Step-by-step polymerization preparation method of fluorinated ethylene propylene

Abstract

The invention discloses a step-by-step polymerization preparation method of fluorinated ethylene propylene, and relates to the technical field of fluorine-containing polymers. According to the method adopted by the invention, ternary polymerization and ternary polymerization are divided into two steps, the first step is copolymerization of tetrafluoroethylene (TFE) and a perfluorovinyl ether monomer, the reaction can be carried out under lower pressure, the concentration of the TFE monomer is lower at the moment, and the polymerization reaction can be carried out according to a preset proportion only by a small amount of the perfluorovinyl ether monomer; the polymer emulsion with very small particle size is obtained. And 2, adding a certain amount of hexafluoropropylene (HFP) into the polymer emulsion, and continuously carrying out polymerization reaction to obtain the final fluorinated ethylene-propylene emulsion. A step-by-step polymerization process is adopted, so that the loss of a perfluorovinyl ether monomer in a fluorinated ethylene propylene ternary polymerization process is reduced, and the production cost is effectively reduced; the step-by-step polymerization can shorten the time of the second-step polymerization reaction and improve the production efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of fluorine-containing polymers, in particular to a method for preparing perfluoroethylene-propylene poly(ethylene propylene) by step polymerization. Background Art

[0002] FEP is a copolymer of tetrafluoroethylene and hexafluoropropylene, with the content of hexafluoropropylene being about 15%. It is a modified material of polytetrafluoroethylene. The full name is fluorinated ethylene propylene copolymer (perfluoroethylene propylene copolymer), referred to as polyperfluoroethylene propylene, also abbreviated as F46.

[0003] Patent document CN112358564A discloses a method for preparing polyperfluoroethylene propylene. With water as the medium, the organic peroxide of the structural formula (1) [RF-COO]2, wherein RF is F(CFCF3CF2O)nCFCF3 (n=1-8) is used as the initiator, and tetrafluoroethylene and hexafluoropropylene are prepared by precipitation polymerization to obtain polyperfluoroethylene propylene. In addition, the particle size and melt viscosity of the polymer are controllable, which can meet the needs of different aspects. The technical solution uses fluorine-containing organic peroxide as the initiator, which can reduce the unstable end groups in the polymer and improve the end group stability of the polymer; it is carried out in the form of precipitation polymerization, and no surfactant is added during the polymerization process, so the polymerization product is cleaner, and at the same time, the post-processing process is reduced, the energy consumption is reduced, and the cost is saved; water is used as the medium, which reduces environmental pollution and saves production costs; it can be carried out at a relatively low temperature and under medium and low pressure conditions, which reduces the risk, has operability, and has the prospect of large-scale industrialization.

[0004] Patent document CN102443092B discloses a method for preparing a polyperfluoroethylene propylene resin, which comprises the following steps: (1) adding deionized water to a reactor, leaving 30 to 50% of the space; (2) filling with nitrogen and boiling the deionized water; (3) adding an emulsifier to the reactor, stirring evenly, adding an initial mixed monomer of tetrafluoroethylene and hexafluoropropylene, and adding an initiator to react; (4) during the reaction, adding a mixed monomer of tetrafluoroethylene and hexafluoropropylene to maintain a constant pressure in the reactor; (5) after the reaction is completed, obtaining a polyperfluoroethylene propylene resin.

[0005] The existing fluorinated ethylene propylene (FEP) polymerization process usually uses water as the medium and persulfate as the initiator to carry out the free radical copolymerization of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) by emulsion polymerization.

[0006] This process is currently widely used, and there are relatively mature industrial products of tetrafluoroethylene (TFE), hexafluoropropylene (HFP) and perfluorovinyl ether (PAVE) terpolymers. And it is widely used. However, the usual terpolymerization process is carried out under the condition of the simultaneous presence of three monomers, which leads to the waste caused by excessive addition of expensive perfluorovinyl ether (PAVE) monomers. Summary of the invention

[0007] The present invention provides a method for preparing polyperfluoroethylene propylene by step-by-step polymerization, so as to overcome at least one technical problem in the above-mentioned background technology.

[0008] The present invention adopts a step-by-step polymerization method, effectively reducing the usage of perfluorovinyl ether in the ternary copolymerization FEP reaction, effectively reducing raw material consumption and reducing waste.

[0009] The method adopted by the present invention is to divide the ternary copolymerization into two steps. The first step is the copolymerization of tetrafluoroethylene (TFE) and perfluorovinyl ether monomers. This reaction can be carried out under a relatively low pressure. At this time, the concentration of TFE monomer is relatively low. Only a small amount of perfluorovinyl ether monomer is needed to carry out the polymerization reaction according to a predetermined ratio to obtain a polymer emulsion with a very small particle size. The second step is to add a certain amount of hexafluoropropylene (HFP) to the polymer emulsion, continue the polymerization reaction, and obtain the final polyperfluoroethylene propylene emulsion.

[0010] The specific plan is as follows: A method for preparing polyperfluoroethylene-propylene by step-by-step polymerization, the operating steps of which are: S1: Add 400-500 parts of pure water to the reactor according to weight parts, start stirring, control the speed to 30-120rpm, add 1-5 parts of surfactant, add 1-5 parts of perfluorovinyl ether monomer, adjust the temperature of the polymerization kettle to 60-100°C, add 15-25 parts of tetrafluoroethylene monomer, make the pressure in the reactor reach 1.2-2.3MPa, add 1-4 parts of initiator solution to the reactor to initiate copolymerization, and control the reaction pressure to 1.2-2.3MPa, maintain the reaction temperature and pressure stable, continue to add 2-8 parts of TFE monomer, and continuously add 2-6 parts of initiator at the same time; after the addition of TFE monomer is completed, recover the unreacted monomer to obtain an initial emulsion; S2: Add 400-500 parts of the initial emulsion into the reactor according to weight parts, start stirring, control the speed to 20-100 rpm, add 1-5 parts of surfactant, 20-40 parts of HFP monomer and 0.5-2 parts of co-emulsifier, and then add 4-8 parts of TFE monomer, adjust the temperature of the polymerization kettle to 80-120°C, add 20-50 parts of tetrafluoroethylene monomer and hexafluoropropylene monomer gas mixture, make the pressure in the reactor reach 2.5-5.0 MPa, add 1-9 parts of initiator solution into the reactor to initiate copolymerization reaction; S3: After the polymerization reaction starts, a gas mixture of tetrafluoroethylene and hexafluoropropylene is continuously added into the polymerization kettle, and the pressure of the reactor is maintained at 2.5-5.0 MPa. At the same time, an initiator solution is continuously added into the reactor at a constant rate of 3-10 parts / h, and the reaction temperature is controlled at 80-120°C. When the amount of mixed monomer added reaches 100-150 parts, the addition of monomers and initiators is stopped to terminate the copolymerization reaction. After the unreacted monomers in the polymerization kettle are recovered, a polyperfluoroethylene-propylene resin emulsion is obtained, which is milky white and opaque.

[0011] Wherein, the surfactant is one or more mixtures of ammonium perfluorooctanoate, ammonium perfluorooctane sulfonate, and perfluorooctane sulfonamide.

[0012] Wherein, the surfactant is an aqueous solution with a concentration of 10-60%.

[0013] Wherein, the perfluoroalkyl vinyl ether is perfluoromethyl vinyl ether or perfluoroethyl vinyl ether or perfluoro-n-propyl vinyl ether.

[0014] Wherein, the particle size of the initial emulsion is 20-50 nm, and the solid content of the initial emulsion is 1-8%.

[0015] Wherein, the concentration of the initiator solution is 2~5wt%.

[0016] Wherein, the initiator is selected from at least one of ammonium persulfate and potassium persulfate.

[0017] Wherein, the molar ratio of tetrafluoroethylene to hexafluoropropylene in the mixed monomer is 8-18:1.

[0018] Wherein, the preparation method of the co-emulsifier is: Step 1: Into a reactor, nitrogen is introduced, and 3-6 parts of 1,′-bis(dimethylsilyl)ferrocene (CAS: 1295-15-4) and 4-8 parts of hexafluorobutyl acrylate, 0.005-0.5 parts of a Castide catalyst, and 200-300 parts of DMF are stirred at 60-80° C. for reaction for 120-180 minutes; Step 2: Add 0.05-0.5 parts of NH2-MIL-101 (CR) metal organic framework, 10-18 parts of dioctadecylamine, and 2-5 parts of sodium ethoxide, and stir the mixture at 60-80° C. for 100-150 minutes; remove DMF by distillation to obtain an emulsifier.

[0019] Wherein, the Castide catalyst is platinum (0)-1,3-diethene-1,1,3,3-tetramethyldisiloxane (CAS: 68478-92-2).

[0020] Technical mechanism of this method: The hydrosilylation reaction between 1,′-bis(dimethylsilyl)ferrocene and hexafluorobutyl acrylate occurs primarily via a hydrosilylation reaction mechanism, in which the hydrogen on the silicon atom reacts with the carbon-carbon double bond in hexafluorobutyl acrylate.

[0021] The NH2-MIL-101(CR) metal organic framework undergoes an amino-ene addition reaction with the propene group, which is a typical Michael addition reaction. The amino group in the MOF acts as a nucleophile to attack the carbon-carbon double bond in the propene group to form a stable CN covalent bond.

[0022] Compared with traditional methods, this method has the following advantages: The amino-ene addition reaction of dioctadecylamine and propenyl also follows the Michael addition reaction mechanism. The long-chain alkylamine attacks the β-carbon atom of the propenyl to generate the corresponding β-amino derivative. The co-emulsifier greatly improves the problem of large amount of the third monomer in the original FEP ternary polymerization reaction by combining the characteristics of ferrocene, hexafluorobutyl ester, dioctadecyl and MOF particles. This co-emulsifier provides better interfacial activity and reduces interfacial tension during emulsion polymerization, making the monomer more stably dispersed in the aqueous phase and reducing the waste caused by excessive local concentration of the monomer. At the same time, the high specific surface area and porosity of MOF may provide more reaction sites for the monomer, thereby accelerating the polymerization reaction and reducing the reaction time and monomer consumption. These factors work together to effectively reduce the consumption of the third monomer by 30%, improving the polymerization efficiency and product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the solid nuclear magnetic spectrum of the polyperfluoroethylene propylene resin prepared in Example 1. DETAILED DESCRIPTION

[0024] The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0025] Example 1 A method for preparing polyperfluoroethylene-propylene by step-by-step polymerization, the operating steps of which are: S1: Add 400g of pure water into the reactor, start stirring, control the speed to 30rpm, add 5g of surfactant, add 1g of perfluorovinyl ether monomer, adjust the temperature of the polymerization kettle to 60°C, add 15g of tetrafluoroethylene monomer, make the pressure in the reactor reach 1.2MPa, add 1g of initiator solution into the reactor to initiate copolymerization, and control the reaction pressure to 1.2MPa, maintain the reaction temperature and pressure stable, continue to add 2g of TFE monomer, and continuously add 2g of initiator; after the addition of TFE monomer is completed, recover the unreacted monomer to obtain an initial emulsion; S2: Add 400 g of the initial emulsion into the reactor, start stirring, control the speed to 20 rpm, add 5 g of surfactant, 20 g of HFP monomer and 0.5 g of co-emulsifier, and then add 4 g of TFE monomer, adjust the temperature of the polymerization kettle to 80°C, add 20 g of a gas mixture of tetrafluoroethylene monomer and hexafluoropropylene monomer, make the pressure in the reactor reach 2.5 MPa, add 1 g of initiator solution into the reactor, and initiate the copolymerization reaction; S3: After the polymerization reaction starts, a gas mixture of tetrafluoroethylene and hexafluoropropylene is continuously added into the polymerization kettle, and the pressure of the reactor is maintained at 2.5 MPa. At the same time, the initiator solution is continuously added into the reactor at a constant rate of 3 g / h, and the reaction temperature is controlled at 80°C. When the amount of mixed monomer added reaches 100 g, the addition of monomers and initiators is stopped to terminate the copolymerization reaction. After the unreacted monomers in the polymerization kettle are recovered, a polyperfluoroethylene propylene resin emulsion is obtained, which is milky white and opaque.

[0026] The surfactant is ammonium perfluorooctanoate.

[0027] The surfactant is an aqueous solution with a concentration of 10%.

[0028] The perfluoroalkyl vinyl ether is perfluoromethyl vinyl ether.

[0029] The solid content of the initial emulsion is 2%.

[0030] The concentration of the initiator solution is 2wt%.

[0031] The initiator is selected from ammonium persulfate.

[0032] The molar ratio of tetrafluoroethylene to hexafluoropropylene in the mixed monomer is 8:1.

[0033] The preparation method of the auxiliary emulsifier is: Step 1: nitrogen was introduced into a reaction kettle, 3 g of 1,′-bis(dimethylsilyl)ferrocene (CAS: 1295-15-4), 4 g of hexafluorobutyl acrylate, 0.02 g of Castaide catalyst, and 200 g of DMF were stirred at 60° C. for 120 minutes; Step 2: Add 0.1 g NH2-MIL-101 (CR) metal organic framework, 10 g dioctadecylamine, and 2 g sodium ethoxide, and stir at 60° C. for 100 minutes; remove DMF by distillation to obtain an emulsifier.

[0034] The Castaide catalyst is platinum (0)-1,3-diethene-1,1,3,3-tetramethyldisiloxane (CAS: 68478-92-2).

[0035] Example 2 A method for preparing polyperfluoroethylene propylene by step-by-step polymerization, the operating steps of which are: S1: Add 450g of pure water into the reactor, start stirring, control the speed to 60rpm, add 4g of surfactant, add 2.5g of perfluorovinyl ether monomer, adjust the temperature of the polymerization kettle to 80°C, add 20g of tetrafluoroethylene monomer, make the pressure in the reactor reach 1.6MPa, add 2g of initiator solution into the reactor to initiate copolymerization, and control the reaction pressure to 1.6MPa, maintain the reaction temperature and pressure stable, continue to add 4g of TFE monomer, and continuously add 4g of initiator; after the addition of TFE monomer is completed, recover the unreacted monomer to obtain an initial emulsion; S2: Add 450 g of the initial emulsion into the reactor, start stirring, control the speed to 45 rpm, add 4 g of surfactant, 30 g of HFP monomer and 1 g of co-emulsifier, and then add 6 g of TFE monomer, adjust the temperature of the polymerization kettle to 100° C., add 35 g of a gas mixture of tetrafluoroethylene monomer and hexafluoropropylene monomer, make the pressure in the reactor reach 3.5 MPa, add 3 g of initiator solution into the reactor, and initiate the copolymerization reaction; S3: After the polymerization reaction starts, a gas mixture of tetrafluoroethylene and hexafluoropropylene is continuously added into the polymerization kettle, the pressure of the reactor is maintained at 3.5 MPa, and the initiator solution is continuously added into the reactor at a constant rate of 6 g / h, and the reaction temperature is controlled at 100°C. When the amount of mixed monomer added reaches 125 g, the addition of monomers and initiators is stopped to terminate the copolymerization reaction. After the unreacted monomers in the polymerization kettle are recovered, a polyperfluoroethylene propylene resin emulsion is obtained, which is milky white and opaque.

[0036] The surfactant is ammonium perfluorooctane sulfonate.

[0037] The surfactant is an aqueous solution with a concentration of 30%.

[0038] The perfluoroalkyl vinyl ether is perfluoromethyl vinyl ether.

[0039] The solid content of the initial emulsion is 3%.

[0040] The concentration of the initiator solution is 3.5wt%.

[0041] The initiator is selected from ammonium persulfate.

[0042] The molar ratio of tetrafluoroethylene to hexafluoropropylene in the mixed monomer is 12:1.

[0043] The preparation method of the auxiliary emulsifier is: Step 1: nitrogen was introduced into a reaction kettle, 4 g of 1,′-bis(dimethylsilyl)ferrocene (CAS: 1295-15-4), 6 g of hexafluorobutyl acrylate, 0.2 g of Castaide catalyst, and 250 g of DMF were stirred at 70° C. for 140 minutes; Step 2: Add 0.2 g NH2-MIL-101 (CR) metal organic framework, 12 g dioctadecylamine, and 3 g sodium ethoxide, and stir at 70° C. for 125 minutes; distill off DMF to obtain an emulsifier.

[0044] The Castaide catalyst is platinum (0)-1,3-diethene-1,1,3,3-tetramethyldisiloxane (CAS: 68478-92-2).

[0045] Example 3 A method for preparing polyperfluoroethylene-propylene by step-by-step polymerization, the operating steps of which are: S1: Add 450g of pure water into the reactor, start stirring, control the speed to 90rpm, add 2.5g of surfactant, add 4g of perfluorovinyl ether monomer, adjust the temperature of the polymerization kettle to 80°C, add 20g of tetrafluoroethylene monomer, make the pressure in the reactor reach 1.9MPa, add 3g of initiator solution into the reactor to initiate copolymerization, and control the reaction pressure to 1.9MPa, maintain the reaction temperature and pressure stable, continue to add 6g of TFE monomer, and continuously add 4g of initiator; after the addition of TFE monomer is completed, recover the unreacted monomer to obtain an initial emulsion; S2: Add 450 g of the initial emulsion into the reactor, start stirring, control the speed to 80 rpm, add 2.5 g of surfactant, 30 g of HFP monomer and 1.5 g of co-emulsifier, and then add 6 g of TFE monomer, adjust the temperature of the polymerization kettle to 100° C., add 40 g of a gas mixture of tetrafluoroethylene monomer and hexafluoropropylene monomer, make the pressure in the reactor reach 4.0 MPa, add 7 g of initiator solution into the reactor, and initiate copolymerization; S3: After the polymerization reaction starts, a gas mixture of tetrafluoroethylene and hexafluoropropylene is continuously added into the polymerization kettle, the pressure of the reactor is maintained at 4.0 MPa, and the initiator solution is continuously added into the reactor at a constant rate of 8 g / h, and the reaction temperature is controlled at 100°C. When the amount of mixed monomer added reaches 140 g, the addition of monomers and initiators is stopped to terminate the copolymerization reaction. After the unreacted monomers in the polymerization kettle are recovered, a polyperfluoroethylene propylene resin emulsion is obtained, which is milky white and opaque.

[0046] The surfactant is perfluorooctane sulfonamide.

[0047] The surfactant is an aqueous solution with a concentration of 45%.

[0048] The perfluoroalkyl vinyl ether is perfluoroethyl vinyl ether.

[0049] The solid content of the initial emulsion is 6%.

[0050] The concentration of the initiator solution is 4wt%.

[0051] The initiator is selected from potassium persulfate.

[0052] The molar ratio of tetrafluoroethylene to hexafluoropropylene in the mixed monomer is 16:1.

[0053] The preparation method of the auxiliary emulsifier is: Step 1: nitrogen was introduced into a reaction kettle, 5 g of 1,′-bis(dimethylsilyl)ferrocene (CAS: 1295-15-4), 6 g of hexafluorobutyl acrylate, 0.35 g of Castaide catalyst, and 250 g of DMF were stirred at 70° C. for 160 minutes; Step 2: Add 0.4 g NH2-MIL-101 (CR) metal organic framework, 16 g dioctadecylamine, and 4 g sodium ethoxide, and stir at 70° C. for 140 minutes; distill off DMF to obtain an emulsifier.

[0054] The Castaide catalyst is platinum (0)-1,3-diethene-1,1,3,3-tetramethyldisiloxane (CAS: 68478-92-2).

[0055] Example 4 A method for preparing polyperfluoroethylene-propylene by step-by-step polymerization, the operating steps of which are: S1: Add 500g of pure water into the reactor, start stirring, control the speed to 120rpm, add 1g of surfactant, add 5g of perfluorovinyl ether monomer, adjust the temperature of the polymerization kettle to 100°C, add 25g of tetrafluoroethylene monomer, make the pressure in the reactor reach 2.3MPa, add 4g of initiator solution into the reactor to initiate copolymerization, and control the reaction pressure to 2.3MPa, maintain the reaction temperature and pressure stable, continue to add 8g of TFE monomer, and continuously add 6g of initiator at the same time; after the addition of TFE monomer is completed, recover the unreacted monomer to obtain an initial emulsion; S2: Add 500 g of the initial emulsion into the reactor, start stirring, control the speed to 100 rpm, add 1 g of surfactant, 40 g of HFP monomer and 2 g of co-emulsifier, and then add 8 g of TFE monomer, adjust the temperature of the polymerization kettle to 120° C., add 50 g of a gas mixture of tetrafluoroethylene monomer and hexafluoropropylene monomer, make the pressure in the reactor reach 5.0 MPa, add 9 g of initiator solution into the reactor, and initiate copolymerization; S3: After the polymerization reaction starts, a gas mixture of tetrafluoroethylene and hexafluoropropylene is continuously added into the polymerization kettle, the pressure of the reactor is maintained at 5.0 MPa, and the initiator solution is continuously added into the reactor at a constant rate of 10 g / h, and the reaction temperature is controlled at 120°C. When the amount of mixed monomer added reaches 150 g, the addition of monomers and initiators is stopped to terminate the copolymerization reaction. After the unreacted monomers in the polymerization kettle are recovered, a polyperfluoroethylene propylene resin emulsion is obtained, which is milky white and opaque.

[0056] The surfactant is perfluorooctane sulfonamide.

[0057] The surfactant is an aqueous solution with a concentration of 60%.

[0058] The perfluoroalkyl vinyl ether is perfluoro-n-propyl vinyl ether.

[0059] The solid content of the initial emulsion is 8%.

[0060] The concentration of the initiator solution is 5wt%.

[0061] The initiator is selected from potassium persulfate.

[0062] The molar ratio of tetrafluoroethylene to hexafluoropropylene in the mixed monomer is 18:1.

[0063] The preparation method of the auxiliary emulsifier is: Step 1: nitrogen was introduced into a reaction kettle, 6 g of 1,′-bis(dimethylsilyl)ferrocene (CAS: 1295-15-4), 8 g of hexafluorobutyl acrylate, 0.5 g of Castaide catalyst, and 300 g of DMF were stirred at 80° C. for 180 minutes; Step 2: Add 0.5 g NH2-MIL-101 (CR) metal organic framework, 18 g dioctadecylamine, and 5 g sodium ethoxide, and stir at 80° C. for 150 minutes; remove DMF by distillation to obtain an emulsifier.

[0064] The Castaide catalyst is platinum (0)-1,3-diethene-1,1,3,3-tetramethyldisiloxane (CAS: 68478-92-2).

[0065] Comparative Example 1 As a comparative example of Example 1, this example is different from Example 1 only in that no auxiliary emulsifier is added.

[0066] Comparative Example 2 As a comparative example of Example 1, the only difference between this example and Example 1 is that 1,′-bis(dimethylsilyl)ferrocene is not added during the preparation of the co-emulsifier.

[0067] Comparative Example 3 As a comparative example of Example 1, the only difference between this example and Example 1 is that hexafluorobutyl acrylate is not added during the preparation of the co-emulsifier.

[0068] The test results of the above specific implementation plan are as follows: Evaluation indicators Product Weight Product solid content Resin product melt index Tensile Strength Elongation at break Evaluation Method Hydrometer Dry weighing Melt Index Tester Universal testing machine Universal testing machine Example 1 1.33g / ml 45%w / w 4.5g / 10min 33.8MPa 359% Example 2 1.38g / ml 50%w / w 7.5g / 10min 32.6MPa 351% Example 3 1.39g / ml 51%w / w 15.0g / 10min 30.4MPa 342% Example 4 1.42g / ml 55%w / w 19.0g / 10min 29.7MPa 348% Comparative Example 1 1.22g / ml 32%w / w 4.6g / 10min 30.2MPa 344% Comparative Example 2 1.29g / ml 39%w / w 4.2g / 10min 31.1MPa 351% Comparative Example 3 1.28g / ml 37%w / w 4.4g / 10min 30.8MPa 348% Although the present invention has been described in detail above by general description, specific implementation methods and experiments, it is obvious to those skilled in the art that some modifications or improvements can be made on the basis of the present invention. Therefore, these modifications or improvements made on the basis of not departing from the spirit of the present invention all belong to the scope of protection claimed by the present invention.

Claims

1. A method for preparing polyperfluoroethylene propylene by step-by-step polymerization, the operating steps of which are: S1: Add 400-500 parts of pure water to the reactor according to weight parts, start stirring, control the speed to 30-120rpm, add 1-5 parts of surfactant, add 1-5 parts of perfluorovinyl ether monomer, adjust the temperature of the polymerization kettle to 60-100°C, add 15-25 parts of tetrafluoroethylene monomer, make the pressure in the reactor reach 1.2-2.3MPa, add 1-4 parts of initiator solution to the reactor to initiate copolymerization, and control the reaction pressure to 1.2-2.3MPa, maintain the reaction temperature and pressure stable, continue to add 2-8 parts of TFE monomer, and continuously add 2-6 parts of initiator at the same time; after the addition of TFE monomer is completed, recover the unreacted monomer to obtain an initial emulsion; S2: Add 400-500 parts of the initial emulsion into the reactor according to weight parts, start stirring, control the speed to 20-100 rpm, add 1-5 parts of surfactant, 20-40 parts of HFP monomer and 0.5-2 parts of co-emulsifier, and then add 4-8 parts of TFE monomer, adjust the temperature of the polymerization kettle to 80-120°C, add 20-50 parts of tetrafluoroethylene monomer and hexafluoropropylene monomer gas mixture, make the pressure in the reactor reach 2.5-5.0 MPa, add 1-9 parts of initiator solution into the reactor to initiate copolymerization reaction; S3: After the polymerization reaction starts, a gas mixture of tetrafluoroethylene and hexafluoropropylene is continuously added into the polymerization kettle, and the pressure of the reactor is maintained at 2.5-5.0 MPa. At the same time, an initiator solution is continuously added into the reactor at a constant rate of 3-10 parts / h, and the reaction temperature is controlled at 80-120°C. When the amount of mixed monomer added reaches 100-150 parts, the addition of monomers and initiators is stopped to terminate the copolymerization reaction. After the unreacted monomers in the polymerization kettle are recovered, a polyperfluoroethylene-propylene resin emulsion is obtained, which is milky white and opaque.

2. The method for preparing perfluoroethylene propylene by step-by-step polymerization according to claim 1, characterized in that: The surfactant is one or more mixtures of ammonium perfluorooctanoate, ammonium perfluorooctane sulfonate and perfluorooctane sulfonamide.

3. The method for preparing perfluoroethylene propylene by step-by-step polymerization according to claim 1, characterized in that: The surfactant is an aqueous solution with a concentration of 10-60%.

4. The method for preparing perfluoroethylene propylene by step-by-step polymerization according to claim 1, characterized in that: The perfluoro vinyl ether monomer is perfluoro methyl vinyl ether, perfluoro ethyl vinyl ether or perfluoro n-propyl vinyl ether.

5. The method for preparing perfluoroethylene propylene by step-by-step polymerization according to claim 1, characterized in that: The particle size of the initial emulsion is 20-50 nm, and the solid content of the initial emulsion is 1-8%.

6. The method for preparing perfluoroethylene propylene by step-by-step polymerization according to claim 1, characterized in that: The concentration of the initiator solution is 2-5wt%.

7. The method for preparing perfluoroethylene-propylene by step-by-step polymerization according to claim 1, characterized in that: The initiator is selected from at least one of ammonium persulfate and potassium persulfate.

8. The method for preparing perfluoroethylene propylene by step-by-step polymerization according to claim 1, characterized in that: The molar ratio of tetrafluoroethylene to hexafluoropropylene in the mixed monomer is 8-18:

1.

9. The method for preparing perfluoroethylene-propylene by step-by-step polymerization according to claim 1, characterized in that: The preparation method of the auxiliary emulsifier is: Step 1: nitrogen is introduced into a reaction kettle, and 3-6 parts of 1,′-bis(dimethylsilyl)ferrocene and 4-8 parts of hexafluorobutyl acrylate, 0.005-0.5 parts of a Castide catalyst, and 200-300 parts of DMF are stirred and reacted at 60-80° C. for 120-180 minutes; Step 2: Add 0.05-0.5 parts of NH2-MIL-101 (CR) metal organic framework, 10-18 parts of dioctadecylamine, and 2-5 parts of sodium ethoxide, and stir the mixture at 60-80° C. for 100-150 minutes; remove DMF by distillation to obtain an emulsifier.

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