JP2020104104A - Porous film, composite film and method for producing porous film - Google Patents

Abstract

To provide a porous film which can achieve both excellent separation performance and permeation performance, has high chemical resistance, and can maintain stable filtration operability even to a liquid to be filtered containing a large amount of suspended matters for a long period of time.SOLUTION: A porous film is formed of a polymer containing a polyvinylidene fluoride-based resin, and has a standard deviation of a deformation volume D to a load of 5 nN of 0.8 nm or more and a coefficient of variation of 0.3 or more.SELECTED DRAWING: None

Description

The present invention relates to a porous membrane, a composite membrane, and a method for producing a porous membrane.

In recent years, porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in various fields such as water treatment fields such as water purification or wastewater treatment, medical fields such as blood purification, and food industry fields. Since the porous membrane in such a field is repeatedly used and is washed or sterilized with various chemicals, it is usually required to have high chemical resistance.

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

JP 2006-263721 A JP, 2016-510688, A

However, in a porous membrane made of a polymer containing a conventional polyvinylidene fluoride resin, which is intended to improve the separation performance or the permeation performance, it is not possible to make both performances in a trade-off relationship compatible with each other. It has been regarded as a problem that one is sacrificed.

Therefore, the present invention can achieve both excellent separation performance and permeation performance, and has high chemical resistance, and is stable even for a liquid to be filtered containing a large amount of suspended matter. An object of the present invention is to provide a porous membrane capable of maintaining a stable filtration drivability for a long time.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, this invention consists of a polymer containing polyvinylidene fluoride system resin, the standard deviation of the amount D of deformation with respect to the load of 5 nN is 0.8 nm or more, and the coefficient of variation is 0.3 or more. A porous membrane is provided.

According to the porous membrane of the present invention, while ensuring high chemical resistance by being composed of a polymer containing a polyvinylidene fluoride resin as a main component, both excellent separation performance and permeation performance are achieved, and stable. It is possible to provide a porous membrane capable of maintaining a stable filtration operability for a long period of time.

It is an example of a force curve for determining the amount of deformation D of the surface of the porous film measured in the tapping mode of an atomic force microscope (AFM). It is an enlarged image of a porous membrane which illustrates a "three-dimensional network structure". It is an enlarged image of a porous membrane which illustrates a "three-dimensional network structure". It is a schematic block diagram of a hollow fiber membrane module for evaluation, such as a filtration resistance increase degree. It is a graph which shows the evaluation result of the porous membrane in each Example/comparative example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto.

The porous film of the present invention is made of a polymer containing a polyvinylidene fluoride resin, has a standard deviation of the deformation amount D with respect to a load of 5 nN of at least one surface of 0.8 nm or more, and a coefficient of variation of 0.3 or more. Need to be.

The polyvinylidene fluoride-based resin contained in the polymer constituting the porous film of the present invention refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer. Here, the vinylidene fluoride copolymer refers to a polymer having a vinylidene fluoride residue structure, and is typically a copolymer of a vinylidene fluoride monomer and other fluorine-based monomers. Examples of such a fluorine-based monomer include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene or trifluoroethylene chloride, but other than the above-mentioned fluorine-based monomers to the extent that the effects of the present invention are not impaired. May be copolymerized.

The weight average molecular weight of the polyvinylidene fluoride resin is preferably 50,000 to 1,000,000 Da because the permeation performance of the porous membrane decreases as the weight average molecular weight increases and the separation performance of the porous membrane decreases as the weight average molecular weight decreases. When the porous membrane is used for a water treatment application exposed to chemical cleaning, the weight average molecular weight is preferably 100,000 to 900,000 Da, more preferably 150,000 to 800,000 Da.

The porous film of the present invention is preferably made of a polymer containing a polyvinylidene fluoride resin as a main component. Here, the phrase "having a polyvinylidene fluoride resin as a main component" means that the proportion of the polyvinylidene fluoride resin in the polymer constituting the porous film is 50% by mass or more. In order to ensure high chemical resistance, the above ratio is preferably 55% by mass or more, and more preferably 60% by mass or more.

The value of a for the polymer constituting the porous membrane of the present invention is 0.32 to 0.41, and the value of b is 0.18 to 0. It is preferably 42.

_{^{<S 2> 1/2 = bM w}} a ··· ( Equation 1)
When the value of a is 0.41 or less, the radius of gyration <S ² > ^1/2 becomes appropriately small with respect to the absolute molecular weight M _w of the polymer, and the polymer is porous when a porous film is formed. It is presumed that by moving to the surface layer of the membrane, the polymer density of the surface layer of the porous membrane increases, whereby the porous membrane exhibits excellent separation performance. On the other hand, when the value of a is 0.32 or more, it is presumed that the polymers are appropriately entangled with each other, the polymer density of the surface layer becomes uniform, and higher separation performance is exhibited. Furthermore, since the polymer density of the inner layer decreases as the polymer density of the surface layer of the porous membrane increases, it is presumed that excellent separation performance and high permeation performance are exhibited at the same time. The value of a is more preferably 0.37 to 0.40, and further preferably 0.37 to 0.39.

The value of b for the above polymer is preferably 0.18 to 0.42, and is preferably 0.20 to 0.38, in order to further enhance the separation performance by homogenizing the polymer density of the surface layer due to the entanglement of the polymers. Is more preferable, and it is further preferable that it is 0.25 to 0.33.

In order to easily adjust the value of a for the above polymer within the range of 0.32 to 0.41, the porous membrane of the present invention preferably contains a branched polyvinylidene fluoride resin as the polyvinylidene fluoride resin. .. The proportion of the branched polyvinylidene fluoride resin in the polyvinylidene fluoride resin is preferably 10 to 100% by mass, more preferably 25 to 100% by mass.

In order to easily adjust the value of a within the range of 0.32 to 0.41, the weight average molecular weight of the branched polyvinylidene fluoride resin is preferably 50,000 to 1,000,000 Da, and 100,000 to 600,000 Da. More preferably, 120,000-300,000 Da is still more preferable.

Further, in order to easily adjust the value of a in the above polymer within the range of 0.32 to 0.41, the branched polyvinylidene fluoride resin preferably has a melt viscosity of 30 kP or less, and 20 kP or less. More preferably, it is more preferably 10 kP or less.

In order to easily adjust the values of a and b for the above-mentioned polymer within a predetermined range, the polymer constituting the porous membrane of the present invention preferably contains a hydrophilic resin. Furthermore, since the polymer constituting the porous membrane of the present invention contains a hydrophilic resin, it becomes difficult for dirt to adhere to the porous membrane.

Here, the “hydrophilic resin” refers to a resin having a high affinity with water and soluble in water, or a resin having a contact angle with water smaller than that of polyvinylidene fluoride resin. Examples of the hydrophilic resin include cellulose ester such as cellulose acetate or cellulose acetate propionate, fatty acid vinyl ester, polyvinyl acetate, polyvinylpyrrolidone, ethylene oxide, propylene oxide, acrylic acid ester such as polymethylmethacrylate, or methacrylic acid. Examples thereof include polymers of esters and copolymers of these polymers.

The deformation amount D (Deformation) with respect to a load of 5 nN on at least one surface of the porous film of the present invention is measured as follows.

The surface of the porous membrane sample is observed in the air in an tapping mode using an atomic force microscope (hereinafter, “AFM”), and a region of 5 μm square is randomly selected. This region is divided into 100 parts in 0.5 μm square, and the deformation amount when a load of 5 nN is applied to the surface of the porous membrane at 100 central points (intersection points of diagonal lines) of the 0.5 μm square divided regions Measure D. The same measurement is repeated for the other two randomly selected 5 μm square areas to calculate the average value, standard deviation, and coefficient of variation for all the measured values of the deformation amount D.

In the tapping mode of the AFM, the deformation amount D with respect to a load of 5 nN is measured more specifically on a force curve in which the horizontal axis represents the chip-sample distance and the vertical axis represents the load, as shown in FIG. In, the point before the tip of the conical cantilever is brought close to the porous membrane sample is the H point, the moment when the load (load) rises is the I point, and the load (load) is 90% of the maximum load (load) Is the J point and the maximum load (load) point is the K point, the distance between the JKs can be the deformation amount D for a load of 5 nN.

The standard deviation and the coefficient of variation of the deformation amount D with respect to the load on the surface of the porous membrane have a relatively hard portion and a soft portion on the surface of the porous membrane, and when the surface hardness distribution is large, It grows proportionally. The porous membrane of the present invention has a standard deviation of the deformation amount D with respect to a load of 5 nN on the surface of 0.8 nm or more, and a coefficient of variation of 0.3 or more, so that the porous membrane can come into contact with the liquid to be filtered or the like. The surface of the porous membrane has an appropriate hardness distribution, and the relatively hard part serves as a holding point for suspended matter, etc. It is supposed to be sustainable.

The average value of the deformation amount D is preferably 3 nm or less in order to suppress the deformation of the entire surface of the porous film.

In addition, the porous membrane of the present invention prevents clogging of the surface of the porous membrane and improves the efficiency of air scrubbing and backwashing (hereinafter, "backwashing"). It is preferable that Ra is 20 nm or less and the root mean square roughness Rq is 20 nm or less.

The arithmetic mean roughness Ra and the root mean square roughness Rq on at least one surface of the porous film of the present invention are measured as follows.

The surface of the porous membrane sample is observed with AFM in the air, and a 5 μm square area is randomly selected. Roughness analysis of this area is performed. The same roughness analysis is repeated for the other four randomly selected 5 μm square areas, and the average value of each value can be taken as the arithmetic mean roughness Ra and the root mean square roughness Rq.

The porous membrane of the present invention preferably has a three-dimensional network structure in order to further enhance the separation performance by homogenizing the polymer density of the surface layer due to the entanglement of the polymers. Here, the “three-dimensional network structure” refers to a structure in which the polymer constituting the porous membrane of the present invention is three-dimensionally spread in a network, as shown in FIGS. 2 and 3. The three-dimensional network structure has pores and voids bounded by a network-forming polymer.

The above values of a and b are measured by gel permeation chromatography (hereinafter, “GPC”) equipped with a multi-angle light scattering detector (hereinafter, “MALS”) and a differential refractometer (hereinafter, “RI”). It can be determined based on the relationship between the radius of gyration <S ² > ^1/2 and the absolute molecular weight M _w measured by a certain GPC-MALS. The GPC-MALS measurement is performed by dissolving the polymer forming the porous film in a solvent. A salt may be added to the solvent in order to improve the solubility of the polymer. When measuring GPC-MALS for a polyvinylidene fluoride-based resin, for example, it is preferable to use N-methyl-2-pyrrolidone (hereinafter, “NMP”) to which 0.1 mol/L of lithium chloride is added. ..

The relationship between the radius of gyration <S ² > ^1/2 and the absolute molecular weight M _w , which is measured by GPC-MALS, is called a conformation plot, and is represented by the following formula 1 according to a method generally used in polymer research. The above values of a and b can be determined by such approximation.

_{^{<S 2> 1/2 = bM w}} a ··· ( Equation 1)
The composite membrane of the present invention comprises the porous membrane of the present invention and another layer, and is characterized in that the porous membrane of the present invention is disposed on the surface portion. Here, the “surface portion” of the composite film refers to a portion from the surface of the composite film to a depth of 20 μm in the thickness direction. Here, when the composite membrane is in the form of hollow fibers, the inner surface and/or the outer surface thereof become the “surface of the composite membrane” here, and the thickness direction of the composite membrane coincides with the radial direction of the hollow fiber membrane. By arranging the porous membrane of the present invention showing excellent separation performance on the surface portion, the components contained in the liquid to be filtered are less likely to enter the inside of the composite membrane, and the composite membrane maintains a high permeation performance for a long period of time. can do.

The above-mentioned other layer is not particularly limited as long as it is a constituent element capable of overlapping with the porous membrane to form a layered structure, but the above-mentioned other layer is preferably a support. Here, the "support" refers to a structure having a higher breaking strength than the porous membrane for physically reinforcing the porous membrane. In order to increase the breaking strength of the support, the breaking strength of the support is preferably 3 MPa or more, more preferably 10 MPa or more. When the composite membrane has a hollow fiber shape, the breaking strength of the support is preferably 300 gf or more, more preferably 800 gf or more. Further, the support preferably has a fibrous structure, a columnar structure or a spherical structure in order to further enhance the strength of the composite membrane.

The breaking strength or breaking strength of the support can be calculated as an average value thereof by using a tensile tester to repeat a tensile test 5 times for a sample having a length of 50 mm under conditions of a tensile speed of 50 mm/min. In addition, when the ratio of the volume of the support to the total volume of the composite membrane is 50% or more, the breaking strength or the breaking strength of the composite membrane is regarded as the breaking strength or the breaking strength of the support which is the constituent element. be able to.

The molecular weight cutoff of the porous membrane or composite membrane of the present invention is preferably 5,000 to 80,000 Da, more preferably 8,000 to 60,000 Da, and more preferably 10,000 to 40,000 Da. It is more preferable that there is. Here, the “fractionated molecular weight” means the minimum molecular weight of the components contained in the liquid to be filtered, which can be removed by 90% by the porous membrane.

The porous membrane of the present invention has an average surface pore diameter of preferably 3 to 16 nm, more preferably 6 to 14 nm, and more preferably 8 to 11 nm in order to increase the polymer density of the surface layer and to exhibit excellent separation performance. Is more preferable. The average surface pore diameter of the porous film can be calculated by observing the surface of the porous film with an electron microscope (hereinafter, “SEM”).

More specifically, the surface of the porous film is observed with an SEM at a magnification of 30,000 to 100,000 times, and the areas of 300 randomly selected holes are measured. From the area of each hole, the diameter when the hole is assumed to be a circle is calculated as the hole diameter, and the average value thereof can be used as the surface average hole diameter.

In the porous membrane or composite membrane of the present invention, the average surface pore diameter is within the above range, and the pure water permeability at 25° C. and 50 kPa is 0.1 to 0.8 m ³ /m ² /hr. It is more preferably 0.3 to 0.7 m ³ /m ² /hr. The pure water permeability of the porous membrane or the composite membrane of the present invention at 50 kPa is measured by measuring the membrane area and the amount of water permeation per hour at a pressure within the range where the porous membrane is not deformed. It may be calculated by converting each value.

The method for producing a porous membrane of the present invention comprises: (A) a polymer solution preparing step of dissolving a polymer containing a polyvinylidene fluoride resin as a main component in a solvent to obtain a polymer solution; and (B) the above polymer solution. A porous film forming step of forming a porous film by coagulating in a non-solvent at 20° C. or lower.

A good solvent is preferable as the solvent used in the polymer solution preparation step. Here, the “good solvent” refers to a solvent capable of dissolving 5% by mass or more of the polyvinylidene fluoride resin even in a low temperature region of 60° C. or lower. Examples of the good solvent include NMP, dimethylacetamide, dimethylformamide, methyl ethyl ketone, acetone, tetrahydrofuran, tetramethyl urea or trimethyl phosphate, or a mixed solvent thereof.

The polymer solution obtained in the polymer solution preparation step may appropriately contain a second resin such as a hydrophilic resin, a plasticizer or a salt, in addition to the polyvinylidene fluoride resin.

In addition, the solubility of the polymer solution is improved because the polymer solution contains the plasticizer or the salt. Examples of the plasticizer include glycerol triacetate, diethylene glycol, dibutyl phthalate, dioctyl phthalate and the like. Examples of the salt include calcium chloride, magnesium chloride, lithium chloride or barium sulfate.

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

Whether or not the polymer is completely dissolved in the solvent in the polymer solution preparation step can be judged by visually confirming that there is no turbidity or insoluble matter, but it is preferable to confirm by using an absorptiometer. If the dissolution of the polymer is insufficient, not only the storage stability of the polymer solution will decrease, but also the porous membrane produced will have an inhomogeneous structure, making it difficult to achieve excellent separation performance. The absorbance of the obtained polymer solution is preferably 0.50 or less, and more preferably 0.09 or less at a wavelength of 500 nm.

The crystallinity of the polyvinylidene fluoride resin that dissolves in the solvent in the polymer solution preparation step is 35 in order to easily adjust the values of a and b for the polymer constituting the porous membrane to be produced within a predetermined range. % Or more, preferably 38% or more, and more preferably 40% or more. The crystallinity of the polyvinylidene fluoride resin can be calculated from the measurement result of a differential scanning calorimeter (hereinafter, "DSC").

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

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

In the formation of an ordinary porous membrane, there is a so-called trade-off relationship that the separation performance is improved as the temperature of the non-solvent for coagulating the polymer solution is lower, but the permeation performance is decreased. The polymer solution for forming the porous membrane has excellent permeation even when the temperature of the non-solvent is further lowered because the values of a and b for the polymer are adjusted within a predetermined range. It is easier to achieve performance. The temperature of the liquid containing the polymer solution and/or the non-solvent in the coagulation bath needs to be 20°C or lower, preferably 0 to 20°C, more preferably 5 to 15°C.

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

When producing a hollow fiber membrane-like porous membrane, the polymer solution can be discharged from the outer peripheral portion of the double-tube mouthpiece, and the core liquid can be simultaneously discharged from the central portion to a coagulation bath containing a non-solvent. As the core liquid, it is preferable to use a good solvent in the polymer solution preparation step. Further, a porous membrane may be formed on the surface of a hollow fiber support made of polymer, metal oxide or metal. As a method for forming a porous film on the surface of a hollow fiber-shaped support made of a polymer, for example, a triple tube mouthpiece is used, and a solution as a raw material of the hollow fiber-shaped support and a polymer solution are simultaneously discharged. Alternatively, a method in which a non-solvent in a coagulation bath is passed through a hollow fiber-shaped support, which has been formed into a film in advance, and a polymer solution applied on the surface of the support.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

The measurement and evaluation in Examples and Comparative Examples were performed as follows.

(1) Deformation amount D of the surface of the porous film with respect to a load of 5 nN
The porous membrane was cut into 1 cm squares and adhered to a sample stand so that the surface to be measured faced up to prepare a porous membrane sample. This porous membrane sample was observed by AFM (manufactured by Bruker AXS; Dimension Fast Scan) to calculate the average value, standard deviation, and coefficient of variation of the deformation amount D, respectively. The specific measurement conditions were as follows. The cantilever was calibrated before each measurement.

Scanning mode: Nanomechanical mapping in the atmosphere Probe: Cantilever (Bruker AXS; RTESPA-150)
Maximum load (load): 5nN
Scanning range: 5μm×5μm
Scanning speed: 0.5Hz
Number of pixels: 256 x 256

Measurement temperature: 25℃
(2) Arithmetic mean roughness Ra and root mean square roughness Rq of the surface of the porous film
Using the porous membrane sample of the above (1), the sample was observed under the same conditions as the above (1), and the arithmetic mean roughness Ra and the root mean square roughness Rq were calculated.

(3) a value and b value of polymer constituting the porous film A porous film or a composite film immersed in distilled water was frozen at -20°C using a cryostat (manufactured by Leica; Jung CM3000) to give a porous film. A section of the membrane (in the case of the composite membrane, a section of the porous membrane on the surface portion) was collected and vacuum dried overnight at 25°C. After vacuum drying, 5 mL of 0.1 M lithium chloride-added NMP was added to 5 mg of the porous membrane, and the mixture was stirred at 50° C. for about 2 hours. The obtained polymer solution was GPC-MALS (column: Showa Denko; Shodex KF-806M φ8.0 mm×30 cm connected in series under the following conditions) and a differential refractometer: manufactured by Wyatt Technology; OptilabrEX, MALS. : Manufactured by Wyatt Technology; DAWN HeLEOS). The injected polymer solution eluted from the column in the range 27-43 minutes.

Column temperature: 50°C
Detector temperature: 23℃
Solvent: NMP with 0.1M lithium chloride
Flow rate: 0.5 mL/min Injection volume: 0.3 mL
From the polymer concentration c _{i at} elution time t _i obtained from RI and the excess Rayleigh ratio R _{θi at} elution time t _i obtained from MALS, sin ² (θ/2) and (K×c) _i /R _θi ) ^1/2 was plotted (Berry plot or Zimm plot; the following formula 3), and the absolute molecular weight M _Wi at each elution time t _i was calculated from the value of θ→0 in the approximate formula. Here, K is an optical constant and is calculated from the following equation 2. Note that dn/dc in Formula 2 is the amount of change in the refractive index of the polymer solution with respect to the change in the polymer concentration, that is, the refractive index increment. If a solvent is used, a value of -0.050 mL/g can be applied as the refractive index increment.

K=4π ² ×n ₀ ² ×(dn/dc) ² /(λ ⁴ ×N ₀ )... (Formula 2)
n ₀ : Refractive index of solvent dn/dc: Refractive index increment λ: Wavelength of incident light in vacuum N ₀ : Avogadro's number Further, the value of radius of gyration <S ² > ^1/2 at each elution time t _i is It was calculated from the slope of Equation 3 below.

(Kc _i /R _θi ) ^1/2 =M _Wi ^−1/2 {1+1/6(4πn ₀ /λ) ² <S ² >sin ² (θ/2) (Equation 3)
The absolute molecular weight M _Wi at each elution time t _i calculated from Equation 3 is plotted on the x-axis, and the radius of gyration <S ² > ^1/2 at each elution time t _i is plotted on the y-axis. The value of a and the value of b of the polymer constituting the porous film were determined by approximating

(4) Crystallinity of Polyvinylidene Fluoride Resin About 5 to 10 mg of polyvinylidene fluoride resin was sampled and set in DSC (manufactured by SEIKO; DSC6200) to raise the temperature from room temperature to 300° C. at 5° C./min. At this time, the endothermic peak seen in the range of 100 to 190° C. is regarded as the heat of fusion of the polyvinylidene fluoride resin, and the amount of heat is divided by 104.6 J/g, which is the complete crystal fusion heat of the polyvinylidene fluoride resin, Calculated as a percentage.

(5) Fractional Molecular Weight of Porous Membrane or Composite Membrane When the shape of the porous membrane was a flat membrane, evaluation was performed on an effective membrane area of 30 cm ² . In addition, when the shape of the porous membrane was a hollow fiber membrane, evaluation was performed for an effective membrane area of 14 cm ² . For the composite membrane including the support in addition to the porous membrane, the evaluation was performed for the entire composite membrane including the support. For evaluation, the following various dextrans were used.

Dextran f1 to f4 (manufactured by Fluka; weight average molecular weights of 1,500 Da, 6,000 Da, 15,000 to 25,000 Da, 40,000 Da, respectively)
Dextran a1 and a2 (manufactured by Aldrich; weight average molecular weights of 60,000 Da and 20,000 Da, respectively)
Dextran a3 and a4 (Aldrich molecular weight standard materials; weight average molecular weights of 5,200 Da and 150,000 Da, respectively)
Dextran a5 to a7 (Molecular weight standard manufactured by Aldrich; weight average molecular weights are 1,300 Da, 12,000 Da and 50,000 Da, respectively)
Dextran f1 to f4 and dextran a1 and a2 were mixed in distilled water at 500 ppm each to prepare a dextran aqueous solution 1. The prepared dextran aqueous solution 1 was supplied to the porous membrane at 10 kPa, cross-flow filtered at a cross-flow linear velocity of 1.1 m/s, and the filtrate was sampled. The dextran aqueous solution 1 and the sampled filtrate were connected to GPC (column: Tosoh; TSKgel G3000PW φ7.5 mm×30 cm 1 and Tosoh; TSKgel α-M φ7.8 mm×30 cm 1 in series, RI: It was measured by injecting it into HLC-8320 manufactured by Tosoh Corporation. The injected dextran eluted from the column in the range of 26-42 minutes.

Column temperature: 40°C
Detector temperature: 40℃
Solvent: 0.5M Lithium nitrate-added 50% by volume aqueous methanol solution Flow rate: 0.5 mL/min Injection volume: 0.1 mL
At each elution time t _i , the removal rate was calculated from the value of the differential refractive index between the filtrate and the dextran aqueous solution 1. In addition, dextran a3 and a4 were each mixed with distilled water in an amount of 500 ppm to prepare a dextran aqueous solution 2. Further, 500 ppm each of dextran a5 to a7 was mixed with distilled water to prepare an aqueous dextran solution 3. These dextran aqueous solutions 2 and 3 were injected into GPC under the same conditions as the dextran aqueous solution 1 for measurement, and a calibration curve for calculating the weight average molecular weight at each elution time t _i was prepared. From the created calibration curve, the removal rate at each elution time t _i was converted to the removal rate at each weight average molecular weight, and the minimum weight average molecular weight at which the removal rate was 90% was calculated as the fraction of the porous membrane to be evaluated. The molecular weight was used.

(6) Average Surface Pore Diameter of Porous Membrane The surface of the porous membrane was observed using SEM (manufactured by HITACHI; S-5500) at a magnification of 30,000 to 100,000, and 300 pores randomly selected were observed. Each area was measured. From the area of each hole, the diameter when assuming that the hole was a circle was calculated as the hole diameter, and the average value thereof was taken as the surface average hole diameter.

(7) Pure Water Permeability of Porous Membrane or Composite Membrane When the porous membrane was a flat membrane, evaluation was performed on an effective membrane area of 30 cm ² . When the porous membrane was in the form of a hollow fiber membrane, the effective membrane area was 14 cm ² . Distilled water was supplied to the porous membrane at a temperature of 25° C. and a filtration differential pressure of 10 kPa for 1 hour to completely filter the amount, and the amount of permeated water (m ³ ) obtained was measured, and the unit time (h) and the unit were measured. The value was converted into a value per membrane area (m ² ) and further converted into a pressure (50 kPa). For the composite membrane including the support in addition to the porous membrane, the evaluation was performed for the entire composite membrane including the support.

(8) Absorbance of polymer solution The polymer solution was put into a polystyrene cell having an optical path length of 10 mm and set in an absorptiometer (Shimadzu; UV-2450) to measure the absorbance at a wavelength of 500 nm.

(9) Increase in Filtration Resistance and Shake Recovery of Porous Membrane or Composite Membrane When the porous membrane is a hollow fiber, 6 hollow fiber-shaped porous membranes were housed in the outer cylinder and fixed at the ends. A hollow fiber membrane module having a length of 150 mm shown in FIG. 4 was produced. In this hollow fiber membrane module, the hollow fiber membrane was opened at the B end and the D end.

Raw water was placed in a 10 L stainless steel pressure tank equipped with a pressure gauge, and distilled water was similarly placed in a 40 L stainless steel pressure tank equipped with a pressure gauge. As raw water, Lake Biwa water (turbidity 1.0 NTU or less, TOC (total organic carbon) 1.2 mg/L, calcium concentration 15 mg/L, silicon concentration 0.5, manganese concentration 0.01 mg/L or less, iron concentration 0 0.01 mg/or less) was used.

The two-way cock connected to the outlet of the pressurized tank containing raw water and the point A of the hollow fiber membrane module were connected with a Teflon (registered trademark) tube through the three-way cock, and the flow of the pressurized tank containing distilled water The two-way cock connected to the outlet and the point B of the hollow fiber membrane module were connected with a Teflon (registered trademark) tube. The point C of the hollow fiber membrane module was sealed with a resin cap so that the permeated water could flow out from the point D.

A pressure of 100 kPa was applied to a raw water-containing pressure tank, the two-way cock was opened, and raw water was supplied to the hollow fiber membrane module. At this time, the three-way cock between the hollow fiber membrane module and the hollow fiber membrane module opens only between the raw water pressure tank and the hollow fiber membrane module. The faucet was closed.

The weight of the permeated water obtained is measured every 5 seconds with an electronic balance (AND; HF-6000) connected to a personal computer, and recorded using a continuous recording program (AND; RsCom ver. 2.40). The filtration resistance was calculated from the following equation 4.

Filtration resistance (1/m)=(filtration pressure (kPa))×10 ³ ×5×(membrane area (m ² ))×10 ⁶ /((permeated water viscosity (Pa·s)×(permeation per 5 seconds) Water weight (g/s))×(permeated water density (g/mL))) (Equation 4)
After continuously supplying raw water until the total amount of permeated water reached 0.03 m ³ /m ² , the two-way cock of the pressurized tank containing raw water was closed to complete the supply of raw water. Next, the three-way cock between the hollow fiber membrane module was opened in all three directions, and the permeated water outlet (point D) of the hollow fiber membrane module was sealed with a resin cap.

A pressure of 150 kPa was applied to a pressurized tank containing distilled water, the two-way cock was opened, and distilled water was supplied to the hollow fiber module to backwash the hollow fiber membrane. After backwashing was continued until the amount of backwash drainage discharged from the 3-way cock was 0.003 m ³ /m ² , the 2-way cock of the pressurized tank containing distilled water was closed and the backwashing was completed.

The above operation was continuously performed 10 times for one hollow fiber membrane module, and the total permeated water amount was plotted on the abscissa and the calculated filtration resistance was plotted on the ordinate, to prepare a graph.

Here, the plot started 30 seconds after the start of the supply of raw water each time. Further, since the amount of permeated water decreases as the filtration resistance increases, the weight of permeated water per 5 seconds decreases. Since the filtration resistance is calculated from the above equation 4 including the permeated water weight per 5 seconds, if the permeated water weight per 5 seconds is reduced, the variation thereof has a great influence on the calculated filtration resistance value. Therefore, when the weight of the permeated water per 5 seconds was remarkably reduced, the graph was corrected by taking the moving average approximation of the graph prepared appropriately.

In the created graph of total permeated water-filtration resistance, and in some cases, in a graph corrected by taking the moving average approximation of the above graph, from the relationship between the total permeated water and filtration resistance, the 2-10th time of starting the supply of raw water was started. The slope of the straight line connecting the nine points of filtration resistance was defined as the degree of increase in filtration resistance. However, when the 9 points do not lie on the straight line, the slope of the straight line was obtained by linear approximation and used as the degree of filtration resistance increase.

Further, the hollow fiber membrane module after 10 times of continuous operation was sealed in a state where distilled water remained, and the hollow fiber membrane module was sealed at 60 cm/sec in the longitudinal direction (horizontal direction to the hollow fiber membrane surface). Shaking was carried out at a speed of 0.5 seconds in one direction and 0.5 seconds in the opposite direction for 15 round trips, and then raw water was supplied in the same manner as above to calculate the filtration resistance. The value obtained by dividing the filtration resistance value after shaking by the filtration resistance value which is an intercept with the vertical axis of the filtration resistance increase degree was defined as the shake recovery property.

When the porous membrane is a flat membrane, the porous membrane is cut into a circle with a diameter of 60 mm and set in a cylindrical filtration holder so that the effective membrane area becomes 0.1 m ^2, and A filtration test was conducted with the same device configuration as that of the thread membrane so that the filtration amount per unit membrane area was equal, and the filtration resistance increase degree and the shake recovery property were calculated.

(10) Scratch resistance test When the porous membrane is a flat membrane, a porous membrane measuring 20 cm in width and 30 cm in length is attached to an ABS plate of the same size with the surface of the porous film facing outside, and 30 cm in width × length. It was set in a tank of 50 cm x 15 cm in width. Then, the inside of the tank was filled with an aqueous solution of kaolin (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) of 5,000 ppm, and 25 L/min of air was continuously supplied for 10 days to perform a scratch resistance test.

When the porous membrane is in the form of a hollow fiber membrane, first, 1500 porous membranes whose ends are sealed are bundled and packed in a cylindrical transparent container having a diameter of 10 cm and a length of 100 cm to obtain a porous membrane module. Was produced. Next, the inside of the porous membrane module was filled with a 5,000 ppm aqueous solution of kaolin, and 100 L/min of air was continuously supplied from the lower part of the cylindrical transparent container for 10 days to perform a scratch resistance test.

With respect to the porous membrane or composite membrane after the abrasion resistance test, the membrane performance was evaluated in the same manner as in the above (5), (7) and (9), and the porous membrane or the non-abrasive resistance test was conducted. The performance ratio with the membrane performance for the composite membrane was calculated.

The raw materials of the polymer solutions used in Examples and Comparative Examples are summarized below.

Branched polyvinylidene fluoride (hereinafter, “branched PVDF”) 1 (Solvey 9009 manufactured by Solvay Specialty Chemicals, crystallinity 44%, melt viscosity 3 kP)
Branched PVDF2 (Made by Solvay Specialty Chemicals; Solef460, crystallinity 38%, melt viscosity 26 kP)
Branched PVDF3 (manufactured by Solvay Specialty Chemicals; Solef 9007, crystallinity 45%, melt viscosity 2 kP)
Linear polyvinylidene fluoride (hereinafter, “linear PVDF”) 1 (Kynar 710 manufactured by Arkema, crystallinity 49%, melt viscosity 6 kP)
Linear PVDF2 (Solf1015 manufactured by Solvay Specialty Chemicals, crystallinity 48%, melt viscosity 22 kP)
Linear PVDF3 (Kureha KF1300)
NMP (manufactured by Mitsubishi Chemical)
Cellulose acetate (hereinafter, "CA") (manufactured by Daicel; LT-35)
Cellulose acetate propionate (hereinafter, "CAP") (manufactured by Eastman Chemical; CAP482-0.5)
(Example 1)
25% by mass of branched PVDF1 and 75% by mass of linear PVDF1 were mixed to obtain “PVDF”, NMP and the like were added, and the mixture was stirred at 120° C. for 4 hours to prepare a polymer solution having a composition ratio shown in Table 1. .. The absorbance of the polymer solution cooled to 25° C. was 0.1.

Next, using a nonwoven fabric made of polyester fiber having a density of 0.42 g/cm ³ as a support, the prepared polymer solution was uniformly applied at 10 m/min using a bar coater (film thickness 2 mil). The support coated with the polymer solution was dipped in distilled water at 6° C. for 60 seconds to be solidified 3 seconds after the coating, to form a porous membrane having a three-dimensional network structure.

The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. The cutoff molecular weight, which is an index of separation performance, pure water permeability, which is an index of permeation performance, and the filtration resistance increase degree, which is an index of filtration operability, all showed excellent values. The average value of the amount of deformation D on the surface of the porous film was 2.34 nm, the standard deviation was 0.82, and the coefficient of variation was 0.35. The change in film performance was small even after the scratch resistance test, and both were excellent values. Indicated.

(Example 2)
Using branched PVDF1 as “PVDF”, NMP and the like were added, and the mixture was stirred at 120° C. for 4 hours to prepare a polymer solution having a composition ratio shown in Table 1. The absorbance of the polymer solution cooled to 25° C. was 0.09.

Then, a porous membrane having a three-dimensional network structure was formed in the same manner as in Example 1 except that the temperature of distilled water was changed to 15°C.

The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. The cutoff molecular weight, which is an index of separation performance, pure water permeability, which is an index of permeation performance, and the filtration resistance increase degree, which is an index of filtration operability, all showed excellent values. The average value of the amount of deformation D on the surface of the porous film was 1.93 nm, the standard deviation was 0.84, and the coefficient of variation was 0.44, and the film performance change was small even after the scratch resistance test. Indicated.

(Example 3)
A polymer solution having a composition ratio shown in Table 1 was prepared in the same manner as in Example 2 except that the branched PVDF3 was used instead of the branched PVDF1 and the CAP was used instead of the CA. The absorbance of the polymer solution cooled to 25° C. was 0.07.

Then, a porous membrane having a three-dimensional network structure was formed in the same manner as in Example 1 except that the temperature of distilled water was changed to 20°C.

The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. The cutoff molecular weight, which is an index of separation performance, pure water permeability, which is an index of permeation performance, and the filtration resistance increase degree, which is an index of filtration operability, all showed excellent values. The average value of the amount of deformation D of the surface of the porous film was 2.58 nm, the standard deviation was 0.86, and the coefficient of variation was 0.33. Even after the scratch resistance test, the change in the film performance was small and excellent values were obtained. Indicated.

(Example 4)
38 mass% of linear PVDF3 and 62 mass% of γ-butyrolactone were mixed and dissolved at 160°C to prepare a film-forming stock solution. This stock solution for film formation was discharged from a double pipe mouthpiece while accommodating an 85% by mass aqueous solution of γ-butyrolactone as a hollow-forming liquid, and an 85% by mass aqueous solution of γ-butyrolactone at a temperature of 20° C., which was installed 30 mm below the die, It was coagulated in a cooling bath to prepare a hollow fiber-shaped support having a spherical structure.

A polymer solution was prepared in the same manner as in Example 5 except that branched PVDF3 was used instead of branched PVDF1. The absorbance of the polymer solution cooled to 25° C. was 0.07.

Next, the polymer solution was uniformly applied to the outer surface of the hollow fiber-like support at 10 m/min (thickness 50 μm). The support coated with the polymer solution was immersed in distilled water at 15° C. for 10 seconds to coagulate the support after 1 second from the application to form a porous film having a three-dimensional network structure.

The results of evaluating the obtained porous membrane are shown in Table 1 and FIG. Moreover, the enlarged image which observed the obtained porous film by SEM is shown in FIG. The molecular weight cutoff, which is an index of separation performance, pure water permeability, which is an index of permeation performance, and the filtration resistance increase degree, which is an index of filtration operability, all showed excellent values. The average value of the amount of deformation D on the surface of the porous film was 2.21 nm, the standard deviation was 0.90, and the coefficient of variation was 0.41. The film performance change was small even after the scratch resistance test, and both were excellent values. Indicated.

(Comparative Example 1)
Linear PVDF2 was used as "PVDF", NMP was added, and the mixture was stirred at 120°C for 4 hours to prepare a polymer solution having a composition ratio shown in Table 2. The absorbance of the polymer solution cooled to 25° C. was 0.01.

Then, a porous membrane having a three-dimensional network structure was formed in the same manner as in Example 1 except that the temperature of distilled water was changed to 25°C.

The results of evaluating the obtained porous film are shown in Table 2 and FIG. Although the value of the molecular weight cut-off which is an index of separation performance was equivalent to the result of the example, the pure water permeability which is an index of permeation performance and the filtration resistance increase degree which is an index of filtration operability are both Was also inferior to the result of the example. The average value of the amount of deformation D on the surface of the porous film was 2.09 nm, the standard deviation was 0.48, and the coefficient of variation was 0.23, and the change in film performance was small even after the scratch resistance test.

(Comparative example 2)
A polymer solution having a composition ratio shown in Table 2 was prepared in the same manner as in Comparative Example 1 except that branched PVDF2 was used instead of linear PVDF2. The absorbance of the polymer solution cooled to 25° C. was 0.1.

Then, a porous membrane having a three-dimensional network structure was formed in the same manner as in Example 1 except that the temperature of distilled water was changed to 40°C.

The results of evaluating the obtained porous film are shown in Table 2 and FIG. The value of pure water permeability, which is an index of permeation performance, was equivalent to the result of the example, but the fractional molecular weight that is an index of separation performance and the filtration resistance increase degree that is an index of filtration operability are both Was also inferior to the result of the example. The average value of the deformation amount D on the surface of the porous film was 2.32 nm, the standard deviation was 0.60, and the coefficient of variation was 0.26, which was more uniform than the results of the examples, and was particularly pure after the scratch resistance test. The changes in water permeability and molecular weight cutoff were large.

(Comparative example 3)
A polymer solution having a composition ratio shown in Table 2 was prepared in the same manner as in Example 4 except that the linear PVDF1 was used instead of the branched PVDF3. The absorbance of the polymer solution cooled to 25° C. was 0.03.

Then, in the same manner as in Example 4, a polymer solution was applied to the outer surface of the hollow fiber-shaped support and solidified to form a porous membrane having a three-dimensional network structure.

The results of evaluating the obtained porous film are shown in Table 2 and FIG. Moreover, the enlarged image which observed the obtained porous film by SEM is shown in FIG. The molecular weight cutoff that is an index of separation performance, pure water permeability that is an index of permeation performance, and the filtration resistance increase degree that is an index of filtration drivability are both inferior to the results of the examples. there were. The average value of the amount of deformation D on the surface of the porous membrane was 1.79 nm, the standard deviation was 0.38, and the coefficient of variation was 0.21. After the scratch resistance test, the pure water permeability and the molecular weight cut-off were determined. The change was great.

(Comparative Example 4)
A polymer solution having the composition ratio shown in Table 2 was prepared in the same manner as in Example 4. The absorbance of the polymer solution cooled to 25° C. was 0.05.

Then, in the same manner as in Example 4, the polymer solution was applied to the outer surface of the hollow fiber-shaped support and solidified to give a porous membrane having a three-dimensional network structure.

The results of evaluating the obtained porous film are shown in Table 2 and FIG. The value of pure water permeability, which is an index of permeation performance, was excellent as compared with the results of the examples, but the fractional molecular weight that is an index of separation performance and the filtration resistance increase degree that is an index of filtration operability are All were inferior to the results of the examples. The average value of the amount of deformation D of the surface of the porous film was 3.11 nm, the standard deviation was 0.55, and the coefficient of variation was 0.18. The film performance change after the scratch resistance test was Comparative Example 2 and Comparative Example 3. It was small compared to.

Claims (9)

Made of a polymer containing polyvinylidene fluoride resin,
A porous membrane having a standard deviation of the deformation amount D for a load of 5 nN on the surface of 0.8 nm or more and a coefficient of variation of 0.3 or more. The porous membrane according to claim 1, wherein the polymer contains a hydrophilic resin. The porous membrane according to claim 1 or 2, wherein the membrane has a three-dimensional network structure. The porous film according to any one of claims 1 to 3, wherein an average value of the deformation amount D is 3 nm or less. The arithmetic average roughness Ra of the surface is 20 nm or less, and
The porous membrane according to any one of claims 1 to 4, wherein the root mean square roughness Rq of the surface is 20 nm or less. A porous membrane according to any one of claims 1 to 5, and another layer,
A composite membrane in which the porous membrane is disposed on the surface portion. The composite membrane according to claim 6, wherein the other layer is a support. (A) a polymer solution preparation step in which a polymer containing a polyvinylidene fluoride resin as a main component is dissolved in a solvent to obtain a polymer solution;
The porous film according to any one of claims 1 to 5, further comprising (B) a step of forming the porous film by coagulating the polymer solution in a non-solvent at 20°C or lower. Membrane manufacturing method. The method for producing a porous membrane according to claim 8, wherein the crystallinity of the polyvinylidene fluoride resin used in the step (A) for preparing the polymer solution is 35% or more.