TW201938703A - Hydrophilized polyvinylidene fluoride-based microporous membrane - Google Patents

Abstract

To hydrophilize a polyvinylidene fluoride-based microporous membrane. Provided are: a coating composition to be used for hydrophilization of the surface of a polyvinylidene fluoride-based microporous membrane, the coating composition containing, as polymerizable monomers, N,N-dimethylacrylamide represented by formula (1) and a trifunctional acrylate compound represented by formula (2); and a hydrophilized polyvinylidene fluoride-based microporous membrane produced using said coating composition.

Description

Hydrophilized polyvinylidene fluoride microporous membrane

The present invention relates to a novel hydrophilizing agent used on the surface of a polyvinylidene fluoride microporous membrane, and a novel polyvinylidene fluoride microporous membrane that is hydrophilized with the hydrophilizing agent.

Polyvinylidene Fluoride (PVDF) -based microporous membranes (PVDF-based microporous membranes) are used for separation or fixation of biochemical substances, drainage treatment, and excellent chemical resistance and heat resistance. Various filters and electrolyte membranes such as gas treatment, dust treatment, and precision filtration are used. As a method for producing a PVDF microporous membrane, there is a non-solvent induced phase separation method (a solution is prepared by dissolving a polymer in a good solvent, and the solution is thinly coated on a glass plate or the like, The winner is a method of obtaining a microporous membrane by immersing in a non-solvent to induce phase separation). The applicant has successfully manufactured a PVDF-based microporous membrane with an asymmetric three-dimensional structure in which relatively uniform pores are formed by a unique method (Patent Document 1).

On the other hand, hydrophobic PVDF has a low affinity for aqueous fluids or biological substances, and therefore, PVDF-based microporous membranes with hydrophilized surfaces are used for liquid filters, biochemical and pharmaceutical separation membranes, and electrolyte membranes. As an initial hydrophilization method, a surface coating with polyvinyl alcohol (PVA) and a surface treatment with ethanol are used. However, these methods have problems in the stability or durability of the hydrophilized surface. Therefore, the applicant has proposed a method of covering the surface of a PVDF-based microporous membrane with a SiO ₂ glass layer in Patent Document 2. However, in this case, there are problems in the adhesion between the glass layer and the PVDF-based microporous membrane.

As another method of hydrophilizing the hydrophobic surface of a microporous membrane, a method of coating the surface of the microporous membrane with a crosslinked polymer has been proposed. As such crosslinked polymers, a nitrogen-containing crosslinked polymer (Patent Document 3), an acrylamide-based polymer (Patent Document 4), a fluorine-containing crosslinked polymer (Patent Document 5), and the like have been proposed in large quantities. However, the compatibility of the surface coating containing these cross-linked polymers with individual microporous membranes varies, and in particular the compatibility of PVDF-based microporous membranes with asymmetric three-dimensional structures developed by the applicant is difficult to say. good.
[Prior Art Literature]
[Patent Literature]

[Patent Document 1] International Publication No. 2014/054658
[Patent Document 2] International Publication No. 2015/0133364
[Patent Document 3] US Patent No. 5,629,084
[Patent Document 4] Japanese Patent Publication No. 2004-532724
[Patent Document 5] Japanese Patent Laid-Open No. 2016-199733

[Problems to be Solved by the Invention]
An object of the present invention is to provide a surface coating agent particularly suitable for hydrophilizing a PVDF-based microporous membrane having an asymmetric three-dimensional structure.
[Means for solving problems]

The present inventors have screened monomers effective as raw materials for surface coating from a large number of candidate raw materials, and surprisingly found that a surface coating agent obtained by cross-linking and polymerizing only two monomers is used as a PVDF for attention The excellent hydrophilizing agent of the microporous membrane is functional. That is, the present invention is as follows.

[Invention 1] A coating composition used for hydrophilization of a microporous membrane, comprising: a polymerizable monomer, a photopolymerization initiator, and a solvent; and a coating composition containing more than 0.75% by mass and 3.0% by mass or less. As the polymerizable monomer, N, N-dimethylacrylamide represented by the formula (1) and a trifunctional acrylate compound represented by the following formula (2) exceeding 0.25% by mass and 1.5% by mass or less are used. body.

[Chemical 1]

[Chemical 2]

[Invention 2] The coating composition according to [Invention 1], wherein the microporous membrane is a fluoropolymer-based microporous membrane.

[Invention 3] The coating composition according to [Invention 1], wherein the microporous film is a polyvinylidene fluoride-based microporous film.

[Invention 4] The coating composition according to [Invention 3], wherein the polyvinylidene fluoride-based microporous film includes a substrate and a microporous film layer, and the microporous film layer is an asymmetric film containing a PVDF-based resin ,
The asymmetric membrane includes a surface layer formed with micropores and a support layer formed with voids larger than the micropores constituting the surface layer,
The surface layer has a plurality of spheroids, and a plurality of linear bonding materials extend from each of the spheroids in a three-dimensional direction, and the adjacent spheroids are connected to each other by the linear bonding material, so that A three-dimensional network structure is formed with the spheroids as intersections, and the polyvinylidene fluoride microporous membrane has the following pore size L (μm): The mode (mode) Lm (μm) of the pore diameter measured by the gas transmission method satisfies (Condition 1): 0.10 ≦ Lm ≦ 0.20, and 95% of all pores of the polyvinylidene fluoride microporous membrane The above satisfies (Condition 2): (Lm × 0.85) ≦ L ≦ (Lm × 1.15).

[Invention 5] A hydrophilized microporous film is a hydrophilized microporous film including a crosslinked polymer layer and a microporous film, and the crosslinked polymer includes N derived from the following formula (1) , A unit of N-dimethylacrylamide and a unit derived from a trifunctional acrylate compound represented by the following formula (2),
In the crosslinked polymer, the unit derived from the N, N-dimethylacrylamide is at least 3.3 mol and 79 mol with respect to 1 mol of the unit derived from the trifunctional acrylate compound. The following ranges exist, and the microporous membrane includes a substrate and a microporous membrane layer.

[Chemical 3]

[Chemical 4]

[Invention 6] The hydrophilized microporous membrane according to [Invention 5], wherein the microporous membrane is a fluoropolymer-based microporous membrane.

[Invention 7] The hydrophilized microporous membrane according to [Invention 5], wherein the microporous membrane is a polyvinylidene fluoride-based microporous membrane.

[Invention 8] The hydrophilized microporous film according to [Invention 7], wherein the polyvinylidene fluoride-based microporous film includes a substrate and a microporous film layer, and the microporous film layer includes a polyvinylidene fluoride-based system An asymmetric membrane of resin, the asymmetric membrane including a surface layer formed with micropores and a support layer formed with voids larger than the micropores constituting the surface layer,
The surface layer has a plurality of spheroids, and a plurality of linear bonding materials extend from each of the spheroids in a three-dimensional direction, and the adjacent spheroids are connected to each other by the linear bonding material, so that A three-dimensional network structure is formed with the spheroids as intersections, and the polyvinylidene fluoride-based microporous membrane has the following pore size L (μm): The mode (mode) Lm (μm) of the pore diameter measured by the gas transmission method satisfies (Condition 1): 0.10 ≦ Lm ≦ 0.20, and 95% of all pores of the polyvinylidene fluoride microporous membrane The above satisfies (Condition 2): (Lm × 0.85) ≦ L ≦ (Lm × 1.15).
[Effect of the invention]

The hydrophilic PVDF microporous membrane of the present invention is a novel industrial material with high surface hydrophilicity and high transmission efficiency.

[Coating composition]
The coating composition of the present invention includes, as a polymerizable monomer, N, N-dimethylacrylamide represented by the following formula (1) and a trifunctional acrylate compound represented by the following formula (2).

[Chemical 5]

[Chemical 6]

The total concentration of the polymerizable monomer contained in the coating composition of the present invention is adjusted within a range that can sufficiently hydrophilize the surface of the PVDF-based microporous membrane without blocking the pores of the PVDF-based microporous membrane. In order to achieve such a concentration of the polymerized monomer, the N, N-dimethylacrylamide and the trifunctional acrylate compound are contained in the coating composition of the present invention in a state of being diluted with a solvent. The preferred solvent is ultrapure water.

That is, in the coating composition of the present invention, the N, N-dimethylacrylamide is present at a concentration exceeding 0.75% by mass and 3.0% by mass or less, preferably 1.5% by mass or more and 2.0% by mass or less, The trifunctional acrylate compound is present at a concentration of more than 0.25% by mass to 1.5% by mass, preferably 0.50% by mass to 1.0% by mass.

That is, the coating composition of the present invention contains 3.3 mol or more and 79 mol or less, preferably 9.8 mol or more and 26 mol or less with respect to 1 mol of the trifunctional acrylate compound. The N, N-dimethylacrylamide is described. The amount ratio of such polymerizable monomers determines the repeating unit ratio of the crosslinked polymer formed by photocuring of the coating composition of the present invention, that is, the N, N-dimethyl group derived from the crosslinked polymer. The ratio of the units of acrylamide to the units derived from the trifunctional acrylate compound. By adjusting the amount ratio of the polymerizable monomers contained in the coating composition of the present invention in this way, the crosslinked polymer produced by the hardening of the coating composition of the present invention can moderately maintain the permeability of the PVDF-based microporous membrane. The surface of the PVDF-based microporous membrane is hydrophilized in a state of.

The coating composition of the present invention contains a photopolymerization initiator in addition to the polymerizable monomer and the solvent. The photopolymerization initiator is not particularly limited. As such a photopolymerization initiator, IRGACURE (registered trademark) series provided by BASF can be used. Among them, preferred photopolymerization initiators are benzoalkyl ketone-based photopolymerization initiators such as the commercial product "IRGACURE 2959".

The coating composition of the present invention may further contain additives such as an antioxidant and a stabilizer at a normal concentration.

The method of mixing the polymerizable monomer with a solvent, a photopolymerization initiator, and an optional additive is not limited. Generally, a polymerization initiator and an optional additive are added to a polymerizable monomer solution dissolved in a solvent at a temperature of 15 ° C or higher and 35 ° C or lower, and the mixture is stirred to obtain the coating composition of the present invention. .

[PVDF series microporous membrane]
The coating composition is applied to the surface of a microporous film. The coating composition is hardened on the surface of the microporous membrane, and the generated crosslinked polymer hydrophilizes the surface of the microporous membrane. Such hydrophilization can be exhibited in any ordinary microporous membrane. However, as a target of application of the coating composition of the present invention, it is possible to select a microporous material which is hydrophobic and which is desired in practical terms and requires active hydrophilization. Porous membrane. As such a microporous membrane, it is preferable to have a microporous membrane layer containing a fluorine-containing resin generally referred to as a "fluoropolymer" (in the present invention, referred to as a "fluoropolymer-based microporous membrane"), It is preferably a PVDF-based microporous membrane.

The preferred PVDF-based microporous membrane used in the present invention includes a substrate and a microporous membrane layer, and the microporous membrane layer is an asymmetric membrane containing a PVDF-based resin. The asymmetric membrane includes a surface layer formed with micropores, and a support layer formed with voids larger than the micropores constituting the surface layer. The surface layer has a plurality of spheroids, and a plurality of linear bonding materials extend from each of the spheroids in a three-dimensional direction. The adjacent spheroids are connected to each other by the linear bonding material, thereby forming a three-dimensional network structure with the spheroids as intersections.

The "surface layer" refers to a layer having a thickness from the surface to the generation of macrovoids in the cross section of the microporous membrane. The "support layer" refers to the thickness of the entire microporous membrane minus the thickness of the surface layer. The value of the thickness of the layer. The "macropore" refers to a large void that is generated in a support layer of a microporous membrane and has a minimum of several μm and a maximum of approximately the same size as the thickness of the support layer. The "spheroid" is a spherical shape formed at the intersection of the three-dimensional network structure of the present invention, and is not limited to a completely spherical shape, and includes a substantially spherical shape.

A typical example of such an asymmetric three-dimensional structure can be understood visually with reference to the drawings.

FIG. 1 is a cross section showing a PVDF-based microporous membrane 1 produced in a reference example. FIG. 2 schematically shows FIG. 1. As shown in FIGS. 1 and 2, a support layer 2 having holes is formed on the surface of the substrate 3. On the surface of the support layer 2, an extremely thin surface layer 1 having holes smaller than the holes of the support layer is further formed. In this way, a microporous film layer including the surface layer 1 and the support layer 2 is formed on the surface of the substrate 3. The PVDF-based microporous membrane of the present invention is a thin film in which the substrate 3 and the microporous membrane layer are integrated.

3 and 4 are enlarged views showing the surface layer 1 of the PVDF-based microporous membrane 1 produced in the reference example. The surface layer 1 includes a spherical body 4 and a bonding material 5 that connects the spherical bodies 4 to each other. The plurality of bonding materials 5 extend from one spheroid 4 in a three-dimensional direction, and each of the bonding materials 5 is connected to other spheroids 4. In this manner, the surface layer 1 is formed of a plurality of spherical bodies 4 and a bonding material 5 arranged in three dimensions.

In the PVDF-based microporous membrane of the present invention, the space between the spheroids and the spheroids is a shape separated by a linear bonding material. Therefore, it is obvious that compared with the existing microporous membranes without spheroids There are microporosity in the shape and size of the voids, and a surface layer having excellent permeability is formed. The linear bonding material cross-links the spheroids and prevents the spheroids from falling off. Therefore, the filter material itself can be prevented from being mixed into the filtrate. The spheroids existing at the intersections of the three-dimensional network structure prevent deformation and breakage of the three-dimensional network structure due to the pressure of the fluid during filtration. Therefore, the PVDF-based microporous membrane of the present invention has high pressure resistance.

In the PVDF-based microporous membrane of the present invention, the spherical body preferably has an average particle diameter of 0.05 μm or more and 0.5 μm or less. The thickness of the surface layer of the microporous membrane of the present invention is preferably 0.5 μm or more and 10 μm or less, and the thickness of the support layer of the microporous membrane of the present invention is preferably 20 μm or more and 500 μm or less.

The PVDF-based resin, which is the main material of the PVDF-based microporous membrane of the present invention, is a preferable resin as a filter membrane material because of its mechanical, thermal, and chemical stability. PVDF-based resins are also advantageous in that they are easier to process than other fluororesins. PVDF-based resins can also be easily cut after a single process or bonded to other raw materials.

The PVDF-based microporous membrane of the present invention is particularly preferably one having a sharp pore size distribution. That is, a preferred PVDF-based microporous membrane has a pore diameter L (μm): the mode (mode) of the pore diameter measured by the gas transmission method satisfies (condition 1): 0.10 ≦ Lm ≦ Lm (μm) of 0.20 and more than 95% of all pores satisfy (Condition 2) (Lm × 0.85) ≦ L ≦ (Lm × 1.15).

In the case where the PVDF-based microporous membrane of the present invention shows a better pore size distribution, the mode (mode) Lm (μm) of the pore size measured by the gas transmission method satisfies (Condition 3): 0.11 ≦ Lm ≦ 0.18, and more than 96% of the total number of pores satisfy (Condition 4) (Lm × 0.85) ≦ L ≦ (Lm × 1.15).

The gas permeation method (permporometry) is one of the common methods for measuring the pore size distribution, and as a material for forming pores as through holes, for example, ceramics, hollow wires, spacers, Non-woven fabrics, membrane filters, and other pore size distribution measurement methods are most commonly used. In this method, the size of the through pores is calculated as the neck caliber using the phenomenon that the gas flow stays in the neck portion (the thinnest point) of the through pores when the gas passes through the through pores of the measurement object. In any measurement equipment (permporometer) based on the gas permeation method, the air filled with an easily wettable organic solvent is slowly raised in pressure while being sent to the through pores of the measurement object. The pore size distribution is calculated based on the relationship between the pressure of the inflow air and the flow rate of the exhaust air.

[Manufacturing method of PVDF-based microporous membrane]
The method for producing such a PVDF-based microporous membrane of the present invention includes the steps of preparing a raw material liquid including a PVDF-based resin, a solvent, a porous agent, and water; and applying the raw material liquid to a substrate film and curing the raw material liquid. A step; and a step of cleaning the film after the curing of the raw material liquid is completed. Generally, dimethylacetamide is used as the solvent, and polyethylene glycol is used as the porous agent.

The method for producing a PVDF-based microporous membrane of the present invention is characterized in that water is added to the raw material liquid applied to the substrate in addition to the PVDF-based resin, solvent, and porogen. By using such a raw material liquid, more uniform pores can be formed in the PVDF-based microporous membrane.

In order to achieve the asymmetric three-dimensional structure unique to the PVDF-based microporous membrane of the present invention, the PVDF-based resin preferably has moderate viscoelasticity.

Such a preferred PVDF-based resin can be judged from the relationship between the shear rate of its good solvent solution and the viscosity of the solution. For this determination, regarding a solution containing 10 parts by weight of PVDF resin, 10 parts by weight of polyethylene glycol, and 80 parts by weight of dimethylacetamide, the shear rate was plotted on the horizontal axis and the solution viscosity was plotted on the vertical axis. reciprocal. In the case where a curve with a quadratic function is used to approximate an area below 40 sec, for example, a curve with a quadratic coefficient of less than -10 ^-8 and a curve with a convex arc on the upper side is obtained, as shown in FIG. 5. This PVDF-based resin was determined to be preferable for the PVDF-based microporous membrane of the present invention.

[Production Example 1 of PVDF-based microporous membrane]
A preferred method for producing a PVDF-based microporous membrane of the present invention includes: (Step 1) preparing a raw material including a PVDF-based resin, dimethylacetamide as a solvent, polyethylene glycol as a porous agent, and water A liquid step; (step 2) a step of coating the raw material liquid obtained in step 1 on a substrate film; (step 3) a step of immersing the film obtained in step 2 in water to solidify the raw material liquid; (Step 4) A step of washing the film passing through Step 3 with water.

(step 1)
Step 1 is a step of preparing a raw material liquid including a PVDF-based resin, a solvent, a porous agent, and water. The PVDF-based resin used here is a raw material of a microporous membrane. As the PVDF-based resin used in step 1, any one or more of vinylidene fluoride homopolymers, one or more vinylidene fluoride copolymers, and mixtures of these are used. As the vinylidene fluoride copolymer, a copolymer of a vinylidene fluoride monomer and a fluorine-based monomer other than the vinylidene fluoride monomer is generally used, for example, one or more selected from the group consisting of vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene. Copolymer of fluorinated monomer and vinylidene fluoride. As the PVDF-based resin used in Step 1, the preferred resin is a vinylidene fluoride homopolymer, and it is desirable that the vinylidene fluoride homopolymer accounts for 50% by weight of the entire PVDF-based resin. In addition, various kinds of vinylidene fluoride homopolymers having different viscosities and molecular weights can also be used.

In step 2 described later, in order that the raw material liquid is not absorbed by the base film and forms a coating film of a uniform raw material liquid, in general, as such a PVDF-based resin, a weight average molecular weight (Mw) of 600,000 to 1.2 million is preferred .

The solvent used in step 1 refers to an organic solvent that can dissolve the PVDF-based resin in a state where the PVDF-based resin is dissolved in the solvent and perform step 2 described later, and that is mixed with water. As such a solvent, N-methyl-2-pyrrolidone (NMP), dimethylsulfinium, N, N-dimethylacetamidamine (DMAc), N, N-diphenyl, which are polar solvents, can be used. Lower alkyl ketones such as methylformamide (DMF), methyl ethyl ketone, acetone, tetrahydrofuran, tetramethylurea, trimethyl phosphate, esters, and amidine. These solvents may be used in combination, or other organic solvents may be included within a range not hindering the effects of the present invention. Among such solvents, N-methyl-2-pyrrolidone, N, N-dimethylacetamide, and N, N-dimethylformamide are preferred.

The porous agent used in step 1 is an organic medium dissolved in the solvent and dissolved in water. In steps 3 and 4 described later, the porous agent and the solvent migrate from the raw material liquid to water. In contrast, the PVDF-based resin does not dissolve in water. Therefore, after passing through steps 3 and 4, it remains on the base film in a solid state, and finally a porous layer is formed on the base film.

As the porous agent used in Step 1, water-soluble polymers such as polyethylene glycol, polypropylene glycol, polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone, and polyacrylic acid are used. The preferred porogen is polyethylene glycol or polyvinylpyrrolidone, and the further preferred porogen is polyethylene glycol. From the pore shape of the obtained vinylidene fluoride microporous membrane, the best The porous agent is polyethylene glycol having a weight average molecular weight of 200 to 1,000.

The amount ratio of the PVDF-based resin, the solvent, and the porous agent is usually 100 parts by weight relative to the total amount of the PVDF-based resin, and the PVDF-based resin accounts for 5 to 20 parts by weight, and the solvent accounts for 70 to 90 parts by weight. The adjustment is performed in such a manner that the porousizing agent accounts for 0.5 to 40 parts by weight.

In the present invention, the following conditions are taken as a necessary condition: In step 1, as the raw material of the raw material liquid, in addition to the PVDF-based resin, the solvent, and the porous agent, water is also used. The water used in step 1 is preferably one having high purity, and ideally, water that can be generally obtained as pure water or ultrapure water. The amount of water added to the raw material liquid is generally adjusted within a range of 6.5% by weight or less, preferably 2% to 6.5% by weight, and more preferably 3% to 5% by weight.

In step 1, the method of mixing the PVDF-based resin, the solvent, the porous agent, and water is not particularly limited. For example, the mixing temperature may be a temperature at which the liquids are completely mixed, and is usually a temperature of room temperature or higher and 100 ° C or lower. The raw material liquid thus obtained is used in Step 2 below.

(Step 2)
Step 2 is a step of applying the raw material liquid obtained in step 1 on the substrate film. The base film is required to promote the formation of pores in the raw material liquid in step 3 described later, and further enhance the function of the obtained PVDF-based microporous film. Therefore, the base film can be used without limitation as long as it is a material that is chemically stable, has mechanical strength, and has excellent affinity or adhesion with a raw material liquid, particularly a PVDF-based resin. As such a base film, for example, papermaking, nonwoven fabric, woven fabric, or porous board obtained by a spunbond method or a meltblown method can be used. As a raw material thereof, polyester, polyolefin, ceramic, cellulose, or the like is used. . Among these substrate films, polypropylene is preferably a spunbond nonwoven fabric in terms of an excellent balance of flexibility, lightness, strength, and heat resistance. When a non-woven fabric is used, the weight per unit area is preferably in the range of 15 g / m ² or more and 150 g / m ² or less, and more preferably 30 g / m ² or more and 70 g / m ² or less. range. When the basis weight exceeds 15 g / m ² , the effect of providing a base material layer can be sufficiently obtained. In addition, if the basis weight is less than 150 g / m ² , it is easy to perform post-processing such as bending or heat bonding.

The coating method of the raw material liquid of the base film is not a method of uniformly coating the raw material liquid on the base film in an amount that can finally generate a PVDF-based microporous film having a thickness of 10 μm to 500 μm. The limitation is, for example, that various coating devices such as a roll coater, a die coater, and a die lip coater, or various film applicators can be selected depending on the area or length of the base film. Step 2 is usually performed at room temperature.

When the base film is a small piece, the base film is placed on a smooth coating table and fixed with a suitable tool, and the raw material liquid is uniformly coated on the film. In this case, the raw material liquid is applied to each sheet of the base material film, and the base material film coated with the raw material liquid is immediately transferred to a container where step 3 described later is performed.

In the case where the base film is long and is typically wound in a roll shape, the rolled base film is pulled out from the end and unrolled, and is conveyed by a transport mechanism such as a roll at a certain tension or The unrolled substrate film is carried into the place where step 2 is performed at a constant speed. In the application section, the raw material liquid is uniformly applied to the surface of the base material film maintained flat and continuously passing through the application section by various application devices. The base material film coated with the raw material liquid carried out from the coating section is immediately transferred to a place where step 3 described later is performed.

(Step 3)
Step 3 is a step of immersing the film obtained in step 2 in water to solidify the raw material liquid. This curing reaction is started by contacting the raw material liquid on the membrane obtained in step 2 with water, and the water-soluble component in the raw material liquid, that is, a fraction mainly containing a solvent and a porous agent, migrates to water and The water-insoluble PVDF-based resin remains and is fixed to the base film to finish. As described above, the raw material liquid is generally present in the raw material liquid in a range of 6.5% by weight or less, preferably 2% by weight or more and 6.5% by weight or less, more preferably 3% by weight or more and 5% by weight or less with respect to the total amount of the raw material liquid. Of course, the water also dissolves out of the membrane. With the migration of the solvent and the porous agent in water, the PVDF-based resin is cured while forming voids in the interior. This step 3 may be referred to as a porosity step or a phase transition step in the formation of a PVDF-based microporous membrane. The water used in step 3 is preferably one having high purity, and ideally, water that can be generally obtained as pure water or ultrapure water.

In this step 3, of course, in order to bring the film obtained in step 2 into contact with water, a container filled with water is required. In the present invention, such a container filled with water is referred to as a curing tank. Curing is performed in a curing tank, and along with this, a water-soluble component, that is, a fraction mainly containing the solvent and a porous agent, moves from the raw material liquid to the water in the curing tank. Increasing or rapidly changing the concentration of such a water-soluble transition component may hinder the curing reaction of step 3 and repeat step 3 with good reproducibility. Therefore, it is desirable to provide an appropriate member that maintains the purity of the water in the curing tank in accordance with the scale of the curing tank or the amount of water in the curing tank.

When a PVDF-based microporous film having a thickness of 10 μm or more and 500 μm or less is formed on the base film, the immersion time (curing time) of the base film coated with the raw material liquid in water is 30 seconds or more It is preferably 1 minute or more and 10 minutes or less, and more preferably 2 minutes or more and 5 minutes or less. In order to generate pores as uniform as possible inside the PVDF-based resin, it is desirable to suppress physical irritation to the surface of the film obtained in step 2 as much as possible during immersion in water. Therefore, in step 3, it is preferable that the water in the curing tank is not stirred or foamed. The water temperature when performing step 3 may be any temperature as long as the curing is performed, and is usually room temperature.

In the case where the film obtained in step 2 is a small piece, step 3 may be performed in a batch manner. Specifically, the film obtained in step 2 is left to stand in a state where the entire film is in contact with water in a curing tank during the curing time. The curing tank used in this case may be appropriately selected according to the shape of the film, and a stainless steel vat or a glass flat bowl may be used if it is a laboratory standard. If the water in the curing tank is changed for each batch of the curing operation in such a way that the purity of the water from the raw material liquid does not greatly change, the curing can be performed with good reproducibility in each step 3 .

In the case where the raw material liquid is applied to the long substrate film in step 2, in step 3, using a conveying member such as a roller, the film obtained in step 2 is first continuously transferred into a curing tank, and secondly, in The film is allowed to pass through the water in the curing tank in such a manner that the film is in contact with water during the curing time, and then discharged from the curing tank. In this way, the curing of the raw material liquid applied to the long film obtained in step 2 is started, performed, and ended. Appropriate drainage and water supply mechanisms can be installed in the curing water tank so that the purity of the water in the curing tank does not change significantly. As such a mechanism, a sensor, a drainage pump, a water supply pump, and the like generally used in a chemical plant can be suitably combined.

The cured film of the raw material liquid whose surface was finished in step 3 is immediately transferred to a place where step 4 described later is performed.

(Step 4)
Step 4 is a step of washing the film after step 3 in water. The water used here is preferably one having the same high purity as in step 3, and ideally, water that can be generally obtained as pure water or ultrapure water.

In such a step 4, of course, a water-filled container for introducing the film through step 3 is required. In the present invention, such a container is referred to as a cleaning tank. In order to improve the cleaning effect, the present invention may also use multiple cleaning tanks, or replace the water in the cleaning tanks to clean the membrane multiple times. In addition, in the present invention, a device that generates water flow or air bubbles may be attached to the washing tank, and the membrane may be cleaned while applying a moderate stimulus. The water flow generating member in this case can be designed by appropriately combining a conventional drainage, water supply mechanism, or stirring mechanism. In addition, the bubble generation device in this case can be suitably selected from components generally called an air diffuser etc. according to the scale of a washing tank. The intensity of the water flow or bubbles is adjusted to such an extent that the PVDF-based resin pores on the surface of the cleaned film will not be deformed. The density of the flow path or air bubbles in the water flow is adjusted in such a way that the water flow or air bubbles are in continuous contact with the membrane in the cleaning tank. In step 4, in order to improve the cleaning efficiency, an appropriate component for maintaining the purity of the water in the cleaning tank may be obtained.

In step 4, the temperature of the water in the cleaning tank may be a temperature that can be cleaned without damaging the membrane, and is usually room temperature.

In the case where the small piece of film is processed in step 4, step 4 may be performed in a batch manner. Specifically, the film obtained in step 3 is allowed to stand (bubble cleaning) in a state where the entire film is in contact with water and air bubbles in a washing tank during the cleaning time. The cleaning tank used in this case may be appropriately selected according to the shape of the film, and if it is a laboratory standard, a stainless steel cylinder or a glass flat bowl may be used. If the water in the cleaning tank is changed for each batch of cleaning in such a way that the purity of the water in the cleaning tank does not change greatly from the components moving from the surface of the membrane, the membrane can be reproducibly performed in step 4 of each time. Cleaning.

In the case where the long film is processed in step 4, in step 4, using a conveying member such as a roller, the film discharged from the curing tank in step 3 is first continuously transferred into a cleaning tank, and secondly, as described above During the cleaning time, the membrane is allowed to pass through the water in the cleaning tank in such a way that the film is in contact with water and air bubbles, and then it is discharged from the cleaning tank (bubble cleaning). In this way, the water-soluble components remaining on the long film during the step 3 are efficiently removed. An appropriate water supply / drainage mechanism can be installed in the washing tank in such a way that the purity of the water in the washing tank does not change greatly. As such a mechanism, a sensor, a drainage pump, a water supply pump, and the like generally used in a chemical plant can be suitably combined.

The film of step 4 is dried, coiled, cut, and packed according to a conventional method and if necessary. In this way, a PVDF-based microporous membrane that can be used as a filtration membrane or a separation membrane is completed.

[Production Example 2 of PVDF-based microporous membrane]
Instead of step 4 of the above-mentioned manufacturing example 1, it is also possible to perform (step 4-1) the step of bubble-washing the film which passed step 3 in water, and (step 4-2) the film which passed the step 4-1 to alcohol. Add the step of bubble cleaning.

(Step 4-1)
Step 4-1 is a step of bubble-cleaning the membrane after step 3 in water. In step 4-1, the water-soluble component in the raw material liquid remaining on the base film, that is, the fraction mainly containing the solvent and the porous agent is subjected to bubble stimulation in water to be efficiently removed from the base film. The water used here is preferably one having the same high purity as in step 3, and ideally, water that can be generally obtained as pure water or ultrapure water.

In such a step 4-1, a container filled with water including a bubble generating device introduced into the film after step 3 is of course required. In the present invention, such a container is referred to as a first cleaning tank. The bubble generating device attached to the first cleaning tank is appropriately selected from parts and the like generally referred to as a diffuser according to the scale of the first cleaning tank. The strength of the air bubbles was adjusted to such an extent that the PVDF-based resin pores on the surface of the washed film were not deformed. The density of the air bubbles is adjusted so that the air bubbles come in contact with the film in the first cleaning tank uniformly and continuously. In step 4-1, in order to improve the cleaning efficiency, an appropriate component for maintaining the purity of the water in the first cleaning tank may be obtained.

In step 4-1, the temperature of the water in the first cleaning tank may be a temperature that can be cleaned without damaging the membrane, and is usually room temperature. The time required for the bubble cleaning in step 4-1, that is, the time during which the film passing through step 3 is in contact with the water in the first cleaning tank is usually 1 minute or more, preferably 2 minutes or more and 20 minutes or less, and more preferably 4 minutes or more and 10 minutes or less.

In the case where the small piece of film is processed in step 4-1, step 4-1 may be performed in a batch manner. Specifically, the film obtained in step 3 is allowed to stand in a state where the entire film is in contact with water and bubbles in the first cleaning tank during the cleaning time. The first cleaning tank used in this case may be appropriately selected according to the shape of the film, and if it is a laboratory standard, a stainless steel cylinder or a glass flat bowl may be used. If the water in the first cleaning tank is changed for each batch of cleaning in such a way that the purity of the water does not change greatly from the components moving from the membrane surface, the reproducibility in each step 4-1 is good. Clean the membrane.

In the case where the long film is processed in step 4-1, in step 4-1, using a transport member such as a roller, the film discharged from the curing tank in step 3 is first continuously transferred into the first cleaning tank Secondly, during the cleaning time, the membrane is allowed to pass through the water in the first cleaning tank in a manner that the film is in contact with water and air bubbles, and then it is discharged from the first cleaning tank. In this way, the water-soluble components remaining on the long film during the step 3 are efficiently removed. An appropriate water supply / drainage mechanism can be installed in the first washing tank in such a manner that the purity of the water in the first washing tank does not greatly change. As such a mechanism, a sensor, a drainage pump, a water supply pump, and the like generally used in a chemical plant can be suitably combined.

The film that has been cleaned for a predetermined period of time in step 4-1 is immediately transferred to a place where step 4-2 described later is performed.

(Step 4-2)
Step 4-2 is a step of bubble-cleaning the membrane in step 4-1 in alcohol. In step 4-2, the alcohol-soluble component in the raw material liquid remaining on the base film, that is, the fraction mainly containing the solvent and the porous agent is bubble-stimulated in the alcohol to be efficiently removed from the base film. As the alcohol used here, a liquid lower alcohol having relatively high fluidity at room temperature is generally used, preferably ethanol, propanols, butanols, and most preferably isopropanol.

In such step 4-2, a container filled with the alcohol including the bubble generating device introduced into the film after step 4-1 is of course required. In the present invention, such a container is referred to as a second cleaning tank. The bubble generating device attached to the second cleaning tank is appropriately selected from parts and the like generally referred to as a diffuser according to the size of the second cleaning tank. The strength of the air bubbles was adjusted to such an extent that the PVDF-based resin pores on the surface of the washed film were not deformed. The density of the air bubbles is adjusted so that the air bubbles come in contact with the film in the second cleaning tank uniformly and continuously. In step 4-2, in order to improve the cleaning efficiency, an appropriate component for maintaining the purity of the alcohol in the second cleaning tank may be obtained.

In step 4-2, the temperature of the alcohol in the second cleaning tank may be a temperature that can be cleaned without damaging the membrane, and is usually room temperature. The time required for the bubble cleaning in step 4-2, that is, the time during which the film passing through step 3 contacts the alcohol in the second cleaning tank is usually 1 minute or more, preferably 5 minutes or more and 120 minutes or less, and more preferably 5 minutes or more and 60 minutes or less.

In the case where the small piece of film is processed in step 4-2, step 4-2 may be performed in a batch manner. Specifically, the film obtained in step 4-1 is left to stand in a state where the entire film is in contact with alcohol and bubbles in the second cleaning tank during the cleaning time. The second cleaning tank used in this case may be appropriately selected according to the shape of the film, and if it is a laboratory standard, a stainless steel cylinder or a glass flat bowl may be used. If the alcohol is changed for each batch of cleaning in such a way that the purity of the alcohol in the second cleaning tank does not change greatly due to the components moving from the membrane surface, the reproducibility in each step 4-2 is good Clean the membrane.

In the case where the long film is processed in step 4-2, in step 4-2, using a conveying member such as a roller, first, the film discharged from the first cleaning tank in step 4-1 is continuously transferred into the first In the second cleaning tank, secondly, the membrane is allowed to pass through the alcohol in the second cleaning tank in a manner that the film is in contact with the alcohol and bubbles during the cleaning time, and then discharged from the second cleaning tank. In this way, the alcohol-soluble components remaining on the long film at the end of step 4-1 are efficiently removed. An appropriate liquid supply / drainage mechanism can be installed in the second cleaning tank in such a manner that the purity of the alcohol in the second cleaning tank does not greatly change. As such a mechanism, a sensor, a drainage pump, a water supply pump, and the like generally used in a chemical plant can be suitably combined.

Dry, wind up, cut, and pack the film that completes step 4-2 as required.

[Hydrophilization of PVDF-based microporous membranes]
The hydrophilized PVDF-based microporous membrane of the present invention is produced by sequentially performing the following steps 5 to 7.
(Step 5) This is a step of bringing the coating composition into close contact with the surface of the PVDF-based microporous membrane. The PVDF-based microporous film is contacted with or impregnated with the coating composition in a batch or continuous manner, so that the coating composition is uniformly adhered to the surface of the PVDF-based microporous film. .

In the batch type, the PVDF-based microporous membrane is impregnated with the coating composition, and then held by a support. Specifically, the PVDF-based microporous membrane impregnated with the coating composition is superimposed on a flat support (the lid side) in a closed container having a transparent lid, and secondly, the Under the condition, the inside of the closed container was replaced with nitrogen.

In the continuous type, the long strips of the PVDF-based microporous membrane are sequentially contacted with the coating composition. Specifically, the PVDF-based microporous film is transferred into a container sealed with the coating composition by a transport mechanism including a series of rollers, and the PVDF-based microporous film is immersed in the coating. The PVDF-based microporous membrane is moved for a fixed time in a state of the composition, and then the PVDF-based microporous membrane is carried out of the container.

(Step 6) Light-irradiating the coating composition on the surface of the PVDF-based microporous membrane, and forming a unit containing the polymerizable monomer on the surface of the PVDF-based microporous membrane. Steps of cross-linking polymer layers. Step 6 can be performed in batch or continuous manner similarly to step 5.

In the batch type, the PVDF-based microporous film on the support is irradiated with ultraviolet rays. Specifically, the surface of the PVDF-based microporous membrane was irradiated with ultraviolet rays through a transparent cover from the outside of the container while the closed container was filled with nitrogen, and the polymerization and crosslinking reaction of the polymerizable monomer was completed.

In the continuous type, the long object of the PVDF-based microporous film to which the coating composition is closely adhered is irradiated with ultraviolet rays. Specifically, the PVDF-based microporous membrane carried out from the container to which the composition is applied is carried into an ultraviolet irradiation region, and is moved within the region for a fixed time. Before the PVDF-based microporous membrane is carried out from the ultraviolet irradiation area, the polymerization and crosslinking reaction of the polymerizable monomer is completed on the surface of the PVDF-based microporous membrane.

(Step 7) The step of drying and cleaning the surface of the PVDF-based microporous membrane that has passed through the step 6 to remove excess components. Step 7 can be performed in batch or continuous manner similarly to steps 5 and 6.

In this way, a hydrophilized PVDF-based microporous film (a PVDF-based microporous film of the present invention) including a crosslinked polymer layer and a PVDF-based microporous film can be obtained. This PVDF-based microporous membrane is dried, coiled, cut, and packed according to a conventional method and if necessary.
[Example]

[Reference example: Production of PVDF-based microporous membranes]
A PVDF-based microporous membrane was produced through the following steps.
(Step 1) A commercial product "Kyner HSV900" manufactured by Arkema, which is a PVDF-based resin, is used in the amount ratio (weight% with respect to the total amount of the raw material liquid) shown in Table 1 as a solvent. The ratio of dimethylacetamide, polyethylene glycol having a weight-average molecular weight of 400 as a porous agent, and ultrapure water were uniformly mixed to produce a raw material liquid.

[Table 1]

(Step 2) As a base film, a 20 cm × 20 cm square spunbond nonwoven fabric (“eltas P03050” manufactured by Asahi Kasei) was used. The base film was placed on a flat glass plate, and the raw material liquid was applied on the surface of the base film with a thickness of 250 μm using a Baker applicator.

(Step 3) As a curing tank, a stainless steel cylinder filled with 2 liters of ultrapure water was used. In this curing tank, the film obtained in step 2 is loaded so that the water surface does not ripple, and the entire film is immersed in water and allowed to stand in the film curing tank for 2 minutes to perform and end the adhesion to the substrate film. Solidification of the raw liquid.

(Step 4-1) A beaker into which a ceramic air filter diffuser made of ceramic sand is inserted is charged with 2.5 liters of ultrapure water, and dry air is supplied to the diffuser from an external tank to make the ultrapure Bubbles of dry air are evenly sprayed out of the water. It was used in the first cleaning tank. The first cleaning tank was filled with the film after step 3. The membrane was cleaned for 6 minutes in a state where the entire surface of the membrane was evenly contacted with water and bubbles.

(Step 4-2) A beaker inserted with a ceramic sand filter porous stone air diffuser is charged with 2.5 liters of isopropanol, and the air diffuser is supplied with dry air from an external tank, and the isopropanol is evenly distributed. Spray bubbles of dry air. It was used in the second cleaning tank. A film having undergone step 4-1 was loaded into the second cleaning tank, and the film was cleaned in a state where the entire surface of the film was evenly contacted with isopropyl alcohol and bubbles. After that, the film was allowed to dry naturally.

The pore diameter of the PVDF-based microporous membrane thus obtained was measured by a gas transmission method. As a measuring device, a vapor pressure gas permeator (manufactured by PMI) supplied by Seika Digital Image Co., Ltd. was used. Table 1 shows the mode (mode) of the pore diameter of the obtained PVDF-based microporous membrane: Lm (μm), and the ratio (%) to the number of pores having a pore diameter within Lm ± 15%. FIG. 6 shows the pore size distribution of the PVDF-based microporous membrane 2 shown in Table 1.

[Manufacture of coating composition]
As the trifunctional acrylate compound represented by the formula (2), "NK ester (registered trademark) A-GLY-9E" manufactured by Shin Nakamura Chemical Industry Co., Ltd. was used. As the photopolymerization initiator, IRGACURE 2959 was used. As a monomer for the coating composition for comparison, N-isopropylacrylamide and N, N'-methylenebisacrylamide were used. The materials shown in Table 2 were mixed to produce a coating composition. The coating composition B, coating composition C, and coating composition D shown in Table 2 are the products of the present invention, the coating composition A, the coating composition E, the coating composition F, and the coating composition G For reference.

[Table 2]

[Manufacture of PVDF-based microporous membranes (hydrophilization of PVDF-based microporous membranes)]
The coating composition and the PVDF-based microporous membrane were selected in a combination shown in Table 3. The surface of the PVDF-based microporous membrane was coated with a crosslinked polymer layer by the following method.
(Step 5) The PVDF-based microporous film that ends the step 4-2 is immersed in the coating composition. A non-woven support and a PVDF-based microporous membrane impregnated with a coating composition were sequentially stacked on a stainless steel mesh bottom surface of a nitrogen replacement box with a lid made of quartz glass. Nitrogen was circulated in the box for 2 minutes to fill the inside of the box with nitrogen. After filling, the box is kept closed.

(Step 6) A light source (Light Hammer 10 (product name)) from the outside of the box is irradiated with ultraviolet rays through the box cover to harden the coating composition.

(Step 7) Take out the PVDF-based microporous membrane from the box, wash and dry it.

The thus-obtained hydrophilic PVDF-based microporous membrane was measured for the following properties. The results are shown in Table 3.
(Water contact angle)
In order to confirm the hydrophilizing effect, the water contact angle (°) on the surface of the obtained hydrophilized PVDF-based microporous membrane was measured. About 2.0 μL of water droplets were infiltrated and the contact angle (°) after 0.5 seconds was measured as the water contact angle.

(Permeability)
A circular sheet having a diameter of 25 mm was cut from the obtained microporous membrane. The sheet was set on a filter holder with an effective filtering area of 3.5 cm ² , and 5 mL of ultrapure water was passed through the set sheet with a filtration pressure of 50 kPa, and the time from the start to the end of the ultrapure water was measured. The flow rate (permeation amount) per unit filtration area of the sheet was obtained by the following formula. The results are shown in Table 2. The greater the amount of water permeation, the lower the degree of occlusion of the pores and the higher the liquid filtration efficiency.

Water permeability (10 ^-9 m ³ / m ² / Pa / sec) = water flow (m ³ ) ÷ effective filtration area (m ² ) ÷ filtration pressure (Pa) ÷ time (sec)

(Change in water permeability)
The change in the amount of water permeation accompanying hydrophilization was evaluated. The change rate (%) of the permeate amount defined by the following formula was calculated. The water permeability of the non-hydrophilized PVDF-based microporous membrane is a value obtained by measuring the PVDF-based microporous membrane 1 or the microporous membrane 2 shown in Table 1 with alcohol.

Change rate of water permeability (%) = | (water permeability of non-hydrophilized PVDF microporous membrane)-(water permeability of obtained microporous membrane) | ÷ (water permeability of non-hydrophilized PVDF microporous membrane) × 100

The balance between the hydrophilicity represented by the water contact angle value and the fluid filterability represented by the water permeability and the water permeability change rate was determined. In Table 3, the results are expressed as "-" (both of which are particularly inferior and have a poor balance) and "+" (moderately having both and well-balanced).

[table 3]

As shown in Table 3, in the examples (Example 2, Example 3, and Example 4) using the coating composition B, the coating composition C, and the coating composition D of the present invention, the water permeability was not significantly impaired. The PVDF-based microporous membrane is made hydrophilic. The hydrophilized PVDF-based microporous membranes of the present invention like Examples 2, 3, and 4 are novel in that they have a special cross-linked polymer layer, and have good balance between hydrophilicity and fluid filtration.
[Industrial availability]

The hydrophilized PVDF-based microporous membrane of the present invention is useful as a filtration membrane or a separation membrane for treating an aqueous fluid or a living body. Furthermore, the hydrophilized PVDF-based microporous membrane of the present invention can also be used for medicinal solution holding materials used in band-aids, surface materials for sanitary materials, battery spacers, and peeled polyvinylidene fluoride with a large surface area and no constituents. Sheet etc. The PVDF-based microporous membrane of the present invention is effective for applications requiring particularly high filter selectivity.

1‧‧‧ surface

2‧‧‧ support layer

3‧‧‧ substrate film

4‧‧‧ spheroid

5‧‧‧Combined materials

FIG. 1 is a cross section showing a PVDF-based microporous membrane 1 produced in a reference example.

FIG. 2 is a cross-sectional view schematically showing a PVDF-based microporous membrane used in the present invention.

FIG. 3 is a scanning electron microscope photograph of the surface layer of the PVDF-based microporous membrane 1 produced in the reference example.

FIG. 4 is a scanning electron microscope photograph of the surface layer of the PVDF-based microporous membrane 1 produced in the reference example.

FIG. 5 shows the relationship between the shear rate (1 / s) (x) and the inverse of its viscosity (1 / mPa · s) (y) with respect to the raw material liquid used in the PVDF-based microporous membrane 2 of the reference example.

FIG. 6 is a graph showing the pore size distribution of the PVDF-based microporous membrane 2 of the reference example.

Claims (8)

A coating composition for use in hydrophilizing a microporous membrane, comprising: a polymerizable monomer, a photopolymerization initiator, and a solvent; and a formula (1) exceeding 0.75% by mass and 3.0% by mass or less As the polymerizable monomer, N, N-dimethylacrylamide and a trifunctional acrylate compound represented by the following formula (2) exceeding 0.25% by mass and 1.5% by mass or less, . The coating composition according to item 1 of the patent application scope, wherein the microporous membrane is a fluoropolymer-based microporous membrane. The coating composition according to item 1 of the patent application scope, wherein the microporous membrane is a polyvinylidene fluoride-based microporous membrane. The coating composition according to item 3 of the scope of patent application, wherein the polyvinylidene fluoride microporous film includes a substrate and a microporous film layer, and The microporous membrane layer is an asymmetric membrane containing a polyvinylidene fluoride resin, The asymmetric membrane includes a surface layer formed with micropores and a support layer formed with voids larger than the micropores constituting the surface layer, The surface layer has a plurality of spheroids, and a plurality of linear bonding materials extend from each of the spheroids in a three-dimensional direction, and the adjacent spheroids are connected to each other by the linear bonding material, so that Forming a three-dimensional network structure with the spheroids as intersections, and The polyvinylidene fluoride-based microporous membrane has the following pore diameter L (μm): the mode (mode) Lm of the pore diameter of the polyvinylidene fluoride-based microporous membrane measured by a gas transmission method (Μm) satisfies (Condition 1): 0.10 ≦ Lm ≦ 0.20, and At least 95% of all the pores of the polyvinylidene fluoride-based microporous membrane satisfy (Condition 2): (Lm × 0.85) ≦ L ≦ (Lm × 1.15). A hydrophilized microporous film is a hydrophilized microporous film including a crosslinked polymer layer and a microporous film, and the crosslinked polymer includes N, N-diamine derived from the following formula (1) The unit of methacrylamide and the unit derived from the trifunctional acrylate compound represented by the following formula (2), in the crosslinked polymer, 1 mole of the unit derived from the trifunctional acrylate compound Ear, the unit derived from the N, N-dimethylacrylamide exists in a range of 3.3 mol to 79 mol, The microporous film includes a substrate and a microporous film layer. The hydrophilized microporous membrane according to item 5 of the patent application scope, wherein the microporous membrane is a fluoropolymer-based microporous membrane. The hydrophilic microporous membrane according to item 5 of the scope of the patent application, wherein the microporous membrane is a polyvinylidene fluoride-based microporous membrane. The hydrophilic microporous membrane according to item 7 of the scope of the patent application, wherein the polyvinylidene fluoride-based microporous membrane includes a substrate and a microporous membrane layer, and The microporous membrane layer is an asymmetric membrane containing a polyvinylidene fluoride-based resin, and the asymmetric membrane includes a surface layer formed with micropores and voids larger than the micropores constituting the surface layer. Support layer, The surface layer has a plurality of spheroids, and a plurality of linear bonding materials extend from each of the spheroids in a three-dimensional direction, and the adjacent spheroids are connected to each other by the linear bonding material, so that Forming a three-dimensional network structure with the spheroids as intersections, and The polyvinylidene fluoride-based microporous membrane has the following pore diameter L (μm): the mode (mode) Lm of the pore diameter of the polyvinylidene fluoride-based microporous membrane measured by a gas transmission method (Μm) satisfies (Condition 1): 0.10 ≦ Lm ≦ 0.20, and At least 95% of all the pores of the polyvinylidene fluoride-based microporous membrane satisfy (Condition 2): (Lm × 0.85) ≦ L ≦ (Lm × 1.15).