Porous film, multilayered object, and filter element

Abstract

Provided is a porous film comprising polytetrafluoroethylene as a main component. The porous film, in differential scanning calorimetry at a heating rate of 10°C / min, gives a first-run melting curve which has an endothermic peak in the range of 340-350°C. The porous film, in differential scanning calorimetry at a heating rate of 10°C / min, gives a second-run melting curve which has an endothermic peak in the range of 320-330°C. The porous film has crystallites, and the crystallites have a length X along the MD of the porous film and a length Y along the TD of the porous film, the product of the length X and the length Y, XY, being 1,200 nm2 to 1,710 nm2.

Description

Porous membranes, laminates, and filter elements

[0001] The present disclosure relates to a porous membrane, a laminate, and a filter element. This application claims priority to Japanese Patent Application No. 2023-084100, filed May 22, 2023. The entire contents of the Japanese patent application are incorporated herein by reference.

[0002] Conventionally, porous membranes containing polytetrafluoroethylene as a main component have been used as dispersion media and substrate precision filters in the semiconductor-related field and the like (Patent Documents 1 to 6, Non-Patent Document 1).

[0003] Japanese Patent Application Publication No. 2021-54892 International Publication No. 2007 / 011492 International Publication No. 2020 / 251909 International Publication No. 2020 / 251912 Japanese Patent Application Publication No. 2015-226877 Japanese Patent Application Publication No. 2021-178948

[0004] Collection of Polymer Papers (Kobunshi Ronbunshu), Vol. 66, No. 12, pp. 585-590 (Dec., 2009)

[0005] A porous membrane according to one embodiment of the present disclosure is a porous membrane comprising polytetrafluoroethylene as a main component, wherein the porous membrane has an endothermic peak in the range of 340°C to 350°C in a first run melting curve obtained by differential scanning calorimetry at a heating rate of 10°C / min, and an endothermic peak in the range of 320°C to 330°C in a second run melting curve obtained by differential scanning calorimetry at a heating rate of 10°C / min, the porous membrane has crystallites, the crystallites have a length X along the MD direction of the porous membrane and a length Y along the TD direction of the porous membrane, and the product XY of the length X and the length Y is 1200 nm. ２ More than 1710 nm ２ The following is the result.

[0006] Fig. 1 is a schematic enlarged cross-sectional view of a porous membrane according to an embodiment of the present disclosure. Fig. 2 is a schematic enlarged cross-sectional view of a laminate according to an embodiment of the present disclosure. Fig. 3 is a perspective view of a filter element (1) according to an embodiment of the present disclosure. Fig. 4 is a perspective view of a filter element (2) according to an embodiment of the present disclosure.

[0007] [Problem that the present disclosure aims to solve] When porous membrane is used as a pleated cartridge filter, it is folded into pleats and then heated to be heat-set, so the porous membrane is required to have thermal stability.This is because stretched polytetrafluoroethylene is inherently prone to shrinkage, and the shrinkage of polytetrafluoroethylene causes the properties of porous membrane (pore size and permeation efficiency) to easily fluctuate.Stretched polytetrafluoroethylene is heated in the sintering process, so that polytetrafluoroethylene melts, and the internal stress caused by stretching is released.Then, it is cooled, and polytetrafluoroethylene is recrystallized, so that porous membrane is formed.However, the porous membrane that contains such polytetrafluoroethylene as the main component has a melting point peak (endothermic peak) of around 345 ° C in the melting curve of the second run obtained by differential scanning calorimetry with a heating rate of 10 ° C / min., so that thermal stability is sometimes insufficient. For the same reason, a laminate comprising one or more porous membranes and a support membrane located on one or both sides of at least one of the porous membranes may also have insufficient thermal stability. Here, excellent "thermal stability" means that "shrinkage of the porous membrane and the laminate due to heating" is unlikely to occur.

[0008] Furthermore, in the sintering process, the internal stress generated by the stretching is released by heating, which acts in a direction that reduces enthalpy, causing the fibers of the porous membrane to aggregate and shrink. In the sintering process, while heating the porous membrane under tension can suppress the shrinkage of the fibers of the porous membrane, it cannot suppress the aggregation of the fibers of the porous membrane, resulting in an enlarged pore size. Therefore, it can be difficult to reduce the pore size of the porous membrane. For the same reason, it can also be difficult to reduce the pore size of a laminate comprising one or more porous membranes and a support membrane located on one or both sides of at least one of the porous membranes.

[0009] For the reasons described above, it may be difficult to provide a laminate having excellent thermal stability and small pore size, the laminate including a porous membrane containing polytetrafluoroethylene as a main component, one or more porous membranes, and a support membrane located on one or both sides of at least one of the porous membranes.

[0010] Therefore, an object of the present disclosure is to provide a porous membrane that combines excellent thermal stability and a small pore size, a filter element including the porous membrane, a laminate that combines excellent thermal stability and a small pore size, and a filter element including the laminate.

[0011] [Effects of the Present Disclosure] According to the present disclosure, it is possible to provide a porous membrane having both excellent thermal stability and a small pore size, a filter element including the porous membrane, a laminate having both excellent thermal stability and a small pore size, and a filter element including the laminate.

[0012] [Description of the embodiment of the present disclosure] First, the embodiments of the present disclosure will be listed and described. [1] A porous film according to one aspect of the present disclosure is a porous film containing polytetrafluoroethylene as a main component, wherein the porous film has an endothermic peak in the range of 340°C to 350°C in a first run melting curve obtained by differential scanning calorimetry at a heating rate of 10°C / min, and an endothermic peak in the range of 320°C to 330°C in a second run melting curve obtained by differential scanning calorimetry at a heating rate of 10°C / min, the porous film has crystallites, and the crystallites have a length X along the MD direction of the porous film and a length Y along the TD direction of the porous film, and the product XY of the length X and the length Y is 1200 nm. ２ More than 1710 nm ２ The following is the result.

[0013] According to the present disclosure, it is possible to provide a porous membrane that combines excellent thermal stability and a small pore size, a filter element including the porous membrane, a laminate that combines excellent thermal stability and a small pore size, and a filter element including the laminate.

[0014] [2] In the above [1], the ratio of the absolute value of the difference between the average bubble point P1a' of the porous membrane after the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1a, relative to the average bubble point P1a of the porous membrane before the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour, is 10% or less, or the ratio of the absolute value of the difference between the average bubble point P1b' of the porous membrane after the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1b, relative to the average bubble point P1b of the porous membrane before the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour, is 10% or less, the average bubble point P1a and the average bubble point P1a' are measured by a bubble point method using a 1a liquid, and the surface tension of the 1a liquid is 13 mN / m, The average bubble point P1b and the average bubble point P1b' may be measured by a bubble point method using liquid 1b, and the surface tension of liquid 1b may be 21 mN / m. This makes it possible to provide a porous membrane having both superior thermal stability and a smaller pore size, a filter element including the porous membrane, a laminate having both superior thermal stability and a smaller pore size, and a filter element including the laminate.

[0015] [3] In the above [2], the average bubble point P1a may be 230 kPa or more and 600 kPa or less, and the average bubble point P1b may be 500 kPa or more and 1130 kPa or less. This makes it possible to provide a porous membrane having a smaller pore size, a filter element including the porous membrane, a laminate having a smaller pore size, and a filter element including the laminate.

[0016] [4] In any of the above [1] to [3], the absolute value of the difference between the Gurley second G1' of the porous membrane after a test in which the porous membrane is left standing in a thermostatic bath at 120°C for 1 hour and the Gurley second G1 of the porous membrane before the test in which the porous membrane is left standing in a thermostatic bath at 120°C for 1 hour and the Gurley second G1 may be 10% or less. This makes it possible to provide a porous membrane having both superior thermal stability and a smaller pore size, a filter element including the porous membrane, a laminate having both superior thermal stability and a smaller pore size, and a filter element including the laminate.

[0017] [5] In any one of the above [1] to [4], the porous membrane may have a mean flow pore size of 80 nm or less. This makes it possible to provide a porous membrane having a smaller pore size, a filter element including the porous membrane, a laminate having a smaller pore size, and a filter element including the laminate.

[0018] [6] A filter element according to one aspect of the present disclosure includes the porous membrane described in any one of [1] to [5] above.

[0019] According to the present disclosure, it is possible to provide a filter element including a porous membrane that combines excellent thermal stability with a small pore size.

[0020] [7] A laminate according to one aspect of the present disclosure is a laminate comprising one or more porous membranes according to the above-mentioned [1] to [5], and a support membrane located on one or both sides of at least one of the porous membranes, wherein the support membrane is porous, and the support membrane contains polytetrafluoroethylene as a main component.

[0021] According to the present disclosure, it is possible to provide a laminate having both excellent thermal stability and a small pore size, and a filter element including the laminate.

[0022] [8] A filter element according to one aspect of the present disclosure includes the laminate described in [7] above.

[0023] According to the present disclosure, it is possible to provide a filter element including a laminate that combines excellent thermal stability with a small pore size.

[0024] [Details of the embodiment of the present disclosure] Hereinafter, one embodiment of the present disclosure (hereinafter referred to as "the present embodiment") will be described. However, the present embodiment is not limited thereto. In this specification, an expression in the form of "A to B" means the upper and lower limits of a range (i.e., A or more and B or less), and when no unit is specified for A and a unit is specified only for B, the unit of A and the unit of B are the same.

[0025] In the present specification, when a compound or the like is represented by a chemical formula, unless the atomic ratio is particularly limited, it is understood to include any conventionally known atomic ratio, and is not necessarily limited to only those within the stoichiometric range.

[0026] [Embodiment 1: Porous Membrane] A porous membrane 1 according to one embodiment of the present disclosure will be described with reference to Fig. 1. One embodiment of the present disclosure (hereinafter also referred to as "the present embodiment") is a porous membrane 1 containing polytetrafluoroethylene as a main component.

[0027] (a) The porous membrane 1 has an endothermic peak in the range of 340°C to 350°C in the melting curve of the first run obtained by differential scanning calorimetry at a heating rate of 10°C / min, and has an endothermic peak in the range of 320°C to 330°C in the melting curve of the second run obtained by differential scanning calorimetry at a heating rate of 10°C / min. This makes it difficult for the fibers of the porous membrane to shrink. As a result, the porous membrane 1 is unlikely to experience "shrinkage due to heating," and therefore has excellent thermal stability.

[0028] (b) The porous film 1 has a crystallite, and the crystallite has a length X along the MD (Machine Direction) direction of the porous film 1 and a length Y along the TD (Transverse Direction) direction of the porous film 1, and the product XY of the length X and the length Y is 1200 nm. ２ More than 1710 nm ２As a result, the polytetrafluoroethylene molecular chains constituting the crystallites are bound together by intermolecular forces, which improves the thermal stability of the porous film and enables the pore size of the porous film 1 to be kept small.

[0029] Therefore, according to the present disclosure, it is possible to provide a porous membrane that combines excellent thermal stability and a small pore size, a filter element including the porous membrane, a laminate that combines excellent thermal stability and a small pore size, and a filter element including the laminate.

[0030] <Composition of Porous Membrane> The porous membrane 1 contains polytetrafluoroethylene as a major component. Here, "major component" refers to the component with the largest content in terms of mass, for example, a component with a content of 90% by mass or more, preferably 95% by mass or more. The porous membrane 1 may be made of polytetrafluoroethylene. The phrase "the porous membrane 1 is made of polytetrafluoroethylene" means that the porous membrane 1 may contain unavoidable impurities, as long as the effects of the present disclosure are achieved. In the present application, "polytetrafluoroethylene" refers to a polymer of tetrafluoroethylene, and is a concept that encompasses both a homopolymer of tetrafluoroethylene and a modified product of the "homopolymer of tetrafluoroethylene." Examples of such modified products include tetrafluoroethylene-hexafluoropropylene copolymer (in other words, perfluoroethylenepropene copolymer; hereinafter, also referred to as "FEP"), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), etc. The modified product may contain hexafluoropropylene (HFP), perfluoro(alkyl vinyl ether) (FVE), or the like in an amount of 0.1 mol % or less.

[0031] In the porous membrane 1, the content of polytetrafluoroethylene is indicated by "-CF ２ -" absorbance at an absorption wavelength of 4.25 μm, and "-CH" indicating FEP ３ The absorbance at the absorption wavelength of 10.18 μm of the "CF group" and the absorbance at the absorption wavelength of 10.18 μm of the "CF group" ３It can be identified based on the absorbance of the "O-group" at an absorption wavelength of 10.07 μm. It has been confirmed that similar results can be obtained even if different measurement ranges are arbitrarily selected for the same porous membrane 1.

[0032] <Shape of porous membrane> The thickness of the porous membrane 1 may be 0.002 mm or more and 0.100 mm or less. If the thickness is less than 0.002 mm, the strength of the porous membrane 1 tends to be insufficient. If the thickness is more than 0.100 mm, the pressure loss during permeation of the filtrate tends to be large. The lower limit of the thickness of the porous membrane 1 may be 0.002 mm or more, 0.005 mm or more, or 0.010 mm or more. The upper limit of the thickness of the porous membrane 1 may be 0.100 mm or less, 0.090 mm or less, or 0.080 mm or less. The thickness of the porous membrane 1 may be 0.005 mm or more and 0.090 mm or less, or 0.010 mm or more and 0.080 mm or less.

[0033] The thickness of the porous membrane 1 can be determined by the following method. First, the thickness is measured at one arbitrary location using a standard digital thickness gauge. Next, the thickness is measured at other arbitrary nine locations using the same standard digital thickness gauge. Next, the average value of the thicknesses at all 10 locations is calculated, thereby determining the thickness of the porous membrane 1.

[0034] It has been confirmed that similar results can be obtained when different measurement points are arbitrarily selected on the same porous membrane 1 and the above measurement is carried out at those measurement points.

[0035] Regarding the shape of the porous membrane 1, it can be understood that the porous membrane 1 is elongated.

[0036] <Density of porous membrane> The density of the porous membrane 1 is 0.05 g / cm ３ 2.00g / cm or more ３ The density may be 0.05 g / cm or less. ３ When the density is less than 2.00 g / cm, the strength of the porous membrane 1 tends to be insufficient. ３If the density is greater than 0.05 g / cm3, the permeation efficiency of the porous membrane 1 tends to decrease. ３ or more, and 0.10 g / cm ３ or more, and 0.15 g / cm ３ The upper limit of the density of the porous membrane 1 is 2.00 g / cm ３ or less, 1.70 g / cm ３ or less, 1.50 g / cm ３ The density of the porous membrane 1 may be 0.10 g / cm or less. ３ 1.70g / cm or more ３ or less, and 0.15 g / cm ３ 1.50g / cm or more ３ It may be the following:

[0037] The density of the porous membrane 1 can be determined by a method in accordance with ASTM-D-792. It has been confirmed that similar results can be obtained when different measurement ranges are arbitrarily selected for the same porous membrane 1 and the above measurements are carried out in those measurement ranges.

[0038] <<Weight of porous membrane>> The weight of the porous membrane 1 is 0.002 mg / mm ２ 0.100mg / mm or more ２ The weight per unit area of ​​the porous membrane 1 may be 0.002 mg / mm ２ When the weight per unit area of ​​the porous membrane 1 is less than 0.100 mg / mm ２ If the permeation efficiency of the porous membrane 1 is more than 0.002 mg / mm ２ or more, and ２ or more, and ２ The upper limit of the basis weight of the porous membrane 1 is 0.100 mg / mm ２ or less, 0.080 mg / mm ２ or less, 0.060 mg / mm ２ The basis weight of the porous membrane 1 may be 0.003 mg / mm ２0.080mg / mm or more ２ or less, 0.004 mg / mm ２ 0.060mg / mm or more ２ It may be the following:

[0039] The basis weight of the porous membrane 1 can be determined by the following method. First, an evaluation sample is obtained by punching out a hole with a diameter of 60 mm at any one location of the porous membrane 1 in the film thickness direction. Next, the mass of the evaluation sample is measured using an analytical balance "AP224X" (trademark) manufactured by Shimadzu Corporation. Next, the mass is calculated by the formula "basis weight [mg / mm ２ ] = {1000 × (mass of the evaluation sample [g])} / {(60 / 2) ２ × π}" to calculate the basis weight of the porous membrane 1.

[0040] It has been confirmed that similar results can be obtained when a different measurement range is arbitrarily selected for the same porous membrane 1 and the above measurement is carried out in that measurement range.

[0041] <<Endothermic peak>> The porous membrane 1 has an endothermic peak in the range of 340 ° C. to 350 ° C. in the melting curve of the first run obtained by differential scanning calorimetry at a heating rate of 10 ° C. / min, and has an endothermic peak in the range of 320 ° C. to 330 ° C. in the melting curve of the second run obtained by differential scanning calorimetry at a heating rate of 10 ° C. / min. This allows the porous membrane 1 to have both excellent thermal stability and a small pore size. The porous membrane 1 may have an endothermic peak in the range of 341 ° C. or higher, an endothermic peak in the range of 342 ° C. or higher, or an endothermic peak in the range of 343 ° C. or higher in the melting curve of the first run obtained by differential scanning calorimetry at a heating rate of 10 ° C. / min. In the melting curve of the first run, the porous membrane 1 may have an endothermic peak in the range of 349° C. or less, may have an endothermic peak in the range of 348° C. or less, or may have an endothermic peak in the range of 347° C. or less. In the melting curve of the first run obtained by differential scanning calorimetry at a heating rate of 10° C. / min, the porous membrane 1 may have an endothermic peak in the range of 341° C. or more and 349° C. or less, may have an endothermic peak in the range of 342° C. or more and 348° C. or less, or may have an endothermic peak in the range of 343° C. or more and 347° C. or less.

[0042] The porous membrane 1 may have an endothermic peak in the range of 321°C or higher, an endothermic peak in the range of 322°C or higher, or an endothermic peak in the range of 323°C or higher in the melting curve of the second run obtained by differential scanning calorimetry at a heating rate of 10°C / min. The porous membrane 1 may have an endothermic peak in the range of 329°C or lower, an endothermic peak in the range of 328°C or lower, or an endothermic peak in the range of 327°C or lower in the melting curve of the second run obtained by differential scanning calorimetry at a heating rate of 10°C / min. In the melting curve of Run, it may have an endothermic peak in the range of 321°C or higher and 329°C or lower, it may have an endothermic peak in the range of 322°C or higher and 328°C or lower, or it may have an endothermic peak in the range of 323°C or higher and 327°C or lower.

[0043] For the porous membrane 1, the endothermic peak temperature (hereinafter also referred to as "endothermic peak temperature") in both the melting curve of the first run obtained by differential scanning calorimetry at a heating rate of 10°C / min and the melting curve of the second run obtained by differential scanning calorimetry at a heating rate of 10°C / min can be determined by the following method. First, a 5 mg porous membrane 1 is heated from room temperature to 400°C at a rate of 10°C / min using a differential scanning calorimeter "DSC-60A" (trademark) manufactured by Shimadzu Corporation to obtain the melting curve of the first run. Next, the porous membrane 1 is cooled from 400°C to 100°C at a rate of -1°C / min. Next, the porous membrane 1 is heated from 100°C to 400°C at a rate of 10°C / min using a differential scanning calorimeter "DSC-60A" (trademark) manufactured by Shimadzu Corporation to obtain the melting curve of the second run. Next, the endothermic peak temperatures are identified in both the melting curves of the first and second runs.

[0044] <<Average Bubble Point>> The average bubble point P1a of the porous membrane 1 before the test of leaving the porous membrane 1 in a thermostatic bath at 120°C for 1 hour may be 230 kPa or more and 600 kPa or less. This allows for a smaller pore size. Therefore, it is possible to have a better capturing performance of fine particles contained in the filtrate. The lower limit of the average bubble point P1a may be 230 kPa or more, 240 kPa or more, or 250 kPa or more. The upper limit of the average bubble point P1a may be 600 kPa or less, 590 kPa or less, or 580 kPa or less. The average bubble point P1a may be 240 kPa or more and 590 kPa or less, or 250 kPa or more and 580 kPa or less.

[0045] The average bubble point P1a of the porous membrane 1 is measured by the bubble point method using liquid 1a, and the surface tension of liquid 1a is 13 mN / m. More specifically, the average bubble point P1a of the porous membrane 1 is determined by the following method. First, for a dry porous membrane 1, the differential pressure applied to the porous membrane 1 and the air flow rate permeating the porous membrane are measured based on the bubble point method (ASTM F316-86, JIS K3832). Next, on a coordinate system with the differential pressure on the horizontal axis and the air flow rate on the vertical axis, a first curve is obtained showing the relationship between the differential pressure and the numerical value obtained by dividing the air flow rate by 2. Next, the porous membrane 1 is immersed in "Opteon SF70" (trademark), a hydrofluoroolefin (liquid 1a) manufactured by Mitsui-Chemours Fluoroproducts, Inc., for about 5 minutes at about 25°C, and then removed from the hydrofluoroolefin (liquid 1a), thereby obtaining a porous membrane 1 wet with hydrofluoroolefin (liquid 1a). Next, for the porous membrane 1 wet with hydrofluoroolefin (liquid 1a), the differential pressure applied to the porous membrane 1 and the air flow rate permeating the porous membrane 1 are measured based on the bubble point method. Next, a second curve showing the relationship between the differential pressure and the air flow rate is obtained on a coordinate system with the differential pressure on the horizontal axis and the air flow rate on the vertical axis. Next, the differential pressure at the intersection of the first curve and the second curve is determined as the average bubble point P1a. The surface tension of the hydrofluoroolefin (liquid 1a) is 13 mN / m.

[0046] The average bubble point P1a' of the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour may be 300 kPa or more and 900 kPa or less. This allows for a smaller pore size. Therefore, the porous membrane 1 can have a better ability to capture fine particles contained in the filtrate. The lower limit of the average bubble point P1a' may be 300 kPa or more, 330 kPa or more, or 350 kPa or more. The upper limit of the average bubble point P1a' may be 900 kPa or less, 890 kPa or less, or 880 kPa or less. The average bubble point P1a' may be 330 kPa or more and 890 kPa or less, or 350 kPa or more and 880 kPa or less.

[0047] In the porous membrane 1, the average bubble point P1a' is measured by a bubble point method using a 1a liquid, and the surface tension of the 1a liquid is 13 mN / m. In the porous membrane 1, the average bubble point P1a' is determined by a method similar to the method for measuring the average bubble point P1a, except that the measurement is performed on the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour.

[0048] The average bubble point P1b of the porous membrane 1 before the test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour may be 500 kPa or more and 1130 kPa or less. This allows for a smaller pore size. Therefore, it is possible to have a better ability to capture fine particles contained in the filtrate. The lower limit of the average bubble point P1b may be 500 kPa or more, 510 kPa or more, or 520 kPa or more. The upper limit of the average bubble point P1b may be 1130 kPa or less, 1120 kPa or less, or 1110 kPa or less. The average bubble point P1b may be 510 kPa or more and 1120 kPa or less, or 520 kPa or more and 1110 kPa or less.

[0049] In the porous membrane 1, the average bubble point P1b is measured by a bubble point method using liquid 1b, and the surface tension of liquid 1b is 21 mN / m. More specifically, in the porous membrane 1, the average bubble point P1b is determined by a method similar to the method for measuring P1a, except that isopropyl alcohol (liquid 1b) manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd. is used instead of liquid 1a.

[0050] The average bubble point P1b' of the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour may be 250 kPa or more and 660 kPa or less. This allows for a smaller pore size. As a result, the porous membrane 1 can have a better ability to capture fine particles contained in the filtrate. The lower limit of the average bubble point P1b' may be 250 kPa or more, 260 kPa or more, or 270 kPa or more. The upper limit of the average bubble point P1b' may be 660 kPa or less, 650 kPa or less, or 640 kPa or less. The average bubble point P1b' may be 260 kPa or more and 650 kPa or less, or 270 kPa or more and 640 kPa or less.

[0051] In the porous membrane 1, the average bubble point P1b' is measured by a bubble point method using the 1b liquid, and the surface tension of the 1b liquid is 21 mN / m. In the porous membrane 1, the average bubble point P1b' is determined by a method similar to the method for measuring the average bubble point P1b, except that the measurement is performed on the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour.

[0052] <Relationship between P1a and P1a' and P1b and P1b'> The ratio of the absolute value of the difference between the average bubble point P1a' of the porous membrane 1 after the test of leaving the porous membrane 1 in a thermostatic bath at 120 ° C. for 1 hour and the average bubble point P1a of the porous membrane 1 before the test of leaving the porous membrane 1 in a thermostatic bath at 120 ° C. for 1 hour and the average bubble point P1a may be 10% or less in percentage, or the ratio of the absolute value of the difference between the average bubble point P1b' of the porous membrane 1 after the test of leaving the porous membrane 1 in a thermostatic bath at 120 ° C. for 1 hour and the average bubble point P1b of the porous membrane 1 before the test of leaving the porous membrane 1 in a thermostatic bath at 120 ° C. for 1 hour and the average bubble point P1b may be 10% or less in percentage. This prevents the fibers of the porous membrane from agglomerating due to heating, and maintains the pore size of the porous membrane at a small size, allowing the porous membrane 1 to have both better thermal stability and a smaller pore size.

[0053] The lower limit of the absolute value of the difference between the average bubble point P1a' of the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1a of the porous membrane 1 before the test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour is preferably as close to 0% as possible, but can be, for example, 0% or more, 1% or more, or 2% or more. The upper limit of this percentage may be 9% or less, or 8% or less. The percentage may be 0% or more and 10% or less, or 0% or more and 9% or less, or 0% or more and 8% or less. The phrase "the ratio of the absolute value of the difference between the average bubble point P1a' of the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1a of the porous membrane 1 before the test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour, and the average bubble point P1a, is 10% or less in percentage" can also be interpreted as meaning that P1a and P1a' satisfy the relationship of the following formula 1: (|P1a'-P1a| / P1a) x 100≦10 Formula 1

[0054] The lower limit of the absolute value of the difference between the average bubble point P1b' of the porous membrane 1 after the test in which the porous membrane 1 is placed in a thermostatic bath at 120°C for 1 hour and the average bubble point P1b of the porous membrane 1 before the test in which the porous membrane 1 is placed in a thermostatic bath at 120°C for 1 hour is preferably as close to 0% as possible, but can be, for example, 0% or more, 1% or more, or 2% or more. The upper limit of this percentage may be 9% or less, or 8% or less. The percentage may be 0% or more and 10% or less, or 0% or more and 9% or less, or 0% or more and 8% or less. The phrase "the ratio of the absolute value of the difference between the average bubble point P1b' of the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1b of the porous membrane 1 before the test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour is 10% or less" can also be interpreted as meaning that P1b and P1b' satisfy the relationship in the following formula 2: (|P1b'-P1b| / P1b) x 100≦10 Formula 2

[0055] The average bubble point is measured by a bubble point method, in which a specific liquid is used. The numerical value of the average bubble point varies depending on the surface tension of the specific liquid. In view of the difficulty of obtaining the specific liquid, "P1a" and "P1a'" measured by the bubble point method using "liquid 1a" and "P1b'" measured by the bubble point method using "liquid 1b" are defined. From the same perspective, "P2a" and "P2b" (described later), and "P3a" and "P3b" (described later) are defined.

[0056] <Gurley seconds> The Gurley seconds G1 of the porous membrane 1 before a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour may be 1 second or more and 100 seconds or less. This allows the flow rate of the filtrate to be increased, thereby increasing the permeation efficiency and improving the ability to capture fine particles contained in the filtrate. The lower limit of the Gurley seconds G1 may be 1 second or more, 3 seconds or more, or 5 seconds or more. The upper limit of the Gurley seconds G1 may be 100 seconds or less, 80 seconds or less, or 60 seconds or less. The Gurley seconds G1 may be 3 seconds or more and 80 seconds or less, or 5 seconds or more and 60 seconds or less.

[0057] The Gurley seconds G1 of the porous membrane 1 can be determined by the following method: That is, in accordance with JIS P 8117, 100 ml of air is suctioned to a membrane with an effective membrane area of ​​6.42 cm. ２ The time required for permeation of the cellulose acetate solution at a differential pressure of 1.22 kPa is measured and specified as Gurley seconds G1.

[0058] The Gurley second G1' of the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic chamber at 120°C for 1 hour may be 1 second or more and 100 seconds or less. This allows the flow rate of the filtrate to be increased, thereby increasing the permeation efficiency and improving the ability to capture fine particles contained in the filtrate. The lower limit of the Gurley second G1' may be 1 second or more, 3 seconds or more, or 5 seconds or more. The upper limit of the Gurley second G1' may be 100 seconds or less, 80 seconds or less, or 60 seconds or less. The Gurley second G1' may be 3 seconds or more and 80 seconds or less, or 5 seconds or more and 60 seconds or less.

[0059] In the porous membrane 1, the Gurley seconds G1' can be determined by a method similar to the measurement method of the Gurley seconds G1, except that the measurement is performed on the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic chamber at 120°C for 1 hour.

[0060] <Relationship between G1 and G1'> The absolute value of the difference between the Gurley seconds G1 of the porous membrane 1 after the test of placing the porous membrane 1 in a thermostatic bath at 120 ° C for 1 hour and the Gurley seconds G1 of the porous membrane 1 before the test of placing the porous membrane 1 in a thermostatic bath at 120 ° C for 1 hour may be 10% or less in percentage. This prevents the porous membrane 1 from flocculating due to heating, and maintains the pore size of the porous membrane in a small state, so that the porous membrane 1 can have both better thermal stability and a smaller pore size. The lower limit of this ratio is preferably as close to 0% as a percentage, but can be, for example, 0% or more, 1% or more, or 2% or more. The upper limit of this ratio may be 9% or less, or 8% or less in percentage. The ratio may be 0% or more and 10% or less, or 0% or more and 9% or less, or 0% or more and 8% or less, in percentage. Note that "the ratio of the absolute value of the difference between the Gurley second G1' of the porous membrane 1 after a test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour and the Gurley second G1 of the porous membrane 1 before the test in which the porous membrane 1 is left standing in a thermostatic bath at 120°C for 1 hour and the Gurley second G1 is 10% or less" can also be interpreted as meaning that G1 and G1' satisfy the relationship of the following formula 5. (|G1'-G1| / G1)×100≦10 Formula 5

[0061] <<Mean Flow Pore Diameter>> The mean flow pore diameter of the porous membrane 1 may be 80 nm or less. This allows the porous membrane 1 to have a smaller pore diameter. Therefore, the porous membrane 1 can have better trapping performance for fine particles contained in the filtrate. The lower limit of the mean flow pore diameter may be 25 nm or more, 26 nm or more, or 27 nm or more. The upper limit of the mean flow pore diameter may be 80 nm or less, 60 nm or less, or 50 nm or less. The mean flow pore diameter may be 25 nm or more and 80 nm or less, 26 nm or more and 60 nm or less, or 27 nm or more and 50 nm or less.

[0062] The mean flow pore size of porous membrane 1 can be determined by the following measurement method. That is, first, for a dry porous membrane 1, the differential pressure applied to porous membrane 1 and the air flow rate permeating through porous membrane 1 are measured based on the bubble point method. Next, in a coordinate system where the horizontal axis is the differential pressure and the vertical axis is the air flow rate, a third curve is obtained showing the relationship between the differential pressure and the numerical value obtained by dividing the air flow rate by 2. Next, the porous membrane 1 is immersed in GALWICK (propylene, 1,1,2,3,3,3-hexafluorofluoric acid oxide, fourth liquid) manufactured by Porous Materials for about 5 minutes at about 25 ° C., and then removed from GALWICK (fourth liquid), thereby obtaining a porous membrane 1 wet with GALWICK (fourth liquid). Next, for the porous membrane 1 wet with GALWICK (fourth liquid), the differential pressure applied to porous membrane 1 and the air flow rate permeating through porous membrane 1 are measured based on the bubble point method. Next, a fourth curve showing the relationship between the differential pressure and the air flow rate is obtained on a coordinate system with the horizontal axis representing the differential pressure and the vertical axis representing the air flow rate. Next, the differential pressure P' at the intersection of the third curve and the fourth curve is identified. The mean flow pore size [nm] of the porous membrane 1 is then determined by dividing the product of the constant 2860 and the surface tension of the fourth liquid, 16 mN, by the differential pressure P'. In the bubble point method, for example, a Perm Porometer "CFP-1500A" manufactured by PMI is used as the pore diameter distribution measuring device.

[0063] <Crystallite> <Crystallite Structure> The porous membrane 1 has a crystallite, and the crystallite has a length X along the MD direction of the porous membrane 1 and a length Y along the TD direction of the porous membrane 1. In the present disclosure, "crystallite" refers to the smallest unit portion of a crystal grain that can be regarded as a single crystal. The upper limit of the length X along the MD direction of the porous membrane 1 may be less than 60 nm. If the length X along the MD direction of the porous membrane 1 is 60 nm or more, the pore size tends to increase and the particle capture performance tends to decrease. The upper limit of the length X along the MD direction of the porous membrane 1 may be less than 59 nm or less than 58 nm. The lower limit of the length X along the MD direction of the porous membrane 1 is not particularly limited, but can be, for example, 5 nm or more, 7 nm or more, or 10 nm or more. In this embodiment, the "MD direction" can be rephrased as the longitudinal direction. In this embodiment, the "TD direction" described below can be rephrased as a direction perpendicular to the "MD direction" and the thickness direction of the porous membrane 1.

[0064] The length X of the porous membrane 1 along the MD direction can be determined by X-ray diffraction measurement. For X-ray diffraction measurement, BL16 at the Synchrotron Radiation Facility SAGA-LS was used. At the beamline, a double-crystal spectrometer using Si (111) diffraction was used to monochromatize the X-rays to a wavelength of 0.124 nm. Measurements were performed using the transmission method, with a double-slit optical system using a NaI scintillation counter as the detector. The receiving slits were all 0.5 mm long (diffraction angle measurement direction) and 3 mm wide (perpendicular). Using the peak integral width B1 of the polytetrafluoroethylene (100) diffraction line in the obtained X-ray diffraction (XRD) profile and θ1, which is 1 / 2 of the Bragg angle 2θ1 and is the peak position, the length X was calculated based on the Scherrer equation (Equation 3 below) with a Scherrer constant of 1. λ1 represents the wavelength of the X-rays and is 0.124 nm. X = λ1 / (B1 cos θ1) Equation 3

[0065] The upper limit of the length Y of the porous membrane 1 along the TD direction may be less than 60 nm. If the length Y of the porous membrane 1 along the TD direction is 60 nm or more, the pore size tends to increase and the particle capture performance tends to decrease. The upper limit of the length Y of the porous membrane 1 along the TD direction may be less than 59 nm or less than 58 nm. The lower limit of the length Y of the porous membrane 1 along the TD direction is not particularly limited, but may be, for example, 5 nm or more, 7 nm or more, or 10 nm or more.

[0066] The length Y of the porous membrane 1 along the TD direction can be determined by X-ray diffraction measurement. For X-ray diffraction measurement, BL16 at the Synchrotron Radiation Facility SAGA-LS was used. At the beamline, a double-crystal spectrometer using Si (111) diffraction was used to monochromatize the X-rays at a wavelength of 0.124 nm. Measurements were performed using the transmission method, with a NaI scintillation counter as the detector and a double-slit optical system. The receiving slits were all 0.5 mm long (diffraction angle measurement direction) and 3 mm wide (perpendicular). X-ray diffraction measurement was performed after measuring the "length X of the porous membrane 1 along the MD direction," and then rotating the porous membrane 1 90° along a virtual plane perpendicular to the film thickness direction of the porous membrane 1. Using the peak integral width B2 of the polytetrafluoroethylene (100) diffraction line in the obtained XRD profile and θ2, which is ½ of the Bragg angle 2θ2 at the peak position, the length Y was calculated based on the Scherrer equation (Equation 4 below) with a Scherrer constant of 1. λ2 means the wavelength of X-rays, which is 0.124 nm. Y=λ2 / (B2cosθ2) Equation 4

[0067] The product XY of length X and length Y is 1200 nm ２ More than 1710 nm ２ This improves the thermal stability of the porous film 1 and reduces the pore size of the porous film 1. The upper limit of the product XY of the length X and the length Y is 1700 nm. ２ or less, 1690 nm ２ The lower limit of the product XY of the length X and the length Y is not particularly limited, but may be, for example, 30 nm. ２ Above, 50nm ２ More than 100 nm ２ It can be more than that.

[0068] <Crystallite Composition> The crystallite may be made of polytetrafluoroethylene. This makes it easier for the crystallite density of the porous film 1 to be high in crystallinity and to be high, so that it can have both better thermal stability and a smaller pore size. Here, the term "made of polytetrafluoroethylene" is not limited to an embodiment made of polytetrafluoroethylene alone, but also encompasses an embodiment containing components other than polytetrafluoroethylene (for example, inevitable impurities) as long as the effects of the present disclosure are achieved.

[0069] <<Method for manufacturing porous film>> The method for manufacturing porous film according to this embodiment includes the steps of: 1-1 step of obtaining a kneaded product of polytetrafluoroethylene powder and liquid lubricant; 1-2 step of extruding the kneaded product to obtain a sheet-like molded body; 1-3 step of biaxially stretching the molded body to obtain an elongated body; and 1-4 step of heat-treating the elongated body to obtain a porous film. In addition, 1-4 step is carried out under the conditions of 345°C or higher and 3 minutes or less.

[0070] <Step 1-1> Step 1-1 is carried out by kneading polytetrafluoroethylene powder and a liquid lubricant to obtain a kneaded product. More specifically, first, polytetrafluoroethylene powder and a liquid lubricant are mixed to obtain a mixture. Next, the mixture is compression-molded into a block shape using a compression molding machine to obtain the kneaded product.

[0071] The polytetrafluoroethylene powder refers to a powder consisting of fine particles of polytetrafluoroethylene. Examples of the polytetrafluoroethylene powder include "PTFE (polytetrafluoroethylene) fine powder" produced by emulsion polymerization and "PTFE molding powder" produced by suspension polymerization.

[0072] The number-average molecular weight of polytetrafluoroethylene in the polytetrafluoroethylene powder may be 12 million or more and 50 million or less, from the viewpoint of being able to promote the growth of the fibrous skeleton while preventing excessive pore size expansion and rupture of the porous film during stretching. Furthermore, the second heat of fusion in the polytetrafluoroethylene powder may be 10 J / g or more and 25 J / g or less, from the viewpoint of being dependent on the number-average molecular weight of the polytetrafluoroethylene powder. Here, the second heat of fusion can be determined by the following method. The polytetrafluoroethylene powder is heated from room temperature to 380°C at a rate of 10°C / min (Pattern 1 (1st Run)), then cooled from 380°C to 100°C at a rate of -1°C / min (Pattern 2), and then heated from 100°C to 380°C at a rate of 10°C / min (Pattern 3 (2nd Run)). The endothermic quantity obtained by integrating a 48°C section starting from the end set temperature of a peak in the range of 300°C to 360°C in the melting curve of Pattern 3 is defined as the second heat of fusion. Here, the "end set temperature" refers to the temperature at the end of melting associated with a temperature rise in the relationship between the melting curve and the peak.

[0073] As the liquid lubricant, various lubricants conventionally used in the extrusion method can be used. Examples of the liquid lubricant include petroleum solvents such as solvent naphtha and white oil, hydrocarbon oils such as undecane, aromatic hydrocarbons such as toluene and xylol, alcohols, ketones, esters, silicone oil, fluorochlorocarbon oil, solutions of polymers such as polyisobutylene and polyisoprene dissolved in these solvents, and water or aqueous solutions containing surfactants. These can be used alone or in combination of two or more. However, from the viewpoint of uniform mixing, a single-component liquid lubricant can also be used.

[0074] The mass ratio of the liquid lubricant to 100 parts by mass of the polytetrafluoroethylene powder may be 10 parts by mass or more and 40 parts by mass or less. If the mass ratio of the liquid lubricant is less than 10 parts by mass, extrusion tends to be difficult. If the mass ratio of the liquid lubricant is more than 40 parts by mass, compression molding tends to be difficult.

[0075] In addition to the polytetrafluoroethylene powder and the liquid lubricant, other additives may be used as materials for the kneaded product. Examples of other additives include pigments for coloring, inorganic fillers such as carbon black, graphite, silica powder, glass powder, glass fiber, silicates and carbonates, metal powder, metal oxide powder, and metal sulfide powder, which are used to improve wear resistance, prevent low-temperature flow, and facilitate pore formation. Furthermore, to aid in the formation of a porous structure, substances that can be removed or decomposed by heating, extraction, dissolution, etc., such as ammonium chloride, sodium chloride, plastics other than polytetrafluoroethylene, rubber, etc., may be used in the form of powder or solution.

[0076] <Step 1-2> Step 1-2 is carried out by extrusion molding the kneaded material to obtain a sheet-like molded body. More specifically, the kneaded material is extruded into a sheet at room temperature (for example, 25°C) or higher and 50°C or lower, and at a speed of, for example, 10 mm / min to 30 mm / min, to obtain a precursor of the sheet-like molded body. Furthermore, the precursor is rolled using a calendar roll or the like to obtain a sheet-like molded body having an average thickness of 0.200 mm to 0.400 mm. The average thickness can be determined in the same manner as the thickness of the porous membrane 1, except that the measurement is carried out on the molded body.

[0077] Furthermore, before carrying out Steps 1-3 described below, the liquid lubricant contained in the compact may be removed. The liquid lubricant can be removed by heating, extracting, dissolving, or the like the compact. When heating, the liquid lubricant can be removed from the compact by rolling the compact with a heated roll at a temperature of 130°C or higher and 220°C or lower. When a liquid lubricant with a relatively high boiling point, such as silicone oil or fluorochlorocarbon oil, is used, removal by extraction is preferred.

[0078] Step 1-3 is carried out by biaxially stretching the molded body to obtain a stretched body. Here, biaxial stretching means stretching the sheet-like molded body in the MD direction (in other words, the flow direction of the molded body) and the TD direction perpendicular to the MD direction.

[0079] The temperature in Step 1-3 may be 60° C. or higher and 300° C. or lower. If the temperature is higher than 300° C., the pore size of the porous membrane tends to be too large (in other words, the average bubble point of the porous membrane tends to be too small). If the temperature is lower than 60° C., the pore size tends to be too small (in other words, the average bubble point of the porous membrane tends to be too large).

[0080] The stretching ratio in MD direction may be 1.5 times or more and 20 times or less.The stretching ratio in MD direction means the value obtained by dividing the average length in MD direction immediately after stretching in MD direction by the average length in MD direction immediately before stretching in MD direction.If the stretching ratio in MD direction is less than 1.5 times, the thickness of the porous film may be outside of the desired range.If the stretching ratio in MD direction is more than 20 times, the thickness of the porous film may be outside of the desired range.In addition, here, "average length in MD direction" means the average value of the length in MD direction at any 10 positions.

[0081] The stretching ratio in TD direction may be 1.5 times or more and 100 times or less. Note that, the stretching ratio in TD direction means the value obtained by dividing the average length in TD direction immediately after stretching in TD direction by the average length in TD direction immediately before stretching in TD direction. If the stretching ratio in TD direction is less than 1.5 times, the thickness of the porous film may be outside of the desired range. If the stretching ratio in TD direction is more than 100 times, the thickness of the porous film may be outside of the desired range. Note that, here, "average length in TD direction" means the average value of the length in TD direction at any 10 positions.

[0082] <Step 1-4> Step 1-4 is performed by heat treating the stretched body to obtain a porous film. Step 1-4 is performed under conditions of 345°C or higher and 3 minutes or shorter. As a result, the porous film has an endothermic peak in the range of 340°C to 350°C in the melting curve of the first run obtained by differential scanning calorimetry at a heating rate of 10°C / min, and an endothermic peak in the range of 320°C to 330°C in the melting curve of the second run obtained by differential scanning calorimetry at a heating rate of 10°C / min. The porous film has crystallites, each of which has a length X along the MD direction of the porous film and a length Y along the TD direction of the porous film, and the product XY of the length X and the length Y is 1200 nm. ２ More than 1710 nm ２ It can be:

[0083] The upper limit of the temperature in Step 1-4 may be 800° C. If the temperature exceeds 800° C., the mechanical strength of the porous film tends to decrease due to thermal decomposition of polytetrafluoroethylene.

[0084] The lower limit of the time for Step 1-4 may be 0.01 minutes or more. If the time is less than 0.01 minutes, the thermal stability of the porous membrane tends to decrease.

[0085] As described above, a porous film containing polytetrafluoroethylene as a main component has an endothermic peak in the range of 340°C to 350°C in a first run melting curve obtained by differential scanning calorimetry at a heating rate of 10°C / min, and has an endothermic peak in the range of 320°C to 330°C in a second run melting curve obtained by differential scanning calorimetry at a heating rate of 10°C / min, the porous film has crystallites, and the crystallites have a length X along the MD direction of the porous film and a length Y along the TD direction of the porous film, and the product XY of the length X and the length Y is 1200 nm. ２ More than 1710 nm ２ A porous membrane can be obtained that is:

[0086] [Embodiment 2: Filter element (1)] A filter element (1) according to one embodiment of the present disclosure will be described with reference to Figure 3. A filter element 500 according to this embodiment includes the porous membrane 570 according to embodiment 1. The filter element 500 according to this embodiment is not particularly limited as long as it includes the porous membrane 570 according to embodiment 1, but for example, in the filter element 500, the porous membrane 570 may have a pleated structure.

[0087] FIG. 3 shows a filter element 500 equipped with a porous membrane 570 having a pleated structure. The porous membrane 570 is sandwiched between two protective materials 520 and 540, then pleated and wrapped around a core 550 having multiple liquid collection ports 590. An outer peripheral guard 510 protects the porous membrane 570. The porous membrane 570 is sealed at both ends of the cylinder by end plates 560a and 560b. The end plates interface with the seals of the filter housing (not shown) via gaskets 600. The filtered liquid is collected through the liquid collection ports 590 of the core 550 and recovered through an outlet 580. Filter elements with outlets at both ends and those at one end are known. Generally, when outlets are provided at both ends, one end is sealed with a fixture, allowing the filtered liquid to be recovered through the outlet at the other end.

[0088] <<Method for manufacturing filter element (1)>> The method for manufacturing the filter element according to this embodiment can be carried out by a method similar to a conventionally known method, except that the porous membrane according to embodiment 1 is used.

[0089] [Embodiment 3: Laminate] The laminate 10 according to this embodiment will be described with reference to FIG.

[0090] <<Laminate Structure>> The laminate 10 includes one or more porous membranes 1 according to embodiment 1. This allows the laminate 10 to have both excellent thermal stability and a small pore size. The laminate 10 also includes a support membrane 2 located on one or both sides of at least one of the porous membranes 1. This allows the support membrane 2 to function as a protective material for the porous membrane 1 according to embodiment 1, thereby improving the capture performance of the laminate 10 and increasing the mechanical strength and lifespan of the laminate 10.

[0091] The thickness of the laminate 10 may be 0.010 mm or more and 0.200 mm or less. If the thickness is less than 0.010 mm, the strength of the laminate 10 tends to be insufficient. If the thickness is more than 0.200 mm, the pressure loss during permeation of the filtrate tends to be large. The lower limit of the thickness of the laminate 10 may be 0.010 mm or more, 0.013 mm or more, or 0.015 mm or more. The upper limit of the thickness of the laminate 10 may be 0.200 mm or less, 0.150 mm or less, or 0.100 mm or less. The thickness of the laminate 10 may be 0.013 mm or more and 0.150 mm or less, or 0.015 mm or more and 0.100 mm or less.

[0092] The thickness of the laminate 10 can be determined by the same method as the method for measuring the thickness of the porous membrane 1, except that the measurement is performed on the laminate 10. It has been confirmed that similar results can be obtained when different measurement points are arbitrarily selected on the same laminate 10 and the above measurement is performed at those measurement points.

[0093] <Density of Laminate> The density of the laminate 10 is 0.10 g / cm ３ 2.00g / cm or more ３ The density may be 0.10 g / cm or less. ３ If the density is less than 2.00 g / cm3, the strength of the laminate 10 tends to be insufficient. ３ If the density exceeds 0.10 g / cm3, the permeability of the laminate 10 tends to decrease. ３or more, and 0.13 g / cm ３ or more, and 0.15 g / cm ３ The upper limit of the density of the laminate 10 is 2.00 g / cm ３ or less, 1.70 g / cm ３ or less, 1.50 g / cm ３ The density of the laminate 10 may be 0.13 g / cm or less. ３ 1.70g / cm or more ３ or less, and 0.15 g / cm ３ 1.50g / cm or more ３ It may be the following:

[0094] The density of the laminate 10 can be determined by the same method as the "method for measuring the density of the porous film 1" described in embodiment 1, except that the measurement is performed on the "laminated body 10." It has been confirmed that similar results can be obtained when a different measurement range is arbitrarily selected for the same laminate 10 and the above measurement is performed in that measurement range.

[0095] <Basis Weight of Laminate> The basis weight of the laminate 10 is 0.005 mg / mm ２ 0.100mg / mm or more ２ The basis weight of the laminate 10 may be 0.005 mg / mm ２ When the weight per unit area of ​​the laminate 10 is less than 0.100 mg / mm, the strength of the laminate 10 tends to be insufficient. ２ If the weight of the laminate 10 exceeds 0.005 mg / mm ２ or more, and ２ or more, and ２ The upper limit of the basis weight of the laminate 10 is 0.100 mg / mm ２ or less, 0.080 mg / mm ２ or less, 0.060 mg / mm ２ The basis weight of the laminate 10 may be 0.007 mg / mm ２ 0.080mg / mm or more ２or less, 0.008 mg / mm ２ 0.060mg / mm or more ２ It may be the following:

[0096] The basis weight of the laminate 10 can be determined by the same method as the "method for measuring the basis weight of the porous membrane 1" described in embodiment 1, except that the measurement is performed on the "laminated body 10." It has been confirmed that similar results can be obtained when a different measurement range is arbitrarily selected for the same laminate 10 and the above measurement is performed in that measurement range.

[0097] <Average Bubble Point> The average bubble point P2a of the laminate 10 may be 400 kPa or more and 900 kPa or less, or the average bubble point P2b of the laminate 10 may be 800 kPa or more and 1300 kPa or less. This allows for both better trapping performance for fine particles contained in the filtrate and better permeation efficiency.

[0098] The average bubble point P2a of the laminate 10 may be 400 kPa or more and 900 kPa or less. This allows for both better trapping performance for fine particles contained in the filtrate and better permeation efficiency. The lower limit of the average bubble point P2a may be 400 kPa or more, 410 kPa or more, or 420 kPa or more. The upper limit of the average bubble point P2a may be 900 kPa or less, 890 kPa or less, or 880 kPa or less. The average bubble point P2a may be 410 kPa or more and 890 kPa or less, or 420 kPa or more and 880 kPa or less.

[0099] In the laminate 10, the average bubble point P2a is measured by a bubble point method using the 2a liquid, and the surface tension of the 2a liquid is 13 mN / m. More specifically, the average bubble point P2a in the laminate 10 is determined by the following method. More specifically, P2a in the laminate 10 is determined by a method similar to the measurement method for P1a, except that the measurement is performed on the "laminate 10" and the name of the liquid "1a liquid" is replaced with "2a liquid."

[0100] The average bubble point P2b of the laminate 10 may be 800 kPa or more and 1300 kPa or less. This allows for both better capture performance of fine particles contained in the filtrate and better permeation efficiency. The lower limit of the average bubble point P2b may be 800 kPa or more, 810 kPa or more, or 820 kPa or more. The upper limit of the average bubble point P2b may be 1300 kPa or less, 1290 kPa or less, or 1280 kPa or less. The average bubble point P2b may be 810 kPa or more and 1290 kPa or less, or 820 kPa or more and 1280 kPa or less.

[0101] In the laminate 10, the average bubble point P2b is measured by a bubble point method using the second liquid b, and the surface tension of the second liquid b is 21 mN / m. More specifically, in the laminate 10, the average bubble point P2b is determined by a method similar to the method for measuring P2a, except that isopropyl alcohol (the second liquid b) manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd. is used instead of the second liquid a.

[0102] <Gurley Second> The laminate 10 has a Gurley second G2, which may be 1 second or more and 100 seconds or less. This allows the flow rate of the filtrate to be increased, thereby increasing the permeation efficiency and improving the ability to capture fine particles contained in the filtrate. The lower limit of the Gurley second G2 may be 1 second or more, 3 seconds or more, or 5 seconds or more. The upper limit of the Gurley second G2 may be 100 seconds or less, 80 seconds or less, or 70 seconds or less. The Gurley second G2 may be 3 seconds or more and 80 seconds or less, or 5 seconds or more and 70 seconds or less.

[0103] In the laminate 10, the Gurley second G2 is determined by the following method. That is, it can be determined by the same method as the "method for measuring the Gurley second G1" in the first embodiment, except that the measurement is performed on the "laminated body 10."

[0104] <<Mean Flow Pore Diameter>> The mean flow pore diameter of the laminate 10 may be 25 nm or more and 100 nm or less. This allows the laminate 10 to have a smaller pore diameter. Therefore, the laminate 10 can have better trapping performance for fine particles contained in the filtrate. The lower limit of the mean flow pore diameter may be 25 nm or more, 26 nm or more, or 27 nm or more. The upper limit of the mean flow pore diameter may be 100 nm or less, 60 nm or less, or 55 nm or less. The mean flow pore diameter may be 26 nm or more and 60 nm or less, or 27 nm or more and 55 nm or less.

[0105] The mean flow pore size of the laminate 10 can be determined in the same manner as the "Method for measuring the mean flow pore size of the porous membrane 1" in embodiment 1, except that the measurement is performed on the "laminated body 10".

[0106] <<Support Membrane>> <Structure of Support Membrane> The support membrane 2 is porous. Here, "porous" means having a fibrous skeleton in which pores are connected in a three-dimensional network. The support membrane 2 has a larger pore size than the porous membrane 1, and does not necessarily inhibit the permeation efficiency. The fact that "the support membrane 2 is porous" can be determined by observing the surface condition and cross-sectional condition of the support membrane 2 using a scanning electron microscope.

[0107] The thickness of the support membrane 2 may be 0.002 mm or more and 0.050 mm or less. If the thickness of the support membrane 2 is less than 0.002 mm, it tends to be difficult for the porous membrane 1 to function as a protective material, and it tends to be difficult to improve the mechanical strength and lifespan of the laminate 10. If the thickness of the support membrane 2 exceeds 0.050 mm, it tends to be difficult for the porous membrane 1 to achieve both the capture performance of the laminate 10 and the permeation efficiency of the laminate 10. The lower limit of the thickness may be 0.002 mm or more, 0.004 mm or more, or 0.006 mm or more. The upper limit of the thickness may be 0.050 mm or less, 0.045 mm or less, or 0.040 mm or less. The thickness may be 0.004 mm or more and 0.045 mm or less, or 0.006 mm or more and 0.040 mm or less.

[0108] The thickness of the support membrane 2 can be determined by a method similar to the method for measuring the "thickness of the porous membrane 1" described in embodiment 1, except that the measurement is performed on the support membrane 2. It has been confirmed that similar results can be obtained when different measurement points are arbitrarily selected on the same support membrane 2 and the above measurement is performed at those measurement points.

[0109] <Density of Support Film> The density of the support film 2 is 0.10 g / cm ３ 2.00g / cm or more ３ The density may be 0.10 g / cm or less. ３ If the density is less than 2.00 g / cm3, the strength of the laminate 10 tends to be insufficient. ３ If the density exceeds 0.10 g / cm3, the permeation efficiency of the laminate 10 tends to decrease. ３ or more, and 0.13 g / cm ３ or more, and 0.15 g / cm ３ The upper limit of the density of the support film 2 is 2.00 g / cm ３ or less, 1.70 g / cm ３ or less, 1.50 g / cm ３ The density of the support film 2 may be 0.13 g / cm or less. ３ 1.70g / cm or more ３ or less, and 0.15 g / cm ３ 1.50g / cm or more ３ It may be the following:

[0110] The density of the support membrane 2 can be determined by a method similar to that for measuring the density of the porous membrane 1, except that the measurement is performed on the "support membrane 2." It has been confirmed that similar results can be obtained when a different measurement range is arbitrarily selected for the same support membrane 2 and the above measurement is performed in that measurement range.

[0111] <Weight of Support Film> The weight of the support film 2 is 0.005 mg / mm ２ 0.100mg / mm or more ２ The weight per unit area of ​​the support film 2 may be 0.005 mg / mm２ When the weight per unit area of ​​the support film 2 is less than 0.100 mg / mm, the strength of the laminate 10 tends to be insufficient. ２ If it exceeds 0.005 mg / mm, the permeation efficiency of the laminate 10 tends to decrease. ２ or more, and ２ or more, and ２ The upper limit of the basis weight of the support film 2 is 0.100 mg / mm ２ or less, 0.080 mg / mm ２ or less, 0.060 mg / mm ２ The basis weight of the support film 2 may be 0.006 mg / mm ２ 0.080mg / mm or more ２ or less, 0.007 mg / mm ２ 0.060mg / mm or more ２ It may be the following:

[0112] The basis weight of the support membrane 2 can be determined by the following method. Except that the measurement is performed on the "support membrane 2", it can be determined by the same method as the method for measuring the basis weight of the porous membrane 1. It has been confirmed that similar results can be obtained when a different measurement range is arbitrarily selected for the same support membrane 2 and the above measurement is performed in that measurement range.

[0113] <Average Bubble Point> The average bubble point P3a of the support film 2 may be 5 kPa or more and 400 kPa or less, or the average bubble point P3b of the support film 2 may be 5 kPa or more and 800 kPa or less, thereby allowing the laminate 10 to have a smaller pore size.

[0114] The average bubble point P3a of the support film 2 may be 5 kPa or more and 400 kPa or less. This allows the laminate 10 to have a smaller pore size. Therefore, the laminate 10 can have better trapping performance for fine particles contained in the filtrate. The lower limit of P3a may be 5 kPa or more, 7 kPa or more, or 10 kPa or more. The upper limit of P3a may be 400 kPa or less, 350 kPa or less, or 300 kPa or less. P3a may be 7 kPa or more and 350 kPa or less, or 10 kPa or more and 300 kPa or less.

[0115] For the support film 2, P3a is measured by the bubble point method using the 3a liquid, and the surface tension of the 3a liquid is 13 mN / m. More specifically, for the support film 2, P3a is determined in a manner similar to that for measuring P1a, except that the measurement is performed on the "support film 2" and the name of the liquid "1a liquid" is replaced with "3a liquid."

[0116] The average bubble point P3b of the support film 2 may be 5 kPa or more and 800 kPa or less. This allows the laminate 10 to have a smaller pore size. Therefore, the laminate 10 can have better trapping performance for fine particles contained in the filtrate. The lower limit of P3b may be 5 kPa or more, 7 kPa or more, or 10 kPa or more. The upper limit of P3b may be 800 kPa or less, 700 kPa or less, or 600 kPa or less. P3b may be 7 kPa or more and 700 kPa or less, or 10 kPa or more and 600 kPa or less.

[0117] In the support film 2, P3b is measured by a bubble point method using the 3b liquid, and the surface tension of the 3b liquid is 21 mN / m. More specifically, in the support film 2, P3b is determined by a method similar to the method for measuring P3a, except that isopropyl alcohol (3b liquid) manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd. is used instead of the 3a liquid.

[0118] <Gurley seconds> The support membrane 2 has a Gurley second G3. In the support membrane 2, G3 may be 0.5 seconds or more and 60 seconds or less. This allows the flow rate of the filtrate to be increased, thereby increasing the permeation efficiency and improving the ability to capture fine particles contained in the filtrate. The lower limit of G3 may be 0.5 seconds or more, 0.7 seconds or more, or 1.0 second or more. The upper limit of G3 may be 60 seconds or less, 50 seconds or less, or 40 seconds or less. G3 may be 0.7 seconds or more and 50 seconds or less, or 1.0 second or more and 40 seconds or less.

[0119] In the support membrane 2, G3 is determined by the following method. Except that the measurement is performed on the "support membrane 2", it can be determined by the same method as the "method for measuring G1 of the porous membrane 1" in embodiment 1.

[0120] <Mean flow pore diameter> The mean flow pore diameter of the support membrane 2 may be 45 nm or more and 600 nm or less. This allows the laminate 10 to have a smaller pore diameter. As a result, the laminate 10 can have better trapping performance for fine particles contained in the filtrate. The lower limit of the mean flow pore diameter may be 45 nm or more, 50 nm or more, or 55 nm or more. The upper limit of the mean flow pore diameter may be 600 nm or less, 550 nm or less, or 500 nm or less. The mean flow pore diameter may be 50 nm or more and 550 nm or less, or 55 nm or more and 500 nm or less.

[0121] The mean flow pore size of the support membrane 2 can be determined by the following measurement method. Except that the measurement is performed on the "support membrane 2", it can be determined by the same method as the "method for measuring the mean flow pore size of the porous membrane 1" in embodiment 1.

[0122] <Composition of Support Film> The support film 2 contains polytetrafluoroethylene as a main component. This can improve the heat resistance, chemical stability, and the like of the support film 2. Here, the term "main component" refers to the component with the largest content in terms of mass, for example, a component with a content of 90 mass% or more, preferably 95 mass% or more. The support film 2 may be made of polytetrafluoroethylene. Note that "the support film 2 is made of polytetrafluoroethylene" means that the support film 2 may contain unavoidable impurities as long as the effects of the present disclosure are achieved.

[0123] The polytetrafluoroethylene content in the support membrane 2 can be determined by a method similar to the measurement method for "the polytetrafluoroethylene content in the porous membrane 1" described in embodiment 1, except that the measurement is performed on the support membrane 2. It has been confirmed that similar results can be obtained when a different measurement range is arbitrarily selected for the same support membrane 2 and the above measurement is performed in that measurement range.

[0124] <<Method for manufacturing laminate>> The method for manufacturing a laminate includes step 2-1 of preparing the porous membrane according to embodiment 1 and a support membrane, and step 2-2 of laminating the porous membrane on the support membrane. Step 2-1 consists of step 2-1-a of preparing the porous membrane and step 2-1-b of preparing the support membrane.

[0125] <Step 2-1> (Step 2-1-a) Step 2-1-a is carried out by preparing a porous membrane. The porous membrane 1 can be prepared by the method described in the first embodiment.

[0126] (Step 2-1-b) Step 2-1-b is carried out by preparing a support film, which can be prepared by a conventionally known method.

[0127] <Step 2-2> Step 2-2 is carried out by laminating a porous membrane on one or both sides of a support membrane. Examples of a method for laminating a porous membrane on one or both sides of a support membrane include a method in which the support membrane and the porous membrane are pressure-bonded together.

[0128] Specifically, the method for pressure-bonding the support membrane and the porous membrane includes, for example, overlapping the support membrane and the porous membrane to obtain a laminate precursor. The laminate precursor is then pressed from above and below with flat plates, or sandwiched between rotating rollers and sent out. The method for pressure-bonding the support membrane and the porous membrane may be performed by pressure bonding at a pressure of 10 kgf or more and 2000 kgf or less. If the pressure is less than 10 kgf, the adhesive strength between the support membrane and the porous membrane tends to be insufficient. If the pressure is more than 2000 kgf, the pores of the support membrane or the porous membrane tend to be crushed, resulting in a decrease in permeation efficiency.

[0129] As a result of the above, a laminate can be obtained which includes one or more porous membranes according to embodiment 1 and a support membrane located on one or both sides of at least one of the porous membranes, wherein the support membrane is porous and contains polytetrafluoroethylene as a main component.

[0130] [Embodiment 4: Filter element (2)] A filter element (2) according to one embodiment of the present disclosure will be described with reference to Fig. 4. The filter element 500 according to this embodiment includes the laminate 670 according to embodiment 3. The filter element 500 according to this embodiment is not particularly limited as long as it includes the laminate 670 according to embodiment 3, but for example, in the filter element 500, the laminate 670 may have a pleated structure.

[0131] The present disclosure provides a filter element including a laminate that combines excellent thermal stability with a small pore size.

[0132] 4 shows a filter element 500 including a laminate 670 having a pleated structure. The structure of the filter element 500 in FIG. 4 is the same as that of the filter element 500 in FIG. 3, except that the porous membrane 570 is replaced with the laminate 670.

[0133] <<Method for manufacturing filter element (2)>> The method for manufacturing the filter element according to this embodiment can be carried out by a method similar to a conventionally known method, except that the laminate according to embodiment 3 is used.

[0134] Hereinafter, the present disclosure will be specifically described based on examples, but the present invention is not limited to the following examples.

[0135] Example 1 <<Production of Porous Membrane>> In Example 1, porous membranes according to Samples 1-1 to 1-6 and Samples 1-101 to 1-104 were produced as follows.

[0136] <Step 1-1> First, a mixture was obtained by mixing "PTFE Fine Powder A (second heat of fusion 15.8 J / g, molecular weight approximately 28 million)," which is a polytetrafluoroethylene powder, and "Supersol FP-25" (trademark), which is a solvent naphtha (liquid lubricant) manufactured by Idemitsu Oil Co., Ltd., in the parts by mass shown in Table 1. Next, the mixture was compression molded into a block using a compression molding machine, thereby obtaining a kneaded product.

[0137] <Step 1-2> The kneaded material was extruded into a sheet under the conditions shown in Table 1 to obtain a precursor of a sheet-like molded product. Next, the precursor was rolled using a calendar roll to obtain a sheet-like molded product having an average thickness as shown in Table 1.

[0138] <Step 1-3> The above molded body was biaxially stretched under the conditions shown in Table 1 to obtain a stretched body.

[0139] <Step 1-4> The stretched body was subjected to a heat treatment under the conditions shown in Table 1 to obtain a porous membrane.

[0140] In this manner, porous membranes according to Samples 1-1 to 1-6 and Samples 1-101 to 1-104 were produced.

[0141]

[0142]

[0143]

[0144]

[0145] <<Evaluation of porous membrane properties>> For the porous membrane of each sample, the polytetrafluoroethylene content, thickness, basis weight, density, average bubble point, Gurley seconds, evaluation items of differential scanning calorimetry, crystallite structure, and mean flow pore size were determined by the method described in embodiment 1. The results obtained are shown in the "PTFE content [mass%]" column in Tables 2 to 4. The measurement error of endothermic peak temperature is ±3°C. A mean flow pore size of 80nm or less means that the pore size is suppressed to be small.

[0146] <Thermal Stability Evaluation Test> Each 100 mm x 100 mm sample was placed between "Pure Wool Felt Buffs" (trademark), a felt product manufactured by Sunflex Corporation, to obtain an evaluation sample. Next, the evaluation sample was placed in a "DH412" (trademark), a precision thermostatic bath manufactured by Yamato Scientific Co., Ltd., and heated at 120°C for 60 minutes. Next, for the evaluation sample after heating, the length of each sample in the MD direction was measured with a metal ruler. Next, the shrinkage rate in the MD direction of each sample was calculated by calculating "{1 - (MD length of evaluation sample after heating [mm] / 100 [mm])} x 100" [%]. The obtained results are shown in the "MD shrinkage rate [%]" column in Table 4. Furthermore, for the evaluation sample after heating, the length in the TD direction of each sample was measured with a metal ruler. Next, the shrinkage rate in the TD direction of each sample was determined by calculating "{1 - (length in the TD direction of the evaluation sample after heating [mm] / 100 [mm])} x 100" [%]. The results obtained are shown in the "TD shrinkage rate [%]" column in Table 4. A "MD shrinkage rate [%]" of 10% or less and a "TD shrinkage rate [%]" of 10% or less means that the porous film has high thermal stability.

[0147] The porous films of Samples 1-1 to 1-6 correspond to Examples. On the other hand, the porous films of Samples 1-101 to 1-104 correspond to Comparative Examples. The porous films of Samples 1-1 to 1-6 have significantly superior thermal stability compared to the porous films of Samples 1-101, 1-102, and 1-103.

[0148] The porous membranes of Samples 1-1 to 1-6 have significantly smaller pore diameters than the porous membrane of Sample 1-104.

[0149] Therefore, the porous films of Samples 1-1 to 1-6 exhibit the particularly excellent effect of being able to combine excellent thermal stability with small pore diameters, compared to the porous films of Samples 1-101 to 1-104.

[0150] From the above, it was found that the porous films of Samples 1-1 to 1-6 exhibited the exceptionally excellent effect of being able to combine excellent thermal stability with small pore diameters.

[0151] Example 2 <<Production of Laminate>> In Example 2, laminates according to Sample 2-1 and Samples 2-101 and 2-102 were produced as follows.

[0152] <Step 2-1-a> One porous film was produced for each sample as follows. First, a mixture was obtained by mixing "PTFE Fine Powder A (second heat of fusion 15.8 J / g, molecular weight approximately 28 million)" which is a polytetrafluoroethylene powder and "Supersol FP-25" (trademark) which is solvent naphtha (liquid lubricant) manufactured by Idemitsu Oil Co., Ltd. in the parts by mass shown in Table 5. Next, the mixture was compression molded into a block using a compression molding machine to obtain a kneaded product.

[0153] The kneaded material was extruded into a sheet under the conditions shown in Table 5 to obtain a precursor of a sheet-like molded product. Next, the precursor was rolled using a calendar roll to obtain a sheet-like molded product having an average thickness as shown in Table 5.

[0154] The molded body was biaxially stretched under the conditions shown in Table 5 to obtain a stretched body.

[0155] The stretched body was subjected to a heat treatment under the conditions shown in Table 5 to obtain a porous membrane.

[0156] As described above, porous films according to Sample 2-1 and Samples 2-101 and 2-102 were prepared.

[0157] <Step 2-1-b> Two support films were prepared for each sample as follows. First, a mixture was obtained by mixing "PTFE Fine Powder B (second heat of fusion 26.0 J / g, molecular weight approximately 5 million)," which is a polytetrafluoroethylene powder, and "Supersol FP-25" (trademark), a solvent naphtha (liquid lubricant) manufactured by Idemitsu Oil Co., Ltd., in the parts by mass shown in Table 6. Next, the mixture was compression-molded into a block using a compression molding machine, thereby obtaining kneaded products according to Sample 2-1 and Samples 2-101 to 2-102.

[0158] The kneaded material was extruded into a sheet under the conditions shown in Table 6 to obtain a precursor of a sheet-like molded product. Next, the precursor was rolled using a calendar roll to obtain a sheet-like molded product having an average thickness as shown in Table 6.

[0159] The molded body was biaxially stretched under the conditions shown in Table 6 to obtain a stretched body.

[0160] The stretched body was subjected to a heat treatment under the conditions shown in Table 6 to obtain a porous support film.

[0161] In this manner, porous support films for Sample 2-1 and Samples 2-101 and 2-102 were prepared.

[0162] <Step 2-2> A support membrane and a porous membrane were laminated in the order of support membrane-porous membrane-support membrane under the conditions shown in Table 7 to obtain a laminate.

[0163] In this manner, laminates according to Sample 2-1 and Samples 2-101 and 2-102 were produced.

[0164]

[0165]

[0166]

[0167]

[0168]

[0169]

[0170]

[0171]

[0172] <<Evaluation of porous membrane properties>> The porous membrane in the laminate of each sample was evaluated for the polytetrafluoroethylene content, thickness, basis weight, density, crystallite structure, mean flow pore size, mean bubble point, Gurley seconds, and differential scanning calorimetry by the method described in embodiment 1. The results obtained are shown in Tables 8 to 10. The measurement error of the endothermic peak temperature was ±3°C.

[0173] <Evaluation of Support Film Properties> For the support film in the laminate of each sample, the polytetrafluoroethylene content, thickness, basis weight, density, mean flow pore size, average bubble point, and Gurley seconds were determined by the method described in embodiment 3. The obtained results are shown in Table 11.

[0174] <<Evaluation of Laminate Properties>> For each sample laminate, the thickness, basis weight, density, average bubble point, Gurley seconds G2, and mean flow pore size were determined by the method described in "Embodiment 3." Furthermore, for each sample laminate, the evaluation items of the thermal stability evaluation test were determined by the same method as in Example 1, except that the measurements were performed on the laminate. The results are shown in Table 12. A "MD shrinkage rate [%]" of 10% or less and a "TD shrinkage rate [%]" of 10% or less indicate high thermal stability of the laminate. A mean flow pore size of 80 nm or less indicates that the pore size is kept small.

[0175] The laminate of Sample 2-1 corresponds to an example. On the other hand, the laminates of Samples 2-101 and 2-102 correspond to comparative examples. The laminate of Sample 2-1 has significantly better thermal stability than the laminate of Sample 1-101.

[0176] The laminate of sample 2-1 has a significantly smaller pore size than the laminate of sample 2-102.

[0177] Therefore, the laminate of sample 2-1 exhibits the particularly excellent effect of being able to combine excellent thermal stability with a small pore size, compared to the laminates of samples 2-101 and 2-102.

[0178] From the above, it was found that the laminate according to Sample 2-1 exhibited the exceptionally excellent effect of being able to combine excellent thermal stability with a small pore size.

[0179] The embodiments disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims, not by the above-described embodiments, and is intended to include any modifications within the scope of the claims and meanings equivalent to the claims.

[0180] 1 porous membrane, 2 support membrane, 10 laminate, 500 filter element, 520, 540 protective material, 590 liquid collection port, 550 core, 510 outer periphery guard, 560a, 560b end plate, 600 gasket, 580 outlet, 570 porous membrane, 670 laminate.

Claims

1. A porous membrane containing polytetrafluoroethylene as a main component, The porous membrane has an endothermic peak in the range of 340°C or more and 350°C or less in a melting curve of a first run obtained by differential scanning calorimetry at a heating rate of 10°C / min, and has an endothermic peak in the range of 320°C or more and 330°C or less in a melting curve of a second run obtained by differential scanning calorimetry at a heating rate of 10°C / min, the porous membrane has crystallites, The crystallite has a length X along the MD direction of the porous membrane and a length Y along the TD direction of the porous membrane, The product XY of the length X and the length Y is 1200 nm ２ More than 1710 nm ２ A porous membrane that is:

2. the ratio of the absolute value of the difference between the average bubble point P1a' of the porous membrane after the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1a of the porous membrane before the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1a is 10% or less, or the ratio of the absolute value of the difference between the average bubble point P1b' of the porous membrane after the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1b of the porous membrane before the test of leaving the porous membrane standing in a thermostatic bath at 120°C for 1 hour and the average bubble point P1b is 10% or less, The average bubble point P1a and the average bubble point P1a' are measured by a bubble point method using a liquid 1a, The surface tension of the first liquid is 13 mN / m; The average bubble point P1b and the average bubble point P1b' are measured by a bubble point method using a 1b liquid, 10. The porous membrane of claim 1, wherein the surface tension of the 1b liquid is 21 mN / m.

3. The average bubble point P1a is 230 kPa or more and 600 kPa or less, The porous membrane according to claim 2 , wherein the average bubble point P1b is 500 kPa or more and 1130 kPa or less.

4. The porous membrane of claim 1 or claim 2, the absolute value of the difference between the Gurley second G1' of the porous membrane after carrying out the test of ...

5. 3. The porous membrane according to claim 1, wherein the mean flow pore size of the porous membrane is 80 nm or less.

6. A filter element comprising the porous membrane of claim 1 or claim 2.

7. 10. A laminate comprising one or more porous membranes according to claim 1 or claim 2 and a support membrane located on one or both sides of at least one of the porous membranes, the support membrane is porous; The laminate, wherein the support film contains polytetrafluoroethylene as a main component.

8. A filter element comprising the laminate of claim 7.