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. 

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.