Process for producing polypropylene film for dry capacitor

Abstract

The production method of polypropylene film for dry capacitor includes the following steps: melt extrusion, casting, stretching, after the casting sheet is peeled off from the cold roller, the casting sheet enters the stretching device for stretching and is stretched into a film; trimming, thickness measuring, corona, trimming the two side edges of the film and measuring the thickness of the film, corona treating the surface of the trimmed film, winding and slitting. The application uses macromolecular polypropylene resin to produce the film, and the whole polypropylene resin isotactic component is classified into long ordered structure with more isotactic concentration, and the produced film has high breakdown strength. Moreover, by controlling the distance between the die and the cold roller and the temperature difference between the die and the cold roller during casting, the first melt can be cooled on the cold roller to form a high proportion of beta crystal. The surface roughness of the film is improved, the dielectric sheet resistance is small after the subsequent gold spraying treatment of the film, the loss of the metalized film capacitor is small, and the service life is long.

Description

Technical Field

[0001] This invention relates to the field of film capacitors, and more particularly to a method for producing polypropylene film for dry capacitors. Background Technology

[0002] Polypropylene film capacitors are electronic components that use polypropylene film as the dielectric and aluminum foil or metallized film as electrodes. They are wound with an inductive structure and encapsulated with epoxy resin. These capacitors have advantages such as low loss, high insulation resistance, excellent dielectric strength, and a wide operating temperature range. Metallized polypropylene film is the core material for capacitor manufacturing. As one of the three major passive components, capacitors are widely used in various electronic products. Film capacitors, in particular, benefit from their high voltage withstand, good temperature characteristics, and long lifespan, showing significant advantages and promising prospects in new energy fields such as new energy vehicles, photovoltaics, and wind power generation. Film capacitors are fundamental electronic components with advantages such as high insulation resistance, low dielectric loss, high dielectric strength, long service life, and excellent frequency characteristics, and are widely used in many industries including home appliances, communications, power grids, rail transportation, industrial control, and new energy.

[0003] The existing polypropylene film used in capacitors has poor thickness uniformity, low breakdown field strength, and insufficient surface roughening, resulting in high dielectric sheet resistance after gold sputtering. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned shortcomings and provide a method for producing polypropylene film for dry capacitors with high breakdown strength.

[0005] To achieve the above objectives, the technical solution of the present invention is: a method for producing polypropylene film for dry-type capacitors, comprising the following steps:

[0006] Melt extrusion involves melting polypropylene resin to form a first melt, which is then extruded from a die. The extrusion temperature of the first melt is 248℃-258℃, and the first melt contains polypropylene resin with an average molecular weight of 1 million to 1.8 million.

[0007] The casting sheet is formed by blowing the first molten material onto a cold roller with an air knife to cool it. The die head is located above the cold roller, and the distance between the die head and the cold roller is 6-7 mm. The temperature of the cold roller is 93℃-93.5℃.

[0008] Stretching involves peeling the cast sheet off the cold rollers and then stretching it into a thin film using a stretching device.

[0009] Edge trimming, thickness measurement, and corona treatment: The two sides of the film are trimmed neatly, the film thickness is measured, and the surface of the trimmed film is subjected to corona treatment.

[0010] Winding: The corona-treated film is wound up to form a film roll;

[0011] Slitting involves cutting the film roll into smaller rolls of film with a narrower width.

[0012] Using high molecular weight polypropylene resin in film production results in a more ordered and concentrated structure of isotactic components within the polypropylene resin, leading to films with high breakdown strength. Furthermore, by controlling the distance between the die and the cold roller, and the temperature difference between them, during casting, the first melt can be cooled on the cold roller to form a high proportion of β-crystals. During the stretching step, the β-crystals transform into larger α-crystals, improving the surface roughness of the film. This results in lower sheet resistance, lower losses in metallized film capacitors, and longer lifespan after subsequent gold sputtering.

[0013] Preferably, the ratio of the weight-average molecular weight to the number-average molecular weight of the polypropylene resin in the melt is 4.2-6.3. High molecular weight polypropylene has a narrow molecular weight distribution, uniform composition, and produces films with good consistency.

[0014] Preferably, the following steps are included prior to the melt extrusion step:

[0015] Separation involves heating and melting polypropylene resin to form a melt, which is then separated into a first melt and a second melt using a separation device.

[0016] The separation device includes a housing and a rotor; the housing is provided with a first inner cavity, a first channel and a second channel, the first channel and the second channel are respectively connected to the two sides of the first inner cavity, the first channel can input hot air into the first inner cavity, and the second channel can discharge the air in the first inner cavity;

[0017] The rotor has a first rotating shaft at its lower end and a second rotating shaft at its upper end. The rotor is located inside a first inner cavity, and the first and second rotating shafts are rotatably connected to the housing. The outer side of the rotor has toothed blades for being propelled by airflow. The rotor has a second inner cavity, the bottom of which has a concave conical surface. The bottom of the conical surface has an upwardly extending guide tube. The conical surface has a discharge port on the outer periphery of the guide tube. The first rotating shaft has a third channel communicating with the discharge port and a fourth channel communicating with the guide tube. The second rotating shaft has a fifth channel communicating with the second inner cavity.

[0018] Polypropylene resin is separated in a separation device to form a first melt with a relatively uniform molecular weight distribution for casting, resulting in films with good consistency.

[0019] Preferably, between the winding and slitting steps, the following steps are also included:

[0020] Aging treatment involves placing the film roll in an environment of 38℃-45℃ and 42%-46% humidity for 45-48 hours. Using high molecular weight polypropylene as raw material results in significant accumulated stress during the stretching process, and some stress remains even after the shaping treatment. Aging treatment can reduce film deformation.

[0021] Preferably, the stretching step includes:

[0022] Longitudinal stretching involves first stretching the cast sheet longitudinally at a temperature of 120℃-145℃.

[0023] Preheating: After longitudinal stretching, the cast sheet is preheated to 165℃-167.5℃.

[0024] Lateral stretching: The cast sheet is laterally stretched at a temperature of 161℃-162℃ to form a thin film.

[0025] The film is then set at a temperature of 162℃-169℃ after being stretched laterally.

[0026] By adopting the above technical solution, the beneficial effects of this invention are as follows: This invention uses high molecular weight polypropylene resin for film production. Overall, the isotactic components of the polypropylene resin are hierarchically structured into a more regular and concentrated long ordered structure, resulting in a film with high breakdown strength. Furthermore, by controlling the distance between the die and the cold roller, and the temperature difference between them, during casting, the first melt can be cooled on the cold roller to form a high proportion of β crystals. In the stretching step, the β crystals transform into larger α crystals, improving the surface roughness of the film. This results in lower sheet resistance, lower loss in the metallized film capacitor, and a longer lifespan after subsequent gold sputtering. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of the present invention;

[0028] Figure 2 This is a schematic diagram of the separation device of the present invention;

[0029] Figure 3 Microscopic image of polypropylene film produced using existing processes;

[0030] Figure 4 This is a microscope image of a polypropylene film produced using the present invention.

[0031] Explanation of key figure labels:

[0032] Housing 11; First inner cavity 111; First channel 112; Second channel 113; Rotor 12; Second inner cavity 121; Guide pipe 122; Discharge port 123; Toothed plate 124; First rotating shaft 125; Second rotating shaft 126; Die head 2; Air knife 3; Cold roller 4; Stretching device 5; Third channel 63; Fourth channel 64; Fifth channel 65. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1:

[0035] like Figure 1 As shown, the method for producing polypropylene film for dry capacitors according to the present invention includes the following steps:

[0036] Melt extrusion: Polypropylene resin is melted to form a first melt and extruded from die 2. The extrusion temperature of the first melt is 248℃-258℃. The first melt contains polypropylene resin with an average molecular weight of 1 million to 1.8 million. The ratio of the weight average molecular weight to the number average molecular weight of the polypropylene resin in the melt is 4.2-6.3.

[0037] The first melt is blown onto the cold roller 4 by the air knife 3 to cool and form a casting sheet. The die head is located above the cold roller, and the distance between the die head and the cold roller is 6-7mm. The temperature of the cold roller is 93℃-93.5℃.

[0038] After the casting is peeled off the cold roller, it enters the stretching device 5 for stretching into a thin film.

[0039] The stretching process includes:

[0040] Longitudinal stretching involves first stretching the cast sheet longitudinally at a temperature of 120℃-145℃.

[0041] Preheating: After longitudinal stretching, the cast sheet is preheated to 165℃-167.5℃.

[0042] Lateral stretching: The cast sheet is laterally stretched at a temperature of 161℃-162℃ to form a thin film.

[0043] The film is then set at a temperature of 162℃-169℃ after being stretched laterally.

[0044] Edge trimming, thickness measurement, and corona treatment: The two sides of the film are trimmed neatly and the thickness of the film is measured. The surface of the trimmed film is then corona treated.

[0045] The corona-treated film is wound up to form a film roll.

[0046] For aging treatment, place the film roll in an environment of 38℃-45℃ and 42%-46% humidity for 45-48 hours.

[0047] Slitting involves cutting the film roll into smaller rolls of film with a narrower width.

[0048] Example 2:

[0049] like Figures 1-2 As shown, the method for producing polypropylene film for dry capacitors according to the present invention includes the following steps:

[0050] Separation involves heating and melting polypropylene resin to form a melt, which is then separated into a first melt and a second melt using a separation device.

[0051] The separation device includes a housing 11 and a rotor 12. The housing 11 contains a first inner cavity 111, a first channel 112, and a second channel 113, which connect to opposite sides of the first inner cavity 111. The first channel 112 is located on the upper right side of the first inner cavity 111, and the second channel 113 is located on the lower left side of the first inner cavity 111. The first channel 112 is connected to a hot gas source and is used to input hot gas into the first inner cavity 111. The second channel 113 can discharge the airflow from the first inner cavity 111.

[0052] The rotor 12 has a first rotating shaft 125 at its lower end and a second rotating shaft 126 at its upper end. The rotor 12 is housed within a first inner cavity 111, and the first and second rotating shafts 125 and 126 are rotatably connected to the housing 11. The rotor can rotate around the first and second rotating shafts within the first inner cavity. The outer side of the rotor 12 is provided with toothed blades 124 for being propelled by airflow. Both the upper and lower ends of the rotor are conical, and the toothed blades are located on the outer periphery of the rotor's middle section. Hot air blown into the first inner cavity from the first channel 112 flows downwards along the rotor's toothed blades and exits through the second channel 113. The hot air pushes the rotor's toothed blades, causing the rotor to rotate and simultaneously heating it.

[0053] The rotor 12 has a second inner cavity 121. The bottom of the second inner cavity 121 has a concave conical surface, and the bottom of the conical surface has an upwardly extending guide pipe 122. A discharge port 123 is located on the outer periphery of the guide pipe. The first rotating shaft has a third channel 63 communicating with the discharge port and a fourth channel 64 communicating with the guide pipe. The second rotating shaft has a fifth channel 65 communicating with the second inner cavity. The melt enters the second inner cavity 121 through the fifth channel and, under the action of the rotor rotation, separates into a first melt and a second melt. The second melt is concentrated in the middle of the second inner cavity 121 and can be discharged by the guide pipe 122, while the first melt can be discharged from the discharge port 123. After centrifugal separation, the remaining first melt exhibits better consistency.

[0054] Melt extrusion involves melting polypropylene resin to form a first melt, which is then extruded from a die. The extrusion temperature of the first melt is 248℃-258℃. The first melt contains polypropylene resin with an average molecular weight of 1 million to 1.8 million. The ratio of the weight-average molecular weight to the number-average molecular weight of the polypropylene resin in the melt is 4.2-6.3.

[0055] The casting process involves using an air knife to blow the first molten material onto a cold roller for cooling and forming a casting sheet. The die head is located above the cold roller, with a distance of 6-7 mm between the die head and the cold roller. The temperature of the cold roller is 93℃-93.5℃.

[0056] Stretching involves peeling the cast sheet from the cold rollers and then stretching it into a thin film using a stretching device.

[0057] The stretching process includes:

[0058] Longitudinal stretching involves first stretching the cast sheet longitudinally at a temperature of 120℃-145℃.

[0059] Preheating: After longitudinal stretching, the cast sheet is preheated to 165℃-167.5℃.

[0060] Lateral stretching: The cast sheet is laterally stretched at a temperature of 161℃-162℃ to form a thin film.

[0061] The film is then set at a temperature of 162℃-169℃ after being stretched laterally.

[0062] Edge trimming, thickness measurement, and corona treatment: The two sides of the film are trimmed neatly and the thickness of the film is measured. The surface of the trimmed film is then corona treated.

[0063] The corona-treated film is wound up to form a film roll.

[0064] For aging treatment, place the film roll in an environment of 38℃-45℃ and 42%-46% humidity for 45-48 hours.

[0065] Slitting involves cutting the film roll into smaller rolls of film with a narrower width.

[0066] like Figure 3 and Figure 4 As shown, the polypropylene film produced by this invention has a higher β crystal ratio compared to polypropylene films produced by existing processes.

[0067] The above description is merely a preferred embodiment of the present invention and does not limit the scope of the present invention. All equivalent changes and modifications made in accordance with the claims of the present invention should still fall within the scope of the present invention.

Claims

1. A method for producing polypropylene film for dry-type capacitors, characterized in that, Includes the following steps: Separation involves heating and melting polypropylene resin to form a melt, which is then separated into a first melt and a second melt using a separation device. The separation device includes a housing and a rotor; the housing is provided with a first inner cavity, a first channel and a second channel, the first channel and the second channel are respectively connected to the two sides of the first inner cavity, the first channel can input hot air into the first inner cavity, and the second channel can discharge the air in the first inner cavity; The rotor has a first rotating shaft at its lower end and a second rotating shaft at its upper end. The rotor is located inside a first inner cavity, and the first and second rotating shafts are rotatably connected to the housing. The outer side of the rotor has toothed plates for being propelled by airflow. The rotor has a second inner cavity, the bottom of which has a concave conical surface. A guide tube extends upward from the bottom of the conical surface, through which the second melt can be discharged. A discharge port is located on the outer periphery of the guide tube on the conical surface, through which the first melt can be discharged. The first rotating shaft has a third channel communicating with the discharge port and a fourth channel communicating with the guide tube. The second rotating shaft has a fifth channel communicating with the second inner cavity. Melt extrusion: The first melt is extruded from the die head. The extrusion temperature of the first melt is 248℃-258℃. The first melt contains polypropylene resin with an average molecular weight of 1 million to 1.8 million. The casting sheet is formed by blowing the first molten material onto a cold roller with an air knife to cool it. The die head is located above the cold roller, and the distance between the die head and the cold roller is 6-7 mm. The temperature of the cold roller is 93℃-93.5℃. Stretching involves peeling the cast sheet off the cold rollers and then stretching it into a thin film using a stretching device. Edge trimming, thickness measurement, and corona treatment: The two sides of the film are trimmed neatly, the film thickness is measured, and the surface of the trimmed film is subjected to corona treatment. Winding: The corona-treated film is wound up to form a film roll; Slitting involves cutting the film roll into smaller rolls of film with a narrower width.

2. The method for producing polypropylene film for dry capacitors according to claim 1, characterized in that, Between the winding and slitting steps, the following steps are also included: For aging treatment, place the film roll in an environment of 38℃-45℃ and 42%-46% humidity for 45-48 hours.

3. The method for producing polypropylene film for dry capacitors according to claim 1, characterized in that, The stretching process includes: Longitudinal stretching involves first stretching the cast sheet longitudinally at a temperature of 120℃-145℃. Preheating: After longitudinal stretching, the cast sheet is preheated to 165℃-167.5℃. Lateral stretching: The cast sheet is laterally stretched at a temperature of 161℃-162℃ to form a thin film. The film is then set at a temperature of 162℃-169℃ after being stretched laterally.

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