Surface characteristics of low-twist worsted yarns and knitted fabrics

Abstract

Wool apparel and other textile products are of high value and popularity in the market all the time because of their aesthetic quality and comfort. With the increasing demand of light, thin and comfortable knitted fabrics, it is desirable to have wool yarns in medium to high counts with better yarn evenness, less hairiness, soft handle and reasonable tenacity, etc. By literature review, ring spinning continues to predominate in worsted yarn manufacturing industry because of its high quality of yarns and good flexibility in materials and yarn count. Many modifications have been conducted to enhance the control of fibers in spinning triangle and reduce yarn hairs, but nearly have no improvement on yarn soft handle and evenness. However,over ten years ago, a modified technology on the ring frame was developed by employing a false twisting device and a strand separator, which was named as Nu-torqueTM or low-torque or low-twist spinning. Since then, the technology has evolved in five versions. The low-twist cotton yarns exceed other types of modified ring spun yarns with respect to softer handle, lower residual torque, and outweigh the conventional ring yarns in aspects of higher tenacity, lower hairiness, etc. Previous versions of low-twist worsted yarn technology have produced low-twist worsted yarns in median and coarse count, however, some problems were found such as worse yarn optical evenness, more neps and tight wrapper fibers, as well as lower yarn tenacity, among which, the wrapper fibers give rise to obvious "bar effect" on the dyed knitted wool fabrics. Hence, this thesis is aimed at investigating these problems and exploring possible solutions from both theoretical and practical points of views. The surface structure of 24Nm low-twist worsted yarns are examined under Microscope Lecia M165 in details and classified into five types including three kinds of wrapping structures and two kinds of unwrapped structures. In particular, the tightly wrapping structures take up almost 60% on the low-twist yarns. These tightly wrapping structures not only bring about quite compact yarn structures resulting in harsh handle of yarns and fabrics,but also deteriorate yarn optical evenness resulted from obviously smaller diameters than conventional yarn structures and higher variations. Using high speed camera system, the formation of wrapper fibers on the low-twist yarn surface has been investigated. It is found that the abrasion between yarns or the protruding fiber ends and the upper false-twister or the lower false-twister, the fiber security of yarns in A zone, as well as the false-twisting effects exerted by the two false-twisters, have effects on the wrapper fiber formation; more importantly, the wrapper fibers have roots in the long protruding fiber ends in A zone on the low-twist spinning system, namely, the hairiness of 3mm and longer of the yarn segment between the spinning triangle and the false-twister. Besides, the bulked yarn segment resulted from excessive twists in A zone is reckoned as the reason for the formation of the curved yarns with tight wrapper fibers.Hypothesizing that the structure of yarn segment in A zone on the low-twist spinning system is similar to that of ring-spun yarns of a high twist without buckling, a hairiness model of such ring yarns is beneficial to understand the origin of hairiness and wrapper fibers as well as the formation of neps. The number of all fiber ends in the out-most layer of ring-spun yarn cross-section, which are already or have potentials to become hairs, is first defined as maximum hairiness in unit length of yarn. Based on Brown and Ly's work on the number of fiber ends in twist-less fiber assembly, a statistical model of the maximum hairiness of ring yarns has been established by considering yarn twist geometry and the contributing surface layer for hairs. In particular, fiber length, fiber cross-section and the number of fibers have been revised with the consideration of yarn twist. Moreover, Hairiness contribution factor (h0) is proposed for model development as the ratio of the number of fiber ends having potentials for hairiness and the total number of fiber ends in yarn cross-section. From the developed model, it can be seen that the maximum hairiness of ring yarns, or the number of long protruding fiber ends in A zone of the low-twist spinning system, relates to fiber length distribution, fiber diameter, yarn count, yarn twist,measured hair length etc. Moreover, the present model provides the length of the predicted maximum hairiness, whereas previous related models fail to do so. The verification by experiments demonstrates that the predicted values of 1mm and longer are in the same order of magnitude as the measured values, which are more accurate than the predicted values from other related models. Whereas, the predicted maximum hairiness of 3mm and longer, that is, the long protruding fiber ends, is almost 1~2 order of magnitude higher than the measured S3 values. Alternatively, there are some other ways to further reduce the number of long protruding fiber ends in A zone, like combining Siro-spinning or Solo-spinning with low-twist spinning, because it is generally believed that surface fiber trapping between the two substrands in Siro-spinning triangle or among several substrands in Solo-spinning triangle contributes to the significant decrease of yarn hairiness. Accordingly, the maximum hairiness model of ring yarns is revised for Siro-spun yarns and Solo-spun yarns with respect to fiber trapping and yarn geometry, respectively. Also, experiments have been carried out to verify the developed hairiness models. As aforementioned before, the fiber security of the yarn segment in A zone also influence the wrapper fiber formation and the resultant yarn surface; and the degree of fiber ends being tucked into yarn bodies directly determines the number of protruding hair of yarns. Whereas, the existing related parameters only describe fiber ends already protruding out of yarns. Similar to the theoretical limit of yarn evenness CVlim, the real yarn hairiness can approach but is always lower than the maximum hairiness of ring yarns. A Relative Hairiness Index (RHI) is accordingly proposed, which has two forms:the theoretical one and the actual one. The theoretical RHI is the ratio of maximum hairiness of certain type of yarn to the maximum hairiness of ring yarns, which can theoretically reveal the effectiveness of different spinning methods in tucking fiber ends into yarn bodies; and the so-called actual RHI is the ratio of the measured yarn hairiness and the maximum hairiness of ring yarns, which can actually demonstrate the degree of fiber ends potential for hairiness being tucked into yarns resulted from various spinning system or their spinning parameters. Both the theoretical RHI and the actual RHI demonstrate that Siro-spinning can most effectively tuck fiber ends into yarn bodies, therefore Siro-spun feeding will give rise to the least long protruding fiber ends. Additionally, winding is employed to mimic the abrasion that yarn will experience, and it is found that the hairiness of ring yarns obviously increases with the increasing winding times and reaches a plateau after the fourth winding, therefore the number of hairs of yarns after four-time winding, is termed as stable hairiness. By analyzing the increment rate of the actual RHI of various yarns in the states of cop, cone and stable, which are the yarns experience zero-time, one-time and four-time winding, respectively, Siro-pun yarns also present the best fiber security.Hence, Siro-spun feeding with a normal roving gap of 14mm is combined with the lately 5th version of low-twist spinning system for further reducing the wrapper fibers and improving the surface of low-twist yarns. However, aiming at yarn evenness and tenacity, as well as proper twists in A zone to avoid bulking as described before, the spinning parameters of 36Nm low-twist yarns (36LT) is first systematically optimized by means of the combination of Fractional Factorial Methodology and Response Surface Methodology, respectively. With a twist multiplier reduced by around 15%, the optimized 36LT yarns show comparable tenacity and similar hairiness, but still a bit worse evenness and more neps than the conventional yarns. Actually, the number of neps (+140%) has been reduced about one order of magnitude on the present low-twist yarns by comparing with that of yarns produced on the previous versions of low-twist spinning system. The blackboard evenness of the optimized 36LT yarns exhibits half grade lower than the conventional yarns with a twist multiplier higher by about 15%, but half grade higher than the counterparts with the same level of twist multiplier, respectively. Then, the spinning parameters of 36Nm low-twist yarns with double-roving feeding (36LT+Siro) are also optimized by using Response Surface Methodology. The tightly wrapped structures on the optimized 36LT yarns only account for 8.9%, whereas the ones on the optimized 36LT+Siro yarns even reduce to 5.8%.Moreover, nearly no tightly wrapped structures with a curved yarn body are found on the two optimized yarn surfaces. Therefore, the optimized spinning parameters of low-twist spinning system, as well as the incorporation of Siro-spinning not only facilitate the reduction of wrapper fibers on the resultant yarns, but also provide suitable twists in A zone to avoid producing buckling and curved yarn segments. Nevertheless, the optimized 36LT yarn has an obviously higher actual RHI than the conventional ring yarns, and it presents similar increment rate when enduring abrasion to that of its counterpart, which indicates that the optimized 36LT yarns possess low fiber tucking and security, in other words, the fiber deformation resulted from false-twisting effect fails to be held in yarns. But the actual RHI of the optimized 36LT+Siro yarn and its increment rate by abrasion are markedly lower than the ones of the conventional yarn, particularly, its increment rate is even similar to that of Solo-spun yarns. It is demonstrated that the fiber tucking and security of low-twist yarns can also be improved by integrating Siro-spinning system.Finally, using the Kawabata Evaluation System of Fabric (KES-F), the knitted fabrics made of the optimized 36LT yarns are examined in terms of fabric surface property, tensile and shear, as well as bending and compression. There are no statistically significant differences in surface property,tensile, shear, bending, compression and bursting strength between the low-twist fabrics and the conventional fabrics made of the worsted yarns with a higher twist multiplier by around 15% (at a significant level of 0.05). However, the low-twist fabric possesses better pilling performance and air permeability, but lower thermal conductivity than its counterpart. Besides, nearly no "bar effect" is found on the resultant fabrics made of the optimized 36LT yarns.