POLYMER SPINNING | DRAWING OR STRETCHING AND HEAT SETTING OF POLYMER YARNS

POLYMER SPINNING 

Polymer spinning is important part of man made fiber and yarn manufacturing technology. Polymer spinning is very popular and result oriented synthetic spinning method. Although melt- spinning, dry-spinning, and wet-spinning techniques are used to form the vast majority of manufactured polymer fibers, several other spinning techniques also exist and may be applied in a limited number of specialized situations. High-molecular-weight polymers, such as those in Spectra@ polyethylene, are formed by solution spinning or gel spinning. As in wet and dry spinning, the polymer is dissolved in a solvent. The polymer and solvent together form a viscous gel that can be processed on conventional melt-spinning equipment to form a gel-like fiber strand. 

Later in the processing, the solvent is extracted and the fibers stretched. Fibers made from polymers that have extremely high melting points and are insoluble present obvious difficulties in spinning. Such materials may be spun by a complex process called emulsion spinning in which small, fibrous polymers are formed into an emulsion, aligned by passing the emulsion through a capillary, then fused or sintered (combined by treating with heat without melting), passed through the spinneret into a coagulating bath, and subsequently stretched. 

DRAWING OR STRETCHING OF POLYMER YARN

Both crystalline and amorphous arrangements of molecules exist within newly formed filaments. It is possible to orient these molecules to make them more parallel to the walls of the filament, and therefore more crystalline and stronger, by stretching the filament before it is completely hardened after polymer spinning. 

Newly formed filaments are, therefore, subjected to drawing or stretching. Depending on the fiber type, this may be done under cold or hot temperature conditions and has the additional effect of making the filament both narrower and longer. Fibers made from polymers that have a low glass transition temperature, such as nylon, can be drawn at room temperature. 

In case of polymer spinning, The polymers are mobile and can be pulled into positions parallel to the fiber length. Polyester, on the other hand, has a higher glass transition temperature and so must be heated to be drawn. Drawing is accomplished by stretching the fibers between two rollers, called Codet rolls, with the second roller rotating faster. 

Not all yarns are drawn to the maximum amount possible, because when a fiber reaches its maximum length, the extensibility of the yarn and fiber are lowered. Yarns that have not been fully drawn are called partially oriented yarns (POY). Those that have been fully drawn are called fully oriented yarns (FOY). Lower speeds in melt spinning produce fibers with lower orientation. As is true of many other textile processes, precise control of the process must be maintained so that the manufacturer can achieve the qualities needed in the final product. 

Other steps may be added, such as texturing (in which crimp is added to the filaments) or heat-setting treatments to ensure very low shrinkage as is required in fibers for automobile tires. Sometimes two or more steps may be combined into consecutive operations to reduce manufacturing costs, so that the fibers may go from spinning directly to drawing or from spinning to drawing to texturing. 

HEAT SETTING AFTER POLYMER SPINNING

Thermoplastic manufactured fibers may shrink when exposed to heat. To prevent shrinkage, such fibers are treated with heat during manufacturing to “set” them into permanent shape. Exposure during use and care to temperatures greater than the heat-setting temperature will counteract the heat setting, resulting in fiber shrinkage or loss of heat-set pleats or creases. 

As the technology for producing manufactured fibers has become more highly developed, manufacturers have turned to increasingly sophisticated techniques for creating new fibers. Different fiber shapes and sizes, as well as unique combinations of polymer types in the same fiber, are but several examples of these techniques. 

Fibre Blends; Properties of fiber Blended Yarns

Fibre Blends 

Fibre Blending is the process of mixing fibers together. As noted earlier, it can take place at any of several points during the preparation of a yarn. The purposes of blending are (1) the thorough intermixing of fibers and/or (2) combining fibers with different properties to produce yarns with characteristics that cannot be obtained by using one type of fiber alone. Self blending of bales of the same fiber is done routinely in processing natural fibers because the fibers may vary from bale to bale. In this type of blending, the mixing of as many bales as possible is done early in the processes preparatory to spinning so that the subsequent steps can help to mix the fiber still more completely. For the same reasons, even when two or more different fiber types are combined, blending is done as early as possible. Carding helps to break up fiber clusters and intermix fibers more thoroughly. However, if the fibers being blended require different techniques for opening, cleaning, and carding, as with polyester and cotton, then slivers can be blended. For blended yarns of different fibers, the blend level is the percentage by weight of each fiber. Blending is not limited to staple-length fibers. Filament fibers of different generic types can be combined into a single yarn. This can be done either by extruding these fibers side by side, during drawing, or during texturing. As described earlier, a blended yarn can be core spun with one fiber at the center and a different fiber as the covering or be wrapped with one fiber making up the central section and another the wrapping yarn. As yarn spinning and texturing technologies grow more sophisticated, we expand the possibilities of combining several different fibers into one yarn. Multiple-input texturing machines can produce specialty yarn blends. 

It should be noted that fabrics woven from two or more yarns each made of different fibers are not considered blends. These fabrics are, instead called combination fabrics. They do not behave in the same way as those fabrics in which, the fibers are more intimately blended and may require special care procedures. Regrettably, the Textile Fibers Products Identification Act (TFPIA) labeling requirements do not distinguish between blended fabrics and combination fabrics when fiber percentage contents of fabrics are given. 

Properties of Blended Yarns 

Fibers with different characteristics, blended into a yarn, can each contribute desirable properties to the final textile material. The ultimate performance is an average of the properties of the component fibers. For example, a fabric of 50 percent cotton and 50 percent polyester would have an absorbency intermediate between that of cotton or polyester. In some cases, however, the observed fabric property is not determined simply by the relative amounts of each fiber in the blend. In blends of nylon with cotton, the tenacity of the blended yarn initially decreases with increasing amounts of nylon because of differences in the breaking elongation of the two fibers. At the breaking elongation of the cotton fibers, the nylon fibers are not assuming their share of the stress, leaving the cotton to bear the load. 

The stage at which blending occurs also affects the properties of the fabrics. In general, the more intimate the mixing of fibers in the blends, the better the resulting properties. Yarns blended at the fiber stage exhibit a more effective averaging of properties than ply-blended yarns. Even though considerable study and evaluation have been made of optimum fiber proportions required to achieve desired results in blends, no certain conclusions have been reached. It is clear that extremely small proportions of fibers have no appreciable influence on performance, although they may have some effect on appearance.

Modern spinning method; Ultra Modern method of manufacturing yarn.

OTHER METHODS OF MANUFACTURING YARNS 

In addition to ring and open-end spinning, techniques that insert true twist into yarn, there are other types of yarn construction. Three of those that have some current commercial application are described in the following sections: false-twist, or self-twist, spinning; yarn wrapping; and splitting or slitting films made from synthetic polymers. The viability of these processes for commercial purposes varies. 

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Air-Jet Spinning 

Air Jet spinning is a ultra modern spinning or yarn manufacturing method The Murata Company, a Japanese firm, has commercialized an air-jet spinning machine that functions as follows. A largely untwisted sliver is fed into the machine. Two nozzles, each forcing an air jet against the sliver from opposite directions, cause fibers from the outer layer of the sliver to wrap around the interior fibers, thereby forming the yarn. 

Hollow Spindle Spinning 

Hollow spindle spinning is another modern yarn manufacturing process. In hollow spindle spinning, a sliver of core fibers is fed through a hollow spindle where it is wrapped by a filament yarn unwinding from the spindle. An interesting application of the technique has been in the manufacture of towels and other fabrics, in which the wrapped yarns are used in the pile. In this instance, the wrapping yarn is made from soluble polyvinyl alcohol (PVA) fibers. After the fabric has been put through the finishing processes, these yarns dissolve, leaving a soft, all-cotton twist less and absorbent yarn in the pile 

Core Spinning 

Core spinning is also a special spinning for yarn manufacturing. Core-spun yarns are usually made with a continuous filament core surrounded by twisted fibers or other yarns. Recently, core spun yarns with a staple core of one fiber and an outer sheath of another fiber have been produced by an adaptation of ring spinning. Two rovings, one of polyester and one of cotton, are fed through drafting rollers and then pass through separate channels before being wound on the spindle. The channel for the cotton sheath is longer, ensuring that it will wrap around the polyester core as the twist is inserted. Fabrics from staple core yarns are more durable and have more easy-care features than those of 100 percent cotton yarns. 

Making Yarns from Films 

Recently, various new techniques have emerged that allow yarns to be formed directly from synthetic polymers without the formation of fibers or the twisting of fibers into yarns. These processes include the formation of yarns by the split-film and slit-film processes. Slit-film yarns could be classified as monofilaments. Yarns made by the split-film process do not fit neatly into the categories of staple or filament yarns. 

Split Films 

In the creation of yarns by the split-film technique, a sheet of polymer is formed. The formed sheet is drawn in the lengthwise direction. Through drawing, the molecules in the polymer are oriented in the direction of the draw, causing the film to be strengthened in the lengthwise direction and weakened in the crosswise direction. This causes a breakdown of the film into a mass of interconnected fibers, most of which are aligned in the direction of the drawing, but some of which also connect in the crosswise direction. The process is known as fibrillation. 

The fibrillated materials can be twisted into strings or twines or other coarse, yarn like materials. The usefulness of split-film yarns is limited because the yarns created are coarse. Olefins are made into split-film yarns for use in making bags, sacks, ropes, and other industrial products. 

Slit Films 

Slit films are made by cutting film into narrow, ribbon like sections. Depending upon the process used for cutting and drawing the film, the tapes may display some degree of fibrillation, like that described for split films. When tapes are made that do not fibrillate, they are flatter and are more suitable for certain uses. Flat tapes are used as warp yarns in weaving and can be made into carpet backings that will be very stable, remaining flat and even. All types of tape yarns are used in making wall coverings, packaging materials, carpet backing, and as a replacement for jute in bags and sacks. 

Lurex@, a flat, ribbon like yarn with a metallic appearance, is a slit film yarn that is often used to add decorative touches to apparel or-household textiles. Lurex@ is made from single or multiple layers of polyester film. Multi-layered types are made by placing a layer of aluminum foil between two layers of polyester film. 

Monoply types are cut from metallized polyester film, protected by a clear or colored resin coating. The natural color of Lurex@ is silver. Other colors are produced by adding pigments to the lacquer coating or to the bonding adhesive. The width of these yarns ranges from 0.069 to 0.010 inch. 

Ply Yarns 

Ply yarns are made from two or more single yarns that are twisted together. Ply yarns are much more expensive than single yarns but are nevertheless often produced to achieve certain benefits. Ply yarns made from identical single yarns are more regular in diameter and are stronger. Ply yarns are often made to achieve particular decorative effects. 

In general, the steps involved in creating ply yarns include: 

1. Winding single yarns and clearing any flaws. 

2. Placing the required number of component yarns alongside each other, in place, ready for supplying to the machine 

3. Insertion of twist to form the ply yarn by any of a number of different machines 

4. Winding the finished yarn on a cone or package for delivery to the customer 

A number of different machines are used in making ply yarns, which may also be referred to as folded yarns. Ring-folding machines, for example, operate on the same principle as ringspinning machines except that instead of a roving being fed to the traveler, the single yarns to be combined are both fed together for twisting.

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Yarn construction; Basic yarn manufacturing process

YARN CONSTRUCTION 

Basic Yarn Manufacturing Processes:
Carding –– Combing –– drafting –– twisting –– winding. 

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As the fibers pass through these processes, they are successively formed into: lap, sliver, roving and finally yarn.
The manufacturing operation in which these stages occurred
(1)Lap to card sliver by the lading process
(2)Card Sliver to Cone sliver by combing process.
(3)Shiver to roving by the drafting, or drawing out process
(4)Roving to yarn by further drafting and twisting process.
(5)Yarn reeled on bobbins, spools or cones by the winding process. 

(1) Bending, Ending, Opening and Cleaning: 
(i) The cotton arrives at the mill in large bales weighing about 500 pounds / 225 kg. The compressed mass of raw fibers must be removed from the bales, blended, opened & cleaned.
(ii) Opening is necessary in order to loosen hard lumps of fibers & disentangle them.
(iii) Cleaning is required to remove trash – such as dirt, leaves, burrs, seeds, etc.
(iv) Blending is necessary to obtain uniformity of fiber quality.
(v) Blending: Mechanical bale pickers pluck thin, even layers of the matted fiber from each of a predetermined number of bales in turn and deposit them on Hooper. The fiber is mixed & passed to an opener.
(vi) Opening: As the mass of fiber passes through the openers, cylinders with protruding fingers open up the lumps & free the trash. The number & kind of cylinder, or beaters, employed depend upon the type of cotton that is being processed.
(vii) Cleaning: As the cotton is opened, trash falls through a series of grid bars. When the cotton emerges from the opener, it still contains small tuffs with about 2/3rd of trash.
(vii) This may be conveyed as a lap, which is loosely entangled mass about 1" thick and about 40" wide. Or it may be fed by chute directly to the card for further cleaning and fiber separation.

Blending

Opening and Cleaning

(2) Carding: 

(i) This is the process of arranging the fibers in a parallel fashion. This is necessary for all staple fibers; otherwise, it would be impossible to produce fine yarns from what is originally a tangled mass.
(ii) Before the raw stock can be made into yarn, the remaining impurities must be removed, the fibers must be disentangled, and they must be straightened.
(iii) The lap is passed through a beater section and drawn on a rapidly revolving cylinder covered with very fine hooks or wire brushes.
(iv) A moving belt of wire brushes slowly moves concentrically above this cylinder. As the cylinder rotate, the cotton is pulled by the cylinder through the small gap under the brushes, the teasing action remove the remaining trash, disentangles the fibers and arranges them in a relatively parallel manner in the form of a thin web.
(v) This web is drawn through a funnel – Shaped device that molds it into a round ropelike mass called the card sliver (about thickness of a broom stick).

Carding

(3) Combing: 
(i) In this operation, fine-toothed combs continue straightening the fibers until they are arranged with such high degree of parallelism that the short fibers called ‘noils’ are combed out and completely separated.
(ii) This procedure is not done when processing man-made staple fibers because they are cut into predetermined uniform length.
(iii) This operation eliminate, as much as 25% of the original card sliver, thus almost one-fourth of the raw cotton becomes waste.
(iv) The combing process forms a comb sliver made of the longest fibers, which, in then, produces a smoother & more even yarn. 

Combing

(4) Drafting / Drawing 

(i) The draw frame has several pairs of rollers, each advance set of which revolves at a progressively faster speed.
(ii) This action pulls the staple lengthwise over each other, thereby producing longer & thinner slivers.
(iii) After several stages of drawing out, the condensed sliver is taken to the slubber, where rollers similar to those in the drawing frame draw out the cotton further.
(iv) The slubbing is passed to the spindles, where it is given its first twist & is then wound on bobbins.

Drawing

(5) Roving: 
(i) Roving is the final product of several drawing out-operation.
(ii) These bobbins are placed on the roving frame, where further drawing out and twisting take place until the cotton stock is about the diameter of a pencil lead
(iii) To this point, only enough twist has been given the stock to hold the fibers together.
(iv) Roving has no tensile strength, it will break apart easily with any slight pull. 

Roving

(6) Spinning: 
(i) The ring spinning frame complete the manufacture of yarn
• By drawing out the roving
• By inserting twist
• By winding the yarn on bobbins.
(ii) Ring Spinning draws; twist s& winds in one continuous process. The traveler carries the yarn as it slides around the ring, thus inserting the twist.

Ring Spinning

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Ring Spinning 

The ring spinning could be defined, the process of manufacturing yarn with flying ring.

The ring spinner is made up of the following parts: 

Ring spinning 

1. Spools on which the roving is wound 

2. A series of drafting rollers through which the roving passes 

3. A guiding ring or eyelet 

4. A stationary ring around the spindle 

5. A traveler-a small, V-shaped clip on the ring 

6. A spindle 

7. A bobbin 

The roving is fed from the spool through the drafting rollers. The rollers elongate the roving, which passes through the eyelet, moving down and through the traveler. The traveler moves freely around the stationary ring. The spindle turns the bobbin at a constant speed. This turning of the bobbin and the movement of the traveler impart the twist to the yarn. The yarn is twisted and wound onto a bobbin in one operation. 

Bobbins must be removed from the machine when full. From here, bobbins are transported to a winding machine where yarn is wound onto packages. Automated systems for doffing and winding have been developed and are widely used. Winding is considered an important step. It provides an opportunity to condition yarn that is, to bring the yarn into equilibrium with the moisture in the atmosphere, and to add wax or other coatings that will facilitate weaving. Winding also allows the identification of flaws in the yarn and formation of larger yarn packages than the spindles on the ring spinning frame. 

1. The value and character of a yarn are determined by 

• Kind and quality of fibers 

• Amount of processing necessary to produce fineness. 

• Amount of twist, which increases tensile strength in the finished yarn. 

2. The purpose of the yarn must be anticipated, as this determines the number and kind & many manufacturing operations. 

3. The formation of yarn from staple fibers by shinning becomes possible when they have surfaces capable of cohesiveness. This quality is exemplified by the natural twist of the cotton fibers, which enables them to entwine around each other, the roughness of the linen fibers, which cause them to cling together, and the scale on the surface of the wool fibers, which cause them to graph each other. 

4. Flexibility permits the fibers to be twisted around one another. 

5. Uniformity & staple give yarn a required evenness & improve the quality. 

Yarn Twist due to ring spinning:

The amount of twist is an important factor in finished consumers’ goods. It determines the appearance as well as the durability and serviceability of a fabric. Fine yarns require more twist than coarse yarns. Warp yarns, which are used for the lengthwise threads in woven fabrics, are given more twist than are filling yarns, which are used for the crosswise threads. To retain the twist in the yarns and prevent any tendency to untwist or kink, the yarns are given a twist-setting finish with heat or moisture, depending upon the kind of fiber used. The direction of twist may be observed by holding the yarn in a vertical position. If the spirals conform to the direction of the slope of the central part of the letter S, the yarn has an S twist; if they conform to the slope of the letter Z, the yarn has a Z twist. 

Yarn Count maintain with ring spinning:

In the spinning process, there is always a fixed relation between the weight of the original quantity of fiber and the length of the yarn produced from that amount of raw material. 

This relation indicates the thickness of the yarn. It is determined by the extent of the drawing process and is designated by numbers, which are called the yarn count. 

The International Organization for Standardization (ISO) fixed relationship between the weight and length of all yarns: one tex equals 1 gram (g) per kilometer. The greater the weight, the thicker the yarn, and consequently the higher the tex number Because of the speed limitations in ring spinning, researchers concentrated on developing techniques for inserting twist into yarns that would permit more rapid production. A result of this search was the introduction, in the 1960s, of the open-end spinning machine, which operated at higher speeds but produced a yarn with slightly different characteristics than conventional ring-spun yarns with ring spinning.

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Rotor spinning; Open-End Spinning

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Self twist spinning or Open-End Spinning 

Open-end spinning omits the step of forming the roving. Instead, a sliver of textile fibers is fed into the spinning machine spinner by a stream of air. The sliver is delivered to a rotary beater that separates the fibers into a thin stream. It is carried into the rotor by a current of air through a duct and is deposited in a V-shaped groove along the outer edge of the rotor. Twist is provided by the turning of the rotor. 

Fibers fed to the rotor are incorporated into the rapidly rotating “open end” of a previously formed yarn that extends out of the delivery tube; hence, the name open- end spinning. As the fibers join the yarn, which is constantly being pulled out of the delivery rube, twist from the movement of the rotor is conveyed to the fibers. A constant stream of new fibers enters the rotor, is distributed in the groove, and is removed at the end of the formed yarn, becoming part of the yarn itself. 

The fineness of the yarn is determined by the rate at which it is drawn out of the rotor relative to the rate at which fibers are being fed into the rotor. In other words, if fewer fibers are being fed in while fibers are being withdrawn rapidly, a thinner yarn will result, and vice versa. The twist is determined by the ratio of the rotor turning speed to the linear or withdrawal speed of the yarn (that is, the higher the speed of the rotor, the greater the twist). 

Theoretically, a variety of different means may be used to form the yarn and insert twist. These have been divided into the following categories: mechanical spinning (of which rotor spinning is an example), electrostatic spinning, fluid spinning, air spinning, and friction spinning. Of these, only rotor and friction open-end spinning machines have been commercialized and most of the open-end spinning machines now in use are of the mechanical rotor spinning type Friction open-end spinning machines are also available. 

Friction spinning systems use friction to insert twist. A mixture of air and fibers is fed to the surface of a moving, perforated drum. Suction holds the fibers against the surface while a second drum rotates in the opposite direction. Twist is inserted and the yarn begins forming as the fibers pass between the two drums. The newly forming yarn is added to the open end of an already formed yarn, and the completed yarn is continuously drawn away. 

The advantages of open-end spinning are that it increases the speed of production, eliminates the step of drawing out the roving before spinning, and permits finished yarns to be wound on any sized bobbin or spool. As a result, it is less expensive. It produces yarns of more even diameter than does ring spinning. Yarns are more uniform in diameter, bulkier, rougher, more absorbent, and less variable in strength than are ring-spun yarns. 

Fabrics made from open-end spun yarns compared with ring spun yarns are more uniform and more opaque in appearance, lower in strength, less likely to pill, and inferior in crease recovery. A number of sources indicated that they are more subject to abrasion. 

Neither friction nor rotor spinning will produce yarns as fine and strong as ring- spun yarns, although recent advances have extended the range of yarn sizes possible. Open-end spun yarns have a handle that has been characterized as “harsh.” Some of the kinds of products that seem to be especially well suited to the use of open-end spun yarns are in filling yarns for fabrics where yarn strength is not a factor, toweling pile yarns, denim, and heavier weights of bed sheeting. The yarns even surface makes them desirable as base fabrics for plastic-coated materials. On the other hand, the more acceptable feel of ring-spun yarns has led knitwear manufacturers to prefer them, and they are better for fine blends of polyester and cotton.

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