YARN OF ETHYLENE/TETRAFLUOROETHYLENE COPOLYMER

Abstract: The present invention relates to yarn of ethylene/tetrafluoroethylene copolymer in which the filaments of the yarn exhibit orientation which is greater in the core of the filament than in the sheath and which further exhibits a tenacity of at least 2 g/d and elongation of at least 15 %.

Description: TITLE YARN OF ETHYLENE/TETRAFLUOROETHYLENE COPOLYMER BACKGROUND OF THE INVENTION Field of the Invention. This invention relates to fluoropolymer yarn. Description of Related Art. Yarn of fluoropolymer is highly desirable for applications requiring chemical inertness, including resistance to weathering, high temperature stability, and slip (non-stick). Japanese Patent Publication 63-219616 (1988) discloses the melt spinning of tetrafluoroethylene copolymer, including ethylene/tetrafluoroethylene copolymer (ETFE), the copolymer having a melt viscosity of 10,000-20,000 poises (1000-2000 Pa » s) , into yarn in which the individual filaments have a cross-section characterized by sharp edges. The melt spinning is carried out at a melt temperature which is about 50°C above the melting point of the copolymer. In the case of ETFE, Example 1 discloses spinning being carried out at 315°C at a yarn windup speed of only 10 m/min. The resultant yarn is then drawn 6X at 150°C to obtain yarn of 150 denier, having a tenacity of 2.9 g/d, and elongation of 18%. The combination of the high tenacity and elongation of at least 15% is desirable so that the yarn is strong without being brittle. Unfortunately, the production rate of this yarn is very slow, amounting to 60 m/min, which greatly increases the cost of the yarn. In addition, the high 6X draw rate increases the likelihood of filament breakage during draw, which leads to production downtime. German OLS 41 31 746 Al (March 25, 1993) discloses a process for melt spinning ETFE at a much faster wind-up rate, disclosing a speed of at least 800 m/min, preferably from 1000 to 3000 m/min, under the following conditions: (a) spinning temperature at least 30°C above the melting point of the copolymer, preferably at 280-310°C, and (b) low melt viscosity of the copolymer, characterized by a melt flow rate of at least 50 g/10 min under a load of 2 kg, which is less than the standard 5 kg load used in the melt flow rate test. Draw of the yarn is also disclosed, as being optional, and if used, then at a draw ratio of 1 :1 to 1.5. The elongation of the yarn is disclosed to be 15 to 60%, preferably 20 to 40%. The tenacity of the yarn is disclosed to be above 8cN/tex, preferably 10 to 15 cN/tex. None of the Examples achieve both highest strength and elongation of at least 15%. The yarn of Example 6 achieves the highest strength, a tenacity of 15.2 cN/tex, which while high for this teaching, is low in an absolute sense, corresponding to a tenacity of only 1.72 g/d (calculation 15.2/100 X 11.33), and the elongation of this yarn is only 12.4%. The problem remains of how to produce ETFE yarn at a high production rate and which is both strong, i.e. having a tenacity of at least 2 g/denier, and has an elongation of at least 15%. BRIEF SUMMARY OF THE INVENTION The present invention solves this problem and in the course of doing so has created an oriented yarn of ethylene/tetrafluoroethylene copolymer (ETFE) which has a novel molecular structure across the thickness of each ETFE filament of the yarn, namely the orientation (axial) of the copolymer molecules making up the filament is greater in the interior of the filament than at the surface of the filament. Thus, in terms of filament cross-section, the core or center of the filament is oriented more than the surface of the filament. Axial orientation of the molecules within the filament occurs upon the drawing of the yarn, either at a high rate of melt draw from the spinneret or such melt draw followed by draw of the yarn after it has solidified, i.e. draw below the melting point of the copolymer (cold draw). Normally, such draw, whether melt draw or melt draw plus cold draw causes the highest orientation of the molecules making up the filament to occur at the surface of the filament, because that is where the shear stress on the copolymer is the greatest, by virtue of the filament cooling from the surface of the filament before the core cools. Such orientation decreases towards the center of the filament. Thus, while the molecules at the surface of the filament become aligned in the length direction of the filament, the molecules in the core of the filament show less to no alignment. Cold draw of the filament maintains the difference between surface and core orientations. This orientation phenomenon is further described in A. Ziabicki and H.Kawai, High- Speed Fiber Spinning, John Wiley & Sons, p. 57 (1985). The ETFE yarn of the present invention has reverse orientation, wherein the molecular orientation is greater in the core than at the surface of the filament(s) making up the yarn. This new orientation in the filaments in the ETFE yarn of the present invention is obtained by the process of the present invention, namely melt spinning the copolymer at a temperature which is at least 90°C greater than the melting point of the copolymer. The melting point of ETFE is generally from 250 to 270°C, depending on the proportion of tetrafluoroethylene and termonomer present, which means that the temperature of the copolymer at the spinneret is at least 340°C, i.e. substantially higher than used in Japanese Patent Publ'n 63- 219616 and German OLS 41 31 746 Al. An extruder/gear pump combination is used to melt the copolymer and to transfer the molten polymer to a spinneret assembly, which includes a transfer tube (adapter) communicating the molten copolymer from the extruder to the spinneret assembly. The 340°C minimum spinning temperature used in the process to make yarn of the present invention would degrade the copolymer if the spinneret assembly were heated to this temperature. Spinning at this minimum temperature is achieved by restricting this temperature essentially to the spinneret face plate, which is that part of the spinneret assembly that has the orifices which form the filament(s) making up the yarn, where the residence time of the copolymer at high yarn formation speed is so short, e.g. fractions of a second , that degradation does not occur. The absence of degradation is observable visually, by the yarn not exhibiting discoloration, especially graying or containing black specs, which would denote carbonization of the copolymer, and also by the yarn exhibiting a tenacity of at least 2 g/d. This high temperature exposure of the ETFE yarn creates the novel reverse orientation in the filament(s) of the yarn of the present invention. In the broadest sense, the process of the present invention comprises melt spinning the ETFE into yarn at a melt spinning temperature which is effective to produce said yarn wherein the filaments thereof have orientation which is greater in the core than at the surface of the filaments. The extremely high melt spinning temperature needed to achieve this reverse orientation without degrading the yarn is accomplished by carrying out the melt spinning at a speed which avoids such degradation. The yarns of the present invention, whether monofilament or multifilament, exhibit high tenacity , i.e. tenacity of at least 2 g/d. The yarns of the present invention can also exhibit high elongation, i.e. elongation of at least 15%, and the combination of this high tenacity and high elongation. The minimum elongation of 15%) enables the yarn to be further processed and used thereafter without brittle breakage. DETAILED DESCRIPTION OF THE INVENTION The ETFE used in the present invention is a copolymer of ethylene and tetrafluoroethylene, usually containing minor proportions of one or more additional monomers to improve the copolymer properties, such as stress crack resistance . U.S. Patent 3,624,250 discloses such polymers. The molar ratio of E (ethylene) to TFE (tetrafluoroethylene) is from about 40:60 to about 60:40, preferably about 45:55 to about 55:45. The copolymer also preferably contains about 0.1 to about 10 mole% of at least one copolymerizable vinyl monomer that provides a side chain containing at least 2 carbon atoms. Perfluoroalkylethylene is such a vinyl monomer, perfluorobutylethylene being a preferred monomer. The polymer has a melting point of from about 250°C to about 270°C, preferably about 255°C to about 270°C. Melting point is determined according to the procedure of ASTM 3159 on melted pellets of the copolymer obtained by melt extrusion of the copolymer and cutting the extrudate into pellets. Preferably, the ETFE used in the present invention has a melt flow rate of less than 40 g/10 min using a 5 kg load in accordance with ASTM D 3159, wherein the melt temperature of 297°C is specified. This ASTM test is the same as DIN Standard 53 735 specified in German OLS 41 31 746 Al, except that in the OLS, a 2 kg load on the molten copolymer is used, whereby it is apparent that maximum 45 g/10 min flow rate applicable to the present invention provides a much more viscous copolymer (more than 2X ) than the minimum 50 g/10 min flow rate required in the OLS. Preferably the ETFE used in the present invention exhibits a melt flow rate of no more than 35 g/10 min and more preferably no greater than 20 g/10 min. The melt flow rate of 30 g/10 min corresponds to a viscosity of 18000 poises (1800 Pa»s) in accordance with the calculation disclosed in U.S. Patent 4,380,618. The melt flow rates (MFR) disclosed herein for the ETFE used in the present invention are the MFR determined using the 5 kg weight to force the molten polymer through the capillary for the collection time for the polymer . Yarn of the present invention can be made using the spinneret assembly described for example in Fig. 2 of WO 00/44967 (published August 3, 2000). As shown in this Fig. 2, the spinneret plate (faceplate) having the orifices which form the filament(s) making up the yarn is heated independently from the heating applied to the spinneret assembly, whereby the temperature of the molten copolymer upstream from the faceplate is cool enough to avoid degradation for the time the copolymer resides in the assembly, while the temperature of the face plate is heated to a higher temperature, which is at least 90°C greater than the melting point of the copolymer. For simplicity, the temperature of the faceplate will be referred to herein in describing the process of the present invention as the melt spinning temperature. Preferably the melt spinning temperature is not more than 150°C above the melting point of the copolymer being melt spun, more preferably, not more than 130°C above the melting point. As disclosed on pages 309 and 306 of J. Scheirs, Modern Fluoropolymers, John Wiley & Sons (1997), ETFE decomposes above 340°C to oligomer and rapidly degrades at temperatures over 380°C. The melt spinning of the present invention is able to operate within this temperature range because of the short time of exposure of the ETFE to this temperature. Because of the rapidity of the decomposition at temperatures above 380°C, and the danger of explosion from pressure build-up with the spinneret, it is preferred that the melt spinning temperature be no greater than 380°C. The melt spinning temperature will generally be at least 20°C greater than the heating applied to the spinneret assembly upstream from the spinneret faceplate. Melt spinning of the yarn of the present invention can be carried out using the apparatus shown in Fig. 9 of WO00/44967, which includes an annealer for the solidified yarn, a wind-up roll and take-up (feed) and draw rolls positioned between the annealer and the wind-up roll. Preferably, the intermediate rolls accomplish heat setting of the yarn as well as any drawing desired. In addition, preferably, the yarn is shielded as it exits the spinneret faceplate, so as to achieve an attenuation of the yarn for a distance of at least 50 diameters from the faceplate before the yarn solidifies. Typically, the spinning speed as determined by the wind-up roll will be at least 500 m/min, and preferably at least 1000 m/min, and more preferably at least 1500 m/min. Speeds up to and above 3000 m min are achievable. The higher the spinning speed as determined by the intermediate take-up rolls, the greater is the orientation of the filament(s) of the yarn without downstream cold draw. Thus, the cold draw used in the present invention will generally be between about 1 :1.1 to 4. Such yarn will exhibit the reverse orientation in its filamentary makeup as described above. The greater orientation in the core of the filament(s) of the yarn of the present invention can be determined several ways. ETFE yarn which is spun at lower temperatures than the present invention, such as 300-320°C, is characterized by the yarn filaments exhibiting a fibrillar surface appearance when viewed under a scanning electron microscope at 10,000X magnification, with the fibrils running in the direction of the longitudinal axis of the filaments, indicative of a high degree of surface orientation. In contrast, under the same conditions of viewing of the yarn filaments of the present invention, the surface of such filaments does not exhibit a fibrillar appearance, indicating the absence of any high degree of orientation. Instead, the surface appearance of such fibers is of a fine texture, free of a striations. While the surface of the filaments does not indicate any high degree of orientation, the core of the filaments indicates high orientation as revealed by the birefringence of the filaments being substantially greater than the birefringence of unoriented ETFE, which has a birefringence of 0.040. Birefringence is a typical way of characterizing orientation. The higher the birefringence, the higher the orientation. The birefringence of the entire filament is the bulk birefringence of the filament and can be determined as disclosed in Col. 4 of U.S. Patent 2,931,068. Birefringence measurements can also be taken at increments along a radius of the filament, so that the birefringence at the surface of the filament can be compared to the birefringence at the core or center of the filament, i.e. differential birefringence, thereby indicating the orientation at the surface of the filament relative to the orientation at the core. Because the orientation or lack of orientation at the filament surface is a surface phenomenon, and birefringence measurement must be taken within the body of the filament, the birefringence measurement for the surface is taken as near as possible to the surface to ascertain the trend of birefringence in the direction from the center of the filament to the filament surface Thus, in addition to birefringence measurement taken at the center of the filament, birefringence measurements are also made along the radius of the filament towards the filament surface, with the region 0.8-0.95 radius (proportion of the distance from the center of the filament to the filament surface) being the region which indicates the birefringence trend towards the surface, or in other words the surface orientation relative to the orientation in the center of the filament. The localized birefringence measurement, as distinguished from the bulk birefringence measurement, is taken on 10 samples of filament, from the center to one side, and the reverse orientation for the yarn filaments of the present invention is indicated by the average of the 10 birefringence measurements at each increment along the filament radius indicating a trend towards lower birefringence, especially in the 0.8-0.95 radius region, as compared to the birefringence measurement for the filament center, thereby indicating that the orientation at the surface is less than in the filament center. Orientation wherein the orientation is greater at the surface than in the center of the filament is determined the same way, wherein the trend towards increasing orientation at the surface is indicated by the trend of increasing birefringence as the measurements along the radius approach the filament surface. These differential birefringences can be determined by the procedure disclosed in British patent 1,406,810 (pp. 5 and 6), except that use of the Leitz Mach-Zehnder interferometer is preferred.. The yarn of the present invention can be monofilament or multifilament, and the melt spinning holes in the spinneret faceplate forming the filaments will generally have a diameter of less than 2000 micrometers. When the yarn is a monofilament, it will generally have a diameter of 50 to 1000 micrometers. When the yarn is multifilament, the individual filaments will generally have a diameter of 8 to 30 micrometers, and the yarn will generally have a denier of 30 to 5000, preferably 100-1000 and contain 20 to 200 filaments. The melt spinning holes in the faceplate are preferably circular to produce filaments having an oval, preferably circular, cross-section, free of sharp edges. The yarn of the present invention is highly uniform, uniformity being characterized by a coefficient of variation of total yarn denier of no greater than 5%, usually less than 2%. Coefficient of variation is the standard deviation divided by the mean weight of 5 consecutive ten meter lengths of the yarn (X 100). This high uniformity of yarn of the present invention enables the yarn to be easily machine handled for the particular application of the yarn. Preferably, the yarn of the present invention has a tenacity of at least 2.4 g/d. The deniers disclosed herein are determined in accordance with the procedure disclosed in ASTM D 1577, and the tensile properties disclosed herein (tenacity, elongation, and modulus) are determined in accordance with the procedure disclosed in ASTM 2256. The yarn of the present invention can also be chopped up into fibers, which can be used for example to form staple fiber, including staple fiber yarn or felt. The multifilament yarn of the present invention will normally be twisted by conventional means for yarn integrity, e.g. 1 to 2 twists per cm, and a plurality of said yarns will be plied or braided together to form such articles as sewing thread, dental floss, and fishing line. To form sewing thread, generally 2-4 yarns of the present invention will be plied together and heat set to form sewing thread having a denier of 800 to 1500. To form dental floss, yarn of the present invention can be plied or braided together to form dental floss having a denier of 800 to 2500. Monofilaments and multifilament yarn of the present invention can be used as fishing line. Such monofilaments will typically have a diameter of 0.12 mm (120 micrometers) to 2.4 mm (2400 micrometers). Such multifilament yarn will generally be braided from 4 to 8 yarns of the present invention, each having a denier of 200 to 600. Colorant can be added to the copolymer prior to yarn formation, so that the yarn will have color, which is especially desirable for many sewing thread, fishing line and dental floss applications. The yarn of the present invention and the products made therefrom, e.g. sewing thread, dental floss, fishing line and fish netting, exhibit excellent chemical and weathering (including UV radiation) resistance, making them especially useful in these applications and other applications requiring exposure to weather and chemicals. The yarn is useful to make woven and knitted fabrics made entirely of such yarn or blended with yarn of other materials Examples of such fabrics include architectural fabrics, fabrics for reinforcement of printed circuit boards and electrical insulation, and for filtration applications. EXAMPLES Fiber spinning is conducted using a 1.5-inch diameter steel single screw extruder connected a gear pump, which is in turn connected through an adapter to the spinneret assembly which includes a screen pack to filter the molten polymer, an extension to essentially thermally isolate the spinneret from the screen pack. The gear pump, adapter, screen pack, and spinneret (faceplate) are heated by external heaters, similar to Fig. 2 of WO 00/44967 except that the adapter is heated.. The spinneret faceplate has 30 holes arranged in a circle, each hole being 30.0 mil (760 μm) in diameter. The spinneret is 90 mils (2.3 mm) thick. Fiber exiting the holes in the spinneret passes six times around a take-up (feed) roll and then around a first and a second set of two rolls for heat setting, and then to a final windup roll. Fiber drawing is done between the feed roll and second roll set, the second roll set speed divided by the feed roll speed being the "draw", except for Comparative A wherein the second roll set is not used, and draw is determined by the feed roll speed relative to the greater speed of the first roll set.. Example 1 Tefzel® ETFE fluoropolymer, MFR 29.6 is spun according to the teachings of this invention. The conditions are summarized in Table 1 Table 1 Extruder Zones Feed # 1 # 2 Gear Adapter Screen Spinneret pump pack 250°C 300°C 300°C 300°C 300°C 300°C 380°C Feed roll First roll set Second roll set Draw 400 m/min 500 m/min 1100 m/min 2.75X 150°C 230°C 150°C The resulting fiber is 435 denier, and has a tenacity of 1.83 g/denier, a modulus of 24.1 g/denier, and an elongation of 28%. The differential birefringence is measured and shows the skin of the fiber to be less oriented than the core, in particular, the birefringence of 0.0468 at the center of the filaments decreases from about this same birefringence to less than 0.044 as the measurement approaches 0.95 radius from 0.8 radius. Example 2 Example 1 is repeated except that the second roll set is run at 1400 m/min, resulting in a draw of 3.5X. The resulting fiber is 350 denier, and has a tenacity of 2.3 g/denier and an elongation of 18%, showing that the tenacity of the yarn produced in Example 1 can be increased, while still obtaining high yarn elongation just by a small amount of additional draw. The differential birefringence is measured and shows the surface of the fiber to be less oriented than the core. Example 3 The conditions of Example 1 are followed generally except that the spinneret temperature is 360°C and the melt temperature before the spinneret is about 270°C. The conditions are summarized in Table 2. Table 2 Extruder Zones Feed # 1 # 2 Gear Adapter Screen Spinneret pump pack 250°C 265°C 270°C 270°C 270°C 270°C 360°C Feed roll First roll set Second roll set Draw 400 m/min 500 m/min 1100 m min 2.75X 150°C 230°C 150°C The resulting fiber is 414 denier, 2.44 g/denier tenacity, has 18.8% elongation, and has a uniformity characterized by a coefficient of variation of 1.6%. The differential birefringence is measured and shows the surface of the fiber to be less oriented than the core. This example shows that 360°C spinneret temperature is sufficient to make fiber according to this invention. Comparative Example A This example is conducted at conditions approximating those disclosed in Japanese Patent Application (Kokai) No. 63-219616 ( 1988), Example 1. The conditions are summarized in Table 3. Table 3 Extruder Zones Feed # 1 # 2 Gear Adapter Screen Spinneret pump pack 250°C 300°C 300°C 300°C 300°C 300°C 300°C Feed roll First roll set Second roll set Draw 20 m/min 120 m/min Not used 6X 150°C 230°C The resulting fiber is 1074 denier, 2.69 g/denier tenacity, and has 15.7% elongation. The differential birefringence is measured and shows the surface of the fiber to be more oriented than the core; in particular, the filament center birefringence is 0.054 and this birefringence increases to 0.055 as the measurement increments move along the filament radius towards the surface of the filament into the 0.8-0.95 radius region. This example demonstrates that fiber spinning according to the teachings of the prior art results in differential birefringence opposite that obtained in this invention. Of course, the spinning speed (120 m/min) is so slow as to be unacceptable from an economic standpoint. Comparative Example B This example is conducted to show the effect of spinning at the same high polymer throughput and wind-up speed as Example 1 , but at a melt spinning temperature of only 300°C. The conditions are summarized in Table 4. Table 4 Extruder Zones Feed # 1 # 2 Gear Adapter Screen Spinneret pump pack 250°C 300°C 300°C 300°C 300°C 300°C 300°C Feed roll First roll set Second roll set Draw 400 m/min 500 m/min 1100 m/min 2.75X 150°C 230°C 150°C The resulting fiber is 423 denier, 2.87 g/denier tenacity, and has 7.5% elongation. The differential birefringence is measured and shows the surface of the fiber to be more oriented than the core. In particular, the birefringence of 0.054 at the center of the filament increases to 0.057 adjacent the surface of the filament. This example demonstrates that absent the high spinneret temperatures of this • invention the fiber has differential birefringence opposite that obtained in this invention. This yarn cannot be drawn further because of the disadvantageous^ low elongation. To increase the elongation to at least 15%, the draw will have to be decreased, resulting in a tenacity of less than 2 g/d. Example 4 Sewing thread of yarn similar to that prepared in Example 3 , the yarn having a denier of 437, is made by (a) applying a twist to the yarn of one twist/cm, (b) plying three ends of such yarn together at a twist of one/cm but in the opposite direction from the twist in the yarn, and (c) heat setting the resultant thread at 150°C under tension. A binder or finish can then be applied to the thread if desired. The resultant sewing thread is a balanced, corded construction having a uniform denier and exhibiting excellent stitch loop formation, without any propensity to knot or snarl. The sewing thread just described can also be used as dental floss. ETFE has a dynamic coefficient of friction (0.4) which is low enough to facilitate slipping the thread though narrow spaces between teeth but higher than the 0.01 coefficient of friction of polytetrafluoroethylene (PTFE) to increase, together with the twisting and plying, the abrasion effectiveness of the ETFE floss. The yarn used to make the sewing thread described above is used to form fishing line by braiding together four of such yarns, the resultant fishing line having a denier of 1750 and break strength of 4 kg.

Claims: CLAIM(S) What is claimed is : 1. Oriented monofilament or multifilament yarn of ethylene/tetrafluoroethylene copolymer wherein the orientation in the filaments is greater in the core of the filaments than at the surface of the filaments. 2. The yarn of claim 1 having a tenacity of at least 2 g/d . 3. The yarn of claim 2 having an elongation of at least 15%). 4. The yarn of claim 1 wherein said copolymer has a melt flow rate of less than 45 g/10 min. 5. Sewing thread containing the yarn of claim 1. 6. Dental floss containing the yarn of claim 1. 7. Fishing line containing the yarn of claim 1. 8. Staple fiber of the yarn of claim 1. 9. Process for making monofilament or multifilament yarn of ethylene/tetrafluoroethylene copolymer comprising melt spinning said copolymer at a temperature above the melting point of said copolymer which is effective to produce said yarn wherein the orientation in the filaments thereof is greater in the core of said filaments than at the surface thereof. 10. The process of claim 9 wherein said melt spinning is carried out at a temperature of at least 90°C greater than the melting point of said copolymer. 11. The process of claim 9 wherein said spinning temperature is no greater than 150°C above the melting point of said copolymer, preferably at a spinning temperature of no greater than 380°C. 12. The process of claim 9 wherein said copolymer is heated to a spinneret melting temperature which is at least 20°C less than said melt spinning temperature. 13. Oriented monofilament or multifilament yarn of ethylene/tetrafluoroethylene copolymer having a tenacity of at least 2 g/d and elongation of at least 15%>. 14. The oriented yarn of claim 13 wherein said copolymer has a melt flow rate of no greater than 45 g/10 min. 15. The oriented yarn of claim 1 containing colorant. ABSTRACT OF THE DISCLOSURE The present invention relates to yarn of ethylene/tetrafluoroethylene copolymer in which the filaments of the yarn exhibit orientation which is greater in the core of the filament than in the sheath and which further exhibits a tenacity of at least 2 g/d and elongation of at least 15%.

Inventors: TOKARSKY EDWARD WILLIAM, TOKARSKY, EDWARD, WILLIAM
Applicants: DU PONT, TOKARSKY EDWARD WILLIAM, E.I. DUPONT DE NEMOURS AND COMPANY, TOKARSKY, EDWARD, WILLIAM
Application year: 2001
Main classification: D01F6
IPC codes: A01K91, A61C15, D01F1, D01F6, D02G3, H05K1