Preparation method of fluorine-containing polymer

Abstract

The invention discloses a preparation method of a fluorine-containing polymer, the preparation method comprises a step of preparing the fluorine-containing polymer by performing a water-based emulsion polymerization reaction on at least one fluorine-containing monomer, the water-based emulsion polymerization reaction is performed in the presence of an emulsifier, and the emulsifier is a compound as shown in a formula (1). The emulsifier is low in chain transfer activity, and the preparation method of the fluorine-containing polymer is short in induction period and high in polymerization rate.

Description

Technical Field

[0001] The present invention relates to the field of polymers, and specifically to a method for preparing a fluoropolymer. Background Art

[0002] Among the current synthesis methods of fluoropolymers, emulsion polymerization is one of the most commonly used polymerization methods. Among them, the emulsifier is one of the most core raw materials. So far, the two types of fluorinated emulsifiers led by PFOA and GenX are the most used emulsifiers in the polymerization of fluorinated monomers. Their advantages mainly lie in: special surface activity and excellent chemical stability. However, these advantages are also the direct reasons for the extremely long degradation half-life and high toxicity of fluorinated emulsifiers. This results in not only high waste treatment costs in the production and use of fluoropolymers, but also inevitable emissions to the natural environment to a certain extent. To solve the above problems, it is an urgent and important issue to develop an emulsifier that is easy to degrade, low-toxic or even non-toxic and applicable to the polymerization of fluorinated monomers.

[0003] Currently, regarding the toxicity problem of fluorinated emulsifiers, the prior art discloses the substitution with non-fluorinated emulsifiers. The design ideas of non-fluorinated emulsifiers are as follows:

[0004] 1) Design into a hydrophilic monomer that can copolymerize with fluorinated monomers, and the oligomer obtained by copolymerization is used as an emulsifier. Patent documents CN104292374A, CN104292378A, US2847404A, and US5859123A adopt this design idea. Although the toxicity problem is solved to a certain extent, there will be hydrophilic groups in the resulting fluoropolymer that cannot be removed, resulting in a decline in product performance and limited application scope. In addition, the polymer solids content prepared by this design idea is generally relatively low.

[0005] 2) Design into a block-type amphiphilic polymer surfactant. Patent documents CN1535989A and CN111148772A adopt this design idea. This design idea brings another technical problem. The mobility of this block-type amphiphilic polymer emulsifier on the surface of fluororesin and at the gas-liquid interface is poor, resulting in a decrease in the mass transfer rate between interfaces.

[0006] In addition, the current non-fluorinated emulsifiers also have problems such as too high chain transfer activity and too long induction period. Summary of the Invention

[0007] To solve the above-mentioned still-existing technical problems, the present invention discovers that the content of primary carbon-carbon hydrogen bonds in the emulsifier reaches 70% or more, and the remaining 30% or less of the carbon-hydrogen bonds are secondary carbon-carbon hydrogen bonds covered by high steric hindrance groups, which greatly reduces the chain transfer activity. In addition, the emulsifier has a relatively low molecular weight and a relatively thin hydration layer of ionic groups, which shortens the polymerization induction period and accelerates the polymerization rate during the aqueous emulsion polymerization reaction.

[0008] The technical solution of the present invention is as follows:

[0009] The present invention provides a method for preparing a fluoropolymer, the preparation method including the step of subjecting at least one fluoromonomer to an aqueous emulsion polymerization reaction to obtain the fluoropolymer, the aqueous emulsion polymerization reaction being carried out in the presence of an emulsifier, and the emulsifier being a compound represented by formula (1),

[0010]

[0011] wherein R 1 is selected from phenyl, methyl, tert-butyl, 2-phenyl-2-propyl, and R 2 is selected from lithium, sodium, potassium, ammonium.

[0012] Further, the R 1 is selected from phenyl, tert-butyl, and R 2 is selected from sodium, potassium, ammonium.

[0013] In the emulsifier of the present invention, the molar content of the hydrophobic monomer structural unit is x, and the molar content of the hydrophilic monomer structural unit is y, and y / x = 1 to 7. Preferably, y / x = 3 to 5, and more preferably, y / x = 3.5 to 4.5.

[0014] When the value of y / x is too low, the ionic groups are not sufficient to provide enough hydrophilicity, resulting in poor water solubility of the overall emulsifier. When the value of y / x is too high, the number of hydrophobic groups is too low, resulting in a decrease in the adsorption ability at the interface of the emulsifier molecule, insufficient surface activity of the aqueous solution, and a decrease in the emulsion stability.

[0015] In the emulsifier of the present invention, the sum of the molar content of the hydrophobic monomer structural unit and the molar content of the hydrophilic monomer structural unit is 0.9 to 1, preferably 0.95 to 1, and more preferably 1.

[0016] The small-molecule fluorinated emulsifier forms thermodynamically stable particles by virtue of its extremely low surface tension. The emulsifier of the present invention can introduce charges on the surface of the polymer by relying on the ionic groups, and prevent the polymer particles from aggregating through the repulsive force between the charges.

[0017] The lower limit value of the surface tension of an aqueous solution containing 1 wt% of the emulsifier of the present invention is 36 mN / m.

[0018] The HLB value of the surfactant described in the present invention is 5 to 12; preferably 5 to 8.

[0019] The number-average molecular weight of the emulsifier described in the present invention is 3000 to 20000. Preferably, the number-average molecular weight is 4000 to 12000. When the number-average molecular weight is lower than 3000, the number of hydrophobic groups in the molecular chain is relatively low, resulting in a decrease in the adsorption strength of the emulsifier. When the number-average molecular weight is higher than 20000, the mobility of the molecular segments decreases, leading to a reduction in surface properties.

[0020] From the perspective of the molecular structure of the emulsifier, the emulsifier described in the present invention is a multi-block copolymer with more than 5 blocks of hydrophobic monomers and hydrophilic monomers. The block length of the hydrophobic monomer structural unit or the hydrophilic monomer structural unit in a single block is 1 to 10. This enables the hydrophobic monomers and hydrophilic monomers to be dispersedly distributed on the main chain of the emulsifier, resulting in better surface activity performance.

[0021] The addition amount of the emulsifier described in the present invention in the aqueous emulsion polymerization reaction is 0.001 to 5 wt% of the amount of the fluoropolymer produced, preferably 0.01 to 3 wt%. Increasing the amount of the emulsifier can reduce the particle size of the emulsion. When the addition amount of the emulsifier is 0.3 to 3 wt% of the amount of the fluoropolymer produced, the particle size of the emulsion can be less than 100 nm. Compared with the current PFOA-based emulsifiers, under the same performance conditions, the usage amount is reduced to 1 / 5 of theirs. Compared with the mainstream non-fluorinated emulsifiers, under the same usage amount, the particle size can be reduced by 40%, and the induction period is reduced by more than 80%. The chain transfer activity of the emulsifier described in the present invention is very low, and an increase in the usage amount will not result in a long induction period or a slow polymerization rate.

[0022] As an embodiment, the aqueous emulsion polymerization reaction described in the present invention is a polymerization reaction of fluorinated monomers. The fluorinated monomers are selected from at least one of vinyl fluoride, vinylidene fluoride, trifluoroethylene, tetrafluoroethylene, hexafluoropropylene, tetrafluoropropylene, trichlorofluoroethylene, 1,1-chlorofluoroethylene, 1,2-chlorofluoroethylene, perfluoroalkyl ethylene, perfluoroalkyl vinyl ether, perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, and perfluoro(2,2-dimethyl-1,3-dioxolene); preferably, the fluorinated monomers are selected from at least one of vinyl fluoride, vinylidene fluoride, trifluoroethylene, hexafluoropropylene, trichlorofluoroethylene, 1,1-chlorofluoroethylene, perfluoropropyl vinyl ether, perfluoromethyl vinyl ether, and perfluoro(2,2-dimethyl-1,3-dioxolene). The above perfluoroalkyl vinyl ethers can be selected from perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoropropyl vinyl ether, and perfluorobutyl vinyl ether.

[0023] As another embodiment, the aqueous emulsion polymerization reaction of the present invention is a polymerization reaction of fluorinated monomers and non-fluorinated monomers. The non-fluorinated monomers are selected from at least one of ethylene, acrylate, methyl acrylate, methacrylate, methyl methacrylate, vinyl ether, vinyl acetate, acrylonitrile, butadiene, isoprene, styrene, maleic anhydride and itaconic acid. The non-fluorinated monomers account for 0 to 50 mol% of the total amount of polymerization monomers. The fluorinated monomers are selected from at least one of vinyl fluoride, vinylidene fluoride, trifluoroethylene, tetrafluoroethylene, hexafluoropropylene, tetrafluoropropylene, chlorotrifluoroethylene, 1,1-difluoroethylene, 1,2-difluoroethylene, perfluoroalkyl ethylene, perfluoroalkyl vinyl ether, perfluoropropyl vinyl ether, perfluoromethyl vinyl ether and perfluoro(2,2-dimethyl-1,3-dioxolene); preferably, the fluorinated monomers are selected from at least one of vinyl fluoride, vinylidene fluoride, trifluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, 1,1-difluoroethylene, perfluoropropyl vinyl ether, perfluoromethyl vinyl ether and perfluoro(2,2-dimethyl-1,3-dioxolene). The above perfluoroalkyl vinyl ethers can be selected from perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoropropyl vinyl ether and perfluorobutyl vinyl ether.

[0024] The fluoropolymer of the present invention is a polymer containing fluorine atoms. Preferably, the fluoropolymer is selected from one of polyvinylidene fluoride (PVDF), polyvinyl fluoride, polytrifluoroethylene, polychlorotrifluoroethylene, polytetrafluoroethylene, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene copolymer, vinylidene fluoride-trifluoroethylene-vinylidene fluoride copolymer, and ethylene-chlorotrifluoroethylene copolymer.

[0025] The aqueous emulsion polymerization reaction of the present invention can be initiated by a water-soluble initiator or an oil-soluble initiator.

[0026] As one embodiment, the aqueous emulsion polymerization reaction is initiated by a water-soluble initiator. The water-soluble initiator includes, but is not limited to, persulfates, ammonium persulfate, potassium persulfate, sodium persulfate, hydrogen peroxide, cumene hydroperoxide, tert-butyl hydroperoxide, etc.

[0027] When a water-soluble initiator is used to initiate the aqueous emulsion polymerization reaction, the average particle size of the primary particles of the prepared fluoropolymer is < 120 nm. Preferably, the average particle size of the primary particles is 60 to 100 nm.

[0028] As another embodiment, the aqueous emulsion polymerization reaction is initiated by an oil-soluble initiator, and the oil-soluble initiator includes, but is not limited to, peroxycarboxylic acid compounds such as benzoyl peroxide and dilauroyl peroxide, peroxycarbonate compounds such as diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, and tert-amyl peroxybenzoate, peroxyester compounds such as tert-butyl peroxyacetate and tert-butyl peroxytrimethylacetate, and azo compounds such as 4,4'-azobis(4-cyanovaleric acid) and azobisisobutyronitrile.

[0029] When an oil-soluble initiator is used to initiate the aqueous emulsion polymerization reaction, the yellowing resistance of the prepared fluoropolymer is enhanced.

[0030] When an oil-soluble initiator is used to initiate the aqueous emulsion polymerization reaction, the average particle size of the primary particles of the prepared fluoropolymer is <120 nm, and preferably, the average particle size of the primary particles is 60 - 100 nm.

[0031] The induction period of the aqueous emulsion polymerization reaction of the present invention, that is, the time from the addition of the initiator to the start of the pressure drop, is 0 - 10 min.

[0032] For the aqueous emulsion polymerization reaction of the present invention, the polymerization temperature is generally 5 - 130 °C, and the polymerization pressure is 0.05 - 10 MPa. Preferably, the polymerization temperature is 60 - 100 °C, and the polymerization pressure is 1 - 6 MPa. The polymerization temperature and polymerization pressure are appropriately determined according to the type of fluoromonomer used, the molecular weight of the target fluoropolymer, and the reaction rate.

[0033] The emulsifier of the present invention can be used together with initiators, chain transfer agents, and detergents commonly used in the polymerization field. It will not decompose under the polymerization temperature and polymerization pressure of the aqueous emulsion polymerization.

[0034] The preparation method of the emulsifier of the present invention includes the step of preparing an emulsifier by polymerizing at least one hydrophobic monomer having a structure as shown in the formula CH2=C(CH3)OOR 1 and at least one hydrophilic monomer having a structure as shown in the formula CH2=C(CH3)OOR 2 in a solvent containing an initiator, where R 1 is selected from phenyl, methyl, tert-butyl, 2-phenyl-2-propyl, and R 2 is selected from hydrogen, lithium, sodium, potassium, and ammonium. Further, R 2 is selected from hydrogen, and the raw material cost is low.

[0035] As one embodiment, when R 2 is hydrogen, after the polymerization reaction is completed, an alkali or an alkali solution is added to neutralize to a pH of 6 - 8 to obtain the emulsifier of the present invention.

[0036] The base of the present invention includes but is not limited to sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide and the like.

[0037] The initiator is not particularly limited, as long as it can achieve monomer polymerization. Preferably, the initiator is selected from at least one of peroxides and azo compounds, and the peroxide may include persulfates, such as ammonium persulfate, potassium persulfate, sodium persulfate, etc., and may also include organic peroxides, such as alkyl, dialkyl or diacyl peroxides, such as di-tert-butyl peroxide or benzoyl peroxide, peroxyesters such as tert-amyl peroxytrimethylacetate, succinic acid peroxide or tert-butyl peroxytrimethylacetate, or peroxydicarbonates such as di-n-propyl peroxydicarbonate or diisopropyl peroxydicarbonate. The azo compound may be selected from dimethyl azobisisobutyrate, azobisisobutyronitrile, 2,2'-azobis-(2,4-dimethyl-4-methoxyvaleronitrile).

[0038] The polymerization reaction temperature is 50-100° C., and the reaction time is 5-25 hours.

[0039] The conversion rate of the hydrophobic monomer and the hydrophilic monomer is above 99%. Preferably, the conversion rate of the hydrophobic monomer and the hydrophilic monomer is close to 100%. More preferably, the conversion rate of the hydrophobic monomer and the hydrophilic monomer is 100%.

[0040] The emulsifier of the present invention is a random copolymer of a hydrophobic monomer and a hydrophilic monomer, and the reactivity ratio of the hydrophobic monomer and the hydrophilic monomer is 0.5 to 2.5, so that the hydrophobic monomer and the hydrophilic monomer are dispersed on the main chain of the emulsifier, and the surface activity performance is better.

[0041] The preparation method of the emulsifier can be selected from solution polymerization, precipitation polymerization and other preparation methods.

[0042] As an embodiment, when solution polymerization is adopted, the solvent is not particularly limited, as long as it can achieve monomer polymerization. Preferably, the solvent is selected from at least one of alcohol small molecule compounds, ester small molecule compounds, and ether small molecule compounds. The alcohol small molecule compound is selected from C1 to C 10 Alcohol compounds, such as isopropanol, tert-butyl alcohol, etc. Ester small molecule compounds are selected from C1~C 10 Ester compounds, such as ethyl acetate, diethyl carbonate, etc. Ether small molecule compounds are selected from C1~C 10 Ether compounds, such as diethyl ether.

[0043] When the solvent is selected from at least one of methanol, ethanol, n-propanol, isopropanol, tert-butanol, ethyl acetate, dimethyl carbonate, tetrahydrofuran, ethylene glycol dimethyl ether, and 1,3-dioxolane, the prepared emulsifier can be directly used without separation from the solvent. When used directly, the solvent therein can play a role in adjusting the molecular weight of the polymer, that is, the role of a chain transfer agent.

[0044] The post-treatment process of the solution polymerization includes neutralization and dilution steps. Further, it includes a solvent removal step.

[0045] As another embodiment, when precipitation polymerization is used, the solvent is not particularly limited as long as it can achieve monomer polymerization. Preferably, the solvent is selected from at least one of n-hexane, isohexane, methylcyclohexane, cyclohexane, petroleum ether, n-heptane, and isooctane.

[0046] The post-treatment process of the precipitation polymerization includes filtration and drying steps.

[0047] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0048] 1) The emulsifier used in the preparation method of the fluoropolymer of the present invention makes the induction period of the aqueous emulsion polymerization reaction short and the polymerization rate fast, comparable to that of PFOA-based emulsifiers.

[0049] 2) The emulsifier used in the preparation method of the fluoropolymer of the present invention has low chain transfer activity, avoiding the presence of inseparable hydrophilic groups in the fluoropolymer obtained by polymerization. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a schematic structural diagram of an emulsifier in the prior art. In the prior art, hydrophilic monomers are distributed in dots. 1 is a hydrophobic chain segment, and 2 is a hydrophilic chain segment.

[0051] Figure 2 It is a schematic structural diagram of the emulsifier of the present invention. 1 is a hydrophobic chain segment, and 2 is a hydrophilic chain segment.

[0052] Figure 3 It is a schematic diagram of the amphiphilic film-like structure formed by the emulsifier of the present invention adsorbed on the fluoropolymer interface. 1 is a hydrophobic chain segment, and 2 is a hydrophilic chain segment.

[0053] Figure 4 It is an SEM image of the fluoropolymer emulsion prepared in Example 4 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0054] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternative, improved, and equivalent solutions that may be included within the scope of the claims.

[0055] In the present invention, Mn refers to the number-average molecular weight of the polymer; PDI refers to the polymer dispersity index. The larger the PDI, the wider the molecular weight distribution; the smaller the PDI, the more uniform the molecular weight distribution.

[0056] PLURONIC 31R1: A bifunctional block copolymer emulsifier with terminal secondary hydroxyl groups, a non-ionic emulsifier, non-toxic, from BASF.

[0057] In the examples of the present invention, the calculation method of HLB adopts the calculation method of surfactants determined based on the mass fraction of groups in the book (Fundamentals of Surfactants, Colloids and Interfacial Chemistry, edited by Zheng-Gang Cui, Chemical Industry Press).

[0058] The specific calculation method of HLB is as follows:

[0059] Where: m(-COONa) is the mass of -COONa groups;

[0060] m[(-CH2-)+(-CH3)+(-C-)] is the total mass of methyl, methylene and sp3 hybridized quaternary carbon;

[0061] M is the mass of the surfactant.

[0062] Preparation of emulsifier

[0063] (1) Preparation of emulsifier A1 from tert-butyl methacrylate and methacrylic acid

[0064] Add tert-butyl methacrylate (5.0 g), methacrylic acid (5.0 g), and isopropanol (5.0 g) into a three-necked flask replaced with vacuum nitrogen, keep stirring and heat to 80 °C. After the temperature is constant, add dimethyl 2,2'-azobis(2-methylpropionate) (0.60 g), continue heating and stirring for 15 h, and the monomers and initiator are completely converted (conversion rate > 99%). Add pure water (85 g) and sodium hydroxide (2.3 g) and keep stirring until completely dissolved. Heat the solution to the boiling state until the solution boiling point reaches 100 °C. Cool and let stand to room temperature, and make up pure water to a total mass of 100 g to obtain a solution containing surfactant. The Mn of the emulsifier = 9570, PDI = 2.63. Dilute the obtained solution to 1%, and measure the surface tension by the droplet method to be 52 mN / m.

[0065] (2) Preparation of emulsifier A2 from tert-butyl methacrylate and methacrylic acid

[0066] Add tert-butyl methacrylate (2.0 g), methacrylic acid (8.0 g), and isopropanol (5.0 g) into a three-necked flask that has been replaced with vacuum nitrogen, keep stirring and heat to 80 °C. After the temperature is constant, add dimethyl 2,2'-azobis(2-methylpropionate) (0.60 g), and continue heating and stirring for 15 h. After that, the monomers and initiator are completely converted (conversion rate > 99%). Add pure water (85 g) and sodium hydroxide (3.7 g) and keep stirring until completely dissolved. Heat the solution to the boiling state until the boiling point of the solution reaches 100 °C. Cool and let it stand at room temperature, and add pure water to make the total mass 100 g to obtain a solution containing a surfactant. The Mn of the emulsifier is 9336, and the PDI is 2.74. Dilute the obtained solution to 1%, and measure the surface tension by the drop method to be 49 mN / m.

[0067] (3) Preparation of emulsifier A3 from tert-butyl methacrylate and methacrylic acid

[0068] Add tert-butyl methacrylate (3.0 g), methacrylic acid (7.0 g), and n-hexane (35.0 g) into a three-necked flask that has been replaced with vacuum nitrogen, keep stirring and heat to 80 °C. After the temperature is constant, add dimethyl 2,2'-azobis(2-methylpropionate) (0.60 g), and continue heating and stirring for 15 h. After that, the monomers and initiator are completely converted (conversion rate > 99%). Filter and dry the resin powder, and add the dry powder into a solution of pure water (85 g) and sodium hydroxide (3.3 g) and keep stirring until completely dissolved. Cool and let it stand at room temperature to obtain a solution containing an emulsifier. The Mn of the emulsifier is 17594, and the PDI is 1.88. Dilute the obtained solution to 1%, and measure the surface tension by the drop method to be 36 mN / m.

[0069] (4) Preparation of emulsifier A4 from tert-butyl methacrylate and methacrylic acid

[0070] Add tert-butyl methacrylate (3.0 g), methacrylic acid (7.0 g), and cyclohexane (35.0 g) into a three-necked flask that has been replaced with vacuum nitrogen, keep stirring and heat to 80 °C. After the temperature is constant, add dimethyl 2,2'-azobis(2-methylpropionate) (0.60 g), and continue heating and stirring for 15 h. After that, the monomers and initiator are completely converted (conversion rate > 99%). Filter and dry the resin powder, and add the dry powder into a solution of pure water (85 g) and sodium hydroxide (3.3 g) and keep stirring until completely dissolved. Cool and let it stand at room temperature to obtain a solution containing an emulsifier. The Mn of the emulsifier is 6504, and the PDI is 1.43. Dilute the obtained solution to 1%, and measure the surface tension by the drop method to be 42 mN / m.

[0071] The properties of emulsifiers A1 - A4 are shown in Table 1.

[0072] Table 1 Performance data of emulsifiers A1 - A4

[0073] x / y HLB value Mn PDI Surface tension A1 1.65 5.7 9570 2.63 52 A2 6.61 9.2 9336 2.74 49 A3 3.86 8.0 17594 1.88 36 A4 3.86 8.0 6504 1.43 42

[0074] Preparation of Fluoropolymer

[0075] Example 1

[0076] Prepare PVDF using emulsifier A1.

[0077] Add pure water (1800 g) into a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate the air with an oil-sealed vacuum pump for 5 minutes, and then backfill high-purity nitrogen to 0.15 MPa. Repeat this operation three times. After the last evacuation, add vinylidene fluoride (150 g) with a high-pressure gas cylinder, add emulsifier A1 (100 g, A1 content 0.4 wt%, isopropanol content 1 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature stabilizes for 5 minutes, add vinylidene fluoride (50 g) with a high-pressure gas cylinder until the pressure reaches 4.50 MPa, and add ammonium persulfate solution (100 g, 0.8 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle (80 ± 0.5 °C), and add vinylidene fluoride to maintain the pressure (4.25 ± 0.25 MPa) until the total feeding target of vinylidene fluoride is reached (600 g). Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2408 g, solid content 20.1 wt%). The total content of demulsifying materials is 0.0 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average particle size of the emulsion is 79 nm, its molecular weight is Mn = 745600, and PDI = 2.83.

[0078] Example 2

[0079] Prepare PVDF using emulsifier A2 as the emulsifier.

[0080] Add pure water (1800 g) to a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. This operation is repeated three times. After the last evacuation, add vinylidene fluoride (150 g) with a high-pressure gas cylinder, add emulsifier A2 (100 g, A2 content 0.4 wt%, isopropanol content 1 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature stabilizes for 5 minutes, add vinylidene fluoride (50 g) with a high-pressure gas cylinder until the pressure reaches 4.50 MPa, and add ammonium persulfate solution (100 g, 0.8 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5) °C, and add vinylidene fluoride to maintain the pressure at (4.25 ± 0.25) MPa until the total feed target of vinylidene fluoride (600 g) is reached. Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2384 g, solid content 20.9 wt%). The total demulsification material content is 0.0 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average emulsion particle size is 82 nm, with a molecular weight of Mn = 751800 and PDI = 2.79.

[0081] Example 3

[0082] Use emulsifier A3 as the emulsifier to prepare PVDF.

[0083] Add pure water (1800 g) to a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. This operation is repeated three times. After the last evacuation, add vinylidene fluoride (150 g) with a high-pressure gas cylinder, add emulsifier A3 (100 g, A3 content 0.4 wt%, isopropanol content 1 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature stabilizes for 5 minutes, add vinylidene fluoride (50 g) with a high-pressure gas cylinder until the pressure reaches 4.50 MPa, and add ammonium persulfate solution (100 g, 0.8 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5) °C, and add vinylidene fluoride to maintain the pressure at (4.25 ± 0.25) MPa until the total feed target of vinylidene fluoride (600 g) is reached. Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2433 g, solid content 19.9 wt%). The total demulsification material content is 0.0 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average emulsion particle size is 108 nm, with a molecular weight of Mn = 745500 and PDI = 2.74.

[0084] Example 4

[0085] Use emulsifier A4 to prepare PVDF.

[0086] Add pure water (1800 g) into a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate the air with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. This operation is repeated three times. After the last evacuation, add vinylidene fluoride (150 g) with a high-pressure gas cylinder, add emulsifier A4 (100 g, A4 content 0.4 wt%, isopropanol content 1 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature is stable for 5 minutes, add vinylidene fluoride (50 g) with a high-pressure gas cylinder until the pressure reaches 4.50 MPa, and add ammonium persulfate solution (100 g, 0.8 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5) °C, and add vinylidene fluoride to maintain the pressure at (4.25 ± 0.25) MPa until the total feeding target of vinylidene fluoride reaches 600 g. Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2398 g, solid content 21.1 wt%). The total content of demulsifying materials is 0.0 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average particle size of the emulsion is 99 nm, with a molecular weight of Mn = 732900 and PDI = 2.84.

[0087] Example 5

[0088] Prepare PVDF using emulsifier A4.

[0089] Add pure water (1800 g) into a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate the air with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. This operation is repeated three times. After the last evacuation, add vinylidene fluoride (150 g) with a high-pressure gas cylinder, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature is stable for 5 minutes, add vinylidene fluoride (50 g) with a high-pressure gas cylinder until the pressure reaches 4.50 MPa, and add the emulsified initiator (200 g, A4 content 0.5 wt%, isopropanol content 2.5 wt%, di-n-propyl peroxydicarbonate 2.5 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5) °C, and add vinylidene fluoride to maintain the pressure at (4.25 ± 0.25) MPa until the total feeding target of vinylidene fluoride reaches 600 g. Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2076 g, solid content 6.8 wt%). The total content of demulsifying materials is 0.8 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average particle size of the emulsion is 75 nm, with a molecular weight of Mn = 165600 and PDI = 2.33.

[0090] Example 6

[0091] Prepare PVF using emulsifier A4.

[0092] Add pure water (1400 g) to a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. This operation is repeated three times. After the last evacuation, add vinyl fluoride (80 g) with a high-pressure gas cylinder, add emulsifier A4 (100 g, A4 content 0.4 wt%, isopropanol content 1 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature stabilizes for 5 minutes, add vinyl fluoride (38 g) with a high-pressure gas cylinder until the pressure reaches 2.75 MPa, and add ammonium persulfate solution (100 g, 0.8 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5 °C), and add vinylidene fluoride to maintain the pressure at (2.75 ± 0.25 MPa) until the total feed target of vinylidene fluoride (500 g) is reached. Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (1926 g, solid content 19.1 wt%). The total demulsifying material content is 0.0 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average emulsion particle size is 102 nm.

[0093] Example 7

[0094] Prepare PVDF-HFP using emulsifier A4.

[0095] Add pure water (1800 g) to a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. This operation is repeated three times. After the last evacuation, add vinylidene fluoride (50 g) with a high-pressure gas cylinder, add emulsifier A4 (100 g, A4 content 0.4 wt%, isopropanol content 1 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature stabilizes for 5 minutes, add vinylidene fluoride (170 g) with a high-pressure gas cylinder until the pressure reaches 4.50 MPa, and add ammonium persulfate solution (100 g, 0.8 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5 °C), and add vinylidene fluoride to maintain the pressure at (4.25 ± 0.25 MPa) until the total feed target of vinylidene fluoride (600 g) is reached. Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2403 g, solid content 20.3 wt%). The total demulsifying material content is 0.01 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average emulsion particle size is 78 nm, and its molecular weight is Mn = 776900, PDI = 2.88.

[0096] Example 8

[0097] Prepare PTFE using emulsifier A4.

[0098] Add pure water (1800 g) into a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate the air with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. Repeat this operation three times. Add emulsifier A4 (100 g, A4 content 0.4 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature is stable for 5 minutes, add tetrafluoroethylene with a high-pressure gas cylinder until the pressure reaches 1.75 MPa, and add ammonium persulfate and sodium bicarbonate solution (100 g, 1.2 wt% ammonium persulfate, 1.2 wt% sodium bicarbonate) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5 °C), and add vinylidene fluoride to maintain the pressure at (1.75 ± 0.25 MPa). Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2084 g, solid content 5.5 wt%). The total content of demulsifying materials is 20 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average particle size of the emulsion is 108 nm.

[0099] Example 9

[0100] Prepare PCTFE using emulsifier A4.

[0101] Add pure water (1800 g) into a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate the air with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. Repeat this operation three times. After the last evacuation, add chlorotrifluoroethylene (604 g) with a high-pressure gas cylinder, add emulsifier A4 (100 g, A4 content 0.4 wt%, isopropanol content 1 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature is stable for 5 minutes, add vinylidene fluoride (50 g) with a high-pressure gas cylinder until the pressure reaches 2.85 MPa, and add ammonium persulfate solution (100 g, 0.8 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5 °C), and wait until the pressure drops to 2.30 MPa. Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2158 g, solid content 7.8 wt%). The total content of demulsifying materials is 0.0 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average particle size of the emulsion is 81 nm.

[0102] Comparative Example 1

[0103] Prepare PVDF using PFOA as an emulsifier.

[0104] Add pure water (1400 g) to a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. Repeat this operation three times. After the last evacuation, add vinylidene fluoride (180 g) with a high-pressure gas cylinder, add a PFOA dilution solution (150 g, surfactant content 0.9 wt%, ethyl acetate content 0.05 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature stabilizes for 5 minutes, add vinylidene fluoride monomer (74 g) to 4.50 MPa with a high-pressure gas cylinder, and add an ammonium persulfate solution (50 g, 1 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5 °C), and add vinylidene fluoride monomer to maintain the pressure at (4.25 ± 0.25 MPa) until the monomer reaches the total feeding target (600 g). Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2022 g, solid content 22.4 wt%). The total demulsifying material content is 0.10 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average emulsion particle size is 132 nm. The viscosity of its 7 wt% NMP solution is 2896 cp (shear rate is 2.325 s-1).

[0105] Comparative Example 2

[0106] Use a block copolymer as an emulsifier to prepare PVDF.

[0107] Add pure water (1400 g) to a 3.4 L polymerization kettle. After closing the kettle, continuously evacuate with an oil-sealed vacuum pump for 5 minutes, and then backfill with high-purity nitrogen to 0.15 MPa. Repeat this operation three times. After the last evacuation, add vinylidene fluoride (180 g) with a high-pressure gas cylinder, add a PLURONIC 31R1 dilution solution (150 g, surfactant content 0.6 wt%, ethyl acetate content 0.05 wt%) with a plunger pump, and start stirring (700 rpm) to heat the mixture to 80 °C. After the temperature stabilizes for 5 minutes, add vinylidene fluoride monomer (82 g) to 4.50 MPa with a high-pressure gas cylinder, and add an ammonium persulfate solution (50 g, 1 wt%) with a plunger pump to initiate the polymerization reaction. During the polymerization reaction, maintain the temperature in the kettle at (80 ± 0.5 °C), and add vinylidene fluoride monomer to maintain the pressure at (4.25 ± 0.25 MPa) until the monomer reaches the total feeding target (600 g). Stop stirring and open the pressure relief valve. After the pressure drops to atmospheric pressure, collect the emulsion (2002 g, solid content 23.2 wt%). The total demulsifying material content is 1.2 wt% calculated based on the polymer mass. The obtained polyvinylidene fluoride is observed by SEM, and the average emulsion particle size is 198 nm. The viscosity of its 7 wt% NMP solution is 1152 cp (shear rate is 2.325 s-1).

[0108] The performance tests of the fluoropolymers prepared in the examples and comparative examples are shown in Table 2.

[0109] Table 2 Performance data of the fluoropolymers prepared in the examples and comparative examples

[0110]

[0111] As can be seen from Table 2: When the usage amount of the emulsifier in the polymerization reaction of the fluoropolymer is 0.01 - 1 wt%, the emulsion particle size is 75 - 108 nm, the demulsification amount is low, and the induction period of the polymerization reaction is short.

[0112] Compared with Example 1, the solid content, demulsification amount and induction period of Comparative Example 1 are comparable, but the usage amount of PFOA in Comparative Example 1 is 3.6 times that of Example 1 (calculated based on the amount of polymer produced), and the emulsion particle size is larger.

[0113] Compared with Example 1, the solid content of Comparative Example 2 is comparable, but for PLURONIC 31R1 in Comparative Example 2, the demulsification amount increases, the induction period is 6 times that of Example 1, the usage amount is 2.3 times that of Example 1, and the emulsion particle size is much larger than that of Example 1.

Claims

1. A method for preparing a fluoropolymer, the preparation method comprising the step of subjecting at least one fluoromonomer to an aqueous emulsion polymerization reaction to obtain the fluoropolymer, the aqueous emulsion polymerization reaction being carried out in the presence of an emulsifier, characterized in that: The emulsifier is a compound represented by formula (1), wherein R 1 is selected from phenyl, methyl, tert-butyl, 2-phenyl-2-propyl, and R 2 is selected from lithium, sodium, potassium, and ammonium.

2. The method for preparing a fluoropolymer according to claim 1, characterized in that: x is the molar content of the hydrophobic monomer structural unit, and y is the molar content of the hydrophilic monomer structural unit, satisfying x + y = 1 and y / x = 1 - 7.

3. The method for preparing a fluoropolymer according to claim 2, characterized in that: y / x = 3 - 5.

4. The method for preparing a fluoropolymer according to claim 1, characterized in that: The lower limit value of the surface tension of an aqueous solution containing 1 wt% of the emulsifier is 36 mN / m.

5. The method for preparing a fluoropolymer according to claim 1, characterized in that: The HLB value of the surfactant is 5 - 12.

6. The method for preparing a fluoropolymer according to claim 1, characterized in that: The number-average molecular weight of the emulsifier is 3000 - 20000.

7. The method for preparing a fluoropolymer according to claim 1, characterized in that: The emulsifier is a multi-block copolymer with more than 5 blocks, and the block length of the hydrophobic monomer structural unit or the hydrophilic monomer structural unit in a single block is 1 - 10.

8. The method for preparing a fluoropolymer according to any one of claims 1-7, characterized in that: The addition amount of the emulsifier in the aqueous emulsion polymerization reaction is 0.001 - 5 wt% of the amount of the fluoropolymer produced.

9. The method for preparing a fluoropolymer according to claim 8, characterized in that: The addition amount of the emulsifier in the aqueous emulsion polymerization reaction is 0.01 - 3 wt% of the amount of the fluoropolymer produced.

10. The method for preparing a fluoropolymer according to claim 1, characterized in that: The fluorine-containing monomer is selected from at least one of vinyl fluoride, vinylidene fluoride, trifluoroethylene, tetrafluoroethylene, hexafluoropropylene, tetrafluoropropylene, chlorotrifluoroethylene, 1,1-chlorofluoroethylene, 1,2-chlorofluoroethylene, perfluoroalkyl ethylene, perfluoroalkyl vinyl ether, perfluoro-n-propyl vinyl ether, perfluoro-methyl vinyl ether, or perfluoro(2,2-dimethyl-1,3-dioxolene).

11. The method for preparing a fluoropolymer according to claim 10, characterized in that: The polymerization monomer further contains a non-fluorine-containing monomer, and the non-fluorine-containing monomer is selected from at least one of ethylene, acrylate, methyl acrylate, methacrylate, methyl methacrylate, vinyl ether, vinyl acetate, acrylonitrile, butadiene, isoprene, styrene, maleic anhydride, or itaconic acid. The non-fluorine-containing monomer accounts for 0 - 50 mol% of the total amount of the polymerization monomer.

12. The method for preparing a fluoropolymer according to claim 1, characterized in that: The fluoropolymer is selected from one of polyvinyl fluoride, polyvinylidene fluoride, polytrifluoroethylene, polychlorotrifluoroethylene, polytetrafluoroethylene, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene copolymer, vinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer, or ethylene-chlorotrifluoroethylene copolymer.

13. The method for preparing a fluoropolymer according to claim 1, characterized in that: The aqueous emulsion polymerization reaction is initiated by an initiator, and the initiator is an oil-soluble initiator.

14. The method for preparing a fluoropolymer according to claim 13, characterized in that: The oil-soluble initiator is selected from at least one of peroxycarboxylic acid compounds, peroxycarbonate compounds, peroxyester compounds, or azo compounds.

15. The method for preparing a fluoropolymer according to claim 13, characterized in that: The oil-soluble initiator is selected from at least one of benzoyl peroxide, dilauroyl peroxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, tert-amyl peroxybenzoate, tert-butyl peroxyacetate, tert-butyl peroxytrimethylacetate, 4,4'-azobis(4-cyanovaleric acid), or azobisisobutyronitrile.

16. The method for preparing a fluoropolymer according to claim 1, characterized in that: The average particle size of the primary particles of the fluoropolymer < 120 nm.

17. The method for preparing a fluoropolymer according to claim 16, characterized in that: The average particle size of the primary particles of the fluoropolymer is 60 - 100 nm.

18. The method for preparing a fluoropolymer according to claim 1, characterized in that: The induction period of the aqueous emulsion polymerization reaction is 0 - 10 min.

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