Tuesday, January 15, 2013

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. 

1 comment:

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