Filtering membrane with aperture sizes in gradient arrangement and preparation method of filtering membrane

Abstract

The invention belongs to the field of separation and filtration, and particularly relates to a filtering membrane with aperture sizes in gradient arrangement and a preparation method thereof. The preparation method comprises the following steps: (1) preparing a membrane casting solution: mixing and dissolving dimethylacetamide, polyether sulfone, polyethylene glycol and an auxiliary agent, and defoaming to obtain the membrane casting solution; (2) preparing core liquid: mixing dimethylacetamide with water to obtain the core liquid; (3) preparing a gel bath: mixing dimethylacetamide with water to obtain the gel bath; the concentration of the gel bath is smaller than that of the core liquid; and (4) preparing the filter membrane: enabling the membrane casting solution to be in contact with the core solution and the gel bath through a spinneret plate, and enabling membrane filaments to pass through the gel bath to a filament collecting tank, so as to obtain the filter membrane of which the pore sizes are arranged in a gradient manner. The pore diameter of the filtering membrane is changed from a large pore diameter on one side to a small pore diameter on the other side in a gradient manner, and the filtering membrane has high retention rate and high flux, high mechanical strength, easy cleaning and low energy consumption.

Description

A filter membrane with gradient pore sizes and its preparation method Technical Field

[0001] This invention belongs to the field of separation and filtration, and specifically relates to a filter membrane with a gradient arrangement of pore sizes and its preparation method. Background Technology

[0002] In the field of separation and filtration, filter membranes, as core separation elements, are widely used in water treatment, biomedicine, food processing, and many other industries. Existing filter membranes are mainly divided into two categories: symmetrical membranes and asymmetrical membranes. Symmetrical membranes have uniform pore sizes but dense structures, resulting in high fluid permeation resistance, low flux, and the easy penetration of contaminants into the membrane interior, causing irreversible blockage. Ordinary asymmetrical membranes, although possessing certain pore size differences, have an unreasonable pore size gradient distribution and poor connection between the separation layer and the support layer, resulting in an inherent contradiction of "high separation accuracy but low flux."

[0003] In addition, the mechanical strength of the support layer of traditional asymmetric membranes is insufficient, making it difficult to withstand transmembrane pressure difference and pressure fluctuations under harsh operating conditions, and it is prone to damage. At the same time, pollutants tend to accumulate inside the membrane, requiring frequent chemical cleaning, which not only shortens the service life of the membrane, but also increases the operating and maintenance costs and energy consumption.

[0004] To address the aforementioned technical challenges, the development of a filter membrane with a reasonable pore size gradient, balancing separation accuracy and flux, high mechanical strength, and easy cleaning has become an urgent need in this field.

[0005] Therefore, based on this, the technical solution of the present invention is proposed. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention provides a method for preparing a filter membrane with a gradient arrangement of pore sizes, the method comprising the following steps:

[0007] (1) Preparation of casting solution: Dimethylacetamide (DMAC), polyethersulfone (PES), polyethylene glycol (PEG-400) and additives are mixed, dissolved and degassed to obtain casting solution; (2) Preparation of core solution: Dimethylacetamide is mixed with water to obtain core solution; (3) Preparation of gel bath: Dimethylacetamide is mixed with water to obtain gel bath; The concentration of the gel bath is less than the concentration of the core solution; (4) Preparation of filter membrane: The casting solution is brought into contact with the core solution and the gel bath through a spinneret, and the membrane fibers pass through the gel bath to the take-up groove to obtain the filter membrane with the pore size arranged in a gradient.

[0008] Preferably, in step (1), the additive is polyvinylpyrrolidone K30 (PVP-K30).

[0009] Preferably, in step (1), the mass ratio of dimethylacetamide, polyethersulfone, polyethylene glycol, and additives is 65-75:15-25:5-15:5-15.

[0010] Preferably, in step (1), the mixing and dissolving temperature is 75-85℃ and the mixing and dissolving time is 7-9h.

[0011] Preferably, in step (2), the concentration of dimethylacetamide in the core fluid is 60-80 wt%.

[0012] Preferably, in step (3), the concentration of dimethylacetamide in the gel bath is 30-50 wt%.

[0013] Preferably, in step (4), the take-up speed is set to 15-25 m / min.

[0014] Preferably, step (4) further includes a water boiling and soaking process: after the membrane fibers pass through the gel bath to the fiber taking tank, the produced membrane fibers are boiled and washed at 55-65°C, and then soaked in a 15-25% glycerol aqueous solution to obtain the filter membrane with the pore size arranged in a gradient.

[0015] Based on the same technical concept, the present invention further provides a filter membrane with a gradient arrangement of pore sizes obtained by the preparation method described above.

[0016] Preferably, the pore size of the filter membrane varies in a gradient from a large pore on one side to a small pore on the other side.

[0017] To facilitate understanding of this invention, the principle is explained as follows: The exchange between low and high solvent concentrations is not instantaneous and uniform. The exchange rate and degree differ at different thicknesses of the membrane. Near the gel bath (outer surface): solvent loss and non-solvent intrusion are faster and more intense, resulting in a higher phase separation rate and a very small polymer-depleted phase region. Consequently, the generated pores are very dense and tiny, resulting in a smaller membrane pore size. Near the core solution (inner membrane): the mass transfer process gradually slows down, allowing more time for the polymer-depleted phase region to grow and coalesce, thus forming increasingly larger pores. Ultimately, a gradient structure of membrane pore size is formed, from a dense separation layer on the outer surface to a macroporous support layer on the inner surface. It should be noted that by changing the DMAC content of the inner and outer solutions—that is, changing the DMAC content of the core solution and the gel bath—the opposite effect occurs: the pore size changes from gradually becoming denser from the inner to the outer side to gradually becoming denser from the outer to the inner side.

[0018] The beneficial effects of this invention are as follows: 1. The small pore side of the filter membrane is extremely thin and dense, mainly playing a separation role, which ensures accurate sieving and efficient retention, effectively blocking target substances (such as proteins, bacteria, and colloids). At the same time, the large pore side of the filter membrane has a porous and loose structure with very low resistance to fluid, providing strong mechanical support for the small pore side that mainly plays a separation role, thus not significantly increasing the overall filtration resistance. This allows the overall filter membrane to achieve a high water flux while ensuring high separation accuracy, solving the traditional contradiction of "high accuracy but low flux".

[0019] 2. In actual use, contaminants are mainly trapped on the smooth, dense, small-pore side of the filter membrane, making it difficult for them to enter the larger-pore side that provides support. This prevents contaminants from accumulating deep inside the membrane and causing irreversible blockage. Since the contaminants only remain on the surface, they can be easily removed by simple physical methods such as backwashing or air-water mixed rinsing, restoring the filter membrane's flux. This significantly extends the chemical cleaning cycle and service life of the filter membrane, reducing operating and maintenance costs. It solves the problems of existing filter membranes being difficult to clean and having a short service life.

[0020] 3. The support layer on the larger pore size side also acts like "scaffolding," providing a backing for the relatively fragile separation layer on the smaller pore size side. This allows it to withstand higher transmembrane pressure differentials and pressure fluctuations during operation, thereby improving the overall mechanical strength and durability of the filter membrane and making it more suitable for long-term stable operation in harsh industrial environments. In contrast, most existing filter membranes are dense and thin, making them relatively fragile and prone to damage under actual operating pressures.

[0021] 4. The filter membrane of this invention has relatively low overall filtration resistance (the main resistance comes from the separation layer on the small pore size side), and the operating pressure required to achieve the designed water production is also relatively low, thereby reducing the energy consumption of power systems such as pumps and meeting the requirements of energy conservation and environmental protection. In contrast, most existing filter membranes are dense layers with small pore sizes, resulting in higher operating pressures and higher energy consumption. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 is a scanning electron microscope image of the filter membrane obtained in Example 1.

[0024] Figure 2 is a pore size distribution diagram of the filter membrane obtained in Example 1. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0026] Example 1 This invention provides a method for preparing a filter membrane with a gradient arrangement of pore sizes. The preparation method includes the following steps: (1) Raw material pretreatment: Polyethersulfone (PES) powder is placed in a forced-air drying oven, the temperature is set to 105℃, dried for 2 hours, and then taken out and cooled to room temperature for later use; the remaining raw materials, dimethylacetamide (DMAC), polyethylene glycol (PEG-400), and povidone K30 (PVP-K30), are all subjected to anhydrous drying to remove impurities and moisture. The moisture adsorbed by the polyethersulfone powder and the trace moisture in the raw materials are removed to avoid moisture affecting the dissolution uniformity of the casting solution and to prevent defects such as bubbles and pinholes in the membrane pores during the subsequent membrane preparation process.

[0027] (2) Preparation of casting solution: (2-1) Weigh 70g of pretreated DMAC, 20g of dried PES powder, 10g of PEG-400, and 10g of PVP-K30 according to a mass ratio of 70:20:10:10; (2-2) Add DMAC to a reaction flask equipped with a temperature control device and a mechanical stirrer, start the stirrer, set the speed to 300r / min, and slowly add PES powder while stirring; (2-3) After the PES powder is completely dispersed, add PEG-400 and PVP-K30, raise the temperature to 80℃, and maintain this temperature while stirring for 8 hours. h; (2-4) Turn off the stirring and transfer the mixture in the reactor to the ultrasonic degassing device. Set the ultrasonic frequency to 40kHz and degas for 30min. Ultrasonic degassing removes tiny air bubbles in the solution to avoid pore defects after the membrane fibers are formed. (2-5) Pour the degassed casting solution into a sealed spinning tank and let it stand at room temperature for 12h after sealing. Standing for 12h can eliminate the mechanical stress inside the casting solution, improve the stability of the casting solution during spinning, and ensure the uniformity of the membrane fiber size.

[0028] (3) Preparation of core solution (3-1) Based on a dimethylacetamide concentration of 70wt%, weigh 70g of DMAC and 30g of deionized water, and add them sequentially to a beaker equipped with a magnetic stirrer; (3-2) Start the magnetic stirrer, set the speed to 200r / min, and stir continuously for 30min until the solution is completely transparent and there is no layering phenomenon, thus obtaining the core solution. After sealing, place it in a constant temperature environment of 20℃ for later use; The high DMAC concentration of the core solution provides slow phase separation conditions for the inner side of the membrane, laying the foundation for the formation of the macroporous support layer on the inner side of the membrane, while maintaining the stability of the internal structure of the membrane fibers.

[0029] (4) Preparation of gel bath (4-1) Weigh 40g of DMAC and 60g of deionized water according to the dimethylacetamide concentration of 40wt%, and add them to the gel bath tank; (4-2) Start the stirring device in the tank, set the speed to 150r / min, and stir for 40min to ensure that DMAC and water are fully mixed; (4-3) Stabilize the temperature of the gel bath at 20℃ through the temperature control system, and continue stirring to keep the gel bath concentration uniform; The gel bath concentration is lower than the core liquid, forming a solvent concentration gradient between the inner and outer sides, driving the rapid solvent-non-solvent exchange between the casting liquid and the gel bath, promoting rapid phase separation on the outer side of the membrane, and forming a dense small-pore separation layer.

[0030] (5) Spinning preparation (5-1) Check whether the spinneret nozzle is smooth and unblocked, install it in the designated position of the spinning equipment, calibrate the coaxiality of the spinneret nozzle and the core liquid outlet to ensure that the casting liquid and the core liquid are in synchronous contact; ensure that the spinneret nozzle is in good condition to avoid problems such as uneven thickness and surface burrs in the film filaments; (5-2) Connect the casting liquid and the spare core liquid in the spinning tank to the corresponding feed channels of the spinning equipment, adjust the casting liquid feed pressure to 0.3MPa and the core liquid feed pressure to 0.1MPa, and perform pressure pre-stabilization for 30min; calibrate the coaxiality and stabilize the feed pressure to ensure that the casting liquid and the core liquid form a uniform initial structure of film filaments, providing a stable premise for the subsequent formation of gradient aperture.

[0031] (6) Preparation of preliminary membrane fibers (6-1) Start the gel bath circulation system to maintain the gel bath liquid level height is stable, turn on the take-up machine and set the take-up speed to 20m / min; (6-2) Start the spinning equipment. After the casting liquid is extruded through the spinneret, it comes into contact with the core liquid and the gel bath simultaneously, and phase separation occurs to form preliminary membrane fibers; (6-3) The preliminary membrane fibers run continuously in the gel bath tank. After solidification and molding, they are pulled into the take-up tank by the take-up machine. The take-up speed is controlled at 20m / min to match the phase separation rate and ensure that the pore size gradient can be fully formed during the membrane fiber forming process. In addition, the length design of the gel bath tank ensures that the membrane fibers have enough solidification time to stabilize the gradient pore size structure.

[0032] (7) Boiling and cleaning: Transfer the initial membrane fibers in the take-up tank to the continuous boiling equipment, set the water temperature to 60℃, control the membrane fiber running speed to 1m / min, and continuously boil and clean for 30min. During the cleaning process, deionized water is continuously introduced to keep the water flowing. By cleaning with 60℃ warm water, the residual DMAC solvent and incompletely volatilized additives inside the membrane fibers are removed to avoid the residual substances affecting the membrane separation performance and biosafety.

[0033] (8) Glycerin Immersion and Preservation: Immediately immerse the membrane fibers after boiling and cleaning in a 20% (mass fraction) glycerin aqueous solution, control the immersion temperature at 25℃, and immerse for 12 hours. After immersion, remove the membrane fibers, allow the surface moisture to drain naturally, vacuum pack them in a sealed bag, and store them in a cool and dry environment (temperature 15-25℃, relative humidity ≤60%). The shelf life is 1 year. Glycerin, as a pore-retaining agent, can maintain the stability of the membrane pore structure and prevent the pore size from shrinking or collapsing after the membrane fibers dry. Vacuum packaging and a specific storage environment can prevent the membrane fibers from getting damp and contaminated, thus extending their shelf life.

[0034] During membrane fabrication, the casting solution comes into contact with the inner core solution and the outer gel bath through the spinneret. The core solution is a 70 wt% DMAC aqueous solution, and the gel bath is a 40 wt% DMAC aqueous solution. Phase-liquid separation occurs in the casting solution. Due to the difference in solvent content (between 20% and 50%), the phase-liquid separation rates on the inner and outer sides are also different, resulting in a gradually denser membrane pore structure arranged in a gradient from the inner to the outer side.

[0035] The scanning electron microscope image of the filter membrane obtained in this embodiment is shown in Figure 1. As can be seen from the figure, the pore size of the filter membrane is arranged in a gradient.

[0036] Figure 2 shows the pore size distribution. As can be seen from Figure 2, the pore size is mainly concentrated in the range of 0.8µm to 1.2µm, indicating that it is a microfiltration membrane with a uniform pore size distribution.

[0037] Example 2 This invention provides a method for preparing a filter membrane with a gradient arrangement of pore sizes. The preparation method is basically similar to that in Example 1. The preparation method includes the following steps: (1) Preparing the casting solution: 70g of dimethylacetamide (DMAC), 20g of polyethersulfone powder (PES), 10g of polyethylene glycol (PEG-400) and 10g of PVP-K30 are stirred at 80°C for 8h. After the polymer is completely dissolved, ultrasonic degassing is performed. After degassing, the casting solution is obtained. The casting solution is poured into a spinning tank and left to stand for 12h; (2) Preparing the core solution: Dimethylacetamide is mixed with water. (2) Prepare a dimethylacetamide aqueous solution with a concentration of 80wt% to obtain the core liquid; (3) Prepare a gel bath: Mix dimethylacetamide with water to prepare a dimethylacetamide aqueous solution with a concentration of 30wt% to obtain a gel bath; (4) Prepare a filter membrane: The casting solution is brought into contact with the core liquid and the gel bath through the spinneret. The take-up speed is set to 20m / min. The membrane fibers pass through the gel bath to the take-up tank. The produced membrane fibers are boiled and washed at 60℃. Then, they are soaked in a 20% glycerol aqueous solution for long-term preservation (validity period of 1 year) to obtain the filter membrane with the pore size arranged in a gradient.

[0038] During the membrane fabrication process, by adjusting the DMAC content on both the inner and outer sides, compared with Example 1, the outer side is adjusted from 40% to 30% and the inner side is adjusted from 70% to 80%, the gradual rate of change of membrane pore size can be controlled, and a high-flux membrane with more macroporous support layers and fewer dense separation layers can be obtained.

[0039] Example 3 This invention provides a method for preparing a filter membrane with a gradient arrangement of pore sizes. The preparation method is basically similar to that in Example 1. The preparation method includes the following steps: (1) Preparing the casting solution: 70g of dimethylacetamide (DMAC), 20g of polyethersulfone powder (PES), 10g of polyethylene glycol (PEG-400) and 10g of PVP-K30 are stirred at 80°C for 8h. After the polymer is completely dissolved, ultrasonic degassing is performed. After degassing, the casting solution is obtained. The casting solution is poured into a spinning tank and left to stand for 12h; (2) Preparing the core solution: Dimethylacetamide is mixed with water. (2) Prepare a 60wt% dimethylacetamide aqueous solution to obtain the core liquid; (3) Prepare a gel bath: Mix dimethylacetamide with water to prepare a 50wt% dimethylacetamide aqueous solution to obtain a gel bath; (4) Prepare a filter membrane: The casting solution is brought into contact with the core liquid and the gel bath through a spinneret. The take-up speed is set to 20m / min. The membrane fibers pass through the gel bath to the take-up tank. The produced membrane fibers are boiled and washed at 60℃, and then soaked in a 20% glycerol aqueous solution for long-term preservation (validity period of 1 year), thus obtaining the filter membrane with the pore size arranged in a gradient.

[0040] During the membrane fabrication process, by adjusting the DMAC content on both the inner and outer sides—from 40% to 50% on the outer side and from 70% to 60% on the inner side compared to Example 1—the rate of change from large to small pore size can be controlled. This results in a fouling-resistant membrane with more dense layers and fewer macroporous support layers.

[0041] The filter membranes obtained in Example 1 and Examples 1-3 had a retention rate of approximately 95% for bovine serum albumin (67 kDa) and a retention rate of 99.6% for PEG 20W (200 kDa).

[0042] 2. The pure water flux of the filter membranes obtained in Examples 1-3, measured by the dead-end filtration method, is 667-807 LMH / bar.

[0043] 3. The mechanical strength of the filter membranes obtained in Examples 1-3 is expressed by the elongation at break tested by a tensile tester: the maximum average force at break is 4.5N, and the average elongation is 93%.

[0044] 4. The pressure resistance test of the filter membranes obtained in Examples 1-3 was conducted in a pure water environment with a pressure of 60 kPa. After 240 minutes of high-pressure operation, the pure water flux decreased from 667 LMH / bar to 543 LMH / bar.

[0045] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a filter membrane with a gradient arrangement of pore sizes, characterized in that, The preparation method includes the following steps: (1) preparing casting solution: dimethylacetamide, polyethersulfone, polyethylene glycol and additives are mixed, dissolved and degassed to obtain casting solution; (2) preparing core solution: dimethylacetamide is mixed with water to obtain core solution; (3) preparing gel bath: dimethylacetamide is mixed with water to obtain gel bath; the concentration of the gel bath is less than the concentration of the core solution; (4) preparing filter membrane: the casting solution is brought into contact with the core solution and the gel bath through a spinneret, and the membrane fibers pass through the gel bath to the take-up groove to obtain the filter membrane with the pore size arranged in a gradient.

2. The method for preparing a filter membrane with gradient pore sizes according to claim 1, characterized in that, In step (1), the additive is povidone K30.

3. The method for preparing a filter membrane with gradient pore sizes according to claim 1, characterized in that, In step (1), the mass ratio of dimethylacetamide, polyethersulfone, polyethylene glycol, and additives is 65-75:15-25:5-15:5-15.

4. The method for preparing a filter membrane with gradient pore sizes according to claim 1, characterized in that, In step (1), the mixing and dissolving temperature is 75-85℃ and the mixing and dissolving time is 7-9h.

5. The method for preparing a filter membrane with gradient pore sizes according to claim 1, characterized in that, In step (2), the concentration of dimethylacetamide in the core fluid is 60-80 wt%.

6. The method for preparing a filter membrane with gradient pore sizes according to claim 1, characterized in that, In step (3), the concentration of dimethylacetamide in the gel bath is 30-50 wt%.

7. The method for preparing a filter membrane with gradient pore sizes according to claim 1, characterized in that, In step (4), the take-up speed is set to 15-25 m / min.

8. The method for preparing a filter membrane with gradient pore sizes according to claim 1, characterized in that, Step (4) also includes a water boiling and soaking process: after the membrane fibers pass through the gel bath to the fiber taking tank, the produced membrane fibers are boiled and washed at 55-65℃, and then soaked in a 15-25% glycerol aqueous solution to obtain the filter membrane with the pore size arranged in a gradient.

9. A filter membrane with a gradient arrangement of pore sizes obtained by the preparation method according to any one of claims 1-8.

10. The filter membrane according to claim 9, characterized in that, The pore size of the filter membrane varies in a gradient from a large pore on one side to a small pore on the other side.

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