Porous polytetrafluoroethylene membrane having a macrotexture surface and method for producing the same

Abstract

To provide compositions and methods directed to the production of single-layer, high-porosity, free-standing polytetrafluoroethylene (PTFE) membranes having macro-textured surfaces.SOLUTION: The macro-textured surfaces are due to the presence of macroscopic nodal aggregates within the membrane that are connected by fibrils. The membranes have high porosity, high airflow, and a bulk density less than 1.0 g / cm3. Articles comprising the high-porosity, single-layer PTFE membranes are also provided.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention generally relates to a single-layer self-supporting highly porous polytetrafluoroethylene (PTFE) membrane having a macrotexture surface resulting from the presence of long strands of macroscopic node aggregates.

Background Art

[0002] Conventional methods for manufacturing expanded PTFE (ePTFE) layers are described in Gore's U.S. Patent No. 3,953,566. In the method described therein, a PTFE paste is formed by combining a PTFE resin and a lubricant. Next, the PTFE paste is extruded. After removing the lubricant from the extruded paste, the PTFE article is expanded to form a porous and high-strength PTFE article. The expanded PTFE article is characterized by a porous open microstructure having nodes interconnected by fibrils.

[0003] ePTFE articles having various microstructures of nodes and fibrils are known in the art. Some such ePTFE articles are described, for example, in Bacino's U.S. Patent No. 4,902,423, Branca et al.'s U.S. Patent No. 5,814,405, Bacino's U.S. Patent No. 5,476,589, and Ruefer et al.'s U.S. Patent No. 6,342,294. However, ePTFE articles having a macrostructure are not described in these documents.

[0004] U.S. Patent Publication No. 2016 / 0367947A1 by Hollenbaugh et al. discloses the preparation of an asymmetric polytetrafluoroethylene composite material having a macrotexture surface. U.S. Patent No. 7,306,841 by Ruefer et al. discloses an asymmetric polytetrafluoroethylene composite material having a plurality of node aggregates separated by relatively long fibrils. In both documents, the disclosed articles are structurally asymmetric composite materials in the z-axis (e.g., having different densities, microstructures, etc.) because the asymmetric composite materials are produced by stacking and co-extruding laminates comprising PTFE tapes having different stretching properties.

[0005] Applications such as scaffolds for in-tissue growth, scaffolds for seeding materials such as prokaryotic and eukaryotic cells, spores and seeds, and high airflow filtration applications may benefit from a single-layer self-supporting highly porous stretched polytetrafluoroethylene membrane having a macrotexture surface / feature. Thus, there is a need for such materials. SUMMARY OF THE INVENTION

[0006] A single-layer self-supporting highly porous polytetrafluoroethylene (PTFE) membrane having a macrotexture surface comprising long strands of macroscopic node aggregates connected by a plurality of long fibrils is provided. A method of manufacturing the same is also provided, along with an article comprising the membrane of the present invention.

[0007] According to one aspect (the "first aspect"), the porous polytetrafluoroethylene (PTFE) membrane has a) a first surface and a second surface, b) a thickness of at least 25 μm, c) a bulk density of 1.0 g / cm 3 the following bulk density, d) a porosity of at least 50%, e) about 2.99 cm 2Measured across the surface area under a differential pressure of 12 millibars (1.2 kPa) with an air flow rate of at least 200 L / h (L / h), and f) including a macrotexture surface on the first or second surface resulting from the presence of a plurality of spaced-apart macroscopic node aggregates within the porous membrane, where adjacent macroscopic node aggregates are connected by a population of long PTFE fibrils, and the macroscopic node aggregates include a plurality of high-density PTFE nodes having a density in the range of 2.0 g / mol to 2.2 g / mol, where i) the average distance between the macroscopic node aggregates is at least 30 μm, ii) the upper quartile average distance between the macroscopic node aggregates is at least 100 μm, iii) the average width of the spaced-apart macroscopic node aggregates is in the range of 10 μm to 200 μm, where at least a portion of the plurality of spaced-apart macroscopic node aggregates extends from the first surface to the second surface of the porous PTFE membrane, and the PTFE membrane is single-layered and self-supporting.

[0008] According to another aspect (Aspect "2"), in addition to Aspect 1, many portions of the macroscopic node aggregates extend from the first surface of the porous PTFE membrane to the second surface of the porous PTFE membrane.

[0009] According to another aspect (Aspect "3"), in addition to any of the preceding aspects, the macroscopic node aggregates have a density lower than the density of the PTFE nodes.

[0010] According to another aspect (Aspect "4"), in addition to any of the above aspects, the macroscopic node aggregates form strands on the first or second surface of the porous PTFE membrane having a length of at least 0.5 cm.

[0011] According to another aspect (Aspect "5"), in addition to Aspect 4, the strands are formed from one or more microscopic gaps connected by a population of short PTFE fibrils having a length of less than 120 μm.

[0012] According to another aspect (Aspect "6"), in addition to the preceding aspects, the porous PTFE membrane is substantially symmetric from the first surface to the second surface.

[0013] According to another aspect (Aspect "7"), the article includes a porous PTFE membrane of any of the preceding aspects.

[0014] According to another aspect (Aspect "8"), in addition to Aspect 7, the article is a vent, a filter, an implantable medical device, a scaffold for in - tissue growth, a scaffold for growing prokaryotic or eukaryotic cells, a scaffold for growing spores, a scaffold for growing plants, or a garment.

[0015] According to another aspect (Aspect "10"), a method of manufacturing a single - layer self - standing porous polytetrafluoroethylene membrane having a macrotexture surface includes: a) providing a first layer including a first PTFE membrane having a matrix tensile strength of less than 800 pounds per square inch (psi) (5.52 MPa) in both the machine direction and the transverse direction, and ii) a second layer including a second PTFE membrane having a matrix tensile strength of more than 800 psi (5.52 MPa) in both the machine direction and the transverse direction; b) stacking the first PTFE membrane on top of the second PTFE membrane; c) applying an appropriate amount of pressure, heat, or a combination thereof to non - permanently bond the first PTFE membrane to the second PTFE membrane to form a laminated product; d) stretching the laminated product at least once in the machine direction and at least once in the transverse direction; e) separating the second layer from the first layer, where the first layer is a single - layer porous PTFE membrane; f) optionally, subjecting the single - layer porous PTFE membrane to at least one additional stretching step; and g) subjecting the single - layer porous PTFE membrane to at least one heat treatment under appropriate conditions to at least partially sinter the single - layer porous PTFE membrane.

[0016] According to another aspect (Aspect "10"), in addition to any of the preceding aspects, the stretching step d) includes a first machine - direction stretch before a first transverse - direction stretch.

[0017] According to another aspect (Aspect "11"), in addition to Aspect 10, the first machine direction stretch includes a stretch ratio of 1.1:1 to 1.7:1.

[0018] According to another aspect (Aspect "12"), in addition to any preceding aspect, step d) or step e) includes biaxial stretching, uniaxial stretching, radial stretching, or any combination thereof.

[0019] According to another aspect (Aspect "13"), in addition to any preceding aspect, the biaxial stretching is sequential biaxial stretching, simultaneous biaxial stretching, or a combination thereof.

[0020] According to another aspect (Aspect "14"), in addition to any preceding aspect, at least one additional stretching step in step f) is biaxial stretching, uniaxial stretching, radial stretching, or any combination thereof.

[0021] According to another aspect (Aspect "15"), in addition to the preceding aspect, the carrier layer is subjected to at least one uniaxial, biaxial, or radial stretching step before stacking the carrier layer on the precursor layer.

[0022] According to another aspect (Aspect "16"), in addition to the preceding aspect, the heat treatment includes a temperature of 345°C to 390°C within 10 minutes.

[0023] According to another aspect (Aspect "17"), in addition to the preceding aspect, the method further includes densifying a single-layer porous PTFE membrane.

[0024] According to another aspect (Aspect "18"), in addition to Aspect 17, the densification is performed before, during, or after at least one heat treatment.

[0025] According to another aspect (Aspect "19"), in addition to the preceding aspect, the method further includes contacting the heat-treated single-layer porous membrane with at least one coating composition.

[0026] According to another aspect (Aspect "20"), in addition to the preceding aspects, this method can further include contacting the heat-treated single-layer porous membrane with a surface modification treatment agent.

[0027] According to another aspect (Aspect "21"), in addition to the preceding aspects, this method further includes laminating or bonding the heat-treated single-layer porous membrane to at least one additional material.

[0028] According to another aspect (Aspect "22"), in addition to Aspect 21, the at least one additional material is not PTFE.

Brief Description of the Drawings

[0029] The accompanying drawings are included to provide a further understanding of the present disclosure, incorporated herein, constituting a part thereof, showing embodiments, and together with the description, serving to explain the principles of the present disclosure.

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[0043] Those skilled in the art will readily understand that various aspects of the present disclosure can be implemented by any number of methods and apparatuses configured to perform the intended functions. The accompanying drawings referred to herein are not necessarily drawn to scale and may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, it should be noted that the drawings should not be construed as limiting.

[0044] The present disclosure is directed to a single-layer self-supporting highly porous polytetrafluoroethylene (PTFE) membrane having a macrotexture surface formed from optically observable node macrostructures (e.g., macroscopic node aggregates). The plurality of macroscopic node aggregates extend across the entire thickness of the membrane (e.g., in the z-direction) and are connected by a plurality of fibrils (i.e., a population of "long" fibrils). It should be understood that the terms "porous PTFE membrane" and "ePTFE membrane" are used interchangeably herein.

[0045] As used herein, the terms "comprises," "comprise," "comprising," "includes," "include," "including," "has," "having," "contains," "contain," or "containing" are intended to cover non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that includes a list of elements is not necessarily limited to only those elements, but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0046] The subordinate clause "consisting of" excludes elements, steps or components that are not specifically identified. If such a clause is used in the claims, such language typically cuts off the claims to exclude materials other than those recited, except for ordinary associated impurities. When the phrase "consisting of" is included in a clause of the body of the claims, rather than immediately following the preamble, it limits only the elements recited in that clause, and other elements are not excluded from the claims as a whole.

[0047] When a quantity, concentration or other value or parameter is given as a range, a preferred range, or a list of preferred values for an upper limit and a preferred value for a lower limit, it is to be understood as specifically disclosing all ranges formed from any pair of any upper limit value or preferred value and any lower limit value or preferred value, whether or not the ranges are individually disclosed. When a numerical range is recited herein, unless otherwise specified, the range is intended to include its endpoints and all integers and fractions within the range.

[0048] As used herein, the term "optically observable" is intended to mean that an object can be observed using an optical microscope and / or with the naked eye.

[0049] As used herein, the term "on" is intended to mean that an element such as a polytetrafluoroethylene (PTFE) film is either directly present on another element or that intervening elements may also be present.

[0050] As used herein, the term "biaxial" or "biaxially oriented" is intended to describe a polymer, film, preform or article that has been stretched in at least two directions, either simultaneously or sequentially. The relative "balance" of a biaxially oriented film can be described using the ratio of the matrix tensile strength (MTS) in two orthogonal directions (i.e., longitudinal / machine direction vs. transverse direction, x / y plane). A balanced film typically exhibits an MTS ratio of about 2:1 or less.

[0051] As used herein, the term "lubricant" is intended to describe a non-compressible fluid that is not a solvent for the polymer under the processing conditions, and in some embodiments consists of a processing aid. The fluid-polymer surface interactions are such that a homogeneous mixture can be formed.

[0052] As used herein, the term "wet state" is intended to describe a PTFE membrane that has not been dried to remove the lubricant.

[0053] As used herein, the term "dry state" is intended to describe a PTFE membrane that has been dried to remove the lubricant.

[0054] As used herein, "micro powder PTFE" is intended to indicate that the PTFE resin was prepared by an aqueous dispersion polymerization technique.

[0055] As used herein, the x, y, or z directions are intended to refer to a Cartesian 3D coordinate system. When referring to an extended membrane, the "x direction" and the "y direction" each mean to indicate the longitudinal direction (i.e., the machine) and the transverse direction, respectively. The term "z direction" is intended to refer to a direction perpendicular to the plane formed by the orthogonal x and y directions. The z direction can be used to describe features related to the thickness of the biaxially oriented membrane.

[0056] As used herein, the term "self-supporting" is intended to refer to the highly porous single-layer PTFE membrane of the present disclosure that has sufficient mechanical strength so as not to require an additional structural support (e.g., a backing or support layer) for use in a desired application.

[0057] As used herein, the term "average engineering strain rate" is the average strain rate of the material over the entire drawing time, assuming either a constant or accelerating rate during drawing. Specifically, the strain rate is defined as the additional strain induced in the material divided by the total time during which the strain was induced (% / sec). [Formula 1] [Final length (lf) - Initial length (lo)] / Initial length (lo) = Strain ε Strain rate = Strain ε / Time (t)

[0058] As used herein, the term "macrotexture surface" refers to an optically observable macroscopic node aggregate having the appearance of macroscopic strands extending on the surface of the porous PTFE membrane disclosed herein.

[0059] As used herein, the term "highly porous" refers to the relative porosity of the porous PTFE membranes described herein, where the membrane has a porosity of at least 50%. The porous PTFE membranes described herein have a porosity of at least about 50%, at least about 60%, at least about 70%, at least about 75%, at least about 80% or at least about 85%. Due to the large pore size and high porosity, the porous PTFE membranes described herein have a high ATEQ air flow rate. In some embodiments, the porous PTFE membrane has an ATEQ air flow rate of at least 200 L / h, at least 300 L / h, about 300 L / h to about 5000 L / h, about 300 L / h to about 2000 L / h, about 300 L / h to about 1700 L / h or about 300 L / h to about 1500 L / h (using a surface area of about 2.99 cm 2 and a differential pressure of 12 mbar (1.2 kPa)).

[0060] As used herein, the terms "macroscopic node aggregates" and "node macrostructures" are used interchangeably to refer to macroscopic structures within a porous PTFE membrane formed from a plurality of PTFE nodes having (1) a density of from about 2.0 g / cc to about 2.2 g / cc aggregated into a macroscopic structure having an average density less than the average density of the individual PTFE nodes, (2) an average width within the ranges described herein, and (3) an average length within the ranges described herein. FIG. 10 provides a view showing elements of a cross-sectional view of a porous PTFE membrane described herein. The cross-section (1000) of the membrane shows the presence of a plurality of macroscopic node aggregates (1020) spanning the width (1010) of the membrane. Many portions of the macroscopic node aggregates of the porous PTFE membrane extend from the first membrane surface through the z-axis (width; 1010) to the second membrane surface (opposite side of the membrane) and are connected by a plurality of long fibrils (1040). The nodes within the node aggregates are closely spaced and can be connected by a plurality of shorter fibrils (shorter as compared to the longer fibrils that connect two or more macroscopic node aggregates).

[0061] In some embodiments, the porous PTFE membrane has some node structures that do not meet the size and length parameters shown herein for macroscopic node aggregates. In some embodiments, many portions of the macroscopic node aggregates extend across the entire thickness of the membrane. As used herein, the term "many portions of the macroscopic node aggregates" means that more than about 50% of the amount of the macroscopic node aggregates (as defined herein) extends across the entire thickness of the porous PTFE membrane, at least about 60%, at least about 70%, at least about 80%, at least about 90% or at least about 95% of the porous PTFE membrane.

[0062] The long strands of macroscopic node aggregates are optically observable on the surface of the porous PTFE membrane. In some embodiments, the macroscopic strands can include narrow gaps / cracks perpendicular to the length of the strand, where multiple short PTFE fibrils bridge the narrow gaps, connecting the macroscopic node aggregates to the extended strands, which are optically observable as a macrotexture surface feature of the porous PTFE membrane.

[0063] Adjacent microscopic node aggregates are separated by various distances. Qualitative image analysis (QIA) (a method described below) was used to generate a distribution of the distance measurements. A statistical analysis was performed on the distribution. The QIA distance data was not normally distributed. Therefore, both the overall average distance (based on all the distances measured) and the average distance based on the upper quartile of the distribution (i.e., excluding the lower 75% of the data points) were determined for each membrane sample to better characterize the macroscopic features. The (overall) average distance (1030) between macroscopic node aggregates (measured by the qualitative image analysis (QIA) method described herein) is at least about 30 μm or at least about 40 μm. In one aspect, the average distance between macroscopic node aggregates is from about 30 μm to about 1000 μm, from about 40 μm to about 500 μm, or from about 40 μm to about 300 μm. The average distance of the upper quartile (upper 25%) between macroscopic node aggregates is at least about 100 μm, from about 100 μm to about 3 mm, from about 100 μm to about 2000 μm, or from about 100 μm to about 1000 μm.

[0064] In another embodiment, the plurality of long fibrils (1040) connecting the macroscopic node aggregates have an average length of from about 30 μm to about 3 mm, preferably from about 100 μm to about 3 mm, from about 500 μm to about 2 mm, and / or from about 500 μm to about 1 mm.

[0065] The average width of the macroscopic node aggregates (1020) can range from about 10 μm to about 200 μm, from about 10 μm to about 100 μm, or from about 10 μm to about 50 μm. The macroscopic node aggregates characteristically form long strands along the x-axis or y-axis (depending on the selected stretching procedure) and can be optically viewed as elongated strands on the surface of the porous PTFE membrane (see, for example, FIGS. 7A and 7B). In some embodiments, the average length of the node aggregate strands (optically observed) is at least about 0.5 cm, at least about 1.0 cm, or at least about 1.5 cm. In some embodiments, the macroscopic strands can include microscopic gaps or cracks (typically perpendicular to the direction of the strand) connected by a plurality of short fibrils (see, for example, FIG. 7C). These microscopic cracks / gaps typically do not alter the optical appearance of the strand and are not considered when describing the average length of the macroscopic strand. The population of short fibrils (if present) connecting the microscopic gaps / cracks typically has a length of about 120 μm or less, a length of about 1 μm to about 50 μm, or a length of about 5 μm to about 40 μm.

[0066] As used herein, the terms "precursor tape", "precursor membrane", "PTFE precursor tape", and "precursor layer" are used interchangeably and refer to the initial PTFE membrane that occupies a majority of the mass of the PTFE membranes of the present disclosure. The precursor membrane is typically a PTFE membrane designed to be highly extensible (i.e., a weaker membrane) relative to the stretching properties of the PTFE carrier layer membrane (i.e., a stronger membrane). The precursor membrane begins as an extruded and calendered single-layer membrane or is formed from a plurality of membranes that are stacked and then calendered under conditions sufficient to durably bond them into a coherent single membrane suitable for use in the methods of the present disclosure. The precursor membrane is relatively weak and is designed to have a matrix tensile strength (MTS) of less than 800 psi (< about 5.52 MPa) in both the machine direction (MD) and the transverse direction (TD).

[0067] As used herein, the terms "carrier layer", "carrier film", "PTFE carrier film", and "carrier tape" are used interchangeably and refer to a stretchable and fibrillatable polytetrafluoroethylene (i.e., not melt processable) film that is reversibly bonded (i.e., removable) to a PTFE precursor film. The carrier film is designed to be much stronger than the precursor film and has a matrix tensile strength (MTS) greater than 800 psi (> about 5.52 MPa) in both the machine direction (MD) and the transverse direction (TD), at least 1000 psi (about 6.89 MPa), at least 1100 psi (about 7.58 MPa), or at least 1500 psi (about 10.34 MPa).

[0068] In some embodiments, the porous PTFE membrane is substantially symmetric. As used herein, the phrase "substantially symmetric membrane" refers to a membrane having similar structural properties (e.g., density, porosity, and micro / macro structure) from the first side of the membrane to the second (opposite) side of the membrane (i.e., through the z-axis). This is in contrast to stretched PTFE membranes or PTFE composites (e.g., open-tite (OT) membranes, etc.) called "asymmetric membranes" that have very different structural properties / characteristics on opposite sides of the membrane or composite material.

[0069] In some embodiments, the relative density across the thickness (z-direction) of the membrane does not substantially change. Thus, the porous PTFE membrane is substantially symmetric across the thickness of the membrane. In at least one embodiment, the porous PTFE membrane is a substantially symmetric single-layer membrane that includes a relative change in density, porosity, and / or pore size (first side relative to the opposite side of the membrane) of about 20% or less, about 10% or less, about 5% or less, or about 1% or less across the entire thickness (z-axis), and thus may be referred to herein as a "symmetric" membrane.

[0070] The film of the present invention has an average thickness of at least about 25 μm, at least about 50 μm, at least about 100 μm, at least about 250 μm, at least about 500 μm, at least about 750 μm, at least about 1000 μm, or from about 50 μm to about 3 mm.

[0071] Furthermore, the porous PTFE film has a bulk density of about 1.0 g / cm 3 or less, about 0.6 g / cm 3 or less, about 0.5 g / cm 3 or less or about 0.4 g / cm 3 or less.

[0072] In some embodiments, composites and / or laminates comprising at least one porous PTFE film are also provided. In other embodiments, the porous PTFE film can be coated, without limitation, with at least one coating composition such as a polymer or a biological coating so that the porous PTFE film remains porous or becomes non-porous. The coating composition can be applied to the porous PTFE film by any conventional coating method such as solvent coating, spray coating, spin coating, vapor deposition, atomic layer deposition (ALD), dip coating or combinations thereof. Further, the coating can be applied to the porous PTFE film by applying heat and pressure between sheets of components such as fluorinated ethylene propylene (FEP), polyfluoroacrylate (PFA) and silicone, without limitation.

[0073] In some embodiments, the coating composition occupies or fills at least a portion of the voids through the thickness of the porous PTFE membrane. The term "voids" is intended to indicate the space between the nodes and fibrils. Suitable polymers and / or biological coatings that can be coated on and / or absorbed by the porous PTFE membrane include, but are not limited to, polyester, polystyrene, polyamide, polyphthalamide, polyamideimide, polycarbonate, polyethersulfone, polysulfone, polyphenylene sulfide, liquid crystalline polymer, polyether ketone, polyether ether ketone, polysiloxane, epoxy, polyurethane, polyimide, polyetherimide, polyacrylate, polyparaxylylene, a terpolymer of tetrafluoroethylene (TFE), VDF (vinylidene fluoride), and HFP (hexafluoropropylene), a copolymer of tetrafluoroethylene (TFE) and perfluoroalkyl vinyl ether (PAVE), a copolymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole, perfluoroalkyl vinyl ether, perfluoroalkyl ether, polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE), polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), polyvinyl alcohol (PVA), CBAS® / heparin coating (commercially available from W.L. Gore & Associates, Inc.), antibacterial agents, antibodies, pharmaceuticals, biological entities, angiogenesis stimulants, and combinations thereof. The amount of coating applied to and / or within the porous PTFE membrane depends on the desired application.

[0074] In some embodiments, the porous PTFE membrane can be surface modified by applying at least one of chemical treatment, plasma treatment, or laser treatment.

[0075] In some embodiments, articles comprising one or more porous PTFE membranes are also provided. In at least one embodiment, the article is a vent, a filter, a garment, an implantable medical device, a scaffold for tissue ingrowth, a scaffold for growing prokaryotic or eukaryotic cells, a scaffold for growing spores, a scaffold for growing plants.

[0076] Method for manufacturing a porous PTFE membrane

[0077] The porous PTFE membrane is formed by a method in which at least two PTFE membranes or tapes are stacked (FIG. 9; 930) and then calendared to form a laminated composite material (FIG. 9; 900). The laminated composite material includes at least one first PTFE membrane (PTFE precursor membrane; FIG. 9; 910) that is reversibly adhered to at least one second PTFE membrane or tape (FIG. 9; 920) that is a PTFE carrier membrane or tape. The PTFE precursor membrane is designed to be mechanically weaker and more extensible than the PTFE carrier membrane. In other words, the PTFE carrier membrane is mechanically stronger and less extensible than the PTFE precursor membrane.

[0078] Next, the laminated composite material is biaxially co-stretched (sequentially and / or simultaneously (FIG. 9; 940)). After biaxial co-stretching (FIG. 9; 950), the PTFE carrier membrane layer is removed. The single-layer porous PTFE membrane remaining after removal of the PTFE carrier membrane (i.e., derived from the PTFE precursor membrane layer) can optionally be subjected to at least one additional stretching step (FIG. 9; 960). The stretching can be in a uniaxial, radial, biaxial, or any combination thereof. In some embodiments, the highly porous single-layer PTFE membrane is subjected to at least one additional stretching step (FIG. 9; 960) after separating the PTFE precursor membrane from the PTFE carrier membrane layer. The additional biaxial stretching steps can be sequential or simultaneous. The highly porous single-layer PTFE membrane is subjected to a final heat treatment (FIG. 9; 980) sufficient to at least partially or completely sinter the porous single-layer PTFE membrane (FIG. 9; 970) to reach the final product (e.g., an article).

[0079] In some embodiments, the laminated composite material is first stretched in the machine direction and subsequently stretched at least once in the transverse direction (i.e., sequential stretching). In some embodiments, the laminated composite material is first stretched in the machine direction at a draw ratio in the range of about 1.1:1 to about 1.7:1 and subsequently stretched at least once in the transverse direction.

[0080] In some embodiments, the method can further include at least one densification step. The densification step can occur after (e.g., immediately after) removal of the carrier layer, or can occur after one or more additional stretching steps of the single-layer porous PTFE membrane. In some embodiments, the densification step can occur before, during, or after the heat treatment step. In some embodiments, densification can occur by plate pressing and / or by calendering the membrane between rollers. In some embodiments, the densification step can also include heating during, before, or after stretching.

[0081] The PTFE starting materials (precursor membranes / tapes and carrier membranes / tapes) used to form the PTFE membranes of the present invention can be of any type of PTFE resin that aids in the formation of fibrils and nodes during stretching, as long as the PTFE precursor membrane is weaker than the PTFE carrier membrane in the machine direction (MD) and / or the transverse direction (TD). Throughout this disclosure, the term "PTFE" is used for convenience, but includes not only polytetrafluoroethylene, but also stretched PTFE, modified PTFE, stretched modified PTFE, and stretched copolymers of PTFE, such as described in U.S. Patent Nos. 5,708,044 to Branca, 6,541,589 to Bailey, 7,531,611 to Sabol et al., 8,637,144 to Ford, and 9,139,669 to Xu et al. As used herein, the term "PTFE" also includes any non-fluoropolymer that can be stretched into a membrane having a node and fibril microstructure and that meets the matrix tensile strength in the machine and transverse directions as described above. Further, the term "PTFE" can be used for both the precursor membranes and carrier membranes described herein, is used for convenience, and is intended to include the broader definition of "PTFE" in this disclosure.

[0082] In one exemplary embodiment, the PTFE starting material can be a PTFE homopolymer or a blend of PTFE homopolymers. In another embodiment, the PTFE starting material can be a blend of a PTFE homopolymer and a PTFE copolymer having no comonomer units in an amount that would cause the non-melt processable characteristics of pure homopolymer PTFE to be lost in the copolymer. Examples of suitable comonomers in the PTFE copolymer include, but are not limited to, olefins such as ethylene and propylene, halogenated olefins such as hexafluoropropylene (HFP), vinylidene fluoride (VDF) and chlorotrifluoroethylene (CFE), perfluoroalkyl vinyl ethers (PPVE) and perfluorosulfonyl vinyl ethers (PSVE). In yet another embodiment, the first PTFE membrane and / or the second PTFE membrane is formed from a blend of a high molecular weight PTFE homopolymer and a low molecular weight modified PTFE polymer.

[0083] In another embodiment, the precursor membrane and / or the carrier layer membrane can be formed from other fibrillatable and expandable materials other than PTFE, such as poly(ethylene-co-tetrafluoroethylene) (ETFE), ultra-high molecular weight polyethylene (UHMWPE), polyethylene, polyparaxylylene (PPX), polylactic acid (PLLA), polyethylene (PE), expanded polyethylene (ePE) and any combination or blend thereof.

[0084] The PTFE film is formed by blending fibrillatable / paste-processable PTFE fine particles with a lubricant. Non-limiting examples of lubricants for use herein include light mineral oil, aliphatic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. The resulting mixture of PTFE resin particles and lubricant is processed into cylindrical pellets and extruded through a die at a reduction ratio of about 10:1 to about 150:1, or about 25:1 to about 90:1, to form a tape. Next, the tape is calendered to a desired thickness at a calender ratio of about 1.1:1 to about 50:1, or about 1.1:1 to about 20:1, between rolls to form a PTFE film / tape (precursor film and / or carrier film / tape).

[0085] In at least one embodiment, the PTFE precursor film is formed without a drying step and laminated with a second PTFE film (carrier film) in a wet state (FIG. 9, 903). It is within the scope of the present disclosure to dry either the PTFE film (pre-calendering) or the first PTFE precursor film (post-calendering) prior to lamination with the PTFE carrier film / tape.

[0086] For ease of discussion, the first PTFE film / tape (i.e., the PTFE precursor layer) and the second PTFE film / tape (PTFE carrier layer) are referred to herein, but it should be noted that more PTFE films / tape can be included in the methods described herein as long as the processing aids are removed and the final product includes the characteristics of the porous PTFE film.

[0087] The second PTFE film / tape (carrier film / tape) can be formed by blending a second suitable PTFE starting material ( fibrillatable / paste-processable resin particles) with a lubricant. Non-limiting examples of lubricants for use herein include light mineral oil, aliphatic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. The resulting mixture can be processed into cylindrical pellets and ram extruded through a die at a reduction ratio of about 10:1 to about 150:1 or about 50:1 to about 120:1 to form a tape. Next, the tape can be calendered to a desired thickness at a calender ratio of about 1.1:1 to about 20:1 or about 1.1:1 to about 10:1 between rolls. Next, the calendered tape can be stretched in one or more directions and dried to remove the lubricant. For example, the calendered tape can be stretched in the longitudinal and / or transverse directions at a draw ratio of about 1.1:1 to about 20:1 or about 1.1:1 to about 10:1. It should be understood that the second PTFE tape / membrane can be formed without drying the PTFE tape and / or membrane and laminated with the first PTFE membrane (precursor) in a wet state. In some embodiments, the PTFE carrier film / tape can be uniaxially or biaxially stretched before contacting the PTFE precursor film / tape. In another embodiment, the PTFE carrier film / tape can be stretched at least once in the transverse direction before contacting the PTFE carrier film / tape with the PTFE precursor film / tape.

[0088] When forming a laminated composite material, a first PTFE film (precursor film) and a second PTFE film (carrier film) are arranged on top of each other in a stack configuration. The first PTFE film and the second PTFE film are arranged in a stack configuration, for example, simply by placing the films on top of each other. Next, the stacked product is subjected to a sufficient amount of pressure (e.g., calendering and / or plate pressing) and / or heat to reversibly bond the PTFE precursor layer to the PTFE carrier layer, thereby forming a laminated composite material. In some embodiments, calendering is used to form the laminated composite material. When forming the laminated composite material, the thickness of the stacked films can be reduced to a desired thickness. However, the bond strength between the PTFE precursor layer and the PTFE carrier layer should be strong enough to facilitate the subsequent co-stretching process (Figure 9, 940), and still provide the ability to separate the PTFE carrier layer from the PTFE precursor layer after biaxially stretching the laminated composite material (950). Therefore, the reduction in thickness (e.g., calendering) when reversibly bonding the PTFE precursor layer to the PTFE carrier layer should be controlled to maintain the ability to remove the PTFE carrier layer after co-stretching the laminated composite material. In one embodiment, the thickness of the laminated composite material is reduced by less than about 20%, less than about 15%, less than about 10%, less than about 5%, less than about 3%, less than about 1% or less than about 0.1% (compared to the thickness of the laminated product before calendering and / or pressing). The thickness of the laminated composite material can be in the range of about 0.01 mm to about 3.0 mm, about 0.01 mm to about 2 mm, about 0.03 mm to about 1.0 mm, about 0.05 mm to about 0.7 mm or about 0.1 mm to about 0.5 mm.

[0089] The laminated composite material can be biaxially stretched sequentially or simultaneously. In one embodiment, the laminated composite material is co-stretched in the machine direction (MD) and subsequently stretched at least once in the transverse direction (TD). For example, the laminated composite material is stretched in the machine direction at an average engineering strain rate of about 0.5% / second to about 300% / second, or about 0.5% / second to about 150% / second and a draw ratio of about 10% to about 350% or about 10% to about 300%, and subsequently stretched in the transverse direction at an average engineering strain rate of about 3% to about 600% or about 10% to about 400% and a draw ratio of about 0% to about 2000% or about 1.0% to about 1600%, or vice versa (e.g., first stretched in the transverse direction and then in the machine direction). In at least one embodiment, the laminated product is simultaneously stretched biaxially at an average engineering strain rate of about 10% / second to about 500% / second, or about 20% / second to about 250% / second and a draw ratio of about 10% to about 2000%.

[0090] Next, the PTFE carrier layer film is separated from the stretched laminated composite material and discarded (Figure 9; 950). In some embodiments, mechanical force is used to separate the carrier layer from the stretched laminated composite material. The remaining product (derived from the PTFE precursor layer) is a single-layer highly porous self-supporting PTFE film having a macrotexture surface.

[0091] This example illustrates the preparation of PTFE precursors and / or carrier membranes using paste processing, although it is envisioned that fibrillating techniques, such as dry blending, can also be used. In alternative embodiments, the PTFE precursor membrane and / or PTFE carrier membrane can be formed by blend fibrillating polymer particles as generally taught in Mitchell et al., U.S. Publication No. 2005 / 0057888, Zhong et al., U.S. Publication No. 2010 / 0119699, Sassa et al., U.S. Patent No. 5,849,235, Rudolf et al., U.S. Patent No. 6,218,000, or Mortimer, U.S. Patent No. 4,985,296. Uniaxial, biaxial, or radial stretching is generally described in Gore, U.S. Patent No. 3,953,566 and Hubis, U.S. Patent No. 4,478,665.

[0092] In another embodiment, the method further includes densifying a single-layer self-supporting porous PTFE membrane. In a further aspect, the densification step is performed before, during, or after at least one heat treatment.

[0093] Those skilled in the art will readily appreciate that various aspects of the present disclosure can be realized by any number of methods and apparatuses configured to perform the intended functions. It should also be noted that the accompanying drawings referred to herein are not necessarily drawn to scale and may be exaggerated in order to illustrate various aspects of the present disclosure, and in that regard, the drawings should not be construed as limiting.

[0094] The invention of the present application has been described above both generally and with respect to specific embodiments. It will be apparent to those skilled in the art that various changes and modifications can be made in the embodiments without departing from the scope of the present disclosure. Accordingly, the embodiments are intended to cover the changes and modifications of the present invention as long as they fall within the scope of the appended claims and their equivalents.

[0095] Test Methods A specific method and apparatus are described below, but it should be understood that other methods or apparatuses determined to be appropriate by those skilled in the art can be alternatively utilized.

[0096] Thickness

[0097] The thickness of the PTFE membrane was determined by placing the PTFE membrane between two plates of a Heidenhain snap gauge (Heidenhain, Schaumburg, Illinois). Thinner samples (thickness less than 140 μm) were determined by SEM cross-section.

[0098] Mass per unit area (mass / area)

[0099] The mass / area of the PTFE membrane was calculated by measuring the mass of a clearly defined area of the sample using a scale. The sample was cut into a defined area using a die or any precision cutting instrument.

[0100] Density

[0101] The density was calculated by dividing the mass per unit area by the thickness.

[0102] Qualitative Image Analysis (QIA)

[0103] Qualitative image analysis was performed to determine the average dimensions of various features of the porous PTFE membrane (macroscopic node aggregates, fibril length, spacing between features, e.g., internal spacing and interspacing).

[0104] Internal spacing of macroscopic node aggregates

[0105] The internal spacing of the macroscopic node aggregates was determined by analyzing SEM images of ImageJ version 1.51h from the National Institutes of Health (NIH) (Schneider, C.A., Rasband, W.S., Eliceiri, K.W., “NIH Image to ImageJ: 25 years of image analysis” Nature Methods 9, 671-675, 2012 or Abramoff, M.D., Magalhaes, P.J., Ram, S.J. "Image Processing with ImageJ". Biophotonics International, volume 11, issue 7, pp.36-42, 2004). The image scale was set based on the scale provided by the SEM image. Features were identified and separated by a combination of size / shading and / or thresholding based on manual identification. Features less than 1% of the area of the largest feature in the image were excluded from the analysis. After separating the features, Delaunay triangulation was performed to identify adjacent features. A line was drawn between the closest edges of adjacent features, and the length was measured to define the spacing between adjacent features (see, for example, FIG. 11A). Lines between 45 degrees and 135 degrees from the horizontal were included in the analysis. The projection of these lines in the vertical direction (taking the orientation of the macroscopic node aggregates to determine the internal spacing).

[0106] As shown pictorially in FIG. 11A, the designated feature (P) is connected to the adjacent solid feature (N) to form a triangle 1100 in which the circumcircle 1110 does not contain the solid feature inside. The solid feature (X) indicates a solid feature that is not an adjacent solid feature to P. Thus, in the example shown in FIG. 11A, the feature spacing 1130 is the straight-line distance between the designated features (P), (N). In contrast, the circumcircle 1150 shown in FIG. 11B is drawn from the triangle 1160 and contains the feature (N) therein, and thus cannot be used to determine the spacing between features.

[0107] The inter-spacing of the macroscopic node aggregates

[0108] The interspacing of macroscopic node aggregates was determined by analyzing SEM images of ImageJ 1.51h from the National Institutes of Health (NIH) (above). The image scale was set based on the scale provided by the SEM image. Features were identified and separated by a combination of size / shading and / or thresholding based on manual identification. Features less than 1% of the area of the largest feature in the image were excluded from the analysis. After separating the features, the features were overlaid with a series of 50 or more parallel and equally spaced lines (Figure 12, 1200) in a direction perpendicular to the macro-aggregates. The lines contacting two nodes were separated and the lengths were measured to determine the average spacing of the macroscopic node aggregates. The mean and median values of the entire dataset were measured (i.e., the overall mean and median of the entire dataset). To further characterize the distribution of the spacing between macroscopic node aggregates, features / distances below the 75th percentile were excluded, and then the mean of the remaining features / distances (interquartile, between the 75th and 100th percentiles) was averaged.

[0109] Matrix Tensile Strength (MTS)

[0110] To determine the Matrix Tensile Strength (MTS), sample PTFE membranes / tapes were cut longitudinally and transversely using ASTM D412 - Dogbone Die Type F (D412F) or ASTM D638 - Standard Test Methods for Tensile Properties of Plastics (D638 - 5). The tensile break load was measured using an INSTRON® 5567 (Illinois Tool Works Inc., Norwood, Massachusetts) tensile tester equipped with flat face grips and a "22 pound" (approx. 100N) load cell. The gauge length of the grips was set to 8.26 cm and the strain rate was 0.847 cm / s. After placing the sample in the grips, the sample was retracted 1.27 cm to obtain a baseline, and then a tensile test was performed at the strain rate described above. Two samples for each condition were tested individually, and the average value of the measured maximum load (i.e., peak force) was used in the calculation of MTS. The longitudinal and transverse MTS were calculated using the following. [Number 2] MTS = (Maximum load / Cross-sectional area) × (Density of resin / Density of film)

[0111] Optical microscope image

[0112] Optical microscope images were generated using an Olympus SZX12 microscope at magnifications of 7× and 32×.

[0113] Scanning electron microscope photograph (SEM) sample image formation

[0114] SEM images were generated using a high-resolution field emission cryo-microscope (Hitachi S4700 FE-SEM).

[0115] ATEQ air flow measurement

[0116] The ATEQ air flow test measures the laminar volumetric flow rate of air passing through a membrane sample. Each membrane sample was fixed between two plates so as to seal a region of 2.99 cm 2 along the entire flow path. Using an ATEQ (registered trademark) (ATEQ Corp., Livonia, Michigan) Premier D Compact flow tester, each membrane sample was challenged with an air differential pressure of 1.2 kPa (12 millibars) to measure the air flow rate (L / hour) through each membrane sample.

[0117] It will be apparent to those skilled in the art that various changes and modifications can be made in the embodiments without departing from the scope of the present disclosure. Accordingly, the embodiments are intended to cover changes and modifications of the present invention as long as they fall within the scope of the appended claims and their equivalents.

Examples

[0118] Example 1 Preparation of the first PTFE membrane (membrane precursor layer) The first polytetrafluoroethylene (PTFE) film was prepared as follows. A blend of high molecular weight polytetrafluoroethylene fine powder and low molecular weight modified polytetrafluoroethylene polymer was prepared according to the teachings of U.S. Patent No. 5,814,405 to Branca et al., and then combined with 0.244 lb / lb of an isoparaffinic hydrocarbon lubricant (Isopar™ K, Exxon, Houston, Texas). The resulting mixture was then blended, compressed into cylindrical pellets, and heat conditioned at a temperature of 25°C for 18 hours. The cylindrical pellets were then extruded through a rectangular orifice die at a reduction ratio of 40:1 to form a tape. The tape was then calendered between rolls at a calender ratio of 1.2:1 and then laminated with a second identical tape at a ratio of 1.1:1 to form the first PTFE film (precursor film).

[0119] Preparation of the second PTFE film (carrier layer) The second PTFE film was prepared as follows. Fine powder of a polytetrafluoroethylene polymer manufactured according to the teachings of U.S. Patent No. 4,576,869 to Malhotra et al. was combined with 0.185 lb / lb of a lubricant (Isopar™ K, Exxon, Houston, Texas). The resulting mixture was then blended, compressed into cylindrical pellets, and then heat conditioned at a temperature of 49°C for 8 hours. The cylindrical pellets were then extruded through a rectangular orifice die at a reduction ratio of 78:1 to form a tape. The tape was then calendered between rolls at a calender ratio of 2.8:1 to form a PTFE carrier film.

[0120] Multilayer treatment (coextrusion) The PTFE precursor film was stacked on the PTFE carrier film, and the laminated product was calendered between rolls to reduce its thickness by 3 - 5% (relative to the thickness of the stacked film before calendering). The obtained laminated laminate was dried at a temperature of 180 °C to remove the lubricant. Next, the dried laminate was stretched at 300 °C in the machine direction (MD) at an average engineering strain rate of 6% / second and a draw ratio of 20%. Next, the MD-stretched laminate was further stretched in the transverse direction (TD) at an average engineering strain rate of 75% / second and a temperature of about 300 °C at a ratio of 3:1. Next, the PTFE carrier layer was mechanically separated from the first PTFE film layer (which started as the precursor film). Next, the remaining first PTFE film layer (derived from the precursor film) was biaxially stretched at 300 °C in the machine direction (MD) at 75% / second at a ratio of 2:1 and in the transverse direction (TD) at a ratio of 2:1 (75% / second). Next, the obtained single-layer highly porous PTFE film was heat-treated at 360 °C for 60 seconds.

[0121] The single-layer highly porous PTFE film had a very prominent three-dimensional structure with nodes extending from the first surface to the second surface (through the z-axis) (Table 1). The optical microscope image of the PTFE film showed a macrotexture surface due to the presence of macroscopic node aggregates (Figure 1, top view; Figure 2, cross-section). The SEM images (Figure 3, top view; Figure 4, cross-section) showed macroscopic node aggregates. Quantitative image analysis was performed as described above and reported in Tables 2 and 3.

[0122] Example 2 Preparation of the First PTFE Film (Film Precursor Layer) The first polytetrafluoroethylene (PTFE) membrane was prepared as follows. A blend of high molecular weight polytetrafluoroethylene fine powder and low molecular weight modified polytetrafluoroethylene polymer was prepared according to the teachings of U.S. Patent No. 5,814,405 to Branca et al., and then combined with 0.244 lb / lb of a lubricant (Isopar™ K, Exxon, Houston, Texas). The resulting mixture was blended and heat conditioned at a temperature of 25 °C for 18 hours. The heat conditioned mixture was compressed into cylindrical pellets. Next, the cylindrical pellets were extruded through a rectangular orifice die at a reduction ratio of 78:1 to form a tape. Next, the tape was calendered between rolls at a calender ratio of 3:1 to form the first PTFE membrane (i.e., the PTFE precursor membrane).

[0123] Preparation of the second PTFE membrane (carrier layer) The second PTFE membrane was prepared as follows. Fine powder of a polytetrafluoroethylene polymer manufactured according to the teachings of U.S. Patent No. 4,576,869 to Malhotra et al. was blended with 0.227 lb / lb of a lubricant (Isopar™ K, Exxon, Houston, Texas). The blended mixture was compressed into cylindrical pellets and then heat conditioned at a temperature of 49 °C for 8 hours. Next, the heat conditioned cylindrical pellets were extruded through a rectangular orifice die at a reduction ratio of 78:1 to form a tape. Next, the tape was calendered between rolls at a calender ratio of 3:1 to form the second PTFE membrane (i.e., the PTFE carrier membrane).

[0124] Multilayer treatment (coextrusion) The PTFE precursor film was laminated onto the PTFE carrier layer film to form a laminated product, which was subsequently calendared between rolls at an appropriate amount of pressure to gently bond the layers without significantly reducing the thickness of the resulting laminate. Next, the laminate was dried at a temperature of 180 °C to remove the lubricant. Next, the laminate was sequentially stretched as follows. The dried laminate was stretched at 330 °C in the machine direction (MD) at an average engineering strain rate of 4% / second and a draw ratio equal to 20%. Next, the MD-stretched laminate was stretched in the transverse direction (TD) at a temperature of about 330 °C at an average engineering strain rate of 50% / second at a ratio of 2:1.

[0125] Next, the sequentially stretched laminate was biaxially stretched at 200% / second at 300 in the transverse direction (TD) at a ratio of 6:1 and in the machine direction (MD) at a ratio of 2:1.

[0126] Next, the second PTFE film layer (i.e., the PTFE carrier layer) was mechanically separated from the first PTFE film layer (i.e., the PTFE precursor film). Next, the single-layer highly porous PTFE film was heat-treated at 360 °C for 180 seconds or less.

[0127] The single-layer highly porous film had a very distinct three-dimensional structure in which the nodes extended from the first surface to the second surface (through the z-axis) (Table 1).

[0128] Example 3 Preparation of the First PTFE Film (Film Precursor Layer) The first polytetrafluoroethylene (PTFE) membrane (i.e., the PTFE precursor membrane layer) was prepared as follows. A blend of high molecular weight polytetrafluoroethylene fine powder and low molecular weight modified polytetrafluoroethylene polymer was prepared according to the teachings of U.S. Patent No. 5,814,405 to Branca et al. Next, it was blended with 0.244 lb / lb of a lubricant (Isopar™ K, Exxon, Houston, Texas), and then the blended product was compressed into cylindrical pellets and heat-conditioned at a temperature of 25°C for 18 hours. Next, the heat-conditioned cylindrical pellets were extruded through a rectangular orifice die at a reduction ratio of 40:1 to form a tape. Next, the tape was calendered between rolls at a calender ratio of 1.2:1, and then laminated with a second identical tape at a ratio of 1.1:1 to form the first PTFE membrane (i.e., the PTFE precursor membrane).

[0129] Preparation of the second PTFE membrane (carrier layer) The second PTFE membrane (i.e., the PTFE carrier membrane) was prepared as follows. A fine powder of a polytetrafluoroethylene polymer manufactured according to the teachings of U.S. Patent No. 4,576,869 to Malhotra et al. was blended with 0.185 lb / lb of a lubricant (Isopar™ K, Exxon, Houston, Texas). The blended mixture was compressed into cylindrical pellets and heat-conditioned at a temperature of 49°C for 8 hours. Next, the heat-conditioned cylindrical pellets were extruded through a rectangular orifice die at a reduction ratio of 78:1 to form a tape. Next, the tape was calendered between rolls at a calender ratio of 2.8:1.

[0130] Multilayer treatment (coextrusion) A PTFE precursor film was stacked on a PTFE carrier film to form a laminated product. Next, the laminated product was calendered between rolls at an appropriate pressure to lightly bond the two PTFE films into a laminate without significantly reducing the thickness. The resulting laminate was dried at a temperature of 180 °C to remove the lubricant. Next, the dried laminate was stretched at 320 °C in the machine direction (MD) at an average engineering strain rate of 1.02% / second and a draw ratio equal to 60% (i.e., a ratio of 1.6:1 MD). Next, the MD-stretched laminate was stretched in the transverse direction (TD) at a ratio of 4:1 at an average engineering strain rate of 75% / second at a temperature of about 300 °C. Next, the second layer (i.e., the PTFE carrier film) was mechanically separated from the first layer (i.e., the PTFE precursor film). The resulting product was a single-layer, highly porous PTFE film (derived from the precursor film). Next, the resulting single-layer, highly porous PTFE film was heat-treated at 350 °C for 120 seconds or less.

[0131] The single-layer, highly porous PTFE film had a very prominent three-dimensional structure in which the nodes extended through the z-axis (Table 1). SEM images at different magnifications showed macroscopic node aggregates (Figure 5A, 5B, and 5C, top views; Figure 6, cross-section). Quantitative image analysis was performed as described above and reported in Tables 2 and 3.

[0132] Example 4 The first polytetrafluoroethylene (PTFE) membrane (i.e., the precursor membrane) was prepared as follows. A blend of high molecular weight polytetrafluoroethylene fine powder and low molecular weight modified polytetrafluoroethylene polymer was prepared according to the teachings of U.S. Patent No. 5,814,405 to Branca et al., and then blended with 0.244 lb / lb of a lubricant (Isopar™ K, Exxon, Houston, Texas). Next, the blended product was compressed into cylindrical pellets and heat conditioned at a temperature of 25°C for 18 hours. Next, the heat conditioned cylindrical pellets were extruded through a rectangular orifice die at a reduction ratio of 40:1 to form a tape. Next, the tape was calendered between rolls at a calender ratio of 1.2:1, and then laminated with a second identical tape at a ratio of 1.1:1 to form the first PTFE membrane (precursor PTFE membrane).

[0133] Preparation of the second PTFE membrane (carrier layer) The second PTFE membrane (carrier layer) was prepared as follows. Fine powder of a polytetrafluoroethylene polymer manufactured according to the teachings of U.S. Patent No. 4,576,869 to Malhotra et al. was blended with 0.185 lb / lb of a lubricant (Isopar™ K, Exxon, Houston, Texas). The blended mixture was compressed into cylindrical pellets and heat conditioned at a temperature of 49°C for 8 hours. Next, the heat conditioned cylindrical pellets were extruded through a rectangular orifice die at a reduction ratio of 78:1 to form a tape. Next, the tape was calendered between rolls at a calender ratio of 2.8:1 to form a PTFE carrier membrane.

[0134] Multilayer treatment (coextrusion) A PTFE precursor film was stacked on a PTFE carrier film to form a laminated product. Next, the laminated product was calendered between rolls at an appropriate amount of pressure to lightly bond the two films into a laminate without significantly reducing the thickness. The resulting laminate was dried at a temperature of 180 °C to remove the lubricant. Next, the dried laminate was stretched at 320 °C in the machine direction (MD) at an average engineering strain rate of 0.29% / second and a draw ratio equal to 20%. Next, the MD-stretched laminate was stretched in the transverse direction (TD) at a ratio of 4:1 at a temperature of about 300 °C at an average engineering strain rate of 75% / second. Next, the second PTFE film (i.e., the carrier film) was mechanically separated from the first PTFE film. The resulting product was a single-layer highly porous PTFE film (derived from the PTFE precursor film). Next, the single-layer highly porous PTFE film was heat-treated at 350 °C for 120 seconds or less.

[0135] The single-layer highly porous PTFE film had a very prominent three-dimensional structure with nodes extending through the z-axis (Table 1). SEM images at different magnifications showed macroscopic node aggregates (Figure 7A, 7B, 7C, and 7D, top views; Figure 8, cross-section). Figure 7D clearly shows that the macroscopic node aggregates are formed from multiple densified PTFE nodes. Quantitative image analysis was performed as described above. It is reported in Tables 2 and 3.

Table 1

Table 2

Table 3

Claims

1. A porous polytetrafluoroethylene (PTFE) membrane, a) a first membrane surface and a second membrane surface opposite to the first membrane surface, b) a thickness of at least 25 μm, c) 1.0 g / cm ３ the following bulk density, d) a porosity of at least 50%, e) about 2.99 cm ２ measured at a differential pressure of 12 mbar (1.2 kPa) across a surface area of and an air flow rate of at least 200 L / hour (L / h), and f) a macrotexture surface on the first or second membrane surface resulting from the presence of a plurality of macroscopic node aggregates extending from the first membrane surface through the z-axis to the second membrane surface, comprising, The macroscopic node aggregates are each formed from PTFE nodes connected by a first PTFE fibril, Adjacent macroscopic node aggregates are connected by a population of second PTFE fibrils longer than the length of the first PTFE fibril, More than 50% of the macroscopic node aggregates extend from the first membrane surface of the porous PTFE membrane through the z-axis to the second membrane surface, and The PTFE membrane is single-layer and is included in an implantable medical device, A porous polytetrafluoroethylene (PTFE) membrane.

2. The porous PTFE membrane according to claim 1, wherein the macroscopic node aggregates have a density lower than that of the PTFE nodes.

3. The porous PTFE membrane according to claim 1, wherein the macroscopic node aggregates form strands on the first membrane surface or the second membrane surface, and the strands have an average length of at least 0.5 cm.

4. The porous PTFE membrane according to claim 3, wherein the first PTFE fibril has a length of less than 120 μm.

5. The porous PTFE membrane according to claim 1, wherein the porous PTFE membrane is substantially symmetric from the first membrane surface to the second membrane surface.

6. The porous PTFE membrane according to claim 1, wherein the macroscopic node aggregates each contain a plurality of high-density PTFE nodes having a density in the range of 2.0 g / mol to 2.2 g / mol.

7. i) The average distance between the macroscopic node aggregates is at least 30 μm, ii) The upper quartile average distance between the macroscopic node aggregates is at least 100 μm, and iii) The average width of the spaced macroscopic node aggregates is in the range of 10 μm to 200 μm. The porous PTFE membrane according to claim 1.

8. The porous PTFE membrane according to claim 1, wherein the second PTFE fibril has an average length of 30 μm to 3 mm.

Images