JP2021186747A - Composite membrane and method for producing composite membrane - Google Patents

Abstract

To provide a composite membrane which achieves both excellent low fouling property and physical durability while maintaining high chemical resistance and a removal ratio of a separation object.SOLUTION: A composite membrane has a porous resin layer on the surface of a porous base material, in which the porous resin layer is formed of a polymer containing a polyvinylidene fluoride-based resin as a main component; an average pore diameter of the surface of the porous resin layer is 0.02-0.05 μm; the porous resin layer has macro voids having a short diameter of 30 μm or more; and the macro voids are present only in a region separated from the surface of the porous resin layer by 5 μm or more.SELECTED DRAWING: Figure 2

Description

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

In recent years, membranes having a porous resin layer 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. There is.

As described above, the membrane having a porous resin layer, which is widely used, impairs the stability of operation when the water permeability is reduced (fouling) due to the accumulation, adhesion, blockage, etc. of the substance to be separated, and aeration cleaning is performed. It is necessary to increase the amount of aeration and the frequency of chemical cleaning, which leads to high operating costs, and therefore a low fouling membrane is required. In the water purification treatment, for the purpose of sterilizing permeated water and preventing biofouling of the membrane, a bactericidal agent such as sodium hypochlorite is added to the membrane module part, or the membrane is coated with acid, alkali, chlorine, surfactant, etc. A film having a porous resin layer is required to have chemical durability (chemical resistance) in order to clean itself. Furthermore, in tap water production, accidents in which pathogenic microorganisms resistant to chlorine such as cryptosporidium derived from livestock manure cannot be completely treated at water purification plants and are mixed into treated water have become apparent since the 1990s. Therefore, in order to prevent such an accident, the film having a porous resin layer is required to have sufficient separation characteristics and high physical durability so that raw water does not mix with the treated water.

Therefore, in order to satisfy the chemical durability and the physical durability among these required performances, a membrane having a porous resin layer using a polyvinylidene fluoride-based resin has come to be used. .. For example, Patent Document 1 discloses a technique for improving low fouling property by controlling the pore size distribution of a porous resin layer made of a polymer containing a polyvinylidene fluoride-based resin. Further, in Patent Document 2, the porous membrane for filtering cell suspension is a membrane having a porous resin layer containing a polyvinylidene fluoride resin and an anionic resin, whereby a negatively charged membrane and a suspended matter are provided. A technique for suppressing a decrease in water permeability (fouling) due to clogging of a membrane by utilizing the repulsive force of the film is disclosed. Further, in Patent Document 3, a porous resin layer made of polyvinylidene fluoride-based resin is blended with an acrylic acid ester-based resin or vinylpyrrolidone-based resin to form a hydrophilic film, thereby forming a clogged component such as hydrophobic microorganisms. A technique for lowering the affinity of a porous resin layer and suppressing a decrease in water permeability (fouling) due to clogging of a membrane has been disclosed. Patent Document 4 discloses a technique for improving the physical durability of a film by making the porous resin layer spherical.

Japanese Patent No. 5310658 Japanese Unexamined Patent Publication No. 2008-229612 Japanese Unexamined Patent Publication No. 2006-205567 Japanese Unexamined Patent Publication No. 2010-75851

However, in the prior art, if the pore size of the porous resin layer is reduced due to the low fouling property, the water permeability is significantly impaired due to a slight pore size closure, or an anionic resin, an acrylic acid ester resin, or a vinylpyrrolidone type. When other resins such as resins were blended, the film strength was weakened, and it was difficult to achieve both low fouling property and physical durability.

Therefore, an object of the present invention is to provide a composite membrane having both low fouling property and physical durability while having excellent removal rate and water permeability.

In order to solve the above problems, the present invention is characterized by the following (1) to (4).
(1) A composite film having a porous resin layer on the surface of a porous base material, wherein the porous resin layer is made of a polymer containing a polyvinylidene fluoride-based resin as a main component, and the porous resin layer is formed. The average pore size of the surface of the quality resin layer is 0.02 to 0.05 μm, the porous resin layer has a macrovoid having a minor axis of 30 μm or more, and the macrovoid is from the surface of the porous resin layer. A composite membrane characterized by being present only in a region separated by 5 μm or more.
(2) The composite film according to (1), wherein the root mean square roughness Rq of the surface of the porous resin layer is 20 nm or more and 40 nm or less.
(3) The polymer containing a polyvinylidene fluoride-based resin as a main component has a branched structure, and the radius of gyration <S2> 1 / measured by GPC-MALS (gel permeation chromatograph equipped with a multi-angle light scattering detector). From the absolute molecular weight Mw of 2 and the polymer, the value of a determined from the relationship of the following formula 1 is 0.29 to 0.33, and the value of b is 0.43 to 0.50. The composite membrane according to 1) or (2).

<S ² > ^1/2 = bM _w ^a ... (Equation 1)
(4) (A) Using a polymer containing a polyvinylidene fluoride-based resin as a main component and a polymer containing polyethylene glycol having a number average molecular weight of 50,000 or more and 300,000 or less as a main component and a solvent, dissolve the polymer to prepare a polymer solution. Obtain, polymer solution preparation process and
(B) The method for producing a composite membrane according to any one of (1) to (3), further comprising a composite film forming step of coagulating the polymer solution in a non-solvent to form a porous resin layer.

According to the present invention, while ensuring high chemical resistance due to the polymer composed of a polyvinylidene fluoride-based resin as a main component, while maintaining the removal rate of the separation target, excellent low fouling property and physical properties are provided. It is possible to provide a composite membrane in which both durability is achieved.

It is a schematic diagram which shows an example of the water treatment apparatus using the composite membrane of this invention. It is a magnified image of a composite membrane exemplifying a "macro void". It is an enlarged image of a composite film exemplifying that "macro voids are present only in a region separated from the surface of the porous resin layer by 5 μm or more".

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 composite film of the present invention is a composite film having a porous resin layer on the surface of a porous base material, and has a porous resin layer made of a polymer containing a polyvinylidene fluoride-based resin as a main component. The average pore size of the surface of the porous resin layer is 0.02 to 0.05 μm, the porous resin layer has macrovoids having a minor axis of 30 μm or more, and the macrovoids are from the surface of the porous resin layer. It needs to be present only in regions separated by 5 μm or more.
The polyvinylidene fluoride-based resin of the present invention refers to a vinylidene fluoride homopolymer or a vinylidene fluoride copolymer. Here, the vinylidene fluoride copolymer means a polymer having a vinylidene fluoride residue structure, and is typically a copolymer of a vinylidene fluoride monomer and another fluorine-based monomer or the like. Examples of such a fluorine-based monomer include vinyl fluoride, ethylene tetrafluoride, propylene hexafluoride, and ethylene trifluoride, but other than the above-mentioned fluorine-based monomer to the extent that the effect of the present invention is not impaired. Ethylene and the like may be copolymerized.

The porous resin layer of the present invention needs to be made of a polymer containing a polyvinylidene fluoride-based resin as a main component. "The main component is polyvinylidene fluoride-based resin" means that the proportion of polyvinylidene fluoride-based resin in the polymer constituting the porous resin layer is 50% by mass or more. The above ratio is preferably 55% by mass or more, and more preferably 60% by mass or more in order to secure high chemical resistance. Examples of the auxiliary component other than the polyvinylidene fluoride-based resin include, but are not limited to, a pore-forming agent described later and an acrylic-based resin that controls hydrophilicity.

The composite film of the present invention needs to have an average pore size of 0.02 to 0.05 μm on the surface of the porous resin layer. If the surface pore diameter is less than 0.02 μm, poor water permeability will occur from the initial stage of operation, which is not preferable.
If it exceeds 0.05 μm, in the formation of a porous structure by phase separation, water rapidly flows in when the porous structure is formed by phase separation, and macrovoids are formed in a region within 5 μm from the surface layer. , It is not preferable because the physical durability is impaired.
In order to keep the average pore size within the above range, a method of lowering the coagulation temperature, a method of increasing the viscosity of the polymer solution, and using a highly branched polyvinylidene fluoride-based resin as the polyvinylidene fluoride-based resin can be mentioned. Among these, a method using a highly branched polyvinylidene fluoride-based resin is preferable in terms of reducing the burden on the film forming process and achieving both fouling property and physical durability. Specifically, the formula (1) described later is used. ) Satisfying the polyvinylidene fluoride-based resin.
The average pore size of the surface of the composite film in the present invention is measured as follows.

The surface of the porous resin layer of the composite film was observed at a magnification of 10,000 times using a scanning electron microscope (hereinafter referred to as SEM), and the diameter of each pore was determined from the area of each pore when it was assumed that the pore was a circle. Each of them can be calculated as, and the average value thereof can be used as the average pore diameter.
The average pore size of the surface of the porous resin layer of the present invention is more preferably in the range of 0.03 to 0.05 μm in terms of both water permeability and fouling property at the initial stage of operation.
Further, the composite film of the present invention needs to have macrovoids having a minor axis of 30 μm or more in the porous resin layer, and the macrovoids need to be present only in a region separated from the surface of the porous resin layer by 5 μm or more. be.
By having the above structure, it is possible to reduce the flow resistance when the permeated water flows in the porous resin layer, and while exhibiting high water permeability, particles and hair in sludge are formed on the surface layer of the porous resin layer. Even if a high-hardness substance comes into contact with the macrovoid, the removal rate can be maintained and the physical durability can be improved without exposing the macrovoid.
Here, as shown in FIG. 2, the macrovoid is a void existing in the porous resin layer having a diameter larger than the pore diameter on the surface. The minor axis of the macrovoid indicates the diameter in the direction parallel to the surface of the porous resin layer. The minor axis of the macrovoid needs to be 30 μm or more. (Hereafter, simply referred to as "macro void")
On the other hand, when the macrovoid becomes extremely large, the strength of the porous resin layer decreases and the physical durability becomes insufficient. Therefore, it is preferable to keep the minor axis and the major axis of the macrovoid to 300 μm or less.

Further, "macro voids are present only in a region separated by 5 μm or more from the surface of the porous resin layer" means that the cross section of the porous resin layer is viewed at a magnification of 10000 times using SEM in a 13 μm × 10 μm square field of view. When 10 or more points are arbitrarily observed and the tip of the macrovoid closest to the surface layer does not exist in the region within 5 μm from the surface layer of the porous resin layer in 8 visual fields or more, “Macrovoid is the porous resin layer”. It exists only in the region 5 μm or more away from the surface of. "
In the porous resin layer, the porous resin layer has a macrovoid having a minor axis of 30 μm or more, and the macrovoid is present only in a region separated from the surface of the porous resin layer by 5 μm or more. When the quality resin layer is formed, the density of the polyvinylidene fluoride-based resin on the surface layer may be improved. Specifically, the polyvinylidene fluoride-based resin satisfying the relationship of the formula 1 described later is contained, and polyethylene glycol is used as a pore-opening agent. Can be achieved by adding.
The composite film of the present invention preferably has a root mean square roughness Rq of 20 nm or more and 40 nm or less on the surface of the porous resin layer. By setting Rq in the above range, it is possible to suppress the contact area between the particles and the film at the convex portion, prevent the entire porous resin layer from being scraped, and improve the physical durability. Further, since the substance to be separated having a low molecular weight does not get caught on the film surface and can suppress the deposition on the film surface, the fouling property is also preferable.
The Rq of the porous resin layer can be adjusted by adjusting the radius of gyration of the polyvinylidene fluoride-based resin, the molecular weight of polyethylene glycol used as a pore-opening agent, and the amount of addition. Specifically, the number average molecular weight of polyethylene glycol is preferably 50,000 to 100,000, and the addition amount is preferably in the range of 3.0 to 7.0% by weight.
In the present invention, among the polyvinylidene fluoride-based resins, it is preferable to use a highly branched polyvinylidene fluoride-based resin, and specifically, the value of a for the polymer, which is determined from the relationship of the following formula 1, is It is preferable to use a polyvinylidene fluoride-based resin having a value of 0.29 to 0.33 and a value of b of 0.43 to 0.50.
<S ² > ^1/2 = bM _w ^a ... (Equation 1)
Here, <S ² > ^1/2 means the radius of gyration of the polymer, Mw means the absolute molecular weight, and by using a highly branched polyvinylidene fluoride-based resin, the above ranges a and b can be satisfied. ..
Here, when the value of a is 0.33 or less, the turning radius <S ² > ^1/2 _{becomes appropriately smaller with respect to the absolute molecular weight M w} of the polymer, and when the porous resin layer is formed. When the polymer moves to the surface layer of the porous resin layer, the polymer density of the surface layer of the porous resin layer is increased, and a porous resin layer in which macrovoids are present in a region of 5 μm or more on the surface of the porous resin layer is obtained. It is presumed that it can exhibit excellent physical durability.
On the other hand, when the value of a is 0.29 or more, the polymers are appropriately entangled with each other, the polymer density of the surface layer becomes homogeneous, and Rq can be controlled in an appropriate range of 20 to 40 nm. It is presumed that high low fouling property and physical durability are exhibited.

The above values a and b are related to the relationship between the ^{radius of gyration <S 2} > ^1/2 and the absolute molecular weight M _w measured by GPC-MALS (gel permeation chromatography equipped with a multi-angle light scattering method). Can be decided based on. The measurement of GPC-MALS is performed by dissolving the polymer constituting the porous resin layer 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 polyvinylidene fluoride-based resin, for example, it is preferable to use N-methyl-2-pyrrolidone (hereinafter, “NMP”) to which 0.1 mol / L lithium chloride is added. ..

The relationship between the radius of gyration <S ² > ^1/2 measured by GPC-MALS and the absolute molecular weight M _w is called a conformation plot, and is of the above formula 1 by a method generally used in polymer research. By approximating as above, the values of a and b can be determined.
The proportion of the highly branched polyvinylidene fluoride-based resin in the polyvinylidene fluoride-based resin of the present invention is preferably 10 to 100% by mass, more preferably 25 to 100% by mass.

Further, in order for macrovoids to be present only in a region 5 μm or more away from the surface of the porous resin layer, the weight average molecular weight of the branched polyvinylidene fluoride-based resin is preferably 100,000 to 1,000,000, preferably 500,000 to 900,000. Is more preferable, and 500,000 to 800,000 is even more preferable.
The composite membrane of the present invention is characterized by having a porous resin layer on the surface of a porous base material.

The porous base material (hereinafter referred to as a base material) supports the porous resin layer and imparts strength to the composite film. The material is not particularly limited, such as an organic material and an inorganic material, but organic fiber is preferable because it is easy to reduce the weight. More preferably, it is such as a woven fabric or a non-woven fabric made of organic fibers such as cellulose fibers, cellulose triacetate fibers, polyester fibers, polypropylene fibers and polyethylene fibers. Among them, a non-woven fabric which is relatively easy to control the density, easy to manufacture, and inexpensive is preferable.

If the thickness of the base material is too thin, it becomes difficult to maintain the strength as a composite film, and if it is extremely thick, the water permeability tends to decrease. Therefore, the thickness is preferably in the range of 50 μm to 1 mm. Most preferably, it is in the range of 70 to 500 μm.

The thickness of the porous resin layer is usually preferably in the range of 1 to 500 μm, and more preferably selected in the range of 5 to 200 μm. If the porous resin layer is too thin, the base material may be exposed, and dirt components may adhere to the base material to increase the filtration pressure, or the filtration performance may not be sufficiently restored even after cleaning. Further, if the porous resin layer is too thick, the amount of water permeation may decrease.

It is preferable that a part of the resin forming the porous resin layer penetrates into at least the surface layer portion of the base material and forms a composite layer with the base material at least in the surface layer portion. When the resin enters the base material, the porous resin layer is firmly fixed to the base material by the so-called anchor effect, and the porous resin layer can be prevented from peeling off from the base material.
In the present invention, the porosity of macrovoids in the porous resin layer within 50 μm from the surface layer of the porous resin layer is preferably 30 to 70%, preferably 30 to 50% or less.

The composite membrane of the present invention described above can typically be produced by a method as described below.

That is, first, a film of a stock solution containing the above-mentioned resin, a pore-opening agent, and a solvent is formed on the surface of the above-mentioned base material, and the base solution is impregnated with the stock solution. After that, the base material is immersed in a coagulation bath containing a non-solvent to solidify the resin and form a porous resin layer on the surface of the base material. It is also preferable that the stock solution further contains a non-solvent. The temperature of the stock solution is usually preferably selected in the range of 15 to 120 ° C. from the viewpoint of film forming property.

The density of the base material ^{is preferably 0.7 g / cm 3} or less, more preferably 0.6 g / cm ³ or less. When the density of the base material is within this range, it is suitable for accepting the resin forming the porous resin layer and forming an appropriate composite layer of the base material and the resin. However, when the density is extremely low, the strength of the composite film tends to decrease, so that the density ^{is preferably 0.3 g / cm 3} or more. The density here is an apparent density and can be obtained from the area, thickness and weight of the base material.

The pore-forming agent is extracted when immersed in a coagulation bath and has an action of making the resin layer porous. The pore-forming agent is preferably one having high solubility in a coagulation bath. For example, inorganic salts such as calcium chloride and calcium carbonate can be used. Further, polyoxyalkylenes such as polyethylene glycol and polypropylene glycol, water-soluble polymers such as polyvinyl alcohol, polyvinyl butyral and polyacrylic acid, and glycerin can be used. The pore-forming agent can be arbitrarily selected, but for example, a polymer containing polyethylene glycol as a main component is preferable.
Further, in the present application, the surface pore size is controlled in the range of 0.02 μm to 0.05 μm by using a polymer containing polyethylene glycol having a weight average molecular weight of 50,000 or more and 100,000 or less as a main component among polyethylene glycols. It has been found that the macrovoid can be present only in a region separated from the surface of the porous resin layer by 5 μm or more, and both physical durability and removal rate can be achieved.
By using a highly branched polyvinylidene fluoride-based resin and polyethylene glycol having a weight average molecular weight in the above range, the coagulation rate can be slowed down and the density of the polyvinylidene fluoride-based resin on the surface layer can be improved. , The physical durability of the porous resin layer can be improved.

The solvent dissolves the resin. The solvent acts on the resin and the pore-forming agent to encourage them to form a porous resin layer. As the solvent, N-methylpyrrolidinone (NMP), N, N-dimethylacetamide (DMAc), N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), acetone, methyl ethyl ketone and the like can be used. Of these, NMP, DMAc, DMF, and DMSO, which have high resin solubility, can be preferably used. In particular, in the present invention, in order to obtain a porous resin layer in which macrovoids are present only in a region separated from the surface of the porous resin layer by 5 μm or more, it is possible to uniformly dissolve a highly branched polyvinylidene fluoride resin. It is more preferable to use a DMF solvent that can be used.

The non-solvent is a liquid that does not dissolve the resin. The non-solvent acts to control the rate of solidification of the resin to control the size of pores and macrovoids. As the non-solvent, water or alcohols such as methanol and ethanol can be used.

In the stock solution, the resin is preferably in the range of 5 to 30% by weight, the pore-forming agent is preferably in the range of 0.1 to 15% by weight, and the solvent is preferably in the range of 40 to 94.9% by weight. If the amount of resin is extremely small, the strength of the porous resin layer will be low, and if it is too large, the water permeability may be reduced. The resin content in the stock solution is more preferably in the range of 8 to 20% by weight. Further, if the amount of the pore-forming agent is too small, the water permeability may decrease, and if it is too large, the strength of the porous resin layer may decrease. Further, if the amount is extremely large, it may remain in the porous resin layer and elute during use, resulting in deterioration of the water quality of the permeated water or fluctuation of the water permeability. A more preferable range of the content of the pore-forming agent in the stock solution is 0.5 to 10% by weight. Further, if the amount of the solvent is too small, the undiluted solution tends to gel, and if the amount is too large, the strength of the porous resin layer may decrease. The solvent content in the stock solution is more preferably in the range of 60 to 90% by weight.

As the coagulation bath, a non-solvent or a mixed solution containing a non-solvent and a solvent can be used. In the coagulation bath, if the amount of non-solvent is large, the solidification rate of the resin becomes high and the surface of the porous resin layer becomes dense, and macrovoids are generated inside, but the surface of the porous resin layer is fine. Cracks may occur. A more preferable range is 60 to 99% by weight. By adjusting the content of the solvent in the coagulation bath, the pore diameter and the size of the macrovoid on the surface of the porous resin layer can be controlled. If the temperature of the coagulation bath is too high, the coagulation rate will be too fast, and conversely, if it is too low, the coagulation rate will be too slow. Therefore, it is usually selected within the range of 5 to 80 ° C. preferable. A more preferable temperature range is 5 to 60 ° C.

The formation of a film of the undiluted solution on the substrate can be performed by applying the undiluted solution to the substrate or by immersing the substrate in the undiluted solution. When the undiluted solution is applied, it may be applied to one side of the base material or to both sides. At this time, although it depends on the composition of the stock solution, ^{it is preferable to use a base material having a density of 0.7 g / cm 3} or less because the base material is appropriately impregnated with the stock solution.

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

(I) Average pore size of the surface of the porous resin layer The surface of the porous resin layer of the composite film is observed from the area of each pore observed under the following observation conditions using SEM (manufactured by HITACHI; S-5500). , The diameter when the hole was assumed to be a circle was calculated as the hole diameter. In addition, arbitrary 5 visual fields were measured, and the average value was taken as the average pore diameter of the surface.
・ Acceleration voltage: 5kV
・ Observation magnification: 10,000 times ・ Image processing software: ImageJ (Wayne Rasband, National Institutes of Health)
(Ii) Prepare a sample for cross-section measurement by the frozen ultra-thin section method so that the short-diameter composite film of macrovoids of the porous resin layer is perpendicular to the longitudinal direction of the film formation, and make it porous using the following conditions. The minor axis of the macrovoid of the resin layer was measured. Measurements were made for any 10 visual fields, the minor axis of the observed macrovoid was measured, and the average value was taken as the minor axis of the macrovoid of the porous resin layer.
-Observation device: Hitachi, S-5500
・ Acceleration voltage: 5kV
-Observation magnification: 500 times (iii) Presence or absence of macrovoids in the region of 5 μm or more on the surface layer A sample for cross-section measurement was prepared by the frozen ultrathin section method so that the composite film was perpendicular to the longitudinal direction of the film formation. The cross-sectional structure of the porous resin layer of the composite film was observed under the following conditions. Here, measurements are made for any 10 visual fields of the composite membrane, and when the tip of the macrovoid closest to the surface layer is present in a region of 5 μm or more from the surface layer of the porous resin layer in 8 visual fields or more, “macro voids”. Is not present in the region 5 μm from the surface of the porous resin layer. "
-Observation device: Hitachi, S-5500
・ Acceleration voltage: 5kV
-Observation magnification: 10000 times (observation field of view 13 μm × 10 μm)
(Iv) A value and b value of the polymer constituting the porous resin layer The composite membrane immersed in distilled water was frozen at −20 ° C. using a cryostat (manufactured by Leica; Jung CM3000) to obtain a porous resin layer. Sections were harvested and vacuum dried at 25 ° C. overnight. To the 5 mg porous resin layer after vacuum drying, 5 mL of 0.1 M lithium chloride-added NMP was added, and the mixture was stirred at 50 ° C. for about 2 hours. The obtained polymer solution was connected to GPC-MALS (column: Showa Denko; Shodex KF-806M φ8.0 mm × 30 cm in series, differential refractometer: Wyatt Technology; Optilab rEX, MALS) under the following conditions. : Wyatt Technology; DAWN HeLEOS) was injected and measured. The injected polymer solution elutes from the column in the range of 27-43 minutes.

Column temperature: 50 ° C
Detector temperature: 23 ° C
Solvent: NMP with 0.1M lithium chloride added
Flow velocity: 0.5 mL / min
Injection volume: 0.3 mL
Obtained from RI, polymer concentration _c i of when the elution time _{t i,} obtained from the MALS, from the excess Rayleigh ratio R _.theta.i when the elution time ^{_{t i, sin 2 (θ /}} 2) and (K × c _i / R _.theta.i) performs plot of ^1/2 (Berry plot or Zimm plot; formula 3), the value of theta → 0 of the approximate expression, to calculate the absolute molecular weight _{M Wi} at each elution time _{t i.} Here, K is an optical constant and is calculated from the following equation 2. Note that dn / dc in Equation 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 increase in the refractive index. When a solvent is used, a value of −0.05 mL / g can be applied as the index of refraction increment.

K = 4π ² × n ₀ ² × (dn / dc) ² / (λ ⁴ × N ₀ ) ・ ・ ・ (Equation 2)
n _0: refractive index dn / dc of the solvent refractive index increment lambda: wavelength N in the incident light in vacuum _0: Avogadro's number and the value of the radius of gyration ^{<S 2> 1/2} at each elution time _{t i} is It was calculated from the inclination 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)
Is calculated from Equation 3, for x-axis the absolute molecular weight _{M wi} at each elution time _{t i,} and plotting the radius of gyration ^{<S 2> 1/2} to y-axis in each elution time _{t i,} the above expression 1 The value of a and the value of b for the polymer constituting the porous resin layer were obtained by approximating with.
(V) Root mean square roughness Rq
The composite membrane was cut into 1 cm squares and adhered to a sample table with the surface of the porous resin layer facing up to prepare a membrane sample. This membrane sample was observed by AFM (manufactured by Bruker AXS; Dimension FastScan), and the root mean square roughness Rq was calculated. The specific measurement conditions are as follows. The silicon cantilever was calibrated before each measurement, and any five fields of view were measured, and the average value was taken as Rq.

Scan mode: ScanAsyst
Probe: Silicon cantilever (manufactured by Bruker AXS; ScanAsyst-Air)
Maximum load (load): 5nN
Scanning range: 2.5 μm × 2.5 μm
Scanning speed: 0.5Hz
Number of pixels: 256 x 256
Measurement temperature: 25 ° C
(Vi) Pure water permeability of composite membrane (initial permeability)
The composite membrane is cut into a circle with a diameter of 50 mm, set in a cylindrical filtration holder (Advantech Toyo, Ultra Holder UHP-43K), and distilled water is pre-permeated at 25 ° C. at a pressure of 5 kPa for 5 minutes, and then continued. Permeated and permeated water was collected for 3 minutes, and calculated by converting it into numerical values per unit time (h) and unit membrane area (m ^2).

(Vii) Differential pressure increase rate of composite membrane (low fouling property)
Using a composite membrane, the composite membrane was attached to both sides of a support frame having a length of 320 mm, a width of 220 mm, and a thickness of 5 mm having a filtered water outlet at the top via a plastic net to obtain an element. This element 2 was placed in a module 1 having a length of 500 mm, a width of 150 mm, and a height of 700 mm, and housed in a water tank 4 to be treated having an air diffuser 3 at the bottom (FIG. 1). When activated sludge with a concentration of 7,000 mg / L is placed in the water tank to be treated and the operation is performed with a permeation flux of 0.9 m / d while supplying air from the air diffuser at 20 L / min, the differential pressure between the films is applied. Was observed over time, and the average rate of increase in differential pressure per day was calculated after 10 days of operation.
(Viii) Polystyrene Latex Fine Particle Blocking Rate A composite film is set in a stirring cell (VHP-43K manufactured by Advantech Co., Ltd.), and a reverse osmosis film (SUL manufactured by Toray Co., Ltd.) is set at an evaluation pressure of 9.8 kPa and a stirring speed of 600 rpm. -The evaluation stock solution prepared by dispersing polystyrene latex fine particles (nominal pore size 0.083 μm manufactured by Ceradin Co., Ltd.) as fine particles having an average particle size of 0.09 μm or less in filtered water according to G10) to a concentration of 25 ppm is filtered. With respect to the evaluation stock solution and the obtained filtered permeate, the absorbance of ultraviolet rays having a wavelength of 250 nm was measured, and the particle blocking rate was determined by the following formula. Here, a spectrophotometer (U-3200 manufactured by Hitachi, Ltd.) was used for the absorbance measurement.
Particle blocking rate = [(absorbance of raw water-absorbance of permeate) / absorbance of undiluted solution] x 100
(IX) Durability (fine particle blocking rate after sand falling wear test)
Using a sand-falling wear tester (ASTM D673 # 80 manufactured by Toyo Seiki Seisakusho), set the porous resin layer of the composite film sample on the pedestal held at an angle of 45 ° to the horizontal plane so that the porous resin layer of the composite film sample comes out on the upper surface, and set the height. From 650 mm to 400 g of SiC (45 #) was dropped from the sample to prepare a composite film after a sand-falling wear test.
Then, the (viii) polystyrene latex fine particle blocking rate was measured using the composite film after the sand falling wear test, and the fine particle blocking rate after the sand falling wear test was determined.
(Example 1)
Branched polyvinylidene fluoride 1 (branched PVDF1, weight average molecular weight 164,000) is used as "PVDF", polyethylene glycol (PEG, weight average molecular weight 100,000) is used as a pore-forming agent, and N, N-dimethylformamide (DMF) is used as a solvent. ), Pure water was added as a non-solvent, and the mixture was sufficiently stirred at a temperature of 90 ° C. to prepare a polymer solution having the composition ratio shown below.
PVDF: 20.0% by weight
PEG: 5.5% by weight
DMF: 71.0% by weight
Pure water: 3.5% by weight
Next, using a polyester fiber non-woven fabric having a density of 0.6 g / cm ³ as a base material, the prepared polymer solution was applied to the surface thereof, and immediately after the application, the mixture was immediately immersed in pure water at 20 ° C. for 5 minutes and further 90 ° C. Was immersed in hot water for 2 minutes to wash away the solvent N, N-dimethylformamide and the polyethylene glycol as a pore-opening agent.

Then, it was immersed in a 20% by weight aqueous solution of a surfactant (polyoxyethylene sorbitan monooleate) for 30 minutes, and then dried in a hot air dryer at 75 ° C. for 30 minutes to form a composite film.

The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.02 μm, the minor axis of the macrovoid was 41 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. In addition, the fine particle blocking rate, which is an index of the removal rate of the substance to be separated, and the fine particle blocking rate after the drop test, which is an index of physical durability, both showed excellent values. In addition, the pure water permeability, which is an index of permeation performance, and the differential pressure increase rate, which is an index of low fouling property, showed practically no problem.

(Example 2)
Polyethylene glycol (PEG, weight average molecular weight 70,000) was used as the pore-forming agent, and a composite film was formed in the same manner as in Example 1 except that the composition ratio was as follows.
PVDF: 18.0% by weight
PEG: 5.5% by weight
DMF: 73.0% by weight
Pure water: 3.5% by weight
The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.03 μm, the minor axis of the macrovoid was 50 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. The pure water permeability, the differential pressure increase rate, the fine particle blocking rate, and the fine particle blocking rate after the drop test all showed excellent values.

(Example 3)
A composite film was formed in the same manner as in Example 1 except that the branched PVDF1 was branched polyvinylidene fluoride 2 (branched PVDF2, weight average molecular weight 550,000) and the composition ratio of the polymer solution was changed to the composition ratio shown below.
PVDF: 18.0% by weight
PEG: 5.5% by weight
DMF: 73.0% by weight
Pure water: 3.5% by weight
The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.04 μm, the minor axis of the macrovoid was 55 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. The pure water permeability, the differential pressure increase rate, the fine particle blocking rate, and the fine particle blocking rate after the drop test all showed excellent values.

(Example 4)
A composite film was formed in the same manner as in Example 2 except that polyethylene glycol (PEG, weight average molecular weight 50,000) was used as the pore-forming agent.

The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.03 μm, the minor axis of the macrovoid was 50 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. The pure water permeability, the differential pressure increase rate, and the fine particle blocking rate all showed excellent values. The particle blocking rate after the drop test showed a value that did not cause any problem in practical use.
(Example 5)
A composite film was formed in the same manner as in Example 4 except that the composition ratio of the polymer solution was changed to the composition ratio shown below.
PVDF: 18.0% by weight
PEG: 8.0% by weight
DMF: 70.5% by weight
Pure water: 3.5% by weight
The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.05 μm, the minor axis of the macrovoid was 60 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. The pure water permeability, the differential pressure increase rate, and the fine particle blocking rate all showed excellent values. The particle blocking rate after the drop test showed a value that did not cause any problem in practical use.
(Example 6)
A composite film was formed in the same manner as in Example 4 except that polyethylene glycol (PEG, weight average molecular weight 200,000) was used as the pore-forming agent.

The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.02 μm, the minor axis of the macrovoid was 41 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Both the water permeability of pure water and the inhibition rate of fine particles showed excellent values. The rate of increase in differential pressure and the rate of blocking fine particles after the drop test showed practically acceptable values.
(Example 7)
A composite film was formed in the same manner as in Example 4 except that the composition ratio of the polymer solution was changed to the composition ratio shown below.
PVDF: 18.0% by weight
PEG: 3.0% by weight
DMF: 75.5% by weight
Pure water: 3.5% by weight
The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.03 μm, the minor axis of the macrovoid was 50 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Both the water permeability of pure water and the inhibition rate of fine particles showed excellent values. The rate of increase in differential pressure and the rate of blocking fine particles after the drop test showed practically acceptable values.

(Example 8)
A composite film was formed in the same manner as in Example 3 except that the branched PVDF1 was a branched polyvinylidene fluoride 3 (branched PVDF3, weight average molecular weight 400,000).

The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.02 μm, the minor axis of the macrovoid was 40 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Both the water permeability of pure water and the inhibition rate of fine particles showed excellent values. The rate of increase in differential pressure and the rate of blocking fine particles after the drop test showed practically acceptable values.
(Example 9)
A composite film was formed in the same manner as in Example 3 except that the branched PVDF1 was a branched polyvinylidene fluoride 4 (branched PVDF4, weight average molecular weight 520,000).

The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.04 μm, the minor axis of the macrovoid was 50 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Pure water permeability, fine particle blocking rate, and differential pressure increase rate all showed excellent values. The particle blocking rate after the drop test showed a value that did not cause any problem in practical use.

(Comparative Example 1)
A composite film was formed in the same manner as in Example 1 except that the composition ratio of the polymer solution was changed to the composition ratio shown below.
PVDF: 21.0% by weight
PEG: 5.5% by weight
DMF: 70.0% by weight
Pure water: 3.5% by weight
The results of evaluating the obtained composite membrane are shown in Table 2. The average pore diameter was 0.01 μm, the minor axis of the macrovoid was 41 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Although the fine particle blocking rate and the fine particle blocking rate after the drop test showed practically acceptable values, the pure water permeability and the differential pressure increase rate were all inferior to the results of the examples.

(Comparative Example 2)
A composite film was formed in the same manner as in Example 2 except that linear polyvinylidene fluoride (linear PVDF, weight average molecular weight 320,000) was used as “PVDF”.

The results of evaluating the obtained composite membrane are shown in Table 2. The average pore diameter was 0.06 μm, the minor axis of the macrovoid was 50 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Although the pure water permeability showed practically no problem, the differential pressure increase rate, the fine particle blocking rate, and the fine particle blocking rate after the drop test were all inferior to the results of the examples.

(Comparative Example 3)
A composite film was formed in the same manner as in Example 5 except that the composition ratio of the polymer solution was changed to the composition ratio shown below.
PVDF: 18.0% by weight
PEG: 10.0% by weight
DMF: 68.5% by weight
Pure water: 3.5% by weight
The results of evaluating the obtained composite membrane are shown in Table 2. The average pore diameter was 0.06 μm, the minor axis of the macrovoid was 60 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Although the pure water permeability, the fine particle blocking rate, and the fine particle blocking rate after the drop test showed practically acceptable values, the differential pressure increase rate was inferior to the results of the examples.
(Comparative Example 4)
A composite film was formed in the same manner as in Example 4 except that polyethylene glycol (PEG, weight average molecular weight 20,000) was used as the pore-forming agent.

The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.03 μm, the minor axis of the macrovoid was 50 μm, and macrovoid was observed 5 μm from the surface of the porous resin layer. Although the pure water permeability, the fine particle blocking rate, and the differential pressure increase rate showed practically acceptable values, the fine particle blocking rate after the drop test was inferior to the results of the examples.
(Comparative Example 5)
A composite film was formed in the same manner as in Example 4 except that the composition ratio of the polymer solution was changed to the composition ratio shown below.
PVDF: 18.0% by weight
DMF: 78.5% by weight
Pure water: 3.5% by weight
The results of evaluating the obtained composite membrane are shown in Table 1. The average pore diameter was 0.02 μm, the minor axis of the macrovoid was 25 μm, and no macrovoid was found 5 μm from the surface of the porous resin layer. Although the fine particle blocking rate, the differential pressure increase rate, and the fine particle blocking rate after the drop test showed practically acceptable values, the pure water permeability was inferior to the results of the examples.

1 Module 2 Element 3 Air diffuser 4 Water tank to be treated 5 Blower 6 Suction pump 7 Water inlet to be treated 8 Water outlet to be treated 9 Permeated water

Claims (4)

A composite film having a porous resin layer on the surface of a porous base material, wherein the porous resin layer is made of a polymer containing a polyvinylidene fluoride-based resin as a main component, and the porous resin layer. The average pore diameter of the surface of the surface is 0.02 to 0.05 μm, the porous resin layer has macrovoids having a minor axis of 30 μm or more, and the macrovoids are separated from the surface of the porous resin layer by 5 μm or more. A composite membrane characterized by being present only in the area. The composite film according to claim 1, wherein the root mean square roughness Rq of the surface of the porous resin layer is 20 nm or more and 40 nm or less. The polymer containing polyvinylidene fluoride-based resin as the main component has a branched structure and has a radius of gyration <S2> 1/2 and a polymer measured by GPC-MALS (gel permeation chromatograph equipped with a multi-angle light scattering detector). 1 or claim 1, wherein the value of a determined from the relationship of the following formula 1 is 0.29 to 0.33, and the value of b is 0.43 to 0.50 from the absolute molecular weight Mw of. 2. The composite membrane according to 2.
<S ² > ^1/2 = bM _w ^a ... (Equation 1) (A) A polymer in which a polymer containing a polyvinylidene fluoride resin as a main component and a polymer containing a polyethylene glycol having a number average molecular weight of 50,000 or more and 300,000 or less as a main component and a solvent are used to dissolve the polymer to obtain a polymer solution. Solution preparation process and
(B) The method for producing a composite membrane according to any one of claims 1 to 3, further comprising a composite film forming step of coagulating the polymer solution in a non-solvent to form a porous resin layer.

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