Porous membrane laminate, filter element, and method for manufacturing a porous membrane laminate

Abstract

The porous membrane laminate of the present application has a support layer having porosity and a porous membrane laminated on one side of the support layer and having polytetrafluoroethylene as a main component, the porous membrane being a uniaxially stretched material, the average pore diameter of the porous membrane being 25 nm or more and 35 nm or less, the maximum pore diameter of the porous membrane being 49 nm or less, and the average thickness of the porous membrane being 0.6 μm or more and 3.5 μm or less.

Description

Technical Field

[0001] This invention relates to porous membrane laminates, filter elements, and methods for manufacturing porous membrane laminates. This application claims priority to Japanese Patent Application No. 2020-089970, filed May 22, 2020, the entire contents of which are incorporated herein by reference. Background Technology

[0002] Porous filters made of polytetrafluoroethylene (PTFE) possess the advantages of PTFE, including high heat resistance, chemical stability, weather resistance, non-flammability, high strength, non-adhesiveness, and low coefficient of friction, as well as the flexibility, dispersion medium permeability, particle trapping ability, and low dielectric constant resulting from its porous structure. Therefore, PTFE porous filters are widely used as precision filters for dispersion media and gases in semiconductor, liquid crystal, and food and medical fields. In recent years, porous filters using porous sheets made of PTFE capable of trapping particles smaller than 0.1 μm have been proposed (Japanese Patent Application Laid-Open No. 2010-94579).

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2010-94579. Summary of the Invention

[0006] One aspect of the present invention is a porous membrane laminate comprising a porous support layer and a porous membrane, which is mainly composed of polytetrafluoroethylene, laminated on one side of the support layer. The porous membrane is a uniaxially stretched material, the average pore size of the porous membrane is 25 nm or more and 35 nm or less, the maximum pore size of the porous membrane is 49 nm or less, and the average thickness of the porous membrane is 0.6 μm or more and 3.5 μm or less.

[0007] In another aspect of the invention, a method for manufacturing a porous membrane laminate includes a porous support layer and a porous membrane laminated on one side of the support layer. The method comprises: a step of coating a porous membrane forming composition, primarily composed of polytetrafluoroethylene (PTFE), onto the surface of a metal foil; a step of sintering the porous membrane forming composition coated in the coating step; a step of laminating a non-porous membrane formed after the sintering step onto one side of the support layer to form a non-porous membrane laminate containing a metal foil; and a step of laminating a non-porous membrane laminate containing a metal foil onto one side of the support layer. The process of removing the metal foil from the non-porous film laminate of the metal foil includes a process of selecting a non-porous film laminate after the removal process that has a pressure resistance of fluorinated solvents of 101.325 kPa or higher, and a process of uniaxially stretching the non-porous film laminate selected by the above selection process at room temperature. The boiling point of the fluorinated solvent is 130°C or lower and the surface tension is 15 mN / m or lower. The average thickness of the porous film in the porous film laminate formed after the uniaxial stretching process is 0.6 μm or higher and 3.5 μm or lower, and the maximum pore size is 49 nm or lower. Attached Figure Description

[0008] Figure 1 This is a schematic partial cross-sectional view illustrating a porous membrane laminate according to one embodiment of the present invention. Detailed Implementation

[0009] [The problem this invention aims to solve]

[0010] In the aforementioned fields, further technological innovation and increasingly stringent requirements necessitate higher-performance precision filters.

[0011] The present invention was made based on the following circumstances, and its purpose is to provide a porous membrane stack with excellent particle capture performance and filtration efficiency.

[0012] [Effects of the Invention]

[0013] The porous membrane laminate of one aspect of the present invention exhibits excellent particle capture performance and filtration efficiency.

[0014] [Description of Embodiments of the Invention]

[0015] First, embodiments of the present invention will be described.

[0016] One aspect of the present invention is a porous membrane laminate comprising a porous support layer and a porous membrane, which is mainly composed of polytetrafluoroethylene, laminated on one side of the support layer. The porous membrane is a uniaxially stretched material, the average pore size of the porous membrane is 25 nm or more and 35 nm or less, the maximum pore size of the porous membrane is 49 nm or less, and the average thickness of the porous membrane is 0.6 μm or more and 3.5 μm or less.

[0017] This porous membrane laminate has a porous membrane with polytetrafluoroethylene (PTFE) as the main component as the uniaxial stretching material, and the porous membrane has a top view of 623.7 cm². 2 The average pore size, maximum pore size, and average thickness of the porous membrane are within the aforementioned ranges, resulting in excellent particle trapping performance and filtration efficiency. Furthermore, "main component" refers to the component with the highest content by mass, for example, a component with a content of 50% by mass or more, preferably 70% by mass or more, and more preferably 90% by mass or more. "Average pore size" refers to the average diameter of the pores on the outer surface of the support layer, which can be measured using a pore diameter distribution measuring device (e.g., PMI's Perm Porometer "CFP-1200A"). "Average thickness" refers to the average thickness at any ten points.

[0018] The porous membrane laminate preferably has an isopropanol bubble point of 600 kPa or higher. By keeping the isopropanol bubble point of the porous membrane laminate within the above range, the particle trapping performance of the porous membrane laminate can be further improved. Here, "isopropanol bubble point" refers to the value measured using isopropanol according to ASTM-F316-86, representing the minimum pressure required to extrude the dispersion medium from the pores, and is an indicator corresponding to the average pore size.

[0019] The preferred surface area of ​​this porous membrane laminate is 623.7 cm² when viewed from above. 2 That's all. According to this embodiment, the area of ​​the porous membrane is 623.7 cm². 2 In the regions described above, the average pore size is above 25 nm and below 35 nm, and the maximum pore size is below 49 nm. Therefore, the particle capture performance and filtration efficiency are excellent over a wide range of regions.

[0020] In existing porous membrane stacks, it is impossible to simultaneously ensure a 623.7 cm⁻¹ diameter while maintaining an average pore size of 25 nm or more and 35 nm or less, and a maximum pore size of 49 nm or less. 2 The area mentioned above. In other words, the area with excellent particle capture performance and filtration efficiency is extremely small.

[0021] The porous membrane laminate of the present invention has a surface with an average pore size of 25 nm or more and 35 nm or less, a maximum pore size of 49 nm or less, and the area of ​​the porous membrane laminate is 623.7 cm². 2 Therefore, it exhibits excellent particle capture performance and filtration efficiency over a wide area.

[0022] Furthermore, another aspect of the present invention is a filter element using this porous membrane laminate. This filter element, using the porous membrane laminate, can therefore provide a precision filter with excellent particle capture performance and filtration efficiency.

[0023] In another aspect of the invention, a method for manufacturing a porous membrane laminate includes a porous support layer and a porous membrane laminated on one side of the support layer. The method comprises: a step of coating a porous membrane forming composition, primarily composed of polytetrafluoroethylene (PTFE), onto the surface of a metal foil; a step of sintering the porous membrane forming composition coated in the coating step; a step of laminating a non-porous membrane formed after the sintering step onto one side of the support layer to form a non-porous membrane laminate containing a metal foil; and a step of laminating a non-porous membrane laminate containing a metal foil onto one side of the support layer. The process of removing the metal foil from the non-porous film laminate of the metal foil includes a process of selecting a non-porous film laminate after the removal process that has a pressure resistance of fluorinated solvents of 101.325 kPa or higher, and a process of uniaxially stretching the non-porous film laminate selected by the above selection process at room temperature. The boiling point of the fluorinated solvent is 130°C or lower and the surface tension is 15 mN / m or lower. The average thickness of the porous film in the porous film laminate formed after the uniaxial stretching process is 0.6 μm or higher and 3.5 μm or lower, and the maximum pore size is 49 nm or lower.

[0024] When the thickness of a PTFE-based film is very thin, the elongation at break is low, making stretching very difficult. Especially when pinholes or other defects exist in the non-porous PTFE-based film before the stretching process, controlling the pore size of the porous film formed after stretching becomes extremely difficult. Furthermore, PTFE-based porous films are transparent, making defect detection difficult; using conventional transmitted light defect inspection equipment, the defect detection limit diameter is approximately 30 μm. However, in this method for manufacturing a porous film laminate, before stretching the non-porous PTFE film, a process is employed to select the non-porous film laminate by evaluating its resistance to pressure in fluorinated solvents with a boiling point below 130°C and a surface tension below 15 mN / m. This allows for easy and precise detection of pinholes and other defects. As a result, the average and maximum pore diameters formed by the uniaxial stretching process can be controlled within a favorable range. Furthermore, by making the porous membrane of the porous membrane stack formed after the above-mentioned uniaxial stretching process have an average thickness of 0.6 μm or more and 3.5 μm or less, and a maximum pore size of 49 nm or less, the filtration efficiency and accuracy of the porous membrane stack can be improved. Therefore, this method for manufacturing porous membrane stacks can easily and reliably produce porous membrane stacks with excellent particle trapping performance and filtration efficiency.

[0025] The non-porous membrane of the non-porous membrane stack selected through the above-mentioned process contains defective pores, preferably with a maximum pore diameter of 600 nm or less. Since the maximum pore diameter of the defective pores in the non-porous membrane stack selected through the above-mentioned process is 600 nm or less, the average and maximum pore diameters of the pores formed after the uniaxial stretching process of the non-porous membrane can be controlled within a good range. When the maximum pore diameter of the defective pores in the non-porous membrane stack is greater than 600 nm, numerous pores with diameters greater than 50 nm are easily scattered after the uniaxial stretching process, thus making pore diameter control potentially difficult.

[0026] Preferably, the non-porous membrane of the non-porous membrane laminate selected through the above-mentioned selection process does not contain defective pores. Since the non-porous membrane of the non-porous membrane laminate selected through the above-mentioned selection process does not contain defective pores, the average and maximum pore diameters of the pores formed after the uniaxial stretching process of the non-porous membrane can be controlled within a good range.

[0027] [Details of the embodiments of the present invention]

[0028] The following is a reference to the appendix. Figure 1 The preferred embodiments of the present invention will be described below.

[0029] <Porous membrane laminate>

[0030] Figure 1The porous membrane laminate 10 shown has a porous support layer 1 and a porous membrane 2 laminated on one side of the support layer 1. In the porous membrane laminate 10, the porous membrane 2 is laminated and supported on one side of the support layer 1, thus improving its strength. Furthermore, the porous membrane laminate 10 can also be used as a filter element.

[0031] [Porous membrane]

[0032] The porous membrane 2 is mainly composed of polytetrafluoroethylene (PTFE). While preventing fine impurities from passing through, the porous membrane 2 allows the filtrate to pass through in the thickness direction.

[0033] The porous membrane 2 is a uniaxially stretched material. A uniaxially stretched material is a material that has undergone uniaxial stretching. Uniaxial stretching means stretching in only one direction. The porous membrane 2 is stretched along the short side direction (the axial direction of the calendering roll perpendicular to the long side direction (the conveying direction)) by the transverse axis.

[0034] The heat of melting of PTFE, the main component of the porous membrane 2, is preferably 25 J / g or more and 29 J / g or less. With the heat of melting of PTFE within this range, it is easy to control the average pore size of the porous membrane 2 within a favorable range.

[0035] Porous membrane 2, viewed from above, per 623.7 cm² 2 The lower limit of the average pore size of the porous membrane 2 is 25 nm. On the other hand, the upper limit of the average pore size is 35 nm, preferably 30 nm. When the average pore size of the porous membrane 2 is less than the lower limit, the pressure loss of the porous membrane stack may increase. On the other hand, when the average pore size of the porous membrane 2 is greater than the upper limit, the particle trapping performance of the porous membrane stack may become insufficient.

[0036] Porous membrane 2, viewed from above, per 623.7 cm² 2 The maximum pore size of the porous membrane 2 is capped at 49 nm, preferably 46 nm. If the maximum pore size of the porous membrane 2 exceeds this upper limit, the particle trapping performance of the porous membrane stack may become insufficient. With the average and maximum pore sizes of the porous membrane 2 within the aforementioned ranges, the porous membrane stack exhibits excellent particle trapping performance and filtration efficiency.

[0037] The lower limit of the average thickness of the porous membrane 2 is 0.6 μm. On the other hand, the upper limit of the average thickness of the porous membrane 2 is 3.5 μm, preferably 3.0 μm. When the average thickness does not meet the lower limit, the strength of the porous membrane 2 may become insufficient. On the other hand, when the average thickness exceeds the upper limit, the porous membrane 2 may become unnecessarily thick, which may increase the pressure loss during filtrate permeation. By keeping the average thickness of the porous membrane 2 within the above range, both the strength of the porous membrane 2 and the filtration efficiency can be balanced.

[0038] The upper limit of the porosity of the porous membrane 2 is preferably 90%, more preferably 85%. On the other hand, the lower limit of the porosity of the porous membrane 2 is preferably 70%, more preferably 75%. When the porosity of the porous membrane 2 is greater than the above-mentioned upper limit, the particle trapping performance of the porous membrane laminate may become insufficient. On the other hand, when the porosity of the porous membrane 2 is less than the above-mentioned lower limit, the pressure loss of the porous membrane laminate may increase. In addition, "porosity" refers to the ratio of the total volume of pores to the volume of the object, which can be determined by measuring the density of the object according to ASTM-D-792.

[0039] In addition to PTFE, the porous membrane 2 may also contain other fluoropolymers and additives, within a range that does not impair the desired effects of the present invention.

[0040] [Support Layer]

[0041] As for the material used as the support layer 1 for the porous structure, there are no particular limitations as long as it is a porous body. Specifically, examples of support layer 1 include foams, nonwoven fabrics, and stretched porous bodies, and examples of materials constituting them include: polyolefin resins such as polyethylene and polypropylene; fluorinated resins such as PTFE and PFA; and polyimide resins such as polyimide and polyamide-imide.

[0042] The lower limit of the average thickness of the support layer 1 is preferably 0.02 mm, more preferably 0.03 mm. On the other hand, the upper limit of the average thickness of the support layer 1 is preferably 0.06 mm, more preferably 0.05 mm. Furthermore, from the viewpoint of balancing the mechanical strength of the support layer 1 and the filtration efficiency of the porous membrane stack 10, the above-mentioned average thickness is preferably 0.020 mm or more and 0.040 mm or less, more preferably 0.025 mm or more and 0.035 mm or less. When the above-mentioned average thickness does not meet the lower limit, the mechanical strength of the support layer 1 may become insufficient. On the other hand, when the above-mentioned average thickness is greater than the upper limit, the porous membrane stack 10 may become unnecessarily thick, which may increase the pressure loss when the filtrate permeates.

[0043] The lower limit of the average pore size of the support layer 1 is preferably 0.5 μm, and more preferably 1 μm.

[0044] On the other hand, the upper limit of the average pore size is preferably 5 μm, more preferably 3 μm. If the average pore size of the support layer 1 is less than the lower limit, the pressure loss of the porous membrane stack 10 may increase. On the other hand, if the average pore size of the porous membrane 2 is greater than the upper limit, the strength of the support layer 1 may become insufficient.

[0045] To the extent that the desired effects of the present invention are not impaired, the support layer 1 may also contain other resins and additives. Examples of such additives include pigments for coloring, inorganic fillers for improving wear resistance, preventing low-temperature flow, and facilitating the formation of pores, metal powders, metal oxide powders, metal sulfide powders, etc.

[0046] The upper limit of the average thickness of the porous membrane stack 10 is preferably 60 μm, more preferably 50 μm. On the other hand, the lower limit of the average thickness of the porous membrane stack 10 is preferably 20 μm, more preferably 25 μm. When the average thickness of the porous membrane stack 10 is greater than the above-mentioned upper limit, the pressure loss of the porous membrane stack 10 may increase. On the other hand, when the average thickness of the porous membrane stack 10 is less than the above-mentioned lower limit, the strength of the porous membrane stack 10 may become insufficient.

[0047] The isopropanol bubble point of the porous membrane stack 10 is preferably 600 kPa or higher and 1310 kPa or lower. If the isopropanol bubble point of the porous membrane stack 10 is lower than the aforementioned lower limit, the dispersion medium retention force of the porous membrane stack 10 may become insufficient. If the isopropanol bubble point of the porous membrane stack 10 is higher than the aforementioned upper limit, gas permeability may decrease, and the degassing efficiency of the porous membrane stack 10 may decline. The closer the isopropanol bubble point is to the average pore size, the better. By ensuring the isopropanol bubble point of the porous membrane stack 10 is within the aforementioned range, the particle trapping performance of the porous membrane stack 10 can be further improved.

[0048] The porous membrane laminate 10 exhibits excellent particle capture performance and filtration efficiency. Therefore, it is suitable as a precision filter for dispersion media and gases used in applications such as cleaning, stripping, and pharmaceutical supply in semiconductor-related fields, liquid crystal-related fields, and food and medical-related fields.

[0049] <Filter element>

[0050] This filter element utilizes the aforementioned porous membrane laminate. Due to the use of this porous membrane laminate, the filter element exhibits excellent particle capture performance and filtration efficiency. It is particularly suitable for purifying pure water used in precision semiconductor-related fields for cleaning and stripping.

[0051] <Manufacturing Method of Porous Membrane Laminates>

[0052] Next, one embodiment of the method for manufacturing this porous membrane laminate will be described. The method for manufacturing this porous membrane laminate comprises a porous membrane laminate having a porous support layer and a porous membrane laminated on one side of the support layer. The method includes: a step of coating a porous membrane forming composition onto the surface of a metal foil; a step of sintering the porous membrane forming composition; a step of laminating a formed non-porous membrane onto one side of the support layer; a step of removing the metal foil; a step of selecting a non-porous membrane laminate after the removal step that has a pressure resistance to fluorinated solvents of 101.325 kPa or higher; and a step of uniaxially stretching the non-porous membrane laminate at room temperature.

[0053] [Process of applying the composition for forming porous membranes]

[0054] In this process, a porous membrane forming composition, primarily composed of polytetrafluoroethylene (PTFE), is applied to the surface of a metal foil. The surface of the metal foil is preferably smooth. The porous membrane forming composition is a dispersion in which PTFE powder is dispersed in a dispersion medium. After application of the porous membrane forming composition, it is dried to remove the dispersion medium. Water or other aqueous media are typically used as the dispersion medium.

[0055] Examples of metals suitable for the foil include aluminum and nickel. Among these, aluminum is particularly preferred from the viewpoints of flexibility, ease of removal, and availability. Furthermore, a smooth foil refers to the absence of pores or unevenness observed on the surface of the foil on the side in contact with the PTFE dispersion during this process. The thickness of the foil is not particularly limited, but a thickness that allows for easy coating without introducing air bubbles into the PTFE dispersion coating film is preferred, and a thickness that does not make subsequent foil removal difficult.

[0056] The lower limit of the number-average molecular weight of the PTFE powder forming the porous membrane 2 is preferably 1 million, more preferably 1.2 million. On the other hand, the upper limit of the number-average molecular weight of the PTFE powder forming the porous membrane 2 is preferably 5 million. If the number-average molecular weight of the PTFE powder forming the porous membrane 2 is less than the above-mentioned lower limit, the porosity and strength of the porous membrane 2 may become insufficient. On the other hand, if the number-average molecular weight of the PTFE powder forming the porous membrane is greater than the above-mentioned upper limit, membrane formation may become difficult. In addition, "number-average molecular weight" refers to the value measured by gel filtration chromatography.

[0057] Drying of the dispersion medium can be achieved by heating it to a temperature close to or above the boiling point of the dispersion medium.

[0058] [Sintering process]

[0059] In this process, the porous membrane forming composition applied in the above-described coating process is sintered. This process forms a non-porous membrane with PTFE as the main component. In this process, a non-porous PTFE membrane is obtained by heating the coated film formed from the porous membrane forming composition to above the melting point of the fluoropolymer. Alternatively, the above-described drying and sintering heating of the dispersion medium can also be performed in this process.

[0060] [Layered processes]

[0061] In this process, a non-porous film layer formed after the above-mentioned sintering process is stacked on one side of the support layer. By stacking the non-porous film layer on one side of the support layer, a non-porous film laminate with a metal foil is formed.

[0062] Methods for fixing the non-porous membrane to the support layer include, for example, bonding with an adhesive or bonding agent, or welding by heating. From the viewpoint of heat resistance and chemical resistance, fluoropolymers or fluororubbers that are soluble in solvents or thermoplastic are preferred as adhesives or bonding agents.

[0063] [The process of removing the metal foil]

[0064] In this process, the metal foil is removed from the non-porous membrane laminate containing the metal foil formed by the above-described lamination process. Methods for removing the metal foil include, for example, dissolving it with acid or mechanical peeling. If the metal foil is not sufficiently removed, pinholes may occur; therefore, it is preferable to wash with water after removing the metal foil to completely remove it. In this way, by coating the metal foil with a fluororesin dispersion obtained by dispersing PTFE powder in a dispersion medium, and then drying and sintering the dispersion medium to remove the metal foil, a non-porous membrane laminate can be obtained.

[0065] [Selected Process]

[0066] In this process, among the non-porous membrane laminates after the aforementioned removal process, a non-porous membrane laminate with a pressure resistance to fluorinated solvents of 101.325 kPa or higher is selected. That is, the non-porous membrane laminate is selected based on its pressure resistance to fluorinated solvents. The aforementioned 101.325 kPa is an atmospheric pressure value.

[0067] As the aforementioned fluorinated solvents, those with low surface tension, viscosity, and quick-drying properties, and which do not affect the material, are preferred. Specifically, fluorinated solvents with a boiling point below 130°C and a surface tension below 15 mN / m are used. For example, fluorinated solvents with a perfluorocarbon framework can be used as such fluorinated solvents. Examples of trade names include, for instance, 3M's perfluorinated solvent (Fluorinert FC-3283).

[0068] The pressure resistance evaluation of the aforementioned non-porous membrane laminate to the aforementioned fluorinated solvent can be performed through the following steps. First, at room temperature and atmospheric pressure, a fluorinated solvent is dropped onto the non-porous membrane surface of the non-porous membrane laminate. If the non-porous membrane is free of pinholes or other defects, the fluorinated solvent is repelled from the surface and does not penetrate into the non-porous membrane and support layer of the non-porous membrane laminate. On the other hand, if the non-porous membrane has pinholes or other defects, when a fluorinated solvent is dropped onto the surface of the non-porous membrane laminate, the fluorinated solvent immediately penetrates from the surface of the non-porous membrane into the support layer. Whether the fluorinated solvent has penetrated can be visually determined by examining the support layer surface on the back side of the aforementioned non-porous membrane laminate.

[0069] Preferably, the non-porous membrane of the non-porous membrane laminate selected through the above-mentioned selection process does not contain defective pores, or although it contains defective pores, the maximum pore size of the defective pores is 600 nm or less. If the non-porous membrane before uniaxial stretching has pores with a maximum pore size greater than 600 nm, these pores are defective pores generated during the manufacturing process. Furthermore, this maximum pore size can be measured using a common defect inspection device using transmitted light. Therefore, by selecting a non-porous membrane laminate with a maximum pore size of 600 nm or less before the uniaxial stretching process, the average and maximum pore sizes of the pores formed after the uniaxial stretching process can be controlled within a good range. When the maximum pore size of the non-porous membrane of the non-porous membrane laminate is greater than 600 nm, numerous pores with a diameter greater than 50 nm are easily scattered after the uniaxial stretching process, thus, pore size control may become difficult.

[0070] [The process of uniaxial tensioning]

[0071] In this process, the non-porous membrane laminate selected through the above-mentioned steps is uniaxially stretched at room temperature. Pores are formed through this process. Alternatively, it can be uniaxially stretched in multiple segments.

[0072] When the thickness of a PTFE-based film is very thin, the elongation at break is low, making stretching very difficult. Especially when pinholes or other defects exist in the non-porous PTFE-based film before the stretching process, controlling the pore size of the porous film formed after stretching becomes extremely difficult. Furthermore, PTFE-based porous films are transparent, making defect detection difficult; using conventional transmitted light defect inspection equipment, the defect detection limit diameter is approximately 30 μm. However, in this method for manufacturing a porous film laminate, a step is performed before stretching the non-porous PTFE film to select the non-porous film laminate based on its pressure resistance to fluorinated solvents with a boiling point below 130°C and a surface tension below 15 mN / m. This allows for easy and precise detection of pinholes and other defects. As a result, the average and maximum pore diameters formed by the uniaxial stretching process can be controlled within a favorable range.

[0073] In this process, uniaxial stretching is performed at room temperature. By performing the stretching at room temperature, the suppression of fractures, pinholes, and other defects caused by uniaxial stretching can be improved. Furthermore, when performing uniaxial stretching in multiple segments, it is preferable to perform uniaxial stretching at room temperature followed by uniaxial stretching at a temperature below 30°C. By keeping the stretching temperature below 30°C, the average pore size of the formed porous membrane 2 can be kept small.

[0074] As described above, the lower limit of the average thickness of the porous membrane 2 in the manufactured porous membrane laminate is 0.6 μm. On the other hand, the upper limit of the average thickness of the porous membrane 2 is 3.5 μm, preferably 3.0 μm. When the average thickness does not meet the lower limit, the strength of the porous membrane 2 may become insufficient. On the other hand, when the average thickness exceeds the upper limit, the porous membrane 2 may become unnecessarily thick, and the pressure loss during filtration may increase. By keeping the average thickness of the porous membrane 2 within the above range, both the strength of the porous membrane 2 and the filtration efficiency can be balanced.

[0075] The porous membrane and other structures of the support layer in the fabricated porous membrane laminate are as described above, so repeated descriptions are omitted.

[0076] According to the manufacturing method of this porous membrane laminate, by having a step of selecting the non-porous membrane laminate by evaluating its pressure resistance to fluorinated solvents with a boiling point of 130°C or less and a surface tension of 15 mN / m or less before stretching the non-porous membrane formed from PTFE, defects such as pinholes can be easily and accurately detected. As a result, the average and maximum pore sizes of the pores formed by the uniaxial stretching step can be controlled within a good range. Furthermore, by ensuring that the average thickness of the porous membrane in the porous membrane laminate formed after the uniaxial stretching step is 0.6 μm or more and 3.5 μm or less, and the maximum pore size is 49 nm or less, the filtration efficiency and accuracy of the porous membrane laminate can be improved. Therefore, this manufacturing method of the porous membrane laminate can easily and reliably manufacture porous membrane laminates with excellent particle trapping performance and filtration efficiency.

[0077] [Other Implementation Methods]

[0078] It should be considered that the embodiments disclosed herein are illustrative in all respects and not restrictive. The scope of the invention is defined by the claims and is not limited to the structure of the above embodiments, and is intended to include all modifications with the same meaning and scope as the claims.

[0079] Explanation of reference numerals in the attached figures

[0080] 1: Support layer;

[0081] 2: Porous membrane;

[0082] 10: Porous membrane stack.

Claims

1. A method for manufacturing a porous membrane laminate, the porous membrane laminate having a porous support layer and a porous membrane laminated on one side of the support layer, The method for manufacturing the porous membrane laminate includes: The process of coating a porous membrane forming composition, with polytetrafluoroethylene as the main component, onto the surface of a metal foil. The process of sintering the porous membrane formed by the coating process described above with the composition. The process of stacking the non-porous film formed after the sintering process onto one side of the support layer to form a non-porous film laminate with metal foil. The process of removing the metal foil from the non-porous film laminate with metal foil formed by the lamination process. The process of selecting a non-porous membrane laminate with a pressure resistance of 101.325 kPa or higher to fluorine-based solvents after the removal process, and... The process of uniaxially stretching the non-porous membrane laminate selected through the chosen process at room temperature. The fluorinated solvent has a boiling point below 130°C and a surface tension below 15 mN / m. The porous membrane in the porous membrane stack formed after the uniaxial stretching process has an average thickness of 0.6 μm or more and 3.5 μm or less, and a maximum pore size of 49 nm or less.

2. The method for manufacturing a porous membrane stack according to claim 1, wherein the non-porous membrane of the non-porous membrane stack selected by the selected process includes defective pores, the maximum pore diameter of which is less than 600 nm.

3. The method for manufacturing a porous membrane laminate according to claim 1, wherein the non-porous membrane of the non-porous membrane laminate selected through the selected process does not contain defective pores.

4. A porous membrane laminate, which is manufactured by the method for manufacturing a porous membrane laminate according to any one of claims 1 to 3. The heat of fusion of the polytetrafluoroethylene (PTFE), the main component of the porous membrane in the porous membrane laminate, is 25 J / g or more and 29 J / g or less. The average pore size of the porous membrane is greater than 25 nm and less than 35 nm. The top-view area of ​​the porous membrane stack is 623.7 cm². 2 above.

5. The porous membrane laminate according to claim 4 has an isopropanol bubble point of 600 kPa or higher.

6. The porous membrane laminate according to claim 4, wherein, The porosity of the porous membrane is 70-90%.

7. A filter element that uses the porous membrane laminate according to any one of claims 4 to 6.

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