Microfiltration membrane and preparation method therefor

Abstract

Provided in the present application are a microfiltration membrane and a preparation method therefor, belonging to the technical field of membrane separation. The preparation method comprises blending a hydrophilic polymer and a bulk polymer to prepare a casting solution, and using a phase inversion method to prepare a microfiltration membrane having a sponge-like porous structure with asymmetric continuous gradient change, without changing the solid content of the casting solution.

Description

Microfiltration membrane and preparation method thereof

[0001] The present application claims priority to the Chinese patent application No. 2025103928231, filed on March 31, 2025, and entitled "A high-toughness and high-flux microfiltration membrane and a preparation method thereof", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the field of membrane separation technology, and relates to a microfiltration membrane and a preparation method thereof. BACKGROUND

[0003] Microfiltration membrane mainly refers to a filtration membrane with a pore size of 0.1-10 μm, especially a microfiltration membrane with a pore size of 0.1 μm, 0.2 μm, 0.45 μm, 0.65 μm, 0.8 μm, 1.0 μm, 2.0 μm and 3.0 μm, etc. Due to its precise filtration accuracy, the microfiltration membrane plays an indispensable role in key links such as mycoplasma, bacterial microorganism interception and microparticle removal. Among them, filtration membranes with different pore sizes or nominal diameters can be used to assemble different series of filters: 0.1 μm double-layer combined membrane filter for mycoplasma removal; 0.2 μm, 0.45 μm / 0.2 μm, 0.65 μm / 0.2 μm, etc. combined membrane filter for bacteria removal; 0.45 μm, 0.65 μm, 0.8 μm, 1.0 μm, 2.0 μm and 3.0 μm, etc. membrane filter for removing large particles. However, the existing traditional filtration materials have deficiencies in filtration efficiency, service life and mechanical strength, etc., and are difficult to meet the requirements of modern industry and medical field for bacteria removal filtration.

[0004] At present, the preparation methods of organic polymer microfiltration membranes mainly include phase inversion method, stretching method, sintering method, etc. Among them, the phase inversion method is widely used due to its relatively simple preparation process, wide application range, diverse membrane materials and controllability of membrane structure, etc., which greatly promotes the development of membrane technology. According to different principles, the phase inversion method can be divided into non-solvent induced phase separation (NIPS), solvent evaporation induced phase separation (VIPS), thermal induced phase separation (TIPS) and reverse thermal induced phase separation (R-TIPS), etc.

[0005] NIPS method is the most commonly used and important method for preparing polymer membranes by phase inversion: after the prepared polymer solution or casting solution is coated or cast on a support (such as a glass plate, non-woven fabric, etc.), it is immediately immersed in a non-solvent coagulation bath to solidify into a film. For example, CN111804148A discloses a high porosity hydrophilic microporous membrane, a preparation method and an application. However, the phase inversion process of NIPS method for membrane preparation is difficult to accurately control due to the great influence of solvent evaporation rate on the structure and performance of the membrane, and the prepared membrane has a wide pore size distribution and the mechanical strength needs to be improved. In particular, microfiltration membranes with larger pore sizes usually use the method of reducing the solid content of the polymer. CN201810044124.8 discloses a preparation method of open-cell fluorine-containing porous material with an average pore size of 2-5 μm based on the induced thickening effect of polylactic acid, which adjusts the pore size by changing the content or molecular weight of polylactic acid. However, due to the trade-off effect between polymer content and mechanical properties of the microporous membrane, the mechanical strength is reduced, which is difficult to meet the application requirements.

[0006] VIPS method is to dissolve the polymer in a specific organic solvent to prepare a homogeneous casting solution, and then place the nascent film in a non-solvent vapor (such as water vapor) atmosphere to induce phase separation of the polymer solution, forming a solid phase rich in polymer and a liquid phase rich in solvent. Finally, the liquid phase is removed by evaporation or extraction to obtain a polymer membrane with microporous structure. However, the air humidity of VIPS method is difficult to control uniformly and stably, and the process running speed is slow, resulting in poor uniformity of the pore structure.

[0007] TIPS method is based on temperature change to induce phase inversion. TIPS method is to mix the polymer and diluent (which can be a solvent or a small molecule additive) at high temperature to form a uniform solution. When the temperature decreases, the compatibility between the polymer and the diluent changes, causing phase separation into a film. CN112316756A discloses a method for preparing a high-strength, high-rejection hollow fiber membrane using polyvinylidene fluoride (PVDF) by TIPS method. TIPS method can effectively enhance the compatibility between fluorine-containing polymer and polyamide and improve the hydrophilicity, strength and retention rate of the membrane, but this method has a relatively slow phase separation process and requires strict temperature control during preparation.

[0008] R-TIPS method is also based on temperature change to induce phase transition. R-TIPS method is the phase separation behavior of polymer at a specific temperature. When the casting solution is heated above its cloud point (phase separation temperature), the solution will undergo phase separation, forming a polymer-rich phase and a polymer-poor phase, and finally solidifying into a film. For example, CN103055724B discloses a method for preparing a polysulfone-based polymer microporous membrane by reverse thermal induced phase separation. The film prepared by R-TIPS method will form a more dense or special pore structure film during the heating process due to the rapid solidification of the polymer. However, the casting solution system suitable for R-TIPS method has limitations and requires high cloud point temperature.

[0009] Currently, the membrane materials used for bacteria removal microfiltration membranes include polyvinylidene fluoride (PVDF), regenerated cellulose (CA), polysulfone (PSF) and polyether sulfone (PES), etc. Among them, PES has high thermal stability and can be used at relatively high temperature without performance degradation, which is suitable for some bacteria removal processes with temperature requirements. It has excellent chemical stability and can resist corrosion of various chemicals. The pore-forming performance of the membrane is good, and it is easy to prepare a bacteria removal microfiltration membrane with appropriate pore size and porosity by phase inversion and other membrane preparation methods, thereby meeting the needs of different precision bacteria retention. However, PES is a hydrophobic polymer, and its poor hydrophilicity leads to serious protein adsorption, low membrane flux and low protein recovery rate. In order to improve the permeation and separation performance and anti-fouling performance of the microfiltration membrane, the microfiltration membrane needs to be properly modified for hydrophilicity.

[0010] Common methods for hydrophilic modification of microfiltration membranes include surface coating, blending modification and irradiation grafting. Among them, surface coating modification is to coat a hydrophilic polymer or coating on the surface of the membrane to improve its hydrophilicity. For example, US4413074A discloses a hydrophobic polymer (PES) membrane matrix, the surface of which is coated with hydroxyalkyl cellulose and treated with water vapor to form a hydrophilic surface, which can have a large pure water flux. However, the effect of surface coating modification will gradually decrease with the extension of storage time, and the stability needs to be improved.

[0011] Blending modification is to add inorganic, organic or amphoteric modified particles to the polymer system, and the modified particles and the bulk polymer are blended and fused to improve the hydrophilicity of the filter membrane. Among them, direct blending of hydrophilic membrane materials is a preferred method. For example, CN109603592A discloses a method for hydrophilic modification of polyether sulfone microporous filter membrane. In the membrane preparation process, a modifier containing free hydroxyl groups is added to the coagulation bath, thereby introducing hydrophilic hydroxyl groups into the polyether sulfone microporous filter membrane, and improving the hydrophilic performance of the filter membrane.

[0012] Irradiation grafting is a method of initiating grafting reactions on the surface of a membrane by high-energy radiation, such as gamma rays, electron beams, and ultraviolet radiation. For example, US5409524 discloses a method for preparing a gas separation membrane. The membrane is made of a plurality of different polymers. The membrane is placed in an oxygen environment and irradiated with ultraviolet (UV) radiation for a period of time that allows grafting reactions to occur on the surface of the membrane. After this treatment, the gas separation selectivity of the membrane is significantly improved.

[0013] Currently, in the filtration process, microfiltration membranes not only face the problems of insufficient hydrophilicity and severe membrane fouling, but also have requirements for mechanical properties. Under high protein concentration and high operating pressure conditions, the hydrophilicity of PES membranes may not be sufficient to effectively resist protein adsorption, leading to flux decay and intensified membrane fouling. At the same time, the breaking strength and elongation at break (toughness) are insufficient, and breakage or deformation easily occurs under high pressure, affecting the stability of filtration and the service life of the membrane. In addition, there are challenges in the durability and chemical stability of the modification effect, increasing the operating cost and maintenance difficulty. These problems limit the long-term stable operation and wide application of PES membranes under harsh conditions such as high protein and high pressure.

[0014] EP1149624B1 discloses a highly asymmetric anion membrane and a method for preparing the same, which can form a polymer membrane with permanent internal anionic charge. The membrane prepared based on this method can be used as a microfiltration membrane, and the average flow pore size range can be less than about 0.1 μm, about 0.2 μm, or between about 0.3-1.0 μm. The membrane has a typical V-shaped asymmetric filter membrane structure, and the flow channel diameter of the porous support structure gradually increases from the first surface to the second surface. The large pore region near the second surface provides a larger space for trapping impurities, which can pre-entrap and accommodate large particles in the feed liquid, thereby effectively reducing the risk of large particles blocking the pore structure of the membrane; while the small pore region near the first surface ensures efficient entrapment of small particle impurities through smaller pore size, realizing the function of graded filtration of impurities of different particle sizes. However, the microfiltration membrane prepared by this method has insufficient hydrophilicity, small flux, and low protein flux.

[0015] In summary, the current microfiltration membranes have the problem of a seesaw effect between insufficient membrane toughness due to large pore size and insufficient hydrophilicity leading to high protein adsorption and low protein flux, which poses a stability risk during filter processing and actual use. The existence of these problems also limits the development of microfiltration membranes to some extent. SUMMARY

[0016] In order to overcome some problems existing in the prior art, the application provides a preparation method of a microfiltration membrane, which adopts a hydrophilic polymer and a bulk polymer to blend and prepare a casting solution, and can prepare a microfiltration membrane with an asymmetric continuous gradient sponge-like pore structure by phase inversion combined with a pretreatment method without changing the solid content of the casting solution.

[0017] It can be understood that the above method can prepare a microfiltration membrane with different pore diameters by phase inversion without changing the solid content of the casting solution, effectively solving the seesaw effect between large pore diameter and insufficient membrane toughness. Specifically, the bulk polymer is blended with a hydrophilic polymer, a solvent, an additive and the like to prepare a casting solution, the bulk polymer is blended with a hydroxyl group and a sulfonic acid group polymer with a hydrophilic group to modify and prepare a casting solution, a microfiltration membrane with an asymmetric sponge-like structure is obtained by phase inversion, and the hydrophilicity is improved. By adjusting the thermodynamics and film-forming kinetics of the casting solution, the asymmetric sponge-like structure of the microfiltration membrane can be used to intercept mycoplasma, bacteria and microparticles. In particular, the intermolecular and intramolecular forces between the bulk polymer and the hydrophilic polymer significantly improve the mechanical properties of the microfiltration membrane, and finally a high-toughness microfiltration membrane with narrow pore size distribution, high bacterial retention rate and high filtration flux of the feed liquid is obtained.

[0018] Optionally, the bulk polymer is selected from at least one of the following group: polyvinyl chloride (PVC), polysulfone (PSF), polyether sulfone (PES) and polyvinylidene fluoride (PVDF); and the hydrophilic polymer is selected from at least one of the following group: sulfonated polyether sulfone (SPES), hydroxyl-terminated polyether sulfone (PES-OH) and polyether sulfone-polyethylene glycol block copolymer (PES-b-PEG).

[0019] When SPES (sulfonation degree: 10-30%) is selected as the hydrophilic modification material, the sulfonic acid group and other hydrophilic groups are introduced by sulfonation reaction, so that the hydrophilicity is significantly improved, and the water contact angle on the surface of the material can be effectively reduced. However, it is theoretically calculated that SPES (sulfonation degree: 10-30%) and PES are a partially compatible system, and the introduction of sulfonic acid groups will reduce the strength of the microfiltration membrane, and too much SPES will cause the strength of the microfiltration membrane to deteriorate. Therefore, the preferred addition amount of SPES in the casting solution includes but is not limited to 1-30 wt.%.

[0020] When PES-OH (or PES-b-PEG) is used alone as the hydrophilic modification material, a higher mass ratio of PES-OH (or PES-b-PEG) is added in the system or pure PES-OH (or PES-b-PEG) is used to prepare the casting solution (PES-OH / PES blending mass ratio > 50 / 50 wt. / wt.) to introduce more hydrophilic groups. However, the molecular weight of PES-OH is relatively low, the casting solution has low viscosity, the film forming effect is poor, and the strength of the film is very low. When a small mass ratio of PES-OH (or PES-b-PEG) (PES-OH / PES blending ratio < 50 / 50 wt. / wt.) is added in the system for blending hydrophilic modification, the number of hydrophilic groups grafted in PES-OH or PES-b-PEG is small, and thus the hydrophilic modification effect is poor.

[0021] Optionally, the hydrophilic polymer is selected from at least two of the following group: SPES, PES-OH and PES-b-PEG; two or more of the hydrophilic polymers are selected to achieve better modification effect.

[0022] Optionally, the mass percentage of each component in the casting solution is as follows: bulk polymer: 14% to 16% (for example, it can be 14%, 15%, 16%, etc.), hydrophilic polymer: 1% to 6% (for example, it can be 2%, 3%, 4%, 5%, etc.), solvent: 30% to 40% (for example, it can be 32%, 34%, 35%, 36%, 38%, etc.), and additive: 40% to 50% (for example, it can be 42%, 44%, 45%, 46%, 48%, etc.). Optionally, the casting solution is composed of the bulk polymer, the hydrophilic polymer, the solvent and the additive.

[0023] Optionally, the solvent in the casting solution is selected from at least one of the following group: N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methyl pyrrolidone (NMP) and dimethyl sulfoxide (DMSO).

[0024] Optionally, the additive in the casting solution is selected from at least one of the following group: polyethylene glycol (PEG), diethylene glycol (DEG), triethylene glycol (TEG), isopropyl alcohol (IPA) and polyvinylpyrrolidone (PVP).

[0025] Optionally, the phase inversion method is selected from at least one of the following methods: non-solvent induced phase separation (NIPS), solvent evaporation induced phase separation (VIPS), thermal induced phase separation (TIPS) and reverse thermal induced phase separation (R-TIPS). The microfiltration membrane prepared by using one or more of the above methods has high mechanical properties and plays a role in bacterial retention.

[0026] Optionally, the microfiltration membrane is prepared by phase inversion method combined with pretreatment method, comprising the following steps:

[0027] The bulk polymer is blended with the hydrophilic polymer, solvent and additives to prepare a casting solution, and the casting solution is coated on a carrier as a support by a doctor blade coating process or a slot die coating process to obtain a nascent membrane;

[0028] The carrier loaded with the nascent membrane is placed in an environment with a certain relative humidity and temperature, and is pretreated in the environment for a certain time to obtain a pretreated membrane;

[0029] The carrier and the pretreated membrane are immersed in a coagulation bath for phase inversion, and then are post-treated and dried, to finally obtain the microfiltration membrane.

[0030] Optionally, the microfiltration membrane is prepared by phase inversion method combined with pretreatment method, comprising the following steps:

[0031] The bulk polymer is blended with the hydrophilic polymer, solvent and additives to prepare a casting solution, and the casting solution is coated on a carrier as a support by a doctor blade coating process or a slot die coating process to obtain a nascent membrane;

[0032] The carrier loaded with the nascent membrane is placed in an environment with a certain relative humidity and temperature, and is pretreated in the environment for a certain time to obtain a pretreated membrane;

[0033] The carrier and the pretreated membrane are immersed in a coagulation bath for phase inversion, and then are post-treated and dried, to finally obtain the microfiltration membrane.

[0034] It can be understood that in the common process for preparing a single-layer microfiltration membrane, the equipment for the doctor blade coating process is relatively simple and easy to operate, and the thickness of the membrane can be accurately controlled by adjusting the gap of the doctor blade and the coating speed; the slot die coating process can also accurately control the coating amount and the thickness of the membrane, ensuring that the thickness of the microfiltration membrane is uniform and the performance is consistent, and the material utilization rate is high, effectively reducing the cost.

[0035] Optionally, more specifically, the prepared casting solution is uniformly coated on the carrier by a coating device (such as a coating machine) at a certain speed. The carrier loaded with the nascent membrane is placed in an air bath with a relative humidity of 30-70% RH and a temperature of 25-40°C, and is pretreated in the air bath for 3-60 s to obtain the pretreated membrane.

[0036] Optionally, then the carrier and the pretreated membrane are immersed in a coagulation bath at 30-70°C for phase inversion, and then are immersed in hot water at 40-60°C for sufficient water washing to remove the residual solvent in the membrane, and then are dried at 75-95°C, to finally obtain the microfiltration membrane with an asymmetric structure.

[0037] Optionally, the viscosity of the casting solution is in the range of 3000 mPa·s-8000 mPa·s at 25°C.

[0038] Optionally, the carrier is selected from at least one of the following group: polyethylene (PE), polypropylene (PP), polycarbonate (PC) and polyester (PET).

[0039] Optionally, the temperature of the coagulation bath is 30-70℃; the composition of the coagulation bath is selected from at least one of the following group: water, ethanol, DMF, DMAc and NMP.

[0040] Optionally, the residence time in the air bath, i.e. the running speed of the coating equipment, includes but is not limited to any speed in the range of 5-15 m / min, which affects the phase inversion rate of the microfiltration membrane by adjusting the running speed of the coating equipment (such as a coating machine), and has a stretching effect on the membrane sheet, thus obtaining microfiltration membranes with different pore size distributions.

[0041] Optionally, the structure of the microfiltration membrane is a non-symmetrical continuous gradient sponge-like pore structure, characterized in that: the microfiltration membrane includes an upper surface and a lower surface in the thickness direction of the membrane, and the pore size of the microfiltration membrane decreases continuously from the upper surface to the lower surface, the upper surface is a loose porous large-pore support layer, the pore size of the intermediate transition layer gradually decreases, and the lower surface forms a dense small-pore functional layer. The large-pore and small-pore are relative; the loose and dense are also relative. The pore size of the microfiltration membrane decreases from the upper surface to the lower surface; wherein, in use, the upper surface is the liquid inlet end and the lower surface is the liquid outlet end; in preparation, relatively, the upper surface is close to the carrier and the lower surface is away from the carrier.

[0042] Optionally, without changing the solid content of the casting solution, microfiltration membranes with different nominal diameters can be prepared, the nominal diameter being selected from any one of the following group: 0.1 μm, 0.2 μm, 0.45 μm, 0.65 μm, 0.8 μm, 1.0 μm, 2.0 μm and 3.0 μm. In this application, not changing the solid content of the casting solution means that the mass percentage of the hydrophilic polymer and the bulk polymer in the casting solution can not be changed (for example, the bulk polymer is maintained at 14% and the hydrophilic polymer is maintained at 5%), and different nominal diameters of microfiltration membranes can be constructed by changing the mass percentage of other components (such as solvents and additives). In addition, nominal diameter is a commonly used term in the art, which is applicable to porous membranes such as microfiltration and ultrafiltration, and describes the nominal pore size of the membrane. Each company can define its own nominal diameter according to the actual pore size of the produced membrane, which is used to distinguish the separation precision of the membrane, and refers to the collection of average pore sizes within a certain average pore size range.

[0043] Optionally, the average pore size of the microfiltration membrane is 0.1 μm-3.5 μm. The thickness of the microfiltration membrane is 130-160 μm, and the porosity is 75-90%.

[0044] It can be understood that the microfiltration membrane with a nominal diameter of 0.1 μm has a particle size range of 1.20-2.78 μm on the upper surface and a particle size range of 0.18-0.25 μm on the lower surface in SEM (Scanning Electron Microscope); the microfiltration membrane with a nominal diameter of 0.2 μm has a particle size range of 1.57-2.94 μm on the upper surface and a particle size range of 0.23-0.40 μm on the lower surface; the microfiltration membrane with a nominal diameter of 0.45 μm has a particle size range of 1.98-3.17 μm on the upper surface and a particle size range of 0.40-0.58 μm on the lower surface; the microfiltration membrane with a nominal diameter of 0.65 μm has a particle size range of 2.03-3.42 μm on the upper surface and a particle size range of 0.53-0.71 μm on the lower surface; the microfiltration membrane with a nominal diameter of 0.8 μm has a particle size range of 2.01-3.54 μm on the upper surface and a particle size range of 0.66-0.89 μm on the lower surface; the microfiltration membrane with a nominal diameter of 1.0 μm has a particle size range of 2.10-3.71 μm on the upper surface and a particle size range of 0.78-1.13 μm on the lower surface; the microfiltration membrane with a nominal diameter of 2.0 μm has a particle size range of 2.12-3.80 μm on the upper surface and a particle size range of 1.60-2.22 μm on the lower surface; and the microfiltration membrane with a nominal diameter of 3.0 μm has a particle size range of 2.92-4.11 μm on the upper surface and a particle size range of 2.62-3.37 μm on the lower surface.

[0045] Optionally, the microfiltration membrane has a pure water permeability of 10,000-70,000 LMH bar -1 , a breaking strength of 3.0-6.0 MPa, and an elongation at break of 20-50%.

[0046] Optionally, the microfiltration membrane has a protein recovery rate of >99% and a bacterial retention rate of 1×10 ^7 CFU / cm 2 above (i.e., a log reduction value LRV≥7).

[0047] Compared with the prior art, the application has the following advantages and positive effects:

[0048] The method for preparing the microfiltration membrane according to at least one embodiment of the application uses bulk polymer blending modification with hydrophilic polymers containing sulfonic acid or hydroxyl groups, significantly improves the mechanical properties of the microfiltration membrane, enhances the compression resistance, and improves the hydrophilicity by means of the intermolecular and intramolecular forces (such as hydrogen bonds) between the bulk polymer and the hydrophilic polymer.

[0049] The preparation method of the microfiltration membrane provided by at least one embodiment of the present application can prepare microfiltration membranes with different pore sizes without changing the solid content of the casting solution by one or more of the NIPS, VIPS, TIPS and R-TIPS methods. By adjusting the thermodynamics to reduce the proportion of closed pores and adjusting the kinetics to change the size of the pores, the seesaw effect between the pore size and the mechanical properties is effectively solved, and the mechanical strength deficiency of the microfiltration membrane prepared by the single NIPS method is compensated.

[0050] The microfiltration membrane provided by at least one embodiment of the present application can effectively improve the hydrophilic performance of the microfiltration membrane by introducing hydrophilic groups into the microfiltration membrane, which can reduce the adsorption of bacteria and proteins and improve the permeation flux, and accordingly the protein yield is also improved.

[0051] The microfiltration membrane provided by at least one embodiment of the present application can improve the bacterial retention rate because the introduction of hydrophilic polymers (acidic groups such as sulfonic acid groups and carboxyl groups or basic groups such as amino groups) into the microfiltration membrane can increase the charge of the microfiltration membrane, the electrostatic interaction between the charged groups and the charge on the surface of the bacteria prevents the bacteria from passing through the membrane pores, thereby improving the bacterial retention rate; and the hydrophilic polymer can reduce the free energy of the membrane surface, making it difficult for bacteria to adhere and grow on the membrane surface, so that the microfiltration membrane has good bacterial retention performance.

[0052] The microfiltration membrane and the preparation method thereof provided by at least one embodiment of the present application have high mechanical strength, the structure is more closely integrated without obvious interface, the filtration resistance can be effectively reduced, the protein permeation rate can be improved, the flux and the maximum processing capacity can be improved while maintaining a high bacterial retention rate and a low protein adsorption amount, and the processing time can be shortened. The microfiltration membrane is widely used in the separation and purification of feed liquid in the field of biopharmaceuticals, such as the removal of microorganisms, mycoplasma and fine particles. BRIEF DESCRIPTION OF DRAWINGS

[0053] FIG. 1 is a SEM schematic diagram of the cross section of the microfiltration membrane of Example 1, with a magnification of 450x;

[0054] FIG. 2 is a SEM schematic diagram of the upper surface of the microfiltration membrane of Example 1, with a magnification of 5,000x;

[0055] FIG. 3 is a SEM schematic diagram of the lower surface of the microfiltration membrane of Example 1, with a magnification of 5,000x;

[0056] FIG. 4 is a SEM schematic diagram of the cross section of the microfiltration membrane of Example 4, with a magnification of 450x;

[0057] FIG. 5 is a SEM schematic diagram of the upper surface of the microfiltration membrane of Example 4, with a magnification of 5,000x;

[0058] Figure 6 is a SEM image of the lower surface of the microfiltration membrane of Example 4 at a magnification of 5,000x;

[0059] Figure 7 is a comparison chart of the pore size distribution of the microfiltration membranes obtained in Example 5;

[0060] Figure 8 is a comparison chart of the mechanical properties of the microfiltration membranes obtained in Examples 1-5;

[0061] Figure 9 is a comparison chart of the mechanical properties of the microfiltration membranes obtained in Comparative Examples 1-3. DETAILED DESCRIPTION

[0062] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0063] The performance and physical parameter testing methods of the examples and comparative examples are as follows:

[0064] (1) Thickness test: A thickness gauge was used to randomly sample 10 points on the prepared microfiltration membrane and calculate the average value.

[0065] (2) Pure water permeability: The microfiltration membrane prepared in the present application was clamped in a membrane cell with an effective area of 7.065 cm 2 , and was pre-pressed for 30 min under a pressure of 1.5 bar to stabilize the pure water permeability, and then the pressure was adjusted to 1 bar for testing. The pure water permeability can be obtained by the following formula:

[0066] wherein PWP is the pure water permeability (LMH bar -1 , i.e. L m -2 h -1 bar -1 ), V is the volume of the filtrate (L), A is the effective area of the membrane (m 2 ), and t is the test time (h).

[0067] (3) Average pore size test: A gas-liquid interface pore size analyzer of Porometer 1000L model was used to test the average pore size and pore size distribution of the microfiltration membrane prepared in the present application.

[0068] (4) Mechanical properties: The microfiltration membrane prepared in the present application was made into a sample of 5x50 mm, and a precision tensile tester was used to test the breaking strength and breaking elongation of the membrane at a stretching speed of 5 mm / min.

[0069] (5) Protein recovery rate: Prepare a BSA (Bovine Serum Albumin) solution with a certain standard concentration, remove particles and protein aggregates through pre-filtration, then use a dead-end filtration device for testing. The concentration of the protein is tested by ultraviolet spectrophotometry (SHIMADZU, UV-2600) at 280 nm, and the recovery rate can be calculated by the following formula:

[0070] Wherein, R is the recovery rate of the protein, C p is the concentration of BSA in the filtrate, and C0is the concentration of BSA in the original solution.

[0071] (6) Bacterial retention challenge: Brevundimonas diminuta (ATCC19146) is used as the retained bacteria for bacterial retention challenge testing using a constant flow filtration device to test the LRV (Log Reduction Value) of the microfiltration membrane. The bacterial retention rate can be calculated by the following formula:

[0072] Wherein, C1is the concentration of bacteria in the original solution (CFU / mL), and C2is the concentration of bacteria in the filtrate after filtration.

[0073] Example 1

[0074] The microfiltration membrane of Example 1 is prepared by a doctor blade coating process. First, the bulk polymer (PES) is blended with the hydrophilic polymer (SPES) in a certain proportion to prepare a polymer solution as the casting solution, which is then coated on the support carrier as the support, and the microfiltration membrane is prepared by phase inversion method. The advantage of Example 1 is that the introduction of hydrophilic polymer can improve the hydrophilicity of the microfiltration membrane and improve the strength of the ultrafiltration membrane. The following is a more specific description of Example 1:

[0075] PES is used as the bulk polymer, SPES (sulfonation degree 20%) is used as the hydrophilic polymer, DMF is used as the solvent, and DEG is used as the additive. The mass ratio of each component is 14:5:33:48. Stir at 70°C to completely dissolve the polymer into a transparent clear viscous solution. The final casting solution viscosity is 8000 mPa·s (25°C).

[0076] The obtained casting solution was blade-coated on a PET carrier to obtain a nascent membrane; the carrier loaded with the nascent liquid membrane was placed in an air bath with a relative humidity of 60% RH and a temperature of 25°C, and was pretreated in the air bath for 15 s to obtain a pretreated membrane; the pretreated membrane and the carrier were then immersed in a coagulation bath at 50°C for phase inversion, and then immersed in hot water at 50°C for sufficient water washing to remove residual solvents in the membrane, and then dried at 80°C to obtain a microfiltration membrane. The SEM diagrams of the cross section, upper surface and lower surface of the obtained microfiltration membrane are shown in Figures 1-3, respectively.

[0077] It was determined that the average pore size of the microfiltration membrane was 0.40 μm, the porosity was 87.0%, the breaking strength was 4.2 MPa, the breaking elongation was 35.2%, and the pure water permeability was 20600 LMH bar -1 ; more specifically, see Table 1 and Figure 8, in which the left side of the two adjacent columnar charts is the breaking strength parameter, and the right side is the breaking elongation. It can be seen that the obtained microfiltration membrane has high toughness and high flux.

[0078] Example 2

[0079] Example 2 is basically the same as Example 1, except that the raw material for the membrane is the bulk polymer (PSF) and the hydrophilic polymer (PES-OH). The following is a more specific description of Example 2:

[0080] The casting solution was prepared: PSF, PES-OH, solvent DMAc, additive DEG, etc. were prepared according to the mass ratio of 16:3:33:48 to prepare the casting solution, and the polymer was completely dissolved into a transparent clear viscous solution by stirring at 70°C. The final viscosity of the casting solution was 7000 mPa·s (25°C). The subsequent processing process is referred to

[0081] Example 1 to obtain a microfiltration membrane.

[0082] It was determined that the average pore size of the microfiltration membrane was 0.38 μm, the porosity was 86.3%, the breaking strength was 4.1 MPa, the breaking elongation was 33.8%, and the pure water permeability was 19200 LMH bar -1 ; more specifically, see Table 1 and Figure 8.

[0083] Example 3

[0084] Example 3 is basically the same as Example 1, except that the raw material for the membrane is the bulk polymer (PES) and the hydrophilic polymer (PES-b-PEG). The following is a more specific description of Example 3:

[0085] Preparation of casting solution: PES, PES-b-PEG, solvent DMF, additive TEG were mixed in a mass ratio of 15:4:33:48 to prepare the casting solution. The polymer was completely dissolved into a transparent clear viscous solution by stirring at 70°C. The viscosity of the casting solution was measured to be 7000 mPa-s (25°C).

[0086] The obtained casting solution was coated on the carrier to obtain a nascent membrane. The carrier with the nascent membrane was placed in an air bath with a relative humidity of 60% RH and a temperature of 25°C, and was pretreated in the air bath for 15 s to obtain a pretreated membrane. The pretreated membrane and the carrier were immersed in a coagulation bath at 50°C for phase inversion, and then were immersed in hot water at 50°C for sufficient water washing to remove residual solvents in the membrane, followed by drying at 80°C to obtain a microfiltration membrane.

[0087] The average pore size of the microfiltration membrane was measured to be 0.38 μm, the porosity was 84.5%, the breaking strength was 4.0 MPa, the breaking elongation was 34.7%, and the pure water permeability was 19800 LMH bar -1 ; more specifically, see Table 1 and Figure 8.

[0088] Example 4

[0089] Example 4 was basically the same as Example 1, except that the air bath had a relative humidity of 30% RH and a temperature of 30°C, and the pretreatment time was reduced to 6 s to obtain a pretreated membrane. The pretreated membrane and the carrier were immersed in a coagulation bath at 50°C for phase inversion, and then were immersed in hot water at 50°C for sufficient water washing to remove residual solvents in the membrane, followed by drying at 80°C to obtain a microfiltration membrane. The pore size of the microfiltration membrane prepared in this way was reduced. The casting solution and the membrane were prepared according to Example 1, except for the environmental temperature and humidity and the pretreatment time.

[0090] The average pore size of the microfiltration membrane was measured to be 0.22 μm, the porosity was 85.6%, the breaking strength was 4.7 MPa, the breaking elongation was 39.1%, and the pure water permeability was 10850 LMH bar -1 ; more specifically, see Table 1 and Figure 8.

[0091] Example 5

[0092] Example 5 is substantially identical to Example 1 except that the ambient humidity is 70% RH, the temperature is 30°C in an air bath, the treatment time is increased to 25 seconds to obtain the pre-treated membrane; the pre-treated membrane and the support are immersed in a coagulation bath at 50°C to perform phase inversion, and then immersed in hot water at 50°C to sufficiently wash the membrane with water to remove residual solvent, and then dried at 80°C to obtain the microfiltration membrane. The pore size of the microfiltration membrane prepared in this way is increased, and the pure water permeability is significantly increased compared with Example 1. The SEM diagrams of the cross section, the upper surface and the lower surface of the high-toughness and high-flux microfiltration membrane obtained are shown in Figures 4-6, respectively. The casting solution is prepared and the membrane is prepared according to Example 1, and the difference is the ambient humidity and the treatment time.

[0093] The average pore size of the microfiltration membrane is 0.59 μm, the porosity is 88.7%, the breaking strength is 3.5 MPa, the breaking elongation is 25.4%, and the pure water permeability is 32,000 LMH bar -1 ; more specifically, see Table 1, Figure 7 and Figure 8. Figure 7 takes five samples of the microfiltration membrane obtained in Example 5, and it is found that the pore sizes are uniform and are distributed at approximately the same position.

[0094] Comparative Example 1

[0095] Comparative Example 1 uses the phase inversion method to prepare a microfiltration membrane by pretreating the nascent liquid membrane, and the difference from Example 1 is mainly that only PES is used as the bulk polymer, and PEG200 is added as an additive, and DMF is used as the solvent. The following is a more specific description of Comparative Example 1:

[0096] Preparation of the casting solution and the membrane: the PES, the additive PEG200, the solvent DMF, etc. are prepared into a casting solution according to the mass ratio of 16:49:35, and the polymer is completely dissolved into a transparent clear viscous solution by stirring at room temperature. The casting solution is sprayed on the support as the support to prepare the nascent membrane by using the equipment;

[0097] The support loaded with the nascent membrane is placed in an air bath with a relative humidity of 60% RH and a temperature of 25°C, and is pretreated in the air bath for 15 seconds to obtain a pretreated membrane; the support loaded with the pretreated membrane is immersed in a water solution at 50°C to perform phase inversion to obtain a microfiltration membrane.

[0098] The average pore size of the microfiltration membrane is 0.37 μm, the porosity is 79.3%, the breaking strength is 3.8 MPa, the breaking elongation is 27.2%, and the pure water permeability is 8,040 LMH bar -1 ; more specifically, see Table 1 and Figure 9, in which the left side of the two adjacent column charts is the breaking strength parameter, and the right side is the breaking elongation.

[0099] Comparative Example 2

[0100] Comparative Example 2 was substantially identical to Example 1 except that the ambient humidity was 20% RH, the temperature was 25°C in an air bath, the treatment time was reduced to 6 seconds to obtain the pre-treated membrane; the pre-treated membrane and the support were immersed in a coagulation bath at 50°C to perform the phase inversion, then immersed in hot water at 50°C for sufficient water washing to remove the residual solvent in the membrane, and then dried at 80°C to obtain the microfiltration membrane. The pore size of the microfiltration membrane prepared thereby was reduced, and the pure water permeability was decreased compared to Example 1.

[0101] The average pore size of the microfiltration membrane was determined to be 0.20 μm, the porosity was 74.6%, the breaking strength was 4.6 MPa, the breaking elongation was 35.3%, and the pure water permeability was 4650 LMH bar -1 ; more specifically, see Table 1 and Figure 9.

[0102] Comparative Example 3

[0103] Comparative Example 3 was substantially identical to Example 1 except that the ambient humidity was 90% RH, the temperature was 30°C in an air bath, the treatment time was increased to 80 seconds to obtain the pre-treated membrane; the pre-treated membrane and the support were immersed in a coagulation bath at 50°C to perform the phase inversion, then immersed in hot water at 50°C for sufficient water washing to remove the residual solvent in the membrane, and then dried at 80°C to obtain the microfiltration membrane. The breaking strength and the breaking elongation of the microfiltration membrane prepared thereby were significantly decreased compared to Example 1.

[0104] The average pore size of the microfiltration membrane was determined to be 0.60 μm, the porosity was 93.2%, the breaking strength was 2.1 MPa, the breaking elongation was 11.6%, and the pure water permeability was 33200 LMH bar -1 ; more specifically, see Table 1 and Figure 9.

[0105] Performance Test

[0106] The performance and physical parameters of the microfiltration membranes of Examples 1-5 and Comparative Examples 1-3 described above are shown in Table 1.

[0107] Table 1: Performance and physical parameters of the microfiltration membranes of Examples 1-5 and Comparative Examples 1-3

[0108] From the data in Table 1, it can be seen that the microfiltration membranes prepared in Examples 1-3, which are prepared by adding hydrophilic polymers to the casting solution and using phase inversion combined with pretreatment, all exhibit excellent separation performance. Among them, the microfiltration membranes prepared in Examples 2 and 3 have a lower hydrophilicity, i.e. pure water permeability, than the microfiltration membrane prepared in Example 1, because the hydrophilic polymers added to the systems in Examples 2 and 3 have fewer hydrophilic groups. In addition, due to the reduction of hydrophilic groups in the system, the breaking strength of the microfiltration membranes also decreases slightly. By comparing Example 4 with Example 1, it can be found that appropriately reducing the humidity and treatment time of the pretreatment environment can result in a decrease in the pore size of the microfiltration membrane, but the microfiltration membrane still has a high porosity. By comparing Example 5 with Example 1, it can be found that appropriately increasing the humidity and treatment time of the pretreatment environment can significantly improve the pore size, pure water permeability and surface opening rate of the microfiltration membrane.

[0109] By comparing Comparative Example 1 with Example 1, it can be found that the pure water permeability and porosity of the microfiltration membrane prepared by adding only the bulk polymer decrease significantly. By comparing Comparative Example 2 with Example 1, it can be found that excessively reducing the humidity and treatment time of the pretreatment environment results in a significant decrease in the pure water flux and porosity of the microfiltration membrane. By comparing Comparative Example 3 with Example 1, it can be found that excessively increasing the humidity and treatment time of the pretreatment environment results in a significant decrease in the breaking strength and breaking elongation of the microfiltration membrane. It can be seen that at least some embodiments of the present application provide a microfiltration membrane with high pure water flux, high bacterial retention rate, uniform pore size distribution, excellent mechanical properties and moderate thickness, which has extremely high commercial value.

[0110] The examples described above merely describe the preferred embodiments of the present application and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art should fall within the protection scope of the present application as defined by the claims.

Claims

1. A method for preparing a microfiltration membrane, wherein, The casting solution is prepared by blending the hydrophilic polymer and the bulk polymer, and a microfiltration membrane with asymmetric continuous gradient sponge-like pore structure is prepared by phase inversion method.

2. The production method according to claim 1, wherein The bulk polymer is selected from at least one of the following group: polyether sulfone, polysulfone, polyvinylidene fluoride, polyvinyl chloride; the hydrophilic polymer is selected from at least one of the following group: hydroxyl-terminated polyether sulfone, sulfonated polyether sulfone, polyether sulfone-polyethylene glycol block copolymer.

3. The production method according to claim 1 or 2, wherein In the casting solution, the bulk polymer is 14%-16%, the hydrophilic polymer is 1%-6%, the solvent is 30%-40%, and the additive is 40%-50% by mass percentage.

4. The production method according to claim 3, wherein The solvent is selected from at least one of the following group: N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide; The additive is selected from at least one of the following group: polyethylene glycol, diethylene glycol, triethylene glycol, isopropyl alcohol and polyvinylpyrrolidone; The phase inversion method is selected from at least one of the following methods: NIPS, VIPS, TIPS and R-TIPS.

5. The production method according to claim 1, wherein The phase inversion method comprises the following steps: The bulk polymer, the hydrophilic polymer, the solvent and the additive are blended to prepare the casting solution, and the casting solution is coated on the carrier as support by using the doctor blade coating process or the slot die coating process to obtain the nascent membrane; The carrier loaded with the nascent membrane is subjected to air bath to obtain the pretreated membrane; The carrier and the pretreated membrane are immersed in the coagulation bath for phase inversion, and then post-treated and dried to obtain the microfiltration membrane.

6. The production method according to claim 5, wherein The relative humidity of the air bath is 30-70% RH, the temperature is 25-40℃, and the time is 3-60s; The temperature of the coagulation bath is 30-70℃; the composition of the coagulation bath is selected from at least one of the following group: water, ethanol, DMF and DMAc; The viscosity of the casting solution is in the range of 3000 mPa·s-8000 mPa·s at 25℃.

7. A microfiltration membrane obtainable by the process according to any one of claims 1 to 6, wherein The microfiltration membrane is a sponge-like pore structure with asymmetric continuous gradient along the thickness direction, and has no obvious interface or delamination phenomenon.

8. The microfiltration membrane according to claim 7, wherein, The nominal diameter of the obtained microfiltration membrane is selected from any one of the following group: 0.1 μm, 0.2 μm, 0.45 μm, 0.65 μm, 0.8 μm, 1.0 μm, 2.0 μm and 3.0 μm; The average pore size of the microfiltration membrane is 0.1 μm-3.5 μm; the thickness of the microfiltration membrane is 130-160 μm, and the porosity is 75-90%.

9. The microfiltration membrane according to claim 7 or 8, wherein, The microfiltration membrane has a pure water permeability of 10,000-70,000 LMH bar -1 , a breaking strength of 3.0-6.0 MPa, and an elongation at break of 20-50%.

10. The microfiltration membrane according to claim 7 or 8, wherein, The microfiltration membrane has a protein recovery rate of >99% and a bacterial retention rate of 1 x 10 7 CFU / cm 2 The above.

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