A method for purifying a polyvinylidene fluoride polymer product

Abstract

This invention provides a method for purifying polyvinylidene fluoride (PVDF) polymerization products. The PVDF slurry, a product of the polymerization reaction, is filtered to obtain a PVDF filter cake. The PVDF filter cake is washed with pure water, then washed again with a mixture of an organic solvent and pure water, filtered, and dried to obtain purified PVDF. The organic solvent is soluble in water and can dissolve the PVDF. The organic solvent accounts for 1-20% of the mixture of organic solvent and pure water. The purification method for PVDF polymerization products provided by this invention can significantly reduce the ion content in the PVDF polymerization products. The obtained PVDF product has high purity, a beautiful appearance, uniform particle size, good flowability, and no unpleasant odor. Furthermore, the method is convenient to operate, rationally designed, and has broad application prospects.

Description

Technical Field

[0001] This invention relates to the field of post-processing of polymer products, and more particularly to a method for purifying polyvinylidene fluoride polymer products. Background Technology

[0002] Polyvinylidene fluoride (PVDF) is a high molecular weight, semi-crystalline thermoplastic with excellent mechanical properties, high and low temperature resistance, weather resistance, radiation resistance and chemical corrosion resistance. It is widely used in the preparation of high value-added powder coatings, membrane materials and other high-performance composite materials.

[0003] The main methods for synthesizing polyvinylidene fluoride (PVDF) are suspension polymerization and emulsion polymerization. Suspension polymerization involves suspending PVDF monomers as microparticles or granules in a suitable medium, typically water, organic solvents, and surfactants. A suitable initiator is then added to generate free radicals, initiating the polymerization reaction. Initiators are usually persulfate or hydrogen peroxide. Emulsion polymerization involves mixing PVDF monomers with emulsifiers, stabilizers, and water, stirring to form a homogeneous emulsion system. The emulsifier is an anionic surfactant, such as sodium hexadecyl sulfonate. An initiator is then added to generate free radicals, initiating the polymerization reaction. Initiators are usually persulfate or hydrogen peroxide.

[0004] After polymerization, due to various reasons, such as ionic impurities introduced by raw materials like initiators, surfactants, and comonomers in the polymerization system, or ionic impurities generated by corrosion of the reactor by initiators, these impurities are adsorbed onto the surface and interior of the polymer resin. In existing technologies, removing ionic impurities from the surface or interior of the polymer resin involves washing with water. For example, CN102336854A discloses a method for preparing high-heat-resistant polyvinylidene fluoride (PVDF), which involves washing the PVDF slurry with deionized water until the foam disappears. Repeated water washing wastes water resources, and because water cannot penetrate the interior of the PVDF, the cleaning effect is poor. Patent CN 105924554A discloses a method for preparing PVDF resin, in which a strong oxidant is added in the washing step at a ratio of 0.01-0.1% by weight relative to the wet PVDF material. This allows residual organic dispersants in the resin to be thoroughly washed out, resulting in a PVDF resin with excellent heat resistance. However, this method cannot remove ionic impurities. Existing technologies cannot meet the growing demand for high-purity polyvinylidene fluoride polymer resins, such as PVDF dielectric films, lithium battery separators, and lithium battery binders. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for purifying polyvinylidene fluoride (PVDF) polymer products, which can significantly reduce the ion content in PVDF polymer products.

[0006] To achieve the above objectives, the purification method for polyvinylidene fluoride polymer provided by this invention adopts the following technical solution:

[0007] A method for purifying polyvinylidene fluoride (PVDF) polymerization products involves filtering a PVDF slurry to obtain a PVDF filter cake; washing the PVDF filter cake with pure water, then washing it with a mixture of an organic solvent and pure water, filtering and drying to obtain purified PVDF; wherein the organic solvent is soluble in water and can dissolve the PVDF, and the organic solvent accounts for 1-20% of the mixture of the organic solvent and pure water.

[0008] This invention involves filtering polyvinylidene fluoride (PVDF) slurry prepared by suspension or emulsion methods. The PVDF filter cake obtained from the initial filtration has a high impurity content. It is first washed with pure water to remove other impurities, and then washed with a mixture of organic solvent and water to specifically remove ionic impurities. The organic solvent is soluble in water and can swell and dissolve PVDF. The mixture of organic solvent and water is a homogeneous system. In this homogeneous system, the organic solvent assists water in penetrating to the surface and interior of the PVDF resin. The water acts as a cleaning agent, removing ionic impurities adsorbed on the surface and interior of the PVDF resin, significantly reducing ionic impurities in the PVDF slurry. During the experiment, the inventors found that organic solvents with a concentration of less than 1% could not penetrate the interior of the PVDF resin, and the washing effect was not significantly different from washing with pure water; organic solvents with a concentration of more than 20% would damage the PVDF resin structure, affecting the appearance and properties of the PVDF resin.

[0009] Preferably, the organic solvent accounts for 1-5% of the mixture of organic solvent and pure water.

[0010] Organic solvents with a concentration greater than 5% do not significantly improve the cleaning effect of ionic impurities in polyvinylidene fluoride resin. From the perspective of cost saving, it is preferable to use organic solvents with a concentration of 1-5%.

[0011] Preferably, the organic solvent is acetone, N-methylpyrrolidone, or / and dimethylformamide.

[0012] Preferably, the polyvinylidene fluoride filter cake is washed with pure water 3 to 5 times, and the amount of water used for each wash is 20 times that of the polyvinylidene fluoride filter cake.

[0013] Polyvinylidene fluoride filter cake prepared by suspension method is generally washed 3 times; polyvinylidene fluoride slurry prepared by emulsion method, because the slurry contains emulsifier, requires the addition of coagulant aluminum nitrate so that the slurry can coagulate to form filter cake, thus increasing the number of washing times to 5 times.

[0014] Preferably, the washing begins from the second time, and the polyvinylidene fluoride filter cake is washed again with the filtered washing water.

[0015] In the pure water washing process, the wash water from the first washing of the polyvinylidene fluoride filter cake is discarded. The wash water from the second pure water washing can be used to wash the filter cake again before being discarded. Similarly, the wash water from the third pure water washing can also be used to wash the filter cake again before being discarded. This method can save 10% to 20% of water consumption overall, which is a significant expense in industrial production.

[0016] Preferably, the polyvinylidene fluoride slurry is stirred and heated before filtration.

[0017] Stirring and heating the polyvinylidene fluoride slurry before filtration can increase the filtration rate and thus improve the filtration efficiency.

[0018] Preferably, the polymerization reaction is a suspension polymerization reaction or an emulsion polymerization reaction.

[0019] Preferably, when the polyvinylidene fluoride slurry is prepared by emulsion polymerization, a coagulant is added after stirring the polyvinylidene fluoride slurry.

[0020] Since the polyvinylidene fluoride slurry prepared by emulsion polymerization contains emulsifiers, a coagulant is added to coagulate the polyvinylidene fluoride dispersed by the emulsifier, making it easier to wash and filter.

[0021] Preferably, the filter screen used for filtration is a nano-sized nylon filter screen.

[0022] The nano-sized nylon filter can effectively intercept polyvinylidene fluoride (PVDF) while filtering out other impurities, and it can be reused multiple times.

[0023] Preferably, the drying temperature is 70-80℃ and the vacuum degree is -0.1 to -0.6 MPa.

[0024] The beneficial effects of this invention are as follows:

[0025] The present invention provides a method for refining polyvinylidene fluoride (PVDF) polymers. By using a mixed system containing organic solvents to wash the PVDF resin, the swelling properties of the organic solvents on the resin are utilized, which helps to compensate for the incompatibility between water and resin. This method removes ionic impurities adsorbed on the surface and / or inside the resin, significantly reducing the ionic impurities in the PVDF resin.

[0026] The purification method for polyvinylidene fluoride polymer of this invention only involves filtration, washing, stirring, and heating, requiring minimal equipment and using readily available and inexpensive organic solvents, thus eliminating the need for additional production costs. Furthermore, the washing water is reused after the second washing cycle, reducing water consumption during the washing process and saving 10-20% of water overall.

[0027] The present invention discloses a method for refining polyvinylidene fluoride (PVDF) polymer. Experimental data proves that this method effectively reduces metal ion impurities in PVDF polymerization products. The resulting PVDF product has high purity, beautiful appearance, uniform particle size, good flowability, and no unpleasant odor. It is widely used in high-end electronic fields such as lithium battery separators, capacitor films, and lithium battery binders. Attached Figure Description

[0028] Figure 1 These are photographs of the polyvinylidene fluoride polymer resin powders prepared in Examples 9, 11, and Comparative Example 2 of this invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0030] The organic solvents and equipment materials used in this invention are all commercially available; including filtration equipment, vacuum pump filtration, ceramic Buchner funnel, and the filter screen used is a 1000-mesh nano-sized nylon filter screen.

[0031] A filter cake is a collection of solid substances separated from a mixture by filtration during the separation of liquids and solids. These solid substances are usually collected in a filter, filter cloth, or filter screen in a cake-like form.

[0032] Example 1

[0033] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0034] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 55℃, maintain for 20 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0035] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and discard the first washing water; then wash the filter cake with 20 times the amount of pure water, and wash the filter cake again with the second filtration washing water, and discard it after filtration; the conductivity of the washing water mixed together after three filtrations was measured to be 8.7μS / cm; compared with Examples 2 and 3, in this example, the filter cake was washed again with the second filtration washing water, and the conductivity of the washing water did not change significantly, indicating that the method is feasible and can also save water.

[0036] (3) Prepare 4L of mixed solvent according to the ratio of N-methylpyrrolidine: pure water = 1:99, stir evenly, wash the above filter cake twice, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 20min, and filter with a ceramic Buchner funnel to obtain the filter cake;

[0037] (4) Place the above filter cake in a vacuum oven with a pressure of -0.1 MPa and a temperature of 80°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0038] Example 2

[0039] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0040] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 50℃, maintain for 30 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0041] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and then wash and filter twice with 20 times the amount of pure water. The conductivity of the filtered washing water was measured to be 6.3μS / cm.

[0042] (3) Prepare 4L of mixed solvent according to the ratio of acetone: pure water = 3:97, wash the above filter cake twice, each time using 2L of mixed solvent, stirring at 500 r / min for 30 min, and filter with a ceramic Buchner funnel to obtain the filter cake;

[0043] (4) Place the above filter cake in a vacuum oven with a pressure of -0.1 MPa and a temperature of 80°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0044] Example 3

[0045] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0046] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 50℃, maintain for 30 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0047] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and then wash and filter twice with 20 times the amount of pure water. The conductivity of the filtered washing water was measured to be 7.5μS / cm.

[0048] (3) Prepare 6L of mixed solvent according to the ratio of N,N-dimethylformamide:pure water = 2:98, stir evenly, wash the above filter cake three times, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 30min, and filter with a ceramic Buchner funnel to obtain filter cake;

[0049] (4) Place the above filter cake in a vacuum oven with a pressure of -0.1 MPa and a temperature of 75°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0050] Example 4

[0051] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0052] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by emulsion method, stir it in a 20 L reactor, set the speed to 120 r / min, add aluminum nitrate coagulant, turn on the high temperature closed circulation tank to heat to the reactor temperature of 50℃, maintain for 30 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0053] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and then wash and filter 5 times with 20 times pure water. The conductivity of the filtered washing water was measured to be 9.1μS / cm.

[0054] (3) Prepare 6L of mixed solvent according to the ratio of N,N-dimethylformamide:pure water = 2:98, stir evenly, wash the above filter cake three times, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 30min, and filter with a ceramic Buchner funnel to obtain filter cake;

[0055] (4) Place the above filter cake in a vacuum oven with a pressure of -0.1 MPa and a temperature of 80°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0056] Example 5

[0057] In this embodiment, the polyvinylidene fluoride slurry is not heated or stirred, unlike in embodiment 3.

[0058] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0059] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0060] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and then wash and filter 5 times with 20 times pure water. The conductivity of the filtered washing water was measured to be 7.7μS / cm.

[0061] (3) Prepare 6L of mixed solvent according to the ratio of N,N-dimethylformamide:pure water = 2:98, stir evenly, wash the above filter cake three times, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 30min, and filter with a ceramic Buchner funnel to obtain filter cake;

[0062] (4) Place the above filter cake in a vacuum oven with a pressure of -0.6 MPa and a temperature of 80°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0063] In this embodiment, the filtration rate is slightly slower than in Example 3 when filtering polyvinylidene fluoride slurry. However, judging from the conductivity of the washing water measured in (2), whether or not stirring and heating are performed in (1) has little impact on the washing results.

[0064] Example 6

[0065] Example 6 differs from Example 1 in that the ratio of the organic solvent N-methylpyrrolidine to pure water is changed to 4:96.

[0066] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0067] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 55℃, maintain for 20 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0068] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, discard the first washing water; then wash the filter with 20 times the amount of pure water, wash the filter cake again with the second washing water, and discard it after filtration.

[0069] (3) Prepare 4L of mixed solvent according to the ratio of N-methylpyrrolidine: pure water = 4:96, stir evenly, wash the above filter cake twice, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 20min, and filter with a ceramic Buchner funnel to obtain the filter cake;

[0070] (4) Place the above filter cake in a vacuum oven with a pressure of -0.1 MPa and a temperature of 80°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0071] Table 1 compares the metal ion content determination of polyvinylidene fluoride resin samples. Example 6 shows slightly better removal of metal ions compared to Example 1.

[0072] Example 7

[0073] Example 7 differs from Example 2 in that the ratio of the organic solvent acetone to pure water is changed to 5:95.

[0074] The specific steps include the following:

[0075] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 50℃, maintain for 30 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0076] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and then wash and filter twice with 20 times the amount of pure water. The conductivity of the filtered washing water was measured to be 6.3μS / cm.

[0077] (3) Prepare 4L of mixed solvent according to the ratio of acetone: pure water = 5:95, stir and dissolve evenly at 55~60°C, wash the above filter cake twice, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 30min, and filter with a ceramic Buchner funnel to obtain the filter cake.

[0078] (4) Place the above filter cake in a vacuum oven with a pressure of -0.1 MPa and a temperature of 80°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0079] As shown in Table 1, the metal ion cleaning results of Example 7 are slightly better than those of Example 2.

[0080] Example 8

[0081] Example 8 differs from Example 2 in that the ratio of organic solvent acetone to pure water is changed to 10:90.

[0082] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0083] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 50℃, maintain for 30 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0084] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and then wash and filter twice with 20 times the amount of pure water. The conductivity of the filtered washing water was measured to be 6.3μS / cm.

[0085] (3) Prepare 4L of mixed solvent according to the ratio of acetone: pure water = 10:90, stir and dissolve evenly at 55~60°C, wash the above filter cake twice, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 30min, and filter with a ceramic Buchner funnel to obtain the filter cake.

[0086] (4) Place the above filter cake in a vacuum oven with a pressure of -0.5MPa and a temperature of 80℃. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0087] As shown in Table 1, the metal ion cleaning results of Example 8 are not significantly different from those of Example 3.

[0088] Example 9

[0089] Example 9 differs from Example 1 in that the ratio of the organic solvent N-methylpyrrolidine to pure water is changed to 3:97, while other steps remain unchanged. The resulting polyvinylidene fluoride resin powder is as follows: Figure 1 As shown in (a), the resin particles are uniform in size, and the actual product is bright white and has a beautiful appearance.

[0090] Example 10

[0091] Example 10 differs from Example 1 in that the ratio of the organic solvent N-methylpyrrolidine to pure water is changed to 15:85, while other aspects remain unchanged. Observing the actual product, the polyvinylidene fluoride resin powder prepared in this example... Figure 1 (a) The difference is not significant.

[0092] Example 11

[0093] Example 11 differs from Example 1 in that the ratio of the organic solvent N,N-dimethylformamide to pure water is changed to 20:80, while other aspects remain unchanged. The washed and dried polyvinylidene fluoride resin powder is as follows: Figure 1 As shown in (b).

[0094] Comparative Example 1

[0095] Compared to Examples 1-11, the polyvinylidene fluoride filter cake in Comparative Example 1 was not washed using an organic solvent and water mixing system.

[0096] A conventional post-processing method for polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0097] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 55℃, maintain for 20 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0098] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and then wash and filter 5 times with 20 times the amount of pure water. The conductivity of the filtered washing water was measured to be 7.3μS / cm.

[0099] (3) The above filter cake was placed in a vacuum oven with a pressure of -0.1 MPa and a temperature of 80°C. After drying for 24 hours, it was taken out to obtain polyvinylidene fluoride resin powder.

[0100] Comparative Example 2

[0101] Comparative Example 2 differs from Example 1 in that the ratio of the organic solvent N,N-dimethylformamide to pure water is changed to 30:70, while other aspects remain unchanged. The washed and dried polyvinylidene fluoride resin powder is as follows: Figure 1 As shown in (C).

[0102] A method for purifying polyvinylidene fluoride (PVDF) polymerization products includes the following specific steps:

[0103] (1) Take 10 kg of polyvinylidene fluoride slurry prepared by suspension method, stir it in a 20 L reactor, set the speed to 120 r / min, turn on the high temperature closed circulation tank to heat to the reactor temperature of 55℃, maintain for 20 min, put the polyvinylidene fluoride slurry into a stainless steel vacuum filter, turn on the vacuum pump to filter, and obtain filter cake.

[0104] (2) Take 100g of the above filter cake, add it to a 2L beaker, add 2000ml of pure water, turn on the mechanical stirring, set the speed to 500 r / min, maintain for 20min, filter with a ceramic Buchner funnel to obtain the filter cake, and discard the first washing water; wash the filter twice with 20 times the amount of pure water, and wash the filter cake again with the washing water after each pure water washing, and then discard it; the conductivity of the washing water mixed together after several filtrations was measured to be 8.7μS / cm;

[0105] (3) Prepare 4L of mixed solvent according to the ratio of N-methylpyrrolidine: pure water = 30:70, stir evenly, wash the above filter cake twice, each time using 2L of mixed solvent, stirring speed 500 r / min, stirring for 20min, and filter with a ceramic Buchner funnel to obtain filter cake;

[0106] (4) Place the above filter cake in a vacuum oven with a pressure of -0.1 MPa and a temperature of 80°C. After drying for 24 hours, take it out to obtain polyvinylidene fluoride resin powder.

[0107] Table 1 compares the metal ion content determination of polyvinylidene fluoride (PVDF) resin samples. Examples 1-8 all achieved good results in removing metal ions. Among them, Example 8 had the highest concentration of organic solvent, but the metal ion removal effect did not improve as expected. As the proportion of organic solvent increases, the penetration into PVDF also increases. The more PVDF penetrates, the more likely the metal ions will be washed out. However, compared with Examples 1-8, an organic solvent proportion of 5% already achieved a relatively good effect and met the production requirements. Therefore, considering the overall production cost, the preferred organic solvent proportion in this invention is 1-5%.

[0108] In Examples 10 and 11, the proportion of organic solvent was further increased. The polyvinylidene fluoride resin powder prepared in Example 10 was not significantly different from that prepared in Example 9; the resin particles were uniform, and the actual product was bright white and aesthetically pleasing. However, the polyvinylidene fluoride resin powder prepared in Example 11... Figure 1 As shown in (b), the particles show slight adhesion. Observing the polyvinylidene fluoride resin powders prepared in Comparative Example 10 and Example 11, the resin powder prepared in Example 11 appears slightly duller in color. This is because some of the resin powder was swollen and destroyed by a higher concentration of organic solvent. The polyvinylidene fluoride prepared in Comparative Example 2... Figure 1 (c) The powder appears somewhat scattered and not completely dried. This is because the organic solvent content is relatively high, resulting in excessive penetration of polyvinylidene fluoride resin and swelling of many particles. Under the same drying conditions, the drying is incomplete. Furthermore, when observing the color of the resin powder prepared in Comparative Example 10 and Example 11, the color of Comparative Example 2 is darker. This is because more resin powder is swollen and destroyed by the higher concentration of organic solvent. Therefore, the organic solvent content in this invention is limited to a maximum of 20%.

[0109] It should be noted that due to lighting conditions during photography, the color of polyvinylidene fluoride resin powder in the image may not fully reflect the actual color of the product; the actual color should be observed.

[0110] Comparison of metal ion content test results

[0111] The metal ion content of the polyvinylidene fluoride samples obtained in Examples 1-4, Examples 6, 7 and 8, and Comparative Examples 1 and 2 were detected using the ICP-MS method described in the invention titled "A Method for Digesting Polyvinylidene Fluoride and Measuring Metal Ions in Polyvinylidene Fluoride" (application number 202310683712.7). The detection data were compared with the data of the comparative examples in Table 1.

[0112] Table 1

[0113] serial number Fe As Na K Ca Mg Ni Cr Cu Al Mn Ba Example 1 0.10 0 0.02 0.15 0 0 0 0 0.01 0.11 0 0 Example 2 0.13 0.02 0.11 0.19 0 0.01 0 0.01 0 0.1 0 0 Example 3 0 0.01 0.05 0.11 0 0.02 0 0 0.03 0.20 0 0 Example 4 0.06 0 0.13 0.05 0 0.06 0 0 0.06 0.11 0.03 0.01 Example 6 0 0.01 0.01 0.1 0 0.01 0 0 0.01 0.10 0 0 Example 7 0 0.01 0.02 0.10 0 0.01 0 0 0.02 0.20 0.01 0.01 Example 8 0.02 0.01 0.01 0.02 0 0.01 0 0 0.01 0.10 0 0 Comparative Example 1 48 2.72 86 56 0.3 14 0.2 0.37 2.36 78 1.26 1.36 Comparative Example 2 0 0 0.01 0.10 0 0 0 0.01 0.01 0.10 0 0

[0114] (Note: The unit for metal ion content in the table is mg / kg)

[0115] As can be seen from the data in Table 1, the metal ion content of polyvinylidene fluoride resin in Examples 1-4, 6, 7 and 8 of the present invention is significantly lower than that in Comparative Example 1. The present invention can clean out the ionic impurities encapsulated inside and / or adsorbed on the surface of polyvinylidene fluoride resin by using a mixed system of organic solvent and water. However, when the proportion of organic solvent is greater than 5%, such as in Comparative Examples 7 and 8, the washing effect is not significantly changed, and too much organic solvent will increase the cost.

[0116] This invention provides a washing scheme for polyvinylidene fluoride slurries prepared by suspension polymerization and emulsion polymerization, because suspension polymerization and emulsion polymerization contain a large number of metal ions, which cannot meet the increasing demand; however, the washing method of this invention is not limited to washing only the polymerization products of suspension polymerization and emulsion polymerization. This invention is also applicable when polyvinylidene fluoride slurries prepared by other polymerization methods contain a large number of metal ions and need to be cleaned.

[0117] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for refining a polyvinylidene fluoride polymer product, characterized by, The polyvinylidene fluoride (PVDF) slurry, a product of the polymerization reaction, is filtered to obtain a PVDF filter cake. The PVDF filter cake is washed with pure water, then washed with a mixture of organic solvent and pure water, filtered, and dried to obtain refined PVDF. The organic solvent is soluble in water and can dissolve the PVDF. The organic solvent accounts for 1-5% of the mixture of organic solvent and pure water.

2. The method for purifying the polyvinylidene fluoride polymer product according to claim 1, characterized in that, The organic solvent is acetone, N-methylpyrrolidone, or / and dimethylformamide.

3. The method for purifying the polyvinylidene fluoride polymer product according to any one of claims 1 to 2, characterized in that, The polyvinylidene fluoride filter cake is washed with pure water 3 to 5 times, and the amount of water used for each wash is 20 times that of the polyvinylidene fluoride filter cake.

4. The method for purifying the polyvinylidene fluoride polymer product according to claim 3, characterized in that, The washing process begins a second time, in which the polyvinylidene fluoride filter cake is washed again with the filtered wash water.

5. The method for purifying the polyvinylidene fluoride polymerization product according to claim 4, characterized in that, Before filtering the polyvinylidene fluoride slurry, the polyvinylidene fluoride slurry is stirred and heated.

6. The method for purifying the polyvinylidene fluoride polymer product according to claim 5, characterized in that, The polymerization reaction is a suspension polymerization reaction or an emulsion polymerization reaction.

7. The method for purifying the polyvinylidene fluoride polymerization product according to claim 6, characterized in that, When the polyvinylidene fluoride slurry is prepared by emulsion polymerization, a coagulant is added after the polyvinylidene fluoride slurry is stirred.

8. The method for purifying the polyvinylidene fluoride polymerization product according to claim 3, characterized in that, The filter used for filtration is a nano-sized nylon filter.

9. The method for purifying the polyvinylidene fluoride polymer product according to claim 3, characterized in that, The drying temperature is 70-80℃, and the vacuum degree is -0.1 to -0.6Mpa.

Images