A polyvinylidene fluoride microporous membrane and its preparation method and application

Abstract

The present invention discloses a polyvinylidene fluoride microporous membrane and a preparation method and application thereof, and relates to the technical field of long-term stable high-performance membrane distillation. The preparation steps include: (1) mixing polyvinylidene fluoride with a first organic solvent, stirring evenly and degassing to obtain a casting liquid; (2) uniformly coating the casting liquid on a hydrophobic polyolefin non-woven fabric, and then transferring it to a mixed solution of a second organic solvent and water to obtain a preformed membrane; (3) transferring the preformed membrane to water, washing and drying to obtain a polyvinylidene fluoride microporous membrane. The prepared polyvinylidene fluoride microporous membrane includes a first hydrophobic surface, an intermediate microporous channel layer and a second hydrophobic surface; wherein the first hydrophobic surface is a polyvinylidene fluoride microporous surface, the intermediate microporous channel layer is a polyvinylidene fluoride microporous structure layer, and the second hydrophobic surface is a composite layer of polyolefin and polyvinylidene fluoride. The membrane is used in membrane distillation, which can simultaneously achieve high flux, low wetting and high resistance to salt crystallization adsorption during the membrane distillation process.

Description

Technical Field

[0001] The invention relates to the technical field of long-acting stable high-performance membrane distillation, and in particular to a polyvinylidene fluoride microporous membrane and a preparation method and application thereof. Background Art

[0002] In recent years, studies have shown that hydrophobicization of the membrane interface can improve the anti-wetting performance during membrane distillation. According to the factors affecting hydrophobicity, the methods for enhancing the hydrophobicity of the material surface mainly include two aspects: one is to increase the roughness of the material surface; the other is to reduce the surface energy of the material surface. Polyvinylidene fluoride is one of the most widely used membrane materials at present, and its intrinsic hydrophobic properties have made it widely studied in the field of membrane distillation. For example, a Chinese patent application (publication number CN116371208A) discloses a method for preparing a super-hydrophobic composite membrane based on modified ZIF-8. First, a macroporous micro-nano membrane surface is constructed, and the membrane surface roughness is further improved by spraying ZIF-8, and then the surface energy of the surface is further reduced by perfluorooctyltrichlorosilane to obtain a super-hydrophobic composite membrane. For example, a Chinese patent application (publication number CN116102759A) discloses a super-hydrophobic modification method based on polyvinylidene fluoride membrane, which, after constructing a macroporous micro-nano membrane surface, performs a hydroxylation treatment on the membrane surface, and then gel-coates inorganic nanoparticles and hydrophobizes the surface to obtain a super-hydrophobic membrane surface. For example, a Chinese patent application (publication number CN116422156A) discloses a method for preparing a dendritic nanofiber composite membrane for membrane distillation, which uses a high-porosity PVDF dendritic nanofiber membrane as a substrate and then in-situ grows Cu nanoparticles to improve the surface roughness of the membrane. The above technologies are all based on the design idea of ​​constructing a super-rough membrane interface, that is, the roughness of the membrane surface is improved by further loading nanoparticles on the macroporous rough membrane substrate, or further grafting low surface energy groups. However, in the actual membrane distillation process, although the super-hydrophobic interface can improve the anti-wetting performance of the membrane surface, the extremely large roughness and rough structure of the membrane surface can easily cause the salt crystals produced during the membrane distillation process to adhere to the membrane surface, thereby accelerating the occurrence of membrane pollution. How to achieve high flux, low wetting and high resistance to salt crystallization adsorption during the membrane distillation process remains the main breakthrough direction for the application and industrial development of membrane distillation technology. Summary of the invention

[0003] The purpose of the present invention is to provide a polyvinylidene fluoride microporous membrane and a preparation method and application thereof, so as to solve the problems raised in the above background technology.

[0004] To achieve the above-mentioned object, the present invention provides a polyvinylidene fluoride microporous membrane, comprising a first hydrophobic surface, an intermediate microporous channel layer and a second hydrophobic surface; wherein the first hydrophobic surface is a polyvinylidene fluoride microporous surface, the intermediate microporous channel layer is a polyvinylidene fluoride microporous structure layer, and the second hydrophobic surface is a composite layer of polyolefin and polyvinylidene fluoride;

[0005] The water drop contact angle of the first hydrophobic surface is 90-110°, the surface roughness of the first hydrophobic surface is less than 2um, the average pore size of the first hydrophobic surface is 0.1-1μm, and the porosity of the first hydrophobic surface is 40%-60%;

[0006] The thickness of the intermediate microporous channel layer is 10-100 μm, and the average pore size of the intermediate microporous channel layer is 0.1-10 μm;

[0007] The water drop contact angle of the second hydrophobic surface is greater than 130°.

[0008] The invention also provides application of the polyvinylidene fluoride microporous membrane in membrane distillation.

[0009] Preferably, the polyvinylidene fluoride microporous membrane is used in membrane distillation by placing the first hydrophobic surface of the polyvinylidene fluoride microporous membrane downward with the hot raw material liquid below, and the second hydrophobic surface of the polyvinylidene fluoride microporous membrane upward with the condensate liquid above.

[0010] The present invention also provides a method for preparing the above-mentioned polyvinylidene fluoride microporous membrane, characterized in that it comprises the following steps:

[0011] (1) mixing polyvinylidene fluoride with a first organic solvent, stirring at 40-100° C. for 6-10 hours, and then degassing to obtain a casting solution;

[0012] (2) preparing a hydrophobic polyolefin nonwoven fabric, uniformly coating the casting solution obtained in step (1) on the hydrophobic polyolefin nonwoven fabric, and then transferring the casting solution into a mixed solution of a second organic solvent and water to obtain a preformed film;

[0013] (3) transferring the preformed membrane obtained in step (2) into water, washing it and drying it to obtain a polyvinylidene fluoride microporous membrane;

[0014] In the step (1), the first organic solvent is one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, and trimethyl phosphate;

[0015] In the step (2), the second organic solvent is one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, and trimethyl phosphate.

[0016] Preferably, in step (1), the mass ratio of polyvinylidene fluoride to the first organic solvent is (13-28): (75-87).

[0017] Preferably, in step (2), the hydrophobic polyolefin non-woven fabric is one or both of polyethylene non-woven fabric and polypropylene non-woven fabric; and the surface water drop contact angle of the hydrophobic polyolefin non-woven fabric is 110°-130°.

[0018] Preferably, in the mixed solution of the second organic solvent and water, the volume ratio of the second organic solvent is ≥50%.

[0019] Therefore, the technology of the present invention realizes the optimization of anti-salt crystallization, mass transfer and anti-wetting of different fluid regions in the membrane distillation process by constructing a three-layer asymmetric structure of a polyvinylidene fluoride microporous membrane, namely, a conventional hydrophobic first hydrophobic surface, an intermediate microporous channel and a strongly hydrophobic or super hydrophobic second hydrophobic surface, through asymmetric structure construction and membrane distillation process design. Among them, different from the traditional super hydrophobic membrane technology of constructing ultra-rough, macroporous and high-porosity membrane surface, the technology of the present invention designs a conventional hydrophobic first hydrophobic surface for the membrane distillation process, that is, by constructing the general hydrophobic membrane surface properties, at the same time, designing a low-roughness, smooth membrane surface, and further designing the membrane surface pore structure to be smaller pore size and medium porosity. Based on this, the technology of the present invention designs the hot raw material liquid to be located below the first hydrophobic surface of the polyvinylidene fluoride microporous membrane, so that the first surface of the membrane and the hot raw material liquid are in a free contact state, that is, the first surface of the membrane and the hot raw material liquid do not have a large interaction force, thereby ensuring the first hydrophobic surface of the membrane against the wetting of the raw material liquid, and at the same time greatly reducing the poor adhesion of the salt crystals produced by the concentration of the raw material liquid on the first surface of the membrane during the membrane distillation process. Further, the technology of the present invention designs a thin layer of polyvinylidene fluoride microporous membrane channel layer with a large pore size and high porosity through the design of the phase separation process, which can greatly improve the mass transfer efficiency in the membrane distillation process and improve the membrane distillation separation rate. Further, the technology of the present invention designs a highly breathable hydrophobic polyolefin non-woven fabric as a membrane support layer, and as the second hydrophobic surface of the polyvinylidene fluoride microporous membrane. At the same time, during the polyvinylidene fluoride coating process and the phase separation process, polyvinylidene fluoride micro-nano fibers are formed on the fiber surface of the polyolefin non-woven fabric, which can further improve the hydrophobicity of the second surface. The second hydrophobic surface described in the present invention can be further grafted with low surface energy groups or super hydrophobic nanomaterials to improve the super hydrophobicity of the second surface. At the same time, the second hydrophobic surface of the polyvinylidene fluoride microporous membrane is designed to face upward, and the condensate is designed to be located on it. The condensate is infinitely close to the second hydrophobic surface of the membrane under the action of gravity, so as to accelerate the condensation of water vapor passing through the membrane layer and improve the mass transfer rate of membrane distillation. At the same time, the second surface of the membrane with strong hydrophobicity or super hydrophobicity can greatly improve the surface's anti-wetting property to the condensate fluid.

[0020] Based on the above beneficial innovative design, the present invention provides a microporous polyvinylidene fluoride membrane with stable performance, a preparation method thereof, and an application in membrane distillation.

[0021] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1This is a scanning electron microscope image of the first hydrophobic surface of the polyvinylidene fluoride microporous membrane prepared in Example 1 of the present invention;

[0023] Figure 2 This is a scanning electron microscope image of the second hydrophobic surface of the polyvinylidene fluoride microporous membrane prepared in Example 1 of the present invention;

[0024] Figure 3 This is a performance change diagram of the polyvinylidene fluoride microporous membrane prepared in Example 1 of the present invention when used for membrane distillation;

[0025] Figure 4 This is a scanning electron microscope image of the surface of the polyvinylidene fluoride microporous membrane after the polyvinylidene fluoride microporous membrane prepared in Example 1 of the present invention was used for membrane distillation for 24 hours;

[0026] Figure 5 This is a scanning electron microscope image of the first hydrophobic surface of the polyvinylidene fluoride microporous membrane prepared in Comparative Example 1 of the present invention;

[0027] Figure 6 This is a scanning electron microscope image of the second hydrophobic surface of the polyvinylidene fluoride microporous membrane prepared in Comparative Example 1 of the present invention;

[0028] Figure 7 This is a performance change diagram of the polyvinylidene fluoride microporous membrane prepared in Comparative Example 1 of the present invention when used for membrane distillation;

[0029] Figure 8 This is a scanning electron microscope image of the surface of the polyvinylidene fluoride microporous membrane prepared in Comparative Example 1 of the present invention after the polyvinylidene fluoride microporous membrane was used for membrane distillation for 24 hours. DETAILED DESCRIPTION

[0030] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. Any other changes, modifications, substitutions, combinations, and simplifications made without violating the spirit and principle of the present invention should all be equivalent replacement methods and are included in the protection scope of the present invention. In addition, it should be understood that after reading the content of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims attached to this application, and all belong to the scope of protection of the present invention.

[0031] Reference to "embodiment" herein means that the specific features, structures or characteristics described in conjunction with the embodiment may be included in at least one embodiment of the present application. The term "embodiment" appearing in various places in the specification does not necessarily refer to the same embodiment, nor does it particularly limit its independence or association with other embodiments. In principle, in the present application, as long as there is no technical contradiction or conflict, the various technical features mentioned in the embodiments can be combined in any way to form a corresponding implementable technical solution.

[0032] Unless otherwise defined, the technical terms used in this document have the same meanings as those generally understood by those skilled in the art to which this application belongs; the use of relevant terms in this document is only for describing specific embodiments and is not intended to limit this application.

[0033] Unless otherwise specified in the present invention, the reagents, instruments, and equipment used are those commonly used by technicians in this field.

[0034] Example 1

[0035] This embodiment provides a method for preparing a polyvinylidene fluoride microporous membrane, comprising the following steps:

[0036] (1) 13 g of polyvinylidene fluoride and 87 g of triethyl phosphate were mixed in a three-necked flask, stirred at 80°C and 200 rpm for 6 hours, vacuum degassed, and allowed to stand for 20 hours to obtain a casting solution.

[0037] (2) A polyethylene nonwoven fabric with a water contact angle of 110° was fixed, and then the casting liquid was coated on the surface of the nonwoven fabric with a 100 μm scraper. The mixture was allowed to stand for 3 seconds, and then transferred to a 60% by volume aqueous solution of dimethylacetamide and allowed to stand for 10 seconds to obtain a preformed film.

[0038] (3) The preformed membrane is transferred into deionized water, washed and dried to obtain a polyvinylidene fluoride microporous membrane.

[0039] Figure 1 The scanning electron microscope photo of the first hydrophobic surface of the polyvinylidene fluoride microporous membrane prepared in this embodiment shows that the average pore size is about 0.3 μm and the surface porosity is about 40% through surface image integral calculation. The surface roughness of the first hydrophobic surface is about 0.8 μm through laser confocal measurement. The contact angle of the water droplet on the first hydrophobic surface is about 100° through contact angle measurement.

[0040] The prepared polyvinylidene fluoride microporous membrane was peeled off from the non-woven fabric and then brittlely broken. Scanning electron microscopy showed that the thickness of the polyvinylidene fluoride channel layer was about 85 μm, and the average pore size was 1.5 μm according to image integration.

[0041] Figure 2 This is a scanning electron microscope photograph of the second hydrophobic surface of the polyvinylidene fluoride microporous membrane obtained in this example. It can be seen that there are a large number of polyvinylidene fluoride nanofibers and nanoparticle structures on the surface of the polyolefin fiber of the non-woven fabric. Through the contact angle test, the water drop contact angle is about 135°.

[0042] The prepared polyvinylidene fluoride microporous membrane was used for membrane distillation. 0.6 mol / L NaCl was used as the raw material liquid, the water bath temperature was set at 70°C; deionized water was used as the coolant, and the low temperature constant temperature bath temperature was set at 15°C; the membrane distillation performance was good and stable, and the results were as follows Figure 3 As shown. The water flux is 17.34L / m 2 After 24 hours of operation, the performance was still stable, with a flux of 16.01 L / m 2 ·h.

[0043] Figure 4 This is a scanning electron microscope photograph of the surface of the polyvinylidene fluoride microporous membrane after membrane distillation was run for 24 hours. It can be seen that there is almost no salt crystal contamination on the surface.

[0044] Comparative Example 1

[0045] This comparative example provides a method for preparing a polyvinylidene fluoride microporous membrane, comprising the following steps:

[0046] (1) 13 g of polyvinylidene fluoride and 87 g of triethyl phosphate were mixed in a three-necked flask, stirred at 80°C and 200 rpm for 6 hours, vacuum degassed, and allowed to stand for 20 hours to obtain a casting solution.

[0047] (2) A polyethylene nonwoven fabric with a water contact angle of 110° was fixed, and then the casting liquid was coated on the surface of the nonwoven fabric with a 100 μm scraper, allowed to stand for 3 seconds, and then transferred to anhydrous ethanol and allowed to stand for 10 seconds to obtain a preformed film.

[0048] (3) The preformed membrane is transferred into deionized water, washed and dried to obtain a polyvinylidene fluoride microporous membrane.

[0049] Figure 5 The scanning electron microscope photo of the first hydrophobic surface of the polyvinylidene fluoride microporous membrane obtained in this comparative example shows that it has large surface pores and high roughness characteristics. Through surface image integral calculation, it can be known that its average pore size is about 3μm and its surface porosity is about 75%. Through laser confocal measurement, the surface roughness of its first hydrophobic surface is about 8μm. Through contact angle measurement, the water drop contact angle of its first hydrophobic surface is about 140°.

[0050] The membrane was peeled off from the non-woven fabric and broke brittlely. Scanning electron microscopy showed that the thickness of the polyvinylidene fluoride channel layer was about 80 μm, and the average pore size was 2 μm based on image integration.

[0051] Figure 6This is a scanning electron microscope photograph of the second hydrophobic surface of the polyvinylidene fluoride microporous membrane prepared in this comparative example. Through scanning electron microscope observation, a large number of polyvinylidene fluoride nanofibers and nanoparticle structures are present on the surface of the polyolefin fiber of the non-woven fabric on the second hydrophobic surface of the polyvinylidene fluoride microporous membrane. Through the contact angle test, the water droplet contact angle is about 135°.

[0052] The prepared polyvinylidene fluoride microporous membrane was used for membrane distillation. 0.6 mol / L NaCl was used as the raw material liquid at a temperature of 70°C; deionized water was used as the cooling liquid at a temperature of 15°C. The membrane distillation performance was slightly worse than that of Example 1, but the stability was also good. Figure 7 As shown. The water flux is 7.16L / m 2 After 24 hours of operation, the membrane flux decreased to 4.91L / m 2 ·h.

[0053] Figure 8 This is a scanning electron microscope photograph of the surface of the polyvinylidene fluoride microporous membrane after membrane distillation was run for 24 hours. It can be seen that a large amount of salt crystal contamination has formed on its surface.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A polyvinylidene fluoride microporous membrane, characterized in that: It includes a first hydrophobic surface, an intermediate microporous channel layer and a second hydrophobic surface; wherein the first hydrophobic surface is a polyvinylidene fluoride microporous surface, the intermediate microporous channel layer is a polyvinylidene fluoride microporous structure layer, and the second hydrophobic surface is a polyolefin and polyvinylidene fluoride composite layer; The water drop contact angle of the first hydrophobic surface is 90-110°, the surface roughness of the first hydrophobic surface is less than 2um, the average pore size of the first hydrophobic surface is 0.1-1μm, and the porosity of the first hydrophobic surface is 40%-60%; The thickness of the intermediate microporous channel layer is 10-100 μm, and the average pore size of the intermediate microporous channel layer is 0.1-10 μm; The water drop contact angle of the second hydrophobic surface is greater than 130°; The above-mentioned method for preparing a polyvinylidene fluoride microporous membrane comprises the following steps: (1) mixing polyvinylidene fluoride with a first organic solvent, stirring at 40-100° C. for 6-10 hours, and then degassing to obtain a casting solution; (2) preparing a hydrophobic polyolefin nonwoven fabric, uniformly coating the casting solution obtained in step (1) on the hydrophobic polyolefin nonwoven fabric, and then transferring the casting solution into a mixed solution of a second organic solvent and water to obtain a preformed film; (3) transferring the preformed membrane obtained in step (2) into water, washing it and drying it to obtain a polyvinylidene fluoride microporous membrane; In the step (1), the first organic solvent is one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, and trimethyl phosphate; In the step (2), the second organic solvent is one or more of dimethylformamide, dimethylacetamide, triethyl phosphate, and trimethyl phosphate.

2. Use of a polyvinylidene fluoride microporous membrane as claimed in claim 1 in membrane distillation.

3. The use of the polyvinylidene fluoride microporous membrane in membrane distillation according to claim 2, characterized in that: The first hydrophobic surface of the polyvinylidene fluoride microporous membrane faces downward, with the hot raw material liquid below, and the second hydrophobic surface of the polyvinylidene fluoride microporous membrane faces upward, with the condensate liquid above.

4. The polyvinylidene fluoride microporous membrane according to claim 1, characterized in that: In the step (1), the mass ratio of polyvinylidene fluoride to the first organic solvent is (13-28): (75-87).

5. The polyvinylidene fluoride microporous membrane according to claim 1, characterized in that: In the step (2), the hydrophobic polyolefin non-woven fabric is one or both of polyethylene non-woven fabric and polypropylene non-woven fabric; and the surface water drop contact angle of the hydrophobic polyolefin non-woven fabric is 110°-130°.

6. A polyvinylidene fluoride microporous membrane according to claim 4, characterized in that: In the mixed solution of the second organic solvent and water, the volume ratio of the second organic solvent is ≥50%.

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