JPH0232008B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing an ultrafiltration membrane that exhibits high water permeability and allows for precise and easy control of pore size distribution, that is, solute separation characteristics. In recent years, membrane separation methods such as reverse osmosis and ultrafiltration have been used for desalination of seawater and underground water, treatment of pulp wastewater, pulp wastewater and various factory wastewater, separation and purification of fermentation products, and food products such as dairy products and fruit juices. It has come to be widely used in industrial concentration, recovery of valuable materials, and other fields such as medical care and pharmaceuticals. Conventionally, ultrafiltration membranes are mainly manufactured by a wet method. This method basically involves dissolving a resin, which is a raw material for the membrane, in a solvent, using this as a membrane-forming stock solution, and then immersing and casting in a non-solvent of the resin to obtain an ultrafiltration membrane. The ultrafiltration membrane prepared by this method is an asymmetric membrane consisting of a double structure of a dense surface active layer and a sponge layer supporting it, and therefore has high water permeability and sharp fractionation characteristics, and is easy to operate. Shows excellent performance with less clogging. As mentioned above, in a wide variety of applications, it is natural that membranes with different supercharacteristics, especially fractionation characteristics, are often required. For this reason, as a general method to change the ultrafiltration characteristics,
It is known to change the resin concentration of the membrane-forming stock solution and to add an appropriate amount of a non-solvent or a swelling agent to the film-forming stock solution. However, method (2) has its limitations because the mechanical strength of the membrane decreases at the same time as the resin concentration is lowered, and water permeability decreases rapidly when the resin concentration is increased, but no drastic change in the fractionation characteristics was observed. I can't. As with the previous method, it was difficult to control the overcharacteristics over a wide range. For this reason, it is currently difficult to obtain an ultrafiltration membrane whose ultrafiltration characteristics can be controlled to a large extent by using a resin with excellent heat resistance and chemical resistance. On the other hand, there is a known method for producing a microporous polymer film with open cells by stretching a non-porous film such as polypropylene by about 50 to 150% at an appropriate temperature (for example, JP-A No. 50-22068). Public notices, etc.). However, since the material of the microporous film produced by this method is strongly hydrophobic, the water permeation rate is low, and therefore a special surface treatment is required to improve water permeability. The present inventors have investigated ultrafiltration membranes that have high substance permeability using various synthetic resins, and have found that ultrafiltration membranes with a conventional asymmetric structure can be stretched with a certain resin stock solution composition. The present invention was achieved based on the discovery that higher permeability can be imparted. In general, with conventional asymmetric membranes that have a dense surface active layer on one side, it seems that stretching can relatively easily enlarge the micropores on the surface and improve substance permeability at the same time. Because of its unique structure, it had low mechanical strength and almost always broke before a sufficient stretching effect was achieved. However, the present inventors focused on the fine structure of the surface dense layer in asymmetric separation membranes, and found that the structure of the surface dense layer is not always constant, and even when the same resin is used, it depends on the composition of the swelling agent, solvent, etc. It is known that the thickness, density, etc. of the stretching layer change, and we have discovered that in certain specific regions, the properties of the separation membrane can be significantly changed even when the stretching ratio is low. Furthermore, as a result of repeated studies on various resins by changing the solution composition of swelling agents, solvents, etc., and the composition of non-solvents that coagulate the resin, we found that resins,
In the solution composition of a ternary system of a swelling agent and a solvent,
A system in which a swelling agent is added to the solution until just before phase separation, specifically, when the swelling agent is added to the solution and the amount of swelling agent added when the solution starts phase separation is 100%,
Separation membranes made from solutions made by adding a swelling agent at a ratio of 60 to 98%, preferably 70 to 95% of the maximum amount added, exhibit properties different from other systems and have a relatively low magnification at room temperature. It has been found that water permeability and substance permeability can also be greatly improved by stretching. In the process of forming a separation membrane using the wet method, the moment the solution comes into contact with the non-solvent of the molecules that make up the resin, the resin solidifies at the interface, forming a dense layer on the surface, and then the solvent and non-solvent are exchanged. progresses, and a sparse structure such as an internal sponge layer is formed. In a solution system immediately before phase separation, the resin is in a state where it is easy to precipitate, so as you can easily imagine, when it comes into contact with a non-solvent, the solidification speed increases and at the same time the thickness of the dense layer on the surface increases. It is understood that, therefore, it can be experimentally confirmed that the increase in water permeability is small or even decreases. On the other hand, the fine structure of the dense layer is fragile to the extent that the cohesive energy of the molecules changes and the structure of the dense layer is affected by stretching. It is thought that the properties will be improved. To explain the present invention in more detail, the resins used in the present invention include cellulose acetate, cellulose triacetate, aromatic polyamide, polyether sulfone,
Examples include, but are not limited to, polysulfone, polyarylate resin, polyacrylonitrile, polymethyl methacrylate, and the like. The solvent must dissolve the resin and be compatible with the non-solvent of the resin, examples of which include methanol, acetone, methyl ethyl ketone, dimethyl formamide, dimethyl sulfoxide, dioxane, and N-methylpyrrolidone. Not limited to these. In addition, the swelling agent must be soluble in the solvent and non-solvent of the resin and reduce the solubility of the resin, and inorganic salts such as sodium chloride, lithium chloride, potassium chloride, sodium carbonate, and magnesium carbonate, Alternatively, organic acids such as tartaric acid, and organic solvents such as methanol, polyhydric alcohols, formamide, and water are suitable, but are not limited thereto. Furthermore, the solubility of the resin can be adjusted by combining two or more of the above-mentioned solvents and swelling agents. Next, to explain the film forming method, it is possible to cast a resin solution prepared with the above composition into a flat plate, and in this case, it is also possible to cast it on a base material such as a nonwoven fabric. It is extruded into a tubular or hollow fiber form through a nozzle, and then immersed in a non-solvent of the resin, such as water, an organic solvent, a mixed solution thereof, or a solution to which salt is added, to solidify it. Next is the stretching step, which can be carried out in an atmosphere at a stretching temperature of -10°C to below the melting point of the resin. Below -10°C, the film becomes brittle, and above the melting point of the resin, the film softens, making stretching difficult. Therefore, at a temperature from -10°C to below the melting point of the resin, preferably from 10°C to 50°C, in air or a heating medium such as glycerin or polyethylene glycol, a flat plate can be biaxially stretched, or a tubular shape can be formed. In this case, it is preferable to carry out uniaxial stretching. The stretching ratio varies depending on the type of resin, but it is stretched at a ratio of 10 to 50%. If the stretching ratio is less than 10%, the effect on water permeability will be small, and if it is more than 50%, the membrane will tend to break during stretching, reducing productivity. Here, the stretching ratio is defined as follows. In uniaxial stretching, stretching ratio (%) = (length after stretching / length before stretching - 1
)×100 In biaxial stretching, it is the average value of the stretching ratio of two sides. The hypercharacteristics of the obtained membrane are defined as follows. Water permeability (/hour・Kg/cm ²・m ² )=water permeability ()/
Rejection time (hours), overpressure (Kg/cm ² ), membrane area (m ² ) Rejection rate (%) = (1 - solute concentration in permeate (%) / solute concentration in permeate) (%)) × 100 For the measurement of rejection rate, dextran (T-500 and T-2000, manufactured by Falnucia Fáiso Chemical Co., Ltd., molecular weight approximately 500,000 and approximately 2 million, respectively) was used.
was used. Comparative examples 1, 2 Polyether sulfone (200P, manufactured by ICI, UK)
A solution consisting of 14 parts of N-methyl-2-pyrrolidone, 78 parts of N-methyl-2-pyrrolidone, and 2 parts of ammonium nitrate (Comparative Example 1) or 4 parts (Comparative Example 2) was heated to a glass plate with a smooth surface at room temperature (22°C). 20℃ after casting on
A flat film with a thickness of 300 μm was prepared by immersion in a water bath. If the amount of ammonium nitrate exceeds 8 parts, the solution will become cloudy and precipitate will form.
It is. The obtained film was stretched to 30% and 50% in a room temperature atmosphere at 30°C using a biaxial stretching machine filled with 60°C water.
% and fixed in hot water at 60°C. Table 1 shows the measurement results of water permeability and inhibition rate against dextran T-500. Thus, it can be seen that when the amount of ammonium nitrate is small, the water permeability and water blocking rate are not significantly improved.

[Table] Examples 1 to 3 5.5 parts, 6.6 parts, and 7.5 parts of ammonium nitrate were added to 14 parts of polyether sulfone (200P) and 78 parts of N-methyl-2-pyrrolidone to produce Examples 1, 2, and 3, respectively. The composition of the stock solution was 3. Thereafter, a flat plate membrane was created in the same process as in the comparative example, and stretched by 30% and 50% in a room temperature atmosphere of 30°C. Here, the amount of ammonium nitrate added is 5.5 parts, 6.6 parts, and
7.5 parts is when the precipitate formation concentration of 8 parts is taken as 100%.
The ratios are 68.8%, 81.3% and 93.8%, respectively. Water permeability and dextran T-500 and T-2000
Table 2 shows the measurement results of the inhibition rate against the above.

[Table] The measurement results of Comparative Examples and Examples are summarized in Fig. 1 (water permeability) and Fig. 2 (rejection rate against dextran T-500). As is clear from the above comparative examples and examples,
The limit addition amount of ammonium nitrate, a precipitant, is 60%.
It is understood that in flat membranes with compositions in which more than % of the above amount is added, the stretching effect appears markedly, and it is an excellent means for modifying membranes.

[Brief explanation of drawings]

Figures 1 and 2 are graphs showing the effects of the present invention, with Figure 1 showing changes in water permeability and Figure 2 showing changes in solute (dextran T-500) rejection.

Claims (1)

[Claims] 1. The resin is coagulated by immersing in a non-solvent of the resin a solution consisting of a resin and a solvent for the resin, and a stock solution consisting of a swelling agent in an amount of 60 to 98% of the amount added to cause phase separation of the solution. At the same time, the solvent and swelling agent are removed to form a semipermeable structure, and then further stretched at a ratio of 10 to 50% at an ambient temperature in the range of 10 to 50 degrees Celsius, resulting in high water permeability. A method for producing an ultrafiltration membrane having properties.