Synthetic fibers are of strands of polymeric materials, such as polyesters and nylons. Their principal use is in the manufacture of textiles. The earliest synthetic fibers were the regenerated fibers, produced by chemical modifications of natural sources of cellulose; newer fibers include fully synthetic materials, such as nylon.
Regenerated cellulose fibers
Modern processes for manufacturing regenerated cellulose, known since 1924 as rayon, all follow a similar pattern. First, wood pulp is treated with chemical reagents that dissolve its cellulose content and free it of impurities. Then, the dissolved and chemically modified cellulose is passed through a perforated disk called a spinneret. The fine streams of solution emerge from the spinneret and pass directly into a bath of chemicals that convert the dissolved cellulose derivative into insoluble cellulose, forming fibers of rayon.
The spinneret was patented in 1883, when a British chemist, Sir Joseph Swan, used such devices to make fibers of nitrocellulose. Swan subsequently converted the fibers into cellulose filaments for use in electric lamps. In 1889, the French chemist Comte Hilaire de Chardonnet used a similar process of nitration, extrusion, drying, and regeneration of cellulose to produce Chardonnet silk fibers. Commercial production of Chardonnet silk began two years later. The process was slow and dangerous, since nitrocellulose is chemically similar to explosive guncotton.
The cuprammonium process, patented in 1890 by the French chemist Louis-Henri Despeissis, overcame the problems of the Chardonnet process and is still in use. In this process, cellulose is dissolved in cuprammonium liquor—a mixture of copper salts and ammonia. Cellulose fibers are formed by extruding the resulting liquid into a bath of alkali solution.
In 1891, a third process developed using sodium hydroxide and carbon disulfide to convert cellulose into sodium cellulose xanthate, a soluble derivative of cellulose called viscose. In the 20th century, the viscose process became the predominant method for manufacturing rayon.
The modern viscose process starts by steeping wood pulp in a caustic soda solution (concentrated sodium hydroxide solution), which converts the pulp into soda cellulose and at the same time extracts certain impurities from the wood pulp. The alkali solution and soluble impurities are squeezed from the swollen pulp, which is then shredded into fine crumbs of soda cellulose.
After aging, the soda cellulose crumbs are treated with liquid carbon disulfide to form bright orange cellulose xanthate. The xanthate dissolves in dilute sodium hydroxide solution to give viscose—an orange–colored syrup. The viscose solution is stored for a while to allow further chemical changes to occur—a process called ripening—before it is extruded into a bath containing dilute sulfuric acid, sodium sulfate, and zinc sulfate. The fibers are stretched as they solidify to promote the alignment of cellulose chains in the fibers, which improves their physical properties.
The main disadvantage of the viscose process is that it generates a great deal of waste. In 1978, the British chemicals company Courtauld's initiated a project whose goal was to develop a rayon-manufacturing process that would be less wasteful and harmful to the environment. In the mid-1980s, the project succeeded in producing a cellulose fiber, trade name Tencel, whose manufacture generates no waste chemicals. In 1996, the U.S. Federal Trade Commission modified the definitions of textiles to include Tencel-type fibers, called lyocells, as a subclass of rayons.
The Tencel process differs from other rayon-manufacturing processes in that it it uses a solvent to dissolve cellulose directly rather than using chemicals to make a soluble derivative. The solvent is an amine oxide—N-methylmorpholine-N-oxide, C4H8N(CH3)(O)O. In the first stage, wood pulp is heated with amine oxide until the cellulose dissolves. Extruding the amine oxide–cellulose solution into dilute amine oxide causes the cellulose to solidify and releases the amine oxide. The dilute amine oxide is concentrated by distillation and reused without waste.
