JPH11539A - Polyacrylonitrile film and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a polyacrylonitrile precision filtration membrane excellent in water permeability by forming a polyacrylonitrile film with a min. pore diameter layer having specified particle blocking performance in one face of the film and specifying the porosity of the min. pore diameter layer in a cross section of the film. SOLUTION: The polyacrylonitrile precision filtration membrane is a polyacrylonitrile film with a min. pore diameter layer having 0.01-1 μm particle blocking performance in one face of the film and the film has a continuous structure from one face to the other face. The porosity of the min. pore diameter layer in a cross section of the film is as high as 40-90%. The film is produced as follows; a film forming stock soln. consisting of an acrylonitrile polymer, an additive and a solvent which dissolves the polymer and causing microphase separation at >=30 deg.C is cast or ejected in a certain form and solidified. The solvent is preferably N,N-dimethylacetamide and the additive is preferably polyethylene glycol.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a polyacrylonitrile-based microfiltration membrane having excellent water permeability.

[0002]

2. Description of the Related Art In recent years, separation techniques using membranes have been developed, and the importance of excellent separation membranes has been further increasing. The performance required for general separation membranes include high separation selectivity, high water permeation rate, high mechanical strength, and chemical or microbial stability. There is no separation membrane that fully satisfies all of the above properties. For example, acetate-based membranes have already been used, but have significant drawbacks in resistance to chemicals and microorganisms. On the other hand, acrylonitrile-based polymers have excellent chemical and microbial stability, and therefore porous membranes prepared by various production methods have been proposed, but all of them have room for improvement.

For example, Japanese Patent Publication No. 60-39404 discloses a membrane structure composed of a dense layer, a porous layer, and huge pores. Although a membrane having this structure is excellent in fractionation performance, Low water permeability. Therefore, in an application for purifying a large amount of water, a large number of membrane modules must be used, which results in an increase in the size of the apparatus, and thus an increase in processing cost.

Japanese Patent Application Laid-Open No. 63-190012 discloses a film using an acrylonitrile polymer having a very high degree of polymerization, having a dense layer only on the outer surface of the film, and having no structure containing large pores. ing. Although this membrane is excellent in mechanical strength, it still has insufficient water permeability.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a polyacrylonitrile-based microfiltration membrane having excellent water permeability and a method for producing the same.

[0006]

The present invention has solved the above-mentioned problems. That is, the present invention relates to (1) a membrane having a minimum pore size layer having a particle blocking performance of 0.01 μm to 1 μm on one surface of the membrane, and having a porosity of 40% to 90% in the cross section of the membrane. %, A polyacrylonitrile-based microfiltration membrane, (2) the polyacrylonitrile-based microfiltration membrane of (1), wherein the membrane is a hollow fiber membrane, and (3) a minimum pore size layer on the outer surface of the membrane. The above (2)
A polyacrylonitrile-based microfiltration membrane of (4) an acrylonitrile-based polymer, an additive and a solvent,
A method for producing a polyacrylonitrile-based microfiltration membrane, characterized by casting or extruding a membrane-forming stock solution that undergoes microphase separation at a temperature of at least ℃, and solidifying it.
(5) The method for producing a polyacrylonitrile-based microfiltration membrane according to (4), wherein the solvent is N, N-dimethylacetamide, and (6) the additive, wherein the additive is polyethylene glycol. 5) a method for producing a polyacrylonitrile-based microfiltration membrane.

Hereinafter, the structure of the film of the present invention will be described. The film of the present invention is a film having a minimum pore size layer having a particle blocking performance of 0.01 μm to 1 μm on one surface of the film, and has a structure integrally and continuously from one surface to the other surface of the film. . The shape of the membrane is not limited, but is preferably a hollow fiber. Since the hollow fiber membrane can increase the effective membrane area per unit volume as compared with the flat membrane, there is an advantage that the membrane filtration device can be downsized.

Further, when filtering a large volume of water using a hollow fiber membrane, if filtration is performed by a method called external pressure filtration, the area of the filtration membrane in the module becomes the outer surface. In comparison, the effective film area can be further increased. In the case of external pressure filtration, in order to reduce clogging of the membrane, it is preferable that the minimum pore size layer of the membrane is located at the outermost layer or in the vicinity of the outermost layer. Therefore, when the membrane of the present invention is in the form of a hollow fiber, it is preferable that the outermost layer of the membrane has a minimum pore size layer.

The minimum pore size layer of the membrane as referred to in the present invention is a layer that contributes to the fractionation performance of the membrane, and has a particle blocking performance of 0.01.
It is not less than μm and not more than 1 μm. If the blocking performance of the particles of the minimum pore size layer is less than 0.01 μm, the water permeability of the membrane tends to decrease, and if it is more than 1 μm, the strength of the membrane tends to decrease. Preferred minimum pore size layer particles have a blocking performance of:
It is 0.1 μm or more and 0.5 μm or less. The term “particle rejection performance” as used herein means the minimum diameter of a spherical particle that can prevent rejection by 90% or more. The thickness of the minimum pore size layer is arbitrary, but if it is too thick, the water permeability is reduced. Therefore, it is usually 30 μm or less, preferably 10 μm or less.

The present invention is characterized in that the porosity of the smallest pore diameter layer in the cross section of the membrane is as high as 40% to 90%. Since the minimum pore size layer has such porosity, the membrane of the present invention exhibits high water permeability. Representative examples of the film of the present invention will be described in more detail with reference to the drawings. FIG. 1 is an electron micrograph showing a cross section perpendicular to the length direction of the hollow fiber membrane, that is, a part of the cross section in the film thickness direction. FIG.
FIG. 2 is an electron microscope photograph in which the vicinity of the outer surface of FIG. 1 is further enlarged. FIG. 3 is an electron micrograph showing the state of the outer surface of the film, and FIG. 4 is an electron micrograph showing the state of the inner surface of the film. As shown in FIGS. 1 and 2, the smallest pore size layer of the membrane is on the outer surface of the membrane and has a high porosity.

Hereinafter, an example of the method for producing a film of the present invention will be described. The membrane of the present invention comprises an acrylonitrile-based polymer, an additive, and a solvent that dissolves the acrylonitrile-based polymer, and a membrane-forming stock solution that undergoes microphase separation at a temperature of 30 ° C. or higher (hereinafter, also simply referred to as phase separation) It is manufactured by casting or discharging into a fixed shape and solidifying.

The acrylonitrile polymer used in the present invention is at least 70% by weight, preferably 85% by weight.
% To 100% by weight of acrylonitrile and 30% by weight or less, preferably 0% by weight or less of one or more kinds of vinyl compounds copolymerizable with acrylonitrile.
15% by weight or less of acrylonitrile homopolymer or acrylonitrile copolymer. The intrinsic viscosity of the acrylonitrile polymer is preferably 0.4 or more and less than 2.0. If the intrinsic viscosity is less than 0.4, the strength of the film is weak,
If it is 2.0 or more, the solubility tends to be poor.

The vinyl compound is not particularly limited as long as it is a known compound having copolymerizability with acrylonitrile. Preferred copolymerization components include acrylic acid, methyl acrylate, ethyl acrylate and itacone. Acid, vinyl acetate, sodium acrylsulfonate, sodium methallylsulfonate, sodium p (para) -styrenesulfonate, hydroxyethyl methacrylate,
Examples thereof include ethyl triethyl ammonium methacrylate, ethyl trimethyl ammonium methacrylate, and vinylpyrrolidone.

Examples of the organic solvent for dissolving the acrylonitrile polymer include propylene carbonate, N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone, ethylene carbonate, N-methyl-2-pyrrolidone, and 2-pyrrolidone. And hexamethylenephosphamide. A preferred solvent is N, N-dimethylacetamide.

The additive may be any as long as it is compatible with the solvent and does not dissolve the acrylonitrile polymer. For the purpose of controlling the viscosity of the stock solution and the dissolution state, water; salts; isopropyl alcohol, methanol, ethanol, propanol, Alcohols such as butanol; ketones such as acetone and methyl ethyl ketone; diethylene glycol, triethylene glycol, tetraethylene glycol and polyethylene glycol (weight average molecular weight 200 to 35.0
Glycols such as 00); glycerin and polyvinylpyrrolidone (weight average molecular weight 1,000 to 2,800,
000) and the like, and a plurality of them can be added, and the kind and the amount of addition may be determined as needed depending on the combination. A preferred additive is polyethylene glycol.

A very important point in the production method of the present invention is that a film forming stock solution comprising an acrylonitrile polymer, an additive and a solvent undergoes microphase separation at a temperature of 30 ° C. or higher. By using the membrane-forming stock solution having this property, the porosity in the smallest pore diameter layer of the membrane cross section is 40% to 90%.
% Can be obtained. If the phase separation temperature is higher, a membrane having higher water permeability tends to be obtained. If the phase separation temperature is lower than 30 ° C., a membrane having higher water permeability tends not to be obtained. A preferred phase separation temperature is 50 ° C. or higher.

The term “microphase separation” as used in the present invention means turbidity (white turbidity) or separation into two phases. In addition, the membrane-forming stock solution of the present invention becomes a turbid state by microphase separation above the phase separation temperature. It has a reversible property, returning to a neat film forming solution. In producing the acrylonitrile-based microfiltration membrane of the present invention, a membrane-forming stock solution having this property is cooled to a phase separation temperature or lower and used as a uniform and transparent membrane-forming stock solution. Further, when the phase separation temperature is 30
The combination of a solvent and an additive for an acrylonitrile-based polymer for preparing a film forming solution having a temperature of at least ℃ is not particularly limited, but typical combinations include N, N-dimethylacetamide as a solvent and polyethylene as an additive. Glycol is used.

The solubility of the acrylonitrile polymer used in the present invention in a solvent and the solubility of additives in a solvent generally increase as the temperature increases. However, in a three-component system composed of an acrylonitrile-based polymer, N, N-dimethylacetamide, and polyethylene glycol, the conversely, the lower the temperature, the greater the compatibility of the mixture and the more uniform the solution. This fact is a completely new finding.

The concentration of the acrylonitrile-based polymer in the film-forming stock solution is not particularly limited as long as it can be formed into a film and the obtained film has a performance as a film.
It is 2% by weight to 50% by weight. If the amount is less than 2% by weight, the viscosity of the film forming stock solution is low, and the film tends to be difficult to form, and 50% by weight.
If it is higher, the viscosity of the film forming stock solution is too high, and film forming tends to be difficult. Preferably it is 5% by weight to 35% by weight.
In order to achieve high water permeability or a high molecular weight cut-off, the lower the acrylonitrile-based polymer concentration, the better.
0-25% by weight is preferred.

The amount of the additive in the film forming solution is from 1% by weight to 4% by weight.
0% by weight, preferably 1% to 30% by weight,
The optimum concentration is determined by the type and molecular weight of the additive used. Hereinafter, a specific example of the method for producing a film will be described. The hollow fiber membrane is manufactured by simultaneously discharging the stock solution of the present invention together with the internal solution from a double annular nozzle into a coagulation bath and coagulating the same. More specifically, the hollow fiber membrane of the present invention is simultaneously discharged from a known tube-in-orifice type double annular nozzle into a coagulation bath and coagulated, and the above-mentioned membrane-forming stock solution and the internal solution described below are coagulated. Can be obtained. It is also possible to manufacture by providing a gap (air gap) between the nozzle and the coagulation bath.

In the case of a flat membrane, the above-mentioned stock solution is uniformly cast into a thin film on a flat plate having a smooth surface, on an endless belt, or on a rotating drum using a knife edge or the like.
It is produced by coagulation in a coagulation bath. The internal liquid used in the production of the hollow fiber membrane is used for forming the hollow portion of the hollow fiber membrane. When a minimum pore size layer is formed on the outer surface, acrylonitrile such as propylene carbonate, N, N-dimethylformamide, N, N-dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone, ethylene carbonate, N-methyl-2-pyrrolidone is used as the internal liquid. An aqueous solution of a good solvent that dissolves the polymer is used. When a minimum pore size layer is formed on the inner surface, those described in a coagulation bath described later are employed as the internal liquid. It is also possible to add glycols such as tetraethylene glycol and polyethylene glycol and non-solvents such as glycerin for the purpose of controlling the viscosity of the internal liquid.

As the coagulation bath, for example, a liquid that does not dissolve the polymer such as water; alcohols such as methanol and ethanol; ethers; and aliphatic hydrocarbons such as n-hexane and n-heptane is used. Preferably, it is used. The solidification rate can also be controlled by adding the good solvent to the coagulation bath. In the case of a planar membrane, a minimum pore size layer is formed on the membrane surface on the side contacting the coagulation bath.

The temperature of the coagulation bath is from -30 ° C to 90 ° C, preferably from 0 ° C to 90 ° C, and more preferably from 0 ° C to 80 ° C. The temperature of the coagulation bath exceeds 90 ° C or -30
If the temperature is lower than ℃, the state of the surface of the film in the coagulation bath is difficult to be stabilized.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below, but the present invention is not limited to these embodiments. Each measuring method is as follows. In addition, the hollow fiber membrane and the planar membrane used as the measurement samples were all completely impregnated with water.

The amount of water permeation of the hollow fiber membrane is determined by permeating ultrafiltration water at 25 ° C. from the inner surface to the outer surface of the sample of the hollow fiber membrane having a length of 50 mm, and the amount is liter / hr · m ² · a.
tm. However, the effective film area was converted to the inner surface.
The film strength was measured using an Autograph AGS- manufactured by Shimadzu Corporation.
The measurement was performed using 5D at a sample length of 50 mm and a pulling speed of 10 mm / min.

The breaking strength is defined as the load at the time of breakage of one hollow fiber membrane per unit area (kgf) of the membrane before pulling.
/ Cm ² ), and the elongation (elongation) was expressed as the length (%) of the original length that elongated before breaking. Fractionation performance (A)
In the case of a hollow fiber membrane, in a 0.2% by weight aqueous solution of sodium dodecyl sulfate, 0.02 μm of uniform latex particles having a particle size of 0.132 μm (polystyrene-based polymer, 0.132 μm, manufactured by Ceradyne Co.) An undiluted solution adjusted to be suspended at a concentration of volume% was applied to a 70 mm hollow fiber membrane at an average pressure of 0.5 kgf /
The rejection after 40 minutes when filtering from the outer surface to the inner surface under the conditions of cross flow of cm ² and a fluid linear velocity of 1 m / sec. The temperature of the solution was adjusted to 25 ° C. The fluid linear velocity was calculated using the area (see FIG. 5) obtained by subtracting the cross-sectional area calculated from the outer diameter of the hollow fiber membrane from the cross-sectional area of the cylindrical container. The concentration of latex particles in the undiluted filtrate was measured at a wavelength of 260 nm using an ultraviolet-visible spectrometer. In the case of a flat membrane, the rejection after 2 minutes is shown when a flat membrane punched to a diameter of 25 mm is incorporated in a filter holder and the stock solution is supplied at a membrane differential pressure of 0.5 kgf / cm ² .

The fractionation performance (B) is such that the latex particles have a particle size of 0.1%.
The same operation as the fractionation performance (A) was performed except that the particle size was 088 μm (manufactured by Dow Chemical Company, polystyrene-based polymer, 0.088 μm). The fractionation performance (C) was the same as the fractionation performance (A) except that the latex particles were 0.042 μm (manufactured by JSR, polystyrene-based polymer, 0.042 μm).
The same operation as described above was performed.

The average pore size of the smallest pore size layer of the membrane is ASTM
It was measured by the air flow method described in F316-86. Further, the porosity of the minimum pore size layer in the film cross section was determined by image analysis of an electron micrograph of the film cross section. The intrinsic viscosity of acrylonitrile polymer is Jo
urnal of polymer science
(A-1) According to the measurement method described in Volume 6, 147 to 157 (1968), measurement was carried out at 30 ° C. using N, N-dimethylformamide as a solvent.

[0029]

Example 1 A copolymer consisting of 91.5% by weight of acrylonitrile, 8.0% by weight of methyl acrylate, and 0.5% by weight of sodium methallylsulfonate having an intrinsic viscosity [η] of 1.2% was obtained. Polyethylene glycol having a weight average molecular weight of 3,000 (PEG4000, manufactured by Wako Pure Chemical Industries, Ltd.) 8
% By weight was dissolved in 76% by weight of N, N-dimethylacetamide to obtain a homogeneous solution. It was confirmed that this membrane forming solution was subjected to microphase separation at 64 ° C. This membrane-forming stock solution was added to 60
C. and a spinneret (double annular nozzle 0.5 mm-0.7 mm-1.3 m) together with an internal solution consisting of a mixed solution of N, N-dimethylacetamide 90% by weight and water 10% by weight.
m) to discharge into a coagulation bath consisting of water at 80 ° C. to complete coagulation. The spinning speed was fixed at 10 m / min. The minimum pore size layer of the membrane was present on the outer surface of the membrane, and the average pore size of the minimum pore size layer of the membrane was 0.12 μm. Table 1 shows the performance of the obtained hollow fiber membrane.

[0030]

Example 2 A film-forming stock solution used in Example 1 was cast on a glass plate to a thickness of 400 μm, and was made of a mixed solution of 90% by weight of N, N-dimethylacetamide and 10% by weight of water.
The casting surface was immersed in a coagulation bath at 0 ° C. and solidified to obtain a planar film. The water permeability of the obtained membrane is 3900 liter / hr.
· M ² · atm (measured at 25 ° C) and fractionation performance (A)
Was 100% and the fractionation performance (B) was 2%. The average pore size of the minimum pore size layer of the membrane was 0.12 μm, and the porosity of the minimum pore size layer in the membrane cross section was 71%.

[0031]

Comparative Example 1 A hollow fiber membrane was obtained in the same manner as in Example 1 except that only N-methyl-2-pyrrolidone was used as a solvent. This membrane-forming stock solution did not show microphase separation with temperature. The average pore size of the obtained hollow fiber membrane is 0.3.
009 μm. The performance of the membrane is shown in Table 1.

[0032]

[Table 1]

[0033]

Since the membrane of the present invention is a polyacrylonitrile-based microfiltration membrane having excellent water permeability, it can be suitably used for purification of water such as tap water which requires a large volume treatment.

[Brief description of the drawings]

FIG. 1 is an electron micrograph (200 × magnification) showing a cross section (part) of a uniform state of a hollow fiber filtration membrane of the present invention.

FIG. 2 is an electron micrograph (magnification: 10,000) of the vicinity of the outer surface of the hollow fiber filtration membrane shown in FIG.

FIG. 3 is an electron micrograph (magnification: 10,000) of the outer surface of the hollow fiber filtration membrane shown in FIG.

FIG. 4 is an electron micrograph (× 1,000) of the inner surface of the hollow fiber filtration membrane shown in FIG. 1.

FIG. 5 is a cross-sectional view showing a positional relationship between a container and a hollow fiber membrane when measuring a linear velocity of a fluid.

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[Submission date] June 13, 1997

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Claims (2)

[Claims] 1. A film having a particle blocking performance of 0.01 on one surface.
A polyacrylonitrile-based microfiltration membrane having a minimum pore size layer of from 1 μm to 1 μm, wherein the porosity of the minimum pore size layer in the membrane cross section is from 40% to 90%. 2. A film-forming stock solution comprising an acrylonitrile-based polymer, an additive, and a solvent and subject to microphase separation at a temperature of 30 ° C. or higher, is cast or extruded into a predetermined shape,
A method for producing a polyacrylonitrile-based microfiltration membrane characterized by coagulation.