Small-pore hydrophilic ultrafiltration membrane and preparation method thereof

Abstract

The present application discloses a method for preparing a small-pore hydrophilic ultrafiltration membrane, wherein a fluorinated polymer, β-cyclodextrin and a diluent are mixed under heating conditions to obtain a casting solution; after the casting solution is extruded through a spinneret or roller-coated, it is immersed in a coagulation bath through an air gap to cool and solidify, and a primary membrane is obtained after winding; the primary membrane is extracted with an extractant and then dried naturally to obtain a small-pore hydrophilic ultrafiltration membrane. The present application introduces β-cyclodextrin, which has excellent hydrophilicity and is hydrophobic inside and hydrophilic outside, into a membrane using a thermally induced phase separation technology, and anchors β-cyclodextrin to the membrane body and the membrane surface through intermolecular forces and chain entanglement, so as to achieve simultaneous hydrophilic modification of the membrane body and the membrane surface, and the prepared ultrafiltration membrane has the characteristics of small pore size, good mechanical properties, and lasting hydrophilicity, and the process is simple, and no equipment needs to be updated, and it has good application prospects in the deep purification process of drinking water. The present application also provides a small-pore hydrophilic ultrafiltration membrane.

Description

Technical Field

[0001] The present application belongs to the technical field of ultrafiltration membranes, and specifically relates to a small-pore hydrophilic ultrafiltration membrane and a preparation method thereof. Background Art

[0002] Phase inversion technology is the main method for preparing polyvinylidene fluoride membranes, including nonsolvent-induced phase separation (NIPS) and thermally induced phase separation (TIPS). Compared with NIPS technology, TIPS technology has the following advantages: (1) the process is highly controllable and easy to promote to industrial scale; (2) the membrane has higher mechanical strength, fewer defects and narrower pore size distribution; (3) the membrane structure has fewer control parameters and the microstructure is more controllable.

[0003] However, the TIPS membrane technology has the following bottlenecks in terms of filtration accuracy and hydrophilic modification: (1) The membrane pore permeability is poor and the membrane pore size is large (microfiltration level, nano calcium carbonate is added as a template during the preparation process to further increase the pore size, and the membrane pore size is usually 50nm or even above 100nm), resulting in low filtration accuracy; (2) Hydrophilic modification is not easy. Known water-soluble and alcohol-soluble hydrophilic agents such as polyethylene glycol will be lost during the extraction process and are not suitable for application in the TIPS process; hydrophilic polymers such as polyvinyl butyral (PVB), nylon (PA), cellulose acetate (CA), polyethylene-vinyl alcohol copolymer (EVOH) have limited hydrophilicity themselves and poor compatibility with fluorinated polymers. They are prone to defective pores, which leads to an increase in membrane pore size, reducing separation performance, and ultimately making the improvement of the hydrophilic effect of the membrane very limited.

[0004] Therefore, it is necessary to provide a small-pore hydrophilic ultrafiltration membrane and a preparation method thereof to solve the above problems. Summary of the invention

[0005] The purpose of the embodiment of the present application is to provide a method for preparing a small-pore hydrophilic ultrafiltration membrane, in which β-cyclodextrin is used as an additive, mixed with a fluorine-containing polymer and a diluent, and a thermally induced phase separation method is used to prepare an ultrafiltration membrane with small pore size, good mechanical properties, excellent hydrophilicity and durability. The problem of low filtration accuracy and difficult hydrophilic modification of polymer porous membranes prepared by existing thermally induced phase separation technology can be effectively solved, and the mechanical properties of the membrane are improved. In addition, the hydroxyl groups of β-cyclodextrin anchored on the membrane surface are convenient for subsequent functional modification.

[0006] In order to solve the above technical problems, this application is implemented as follows:

[0007] A method for preparing a small-pore hydrophilic ultrafiltration membrane comprises the following steps:

[0008] S1: according to the weight ratio, 15-40 parts of fluorine-containing polymer, 1-10 parts of β-cyclodextrin and 50-84 parts of diluent are placed in a reaction kettle, and mixed evenly under heating conditions to obtain a casting solution;

[0009] S2: extruding the casting solution through a spinneret or roller coating, immersing the solution in a coagulation bath through an air gap to cool and solidify the solution, and winding the solution to obtain a primary film;

[0010] S3: extracting the diluent from the primary membrane with an extractant, and obtaining a small-pore hydrophilic ultrafiltration membrane after natural drying.

[0011] Preferably, the fluorine-containing polymer is a homopolymer or copolymer of polyvinylidene fluoride; the diluent is a high-boiling point organic substance that is insoluble in water but soluble in ethanol, including at least one of dimethyl phthalate, dibutyl phthalate, dioctyl phthalate, dibutyl sebacate, dioctyl sebacate, acetyl tributyl citrate, and tributyl citrate.

[0012] Preferably, in step S1, the heating temperature is 150-190°C.

[0013] Preferably, in step S2, the air gap is 5-20 cm.

[0014] Preferably, in step S2, the coagulation bath is deionized water at 20-60° C., and the residence time in the coagulation bath is less than 30 s.

[0015] Preferably, in step S2, the winding speed is not higher than 10 m / min.

[0016] Preferably, in step S3, the extractant is ethanol with a purity of not less than 99%.

[0017] The present invention also provides a small-pore hydrophilic ultrafiltration membrane, which is prepared by the above-mentioned preparation method.

[0018] Preferably, the pore size of the small-pore hydrophilic ultrafiltration membrane is 5-10 nm.

[0019] The present application is to mix a fluorinated polymer, β-cyclodextrin and a diluent under heating conditions to obtain a casting liquid; after the casting liquid is extruded through a spinneret or roller-coated, it is immersed in a coagulation bath through an air gap to cool and solidify, and a primary membrane is obtained after winding; the diluent is then extracted with an extractant, and a small-pore hydrophilic ultrafiltration membrane is obtained after natural drying. The present application introduces β-cyclodextrin, which has excellent hydrophilicity and is hydrophobic inside and hydrophilic outside, into a membrane by thermally induced phase separation technology, and anchors β-cyclodextrin to the membrane body and the membrane surface through intermolecular forces and chain entanglement, and simultaneously realizes hydrophilic modification of the membrane body and the membrane surface; at the same time, β-cyclodextrin can play a role in causing small pores, and the prepared membrane has a small pore size; β-cyclodextrin acts as a cross-linking node in the three-dimensional network of the polymer, plays a bridging role, thereby enhancing the interaction between polymer chains and improving the mechanical properties of the membrane; in addition, the β-cyclodextrin anchored on the membrane surface is rich in hydroxyl groups, which can be further functionalized and modified later. The small-pore hydrophilic ultrafiltration membrane prepared by the present invention has the characteristics of small pore size, good mechanical properties, excellent and durable hydrophilicity, etc., and the process is simple and does not require equipment renewal. It has good application prospects in the deep purification process of drinking water. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application are described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0021] The present application provides a method for preparing a small-pore hydrophilic ultrafiltration membrane, comprising the following steps:

[0022] S1: According to the weight ratio, 15-40 parts of fluorine-containing polymer, 1-10 parts of β-cyclodextrin and 50-84 parts of diluent are placed in a reaction kettle, and mixed evenly under heating conditions to obtain a casting solution.

[0023] Preferably, the fluorine-containing polymer is a homopolymer or copolymer of polyvinylidene fluoride; the diluent is a high-boiling point organic substance that is insoluble in water but soluble in ethanol, including at least one of dimethyl phthalate, dibutyl phthalate, dioctyl phthalate, dibutyl sebacate, dioctyl sebacate, acetyl tributyl citrate, and tributyl citrate.

[0024] In the step S1, the heating temperature is 150-190°C.

[0025] S2: The casting solution is extruded through a spinneret or roller-coated, immersed in a coagulation bath through an air gap to be cooled and solidified, and then wound up to obtain a primary film.

[0026] In step S2, the air gap is 5-20 cm; the coagulation bath is deionized water at 20-60° C., and the residence time in the coagulation bath is less than 30 s; the winding speed of the coil is not higher than 10 m / min, and the residence time in the coagulation bath can be controlled by controlling the winding speed.

[0027] S3: extracting the diluent from the primary membrane with an extractant, and obtaining a finished membrane after natural drying.

[0028] In step S3, the extractant is ethanol with a purity of not less than 99%.

[0029] The present invention also provides a small-pore hydrophilic ultrafiltration membrane, which is prepared by the above-mentioned preparation method.

[0030] The pore size of the small-pore hydrophilic ultrafiltration membrane is 5-10 nm.

[0031] β-cyclodextrin is a green, water-insoluble cyclic oligosaccharide with a molecular weight of 1135 g / mol. It has the shape of a hollow truncated cone with secondary hydroxyl groups distributed on the wide end and primary hydroxyl groups distributed on the narrow end. It is hydrophilic outside the cavity and hydrophobic inside the cavity. The numerous hydroxyl groups on the surface make it extremely hydrophilic and have a small-pore effect, thus giving it excellent water permeability and anti-pollution properties, while the hydrophobic cavity inside makes it more compatible with fluorinated polymers.

[0032] The present application adopts thermally induced phase separation technology to prepare membranes. Since β-cyclodextrin and fluorinated polymers have different hydrophilicities, their curing speeds are different during the cooling and molding process, so that the β-cyclodextrin with excellent hydrophilicity is post-cured and precipitated to be enriched on the membrane surface and the pores of the membrane body, thereby simultaneously anchoring the β-cyclodextrin to the membrane body and the cortex surface, and synchronously realizing the hydrophilic modification of the membrane body and the membrane surface, so that the permeability of the membrane is significantly improved; in addition, the numerous hydroxyl groups of β-cyclodextrin can combine with water, and its own molecular weight is low, so it can play a role in causing small holes, thereby improving the retention performance of the membrane; the β-cyclodextrin anchored on the membrane surface is rich in hydroxyl groups, which provides the possibility for further functional modification in the future.

[0033] It should be noted that although the present application is based on the TIPS method for preparing hollow fiber membranes or rolled membranes, it is also applicable to improved TIPS technology (such as composite thermotropic phase, low-temperature thermotropic phase, etc.).

[0034] Example 1

[0035] (1) adding β-cyclodextrin, polyvinylidene fluoride, and dibutyl phthalate into a reaction kettle in a mass ratio of 5:22:73, stirring and mixing at 150° C. for 3 h, and then vacuum degassing for 3 h to obtain a uniform casting solution;

[0036] (2) The casting solution is continuously extruded through a hollow fiber spinneret, and after passing through an air gap of 5 cm, it is immersed in a 25°C coagulation bath (deionized water) to cool and set. The winding speed is controlled to be 10 m / min to control the time in the coagulation bath to be less than 30 s, and the primary membrane is obtained by winding. Then the primary membrane is soaked in anhydrous ethanol for 48 hours to extract the diluent remaining in the membrane, and finally the membrane fibers are naturally dried at room temperature to obtain a small-pore hydrophilic ultrafiltration membrane.

[0037] Example 2

[0038] (1) adding β-cyclodextrin, polyvinylidene fluoride, and dibutyl sebacate into a reaction kettle in a mass ratio of 8:22:70, stirring and mixing at 170° C. for 3 h, and then vacuum degassing for 3 h to obtain a uniform casting solution;

[0039] (2) The casting liquid is coated by roller pressure, and after passing through an air gap of 5 cm, it is immersed in a 40°C coagulation bath (deionized water) to cool and set. The time in the coagulation bath is controlled to be less than 30 seconds by controlling the winding speed to 10 m / min, and the primary membrane is obtained by rolling. Then the primary membrane is immersed in anhydrous ethanol for 48 hours to extract the diluent remaining in the membrane, and finally the membrane is naturally dried at room temperature to obtain a small-pore hydrophilic ultrafiltration membrane.

[0040] Example 3

[0041] (1) β-cyclodextrin, polyvinylidene fluoride, and tributyl citrate were added into a reaction kettle in a mass ratio of 10:22:68, stirred and mixed at 190° C. for 3 h, and then vacuum degassed for 3 h to obtain a uniform casting solution;

[0042] (2) The casting liquid is continuously extruded through a hollow fiber spinneret, and after passing through an air gap distance of 20 cm, it is immersed in a 50°C coagulation bath (deionized water) to cool and set. The time in the coagulation bath is controlled to be less than 30 seconds by controlling the winding speed to 10 m / min, and the primary membrane is obtained by winding. Then the primary membrane is soaked in anhydrous ethanol for 48 hours to extract the diluent remaining in the membrane, and finally the membrane filament is naturally dried at room temperature to obtain a small-pore hydrophilic ultrafiltration membrane.

[0043] Comparative Example 1

[0044] (1) adding polyvinylidene fluoride and dibutyl phthalate in a mass ratio of 22:78 into a reaction kettle, stirring and mixing at 170° C. for 3 h, and then vacuum degassing for 3 h to obtain a uniform casting solution;

[0045] (2) The casting solution is continuously extruded through a hollow fiber spinneret, and after passing through an air gap of 5 cm, it is immersed in a 25°C coagulation bath (deionized water) to cool and set. The winding speed is controlled to be 10 m / min to control the time in the coagulation bath to be less than 30 s, and the primary membrane is obtained by winding. Then the primary membrane is soaked in anhydrous ethanol for 48 hours to extract the diluent remaining in the membrane, and finally the membrane fiber is naturally dried at room temperature to obtain an ultrafiltration membrane.

[0046] Comparative Example 2

[0047] (1) adding polyvinylidene fluoride and dibutyl sebacate into a reaction kettle at a mass ratio of 22:78, stirring and mixing at 170° C. for 3 h, and then vacuum degassing for 3 h to obtain a uniform casting solution;

[0048] (2) The casting solution is continuously extruded through a hollow fiber spinneret, and after passing through an air gap of 5 cm, it is immersed in a 25°C coagulation bath (deionized water) to cool and set. The winding speed is controlled to be 10 m / min to control the time in the coagulation bath to be less than 30 s, and the primary membrane is obtained by winding. Then the primary membrane is soaked in anhydrous ethanol for 48 hours to extract the diluent remaining in the membrane, and finally the membrane fiber is naturally dried at room temperature to obtain an ultrafiltration membrane.

[0049] The films prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to the following performance tests, respectively. The test results are shown in Table 1:

[0050] (1) Use 25℃ deionized water at 0.1MPa pressure to test for 2h, then calculate the pure water flux per unit area and time, and take the average value of the test five times;

[0051] (2) Use a capillary flow pore size analyzer to measure the average pore size of the membrane, and take the average value after three tests;

[0052] (3) Measure the contact angle of the film surface, and randomly measure 5 times at different positions on each film to obtain the average value;

[0053] (4) A tensile test was performed on a universal testing machine at a linear speed of 10 mm / s, and the maximum force when the membrane filament broke was recorded. The average value was obtained after five tests.

[0054] Table 1 Membrane performance data of Examples 1-3 and Comparative Examples 1-2

[0055]

[0056] As can be seen from Table 1, compared with Comparative Examples 1-2, the small-pore hydrophilic ultrafiltration membrane obtained by the preparation method of the present application greatly improves the pure water flux, and the average pore size is smaller; the contact angle is smaller, indicating that the hydrophilicity of the membrane is better, the wettability of the membrane surface is better, and at the same time, the mechanical properties of the membrane are greatly improved.

[0057] The principle of improving the hydrophilicity and interception performance of the small-pore hydrophilic ultrafiltration membrane prepared in this application is as follows: the hydrophilic hydroxyl groups of β-cyclodextrin form intermolecular forces such as hydrogen bonds with the highly electronegative fluorine atoms in the fluoropolymer, and at the same time, the hydrophobic cavity of β-cyclodextrin and the fluoropolymer molecules are chain entangled, and the hydrophobic fluoropolymer part of the chain segments enters the hydrophobic cavity of β-cyclodextrin, while anchoring β-cyclodextrin in the polymer membrane body and the membrane surface. The outside of β-cyclodextrin is a hydrophilic hydroxyl group, so the hydrophilicity of the membrane body and the membrane surface is improved simultaneously, which helps to filter more efficiently. At the same time, the molecular weight of β-cyclodextrin is low, the hydrophobic part has good compatibility with the fluoropolymer, and the amount of addition can be large, so that β-cyclodextrin can play a role in causing small pores and improve the interception performance of the membrane.

[0058] The principle of improving the mechanical strength of the small-pore hydrophilic ultrafiltration membrane prepared in the present application is as follows: first, the hydrophobic cavity of β-cyclodextrin and the fluorinated polymer molecules are chain entangled, and β-cyclodextrin acts as a cross-linking node in the three-dimensional network of the polymer, playing a bridging role, thereby enhancing the interaction force between the polymer chains and producing a reinforcing effect on the polymer; secondly, the functional group hydroxyl group on the surface of β-cyclodextrin forms hydrogen bonds with water molecules and H and F atoms of the fluorinated polymer, which can strengthen the interface bonding between β-cyclodextrin and the polymer matrix, effectively promote stress transfer, and further improve the toughening effect.

[0059] The embodiments of the present application are described above, but the present application is not limited to the above-mentioned specific implementation modes. The above-mentioned specific implementation modes are merely illustrative and not restrictive. Under the guidance of the present application, ordinary technicians in this field can also make many forms without departing from the purpose of the present application and the scope of protection of the claims, all of which are within the protection of the present application.

Claims

1. A method for preparing a small-pore hydrophilic ultrafiltration membrane, characterized in that: The steps include: S1: according to the weight ratio, 15-40 parts of fluorine-containing polymer, 1-10 parts of β-cyclodextrin and 50-84 parts of diluent are placed in a reaction kettle, and mixed evenly under heating conditions to obtain a casting solution; S2: extruding the casting solution through a spinneret or roller coating, immersing the solution in a coagulation bath through an air gap to cool and solidify the solution, and winding the solution to obtain a primary film; S3: extracting the diluent from the primary membrane with an extractant, and naturally drying to obtain a small-pore hydrophilic ultrafiltration membrane, wherein the pore size of the small-pore hydrophilic ultrafiltration membrane is 5-10 nm; The fluorine-containing polymer is a homopolymer or copolymer of polyvinylidene fluoride; the diluent is a high-boiling point organic substance that is insoluble in water but soluble in ethanol, including at least one of dimethyl phthalate, dibutyl phthalate, dioctyl phthalate, dibutyl sebacate, dioctyl sebacate, acetyl tributyl citrate, and tributyl citrate; In the step S1, the heating temperature is 150-190°C; In step S2, the coagulation bath is deionized water at 20-60° C., and the residence time in the coagulation bath is less than 30 seconds; In step S3, the extractant is ethanol with a purity of not less than 99%.

2. The preparation method according to claim 1, characterized in that: In step S2, the air gap is 5-20 cm.

3. The preparation method according to claim 1, characterized in that: In the step S2, the winding speed is not higher than 10 m / min.

4. A small pore hydrophilic ultrafiltration membrane, characterized in that: The preparation method is described in any one of claims 1 to 3.

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