HIGH SPEED MELT SPINNING OF FLUOROPOLYMER FIBERS

Abstract: The processes and apparatus of the present invention concern melt spinning high viscosity fluoropolymers into single filaments or multi-filament yarns at high spinning speeds.

Description: TITLE HIGH SPEED MELT SPINNING OF FLUOROPOLYMER FIBERS BACKGROUND OF THE INVENTION The processes and apparatus of the present invention concern melt spinning fluoropolymers into single filaments or multi-filament yarns at high spinning speeds. Melt spinning of thermoplastic copolymers based on tetrafluoroethylene is known. However, there is considerable economic incentive to drive fiber spinning rates ever higher for these high value polymers. One problem facing processes of melt spinning is that at high shear rates, melt fracture occurs which becomes evident as surface roughness in the extruded fibers. Since the critical shear rate for the onset of melt fracture decreases with increasing melt viscosity, ways to decrease melt viscosity have centered on raising the temperature of the melt. However, in many polymers including thermoplastic copolymers based on tetrafluoroethylene, the polymer exhibits thermal degradation before any significant decrease in melt viscosity can be achieved. Fibers of polytetrafluoroethylene (PTFE) homopolymer are also highly valued, particularly for their chemical and mechanical properties, such as low coefficient of friction, thermal stability and chemical inertness. However, processing by melt spinning has proved elusive. Since polytetrafluoroethylene homopolymer fibers are conventionally formed by a disperson spinning process involving many steps and complicated equipment, there is great economic incentive to find a method for melt spinning such fibers. The problem of spinning fibers from high viscosity polymer melts has been previously addressed for polyesters. In U.S. Patent 3,437,725 a spinneret assembly is described having a top plate, a heating plate and a lower plate with a spacer providing air space between the top plate and the heating plate. Hollow inserts, one for each filament to be spun, are placed in the top plate and extend to the bottom face of the lower plate. Molten polymer is fed into the inserts for spinning through capillaries. An electrical heater supplies heat to maintain the lower plate, heating plate and lower portions of the inserts at a temperature at least 60°C higher than the temperature of the supplied molten polymer. Heated capillary temperatures ranging between 290 and 430°C were listed in examples for spinning polyesters. No mention is made of any fluoropolymer or temperatures needed to melt spin fluoropolymers at high spinning speeds. SUMMARY OF THE INVENTION The present invention provides a process for melt spinning a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers, comprising the steps of melting a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers to form a molten fluoropolymer composition; conveying said molten fluoropolymer composition under pressure to an extrusion die of an apparatus for melt spinning; and extruding the molten fluoropolymer composition through the extrusion die to form molten filaments, said die being at a temperature of at least 450°C, at a shear rate of at least 100 sec 1 , and at a spinning speed of at least 500 m/min. The present invention also provides a process for melt spinning a composition comprising polytetrafluoroethylene homopolymer, comprising the steps of melting a composition comprising a polytetrafluoroethylene homopolymer to form a molten polytetrafluoroethylene composition; conveying said molten polytetrafluoroethylene composition under pressure to an extrusion die of an apparatus for melt spinning; and extruding the molten polytetrafluoroethylene composition through the extrusion die to form molten filaments. The present invention further provides an apparatus for melt-spinning fibers comprising a spinneret assembly comprising means for filtering; a spinneret; an elongated transfer line, said transfer line being disposed between said filtration means and said spinneret; means for heating said elongated transfer line; means for heating said spinneret; and an elongated annealer disposed beneath said spinneret assembly. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a portion of a conventional apparatus for melt spinning. Figure 2 is a cross-sectional view of one embodiment of a portion of a melt spinning apparatus of the present invention having an elongated spinneret. Figure 3 is a cross-sectional view of one embodiment of a portion of a melt spinning apparatus having a shortened elongated spinneret. Figure 4 is a cross-sectional view of one embodiment of a portion of a melt spinning apparatus of the present invention having a shortened elongated spinneret with heating means disposed within a center cavity thereof and heating means disposed on an outer surface thereof. Figure 5 is an exploded cross-sectional view of one embodiment of a melt spinning apparatus of the present invention featuring an elongated transfer line disposed between a pack filter and a spinneret disc. Figure 6 is an assembled cross-sectional view of the melt spinning apparatus of Fig. 5. Figure 7 is an exploded cross-sectional view one embodiment of a melt spinning apparatus of the present invention featuring another embodiment of an elongated transfer line and spinneret disc. Figure 8 is an assembled cross-sectional view of the melt spinning apparatus of Fig. 7. Figure 9 is a schematic of one embodiment of a melt spinning apparatus of the present invention. Figures 10A and 10B are cross-sectional views of one embodiment of an annealer useful in the present invention. Fig. 1 OB is an enlarged view of a portion of Fig. 10A. Figure 11 is a graph plotting shear rate (1/sec) vs. SSF at 500°C for a composition of Example 1, wherein the darkened triangle represents the spin stretch factor (SSF) at first filament break and the open triangle represents the SSF at the last filament break. Included is some data for denier/tenacity/speed/gpm. Figure 12 is a graph demonstrating that temperature exerts a positive effect on SSF at first filament break at constant shear rate. The circle represents SSF at 420°C; the square represents SSF at 460°C; and the triangle represents SSF at 500°C (see also Example 1). Figure 13 is a graphical representation of throughput vs. solidification distance from a spinneret with and without an annealer using FEP-5100, a 30- mil/30-filament spinneret, a 3-in diameter, 41 -in long annealer, and spinneret temperatures of 380°C (triangle), 430°C (square) and 480°C (circle), wherein the open symbols represent no annealer and the darkened symbols represent use of an annealer. Figure 14 is a graphical representation of distance from a spinneret (inch) vs. yam temperature with an annealer (darkened symbols) and without an annealer (open symbols) using FEP-5100, a 39.4-mil/30-fιlament spinneret, a spinneret temperature of 480°C, at 45.4 gpm/6.0 pph, wherein the square represents the yam temperature at a spinning speed of 400 mpm, the circle represents the yam temperature at 500 mpm, and the triangle represents the yam temperature at 700 mpm. Figure 15 is a graphical representation of length of annealer (inch) vs. first-filament-break speed in meters/minute (mpm). The following were used: FEP-5100 fluoropolymer, a 30-mil/30-filament spinneret, a spinneret temperature of 480°C, and 44.8 grams/minute (gpm). Figure 16 is a graphical representation of temperature vs. first filament break speed (mpm) for Example 23, wherein the darkened circle represents the sample of the present invention and the square represents the comparative sample. DETAILED DESCRIPTION The process of the present invention affords the benefits of high temperature spinning while avoiding the pitfalls thereof. In the process of the present invention, the composition comprising highly fluorinated thermoplastic polymer or blend of such polymers can be exposed to temperatures above the degradation temperature of the polymers for times sufficient to cause a decrease in melt viscosity but insufficient for significant polymer degradation to occur. In melt spinning, the molten composition experiences the highest shear rate during its transit through the extrusion die, e.g. capillaries, of the spinneret of the melt spinning apparatus. In the process of the present invention, it is at that point that the molten composition can be heated to a temperature above the degradation temperature of the highly fluorinated polymer. Because of the high throughput speed achievable in the present invention due to the elevated temperature, the residence time of the composition in the extrusion die is kept to a minimum. Accordingly, the present invention provides a first process for melt spinning a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers, comprising the steps of melting a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers to form a molten fluoropolymer composition; conveying said molten fluoropolymer composition under pressure to an extrusion die of an apparatus for melt spinning; and extruding the molten fluoropolymer composition through the extrusion die to form molten filaments, said die being at a temperature of at least 450°C, at a shear rate of at least 100 sec -1 , and at a spinning speed of at least 500 m/min. In the melting step, a composition including a highly fluorinated thermoplastic polymer or a blend of such polymers is melted. Highly fluorinated thermoplastic polymers for the purpose of this first process include homopolymers other than polytetrafluoroethylene (PTFE), such as polyvinylidene fluoride (PVDF), and copolymers, such as copolymers of tetrafluoroethylene (TFE) prepared with comonomers including perfluoroolefins, such as a perfluorovinyl alkyl compound, a perfluoroalkyl vinyl ether, or blends of such polymers. The term "copolymer", for purposes of this invention, is intended to encompass polymers comprising two or more comonomers in a single polymer. A representative perfluorovinyl alkyl compound is hexafluoropropylene. Representative perfluoroalkyl vinyl ethers are perfluoromethyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), and perfluoropropyl vinyl ether (PPVE). Preferred highly fluorinated polymers are the copolymers prepared from tetrafluoroethylene and perfluoroalkyl vinyl ether and the copolymers prepared from tetrafluoroethylene and hexafluoropropylene. Most preferred copolymers are TFE with 1-20 mol% of a perfluorovinyl alkyl comonomer, preferably 3-10 mol% hexafluoropropylene or 3-10 mol% hexafluororpopylene and 0.2-2 mol% PEVE or PPVE, and copolymers of TFE with 0.5-10 mol% perfluoroalkyl vinyl ether, including 0.5-3 mol% PPVE or PEVE. Also suitable for the practice of this invention are blends of the highly fluorinated thermoplastic polymers including blends of TFE copolymers. The fluoropolymers suitable for the practice of the present invention preferably exhibit a melt flow rate (MFR) of 1 to about 50 g/10 minutes as determined at 372°C according to ASTM D2116, D3307, D1238, or corresponding tests available for other highly fluorinated thermoplastic polymers. The composition comprising the highly fluorinated thermoplastic polymer or a blend of such polymers can further comprise additives. Such additives can include, for example, pigments and fillers. In the present process the composition comprising the highly fluorinated polymer or blend of such polymers, discussed above, is melted to form a molten fluoropolymer composition. Any means known in the art for providing a melt can be used. A representative method can include introducing the fluoropolymer composition to an extruder which is heated to a temperature sufficient to melt the composition but below the degradation temperature of the highly fluorinated thermoplastic polymer or blend of such polymers. This temperature is dependent upon the particular polymers used. Once the composition is in a molten state, it is conveyed under pressure to an extrusion die, such as a spinneret, of an apparatus for melt spinning. Means of conveying compositions to the extrusion die are well known in the art and include apparatus with a ram or piston, a single screw or a twin-screw. In a preferred embodiment of the process of the present invention, an extruder is employed to melt and convey the molten composition suitable for the practice of this invention to a single or multi-aperture strand extrusion die to form, respectively a monofilament or multifilament fiber product. The extruder barrel and screw, and the die are preferably made from corrosion resistant materials including high nickel content corrosion resistant steel alloy, such as Hastelloy C-276 (Cabot Corp., Kokomo, IN). Many suitable extraders, including screw-type and piston type, are know in the art and are available commercially. A metering device, such as a gear pump, may also be included to facilitate the metering of the melt between the screw and the spinneret. In the process of the present invention, after the molten fluoropolymer composition is conveyed to the extrusion die, it is extruded through the apertures of the extrusion die, said die being at a temperature of at least 450°C, at a shear rate of at least 100 sec -1 , and at a spinning speed of at least 500 m/min. The apertures of the extrusion die can be of any desired cross-sectional shape, with a circular cross-sectional shape preferred. The diameter of a circular cross-sectional aperture found suitable for use in the process of the present invention can be in the range of about 0.5 to 4.0 mm, but the practice of this invention is not limited to that range. The length to diameter ratio of the extrusion die aperture useful in the present invention is preferably in the range of about 1 : 1 to about 8:1. Although the hole pattern is not critical, it is preferred if the holes are arranged in one or two concentric circles, with a single circle arrangement being more preferred. Fig. 1 depicts a portion of a conventional melt spinning apparatus for thermoplastic polymers, spinneret assembly 10. Shown are adaptor 1 which may be heated with a cartridge heater inserted within space 9 located between the dotted lines along adaptor 1, which is attached to means for conveying and melting the fluoropolymer composition (not shown), filter pack 2 containing melt filtration means 3, typically screens, and conventional spinneret 4 having face plate 5, face plate 5 being disposed at one end of spinneret 4 at a distance, h, from the opposite end of spinneret 4. Spinneret 4 is disposed adjacent bottom face 8 of filter pack 2, and together with filter pack 2 is affixed to adaptor 1 by retaining nut 6. Spinneret assembly 10 is heated by band heater 7 circumferentially disposed around retaining nut 6. In Fig. 1, spinneret 4 is generally heated by its conductive contact with retaining nut 6. In the conventional spinneret assembly design of Fig. 1, there is no convenient way to heat only face plate 5 of spinneret 4 because spinneret 4 resides entirely within retaining ring 6. Any attempt to super-heat face plate 5 would result in heating a considerable portion of other areas of spinneret assembly 10 to a similar if somewhat lower temperature. This undesirable heating of areas besides face plate 5 of spinneret assembly 10 to temperatures at or above the degradation temperature of the fluoropolymer composition would result in an undesirably long duration of exposure of the fluoropolymer composition to high temperature and could lead to excessive polymer degradation under some circumstances. During extrusion in the present invention, the extrusion die is heated to a temperature of at least 450°C. For certain fluoropolymer compositions herein, the extrusion die can be heated to temperatures greater than about 500°C. Heating to these temperatures without degradation of the fluoropolymer composition can be done by thermally isolating the extrusion die from other areas of the melt spinning apparatus that may contain the fluoropolymer composition. When the molten fluoropolymer composition begins to pass through the extrusion die, the elevated temperature of the die thereof induces a rapid decrease in polymer melt viscosity, permitting a high rate of transmission through the extrusion die. To avoid thermal degradation, it is necessary to reduce the residence time of the melt at the high temperatures. Since degradation is a function not only of temperature but also of time, if the temperature is high, it is preferred that the residence time be minimized. Thus, the present invention provides the highest temperature in the area where it would be most beneficial, namely the extrusion die, e.g. the walls of the spinneret capillary holes, which are in the face plate of the spinneret. Therefore, the extrusion die can be kept thermally isolated from other areas of the melt spinning apparatus that may be in contact with the fluoropolymer composition. The spinneret or a portion thereof that includes the face plate can be heated independently of other areas of the spinneret assembly. Any means for providing highly localized heating to a temperature of at least 450°C can be employed for the practice of the invention. Such means includes a coil heater, a cartridge heater, a band heater, and apparatus for radio frequency, conduction, induction or convective heating, such as an induction heater. Insulation may be used, such as ceramic insulation, to provide off-sets and thereby thermal isolation between the face plate and other areas of the melt spinning apparatus that may be in contact with the fluoropolymer composition. Use of one or more cooling jackets can also be used on areas of the spinneret or spinneret assembly other than the extrasion die to provide thermal isolation of the extrusion die. In order to facilitate the thermal isolation of the extrusion die, it has been found satisfactory in one embodiment of the present invention to offset the spinneret face plate from the spinneret body by simply increasing the distance, h, between the ends of the conventional spinneret shown in Fig. 1. Increasing the distance in this manner, shown in Fig. 2 as h', enables separate heating of the spinneret face plate from the bulk of the remainder of the spinneret assembly. Thus, the spinneret face plate of the present invention in one embodiment is separated from the bottom face of the filter pack by distance h' which distance is sufficient to allow separate heating of the spinneret face plate. In Fig. 2 is shown spinneret assembly 20 having adapter 21 which is attached to means for melting and/or conveying the fluoropolymer composition (not shown), filter pack 22 containing screen 23 and bottom face 28, elongated spinneret 24 having face plate 25 being disposed at one end of spinneret 24 at a distance, h', from the opposite end of spinneret 24 at bottom face 28 of filter pack 22, wherein h'>h other measurements of Fig. 1 and 2 held equal, to enable face plate 25 to extend outside of retaining nut 26. With face plate 25 thus protruding from retaining nut 26, heating means 29 can be used to separately heat face plate 25, and thus face plate 25 is thermally isolated from the remainder of the spinneret assembly. Heating means 27, such as a band or coil heater, is disposed circumferentially around retaining nut 26. An alternative embodiment of a spinneret assembly useful in the present invention is shown in Fig. 3 as spinneret assembly 30. In this embodiment, the bottom part of retaining nut 26 of Fig. 2 is reduced in size, e.g. the retaining nut is thinner, see retaining nut 36 in Fig. 3. Here, the body of elongated spinneret 34 is shortened relative to the length of spinneret 24 of Fig. 2, and yet spinneret 34 is elongated (relative to spinneret 4 of Fig. 1) so as to extend beyond retaining nut 46 enabling face plate 35 to be heated separately, by means 39, from means 37 shown for heating another area of the spinneret assembly. Also shown is adapter 31 which is attached to means for melting and/or conveying the fluoropolymer composition (not shown), filter pack 32 and filtration means 33, and channel 38. In the above embodiments of the present invention, molten composition conveyed into the spinneret can be heated by means disposed around the outside wall of the spinneret, and thus the temperature of the melt adjacent the walls of the apertures is higher than the temperature in the center of the melt. The effect of this temperature non-uniformity, highest at the outside and cooling toward the center of the melt, can cause extruding filaments to bend toward the center of the spinneret. The bent angle has been observed higher than 45 degrees at high jet velocity for certain fluoropolymer compositions. The impact of this phenomenon can be reduction in attainable high speed filament continuity. In order to reduce any temperature gradient between the outermost and innermost parts of the polymer melt, a heating means is provided within aperture 48, such as a cartridge heater, can be introduced into the center of elongated spinneret 44, as shown in the spinneret assembly 40 of Fig. 4. Also shown in Fig. 4 are adapter 41 which is attached to means for melting and/or conveying the fluoropolymer composition (not shown), filter pack 42, filtration means 43, retaining nut 46, heating means 47 and 49, and face plate 45. A further embodiment provided by the present invention, shown in Figs. 5 and 6 as spinneret assembly 50, is to heat the melt faster and through narrow channel 62 (relative to channel 38 of Fig. 3) provided within transfer line 58, and reduce the volume directly upstream to spinneret face plate 55. By reducing the volume, the residence time is reduced. This embodiment also provides the opportunity to provide an intermediate temperature zone for the composition while in channel 62 of transfer line 58 through use of heating means 60. Thus, the present process can further include exposing the fluoropolymer composition to an intermediate temperature ranging from the melt temperature of the fluoropolymer composition to a temperature less than the temperature of the extrusion die, e.g. at the face plate of the spinneret. As shown, the portion of transfer line 58 adjacent filter pack 52 can be heated via heating means 57 disposed circumferentially around retaining nut 56. The fluoropolymer composition within channel 62 of transfer line 58 can be pre-heated to at least one intermediate temperature which can range from above the melting temperature of the fluoropolymer composition to a temperature lower than the temperature at face plate 55 via heating means 57 and/or heating means 60. Face plate 55 is shown in this embodiment as being separately heated via heating means 61 held in spinneret sleeve 59. Transfer line 58 is disposed downstream of filter pack 52 and filtration means 53 and followed by spinneret 54, shown having a disc shape. Spinneret 54 can be removable for cleaning and replacement without removal of pack filter 52. Also shown is adapter 51 which is attached to means for melting and/or conveying the fluoropolymer composition (not shown). Figs. 7 and 8 show spinneret assembly 70 of the present invention which embodiment permits removal of transfer line 78 and can accomodate larger diameter disc spinnerets relative to the embodiment shown in Figs. 5 and 6, such as spinneret 74. Spinneret nut 79 holds disc spinneret 74 having face plate 75 to the bottom of face 82 of transfer line 78. Narrow internal flow channel 83 in transfer line 78 reduces the volume and residence time of the fluoropolymer composition at high temperature to further reduce the chance of degradation. Transfer line 78 also provides a means of stepping up to an intermediate temperature between filtration means 73 and spinneret 74 via its separate heating means 80. At the same time, the transfer line embodiment shown provides more uniform and faster heat transfer. An additional advantage of this embodiment is that disc spinneret 74 can be replaced without having to remove the filter pack, and the disc can be easier to fabricate. Also shown are adapter 71, which is attached to means for melting and/or conveying the fluoropolymer composition (not shown), plate 72 which has multiple distribution channels providing support for filtration means 73, retaining nut 76 surrounded by heating means 77, chamber 84 disposed between filtration means 73 and transfer line 78, and face plate 75. It is believed that the present process provides self-melt lubricated extrusion. By "self-melt lubricated extrasion" is meant that only the skin of the extrudate, the portion of the melt directly adjacent the walls of the apertures, becomes heated to extremely high temperature by the very hot die aperture surface resulting in very low viscosity of this portion of the melt while keeping the bulk of the extrudate to a lower temperature due to the short contact or residence time. The considerably reduced viscosity of the outer layer skin behaves like a thin lubricating film thus permitting the extrusion to become plug flow, wherein the bulk of the extrudate experiences uniform velocity. As used herein "shear rate" refers to the apparent wall shear rate, calculated as 4Q/πR (Q = volumetric flow rate, R= radius of capillary). In the process of the present invention, the shear rate is at least 100/sec. The shear rate range over which satisfactory fiber melt-spinning can be achieved in a given configuration and at a given temperature grows progressively narrower with increasing polymer melt viscosity. The operating window can be expanded by increasing the temperature which displaces the critical shear rate for the onset of melt fracture to higher rates, but care must be taken to avoid polymer degradation. The critical temperature/shear rate for melt fracture is determined herein by increasing the throughput rate for a given temperature and die dimension until surface roughness is visible as shown by the change in molten extrudate from a transparent to a slightly opaqueness indicating the onset of melt fracture. Further increase in throughput rate would give an undesirable coarser surface roughness and poorer spinning performance and properties. The spinning speed of the process of the present invention is at least 500 m/min and is determined herein as the spinning speed at the last roll, which depending on the configuration of the melt-spinning apparatus may be a take-up roll or may be a wind-up roll. It is found in the practice of the present invention that both shear rate and SSF have a large effect on the strength of the spun filament. The same strength can be maintained as the shear rate increases while the SSF decreases and vice versa as demonstrated in Example 1 and shown graphically in Fig. 11. The process of the present invention can further comprise shielding the filaments. By shielding the filaments, the air surrounding the filaments remains warmer than if the filaments were exposed to unrestricted ambient air and thus prevents rapid cooling of the filaments. Unrestricted ambient air, and in particular, turbulent air can result in rapid cooling of the filaments which is undesirable because it can be detrimental to the amount of draw the filament may have. Thus shielding the filaments can permit higher attenuation of spin stretch. It has been observed herein that the achievement of high SSF for high spinning can be obtained if the solidification of the molten threadline occurs at a distance greater than 50 times the diameter of the extrasion die (capillary diameter) (see also Fig. 13). Preferably, the solidification distance is greater than 500 times the diameter of the capillary diameter. Shielding can be accomplished by running the molten filaments through an annealer. An annealer permits the high speed extruded molten filaments to be spin stretched to a high degree and thus increases the spinning speed. Although a gentle suction of air can be generated by the fast moving yam through the bottom of the annealer, the annealer still provides a relatively quiescent environment against surrounding air turbulence which partially cools but prevents rapid cooling of the extremely hot molten filaments, maintaining the filaments above their melting point for a much further distance from the spinneret than without an annealer. This is shown graphically in Fig. 13. The use of an annealer also maintains the solidified yam at a higher temperature than without the use of an annealer as shown in Fig. 14. In addition, the use of an annealer can permit higher spinning speeds as shown in Fig. 15 (note: 0-inch represents no annealer). One embodiment of an annealer useful in the present invention is shown in Figs. 10A and 10B. As shown, annealer 200 includes inner tube 202 which is a long tube concentrically disposed inside outer tuber 204, a slightly larger diameter tube which can be of substantially the same length. Inner tube 202 can be positioned within outer tube 204 to extend below outer tube 204 and thus provides an exit for the molten filaments and further creates a cylindrical opening 205 at the top of outer tube 204. Opening 205 permits air to be sucked into inner chamber 206 of inner tube 202 which may have been pre-heated in annular space 208 between inner tube 202 and outer tube 204. Although external heat is not provided, annular space 208 can be heated during spinning by the heat radiating from the extruded hot molten filaments. Top flange 210, which can have a circular peripheral lip, sits on top of outer tube 204. Mesh tubing 212, preferably composed of a fine mesh screen, such as 20-mesh, can be attached to top flange 210 and is disposed adjacent the inner walls of inner tube 202. Mesh tubing 212 extends axially through inner chamber 206 beyond opening 205, but it is not necessary to provide the mesh tubing for the entire length of the inner tube. Mesh tubing 212, which can further include a second finer mesh, such as 100-mesh, attached to or in close proximity to the first mesh, serves to reduce incoming air turbulence and also facilitates a substantially uniform distribution of the air so that the air travels radially into inner chamber 206 through opening 205. There is also shown perforated annular plate spacers 214, disposed between inner tube 202 and outer tube 204, and connected either to the outer surface of inner tube 202 or to the inner surface of outer tube 204, and can serve to prevent inner tube 202 from falling out of outer tube 204. Screens 216 of fine mesh can be placed on top of plate 214 to diffuse and distribute the air traveling up and into opening 205. Such spacers 214 and 216 are optional. An optional glass ring 220 permits visual observation of the molten threadlines and spinneret face. The inner and outer tubes of the annealer can be fabricated from materials including metal, such as aluminum, or plastic, such as Lucite®. The annealer can be self-standing or held stable with a suitable mounting mechanism which can be attached to other elements of a melt spinning apparatus or affixed to other materials to keep it held steady. The process of the present invention can further comprise passing the extrudate in the form of one or more strands through a quench zone to means for accumulating the spun fiber. The quench zone may be at ambient temperature, or heated or cooled with respect thereto, depending upon the requirement of the particular process configuration employed. Any means for accumulating the fiber is suitable for the practice of the present invention. Such means include a rotating dram, a piddler, or a wind-up, preferably with a traverse, all of which are known in the art. Other means include a process of chopping or cutting the continuous spun-drawn fiber for the purpose of producing a staple fiber tow or a fibrid. Still other means include a direct on- line incorporation of the spun-drawn fiber into a fabric structure or a composite structure. One means found suitable in the embodiments here in below described is a high-speed textile type wind-up, of the sort commercially available from Leesona Co., Burlington, NC. Such other means as are known in the art of fiber spinning to assist in conveying the fiber may be employed as warranted. These means include the use of guide pulleys, take-up rolls, air bars, separators and the like. An anti-static finish can be applied to the fiber. Such finish application is well known in the trade. The process of the present invention can further comprise drawing the fiber, a relaxing stage, or both. The fiber can be drawn between take-up rolls and a set of draw-rolls. Such drawing is well known in the trade to increase the fiber tenacity and decrease the linear density. The take-up rolls may be heated to impart a higher degree of draw to the fiber, the temperature and the degree of draw depending on the desired final fiber properties. Likewise additional steps, known to those of ordinary skill in the art, may be added to the present process to relax the fiber. The present invention also provides a second process for melt spinning a composition comprising polytetrafluoroethylene homopolymer, comprising the steps of melting a composition comprising a polytetrafluoroethylene homopolymer to form a molten polytetrafluoroethylene composition; conveying said molten tetrafluoroethylene composition under pressure to an extrasion die of an apparatus for melt spinning; and extruding the molten polytetrafluoroethylene composition through the extrasion die to form molten filaments. In the method of melt spinning the homopolymer, polytetrafluoroethylene (PTFE), preferred PTFE homopolymers are those that give a melt flow at temperatures below 480°C. Preferred homopolymers include Zonyl® fluoro- additives, PTFE granular molding powder grades, such as Teflon® PTFE TE- 6472, and PTFE lubricated paste extrasion resins, such as Teflon® PTFE 62, all available from E. I. du Pont de Nemours and Co., Wilmington, DE. Because of the extreme temperatures required to exhibit melt flow characteristics which border on the verge of thermal degradation, the present process is of particular importance in the successful melt processing and fiber spinning of PTFE homopolymers. The description above pertaining to the steps in the first process of melt spinning the highly fluorinated thermoplastic composition and the apparatus useful therefor are applicable to the process of melt spinning the polytetrafluoroethylene composition. However, the same limitations on extrusion die temperature or shear rate or spinning speed found in the first process may not be applicable in the present PTFE process. Preferably, the temperature of the extrusion die is at least 450°C. The spinning speed is preferably at least 50 mpm; more preferably at least 200 mpm; and most preferably at least 500 mpm. The present invention further provides an apparatus for melt-spinning fibers comprising a spinneret assembly comprising means for filtering; a spinneret; an elongated transfer line, said transfer line being disposed between said filtration means and said spinneret; means for heating said elongated transfer line; means for heating said spinneret; and an elongated annealer disposed beneath said spinneret assembly. Any means for filtering melt-spun fiber conventionally used in the art for melt-spinning can be used in the present apparatus. The spinneret is constructed to allow separate heating of the face of the spinneret, e.g. the portion of the spinneret which includes the walls of the capillaries, which face may comprise a separate plate or be integral part of the body of the spinneret, from other areas of the melt-spinning apparatus. The length to diameter ratio of the capillaries within the spinneret are preferably about 1 : 1 to about 8:1. The capillary holes of the spinneret are preferably arranged to achieve uniform heating among all of the holes. Preferably, the capillary holes are arranged in two concentric circles or in one circle. Preferably the spinneret is separately removable from the transfer line to allow easy cleaning or replacement. Likewise, the transfer line is preferably removable from the filter pack and the spinneret. Means for heating the transfer line and means for heating the spinneret can include a band heater, a coil heater, or other conduction, convection or induction heaters known to those of skill in the art. The elongated annealer, described in more detail above and in the examples, preferably comprises an inner tube and an outer tube separated by an annular space. Preferably the inside diameter of the inner tubes ranges from about 3-inches to about 8-inches. The elongated annealer can further comprise a mesh tube disposed adjacent the inner wall of the inner tube extending at least partially down the length of the inner tube. The elongated annealer can further comprise at least one perforated plate disposed within the annular space, extending radially with respect to the circumference of said outer tube, and attached to the outer wall of said inner tube, the inner wall of said outer tube, or to both tubes. Screens may be positioned on or in close proximity to these perforated plates. Air can enter the annular space of the annealer through an opening or port. The annealer can further comprise means for measuring or controlling the air flow rate, such as via a needle valve or a flow meter. The present apparatus can further comprise means for accumulating the spun filaments. Any means conventionally known in the art can be used, including but not limited to, a take-up roll, a draw-roll, and a wind-up roll. One embodiment of an apparatus of the present invention for melt- spinning is shown, as melt spinning apparatus 100 in Fig. 9. Shown are feed hopper 102 into which the polymer composition is fed, preferably in the form of pellets. These pellets are heated and conveyed through screw extruder 103. After the polymer or blend composition is melted, it is conveyed under pressure to pump block 104, through filter pack 105, transfer line 106 to spinneret 107 having face 108. Glass sleeve 109 permits viewing of the molten filaments. Molten fluoropolymer composition is extruded through one or more apertures of face plate 108 in spinneret 107 to form a continuous strand which is directed through elongated annealer 110 wherein the strand is shielded to prevent rapid cooling. Upon leaving the annealer, the spun fiber travels through pigtail guides 111, change of direction guides 116 to kiss roll 112 for an optional finish application, to a pair of take-up rolls 113, a pair of draw rolls 114, and a windup 115. Additional draws may be added as well as relaxation rolls. Fibers made by the process and apparatus of the present invention can be useful in textiles. Such textiles can be used in high performance sporting apparel, such as socks. Such fibers can be combined with other fibers in fabrics. Fibers of PTFE can be used for industrial quality yam for wet filtration. PTFE fiber can also be chopped for dry lubricant bearings. EXAMPLES In the examples the following polymers (all available from E. I. du Pont de Nemours and Company, Wilmington, DE) were used: Teflon® PFA 340, a copolymer of TFE and perfluoropropyl vinyl ether Teflon® FEP 5100, a copolymer of TFE, hexafluoropropylene, and perfluoroethyl vinyl ether Zonyl® MP-1300 PTFE Teflon® TE-6462 PTFE Teflon® PTFE TE-6472, a granular molding powder Teflon® PTFE 62, a lubricated paste extrasion resin Zonyl® MP-1600N, PTFE Unless otherwise indicated, the polymer used was Teflon® PFA 340. EXAMPLE 1 The effects of spinneret temperature, shear rate and spin stretch factor (SSF) on spinning speed and fiber properties were tested. Spinning was conducted using a 1.0-inch diameter steel single screw extrader, to which was connected a spin pump block, which was in turn connected to a spinneret pack adapter with the following features: a by-pass plate was used in place of a spin pump. An elongated spinneret was used, such as is depicted in Fig. 2, wherein "h"' was 2.0 in. A 30-mil 39-hole spinneret, wherein all of the holes were in only one circle, was used to cover the shear rate from low to medium shear rates, e.g. about 60/sec to about 180/sec, while a 15-mil 25-hole spinneret was used to cover the medium to high shear rates, e.g. about 350/sec to about 1,150/sec. A 1-inch high, 1.25-inch inside diameter coil heater (Industrial Heater Corp.) was wrapped around the lower 1-inch part of the elongated spinneret and was used to separately heat a portion of the spinneret that included the face plate. Conventional take-up rolls were used along with a Leesona wind- up. The temperature profile prior to the spinneret was 350°C in the screw extrader, 380°C in the pump block to the pack filter located between the extrader and the spinneret. Three spinning operations were performed using Teflon® PFA 340. The spinneret temperature was set at 420°C, 460°C, or 500°C. At 420°C melt fracture (M.F.) occurred at about 180/sec shear rate. The highest possible spinning speed with all filaments intact without melt fracture was slightly less than 219 mpm at a shear rate of about 90/sec. The fiber tenacity at this speed and shear was 1.02 gpd. The highest spinning speed at last filament break was 490 mpm at a shear rate of about 60/sec, and the fiber tenacity was 1.68 gpd with a filament denier of 4.0. At 460°C the spinnable shear rate increased to slightly less than 720/sec before the onset of melt fracture. The highest measured spinning speed at first filament break was 435 mpm at a shear rate of 160/sec, and the fiber possessed a tenacity of 1.13 gpd. The highest spinning speed at last filament break was 850 mpm also at a shear rate of about 160/sec. The highest fiber tenacity for fiber spun to last filament break was 1.61 gpd spun at 580 mpm with a filament denier of2.0. A graph of shear rate vs. spin stretch factor for the 500°C spinneret sample is shown in Fig. 11. The darkened triangle represents data at first filament break and the open triangle is data at last filament break. At 500°C, the spinnable shear rate was pushed to slightly less than 1,150/sec before the onset of melt fracture. The highest spinning speed at first filament break was 933 mpm at a shear rate of about 180/sec, and the fiber possessed a tenacity of 1.04 gpd. The highest spinning speed at last filament break was 930 mpm also about 180/sec, and the tenacity at this speed was of 1.15 gpd. Thus, it is seen that as the temperature of spinneret increased from 420°C to 500°C, the attainable spinning speed increased by a factor of 4.3X. Temperature also exerted a positive effect on the SSF at first filament break at constant shear rate, as shown in Fig. 12. The darkened circles show SSF at 420°C; the darkened squares show SSF at 460°C; and the darkened triangles show SSF at 500°C. A higher SSF meant that at the same throughput rate and given spinneret hole size, the take-up roll speed was higher in spinning speed. Unless otherwise stated in the remaining examples, spinning was conducted using the equipment described above except that a 1.5 -inch diameter corrosion resistant single screw extruder, made by Killion Extraders, Inc., Cedar Grove, N.J, was used. This extrader had three separate heating zones designated "Screw Zone 1, 2 and 3" in the temperature profiles below. A clamp ring was used to attach the extrader to a screw adapter holding them together, and the screw adapter was, in turn, attached to a spinneret adapter. The clamp ring was heated using a cylindrical rod cartridge heater, and the screw adapter and spinneret adapters were heated using cartridge heaters. A band heater was used to heat the filter pack. Unless otherwise indicated, a band or coil heater was used for heating any transfer line present, and the spinneret face. Conventional take-up and wind- up equipment was used, including a Leesona wind-up. EXAMPLE 2 Spinning was conducted at a throughput rate of 1.3 grams per minute per hole using a 30-mil 30-hole elongated spinneret at a jet velocity of 1.9 mpm. The equipment spinning temperature (°C) profile was: Screw Zone Clamp Screw Spinneret Pack 1 2 3 Ring Adapter Adapter Filter Spinneret 350 350 350 380 353 480 480 500 The shear rate was 328/sec, and the maximum spinning speed achieved was 1,100 mpm for a spin-stretch factor at first filament break (FFB) of 580. The denier, tenacity, elongation, and modulus of the resultant fibers were, respectively: 11 d 0.76 gpd/61%/5.6 gpd. EXAMPLE 3 This spin was done similar to Example 2 except that a 5 -foot tall tapered aluminum annealer was added to the equipment downstream of the spinneret to shield the molten filaments after their exit from the spinneret. The annealer had a square cross section, 12-inch square at the top and tapering down to a 1.0-inch square at the bottom. The same temperature profile was used as in Example 2 except for the following changes: 380°C screw adapter, 470°C spinneret adapter, 470°C pack filter. The shear rate was 328/sec. At the same throughput rate of 1.3 grams per minute per hole and using the same 30-mil, 30-hole elongated spinneret as was used in Example 2, the maximum spinning speed increased by 35%, or 385 mpm to 1,485 mpm, for a SSF at FFB of 782. The denier, tenacity, elongation and modulus of the resultant fibers were, respectively: 9.4 d/0.72 gpd/76%/5.1 gpd. EXAMPLE 4 This spin was done similar to Examples 2 and 3 except that a different annealer was used. For this spin, a 6-ft 3-in high self-standing Lucite® annealer was used which had a 12-in x 12-in square cross section. The same temperature profile was used as in Example 3. The shear rate was 328/sec. The maximum spinning speed was increased to 1,756 mpm for a SSF at FFB of 924. This was a 60% increase in spinning speed compared to Example 2, or an 18% increase in spinning speed compared to Example 3. The denier, tenacity, elongation and modulus of the resultant fibers were respectively: 6.0 d/1.16 gpd/28%/10 gpd. EXAMPLE 5 A spinneret assembly, such as shown in Fig. 3, having a shortened elongated spinneret was used in this example. The distance between the bottom face of the filter pack and the face plate of the spinneret was 1.25-inch. The same temperature profile and the same 6-ft 3-in Lucite® annealer was used as in Example 4. The shear rate was 328/sec. The maximum spinning speed achieved was 1,860 mpm for a SSF at FFB of 979. This high speed sample was not tested for fiber properties, but another sample spun under the same conditions at a shear rate of 342/sec with a spinning speed of 1,701 mpm had fiber properties (denier, tenacity, elongation and modulus, respectively) of: 7.6 d/1.01 gpd/68%/6.2 gpd. EXAMPLE 6 Spinning was conducted as in Example 5, except that the shortened elongated spinneret was heated using an induction heating coil, and the following changes in the temperature profile were used: 440°C pack filter, 522-531°C spinneret. The shear rate was 342/sec. The maximum spinning speed at FFB was 1,860 mpm. The denier, tenacity, elongation and modulus of the resultant fibers were, respectively: 9.6 d/1.06 gpd/49%/8.7 gpd. EXAMPLE 7 Spinning was conducted as in Example 6, except that the annealer used was the same tapered aluminum annealer used in Example 3. A 12-in cube clear Lucite® box was added on top on the annealer for the purpose of viewing the threadlines. The shear rate was 342/sec. The maximum spinning speed at FFB was 1,860 mpm. The denier, tenacity, elongation and modulus of the resultant fibers were, respectively: 9.0 d/1.02 gpd/54%/7.7 gpd. EXAMPLE 8 Spinning was conducted using a spinneret, such as is shown in Fig. 4, having a cartridge heater (available from Industrial Heater Corp. Stratford, CT) in the center of the spinneret and a standard band heater on the outside of the spinneret. The length of the spinneret from the bottom face of the filter pack to the face plate of the spinneret was 1.25-inch. The temperature profile used was: Screw Zone Clamp Screw Spinneret Pack Spinneret 1 2 3 Ring Adapter Adapter Filter Center Spinneret 350 350 350 380 380 411 410 496 500 The spinneret used had 26 holes; however, the throughput per hole was kept constant as in Examples 2 to 7. Thus, the shear rate was about the same, i.e. 342/sec. The maximum spinning speed was 1,976 mpm for a SSF of 1,040. The 6% increase in speed compared to Example 5 was attributed to the more uniform heating of the melt across the spinneret. The fiber properties of denier, tenacity, elongation and modulus were, respectively: 5.6 d/1.09 gpd/55%/7.0 gpd. Another sample spun with a 400°C temperature in the spinneret adapter and pack filter and the same 500°C in the spinneret gave a maximum speed of 1 ,920 mpm for a SSF of 1 ,010. Fiber tenacity was higher with the fiber properties of denier, tenacity, elongation and modulus measured as follows: 5.6 d 1.25 gpd/54%/8.7 gpd. EXAMPLE 9 A spinneret assembly, such as is shown in Fig. 6, was used to test the effectiveness of this embodiment in achieving high spinning speed. A 15-hole 1.0 in diameter disc spinneret with 30-mil diameter holes was used. The annealer used was the 6-ft 3-in Lucite® annealer used in Example 4. A band heater was used for the pack filter. The transfer line measured from the bottom face of the filter pack to the spinneret disc was 3.125-inch. At a screw rpm of 4.0, the total throughput rate was 20.3 grams per minute (2.7 lbs/hr) or 1.35 gpm per hole. This is substantially the same throughput rate per hole for the previous examples. A spinning speed of 1 ,816 mpm was achieved with all filaments intact under the following conditions: the screw extrader temperature was set at 350°C in all three zones; the clamp ring and the screw adapter were set at 380°C for a measured melt temperature of 389°C; the spinneret adapter and pack filter were set at 430°C; the transfer line was set at 470°C; and the spinneret was set at 500°C. Decreasing the temperature of the spinneret adapter and pack filter and increasing the transfer line temperature further improved the spinning speed: Spinneret Adapter Transfer Maximum Properties and Pack Filter Line Spinneret Speed Den/Ten/E/Mod 430°C 474°C 500°C 1,816 mpm 6.5/1.20/45%/10 420°C 471°C 500°C 1,969 mpm 5.5/1.24/24%/12 410°C 471 °C 500°C 1,965 mpm 5.6/1.38/35%/13 400°C 470°C 500°C 1,950 mpm 5.8/1.27/32%/12 400°C 480°C 500°C 1,994 mpm 5.3/1.48/48%/12 A spinning speed of 1,994 mpm was achieved which was a 14% improvement from the spinning speed of 1,756 mpm in Example 4. The shear rate was 347/sec. Fiber tenacity improved by 28% from 1.16 gpd to 1.48 gpd. This improvement in strength was attributed, besides the higher speed, to a lesser or no polymer degradation. Several samples of yam were collected at 1,000 mpm to test the long term stability of the spinning process. Filament spinning continuity was excellent allowing for a wind up of 60 minutes and 105 minutes with both voluntarily doffed. The fiber properties of denier/tenacity/elongation and modulus were: 1 ld/0.94-1.01gpd/68-80%/7.5gpd, respectively. A sample, spun at 1,500 mpm and lasting 4 minutes, had filament properties of denier/tenacity/elongation/modulus of 7.2d/1.20gpd/39%/l lgpd, respectively. Another sample, spun at 1 ,000 mpm and drawn in-line at 1.4x at 280°C, had the fiber properties of denier/tenacity/elongation/modulus of 7.6d/l .41gρd/25%/14gpd, respectively. Measurements made on air samples collected at the annealer exit, along the yam path above the heated take-up rolls, and above the wind-up did not detect any evolved gases. Thermal polymer degradation would have produced gases. Since evolved gases could also have been trapped or dissolved inside the fibers, the fibers were collected in vials and their head spaces, checked at various time intervals using infra-red spectroscopy, gas chromatograph/mass spectrometry, and ion chromatography, also did not contain any evolved gases. Additionally, the fiber samples were heated to 200°C to release any dissolved gases, but none were detected. These results confirmed that in the present process, despite using temperatures as high as 500°C to facilitate high shear rate, high spinning speed and high SSF, there was no polymer degradation. PFA polymer would have degraded easily if subjected to a temperature as low as 425°C for more than 1.0 minute. EXAMPLE 10 This spin was similar to Example 9 except that an induction heater coil of about 1/8-in was wrapped twice around the face of the spinneret. The temperature profile in the screw extrader up to the screw adapter were kept the same as in Example 9. The shear rate was 347/sec. There was a 3.6% improvement in maximum spinning speed (from 1 ,994 mpm in Example 9) to 2,065 mpm for a SSF at FFB of 1,087. Maximum speed and properties obtained are shown below: Spinneret Adapter Transfer Maximum Properties and Pack Filter Line Spinneret Speed Den/Ten/E/Mod 430°C 470°C 520°C 1,910 mpm 6.9/1.04/59%/6.5 400°C 480°C 525°C 2,065 mpm 5.6/1.21/32%/l l Spinning continuity proved excellent when a sample was spun for 90 minutes at 997 mpm and voluntarily doffed. Fiber properties of denier/tenacity/elongation/ modulus were: 10.3d/0.97gpd/68%/3.6gpd, respectively. EXAMPLE 11 A spinneret assembly, as shown in Fig. 8, was used. The spinneret face had a diameter of 1.75" and 60 holes of 30-mil diameter. Throughput rate per hole was 1.35 gpm for a total throughput of 81 gpm or 10.7 pounds per hour (pph). The tapered aluminum annealer with the 12-in cube Lucite® box on top of Example 7 was used. The temperature (°C) profile used was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 350 350 350 380 380 400 400 477 500 The maximum spinning speed was 1,359 mpm. The shear rate was 347/sec. The fiber properties of denier/tenacity/elongation/modulus were 8.0 d/1.04 gρd 67%/7.1 gpd, respectively. The cause of the decrease in spinning speed, compared to the spinneret with 30 holes, such as in Example 7, was thought to be due to too much heat retention in the annealer due to the 2x higher total throughput. The annealer was replaced with the larger capacity 6-ft 3-in Lucite® box annealer, and the maximum spinning speed increased to 1,500 mpm. The temperature (°C) profile used was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 350 350 350 380 380 420 420 500 520 The fiber properties of denier/tenacity/elongation/modulus were: 7.2 d/1.20 gpd/48%/9.4 gpd. In order to reduce excessive heat retention within the annealer, the annealer door, which ran lengthwise and nearly encompassed one side of the annealer, was opened full and covered with a perforated screen to provide quiescent air movement without turbulence. Using a perforated metal sheet with 3/32-inch diameter holes separated by 3/16-inch center-to center improved the maximum spinning speed by 8% to 1 ,623 mpm, compared to using the annealer with the door closed, using the slightly different temperature (°C) profile: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 350 350 350 380 380 400 400 500 520 The fiber properties of denier/tenacity/elongation/modulus were 7.5 d/1.18 gpd/50%/8.9 gpd, respectively. Some non-uniform air movement was observed in the perforated metal sheet covered front annealer, described above, because there was diffused air movement going in and out at the front while none at the other three sides. A thermocouple placed near the spinneret face showed the temperature fluctuating from 368°C to 390°C or a change of 22°C. A larger Lucite® annealer was used which measured 20-in x 24-in cross- section and 71.5 -inch in height with an opening at the top for the spinneret and at the bottom for access to threadline. During spinning, there was too much up and down air motion and the spinning speed was reduced. Inserts were placed at the bottom of the annealer to reduce the 20-in x 24- in opening to a 20-in square. These inserts were tapered down so that the yam would fall out. The measured temperature fluctuation was still high at 25°C, but the actual temperatures were significantly cooler, 240°C to 265°C (Note: while the measured temperature was lower than in the smaller annealer, comparison between the absolute temperature between the two annealers should not be taken too exactly as the location of the thermocouple may not be exactly situated.) The air stability was visibly more quiescent. With the same temperature profile, the maximum spinning speed was improved and was slightly higher than that recorded for the smaller annealer: 1,680 mpm. The fiber properties of denier/tenacity/elongation/modulus were 8.2 d/0.84 gpd/59%/5.9 gpd, respectively. EXAMPLE 12 With the preceeding designs for an annealer there was some difficulty in reaching the yam at the bottom of the annealer in order to bring it to a sucker gun for stringing up the yam through all the yam processing path to the wind-up. In addition, annealing of the molten threadline depended entirely on natural air convection with no means of control. These two problems were solved with an annealer design, such as is shown in Figs. 10A and 10B. This annealer easily permitted picking up of the yam at its bottom conical exit, incoming air from a compressed air source flowed through the annular space between the inner and outer tubes and up through several fine mesh screens to eliminate eddy's current and into the top and radially toward the molten filaments. Air was allowed to enter through a lower port in the annealer, and the air flow rate was controlled with a needle valve and measured by a flow meter. Temperatures within the inner tube along the top six inches could be monitored by thermocouples placed an inch apart. The height of the air inlet port between the inside and outside tube was adjustable between 1.0 in to 4.0 in. A 1.0 in high glass ring permitted visual observation of the molten threadlines and the spinneret face. Spinning was conducted using a spinneret assembly configured as in Fig. 8 and a 30-hole 39.4-mil diameter with a length/diameter of 3.0 spinneret. Spinning occurred at a throughput of 1.3 gpm with the following temperature profile: 350°C from the screw extrader to the pack filter, 450°C in the transfer line and 500°C in the spinneret. The temperatures inside the annealer were: 268°C at 1.0-in from the spinneret face, 252°C at 2-in from the spinneret face, and 222°C at 6-in from the spinneret face. The temperature fluctuation was negligible with a change of only 2°C versus up to 25°C observed in the annealers of the previous examples herein. The shear rate was 151/sec. Maximum spinning speed achieved was 1,737 mpm. The fiber properties of denier/tenacity/elongation/modulus were: 4.2 d/1.17 gpd/57%/7.8 gpd, respectively. The robustness of this spinning system was confirmed when excellent spinning continuity was demonstrated with a 3.5-hour package of yam at 1,005 mpm with a 1.4x in-line draw from a 702 mpm at 240°C take-up roll speed. The yam package had a net weight of over 20 pounds and a 2.0-in thick cake on a 6.0- in diameter bobbin. The temperature (°C) profile was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 350 350 350 350 350 350 350 448 500 The fiber properties of denier/tenacity/elongation/modulus were 12.6 d/0.80 gpd/92%/3.8 gpd, respectively. EXAMPLE 13 Spinning was conducted as in Example 12 but instead of PFA 340, Teflon® FEP 5100 fluoropolymer was used. The temperature (°C) profile was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 315 319 325 325 325 325 325 401 480 The temperatures used were lower in this example than for the PFA polymer because FEP is less stable than PFA. The shear rate was 161/sec. The maximum spinning speed achieved was 1,290 mpm. The fiber properties of denier/tenacity/elongation/modulus were 7.3 d/1.04 gpd/36%/10 gpd, respectively. EXAMPLE 14 This spin was made to test the process robustness developed in Example 13 for the Teflon® FEP 5100 polymer. Excellent spinning continuity, using the same equipment design as in Examples 12 and 13, was demonstrated with a 3.5- hour bobbin obtained at the same take-up speed of 700 mpm as in Example 12 for the PFA polymer. The yam was drawn off-line at the same draw ratio of 1.4x but at a lower temperature of 200°C because the melting point of FEP (260°C ) is lower than the melting point of PFA (305°C). The yam package was similar to that of the PFA 340 polymer spin in Example 12. The temperature (°C) profile used was lower than the one used in Example 13, namely: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 305 310 315 315 315 315 315 393 480 The shear rate was 163/sec. The drawn fiber properties of denier/tenacity/elongation/modulus were 12.2 d/0.97 gpd/45%/5.8 gpd, respectively. EXAMPLE 15 A spin of PTFE homopolymer was made using pelletized Zonyl® MP- 1300 PTFE. The pelletized form of the homopolymer was compacted from fine PTFE powder using a pelletizer comprising a male mold with 1,013 of 0.257-inch diameter imbedded rods and a female mold, 2.0-inch thick. The powder which had a density of about 0.36 g/ml was compacted under over 30 tons of pressure in a press to produce pellets having a 0.28-inch diameter, 0.50 inch length and a density of 1.58 g/ml. The same equipment and 30-hole spinneret as in Example 14 was used. The temperature (°C) profile used was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 400 400 400 400 400 410 410 450 520 The molten filaments exiting from the spinneret face appeared translucent and glittering, an indication of some degradation. The filaments, however, did not come out of the annealer in continuous form but rather in bits and pieces. Varying the throughput rate from 0.17 g/min/hole to 1.33 g/min/hole did not result in continuous filaments. After the MP-1300 pellets ran out in the feed hopper, about 200 grams of PTFE homopolymer TE-6462 in powder form was fed into the hopper and extrudedjesulting in long, continuous filaments. The free-fall continuous filaments were ductile and could be handled or gently pulled between fingers without breaking. The measured denier of a filament was 349. EXAMPLE 16 In order to spin Teflon® PTFE TE-6472, the extrader and spinning apparatus used in Example 15 was brought to the following high temperature (°C) profile, and PFA 340 was used first to avoid degradation of the PTFE homopolymer to follow due to stagnation during the heating-up process which lasted 2.5 hrs: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 470 470 470 470 470 470 470 450 510 Compressed powder pellets of Teflon® PTFE TE-6472, classified as a granular molding powder, were added after the PFA pellets feed were gone and the screw was turning at 14.0 rpm. Six minutes after the Teflon® PTFE TE-6472 pellets were added, the pack pressure was found rapidly rising from 204 psi to over 1,000 psi indicating that the Teflon® PTFE TE-6472 had reached the pack. Screw speed was constantly adjusted and spinneret temperature raised to 550°C to maintain pack pressure at 1,000 psi. Continuous transparent molten filaments were extruding but contained gas bubbles, an indication of thermal degradation, and solidifying into white filaments. At 2.0 φm, the measured throughput was 7.6 gpm versus an expected 10.5 gpm. Even though the screw φm was maintained at 2.0 φm, the throughput was found to continuously decrease to as low as 0.4 gpm, and the continuous filaments began to break up into drips connected between long (as long as 48-in) and very fine filaments. These very fine filaments were visually similar to a_light spider web, so light that they floated in the air. Measured filaments denier was between less than 0.6 and 18. This clearly demonstrated that PTFE could be melt spun even to very fine filament denier. The cause of the reduction in throughput was ring pluggage at the entrance to the barrel of the extrader, which effectively prevented the feeding of the fluoropolymer pellets. In order to clear the pluggage, all of the polymer was vacuummed out until the screw was visible. Then PFA pellets were added and pushed down using a specially made rectangular plate, attached to a 0.5 -inch rod, which had the dimension of the barrel opening. Turning the screw caused the small PFA pellets to scrape off the stuck PTFE compressed powder from the screw surface. After the ring pluggage was cleared and feeding resumed, the PTFE compressed powder pellets were added again. At a screw speed of 5.0 φm, with a measured throughput of 9.3 gpm, continuous filaments from all 30 holes were spun and taken up on take-up rolls at 30 mpm. Excellent spinning continuity lasted about 15 minutes before ring pluggage occurred again as evidenced by a drop in pack pressure. This experiment clearly demonstrated that homopolymer PTFE can be melt-spun. The temperature (°C) profile was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 420 440 480 485 485 485 485 495 500 The PTFE fiber samples were ductile permitting handling without brittle failure and permitted tensile testing. Sample Filament Strength Tenacity Identification Denier (grams) (gpd) Free fall 686 36.0 0.05 Free fall 1,042 71.8 0.07 30 mpm 332 14.0 0.04 EXAMPLE 17 Spinning was conducted on Teflon® PTFE 62, classified as a lubricated paste extrasion resin. The powder was similarly compressed under 50 tons of pressure into cylindrical pellets 0.28-inch in diameter and 0.52-inch in length and with a density of about 1.6 g/cc. The same equipment and start-up procedure was used as in Example 16. The Teflon® PTFE 62 pellets were added at 3.8 φm screw speed. Good feeding was obtained at beginning and measured throughput was 9.9 gpm versus 20 gpm expected. Screw speed was increased to 7.7 φm. Pack pressure was found to rise continuously and was held at 1,200 psi by reducing the screw speed indicating good feeding. Ring pluggage occured and pack pressure dropped. Revving up the screw to 30 φm loosened the pluggage and the pack pressure rose. At 10 φm, the pack pressure climbed to as high as 2, 150 psi when continuous filaments were spun at 55 mpm. Spinning continuity lasted about 5 minutes before ring pluggage occurred. EXAMPLE 18 The fibers spun in Examples 16 and 17 were hot drawn in a heated salt bath. Filaments were cut to about one inch in length and were held between pointed tweezers and drawn while briefly immersed in a salt bath. Draw temperature ranged from 330°C to 400°C. The fiber could not be drawn at 320°C. The melting point of PTFE homopolymer ranged from 325°C to 342°C, thus the fibers were drawn in the molten state. The filaments were easily drawn between 5.0x to 8. Ox draw ratio. The filaments changed from a bright with no preferred orientation, under cross-polars, to a intense blue color in one direction and pinkish red in a direction 90° to it, indicating preferred molecular orientation along fiber axis. A 340°C draw temperature gave the highest degree of orientation. A drawn filament with a measured denier of 7.7 gave 0.2 gpd in tenacity. EXAMPLE 19 The spinneret assembly described in Example 9 and shown in Fig. 6 was used to spin Teflon® PFA 340 and to compare the spinning conditions found with a conventional spinneret assembly design (see Fig. 1), where the spinneret cannot be heated separately, with spinning conditions in which the spinneret is thermally isolated from the pack filter. Thermal isolation was obtained in part in this embodiment by adding a transfer line between the bottom face of the pack filter and the spinneret face. Two control runs were made using the same spinneret system but keeping the spinneret at the same constant temperature. A 10-hole 30-mil spinneret was used. The first control spin was made by keeping the temperature (°C) profile at 350°C as shown below: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 350 350 350 350 350 350 350 350 350 The throughput was increased until a slight melt fracture was observed at 0.178 gpm per hole. The shear rate at this maximum throughput was 45.7/sec, and the maximum spinning speed achieved was 58 mpm having a jet velocity of 0.26 mpm and a SSF of 223. The second control spin was made at a higher temperature profile of 400°C as shown below: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 350 350 350 350 350 350 400 400 400 The higher temperature of 400°C permitted higher throughput of 0.370 gpm per hole before melt fracture. At a lower throughput, before melt fracture, of 0.238 gpm per hole, a maximum spinning speed of 206 mpm was obtained. At the highest throughput and at the edge of melt fracture, the achieved maximum spinning speed was 381 mpm at a shear rate of 95/sec, jet velocity of 0.54 mpm and a SSF of 704. The following temperature (°C) profile was used: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 325 330 335 335 335 335 335 450 500 With this temperature profile, the throughput could be pushed to as high as 1.125 gpm per hole, 3 times higher than the uniform 400°C control, and still without melt fracture. Achieved maximum spinning speed was 1,956 mpm, 5 times higher than the uniform 400°C control, at a shear rate of 289/sec, jet velocity of 1.645 mpm and a SSF of 1,189. A control ran was not simulated at 500°C because in a conventional spinneret system, the pack filter has to be heated to the same 500°C temperature. With the pack filter at 500°C, the polymer would seriously degrade due to the long residence time, 10.1 minutes, in the pack filter. At 425°C, the polymer would begin degrading in less than 1.3 minutes. EXAMPLE 20 The following experiment was conducted to determine the distance from the spirmeret face when the molten filaments would solidify. Solidification was determined to have occurred when it was visually observed that the transparent molten filaments turned opaque. This observation was more clearly observed with a high intensity lamp shining directly at the bundle of filaments. The transition from transparent to opaque was observable from free-fall (by gravity) to speeds up to 200 mpm. Extrasion of the molten filaments were conducted with and without an annealing tube. In the case where an annealing tube was used, a special clear glass annealing tube was used in order to permit visual observation and which measured 3.0-inch in diameter and 41 -inch long. The spinneret used had 30 holes of 30-mil diameter. Teflon® FEP-5100 polymer was used. The results plotted in Fig. 13 show the data without an annealer in opened symbols while those using an annealer in filled symbols. The plot shows the free- fall distance as an increasing function of total throughput at three constant spinneret temperatures: 380°C (triangle symbol), 430°C (square symbol) and 480°C (circle symbol). It shows that the solidification distance increases with total throughput at constant spinneret temperature. It also shows that the solidification distance increases with increasing spinneret temperature at the same throughput. Furthermore, it shows that with an annealing tube, the solidification distance is about twice as far as that without an annealing tube. The effects of stringing up the filaments were shown in another experiment to increase the solidification distance from about 6 inches to about 15 inches without an annealing tube at a take-up speed of 200 mpm. Therefore, the solidification distance shown in the Fig. 13 represents the shortest solidification distance. The following temperature (°C) profile was used: Screw Zones Clamp Screw Spinneret Pack Tranfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 275 285 295 315 315 315 315 380 380, 430, 480 EXAMPLE 21 PTFE homopolymer grade, Zonyl® MP-1600N, was melt-processed and spun into fibers, using a spinneret assembly as depicted in Fig. 8. The polymer powder was compressed in a 0.5-in high female mold with 0.25-in diameter holes, which were filled with the polymer powder, using less than 0.25-in diameter rods into thin discs of about 0.1 -in thick. About two pounds of these thin disc pellets were made. The pellets were hand fed into the screw extrader just enough to fill the threads section of the screw as a precaution against being crashed and causing sticking and ring pluggage in the screw. The following temperature profile was used. Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 380 385 390 390 390 390 390 450 500°C At a screw speed of 1.94 φm, the throughput was at 9.4 grams per minute with a pack pressure of 238-246 psi using a 10 holes 30-mil diameter spinneret. The shear rate was 242/sec. The annealer used in Example 12 and shown in Figs. 10A and 10B, was used. No ring pluggage problems were experienced. The spin was cut short after running out of pellets. The 10 filaments was initially picked up by hand and went over to the take-up roll and after one wrap, a sucker gun was used to string up the yam all the way to the Leesona windup. The initial spinning speed was 30 mpm and speed was gradually increased to a maximum of 202 mpm. Filament denier measurement on three filaments were: 33, 36 and 41. The measured as-spun filament fiber properties for the 41 denier filament (denier/tenacity/break elongation/modulus) were: 41 denier/0.05 gpd 1.3%/3.7 gpd. Teflon® PTFE 62 was spun using cut-up pieces and thin disc pellets to avoid the ring pluggage. The temperature (°C) profile used was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 440 445 450 450 450 450 450 450 500°C The cut-up pellets fed well with no pluggage. However, the pellet discs eventually developed a ring pluggage problem. Spinning at up to 60 mpm was achieved before the pluggage occurred at shear rate ranging from 183/sec to 614/sec. EXAMPLE 22 Pellets of Zonyl® MP-1600N PTFE homopolymer powder were similarly prepared as in Example 21, using the same spinneret assembly. At the following temperature profile, the effects of an annealer were studied by spinning without and with the annealer. Throughput rate was at 8.4 grams per minute through a 30- mil diameter, 30-hole spinneret for a shear rate of 72/sec. Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 315 330 340 340 340 340 340 400 400°C Without annealer. About 15% of these extruding filaments could not sustain their own weight at a vertical free fall distance of 5-ft 8-in. For those surviving filaments, they were able to be spun at a maximum speed of only 15 mpm before they broke. With a 48-in long annealer: All filaments were free falling continuously to the floor. The first filament-break (FFB) spinning speed was 50 mpm and the maximum spinning speed (MSS) attained was 480 mpm. By raising the temperature of the transfer line and spinneret to 450°C and 500°C, the FFB was improved to 85 mpm and the MSS was at 250 mpm. The yam was visibly thick and thin. The yam uniformity was found to improve with the introduction of room temperature air through the annealer jacket into the top of the annealer. At 250 cfh (cubic feet per hour), the yam became uniform. Under this condition of spinning, the MSS was improved to 404 mpm. Filament fiber properties (denier/tenacity/break elongation/ modulus) were 5.8/0.16gpd/12%/8 gpd. EXAMPLE 23 This experiment used Teflon® FEP-5100 as the fluoropolymer composition and demonstrated the advantage of thermally isolating the spinneret. A spinneret assembly as depicted Fig. 8 was used. The control was ran in the same assembly but keeping the temperature the same for all parts. The temperature(°C) profiles for the controls were: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 275 300 350 350 350 350 350 350 350 275 350 400 400 400 400 400 400 400 275 350 400 400 450 450 450 450 450 The temperature profile in the Screw Zones 1 and 2 was kept low and not at test temperature until Screw Zone 3 or Clamp Ring. The degradation would hve been worse had Screw Zones 1 and 2 been at test temperature. The temperature profile for the sample of the present invention was: Screw Zone Clamp Screw Spinneret Pack Transfer 1 2 3 Ring Adapter Adapter Filter Line Spinneret 275 295 300 300 300 300 300 380 480 The shear rates were: 86/sec at 10 gpm, 232/sec at 27.2 gpm, 359/sec at 42 gpm, and 385/sec at 45 gpm. As seen in Fig. 16, a spinning speed of 1,900 mpm, without any noticeable degradation, was achieved at a spinneret temperature of about 480°C. However, the control experienced slight thermal degradation at a spinneret temperature of 400°C attaining a spinning speed of about 600 mpm at that temperature and severe thermal degradation at about 450°C with a spinning speed of 900 mpm.

Claims: WHAT IS CLAIMED IS: 1. A process for melt spinning a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers, comprising the steps of: melting a composition comprising a highly fluorinated thermoplastic polymer or a blend of such polymers to form a molten fluoropolymer composition; conveying said molten fluoropolymer composition under pressure to an extrasion die of an apparatus for melt spinning; and extruding the molten fluoropolymer composition through the extrusion die to form filaments, said die being at a temperature of at least 450°C, at a shear rate of at least 100 sec '1 , at a spinning speed of at least 500 m/min. 2. The process of Claim 1 further comprising shielding the filaments. 3. The process of Claim 1 further comprising exposing the molten fluoropolymer composition to an intermediate temperature ranging between the melting temperature of said composition and a temperature less than the temperature of the extrasion die prior to extruding said composition through the extrasion die. 4. The process of Claim 1 wherein the highly fluorinated polymer has a melt flow rate of 1 to 50 g/10 minutes at 372°C. 5. The process of Claim 1 wherein the fluorinated polymer is a copolymer of tetrafluoroethylene and a perfluoroolefin. 6. The process of Claim 5 wherein the fluorinated polymer is a copolymer of tetrafluoroethylene and hexafluoropropylene. 7. The process of Claim 5 wherein the fluorinated polymer is a copolymer of tetrafluoroethylene and a perfluoroalkyl vinyl ether. 8. The process of Claim 7 wherein the perfluoroalkyl vinyl ether is perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, or and perfluoropropyl vinyl ether. 9. The process of Claim 1 wherein the temperature of the die is at least 500°C. 10. The process of Claim 1 wherein the extrasion die is thermally isolated from other areas of the apparatus that may contain the fluoropolymer composition. 11. The process of Claim 1 wherein the spinning speed is at least 1000 m/min. 12. The process of Claim 1 wherein the shear rate is at least 500 sec "1 . 13. The process of Claim 1 further comprising drawing the fibers. 14. The process of Claim 1 further comprising a relaxation stage. 15. A process for melt spinning a composition comprising polytetrafluoroethylene homopolymer, comprising the steps of: melting a composition comprising polytetrafluoroethylene homopolymer to form a molten polytetrafluoroethylene composition; conveying said molten polytetrafluoroethylene composition under pressure to an extrasion die of an apparatus for melt spinning; and extruding the molten polytetrafluoroethylene composition through the extrusion die to form molten filaments. 16. The process of Claim 15 wherein the temperature of the extrasion die is at least 450°C. 17. The process of Claim 15 wherein the spinning speed is at least 50 mpm. 18. The process of Claim 17 wherein the spinning speed is at least 200 mpm. 19. The process of Claim 18 wherein the spinning speed is at least 500 mpm. 20. The process of Claim 15 further comprising shielding the filaments. 21. An apparatus for melt-spinning fibers, comprising: a spinneret assembly comprising: means for filtering; a spinneret; an elongated transfer line, said transfer line being disposed between said filtration means and said spinneret; means for heating said elongated transfer line; means for heating said spinneret; and an elongated annealer disposed beneath said spinneret assembly. 22. The apparatus of Claim 21 wherein the elongated annealer comprises an inner tube disposed within an outer tube, said inner tube and said outer tube separated from each other by an annular space. 23. The apparatus of Claim 22 further comprising a mesh tube disposed adjacent the inner wall of said inner tube extending at least partially down the length of said inner tube. 24. The apparatus of Claim 22 further comprising at least one perforated plate disposed within said annular space, extending radially with respect to the circumference of said outer tube, and attached to the outer wall of said inner tube or the inner wall of said outer tube, or to both tubes. 25. The apparatus of Claim 24 further comprising a screen placed on or in close proximity to the at least one perforated plate. 26. The apparatus of Claim 21 wherein the elongated annealer further comprises means for measuring or controlling air flow rate. 27. The apparatus of Claim 21 wherein the spinneret is removable. 28. The apparatus of Claim 21 wherein the transfer line is removable. 29. The apparatus of Claim 21 wherein said means for heating the spinneret is a conduction heater, a convection heater or an induction heater. 30. The apparatus of Claim 21 wherein the spinneret has a plurality of extrusion holes all of which are arranged in one circle. 31. The apparatus of Claim 21 further comprising means for accumulating the spun filaments.

Inventors: UY WILLIAM CHENG, UY, WILLIAM, CHENG
Applicants: DU PONT, UY WILLIAM CHENG, E.I. DU PONT DE NEMOURS AND COMPANY, UY, WILLIAM, CHENG
Application year: 2000
Main classification: D01F6
IPC codes: D01D4, D01D5, D01F6