Three-layer coextruded multi-layer corona-resistant PI film combining thermosetting and thermoplastic and its preparation method

Abstract

The present invention discloses a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film and its preparation method, relating to the technical field of polyimide films. The present invention uses titanium dioxide and aluminum hydroxide as inorganic fillers, and utilizes the structural characteristics of the aluminum hydroxide structure stacked and the densely packed structure of the additional spherical titanium dioxide to form a layered shield that hinders the movement of charges. Furthermore, the present invention utilizes a three-layer film structure, with a high-performance thermosetting polyimide film as a supporting carrier and thermoplastic polyimide resin containing aluminum hydroxide and titanium dioxide as functional layers attached to the surface of the thermosetting polyimide film layer. This further improves the film's resistance to high-temperature and high-voltage breakdown while maintaining excellent tensile strength and elongation at break.

Description

Technical Field

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

[0002] Polyimide (PI) film is a new type of high-temperature-resistant organic polymer film. It features high modulus, low shrinkage, high strength, low water absorption, resistance to hydrolysis and radiation, non-toxicity, excellent insulation, and thermal and oxidative stability. Because it is one of the most expensive film materials internationally, it is known as "golden film." It is a key insulation material for high-speed motors and a critical voltage-resistant material for ultra-high-voltage electronic coils.

[0003] Inorganic nanofillers play a significant role in the performance of PI films. For example, the interfacial interaction between inorganic nanofillers such as titanium dioxide and aluminum hydroxide and the PI matrix can enhance the overall voltage breakdown strength of PI films. However, due to the large specific surface area and surface energy of nanoparticles, strong interactions between the particles can easily lead to agglomeration. This makes it difficult to achieve an ideal nanoscale composite between nanoparticles and highly viscous polymers, impacting the overall performance and product quality of the resulting PI film.

[0004] Chinese invention patent CN111087633B discloses a corona-resistant polyimide film and its preparation method. By introducing a first inorganic filler and a second inorganic filler of different particle sizes to maximize the filler packing, combined with a chemical imidization process, a uniform thermal conductivity network is established within the film. This method can improve the dispersion of the inorganic filler in the system to a certain extent, preventing agglomeration or sedimentation of the inorganic filler, but the effect on improving 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 shortcomings of the existing technology, provide a three-layer co-extruded multi-layer thermosetting and thermoplastic combined corona-resistant PI film and a preparation method thereof, use titanium dioxide and aluminum hydroxide as inorganic fillers, utilize the aluminum hydroxide structure stacking, and the tightly packed structural characteristics of additional spherical titanium dioxide to form a layered shield to hinder the movement of electric charges; at the same time, utilize a three-layer film structure, use a high-performance thermosetting polyimide film as a supporting carrier, and use thermoplastic polyimide resin containing aluminum hydroxide and titanium dioxide as functional layers attached to the surface of the thermosetting polyimide film layer on the upper and lower sides, so as to further improve the high-temperature resistance and high-voltage breakdown resistance of the film surface while maintaining excellent tensile strength and elongation at break.

[0006] The technical solution of the present invention is:

[0007] In one aspect, the present invention provides a method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film, comprising the following steps:

[0008] S1: 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 are stirred and mixed;

[0009] S2: adding 15-20 parts by weight of p-phenylenediamine to the material obtained in step S1, stirring and mixing until the designed viscosity is reached to obtain a mixture 1;

[0010] S3: stirring and mixing 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;

[0011] S4: adding 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, stirring and mixing until the designed viscosity is reached, to obtain a second mixture;

[0012] S5 mixture 1 is mixed with 750-1000 parts by weight of acetic anhydride, and then mixed with 20-25 parts by weight of pyridine to obtain mixture 3;

[0013] S6 mixture 2 is mixed with 1000-1250 parts by weight of acetic anhydride, and then mixed with 18-22 parts by weight of isoquinoline to obtain mixture 4;

[0014] S7 extrudes mixture three and mixture four simultaneously through a three-layer slit co-extrusion die head, and coats them onto the annular mirror steel belt of the 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; the film is then rolled up after imidization to obtain a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

[0015] In the present invention, acetic anhydride is a dehydrating agent, pyridine and isoquinoline are catalysts, and a pin-bar mixer is provided in front of a die head above a tape casting machine. The pin-bar mixer operates at a low temperature of -5 to 0°C. The low temperature ensures that the first and second mixtures are only mixed with the catalyst without causing a catalytic reaction. In the pin-bar mixer, the first and second mixtures are uniformly mixed with pyridine, acetic anhydride or isoquinoline, and then extruded through the die head to form a rubber layer on the surface of a steel strip. The steel strip enters the inner chamber of the tape casting machine with the rubber layer. After being heated, the pyridine or isoquinoline catalyzes the first and second mixtures to cause an imidization reaction. The imidization reaction is a dehydration reaction. The dehydration process is slow at a low temperature, so the dehydration process needs to be accelerated. In the present invention, acetic anhydride is added to accelerate the removal of water, so that sufficient imidization can be performed at a low temperature, and the imidization completion degree is greater than or equal to 95%.

[0016] Preferably, the specific operation of step S1 is as follows: first, dimethylformamide is added into the reactor and stirred for 5-10 minutes at a temperature of 8-10°C and a stirring speed of 10-20 r / min; then 4,4-diaminodiphenyl ether and pyromellitic dianhydride are added and stirred together for 20-40 minutes at a temperature of 8-15°C and a stirring speed of 20-50 r / min.

[0017] 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.

[0018] Preferably, the specific operation of step S3 is as follows: first, dimethylformamide is added into the reactor and stirred for 5-10 minutes at a temperature of 8-10°C and a stirring speed of 10-20 r / min; then, bisphenol A diether dianhydride and m-phenylenediamine are added and stirred together for 20-40 minutes at a temperature of 8-15°C and a stirring speed of 20-50 r / min.

[0019] 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.

[0020] 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.

[0021] Preferably, in steps S1-S4, defoaming treatment is performed during the stirring process.

[0022] Preferably, the specific operations of steps S5, S6, and S7 are as follows: the mixture 1 and the mixture 2 are respectively transported to the storage tank of the corresponding casting nozzle, and the temperature is controlled to be maintained at 0-5°C. Then, the mixture 1 and the mixture 2 in the storage tank are respectively transported to the corresponding die head pin bar mixer, wherein the mixture 1 is mixed with acetic anhydride and pyridine, and the mixture 2 is mixed with acetic anhydride and isoquinoline. The mixing temperature is -5~0°C. The mixing temperature can ensure that the mixture 1 and the mixture 2 are only mixed with the catalyst pyridine and isoquinoline without catalysis. If the temperature is too high, catalysis and dehydration reactions will occur during mixing. Mixture 1 and Mixture 2 will produce water in the die head, which will flow onto the steel belt and become imidized film and water coexisting. The product quality cannot be guaranteed. It is then extruded through the three-layer co-extrusion die lip of the cast-film machine, evenly coated on the annular mirror steel belt, and dried in the cast-film machine on the annular mirror steel belt. After obtaining the film, it is introduced into the stretching machine furnace for imidization to form an integrated film, and then wound up to obtain a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

[0023] Preferably, 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.

[0024] On the other hand, the present invention provides a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film, which is prepared by the above-mentioned method for preparing the three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention adds titanium dioxide and aluminum hydroxide inorganic fillers during the synthesis of thermoplastic polyamic acid solution to improve the corona resistance of the film, wherein aluminum hydroxide has strong charge conductivity, which can effectively alleviate the charge accumulation under partial discharge and quickly dissipate the charge; titanium dioxide has strong heat resistance and heat conductivity, which can effectively alleviate the heat accumulation under partial discharge and improve the high-voltage breakdown resistance. At the same time, the present invention utilizes a three-layer film structure, uses a high-performance thermosetting polyimide film as a supporting carrier, and uses 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 be attached to the surface of the thermosetting polyimide film layer. While maintaining the excellent tensile strength and elongation at break of the middle layer thermosetting polyimide film, the high-temperature resistance and high-voltage breakdown resistance of the film surface are further improved by the upper and lower layers of corona-resistant and high-temperature-resistant thermoplastic polyimide resins. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the technical solutions of 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.

[0028] Example 1

[0029] The method for preparing the corona-resistant PI film of the present embodiment by three-layer co-extruded multi-layer thermosetting and thermoplastic combination comprises the following steps:

[0030] S1: Prepare a reactor with nitrogen protection, add 1400 parts by weight of dimethylformamide into the reactor, turn on the stirring device of the reactor, pre-stir the dimethylformamide for 10 minutes, control the temperature at 8°C, and stir at a 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, continue stirring and mixing for 40 minutes, control the temperature at 8°C, and stir at a speed of 50 r / min. During the stirring process, nitrogen is continuously introduced for defoaming treatment;

[0031] S2: adding 20 parts by weight of p-phenylenediamine to the material obtained in step S1, stirring and mixing while adding; the stirring time is 60 minutes, the temperature is maintained at 8°C, the stirring speed is 40 r / min, and nitrogen is continuously introduced during the stirring process to control the viscosity at 300,000 mPa·s (45°C), to obtain a mixture 1;

[0032] S3: Prepare a reactor with nitrogen protection, add 1350 parts by weight of dimethylformamide into the reactor, turn on the stirring device of the reactor, pre-stir the dimethylformamide for 8 minutes, control the temperature at 8°C, and stir at a speed of 20 r / min; then add 90 parts by weight of bisphenol A diether dianhydride and 18 parts by weight of m-phenylenediamine, continue stirring and mixing for 30 minutes, control the temperature at 8°C, and stir at a speed of 40 r / min, and continuously introduce nitrogen during the stirring process;

[0033] S4: 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) are stirred and mixed uniformly to obtain a premixed inorganic material; the premixed inorganic material is then added to the material obtained in step S3, and stirred for 130 minutes. The temperature is controlled at 8° C., the stirring speed is 50 r / min, and nitrogen is continuously introduced during the stirring process to control the viscosity at 700,000 mPa·s (45° C.), thereby obtaining a second mixture;

[0034] S5: Mixture 1 and mixture 2 are respectively conveyed into storage tanks of corresponding casting nozzles, and the system temperature is controlled to be maintained at 0° C., and then mixture 1 and mixture 2 in the storage tanks are respectively conveyed into corresponding die pin-bar mixers, wherein mixture 1 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 3; mixture 2 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 4;

[0035] S6 mixture 3 and mixture 4 are extruded through the three-layer co-extrusion die lip of the casting machine, evenly coated on the annular mirror steel belt, and then sent to the drying channel (the annular mirror steel belt in the casting machine is 30 meters in total) for drying. The film speed is 18m / min, the drying temperature of the first 15m drying channel is 165℃, and the drying temperature of the last 15m drying channel is 145℃. The temperature of the film is gradually increased during drying, and the solvent is gradually evaporated to form a film with a solid content of 70wt.%;

[0036] S7 introduces the film into the stretching machine furnace at a speed of 18m / min, 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 in each temperature zone are 220℃, 300℃, 350℃, 450℃, 550℃ and 500℃ in sequence. The heating time in each temperature zone is 10s, and then it is rolled up to obtain a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

[0037] 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.

[0038] Comparative Example 1

[0039] The difference from Example 1 is that in step S4, the amount of titanium dioxide added is 2.5 parts by weight, and the amount of aluminum hydroxide added is 10 parts by weight.

[0040] Comparative Example 2

[0041] The difference from Example 1 is that in step S4, the amount of titanium dioxide added is 7.5 parts by weight, and aluminum hydroxide is not added.

[0042] Example 2

[0043] The method for preparing the corona-resistant PI film of the present embodiment by three-layer co-extruded multi-layer thermosetting and thermoplastic combination comprises the following steps:

[0044] S1: Prepare a reactor with nitrogen protection, add 1280 parts by weight of dimethylformamide into the reactor, turn on the stirring device of the reactor, pre-stir the dimethylformamide, and stir for 8 minutes. The temperature is controlled at 9°C and the stirring speed is 15 r / min. Then, add 65 parts by weight of 4,4-diaminodiphenyl ether and 80 parts by weight of pyromellitic dianhydride in six portions. Continue stirring and mixing for 30 minutes. The temperature is controlled at 10°C and the stirring speed is 40 r / min. During the stirring process, nitrogen is continuously introduced for defoaming treatment.

[0045] S2: adding 17 parts by weight of p-phenylenediamine to the material obtained in step S1, stirring and mixing while adding; the stirring time is 45 minutes, the temperature is maintained at 10°C, the stirring speed is 35 r / min, and nitrogen is continuously introduced during the stirring process to control the viscosity at 260,000 mPa·s (45°C), to obtain a mixture 1;

[0046] S3: Prepare a reactor with nitrogen protection, add 1400 parts by weight of dimethylformamide into the reactor, turn on the stirring device of the reactor, pre-stir the dimethylformamide for 10 minutes, control the temperature at 9°C, and stir at a speed of 15 r / min; then add 100 parts by weight of bisphenol A diether dianhydride and 20 parts by weight of m-phenylenediamine, continue stirring and mixing for 40 minutes, control the temperature at 10°C, and stir at a speed of 50 r / min, and continue to introduce nitrogen during the stirring process;

[0047] S4: 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) are stirred and mixed uniformly to obtain a premixed inorganic material; the premixed inorganic material is then added to the material obtained in step S3, and stirred for 100 minutes. The temperature is controlled at 10° C., the stirring speed is 45 r / min, and nitrogen is continuously introduced during the stirring process to control the viscosity at 650,000 mPa·s (45° C.), thereby obtaining a second mixture;

[0048] S5: Mixture 1 and mixture 2 are respectively conveyed to the storage tank of the corresponding casting nozzle, and the system temperature is controlled to be maintained at 2°C. Thereafter, mixture 1 and mixture 2 in the storage tank are respectively conveyed to the corresponding die head pin bar mixer, wherein mixture 1 is mixed with 1000 parts by weight of acetic anhydride at -3°C and then with 25 parts by weight of pyridine to obtain mixture 3; mixture 2 is mixed with 1250 parts by weight of acetic anhydride at -3°C and then with 22 parts by weight of isoquinoline to obtain mixture 4; then mixture 3 and mixture 4 are extruded through the three-layer co-extrusion die lip of the casting machine, uniformly coated on the annular mirror steel belt, and then conveyed to the drying channel (the annular mirror steel belt in the casting machine is 30m in total) for drying, the film speed is 18m / min, the drying temperature of the first 15m drying channel is 165°C, and the drying temperature of the last 15m drying channel is 145°C, so that the temperature of the film gradually increases during drying, the solvent gradually evaporates, and a film with a solid content of 72wt.% is formed;

[0049] S6 introduces the film into the stretching machine furnace at a speed of 18m / min, 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 in each temperature zone are 210℃, 275℃, 325℃, 425℃, 525℃ and 480℃ in sequence. The heating time in each temperature zone is 15s. Then it is rolled up to obtain a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

[0050] 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.

[0051] Comparative Example 3

[0052] The difference from Example 2 is that in step S4, the amount of titanium dioxide added is 1.8 parts by weight, and the amount of aluminum hydroxide added is 8 parts by weight.

[0053] Comparative Example 4

[0054] The difference from Example 2 is that in step S4, the added amount of aluminum hydroxide is 6.1 parts by weight, and titanium dioxide is not added.

[0055] Example 3

[0056] The method for preparing the corona-resistant PI film of the present embodiment by three-layer co-extruded multi-layer thermosetting and thermoplastic combination comprises the following steps:

[0057] S1: Prepare a reactor with nitrogen protection, add 1200 parts by weight of dimethylformamide into the reactor, turn on the stirring device of the reactor, pre-stir the dimethylformamide, and stir for 5 minutes. The temperature is controlled at 10°C and the stirring speed is 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 stirring and mixing for 20 minutes. The temperature is controlled at 15°C and the stirring speed is 20 r / min. During the stirring process, nitrogen is continuously introduced for defoaming treatment.

[0058] S2: adding 15 parts by weight of p-phenylenediamine to the material obtained in step S1, stirring and mixing while adding; the stirring time is 30 minutes, 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 a mixture 1;

[0059] S3: Prepare a reactor with nitrogen protection, add 1200 parts by weight of dimethylformamide into the reactor, turn on the stirring device of the reactor, pre-stir the dimethylformamide for 5 minutes, control the temperature at 10°C, and stir at a 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 stirring and mixing for 20 minutes, control the temperature at 15°C, and stir at a speed of 20 r / min, and continue to introduce nitrogen during the stirring process;

[0060] S4: 50 parts by weight of dimethylformamide, 0.5 parts by weight of titanium dioxide (average particle size of 0.5 μm), and 3 parts by weight of aluminum hydroxide (average particle size of 500 nm) are stirred and mixed uniformly to obtain a premixed inorganic material; the premixed inorganic material is then added to the material obtained in step S3, and stirred for 60 minutes. The temperature is controlled 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.), thereby obtaining a second mixture;

[0061] S5: Mixture 1 and mixture 2 are respectively conveyed to the storage tank of the corresponding casting nozzle, and the system temperature is controlled to be maintained at 5°C. Thereafter, mixture 1 and mixture 2 in the storage tank are respectively conveyed to the corresponding die head pin bar mixer, wherein mixture 1 is mixed with 750 parts by weight of acetic anhydride at 0°C and then 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 with 18 parts by weight of isoquinoline to obtain mixture 4; then mixture 3 and mixture 4 are extruded through the three-layer co-extrusion die lip of the casting machine, uniformly coated on the annular mirror steel belt, and then conveyed to the drying channel (the annular mirror steel belt in the casting machine is 30m in total) for drying, the film speed is 18m / min, the drying temperature of the first 15m drying channel is 135°C, and the drying temperature of the last 15m drying channel is 145°C, so that the temperature of the film gradually increases during drying, the solvent gradually evaporates, and a film with a solid content of 70wt.% is formed;

[0062] S6 introduces the film into the stretching machine furnace at a speed of 18m / min, 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 in each temperature zone are 200℃, 250℃, 300℃, 400℃, 500℃ and 450℃ in sequence. The heating time in each temperature zone is 20s. Then it is rolled up to obtain a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

[0063] 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.

[0064] Comparative Example 5

[0065] The difference from Example 3 is that in step S4, the amount of titanium dioxide added is 3.5 parts by weight, and the amount of aluminum hydroxide added is 10 parts by weight.

[0066] Comparative Example 6

[0067] The difference from Example 3 is that in step S4, the amount of aluminum hydroxide added is 3.5 parts by weight, and titanium dioxide is not added.

[0068] Comparative Example 7

[0069] The preparation method of the PI film of Comparative Example 7 comprises the following steps:

[0070] Steps S1 and S2 are the same as those in Example 3;

[0071] S3: 50 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 300 nm) are stirred and mixed to obtain a premixed inorganic material;

[0072] S4: Add the premixed inorganic material to the mixture 1, stir for 60 minutes, control the temperature at 15°C, stir at a 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 the mixture 2;

[0073] S5: Mixture 2 is conveyed to the storage tank of the corresponding casting nozzle, and the system temperature is controlled to be maintained at 5°C. Thereafter, Mixture 2 in the storage tank is conveyed to the corresponding die pin-bar mixer, and Mixture 2 is mixed with 1000 parts by weight of acetic anhydride at 0°C and then with 18 parts by weight of isoquinoline to obtain Mixture 3; Mixture 3 is then extruded through the middle layer at the three-layer co-extrusion die lip of the casting machine, uniformly coated on an annular mirror steel belt, and then conveyed to a drying channel (the annular mirror steel belt in the casting machine is 30m in total) for drying. The film speed is 18m / min, the drying temperature of the first 15m of the drying channel is 135°C, and the drying temperature of the last 15m of the drying channel is 145°C, so that the temperature of the film gradually increases during drying, and the solvent gradually evaporates, forming a film with a solid content of 70wt.%;

[0074] S6 introduces the film into the stretching machine furnace at a speed of 18m / min, 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 in each temperature zone are 210℃, 275℃, 325℃, 425℃, 525℃ and 480℃ in sequence. The heating time in each temperature zone is 15s, and then it is rolled up to obtain the PI film.

[0075] The PI film prepared in Comparative Example 7 is only a layer of thermosetting polyimide film structure.

[0076] According to the provisions of "GB / T13542.2-2021 Electrical Insulation Film 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 were tested. The test conditions for corona resistance time were: test voltage 2kV, test frequency 20kHz, pulse rise time 50ns, bipolar pulse square wave, and 6mm cylindrical electrode. The test results are shown in Table 1:

[0077] Table 1 Performance test results of PI films prepared in Examples 1-3 and Comparative Examples 1-7

[0078]

[0079] As shown in Table 1, Examples 1-3 and Comparative Examples 1, 3, and 5 show, excessive inorganic filler addition leads to titanium dioxide accumulation, hindering the extension of molecular chains and causing agglomeration of nano-aluminum hydroxide, resulting in poor localized corona resistance. Excessive addition of inorganic filler also degrades the film's physical properties, leading to abnormal decreases in tensile strength and elongation at break. This indicates that excessive inorganic filler hinders the growth of imidized molecular chains within the film. This demonstrates that the amount of inorganic filler added is a key factor influencing the film's physical properties.

[0080] As shown in Table 1, Examples 1-3 and Comparative Examples 2, 4, and 6 demonstrate that, compared to the addition of either titanium dioxide or aluminum hydroxide alone as an inorganic filler, the PI films prepared using Examples 1-3 of the present invention, which incorporate both aluminum hydroxide and titanium dioxide as inorganic fillers, exhibit significantly improved corona resistance and thermal conductivity at the same film thickness. This indicates that aluminum hydroxide and titanium dioxide synergistically form a superior charge barrier, effectively mitigating charge and heat accumulation.

[0081] 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 "mother-child" relationship. The inorganic filler in the "child" belongs to the presence of isomers in the film. Such isomers will affect the length of the molecular chain during the imidization of the film. Short molecular chains 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 utilizes the thermosetting polyimide film without isomers in the middle layer to complete 100% imidization, improve the overall physical properties, and use high-intensity tensile strength and elongation at break to carry the upper and lower layers of functional thermoplastic polyimide films. After fusion at high temperature, the overall dimensional stability is maintained. The membrane structure of Comparative Example 7 is equivalent to a separate "child" layer. Such a "child" layer becomes brittle after being pulled and deformed due to the presence of inorganic fillers in the isomers. Cracks and gaps also appear in the functional layer formed by the inorganic fillers. High-voltage current will penetrate through the gaps, and corona resistance will be reduced.

Claims

1. A method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film, characterized in that: The following steps are involved: S1: 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 are stirred and mixed; S2: adding 15-20 parts by weight of p-phenylenediamine to the material obtained in step S1, stirring and mixing until the designed viscosity is reached to obtain a mixture 1; S3: stirring and mixing 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: adding 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, stirring and mixing until the designed viscosity is reached, to obtain a second mixture; S5 mixture 1 is mixed with 750-1000 parts by weight of acetic anhydride, and then mixed with 20-25 parts by weight of pyridine to obtain mixture 3; S6 mixture 2 is mixed with 1000-1250 parts by weight of acetic anhydride, and then mixed with 18-22 parts by weight of isoquinoline to obtain mixture 4; S7 extrudes mixture three and mixture four simultaneously through a three-layer slit co-extrusion die head, and coats them onto a circular mirror steel belt of a casting machine to obtain a film, wherein the middle of the film is a thermosetting polyimide film and the two sides are thermoplastic polyimide films; the film is then rolled up after imidization to obtain a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

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

3. The method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film according to claim 1, wherein: 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 method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film according to claim 1, wherein: The specific operation of step S3 is as follows: first, dimethylformamide is added into the reactor and stirred for 5-10 minutes at a temperature of 8-10°C and a stirring speed of 10-20 r / min; then, bisphenol A diether dianhydride and m-phenylenediamine are added and stirred together for 20-40 minutes at a temperature of 8-15°C and a stirring speed of 20-50 r / min.

5. The method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film according to claim 1, 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 method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film according to claim 1, wherein: 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 method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film according to claim 1, characterized in that: In steps S1-S4, defoaming treatment is performed during the stirring process.

8. The method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film according to claim 1, characterized in that: The specific operations of steps S5, S6, and S7 are as follows: Mixture 1 and mixture 2 are respectively conveyed to the storage tank of the corresponding casting nozzle, and the temperature is controlled to be maintained at 0-5°C, and then mixture 1 and mixture 2 in the storage tank are respectively conveyed to the corresponding die head needle bar mixer, wherein mixture 1 is mixed with acetic anhydride and pyridine, and mixture 2 is mixed with acetic anhydride and isoquinoline, and the mixing temperature is -5~0°C; then, they are extruded through the three-layer co-extrusion die lip of the casting machine, uniformly coated on the annular mirror steel belt, dried in the casting machine through the annular mirror steel belt, and after obtaining the film, introduced into the stretching machine furnace for imidization into an integrated film, and wound to obtain a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film.

9. The method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film according to claim 8, characterized in that: 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 three-layer co-extruded multi-layer thermosetting and thermoplastic corona-resistant PI film, characterized in that: The corona-resistant PI film is prepared by the method for preparing a three-layer co-extruded multi-layer thermosetting and thermoplastic combined corona-resistant PI film according to any one of claims 1 to 9.

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