JPH0647066B2 - Porous separation membrane and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a porous separation membrane having excellent separability and filtration performance and a method for producing the same, and in particular, the pore size distribution of the fine through-holes in the separation direction. The present invention relates to a novel asymmetric membrane, that is, a porous membrane having a plurality of layers having different pore diameters in the thickness direction of the membrane and a method for producing the same.

[Conventional technology]

A porous membrane having a configuration in which a large number of fine through holes are formed in the circumferential wall portion of a polymeric material or in the thickness direction of a film,
For example, it is used in various fields as a plasma separation membrane in the medical field, a filtration membrane or separation membrane for artificial lungs and water purification.

By the way, as for the performance of such a separation membrane, the separation ability (the ability to separate an object to be separated having a specific pore size or more corresponding to the purpose) is good, and the amount of permeation (the amount of the specific object to be permeated corresponding to the object) It is desired that the film has a large film thickness and a large mechanical strength.

Here, the separability depends on the pore size of the membrane, and it is necessary to have a predetermined pore size depending on the purpose of use. On the other hand, the permeation amount is related to the thickness of the membrane if the pore diameter of the membrane is the same, and becomes larger as the thickness of the membrane becomes thinner. The larger the pore size, the larger the film thickness. Further, the mechanical strength of the film increases as the film thickness increases.

Therefore, regarding the above-mentioned separation ability, permeation amount and mechanical strength,
It is desirable to obtain a separation membrane in which the pore size distribution of through holes and the film thickness are adjusted so as to satisfy all of them. By the way, as a method for producing a porous membrane (a porous hollow fiber or a porous film) which is a separation membrane, a polymer membrane material is conventionally dissolved in a solvent and a mixed solvent system of a swelling agent or a non-solvent to form a uniform solution. Is used as a stock solution, and this stock solution is cast into a film to partially or completely evaporate the volatile solvent, and then immersed in a coagulation bath to extract and remove the solvent to form a porous film. Phase conversion, or a method of extracting the substance to be eluted from the film to form a porous film after mixing the substance to be eluted into a polymer film material to form a film, and further using an unstretched hollow After spinning or film forming the yarn or film,
A method of making porous by stretching in a specific temperature condition and / or a specific medium is known.

[Problems to be solved by the invention]

However, although it is possible to have a pore size distribution in the thickness direction of the porous membrane by the above-mentioned phase conversion method or extraction method, it is possible to accurately and precisely control these distributions in order to improve the resolution and the permeability. It was extremely difficult to manufacture a porous membrane having excellent quantity and mechanical strength, and the manufacturing process itself was complicated. In addition, since a solvent is used in these methods, there is a problem that the post-treatment of the solvent remaining on the membrane is troublesome.

On the other hand, in the method by stretching, there is no problem of residual solvent, but it was almost impossible to have a distribution of pore diameters in the thickness direction or control it.

[Means for solving problems]

The present invention has been made in view of the problems of the above-mentioned conventional porous membrane and a method for producing the same, and in order to obtain a porous membrane having excellent separation ability, permeation amount and mechanical strength, it is involved in the separation ability. It is necessary that the thin portion having a predetermined pore diameter and the portion having a larger pore diameter are porous membranes having a multi-layered structure or a heterogeneous membrane existing in the thickness direction of the membrane. In order to produce a membrane, by stretching an unstretched membrane (hollow fiber or film) having a multi-layer structure by combining thermoplastic resin layers of the same type with different molecular weights, the thickness direction of the peripheral wall of the hollow fiber or the film The inventors have found that a layer having different pore diameters in the thickness direction, that is, a porous membrane having different pore diameter distributions can be obtained, and arrived at the present invention.

That is, according to the present invention, the first porous layer made of a thermoplastic resin having fine pores having a predetermined pore diameter necessary for separation has fine pores having a pore diameter larger than the predetermined pore diameter and the first porous layer. There is provided a porous separation membrane having a multilayer structure in which at least one second porous layer made of a thermoplastic resin having the same kind as the thermoplastic resin of the porous layer and having a larger molecular weight than that is laminated.

Further, according to the present invention, the unstretched membrane material has a multilayer structure composed of thermoplastic resin layers of the same kind having different molecular weights, and then this unstretched membrane material is stretched to obtain fine particles having different pore sizes in the thickness direction of the membrane. A method for producing a porous separation membrane having perforations is provided.

By the porosity of the present invention, the thermoplastic resin layer having a large molecular weight becomes a porous membrane having a smaller average pore size than the thermoplastic resin layer having a small molecular weight.

The thermoplastic resin used in the present invention is not particularly limited, but for example, polypropylene, polyethylene, poly (4-methyl-pentene-1), polyoxymethylene,
Examples thereof include polyvinylidene fluoride and ethylene tetrafluoroethylene copolymer.

When the molecular weight of the thermoplastic resin used is expressed by melt viscosity, the melt viscosity [melt flow index (MFI) or melt index (MI)] is
The hollow fiber is not particularly limited as long as it can be spun. For example, when polypropylene is used, it is preferable to use one having an MFI of 0.5 to 40 g / 10 minutes in consideration of the efficiency of spinning a hollow fiber or film forming, or productivity. Among the thermoplastic resins having such a melt viscosity, those having different melt viscosities (that is, different molecular weights) are used in combination in the present invention. However, in the case of the combination having the same MFI, there is no difference in the pore diameters of the respective porous layers, and if there is too much difference between them, it may be difficult to spin the hollow fibers or form the film. Considering these points, when polypropylene is used as the thermoplastic resin, the MFI
And a combination having a difference of 1 to 35, preferably 2 to 35 is used.

In addition, a thermoplastic resin containing an additive (material) such as a plasticizer, a colorant, a flame retardant, and a filler can also be used.

In the present invention, the thermoplastic resin as described above is first spun or film-formed by using a multi-layer hollow fiber nozzle or a multi-layer die to obtain an unstretched thermoplastic resin hollow fiber or film having a multilayer structure.

The multi-layer structure has two or more layers, and the number of layers can be determined according to its purpose, application field, and the like. The layer ratio (ratio of layer thickness) can also be determined according to the purpose, application field, etc., and any combination of layer ratios can be used, except when at least one layer is so thin that it cannot be molded. Can be taken.

Spinning or film forming conditions can be appropriately selected from known techniques. For example, the spinning temperature may be higher than the temperature at which the thermoplastic resin used can be discharged and lower than the thermal decomposition temperature of the resin.
When polypropylene is used as the thermoplastic resin, the spinning or film forming temperature is, for example, not less than the temperature at which polypropylene can be discharged and not more than the thermal decomposition temperature of polypropylene, and usually 170 to 300. C., preferably 190 to 270.degree.

When high density polyethylene is used, it is usually 150 to 300 ° C., preferably 160 to 270 ° C. When poly (4-methyl-pentene-1) is used, it is usually 260 to 330 ° C., preferably Is 270-300
℃, when using ethylene tetrafluoroethylene copolymer, usually 290 ~ 350 ℃, preferably 1
90 to 280 ° C. When using polyvinylidene fluoride, it is usually 190 to 300 ° C., preferably 190 to 300 ° C.
280 ° C.

The elastic recovery rate (or draft ratio) of the unstretched thermoplastic resin film (hollow fiber or film) having a multilayer structure obtained by spinning or film forming is not particularly limited. However, when an unstretched thermoplastic resin film having an elastic recovery rate (or a draft ratio) of zero (%) to extremely low, that is, an unstretched thermoplastic resin film having extremely low crystal orientation is used, the porosity obtained is In some cases, it may be difficult to give satisfactory properties to the thermoplastic resin film.

As described above, the elastic recovery rate of the unstretched thermoplastic resin film is not particularly limited, but for the above reason, the stretched thermoplastic resin film represented by the following formula has a 50% at 25 ° C. and a relative humidity of 65%.
The elastic recovery rate at the time of% elongation is, for example, preferably 20% or more in the case of using polypropylene, and is 30 to 30 in consideration of the productivity in the case of using an ordinary molding apparatus. The range of 95% is particularly preferable.

Elastic recovery rate (%) = [length when stretched-length after stretch] / [length when stretched-length of raw membrane (hollow fiber or film)] x 100 Also, the above requirements and productivity In consideration of factors such as the above, the draft ratio of the unstretched thermoplastic resin film used in the present invention (ratio between the drawing speed of the unstretched thermoplastic resin film and the discharge speed from the nozzle or die: take-up speed / discharge The speed is, for example, 5 when polypropylene is used.
It is desirable to be in the range of up to 6000.

The unstretched thermoplastic resin film may be heat-treated before being subjected to the stretching step. By performing the heat treatment before this stretching,
Since the crystallization of the unstretched thermoplastic resin film can be enhanced, the properties of the porous thermoplastic resin film obtained by stretching are further improved.

The above heat treatment is carried out by a method of heating the unstretched thermoplastic resin film for 3 seconds or more in air heated to a temperature lower by 30 to 5 ° C. than the melting temperature of the thermoplastic resin, for example.

Next, the stretching step in the present invention may use a known method, such as a method of stretching in a single stage or multiple stages in a specific temperature range,
For example, after stretching at a temperature near room temperature, a method of further stretching in a temperature range of 140 to 150 ° C., nitrogen, oxygen, argon,
A method of stretching in a medium selected from the group consisting of carbon monoxide, methane and ethane at a stretching temperature of -100 ° C or lower and at a temperature of 50 ° C higher than the boiling point of the medium. A method of making the thermoplastic resin film having a multilayer structure porous by stretching in a specific temperature range and / or a specific medium is appropriately adopted.

The stretching step is performed so that the first porous layer holds fine through holes having a predetermined pore size necessary for separating a specific substance to be separated. It is necessary to set the conditions. (Of course,
The setting of this condition is known in the conventional stretching technology. ) It is preferable that the thermoplastic resin film which has been made porous through the stretching step is then subjected to heat setting treatment. This heat setting treatment is mainly intended for heat setting to hold the formed fine through holes. The heat setting treatment is carried out by a method of heating the porous thermoplastic resin film for 3 seconds or more in air heated to a temperature 70 to 5 ° C. lower than the melting temperature of the thermoplastic resin used.

Thus, in the resulting porous membrane, a layer having a smaller molecular weight (that is, a larger MFI or MI) becomes a porous layer having a larger average pore size, and a layer having a larger molecular weight (that is, M
Figure 4 shows a multi-layered structure in which a layer (smaller in FI or MI) is combined with a porous layer having a smaller average pore size. Further, the difference in average pore diameter becomes larger as the difference in molecular weight (that is, difference in MFI or MI) becomes larger, and the layer ratio becomes a multi-layer in the case of spinning or molding an unstretched thermoplastic resin membrane (hollow fiber or film). It is completely controlled by the layer ratio in the structure.

Next, the structure of the porous separation membrane according to the present invention will be described with reference to the drawings.

The drawing schematically shows a partially enlarged cross-section of the peripheral wall portion of the porous hollow fiber. A porous layer having a small pore size (first porous layer) is obtained by stretching a thermoplastic resin layer having a large molecular weight. 1) and a porous layer (second porous layer) 2 having a large pore size obtained by stretching a thermoplastic resin layer having a small molecular weight are laminated.

〔Example〕

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

(Example 1) A polypropylene (UBE-PP-J130G, trade name: Ube Kosan Co., Ltd.) having a MFI of 30 g / 10 min was used by using a two-layer nozzle for producing a hollow fiber equipped with a gas supply pipe having a diameter of 33 mm and an inner diameter of 27 mm. )) As an outer layer and polypropylene having an MFI of 5 g / 10 min (UBE-PP-Y105J, trade name: Ube Industries, Ltd.) as an inner layer, and the layer ratio of the outer layer to the inner layer is 9: 1 (extrusion of resin). The spinning temperature is 200)
Spinning was carried out under the conditions of ℃ and take-off speed of 116 m / min. The obtained polypropylene hollow fiber was placed in a heated air tank at 145 ° C for 6 minutes.
Heat for 1 minute, then in liquid nitrogen (-195 ° C)
Stretched 20% of the initial length, keeping the stretched state 1
Heat treatment was performed for 2 minutes in a heated air tank at 45 ° C. to produce a porous polypropylene hollow fiber.

This hollow fiber was subjected to 200% hot drawing in an air atmosphere at 145 ° C, and then heat-treated for 15 minutes in a heated air tank at 145 ° C while maintaining the drawn state to produce a porous polypropylene hollow fiber. This porous hollow fiber has an outer diameter of 400
The micron had an inner diameter of 300 microns.

When the cross section of the peripheral wall of the above-mentioned porous polypropylene hollow fiber was observed with a scanning electron microscope (X-650, manufactured by Hitachi, Ltd.), it was found that MFI had a large pore diameter of MFI of 30 g / 10 min. Of 5 g / 10 min., Pores having a small pore size were clearly present at the desired layer ratio of 9: 1, and most of the pores at the boundary of the layers were also penetrated. When the hole diameter of the through holes in each layer is measured by an electron micrograph, the MFI
30g / 10min layer average pore size is 1.4 microns, MF
The average pore size of the I5 g / 10 min layer was 0.67 microns.

A 0.1% γ-globulin physiological saline solution (pH 7.4) was filtered using the obtained porous hollow fiber, and its transmittance (separation rate was expressed by 100-transmittance) was 60.
It was 3%. In addition, the ratio of the filtration amount to the total flow rate (v ₁ / v
₁ + v ₂ ) was 0.058, and the water permeability of this porous hollow fiber was 12.9 / m ² · min · atm.

(Comparative Example 1) Polypropylene (UBE-PP-J105G, trade name:
Ube Industries, Ltd., MFI = 5g / 10 minutes), diameter 3
Using a hollow fiber manufacturing nozzle equipped with a gas supply pipe of 3 mm and an inner diameter of 27 mm, the spinning temperature is 200 ° C., the take-up speed is 116 m.
Spinning was performed under the condition of / min. The obtained polypropylene hollow fiber was heat-treated in a heated air bath at 145 ° C. for 6 minutes, and then stretched in liquid nitrogen (−195 ° C.) by 20% with respect to the initial length, and the stretched state was maintained at 145 ° C. Heat treatment was performed for 6 minutes in the heated air bath.

This hollow fiber was subjected to 200% hot drawing in an air atmosphere at 145 ° C, and then heat-treated for 15 minutes in a heated air tank at 145 ° C while maintaining the drawn state to produce a porous polypropylene hollow fiber.

This porous hollow fiber has an outer diameter of 400 microns and an inner diameter of 30.
It was 0 micron.

When the cross section of the peripheral wall of the above-mentioned porous polypropylene hollow fiber was observed with an electron microscope, a large number of large through holes were formed uniformly in the peripheral wall, and the diameter of the through holes was almost constant throughout. When the pore diameter was measured by an electron micrograph, the average pore diameter was 0.67 micron.
When the 0.1% γ-globulin physiological saline solution (pH 7.4) was filtered using the obtained porous hollow fiber, its transmittance was 58.6%. The ratio (v ₁ / v ₁ + v ₂ ) of the filtration amount to the total flow rate was 0.025. Also,
The water permeability of this porous hollow fiber is 3.8 / m ² · min · atm
It was.

(Example 2) A polypropylene (UBE-PP-J130G, trade name: Ube Industries, Ltd.) having a MFI of 30 g / 10 min was used by using a two-layer nozzle for producing a hollow fiber equipped with a gas supply pipe having a diameter of 33 mm and an inner diameter of 27 mm. )) As the outer layer, and MFI having a MFI of 9 g / 10 min (UBE-PP-Y109J, trade name: Ube Industries, Ltd.) as the inner layer, and the layer ratio of the outer layer to the inner layer is 9: 1 (extrusion of resin). The spinning temperature is 200)
Spinning was carried out under the conditions of ℃ and take-off speed of 116 m / min. The obtained polypropylene hollow fiber was heated in a heated air tank at 145 ° C for 5 hours.
Heat treatment for 15 minutes, then stretch at a temperature of 145 ° C. with an initial length of 200% and a strain rate of 8.33% / min, and perform a heat treatment for 15 minutes in a heated air tank at 145 ° C. while maintaining the stretched state. A porous polypropylene hollow fiber was produced.

This porous hollow fiber has an outer diameter of 400 microns and an inner diameter of 300.
It was micron.

When the cross section of the peripheral wall of the porous polypropylene hollow fiber was observed with a scanning electron microscope (X-650, manufactured by Hitachi, Ltd.), MFI had a large pore diameter of 30 g / 10 min. Through holes having a small pore size were clearly present in the layer having an MFI of 9 g / 10 min at a desired layer ratio of 9: 1, and most of the pores at the boundary of the layers were also penetrated. When the hole diameter of the through hole in each layer is measured by an electron micrograph, MFI
30g / 10min layer average pore size is 1.4 microns, MF
The average pore size of the I9 g / 10 min layer was 0.96 microns.

When a 0.1% γ-globulin physiological saline solution (pH 7.4) was filtered using the obtained porous hollow fiber, its permeation rate (separation rate was expressed as 100-permeability) was 69.
It was 6%. In addition, the ratio of the filtration amount to the total flow rate (v ₁ / v
₁ + v ₂ ) was 0.064. The water permeability of this porous hollow fiber was 12.3 / m ² · min · atm.

(Comparative Example 2) Polypropylene (UBE-PP-F109K, trade name:
Ube Kosan Co., Ltd., MFI = 9g / 10 minutes), diameter 3
Using a hollow fiber manufacturing nozzle equipped with a gas supply pipe of 3 mm and an inner diameter of 27 mm, the spinning temperature is 200 ° C., the take-up speed is 116 m.
Spinning was performed under the condition of / min. The obtained polypropylene hollow fiber was heat-treated in a heated air bath at 145 ° C. for 5 minutes, then at a temperature of 145 ° C., 200% of the initial length, and a strain rate of
It was stretched at 33% / min, and heat-treated for 15 minutes in a heated air tank at 145 ° C. while maintaining the stretched state to produce a porous polypropylene hollow fiber.

This porous hollow fiber has an outer diameter of 400 microns and an inner diameter of 30.
It was 0 micron.

When the cross section of the peripheral wall of the above-mentioned porous polypropylene hollow fiber was observed with an electron microscope, a large number of large through holes were formed uniformly in the peripheral wall, and the diameter of the through holes was almost constant throughout. When the pore diameter was measured by an electron micrograph, the average pore diameter was 0.96 micron.
A 0.1% γ-globulin physiological saline solution (pH 7.4) was filtered using the obtained porous hollow fiber, and the transmittance was 66.3%. The ratio (v ₁ / v ₁ + v ₂ ) of the filtration amount to the total flow rate was 0.020. The water permeability of this porous hollow fiber was 3.7 / m ² · min · atm.

(Example 3) Using an inflation molding machine equipped with an inflation two-layer molding die having a diameter of 50 mm and a slit gap of 0.7 mm, polypropylene having a MFI of 9 g / 10 min (UBE-
PP-F109K, trade name: Ube Industries, Ltd. outer layer, MFI 1g / 10min polypropylene (UBE-
PP-B101H, the same) was used as the inner layer, and the layer ratio of the outer layer to the inner layer was set to 5: 5 (controlled by the extrusion amount of the resin) to form an unstretched polypropylene film having a multilayer structure. The molding operation was performed by blowing polypropylene at a resin discharge temperature of 220 ° C. while blowing air into the valve so that the blow ratio was 0.7, and at room temperature on the outer wall surface of the film discharged at a position 5 cm above the die. It is cooled by blowing air, and the take-up speed is 36 m by a nipple roll at the position of 1.8 m on the die.
The desired unstretched polypropylene film was molded by the method of taking up at a rate of 1 / min.

The thickness of the obtained unstretched film was 20 μm.

This unstretched film was heated at a temperature of 145 ° C. and a strain rate of 8.3.
3% / min, 300% stretching to the initial length,
With this stretched state maintained, 1
Heat setting was performed for 0 minutes to produce a porous polypropylene film.

When the obtained porous film was observed with an electron microscope (X-650, manufactured by Hitachi Ltd.), the MFI was 9
Large pore size pores in M / 10 min layer, MFI is 1 g /
Through holes having a small pore size were clearly present in the layer of 10 minutes at a desired layer ratio of 5: 5, and most of the holes at the boundary of the layers were also penetrated. When the hole diameter of the through holes in each layer was measured by an electron micrograph, the average hole diameter of the layer having MFI of 9 g / 10 min was 0.98 μm, and the mean hole diameter of the layer having MFI of 1 g / 10 min was 0.3 μm.

From the above examples, the multi-layered porous polypropylene membrane according to the present invention is similar in separability (represented by 100-transmittance) as compared with the single-layered porous polypropylene membrane shown in the comparative example. The performance is that the ratio of filtration rate to total flow rate is about 2-3 times, and the water permeability is about 3-4.
It turns out that it shows double the ability. The mechanical strength did not differ between the two.

(Effect of the invention) As described above, according to the method for producing a porous membrane of the present invention, it is possible to produce a porous membrane having a multilayer structure in which the pore diameters are accurately distributed in the thickness direction, And not only is the porous membrane excellent in separation ability and permeation amount,
It also has mechanical strength.

[Brief description of drawings]

The drawings are partially enlarged cross-sectional explanatory views showing the structure of the porous membrane according to the present invention. 1 ... First porous layer 2 ... Second porous layer

Claims (4)

[Claims] 1. A first porous layer made of a thermoplastic resin having fine pores having a predetermined pore size required for separation, wherein the first porous layer has fine pores having a pore size larger than the predetermined pore size. A porous separation membrane comprising a multilayer structure in which at least one second porous layer made of a thermoplastic resin of the same type as the thermoplastic resin and having a larger molecular weight than that is laminated to form a multilayer structure. 2. The thermoplastic resin is selected from the group consisting of polypropylene, polyethylene, poly (4-methyl-pentene-1), polyoxymethylene, polyvinylidene fluoride, and ethylene-tetrafluoroethylene copolymer. The porous separation membrane according to claim 1. 3. A non-stretched membrane material having a multilayer structure composed of thermoplastic resin layers of the same kind having different molecular weights from each other, and then the unstretched membrane material is stretched to have fine pores having different pore diameters in the thickness direction of the membrane. A method for producing a porous separation membrane, comprising: 4. The thermoplastic resin is selected from the group consisting of polypropylene, polyethylene, poly (4-methyl-pentene-1), polyoxymethylene, vinylidene fluoride, and ethylene-tetrafluoroethylene copolymer. The method for producing a porous separation membrane according to claim 3.