JPS6151928B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a selectively permeable membrane, and more particularly to an aromatic polyamide selectively permeable membrane that can be suitably used as an ultrafiltration membrane. In general, membranes that selectively permeate specific components in liquid mixtures such as solutions, emulsions, and suspensions are called selectively permeable membranes.
Since it is possible to separate the solvent and dispersion medium from solutions and emulsions containing synthetic polymer substances and microorganisms, it is suitable for purification and concentration processes in fields such as factory wastewater treatment, sewage purification, food, medicine, brewing, and fermentation. It is used. Conventionally, such selective permeation membranes have mainly been made of cellulose acetate, which has a high permeation rate and rejection rate. However, this is not always sufficient, so recently, a selectively permeable membrane made of aromatic polyamide, which is superior in these respects, has been proposed. Generally, in order for a selectively permeable membrane to be put to practical use in membrane separation processing, the permeable membrane must have a high permeation rate for solvents and dispersion media, as well as a high rejection rate for solutes and dispersoids. However, there is a contradictory relationship between the permeation rate and the rejection rate, and in order to increase the rejection rate, the permeation rate must be sacrificed to some extent.
Therefore, none of the aromatic polyamide membranes that have been proposed so far have both a high permeation rate and a high rejection rate, and there are still difficulties in putting them into practical use. In particular, conventional aromatic polyamide membranes have a wide pore size distribution in the membrane surface layer, the so-called skin layer, and low fractionation with respect to the molecular weight of the solute, which is the reason why it is not possible to increase the rejection rate without impairing the permeation rate. It can be said. In order to solve these problems, ultrafiltration of aromatic polyamide from dope containing an inorganic swelling agent and dimethyl sulfoxide as an organic swelling agent and using N-methyl-2-pyrrolidone, dimethylformamide, etc. as a solvent was developed. A method for manufacturing membranes has been proposed (Japanese Patent Application Laid-Open No. 56-2804). According to this method, compared to conventional aromatic polyamide ultrafiltration membranes, the fractionation performance for the molecular weight of solutes and the water permeation rate are improved, but the molecular weight fractionation performance remains at around tens of thousands. , it has almost no ability to exclude solutes with molecular weights as small as several thousand to 10,000. However, in membrane separation processes, selectively permeable membranes having the ability to remove such low molecular weight solutes are often required for processes such as concentration and purification. The present inventors have conducted extensive research in order to solve the above-mentioned problems in aromatic polyamide membranes. As a result, in the production of aromatic polyamide membranes, the membrane-forming solution (hereinafter referred to as dope) contains By containing the specified organic swelling agent and inorganic swelling agent in a specific ratio, it is possible to obtain a selectively permeable membrane having a significantly smaller molecular weight fractionation than conventional aromatic polyamide selectively permeable membranes. This is what led to the discovery of the present invention. The method for producing a selectively permeable membrane according to the present invention is as follows: (A) General formula [-R ¹ -X ¹ -R ² -X ² -] () (However, R ¹ and R ² are each independently a divalent aromatic X ¹ and X ² each independently represent an amide bond.) An aromatic polyamide consisting of repeating units represented by Inorganic swelling agent selected from metal halides, nitrates, sulfates, perchlorates, and mixtures of two or more thereof
15 to 90 parts by weight, and (C) a coagulation value of 90 to 90 for the above aromatic polyamide.
A dope consisting of 800 w/v% and 10 to 500 parts by weight of an organic swelling agent having a boiling point of 50 to 120°C under normal pressure is applied to the supporting substrate, and then,
It is characterized in that the aromatic polyamide is coagulated and formed into a film by immersing it in a coagulating solvent that does not dissolve the aromatic polyamide but can dissolve the organic solvent, inorganic swelling agent, and organic swelling agent. However, the coagulation value of the organic swelling agent here refers to the polymer dissolved in a 5% by weight N-methyl-2-pyrrolidone solution of lithium chloride to prepare a solution with a polymer concentration of 2% by weight, and 5 ml of this solution. The volume V (ml) of the minimum amount of organic swelling agent that must be added to produce white turbidity due to polymer precipitation at a temperature of 25° C. is measured, and is a value defined according to the following formula: Coagulation value = V (ml) × Specific gravity of organic swelling agent / 5 (ml) × 100 (w/v%) The aromatic polyamide used in the present invention consists of repeating units represented by the above general formula,
Examples of R ¹ and R ² include 1,3-phenylene, 1,4-phenylene, 3,3'-diphenylene, 4,4'-biphenylene, 1,4-naphthylene, 1,5-naphthylene, 1, 6-naphthylene,
4,4'-oxadiphenylene, 4,4'-methylenediphenylene, 4,4'-carbonyldiphenylene, 3,3'-sulfonyldiphenylene, etc., and these aromatic groups may have a portion of its hydrogen atoms substituted with a halogen atom, an amino group, a nitro group, a carboxyl group, a sulfonic acid group, a lower alkyl group, or the like. Among these aromatic polyamides, in the present invention, substantially the following formula is used: It is preferable to use repeating units represented by
Those having a mol% or less are preferably used. The aromatic polyamide as described above can be produced, for example, by the method disclosed in Japanese Patent Publication No. 35-13247 and Japanese Patent Publication No. 35-14399. Typically, a dicarboxylic acid chloride and a diamine are condensed and polymerized in a solvent such as dimethylacetamide at low temperature. The aromatic polyamide obtained in this way varies depending on the starting materials, reaction conditions, etc.
Generally, the intrinsic viscosity (solvent N-methyl-2-pyrrolidone, hereinafter referred to as NMP) at a temperature of 30℃ is
It is about 0.3 to 2.0. In the present invention, so that the resulting permeable membrane has sufficient mechanical strength,
Aromatic polyamides having an intrinsic viscosity of 0.9 to 2.0 are preferably used. In the present invention, the dope solvent must be able to dissolve the aromatic polyamide, the inorganic swelling agent, and the organic swelling agent, and be compatible with the coagulating solvent, usually water, such as NMP, dimethylformamide (hereinafter referred to as DMF). ), dimethylacetamide (hereinafter referred to as DMAC), hexamethylene phosphoramide, or a mixture of two or more of these are used, and the aromatic polyamide concentration in the dope (hereinafter referred to as dope concentration) is usually It is 5 to 30% by weight, preferably 8 to 20% by weight. If the dope concentration is too low, the resulting permeable membrane will have poor selective separation ability, while if the dope concentration is too high, it will not only be difficult to coat it uniformly on the supporting substrate, but also the resulting permeable membrane will have poor selective separation ability. This is because the permeation rate of the permeable membrane becomes lower. In relation to the dope concentration, the viscosity of the dope when applied to the support substrate is usually adjusted to 20 to 5000 poise (20°C), preferably 50 to 1000 poise (20°C). In the present invention, an inorganic swelling agent and an organic swelling agent are used together as additives constituting the dope. Inorganic swelling agents are halides of alkali metals and alkaline earth metals, preferably lithium, potassium, sodium, calcium and magnesium, especially chlorides and bromides, nitrates, sulfates, perchloroxygen salts and mixtures of two or more thereof. selected from. Specific examples include lithium chloride, potassium chloride, calcium chloride, lithium bromide, potassium bromide, lithium nitrate, potassium nitrate, magnesium sulfate, and magnesium perchlorate. The amount of these inorganic swelling agents used varies depending on the dope solvent and dope concentration, but in general, it is
The amount is 15 to 90 parts by weight, preferably 30 to 80 parts by weight, per 100 parts by weight. If the amount of inorganic swelling agent used is too large, the dissolution state of the dope tends to deteriorate, and the resulting membrane becomes inferior in selective separation ability. As the solubility in the
This is because not only is membrane formation difficult, but also the permeation rate of the resulting permeable membrane tends to be low. As described above, the organic swelling agent used in combination with the inorganic swelling agent in the present invention is a liquid organic compound having a specific coagulation value and a specific boiling point range for the aromatic polyamide used. Specific examples of such organic swelling agents include tetrahydrofuran, dioxane, diisopropyl ether, di-n-butyl ether, acetone, methyl ethyl ketone, diethyl ketone, cyclohexanone, methyl formate, ethyl formate, methyl acetate, ethyl acetate, methanol, ethanol, n-propanol, i-propanol, n-butanol, sec
-butanol, t-butanol, etc., and one or a mixture of two or more of these can be used. The organic swelling agent also needs to be soluble in the dope-forming solvent and the coagulation solvent described below. The amount of organic swelling agent used according to the invention is from 10 to 500 parts by weight, preferably from 30 to 250 parts by weight, per 100 parts by weight of aromatic polyamide. Similar to inorganic swelling agents, organic swelling agents also tend to not produce a uniform dope if used in too large a quantity;
This is because if the amount is too small, it tends to be difficult to obtain a permeable membrane having a sufficient solvent permeation rate. The method for preparing the dope containing such an organic swelling agent and an inorganic swelling agent is not particularly limited, but preferably, the aromatic polyamide is added and dissolved in a dope solvent in which an inorganic swelling agent has been dissolved in advance, and then the aromatic polyamide is dissolved therein. Add an organic swelling agent to the room temperature or 80~80℃ as needed.
Stir while heating to a temperature of about 150°C to make a uniform dope. In the method of the present invention, the dope thus prepared is applied to a supporting substrate, and then immersed in a coagulating solvent that coagulates the aromatic polyamide to coagulate the polymer.
Surprisingly, it has been found that the above-mentioned organic swelling agent has a large effect on the coagulation properties of the aromatic polyamide when the doped support substrate is immersed in a coagulation solvent. That is, it has been found that when the organic swelling agent is used in the above-mentioned range for aromatic polyamide, the aromatic polyamide is rapidly coagulated in the coagulation solvent. In other words, the aromatic polyamide rapidly solidifies while remaining relatively uniformly dissolved in the dope solvent, and in terms of the film structure, the surface layer is quickly and uniformly formed. . As a result, according to the method of the present invention, the skin layer has a microporous structure with a significantly narrow pore size distribution width, so that the fractionation property with respect to the molecular weight of the solute is high. An ultrafiltration membrane with a high rejection rate can be obtained. On the other hand, when the amount of organic swelling agent added is too small,
When not used as an additive, the solidification rate of aromatic polyamide in the immersion solvent is slow, so the membrane surface layer has an uneven structure during the solidification process, resulting in a skin layer with a wide pore size distribution. All that can be obtained is a membrane. Generally, the solubility of an aromatic polyamide in a dope solvent depends not only on the type of solvent but also on the content of the inorganic swelling agent in the dope. If the amount of the organic swelling agent added is too large, the state of dissolution of the dope will deteriorate when the dope concentration is increased enough to be suitable for film formation. Therefore, if the amount of the inorganic swelling agent that helps dissolve the aromatic polyamide in the dope is increased more than necessary, the above-mentioned disadvantages will occur. Therefore, in the method of the present invention, the amounts of the organic swelling agent and the inorganic swelling agent added are critical as they are mutually related. In the method of the present invention, by providing a certain balance to the dope composition, as described above, the fractionation property can be significantly improved, and a permeable membrane with a high rejection rate for solutes with a molecular weight of about 1,000 to 10,000 can be obtained. It was extremely successful. As is clear from a comparison of the Examples and Comparative Examples described later, for example, when a polyethylene glycol aqueous solution with a molecular weight of 2,000 or 6,000 is treated, the limit obtained from a dope that does not contain the organic swelling agent of the present invention as an additive is While the ultrafiltration membrane has almost no exclusion ability under the conditions described below, the ultrafiltration membrane obtained according to the present invention has an exclusion rate of 80% or more, preferably 90% or more. This difference in rejection rate is extremely important in practical ultrafiltration. Moreover, since the membrane according to the present invention is also superior in terms of water permeation rate, the superiority of the membrane according to the present invention is clear. The dope is usually applied onto the supporting substrate at room temperature of about 15 to 35°C. This doped support base material is
After application, it is contacted with a coagulating solvent, typically within a few minutes, for example within 5 minutes. This is because if the membrane is left for too long after dope application, the resulting permeable membrane will be inferior in selective separation ability. The coagulating solvent must not dissolve the aromatic polyamide, be compatible with the dope solvent, and be capable of dissolving the organic swelling agent and the inorganic swelling agent. Water is most preferably used as the coagulating solvent, but methanol, ethanol, acetone, ethylene glycol, and mixed solvents of these and water can also be used. Although the coagulation temperature is not particularly limited, the coagulation is generally carried out at a temperature below the boiling point of the coagulation solvent. When water is used as a coagulating solvent, the temperature is usually 0 to 80°C, preferably 0 to 50°C. The time required for coagulation molding varies depending on the coagulation solvent and its temperature, but
Usually 1 to 10 hours. The supporting base material to which the dope is applied is not particularly limited. When a plate or tube with a smooth surface, such as glass, stainless steel, aluminum, polypropylene, polyethylene, etc., is used as a support base material, after the aromatic polyamide is solidified in a coagulation solvent,
Since it is easily peeled off from these supporting substrates, sheet-like and tubular permeable membranes can be obtained, respectively. In addition, by using sheet-like or tubular support base materials of woven or non-woven fabrics made of organic fibers such as polyester fibers and acrylic fibers, and inorganic fibers such as glass fibers and carbon fibers, these support base materials can be A composite permeable membrane can be obtained integrally formed thereon. Furthermore, a hollow fiber-like permeable membrane can also be obtained by extruding the dope from a hollow spinneret into a coagulating solvent. In the method of the present invention, after the dope is applied to the support substrate, the dope can be heated to force a portion of the dope solvent to evaporate from the dope surface, and then immersed in a coagulation solvent. This heat treatment can usually be carried out by blowing hot air at 50 to 200°C onto the dope-coated surface, for example, for 1 to 120 seconds. In the method of the present invention, for the purpose of improving the mechanical toughness at high temperatures of the selectively permeable membrane obtained as described above, the membrane is heated at a temperature of 100 to 400°C after film formation. Heat treatment can be performed for about 5 seconds to 30 minutes. This treatment is carried out by placing the permeable membrane in heated air or by immersing it in heated water or glycerin. The treatment time may be short if the treatment temperature is high, and long if the treatment temperature is low. The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples in any way. In addition, in the following examples, film characteristics were evaluated as follows. That is, the permeable membrane is attached to a pressurized filter holder, and in the case of pure water, 2 kg/cm ² and polyethylene glycol (hereinafter referred to as
It's called PEG. ) In the case of an aqueous solution (concentration 5000 ppm), each was supplied at an operating pressure of 4 Kg/cm ² and the permeation rate and rejection rate defined by the following formula were determined. Permeation rate = permeated water amount (ml) / effective membrane area (cm ² ) x permeation time (min) Rejection rate = (1 - PEG concentration in permeate / PEG concentration in feed liquid) x 100 (%) Example Said In the general formula (), an aromatic polyamide containing 70 mol% of m-phenylene groups and 30 mol% of p-phenylene groups, and an intrinsic viscosity of an NMP solution at 30°C of 1.83 is used, and the solvent is NMP. Dopes A to G having the compositions shown in the table were prepared.

[Table] Each dope was cast onto a glass plate at room temperature, immediately poured into water at 0°C, and immersed for 24 hours to obtain an ultrafiltration membrane with a thickness of 150 μm. Table 2 shows the physical properties of each film. The polyethylene glycol molecular weight fractionation properties of the ultrafiltration membranes obtained from Dopes A and F, respectively, are shown in the attached drawings. The former is the molecular weight
It is shown that the latter has fractionation in the 6000 region, and the latter has fractionation in the 2000 region. Comparative Example Dope H was prepared in the same manner as in Example, except that 41.7 parts by weight of diethylene glycol (coagulation value: 898 w/v%, boiling point: 245°C) was used instead of methanol in Dope A of Example. This dope was cast onto a glass plate at room temperature, immediately poured into water at 20°C, and immersed for 24 hours to obtain an ultrafiltration membrane with a thickness of 150 μm. The physical properties of this membrane are shown in Table 2, and the molecular weight
It has almost no ability to eliminate 6000 polyethylene glycol. The molecular weight fractionation of this membrane was approximately 20,000 as shown in the drawing.

[Table] In addition, an ultrafiltration membrane with a thickness of 150 μm was obtained in the same manner as in the above comparative example using an NMP solution containing 5 parts by weight of lithium chloride and 12 parts by weight of aromatic polyamide as a dope. The physical properties of this film are shown in Table 2.

[Brief explanation of the drawing]

The drawing shows the polyethylene glycol molecular weight fractionation properties of ultrafiltration membranes obtained in Examples and Comparative Examples of the present invention.

Claims (1)

[Claims] 1 (A) General formula [-R ¹ -X ¹ -R ² -X ² -] () (However, R ¹ and R ² each independently represent a divalent aromatic group, (X ¹ and X ² each independently represent an amide bond.) An aromatic polyamide consisting of a repeating unit represented by (B) an alkali metal and alkaline earth metal halide, Inorganic swelling agent selected from nitrates, sulfates, perchlorates, and mixtures of two or more of these
15 to 90 parts by weight, and (C) a coagulation value of 90 to 90 for the above aromatic polyamide.
A dope consisting of 800 w/v% and 10 to 500 parts by weight of an organic swelling agent having a boiling point of 50 to 120°C under normal pressure is applied to the supporting substrate, and then,
A selection characterized in that the aromatic polyamide is coagulated and formed into a film by immersing it in a coagulation solvent that does not dissolve the aromatic polyamide but can dissolve the organic solvent, inorganic swelling agent, and organic swelling agent. Method for manufacturing a sexually permeable membrane. 2 Aromatic polyamide is substantially of the formula Claim 1 consisting of repeating units represented by, and characterized in that out of all phenylene groups, m-phenylene groups account for 70 mol% or more, and p-phenylene groups account for 30 mol% or less The method for producing the selectively permeable membrane described above. 3. Claim 1, wherein the organic swelling agent is at least one selected from tetrahydrofuran, dioxane, acetone, methyl ethyl ketone, diethyl ketone, cyclohexanone, methyl formate, ethyl formate, methanol, ethanol, propanol, and butanol. The method for producing the selectively permeable membrane described above.