Fibers, Chemical

fibers made from natural and synthetic organic polymers. Depending on the type of raw material, chemical fibers are divided into synthetic (from synthetic polymers) and natural (from natural polymers) categories. Fibers prepared from inorganic compounds (glass, metal, basalt, or quartz) are sometimes also regarded as chemical fibers. Chemical fibers are manufactured industrially in the form of monofilament (a single, very long fiber), staple fiber (short pieces of fine fibers), and filament threads (a packet consisting of a large number of fine and very long fibers that are twisted together). Depending on the purpose for which they are intended, filament threads are divided into textile and industrial, or cord, threads (thicker threads of greater strength and twist).

History. The possibility of making chemical fibers from various substances (glue or resins) was predicted as early as the 17th and 18th centuries, but it was not until 1853 that the Englishman Audemars first proposed the formation of endless threads from a solution of cellulose nitrate in a mixture of ethanol and ether. In 1891 the French engineer H. de Chardonnet was the first to organize the manufacture of such threads on an industrial scale. From that time the manufacture of chemical fibers developed rapidly. In 1896 cuprammonium fiber was manufactured from cellulose dissolved in aqueous ammonia containing cupric hydroxide. In 1893 the Englishmen Cross, Bevan, and Beadle proposed a method of making viscose fibers from aqueous alkaline cellulose xanthate solutions; this was performed on an industrial scale in 1905. In 1918-20 a method for making acetate fiber from partly hydrolyzed cellulose acetate dissolved in acetone was developed, and in 1935 the production of protein fiber from milk casein was begun. Production of synthetic fibers began in 1932 with the manufacture of polyvinyl chloride fiber (Germany). In 1940 the best-known synthetic fiber, poly-amide fiber (USA), was made on an industrial scale. From 1954 to 1960 polyester, polyacrylonitrile, and polyolefin synthetic fibers were produced on an industrial scale.

Properties. Chemical fibers often have high tensile strength (up to 1,200 meganewtons per sq m, or 120 kilograms-force per sq mm), high ultimate elongation, good shape retention and crease resistance, and high resistance to repeated and alternating load and to the action of light, moisture, mold, bacteria, chemicals, and heat. The physicomechanical and physicochemical properties of chemical fibers can be changed during formation, stretching, finishing, and heat treatment, as well as through modification of both the raw material (the polymer) and the actual fiber. That makes possible the creation, even from a single fiber-forming polymer, of chemical fibers with diverse textile and other properties (see Table 1). Chemical fibers can be used in combination with natural fibers to make new varieties of textile articles, with considerable improvement in the latter’s quality and appearance.

Production. Of the large number of existing polymers, only those that consist of flexible, long macromolecules that are linear or only slightly branched and that have a sufficiently high molecular weight and can melt without decomposition or dissolve in available solvents are used in the manufacture of chemical fibers. Such polymers are called fiber-forming polymers. The process of producing fibers consists of the following operations: (1) preparation of the spinning solutions or melts, (2) formation of the fiber, and (3) finishing of the formed fiber.

The preparation of the spinning solutions (melts) starts with the passage of the raw polymer into a state of viscous flow (solution or melt). Then the solution (melt) is cleansed of mechanical impurities and air bubbles, and various additives are mixed in to make the fibers resistant to heat and light and to give them a dull polish. The solution (melt) thus made is fed into a spinning machine to form the fibers.

The formation of the fibers involves pressing the spinning solution (melt) through the fine holes of a spinneret into a medium that causes the polymer to solidify into fine fibers. The number and diameter of the holes in a spinneret can vary depending on the intended use and thickness of the formed fiber. In forming chemical fibers from a polymer melt (for example, polyamide fibers), cold air is the medium used to solidify the polymer. If the fibers are formed from a solution of a polymer in a volatile solvent (in the case of acetate fibers), a suitable medium is hot air, in which the solvent

evaporates (the so-called dry forming method). When forming fibers from a polymer solution in a nonvolatile solvent (for example, viscose fiber), the threads solidify upon falling from the spinneret into a special solution—the so-called precipitating bath (the wet forming method). The forming rate depends on the thickness and intended use of the fibers, as well as on the forming method. In the case of forming from a melt, speeds reach 600-1,200 m/min; with a solution by the dry method, 300-600 m/min; and by the wet method, 30-130 m/min. In the process of conversion of a small jet of viscous liquid into a fine fiber, the spinning solution (melt) is stretched simultaneously (spinneret stretching). In some cases the fiber undergoes additional stretching immediately after emerging from the spinning machine (plastification stretching), resulting in increased strength of the chemical fibers and an improvement in their textile properties.

The finishing of chemical fibers involves treating freshly formed fibers with various reagents. The nature of the finishing operations depends on the conditions of formation and on the fiber type. During finishing, low-molecular compounds (for example, from polyamide fibers) and solvents (from polyacrylonitrile fibers) are removed, and acids, salts, and other substances drawn out of the precipitating bath by the fibers (for example, viscose fibers) are washed away. To give fibers such properties as pliability, improved sliding, and surface adhesion of separate fibers, they are given a dressing treatment or are lubricated after washing and cleaning. The fibers are then dried on drying rollers or cylinders or in drying chambers. After finishing and drying, some chemical fibers are given an additional heat treatment, called heat setting (usually under stress, at 100°-180° C), which results in stabilization of the thread form and a decrease in shrinkage—both of the actual fibers and of articles made from them—during dry and wet treatments at high temperatures.

The world production of chemical fibers is growing rapidly; this is primarily due to reasons of economics (lower labor costs and capital investment) and to the high quality of chemical fibers relative to natural fibers. In 1968 world production of chemical fibers reached 36 percent (7,287,000 tons) of the total volume of production of all fibers. In various fields chemical fibers are displacing natural silk, flax, and even wool. It is thought that by 1980 the annual output of chemical fibers will reach 9 million tons and by 2000, 20 million tons, and that production will be comparable with that of natural fibers. In 1966 the output of the USSR was about 467,000 tons, and in 1970, 623,000 tons.

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