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 investigate how spinning processes influence yarn structure, we conducted trial spinning with 3 dtex, 38 mm viscose staple fibers 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) Scanning electron microscopy reveals the external yarn structure described above. In compact ring-spun yarn, the yarn body incorporates more fibers, and nearly all fibers fully integrate into the yarn structure, thereby optimizing the staple-fiber yarn structure. The twist structure appears clearly, with one end of each fiber twisting into the yarn along its length. As a result, compact ring-spun yarn shows the best fiber orientation.
(2) Conventional ring spinning: At the same twist level, conventional ring-spun yarn shows a relatively disordered surface structure. Numerous fiber ends fail to fully integrate into the yarn body, and some individual fibers protrude from the yarn surface. The ring, traveller, or yarn guide during spinning may contribute to this phenomenon.
(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 differs essentially 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 remain stretched and untwisted. The outer fibers wrap around the yarn core more tightly than those in vortex yarn.
(5) Rotor spinning: Regardless of whether rotor-spun yarn belongs to the category of true-twist yarns, its fiber arrangement remains disordered. Fibers in the middle section show no distinct spiral structure in either the Z- or S-twist direction and remain mostly straight. The wrapped yarn clearly presents a non-entangled state, which serves as 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 shows a superior fiber arrangement compared with conventional ring-spun yarn. However, all non-traditional spinning technologies have certain limitations. Yarns produced by these newer spinning methods require special treatment when used as warp yarns. In air-jet spinning, fewer fibers undergo tight twisting than in true-twist yarns, and the fibers mainly arrange themselves in straight and wrapped forms. This structure leads to uneven mechanical and physical properties, especially during winding and cone winding. It also represents a key difference between non-traditional spun yarns and ring-spun yarns.
5、Internal Structure of Yarn
The formation of internal yarn fibers closely relates to the external yarn structure. Scanning electron microscopy during the drafting process reveals the arrangement of the inner yarn core and the extension of fibers along the yarn length. The cross-sectional view shows a highly parallel fiber distribution, which directly affects yarn tenacity. Yarn tenacity characteristics also depend closely on the gauge length used 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.
6、Effects of Yarn Formation on Yarn Deformation Characteristics
A novel method has recently been developed to evaluate yarn bending resistance. The test results show that, when the bending strength of compact yarn is defined as 100%, rotor-spun yarn and vortex-spun yarn reach 200%, while double-nozzle air-jet yarn reaches 300%. These proportional differences can be verified by comparing woven and knitted fabrics made from non-traditional spun yarns with those made from ring-spun yarns. Fabrics produced from non-traditional yarns show a coarser and stiffer hand feel than fabrics made from conventional ring-spun yarns. Conventional ring-spun yarn and compact yarn also show a slight difference in bending strength.
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.
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.
