1. The preparation method of the meltable polytetrafluoroethylene is characterized by comprising the following steps of:

(1) adding a premix into a deoxidation polymerization kettle in advance before the reaction starts, wherein the premix comprises water, a dispersing agent, an organic solvent, perfluoroalkyl vinyl ether, tetrafluoroethylene, a modified monomer and a chain transfer agent, adding an initiator into the premix when the reaction starts, carrying out polymerization reaction at 20-50 ℃, continuously adding tetrafluoroethylene to control the reaction pressure to be 1.5-2.5 MPa, adjusting the melt flow rate of a target polymer by adjusting the ratio of the organic solvent to the perfluoroalkyl vinyl ether, and the melt flow rate of the prepared PFA resin is 1-80 g/10min, wherein the weight ratio of the organic solvent to the perfluoroalkyl vinyl ether is 1: 0.2-1: 3, the dispersing agent is a single surfactant or a mixed surfactant, and the weight ratio of the added dispersing agent to the water is 1: 100-1: 1000, parts by weight;

(2) after the tetrafluoroethylene monomer participating in the reaction reaches the preset quality, increasing the reaction temperature to 70-90 ℃, controlling the adding speed of the tetrafluoroethylene to automatically reduce the reaction pressure to 0.8-1.2 MPa, and continuing the polymerization reaction;

(3) evacuating the unreacted polymerized monomers to obtain a target polymer emulsion;

(4) and carrying out post-treatment on the obtained polymer emulsion to obtain a concentrated dispersion liquid of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer or a pelletized material of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer with stabilized terminal groups.

2. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: the perfluoroalkyl vinyl ether is one or the mixture of more than two of perfluoromethyl vinyl ether, perfluoroethyl vinyl ether and perfluoro-n-propyl vinyl ether.

3. The process for preparing fusible polytetrafluoroethylene according to claim 2, characterized in that: the weight ratio of the added perfluoroalkyl vinyl ether to the tetrafluoroethylene monomer is 1: 5-1: 30.

4. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: the modified monomer is one or a mixture of more than two of hexafluoropropylene and perfluoroalkoxyalkyl vinyl ether, the mass fraction of the modified monomer in the product is not higher than 5%, and the perfluoroalkoxyalkyl vinyl ether has the following structure:

CF₂＝CF-O-(C₃F₆O)_n-C₃F₇

wherein: n is an integer of 1 to 3.

5. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: the added organic solvent is one or the mixture of more than two of perfluorinated alkane, perfluorinated cycloalkane, perfluorinated oxygen-containing cycloalkane, perfluoroketone, fluorochlorohydrocarbon and perfluorinated polyether,

the molecular formula of the perfluoroalkane is as follows:

C_nF_(2n+2)

wherein: n is an integer of 5-9;

the molecular formula of the perfluorocycloalkane is as follows:

C_nF_2n

wherein: n is an integer of 4-9;

the molecular formula of the perfluorinated oxygen-containing cycloalkane is as follows:

C_nF_2nO

wherein: n is an integer of 6-9;

the molecular formula of the perfluoro ketone is as follows:

C_nF_2nO

wherein: n is an integer of 6-9;

the molecular formula of the chlorofluorocarbon is as follows:

C_nF_mCl_(2n+2-m)

wherein: n is an integer of 1 to 3, and m is an integer of 1 to 7;

the molecular formula of the perfluoropolyether is as follows:

CF₃-(-CF₂-)_n-O-(-Rf₁-O-)_m-(-CF₂-O-)_q-Rf₂

wherein: rf₁、Rf₂Is a linear or branched perfluoroalkyl group having 1 to 3 carbon atoms, n is an integer of 0 to 2, and m and q are integers of 0 to 40.

6. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: the weight ratio of the added organic solvent to the perfluoroalkyl vinyl ether is 1: 0.2-1: 1.5.

7. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: the chain transfer agent is hydrogen, methane, ethane, methanol, cyclohexane or difluoromethane, and the weight ratio of the added chain transfer agent to the tetrafluoroethylene monomer is 1: 250-1: 8000.

8. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: in the step (2), the mass of the tetrafluoroethylene monomer participating in the reaction before the reaction temperature is increased is not more than 20% of the total reaction amount of the tetrafluoroethylene monomer.

9. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: in the step (2), the reaction temperature is increased to 70-80 ℃, so that the reaction pressure is automatically reduced to 1-1.2 MPa.

10. The process for preparing fusible polytetrafluoroethylene according to claim 1, characterized in that: the dispersing agent is perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, perfluorodecanoic acid, perfluorooctanesulfonic acid, perfluoroheptanesulfonic acid or perfluoro-2, 5-dimethyl-3, 6-dioxanonanoic acid.

Background

Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer is commonly called fusible Polytetrafluoroethylene (PFA) because it has excellent chemical stability, physical and mechanical properties, electrical insulation properties, lubricity, non-stick properties, aging resistance, non-flammability and thermal stability, which are the same as those of Polytetrafluoroethylene (PTFE), and can be processed by a general thermoplastic molding method. Fusible polytetrafluoroethylene resins were first developed successfully by DuPont, USA in 1973 under the trade name Teflon PFA. The major manufacturers of fusible polytetrafluoroethylene resins in the world are DuPont, Japan Dajin, Japan Asahi glass, 3M and Suwei, and only Shandong Huaxia Shenzhou new materials are currently produced in China.

Based on the advantages of the performance of the meltable polytetrafluoroethylene resin, the high-frequency ultrahigh-frequency polytetrafluoroethylene resin can be used for manufacturing the corrosion-resistant linings of wire and cable insulating sheaths, high-frequency ultrahigh-frequency insulating parts, chemical pipeline valves and pumps at present; welding rods such as special parts for the mechanical industry, various anticorrosive materials for the light textile industry, polytetrafluoroethylene anticorrosive linings and the like; and the method has wide application in the fields of semiconductor industry, medicine industry, electronic and electrical equipment industry, national defense military industry, aerospace and the like.

Many patents have been described in greater detail in the synthesis of fusible polytetrafluoroethylene resins. For example, patent RU2195465 discloses a method for synthesizing fusible polytetrafluoroethylene in which the reaction medium is a perfluorinated organic solvent. The solvent adopts octafluorocyclobutane, the initiator adopts perfluorodiacyl peroxide, tetrafluoroethylene and perfluoropropyl vinyl ether monomers are added into a polymerization kettle according to a certain proportion for copolymerization reaction, and the copolymerization reaction is continuously supplemented to keep the reaction pressure. And after the reaction is finished, separating the polymer from the monomer and the solvent by distillation to obtain the PFA resin product. In patent US3528954, a method of copolymerizing tetrafluoroethylene and perfluoroalkyl vinyl ether monomers in a chlorofluorocarbon or hydrochlorofluorocarbon solvent medium is disclosed, wherein perfluoropropionyl peroxide is used as an initiator, and a meltable polytetrafluoroethylene resin suspension is obtained by a suspension polymerization method. Patent US3635926 discloses a copolymerization method of tetrafluoroethylene and perfluoroalkyl vinyl ether in an aqueous medium containing a small amount of fluorocarbon solvent, wherein a chain transfer agent is added into the reaction system to control the die swell ratio of the PFA resin product and improve the flex life of the PFA resin product, but the problem of too wide molecular weight distribution caused by the introduction of the fluorocarbon solvent cannot be completely counteracted by the addition of the chain transfer agent. The patent CN1774457 adopts a method of aqueous suspension polymerization to polymerize and obtain a meltable polytetrafluoroethylene resin product; patent CN106519100 adopts a mixed medium process in which the reaction early stage is organic phase suspension polymerization and the reaction late stage is added with water, which is tedious in process and not beneficial to large-scale popularization and application.

Generally, there are two main processes for producing fusible polytetrafluoroethylene resin: dispersion polymerization and suspension polymerization. The suspension polymerization is widely applied, but the problems of slow reaction rate and easy adhesion of polymer products in the later reaction period exist in any reaction medium. And the organic medium polymerization and the mixed medium polymerization need solvent recovery, and the process is complicated. The aqueous dispersion polymerization has problems of large number of unstable terminal groups, uneven molecular weight distribution and the like, thereby affecting the processing performance of products. Meanwhile, the mechanical property of the product is reduced due to uneven distribution of the perfluoroalkyl vinyl ether in the copolymer in many products, and the problem is solved by adopting a method of continuously supplementing the perfluoroalkyl vinyl ether in most production processes. However, the method has complex control process and high requirements on equipment and operators, and is not easy to popularize and use on a large scale.

Disclosure of Invention

Aiming at the technical problems existing at present, the invention aims to provide a preparation method of meltable polytetrafluoroethylene.

In order to solve the technical problems, the invention adopts the following technical scheme: the preparation method of the meltable polytetrafluoroethylene comprises the following steps:

(1) adding a premix into a deoxidation polymerization kettle in advance before the reaction starts, wherein the premix comprises water, a dispersing agent, an organic solvent, perfluoroalkyl vinyl ether, tetrafluoroethylene, a modified monomer and a chain transfer agent, adding an initiator into the premix when the reaction starts, carrying out polymerization reaction at 20-50 ℃, continuously adding tetrafluoroethylene to control the reaction pressure to be 1.5-2.5 MPa, adjusting the melt flow rate of a target polymer by adjusting the ratio of the organic solvent to the perfluoroalkyl vinyl ether, and the melt flow rate of the prepared PFA resin is 1-80 g/10min, wherein the weight ratio of the organic solvent to the perfluoroalkyl vinyl ether is 1: 0.2-1: 3, the dispersing agent is a single surfactant or a mixed surfactant, and the weight ratio of the added dispersing agent to the water is 1: 100-1: 1000, parts by weight;

(2) after the tetrafluoroethylene monomer participating in the reaction reaches the preset quality, increasing the reaction temperature to 70-90 ℃, controlling the adding speed of the tetrafluoroethylene to automatically reduce the reaction pressure to 0.8-1.2 MPa, and continuing the polymerization reaction;

(3) evacuating the unreacted polymerized monomers to obtain a target polymer emulsion;

(4) and carrying out post-treatment on the obtained polymer emulsion to obtain a concentrated dispersion liquid of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer or a pelletized material of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer with stabilized terminal groups.

Preferably, the perfluoroalkyl vinyl ether is one or a mixture of any two or more of perfluoromethyl vinyl ether, perfluoroethyl vinyl ether and perfluoro-n-propyl vinyl ether.

Preferably, the weight ratio of the added perfluoroalkyl vinyl ether to the tetrafluoroethylene monomer is 1: 5-1: 30.

Preferably, the modified monomer is one or a mixture of any two or more of hexafluoropropylene and perfluoroalkoxyalkyl vinyl ether, the mass fraction of the modified monomer in the product is not higher than 5%, and the perfluoroalkoxyalkyl vinyl ether has the following structure:

CF2＝CF-O-(C3F6O)n-C3F7

wherein: n is an integer of 1 to 3.

Preferably, the added organic solvent is one or the mixture of any two or more of perfluoroalkane, perfluorocycloalkane, perfluorooxygenated cycloalkane, perfluoroketone, chlorofluorocarbon and perfluoropolyether,

the molecular formula of the perfluoroalkane is as follows:

CnF(2n+2)

wherein: n is an integer of 5-9;

the molecular formula of the perfluorocycloalkane is as follows:

CnF2n

wherein: n is an integer of 4-9;

the molecular formula of the perfluorinated oxygen-containing cycloalkane is as follows:

CnF2nO

wherein: n is an integer of 6-9;

the molecular formula of the perfluoro ketone is as follows:

CnF2nO

wherein: n is an integer of 6-9;

the molecular formula of the chlorofluorocarbon is as follows:

CnFmCl(2n+2-m)

wherein: n is an integer of 1 to 3, and m is an integer of 1 to 7;

the molecular formula of the perfluoropolyether is as follows:

CF3-(-CF2-)n-O-(-Rf1-O-)m-(-CF2-O-)q-Rf2

wherein: rf1 and Rf2 are linear or branched chain perfluoroalkyl groups having 1-3 carbon atoms, n is an integer of 0-2, and m and q are integers of 0-40.

Preferably, the weight ratio of the added organic solvent to the perfluoroalkyl vinyl ether is 1: 0.2-1: 1.5.

Preferably, the chain transfer agent is hydrogen, methane, ethane, methanol, cyclohexane or difluoromethane, and the weight ratio of the added chain transfer agent to the tetrafluoroethylene monomer is 1: 250-1: 8000.

Preferably, in the step (2), the mass of the tetrafluoroethylene monomer participating in the reaction before the reaction temperature is increased is not more than 20% of the total reaction amount of the tetrafluoroethylene monomer.

Preferably, in the step (2), the reaction temperature is increased to 70-80 ℃ so that the reaction pressure is reduced to 1-1.2 MPa.

Preferably, the dispersing agent is perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, perfluorodecanoic acid, perfluorooctanesulfonic acid, perfluoroheptanesulfonic acid, or perfluoro-2, 5-dimethyl-3, 6-dioxanonanoic acid.

By adopting the technical scheme, compared with the prior art, the invention has the following beneficial effects:

1. the melt flow rate of the target polymer can be easily adjusted by adjusting the ratio of the organic solvent to the perfluoroalkyl vinyl ether, the method is simple and easy to implement, and the problem of great change of reaction speed caused by changing the dosage of the initiator in the traditional method is avoided.

2. The reaction temperature in the early stage of the reaction is low, chain termination caused by intramolecular rearrangement of the perfluoroalkyl vinyl ether structure is inhibited, the formation of molecular chains with extremely low molecular weight is avoided, the molecular weight distribution of the product is narrowed, and the number of unstable terminal groups can be reduced to a certain extent.

3. Compared with an organic initiator, the redox system is adopted for initiation at a lower reaction temperature, so that the initiation efficiency is high, the reaction speed is high, and the defects that the organic initiator is flammable and explosive and the storage condition is harsh are overcome.

4. The perfluoroalkyl vinyl ether is added at one time, in the middle reaction period, after a certain mass of tetrafluoroethylene monomer is reacted, the adding speed of the tetrafluoroethylene monomer is adjusted, the reaction pressure is reduced, and the partial pressure of the tetrafluoroethylene in the reaction gas phase is reduced by reducing the reaction pressure, so that the amount of the perfluoroalkyl vinyl ether participating in the polymerization reaction in the later reaction period is increased, the problems that the perfluoroalkyl vinyl ether is fast consumed in the early reaction period and is unevenly distributed in the copolymer due to slow consumption in the later reaction period are solved, the mechanical property of the product is improved, and the utilization rate of the perfluoroalkyl vinyl ether is improved; the operation is simple and easy, and compared with continuous feeding, the one-time feeding operation is simple and the process is controllable.

5. The reaction temperature is increased in the middle stage of the reaction, so that the reaction speed can be maintained at a higher level, and the problem of reaction speed reduction caused by reaction pressure reduction is solved.

6. The added organic solvent has little amount and does not need to be recycled, the high boiling point solvent can enter a sewage system after coagulation washing, and the low boiling point solvent can directly enter a reaction residual gas treatment system.

The following detailed description will explain the present invention and its advantages.

Detailed Description

The technical solutions in the embodiments of the present invention are clearly and completely described below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The invention provides a preparation method of tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, which adopts a mixed medium dispersion polymerization process using a water phase containing a small amount of organic solvent as a dispersed phase.

The operation steps of the invention are as follows:

(1) adding an initiator into a mixture of water, a dispersing agent, an organic solvent, perfluoroalkyl vinyl ether, tetrafluoroethylene, a modified monomer and a chain transfer agent in a deoxidation polymerization kettle, carrying out polymerization reaction at 20-50 ℃, and continuously adding tetrafluoroethylene to control the reaction pressure to be 1.5-2.5 MPa;

the perfluoroalkyl vinyl ether suitable for use in the present invention may be one or a mixture of more, for example, any two or three, of perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, and perfluoro-n-propyl vinyl ether.

In the step (1) of the method, the weight ratio of the perfluoroalkyl vinyl ether to the tetrafluoroethylene monomer is 1: 5-1: 30, preferably 1: 5-1: 20, and more preferably 1: 10-1: 20.

The modified monomer suitable for the invention can be one or a mixture of more of hexafluoropropylene and perfluoroalkoxy alkyl vinyl ether, and the mass fraction of the modified monomer in the product is not higher than 5%. Wherein the structural formula of the perfluoroalkoxyalkyl vinyl ether is CF₂＝CF-O-(C₃F₆O)_n-C₃F₇And n is an integer of 1 to 3.

In step (1) of the process of the present invention, the organic solvent to be added may be one or a mixture of several of perfluoroalkane, perfluorocycloalkane, perfluorooxocycloalkane, perfluoroketone, chlorofluorocarbon, perfluoropolyether, non-limiting examples of which include, for example, perfluoropentane, perfluorohexane, perfluoroheptane, perfluorooctane, perfluorocyclobutane, perfluoromethylcyclopentane, perfluoromethylcyclohexane, 1, 2-trichloro-1, 2, 2-trifluoroethane, dichlorotetrafluoroethane, tetrachlorodifluoroethane, perfluoroheptalene ether, perfluorooctalene ether, perfluorohexanone and the like, preferably perfluoroalkane, perfluorocycloalkane, such as perfluorooctane, perfluorocyclobutane and the like.

In the step (1) of the method, the weight ratio of the organic solvent to the perfluoroalkyl vinyl ether is 1: 0.2-1: 3, preferably 1: 0.2-1: 1.5.

The organic solvent is introduced into the aqueous phase dispersion polymerization system, and has the functions of dissolving aid and slow release for the perfluoroalkyl vinyl ether. The proportion of the organic solvent to the perfluoroalkyl vinyl ether in the reaction system is controlled, so that the speed of the perfluoroalkyl vinyl ether participating in the polymerization reaction can be controlled, and the content of the perfluoroalkyl vinyl ether in the polymer product can be controlled. It is shown by experimental data that when the content of the perfluoroalkyl vinyl ether in the PFA resin is within a certain range, changing the content thereof does not greatly affect the properties of the product, but greatly affects the melt flow rate of the product. Thus, the melt flow rate of the PFA resin product can be controlled by controlling the ratio of the added organic solvent and the perfluoroalkyl vinyl ether. The conventional method for controlling the melt flow rate of the polymer product is to adjust the amount of the initiator, but this method usually has a large influence on the reaction rate, and this problem can be avoided by the method of the present invention.

In the step (1) of the method, the weight ratio of the organic solvent to the water is 1: 10-1: 200, preferably 1: 20-1: 120, and more preferably 1: 50-1: 120.

In the present invention, the perfluoroalkyl vinyl ether and the organic solvent are added in one portion before the start of the reaction.

The perfluoroalkyl vinyl ether and the organic solvent are added once before the reaction begins, and the method has the advantages that the method has high requirements on equipment and operators, is simpler in one-time investment and operation, is easy to control the process, and is more favorable for industrial scale-up production.

The dispersing agent used in the present invention is a known fluorine-containing surfactant, and may be a single surfactant or a mixture of surfactants, and non-limiting examples thereof include perfluorohexanoic acid, perfluoroheptanoic acid, perfluorooctanoic acid, perfluorodecanoic acid, perfluorooctanesulfonic acid, perfluoroheptanesulfonic acid, perfluoro-2, 5-dimethyl-3, 6-dioxanonanoic acid, the mixed surfactant described in patent CN 106366230, and the like, and other commercially available fluorine-containing surfactants can be used as the dispersing agent used in the method of the present invention. The weight ratio of the added dispersant to water is 1: 100-1: 1000, preferably 1: 500-1: 1000.

the chain transfer agents employed in the present invention are conventional chain transfer agents known in the art, non-limiting examples of which include, for example, hydrogen, methane, ethane, methanol, cyclohexane, difluoromethane, etc., and other hydrogen-containing species may also be employed as chain transfer agents in the polymerization reaction. The weight ratio of the added chain transfer agent to the tetrafluoroethylene monomer is 1: 250-1: 8000.

The initiator adopted by the invention is a redox system initiator, wherein the oxidant is one or a mixture of ammonium persulfate and potassium persulfate, the reducing agent is one of sodium metabisulfite, sodium thiosulfate, sodium bisulfite, sodium sulfite, potassium metabisulfite, potassium thiosulfate, potassium bisulfite, potassium sulfite, ammonium thiosulfate, ammonium sulfite, ammonium bisulfite and ferrous sulfate, and the oxidant is added in an excessive manner.

The initiator adding mode with excessive oxidant is adopted in the invention to reduce the introduction of hetero atoms as much as possible, ensure the purity of the product and reduce the washing times of post-treatment.

In the step (1) of the method, the temperature for polymerization is 20-50 ℃, and the lower reaction temperature can inhibit intramolecular rearrangement of the perfluoroalkyl vinyl ether structure, so that chain termination caused by rearrangement is avoided, the problem of uneven molecular weight distribution can be solved, and the number of unstable terminal groups can be reduced.

In the step (1) of the method, the reaction pressure is controlled by continuously adding the tetrafluoroethylene monomer, and the reaction pressure is 1.5-2.5 MPa, preferably 1.5-2.0 MPa.

In step (1) of the process of the present invention, the mass of the tetrafluoroethylene monomer introduced during the reaction is not more than 20% of the total reaction amount of the tetrafluoroethylene monomer.

In the present invention, the term "deoxidation reaction vessel" means a reaction vessel in which the oxygen content in the vessel is 30ppm or less, preferably 20ppm or less, by evacuation substitution.

(2) After the tetrafluoroethylene monomer participating in the reaction reaches the preset quality, increasing the reaction temperature to 70-90 ℃, controlling the adding speed of the tetrafluoroethylene to automatically reduce the reaction pressure to 0.8-1.2 MPa, and continuing the polymerization reaction;

it is known that in the copolymerization of tetrafluoroethylene and perfluoroalkyl vinyl ether, the reactivity ratio of tetrafluoroethylene is much higher than that of perfluoroalkyl vinyl ether, so that the homopolymerization structure of tetrafluoroethylene tends to occur in the molecular chain segment of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer. The uneven distribution of the perfluoroalkyl vinyl ether in the copolymer can cause the mechanical property of the product to be reduced, so that the continuous supplement of the perfluoroalkyl vinyl ether is usually adopted to maintain the proportion of the perfluoroalkyl vinyl ether in the reaction system. However, this method is difficult to operate and has high requirements for the personnel, so it is not suitable for large-scale industrial application.

In the present invention, in order to avoid the above problems and to obtain more excellent performance of the PFA resin, after reacting a certain mass of tetrafluoroethylene monomer, the rate of addition of the tetrafluoroethylene monomer is adjusted to reduce the reaction pressure to 0.8 to 1.2MPa, preferably to 1.0 to 1.2 MPa. By reducing the reaction pressure, the partial pressure of tetrafluoroethylene in the reaction gas phase is reduced, so that the amount of perfluoroalkyl vinyl ether participating in the polymerization reaction in the later reaction stage is increased, the operation is simple and easy to implement, the problem of uneven distribution of perfluoroalkyl vinyl ether in the copolymer is solved, and the utilization rate of perfluoroalkyl vinyl ether can be improved.

In the reaction process, the reaction rate generally changes according to a law that the reaction rate continuously increases about 1 hour from the beginning of the reaction and tends to be stable after 1 hour. Therefore, in the present invention, when the reaction rate tends to be stable, the reaction temperature is increased and the reaction pressure is decreased to achieve the desired effect. The mass of tetrafluoroethylene monomer which is reacted at this time is generally not more than 20% of the total reaction mass.

In the step (2) of the method of the present invention, the polymerization reaction is carried out at a temperature of 70 to 90 ℃, preferably 70 to 80 ℃. Because the reaction pressure is reduced, and the excessive oxidant is adopted when the initiator is added, the reaction temperature is increased to maintain the reaction speed at a higher level, otherwise, the reducing agent in the stage is basically ineffective, and if the reaction temperature is not increased, the concentration of active free radicals in the reaction system is reduced seriously to cause that the reaction can not be continued. Further, in the middle stage of the polymerization reaction, the molecular chain growth of the polymer is completed to a certain extent, and even if there is chain termination due to intramolecular rearrangement of the perfluoroalkyl vinyl ether structure caused by temperature rise, the molecular chain having an extremely low molecular weight is not generated, and therefore the molecular weight distribution thereof is not widened seriously.

(3) And evacuating the unreacted polymerization monomer to obtain the target polymer emulsion.

In the present invention, the term "target polymer" means tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, and meltable polytetrafluoroethylene, PFA resin means the final target product, and they are used interchangeably.

In the present invention, the term "emulsion" means a milky white liquid containing a target polymer, a dispersant, water and the like obtained by emulsion polymerization, i.e., dispersion polymerization as described above.

(4) And carrying out post-treatment on the obtained polymer emulsion to obtain a concentrated dispersion liquid of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer or a pelletized material of the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer with stabilized terminal groups.

The method for post-treating the polymer emulsion in the step (4) of the process of the present invention is not particularly limited and is any conventional method known in the art.

The method for post-treating the polymer emulsion in step (4) of the method of the present invention may be a thermal concentration method to obtain a concentrated PFA resin dispersion.

The method for post-treating the polymer emulsion in the step (4) of the method can adopt a high-speed shearing and coagulation method to obtain PFA resin powder.

The PFA resin powder obtained after coagulation needs to be washed by water, and the conductivity of the water after washing is generally required to be lower than 5 mu s/cm.

Generally speaking, the mixed medium polymerization process needs to recover the organic solvent, but in the invention, the organic solvent does not serve as a reaction site and has the function of helping the perfluoroalkyl vinyl ether to be better dispersed in a reaction system, so the amount of the added organic solvent is very small, the recovery is not needed, the high-boiling point solvent can enter a sewage system after coagulation washing, and the low-boiling point solvent can directly enter a reaction residual gas treatment system.

PFA resin powder obtained after coagulation washing is dried, fluorinated and then granulated by a conventional thermoplastic resin granulation method to obtain a PFA resin particle product with stable end groups and less fluorine ion dissolution.

In the invention, the melt flow rate of the prepared PFA resin is 1-80 g/10 min. The size of the melt flow rate can be adjusted by adjusting the adding proportion of the perfluoroalkyl vinyl ether and the organic solvent, and the problem of large reaction speed change caused by the traditional method for adjusting the adding amount of the initiator is avoided.

In the present invention, the content of the perfluoroalkyl vinyl ether in the prepared PFA resin is 1 to 20 percent, preferably 1 to 15 percent, and more preferably 3 to 10 percent, based on the total weight of the resin.

In the invention, the tensile strength of the PFA resin is 28-36 MPa.

In the invention, the elongation at break of the prepared PFA resin is 340-420%.

In the invention, the decomposition temperature of the prepared PFA resin is 480-510 ℃.

In the invention, the melting point of the prepared PFA resin is 302-310 ℃.

In the present invention, the obtained PFA resin has a bending resistance of 250000 or more times.

The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way.

Measuring method

1. Determination of melt flow Rate

According to the method described in ASTM D1238, a melt flow Rate Meter (RL-Z1B) was used₁"shanghai srda scientific instruments ltd). The test temperature was 372 ℃ and the test load was 5 kg.

2. Measurement of mechanical Properties

The tensile strength and elongation at break of the molded specimens were measured according to the method described in ASTM D638 using an universal tensile machine (ETM503A, Shenzhen Wan Messaging device Co., Ltd.). The experimental environment temperature is 23 +/-2 ℃, the stretching speed is 50mm/min +/-5 mm/min, and the clamp spacing is 24 mm.

3. Determination of decomposition temperature

A sample of PFA resin of about 10mg was taken, and heated from 50 ℃ to 600 ℃ at a heating rate of 10 ℃/min under a nitrogen atmosphere using a thermogravimetric analyzer (TGA550, TA), and a thermogravimetric loss curve was recorded, and the temperature at which weight loss was 1% (wt) was taken as the decomposition temperature.

4. Melting point measurement and molecular weight distribution determination

The melting point of PFA was determined using a differential scanning calorimeter (DSC823e, METTLER) according to the method described in ASTM D3418: weighing 20mg +/-0.5 mg of sample, heating to 400 ℃ at a heating rate of 10 ℃/min in a nitrogen atmosphere, and taking the peak top temperature of a DSC spectrum melting peak as the melting point of the polymer. Meanwhile, by observing the width of the melting peak, namely the difference between the melting termination temperature and the melting initiation temperature, the width of the molecular weight distribution of PFA can be judged, and the molecular weight distribution with the narrow melting peak is also narrow.

5. Measurement of perfluoroalkyl vinyl Ether content

Weighing about 1g of PFA sample, pressing the PFA sample into a sheet with the thickness of 0.2-0.3 mm at 350 ℃, scanning the PFA sample by a Fourier transform infrared spectrometer (Spectrum Two, PerkinElmer), and calculating the content of the perfluoroalkyl vinyl ether according to the absorbance (A) of a characteristic peak by a formula, wherein the content of the perfluoroalkyl vinyl ether is obtained by the wave number of 893cm^-1The absorbance was determined and the perfluoroethyl vinyl ether content was determined by the wavenumber 1089cm^-1The absorbance was determined and the perfluoropropyl vinyl ether content was determined by means of a wave number of 990cm^-1The absorbance was determined, the formula is as follows:

PMVE content wt% -7 × (A)₁/A₀)

PEVE content wt% 0.75+1.28 × (A)₂/A₀)

PPVE content wt% 0.97 × (A)₃/A₀)

Wherein: a. the₀At a wave number of 2353cm^-1Absorbance of A₁Wave number is 893cm^-1Absorbance of A₂Wave number 1089cm^-1Absorbance of A₃Wave number 990cm^-1And (4) absorbance.

When other modified monomers exist, the determination of the characteristic absorbance of the perfluoro-n-propyl vinyl ether can be influenced, and the characteristic absorbance of the perfluoro-n-propyl vinyl ether is determined by using the deconvolution function carried by software.

6. Measurement of number of times of bending resistance

A PFA sample of about 1g was weighed, pressed at 350 ℃ to form a sheet of 0.2mm thickness, and cut into a 120mm × 15mm long piece. Measured according to the method described in ASTM D2176 using MIT refractometer (PN-NZ135, Hangzhou Providence technologies Co., Ltd.). The load was 1kg and the bending speed was 175 times/min.

Example 1

1. 11L of deionized water and 20g of a mixed surfactant (hereinafter referred to as dispersant X) described in patent CN 106366230 of this company were charged in a 20L horizontal reactor equipped with a stirrer, and the reactor was evacuated until the oxygen content in the reactor became 30ppm or less.

2. Adding 1.5g of ammonium persulfate, 300g of perfluoro-n-propyl vinyl ether, 150g of perfluoroheptane, perfluorooctane and a mixture of perfluoroheptyl ether into a reaction kettle, wherein the weight ratio of the perfluoroheptane to the perfluorooctane to the perfluoroheptyl ether is 1:1:1, adding 10g of high-purity hydrogen, opening a stirring device, stirring at the rotating speed of 60rpm, and heating the reaction kettle to 40 ℃.

3. Adding a mixed gas of tetrafluoroethylene and hexafluoropropylene (hereinafter referred to as mixed gas) until the gauge pressure of the reaction kettle is 1.6MPa, wherein the mass fraction of hexafluoropropylene is 3%, adding 50ml of a reducing agent (sodium thiosulfate aqueous solution containing 1% mass fraction), starting a polymerization reaction, and maintaining the pressure in the reaction kettle to be constant by supplementing the mixed gas.

4. When the mixed gas is supplemented to 500g, the adding speed of the mixed gas is controlled to ensure that the pressure in the reaction kettle is automatically reduced to 1.0MPa, and the reaction temperature is increased to 75 ℃.

5. When the mixed gas is supplemented to 3000g, stopping stirring, emptying the unreacted monomers, and collecting the polymerization materials to obtain the PFA resin emulsion with the specific gravity of 1.169.

6. The PFA resin emulsion obtained by collection is coagulated, washed, dried, fluorinated and granulated to obtain 2264g of PFA granules, and the analysis and test results are shown in Table 1.

Example 2

1. Adding 11L of deionized water and 20g of dispersing agent X into a 20L horizontal reaction kettle with a stirring device, and evacuating the reaction kettle until the oxygen content in the reaction kettle is less than or equal to 30 ppm.

2. Adding 1.5g of ammonium persulfate, 80g of 2-perfluoropropoxy perfluoropropyl trifluorovinyl ether (PPVE-2), 300g of perfluoro-n-propyl vinyl ether, 150g of perfluoroheptane, perfluorooctane and a mixture of perfluoroheptyl ether into a reaction kettle, wherein the weight ratio of the perfluoroheptane to the perfluorooctane to the perfluoroheptyl ether is 1:1:1, adding 10g of high-purity hydrogen, opening a stirring device, stirring at the rotating speed of 60rpm, and heating the reaction kettle to 40 ℃.

3. Adding the mixed gas until the gauge pressure of the reaction kettle is 1.6MPa, adding 50ml of reducing agent (containing 1% of sodium thiosulfate aqueous solution by mass fraction), starting polymerization reaction, and maintaining the pressure in the reaction kettle to be constant by replenishing the mixed gas.

4. When the mixed gas is supplemented to 500g, the adding speed of the mixed gas is controlled to ensure that the pressure in the reaction kettle is automatically reduced to 1.0MPa, and the reaction temperature is increased to 75 ℃.

5. When the mixed gas is supplemented to 3000g, stopping stirring, emptying the unreacted monomers, and collecting the polymerization materials to obtain the PFA resin emulsion with the specific gravity of 1.167.

6. The PFA resin emulsion obtained by collection is coagulated, washed, dried, fluorinated and granulated to obtain 2199gPFA granules, and the analysis and test results are shown in Table 1.

Example 3

1. Adding 11L of deionized water and 20g of dispersing agent X into a 20L horizontal reaction kettle with a stirring device, and evacuating the reaction kettle until the oxygen content in the reaction kettle is less than or equal to 30 ppm.

2. Adding 1.5g of ammonium persulfate, 300g of perfluoro-n-propyl vinyl ether, 300g of perfluoroheptane, perfluorooctane and a mixture of perfluoroheptyl ether into a reaction kettle, wherein the weight ratio of the perfluoroheptane to the perfluorooctane to the perfluoroheptyl ether is 1:1:1, adding 10g of high-purity hydrogen, opening a stirring device, stirring at the rotating speed of 60rpm, and heating the reaction kettle to 40 ℃.

3. Adding the mixed gas until the gauge pressure of the reaction kettle is 1.6MPa, adding 50ml of reducing agent (containing 1% of sodium thiosulfate aqueous solution by mass fraction), starting polymerization reaction, and maintaining the pressure in the reaction kettle to be constant by replenishing the mixed gas.

4. When the mixed gas is supplemented to 500g, the adding speed of the mixed gas is controlled to ensure that the pressure in the reaction kettle is automatically reduced to 1.0MPa, and the reaction temperature is increased to 75 ℃.

5. When the mixed gas is supplemented to 3000g, stopping stirring, emptying the unreacted monomers, and collecting the polymerization materials to obtain the PFA resin emulsion with the specific gravity of 1.169.

6. The PFA resin emulsion obtained by collection is coagulated, washed, dried, fluorinated and granulated to obtain 2462g of PFA granules, and the analysis and test results are shown in Table 1.

Example 4

1. Adding 11L of deionized water and 20g of dispersing agent X into a 20L horizontal reaction kettle with a stirring device, and evacuating the reaction kettle until the oxygen content in the reaction kettle is less than or equal to 30 ppm.

2. Adding 1.5g of ammonium persulfate, 300g of perfluoro-n-propyl vinyl ether, 100g of perfluoroheptane, perfluorooctane and a mixture of perfluoroheptyl ether into a reaction kettle, wherein the weight ratio of the perfluoroheptane to the perfluorooctane to the perfluoroheptyl ether is 1:1:1, adding 10g of high-purity hydrogen, opening a stirring device, stirring at the rotating speed of 60rpm, and heating the reaction kettle to 40 ℃.

3. Adding the mixed gas until the gauge pressure of the reaction kettle is 1.6MPa, adding 50ml of reducing agent (containing 1% of sodium thiosulfate aqueous solution by mass fraction), starting polymerization reaction, and maintaining the pressure in the reaction kettle to be constant by replenishing the mixed gas.

4. When the mixed gas is supplemented to 500g, the adding speed of the mixed gas is controlled to ensure that the pressure in the reaction kettle is automatically reduced to 1.0MPa, and the reaction temperature is increased to 75 ℃.

5. When the mixed gas is supplemented to 3000g, the stirring is stopped, unreacted monomers are emptied, and the polymerization materials are collected to obtain the PFA resin emulsion with the specific gravity of 1.171.

6. The PFA resin emulsion obtained by collection is coagulated, washed, dried, fluorinated and granulated to obtain 2358g of PFA granules, and the analysis and test results are shown in Table 1.

Example 5

1. Adding 11L of deionized water and 20g of dispersing agent X into a 20L horizontal reaction kettle with a stirring device, and evacuating the reaction kettle until the oxygen content in the reaction kettle is less than or equal to 30 ppm.

2. Adding 1.5g of ammonium persulfate, 300g of perfluoro-n-propyl vinyl ether, 150g of 1, 1, 2-trichloro-1, 2, 2-trifluoroethane into a reaction kettle, adding 10g of high-purity hydrogen, opening a stirring device, wherein the stirring speed is 60rpm, and heating the reaction kettle to 40 ℃.

3. Adding the mixed gas until the gauge pressure of the reaction kettle is 1.6MPa, adding 50ml of reducing agent (containing 1% of sodium thiosulfate aqueous solution by mass fraction), starting polymerization reaction, and maintaining the pressure in the reaction kettle to be constant by replenishing the mixed gas.

4. When the mixed gas is supplemented to 500g, the adding speed of the mixed gas is controlled to ensure that the pressure in the reaction kettle is automatically reduced to 1.0MPa, and the reaction temperature is increased to 75 ℃.

5. When the mixed gas is added to 3000g, the stirring is stopped, unreacted monomers are emptied, and the polymerization materials are collected to obtain the PFA resin emulsion with the specific gravity of 1.166.

6. The collected PFA resin emulsion is subjected to coagulation washing, drying fluorination and granulation to obtain 2392g of PFA granules, and the analysis and test results are shown in Table 1.

Comparative example 1

1. Adding 11L of deionized water and 20g of dispersing agent X into a 20L horizontal reaction kettle with a stirring device, and evacuating the reaction kettle until the oxygen content in the reaction kettle is less than or equal to 30 ppm.

2. Adding a mixture of 300g of perfluoro-n-propyl vinyl ether, 150g of perfluoroheptane, perfluorooctane and perfluoroheptyl ether into a reaction kettle, wherein the weight ratio of the perfluoroheptane to the perfluorooctane to the perfluoroheptyl ether is 1:1:1, adding 10g of high-purity hydrogen, opening a stirring device, stirring at the rotating speed of 60rpm, and heating the reaction kettle to 75 ℃.

3. Adding mixed gas until the gauge pressure of the reaction kettle is 1.6MPa, adding 60ml of initiator (containing 2.5 mass percent of ammonium persulfate aqueous solution), starting polymerization reaction, and maintaining the pressure in the reaction kettle to be constant by replenishing the mixed gas.

4. When the mixed gas is supplemented to 500g, the adding speed of the mixed gas is controlled to lead the pressure in the reaction kettle to automatically drop to 1.0 MPa.

5. When the mixed gas is supplemented to 3000g, stopping stirring, emptying the unreacted monomers, and collecting the polymerization materials to obtain the PFA resin emulsion with the specific gravity of 1.168.

6. The PFA resin emulsion obtained by collection is coagulated, washed, dried, fluorinated and granulated to obtain 2358g of PFA granules, and the analysis and test results are shown in Table 1.

Comparative example 2

1. Adding 11L of deionized water and 20g of dispersing agent X into a 20L horizontal reaction kettle with a stirring device, and evacuating the reaction kettle until the oxygen content in the reaction kettle is less than or equal to 30 ppm.

2. Adding 1.5g of ammonium persulfate, 300g of perfluoro-n-propyl vinyl ether, 150g of perfluoroheptane, perfluorooctane and a mixture of perfluoroheptyl ether into a reaction kettle, wherein the weight ratio of the perfluoroheptane to the perfluorooctane to the perfluoroheptyl ether is 1:1:1, adding 10g of high-purity hydrogen, opening a stirring device, stirring at the rotating speed of 60rpm, and heating the reaction kettle to 40 ℃.

3. Adding the mixed gas until the gauge pressure of the reaction kettle is 1.6MPa, adding 50ml of reducing agent (containing 1% of sodium thiosulfate aqueous solution by mass fraction), starting polymerization reaction, and maintaining the pressure in the reaction kettle to be constant by replenishing the mixed gas.

4. When the mixed gas is supplemented to 500g, the reaction temperature is increased to 75 ℃, the adding speed of the mixed gas is controlled, the monomer concentration in the reaction kettle is kept unchanged, and the pressure in the reaction kettle is 1.78 MPa.

5. When the mixed gas is added to 3000g, the stirring is stopped, unreacted monomers are emptied, and the polymerization materials are collected to obtain the PFA resin emulsion with the specific gravity of 1.166.

6. The PFA resin emulsion obtained was collected, coagulated, washed, dried, fluorinated and pelletized to obtain 2179g of PFA pellets, and the results of the analysis and the measurement are shown in Table 1.

TABLE 1 Properties of PFA resins obtained in examples 1 to 5 and comparative examples 1 and 2

Comparative example 2DSC curve melting peaks with shoulders, indicating the presence of regions of tetrafluoroethylene enrichment in the PFA.

While the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that the invention is not limited thereto, and may be embodied in other forms without departing from the spirit or essential characteristics thereof. Any modification which does not depart from the functional and structural principles of the present invention is intended to be included within the scope of the claims.