Three-layer co-extrusion multi-layer thermosetting and thermoplastic combined corona-resistant PI film and preparation method thereof

Abstract

The invention discloses a three-layer co-extrusion multi-layer thermosetting and thermoplastic combined corona-resistant PI film and a preparation method thereof, and relates to the technical field of polyimide films. According to the invention, titanium dioxide and aluminum hydroxide are used as inorganic fillers, and layered shielding is formed by using the structural characteristics of aluminum hydroxide structure lamination and additional tight filling of spherical titanium dioxide, so that the movement of charges is hindered; meanwhile, a three-layer film layer structure is utilized, a high-performance thermosetting polyimide film is used as a supporting carrier, and thermoplastic polyimide resin containing aluminum hydroxide and titanium dioxide is used on the upper portion and the lower portion as functional layers to be attached to the surfaces of the thermosetting polyimide film layers, so that the tensile strength and the elongation at break are kept excellent; and the high-temperature resistance and high-voltage breakdown resistance of the film surface are further improved.

Description

Technical Field

[0001] The present invention relates to the technical field of polyimide films, and particularly to a corona-resistant PI film combining multi-layer thermosetting and thermoplastic by three-layer co-extrusion and a preparation method thereof. Background Art

[0002] Polyimide (PI) film is a new type of high-temperature resistant organic polymer film. It has high modulus, low shrinkage, high strength, low water absorption, hydrolysis resistance, radiation resistance, non-toxicity, excellent insulation and heat-oxidation stability. Because it is one of the most expensive film materials in the world at present, it is called the "gold film". It is a key insulating material for high-speed motors and also a key voltage-resistant material for extra-high voltage electronic coils.

[0003] Inorganic nano-fillers play an important role in the performance of PI films. For example, through the interfacial interaction between inorganic nano-fillers such as titanium dioxide and aluminum hydroxide and the PI matrix, the overall voltage breakdown strength of the PI film can be enhanced. However, due to the large specific surface area and surface energy of nano-particles, there is a strong interaction between the particles, which is prone to agglomeration. It is difficult to achieve an ideal nano-scale composite between nano-particles and polymers with high viscosity, affecting the comprehensive performance and product quality of the finally obtained PI film.

[0004] Chinese invention patent CN111087633B discloses a corona-resistant polyimide film and a preparation method thereof. By introducing first inorganic fillers and second inorganic fillers with different particle sizes, the fillers form the maximum packing degree, and then combined with the chemical imidization method, a uniform heat conduction network is established inside the film. This method can improve the dispersion of inorganic fillers in the system to a certain extent and avoid the occurrence of agglomeration or sedimentation of inorganic fillers, but the improvement effect on the performance of the PI film is limited. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: to overcome the deficiencies of the prior art and provide a corona-resistant PI film combining multi-layer thermosetting and thermoplastic by three-layer co-extrusion and a preparation method thereof. Using titanium dioxide and aluminum hydroxide as inorganic fillers, utilizing the structural lamination of aluminum hydroxide and the structural characteristics of close packing of spherical titanium dioxide to form a layered shield to hinder the movement of charges; at the same time, using a three-layer film structure, using a high-performance thermosetting polyimide film as a support carrier, and using thermoplastic polyimide resins containing aluminum hydroxide and titanium dioxide as functional layers on the upper and lower surfaces attached to the surface of the thermosetting polyimide film layer, to further improve the high-temperature resistance and high-voltage breakdown performance of the film surface while maintaining excellent tensile strength and elongation at break.

[0006] The technical solution of the present invention is as follows: On the one hand, the present invention provides a method for preparing a corona-resistant PI film with a combination of three-layer co-extruded multi-layer thermosetting and thermoplastic, comprising the following steps: S1 Stir and mix 1200 - 1400 parts by weight of dimethylformamide, 50 - 70 parts by weight of 4,4-diaminodiphenyl ether, and 75 - 100 parts by weight of pyromellitic dianhydride; S2 Add 15 - 20 parts by weight of p-phenylenediamine to the material obtained in step S1, stir and mix until the designed viscosity is reached to obtain mixture one; S3 Stir and mix 1200 - 1400 parts by weight of dimethylformamide, 75 - 100 parts by weight of bisphenol A diether dianhydride, and 15 - 20 parts by weight of m-phenylenediamine; S4 Then add 3 - 6 parts by weight of aluminum hydroxide, 0.5 - 1.5 parts by weight of titanium dioxide, and 50 - 100 parts by weight of dimethylformamide to the material obtained in step S3, stir and mix until the designed viscosity is reached to obtain mixture two; S5 After mixing mixture one with 750 - 1000 parts by weight of acetic anhydride, then mix with 20 - 25 parts by weight of pyridine to obtain mixture three; S6 After mixing mixture two with 1000 - 1250 parts by weight of acetic anhydride, then mix with 18 - 22 parts by weight of isoquinoline to obtain mixture four; S7 Synchronously extrude mixture three and mixture four through a three-layer slit co-extrusion die head, coat them onto the annular mirror steel belt of a casting machine to obtain a film. The middle of the film is a thermosetting polyimide film formed by mixture three, and the two sides are thermoplastic polyimide films formed by mixture four; then imidize the film and wind it up to obtain the corona-resistant PI film with a combination of three-layer co-extruded multi-layer thermosetting and thermoplastic.

[0007] In the present invention, acetic anhydride is a dehydrating agent, and pyridine and isoquinoline are catalysts. In front of the die head above the casting machine, there is a needle bar mixer, which works under the low-temperature condition of -5 to 0 °C. The low temperature can ensure that only mixing occurs between mixture one, mixture two and the catalyst without catalytic reaction; in the needle bar mixer, after mixture one, mixture two are mixed evenly with pyridine, acetic anhydride or isoquinoline, they are extruded through the die head and reach the surface of the steel belt to form a glue layer. The steel belt carries the glue layer into the inner chamber space of the casting machine. After being heated, pyridine or isoquinoline catalyzes the imidization reaction of mixture one and mixture two. The imidization reaction is a dehydration reaction. If the temperature is low, the dehydration process is slow. Therefore, it is necessary to accelerate the dehydration process. In the present invention, acetic anhydride is added to accelerate the removal of water, so that sufficient imidization can be carried out at low temperature, and the imidization completion degree is ≥95%.

[0008] Preferably, the specific operation of step S1 is as follows: First, put dimethylformamide into the reactor and stir for 5 - 10 min at a temperature of 8 - 10 °C and a stirring speed of 10 - 20 r / min; then add 4,4'-diaminodiphenyl ether and pyromellitic dianhydride and stir together for 20 - 40 min at a temperature of 8 - 15 °C and a stirring speed of 20 - 50 r / min.

[0009] Preferably, in step S2, the stirring time of p-phenylenediamine is 30 - 60 min, the temperature is 8 - 15 °C, and the stirring speed is 30 - 40 r / min.

[0010] Preferably, the specific operation of step S3 is as follows: First, put dimethylformamide into the reactor and stir for 5 - 10 min at a temperature of 8 - 10 °C and a stirring speed of 10 - 20 r / min; then add bisphenol A diether dianhydride and m-phenylenediamine and stir together for 20 - 40 min at a temperature of 8 - 15 °C and a stirring speed of 20 - 50 r / min.

[0011] Preferably, in step S4, the average particle size of titanium dioxide is 0.5 - 1 μm, and the average particle size of aluminum hydroxide is 200 - 500 nm.

[0012] Preferably, in step S4, the stirring time is 60 - 130 min, the temperature is 8 - 15 °C, and the stirring speed is 40 - 50 r / min.

[0013] Preferably, during the stirring process in steps S1 - S4, defoaming treatment is carried out.

[0014] Preferably, the specific operations of steps S5, S6, and S7 are as follows: Transport mixture one and mixture two to the storage tanks of the corresponding casting nozzles respectively, control the temperature to remain at 0 - 5 °C, and then transport mixture one and mixture two in the storage tanks to the corresponding die head pin bar mixers respectively. Among them, mixture one is mixed with acetic anhydride and pyridine, and mixture two is mixed with acetic anhydride and isoquinoline. The mixing temperature is -5~0 °C. This mixing temperature can ensure that mixture one and mixture two only mix with the catalysts pyridine and isoquinoline without undergoing catalytic reactions. If the temperature is too high, then catalytic and dehydration reactions will occur during mixing. Water will be generated in mixture one and mixture two in the die head and turn into a coexistence of imidized film and water after flowing onto the steel belt, and the product quality cannot be guaranteed. Subsequently, it is extruded through the three-layer coextrusion die lip of the casting machine, uniformly coated on the annular mirror steel belt, dried in the casting machine by the annular steel belt, and after obtaining the film, it is introduced into the stretching machine furnace for imidization into an integral film, and then wound up to obtain a three-layer coextruded multilayer heat-resistant corona PI film combining thermosetting and thermoplastic properties.

[0015] Preferably, in step S7, the adhesive film enters the stretching furnace and successively passes through the first temperature zone, the second temperature zone, the third temperature zone, the fourth temperature zone, the fifth temperature zone, and the sixth temperature zone. The heating temperatures of each temperature zone are 200 - 220 °C, 250 - 300 °C, 300 - 350 °C, 400 - 450 °C, 500 - 550 °C, and 450 - 500 °C in sequence, and the heating time of each temperature zone is 10 - 20 s.

[0016] On the other hand, the present invention provides a corona-resistant PI film combining multilayer thermosetting and thermoplastic through three-layer coextrusion, which is prepared by the preparation method of the corona-resistant PI film combining multilayer thermosetting and thermoplastic through three-layer coextrusion described above.

[0017] Compared with the prior art, the present invention has the following beneficial effects: In the process of synthesizing the thermoplastic polyamic acid solution of the present invention, titanium dioxide and aluminum hydroxide inorganic fillers are added to improve the corona resistance of the film. Among them, aluminum hydroxide has strong charge conductivity, which can effectively alleviate the charge accumulation under partial discharge and make the charge dissipate quickly; titanium dioxide has strong heat resistance and heat transfer performance, which can effectively alleviate the heat accumulation under partial discharge and improve the high-voltage breakdown resistance. At the same time, the present invention uses a three-layer film structure, using a high-performance thermosetting polyimide film as a support carrier, and using thermoplastic polyimide resins containing aluminum hydroxide and titanium dioxide inorganic fillers as functional layers on the upper and lower layers, so that the thermoplastic polyimide can adhere to the surface of the thermosetting polyimide film layer. While maintaining the excellent tensile strength and elongation at break of the middle thermosetting polyimide film, the high-temperature resistance and high-voltage breakdown resistance of the film surface are further improved through the upper and lower layers of corona-resistant and high-temperature-resistant thermoplastic polyimide resins. Detailed Embodiments

[0018] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0019] Example 1 The preparation method of the corona-resistant PI film combining multilayer thermosetting and thermoplastic through three-layer coextrusion in this example includes the following steps: S1 Prepare a reactor with nitrogen protection, put 1400 parts by weight of dimethylformamide into the reactor, and turn on the stirring device of the reactor to pre-stir the dimethylformamide for 10 min at a temperature of 8 °C and a stirring speed of 20 r / min; then add 70 parts by weight of 4,4-diaminodiphenyl ether, and add 100 parts by weight of pyromellitic dianhydride in six portions, and continue to stir and mix for 40 min at a temperature of 8 °C and a stirring speed of 50 r / min. During the stirring process, nitrogen is continuously introduced for defoaming treatment; Add 20 parts by weight of p-phenylenediamine to the material obtained in step S1, while adding and stirring and mixing. The stirring time is 60 min, the temperature is maintained at 8 °C, the stirring speed is 40 r / min, and nitrogen is continuously introduced during the stirring process. Control the viscosity at 300,000 mPa·s (45 °C) to obtain mixture one; Prepare a reactor with nitrogen protection, put 1350 parts by weight of dimethylformamide into the reactor, and turn on the stirring equipment of the reactor to pre-stir the dimethylformamide. The stirring time is 8 min, the temperature is controlled at 8 °C, and the stirring speed is 20 r / min; then add 90 parts by weight of bisphenol A diether dianhydride and 18 parts by weight of m-phenylenediamine, and continue to stir and mix for 30 min. The temperature is controlled at 8 °C, the stirring speed is 40 r / min, and nitrogen is continuously introduced during the stirring process; Mix 100 parts by weight of dimethylformamide, 1.5 parts by weight of titanium dioxide (average particle size of 1 μm), and 6 parts by weight of aluminum hydroxide (average particle size of 200 nm) evenly to obtain a premixed inorganic material; then add the premixed inorganic material to the material obtained in step S3, stir for 130 min, the temperature is controlled at 8 °C, the stirring speed is 50 r / min, and nitrogen is continuously introduced during the stirring process. Control the viscosity at 700,000 mPa·s (45 °C) to obtain mixture two; Transport mixture one and mixture two to the storage tanks of the corresponding casting nozzles respectively, control the system temperature to remain at 0 °C, and then transport mixture one and mixture two in the storage tanks to the corresponding die head pin bar mixers respectively. Among them, mixture one is mixed with 900 parts by weight of acetic anhydride at -5 °C and then mixed with 24 parts by weight of pyridine to obtain mixture three; mixture two is mixed with 1200 parts by weight of acetic anhydride at -5 °C and then mixed with 20 parts by weight of isoquinoline to obtain mixture four; Mixture three and mixture four are extruded through the three-layer coextrusion die lip of the casting machine, evenly coated on the annular mirror steel belt, and then sent into the drying channel (the annular mirror steel belt in the casting machine is 30 m in total) for drying. The film speed is 18 m / min, the drying temperature of the first 15 m drying channel is 165 °C, and the drying temperature of the last 15 m drying channel is 145 °C, so that the temperature of the film gradually rises during drying and the solvent gradually volatilizes to form a gel film with a solid content of 70 wt.%; Introduce the gel film into the stretching machine furnace at a speed of 18 m / min, and successively pass through the first temperature zone, the second temperature zone, the third temperature zone, the fourth temperature zone, the fifth temperature zone and the sixth temperature zone. The heating temperatures of each temperature zone are 220 °C, 300 °C, 350 °C, 450 °C, 550 °C and 500 °C in turn, and the heating time of each temperature zone is 10 s, and then wind up to obtain a three-layer coextruded multilayer heat-resistant corona PI film combining thermosetting and thermoplasticity.

[0020] The PI film prepared in this embodiment has a three-layer structure, with a thermosetting polyimide film in the middle and thermoplastic polyimide films on both sides.

[0021] Comparative Example 1 The difference from Example 1 is that in step S4, the addition amount of titanium dioxide is 2.5 parts by weight, and the addition amount of aluminum hydroxide is 10 parts by weight.

[0022] Comparative Example 2 The difference from Example 1 is that in step S4, the addition amount of titanium dioxide is 7.5 parts by weight, and aluminum hydroxide is not added.

[0023] Example 2 The preparation method of the corona-resistant PI film with a three-layer co-extruded multi-layer combination of thermosetting and thermoplastic in this embodiment includes the following steps: S1 Prepare a reactor with nitrogen protection, put 1280 parts by weight of dimethylformamide into the reactor, and turn on the stirring equipment of the reactor to pre-stir the dimethylformamide for 8 minutes at a temperature controlled at 9 °C and a stirring speed of 15 r / min; then add 65 parts by weight of 4,4-diaminodiphenyl ether, and add 80 parts by weight of pyromellitic dianhydride in six portions, and continue to stir and mix for 30 minutes at a temperature controlled at 10 °C and a stirring speed of 40 r / min. During the stirring process, nitrogen is continuously introduced for defoaming treatment; S2 Add 17 parts by weight of p-phenylenediamine to the material obtained in step S1 while stirring and mixing. The stirring time is 45 minutes, the temperature is maintained at 10 °C, and the stirring speed is 35 r / min. Nitrogen is continuously introduced during the stirring process to control the viscosity at 260,000 mPa·s (45 °C) to obtain mixture one; S3 Prepare a reactor with nitrogen protection, put 1400 parts by weight of dimethylformamide into the reactor, and turn on the stirring equipment of the reactor to pre-stir the dimethylformamide for 10 minutes at a temperature controlled at 9 °C and a stirring speed of 15 r / min; then add 100 parts by weight of bisphenol A diether dianhydride and 20 parts by weight of m-phenylenediamine, and continue to stir and mix for 40 minutes at a temperature controlled at 10 °C and a stirring speed of 50 r / min. Nitrogen is continuously introduced during the stirring process; S4 Stir and mix 80 parts by weight of dimethylformamide, 1.1 parts by weight of titanium dioxide (average particle size of 0.8 μm), and 5 parts by weight of aluminum hydroxide (average particle size of 300 nm) evenly to obtain a premixed inorganic material; then add the premixed inorganic material to the material obtained in step S3, stir for 100 minutes at a temperature controlled at 10 °C and a stirring speed of 45 r / min. Nitrogen is continuously introduced during the stirring process to control the viscosity at 650,000 mPa·s (45 °C) to obtain mixture two; S5 transports mixture one and mixture two to the storage tanks of the corresponding casting nozzles respectively, controls the system temperature to be maintained at 2 °C, and then transports mixture one and mixture two in the storage tanks to the corresponding die head needle bar mixers respectively. Among them, mixture one is mixed with 1000 parts by weight of acetic anhydride at -3 °C and then mixed with 25 parts by weight of pyridine to obtain mixture three; mixture two is mixed with 1250 parts by weight of acetic anhydride at -3 °C and then mixed with 22 parts by weight of isoquinoline to obtain mixture four; subsequently, mixture three and mixture four are extruded at the three-layer coextrusion die lip of the casting machine, evenly coated on the annular mirror steel belt, and then sent into the drying channel (the annular mirror steel belt in the casting machine is 30 m in total) for drying. The film speed is 18 m / min. The drying temperature of the first 15 m of the drying channel is 165 °C, and the drying temperature of the last 15 m of the drying channel is 145 °C, so that the temperature of the film gradually rises during drying and the solvent gradually volatilizes to form a gelatinous film with a solid content of 72 wt.%. S6 introduces the gelatinous film into the stretching machine furnace at a speed of 18 m / min, successively passes through the first temperature zone, the second temperature zone, the third temperature zone, the fourth temperature zone, the fifth temperature zone and the sixth temperature zone. The heating temperatures of each temperature zone are 210 °C, 275 °C, 325 °C, 425 °C, 525 °C and 480 °C in sequence, and the heating time of each temperature zone is 15 s, and then it is wound up to obtain a three-layer coextruded multilayer corona-resistant PI film combining thermosetting and thermoplasticity.

[0024] The PI film prepared in this example has a three-layer structure, with a thermosetting polyimide film in the middle and thermoplastic polyimide films on both sides.

[0025] Comparative Example 3 The difference from Example 2 is that in step S4, the addition amount of titanium dioxide is 1.8 parts by weight, and the addition amount of aluminum hydroxide is 8 parts by weight.

[0026] Comparative Example 4 The difference from Example 2 is that in step S4, the addition amount of aluminum hydroxide is 6.1 parts by weight, and titanium dioxide is not added.

[0027] Example 3 The preparation method of the three-layer coextruded multilayer corona-resistant PI film combining thermosetting and thermoplasticity in this example includes the following steps: S1 Prepare a reactor with nitrogen protection, put 1200 parts by weight of dimethylformamide into the reactor, turn on the stirring equipment of the reactor, pre-stir the dimethylformamide for 5 min at a temperature controlled at 10 °C and a stirring speed of 10 r / min; then add 50 parts by weight of 4,4'-diaminodiphenyl ether, and add 75 parts by weight of pyromellitic dianhydride in six portions, continue to stir and mix for 20 min at a temperature controlled at 15 °C and a stirring speed of 20 r / min, and continuously introduce nitrogen during the stirring process for defoaming treatment; S2 Add 15 parts by weight of p-phenylenediamine to the material obtained in step S1, add and stir and mix at the same time, the stirring time is 30 min, the temperature is maintained at 15 °C, the stirring speed is 30 r / min, and nitrogen is continuously introduced during the stirring process to control the viscosity at 250,000 mPa·s (45 °C) to obtain mixture one; S3 Prepare a reactor with nitrogen protection, put 1200 parts by weight of dimethylformamide into the reactor, turn on the stirring equipment of the reactor, pre-stir the dimethylformamide for 5 min at a temperature controlled at 10 °C and a stirring speed of 10 r / min; then add 75 parts by weight of bisphenol A diether dianhydride and 15 parts by weight of m-phenylenediamine, continue to stir and mix for 20 min at a temperature controlled at 15 °C and a stirring speed of 20 r / min, and continuously introduce nitrogen during the stirring process; S4 Stir and mix 50 parts by weight of dimethylformamide, 0.5 part by weight of titanium dioxide (average particle size is 0.5 μm), and 3 parts by weight of aluminum hydroxide (average particle size is 500 nm) evenly to obtain a premixed inorganic material; then add the premixed inorganic material to the material obtained in step S3, stir for 60 min at a temperature controlled at 15 °C and a stirring speed of 40 r / min, and continuously introduce nitrogen during the stirring process to control the viscosity at 180,000 mPa·s (45 °C) to obtain mixture two; S5 conveys Mixture 1 and Mixture 2 to the storage tanks of the corresponding casting nozzles respectively, controls the system temperature to remain at 5°C, and then conveys Mixture 1 and Mixture 2 in the storage tanks to the corresponding die head needle bar mixers respectively. Among them, Mixture 1 is mixed with 750 parts by weight of acetic anhydride at 0°C and then mixed with 20 parts by weight of pyridine to obtain Mixture 3; Mixture 2 is mixed with 1000 parts by weight of acetic anhydride at 0°C and then mixed with 18 parts by weight of isoquinoline to obtain Mixture 4; Subsequently, Mixture 3 and Mixture 4 are extruded at the three-layer coextrusion die lip of the casting machine, uniformly coated on the annular mirror steel belt, and then sent into the drying channel (the annular mirror steel belt in the casting machine is 30 m in total) for drying. The film speed is 18 m / min, the drying temperature of the first 15 m drying channel is 135°C, and the drying temperature of the last 15 m drying channel is 145°C, so that the temperature of the film gradually increases during drying and the solvent gradually volatilizes to form a gel film with a solid content of 70 wt.%. S6 introduces the gel film into the stretching machine furnace at a speed of 18 m / min, successively passes through the first temperature zone, the second temperature zone, the third temperature zone, the fourth temperature zone, the fifth temperature zone and the sixth temperature zone. The heating temperatures of each temperature zone are 200°C, 250°C, 300°C, 400°C, 500°C and 450°C in sequence, and the heating time of each temperature zone is 20 s, and then it is wound up to obtain a three-layer coextruded multilayer corona-resistant PI film combining thermosetting and thermoplastic properties.

[0028] The PI film prepared in this example has a three-layer structure, with a thermosetting polyimide film in the middle and thermoplastic polyimide films on both sides.

[0029] Comparative Example 5 The difference from Example 3 is that in step S4, the addition amount of titanium dioxide is 3.5 parts by weight, and the addition amount of aluminum hydroxide is 10 parts by weight.

[0030] Comparative Example 6 The difference from Example 3 is that in step S4, the addition amount of aluminum hydroxide is 3.5 parts by weight, and titanium dioxide is not added.

[0031] Comparative Example 7 The preparation method of the PI film of Comparative Example 7 includes the following steps: Steps S1 and S2 are the same as those in Example 3; S3 stirs and mixes 50 parts by weight of dimethylformamide, 1.5 parts by weight of titanium dioxide (average particle size is 1 μm), and 6 parts by weight of aluminum hydroxide (average particle size is 300 nm) evenly to obtain a premixed inorganic material; S4 adds the premixed inorganic material to Mixture 1, stirs for 60 min, controls the temperature at 15°C, the stirring speed is 40 r / min, and nitrogen is continuously introduced during the stirring process to control the viscosity at 180,000 mPa·s (45°C) to obtain Mixture 2; S5 transports the second mixture to the storage tank of the corresponding drooling nozzle, controls the system temperature to remain at 5°C, and then transports the second mixture in the storage tank to the corresponding die needle bar mixer. The second mixture is mixed with 1000 parts by weight of acetic anhydride at 0°C and then mixed with 18 parts by weight of isoquinoline to obtain the third mixture. Subsequently, the third mixture is extruded through the middle layer at the three-layer coextrusion die lip of the drooling machine, uniformly coated on the annular mirror steel belt, and then sent into the drying tunnel (the annular mirror steel belt in the drooling machine is 30 m in total) for drying. The film speed is 18 m / min. The drying temperature of the first 15 m of the drying tunnel is 135°C, and the drying temperature of the last 15 m of the drying tunnel is 145°C, so that the temperature of the film gradually increases during drying and the solvent gradually volatilizes to form a gel film with a solid content of 70 wt.%. S6 Introduce the gel film into the stretching machine furnace at a speed of 18 m / min, and successively pass through the first temperature zone, the second temperature zone, the third temperature zone, the fourth temperature zone, the fifth temperature zone, and the sixth temperature zone. The heating temperatures of each temperature zone are 210°C, 275°C, 325°C, 425°C, 525°C, and 480°C in sequence, and the heating time of each temperature zone is 15 s, and then wind up to obtain the PI film.

[0032] The PI film prepared in Comparative Example 7 is only a single-layer thermosetting polyimide film structure.

[0033] According to the regulations of "GB / T 13542.2-2021 Electrical insulating films - Part 2: Test methods", the thickness, tensile fracture resistance, corona resistance time, and thermal conductivity of the PI films prepared in Examples 1-3 and Comparative Examples 1-7 are tested. The test conditions for the corona resistance time are: experimental voltage 2 kV, experimental frequency 20 kHz, pulse rise time 50 ns, bipolar pulse square wave, 6 mm cylindrical electrode. The test results are shown in Table 1: Table 1 Performance test results of the PI films prepared in Examples 1-3 and Comparative Examples 1-7

[0034] It can be seen from Examples 1-3 and Comparative Examples 1, 3, and 5 in Table 1 that after excessive input of inorganic fillers, titanium dioxide accumulates, hindering the extension length of molecular chains, and nano-scale aluminum hydroxide agglomerates, resulting in poor local corona resistance. And when the inorganic fillers are added too much, the physical properties of the film will also decline, resulting in abnormal decreases in tensile strength and elongation at break, indicating that when the inorganic fillers are added too much, they will hinder the growth of the length of imidized molecular chains in the gel film. This shows that the addition amount of inorganic fillers is a key factor sufficient to affect the physical properties of the film.

[0035] As can be seen from Examples 1-3 and Comparative Examples 2, 4, and 6 in Table 1, compared with adding only a single inorganic filler such as titanium dioxide or aluminum hydroxide, under the same film thickness, in Examples 1-3 of the present invention, aluminum hydroxide and titanium dioxide are simultaneously added as inorganic fillers, and the corona resistance time and thermal conductivity of the prepared PI film are both greatly improved. It shows that aluminum hydroxide and titanium dioxide synergistically form a good charge barrier and can effectively alleviate the aggregation of charges and heat.

[0036] As can be seen from Example 3 and Comparative Example 7 in Table 1, the advantage of the three-layer co-extruded product is the "son-mother-son" relationship. The inorganic filler in the "son" exists as an isomer within the adhesive film. Such isomers will affect the length of the molecular chain during the imidization of the adhesive film. A short molecular chain will lead to a decrease in tensile strength and elongation at break, and the brittleness of the film will increase. The three-layer co-extrusion uses a thermosetting polyimide film without isomers in the middle layer to complete 100% imidization, improving the overall physical properties. The upper and lower two functional thermoplastic polyimide films are carried by the high tensile strength and elongation at break. After fusion at high temperature, the overall dimensional stability is maintained. The film structure of Comparative Example 7 is equivalent to a single "son" layer. Due to the presence of inorganic fillers in the isomers in such a "son" layer, after being pulled and deformed and becoming brittle, cracks and voids also appear in the functional layer formed by the inorganic fillers, and high-voltage current will penetrate through the gaps, resulting in a decrease in corona resistance performance.

Claims

1. Preparation method of corona-resistant PI film combining multi-layer thermosetting and thermoplastic by three-layer co-extrusion, characterized in that, It includes the following steps: S1 Stir and mix 1200 - 1400 parts by weight of dimethylformamide, 50 - 70 parts by weight of 4,4 - diaminodiphenyl ether, and 75 - 100 parts by weight of pyromellitic dianhydride; S2 Add 15 - 20 parts by weight of p - phenylenediamine to the material obtained in step S1, stir and mix until the designed viscosity is reached to obtain mixture one; S3 Stir and mix 1200 - 1400 parts by weight of dimethylformamide, 75 - 100 parts by weight of bisphenol A diether dianhydride, and 15 - 20 parts by weight of m - phenylenediamine; S4 Then add 3 - 6 parts by weight of aluminum hydroxide, 0.5 - 1.5 parts by weight of titanium dioxide, and 50 - 100 parts by weight of dimethylformamide to the material obtained in step S3, stir and mix until the designed viscosity is reached to obtain mixture two; S5 After mixing mixture one with 750 - 1000 parts by weight of acetic anhydride, then mix it with 20 - 25 parts by weight of pyridine to obtain mixture three; S6 After mixing mixture two with 1000 - 1250 parts by weight of acetic anhydride, then mix it with 18 - 22 parts by weight of isoquinoline to obtain mixture four; S7 Co - extrude mixture three and mixture four synchronously through a three - layer slit die and coat them on the annular mirror - surface steel belt of a casting machine to obtain a film. The middle of the film is a thermosetting polyimide film, and the two sides are thermoplastic polyimide films; then imidize the film and wind it up to obtain a corona - resistant PI film with a three - layer co - extrusion structure combining thermosetting and thermoplastic properties.

2. The preparation method of the corona-resistant PI film with three-layer co-extrusion of multi-layer thermosetting and thermoplastic combination according to claim 1, characterized in that The specific operation of step S1 is as follows: First, put dimethylformamide into the reactor and stir for 5 - 10 min at a temperature of 8 - 10 °C and a stirring speed of 10 - 20 r / min; then add 4,4 - diaminodiphenyl ether and pyromellitic dianhydride and stir together for 20 - 40 min at a temperature of 8 - 15 °C and a stirring speed of 20 - 50 r / min.

3. The preparation method of the corona-resistant PI film with a combination of thermosetting and thermoplastic properties by three-layer coextrusion as described in claim 1, characterized in that, In step S2, the stirring time of p - phenylenediamine is 30 - 60 min, the temperature is 8 - 15 °C, and the stirring speed is 30 - 40 r / min.

4. The preparation method of the corona-resistant PI film with a combination of thermosetting and thermoplastic properties by three-layer co-extrusion as described in claim 1, characterized in that, The specific operation of step S3 is as follows: First, put dimethylformamide into the reactor and stir for 5 - 10 min at a temperature of 8 - 10 °C and a stirring speed of 10 - 20 r / min; then add bisphenol A diether dianhydride and m - phenylenediamine and stir together for 20 - 40 min at a temperature of 8 - 15 °C and a stirring speed of 20 - 50 r / min.

5. The preparation method of the corona-resistant PI film with three-layer co-extrusion and combination of thermosetting and thermoplastic, as claimed in claim 1, is characterized in that, In step S4, the average particle size of titanium dioxide is 0.5 - 1 μm, and the average particle size of aluminum hydroxide is 200 - 500 nm.

6. The preparation method of the corona-resistant PI film with a combination of thermosetting and thermoplastic layers by three-layer coextrusion according to claim 1, characterized in that, In step S4, the stirring time is 60 - 130 min, the temperature is 8 - 15 °C, and the stirring speed is 40 - 50 r / min.

7. The preparation method of the corona-resistant PI film with a three-layer co-extruded multi-layer combination of thermosetting and thermoplastic, characterized in that, During the stirring process in steps S1 - S4, defoaming treatment is carried out.

8. The preparation method of the corona-resistant PI film with a combination of thermosetting and thermoplastic properties by three-layer co-extrusion as claimed in claim 1, wherein, The specific operations of steps S5, S6, and S7 are as follows: Feed mixture one and mixture two into the storage tanks of the corresponding casting nozzles respectively, control the temperature to be maintained at 0 - 5°C, and then feed mixture one and mixture two in the storage tanks into the corresponding die head pin-bar mixers respectively, where mixture one is mixed with acetic anhydride and pyridine, and mixture two is mixed with acetic anhydride and isoquinoline, and the mixing temperature is -5~0°C; then extrude through the three-layer coextrusion die lip of the casting machine, uniformly coat on the annular mirror steel belt, dry in the casting machine through the annular steel belt, obtain a film, introduce it into the stretching machine furnace for imidization into an integral film, and wind it up to obtain a three-layer coextruded multilayer corona-resistant PI film combining thermosetting and thermoplastic properties.

9. The preparation method of the corona-resistant PI film with a combination of thermosetting and thermoplastic properties by three-layer coextrusion as claimed in claim 8, wherein, In step S7, the film enters the stretching machine furnace and passes through the first temperature zone, the second temperature zone, the third temperature zone, the fourth temperature zone, the fifth temperature zone, and the sixth temperature zone in sequence. The heating temperatures of each temperature zone are 200 - 220°C, 250 - 300°C, 300 - 350°C, 400 - 450°C, 500 - 550°C, and 450 - 500°C in sequence, and the heating time of each temperature zone is 10 - 20s.

10. A corona-resistant PI film with a three-layer co-extruded combination of thermosetting and thermoplastic properties, characterized in that, It is prepared by the preparation method of the three-layer coextruded multilayer corona-resistant PI film combining thermosetting and thermoplastic properties according to any one of claims 1 - 9.

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