A method for preparing a polyvinylidene fluoride powder

Abstract

The present application relates to a kind of preparation methods of polyvinylidene fluoride powder, the preparation method includes the preparation step of large particle size polyvinylidene fluoride emulsion and washing step, the preparation step of large particle size polyvinylidene fluoride emulsion is: in the presence of ionic strength regulator, in the aqueous medium containing initiator, chain transfer agent and ionic emulsifier, polyvinylidene fluoride is homo-polymerized, or polyvinylidene fluoride and fluorine-containing comonomer is copolymerized, and polyvinylidene fluoride emulsion with average particle size ≥300nm is prepared;Large particle size polyvinylidene fluoride emulsion is washed, dried to prepare polyvinylidene fluoride powder.The polyvinylidene fluoride powder prepared in the present application, its impurity content is ≤700ppm, can effectively improve the processing thermal stability of polyvinylidene fluoride.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of polymers, in particular to a method for preparing polyvinylidene fluoride powder. BACKGROUND

[0002] In the production of polyvinylidene fluoride, the commonly used polymerization methods are emulsion polymerization and suspension polymerization, among which emulsion polymerization is the most efficient method for preparing polyvinylidene fluoride. In the emulsion polymerization process, emulsifiers such as perfluorooctanoic acid or its ammonium salt, perfluorosulfonic acid or its ammonium salt, polyfluoroalkoxy ether acid or its ammonium salt, and perfluoropolyether acid or its ammonium salt need to be added.

[0003] Although emulsifiers and other polymerization aids are essential in the production of polyvinylidene fluoride, the content of the aids in polyvinylidene fluoride is required to be as low as possible, preferably zero, because the aids have a negative impact on the heat resistance stability of polyvinylidene fluoride. In order to reduce the content of the aids in polyvinylidene fluoride, the existing technology often uses washing methods, but when the polymer latex particle size is very small and the interaction between the latex particles is very strong, the washing liquid has great resistance to enter the gap between the latex particles, resulting in ineffective removal of the polymerization aids.

[0004] In order to increase the particle size of the polymer latex, the existing technology often uses the following methods:

[0005] CN108047374A discloses a preparation method of a polytetrafluoroethylene / polyacrylate core-shell emulsion, the preparation method of the core-shell emulsion uses polytetrafluoroethylene pre-emulsion as a seed and a copolymer of acrylate monomers as a shell to prepare a core-shell structure composite modified emulsion, and the final latex particle size is between 254 and 323 nm, and the particle size polydispersity index (PDI) is less than 0.02.

[0006] CN112480437A discloses a preparation method of large-particle-size high-solid-content styrene-butadiene latex, the preparation method performs secondary emulsification on the styrene-butadiene rubber, controls the latex particle size by controlling parameters such as shear strength and emulsifier content in the emulsification process, and prepares a styrene-butadiene latex with a latex particle size greater than 500 nm.

[0007] CN110982017A discloses a preparation method of a self-extinction water-based acrylic-polyurethane core-shell emulsion, the preparation method controls the particle size of the water dispersion by changing the proportion of the hydrophilic group and the molar mass of the polymer chain, so that the polymerized latex has a relatively large particle size and a wide particle size distribution, but the specific latex particle size and distribution are not disclosed.

[0008] CN110330587A discloses a preparation method of a wide particle size distribution acrylic emulsion, which continuously returns a polymerized seed emulsion to a pre-emulsion tank and continuously drops pre-emulsified monomers into a reaction tank, adjusts the particle size of the seed emulsion, and obtains an acrylic emulsion with a particle size of 50 nm to 5 μm and a particle size distribution coefficient of 1 to 1.5.

[0009] CN101434674A discloses a fluorine-containing water-based resin prepared by a core-shell emulsion multi-copolymerization method, which uses trifluorochloroethylene or tetrafluoroethylene as a polymerization monomer, and methylcycloalkyl methacrylate, alkenyl fatty acid ester, unsaturated organosiloxane, and diol alkenyl ether as modification monomers, and is prepared by a two-step core-shell emulsion multi-copolymerization method to have an average particle size of 100-200 nm.

[0010] In summary, the synthesis of large particle size polymer latex mainly relies on agglomeration technology. Such a method is characterized in that a small particle size seed latex is first synthesized, and then a copolymerization monomer is added to prepare a core-shell structure composite modified latex with a copolymer as a shell. In addition, there are also physical or chemical methods to destroy the stability of the latex, so that the latex itself is coagulated to form particle clusters, thereby increasing the particle size. However, although such a method can achieve the preparation of large particle size latex, the stability of the latex is poor, which seriously limits the application of the latex in many fields. SUMMARY

[0011] The purpose of the present application is to provide a preparation method of polyvinylidene fluoride powder, which slowly adds an ionic strength regulator during polymerization to obtain a polyvinylidene fluoride emulsion with an average particle size of not less than 300 nm, and after water washing and drying, a polyvinylidene fluoride powder with a very low impurity residue is obtained.

[0012] The technical solution of the present application is as follows:

[0013] A preparation method of polyvinylidene fluoride powder, which comprises a large particle size polyvinylidene fluoride emulsion preparation step and a washing step, wherein the large particle size polyvinylidene fluoride emulsion preparation step is: in an aqueous medium containing an initiator, a chain transfer agent and an ionic emulsifier, in the presence of an ionic strength regulator, polyvinylidene fluoride is subjected to homopolymerization, or polyvinylidene fluoride and a fluorine-containing comonomer are subjected to copolymerization, to prepare a large particle size polyvinylidene fluoride emulsion with an average particle size of ≥300 nm.

[0014] The addition amount of the ionic strength regulator is 0.001 wt% to 0.1 wt% of the total amount of the polymerization monomers. As a preferred, the addition amount of the ionic strength regulator is 0.01 wt% to 0.05 wt% of the total amount of the polymerization monomers. When the addition amount of the ionic strength regulator is too much, the stability of the emulsion will be poor, and coagulum will be generated; when the addition amount of the ionic strength regulator is too little, the purpose of the present application cannot be achieved.

[0015] The ionic strength modifier of the present invention is selected from at least one of potassium trihydrogen oxalate, potassium hydrogen phthalate, dipotassium hydrogen phosphate, sodium acetate trihydrate, sodium tetraborate, sodium citrate, or trimethylolpropane; preferably, the ionic strength modifier is selected from at least one of potassium trihydrogen oxalate, dipotassium hydrogen phosphate, sodium acetate trihydrate, sodium tetraborate, or sodium citrate.

[0016] The initiator used in this invention is an initiator that can decompose in a short time. This initiator allows the nucleation process of latex particles to be completed in a short time, resulting in latex particles with basically the same particle size. Preferably, the initiator is selected from at least one of the following: dicyclohexyl peroxide, di-tert-butylcyclohexyl peroxide, phenoxyethyl peroxide, cyclooctyl peroxide, dioctyl peroxide, ditetradecyl peroxide, di(hexadecyl)dicarbonate, di-2-ethoxyethanol peroxide, or di-3-methoxybutyl peroxide.

[0017] The amount of initiator used in this invention is 0.03 wt% to 0.3 wt% of the total amount of polymeric monomers; preferably, the amount of initiator used is 0.05 wt% to 0.2 wt% of the total amount of polymeric monomers.

[0018] The chain transfer agent used in this invention is a commonly used chain transfer agent in the art. Preferably, the chain transfer agent is selected from at least one of isopropanol, ethyl acetate, diethyl malonate, diethyl carbonate, HCFC-21, HCFC-22, HCFC-123, HCFC-225 or HFC-4310.

[0019] The chain transfer agent of the present invention is used in an amount of 0.01 wt% to 2.0 wt% of the total amount of polymeric monomers; preferably, the chain transfer agent is used in an amount of 0.05 wt% to 1.0 wt% of the total amount of polymeric monomers.

[0020] The emulsifier used in this invention is an ionic emulsifier. Preferably, the ionic emulsifier is selected from perfluorooctanoic acid or its ammonium salt, perfluorooctanoic acid sulfonic acid or its ammonium salt, or CF3CF2CF2-O-(CF2CF(CF3)O). m (CF(CF3)O) n -O-CF2CF2COOH or its ammonium salt, m is 1 or 2, n is an integer from 0 to 4, CF3CF2CF2-O-(CF(CF3)CF2-O-) p CF(CF3)COOH or its ammonium salt, p being an integer from 1 to 4, CF2=CFCF2-O-CF2CF3CF2-O-CF2CF3COOH or its ammonium salt, CF3CF2CF2-O-(CF2CF2-O-) qCF2COOH or its ammonium salt, q is an integer from 0 to 3, CF3CF2(CF2OCFCF3) r COOH or its ammonium salt, r is an integer from 0 to 3, F3COOCF2(CF2OCFCF3) o COOH or its ammonium salt, o is an integer from 0 to 3, or CF3CF2((CFCF3) s CF2) t CF2COOH or its ammonium salt, s is 0 or 1, t is at least one of an integer from 0 to 3; more preferably, the ionic emulsifier is selected from CF3CF2CF2-O-(CF2CF(CF3)O) m (CF(CF3)O) n -O-CF2CF2COOH or its ammonium salt, m is 1 or 2, n is an integer from 0 to 4, CF3CF2CF2-O-(CF(CF3)CF2-O- p CF(CF3)COOH or its ammonium salt, p is an integer from 1 to 4, CF2=CFCF2-O-CF2CF3CF2-O-CF2CF3COOH or its ammonium salt, CF3CF2CF2-O-(CF2CF2-O- q CF2COOH or its ammonium salt, q is an integer from 0 to 3, CF3CF2(CF2OCFCF3) r COOH or its ammonium salt, r is an integer from 0 to 3, F3COOCF2(CF2OCFCF3) o COOH or its ammonium salt, o is an integer from 0 to 3, or CF3CF2((CFCF3) s CF2) t CF2COOH or its ammonium salt, s is 0 or 1, t is at least one of an integer from 0 to 3.

[0021] The amount of the ionic emulsifier used in the present application is 0.03wt% to 1.0wt% of the total amount of the polymerized monomers; preferably, the amount of the ionic emulsifier used is 0.06wt% to 0.6wt% of the total amount of the polymerized monomers.

[0022] The force between the polymerized latex particles prepared by the ionic emulsifier is mainly the electrostatic repulsion of the latex particle surface, and the ionic strength in the water phase medium has a very important influence on the particle size of the polymerized latex particles. When the amount of the emulsifier is large, the ionic strength in the reaction system is large, the electrostatic repulsion of the latex particle surface is large, and then the latex particle size is small; when the amount of the emulsifier is reduced, the ionic strength in the reaction system becomes small, the electrostatic repulsion of the latex particle surface becomes small, and the latex particle size becomes large, but the problem brought by this is that the stability of the reaction system is reduced, and the latex particles will coalesce.

[0023] In order to obtain large-diameter polymer latex particles without destroying the stability of the reaction system, the application adds an ionic strength regulator during polymerization without reducing the amount of emulsifier.

[0024] The polyvinylidene fluoride powder of the application can be prepared by homopolymerization of vinylidene fluoride or by copolymerization of vinylidene fluoride and a fluorine-containing comonomer. When obtained by copolymerization of vinylidene fluoride and a fluorine-containing comonomer, the fluorine-containing comonomer is selected from at least one of hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluorochloroethylene, trifluoroethylene, fluoroethylene or perfluoroalkyl vinyl ether; preferably, the fluorine-containing comonomer is selected from at least one of hexafluoropropylene, trifluorochloroethylene, trifluoroethylene, fluoroethylene or perfluoroalkyl vinyl ether; the mass ratio of the vinylidene fluoride and the fluorine-containing comonomer is 60 / 40 to 99.99 / 0.01; preferably, the mass ratio of the vinylidene fluoride and the fluorine-containing comonomer is 80 / 20 to 95 / 5.

[0025] The washing step of the application includes atmospheric pressure stirring washing and / or vacuum stirring washing. It can be atmospheric pressure stirring washing alone, vacuum stirring washing alone, or both atmospheric pressure stirring washing and vacuum stirring washing.

[0026] In order to reduce the impurity content of the polyvinylidene fluoride powder, it is preferable to alternately perform atmospheric pressure stirring washing and vacuum stirring washing. Specifically, the large-diameter polyvinylidene fluoride emulsion and water are placed in a reactor, and atmospheric pressure stirring washing and vacuum stirring washing are alternately performed. The time for a single atmospheric pressure stirring washing and a single vacuum stirring washing is 5 to 40 min, the washing temperature is 20 to 75℃, the conductivity of the discharged waste water after each washing is monitored, and when the conductivity is reduced to 1 to 2 μS / cm, the washing is stopped, and the polyvinylidene fluoride powder is obtained by drying.

[0027] The volume ratio of the large-diameter polyvinylidene fluoride emulsion to water of the application is 0.1:1 to 1:1.

[0028] The water of the application is pure water, preferably ultrapure water with a conductivity of less than 1 μS / cm.

[0029] After a single atmospheric pressure stirring washing of the application is completed, the polyvinylidene fluoride emulsion is allowed to float, and the lower layer of waste water is discharged. After a single vacuum stirring washing is completed, the polyvinylidene fluoride emulsion is allowed to sink, and the upper layer of waste water is discharged.

[0030] The impurity content of the polyvinylidene fluoride powder of the application is ≤700 ppm.

[0031] The washing method of the present application alternately carries out atmospheric agitation washing and vacuum agitation washing, air and water alternately enter the particle pores of polyvinylidene fluoride latex particles, and the remaining additives in the particle pores are washed away. In atmospheric agitation washing, air enters the particle pores of polyvinylidene fluoride latex particles, the dispersion containing polyvinylidene fluoride latex particles floats, and after standing, the dispersion containing polyvinylidene fluoride latex particles and waste water can be layered, the upper layer is the dispersion containing polyvinylidene fluoride latex particles, and the lower layer is waste water; in vacuum agitation washing, the air in the particle pores of polyvinylidene fluoride latex particles is removed, and after water enters the particle pores of polyvinylidene fluoride latex particles, the dispersion containing polyvinylidene fluoride latex particles sinks, and after standing, the dispersion containing polyvinylidene fluoride latex particles and waste water can be layered, the upper layer is waste water, and the lower layer is the dispersion containing polyvinylidene fluoride latex particles; in atmospheric agitation washing again, air enters the particle pores of polyvinylidene fluoride latex particles, and the remaining additives in the original particle pores are removed with water. Compared with the traditional high-temperature-mechanical agitation washing method, the washing method of alternately carrying out atmospheric agitation washing and vacuum agitation washing can effectively remove the remaining additives in the particle pores.

[0032] The technical solution of the present application has the following technical effects compared with the prior art:

[0033] (1) The present application adds an ionic strength regulator during polymerization without reducing the amount of emulsifier, and polyvinylidene fluoride emulsion with an average particle size of ≥300 nm is prepared.

[0034] (2) The washing method of the present application alternately carries out atmospheric agitation washing and vacuum agitation washing, and the impurity content of the polyvinylidene fluoride powder prepared is ≤700 ppm, which improves the processing thermal stability of polyvinylidene fluoride. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 The scanning electron microscope image of the polyvinylidene fluoride powder prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0036] The present application will be further described below in conjunction with specific examples, but the present application is not limited to these specific examples. Those skilled in the art should recognize that the present application encompasses all alternatives, improvements and equivalents within the scope of the claims.

[0037] Example 1

[0038] First, 1.6 g of CF3CF2CF2-O-(CF2CF(CF3)O)2-O-CF2CF2COONH4 and ultrapure water were mixed to form a 100 g emulsifier aqueous solution. In a 5 L stainless steel reactor, 3000 g of ultrapure water, 10 g of paraffin and 100 g of the above emulsifier aqueous solution were added. The reactor was opened for stirring, vacuum was replaced with nitrogen three times, the reactor temperature was raised to 100℃, the stirring speed was 500 rpm / min, and the reactor pressure was increased to 4.0 MPa by adding vinylidene fluoride monomer. The polymerization reaction started when 1.8 g of peroxydicarbonic acid dioctyl ester and 1.5 g of ethyl acetate were added by a metering pump. The aqueous solution of dipotassium hydrogen phosphate and ethyl acetate (0.24 g of dipotassium hydrogen phosphate, 8 g of ethyl acetate and ultrapure water were mixed to form a 360 g aqueous solution) was continuously added by a peristaltic pump at a flow rate of 3 g / min. The reactor pressure was kept constant at 4.0 MPa by continuously adding vinylidene fluoride monomer. When the consumption of vinylidene fluoride reached 800 g, the stirring was stopped, the remaining reaction gas in the reactor was emptied, and the polyvinylidene fluoride emulsion was discharged through the bottom valve. The average particle size of the emulsion was 320 nm measured by a laser particle size analyzer.

[0039] The polyvinylidene fluoride emulsion was cooled to 40℃, and the floating or precipitated material was removed by filtration to obtain 4.3 Kg of emulsion with a solid content of 18.0 wt%. The emulsion was added to a 20 L volume coagulation reactor, and the stirring was started at a speed of 500 rpm / min. The emulsion was coagulated by adding 500 g of 1 wt% sodium chloride solution. After demulsification, the mixture was allowed to stand for 10 min, the lower layer of waste water was discharged, and the conductivity of the sample was tested. The coagulation reactor was added with ultrapure water at a volume ratio of 0.2:1 of paste to ultrapure water, the washing temperature was maintained at 56℃, the stirring speed was 150 rpm / min, and the washing was carried out for 20 min. After standing for 10 min, the lower layer of waste water was discharged, and the conductivity of the sample was tested. The coagulation reactor was continuously added with ultrapure water at a volume ratio of 0.2:1 of paste to ultrapure water, the vacuum degree was -0.08 MPa, and the stirring was carried out for 20 min at a speed of 50 rpm / min. After standing for 10 min, the upper layer of waste water was discharged, and the conductivity of the sample was tested. The alternating washing under normal pressure and vacuum was carried out once, and the upper layer of waste water was discharged after standing. The conductivity was tested by a DDS-307 conductivity meter at 25℃, and the value was lower than 2.0 μS / cm. The product was discharged through the bottom valve, and the polyvinylidene fluoride powder was obtained after air flow drying.

[0040] The paraffin residue in the polyvinylidene fluoride powder was 100 ppm, the initiator residue was 230 ppm, and the emulsifying aid residue was 350 ppm, which were measured by a 600 mega Bruker hydrogen nuclear magnetic resonance spectrometer and a HPLC-MS / MS method, respectively. The yellowness index of the polyvinylidene fluoride powder was 3.5, which was measured by a HunterLab ColorFlex EZ colorimeter after the powder was placed in a 250℃ oven for half an hour.

[0041] Example 2

[0042] The same as example 1, except that vinylidene fluoride and hexafluoropropylene were used as the 90:10 mass ratio make-up monomers to reach a kettle pressure of 4.0 MPa. The metering pump was used to add dioctyl peroxydicarbonate 1.8 g and ethyl acetate 1.5 g, and the polymerization reaction started. The peristaltic pump was used to add a potassium phosphate dibasic, ethyl acetate aqueous solution (0.16 g of potassium phosphate dibasic, 8 g of ethyl acetate, and 360 g of an aqueous solution of ultrapure water) at a flow rate of 3 g / min, and vinylidene fluoride and hexafluoropropylene were continuously added as make-up monomers at a mass ratio of 90:10 to maintain the kettle pressure at 4.0 MPa. When the monomer consumption reached 800 g, the stirring was stopped, the remaining reaction gas in the kettle was emptied, and the vinylidene fluoride and hexafluoropropylene copolymer emulsion was discharged through the bottom valve. The average particle size of the emulsion was 340 nm as measured by a laser particle size analyzer.

[0043] The vinylidene fluoride and hexafluoropropylene copolymer emulsion was cooled to 40°C, filtered to remove floating or precipitated substances, and 4.4 kg of an emulsion with a solid content of 17.9 wt% was obtained. The emulsion was placed in a 20 L volume coagulation kettle, the stirring was started at a speed of 500 rpm / min, and the emulsion was added to the coagulation kettle for coagulation treatment with 500 g of a 1 wt% sodium chloride solution. After demulsification, the sample was placed for 10 min for layering, the lower layer of waste water was discharged, and the conductivity test was performed. The paste, ultrapure water was added to the coagulation kettle at a volume ratio of 0.2:1, the washing temperature was maintained at 56°C, the stirring was performed at a speed of 150 rpm / min for 20 min, and after the sample was placed for 10 min for layering, the lower layer of waste water was discharged and the conductivity test was performed. The paste, ultrapure water was continuously added to the coagulation kettle at a volume ratio of 0.2:1, the vacuum degree was -0.08 MPa, and the stirring was performed at a speed of 50 rpm / min for 20 min. After the sample was placed for 10 min for layering, the upper layer of waste water was discharged and the conductivity test was performed. The alternating washing was performed once under normal pressure and vacuum, the upper layer of waste water was discharged after the sample was placed for layering, and the conductivity was less than 2.0 μS / cm as measured by a DDS-307 conductivity meter at a temperature of 25°C. The bottom valve was discharged, and the vinylidene fluoride and hexafluoropropylene copolymer powder was obtained after air flow drying.

[0044] The vinylidene fluoride and hexafluoropropylene copolymer powder was measured by a 600 mega Bruker hydrogen nuclear magnetic resonance spectrometer to have a paraffin residue of 90 ppm, an initiator residue of 200 ppm, and an emulsifying aid residue of 320 ppm as measured by an HPLC-MS / MS method. The vinylidene fluoride and hexafluoropropylene copolymer powder was placed in an oven at 250°C for half an hour, taken out, and measured by a HunterLab ColorFlex EZ colorimeter to have a yellowness index of 3.0.

[0045] Example 3

[0046] The same as example 1, except that: the peristaltic pump was used to supplement sodium acetate trihydrate, ethyl acetate aqueous solution (0.16 g of sodium acetate trihydrate, 8 g of ethyl acetate and water to form a 360 g aqueous solution) at a flow rate of 3 g / min, and vinylidene fluoride was continuously supplemented to maintain the kettle pressure constant at 4.0 MPa. The reaction monomer consumption reached 800 g. The stirring was stopped, the remaining reaction gas in the kettle was emptied, and the polyvinylidene fluoride emulsion was discharged through the bottom valve. The laser particle size analyzer measured that the average particle size of the emulsion was 310 nm.

[0047] The polyvinylidene fluoride emulsion was cooled to 40°C, and the floating or precipitated material was removed by filtration to obtain 4.4 kg of emulsion with a solid content of 17.8 wt%. The discharge was placed in a volume of 20 L coagulation kettle, and the stirring was started to a speed of 500 rpm / min. The emulsion was added with 500 g of 1 wt% sodium chloride solution for coagulation treatment. After demulsification, the sample was placed for 10 min, the lower layer of waste water was discharged, and the conductivity test was performed. The paste, ultrapure water was added to the coagulation kettle at a volume ratio of 0.2:1, the washing temperature was maintained at 56°C, the stirring speed was 150 rpm / min, and the washing was performed for 20 min. After standing for 10 min, the lower layer of waste water was discharged, and the conductivity test was performed. The paste, ultrapure water was continuously added to the coagulation kettle at a volume ratio of 0.2:1, the vacuum degree was -0.08 MPa, and the stirring speed was 50 rpm / min. The washing was performed for 20 min, and after standing for 10 min, the upper layer of waste water was discharged, and the conductivity test was performed. The normal pressure and vacuum stirring were alternately washed once, and after standing for 10 min, the upper layer of waste water was discharged. The conductivity was tested by DDS-307 conductivity meter at 25°C, and the conductivity was less than 2.0 μS / cm. The bottom valve was discharged, and the polyvinylidene fluoride powder was obtained after air drying.

[0048] The paraffin residue of the polyvinylidene fluoride powder was 110 ppm, the initiator residue was 230 ppm, and the emulsifying aid residue was 350 ppm, which were measured by 600 mega Bruker hydrogen nuclear magnetic resonance spectrometer and HPLC-MS / MS method, respectively. The HunterLab ColorFlex EZ colorimeter measured that the yellowness index of the polyvinylidene fluoride powder placed in a 250°C oven for half an hour was 3.0.

[0049] Comparative example 1

[0050] The same as example 1, except that the kettle temperature was raised to 70°C, the stirring speed was 500 rpm / min, and the perfluorovinyl ether monomer was added to the kettle until the pressure reached 4.0 MPa. The metering pump was used to add 1.3 g of di-n-propyl peroxydicarbonate and 1.0 g of ethyl acetate, and the polymerization reaction began. The peristaltic pump was used to continuously add an aqueous solution of ethyl acetate (5 g of ethyl acetate was dissolved in 360 g of water) at a flow rate of 3 g / min, and the perfluorovinyl ether monomer was continuously added to maintain the kettle pressure at 4.0 MPa. When the consumption of perfluorovinyl ether reached 800 g, the stirring was stopped, the remaining reaction gas in the kettle was emptied, and the polyvinylidene fluoride emulsion was discharged through the bottom valve. The average particle size of the emulsion was 160 nm, as measured by a laser particle size analyzer.

[0051] The polyvinylidene fluoride emulsion was cooled to 40°C, filtered to remove floating or precipitated substances, and 4.4 kg of an emulsion with a solid content of 18 wt% was obtained. The emulsion was placed in a coagulation kettle with a volume of 20 L, and the stirring was started at a speed of 500 rpm / min. The emulsion was coagulated by adding 500 g of a 1 wt% sodium chloride solution, and after the emulsion was broken, it was allowed to stand for 10 min to separate into layers. The lower layer of waste water was discharged, and a sample was taken for conductivity testing. The coagulation kettle was filled with ultrapure water at a volume ratio of paste: ultrapure water of 0.2:1, and the washing temperature was maintained at 56°C. The stirring was performed at a speed of 150 rpm / min for 20 min, and after the emulsion was allowed to stand for 10 min to separate into layers, the lower layer of waste water was discharged, and a sample was taken for conductivity testing at a temperature of 25°C using a DDS-307 conductivity meter. The stirring was continued until the conductivity of the waste water was less than 2.0 μS / cm, and the bottom valve was used to discharge the material. After air flow drying, polyvinylidene fluoride powder was obtained.

[0052] The paraffin residue in the polyvinylidene fluoride powder was 300 ppm, the initiator residue was 430 ppm, and the emulsifying aid residue was 950 ppm, as measured by a 600-megahertz Bruker hydrogen nuclear magnetic resonance spectrometer and an HPLC-MS / MS method. The yellowness index of the polyvinylidene fluoride powder was 13.8, as measured by a HunterLab ColorFlex EZ colorimeter after the powder was placed in an oven at 250°C for half an hour.

[0053] Comparative Example 2

[0054] The same as Example 1, except that 1.2 g of CF3CF2CF2-O-(CF2CF(CF3)O)2-O-CF2CF2COONH4 and ultra-pure water were prepared into a 100 g emulsifier aqueous solution in advance, 3000 g of ultra-pure water, 10 g of paraffin and 100 g of the above emulsifier aqueous solution were added into a 5 L stainless steel reaction kettle, the kettle was opened for stirring, vacuum was extracted and replaced with nitrogen for three times, the kettle temperature was raised to 100℃, the stirring speed was 500 rpm / min, and the kettle pressure was increased to 4.0 MPa by adding vinylidene fluoride monomer. The polymerization reaction started after 1.8 g of peroxydicarbonic acid dioctyl ester and 1.5 g of ethyl acetate were added by a metering pump. The ethyl acetate aqueous solution (8 g of ethyl acetate was prepared into a 360 g aqueous solution with water) was continuously added by a peristaltic pump at a flow rate of 3 g / min, the kettle pressure was kept constant at 4.0 MPa by continuously adding vinylidene fluoride monomer, the vinylidene fluoride consumption reached 800 g, the stirring was stopped, the remaining reaction gas in the kettle was emptied, the polyvinylidene fluoride emulsion was discharged through the bottom valve, and polymeric coagulum was generated on the bottom of the kettle and the bottom of the collection tank. The average particle size of the emulsion was 300 nm measured by a laser particle size analyzer.

[0055] The polyvinylidene fluoride emulsion was cooled to 40℃, and the floating or precipitated material was removed by filtration to obtain 3.5 Kg of emulsion with a solid content of 17.3 wt%, which was placed in a 20 L volume coagulation kettle, the stirring was started at a speed of 500 rpm / min, and the emulsion was added with 500 g of 1 wt% sodium chloride solution for coagulation treatment, the emulsion was separated into layers after standing for 10 min, the lower layer waste water was discharged, and the conductivity test was performed on the sample. The paste, ultra-pure water and the coagulation kettle were added with ultra-pure water at a volume ratio of 0.2:1, the washing temperature was maintained at 56℃, the stirring was performed at a speed of 150 rpm / min for 20 min, the lower layer waste water was discharged after standing for 10 min, and the conductivity test was performed on the sample; the paste, ultra-pure water and the coagulation kettle were continuously added with ultra-pure water at a volume ratio of 0.2:1, the vacuum degree was-0.08 MPa, and the stirring was performed at a speed of 50 rpm / min for 20 min, the upper layer waste water was discharged after standing for 10 min, and the conductivity test was performed on the sample; the normal pressure and vacuum stirring were alternately washed once, the upper layer waste water was discharged after standing for 10 min, and the conductivity was tested at 25℃ by using a DDS-307 type conductivity meter, which was lower than 2.0 μS / cm, and the bottom valve was discharged, and the polyvinylidene fluoride powder was obtained after air flow drying.

[0056] The polyvinylidene fluoride powder was measured by using a 600 mega Bruker hydrogen nuclear magnetic resonance spectrometer to determine that the paraffin residual amount was 320 ppm, the initiator residual amount was 460 ppm, and the emulsifying auxiliary residual amount was 1280 ppm by using an HPLC-MS / MS method.

[0057] The polyvinylidene fluoride powder was placed in a 250℃ oven for half an hour, and the yellowness index was 13.5 measured by using a HunterLab ColorFlex EZ colorimeter.

[0058] Table 1 Test results of emulsion particle size, yellow index, and residual amount of auxiliary of examples and comparative examples

[0059]

[0060] Note:

[0061] Emulsion particle size d 0.5 1 0.2 ml of the emulsion was taken in a sample cell by using a pipette, and a Malvern Mastersizer 2000 laser particle size instrument was used for testing;

[0062] Yellow index 2 The polymer powder was placed in an aluminum foil, and was placed in an oven at 250°C for half an hour, and then was taken out and measured for yellowness index by using a HunterLab ColorFlex EZ colorimeter;

[0063] Residual amount of initiator 3 Residual amount of paraffin 3 Test method: The residual amount of initiator and paraffin was determined by hydrogen spectrum nuclear magnetic resonance by using a 600-megahertz Bruker hydrogen spectrum nuclear magnetic resonance spectrometer;

[0064] Residual amount of emulsifier 4 Test method: The residual amount of auxiliary was detected by using a Waters ACQUITY UPLC high-performance liquid chromatography-tandem quadrupole mass spectrometer HPLC-MS / MS after pretreatment of the polymer powder sample and Soxhlet extraction. The mobile phase of the mass spectrometer was chromatographically pure methanol and water, the elution mode was gradient elution, the chromatographic column was an ACQUITY BEH C18 column with a specification of 2.1 mm x 50 mm x 1.7 μm, the column temperature was 30-50°C, and the flow rate was 0.4 mL / min. The mass spectrometry detection conditions were: ionization mode was ESI, monitoring mode was MRM (multiple reaction monitoring), and negative ion scanning.

Claims

1. A process for the preparation of polyvinylidene fluoride powder, characterized in that: The preparation method comprises a large-particle-size polyvinylidene fluoride emulsion preparation step and a washing step, wherein the large-particle-size polyvinylidene fluoride emulsion preparation step is that, in an aqueous medium containing an initiator, a chain transfer agent and an ionic emulsifier, and in the presence of an ionic strength regulator, polyvinylidene fluoride is homopolymerized or polyvinylidene fluoride and a fluorine-containing comonomer are copolymerized to obtain a large-particle-size polyvinylidene fluoride emulsion with an average particle size of ≥ 300 nm, The ionic strength regulator is at least one selected from potassium trihydrogen oxalate, potassium hydrogen phthalate, dipotassium hydrogen phosphate, sodium acetate trihydrate, sodium tetraborate and sodium citrate, and the addition amount of the ionic strength regulator is 0.001 wt% to 0.1 wt% of the total amount of the polymerized monomers, The amount of the ionic emulsifier is 0.03 wt% to 1.0 wt% of the total amount of the polymerized monomers, The washing step comprises atmospheric agitation washing and vacuum agitation washing, and the atmospheric agitation washing and the vacuum agitation washing are alternately performed.

2. The method of claim 1, wherein the polyvinylidene fluoride powder is prepared by: The initiator is at least one selected from dicyclohexyl peroxydicarbonate, di-p-t-butylcyclohexyl peroxydicarbonate, phenoxyethyl peroxydicarbonate, cyclooctyl peroxydicarbonate, dioctyl peroxydicarbonate, ditetradecyl peroxydicarbonate, dicetyl peroxydicarbonate, di-2-ethoxyethanol peroxydicarbonate and di-3-methoxybutanol peroxydicarbonate.

3. The method of claim 1, wherein the polyvinylidene fluoride powder is prepared by: said ionic emulsifier is selected from the group consisting of perfluorooctanoic acid or its ammonium salt, perfluorooctane sulfonic acid or its ammonium salt, CF3CF2CF2-O-(CF2CF(CF3)O) m (CF(CF3)O) n -CF2CF2COOH or its ammonium salt, m is 1 or 2, n is an integer from 0 to 4, CF3CF2CF2-O-(CF(CF3)CF2-O- p CF(CF3)COOH or its ammonium salt, p is an integer from 1 to 4, CF3CF2CF2-O-(CF2CF2-O- q CF2COOH or its ammonium salt, q is an integer from 0 to 3, CF3CF2(CF2OCF(CF3)) r COOH or its ammonium salt, r is an integer from 0 to 3 or CF3CF2((CF(CF3)) s CF2) t CF2COOH or its ammonium salt, s is 0 or 1, t is an integer from 0 to 3.

4. The method of claim 1, wherein the polyvinylidene fluoride powder is prepared by: The fluorine-containing comonomer is at least one selected from hexafluoropropylene, pentafluoropropylene, tetrafluoropropylene, trifluorochloroethylene, trifluoroethylene, fluoroethylene and perfluoroalkyl vinyl ether.

5. The method for preparing polyvinylidene fluoride powder according to claim 1, characterized in that: The large-particle-size polyvinylidene fluoride emulsion and water are placed in a reactor, the atmospheric agitation washing and the vacuum agitation washing are alternately performed, the time for a single atmospheric agitation washing and a single vacuum agitation washing is 5 to 40 min, the washing temperature is 20 to 75 ℃, the conductivity of the discharged waste water after each washing is monitored, and the washing is stopped when the conductivity is reduced to 1 to 2 μS / cm, and a polyvinylidene fluoride powder is obtained by drying.

6. The method of claim 5, wherein the polyvinylidene fluoride powder is prepared by: The volume ratio of the large-particle-size polyvinylidene fluoride emulsion to water is 0.1:1 to 1:

1.

7. The method of claim 6, wherein the polyvinylidene fluoride powder is prepared by: The water is ultrapure water with a conductivity of less than 1 μS / cm.

8. The method for preparing polyvinylidene fluoride powder according to claim 5, characterized in that: After a single atmospheric agitation washing, the polyvinylidene fluoride emulsion is allowed to float, and the lower layer of waste water is discharged; after a single vacuum agitation washing, the polyvinylidene fluoride emulsion is allowed to sink, and the upper layer of waste water is discharged.

9. The method of producing a polyvinylidene fluoride powder according to any one of claims 1 to 8, characterized by: The impurity content of the polyvinylidene fluoride powder is ≤ 700 ppm.

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