WO2007142726A1

WO2007142726A1 - Non-woven fabric stable to gamma-radiation sterilization

Info

Publication number: WO2007142726A1
Application number: PCT/US2007/008072
Authority: WO
Inventors: Smita Kacker; Narayanaswami Raja Dharmarajan; Chia Yung Cheng
Original assignee: Exxonmobil Chemical Patents Inc.
Priority date: 2006-05-30
Filing date: 2007-04-03
Publication date: 2007-12-13

Abstract

Disclosed herein is a non-woven fabric comprising propylene and a random propylene copolymer, wherein a difference in tensile strength of the fabric is less than or equal to about 40%, when the difference is determined before and after gamma-radiation sterilization of the fabric at between about 0.5 and 15 Mrads.

Description

Non-Woven Fabric Stable to Gamma-Radiation Sterilization

Technical Field

[0001] The present invention generally relates to a nonwoven fabric comprising polypropylene and a random propylene copolymer, wherein the fabric is stable to gamma radiation sterilization. The random propylene polymer exhibits significant retention of fabric elongation and tensile strength even after gamma radiation.

Background of the Invention

[0002] Polypropylene (PP) is a well-known article of commerce, and is utilized in a wide variety of applications. Polypropylene is utilized widely in many fiber, fabric, and similar product applications. However, upon exposure to gamma radiation, propylene polymer fibers (PP fibers) develop objectionable color and lose their desirable physical properties due to degradation. [0003] Recent advances in PP technology have been instrumental in propagating the use of the polymer for shaped articles for a wide variety of uses. The chemical inertness and lack of toxicity of PP fibers, their low weight, and the relatively low cost of producing such articles of arbitrary size and shape, make the PP fibers peculiarly well-adapted for use in the medical and institutional maintenance fields. Thus, PP is the material of choice for laboratory filtration fabric, and a variety of disposable articles including curtains, bedsheets, surgical gowns, and the like. Such articles, stabilized with hindered phenols, were routinely sterilized with ethylene oxide, but the toxicity of ethylene oxide resulted in a gradual withdrawal of the sterilant. Ethylene oxide has been replaced in large part by gamma radiation sterilization, wherein the item to be sterilized is exposed to gamma radiation at a dosage level in the range from about 0.5 to about 15 MRads (megarads). Items are exposed to the gamma radiation for a particular time at a particular intensity for a given dosage, e.g., a short exposure time is given at a high intensity. Typical exposure times are from about 1 min (minute) to about 12 hr (hours) at an intensity to yield a dosage from about 0.5 to about 15 Mrads. [0004] PP fibers degrade rapidly when exposed to gamma radiation. PP so sterilized, develops an objectionable yellow color, and suffers a severe loss of physical integrity. The higher the intensity of radiation, the worse the degradation. The degradation, referred to as "oxidative degradation", is particularly noticeable upon storage of a sterilized article at room temperature, wherein the longer the storage period, the worse the degradation. For more information, see "Recent Developments in the Oxidative Degradation of Polypropylene by Gamma Radiation" by Wiles, D. M. and Carlsson, D. J., wherein theoretical equations are presented to explain the mechanisms and kinetics.

[0005] Various stablizers have been developed to stabilize PP against gamma radiation, which have been partially effective. However, a PP non-woven fabric, which is stable to gamma radiation, remains an elusive goal in the art. Because of the large surface area of the fibers, the oxidative degradation is much more severe compared to the injection molded article which has a smaller surface area compared to the woven and nonwoven fabrics.

Summary of the Invention

[0006] In an embodiment, a non-woven fabric comprises polypropylene and a random propylene copolymer, wherein a difference in tensile strength of the fabric is less than or equal to about 40%, when the difference is determined before and after gamma-radiation sterilization of the fabric at between about 0.5 and 15

Mrads.

Brief Description of the Drawings

[0007] These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description, appended claim, and accompanying drawings where:

[0008] Figure 1 is a graphical representation of tensile strength for spunbond fabrics of the instant disclosure.

[0009] Figure 2 is a graphical representation of tensile strength for melt blown fabrics of the instant disclosure. [0010] Figure 3 is a graphical representation of elongation at break for spunbond fabrics of the instant disclosure.

[0011] Figure 4 is a graphical representation of elongation at break for meltblown fabrics of the instant disclosure.

[0012] Figure 5 is a graphical representation of hysteresis for an embodiment of a spunbond fabric of the instant disclosure.

Detailed Description of the Invention

[0013] While the instant disclosure will be described in connection with preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the disclosure as defined by the appended claims. Embodiments of the instant disclosure can comprise, consist of, or consist essentially of the disclosed elements and limitations of the embodiments described herein, as well as any of additional or optional ingredients, components, or limitations known or otherwise effective for use in such compositions, unless otherwise stated. [0014] For purposes of this disclosure, the term polymer may include homopolymers, copolymers (any polymer comprising two or more monomers), terpolymers, and the like, unless explicitly stated otherwise. When a polymer is referred to as comprising a monomer, the monomer present in the polymer is the polymerized form of the monomer. Likewise, when catalyst components are described as comprising neutral stable forms of the components, it is to be understood that the active form of the component is the form that reacts with the monomers to produce polymers. The new notation numbering scheme for the Periodic Table Groups is used herein as set out in CHEMICAL AND ENGINEERING NEWS, 63(5), 27 (1985).

[0015] As used herein, the term "nonwoven" or "nonwoven fabric" refers to any material made from the aggregation of fibers fabricated by methods such as, for example, spunbonding, melt blowing, thermobonding, or combinations thereof. [0016] As used herein, anisotropic behavior refers to fabrics having different properties in different directions. For example, a fabric demonstrating anisotropic elongation would have an elongation in the machine direction (MD) different from its elongation measured in the transverse direction (TD). The same fabric may also be characterized as having an asymmetric stretch. In this example, the anisotropic behavior typically has elongation in the machine direction (MD) substantially less than the elongation in the transverse direction (TD). [0017] As used herein, the term "polypropylene", "propylene polymer," or "PP" refers to homopolymers, copolymers, terpolymers, and interpolymers, comprising at least 50 wt % propylene.

[0018] As used herein, "metallocene" means one or more compounds represented by the formula Cp_mMR_nXq, wherein Cp is a cyclopentadienyl ring which may be substituted, or derivative thereof (such as indene or fluorene) which may be substituted; M is a Group 4, 5, or 6 transition metal, for example titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten; R is a substituted or unsubstituted hydrocarbyl group or hydrocarboxy group having from one to 20 carbon atoms; X may be a halide, a hydride, an alkyl group, an alkenyl group or an arylalkyl group; and m=l-3; n=0-3; q=0-3; and the sum of m+n+q is equal to the oxidation state of the transition metal. If m is 2 or 3, then any two Cp groups may be bound to one another through a bridging group T, which is typically a group 14 atom which may be substituted with one or two hydrocarbyl groups (a preferred example includes (CHs)₂-Si). If m is 1, then the Cp group may be bound to R via a bridging group T which is typically a group 14 atom which may be substituted with one or two hydrocarbyl groups (a preferred example includes (CHs)₂-Si).

[0019] Abbreviations may be used including: Me = methyl, Et = ethyl, Bu = butyl, Ph = phenyl, Cp = cyclopentadienyl, Cp* = pentamethyl cyclopentadienyl, Ind = indenyl, and Flu = fluorene.

[0020] As used herein, "metallocene catalyst system" is the product of contacting one or more metallocenes; one or more activators; and optionally, one or more support compositions. Preferred activators include alumoxanes (including methylalumoxane and modified-methylalumoxane), stoichiometric activators, ionic activators, non-coordinating anions, and the like. [0021] As used herein, molecular weight (Mn and Mw) and molecular weight distribution (MWD or Mw/Mn) are determined by gel permeation chromatography using polystyrene standards, unless otherwise specified. The GPC data were taken on a Waters 150 GPC using three Shodex mixed bed AT- 80M/S columns. The solvent used was 1,2,4 trichlorobenzene that contains 300 ppm of the antioxidant Santonox R. The run conditions were an operating temperature of 145 ⁰C, a nominal flow rate of 1.0 ml/min and a 300 microliter injection volume. Solutions for injection were typically 1.0 to 1.5 mg/ml. The columns were calibrated by running a series of narrow molecular weight polystyrene (PS) standards and recording their retention volumes. Polypropylene (PP) molecular weight values were calculated using the "universal calibration" approach and the following Mark-Houwink coefficients:

k (dL/g) a

PS 1.75 X lO-⁴ 0.67

PP 8.33 x IQ-⁵ 0.80

[0022] A third order fit is used to fit the Log (MW) vs Retention volume points. The data were taken and analyzed by Waters Millenium software.

Propylene Polymer

[0023] In an embodiment, a non-woven fabric comprises a blend or alloy of polypropylene and a random propylene copolymer, wherein the fabric is stable to sterilization by gamma radiation. The propylene polymer of the instant disclosure is typically polypropylene homopolymer, but may be a random or block copolymer of propylene and a monoolefinically unsaturated monomer X, (P-co-X) with up to about 30 wt% of X, wherein X represents vinyl acetate, or a lower Ci - C₄ alkyl acrylate or methacrylate. Blends of such propylene polymers with other polymers such as polyethylene are also included within the scope of the instant disclosure. For convenience, homopolymer PP and copolymer P-co-X are together referred to herein as "propylene polymer" PP. The PP preferably has a number average molecular weight (Mn) in the range from about 10,000 to about 1,000,000, preferably about 30,000 to about 300,000 with a melt flow index from about 0.1 to 2000 g/10 min. when measured according to ASTM D-1238 @ 230⁰C₅ 2.16 kg, more preferably about 10 to 150 g/10 min.

Random Propylene Copolymer

[0024] The random propylene copolymer of the instant non-woven fabric comprises propylene, at least one comonomer selected from ethylene, C₄-C n α- oleflns, and optionally a non-conjugated C12 diene. In a particular aspect of this embodiment, the copolymer includes ethylene-derived units in an amount ranging from a lower limit of 2%, 5%, 6%, 8%, or 10% by weight, to an upper limit of

15%, 16%, 17%, 20%, 25%, or 28% by weight.

[0025] The ethylene content is measured as follows. A thin homogeneous film is pressed according to sub-method A of ASTM D-3900. It is then mounted on a

Perkin Elmer Spectrum 2000 infrared spectrophotometer. A full spectrum is recorded using the following parameters: resolution: 4.0 cm~l, spectral range: 4500 to 450 cm~l. Ethylene content is determined by taking the ratio of the propylene band area at 1155 cm"l to the ethylene band area at 732 cm"* (C3/C2 = AR) and applying it to the following equation

Wt% Ethylene = 73.492 - 89.298X 4- 15.637X² Where X = AR/(AR+1) and AR is the peak area ratio (1155 cm"¹ / 722 cm"¹)

[0026] This embodiment will also include propylene-derived units present in the copolymer in an amount ranging from a lower limit of 72%, 75%, 80%, 83%, 84%, or 85% by weight, to an upper limit of 98%, 95%, 94%, 92%, or 90% by weight. These percentages by weight are based on the total weight of the propylene and ethylene-derived units present in the random propylene copolymer; i.e., based on the sum of weight percent propylene-derived units and weight percent ethylene-derived units being 100%. [0027] In embodiments containing a diene, the random propylene copolymer may optionally comprise < 10 wt% diene, preferably less than or equal to about 5 wt% diene, more preferably less than or equal to about 3 wt% diene, preferably from about 0.1 to about 3 wt%, more preferably from about 0.1 to about 2 wt%, and more preferably from about 0.1 to about 1 wt% diene. Suitable dienes useful as co-monomers are, for example: 1,4-hexadiene, 1,6 octadiene, 5-methyl-l,4- hexadiene, 3,7-dimethyl-l,6-octadiene, dicyclopentadiene (DCPD), ethylidiene norbornene (ENB)₃ norbornadiene, 5-vinyl-2-norbornene (VNB), and combinations thereof. The diene is most preferably ENB.

[0028] Comonomer content of discrete molecular weight ranges can be measured by Fourier Transform Infrared Spectroscopy (FTIR) in conjunction with samples collected by GPC. One such method is described in Wheeler and Willis, Applied Spectroscopy, 1993, vol. 47, pp. 1128-1130. Similar methods may be equally functional for this purpose, and are well known to those skilled in the art. [0029] Comonomer content and sequence distribution of the polymers can be measured by ¹³C nuclear magnetic resonance (¹³C NMR)₅ and such methods are well known to those skilled in the art.

[0030] In an embodiment, the random propylene copolymer has a narrow compositional distribution. In another embodiment, the random propylene copolymer has a narrow compositional distribution and a melting point as determined by DSC of from 25°C to HO⁰C. The copolymer is described as random because for a polymer comprising propylene, comonomer, and optionally diene, the number and distribution of comonomer residues is consistent with the random statistical polymerization of the monomers. In stereoblock structures, the number of block monomer residues of any one kind adjacent to one another is greater than predicted from a statistical distribution in random copolymers with a similar composition. Historical ethylene-propylene copolymers with stereoblock structure have a distribution of ethylene residues consistent with these blocky structures rather than a random statistical distribution of the monomer residues in the polymer. The intramolecular composition distribution (i.e., randomness) of the copolymer may be determined by ¹³C NMR, which locates the comonomer residues in relation to the neighbouring propylene residues. [0031] The intermolecular composition distribution of the copolymer is preferably determined by thermal fractionation in a solvent, by methods known to those skilled in the art. For purposes herein, a copolymer is considered to have a narrow compositional distribution if approximately 75% by weight, preferably 85% by weight of the copolymer, is isolated as one or two adjacent, soluble fractions with the balance of the copolymer in immediately preceding or succeeding fractions. Each of these fractions has a composition (wt% comonomer such as ethylene or other α-olefin) with a difference of no greater than 20% (relative), preferably 10% (relative), of the average weight % comonomer of the copolymer.

[0032] To produce the instant random propylene copolymer having the desired randomness and narrow composition, it is beneficial if (1) a single sited metallocene catalyst is used which allows only a single statistical mode of addition of the first and second monomer sequences and (2) the copolymer is well-mixed in a continuous flow stirred tank polymerization reactor which allows only a single polymerization environment for substantially all of the polymer chains of the copolymer.

[0033] The crystallinity of the polymers may be expressed in terms of heat of fusion. Embodiments of the instant disclosure include random propylene copolymers having a heat of fusion, as determined by differential scanning calorimetry (DSC), ranging from a lower limit of 1.0 J/g, 3 J/g, 5 J/g, or 10.0 J/g, to an upper limit of 50 J/g, 55 J/g, 60 J/g, 65 J/g, or 75 J/g. Without wishing to be bound by theory, it is believed that the polymers of embodiments of the instant disclosure have generally isotactic crystallizable propylene sequences, and the above heats of fusion are believed to be due to the melting of these crystalline segments.

[0034] The crystallinity of the polymer may also be expressed in terms of crystallinity percent. The thermal energy for the highest order of polypropylene is estimated at 189 J/g. That is, 100% crystallinity is equal to 189 J/g. Therefore, according to the aforementioned heats of fusion, the polymer has a polypropylene crystallinity within the range having an upper limit of 65%, 40%, 30%, 25%, or 20%, and a lower limit of 1%, 3%, 5%, 7%, or 8%. [0035] The level of crystallinity is also reflected in the melting point. The term "melting point," as used herein, is the highest peak among principal and secondary melting peaks as determined by DSC. In an embodiment of the instant disclosure, the polymer has a single melting point. Typically, a sample of the random propylene copolymer will show secondary melting peaks adjacent to the principal peak, which are considered together as a single melting point. The highest of these peaks is considered the melting point. The polymer preferably has a melting point by DSC ranging from an upper limit of 105⁰C, 9O⁰C, 8O⁰C, 75°C, or 70⁰C, to a lower limit of 0⁰C, 20⁰C, 25°C, 3O⁰C, 35°C, 40⁰C, or 45°C. [0036] The random propylene copolymers used in the instant disclosure have a weight average molecular weight (Mw) within the range having an upper limit of 5,000,000 g/mol, 1,000,000 g/mol, or 500,000 g/mol, and a lower limit of 10,000 g/mol, 20,000 g/mol, or 80,000 g/mol, and a molecular weight distribution Mw/Mn (MWD), sometimes referred to as a "polydispersity index" (PDI), ranging from a lower limit of 1.5, 1.8, or 2.0 to an upper limit of 40, 20, 10, 5, 4.5, 3.5, or 3.0. The Mw and MWD, as used herein, can be determined by a variety of methods, including those in U.S. Patent No. 4,540,753 to Cozewith, et al., and references cited therein, or those methods found in Verstrate et al., Macromolecules, v. 21, p. 3360 (1988), the descriptions of which are incorporated by reference herein for purposes of U.S. practice.

[0037] In another embodiment, the random propylene copolymer has a MFR, @ 230⁰C, 2.16 Kg of 400 or less, 300 or less, 200 or less, or 80 or less, as used according to ASTM D-1238, unless otherwise specified.

[0038] The propylene-based polymer can have a triad tacticity of three propylene units, as measured by ¹³C NMR of 75% or greater, 80% or greater, 82% or greater, 85% or greater, or 90% or greater. Preferred ranges include from about 50 to about 99 %, more preferably from about 60 to about 99%, more preferably from about 75 to about 99% and more preferably from about 80 to about 99%; and in other embodiments from about 60 to about 97%. Triad tacticity is determined by the methods described in U.S. Patent Application Publication 20040236042. [0039] The random propylene copolymer may be produced by any process that provides the desired polymer properties, in heterogeneous polymerization on a support, such as slurry or gas phase polymerization, or in homogeneous conditions in bulk polymerization in a medium comprising largely monomer or in solution with a solvent as diluent for the monomers. For industrial uses, continuous polymerization processes are preferred. Homogeneous polymers are often preferred in the instant disclosure. For these polymers, preferably the polymerization process is a single stage, steady state, polymerization conducted in a well-mixed continuous feed polymerization reactor. The polymerization can be conducted in solution, although other polymerization procedures such as gas phase or slurry polymerization, which fulfil the requirements of single stage polymerization and continuous feed reactors, are contemplated. [0040] The random propylene copolymer may be made advantageously by the continuous solution polymerization process described in WO 02/34795, advantageously in a single reactor and separated by liquid phase separation from an alkane solvent. Preferred methods for producing the propylene-based polymers are found in U.S. Patent Application Publication 20040236042 and U.S. Patent 6,881,800, which are incorporated by reference herein. [0041] Pyridine amine complexes, such as those described in WO03/040201 are also useful to produce the propylene-based polymers useful herein. The catalyst can involve a fluxional complex, which undergoes periodic intramolecular re-arrangement so as to provide the desired interruption of stereoregularity as in U.S. 6,559,262. The catalyst can be a stereorigid complex with mixed influence on propylene insertion, see Rieger EP 1070087. [0042] Random propylene copolymers of the instant disclosure may be produced in the presence of a chiral metallocene catalyst with an activator and optional scavenger. The use of single site catalysts is preferred to enhance the homogeneity of the polymer. As only a limited tacticity is needed, many different forms of single site catalyst may be used. Possible single site catalysts are metallocenes, such as those described in U.S. Patent No. 5,026,798, which have a single cyclopentadienyl ring, advantageously comprising alkyl group substitutions, aryl group substitutions, substituted alkyl group substitutions, and/or forming part of a polycyclic structure, and a hetero-atom, generally a nitrogen atom, but possibly also a phosphorus atom or phenoxy group connected to a group 4 transition metal, preferably titanium but possibly zirconium or hafnium. A further example is Me_SCpTiMe₃ activated with B(CF)₃ as used to produce elastomeric polypropylene with an Mn of up to 4,000,000 g/mol (See Sassmannshausen, Bochmann, Rosch, Lilge, J.Organomet. Chem. (1997) 548, 23- 28.)

[0043] Other possible single site catalysts are metallocenes which are bis cyclopentadienyl derivatives having a group transition metal, preferably hafnium or zirconium. Such metallocenes may be unbridged as in U.S. Patent No. 4,522,982 or U.S. Patent No. 5,747,621. The metallocene may be adapted for producing a polymer comprising predominantly propylene derived units as in U.S. Patent No. 5,969,070 which uses an unbridged bis(2-phenyl indenyl) zirconium dichloride to produce a homogeneous polymer having a melting point of above 79°C. The cyclopentadienyl rings may comprise alkyl substitutions, aryl substitutions, substituted alkyl groups, haloalkyl substitutions, and/or be part of polycyclic systems as described in the above U.S. Patents.

[0044] Other possible metallocenes include those in which the two cyclopentadienyl groups are connected through a bridge, generally a single atom bridge, such as a silicon or carbon atom, with a choice of alkyl groups to occupy the two remaining valencies. Such metallocenes are described in U.S. Patent No. 6,048,950 which discloses bis(indenyl)bis(dimethylsilyl) zirconium dichloride and MAO; WO 98/27154 which discloses a dimethylsilyl bridged bisindenyl hafnium dimethyl together with a non-coordinating anion activator; EP 1070087 which discloses a bridged biscyclopentadienyl catalyst which has elements of asymmetry between the two cyclopentadienyl ligands to give a polymer with elastic properties; and the metallocenes described in U.S. Patent Nos. 6,448,358 and 6,265,212.

[0045J The manner of activation of the single site catalyst can vary. Alumoxane and preferably methyl alumoxane can be used. Higher molecular weights can be obtained using non-or weakly coordinating anion activators (NCA) derived and generated in any of the ways amply described in the art, including EP277004, EP426637, and others. Activation generally is believed to involve abstraction of an anionic group such as the methyl group to form a metallocene cation, although according to some literature, zwitterions may be produced. The NCA precursor can be an ion pair of a borate or aluminate in which the precursor cation is eliminated upon activation in some manner, e.g. trityl or ammonium derivatives of tetrakis pentafluorophenyl boron (See EP277004). The NCA precursor can be a neutral compound such as a borane, which is formed into a cation by the abstraction of and incorporation of the anionic group abstracted from the metallocene (See EP426638.)

[0046] In one embodiment, the random propylene copolymer used in the instant disclosure is described in detail as the "Second Polymer Component (SPC)" in WO 00/69963, WO 00/01766, WO 99/07788, WO 02/083753, and described in further detail as the "Propylene Olefin Copolymer" in WO 00/01745, all of which are fully incorporated by reference herein for purposes of U.S. patent practice.

[0047] Commercially available random propylene polymers of the instant disclosure are available under the tradename Vistamaxx™ from ExxonMobil Chemical' Company and Versify™ from Dow Chemical Company. Suitable grades of Vistamaxx™ polymers include VMIlOO, VM1120, VM2100, VM2125, VM2210, VM2120, VM2320, VM2330, VM3000, VM6100, and VM6200.

Polymer Alloy Composition

[0048] An essentially uniform blend or alloy of the polypropylene and the random propylene copolymer, referred to herein as the copolymer alloy, is preferably made into filaments, fibers, or the like.

[0049] The polymer alloy composition may comprise from about 0.1 to 99.9 wt% of random copolymer. The polymer alloy composition preferably has an MFR ranging from about 20 to 400 g/10 min. Polypropylene is also preferably the next major component in the blend with an MFR ranging from about 10 to 2000. The polymer alloy composition may or may not be vis-cracked. Accordingly, the copolymer alloy preferably comprises polypropylene, and greater than or equal to about 0.5 wt% of the instant random propylene copolymer. Preferably, the copolymer alloy comprises at least lwt%, preferably at least 2 wt%, preferably at least 3 wt%, preferably at least 4 wt%, preferably at least 5 wt%, preferably at least 6 wt%, preferably at least 7 wt%, preferably at least 8 wt%, preferably at least 9 wt%, preferably at least 10 wt%, preferably at least 15 wt%, preferably at least 20 wt%, preferably at least 25 wt%, preferably at least 30 wt%, preferably at least 35 wt%, preferably at least 40 wt%, preferably at least 45 wt%, preferably at least 50 wt% of the random propylene copolymer, at least 60 wt% of the random propylene copolymer, at least 70 wt% of the random propylene copolymer, at least 80 wt% of the random propylene copolymer, at least 90 wt% of the random propylene copolymer, at least 95 wt% of the random propylene copolymer, based on the total amount of random propylene copolymer, polypropylene, and optionally other polymers present.

[0050] In addition to polypropylene and the random propylene copolymer, the copolymer alloy may further comprise: isotactic polypropylene, highly isotactic polypropylene (e.g., having greater than about 50% m-pentads), syndiotactic polypropylene, copolymers of propylene and ethylene and/or butene and/or hexene, polybutene, ethylene vinyl acetate, low density polyethylene (density 0.915 to less than 0.935 g/cm³) linear low density polyethylene, ultra low density polyethylene (density 0.86 to less than 0.90 g/cm³), very low density polyethylene (density 0.90 to less than 0.915 g/cm³), medium density polyethylene (density 0.935 to less than 0.945 g/cm³), high density polyethylene (density 0.945 to 0.98 g/cm³), ethylene vinyl acetate, ethylene methyl acrylate, copolymers of acrylic acid, terpolymers of ethylene acrylic acid and methyl methacrylate, zinc, magnesium or sodium ionomers, polymethylmethacrylate or any other polymers polymerizable by a high-pressure free radical process, polyvinylchloride, polybutene- 1, isotactic polybutene, ABS resins, elastomers such as ethylene- propylene rubber (EPR), vulcanized EPR, EPDM, block copolymer elastomers such as SBS, nylons (polyamides), polycarbonates, PET (polyester resins), crosslinked polyethylene, copolymers of ethylene and vinyl alcohol (EVOH), polymers of aromatic monomers such as polystyrene, poly-1 esters, graft copolymers generally, polyacrylonitrile homopolymer or copolymers, thermoplastic polyamides, polyacetal, polyvinylidine fluoride and other fluorinated elastomers, polyethylene glycols and copolymers of isobutylene and para methyl styrene, polybutadiene, polyisoprene, block copolymers, copolymers of styrene and butadiene of isoprene, hydrogenated block copolymers of styrene and butadiene (SEBS), and/or combinations thereof.

[0051] In a preferred embodiment, the alloy may be subjected to a controlled rheology (CR) process well known in the art, whereby the copolymer is visbroken into a resin having a narrower molecular weight distribution and lower average molecular weight in order to facilitate fiber spinning. The molecular weight (MW) of the visbroken alloy determines the level of melt viscosity and the ultimate desirable physical properties of the fiber. The MW of the visbroken alloy as determined by the MFR test (ASTM D1238, Condition L) may vary within a wide range from about 0.5 to about 2000 g/10 minutes, preferably between about 3 to about 400 and most preferably between about 50 to about 300. The MWD of the visbroken alloy may also vary within a wide range, but a generally narrow overall MWD is preferred for fiber applications. MWD plays a role in melt processability as well as the level and balance of physical properties achievable. The MWD of the visbroken alloy may vary from extremely narrow (as in a polydispersity, Mw/Mn, of about 2), to broad (as in a polydispersity of about 12). A polydispersity in the range of from about 2 to about 6 is preferred, with a polydispersity of about 2 to about 4 being most preferred.

[0052] Additives such as stabilizers, pigments, fillers, antioxidants, ultraviolet screening agents, nucleating agents, processing oils, and the like, may optionally be added. CR processes are described in U.S. Pat. No. 4,143,099 and are incorporated herein by reference for purposes of US patent practice.

Fibers

[0053] The copolymer alloy can be drawn to a fine diameter fiber by one of several well known in the art modifications of the basic melt-extrusion fiber process. This process consists of the steps of (1) continuously feeding the copolymer alloy to a melting screw extruder; (2) simultaneously melting and forcing the copolymer alloy through a spinneret whereby the alloy is extruded into fibers under pressure through holes that, depending upon the desired fiber product, may vary widely in number, size and shape; (3) solidifying the fibers by transferring the heat to a surrounding medium; and (4) winding of the solidified fibers onto packages. Further processing typically includes orienting the fibers by drawing it to many times its original length. Also, a variety of thermal and texturing treatments well known in the art may be employed, depending on the desired final properties of the fiber. Embodiments of the present invention copolymer alloy are drawn into fine diameter fibers at generally high drawdown speed, without the individual fibers sticking together below the crystallization point.

[0054] Although the terms of "draw-down speed" and "crystallization point" are well known among those skilled in the art, a brief explanation is provided herein in the interest of clarity. The drawdown speed is measured by extruding the polymer through a capillary at a given rate throughout, typically 0.3-1.2 g/hole/min. The take up speed of the fiber is increased until the fibers break. The maximum take up speed at which the fiber breaks is defined as the drawdown speed. For effective spinning in a fiber spinning or spunbond processes, a resin should have at least 1,000 meter/minutes of drawdown speed capability. Homopolymer and conventional random copolymer resins used in spunbond applications are processed at a drawdown speed of from about 1,000 to about 5,000 meters per minute.

[0055] The crystallization point is the point at some distance below the spinneret where the fibers solidify. Fibers made from the resin of the present invention crystallize faster than corresponding conventional random copolymers (i.e. random copolymers having the same ethylene content.) This characteristic in combination of their overall high ethylene content results in the making of fabrics having exceptional balance of softness, spinning capability, and physical properties. Fibers prepared from embodiments of the present invention copolymer alloy exhibit excellent characteristics including tensile strength comparable to that of polypropylene. Moreover the fibers of the instant disclosure are stable to gamma radiation sterilization.

Spunbonded Fabrics

[0056] In an embodiment, an article or fabric of woven or non-woven fibers comprises the instant copolymer alloy, which article is sterilϊzable by gamma radiation, preferably from a cobalt-60 source. Non- woven fabrics of PP may have a carded fiber structure or comprise a mat in which the fibers or filaments are distributed in a. random array. The fibers may be bonded with a bonding agent such as a polymer, or the fibers may be thermally bonded without a bonding agent. The fabric may be formed by any one of numerous known processes including hydroentanglement, spun-lace techniques, air laying, melt-blowing, batt drawing, stitchbonding, and/or the like, depending upon the end use of the fabric. [0057] A preferred embodiment includes the use of the instant copolymer alloy in the making of spunbonded fabrics. Conventional spunbond processes are illustrated in U.S. Pat. Nos. 3,825,379; 4,813,864; 4,405,297; 4,208,366; and 4,334,340 all hereby incorporated by reference for purposes of US patent practice. The spinbonding process is one which 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, and often times to a second layer such as a melt blown layer, often by a heated calender roll, or addition of a binder. An overview of spunbonding may be obtained from L. C. Wadsworth and B. C. Goswami, Nonwoven Fabrics: "Spunbonded and Melt Blown Processes" proceedings Eight Annual Nonwovens Workshop, JuI. 30-Aug. 3, 1990, sponsored by TANDEC, University of Tennessee, Knoxville, Tenn.

[0058] A typical spunbond process consists of a continuous filament extrusion, followed by drawing, web formation by the use of some type of ejector, and bonding of the web. Optionally, the copolymer alloy may be visbroken using peroxide into a resin having a narrower molecular weight distribution and about 35 MFR. During this step the polymer granules may be converted into pellets. The copolymer alloy resin is then fed into an extruder. In the extruder, the pellets simultaneously are melted and forced through the system by a heating melting screw. At the end of the screw, a spinning pump meters the melted polymer through a filter to a spinneret where the melted polymer is extruded under pressure through capillaries, at a rate of 0.3-1.0 grams per hole per minute. The spinneret contains a few hundred capillaries, measuring 0.4-0.6 mm in diameter. The polymer is melted at about 3O°C-8O°C above its melting point to achieve sufficiently low melt viscosity for extrusion. The fibers exiting the spinneret are quenched and drawn into fine fibers measuring 10-40 microns in diameter by cold, 1000-6000 m/minutes velocity air jets. The solidified fiber is laid randomly on a moving belt to form a random netlike structure known in the art as web. After web formation, the web may be bonded to achieve its final strength using a heated textile calender known in the art as thermobond calender. The calender consists of two heated steel rolls; one roll is plain ant the other bears a pattern of raised points. The web is conveyed to the calender wherein a fabric is formed by pressing the web between the rolls at a bonding temperature up to about 150⁰C. [0059] The aforementioned fabric embodiments may be made from fibers of about 15 to about 30 microns in diameter. The fabric basis weight may vary from about 20 to about 1,500 g/m².

Gamma-Radiation Sterilization

[0060] Gamma-radiation sterilization is preformed on the instant non-woven fabrics in a manner known in the art. The non-woven fabrics comprising the instant copolymer alloys are stable to gamma radiation sterilization, as compared to non-woven fabrics consisting essentially of polypropylene. Polypropylene tends to disintegrate when the total dosage of gamma radiation exceeds about 5 Mrad. For sterilization of the fibers, a total dosage in the range from about 0.5 to 15 Mrad, more preferably from about 1.5 to 5 Mrad, is generally delivered and accumulated by the fibers over a period of from about 1 minute to about 12 hrs, the most preferred rate being about 0.5 Mrad/hr. The physical properties of the instant non-woven fibers show little degradation after gamma radiation sterilization.

[0061] In an embodiment, the difference in tensile strength of a non-woven fabric comprising the instant random propylene copolymer, determined before and after gamma radiation sterilization, is less than a 30%, preferably less than a 20%, more preferably less than 10 %.

[0062] In an embodiment, the difference in the load at break of a non-woven fabric comprising the instant random propylene copolymer, determined before and after gamma radiation sterilization, is less than a 30%, preferably less than a 20%, more preferably less than 10 %. [0063] In an embodiment, the difference in the elongation at break of a non- woven fabric comprising the instant random propylene copolymer, determined before and after gamma radiation sterilization, is less than a 30%, preferably less than a 20%, more preferably less than 10 %.

[0064] In, an embodiment, the difference in the maximum mechanical hysteresis of a non-woven fabric comprising the instant random propylene copolymer, determined before and after gamma radiation sterilization, is less than a 30%, preferably less than a 20%, more preferably less than 10 %. [0065] In an embodiment, the difference in the permanent set hysteresis of a non-woven fabric comprising the instant random propylene copolymer, determined before and after gamma radiation sterilization, is less than a 30%, preferably less than a 20%, more preferably less than 10 %.

[0066] The difference in tensile strength determined before and after gamma radiation sterilization TS, is determined by subtracting the tensile strength determined on a sample of the fabric after gamma-radiation sterilization (TS-after) from the tensile strength determined on a sample of the fabric prior to gamma- radiation sterilization, (TS-prior) and dividing this difference by the value of the tensile strength determined on the sample of the fabric prior to gamma-radiation sterilization (TS-prior).

TS = [(TS-prior) - (TS-after)] ÷ (TS-prior) * 100%.

[0067] The difference in other disclosed properties before and after gamma- radiation sterilization (i.e., load at break, elongation at break, maximum mechanical hysteresis, and permanent set hysteresis) are determined in the above manner.

[0068] In an embodiment, a sterilized article comprising the instant non- woven fabric having any or all of the above disclosed properties, may be produced by irradiating an article comprising the instant non-woven fabric with gamma radiation at an exposure level, and for a period of time suitable to produce the sterilized article. EXAMPLES

[0069] In order to provide a better understanding of the instant disclosure, including representative advantages thereof, particular embodiments of fabrics comprising the instant random propylene copolymer are provided. These examples are not in any way intended as a limitation on the scope of the instant disclosure.

[0070] Samples of nonwoven fabrics were prepared using Vistamaxx™

VM2120, VM2125, VM2320, and VM2330 were prepared using Reicofϊl® machines. Details around Reicofil® machines are given below.

[0071] Table 1 illustrates the key processing parameters of either SPE 2120 or

SPE 2125 on a 1 m wide Reicofil ® 2 pilot line at the Textile and Nonwovens

Development Center (TANDEC) located at the University of Tennessee.

Table 1

Typical process conditions for SPE 2120 or SPE 2125 on Reicofil 2 spunbond process.

[0072] The extruder amperage (related to the screw torque and viscous shear heating), die pressure and fiber diameter of the spunbond fabric at two output rates (0.2 gram per die hole per minute, or ghm and 0.4 ghm) were compared. The fiber diameter at 0.4 ghm was higher than the 0.2 ghm because of the limited drawing air capability of the system.

[0073] Table 2 compares the key processing parameter of the two 80 MFR specialty elastomers against the polypropylene spunbond resin on a 1 m Reicofil 3 pilot line under the same output rate. Table 2

Comparison of process conditions for SPE 2120, SPE 2125 and PP in Reicofil 3 spunbond process.

Note: PP 3155. is a 36 MFR polypropylene homopolymer.

[0074] The 36 MFR PP 3155 resin is typically quenched with a 20 ⁰C air and processed at 240 ⁰C melt temperature. The higher MFR SPE resins were successfully processed at a lower melt temperature using a 14 ⁰C (57 ⁰F) quench air temperature. The cabin pressure was a measure of quench airflow rate and consequently related to the draw force imposed on the fiber. The system had a maximum cabin pressure of approximately 5000 Pa. As seen in Table 3, the SPE resins could be drawn at the same cabin pressure as the PP homopolymer without fiber breaks.

[0075] Spunbond fabrics comprising 50 gsm of VM2125 and VM2120, were prepared using a Reicofil III spunbond pilot line manufactured by Reifenhauser GmbH &. Co. The Reicofil III spunbond system is a third generation of Reicofil spunbond system used widely by the polyolefm spunbond producers. It used a slot draw system, where a curtain of fibers was quenched in a quenching chamber as the fibers exited from the spinneret. Below the quenching chamber, the fibers and the quenching air entered into a narrow slot. The gap of the slot ranged from 15 to 30 mm. The quenching air velocity increased as it entered the slot because of the reduced cross sectional area. The high velocity air drew the fibers and increases the fiber velocity, producing a curtain of fine fibers. The fibers were then deposited onto a moving belt to form a nonwoven web. The web was then passes through a calendar to bond the fibers and form a nonwoven web. [0076] The spunbond line used to produce the spunbond fabric samples was a one meter wide Reifocil III spunbond system located in the laboratory of Rifenhauser GmbH & Co., Troisdorf, Germany. The propylene copolymer was fed to the spunbond extruder hopper using conventional material transfer system. The extruder melted the polymer and delivered the polymer to the melt pump, which in turn metered the polymer to the spuhbond die. The die had a total of approximately 5600 capillaries (die holes) over an approximately 1 meter wide die face. The melt temperature was set at 200 to 220 ⁰C and quench air temperature at 14°C. The output rate ranged from 164 to 202 kg/hr and fabric basis weight ranges from 35 to 100 gram/m . The take up speed was used to adjust the basis weight of the fabric. The cabin pressure, which is related to the quenching air flow rate, ranged from 4000 to 5000 Pa and was used to control the draw down of the fibers. Most of the fibers produced had a diameter of 18 to 22 microns. The fabric was bonded at 90 to 100 ⁰C calendar temperature. The fabric so produced was soft, elastic and has good web uniformity.

[0077] The melt blown system used to produce the sample fabric was based on design which is used widely by the nonwoven industry. It utilized a single row of die holes to produce fibers rather than a multi-row die hole used by some equipment manufacturers.

[0078] The fabrics were produced on a 0.5 meter wide melt blown line manufactured by Rifenhauser GmbH & Co., Troisdorf, Germany. The line was located at Textile and Nonwovens Development Center (TANDEC) at the University of Tennessee. The process was very similar to the condition when conventional isotactic polypropylene is used. The melt temperature and process air temperatures were set at 230 to 250 ⁰C. The die-to-collector distance (DCD) was set at 10 to 15". The process air flow rate was set at maximum before the "fly" (loose fibers) was generated. The web has sufficient integrity and no additional thermal bonding was required.

[0079] Tensile testing (load at break and elongation at break) were conducted on 2" x 10" (250 mm x 50 mm) specimens with a gage length of 8" (200 mm) and at crosshead speed of 4"/min (100 mm/min). An average of 3 test for each sample were conducted. [0080] Hysteresis testing was preformed on 2" x 7" specimens with a gage length of 3" at a crosshead speed of 20"/min. An average of 3 specimens each were tested using 2 cycle hysteresis to 100% elongation. The data are shown in Figures 1, 2, 3, and 4 and in Table 3.

Table 3. Hysteresis of VM2125 Spunbond Fabric

* Original Length = 1 in.

300 % Extension. Hold 10 min. Measure after 10 min

* Original Length = lin. Procedure:

Step 1 300 % Extension;

Step 2 Hold 10 min.

Step 3 Measure after 10 min.

Table 4 Hysteresis of VM2125 Spunbond Fabric

Table 5 Hysteresis of VM2330

Table 7 Hysteresis of VM2120

[0081] All patents and patent applications, test procedures (such as ASTM methods), and other documents cited herein, including priority documents, are fully incorporated by reference for purposes of U.S. practice. When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated.

Claims

We claim:

L A non-woven fabric comprising polypropylene and a random propylene copolymer comprising greater than or equal to 50 wt% propylene, wherein a difference in tensile strength of the fabric is less than or equal to about 40%, when the difference is determined before and after gamma-radiation sterilization of the fabric at between about 0.5 and 15 Mrads.

2. The fabric according to Claim 1 wherein the random propylene copolymer has: a propylene content of 75 wt% or greater; a melting point < .about 105⁰C; and a heat of fusion < about 75 J/g.