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 of related applications This application claims the interests relating to U.S. Provisional Patent Application No. 62 / 780,776, filed on 17 December 2018, the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002] Porous polytetrafluoroethylene (PTFE) membranes are known to have asymmetrical 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 symmetrical strength are often desirable. [Overview of the Initiative] [Means for solving the problem]

[0003] This disclosure provides a porous polytetrafluoroethylene (PTFE) membrane comprising a nonwoven web having a microstructure substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10% or less. Preferably, the membrane has a tensile peak stress (peak force / cross-sectional area) in the longitudinal direction and a tensile peak stress in the transverse direction, where the tensile peak stress in the longitudinal direction is 10% or less of the tensile peak stress in the transverse direction.

[0004] In one embodiment, the disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of no more than 10% (preferably, the membrane has longitudinal tensile peak stress and transverse tensile peak stress, where the longitudinal tensile peak stress is no more than 10% of the transverse tensile peak stress); and the membrane further has an IPA flux to pore diameter ratio of at least 3, and typically 30 or less, for pore diameters of 1 micron or less.

[0005] In one embodiment, the disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10% or less (preferably, the membrane has longitudinal and transverse tensile peak stresses, where the longitudinal tensile peak stress is 10% or less of the transverse tensile peak stress); and the membrane further has: a thickness of 1 to 30 microns; an airflow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); a pore size of 0.05 to 2 microns; a foaming point of 10 psi (68.9 kPa) to 60 psi (413.7 kPa); and an IPA flux to pore size ratio of 3 to 30 (i.e., 3:1 to 30:1) for pore sizes of 1 micron or less.

[0006] In one embodiment, the disclosure provides a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10%, 5%, or 1% (preferably, the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, where the tensile peak stress in the longitudinal direction is greater than the tensile peak stress in the transverse direction). The membrane has a tensile peak stress of 10%, 5%, or 1% or less; and the membrane further has a thickness of 1 to 30 microns; an airflow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); a pore size of 0.05 to 2 microns; a foaming point of 10 psi (68.9 kPa) to 60 psi (413.7 kPa); and an air permeability to foaming 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 cells and filtration applications where higher permeability of air and liquid to a given pore size is often desired.

[0008] The term “comprises” and its variations are not restrictive when they appear in the specification and claims. Such terms are understood to mean encompassing the steps or elements or groups of steps or elements described, but not to exclude any other steps or elements or groups of steps or elements. “Consisting of” means encompassing and being limited to any of the following phrases. Thus, the phrase “consisting of” indicates that the enumerated elements are necessary or essential, and no other elements can exist. “Consisting essentially of” means encompassing any of the elements enumerated after this phrase, and being limited to other elements that do not interfere with or contribute to the actions or functions specified for the elements enumerated in this disclosure. Therefore, the phrase "consisting essentially of" indicates that the listed elements are necessary or essential, while the other elements are optional and may or may not be present, depending on whether they substantially affect the actions or actions related to the listed elements.

[0009] The terms “preferred” and “preferred” refer to embodiments of the disclosure that may provide certain benefits under certain circumstances. However, other embodiments may also be preferred under the same or other circumstances. Furthermore, the description of one or more preferred embodiments does not mean that other embodiments are not useful, nor is it intended to exclude other embodiments from the scope of the 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 singular entities, but include general classifications for which specific examples may be used. 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 ~” that follow an enumeration refer to any one of the items in the enumeration, or any combination of two or more items in the enumeration.

[0013] As used herein, the term “or” is used in its usual sense, including “and / or,” unless otherwise explicitly stated. The term “and / or” means one or all of the listed elements, or any combination of two or more of the listed elements.

[0014] Furthermore, in this specification, all figures are considered to be modified by the terms “approximately,” and preferably the terms “exactly.” As used herein with respect to measured quantities, “approximately” means the variation in the measured quantity that can be expected by a person skilled in the art who performs the measurement and pays a reasonable level of attention to the purpose of the measurement and the precision of the measuring instrument used.

[0015] Furthermore, in this specification, an enumeration of numerical ranges by endpoints includes all numerical values ​​contained within that range, as well as its endpoints (for example, 1-5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.) and any of the lower ranges (for example, 1-5 includes 1-4, 1-3, 2-4, etc.).

[0016] In this specification, "less than or equal to" a given number (for example, 50 or less) includes that number (for example, 50).

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

[0018] The above summary of the present disclosure is not intended to describe every disclosed embodiment or every implementation of the present invention. The following description more particularly exemplifies illustrative embodiments. Guidance is provided throughout this application in numerous places by way of listing examples, and these examples may be used in various combinations. In each case, the listed descriptions are merely representative groups and should not be construed as exclusive listings. Therefore, the scope of the present disclosure should not be limited to the specific illustrative structures described herein, but rather extends at least to the structures described by the terms of the claims and their equivalents. Alternatively, any of the elements positively described in this specification may be explicitly included in or excluded from the claims in any combination as desired. Various theories and possible mechanisms may be considered in this specification, but in no case should such considerations be construed as limiting the patentable subject matter.

Brief Description of the Drawings

[0019] [Figure 1] FIG. 1 is a SEM of the membrane of Example 1. [Figure 2] FIG. 2 is a SEM of the membrane of Example 2.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present disclosure provides a unique porous polytetrafluoroethylene (PTFE) membrane that consists essentially of microfibrils only (i.e., having substantially no nodes) and has a substantially symmetric tensile peak stress (i.e., a tensile peak stress in a longitudinal direction substantially the same as the transverse direction). In this context, "consisting essentially of microfibrils only" (i.e., having substantially no nodes) means less than 100 nodes / 100 microns, and "substantially symmetric" tensile peak stress means that the membrane has a tensile peak stress in the longitudinal direction within 10% of the tensile peak stress in the transverse direction. When the longitudinal and transverse directions cannot be distinguished, substantially symmetric tensile peak stress means that the membrane has a balance ratio of orthogonal dimensions within 10%. 2 More specifically, the present disclosure provides a porous PTFE membrane comprising a non-woven web having a microstructure formed essentially only of microfibrils joined at tissue points, the membrane having a balance ratio of orthogonal dimensions within 10%. This means that for a membrane having a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, the tensile peak stress in the longitudinal direction is within 10% of the tensile peak stress in the transverse direction.

[0021] In certain embodiments, the PTFE membrane has a balance ratio of orthogonal dimensions within 5% or within 1%. That is, the membrane has a tensile peak stress in the longitudinal direction within 5% or within 1% of the tensile peak stress in the transverse direction.

[0022]

[0023] Peak stress is defined as the peak force relative to the cross-sectional area of ​​the specimen. The ratio used to determine the balance between orthogonal dimensions (longitudinal ("MD") versus transverse ("CD" or "CMD")) is calculated as the ratio of the difference in peak stress between the orthogonal dimensions relative to the longitudinal direction. Alternatively, the balance between orthogonal dimensions (longitudinal vs. transverse) can be calculated as the ratio of the difference in peak stress between the orthogonal dimensions relative to the dimension with higher strength (either longitudinal or transverse).

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

number

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

number

[0026] The properties of substantially symmetrically strong membranes with virtually no nodes are often desirable for improving membrane performance in applications such as fuel cells, filters (e.g., for aeration and microfiltration), and semiconductors. For example, symmetrically strong membranes 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 tensile peak stress of at least 5 MPa in both the longitudinal and transverse directions at room temperature. In certain embodiments, the membrane has a tensile peak stress of 130 MPa or less in both the longitudinal and transverse directions at room temperature. The tensile peak stress can be measured using an EJA series tensile testing machine (available from Thwing-Albert, WestBerlin, NJ) according to ASTM D882-18 (grip spacing of 60 mm at the start of the test; sample cut to a width of 15 mm; and fracture strength tested at 0.24 inches / minute (6.09 mm / min)).

[0028] In certain embodiments, the PTFE membrane has a thickness of at least 1 micron. In 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), its strength is lost. In certain embodiments of the membranes described herein, when the membrane is made thinner (e.g., by stretching), its strength increases up to a maximum value and then decreases.

[0030] In certain embodiments, the PTFE membrane has a pore size of at least 0.05 microns (i.e., 1 micrometer). In certain embodiments, the membrane has a pore size of 2 microns or less. The pore size is the average of the average pore sizes of multiple membrane samples, where the average pore size is the average pore size of one membrane sample. The pore size was measured using a PMI Advanced Perm Porometer manufactured by Porous Materials Inc., Ithaca, NY, with the Capillary Flow Procedure pre-programmed in the PMI Advanced Perm Porometer set to the following setting: 100,000 cm². 3 Maximum flow rate per minute: 27.9 cm³ 3The bubble flow rate can be measured using a value of 50 past bubble times per minute (F / PT).

[0031] In certain embodiments, the PTFE membrane has an airflow of at least 0.5 cubic feet / square feet / minute (fpm) (0.00254 m / sec). In certain embodiments, the membrane has an airflow of 10 fpm (0.0508 m / sec) or less, or 5 fpm (0.0254 m / sec) or less. This can be measured using the airflow test described in ASTM D737-18.

[0032] Liquids with a lower surface free energy than stretched porous PTFE can be forcibly removed from the structure by applying differential pressure. This removal first occurs through the largest passage, then a passage is formed through which a bulk flow of air can pass. The air flow manifests as a steady flow of small bubbles permeating the liquid layer at the top of the sample. The pressure at which the bulk flow of air first occurs is called the foaming point, which depends on the surface tension of the test fluid and the size of the largest opening. The foaming point can be used as a relative measure of the membrane's structure and is often corrected by several other types of performance criteria, such as filtration efficiency.

[0033] The foaming point can be measured according to the ASTM F316-03 (2011) method. Isopropyl alcohol (IPA) is typically used as the wetting fluid to fill the pores of the specimen. The foaming point is the air pressure required to replace the IPA from the largest pore in the specimen, forming a continuous flow of the first bubbles detectable by rising through the IPA layer covering the porous medium. This measurement allows for an estimation of the largest pore diameter.

[0034] In certain embodiments, the PTFE membrane has a foaming point of at least 10 psi (68.9 kPa). In certain embodiments, the PTFE membrane has a foaming point of at least 20 psi (137.9 kPa). In certain embodiments, the PTFE membrane has a foaming 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 sizes 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 sizes of 1 micron or less. The IPA flux can be measured using the volumetric flow rate of 100% IPA at 10 psi (68.9 kPa) in a 42 mm sample. A fixed amount of IPA can be collected in a graduated cylinder, and the time required for that fixed amount of IPA can be calculated. For example, when an upstream pressure of 10 psi (68.9 kPa) is applied to the membrane, the time required to flow 100 mL of IPA can be measured using a membrane sample. The pore size can be measured using the PMI Advanced Perm Porometer manufactured by Porous Materials Inc., Ithaca, NY.

[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 200 nm or less, or 150 nm or less. This is measurable by SEM.

[0037] Therefore, in certain embodiments, the PTFE membrane has a tensile peak stress for a microfibril diameter of at least 190 MPa / micron. There is no upper limit to this, but in some embodiments, the PTFE membrane has a tensile peak stress for a microfibril diameter of 900 MPa / micron or less.

[0038] Therefore, in certain embodiments, the PTFE membrane has an airflow-to-foaming point ratio of at least 0.008 (fpm / psi) (0.0059 m / sec / MPa). In some embodiments, the PTFE membrane has an airflow-to-foaming point ratio of 0.05 (fpm / psi) (0.0037 m / sec / MPa) or less.

[0039] The symmetry of the membrane structure can be created by controlling the fibril diameter, internode distance, and node size of the precursor material.

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

[0041] The PTFE raw material (i.e., resin) is typically then homogeneously mixed with a hydrocarbon extrusion aid, such as mineral spirits (e.g., ISOPAR K, available from Exxon Mobil), naphtha oil, or other such lubricants, to form a paste. In certain embodiments, the amount of the extrusion aid is typically at least 15 wt% of the total weight of the paste. In certain embodiments, the amount of the extrusion aid is typically 20 wt% or less of 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, by evaporation.

[0042] Next, the paste is molded into the shape required by the intended use of the final product by a molding method that imparts shear deformation, such as extrusion molding or calendering. Examples of extrusion molds and extrusion processes that can be used in this first step are described in U.S. Patent No. 3,315,020 (Gore) and U.S. Patent No. 3,953,566 (Gore); however, to form the tape used in the manufacture of the membrane described herein, a different stretch difference is used than that used in those patents (for example, the stretch difference in Gore is 1.4+ / -0.3), and a balanced fibrillated tape is not used initially, but the final product is a balanced membrane.

[0043] This paste is typically compressed in a cylinder and formed into a tape (e.g., by ram extrusion), but the shape is not necessarily limited to this, and the article may be formed into various 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. This calendering process can be carried out multiple times in a wet state, a dry state, or a combination of wet and dry states to achieve the desired thickness. Typically, the desired thickness is 20 mils (508 micrometers) or less. In certain embodiments, the desired thickness of this intermediate calendered product is at least 11 mils (279 micrometers). The density of this intermediate calendered product (e.g., tape) is typically at least 1.5 grams / cubic centimeter (g / cc), or at least 1.8 g / cc. In certain embodiments, the density of this intermediate calendered product is 2.2 g / cc or less.

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

[0046] Typically, the wet intermediate calender product is not stretched, in contrast to the process described in U.S. Patent No. 5,476,589 (Bacino).

[0047] To form an extended membrane (ePTFE membrane), a dry intermediate calender product (e.g., tape) is first stretched longitudinally in a single step or in multiple steps to a composite stretch of sufficient strength to yield 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 extension) ratio is 40:1 or less, 30:1 or less, or 20:1 or less. In an exemplary embodiment, the composite longitudinal stretch ratio is 18:1. In some embodiments, the composite longitudinal stretch ratio is 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] Next, after longitudinal stretching, the material is stretched transversely at a ratio high enough to yield a fibril diameter of 1 micron or less. Typically, transverse stretching (i.e., transverse stretching) is at a temperature of 500°F (260°C) or higher and at a ratio of at least 10:1. For example, the transverse stretching ratio is 21:1. In some embodiments, transverse stretching is at a temperature of 500°F (260°C) or higher and below the melting point of PTFE, and at a ratio of 40:1 or less.

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

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

[0051] This process provides a rigid, open or porous structure that results in a highly air-permeable membrane. Despite the presence of numerous pores and its thinness, this membrane is remarkably strong.

[0052] The ePTFE membranes of this disclosure have many applications, such as cell diaphragms in air filters, diaphragms in humidifiers, or pervaporation diaphragms. They can also be used as textile materials in applications where a clean environment is required.

[0053] Exemplary Embodiments Embodiment 1 is a porous polytetrafluoroethylene membrane comprising (or substantially comprising, or comprising) a nonwoven web having a microstructure substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10% or less. Preferably, the membrane has a longitudinal tensile peak stress and a transverse tensile peak stress, where the longitudinal tensile peak stress is 10% or less of the transverse tensile peak stress.

[0054] Embodiment 2 is a membrane of Embodiment 1 having a balance ratio of orthogonal dimensions of 5% or less. Preferably, the membrane has a tensile peak stress in the longitudinal direction that is 5% or less of the tensile peak stress in the transverse direction.

[0055] Embodiment 3 is a membrane of Embodiment 2 having a balance ratio of orthogonal dimensions of 1% or less. Preferably, the membrane has a tensile peak stress in the longitudinal direction that is 1% or less of the tensile peak stress in the transverse direction.

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

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

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

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

[0060] Embodiment 8 is a membrane according to any of the embodiments described above, having a pore size of at least 0.05 microns.

[0061] Embodiment 9 is a membrane according to any of the embodiments described above, having a pore size of 2 microns or less.

[0062] Embodiment 10 is a membrane according to any of the embodiments described above, having an airflow of at least 0.5 fpm (0.00254 m / sec).

[0063] Embodiment 11 is a membrane according to any of the embodiments described above, having an airflow of 10 fpm (0.0508 m / sec) or less.

[0064] Embodiment 12 is a membrane according to any of the embodiments described above, having a foaming 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 embodiments described above, having a foaming point of 60 psi (413.7 kPa) or less.

[0066] Embodiment 14 is a membrane according to any of the embodiments described above, having at least 3 IPA flux-to-pore diameter ratios for pore diameters of 1 micron or less.

[0067] Embodiment 15 is a membrane according to any of the embodiments described above, having an IPA flux-to-pore diameter ratio of 30 or less for pore diameters of 1 micron or less.

[0068] Embodiment 16 is a membrane according to any of the embodiments described above, 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 embodiments described above, 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 substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10% or less, 5% or less, or 1% or less (preferably, the membrane has a longitudinal tensile peak stress and a transverse tensile peak stress, where the longitudinal tensile peak stress is 10% or less, 5% or less, or 1% or less of the transverse tensile peak stress); and further wherein the membrane has an IPA flux to pore diameter ratio of 3 to 30 for pore diameters of 1 micron or less.

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

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

[0073] Embodiment 21 is a porous polytetrafluoroethylene membrane comprising a nonwoven web having a microstructure substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10% or less, 5% or less, or 1% or less (preferably, the membrane has a longitudinal tensile peak stress and a transverse tensile peak stress, where the longitudinal tensile peak stress is 10% or less, 5% or less, or 1% or less of the transverse tensile peak stress); and wherein the membrane is: Thickness of 1 to 30 microns; Airflow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); Pore ​​size range: 0.05 microns to 2 microns; A foaming point of 10 psi (68.9 kPa) to 60 psi (413.7 kPa); and For pore sizes of 1 micron or less, the ratio of IPA flux to pore size is 3 to 30. It also possesses.

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

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

[0076] Embodiment 24 is a membrane according to any of the embodiments described above, having an airflow-to-foaming 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 above embodiments having an airflow-to-foaming 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 substantially formed solely from microfibrils joined at structural points, wherein the membrane has a balance ratio of orthogonal dimensions of 10% or less, 5% or less, or 1% or less (preferably, the membrane has a longitudinal tensile peak stress and a transverse tensile peak stress, where the longitudinal tensile peak stress is 10% or less, 5% or less, or 1% or less of the transverse tensile peak stress); and wherein the membrane is: Thickness of 1 to 30 microns; Airflow of 0.5 fpm (0.00254 m / sec) to 10 fpm (0.0508 m / sec); Pore ​​size range: 0.05 microns to 2 microns; A foaming point of 10 psi (68.9 kPa) to 60 psi (413.7 kPa); and Air permeability to foaming point ratio of 0.05 (fpm / psi) (0.0037 m / sec / MPa) or less It also possesses.

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

[0080] Embodiment 28 is a fuel cell containing a porous polytetrafluoroethylene membrane according to any of Embodiments 1 to 27.

[0081] Embodiment 29 is a filter containing a porous polytetrafluoroethylene membrane according to any of Embodiments 1 to 27.

[0082] Embodiment 30 is a semiconductor comprising a porous polytetrafluoroethylene membrane according to any of Embodiments 1 to 27. [Examples]

[0083] The objects and advantages of the present disclosure are further illustrated by the following examples, but the specific materials and their amounts described in these examples, as well as other conditions and details, should not be construed as unduly limiting the present disclosure.

[0084] Example 1 PTFE fine powder (601X, Chemours Co., Wilmington, DE) was mixed with an extrusion aid (Exxon Mobil D - 80) at 15.25 wt% and pre - formed 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). This tape was calendered between two steel rollers and then dried to remove the lubricant used during the mixing step. The resulting tape had a longitudinal tensile stress of approximately 2511 lb f / in 2 (psi) (17312 KPa) and a width - wise tensile strength of approximately 1506 lb f / in 2 (10383 KPa). The dried tape was then stretched longitudinally at a temperature of 232 °C and a draw ratio of 9:1 and drawn in the opposite direction at a temperature of 260 °C to 380 °C and a draw ratio of 21.6. The final membrane produced had a longitudinal tensile stress of 4429 lb f / in 2 (30.53 MPa) (the "MD peak stress") and a width - wise tensile stress of 4702 lb f / in 2 (32.42 MPa) (the "CMD peak stress"). The SEM of the membrane is shown in Figure 1. Characteristics such as air permeability, bubble point, pore size, and IPA flux are listed in Table 1.

[0085] Table 2 demonstrates that thinning the membrane of Example 1 increases its strength up to a maximum value, and then decreases it. Sample 1 = Example 1. Compared to Sample 1 prepared as described above, Sample 2 was stretched to MD at a stretch ratio of 12.75 and to CMD at a stretch ratio of 30.2. All other aspects remained the same between these two samples. Compared to Sample 1, Sample 3 was stretched to MD at a stretch ratio of 18 and to CMD at a stretch ratio of 38.8.

[0086] Example 2 PTFE fine powder (601X, Chemours) was mixed with an extrusion aid (Exxon Mobil D-80) at 20 wt% and preformed and ram-extruded into a tape with a thickness of 47 mils (1193.8 microns) and a width of 8.5 inches (21.6 cm). This 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 A longitudinal tensile stress of 3.7 MPa and approximately 420 lb f / in 2 It had a tensile strength in the width direction of (2.9 MPa). The dried tape was then stretched longitudinally at a temperature of 260°C and a stretch ratio of 10.4:1, and then stretched in the opposite direction at a temperature of 260°C and a stretch ratio of 10.4. The final membrane produced had a strength of 2450 lb. f / in 2 A longitudinal tensile stress of 16.9 MPa and 2234 lb f / in 2 It had a tensile stress in the width direction of (15.4 MPa). The 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 patents, patent documents and publications cited herein are invoked in whole by reference as each of them is invoked individually. In the event of any inconsistency or conflict between this Specified Publication and any disclosures invoked by reference herein, this Specified Publication shall prevail. Various modifications and changes to this Disclosure will be apparent to those skilled in the art who deviate from the scope and intent of this Disclosure. It should be understood that this Disclosure is not intended to be unduly limited by the exemplary embodiments and examples described herein, and that such embodiments and examples are presented merely as examples, and the scope of the Disclosure is intended to be limited only by the claims set forth below herein.

Claims

【Request Item 1】 A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membranes are joined at structural points, with a density of 100 nodes / 100 μm. ２ A porous polytetrafluoroethylene membrane having a microstructure of microfibrils having less than 130 MPa, wherein the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ±10% of the tensile peak stress in the transverse direction, and the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction are 130 MPa or less at room temperature. 【Request Item 2】 A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membranes are joined at structural points, with a density of 100 nodes / 100 μm. ２ A porous polytetrafluoroethylene membrane having a microstructure of microfibrils less than , wherein the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, and the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ±5% of the tensile peak stress in the transverse direction. 【Request Item 3】 A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membranes are joined at structural points, with a density of 100 nodes / 100 μm. ２ A porous polytetrafluoroethylene membrane having a microstructure of microfibrils less than 1.5 mm, wherein the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ±10% of the tensile peak stress in the transverse direction, and the membrane has a thickness of 1 to 3.8 micrometers. 【Request Item 4】 A porous polytetrafluoroethylene membrane made of polytetrafluoroethylene, wherein the porous polytetrafluoroethylene membranes are joined at structural points, with a density of 100 nodes / 100 μm. ２ A porous polytetrafluoroethylene membrane having a microstructure of microfibrils less than 100, wherein the membrane has a tensile peak stress in the longitudinal direction and a tensile peak stress in the transverse direction, the difference between the tensile peak stress in the longitudinal direction and the tensile peak stress in the transverse direction is within ±10% of the tensile peak stress in the transverse direction, and the membrane has a thickness of 7.8 to 10 micrometers.

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