Porous PTFE membrane

Abstract

To provide a membrane having symmetrical strength for certain applications such as a fuel cell and the like.SOLUTION: Porous polytetrafluoroethylene membranes comprising polytetrafluoroethylene are joined at texture points and are nonwoven webs having microfibril microstructures of less than 100 node / 100 μm2. The membrane has tensile peak stress in a longitudinal direction and tensile peak stress in a lateral direction. The tensile peak stress in the longitudinal direction is 10% or below the tensile peak stress in the lateral direction. The tensile peak stress in the longitudinal direction and the tensile peak stress in the lateral direction are 130 MPa or less at room temperature.SELECTED DRAWING: None

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application No. 62 / 780,776, filed December 17, 2018, the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002] Porous polytetrafluoroethylene (PTFE) membranes are known to have asymmetric strength (i.e., in the XY plane or longitudinal and transverse directions) and related properties. For certain applications, such as in fuel cells, membranes with symmetric strength are often desirable. Summary of the Invention [Means for solving the problem]

[0003] The present disclosure provides a porous polytetrafluoroethylene (PTFE) membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, said membrane having a balance ratio in orthogonal dimensions that is within 10%. Preferably, said membrane has a peak tensile stress (peak force / cross-sectional area) in the machine direction and a peak tensile stress in the transverse direction, wherein the peak tensile stress in the machine direction is within 10% of the peak tensile stress in the transverse direction.

[0004] In one embodiment, the present disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, said membrane having orthogonal dimension balance percentages within 10% (preferably, said membrane has a peak tensile stress in the longitudinal direction and a peak tensile stress in the transverse direction, wherein the peak tensile stress in the longitudinal direction is within 10% of the peak tensile stress in the transverse direction); and said membrane further has an IPA flux to pore size ratio of at least 3, and typically no greater than 30, for pore sizes of 1 micron or less.

[0005] In one embodiment, the present disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, said membrane having orthogonal dimension balance percentages within 10% (preferably, said membrane has a peak tensile stress in the machine direction and a peak tensile stress in the cross direction, wherein the peak tensile stress in the machine direction is within 10% of the peak tensile stress in the cross direction); and said membrane further has: a thickness between 1 micron and 30 microns; an air flow between 0.5 fpm (0.00254 m / sec) and 10 fpm (0.0508 m / sec); a pore size value between 0.05 microns and 2 microns; a bubble point between 10 psi (68.9 KPa) and 60 psi (413.7 KPa); and an IPA flux to pore size value ratio between 3 and 30 (i.e., 3:1 to 30:1) for pore sizes of 1 micron and smaller.

[0006] In one embodiment, the present disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, the membrane having a balance percentage in the orthogonal dimensions that is within 10%, or within 5%, or within 1% (preferably, the membrane has a peak tensile stress in the machine direction and a peak tensile stress in the transverse direction, where the peak tensile stress in the machine direction is less than the peak tensile stress in the transverse direction). and the membrane further has: a thickness between 1 micron and 30 microns; an air flow between 0.5 fpm (0.00254 m / sec) and 10 fpm (0.0508 m / sec); a pore size value between 0.05 microns and 2 microns; a bubble point between 10 psi (68.9 KPa) and 60 psi (413.7 KPa); and an air permeability to bubble point ratio of 0.05 (fpm / psi) (0.0037 m / sec / MPa) or less.

[0007] Such porous polytetrafluoroethylene membranes can be used, for example, in fuel cell and filtration applications where higher air and liquid permeability for a given pore size is often a desired feature.

[0008] The term "comprises" and variations thereof do not have a limiting meaning where these terms appear in the specification and claims. Such terms will be understood to mean the inclusion of the recited step or element or group of steps or elements, but not the exclusion of any other step or element or group of steps or elements. "Consisting of" means inclusive and limited to whatever follows the phrase "consisting of". Thus, the phrase "consisting of" indicates that the recited elements are necessary or essential, and no other elements may be present. "Consisting essentially of" means including any of the elements recited following the phrase, and limited to other elements that do not interfere with or contribute to the action or function specified for the recited elements in this disclosure. Thus, the phrase "consisting essentially of" indicates that the recited elements are required or essential, but that other elements are optional and may or may not be present depending on whether they substantially affect the act or action of the recited elements.

[0009] The terms "preferred" and "preferably" refer to embodiments of the present disclosure that may provide certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the present disclosure.

[0010] As used herein, the term "room temperature" refers to a temperature of 20°C to 25°C or 22°C to 25°C.

[0011] In this application, terms such as "a," "an," and "the" are not intended to refer only to a singular entity, but include general classifications for which specific examples may be used for illustration. The terms "a," "an," and "the" are used synonymously with the term "at least one."

[0012] The phrases "at least one of" and "including at least one of" followed by a list refer to any one of the items in the list, as well as any combination of two or more items in the list.

[0013] As used herein, the term "or" is generally used in its ordinary sense, including "and / or," unless the content clearly dictates otherwise. The term "and / or" refers to one or all of the listed elements or a combination of two or more of any of the listed elements.

[0014] Also, all numbers herein are considered to be modified by the term "about," and preferably, the term "exactly." As used herein in reference to a measured quantity, the term "about" refers to the variation in the measured quantity that would be expected by one of ordinary skill in the art using a reasonable level of care in making the measurement and for the purpose of the measurement and the precision of the measuring instrument being used.

[0015] Also herein, the recitation of numerical ranges by endpoints includes all numbers subsumed within that range, as well as the endpoints (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.) and any sub-ranges (e.g., 1 to 5 includes 1 to 4, 1 to 3, 2 to 4, etc.).

[0016] As used herein, a reference to "less than or equal to" a numerical value (eg, less than or equal to 50) includes that numerical value (eg, 50).

[0017] References throughout this specification to "one embodiment," "an embodiment," "certain embodiments," or "some embodiments" mean that the particular features, configurations, compositions, or characteristics described in connection with an embodiment are included in at least one embodiment of the present invention. Thus, the appearances of such phrases in various places throughout this specification are not necessarily referring to the same embodiment of the present invention. Furthermore, the particular features, configurations, compositions, or characteristics may be combined in any suitable manner in one or more embodiments.

[0018] The above summary of the disclosure is not intended to describe each disclosed embodiment or every implementation of the invention. The following description more particularly illustrates exemplary embodiments. In numerous places throughout this application, guidance is provided through the enumeration of examples, which can be used in various combinations. In each instance, the recited enumeration is merely intended to be a representative group and should not be construed as an exclusive enumeration. Thus, the scope of the disclosure should not be limited to the specific exemplary structures described herein, but rather should extend to at least the structures described by the terms of the claims and equivalents of these structures. In the alternative, any of the elements expressly described in this specification may be expressly included in or excluded from the claims, in any combination as desired. Although various theories and possible mechanisms may be discussed herein, in no event should such discussion be taken as a limitation of the claimed subject matter. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is an SEM of the membrane of Example 1. [Diagram 2] FIG. 2 is an SEM of the membrane of Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present disclosure provides a unique porous polytetrafluoroethylene (PTFE) membrane having a microstructure consisting essentially of microfibrils (i.e., having substantially no nodes) and a substantially symmetrical peak tensile stress (i.e., a peak tensile stress in the longitudinal direction that is substantially the same as the transverse direction). In this context, "consisting essentially of microfibrils" (i.e., having substantially no nodes) refers to a membrane having 100 nodes / 100 microns. 2 "Substantially symmetric" tensile peak stress means that the membrane has a peak tensile stress in the longitudinal direction that is within 10% of the peak tensile stress in the transverse direction. If the longitudinal and transverse directions cannot be distinguished, substantially symmetric tensile peak stress means that the membrane has a balance percentage in the orthogonal dimension that is within 10%.

[0021] More specifically, the present disclosure provides a porous PTFE membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, said membrane having a balance percentage in the orthogonal dimension within 10%, meaning that for a membrane having a peak tensile stress in the machine direction and a peak tensile stress in the transverse direction, the peak tensile stress in the machine direction is within 10% of the peak tensile stress in the transverse direction.

[0022] In certain embodiments, the PTFE membrane has a balance percentage in the orthogonal dimension within 5% or within 1%, i.e., the membrane has a peak tensile stress in the longitudinal direction within 5% or within 1% of the peak tensile stress in the transverse direction.

[0023] Peak stress is defined as the peak force relative to the cross-sectional area of ​​the specimen. The ratio to determine cross-dimensional (machine direction ("MD") vs. cross-sectional ("CD" or "CMD")) balance is calculated as the ratio of the difference in peak stress between the cross-dimensional to the machine direction. Alternatively, cross-dimensional (machine direction vs. cross-sectional) balance can be calculated as the ratio of the difference in peak stress between the cross-dimensional to the one of the two cross-dimensional dimensions (machine direction or cross-sectional) that has the higher strength.

[0024] The balance ratio is described below in the form of a formula.

number

[0025] If the longitudinal strength is higher than the transverse strength, the strength balance percentage can instead be calculated as:

number

[0026] The property of a substantially symmetrically strong membrane with substantially no nodes is often desirable for improving membrane performance in applications such as fuel cells, filters (e.g., for ventilation and microfiltration), and semiconductors. For example, a symmetrically strong membrane can improve membrane permeation flux by reducing resistance to the flow of fluid ions. Similarly, for applications where the membrane can be cut to smaller sizes, balanced strength provides dimensional stability under thermal and mechanical stress.

[0027] In certain embodiments, the membrane has a peak tensile stress in each of the longitudinal and transverse directions of at least 5 MPa at room temperature. In certain embodiments, the membrane has a peak tensile stress in each of the longitudinal and transverse directions of no more than 130 MPa at room temperature. The peak tensile stress can be measured using an EJA series tensile tester (available from Thwing-Albert, West Berlin, NJ) according to ASTM D882-18 (grip separation at start of test is 60 millimeters (mm); sample cut to 15 mm wide and tested for breaking strength at 0.24 inches / minute (6.09 mm / minute)).

[0028] In certain embodiments, the PTFE membrane has a thickness of at least 1 micron, hi certain embodiments, the membrane has a thickness of 30 microns or less, or 20 microns or less, or 10 microns or less.

[0029] Typically, when a membrane is made thinner (e.g., by stretching), it loses strength. In certain embodiments of the membranes described herein, when the membrane is made thinner (e.g., by stretching), the strength increases until it reaches a maximum value, after which it decreases.

[0030] In certain embodiments, the PTFE membrane has a pore size value of at least 0.05 microns (i.e., micrometers). In certain embodiments, the membrane has a pore size value of 2 microns or less. The pore size value is an average of the average pore size of multiple samples of the membrane, where the average pore size is the average pore size of one membrane sample. The pore size value was determined using a PMI Advanced Perm Porometer manufactured by Porous Materials Inc., Ithaca, NY, using the Capillary Flow Procedure preprogrammed into the PMI Advanced Perm Porometer with the following settings: 100,000 cm 3 Maximum flow rate per minute: 27.9 cm 3 / min bubble flow; and a past bubble time F / PT of 50.

[0031] In certain embodiments, the PTFE membrane has an air flow of at least 0.5 cubic feet per square foot per minute (fpm) (0.00254 m / sec). In certain embodiments, the membrane has an air flow of no more than 10 fpm (0.0508 m / sec), or no more than 5 fpm (0.0254 m / sec), which can be measured using the air flow test described in ASTM D737-18.

[0032] Liquids with a lower surface free energy than the expanded porous PTFE can be forced out of the structure by application of a pressure differential. This removal will occur from the largest passages first. Passages are then formed through which the bulk flow of air can pass. The air flow appears as a steady stream of small air bubbles penetrating the liquid layer on top of the sample. The pressure at which the bulk flow of air first occurs is called the bubble point, which depends on the surface tension of the test fluid and the size of the largest opening. The bubble point can be used as a relative measure of the structure of the membrane and is often corrected for some other type of performance criterion, such as filtration efficiency.

[0033] Bubble point can be measured according to the methodology of ASTM F316-03(2011). Isopropyl alcohol (IPA) is typically used as the wetting fluid to fill the pores of the specimen. The bubble point is the pressure of air required to displace the IPA from the largest pore of the specimen and form the first continuous flow of air bubbles that are detectable by rising through the IPA layer covering the porous media. This measurement provides an estimate of the largest pore size.

[0034] In certain embodiments, the PTFE membrane has a bubble point of at least 10 psi (68.9 KPa). In certain embodiments, the PTFE membrane has a bubble point of at least 20 psi (137.9 KPa). In certain embodiments, the PTFE membrane has a bubble point of 60 psi (413.7 KPa) or less.

[0035] In certain embodiments, the PTFE membrane has an IPA flux to pore size ratio of at least 3:1 for pore size values ​​of 1 micron or less. In certain embodiments, the PTFE membrane has an IPA flux to pore size ratio of 30:1 or less for pore size values ​​of 1 micron or less. IPA flux can be measured using a volumetric flow rate of 100% IPA at 10 psi (68.9 kPa) on a 42 mm sample. A fixed amount of IPA can be collected in a graduated cylinder and the time for a fixed amount of IPA can be calculated. For example, a membrane sample can be measured for the time required for 100 mL of IPA to flow when an upstream pressure of 10 psi (68.9 kPa) is applied to the membrane. Pore size values ​​can be measured using a PMI Advanced Perm Porometer from Porous Materials Inc., Ithaca, NY, as mentioned above.

[0036] In certain embodiments, the microfibrils of the PTFE membrane have an average fiber diameter of at least 5 nanometers (nm), or at least 10 nm. In certain embodiments, the microfibrils have an average fiber diameter of no more than 200 nm, or no more than 150 nm, as can be measured by SEM.

[0037] Thus, in certain embodiments, the PTFE membrane has a peak tensile stress relative to microfibril diameter of at least 190 MPa / micron, and although there is no upper limit, in some embodiments the PTFE membrane has a peak tensile stress relative to microfibril diameter of 900 MPa / micron or less.

[0038] Thus, in certain embodiments, the PTFE membrane has an airflow (i.e., air permeability) to bubble point ratio of at least 0.008 (fpm / psi) (0.0059 m / sec / MPa). In some embodiments, the PTFE membrane has an airflow to bubble point ratio of no more than 0.05 (fpm / psi) (0.0037 m / sec / MPa).

[0039] Symmetry of the membrane structure can be created by controlling the fibril diameter, internodal distance, and node size of the precursor materials.

[0040] To manufacture PTFE membranes, PTFE raw materials are typically used that have a low amorphous content and at least 98% crystallinity. The polytetrafluoroethylene used can be in the form of a coagulated dispersion or fine powder. Suitable commercially available resins include those available under the trade names 601x from Chemours (Wilmington, DE) and F131 from Daikin (Orangeburg, NY).

[0041] This PTFE raw material (i.e., resin) is typically then made into a paste by intimately mixing with a hydrocarbon extrusion aid, such as mineral spirits (e.g., ISOPAR K available from Exxon Mobil), naphtha oil, or other such lubricants. In certain embodiments, the amount of extrusion aid is typically in an amount of at least 15 weight percent (wt%) based on the total weight of the paste. In certain embodiments, the amount of extrusion aid is typically in an amount of 20 wt% or less based on the total weight of the paste. The final product may contain less of the extrusion aid (e.g., 15.25 wt%) than the amount added (e.g., 18 wt%), for example, due to evaporation.

[0042] The paste is then formed into the shape required by the intended use of the final product by a forming method that imparts shear deformation, such as extrusion or calendering. An example of an extrusion die and process that can be used for this first step is described in U.S. Pat. No. 3,315,020 (Gore) and U.S. Pat. No. 3,953,566 (Gore); however, a different stretch differential than that used in these patents (e.g., the stretch differential in Gore is 1.4 + / - 0.3) is used to form the tapes used to make the membranes described herein, and a balanced fibrillated tape is not used initially, but the final product is a balanced membrane.

[0043] The paste is typically compressed into a cylinder and formed (e.g., by ram extrusion) into a tape form, although the shape need not be this way and the article can be formed into a variety of cross-sectional shapes, such as rods or tubes, at temperatures between 75°F and 115°F (23.9°C and 46.1°C) depending on the intended use of the final product.

[0044] If desired, two or more layers of tape can be stacked together and calendered between two rollers. The calendering can be performed multiple times in wet, dry, or a combination of wet and dry forms to achieve the desired thickness. Typically, the desired thickness is 20 mils (508 micrometers) or less. In certain embodiments, the desired thickness of the intermediate calender product is at least 11 mils (279 micrometers). The density of the intermediate calender product (e.g., tape) is typically at least 1.5 grams per cubic centimeter (g / cc), or at least 1.8 g / cc. In certain embodiments, the density of the intermediate calender product is 2.2 g / cc or less.

[0045] The extrusion aid is typically removed with heat (e.g., at a temperature of 293° F. (145° C.)) after the first or second step of calendering. This intermediate calendered product has a microstructure of microfibrils and nodes.

[0046] Typically, the wet intermediate calender product is not oriented, in contrast to the process described in US Pat. No. 5,476,589 (Bacino).

[0047] To form an expanded membrane (ePTFE membrane), the dry intermediate calender product (e.g., tape) is first stretched longitudinally, in a single step or in multiple steps, to a composite stretch sufficient to produce an internode distance of at least 1.5 microns and a fibril diameter of 1 micron or less. Typically, the composite (i.e., multiplicative) longitudinal stretch (i.e., longitudinal stretch) is in a ratio of 40:1 or less, 30:1 or less, or 20:1 or less. In exemplary embodiments, the composite longitudinal stretch ratio is 18:1. In some embodiments, the composite longitudinal stretch is in a ratio of at least 6:1. Such values ​​are obtained at temperatures above 400°F (204.4°C) and typically below 665°F (351.7°C).

[0048] After longitudinal stretching, the material is then stretched in the transverse direction at a ratio high enough to result in fibril diameters of 1 micron or less. Typically, the transverse stretching (i.e., cross-stretching) is at a ratio of at least 10:1 at a temperature of 500°F (260°C) or greater. For example, the transverse stretch ratio is 21:1. In some embodiments, the transverse stretching is at a ratio of 40:1 or less at a temperature of 500°F (260°C) or greater and below the melting point of PTFE.

[0049] After spreading, in certain embodiments, the membrane is cured (i.e., sintered as described in U.S. Pat. No. 3,953,566 (Gore)) at a temperature of up to 716° F. (380° C.). In certain embodiments, the membrane is cured at a temperature of at least 617° F. (335° C.).

[0050] The resulting membrane has substantially similar peak tensile stresses in both directions of at least 19 MPa at a temperature of 72°F (22°C).

[0051] This process provides an open or porous but strong structure that results in a highly air permeable membrane. Despite the presence of numerous pores and despite its thinness, the membrane is very strong.

[0052] The ePTFE membranes of the present disclosure find many applications, such as cell diaphragms in air filters, humidifier diaphragms or pervaporation diaphragms. They can also be used as woven materials in applications where a clean environment is required.

[0053] Exemplary embodiments Embodiment 1 is a porous polytetrafluoroethylene membrane comprising (or consisting essentially of, or consisting of) a nonwoven web having a microstructure formed substantially only of microfibrils joined at texture points, said membrane having a balance percentage in orthogonal dimensions that is within 10%. Preferably, said membrane has a peak tensile stress in the machine direction and a peak tensile stress in the transverse direction, where the peak tensile stress in the machine direction is within 10% of the peak tensile stress in the transverse direction.

[0054] Embodiment 2 is the membrane of embodiment 1 having a balance percentage in the orthogonal dimension within 5%. Preferably, the membrane has a peak tensile stress in the longitudinal direction that is within 5% of the peak tensile stress in the transverse direction.

[0055] Embodiment 3 is the membrane of embodiment 2 having a balance percentage in the orthogonal dimension that is within 1%. Preferably, the membrane has a peak tensile stress in the longitudinal direction that is within 1% of the peak tensile stress in the transverse direction.

[0056] Embodiment 4 is a membrane according to any of the previous embodiments, having a peak tensile stress in each of the longitudinal and transverse directions of at least 5 MPa at room temperature.

[0057] Embodiment 5 is a membrane according to any of the previous embodiments, having a peak tensile stress in each of the longitudinal and transverse directions of 130 MPa or less at room temperature.

[0058] Embodiment 6 is a membrane according to any of the previous embodiments having a thickness of at least 1 micron.

[0059] Embodiment 7 is a membrane according to any of the previous embodiments having a thickness of 30 microns or less, 20 microns or less, or 10 microns or less.

[0060] An eighth embodiment is a membrane according to any of the previous embodiments, having a pore size value of at least 0.05 microns.

[0061] A ninth embodiment is a membrane according to any of the previous embodiments, having a pore size value of 2 microns or less.

[0062] Embodiment 10 is a membrane according to any of the previous embodiments having an air flow of at least 0.5 fpm (0.00254 m / sec).

[0063] An eleventh embodiment is a membrane according to any of the previous embodiments having an air flow of 10 fpm or less.

[0064] Embodiment 12 is a membrane according to any of the previous embodiments having a bubble point of at least 10 psi (68.9 KPa), or at least 20 psi (137.9 KPa).

[0065] Embodiment 13 is a membrane according to any of the previous embodiments having a bubble point of 60 psi (413.7 KPa) or less.

[0066] Embodiment 14 is a membrane according to any of the previous embodiments having an IPA flux to pore size ratio of at least 3 for pore sizes of 1 micron or less.

[0067] Embodiment 15 is a membrane according to any of the previous embodiments, having a ratio of IPA flux to pore size value of 30 or less for pore sizes of 1 micron or less.

[0068] Embodiment 16 is a membrane according to any of the previous embodiments, wherein the microfibrils have an average fiber diameter of at least 5 nm (or at least 10 nm).

[0069] Embodiment 17 is a membrane according to any of the previous embodiments, wherein the microfibrils have an average fiber diameter of 200 nm or less (or 150 nm or less).

[0070] Embodiment 18 is a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at structural points, the membrane having orthogonal dimension balance percentages that are within 10%, or within 5%, or within 1% (preferably, the membrane has a peak tensile stress in the machine direction and a peak tensile stress in the transverse direction, where the peak tensile stress in the machine direction is within 10%, or within 5%, or within 1% of the peak tensile stress in the transverse direction); and wherein the membrane further has an IPA flux to pore size ratio of 3 to 30 for pore sizes of 1 micron and smaller.

[0071] Embodiment 19 is the membrane of embodiment 18, having a peak tensile stress in each of the longitudinal and transverse directions of at least 5 MPa at room temperature.

[0072] Embodiment 20 is the membrane of embodiment 18 or 19, having a peak tensile stress in each of the longitudinal and transverse directions of 30 MPa or less at room temperature.

[0073] Embodiment 21 is a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, said membrane having a balance percentage in orthogonal dimensions that is within 10%, or within 5%, or within 1% (preferably, said membrane has a peak tensile stress in the machine direction and a peak tensile stress in the transverse direction, wherein the peak tensile stress in the machine direction is within 10%, or within 5%, or within 1% of the peak tensile stress in the transverse direction); and wherein said membrane: Thickness from 1 micron to 30 microns; Airflow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); Pore ​​size values ​​from 0.05 microns to 2 microns; Bubble point of 10 psi (68.9 KPa) to 60 psi (413.7 KPa); and IPA flux vs. pore size ratios from 3 to 30 for pore sizes below 1 micron It further has:

[0074] Embodiment 22 is a membrane according to any of embodiments 21, having a peak tensile stress in each of the longitudinal and transverse directions of 5 MPa to 130 MPa at room temperature.

[0075] Embodiment 23 is a membrane according to any of the previous embodiments having a peak tensile stress relative to microfibril diameter of at least 190 MPa / micron.

[0076] Embodiment 24 is a membrane according to any of the previous embodiments having an airflow to bubble point ratio of at least 0.008 (fpm / psi) (0.0059 m / sec / MPa).

[0077] Embodiment 25 is a membrane according to any of the previous embodiments having an airflow to bubble point ratio of 0.05 (fpm / psi) (0.0037 m / sec / MPa) or less.

[0078] Embodiment 26 is a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, said membrane having a balance percentage in orthogonal dimensions that is within 10%, or within 5%, or within 1% (preferably, said membrane has a peak tensile stress in the machine direction and a peak tensile stress in the transverse direction, wherein the peak tensile stress in the machine direction is within 10%, or within 5%, or within 1% of the peak tensile stress in the transverse direction); and wherein said membrane: Thickness from 1 micron to 30 microns; Airflow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); Pore ​​size values ​​from 0.05 microns to 2 microns; Bubble point of 10 psi (68.9 KPa) to 60 psi (413.7 KPa); and Air permeability to bubble point ratio of 0.05 (fpm / psi) (0.0037 m / sec / MPa) or less It further has:

[0079] Embodiment 27 is a membrane according to any of the previous embodiments having a thickness between 1 micron and 30 microns, where making the membrane thinner increases the strength until it reaches a maximum value and then decreases.

[0080] A twenty-eighth embodiment is a fuel cell including the porous polytetrafluoroethylene membrane according to any one of the first to twenty-seventh embodiments.

[0081] The twenty-ninth embodiment is a filter including the porous polytetrafluoroethylene membrane according to any one of the first to twenty-seventh embodiments.

[0082] A thirtyth embodiment is a semiconductor including the porous polytetrafluoroethylene membrane according to any one of the first to twenty-seventh embodiments. EXAMPLES

[0083] Objects and advantages of the present disclosure are further illustrated by the following examples, in which the particular materials and amounts thereof recited, as well as other conditions and details, should not be construed as unduly limiting the disclosure.

[0084] Example 1 Fine PTFE powder (601X, Chemours Co., Wilmington, Del.) was mixed with an extrusion aid (Exxon Mobil D-80) at 15.25 wt %, preformed and ram extruded into a tape having a thickness of 47 mils (1193.8 microns) and a width of 8.75 inches (22.23 cm). The tape was calendered between two steel rollers and then dried to remove the lubricant used during the mixing step. The resulting tape weighed approximately 2511 lb. f / in 2 (psi)(17312 KPa) longitudinal tensile stress and approximately 1506 lb f / in 2 (10,383 KPa). The dried tape was then stretched in the machine direction at a temperature of 232° C. with a stretch ratio of 9:1, and stretched in the opposite direction at temperatures of 260° C. to 380° C. with a stretch ratio of 21.6. The final membrane produced had a cross-machine tensile strength of 4429 lb. f / in 2 (30.53 MPa) longitudinal tensile stress ("MD peak stress") and 4702 lb f / in 2 (32.42 MPa). An SEM of the membrane is shown in Figure 1. Properties such as air permeability, bubble point, pore size, and IPA flux are listed in Table 1.

[0085] Table 2 demonstrates that making the membrane of Example 1 thinner increases the strength until it reaches a maximum value and then decreases. Sample 1 = Example 1. Compared to Sample 1 prepared as above, Sample 2 was stretched in the MD with a stretch ratio of 12.75 and in the CMD with a stretch ratio of 30.2. Everything else remained the same between these two samples. Sample 3 was stretched in the MD with a stretch ratio of 18 and in the CMD with a stretch ratio of 38.8 compared to Sample 1.

[0086] Example 2 PTFE fine powder (601X, Chemours) was mixed with an extrusion aid (Exxon Mobil D-80) at 20 wt%, preformed and ram extruded into a tape having a thickness of 47 mils (1193.8 microns) and a width of 8.5 inches (21.6 cm). The tape was calendered between two steel rollers and then dried to remove the lubricant used during the mixing step. The resulting tape weighed approximately 536 lb. f / in 2 (3.7MPa) longitudinal tensile stress and approximately 420 lb f / in 2 (2.9 MPa). The dried tape was then stretched in the machine direction at a temperature of 260° C. with a stretch ratio of 10.4:1, and stretched in the reverse direction at a temperature of 260° C. with a stretch ratio of 10.4. The final membrane produced had a cross tensile strength of 2450 lb. f / in 2 (16.9MPa) longitudinal tensile stress and 2234lb f / in 2 (15.4 MPa). SEM of the membrane is shown in Figure 2. The properties of the membrane are listed in Table 1.

[0087] [Table 1]

[0088] [Table 2]

[0089] The complete disclosures of the patents, patent documents and publications cited herein are incorporated by reference in their entirety as if each of them were individually incorporated. In the event of any conflict or inconsistency between the specification as set forth herein and the disclosure in any document incorporated by reference herein, the specification as set forth herein shall control. Various modifications and alterations to the present disclosure will become apparent to those skilled in the art that would depart from the scope and spirit of the present disclosure. It should be understood that the present disclosure is not intended to be unduly limited by the exemplary embodiments and examples described herein, and that such examples and embodiments are presented merely by way of example, and that the scope of the disclosure is intended to be limited only by the claims set forth herein below.

Claims

1. A porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at tissue points, the porous polytetrafluoroethylene membrane having a balance ratio in orthogonal dimensions within 10%.

2. 2. The membrane of claim 1 having a peak tensile stress in a longitudinal direction and a peak tensile stress in a transverse direction, said peak tensile stress in the longitudinal direction being within 10% of said peak tensile stress in the transverse direction.

3. 3. The membrane of claim 1 or 2 having a peak tensile stress in each of the longitudinal and transverse directions of at least 5 MPa.

4. The membrane according to any one of claims 1 to 3, having a thickness of from 1 micron to 30 microns.

5. A membrane according to any one of claims 1 to 4, having a pore size value between 0.05 microns and 2 microns.

6. The membrane of any one of claims 1 to 5 having an air flow of between 0.5 fpm (0.00254 m / sec) and 10 fpm (0.0508 m / sec).

7. The membrane of any one of claims 1 to 6 having a bubble point of from 10 psi (68.9 KPa) to 60 psi (413.7 KPa).

8. A membrane according to any one of claims 1 to 7 having an IPA flux to pore size ratio of 3 to 30 for pore sizes of 1 micron and smaller.

9. The membrane according to any one of claims 1 to 8, wherein the microfibrils have an average fibre diameter of from 5 nm to 200 nm.

10. A porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at texture points, the nonwoven web having a balance percentage in orthogonal dimensions that is within 10%; and a ratio of IPA flux to pore size value of 3 to 30 for pore sizes of 1 micron or less.

11. 11. The membrane of claim 10, having a peak tensile stress in each of the longitudinal and transverse directions of at least 5 MPa at room temperature.

12. 12. The membrane of claim 10 or 11, having a peak tensile stress in each of the longitudinal and transverse directions of less than or equal to 130 MPa at room temperature.

13. A porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at points of organization, the nonwoven web having a balance percentage in an orthogonal dimension that is within 10%; and A thickness between 1 micron and 30 microns; Air flow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); Pore ​​size values ​​between 0.05 microns and 2 microns; A bubble point of 10 psi (68.9 KPa) to 60 psi (413.7 KPa); and IPA flux vs. pore size ratios of 3 to 30 for pore sizes of 1 micron and smaller The porous polytetrafluoroethylene membrane further comprises:

14. A porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure formed substantially solely of microfibrils joined at points of organization, the balance percentage of the orthogonal dimensions being within 10%; and A thickness between 1 micron and 30 microns; Air flow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); Pore ​​size values ​​between 0.05 microns and 2 microns; A bubble point of 10 psi (68.9 KPa) to 60 psi (413.7 KPa); and Air permeability to bubble point ratio of 0.05 (fpm / psi) (0.0037 m / sec / MPa) or less The porous polytetrafluoroethylene membrane further comprises:

15. A fuel cell comprising a porous polytetrafluoroethylene membrane according to any one of claims 1 to 14.

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