Method for purifying fluorine-containing polymer

Abstract

The invention relates to a method for purifying a fluorine-containing polymer, belongs to the field of fluorine-containing polymers, and aims to solve the problem that the conventional method for removing impurities in the fluorine-containing polymer cannot achieve a relatively ideal effect, the method for purifying the fluorine-containing polymer comprises the following specific steps: adding the fluorine-containing polymer and a fluorinated solvent into a reactor, and stirring and mixing; a swollen fluorine-containing polymer is obtained; standing until the upper layer and the lower layer are layered, then discharging the solvent on the upper layer, and freezing the swelled fluorine-containing polymer in the lower layer; the frozen fluorine-containing polymer is subjected to crushing treatment, and fluorine-containing polymer powder is obtained; and recovering the solvent, sequentially carrying out acid pickling and water washing on the crushed powder, and drying to obtain the fluorine-containing polymer. The method comprises the following steps: enabling a fluorinated solvent to enter pores of fluorine-containing polymer particles, freezing and crushing to obtain fluorine-containing polymer powder with smaller particle size, and carrying out acid pickling, water washing and other treatments to reduce the impurity content in the fluorine-containing polymer to ppb level, so that the purification effect is obviously improved.

Description

Technical Field

[0001] The invention belongs to the field of fluorine-containing polymers, and in particular relates to a method for purifying fluorine-containing polymers. Background Art

[0002] Fluoropolymers have excellent solvent resistance, high and low temperature resistance, weather resistance and other properties, and are used in chemical, mechanical, electronic, semiconductor and other fields. In industrial production, they are usually prepared by emulsion polymerization, suspension polymerization or solution polymerization.

[0003] However, no matter what polymerization method is used, the fluorinated polymers obtained contain impurities. These impurities include residues of the reaction system, such as initiators, emulsifiers, buffers, etc., as well as metal ions and fluoride ions introduced by the corrosion of the reactor caused by hydrogen fluoride produced during the polymerization process. If the impurities are not effectively removed, they will not only reduce the purity of the polymer, but also have a serious negative impact on the performance, processing and application of the polymer. For example, impurities can cause changes in the transparency, refractive index and other optical properties of the polymer, affecting the application of the polymer in the optical field; impurities can easily become stress concentration points during the processing process, causing the polymer to break or deform during processing such as extrusion and injection molding; the presence of impurities causes fluorinated polymers to be unable to meet the strict requirements for material purity in the medical device and semiconductor fields.

[0004] The prior art proposes many methods for reducing the impurity content of fluoropolymers. For example, Chinese patent CN114437255A discloses a method for purifying fluoropolymers, which removes impurities in fluoropolymers by normal pressure stirring washing and vacuum stirring washing, monitors the conductivity of wastewater discharged after washing, and can effectively remove residual emulsifiers in the particle gaps, but does not control the amount of ion precipitation, so it cannot ensure that the amount of ion precipitation in the product meets the requirements of the high-purity industry.

[0005] Chinese patent CN101624432B discloses a method for reducing impurities in fluorine-containing polymers. The method combines gas-phase medium crushing with liquid-phase medium crushing to control the impurity content in the fluorine-containing polymer within an acceptable range. However, the high-purity product obtained cannot meet the requirements of high-end application fields such as semiconductors.

[0006] Chinese patent CN102977236B discloses a method for purifying a fluorinated polymer, which comprises adding C6~C 10 Aliphatic alcohol, and then the fluorinated polymer aqueous dispersion flows through the cross-flow microfiltration membrane separation unit, which greatly reduces the amount of water used while washing and removing impurities in the polymer. However, the impurity content in the final product is still at the ppm level and cannot stably meet the requirements of the semiconductor industry and other industries. Summary of the invention In view of the problem that existing methods for removing impurities in fluorine-containing polymers cannot achieve relatively ideal results, the present invention provides a method for purifying fluorine-containing polymers. The method of the present application can control the impurity content of the fluorine-containing polymer to the ppb level, and the purity of the product is stable, meeting the requirements of the semiconductor, medical device and other industries.

[0007] The present invention adopts the following technical solution: a method for purifying a fluorine-containing polymer, the specific steps are as follows: S1, adding a fluorinated polymer and a fluorinated solvent into a reactor and stirring and mixing them to obtain a swollen fluorinated polymer; S2, standing until the upper and lower layers are separated, then draining the solvent in the upper layer, and freezing the swollen fluorine-containing polymer in the lower layer; S3, crushing the frozen fluorine-containing polymer to obtain fluorine-containing polymer powder; S4, recovering the solvent, washing the crushed powder with acid and water in sequence, and obtaining the fluorine-containing polymer after drying.

[0008] In the present application, since the fluorinated solvent forms crystals inside the fluorinated polymer after freezing treatment, these crystals serve as stress concentration points in the subsequent crushing process, promoting the crushing of the fluorinated polymer. Because of the internal stress of the fluorinated polymer, it is easier to form microcracks on the surface when it is impacted by external force, resulting in the roughening of the surface of the fluorinated polymer powder, thereby increasing the specific surface area. It is well known that there is a positive correlation between the specific surface area of ​​the material and the ion diffusion rate. The larger the specific surface area, the higher the rate at which ions diffuse on the active sites on the surface of the material. For polymers, this means that the diffusion and migration of ions inside the polymer will be faster, which is conducive to the precipitation of ions. In addition, it is known that the particle size of the fluorinated polymer powder determines the impurity content of the final product. The particle size is reduced, the specific surface area of ​​the fluorinated polymer powder is increased, and it is easier to remove impurities. Therefore, the present application allows the fluorinated solvent to enter the pores of the fluorinated polymer particles, and then freezes and crushes them to obtain a fluorinated polymer powder with a particle size of 0.6 to 20 microns, and then treats it by conventional acid washing, water washing, etc., to obtain a fluorinated polymer with fewer impurities and a more ideal one.

[0009] Further, the fluorine-containing polymer is a fluorine-containing polymer obtained by emulsion polymerization, suspension polymerization or solution polymerization, including but not limited to one of polytetrafluoroethylene (PTFE), polyperfluoroethylene propylene (FEP), fusible polytetrafluoroethylene (PFA), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), and perfluoroether rubber (FFKM); The fluorinated agent includes at least one of fluorinated alcohols, fluorinated ethers, fluorinated alkanes or fluorinated cycloalkanes. Specifically, the fluorinated agent is one or a mixture of perfluorooctane, hexafluoroisopropanol, trifluorotrichloroethane, and perfluorodimethylcyclobutane.

[0010] Furthermore, the amount of the fluorinated solvent added is 1.2 to 4 times, preferably 1.5 to 2 times, the solid amount of the fluorinated polymer by weight. The amount of the fluorinated solvent added should be excessive to ensure that the solvent fully penetrates into the pores inside the polymer.

[0011] Furthermore, the stirring speed of the reactor is 200-660 r / min, preferably 300-500 r / min, and the stirring time is 1-5 hours, preferably 2-4 hours. The purpose of stirring is to avoid agglomeration of the fluorine-containing polymer material and to improve the contact effect between the fluorine-containing polymer and the fluorinated solvent.

[0012] Furthermore, the mixture is allowed to stand for 5 to 25 minutes, preferably for 10 to 15 minutes.

[0013] Furthermore, the freezing treatment is carried out by injecting a low-temperature cooling medium into the reactor to make the temperature below the freezing point of the fluorinated solvent, and the fluorinated solvent exists in the fluorinated polymer in the form of crystals. When hexafluoroisopropanol is used as the solvent, the freezing temperature is -19 to -30°C. When perfluorodimethylcyclobutane is used as the solvent, the freezing temperature is -38 to -50°C.

[0014] Furthermore, the crushing treatment is completed by high-speed shearing of the reactor stirring, with a rotation speed of 1500-3500r / min, preferably 2500-3000r / min. After the crushing treatment, the average particle size of the fluorine-containing polymer is 0.6-20 microns, preferably 0.8-10 microns.

[0015] Furthermore, the fluorinated solvent is recovered by distillation, and the solvent can be reused. The acid washing comprises the following steps: adding the fluorinated polymer powder and 5-20wt% nitric acid solution in a mass ratio of 3-5:1 into a reactor for stirring and washing, and then draining the upper washing wastewater.

[0016] Furthermore, the water washing comprises the following steps: adding fluorine-containing polymer powder and ultrapure water in a mass ratio of 0.5 to 2:1 into a reactor for stirring and washing, and then draining the upper washing wastewater.

[0017] Furthermore, the acid washing and water washing steps are repeated alternately at least three times.

[0018] Furthermore, the washed fluorine-containing polymer powder is dried at a temperature of 130 to 280° C., preferably 150 to 240° C. The dried fluorine-containing polymer powder can be used directly or granulated into pellets before use.

[0019] The beneficial effects of the present invention are as follows: The present application is a method for purifying fluoropolymers, in which a fluorinated solvent is allowed to enter the pores of the fluoropolymer particles, which are then frozen and crushed to obtain fluoropolymer powder with a particle size of 0.6 to 20 microns. The fluoropolymer powder is then treated by conventional acid washing, water washing, etc. to reduce the impurity content in the fluoropolymer to the ppb level. The final product has a high purity and can be used in medical devices, semiconductors and other fields. DETAILED DESCRIPTION

[0020] The technical solutions of the embodiments of the present invention are explained and described below, but the following embodiments are only preferred embodiments of the present invention, not all. Based on the embodiments in the implementation mode, other embodiments obtained by those skilled in the art without creative work all belong to the protection scope of the present invention.

[0021] Example 1 Add 25kg of fluorinated polymer PFA and 39.5kg of hexafluoroisopropanol to a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -19°C and keep it for 3h. Then crush the PFA into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 60°C and recover the hexafluoroisopropanol. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater, and then add 12.5kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 260°C to obtain the final product.

[0022] Example 2 Add 15kg of fluorinated polymer PFA and 60kg of perfluorodimethylcyclobutane into a 100L reactor, stir and mix at a speed of 200r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -40℃ and keep it for 3h. Then crush the PFA into powder at a speed of 1500r / min, and test the average particle size of the powder. Heat the reactor to 50℃ and recover the perfluorodimethylcyclobutane. After recovery, add 5kg of 5wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater and add 30kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 260℃ to obtain the final product.

[0023] Example 3 Add 27kg of fluorinated polymer FEP and 32.4kg of perfluorodimethylcyclobutane into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -40℃ and keep it for 3h. Then crush the FEP into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 50℃ and recover the perfluorodimethylcyclobutane. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater and add 25kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 240℃ to obtain the final product.

[0024] Example 4 Add 26.5kg of fluoropolymer PCTFE and 44.3kg of trifluorotrichloroethane into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -40℃ and keep it for 3h. Then crush the PCTFE into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 50℃ and recover perfluorodimethylcyclobutane. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower washing wastewater and add 30kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 130℃ to obtain the final product.

[0025] Example 5 Add 30kg of fluoropolymer PTFE and 41kg of perfluorooctane into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -40℃ and keep it for 3h. Then crush the PTFE into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 50℃ and recover perfluorodimethylcyclobutane. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater and add 30kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 280℃ to obtain the final product.

[0026] Example 6 Add 28kg of fluoropolymer PVDF and 42kg of hexafluoroisopropanol into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -19°C and keep it for 3h. Then crush the PVDF into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 60°C and recover the hexafluoroisopropanol. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater, and then add 30kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 170°C to obtain the final product.

[0027] Example 7 Add 27kg of fluoropolymer FFKM and 33.4kg of hexafluoroisopropanol into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -19°C and keep it for 3h. Then crush FFKM into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 60°C and recover hexafluoroisopropanol. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater, and then add 30kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 150°C to obtain the final product.

[0028] Comparative Example 1 Add 25kg of fluorinated polymer PFA and 25kg of hexafluoroisopropanol into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the lower layer of solvent, cool the reactor to -19°C and keep it for 3h. Then crush the PFA into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 60°C and recover the hexafluoroisopropanol. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing. After draining the lower layer of washing wastewater, add 12.5kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 260°C to obtain the final product.

[0029] Comparative Example 2 Add 25kg of fluorinated polymer PFA and 39.5kg of hexafluoroisopropanol into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then let stand for 10min. After draining the upper solvent, crush the PFA into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 60°C and recover the hexafluoroisopropanol. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater, and then add 12.5kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 260°C to obtain the final product.

[0030] Comparative Example 3 Add 25kg of fluoropolymer PFA and 39.5kg of chloroform into a 100L reactor, stir and mix at a speed of 660r / min for 2.5 hours and then stand for 10min. After draining the upper solvent, cool the reactor to -66°C and keep it for 3h, then crush the PFA into powder at a speed of 3500r / min, and test the average particle size of the powder. Heat the reactor to 65°C and recover the chloroform. After recovery, add 5kg of 20wt% nitric acid solution to the reactor for stirring and washing, drain the lower layer of washing wastewater and add 12.5kg of ultrapure water to the reactor for stirring and washing. After repeating the above pickling and water washing steps three times, test the conductivity of the washing wastewater, and dry the powder at 260°C to obtain the final product.

[0031] The fluorine-containing polymer products obtained in the above examples and comparative examples were subjected to performance tests, and the test results are listed in Table 1.

[0032] The conductivity test method is as follows: take 100 mL of the solution to be tested and test the conductivity using a conductivity meter at 25°C.

[0033] The average particle size test method is as follows: weigh 2 g of polymer sample and put it into a laser particle size analyzer for measurement, and take the D50 value as the average particle size of the sample.

[0034] The test method for the amount of metal ion precipitation is as follows: place 1g of polymer sample in a PFA bottle, add 100mL of high-purity concentrated nitric acid, heat and soak at 100°C for 24h, and after cooling, use ICP-MS to test the amount of metal ion precipitation.

[0035] The test method for the fluoride ion precipitation amount is as follows: 10 g of polymer sample is placed in a PFA bottle, 10 mL of a 1:1 mixture of methanol / water is added, and 10 mL of a total ionic strength buffer is added. After stirring, the mixture is allowed to stand for 24 hours, and the fluoride ion precipitation amount is tested using an ion chromatograph.

[0036] The TOC dissolution value test method is as follows: place 10g of polymer sample in a PFA bottle, add 100mL of ultrapure water, soak at 90℃ for 24h, and after cooling, use a TOC meter to test the TOC dissolution amount.

[0037] Table 1

[0038] It can be seen from the data in Table 1 that the method of the present invention (Examples 1 to 7) can reduce the impurity content in the fluorine-containing polymer to the ppb level, and the final product obtained by the present invention can be used in the fields of medical devices, semiconductors, etc.

[0039] Compared with Example 1, the fluorinated solvent used in Comparative Example 1 was not excessive, resulting in insufficient or inadequate crystallization in the internal pores of the polymer; Comparative Example 2 was not subjected to freezing treatment, and although the solvent penetrated into the polymer, it could not crystallize; Comparative Example 3 used a conventional low surface tension organic solvent instead of the fluorinated solvent, which could wet the polymer but could not enter the pores of the polymer particles and only crystallized on the surface of the polymer. The methods of Comparative Examples 1 to 3 all resulted in poor subsequent crushing effects, large polymer particle sizes, and high conductivity of washing wastewater, indicating that internal impurities could not be fully released during the cleaning process, and the impurity content of the final fluorinated polymer was high.

[0040] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes but is not limited to the contents described in the above specific embodiments. Any modification that does not deviate from the functional and structural principles of the present invention will be included in the scope of the claims.

Claims

1. A method for purifying a fluorine-containing polymer, characterized in that: The specific steps are as follows: S1, adding a fluorinated polymer and a fluorinated solvent into a reactor and stirring and mixing them to obtain a swollen fluorinated polymer; S2, standing until the upper and lower layers are separated, then draining the solvent in the upper layer, and freezing the swollen fluorine-containing polymer in the lower layer; S3, crushing the frozen fluorine-containing polymer to obtain fluorine-containing polymer powder; S4, recovering the solvent, sequentially washing the crushed powder with acid and water, and obtaining the fluorine-containing polymer after drying.

2. The method for purifying a fluorinated polymer according to claim 1, characterized in that: The fluorine-containing polymer is a fluorine-containing polymer obtained by emulsion polymerization, suspension polymerization or solution polymerization, including but not limited to one of polytetrafluoroethylene, polyperfluoroethylene propylene, fusible polytetrafluoroethylene, polyvinylidene fluoride, polychlorotrifluoroethylene or ethylene-tetrafluoroethylene copolymer, and perfluoropolyether; The fluorination agent includes at least one of fluorinated alcohols, fluorinated ethers, fluorinated alkanes or fluorinated cycloalkanes.

3. The method for purifying a fluorinated polymer according to claim 1 or 2, characterized in that: The amount of the fluorinated solvent added is 1.2 to 4 times the solid amount of the fluorinated polymer by weight.

4. The method for purifying a fluorinated polymer according to claim 1, characterized in that: The stirring speed is 200-660 r / min, and the stirring time is 1-5 hours.

5. The method for purifying a fluorinated polymer according to claim 1 or 4, characterized in that: The freezing treatment is carried out by injecting a low-temperature cooling medium into the reactor to make the temperature below the freezing point of the fluorinated solvent, and the fluorinated solvent exists in the fluorinated polymer in the form of crystals.

6. The method for purifying a fluorinated polymer according to claim 1 or 4, characterized in that: The crushing treatment is completed by high-speed shearing of the reactor stirring, with a rotation speed of 1500-3500 r / min. After the crushing treatment, the average particle size of the fluorine-containing polymer is 0.6-20 microns.

7. The method for purifying a fluorinated polymer according to claim 1, characterized in that: The acid washing comprises the following steps: adding fluorine-containing polymer powder and 5-20wt% nitric acid solution into a reactor at a mass ratio of 3-5:1 for stirring and washing, and then draining the upper washing wastewater.

8. The method for purifying a fluorinated polymer according to claim 1, characterized in that: The water washing comprises the following steps: adding fluorine-containing polymer powder and ultrapure water in a mass ratio of 0.5-2:1 into a reactor for stirring and washing, and then draining the upper washing wastewater.

9. The method for purifying a fluorinated polymer according to claim 1, characterized in that: The acid washing and water washing steps are repeated alternately at least three times.

10. The method for purifying a fluorinated polymer according to claim 1, characterized in that: The washed fluorine-containing polymer powder is heated to 130-280°C until it is dried.

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