JPS6039402B2 - High performance semipermeable membrane and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-performance acrylonitrile semipermeable membrane that has high mechanical strength, is easy to handle, has no defects such as pinhole cracks, and has excellent water permeability, and a method for producing the same. be.

In recent years, semipermeable membranes have been used in wastewater treatment, water production, pharmaceutical industry, food industry,
Applied research has progressed in the separation of bacteria, yeast, viruses, etc., and it has come to be widely used.

As they are widely used industrially and become larger, improvements in separation precision, throughput, and durability are desired. Conventional semipermeable membranes vary widely in nature, and uniformity of quality is often difficult to guarantee, especially as the membrane area used increases. This is mostly due to the weak mechanical strength of the semipermeable membrane, which is caused by the transportation of the semipermeable membrane, installation into the equipment, and physical and chemical cleaning to restore water permeability during operation. This is thought to be due to cracks and tears occurring in the shadows due to the abnormal force applied. Yet another reason is that in conventional film forming methods, local defects and invisible pinholes exist in the film during film forming. If defects occur in the membrane due to these causes, the membrane cannot be used in fields that require high-precision separation, such as blood separation, removal of viruses, bacteria, etc., and ultrapure water production. Also, in the general water treatment field, if the performance of semipermeable membranes is unreliable, accurate equipment design is not possible. As a result of intensive studies aimed at eliminating these drawbacks, the present inventors have arrived at the present invention, which makes it possible to industrially obtain a highly reliable membrane with high mechanical strength, no defects, and excellent water permeability. Regarding the method of obtaining a semipermeable membrane with excellent semipermeability from a copolymer mainly composed of acrylonitrile, please refer to Hiiragi Publications 48-
No. 86163, Special Publication No. 49-278, Special Publication No. 49-
It has already been disclosed in No. 43878, Japanese Patent Publication No. 50-178576, etc.

According to these methods, a polymer is dissolved in a solvent, the polymer solution is cast onto a flat surface such as a glass plate or a metal plate, and
A semipermeable membrane is obtained by coagulation and gelation in a non-solvent. The semipermeable membrane obtained from these methods has a thickness of 100
~200r, and since the entire membrane is a gel-like semipermeable membrane, its mechanical strength is low and it is easily torn. On the other hand, Japanese Patent Publication No. 15398/1983 discloses a membrane manufacturing method based on cellulose acetate in which a woven fabric is used as a membrane support to impart strength to the semipermeable membrane. however,
When this method was applied to the production of polyacrylonitrile semipermeable membranes with high water permeability, the following drawbacks were encountered. 1
Pinholes are generated in the membrane. These pinholes are small and cannot be seen with the naked eye. For example, pinholes are small that cannot be seen with the naked eye.
In the case of the tube membrane on the 4th side, seal one end of the tube membrane and send pressurized air of 0.2k9/c into the tube membrane from the other end to pressurize the inside of the tube membrane and immerse it in water.
This can be easily confirmed by the appearance of small bubbles from the defective location.

2. When the support is a nonwoven fabric, single fibers of the nonwoven fabric fall off after long-term use, reducing the strength of the membrane and making it more likely to tear.

3. When the support is made of woven fabric, it is not possible to obtain a thin woven fabric, resulting in an increase in the overall thickness of the fabric, resulting in poor handling. 1) above
In order to solve the drawbacks of the film, known film forming methods involve directly casting a film-forming polymer solution onto a non-woven fabric or woven fabric. cause defects to form.

It is also believed that the gas contained between the fibers of these supports remains as bubbles in the casting solution, causing film defects. The defect 2) is considered to be due to poor adhesion between the membrane and the support.

Regarding 3), it may be solved by using a thin non-woven fabric with a smooth surface.

As a result of studies from the above viewpoints, the present inventors solved these drawbacks by using a nonwoven fabric as a membrane support and performing two-stage membrane formation. That is, in the first step, a solution having a relatively low polymer concentration of 2 to 15 wt %, preferably 4 to 1 t % is used to form a belly inside the nonwoven fabric. In this case, a film can be easily formed inside the nonwoven fabric by impregnating the nonwoven fabric in the solution, taking it out from the impregnation tube, and immersing it in a non-solvent to gel it. At the same time, a thin film is formed on the front and back surfaces of the nonwoven fabric. This membrane has many pinholes and has a very high water permeation rate, so it can be used as a rough filtration membrane, but it cannot be used as a precise filtration membrane or an ultrafiltration membrane. The gelled membrane is treated in 70-100 oo warm water and then dried to complete the first stage of membrane formation. This film is hereinafter referred to as a precursor film. In the second stage of film formation, the surface of this precursor film is coated with a relatively high concentration of the same polymer used in the production of the precursor film.
This is achieved by casting a solution of ~25 M%, preferably 8-23 M%, to a uniform thickness using an appropriate nozzle or die, and immersing it in a non-solvent to gel it.
The performance required of a high-performance semipermeable membrane is not only the mechanical strength and performance stability mentioned above, but also a high water permeation rate.
It must also have excellent lacquer rejection.

These performances vary greatly depending on the polymer concentration during film formation and gelation conditions. Cross-sectional electron micrographs of semipermeable membranes obtained according to the present invention are shown in FIGS. 1 to 5. The photograph in FIG. 1 is an overall photograph of the cross section of the membrane, and FIG. 2 is an enlarged photograph of the cross section near the membrane surface. FIG. 3 is a further enlarged photograph of the vicinity of the membrane surface, and FIG. 4 is an enlarged photograph of microvoids inside the pus. FIG. 5 is an enlarged cross-sectional photograph of the vicinity of the back surface of the membrane. From these photographs, it can be seen that the film of the present invention has a four-layer structure. That is, from the front surface to the back surface of the membrane, there are a surface dense layer, a gradient microvoid layer, a macrovoid layer, and a reinforcing macrovoid layer. Figure 3 is an enlarged view of the surface dense layer, and the size of the pores (called voids) that exist here is 50
It is very small below 0A, and it is thought that solutes are blocked in this layer. On the other hand, since the solvent molecules are 4.0 cm in size, they pass between these poids. Therefore, in order to increase the water permeability rate of the membrane (when the solvent is water), it is necessary to make the thickness of this dense layer as thin as possible, and to make it a practical semipermeable membrane, it should be less than 2 mm. Should. Next, as seen in FIGS. 2 and 3, in contact with the surface dense layer, there is a gradient microvoid layer having voids whose diameter gradually increases as the film progresses into the interior of the membrane. In other words, the pore size near the surface dense layer is several hundred amps, but as it goes inside, the average pore size decreases to about 200 amps.
It becomes a void of 0A. 2000A as seen in Figure 4
The small voids at the front and rear are referred to as microvoids in this specification. There are no macrovoids, which will be explained later, in this gradient microvoid layer. The role of this layer is thought to be to support the thin dense layer on the surface. Since the size of the voids inside the gradient microvoid layer is small,
Relatively thinner materials are preferred, as they offer significant resistance to water penetration. In other words, it is desirable that it is 20 Buddhas or less. As seen in FIGS. 1 and 2, a macrovoid layer exists in contact with the gradient microvoid layer. As is clear from FIG. 2, the macrovoid layer is a layer containing many large voids with a diameter of 5 mm or more within the microvoid layer. In this specification, diameter 5. A void larger than a Buddha will be called a macro void.
The presence of this macrovoid layer plays a major role in increasing the water permeability rate in the belly. However, on the other hand it reduces the strength of the membrane. When used as an ultra-furnace filtration membrane, the operating pressure is a few k9/min at most, but macrovoids collapse after long-term use, resulting in a decrease in water permeation rate. A major feature of the horizontal membrane structure of the present invention is that a reinforcing macrovoid layer reinforced with single fibers is formed in contact with this macrovoid layer. FIG. 5 is an enlarged view of this layer, showing that the paper is embedded within the microvoids that make up the membrane and is firmly bonded to the membrane. Furthermore, macrovoids and single fibers are formed which help improve water permeability. In a membrane having such a structure, the reinforcing single fibers do not peel off from the membrane, making it possible to use it stably for a long time. This reinforcing macropoid layer occupies about 1/2 of the total macrovoid layer in the photograph of FIG. 1, and one end forms the back surface of the membrane. It is highly desirable for the reinforcing macrovoid layer to form the back surface of the membrane, considering that the membrane is generally used under pressure on a highly water-permeable porous support. If the reinforcing macrovoid layer is too thin, the reinforcing effect of the membrane cannot be exhibited, and if it is too thick, the membrane will lack flexibility, making handling inconvenient. It is desirable that the film has a total thickness of 14 to 34 mm. FIG. 6 is a cross-sectional photograph of a commercially available semipermeable membrane reinforced with a nonwoven fabric support for comparison. It can be seen that the film structure is significantly different from the film structure of the present invention shown in FIG. That is, in the known membrane shown in FIG. 6, there is no macrovoid layer, and although the nonwoven fabric single fibers and the membrane are firmly bonded at their boundaries, the bonding properties with the membrane are poor near the back surface of the membrane. When such a membrane is used for a long time as a water treatment membrane,
Single fibers separate from the membrane, significantly reducing the strength of the waist and making it more likely to tear. A semipermeable membrane having the structure shown in FIG. 1 can be obtained by the two-stage membrane forming method described above.

Furthermore, in order to obtain a semipermeable membrane with excellent water permeability, the polymer concentration of the polymer solution A used in the first stage membrane formation must be adjusted to the second stage.
It is desirable that the polymer concentration be lower than the polymer concentration of polymer solution B used during step film formation. Furthermore, in order for polymer solution B to be uniformly cast onto the precursor film and for the film obtained by forming the precursor film in two stages to be bonded with a continuous microvoid layer, the polymers in solutions A and B must be the same. It is preferable to choose. The acrylonitrile polymer for film formation preferably contains 80 mol% or more of acrylonitrile residues.

If the acrylonitrile component is less than 80 mol %, it becomes difficult to form a dense surface layer, or open-pore microvoids do not develop inside the membrane, resulting in a large number of closed-pore voids. The former significantly increases the water permeation rate of the membrane, but significantly impairs the semipermeability of the membrane. The latter significantly reduces the water permeation rate of the membrane. These phenomena vary depending on the type of copolymerized component with acrylonitrile and gelation conditions. However, if a copolymer containing 80 mol % or more of an acrylonitrile component is used, it is possible to obtain a membrane with excellent water permeability and good semipermeability, having a structure similar to that of the present invention. As the comonomer constituting the copolymer with acrylonitrile, known comonomers that can be copolymerized with acrylonitrile can be used. Moreover, one or more types of copolymerization component monomers can be used. As these monomers, acrylamide, N-pynyl-2-pyrrolidone, acrylic acid esters, vinyl acetate, styrene, etc. can be used as nonionic monomers. Ionic monomers include acrylic acid, methacrylic acid, methallylsulfonic acid, vinylbenzenesulfonic acid and salts thereof, tertiary amines such as 2-vinyl and 4-vinylpyridine, dimethylaminoethyl methacrylate, and more. There are salts of quaternary ammonium salts obtained by alkylating these. As a solvent for forming film forming solutions A and B using these polymers, organic solvents such as dimethylformamide, dimethylacetamide, dimethyl sulfoxide, and r-butyrolactone are preferable. Inorganic solvents such as nitric acid and concentrated aqueous sodium rhodanate solutions also exist, but it is important to prepare a uniform solution.
In order to uniformly introduce the solution between all of the reinforcing single fibers, including air bubbles, it is desirable to use an organic solvent with a low surface tension. As the gelling non-solvent, it is desirable to use a mixed aqueous solution of a solvent and water.

The gelled membrane can be washed with water and used as is, but depending on the purpose, it can also be used after heat treatment in warm water. Although the water permeation rate of the membrane is reduced by heat treatment, the solute rejection rate is improved and the membrane becomes a semipermeable membrane that can be handled in a dry state. As the nonwoven fabric used in the membrane of the present invention, known nonwoven fabrics can be used.

In addition, a paper-like sheet-like structure made of fibers that cannot be defined as a non-woven fabric, that is, a sheet-like fiber aggregate can be used. When the membrane is applied to the field of water treatment, nonwoven fabrics or sheet-like fiber aggregates made of synthetic fibers are preferred over paper or nonwoven fabrics made of cellulose fibers. The nonwoven fabric made of synthetic fibers is preferably one made of polyester, nylon, polypropylene, etc. from the viewpoint of chemical resistance. The form of the membrane of the present invention includes a flat membrane and a tube.

For tube-shaped tubes, there are two possible methods of forming a second-stage film on the inner and outer surfaces of the tube. As for the diameter of the tube, it is possible to stably form a film with three or more legs, and it becomes difficult to form a film with a diameter smaller than this. With the membrane formed on the inner surface of the tube in the second stage, the treated liquid can be introduced into the tube under pressure and treated water can be obtained from the outer wall of the tube. In such an internal pressure type tubular module, if the tube inner diameter of the tube membrane and the tube inner diameter of the tubular module are not equal, abnormal pressure on the membrane may cause the membrane to rupture. Since the membrane obtained by the present invention has high strength, it exhibits its effectiveness in such usage. The present invention will be explained in more detail below using Examples. Example 1 A semipermeable membrane with no defects, strength, and excellent water permeability was obtained using a polyester nonwoven fabric as a support and a copolymer containing acrylonitrile as a main component.

A polymer containing 95 mol% of acrylonitrile and 5 mol% of methyl acrylate was uniformly dissolved in dimethylacetamide (DMAC) to a concentration of 5M%. A polyester non-woven fabric with a basis weight of 40mm/cm and a thickness of 110A was impregnated with the solution, soaked in water at room temperature to gel, heat-treated in 75o0 hot water, and dried in a hot air dryer at 13,000mm. One-stage film formation was performed. Next, add 2 of the above polymers.
A DMAC solution containing t% of the skin was prepared and cast on one side of the precursor film obtained by the first stage production process, and immediately the DMA
The mixture was immersed in a non-solvent mixture of C and water to form a gel, and the gelled product was washed with water to obtain the desired semipermeable membrane. A pinhole test of the obtained semipermeable membrane was performed using the following method.
The semi-permeable membrane was cut into a rectangle of 8 skins x 30 arcs, and the membrane was fixed in a metal frame around the membrane, and the surface of the membrane (i.e. the side where the second membrane was formed) was cut.
was in contact with a closed chamber into which pressurized air was introduced, and the back side of the membrane was in contact with water. Pressurized air of 0.2 k9/h was sent into the sealed chamber and the ventral surface in contact with the water surface was observed, and no bubbles were observed due to the waist defect. Next, the strength of the membrane was measured using a tensile tester, and the breaking strength was 1.
It was very large at 23 (kg'). Furthermore, the water permeation rate of the membrane was determined using an ultrafiltration device and was found to be 0.190 (desktop/ground, mln, stm). Furthermore, when an aqueous solution containing 0.1 wt% egg albumin was treated as a stock solution, the solute rejection rate was 100%. Example 2 A precursor film was obtained in the same manner as in Example 1 using a polymer consisting of 96 mol % of acrylonitrile and 4 mol % of vinyl acetate.

Next, a DMAC solution containing 1 t% of the above polymer was prepared.
The mixture was uniformly cast on one side of the precursor membrane, immersed in a non-solvent mixture of DMAC and water, and gelled, thereby obtaining a tubular semipermeable membrane with an inner diameter of 24 ribs formed on the inner surface of the tube. The structure of the semipermeable membrane thus obtained had the structure shown in FIGS. 1 to 5. The tube membrane was cut into 3 lengths, pressurized air of 0.2K9' was sent from inside the tube, and the tube membrane was immersed in water for a pinhole test. 3 There was no spot in the length of the skin. Furthermore, as a result of measuring the strength of the membrane using a tensile tester, the breaking strength was extremely high at 1.02 (k9/fence). Furthermore, this tube membrane was attached to a tube-shaped module made of dawn resin and an outer cylinder, and pressurized water was pumped from inside, and the water permeation rate of water permeating through the membrane outer wall was determined to be 0.250 (secretion/ Akira, mln, sin). In addition, when an aqueous solution containing 0.1 M% egg albumin was used as a stock solution in the furnace, the inhibition rate of clan trade was 100%. Comparative Example 1 Using the polyester nonwoven fabric used in Example 1, the second stage film formation was directly performed without performing the first stage film formation of Example 2.

That is, a tubular semipermeable membrane was obtained in the same manner as in Example 2 by directly casting a solution containing 1 t% of the polymer onto a polyester nonwoven fabric. A pinhole test of this film was carried out in the same manner as in Example 2, and as a result, there were 13 bubble generation locations with a length of 3 mm. The water permeability rate and egg albumin rejection rate of this membrane were determined in the same manner and were found to be 0.285 (Y/J, mln, atm)
and the inhibition rate was 93%. The rejection rate was not 100% due to small film defects. Example 3 The tubular semipermeable membrane obtained in Example 2 was installed in a tubular module consisting of a sintered body and an outer cylinder, and an experiment was conducted for 6 months to obtain recycled water from a furnace filtering building wastewater.

For comparison, a commercially available semipermeable membrane having the structure shown in FIG. 6 was simultaneously tested. The operation results are shown in Table 1. Table 1 A commercially available tube membrane was removed from the module after 6 months of operation and examined, and it was found that many of the membrane supports had fallen off and the membrane was partially torn.

Example 4 The semipermeable membrane obtained in Example 1 was heat treated in hot water at 75oC.

The obtained membrane was dried in a hot air dryer at 60 °C to obtain a dry semipermeable membrane. The water permeation rate was determined in the same manner as in Example 1, and the result was 0.115 (Desk, mln, atm).

Further, when an aqueous solution containing 0.1 wt% of cytochrome C was passed through an ultra-furnace as a stock solution, the solute rejection rate was 100%. Note that there was no change in performance of this membrane between wet and dry conditions.

[Brief explanation of the drawing]

FIG. 1 is a scanning electron micrograph of a cross section of a semipermeable membrane obtained according to the present invention. FIG. 2 is an enlarged cross-sectional photograph of the vicinity of the membrane surface in FIG. FIG. 3 is an enlarged photograph of a cross section near the membrane surface taken with a transmission electron microscope. FIG. 4 is an enlarged photograph of the internal cross section of the membrane taken with a transmission electron microscope. FIG. 5 is an enlarged photograph of a cross section near the back surface of the pus.
FIG. 6 is a cross-sectional photograph of a commercially available semipermeable membrane with a support shown for reference. Figure' Figure 2 Figure 3 , Figure 4 Figure; Figure 6

Claims (1)

[Claims] 1 Having a dense layer with a thickness of 2μ or less on the membrane surface, followed by a gradient type microvoid layer with a thickness of 2μ or more and 20μ or less, with the pore size increasing as it goes inside the membrane, and following the gradient type microvoid layer. , an acrylonitrile-based semipermeable membrane 2, characterized in that it has a macrovoid layer, and continues to the macrovoid layer and has a reinforcing macrovoid layer extending to the surface of the membrane in which single fibers are embedded in the macrovoid layer.Macrovoid layer A semipermeable membrane 3 according to claim 1, characterized in that the diameter of the macrovoids in the membrane is 5μ or more.A claim characterized in that a copolymer containing 80 mol% or more of an acrylonitrile component is used. Semipermeable membrane 4 according to claim 1, characterized in that it has a tube-like shape with an inner diameter of 3 mm or more, has a dense layer on the inside of the tube, and has a reinforcing macrovoid layer on the outside. Semipermeable membrane 5 according to item 1
After impregnating or casting the solution A of the acrylonitrile polymer on a nonwoven fabric or sheet-like fiber aggregate, immersing it in a non-solvent to gel it, heat treating it in warm water, and drying it to perform the first stage film formation, Next, solution B containing the same polymer as solution A is cast onto the membrane, and the membrane is immersed in a non-solvent to gel. Method 6 for producing a semipermeable membrane Polymer concentration of solution A is lower than the polymer concentration of solution B. Method 7 for producing a semipermeable membrane according to claim 5. A patent characterized in that the solvents constituting solutions A and B are organic solvents. Method 8 for producing a semipermeable membrane according to claim 5 The single fibers constituting the nonwoven fabric or sheet-like fiber aggregate are made of polymer solution A,
The method for manufacturing a semipermeable membrane according to claim 5, characterized in that solution A of B and solution B that do not dissolve in the solvent are used.