A method for preparing a hydrophilic pvdf emulsion, polyvinylidene fluoride prepared by the method and applications thereof

Abstract

The application discloses a hydrophilic PVDF emulsion polymerization method, polyvinylidene fluoride prepared by the method and application, and relates to the technical field of lithium battery materials.The hydrophilic PVDF emulsion polymerization method comprises the following steps: S1, water and an emulsifier are added into a reaction device, vacuum is drawn, and inert gas is filled into the reaction device space until the oxygen content is less than or equal to 26 ppm; S2, vinylidene fluoride, an initiator and a chain transfer agent are added, and a polymerization reaction occurs at a temperature of 70-95 DEG C and a pressure of 3.5-5 MPa, wherein the chain transfer agent is a carboxyl functional group-containing thiocarbonic ester; and S3, the polymerization reaction is stopped, and the emulsion obtained through the polymerization reaction is subjected to coagulation, washing and drying to obtain the hydrophilic PVDF.Compared with traditional PVDF emulsion polymerization, more carboxyl groups can be introduced into the hydrophilic PVDF emulsion polymerization method, so that the PVDF product itself has good hydrophilicity.

Description

A hydrophilic PVDF emulsion polymerization method, the prepared polyvinylidene fluoride and its applications Technical Field

[0001] This invention relates to the field of lithium battery materials technology, and more specifically, to a hydrophilic PVDF emulsion polymerization method, the polyvinylidene fluoride prepared therefrom, and its applications. Background Technology

[0002] Polyvinylidene fluoride (PVDF) possesses excellent chemical, thermal, mechanical, and weather resistance properties, making it suitable for use as a lithium-ion battery separator and in microfiltration and ultrafiltration membrane materials, with wide applications in domestic water treatment. However, due to the strong hydrophobicity of the -CF- and -CH- bonds in the PVDF structure, PVDF exhibits poor wettability and strong hydrophobic interactions with water molecules, making it difficult for water molecules to adhere to the PVDF membrane surface and resulting in poor water flux. Furthermore, its poor hydrophilicity and good oleophilicity make it prone to oil adsorption on the membrane surface and within the pores when used as a microfiltration and ultrafiltration membrane material, further reducing water flux, complicating cleaning, and hindering its ability to remove oil contaminants.

[0003] Existing technology CN112029057A provides a hydrophilic modified polyvinylidene fluoride (PVDF) block copolymer, the preparation method of which is as follows: S1. Preparation of PVDF: PVDF monomer and a bifunctional peroxide initiator are polymerized in supercritical carbon dioxide conditions to obtain PVDF with peroxy groups at the end groups; S2. Preparation of hydrophilic modified PVDF block copolymer: Under an inert atmosphere, hydrophilic monomers and organic solvents are added to the reaction system of step S1 for copolymerization. After the reaction, post-treatment is performed to obtain the hydrophilic modified PVDF block copolymer. Its hydrophilic segments give it good hydrophilic properties, but the final water flux of the hydrophilic modified PVDF block copolymer membrane only reaches 1000 L (m³). 2 *h), that is, only 1.0m 3 / (㎡*h), the improvement in hydrophilicity of polyvinylidene fluoride materials is limited and still needs to be further improved to meet the application requirements. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects and shortcomings of existing polyvinylidene fluoride (PVDF) with poor water flux and to provide a hydrophilic PVDF emulsion polymerization method. By using a chain transfer agent with carboxyl groups during the emulsion polymerization reaction, carboxyl groups are added to the ends of PVDF through the chain transfer stage, so that the PVDF product itself has better hydrophilicity.

[0005] Another object of the present invention is to provide a hydrophilic polyvinylidene fluoride.

[0006] Another object of the present invention is to provide an application of hydrophilic polyvinylidene fluoride in the preparation of lithium battery separators, microfiltration and ultrafiltration membrane materials.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution:

[0008] A method for hydrophilic PVDF emulsion polymerization includes the following steps:

[0009] S1. Add water and emulsifier to the reaction equipment, evacuate, and fill with inert gas until the oxygen content in the reaction equipment space is ≤26ppm;

[0010] S2. Add vinylidene fluoride, initiator, and chain transfer agent, and polymerize under conventional conditions of 70-95℃ and 3.5-5MPa for 1.0-2.5 hours.

[0011] The chain transfer agent is a thiocarbonate containing a carboxyl functional group, and the amount of chain transfer agent used is 1 to 3‰ of the vinylidene fluoride monomer.

[0012] S3. Stop the polymerization reaction, and coagulate, wash and dry the emulsion obtained from the polymerization reaction to obtain hydrophilic PVDF.

[0013] It should be noted that:

[0014] In order to solve the problem of poor hydrophilicity of PVDF, the polyvinylidene fluoride emulsion polymerization of the present invention adopts a synergistic action of a thiocarbonate chain transfer agent and an initiator. After the initiator decomposes, the chain transfer agent initiates monomer polymerization to form chain-growing free radicals, which can reversibly add to -C=S- in the chain transfer agent, while -SR breaks to form new free radicals that can continue to initiate polymerization. This allows R groups containing -COOH to be attached to PVDF, increasing the carboxyl content of PVDF and improving its hydrophilicity.

[0015] Furthermore, since the chain transfer agent is acidic, it can accelerate the decomposition rate of the initiator. After the chain transfer agent decomposes, it can generate new free radicals to continue initiating the reaction, thereby increasing the reaction rate.

[0016] Chain transfer agents often reduce molecular weight, but they can also introduce carboxyl groups onto PVDF, increasing its hydrophilicity. Conversely, only a suitable ratio can control the overall effect of molecular weight and carboxyl group modification.

[0017] In a specific implementation, for example, the amount of chain transfer agent used is 2‰ of the vinylidene fluoride monomer;

[0018] For example, the amount of chain transfer agent used is 1.55‰ of the vinylidene fluoride monomer.

[0019] The hydrophilic PVDF emulsion polymerization method of the present invention improves the hydrophilicity problem by grafting carboxyl groups onto PVDF during the reaction, optimizes the production process, reduces costs, and is more environmentally friendly.

[0020] In a specific embodiment, the chain transfer agent of the present invention may preferably be 2-(dodecyltrithiocarbonate)-2-butanoic acid.

[0021] In a specific embodiment, the chain transfer agent and initiator described in step S2 of this invention are added every 35 minutes after the reaction begins, and the addition is completed in three batches. Adding them in batches allows for better control of the initiator's effect, further controlling the appropriate reaction rate.

[0022] In a specific embodiment, the initiator of the present invention is preferably an inorganic peroxide, such as ammonium persulfate or potassium persulfate, which is prepared into a solution with a concentration of 10-15 wt% and injected intermittently.

[0023] More preferably, the amount of initiator used in S2 is 0.5 to 1‰ of the mass of the vinylidene fluoride monomer. More preferably, the amount of initiator used is 1‰ of the mass of the vinylidene fluoride monomer.

[0024] In a specific embodiment, the emulsifier of the present invention is a fluorocarbon emulsifier, such as perfluorooctanoic acid (PFOA).

[0025] More preferably, the amount of emulsifier used in S1 is 1 to 2‰ of the water mass. Insufficient emulsifier will result in an unstable emulsion state.

[0026] The present invention also specifically protects a hydrophilic polyvinylidene fluoride prepared by the above-mentioned hydrophilic PVDF emulsion polymerization method.

[0027] Preferably, the carboxyl content of the hydrophilic polyvinylidene fluoride is 0.15–0.19 NaOH mol / L.

[0028] This invention also specifically protects the application of a hydrophilic polyvinylidene fluoride in the preparation of lithium battery separators, microfiltration and ultrafiltration membrane materials.

[0029] The hydrophilic polyvinylidene fluoride of this invention improves the hydrophilicity of PVDF, which can increase the wettability of water molecules on the membrane surface and pores, greatly increase water flux and oil treatment capacity, and simplify the cleaning process. It can be widely used in the preparation of lithium battery separators, microfiltration and ultrafiltration membrane materials.

[0030] The present invention also specifically protects a PVDF membrane, which is prepared from the above-mentioned hydrophilic polyvinylidene fluoride.

[0031] Preferably, the PVDF membrane comprises, by weight, the following components:

[0032] 15 parts PVDF, 6-8 parts PVP, 70-80 parts DMF.

[0033] PVDF is the hydrophilic polyvinylidene fluoride prepared in this invention. DMF is N,N-dimethylformamide.

[0034] The preparation method of PVDF membrane can be a conventional method in this field. For example, the specific preparation method can be found as follows:

[0035] PVP was added to DMF at 60-80℃ and stirred until completely dissolved. PVDF was then added and stirred at 60-80℃ for 8-12 hours. After defoaming under negative pressure, the mixture was spun to obtain a PVDF membrane. The water flux was tested using a 3H-2000PB series water flux meter.

[0036] The hydrophilic polyvinylidene fluoride of this invention increases the carboxyl content of PVDF, thus improving its hydrophilicity. The carboxyl content can reach 0.15–0.19 NaOH mol / L. The water flux of the PVDF membrane prepared using the hydrophilic PVDF of this invention can reach 57.6–62.4 m³ / L. 3 / (m²*h), water flux relative to the existing technology of 1000 L(m²) 2 *h), which is 1m3 / (㎡*h), is significantly improved, increasing the water flux of PVDF, and can be widely used in lithium battery separators as well as microfiltration and ultrafiltration membrane materials.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] This invention utilizes a chain transfer agent with carboxyl groups during the emulsion polymerization process. Through the chain transfer stage, carboxyl groups can be added to the ends of PVDF. Compared with traditional PVDF emulsion polymerization, this invention can add more carboxyl groups, giving the PVDF product itself better hydrophilicity.

[0039] The hydrophilic polyvinylidene fluoride of this invention has a carboxyl group content of 0.15–0.19 NaOH mol / L and a water flux of 57–63 m³ / L. 3 / (㎡*h). Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0041] Example 1

[0042] A method for hydrophilic PVDF emulsion polymerization includes the following steps:

[0043] S1. In a 150L polymerization reactor, add 90 kg of deionized water and 100 g of perfluorooctanoic acid (PFOA). Close the reactor, evacuate it, and replace it with nitrogen several times until the oxygen content in the reactor is less than 26 ppm.

[0044] S2. Turn on the stirrer in the reactor and heat it to 90°C. Add VDF monomer until the reactor pressure reaches 4.0 MPa. Use a metering pump to add 10 g of potassium persulfate initiator (dissolved in water to form a 10% solution) and 20 g of 2-(dodecyltrithiocarbonate)-2-succinic acid chain transfer agent to the polymerization reactor to begin the polymerization reaction.

[0045] When the pressure in the reactor decreases, add VDF monomer to the polymerization reactor to maintain the pressure between 4.0 and 4.5 MPa. After reacting for 35 minutes, continue to add 5 grams of potassium persulfate initiator (prepared with water to form a 10% solution) and 10 grams of chain transfer agent. Add once every 35 minutes, and after adding twice, stop adding initiator and chain transfer agent. Stop the reaction when the total amount of VDF monomer added reaches 20 kg.

[0046] S3. The collected polymer emulsion is filtered, demulsified, washed and dried to obtain PVDF solid powder.

[0047] The amount of chain transfer agent used is 2‰ of the total VDF monomer.

[0048] The amount of initiator used is 1‰ of the total amount of VDF monomer.

[0049] The amount of emulsifier used is 1‰ of the mass of pure water.

[0050] Example 2

[0051] A method for hydrophilic PVDF emulsion polymerization includes the following steps:

[0052] S1. In a 150L polymerization reactor, add 90 kg of deionized water and 100 g of perfluorooctanoic acid (PFOA). Close the reactor, evacuate it, and replace it with nitrogen several times until the oxygen content in the reactor is less than 26 ppm.

[0053] S2. Turn on the stirrer in the reactor and heat it to 90°C. Add VDF monomer until the reactor pressure reaches 4.0 MPa. Use a metering pump to add 8 grams of potassium persulfate initiator (dissolved in water to form a 10% solution) and 15 grams of 2-(dodecyltrithiocarbonate)-2-succinic acid chain transfer agent to the polymerization reactor to start the polymerization reaction.

[0054] As the pressure in the reactor decreases, VDF monomer is added to maintain the pressure between 4.0 and 4.5 MPa. After 35 minutes of reaction, 3 grams of potassium persulfate initiator (dissolved in water to form a 10% solution) and 8 grams of chain transfer agent are added every 35 minutes, for a total of two additions. Then, the addition of initiator and chain transfer agent is stopped. The reaction is stopped when the cumulative amount of VDF monomer added reaches 20 kg.

[0055] S3. The collected polymer emulsion is filtered, demulsified, washed and dried to obtain PVDF solid powder.

[0056] The amount of chain transfer agent used is 1.55‰ of the total VDF monomer.

[0057] The amount of initiator used is 10.7‰ of the total VDF monomer.

[0058] The amount of emulsifier used is 1‰ of the mass of pure water.

[0059] Example 3

[0060] A PVDF membrane, by weight, comprises the following components:

[0061] 15 parts PVDF, 7 parts PVP, 75 parts DMF

[0062] PVDF is the hydrophilic polyvinylidene fluoride prepared in Example 1 of this invention. DMF is N,N-dimethylformamide.

[0063] The preparation method of PVDF membrane is as follows:

[0064] PVP was added to DMF at 60-80℃ and stirred until completely dissolved. PVDF was then added and stirred at 60-80℃ for 8-12 hours. After defoaming under negative pressure, the mixture was spun to obtain a PVDF membrane. The water flux was tested using a 3H-2000PB series water flux meter.

[0065] Example 4

[0066] A PVDF membrane, by weight, comprises the following components:

[0067] 15 parts PVDF, 7 parts PVP, 75 parts DMF

[0068] PVDF is the hydrophilic polyvinylidene fluoride prepared in Example 2 of this invention. DMF is N,N-dimethylformamide.

[0069] The preparation method of PVDF membrane is as follows:

[0070] PVP was added to DMF at 60-80℃ and stirred until completely dissolved. PVDF was then added and stirred at 60-80℃ for 8-12 hours. After defoaming under negative pressure, the mixture was spun to obtain a PVDF membrane. The water flux was tested using a 3H-2000PB series water flux meter.

[0071] Comparative Example 1

[0072] A method for PVDF emulsion polymerization includes the following steps:

[0073] S1. In a 150L polymerization reactor, add 90 kg of deionized water and 100 g of perfluorooctanoic acid (PFOA). Close the reactor, evacuate it, and replace it with nitrogen several times until the oxygen content in the reactor is less than 26 ppm.

[0074] S2. Turn on the stirrer in the reactor and heat the reactor to 90°C. Add VDF monomer until the reactor pressure reaches a certain level. Use a metering pump to add 10 grams of potassium persulfate initiator (dissolved in water to make a 10% solution) and 20 grams of ethyl acetate chain transfer agent to the polymerization reactor to start the polymerization reaction.

[0075] As the pressure in the reactor decreases, VDF monomer is added to maintain the pressure between 4.0 and 4.5 MPa. After reacting for 35 minutes, 5 grams of potassium persulfate initiator (dissolved in water to form a 10% solution) and 10 grams of chain transfer agent are added every 35 minutes, for a total of two additions. Then, the addition of initiator and chain transfer agent is stopped. The reaction is stopped when the cumulative amount of VDF monomer added reaches 20 kg.

[0076] S3. The collected polymer emulsion is filtered, demulsified, washed and dried to obtain PVDF solid powder.

[0077] Comparative Example 2

[0078] A method for PVDF emulsion polymerization includes the following steps:

[0079] S1. In a 150L polymerization reactor, add 90 kg of deionized water and 100 g of perfluorooctanoic acid (PFOA). Close the reactor, evacuate it, and replace it with nitrogen several times until the oxygen content in the reactor is less than 26 ppm.

[0080] S2. Turn on the stirrer in the reactor and heat the reactor to 90°C. Add VDF monomer until the reactor pressure reaches a certain level. Use a metering pump to add 8 grams of potassium persulfate initiator (dissolved in water to make a 10% solution) and 15 grams of ethyl acetate chain transfer agent to the polymerization reactor to start the polymerization reaction.

[0081] When the pressure in the reactor decreases, add VDF monomer to the polymerization reactor to maintain the pressure between 4.0-4.5 MPa. After reacting for 35 minutes, continue to add 3 grams of potassium persulfate initiator (prepared into a 10% solution with water) and 8 grams of chain transfer agent. Add once every 35 minutes. After adding twice, stop adding initiator and chain transfer agent. Stop the reaction when the total amount of VDF monomer added is 20 kg.

[0082] S3. The collected polymer emulsion is filtered, demulsified, washed and dried to obtain PVDF solid powder.

[0083] Comparative Example 3

[0084] A PVDF membrane, by weight, comprises the following components:

[0085] 15 parts PVDF, 7 parts PVP, 75 parts DMF

[0086] PVDF is the hydrophilic polyvinylidene fluoride prepared in Comparative Example 1 of this invention. DMF is N,N-dimethylformamide.

[0087] The preparation method of PVDF membrane is as follows:

[0088] PVP was added to DMF at 60-80℃ and stirred until completely dissolved. PVDF was then added and stirred at 60-80℃ for 8-12 hours. After defoaming under negative pressure, the mixture was spun to obtain a PVDF membrane. The water flux was tested using a 3H-2000PB series water flux meter.

[0089] Comparative Example 4

[0090] A PVDF membrane, by weight, comprises the following components:

[0091] 15 parts PVDF, 7 parts PVP, 75 parts DMF

[0092] PVDF is the hydrophilic polyvinylidene fluoride prepared in Comparative Example 2 of this invention. DMF is N,N-dimethylformamide.

[0093] The preparation method of PVDF membrane is as follows:

[0094] PVP was added to DMF at 60-80℃ and stirred until completely dissolved. PVDF was then added and stirred at 60-80℃ for 8-12 hours. After defoaming under negative pressure, the mixture was spun to obtain a PVDF membrane. The water flux was tested using a 3H-2000PB series water flux meter.

[0095] Result detection

[0096] The hydrophilicity of the PVDs prepared in the above examples and comparative examples was tested, and the specific testing methods are as follows:

[0097] (1) Water flux test: Apply a given pressure to both sides of the filter surface of the filter material to be tested, and measure the cumulative water flow rate within 1 hour to obtain the water flux of the filter material. PVDF membrane preparation method: 15 parts PVDF, 7 parts PVP, and 75 parts DMF. Add PVP to DMF at 70 degrees and stir until completely dissolved. Add PVDF and stir at 70 degrees for 10 hours. After defoaming under negative pressure, spin to obtain PVDF membrane. Test the water flux using a 3H-2000PB series water flux tester.

[0098] (2) Carboxyl content test: The carboxyl content of PVDF is determined by acid-base potentiometric titration of the PVDF product. The test method is as follows: 1 part PVDF, 200 parts acetone, and 20 parts deionized water are stirred for 20 minutes. A 0.1 mol / L NaOH titrant is prepared, and the potential is measured using a potentiometric titrator. Titration is performed to -270 Mv. The titration amount is the carboxyl content.

[0099] The specific test results are shown in Table 1 below:

[0100] Table 1 Performance Test Results

[0101]

[0102]

[0103] As can be seen from the data in Table 1 above, the hydrophilic PVDF emulsion polymerization method of the present invention solves the problem of poor hydrophilicity of PVDF. It employs a synergistic effect of a thiocarbonate chain transfer agent and an initiator. After the initiator decomposes, this type of chain transfer agent initiates monomer polymerization to form chain-growing free radicals, which can reversibly add to -C=S- in the chain transfer agent. Meanwhile, the -SR group breaks to form new free radicals that can continue to initiate polymerization. This allows R groups containing -COOH to be attached to the PVDF, increasing the carboxyl content of the PVDF and improving its hydrophilicity. The carboxyl content can reach 0.15–0.19 NaOH mol / L.

[0104] Furthermore, the water flux of the PVDF membrane prepared using the hydrophilic PVDF of this invention can reach 57.6–62.4 m³ / (m²*h), which is significantly higher than the 1000 L / (m²*h) of the prior art. 2 *h), which is 1m3 / (㎡*h), is significantly improved, increasing the water flux of PVDF, and can be widely used in lithium battery separators as well as microfiltration and ultrafiltration membrane materials.

[0105] In Comparative Examples 1 and 2, the type of chain transfer agent was changed. The chain transfer agent used in Comparative Examples 1 and 2 was ethyl acetate, instead of the thiocarbonate chain transfer agent protected in this invention. Obviously, it cannot introduce -COOH during monomer polymerization, and the final carboxyl content is 0.1 NaOH mol / L or less. Correspondingly, the water flux of the PVDF membranes prepared in Comparative Documents 1 and 2 does not achieve the water flux improvement effect of this invention, only reaching 39.1–41.5 m. 3 / (㎡*h) indicates that the chain transfer agent of the present invention has unexpected beneficial effects.

[0106] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for hydrophilic PVDF emulsion polymerization, characterized in that, The process includes the following steps: S1. Add water and emulsifier to the reaction equipment, evacuate, and fill with inert gas until the oxygen content in the reaction equipment space is ≤26ppm; S2. Add vinylidene fluoride monomer, initiator, and chain transfer agent, and carry out a polymerization reaction at a temperature of 70-95℃ and a pressure of 3.5-5MPa for 1.0-2.5h; S3. Stop the polymerization reaction, and coagulate, wash, and dry the emulsion obtained from the polymerization reaction to obtain hydrophilic PVDF; the chain transfer agent mentioned in step S2 is 2-(dodecyltrithiocarbonate)-2-butanoic acid; the amount of the chain transfer agent is 1-3‰ of the vinylidene fluoride monomer.

2. The hydrophilic PVDF emulsion polymerization method as described in claim 1, characterized in that, The chain transfer agent and initiator described in S2 are added every 20-50 minutes after the reaction begins, in three separate additions.

3. The hydrophilic PVDF emulsion polymerization method as described in claim 1, characterized in that, The initiator mentioned in S2 is an inorganic peroxide.

4. The hydrophilic PVDF emulsion polymerization method as described in claim 1, characterized in that, The amount of initiator used in S2 is 0.5 to 1‰ of the mass of vinylidene fluoride monomer.

5. The hydrophilic PVDF emulsion polymerization method as described in claim 1, characterized in that, The emulsifier mentioned in S1 is a fluorocarbon emulsifier.

6. The hydrophilic PVDF emulsion polymerization method according to claim 1, characterized in that, The amount of emulsifier used in S1 is 1 to 2‰ of the water mass.

7. A hydrophilic polyvinylidene fluoride prepared by the hydrophilic PVDF emulsion polymerization method according to claims 1 to 6.

8. The application of the hydrophilic polyvinylidene fluoride of claim 7 in the preparation of lithium battery separators, microfiltration and ultrafiltration membrane materials.

9. A PVDF membrane, characterized in that, The PVDF membrane is prepared from the hydrophilic polyvinylidene fluoride described in claim 7.

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