WO2021132397A1

WO2021132397A1 - Separation membrane and method for producing same

Info

Publication number: WO2021132397A1
Application number: PCT/JP2020/048296
Authority: WO
Inventors: 皓一高田; 弘希栄村; 万里奈大塚; 花川　正行
Original assignee: 東レ株式会社
Priority date: 2019-12-23
Filing date: 2020-12-23
Publication date: 2021-07-01
Also published as: JPWO2021132397A1; CN114828992B; KR20220112792A; CN114828992A; JP7107429B2

Abstract

The present invention provides a separation membrane which contains a cellulose ester, while having a plurality of voids in a cross-section that is parallel to the longitudinal direction and the membrane thickness direction of the membrane, wherein: the average depth D of the plurality of voids is from 0.7 to 20 μm; the average length L of the plurality of voids is 3 μm or more; and the average of the ratios of respective lengths l to respective depth d of the voids, namely (l/d)_a is from 2 to 40.

Description

Separation membrane and its manufacturing method

The present invention relates to a separation membrane and a method for producing the same.

In recent years, separation membranes include water treatment membranes for water purification and wastewater treatment, medical membranes for blood purification, food industry membranes, battery separator membranes, charged membranes, electrolyte membranes for fuel cells, and the like. It is used in various fields.

Most separation membranes are made of polymer. Among them, cellulosic resins such as cellulose esters have permeation performance due to their hydrophilicity and chlorine resistance that is resistant to chlorine-based bactericides, so separation membranes such as water treatment membranes. It is widely used as a material for.

For example, Patent Document 1 discloses a technique for obtaining a hollow filament-shaped separation membrane by discharging a membrane-forming stock solution containing cellulose triacetate into a coagulating liquid consisting of a solvent, a non-solvent, and water for phase separation. ..
Further, Patent Document 2 discloses a hollow filament-like separation membrane in which hydroxyalkyl cellulose is fixed on the surface in the state of fine particles.

Japanese Patent Application Laid-Open No. 2011-235204 Japanese Patent Application Laid-Open No. 2015-157278

However, in the conventional separation membrane using cellulose ester as a material, the size of the voids is reduced in order to improve the separation performance, so that it is necessary to reduce the thickness of the membrane in order to improve the permeation performance, and as a result. As a result, there is a problem that defects are likely to occur in the separation membrane.

Therefore, an object of the present invention is to provide a separation membrane and a method for producing the same, which have both high separation performance and permeation performance.

As a result of diligent studies to solve the above problems, the present inventors have made it possible to improve the permeation performance while maintaining high separation performance by having the separation membrane containing the cellulose ester having voids satisfying specific conditions. It was found that the present invention was completed.

That is, the present invention relates to the following [1] to [10].
[1] A separation membrane containing a cellulose ester.
The separation membrane has a plurality of voids in a cross section parallel to the longitudinal direction and the film thickness direction of the membrane.
The average depth D of the plurality of voids is 0.7 to 20 μm.
The average length L of the plurality of voids is 3 μm or more, and
_{A separation membrane in which the average value (l / d) a} of the ratio of the length l to the depth d of each void is 2 to 40.
[2] The separation membrane according to the above [1], wherein the occupancy rate of the plurality of voids in the cross section is 15 to 55%.
[3] The separation membrane according to the above [1] or [2], wherein the average thickness of the wall portion in the cross section is 0.7 to 5.0 μm.
[4] The separation membrane according to any one of [1] to [3] above, wherein the average pore diameter of the surface pores is 0.050 to 0.500 μm on at least one surface.
[5] On at least one surface, the average minor axis X of the surface holes is 0.030 to 0.250 μm, and the average major axis Y of the surface holes is 0.060 to 0.450 μm. The separation membrane according to any one of the above [1] to [4], wherein the average value (y / x) _{a of the diameter ratio is 1.00 to 1.50.}
[6] The separation membrane according to any one of [1] to [5], wherein the longitudinal direction of the plurality of voids is along the longitudinal direction of the separation membrane.
[7] The separation membrane according to any one of [1] to [6] above, which contains cellulose acetate propionate and / or cellulose acetate butyrate as the cellulose ester.
[8] The separation membrane according to any one of the above [1] to [7], which has a hollow fiber shape.
[9] (1) A mixture containing 10 to 80% by mass of cellulose ester, 10 to 80% by mass of a structure forming agent, and 2 to 20% by mass of a void forming agent is melt-kneaded to form a resin composition. Getting things, the preparation process,
(2) A molding step of using a filter having a pore size of 40 to 200 μm and discharging the resin composition from a discharge port to obtain a resin molded product at a draft ratio of 30 to 200.
(3) said resin molded product is immersed in a solvent in the range of solubility parameters distance D _S 10-25 to cellulose ester, and a dipping process, a manufacturing method of the separation membrane.
[10] The method for producing a separation membrane according to the above [9], wherein the temperature of the resin molded product in the dipping step is 50 to 80 ° C.

According to the present invention, it is possible to provide a separation membrane and a method for producing the same, which have both high separation performance and permeation performance.

FIG. 1A is a drawing schematically showing a cross section Z and an internal structure of a separation membrane, FIG. 1B is a side view of FIG. 1A, and FIG. 1C is a top view of FIG. 1A. .. FIG. 2 is an example of an image in which the cross section Z is captured by SEM. FIG. 3 is an image obtained by removing noise from the image of FIG. 2, binarizing the image, and extracting voids. FIG. 4 is an image obtained by further extracting the outline of the void from the image of FIG.

The separation membrane of the present invention is a separation membrane containing a cellulose ester, and has a plurality of voids in a cross section parallel to the longitudinal direction and the film thickness direction of the membrane, and the average depth D of the plurality of voids is determined. The average length L of the plurality of voids is 0.7 to 20 μm, the average length L of the plurality of voids is 3 μm or more, and the average value (l / d) _a of the ratio of the length l and the depth d of each void is It is characterized by being 2 to 40. In the present specification, the mass-based ratio (percentage, parts, etc.) is the same as the weight-based ratio (percentage, parts, etc.).

(Resin composition constituting the separation membrane)
The resin composition constituting the separation membrane of the present invention contains the cellulose ester shown in (1) below. In addition to (1), the following components (2) to (6) can be contained.

(1) Cellulose ester The separation membrane of the present invention needs to contain a cellulose ester. In order to further enhance the effect of the present invention, the separation membrane of the present invention preferably contains cellulose ester as a main component. The main component here means the component contained most in terms of mass among all the components of the resin composition constituting the separation membrane.

Examples of the cellulose ester include a cellulose ester such as cellulose acetate, cellulose propionate or cellulose butyrate, or a cellulose mixed ester such as cellulose acetate propionate or cellulose acetate butyrate. Among them, from the viewpoint of processability of the resin molded product and the film strength of the obtained separation film, a cellulose mixed ester is preferable, a cellulose acetate propionate and / or a cellulose acetate butyrate is more preferable, and a cellulose acetate propionate is further preferable. .. The cellulose acetate propionate here is a cellulose ester having an average degree of substitution of an acetyl group and a propionyl group of 0.1 or more, respectively.

The weight average molecular weight (Mw) of the cellulose ester is preferably 50,000 to 250,000. When the weight average molecular weight (Mw) is 50,000 or more, the thermal decomposition of the cellulose ester at the time of melting during the production of the separation membrane is suppressed, and the membrane strength of the separation membrane can be easily reached to a practical level. .. When the weight average molecular weight (Mw) is 250,000 or less, the melt viscosity does not become too high, so that stable melt film formation becomes possible. The weight average molecular weight (Mw) is a value calculated by GPC (gel permeation chromatography) measurement. The calculation method will be described in detail in Examples.

Each of the exemplified cellulose mixed esters has an acetyl group and another acyl group (propionyl group, butyryl group, etc.). In the cellulose mixed ester contained in the separation membrane, the average degree of substitution of the acetyl group with another acyl group preferably satisfies the following formula.
1.0 ≤ (average degree of substitution of acetyl group + average degree of substitution of other acyl groups) ≤ 3.0
0.1 ≤ (average degree of substitution of acetyl groups) ≤ 2.6
0.1 ≤ (average degree of substitution of other acyl groups) ≤ 2.6

When the above formula is satisfied, the permeation performance of the separation membrane and the heat fluidity when melting the resin composition constituting the separation membrane are improved. The average degree of substitution refers to the number of acyl groups (acetyl group + other acyl groups) chemically bonded among the three hydroxyl groups existing per glucose unit of cellulose.
The separation membrane may contain only one type of cellulose ester, or may contain two or more types of cellulose esters.

The content of the cellulose ester in the separation membrane is preferably 70 to 100% by mass, more preferably 80 to 100% by mass, still more preferably 90 to 100% by mass, when all the components of the separation membrane are 100% by mass. .. When the content of the cellulose ester in the separation membrane is 70% by mass or more, the membrane strength of the separation membrane becomes sufficient.

Further, the content of the cellulose ester in the raw material for producing the separation membrane is preferably 10 to 80% by mass when the total of the components constituting the raw material is 100% by mass. When the content is 10% by mass or more, the membrane strength of the separation membrane becomes good. On the other hand, when the content is 80% by mass or less, the thermoplasticity and permeation performance of the separation membrane are improved. The content is more preferably 15% by mass or more, and further preferably 20% by mass or more. The content is more preferably 70% by mass or less, further preferably 60% by mass or less, and particularly preferably 45% by mass or less.

(2) Cellulose Ester Plasticizer The resin composition constituting the separation membrane of the present invention may contain a cellulose ester plasticizer.
The plasticizer of the cellulose ester is not particularly limited as long as it is a compound that thermally plasticizes the cellulose ester. Further, not only one kind of plasticizer but also two or more kinds of plasticizers may be used in combination.

Examples of the plasticizer for the cellulose ester include polyalkylene glycol compounds such as polyethylene glycol and polyethylene glycol fatty acid ester, glycerin compounds such as glycerin fatty acid ester and diglycerin fatty acid ester, citric acid ester compounds, and phosphoric acid ester compounds. Alternatively, a fatty acid ester compound such as adipic acid ester or a caprolactone compound, or a derivative thereof and the like can be mentioned.

Examples of the polyalkylene glycol-based compound include polyethylene glycol, polypropylene glycol, polybutylene glycol, and the like having a weight average molecular weight (Mw) of 400 to 4,000.

After forming the separation membrane, the cellulose ester plasticizer may remain in the separation membrane or may be eluted from the separation membrane. The content of the plasticizer of the cellulose ester is preferably 5 to 40% by mass when the total amount of the components constituting the raw material is 100% by mass.
When the content is 5% by mass or more, the thermoplasticity of the cellulose ester becomes good. On the other hand, when the content is 40% by mass or less, the membrane strength of the separation membrane becomes good. The content of the plasticizer of the cellulose ester is more preferably 5 to 35% by mass, further preferably 5 to 30% by mass.

(3) Antioxidant The resin composition constituting the separation membrane of the present invention preferably contains an antioxidant. When the resin composition contains an antioxidant, thermal decomposition at the time of melting the polymer during the production of the separation film is suppressed, the membrane strength of the resulting separation film is improved, and the coloring of the separation film is suppressed. Will be done.

As the antioxidant, a phosphorus-based antioxidant is preferable, and a pentaerythritol-based compound is more preferable. Examples of the pentaerythritol-based compound include bis (2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite and the like.

The content of the antioxidant is preferably 0.005 to 0.500% by mass when the total amount of the components constituting the raw material is 100% by mass. When the content of the antioxidant is in the above range, a uniform resin composition can be obtained in the preparation step.

(4) Structure-forming agent The resin composition constituting the separation membrane of the present invention may contain a structure-forming agent.
The structure-forming agent in the present invention is not particularly limited as long as it is partially compatible with the cellulose ester or a mixture of the cellulose ester and its plasticizer and can be eluted or decomposed by a solvent that does not dissolve the cellulose ester. The weight average molecular weight of the structure-forming agent is preferably 1000 or more from the viewpoint of appropriately controlling the values of _{L, D, and (l / d) a, which will be described later.}
Partial compatibility means that two or more substances are completely compatible under certain conditions, but phase-separated under different conditions. The structure-forming agent is a substance that phase-separates from the cellulose ester when it comes into contact with a solvent that satisfies specific conditions in the dipping step described later. Specific conditions will be described later.

The structure-forming agent in the present invention is preferably a hydrophilic compound because it can be easily eluted. Here, the hydrophilic compound means a compound that is soluble in water or has a contact angle with water smaller than that of the cellulose ester contained in the separation membrane. Among the hydrophilic compounds, a compound that dissolves in water is particularly preferable because it can be easily eluted.

Examples of the structure-forming agent include PVP-based copolymers such as polyvinylpyrrolidone (hereinafter, “PVP”), PVP / vinyl acetate copolymer, PVP / methyl methacrylate copolymer, polyvinyl alcohol, or Examples include polyester compounds.
When PVP is used as a structure-forming agent, it is difficult to elute it from the separation membrane when thermal cross-linking occurs, so that intermolecular cross-linking is relatively difficult to proceed and it is possible to elute even if cross-linked. The weight average molecular weight (Mw) is preferably 20,000 or less. It is also preferable to use the above-mentioned PVP-based copolymer because thermal cross-linking is suppressed.

By eluting at least a part of the structure-forming agent in the steps after the dipping step described later, the traces of the structure-forming agent being removed (eluted) become voids in the film, and as a result, the permeation performance is improved.

The content of the structure-forming agent is preferably 10 to 80% by mass when the total content of the components constituting the raw material is 100% by mass.
When the content is 10% by mass or more, the permeation performance of the separation membrane becomes good. On the other hand, when the content is 80% by mass or less, the film strength becomes good. The content of the structure forming agent is more preferably 20% by mass or more, further preferably 25% by mass or more. The content of the structure-forming agent is more preferably 75% by mass or less, and further preferably 70% by mass or less.

(5) Void-forming agent The resin composition constituting the separation membrane of the present invention may contain a void-forming agent. Here, the void forming agent refers to a compound that is incompatible with the cellulose ester and is plasticized or melted by heat. By eluting the void-forming agent that is incompatible with the cellulose ester, voids are formed at the site where the void-forming agent was present. Further, when the void forming agent is plasticized or melted by heat, the value of (l / d) _a , which is the average value of the ratio of the length l and the depth d of each void formed, can be increased.

Examples of the void forming agent include phthalate ester compounds, trimellitic acid ester compounds, polyalkylene glycol compounds such as polyethylene glycol, polypropylene glycol and polybutylene glycol, and derivatives of these compounds. The weight average molecular weight (Mw) of these compounds is preferably 100,000 to 1,000,000. Since it is easy to control the values of L, D, and (l / d) _a described later by showing an appropriate viscosity when heated, the weight average molecular weight (Mw) of the void forming agent is preferably 100,000 to 1,000,000. 10,000 to 500,000 is more preferable, and 100,000 to 300,000 is particularly preferable.

The content of the void forming agent is preferably 2 to 20% by mass when the total content of the components constituting the raw material is 100% by mass. When the content is 2% by mass or more, the permeation performance of the separation membrane is good. On the other hand, when the content is 20% by mass or less, the separation performance becomes good. The content of the void forming agent is more preferably 3% by mass or more, further preferably 5% by mass or more, and particularly preferably 10% by mass or more. The content is more preferably 18% by mass or less, and further preferably 15% by mass or less.

(6) Additives The resin composition constituting the separation membrane of the present invention may contain additives other than those described in (2) to (5) as long as the effects of the present invention are not impaired.

Additives include, for example, cellulose ether, polyacrylonitrile, polyolefin, polyvinyl compound, polycarbonate, poly (meth) acrylate, polysulfone, resin such as polyethersulfone, organic lubricant, crystal nucleating agent, organic particles, inorganic particles, terminal closure. Agents, chain extenders, UV absorbers, infrared absorbers, color inhibitors, matting agents, antibacterial agents, antistatic agents, deodorants, flame retardants, weather resistant agents, antistatic agents, antioxidants, ion exchangers , Antifoaming agents, color pigments, fluorescent whitening agents, dyes and the like.

(Shape of separation membrane)
The shape of the separation membrane of the present invention is not particularly limited, but a hollow fiber-shaped separation membrane (hereinafter, “hollow fiber membrane”) or a flat-shaped membrane (hereinafter, “flat membrane”) is preferably adopted. Above all, the hollow fiber membrane can be efficiently filled in the module, and the effective film area per unit volume of the module can be increased, which is more preferable.

The thickness of the separation membrane is preferably 10 to 500 μm from the viewpoint of achieving both permeation performance and membrane strength. Further, the thickness is more preferably 30 μm or more, and further preferably 50 μm or more. The thickness is more preferably 400 μm or less, further preferably 300 μm or less.

In the case of a hollow fiber membrane, the outer diameter of the hollow fiber membrane is preferably 50 to 2500 μm from the viewpoint of achieving both the effective membrane area when filled in the module and the membrane strength. The outer shape of the hollow fiber membrane is more preferably 100 μm or more, further preferably 200 μm or more, and particularly preferably 300 μm or more. Further, the outer shape is more preferably 2000 μm or less, further preferably 1500 μm or less, and particularly preferably 1000 μm or less.

Further, in the case of the hollow fiber membrane, the hollow ratio of the hollow fiber is preferably 15 to 70% from the relationship between the pressure loss of the fluid flowing through the hollow portion and the buckling pressure. The hollow ratio is more preferably 20% or more, further preferably 25% or more. Further, the hollow ratio is more preferably 65% or less, and further preferably 60% or less.

The method of setting the outer diameter and the hollow ratio of the hollow fiber in the hollow fiber membrane in the above range is not particularly limited, but can be calculated by, for example, the shape of the discharge hole of the spinneret for producing the hollow fiber, or the take-up speed / discharge speed. It can be adjusted by changing the draft ratio as appropriate.

(Cross-sectional structure of separation membrane)
In the separation membrane of the present invention, the average depth D, the average length L, and the average value of l / d (l / d) _a , which is the ratio of the depth d and the length l of each void, are in a specific range. It has a plurality of voids. The depth and length of the voids are values measured in a cross section (hereinafter, "cross section Z") parallel to the longitudinal direction and the film thickness direction of the separation membrane to be measured. Here, the longitudinal direction of the membrane is the direction parallel to the central axis in the hollow fiber membrane, and the mechanical direction at the time of manufacture in the flat membrane. FIG. 1A is a drawing schematically showing a cross section Z and an internal structure of the separation membrane when the separation membrane has a hollow fiber shape. FIG. 1B is a side view of FIG. 1A, and FIG. 1C is a top view of FIG. 1A. In FIG. 1, C indicates a central axis, and the central axis C is parallel to the longitudinal direction of the film. Further, in FIG. 1 (c), the bidirectional arrow exemplifies the film thickness direction of the hollow fiber membrane, and the dotted line indicates the direction parallel to the film thickness direction.

Here, the “void” refers to a recess having ^{an area of 1 μm 2} or more when the cross section Z is observed with a scanning electron microscope (hereinafter, “SEM”) at a magnification of 2,000 times. The detailed observation method is described in (7) Measurement for a plurality of voids and walls in Examples. The "recess" here means a dark part in the image observed by SEM, and the outline of the image captured by SEM is extracted by binarizing (binarizing Huang) using image analysis software. can do.
Specifically, first, using imageJ, which is an image analysis software, an image captured by SEM is converted into 8 bits, and all pixels are replaced with the center value of 3 × 3 pixels in the vicinity of the pixel. After 10 times, Hung is binarized. Subsequently, the obtained image can be processed as a Mask display by setting Size to 0-Infinity and Circularity to 0-1 in the ImageJ Analysis Particles command, so that an image in which the recesses are extracted can be acquired. Based on the image obtained in this way, the contour of the concave portion can be extracted. Specifically, in the ImageJ Analysis Particles command, the size is set to 0-Infinity and the Circularity is set to 0-1 and the contour of the concave portion can be extracted by processing as a Barre Outline display.
Further, the void extraction can be carried out by setting the lower limit of the size so that the recesses of ^{1 μm 2} or more are included in the above-mentioned extraction of the recesses. For example, in ^{an image of 1 μm 2} = 100 pixels ² , the void can be extracted by setting the lower limit to 100 pixels ^2. In the image thus obtained, the contour of the void can be extracted by performing the same processing as the extraction of the contour of the concave portion described above. In the present application, the contour may be referred to as an outer edge.

An example of an image captured by SEM is shown in FIG. 2, an image of FIG. 2 is noise-removed and binarized to extract voids in FIG. 3, and an image obtained by extracting the outline of voids from the image of FIG. 3 is shown in FIG. , Respectively.

When the cross section Z is observed at a magnification of 5,000 times using SEM, when the ratio of the sum of the areas of all the recesses to the entire observation range is defined as the "recess area ratio", the permeation flux and separation performance The recessed area ratio of the separation membrane of the present invention is preferably 50 to 85%, more preferably 60 to 80%. The recesses here are not limited to recesses having an area of 1 μm ² or more, that is, voids, and recesses having an area of less ^{than 1 μm 2} , that is, pores are also targeted. Similarly to the above, the concave portion here can also be extracted by removing noise and binarizing (binarizing Huang) the image captured by SEM using image analysis software such as ImageJ. it can.

"Void depth d" refers to a void to be measured when the cross-section Z is observed at a magnification of 2,000 times using an SEM and the film thickness direction of the separation membrane is the depth direction. The maximum length in the depth direction. Further, the "gap length l" is a straight line capable of directly connecting two points on the outer edge of the gap to be measured when the cross section Z is observed at a magnification of 2,000 times using the same SEM. Of these, the length of the longest straight line. Here, the straight line that can directly connect two points on the outer edge means a straight line that connects two points on the outer edge and does not pass over the other outer edge. _{The average value (l / d) a} of l / d, which is the ratio of the length l to the depth d of each void, is calculated as l / d for each void in 30 randomly selected voids. , The value obtained by taking the arithmetic mean value.

The average depth D of a plurality of voids is an arithmetic mean value obtained by measuring the depths of 30 randomly selected voids when the cross section Z is observed at a magnification of 2,000 times using an SEM. The value calculated as. Further, the average length L of a plurality of voids is calculated by measuring the lengths of 30 randomly selected voids when the cross section Z is observed at a magnification of 2,000 times using the same SEM. A value calculated as an average value.

_{The value of the average value (l / d) a} of l / d, which is the ratio of the length l to the depth d of each void, needs to be 2 to 40. When the value of (l / d) _a is in this range, the voids are appropriately dispersed while reducing the substantial thickness of the separation membrane, so that the separation membrane exhibits excellent permeation performance and separation performance. It is presumed to be. The value of (l / d) _a is preferably 3 to 20, more preferably 4 to 20, and even more preferably 8 to 20. Among them, setting it to 4 to 20 makes it possible to achieve both particularly high transmission performance and high separation performance, and setting it to 8 to 20 achieves both extremely high transparency and high separation performance. be able to.

The average depth D of the plurality of voids needs to be 0.7 to 20 μm in order to appropriately disperse the voids and reduce the substantial thickness of the separation membrane. The average depth D of the plurality of voids is preferably 0.8 μm or more, more preferably 1.0 μm or more. The average depth D of the plurality of voids is preferably 5.0 μm or less, more preferably 2.0 μm or less.

The average length L of the plurality of voids needs to be 3 μm or more in order to appropriately disperse the voids. The average length L of the plurality of voids is preferably 4 μm or more, more preferably 5 μm or more, and further preferably 10 μm or more. Further, the average length L of the plurality of voids is preferably 50 μm or less, and more preferably 30 μm or less.

The longitudinal direction of the plurality of voids is preferably along the longitudinal direction of the separation membrane. Since the longitudinal directions of the plurality of voids are substantially parallel, even if the separation membrane is bent in the longitudinal directions of the plurality of voids, the stress is easily dispersed and the membrane strength of the separation membrane is increased. The cross section Z was observed at a magnification of 2,000 times using SEM, the direction of the length l of each void in the 30 randomly selected voids and the angle formed by the longitudinal direction of the membrane were calculated, and the arithmetic was performed. When the average value (hereinafter, sometimes referred to as "angle in the longitudinal direction of a plurality of voids") is within 20 °, it is determined that the longitudinal direction of the plurality of voids is along the longitudinal direction of the separation membrane. can do. The longitudinal angle of the plurality of voids is preferably within 15 °, more preferably within 10 °.

The occupancy of a plurality of voids in the cross section of the separation membrane of the present invention parallel to the longitudinal direction and the film thickness direction is 15 to 15 in order to appropriately disperse the voids while further suppressing the substantial thickness of the separation membrane. It is preferably 55%, more preferably 18 to 50%, even more preferably 20 to 50%, particularly preferably 30 to 50%, and most preferably 40 to 50%. preferable. Here, the "occupancy rate of a plurality of voids" is the sum of the areas of all the voids occupying _{the area S 0} of the entire observation range when the cross section Z is observed at a magnification of 2,000 times using SEM. It refers to the percentage of a certain _{S 1.}

The average thickness of the wall portion in the cross section Z of the separation membrane is 0.7 to 5.0 μm in order to obtain good separation performance by appropriately dispersing the voids while further suppressing the substantial thickness of the separation membrane. Preferably, 1.0 to 4.0 μm is more preferable, and 1.0 to 3.0 μm is even more preferable. Above all, by setting it to 1.0 to 3.0 μm, it is possible to achieve both particularly excellent transmission performance and separation performance. Here, the "wall portion" of the hollow fiber membrane refers to a portion other than the void when the cross section Z is observed at a magnification of 2,000 times using an SEM (FIG. 1). In the above observation, the "average thickness of the wall portion" is a straight line that passes through the center of the observed image and is perpendicular to the longitudinal direction of the separation membrane, and is parallel to each other at intervals of 20 μm on both sides of the straight line. When subtracted, it means the average value of the length of each wall on these straight lines.

(Shape of surface hole)
The separation membrane of the present invention preferably has an average pore diameter of 0.050 to 0.500 μm on at least one surface in order to further enhance the separation performance and the water permeability. The average pore diameter of the surface pores is more preferably 0.080 μm or more, further preferably 0.090 μm or more, particularly preferably 0.095 μm or more, and most preferably 0.100 μm or more. The average pore diameter of the surface pores is more preferably 0.450 μm or less, further preferably 0.400 μm or less. Here, the surface hole refers to a recess in an image obtained by imaging the surface of the separation membrane at a magnification of 10,000 using SEM. The "recess" here means a dark part in the image observed by SEM, and the image captured by SEM is noise-removed and binarized (Hang binarized) by using image analysis software such as ImageJ. This makes it possible to extract the outline. The specific method for extracting the contour of the recess is as described above. The detailed observation method is described in (3) Shape of surface hole of the example. The average pore diameter of the surface pores may be referred to as the surface pore diameter.

In the separation membrane of the present invention, the average minor axis X, the average major axis Y, and the average value (y / x) _a of the ratio of the major axis to the minor axis of the surface holes are in a specific range on at least one surface. Is preferable. The average minor axis X is the arithmetic mean of the minor axis x when each surface hole is regarded as an ellipse. The average major axis Y is the arithmetic mean of the major axis when each surface hole is regarded as an ellipse. The average value (y / x) _a is the arithmetic mean of the minor axis x of each surface hole divided by the major axis y. The average minor axis X, the average major axis Y, and the average value (y / x) _a of the ratio of the major axis to the minor axis of the surface holes are images obtained by imaging the surface of the separation membrane at a magnification of 10,000 using an SEM. Is obtained by analyzing using image analysis software such as ImageJ. Specifically, in ImageJ's Set Measurements, after selecting Fit Ellipse, the ImageJ Analysis Particles command is executed for the recesses extracted in the same manner as described above. As a result, the minor axis x and the major axis y of each surface hole are calculated, and the average minor axis X and the average major axis Y can be obtained by arithmetically averaging each of them. Further, by obtaining y / x for each surface hole and performing an arithmetic mean, the average value (y / x) _a of the ratio of the major axis to the minor axis can be obtained.

The average minor diameter X of the surface holes is preferably 0.030 to 0.250 μm in order to further enhance the separation performance and the water permeability. It is more preferably 0.040 to 0.160 μm, and even more preferably 0.045 to 0.160 μm.

The average major axis Y of the surface holes is preferably 0.060 to 0.450 μm, more preferably 0.070 to 0.240 μm, in order to further enhance the separation performance and the water permeability. It is more preferably 075 to 0.240 μm, and particularly preferably 0.085 to 0.240 μm.

_{The average value (y / x) a} of the ratio of the major axis to the minor axis of the surface hole is preferably 1.00 to 1.50 in order to further enhance the separation performance and the water permeability. It is more preferably 00 to 1.40, further preferably 1.00 to 1.35, and particularly preferably 1.30 to 1.35.

(Membrane permeation flux)
The separation membrane of the present invention preferably has a membrane permeation flux of 0.10 to 20 m ³ / m ² / h at 50 kPa and 25 ° C., preferably 0.25 to 15 m ³ / m ² / h. It is more preferably 0.30 to 10 m ³ / m ² / h, and ^{particularly preferably 0.50 to 7.00 m 3} / m ² / h. The calculation method will be described in detail in Examples.

(Separation performance)
In the separation membrane of the present invention, the separation performance of polystyrene latex particles having an average particle size of 0.2 μm is preferably 50% or more, more preferably 90% or more, further preferably 95% or more. It is particularly preferably 99% or more. The calculation method will be described in detail in Examples.

(Manufacturing method of separation membrane)
The method for producing a separation membrane of the present invention includes the following (1) to (3).
(1) A mixture containing 10 to 80% by mass or less of cellulose ester, 10 to 80% by mass of a structure forming agent, and 2 to 20% by mass of a void forming agent is melt-kneaded to obtain a resin composition. , Preparation process.
(2) A molding step of using a filter having a diameter of 40 to 200 μm and discharging the resin composition from a discharge port to obtain a resin molded product at a draft ratio of 30 to 200.
(3) The resin molded product, the solubility parameter distance D _S for the cellulose ester is immersed in a solvent in the range of 10 to 25, the immersion step.

Next, the method for producing the separation membrane of the present invention will be specifically described by taking the case where the separation membrane is a hollow fiber membrane as an example.
In the preparation step for obtaining the resin composition for producing the separation membrane of the present invention, 10 to 80% by mass of cellulose ester, 10 to 80% by mass of a structure forming agent, and 2 to 20% by mass of a void forming agent were used. The mixture containing is melt-kneaded. The mixture preferably contains 15 to 75% by mass of a cellulose ester, 20 to 75% by mass of a structure forming agent, and 3 to 18% by mass of a void forming agent, and 20 to 60% by mass of the cellulose ester. More preferably, it contains 25 to 70% by mass of a structure forming agent and 5 to 15% by mass of a void forming agent, 20 to 60% by mass of a cellulose ester, 25 to 70% by mass of a structure forming agent, and 10%. It is particularly preferable to contain ~ 15% by mass of the void forming agent.

The apparatus used for melting and kneading the mixture is not particularly limited, and a kneader, a roll mill, a Banbury mixer, or a mixer such as a single-screw or twin-screw extruder can be used. Above all, it is preferable to use a twin-screw extruder from the viewpoint of improving the dispersibility of the structure-forming agent and the plasticizer, and from the viewpoint of being able to remove volatile substances such as water and low molecular weight substances, a twin-screw extruder with a vent hole is used. Is more preferred. Further, a twin-screw extruder having a screw having a flight portion and a kneading disc portion may be used, but in order to reduce the strength of kneading, a twin-screw extruder having a screw having only the flight portion may be used. It is preferable to use it.

The resin composition obtained in the preparation step may be pelletized once and melted again to be used for melt film formation, or may be directly led to a mouthpiece and used for melt film formation. When pelletizing once, it is preferable to dry the pellets and use a resin composition having a water content of 200 ppm (mass basis) or less. Deterioration of the resin can be suppressed by setting the water content to 200 ppm (mass standard) or less.

The molding step is a step of forming a resin molded product by discharging the resin composition obtained in the preparation step from the discharge port. The molding step may be, for example, a step of discharging into air from a discharge port having a double annular nozzle having a gas flow path in the center and cooling with a cooling device to form a resin molded product. Absent.

It is preferable to discharge the resin composition that has passed through the filter in advance from the discharge port. The pore size of the filter is preferably 40 to 200 μm, more preferably 70 to 150 μm, in order to increase the values of l, L and (l / d) _{a and suppress the bonding between voids.} It is more preferably 70 to 120 μm. When the resin composition passes through the filter, the void forming agent contained in the resin composition is stretched, _{the values of l, L and (l / d) a} are increased, and the binding between the voids is suppressed. It is presumed that the effect will be obtained.

The resin molded product or hollow fiber cooled by the cooling device may be wound by the winding device. In this case, the draft ratio values calculated by the winding device (winding speed) / (discharge speed from the discharge port) increase the values of l, L and (l / d) _a , and also. In order to prevent the average thickness of the wall portion in the longitudinal direction of the separation membrane from being excessively lowered, it is preferably 30 to 200, more preferably 50 to 150, and particularly preferably 100 to 150. .. By stretching the resin composition discharged from the base under the above draft ratio conditions, the void forming agent contained in the resin composition is stretched, and the values of _{l, L and (l / d) a are set.} It is presumed that the effect of increasing the size and suppressing the binding between the voids can be obtained. In solution film formation carried out by mixing 50% by weight or more of a low molecular weight compound having a molecular weight of less than 1000 and a polymer, spinning at a high draft ratio as in the present invention is difficult, and the void forming agent is sufficiently drawn. It is difficult to obtain such an effect because it is not extended.

Immersion step, the solubility parameter distance D _S to cellulose ester as a raw material is in a solvent 10-25, a step of impregnating the resin molded product. At this time, by using a solvent or a mixed solvent having an appropriate affinity for the cellulose ester, it is possible to suppress extreme swelling and plasticization of the resin. Therefore, the solvent permeates the resin molded product while maintaining the shape of the resin. At this time, it is presumed that the plasticizer and the structure-forming agent are eluted while phase separation occurs in the resin molded product. The longer or higher the immersion time and temperature of the solvent, the larger the surface pore diameter, and the larger the abundance ratio and size of the voids and pores in the cross section Z tend to be. In the present invention, it is preferable to select a solvent having a certain affinity with the cellulose ester. The affinity between the cellulose ester and the solvent can be estimated by the three-dimensional Hansen solubility parameter (Non-Patent Document 1). Specifically, the smaller solubility parameter distance D _S obtained from the following equation (1), the cellulose ester, a high affinity for the solvent.

However, δ _Ad , δ _Ap and δ _Ah are the dispersion term, the polarity term and the hydrogen bond term of the solubility parameter of the cellulose ester, and δ _Bd , δ _Bp and δ _Bh are the dispersion terms of the solubility parameter of the solvent or the mixed solvent. , Polarity term and hydrogen bond term. The solubility parameter (δ _Mixture ) of the mixed solvent can be obtained by the following formula (2).

However, φ _i and δ _i are volume fractions and solubility parameters of the component i, and hold for each of the dispersion term, the polarity term, and the hydrogen bond term. Here, the "volume fraction of the component i" means the ratio of the volume of the component i before mixing to the sum of the volumes of all the components before mixing. As the three-dimensional Hansen solubility parameter of the solvent, the value described in Non-Patent Document 1 was used. For the solvent parameters not described, the values contained in the software "Hansen Solubility Parameter in Practice" developed by Charles Hansen et al. Were used. The three-dimensional Hansen solubility parameter of a solvent or polymer not described in the above software can be calculated by the Hansen sphere method using the above software.

The present inventors have found that the solvent having the above solubility parameter distance D _S 10-25, by impregnating the resin molded product, average depth D of the depth d and a plurality of voids of the void is increased, We obtained the unexpected finding that large films with d and D can be obtained. Then, it was found that the effect of substantially reducing the film thickness can be obtained more remarkably. The reason why such an effect is obtained is not clear, but it is presumed as follows. That is, since the void forming agent is incompatible with the cellulose ester, the void forming agent is dispersed in the cellulose ester after the molding step and before the dipping step, and the solubility in the cellulose ester in the dipping step. It is presumed that the void-forming agent swells with a solvent having a parameter distance D _{s of 10 to 25 to obtain a film having a large d and D.}

The temperature of the resin molded product in the dipping step is preferably 50 to 80 ° C. When the temperature of the resin molded product in the dipping step is 50 to 80 ° C., surprisingly, the voids in the cross section Z have (l / d) _a of 2 to 40, but the ratio of the major axis to the minor axis of the surface hole. It was found that the average value (y / x) _{a of} is as low as 1.0 to 1.5, that is, close to a circle. The reason for this is presumed as follows. That is, in the resin molded product, when the thread temperature is 50 to 80 ° C., the molecules are relatively easy to move, but at this time, the surface is in a state where the molecules are particularly easy to move as compared with the inside, so that the resin molded product is immersed. It is presumed that when the plasticizing is further promoted by immersing in a solvent in the process, the structure-forming agent stretched by the filter holes and the draft returns to the original shape on the surface and approaches a circular shape. ..

In the present invention, the solvent for immersing the resin molded product, a solvent such as D _S of 13 to 25 preferred. As such solvent, a solvent such as _{D S} is 4 to 12, a mixed solvent of water Preferably, for example, .gamma.-butyrolactone (hereinafter, gamma-BL), acetone, acetonitrile, 1,4 Examples thereof include a mixed solvent of water and at least one selected from the group consisting of dioxane, methyl acetate and tetrahydrofuran. And solvents such as D _S is 4 to 12, by using a mixed solvent of water, the film strength of the separation membrane to be obtained becomes good.

The obtained separation membrane can be used as it is, but it is preferable to hydrophilize the surface of the membrane with, for example, an alcohol-containing aqueous solution or an alkaline aqueous solution before use.
If the void forming agent remains even after the steps up to this point, it is preferable to provide a step of removing the void forming agent. As a method for removing the void forming agent, for example, the cellulose ester is not dissolved or decomposed, but is immersed in a solution that dissolves or decomposes the void forming agent.

The present invention will be described in more detail with reference to Examples below, but the present invention is not limited thereto.

[Measurement and evaluation method]
Each characteristic value in the example was obtained by the following method.
(1) Average Degree of Substitution of Cellulose Mixed Ester The method for calculating the average degree of substitution of a cellulose mixed ester in which an acetyl group and other acyl groups are bonded to cellulose is as follows.
0.9 g of the cellulose mixed ester dried at 80 ° C. for 8 hours was weighed, 35 mL of acetone and 15 mL of dimethyl sulfoxide were added and dissolved, and then 50 mL of acetone was further added. 30 mL of 0.5 N-sodium hydroxide aqueous solution was added with stirring, and saponification was performed for 2 hours. After adding 50 mL of hot water and washing the side surface of the flask, titration was performed with 0.5 N-sulfuric acid using phenolphthalein as an indicator. Separately, a blank test was performed by the same method as the sample. The supernatant of the titrated solution was diluted 100-fold, and the composition of the organic acid was measured using an ion chromatograph. From the measurement results and the acid composition analysis results by ion chromatography, the degree of substitution was calculated by the following formulas (3) to (5).
TA = (BA) x F / (1000 x W) ... (3)
DSace = (162.14 × TA) / [{1- (Mwace- (16.00 + 1.01)) × TA} + {1- (Mwacy- (16.00 + 1.01)) × TA} × (Acy / Ace)] ・・・・・・ (4)
DSacy = DSace × (Acy / Ace) ・・・・・・ (5)
TA: Total organic acid amount (mL)
A: Sample titration (mL)
B: Blank test titration (mL)
F: Sulfuric acid titer W: Sample mass (g)
DSace: Average degree of substitution of acetyl group DSacy: Average degree of substitution of other acyl groups Mwace: Molecular weight of acetic acid Mwacy: Molecular weight of other organic acids Acy / Ace: Mol of acetic acid (Ace) and other organic acids (Acy) Ratio 162.14: Molecular weight of repeating unit of cellulose 16.00: Atomic weight of oxygen 1.01: Atomic weight of hydrogen

(2) Weight average molecular weight of cellulose ester (Mw)
It was completely dissolved in tetrahydrofuran so that the concentration of the cellulose ester was 0.15% by mass, and used as a sample for GPC measurement. Using this sample, GPC measurement was performed using a GPC device (Waters2690) under the following conditions, and the weight average molecular weight (Mw) was determined by polystyrene conversion.
Column: Two Tosoh TSK gel GMHHR-H connected Detector: Water2410 Differential refractometer RI
Moving layer Solvent: Tetrahydrofuran Flow rate: 1.0 mL / min Injection amount: 200 μL

(3) Shape of surface pores The outer surface of the separation membrane sputtered with platinum was observed at a magnification of 10,000 times using SEM, and the pore diameters r of 50 randomly selected surface pores were measured. The arithmetic mean value was taken as the surface pore diameter r in Tables 1 and 2.
Here, the hole diameter r of each surface hole was calculated by the following formula (6), assuming that the area of the surface hole was measured by image processing and a perfect circular hole having the same area was assumed.
r = (4 x A / π) ^0.5 ... (6)
A: Hole area The observation conditions using sputtering and SEM are as follows.
(Sputtering conditions)
Equipment: Hitachi High-Tech Co., Ltd. (E-1010)
Deposition time: 40 seconds Current value: 20mA
(SEM condition)
Equipment: Made by Hitachi High-Tech Co., Ltd. (SU1510)
Acceleration voltage: 5kV
Probe current: 30
Further, the outer surface of the separation membrane was observed under the same observation conditions as described above, and the average minor axis X, the average major axis Y, and the average value (y / x) of the ratio of the major axis to the minor axis of the surface holes were observed by the above-mentioned analysis method. ) was determined by the value of _a.

(4) Thickness of Hollow Fiber Membrane After freezing the hollow fiber membrane with liquid nitrogen, stress was applied (using a razor or a microtome as necessary), and the hollow fiber membrane was cut so as to expose the radial cross section. The obtained cross section was observed with an optical microscope, and the average value of the thicknesses of 10 randomly selected points was taken as the thickness (film thickness) of the hollow fiber membrane.

(5) Outer Diameter of Hollow Fiber Membrane The cross section of the above (4) was observed with an optical microscope, and the average value of the outer diameters of 10 randomly selected points was taken as the outer diameter of the hollow fiber membrane.

(6) Membrane Permeation Flux of Hollow Fiber Membrane A small module having an effective length of 100 mm made of one hollow fiber membrane was produced. Distilled water was sent to this small module under the conditions of a temperature of 25 ° C. and a filtration differential pressure of 16 kPa for 30 minutes by total external pressure filtration, and the obtained permeated water amount (m ³ ) was measured and measured for a unit time (h). ) And the value per unit film area (m ² ), and further converted to pressure (50 kPa) to obtain the permeation performance of pure water (unit = m ³ / m ² / h).

(7) Measurement of multiple voids and walls After freezing the separation membrane in liquid nitrogen, stress is applied (using a razor or microtome as necessary), and the separation membrane is parallel to the longitudinal direction and the film thickness direction. It was cut so that the cross section Z, which is a smooth cross section, was exposed. Subsequently, after sputtering with platinum and pretreating the cross section Z, the field of view is set so that the distances from both surfaces are equal at the center of the field of view using SEM, and the field of view is observed at a magnification of 2,000. did. In the same manner, 5 visual fields were observed, and 30 voids were randomly extracted in each visual field, and then the average depth D (μm) and average length L (μm) of the plurality of voids and l / of each void. The average value (l / d) _a of d was calculated, and further, the occupancy rate (%) of the plurality of voids, the average thickness of the wall portion (μm), and the longitudinal angles (°) of the plurality of voids were calculated. .. The observation conditions using sputtering and SEM are as follows.
(Sputtering conditions)
Equipment: Hitachi High-Tech Co., Ltd. (E-1010)
Deposition time: 40 seconds Current value: 20mA
(SEM condition)
Equipment: Made by Hitachi High-Tech Co., Ltd. (SU1510)
Acceleration voltage: 5kV
Probe current: 30

(8) Area ratio of recesses The separation membrane is cut in the same manner as in (7) above, and the exposed cross section Z is observed using SEM at a magnification of 5,000 times to determine the area ratio of the recesses (recess area ratio). Calculated.

(9) Separation performance A small module was produced in the same manner as in (6) above. An aqueous solution containing 20 ppm of polystyrene latex particles (manufactured by Magsphere) having an average particle size of 0.2 μm as a turbid component under the conditions of a temperature of 25 ° C. and a filtration differential pressure of 16 kPa was added to this small module over 30 minutes by external pressure total filtration. The turbidity component concentrations of the supplied water and the permeated water were calculated from the ultraviolet absorption coefficient with a wavelength of 234 nm measured using a spectrophotometer (manufactured by Hitachi, Ltd .; U-3200), and the following formula (7) ).
Separation performance (%) = [1-2 x (concentration of turbid component of permeated water) / {(concentration of turbid component of supply water at start of filtration) + (concentration of turbid component of supply water at end of filtration)} ] × 100 ・・・・・・ Equation (7)

[Cellulose ester (A)]
The following were prepared as cellulose esters.
Cellulose ester (A1)
To 100 parts by mass of cellulose (cotton linter), 240 parts by mass of acetic acid and 67 parts by mass of propionic acid were added and mixed at 50 ° C. for 30 minutes. After cooling the mixture to room temperature, 172 parts by mass of acetic anhydride and 168 parts by mass of propionic anhydride cooled in an ice bath were added as an esterifying agent, and 4 parts by mass of sulfuric acid was added as an esterification catalyst, and the mixture was stirred for 150 minutes. An esterification reaction was carried out. In the esterification reaction, when it exceeded 40 ° C., it was cooled in a water bath.
After the reaction, a mixed solution of 100 parts by mass of acetic acid and 33 parts by mass of water was added as a reaction terminator over 20 minutes to hydrolyze the excess anhydride. Then, 333 parts by mass of acetic acid and 100 parts by mass of water were added, and the mixture was heated and stirred at 80 ° C. for 1 hour. After completion of the reaction, an aqueous solution containing 6 parts by mass of sodium carbonate was added, and the precipitated cellulose acetate propionate was filtered off, subsequently washed with water, and then dried at 60 ° C. for 4 hours. The average degree of substitution of the acetyl group and the propionyl group of the obtained cellulose acetate propionate was 1.9 and 0.7, respectively, and the weight average molecular weight (Mw) was 178,000.

Cellulose ester (A2): Cellulose acetate propionate (average degree of substitution of acetyl group: 0.2, average degree of substitution of propionyl group: 2.5, weight average molecular weight (Mw): 185,000)

[Other raw materials]
The following were prepared as other raw materials.
Cellulose ester plasticizer (B): Polyethylene glycol (weight average molecular weight (Mw) 600)
Structure-forming agent (C): PVP / vinyl acetate copolymer (PVP / vinyl acetate = 6/4 (molar ratio), weight average molecular weight 50,000)
Void forming agent (D): Polyethylene glycol (weight average molecular weight (Mw) 300,000)
Antioxidant (E): Bis (2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite

(Example 1)
40% by mass of cellulose ester (A1), 26.9% by mass of plasticizer (B), 30% by mass of structure forming agent (C), 3% by mass of void forming agent (D), and antioxidant (E). 0.1% by mass was melt-kneaded at 220 ° C. with a twin-screw extruder, homogenized and then pelletized to obtain a resin composition. This resin composition was vacuum dried at 80 ° C. for 8 hours.

The dried resin composition is supplied to a twin-screw extruder equipped with a screw consisting only of a flight portion, melt-kneaded at 220 ° C., and then introduced into a melt-spinning pack having a spinning temperature of 220 ° C. to discharge the amount. Under the condition of 10 g / min, the spinner was spun below the outer annular portion of the discharge mouthpiece having one mouthpiece hole (double circular tube type, discharge hole diameter 2.6 mm, slit width 0.35 mm). The spun hollow fiber was guided to a cooling device, cooled by a cooling air at 25 ° C. and a wind speed of 1.5 m / sec, and wound with a winder so that the draft ratio was 30. Here, as the filter in the molten spinning pack, a metal filter having a pore diameter (filter diameter) of 200 μm was used. The wound hollow fiber (resin molded product) is heated to 30 ° C., immersed in an aqueous acetone solution having a volume fraction of 40% for 1 hour, and further immersed in water for 1 hour or more to obtain a plasticizer (B) and a structure. The forming agent (C) and the void forming agent (D) were eluted to obtain a separation membrane. The physical characteristics of the obtained separation membrane are shown in Table 1.

(Examples 2 to 9 and Comparative Examples 1 to 6)
A separation membrane was obtained in the same manner as in Example 1 except that the composition and production conditions of the resin composition were changed as shown in Tables 1 and 2, respectively. The physical characteristics of the obtained separation membrane are shown in Tables 1 and 2. In Comparative Example 1, no voids were observed, and in Comparative Example 2, spinning was not possible due to yarn breakage.

The separation membranes obtained in Examples 1 to 9 all have a membrane permeation flux of 0.1 m ³ / m ² / h or more and a separation performance of 50% or more, and have a high membrane permeation flux. It had both separation performance. On the other hand, in Comparative Example 2, spinning was not possible due to yarn breakage, and a separation membrane could not be obtained. Further, the separation membranes of Comparative Examples 1, 3 to 6 in which the shapes of the plurality of voids do not satisfy the requirements of the present invention show a low value in at least one of the membrane permeation flux and the separation performance, and are separated from the high membrane permeation flux. It was not possible to achieve both performance.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on a Japanese patent application filed on December 23, 2019 (Japanese Patent Application No. 2019-231579), the contents of which are incorporated herein by reference.

The separation membrane of the present invention is a water treatment membrane for producing industrial water or drinking water from seawater, irrigation, sewage, drainage, etc., a medical membrane for artificial kidney, plasma separation, etc., for fruit juice concentration, etc. It can be suitably used as a membrane for the food / beverage industry, a gas separation membrane for separating exhaust gas, carbon dioxide gas, etc., or a membrane for the electronic industry such as a fuel cell separator.

Claims

Separation membrane containing cellulose ester
The separation membrane has a plurality of voids in a cross section parallel to the longitudinal direction and the film thickness direction of the membrane.
The average depth D of the plurality of voids is 0.7 to 20 μm.
The average length L of the plurality of voids is 3 μm or more, and
A separation membrane in which the average value (l / d) a of the ratio of the length l to the depth d of each void is 2 to 40.
The separation membrane according to claim 1, wherein the occupancy rate of the plurality of voids in the cross section is 15 to 55%.
The separation membrane according to claim 1 or 2, wherein the average thickness of the wall portion in the cross section is 0.7 to 5.0 μm.
The separation membrane according to any one of claims 1 to 3, wherein the average pore diameter of the surface pores is 0.050 to 0.500 μm on at least one surface.
On at least one surface, the average minor axis X of the surface holes is 0.030 to 0.250 μm, and the average major axis Y of the surface holes is 0.060 to 0.450 μm, which is the ratio of the major axis to the minor axis. The separation membrane according to any one of claims 1 to 4, wherein the average value (y / x) of a is 1.00 to 1.50.
The separation membrane according to any one of claims 1 to 5, wherein the longitudinal direction of the plurality of voids is along the longitudinal direction of the separation membrane.
The separation membrane according to any one of claims 1 to 6, which contains cellulose acetate propionate and / or cellulose acetate butyrate as the cellulose ester.
The separation membrane according to any one of claims 1 to 7, which has a hollow fiber shape.
(1) A mixture containing 10 to 80% by mass of cellulose ester, 10 to 80% by mass of a structure forming agent, and 2 to 20% by mass of a void forming agent is melt-kneaded to obtain a resin composition. , Preparation process and
(2) A molding step of using a filter having a pore size of 40 to 200 μm and discharging the resin composition from a discharge port to obtain a resin molded product at a draft ratio of 30 to 200.
(3) said resin molded product is immersed in a solvent in the range of solubility parameters distance D S 10-25 to cellulose ester, and a dipping process, a manufacturing method of the separation membrane.
The method for producing a separation membrane according to claim 9, wherein the temperature of the resin molded product in the dipping step is 50 to 80 ° C.