The properties of TEFLON® PTFE fluoropolymer resin used in fiber production do not permit solution or melt spinning as practiced with other synthetic fibers. Rather, natural TEFLON® PTFE fiber is formed by blending finely divided polymer particles with a solubilized form of cellulose to obtain a spinnable mixture. When fine streams of this mixture are extruded into a chemical bath, the water-insoluble form of cellulose is regener­ated, creating filaments suitable for further processing. These are conveyed to a heating means and progressively raised to a temperature sufficient to degrade the cellulose matrix and coagulate, or "sinter", the resin particles into continuous filaments. Finally, the filaments are oriented through stretching in the length direction to enhance their physical properties. The resulting natural TEFLON® fiber is approxi­mately 95% pure PTFE, the balance being a complex mixture of thermal degradation products of cellulose. This residue, which is largely on the surfaces of the filaments, but also dis­persed within them, accounts for the characteris­tic dark brown color of natural TEFLON® fiber. [Methods for complete removal of this residue by thermal or chemical bleaching processes and the effect of these processes on the physical proper­ties of the bleached fiber are discussed in the section entitled "Processing Information.")