JPS62289202A - Production of microporous membrane - Google Patents

Abstract

PURPOSE:To obtain the title anisotropic microporous membrane with the surface layer having uniform pore diameter by forming a liq. film from a stable dope, and providing a temp. difference between the front and rear surfaces of the liq. film. CONSTITUTION:Dimethylformamide, dimethylacetamide, dimethyl sulfoxide, etc., for example, are used as the solvent, when polysulfone is used as the polymer, to prepare a 5-30wt% dope. A nonsolvent or a swelling agent is added, as required, to improve the permeability of the microporous membrane. The dope is used to make the liq. membrane. The thickness is preferably controlled to 50-500mum. A temp. difference is provided between the front and rear surfaces of the liq. film of the dope. The temp. difference is adjusted to >=5 deg.C. The temp. difference is preferably kept for 5sec-10min. Then film is the dipped in a coagulation bath, then washed, and dried, and the anisotropic microporous membrane wherein the pore diameter is increased from the front surface layer toward the rear surface layer is obtained.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Technical Field] The present invention relates to a method for producing a microporous membrane having anisotropy and suitable for precision filtration.

[Background technology]

Microporous membranes are used for sterilization and clarification of beer, wine, sake, etc., sterilization of injection drugs and infusions, microbial testing and analysis, sterilization and particle removal of various types of water, and removal of dust and bacteria in the air. It is used in many fields that require high-precision filtration.

Conventionally, microporous membranes are produced by dissolving the raw material polymer in a solvent and casting this solution (dope) onto a casting table (support plate). The film is produced by a wet method, in which the membrane is obtained by immersing it in a coagulation bath consisting of a solvent in which it does not dissolve.

The microporous membrane obtained by the above wet method has a dense membrane surface or a small pore size on the membrane surface, and the inside of the membrane and the surface in contact with the casting table are spongy with larger pores than the surface, or It has an elongated finger-like megaventricular foramen,
It has so-called "anisotropy". The water permeability of a microporous membrane is mainly controlled by the layer with small pores on the membrane surface, so a membrane with the above-mentioned anisotropy has the ability to trap particles and has relatively high water permeability. show.

However, conventional microporous membranes are unsatisfactory in terms of liquid permeability, bacterial and particle retention, etc. This is due to the fact that the pore size on the surface is not uniform.
Publication No. 51 describes a method of obtaining a microporous membrane by dissolving a polymer in a solvent and adding a non-solvent to this solution until the solution becomes slightly cloudy, and using this as a membrane-forming stock solution (dope). Proposed. In this method, since the dope is in a slightly cloudy and unstable state, separation of the dope is likely to occur due to changes over time (separation occurs within two weeks with the dope described in the above publication), and the dope cannot be stored for a long period of time. After that, there is a problem that it is difficult to obtain a microporous membrane with uniform pore size.

[Purpose of the invention]

In view of the above, an object of the present invention is to provide a method for producing a microporous membrane that is easy to control dope, has a uniform pore size in the surface layer, and has anisotropy.

[Disclosure of the invention]

In the case of the above wet method, when the dope in the form of a liquid film (film-forming stock solution) is immersed in a non-solvent bath, there is a large concentration gradient between the solvent and the non-solvent at the interface with the non-solvent, and the rate of substitution between the solvent and the non-solvent is Since the is large, the precipitation rate of the polymer is fast. Therefore, the pore size of the membrane near the interface with the non-solvent becomes extremely small.

On the other hand, the diffusion rate of the non-solvent becomes slower in the interior of the membrane (that is, the further away from the interface with the non-solvent and closer to the casting table) the more a layer with larger pores is formed. In short, in general, the anisotropy of a microporous membrane in a wet method is controlled by the infiltration rate of a non-solvent.

The inventors thought that by using a stable dope, the pore diameter of the resulting microporous membrane would not change depending on the length of the storage period of the dope, and dope management would become easier. However, simply using a stable dope is insufficient to give the obtained microporous membrane anisotropy. If the anisotropy of the film is controlled by creating a temperature difference between the front and back sides of the dope, it is possible to obtain a microporous film with uniform pore size and anisotropy. As a result of researching this idea, we completed this invention.

Therefore, the present invention is characterized in that, in a method for manufacturing a microporous membrane that includes a step of forming a liquid film from a dope, a stable dope is used and a temperature difference is created between the front and back surfaces of the liquid film. The gist of this paper is a method for manufacturing microporous membranes.

The present invention will be explained in detail below.

The dope is usually prepared by dissolving a polymer in a solvent and adding, if necessary, a swelling agent and/or a non-solvent for the polymer. However, since the stability is not good as it is, in this invention, the type and blending ratio of these blends are appropriately adjusted to make them stable. Here, "stable" means that separation does not occur even if left for two weeks, for example. If a stable dope is used, the dope can be easily controlled in the film forming process, and the resulting microporous film will be uniform and highly reliable.

Various polymers are used, for example,
Natural resins, semi-synthetic resins such as cellulose acetate and nitrocellulose, polysulfone, polycarbonate, polyvinyl chloride, polystyrene, polyamide, halogenated polymers such as polyvinylidene fluoride, polyvinyl fluoride, polyfluorohalocarbon, polychloroether, etc. Acetal polymers such as polyether and polyformaldehyde, polyacrylonitrile, polymethyl methacrylate,
Acrylic resins such as polyn-butyl methacrylate, polyurethane, polyimide, polybenzimidazole, polyvinyl acetate, aromatic and aliphatic polyethers, and the like can be used alone or in combination, but are not limited to these.

Polysulfone has excellent heat resistance and chemical resistance, so if you use polysulfone to make a microporous membrane,
A film with excellent heat resistance and chemical resistance can be obtained. Polysulfone is a polymer containing an -8O□- group, and includes, but is not limited to, polysulfone composed of repeating units of and polyethersulfone composed of repeating units of In addition, it is possible to use a material that is compatible with a non-solvent that has coagulation ability with respect to the polymer. For example, when polysulfone is used as the polymer, dimethylformamide, dimethylacetamide, N
-Methyl-2-pyrrolidone, dimethyl sulfoxide, etc. can be used alone or in combination,
Not limited to these. In addition, when polyvinyl chloride, polycarbonate, polystyrene, etc. are used as a polymer, dimethylformamide etc. are used as a solvent, when polyamide etc. are used as a polymer, m-cresol etc. are used as a solvent, and polyacrylonitrile etc. are used as a polymer. When using, dimethylacetamide or the like is used as a solvent, but the solvent is not limited to the above.

A dope is prepared by dissolving the polymer in the above solvent. When using polysulfone as the polymer, the concentration of polysulfone is preferably 5 to 30% by weight, more preferably 8 to 15% by weight, although it is not particularly limited. If the concentration of polysulfone is too high, the water permeability of the resulting microporous membrane may be reduced; if it is too low, a microporous membrane with sufficient strength may not be obtained. In the case of other polymers, the concentration may be similar to that of polysulfone.

Note that a non-solvent and/or a swelling agent may be added to the dope as necessary in order to improve the permeation performance of the microporous membrane. For example, inorganic salts such as halides, nitrates, and sulfates of alkali metals or alkaline earth metals such as lithium nitrate and potassium chloride; alcohols such as methanol, ethanol, and isopropanol;
Glycols and their ethers such as ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, polyoxyethylene octylphenol ether, polyhydric alcohols such as glycerin, ketones such as acetone and methyl ethyl ketone, etc. may be added, but are not limited to these. . If the amount of non-solvent and/or swelling agent added is large, phase separation of the dope will occur, so it must be added within a range that is homogeneous at room temperature and does not separate into two phases even if left standing.

The amount of the swelling agent and/or non-solvent to be added is not particularly limited as it varies depending on the type and concentration of the polymer, the type of solvent, etc. For example, in Example 1 below, the ratio of polymer: swelling agent is 100:580, and in Example 2, the ratio of polymer: swelling agent = 100:286.

The dope is made into a liquid film by appropriate means.

A liquid film of dope is formed by, for example, casting or coating the dope on a casting table in a gas phase, but methods other than these methods may also be used. It is also possible to extrude the dope from an annular nozzle for producing hollow fibers. In this case,
It is best to create a temperature difference between the inside and outside of the hollow dope.

The thickness of the dope when forming it into a liquid film (thickness of the liquid film,
(not the dry film thickness) is not particularly limited, but
The range is preferably 50 to 500 μm, and 100 to 400 p
A range of m is more preferable. If the upper limit of this 1n range is exceeded, the selectivity of the obtained microporous membrane will increase, but the water permeation rate may decrease; if it is below the lower limit, the strength of the obtained microporous membrane may be insufficient. be. Note that the gas phase usually refers to air, but is not limited to this.

A temperature difference is created between the front and back surfaces of the dope, which has been turned into a liquid film. When the temperature is high, the dope becomes unstable and separates, so this separation is used to control the anisotropy. That is, the pore diameter on the side where the temperature is higher is larger, and the pore diameter is smaller on the side where the temperature is lowered, thereby imparting anisotropy to the membrane. Moreover, the pore diameter on the side where the temperature is lower can be made uniform. Note that it is preferable that the temperature difference is 5° C. or more. If the temperature difference is smaller than this, the effect of creating a temperature difference may not be achieved. Further, the temperature difference may be applied either at the same time as forming the dope liquid film or after forming the liquid film.

The dope formed into a liquid film on the casting table is preferably placed in a gas phase with the side in contact with the casting table having a higher temperature than the side in contact with the gas phase. This is because when solidifying the polymer by immersing it in a non-solvent, it is convenient to provide anisotropy by also utilizing the effect of the penetration rate of the non-solvent. There are no particular limitations on the method of making the side of the dope in contact with the casting table higher than the side in contact with the gas phase, but for example, ■ Turn room-temperature dope into a liquid film on a casting table at a higher temperature, and then heat it with air at the same temperature as the dope. Methods such as exposing the dope to a liquid film at a temperature lower than room temperature on a casting stand at room temperature or higher.

The time period for which the dope formed into a liquid film is kept at a temperature difference as described above is not particularly limited, but is preferably 5 seconds to 10 minutes. Below the lower limit of this range? & There is a possibility that a sufficient temperature difference will not be generated within the film, and even if the temperature exceeds the upper limit, it will not lead to much improvement in characteristics, and there is a possibility that time will be wasted.

The above-mentioned casting table is not particularly limited, but metal plates and tubes such as stainless steel plates, copper plates, Ni-Kel plated iron plates, glass plates and tubes, plates and tubes such as polyethylene, polypropylene, polyester, polytetrafluoroethylene, etc.・Film etc. are used. Also, instead of turning the dope into a liquid film on a casting table, it can be turned into a liquid film on woven or non-woven fabrics made of organic fibers such as polyester fibers and acrylic fibers, and inorganic fibers such as glass fibers.
Composite permeable membranes can also be formed.

The dope formed into a liquid film is immersed in a coagulation bath after creating a temperature difference between the front and back surfaces.

The coagulation bath used is a non-solvent for the polymer that has good compatibility (miscibility) with the solvent, swelling agent, and non-solvent contained in the dope. Typically, but not limited to, water or a mixture of water and a small amount of an organic solvent is used.

Examples of the organic solvents used in combination with water include, but are not limited to, methanol, ethanol, and ethylene glycol. Although the temperature of the coagulation bath is not particularly limited, it is preferably a temperature below the boiling point of the solvent used in the coagulation bath.

The time for immersion in the coagulation bath is not particularly limited, but is preferably 30 seconds to 15 minutes. If it is below the lower limit of this range, there is a possibility that the polymer will not be sufficiently coagulated, and if it is longer than 15 minutes, the polymer will usually coagulate sufficiently, so there is no need to leave it in the coagulation bath any longer. Note that when washing is performed subsequently, there may not be a clear distinction between coagulation and washing in the actual process.

After being immersed in a coagulation bath to solidify, thoroughly wash and remove the solvent.
After removing the non-solvent and drying, a microporous membrane is obtained.

Alternatively, the microporous membrane may be obtained by volatilizing the solvent or non-solvent. It has anisotropy in that the pore diameter on the surface layer is small and uniform, and the pore diameter increases from this surface layer to the back surface (the surface where the temperature was high when it was turned into a liquid film). Therefore, when filtration is performed with the back side facing upstream and the front side facing downstream, the surface layer becomes a barrier layer during filtration, making it highly reliable for filtering out particles, bacteria, etc. Good transparency.

Next, examples will be shown, but the present invention is not limited to the examples.

(Example 1) 10 parts by weight of polyether sulfone and 58 parts by weight of polyoxyethylene octylphenol ether were added to 32 parts by weight of dimethyl sulfoxide, and the mixture was stirred and melted at 30° C. to prepare a dope.

This dope is placed on a glass plate kept at 40°C to a thickness of 3.
00III! 30℃, RH
It was left standing in 75% air for 5 minutes. Thereafter, it was immersed in water for 5 minutes to solidify, thoroughly washed, and then dried to obtain a microporous membrane.

When the surface of the obtained microporous membrane was observed with a scanning electron microscope, the pore diameter on the surface in contact with the glass plate was 15 μm, and the pore diameter on the surface in contact with air was 0.4 μm, which were uniform.

(Example 2) Polyether sulfone 1). 0 parts by weight and 31.5 parts by weight of diethylene glycol, 5 parts by weight of dimethyl sulfoxide
A dope was prepared by adding 7.5 parts by weight and stirring and dissolving at 30°C.

This dope is spread on a glass plate kept at 45°C to a thickness of 3.
After casting to a thickness of 50 μm, 25°C, RH80%
It was left standing in the air for 5 minutes. Thereafter, it was immersed in water for 10 minutes to solidify, thoroughly washed, and then dried to obtain a microporous membrane.

When the surface of the obtained microporous membrane was observed with a scanning electron microscope, the pore diameter on the surface in contact with glass was 12 μm, and the pore diameter on the surface in contact with air was 0.3 μm, which were uniform.

It can be seen that the microporous membranes obtained in Examples 1 and 2 exhibit anisotropy in which the pore diameter on one surface and the pore diameter on the other surface are approximately 1:40.

(Comparative Example 1) Using the same dope as in Example 1, this dope was cast onto a glass plate kept at 30°C to a thickness of 300 pm, and then left standing in air at 30°C and 75% RH for 5 minutes. I placed it. After this, a microporous membrane was obtained in the same manner as in Example 1.

When the surface of the obtained microporous membrane was observed with a scanning electron microscope, the pore diameter on the surface in contact with the glass plate was 3 μm, and the pore diameter on the surface in contact with air was 0.4 μm, and it was found that they were nonuniform. .

(Example 3) After storing the dope prepared in Example 1 at 30° C. for 3 weeks, a microporous membrane was obtained in the same manner as in Example 1 using this dope.

When the surface of the obtained microporous membrane was observed with a scanning electron microscope, the pore diameter on the surface in contact with the glass plate was 15 μm, and the pore diameter on the surface in contact with air was 0.4 μm, which were uniform.

(Comparative Example 2) Parts by weight of polyether sulfone 1) were dissolved in 30 parts by weight of dimethyl sulfoxide, and 59 parts by weight of polyoxyethylene octylphenol ether were added and mixed with stirring to obtain a dope.

After storing this dope at 30°C for 3 weeks, a microporous membrane was obtained in the same manner as in Example 1 using this dope.

The microporous membrane obtained was extremely non-uniform and had low strength.

As seen in the results of Example 3 and Comparative Example 2, it can be seen that the stable dope used in the present invention is stable even when stored for two weeks or more, and the dope can be easily controlled.

[Effects of the Invention] As described above, the method for producing a microporous membrane of the present invention is characterized by forming a liquid film using a stable dope and creating a temperature difference between the front and back surfaces of this liquid film. Therefore, it is possible to easily control the doping, and to obtain a microporous membrane with uniform pore diameter in the surface layer and anisotropy.

Claims (1)

[Claims] (1) In a method for manufacturing a microporous membrane that includes a step of forming a liquid film from a dope, a stable material is used as the dope, and a temperature difference is created between the front and back surfaces of the liquid film. (2) Forming a liquid film is performed on a casting table in a gas phase, and creating a temperature difference makes the temperature of the casting table lower than the temperature of the dope. A method for producing a microporous membrane according to claim 1, which is achieved by raising the temperature to a high temperature. (3) The method for producing a microporous membrane according to claim 1 or 2, wherein the temperature difference is 5° C. or more.