A method for producing a low molecular weight polytetrafluoroethylene ultrafine powder

Abstract

The application belongs to the field of fluorine-containing polymer materials, and particularly relates to a preparation method of low-molecular-weight polytetrafluoroethylene superfine powder. The method is to first prepare core-shell structure low-molecular-weight polytetrafluoroethylene resin by using a seed polymerization method, wherein the inner core is low-molecular-weight polytetrafluoroethylene with a melting point of 327-330 DEG C, and the shell layer is modified low-molecular-weight polytetrafluoroethylene with a melting point of 305-315 DEG C; then the core-shell structure low-molecular-weight polytetrafluoroethylene resin is heated and treated at 310-320 DEG C; and then airflow crushing is performed to obtain low-molecular-weight polytetrafluoroethylene superfine powder with an average particle size of 2-4 mu m. The application utilizes the characteristics that low-molecular-weight polytetrafluoroethylene becomes hard and brittle after heat treatment at a temperature higher than the melting point, and through a temperature-controllable heat treatment process, the shell layer is melted, hardened and embrittled, so that the surface is smooth, thereby improving the flowability and dispersibility of the particles, and the core low-molecular-weight polytetrafluoroethylene powder does not change in properties.

Description

Technical Field

[0001] This invention relates to the field of fluorine-containing polymer materials, specifically to a method for preparing low molecular weight polytetrafluoroethylene ultrafine powder. Background Technology

[0002] Low molecular weight polytetrafluoroethylene (PTFE) ultrafine powder has an average particle size of 2-4 μm. Due to its unique helical structure and tight fluorine shell configuration, it possesses an extremely low coefficient of friction, excellent chemical stability, and outstanding thermal stability, making it widely used in fluorochemicals, including lubricants and waterproofing materials. Low molecular weight PTFE ultrafine powder can also be used as an additive to improve the slip properties of coating surfaces or reduce surface sliding resistance, and is widely used in industrial and daily chemical fields such as coatings, inks, plastics, lubricants, and cosmetics.

[0003] Low molecular weight polytetrafluoroethylene (PTFE) ultrafine powder can be prepared by physical pulverization after high molecular weight PTFE undergoing high-temperature thermal pyrolysis or irradiation pyrolysis. However, this process will generate a high content of PFAS in the product (generally, the total amount of PFAS is greater than 1000 ppb; PFAS has the molecular formula C). n F 2n+1 COOH is a small-molecule straight-chain or branched perfluorocarboxylic acid, where n = 8-13, including its salts and any other substances, and the product has a very low specific surface area (generally less than 2m). 2 / g), which is not applicable in the fields of coatings, greases, inks, etc.

[0004] To obtain PFAS-free, high-specific-surface-area polytetrafluoroethylene (PTFE) ultrafine powder, many manufacturers have developed methods for preparing low-molecular-weight PTFE using dispersion polymerization. For example, CN102443086A discloses a method for preparing low-molecular-weight PTFE resin, which includes the following steps: using tetrafluoroethylene (TFE) as raw material and perfluoroalkoxycarboxylate as dispersant, a polymerization reaction is carried out in an aqueous suspension system in the presence of specific initiators and stabilizers to obtain an aqueous emulsion. Then, under specific conditions, coagulation and drying are performed to obtain low-molecular-weight PTFE resin with an average particle size of 5-14 μm. However, the low-molecular-weight PTFE D obtained by this method... 90 Generally, particles larger than 50-100 μm need to be physically pulverized to obtain an average particle size of 2-4 μm. 90 The powder is an ultrafine powder with a particle size distribution of less than 10 μm. However, after pulverization, the ultrafine powder particles are fibrous in shape, easily agglomerated, and have extremely poor flowability and miscibility. It is very difficult to mix evenly in coatings, inks, plastics, and greases. It also has strong electrostatic adsorption, causing it to stick to packaging bags. This results in a large amount of resin residue (around 100-200g) remaining in the bags after use, increasing the manufacturer's product costs in order to ensure that customers receive sufficient resin. Summary of the Invention

[0005] This invention addresses the problems of high PFAS content and poor flowability in existing methods for preparing low molecular weight polytetrafluoroethylene (PTFE) ultrafine powder by providing a method for preparing such powder. The low molecular weight PTFE ultrafine powder prepared by this invention has an average particle size of 2-4 μm, maintains a high specific surface area and is PFAS-free, while exhibiting excellent flowability and miscibility, and significantly reduces electrostatic adsorption.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] A method for preparing low molecular weight polytetrafluoroethylene ultrafine powder includes the following steps:

[0008] (4) A core-shell structured low molecular weight polytetrafluoroethylene resin was obtained by seed polymerization, wherein the mass ratio of the core to the shell was 80:20-95:5, the core was low molecular weight polytetrafluoroethylene with a melting point of 327-330℃, the shell was modified low molecular weight polytetrafluoroethylene with a melting point of 305-315℃, the modifier was perfluoropropyl vinyl ether (PPVE), and the content of the modifier accounted for 0.5%-10% of the mass of the shell polytetrafluoroethylene;

[0009] (5) Heat-treat the core-shell structured low molecular weight polytetrafluoroethylene resin at 310-320℃;

[0010] (6) The polytetrafluoroethylene resin after heat treatment in step (2) is subjected to air jet pulverization to obtain low molecular weight polytetrafluoroethylene ultrafine powder with an average particle size of 2-4 μm.

[0011] The melt index of the low molecular weight polytetrafluoroethylene ultrafine powder obtained by the method of the present invention is 1~3 g / (372℃ / 5kg / 10min).

[0012] Preferably, the shell mass ratio of the low molecular weight polytetrafluoroethylene resin in step (1) is 7-17%; the specific surface area is 8-11 m². 2 / g; average particle size 5-30μm.

[0013] Preferably, the specific method of step (2) is as follows: the low molecular weight polytetrafluoroethylene resin is heated to 310-320℃ in a sintering furnace and held at that temperature for 10-20 minutes, then cooled to 250℃ in the furnace at a rate of 0.5-1℃ / min, and then cooled to room temperature. The heating temperature in step (2) is 310-320℃, which is higher than the melting point of the outer shell polytetrafluoroethylene and lower than the melting point of the inner core polytetrafluoroethylene. Sintering at this temperature can melt, harden, and embrittle the outer shell low molecular weight polytetrafluoroethylene, and the particle surface is smooth, while the inner core polytetrafluoroethylene does not melt and the specific surface area does not change.

[0014] The low molecular weight polytetrafluoroethylene ultrafine powder prepared by the above method can maintain a high specific surface area and have no PFAS content while possessing excellent flowability and blendability, and greatly reducing electrostatic adsorption.

[0015] The present invention also provides a method for preparing the above-mentioned core-shell structured low molecular weight polytetrafluoroethylene resin, comprising the following steps:

[0016] ① Add deionized water, stabilizer, and dispersant to the reactor. Seal the reactor and heat it to 50-55℃, controlling the oxygen content to ≤20ppm. Maintain the temperature inside the reactor, introduce tetrafluoroethylene monomer to increase the pressure to 0.7-1MPa, and then heat it to 70-90℃. Add the initiator persulfate and chain transfer agent to initiate the polymerization reaction. Maintain the constant temperature reaction, keep the stirring speed constant, and continuously introduce tetrafluoroethylene monomer to maintain the pressure inside the reactor constant.

[0017] ② When the amount of TFE consumed in the reaction reaches the required amount, stop the supply of TFE and stirring, keep the temperature inside the reactor constant, drain the TFE monomer and chain transfer agent from the high-pressure reactor, reduce the pressure inside the reactor to 0.05-0.1 MPa, then introduce tetrafluoroethylene monomer to increase the pressure to 1.5-2.5 MPa, and inject the initiator, chain transfer agent and modified monomer into the reactor again, restart the reaction by stirring, and stop the reaction when the specified amount of tetrafluoroethylene monomer is reached; after the reaction is completed, collect the polytetrafluoroethylene dispersion;

[0018] ③ The obtained polytetrafluoroethylene dispersion is coagulated, washed, and dried to obtain low molecular weight polytetrafluoroethylene resin.

[0019] According to the present invention, preferably, the stabilizer in step ① is paraffin wax, and the amount of stabilizer added is 2% to 7% of the mass of deionized water. The stirring speed is 150-200 r / min.

[0020] According to the present invention, preferably, the dispersant in step ① is a fluorinated carboxylic acid and its fluorinated carboxylic acid salt; more preferably, it is one of C3F7OCF2CF2COOH, C4F9OCF2COOH and its ammonium salt and alkali metal salt, and the amount of dispersant added is 0.05-0.2% of the mass of deionized water.

[0021] According to the present invention, preferably, the initiator persulfate in steps ① and ② is ammonium persulfate or potassium persulfate; the amount of persulfate added is 0.0002-0.004% of the mass of deionized water.

[0022] According to the present invention, preferably, the chain transfer agent in steps ① and ② is ethane; the amount of ethane added in step ① is 0.05-5% of the amount of polytetrafluoroethylene reacted in the core layer; the amount of ethane added in step ② is 0.05-5% of the amount of polytetrafluoroethylene reacted in the shell layer.

[0023] According to the present invention, preferably, the modifier monomer in step ② is perfluoropropyl vinyl ether (PPVE); the amount of perfluoropropyl vinyl ether added is 0.5-10% of the amount of polytetrafluoroethylene in the shell.

[0024] According to the present invention, the post-treatment technology (coagulation, washing, and drying) of the low molecular weight polytetrafluoroethylene dispersion resin in step ③ is existing technology.

[0025] The technical features and beneficial effects of this invention are as follows:

[0026] This invention designs the primary particles of low molecular weight polytetrafluoroethylene (PTFE) resin into a core-shell structure. The core is low molecular weight PTFE with a high melting point and high specific surface area (327-330℃), and the shell is low molecular weight modified PTFE with a low melting point (305-315℃). Taking advantage of the characteristic that low molecular weight PTFE becomes hard and brittle after heat treatment at temperatures above its melting point, the shell is melted, hardened, and embrittled through a temperature-controlled heat treatment process, resulting in a smooth surface and improved particle flowability and dispersibility, while the core low molecular weight PTFE powder does not undergo any change in properties.

[0027] During air jet milling, powder particles are more easily broken at the embrittled shell, resulting in regular blocky or spherical particle shapes instead of fibrous formation. This results in powders with excellent flowability and miscibility, preventing clumping and significantly reducing electrostatic adsorption. The final low-molecular-weight polytetrafluoroethylene (PTFE) ultrafine powder maintains a high specific surface area, is PFAS-free, and possesses excellent flowability and miscibility, while also significantly reducing electrostatic adsorption. Attached Figure Description

[0028] Figure 1 This is a scanning electron microscope image of the low molecular weight polytetrafluoroethylene ultrafine powder obtained in Example 1;

[0029] Figure 2 The image shows a scanning electron microscope (SEM) image of the low molecular weight polytetrafluoroethylene (PTFE) ultrafine powder obtained in Comparative Example 1.

[0030] Figure 3 The image shows the DSC diagram of the low molecular weight polytetrafluoroethylene ultrafine powder obtained in Example 1.

[0031] Figure 4 Examples 1 show the electrostatic adsorption test results and macroscopic morphology of the powder.

[0032] Figure 5 The electrostatic adsorption test and macroscopic morphology of the powder are shown in Comparative Example 1. Detailed Implementation

[0033] The present invention will be further described below through specific embodiments, but is not limited thereto.

[0034] In addition, the experimental methods described in the following embodiments are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0035] Test method:

[0036] Melt flow index: The melt flow index is tested according to the ASTM D1238 standard. The melt flow index represents the molecular weight of low molecular weight polytetrafluoroethylene. The larger the melt flow index, the smaller the molecular weight.

[0037] Particle size: The average particle size was measured using a laser diffraction particle size distribution measuring device at a dispersion pressure of 3.5 bar without using cascade. The average particle size is equal to the particle size corresponding to 50% of the cumulative particle size distribution.

[0038] Flowability: The flowability of the powder was determined using the angle of repose method.

[0039] Melting point and shell-to-core mass ratio: Determined using differential scanning calorimetry (DSC). After temperature calibration using indium and lead as standard samples, approximately 3 mg of low molecular weight PTFE powder was placed in an aluminum pan (capped container). The temperature was increased at a rate of 10 °C / min within a 50-380 °C temperature range under a nitrogen flow of 200 ml / min. The point of minimum heat of fusion within this range was taken as the melting point. The shell-to-core mass ratio corresponds to the proportion of the enthalpy values ​​fitted to the two melting point regions (327-330 °C, 305-315 °C).

[0040] Specific surface area: The specific surface area of ​​low molecular weight polytetrafluoroethylene was measured using a NOVA 2200E specific surface area meter.

[0041] Electrostatic adsorption: A certain amount of material is placed in a packaging bag, shaken vigorously for 5 minutes, and then left for 1 hour. The adsorption on the inner wall of the packaging bag is then observed.

[0042] Example 1

[0043] A method for preparing low molecular weight polytetrafluoroethylene ultrafine powder includes the following steps:

[0044] (1) Preparation of core-shell structured low molecular weight polytetrafluoroethylene dispersion

[0045] In the first stage, 6 kg of deionized water, 0.35 kg of paraffin wax, and 6 g of dispersant C3F7OCF2CF2COOH were added to a 10 L reactor. The reactor was sealed and heated to 55 °C, then evacuated and purged with nitrogen to ensure an oxygen content of ≤20 ppm. The stirring speed was 150 r / min, and the reactor temperature was maintained. Tetrafluoroethylene monomer was introduced to increase the pressure to 0.9 MPa, and then the temperature was increased to 70 °C. Subsequently, 3.2 g of 5% ammonium persulfate aqueous solution and 72 g of chain transfer agent ethane were added to initiate the polymerization reaction. The reaction was maintained at a constant temperature and stirring speed, and tetrafluoroethylene monomer was continuously introduced to maintain a constant pressure inside the reactor.

[0046] In the second stage, when the amount of tetrafluoroethylene monomer reacted reached 2.4 kg, the supply of TFE and stirring were stopped. The temperature inside the reactor was kept constant, and the TFE monomer and chain transfer agent in the high-pressure reactor were emptied. The pressure inside the reactor was reduced to 0.05 MPa, and then tetrafluoroethylene monomer was introduced to increase the pressure to 1.8 MPa. 1.6 g of 5% ammonium persulfate aqueous solution, 30 g of ethane, and 18 g of modified monomer PPVE were injected into the reactor again. The reaction was restarted with stirring. The reaction was stopped when the amount of tetrafluoroethylene monomer reacted reached 3 kg. After the reaction was completed, the low molecular weight polytetrafluoroethylene dispersion was collected.

[0047] (2) The obtained low molecular weight polytetrafluoroethylene dispersion was coagulated, washed and dried to obtain low molecular weight polytetrafluoroethylene resin with an average particle size of 17 μm.

[0048] (3) Spread the low molecular weight polytetrafluoroethylene resin in a tray with aluminum foil, then heat it to 315°C in an electric furnace and keep it at that temperature for 15 minutes. Then cool it down to 250°C in the furnace at a rate of 0.5-1°C / min. Finally, remove it from the electric furnace and cool it to room temperature.

[0049] (4) The cooled polytetrafluoroethylene resin is pulverized by air jet to obtain low molecular weight polytetrafluoroethylene ultrafine powder.

[0050] Tests showed that its average particle size was 3 μm, the core melting point was 327.36℃, and the shell melting point was 307.2℃. Figure 3 The shell accounts for 7.29% of the total mass, with a specific surface area of ​​10.5 m². 2 / g, with an angle of repose of 37°, and weak electrostatic adsorption of the powder. The modifier content accounts for 3% of the mass of the shell polytetrafluoroethylene, and the melt index of the low molecular weight polytetrafluoroethylene ultrafine powder is 2.6g (372℃ / 5kg / 10min).

[0051] Scanning electron microscope image of the obtained low molecular weight polytetrafluoroethylene ultrafine powder is shown below. Figure 1As shown in the figure, the low molecular weight polytetrafluoroethylene (PTFE) ultrafine powder prepared in Example 1 has regular particles, good particle dispersibility, and little agglomeration. The electrostatic adsorption test diagram and macroscopic morphology diagram of the obtained low molecular weight PTFE ultrafine powder are shown below. Figure 4 As shown in the figure, it can be seen that the low molecular weight polytetrafluoroethylene ultrafine powder prepared in Example 1 has weak electrostatic adsorption and does not agglomerate in macroscopic morphology.

[0052] Example 2

[0053] A method for preparing low molecular weight polytetrafluoroethylene ultrafine powder includes the following steps:

[0054] (1) Preparation of core-shell structured low molecular weight polytetrafluoroethylene dispersion

[0055] In the first stage, 6 kg of deionized water, 0.3 kg of paraffin wax, and 4 g of dispersant C4F9OCF2COOH were added to a 10 L reactor. The reactor was sealed and heated to 55 °C, then evacuated and purged with nitrogen to ensure an oxygen content of ≤20 ppm. The stirring speed was 180 r / min, and the reactor temperature was maintained. Tetrafluoroethylene monomer was introduced to increase the pressure to 0.7 MPa, and then the temperature was increased to 90 °C. Subsequently, 2.4 g of 5% ammonium persulfate aqueous solution and 114 g of chain transfer agent ethane were added to initiate the polymerization reaction. The reaction was maintained at a constant temperature and stirring speed, and tetrafluoroethylene monomer was continuously introduced to maintain a constant pressure inside the reactor.

[0056] In the second stage, when the amount of tetrafluoroethylene monomer reacted reached 2.85 kg, the supply of TFE and stirring were stopped. The temperature inside the reactor was kept constant, and the TFE monomer and chain transfer agent in the high-pressure reactor were emptied. The pressure inside the reactor was reduced to 0.1 MPa, and then tetrafluoroethylene monomer was introduced to increase the pressure to 2.5 MPa. 1.6 g of 5% ammonium persulfate aqueous solution, 6 g of ethane, and 15 g of modified monomer PPVE were injected into the reactor again. The reaction was restarted with stirring. The reaction was stopped when the amount of tetrafluoroethylene monomer reacted reached 3 kg. After the reaction was completed, the polytetrafluoroethylene dispersion was collected.

[0057] (2) The low molecular weight polytetrafluoroethylene dispersion was coagulated, washed and dried to obtain a low molecular weight polytetrafluoroethylene resin with an average particle size of 21 μm.

[0058] (3) Spread the low molecular weight polytetrafluoroethylene resin in a tray with aluminum foil, then heat it to 320°C in an electric furnace and keep it at that temperature for 15 minutes. Then cool it down to 250°C in the furnace at a rate of 0.5-1°C / min. Finally, remove it from the electric furnace and cool it to room temperature.

[0059] (4) The cooled polytetrafluoroethylene resin is pulverized by air jet to obtain low molecular weight polytetrafluoroethylene ultrafine powder.

[0060] Tests showed that its average particle size was 3 μm, the core layer melting point was 328℃, the shell layer melting point was 313℃, the shell layer accounted for 16.7% of the total mass, and the specific surface area was 9.2 m². 2 / g, with an angle of repose of 34°, indicating weak electrostatic adsorption of the powder.

[0061] The modifier content accounts for 10% of the mass of the shell polytetrafluoroethylene, and the melt index of the low molecular weight polytetrafluoroethylene ultrafine powder is 2.1g (372℃ / 5kg / 10min).

[0062] Comparative Example 1 (shell layer not modified with modified monomers)

[0063] A method for preparing low molecular weight polytetrafluoroethylene ultrafine powder includes the following steps:

[0064] (1) Preparation of low molecular weight polytetrafluoroethylene dispersion

[0065] In the first stage, 6 kg of deionized water, 0.35 kg of paraffin wax, and 6 g of dispersant C3F7OCF2CF2COOH were added to a 10 L reactor. The reactor was sealed and heated to 55 °C, then evacuated and purged with nitrogen to ensure an oxygen content of ≤20 ppm. The stirring speed was 150 r / min, and the reactor temperature was maintained. Tetrafluoroethylene monomer was introduced to increase the pressure to 0.9 MPa, and then the temperature was increased to 70 °C. Subsequently, 3.2 g of 5% ammonium persulfate aqueous solution and 72 g of chain transfer agent ethane were added to initiate the polymerization reaction. The reaction was maintained at a constant temperature and stirring speed, and tetrafluoroethylene monomer was continuously introduced to maintain a constant pressure inside the reactor.

[0066] In the second stage, when the amount of tetrafluoroethylene monomer reacted reached 2.4 kg, the supply of TFE and stirring were stopped. The temperature inside the reactor was kept constant, and the TFE monomer and chain transfer agent in the high-pressure reactor were emptied. The pressure inside the reactor was reduced to 0.05 MPa, and then tetrafluoroethylene monomer was introduced to increase the pressure to 1.8 MPa. 1.6 g of 5% ammonium persulfate aqueous solution and 30 g of ethane were injected into the reactor again, and the reaction was restarted with stirring. The reaction was stopped when 3 kg of tetrafluoroethylene monomer had reacted. After the reaction was completed, the polytetrafluoroethylene dispersion was collected.

[0067] (2) The low molecular weight polytetrafluoroethylene dispersion was coagulated, washed and dried to obtain a low molecular weight polytetrafluoroethylene resin with an average particle size of 17 μm.

[0068] (3) Spread the low molecular weight polytetrafluoroethylene resin in a tray with aluminum foil, then heat it to 315°C in an electric furnace and keep it at that temperature for 15 minutes. Then cool it down to 250°C in the furnace at a rate of 0.5-1°C / min. Finally, remove it from the electric furnace and cool it to room temperature.

[0069] (4) The cooled polytetrafluoroethylene resin was pulverized by air jet milling to obtain low molecular weight polytetrafluoroethylene ultrafine powder. The melt index of the low molecular weight polytetrafluoroethylene ultrafine powder was 2.5 g (372℃ / 5kg / 10min).

[0070] Tests showed that the average particle size was 3 μm, the melting point was 328℃, and the specific surface area was 12.8 m². 2 / g, unable to test the angle of repose of powder, powder agglomeration clogging the testing equipment, powder has strong electrostatic adsorption.

[0071] Scanning electron microscope image of the obtained low molecular weight polytetrafluoroethylene ultrafine powder is shown below. Figure 2 As shown in the figure, it can be seen that the low molecular weight polytetrafluoroethylene (PTFE) ultrafine powder prepared in Comparative Example 1 is flocculent, with severe inter-particle agglomeration and poor particle dispersibility. The electrostatic adsorption test diagram and macroscopic morphology diagram of the obtained low molecular weight PTFE ultrafine powder are shown below. Figure 5 As shown in the figure, it can be seen that the low molecular weight polytetrafluoroethylene ultrafine powder prepared in Comparative Example 1 has strong electrostatic adsorption, leaving a large amount of raw material residue on the packaging bag, and exhibits agglomeration in its macroscopic morphology.

[0072] Comparative Example 2 (without heat treatment)

[0073] A method for preparing low molecular weight polytetrafluoroethylene ultrafine powder includes the following steps:

[0074] (1) Preparation of low molecular weight polytetrafluoroethylene dispersion

[0075] In the first stage, 6 kg of deionized water, 0.35 kg of paraffin wax, and 6 g of dispersant C3F7OCF2CF2COOH were added to a 10 L reactor. The reactor was sealed and heated to 55 °C, then evacuated and purged with nitrogen to ensure an oxygen content of ≤20 ppm. The stirring speed was 150 r / min, and the reactor temperature was maintained. Tetrafluoroethylene monomer was introduced to increase the pressure to 0.9 MPa, and then the temperature was increased to 70 °C. Subsequently, 3.2 g of 5% ammonium persulfate aqueous solution and 72 g of chain transfer agent ethane were added to initiate the polymerization reaction. The reaction was maintained at a constant temperature and stirring speed, and tetrafluoroethylene monomer was continuously introduced to maintain a constant pressure inside the reactor.

[0076] In the second stage, when the amount of tetrafluoroethylene monomer reacted reached 2.4 kg, the supply of TFE and stirring were stopped. The temperature inside the reactor was kept constant, and the TFE monomer and chain transfer agent in the high-pressure reactor were emptied. The pressure inside the reactor was reduced to 0.05 MPa, and then tetrafluoroethylene monomer was introduced to increase the pressure to 1.8 MPa. 1.6 g of 5% ammonium persulfate aqueous solution, 30 g of ethane, and 18 g of modified monomer PPVE were then injected into the reactor again. The reaction was restarted by stirring. The reaction was stopped when the amount of tetrafluoroethylene monomer reacted reached 3 kg. After the reaction was completed, the polytetrafluoroethylene dispersion was collected.

[0077] (2) The low molecular weight polytetrafluoroethylene dispersion was coagulated, washed and dried to obtain a low molecular weight polytetrafluoroethylene resin with an average particle size of 17 μm.

[0078] (3) The cooled polytetrafluoroethylene resin is pulverized by air jet to obtain low molecular weight polytetrafluoroethylene ultrafine powder.

[0079] Tests showed an average particle size of 3 μm, a core melting point of 328℃, a shell melting point of 311℃, a shell mass percentage of 15.3%, and a specific surface area of ​​12.6 m². 2 / g, unable to test powder angle of repose, powder agglomeration clogging the testing equipment, powder has strong electrostatic adsorption. Modifier content accounts for 3% of the shell PTFE mass. The melt index of low molecular weight PTFE ultrafine powder is 2.6g (372℃ / 5kg / 10min).

[0080] Comparative Example 3 (no modified shell, no heat treatment)

[0081] A method for preparing low molecular weight polytetrafluoroethylene ultrafine powder includes the following steps:

[0082] (1) Preparation of low molecular weight polytetrafluoroethylene dispersion

[0083] In the first stage, 6 kg of deionized water, 0.35 kg of paraffin wax, and 6 g of dispersant C3F7OCF2CF2COOH were added to a 10 L reactor. The reactor was sealed and heated to 55 °C, then evacuated and purged with nitrogen to ensure an oxygen content of ≤20 ppm. The stirring speed was 150 r / min, and the reactor temperature was maintained. Tetrafluoroethylene monomer was introduced to increase the pressure to 0.9 MPa, and then the temperature was increased to 70 °C. Subsequently, 3.2 g of 5% ammonium persulfate aqueous solution and 72 g of chain transfer agent ethane were added to initiate the polymerization reaction. The reaction was maintained at a constant temperature and stirring speed, and tetrafluoroethylene monomer was continuously introduced to maintain a constant pressure inside the reactor.

[0084] In the second stage, when the amount of tetrafluoroethylene monomer reacted reached 2.4 kg, the supply of TFE and stirring were stopped. The temperature inside the reactor was kept constant, and the TFE monomer and chain transfer agent in the high-pressure reactor were emptied. The pressure inside the reactor was reduced to 0.05 MPa, and then tetrafluoroethylene monomer was introduced to increase the pressure to 1.8 MPa. 1.6 g of 5% ammonium persulfate aqueous solution and 30 g of ethane were injected into the reactor again, and the reaction was restarted with stirring. The reaction was stopped when 3 kg of tetrafluoroethylene monomer had reacted. After the reaction was completed, the polytetrafluoroethylene dispersion was collected.

[0085] (2) The low molecular weight polytetrafluoroethylene dispersion was coagulated, washed and dried to obtain a low molecular weight polytetrafluoroethylene resin with an average particle size of 17 μm.

[0086] (3) The cooled polytetrafluoroethylene resin was pulverized by air jet milling to obtain low molecular weight polytetrafluoroethylene ultrafine powder. The melt index of the low molecular weight polytetrafluoroethylene ultrafine powder was 2.6 g (372℃ / 5kg / 10min).

[0087] After testing, the average particle size was 3μm, the powder melting point was 328℃, and the specific surface area was 12.7m². 2 / g, unable to test the angle of repose of powder, powder agglomeration clogging the testing equipment, powder has strong electrostatic adsorption.

[0088] Table 1. Properties of low molecular weight polytetrafluoroethylene ultrafine powder obtained in the examples and comparative examples.

[0089]

[0090] From Table 1 and Figure 1 , Figure 2 , Figure 4 , Figure 5 As can be seen, the PTFE ultrafine powder particles obtained by the preparation method of this invention are dispersed in micron-sized spherical or blocky shapes, exhibiting strong flowability, non-agglomeration, and a specific surface area that can reach 9.2 m². 2 / g or above, the specifications of high specific surface area ultrafine powder products on the market are currently between 9 and 11 μm. 2 The concentrations were between / g, while the ultrafine powder particles obtained in comparative examples 1, 2, and 3 were flocculent, with strong electrostatic effects, easy agglomeration, and poor flowability.

Claims

1. A process for the production of low molecular weight polytetrafluoroethylene ultrafine powder, characterized by, The method comprises the following steps: (1) obtaining a core-shell structure low molecular weight polytetrafluoroethylene resin by seed polymerization, wherein the mass ratio of the inner core to the shell layer is 80:20-95:5, the inner core is low molecular weight polytetrafluoroethylene with a melting point of 327-330℃, and the shell layer is modified low molecular weight polytetrafluoroethylene with a melting point of 305-315℃, the modifier is perfluoro-n-propyl vinyl ether, and the modifier content accounts for 0.5%-10% of the mass of the shell layer polytetrafluoroethylene; The preparation method of the core-shell structure low molecular weight polytetrafluoroethylene resin comprises the following steps: ①adding deionized water, a stabilizer and a dispersant into a reaction kettle, heating the reaction kettle to 50-55℃, controlling the oxygen content to be ≤20ppm, maintaining the temperature in the kettle, and then adding tetrafluoroethylene monomer to increase the pressure to 0.7-1MPa and the temperature to 70-90℃, and then adding an initiator persulfate and a chain transfer agent to initiate polymerization; ②when the consumed amount of TFE reaches the required amount, stopping the supply of TFE and stirring, maintaining the temperature in the kettle, emptying the TFE monomer and the chain transfer agent in the high-pressure kettle, reducing the pressure in the reaction kettle to 0.05-0.1MPa, then adding tetrafluoroethylene monomer to increase the pressure to 1.5-2.5MPa, and then adding an initiator, a chain transfer agent and a modified monomer into the reaction kettle again, and starting the reaction again by stirring, and stopping the reaction when the amount of tetrafluoroethylene monomer reaches the specified amount; after the reaction is completed, collecting the polytetrafluoroethylene dispersion; ③obtaining low molecular weight polytetrafluoroethylene resin by coagulation, washing and drying the obtained polytetrafluoroethylene dispersion; (2) heating the core-shell structure low molecular weight polytetrafluoroethylene resin at 310-320℃; (3) obtaining low molecular weight polytetrafluoroethylene ultrafine powder with an average particle size of 2-4μm by airflow crushing the polytetrafluoroethylene resin after the heating treatment in step (2).

2. The method of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder is prepared by the process of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder has a particle size of 0.1 to 0.5 μm. The specific surface area of the low molecular weight polytetrafluoroethylene resin in step (1) is 8-11 m 2 / g; and the average particle diameter of the low molecular weight polytetrafluoroethylene resin is 5-30 μm.

3. The method for preparing low molecular weight polytetrafluoroethylene ultrafine powder according to claim 1, characterized in that, The specific method of step (2) is as follows: heating the core-shell structure low molecular weight polytetrafluoroethylene resin to 310-320℃ in a sintering furnace, maintaining the temperature for 10-20min, reducing the temperature to 250℃ in the furnace at a speed of 0.5-1℃ / min, and then cooling to room temperature.

4. The method of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder is prepared by the process of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder has a particle size of 0.1 to 0.5 μm. The stabilizer in step ① is paraffin wax, and the amount of the stabilizer added is 2%-7% of the mass of the deionized water.

5. The method of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder is prepared by the process of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder has a particle size of 0.1 to 0.5 μm. The dispersant in step ① is fluorine-containing carboxylic acid and its fluorine-containing carboxylic acid salt, and the amount of the dispersant added is 0.05-0.2% of the mass of the deionized water.

6. The method of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder is prepared by the process of claim 1, wherein the low molecular weight polytetrafluoroethylene ultrafine powder has a particle size of 0.1 to 0.5 μm. The initiator persulfate in steps ① and ② is ammonium persulfate or potassium persulfate, and the amount of the initiator added is 0.0002-0.004% of the mass of the deionized water.

7. The method for preparing low molecular weight polytetrafluoroethylene ultrafine powder according to claim 1, characterized in that, The chain transfer agent in steps ① and ② is ethane, and the amount of the ethane added in step ① is 0.05-5% of the reaction amount of the core layer polytetrafluoroethylene, and the amount of the ethane added in step ② is 0.05-5% of the reaction amount of the shell layer polytetrafluoroethylene.

8. The method for preparing low molecular weight polytetrafluoroethylene ultrafine powder according to claim 1, characterized in that, The modified monomer in step ② is perfluoro-n-propyl vinyl ether, and the amount of the perfluoro-n-propyl vinyl ether added is 0.5-10% of the reaction amount of the shell layer polytetrafluoroethylene.

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