JP7194656B2 - porous membrane - Google Patents

Description

The present invention relates to porous membranes.

Porous membranes made of polymers such as polysulfone are industrially useful as filtration membranes for water purification applications, etc. Products that are pleated and housed in cartridges of a certain capacity are also commercially available. However, since polymers such as polysulfone are hydrophobic, proteins are likely to be adsorbed on the surface, and fouling that clogs the filtration membrane is likely to occur. Therefore, conventionally, various methods for hydrophilizing porous membranes have been studied.

In Patent Document 1, a separation membrane containing a polysulfone-based polymer and polyvinylpyrrolidone is coated with a solution in which polymethoxyacrylate (PMEA) is dissolved to make it hydrophilic, thereby providing blood treatment with excellent blood compatibility. An example of forming a separation membrane for liquid crystals is disclosed.

Patent Document 2 describes a hollow fiber membrane containing an acrylic copolymer consisting of a polyarylsulfone polymer, polyvinylpyrrolidone, and specific hydrophobic and hydrophilic side chain structures.

JP 2016-77570 A JP 2019-18193 A

In the method described in Patent Document 1, since PMEA is coated after forming the separation membrane, PMEA tends to flow out during use of the membrane, resulting in low durability.
In the method described in Patent Document 2, due to the hydrophobic structure introduced into the copolymer to prevent polymer elution into the blood, fouling resistance is low, and water permeability tends to decrease with use.
An object of the present invention is to provide a porous membrane that has high anti-fouling performance and whose performance is less likely to deteriorate with use.

The inventors of the present invention have made extensive studies to solve the above problems, and found that a hydrophilic material having a specific structure was added to the undiluted solution for producing a porous membrane containing a polysulfone-based polymer (undiluted solution addition ), and found that the above-mentioned problems were solved in the porous membrane produced by the above method.

That is, the present invention provides the following <1> to <10>.
<1> A porous membrane having a skeleton made of a polymer selected from the group consisting of polysulfone and polyethersulfone and further comprising polyvinylpyrrolidone,
A hydrophilic material comprising a polymer containing a monomer unit represented by general formula (I) or a polymer represented by general formula (II) or general formula (III) is included in the skeleton,
The porous membrane in which the hydrophilic material is unevenly distributed on the pore surfaces;

In general formula (I),
R ₁ represents a hydrogen atom or a methyl group;
_R2 represents a hydrogen atom, a methyl group, an ethyl group, a phenyl group _, or _{PO3C2H4N ( CH3} ) ₃ ;
R ₃ represents a hydrogen atom, provided that R ₂ and R ₃ may combine to form a five-membered ring together with the oxygen atom to which R ₂ is bonded and the carbon atom to which R ₃ is bonded,
t is an integer from 1 to 9,
A polymer containing a monomer unit represented by general formula (I) is a polymer containing 60 mol% or more of a monomer unit represented by general formula (I),
In general formula (II), m and n are each independently an integer and satisfy the following formula;
m/(n+m)=0.3-0.7
In general formula (III), n is an integer.

<2> In the depth direction analysis from the pore surface using a time-of-flight secondary ion mass spectrometer, from the depth at which the ions of the polymer selected from the group consisting of polysulfone and polyethersulfone were observed for the first time I _A is the average value of the ionic strength of the polymer from a depth to a depth of 140 nm, I _B0 is the ionic strength of the hydrophilic material at the depth where the polymer ions are observed for the first time, and the polymer ions are first observed. The porous membrane according to <1>, which satisfies the following formula, where I _B100 is the ionic strength of the hydrophilic material at a depth 100 nm deeper than the observed depth;
I _B0 /I _B100 >2;
I _B100 /I _A >0.05.

<3> The porous membrane according to <1> or <2>, wherein the hydrophilic material has a weight average molecular weight of 10,000 or more.
<4> The porous membrane according to any one of <1> to <3>, wherein the hydrophilic material comprises a polymer containing a monomer unit represented by formula (I).
<5> The porous membrane according to any one of <1> to <3>, wherein the hydrophilic material comprises a polymer represented by general formula (II).
<6> The porous membrane according to any one of <1> to <3>, wherein the hydrophilic material comprises a polymer represented by general formula (III).
<7> The porous membrane according to any one of <1> to <6>, which has a thickness of 10 to 1000 μm.
<8> The porous membrane according to any one of <1> to <7>, which has a pore size distribution in the thickness direction.
<9> Having a layered dense portion with the smallest pore size inside,
The porous membrane according to any one of <1> to <8>, wherein the pore diameter continuously increases in the thickness direction from the dense portion toward at least one membrane surface of the porous membrane.
<10> The porous membrane according to <9>, wherein the dense portion has an average pore size of 0.01 to 10 μm.

INDUSTRIAL APPLICABILITY The present invention provides a porous membrane that has high anti-fouling performance and whose performance is less likely to deteriorate with use.

1 is an example of a TOF-SIMS spectrum of the porous membrane of the present invention;

The present invention will be described in detail below.
In the present specification, the term "~" is used to include the numerical values before and after it as lower and upper limits.

<Porous membrane>
A porous membrane refers to a membrane having a plurality of pores. The pores can be confirmed, for example, in a scanning electron microscope (SEM) image or a transmission electron microscope (TEM) image of the cross section of the membrane.
The hydrophilic porous membrane of the present invention comprises a porous membrane framework and a hydrophilic material inside the framework, and the hydrophilic material is unevenly distributed on the surface of each pore of the porous membrane. In the present specification, the skeleton of the porous membrane means the part that can form the porous membrane by itself, and means the part that functions as a base material that holds the hydrophilic material and the like. In this specification, including a hydrophilic material inside the skeleton means that at least part of the hydrophilic material is present inside the skeleton, and when the hydrophilic material merely covers the skeleton surface, i.e., the pore surface And it does not include the case where the film surface is only coated. Each pore surface means the surface of the porous membrane facing each pore inside the porous membrane, and is distinguished from the membrane surface of the porous membrane (the front side or the back side of the membrane). As used herein, the term "ubiquitously distributed" means that it is abundantly present compared to other parts, and includes being present only in that part.

The hydrophilic porous membrane of the present invention contains a hydrophilic material inside the skeleton of the porous membrane, so that the hydrophilic material is less likely to be eluted even when the porous membrane is used. In addition, since the hydrophilic material is unevenly distributed on the surface of each pore, the surface of each pore can be efficiently hydrophilized.
The above configuration can be realized, for example, by adding a hydrophilic material described below to a membrane-forming stock solution containing a polymer and polyvinylpyrrolidone (addition of the stock solution) to form a porous membrane skeleton.

[Skeleton of porous membrane]
The porous membrane of the present invention is formed with a skeleton made of a polymer selected from the group consisting of polysulfone and polyethersulfone.
Polysulfone is a polymer consisting of repeating structural units represented by general formula (1), and polyethersulfone is a polymer consisting of repeating structural units represented by general formula (2).

The polymer selected from the group consisting of polysulfone and polyethersulfone, which forms the skeleton of the porous membrane of the present invention, is a structural unit represented by general formula (1) and a structural unit represented by general formula (2). It may be a copolymer containing Also, the polymer forming the skeleton of the porous membrane of the present invention may be a mixture of polysulfone and polyethersulfone.
Porous membranes formed from water-insoluble resins generally have high water resistance, chemical resistance, and mechanical resistance, and are suitable for industrial use as filters. When a polymer selected from the group consisting of polysulfone and polyethersulfone, particularly polysulfone, is used as the water-insoluble resin, a porous membrane having higher water resistance, chemical resistance and mechanical resistance can be obtained.

The polymer selected from the group consisting of polysulfone and polyethersulfone preferably has a number average molecular weight (Mn) of 1,000 to 10,000,000, preferably 5,000 to 1,000,000. more preferred. Here, the number average molecular weight (Mn) of the polymer is determined by gel permeation chromatography (GPC).

[Hydrophilic material]
In the porous membrane of the present invention, as the hydrophilic material, a polymer containing a monomer unit represented by general formula (I) or a polymer represented by general formula (II) or general formula (III) is used. . These polymers are selected from the group consisting of a hydroxyl group, an ether bond, an ester bond, a carboxyl group, a carbonyl group, an amide bond, an amino group, and a betaine structure as relatively hydrophilic sites in the same molecule. and a structure selected from the group consisting of polyethylene, polypropylene, polyfluoroethylene, polyacrylonitrile, polyethylene terephthalate, polysulfone, polyamide, and polyimide as a relatively hydrophobic site. When the hydrophilic material having such a structure is added to the membrane-forming stock solution at the time of forming the porous membrane, it is positioned between the porous membrane skeleton, which is hydrophobic, and the water and polyvinylpyrrolidone taken in, It is uniformly immobilized and unevenly distributed on the pore surface side inside the porous membrane skeleton. In addition, at least part of the hydrophilic material is contained inside the skeleton, so that it is difficult for the hydrophilic material to flow out, and a highly durable film can be formed.
The polymer containing the monomer unit represented by general formula (I) or the polymer represented by general formula (II) or general formula (III) will be described below.

In general formula (I), R ₁ represents a hydrogen atom or a methyl group.
_R2 represents a hydrogen atom, a methyl group, an ethyl group, a phenyl group _, or _{PO3C2H4N ( CH3} ) ₃ .
R ₃ represents a hydrogen atom, provided that R ₂ and R ₃ may combine to form a 5-membered ring together with the oxygen atom to which R ₂ is bonded and the carbon atom to which R ₃ is bonded. Five-membered rings include tetrahydrofuran. When t is 2 or more, R ₃ that forms the 5-membered ring by combining with R ₂ is the R ₃ closest to the R ₂ -terminal. When R ₂ and R ₃ combine to form a 5-membered ring together with the oxygen atom to which R ₂ is bonded and the carbon atom to which R ₃ is bonded, t is particularly preferably 1.
t is an integer of 1-9, preferably 1-3.

The polymer containing the monomer unit represented by general formula (I) is a polymer containing 60 mol% or more, preferably 80 mol% or more, of the monomer unit represented by general formula (I), more preferably general formula (I) The polymer containing the monomer unit represented by is preferably composed of repeating monomer units represented by general formula (I). As a monomer unit other than the monomer unit represented by general formula (I) contained in the polymer containing the monomer unit represented by general formula (I) (hereinafter referred to as "another monomer unit"), methyl acrylate , acrylates such as ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate; and methacrylates such as methyl methacrylate, ethyl methacrylate, butyl acrylate and 2-ethylhexyl methacrylate.

The polymerization of the monomer units represented by general formula (I) and other monomer units may be block copolymerization, random copolymerization, or, for example, two or more general formula (I) A continuous unit containing a monomer unit represented by and another monomer unit may be alternately polymerized.

Examples of polymers containing monomer units represented by general formula (I) include the following.

In general formula (II), m and n are each independently integers (integers indicating the number of structural units in the polymer (degree of polymerization)) and satisfy the following formula.
m/(n+m)=0.3-0.7
The polymer represented by the general formula (II) is a random copolymer or a block copolymer of n monomer units represented by the formula II-1 and m monomer units represented by the formula II-2. It is a polymerized polymer.

In general formula (III), n is an integer (an integer indicating the number of structural units (degree of polymerization) in the polymer).

The weight average molecular weight of the polymer containing a monomer unit represented by general formula (I) or the polymer represented by general formula (II) or general formula (III) is all 10,000 or more. It is preferably 30,000 or more, and even more preferably 50,000 or more.

The weight average molecular weight of the polymer can be evaluated by GPC measurement under the following analysis conditions.
Column: Shodex OHpak KB805HQ
Mobile phase: 0.1M sodium acetate buffer Flow rate: 1.0 mL/min
Temperature: 40°C
Detector: RI (differential refractometer)
In calculating the molecular weight, standard pullulan preparations Shodex Pullulan P-5, P-10, P-20, P-50, P-82, P-100, P-200, P-400, P-800, P -1600 can be used.

[Time-of-flight secondary ion mass spectrometry: TOF-SIMS]
Regarding the fact that the porous membrane of the present invention contains the hydrophilic material that is any of the above polymers inside the skeleton of the porous membrane and that the hydrophilic material is unevenly distributed on the surface of each pore, the following means You can check with
In addition, as described above, the pore surface of the porous membrane refers to the surface of the pores inside the membrane, not the portion that can be directly touched by hand such as the front surface or the back surface of the porous membrane. Pore surfaces can be exposed by breaking the porous membrane. The destruction may be performed, for example, by freezing the porous membrane with liquid nitrogen or the like, and then cutting the porous membrane so that the pore surfaces are exposed.

The pore surface exposed as described above is subjected to depth profile analysis using a time-of-flight secondary ion mass spectrometry (TOF-SIMS) device. In this analysis, the average value of the ionic strength of the polymer from the depth at which ions of the polymer selected from the group consisting of polysulfone and polyethersulfone were observed for the first time to a depth of 140 nm from that depth is I _A , I _B0 is the ionic strength of the hydrophilic material at the depth at which ions are first observed, and I _B100 is the ionic strength of the hydrophilic material at a depth 100 nm deeper than the depth at which the polymer ions are first observed. Then, when the following two formulas are satisfied, it can be determined that at least the hydrophilic material is included in the skeleton of the porous membrane and the hydrophilic material is unevenly distributed on the surface of each pore.
I _B0 /I _B100 >2
I _B100 /I _A >0.05
I _B0 /I _B100 >2 means that the hydrophilic material is unevenly distributed on the surface of each pore, the surface is efficiently hydrophilized, and the effect of suppressing protein adhesion to the porous membrane is high. .
In addition, I _B100 /I _A >0.05 means that the hydrophilic material is present inside the skeleton of the porous membrane, and the hydrophilic material is less likely to flow out during use of the membrane, improving durability. do.

For example, in the porous membrane described in Patent Document 1, since the hydrophilic material is coated after forming the skeleton, the hydrophilic material does not exist in the base material and I _B100 /I _A >0.05 is not satisfied.

Examples of depth direction analysis results (TOF-SIMS spectra) using a time-of-flight secondary ion mass spectrometer for porous membranes that satisfy the above two equations, and measured and calculated I _A , I _B0 , and I _B100 is shown in FIG.

As a time-of-flight secondary ion mass spectrometry (TOF-SIMS) device, for example, TRIFT V nano TOF manufactured by Ulvac-Phi, Inc. can be used.
The primary ion source can be Au or Bi, preferably Bi.
Analysis in the depth direction can be realized by performing a sputtering process using a sputter ion gun. Fullerene (C60) or argon gas cluster ions (Ar-GCIB) can be used as sputter ions, and Ar-GCIB is preferably used. For example, TOF-SIMS measurement is performed at a sputtering speed of about 10 to 100 nm/min, and the sputtering speed is converted into depth, thereby enabling analysis of the intensity of ions derived from each component in the depth direction.

Sputtering and TOF-SIMS for analysis in the depth direction are performed from the pore surface, but the pore at this time is not the surface of the porous membrane, but is porous depending on the thickness of the porous membrane, for example. It is preferable to select pores at a site of about 1 to 20 μm, preferably about 10 μm from the surface of the membrane. Further, in the case of a porous membrane having a pore size portion as described later, it is preferable to select the primary surface as the surface of the porous membrane and select the pores of the portion other than the dense portion.
Moreover, as each value of I _A , I _B0 , and I _B100 , it is preferable to use the average value of the values obtained by analyzing a plurality of pores in the above-described region for one porous membrane.

[Polyvinylpyrrolidone]
The porous membrane of the present invention further contains polyvinylpyrrolidone. At this time, the polyvinylpyrrolidone may be in a state of being retained by the porous membrane. The inclusion of polyvinylpyrrolidone further increases the hydrophilicity of the porous membrane of the present invention. Polyvinylpyrrolidone is added as a pore-forming agent to the stock solution for forming a polysulfone membrane or a polyethersulfone membrane, as described in, for example, JP-A-64-34403. Most of the polyvinylpyrrolidone in the membrane-forming stock solution is dissolved in the coagulating water and removed during the membrane-forming process, but part of it remains on the membrane surface.

[Other additives]
The porous membrane may contain, as additives, components other than the polymer, hydrophilic material, and polyvinylpyrrolidone.
Examples of the above additives include salt, metal salts of inorganic acids such as lithium chloride, sodium nitrate, potassium nitrate, sodium sulfate and zinc chloride, metal salts of organic acids such as sodium acetate and sodium formate, polymers such as polyethylene glycol, and polystyrene. Examples include polymer electrolytes such as sodium sulfonate and polyvinylbenzyltrimethylammonium chloride, and ionic surfactants such as sodium dioctylsulfosuccinate and sodium alkylmethyltaurate. Additives may act as swelling agents for the porous structure.

[Structure of porous membrane]
As mentioned above, a porous membrane is a membrane having a plurality of pores.
The pore diameter of the pores of the porous membrane can be appropriately selected depending on the size of the object to be filtered, but it may be 0.005 μm to 25 μm, and more preferably 0.01 μm to 20 μm. When having a pore size distribution, it may be distributed within this range. The pore size can be measured from a photograph of a membrane cross section obtained by an electron microscope. The porous membrane is cut with a microtome or the like, and a photograph of the cross section of the porous membrane can be obtained as a slice of the thin film in which the cross section can be observed.

The porous membrane may have a structure having a pore size distribution in the thickness direction or a homogeneous structure having no pore size distribution in the thickness direction. In addition, the structure having a pore size distribution in the thickness direction may be an asymmetric structure (asymmetric structure) having a pore size distribution in the thickness direction such that the pore sizes on the front surface and the pore size on the back surface of the membrane are different. Examples of asymmetric structures include a structure in which the pore size continuously increases in the thickness direction from one membrane surface to the other, Examples include a structure in which the pore diameter continuously increases in the thickness direction from the site toward at least one membrane surface of the porous membrane.
In the present specification, the surface of the porous membrane having the asymmetric structure with the larger pore size is called the primary surface, and the surface with the smaller pore size is called the secondary surface.

In particular, the porous membrane has a layered dense portion with the smallest pore size inside, and the pore size continuously increases in the thickness direction from this dense portion toward at least one membrane surface of the porous membrane. A structure is preferred. A polymer selected from the group consisting of polysulfone and polyethersulfone is well known as a polymer that can be preferably used when producing such a porous membrane.

In the present specification, when comparing the pore diameters in the thickness direction of the membrane, the SEM photograph of the cross section of the membrane is divided in the thickness direction of the membrane. The number of divisions can be appropriately selected according to the thickness of the film. The number of divisions should be at least 5 or more. For example, in the case of a film with a thickness of 200 μm, 19 division lines are drawn to divide the surface X into 20 parts, which will be described later. Calculate the average pore diameter of each pore. The division width means the width in the thickness direction of the film, and does not mean the width in the photograph. In comparing the pore sizes in the thickness direction of the membrane, the pore sizes are compared as the average pore size of each section. The average pore size of each segment may be, for example, the average value of 50 pores in each segment of the membrane cross-sectional view. A cross-sectional view of the film in this case may be obtained, for example, with a width of 80 μm (a distance of 80 μm in the direction parallel to the surface). At this time, for sections with large holes that cannot be measured by 50, it is sufficient to measure only the number that can be taken in the section. At this time, if the hole is too large to fit in the section, the size of the hole is measured across other sections.

The layered dense portion with the smallest pore size refers to the layered portion of the porous membrane corresponding to the section with the smallest average pore size among the sections of the membrane cross section. Even if the dense part consists of a part corresponding to one section, it is composed of two, three, etc., parts corresponding to multiple sections having an average pore size within 1.1 times the section with the smallest average pore size. It may be. The thickness of the dense portion may be from 0.5 μm to 50 μm, preferably from 0.5 μm to 30 μm.

For porous membranes used as filtration membranes for industrial applications, especially for filtration of beverages such as beer, it is important to achieve both a high flow rate and reliable capture of contaminants in the liquid. Therefore, an appropriate setting of the minimum pore size is required.
In this specification, the average pore size of the dense portion is defined as the minimum pore size of the porous membrane. The minimum pore size of the porous membrane is preferably 0.01 μm or more, more preferably 0.03 μm or more, still more preferably 0.05 μm or more, and particularly preferably 0.1 μm or more, Also, it is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less.
Here, it is assumed that the average pore diameter of the dense portion is measured according to ASTM F316-80.

The porous membrane preferably has a dense portion inside. Internal means not in contact with the surface of the membrane, and "having a dense area internally" means that the dense area is not the section closest to either surface of the membrane. By using a porous membrane having a structure having a dense portion inside, the permeability of the substance intended to permeate is lower than in the case of using a porous membrane having the same dense portion in contact with the surface. Hateful. Without adhering to any theory, it is believed that the presence of dense sites inside makes it difficult for substances such as proteins to be adsorbed.

It is preferable that the dense portion is biased toward one of the surfaces of the porous membrane relative to the central portion of the thickness of the porous membrane. Specifically, it is preferable that the dense portion is within a distance of one-third of the thickness of the porous membrane from either surface of the porous membrane, and more preferably within two-fifths of the thickness of the porous membrane. , more preferably within a quarter of the distance. This distance can be determined from the film cross-sectional photograph described above. In this specification, the surface of the porous membrane closer to the dense portion is referred to as "surface X".

In the porous membrane, it is preferable that the pore diameter continuously increases in the thickness direction from the dense portion toward at least one of the surfaces. In the porous membrane, the pore size may continuously increase in the thickness direction from the dense portion toward the surface X, and the pore size may continuously increase in the thickness direction from the dense portion toward the surface opposite to the surface X. Alternatively, the pore diameter may continuously increase in the thickness direction from the dense portion to any surface of the porous membrane. Among these, it is preferable that the pore diameter continuously increases in the thickness direction at least from the dense portion toward the surface opposite to the surface X, and the pore size is preferably increased in the thickness direction from the dense portion to any surface of the porous membrane. It is more preferable that the pore size increases continuously. "Pore diameter continuously increases in the thickness direction" means that the difference in average pore diameter between adjacent sections in the thickness direction is 50% or less of the difference between the maximum average pore diameter (maximum pore diameter) and the minimum average pore diameter (minimum pore diameter) , preferably 40% or less, more preferably 30% or less. "Continuously increasing" essentially means uniformly increasing without decreasing, but the site of decrease may occur accidentally. For example, when combining two sections from the surface, if the average value of the combinations increases uniformly (decreases uniformly when going from the surface to the dense area), then "from the dense area to the surface of the membrane The pore diameter increases continuously in the thickness direction."

The maximum pore size of the porous membrane is preferably greater than 0.1 μm, more preferably 1.0 μm or more, even more preferably greater than 1.5 μm, and preferably 25 μm or less, and 23 μm. It is more preferably 21 μm or less, and further preferably 21 μm or less. In the present specification, the average pore size of the section having the largest average pore size among the sections of the membrane cross section is defined as the maximum pore size of the porous membrane.

The ratio of the average pore size of the dense portion to the maximum pore size of the porous membrane (the ratio of the minimum pore size to the maximum pore size of the porous membrane, the maximum pore size divided by the minimum pore size, herein referred to as the "anisotropic ratio ) is preferably 3 or more, more preferably 4 or more, and even more preferably 5 or more. This is to increase the average pore size of areas other than the dense portions, thereby increasing the substance permeability of the porous membrane. Also, the anisotropy ratio is preferably 25 or less, more preferably 20 or less. This is because the effect of multi-stage filtration can be obtained efficiently in the range of the anisotropic ratio of 25 or less.
Preferably, the section with the largest average pore size is the section closest to or in contact with either surface of the membrane.

In the section closest to either surface of the membrane, the average pore size is preferably greater than 0.05 μm and no greater than 25 μm, more preferably greater than 0.08 μm and no greater than 23 μm, and greater than 0.1 μm and no greater than 21 μm. is more preferred. In addition, the ratio of the average pore size of the section closest to either surface of the membrane to the average pore size of the dense portion is preferably 1.2 or more and 20 or less, more preferably 1.5 or more and 15 or less. , 2 or more and 13 or less.

The thickness of the porous membrane is not particularly limited, but may be 10 μm to 1000 μm, preferably 10 μm to 500 μm, and 30 μm to 300 μm from the viewpoint of membrane strength, handleability, filtration performance, and filtration life. is more preferable.

The porous membrane is preferably a membrane formed from one composition as a single layer, and preferably not a laminated structure of multiple layers.

<Method for producing porous membrane>
The method for producing the porous membrane of the present invention is not particularly limited, and any ordinary method for forming a polymer membrane can be used. Examples of methods for forming a polymer film include a drawing method and a casting method, and it is particularly preferable to use a casting method for producing a porous film. In the casting method, a porous membrane having a pore size distribution can be produced by adjusting the type and amount of the solvent used in the membrane-forming stock solution and the drying method after casting.

The production of the porous membrane by the casting method can be carried out, for example, by a method including the following (1) to (4) in this order.
(1) A membrane-forming stock solution containing a polymer, a hydrophilic material, polyvinylpyrrolidone, and, if necessary, other additives and a solvent is cast on a support in a dissolved state.
(2) The surface of the cast liquid film is exposed to a temperature-controlled humid air.
(3) The film obtained after being exposed to temperature-controlled moist air is immersed in a coagulating liquid.
(4) Peel off the support if necessary.

The temperature of the temperature-controlled humid air may be 4°C to 60°C, preferably 10°C to 40°C. The relative humidity of the temperature-controlled humid air may be 15% to 100%, preferably 25% to 95%. The temperature-controlled wet air may be applied at a wind speed of 0.1 m/sec to 10 m/sec for 0.1 sec to 30 sec, preferably 1 sec to 10 sec.

In addition, the average pore size and position of the dense portion can also be controlled by the moisture concentration contained in the temperature-controlled humid air and the time for which the temperature-controlled and humid air is applied. The average pore size of the dense portion can also be controlled by the amount of water contained in the membrane-forming stock solution.

By applying temperature-controlled moist air to the surface of the liquid film as described above, evaporation of the solvent can be controlled and coacervation can occur from the surface of the liquid film toward the inside of the film. In this state, the coacervation phase is fixed as micropores and pores other than the micropores are formed by immersing the polymer in a coagulation liquid containing a solvent having low polymer solubility but compatibility with the polymer solvent. can do.

The temperature of the coagulating liquid may be -10°C to 80°C during the immersion in the coagulating liquid. By changing the temperature in this range, the time from the formation of the coacervation phase toward the support surface side from the dense portion to solidification is adjusted, and the size of the pore diameter up to the support surface side is controlled. It is possible. When the temperature of the coagulating liquid is increased, the formation of the coacervation phase is accelerated and the time until solidification is lengthened, so that the pore diameter toward the surface of the support tends to increase. On the other hand, when the temperature of the coagulating liquid is lowered, the formation of the coacervation phase is delayed and the time until solidification is shortened, so that the pore diameter toward the surface of the support is less likely to increase.

A plastic film or a glass plate may be used as the support. Examples of plastic film materials include polyesters such as polyethylene terephthalate (PET), polycarbonates, acrylic resins, epoxy resins, polyurethanes, polyamides, polyolefins, cellulose derivatives, and silicones. A glass plate or PET is preferable as the support, and PET is more preferable.

The membrane-forming stock solution may contain a solvent. A solvent in which the polymer to be used is highly soluble (hereinafter sometimes referred to as a "good solvent") may be used. It is preferable that the solvent is quickly replaced with the coagulating liquid when the membrane is immersed in the coagulating liquid. Examples of solvents include N-methyl-2-pyrrolidone, dioxane, tetrahydrofuran, dimethylformamide, dimethylacetamide, and mixed solvents thereof. Among these, it is preferable to use N-methyl-2-pyrrolidone.

In addition to the good solvent, the membrane-forming stock solution preferably contains a solvent in which the polymer has low solubility but is compatible with the good solvent (hereinafter sometimes referred to as "non-solvent"). Non-solvents include water, cellosolves, methanol, ethanol, propanol, acetone, tetrahydrofuran, polyethylene glycol, glycerin and the like. Among these, it is preferable to use water.

The concentration of the polymer in the membrane-forming solution may be 5% by mass or more and 35% by mass or less, preferably 10% by mass or more and 30% by mass or less. When the amount is 35% by mass or less, sufficient permeability (for example, water permeability) can be imparted to the resulting porous membrane, and when the amount is 5% by mass or more, a porous membrane that selectively permeates substances can be obtained. can ensure the formation of The amount of the additive to be added is not particularly limited as long as the uniformity of the membrane-forming stock solution is not lost by the addition, but it is usually 0.5% by volume or more and 10% by volume or less based on the solvent. When the film-forming stock solution contains a non-solvent and a good solvent, the ratio of the non-solvent to the good solvent is not particularly limited as long as the mixed solution can maintain a uniform state, but 1.0% by mass to 50% by mass. It is preferably 2.0% by mass to 30% by mass, and even more preferably 3.0% by mass to 10% by mass.

In addition, in the membrane-forming stock solution for producing the porous membrane, the hydrophilic material is preferably contained in an amount of 0.1% by mass to 10% by mass with respect to the total mass of the polymer, and 0.3% by mass. More preferably, it is contained in a mass % to 5 mass %.

In addition, in the membrane-forming stock solution for producing the porous membrane, polyvinylpyrrolidone is preferably contained in an amount of 50% by mass to 120% by mass, more preferably 80% by mass to 110% by mass, with respect to the total mass of the polymer. % is more preferable. By using such a film forming stock solution, a porous film containing polyvinylpyrrolidone in an amount of about 0.05 to 8.0% by mass can be obtained. The amount of polyvinylpyrrolidone is reduced because the polyvinylpyrrolidone is largely removed during the washing process.
Furthermore, when the membrane-forming stock solution contains lithium chloride as an additive, lithium chloride is preferably contained in an amount of 5% to 20% by weight, preferably 10% by weight to 20% by weight, based on the total weight of polysulfone and polyethersulfone. It is more preferably contained at 15% by mass.

As the coagulating liquid, it is preferable to use a solvent in which the polymer has low solubility. Examples of such solvents include alcohols such as water, methanol, ethanol and butanol; glycols such as ethylene glycol and diethylene glycol; aliphatic hydrocarbons such as ether, n-hexane and n-heptane; Examples include glycerols. Examples of preferred coagulating liquids include water, alcohols, or mixtures of two or more thereof. Among these, it is preferable to use water.

After immersion in the coagulating liquid, it is also preferable to wash with a solvent different from the coagulating liquid used. Washing can be performed by immersion in a solvent. Diethylene glycol is preferred as the washing solvent. When polyvinylpyrrolidone is added to the undiluted solution for forming a porous membrane, diethylene glycol is used as a washing solvent, and either one or both of the temperature and immersion time of diethylene glycol for immersing the membrane are adjusted to allow polyvinylpyrrolidone to adhere to the membrane. can control the remaining amount. After washing with diethylene glycol, it may be washed with water.

Methods for producing porous membranes are described in JP-A-63-141610, JP-A-4-349927, JP-B-4-68966, JP-A-04-351645, JP-A-9-227714, Japanese Patent Application Laid-Open No. 2010-235808 can be referred to.

[Sterilization]
The porous membrane may be sterilized. As the sterilization treatment of the porous membrane, for example, autoclave sterilization treatment can be performed. In particular, it is preferable to perform treatment with high-temperature, high-pressure steam using an autoclave. Normally, high-pressure steam sterilization treatment for plastics is performed by pressurizing with saturated steam and treating in an environment of about 110 to 140 ° C. for 10 to 30 minutes, but the sterilization treatment of the hydrophilic porous membrane of the present invention is the same. can be done under the following conditions: Autoclaves used for sterilization include, for example, SS325 manufactured by Tomy Seiko Co., Ltd.

<Application of porous membrane>
The porous membrane of the present invention can be used in various applications as a filtration membrane. Filtration membranes are applied to separation, purification, recovery, and concentration of liquids containing or suspending various macromolecules, microorganisms, yeast, and fine particles. can be applied when it is necessary to separate For example, a filtration membrane can be used when separating fine particles from various suspensions containing fine particles, fermentation liquids, culture solutions, pigment suspensions, and the like. Specifically, the porous membrane of the present invention is used in the production of drugs in the pharmaceutical industry, the production of alcoholic beverages such as beer in the food industry, the fine processing in the electronics industry, and the precision filtration required in the production of purified water. It can be used as a membrane.

When the porous membrane of the present invention having a pore size distribution is used as a filtration membrane, fine particles are efficiently removed by arranging the portion with the smaller pore size close to the outlet side (outlet side) of the filtrate and performing filtration. can be captured. Also, in a porous membrane having a pore size distribution, fine particles introduced from the surface are removed by adsorption or adhesion before reaching the minimum pore size portion. Therefore, clogging is unlikely to occur, and high filtration efficiency can be maintained over a long period of time.

The porous membrane of the present invention can be processed into a shape according to the application and used in various applications. Examples of the shape of the porous membrane include flat membrane, tubular, hollow fiber, pleated, fibrous, spherical particles, crushed particles, and continuous lumps. The porous membrane may be processed into a shape according to the application before the hydrophilization treatment of the porous membrane, or may be processed into a shape according to the application after the hydrophilization treatment of the porous membrane.

The porous membrane may be mounted in a cartridge that is easily removable in equipment for various applications. The porous membrane is preferably retained in the cartridge in a form capable of functioning as a filtration membrane. A cartridge holding a porous membrane can be manufactured in the same manner as known porous membrane cartridges, and for example, reference can be made to International Publication No. 2005/037413 and Japanese Patent Laid-Open Publication No. 2012-045524.

For example, a filter cartridge can be manufactured as follows.
A long porous membrane is pleated so that creases are formed in the short side (width) direction. For example, it can be sandwiched, usually between two membrane supports, and pleated by known methods. A non-woven fabric, a woven fabric, a net, or the like may be used as the membrane support. The membrane support functions to reinforce the filtration membrane against filtration pressure fluctuations and at the same time to introduce liquid into the depths of the pleats. The width of the pleat folds may be, for example, 5 mm to 25 mm. The pleated porous membrane may be rolled into a cylindrical shape and the joints thereof may be sealed.

The cylindrical porous membrane is end-sealed to the end plate. The end-sealing may be performed by a known method according to the material of the end plate. When using a thermosetting epoxy resin for the end plate, pour the prepared epoxy resin adhesive liquid into the potting mold and pre-cure it so that the viscosity of the adhesive becomes moderately high. One end face can be inserted into this epoxy and then heated to fully cure. When the material of the end plate is a thermoplastic resin such as polypropylene or polyester, a method of inserting one end face of the cylindrical filter medium into the resin immediately after pouring the melted resin into the mold may be performed. On the other hand, only the seal surface of the already formed end plate is brought into contact with a hot plate or irradiated with an infrared heater to melt only the plate surface, and one end surface of the cylindrical filter medium is pressed against the melted surface of the plate and welded. may

The assembled filter cartridge may be subjected to further washing steps known in the art.
A part or all of the hydroxyalkyl cellulose in the porous membrane may be dissolved in a solvent used in a washing step or the like to be removed in the filter cartridge.

EXAMPLES The characteristics of the present invention will be described more specifically below with reference to examples and comparative examples. The materials, amounts used, proportions, treatment details, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the gist of the present invention. Therefore, the scope of the present invention should not be construed to be limited by the specific examples shown below.

[Formation of porous membrane]
(Examples 1 to 18)
0.3% by mass of the hydrophilic material shown in Table 1 was mixed with 68.7% by mass of N-methylpyrrolidone (manufactured by Fujifilm Wako Pure Chemical Industries, special grade) and stirred at 200 rpm for 30 minutes. Next, 15.6% by mass of polyvinylpyrrolidone (Pittscol k-50) was mixed and stirred at 300 rpm for 25 minutes. Next, polysulfone (Solvay, Udel P-3500LCD, Example 18 only, polyethersulfone (Sumika Excel PES 5200P)) was mixed at 14.5% by mass and stirred at 300 rpm for 120 minutes. Finally, 1% by mass of lithium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for molecular biology) was added and stirred at 300 rpm for 30 minutes. All stirring was performed using a three-one motor (manufactured by Shinto Kagaku, BL600).

The prepared liquid was allowed to stand at room temperature for 15 hours to defoam. The liquid was applied onto a PET substrate (A4300, manufactured by Toyobo Co., Ltd.) using an applicator so that the dry thickness would be 140 μm. While applying, a humid wind with a dew point of 15°C was applied for 15 seconds at a wind speed of 1.9 m/s. Next, it was immersed in a water bath at 4°C for 20 minutes to solidify the film. The porous membrane was peeled off from the PET substrate and dried in an oven at 80° C. for 10 minutes to obtain a porous membrane. The thickness of the porous membrane after drying, as measured from the cross-sectional scanning electron microscope (SEM) image, was approximately 140 μm, and the porous membrane had an asymmetric structure with a dense portion inside. rice field.

(Comparative example 1)
15.6% by mass of polyvinylpyrrolidone (Pittscol k-50) was mixed with 68.9% by mass of N-methylpyrrolidone (manufactured by Fujifilm Wako Pure Chemical Industries, special grade) and stirred at 300 rpm for 25 minutes. Next, 14.5% by mass of polysulfone (Solvay, Udel P-3500LCD) was mixed and stirred at 300 rpm for 120 minutes. Finally, 1% by mass of lithium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for molecular biology) was added and stirred at 300 rpm for 30 minutes. All stirring was performed using a three-one motor (manufactured by Shinto Kagaku, BL600).
The hydrophilic material solution was prepared by mixing 99.3% by mass of water (or ethanol) and 0.7% by mass of the hydrophilic material shown in Table 1 and stirring the mixture for 30 minutes with a stirrer.

The prepared liquid was allowed to stand at room temperature for 15 hours to defoam. The liquid was applied onto a PET substrate (A4300, manufactured by Toyobo Co., Ltd.) using an applicator so that the dry thickness would be 140 μm. While applying, a humid wind with a dew point of 15°C was applied for 20 seconds at a wind speed of 1.9 m/s. Next, it was immersed in a water bath at 4°C for 20 minutes to solidify the film. The porous membrane was peeled off from the PET substrate and dried in an oven at 80° C. for 10 minutes to obtain a porous membrane. The resulting porous membrane was immersed in the hydrophilic material solution for 30 seconds and pulled out. Excess solution on the surface of the porous membrane was wiped off with Kimtowel so that the membrane weight was such that the pores were 100% filled with the solution. This was dried in an oven at 80° C. for 10 minutes to obtain a porous membrane. The thickness of the dried porous membrane measured from a scanning electron microscope (SEM) image of the cross section of the membrane was approximately 140 μm, and the porous membrane had an asymmetric structure with a dense portion inside.

(Comparative Examples 2 to 6)
0.3% by mass of the hydrophilic material shown in Table 1 was mixed with 68.7% by mass of N-methylpyrrolidone (manufactured by Fujifilm Wako Pure Chemical Industries, special grade) and stirred at 200 rpm for 30 minutes. Next, 15.6% by mass of polyvinylpyrrolidone (Pittscol k-50) was mixed and stirred at 300 rpm for 25 minutes. Next, 14.5% by mass of polysulfone (Solvay, Udel P-3500LCD) was mixed and stirred at 300 rpm for 120 minutes. Finally, 1% by mass of lithium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for molecular biology) was added and stirred at 300 rpm for 30 minutes. All stirring was performed using a three-one motor (manufactured by Shinto Kagaku, BL600).

The prepared liquid was allowed to stand at room temperature for 15 hours to defoam. The liquid was applied onto a PET substrate (A4300, manufactured by Toyobo Co., Ltd.) using an applicator so that the dry thickness would be 140 μm. While applying, a humid wind with a dew point of 15°C was applied for 15 seconds at a wind speed of 1.9 m/s. Next, it was immersed in a water bath at 4°C for 20 minutes to solidify the film. The porous membrane was peeled off from the PET substrate and dried in an oven at 80° C. for 10 minutes to obtain a porous membrane. The thickness of the porous membrane after drying, as measured from the cross-sectional scanning electron microscope (SEM) image, was approximately 140 μm, and the porous membrane had an asymmetric structure with a dense portion inside. rice field.

(Comparative Example 7)
15.6% by mass of polyvinylpyrrolidone (Pittscol k-50) was mixed with 68.9% by mass of N-methylpyrrolidone (manufactured by Fujifilm Wako Pure Chemical Industries, special grade) and stirred at 300 rpm for 25 minutes. Next, 14.5% by mass of polysulfone (Solvay, Udel P-3500LCD) was mixed and stirred at 300 rpm for 120 minutes. Finally, 1% by mass of lithium chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for molecular biology) was added and stirred at 300 rpm for 30 minutes. All stirring was performed using a three-one motor (manufactured by Shinto Kagaku, BL600).

The prepared liquid was allowed to stand at room temperature for 15 hours to defoam. The liquid was applied onto a PET substrate (A4300, manufactured by Toyobo Co., Ltd.) using an applicator so that the dry thickness would be 140 μm. While applying, a humid wind with a dew point of 15°C was applied for 15 seconds at a wind speed of 1.9 m/s. Next, it was immersed in a water bath at 4°C for 20 minutes to solidify the film. The porous membrane was peeled off from the PET substrate and dried in an oven at 80° C. for 10 minutes to obtain a porous membrane. The thickness of the dried porous membrane measured from a scanning electron microscope (SEM) image of the cross section of the membrane was approximately 140 μm, and the porous membrane had an asymmetric structure with a dense portion inside.

[TOF-SIMS measurement]
The porous membrane of each example and comparative example was frozen with liquid nitrogen, and cut by pressing with a razor at about 10 μm from the primary surface in the thickness direction to expose the surface of the pores inside the membrane. Surface samples of five holes arbitrarily selected from the exposed holes were introduced into a TOF-SIMS device (TRIFT V nano TOF manufactured by ULVAC-PHI), a measurement area of 1 × 1 μm, a sputtering speed of 70 nm / min, primary Ion gun: Bi3 ⁺⁺ (30 kV, DC 2.0 nA) irradiation, secondary ion mass spectrometry, ionic strength and hydrophilic material derived from polysulfone or polyethersulfone versus depth (nm) from the pore surface A TOF-SIMS spectrum of ionic strength was obtained. The observed fragments are as follows.
Polysulfone or polyethersulfone: C ₆ H ₅ ⁺ (m/z=77)
General formula (I): _C3H3O ⁺ ₍ m/Z=55) or _C4H5O ⁺ ₍ m/z=69)
General formula (II): _C6H10NO ⁺ (m/z= _112.3 )
General formula ( _III ): _C3H3O ⁺ (m/z=55)

From the above TOF-SIMS spectrum, the average value (I _A ) of the ionic strength of the polymer from the depth where polysulfone or polyethersulfone ions were observed for the first time to a depth of 140 nm from that depth, polysulfone or polyethersulfone The ionic strength of the hydrophilic material (I _B0 ) at the depth where the ions of polysulfone or polyethersulfone were first observed, and the ionic strength of the hydrophilic material at a depth of 100 nm from the depth at which the ions of polysulfone or polyethersulfone were first observed ( I _B100 ) was obtained and the average value of the above five sample values was determined for each value. Table 1 shows the values of I _A , I _B0 , and I _B100 , and “I _B0 /I _B100 ” and “I _B100 /I _A ” calculated based on these values.
All the examples satisfied both of the following two formulas.
I _B0 /I _B100 >2
I _B100 /I _A >0.05

[Evaluation of fouling resistance]
Bovine serum albumin (manufactured by Sigma-Aldrich) was added to ultrapure water so as to be 4 g/L, and the mixture was stirred for 30 minutes. The temperature of this liquid was adjusted to 27° C., and 100 ml of each liquid was passed through the porous membrane from the primary surface ten times at a pressure of 50 kPa. The first water flow rate (initial water flow rate) R _I and the tenth water flow rate (late water flow rate) R _F were compared, and R _F /R _I was evaluated as an index of anti-fouling performance. The larger the R _F /R _I value, the higher the anti-fouling performance of the porous membrane. Table 1 shows R _I , R _F , and R _F /R _I .

Claims (10)

A porous membrane having a backbone made of a polymer selected from the group consisting of polysulfone and polyethersulfone and further comprising polyvinylpyrrolidone,
A hydrophilic material comprising a polymer containing a monomer unit represented by general formula (I) or a polymer represented by general formula (II) or general formula (III) is included in the skeleton,
The porous membrane in which the hydrophilic material is unevenly distributed on the pore surfaces;

In general formula (I),
R ₁ represents a hydrogen atom or a methyl group;
_R2 represents a hydrogen atom, a methyl group, an ethyl group, a phenyl group _, or _{PO3C2H4N ( CH3} ) ₃ ;
R ₃ represents a hydrogen atom, provided that R ₂ and R ₃ may combine to form a five-membered ring together with the oxygen atom to which R ₂ is bonded and the carbon atom to which R ₃ is bonded,
t is an integer from 1 to 9,
A polymer containing a monomer unit represented by general formula (I) is a polymer containing 60 mol% or more of a monomer unit represented by general formula (I),
In general formula (II), m and n are each independently an integer and satisfy the following formula;
m/(n+m)=0.3-0.7
In general formula (III), n is an integer. From the depth at which ions of the polymer selected from the group consisting of polysulfone and polyethersulfone are observed for the first time in the depth direction analysis from the pore surface using a time-of-flight secondary ion mass spectrometer. I _A is the average value of the ionic strength of the polymer up to a depth of 140 nm, I _B0 is the ionic strength of the hydrophilic material at the depth at which the polymer ions are observed for the first time, and the polymer ions are first observed. The porous membrane according to claim 1, which satisfies the following formula when the ionic strength of the hydrophilic material at a depth of 100 nm from the depth is I _B100 ;
I _B0 /I _B100 >2;
I _B100 /I _A >0.05. 3. The porous membrane according to claim 1, wherein the hydrophilic material has a weight average molecular weight of 10,000 or more. The porous membrane according to any one of claims 1 to 3, wherein the hydrophilic material comprises a polymer containing a monomer unit represented by general formula (I). The porous membrane according to any one of claims 1 to 3, wherein the hydrophilic material comprises a polymer represented by general formula (II). The porous membrane according to any one of claims 1 to 3, wherein the hydrophilic material comprises a polymer represented by general formula (III). The porous membrane according to any one of claims 1 to 6, which has a thickness of 10 to 1000 µm. The porous membrane according to any one of claims 1 to 7, which has a pore size distribution in the thickness direction. It has a layered dense portion with the smallest pore size inside,
The porous membrane according to any one of claims 1 to 8, wherein the pore diameter continuously increases in the thickness direction from the dense portion toward at least one membrane surface of the porous membrane. 10. The porous membrane according to claim 9, wherein the average pore size of the dense portion is 0.01 to 10 μm.

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