Manufacturing method for positive permeable membrane

Abstract

To retain strength even when a supporting body itself is thinned and to achieve low salt transmittance while securing enough water permeability by thinning.SOLUTION: When a track-etched membrane which has 10 to 50μkm film thickness and whose vertical hole diameter is 0.05 to 1.0 μm is used as a porous supporting body and a composite polyamide layer to be an ultrathin film is manufactured on the surface of the track-etched membrane as a skin layer, an aromatic polyfunctional amine solution prepared in a prescribed concentration is prepared as a first solution, an aromatic polyfunctional acid halide solution prepared in a prescribed concentration is prepared as a second solution, extra solution is removed by flowing the first solution on the surface of the track-etched membrane and thinning while forming a solution layer, and then the second solution is flown in, so that an ultrathin film-like composite polyamide layer that is condensed and polymerized is manufactured by polymerization reaction on an interface with a solution layer by the first solution.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a forward osmosis membrane. [Background technology]

[0002] As composite semipermeable membranes used in the production of ultrapure water, desalination of seawater, desalination of brackish water, reuse of wastewater, and the like, composite semipermeable membranes having a polyamide layer formed on a porous polysulfone membrane and cellulose acetate membranes have been known (Patent Document 1, Patent Document 2). One example of a composite semipermeable membrane reported in recently published Patent Documents 1 and 2 is a composite semipermeable membrane having a two-layer structure consisting of a support and a skin layer formed thereon, in which an epoxy resin is used as the porous membrane and a polyamide resin is used as the skin layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2013-13888 A [Patent Document 2] JP 2015-85234 A Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned Patent Documents 1 and 2 disclose the use of a porous epoxy resin with a thickness of 10 to 250 μm as a support (support film). In order to provide mechanical strength to a sponge-like material, a certain degree of thickness is necessary, and if the film thickness is made extremely thin, such as with a lower limit of 10 μm, there is a risk that the strength will be insufficient. In reality, commercially available products only have a film thickness of about 110 to 150 μm. Looking directly at this reality, even if epoxy resin is used, the film thickness value is thought to be limited to about 100 μm. Even if the epoxy resin is made into a membrane with a thickness of about 100 μm, the water permeability becomes an issue. It is known that a membrane thickness of about 100 to 150 μm imposes a large burden on the transport of water molecules, resulting in poor water permeability. Even with a membrane thickness of about 100 μm, the water permeability remains poor, so it is difficult to say that it is an optimal semipermeable membrane material for liquid separation devices that separate water molecules. The present invention has been made to solve such conventional problems, and proposes a method for manufacturing a forward osmosis membrane that can reduce the thickness of the support as much as possible in order to increase the water molecule permeation flux and can effectively remove impurities. [Means for solving the problem]

[0005] The method for producing a forward osmosis membrane according to the present invention described in claim 1 comprises using a track-etched membrane having a thickness of 10 to 50 μm and a pore size of 0.05 to 1.0 μm as a porous support, and forming an extremely thin composite polyamide layer as a skin layer on the surface of the track-etched membrane, A solution of an aromatic polyfunctional amine having a predetermined concentration is prepared as a first solution, and a solution of an aromatic polyfunctional acid halide having a predetermined concentration is prepared as a second solution; The method is characterized in that the first solution is poured onto the surface of the track-etched film to form a thin coating layer while excess solution is poured off the surface of the coating layer, and then the second solution is poured onto the surface of the coating layer to form a very thin composite polyamide layer that is condensed and polymerized by a polymerization reaction with the second solution at the interface of the coating layer. When the excess first solution is washed away from the surface of the coating layer, the excess solution is removed without touching the coating layer of the first solution, so that the coating layer can be produced without being damaged. In the method for producing a forward osmosis membrane according to the present invention described in claim 2, the first solution used is characterized in that m-phenylenediamine, an aromatic polyfunctional amine, is mixed in an amount of 0.5 to 1.5 weight % (wt %) in 100 mL of pure water. In the method for producing a forward osmosis membrane according to the present invention described in claim 3, the second solution used is characterized in that it contains 0.1 to 0.2 wt % of 1,3,5-benzenetricarboxylic acid chloride, among aromatic polyfunctional acid halides, in 100 mL of hexane. Effect of the Invention

[0006] According to this invention, a track-etched membrane with a thickness of about 10 to 50 μm and a through-hole diameter of 0.05 to 1.0 μm is used as a support for the skin layer, and a composite polyamide layer with a thickness of 0.1 to 0.2 μm is formed on the surface of this track-etched membrane as a skin layer, so that the strength of the support itself can be maintained even if it is made thin, and sufficient water permeability can be secured by making the membrane thin, and further, low salt permeability can be achieved. The membrane formation process is also relatively simple. [Brief description of the drawings]

[0007] [Figure 1] FIG. 2 is an enlarged cross-sectional view of a main portion of a forward osmosis membrane produced in a flat membrane shape according to the present invention. [Diagram 2] FIG. 1 is a first process chart showing a schematic diagram of steps for explaining a film forming method according to the present invention. [Diagram 3] FIG. 2 is a second process diagram showing a schematic diagram of the process for explaining the film forming method according to the present invention. [Figure 4] FIG. 3 is a third process diagram showing a schematic diagram of the process for explaining the film forming method according to the present invention. [Diagram 5] FIG. 4 is a fourth process diagram showing a schematic diagram of the process for explaining the film forming method according to the present invention. [Figure 6] 1 is a micrograph (a drawing substitute photograph) showing a cross section of a forward osmosis membrane according to the present invention. [Figure 7] 7 is a micrograph (a drawing substitute photograph) showing an enlarged portion of the central portion of FIG. 6. [Figure 8] 8 is a micrograph (a drawing substitute photograph) showing a further enlarged portion of FIG. 7. [Figure 9] FIG. 1 is a schematic diagram showing an example of an evaluation model used in an evaluation experiment. [Figure 10] This is a characteristic diagram at that time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Next, the method for producing a forward osmosis membrane according to the present invention will be described based on examples.

[0009] [Structure of forward osmosis membrane] 1 is an enlarged cross-sectional view of a main portion showing a simplified structure of a forward osmosis membrane 10 produced according to the present invention. For convenience of explanation, the shape of the forward osmosis membrane 10 will be described as a flat membrane. The forward osmosis membrane 10 uses a track-etched membrane (TE membrane) as a porous support 12, and has a composite polyamide layer formed on one side (surface) of the membrane as a skin layer 14. The track-etched membrane has a large number of through-holes 16 formed as vertical holes, and its thickness can be selected within the range of 10 to 50 μm, with a thickness of about 10 to 25 μm being preferred from the viewpoints of water permeability and salt permeability. In the embodiment, a thickness of 25 μm was used. The track-etched membrane is stronger than the epoxy resin membrane that has been used conventionally, and can maintain a certain level of pressure resistance even with a film thickness of about 10 μm. The pore size is preferably 0.05 to 1.0 μm, and more preferably 0.1 to 0.2 μm. The reason for making the membrane thinner is to increase the permeation flux of the permeating water, and the reason for making the through-hole diameter 0.1 to 0.2 μm is to selectively allow water molecules to pass through.

[0010] The track-etched film used as the support 12 can be prepared through the following steps. The material is polycarbonate resin, and a 25 μm thick polycarbonate film is irradiated with radiation to form numerous tracks, which is then etched to create uniform vertical holes (vertical holes) that are extremely thin and have a pore diameter of 0.2 μm or less.

[0011] This track-etched film is characterized by an extremely narrow pore size distribution, about 0.1 to 0.2 μm, and can form through-holes that are drilled vertically. As the material for the track etched film, in addition to polycarbonate, polyester, polyimide, etc. can be used. The composite polyamide layer that functions as the skin layer 14 is formed as an extremely thin film of about 0.1 to 0.2 μm by utilizing a condensation polymerization reaction at the interface between two types of solutions. In this film formation, it is necessary to remove excess solution so that the initial solution poured onto the surface of the track-etched film does not peel off from the surface (coating surface). The formation of the composite polyamide layer will be described later. When the forward osmosis membrane 10 thus produced is applied to a liquid separation device, the elements of the forward osmosis membrane 10 are arranged so that the composite polyamide layer side is the supply liquid (feed solution) side and the track-etched membrane is the high osmotic pressure solution (draw solution) side. Conversely, the track-etched membrane may be arranged on the feed solution side. A cross-flow operation method or the like may be considered as a processing method.

[0012] When the forward osmosis membrane 10 according to the present invention is applied as a composite semipermeable membrane to such a liquid separation device, the following advantages are expected. Since a track-etched film is used as the support 12, unlike a sponge-like support, even if the film is an extremely thin film with a thickness of about 10 μm, it has strength, and therefore can ensure self-supporting and durability. Since track-etched membranes are used as extremely thin porous films without nonwoven fabric, they can be used even with an extremely thin film thickness of about 10 μm, and even if vertical pores overlap, there is no problem, so the porosity can be increased. In forward osmosis membranes, molecular diffusion in the membrane accounts for most of the water flow resistance, so the thinner the membrane, the lower the water filtration resistance and the higher the water permeability.

[0013] [Method for producing composite polyamide layer] (1) Prepare a track-etched film. Thickness: 10 to 30 μm. In this example, a little extra space is provided from the viewpoint of strength and durability. A track-etched polycarbonate membrane with a thickness of 25 μm and a uniform pore size of 0.2 μm (track-etched membrane) is prepared.

[0014] (2) Prepare a first solution for the composite polyamide layer. An aromatic polyfunctional amine is used as the first solution. In this example, m-phenylenediamine (hereinafter referred to as MPD solution) is used as the aromatic polyfunctional amine. The MPD solution contains the following three types of additive liquids to promote dissolution and dispersion of m-phenylenediamine. In this example, the additive liquids are: CSA (10-camphorsulfone solution) ··· 4wt% TEA (triethylamine liquid) ··· 2wt% SDS (sodium dodecyl sulfate solution) ··· 0.25 wt% Three types of solutions were used: These additive solutions were mixed with 0.5 wt % MPD (m-phenylenediamine solution) in 100 mL of pure water (Milly-Q water) by stirring to prepare an aqueous MPD solution. This MPD aqueous solution was then placed in an amber bottle with a lid and thoroughly stirred and mixed at a rotation speed of 70 rpm for 1 hour to obtain the final MPD aqueous solution. In addition, a total of three types of MPD aqueous solutions were prepared for evaluation, with only the concentrations changed to 1.0 wt% and 1.5 wt%. The mixing speed and mixing time of the solutions are examples.

[0015] (3) A second solution (TMC solution) is prepared for forming a composite polyamide layer. The second solution is an aromatic polyfunctional acid halide. Examples of aromatic polyfunctional halides include trimesoyl chloride solution and 1,3,5-benzene tricarboxylic acid chloride solution, and the latter solution was used in the following example. The TMC solution was prepared by adding 0.1 wt% TMC to 100 mL of hexane. This TMC solution is placed in a mixing bottle (amber bottle with lid) and stirred at a rotation speed of 300 rpm for about an hour to use as the final TMC solution. In addition to this, a total of three types of TMC solutions were prepared by changing the concentration of the TMC solution to 0.15 wt% and 0.2 wt%. The rotation speed and stirring time used in the preparation are examples. A total of six types of solutions prepared in this way will be used for evaluation.

[0016] [Preparation of forward osmosis membrane 10] The process for producing the forward osmosis membrane 10 will be described with reference to schematic drawings. First, the track-etched membrane described above is prepared as the porous support 12, and the first solution, an MPD aqueous solution (1.0 wt% MPD aqueous solution in this example), is dripped uniformly and slowly onto the surface (see the first process diagram in FIG. 2). This results in a coating layer 20 (shown with diagonal lines) in which the surface of the track-etched membrane is thinly covered with the MPD aqueous solution. In FIG. 2, 22 denotes a container containing the MPD aqueous solution.

[0017] At this time, it is necessary to remove the excess MPD aqueous solution. Extreme care must be taken when removing it. This is because, while the MPD aqueous solution penetrates into the polysulfone membrane, which has a sponge-like internal structure, and the necessary MPD aqueous solution remains on the membrane surface, if the coating layer 20, which is only on the surface of the track-etched membrane, is wiped off with tissue paper or the like, all of the MPD aqueous solution will be wiped off, and no MPD will remain at all.

[0018] Therefore, in this invention, instead of wiping it off with tissue or the like, we decided to add a step of tilting the frame supporting the track-etched film (e.g., a frame using a PTFE film) for a specified period of time so that the excess solution can be removed without touching the coating layer of the first MPD aqueous solution (see the second process diagram in Figure 3).

[0019] By adding this step, the excess MPD aqueous solution can be washed away without damaging the coating layer 20, and a uniform and even coating layer (liquid layer surface) can be produced. The track-etched film may be tilted in advance or may be tilted during application. If the track-etched film is a flat film, instead of tilting the track-etched film, a spin coater may be used to disperse and wash away excess liquid by centrifugal force, or a bar coater may be used to wash away excess liquid.

[0020] After pouring off excess solution, leave the track-etched film to stand for 1 to 2 minutes. If the track-etched film was tilted during processing, return it to a horizontal position before leaving it to stand. Then, 15 mL of TMC solution (TMC concentration is 0.1 wt%) is slowly poured onto the surface of the track-etched film (the surface of the coating layer of the MPD aqueous solution) for 30 seconds (see the third process diagram shown in Figure 4). As a result, the surface of the coating layer 20 is covered with a second solution layer 24 (shown with diagonal lines), and the excess TMC solution is drained. In Figure 4, 26 denotes a container containing the TMC solution.

[0021] By pouring the TMC solution onto the surface of the coating layer 20 of the MPD aqueous solution, an interfacial polymerization reaction (condensation polymerization reaction) occurs at the interface between the coating layer 20 of the MPD aqueous solution and the solution layer 24 of the TMC solution, and as a result, a composite polyamide layer (composite polyamide film) 14 (shown by reticulated hatching) having bag-like cavities of various sizes is produced on the surface of the track-etched film (see the fourth process diagram in Figure 5 and Figure 1).

[0022] The thickness of the composite polyamide layer can be adjusted appropriately by adjusting the reaction conditions of the chemical composite polyamide layer, such as the concentration of the MPD aqueous solution or TMC solution to be polymerized, the reaction time, the reaction temperature, etc. In this example, the thickness of the composite polyamide layer was adjusted to about 0.1 to 0.2 μm by appropriately selecting these conditions. The composite semipermeable membrane, which is made by forming a composite polyamide layer on the surface of the track-etched membrane, is stabilized by heating at 100°C for about 5 minutes, and then cooled to room temperature (22°C). The cooled composite semipermeable membrane is immersed in pure water (Milli-Q water) for washing to remove excess unreacted chemicals (TMC solution and MPD aqueous solution), thereby obtaining the desired forward osmosis membrane 10 consisting of the composite semipermeable membrane (see Figure 1).

[0023] In addition to producing a forward osmosis membrane as a flat membrane, it is also possible to produce a track-etched membrane using a material wound in a roll, or to produce a forward osmosis membrane using a roll of material on which a track-etched membrane has already been produced. An example of the membrane production process for producing a track-etched membrane from a material is described below.

[0024] A polycarbonate film of a predetermined width wound in a roll is transported at a predetermined speed, and the film surface is continuously or intermittently irradiated with radiation (e.g., gamma rays) to form a large number of tracks, and then immersed in an etching solution bath for etching to form a track-etched film with a large number of through holes. The track-etched film that has passed through the etching bath is wound up. The track-etched film is then transported in an oblique state, and a first solution, an MPD aqueous solution, is slowly poured onto the oblique track-etched film surface from above at an angle, so that the solution spreads over the entire film surface. By pouring the solution from above while transporting it in this oblique manner, excess solution can be drained off from the track-etched film surface without touching it. The track-etched film from which excess MPD aqueous solution has been removed is returned to a horizontal state and transported, and a second solution, a TMC solution, is slowly poured onto the entire surface of the MPD aqueous solution layer.

[0025] This pouring causes a condensation polymerization reaction at the interface between the MPD aqueous solution and the TMC solution, forming a composite polyamide membrane. After that, the membrane is heated, cooled, washed, etc. while being wound up, and a composite semipermeable membrane is completed with a polyamide membrane formed on the track-etched membrane. This membrane formation process is just one example.

[0026] [Cross-sectional observation of forward osmosis membrane 10] Next, a forward osmosis membrane 10 made using a 1.0 wt% MPD aqueous solution and a 0.1 wt% TMC solution was used as a sample, and its cross section was observed. This sample was freeze-fractured in liquid nitrogen while it was wet, and then fixed to a holder after drying. The cross section of the sample was then subjected to a conductive treatment, and used as a sample for cross-sectional FE-SEM observation. FE-SEM is an abbreviation for field emission scanning electron microscope, and allows the observation of the sample morphology at high resolution and high magnification.

[0027] Figures 6 to 8 are micrographs (sample cross section) taken when observing secondary electron images using a Hitachi (SU-8220) observation device with an accelerating voltage set to 1.0 kV. Figure 7 is an enlarged photograph of the interface near the center of Figure 6, and Figure 8 is an enlarged photograph of a further important portion of Figure 7. Although the size and shape of the cavities 16 in the skin layer 14 are not uniform, a large number of cavities 16 having a thickness of 30 to 40 nm and a cross-sectional diameter of several tens to several hundreds of nm are formed over the entire surface of the skin layer 14. The cavities 16 formed near the interface of the support 12 are distributed in relatively small sizes, and although there is variation in the shape and overlap of the cavities 16 in the thickness direction, the film is formed in a state in which the cavities 16 are uniformly distributed over the entire interface. It is believed that the presence of these cavities 16 ensures high transmission efficiency.

[0028] [Evaluation test of forward osmosis membrane 10] Next, an evaluation test of the above-mentioned forward osmosis membrane 10 will be described using an evaluation model of FIG. A forward osmosis membrane 10 is placed vertically in the center of the tank (container) used in the evaluation model to separate the tank into left and right. In this example, salt water is stored on the left side as a solution with high osmotic pressure (draw solution), and pure water is stored on the right side as a solution with low osmotic pressure (feed solution). Water flows (permeates) into the draw solution with high osmotic pressure due to the phenomenon of forward osmosis. The permeation flux (L / m 2 H) and salt permeability (salt back diffusion flux) (g / m 2 The temperature of the solution is 25°C, the flow rate of the draw solution is 0.9 L / min, and the flow rate of the feed solution is 1.0 L / min.

[0029] The results are shown in Figure 10. As a comparative example, the case where cellulose triacetate (CTA) was used as the semipermeable membrane is shown. Bar graph 30 showing the permeation flux when cellulose triacetate was used is 9.1 L / m 2 H, and the salt back-diffusion flux is 1.7 g / m as shown in bar 32. 2 It's H. In contrast, in the case of the forward osmosis membrane 10 according to the present invention, the permeation flux is 21.0 L / m, as shown by bar graph 40. 2 H, and the salt back-diffusion flux is 8.1 g / m as shown in bar 42. 2 It became H. As is clear from this, in the case of the forward osmosis membrane 10 of the present invention, the permeation flux is about twice as high as that of cellulose triacetate, and the permeation flux is significantly improved. On the other hand, the salt back diffusion flux was about four times higher. From this, it is evident that the forward osmosis membrane 10 according to the present invention is extremely suitable for use in a water separation device since it can efficiently separate water molecules. [Industrial Applicability]

[0030] From the above evaluation tests, it was found that the forward osmosis membrane 10 produced according to the present invention has high water permeability and low salt permeability, and is therefore suitable for use as the forward osmosis membrane 10 for the above-mentioned liquid separation device. [Explanation of symbols]

[0031] 10 Forward osmosis membrane 12 Support (track etched film) 14 Skin layer (composite polyamide layer) 16 Through hole 20 Coating layer made of first solution 24 Second solution layer

Claims

1. A track-etched membrane having a thickness of 10 to 50 μm and a pore size of 0.05 to 1.0 μm is used as a porous support, and an extremely thin composite polyamide layer is formed as a skin layer on the surface of the track-etched membrane, A solution of an aromatic polyfunctional amine having a predetermined concentration is prepared as a first solution, and a solution of an aromatic polyfunctional acid halide having a predetermined concentration is prepared as a second solution; The first solution is poured onto the surface of the track-etched film to form a thin coating layer while excess solution is poured off the surface of the coating layer, and then the second solution is poured onto the surface of the coating layer to form a condensation-polymerized, extremely thin composite polyamide layer at the interface of the coating layer through a polymerization reaction with the second solution. A method for producing a forward osmosis membrane.

2. The first solution used in the method for producing a forward osmosis membrane is a solution in which m-phenylenediamine, one of aromatic polyfunctional amines, is mixed in an amount of 0.5 to 1.5% by weight in 100 mL of pure water.

2. The method for producing a forward osmosis membrane according to claim 1.

3. The second solution used in the method for producing a forward osmosis membrane contains 0.1 to 0.2% by weight of 1,3,5-benzenetricarboxylic acid chloride, one of aromatic polyfunctional acid halides, in 100 mL of hexane.

2. The method for producing a forward osmosis membrane according to claim 1.

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