Manmade Fibres
| Fibre whose chemical composition, structure, and properties are significantly modified during the manufacturing process. Man-made fibres are spun and woven into a huge number of consumer and industrial products, including garments such as shirts, scarves, and hosiery; home furnishings such as upholstery, carpets, and drapes; and industrial parts such as tire cord, flame-proof linings, and drive belts. The chemical compounds from which man-made fibres are produced are known as polymers, a class of compounds characterized by long, chainlike molecules of great size and molecular weight. Many of the polymers that constitute man-made fibres are the same as or similar to compounds that make up plastics, rubbers, adhesives, and surface coatings. Indeed, polymers such as regenerated cellulose, polycaprolactam, and polyethylene terephthalate, which have become familiar household materials under the trade names rayon, nylon, and Dacron (trademark), respectively, are also made into numerous nonfibre products, ranging from cellophane envelope windows to clear plastic soft-drink bottles. As fibres, these materials are prized for their strength, toughness, resistance to heat and mildew, and ability to hold a pressed form.
Man-made fibres are to be distinguished from natural fibres such as silk, cotton, and wool. Natural fibres also consist of polymers (in this case, biologically produced compounds such as cellulose and protein), but they emerge from the textile manufacturing process in a relatively unaltered state. Some man-made fibres, too, are derived from naturally occurring polymers. For instance, rayon and acetate, two of the first man-made fibres ever to be produced, are made of the same cellulose polymers that make up cotton, hemp, flax, and the structural fibres of wood. In the case of rayon and acetate, however, the cellulose is acquired in a radically altered state (usually from wood-pulp operations) and is further modified in order to be regenerated into practical cellulose-based fibres. Rayon and acetate therefore belong to a group of man-made fibres known as regenerated fibres.
Another group of man-made fibres (and by far the larger group) is the synthetic fibres. Synthetic fibres are made of polymers that do not occur naturally but instead are produced entirely in the chemical plant or laboratory, almost always from by-products of petroleum or natural gas. These polymers include nylon and polyethylene terephthalate, mentioned above, but they also include many other compounds such as the acrylics, the polyurethanes, and polypropylene. Synthetic fibres can be mass-produced to almost any set of required properties. Millions of tons are produced every year.
This article reviews the composition, structure, and properties of man-made fibres, both regenerated and synthetic, and then describes the ways in which they are spun, drawn, and textured into useful fibres. For a full understanding of the material from which these fibres are made, it is recommended that the reader begin with the article industrial polymers, chemistry of. For an indication of the place occupied by man-made fibres in the overall coverage of polymers in Britannica Online, see Industrial Polymers: Outline of Coverage.
Chemical composition and molecular structure Linear, branched, and network polymers One of the features common to all the fibre-forming polymers is a linear structure. As explained in the article industrial polymers, chemistry of, polymers are built up by the joining together, through strong covalent bonds, of smaller molecular units known as monomers. When these monomers are joined end-to-end like links along a chain, a polymer with a simple linear structure is formed. In some polymers shorter chains grow off the long chain at certain intervals, so that a branched structure is formed. In other polymers the branches become numerous and cross-link to other polymer chains, thus forming a network structure. (These three polymer structures are illustrated in Figures 1A, 1B, and 1C of industrial polymers, chemistry of.) Materials made of linear and branched polymers will hold their shape when cooled, owing to the considerable attraction (known as intermolecular forces, or van der Waals forces) that such large molecules exert upon one another. With the application of heat, however, these materials will soften and eventually become molten, as the molecules, which are not cross-linked by covalent bonds, overcome the intermolecular forces and flow past one another. Linear and branched polymers will also dissolve in suitable solvents. Such behaviour makes linear polymers especially suitable for forming into fibres, which, as is explained below, are usually spun from a molten state or from solution. Few highly branched polymers are suitable for fibres, because they do not crystallize readily and have relatively poor mechanical properties. Network polymers form enormous, complex, chemically bonded structures that do not melt without undergoing chemical decomposition. In addition, while network polymers may soften and swell upon treatment with solvents, they do not readily dissolve. Such properties render most network polymers unsuitable for forming into fibres. Influence of chemical structure on properties The most important fibre-forming polymers are shown in Table 1. For details on their composition, properties, and applications, links are provided from the table to entries on the materials. An important requirement of these polymers is that they have melting points which are sufficiently high to make the fibres useful--for instance, so that clothing made from them can be ironed or pressed--but which also fall within a range that permits melt-spinning without decomposition of the polymer. Alternatively, polymers that melt at too high a temperature for practical melt-spinning or polymers that decompose at melt-spinning temperatures may be suitable for fibre forming if they can be dissolved and then spun from solution. The extent to which a polymer possesses these essential properties is often determined by the structure of its repeating units. To illustrate the manner in which these structural units can result in either good or poor fibre-forming properties, several basic polymer structures are discussed below , along with variations in chemical structure that cause variations in fibre-forming properties. |
