Man-made fiber formation

Chemistry of Man-Made Fiber Formation Polyesters Nylons... 

The remainder of this chapter is concerned with briefly summarizing the chemistry and technology of man-made fiber formation. An attempt has been made to place the emphasis on those synthetic and cellulosic fibers that represent significance either in terras of world production or, in the authors view, in terms of unusual or unique polymer chemistry. More detailed and comprehensive general reviews on various aspects of these topics have been published elsewhere (4-10). 

H. F. Mark, S. M. Adas, and E. Cemia, eds., Man-Made Fibers Science and Technology, 3 Vols., Wiley-Interscience, New York, 1967—1968. Z. K. Walczak, Formation of Synthetic Fibers, Gordon Breach, New York, 1977. 

In general terms a man-made fiber polymerization scheme can be classified as either a batch or a continuous process. In a pure batch process the polymerization step is carried out separately from fiber formation in reactors that receive discrete charges of monomer(s). In a continuous polymerization (CP), monomer is fed continually into the reactors, and polymer is continually removed downstream. For some polyamides and polyesters, fiber formation may or may not be an integral part of the CP line. Most modern polymerization schemes are continuous processes, and these are slowly replacing much of the older batch technology. 

Fiber spinning is the one area of man-made fiber production in which a major change in technology is most likely to occur in the near future. Ultra high-speed spinning with nonmechanical devices could dramatically affect all phases of pre- and postfiber formation chemistry and technology. 

A. Coulsey and S. B. Smith, The formation and structure of a new cellulosic fiber . International Man-Made Fiber Conference, Austria, 1995. 

Textile fibers are normally broken down into two main classes, natural and man-made fibers. All fibers which come from natural sources (animals, plants, etc.) and do not require fiber formation or reformation are classed as natural fibers. Natural fibers include the protein fibers such as wool and silk, the cellulose fibers such as cotton and linen, and the mineral fiber asbestos. Man-made fibers are fibers in which either the basic chemical units have been formed by chemical synthesis followed by fiber formation or the polymers from natural sources have been dissolved and regenerated after passage through a spinneret to form fibers. Those fibers made by chemical synthesis are often called synthetic fibers, while fibers regenerated from natural polymer sources are called regenerated fibers or natural polymer fibers. In other words, all synthetic fibers and regener-... 

Fiber Length to Width Ratio Fibrous materials must have sufficient length so that they can be made into twisted yarns. In addition, the width of the fiber (the diameter of the cross section) must be much less than the overall length of the fiber, and usually the fiber diameter should be 1/100 of the length of the fiber. The fiber may be "infinitely" long, as found with continuous filament fibers, or as short as 0.5 inches (1.3 em), as found in staple fibers. Most natural fibers are staple fibers, whereas man-made fibers come in either staple or filament form depending on processing prior to yam formation. 

Common cross sections of man-made fibers include round, trilobal, pentalobal, dog-bone, and crescent shapes. Whai two polymers are used in fiber formation as in bicomponent or biconstituent fibers, the two components can be arranged in a matrix, side-by-side, or sheath-core configuration. Round cross sections are also found where skin formation has caused fiber contraction and puckering (as with rayons) has occurred or where the spinneret shape has provided a hollow fiber. Complex fiber cross-sectional shapes with special properties are also used. See Figure 1-5. 

Often fibers in textile substrates are deficient in one or more properties, or improved properties are desired for the substrate. Textile finishing provides a method whereby deficiencies in the textile can be corrected or specific properties can be introduced. Physical finishing techniques (dry finishing processes) or chemical finishing methods (wet finishing) are used. Physical finishing is usually carried out on the yarn or formed textile substrate, whereas chemical finishes can be added to the spinning bath prior to fiber formation for man-made fibers or appl ied to individual fibers, yarns, or completed textile structures. 

Web formation Man-made fibers are processed on roller cards into webs and then plaited... 

Manufactured protein fibers, often called azlons, are man-made fibers produced from animal or plant proteins. Examples of protein sources are milk, chicken feathers, soy beans, peanuts, corns, etc. Traditionally, most manufactured protein fibers were made directly from proteins dissolved in solvents. Recent trends in the research and development of manufactured protein fibers include the use of biochemistry to modify the source proteins and the introduction of synthetic polymers such as polyvinyl alcohol and polyacrylonitrile to improve the fiber mechanical properties. Antibacterial agents are often being added during the fiber formation process to provide health benefits to the manufactured protein fibers. As a result, the chemical structure of manufactured protein fibers is becoming more complex. 

Fig. 10. Formation of fibers used in Kuraray man-made leather (a) porous fiber, and (b) a bundle of microfibers.

Many molecules contain more than one function group (either the same or different). In such cases, condensation reactions involving two or more groups per molecule can lead to the formation of polymers, as mentioned above in the formation of proteins from amino acids (—NH2 and —COOH groups). From the standpoint of terminology, the unit or units that are joined together to produce a polymer are monomers with multiple units possible (dimer, trimer, etc.). An example of a man-made polymer involves the synthesis of polyester fiber, such as Dacron polyester on which the textile industry depends. 

Rayon. Virtually all current commercial production of man-made cellulosic fibers uses the viscose process, or some modification of it (19,20) (Fig. 1). The basis for this process is the formation of a metastable, water-soluble derivative from which cellulose can be regenerated after the filament is formed. Similar technology is used to manufacture cellophane film. Traditionally the process, as outlined in the following, is a batch operation involving discrete steps more recently semlcontinuous or continuous systems have been developed. 

Cellulose 11 as the most important from a technical and commercial point of view is formed from cellulose I by precipitating cellulose firom solution into an aqueous medium at room or slightly elevated temperature, i.e., in technical spinning processes for man-made cellulose fibers. It is also obtained in the large-scale mer-cerization process of cotton, which proceeds via the formation of sodium cellulose by interaction of the polymer with aqueous sodium hydroxide and subsequent decomposition of this intermediate by neutralization or washing out of the sodium hydroxide. It is not yet understood how the parallel chain arrangement of cellulose I undergoes transition into the antiparallel orientation of cellulose II without an intermediate dispersion of cellulose molecules. The crystalline structure of cellulose I and cellulose 11 are shown in Fig. 2. 

The uniformity of fibers affects the properties and quality of end-products. The strength of an individual fiber is determined by its weakest point. For synthetic polymer fibers, man-made inorganic fibers and nanofibers, the uniformity can be controlled during the fiber formation process to minimize the stmctural irregularities. However, it is difficult to control the uniformity of natural fibers since the structure of these fibers is affected by many environmental factors. Although the uniformity of an individual natural fiber is uncontrollable, it is possible to improve the overall uniformity of fibers in end-products by blending natural fibers from many different batches. 

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