JP2011194272A - Polysulfone film and composite membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composite reverse osmosis membrane which is made to have both of high water permeability and high solute removal properties without carrying out a modification step after the composite reverse osmosis membrane is made.SOLUTION: A polysulfone film is characterized in that a layer (A layer) having the depth of 1 μm from the surface has ≤200 nm average pore diameter, 40-75% average porosity and the diffused amount of meta-phenylenediamine of 1×10to 2×10mol/m.

Description

  The present invention relates to a polysulfone membrane useful for selective separation of a liquid mixture and a composite membrane comprising a polysulfone membrane.

  Known composite semipermeable membranes include composite reverse osmosis membranes and composite nanofiltration membranes in which a separation functional membrane having substantially selective separation properties is formed on a microporous support membrane. In the composite membrane, it is possible to select an optimal material for each of the separation functional membrane and the microporous support membrane, and various methods can be selected for the membrane production technique. Examples of the material for the microporous support membrane include polysulfone, polyphenylene sulfide sulfone, and polyphenylene sulfone. Among these materials, polysulfone that has high chemical, mechanical, and thermal stability and is easy to mold is common. Is used.

  When separation is performed using a composite semipermeable membrane, it is necessary to apply a pressure higher than the difference between the osmotic pressure of the supply liquid and the osmotic pressure of the permeate to the supply liquid side. When the pressure is high, a high pressure is required as the operation pressure. In general, seawater desalination is operated at a pressure of about 60 to 65 atm. In this case, since a strong pressure is applied to the membrane surface, the membrane is consolidated during operation, and voids in the microporous support membrane are crushed. Or the separation functional membrane is further densified to change the membrane form and membrane performance.

  Under such circumstances, as a polysulfone membrane used for the microporous support membrane of the composite semipermeable membrane, a pressure-resistant polysulfone support membrane is proposed in which the change in water permeation performance and desalting performance is small even during high-pressure operation (Patent Literature). 1). This pressure-resistant polysulfone support membrane can be obtained by controlling the porosity and pore diameter in the membrane, but the water permeability and desalination performance of the composite semipermeable membrane made from the support membrane is the same as before. No pressure-resistant polysulfone membrane that improves the membrane performance more than before has been known so far.

  There are two types of composite semipermeable membranes: those having an active layer obtained by crosslinking a gel layer and a polymer on a microporous support membrane, and those having an active layer obtained by polycondensing monomers on a microporous support membrane. is there. Among these, composite semipermeable membranes obtained by coating a separation function membrane made of a crosslinked polyamide obtained by polycondensation reaction of a polyfunctional amine and a polyfunctional acid halide on a microporous support membrane are permeable and selective. It is widely used as a separation membrane with high separability. The composite semipermeable membrane has high desalting performance and water permeation performance, but further improving water permeability while maintaining high desalting performance can reduce operating costs, equipment costs, and efficiency. It is desired from.

  In response to these requirements, various additives (Patent Document 2) have been proposed and improved in performance, but are still insufficient. Further, (i) a method of contact treatment with an aqueous solution containing chlorine (Patent Document 3) and (ii) a method of contact treatment with an aqueous solution containing nitrous acid for a composite semipermeable membrane provided with a crosslinked polyamide polymer as a separation functional membrane ( Patent document 4) is known. Although the water permeation performance can be improved by using the treatment method as described in (i) and (ii) above, it is still insufficient from the viewpoint of economic aspect and global environment, and further while maintaining high desalination performance. Improvement of water permeability is desired. Furthermore, these methods have problems such as an increase in the amount of chemicals required for film formation and an increase in economic burden and burden on waste liquid treatment.

JP-A-8-168658 JP 63-12310 A JP 2005-246207 A JP 2005-186059 A

  An object of the present invention is to provide a composite semipermeable membrane that maintains a high salt rejection and reduces water load while reducing economic burden and waste liquid treatment.

The present invention for achieving the above object is characterized by the following configurations (1) to (2).
(1) The average pore diameter of the layer (A layer) having a depth of 1 μm from the surface is 200 nm or less, the average porosity is 40 to 75%, and the metaphenylenediamine diffusion amount is 1 × 10 ⁻³ mol / m ² or more and 2 × 10 ⁻³ mol / m ² or less.
(2) A composite membrane, wherein a separation functional membrane is provided on the polysulfone membrane according to (1).

  According to the present invention, a composite semipermeable membrane having both high water permeability and high solute removability can be obtained without using a modification treatment step after film formation. In addition, since it is not necessary to add a new chemical agent, it is possible to reduce the economic burden and the waste liquid treatment load, and to achieve this by a simpler and safer method.

It is a fragmentary sectional view showing an example of a composite reverse osmosis membrane. It is the schematic which shows the metaphenylenediamine diffusion amount measuring method.

  The average porosity of the polysulfone membrane referred to in the present invention is a value measured as follows. In the composite membrane shown in FIG. 1, for example, the pore diameter is measured by a mercury intrusion method with respect to a polysulfone membrane mechanically separated from the substrate using tweezers. In the mercury intrusion method, a pressure p is applied to mercury so that mercury is intruded into the communication hole of the polysulfone membrane, and the volume change dV of mercury in the cell with respect to the pressure increment dp is measured. The pore distribution function F (r) is obtained.

(Where r is the pore radius, σ is the surface tension of mercury (0.480 N / m), and θ is the contact angle (140 °).) The average pore diameter can be determined by the following equation.

On the other hand, the average porosity is obtained from the following equation by determining the bulk density Da before mercury is press-fitted into the communication hole for the polysulfone membrane by the mercury intrusion method.
Da = (m0−m1) / ρ (3)
(Where m0 is the weight of mercury when the empty measurement cell is filled with mercury, m1 is the weight of mercury when the sample is placed in the cell and mercury is introduced into the cell, and ρ is the mercury density. To express.)
Further, from the volume change V1 before and after the communication hole is completely replaced with mercury by injecting mercury, the true density Dt is obtained by the following equation:
Dt = W / (W / Da-V1) (4)
(W represents the sample weight.)
The average porosity is: average porosity = (Dt−Da) × 100 / Dt (5)
Is the calculated value.

  In the polysulfone membrane of the present invention characterized by the above-mentioned parameter characteristics, when the average pore diameter is assumed to be approximately the same, the smaller the average porosity, the larger the occupation ratio of the skeleton portion of the membrane and the greater the strength of the membrane itself Therefore, when a composite membrane having a polysulfone membrane as a supporting membrane is used for seawater desalination, consolidation of the supporting membrane hardly occurs even when the operating pressure increases. However, on the other hand, the permeation resistance of the permeated water is increased, causing a decrease in water permeation performance. For this reason, the lower limit value of the average porosity is set to 40%. On the other hand, as the average porosity increases, the permeation resistance with respect to the permeate decreases and the water permeation performance is improved. However, since the occupying ratio of the skeleton portion of the membrane is small and the strength thereof is low, consolidation tends to occur when the operating pressure is high. For this reason, the upper limit value of the average porosity is set to 75%.

  In the present invention, the term “pore” refers to a void (void) present in the inside and a gap between the polymer particulate matter or columnar matter, and the pore diameter and average pore diameter can be determined by the method described below. . First, it cut | disconnects by the freezing cleaving method and it is set as the cross-sectional observation sample for scanning electron microscopes (SEM), and the cross-sectional photograph of the obtained sample is image | photographed using SEM. The observation magnification is preferably about 1,000 to 50,000 times. In particular, when the pore diameter and the layer thickness are measured, 5,000 to 20,000 times is preferable. Finally, the photographed cross-sectional photograph is read into image analysis software and analyzed to obtain the pore diameter and average pore diameter. The average pore diameter of the A layer of the polysulfone membrane of the present invention obtained by the above method is 200 nm or less, more preferably 150 nm or less.

  The polysulfone membrane of the present invention is preferably an asymmetric membrane. An asymmetric structure is a structure that has fine pores on one side and the pores gradually increase from the other side, and the pore diameter continuously changes from one side to the other. preferable. In the case of an asymmetric polysulfone membrane, the A layer represents a layer on the side having dense pores.

  In the present invention, the average particle diameter of the polysulfone membrane particulate or columnar is 5 to 80 nm, preferably 10 to 70 nm, and more preferably 15 to 50 nm. If the average diameter is too small, the permeability of the separation membrane decreases, which is not preferable. On the other hand, if the average diameter is too large, the film surface tends to be uneven, which is not preferable.

  The polysulfone membrane in the present invention is usually reinforced with a fabric (base material) mainly composed of at least one selected from polyester or aromatic polyamide, and used as a polysulfone support membrane.

  The thickness of the polysulfone layer in the present invention is 10 μm to 5 mm, preferably 10 μm or more from the viewpoint of membrane strength, and 400 μm or less from the viewpoint of ease of handling and module processing.

  In the polysulfone membrane according to the present invention, by setting each parameter in the above range, the composite reverse osmosis membrane using the membrane as a support membrane is less likely to cause consolidation even under high pressure operation such as seawater desalination, and It functions as a support membrane that achieves high water permeability and high solute removal.

The “metaphenylenediamine diffusion amount” referred to in the present invention means a value measured as follows. That is, first, the polysulfone membrane was immersed in a 5 wt% aqueous solution of metaphenylenediamine for 10 seconds to remove excess aqueous solution from the surface, and then the polysulfone membrane was set in the apparatus shown in FIG. Fig. 2-8). Next, a unit that diffuses from layer A to isooctane in the initial 10 seconds when isooctane is brought into contact with the surface of layer A of the polysulfone membrane at a rate of 25 L / m ² and isooctane is stirred with a stirring blade (rotation speed: 100 rpm). The amount of metaphenylenediamine per area is expressed as the diffusion amount (mol / m ² ). In addition, the isooctane temperature at the time of performing a diffusion experiment shall be 25 degreeC, and the measurement of the amount of metaphenylenediamine in isooctane is calculated | required by an ultraviolet absorption visible spectrum, chromatography, mass spectrometry, etc.

By improving the diffusion amount, it is possible to improve the water permeability of the composite semipermeable membrane in which the separation functional membrane is provided on the polysulfone membrane. Normally, there is a trade-off between water permeability and removal rate. If water permeability improves, there is a concern that the removal rate will decrease. However, by increasing the diffusion amount, the metabolites involved in the polycondensation reaction during the formation of the separation functional membrane are concerned. The amount of phenylenediamine is also improved, and the density of the film, which is one of the important factors that determine the removal rate, can be maintained or improved. As a result, the removal rate is not reduced. However, even if the amount of diffusion is too large, the formation of the separation functional film is not successful, resulting in a film with a reduced removal rate or inferior pressure resistance. From these things, the metaphenylenediamine diffusion amount of the polysulfone membrane in the present invention is set to 1 × 10 ⁻³ mol / m ² or more and 2 × 10 ⁻³ mol / m ² or less.

  The polysulfone membrane in the present invention can also be used as a support membrane for producing a composite membrane having a separation functional membrane thereon. Such support membranes are described in “Office of Saleen Water Research and Development Progress Report” No. 359 (1968). That is, a predetermined amount of polysulfone is dissolved in dimethylformamide (hereinafter referred to as DMF) to prepare a polysulfone resin solution having a predetermined concentration. Next, this polysulfone resin solution is applied to a substrate made of polyester cloth or nonwoven fabric to a substantially constant thickness, and after removing the surface solvent in the air for a certain period of time, the polysulfone is coagulated in the coagulation liquid. Can be obtained. At this time, DMF as a solvent is rapidly volatilized on the surface portion that comes into contact with the coagulation liquid, and polysulfone coagulates rapidly, and fine communication holes having the portion where DMF is present as a core are generated.

  Further, in the interior from the surface portion toward the base material, the volatilization of DMF and the solidification of polysulfone proceed more slowly than the surface, so that the DMF tends to aggregate and form large nuclei, and thus generate. The communication hole becomes larger in diameter. Of course, the above nucleation conditions gradually change depending on the distance from the film surface, so that a support film having a smooth pore size distribution without a clear boundary is formed. The present invention relates to the polysulfone resin solution used in this forming step, the temperature of the polysulfone resin solution, the concentration of polysulfone, the relative humidity of the atmosphere in which it is applied, the time from application to immersion in the coagulation liquid, the temperature and composition of the coagulation liquid, etc. By adjusting the ratio, it is possible to obtain a polysulfone membrane in which the average porosity and the average pore diameter are controlled.

  Specifically, first, in the application of the polysulfone resin solution, it is important to control the temperature of the polysulfone resin solution, the concentration of the polymer material, the relative humidity of the atmosphere in which the application is performed, and the time until dipping in the coagulation liquid. The temperature is good to apply | coat the polysulfone resin solution which exists in the range of 20-30 degreeC. When this temperature is lower than 20 ° C., solidification starts before the phase separation between the resin and the solvent is sufficiently advanced, so that the pore diameter of the generated communication holes tends to be small. Further, when the temperature exceeds 30 ° C., the phase separation proceeds and the solvent phase that becomes communication holes grows large, and the communication holes tend to increase, and it is difficult to obtain a support membrane in which the change in average pore diameter is within a predetermined range. Become.

  Regarding the resin concentration, it is preferable to use a polysulfone resin solution in the range of 12 to 20% by weight. If the amount is less than 12% by weight, the hole diameter of the communication hole tends to increase, and if the amount exceeds 20% by weight, the hole diameter of the communication hole tends to decrease. In any case, the change in the average hole diameter is within a predetermined range. It is difficult to obtain a support membrane in

  Furthermore, it is good to control the relative humidity of the atmosphere at the time of apply | coating said polysulfone resin solution in the range of 40 to 70%. This is because when the relative humidity exceeds 70%, solidification of the polysulfone resin solution on the surface side in contact with the atmosphere proceeds rapidly and the average pore size tends to decrease, and when the relative humidity is less than 40%, the polysulfone resin solution on the surface side The progress of solidification is delayed, and the progress of solidification in the film thickness direction is likely to be uniform. In any case, it is difficult to obtain a support film having an average pore size distribution within a predetermined range.

  It is preferable to control the time until the polysulfone resin solution is immersed in the coagulation liquid after being applied to the base material to be 1 to 10 seconds. Since this also affects the average porosity and average pore diameter of the support membrane, if this time is less than 1 second, solidification starts before the phase separation between the polysulfone and the organic solvent proceeds sufficiently, so the average pore diameter is small. In addition, the phase separation proceeds after 10 seconds, and the solvent phase that becomes the communication hole later grows large and the average pore diameter becomes too large. It becomes difficult to obtain the support film inside.

  Further, as the coagulation liquid, pure water or a mixed solution composed of pure water and an organic solvent can be used by using the coagulation liquid. Examples of organic solvents include tetrahydrofuran, 1,4-dioxane, acetonitrile, N, N-dimethylacetamide, N-methyl-2-pyrrolidone, 2-pyrrolidone, DMF, formamide, dimethyl sulfoxide, and the like. A good solvent that dissolves 10% by weight or more and dissolves polysulfone by 10% by weight or more is preferable. As content of an organic solvent, 15 to 45 weight% is preferable, More preferably, it is 20 to 35 weight%.

  As the polysulfone, it is preferable to use a polysulfone composed of a repeating unit represented by the following chemical formula, which has high chemical, mechanical and thermal stability, can easily control the pore size, and has high dimensional stability.

  After forming the polysulfone support membrane as described above, a composite membrane is produced by coating the separation functional membrane. In the present invention, the separation functional membrane is a very thin layer having a separation function substantially in the separation membrane, and may be called a separation functional layer, an active layer, an ultra thin film, or an ultra thin film layer. As a material for the separation functional membrane, a crosslinked or linear organic polymer can be used. In order for the separation membrane to exhibit high separation performance, the polymer is preferably a polyamide, polyurethane, polyether, polyester, cellulose ester, polyimide, polyamic acid, or vinyl polymer, more preferably a polyamide, particularly an aromatic polyamide. Further, in order to further increase the pressure resistance of the entire separation membrane and further maintain a high rejection rate even at a pressure of 70 atm or higher, these polymers are preferably crosslinked polymers. In particular, crosslinked aromatic polyamides and copolymers thereof are preferred.

  “Separation membrane” refers to a membrane having a separation function such as an ultrafiltration membrane or a reverse osmosis membrane, and indicates a membrane having a microporous layer or a composite membrane using the membrane as a microporous support membrane. Shall. In particular, the separation membrane used in the reverse osmosis method for seawater desalination is preferably a composite membrane in which the surface of the microporous support membrane is coated with a separation functional membrane that substantially controls separation performance with different materials. At this time, the separation functional membrane is preferably formed on the dense layer surface, and the average pore diameter of the dense layer surface is preferably 100 nm or less.

  The thickness of the separation functional membrane in the composite membrane of the present invention is 1 to 1,000 nm, preferably 5 to 800 nm, more preferably 10 to 500 nm. If the thickness of the separation functional membrane is too small, the occurrence of defects during film formation increases or the film is easily damaged during handling, and defects are also generated when pressure is applied, leading to a reduction in the rejection rate. On the other hand, if the thickness of the separation functional membrane is too large, the permeation rate coefficient is extremely lowered and a sufficient amount of permeation cannot be obtained.

In the present invention, the separation functional membrane is coated by a method of coating a polymer, a method of further crosslinking the coated polymer, a method of polymerizing monomers on the membrane surface of the microporous support membrane, or an interface on the membrane surface of the microporous support membrane. It can be carried out by a polycondensation method. In particular, the interfacial reaction method is preferable because a thin and uniform separation functional membrane can be obtained.

  The form of the separation membrane may be a flat membrane or a hollow fiber. In the case of a flat membrane, the separation membrane may be lined with cloth, nonwoven fabric, paper or the like. In addition, when the obtained separation membrane is a flat membrane, it can be incorporated into a spiral, tubular, plate and frame module, and in the case of a hollow fiber, it can be bundled and incorporated into the module. The present invention does not depend on the usage pattern of these membranes.

  The present invention will be described in more detail with reference to the following examples. However, the present invention is not limited to these examples.

  In Examples and Comparative Examples, the amount of metaphenylenediamine diffused, the average porosity, and the average pore size of the A layer were measured as follows.

(Metaphenylenediamine diffusion amount)
A support membrane composed of a base material and a polysulfone layer is immersed in a 5% by weight aqueous solution of metaphenylenediamine for 10 seconds to remove excess aqueous solution from the surface, and then the membrane shown in FIG. 2 is placed with the polysulfone layer side up. I set it. Next, isooctane was brought into contact with the surface of the polysulfone membrane at a rate of 25 L / m ² , and isooctane was stirred for 10 seconds with a stirring blade (rotation speed: 100 rpm). The amount of metaphenylenediamine diffusion was calculated from the absorbance at 294.8 nm. The diffusion experiment was conducted at 25 ° C.

(Average porosity)
Three sample pieces of about 12 mm × 20 mm square were cut out from the polysulfone membrane peeled from the substrate, and after precise weighing, put into a measuring cell so as not to overlap, and mercury was injected under reduced pressure. Next, the pore size distribution of this sample was measured with a pore sizer 9320 manufactured by Micromeritec. The number of measurements was one.

(Average pore diameter of layer A)
A polysulfone membrane is cut by a freeze cleaving method to prepare a cross-sectional observation sample, and this sample is thinly coated with platinum and then 3 to 6 kV using a high resolution field emission scanning electron microscope (Hitachi S-900 electron microscope). A cross-sectional photograph was taken at an acceleration voltage of. A cross-sectional photograph taken by SEM was taken into image analysis software Image Pro and analyzed, and the average pore diameter of layer A was determined.

Example, various characteristics of the composite semipermeable membrane in the comparative example, the composite semipermeable membrane, the temperature 25 ° C., seawater was adjusted to pH6.5 with (TDS (T otal D issolved S olids) concentration of about 3.5%) Membrane filtration was carried out for 24 hours while supplying at an operating pressure of 5.5 MPa, and the water quality was determined by measuring the quality of the permeated water and the supplied water.

(TDS removal rate (desalting rate))
TDS removal rate (%) = 100 × {1− (TDS concentration in permeated water / TDS concentration in feed water)}
(Membrane permeation flux)
Membrane permeation flux (m ³ / m ² / day) was expressed in terms of the permeation amount of the feed water (seawater) per square meter of the membrane surface with the permeation amount per day (cubic meter).
Example 1
At room temperature (25 ° C.), a 15.7 wt% DMF solution of polysulfone was cast to a thickness of 200 μm on a polyester nonwoven fabric (air permeability 0.5 to 1 cc / cm ² / sec), and immediately 20 wt% DMF was added. A polysulfone supporting membrane (thickness: 210 to 215 μm) was prepared by immersing it in the aqueous solution containing it and allowing it to stand for 5 minutes. Next, the support membrane was immersed in a 5 wt% aqueous solution of m-PDA for 2 minutes, slowly pulled up in the vertical direction, nitrogen was blown from an air nozzle to remove excess aqueous solution from the surface of the support membrane, and then trimesic acid chloride. An n-decane solution at 25 ° C. containing 0.175% by weight was applied so that the surface was completely wetted and allowed to stand for 1 minute. Next, in order to remove excess solution from the membrane, the membrane was held vertically for 1 minute and drained. Then, it wash | cleaned for 2 minutes with 90 degreeC hot water, and obtained the composite semipermeable membrane. The amount of metaphenylenediamine diffused in the polysulfone support membrane thus obtained, the average porosity, the average pore diameter of the layer A, the membrane permeation flux of the composite semipermeable membrane, and the TDS removal rate are the values shown in Table 1, respectively. Met.
(Example 2)
A polysulfone support membrane was prepared in the same manner as in Example 1 except that the DMF concentration in the coagulation bath was changed to 26%, and a composite semipermeable membrane was prepared from the support membrane in the same manner as in Example 1. The amount of metaphenylenediamine diffused in the polysulfone support membrane thus obtained, the average porosity, the average pore diameter of the layer A, the membrane permeation flux of the composite semipermeable membrane, and the TDS removal rate are the values shown in Table 1, respectively. Met.
(Example 3)
A polysulfone support membrane was prepared in the same manner as in Example 1 except that the DMF concentration in the coagulation bath was changed to 35%, and a composite semipermeable membrane was prepared from the support membrane in the same manner as in Example 1. The amount of metaphenylenediamine diffused in the polysulfone support membrane thus obtained, the average porosity, the average pore diameter of the layer A, the membrane permeation flux of the composite semipermeable membrane, and the TDS removal rate are the values shown in Table 1, respectively. Met.
Example 4
A polysulfone support membrane was prepared in the same manner as in Example 2 except that the polysulfone concentration was changed to 17%, and a composite semipermeable membrane was prepared from the support membrane in the same manner as in Example 1. The amount of metaphenylenediamine diffused in the polysulfone support membrane thus obtained, the average porosity, the average pore diameter of the layer A, the membrane permeation flux of the composite semipermeable membrane, and the TDS removal rate are the values shown in Table 1, respectively. Met.
(Comparative Example 1)
A polysulfone support membrane was prepared in the same manner as in Example 1 except that the coagulation bath was changed to pure water, and a composite semipermeable membrane was prepared from the support membrane in the same manner as in Example 1. The amount of metaphenylenediamine diffused in the polysulfone support membrane thus obtained, the average porosity, the average pore diameter of the layer A, the membrane permeation flux of the composite semipermeable membrane, and the TDS removal rate are the values shown in Table 1, respectively. Met.

As is apparent from the results in Table 1, by using a polysulfone membrane having a metaphenylenediamine diffusion amount to isooctane of 1 × 10 ⁻³ mol / m ² or more and 2 × 10 ⁻³ mol / m ² or less as a support membrane. Thus, it is possible to obtain a composite semipermeable membrane having a higher permeation flux than the conventional one while maintaining high salt blocking performance.

  The polysulfone membrane of the present invention and the composite membrane comprising a polysulfone membrane can be suitably used for desalination of seawater, desalination of brine, wastewater treatment, concentration of valuable materials, and recovery. In particular, when producing a composite semipermeable membrane having both a high salt rejection and a high permeation flux, it is effective to use the polysulfone membrane of the present invention as a support membrane for the separation functional membrane.

1 Separation function membrane 2 Microporous support membrane (polysulfone membrane)
3 Micropore 4 A layer
5 Voids of 200 nm or more 6 Stirring bar 7 Isooctane 8 Metaphenylenediamine impregnated polysulfone membrane 9 Stirrer

Claims (2)

The layer (A layer) having a depth of 1 μm from the surface has an average pore diameter of 200 nm or less, an average porosity of 40 to 75%, and a metaphenylenediamine diffusion amount of 1 × 10 ⁻³ mol / m ² or more. A polysulfone membrane, which is 2 × 10 ⁻³ mol / m ² or less.   A composite membrane, wherein a separation functional membrane is provided on the polysulfone membrane according to claim 1.

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