JPH1170326A - Polyacrylonitrile-based membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyacrylonitrile-based filter membrane in which the balance of water permeable performance and membrane strength is excellent. SOLUTION: The polyacrylonitrile-based filter membrane consists of an acrylonitrile-based polymer in which ultimate viscosity is below 2.0 and consists of spongy structure free from a polymer-deficient part such as a void. A minimum pore diameter layer in which blocking performance is <=0.05 μm favorably (0.01-0.05) μm is provided on only one surface. The above-mentioned void is a cavity having a diameter of >=5 μm, furthermore >=10 μm. It is desirable that the membrane is a hollow fiber state with no restriction to the shape of the membrane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyacrylonitrile filtration membrane having an excellent balance between water permeability and membrane strength.

[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
Because of their excellent chemical and microbial stability, porous membranes prepared by various production methods have been proposed, but all 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-based polymer having a very high degree of polymerization, having a dense layer only on the outer surface of the film, and free from giant pores. Have been. 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 filtration membrane having an excellent balance between water permeability and strength.

[0006]

The present invention has solved the above-mentioned problems. That is, the present invention relates to (1) a film having a sponge structure which is made of an acrylonitrile-based polymer having an intrinsic viscosity of less than 2.0 and does not have a defect portion of a polymer such as a void, and has a particle blocking performance of 0.05 μm
A polyacrylonitrile-based filtration membrane characterized by having the following minimum pore size layer on only one surface; (2) the polyacrylonitrile-based filtration membrane of (1) above, wherein the membrane is a hollow fiber membrane;
(3) The polyacrylonitrile-based filtration membrane according to (2), having a minimum pore size layer on the outer surface of the membrane.

Hereinafter, the structure of the film of the present invention will be described. The film of the present invention is made of an acrylonitrile-based polymer having an intrinsic viscosity of less than 2.0, and has a structure integrally and continuously from one surface to the other surface of the film. The acrylonitrile-based polymer having an intrinsic viscosity of 2.0 or more tends to have poor solubility, and the concentration of the acrylonitrile-based polymer in the stock solution for film formation cannot be increased. Is difficult. Further, the film of the present invention has a sponge (mesh) structure which does not substantially include a defect such as a void (void) inside the film. The void referred to here is a cavity having a diameter of 5 μm or more, and more preferably 10 μm or more. Then, one of the surfaces of the film has a particle blocking performance of 0.1.
05 μm or less, preferably 0.01 μm to 0.05 μm
Having a minimum pore diameter layer. The shape of the membrane is not limited, but is preferably a hollow fiber. The hollow fiber membrane is
Since the effective membrane area per unit volume can be increased as compared with the flat membrane, there is an advantage that the membrane filtration device can be reduced in size.

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 diameter layer of the membrane referred to in the present invention is a layer which has small pores in the polymer constituting the membrane in a cross section in the film thickness direction when viewed microscopically, that is, a layer which contributes to the fractionation performance of the membrane. Yes, the particle blocking performance is 0.05 μm or less, preferably 0.01 μm or more and 0.05 μm or less. 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,
A pore size larger than m is not preferable in removing fine particles. 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, water permeability is reduced.
μm or less.

A typical example of the film of the present invention will be described in more detail with reference to the drawings. 1 is an electron micrograph showing a cross section perpendicular to the length direction of the hollow fiber membrane, that is, a part of a cross section in the film thickness direction, and FIG. 2 is an electron micrograph showing a state of the outer surface of the membrane. It is. As shown in FIG. 1, the membrane has a sponge structure in which the entire thickness does not include voids, and the minimum pore size layer of the membrane is on the outer surface of the membrane.

The membrane of the present invention has a performance that is significantly better than the balance between strength and water permeability of the prior art membrane. Generally, in order to increase the strength of the membrane, the polymer concentration in the membrane-forming stock solution is increased, so that the pore diameter tends to be small and the water permeability tends to be low. On the other hand, the membrane of the present invention satisfies the following equation in FIG. 3 showing the relationship between the breaking strength of the membrane and the water permeability.

Y ≧ AX + B (where Y: water permeability of the membrane (liter / hr · m ² · atm (25
A: -18.5 X: Breaking strength (kgf / cm ² ) B: 2870 That is, the membrane of the present invention is located in the region above the straight line in FIG. 3 and has an excellent balance between membrane strength and water permeability.

[0013] The hollow fiber membrane of the present invention comprises an acrylonitrile-based polymer comprising propylene carbonate and another organic solvent,
It is manufactured by discharging a film forming stock solution dissolved in a mixed solvent with an additive and an additive from a double annular nozzle together with an internal solution, passing through an air gap, and then coagulating in a coagulation bath. In addition, a flat film is obtained by uniformly casting the above-mentioned stock solution on a flat plate having a smooth surface, on an endless belt, or on a rotating drum in a thin film form using a knife edge or the like, and coagulating in a coagulation bath. It is manufactured by

The acrylonitrile-based polymer used in the present invention is at least 70% by weight, preferably 8% by weight.
5% to 100% by weight of acrylonitrile and 30% by weight or less, preferably 0% by weight, of one or more vinyl compounds copolymerizable with acrylonitrile
It is an acrylonitrile homopolymer or an acrylonitrile-based copolymer of up to 15% by weight. The intrinsic viscosity of the acrylonitrile-based 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 tends to be low, and 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.

As the organic solvent for dissolving the acrylonitrile-based polymer, propylene carbonate, N, N-dimethylformamide, N, N-dimethylacetamide,
Examples include dimethyl sulfoxide, γ-butyrolactone, ethylene carbonate, N-methyl-2-pyrrolidone, 2-pyrrolidone, hexamethylene phosphoamide and the like. Also, important for obtaining the film of the present invention, a mixed solvent of propylene carbonate and another organic solvent,
A mixture of propylene carbonate and one or more organic solvents other than the solvent (propylene carbonate). Unless propylene carbonate is used, the membrane of the present invention is difficult to obtain.

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. %. If it is less than 2% by weight, the viscosity of the film forming solution tends to be low and the film tends to be difficult to form, and if it is more than 50% by weight, the viscosity of the film forming solution tends to be 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 cutoff, the acrylonitrile-based polymer concentration is preferably low, and is preferably from 10 to 25% by weight.

The additive may be any as long as it is compatible with the solvent and does not dissolve the acrylonitrile-based polymer. For the purpose of controlling the viscosity of the stock solution and the dissolution state, water; salts; isopropyl alcohol, methanol, ethanol, propanol, butanol Alcohols such as acetone; ketones such as acetone and methyl ethyl ketone; diethylene glycol, triethylene glycol, tetraethylene glycol, 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 polyvinylpyrrolidone.

The amount of the additive in the stock 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. The internal liquid used in the production of the hollow fiber membrane is used for forming the hollow portion of the hollow fiber membrane.
As the internal liquid, propylene carbonate, N, N-dimethylformamide, N, N-dimethylacetamide,
An aqueous solution of a good solvent that dissolves acrylonitrile-based polymers such as dimethyl sulfoxide, γ-butyrolactone, ethylene carbonate, and N-methyl-2-pyrrolidone is used.

The aqueous solution of a good solvent is preferably an aqueous solution containing 50% by weight or more of a good solvent, preferably 75% by weight or more of a good solvent. In order to further reduce the filtration resistance in the inner surface layer of the membrane, the content is preferably 90% by weight or more. 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.

The hollow fiber membrane can be formed using a known tube-in-orifice type double annular nozzle. More specifically, the hollow fiber-like membrane of the present invention is obtained by simultaneously discharging the above-mentioned stock solution and the internal solution from the double annular nozzle, passing through an air gap, and coagulating in a coagulation bath. Can be. Here, the air gap means a gap between the nozzle and the coagulation bath. When the air gap is surrounded by a cylindrical tube or the like and a gas having a certain temperature and humidity flows through the air gap at a certain flow rate, the hollow fiber membrane can be manufactured in a more stable state.

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 (polystyrene-based polymer, 0.042 μm, manufactured by JSR Corporation) having a particle size of 0.042 μm are used. An undiluted solution adjusted to be suspended at a concentration of% by volume was applied to a 70 mm hollow fiber membrane at an average pressure of 0.5 kgf / cm of the input pressure and the output pressure.
^2. Shows the rejection after 40 minutes of filtration from the outer surface to the inner surface under the condition of cross flow at 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. 4) 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, a flat membrane punched out to a diameter of 25 mm is incorporated into a filter holder, and the above-mentioned stock solution is applied to a membrane having a differential pressure of 0.1 mm.
The rejection after 2 minutes when supplied at 5 kgf / cm ² is shown.

The fractionation performance (B) is such that the latex particles have a particle size of 0.1%.
The same operation as in the fractionation performance (A) was performed except that the particle size was 028 μm (manufactured by Magsphere, polystyrene-based polymer, 0.028 μm). The average pore size of the smallest pore size layer of the membrane was measured according to the air flow method described in ASTM F316-86.

The intrinsic viscosity of the acrylonitrile-based polymer is determined by Journal of Polymer Scie.
nce (A-1) Vol. 6, 147-157 (1968)
The measurement was carried out at 30 ° C. using N, N-dimethylformamide as a solvent according to the measuring method described in (1).

[0029]

EXAMPLE 1 18% by weight of a copolymer having an intrinsic viscosity [η] = 1.2 and a weight average of 91.5% by weight of acrylonitrile, 8.0% by weight of methyl acrylate and 0.5% by weight of sodium methallylsulfonate Polyvinylpyrrolidone having a molecular weight of 9,000 (K17, manufactured by BASF) 16% by weight
Was dissolved in a mixed solvent composed of 33% by weight of dimethyl sulfoxide and 33% by weight of propylene carbonate to obtain a homogeneous solution.

The solution was kept at 60 ° C., and a spinning nozzle (a double annular nozzle 0.5 mm-
(0.7 mm-1.3 mm), passed through a 20 mm air gap, and immersed in a coagulation bath made of water at 80 ° C. to complete coagulation. At this time, the area from the spinneret to the coagulation bath was surrounded by a cylindrical tube, and the humidity of the air gap in the tube was controlled to 100%, and the temperature was controlled to 60 ° C. Spinning speed is 10m
/ Min.

When the cross section of the obtained hollow fiber membrane was observed, there was no void between both surfaces of the membrane, and the membrane had a complete sponge structure. Table 1 shows the structure and performance of this film. Also, the average pore size of the smallest pore size layer formed on the outer surface of the membrane,
It was 0.037 μm.

[0032]

Example 2 The stock solution used in Example 1 was cast on a glass plate to a thickness of 200 μm, and dimethyl sulfoxide 90
The casting surface was immersed in a coagulation bath at 80 ° C. made of a mixed solution of 10% by weight of water and 10% by weight of water, and solidified to obtain a planar film.
The resulting membrane has a void-free sponge structure, a water permeability of 2500 l / hr · m ² · atm (measured at 25 ° C.), a fractionation performance (A) of 100%, and a fractionation performance (B) of 9
2%. The average pore size of the smallest pore size layer of the membrane formed on the side that was in contact with the coagulation bath was 0.035 μm.

[0033]

Comparative Example 1 18% by weight of the polymer used in Example 1 and polyvinylpyrrolidone 16 used in Example 1
% By weight was dissolved in 66% by weight of dimethyl sulfoxide to obtain a homogeneous solution. Example 1 was repeated except that this stock solution was used.
The same operation as described above was performed to obtain a hollow fiber membrane. Table 1 shows the structure and performance of the obtained hollow fiber membrane.

[0034]

[Table 1]

[0035]

Since the membrane of the present invention is a polyacrylonitrile-based filtration membrane having an excellent balance between water permeability and strength, it can be suitably used for purification of water such as waterworks which requires a large volume treatment. it can.

[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 outer surface of the hollow fiber filtration membrane shown in FIG.

FIG. 3 is a diagram showing the relationship between the breaking strength and the water permeability of the membrane of the present invention.

FIG. 4 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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[Procedure amendment]

[Submission date] March 5, 1998

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[Document name to be amended] Drawing

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[Correction method] Change

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[Procedure amendment 2]

[Document name to be amended] Drawing

[Correction target item name] Figure 2

[Correction method] Change

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Claims (1)

[Claims] 1. A film comprising an acrylonitrile-based polymer having an intrinsic viscosity of less than 2.0 and a sponge structure having no voids or other polymer deficient portions, and having a minimum particle blocking performance of 0.05 μm or less. A polyacrylonitrile-based filtration membrane having a pore size layer on only one surface.