Porous ptfm membrane

Abstract

A porous polytetrafluoroethylene (PTFE) membrane comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersections, the membrane having a percent balance of orthogonal dimensions within 10%.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 780,776, filed December 17, 2018, the disclosure of which is incorporated by reference herein in its entirety. BACKGROUND

[0003] Porous polytetrafluoroethylene (PTFE) membranes are known to have asymmetric strength (i.e., in the X-Y plane or machine direction and transverse direction) and related properties. For certain applications, such as in fuel cells, it is often desirable to have a membrane with symmetric strength. SUMMARY

[0004] The present disclosure provides a porous polytetrafluoroethylene (PTFE) membrane comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersections, the membrane having a percent balance of orthogonal dimensions within 10%. Preferably, the membrane has a tensile peak stress (peak force / cross-sectional area) in the machine direction and a tensile peak stress in the transverse direction, wherein the tensile peak stress in the machine direction is within 10% of the peak tensile stress in the transverse direction.

[0005] In one embodiment, the present disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersections, the membrane having a percent balance of orthogonal dimensions within 10% (preferably, the membrane has a tensile peak stress in the machine direction and a tensile peak stress in the transverse direction, wherein the tensile peak stress in the machine direction is within 10% of the tensile peak stress in the transverse direction); and further wherein the membrane has a ratio of IPA flux to pore size value of at least 3 and typically up to 30 for a pore size of 1 micron or less.

[0006] In one embodiment, the present disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersection points, the membrane having a percent balance of orthogonal dimensions within 10% (preferably, the membrane has a tensile peak stress in the machine direction and a tensile peak stress in the cross direction, wherein the tensile peak stress in the machine direction is within 10% of the tensile peak stress in the cross direction); and further wherein the membrane has: a thickness of 1 micron to 30 microns; an air flow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); a pore size value of 0.05 microns to 2 microns; a bubble point of 10 psi (68.9 KPa) to 60 psi (413.7 KPa); and a ratio of IPA flux to pore size value of 3 to 30 (i.e., 3: 1 to 30: 1) for a pore size of 1 micron or less.

[0007] In one embodiment, the present disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersection points, the membrane having a percent balance of orthogonal dimensions within 10%, or within 5%, or within 1% (preferably, the membrane has a tensile peak stress in the machine direction and a tensile peak stress in the cross direction, wherein the tensile peak stress in the machine direction is within 10%, or within 5%, or within 1% of the tensile peak stress in the cross direction); and further wherein the membrane has: a thickness of 1 micron to 30 microns; an air flow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); a pore size value of 0.05 microns to 2 microns; a bubble point of 10 psi (68.9 KPa) to 60 psi (413.7 KPa); and a ratio of air permeability to bubble point of up to 0.05 (fpm / psi) (0.0037 m / sec / MPa).

[0008] Such porous polytetrafluoroethylene membranes can be used in fuel cell and filtration applications, for example, where higher air and liquid permeability for a given pore size is generally desirable.

[0009] Where the term "comprising" or variations such as "comprise" or "comprises" is used in the specification, these terms are open-ended and do not exclude other steps or elements not expressly included. The term "consisting of" is intended to mean an exclusive listing, i.e., a listing without any elements other than those recited. The term "consisting essentially of, as used herein, is intended to refer to a listing of elements that is the same as the term "comprising," except that it does not recite any elements that are not specifically listed.

[0010] The words "preferred" and "preferably" refer to embodiments of the disclosure that can provide certain benefits under certain circumstances. However, other embodiments can also be preferred under the same or other circumstances. Additionally, 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 disclosure.

[0011] As used herein, the term "room temperature" refers to a temperature of 20 °C to 25 °C or 22 °C to 25 °C.

[0012] In this application, the terms "a," "an," and "the" are not intended to refer to only a singular entity but include the general class of which a specific example can be used for illustration. The terms "a" (or "an"), "the" and "at least one" are used interchangeably.

[0013] The phrases "at least one of" and "comprises at least one of," as used herein, refers to any one of the items in the list and any combination of two or more items in the list.

[0014] As used herein, the term "or" is generally employed in its usual sense, including "and / or," unless the context clearly indicates otherwise. The term "and / or" means one or all of the listed elements or a combination of any two or more of the listed elements.

[0015] Also herein, it is assumed that all numbers are modified by the term "about" and preferably by the term "exactly." As used herein in connection with a measured quantity, the term "about" refers to a variation in the measured quantity, as expected by a person of skill in the art, such as by a level of care taken in making the measurement and operating with an accuracy commensurate with the purpose of the measurement and the precision of the measuring equipment used.

[0016] Also herein, recitation of ranges of values by endpoints includes all values falling within the range and 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.).

[0017] Herein, "up to" a certain number (e.g., up to 50) includes the number (e.g., 50).

[0018] References throughout this specification to "one embodiment," "an embodiment," "certain embodiments," or "some embodiments," and the like, mean that a particular feature, structure, constituent, or characteristic described in connection with the embodiment is included in at least one embodiment of the application. Therefore, appearances of such phrases in various places throughout this specification are not necessarily intended to refer to the same embodiment. Further, when a particular feature, structure, constituent, or characteristic is described in connection with an embodiment, it is submitted that it is within the purview of one of ordinary skill in the art to effect such feature, structure, constituent, or characteristic in connection with another embodiment, whether or not the other embodiment is described or otherwise presented in this specification. In addition, it is within the purview of one of ordinary skill in the art to make

[0019] The above summary of the disclosure of the application is not intended to describe each disclosed embodiment or every implementation of the present application. The description which follows more particularly exemplifies illustrative embodiments. At several places throughout this application, guidance is provided through lists of examples, which examples can be used individually or in any combination. In each instance, the list of examples is not meant to be exhaustive of possible combinations. Thus, the scope of the disclosure should not be limited to the specific illustrative structures described herein, but rather extends at least to the structures described in the language of the claims, and equivalents of those structures. Any element expressed in this specification as a choice of alternatives positive ly recited as an option in the specification can be explicitly included in or excluded from the claims in any desired combination. Although various theories and possible mechanisms have been discussed throughout this document, in no case is such discussion to be construed as a limitation on the claimed subject matter. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 SEM of the film of Example 1.

[0021] Figure 2 SEM of the film of Example 2. DETAILED DESCRIPTION

[0022] The present disclosure provides a unique porous polytetrafluoroethylene (PTFE) film having a microstructure that is substantially only microfibers (i.e., substantially no nodes) and substantially symmetric tensile peak stress (i.e., the tensile peak stress in the machine direction is substantially the same as the tensile peak stress in the cross direction). In this context, "substantially only microfibers (i.e., substantially no nodes)" means less than 100 nodes per 100 microns 2Also, "substantially symmetric" tensile peak stress means that the tensile peak stress of the film in the machine direction is within 10% of the tensile peak stress in the cross direction. If the machine direction and cross direction cannot be distinguished, substantially symmetric tensile peak stress means that the film has a percent balance of orthogonal dimensions within 10%.

[0023] More specifically, the present disclosure provides a porous PTFE film comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersections, the film having a percent balance of orthogonal dimensions within 10%. This means that for a film having a tensile peak stress in the machine direction and a tensile peak stress in the cross direction, the tensile peak stress in the machine direction is within 10% of the peak tensile stress in the cross direction.

[0024] In certain embodiments, the PTFE film has a percent balance of orthogonal dimensions within 5% or within 1%. That is, the tensile peak stress of the film in the machine direction is within 5% or within 1% of the tensile peak stress in the cross direction.

[0025] Peak stress is defined as the peak force of a test sample divided by the cross-sectional area. The percent balance of orthogonal dimensions (machine direction ("MD") versus cross direction ("CD" or "CMD")) is calculated as the difference in peak stress between the orthogonal dimensions divided by the ratio of the machine direction. Alternatively, the percent balance of orthogonal dimensions (machine direction versus cross direction) can be calculated as the difference in peak stress between the orthogonal dimensions divided by the ratio of one of the dimensions with the higher strength (machine direction or cross direction) of the two orthogonal dimensions.

[0026] The percent balance is described in equation form below.

[0027]

[0028]

[0029] If the strength in the machine direction is higher than the cross direction, the % balance of strength can alternatively be calculated as:

[0030]

[0031] The properties of a substantially symmetric strong film with substantially no nodes are generally desirable in applications such as fuel cells, filters (e.g., for exhaust and microfiltration), and semiconductors in enhancing the performance of the film. For example, a symmetric strong film can improve the cross-membrane flux by reducing the resistance to fluid ion flow. Similarly, for applications where the film can be cut into smaller sizes, the balanced strength provides dimensional stability under thermal and mechanical stresses.

[0032] In certain embodiments, the film has a tensile peak stress of at least 5 MPa in each of the machine direction and the transverse direction at room temperature. In certain embodiments, the film has a tensile peak stress of up to 130 MPa in each of the machine direction and the transverse direction at room temperature. The tensile peak stress can be determined according to ASTM D882-18 using an EJA series tensile testing machine (available from Thwing-Albert, West Berlin, NJ) with a crosshead speed of 0.24 inch / minute (6.09 mm / min) and a gauge length of 60 millimeters (mm) (at the start of the test; the sample is cut to a 15 mm width).

[0033] In certain embodiments, the PTFE film has a thickness of at least 1 micron. In certain embodiments, the film has a thickness of up to 30 microns, or up to 20 microns, or up to 10 microns.

[0034] Typically, as a film becomes thinner and thinner (e.g., by stretching), it loses strength. In certain embodiments of the films described herein, if the film becomes thinner and thinner (e.g., by stretching), its strength increases until the strength reaches a maximum and then decreases.

[0035] In certain embodiments, the PTFE film has a pore size value of at least 0.05 microns (i.e., micrometers). In certain embodiments, the film has a pore size value of up to 2 microns. The pore size value is the average of the average pore sizes of a plurality of samples of the film, where the average pore size is the average pore size of one sample of the film. The pore size value can be measured using a PMI Advanced Perm Porosimeter from Porous Materials Inc., Ithaca, NY, using the preprogrammed capillary flow program in the PMI Advanced Perm Porosimeter with the following settings: a maximum flow of 100,000 cm 3 / min; a bubble flow of 27.9 cm 3 / min; and an old bubble time F / PT of 50.

[0036] 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 up to 10 fpm (0.0508 m / sec) or up to 5 fpm (0.0254 m / sec). This can be determined using the air flow test described in ASTM D737-18.

[0037] By applying a pressure differential, a liquid with a surface free energy less than that of the stretched porous PTFE can be extruded from the structure. This purging will occur first from the largest channels. Then channels are created through which a significant air flow can occur. The air flow appears as a steady stream of small bubbles through the liquid layer on top of the sample. The pressure at which the first significant air flow occurs is called the bubble point, and this pressure 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 membrane structure and is often correlated with some other type of performance criteria, such as filtration efficiency.

[0038] The bubble point can be measured according to the procedure in ASTM F316-03 (2011). Isopropyl alcohol (IPA) is typically used as the wetting fluid to fill the pores of the test sample. The bubble point is the air pressure required to displace IPA from the largest pores of the test sample and to create the first continuous stream of air bubbles (noticeable by their rising through the IPA layer covering the porous media). This measurement provides an estimate of the largest pore size.

[0039] 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 up to 60 psi (413.7 KPa).

[0040] In certain embodiments, the PTFE membrane has a ratio of IPA flux to pore size value of at least 3: 1 for a pore size value of 1 micron or less. In certain embodiments, the PTFE membrane has a ratio of IPA flux to pore size value of up to 30: 1 for a pore size of 1 micron or less. The IPA flux can be determined using the volumetric flow of 100% IPA through a 42 mm sample at 10 psi (68.9 kPa). The volume of IPA can be collected in a graduated cylinder and the time for a fixed volume of IPA can be calculated. For example, the time required for 100 mL of IPA to flow over a membrane sample when the membrane is subjected to an upstream pressure of 10 psi (68.9 kPa) can be measured. The pore size value can be measured using a PMI Advanced Perm Porosimeter from Porous Materials, Inc. of Ithaca, New York as described above.

[0041] In certain embodiments, the microfibers of the PTFE membrane have an average fiber diameter of at least 5 nanometers (nm), or at least 10 nm. In certain embodiments, the microfibers have an average fiber diameter of up to 200 nm, or up to 150 nm. This can be determined with SEM.

[0042] Thus, in certain embodiments, the PTFE membrane has a tensile peak stress ratio microfiber diameter of at least 190 MPa / micron. Although there is no upper limit to this, in some embodiments, the PTFE membrane has a tensile peak stress ratio microfiber diameter of up to 900 MPa / micron.

[0043] Thus, in certain embodiments, the PTFE membrane has a ratio of air flow (i.e., air permeability) to bubble point of at least 0.008 (fpm / psi) (0.0059 m / sec / MPa). In some embodiments, the PTFE membrane has a ratio of air flow to bubble point of up to 0.05 (fpm / psi) (0.0037 m / sec / MPa).

[0044] The symmetry of the membrane structure can be created by controlling the fiber diameter, the inter-nodal distance, and the nodal size of the precursor material.

[0045] To make the PTFE membrane, a PTFE raw material, typically having a low amorphous content and a crystallinity of at least 98%, is used as a raw material. The polytetrafluoroethylene used can be in the form of a coagulated dispersion or a fine powder. Suitable commercially available resins include those available from Chemours (Wilmington, DE) under the trade designation 601x and from Daikin (Orangeburg, NY) under the trade designation F131.

[0046] This PTFE raw material (i.e., resin) is then typically made into a paste by uniformly mixing it with a hydrocarbon extrusion aid, such as mineral spirits (e.g., ISOPAR K available from Exxon Mobil), naphtha, 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 up to 20 wt-% based on the total weight of the paste. Due to, for example, evaporation, the final product can include an amount of extrusion aid (e.g., 15.25 wt-%) that is less than the amount added (e.g., 18 wt-%).

[0047] The paste is then molded into a shape dictated by the intended use of the finished product by a molding process that imparts shear deformation, such as extrusion molding or calendering molding. Examples of extrusion dies and extrusion molding processes that can be used for this initial step are described in U.S. Patent Nos. 3,315,020 (Gore) and 3,953,566 (Gore); however, to produce the tape used to produce the films described herein, a different draw ratio is used than that used in these patents (e.g., the draw ratio for Gore is 1.4 + / - 0.3) and the initially unbalanced fibrillated tape is not used, but the finished product is a balanced film.

[0048] The paste is typically compressed into a cylinder and molded into the form of a tape (e.g., by plunger extrusion), but the shape is not necessarily limited to this, and the article can be molded into various cross-sectional shapes, such as a rod or tube, at temperatures of 75°F to 115°F (23.9°C to 46.1°C), depending on the intended use of the finished product.

[0049] If desired, two or more layers of tape can be laminated together and calendered between two rollers. This calendering can be done multiple times, either wet, dry, or a combination of wet and dry, to achieve the desired thickness. Typically, the desired thickness is 20 mils (508 microns) or less. In certain embodiments, the desired thickness of this intermediate calendered product is at least 11 mils (279 microns). The density of this intermediate calendered 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 this intermediate calendered product is up to 2.2 g / cc.

[0050] After the first or second calendering step, the extrusion aid is typically driven off with heat (e.g., at a temperature of 293°F (145°C)). This intermediate calendered product has a microstructure of microfibers and nodes.

[0051] Typically, the wet intermediate calendered product is not stretched compared to the process described in U.S. Patent No. 5,476,589 (Bacino).

[0052] To form an expanded film (ePTFE film), the dry intermediate calendered product (e.g., tape) is initially stretched longitudinally in a single step or multiple steps to a sufficiently high combined stretch to produce an inter-nodal distance of at least 1.5 microns and a fiber diameter of 1 micron or less. Typically, the combined (i.e., multiplicative) longitudinal stretch (i.e., longitudinal expansion) is at a ratio of 40: 1 or less, 30: 1 or less, or 20: 1 or less. In an example embodiment, the combined longitudinal stretch ratio is 18: 1. In some embodiments, the combined longitudinal stretch is at a ratio of at least 6: 1. These values are obtained at temperatures greater than 400°F (204.4°C) and typically up to 665°F (351.7°C).

[0053] Next, after the longitudinal expansion, the material is expanded laterally at a sufficiently high ratio to produce a fiber diameter of 1 micron or less. Typically, the lateral stretch (i.e., lateral expansion) is at a ratio of at least 10: 1 at temperatures of 500°F (260°C) or greater. For example, the lateral stretch ratio is 21: 1. In some embodiments, the lateral stretch is at a ratio of up to 40: 1 at temperatures of 500°F (260°C) or greater up to the melting temperature of PTFE.

[0054] After expansion, in certain embodiments, the film is set (i.e., sintered, as described in U.S. Patent No. 3,953,566 (Gore)) at temperatures of 716°F (380°C) or less. In certain embodiments, the film is set at temperatures of at least 617°F (335°C).

[0055] The resulting film has a tensile peak stress in both directions that is substantially similar of at least 19 MPa at a temperature of 72°F (22°C).

[0056] By this method, an open or porous but strong structure is provided that provides a high air permeable film. The film is exceptionally strong despite the large number of pores and despite being very thin.

[0057] The ePTFE films of the present disclosure have many uses, such as a cell separator in air filters, a humidifier separator, or a pervaporation separator. They can also be used as a fabric material for use in applications where a clean environment is desired.

[0058] Exemplary Embodiments

[0059] Example 1 is a porous polytetrafluoroethylene membrane comprising (or consisting essentially of, or consisting of) a nonwoven web having a microstructure with substantially only microfibers fused at intersections, the membrane having a percent balance of orthogonal dimensions within 10%. Preferably, the membrane has a tensile peak stress in the machine direction and a tensile peak stress in the cross direction, wherein the tensile peak stress in the machine direction is within 10% of the peak tensile stress in the cross direction.

[0060] Example 2 is the membrane of Example 1 having a percent balance of orthogonal dimensions within 5%. Preferably, the tensile peak stress in the machine direction of the membrane is within 5% of the tensile peak stress in the cross direction.

[0061] Example 3 is the membrane of Example 2 having a percent balance of orthogonal dimensions within 1%. Preferably, the tensile peak stress in the machine direction of the membrane is within 1% of the tensile peak stress in the cross direction.

[0062] Example 4 is the membrane of any of the preceding examples having a tensile peak stress in each of the machine direction and the cross direction of at least 5 MPa at room temperature.

[0063] Example 5 is the membrane of any of the preceding examples having a tensile peak stress in each of the machine direction and the cross direction of up to 130 MPa at room temperature.

[0064] Example 6 is the membrane of any of the preceding examples having a thickness of at least 1 micron.

[0065] Example 7 is the membrane of any of the preceding examples having a thickness of up to 30 microns, up to 20 microns, or up to 10 microns.

[0066] Example 8 is the membrane of any of the preceding examples having a pore size value of at least 0.05 microns.

[0067] Example 9 is the membrane of any of the preceding examples having a pore size value of up to 2 microns.

[0068] Example 10 is the membrane of any of the preceding examples having an air flow of at least 0.5 fpm (0.00254 m / sec).

[0069] Example 11 is the membrane of any of the preceding examples having an air flow of up to 10 fpm (0.0508 m / sec).

[0070] Example 12 is the film of any of the preceding examples having a bubble point of at least 10 psi (68.9 KPa) or at least 20 psi (137.9 KPa).

[0071] Example 13 is the film of any of the preceding examples having a bubble point of up to 60 psi (413.7 KPa).

[0072] Example 14 is the film of any of the preceding examples having a ratio of IPA flux to pore size value of at least 3 for a pore size of 1 micron or less.

[0073] Example 15 is the film of any of the preceding examples having a ratio of IPA flux to pore size value of up to 30 for a pore size of 1 micron or less.

[0074] Example 16 is the film of any of the preceding examples, wherein the microfibers have an average fiber diameter of at least 5 nm (or at least 10 nm).

[0075] Example 17 is the film of any of the preceding examples, wherein the microfibers have an average fiber diameter of up to 200 nm (or up to 150 nm).

[0076] Example 18 is a porous polytetrafluoroethylene film comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersection points, the film having a percent balance in the orthogonal dimensions within 10%, or within 5%, or within 1% (preferably, the film has a tensile peak stress in the machine direction and a tensile peak stress in the cross direction, wherein the tensile peak stress in the machine direction is within 10%, or within 5%, or within 1% of the tensile peak stress in the cross direction); and further wherein the film has a ratio of IPA flux to pore size value of 3 to 30 for a pore size of 1 micron or less.

[0077] Example 19 is the film of Example 18 having a tensile peak stress in each of the machine and cross directions of at least 5 MPa at room temperature.

[0078] Example 20 is the film of Example 18 or 19 having a tensile peak stress in each of the machine and cross directions of up to 30 MPa at room temperature.

[0079] Example 21 is a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersection points, the membrane having a percent balance of orthogonal dimensions within 10%, or within 5%, or within 1% (preferably, the membrane has a tensile peak stress in the machine direction and a tensile peak stress in the cross direction, wherein the tensile peak stress in the machine direction is within 10%, or within 5%, or within 1% of the tensile peak stress in the cross direction); and further wherein the membrane has:

[0080] a thickness of 1 micron to 30 microns;

[0081] an air flow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec);

[0082] a pore size value of 0.05 microns to 2 microns;

[0083] a bubble point of 10 psi (68.9 KPa) to 60 psi (413.7 KPa); and

[0084] a ratio of IPA flux to pore size value of 3 to 30 for a pore size of 1 micron or less.

[0085] Example 22 is the membrane of Example 21 having a tensile peak stress in each of the machine direction and the cross direction of 5 MPa to 130 MPa at room temperature.

[0086] Example 23 is the membrane of any of the preceding examples having a tensile peak stress ratio microfiber diameter of at least 190 MPa / micron.

[0087] Example 24 is the membrane of any of the preceding examples having a ratio of air flow to bubble point of at least 0.008 (fpm / psi) (0.0059 m / sec / MPa).

[0088] Example 25 is the membrane of any of the preceding examples having a ratio of air flow to bubble point of up to 0.05 (fpm / psi) (0.0037 m / sec / MPa).

[0089] Example 26 is a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure with substantially only microfibers fused at intersections, the membrane having a percent balance of orthogonal dimensions within 10%, or within 5%, or within 1% (preferably, the membrane has a tensile peak stress in the machine direction and a tensile peak stress in the cross direction, wherein the tensile peak stress in the machine direction is within 10%, or within 5%, or within 1% of the tensile peak stress in the cross direction); and further wherein the membrane has:

[0090] 1 micron to 30 microns in thickness;

[0091] 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec) air flow rate;

[0092] 0.05 micron to 2 microns in pore size value;

[0093] 10 psi (68.9 KPa) to 60 psi (413.7 KPa) bubble point; and

[0094] a ratio of air permeability to bubble point of up to 0.05 (fpm / psi) (0.0037 m / sec / MPa).

[0095] Example 27 is the membrane of any of the preceding examples having a thickness of 1 micron to 30 microns, and as the membrane becomes thinner and thinner, the strength increases until the strength reaches a maximum and then decreases.

[0096] Example 28 is a fuel cell comprising the porous polytetrafluoroethylene membrane of any of Examples 1 to 27.

[0097] Example 29 is a filter comprising the porous polytetrafluoroethylene membrane of any of Examples 1 to 27.

[0098] Example 30 is a semiconductor comprising the porous polytetrafluoroethylene membrane of any of Examples 1 to 27.

[0099] Examples

[0100] Objects and advantages of the disclosure will be further understood by those skilled in the art upon consideration of the following examples, the specification being deemed not to limit the disclosure to the specific materials or conditions recited in these examples, but rather the specific examples being included solely to illustrate the disclosure.

[0101] Example 1

[0102] PTFE fine powder (601X, Chemours, Wilmington, DE) was mixed with extrusion aid (D-80 by ExxonMobil) at 15.25 wt-% and preformed and plod 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 had a weight of approximately 2511 lb f / in 2 (psi) (17312 KPa) tensile stress in the machine direction and a tensile strength of approximately 1506 lb f / in 2 (10383 KPa) in the cross-machine direction. The dried tape was then stretched in the machine direction at a temperature of 232 °C at a stretch ratio of 9: 1 and elongated in the opposite direction at a temperature of 260 °C to 380 °C at a stretch ratio of 21.6. The resulting final film had a tensile stress in the machine direction of 4429 lb f / in 2 (30.53 MPa) ("MD peak stress") and a tensile stress in the cross-machine direction of 4702 lb f / in 2 (32.42 MPa) ("CMD peak stress"). Figure 1 SEM of the film is shown. Table 1 lists properties such as air permeability, bubble point, pore size, and IPA flux.

[0103] Table 2 demonstrates that as the film of Example 1 becomes thinner, its strength increases until the strength reaches a maximum and then decreases. Sample 1 = Example 1. Sample 2 (prepared as described above) was stretched in the MD at a stretch ratio of 12.75 and in the CMD at a stretch ratio of 30.2 as compared to Sample 1. Everything else between the two samples remained the same. Sample 3 was stretched in the MD at a stretch ratio of 18 and in the CMD at a stretch ratio of 38.8 as compared to Sample 1.

[0104] Example 2

[0105] PTFE fine powder (601X by Chemours) was mixed with extrusion aid (D-80 by ExxonMobil) at 20 wt-% and preformed and plod 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 had a weight of approximately 536 lb f / in 2 (3.7 MPa) tensile stress in the machine direction and a tensile strength of approximately 420 lb f / in 2 The dried tape was then stretched in the cross-machine direction at a temperature of 260 °C at a stretch ratio of 10.4:1 and in the opposite direction at a temperature of 260 °C at a stretch ratio of 10.4. The resulting final film had a tensile strength of 2450 lb f / in 2 A tensile stress of 2234 lb f / in 2 A tensile stress of 2234 lb Figure 2 SEM of the film is shown. Table 1 lists the properties of the film.

[0106] Table 1:

[0107]

[0108] Table 2:

[0109] Thickness Peak Stress in the Machine Direction Peak Stress in the Cross Direction Sample 1 7.8 microns 30.53 MPa 32.42 MPa Sample 2 3.8 microns 78.06 MPa 79.46 MPa Sample 3 3.04 microns 52.20 MPa 56.29 MPa

[0110] The complete disclosure of the patents, patent documents, and publications cited herein are incorporated by reference herein in their entirety. In the event of any conflict between the disclosure of the specification and that of any document incorporated by reference, the specification shall control. Various modifications and alterations to this disclosure will become apparent to those of ordinary skill in the art without departing from the scope and spirit of this disclosure. It should be understood that this disclosure is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the disclosure intended to be limited only by the claims set forth herein as follows.

Claims

1. A porous polytetrafluoroethylene membrane composed of polytetrafluoroethylene, said porous polytetrafluoroethylene membrane being a nonwoven web having a microstructure of microfibers with less than 100 nodes / 100 µm² fused at intersections, said membrane having 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% of the peak tensile stress in the transverse direction, and wherein the peak tensile stress in the machine direction and the peak tensile stress in the transverse direction reach a maximum of 130 MPa at room temperature.

2. A porous polytetrafluoroethylene membrane composed of polytetrafluoroethylene, the porous polytetrafluoroethylene membrane being a nonwoven mesh having a microstructure of microfibers with less than 100 nodes / 100 µm² fused at intersections, the membrane having 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% of the peak tensile stress in the transverse direction, and wherein the thickness of the membrane is from 1 micrometer to 3.8 micrometers.

3. A porous polytetrafluoroethylene membrane composed of polytetrafluoroethylene, the porous polytetrafluoroethylene membrane being a nonwoven mesh having a microstructure of microfibers with less than 100 nodes / 100 µm² fused at intersections, the membrane having 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% of the peak tensile stress in the transverse direction, and wherein the thickness of the membrane is 7.8 micrometers to 10 micrometers.

4. A porous polytetrafluoroethylene membrane composed of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membrane is a nonwoven mesh having a microstructure in which microfibers of less than 100 nodes / 100 µm² are fused at their intersections, the membrane having 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 5% of the peak tensile stress in the transverse direction.

5. The membrane according to any one of claims 1-4, having a tensile peak stress of at least 5 MPa in each of the machine direction and the transverse direction.

6. The membrane according to any one of claims 1-4, having a pore size of 0.05 micrometers to 2 micrometers as measured using a porosimeter, said porosimeter having a maximum flow rate of 100,000 cm³. 3 / min, bubble flow rate is 27.9 cm 3 / min, the original bubble time F / PT is 50.

7. The membrane according to any one of claims 1-4, having an air flow rate of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec).

8. The membrane according to any one of claims 1-4, having a bubble point of 10 psi (68.9 kPa) to 60 psi (413.7 kPa).

9. The membrane according to any one of claims 1-4, for a pore size of 1 micrometer or smaller, the membrane has an IPA flux to pore size value ratio of 3 to 30, wherein the pore size is measured using a porosimeter having a maximum flow rate of 100,000 cm⁻². 3 / min, bubble flow rate is 27.9 cm 3 / min, the original bubble time F / PT is 50, and the IPA flux is determined by the volumetric flow rate of 100% IPA through a 42 mm sample at 10 psi (68.9 kPa).

10. The membrane according to any one of claims 1-4, wherein, The microfibers have an average fiber diameter of 5 nm to 200 nm.

11. The membrane of claim 9, having at room temperature a peak tensile stress of at least 5 MPa in each of the machine direction and the transverse direction.

12. A fuel cell comprising a porous polytetrafluoroethylene membrane as described in any of the preceding claims.

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