A textile yarn is an assembly of fibers of substantial length and relative cross-section and/orfilaments with or without twist. All textile yarns are made from either man-made or naturalfibers. The method by which a yarn is made depends on thefiber type from which it is made. The process of making a yarn from afiber is calledspinning. Based on thefibers from which they are made yarns can be divided into two broad groups, namely, staple spurn yarns and continuousfilament yarns.
Several examples in each group abound are shown in Fig.3.5.
Fig. 3.4 A generalized flowchart for the preparation of yarns from asbestos
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3.4.1 Spurn-Staple Yarn
Spurn-staple yarns are spun from staplefibers, i.e.fibers of definite length (usually 10–500 mm). All naturalfibers are staplefibers. The only exception is silk. Some man-made fibers also belong to this stock. The basic principles underlying the manufacture of all staple spurn yarns are the same. It is in the detailed application of the principles that vary based on the yarn type. The process consists essentially of drawing (aligning the individual fibers of the sliver-the product from the carding process) and then subjecting them to drafting and finally spinning the drafted material. A simpleflowchart of the process is shown in Fig.3.6.
3.4.1.1 Drawing
Drawing is the process of straightening and aligning the individual fibers in the sliver from the carding process. Usually, thefibers in a carded sliver are arranged in a disorganized and random way. They give mostly weak yarn if not pretreated before spinning. If strong and useful yarns are to be obtained from them, it is important that thefibers in the sliver are properly orientated prior to spinning. The drawing process helps achieve this. This process is carried out in machines called drawframes. During drawing the slivers are made to travel in-between a series of pairs of rollers of increasingly greater surface speed. In so doing, thefibers in the sliver slide past one another and hence become straighter and more parallel to each other. Another outcome of the drawing process is that it reduces the irregularity present in individual slivers; it also improves the uniformity of slivers made from blendedfibers.
Fig. 3.5 Examples in each of the two broad groups of yarns
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3.4.1.2 Drafting
This is a process during which the cross-section of the sliver is reduced, and its length increased. In the process, the linear density of the sliver is significantly reduced. This is achieved by causing thefibers in a cross-section of a sliver to slide along the axis of the sliver relative to each other. The main objective of the drafting process is to draw out the material to the requiredfineness before spinning. The most common method of drafting is roller drafting. In this method, a roving or sliver is passed between a set of two pairs of rotating rollers one after the other. The second set of rollers (the output rollers) is made to move faster than thefirst set of rollers (input rollers). The relative speeds of the two sets of rollers determine the extent of reduction of the thickness and the increase in length of the product material relative to the feed sliver/roving. For example, if the second set of rollers moves six times faster than thefirst set then the thickness of the product material would be one-sixth of the feed material and its length would be six times that of the feed material. Drafting is often carried out not as an individual stand-alone oper- ation but as a component of carding, combing, drawing or reduction.
3.4.1.3 Spinning
The process of turning the loosely heldfibers in the drafted material into a yarn with the strength and characteristics required for the intended application is called spinning. This is achieved by the insertion of a twist into a strand offibers. The Fig. 3.6 A simplified
flowchart for yarn formation
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twisting process generates the forces necessary to prevent thefibers from slipping past each other. It is this resistance to slipping that gives the yarn its axial strength.
It forces the generally parallel arrangement offibers into a spiral of helical for- mation. Twist is inserted by rotating one end of a strand offibers relative to the other end. One complete rotation of 360° inserts one turn of twist. Spinning is the final stage in the process of yarn formation from staple fibers. During staple spinning, three objectives are achieved namely;
(i) the input material is drafted (stretched) to the required linear density;
(ii) the required amount of twist is inserted in the yarn; and
(iii) to wind the yarn onto a package, which is suitable for handling, storage and transport and is capable of being unwound at high speed during subsequent processing.
3.4.2 Continuous Filament Yarn
The product from the man-madefiber processing is, in thefirst stage, a continuous filament yarn or tow. During the man-made fiber processing, the fiber product comes off as afiber of indefinite length called afilament. This product, by its nature and characteristics, is for all intents and purposes, a yarn. It is appropriately, therefore, called acontinuousfilament yarnorflatfilamentbecause it does not have the feel and look associated with staple spurn yarns of natural origin. A tow is a collection of thousands of parallel-lying continuousfilaments of man-madefibers.
To be used in fabrics and other applications, the flat filaments are thereforefirst processed to confer on them such characteristics as bulkiness, better cover when used in fabrics, greater comfort, softness of handle, opacity, improved thermal insulation, better moisture absorption, and improve surface characteristics, which are associated with staple spurn yarns. To achieve this, two approaches are used, namely, to introduce crimp and deformation (texture) to the yarn by a process called texturingor to cut up thefilaments into short lengths (staplefiber) and subsequently spin them on conventional spinning machines. The latter process is called tow-to-top conversion.
3.4.2.1 Texturing
The main objective of the texturing process is to confer on the yarn properties similar to those exhibited by yarns made from natural fibers. This is attained by using mechanical means to deform continuousfilament yarns and at the same time create crimps in the yarns. The deformation in the yarn is made irreversible by taking the deformed yarn through a heating and cooling cycle. The four main methods used for making textured yarns are described below.
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i.False Twist Texturing
In this process, which is a continuous one, the yarn is passed through a twisting mechanism, which imparts temporary twist to the yarn traveling towards it and removes it as it passes beyond it. A heat source placed in the path of the twisted yarn sets the twist before it is removed at the spindle. Textured yarns produced by this method are mostly nylon and polyester.
ii.Stuffer Box Method
The method is based on the principle of heat settingfilaments, which are held in a confined space in a compressed state and then withdrawing them in a crimpled form. In practice, this is achieved by using a pair of feed rollers to feed a yarn into a tube where it is restrained at the exit. In this manner, the individualfilaments are caused to bend or fold into a zigzag configuration. When the compressedfilament is set by heating, deformation is set into the yarn. The product of this process has high bulk, soft handle and less stretch than yarn made by the false twist method.
iii.Crinkle-Type Textured Yarns
This type of yarn can be produced in two ways as follows; namely,the Knit de Knit methodandthe Gear Crimpling method. In the former method, aflat yarn isfirst knit and subsequently the knitted fabric is heated, and the fabric is then unraveled (de knit). It is possible to vary the crimp frequency and shape by changing the needle gauge and fabric structure. Fabric produced from such yarn has a pro- nounced sparkle, boucle-type texture, good stretch and recovery, and good handle.
In the Gear Crimpling methodcrinkle type, textured yarns can also be produced by passing a yarn through closely meshed gears. In the process, bulk is generated in the structure. The gear head is heated so that the crinkle produced from the yarn is permanent.
iv.Air-Jet Texturing
Texturing using this method is achieved by blowing a jet of air stream into a yarn while it is being delivered at a higher rate than when it is being taken up. The air produces turbulence, which causes the formation of random loops in the overfed yarn. The loop structures are consolidated in the yarn by passing through a heat stabilization zone. Polyesters, polyamides, viscose, acetate, polypropylene and combinations of these materials are mostly used under this method to produce a wide variety of yarns. The process is capable of producing blended yarns. It is noteworthy that this is the only process that can be used to texture non-thermoplasticfibers. Another feature of this process is the possibility of mixing several filaments in a jet. This paves the way for mixtures of different basic materials such as color mixes, and structural effects. Fabrics produced from yarns processed by this method have a soft and a pleasant handle with an appearance similar to spurn products but with a high bulk. Yarns from polyesters and
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polyamides filaments are used in the sports and leisure wear industry to produce fabrics with good handle and appearance. Some of the areas of applications of air-textured yarns are outer wear, sportswear, furnishing fabrics, curtaining, wall covering, blankets, car seat fabrics and many more.
3.4.2.2 Tow-to Top Conversion
One other way of producing useful yarns from continuousflatfilamentsfibers is, as a first step, to turn them into a staple and subsequently spinning them on con- ventional staple spinning machines. Staplefiber is produced from thefilaments by two methods namely by cutting thefilaments into short lengths in a process called crush cut or by stretching them until they break into pieces in a process called stretch-break. Thefilaments processed by this method are invariable in a tow (a tow is a collection of thousands of parallel lying continuousfilaments). The tow is the direct product of the man-madefiber processing. To cut production costs and time and to get a quality sliver andfiner yarns, the cutting or breaking into staple of the tow is carried out such that the parallel arrangements of the individualfilaments in the tow are maintained. In this manner, the resulting short staple is called a top (sliver), hence the name of the process tow-to-top conversion. In the crush cut method, the cutting machines shear the tows at an angle thereby ensuring that the continuity and parallel arrangement of thefibers are maintained. On the other hand, when the stretch break method is used, drafting rollers are used to stretch the tows beyond their breaking such that they will be broken. Since not allfilaments will break in the same place, the sliver produced has variable length. The top from these processes is subsequently taken through similar process as described previously for the spinning of natural staplefiber. Man-made top can be processed to form 100%
man-madefiber yarns; it can be blended with wool for a wool/man-made blended yarn.