CN111269342B - Preparation method of high-purity polytetrafluoroethylene - Google Patents
Preparation method of high-purity polytetrafluoroethylene Download PDFInfo
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Abstract
Discloses a preparation method of high-purity polytetrafluoroethylene, which comprises the following steps: (1) adding water into a reaction container, adding an auxiliary agent and a tetrafluoroethylene monomer after deoxygenation to initiate a polymerization reaction, and then supplementing the tetrafluoroethylene monomer to maintain the reaction pressure; (2) stopping adding the tetrafluoroethylene monomer, slowly reducing the reaction pressure due to the consumption of the monomer, and adding an inorganic acid aqueous solution when the reaction pressure is reduced to 10-30% of the original pressure; (3) the polymerization was terminated to obtain polytetrafluoroethylene particles.
Description
Technical Field
The invention relates to a preparation method of high-purity Polytetrafluoroethylene (PTFE). The method of the present invention can prepare PTFE with low metal ion content and high cleanliness, and the filtrate after water washing has conductivity of 3 mus/cm or below and metal ion migration amount of 1-20 ppb.
Background
Polytetrafluoroethylene has a wide range of industrial applications and can be prepared by a variety of conventional polymerization processes. For example, CN103665240A discloses a method for preparing polytetrafluoroethylene dispersion resin, which comprises adding deionized water, initiator, pH regulator, perfluorobutylethylene, dispersant and stabilizer into a reaction kettle, introducing nitrogen gas to make the oxygen content less than or equal to 30ppm, controlling the pressure of the reaction kettle at 1.5-3.0MPa and the temperature at 70-105 ℃, adding gas phase tetrafluoroethylene monomer, carrying out polymerization reaction under stirring, collecting polytetrafluoroethylene polymerization solution after the reaction is finished, coagulating, cleaning and drying to obtain polytetrafluoroethylene dispersion resin.
At present, the polytetrafluoroethylene varieties in China are few, the polytetrafluoroethylene resin is mainly molded by a suspension method, and the high-end application occupation ratio is small. With the continuous development of PTFE application fields in China, the development of fluororesins faces the problems of adjusting product structures and improving grades.
5G is affecting the development of the semiconductor industry and changing the needs of the industry. PTFE products also play an important role in semiconductor manufacturing processes in high-end applications. PTFE has excellent high and low temperature performance and chemical stability, good electrical insulation, non-adhesiveness, weather resistance, non-inflammability and good lubricity, and is a material which is indispensable for solving a plurality of key technologies in modern scientific technology, military industry and civil use from the aerospace field to wide daily commodities in the traditional application. PTFE products are increasingly used for high-end application, the existing domestic PTFE industry is mainly based on basic products, the requirement of innovative products of downstream enterprises cannot be met, and the situations that domestic general PTFE basic materials have excess capacity and medium-high-end products of functional new materials are insufficient are caused.
The conductivity and metal ion content of the washing filtrate of PTFE are key indexes for characterizing the purity of polytetrafluoroethylene. At present, the conductivity of filtrate obtained after washing PTFE resin is 5 mu s/cm or more, and the content of PTFE metal ions is relatively high, so that the requirement of innovative products of downstream enterprises is difficult to meet.
In order to reduce the decrease and fluctuation of the polymerization rate and the decrease and fluctuation of the physical properties of the obtained PTFE, CN1461313A discloses a process for producing polytetrafluoroethylene by polymerizing tetrafluoroethylene in an aqueous medium in the presence of a water-soluble initiator, wherein the TOC of water used for the polymerization, i.e., the total organic carbon amount, is 100ppb or less and/or the electrical conductivity is 1.0 μ s/cm or less. It is not mentioned that lowering the TOC of the water used for polymerization contributes to lowering the conductivity and metal ion content of the finally produced PTFE resin.
In order to reduce the metal ion content of the finally prepared PTFE resin, CN109575157A discloses a fluorine-containing polymer with low metal ion content and a preparation method thereof. The preparation method comprises the steps of (a) providing a fluorine-containing polymer dispersion liquid, (b) adding an organic solution and an alkaline reagent into the fluorine-containing polymer dispersion liquid, (c) removing the organic solvent after washing, extruding to obtain fluorine-containing polymer powder particles, and (d) adding the obtained fluorine-containing polymer powder particles into an acid solution, stirring, and washing to obtain the fluorine-containing polymer granular particles with low metal ion content. The fluoropolymer produced by this method has a filtrate conductivity of about 5. mu.s/cm after washing with water.
Therefore, there is a need in the art to develop a process for producing high purity polytetrafluoroethylene, wherein the filtrate obtained by washing the high purity polytetrafluoroethylene resin obtained by this process with water has an electrical conductivity of 3. mu.s/cm or less, and has a low content of PTFE metal ions and a migration amount of 1 to 20 ppb.
Disclosure of Invention
An object of the present invention is to provide a process for producing high purity polytetrafluoroethylene, which is characterized in that the polytetrafluoroethylene having a high cleanliness obtained by the process has an electric conductivity of 3 μ s/cm or less in the filtrate obtained by washing the PTFE resin with water, as represented by the electric conductivity, and the PTFE metal ion content is low and the transfer amount thereof is 1 to 20 ppb.
Accordingly, one aspect of the present invention is directed to a process for preparing polytetrafluoroethylene, comprising the steps of:
(1) adding water into a reaction container, adding an auxiliary agent and a tetrafluoroethylene monomer after deoxygenation to initiate a polymerization reaction, and then supplementing the tetrafluoroethylene monomer to maintain the reaction pressure;
(2) stopping adding the tetrafluoroethylene monomer, slowly reducing the reaction pressure due to the consumption of the monomer, and adding an inorganic acid aqueous solution when the reaction pressure is reduced to 10-30% of the original pressure;
(3) the polymerization was terminated to obtain polytetrafluoroethylene particles.
Detailed Description
The preparation method of the polytetrafluoroethylene comprises the following steps:
(1) adding water into a reaction vessel, adding an auxiliary agent and a tetrafluoroethylene monomer after deoxygenation to initiate polymerization, and then supplementing the tetrafluoroethylene monomer to maintain the reaction pressure
The polymerization water suitable for the process of the present invention preferably has a high purity. In one embodiment of the present invention, the water for polymerization has a TOC content (total organic carbon content by weight) of 5 to 50ppb, preferably 5 to 20ppb, more preferably 1 to 15ppb, and/or a conductivity of water of 2.5. mu.s/cm, preferably 2.0. mu.s/cm, more preferably 1.0. mu.s/cm.
In order to prevent oxygen from interfering with the polymerization reaction, it is necessary to remove oxygen from the reaction vessel containing water until the oxygen content is in compliance before the reaction. The oxygen-removing means suitable for use in the method of the present invention is not particularly limited and may be conventional means known in the art. In one embodiment of the invention, the means for removing oxygen is selected from the group consisting of inert gas purging, vacuum pumping, and a combination of both. The step of purging the reaction vessel with an inert gas is conventional in the art. For example, the reaction vessel may be purged with an inert gas until the oxygen content therein is below 30 ppm.
The inert gas to be used is not particularly limited, and may be any inert gas known in the art, such as nitrogen, argon, helium, or a mixture of two or more thereof, and the like. From the viewpoint of cost, nitrogen is preferable.
In the present invention, the term "oxygen content is acceptable" means that the oxygen content in the gas phase portion of the reaction vessel containing water after purging with an inert gas and/or evacuation is less than 30ppm by volume, preferably less than 20ppm by volume.
The initiator suitable for use in the process of the present invention is not particularly limited and may be a conventional initiator known in the art. No fluorosurfactant is added.
The initiator may be alkali metal or alkaline earth metal persulfates, such as potassium persulfate, sodium persulfate, ammonium persulfate, etc., redox initiator systems may also be used, the oxidant is likewise alkali metal or alkaline earth metal persulfates, such as potassium persulfate, ammonium persulfate, and the reducing agent is sulfite, such as sodium sulfite, sodium bisulfite, sodium metabisulfite, etc.
The pressure of the tetrafluoroethylene monomer in the polymerization process of the present invention is not particularly limited, and may be a conventional monomer pressure known in the art. In one embodiment of the present invention, the pressure of the tetrafluoroethylene monomer is controlled to 0.5 to 1.5MPa, preferably 0.6 to 1.4MPa, and more preferably 0.8 to 1.2 MPa.
Additional tetrafluoroethylene monomer is required to maintain the pressure of the reaction system during the polymerization at, for example, 0.5 to 1.5MPa, preferably 0.6 to 1.4MPa, more preferably 0.8 to 1.2MPa, due to the consumption of the monomer. The process of the present invention therefore includes the step of replenishing tetrafluoroethylene monomer during the polymerization to maintain the reaction pressure.
The polymerization temperature in the present invention is not particularly limited and may be a conventional temperature known in the art. In one embodiment of the present invention, the polymerization temperature is controlled to 10 to 70 ℃ and preferably 20 to 60 ℃.
(2) Stopping adding tetrafluoroethylene monomer, slowly reducing reaction pressure due to monomer consumption, and adding inorganic acid aqueous solution when reaction pressure is reduced to 10-30% of original pressure
After the tetrafluoroethylene monomer is stopped being supplemented, the pressure of the whole reaction system is reduced along with the consumption of polymerization. When the pressure of the reaction system is reduced to 10 to 30% of the original pressure, preferably 12 to 28% of the original pressure, more preferably 15 to 25% of the original pressure, and preferably 18 to 22% of the original pressure, an aqueous solution of an inorganic acid is added to the reaction system.
The inorganic acid to be used is not particularly limited as long as it can be dissolved in the reaction system of the present invention and does not adversely affect the PTFE resin product. In one embodiment of the present invention, the inorganic acid is selected from hydrochloric acid, nitric acid, sulfuric acid, acetic acid, phosphoric acid or a mixture of two or more thereof at any ratio, preferably, the inorganic acid is selected from nitric acid, sulfuric acid, acetic acid or a mixture thereof.
In one embodiment of the invention, the inorganic acid is present in a concentration of 0.1 to 40% by weight, preferably 10 to 30% by weight, more preferably 15 to 25% by weight, preferably 15 to 22% by weight, and most preferably 16 to 20% by weight.
In one embodiment of the invention, the concentration of mineral acid is added in an amount of 0.5 to 2 x 10 of the volume of water in the reaction vessel-3Preferably 0.6-1.6 x 10-3Preferably 0.6-1.4 x 10-3Preferably 0.8-1.2 x 10-3And (4) doubling.
After the addition of the aqueous solution of the inorganic acid, the process of the present invention further comprises the step of heating and stirring the mixture of the PTFE suspension and the inorganic acid solution in the reaction vessel. The temperature of heating is not particularly limited as long as the safety of the reaction system is ensured and no adverse effect is exerted on the PTFE resin product. In one embodiment of the invention, the mixture of PTFE suspension and mineral acid solution in the reaction vessel is stirred with heating at a temperature of from 40 to 95 deg.C, preferably from 45 to 90 deg.C, more preferably from 50 to 85 deg.C, preferably from 50 to 70 deg.C, preferably from 55 to 65 deg.C.
The time for heating and stirring in the method of the present invention is not particularly limited. In one embodiment of the invention, the stirring time is from 1 to 180min, preferably from 5 to 120min, more preferably from 10 to 100min, preferably from 20 to 80min, most preferably from 30 to 60 min.
The prior art, for example, CN109575157A, proposed that in order to reduce the amount of metal ions released from fluoropolymer pellets, the resulting fluoropolymer powder particles are added to an acidic solution after the polymerization reaction is completed and stirred. The inventors of the present invention have found that, although the method proposed in CN109575157A can advantageously reduce the conductivity of the filtrate after washing the PTFE resin with water, expressed as conductivity, to around 5 μ S/cm, if the acid washing step of the fluoropolymer is advanced to the end of the polymerization reaction rather than after the polymerization reaction is completed, the conductivity of the filtrate after washing the PTFE resin with water, expressed as conductivity, can be further reduced to 3 μ S/cm or less, without adding a fluorosurfactant in the present process. The present invention has been completed based on this finding.
(3) The polymerization was terminated to obtain polytetrafluoroethylene particles.
After the polymerization reaction is completed, PTFE is insoluble in water and precipitates out of the water to form primary particles. Usually, these large-particle-size suspended PTFE carry impurities from the polymerization aid, and therefore, it is necessary to pulverize it into small-particle-size products to release the impurities, and the products are subjected to multiple pulverization and washing processes before entering the drying process. Thus, the process of the present invention further comprises pulverizing, washing, drying, and separating the resulting polytetrafluoroethylene resin to form a suspended PTFE resin.
The method for pulverizing, washing, drying and separating the polytetrafluoroethylene resin obtained as described above is not particularly limited, and may be a conventional method in the art. After reading the present disclosure, one of ordinary skill in the art can readily perform the pulverization, washing, drying, and separation operations on the polymerized PTFE resin.
In one embodiment of the present invention, the washing step comprises washing the pulverized polymer powder with deionized water having an electric conductivity of less than 0.5. mu.S/cm until the washing water has an electric conductivity of 0.5 to 4.5. mu.S/cm, preferably 0.5 to 4. mu.S/cm, more preferably 0.5 to 3.5. mu.S/cm, further preferably 0.5 to 3. mu.S/cm, particularly preferably 0.5 to 2.5. mu.S/cm.
In one embodiment of the invention, the PTFE after drying has a water content of 0.05 to 0.5 wt.%, preferably 0.05 to 0.3 wt.%, more preferably 0.05 to 0.2 wt.%, and most preferably 0.05 to 0.15 wt.%, based on the mass of the polymer.
In a preferred embodiment of the present invention, the method comprises the steps of: adding deionized water into a polymerization reaction kettle, repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using inert gas, and detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated, then an initiator and an auxiliary agent are added, the pressure of the tetrafluoroethylene monomer in the kettle is increased to the pressure of the polymerization reaction, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 4 ℃ during the reaction. After the reaction is completed, the unreacted tetrafluoroethylene monomer is recovered. And adding an inorganic acid solution at the final stage of polymerization, heating and stirring to dissolve impurities such as metal ions in the PTFE particles to the maximum extent. Washing with deionized water to obtain the high-cleanliness suspended PTFE resin.
The method can prepare the high-cleanliness PTFE with low metal ion content, the metal ion migration amount of the PTFE is 1-30ppb, more preferably 1-20ppb, the PTFE meets the application field with strict requirements on metal ions, and the PTFE can be used in the high-end application fields of semiconductors and the like. The method has strong operability and can realize large-scale production.
The present invention is further illustrated below with reference to examples, but the scope of the present invention is not limited to the following examples.
Examples
Amount of metal ion transferred
The test of the metal ion migration amount refers to that 1g of a sample is put into 10ml of concentrated nitric acid, heated for 2 hours at 120 ℃, and then soaked for 12 hours; after cooling, ICP-MS is used for testing the migration amount of metal ions in the nitric acid solution.
Electrical conductivity of
The method for measuring the electrical conductivity is a conductivity meter method, and 25 ℃ is defined as a standard temperature for measuring the electrical conductivity.
Determination of tensile Strength and elongation at Break
The tensile strength and elongation at break of the samples were tested according to HG/T2903-1997 standard.
Example 1
Adding 30L of deionized water with the metal ion content less than 10ppb into a 50L polymerization reaction kettle, then repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using nitrogen, and then detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated to 20 ℃, then 6.7g of an oxidation-reduction initiation system consisting of potassium persulfate and sodium sulfite is added, the pressure of a tetrafluoroethylene monomer in the kettle is increased to 0.8MPa, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 4 ℃ during the reaction.
Stopping supplying the tetrafluoroethylene monomer into the polymerization reaction kettle, and gradually reducing the pressure of the polymerization reaction kettle. When the pressure was reduced to about 0.16MPa, 200ml of a 20 wt% aqueous nitric acid solution was added and stirred at 30 ℃ for 40 minutes.
After the reaction is finished, recovering unreacted tetrafluoroethylene monomer, filtering out polytetrafluoroethylene solid, crushing the polytetrafluoroethylene solid into particles with the particle size of less than 50 microns, washing the particles for 5 times by using deionized water with the conductivity of less than 2 mu s/cm to obtain high-cleanliness suspended PTFE resin, and testing the conductivity of the washing water solution at the last time. The washed PTFE was then dried in an oven at 160 ℃ for 20 hours and tested for metal ion content.
Example 2
Adding 30L of deionized water with the metal ion content less than 10ppb into a 50L polymerization reaction kettle, then repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using nitrogen, and then detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated to 20 ℃, then 6.7g of an oxidation-reduction initiation system consisting of potassium persulfate and sodium sulfite is added, the pressure of a tetrafluoroethylene monomer in the kettle is increased to 0.8MPa, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 4 ℃ during the reaction.
Stopping supplying the tetrafluoroethylene monomer into the polymerization reaction kettle, and gradually reducing the pressure of the polymerization reaction kettle. When the pressure was reduced to about 0.16MPa, 300ml of a 20 wt% aqueous nitric acid solution was added and stirred at 50 ℃ for 1 hour.
After the reaction is finished, recovering unreacted tetrafluoroethylene monomer, filtering out polytetrafluoroethylene solid, crushing the polytetrafluoroethylene solid into particles with the particle size of less than 50 microns, washing the particles for 5 times by using deionized water with the conductivity of less than 2 mu s/cm to obtain high-cleanliness suspended PTFE resin, and testing the conductivity of the washing water solution at the last time. The washed PTFE was then dried in an oven at 160 ℃ for 20 hours and tested for metal ion content.
Example 3
Adding 30L of deionized water with the metal ion content less than 10ppb into a 50L polymerization reaction kettle, then repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using nitrogen, and then detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated to 60 ℃, then 6.2g of an oxidation-reduction initiation system consisting of potassium persulfate and sodium sulfite is added, the pressure of the tetrafluoroethylene monomer in the kettle is increased to 0.8MPa, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 4 ℃ during the reaction.
Stopping supplying the tetrafluoroethylene monomer into the polymerization reaction kettle, and gradually reducing the pressure of the polymerization reaction kettle. When the pressure had dropped to about 0.16MPa, 300ml of a 20 wt% aqueous nitric acid solution was added and stirred at 60 ℃ for 1 hour.
After the reaction is finished, recovering unreacted tetrafluoroethylene monomer, filtering out polytetrafluoroethylene solid, crushing the polytetrafluoroethylene solid into particles with the particle size of less than 50 microns, washing the particles for 10 times by using deionized water with the conductivity of less than 2 mu s/cm to obtain high-cleanliness suspended PTFE resin, and testing the conductivity of the washing water solution at the last time. The washed PTFE was then dried in an oven at 160 ℃ for 20 hours and tested for metal ion content.
Example 4
Adding 30L of deionized water with the metal ion content less than 10ppb into a 50L polymerization reaction kettle, then repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using nitrogen, and then detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated to 70 ℃, then 6.2g of an oxidation-reduction initiation system consisting of potassium persulfate and sodium sulfite is added, the pressure of a tetrafluoroethylene monomer in the kettle is increased to 0.8MPa, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 4 ℃ during the reaction.
Stopping supplying the tetrafluoroethylene monomer into the polymerization reaction kettle, and gradually reducing the pressure of the polymerization reaction kettle. When the pressure had dropped to about 0.16MPa, 300ml of a 20 wt% aqueous nitric acid solution was added and stirred at a temperature of 78 ℃ for 1 hour.
After the reaction is finished, recovering unreacted tetrafluoroethylene monomer, filtering out polytetrafluoroethylene solid, crushing the polytetrafluoroethylene solid into particles with the particle size of less than 50 microns, washing the particles for 10 times by using deionized water with the conductivity of less than 2 mu s/cm to obtain high-cleanliness suspended PTFE resin, and testing the conductivity of the washing water solution at the last time. The washed PTFE was then dried in an oven at 160 ℃ for 20 hours and tested for metal ion content.
Comparative example 1
Adding 30L of deionized water with the metal ion content less than 10ppb into a 50L polymerization reaction kettle, then repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using nitrogen, and then detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated to 20 ℃, then 6.7g of an oxidation-reduction initiation system consisting of potassium persulfate and sodium sulfite is added, the pressure of a tetrafluoroethylene monomer in the kettle is increased to 0.8MPa, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 5 ℃ during the reaction.
After the reaction is completed, the unreacted tetrafluoroethylene monomer is recovered. And washing the resin with deionized water with the conductivity of less than 5 mu s/cm for 5 times to obtain the suspension PTFE resin, and testing the conductivity of the water solution in the last time. The washed PTFE was then dried in an oven at 160 ℃ for 20 hours, crushed to particles less than 50 microns in size, and tested for metal ion content.
Comparative example 2
Adding 30L of deionized water with the metal ion content less than 10ppb into a 50L polymerization reaction kettle, then repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using nitrogen, and then detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated to 60 ℃, then 6.2g of an oxidation-reduction initiation system consisting of potassium persulfate and sodium sulfite is added, the pressure of the tetrafluoroethylene monomer in the kettle is increased to 0.8MPa, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 5 ℃ during the reaction.
After the reaction is completed, the unreacted tetrafluoroethylene monomer is recovered. And washing the resin with deionized water with the conductivity of less than 5 mu s/cm for 8 times to obtain the suspension PTFE resin, and testing the conductivity of the water solution in the last time. The washed PTFE was then dried in an oven at 160 ℃ for 20 hours, crushed to particles less than 50 microns in size, and tested for metal ion content.
Comparative example 3
Adding 30L of deionized water with the metal ion content less than 10ppb into a 50L polymerization reaction kettle, then repeatedly performing replacement, vacuum pumping and the like on the polymerization kettle by using nitrogen, and then detecting the oxygen content in the kettle until the oxygen content is below 30 ppm. After the oxygen content is qualified, the reaction kettle is heated to 20 ℃, then 6.7g of an oxidation-reduction initiation system consisting of potassium persulfate and sodium sulfite is added, the pressure of a tetrafluoroethylene monomer in the kettle is increased to 0.8MPa, and stirring is started to start the reaction. Tetrafluoroethylene monomer initiates polymerization. As the polymerization proceeds, it is necessary to constantly replenish the polymerization pressure with tetrafluoroethylene monomer. The reaction temperature was kept fluctuating within a range of. + -. 4 ℃ during the reaction.
After the reaction is finished, recovering unreacted tetrafluoroethylene monomer, filtering out polytetrafluoroethylene solid, crushing the polytetrafluoroethylene solid into particles with the particle size of less than 50 microns, adding 3kg of deionized water with the metal ion content of less than 10ppb and 0.05kg of nitric acid, stirring the mixture for 2 hours at 70 ℃, filtering out the solid, washing the solid for 5 times by using the deionized water with the conductivity of less than 5 mu s/cm to obtain high-cleanliness suspended PTFE resin, and testing the conductivity of the washing water solution at the last time. The washed PTFE was then dried in an oven at 160 ℃ for 20 hours and tested for metal ion content.
TABLE 1 Performance index of samples in examples and comparative examples
The test results show that the method for adding the acid washing step to the polymerization part to obtain the high-cleanliness PTFE can be used for analyzing the conductivity and the metal ion content in a combined manner, so that the conductivity of the washing water and the metal ion elution amount are low, and the method meets the application fields of high-end industries with strict requirements on metal ions, such as semiconductors and the like.
The above embodiments are merely illustrative of the technical ideas and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the protection scope of the present invention. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.
Claims (27)
1. A preparation method of polytetrafluoroethylene comprises the following steps:
(1) adding water into a reaction container, adding an initiator and a tetrafluoroethylene monomer after deoxygenation to initiate polymerization reaction, and then supplementing the tetrafluoroethylene monomer to maintain reaction pressure; the TOC content of the water added to the reaction vessel is 5-50ppb by weight and/or the electrical conductivity of the water is 3.0 [ mu ] S/cm or less;
(2) stopping adding the tetrafluoroethylene monomer, slowly reducing the reaction pressure due to the consumption of the monomer, and adding inorganic acid or acetic acid aqueous solution when the reaction pressure is reduced to 10-30% of the original pressure;
(3) the polymerization was terminated to obtain polytetrafluoroethylene particles.
2. The process for producing polytetrafluoroethylene according to claim 1, wherein the water fed into the reaction vessel has a TOC content of 5 to 20ppb by weight and/or an electrical conductivity of 2.0 μ S/cm or less.
3. The process for producing polytetrafluoroethylene according to claim 1, wherein the water fed into the reaction vessel has a TOC content of 5 to 15ppb by weight and/or an electrical conductivity of 1.0 μ S/cm or less.
4. The process for producing polytetrafluoroethylene according to claim 1, wherein in step (2), when the reaction pressure is lowered to 12 to 28% of the original pressure, an aqueous solution of an inorganic acid or acetic acid is added to the reaction system.
5. The process for producing polytetrafluoroethylene according to claim 1, wherein in step (2), when the reaction pressure is lowered to 15 to 25% of the original pressure, an aqueous solution of an inorganic acid or acetic acid is added to the reaction system.
6. The process for producing polytetrafluoroethylene according to claim 1, wherein in step (2), when the reaction pressure is lowered to 18 to 22% of the original pressure, an aqueous solution of an inorganic acid or acetic acid is added to the reaction system.
7. The process for producing polytetrafluoroethylene according to claim 1, wherein the inorganic acid is selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and a mixture of two or more thereof in any ratio.
8. The process for producing polytetrafluoroethylene according to claim 1, wherein the weight concentration of the inorganic acid or acetic acid is 0.1 to 40%.
9. The process for producing polytetrafluoroethylene according to claim 1, wherein the weight concentration of the inorganic acid or acetic acid is 10 to 30%.
10. The process for producing polytetrafluoroethylene according to claim 1, wherein the weight concentration of the inorganic acid or acetic acid is 15 to 25%.
11. The process for producing polytetrafluoroethylene according to claim 1, wherein the weight concentration of the inorganic acid or acetic acid is 15 to 22%.
12. The process for producing polytetrafluoroethylene according to claim 1, wherein the weight concentration of the inorganic acid or acetic acid is 16 to 20%.
13. The process for producing polytetrafluoroethylene according to any of claims 8 to 12, wherein the concentration of the inorganic acid or acetic acid is 0.5 to 2 x 10 times the volume of water in the reaction vessel-3And (4) doubling.
14. The process for producing polytetrafluoroethylene according to any of claims 8 to 12, wherein the concentration of the inorganic acid or acetic acid is 0.6 to 1.6 x 10 times the volume of water in the reaction vessel-3And (4) doubling.
15. The process for producing polytetrafluoroethylene according to any of claims 8 to 12, wherein the concentration of the inorganic acid or acetic acid is 0.6 to 1.4 x 10 times the volume of water in the reaction vessel-3And (4) doubling.
16. The process for producing polytetrafluoroethylene according to any of claims 8 to 12, wherein the concentration of the inorganic acid or acetic acid is 0.8 to 1.2 x 10 times the volume of water in the reaction vessel-3And (4) doubling.
17. The process for producing polytetrafluoroethylene according to any of claims 1 to 7, further comprising the step of heating and stirring a mixture of the PTFE suspension and the solution of the inorganic acid or acetic acid in the reaction vessel.
18. The process for producing polytetrafluoroethylene according to claim 17, wherein the heating temperature is 40 to 95 ℃.
19. The process for producing polytetrafluoroethylene according to claim 17, wherein the heating temperature is 50 to 90 ℃.
20. The process for producing polytetrafluoroethylene according to claim 17, wherein the heating temperature is 60 to 85 ℃.
21. The process for producing polytetrafluoroethylene according to claim 17, wherein the heating temperature is 65 to 80 ℃.
22. The process for producing polytetrafluoroethylene according to claim 17, wherein the heating temperature is 50 to 60 ℃.
23. The process for producing polytetrafluoroethylene according to claim 17, wherein the stirring time is 1 to 180 min.
24. The process for producing polytetrafluoroethylene according to claim 17, wherein the stirring time is 5 to 120 min.
25. The process for producing polytetrafluoroethylene according to claim 17, wherein the stirring time is 10 to 100 min.
26. The process for producing polytetrafluoroethylene according to claim 17, wherein the stirring time is 20 to 80 min.
27. The process for producing polytetrafluoroethylene according to claim 17, wherein the stirring time is 30 to 60 min.
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