Method for manufacturing a porous membrane laminate

Abstract

A porous film laminate according to the present disclosure comprises: a porous supporting layer; and a porous film which is stacked on one surface of the supporting layer and is composed primarily of polytetrafluoroethylene. The porous film is made of a uniaxially drawn material, the porous film has an average pore diameter of 25-35 nm and a maximum pore diameter of 49 nm or less, and the porous film has an average thickness of 0.6-3.5 μm.

Description

Technical Field

[0001] The present disclosure relates to a porous membrane laminate, a filter element, and a method for manufacturing a porous membrane laminate. This application claims priority based on Japanese Application No. 2020-089970 filed on May 22, 2020, and incorporates all the descriptions described in the above Japanese application.

Background Art

[0002] A porous filter using polytetrafluoroethylene (PTFE) has characteristics such as high heat resistance, chemical stability, weather resistance, incombustibility, high strength, non-stickiness, and low friction coefficient of PTFE, as well as characteristics such as flexibility due to porosity, dispersion medium permeability, particle capture ability, and low dielectric constant due to porosity. Therefore, porous filters made of PTFE are widely used as precision filters for dispersion media and gases in semiconductor-related fields, liquid crystal-related fields, and food and medical-related fields. As such a filter, in recent years, a porous filter using a porous sheet made of PTFE capable of capturing fine particles with a particle diameter of less than 0.1 μm has been proposed (see Japanese Patent Application Laid-Open No. 2010-94579).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] A porous membrane laminate according to one aspect of the present disclosure includes a porous support layer and a porous membrane laminated on one side of the support layer and mainly composed of polytetrafluoroethylene. The porous membrane is a uniaxially stretched material, the average pore diameter in the porous membrane is 25 nm or more and 35 nm or less, the maximum pore diameter 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.

[0005] A method for manufacturing a porous membrane laminate according to another aspect of the present disclosure is a method for manufacturing a porous membrane laminate including a porous support layer and a porous membrane laminated on one side of the support layer, the method comprising: a step of applying a composition for forming a porous membrane containing polytetrafluoroethylene as a main component onto the surface of a metal foil; a step of sintering the composition for forming a porous membrane applied in the applying step; a step of laminating the non-porous membrane with the metal foil formed after the sintering step onto one side of the support layer; a step of removing the metal foil from the non-porous membrane laminate with the metal foil formed in the laminating step; a step of selecting, from among the non-porous membrane laminates after the removing step, a non-porous membrane laminate having a pressure resistance of 101.325 kPa or more with respect to a fluorine-based solvent; and a step of uniaxially stretching the selected non-porous membrane laminate at room temperature, wherein the fluorine-based solvent has a boiling point of 130° C. or less and a surface tension of 15 mN / m or less, and the average thickness of the porous membrane of the porous membrane laminate formed after the uniaxially stretching step is 0.6 μm or more and 3.5 μm or less, and the maximum pore diameter is 49 nm or less.

Brief Description of the Drawings

[0006]

Figure 1

Embodiments for Carrying Out the Invention

[0007] [Problems to be Solved by the Present Disclosure] In the fields as described above, due to further technological innovations and increasing requirements, a more high-performance precision filtration filter is desired.

[0008] The present disclosure has been made based on such circumstances, and an object thereof is to provide a porous membrane laminate excellent in particulate capture performance and filtration treatment efficiency.

[0009] [Effects of the Present Disclosure] A porous membrane laminate according to one aspect of the present disclosure is excellent in particulate capture performance and filtration treatment efficiency.

[0010] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0011] The porous membrane laminate according to one aspect of the present disclosure includes a porous support layer and a porous membrane laminated on one side of the support layer and mainly composed of polytetrafluoroethylene. The porous membrane is a uniaxially stretched material, the average pore diameter in the porous membrane is 25 nm or more and 35 nm or less, the maximum pore diameter 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.

[0012] The porous membrane laminate includes a porous membrane that is a uniaxially stretched material mainly composed of polytetrafluoroethylene (hereinafter also referred to as PTFE). The average pore diameter, maximum pore diameter, and average thickness per area of 623.7 cm 2 in the plane view of the porous membrane are within the above ranges, so that the microparticle capture performance and filtration efficiency of the porous membrane are excellent. Here, the "main component" refers to the component with the largest content in terms of mass conversion. For example, it refers to 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. The "average pore diameter" means the average diameter of the pores on the outer surface of the support layer and can be measured by a pore diameter distribution measuring device (for example, the Palm Porometer "CFP-1200A" manufactured by PMI). The "average thickness" refers to the average value of the thicknesses at any 10 points.

[0013] The porous membrane laminate preferably has an isopropanol bubble point of 600 kPa or more. Thus, when the isopropanol bubble point of the porous membrane laminate is within the above range, the porous membrane laminate can enhance the microparticle capture performance. Here, the "isopropanol bubble point" is a value measured using isopropyl alcohol in accordance with ASTM-F316-86, indicates the minimum pressure required to extrude the dispersion medium from the pores, and is an index corresponding to the average pore diameter.

[0014] It is preferable that the area of the porous membrane laminate in plan view is 623.7 cm 2 or more. According to this form, in the region where the area of the porous membrane is 623.7 cm 2 or more, the average pore diameter is 25 nm or more and 35 nm or less, and the maximum pore diameter is 49 nm or less. Therefore, it is excellent in the capture performance of fine particles and the filtration efficiency in a wide range of regions. In the conventional porous membrane laminate, it was not possible to secure an area of 623.7 cm 2 or more while the average pore diameter was 25 nm or more and 35 nm or less, and the maximum pore diameter was 49 nm or less. In other words, the area of the region excellent in the capture performance and the filtration efficiency was very small. The porous membrane laminate of the present disclosure has a surface with an average pore diameter of 25 nm or more and 35 nm or less, and a maximum pore diameter of 49 nm or less, and its area is 623.7 cm 2 or more. Therefore, it is excellent in the capture performance of fine particles and the filtration efficiency in a wide range of regions.

[0015] Another aspect of the present disclosure is a filter element using the porous membrane laminate. Since the filter element uses the porous membrane laminate, it is possible to provide a precision filtration filter excellent in the capture performance of fine particles and the filtration efficiency.

[0016] A method for manufacturing a porous membrane laminate according to another aspect of the present disclosure is a method for manufacturing a porous membrane laminate including a porous support layer and a porous membrane laminated on one side of the support layer. The method includes a step of coating a surface of a metal foil with a composition for forming a porous membrane containing polytetrafluoroethylene as a main component, a step of sintering the composition for forming a porous membrane coated in the coating step, a step of laminating the non-porous membrane with the metal foil formed after the sintering step on one side of the support layer, a step of removing the metal foil from the non-porous membrane laminate with the metal foil formed in the laminating step, a step of selecting a non-porous membrane laminate having a pressure resistance of 101.325 kPa or more with respect to a fluorine-based solvent from among the non-porous membrane laminates after the removing step, and a step of uniaxially stretching the non-porous membrane laminate selected by the selecting step at room temperature. The fluorine-based solvent has a boiling point of 130° C. or less and a surface tension of 15 mN / m or less. The average thickness of the porous membrane of the porous membrane laminate formed after the uniaxially stretching step is 0.6 μm or more and 3.5 μm or less, and the maximum pore diameter is 49 nm or less.

[0017] When the thickness of a membrane mainly composed of PTFE is extremely thin, the elongation at break is small and stretching processing becomes extremely difficult. In particular, when there are defective holes such as pinholes in the non-porous membrane mainly composed of PTFE before the stretching process for forming pores, it becomes very difficult to control the pore size of the porous membrane formed after the stretching process. On the other hand, since the porous membrane mainly composed of PTFE is transparent, it is difficult to detect defective holes, and in a general defect inspection device using transmitted light, the defect detection limit diameter is about 30 μm. However, the manufacturing method of the porous membrane laminate includes a step of selecting a non-porous membrane laminate by using a pressure resistance evaluation against a fluorine-based solvent having a boiling point of 130 °C or lower and a surface tension of 15 mN / m or lower before stretching the non-porous membrane made of PTFE, whereby defective holes such as pinholes can be easily and accurately detected. As a result, the average pore diameter and the maximum pore diameter of the pores formed by the uniaxial stretching step can be controlled within a good range. Further, by making the average thickness of the porous membrane of the porous membrane laminate formed after the above uniaxial stretching step 0.6 μm or more and 3.5 μm or less, and the maximum pore diameter 49 nm or less, the efficiency and accuracy of the filtration treatment of the above porous membrane laminate can be improved. Therefore, the manufacturing method of the porous membrane laminate can easily and surely manufacture a porous membrane laminate excellent in the microparticle capturing performance and the filtration treatment efficiency.

[0018] It is preferable that the non-porous membrane of the non-porous membrane laminate selected by the above selection step contains defective holes and the maximum pore diameter of the defective holes is 600 nm or less. By the maximum pore diameter of the defective holes of the non-porous membrane of the non-porous membrane laminate selected by the above selection step being 600 nm or less, the average pore diameter and the maximum pore diameter of the pores formed after the uniaxial stretching step of the non-porous membrane can be controlled within a good range. When the maximum pore diameter of the defective holes of the non-porous membrane of the non-porous membrane laminate exceeds 600 nm, there is a risk that countless holes with a pore diameter of 50 nm or more will be easily scattered after the uniaxial stretching step, making it difficult to control the pore diameter.

[0019] The porous membrane of the porous membrane laminate selected by the above selection step preferably does not contain defective holes. Since the porous membrane of the porous membrane laminate selected by the above selection step does not contain defective holes, the average pore diameter and the maximum pore diameter of the pores formed after the uniaxial stretching step of the porous membrane can be controlled within a favorable range.

[0020] [Details of Embodiments of the Present Disclosure] Hereinafter, preferred embodiments of the present disclosure will be described with reference to the drawings.

[0021] <Porous Membrane Laminate> The porous membrane laminate 10 shown in FIG. 1 includes 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, since the porous membrane 2 is laminated and supported on one side of the support layer 1, the strength can be improved. Also, the porous membrane laminate 10 can be applied as a filter element.

[0022] [Porous Membrane] The porous membrane 2 is mainly composed of polytetrafluoroethylene (PTFE). The porous membrane 2 allows the filtrate to permeate in the thickness direction while preventing the permeation of fine impurities.

[0023] The porous membrane 2 is a uniaxially stretched material. A uniaxially stretched material refers to a material that has been uniaxially stretched. Uniaxial stretching means stretching only in one direction, and the porous membrane 2 is horizontally stretched in the short direction (the axial direction of the rolling roll perpendicular to the longitudinal direction (transport direction)).

[0024] The heat of fusion of PTFE, which is the main component of the porous membrane 2, is preferably 25 J / g or more and 29 J / g or less. When the heat of fusion of the PTFE is within the above range, it becomes easier to control the average pore diameter range of the porous membrane 2 within a favorable range.

[0025] The area in plan view of the porous membrane 2 is 623.7 cm 2The lower limit of the average pore diameter per unit area is 25 nm. On the other hand, the upper limit of the average pore diameter is 35 nm, and 30 nm is preferable. If the average pore diameter of the porous membrane 2 is less than the lower limit, the pressure loss of the porous membrane laminate may increase. On the other hand, if the average pore diameter of the porous membrane 2 exceeds the upper limit, the microparticle capture performance of the porous membrane laminate may be insufficient.

[0026] The area in plan view of the porous membrane 2 is 623.7 cm 2 The upper limit of the maximum pore diameter per unit area is 49 nm, and 46 nm is preferable. If the maximum pore diameter of the porous membrane 2 exceeds the upper limit, the microparticle capture performance of the porous membrane laminate may be insufficient. When the average pore diameter and the maximum pore diameter of the porous membrane 2 are within the above ranges, the porous membrane laminate is excellent in microparticle capture performance and filtration treatment efficiency.

[0027] 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, and 3.0 μm is preferable. If the average thickness is less than the lower limit, the strength of the porous membrane 2 may be insufficient. On the other hand, if the average thickness exceeds the upper limit, the porous membrane 2 becomes unnecessarily thick, and the pressure loss when allowing the filtrate to pass through may increase. When the average thickness of the porous membrane 2 is within the above range, both the strength of the porous membrane 2 and the filtration treatment efficiency can be achieved.

[0028] 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%. If the porosity of the porous membrane 2 exceeds the upper limit, the microparticle capture performance in the porous membrane laminate may be insufficient. On the other hand, if the porosity of the porous membrane 2 is less than the lower limit, the pressure loss of the porous membrane laminate may increase. Note that the "porosity" refers to the ratio of the total volume of pores to the volume of the object, and can be obtained by measuring the density of the object in accordance with ASTM-D-792.

[0029] In addition to PTFE, the porous membrane 2 may contain other fluororesins and additives as long as the desired effects of the present disclosure are not impaired.

[0030] [Support layer] What is used for the porous support layer 1 may be any porous body and is not particularly limited. Specifically, examples of the support layer 1 include foams, nonwoven fabrics, stretched porous bodies, etc. Examples of the materials constituting them include polyolefin resins such as polyethylene and polypropylene, fluororesins such as PTFE and PFA, and polyimide resins such as polyimide and polyamideimide.

[0031] As the lower limit of the average thickness of the support layer 1, 0.02 mm is preferable, and 0.03 mm is more preferable. On the other hand, as the upper limit of the average thickness of the support layer 1, 0.06 mm is preferable, and 0.05 mm is more preferable. Furthermore, from the viewpoint of achieving both the mechanical strength of the support layer 1 and the filtration efficiency of the porous membrane laminate 10, the average thickness is preferably 0.020 mm or more and 0.040 mm or less, and more preferably 0.025 mm or more and 0.035 mm or less. If the average thickness is less than the lower limit, the mechanical strength of the support layer 1 may be insufficient. On the other hand, if the average thickness exceeds the upper limit, the porous membrane laminate 10 may become unnecessarily thick, and the pressure loss when permeating the filtrate may increase.

[0032] As the lower limit of the average pore diameter of the support layer 1, 0.5 μm is preferable, and 1 μm is more preferable. On the other hand, as the upper limit of the average pore diameter, 5 μm is preferable, and 3 μm is more preferable. If the average pore diameter of the support layer 1 is less than the lower limit, the pressure loss of the porous membrane laminate 10 may increase. On the other hand, if the average pore diameter of the porous membrane 2 exceeds the upper limit, the strength of the support layer 1 may be insufficient.

[0033] The support layer 1 may contain other resins and additives as long as the desired effects of the present disclosure are not impaired. Examples of the above additives include pigments for coloring, inorganic fillers for improving wear resistance, preventing low-temperature flow, and facilitating pore formation, metal powders, metal oxide powders, metal sulfide powders, and the like.

[0034] As the upper limit of the average thickness of the porous membrane laminate 10, 60 μm is preferable, and 50 μm is more preferable. On the other hand, as the lower limit of the average thickness of the porous membrane laminate 10, 20 μm is preferable, and 25 μm is more preferable. When the average thickness of the porous membrane laminate 10 exceeds the above upper limit, the pressure loss of the porous membrane laminate 10 may increase. On the other hand, when the average thickness of the porous membrane laminate 10 is less than the above lower limit, the strength of the porous membrane laminate 10 may be insufficient.

[0035] The isopropanol bubble point of the porous membrane laminate 10 is preferably 600 kPa or more and 1310 kPa or less. When the isopropanol bubble point of the porous membrane laminate 10 is less than the above lower limit, the dispersion medium holding power of the porous membrane laminate 10 may be insufficient. When the isopropanol bubble point of the porous membrane laminate 10 exceeds the above upper limit, the gas permeability becomes small, and the degassing efficiency of the porous membrane laminate 10 may decrease. The closer the isopropanol bubble point is to the value at the average pore diameter, the more preferable it is. When the isopropanol bubble point of the porous membrane laminate 10 is within the above range, the porous membrane laminate 10 can enhance the capture performance of fine particles.

[0036] According to the porous membrane laminate 10, it is excellent in the capture performance of fine particles and the filtration treatment efficiency. Therefore, it is suitable for a precision filtration filter for a dispersion medium and a gas used in applications such as cleaning, peeling, and chemical solution supply in the semiconductor-related field, the liquid crystal-related field, and the food and medical-related field.

[0037] <Filter element> The filter element uses the above-mentioned porous membrane laminate. Since the filter element uses the porous membrane laminate, it has excellent microparticle capture performance and filtration efficiency. In particular, it is suitable for the purification of ultrapure water for cleaning and stripping in the semiconductor-related fields where high precision is required.

[0038] <Method for manufacturing a porous membrane laminate> Next, an embodiment of the method for manufacturing the porous membrane laminate will be described. The method for manufacturing the porous membrane laminate is a method for manufacturing a porous membrane laminate including 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 a step of coating a composition for forming a porous membrane on the surface of a metal foil, a step of sintering the composition for forming a porous membrane, a step of laminating the formed non-porous membrane with the metal foil on one side of the support layer, a step of removing the metal foil, a step of selecting a non-porous membrane laminate having a pressure resistance of 101.325 kPa or more with respect to a fluorine-based solvent from the non-porous membrane laminate after the removing step, and a step of uniaxially stretching the non-porous membrane laminate at room temperature.

[0039] [Step of coating the composition for forming a porous membrane] In this step, a composition for forming a porous membrane mainly composed of polytetrafluoroethylene is coated on the surface of a metal foil. The surface of the metal foil is preferably smooth. The composition for forming a porous membrane is a dispersion in which PTFE powder is dispersed in a dispersion medium. In this step, after coating the composition for forming a porous membrane, it is dried to remove the dispersion medium. As the dispersion medium, an aqueous medium such as water is usually used.

[0040] Examples of the metal of the metal foil include aluminum and nickel. Among these, aluminum is preferable from the viewpoints of flexibility, ease of removal, and ease of availability. Further, the smoothness of the metal foil means that no holes or irregularities are observed on the surface of the metal foil on the side in contact with the PTFE dispersion in this step. The thickness of the metal foil is not particularly limited, but it is preferably a thickness having flexibility such that the operation of applying the PTFE dispersion can be easily performed so that no air bubbles enter the coating film, and a thickness such that the subsequent removal of the metal foil is not difficult.

[0041] The lower limit of the number average molecular weight of the PTFE powder for forming the porous membrane 2 is preferably 1,000,000, more preferably 1,200,000. On the other hand, the upper limit of the number average molecular weight of the PTFE powder for forming the porous membrane 2 is preferably 5,000,000. If the number average molecular weight of the PTFE powder for forming the porous membrane 2 is less than the above lower limit, the porosity and strength of the porous membrane 2 may be insufficient. On the other hand, if the number average molecular weight of the PTFE powder for forming the porous membrane exceeds the above upper limit, membrane formation may become difficult. The "number average molecular weight" is a value measured by gel filtration chromatography.

[0042] The drying of the dispersion medium can be carried out by heating to a temperature close to or above the boiling point of the dispersion medium.

[0043] [Sintering step] In this step, the porous membrane-forming composition coated in the above coating step is sintered. By this step, a non-porous membrane mainly composed of PTFE is formed. In this step, a non-porous membrane of PTFE can be obtained by heating and sintering a coating film composed of the porous membrane-forming composition to a temperature equal to or higher than the melting point of the fluororesin. Incidentally, the drying and sintering heating of the above-described dispersion medium may be performed in this step.

[0044] [Laminating step] In this step, the non-porous membrane with the metal foil formed after the above sintering step is laminated on one side of the support layer. By laminating the non-porous membrane with the metal foil on one side of the support layer, a non-porous membrane laminate is formed.

[0045] As a method for fixing the porous membrane to the support layer, for example, a method of bonding using an adhesive or a pressure-sensitive adhesive, a method of fusion bonding by heating, etc. can be mentioned. As the adhesive or pressure-sensitive adhesive, a fluororesin or fluororubber having solvent solubility or thermoplasticity is preferable from the viewpoints of heat resistance, chemical resistance, etc.

[0046] [Step of removing the metal foil] In this step, the metal foil is removed from the porous membrane laminate with the metal foil formed in the above laminating step. Examples of the method for removing the metal foil include dissolution and removal with an acid or the like, and mechanical peeling. If the removal of the metal foil is insufficient, there is a possibility of pinholes occurring. Therefore, after removing the metal foil, it is preferable to perform water washing to completely remove the metal foil. Thus, the porous membrane laminate can be obtained by applying a fluororesin dispersion in which PTFE powder is dispersed in a dispersion medium on the metal foil, followed by drying and sintering of the dispersion medium and removing the metal foil.

[0047] [Step of selection] In this step, among the porous membrane laminates after the above removing step, a porous membrane laminate having a pressure resistance of 101.325 kPa or more against a fluorine-based solvent is selected. That is, the porous membrane laminate is selected by evaluating the pressure resistance against a fluorine-based solvent. The 101.325 kPa is the value of atmospheric pressure.

[0048] As the fluorine-based solvent, a fluorine-based solvent having low surface tension, viscosity, and quick drying property and not affecting the material is preferable. Specifically, a fluorine-based solvent having a boiling point of 130°C or lower and a surface tension of 15 mN / m or lower is used. As such a fluorine-based solvent, for example, a fluorine-based solvent having a perfluorocarbon skeleton can be used. Examples of the trade name include Fluorinert (FC-3283) of 3M Company.

[0049] The pressure resistance evaluation of the above-mentioned porous membrane laminate against the fluorinated solvent can be specifically carried out according to the following procedure. First, under the conditions of room temperature and atmospheric pressure, the fluorinated solvent is dropped onto the surface of the porous membrane of the porous membrane laminate. When there are no defect holes such as pinholes in the porous membrane, the fluorinated solvent is repelled on the surface of the porous membrane, and the fluorinated solvent does not penetrate into the porous membrane and the support layer of the porous membrane laminate. On the other hand, if there are defect holes such as pinholes in the porous membrane, when the fluorinated solvent is dropped onto the surface of the porous membrane of the porous membrane laminate, the fluorinated solvent immediately penetrates from the surface of the porous membrane to the support layer. The presence or absence of the penetration of this fluorinated solvent can be visually determined from the surface of the support layer on the back side of the above-mentioned porous membrane laminate.

[0050] The porous membrane of the porous membrane laminate selected by the above-mentioned selection step does not contain defect holes, or may contain defect holes, but the maximum pore diameter of the defect holes is preferably 600 nm or less. If there are holes with a maximum pore diameter exceeding 600 nm in the porous membrane before uniaxial stretching, they are defect holes generated in the manufacturing process. The maximum pore diameter can be measured by a general defect inspection device using transmitted light. Therefore, by selecting the porous membrane of the porous membrane laminate so that the maximum pore diameter is 600 nm or less before the uniaxial stretching step, the average pore diameter and the maximum pore diameter of the pores formed after the uniaxial stretching step of the porous membrane can be controlled within a good range. If the maximum pore diameter of the porous membrane of the porous membrane laminate exceeds 600 nm, countless holes with a pore diameter of 50 nm or more are likely to be scattered after the uniaxial stretching step, so it may be difficult to control the pore diameter.

[0051] [Uniaxial stretching step] In this step, the porous membrane laminate selected by the above-mentioned selection step is uniaxially stretched at room temperature. By this step, pores are formed. Also, the uniaxial stretching may be performed in multiple stages.

[0052] When the thickness of a film mainly composed of PTFE is very thin, the elongation at break is small and the stretching process becomes very difficult. In particular, when there are defective holes such as pinholes in the non-porous film mainly composed of PTFE before the stretching process for forming pores, it becomes extremely difficult to control the pore size of the porous film formed after the stretching process. On the other hand, since the porous film mainly composed of PTFE is transparent, it is difficult to detect defective holes, and in a general defect inspection device using transmitted light, the defect detection limit diameter is about 30 μm. However, the method for manufacturing the porous film laminate includes a step of selecting, by using a pressure resistance evaluation against a fluorine-based solvent having a boiling point of 130 °C or lower and a surface tension of 15 mN / m or lower, before stretching the non-porous film made of PTFE, so that defective holes such as pinholes can be easily and accurately detected. As a result, the average pore diameter and the maximum pore diameter of the pores formed by the uniaxial stretching process can be controlled within a favorable range.

[0053] In this step, uniaxial stretching is performed at room temperature. By performing it at room temperature, the inhibitory effect on breakage and the occurrence of pinholes or the like due to uniaxial stretching can be improved. Further, when performing uniaxial stretching in multiple stages, it is preferable that uniaxial stretching is performed at a temperature of less than 30 °C after uniaxial stretching at room temperature. By setting the stretching temperature to less than 30 °C, the average pore diameter of the formed porous film can be kept small.

[0054] As described above, the lower limit of the average thickness of the porous film 2 of the manufactured porous film laminate is 0.6 μm. On the other hand, the upper limit of the average thickness of the porous film 2 is 3.5 μm, and 3.0 μm is preferable. If the above average thickness is less than the above lower limit, the strength of the porous film 2 may be insufficient. On the other hand, if the above average thickness exceeds the above upper limit, the porous film 2 becomes unnecessarily thick, and there is a possibility that the pressure loss when allowing the filtrate to permeate increases. By the average thickness of the porous film 2 being within the above range, both the strength and the filtration treatment efficiency of the porous film 2 can be achieved.

[0055] Regarding the other configurations of the porous film and the support layer of the manufactured porous film laminate, they are as described above, so overlapping explanations are omitted.

[0056] According to the method for manufacturing the porous membrane laminate, before stretching the non-porous membrane made of PTFE, by providing a step of selecting using the pressure resistance evaluation for a fluorine-based solvent having a boiling point of 130 ° C or lower and a surface tension of 15 mN / m or lower, defective holes such as pinholes can be easily and accurately detected. As a result, the average pore diameter and the maximum pore diameter of the pores formed by the uniaxial stretching step can be controlled within a favorable range. Further, by making the average thickness of the porous membrane of the porous membrane laminate formed after the uniaxial stretching step 0.6 μm or more and 3.5 μm or less, and the maximum pore diameter 49 nm or less, the efficiency and accuracy of the filtration treatment of the porous membrane laminate can be improved. Therefore, the method for manufacturing the porous membrane laminate can easily and surely manufacture a porous membrane laminate excellent in the capturing performance of fine particles and the filtration treatment efficiency.

[0057] [Other Embodiments] The embodiments disclosed this time should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is not limited to the configuration of the above embodiments, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

Explanation of Reference Numerals

[0058] 1 Support layer 2 Porous membrane 10 Porous membrane laminate

Claims

1. A method for manufacturing a porous membrane laminate comprising a porous support layer and a porous membrane laminated on one side of the support layer, a step of applying a composition for forming a porous membrane mainly composed of polytetrafluoroethylene on the surface of a metal foil, a step of sintering the composition for forming a porous membrane applied in the applying step, a step of laminating the non-porous membrane with the metal foil formed after the sintering step on one side of the support layer, a step of removing the metal foil from the non-porous membrane laminate with the metal foil formed in the laminating step, a step of selecting a non-porous membrane laminate having a pressure resistance of 101.325 kPa or more against a fluorine-based solvent among the non-porous membrane laminates after the removing step, a step of uniaxially stretching the non-porous membrane laminate selected by the selecting step and comprising, wherein the fluorine-based solvent has a boiling point of 130°C or less and a surface tension of 15 mN / m or less, the average thickness of the porous membrane of the porous membrane laminate formed after the uniaxially stretching step is 0.6 μm or more and 3.5 μm or less, and the maximum pore diameter is 49 nm or less, a method for manufacturing a porous membrane laminate, wherein the uniaxially stretching step is performed at room temperature, or when uniaxially stretching is performed in multiple stages, uniaxially stretching is performed at a temperature of less than 30°C after uniaxially stretching at room temperature.

2. The method for manufacturing a porous membrane laminate according to claim 1, wherein the non-porous membrane of the non-porous membrane laminate selected by the selecting step contains defect holes, and the maximum pore diameter of the defect holes is 600 nm or less.

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 by the selecting step does not contain defect holes.

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