Preparation method and device of ultrafiltration membrane with low molecular weight cutoff

Abstract

The invention discloses a preparation method and device of a low molecular weight cutoff ultrafiltration membrane, and belongs to the technical field of ultrafiltration membrane preparation. A thermally induced phase separation method and a non-solvent induced phase separation method are combined to prepare the membrane, the relationship between the flux and the retention rate of the ultrafiltration membrane is balanced, high flux is considered while high retention of low-molecular-weight substances is provided, an upper-layer membrane casting solution and a lower-layer membrane casting solution are used for double-layer simultaneous coating, a substrate is coated with the membrane casting solutions, and the membrane is obtained through a dry process section. Phase separation is carried out after entering a coagulating bath, a finished membrane is obtained through cleaning, the obtained membrane is of an integrated structure, the coagulating bath used in the membrane casting process is pure water, and recycling and aftertreatment of the ultrafiltration membrane are facilitated.

Description

Technical Field

[0001] The invention belongs to the technical field of ultrafiltration membrane preparation, and relates to a method and a device for preparing a low molecular weight cut-off ultrafiltration membrane. Background Art

[0002] With the development of economy and the increase of application scenarios, the market demand for low molecular weight cut-off (MWCO ≤ 5000) ultrafiltration membranes is growing, especially for protein concentration and purification in the biomedical field. At present, there is little research on ultrafiltration membranes with a molecular weight cut-off of ≤ 5000 in China, and the products used in experiments and production mainly rely on imports. The reported methods for preparing low molecular weight cut-off ultrafiltration membranes mostly use a single casting solution and adopt a non-solvent-induced phase inversion method to prepare the finished membrane.

[0003] Patent application CN103816810A reports a method for preparing a low molecular weight cut-off rolled ultrafiltration membrane and its product, which uses a single casting solution to be scraped onto a polyester non-woven fabric, and then undergoes phase inversion to obtain a low molecular weight cut-off ultrafiltration membrane. In order to improve the retention effect of the ultrafiltration membrane on low molecular weight substances, a cross-linking reaction is usually performed on the membrane surface to reduce the membrane surface pore size.

[0004] Patent application CN109126489A discloses a method for preparing a polysulfone ultrafiltration membrane with a small molecular weight cutoff, the steps comprising: preparing a polysulfone ultrafiltration membrane base membrane, then coating PVA on the surface of the base membrane, and placing it in an oven for cross-linking reaction to obtain the desired ultrafiltration membrane.

[0005] Patent application publication number CN117463153A proposes a low molecular weight cut-off ultrafiltration membrane and a preparation method thereof, with similar steps, using water and an organic solvent in the coagulation bath to obtain a low molecular weight cut-off ultrafiltration membrane. These technologies have good interception effects on specific low molecular weight substances, but while maintaining a high interception rate, the membrane flux is low, which is not conducive to the efficient separation process; and the added cross-linking coating process increases the process control points, and the obtained separation layer has the risk of falling off during use; in addition, the use of water and organic solvents in the coagulation bath is not easy to separate and recover, which also increases the cleaning time of the membrane.

[0006] The existing methods have the following disadvantages: (1) Most of them use a single casting method, such as the non-solvent induced phase separation method. Due to the "trade-off" effect of the membrane, this single casting method often significantly reduces the flux when improving the retention of low molecular weight substances by the ultrafiltration membrane, which reduces the separation efficiency and is not conducive to production. (2) The ultrafiltration membrane base membrane is first prepared, and then the cross-linking solution is coated on its surface. The preparation process is relatively complicated, and the separation layer of the ultrafiltration membrane obtained in the end is at risk of falling off during use. (3) The coagulation bath mostly uses a mixture of water and organic solvents, which is not conducive to recovery. The ultrafiltration membrane must be fully washed during post-treatment, which wastes water resources. Summary of the invention

[0007] The present invention aims at solving the problems in the prior art and provides a method and a device for preparing a low molecular weight cut-off ultrafiltration membrane.

[0008] A low molecular weight cut-off ultrafiltration membrane, the membrane layer thickness of the ultrafiltration membrane is 40-70 μm, the membrane structure surface is a dense cortex, the upper layer of the cross section is a sponge pore structure, and the lower layer is a finger-like pore structure.

[0009] The pure water flux of ultrafiltration membrane is 40-60L / m 2 / h, the retention rate for PEG2000 is 97.2%, and the retention rate for PEG5000 is 99.3%.

[0010] A method for preparing a low molecular weight cut-off ultrafiltration membrane comprises the following steps: preparing the membrane by combining a thermally induced phase separation method with a non-solvent induced phase separation method, balancing the relationship between the ultrafiltration membrane flux and the interception rate, taking into account high flux while providing high interception of low molecular weight substances, using an upper layer casting liquid and a lower layer casting liquid for double-layer simultaneous coating, coating the casting liquid on a substrate, passing through a dry process section, entering a coagulation bath for phase separation, and obtaining a finished membrane after washing. The obtained membrane is an integrated structure, and the coagulation bath used in the casting process is pure water, which is conducive to recovery and post-treatment of the ultrafiltration membrane.

[0011] A preparation device for a low molecular weight cut-off ultrafiltration membrane, wherein the inner cavity walls of a gel tank (pure water) are respectively connected to guide rollers, a reeling device, a guide roller, a coating support roller and a reeling device are respectively connected to a substrate, and the periphery of the coating support roller is respectively provided with a scraper blade for coating an upper layer of casting liquid, a scraper blade for coating a lower layer of casting liquid, an upper layer of casting liquid storage tank and a lower layer of casting liquid storage tank, wherein the scraper blade for coating the upper layer of casting liquid is placed at the lower part of the upper layer of casting liquid storage tank, and the scraper blade for coating the lower layer of casting liquid is placed at the lower part of the lower layer of casting liquid storage tank.

[0012] The advantages of the present invention are: the membrane is prepared by combining the thermally induced phase separation method with the non-solvent induced phase separation method, which effectively balances the relationship between the ultrafiltration membrane flux and the retention rate, and provides a high retention of low molecular weight substances while taking into account a relatively high flux. Two casting liquids are used for double-layer simultaneous coating, and the casting liquid is coated on the substrate, passes through the dry process section, enters the coagulation bath, and phase separation occurs. The finished membrane is obtained after washing, and the process is simple, and the obtained membrane is an integrated structure, and there is no risk of separation layer falling off. The coagulation bath used in the casting process is pure water, which is conducive to recovery and post-treatment of the ultrafiltration membrane.

[0013] The present invention adopts two different casting solutions to prepare a double-layer ultrafiltration membrane. The prepared ultrafiltration membrane has a relatively high flux while maintaining a high retention rate. Moreover, the preparation process and post-treatment process are simple, saving water resources.

[0014] The membrane is prepared by a single thermally induced phase separation method or a non-solvent induced phase separation method, and the cross-sectional structure of the obtained membrane is a sponge hole or a finger-like pore structure, or a sponge hole plus a finger-like pore, but the morphology is uncontrollable. The double-layer membrane prepared by combining the thermally induced phase separation method and the non-solvent induced phase separation method can effectively adjust the membrane structure. The upper layer of the prepared membrane is a sponge hole, and the lower layer is a finger-like pore. The thickness of the upper and lower layers can be adjusted to control the membrane performance. Compared with the single finger-like pore structure, this structure reduces membrane defects and maintains a high retention performance; compared with the single sponge pore structure, the lower finger-like pore structure reduces the filtration resistance and increases the flux of the membrane. Therefore, the ultrafiltration membrane with this structure has a relatively high flux while maintaining a high retention rate. In addition, the two phase separation methods are carried out simultaneously during the phase separation process. After the two casting solutions come into contact, they move against each other and separate at the same time. The obtained membrane is a stable integrated structure, and the finished membrane is obtained after washing with pure water. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the prior art descriptions. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. As shown in the figure:

[0016] Figure 1 It is a schematic diagram of the double-layer coating cast film of the present invention.

[0017] Figure 2 It is a schematic diagram of the position of the casting liquid during the coating process of the present invention.

[0018] Figure 3 This is a field emission scanning electron microscope image of the ultrafiltration membrane surface of the present invention.

[0019] Figure 4 This is a field emission scanning electron microscope image of the cross section of the ultrafiltration membrane of the present invention. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] Example 1: Figure 1 , Figure 2 , Figure 3 and Figure 4As shown, a device for ultrafiltration membrane with low molecular weight cut-off comprises an unwinding device 1, a substrate 2, a guide roller 3, a coating support roller 4, a scraper blade 5 for coating an upper layer of casting liquid, a scraper blade 6 for coating a lower layer of casting liquid, an upper layer of casting liquid storage tank 7, a lower layer of casting liquid storage tank 8, a gel tank (pure water) 9, a cleaning tank (pure water) 10, and a winding device 11. The inner cavity walls of the gel tank (pure water) 9 are respectively connected to the guide roller 3, the unwinding device 1, the guide roller 3, the coating support roller 4, the scraper blade 5 for coating an upper layer of casting liquid, the scraper blade 6 for coating a lower layer of casting liquid, the upper layer of casting liquid storage tank 7, the lower layer of casting liquid storage tank 8, the gel tank (pure water) 9, the cleaning tank (pure water), and the winding device 11. The roller 3, the coating support roller 4 and the winding device 11 are respectively connected to the substrate 2. The periphery of the coating support roller 4 is respectively provided with a scraper blade 5 for coating the upper layer casting liquid, a scraper blade 6 for coating the lower layer casting liquid, an upper layer casting liquid storage tank 7 and a lower layer casting liquid storage tank 8. The scraper blade 5 for coating the upper layer casting liquid is placed at the lower part of the upper layer casting liquid storage tank 7, and the scraper blade 6 for coating the lower layer casting liquid is placed at the lower part of the lower layer casting liquid storage tank 8. The unwinding device 1, the guide roller 3, the coating support roller 4 and the winding device 11 are respectively connected to the frame.

[0022] A method for preparing a low molecular weight cut-off ultrafiltration membrane comprises the following steps: firstly, a scraper blade is pressed onto a substrate, and a feeler gauge of a certain thickness is placed on both sides of the scraper blade 5 coated with the upper layer casting liquid and the scraper blade 6 coated with the lower layer casting liquid, and the blades are pressed tightly. Then, the film scraping machine control system is turned on, and the unwinding device 1 starts to start, and the substrate 2 passes through the guide roller 3 and enters the surface of the coating support roller 4, and the lower layer casting liquid is first poured into the lower layer casting liquid storage tank 8, so that the lower layer casting liquid is coated on the substrate, and then the upper layer casting liquid is poured into the upper layer casting liquid storage tank 7, so that the upper layer casting liquid is coated on the lower layer casting liquid, and the casting liquid passes through the dry section together with the substrate, enters the gel tank (pure water) 9 to separate phases, and then passes through three guide rollers 3 to enter the cleaning tank (pure water) 10 for cleaning, passes through multiple guide rollers, and after cleaning in the cleaning tank, the finished product is collected on the winding device 11.

[0023] A method for preparing a low molecular weight cut-off ultrafiltration membrane further comprises the following steps:

[0024] Step 1: dissolve the polymer in a solvent, stir evenly at 70-100°C for 8-10h, stop stirring after complete dissolution, keep warm, and stand for degassing for 6-8h to obtain a clear and transparent upper casting solution, and use the same method to prepare the lower casting solution;

[0025] Step 2: Install the substrate on the scraper, place feeler gauges of a certain thickness on both sides of the scraper blade, turn on the scraper machine control system, pour the lower layer casting liquid into the side of the scraper close to the substrate unwinding, and evenly apply the lower layer casting liquid on the substrate, then pour the upper layer casting liquid into the other side of the scraper, and evenly scrape the upper layer casting liquid on the lower layer casting liquid, then pass through the dry process section, enter the coagulation bath, and get the finished film after washing with pure water.

[0026] In step one, the polymer includes a polysulfone substance, a mixture of a polysulfone substance and a sulfonated polysulfone substance, wherein the polysulfone substance includes one of polysulfone and polyethersulfone, and the sulfonated polysulfone substance includes one of sulfonated polysulfone and sulfonated polyphenylsulfone; in step one, the mass fraction of the polymer is 20-35 parts.

[0027] The sulfonation degree of the sulfonated polysulfone substance is 10-30%; the mass ratio of the polysulfone substance to the sulfonated polysulfone substance is 3:1-5:1.

[0028] In step one, the solvent includes one or a mixture of solvents selected from the group consisting of N,N-dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, caprolactam, dimethyl sulfoxide, butyrolactone and triethyl phosphate; in step one, the mass fraction of the solvent is 65-80 parts.

[0029] In step 1, the upper casting liquid and the lower casting liquid have different compositions, wherein the polymer content of the upper casting liquid is greater than that of the lower casting liquid.

[0030] In step 2, the substrate is one of polyolefin non-woven fabric and polyester non-woven fabric.

[0031] In step 2, the scraper coating is single-layer scraper double-layer coating, double-layer scraper double-layer coating or coating head coating.

[0032] In step 2, the temperature of the drying process is 10-25° C., and the humidity is 0-30%.

[0033] In step 2, the coagulation bath is pure water at a temperature of 5-15°C.

[0034] The ultrafiltration membrane has a double-layer structure, in which the upper layer is a sponge-like structure and the lower layer is a finger-like pore structure.

[0035] Example 2: Ultrafiltration membrane: It is composed of an extremely thin epidermal layer for separation and a thicker sponge pore or finger-like pore layer for support, and has a cut-off molecular weight of several hundred to several million.

[0036] Double-layer ultrafiltration membrane: refers to an ultrafiltration membrane prepared by using two casting liquids to cast the upper and lower layers. One casting liquid (referred to as the lower casting liquid) is coated on the substrate, and another casting liquid (referred to as the upper casting liquid) is coated on the casting liquid.

[0037] like Figure 1 , Figure 2 , Figure 3 and Figure 4As shown, a method for preparing a low molecular weight cut-off ultrafiltration membrane adopts a combination of thermally induced phase separation and non-solvent induced phase separation to effectively balance the relationship between the flux and the retention rate of the ultrafiltration membrane, and takes into account a relatively high flux while providing high retention of low molecular weight substances. Two casting liquids are used for double-layer simultaneous coating, and the casting liquid is coated on a substrate. After a dry process section, phase separation occurs after entering a coagulation bath, and a finished membrane is obtained after washing. The process is simple, and the obtained membrane is an integrated structure, and there is no risk of separation layer falling off. The coagulation bath used in the casting process is pure water, which is conducive to recovery and post-treatment of the ultrafiltration membrane.

[0038] Embodiment 3: A method for preparing a low molecular weight cut-off ultrafiltration membrane comprises the following steps:

[0039] Step 1: prepare the upper casting liquid and the lower casting liquid separately. The upper casting liquid consists of 25.8 parts of polyethersulfone, 5.2 parts of sulfonated polysulfone and 69 parts of N,N-dimethylacetamide, and the lower casting liquid consists of 22 parts of polyethersulfone, 7.8 parts of triethyl phosphate and 70.2 parts of N,N-dimethylacetamide. Dissolve the polymer in the solvent, mix well, stir at 100°C for 10 hours, stop stirring after complete dissolution, keep warm, and let stand for degassing for 8 hours to obtain two uniform and clear casting liquids.

[0040] Step 2: Install the substrate on the scraper, pad the blades of the double-layer scraper with a certain thickness of feeler gauges, turn on the scraper control system, control the speed to 10m / min, first pour the lower layer of casting liquid on the side of the scraper close to the substrate unwinding, at this time the lower layer of casting liquid contacts the substrate, then pour the upper layer of casting liquid on the other side of the scraper, at this time the upper layer of casting liquid contacts the lower layer of casting liquid, then, through the dry process section, enter the pure water coagulation bath for phase separation, and wash with pure water to obtain the finished film. The casting liquid is evenly coated on the substrate by using the scraper, and the thickness of the upper and lower layers of casting liquid is controlled by controlling the thickness of the feeler gauges on both sides of the scraper blade.

[0041] The ultrafiltration membrane obtained by the present invention has a membrane layer thickness of 40-70 μm and a membrane structure such as Figure 3 and Figure 4 As shown, the surface is a dense cortex, the upper layer of the cross section is a sponge pore structure, and the lower layer is a finger-like pore structure. The pure water flux of the ultrafiltration membrane is 40-60L / m 2 / h, the retention rate for PEG2000 is 97.2%, and the retention rate for PEG5000 is 99.3%.

[0042] Test method:

[0043] Pure water flux: water temperature 25℃, pressure 0.2MPa, pre-pressing 10min, test three groups and take the average value, the calculation formula is as follows:

[0044] F=V / (S*t),

[0045] F pure water flux, unit L / m 2 / h,

[0046] V permeate volume, unit L,

[0047] S membrane effective filtration area, unit: m 2 ,

[0048] t is the time taken to pass through a volume of water V, in h.

[0049] Retention rate: water temperature 25℃, polyethylene glycol (PEG) solution concentration 1000ppm, pressure 0.2MPa, pre-pressing 10min, test three groups and take the average value, the calculation formula is as follows:

[0050] R=(1-C p / C f )*100,

[0051] R retention rate, unit: %

[0052] C p Permeate concentration, mg / L,

[0053] C f Stock solution concentration, unit: mg / L.

[0054] The present invention mainly protects the method of double-layer coating casting and its simple deformation mode. The prepared membrane has a double-layer structure, the upper layer is a sponge pore structure, and the lower layer is a finger-like pore structure. Using two casting liquids, the thermally induced phase separation method and the non-solvent induced phase separation method are combined to form the membrane, which can effectively adjust the structure-activity relationship of the ultrafiltration membrane. The method and equipment form include but are not limited to scraper coating and coating head coating; the coating form includes but is not limited to double-layer coating and multi-layer coating (greater than double layers).

[0055] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A low molecular weight cut-off ultrafiltration membrane, characterized in that: The thickness of the ultrafiltration membrane is 40-70μm, the surface of the membrane structure is a dense cortex, the upper layer of the cross section is a sponge-like structure, and the lower layer is a finger-like pore structure. The pure water flux of the ultrafiltration membrane is 40-60L / m 2 / h, the retention rate for PEG2000 is 97.2%, and the retention rate for PEG5000 is 99.3%.

2. A method for preparing a low molecular weight cut-off ultrafiltration membrane, characterized in that: The invention comprises the following steps: preparing a membrane by combining a thermally induced phase separation method with a non-solvent induced phase separation method, balancing the relationship between the flux and the interception rate of an ultrafiltration membrane, taking high flux into consideration while providing high interception of low molecular weight substances, using an upper layer casting liquid and a lower layer casting liquid for double-layer simultaneous coating, coating the casting liquid on a substrate, passing through a dry process section, entering a coagulation bath to cause phase separation, and obtaining a finished membrane after washing. The obtained membrane is an integrated structure, and the coagulation bath used in the casting process is pure water, which is conducive to recovery and post-treatment of the ultrafiltration membrane.

3. The method for preparing a low molecular weight cut-off ultrafiltration membrane according to claim 2, characterized in that: The following steps are also included: Step 1: prepare an upper casting liquid and a lower casting liquid respectively, the upper casting liquid comprises: 25.8 parts of polyethersulfone, 5.2 parts of sulfonated polysulfone and 69 parts of N, N-dimethylacetamide, the lower casting liquid comprises: 22 parts of polyethersulfone, 7.8 parts of triethyl phosphate and 70.2 parts of N, N-dimethylacetamide, dissolve the polymer in a solvent, stir evenly at 70-100°C for 8-10h, stop stirring after complete dissolution, keep warm, and stand for degassing for 6-8h to obtain a clear and transparent upper casting liquid, and prepare the lower casting liquid using the same method; The polymer includes a polysulfone substance, a mixture of a polysulfone substance and a sulfonated polysulfone substance, wherein the polysulfone substance includes one of polysulfone and polyethersulfone, and the sulfonated polysulfone substance includes one of sulfonated polysulfone and sulfonated polyphenylsulfone; the mass fraction of the polymer is 20-35 parts, The sulfonation degree of the sulfonated polysulfone substance is 10-30%; the mass ratio of the polysulfone substance to the sulfonated polysulfone substance is 3:1-5:1, Step 2: Install the substrate on the scraper, place feeler gauges of a certain thickness on both sides of the blade of the double-layer scraper, turn on the scraper control system, control the speed to 10m / min, pour the lower layer of casting liquid on the side of the scraper close to the substrate unwinding, and evenly apply the lower layer of casting liquid on the substrate, then pour the upper layer of casting liquid on the other side of the scraper, and evenly scrape the upper layer of casting liquid on the lower layer of casting liquid, then go through the dry process section, enter the coagulation bath, and get the finished film after washing with pure water.

4. The method for preparing a low molecular weight cut-off ultrafiltration membrane according to claim 3, characterized in that: The casting liquid is evenly coated on the substrate by using a scraper, and the thickness of the upper and lower layers of the casting liquid is controlled by controlling the thickness of the feeler gauge on both sides of the scraper blade.

5. A device for preparing a low molecular weight cut-off ultrafiltration membrane, characterized in that: The inner cavity walls of the gel tank (pure water) are connected to guide rollers respectively, and the unwinding device, guide rollers, coating support rollers and winding device are connected to the substrate respectively. The periphery of the coating support rollers are respectively provided with a scraper blade for coating the upper layer casting liquid, a scraper blade for coating the lower layer casting liquid, an upper layer casting liquid storage tank and a lower layer casting liquid storage tank. The scraper blade for coating the upper layer casting liquid is placed at the lower part of the upper layer casting liquid storage tank, and the scraper blade for coating the lower layer casting liquid is placed at the lower part of the lower layer casting liquid storage tank.

6. The device for preparing a low molecular weight cut-off ultrafiltration membrane according to claim 5, characterized in that: The unwinding device, the guide roller, the coating support roller and the winding device are respectively connected to the frame.

Images