Different staple fiber spinning processes and technologies exert varying effects on the physical properties and appearance of yarns, and even further differentiate the characteristics of final finished products.
(1) Different spinning methods affect yarn structure. The structure of staple fiber yarns differs from that of filaments, primarily in the arrangement of fibers on the yarn surface and within the interior. The external structure includes the yarn’s appearance and surface morphology, such as the arrangement of fibers on the yarn surface, yarn hairiness, friction coefficient, comprehensive characteristics, wear resistance, and other surface properties.
(2) The internal structure of yarn mainly refers to the cross-sectional and longitudinal arrangement of fibers within the yarn body, including fiber orientation, elongation, displacement and twist degree. Internal structural indicators cover yarn strength, fiber blending uniformity, bending resistance, compressibility and resilience (tendency to crimp/kink).
2、Spinning Process
To understand the influence of spinning processes on yarn structure, 3 dtex, 38 mm viscose staple fibers were used for trial spinning on five different spinning systems.
A. Conventional Ring Spinning Process B. Compact Ring Spinning Process C. Double-nozzle (MTS) False-twist Wrapped Spinning Process D. Vortex Spinning (MVS)E. Rotor Spinning
(1) The external yarn structure mentioned above can be observed under a scanning electron microscope. For compact ring-spun yarn, more fibers are incorporated into the yarn body, with nearly all fibers fully integrated inside the yarn, which optimizes the structure of staple fiber yarns. The twist structure is clearly visible, and one end of each fiber is twisted into the yarn along the yarn length. Compact ring-spun yarn exhibits the best fiber orientation.
(2) Conventional ring spinning: under the same twist level, the yarn surface of conventional ring-spun yarn is disordered. A large number of fiber ends are not twisted into the yarn body, with individual fibers protruding outside the yarn structure, which may be caused by the steel ring, traveller or yarn guide.
(3) Murata Vortex Spinning (MVS) is similar to ring spinning, with fibers well-aligned in the yarn body. At a spinning speed of 350 meters per minute, the wrapping fibers present a fine spiral shape. Its twist level is basically equivalent to that of ring-spun yarn.The proportion of wrapping fibers to untwisted core fibers is relatively high. Therefore, vortex yarn has an appearance generally similar to ring-spun yarn. The outer wrapping fibers together with the untwisted yarn core form true twist.
(4) Double-nozzle false-twist yarn is essentially different from vortex yarn. In double-nozzle air-jet yarn, wrapping fibers account for only 6%–8% of the total fibers, while approximately 90% of the fibers are stretched and untwisted. It can be clearly observed that the wrapping tightness of the outer fibers around the yarn core is higher than that of vortex yarn.
(5) Rotor spinning. Regardless of whether rotor-spun yarn belongs to the category of true twist, the fiber arrangement in rotor yarn is disordered. Fibers in the middle section of the yarn show no distinct spiral structure in either Z-twist or S-twist direction and remain mostly straight. It can be clearly observed that the wrapped yarn presents a non-tangled state, which is an advantage of rotor-spun yarn and forms the basis of its unique characteristics.
3、Yarn Hairiness
Fly waste and yarn hairiness generated during spinning are a critical and problematic issue. Yarn hairiness exerts numerous adverse effects in downstream processing and negatively influences the hand feel of textiles as well as the performance of final products. The Zweigle hairiness tester is used to classify hairiness of 1 mm to 2 mm in length and identify harmful hairiness longer than 3 mm. Taking the hairiness of conventional ring-spun yarn as the reference of 100%, compact ring-spun yarn, vortex yarn and rotor-spun yarn all show reduced hairiness in the range of 1 mm to 2 mm. By contrast, double-nozzle false-twist wrapped yarn exhibits insufficient wrapping effect and higher hairiness. The SO-Called testing instrument can measure fly waste produced by friction during subsequent processing, while friction force is tested using rubber rings. Compact yarn delivers better friction resistance than conventional yarn. Rotor-spun yarn features fewer protruding hairiness, which is particularly prominent for viscose fibers.Surface fibers hardly break, and most loose hairs are wrapped and fixed inside the yarn structure by wrapping fibers, resulting in the low hairiness property of rotor-spun yarn.
4、Yarn Bulk
Yarn bulk is an important index reflecting the covering power of yarn. At the same twist level, conventional ring-spun yarn has lower covering performance in final products compared with compact ring-spun yarn. Under the condition of equal yarn tenacity, the twist of compact ring-spun yarn can be appropriately reduced to increase yarn bulk, so as to achieve the same covering capacity as conventional ring-spun yarn, with the twist reduced by 5%–10%. The Denkenolorf yarn structure tester can measure the actual yarn bulk at a test length of 0.3 mm.
Ideal compact ring-spun yarn presents a superior fiber arrangement structure to conventional ring-spun yarn. All non-traditional spinning technologies have certain deficiencies. Yarns produced by these new spinning methods must be specially treated when used as warp yarns. In air-jet spinning, compared with true-twist yarns, fewer fibers are tightly twisted, and fibers are arranged in a straight and wound state. This results in inconsistent mechanical and physical properties of the yarn, which is particularly prominent during winding (winding and cone winding), and it is also a key difference between non-traditional spun yarns and ring-spun yarns.
5、Internal Structure of Yarn
The formation of internal yarn fibers is closely related to the external yarn structure. The arrangement of the inner yarn core and the extension of fibers along the yarn length can be observed by scanning electron microscopy during the drafting process. The cross‑sectional view shows highly parallel fiber distribution, which directly affects yarn tenacity. The tenacity characteristics of yarn are closely associated with the gauge length adopted in testing.
Possibility of reducing thin places and weak tenacity points: Conventional yarn tenacity is tested on a strength tester with a gauge length of 520 mm, while 100 mm and 18 mm gauge lengths are also applied in some tests. As the gauge length decreases, yarn breaking strength increases. A shorter gauge length lowers the probability of weak tenacity points and thin places, reduces the effective fracture interval, and minimizes the chance of breakage.
The tenacity of air‑jet yarn falls between that of ring‑spun yarn, compact yarn and rotor‑spun yarn. The yarn core of air‑jet yarn features higher fiber parallelism than rotor‑spun yarn, yet with fewer wrapping fibers, resulting in moderately higher tenacity than rotor‑spun yarn.
6、Effects of Yarn Formation on Yarn Deformation Characteristics
A novel method for testing the bending resistance of yarn has recently been developed. The test results show that if the bending strength of compact yarn is defined as 100%, the values of rotor-spun yarn and vortex-spun yarn reach 200%, while double-nozzle air-jet yarn registers 300%. These proportional differences can be verified by comparing woven and knitted fabrics made from non-traditional spun yarns with ring-spun yarns. Fabrics produced from non-traditional yarns exhibit a coarser and stiffer hand feel than those of conventional ring-spun yarns. There is also a slight difference in bending strength between conventional ring-spun yarn and compact yarn.
Another concern involves the flattening deformation of warp and weft yarn cross-sections under compressive load during yarn deformation. Slub testing reveals that yarn slubs decrease as compressive force increases.
The unique structural characteristics of rotor-spun yarn determine its fiber distribution. Wrapped fiber regions deliver a stiffer hand feel and require lower deformation force than unwrapped sections. Woven and knitted fabrics made of rotor-spun yarn generally present uneven appearance and a harsh touch compared with ring-spun yarn fabrics.
For comparative analysis, yarn slubs were tested under a pressure of 100 cN. The results indicate optimum twist structure under this condition. Among MTS double-nozzle air-jet yarns with identical deformation, approximately 95% of fibers are parallel and untwisted, making the yarn more susceptible to deformation. Based on the tests, rotor-spun yarn features a relatively stiff hand feel. Yarn deformation during weaving and knitting can be evaluated by observing the structural compactness of the finished yarn.
7、Yarn Resilience
Yarn resilience is of great importance to textile processing. Resilience is measured by testing the yarn kink force. Significant differences in resilience exist between true-twist yarns (such as compact ring‑spun yarn and conventional ring‑spun yarn) and non‑traditional yarns. Rotor‑spun yarns show low kink tendency in both S-twist and Z-twist directions.
Air‑jet spun yarns feature low resilience, mainly due to the high proportion of parallel untwisted fibers. True-twist yarns possess higher resilience than non-traditional yarns, resulting in less fabric skew during subsequent knitted fabric production.
8、Conclusion
Yarn structure is one of the essential properties of yarn. Yarn appearance is closely related to its performance, and the internal fiber arrangement exerts a profound influence on yarn characteristics, especially on subsequent processing performance and the properties of final textile products.Yarns with optimized structure possess superior comprehensive functions and wide application prospects. In terms of comprehensive adaptability, textiles made of compact yarn present the most desirable structural appearance.
