Environment-friendly polytetrafluoroethylene and preparation method thereof

Abstract

The invention discloses environment-friendly polytetrafluoroethylene and a preparation method thereof, and relates to the field of fluorine-containing polymers. The method comprises the following steps: adding an emulsifier solution, a stabilizer and deoxidized deionized water into a reaction kettle; introducing nitrogen and vacuumizing until the requirement is met; heating to a specified temperature, introducing a tetrafluoroethylene monomer to a set reaction pressure, adding a potassium persulfate solution to initiate a polymerization reaction, and continuously introducing tetrafluoroethylene during the period to maintain the reaction pressure; and when the solid content reaches 35-45%, stopping the reaction, recovering the unreacted tetrafluoroethylene monomer, and carrying out condensation, washing and drying to obtain the polytetrafluoroethylene resin. According to the method, the emulsifying performance is improved, environment-friendly substitution of PFOA is realized, and the polymerization process is optimized.

Description

Technical Field

[0001] The invention relates to the field of fluorine-containing polymers, in particular to environmentally friendly polytetrafluoroethylene and a preparation method thereof. Background Art

[0002] Polytetrafluoroethylene (PTFE), abbreviated as PTFE, is commonly known as the "King of Plastics". PTFE is a polymer made by polymerizing tetrafluoroethylene as a monomer. It has strong corrosion resistance and high temperature resistance and is a colorless, transparent polymer material. The chemical structure of PTFE is formed by replacing all hydrogen atoms in polyethylene with fluorine atoms.

[0003] PTFE has the following characteristics: extremely high chemical stability, resistant to almost all acids, alkalis and organic solvents; excellent electrical insulation performance, not affected by the environment and frequency; outstanding heat resistance and cold resistance, with a long-term use temperature range of -200℃ to +250℃; self-lubricating, with the smallest friction coefficient among plastics; non-sticky surface, no known solid materials can adhere to its surface; non-flammable, with a limited oxygen index below 90; resistance to atmospheric aging, radiation resistance and low permeability.

[0004] These characteristics of PTFE make it widely used in atomic energy, national defense, aerospace, electronics, electrical, chemical, machinery, instrumentation, construction, textile, metal surface treatment, pharmaceutical, medical, textile, food, metallurgy and smelting industries. It can be processed by compression or extrusion; it can also be made into a water dispersion for coating, impregnation or fiber. The discovery and industrial production of PTFE began in the late 1930s when Roy Plunkett of DuPont in the United States accidentally discovered it when he was studying substitutes for Freon.

[0005] A method for producing low molecular weight polytetrafluoroethylene and powder (patent application number: JP2018020459) provides a method for producing low molecular weight polytetrafluoroethylene that is not prone to producing perfluorocarboxylic acids and salts thereof having 6 to 14 carbon atoms. The method for producing low molecular weight polytetrafluoroethylene is characterized in that it comprises: step (1), adding high molecular weight polytetrafluoroethylene and a mixed gas containing an inert gas and oxygen, wherein the oxygen content is 1 to 10% by volume relative to the total amount of the inert gas and oxygen, into a sealed container; and step (2), irradiating the high molecular weight polytetrafluoroethylene with radiation to obtain a melt viscosity of 1.0×10 2 ~7.0×10 5 Pa·s of low molecular weight polytetrafluoroethylene.

[0006] A method for producing low molecular weight polytetrafluoroethylene resin (Chinese patent application number: CN200610022250.0) adopts an emulsion polymerization method, uses deionized water as a polymerization medium, ammonium perfluorooctanoate as a dispersant, and uses dichloromethane as a chain transfer agent.

[0007] The PTFE fine powder produced by the dispersion polymerization method is generally called "dispersion material". The most widely used fluorinated surfactant in the field of fluorinated polymer production by the dispersion method is perfluorooctanoic acid, which is an artificially synthesized rather than naturally occurring industrial raw material. The molecular structure of ammonium perfluorooctanoate (PFOA) gives it excellent surface properties, but also determines its persistence and accumulation in the environment. Because the PFOA molecule contains perfluorinated groups and the interconnected carbon chains are relatively long, PFOA ultimately has strong thermal and kinetic stability, and it has become one of the most difficult organic pollutants to degrade today.

[0008] On June 14, 2017, the European Union published (EU) 2017 / 1000 in its official gazette, adding a new restriction clause on perfluorooctanoic acid (PFOA) in Annex XVII, Item 68 of the REACH Regulation, and officially included PFOA and its salts and related substances in the REACH Regulation restriction list. The regulation came into effect on July 4, 2017. This restriction clause means that the production and use of perfluorooctanoic acid are subject to comprehensive restrictions. Therefore, the long-term development of fluorinated polymers in the future must achieve green substitution of PFOA. Summary of the invention

[0009] The main purpose of the present invention is to provide an environmentally friendly polytetrafluoroethylene and a preparation method thereof, so as to overcome the deficiencies in the prior art.

[0010] Another object of the present invention is to provide polytetrafluoroethylene prepared by the above method.

[0011] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention includes:

[0012] The embodiment of the present invention provides an environmentally friendly polytetrafluoroethylene and a preparation method thereof, and the operation steps are as follows:

[0013] Step 1: Add 60-100 parts of emulsifier solution, 50-85 parts of stabilizer and 8000-12000 parts of deoxygenated deionized water in a reaction kettle with a stirrer according to the weight ratio, and start the reaction kettle for stirring;

[0014] Step 2: nitrogen is introduced into the reactor, and then vacuum is drawn until the oxygen content in the reactor meets the requirements, and then tetrafluoroethylene monomer is introduced, and then vacuum is drawn until the nitrogen content in the reactor meets the requirements;

[0015] Step 3: The reactor is heated to a specified temperature, tetrafluoroethylene monomer is introduced to the reaction pressure, 200-350 parts of potassium persulfate solution are added to initiate polymerization reaction, and tetrafluoroethylene is continuously introduced during the reaction to maintain the reaction pressure;

[0016] Step 4: When the solid content in the kettle reaches 35-45%, the reaction is stopped, the unreacted tetrafluoroethylene monomer is recovered in the reactor, and the dispersion in the reactor is collected, and polytetrafluoroethylene resin is obtained after condensation, washing and drying.

[0017] Furthermore, the concentration of the emulsifier solution in step 1 is 20-35wt%.

[0018] Furthermore, the emulsifier solution in step 1 is a boron-based emulsifier solution, and its preparation method is:

[0019] Add 0.3-3 parts of potassium allyl trifluoroborate, 10-20 parts of 1H,1H,2H,2H-perfluorodecyl acrylate, 90-120 parts of perfluorooctane sulfonamide and 2-4 parts of ethylenediamine to a reaction container in order by mass, start a stirring device to fully mix the raw materials, and stir at 70-80° C. for 0.5-2 hours;

[0020] Weigh 10-20 parts of sodium 1-pyrrolidine dithiocarboxylate and 0.02-0.2 parts of a photoinitiator and add them into a reaction container, continue stirring and reacting for 1-3 hours, and after the reaction is completed, a boron-based emulsifier solution is obtained.

[0021] Furthermore, the photoinitiator is selected from one or more of 2,4,6-trimethylbenzoyldiphenylphosphine oxide, benzophenone, 1-hydroxycyclohexylbenzophenone, and 2-hydroxy-2-methyl-1-phenylacetone.

[0022] Furthermore, the photo-initiating conditions are: under ultraviolet light irradiation, the wavelength range of the ultraviolet light is 350-400nm, and the irradiation time is 10-20 minutes.

[0023] Furthermore, in the step 1, the stabilizer is paraffin.

[0024] Furthermore, in the step 2, the oxygen content index is not higher than 50 ppm, and the nitrogen content index is not higher than 5%.

[0025] Furthermore, the temperature in step three is specified to be 50-80°C.

[0026] Furthermore, the reaction pressure in step three is 0.8-1.2 MPa.

[0027] Furthermore, the concentration of the potassium persulfate solution is 0.1-1 wt %.

[0028] Emulsifier preparation mechanism:

[0029] 1. Amino-acryl addition reaction mechanism:

[0030] During the preparation process, potassium propenyl trifluoroborate and 1H,1H,2H,2H-perfluorodecyl acrylate react with perfluorooctane sulfonamide to undergo amino-propenyl addition reaction. This reaction involves nucleophilic addition, that is, the amino group in perfluorooctane sulfonamide attacks the double bond in the propenyl group to form a new carbon-nitrogen bond. Due to the special properties of perfluorinated compounds, this addition reaction may have higher stability and reactivity.

[0031] 2. Chemical click reaction mechanism:

[0032] Sodium 1-pyrrolidine dithiocarboxylate continues to undergo a chemical click reaction with the propenyl group. In this reaction, the pyrrolidine group is click-linked with the double bond in the propenyl group to generate an emulsifier containing potassium trifluoroborate, perfluorodecyl and pyrrolidine.

[0033] Technical effects:

[0034] 1. Improve emulsification performance:

[0035] By introducing functional groups such as perfluorodecyl and pyrrolidinyl, the prepared emulsifier has better emulsification effect and stability. These functional groups can enhance the interaction between the emulsifier and the polymer chain, thereby improving the emulsification efficiency and making the tetrafluoroethylene polymerization process more uniform and controllable.

[0036] 2. Environmentally friendly alternative:

[0037] This emulsifier uses perfluorodecyl to replace traditional perfluorooctanoic acid, reducing potential harm to the environment. Perfluorooctanoic acid has attracted attention due to its bioaccumulation and potential toxicity, while perfluorodecyl, as a longer carbon chain compound, may have lower bioavailability and toxicity, and is therefore more in line with environmental protection requirements.

[0038] 3. Optimize the polymerization process:

[0039] Emulsifiers containing potassium trifluoroborate may play a special catalytic or stabilizing role in the polymerization process of tetrafluoroethylene, which helps to optimize the conditions and process of the polymerization reaction. This can not only improve the yield and quality of the polymer, but also reduce production costs and energy consumption, with significant economic benefits. DETAILED DESCRIPTION

[0040] In view of the deficiencies in the prior art, the inventor of this case, after long-term research and extensive practice, was able to propose the technical solution of the present invention. The technical solution, its implementation process and principle, etc. will be further explained as follows.

[0041] Example 1

[0042] An environmentally friendly polytetrafluoroethylene and a preparation method thereof, wherein the operation steps are as follows:

[0043] Step 1: Add 60g of emulsifier solution, 50g of stabilizer and 8000g of deoxygenated deionized water into a reactor with a stirrer, and start stirring the reactor;

[0044] Step 2: nitrogen is introduced into the reactor, and then vacuum is drawn until the oxygen content in the reactor meets the requirements, and then tetrafluoroethylene monomer is introduced, and then vacuum is drawn until the nitrogen content in the reactor meets the requirements;

[0045] Step 3: The reactor is heated to a specified temperature, tetrafluoroethylene monomer is introduced to the reaction pressure, 200 g of potassium persulfate solution is added to initiate the polymerization reaction, and tetrafluoroethylene is continuously introduced during the reaction to maintain the reaction pressure;

[0046] Step 4: When the solid content in the kettle reaches 35%, the reaction is stopped, the unreacted tetrafluoroethylene monomer is recovered in the reactor, and the dispersion in the reactor is collected, and polytetrafluoroethylene resin is obtained after condensation, washing and drying.

[0047] The concentration of the emulsifier solution in step 1 is 20 wt %.

[0048] The emulsifier solution in step 1 is a boron-based emulsifier solution, and its preparation method is as follows:

[0049] 0.3 g of potassium allyl trifluoroborate, 10 g of 1H,1H,2H,2H-perfluorodecyl acrylate, 90 g of perfluorooctane sulfonamide and 2 g of ethylenediamine were added to the reaction container in sequence, the stirring device was turned on to fully mix the raw materials, and stirred at 70° C. for 0.5 hour;

[0050] 10 g of sodium 1-pyrrolidine dithiocarboxylate and 0.02 g of a photoinitiator were weighed and added to a reaction container, and the reaction was continued with stirring for 1 hour. After the reaction was completed, a boron-based emulsifier solution was obtained.

[0051] The photoinitiator is selected from 2,4,6-trimethylbenzoyldiphenylphosphine oxide.

[0052] The photoinitiating conditions are: under ultraviolet light irradiation, the wavelength range of the ultraviolet light is 350nm, and the irradiation time is 10 minutes.

[0053] In the step 1, the stabilizer is paraffin.

[0054] In the step 2, the oxygen content index is not higher than 50 ppm, and the nitrogen content index is not higher than 5%.

[0055] The temperature in step 3 is set to 50°C.

[0056] The reaction pressure in step 3 is 0.8 MPa.

[0057] The concentration of the potassium persulfate solution is 0.1 wt %.

[0058] Example 2

[0059] An environmentally friendly polytetrafluoroethylene and a preparation method thereof, wherein the operation steps are as follows:

[0060] Step 1: Add 75g of emulsifier solution, 65g of stabilizer and 10000g of deoxygenated deionized water into a reactor with a stirrer, and start stirring the reactor;

[0061] Step 2: nitrogen is introduced into the reactor, and then vacuum is drawn until the oxygen content in the reactor meets the requirements, and then tetrafluoroethylene monomer is introduced, and then vacuum is drawn until the nitrogen content in the reactor meets the requirements;

[0062] Step 3: The reactor is heated to a specified temperature, tetrafluoroethylene monomer is introduced to the reaction pressure, 250 g of potassium persulfate solution is added to initiate the polymerization reaction, and tetrafluoroethylene is continuously introduced during the reaction to maintain the reaction pressure;

[0063] Step 4: When the solid content in the kettle reaches 40%, the reaction is stopped, the unreacted tetrafluoroethylene monomer is recovered in the reactor, and the dispersion in the reactor is collected, and polytetrafluoroethylene resin is obtained after condensation, washing and drying.

[0064] The concentration of the emulsifier solution in step 1 is 25 wt %.

[0065] The emulsifier solution in step 1 is a boron-based emulsifier solution, and its preparation method is as follows:

[0066] 1.5 g of potassium allyl trifluoroborate, 13.5 g of 1H,1H,2H,2H-perfluorodecyl acrylate, 100 g of perfluorooctane sulfonamide and 3 g of ethylenediamine were added to the reaction container in sequence, and the stirring device was turned on to fully mix the raw materials, and stirred at 75° C. for 1 hour;

[0067] 15 g of sodium 1-pyrrolidine dithiocarboxylate and 0.09 g of a photoinitiator were weighed and added to a reaction container, and the reaction was continued with stirring for 2 hours. After the reaction was completed, a boron-based emulsifier solution was obtained.

[0068] The photoinitiator is selected from benzophenone.

[0069] The photoinitiating conditions are: under ultraviolet light irradiation, the wavelength range of the ultraviolet light is 350nm, and the irradiation time is 15 minutes.

[0070] In the step 1, the stabilizer is paraffin.

[0071] In the step 2, the oxygen content index is not higher than 50 ppm, and the nitrogen content index is not higher than 5%.

[0072] The temperature in step 3 is set to 60°C.

[0073] The reaction pressure in step 3 is 0.9 MPa.

[0074] The concentration of the potassium persulfate solution is 0.4 wt %.

[0075] Example 3

[0076] An environmentally friendly polytetrafluoroethylene and a preparation method thereof, wherein the operation steps are as follows:

[0077] Step 1: Add 90g of emulsifier solution, 75g of stabilizer and 10000g of deoxygenated deionized water into a reactor with a stirrer, and start stirring the reactor;

[0078] Step 2: nitrogen is introduced into the reactor, and then vacuum is drawn until the oxygen content in the reactor meets the requirements, and then tetrafluoroethylene monomer is introduced, and then vacuum is drawn until the nitrogen content in the reactor meets the requirements;

[0079] Step 3: The reactor is heated to a specified temperature, tetrafluoroethylene monomer is introduced to the reaction pressure, 300 g of potassium persulfate solution is added to initiate the polymerization reaction, and tetrafluoroethylene is continuously introduced during the reaction to maintain the reaction pressure;

[0080] Step 4: When the solid content in the kettle reaches 40%, the reaction is stopped, the unreacted tetrafluoroethylene monomer is recovered in the reactor, and the dispersion in the reactor is collected, and polytetrafluoroethylene resin is obtained after condensation, washing and drying.

[0081] The concentration of the emulsifier solution in step 1 is 30 wt %.

[0082] The emulsifier solution in step 1 is a boron-based emulsifier solution, and its preparation method is as follows:

[0083] 2.4 g of potassium allyl trifluoroborate, 17 g of 1H,1H,2H,2H-perfluorodecyl acrylate, 110 g of perfluorooctane sulfonamide and 3 g of ethylenediamine were added to the reaction container in sequence, and the stirring device was turned on to fully mix the raw materials, and stirred at 75° C. for 1.5 hours;

[0084] 15 g of sodium 1-pyrrolidine dithiocarboxylate and 0.15 g of a photoinitiator were weighed and added to a reaction container, and the reaction was continued with stirring for 2 hours. After the reaction was completed, a boron-based emulsifier solution was obtained.

[0085] The photoinitiator is selected from 1-hydroxycyclohexyl benzophenone.

[0086] The photoinitiating conditions are: under ultraviolet light irradiation, the wavelength range of the ultraviolet light is 400nm, and the irradiation time is 15 minutes.

[0087] In the step 1, the stabilizer is paraffin.

[0088] In the step 2, the oxygen content index is not higher than 50 ppm, and the nitrogen content index is not higher than 5%.

[0089] The temperature in step 3 is set to 70°C.

[0090] The reaction pressure in step 3 is 1.1 MPa.

[0091] The concentration of the potassium persulfate solution is 0.8 wt %.

[0092] Example 4

[0093] An environmentally friendly polytetrafluoroethylene and a preparation method thereof, wherein the operation steps are as follows:

[0094] Step 1: Add 100g of emulsifier solution, 85g of stabilizer and 12000g of deoxygenated deionized water into a reactor with a stirrer, and start stirring the reactor;

[0095] Step 2: nitrogen is introduced into the reactor, and then vacuum is drawn until the oxygen content in the reactor meets the requirements, and then tetrafluoroethylene monomer is introduced, and then vacuum is drawn until the nitrogen content in the reactor meets the requirements;

[0096] Step 3: The reactor is heated to a specified temperature, tetrafluoroethylene monomer is introduced to the reaction pressure, 350 g of potassium persulfate solution is added to initiate the polymerization reaction, and tetrafluoroethylene is continuously introduced during the reaction to maintain the reaction pressure;

[0097] Step 4: When the solid content in the kettle reaches 45%, the reaction is stopped, the unreacted tetrafluoroethylene monomer is recovered in the reactor, and the dispersion in the reactor is collected, and polytetrafluoroethylene resin is obtained after condensation, washing and drying.

[0098] The concentration of the emulsifier solution in step 1 is 35 wt %.

[0099] The emulsifier solution in step 1 is a boron-based emulsifier solution, and its preparation method is as follows:

[0100] 3 g of potassium allyl trifluoroborate, 20 g of 1H,1H,2H,2H-perfluorodecyl acrylate, 120 g of perfluorooctane sulfonamide and 4 g of ethylenediamine were added to the reaction container in sequence, the stirring device was turned on to fully mix the raw materials, and stirred at 80° C. for 2 hours;

[0101] 20 g of sodium 1-pyrrolidine dithiocarboxylate and 0.2 g of a photoinitiator were weighed and added to a reaction container, and the reaction was continued with stirring for 3 hours. After the reaction was completed, a boron-based emulsifier solution was obtained.

[0102] The photoinitiator is selected from 2-hydroxy-2-methyl-1-phenylpropanone.

[0103] The photoinitiating conditions are: under ultraviolet light irradiation, the wavelength range of the ultraviolet light is 400nm, and the irradiation time is 20 minutes.

[0104] In the step 1, the stabilizer is paraffin.

[0105] In the step 2, the oxygen content index is not higher than 50 ppm, and the nitrogen content index is not higher than 5%.

[0106] The temperature in step 3 is set to 80°C.

[0107] The reaction pressure in step 3 is 1.2 MPa.

[0108] The concentration of the potassium persulfate solution is 1 wt %.

[0109] Comparative Example 1

[0110] In this example, potassium allyl trifluoroborate was not added during the preparation of the emulsifier solution, and the remaining steps were the same as those in Example 1, so they will not be described in detail.

[0111] Comparative Example 2

[0112] In this example, sodium 1-pyrrolidine dithiocarboxylate was not added during the preparation of the emulsifier solution, and the remaining steps were consistent with those in Example 1, so they are not described again.

[0113] The polytetrafluoroethylene prepared in the above embodiment was tested with reference to GB7136-86, and the results are as follows:

[0114] Tensile strength MPa Elongation at break % <![CDATA[Relative density g / cm 3 > Average particle size μm Example 1 305 610 2.191 477 Example 2 317 622 2.186 471 Example 3 322 627 2.220 465 Example 4 331 634 2.198 454 Comparative Example 1 288 485 2.188 521 Comparative Example 2 296 512 2.193 509

[0115] It can be seen from the above specific implementation scheme that the method improves the mechanical properties of the resin to a certain extent and improves the dispersion performance of the emulsion.

[0116] It should be understood that the above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, so as to clarify the protection scope. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. An environmentally friendly polytetrafluoroethylene and a preparation method thereof, wherein the operation steps are: Step 1: Add 60-100 parts of emulsifier solution, 50-85 parts of stabilizer and 8000-12000 parts of deoxygenated deionized water according to the weight ratio into a reaction kettle with a stirrer, and start the reaction kettle for stirring; Step 2: nitrogen is introduced into the reactor, and then vacuum is drawn until the oxygen content in the reactor meets the requirements, and then tetrafluoroethylene monomer is introduced, and then vacuum is drawn until the nitrogen content in the reactor meets the requirements; Step 3: The reactor is heated to a specified temperature, tetrafluoroethylene monomer is introduced to the reaction pressure, 200-350 parts of potassium persulfate solution are added to initiate polymerization reaction, and tetrafluoroethylene is continuously introduced during the reaction to maintain the reaction pressure; Step 4: When the solid content in the kettle reaches 35-45%, the reaction is stopped, the unreacted tetrafluoroethylene monomer is recovered in the reactor, and the dispersion in the reactor is collected, and polytetrafluoroethylene resin is obtained after condensation, washing and drying.

2. The environmentally friendly polytetrafluoroethylene and preparation method thereof according to claim 1, characterized in that: The concentration of the emulsifier solution in step 1 is 20-35 wt %.

3. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 1, characterized in that: The emulsifier solution in step 1 is a boron-based emulsifier solution, and its preparation method is as follows: Add 0.3-3 parts of potassium allyl trifluoroborate, 10-20 parts of 1H,1H,2H,2H-perfluorodecyl acrylate, 90-120 parts of perfluorooctane sulfonamide and 2-4 parts of ethylenediamine to a reaction container in order by mass, start a stirring device to fully mix the raw materials, and stir at 70-80° C. for 0.5-2 hours; Weigh 10-20 parts of sodium 1-pyrrolidine dithiocarboxylate and 0.02-0.2 parts of photoinitiator and add them into the reaction container, continue to stir and react for 1-3 hours, and after the reaction is completed, a boron-based emulsifier solution is obtained.

4. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 3, characterized in that: The photoinitiator is selected from one or more of 2,4,6-trimethylbenzoyldiphenylphosphine oxide, benzophenone, 1-hydroxycyclohexylbenzophenone, and 2-hydroxy-2-methyl-1-phenylacetone.

5. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 3, characterized in that: The photoinitiating conditions are: under ultraviolet light irradiation, the wavelength range of the ultraviolet light is 350-400nm, and the irradiation time is 10-20 minutes.

6. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 1, characterized in that: In the step 1, the stabilizer is paraffin.

7. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 1, characterized in that: In the step 2, the oxygen content index is not higher than 50 ppm, and the nitrogen content index is not higher than 5%.

8. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 1, characterized in that: The temperature in step 3 is specified to be 50-80°C.

9. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 1, characterized in that: The reaction pressure in step 3 is 0.8-1.2 MPa.

10. The environmentally friendly polytetrafluoroethylene and the preparation method thereof according to claim 1, characterized in that: The concentration of the potassium persulfate solution is 0.1-1 wt %.