The yarn is the basic component of all the textile fabrics, the engineering of a fabric relies on the yarn used and its characteristics, thus the study of yarn structure and properties has a crucial importance.
In general, yarn, being the building block of fabric, contributes to end product performance through three key structural aspects:
- Fiber compactness,
- Fiber arrangement,
- Fiber mobility
1 Fiber Compactness
The integrity of a yarn structure is maintained partially by some form of compactness of fibers or filaments in the yarn. In case of spun yarns, fibers are compacted together by lateral forces imposed by the twist inserted to form the yarn. The discrete nature of the fibers provides variable compactness with plenty of air pockets inside the yarn structure filling in the inter-fiber spaces.
The importance of fiber compactness stems from two important points that should be realized in the design of fibrous products. For a given fiber type, high fiber compactness is likely to result in low yarn compressibility (or low softness), high strength, low yarn flexibility, low yarn porosity, and more moisture wicking along the yarn surface than along the fibers in the yarn.
On the other hand, low fiber compactness is likely to result in high yarn compressibility (or high softness), high flexibility, high porosity, and more moisture wicking along the fibers in the yarn. Between these two situations, yarns of different levels of fiber compactness can be produced to meet key fabric performance characteristics such as strength, hand, drape, moisture management, and comfort.
Furthermore, fiber compactness is a key factor in determining the yarn dimensional stability; a yarn can be considered dimensionally stable if the fiber compactness is the same in the relaxed state and under low levels of stress.
2 Fibre Arrangement
The way fibers are arranged in the yarn structure can have a great impact on a number of yarn and fabric performance characteristics such as yarn liveliness, yarn appearance, yarn strength, fabric dimensional stability, and fabric cover. In spun yarns, fiber arrangement is quite different from the simple arrangement of continuous-filament yarns.
The discrete nature of staple fibers makes it difficult to fully control the fiber flow thus restricting the possibility to produce a well-defined fiber arrangement. As a result, the fibre arrangement in spun yarns is typically an idealized twisted yarn structure. The arrangement of fibres results in a self-locking structure that provides the necessary integrity to the yarn.
Some yarn types may even have a greater deviation from the idealized twisted structure. For example, air-jet spun yarn exhibits no true twist with fibers in the core being largely parallel and outer fibers forming random wrapping around the core fibers.
3 Fibre Mobility
Fibre mobility is the term used for the change in physical forms that a yarn takes during the various stages of processing. Fiber mobility is a key design aspect of yarn as it determines the dimensional stability of both the yarn and the fabric is woven or knitted from it.
In the case of spun yarns, the discrete nature of staple fibers can result in many modes of fiber mobility depending on the external effects encountered. Under minimum yarn strain, fibers will tend to settle down and cohere. If the yarn is relaxed or slack, the fibers will likely be in a bending mode; if the yarn is tensioned, the fibers will follow the path of minimum length or a straight line between fiber ends; if the yarn is twisted, the fibers may be buckled or tend to snarl; and when a yarn end is set free, the fibers will tend to untwist.
The point is, most spun yarns exhibit a great deal of fiber mobility that in one way or another can influence yarn performance during the conversion of yarn into fabrics and during the use of fabric in various applications. Perhaps, fiber mobility is observed in a yarn only for the fiber segments or filaments near the yarn surface.
The concept of yarn hairiness: In spun yarns, an inevitable phenomenon is the existence of fiber segments protruding from the yarn body. This is called ‘yarn hairiness’ and it is measured by the number of hairs projecting from the yarn and their lengths.
Typically, short hairs (less than two millimeters) adhered to the yarn body are acceptable as they provide a nice fuzzy feeling in apparel fabrics against the skin. This is particularly true when the protruding hairs are flexible so that they can bend easily against the skin. They may also create an opportunity to create tiny fiber pockets that can entrap air for good thermal insulation.
On the other hand, long hairs protruding from the yarn body can be of major adverse effect on yarn performance. During the conversion of yarns to fabrics, long hairs of different yarns will tend to entangle together creating fiber bridges that hinder a smooth flow of the yarn during processing.
Staple Yarns Spun On Different Technologies
The above discussed three aspects are unique for the different yarn forming technologies used, thus as a combination together results into a yarn having a specific set of characteristics which suits specific end uses.
The selection of staple yarns for a specific end use can be done using a three-step comparative study according to the Rieter philosophy.
- Yarn properties
- Downstream Processability
- Delivered fabric properties
This is an attempt to lay the staple spinning technologies according to their suitability to the end use requirements.
1 Ring Yarn
The ring-spun yarn comprises on fibres following a helical path around the axis, these fibres are in layers wrapped around the core, while some fibres also migrate between the layers to give a self-locking effect to the yarn body. The helix angle (angle between fibre and yarn axis) depends upon the degree of twist inserted during spinning.
The inclination or the helix angle is responsible for the binding of the fibres and contribution to yarn strength up to a certain level. Beyond which, the individual fibre contribution towards tenacity decreases due to the increase in obliquity. Also, with the increase in the twist, the yarn becomes more compact and rigid.
The ring spinning machine is the most versatile system, as all types of spinnable staple fibres can be spun on the spinning machine, in addition to that one can spin counts ranging from 1sNe–200sNe. This is the reason why in spite of its low production rates the ring spinning machine still exists in the spinning industry.
The ring-spun yarn has sufficient tensile characteristics, but it exhibits high hairiness values along with high mass variation. The hairs protruding out from the yarn body make the yarn bulkier and result in improved comfort characteristics. The ring-spun yarn being the oldest system will be used as the standard for comparison with the other systems that we will see further.
In the downstream processing, the ring spun yarns can be processed at high production rates owing to their excellent strength. They are also the most suitable weft yarns for the Air-jet weaving technologies, due to their high hairiness values.
The ring-spun yarn fabrics have the high degree of opacity, soft feel to touch and thus higher comfort characteristics. Majority of the requirements for staple yarns are fulfilled by the ring spun yarns due to the versatile nature of this technology. These yarns are used universally viable for the majority of the applications and are generally used for Outerwear, Innerwear, and Home-textiles.
2 Compact Spun Yarn
As the compact yarns are spun on the ring spinning machine itself with an auxiliary compacting attachment, the structure of a compact spun yarn is similar to the ring spun yarn. The fibres in compact spun yarns follow the typical helical path.
The only difference between the normal ring spun and compact ring spun yarn, is the compactness of the yarn body. This is due to the reduction in the spinning triangle by the compacting arrangement. The spinning triangle exerts unnecessary tension on the fibres, these fibres then migrate trying to follow the minimum energy path.
These migrated fibres deviate the structure from the ideal helical yarn geometry and result into a bulky ring spun yarn which is avoided in case of compact yarns.
Due to the condensation of fibre flow and elimination of the spinning triangle, the fibres almost lay close to each other and are twisted into the yarn body.The packing fraction calculated from the yarn diameter is found to be higher for compact yarn than that of normal ring spun yarn. There are sufficient evidence to prove that the yarn produced on this system have comparatively lesser long hairs as discussed earlier, better tensile characteristics, lesser unevenness, and imperfections.
But, along with this, the compact yarn is also found to have higher flexural and torsional rigidity, low bulk and lesser moisture absorbency compared to the ring spun yarns. The compact spun yarns have great advantages in relation to the processing speeds. The high tenacity and low hairiness values make these yarns resistant to any probable damages in the processing.
Low-end breakage rates are recorded with the compact spun yarns. Generally, every finishing stage deteriorates the fabric properties, but with the strong compact yarns, the finishing potential can be further exploited. The compact spun yarn fabrics possess the highest strength, absolutely clear defined structures with printed products.
Fewer hairs result in less light refraction and higher luster values. These properties make the yarns suitable to manufacture top quality shirting fabrics, high quality knitted garments, Fine bed linen and so on.
3 Rotor Yarn
The structure of the rotor spun yarn can be understood as a combination of core fibres, outer layer, and wrapper fibres. The core fibres are helically twisted around the yarn axis, the outer layer has a partial twist and some of the fibres from the outer layer are enveloped around the yarn body in the form of random wrapper/belt fibres.
The term spinning- in coefficient has been used for the mean fibre extent, it was observed that for viscose fibres of 1.5 deniers and 44mm length, the spinning-in coefficient for rotor yarns is 0.46 against a value of 0.69 for ring yarn. The rotor spun yarns are reported to be weaker than the ring spun yarns due to their structural difference.
The rotor yarns are mainly used for the spinning of coarser yarn counts, for the coarser count yarns even with the short fibre content, rotor spun yarns are more uniform compared to their counterparts, they exhibit low variation in tensile characteristics.
These properties are with regards to the back doubling of fibres that takes place in the rotor, where every single fibre in naturally picked up by the rotating parent yarn producing a very even structure.
In accordance with the unique structure of the rotor yarn, the yarns require lesser sizing agent and absorbs the sizing agent more rapidly. The rotor yarns are found to exhibit lesser fly and fluff generation during the processing, this means lesser cleaning of the machines is required.
Even and uniform fabric surface is obtained by the usage of rotor yarn for fabric forming, particularly with knits. These fabrics also possess higher abrasion resistance. The rotor yarn fabrics are found to have a rougher surface compared to its counterparts.
The rotor yarns find application in Denim, Workwear, Terry fabrics including towels, napkins, upholstery and so on. They are also used to manufacture various technical textile products like the canvas fabrics.
4 Air-Jet Yarn
It has been observed that the air-jet spun yarn has a core of parallel laid fibres, whereas the sheath/surface fibres are evenly or unevenly wrapped around the yarn body. This structure is said to be a fascinated yarn structure.
The air-jet yarn gets its strength from the lateral force exerted by the wrapper fibres on the surface which are created by wrapping the selvedge fibres around the core fibres by the high velocity compressed air being whirled around the yarn.
The tenacity value of cotton air-jet spun yarn is 55-60% of that of similar ring spun yarn and this value is 80-85% for polyester and polyester/cotton blended yarns, this low tenacity difference in case of synthetic staple spun yarns was observed in case of viscose and acrylic fibres as well.
Although the tenacity values are inferior to ring spun yarns the elongation at break values of air-jet spun yarns are similar to that of ring spun yarns. Air-jet spun yarns show the lowest unevenness values followed by friction, wrap-spun, rotor spun and ring spun yarns.
Air-jet yarns are characterized as the voluminous yarns that yield high opacity in fabrics that are manufactured. The unique fibre bonding in the form of wrapper fibres the yarn has low hairiness and high abrasion resistance values.
An Air-jet spun yarn with a similar linear density requires comparatively lesser dyestuff for the same shade, thus the potential for savings in the dyeing stage. Similar to the rotor yarns by the virtue of wrapper fibres the Air-jet yarns have lesser fly generation and higher abrasion resistance.
The problem of pilling which is very common with the P/C blend knitted goods is diminished to a significant extent in the rotor yarns making them popular for knitting application. These yarns also possess higher water absorbency compared to their counterparts.
The Air-jet spun yarns due to their low pilling tendency, low spirally and dimensional stability are majorly used in the manufacture of knit goods, apart from that they are also used for outerwear and bed linen.
The properties exhibited by the fabrics woven from different staple yarns can be highlighted as below
5 Friction Spun Yarn (DREF-II)/Open End Friction Yarn
The yarn is characterized by helical twisting of fibres with each fibre following a conical helix with differential twist depending upon its contact with the drum.
|Parameters||Ring yarn||Rotor Yarn||Air-jet||Friction|
|Count range||1-100s Ne||2-40s Ne||20-60s Ne||1-40s Ne|
|Production speed||2-25 mpm||100-200 mpm||150-200 mpm||150-200 mpm|
|Elongation at break||100%||100%|
|U %||D (maximum)||C||A (lowest)||B|
|Power consumption||D (maximum)||C||A||B|
|Yarn Tension||D (maximum)||A||B||C|
The open end friction spun yarn is bulkier and fibres in this yarn are most loosely packed, followed by the rotor, ring and air-jet spun yarns. The open end friction yarn is the weakest amongst all due to its poor fibre orientation which means its spinning-in coefficient is lowest amongst all.
The friction spun yarn has lower unevenness values compared to rotor spun yarn and ring spun yarns. Friction spun yarns are used in technical applications where the end product is not subjected to much movements and external forces.
6 Wrap Spun Yarns
The wrap spun yarn contains parallel fibre core, wrapped with filament or yarn. The wrapping element exerts a radial tension on the parallel fibre core, thereby providing the necessary frictional force between the individual fibres in the core.
The breakage of a wrap spun yarn may be catastrophic or stick-slip type depending upon the physical properties of the wrapping element, wrap density and other related parameters. The breaking strength and elongation of a wrap spun yarn increases with the increase in wrap density up to a certain level and then decreases.
With so many alternatives in hand, the knowledge of the structural properties relationship helps us to choose ideal spinning system and parameters, for a specific end-use or application.
For example, the rotor spun yarns are preferred for the use in denim due to the better abrasion resistance, shirting fabrics are made using the compact spun yarns due to their fine and crisp appearance.
This comparative analysis gives us a gross idea about the structure and properties of yarns produced on different spinning systems and their implications.
The yarn structure generally changes with yarn production technology, process parameters and fibre parameters which means one has to look into all these variables while commenting on the yarn characteristics.
This is because a comparative comment made for cotton yarns may not be true for polyester yarn, a comment made for coarser yarn may not be true for fine yarns and so on.
- Arindam Basu, Yarn structure-properties relationship, Indian Journal of fibre & Textile Research Vol.34, September 2009, pp. 287-294.
- El-Mogahzy, Auburn University, Structure and mechanics of yarns, pp 190-211, Structure, and mechanics of textile fibre assemblies, Edited by P.Schwartz.
- The Rieter Manual of spinning, Vol.6 Alternative spinning systems
- The Rieter Manual of spinning, Vol.5, Heinz Ernst
- Rieter com4 Yarns special print