparison was carried out between the optimizations of the yarns with different yarns counts (16Ne, 20Ne and 30Ne). The effect of spinning machines on the properties of the TBS yarns was evaluated by conducting three optimizations of the 20Ne yarn on Spin tester, Zinser 319SL and Toyota RY spinning machines. Finally, TBS yarns and plain knitted fabrics were produced for evaluating the effect of the modified spinning system on the performance of the yarn and resultant fabrics. The results show that the TBS yarn has a lower residual torque and relatively higher strength simultaneously. Others such as hairiness and evenness are similar with or not worse than their corresponding conventional ring spun yarns. Plain knitted fabrics were produced by using greige yarns and package dyed yarns on two commercial circular knitting machines in a factory. The results of the measurement of the greige, piece dyed and yarn dyed stripe fabrics show that the fabric spirality can be dramatically reduced by using the TBS yarns. Compared to their control fabrics, the TBS fabrics have better or similar properties in dimensional stability, fabric weight, bursting strength and pilling resistance etc. The laboratory results are in agreement with the feedback from a knitting factory who conducted a large scale trial of TBS yarns.