A tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin and a method for producing the same

Abstract

The present application belongs to the field of fluorine-containing resin, and relates to a tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin and a preparation method thereof. The resin is prepared by using a fluorine-containing emulsifier shown in formula (1) to perform emulsion copolymerization of tetrafluoroethylene and PAVE monomers in an aqueous medium, and then performing condensation, washing and drying on the emulsion, and the structure of the emulsifier is shown in general formula (1) R f1 (OCF2) x -(OR f2 ) y -OCF(CF3)COOA, in formula (1), the R f1 is a perfluoro linear alkyl with a carbon number of 1-2, the R f2 is a perfluoro linear or side chain alkyl with a carbon number of 2-3, x and y are independently 0 or 1, x and y cannot be 0 at the same time, A is a hydrogen atom, an alkali metal or NH4, and the residual amount of the fluorine-containing emulsifier in the resin is less than 0.2 ppm.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of fluorine-containing resins, and relates to a tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin and a preparation method thereof. BACKGROUND

[0002] Tetrafluoroethylene (hereinafter referred to as TFE) homopolymer and its copolymer have outstanding heat resistance, chemical corrosion resistance, weather resistance, low flammability and low surface energy, and can be used as functional films, cable insulation protective layers, corrosion-resistant coatings and seals, etc., and are often applied in aerospace, automotive industry, electronic information and new energy fields, to resist harsh application environments and play a special role.

[0003] Through aqueous emulsion polymerization of TFE and other fluorine-containing monomers, polytetrafluoroethylene (PTFE) or modified PTFE emulsion can be prepared, and further through emulsion coagulation and drying, fluorine resin in the form of powder, etc. can be obtained as raw materials for various shaped products. This polymerization method generally uses fluorine-containing emulsifiers, and ammonium perfluorooctanoate (structural formula CF3(CF2)6COONH4, abbreviated as PFOA) with a main chain composed of only 8 carbon atoms is usually used. However, if the emulsifier remains in the fluorine resin material, it cannot meet the application requirements of the semiconductor, medical and other fields which have particularly high requirements for purity. Moreover, PFOA has strong environmental persistence, biological accumulation, long-distance migration ability and biological toxicity, and is gradually controlled by the global community, and has been included in the Annex A of the POPs Convention for control in 2020.

[0004] In order to solve the above problems, early solutions were proposed to obtain higher purity fluororesin by ion exchange or washing or refining of fluororesin emulsion. With the progress of technology and the enhancement of people's environmental awareness, the skilled in the art expect to provide a solution without PFOA or using a new emulsifier to achieve the purpose of no PFOA or low residual emulsifier in the polymer. Chinese patent document CN1505644A provides a method for preparing tetrafluoroethylene homopolymer or copolymer without emulsifier, however, the fluoropolymer emulsion prepared by this method has a particle size of about 500 nm, which is 3-5 times the particle size of the emulsifier system, and the emulsion stability is poor. Chinese patent document CN102264684A proposes a technology for preparing fluoropolymer using a partially fluorinated carboxylate salt containing one or more ethylene-tetrafluoroethylene moieties (-CH2CH2CF2CF2-) as an emulsifier, but the above-mentioned partially fluorinated carboxylate salt has the problem of difficult structure control in the preparation process, and the H atoms on the molecular chain of the emulsifier have a strong chain transfer tendency, resulting in problems of insufficient high molecular weight and uneven molecular weight distribution of the polymer; at the same time, there are problems of low solid content of the polymer emulsion, only 15-16wt%, low production efficiency and high manufacturing cost. Patent CN106084104A provides a method for preparing PTFE resin emulsion using perfluorohexanoate as an emulsifier, however, perfluorohexanoate is a compound containing a long perfluoro segment, which has been restricted from being used as an emulsifier due to its bioaccumulation and non-degradability. Patent CN101296950A uses a specific CF3CF2OCF2CF2OCF2COONH4(EEA) structure emulsifier to prepare fluoropolymer emulsion. The patent uses the emulsifier to prepare PTFE homopolymer emulsion, and the emulsifier has a short straight chain group containing only 6 carbon atoms in the main chain, but it has the disadvantage of low emulsion solid content, and the residual amount of emulsifier in the resin is still high, which is not conducive to the application in the field with low residual amount requirement. SUMMARY

[0005] The present application aims to provide a fluororesin with low environmental load, no PFOA, and less fluorine-containing emulsifier residual compared with PFOA, and a preparation method thereof. The present inventors found that by selecting one or more fluorine-containing emulsifiers of fluorine-containing carboxylic acid and its salt with carbon number 6-8 and main chain having 1-4 ether oxygen atoms for TFE and perfluoroalkyl vinyl ether monomer (hereinafter referred to as PAVE) emulsion polymerization, a stable polymer emulsion can be obtained. Further, the emulsion can be used to prepare fluoropolymer powder or melt granules through processes such as coagulation, washing, drying, crushing or granulation.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] A tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin (PFA) comprising tetrafluoroethylene structural units (TFE units) and perfluoroalkyl vinyl ether structural units (PAVE units) at a molar ratio of 70 to 99.5 / 30 to 0.5;

[0008] The resin is prepared by emulsion copolymerization of tetrafluoroethylene and PAVE monomers in an aqueous medium using a fluorine-containing emulsifier represented by formula (1), followed by coagulation, washing and drying of the emulsion, and the residual amount of the fluorine-containing emulsifier of formula (1) in the resin is less than 0.2 ppm,

[0009] The emulsifier structure is represented by general formula (1) R f1 (OCF2) x -(OR f2 ) y -OCF(CF3)COOA, in formula (1), the R f1 is a perfluoro linear alkyl group with a carbon number of 1 to 2, R f2 is a perfluoro linear or branched alkyl group with a carbon number of 2 to 3, x and y are independently 0 or 1, and x and y are not simultaneously 0, and A is a hydrogen atom, an alkali metal or NH4. Further preferably, A is NH4.

[0010] Such a resin can solve the problem of high-purity fluorine-containing resins required in the fields of semiconductors, medical treatment, etc. and actually containing no PFOA.

[0011] The fluorine-containing emulsifier of formula (1) has good emulsion stabilization effect and small chain transfer tendency to monomer free radicals, and the residual amount of the fluorine-containing emulsifier in the prepared polymer is 0.2 ppm or less. Preferably, the residual amount of the emulsifier in the fluorine resin is less than 50 ppb, and more preferably less than 25 ppb.

[0012] Further preferably, the fluorine-containing emulsifier comprises:

[0013] (a) CF3OCF2CF2OCF(CF3)COONH4,

[0014] (b) CF3OCF2OCF2CF2OCF(CF3)COONH4,

[0015] (c) CF3CF2OCF(CF3)CF2OCF(CF3)COONH4, and combinations thereof.

[0016] In particular, the above-mentioned preferred fluorine-containing carboxylic acid containing ether oxygen atom with total carbon atom number of 6-8 and its salt or composition thereof have similar perfluoroether bond structure as PAVE, thus having better solubility and dispersion effect on PAVE. On one hand, it can reach higher solubility concentration in reaction phase under low PAVE addition condition, which is beneficial to reduce the addition amount of PAVE, on the other hand, it is particularly beneficial to obtain PFA polymer with uniform segment distribution. At the same time, compared with PFOA, the fluororesin latex particles obtained have better water wettability due to the presence of ether oxygen atom or shorter fluorocarbon segment, thus realizing the effect of using less washing water to achieve lower emulsifier residue in the post-processing process, and improving the washing efficiency. This effect is particularly beneficial to commercial production.

[0017] Preferably, the molar ratio of TFE unit to PAVE unit in the resin is 80-99 / 20-1, more preferably 90-98 / 10-2, and further more preferably 96-98 / 2-4. If the PAVE monomer unit is too little, the melting point is too high, which tends to reduce the formability, and if the PAVE monomer unit is too much, the mechanical properties and temperature resistance tend to decrease. The addition method of the PAVE monomer is not particularly limited, which can be added once before reaction or added continuously or intermittently during reaction. From the perspective of improving the uniformity of the resin structure, it is preferred to use continuous addition during reaction.

[0018] The melt flow rate (MFR) of the PFA resin is 0.1-100 g / 10 min, preferably 0.5-80 g / 10 min, and more preferably 1-30 g / 10 min. The emulsion particle size is 250-290 nm.

[0019] Preferably, the above-mentioned PFA resin further comprises monomer units copolymerizable with TFE and PAVE, which account for 0.1-10% of the molar percentage of the structural units of the PFA resin. The monomer copolymerizable with TFE and PAVE can be exemplified by hexafluoropropylene (HFP) and the like.

[0020] Preferably, the PAVE monomer has the general structure of: CF2=CF-O-R f3 (2),

[0021] In the formula, R f3a straight-chain or branched perfluoroalkyl group having 1 to 6 carbon atoms, examples of such comonomer can be CF2=CF-O-CF3 (PMVE), CF2=CF-O-CF2-CF3 (PEVE), CF2=CF-O-CF2-CF2-CF3 (PPVE); preferably CF2=CF-O-CF2-CF3 (PEVE) or CF2=CF-O-CF2-CF2-CF3 (PPVE). One or a combination of them can be used in the polymerization.

[0022] The present application also provides a copolymer emulsion of the above-mentioned tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin (PFA) resin, the solid content of the copolymer resin in the emulsion can be 8 to 38%, preferably 10 to 35% and more preferably 20 to 30% from the aspects of emulsion stability, resin molecular weight uniformity and commercial cost.

[0023] The present application provides a preparation method of the above-mentioned tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin (PFA), comprising the following steps: emulsion polymerization of TFE and PAVE comonomers in the presence of an aqueous medium, an emulsifier, a free radical polymerization initiator and a molecular weight regulator, and performing the processes of coagulation, washing and drying on the prepared copolymer emulsion to obtain PFA resin powder.

[0024] Preferably, the amount of the emulsifier is generally 500 to 15000 ppm, preferably 1000 to 12000 ppm, more preferably 2000 to 10000 ppm, and most preferably 3000 to 5000 ppm, based on the mass of the finally obtained PFA polymer. The emulsifier can be added in any manner without particular limitation, and can be added at once before the reaction or continuously or intermittently during the reaction.

[0025] As the free radical initiator used in the emulsion polymerization of the tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin (PFA), there is no particular limitation, and water-soluble redox initiators can be exemplified, including but not limited to persulfate systems such as ammonium persulfate, potassium persulfate or sodium persulfate and reducing agents such as sodium sulfite, sodium metabisulfite, sodium thiosulfate, peroxide systems such as di-succinic acid peroxide, di-glutaric acid peroxide, tert-butyl peroxide and the like, manganate systems such as permanganate compounds, and oxalic acid, ascorbic acid and the like can also be used as reducing agents to improve the initiation efficiency, azo salt compounds such as azobisdiisobutylamidine dihydrochloride, azobisdiisopropylimidazoline hydrochloride and the like.

[0026] The amount of the initiator added is preferably 50 to 5000 ppm, more preferably 50 to 1000 ppm, relative to the total mass of the polymerized monomers. The initiator can be added in any manner without particular limitation, and can be added at the beginning of the emulsion polymerization or added in the middle of the emulsion polymerization.

[0027] As a chain transfer agent used in emulsion polymerization of tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer (PFA), an alcohol such as methanol or ethanol; a chlorofluorocarbon such as 1,3-dichloro-l,l,2,2,3-pentafluoropropane or 1,1-dichloro-l-fluoroethane; or a hydrocarbon such as methane, ethane, propane, pentane, cyclohexane; or an ether such as dimethyl ether, t-butyl ether, methyl t-butyl ether, etc. can be used to adjust the molecular weight of the polymer. Preferably, the chain transfer agent can be an alcohol such as methanol or ethanol; a chlorofluorocarbon such as 1,3-dichloro-l,l,2,2,3-pentafluoropropane or 1,1-dichloro-l-fluoroethane; or a hydrocarbon such as methane, ethane, propane, pentane, cyclohexane; or an ether such as dimethyl ether, t-butyl ether, methyl t-butyl ether, etc.

[0028] The amount of the chain transfer agent added is preferably 10 to 15,000 ppm, more preferably 100 to 12,000 ppm, relative to the total mass of the polymerized monomers. The chain transfer agent can be added at the beginning of the emulsion polymerization or during the emulsion polymerization.

[0029] The conditions for emulsion polymerization of tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer (PFA) can be appropriately selected depending on the type of monomer used, the decomposition temperature of the initiator, etc. The emulsion polymerization temperature is preferably 10 to 95°C, more preferably 30 to 80°C. The polymerization pressure is preferably 0.5 to 4.0 MPa, more preferably 0.6 to 3.5 MPa. The polymerization time is preferably 50 to 600 minutes, more preferably 90 to 480 minutes.

[0030] The method for coagulating the emulsion of tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer (PFA) is not particularly limited and can be exemplified by strong mechanical stirring coagulation, or addition of a coagulant, or freezing. The preferred method is the addition of a coagulant.

[0031] The coagulant can include a water-soluble salt such as sodium chloride, calcium chloride, magnesium chloride, etc.; a mineral acid such as nitric acid, hydrochloric acid, sulfuric acid, etc.; an organic compound such as ethanol, acetone, etc. The amount of the coagulant added is 0.01 to 20%, more preferably 0.01 to 10%, relative to the mass of the copolymer emulsion.

[0032] The PFA resin after coagulation is washed with deionized water and then dried to prepare a polymer powder. The drying method can be exemplified by vacuum drying, hot air drying, etc. The drying temperature is preferably 30 to 230°C, more preferably 100 to 230°C.

[0033] Advantages of the present application:

[0034] The tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer (PFA) prepared using the present application has a lower residual amount of fluorine-containing emulsifier, which is less than 15 ppb, and can be further screw-extruded to produce resin pellets or solvent granulated or roll-pressed to produce coating powder, etc., as a raw material for molded bodies such as cable coating layers, injection-molded parts, tubes, films, and coated parts, and optical parts, etc. DETAILED DESCRIPTION

[0035] The application will be further described in detail below in connection with examples, but the application is not limited to these examples only.

[0036] Test method:

[0037] (1) Monomer unit content test: The monomer unit content of the PFA resin is obtained according to monomer type combined with NMR, FTIR, elemental analysis, etc.

[0038] (2) Emulsion particle size: The PFA emulsion particle size is tested and characterized by a scanning electron microscope energy dispersive spectrometer (SEM-EDS).

[0039] (3) Melting point: The melting point of the PFA resin is the temperature corresponding to the maximum value on the melting heat curve under the condition of nitrogen atmosphere and 10 ℃ / min speed of temperature rise by using a differential scanning calorimeter (DSC).

[0040] (4) Melt index: The MFR of the PFA is the mass of the resin melted and flowed out in 10 minutes under the condition of 372 ℃, 5 kgf load, 2.1 mm diameter and 8 mm length of the die, which is the melt flow rate, according to the method of ASTM D 3307.

[0041] (5) Fluorine-containing emulsifier residual amount: 5 portions of 1 g of powder sample are accurately weighed, respectively placed in 5 clean glass vials with lids and 3 ml of extraction solvent is added, wherein the extraction solvent is composed of 80 parts of methanol and 20 parts of ammonium acetate-acetic acid solution (solution concentration 20 mmol / L); the sample vials are sealed and placed in a shaker under the condition of 250 rpm frequency and 40 ℃, and vibrated for 1 h. The extract is filtered through an organic microporous membrane, and then loaded into a liquid phase sample bottle for LC-MSMS liquid chromatography-mass spectrometry detection. The fluorine-containing emulsifier content of the sample to be tested is determined by using the standard solution curve made.

[0042] The emulsifiers used in the specific embodiments and comparative examples of the application are as follows:

[0043] (a) CF3OCF2CF2OCF(CF3)COONH4,

[0044] (b) CF3OCF2OCF2CF2OCF(CF3)COONH4,

[0045] (c) CF3CF2OCF(CF3)CF2OCF(CF3)COONH4,

[0046] (d) CF3CF2CF2OCF(CF3)CF2OCF(CF3)COONH4,

[0047] (PFOA) CF3CF2CF2CF2CF2CF2CF2COONH4,

[0048] (EEA) CF3CF2OCF2CF2OCF2COONH4.

[0049] Example 1

[0050] Into a 5L stainless steel autoclave equipped with stirring, temperature control device, 3L deionized water, 20g CF3OCF2CF2OCF(CF3)COONH4(a) aqueous solution with a mass concentration of 20% were added, and vacuum and nitrogen replacement were repeated until the oxygen content was below 10ppm. 10g of methanol was added, and the temperature was raised to 60°C. TFE was added to the system until the pressure reached 2.0MPa, and 48g of PPVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization kettle to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours, wherein 80g of PPVE monomer and 860g of TFE monomer were added cumulatively, and the unreacted monomers in the kettle were vented. An emulsion with a solid content of 23.6% was obtained. SEM-EDS test showed that the average particle size of the emulsion was 256nm.

[0051] The PFA emulsion after reaction was coagulated, washed with water, and dried to obtain 980g of PFA resin powder. The melt index of the PFA was measured to be 16.8g / 10min by a melt index tester, the melting point was measured to be 302°C by a differential scanning calorimeter DSC, and the TFE / PPVE content (molar ratio) in the PFA polymer was measured to be 97.5 / 2.5 by infrared FT-IR. The content of fluorine-containing emulsifier in the PFA powder was measured to be 10.2ppb.

[0052] Example 2

[0053] Into a 5L stainless steel autoclave equipped with stirring and temperature control device, 3L deionized water, 20g of CF3OCF2OCF2CF2OCF(CF3)COONH4(b) aqueous solution with a mass concentration of 20% were added. Vacuum and nitrogen replacement were repeated until the oxygen content was below 10 ppm. 10g of methanol was added, and the temperature was raised to 60°C. TFE was added to the system until the pressure reached 2.0 MPa, and 52g of PPVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization reactor to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0 MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours, wherein 108g of PPVE monomer and 900g of TFE monomer were added cumulatively, and the unreacted monomers in the reactor were discharged. An emulsion with a solid content of 25.6% was obtained. SEM-EDS test showed that the average particle size of the emulsion was 285nm.

[0054] The PFA emulsion after the reaction was coagulated, washed with water, and dried to obtain PFA resin powder 1038g. The PFA melt index was measured by a melt index tester to be 18.3g / 10min, the melting point was measured by a differential scanning calorimeter DSC to be 302°C, and the TFE / PPVE content (mol ratio) in the PFA polymer was measured by infrared FT-IR to be 96.9 / 3.1. The content of fluorine-containing emulsifier in the PFA powder was tested to be 8.5ppb.

[0055] Example 3

[0056] Into a 5L stainless steel autoclave equipped with stirring and temperature control device, 3L deionized water, 20g of CF3OCF2OCF2OCF(CF3)COONH4(b) aqueous solution with a mass concentration of 20% were added. Vacuum and nitrogen replacement were repeated until the oxygen content was below 10 ppm. 10g of methanol was added, and the temperature was raised to 60°C. TFE was added to the system until the pressure reached 2.0 MPa, and 52g of PPVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization reactor to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0 MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours, wherein 108g of PPVE monomer and 900g of TFE monomer were added cumulatively, and the unreacted monomers in the reactor were discharged. An emulsion with a solid content of 25.6% was obtained. SEM-EDS test showed that the average particle size of the emulsion was 285nm.

[0057] The PFA emulsion after reaction was condensed, washed with water and dried to obtain PFA resin powder 1056 g. The PFA melt index was measured to be 18.5 g / 10 min by using a melt index tester, the melting point was measured to be 302°C by using a differential scanning calorimeter DSC, and the TFE / PPVE content (mol ratio) in the PFA polymer was measured to be 97.2 / 2.8 by using infrared FT-IR. The content of fluorine-containing emulsifier in the PFA powder was tested to be 6.7 ppb.

[0058] Example 4

[0059] A 5L stainless steel high-pressure reaction kettle equipped with stirring, temperature control device was added with 3L deionized water, 16g of 20% CF3CF2OCF(CF3)CF2OCF(CF3)COONH4(c) aqueous solution, repeatedly vacuumed and replaced with nitrogen until the oxygen content was below 10 ppm. 10g of ethane was added, and the temperature was raised to 60°C. TFE was added to the system until the pressure reached 2.0 MPa, and 52g of PPVE monomer was added by a metering pump. 10g of 2g / L ammonium persulfate solution was added to the polymerization kettle to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0 MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours. Among them, 109g of PPVE monomer and 980g of TFE monomer were added cumulatively, and the unreacted monomers in the kettle were discharged to obtain an emulsion with a solid content of 28.4%. The average particle size of the emulsion was 274nm by SEM-EDS test.

[0060] The PFA emulsion after reaction was condensed, washed with water and dried to obtain PFA resin powder 1038 g. The PFA melt index was measured to be 17.6 g / 10 min by using a melt index tester, the melting point was measured to be 303°C by using a differential scanning calorimeter DSC, and the TFE / PPVE content (mol ratio) in the polymer was measured to be 97.3 / 2.7 by using infrared FT-IR combined with solid NMR. The content of fluorine-containing emulsifier in the PFA powder was tested to be 4.8 ppb.

[0061] Example 5

[0062] Into a 5L stainless steel autoclave equipped with stirring, temperature control device, 3L deionized water, 6g CF3OCF2OCF2CF2OCF(CF3)COONH4(b) aqueous solution with a mass concentration of 20%, 14g CF3CF2OCF(CF3)CF2OCF(CF3)COONH4(c) aqueous solution with a mass concentration of 20% were added. Vacuum and nitrogen were repeatedly replaced until the oxygen content was below 10ppm. 10g of ethane was added, and the temperature was raised to 60℃. TFE was added to the system until the pressure reached 2.0MPa, and 50g of PPVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization reactor to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours. Among them, 96g of PPVE monomer and 925g of TFE monomer were added, and the unreacted monomers in the reactor were discharged. An emulsion with a solid content of 26.1% was obtained. The average particle size of the emulsion was 263nm by SEM-EDS test.

[0063] The PFA emulsion after reaction was coagulated, washed with water and dried to obtain PFA resin powder 1040g. The PFA melt index was measured by a melt index tester to be 19.5g / 10min, the melting point was measured by a differential scanning calorimeter DSC to be 303℃, and the TFE / PPVE content (mol ratio) in the polymer was measured by infrared FT-IR combined with solid NMR to be 96.8 / 3.2. The content of fluorine-containing emulsifier in the PFA powder was 5.5ppb.

[0064] Comparative Example 1

[0065] Into a 5L stainless steel autoclave equipped with stirring, temperature control device, 3L deionized water, 16g ammonium perfluorooctanoate CF3(CF2)6COONH4(PFOA) aqueous solution with a concentration of 20% was added. Vacuum and nitrogen were repeatedly replaced until the oxygen content was below 10ppm. 10g of methanol was added, and the temperature was raised to 60℃. TFE was added to the system until the pressure reached 2.0MPa, and 48g of PPVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization reactor to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours. Among them, 88g of PPVE monomer and 1045g of TFE monomer were added, and the unreacted monomers in the reactor were discharged. An emulsion with a solid content of 30.8% was obtained. The average particle size of the emulsion was 251nm by SEM-EDS test.

[0066] The PFA emulsion after reaction was condensed, washed with water and dried to obtain PFA resin powder 1167 g. The PFA melt index was 13.5 g / 10 min, the melting point was 302°C, and the TFE / PPVE content (mol ratio) in the PFA polymer was 98.3 / 1.7, as measured by differential scanning calorimeter DSC and infrared FT-IR, respectively. The fluorine-containing emulsifier content in the PFA powder was 75.5 ppb.

[0067] Comparative Example 2

[0068] A 5L stainless steel high-pressure reaction kettle equipped with stirring, temperature control device was added with 3L deionized water, 20g ammonium perfluorooctanoate CF3(CF2)6COONH4 (PFOA) aqueous solution with a concentration of 20%, and repeatedly vacuumed and replaced with nitrogen until the oxygen content was below 10 ppm. 10g of ethane was added, and the temperature was raised to 60°C. Tetrafluoroethylene was added to the system until the pressure reached 2.0 MPa, and 52g of PPVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization kettle to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0 MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours. Among them, 131g of PPVE monomer and 1080g of TFE monomer were added cumulatively, and the unreacted monomers in the kettle were discharged to obtain an emulsion with a solid content of 30.4%. The average particle size of the emulsion was 259nm, as measured by SEM-EDS.

[0069] The PFA emulsion after reaction was condensed, washed with water and dried to obtain PFA resin powder 1271 g. The PFA melt index was 15.2 g / 10 min, the melting point was 301°C, and the TFE / PEVE content (mol ratio) in the polymer was 98.4 / 1.6, as measured by differential scanning calorimeter DSC and infrared FT-IR combined with solid NMR. The fluorine-containing emulsifier content in the PFA powder was 96.0 ppb.

[0070] Comparative Example 3

[0071] Into a 5L stainless steel autoclave equipped with stirring, temperature control device, 3L deionized water, 20g CF3CF2OCF2CF2OCF2COONH4(EEA) aqueous solution with a concentration of 20%, vacuum-nitrogen repeatedly until the oxygen content is below 10ppm. 10g methanol was added, and the temperature was raised to 60°C. TFE was added until the pressure reached 2.0MPa, and 48g of PPVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization reactor to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours, wherein 74g of PPVE monomer and 728g of TFE monomer were added cumulatively, and the unreacted monomers in the reactor were vented. An emulsion with a solid content of 21.2% was obtained. The average particle size of the emulsion was 297nm by SEM-EDS test.

[0072] The PFA emulsion after the reaction was coagulated, washed with water, and dried to obtain PFA resin powder 812g. The PFA melt index was measured by a melt index tester to be 18.5g / 10min, the melting point was measured by a differential scanning calorimeter DSC to be 302°C, and the TFE / PPVE content (molar ratio) in the PFA polymer was measured by infrared FT-IR to be 98.5 / 1.5. The fluorine-containing emulsifier content in the PFA powder was tested to be 87.5ppb.

[0073] Comparative Example 4

[0074] Into a 5L stainless steel autoclave equipped with stirring, temperature control device, 3L deionized water, 20g CF3CF2CF2OCF(CF3)CF2OCF(CF3)COONH4(d) aqueous solution with a concentration of 20%, vacuum-nitrogen repeatedly until the oxygen content is below 10ppm. 10g methanol was added, and the temperature was raised to 60°C. TFE was added until the pressure reached 2.0MPa, and 52g of PEVE monomer was added by a metering pump. 10g of ammonium persulfate solution with a concentration of 2g / L was added to the polymerization reactor to initiate the polymerization reaction. After the reaction started, TFE monomer and PPVE monomer were continuously added to maintain the polymerization pressure at 2.0MPa, and ammonium persulfate solution was continuously added at a rate of 0.15g / min. The polymerization reaction was stopped after 2.5 hours, wherein 86g of PEVE monomer and 720g of TFE monomer were added cumulatively, and the unreacted monomers in the reactor were vented. An emulsion with a solid content of 22.8% was obtained. The average particle size of the emulsion was 298nm by SEM-EDS test.

[0075] The PFA emulsion after the reaction was coagulated, washed with water, and dried to obtain 833 g of PFA resin powder. The PFA melt index was measured to be 21.6 g / 10 min using a melt index tester, the melting point was measured to be 252°C using a differential scanning calorimeter DSC, and the TFE / PPVE content (molar ratio) in the polymer was measured to be 97.7 / 2.3 using infrared FT-IR combined with solid NMR. The fluorine-containing emulsifier content in the PFA powder was measured to be 96.5 ppb.

[0076] The results of each example and comparative example are summarized in Table 1.

[0077] Table 1

[0078]

[0079]

Claims

1. A copolymer resin of tetrafluoroethylene and perfluoroalkyl vinyl ether, characterized in that, It includes tetrafluoroethylene structural units and perfluoroalkyl vinyl ether structural units, with a molar ratio of tetrafluoroethylene structural units to perfluoroalkyl vinyl ether structural units of (96~98) / (2~4). The resin is prepared by emulsion copolymerization of tetrafluoroethylene and perfluoroalkyl vinyl ether monomers in an aqueous medium using a fluorinated emulsifier, followed by coagulation, washing, and drying of the emulsion. The residual amount of fluorinated emulsifier in the resin is less than 25 ppb. The fluorinated emulsifier is a combination of (a) CF3OCF2CF2OCF(CF3)COONH4 and (b) CF3OCF2OCF2CF2OCF(CF3)COONH4.

2. The resin according to claim 1, characterized in that, The melt index of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin is 1~30 g / 10 min, and the emulsion particle size is 250~290 nm.

3. A method for preparing the resin according to any one of claims 1 to 2, characterized in that, Includes the following steps: In the presence of an aqueous medium, an emulsifier, a free radical polymerization initiator, and a molecular weight regulator, tetrafluoroethylene and perfluoroalkyl vinyl ether comonomers are subjected to emulsion polymerization. The prepared copolymer emulsion is then subjected to coagulation, washing, and drying processes to obtain tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin powder.

4. The preparation method according to claim 3, characterized in that, The amount of emulsifier used is 500-15000 ppm based on the mass of the final tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin.

5. The preparation method according to claim 4, characterized in that, The amount of emulsifier used is 2000~10000 ppm based on the mass of the final tetrafluoroethylene and perfluoroalkyl vinyl ether copolymer resin.

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