Melt-spun process

The irregular shapes and fine diameters ofthese fibers make them difficult to characterize and most available data concerns the properties of finished products. However, Table 6 gives some data on typical fibers made by the melt spun process (20). 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

The process competes with the traditional method of fiber production in which the precursor material is melted, usually in an arc furnace, then drawn through spinnerets and spun or impinged by high pressure air. The melt-spin process is not well suited to materials with high melting points such as zirconia, silicon carbide, or pure alumina. 

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

Nitrogen is pumped in and molten polymer is extruded and cooled on a drum to form ribbons. Nylon 6, 10 is formed in similar way at a lower temperature. Nylon 6, 6 can also be prepared in a continuous process in three separate tubes wherein reaction is started, steam removed and polymerisation completed. The product is directly melt spun into fibres. 

In 1974 Verbeek and Winter44,45 described a related process in which methyl-trichlorosilane reacts with methylamine or ammonia to yield a complex oligomer with a structure approximating to 9.15. This species could be melt-spun to form preceramic fibers which, when cross-linked, were pyrolyzed to amorphous silico-carbo-nitride fibers. The sequence of steps is shown in reaction (12). 

Because of its extremely low solubility in low-boiling and inexpensive organic solvents, nylon 66 required a new technique for converting the solid polymer into fibers hence the development of melt spinning, the third basic method for producing manufactured fibers. The following description refers essentially to nylon 66 because it was the first to use the method, but the process applies, in general, to all melt-spun manufactured fibers. 

Another polyester that has reached longterm commercialization is now produced in limited volume as Kodel 200 by Tennessee Eastman Co. and is considered to be 1-4 cyclohexylene dimethylene terephthalate. The glycol that is used instead of ethylene glycol in this process exists in two isomeric forms, one melting at 43°C and the other at 67°C. This makes possible their separation by crystallization, to secure the desired ratio of the two forms for conversion to the polymer. This ratio determines the melting point of the polymer, a most important property for a material that is to be melt-spun. The polymer from the 100 percent cis form melts at 275°C, and that from the 100 percent trans form at 318°C. Indications are that the commercial product is about 30/70 cis-trans. 

The worldwide production of acrylic fiber has declined significantly over the years because of the environmental concerns associated with the solution-spun process. In view of this, scientists have sought over the years a method that could render the high acrylics melt-spinnable. Such a method would not... 

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