Preparation method of casting solution and preparation method of amphiphilic PVDF-based membrane sheet

Abstract

The application discloses a preparation method of a casting solution and a preparation method of an amphiphilic PVDF-based film sheet, solves the technical problems of easy elution and instability of the casting solution in the blending modification method in the prior art, and has the advantages that the prepared PVDF film sheet has good quality, good hydrophilicity and high water flux. The preparation method of the casting solution for preparing the amphiphilic PVDF-based film sheet comprises the following steps: (1) dispersing PVDF in an organic solvent to obtain a first solution; (2) adding a polymerization monomer to the first solution, and then performing a polymerization reaction to obtain a second solution; the polymerization monomer has a hydrophilic group and a hydrophobic group; and (3) adding a pore-forming agent to the casting solution to obtain the casting solution. The preparation method of the amphiphilic PVDF-based film sheet comprises the following steps: (1) coating the casting solution prepared by the preparation method in the first aspect on a carrier to form a film blank; and (2) performing a coagulation treatment on the film blank to obtain the amphiphilic PVDF-based film sheet.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of membrane materials for sewage treatment, in particular to a preparation method of a casting solution and a preparation method of an amphiphilic PVDF-based membrane. BACKGROUND

[0002] Membrane technology has developed rapidly in recent years due to its small footprint, high separation efficiency, good water treatment quality, and automatic operation. In particular, low-pressure membrane technology, including microfiltration and ultrafiltration, has become a hot topic in water treatment research.

[0003] Membrane fouling is the biggest problem in membrane separation technology, which seriously restricts the feasibility and operating cost of membrane technology. Membrane fouling is caused by the accumulation and deposition of substances on the membrane surface and inside, which can cause the membrane pore size to decrease or be blocked, further causing a decrease in membrane permeation flux and an increase in transmembrane pressure, ultimately significantly reducing water treatment performance. During use, as membrane fouling accumulates, the separation performance and permeation flux of the membrane will deteriorate irreversibly. Therefore, reducing membrane fouling not only improves the treatment efficiency of membrane technology, but also further reduces the frequency of membrane cleaning and other operations, thereby saving costs.

[0004] In the development of membrane separation technology, the main membrane materials used are organic polymers. Common polymer membrane materials include: polysulfone (PES), polyacrylonitrile (PAN), polysulfone (PSf), polytetrafluoroethylene (PTFE), polypropylene (PP), and polyvinylidene fluoride (PVDF). Among them, PVDF material has excellent film-forming properties, stability, and mechanical strength, making it the preferred material for preparing porous separation membranes.

[0005] However, due to the low surface energy of PVDF membranes, the wettability of the PVDF membrane surface is poor, and water droplets are difficult to spread on the PVDF membrane surface. The hydrophobicity of PVDF itself makes it easy to be adsorbed by pollutants when treating wastewater containing organic pollutants, especially for proteins and other organic matter, which are easily adsorbed on the surface and membrane pores of the PVDF membrane, resulting in a decrease in water flux of the separation membrane, a shortened service life, and an increased cost due to frequent cleaning. Therefore, how to improve the water flux and anti-pollution performance of PVDF membranes is a research focus in the field of membrane separation.

[0006] Generally, the higher the hydrophilicity of the membrane surface, the stronger the anti-pollution performance. Therefore, forming a highly hydrophilic hydrophilic layer on the surface of the PVDF membrane can effectively prevent the adsorption and deposition of pollutants, thereby increasing the anti-pollution ability. Currently, the main modification methods to improve the hydrophilicity of PVDF membranes include surface modification and blending modification.

[0007] Surface modification refers to functional modification of the surface of the separation membrane sheet, which can be divided into surface coating and surface grafting, etc. Surface coating is a method of improving hydrophilicity by physically coating or depositing hydrophilic reagents on the surface of the membrane sheet. This method is simple and convenient. However, the binding force between the general hydrophilic polymer and the hydrophobic membrane substrate is poor, which leads to that the hydrophilic polymer coating is easily detached during use, and it is difficult to maintain long-term stability. Surface grafting is to fix the hydrophilic polymer on the surface of the membrane sheet through chemical bonds, but this method is complex and high in production cost, which is not suitable for industrialization.

[0008] Blending modification refers to dispersing or dissolving the hydrophilic modifier and the membrane substrate in the same solvent to form a uniformly dispersed casting solution, and then obtaining a hydrophilic composite modified membrane through phase inversion. The commonly used hydrophilic modifier can be divided into two categories, namely hydrophilic polymer material and hydrophilic inorganic nanoparticles. Blending modification has both longer service life than surface coating and more industrialized process than surface grafting. However, the hydrophilic modifier has the following shortcomings: (1) The hydrophilic polymer material has poor compatibility with the hydrophobic PVDF, which is easily dissolved out during phase transition. (2) The hydrophilic inorganic nanoparticles have the shortcomings of easy agglomeration and uneven distribution in the membrane matrix, which causes instability of the casting solution and easily causes changes in the morphology, structure and performance of the membrane, and potential weakening of the anti-pollution performance of the inorganic nanoparticles. SUMMARY

[0009] The main purpose of the present application is to provide a preparation method of casting solution and a preparation method of amphiphilic PVDF membrane sheet, so as to solve the technical problems of easy dissolution and instability of the casting solution in the blending modification method in the prior art. The prepared PVDF membrane sheet has good quality, good hydrophilicity and high water flux.

[0010] In order to achieve the above purpose, according to the first aspect of the present application, a preparation method of casting solution for preparing amphiphilic PVDF membrane sheet is provided, and the technical scheme is as follows:

[0011] The preparation method of casting solution for preparing amphiphilic PVDF membrane sheet comprises the following steps:

[0012] (1) dispersing PVDF in an organic solvent to obtain a first solution;

[0013] (2) adding a polymer monomer to the first solution, and then performing a polymerization reaction to obtain a second solution; the polymer monomer has a hydrophilic group and a hydrophobic group;

[0014] (3) adding a pore-forming agent to the second solution to obtain a casting solution.

[0015] As a further improvement of the above first aspect: in step (1), the organic solvent is first heated to 35-60℃, then PVDF is added, and constant temperature stirring is carried out for 12-24 hours to obtain the first solution.

[0016] As a further improvement of the above first aspect: the organic solvent is any of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, triethyl phosphate, N-methyl pyrrolidone, sulfolane, and glyceryl triacetate.

[0017] As a further improvement of the above first aspect: the polymerization monomer includes hydrophilic substances and hydrophobic substances, the hydrophilic substances are any of maleic anhydride, acrylic acid, and acrylamide, and the hydrophobic substances are any of styrene, methyl methacrylate, and butyl acrylate; the weight ratio of the hydrophobic substances to the hydrophilic substances is 1: (1.5-3).

[0018] As a further improvement of the above first aspect: in step (2), an initiator is also added to the first solution; the initiator is any of benzoyl peroxide, lauryl peroxide, t-butyl peroxybenzoate, di-t-butyl peroxide, potassium persulfate, and sodium persulfate.

[0019] As a further improvement of the above first aspect: in step (2), the temperature of the first solution is heated to 70-100℃, the polymerization monomer and the initiator are first added and stirred for 12-24 hours, then naturally cooled to 35-50℃ and stirred for 8-16 hours, and finally heated to 70-100℃ and stirred for 12-24 hours to obtain the second solution.

[0020] As a further improvement of the above first aspect: in step (3), the pore-forming agent is PVP and / or PEG, and the pore-forming agent is added to the second solution at 70-80℃ and stirred for 12-24 hours to obtain the casting solution.

[0021] As a further improvement of the above first aspect: it further includes step (4): the casting solution is subjected to vacuum degassing treatment, the temperature of the vacuum degassing treatment is 50-70℃, and the treatment time is 10-16 hours.

[0022] As a further improvement of the above first aspect: the weight ratio of the organic solvent, PVDF, polymerization monomer, and pore-forming agent is (60-80):(10-15):(5-9):(5-12).

[0023] In order to achieve the above purpose, according to the second aspect of the present application, a preparation method of an amphiphilic PVDF membrane is provided, and the technical scheme is as follows:

[0024] The preparation method of the amphiphilic PVDF base membrane includes the following steps:

[0025] (1) coating the casting solution prepared by the preparation method of the first aspect on a carrier to form a membrane blank;

[0026] (2) performing coagulation treatment on the membrane blank to obtain the amphiphilic PVDF membrane sheet.

[0027] As a further improvement of the second aspect, the coagulation treatment is placing the membrane blank in pure water, the temperature of the pure water is 8-85℃, and the coagulation time is 30-120 minutes.

[0028] The present application has the following advantages: (1) the polymerized monomer has hydrophilic groups and hydrophobic groups, the hydrophilic groups can be enriched on the pore surface of the membrane sheet through the effect of fractionation during the phase inversion process, thereby maximizing the hydrophilicity of the membrane sheet, and the hydrophobic groups are combined with PVDF through hydrophobic effect, ensuring the uniformity and stability of the dispersing of the modifier in the membrane matrix, so that it will not be dissolved out during the membrane separation process. (2) The polymerized monomer is polymerized in situ in the PVDF matrix, further improving the homogeneity of the casting solution and the binding force between the modifier and the PVDF matrix, thereby significantly improving the stability of the casting solution, helping to improve the forming rate of the membrane blank and the membrane sheet, and significantly improving the production efficiency. (3) The raw materials used are green and environmentally friendly, easy to obtain and low in cost. (4) The preparation process is simple and controllable, does not increase the original PVDF membrane sheet production process, has low production cost, and is suitable for industrial application. It has been verified that the PVDF membrane sheet prepared by the preparation method of the present application has both hydrophilicity and hydrophobicity, wherein the pure water contact angle is as low as 10°, the membrane sheet water flux is above 477LMH, which is very suitable for solid-liquid membrane separation treatment of industrial wastewater, and the forming rate of the membrane blank and the membrane sheet is good, and the production efficiency is high, therefore, the present application has strong practicability.

[0029] The present application will be further described by specific embodiments. Additional aspects and advantages of the present application will be partially given in the following description, partially will become obvious from the following description, or will be understood by the practice of the present application. DETAILED DESCRIPTION

[0030] The present application will be further described by specific embodiments. Additional aspects and advantages of the present application will be partially given in the following description, partially will become obvious from the following description, or will be understood by the practice of the present application.

[0031] The technical solutions and technical features provided in each part of the present application, including the following description, can be combined with each other without conflict.

[0032] In addition, the embodiments of the present application involved in the following description are generally only a part of the embodiments of the present application, rather than all the embodiments. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments in the present application without creative labor should belong to the scope of protection of the present application.

[0033] Regarding the terms and units in the present application. The terms "comprise", "have" and any variations thereof in the description and claims of the present application and related parts are intended to cover non-exclusive inclusion.

[0034] The specific embodiment of the preparation method of the casting solution for preparing the amphiphilic PVDF-based membrane sheet of the present application comprises the following steps:

[0035] (1) dispersing PVDF in an organic solvent to obtain a first solution;

[0036] First, heat the organic solvent to 35-60℃, then add PVDF, and stir at constant temperature for 12-24 hours to obtain the first solution; the organic solvent is any of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, triethyl phosphate, N-methyl pyrrolidone, sulfolane, and glyceryl triacetate.

[0037] (2) adding a polymerization monomer to the first solution, and then performing a polymerization reaction to obtain a second solution; the polymerization monomer has a hydrophilic group and a hydrophobic group;

[0038] The polymerization monomer includes a hydrophilic substance and a hydrophobic substance, the hydrophilic substance is any of maleic anhydride, acrylic acid, and acrylamide, and the hydrophobic substance is any of styrene, methyl methacrylate, and butyl acrylate.

[0039] It is preferred that the use amount of the hydrophilic substance is greater than that of the hydrophobic substance, so as to ensure the hydrophilicity of the obtained membrane sheet. When the weight ratio of the hydrophobic substance to the hydrophilic substance is 1: (1.5-3), not only the hydrophilicity is good, but also the obtained casting solution does not show a dissolution phenomenon.

[0040] In order to improve the hydrophilicity, it is preferred to use at least two kinds of hydrophilic substances.

[0041] In order to improve the reaction efficiency, it is preferred to add an initiator to the first solution; the initiator is any of benzoyl peroxide, lauroyl peroxide, tert-butyl benzene hydroperoxide, di-tert-butyl peroxide, potassium persulfate, and sodium persulfate. When an initiator is used, the use amount of the initiator is preferably 0.2-3% of the total weight of the casting solution.

[0042] The temperature of the first solution is heated to 70-100℃, the polymerization monomer and initiator are first added and stirred for 12-24 hours, then naturally cooled to 35-50℃ and stirred for 8-16 hours, and finally heated to 70-100℃ and stirred for 12-24 hours, to obtain the second solution. It has been verified that when the above special heat treatment process is used, the polymerization reaction efficiency can be significantly improved, the polymerization reaction degree is appropriate, and the performance of the obtained PVDF-based film sheet is better.

[0043] (3) adding a pore-forming agent to the second solution to obtain a casting solution;

[0044] The pore-forming agent is PVP (polyvinylpyrrolidone) and / or PEG (polyethylene glycol), and when PVP and PEG are used together, it helps to obtain a film sheet with uniform pore distribution and high water flux.

[0045] Preferably, the pore-forming agent is added to the second solution at 70-80℃, and after stirring for 12-24 hours, a more homogeneous casting solution is obtained.

[0046] In order to improve the stability of the casting solution, the casting solution is preferably subjected to vacuum degassing treatment, and the temperature of the vacuum degassing treatment is 50-70℃, and the treatment time is 10-16 hours.

[0047] In the above preparation method of the casting solution, when the weight ratio of the organic solvent, PVDF, polymerization monomer, and pore-forming agent is (60-80):(10-15):(5-9):(5-12), the obtained casting solution has very uniform dispersibility and stability, and the obtained film sheet not only has high quality and long service life, but also has good hydrophilicity and water flux. When the pore-forming agent is PVP and PEG, the weight ratio of the organic solvent, PVDF, polymerization monomer, PVP, and PEG is (60-80):(10-15):(5-9):(3-7):(2-5).

[0048] The specific embodiment of the preparation method of the amphiphilic PVDF-based film sheet of the present application comprises the following steps:

[0049] (1) using a doctor blade to coat the casting solution prepared by the above preparation method on a non-woven fabric to form a film blank;

[0050] (2) subjecting the film blank to coagulation treatment to obtain the amphiphilic PVDF-based film sheet.

[0051] The coagulation treatment is to place the film blank in pure water. After the film blank is placed in pure water, the casting solution on the non-woven fabric will complete phase separation in a short time. In order to ensure complete phase separation and good uniformity of the obtained film, the temperature of the pure water is preferably 8-85℃, and the coagulation time is 30-120 minutes.

[0052] The advantageous effects of the present application are illustrated below through specific examples.

[0053] Example 1

[0054] Preparation of the casting solution: (1) The organic solvent was heated to 60°C, and then PVDF was added, and constant temperature stirring was performed for 12 hours, to obtain a first solution. (2) The temperature of the first solution was heated to 80°C, and the polymerization monomer and the initiator were first added and stirred for 12 hours, and then the temperature was naturally lowered to 40°C and stirred for 12 hours, and finally the temperature was raised to 80°C and stirred for 24 hours, to obtain a second solution. (3) PVP and PEG were added to the second solution at 75°C, and vacuum degassing treatment was performed after stirring for 24 hours, with a temperature of 60°C and a treatment time of 12 hours, to obtain the casting solution.

[0055] The weight fractions of the organic solvent, PVDF, the polymerization monomer, PVP and PEG were 73 parts, 12 parts, 7.5 parts, 4.5 parts and 2.5 parts, respectively. The organic solvent was dimethyl sulfoxide. The polymerization monomer was styrene and maleic anhydride, with weight fractions of 3 parts and 4.5 parts, respectively. The initiator was tert-butyl peroxybenzoate, and the usage amount of the initiator was 0.5% of the total weight of the casting solution.

[0056] Preparation of the amphiphilic PVDF-based membrane sheet: The casting solution was blade-coated onto the non-woven fabric using a 200μm knife gap, with the knife moving at a speed of 2 m / min, to obtain a membrane blank. The membrane blank was placed into a gel bath after volatilizing in air for 15s, and the gelation time was 60 minutes, to obtain the amphiphilic PVDF-based membrane sheet.

[0057] Example 2

[0058] Compared with Example 1, the difference of the present example is that the polymerization monomer is styrene, maleic anhydride and acrylamide, with weight fractions of 3 parts, 2 parts and 2.5 parts, respectively.

[0059] Example 3

[0060] Compared with Example 1, the difference of the present example is that the weight fractions of the organic solvent, PVDF, the polymerization monomer, PVP and PEG are 73.5 parts, 11 parts, 7 parts, 5 parts and 3 parts, respectively. The organic solvent is dimethylacetamide. The polymerization monomer is styrene, maleic anhydride and acrylamide, with weight fractions of 2 parts, 3 parts and 2 parts, respectively.

[0061] Example 4

[0062] Compared with Example 1, the difference of the present example is that the weight fractions of organic solvent, PVDF, polymerizable monomer, PVP and PEG are 71 parts, 13 parts, 8 parts, 4.5 parts and 3 parts respectively. The polymerizable monomers are methyl methacrylate, acrylic acid and acrylamide, and the weight fractions are 2 parts, 3 parts and 3 parts respectively.

[0063] Example 5

[0064] Compared with Example 1, the difference of the present example is that the weight fractions of organic solvent, PVDF, polymerizable monomer, PVP and PEG are 60 parts, 10 parts, 5 parts, 3 parts and 2 parts respectively. The polymerizable monomers are butyl acrylate, maleic anhydride, acrylic acid and acrylamide, and the weight fractions are 2 parts, 1 part, 1 part and 1 part respectively.

[0065] Example 6

[0066] Compared with Example 1, the difference of the present example is that the weight fractions of organic solvent, PVDF, polymerizable monomer, PVP and PEG are 80 parts, 15 parts, 9 parts, 7 parts and 5 parts respectively. The polymerizable monomers are styrene, maleic anhydride, acrylic acid and acrylamide, and the weight fractions are 3 parts, 2 parts, 2 parts and 2 parts respectively.

[0067] The contact angle of the amphiphilic PVDF-based membrane sheet of Examples 1-6 was tested by using “GB T 30447-2013 Nanometer Thin Film Contact Angle Measurement Method”.

[0068] The water flux of the amphiphilic PVDF-based membrane sheet of Examples 1-6 was tested by using an ultrafiltration membrane test platform. Specifically, the water inlet pressure was slowly adjusted to 0.01 MPa, and after the pressure was stable for 10 min, a certain volume of produced water was collected by using a graduated cylinder, and the time taken was recorded by using a stopwatch. The water flux was calculated by using the following formula:

[0069]

[0070] Wherein, J is the water flux, unit is L / (m 2 ·h); V is the volume of the graduated cylinder, unit is L; T is the sampling time, unit is h; A is the effective area of the membrane, unit is m 2 .

[0071] The test results of the contact angle and the water flux are shown in Table 1.

[0072] Table 1

[0073]

[0074] As can be seen from Table 1, the amphiphilic PVDF-based membrane pieces of Examples 1-6 all exhibit excellent hydrophilicity and water flux. It is found in practice that the preparation method of the amphiphilic PVDF-based membrane piece of the present application exhibits very good production efficiency in actual production, and the obtained membrane piece not only has good apparent quality and high forming rate, but also has significantly improved anti-pollution ability when applied to flat membrane equipment, roll membrane equipment and disc tube equipment, etc., and has good economic value.

[0075] The above describes the relevant content of the present application. Those skilled in the art will be able to implement the present application based on these descriptions. Based on the above content of the present application, all other embodiments obtained by those skilled in the art without creative labor shall fall within the scope of protection of the present application.

Claims

1. A method for preparing a casting solution for preparing amphiphilic PVDF-based films, characterized in that: Includes the following steps: (1) Disperse PVDF in an organic solvent to obtain the first solution; (2) Add the polymerizing monomer and initiator to the first solution, and then carry out the polymerization reaction to obtain the second solution; the polymerizing monomer has hydrophilic groups and hydrophobic groups; (3) Add a pore-forming agent to the second solution to obtain a casting solution; In step (2): The polymer monomer includes hydrophilic and hydrophobic substances, wherein the hydrophilic substance is any of maleic anhydride, acrylic acid, and acrylamide, and the hydrophobic substance is any of styrene, methyl methacrylate, and butyl acrylate. The initiator is any of the following: benzoyl peroxide, lauroyl peroxide, tert-butyl peroxide, di-tert-butyl peroxide, potassium persulfate, and sodium persulfate. The temperature of the first solution is heated to 70-100℃. First, the monomer and initiator are added and stirred for 12-24 hours. Then, the temperature is naturally cooled to 35-50℃ and stirred for 8-16 hours. Finally, the temperature is raised to 70-100℃ and stirred for 12-24 hours to obtain the second solution.

2. The method for preparing the casting solution for preparing amphiphilic PVDF-based films as described in claim 1, characterized in that: In step (1), the organic solvent is first heated to 35-60°C, then PVDF is added, and the mixture is stirred at a constant temperature for 12-24 hours to obtain the first solution; the organic solvent is any of the following: dimethylformamide, dimethylacetamide, dimethyl sulfoxide, triethyl phosphate, N-methylpyrrolidone, sulfolane, and triacetylglycerol.

3. The method for preparing the casting solution for preparing amphiphilic PVDF-based films as described in claim 1, characterized in that: The weight ratio of the hydrophobic substance to the hydrophilic substance is 1:(1.5-3).

4. The method for preparing the casting solution for preparing amphiphilic PVDF-based films as described in claim 1, characterized in that: In step (3), the pore-forming agent is PVP and / or PEG. The pore-forming agent is added to the second solution at 70-80°C and stirred for 12-24 hours to obtain the casting solution.

5. The method for preparing the casting solution for preparing amphiphilic PVDF-based films as described in claim 1, characterized in that: It also includes step (4): vacuum degassing treatment of the casting solution, the temperature of which is 50-70℃ and the treatment time is 10-16 hours.

6. The method for preparing the casting solution for preparing amphiphilic PVDF-based films as described in claim 1, characterized in that: The weight ratio of organic solvent, PVDF, polymer monomer, and pore-forming agent is (60-80): (10-15): (5-9): (5-12).

7. A method for preparing an amphiphilic PVDF-based film, characterized in that: Includes the following steps: (1) The casting solution prepared by any one of claims 1-6 is coated onto a carrier to form a film preform; (2) The membrane blank is solidified to obtain an amphiphilic PVDF-based membrane.

8. The method for preparing the amphiphilic PVDF-based film as described in claim 7, characterized in that: The coagulation process involves immersing the film preform in pure water at a temperature of 8–85°C for 30–120 minutes.

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