Microfiltration membrane and preparation method therefor

Abstract

A microfiltration membrane and a preparation method therefor. In the method, the upper surface and the lower surface of a porous-structure supporting layer coated with a membrane casting liquid are simultaneously subjected to steam-induced molding by means of a first atmosphere in an upper space and a second atmosphere in a lower space, such that the membrane preforming time is greatly shortened. Moreover, by adjusting the composition and temperature of the first atmosphere in the upper space and the second atmosphere in the lower space of the porous-structure supporting layer, synchronous control over the pore diameters of the upper and lower surfaces of a preformed membrane is achieved, such that the micro-filtration membrane has a high porosity, and the pore diameters of the upper and lower surfaces thereof are controllable.

Description

A microfiltration membrane and its preparation method

[0001] This application claims priority to the Chinese patent application filed with the State Intellectual Property Office on June 12, 2023, with application number CN202310691553.5 and invention name “A microfiltration membrane and its preparation method”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of polymer materials, and in particular to a microfiltration membrane and a preparation method thereof. Background Art

[0003] Membrane technology is a new, highly efficient separation technology. Compared to traditional distillation and rectification techniques, it offers advantages such as high separation efficiency, low energy consumption, and a small footprint. The core of membrane separation technology lies in the separation membrane. Polymer membranes are a type of separation membrane made from organic high-molecular-weight polymers using a specific process. Depending on the type of polymer, polymer membranes can be further categorized into cellulose, polyamide, sulfone, and polytetrafluoroethylene. Furthermore, membrane pore size can be further categorized into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes.

[0004] Microfiltration membranes can retain particles between 0.1 and 1 μm, allowing macromolecules and dissolved solids (inorganic salts) to pass through, but retain suspended solids, bacteria, and high-molecular-weight colloids. Currently, microfiltration membranes are primarily used for filtration and sterilization in the pharmaceutical industry, including the production of sterile liquids, separation of biological preparations, preparation of ultrapure water, air filtration, and biological and microbiological testing and analysis. Microfiltration membranes typically operate at pressures between 0.3 and 7 bar. Microfiltration is one of the earliest membrane technologies developed and applied worldwide, using natural or synthetic polymers as membrane materials. The separation mechanism for microfiltration membranes is primarily sieving and retention. Common methods for developing microfiltration membranes with higher porosity and water flux include non-solvent-induced phase separation (NIPS), vapor-induced phase separation (VIPS), and atomization pretreatment-induced phase separation.

[0005] Traditional steam-induced phase separation, such as patent CN1170620C, can significantly improve the porosity of the membrane and increase the water flux of the membrane, but generally requires being placed in high-temperature steam (greater than 100°C) for a long time (greater than 30 minutes). Methods combining atomization pretreatment and non-solvent-induced phase separation, such as CN112973451B and CN114797484B, can to a certain extent solve the problem of high temperature and long processing time during traditional steam-induced phase separation. However, the pre-atomization process will produce obvious liquid droplets, and the falling liquid droplets will contaminate the membrane surface during long-term batch film formation. At the same time, both methods involve the diffusion of steam or liquid droplets from the surface of the casting liquid to the bottom layer, which will result in a large difference in the pore size morphology between the surface and the bottom layer, and poor controllability of the bottom layer pore size.

[0006] Therefore, there is an urgent need to develop a microfiltration membrane with high porosity and controllable pore sizes on the upper and lower surfaces.

[0007] Summary of the Invention

[0008] In view of this, the technical problem to be solved by this application is to provide a microfiltration membrane and a preparation method thereof. The microfiltration membrane has the advantages of high porosity and controllable pore sizes on both the upper and lower surfaces.

[0009] To achieve the above objectives, the technical solutions adopted in this application are as follows:

[0010] The present application provides a method for preparing a microfiltration membrane, comprising the following steps:

[0011] In a closed space, a support layer divides the space into an upper space and a lower space. The upper and lower surfaces of the support layer are simultaneously induced to form by a first atmosphere in the upper space and a second atmosphere in the lower space to obtain a pre-film, which is then phase-separated and formed to prepare a microfiltration membrane; the first atmosphere and the second atmosphere are different; the support layer is a porous structure support layer coated with a casting liquid.

[0012] Preferably, the support layer is placed on a rubber ring, and the rubber ring is hollow.

[0013] The hollow rubber ring is conducive to the atmosphere in the upper and lower spaces to simultaneously induce the upper and lower surfaces of the porous structure support layer coated with the casting liquid to form.

[0014] The preparation method controls the composition and temperature of the atmosphere in the upper and lower spaces, thereby controlling the pore diameters of the upper and lower surfaces of the membrane.

[0015] Preferably, the first atmosphere and the second atmosphere are independently selected from one or more of air, water vapor, ethanol vapor, ethylene glycol vapor, vapor of a polar aprotic solvent, salt solution vapor, acid solution vapor and alkaline solution vapor; more preferably, the atmosphere is selected from one or more of air, water vapor and ethanol vapor.

[0016] Preferably, the temperature of the upper space and the lower space are independently 5°C to 125°C; more preferably 30°C to 125°C.

[0017] In some specific embodiments of the present application, the first atmosphere and the second atmosphere are independently preferably one or more of 60°C air with 70% humidity, 30°C air with 30% humidity, 125°C water vapor, and a 60°C mixed gas of water and ethanol with a mass ratio of 1:1.

[0018] Preferably, the pre-film forming time is 10 to 600 seconds, more preferably 50 to 300 seconds, and even more preferably 100 to 200 seconds. In some specific embodiments of the present application, the pre-film forming time of the microfiltration membrane is 120 seconds or 180 seconds.

[0019] Preferably, the casting solution is a solution containing a polymer;

[0020] Preferably, the polymer is selected from one or more of polysulfone, polyethersulfone, polyvinylidene fluoride, cellulose acetate, polyethylene oxide, polyacrylonitrile, polyamide and polyimide; more preferably, polyethersulfone or polyvinylidene fluoride.

[0021] Preferably, the solution containing the polymer further contains a pore-forming additive and a solvent for the polymer;

[0022] The solvent of the polymer is a good solvent and / or a poor solvent for the polymer;

[0023] The pore-forming additive is selected from one or more of polyvinyl pyrrolidone, polyethylene glycol, zinc chloride and lithium chloride.

[0024] Preferably, the good solvent for the polymer is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, acetone, N-methyl-2-pyrrolidone, dimethyl sulfoxide, tetramethyl sulfoxide, tetrahydrofuran, dioxane, acetonitrile, chloroform and tetramethyl sulfoxide; more preferably, N,N-dimethylacetamide or N-methyl-2-pyrrolidone.

[0025] Preferably, the poor solvent for the polymer is selected from water or alcohol solvent.

[0026] The alcohol solvent is preferably a small molecule alcohol, specifically including but not limited to one or more of methanol, ethanol, benzyl alcohol, and isopropanol.

[0027] Preferably, the solid content of the casting solution is 6 to 30 wt %, more preferably 10 to 25 wt %. In some specific embodiments of the present application, the solid content of the casting solution is 19 wt %.

[0028] Preferably, the thickness of the casting solution on the porous support layer is 50-500 μm, more preferably 75-300 μm, and even more preferably 100-150 μm. In some specific embodiments of the present application, the thickness of the casting solution on the porous support material is 100 μm or 110 μm.

[0029] The present application has no particular limitation on the stirring time and temperature of the casting solution. The stirring time is preferably 24 to 48 hours, and the stirring temperature is preferably 60 to 100°C.

[0030] In the above preparation method, the porous structure support layer (porous membrane) includes but is not limited to porous structure support materials such as non-woven fabrics, woven fabrics and porous ceramics.

[0031] In the present application, in order to fully carry out steam induction, it is preferred to first use a partition to divide the enclosed space into two parts, upper and lower parts. After the atmosphere in the upper and lower parts is stable, the partition is quickly replaced with a porous membrane coated with casting liquid on the surface, so that the upper and lower atmospheres can simultaneously steam-induce the porous membrane coated with casting liquid on the surface to obtain a pre-film.

[0032] Before the steam-induced molding, the partition is placed on the hollow rubber ring.

[0033] In the present application, the phase separation molding is preferably carried out by a non-solvent induced phase separation method.

[0034] The non-solvent-induced phase separation method is specifically to immerse the pre-film prepared above in a non-solvent of the polymer for a period of time and then shape it into a microfiltration membrane, and utilize the physical exchange between the solvent in the pre-film and the non-solvent of the polymer to cause phase separation of the pre-film.

[0035] The non-solvent for the polymer includes, but is not limited to, single-component solvents such as water, ethanol, ethylene glycol, and sodium hydroxide aqueous solution, or mixed solutions of water and polar aprotic solvents, or mixed solutions of water and other solvents.

[0036] When pre-forming a membrane for phase separation, the temperature is preferably 10 to 80°C, the time is preferably 1 to 60 minutes, more preferably 3 to 10 minutes;

[0037] In this application, the casting liquid is coated on the porous structure support layer substrate by scraping or casting technology, and the steam-induced phase separation is completed on the upper and lower sides of the porous structure support layer by controlling the atmosphere composition and temperature to complete the preparation of the pre-film, and then the pre-film is immersed in a non-solvent bath to further complete the phase separation, thereby obtaining a microfiltration membrane with high porosity and controllable pore size on the upper and lower surfaces.

[0038] The method for preparing the microfiltration membrane described in this application specifically comprises the following steps:

[0039] 1) In a closed space, a partition is placed on a hollow rubber ring to divide the space into an upper space and a lower space, and atmosphere is introduced into the upper and lower spaces, while the temperatures of the upper and lower spaces are controlled separately;

[0040] 2) Remove the separator, place the porous structure support layer coated with the casting solution on the gasket, and prepare a pre-film by simultaneously inducing molding in the upper and lower spaces;

[0041] 3) The preformed membrane is subjected to phase separation and molding to prepare a microfiltration membrane.

[0042] In the above preparation method, after the phase separation and forming in step 3) is completed, the fully formed porous membrane is soaked in deionized water for a certain period of time to remove the residual solvent, and then dried to prepare the microfiltration membrane described in the present application.

[0043] The deionized water soaking time is preferably 24 to 48 hours, and the soaking temperature is preferably 30 to 80°C.

[0044] The present application does not specifically limit the drying method, and can be conventional heating drying, vacuum drying, or other methods well known to those skilled in the art. In the present application, the heating drying method is preferably adopted, and the heating drying temperature is preferably 60 to 120° C., and the heating drying time is preferably 24 to 48 hours.

[0045] The present application also provides a microfiltration membrane prepared by the above-mentioned preparation method.

[0046] Preferably, the interior of the microfiltration membrane has a sponge pore structure.

[0047] The present application adopts a steam induction method to simultaneously induce the molding of the upper and lower surfaces of the porous structure support layer coated with the casting liquid film, which greatly shortens the pre-film forming time.

[0048] Compared to existing technologies, the microfiltration membrane provided by this application utilizes the permeability of the porous support layer to atmosphere. This allows for simultaneous steam-induced shaping of the upper and lower surfaces of the porous support layer coated with the liquid casting film, using a first atmosphere in the upper space and a second atmosphere in the lower space. This significantly shortens the pre-film formation time. Furthermore, by adjusting the composition and temperature of the first atmosphere in the upper space and the second atmosphere in the lower space of the porous support layer, the pore sizes of the upper and lower surfaces of the pre-film can be simultaneously controlled, resulting in the microfiltration membrane having the advantages of high porosity and controllable pore sizes on both the upper and lower surfaces. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG1 is a schematic diagram of the preparation process of a microfiltration membrane;

[0050] Figure 2 is a cross-section of the microfiltration membrane in Example 1, wherein a is a longitudinal cross-section, b is the upper surface, and c is the lower surface;

[0051] FIG3 is a cross section of the microfiltration membrane in Example 2, wherein d is a longitudinal cross section, e is an upper surface, and f is a lower surface. DETAILED DESCRIPTION

[0052] In order to further illustrate the present application, the microfiltration membrane and the preparation method thereof provided in the present application are described in detail below with reference to the examples.

[0053] Example 1

[0054] Step 1: Place the partition on the hollow rubber ring to separate the space into an upper space and a lower space. Inject 60°C air with 70% humidity into the upper space as atmosphere A, and inject 30°C air with 30% humidity into the lower space as atmosphere B. Wait until the atmosphere composition and temperature of the upper and lower spaces are stable.

[0055] Step 2: Dissolve 12g of polyethersulfone and 4g of polyvinylpyrrolidone in 84g of N,N-dimethylacetamide and stir at 60°C for 24h to obtain a uniform casting solution;

[0056] Step 3: The obtained casting solution is coated on the porous ceramic by using a doctor blade technique, and the coating thickness is controlled to be 100 μm;

[0057] Step 4: Quickly remove the separator, and then place the porous ceramic coated with the casting liquid between atmospheres A and B. The upper layer of the casting liquid is affected by the upper space atmosphere A, and the bottom layer is affected by the lower space atmosphere B. At the same time, steam-induced phase separation is carried out. The control time is 2 minutes to obtain a pre-film.

[0058] Step 5: Immerse the pre-formed film in 40% DMAc solution for 7 minutes to further perform phase separation molding;

[0059] Step 6: Soak the fully formed porous membrane in deionized water at 30°C for 24 hours to remove residual solvent, and dry it at 80°C for 24 hours to obtain a microfiltration membrane.

[0060] Example 2

[0061] Step 1: Place a partition on the hollow rubber ring to separate the space into an upper space and a lower space. Inject 125°C water vapor into the upper space as atmosphere A, and inject a mixed gas of 60°C water and ethanol (100g:100g) into the lower space as atmosphere B. Wait until the atmosphere composition and temperature of the upper and lower spaces are stable.

[0062] Step 2: Dissolve 10g of polyvinylidene fluoride and 6g of polyvinyl pyrrolidone in 84g of N-methyl pyrrolidone and stir at 80°C for 24h to obtain a uniform casting solution;

[0063] Step 3: The obtained casting solution is coated on the non-woven fabric by using a doctor blade technique, and the coating thickness is controlled to be 110 μm;

[0064] Step 4: Quickly remove the partition, and then place the non-woven fabric coated with the casting solution between atmospheres A and B. The upper layer of the casting solution is affected by the upper space atmosphere A, and the bottom layer is affected by the lower space atmosphere B. At the same time, steam-induced phase separation is carried out. The control time is 3 minutes to obtain a pre-film.

[0065] Step 5: Immerse the pre-formed film in 30% DMAc solution for 3 minutes to further phase separate and form;

[0066] Step 6: Soak the fully formed porous membrane in deionized water at 60°C for 24 hours to remove residual solvent, and dry it at 60°C for 36 hours to obtain a microfiltration membrane.

[0067] The following relevant tests were performed on the microfiltration membrane prepared in the above examples, and the specific data are shown in Table 1.

[0068] 1) Scanning electron microscope test

[0069] The pore sizes of the upper and lower surfaces of the microfiltration membrane were scanned and counted using a Hitachi SU3800 to obtain the average pore size of the upper and lower surfaces (see Figure 2 (microfiltration membrane prepared in Example 1) and Figure 3 (microfiltration membrane prepared in Example 2) for details).

[0070] 2) Water flux test

[0071] Water flux, short for the volume or mass of water passing through a unit membrane area per unit time, is an important indicator for evaluating membrane separation efficiency and a key indicator for measuring microfiltration membrane performance. A higher water flux indicates a higher separation efficiency and more permeable membrane pores. The microfiltration membranes prepared in Examples 1 and 2 exhibited high water flux. Testing was performed in accordance with GB / T 34242-2017.

[0072] 3) Most probable aperture

[0073] The most probable pore size refers to the most likely pore size, that is, the pore size with the most pores. It is an important indicator for evaluating membrane separation efficiency and is also one of the key indicators for measuring microfiltration membrane performance. The most probable pore size determines the size of the particles separated by the membrane. Table 1 shows that the microfiltration membranes prepared in Examples 1 and 2 have excellent separation performance. The tests were conducted with reference to the GB / T 5249-2013 standard.

[0074] 4) Porosity

[0075] Porosity refers to the ratio of the total volume of interconnected microscopic voids within a porous medium to the surface volume of the porous medium. Generally speaking, the higher the porosity, the more permeable the membrane interior and the higher the separation efficiency. A porosity greater than 80% is considered high porosity. Testing is performed using the GB / T 33052-2016 standard.

[0076] Table 1 Water flux, pore size and porosity of the examples

[0077] The data in Table 1, combined with Figures 2 and 3, show that this application controls the pore size of the upper and lower surfaces of the pre-formed membrane by varying the composition and temperature of the atmosphere within the upper and lower spaces. The resulting microfiltration membrane exhibits a sponge-like internal pore structure and exhibits excellent separation performance and high porosity. Furthermore, the microfiltration membrane preparation method described in this application can be used to prepare microfiltration membranes with varying pore sizes and adjustable pore size on the upper and lower surfaces, demonstrating strong practical applicability. Furthermore, the preparation method is simple, and the use of simultaneous steam induction on the upper and lower surfaces significantly shortens the pre-formed membrane time, meeting diverse industrial production needs and possessing broad market application prospects.

[0078] The above embodiments are only intended to help understand the method and core concept of the present application. It should be noted that, for those skilled in the art, without departing from the principles of the present application, several improvements and modifications may be made to the present application, and such improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A method for preparing a microfiltration membrane, characterized in that: The following steps are involved: In a closed space, the support layer divides the space into an upper space and a lower space, and the upper and lower surfaces of the support layer are simultaneously induced to form by a first atmosphere in the upper space and a second atmosphere in the lower space to obtain a pre-film, and then phase separation forming is performed to prepare a microfiltration membrane; the first atmosphere and the second atmosphere are different; The support layer is a porous structure support layer coated with a casting liquid.

2. The preparation method according to claim 1, characterized in that: The supporting layer is placed on the rubber ring, and the rubber ring is hollow.

3. The preparation method according to claim 1, characterized in that: The first atmosphere and the second atmosphere are independently selected from one or more of air, water vapor, ethanol vapor, ethylene glycol vapor, vapor of a polar aprotic solvent, salt solution vapor, acid solution vapor and alkaline solution vapor.

4. The method for preparing a microfiltration membrane according to claim 1, characterized in that: The temperatures of the upper space and the lower space are independently 5 to 125°C.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The phase separation molding time is 10 to 600 seconds.

6. The preparation method according to claim 1, characterized in that: The casting solution is a solution containing a polymer; The polymer is selected from one or more of polysulfone, polyethersulfone, polyvinylidene fluoride, cellulose acetate, polyethylene oxide, polyacrylonitrile, polyamide and polyimide; The solution containing the polymer also contains a pore-forming additive and a solvent for the polymer; The solvent of the polymer is a good solvent of the polymer and / or a poor solvent of the polymer; The pore-forming additive is selected from one or more of polyvinyl pyrrolidone, polyethylene glycol, zinc chloride and lithium chloride; The good solvent of the polymer is selected from one or more of N,N-dimethylformamide, N,N-dimethylacetamide, acetone, N-methyl-2-pyrrolidone, dimethyl sulfoxide, tetramethyl sulfoxide, tetrahydrofuran, dioxane, acetonitrile, chloroform and tetramethyl sulfoxide; The poor solvent for the polymer is selected from water or an alcohol solvent.

7. The preparation method according to claim 1 or 6, characterized in that: The solid content of the casting solution is 6-30wt%.

8. The preparation method according to claim 1 or 6, characterized in that: The thickness of the casting liquid on the porous structure support layer is 50-500 μm.

9. The preparation method according to claim 3 or 4, characterized in that: The first atmosphere and the second atmosphere are independently selected from one or more of 60°C air with 70% humidity, 30°C air with 30% humidity, 125°C water vapor, and a 60°C mixed gas of water and ethanol in a mass ratio of 1:

1.

10. The preparation method according to claim 6, characterized in that: The alcohol solvent is a small molecule alcohol; The small molecule alcohol includes, but is not limited to, one or more of methanol, ethanol, benzyl alcohol and isopropanol.

11. The preparation method according to claim 1, characterized in that: The porous structure support layer includes a porous structure support material; The porous structure supporting material is one or more of non-woven fabric, woven fabric and porous ceramic.

12. The preparation method according to claim 1, characterized in that: After the phase separation and forming, the fully formed porous membrane is soaked in deionized water to remove residual solvent, and dried to obtain a microfiltration membrane; The deionized water is immersed for 24 to 48 hours at a temperature of 30 to 80°C.

13. A microfiltration membrane, characterized in that: The compound is prepared by the preparation method according to any one of claims 1 to 12.

14. The microfiltration membrane according to claim 13, characterized in that The interior of the microfiltration membrane presents a sponge pore structure.

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