WO2018021545A1 - Separation membrane and production process therefor - Google Patents

Abstract

The separation membrane of the present invention comprises a hydrophilic polymer as a main component, has a co-continuous structure in which a phase including the hydrophilic polymer and pores have a structural period of 0.001-10 µm, and has a thickness of 10-500 µm. The separation membrane has a thickness-direction cross-section that satisfies relationship (1) and relationship (2), where Ha is the porosity of region a which extends from 1 µm to 4 µm in terms of depth-direction distance from one of the surfaces, Hb is the porosity of region b which extends from 1 µm to 4 µm in terms of depth-direction distance from the other surface, and Hc is the porosity of 3 µm-thick region c located at the same distance from both surfaces. (1) 1.00≤Ha/Hc≤1.50 (2) 1.05≤Hb/Hc≤1.50

Description

Separation membrane and manufacturing method thereof

The present invention relates to a separation membrane mainly composed of a hydrophilic polymer and a method for producing the same.

Separation membrane removes turbidity and ions from rivers, seawater and sewage wastewater, water treatment membrane to produce industrial water and drinking water, medical membranes such as artificial kidney and plasma separation, food concentrate such as fruit juice It is used in a wide range of fields such as membranes for the beverage industry and gas separation membranes for separating carbon dioxide and the like.

大 Most separation membranes are made of polymer. For example, in Patent Document 1, a membrane forming solution (spinning stock solution) composed of cellulose triacetate, a solvent, and a non-solvent is discharged into a coagulating solution composed of a solvent, a non-solvent, and water to obtain a hollow fiber membrane. Technology is disclosed.

Patent Document 2 discloses a technique for obtaining a porous film by immersing a molten film-forming sheet of polymethyl methacrylate and polylactic acid in an aqueous potassium hydroxide solution and hydrolyzing and removing polylactic acid.

In Patent Literature 3, a resin composition comprising a cellulose ester and a plasticizer is melt-kneaded, and then a plasticizer is eluted from a hollow fiber that is discharged from the die into the air and wound up to obtain a hollow fiber membrane. Is disclosed.

Japanese Unexamined Patent Publication No. 2011-235204 Japanese Unexamined Patent Publication No. 2012-092318 International Publication No. 2016/52675

With the technique described in Patent Document 1, a so-called asymmetric membrane is obtained in which the pore diameter varies greatly in the film thickness direction. An asymmetric membrane has a dense layer with a small pore diameter that exhibits separation performance near the surface layer of the membrane. In order to develop high transmission performance, it is necessary to sufficiently reduce the thickness of the dense layer, and it is necessary to sufficiently increase the pore diameter other than the dense layer. Due to the former, there is a problem that the film is likely to be defective during manufacture and use, and the latter causes a problem that the film strength is low. That is, a separation membrane that satisfies all of the permeability, stability of membrane performance, and membrane strength could not be obtained.

In the technique described in Patent Document 2, a separation membrane having continuous pores uniformly controlled in the film thickness direction can be obtained. Because of its uniform structure, it becomes a separation membrane with excellent membrane performance stability and membrane strength, but due to the thick thickness of the uniform structure portion that exhibits separation performance, it was a separation membrane with low permeability. .

In the technique described in Patent Document 3, a separation membrane with a high membrane strength having a uniform structure in the film thickness direction can be obtained. However, since it is a so-called reverse osmosis membrane, a very high pressure is required to develop permeation performance. In addition, the permeation performance obtained was also low.

In view of the background of such prior art, the present invention provides a separation membrane mainly composed of a hydrophilic polymer having excellent permeability and stability of membrane performance and high membrane strength, and a method for producing the same. is there.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have a hydrophilic polymer as a main component, a phase having the hydrophilic polymer and a co-continuous structure having a structure period of a constant size, and And in the cross section of the thickness direction, it has been found that a separation membrane that can solve the problem can be provided by setting the ratio of the porosity of the surface layer portion and the inner layer portion within a certain range, and has completed the present invention. .

That is, the separation membrane of the present invention is a separation membrane mainly comprising a hydrophilic polymer,
Having a co-continuous structure in which the structural period of the phase having the hydrophilic polymer and the pores is 0.001 μm or more and 10 μm or less;
The thickness of the separation membrane is 10 μm or more and 500 μm or less,
In the cross section in the thickness direction, the porosity of the region a having a depth of 1 to 4 μm from any one surface (A surface) is Ha, and the porosity of the region b having a depth of 1 to 4 μm from the other surface (B surface). Is a separation membrane satisfying the following formulas (1) and (2), where Hb is the porosity of the region c having a thickness of 3 μm and the same depth from both surfaces.
(1) 1.00 ≦ Ha / Hc ≦ 1.50
(2) 1.05 ≦ Hb / Hc ≦ 1.50

According to a preferred embodiment of the present invention, an average porosity calculated by a simple average of the porosity Ha, the porosity Hb, and the porosity Hc is 20% or more and 80% or less.
According to a further preferred embodiment of the present invention, the hydrophilic polymer is at least one selected from the group consisting of polyester, polyamide and cellulose ester.
According to a further preferred embodiment of the present invention, the separation membrane has a hollow fiber shape.
According to a further preferred embodiment of the present invention, the hollow fiber has an outer diameter of not less than 50 μm and not more than 2500 μm.

In addition, according to the present invention, there is provided a method for producing a separation membrane having a hydrophilic polymer as a main component, wherein the separation membrane is produced by performing at least the following steps 1 to 5.
1. Melt-kneading step of obtaining a resin composition by melt-kneading 20% by weight to 80% by weight of hydrophilic polymer and 20% by weight to 80% by weight of a structure-forming agent. 2. A film forming process for forming a film by discharging and cooling the resin composition from a die. 3. Stretching process for stretching the film at a stretch ratio of 1.1 to 5.0. 4. Heat treatment step of heating the stretched film obtained in the stretching step at 50 ° C. or more and 300 ° C. or less. An elution step of eluting the structure-forming agent from the film obtained in the heat treatment step

According to the present invention, a separation membrane having excellent permeability and membrane performance stability and high membrane strength is provided. The separation membrane of the present invention can be preferably used for applications that require permeation performance and high membrane strength.
Specifically, water treatment membranes for removing turbidity from river water, seawater, brine, sewage, drainage, medical membranes such as artificial kidneys and plasma separation, membranes for food and beverage industries such as fruit juice concentration, It can be used for gas separation membranes for separating exhaust gas, carbon dioxide gas, etc., and membranes for electronic industries such as fuel cell separators. The type of the water treatment membrane can be preferably used for microfiltration, ultrafiltration and the like.

FIG. 1 is a cross-sectional view of a separation membrane and an enlarged view thereof.

The separation membrane of the present embodiment is a separation membrane containing a hydrophilic polymer as a main component,
Having a co-continuous structure in which the structural period of the phase having the hydrophilic polymer and the pores is 0.001 μm or more and 10 μm or less;
The thickness of the separation membrane is 10 μm or more and 500 μm or less,
In the cross section in the thickness direction, the porosity of the region a having a depth of 1 to 4 μm from any one surface (A surface) is Ha, and the porosity of the region b having a depth of 1 to 4 μm from the other surface (B surface). Where Hb is Hb, and the porosity of the region c having a thickness of 3 μm having the same depth from both surfaces is Hc, the following expressions (1) and (2) are satisfied.
(1) 1.00 ≦ Ha / Hc ≦ 1.50
(2) 1.05 ≦ Hb / Hc ≦ 1.50
The separation membrane of the present invention may contain a liquid such as water in order to maintain its shape. However, in the following description, these liquids for maintaining the shape are not considered as components of the separation membrane.

(Resin composition constituting separation membrane)
The separation membrane of the present invention can contain the components shown in the following (1) to (5).

(1) Hydrophilic polymer It is important that the separation membrane of the present invention contains a hydrophilic polymer as a main component. As used herein, the main component refers to a component that is contained most in weight among all the components of the resin composition constituting the separation membrane.

In the present invention, the hydrophilic polymer means a polymer containing a hydrophilic group as a constituent component of the polymer and having a contact angle with water of 60 ° or less with respect to the polymer film. Here, the hydrophilic group is a hydroxyl group, a carboxyl group, a carbonyl group, an amino group, or an amide group.

Specific examples of the hydrophilic polymer include polyester, polyamide, polymethyl acrylate, polyvinyl acetate, cellulose ester, polyester, and the like. Among these, it is preferable that it is at least 1 sort (s) chosen from the group which consists of polyester, polyamide, and a cellulose ester.

Specific examples of polyamide include nylon 6 and nylon 11.

Specific examples of the cellulose ester include cellulose esters such as cellulose acetate, cellulose propionate, and cellulose butyrate, and cellulose mixed esters such as cellulose acetate propionate and cellulose acetate butyrate.

The weight average molecular weight (Mw) of the cellulose ester is preferably 50,000 to 250,000. When Mw is 50,000 or more, the thermal decomposition of the cellulose ester when melted during the production of the separation membrane is suppressed, and the membrane strength of the separation membrane can reach a practical level. When the Mw is 250,000 or less, the melt viscosity does not become too high, so that stable melt film formation is possible.

Mw is more preferably 60,000 to 220,000, and further preferably 80,000 to 200,000. The weight average molecular weight (Mw) is a value calculated by GPC measurement. The calculation method will be described in detail in Examples.

Each illustrated cellulose mixed ester 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 between the acetyl group and other acyl groups 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 group) ≦ 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 good thermal fluidity when the resin composition constituting the separation membrane is melted are realized. The average degree of substitution refers to the number of chemically bonded acyl groups (acetyl group + other acyl groups) among the three hydroxyl groups present per glucose unit of cellulose.

Only one kind of hydrophilic polymer may be contained, or two or more kinds thereof may be contained.

The content of the hydrophilic polymer in the resin composition constituting the separation membrane is preferably from 70 to 100% by weight, and preferably from 80 to 100% by weight, assuming that all components of the resin composition constituting the separation membrane are 100% by weight. % Is more preferable, and 90 to 100% by weight is particularly preferable.

Further, the hydrophilic polymer is preferably contained in an amount of 20% by weight to 80% by weight, based on 100% by weight of the total components constituting the raw material for film formation.

When the content is 20% by weight or more, the membrane strength of the separation membrane is improved. When the content is 80% by weight or less, the thermoplasticity and permeation performance of the separation membrane are improved. The content is more preferably 25% by weight or more, and further preferably 30% by weight or more. Further, the content is more preferably 70% by weight or less, and further preferably 60% by weight or less.

(2) Plasticizer for hydrophilic polymer The resin composition constituting the separation membrane of the present invention may contain a plasticizer for hydrophilic polymer.

The plasticizer for the hydrophilic polymer is not particularly limited as long as it is a compound that thermoplasticizes the hydrophilic polymer. Moreover, not only one type of plasticizer but also two or more types of plasticizers may be used in combination.

Specific examples of hydrophilic polymer plasticizers include polyalkylene glycol compounds such as polyethylene glycol and polyethylene glycol fatty acid esters, glycerin compounds such as glycerin fatty acid esters and diglycerin fatty acid esters, citrate ester compounds, and phosphate esters. And fatty acid ester compounds such as adipic acid esters, caprolactone compounds, and derivatives thereof.

Specific preferred examples of the polyalkylene glycol compound include polyethylene glycol, polypropylene glycol, and polybutylene glycol having a weight average molecular weight of 400 to 4,000.

The hydrophilic polymer plasticizer may remain in the separation membrane after the separation membrane is formed or may be eluted from the separation membrane. In the case of elution, traces from which the plasticizer has fallen may become pores in the film, and as a result, the permeation performance is improved.

The hydrophilic polymer plasticizer is preferably contained in an amount of 5% by weight to 40% by weight, based on 100% by weight of the total components constituting the raw material for film formation.

When the content is 5% by weight or more, the thermoplasticity of the hydrophilic polymer and the permeation performance of the separation membrane are improved. By setting the content to 40% by weight or less, the membrane strength of the separation membrane is improved. The content of the plasticizer of the hydrophilic polymer is more preferably 10 to 35% by weight, still more preferably 15 to 30% by weight.

(3) Antioxidant The resin composition constituting the separation membrane of the present invention preferably contains an antioxidant.

As a specific example of the antioxidant, it is preferable to contain a phosphorus-based antioxidant, and a pentaerythritol-based compound is more preferable. Specific examples of the pentaerythritol compound include bis (2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite. When the antioxidant is contained, thermal decomposition at the time of melt film formation is suppressed, and as a result, film strength can be improved and coloring of the film can be prevented.

The content of the antioxidant is preferably 0.005 to 0.500% by weight when the total of the components constituting the raw material for film formation is 100% by weight.

(4) Structure-forming agent The resin composition constituting the separation membrane of the present invention may contain a structure-forming agent. In the present invention, the structure-forming agent refers to a compound that is partially compatible with a mixture of a hydrophilic polymer and a plasticizer. Partially compatible means that two or more substances are completely compatible under certain conditions but phase-separated under other conditions. The structure-forming agent is a substance that undergoes phase separation from the hydrophilic polymer by being placed under specific conditions in the below-described co-continuous structure forming step.

The structure-forming agent in the present invention is preferably hydrophilic, and refers to a resin that dissolves in water or has a smaller contact angle with water than the main hydrophilic polymer.

Specific examples of the structure-forming agent include polyvinylpyrrolidone (PVP), PVP / vinyl acetate copolymer, PVP-based copolymers such as PVP / methyl methacrylate copolymer, polyvinyl alcohol, and polyester compounds. can give. These can be used alone or in combination.

Since it is difficult to elute PVP from the separation membrane when thermal crosslinking occurs, the weight average molecular weight is 20,000 from the viewpoint that intermolecular crosslinking is relatively difficult to proceed and can be eluted even after crosslinking. The following is preferable. It is also preferable to use the PVP-based copolymer described in the previous paragraph because thermal crosslinking is suppressed.

The structure forming agent is eluted from the separation membrane after the separation membrane is formed. By removing the structure-forming agent by elution, traces from which the structure-forming agent has been removed may become pores in the film, resulting in good permeation performance. In addition, although the said effect is acquired by removal of a structure formation agent, a structure formation agent may remain | survive in a separation membrane, without removing completely.

The content of the structure-forming agent is preferably 20% by weight or more and 80% by weight or less when the total of the components constituting the film-forming raw material is 100% by weight.

When the content is 20% by weight or more, the permeation performance of the separation membrane is improved. By setting the content to 80% by weight or less, the film strength is improved. The content of the structure forming agent is more preferably 30% by weight or more, and still more preferably 40% by weight. Further, the content of the structure forming agent is more preferably 75% by weight or less, and still more preferably 70% by weight or less.

(5) Additive The resin composition constituting the separation membrane of the present invention may contain additives other than those described in (1) to (4) as long as the effects of the present invention are not impaired.

Specific examples of additives include resins such as cellulose ether, polyacrylonitrile, polyolefin, polyvinyl compound, polycarbonate, poly (meth) acrylate, polysulfone, polyethersulfone, organic lubricant, crystal nucleating agent, organic particles, inorganic particles, terminal Blocking agent, chain extender, ultraviolet absorber, infrared absorber, anti-coloring agent, matting agent, antibacterial agent, antistatic agent, deodorant, flame retardant, weathering agent, antistatic agent, antioxidant, ion exchange Agents, antifoaming agents, color pigments, fluorescent brighteners, dyes and the like.

(Membrane shape)
The shape of the separation membrane of the present invention is not particularly limited, but a hollow fiber-shaped separation membrane (hereinafter also referred to as a hollow fiber membrane) or a planar membrane (hereinafter also referred to as a flat membrane) is preferably employed. Among these, the hollow fiber membrane is more preferable because it can be efficiently filled into the module and the effective membrane area per unit volume of the module can be increased. A hollow fiber membrane is a filamentous membrane having a hollow.

The thickness of the separation membrane is preferably 10 μm or more and 500 μm or less from the viewpoint of achieving both permeation performance and membrane strength. The thickness of the separation membrane is more preferably 20 μm or more, further preferably 30 μm or more, and particularly preferably 40 μm or more. The thickness of the separation membrane is more preferably 400 μm or less, further preferably 300 μm or less, and particularly preferably 200 μm or less.

In the case of a hollow fiber membrane, it is preferable that the outer diameter of the hollow fiber membrane is 50 μm or more and 2500 μm or less from the viewpoint of achieving both effective membrane area when the module is filled and membrane strength. The outer diameter 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 diameter of the hollow fiber membrane is more preferably 2000 μm or less, further preferably 1500 μm or less, and particularly preferably 1000 μm or less.

In the case of a hollow fiber membrane, the hollow ratio of the hollow fiber is preferably 15 to 70%, more preferably 20 to 65%, from the relationship between the pressure loss of the fluid flowing through the hollow part and the buckling pressure. Preferably, it is 25 to 60%.

The method of setting the outer diameter and hollow ratio of the hollow fiber in the hollow fiber membrane in the above range is not particularly limited. For example, the shape of the discharge hole of the spinneret for producing the hollow fiber, or the draft ratio that can be calculated by the winding speed / discharge speed , Can be adjusted by appropriately changing.

(Internal structure of membrane)
In the separation membrane of the present invention, it is important to have a co-continuous structure in which the structure period of the phase having the hydrophilic polymer and the pores is 0.001 μm or more and 10 μm or less. Here, the co-continuous structure means, for example, a phase and pores having a hydrophilic polymer when a cross section obtained by applying stress to a separation membrane sufficiently cooled in liquid nitrogen and observing it with a scanning electron microscope (SEM) or the like. However, it means that a continuous structure is observed in the depth direction. In addition, the measuring method of a structure period is described in an Example. In the present invention, the width of the structural period is sometimes simply referred to as the hole diameter.

From the viewpoint of achieving both permeability and membrane strength, the structural period is preferably 0.005 μm or more, more preferably 0.010 μm or more, further preferably 0.015 μm or more, and particularly preferably 0.020 μm or more. Similarly, the structural period is preferably 5.0 μm or less, more preferably 1.0 μm or less, further preferably 0.5 μm or less, particularly preferably 0.3 μm or less or 0.2 μm or less, and most preferably 0.1 μm or less. .

Although the method of setting the structural period in the above range is not particularly limited, it is possible to employ stretching under the conditions described later and then heat treatment under the conditions described later when manufacturing the separation membrane.

(Porosity)
In the separation membrane of the present invention, referring to FIG. 1, in the cross section in the thickness direction of the separation membrane 1, a region having a depth of 1 to 4 μm from any one surface (A surface) (corresponding to reference numeral 2 in FIG. 1). The porosity of a (corresponding to reference numeral 11 in FIG. 1) is Ha, and the area b (corresponding to reference numeral 12 in FIG. 1) having a depth of 1 to 4 μm from the other surface (surface B) (corresponding to reference numeral 3 in FIG. 1). ) In the region c (FIG. 1) where the porosity is Hb and the depth from both surfaces is the same (that is, the distance 8 from the A surface 2 is equal to the distance 9 from the B surface 3 in FIG. 1). It is important to satisfy the following expressions (1) and (2) when the porosity of (corresponding to reference numeral 13) is Hc.
(1) 1.00 ≦ Ha / Hc ≦ 1.50
(2) 1.05 ≦ Hb / Hc ≦ 1.50

From the viewpoint of achieving both permeability and film strength, Ha / Hc is preferably 1.05 or more, more preferably 1.10 or more, and even more preferably 1.15 or more. It is especially preferable that it is 20 or more. Further, Ha / Hc is preferably 1.45 or less, more preferably 1.40 or less, further preferably 1.35 or less, and particularly preferably 1.30 or less. Similarly, Hb / Hc is preferably 1.10 or more, more preferably 1.15 or more, further preferably 1.20 or more, and particularly preferably 1.25 or more. . Further, Ha / Hc is preferably 1.45 or less, more preferably 1.40 or less, further preferably 1.35 or less, and particularly preferably 1.30 or less.

The method of setting Ha / Hc and Hb / Hc in the above ranges is not particularly limited, but it is possible to use stretching under the conditions described later and then heat treatment under the conditions described later when manufacturing the separation membrane.

In the separation membrane of the present invention, it is preferable that the average porosity calculated by the simple average of the porosity Ha, Hb, Hc is 20% or more and 80% or less. The average porosity is more preferably 25% or more, further preferably 30% or more, and particularly preferably 35% or more. Similarly, the average porosity is more preferably 75% or less, further preferably 70% or less, and particularly preferably 65% or less.

When the average porosity is 20% or more, the permeability is good, and when the average porosity is 80% or less, the film strength is good. The method for setting the average porosity in the above range is not particularly limited, but a method for setting the content of the structure forming agent in the whole components constituting the raw material for film formation in the above preferable range can be used.

In addition, the measuring method of the porosity is described in the examples.

By satisfying the above-described structural period and formulas (1) and (2), the polymer phase and the pores have a co-continuous structure having a constant structural period in the cross section in the thickness direction, and In the cross section in the film thickness direction, there is a relationship that the porosity in the vicinity of at least one surface layer is larger than the porosity in the central portion in the thickness direction.

Especially, the former co-continuous structure makes it possible to develop high film strength. Further, the latter relationship between the porosity in the thickness direction of the film makes it possible to express high permeability and stability of the film performance.

(Membrane permeation flux)
The separation membrane of the present invention preferably has a membrane permeation flux at 50 kPa and 25 ° C. of 0.1 m ³ / m ² / h or more and 10 m ³ / m ² / h or less. The membrane permeation flux is more preferably 0.3 m ³ / m ² / h or more, and further preferably 0.5 m ³ / m ² / h or more. The measurement conditions of the membrane permeation flux will be described in detail in Examples.

(Membrane strength)
The separation membrane of the present invention preferably has a tensile strength in the longitudinal direction of 30 MPa or more in order to exhibit membrane strength against tensile in the longitudinal direction. The conditions for measuring the tensile strength will be described in detail in Examples. The tensile strength is more preferably 50 MPa or more, further preferably 70 MPa or more, and particularly preferably 90 MPa or more. Higher tensile strength is preferable, but it is preferably 300 MPa or less from the viewpoint of balance with elongation.

(Production method)
The method for producing the separation membrane of the present invention comprises:
1. Melt-kneading step of obtaining a resin composition by melt-kneading 20% by weight to 80% by weight of hydrophilic polymer and 20% by weight to 80% by weight of a structure-forming agent. 2. A film forming process for forming a film by discharging and cooling the resin composition from a die. 3. Stretching process for stretching the film at a stretch ratio of 1.1 to 5.0. 4. Heat treatment step of heating the stretched film obtained in the stretching step at 50 ° C. or more and 300 ° C. or less. An elution step of eluting the structure-forming agent from the film obtained in the heat treatment step.

Next, the method for producing the separation membrane of the present invention will be specifically described by taking a case where the separation membrane is a hollow fiber membrane as an example, but is not limited thereto.

In obtaining the resin composition for forming a separation membrane of the present invention, a method of melt-kneading 20% by weight to 80% by weight of a hydrophilic polymer and 20% by weight to 80% by weight of a structure forming agent is used. It is done. If necessary, a hydrophilic polymer plasticizer, antioxidant, and additive of the above-described types and contents can be contained.

The apparatus to be used is not particularly limited, and a known mixer such as a kneader, roll mill, Banbury mixer, single-screw or twin-screw extruder can be used. Among these, from the viewpoint of improving the dispersibility of the structure forming agent and the plasticizer, it is preferable to use a twin screw extruder. From the viewpoint of removing volatiles such as moisture and low molecular weight substances, it is more preferable to use a twin screw extruder with a vent hole.

The obtained resin composition may be once pelletized and melted again and used for melt film formation, or may be directly guided to a die and used for melt film formation. Once pelletized, it is preferable to use a resin composition in which the pellet is dried to have a moisture content of 200 ppm (weight basis) or less.

A hollow fiber membrane is formed by discharging the resin composition melted by the above method into the air from a spinneret having a double annular nozzle having a gas flow path at the center, and cooling with a cooling device. The draft ratio that can be calculated by the winding speed / discharge speed is preferably 50 or more and 500 or less. The draft ratio is more preferably 400 or less, and further preferably 300 or less.

The formed hollow fiber membrane may be wound up once, unwound again and used for stretching, or may be directly guided to a stretching process for stretching. The stretching process is not only preferable in terms of improving the film strength by increasing the orientation of the hydrophilic polymer, but is also important in controlling the internal structure and porosity of the film formed by the subsequent heat treatment within the above-mentioned range. is there. Although the stretching method is not particularly limited, for example, the temperature may be increased to a temperature at which stretching is performed by conveying the hollow fiber membrane before stretching on a heated roll, and a method of stretching using a peripheral speed difference between the rolls may be used, A method may be used in which the hollow fiber membrane before stretching is heated to a temperature at which stretching is performed by transporting it in a dry heat oven, and stretched using a peripheral speed difference between rolls. In addition, the stretching may be performed in one stage, or may be performed in two or more stages.

The preferable range of the temperature of the hollow fiber membrane in the stretching step is 40 to 180 ° C, more preferably 60 to 160 ° C, still more preferably 80 to 140 ° C. The total draw ratio is preferably 1.2 times or more, more preferably 1.4 times or more, and further preferably 1.6 times or more. The total draw ratio is preferably 5.0 times or less, more preferably 4.5 times or less, and further preferably 4.0 times or less.

Subsequently, the hollow fiber membrane (stretched membrane) is heat-treated by heating at 50 to 300 ° C. Through this heat treatment step, phase separation between the hydrophilic polymer and the structure forming agent is induced. The heat treatment may be carried out on a heated roll, may be carried in a dry heat oven, or may be put into a dry heat oven in a roll wound around a bobbin or a paper tube. .

The heat treatment temperature is preferably 80 to 300 ° C, more preferably 100 to 250 ° C, and further preferably 120 to 250 ° C. The heat treatment time is preferably 10 to 600 seconds, more preferably 20 to 480 seconds, and further preferably 30 to 360 seconds.

The hollow fiber membrane after the heat treatment is immersed in water, an acid aqueous solution, an alkaline aqueous solution, an alcohol, or an alcohol aqueous solution, and the step of eluting the structure-forming agent, and then the separation membrane (hollow fiber membrane) of the present invention To do. Since the present invention is a method of eluting the structure-forming agent after inducing phase separation by performing heat treatment, voids exceeding 10 μm are not easily formed on the resulting separation membrane.

Although the separation membrane thus obtained can be used as it is, 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.

Thus, the separation membrane of the present invention having a hydrophilic polymer as a main component can be produced.

(module)
The separation membrane of the present invention may be incorporated into a separation membrane module at the time of use. The separation membrane module includes, for example, a membrane bundle composed of a plurality of hollow fiber membranes and a housing that accommodates the membrane bundle.

Also, if it is a flat membrane, it is fixed to a support, or an envelope-like membrane is formed by bonding the membranes together, and if necessary, it is modularized by being attached to a water collection pipe or the like.

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

[Measurement and evaluation method]
Each characteristic value in the examples is obtained by the following method. In the following (3) to (7), the separation membrane was measured and evaluated in a state of being vacuum-dried at 25 ° C. for 8 hours.

(1) Average substitution degree of cellulose mixed ester The calculation method of the average substitution degree of the cellulose mixed ester in which an acetyl group and an acyl group are bonded to cellulose is as follows.
After 0.9 g of cellulose ester mixed for 8 hours at 80 ° C. was weighed and dissolved by adding 35 ml of acetone and 15 ml of dimethyl sulfoxide, 50 ml of acetone was further added. While stirring, 30 ml of 0.5N sodium hydroxide aqueous solution was added and saponified for 2 hours. After adding 50 ml of hot water and washing the side of the flask, it was titrated with 0.5 N sulfuric acid using phenolphthalein as an indicator. Separately, a blank test was performed in the same manner as the sample. The supernatant of the solution after titration was diluted 100 times, and the composition of the organic acid was measured using an ion chromatograph. From the measurement result and the acid composition analysis result by ion chromatography, the substitution degree was calculated by the following formula.
TA = (BA) × F / (1000 × W)
DSace = (162.14 × TA) / [{1− (Mwace− (16.00 + 1.01)) × TA} + {1− (Mwacy− (16.00 + 1.01)) × TA} × (Acy / Ace)]
DSacy = DSace × (Acy / Ace)
TA: Total organic acid amount (ml)
A: Sample titration (ml)
B: Blank test titration (ml)
F: titer of sulfuric acid W: sample weight (g)
DSace: average substitution degree of acetyl group DSacy: average substitution degree of other acyl groups Mwash: molecular weight of acetic acid Mwacy: molecular weight of other organic acids Acy / Ace: mole of acetic acid (Ace) and other organic acids (Acy) Ratio 162.14: Molecular weight of cellulose repeating unit 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 weight, and used as a sample for GPC measurement. Using this sample, GPC measurement was performed with Waters 2690 under the following conditions, and the weight average molecular weight (Mw) was determined in terms of polystyrene.
Column: Two Tosoh TSK gel GMHHR-H connected Detector: Waters2410 Differential refractometer RI
Moving bed solvent: Tetrahydrofuran Flow rate: 1.0 ml / min Injection volume: 200 μl

(3) Structural period Measurement method 1
After the separation membrane immersed in a 50% aqueous solution of glycerin for 1 hour was frozen with liquid nitrogen, it was cleaved by applying stress so that a section perpendicular to the longitudinal direction of the separation membrane and a thickness direction of the membrane appeared. At this time, if the longitudinal direction of the separation membrane is unknown, it is cleaved in an arbitrary direction. Moreover, when cleaving, a razor or a microtome is used as necessary. The separation membrane having a cross-section was vacuum-dried at 25 ° C. for 8 hours, and the obtained cross-section was observed with a scanning electron microscope. At this time, the film cross section was observed with the center in the film thickness direction as the center of the microscope field. The obtained scanning electron microscope image was subjected to Fourier transform, and the presence or absence of a maximum peak when the wave number was plotted on the horizontal axis and the intensity was plotted on the vertical axis was confirmed. When the maximum peak exists, that is, when the plot result has a maximum value, the structural period λ (= 1 / q) was derived from the wave number q corresponding to the maximum value. At this time, the image size of the scanning electron microscope image was a square with one side having a length of 10 to 100 times the hole diameter. When the measurement method 1 shows the maximum value, the uniformity of the pore diameter is high, and higher film strength can be expressed, which is a preferable mode.
Measurement method 2
When the periodic structure is not measured by the measurement method 1, the structural period is measured by the following method.
After the separation membrane immersed in a 50% aqueous solution of glycerin for 1 hour was frozen with liquid nitrogen, it was cleaved by applying stress so that a section perpendicular to the longitudinal direction of the separation membrane and a thickness direction of the membrane appeared. At this time, if the longitudinal direction of the separation membrane is unknown, it is cleaved in an arbitrary direction. Moreover, when cleaving, a razor or a microtome is used as necessary. The separation membrane having a cross-section was vacuum-dried at 25 ° C. for 8 hours, and the obtained cross-section was observed with a scanning electron microscope. At this time, the film cross section was observed with the center in the film thickness direction as the center of the microscope field. In the obtained image, the diameters of 20 pores were measured and number averaged to obtain the structural period. At this time, the image size of the scanning electron microscope image is a square with one side having a length of 10 to 100 times the hole diameter.

(4) Separation membrane thickness (μm)
The cross section of the membrane produced in (3) above was observed and photographed with an optical microscope, and the thickness (μm) of the separation membrane was calculated. In addition, the thickness of the separation membrane was calculated by observing arbitrary 10 locations, and was taken as an average value.

(5) Porosity (Ha, Hb, Hc, average porosity) (%)
Regarding the cross section of the film prepared in (3), a region a having a depth of 1 to 4 μm from one surface (A surface), a region b having a depth of 1 to 4 μm from the other surface (B surface), and both surfaces A region c having a thickness of 3 μm having the same depth from the center was observed and photographed at arbitrary five points with a scanning electron microscope. A transparent film or sheet was overlaid on the obtained photograph, and the portion corresponding to the gap was painted with oil-based ink or the like. Next, the ratio of the area corresponding to the air gap is obtained using an image analyzer. This measurement was carried out at five locations where the images were taken and averaged to determine the void ratios Ha (%), Hb (%), and Hc (%) of each region.
The average porosity was calculated by simply averaging Ha, Hb, and Hc.
At this time, the image size of the scanning electron microscope image is a square with one side having a length of 10 to 100 times the hole diameter.

(6) Outer diameter of hollow fiber membrane (μm)
The cross section of the membrane produced in (3) was observed and photographed with an optical microscope, and the outer diameter (μm) of the hollow fiber membrane was calculated. In addition, the outer diameter of the hollow fiber membrane was calculated by observing arbitrary 10 locations, and was taken as an average value.

(7) Tensile strength (MPa)
The tensile strength in the longitudinal direction of the separation membrane that was vacuum-dried at 25 ° C. for 8 hours was measured using a tensile tester (Orientec Tensilon UCT-100) in an environment of 20 ° C. and 65% humidity. . Specifically, measurement was performed under the conditions of a sample length of 100 mm and a tensile speed of 100 mm / min, and the tensile strength (breaking strength) (MPa) was calculated from the tensile strength. The number of measurements was 5, and the average value was used.

(8) Membrane permeation flux (m ³ / m ² / h)
When the separation membrane was a hollow fiber membrane, a small module having an effective length of 200 mm consisting of four hollow fiber membranes was produced. The amount of permeated water (m ³ ) obtained by feeding distilled water over 30 minutes to the module at a temperature of 25 ° C. and a filtration differential pressure of 16 kPa was measured, and the unit time (h) and unit membrane area (m ² ) were measured. ) Per unit value, and further converted into pressure (50 kPa) to obtain permeation performance of pure water (unit = m ³ / m ² / h).

(9) Stability of membrane performance Twenty small modules similar to (8) were produced, and the membrane permeation flux was determined by the method described in (8). The standard deviation was calculated for the 20 data obtained. Using the standard deviation, the following criteria were used for evaluation.
◎: Less than 0.02 ○: 0.02 or more and less than 0.05 Δ: 0.05 or more and less than 0.1 ×: 0.1 or more

[Hydrophilic polymer (A)]
Cellulose ester (A1): Cellulose acetate propionate obtained by the following method To 100 parts by weight of cellulose (cotton linter), 240 parts by weight of acetic acid and 67 parts by weight of propionic acid were added and mixed at 50 ° C. for 30 minutes. After the mixture was cooled to room temperature, 172 parts by weight of acetic anhydride cooled in an ice bath and 168 parts by weight of propionic anhydride were added as an esterifying agent, and 4 parts by weight of sulfuric acid was added as an esterification catalyst, followed by stirring for 150 minutes. An esterification reaction was performed. 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 weight of acetic acid and 33 parts by weight of water was added as a reaction terminator over 20 minutes to hydrolyze excess anhydride. Thereafter, 333 parts by weight of acetic acid and 100 parts by weight 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 weight of sodium carbonate was added, and the precipitated cellulose acetate propionate was filtered off, subsequently washed with water, and dried at 60 ° C. for 4 hours. The average substitution degree of the acetyl group and 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.

[Plasticizer for hydrophilic polymer (B)]
Plasticizer (B1): polyethylene glycol, weight average molecular weight 600

[Structure forming agent (C)]
Structure forming agent (C1): PVP / vinyl acetate copolymer (Kollidon VA 64 (BASF Japan Ltd.))
Structure forming agent (C2): Polyvinylpyrrolidone (PVP K17)

[Antioxidant (D)]
Antioxidant (D1): Bis (2,6-di-t-butyl-4-methylphenyl) pentaerythritol diphosphite

[Manufacture of separation membrane]
Example 1
45% by weight of the hydrophilic polymer (A1), 24.9% by weight of the plasticizer (B1), 30% by weight of the structure forming agent (C1) and 0.1% by weight of the antioxidant (D1) in a twin screw extruder The mixture was melt-kneaded at 220 ° C., homogenized and then pelletized to obtain a resin composition for melt spinning. This resin composition was vacuum-dried at 80 ° C. for 8 hours.

The dried resin composition is supplied to a twin screw extruder, melted and kneaded at 220 ° C., and then introduced into a melt spinning pack having a spinning temperature of 220 ° C. A double tube type, a discharge hole diameter of 8.3 mm, and a slit width of 1.1 mm was spun downward from the outer annular portion of the die having one hole. The spun hollow fiber was guided to a cooling device, cooled with 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 60. The spun yarn was heated to 100 ° C. by passing through a dry heat oven, and wound up at a draw ratio of 1.3 times using a difference in peripheral speed between rolls. Subsequently, after heat treatment at 150 ° C. for 300 seconds, the separation membrane was immersed in a 50% ethanol aqueous solution for 12 hours to elute the plasticizer and the structure forming agent. Table 1 shows the physical properties of the obtained separation membrane. The structure period was measured using the method described in Measurement Method 1 above.

(Examples 2 to 13, Comparative Example 1)
A separation membrane was obtained in the same manner as in Example 1 except that the production conditions were as shown in Table 1. Table 1 shows the physical properties of the obtained separation membrane. The structural period was measured using Examples 2 to 10 and Example 13 using the method described in Measurement Method 1 above, and Examples 11 and 12 using the method described in Measurement Method 2 above. .

From the results in Table 1, since the separation membranes of Examples 1 to 13 have a membrane permeation flux of 0.1 m ³ / m ² / h or more and a tensile strength of 30 MPa or more, good permeation performance And the film strength was expressed. The separation membranes of Examples 1 to 10 and Example 13 had maximum values in the structural period measurement method 1. This represents that the uniformity of the hole diameter is high. It is considered that the strength of the separation membrane is increased when the uniformity of the pore diameter is high, and in fact, the membranes of these examples showed particularly high strength as shown in Table 1. The separation membranes of Examples 1 to 13 all had a co-continuous structure.
On the other hand, in the separation membrane of Comparative Example 1, both the membrane permeation flux and the stability of the membrane performance were insufficient.

The present invention is a separation membrane mainly composed of a hydrophilic polymer having excellent permeability and membrane performance stability and high membrane strength. The separation membrane of the present invention is a water treatment membrane for producing industrial water, drinking water, etc. from seawater, brine, sewage, drainage, etc., a medical membrane such as an artificial kidney or plasma separation, and a food / beverage such as fruit juice concentrate It can be used for industrial membranes, gas separation membranes for separating exhaust gas, carbon dioxide gas, etc., and membranes for electronic industries such as fuel cell separators. The water treatment membrane can be preferably used for microfiltration membranes, ultrafiltration membranes, and the like.

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 July 29, 2016 (Japanese Patent Application No. 2016-149351), the contents of which are incorporated herein by reference.

DESCRIPTION OF SYMBOLS 1 Separation membrane 2 A surface 3 B surface 8 Distance from A surface 9 Distance from B surface 11 Area a
12 region b
13 region c

Claims (6)

A separation membrane mainly composed of a hydrophilic polymer,
Having a co-continuous structure in which the structural period of the phase having the hydrophilic polymer and the pores is 0.001 μm or more and 10 μm or less;
The thickness of the separation membrane is 10 μm or more and 500 μm or less,
In the cross section in the thickness direction, the porosity of the region a 1 to 4 μm deep from either surface is Ha, the porosity of the region b 1 to 4 μm deep from the other surface is Hb, and the depth from both surfaces A separation membrane satisfying the following formulas (1) and (2), where Hc is the porosity of the region c having the same thickness of 3 μm.
(1) 1.00 ≦ Ha / Hc ≦ 1.50
(2) 1.05 ≦ Hb / Hc ≦ 1.50 The separation membrane according to claim 1, wherein an average porosity calculated by a simple average of the porosity Ha, the porosity Hb, and the porosity Hc is 20% or more and 80% or less. The separation membrane according to claim 1 or 2, wherein the hydrophilic polymer is at least one selected from the group consisting of polyester, polyamide and cellulose ester. The separation membrane according to any one of claims 1 to 3, wherein the separation membrane has a hollow fiber shape. The separation membrane according to claim 4, wherein the hollow fiber has an outer diameter of 50 µm or more and 2500 µm or less. A method for producing a separation membrane comprising a hydrophilic polymer as a main component, wherein at least the following steps 1 to 5 are carried out.
1. Melt-kneading step of obtaining a resin composition by melt-kneading 20% by weight to 80% by weight of hydrophilic polymer and 20% by weight to 80% by weight of a structure-forming agent. 2. A film forming process for forming a film by discharging and cooling the resin composition from a die. 3. Stretching process for stretching the film at a stretch ratio of 1.1 to 5.0. 4. Heat treatment step of heating the stretched film obtained in the stretching step at 50 ° C. or more and 300 ° C. or less. An elution step of eluting the structure-forming agent from the film obtained in the heat treatment step

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