Synthetic Polymers—Cellulosics, Other Polysaccharides, Polyamides, and Polyimides

Natural polymers are biological macromolecules and are as old as life itself. Hemp, flax, cotton, and wood fibers from plants and silk and wool fibers from animals have been utilized since the dawn of civilization to make rope, paper, clothing, and furniture. What child has not been fascinated by a spider's web? The natural polymers include such diverse materials as proteins, polysaccharides, DNA, and polypeptides. The word polymer is derived from the Greek words poly, or many, and meros, or parts. This chapter will focus on cellulose and it derivatives, polyamides such as nylon and polyimides. Cellulose plastics are produced by the chemical modification of cellulose. Raw cellulose is not a thermoplastic: it does not melt. Cellulose is a substance that forms the cell walls of many trees and other plants. Raw cellulose can be made into a fiber or film, but it must be chemically modified to produce a thermoplastic material. Regenerating cellulose to yield the products rayon and cellophane removes the natural impurities. These regenerated products are essentially inert unless potential toxicants such as finishes and plasticizers are added in sufficient quantities to cause injury. Many cellulose derivatives would appear to be similarly inert. “Rayon” is, by definition, established by the Federal Trade Commission, the “generic name for a manufactured fiber composed of regenerated cellulose as well as manufactured fibers composed of regenerated cellulose in which substitutes have replaced not more than 15% of the hydrogens of the hydroxyl groups.” Cellophane is “regenerated cellulose, chemically similar to rayon, made by mixing cellulose xanthate with a dilute sodium hydroxide solution to form a viscose, then extruding this viscose into an acid for regeneration. The term rayon is used when the material is in fibrous form.” Rayon is made from regenerated cellulose by forcing it through small holes into the coagulating acid bath at the end of the process, while cellophane is the film form of regenerated cellulose that has been forced through a thin slit into an acid bath. All methods of preparation essentially depend upon solubilizing thin, short-fibered forms of natural cellulose, reshaping it into long fibers or film by extrusion through a spinneret or slit aperture, then immediately converting the extruded product back into solid cellulose. Rayon was first commercialized in the nineteenth century by the now discarded Chardonnet process that used highly flammable cellulose nitrate. The cuprammonium process replaced the Chardonnet process and is still used to a limited extent to produce extremely fine, silk-like filaments. Today the most widely used process is the xanthate or viscose process. Generally, alkali cellulose is prepared by reacting wood pulp with excess sodium hydroxide (or other alkali), followed by aging to permit separation of the pulp fibers. The alkali cellulose is reacted with carbon disulfide to form sodium cellulose xanthate, which is then dissolved in alkali and extruded into an acid bath that converts the filaments or film into rayon or cellophane. These filaments and films may be stretched, desulfurized, washed, dried, or otherwise finished. Industrial uses of rayon include reinforcing cords for tires, belts, and hoses, as well as in “disposable,” nonwoven fabrics. At one time, rayon was widely used in the textile industry. Extruding the viscose through a thin slit into an acid bath yields cellophane. Cellophane can be plasticized by washing the product with glycerol, propylene glycol, or polyethylene glycol. Regenerated cellulose may also be prepared by saponification of cellulose acetate. Cellophane films are widely used in the food industry. These products are so diverse that the references given here are primarily limited to reviews. Unprocessed rayon does not cause dermatitis. Commercial fabrics may contain free formaldehyde or formaldehyde resins. Analyses of 12 samples of 100% rayon clothing showed free formaldehyde levels ranging from 15 to 3517 ppm; formaldehyde is present in the finishing agents of fabrics. In the general population, harm is more likely to result from the flammability of these cellulosics or the means used to retard flammability than any other factor associated with their use. Untreated cellulosic materials exposed to smoldering flames readily generate lethal amounts of carbon monoxide. The synthetic polyamides and polyimides are all step-growth or condensation polymers. As a group, they are considered performance polymers, whereas the chain growth or addition polymers include the typical commodity polymers of polyethylene, polyvinyl chloride, and polystyrene as well as the high-performance fluoropolymers. Polyamides are linked with the word nylon, the first major synthetic polyamide. Nylon was developed as a fiber in the 1930s and as a plastic in the 1940s. Polyamide polymers also include protein fibers such as wool and silk that have been an important commodity throughout recorded history. These natural protein fibers are not discussed in this section. Nylon is a generic term for a synthetic aliphatic polyamide of well-defined structure and certain typical properties either as a fiber or as a plastic. The name system reflects the chemical structure and preparation. Nylons 66, 610, and 612 are all prepared from a six-carbon diamine and a 6-, 10-, or 12-carbon dibasic acid, respectively. [The names can also be written in the style nylon 6/6, nylon 6.6, or nylon 6,6 to reflect the two-monomer origin; the simpler style of nylon 66—always “six six,” or “six ten” for nylon 610—is usually preferred.] Nylons 6, 11, and 12 are prepared from an amino acid or derivative thereof with 6, 11, or 12 carbons, respectively. Nylon 66 was developed in the United States and nylon 6 was developed abroad. Both are now produced throughout the world. As a group, the nylons are tough, strong, abrasion resistant, and resistant to alkalies, hydrocarbons, ketones, and esters. Aromatic polyamides such as Nomex were formerly called nylon, but aramid is now the official generic classification of the U.S. Federal Trade Commission and the International Standards Organization. Aramid denotes a long-chain synthetic polyamide fiber in which at least 85% of the amide linkages are attached directly to two aromatic rings, whereas nylon now indicates that less than 85% of the amide linkages are so attached. Aromatic polyamide fibers typically have many desirable properties of nylon fibers plus improved heat resistance and strength. Polyimides are all synthetic polymers developed as a variation on polyamides to provide increased resistance to high temperature. Aromatic polyimides have exceptional heat resistance. Conventional tensile strength has been measured up to 500°C. Thermoplastic varieties, or those that become rubbery rather than melt at the glass-transition temperature of approximately 310°C, retain high strength at almost 300°C. Available data concerning residual reactions or solvents in the synthetic polymers of this group are meager. Negligible amounts of residual reactants would be expected on a stoichiometric basis in nylons formed by polymerization of a nylon salt and the known aromatic polyamides or polyimides. In the case of nylon 6, residual caprolactam is present in the polymer, but little concern on this point appears to have developed. Residual solvent might be a concern in view of the powerful solvent systems required for polymerization and processing of the high-melting aromatic polymers. Examples of such solvents for aromatic polyamides are dimethylacetamide, N-methylpyrrolidone, hexamethylphosphoramide, tetramethylurea, and mixtures of these solvents, which may be used with inorganic salts to increase solvating power. Subsequent processing as a textile would generally remove most or all polymerization solvent. Allergic dermatitis from currently manufactured, commercial nylon fabric is rare and associated with the dyes or other finishing products. Thermal degradation of polyamides and polyimides can yield toxic gases, particularly carbon monoxide, hydrogen cyanide, and/or ammonia. The temperatures at which these gaseous products are released can varies appreciably with specific polymers. While the field of polymer chemistry continues to expand, the use of these materials is not expanding, primarily do to their low biodegradation. Most of the research papers on these materials relates to environmental degradation as can be appreciated in a search of the National Library of Medicine's TOXNET databases 15. Keywords: cellulosics; dusts; fire; inhalation; polyamides; polyimides; nylon; thermal degradation