Preparation of FEP (fluorinated ethylene propylene) resin with low crystal point content

Abstract

The invention discloses preparation of FEP (fluorinated ethylene propylene) resin with low crystal point content, and relates to the technical field of fluorine-containing polymers. The preparation method comprises the following specific steps: mixing gas before feeding, controlling the proportion, adding deionized water and a surfactant into a reaction kettle, stirring at a high speed, then adding the mixed gas, keeping stirring, heating, adding a low-crystal-point accelerant, adding an initiator solution to initiate a copolymerization reaction, continuously supplementing the mixed gas and an initiator during the period, maintaining the pressure and the temperature, and continuously adding the initiator; and at the later stage of the reaction, reducing the stirring speed and continuously supplementing until the reaction is finished to finally obtain the copolymer emulsion. Compared with an original FEP water-phase reaction system, the method has the advantages that the content of crystal points in an FEP product is effectively reduced; the emulsion breaking phenomenon in the original FEP water phase polymerization process is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of fluoropolymer, in particular to the preparation of perfluoroethylene - propylene resin (FEP) with a low crystal point content. Background Art

[0002] Currently, the existing polymerization processes of perfluoroethylene - propylene (FEP) mainly include two methods: solvent - phase polymerization and emulsion polymerization (also known as aqueous - phase polymerization). Among them, emulsion polymerization has become the current mainstream production process due to advantages such as fast reaction rate and short production cycle. FEP emulsion polymerization is divided into two major types. One is the copolymerization of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP), also known as the binary copolymerization system. The other is the copolymerization system of three monomers: tetrafluoroethylene (TFE), hexafluoropropylene (HFP), and perfluoroalkyl vinyl ether (PAVE), also known as the ternary copolymerization system.

[0003] FEP emulsion polymerization usually uses water as the medium, adds an appropriate amount of fluorosurfactant, uses persulfate as the initiator, and initiates the free - radical polymerization of comonomers in the aqueous phase. Under the action of the stirring impeller, the monomers are inhaled from the gas phase into the liquid phase, and the chain - growth reaction occurs with the molecular chain with active free radicals as the center in the liquid phase until the chain - termination reaction ends.

[0004] In the FEP products obtained by the above emulsion polymerization method, due to the large difference in the reactivity ratios of TFE and HFP (r TFE = 15, r HFP = 0), TFE reacts better than HFP, and components with a high TFE content are easily formed in the product. The melting point and crystallization point of these high - TFE - content components are higher than those of normal FEP, and crystal points are easily formed during the molding process, resulting in an increase in the defective rate of FEP products.

[0005] CN110480982A discloses an extruded perfluoroethylene - propylene resin and its preparation method, which uses emulsion polymerization to prepare the extruded perfluoroethylene - propylene resin. This method for preparing extruded perfluoroethylene - propylene resin has problems such as a high content of unstable end - groups, strong acidity, and poor corrosion resistance.

[0006] The purpose of the present invention is to provide an improved method for copolymerizing tetrafluoroethylene and hexafluoropropylene to prepare FEP resin, so as to reduce the components with a high TFE content in the product, thereby reducing the crystal point content and improving the yield of FEP products. Summary of the Invention

[0007] The purpose of the present invention is to provide an improved method for preparing a copolymer of tetrafluoroethylene and hexafluoropropylene, which can improve the reaction rate of HFP in the initial stage of the polymerization reaction, reduce the proportion of components with a high TFE content in the obtained polymer, and thus reduce the crystal point content in the product.

[0008] The invention uses free radical initiators such as ammonium persulfate or potassium persulfate in a water phase system with a surfactant, and copolymerizes tetrafluoroethylene and hexafluoropropylene at a temperature of 90-115° C. and a pressure of 1.5-5.0 MPa.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A method for preparing a low-crystal content polyperfluoroethylene-propylene resin (FEP), comprising the following steps:

[0011] (1) Before feeding, tetrafluoroethylene and hexafluoropropylene gases are introduced and fully mixed, and the mixing ratio of tetrafluoroethylene and hexafluoropropylene is strictly controlled.

[0012] The high molar ratio of hexafluoropropylene ensures that the partial pressure of hexafluoropropylene is much higher than that of tetrafluoroethylene at the initial stage of polymerization. According to Henry's law, at a certain temperature, the higher the partial pressure of a gas, the higher the solubility of the gas in the solution. Therefore, the solubility of high partial pressure hexafluoropropylene in the water phase is higher than that of low partial pressure tetrafluoroethylene monomer, ensuring that the concentration of hexafluoropropylene in the water phase reaction field is much higher than that of tetrafluoroethylene during the subsequent low-pressure initiation, which is beneficial to the reaction of hexafluoropropylene.

[0013] (2) After adding deionized water into the reactor, high-speed stirring is started, and the relative stirring speed is 4.0-6.0. The high-speed stirring is maintained and a prescribed amount of surfactant is slowly added, and the amount added is controlled within the range of 0.1-1.0wt%.

[0014] High-speed stirring is beneficial to better dispersion of surfactants and formation of smaller droplets, which is beneficial to increase the number of reaction fields in the water phase and promote the copolymerization reaction.

[0015] (3) Add the tetrafluoroethylene and hexafluoropropylene mixed gas premixed in step 1, control the pressure in the reactor, maintain high-speed stirring for 5-15 minutes before initiating the polymerization reaction, heat the reactor, and add a low crystal point accelerator at the same time.

[0016] Before the polymerization reaction is initiated, high-speed stirring is maintained for a certain period of time, and the high-speed stirring time must be at least greater than 10 minutes. If the time is too short, the effect of uniform mixing and dispersion cannot be achieved, and if the time is too long, the production cycle will be affected.

[0017] The reaction mechanism of the low crystal point accelerator is:

[0018] Acylation reaction: Sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate reacts with 2-mercaptobenzoyl chloride to form an intermediate. The reaction proceeds via a nucleophilic substitution mechanism, in which di-tert-butylphenyl undergoes acylation of the acyl chloride to form an intermediate with a thiol group.

[0019] Thiol-ene addition: The intermediate obtained in the previous step is subjected to a thiol-ene addition reaction with allyl(cyclopentadienyl)nickel. This step involves the Michael addition of the thiol group to the olefin, forming a new carbon-sulfur bond, and simultaneously undergoing olefin polymerization or rearrangement under the action of a nickel catalyst.

[0020] Thiol-epoxy addition: Finally, the product of the previous step is subjected to a thiol-epoxy ring-opening addition reaction with sodium 2-(oxiran-2-yl)methanesulfonate. This is a typical nucleophilic ring-opening reaction, in which the thiolate acts as a nucleophile to attack the oxirane oxygen heterocycle of ethylene oxide, resulting in ring opening and the formation of new hydroxyl and thioether bonds.

[0021] The technical effects and principles of the above low-crystallization-point promoters

[0022] Reducing the crystallization-point content: The low-crystallization-point promoters containing nickelocene and sodium sulfonate can significantly reduce the crystallization-point content of perfluoroethylene-propylene copolymer (FEP). The principle lies in that these promoters introduce branched-chain structures during the polymerization process, disrupting the regularity of the polymer chains, thereby inhibiting the formation and growth of crystals. The introduction of this branched-chain structure is achieved through the above-mentioned thiol-ene addition and thiol-epoxy ring-opening addition reactions.

[0023] The role of the promoter: In these reactions, the nickelocene-containing compound participates in the reaction as a catalyst or ligand, while sodium sulfonate exists as a leaving group or stabilizer. They act together to promote the addition reaction between the thiol group and the olefin and epoxy group, thereby introducing the required branched-chain structure onto the polymer chain.

[0024] Technical advantages: Compared with traditional nucleating agents, this new type of low-crystallization-point promoter has higher efficiency and better performance. They can not only reduce the crystallization-point content of the polymer but also improve the transparency and other physical properties of the polymer. In addition, the synthesis method of these promoters is relatively simple, and the raw materials are easily available, having high industrial application value.

[0025] (4) Add the pre-prepared initiator solution, and initiate the copolymerization reaction under high-speed stirring. After the copolymerization reaction starts, continue to maintain high-speed stirring and slowly add the pre-mixed mixed gas in step one into the reaction kettle, controlling the pressure in the reaction kettle not to exceed 2.5 Mpa, and maintaining high-speed stirring for 5 - 15 minutes.

[0026] This step is a key step in controlling the formation of the number of reaction fields in the aqueous phase. The increase in the number of reaction fields is beneficial to obtaining an emulsion with a smaller primary particle size in the subsequent copolymerization reaction.

[0027] (5) Continuously add the pre-mixed mixed gas in step (1) into the reaction kettle, increase the pressure in the kettle, continuously supplement tetrafluoroethylene and initiator solution into the kettle to maintain a stable reaction pressure and temperature. After the addition amount of tetrafluoroethylene reaches 10% of the total reaction amount, reduce the relative stirring speed from 4.0 - 6.0 to the normal stirring speed of 1.0, and continuously supplement tetrafluoroethylene and initiator solution until the reaction ends. After the reaction ends, a copolymer emulsion of tetrafluoroethylene and hexafluoropropylene can be obtained.

[0028] In step (1), the molar ratio of tetrafluoroethylene to hexafluoropropylene in the mixing ratio is controlled at 20:80 - 5:95.

[0029] In step (2), the surfactant is selected from at least one of ammonium perfluorooctanoate and perfluorooctanoic acid.

[0030] In step (3), the pressure in the kettle is controlled at 1.5 - 2.5 Mpa.

[0031] In step (3), the temperature in the kettle is controlled at 90 - 115 °C.

[0032] In step (3), the addition amount of the low-crystallinity promoter is 2 - 6% of the total mass percentage of the monomers, and its preparation method is as follows:

[0033] Step 1: Acylation reaction: Add 51 - 102 parts of sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate, 500 - 800 parts of toluene, and 17 - 34 parts of 2-mercaptobenzoyl chloride into the reaction kettle, mix and stir, and raise the temperature to 80 - 90 °C for reaction for 2 - 5 hours;

[0034] Step 2: Addition reaction: Then add 0.02 - 0.2 parts of allyl(cyclopentadienyl)nickel, CAS: 12107 - 46 - 9, 0.05 - 0.5 parts of sodium 2-(2-methoxyethoxy)ethanesulfonate, and 2 - 5 parts of sodium ethoxide, react at 80 - 90 °C for 100 - 150 minutes, and distill off toluene to obtain the low-crystallinity promoter.

[0035] In step (4), the initiator solution is selected from ammonium persulfate or potassium persulfate, and is prepared into an aqueous solution of 1 - 10 wt%.

[0036] In step (5), the pressure in the reaction kettle is increased to 3.5 - 5.0 Mpa.

[0037] The method is applicable to the FEP binary (tetrafluoroethylene and hexafluoropropylene) copolymerization and ternary (tetrafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether) copolymerization systems. In the case of the ternary copolymerization system, only need to add perfluoroalkyl vinyl ether into the reaction kettle before adding the mixed gas of tetrafluoroethylene and hexafluoropropylene.

[0038] The FEP resin particles obtained by using the present invention have basically the same basic physical properties as the FEP products obtained by traditional methods. However, for the thin films prepared from the FEP resin particles of the present invention by the casting method, the number of crystal points (also known as fish eyes / Fisheye) is significantly reduced; the number of crystal points or protrusions on the insulating outer skin of the signal transmission cable prepared by the extrusion molding method is also greatly decreased, effectively increasing the yield rate of FEP products.

[0039] Compared with the prior art solutions, the present invention has the following beneficial effects:

[0040] 1. Compared with the original FEP aqueous reaction system, the present invention effectively reduces the content of crystal points in FEP products, and the reduction rate reaches more than 80%.

[0041] 2. The present invention effectively reduces the phenomenon of emulsion demulsification during the original FEP aqueous polymerization process, and the reduction rate is as high as more than 80%. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a flow chart of the present invention.

[0043] Figure 2 It is a comparison of the amounts of demulsified powder in each example of the specific implementation manner.

[0044] Figure 3 It is a graph showing the relationship between the relative stirring speed and the relative primary particle size in each example of the specific implementation manner.

[0045] Figure 4 It is a graph showing the relationship between the relative stirring speed and the reaction time in each example of the specific implementation manner. DETAILED DESCRIPTION OF THE INVENTION

[0046] The following examples are provided to better further understand the present invention, which is not limited to the described optimal implementation manner, and does not limit the content and protection scope of the present invention. Any product that is the same as or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior art features falls within the protection scope of the present invention.

[0047] COMPARATIVE EXAMPLE

[0048] Add 6500 g of high-purity water to a 10 L stainless steel reaction kettle, start stirring, with a relative stirring speed of 1.0, add 30 g of the surfactant ammonium perfluorooctanoate to the kettle; add 200 g of linear alkane (C n H 2n+2, a mixture with n = 16 - 30) as a co-solvent (the co-solvent is a straight-chain alkane, a branched-chain alkane, or a monocyclic cycloalkane; the straight-chain alkane has the molecular formula CnH2n+2, where n = 8 - 40; the branched-chain alkane has the molecular formula CnH2n+2, where n = 8 - 40; the monocyclic cycloalkane with a single ring has the molecular formula CnH2n, where n = 8 - 40;), and the reaction kettle is heated to 90°C, and a low-crystallization-point promoter accounting for 2% of the total mass percentage of the monomers is added.

[0049] Among them, the preparation method of the low-crystallization-point promoter is as follows:

[0050] Step 1: Acylation reaction: Add 51 g of sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate, 500 g of toluene, and 17 g of 2-mercaptobenzoyl chloride into the reaction kettle, mix and stir, and heat up to 80°C for reaction for 2 hours;

[0051] Step 2: Addition reaction: Then add 0.02 g of allyl(cyclopentadienyl)nickel, CAS: 12107-46-9, 0.05 g of 2-methanesulfonatoethyl oxirane, and 2 g of sodium ethoxide, react at 80°C for 100 minutes, distill off toluene, and obtain the low-crystallization-point promoter.

[0052] After maintaining the temperature for 30 minutes, add a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 25:75, molar ratio) until the pressure in the kettle reaches 3.5 Mpa. Add 200 g of initiator solution into the reaction kettle to initiate the polymerization reaction. When the pressure in the kettle drops to 3.3 Mpa, it is judged that the polymerization reaction has started. Subsequently, continuously introduce tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 75:25, molar ratio) into the kettle, and at the same time continuously supplement the initiator solution into the kettle at a feeding rate of 80 g / hr until the reaction ends. Maintain the pressure in the reaction kettle at 3.5 ± 0.1 Mpa and maintain the reaction temperature at 90 ± 0.5°C. When the total amount of the cumulative introduced mixed monomers reaches 2500 g, stop supplementing the mixed monomers and the initiator solution. Release the pressure of the reaction kettle to atmospheric pressure, and then purge with nitrogen to discharge the FEP emulsion product.

[0053] The above FEP emulsion is obtained as an FEP pellet product after demulsification, washing, drying, extrusion granulation, and end-group treatment.

[0054] The number of crystallization points of the pellet product is detected by the casting film detection method, and the number of crystallization points above 50 μm is recorded, with the unit of pieces / m 2 .

[0055] Example 1

[0056] In the pre-mixing tank, tetrafluoroethylene and hexafluoropropylene are fully mixed according to a molar ratio of 10:90.

[0057] Add 6500 g of high-purity water to a 10 L stainless steel reactor, start stirring, with a relative stirring speed of 5.0, and slowly add 30 g of the surfactant ammonium perfluorooctanoate into the reactor; add a straight-chain alkane (C8H 18 ) as a co-solvent into the reactor, and heat the reactor to 90 °C, then add a low-crystallization point promoter accounting for 2% of the total mass percentage of the monomers.

[0058] Among them, the preparation method of the low-crystallization point promoter is as follows:

[0059] Step 1: Acylation reaction: Add 51 g of sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate, 500 g of toluene, and 17 g of 2-mercaptobenzoyl chloride into the reactor, mix and stir, and heat up to 80 °C for reaction for 2 hours;

[0060] Step 2: Addition reaction: Then add 0.02 g of allyl(cyclopentadienyl)nickel, CAS: 12107-46-9, 0.05 g of 2-methanesulfonate of epoxyethane, and 2 g of sodium ethoxide, react at 80 °C for 100 minutes, distill off toluene, and obtain the low-crystallization point promoter.

[0061] Maintain high-speed stirring and constant temperature for 30 minutes, then add a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) until the pressure in the reactor reaches 2.0 Mpa. Continue to maintain high-speed stirring with a relative stirring speed of 5.0 for 10 minutes. Subsequently, add 200 g of an initiator solution into the reactor to initiate the polymerization reaction. When the pressure in the reactor drops to 1.8 Mpa, it is judged that the polymerization reaction has started. Subsequently, continuously introduce a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) into the reactor until the pressure is 3.5 ± 0.1 Mpa. Continue to maintain a relative stirring speed of 5.0, and simultaneously continuously supplement TFE monomer and the initiator solution into the reactor until the reaction ends. The feeding speed of the initiator solution is 80 g / hr, and the feeding speed of TFE must be able to maintain the pressure in the reactor at 3.5 ± 0.1 Mpa and maintain the reaction temperature at 90 ± 0.5 °C. When the total cumulative amount of tetrafluoroethylene monomer added reaches 250 g, reduce the stirring speed to the normal stirring speed (i.e., relative stirring speed 1.0); when the total cumulative amount of tetrafluoroethylene monomer added reaches 2500 g, stop adding the mixed monomer and the initiator solution. Release the pressure of the reactor to atmospheric pressure, then purge with nitrogen, and discharge the FEP emulsion product.

[0062] The above-mentioned FEP emulsion is obtained as an FEP pellet product after demulsification, washing, drying, extrusion granulation, and end-group treatment.

[0063] The pellet product is detected for the number of crystallization points by the casting film detection method, record the number of crystallization points above 50 μm, and the unit is pieces / m 2 .

[0064] Example 2

[0065] Tetrafluoroethylene and hexafluoropropylene are fully mixed in a premixing tank at a molar ratio of 10:90.

[0066] Add 6500 g of high-purity water to a 10 L stainless steel reactor, start stirring with a relative stirring speed of 4.0, slowly add 30 g of the surfactant perfluorooctanoic acid into the reactor; add 200 g of linear alkane (C 15 H 32 ) as a cosolvent, and heat the reactor to 90 °C, then add a low-crystallization point promoter accounting for 3% of the total mass percentage of the monomers.

[0067] Among them, the preparation method of the low-crystallization point promoter is as follows:

[0068] Step 1: Acylation reaction: Add 63 g of sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate, 600 g of toluene, and 22 g of 2-mercaptobenzoyl chloride into the reactor, mix and stir, and heat up to 80 °C for reaction for 3 hours;

[0069] Step 2: Addition reaction: Then add 0.06 g of allyl(cyclopentadienyl)nickel, CAS: 12107-46-9, 0.12 g of 2-(methanesulfonyloxy)ethyl oxirane, and 3 g of sodium ethoxide, react at 80 °C for 110 minutes, distill off toluene to obtain the low-crystallization point promoter.

[0070] Maintain high-speed stirring and constant temperature for 30 minutes, then add a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) until the pressure in the reactor reaches 2.0 Mpa. Continue to maintain high-speed stirring at a relative stirring speed of 4.0 for 10 minutes. Subsequently, add 200 g of initiator solution into the reactor to initiate the polymerization reaction. When the pressure in the reactor drops to 1.8 Mpa, it is judged that the polymerization reaction has started. Subsequently, continuously introduce a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) into the reactor until the pressure is 3.5 ± 0.1 Mpa. Continue to maintain the relative stirring speed at 4.0, and simultaneously continuously supplement TFE monomer and initiator solution into the reactor until the reaction ends. The supplementary speed of the initiator solution is 80 g / hr, and the supplementary speed of TFE must be able to maintain the pressure in the reactor at 3.5 ± 0.1 Mpa and maintain the reaction temperature at 90 ± 0.5 °C. When the total cumulative supplementary amount of tetrafluoroethylene monomer reaches 250 g, reduce the stirring speed to the normal stirring speed (i.e., relative stirring speed 1.0); when the total cumulative supplementary amount of tetrafluoroethylene monomer reaches 2500 g, stop supplementing the mixed monomer and initiator solution. Release the pressure of the reactor to atmospheric pressure, then purge with nitrogen and discharge the FEP emulsion product.

[0071] The above-mentioned FEP emulsion is subjected to demulsification, washing, drying, extrusion granulation, and end-group treatment to obtain an FEP pellet product.

[0072] The number of crystal points of the pellet product is detected by the cast film detection method, and the number of crystal points above 50 μm is recorded, with the unit being pieces / m. 2 。

[0073] Example 3

[0074] In a premixing tank, tetrafluoroethylene and hexafluoropropylene are fully mixed in a molar ratio of 10:90. 6500 g of high-purity water is added to a 10 L stainless steel reaction kettle, the stirring is started, and the relative stirring speed is 3.0. 30 g of the surfactant perfluorooctanoic acid is slowly added into the kettle; 200 g of a straight-chain alkane (C 20 H 42 ) is added as a cosolvent, and the reaction kettle is heated to 90 °C, and a low crystal point promoter accounting for 4% of the total mass percentage of the monomers is added.

[0075] Among them, the preparation method of the low crystal point promoter is as follows:

[0076] Step 1: Acylation reaction: 78 g of sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate, 650 g of toluene, and 26 g of 2-mercaptobenzoyl chloride are added to a reaction kettle, and the mixture is stirred and heated to 85 °C for reaction for 3 hours;

[0077] Step 2: Addition reaction: Then 0.1 g of allyl(cyclopentadienyl)nickel, CAS: 12107-46-9, 0.25 g of 2-(methanesulfonyloxy)ethyl oxirane, and 3 g of sodium ethoxide are added, and the reaction is carried out at 85 °C for 120 minutes. Toluene is distilled off to obtain the low crystal point promoter.

[0078] Maintain high-speed stirring and keep the temperature constant at 30 minutes, then add the mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) until the pressure in the autoclave reaches 2.0 Mpa. Continue to maintain high-speed stirring at a relative stirring speed of 3.0 for 10 minutes. Subsequently, add 200 g of initiator solution into the reaction autoclave to initiate the polymerization reaction. When the pressure in the autoclave drops to 1.8 Mpa, it is judged that the polymerization reaction has started. Subsequently, continuously introduce the mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) into the autoclave until the pressure is 3.5 ± 0.1 Mpa. Continue to maintain the relative stirring speed at 3.0, and simultaneously continuously supplement the TFE monomer and the initiator solution into the autoclave until the reaction ends. The feeding speed of the initiator solution is 80 g / hr, and the feeding speed of TFE must be able to maintain the pressure in the reaction autoclave at 3.5 ± 0.1 Mpa and maintain the reaction temperature at 90 ± 0.5 °C. When the total cumulative amount of tetrafluoroethylene monomer added reaches 250 g, reduce the stirring speed to the normal stirring speed (i.e., relative stirring speed 1.0); when the total cumulative amount of tetrafluoroethylene monomer added reaches 2500 g, stop supplementing the mixed monomer and the initiator solution. Release the pressure of the reaction autoclave to atmospheric pressure, then purge with nitrogen and discharge the FEP emulsion product.

[0079] The above FEP emulsion is obtained by demulsification, washing, drying, extrusion granulation, and end group treatment to obtain the FEP pellet product.

[0080] The number of crystal points of the pellet product is detected by the cast film detection method, and the number of crystal points above 50 μm is recorded, with the unit of pieces / m 2 。

[0081] Example 4

[0082] In the pre-mixing tank, tetrafluoroethylene and hexafluoropropylene are fully mixed according to the molar ratio of 10:90. Add 6500 g of high-purity water into a 10 L stainless steel reaction autoclave, start stirring, with a relative stirring speed of 1.0, slowly add 30 g of the surfactant ammonium perfluorooctanoate into the autoclave; add 200 g of linear alkane (C 30 H 62 ) as a co-solvent, and heat the reaction autoclave to 90 °C, and add a low crystal point promoter at 5% of the total monomer mass percentage.

[0083] Among them, the preparation method of the low crystal point promoter is as follows:

[0084] Step 1: Acylation reaction: Add 92 g of sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate, 700 g of toluene, and 30 g of 2-mercaptobenzoyl chloride into the reaction autoclave, mix and stir, heat up to 85 °C and react for 4 hours;

[0085] Step 2: Addition reaction: Then add 0.16 g of allyl(cyclopentadienyl)nickel, CAS: 12107-46-9, 0.4 g of 2-(methanesulfonyloxy)ethanol sodium salt and 4 g of sodium ethoxide, and react at 85 °C for 140 minutes. Distill off toluene to obtain a low-crystallization-point promoter.

[0086] After maintaining a constant temperature for 30 minutes, add a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) until the pressure in the autoclave reaches 2.0 Mpa. Continue stirring for 10 minutes, then add 200 g of initiator solution to the autoclave to initiate the polymerization reaction. When the pressure in the autoclave drops to 1.8 Mpa, it is judged that the polymerization reaction has started. Subsequently, continuously introduce a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) into the autoclave until the pressure is 3.5 ± 0.1 Mpa. Then continuously add TFE monomer and initiator solution to the autoclave until the reaction ends. The addition rate of the initiator solution is 80 g / hr, and the addition rate of TFE must be able to maintain the pressure in the autoclave at 3.5 ± 0.1 Mpa and maintain the reaction temperature at 90 ± 0.5 °C. When the total cumulative addition amount of tetrafluoroethylene monomer reaches 2500 g, stop adding the mixed monomer and initiator solution. Release the pressure in the autoclave to atmospheric pressure, then purge with nitrogen to discharge the FEP emulsion product.

[0087] The above FEP emulsion is subjected to demulsification, washing, drying, extrusion granulation, and end-group treatment to obtain an FEP pellet product.

[0088] The number of crystallization points of the pellet product is detected by the cast film detection method, and the number of crystallization points above 50 μm is recorded, with the unit of pieces / m. 2 。

[0089] Example 5

[0090] In the pre-mixing tank, tetrafluoroethylene and hexafluoropropylene are fully mixed according to a molar ratio of 10:90.

[0091] Add 6500 g of high-purity water to a 10 L stainless steel autoclave, start stirring, with a relative stirring speed of 5.0, and slowly add 30 g of the surfactant ammonium perfluorooctanoate to the autoclave; add 200 g of linear alkane (C 40 H 82 ), and heat the autoclave to 90 °C, then add a low-crystallization-point promoter accounting for 6% of the total monomer mass.

[0092] Among them, the preparation method of the low-crystallization-point promoter is as follows:

[0093] Step 1: Acylation reaction: Add 102 g of sodium 2,2'-methylenebis(4,6-di-tert-butylphenyl) phosphate, 800 g of toluene, and 34 g of 2-mercaptobenzoyl chloride into the reaction kettle, mix and stir, heat up to 90 °C and react for 5 hours;

[0094] Step 2: Addition reaction: Then add 0.2 g of allyl(cyclopentadienyl)nickel, CAS: 12107-46-9, 0.5 g of sodium 2-(methanesulfonyl)ethyl oxide, and 5 g of sodium ethoxide, react at 90 °C for 150 minutes, distill off toluene to obtain a low-crystallization-point promoter.

[0095] Maintain high-speed stirring and constant temperature for 30 minutes, then add 50 g of perfluoropropyl vinyl ether (PPVE), and then add a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) until the pressure in the kettle reaches 2.0 Mpa. Continue to maintain high-speed stirring at a relative stirring speed of 5.0 for 10 minutes. Then add 200 g of initiator solution into the reaction kettle to initiate the polymerization reaction. When the pressure in the kettle drops to 1.8 Mpa, it is judged that the polymerization reaction has started. Then continuously introduce a mixed monomer gas of tetrafluoroethylene and hexafluoropropylene (TFE:HFP = 10:90, molar ratio) into the kettle until the pressure is 3.5 ± 0.1 Mpa. Continue to maintain a relative stirring speed of 5.0, and simultaneously continuously supplement TFE monomer and initiator solution into the kettle until the reaction ends. The feeding speed of the initiator solution is 80 g / hr, and the feeding speed of TFE must be able to maintain the pressure in the reaction kettle at 3.5 ± 0.1 Mpa and the reaction temperature at 90 ± 0.5 °C. When the total cumulative amount of tetrafluoroethylene monomer added reaches 250 g, reduce the stirring speed to the normal stirring speed (i.e., relative stirring speed 1.0); when the total cumulative amount of tetrafluoroethylene monomer added reaches 2500 g, stop feeding the mixed monomer and initiator solution. Release the pressure of the reaction kettle to atmospheric pressure, then purge with nitrogen and discharge the FEP emulsion product.

[0096] The above FEP emulsion is obtained as an FEP pellet product after demulsification, washing, drying, extrusion granulation, and end-group treatment.

[0097] The pellet product is detected for the number of crystallization points by the casting film detection method, and the number of crystallization points above 50 μm is recorded, with the unit of number / m 2 。

[0098] Table: Evaluation data of intermediate products and products in the comparative examples and examples

[0099]

[0100]

[0101] Obviously, the above embodiments are merely examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A method for preparing a low-crystal content polyperfluoroethylene-propylene resin (FEP), comprising the following steps: (1) Before feeding, tetrafluoroethylene and hexafluoropropylene gases are introduced and fully mixed, and the mixing ratio of tetrafluoroethylene and hexafluoropropylene is strictly controlled; (2) After adding deionized water into the reactor, high-speed stirring is started, and the relative stirring speed is 4.0-6.

0. The high-speed stirring is maintained and a prescribed amount of surfactant is slowly added, and the amount added is controlled within the range of 0.1-1.0 wt %; (3) adding the tetrafluoroethylene and hexafluoropropylene mixed gas premixed in step 1, controlling the pressure in the reactor, maintaining high-speed stirring for 5-15 minutes before initiating the polymerization reaction, heating the reactor, and adding a low crystal point accelerator; (4) adding a pre-prepared initiator solution to initiate a copolymerization reaction under high-speed stirring. After the copolymerization reaction begins, continue to maintain high-speed stirring and slowly add the mixed gas pre-mixed in step 1 into the reactor, control the pressure in the reactor to be no higher than 2.5 MPa, and maintain high-speed stirring for 5-15 minutes; (5) Continue to add the mixed gas premixed in step 1 into the reactor to increase the pressure in the reactor, and continue to add tetrafluoroethylene and initiator solution into the reactor to maintain stable reaction pressure and temperature. After the amount of tetrafluoroethylene added reaches 10% of the total reaction amount, reduce the relative stirring speed from 4.0-6.0 to the normal stirring speed of 1.0, and continue to add tetrafluoroethylene and initiator solution until the reaction is completed. After the reaction is completed, a tetrafluoroethylene and hexafluoropropylene copolymer emulsion can be obtained.

2. Preparation of a perfluoroethylene propylene resin (FEP) with a low crystal point content according to claim 1, characterized in that: The molar ratio of tetrafluoroethylene to hexafluoropropylene in step (1) is controlled within the range of 20:80-5:

95.

3. Preparation of a perfluoroethylene-propylene resin (FEP) with a low crystal point content according to claim 1, characterized in that: The surfactant in step (2) is selected from at least one of ammonium perfluorooctanoate and perfluorooctanoic acid.

4. Preparation of a perfluoroethylene-propylene resin (FEP) with a low crystal point content according to claim 1, characterized in that: In step (3), the pressure in the kettle is controlled at 1.5-2.5 MPa.

5. Preparation of a perfluorinated ethylene propylene resin (FEP) with a low crystal point content according to claim 1, characterized in that: In step (3), the temperature in the kettle is controlled at 90-115°C.

6. The preparation of a perfluorinated ethylene propylene resin (FEP) with a low crystal point content according to claim 1, characterized in that: The low crystal point accelerator added in step (3) is 2-6% of the total weight percentage of the monomers, and its preparation method is: Step 1: Acylation reaction: add 51-102 parts of 2,2'-methylenebis(4,6-di-tert-butylphenyl) sodium phosphate, 500-800 parts of toluene, and 17-34 parts of 2-mercaptobenzoyl chloride into a reaction kettle, stir and heat to 80-90°C for 2-5 hours; Step 2: Addition reaction: add 0.02-0.2 parts of allyl(cyclopentadienyl)nickel, 0.05-0.5 parts of sodium oxirane-2-ylmethanesulfonate and 2-5 parts of sodium ethoxide, react at 80-90°C for 100-150 minutes, remove toluene by distillation, and obtain a low crystal point promoter.

7. Preparation of a perfluoroethylene-propylene resin (FEP) with a low crystal point content according to claim 1, characterized in that: The initiator solution in step (4) is selected from ammonium persulfate or potassium persulfate and is prepared into a 1-10 wt % aqueous solution.

8. Preparation of a perfluoroethylene-propylene resin (FEP) with a low crystal point content according to claim 1, characterized in that: In step (5), the pressure of the reactor is increased to 3.5-5.0 MPa.

9. Preparation of a perfluoroethylene-propylene resin (FEP) with a low crystal point content according to claims 1-8, characterized in that: The method is applicable to the FEP binary copolymerization system (tetrafluoroethylene and hexafluoropropylene) and the ternary copolymerization system (tetrafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether). For the ternary copolymerization system, it is only necessary to add the perfluoroalkyl vinyl ether into the reaction kettle before adding the mixed gas of tetrafluoroethylene and hexafluoropropylene.

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