FIBERS AND FABRICS OF HIGH DENSITY POLYETHYLENE AND METHOD OF MAKING SAME

Abstract: Fiber produced from high density (>0.940 g/cm<3>) polyethylene resins. The resins have a MWD less than 3.6, a melt index in the range from 4 to 1000, and a Mz/Mw ratio less than 2.2. Both homopolymers and copolymers of ethylene may be used in this invention. The fibers may be formed by the spunbonded, meltblown or melt spinning processes and may be used to form fabrics and garments.

Description: FIBERS AND FABRICS OF HIGH DENSITY POLYETHYLENE AND METHOD OF MAKING SAME FIELD OF THE INVENTION This invention relates generally to fibers and fabrics made from polyethylene. This invention also relates to a method of making such fibers and fabrics. More specifically, this invention relates to fibers and fabrics, and their methods of production, made from high density (greater than 0.940 g/cm 3 ) polyethylene. BACKGROUND OF THE INVENTION Until now the use of polyethylene for the formation of fibers and textiles has largely been limited to linear low density polyethylenes (LLDPE). LLDPE is a copolymer of ethylene and another olefin or diene and typically has a density of less than 0.940 g/cm 3 . Generally, polyethylene fibers and fabrics lack the strength properties of fibers and fabrics made from polypropylene. As is well known in the art, certain mechanical properties, such as tensile strength, of a given polymer change with the polymer's density and molecular weight. Thus, the strength differential between polyethylene and polypropylene could be partly overcome by using higher density and/or higher molecular weight polyethylene resins to produce fibers and fabrics. Moreover, in certain applications, polyethylene fabrics are preferred over polypropylene fabrics. For example, medical garments are often sterilized by using gamma radiation. It is well known that polypropylene fabrics tend to become brittle and produce odors when subjected to gamma radiation, whereas polyethylene does not. Polyethylene fabrics are also more suitable for use in ultraviolet and beta radiation environments than polypropylene fabrics. A variety of methods have been developed for the polymerization of olefins. In the 1940's the free-radical polymerization of olefins was developed. This technique uses high pressures, high temperatures, and a free-radical initiator - l - such as peroxides, to produce polymers such as polyethylene. However, the free- radical process generally produces low density polyethylene (LDPE) having a level of random branching of varying length, and densities in the range of from about 0.910 to about 0.935 g/cm 3 . In the late 1950's and early 1960's the use of "Ziegler-Natta" catalysts became common. These catalysts are used in a wide range of processes including low, medium, and high-pressure processes. Generally, when ethylene is polymerized using a Ziegler-Natta catalyst, a "linear" product will result whose polymer molecules will be substantially unbranched. Such linear polyolefins generally have relatively high densities, in the ranges of about 0.941 to about 0.965 g/cm 3 , which result from closer packing of the polymer molecules and minimal chain entanglement compared with the more highly branched and less dense materials. When ethylene is copolymerized using a Ziegler-Natta catalyst and a higher α-olefin as a comonomer, a product can be produced that has no detectable long chain branching, but which decreases in density as the amount of higher α- olefin comonomer incorporation is increased. Such linear low density polyethylenes (LLDPE's) may range in density from 0.860-0.940 g/cm 3 . One characteristic of the polymeric species produced using the Ziegler-Natta catalysts is their very broad molecular weight distribution (MWD). A difficulty with producing fibers from high density polyethylene resins in the past has been that, at a given MWD, such fibers tend to break or "slub" during processing more often then lower density polyethylene fibers. "Slubs" are globules of polymer or foreign matter that form on the surface of the die face or on the fiber as it is formed through the die. Such fiber breaks or slubs result in poor quality webs, and can cause the shutdown of the processing equipment, resulting in lost production time and higher processing costs. The presence of relatively large amounts of very low molecular weight species and very high molecular weight species in Ziegler-Natta type polyethylene is a cause of the processing difficulties experienced in processing high density polyethylene. The presence of relatively large amounts of very high molecular weight material also creates difficulties in drawing down the fibers to an acceptably small diameter, because of increased chain entanglements that lead to fiber breakage. Another problem with producing fiber and fabrics from high density polyethylene has been that such fibers tend to be stiff or rigid and yield a coarse or "boardy" fabric. Such fibers can also be brittle. Thus, fiber production from high density polyethylene resins produced by Ziegler-Natta catalysis has historically been commercially impracticable. Sawyer, et al. describe, in U.S. 4,830,907, the fabrication of LLDPE produced by Ziegler-Natta catalysis into fine denier fibers. Sawyer, et al. claim a multi-filament composition of less than about 15 denier produced from a LLDPE copolymer having a density in the range of from about 0.86 to about 0.95 g/cm 3 ; however, they do not provide an example of producing such a composition from a LLDPE resin having a density greater than 0.926 g/cm 3 . Kubo, et al., U.S. 5,068,141, describe fabric produced from ethylene/octene-1 LLDPE copolymers having a density of 0.900 to 0.940 g/cm 3 . However, Kubo, et al. expressly limit the density of the LLDPE used in their invention to 0.940 in order to achieve acceptable reduction in the weight of the filaments formed, and only exemplify producing fibers from 0.937 g/cm 3 resins. The use of single-site catalysts such as metallocenes instead of the Ziegler- Natta catalysts in the polymerization process solves some of the problems in processing high density polyethylene. Polyethylene produced using a metallocene catalyst system has a more narrow molecular weight distribution and a more uniform comonomer distribution compared to polyethylene produced by Ziegler- Natta catalysis. Polyethylenes having such a narrow molecular weight distribution, while being of generally high average molecular weight, effectively provide a polymer which does not have the low molecular weight fraction which causes difficulty in strand formation. These narrow molecular weight distribution products have a generally higher level of crystallinity since they lack the low molecular weight fraction. However, because these polyethylenes do not have a large fraction of high molecular weight species, fibers may more readily be drawn to smaller diameters than high density polyethylenes in the prior art and thus are acceptably soft and yielding to body movement. Davey, et al., U.S. 5,322,728, describe the use of LLDPE produced using a metallocene catalyst system in the formation of fibers, but limit the density of the resin to between about 0.86 to about 0.93 g cm 3 . It would be desirable to produce fibers and fabrics from a high density polyethylene resin to achieve an improvement in mechanical properties over fibers produced from lower density polyethylene resins without sacrificing processability or fabric quality. SUMMARY OF THE INVENTION This invention provides novel fibers, either of a homopolymer of ethylene or a copolymer of ethylene and a comonomer, the polymer having a density of at least 0.940 g/cm 3 , a MWD less than about 3.6, a melt index in the range from about 4 to about 1000, and a M z /M w ratio less than about 2.2, along with fabrics incorporating these fibers. This invention also provides novel processes for producing such fibers and fabrics. These fibers and fabrics have improved processability and mechanical properties, and have a better "feel" than fibers and fabrics produced from polyethylene resins having a similar melt index and density but manufactured with a Ziegler-Natta catalyst. Unlike polypropylene fibers, these fibers can withstand gamma radiation sterilization, and so are particularly useful in medical applications. These fibers and the fabrics produced from them can also withstand ultraviolet and beta radiation environments, unlike polypropylene fibers and fabrics. The good processability allows the formation of finer fibers than Ziegler-Natta catalyzed polyethylene fibers, which results in a softer, more drapeable fabric. The high density allows certain mechanical properties to be maximized, without sacrificing processability or fabric quality. These fibers may be produced by a number of methods, including melt spinning, meltblown, and spunbond processes. These fibers also exhibit fewer fiber breaks and fewer slubs at relatively high processing speeds than fibers produced from Ziegler-Natta catalyzed polyethylene. It is surprising that such polyethylene fibers may be so easily formed from such high density resins. It is also surprising that the inventive fibers form such a soft, drapeable fabric, and exhibit improved strength and elongation compared to polymers having similar melt indices and densities made using Ziegler-Natta or other multi-site catalyst systems. The improved process performance when compared to like material produced using multi-site catalyzed polymers is also an advantage. DESCRIPTION OF THE PREFERRED EMBODIMENTS The Catalyst Used in the Production of the Resins The polyethylene resins used in this invention are preferably produced using a supported metallocene catalyst. Metallocene catalysts are typically those bulky ligand transition metal compounds derivable from the formula: [L] m M[A] n where L is a bulky ligand; A is at least one halogen leaving group, M is a transition metal and m and n are such that the total ligand valency corresponds to the transition metal valency. Preferably the catalyst is four coordinate such that the compound is ionizable to a 1 + valency state. The ligands L and A may be bridged to each other, and if two ligands L and/or A are present, they may be bridged. The metallocene compound may be full-sandwich compounds having two or more ligands L which may be cyclopentadienyl ligands or cyclopentadiene derived ligands or half-sandwich compounds having one ligand L, which is a cyclopentadienyl ligand or derived ligand. The metallocene compounds contain a multiplicity of bonded atoms, preferably carbon atoms, forming a group which can be cyclic. The bulky ligand can be a cyclopentadienyl ligand or cyclopentadienyl derived ligand which can be mono- or poly-nuclear or any other ligand capable of η-5 bonding to the transition metal. One or more bulky ligands may be π -bonded to the transition metal atom. The transition metal atom may be a Group 4, 5, or 6 transition metal and/or a transition metal from the lanthanide and actinide series. Other ligands may be bonded to the transition metal, such as at least one halogen as a leaving group that is detachable from the transition metal. Non-limiting examples of metallocene catalysts and catalyst systems are discussed in for example, U.S. Patent Nos. 4,530,914, 5,124,418, 4,808,561, 4,897,455, EP-A-0129,368, EP-A-0520732, EP- A-0277003, EP-A-0277004, EP-A-0420436, WO 91/04257, WO 92/00333, WO 93/08221, and WO 93/08199. Various forms of the catalyst system of the metallocene type may be used in the polymerization process of this invention. Exemplary of the development of metallocene catalysts in the art for the polymerization of ethylene is the disclosure of U.S. Patent No. 4,871,705 to Hoel, U.S. Patent No. 4,937,299 to Ewen, et al. and EP-A-0 129 368 published July 26, 1989, and U.S. Patent Nos. 5,017,714 and 5,120,867 to Welborn, Jr. These publications teach the structure of the metallocene catalysts and include alumoxane as the cocatalyst. There are a variety of methods for preparing alumoxane; one of which is described in U.S. Patent 4,665,208. Further, the metallocene catalyst component of the invention can be a monocyclopentadienyl heteroatom containing compound. This heteroatom is activated by either an alumoxane alone or an alumoxane and an ionic activator to form an active polymerization catalyst system to produce polymers useful in this present invention. These types of catalyst systems are described in, for example, PCT International Publications WO 92/00333, WO 94/07928, and WO 91/04257, U.S. Patent Nos. 5,057,475, 5,096,867, 5,055,438 and 5,227, 440 and EP-A-0 420 436. In addition, the metallocene catalysts useful in this invention can include non- cyclopentadienyl catalyst components, or ancillary ligands such as boroles or carbollides in combination with a transition metal. Additionally it is not beyond the scope of this invention that the catalysts and catalyst systems may be those described in U.S. Patent No. 5,064,802 and PCT publications WO 93/08221 and WO 93/08199 published April 29, 1993. The preferred transition metal components of the catalyst of the invention are those of Group 4, particularly, zirconium, titanium and hafnium. The transition metal may be in any oxidation state, preferably +3 or +4 or a mixture hereof. All the catalyst systems of the invention may be prepolymerized or used in conjunction with an additive or scavenging component to enhance catalytic productivity. For purposes of this patent specification the term "metallocene" is defined to contain one or more unsubstituted or substituted cyclopentadienyl or cyclopentadienyl moiety in combination with a transition metal. In one embodiment the metallocene catalyst component is represented by the general formula (Cp) m MeR n R'p wherein at least one Cp is an unsubstituted or, preferably, a substituted cyclopentadienyl ring even more preferably a monosubstituted cyclopentadienyl ring; Me is a Group 4, 5 or 6 transition metal; R and R' are independently selected halogen, hydrocarbyl group, or hydrocarboxyl groups having 1-20 carbon atoms; m = 1-3, n = 0-3, p = 0-3, and sum of m + n + p equals the oxidation state of Me. In another embodiment the metallocene catalyst component is represented by the formulas: (C5R , m)pR H s(C 5 R , m) eQ3_p_ x and R"s(C5R , m)2MeQ' wherein Me is a Group 4, 5, 6 transition metal, C5R' m is a substituted cyclopentadienyl, each R', which can be the same or different is hydrogen, alkyl, alkenyl, aryl alkylaryl or arylalkyl radical having from 1 to 20 carbon atoms or two carbon atoms joined together to form a part of a C4 to C20 ring. R" is one or more of or a combination of a carbon, a germanium, a silicon, a phosphorous or a nitrogen atom containing radical bridging two (C5R' m ) rings, or bridging one (C5R' m ) ring back to Me, when p = 0 and x = 1, otherwise "x" is always equal to 0, each Q which can be the same or different is an aryl, alkyl, alkenyl, alkylaryl, or arylalkyl radical having from 1 to 20 carbon atoms or halogen, Q' is an alkylidene radical having from 1-20 carbon atoms, s is 0 or 1 and when s is 0, m is 5 and p is 0, 1 or 2 and when s is 1, m is 4 and p is 1. While any metallocene catalyst component can be used in the invention the monosubstituted metallocenes are preferred over the disubstituted. However, the disubstituted and polysubstituted metallocenes still are better than counterpart catalyst systems, such as Ziegler-Natta systems, produced in accordance with prior art methods. In a further embodiment the preferred metallocene catalyst component of the invention is represented by the formulas: (C 5 H n R')R" s (C 5 H n R')MeQ2 and wherein Me is a Group 4, 5, 6 transition metal, each R', which can be the same or different, is hydrogen, alkyl, alkenyl, aryl, alkylaryl or arylalkyl radical having from 1 to 20 carbon atoms, R" is one or more of or a combination of a carbon, a germanium, a silicon, a phosphorous or a nitrogen atom containing radical bridging two (C5H4R') rings, each Q which can be the same or different is an aryl, alkyl, alkenyl, alkylaryl, or arylalkyl radical having from 1 to 20 carbon atoms or halogen, Q 1 is an alkylidene radical having from 1-20 carbon atoms; s is 0 or 1, when s = 1, then n = 3, when s = 0, n = 4. In another embodiment the metallocene catalyst component is represented by the formula: R"(C 5 H3R')2MeQ2 wherein Me is a Group 4, 5, 6 transition metal, each R', which can be the same or different, is hydrogen, alkyl, alkenyl, aryl, alkylaryl or arylalkyl radical having from 1 to 20 carbon atoms, R" is one or more of a combination of carbon, a germanium, a silicon, a phosphorous or a nitrogen atom containing radical bridging the (C5R' m ) ring back to Me, each Q which can be the same or different is an aryl, alkyl, alkenyl, alkylaryl, or arylalkyl radical having from 1 to 20 carbon atoms or halogen. For the purposes of this patent specification, the terms "cocatalysts" and "activators" are used interchangeably and are defined to be any compound or component which can activate a bulky ligand transition metal compound or a metallocene, as defined above. It is within the scope of this invention to use, in addition to using alumoxane, ionizing ionic activators or compounds such as tri (n- butyl) ammonium tetra (pentafluorophenyl) boron, which ionize the neutral metallocene compound. Such ionizing compounds may contain an active proton, or some other cation associated with but not coordinated or only loosely coordinated to the remaining ion of the ionizing ionic compound. Such compounds and the like are described in EP-A-0520 732, EP-A-0277 003 and EP- A-0277 004, and U.S. Patent Nos. 5,153,157, 5,198,401 and 5,241,025. For purposes of this patent specification the terms "carrier" and "support" are interchangeable and can be any support material, preferably a porous support material, capable of containing water, absorbed or adsorbed, such for example, talc, inorganic oxides, inorganic chlorides and resinous support materials such as polyolefin or polymeric compounds or other organic support materials. The preferred support materials are inorganic oxide materials which include those from the Periodic Table of Elements of Groups 2, 3, , 5, 13 or 14 metal oxides. In a preferred embodiment, the catalyst support material include silica, alumina, silica-alumina, and mixtures thereof. Other inorganic oxides that may be employed either alone or in combination with the silica, alumina or silica-alumina are magnesia, titania, zirconia, and the like. Other suitable support materials can be employed such as, finely divided polyolefins, such as polyethylene or polymeric compounds and inorganic compounds such as magnesium dichloride and the like. In accordance with this invention the support material preferably has a water content in the range of from about 3 weight percent to about 27 weight percent based on the total weight of the support material and water contained therein, preferably in the range of from about 7 weight percent to about 15 weight percent, and most preferably in the range of from about 9 weight percent to about 14 weight percent. The amount of water contained within the support material can be measured by techniques well known in the art, such as by loss on ignition (LOI). Preparation of the Catalyst Used to Produce the Resins In the method of making the preferred catalyst system of the invention, the support material is first contacted with a component capable of forming an activator for the metallocene catalyst component, as previously discussed. In one embodiment, the preferred component is an organometallic compound of Group 1, 2, 3 and 4 organometallic alkyls, alkoxides, and halides. The preferred organometallic compounds are lithium alkyls, magnesium alkyls, magnesium alkyl halides, aluminum alkyls, silicon alkyl, silicon alkoxides and silicon alkyl halides. The more preferred organometallic compounds are aluminum alkyls and magnesium alkyls. The most preferred organometallic compounds are aluminum alkyls, for example, triethylaluminum (TEAL), trimethylaluminum (TMAL), tri-isobutylaluminum (TIBAL) and tri-n-hexylaluminum (TNHAL) and the like. The most preferred organometallic compounds are those that when contacted with the water containing support material of the invention form an oxy- containing organometallic compound represented by the following general formula: (R-Al-O) n which is a cyclic compound and R (R-Al-O) n AlR 2 which is a linear or non-cyclic compound and mixtures thereof including multi¬ dimensional structures. In the general formula R is a Cj to Cj2 alkyl group such as for example methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, nonyl and n is an integer from about 1 to 20. The most preferred oxy containing organometallic compounds are alumoxanes, for example methyl alumoxane and/or ethylalumoxane. In the preferred embodiment the support material is introduced to a solution of an organometallic compound such that the temperature of the solution containing the organometallic compound remains substantially constant throughout the introduction of the support material such that the temperature is always within the temperature ranges described below. Into a 1 liter flask equipped with mechanical stirrer, 180 ml of TMAL in heptane solution (15 wt%) and 90 ml of heptane were charged. The solution was cooled and maintained at a temperature of 45°F (7.2°C). A 40 g sample of silica gel (Davison D-948 with average particle size of 70 micron) which contained 12.5 wt% of water was slowly added into the flask over 70 minutes. The mole ratio TMAL/Η2O was 0.91. Next, 0.9 g of (n-butylcyclopentyldienyl)2ZrCl2 was slurried in 20 ml of heptane and then added into the vessel. The mixture was allowed to react at 165°F (74°C) for 1 hour. At the end of the reaction, the solid was dried by nitrogen purging. A free flowing solid was obtained at the end of the preparation. The Process Used to Produce the Resins The resins used in the present invention are preferably produced using a continuous slurry process. Such continuous, slurry polymerization processes are well known to those skilled in the art. A slurry polymerization process generally uses pressures in the range of about 1 to about 500 atmospheres and even greater and temperatures in the range of -60°C to about 280°C. In a slurry polymerization, a suspension of solid, particulate polymer is formed in a liquid polymerization medium to which ethylene and comonomers and often hydrogen along with catalyst are added. The liquid employed in the polymerization medium is preferably an alkane or cycloalkane, or an aromatic hydrocarbon such as toluene, ethylbenzene or xylene. The medium employed should be liquid under the conditions of polymerization and relatively inert. Preferably, hexane or isobutane is employed. Table I sets forth the operating parameters used in producing the polyethylene resin grades (Resin A and Resin B) of the present invention. Note that Resin A and Resin B contain a small amount of hexene comonomer. Those skilled in the art will appreciate that other polymerization processes may be used to produce the resins, such as high pressure, medium pressure, low pressure, bulk phase, gas phase and solution phase polymerization. Characteristics of the Resins In the preferred embodiment of the present invention, there is provided a fiber comprising a copolymer of ethylene and one or more comonomers, which copolymer has a density greater than 0.940 g/cm 3 , preferably greater than 0.945 g/cm 3 , and most preferably, greater than 0.950 g/cm 3 , a MWD of less than 3.6, and a melt index (MI, ASTM D- 1238(E)) of from about 4 to about 1000. Those skilled in the art will recognize that a homopolymer having these same characteristics may also be used. Where a comonomer is used, each comonomer preferably has 3 to 20, more preferably 3 to 10 carbon atoms and may comprise, for example, propylene, butene, hexene, octene, 4-methyl-l-pentene, styrene or combinations thereof. The preferred resins will have no detectable long chain branching. In general, the narrower the MWD or polydispersity index /L Mm) the better for fiber production, so the preferred MWD range is less than about 3.6; a MWD of 1.8 to 3.5 is more preferred, and a MWD of 2.0 to 3.1 is most preferred. The ratio of the third moment (M ) to the second moment (M w ) of the molecular weight distribution curve provides a measure of the portion of very high molecular weight and very low molecular weight chains present in a polymer. As previously described, a polymer with a large portion of relatively very low molecular weight chains tends to form slubs during fiber processing, while the presence of a large portion of relatively very high molecular weight chains leads to poor fabric quality and difficulties in drawing down the fibers to acceptably small diameters. Thus, the smaller the M z /M w ratio the better for fiber processing. The preferred M z /M w ratio for the fibers and fabrics of the present invention is less than about 2.2. A M z /M w ratio less than about 2.0 is more preferred, and a M z /M w ratio of less than about 1.9 is most preferred. In addition to creating the highly desirable characteristic of being able to tailor the molecular weight distribution of the molecules of the polymer resin, metallocene-type catalysts also have the desirable characteristic of being able to incorporate comonomers of varying size more uniformly within the backbone of the polymer than Ziegler-Natta catalysts. Also, metallocene-type catalysts may be advantageously employed in several different polymerization processes including, for example, high pressure, medium pressure, low pressure, solution phase, bulk phase, slurry phase, and gas phase polymerization. The preferred MI of the polymer depends on the manner in which the fiber is to be formed. The fibers of the present invention may be formed by various processes well known in the art, including spunbond, meltblown, and melt spinning processes. For production by the spunbond process, the preferred range is from 4 to 60; a MI of from about 15 to about 35 is more preferred; and a MI of from about 25 to about 30 is most preferred. For the meltblown process, the preferred MI is from about 10 to about 1000. For the melt spinning process, the preferred MI is from about 4 to about 150. Table II provides melting point, molecular weight, and molecular weight distribution data for the resins used in the present invention, and also for some comparative commercially available resins. All melt indices and densities were determined in accordance with ASTM D-1238 (E) and ASTM D- 1505, respectively. As Table II shows, the MWD of Resin A and Resin B is 3.1 and 3.3, respectively, compared to a value of 4.0 for the Ziegler-Natta high density polyethylene resin (HD-6705, commercially available from Exxon Chemical Company, Houston, Texas). Also, the "tails" or high and low molecular weight ends of the molecular weight distribution curves for Resins A and B are considerably smaller than those of the Ziegler-Natta polyethylene. Note that Resin A and Resin B have M- M y , values of 1.81 and 1.86, respectively, while the HD- 6705 resin has a M-ZM ^ , value of 2.74. Those skilled in the art will appreciate that there are several methods available for determining the MWD of a polyethylene sample. The molecular weights listed in Table II were determined with a Waters Gel Permeation Chromatograph equipped with ultrastyro gel columns operated at 145° C. Trichlorobenzene was used as the eluting solvent. The calibration standards were 16 polystyrenes of precisely known molecular weight, ranging from a molecular weight of 500 to a molecular weight 5.2 million. NBS 1475 polystyrene was also used as a calibration standard. The preferred use of the inventive fibers is in the formation of fabric, especially non-woven fabrics. Fabrics formed from the fibers have been found to have good mechanical properties and good drapeability. Such fibers may be used to make garments, for example, surgical drapes, medical gowns, and diaper backing, as well as for filters and absorbants. Properties of Fibers and Fabrics Produced From the Resins Fibers were formed from Resins A and B by the spunbond and meltblown processes. For comparative purposes, fibers were also formed from HD-6705 and EXACT™ 4023 resins. EXACT™ 4023 is an ethylene/butene copolymer produced using a metallocene catalyst, and is commercially available from Exxon Chemical Company, Houston, Texas. Table III sets forth the process conditions for the spunbond process. The spunbonded non- woven fabrics were produced using a 1 meter Reicofil line made by the Reifenhauser Company. The extruder size was 7 cm. (2.75 in.) with a 30:1 length: diameter ratio. There were 3719 die plate holes, each having a diameter of 0.4 millimeters with IJD=4/1. The spunbond process is well known in the art of fabric production. Generally, continuous fibers are extruded, laid on an endless belt, and then bonded to each other, often by a heated calendar roll. An overview of spunbonding may be obtained from Wadsworth, L.C. and Goswami, B.C., "Non-Woven Fabrics: Spunbonded and Meltblown Processes", Proceedings Eighth Annual Non-Wovens Workshop, July 30 - August 3, 1990, sponsored by TANDEC, University of Tennessee, Knoxville. The spunbonded fibers produced from Resin A and Resin B demonstrated advantages over the Ziegler-Natta based fibers and fabrics in a number of ways. Table III gives spunbond process data for two samples of each of the resins tested. The data in Table III show that the high density polyethylene resins of the present invention ran longer and with fewer processing difficulties than the comparable Ziegler-Natta based material (0 slubs for Resin A, compared to 2 slubs for HD- 6705, with longer run times for Resin A). Overall, the resins of the present invention spun into fibers well and demonstrated qualitatively easier fiber formation over the Ziegler-Natta based resin. Also, fabric made from the resultant fibers was less coarse than fabric made from the Ziegler-Natta based fibers and lacked the "greasy" feeling usually associated with lower density polyethylene fibers and fabric. Table IV sets forth the results of physical property testing on the fabrics produced by the spunbond process. Strip tensile force and elongation to break were measured using ASTM D- 1682-75. Measurements for spunbonded fabrics were made on a United Model SSTM-1-E-PC tensile testing machine, using a 13 cm (5 in.) jaw gap and a crosshead speed of 13 cm/min (5 in./min.). Total hand was measured using a Thwing Albert Handle-O-Meter, Model 211-5 according to TAPPI 4998 CM-85 test method using a 0.64 cm (0.25 in) slot with a sample of 20 cm x 20 cm (8 in x 8 in). It is readily apparent from Table IV that for a given basis weight of fabric, the resins of the present invention produce fibers having a higher tensile strength (tensile force at break) and greater elongation to break than the Ziegler-Natta based counterpart at the same melt index and density. Thus, fabrics of the present invention are stronger and more "stretchable" than fabrics made from Ziegler-Natta catalyzed resins. The inventive fibers also have improved mechanical properties over the EXACT™ fibers. Further testing was done by spinning fibers using a meltblown process. Table V sets forth the process conditions for the meltblown process. Meltblown technology is also well known in the art of fabric production. An overview of the process may be obtained from Wadsworth, L.C. and Goswami, B.C., "Non-Woven Fabrics: Spunbonded and Meltblown Processes", Proceedings Eighth Annual Non- Wovens Worfcshop, July 30 - August 3, 1990, sponsored by TANDEC, University of Tennessee, Knoxville, and from "Meltblown Process", Meltblown Technology Today, Miller Freeman Publications, Inc., San Francisco, California, 1989, pgs. 7- 12. Generally, in a meltblown process, noncontinuous fibers are extruded, typically with high velocity, hot attenuating air, and then collected on a collector drum. The fibers are typically held together by a combination of fiber interlacing and thermal bonding resulting from the residual heat of extrusion and hot attenuating air. Additional post-extrusion bonding may or may not be necessary depending upon the fabric's end use. This testing was accomplished using a 51 cm. (20 in.) Accurate Products Meltblown line. The extruder was a 5 cm. (2 in.) Davis Standard with a 30:1 length: diameter ratio. The die nozzle had 501 die holes. The diameter of each die hole was 0.4 mm (0.015 in ). Die length was 15:1 and the air gap was set to 1.5 mm (0.060 in.). Table VI sets forth physical property results for the fabrics produced by the meltblown process. Testing of tensile strength and elongation of the meltblown fabric was done according to ASTM method 1682-75 (same procedure as for the spunbonded fabrics) using a C station United model 7-VI tensile testing machine. Total hand was measured by the same apparatus and method used for the spunbonded fabrics. As Table VI shows, the fabrics produced from resins A and B have similar or improved mechanical properties relative to the Ziegler-Natta based fabric, even at a much higher melt index (64 MI for Resin B compared to 20 MI for HD-6705). In addition, the fibers of the present invention were finer than fibers produced from the Ziegler-Natta based resins. This is a result of the narrower MWD and lower M z /M w ratio for Resin A when compared to its like multi-site catalyzed counterpart (HD-6705). The lower viscosity of Resin B also is a contributor to finer fiber production. The combination of both narrow MWD and low viscosity (MI > 100 ) would be ideal in this process. The use of substantially 100% by weight of polyethylene homopolymers or copolymers is preferred in producing the fibers. It will be appreciated, however, that Resins A and B could be blended with a variety of polar and nonpolar polyolefins and thermoplastic rubbers, such as LLDPE, PP, EVA, EMA, EPR, EPDM, SBS, SIS, SEBS, and the like, and remain within the scope of this invention. The high density polyethylene resins of the present invention may also contain additives such as processing aids, pigments, dyes, stabilizers and flame retardants. Those skilled in the art will appreciate that numerous modifications to the preferred embodiments described herein can be made without departing from the scope of the invention. TABLE I Resin A Resin B Melt Index (dg/min.) 20 64 Density (g/cm 3 ) .953 .958 Slurry Concentration (wt%) 42 . 42 Ethylene (wt%) 5.5 5.5 Reactor Pressure (kg/cm^) 39 39 Production rate (kg/hr) 1814 1814 H 2 (lbs/Klbs iC 4 ) .016 0.03 Reactor Temperature (°C) 102 102 Hexene (gal/Klbs iC 4 ) 2.0 2.0 Hexene/Ethylene ((gal hexene/Klbs 0.36 0.36 iC 4 )/(wt% ethylene)) No alkyl was fed to the reactor. TABLE π HD-6705 Resin A Resin B Exact 4023 MI (dg/min.) 20 20 64 35 Density (g/cm- ) .953 .953 .958 .882 belting Point (°C) 129 131 129 68 1 Crystallization Point (°C) 113 113 114 52 2 Mw 52,700 47,100 34,900 37,100 2 M 13,100 15,300 10,500 16,800 M- Mn 4.0 3.1 3.3 2.2 2 M Z 144,400 85,200 64,700 60,200 M z M w 2.74 1.81 1.86 1.62 Catalyst Family Ziegler-Natta Metallocene Metallocene Metallocene Production Process Gas Phase Slurry Slurry High Pressure 1. Measured by Differential Scanning Calorimetry (DSC) 2. Measured by Gel Permeation Chromatography (GPC) TABLE m Spunbond Process Results Resin Data Grade HD-6705 HD-6705 Resin A Resin A Exact 4023 Exact 4023 MI (dg/min.) 20 20 20 20 35 35 Density (g cm*') .953 .953 .953 .953 .882 .882 Pertinent Process Parameters Rate (g/hole/min) .35 .35 .35 .35 .20 .20 Basis Wt. (g/m 2 ) 70 40 70 40 70 38 Die Melt (°C) 212 212 212 212 186 186 Spin Pump (rpm) 15.5 15.5 15.6 15.6 8.9 8.9 Extruder rpm 88 88 85 85 45 45 Spin Pump Press, (kg/cm 2 ) 137 138 158 177 124 124 Extruder Press, (kg/cm**-) 85 85 87 85 85 85 Die Press, (kg/cm 2 ) 53 53 58 59 39 39 Suction Speed (rpm) 1713 1713 1714 1714 1840 1840 Cooling Air Speed (rpm) 786 787 787 787 785 786 Cooling Air Temp. (°C) 8 8 8 8 11 8 Room Air Temp. (°C) 31 31 32 31 29 29 Spin Belt Speed (mpm) 15.3 27.3 15.4 26.9 8.4 15.7 Calendar Speed (mpm) 14.9 26.5 18.0 26.2 8.2 15.4 Winder Speed (mpm) 15.5 27.8 15.7 27.6 9.5 17.1 Bond Press, (kg/cm* ) 101 69 103 68 103 59 Bond Temp. (°C) 122 123 123 123 65 63 Prod. Run Time (hrs) 2.5 3.5 5.0 "Slubs" 2 0 0 TABLE IV Spunbond Property Results Resin Data Grade HD-6705 HD-6705 Resin A Resin A Exact 40 MI (dg/min.) 20 20 20 20 35 Density (g/cm 3 ) .953 .953 .953 .953 .882 Physical Property Results Tensile Force at Break (g) MD 2043.0 1180.4 2315.4 1407.4 726.4 TD 862.6 363.2 862.6 454.0 499.4 Break Elong (%) MD 375 200 515 400 350 TD 330 170 400 260 350 Basis Weight (g/m 2 ) 70 40 70 40 70 Fiber Diameter (μm) 33.9 n/a 1 32.7 n/a 1 29.4 Total Hand (g) 71.23 19.97 82.75 21.98 18.23 Bond Temp. (°C) 122 123 123 123 65 1. Not measured TABLE V Meltblown Process Results Resin Data Grade HD-6705 Resin A Resin B Exact 4023 MI (dg/min.) 20 20 64 35 Density (g/cm 3 ) .953 .953 .958 .882 Pertinent Process Parameters Rate (g/hole/min) .25* .25* .4 .4** Basis Wt. (g/m 2 ) 68 68 68 68 Melt Temp. (°C) 228 227 226 216 % Air -40 -40 -40 -40 Air Rate (m 3 /min) 9.20 9.20 9.20 9.20 Air Temp. (°C) 226 226 226 213 !DCD (cm) 30 30 30 30 Die Press, (kg/cm 2 ) 46 47 24 28 Fiber Fly (Y/N) Y Y N N Web Rating (1-5) 5 4 2 2 Note: * Had to reduce rate due to die pressure ** Had to reduce temp, to 216° C and add H2O spray due to tack Web Rating: Extrusion Line = 20" Accurate Meltblown Line 1 = Excellent Die Hole Diameter = 0.4 mm 2 = Good # Die Holes = 501 3 = Fair Air Gap = 1.5 mm 4 = Poor Set Back = 1.5 mm 5 = Very Poor 1. Distance to collector drum TABLE VI Meltblown Property Results Resin Data Grade HD-6705 Resin A Resin B Exact 4023 MI (dg/min.) 20 20 64 35 Density (g/cc) .953 .953 .958 .882 Phvsical Properties Tensile Force at Break (g) MD 272.4 281.5 317.8 272.4 TD 281.5 272.4 544.8 249.7 Break Energy (kg-cm) MD 0.15 0.39 0.70 2.14 TD 0.39 0.39 2.57 1.84 Break Elongation (%) MD 31 74 108 250 TD 74 72 120 300 Basis Weight (g/m 2 ) 68 68 68 68 Total Hand (g) 140 98 136 30 SEM Fiber Dia. (μm) 30 24.5 9.1 20.1

Claims: CLAIMS We claim: 1. Fibers comprised of polyethylene characterized in that said polyethylene has a density greater than 0.940 g/cm 3 , preferably at least 0.945 g/cm 3 , and more preferably at least 0.950 g/cm 3 , a MWD less than 3.6, preferably in the range from 1.8 to 3.5, more preferably in the range from 2.0 to 3.1, a melt index in the range from 4 to 1000, and a M z /M w ratio less than 2.2, preferably less than 2.0, and more preferably less than 1.9. 2. Fibers as set forth in claim 1, wherein said polymer comprises a copolymer of ethylene with at least one alpha-olefin comonomer of C 3 to C 20 , and wherein said copolymer was produced using a metallocene catalyst. 3. Fibers as set forth in claim 2, wherein said comonomer is selected from the group consisting of propylene, butene, hexene, octene, and 4-methyl-l- pentene. 4. Fibers as set forth in any of the preceding claims, wherein said polyethylene is produced in a slurry process. 5. Fibers as set forth in any of the preceding claims, wherein said polyethylene has no detectable long chain branching. 6. A process for producing a polymeric fiber, said process comprising the step of extruding a polymer comprised of polyethylene through an orifice, characterized in that said polyethylene has a density greater than 0.940 g/cm 3 , preferably at least 0.945 g/cm 3 , and more preferably at least 0.950 g/cm 3 , a MWD less than 3.6, preferably in the range from 1.8 to 3.5, and more preferably in the range from 2.0 to 3.1 , a melt index in the range from 4 to 1000, and a M z /M w ratio less than 2.2, preferably less than 2.0, and more preferably less than 1.9. 7. A process for producing a fabric, comprising the steps of extruding a polymer through a plurality of orifices to produce a plurality of fibers, collecting said fibers on a collecting means and forming a fabric comprising said fibers, said polymer comprising polyethylene, characterized in that said polyethylene has a density greater than 0.940 g/cm 3 , preferably at least 0.945 g/cm 3 , and more preferably at least 0.950 g/cm 3 , a MWD less than 3.6, preferably in the range from 1.8 to 3.5, more preferably in the range from 2.0 to 3.1, a melt index in the range from 4 to 1000, and a M z /M w ratio less than 2.2. 8. The process as set forth in any of claims 6-7, wherein said polyethylene is produced in a slurry process. 9. The process as set forth in any of claims 6-8, wherein said polyethylene has no detectable long chain branching. 10. The process as set forth in any of claims 6-9, wherein said polymer comprises a copolymer of ethylene with at least one alpha-olefin comonomer of C 3 to C 20 , and wherein said copolymer was produced using a metallocene catalyst. 11. The process as set forth in any of claims 7- 10, wherein said process is a spunbonded process, and said melt index is in the range from 4 to 60, preferably from 15 to 35, and more preferably 25 to 30. 12. The process as set forth in any of claims 9-10, wherein said process is a meltblown process, and said melt index is in the range from 10 to 1000, preferably from 4 to 150. 13. Fabric comprising fibers of any of claims 1 -5. 14. An article comprised of fabric as set forth in claim 13.

Inventors: BRANDENBURGER WAYNE ROBERT, ERDERLY THOMAS CRAIG, TRUDELL BARRY COLIN, BRANDENBURGER, WAYNE, ROBERT, ERDERLY, THOMAS, CRAIG, TRUDELL, BARRY, COLIN
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1995
Main classification: D01F6
IPC codes: D01F6, D04H3