Production of filament yarn with man-made fibre by Melt spinning and dry spinning

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Man Made Fiber Formation 

Man-made fibers are polymeric forms that are produced by some type of chemical which or by the regeneration & natural polymers in a new physical form. The polymer is connected into some type & liquid / fluid state and forced through a spinnerette. Although most spinnerette are made with round openings, some may use orifices & other shape is order to produce fiber with special characteristics. 

The basic steps in producing manufactured fibers are as; 

The liquid polymer is then extruded through a spinneret. Each spinneret has a number of holes, and each hole produces one filament. As they exit the spinneret the filament fibers are solidified by cooling of melted polymers, by evaporating the solvent, or by precipitating the polymer from solution. These methods of solidification are the bases of the three primary fiber manufacturing processes. Other spinning methods that have been developed are described later in this chapter. 

Filament yarns are described by denier (that is, size) and number of filaments; for example, filaments described as 70/34 represent 70 denier/34 filaments. When fibers being extruded are intended for conversion into staple lengths, spinnerets with larger numbers of holes are used to produce more filaments that are later cut into staple lengths. Spinneret holes are spaced to allow the filaments to be extruded without touching each other. The holes must be exactly the same size to produce uniform fibers. The metal used in the plate must be capable of withstanding high pressures or corrosive spinning solutions. 

Most fiber spinning processes include a final step of drawing in which the filaments are stretched around rollers. 

Melt Spinning 

Melt spinning take advantage of the thermoplastic characteristics of polymers. Chips of solid polymer about the size of rice grains are dropped from a hopper into a melter where heat converts the solid polymer into a viscous liquid. The liquid forms a “melt pool” that is pumped through filters to remove any impurities that, would clog the spinneret and is delivered to the spinneret at a carefully controlled rate of Row. Melt spinning is simpler and cheaper than other spinning methods; therefore, it is used except when polymers cannot be melt spun. 

The spinneret holes are usually round, but noncircular holes are also used to make filaments of various cross-sectional shapes. Melt-spun fibers may be made through Y-shaped holes that yield a three-lobed fiber or C-shaped holes to produce a hollow filament, for example; The diameter of the fiber is determined by the rate’ at which the polymer is supplied to the hole in the spinneret and the windup speed, not by the diameter of the hole. When the molten polymer emerges from the spinneret hole, a cool air current is passed over the fiber, causing it to harden. Failure to maintain constant feeding speed of molten polymer or changes in the temperature of cooling will cause irregularities in the diameter of the fiber. Nylon and polyester are the most common melt-spun fibers. One of the latest developments in melt spinning has been the significant increase in spinning speeds. Processing speed has increased from less than 1,000 meters per minute in the 1960s to over 7,000 meters per minute today. This is the equivalent of a car traveling over 250 miles per hour. Higher-speed spinning is cost-effective and up to a certain point increases the orientation of the polymers in the fibers. Beyond a speed of about 6500 meters per minute, however, this advantage disappears as there is not enough time for the polymers to crystallize and the fibers may break. 

Dry Spinning 

In dry spinning the fiber solution is forced through the spinneret into a warm air chamber. The warm air causes the solvent used to make the fiber solution evaporate & the filament fibers are formed & hardened. This process, too may involve converting the fiber polymer into a different chemical form that is soluble in a suitable liquid As the solvent evaporate, the fiber polymer is reconstituted & return to its original chemical form, but now it is in a filament shape. 

Many polymers are adversely affected by heat at or close to their melting temperatures. Polymers that cannot be melt spun undergo other methods of spinning, such as dry spinning, to produce filaments. Dry spinning requires the dissolving of the polymer in a solvent to convert it into liquid form. Substances used as solvents are chosen not only because they will dissolve the polymer but also because they are safe and can be reclaimed and reused. 

The polymer and solvent are extruded through a spinneret into a circulating current of hot gas that evaporates the solvent from the polymer and causes the filament to harden. The solvent is removed and recycled to be used again. Dry-spun filaments generally have an irregular cross section. Because the solvent evaporates first from the outside of the fiber, a hard surface skin of solid polymer forms. As the solvent evaporates from the inner part of the fiber, this skin “collapses” or folds to produce an irregular shape. If the rate of evaporation is slowed, the cross section of the filament will be more nearly round. Acetate fibers and some acrylic fibers are dry spun. 

 

Fibers formed are: acetate, triacetate, acrylic, modacrylic, aramid fibers. 

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HOLLOW FIBERS | BI-COMPONENT FIBERS

HOLLOW FIBERS 

Hollow fibers are made of a sheath of fiber material and one or more hollow spaces at the center. These hollows may be formed in a number of different ways. The fiber may be made with a core of one material and a sheath of another, and then the central material is dissolved out. Alternatively, an inert gas may be added to the solution from which the fiber is formed, with the gas bubbles creating a hollow area in the fiber. Other experimental or proprietary techniques have been used to make hollow fibers. One involves spinneret holes with solid cores around which the polymer flows. 

Hollow fibers provide greater bulk with less weight. They are therefore, often used to make insulated clothing. For absorbent fibers such as rayon, hollow fibers provide increased absorbency. Some have been put to such specialized uses as filters or as carriers for carbon particles in safety clothing for persons who come into contact with toxic fumes. The carbon serves to absorb the fumes Bi-component Fibers 

As the technology for producing manufactured fibers has become more highly developed, manufacturers have turned to increasingly sophisticated techniques for creating new fibers. Not only are new generic fibers being created but also different polymers or variants of the same polymer can be combined into a single fiber to take advantage of the special characteristics of each polymer. Such fibers are known as bi-component fibers. 

The American Society for Testing and Materials (ASTM) defines a bi-component fiber as “a fiber consisting of two polymers which are chemically different, physically different, or both. Bicomponent fibers can be made from two variants of the same generic fiber (for example, two types of nylon, two types of acrylic) or from two generically different fibers (for example, nylon and polyester or nylon and spandex). The latter are called bi-component bi-generic fibers. 

Components in bi-component fibers may be arranged either side by side or as a sheath core. In making a side-by-side bicomponent fiber, the process requires that the different polymers be fed to the spinneret together so that they exit from the spinneret opening, side by side. Sheath-core bi-component fibers require that one component be completely surrounded by the other, so that the polymer is generally fed into the spinneret as shown in Figure. Variation in the shape of the orifice that contains the inner core can produce fibers with different behavioral characteristics. 

BI-COMPONENT FIBERS 

As the technology for producing manufactured fibers has become more highly developed, manufacturers have turned to increasingly sophisticated techniques for creating new fibers. Not only are new generic fibers being created but also different polymers or variants of the same polymer can be combined into a single fiber to take advantage of the special characteristics of each polymer. Such fibers are known as bi-component fibers. The American Society for Testing and Materials (ASTM) defines a bi-component fiber as “a fiber consisting of two polymers which are chemically different, physically different, or both. Bi-component fibers can be made from two variants of the same generic fiber (for example, two types of nylon, two types of acrylic) or from two generically different fibers (for example, nylon and polyester or nylon and spandex). The latter are called bi-component bi-generic fibers. 

Bi-component fibre

Components in bi-component fibers may be arranged either side by side or as a sheath core. In making a side-by-side bi-component fiber, the process requires that the different polymers be fed to the spinneret together so that they exit from the spinneret opening, side by side. Sheath-core bi-component fibers require that one component be completely surrounded by the other, so that the polymer is generally fed into the spinneret as shown in Figure. Variation in the shape of the orifice that contains the inner core can produce fibers with different behavioral characteristics.

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.