WO2020262490A1

WO2020262490A1 - Porous membrane and composite porous membrane

Info

Publication number: WO2020262490A1
Application number: PCT/JP2020/024906
Authority: WO
Inventors: 稜佐藤; 花川　正行; 健太岩井; 俊志村
Original assignee: 東レ株式会社
Priority date: 2019-06-27
Filing date: 2020-06-24
Publication date: 2020-12-30
Also published as: JPWO2020262490A1

Abstract

A porous membrane according to the present invention contains a branched polyvinylidene fluoride-based resin and a hydrophilic resin, wherein regarding the intensity ratio A/A' of a hydrophilic resin-derived peak intensity A to a polyvinylidene fluoride-based resin-derived peak intensity A', as detected by infrared spectroscopic measurement at intervals of 5 μm from the surface of the porous membrane in the thickness direction, the maximum value of the intensity ratios A/A' at four points of 5, 10, 15, and 20 μm from the dense surface of the porous membrane in the thickness direction is greater than the maximum value of the intensity ratios A/A' at three points of 25, 30, and 35 μm in the thickness direction, and the average value of the intensity ratios A/A' at five points of 5, 10, 15, 20, and 25 μm from the dense surface in the thickness direction is 0.90-1.20.

Description

Porous Membrane and Composite Porous Membrane

The present invention relates to a porous membrane and a composite porous membrane.

In recent years, porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in various fields such as water treatment fields such as water purification or wastewater treatment, medical fields such as blood purification, and food industry fields. Porous membranes in such fields are used repeatedly and are washed or sterilized with various chemicals, so that high chemical resistance is usually required.

As a porous membrane exhibiting excellent chemical resistance, a porous membrane made of a polymer containing a polyvinylidene fluoride resin is known. For example, Patent Document 1 discloses a technique for improving separation performance by reducing the pore size distribution in the cross-sectional structure of a porous membrane made of a polymer containing a polyvinylidene fluoride resin. Further, Patent Document 2 discloses a technique for expanding the pore size of a porous membrane and improving the permeation performance by selecting a long-chain branched fluoropolymer as the polyvinylidene fluoride-based resin contained in the porous membrane. There is.

Japanese Patent Application Laid-Open No. 2006-263721 Japan Special Table 2016-510688

However, in the conventional porous membrane made of a polymer containing a polyvinylidene fluoride resin, a membrane having both separation performance and permeation performance has not been obtained.
Therefore, an object of the present invention is to provide a porous membrane capable of achieving both excellent separation performance and permeation performance and having high chemical resistance.

In order to solve the above problems, the present invention includes the following aspects.
[1] A porous film containing a branched polyvinylidene fluoride resin and a hydrophilic resin, which is derived from a hydrophilic resin detected by infrared spectroscopic measurement at intervals of 5 μm in the thickness direction from the surface of the porous film. Regarding the intensity ratio A / A'of the peak intensity A and the peak intensity A'derived from the polyvinylidene fluoride resin, 5 μm, 10 μm, and 15 μm in the thickness direction from the dense surface on any one surface of the porous film. The maximum value of the intensity ratio A / A'at four points of 20 μm is larger than the maximum value of the intensity ratio A / A'at three points of 25 μm, 30 μm and 35 μm in the thickness direction from the dense surface, and , A porous film in which the average value of the strength ratio A / A'at 5 points of 5 μm, 10 μm, 15 μm, 20 μm and 25 μm in the thickness direction from the dense surface is in the range of 0.90 to 1.20.
[2] The porous membrane according to the above [1], wherein the hydrophilic resin is a water-insoluble resin.
[3] When an electron microscope image (SEM image) having an average surface pore diameter of 3 to 16 nm on the dense surface and a cross section perpendicular to the surface of the porous film is taken at intervals of 5 μm in the thickness direction from the surface. The average cross-sectional hole diameter B in the region 25 to 30 μm in the thickness direction from the dense surface is within a range of 5 to 25 times the average cross-sectional hole diameter B in the region 0 to 5 μm in the thickness direction from the dense surface. The porous membrane according to [1] or [2].
[4] SEM images of a cross section perpendicular to the surface of the porous film are taken at intervals of 5 μm in a range of up to 30 μm in the thickness direction from the dense surface, and the distance C (μm) from the dense surface and the above. The slope k in the following approximate formula (1) obtained by the minimum square method from the measured value of the average cross-sectional pore diameter B (μm) of the pores in each cross section obtained from the SEM image is within the range of 0.01 to 0.2. The porous film according to any one of the above [1] to [3].
B = kC + d ... (1)
(D is a constant)
[5] The porous film according to any one of [1] to [4] above, wherein the standard deviation of the average surface pore diameter of the dense surface is 8.0 nm or less.
[6] With respect to the polymer constituting the porous membrane, the radius of gyration <S ² > ^1/2 and the absolute molecular weight M _w obtained by using a gel permeation chromatograph equipped with a multi-angle light scattering detector and a differential refractometer. The value of a in the following approximate formula (2) obtained by the conformation plot is in the range of 0.32 to 0.41, and the value of b is 0.18 to 0.42. The porous membrane according to any one of the above [1] to [5], which is within the range of.
<S ² > ^1/2 = bM _w ^a ... (2)
[7] A composite porous membrane in which the porous membrane according to any one of [1] to [6] above and a support layer containing a polyvinylidene fluoride-based resin as a main component are laminated. In the SEM image of the cross section perpendicular to the surface of the porous film, the average cross-sectional pore diameter D of the pores existing in the region from the surface of the support layer not in contact with the porous film to 30 μm in the thickness direction is dense. A composite porous membrane within the range of 50 to 500 times the average surface pore size of the surface.
[8] In the SEM image of the cross section perpendicular to the surface of the composite porous membrane, the average of the pores existing in the range from the surface where the porous membrane and the support layer are in contact to 25 μm in the thickness direction of the support layer. The composite porous material according to the above [7], wherein the cross-sectional pore diameter E is within a range of 2 to 10 times the average cross-sectional pore diameter F of the porous membrane in the SEM image of the cross section perpendicular to the surface of the composite porous membrane. film.

According to the present invention, it is possible to provide a porous membrane in which both excellent separation performance and permeation performance are achieved while ensuring high chemical resistance by containing a branched polyvinylidene fluoride resin.

FIG. 1 is a graph showing the evaluation results of the porous membrane in each Example and Comparative Example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto. In the present specification, the mass-based ratio (percentage, etc.) is the same as the weight-based ratio (percentage, etc.).

1. 1. Porous Membrane (1-1) Composition of Porous Membrane (a) Polymer The porous membrane of the present invention needs to contain a branched polyvinylidene fluoride resin and a hydrophilic resin.
The "polyvinylidene fluoride-based resin" refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer. The vinylidene fluoride copolymer refers to a polymer having a vinylidene fluoride residue structure, and is typically a copolymer of a vinylidene fluoride monomer and another fluoromonomer or the like. Examples of such a fluorine-based monomer include vinyl fluoride, ethylene tetrafluoroethylene, propylene hexafluoride, and ethylene trifluoride chloride. The vinylidene fluoride copolymer may be copolymerized with ethylene or the like other than the above-mentioned fluorine-based monomer to the extent that the effect of the present invention is not impaired.

Here, the "branched polyvinylidene fluoride resin" is a gel permeation chromatograph (hereinafter, "RI") equipped with a multi-angle light scattering detector (hereinafter, "MALS") and a differential refractometer (hereinafter, "RI"). The value of a in the following approximate formula (2) obtained by the conformation plot from the measurement results of the radius of gyration <S ² > ^1/2 and the absolute molecular weight M _w measured by GPC-MALS which is "GPC"). However, it refers to a polyvinylidene fluoride-based resin of 0.41 or less. When the value of a is 0.41 or less, the radius of gyration <S ² > ^1/2 is appropriately small with respect to the absolute molecular weight M _w of the polymer, so that the polymer is porous when the porous film is formed. It becomes easy to move to the vicinity of the surface of the quality film. As a result, the polymer density of the layer near the surface of the porous membrane tends to increase, and it is presumed that the porous membrane exhibits excellent separation performance.
<S ² > ^1/2 = bM _w ^a ... (2)

The measurement using GPC-MALS is performed by dissolving the polymer constituting the porous membrane in a solvent. A salt may be added to the solvent in order to improve the solubility of the polymer. When measuring a polyvinylidene fluoride resin using GPC-MALS, for example, N-methyl-2-pyrrolidone (hereinafter, “NMP”) to which 0.1 mol / L lithium chloride is added should be used. Is preferable.

Conformation plots are a commonly used technique in polymer research. That is, the radius of gyration <S ² > ^1/2 measured by GPC-MALS and the absolute molecular weight M _w can be plotted in both logarithms, and the value of a in the above approximate expression (2) can be determined from the slope. it can.

"Hydrophilic resin" refers to a resin that has a high affinity for water. Since the porous membrane contains a hydrophilic resin, it has a high affinity with water and exhibits high permeation performance. Examples of the hydrophilic resin include cellulose esters such as cellulose acetate and cellulose acetate propionate, fatty acid vinyl esters, polyvinyl acetate, polymers of acrylic acids such as polymethyl methacrylate, or polymers of methacrylic esters, or their weights. A coalesced copolymer can be mentioned.

The hydrophilic resin is preferably a water-insoluble resin in order to retain a large amount of the hydrophilic resin in the porous membrane when forming the porous membrane. As the water-insoluble hydrophilic resin, it is preferable that the hydrophilic resin itself is insoluble in water or water-insoluble by appropriate treatment. As such a treatment, for example, in the case of a hydrophilic resin having vinylpyrrolidone, ethylene oxide or propylene oxide in the main chain and / or side chain, a method of copolymerizing with another monomer to make it water-insoluble is preferably adopted. For example, a random copolymer of vinylpyrrolidone and methylmethacrylate (PMMA-co-PVP) or a graft copolymer of vinylpyrrolidone to polymethylmethacrylate (PMMA-g-PVP) has an appropriate molar copolymerization ratio. By doing so, it can be obtained as a water-insoluble hydrophilic resin.
Here, insoluble in water means that it does not dissolve in water in an amount of 5% by mass or more even in a high temperature region of 60 ° C. or higher.

The porous membrane of the present invention is composed of a polymer containing a branched polyvinylidene fluoride resin and a hydrophilic resin. Further, the value of a in the above approximate formula (2) for the polymer constituting the porous membrane is 0.32 to 0.41, and the value of b is 0.18 to 0.42. Is preferable.

Since the value of a for the polymer constituting the porous membrane of the present invention is 0.41 or less, the radius of gyration <S ² > ^1/2 is appropriately smaller than the absolute molecular weight M _w of the polymer. When the porous membrane is formed, the polymer easily moves to the vicinity of the surface of the porous membrane. As a result, the polymer density of the layer near the surface of the porous membrane tends to increase, and it is presumed that the porous membrane exhibits excellent separation performance. In addition, when the value of a is 0.32 or more, the polymers are appropriately entangled with each other, and the polymer density of the layer near the surface tends to be uniform, so that it is presumed that even higher separation performance is exhibited. Further, as the polymer density of the layer near the surface of the porous membrane increases, the polymer density of the inner layer decreases, so that it is presumed that high separation performance and high permeation performance are exhibited. The value of a is more preferably 0.37 to 0.40, and even more preferably 0.37 to 0.39.

The value of b for the polymer constituting the porous membrane of the present invention is 0.18 to 0.42 in order to further enhance the separation performance by homogenizing the polymer density of the layer near the surface due to the entanglement of the polymers. It is preferably 0.20 to 0.38, more preferably 0.25 to 0.33, and particularly preferably 0.25 to 0.33.

(B) Composition distribution The porous membrane of the present invention has a peak intensity A derived from a hydrophilic resin detected when infrared spectroscopic measurement is performed at intervals of 5 μm in the thickness direction from the surface of the porous membrane, and polyvinylidene fluoride. Regarding the strength ratio A / A'with the peak strength A'derived from the system resin, the above-mentioned strength ratios at four points of 5 μm, 10 μm, 15 μm and 20 μm in the thickness direction from the dense surface on any one surface of the porous film. The maximum value of A / A'is larger than the maximum value of the above-mentioned strength ratio A / A'at three points of 25 μm, 30 μm and 35 μm in the thickness direction from the dense surface, and 5 μm in the thickness direction from the dense surface. It is required that the average value of the above-mentioned intensity ratio A / A'at 5 points of 10 μm, 15 μm, 20 μm and 25 μm is in the range of 0.90 to 1.20. Here, the dense surface means the surface of the front and back surfaces of the porous film, whichever has the smaller average surface pore diameter. When the porous membrane is a composite porous membrane provided with a support layer, the surface on the side where the porous membrane and the support layer are not in contact is defined as a dense surface.

As described above, the porous membrane of the present invention tends to increase the polymer density near the surface of the porous membrane by using the branched polyvinylidene fluoride resin, whereby the porous membrane exhibits excellent separation performance. On the other hand, the permeation performance tends to decrease in the vicinity of the surface where the polymer density is high. The maximum value of the strength ratio A / A'at the four points of 5 μm, 10 μm, 15 μm and 20 μm in the thickness direction from the dense surface of the porous membrane is the above strength ratio at the three points of 25 μm, 30 μm and 35 μm in the thickness direction. When it is larger than the maximum value of A / A', the hydrophilic resin is present at a high density in a portion having a high polymer density near the dense surface of the porous membrane. As a result, the affinity with water in the vicinity of the dense surface of the porous membrane tends to be high, and it is presumed that high permeation performance is exhibited. Further, when the average value of the intensity ratio A / A'at 5 points of 5 μm, 10 μm, 15 μm, 20 μm and 25 μm from the dense surface in the thickness direction is 0.90 or more, water is formed in the vicinity of the dense surface of the porous film. It is presumed that it exhibits high permeation performance because it tends to have a high affinity with. On the other hand, when the average value of the strength ratio A / A'is 1.20 or less, the ratio of the branched polyvinylidene fluoride-based resin near the dense surface of the porous membrane becomes moderately high, and high chemical resistance is achieved. Easy to show. The average value of the intensity ratio A / A'is more preferably in the range of 1.00 to 1.20, and particularly preferably in the range of 1.05 to 1.20.

Infrared spectroscopy is performed using a microscopic FT-IR. The porous membrane immersed in distilled water is frozen using a cryostat and cut perpendicular to the surface of the porous membrane. The cut porous membrane is placed on a CaF ₂ plate, and the measurement range is set to a range of 10 μm centered on the measurement position in the thickness direction of the porous membrane, and the measurement is performed at intervals of 5 μm from the surface of the porous membrane in the thickness direction. To do.

From the measured absorption spectrum, a peak derived from polyvinylidene fluoride resin and a peak derived from hydrophilic resin were detected, and the intensity ratio A / A'from the peak intensity derived from hydrophilic resin / the peak intensity derived from polyvinylidene fluoride resin. Is calculated. The peak intensity is a value obtained from the absorbance of the absorption spectrum. The position of the peak derived from the polyvinylidene fluoride resin is around 1400 cm ^-1 . The position of the peak derived from the hydrophilic resin is, for example, is around 1750 cm ^-1 if cellulose acetate, is around 1250 cm ^-1 if polyvinyl acetate.

The porous membrane in the present invention preferably has a thickness of 35 μm or more in order to exhibit separation performance without defects. On the other hand, in order to exhibit excellent permeation performance, the thickness of the porous membrane is preferably 35 to 80 μm, more preferably 35 to 50 μm.

(1-2) Structure of Porous Membrane (a) Average Surface Pore Diameter The porous membrane of the present invention preferably has an average surface pore diameter of 3 to 16 nm on a dense surface in order to exhibit excellent separation performance. Further, the porous film of the present invention has an average surface pore diameter of 3 to 16 nm on a dense surface, and an electron microscope image (SEM image) of a cross section perpendicular to the surface of the porous film is 5 μm in the thickness direction from the surface. When photographed at intervals, the average cross-sectional hole diameter B in the region 25 to 30 μm in the thickness direction from the dense surface is 5 to 25 times the average cross-sectional hole diameter B in the region 0 to 5 μm in the thickness direction from the dense surface. It is more preferable that it is within the range.

Further, in the porous film of the present invention, SEM images of a cross section perpendicular to the surface of the porous film are photographed at intervals of 5 μm in a range of up to 30 μm in the thickness direction from the dense surface, and a distance C from the dense surface. The inclination k in the following approximate formula (1) obtained by the minimum square method from the measured values of (μm) and the average cross-sectional pore diameter B (μm) of the pores in each cross section obtained from the SEM image is 0.01 to 0.2. It is preferably within the range of. Further, in the porous film of the present invention, it is more preferable that the average surface pore diameter of the dense surface is 3 to 16 nm and the inclination k is in the range of 0.01 to 0.2.
B = kC + d ... (1)
(D is a constant)

The average surface pore size can be calculated by observing the surface of the porous membrane with an electron microscope (hereinafter, "SEM"). More specifically, the surface of the porous membrane is observed using SEM at a magnification of 30,000 to 100,000 times, and the areas of 300 randomly selected pores are measured respectively. From the area of each hole, the diameter when it is assumed that the hole is a circle is calculated as the hole diameter, and the average value thereof can be used as the average surface hole diameter. In order to exhibit particularly excellent separation performance and permeation performance, the average surface pore size of the dense surface is more preferably 6 to 14 nm, and particularly preferably 8 to 11 nm.

In the porous membrane or composite porous membrane of the present invention, the average surface pore size of the dense surface is in the above range, and the pure water permeability at 25 ° C. and 50 kPa is 0.1 to 0.8 m ³ / m ² /. It is preferably hr, and more preferably 0.2 to 0.4 m ³ / m ² / hr. For the pure water permeability of the porous membrane or composite porous membrane of the present invention at 50 kPa, the membrane area and the amount of permeated water per hour are measured at a pressure within the range where the porous membrane is not deformed, and these values are measured under a pressure of 50 kPa. It may be calculated by converting each of the values into. A proportional relationship is established when converting the pressure.

Here, the standard deviation of the average surface pore diameter of the dense surface is preferably 8.0 nm or less. When the standard deviation is 8.0 nm or less, high separation performance is likely to be exhibited due to the uniformity of the surface structure of the porous membrane. In particular, in order to exhibit excellent separation performance, the standard deviation of the average surface pore size is more preferably 5.0 nm or less.

The fractional molecular weight of the porous membrane or the composite porous membrane of the present invention is preferably 5,000 to 80,000 Da, more preferably 8,000 to 50,000 Da, and 10,000 to 29, It is more preferably 000 Da. Here, the "molecular weight cut-off" refers to the minimum molecular weight that can be removed by 90% of the molecular weights of the components contained in the liquid to be filtered by the porous membrane.

(B) Average cross-sectional hole diameter The average cross-sectional hole diameter B is calculated by a method using image analysis in the same manner as the average surface hole diameter. More specifically, the porous membrane immersed in distilled water is sectioned by cutting with a microtome. The cross section of the obtained section was observed using SEM at a magnification of 10,000 times, and in the cross-sectional image, (130 μm parallel to the surface of the porous membrane) × (relative to the thickness direction of the porous membrane). A region of 5 μm) in parallel is divided into one segment and continuously divided in the thickness direction. Each of the divided segments is binarized with a structural portion made of resin and a pore portion. The contour of the hole can be discriminated using image processing software, and the structural part made of resin and the pore part are binarized, and the part and noise that cannot be discriminated are corrected with a freehand tool. can do. It is possible to use general image processing software for the binarization processing. Examples of the image processing software include ImageJ (Wayne Rasband, National Institutes of Health) and the like. Using the image after the binarization process, the areas of 30 holes randomly selected from the locations corresponding to the holes are measured by image processing software. From the area of each hole, the diameter when it is assumed that the hole is a circle is calculated as the hole diameter, and the average value thereof is defined as the average cross-sectional hole diameter B.
Let the average value of the average cross-sectional hole diameter B in each segment be the average cross-sectional hole diameter F. The average cross-sectional pore diameter B and the average cross-sectional pore diameter F of the composite porous membrane described later are the same as the definitions of the average cross-sectional pore diameter B and the average cross-sectional pore diameter F of the porous membrane.

In the porous membrane of the present invention, the average cross-sectional pore diameter B in the region 25 to 30 μm in the thickness direction from the dense surface is 5 to 25 times the average cross-sectional pore diameter B in the region 0 to 5 μm in the thickness direction from the dense surface. It is preferably within the range, more preferably 5 to 15 times, still more preferably 5 to 10 times. High separation performance is maintained because the average cross-sectional hole diameter B in the region 25 to 30 μm in the thickness direction from the dense surface is 5 times or more the average cross-sectional hole diameter B in the region 0 to 5 μm in the thickness direction from the dense surface. On the other hand, it is easy to suppress the deterioration of the permeation performance, and it is easy to maintain the separation performance when it is 25 times or less.

(C) Inclination k
The slope k in the above approximate expression (1) is obtained by photographing SEM images of a cross section perpendicular to the surface of the porous film at intervals of 5 μm in the range of 30 μm in the thickness direction from the dense surface, and from the dense surface of the porous film. It is calculated by approximating all points when the distance C (μm) is plotted on the horizontal axis and the average cross-sectional hole diameter B (μm) measured in the cross section is plotted on the vertical axis. The slope k is preferably in the range of 0.01 to 0.2. When k is 0.01 or more, an inclined asymmetric structure is formed in which the average cross-sectional pore diameter increases as the distance from the surface of the porous membrane increases in the thickness direction, and the permeation performance deteriorates while maintaining high separation performance. Is easy to suppress. Further, when k is 0.2 or less, it is easy to maintain the separation performance of the porous membrane. The slope k is more preferably in the range of 0.01 to 0.08, and even more preferably in the range of 0.01 to 0.04.

2. 2. Composite Porous Membrane (2-1) Structure of Support Layer The composite porous membrane of the present invention tends to exhibit physical strength when the support layer is in contact (laminated) with one surface of the porous membrane. .. The "support layer" refers to a structure for physically reinforcing the porous membrane, which has a higher breaking strength than the porous membrane. It is preferable that the support layer contains a polyvinylidene fluoride-based resin as a main component from the viewpoint of adhesiveness to the porous film. In order to reinforce the porous film, the breaking strength of the support layer, that is, the breaking strength per unit area is preferably 3 MPa or more, and more preferably 10 MPa or more. When the composite porous film is hollow filamentous, the breaking strength of the support layer is preferably 300 gf or more, and more preferably 800 gf or more. Further, the support layer preferably has a fibrous structure, a columnar structure or a spherical structure in order to further enhance the strength of the composite porous film.

The breaking strength or breaking strength can be calculated by repeating the tensile test 5 times on a sample having a length of 50 mm under the condition of a tensile speed of 50 mm / min using a tensile tester and averaging those values. When the ratio of the volume of the support layer to the total volume of the composite porous membrane is 50% or more, the breaking strength or breaking strength of the composite porous membrane is adjusted to the breaking strength or breaking strength of the supporting layer which is a component thereof. It can be regarded as breaking strength.

The support layer preferably has a thickness of 30 μm or more in order to exhibit physical strength. On the other hand, in order to exhibit excellent permeation performance, the thickness of the support layer is preferably 30 to 400 μm, more preferably 30 to 200 μm.

Here, in order to exhibit excellent breaking strength and permeation performance, the support layer preferably has an average cross-sectional pore diameter D in the range of 50 to 500 times the average surface pore diameter of the dense surface. The average cross-sectional hole diameter D is more preferably in the range of 100 to 400 times, and further preferably in the range of 150 to 300 times. When the average cross-sectional hole diameter D is 50 times or more, excellent permeation performance is likely to be exhibited. When the average cross-sectional hole diameter D is 500 times or less, excellent breaking strength is likely to be exhibited.

The average cross-sectional pore diameter D of the support layer is calculated by a method using image analysis in the same manner as the average cross-sectional pore diameter B of the porous film. Specifically, the cross section of the composite porous film was photographed using SEM at a magnification of 10,000 times, and in the cross-sectional image, from the surface of the support layer not in contact with the porous film (relative to the surface of the support layer). A region of (130 μm in parallel) × (30 μm in parallel with the thickness direction of the support layer) was cut out, binarized using image processing software, and randomly selected from the locations corresponding to the holes 30. The area of each pore is measured. From the area of each hole, the diameter when it is assumed that the hole is a circle is calculated as the hole diameter, and the average value thereof can be taken as the average cross-sectional hole diameter D.

Further, it is preferable that the average surface pore size of the surface of the support layer on the side not in contact with the porous film is larger than the average surface pore size of the dense surface of the porous film in the composite porous film. Specifically, the average surface pore size of the surface of the support layer on the side not in contact with the porous film is preferably 0.01 to 120 μm, more preferably 0.01 to 10 μm, and even more preferably 0.01 to 5 μm. When the average surface pore diameter is at least the above lower limit value, excellent permeation performance is likely to be exhibited. Further, when the average surface pore diameter is not more than the above upper limit value, excellent breaking strength and breaking strength are likely to be exhibited. The average surface pore size of the support layer can be calculated by the same method as the average surface pore size of the porous film.

(2-2) Structure of the surface of the support layer Generally, when the layers are stacked in multiple stages, the layers become dense because the layers enter each other at the interface of each layer, and the permeation performance tends to deteriorate. Therefore, when the density of the surface portion of the support layer is high, the ratio of the porous membrane in contact with the surface of the support layer entering the support layer side decreases, and high permeation performance is likely to be exhibited.

In the support layer of the present invention, the average cross-sectional pore size E in the range from the surface where the porous film and the support layer contact to 25 μm in the thickness direction of the support layer is in the range of 2 to 10 times the average cross-sectional pore size F of the porous film. It is preferable to be inside. When the average cross-sectional hole diameter E is twice or more the average cross-sectional hole diameter F, high permeation performance can be easily exhibited. When the average cross-sectional hole diameter E is 10 times or less the average cross-sectional hole diameter F, it is easy to prevent the porous membrane from entering the support layer and deteriorating the permeation performance.

The average cross-sectional hole diameter E is calculated as follows. That is, a cross section of the composite porous film is photographed using SEM at a magnification of 10,000 times, and in the cross-sectional image, from the surface where the porous film and the support layer are in contact (130 μm parallel to the surface of the support layer) ×. A region (25 μm parallel to the thickness direction of the support layer) is cut out. This was binarized using image processing software, the areas of 30 holes randomly selected from the locations corresponding to the holes were measured, and it was assumed that the holes were circular from the area of each hole. The diameter at the time of this is calculated as the hole diameter. The average value of these hole diameters can be defined as the average cross-sectional hole diameter E.

3. 3. Production Method The method for producing a porous film of the present invention includes (A) a polymer solution preparation step of dissolving a polymer containing a branched polyvinylidene fluoride resin and a hydrophilic resin in a solvent to obtain a polymer solution, and (B). The present invention comprises a porous film forming step of coagulating the polymer solution in a non-solvent to form a porous film. When the value of a for the polymer containing the branched polyvinylidene fluoride resin and the hydrophilic resin dissolved in the solvent in the polymer solution preparation step is 0.37 to 0.41, the porous membrane is formed. In addition, the polymer easily moves to the vicinity of the surface of the porous membrane, and the polymer density near the surface of the porous membrane tends to increase. As a result, the maximum value of the strength ratio A / A'at the four points of 5 μm, 10 μm, 15 μm and 20 μm in the thickness direction from the dense surface of the porous membrane is 25 μm, 30 μm and 35 μm in the thickness direction from the dense surface. It is presumed that the intensity ratio A / A'at the three points is larger than the maximum value.

Further, when the value of b for the polymer is 0.18 to 0.42, the polymer density on the surface side is more likely to be homogenized due to the entanglement of the polymers, and the average value of A / A'is 0.90. It is estimated that it tends to be ~ 1.20.

A good solvent is preferable as the solvent used in the polymer solution preparation step. Here, the "good solvent" means a solvent capable of dissolving 5% by mass or more of the polyvinylidene fluoride-based resin even in a low temperature region of 60 ° C. or lower. Examples of a good solvent include NMP, dimethylacetamide, dimethylformamide, methylethylketone, acetone, tetrahydrofuran, tetramethylurea or trimethyl phosphate, or a mixed solvent thereof.

The polymer solution obtained in the polymer solution preparation step may appropriately contain a third resin, a plasticizer, a salt, or the like in addition to the branched polyvinylidene fluoride resin and the hydrophilic resin.
Further, when the polymer solution contains a plasticizer or a salt, the solubility of the polymer solution is improved. Examples of the plasticizer include glycerol triacetate, diethylene glycol, dibutyl phthalate, dioctyl phthalate and the like. Examples of the salt include calcium chloride, magnesium chloride, lithium chloride or barium sulfate.

The concentration of the polymer solution obtained in the polymer solution preparation step is preferably 15 to 30% by mass, more preferably 20 to 25% by mass, in order to achieve both high separation performance and permeation performance.

The "non-solvent" in the porous film forming step means a solvent that does not dissolve or swell the fluororesin polymer up to the melting point of the polyvinylidene fluoride resin or the boiling point of the solvent. Examples of the non-solvent include water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol and pentane. Aliphatic hydrocarbons such as diols, hexanediols or low molecular weight polyethylene glycols, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, or other chlorinated organic liquids or theirs. A mixed solvent can be mentioned.

When the porous film is continuously formed in the porous film forming step, the solvent of the polymer solution is mixed with the non-solvent in the coagulation bath in which the polymer solution and the non-solvent are brought into contact with each other, and the solvent derived from the polymer solution is used. The concentration increases. Therefore, it is preferable to replace the non-solvent in the coagulation bath so that the composition of the liquid in the coagulation bath is maintained in a certain range. The lower the concentration of the good solvent in the coagulation bath, the faster the polymer solution coagulates, so that the structure of the porous membrane is homogenized and excellent separation performance is likely to be exhibited. Further, since the polymer solution coagulates faster, the film forming speed can be increased, and the productivity of the porous film can be improved. The concentration of the good solvent in the coagulation bath is preferably 20% by mass or less, more preferably 15% by mass or less, and further preferably 10% by mass or less.

In the formation of a normal porous membrane, the lower the temperature of the non-solvent that coagulates the polymer solution, the better the separation performance, but on the other hand, the permeation performance deteriorates, so there is a so-called trade-off relationship. The polymer solution for forming the porous film of the present invention increases the density of the hydrophilic resin on the surface side, which greatly affects the permeation performance, so that even when the temperature of the non-solvent is lowered further. It is possible to realize excellent transmission performance. The temperature of the polymer solution and / or the liquid containing the non-solvent in the coagulation bath is preferably 0 to 25 ° C, more preferably 0 to 20 ° C, still more preferably 5 to 15 ° C.

The shape of the produced porous membrane can be controlled by the mode of solidification of the polymer solution in the porous membrane forming step. In the case of producing a flat film-like porous film, for example, a film-like support layer made of a non-woven fabric, a metal oxide, a metal or the like coated with a polymer solution can be immersed in a coagulation bath.

When producing a hollow fiber membrane-like porous film, the polymer solution can be discharged from the outer peripheral portion of the double tube cap, and the core solution can be discharged from the central portion into a coagulation bath containing a non-solvent at the same time. As the core liquid, it is preferable to use a good solvent or the like in the polymer solution preparation step. Further, a porous film may be formed on the surface of a hollow thread-like support layer made of a polymer, a metal oxide, a metal or the like. As a method of forming a porous film on the surface of a hollow thread-like support layer made of a polymer, for example, a method of simultaneously discharging a solution as a raw material of the hollow thread-like support layer and a polymer solution using a triple tube mouthpiece. Alternatively, a method in which a polymer solution is applied to the outer surface of a hollow thread-like support layer formed in advance and passed through a non-solvent in a coagulation bath can be mentioned. The surface of the hollow thread-like support layer is preferably dense, and as a method for producing the hollow thread-like support layer, the concentration of the polymer solution as a raw material for the support layer is increased, and the temperature of the coagulation bath is lowered.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

(I) A value and b value of the polymer constituting the porous membrane The porous membrane or composite porous membrane immersed in distilled water is frozen at −20 ° C. using a cryostat (manufactured by Leica; Jung CM3000). , Porous membrane sections were harvested and vacuum dried overnight at 25 ° C. To the 5 mg porous membrane after vacuum drying, 5 mL of NMP containing 0.1 M lithium chloride was added, and the mixture was stirred at 50 ° C. for about 2 hours. GPC-MALS (column: manufactured by Showa Denko KK; Shodex KF-806M φ8.0 mm × 30 cm, connected in series, differential refractometer: manufactured by Waitt Technology; Optilab) of the obtained polymer solution under the following conditions. rEX, MALS: manufactured by Wyatt Technology; DAWN HeLEOS) was injected and measured. The injected polymer solution eluted from the column in the range of 27-43 minutes.
Column temperature: 50 ° C
Detector temperature: 23 ° C
Solvent: NMP with 0.1M lithium chloride added
Flow velocity: 0.5 mL / min
Injection volume: 0.3 mL

Obtained from RI, polymer concentration _c i of when the elution time _{t i,} obtained from the MALS, from the excess Rayleigh ratio R _.theta.i when the elution time _{^{t i, sin 2 (θ /}} 2) and (K × c _i / R _.theta.i) performs a plot of ^1/2; calculated (Berry plot or Zimm plot formula (4)), the value of theta → 0 of the approximate expression, the absolute molecular weight _{M Wi} at each elution time _{t i} did. Here, K is an optical constant and is calculated from the following equation (3). Note that dn / dc in the formula (3) is the amount of change in the refractive index of the polymer solution with respect to the change in the polymer concentration, that is, the increase in the refractive index. However, the branched polyvinylidene fluoride-based resin is the measurement target, and the above solvent is used. In some cases, a value of −0.05 mL / g can be applied as the index of refraction increment.
K = 4π ² × n ₀ ² × (dn / dc) ² / (λ ⁴ × N ₀ ) ・・・ (3)
n ₀ : Refractive index of solvent dn / dc: Refractive index increment λ: Wavelength of incident light in vacuum N ₀ : Avogadro's number

The value of the radius of gyration ^<S ^{2> 1/2} at each elution time _{t i} was calculated from the slope of the following formula (4).
(Kc _i / R _θi ) ^1/2 = M _Wi ^{-1 / 2} {1 + 1/6 (4πn ₀ / λ) ² <S ² > sin ² (θ / 2)} ・・・ (4)
Is calculated from equation (4), for the x-axis the absolute molecular weight _{M wi} at each elution time _{t i,} and, to a log-log plot of radius of gyration ^<S ^{2> 1/2} to y-axis in each elution time _{t i,} By approximating with the above formula (2), the values of a and b for the polymer constituting the porous membrane were obtained.

(Ii) Infrared spectroscopic measurement of porous membrane or composite porous membrane The porous membrane or composite porous membrane immersed in distilled water is frozen at −20 ° C. using a cryostat (manufactured by Leica; Jung CM3000) and frozen at −20 ° C. Sections were prepared by cutting at intervals of 30 μm perpendicular to the surface of the porous membrane or the composite porous membrane. Sections of the cut porous membrane or composite porous membrane, placed as one of its cut surfaces on the CaF ₂ plate is in contact with the CaF ₂ plate, microscopic FT-IR (manufactured by JASCO Corporation, IRT-3000) Was measured under the following conditions.
Wavelength range: 4000-650 cm ^-1
Resolution: 4 cm ^-1
Number of integrations: 512 times Measurement method: Transmission observation method: Reflection aperture: Width 10, height 90, angle 0

Measured at 5 μm intervals in the thickness direction from the dense surface of the porous film or composite porous film, and the peak derived from polyvinylidene fluoride resin and the peak derived from hydrophilic resin were detected from the obtained absorption spectrum, and the hydrophilic resin was detected. The intensity ratio A / A'was determined from the peak intensity A'derived from the peak intensity A / derived from the polyvinylidene fluoride resin. The position of the peak derived from the polyvinylidene fluoride resin is around 1400 cm ^-1 . The position of the peak derived from the hydrophilic resin is, for example, is around 1750 cm ^-1 if cellulose acetate, is around 1250 cm ^-1 if polyvinyl acetate.
The average value of the intensity ratio A / A'was determined from the average of 5 points of 5 μm, 10 μm, 15 μm, 20 μm and 25 μm in the thickness direction from the dense surface of the porous film or the composite porous film.

(Iii) Fractional molecular weight of the porous membrane or the composite porous membrane When the shape of the porous membrane is flat membrane, evaluation was performed for an effective membrane area of 30 cm ² . When the shape of the porous membrane was a hollow fiber membrane, the effective film area of 14 cm ^{2 was} evaluated. Regarding the composite porous membrane having a support layer in addition to the porous membrane, the entire composite porous membrane including the support layer was evaluated. The following various dextrans were used for the evaluation.

Dextran f1 to f4 (manufactured by Fluka; weight average molecular weights of 1,500 Da, 6,000 Da, 15,000 to 25,000 Da, 40,000 Da, respectively)
Dextran a1 and a2 (manufactured by Aldrich; weight average molecular weight is 60,000 Da, 20,000 Da, respectively)
Dextran a3 and a4 (Molecular weight standard material manufactured by Aldrich; weight average molecular weights of 5,200 Da and 150,000 Da, respectively)
Dextran a5 to a7 (Molecular weight standard substance manufactured by Aldrich; weight average molecular weights are 1,300 Da, 12,000 Da, 50,000 Da, respectively)

Dextran f1 to f4 and dextran a1 and a2 were mixed with distilled water in an amount of 500 ppm each to prepare an aqueous dextran solution 1. The prepared dextran aqueous solution 1 was supplied to the porous membrane at 10 kPa, cross-flow filtered at a cross-flow linear velocity of 1.1 m / s, and the filtrate was sampled. Dextran aqueous solution 1 and sampled filtrate are combined in series with one GPC (column: manufactured by Tosoh Corporation; TSKgel G3000PW φ7.5 mm x 30 cm and one manufactured by Tosoh Corporation; TSKgel α-M φ7.8 mm x 30 cm). Connection, RI: manufactured by Tosoh Corporation; HLC-8320) was injected and measured. The injected dextran eluted from the column in the range of 26-42 minutes.
Column temperature: 40 ° C
Detector temperature: 40 ° C
Solvent: 0.5M lithium nitrate added 50% by volume methanol aqueous solution Flow rate: 0.5mL / min
Injection volume: 0.1 mL

At each elution time t _i, was calculated removal rate from the value of the differential refractive index between the filtrate and the aqueous dextran solution 1. Further, 500 ppm each of dextran a3 and a4 was mixed with distilled water to prepare a dextran aqueous solution 2. Further, 500 ppm each of dextran a5 to a7 was mixed with distilled water to prepare a dextran aqueous solution 3. These dextran solution 2 and 3, measured and injected into GPC under the same conditions as dextran solution 1, to calculate the molecular weight at each elution time t _i, a calibration curve was prepared. A calibration curve prepared, the removal rate of each elution time t _i, in terms of the removal rate in each molecular weight, removal rate of the minimum molecular weight is 90% and the molecular weight cutoff of the porous membrane to be evaluated ..

(Iv) Average surface pore size of the porous membrane The surface of the porous membrane was observed using SEM (manufactured by Hitachi High-Technologies Corporation; S-5500) at a magnification of 30,000 to 100,000 times, and was randomly selected. The area of each of the 300 holes was measured. From the area of each hole, the diameter when the hole was assumed to be a circle was calculated as the hole diameter, and the average value thereof was taken as the average surface hole diameter.

(V) Average cross-sectional pore size of the porous membrane The cross-section of the porous membrane is 10,000 times larger in the range from the dense surface to 30 μm in the thickness direction using SEM (manufactured by Hitachi High-Technologies Corporation; SU-1510). Observed at magnification, a region of (130 μm parallel to the surface) × (5 μm parallel to the thickness direction) was defined as one segment, continuously divided in the thickness direction, and randomly divided for each divided segment. The area of each of the 30 holes selected in 1 was measured. From the area of each hole, the diameter when the hole was assumed to be a circle was calculated as the hole diameter, and the average value thereof was taken as the average cross-sectional hole diameter B.
The average value of the average cross-sectional hole diameter B of each segment was defined as the average cross-sectional hole diameter F.
The following equation (1) was obtained by approximating the least squares method for all points when the average cross-sectional pore diameter B was plotted on the vertical axis and the distance C (μm) from the surface of the porous film was plotted on the horizontal axis. Note that k is the slope.
B = kC + d ... (1)
(D is a constant)

(Vi) Average surface pore size of the support layer For the composite porous film, the surface of the support layer on the side not in contact with the porous film is 30,000 to 10 using SEM (Hitachi High-Technologies Corporation; S-5500). The area of 300 randomly selected holes was measured by observing at a magnification of 10,000 times. From the area of each hole, the diameter when the hole was assumed to be a circle was calculated as the hole diameter, and the average value thereof was taken as the average surface hole diameter.

(Vii) Average cross-sectional pore size of the support layer A cross-section in the thickness direction of the support layer was observed using SEM (manufactured by Hitachi High-Technologies Corporation; SU-1510) at a magnification of 10,000 times, and the porous film of the support layer was observed. A region (130 μm parallel to the surface) × (30 μm parallel to the thickness direction) was cut out from the surface not in contact with the surface, and the areas of 30 randomly selected holes were measured for each of the cut images. did. From the area of each hole, the diameter when the hole was assumed to be a circle was calculated as the hole diameter, and the average value thereof was taken as the average cross-sectional hole diameter D.

(Viii) Average cross-sectional hole diameter of the surface of the support layer The cross-section of the support layer in the thickness direction was observed using SEM (Hitachi High-Technologies Corporation; SU-1510) at a magnification of 10,000 times, and from the vicinity of the surface. , (130 μm parallel to the surface) × (25 μm parallel to the thickness direction) were cut out, and the areas of 30 randomly selected holes were measured for each of the cut out images. From the area of each hole, the diameter when the hole was assumed to be a circle was calculated as the hole diameter, and the average value thereof was taken as the average cross-sectional hole diameter E.

(Ix) Pure water permeability of the porous membrane or the composite porous membrane When the porous membrane has a flat membrane shape, an evaluation was performed for an effective membrane area of 30 cm ² . When the porous membrane was in the form of a hollow fiber membrane, an evaluation was performed on an effective membrane area of 14 cm ² . Distilled water was sent to the porous membrane at a temperature of 25 ° C. and a filtration differential pressure of 10 kPa for 1 hour to filter the entire amount, and the obtained permeated water amount (m ³ ) was measured, and the unit time (h) and It was calculated by converting it into a numerical value per unit film area (m ² ) and further converting it into a pressure (50 kPa). Regarding the composite porous membrane having a support layer in addition to the porous membrane, the entire composite porous membrane including the support layer was evaluated.

The raw materials of the polymer solutions used in Examples and Comparative Examples are summarized below.
Branched polyvinylidene fluoride (hereinafter, "branched PVDF") 1 (Solvay Solef9009, crystallinity 44%, melt viscosity 3 kP)
Branched PVDF2 (manufactured by Solvay; Solef9007, crystallinity 38%, melt viscosity 26 kP)
Branched PVDF3 (manufactured by Solvay; Solef460, crystallinity 45%, melt viscosity 2 kP)
Linear polyvinylidene fluoride (hereinafter, "linear PVDF") 1 (Kynar710 manufactured by Arkema, crystallinity 49%, melt viscosity 6 kP)
Linear PVDF2 (Solvay Solef 1015, crystallinity 48%, melt viscosity 22 kP)
NMP (manufactured by Mitsubishi Chemical Corporation)
Cellulose Acetate (hereinafter "CA") (manufactured by Daicel Corporation; LT-35)

(Example 1)
12% by mass of branched PVDF1 and 7% by mass of CA were mixed, NMP and the like were added, and the mixture was stirred at 120 ° C. for 4 hours to prepare a polymer solution having the composition ratio shown in Table 1. Next, using a polyester fiber non-woven fabric having a density of 0.42 g / cm ³ as a support layer, the prepared polymer solution was uniformly applied to the surface thereof at 10 m / min using a bar coater (thickness 2 mil). The support layer coated with the polymer solution held at 40 ° C. for 1 hour was immersed in distilled water at 15 ° C. for 60 seconds to solidify the support layer 3 seconds after the coating, to form a porous film (thickness 45 μm).

The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. The maximum value of the strength ratio A / A'was located at a position of 15 μm in the thickness direction from the dense surface of the porous film, and the average value of the strength ratio A / A'was 1.06. The average cross-sectional hole diameter B (μm) is 0.07, 0.13, 0.16, 0.22, 0.30, 0.69 from the dense surface side, and the value of k in the above formula (1) is 0. It was 0.02. The value of a in the above formula (2) was 0.40, and the value of b was 0.19. The molecular weight cut-off, which is an index of separation performance, and the water permeability of pure water, which is an index of permeation performance, both showed excellent values.

(Example 2)
A polymer solution having a composition ratio shown in Table 1 was prepared in the same manner as in Example 1 except that branched PVDF2 was used instead of branched PVDF1. Next, a porous film (thickness 40 μm) was formed in the same manner as in Example 1 except that the temperature of the distilled water was changed to 10 ° C.
The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. The maximum value of the strength ratio A / A'was located at a position of 15 μm in the thickness direction from the dense surface of the porous film, and the average value of the strength ratio A / A'was 1.06. The average cross-sectional hole diameter B (μm) is 0.08, 0.17, 0.21, 0.24, 0.30, 0.55 from the dense surface side, and the value of k in the above formula (1) is 0. It was 0.02. Further, the value of a in the above formula (2) was 0.37, and the value of b was 0.28. Both the molecular weight cut-off and the permeability of pure water showed excellent values.

(Example 3)
A polymer solution having a composition ratio shown in Table 1 was prepared in the same manner as in Example 1. Next, a porous film (thickness 45 μm) was formed in the same manner as in Example 1 except that the temperature of the distilled water was changed to 40 ° C.
The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. The maximum value of the strength ratio A / A'was located at a position of 10 μm in the thickness direction from the dense surface of the porous membrane, and the average value of the strength ratio A / A'was 1.04. The average cross-sectional hole diameter B (μm) is 0.17, 0.25, 0.77, 0.65, 1.20, 1.52 from the dense surface side, and the value of k in the above formula (1) is 0. It was 0.05. The value of a in the above formula (2) was 0.40, and the value of b was 0.19. Both the molecular weight cut-off and the permeability of pure water showed excellent values.

(Example 4)
25% by mass of branched PVDF3 and 75% by mass of linear PVDF1 were mixed to prepare "PVDF", NMP and the like were added, and the mixture was stirred at 120 ° C. for 4 hours to prepare a polymer solution having the composition ratio shown in Table 1.
Next, a porous film (thickness 45 μm) was formed in the same manner as in Example 1.
The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. The maximum value of the strength ratio A / A'was located at a position of 15 μm in the thickness direction from the dense surface of the porous film, and the average value of the strength ratio A / A'was 0.99. The average cross-sectional hole diameter B (μm) is 0.08, 0.14, 0.24, 0.31, 1.20, 1.61 from the dense surface side, and the value of k in the above formula (1) is 0. It was .06. Further, the value of a in the above formula (2) was 0.40, and the value of b was 0.18. Both the molecular weight cut-off and the permeability of pure water showed excellent values.

(Example 5)
42% by mass of linear PVDF2 and 58% by mass of γ-butyrolactone were mixed and dissolved at 160 ° C. to prepare a membrane-forming stock solution. This film-forming stock solution is discharged from the double tube mouthpiece with an 85% by mass γ-butyrolactone aqueous solution accompanied as a hollow portion forming liquid, and an 85% by mass γ-butyrolactone aqueous solution at a temperature of 5 ° C. installed 30 mm below the base is contained. By solidifying in a cooling bath, a hollow filament-like support layer containing a polyvinylidene fluoride-based resin as a main component was prepared.

A polymer solution having a composition ratio shown in Table 1 was prepared in the same manner as in Example 1.
Next, a polymer solution held at 50 ° C. for 1 hour was uniformly applied at 10 m / min to the surface of the support layer containing the hollow filament-like polyvinylidene fluoride resin as a main component (thickness 50 μm). One second after the application of the polymer solution, the support layer is immersed in distilled water at 15 ° C. for 10 seconds to coagulate to form a composite porous film (porous film thickness 40 μm, support layer thickness 200 μm). did.

The results of evaluating the obtained composite porous membrane are shown in Table 1 and FIG. The maximum value of the strength ratio A / A'was located at a position of 15 μm in the thickness direction from the dense surface of the porous film, and the average value of the strength ratio A / A'was 1.06. The average cross-sectional hole diameter B (μm) is 0.07, 0.13, 0.16, 0.21, 0.30, 0.70 from the dense surface side, and the value of k in the above formula (1) is 0. It was 0.02. The average cross-sectional hole diameter D (μm) was 1.80, which was 200 times the average surface hole diameter of the dense surface. The average cross-sectional pore diameter E (μm) was 0.84, and the average cross-sectional pore diameter F (μm) of the porous membrane was 0.41. The value of a in the above formula (2) was 0.40, and the value of b was 0.19. Both the molecular weight cut-off and the permeability of pure water showed excellent values. Further, the surface on the porous film side where the porous film and the support layer are not in contact, that is, the average surface pore diameter of the dense surface is 9 nm, and the average of the surfaces on the support layer side where the porous film and the support layer are not in contact. The surface pore diameter was 0.3 μm. It can be seen that the average surface pore size of the porous film is denser than the average surface pore size of the support layer. The breaking strength of the composite porous membrane was 10 MPa, and the breaking strength was 970 gf.

(Comparative Example 1)
Using linear PVDF1 as "PVDF", NMP was added and the mixture was stirred at 120 ° C. for 4 hours to prepare a polymer solution having the composition ratio shown in Table 2. Next, a porous film (thickness 40 μm) was formed in the same manner as in Example 1.
The results of evaluating the obtained porous membrane are shown in Table 2 and FIG. The maximum value of the strength ratio A / A'was located at a position of 30 μm in the thickness direction from the dense surface of the porous film, and the average value of the strength ratio A / A'was 1.08. The average cross-sectional hole diameter B (μm) is 0.30, 0.31, 0.31, 0.34, 0.33, 0.35 from the dense surface side, and the value of k in the above formula (1) is 0. It was .002. Further, the value of a in the above formula (2) was 0.43, and the value of b was 0.17. Both the molecular weight cut-off and the water permeability of pure water were inferior to the results of Examples.

(Comparative Example 2)
A polymer solution having a composition ratio shown in Table 2 was prepared in the same manner as in Comparative Example 1 except that branched PVDF3 was used.
Next, a porous film (thickness 35 μm) was formed in the same manner as in Example 1 except that the temperature of the distilled water was changed to 20 ° C.
The results of evaluating the obtained porous membrane are shown in Table 2 and FIG. The maximum value of the strength ratio A / A'was located at a position of 10 μm in the thickness direction from the dense surface of the porous film, and the average value of the strength ratio A / A'was 0.57. The average cross-sectional hole diameter B (μm) is 0.18, 0.22, 5.85, 6.02, 6.11, 8.90 from the dense surface side, and the value of k in the above formula (1) is 0. It was 0.4. Further, the value of a in the above formula (2) was 0.36, and the value of b was 0.27. Both the molecular weight cut-off and the water permeability of pure water were inferior to the results of Examples.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on a Japanese patent application filed on June 27, 2019 (Japanese Patent Application No. 2019-119656), the contents of which are incorporated herein by reference.

Claims

A porous film containing a branched polyvinylidene fluoride resin and a hydrophilic resin, and the peak intensity A derived from the hydrophilic resin detected by infrared spectroscopic measurement at intervals of 5 μm in the thickness direction from the surface of the porous film. With respect to the intensity ratio A / A'with the peak intensity A'derived from the polyvinylidene fluoride resin, 5 μm, 10 μm, 15 μm and 20 μm in the thickness direction from the dense surface on any one surface of the porous film. The maximum value of the strength ratio A / A'at four points is larger than the maximum value of the strength ratio A / A'at three points of 25 μm, 30 μm, and 35 μm in the thickness direction from the dense surface, and the density is high. A porous film in which the average value of the strength ratio A / A'at 5 points of 5 μm, 10 μm, 15 μm, 20 μm and 25 μm from the surface in the thickness direction is in the range of 0.90 to 1.20.
The porous membrane according to claim 1, wherein the hydrophilic resin is a water-insoluble resin.
When an electron microscope image (SEM image) having an average surface pore diameter of 3 to 16 nm and a cross section perpendicular to the surface of the porous film is taken at 5 μm intervals in the thickness direction from the surface, the dense surface 1 or claim 1, wherein the average cross-sectional hole diameter B in the region from 25 to 30 μm in the thickness direction is within a range of 5 to 25 times the average cross-sectional hole diameter B in the region from 0 to 5 μm in the thickness direction from the dense surface. 2. The porous membrane according to 2.
SEM images of a cross section perpendicular to the surface of the porous film were taken at intervals of 5 μm in a range of up to 30 μm in the thickness direction from the dense surface, and the distance C (μm) from the dense surface and the SEM image were taken. The slope k in the following approximate formula (1), which is obtained by the minimum square method from the measured value of the average cross-sectional pore diameter B (μm) of the pores in each obtained cross section, is in the range of 0.01 to 0.2. The porous film according to any one of claims 1 to 3.
B = kC + d ... (1)
(D is a constant)
The porous film according to any one of claims 1 to 4, wherein the standard deviation of the average surface pore diameter of the dense surface is 8.0 nm or less.
Measurement results of radius of gyration <S 2 > 1/2 and absolute molecular weight M w obtained by using a gel permeation chromatograph equipped with a multi-angle light scattering detector and a differential refractometer for the polymer constituting the porous membrane. Therefore, the value of a in the following approximate formula (2) obtained by the conformation plot is in the range of 0.32 to 0.41, and the value of b is in the range of 0.18 to 0.42. The porous membrane according to any one of claims 1 to 5.
<S 2 > 1/2 = bM w a ... (2)
A composite porous membrane in which the porous membrane according to any one of claims 1 to 6 and a support layer containing a polyvinylidene fluoride-based resin as a main component are laminated, and the surface of the composite porous membrane. In the SEM image of the cross section perpendicular to, the average cross-sectional pore diameter D of the pores existing in the region from the surface of the support layer not in contact with the porous film to 30 μm in the thickness direction is the average surface pore diameter of the dense surface. A composite porous membrane in the range of 50 to 500 times that of.
In the SEM image of the cross section perpendicular to the surface of the composite porous film, the average cross-sectional pore diameter E of the pores existing in the range from the surface where the porous film and the support layer are in contact to 25 μm in the thickness direction of the support layer. The composite porous film according to claim 7, wherein the composition is within a range of 2 to 10 times the average cross-sectional pore diameter F of the porous film in the SEM image of the cross section perpendicular to the surface of the composite porous film.