WO2017014130A1 - Porous membrane, water treatment membrane and method for producing porous membrane - Google Patents

Abstract

This method for producing a porous membrane comprises: a step wherein a polyimide resin or a precursor thereof is dissolved into a solvent, thereby obtaining a casting liquid that contains the polyimide resin or a precursor thereof; a step wherein the casting liquid is applied over a base; a step wherein the casting liquid is brought into contact with a non-solvent material, thereby forming a porous structure having a skin layer on a surface that is not in contact with the base; a step wherein the porous structure is dried; and a step wherein the skin layer is removed from the dried porous structure.

Description

Porous membrane, water treatment membrane and method for producing porous membrane

The present invention relates to a porous membrane, a water treatment membrane, and a method for producing a porous membrane. This application claims priority based on Japanese Patent Application No. 2015-146013 for which it applied to Japan on July 23, 2015, and uses the content here.

Separation membranes (water treatment membranes) used for water treatment include UF membranes (Ultrafiltration Membrane), NF membranes (Nanofiltration Membrane), RO membranes (Reverse Osmosis Membrane), FO There are membranes (Forward Osmosis Membrane).
The NF membrane and the RO membrane usually have a three-layer structure of a nonwoven fabric, a porous membrane, and a membrane that exhibits the function of NF or RO.
Hereinafter, the UF film, the film exhibiting the function of NF or RO may be collectively referred to as a functional film.

Since polyimide resins are excellent in heat resistance and chemical resistance, they are attracting attention as resins constituting porous membranes, and methods for producing porous membranes are also being studied.

Patent Document 1 discloses that a porous film is obtained by precipitating a polyimide precursor solution in a poor solvent. Patent Document 2 discloses that a polyamideimide solution is precipitated in a poor solvent to obtain a porous film. The membranes obtained in Patent Documents 1 and 2 have pore diameters and surface irregularities on the order of μm.

Patent Document 3 discloses that a fluorine-containing polyimide is precipitated from a solution with a poor solvent to produce a porous film having a skin layer, and further, an NF film is obtained by ion irradiation and plasma treatment.

JP 2002-201305 A Japanese Patent Application Laid-Open No. 2014-094501 Japanese Patent Laying-Open No. 2005-081226 JP 2013-202461 A

In the porous membranes described in Patent Documents 1 and 2, since the pore diameter is on the order of μm, the function of a UF membrane (Ultrafiltration Membrane: ultrafiltration membrane) cannot be obtained.
Moreover, the pore diameter of the porous membrane being on the order of μm means that the irregularities on the surface of the porous membrane are on the order of μm. As a support material (porous membrane) for NF membrane (Nanofiltration Membrane) and RO membrane (Reverse Osmosis Membrane), a thin NF or RO with a thickness of 500 nm or less on the surface having irregularities of μm order. It is difficult to mount a film having a function (hereinafter referred to as NF function film or RO function film). Even if the membrane is successfully placed, the membrane is easily broken when pressure is applied from above the NF functional membrane or RO functional membrane.

When a porous polyimide membrane is applied to a water treatment membrane, that is, to form a UF membrane or an NF functional membrane or an RO functional membrane to form an NF membrane or an RO membrane, the pore diameter and surface of the porous membrane It is required that the unevenness is small (for example, 1 μm or less) and the water permeability is high. If the water permeability is low, the porous membrane cannot be used as a water treatment membrane. When the pore diameter and surface irregularities of the porous membrane are large, the thin NF functional membrane or RO functional membrane tends to be damaged.

Also, the porous membrane of Patent Document 3 requires special ion irradiation. Moreover, the resulting porous membrane has low water permeability and is not practical.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a porous film having both high water permeability and low surface roughness, a water treatment film provided with the porous film, and a method for producing the porous film. And

In order to solve the above problems, the following means were adopted.

The method for producing a porous membrane according to one aspect of the present invention includes a step of dissolving a polyimide resin or a precursor thereof in a solvent to obtain a cast solution containing the polyimide resin or a precursor thereof, and a method based on the cast solution. A step of coating on a material, a step of forming a porous structure having a skin layer on the surface on the opposite side of the base material by bringing the casting solution into contact with a non-solvent, and the porous structure. A drying step, and a step of removing the skin layer from the dried porous structure by an etching process.

In the method for producing a porous film, the step of removing the skin layer may be performed by an etching process.

In the method for producing a porous film, a curing treatment may be performed between the drying step and the skin layer removing step.

In the method for manufacturing a porous membrane, the curing treatment may be a heat treatment.

In the porous film manufacturing method, the etching process may be a plasma process.

In the above porous membrane manufacturing method, the plasma may be oxygen plasma generated at a high frequency.

In the above porous membrane production method, the casting solution may contain a water-soluble compound.

A porous film according to one embodiment of the present invention includes a porous layer that includes a polyimide resin as a main component and includes a layer having finger-like pores and two layers having no finger-like pores sandwiching the layer. Is.

A porous film according to one embodiment of the present invention includes a porous layer that includes a polyimide resin as a main component and includes a layer having finger-like pores and two layers having no finger-like pores sandwiching the layer. The surface of one of the two layers is oxidized.

The porous membrane may have a water permeability of 0.5 [m ³ / (m ² · day · MPa)] or more.

The porous membrane may have a water permeability of 200 [m ³ / (m ² · day · MPa)] or less.

The porous membrane may be capable of separating Blue Dextran 2000 (trade name) in the solute removal test.

The surface of the porous film may be oxidized.

A water treatment membrane according to one embodiment of the present invention includes the porous membrane.

According to the porous film of one embodiment of the present invention, a porous film having both high water permeability and small surface irregularities can be provided.
According to the water treatment membrane of one embodiment of the present invention, it is possible to provide a water treatment membrane including a porous membrane that has both high water permeability and small surface irregularities.
According to the method for producing a porous membrane according to one embodiment of the present invention, a method for producing a porous membrane in which high water permeability and small surface irregularities are compatible can be provided.

It is a SEM image of the cross section of the porous structure made from a polyamideimide which consists of a skin layer and a porous layer. (A) It is a cross-sectional schematic diagram of the porous structure produced by the non-solvent induced phase separation method, (b) shows the cross-sectional schematic diagram of the porous film from which the skin layer was removed from the porous structure. (A) is an SEM image before removing a skin layer by oxygen plasma etching from a porous structure produced by a non-solvent induced phase separation method, and (b) is an SEM image after removing the skin layer. It is the flowchart which showed an example of the process of the manufacturing method of a porous membrane with the schematic diagram. (A) is the SEM image taken from the surface (surface which was not in contact with the base material) side of the porous membrane manufactured in Example 1, and (b) is simply in an argon atmosphere without plasma etching treatment. 2 is an SEM image taken from the surface side of the porous film that was heat-treated for the same time as in Example 1. (A) XPS measurement was performed on the surface side surface that was not in contact with the base material before the oxygen plasma etching treatment, and (b) XPS was measured on the surface after the oxygen plasma etching treatment (after removal of the skin layer). The result of the measurement is shown. (A) is an SEM image taken from the surface side of the porous membrane produced in Example 2, and (b) is produced under the same conditions as in Example 2 except that the oxygen plasma etching time was 60 seconds. It is a SEM image taken from the surface side of the made porous membrane.

Hereinafter, the configuration of the porous membrane, the water treatment membrane including the porous membrane, and the manufacturing method thereof will be described with reference to the drawings. In the drawings used in the following description, in order to make the features easy to understand, the portions that become the features may be shown in an enlarged manner for the sake of convenience. The materials, dimensions, and the like exemplified in the following description are merely examples, and the present invention is not limited to them. Additions, omissions, substitutions, and other configurations are not limited to the scope of the present invention. It can be changed.

(Porous membrane)
The porous film according to the first embodiment has a polyimide-based resin as a main component, a layer having finger-like holes (corresponding to a “second layer” described later), and no finger-like holes sandwiching the layer. It consists of a porous layer having two layers (corresponding to “first layer” and “third layer” described later). The porous membrane which concerns on one Embodiment can be manufactured by removing a skin layer from the porous structure which consists of the skin layer and porous layer which were produced by the below-mentioned method.
The polyimide resin in the present embodiment includes a wide range of polymer compounds containing imide bonds such as polyamideimide, polyesterimide, and polyetherimide, in addition to polyimide, which is a polymer compound containing imide bonds in the repeating unit.

The porous film contains a polyimide resin as a main component of the polymer component. Here, “including a polyimide resin as a main component” means that 60% by mass or more of the polymer component includes a polyimide resin. The case where 100 mass% of polyimide resin is included is also included. The porous membrane is not particularly limited, but it preferably contains 60% by mass or more and 100% by mass or less of a polyimide resin as a main component of the polymer component, and contains 80% by mass or more and 100% by mass or less. It is more preferable. The porous membrane may contain other polymer components. The other polymer component is not particularly limited, but is preferably compatible with the polyimide resin.

FIG. 1 shows an electron microscope (SEM) image of a cross section of a porous structure made of polyamideimide composed of a skin layer and a porous layer. In FIG. 1, the leftmost SEM image is a cross-sectional SEM image of almost the entire porous structure, and the SEM image at the tip of the arrow is an enlarged SEM image of the part surrounded by the circle at the base of the arrow. In the example of the porous structure shown in FIG. 1, the skin layer has a thickness of about 2 μm.
From the SEM image of FIG. 1, the presence of the skin layer is clear, and the skin layer clearly differs from the subsequent porous layer in the appearance of the SEM image.

The porous structure shown in FIG. 1 is produced using a non-solvent induced phase separation method (NIPS: Nonsolvent Induced Phase Separation). When this method is used, it is known that a porous structure having a skin layer without pores and a continuous porous layer can be formed from the surface side (see, for example, Patent Document 3). Here, the skin layer having no pores means a skin layer having no pore structure exhibiting water permeability that can be used as a porous membrane constituting the water treatment membrane. The skin layer has almost no pores of a size (diameter) through which water can permeate, and is almost dense.

As shown in FIG. 1, the presence of the skin layer is clear in the SEM image of the porous structure produced by the non-solvent induced phase separation method. In general, the porous structure tends to have an inclined structure in which the pore diameter gradually increases from the skin layer toward the inside of the porous layer (for example, Patent Document 4). In other words, the pore diameter changes continuously from the skin layer to the inside of the porous layer, and the skin layer and the porous layer cannot be simply distinguished by the presence or absence of pores. Here, the pores in the porous structure communicate with each other.
For example, the skin layer and the porous layer can be distinguished from the viewpoint of water permeability. In a water treatment membrane, for example, an RO membrane, the RO function membrane assumes the separation function. On the other hand, the porous membrane is required to rapidly pass the water separated by the RO functional membrane, that is, to have high water permeability. From this point of view, the skin layer is a layer that prevents the predetermined water permeability in the water treatment film, and is a portion of the porous structure that should be removed in order to ensure the predetermined water permeability.
The porous membrane according to an embodiment of the present invention is obtained by removing a portion (skin layer) having a certain depth from the surface of the porous structure in order to ensure a predetermined water permeability.
In the schematic diagram shown in FIG. 2, which will be described later, the porous membrane is manufactured by removing a portion of a certain depth from the surface of the porous structure to a portion having a large number of pores having a pore diameter of the order of 100 nm. Can do. Below the portion with many holes of the order of 100 nm (corresponding to the “first layer” described later), it has a finger-like shape (finger-like shape) and the maximum diameter in the minor axis direction is about 5 to 30 μm. There is a portion (corresponding to a “second layer” described later) including a large hole having a length of about 40 to 100 μm. If the portion including the large hole is removed, the unevenness of the scraped surface is too large, and the functional film placed on the surface is easily damaged.

Factors that determine water permeability include not only the diameter of the holes, but also the degree of bending of the passages that connect the holes (curvature, bending coefficient), hydrophilicity / hydrophobicity, and the like. These elements are related to each other, and the magnitude of water permeability cannot be determined only by the value or degree of each element. However, the size of the pore size is a major factor that determines the size of the water permeability, and the pore size is a measure of the size of the water permeability.

FIG. 2A shows a schematic cross-sectional view of a porous structure prepared by a non-solvent induced phase separation method. FIG. 2B shows a schematic cross-sectional view of a porous film in which the skin layer is removed from the porous structure.
As shown in FIG. 2A, a skin layer and a porous layer are formed in order from the surface side on which the functional film is placed.
In the example shown in FIG. 2 (a), the porous layer has a portion having a large number of pores having a typical pore diameter of about 0.05 to 0.2 μm (first layer) and a long axis in the direction from the skin layer to the porous layer. A portion including a large hole having a typical maximum diameter in the short axis direction of about 5 to 30 μm and a typical length of about 40 to 100 μm (second layer) ) And a portion (third layer) having many pores having a pore diameter of about 0.1 to 1 μm. In FIG. 1, three SEM images vertically arranged in the center row are SEM images of the first layer, the second layer, and the third layer in order from the top.
That is, the porous layer has a three-layer structure of a first layer, a second layer, and a third layer. The three SEM images in the second column in FIG. 1 show typical structures of the first layer, the second layer, and the third layer in order from the top. The average size of the holes in the first layer is usually smaller than the average size of the holes in the third layer.
Moreover, when the average hole diameter of the hole exposed on the surface of the first layer (the surface on which the functional film is placed when the porous film is used as the water treatment film) is exemplified, it is 0.05 μm or more and 0.2 μm or less.
In addition, the “finger-like hole” described in the present specification is a hole having the above finger-like shape.

Here, the hole diameter in the present specification is, for example, the hole diameter of a surface hole that can be measured using an SEM image.
For example, if necessary, the surface of the porous film subjected to antistatic treatment such as Pt sputter deposition is observed, imaged, and observed so that a pore diameter of about 10 nm to 1000 nm can be discriminated with a field of view of about 2 μm square. 10 SEM images are taken at different positions, and an average pore diameter can be obtained by image analysis. Image analysis can be performed in accordance with the method described in the standard of particle size analysis (JIS Z 8827-1 Particle size analysis-Image analysis method-Part 1: Static image analysis method).

FIG. 3 shows SEM images before and after removing the skin layer by oxygen plasma etching from the porous structure produced by the non-solvent induced phase separation method. (A) is an SEM image before removing the skin layer, and (b) is an SEM image after removing the skin layer. Both (a) and (b) are, in order from the left, a cross-sectional SEM image, an enlarged SEM image near the skin layer, and an SEM image taken from the surface side. The cross-sectional SEM image has a scale of 10 μm, and the SEM image obtained by enlarging the vicinity of the skin layer and the SEM image taken from the surface side have a scale of 0.1 μm. In this example, the skin layer was 0.1 μm.

The surface before removing the skin layer (skin layer surface) looks flat in the SEM image, but after removing the skin layer, the porous layer is exposed and many pores with a maximum diameter of about 500 nm are visible.

The thickness of the skin layer depends on the conditions of the non-solvent induced phase separation method for producing the porous structure, but as an example, it is about 0.1 to 3 μm.

The thickness of the porous layer also depends on the conditions of the non-solvent induced phase separation method for producing the porous structure, but as an example, it is about 30 to 150 μm. The breakdown is that the portion with many holes having a hole diameter of about 0.05 to 0.5 μm has a thickness of about 1 to 5 μm, the part including a larger hole has a thickness of about 40 to 130 μm, and the hole diameter is 0.1 The portion with many holes of about 1 μm is about 2-20 μm thick.

The maximum diameter of the pores exposed on the surface after removing the skin layer from the porous structure is preferably 500 nm or less. When the pore size is small, the surface irregularities are also small, and therefore it is possible to prevent the functional film placed on the porous film from being damaged. The maximum pore size is more preferably 300 nm or less, and the maximum pore size is more preferably 200 nm or less.

The porous membrane according to one embodiment of the present invention preferably has a water permeability of 0.5 [m ³ / (m ² · day · MPa)] or more. This is to allow water separated by the functional membrane to pass quickly at a practical level. From this viewpoint, the water permeability is more preferably 1 [m ³ / (m ² · day · MPa)] or more, and 1.5 [m ³ / (m ² · day · MPa)] or more. Is more preferable.

Moreover, it is preferable that the porous membrane which concerns on one Embodiment of this invention is 200 [m < ³ > / (m < ² > * day * MPa)] or less. When the water permeability is larger than this, the pore diameter is too large. This also means that the unevenness of the surface is large, and as a result, the functional film placed thereon is easily damaged. From this viewpoint, the water permeability is more preferably 100 [m ³ / (m ² · day · MPa)] or less, and further 70 [m ³ / (m ² · day · MPa)] or less. preferable.

Here, the water permeability means a proportionality constant (Lp) in the Hagen-Poiseuille equation shown below.

In this equation, Jv is water permeability (flux) [m ³ / (m ² · day)], A is a coefficient for unit conversion [dimensionless], ε is surface porosity [dimensionless], and η is kinematic viscosity Coefficient [Pa · s], τ is the flexibility coefficient [dimensionless], Δx is the film thickness [m], ΔP is the transmembrane pressure difference [MPa], Lp is the water permeability [m ³ / (m ² · day · MPa )].

Water permeability is evaluated as follows, for example. First, the porous membrane is filled with pure water, and pressurized with gas from above. And water permeability is evaluated by measuring the permeation | transmission speed | rate of water when carrying out pressure filtration.

Moreover, the porous membrane which concerns on one Embodiment of this invention can isolate | separate Blue Dextran 2000 (brand name) (Molecular weight 2 million or more, GE Healthcare Japan Co., Ltd.) in a solute removal test (in other words, porous It is preferred that it cannot pass through the membrane. If the pore size is so large that the solute cannot be separated (in other words, passes through the porous membrane), the surface irregularities are too large. When the unevenness on the surface is large, the functional film placed on the surface tends to be damaged.

Here, “in the solute removal test, Blue Dextran 2000 (trade name) can be separated” means that when a porous film is filled with a 100 mass ppm aqueous solution of Blue Dextran 2000 and subjected to pressure filtration, the rejection rate is 95. % Or more. More specifically, the calculation is performed as follows. First, a membrane sample is cut into a circle having a diameter of 25 mm and mounted on a filtration device (stirring type ultra holder UHP-25K, manufactured by Advantech). Then, about 5 ml of 100 mass ppm blue dextran 2000 aqueous solution (stock solution) is placed on the attached membrane, and the pressure is increased to 0.3 MPa with nitrogen gas from above, and the filtrate is recovered by filtration. The absorbance (wavelength 620 nm) of the stock solution and the filtrate is measured, and the concentration of Blue Dextran 2000 in the filtrate is calculated. The rejection rate R [%] is calculated by the following formula from the concentration Cb [ppm] in the stock solution and the concentration Cp [ppm] in the filtrate.

There is no particular limitation on the method for removing the skin layer. There are dry process etching using plasma and wet process etching using an alkaline solution or chromic acid solution. It is preferable to remove the skin layer by etching using oxygen plasma. In this case, the surface of the porous membrane is oxidized. In other words, the surface of the porous film has a higher oxygen concentration than when the skin layer is removed by a method other than etching using oxygen plasma. Although the degree of oxidation is a standard, the removal of the skin layer by oxygen plasma etching increases the surface O / C ratio (the ratio of oxygen to carbon) by about 30%. For this reason, there is an effect that the surface can be hydrophilized by removing the skin layer by oxygen plasma etching.

As described above, in the SEM image, the skin layer is different from the subsequent porous layer, but the skin layer and the porous layer are integrally formed. The size of the pores changes continuously from the skin layer to the inside of the porous layer, and a clear line cannot be drawn between the skin layer and the porous layer. Therefore, “a porous film formed by removing a skin layer” means that a material that cannot be used as a porous film (support for a functional film) of a water treatment film because of having a skin layer is water It means a porous film obtained by scraping the surface layer to such an extent that it can be used as a porous film for a treatment film.

In fact, when removing the skin layer from the porous structure, change the etching conditions to change the depth of the surface layer, and measure the water permeability of the porous membrane for each porous membrane with a different depth. Thus, the shaving depth (or the etching process conditions) at which a predetermined water permeability that can be used as the porous film of the water treatment film is obtained is determined. This scraped surface layer corresponds to the skin layer. By removing the skin layer from the porous structure based on the etching treatment conditions thus obtained, a porous film having practically the same water permeability can be obtained.

The porous film according to the second embodiment is composed of a porous layer having a polyimide resin as a main component, a layer having finger-like holes, and two layers having no finger-like holes sandwiching the layer. The surface of one of the two layers (first layer) is oxidized. Compared to the porous film according to the first embodiment, the porous film according to the second embodiment is one of the two layers having no finger-like holes (the surface of the first layer is oxidized). The porous film according to the second embodiment can be manufactured by removing the skin layer of the porous structure composed of the skin layer and the porous layer by oxygen plasma etching.
Since the porous film according to the second embodiment is the same as the porous film according to the first embodiment except that the skin layer of the porous structure is removed, the porous film according to the first embodiment. The contents described above can be applied.

(Water treatment membrane)
The water treatment film | membrane which concerns on one Embodiment of this invention is equipped with the above-mentioned porous membrane. Specifically, for example, in the UF membrane, the above-described porous membrane is used as the functional membrane, and in the NF membrane, the RO membrane, and the FO membrane, the above-mentioned porous membrane is used as a support material on which the functional membrane is placed. A film is used. It is not limited to these water treatment membranes, but can be used in water treatment membranes to which the functions of the porous membrane described above can be applied.

(Method for producing porous membrane)
The method for producing a porous membrane according to an embodiment of the present invention includes a step of dissolving a polyimide resin or a precursor thereof in a solvent to obtain a cast solution containing the polyimide resin or a precursor thereof, A step of coating on a base material, a step of forming a porous structure having a skin layer on the surface on the opposite side of the base material by bringing the casting solution into contact with a non-solvent, and the porous structure body And drying the skin layer by an etching process.

FIG. 4 is a flow diagram schematically showing an example of the steps of the method for producing the porous membrane. A method for producing a porous membrane will be described with reference to the flowchart shown in FIG.

First, a polyimide resin or a precursor thereof is dissolved in a solvent to produce a porous membrane solution (cast solution) containing the polyimide resin or a precursor thereof ((a) in FIG. 4). It is preferable to dissolve the additives described later in the porous membrane solution.

Next, the obtained porous membrane solution is applied onto a substrate ((b) of FIG. 4). The schematic diagram shown in FIG. 4 shows a state of application by a doctor blade.

Next, a porous structure having a skin layer on a surface not in contact with the substrate by a non-solvent induced phase separation method by immersing the substrate coated with the solution for the porous membrane in a coagulation bath containing a non-solvent A body is formed ((c) of FIG. 4). It is preferable that the time from the substrate coated with the porous membrane solution to the immersion in the coagulation bath is 10 seconds or less. If it is 10 seconds or less, the solvent evaporation amount of the applied porous membrane solution is small, and the membrane structure can be easily controlled.

Next, the obtained porous structure is dried at a temperature of 50 to 90 ° C. to remove the solvent ((d) in FIG. 4). Next, although not essential, in this example, the dried porous structure is cured at a temperature of 200 to 300 ° C. ((e) of FIG. 4). Next, the skin layer is removed from the cured porous structure by plasma etching (FIG. 4F). The curing process in FIG. 4E can also be performed after the skin layer removing process in FIG.

(D) Drying step, (e) Curing step, (f) Porous structure or porous membrane from which skin layer has been removed from porous structure by SEM observation or the like after each removal of skin layer The conditions of each step can be optimized so as to obtain a desired porous structure and porous membrane.

<Polyimide resin, precursor>
As the resin which is a material constituting the porous film, a polyimide resin having heat resistance and excellent mechanical strength and chemical resistance is used. As described above, the polyimide resin includes a wide range of polymer compounds containing imide bonds such as polyamideimide, polyesterimide, and polyetherimide, in addition to polyimide, which is a polymer compound containing imide bonds in the repeating unit. Polyamideimide resin can be usually produced by polymerizing by reaction of trimellitic anhydride and diisocyanate, or by reaction of trimellitic anhydride chloride and diamine, and then imidizing. Moreover, a polyimide resin can be manufactured by obtaining a polyamic acid (polyimide-type resin precursor) by reaction with a tetracarboxylic-acid component and a diamine component, and imidating it further, for example. When the porous layer is composed of a polyimide resin, the solubility becomes worse when imidized. Therefore, first, a porous structure is formed at the polyamic acid stage and then imidized (thermal imidization, chemical imidization, etc.). It is preferable.

<Solvent>
The solvent for the porous membrane solution (cast solution) is not particularly limited as long as it has solubility (good solvent for the resin component) according to the chemical skeleton of the resin component to be dissolved. For example, using dimethyl sulfoxide, N, N-dimethylformamide, N, N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), 2-pyrrolidone, γ-butyrolactone, and mixtures thereof Can do.

<Additives>
Addition of a water-soluble additive (water-soluble organic compound or water-soluble polymer) or water to the porous membrane solution is effective for making the porous structure porous.
Examples of the water-soluble additive include diethylene glycol, triethylene glycol, polyethylene glycol, poly N-vinyl pyrrolidone, polyethylene oxide, polyvinyl alcohol, polyacrylic acid, polysaccharides and derivatives thereof, and mixtures thereof. Of these, diethylene glycol, triethylene glycol, polyethylene glycol and poly N-vinyl pyrrolidone are preferable in that they can suppress the formation of coarse pores in the porous structure and improve the mechanical strength of the porous membrane. These water-soluble additives can be used alone or in combination of two or more. In addition, the pore diameter can be adjusted by adding water to the porous membrane solution.

The water-soluble additive is very effective for making the porous structure into a uniform sponge-like porous structure, and various structures can be obtained by changing the type and amount of the water-soluble additive. .
For this reason, a water-soluble additive is very suitably used as an additive for forming a porous film for the purpose of imparting desired pore characteristics.

On the other hand, the water-soluble additive is an unnecessary component that should not be removed and does not ultimately constitute a porous membrane. These unnecessary components are removed by washing in the step of phase conversion by immersion in the coagulation liquid. Furthermore, unnecessary components are removed by heating in a heat treatment step for curing, which will be described later.

However, when the amount of the water-soluble additive is increased, the pore connectivity tends to increase. Therefore, when it is preferable that the connectivity is low, the amount of the water-soluble additive is preferably the minimum amount.
There is a tendency for the strength to decrease as the connectivity increases. Therefore, it is not preferable to add the water-soluble additive in excess of 10 times or more the resin that dissolves the water-soluble additive. Further, an excessive addition of 10 times or more of the resin to be dissolved is not preferable because it requires a longer washing time. The water-soluble additive is not essential and may not be used.

<Porous membrane solution (cast liquid)>
When the compounding quantity of each component in the solution for porous membranes is illustrated, the water-soluble additive is 0 parts by mass or more and 500 parts by mass or less, the water is 0 parts by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the resin component. And the solvent of a resin component is 200 mass parts or more and 900 mass parts or less.
The water-soluble additive is preferably 0 to 400 parts by mass, more preferably 50 to 300 parts by mass, with respect to 100 parts by mass of the resin component. As described above, the pore connectivity increases as the amount of the water-soluble additive is increased. On the other hand, when the amount of the water-soluble additive is less than this value, the possibility that the resin component is precipitated from the solvent is reduced.
Water is preferably 0 to 10 parts by mass, more preferably 0 to 5 parts by mass, relative to 100 parts by mass of the resin component. By making the amount of water below this value, the possibility that the resin component will precipitate from the solvent is reduced.
The solvent of the resin component is preferably 250 parts by mass or more and 700 parts by mass or less, and more preferably 300 parts by mass or more and 500 parts by mass or less with respect to 100 parts by mass of the resin component. By setting the amount of the solvent of the resin component to 250 parts by mass or more, or 300 parts by mass or more and the amount of the solvent of the resin component to 700 parts by mass or less, or 500 parts by mass or less, the viscosity of the casting liquid is lowered. (Specifically, it is difficult to apply by repelling a non-porous substrate, and it is difficult to apply by permeating too much in the case of a porous substrate). It can suppress that the porosity of the made porous structure is too high, and becomes brittle.

If the concentration of the resin component is too low, the thickness of the porous structure becomes insufficient, or desired pore (hole) characteristics are difficult to obtain. On the other hand, if the concentration of the resin component is too high, the porosity tends to decrease. If the concentration of the water-soluble additive is too high, problems such as poor solubility of the resin component in the solution for the porous membrane and a decrease in the strength of the porous membrane are likely to occur. For these reasons, the concentration of the resin component in the porous membrane solution is preferably 10 to 30% by mass. The amount of water added can be used to adjust the pore size.

<Base material>
As the substrate on which the porous membrane solution (cast solution) is applied, for example, a nonporous substrate, a porous substrate, or the like can be used.
The type and roughness of the surface material of the base material affect the ease of peeling of the porous film, the pore diameter, the open area ratio, and the smoothness of the porous film, and therefore it is preferable to select appropriately according to the purpose.

When using a non-porous substrate, for example, a porous membrane solution is applied in a film form on the non-porous substrate. Then, this is immersed in a coagulation liquid (liquid containing a non-solvent), and the film-like porous structure is peeled from the substrate. Then, the obtained film-like porous structure is subjected to drying. Examples of non-porous substrates include glass plates; polyolefin resins such as polyethylene and polypropylene; polyester resins such as polyethylene terephthalate (PET); polycarbonate resins, styrene resins, polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF). Examples include plastic sheets made of fluororesin, vinyl chloride resin, and other resins; metal plates such as stainless steel and aluminum. You may use the composite board which combined the surface material and the internal material with what is different.

When using a porous base material, since this porous base material is used as a constituent member of the water treatment film as it is, it is preferable that the porous base material also has high heat resistance. For example, there are woven fabrics and nonwoven fabrics made of heat-resistant substances such as natural fibers, synthetic fibers, and inorganic fibers.
Examples of porous substrates having heat resistance include aromatic polyamide resins (aramid resins), polyphenylene sulfide resins, liquid crystalline polyester resins, polyimide resins, polyether ether ketone resins (PEEK resins), poly Examples thereof include those made of benzoxazole resin (PBO resin), cellulosic fibers, glass fibers, ceramic fibers, and metal fibers including stainless steel fibers.

<Application>
As means for applying the porous membrane solution (cast solution) onto the substrate, for example, a doctor blade, an applicator, or the like can be used. Depending on the purpose, methods such as spin coating and dip coating can also be used.

<Coagulation liquid (including non-solvent)>
The coagulation-rich liquid used in the non-solvent-induced phase separation method contains a solvent (non-solvent of the resin component) that coagulates the dissolved polyimide resin or its precursor component, and the solvent of the polyimide resin component If it is a solvent miscible with, there will be no restriction | limiting in particular. For example, any solvent that coagulates polyamideimide resin or polyamic acid may be used. For example, water-alcohol such as monohydric alcohol such as methanol and ethanol, polyhydric alcohol such as glycerin; Molecules; water-soluble coagulation liquids such as a mixture thereof can be used.

<Dry>
After the step of forming the porous structure, before removing the skin layer from the porous structure, the porous structure is dried by heating.
The heating temperature and the heating time only have to be able to remove the solvent. For example, it can be 2 to 10 hours at 50 to 90 ° C. Drying can be performed in air | atmosphere, for example. You may carry out in pressure reduction or the atmosphere of inert gas, such as nitrogen and argon.

<Curing>
A curing treatment may be performed between the step of drying the porous structure and the step of removing the skin layer.
Although the dried porous structure can be self-supporting, if the curing process is performed before the etching process for removing the skin layer, there is an advantage that the handling becomes stable.

The curing process may be performed by heat treatment.
What is the heating temperature and heating time for curing? For example, it can be performed at 200 to 300 ° C. for 1 to 24 hours.
The heat treatment for curing can be performed in an argon atmosphere, for example. You may carry out under reduced pressure or nitrogen gas and an atmospheric condition.

<Etching>
The etching method is not particularly limited as long as the skin layer can be removed from the porous structure. For example, plasma etching or etching with a chemical such as an alkaline solution can be used. Plasma etching is easy to control the amount of scraping from the porous structure.
As described above, in the porous structure produced by the non-solvent induced phase separation method, the pore diameter continuously changes from the skin layer to the inside of the porous layer (first layer). Therefore, by controlling the scraping amount, the average hole diameter exposed on the scraped surface can be controlled. For example, a porous film having a desired water permeability can be obtained by controlling the amount to be scraped off.

Plasma etching can be performed using parallel plate electrodes. According to the plasma etching using parallel plate electrodes, the skin layer parallel to the surface of the porous structure can be removed.

As plasma etching, oxygen plasma etching, plasma etching with argon or nitrogen can be used.
When oxygen plasma etching is used, the surface of the porous film can be hydrophilized and the water permeability can be further improved.

According to the method for producing a porous membrane described above, a porous membrane in which the pore diameter of the pores on the surface is controlled can be produced by controlling the amount scraped from the surface of the porous structure. That is, since the porous structure includes pores having a large average pore diameter gradually from the surface toward the inside of the porous film, the pore diameter on the surface can be increased by increasing the amount of scraping.

Hereinafter, the present invention will be specifically described by way of examples. However, the present invention is not limited only to these examples.

Example 1
Polyamideimide resin (Toluon 4000T-LV (trade name) manufactured by Solvay Advanced Polymers) was dried in a vacuum dryer at 170 ° C. for 12 hours. 100 parts by weight of polyamideimide resin after drying is put into 350 parts by weight of NMP (Wako Pure Chemical Industries, Ltd., Wako Special Grade), mixed in a glass bottle for 5 days with a rotary mixer at 50 ° C., dissolved, and polyamide An imide resin solution was prepared.

Next, a predetermined amount of 100 parts by mass of a polyamideimide resin solution and PEG200 ((polyethylene glycol 200) (manufactured by Wako Pure Chemical Industries, Ltd., Wako First Grade)) is weighed into a PP (polypropylene) container, and a planetary agitator (stock) A porous membrane solution (casting solution) was prepared by mixing and defoaming using a product name “Awatori Nertaro (registered trademark) Shinky ARE-310” manufactured by Shinky Co., Ltd.

Next, using a PTFE (polytetrafluoroethylene) tape (thickness 180 μm, width 5 mm) as a gap spacer on a glass plate (base material), the prepared porous membrane solution was applied in a range of 70 mm × 100 mm using a doctor blade. did. Immediately after coating, a glass plate was put into a tank filled with pure water, and the coating film was gelled by non-solvent induced phase separation (NIPS). The coating film naturally separated from the glass plate by gelation.

Next, the peeled coating film was sandwiched between cellulose 5C filter papers, put into a dryer with glass plates sandwiched from above and below, and dried at 80 ° C. for 4 hours in an air atmosphere. Thereafter, the sheet was taken out from the filter paper, sandwiched between grapho foils GTA (trade name), transferred to a gas purge furnace, and cured by being kept at 280 ° C. for 1 hour in an argon atmosphere.

Through the above-described steps, a polyamideimide porous structure having a skin layer (pores communicated) was obtained.
The polyamideimide porous structure having this skin layer was subjected to 50 W of high frequency plasma treatment in an oxygen atmosphere using a parallel plate high frequency plasma processing apparatus (manufactured by Samco, RF plasma etching system FA-1 (product name)). For 60 seconds. The results are shown in Table 1.

(Comparative Example 1)
The production conditions were the same as those of Example 1 except that oxygen plasma etching was not performed.

As shown in Table 1, the porous film that was not subjected to oxygen plasma etching (the porous structure as it was) could not permeate water and did not pass solute.

In Table 1, the abbreviations are as follows.
(A) Solvent A-1: NMP (N-methyl-2-pyrrolidone)
(B) Polyimide resin B-1: LV is a polyamideimide manufactured by Solvay Specialty Polymers (Torlon 4000T-LV (trade name)).
B-2: AI-002 is a polyamideimide manufactured by Hitachi Chemical Co., Ltd.
(C) Additive C-1: DEG is a Wako special grade diethylene glycol manufactured by Wako Pure Chemical Industries, Ltd.
C-2: PEG200 (trade name) is a polyethylene glycol having a mean molecular weight of 180 to 220, manufactured by Wako Pure Chemical Industries, Ltd.
C-3: PVPK30 is poly N-vinylpyrrolidone manufactured by Nippon Shokubai Co., Ltd.
C-4: PVPK85 is poly N-vinylpyrrolidone manufactured by Nippon Shokubai Co., Ltd.
[Base material]
SUS is a stainless steel fiber filter manufactured by Nippon Seisen Co., Ltd. (Naslon filter (trade name)).
[Solute]
BD is Blue Dextran 2000 (trade name) having a molecular weight of 2,000,000 manufactured by GE Healthcare Japan. The hydrodynamic diameter is 50-60 nm.
Cytochrome C is Cytochrome c manufactured by Wako Pure Chemical Industries, Ltd. for biochemistry. It is derived from the equine heart and is TCA treated. The hydrodynamic diameter is 3-5 nm.
DR80 is Direct Red 80 manufactured by Tokyo Chemical Industry Co., Ltd. The hydrodynamic diameter is 1-2 nm.

FIG. 5A shows an SEM image taken from the surface side (the side not in contact with the substrate) of the porous membrane produced in Example 1. FIG.
FIG. 5B is an SEM image of Comparative Example 1 taken from the surface side of a porous film that was simply subjected to heat treatment in an argon atmosphere for the same time as Example 1 without an oxygen plasma etching process.
In both FIGS. 5A and 5B, the lower SEM image is enlarged 10 times the upper SEM image. The upper SEM image has a scale of 1 μm, and the lower SEM image has a scale of 0.1 μm.
From the SEM image of FIG. 5 (a), the pores on the surface of the porous membrane produced in Example 1 have an exposed maximum pore size of about 200 nm and an average pore size of about 50 nm. I understand.
On the other hand, from the SEM image of Comparative Example 1 in FIG. 5 (b), without performing the oxygen plasma etching process, simply performing the heat treatment in the argon atmosphere for the same time as Example 1, It turned out that the surface of the hole diameter required for a porous membrane is not obtained.

In the porous membrane produced in Example 1, the water permeability was 40 [m ³ / (m ² · day · MPa)]. In addition, BD could be separated, but cytochrome C and DR80 could not be separated.

FIG. 6A shows the result of XPS measurement on the surface before the oxygen plasma etching process (Comparative Example 1). FIG. 6B shows the surface after the oxygen plasma etching process (after removing the skin layer; Example 1). The result of having performed XPS measurement about is shown. In the graphs of FIG. 6A and FIG. 6B, the horizontal axis is the binding energy (eV), and the vertical axis is the intensity (cps: counts per second). Table 2 shows the results of calculating the concentrations of C, N, and O on the surface based on the XPS measurement results.

Before and after the oxygen plasma etching treatment, the O / C on the surface was 70% or more, from 0.17 to 0.30.
It was found that the surface was oxidized by the oxygen plasma etching performed for removing the skin layer.

(Example 2)
The production conditions were the same as those of Example 1 except that the additive was DEG and the oxygen plasma etching time was 30 seconds.

In the porous membrane produced in Example 2, the water permeability was 2.5 [m ³ / (m ² · day · MPa)].
Moreover, all of BD, cytochrome C, and DR80 were separable.
Thus, the porous film of the present invention could be produced even when the additive and plasma etching time were changed.

FIG. 7A shows an SEM image taken from the surface side of the porous membrane produced in Example 2. FIG.
FIG. 7B is an SEM image taken from the surface side of the porous membrane manufactured under the same conditions as in Example 2 except that the oxygen plasma etching time was 60 seconds.
In both FIGS. 7A and 7B, the lower SEM image is 10 times larger than the upper SEM image. The upper SEM image has a scale of 1 μm, and the lower SEM image has a scale of 0.1 μm.
From the SEM image of FIG. 7 (a), the pores on the surface of the porous membrane produced in Example 2 are exposed with a maximum pore size of about 70 nm and an average pore size of about 20 nm. I understand.
On the other hand, from the SEM image of FIG. 7B, by making the oxygen plasma etching time twice that of Example 2, the diameter of the holes on the exposed surface is considerably larger than that of Example 2. You can see that it is getting bigger. Thus, it can be seen that the diameter of the surface holes and the surface irregularities can be controlled by changing the depth to be scraped depending on the plasma etching time.

(Example 3)
The production conditions were the same as those of Example 1 except that the polyimide resin was AI002.

The porous membrane prepared in Example 3, water permeability was ^{5 [m 3 / (m 2} · day · MPa)]. Further, BD and cytochrome C could be separated, but DR80 could not be separated.
Thus, even if it changed the polyimide-type resin, the predetermined porous film was able to be manufactured.

Example 4
The production conditions were the same as those of Example 1 except that the additive was PVPK85 in addition to PEG200 and the oxygen plasma etching time was 180 seconds.

In the porous membrane produced in Example 4, the water permeability was 10 [m ³ / (m ² · day · MPa)]. Further, BD and cytochrome C could be separated, but DR80 could not be separated.
Thus, even if the additive and the plasma etching time were changed, a predetermined porous film could be produced.

(Example 5)
The production conditions were the same as those of Example 1 except that the additive was PVPK30 in addition to PEG200 and the oxygen plasma etching time was 120 seconds.

In the porous membrane produced in Example 5, the water permeability was 1.5 [m ³ / (m ² · day · MPa)]. Further, BD and cytochrome C could be separated, but DR80 could not be separated.
Thus, even if the additive and the plasma etching time were changed, a predetermined porous film could be produced.

(Example 6)
The production conditions were the same as those in Example 1 except that the coated substrate was porous SUS (stainless fiber). Unlike a glass plate substrate, the porous SUS substrate does not naturally peel off the coating film due to NIPS. Therefore, drying, curing, and etching were performed in a state where the coating film was attached to a porous SUS substrate.

In the porous membrane produced in Example 6, the water permeability was 30 [m ³ / (m ² · day · MPa)]. In addition, BD could be separated, but cytochrome C and DR80 could not be separated.
Thus, even if the kind of base material was changed, the predetermined porous membrane could be manufactured.

Claims (13)

1. Dissolving a polyimide resin or a precursor thereof in a solvent to obtain a casting liquid containing the polyimide resin or a precursor thereof;
   Applying the casting solution onto a substrate;
   Forming a porous structure having a skin layer on the surface not in contact with the substrate by bringing the casting liquid into contact with a non-solvent;
   Drying the porous structure;
   Removing the skin layer from the dried porous structure.
2. The method for producing a porous film according to claim 1, wherein the step of removing the skin layer is performed by an etching process.
3. The method for producing a porous film according to claim 1 or 2, wherein a curing treatment is performed between the drying step and the step of removing the skin layer.
4. The method for producing a porous membrane according to claim 3, wherein the curing treatment is a heat treatment.
5. The method for producing a porous film according to any one of claims 2 to 4, wherein the etching treatment is a plasma treatment.
6. The method for producing a porous film according to claim 5, wherein the plasma is oxygen plasma generated at a high frequency.
7. The method for producing a porous membrane according to any one of claims 1 to 6, wherein the casting solution contains a water-soluble compound.
8. A layer mainly composed of a polyimide resin and having finger-like holes;
   The porous membrane which consists of a porous layer which has two layers which do not have a finger-like hole which pinches | interposes this layer.
9. It is composed of a polyimide resin as a main component, a porous layer having a layer having finger-like holes, and two layers having no finger-like holes sandwiching the layer, and one of the two layers. A porous membrane with an oxidized surface.
10. The porous membrane according to claim 8 or 9, wherein water permeability is 0.5 [m ³ / (m ² · day · MPa)] or more.
11. The porous membrane according to any one of claims 8 to 10, which has a water permeability of 200 [m ³ / (m ² · day · MPa)] or less.
12. 12. The porous membrane according to claim 8, wherein Blue Dextran 2000 (trade name) can be separated in a solute removal test.
13. A water treatment membrane comprising the porous membrane according to any one of claims 8 to 12.

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