High-purity fusible polytetrafluoroethylene resin and preparation method thereof

Abstract

The invention relates to high-purity fusible polytetrafluoroethylene resin and a preparation method thereof. The method comprises the following steps: adding deionized water and a buffering agent into a high-pressure reaction kettle, adding a part of perfluoroalkyl vinyl ether and a chain transfer agent, adding an organic initiator at a reaction temperature, introducing a tetrafluoroethylene monomer to a reaction pressure, carrying out a copolymerization reaction, maintaining the reaction pressure, continuously dropwise adding the perfluoroalkyl vinyl ether, and carrying out a polymerization reaction to obtain the perfluoroalkyl vinyl ether copolymer. And carrying out high-temperature water washing on polymer particles obtained by the reaction, drying, and carrying out fluorine gas end-capping treatment on the obtained fusible polytetrafluoroethylene resin powder. According to the fusible polytetrafluoroethylene resin powder and granulation material produced by the method, tetrafluoroethylene and perfluoroalkyl vinyl ether are copolymerized in the polymerization process, the number of introduced metal ion impurities is effectively avoided through process and process control, the product is subjected to fluorine high-temperature end sealing after high-temperature water washing, the number of unstable end groups of a molecular chain is reduced, and the product quality is improved. The fluorine ion precipitation amount is reduced, and the use requirements in the high-purity field are met.

Description

Technical Field

[0001] The invention relates to the technical field of polytetrafluoroethylene resins, and in particular to a high-purity fusible polytetrafluoroethylene resin and a preparation method thereof. Background Art

[0002] Fusible polytetrafluoroethylene resin (PFA) is a very important new variety, which is a copolymer of tetrafluoroethylene (TFE) and perfluoroalkyl vinyl ether (PAVE), commonly known as fusible polytetrafluoroethylene. Its chemical stability, physical and mechanical properties, electrical insulation properties, lubricity, non-stickiness, aging resistance, non-flammability and thermal stability are very good, similar to ordinary PTFE, and its high-temperature mechanical strength is about 2 times higher than that of ordinary PTFE. Among them, PAVE mainly includes perfluoropropyl vinyl ether (PPVE), perfluoroethyl vinyl ether (PEVE) and perfluoromethyl vinyl ether (PMVE). PFA contains 1-15% (mass fraction) of PAVE, which significantly improves the flexibility of the polymer chain, reduces the crystallinity of PFA, and makes PFA have good thermoplasticity, solving the problem that traditional polytetrafluoroethylene cannot be melt-processed due to its high melt viscosity, and can be processed by the molding process of general thermoplastic plastics.

[0003] Patent application US3635926 discloses a copolymerization reaction of tetrafluoroethylene and perfluoroalkyl vinyl ether in an aqueous medium containing a small amount of fluorocarbon solvent. The reaction is started by heating up the reaction vessel in an anaerobic reactor using water, solvent, comonomer, initiator and surfactant. A small amount of ammonium carbonate is also used as a buffer. After the reaction, a fusible tetrafluoropolymer emulsion is obtained, and a powder is obtained through a post-treatment process. The powder uses a small amount of fluorocarbon solvent in the production process, which is easy to cause adverse effects on the environment. At the same time, the powder has a large number of unstable end groups. In high-temperature processing projects, the unstable end groups decompose, corrode equipment, and pollute products, making it difficult to meet high purity requirements.

[0004] Patent application CN106519100A discloses a method for preparing fusible polytetrafluoroethylene, which is characterized in that a solvent, a polymerizing monomer and a chain transfer agent are added to a polymerization kettle in a certain proportion, an initiator is added during heating, a reaction is started, and a constant pressure is maintained by adding tetrafluoromonomer. When a predetermined amount is reached, deionized water and an initiator are continuously added to the kettle, and the temperature is increased and kept warm under stirring to obtain a solid material, which is filtered, washed and dried to obtain a fusible polytetrafluoroethylene powder. The powder has a uniform particle size and a significantly reduced number of unstable end groups. However, a large amount of restricted organic solvents, such as F113, are used in the production process of the powder, which is likely to have an adverse effect on the environment. At the same time, the number of unstable end groups is too large, and direct melt extrusion may result in problems such as gray appearance and bubbles in the product, making it difficult to meet high purity requirements.

[0005] Patent application CN100503669C introduces a copolymer to which tetrafluoroethylene and fluorinated vinyl ether can be melt-added and produced by a suspension polymerization method. The copolymerization is carried out in water, a free radical initiator, and a stirred reactor without using a fluorinated organic solvent. After the reaction, a solid copolymer is directly obtained. This method does not use surfactants and solvents, and can easily separate a solid product. However, the product has a large number of unstable end groups, and direct melt extrusion may result in problems such as gray appearance and bubbles in the product.

[0006] The commonly used polymerization methods for PFA are solution polymerization, emulsion polymerization and suspension polymerization. Among them, aqueous suspension polymerization has the advantages of using little or no surfactant (dispersant), no organic solvent, and the ability to directly separate the polymer from the aqueous medium. However, emulsion polymerization requires steps such as coagulation and washing, and the surfactant is adsorbent and difficult to completely handle, which can easily affect the appearance of the product; solvent polymerization requires the treatment of organic solvents and the washing of polymers, which can easily cause adverse effects on the environment. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a high-purity fusible polytetrafluoroethylene resin and a preparation method thereof.

[0008] In the first aspect, the present invention provides a method for preparing a high-purity fusible polytetrafluoroethylene resin, comprising the following steps: copolymerizing tetrafluoroethylene monomer, perfluoroalkyl vinyl ether, and chain transfer agent by suspension polymerization; then washing the obtained polymer particles with high temperature water, drying, and performing fluorine gas end-capping treatment; the chain transfer agent is methane, ethane, or propane. In the present invention, by adding perfluoroalkyl vinyl ether, chain transfer agent and other raw materials in a high-pressure reactor and introducing tetrafluoroethylene monomer by suspension polymerization, the polymer particles obtained by the reaction are washed with high temperature water and then dried, and the obtained fusible polytetrafluoroethylene resin powder is subjected to fluorine gas end-capping treatment. This method can be used to produce fusible polytetrafluoroethylene resin powder or obtain granulated materials by screw granulation. The polymerization process adopts tetrafluoroethylene and perfluoroalkyl vinyl ether copolymerization, and the number of metal ion impurities introduced is effectively avoided through process and process control. The product is high-temperature water washed and then fluorine gas high-temperature end-capped, which reduces the number of unstable end groups of the molecular chain and the amount of fluorine ion precipitation, thereby meeting the use requirements in the high-purity field.

[0009] The polymerization process of the invention adopts a suspension polymerization method and low-pressure copolymerization to increase the effective linking ratio of copolymer monomers; adopts a preferred gas chain transfer agent to effectively control the molecular weight and molecular weight distribution of the polymer and reduce the introduction of impurity ions; the organic initiation system can effectively reduce the introduction of inorganic ions and the number of unstable end groups, and make the reaction rate stable and effectively controllable; post-treatment high-temperature water washing can effectively remove residual reagents and impurities; high-temperature end-capping with fluorine gas can reduce the number of unstable end groups of the molecular chain; the obtained product has the characteristics of transparent appearance, no yellowing, low fluoride ion content and metal ion content, and the product meets the use requirements in the high-purity field.

[0010] Preferably, the chain transfer agent is propane; and / or, the amount of the chain transfer agent is 0.04-0.1% of the mass of deionized water. In the suspension polymerization process system of the present invention, the use of propane, which is a preferred type of chain transfer agent, to interact with other raw materials can significantly improve the quality of the product, more effectively control the molecular weight and molecular weight distribution of the polymer, and greatly reduce the introduction of impurity ions. When propane is used, the amount required is less than that of reagents such as ethane, providing the highest polymerization rate for each initiator consumption at a specified degree of polymerization. Propane is also inexpensive and not dangerous.

[0011] Preferably, the fluorine gas end-capping treatment includes fluorine gas treatment at 220-240°C for 2-4h, fluorine gas concentration of 15-25%, and fluorine gas flow rate of 0.8-1.2 L / min; preferably, fluorine gas treatment at 230±5°C for 3±0.5h, fluorine gas concentration of 20±2%, and fluorine gas flow rate of 1±0.1 L / min. The fluorine gas end-capping treatment under the preferred conditions has a better effect.

[0012] More preferably, the high-temperature water washing is performed at least twice, preferably 2-3 times, and the temperature of the high-temperature water washing is ≥88°C, preferably above 90°C.

[0013] In the present invention, by adopting high temperature water washing and fluorine gas end-capping treatment, and by process and process control, the number of metal ion impurities and the residual amount of reagents are effectively avoided and removed, the number of unstable end groups of molecular chains is reduced, the amount of fluorine ion precipitation is reduced, and the use demand in the high-purity field is met. The present invention designs the copolymerization pressure and temperature by a suspension polymerization method, especially in the system of the present invention, the overall optimization of conditions such as process and raw materials, further increases the effective link ratio of perfluoroalkyl vinyl ether, uses an organic initiation system, effectively reduces the use of inorganic ions, and the main chain end group structure is more stable, so that the reaction speed is stable and effectively controllable; in the post-processing stage, by high temperature water washing, and low molecular weight substances are removed at high temperature, through fluorine gas end-capping treatment, the unstable end group decomposition of the product during hot melt processing is avoided, and the demand in the high-purity field is met.

[0014] Preferably, the reaction pressure is 0.8-1.8 MPa; and / or the reaction temperature is 70-75°C. For example, the reaction pressure is 0.8, 0.9, 1.0, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 MPa and any values ​​therebetween, and the reaction temperature is 70, 71, 72, 73, 74, 75°C and any values ​​therebetween. In the process and raw material system of the present invention, the high-purity fusible polytetrafluoroethylene resin prepared by the preferred reaction pressure and temperature has better effect. The effect of the reaction pressure and temperature being too high or too low is poor.

[0015] As a preferred method, the preparation method of high-purity fusible polytetrafluoroethylene resin provided by the present invention comprises: adding deionized water and a buffer into a high-pressure reactor, adding a part of perfluoroalkyl vinyl ether and a chain transfer agent, adding an initiator at a reaction temperature, introducing tetrafluoroethylene monomer to the reaction pressure, carrying out copolymerization, maintaining the reaction pressure and continuously dripping perfluoroalkyl vinyl ether, the polymer particles obtained by the reaction are washed with high temperature water at least once and then dried, and the obtained fusible polytetrafluoroethylene resin powder is subjected to fluorine gas end-capping treatment. The present invention adds deionized water and a buffer into a high-pressure reactor with stirring and a jacket, after the oxygen analysis is qualified, adds a certain amount of perfluoroalkyl vinyl ether and a chain transfer agent, adds an initiator at a reaction temperature, introduces tetrafluoroethylene monomer to the reaction pressure, starts copolymerization, maintains a specified reaction pressure and continuously drips perfluoroalkyl vinyl ether, when the TFE feed reaches a certain amount, stops the reaction to obtain polymer particles, washes with high temperature water, and then vacuum-dries at 200°C to obtain fusible polytetrafluoroethylene resin powder. The powder is treated with fluorine gas at 230°C to complete the end-capping reaction. The obtained powder product can meet the needs of electrostatic spraying in the high-purity field, and can also be further melt-extruded and granulated to obtain a granular product that can meet the needs of preparing pipes, valves, containers, etc. in the high-purity field. The obtained product has a transparent appearance, and the fluorine ion precipitation of the product is less than 4ppm, and the metal ion precipitation is less than 0.2ppm.

[0016] Preferably, the perfluoroalkyl vinyl ether is one or more of perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, and perfluoropropyl vinyl ether, and the amount used is 2.5-5.5% of the mass of deionized water.

[0017] More preferably, the organic initiator is succinic acid peroxide, and the added amount is 2.0-4.5% of the mass of deionized water.

[0018] Preferably, the buffer is ammonium carbonate or ammonium bicarbonate, and the amount used is 0.1-0.3% of the mass of deionized water.

[0019] In the process of the present invention, the preferred raw materials are used to interact with each other so that the comprehensive effect of the prepared high-purity fusible polytetrafluoroethylene resin product is better.

[0020] Preferably, the amount of deionized water used is 50-70% of the volume of the reaction container.

[0021] More preferably, the drying temperature after high-temperature water washing is 185-205°C, preferably 200±0.2°C, and the drying time is 12-24h.

[0022] In a second aspect, the present invention provides a high-purity fusible polytetrafluoroethylene resin obtained by the above-mentioned preparation method.

[0023] The beneficial effects of the present invention are at least that: the fusible polytetrafluoroethylene resin powder and granulated material produced by the method adopts tetrafluoroethylene and perfluoroalkyl vinyl ether copolymerization in the polymerization process, and the introduction of metal ion impurities is effectively avoided through process and process control. The product is capped with fluorine gas at high temperature to reduce the number of unstable end groups of the molecular chain and the amount of fluoride ion precipitation, thereby meeting the use requirements in the high-purity field. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products or can be prepared by known methods. If no specific techniques or conditions are specified in the examples, they are all conventional methods or the techniques or conditions described in the literature in this field, or according to the product instructions. If the manufacturers of the reagents and instruments are not specified, they are all conventional products that can be purchased through regular channels.

[0026] Example 1 In a 20L stainless steel autoclave equipped with a jacket and stirring, add 11L deionized water, 15g ammonium carbonate, start stirring, evacuate, and replace with nitrogen. When the oxygen content is less than 30ppm, add 5g propane and 110g perfluoropropyl vinyl ether. When the temperature in the autoclave is controlled to 70°C, add 300g peroxysuccinic acid initiator through a metering pump. Pass tetrafluoroethylene monomer, control the reaction pressure to 1.2MPa, and after the reaction starts, continuously add 240g perfluoropropyl vinyl ether at a certain speed. During the reaction, control the temperature in the autoclave to 70-71°C. When the TFE feed reaches 3Kg, stop the reaction, recover the unreacted monomer, and obtain 1846g of polymerization product.

[0027] The obtained wet material was placed in a washing bucket, deionized water was added, and the temperature was raised to above 90°C for washing three times. After being dried in an oven at 200°C and treated with fluorine gas at 230°C for 3 hours, the melting point, melt index, metal ion precipitation, fluoride ion precipitation, etc. were tested.

[0028] Example 2 The process is the same as that of Example 1, except that the reaction temperature is 74-75°C, the reaction pressure is 1.2 MPa, and when the TFE feed amount reaches 3 kg, 1828 g of polymer product is obtained. The obtained wet material is placed in a washing bucket, deionized water is added, and the temperature is raised to above 90°C for washing twice, and then dried in an oven at 200°C and treated with fluorine gas at 230°C for 3 hours, and then the melting point, melt index, metal ion precipitation, fluorine ion precipitation, etc. are tested.

[0029] Example 3 The process was the same as in Example 1, except that the reaction pressure was 1.8 MPa. When the TFE feed amount reached 3 kg, 1816 g of polymer product was obtained. The obtained wet material was placed in a washing bucket, deionized water was added, and the temperature was raised to above 90°C for washing 3 times. After drying in an oven at 200°C and treated with fluorine gas at 230°C for 3 hours, the melting point, melt index, metal ion precipitation, fluorine ion precipitation, etc. were tested.

[0030] Example 4 The process was the same as in Example 1, except that the reaction pressure was 0.8 MPa. When the TFE feed amount reached 3 kg, 1835 g of polymer product was obtained. The obtained wet material was placed in a washing bucket, deionized water was added, and the temperature was raised to above 90°C for washing 3 times. After drying in an oven at 200°C and treated with fluorine gas at 230°C for 3 hours, the melting point, melt index, metal ion precipitation, fluorine ion precipitation, etc. were tested.

[0031] Comparative Example 1 The process is the same as that of Example 1, except that the reaction pressure is 2.0 MPa. After the resin powder is fluorinated, the melting point, melt index, metal ion precipitation, fluoride ion precipitation and other tests are performed.

[0032] Comparative Example 2 The process is the same as that of Example 1, except that 10 g of dichloromethane is added as a chain transfer agent. After the obtained resin powder is fluorinated, the melting point, melt index, metal ion precipitation, fluoride ion precipitation and the like are tested.

[0033] Comparative Example 3 The process is the same as that of Example 1, except that the reaction pressure is 0.5 MPa. After the resin powder is fluorinated, the melting point, melt index, metal ion precipitation, fluoride ion precipitation and other tests are performed.

[0034] Comparative Example 4 Emulsion polymerization method: In a 5L stainless steel high-pressure reactor equipped with a jacket and stirring, add 2.8L deionized water, 8.4g ammonium carbonate, and 2.8g perfluorooctanoic acid ammonium substitute, start stirring, evacuate, and replace with nitrogen. Until the oxygen content is less than 30ppm, control the temperature in the reactor to 68°C, add 84g perfluoropropyl vinyl ether and 0.36g ammonium persulfate initiator, pass tetrafluoroethylene monomer, control the reaction pressure to 0.6MPa, start the reaction, and control the maximum temperature in the reactor to 70°C during the reaction. After 6 hours of reaction, stop the reaction to obtain a polymer emulsion with a solid content of 27%.

[0035] Deionized water was added to the polymer emulsion to adjust the solid content to 15%, and mechanical stirring and condensation operation was carried out. The wet material was added with deionized water and washed three times, and then dried in an oven at 240°C to obtain 1150g of fusible polytetrafluororesin powder. After the powder was treated with fluorine gas at 200°C for 20 hours, the melting point, melt index, metal ion precipitation, fluoride ion precipitation and other tests were carried out.

[0036] Comparative Example 5 The process is the same as that of Example 1, except that no fluorination treatment is performed, and the melting point, melt index, metal ion precipitation, fluoride ion precipitation and other tests are directly performed.

[0037] Comparative Example 6 The process is the same as that of Example 1, except that the fluorination treatment is performed directly without high-temperature water washing, and the melting point, melt index, metal ion precipitation, fluoride ion precipitation and other tests are directly performed.

[0038] Performance Testing: The fusible polytetrafluoroethylene resin provided in Examples 1-4 and Comparative Examples 1-6 was subjected to performance tests, and its melting point was tested according to the ASTM D4591 test standard, its melt index was tested according to the ASTM D3307 test standard, fluoride ion precipitation was tested according to the SEMI F57 standard, metal ion precipitation was tested by ICP-OES, relative density and tensile strength were tested according to T / FSI 028-2019, and the appearance of the sheet was visually inspected. The above test data are shown in the following table.

[0039] Table 1

[0040] It can be seen from the embodiments and performance tests that the melting point of the fusible polytetrafluoroethylene resin provided by the present invention is about 306°C, the tensile strength is greater than 32MPa, the sheet appearance is transparent and does not yellow, and the metal ion precipitation is below 0.2ppm, and the fluorine ion precipitation is below 3ppm. The products processed by the resin can meet the needs of the high-purity field.

[0041] From the comparison between Examples 1-4 and Comparative Examples 1-6, it can be seen that only by adopting the solution provided by the present invention can a resin product with excellent performance and meeting the application requirements in the high-purity field be obtained.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a high-purity fusible polytetrafluoroethylene resin, characterized in that: The following steps are involved: Tetrafluoroethylene monomer, perfluoroalkyl vinyl ether and chain transfer agent are copolymerized by suspension polymerization; the obtained polymer particles are then washed with high-temperature water, dried and subjected to fluorine gas end-capping treatment; the chain transfer agent is methane, ethane or propane.

2. The method for preparing high-purity fusible polytetrafluoroethylene resin according to claim 1, characterized in that: The chain transfer agent is propane; the amount of the chain transfer agent is 0.04-0.1% of the mass of deionized water.

3. The method for preparing high-purity fusible polytetrafluoroethylene resin according to claim 1 or 2, characterized in that: The fluorine gas end-capping treatment comprises fluorine gas treatment at 220-240° C. for 2-4 hours, fluorine gas concentration of 15-25%, and fluorine gas flow rate of 0.8-1.2 L / min; preferably, fluorine gas treatment at 230±5° C. for 3±0.5 hours, fluorine gas concentration of 20±2%, and fluorine gas flow rate of 1±0.1 L / min.

4. The method for preparing a high-purity fusible polytetrafluoroethylene resin according to any one of claims 1 to 3, characterized in that: The high temperature water washing is performed at least twice, preferably 2-3 times, and the temperature of the high temperature water washing is ≥88°C, preferably above 90°C.

5. The method for preparing a high-purity fusible polytetrafluoroethylene resin according to any one of claims 1 to 4, characterized in that: The reaction pressure is 0.8-1.8 MPa; and / or the reaction temperature is 70-75°C.

6. The method for preparing a high-purity fusible polytetrafluoroethylene resin according to any one of claims 1 to 5, characterized in that: Deionized water and a buffer are added into a high-pressure reactor, a portion of perfluoroalkyl vinyl ether and a chain transfer agent are added, an organic initiator is added at the reaction temperature, tetrafluoroethylene monomer is introduced to the reaction pressure, a copolymerization reaction is carried out, the reaction pressure is maintained and the perfluoroalkyl vinyl ether is continuously added dropwise, the polymer particles obtained by the reaction are washed with high-temperature water at least once and then dried, and the obtained fusible polytetrafluoroethylene resin powder is subjected to fluorine gas end-capping treatment.

7. The method for preparing high-purity fusible polytetrafluoroethylene resin according to claim 6, characterized in that: The perfluoroalkyl vinyl ether is one or more of perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, and perfluoropropyl vinyl ether, and the amount used is 2.5-5.5% of the mass of deionized water; And / or, the organic initiator is succinic acid peroxide, and the added amount is 2.0-4.5% of the mass of deionized water.

8. The method for preparing high-purity fusible polytetrafluoroethylene resin according to claim 6 or 7, characterized in that: The buffer is ammonium carbonate or ammonium bicarbonate, and the amount used is 0.1-0.3% of the mass of deionized water; And / or, the amount of deionized water used is 50-70% of the volume of the reaction container.

9. The method for preparing a high-purity fusible polytetrafluoroethylene resin according to any one of claims 1 to 8, characterized in that: The drying temperature after high-temperature water washing is 185-205°C, preferably 200±0.2°C, and the drying time is 12-24h.

10. High-purity fusible polytetrafluoroethylene resin obtained by the preparation method according to any one of claims 1 to 9.

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