JP2021003684A - Separation film and manufacturing method thereof - Google Patents

Abstract

To provide a separation film having excellent water permeable performance due to improvement in surface hydrophilicity, increase in a specific surface area and a substantial reduction in film thickness.SOLUTION: The present invention provides a separation film containing cellulose acetate butylate and polyvinyl alcohol, where a ratio of the polyvinyl alcohol to the cellulose acetate butyl is within a range of 10-50 mass%.SELECTED DRAWING: None

Description

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, separator membranes for batteries, charged membranes, electrolyte membranes for fuel cells, etc. It is used in various fields.

Most separation membranes are made of polymer. Among them, cellulosic resins such as cellulose esters have water permeability 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 Unexamined Patent Publication No. 2011-235204 Japanese Unexamined Patent Publication No. 2015-157278

Ind. Eng. Chem. Res. 2011, 50, 3798-3817.

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 water permeability, and as a result. However, there is a problem that defects are likely to occur in the separation membrane. On the other hand, if the separation layer is made thicker in order to suppress the occurrence of defects, there is a problem that the water permeability is lowered.

Therefore, an object of the present invention is to provide a separation membrane or the like having high water permeability.

As a result of diligent studies to solve the above problems, the present inventors have found that the inclusion of cellulose acetate butyrate and polyvinyl alcohol in a specific ratio is extremely important for improving the water permeability, and the present invention has been made. I arrived.

That is, the separation membrane of the present invention is characterized by containing cellulose acetate butyrate and polyvinyl alcohol, and the ratio of polyvinyl alcohol to cellulose acetate butyrate is in the range of 10 to 50% by mass.

According to the present invention, it is possible to provide a separation membrane having high water permeability.

The separation membrane of the present invention is characterized by containing cellulose acetate butyrate and polyvinyl alcohol, and the ratio of polyvinyl alcohol to cellulose acetate butyrate is in the range of 10 to 50% by mass.

(Resin composition constituting the separation membrane)
The resin composition constituting the separation membrane of the present invention contains the following components (3) to (5) in addition to the following (1) cellulose acetate butyrate and (2) polyvinyl alcohol. be able to.

(1) Cellulose Acetate Butyrate The separation membrane of the present invention needs to contain cellulose acetate butyrate. In order to further enhance the effect of the present invention, the separation membrane of the present invention preferably contains a cellulose ester as a main component. The main component referred to here is a component that is contained in the largest amount by mass among all the components of the resin composition constituting the separation membrane.

The cellulose acetate butyrate here is a cellulose ester having an average degree of substitution between an acetyl group and a butyryl group of 0.1 or more, respectively.

The weight average molecular weight (Mw) of cellulose acetate butyrate is preferably 20,000 to 250,000. When the weight average molecular weight (Mw) is 20,000 or more, the thermal decomposition of cellulose acetate butyrate during 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. Can be done. 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 more preferably 20,000 to 100,000, and even more preferably 20,000 to 60,000. The weight average molecular weight (Mw) is a value calculated by GPC measurement. The calculation method will be described in detail in Examples.

The exemplified cellulose acetate butyrate has an acetyl group and another acyl group (propionyl group, butyryl group, etc.). The average degree of substitution between the acetyl group and the butyryl group preferably satisfies the following formula.

2.1 ≤ (average substitution of acetyl group + average substitution of butyryl group) ≤ 3.0
0.1 ≤ (average degree of substitution of acetyl groups) ≤ 1.0
2.0 ≤ (average degree of substitution of butyryl groups) ≤ 2.6
By satisfying the above formula, the water permeability of the separation membrane and good heat fluidity when melting the resin composition constituting the separation membrane are realized. The average degree of substitution refers to the number of chemically bonded acyl groups among the three hydroxyl groups present per glucose unit of cellulose. The detailed measurement method of the average degree of substitution will be described in Examples.
The melting temperature of the cellulose acetate butyrate contained in the separation membrane, which is determined by a differential operating calorimeter, is preferably 150 to 180 ° C, more preferably 160 to 175 ° C. When the melting temperature is 150 ° C. or higher, the melting viscosity does not become too low, so that stable melt forming is possible. On the other hand, when the melting temperature is 180 ° C. or lower, thermal decomposition of polyvinyl alcohol during melt film formation can be suppressed.
The separation membrane may contain only one type of cellulose acetate butyrate, or may contain two or more types of cellulose acetate butyrate.

The content of cellulose acetate butyrate in the separation membrane is preferably 40 to 87% by mass, more preferably 50 to 87% by mass, and 60 to 87% by mass when all the components of the separation membrane are 100% by mass. More preferred. When the content of cellulose acetate butyrate in the separation membrane is 40% by mass or more, the membrane strength of the separation membrane becomes sufficient. On the other hand, when the content of cellulose acetate butyrate in the separation membrane is 90% by mass or less, appropriate water permeability can be obtained.

The content of cellulose acetate butyrate in the raw material for producing the separation membrane is preferably 20 to 65% by mass, assuming that the total components constituting the raw material are 100% by mass. When the content is 20% by mass or more, the membrane strength of the separation membrane becomes good. On the other hand, when the content is 65% by mass or less, the thermoplasticity and water permeability of the separation membrane are improved. The content is more preferably 20 to 50% by mass, further preferably 20 to 40% by mass.
(2) Polyvinyl alcohol The resin composition constituting the separation membrane of the present invention needs to contain polyvinyl alcohol as a structure-forming agent.

The polyvinyl alcohol in the present invention can be eluted or decomposed by a solvent that is compatible or partially compatible with cellulose acetate butyrate or a mixture of cellulose acetate butyrate and a plasticizer thereof, and does not dissolve cellulose acetate butyrate. If there is, there is no particular limitation.

Partial compatibility 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 phase-separates from cellulose acetate butyrate 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 weight average molecular weight (Mw) of polyvinyl alcohol is preferably 20000 to 50000, more preferably 20000 to 40,000, and even more preferably 20000 to 30000. When the weight average molecular weight (Mw) is 50,000 or less, the melt viscosity does not become too high, so that the compatibility with cellulose acetate butyrate becomes high, and a film structure having good water permeability can be obtained. On the other hand, when the weight average molecular weight (Mw) is 20000 or more, the melt viscosity does not become too low, so that stable melt film formation becomes possible.
The degree of saponification of polyvinyl alcohol is preferably 50 to 99 mol%, more preferably 60 to 95 mol%, still more preferably 70 to 92 mol%. Here, the "degree of saponification" refers to the ratio of hydroxy groups to the total of hydroxy groups and acetyl groups in polyvinyl alcohol. When the degree of saponification of polyvinyl alcohol is 50 mol% or more, the compatibility with cellulose acetate butyrate becomes high, and the hydrophilicity of the membrane surface becomes high, so that the water permeability becomes good. On the other hand, when it is 99 mol% or less, it is possible to prevent a decrease in water permeability due to swelling of polyvinyl alcohol.

The content of polyvinyl alcohol with respect to cellulose acetate butyrate in the separation membrane is preferably 10 to 50% by mass, more preferably 10 to 35% by mass, when the cellulose acetate butyrate of the separation membrane is 100% by mass. ~ 25% by mass is more preferable. When the content of polyvinyl alcohol with respect to cellulose acetate butyrate of the separation membrane is 10% by mass or more, the hydrophilicity of the membrane surface becomes high, so that the water permeability becomes good. On the other hand, when it is 50% by mass or less, the decrease in water permeability due to the swelling of polyvinyl alcohol is prevented, and the membrane strength of the separation membrane becomes good, which is preferable.

The content of polyvinyl alcohol is preferably 15 to 45% by mass when the total content of the components constituting the raw material is 100% by mass.

When the content is 15% by mass or more, the water permeability of the separation membrane becomes good. On the other hand, when the content is 45% by mass or less, the film strength becomes good. The content of polyvinyl alcohol is more preferably 15 to 35% by mass, further preferably 25 to 35% by mass.
(3) Plasticizer The resin composition constituting the separation membrane of the present invention may contain a plasticizer.

The plasticizer is not particularly limited as long as it is a compound that thermally plasticizes cellulose acetate butyrate. Further, not only one kind of plasticizer but also two or more kinds of plasticizers may be used in combination.

Examples of the plasticizer used for the separation film of the present invention include polyalkylene glycol-based compounds such as polyethylene glycol and polyethylene glycol fatty acid ester, glycerin-based compounds such as glycerin fatty acid ester and diglycerin fatty acid ester, citric acid ester-based compounds, and phosphorus. Examples thereof include fatty acid ester compounds such as acid ester compounds and adipic acid esters, caprolactone compounds, and derivatives thereof.

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.

The cellulose acetate butyrate plasticizer may remain in the separation membrane after the separation membrane is formed, or may be eluted from the separation membrane.

The content of the plasticizer of cellulose acetate butyrate is preferably 20 to 55% by mass when the total content of the components constituting the raw material is 100% by mass.

When the content is 20% by mass or more, the thermoplasticity of the cellulose acetate butyrate becomes good. On the other hand, when the content is 55% by mass or less, the membrane strength of the separation membrane becomes good. The content of the plasticizer of cellulose acetate butyrate is more preferably 25 to 45% by mass, further preferably 35 to 45% by mass.

(4) Antioxidant The resin composition constituting the separation membrane of the present invention may contain 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 phosphite esters of pentaerythritol-based compounds are more preferable. Examples of the phosphite esters 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 content 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.

(5) Additives The resin composition constituting the separation membrane of the present invention may contain additives other than those described in (2) to (4) as long as the effects of the present invention are not impaired.

Examples of the additive include cellulose ether, polyacrylonitrile, polyolefin, polyvinyl compound, polycarbonate, poly (meth) acrylate, polysulfone, resin such as polyethersulfone, organic lubricant, crystal nucleating agent, organic particles, inorganic particles, and end-sealing. 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 agent, color pigment, fluorescent whitening agent, dye 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 is more preferable because it can be efficiently filled in the module and the effective film area per unit volume of the module can be increased.

The thickness of the separation membrane is preferably 10 to 500 μm, more preferably 30 to 400 μm, and even more preferably 50 to 300 μm or less, from the viewpoint of achieving both water permeability and film strength.

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

Further, in the case of the 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 portion and the buckling pressure. It is preferably 25 to 60%, more preferably 25 to 60%. Here, the "hollow fiber ratio" refers to the ratio of the area of the inner peripheral circle (hollow portion) to the area of the outer peripheral circle when the cross section of the hollow fiber membrane is observed at a magnification of 200 times using a microscope microscope. ..

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.

(Convex part on the film surface)
The separation membrane of the present invention has a convex portion on the membrane surface. Here, the "convex portion" means two portions (hereinafter, "bottom") where the angle of the membrane surface is 135 ° or less when the cross section of the separation membrane is observed at a magnification of 400 times using a microscope microscope. Yes, and the height of the convex part is 5 μm or more. Here, the "convex height" is the distance at which the distance from the bottom to the film surface is the longest when the bottoms of two points are connected by a straight line and a straight line is drawn vertically from the straight line toward the film surface. To say.
The value X / S obtained by dividing the average value X of the distances between adjacent vertices of the convex portion by the average value S of the film thickness is preferably 0.5 to 1.0. When the value obtained by dividing X by S is 0.5 or more, the protrusions are prevented from overlapping with each other, and the increase in the specific surface area with respect to the film thickness is effective in improving the water permeability. On the other hand, when the value obtained by dividing X by S is 1.0 or less, the film shape can be maintained and the frequency of protrusions can be increased, which is preferable. The method of obtaining the average value X of the distance between adjacent vertices of the convex portion and the average value S of the film thickness will be described in Examples.

The average value of the height of the convex portion is indicated by the average value of the convex portion height measured using a microscope microscope "Z ₁ and the average value of the convex portion height measured from the scanning electron microscope image Z _2. .. Here, Z ₁ is the average value of the heights measured at any 10 points among all the convex parts observed when the cross section of the separation membrane is observed at a magnification of 400 times using a microscope microscope. Say. On the other hand, Z ₂ refers to any 10 points among the convex portions existing on the cross section when the cross section of the separation membrane is observed at a magnification of 350 times using a scanning electron microscope (hereinafter, “SEM”). The average value of the measured heights.
The value Z ₁ / S obtained by dividing the average value Z ₁ of the height of the convex portion on the film surface by the average value S of the film thickness is preferably 0.1 to 0.35. When the value obtained by dividing Z ₁ by S is 0.1 or more, an increase in the specific surface area with respect to the film thickness is effective in improving the water permeability. On the other hand, when the value obtained by dividing Z ₁ by S is 0.35 or less, the film shape can be maintained, and the convex portion is crushed, dropped, or damaged when external pressure filtration is performed. This is preferable because the decrease in film area can be suppressed.
(Void inside the convex part)
The separation membrane of the present invention has a void inside the convex portion. Here, the void means a concave portion having an area of 20 μm ² or more when the inside of the convex portion of the separation membrane is observed at a magnification of 350 times using SEM. The "recess" referred to here means a dark part in the image observed by the SEM, and the outline can be extracted by binarizing the image captured by the SEM using image analysis software. When the diameter of the void is 10 μm or more, good water permeability can be exhibited. The diameter of the voids is measured by image processing to measure the area of all voids with an area of 20 μm ² or more inside the convex portion included in the image captured by SEM, and assuming a perfect circular hole of the same area, the following formula ( Calculate from 1).

r = (4 x A / π) ^0.5 ... (1)
A: The area of the voids The arithmetic mean of the _n values r _{1 to} rn is defined as the average diameter R of the voids. The value obtained by dividing the average diameter R of the voids by the average value Z ₂ of the convex heights measured from the scanning electron microscope image is preferably 2.5 to 6.0, more preferably 2.5 to 4.0. When the value obtained by dividing R by Z ₂ is 2.5 or more, the film thickness is substantially reduced, and good water permeability is exhibited. On the other hand, when it is 6.0 or less, it is possible to suppress the crushing of the convex portion when the external pressure type filtration is performed.

(Average pore size on the membrane surface)
The separation membrane of the present invention has a plurality of pores. Here, the “pore” refers to a recess having an area of 2 μm ² or less when the surface of one surface of the separation membrane is observed at a magnification of 10,000 times using SEM. The "recess" referred to here refers to a dark part in an 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. When the diameter of the pores is 1 nm or more, good water permeability can be exhibited. The diameter of the pores is measured by image processing to measure the area of all voids with an area of 2 μm ² or less contained in the image captured by SEM, and assuming a perfect circular hole of the same area, from the following equation (2). calculate.

d = (4 x B / π) ^0.5 ... (2)
B: The area of the pores The arithmetic mean of the n values d _{1 to} d ₂ is defined as the average pore diameter D ₁ . The average pore diameter D ₁ on the film surface is preferably ₁ to 1000 nm. High water permeability can be obtained when the thickness is 1 nm or more. On the other hand, when it is 1000 nm or less, good separation performance can be exhibited as a separation membrane. The average pore diameter D ₁ is preferably 100 to 700 nm, more preferably 200 to 500 nm, and even more preferably 300 to 500 nm. In the present specification, the width of the pores may be simply referred to as the pore diameter.

The details of the method for measuring the pore diameter will be described in Examples.
(Aperture ratio)
The separation membrane of the present invention preferably has a surface opening ratio (hereinafter, “opening ratio”) of 3% or more and 50% or less. When it is 3% or more, a good permeation flux can be obtained, and when it is 50% or less, a good membrane strength can be obtained.

The aperture ratio is the ratio of the area of the voids to the observed area when the surface is observed, and is represented by the aperture ratio (%) = the area of the pores on the surface / the observed area × 100.

The aperture ratio described in the present specification is a value measured and calculated using a scanning electron microscope under the conditions described in Examples described later.

(Water infiltration time)
The separation membrane of the present invention is made of the above-mentioned cellulose acetate butyrate and polyvinyl alcohol, and has excellent hydrophilicity on the membrane surface. As an index of hydrophilicity, the separation membrane of the present invention preferably has a membrane surface permeation time into water of 5 seconds or less, more preferably 3 seconds or less, and even more preferably 1 second or less. Here, the "water permeation time" is defined as the water droplets of 1.5 μL of pure water gently contacting the surface of the membrane, and the angle formed by the tangent line at the end point of the water droplet formed on the surface of the membrane and the surface of the membrane is contacted. It shows the time until the contact angle reaches 10 ° when measured with an angle meter. Therefore, it can be said that the shorter this time is, the higher the hydrophilicity is. The more hydrophilic it is, the more it is possible to prevent the adsorption of organic substances such as proteins contained in the filtered raw water to the membrane, so that the deterioration of the filtration performance due to the contamination of the membrane can be suppressed.

(Manufacturing method of separation membrane)
The method for producing a separation membrane of the present invention is
(1) Preparation step for obtaining a resin composition by melt-kneading a mixture containing 20 to 65% by mass of cellulose acetate butyrate, 15 to 45% by mass of polyvinyl alcohol, and 20 to 55% by mass of a plasticizer. ..
(2) A molding step of discharging the above resin composition from a discharge port to obtain a resin molded product.
(3) Immersion step of immersing the resin molded product in a solvent. To be equipped.

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, 20 to 65% by mass of cellulose acetate butyrate, 15 to 45% by mass of polyvinyl alcohol, and 20 to 55% by mass of a plasticizer were used. The mixture containing the above is melt-kneaded. The mixture preferably contains 20 to 50% by mass of cellulose acetate butyrate, 15 to 35% by mass of polyvinyl alcohol, and 25 to 45% by mass of a plasticizer, and 20 to 40% by mass of cellulose acetate butyrate. It is more preferable to contain 25 to 35% by mass of polyvinyl alcohol and 35 to 45% by mass of a plasticizing agent.

The apparatus used for melt-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 the air from a discharge port having a double annular nozzle having a gas flow path arranged in the center and cooling with a cooling device to form a resin molded product. Absent.

Immersion step, the solvent solubility parameter distance D _S is 10 to 25 with respect to the cellulose ester as a raw material, the step of impregnating the resin molded product is preferred. 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 composition while maintaining the shape of the resin. At this time, it is presumed that the plasticizer and the plasticizer are eluted while the phase separation of the resin composition occurs. The longer or higher the immersion time and temperature of the solvent, the larger the abundance ratio of voids and pores and the pore size 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 solubility parameter distance D _S of the formula (3)

The smaller the value, the higher the affinity of the solvent for the cellulose ester. However, δ _Ad , δ _Ap and δ _Ah are dispersion terms, polar terms and hydrogen bond terms of the solubility parameter of the cellulose ester, and δ _Bd , δ _Bp and δ _Bh are dispersion terms of the solubility parameter of the solvent or mixed solvent. , Polarity term and hydrogen bond term. The solubility parameter (δ _Mixture ) of the mixed solvent can be obtained by the following formula (4).

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 component i" means the ratio of the volume of component i before mixing to the sum of the volumes of all 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 which is not described in the above software can be calculated by the Hansen sphere method using the above software.

In the present invention, the solvent for immersing the resin molded product, D _S is preferably a solvent such as a 13 to 25. As such solvent, a solvent such as D _S is 4 to 12, preferably a mixed solvent of water, for example, .gamma.-butyrolactone, acetone, acetonitrile, 1,4-dioxane, methyl acetate and tetrahydrofuran A mixed solvent of at least one and water selected from the above group. 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.

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 examples was obtained by the following method.

(1) Ratio of polyvinyl alcohol to cellulose acetate butyrate Cellulose acetate butyrate and polyvinyl alcohol are melt-kneaded at 200 ° C. in a single-screw extruder at ratios of 100:50, 100:30, and 100:10 to homogenize. After the formation, pelletization was performed to obtain a resin composition. After that, three types of press films having a film thickness of about 160 μm were prepared on a hot stage at 200 ° C. Using this press film as a standard sample, FT-IR measurement was performed at any three points on the surface of each film using an FT-IR device (IRtracer100) under the following conditions, and the cellulose acetate butyrate in the obtained spectrum was obtained. a calibration curve was prepared for the average value of the peak intensity ratio of the 1420～1470Cm ^-1 representative of peaks and polyvinyl alcohol representative 1720~1770cm ^-1. After that, FT-IR measurement was performed on the separation membrane at any three points on the membrane surface, and based on the prepared calibration curve, polyvinyl alcohol (secondary component) for cellulose acetate butyrate (A), which is the main component of the separation membrane, was used. The ratio of B) was calculated.
Measurement mode: Absorbance apodizing function: Happ-Genzel
Integration function: 16 times Resolution: 4 cm ^-1
Wavenumber measured: 600-4000 cm ^-1
(2) 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 the mixture was saponified 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 (5) to (7).

TA = (BA) x F / (1000 x W) ... (5)
DSace = (162.14 × TA) / [{1- (Mwace- (16.00 + 1.01)) × TA} + {1- (Mwacy- (16.00 + 1.01)) × TA} × (Acy / Ace)] ・ ・ ・ ・ ・ ・ (6)
DSacy = DSace × (Acy / Ace) ・ ・ ・ ・ ・ ・ (7)
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 substitution degree of acetyl group DSacy: Average substitution degree 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 (3) Weight average molecular weight of cellulose acetate butyrate and polyvinyl alcohol (Mw)
It was completely dissolved in tetrahydrofuran or another solvent so that the concentration of cellulose acetate butyrate or polyvinyl alcohol 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 are connected Detector: Water2410 Differential refractometer RI
Flow velocity: 1.0 mL / min Injection volume: 200 μL
(4) Distance X between adjacent vertices of the convex portion
After freezing the separation membrane with liquid nitrogen, the separation membrane was cut by applying stress so that the cross section of the separation membrane would appear. At this time, if the longitudinal direction of the separation membrane is unknown, it shall be cut in any direction. When cutting, use a razor, microtome, etc., if necessary. Next, the cross section of the obtained membrane was observed at a magnification of 400 times using a microscope microscope. At this time, observation was performed with one of the film surfaces as the center of the field of view of the microscope. Of all the adjacent convex parts contained in the observed square microscopic image, the distance between vertices is calculated for any 10 points, and the arithmetic mean of the ₁₀ values x _{1 to} x _{10 is} the average of the separation membranes. Let X be the distance between adjacent vertices.
(5) Mean value of convex height measured by a microscope microscope Z ₁
The cross section of the film created in (4) above was observed with a microscope microscope at a magnification of 400 times, and the heights of any 10 points among all the convex parts included in the microscope image were calculated, and the heights of the 10 points were calculated. The arithmetic mean of the values z _{11 to} z ₁₁₀ was defined as Z ₁ .
(6) Mean value Z _{2 of the} height of the convex portion measured from the scanning electron microscope image
The cross section of the film prepared in (4) above was sputtered with platinum and observed at a magnification of 350 times using SEM. At this time, observation was performed with one of the film surfaces as the center of the field of view of the microscope. The height was calculated for any 10 points among the convex parts existing on all the cross sections included in the image captured by SEM, and the arithmetic mean of the 10 values z _{21 to} z ₂₁₀ was defined as Z ₂ . ..
(7) Thickness of separation membrane S
The cross section of the membrane prepared in (4) above was photographed with a microscope, and the thickness of the separation membrane excluding the convex portion was calculated. The thickness of the separation membrane was calculated by observing 10 arbitrary points, and the arithmetic mean thereof was defined as the film thickness S.
(8) Diameter R of the void inside the convex part
The cross section of the film prepared in (4) above was sputtered with platinum and observed at a magnification of 350 times using SEM. At this time, observation was performed with one of the film surfaces as the center of the field of view of the microscope. The diameters of all the concave parts with an area of 20 μm ² or more inside the convex part included in the image captured by SEM are measured by image processing, and it is calculated by the following formula (1) assuming a perfect circular hole of the same area. To do.

r = (4 x A / π) ^0.5 ... (1)
A: The area that n values _r 1 ~r _n arithmetic mean of the gap and the average diameter R of the gap.

(9) Average Pore Diameter of Membrane Surface The outer surface of the separation membrane sputtered with platinum was observed at a magnification of 10,000 times using an SEM, and the area included in the image imaged by the SEM was 2 μm. The areas of all the recesses of ² or less were measured by image processing, and calculated from the following formula (2) assuming a perfect circular hole having the same area.

d = (4 x B / π) ^0.5 ... (2)
B: Area of pores (10) Surface opening ratio of outer surface The outer surface of the separation membrane sputtered with platinum was observed at a magnification of 5,000 times using SEM, and the area ratio of the recesses was calculated.

(11) Melting temperature of cellulose acetate butyrate Using a differential scanning calorimeter DSC-6200 manufactured by Seiko Instruments Co., Ltd., using a separation membrane containing 40% or more of cellulose acetate butyrate as a sample by the same measurement method as in (1) above. About 5 mg of the sample that had been vacuum-dried at 25 ° C. for 8 hours was set in an aluminum saucer, heated from −70 ° C. to 350 ° C. at a heating rate of 20 ° C./min, and then melted and held at 350 ° C. for 5 minutes. The crystal melting peak observed at this time was defined as the melting temperature (° C.). When a plurality of crystal melting peaks appear, the crystal melting peak that appears on the highest temperature side is adopted.

[Cellulose ester (A)]
The following were prepared as cellulose esters.

Cellulose acetate butyrate (A1): Cellulose acetate butyrate (average degree of substitution of acetyl group: 0.30, average degree of substitution of butyryl group: 2.47, weight average molecular weight: 57,000)
Cellulose acetate butyrate (A2): Cellulose acetate butyrate (average degree of substitution of acetyl group: 1.00, average degree of substitution of butyryl group: 1.67, weight average molecular weight: 7,000,000)
Cellulose acetate propionate (A3): Cellulose acetate propionate (average degree of substitution of acetyl group: 0.18, propionyl group: 2.51, weight average molecular weight: 75,000)
[Polyvinyl alcohol B]
Polyvinyl alcohol: Degree of saponification: 86.7%, weight average molecular weight: 22000)
[Other raw materials]
The following were prepared as other raw materials.

Cellulose Acetate Butyrate Plasticizer (C): Polyethylene Glycol (Weight Average Molecular Weight: 600)
(Example 1)
Cellulose acetate butyrate (A1) 30.8% by mass, polyvinyl alcohol (B) 28.8% by mass, and structure forming agent (C) 40.4% by mass are melted at 200 ° C. in a twin-screw extruder. It was kneaded, homogenized and then pelletized to obtain a resin composition. The resin composition was vacuum dried at 80 ° C. for 8 hours.

The dried resin composition is supplied to a twin-screw extruder, melt-kneaded at 200 ° C., and then introduced into a melt-spinning pack having a spinning temperature of 220 ° C. under the condition of a discharge rate of 10 g / min. A heavy circular tube type, a discharge hole diameter of 8.3 mm, a slit width of 1.1 mm) was spun below the outer annular portion of the discharge port having one hole. The spun hollow fiber was guided to a cooling device, cooled by 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 diameter of 200 μm was used. The wound hollow fiber is immersed in an aqueous acetone solution at 30 ° C. having a volume fraction of 55% for 10 minutes, and further immersed in water at 25 ° C. for 1 hour or more to form a plasticizer (B) and a structure forming agent (C). ) Was eluted to obtain a separation membrane. The dipping step was carried out by immersing 20 mg of the separation membrane in 100 mL of an acetone solution or water. The physical characteristics of the obtained separation membrane are shown in Table 1.

(Examples 2 to 4 and Comparative Examples 1 to 3)
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 Table 1. The physical characteristics of the obtained separation membrane are shown in Table 1. In Comparative Example 3, the film shape could not be maintained after the plasticized phase separation.

The separation membranes obtained in Examples 1 to 4 all contained 10 to 50% by mass of polyvinyl alcohol in the separation membrane, and showed a high membrane permeation flux. On the other hand, the separation membranes of Comparative Examples 1 to 3 whose membrane composition does not satisfy the requirements of the present invention could not realize a high membrane permeation flux.

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

Claims (7)

A separation membrane containing cellulose acetate butyrate and polyvinyl alcohol, and the ratio of polyvinyl alcohol to cellulose acetate butyrate is in the range of 10 to 50% by mass. The separation membrane according to claim 1, wherein the cellulose acetate butyrate has an average degree of substitution of butyryl groups of 2.0 to 2.6 and a melting temperature in the range of 150 to 180 ° C. The separation film according to claim 1 or 2, wherein the average value X of the distances between adjacent vertices on at least one surface having a convex portion and the film thickness S of the separation film satisfy the following relationship.
0.5 ≤ X / S ≤ 1.0 Claims 1 to _{1 in} which the average value Z _{1 of the} height of the convex portion measured by using a microscope microscope on at least one surface having the convex portion and the film thickness S of the separation membrane described above satisfy the following relationship. The separation membrane according to any one of 3.
0.1 ≤ Z _{1 /} S ≤ 0.35 A claim that includes a void inside in the cross section of the convex portion, and the average value R of the diameters of the void and the average value Z ₂ of the convex portion height measured from the scanning electron microscope image satisfy the following relationship. Item 3. The separation membrane according to any one of Items 1 to 4.
2.5 ≤ R / Z ₂ ≤ 6.0 The invention according to any one of claims 1 to 5, wherein the surface opening ratio of the membrane surface of the separation membrane is 3 to 50%, and the average pore diameter of the membrane surface is 0.001 to 1.000 μm. Separation membrane. (1) Preparation for obtaining a resin composition by melt-kneading a mixture containing 20 to 65% by mass of cellulose acetate butyrate, 15 to 45% by mass of polyvinyl alcohol, and 20 to 55% by mass of a plasticizer. Process and
(2) A molding step of discharging the resin composition from a discharge port to obtain a resin molded product, and
(3) A method for producing a separation membrane, comprising a dipping step of immersing the resin molded product in a solvent.