Polypropylene film and industrial production method thereof

Abstract

The invention discloses a polypropylene film and an industrial production method thereof. The method comprises the following steps: 1) providing blended granules of polypropylene and a cycloolefin copolymer; wherein the cycloolefin copolymer accounts for 0-15% of the mass of the blended granules; (2) carrying out melt casting on the blended granules by adopting a casting machine to obtain a polypropylene film; wherein in the casting machine, the temperature of a mold heating area for spreading the melt of the blended granules to form a thin film shape is 195 DEG C or above. According to the industrial production method, the high-temperature breakdown field strength of the obtained polypropylene film can be effectively improved.

Description

Technical Field

[0001] This invention relates to a polypropylene film and its industrial production method. Background Technology

[0002] Dielectric polypropylene film is the core of dry-type DC capacitors, serving as the "heart" of new energy power generation and flexible DC transmission, directly affecting the quality and safety of power conversion. Commercially available polypropylene film capacitors operate at only 85℃. When the operating temperature exceeds 105℃, the capacitors face failure issues due to decreased breakdown field strength and a sharp increase in leakage current. Therefore, it is necessary to develop capacitor film materials with higher breakdown field strength at higher temperatures (e.g., 125℃).

[0003] CN110914939A discloses a film capacitor comprising a film of a blend of polypropylene and a cyclic olefin copolymer (COC). This patent document uses a relatively large amount of the cyclic olefin copolymer, which improves the dissipation factor, but its breakdown field strength still needs further improvement. The preparation method employs a flat die extrusion process.

[0004] CN114148004A discloses a method for improving the high-temperature dielectric properties of polypropylene film for capacitors, comprising the following steps: drying cyclic olefin copolymer particles with different glass transition temperatures in an oven; mixing the dried cyclic olefin copolymer with polypropylene on a two-roll mill; hot-pressing the mixture of polypropylene and cyclic olefin copolymer in a flat vulcanizing machine; cooling using a cooling device; after cooling, removing the cyclic olefin copolymer-modified polypropylene film from the flat vulcanizing machine and cooling the film using the cooling device to obtain the cyclic olefin copolymer-modified polypropylene film. The high-temperature breakdown field strength of the polypropylene film obtained by this patent document is still relatively low.

[0005] CN116515201A discloses a polymer film, its preparation method, and a capacitor. The polymer film is formed from raw materials including polypropylene and a cyclic olefin copolymer; wherein, based on 100 parts by weight of the polymer film, the cyclic olefin copolymer comprises 4 to 9 parts by weight. The preparation method includes: extruding granules comprising polypropylene and the cyclic olefin copolymer using a screw extruder, followed by pelletizing using a pelletizer to obtain a blend of granules; and then extruding the blend of granules using a hot press to obtain the polymer film. The high-temperature breakdown electric field strength of this polymer film still needs improvement.

[0006] CN107383740A discloses a high-strength, low-shrinkage PP / COC alloy material and its preparation method. The alloy material comprises the following raw materials in parts by weight: 20-80 parts polypropylene, 20-50 parts cyclic olefin copolymer, 1-5 parts compatibilizer, 1-5 parts toughening agent, 0.1-0.5 parts dispersant, 0.2-0.8 parts lubricant, and 0.1-0.3 parts antioxidant. However, the high-temperature breakdown field strength of this alloy material remains relatively low. Summary of the Invention

[0007] In view of this, one object of the present invention is to provide an industrial production method for polypropylene film. This method produces a polypropylene film with high high-temperature breakdown field strength by melt casting of blended granules in a specific ratio. Another object of the present invention is to provide a polypropylene film produced according to the industrial production method described above. The present invention achieves the above objects using the following technical solutions.

[0008] This invention provides an industrial production method for polypropylene film, comprising the following steps:

[0009] 1) Provides a blend of polypropylene and cyclic olefin copolymer granules; wherein the cyclic olefin copolymer accounts for 0-15% of the mass percentage of the blend granules;

[0010] 2) The blended granules are melt-cast using a casting machine to obtain a polypropylene film; wherein, the temperature of the mold heating zone in the casting machine where the melt of the blended granules is spread to form a film shape is above 195°C.

[0011] According to the industrial production method of the present invention, preferably, the cyclic olefin copolymer accounts for 5-12% of the mass percentage of the blended granules.

[0012] According to the industrial production method of the present invention, preferably, the casting machine further includes a feeding zone, a feeding cooling zone, and a barrel heating zone; wherein,

[0013] The feeding zone is used to add the blended granules;

[0014] The feeding cooling zone is connected to the feeding zone and is used to receive the blended granules and stir the blended granules.

[0015] The barrel heating zone is horizontally arranged, with one end connected to the feeding cooling zone, and is used to heat the blended granules and push the melt of the heated blended granules into the mold heating zone;

[0016] The mold heating zone is connected to the other end of the barrel heating zone, and has an outlet for spreading the melt into a film shape. This outlet is used to output the melt spread into a film shape, which forms an initial polypropylene film after being output.

[0017] According to the industrial production method of the present invention, preferably, the feeding cooling zone is located below the feeding zone and its temperature is set to below 150°C; the temperature of the barrel heating zone is set to above 190°C; a screw is provided in the barrel heating zone, and the screw is configured to push the melt of the heated blended granules into the mold heating zone when rotating.

[0018] According to the industrial production method of the present invention, preferably, the temperature of the feeding cooling zone is set to 90-150°C; the temperature of the barrel heating zone is set to 190-240°C; and the temperature of the mold heating zone is set to 195-240°C.

[0019] According to the industrial production method of the present invention, preferably, the casting machine further includes a casting roller, a traction roller, and a take-up roller; the casting roller is disposed near the mold heating zone, and the casting roller includes an upper casting roller and a lower casting roller symmetrically arranged vertically; the upper casting roller and the lower casting roller are configured to stretch the initial polypropylene film by rotation; the traction roller includes an upper traction roller and a lower traction roller symmetrically arranged vertically, and the upper traction roller and the lower traction roller are configured to further stretch the initial polypropylene film by rotation to obtain a polypropylene film, and convey the polypropylene film toward the take-up roller; the take-up roller is used to collect the obtained polypropylene film into a roll.

[0020] According to the industrial production method of the present invention, preferably, in step 1), a co-rotating twin-screw extruder is used to prepare blended granules, and the temperatures of the feeding section, compression section and metering section of the twin-screw extruder are set to 160-170°C, 175-185°C and 175-185°C, respectively.

[0021] According to the industrial production method of the present invention, preferably, in step 1), a co-rotating twin-screw extruder is used to prepare blended granules, and the temperatures of the feeding section, compression section and metering section of the twin-screw extruder are set to 160-165°C, 180-185°C and 180-185°C, respectively.

[0022] According to the industrial production method of the present invention, preferably, the cyclic olefin copolymer is formed from bicyclo[2,2,1]-2-heptene and ethylene; the weight ratio of ethylene to the sum of the weights of bicyclo[2,2,1]-2-heptene and ethylene is 30-40:100; the cyclic olefin copolymer is an amorphous copolymer; the glass transition temperature of the cyclic olefin copolymer is 132-136°C; and the melting point of the polypropylene is 160-168°C.

[0023] On the other hand, the present invention also provides a polypropylene film produced according to the industrial production method described above, wherein the breakdown field strength of the polypropylene film is 456MV / m or more at a temperature of 125°C.

[0024] The industrial production method of this invention is beneficial for improving the high-temperature breakdown field strength of the obtained polypropylene film. This method is suitable for industrial production and can mass-produce polypropylene films of different thicknesses. The high-temperature breakdown field strength of the polypropylene film obtained by this invention is effectively improved. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of polypropylene film produced by melt casting using a casting machine according to the present invention.

[0026] Figure 2 This is a schematic diagram of the mold heating zone of the casting machine used in this invention. Figure 1 (A magnified view of a portion of the image).

[0027] Figure 3 This is a schematic diagram illustrating the relationship between breakdown field strength and temperature.

[0028] Explanation of reference numerals in the attached figures:

[0029] 100-Feeding zone, 200-Feeding cooling zone, 300-Barrel heating zone, 301-Heating cylinder, 302-Screw, 400-Die heating zone, 401-Connecting part, 402-Outlet part, 500-Casting roller, 501-Upper casting roller, 502-Lower casting roller, 600-Traction roller, 601-Upper traction roller, 602-Lower traction roller, 700-Rewinding roller; A-Blended granules, C-Melted material, B-Polypropylene film. Detailed Implementation

[0030] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0031] This invention reveals that the high-temperature breakdown field strength (breakdown field strength at 125°C) of existing polypropylene films still needs improvement. Furthermore, the current technology generally employs hot pressing, which is not conducive to industrialization. In addition, the resulting polypropylene films lack uniform performance. Although hot pressing and casting are both conventional polymer processing methods, those skilled in the art have no incentive to use casting instead of hot pressing to improve the high-temperature breakdown field strength of polypropylene films. Therefore, this invention provides an industrial production method for polypropylene films. This method involves extruding a specific ratio of polypropylene and cyclic olefin copolymer through a twin-screw extruder to obtain mixed granules, followed by melt casting using a casting machine to obtain a polypropylene film. The high-temperature breakdown field strength of the polypropylene film obtained by this method is effectively improved, and this preparation method is conducive to industrial-scale production.

[0032] <Production Methods>

[0033] The present invention provides a method for producing a polypropylene film, comprising the following steps: providing a blended granule; and casting. These are described in detail below.

[0034] Steps for providing blended granules

[0035] A blend granule of polypropylene and a cyclic olefin copolymer is provided. The cyclic olefin copolymer constitutes 0-15% of the mass percentage of the blend granule, preferably 5-15%, more preferably 9-15%, further preferably 10-13%, and even more preferably 10-12%. This invention has found that polypropylene films obtained with the cyclic olefin copolymer within a specific range exhibit higher high-temperature breakdown field strength; outside this range, the high-temperature breakdown field strength decreases.

[0036] In this invention, no other additives, including anti-blocking agents and nucleating agents, are added to the blended granules. In some embodiments, the blended granules are formed solely from a blend of polypropylene and a cyclic olefin copolymer. That is, in the formation of the polypropylene film of this invention, only polypropylene and a cyclic olefin copolymer are used, without the addition of any other additives.

[0037] In this invention, the melting point of polypropylene is 160–168°C, preferably 162–166°C, and more preferably 163.3–165.5°C. The density is 0.89–0.91 g / cm³. 3 Preferably, it is 0.9–0.91 g / cm³. 3 More preferably, it is 0.9–0.906 g / cm³. 3 .

[0038] In this invention, the cyclic olefin copolymer is formed from bicyclo[2,2,1]-2-heptene and ethylene. The weight ratio of ethylene to the sum of the weights of bicyclo[2,2,1]-2-heptene and ethylene is 30-40:100, preferably 33-38:100, and more preferably 35-37:100. The cyclic olefin copolymer is an amorphous copolymer, preferably 100% amorphous. The glass transition temperature (Tg) of the cyclic olefin copolymer is 132-136°C, preferably 133-135°C, and more preferably 133.5-134°C. The glass transition temperature (Tg) of the cyclic olefin copolymer increases with the increase of the content of the comonomer norbornene; the COC molecular chain is rigid, and the Tg is much higher than room temperature. The heat distortion temperature (Tg) of the cyclic olefin copolymer is 124-129°C, preferably 125-128°C, and more preferably 126-127°C. The density of the cyclic olefin copolymer is 1.0-1.05 g / cm³. 3 The preferred concentration is 1.01–1.04 g / cm³. 3More preferably, it is 1.01–1.02 g / cm³. 3 Such polypropylene and cyclic olefin copolymers are beneficial for improving the high-temperature breakdown field strength of the resulting polypropylene film.

[0039] In this invention, a twin-screw extruder can be used for blending. According to one embodiment of the invention, a co-rotating twin-screw extruder is used to prepare blended granules, with the temperatures of the feeding section, compression section, and metering section of the twin-screw extruder set to 160–170°C, 175–185°C, and 175–185°C, respectively. In this invention, the temperature of the feeding section is preferably 160–165°C. The temperature of the compression section is preferably 180–185°C. The temperature of the metering section is preferably 180–185°C.

[0040] According to a preferred embodiment of the present invention, polypropylene and cyclic olefin copolymer are added to the hopper of a co-rotating twin-screw extruder for blending, and the extruded sample is pelletized by a pelletizer to obtain blended granules (which can be referred to as PP / COC granules); wherein the temperatures of the feeding section, compression section and metering section of the twin-screw extruder are set to 160-165°C, 180-185°C and 180-185°C, respectively.

[0041] Casting steps

[0042] The blended granules are melt-cast using a casting machine to obtain a polypropylene film. The temperature of the heating zone in the mold within the casting machine, where the melt of the blended granules is spread to form the film shape, is above 195°C. This results in a polypropylene film with a high high-temperature breakdown field strength.

[0043] In this invention, the casting machine includes a feeding zone, a feeding cooling zone, a barrel heating zone, a mold heating zone, casting rollers, traction rollers, and take-up rollers.

[0044] The feeding zone is used to add the blended granules. The feeding zone may include a feeding tank. The feeding tank has upper and lower openings, with the lower opening connected to the feeding cooling zone.

[0045] The feeding cooling zone is connected to the feeding zone and is used to receive and mix the blended granules. The feeding cooling zone can be located directly below the feeding zone. The temperature of the feeding cooling zone is set below 150°C, preferably 90–150°C, for example, 100–148°C. The temperature display in the feeding cooling zone mainly serves a monitoring function. The mixing speed can be 300–800 rpm, preferably 350–500 rpm, for example, 390 rpm.

[0046] The barrel heating zone is horizontally positioned, with one end connected to the feeding cooling zone. It is used to heat the blended granules and push the heated granules into the mold heating zone. The portion of the barrel heating zone near the feeding cooling zone contains the blended granules, while the portion further away from the feeding cooling zone is primarily composed of the melt formed from the blended granules. The barrel heating zone includes a heating cylinder and a screw. The heating cylinder can be cylindrical and is configured to heat the blended granules within it to form a melt. The screw is rotatable, specifically driven by a motor. The screw, during rotation, pushes the heated melt of the blended granules into the mold heating zone. The temperature of the barrel heating zone is set to above 190°C, preferably between 190°C and 240°C, for example, between 200°C and 220°C.

[0047] The mold heating zone is connected to the barrel heating zone at one end and has an outlet for spreading the melt into a film shape. This outlet is used to output the melt spread into a film shape, forming an initial polypropylene film. The mold heating zone includes a connecting part and an outlet part. The cross-section of the connecting part is approximately triangular; the outlet part is flat and similar to the shape of the film. The height of the outlet part can be set to a fixed value. The temperature of the mold heating zone is set to 195°C or higher, preferably 195–240°C, for example, 200–220°C. This is beneficial for forming the desired polypropylene film.

[0048] The casting rollers are positioned close to the mold heating zone. The casting rollers include an upper casting roller and a lower casting roller, symmetrically arranged vertically. The upper and lower casting rollers are configured to stretch the initial polypropylene film by rotation.

[0049] The traction roller is positioned between the casting roller and the take-up roller. The traction roller includes an upper traction roller and a lower traction roller arranged symmetrically. The upper and lower traction rollers are configured to further stretch the initial polypropylene film by rotation, obtaining a polypropylene film, and then convey the polypropylene film towards the take-up roller.

[0050] The take-up roller is used to take the obtained polypropylene film into a roll.

[0051] This invention uses melt casting, and there are currently no reports of using melt casting to form polypropylene films.

[0052] <Polypropylene film>

[0053] The present invention also provides a polypropylene film produced according to the industrial production method described above. The thickness of the polypropylene film can be 30 to 60 μm. The breakdown field strength of the polypropylene film at a temperature of 125°C is 456 MV / m or more, preferably 456.5 MV / m or more.

[0054] The following describes the raw materials and testing methods used in the example:

[0055] Polypropylene: Granules, Korean Oil & Chemical Corporation, grade 5014L HPT. Polypropylene has a melting point of 165.5℃ and a density of 0.906 g / cm³. 3 .

[0056] Cyclic olefin copolymer: granules, TOPAS, grade 5013L-10; the glass transition temperature of the cyclic olefin copolymer is 134℃, and the density is 1.02 g / cm³. 3 .

[0057] Example 1

[0058] 1) Polypropylene and cyclic olefin copolymers were added to the hopper of a co-rotating twin-screw extruder for blending. The extruded sample was pelletized by a pelletizer to obtain blend pellet A. The temperatures of the feeding section, compression section, and metering section of the twin-screw extruder were set to 160℃, 180℃, and 180℃, respectively. The cyclic olefin copolymer accounted for 10% of the mass percentage of blend pellet A.

[0059] 2) The blended granules A are melt-cast using a casting machine to obtain a polypropylene film. For example... Figure 1 and Figure 2 As shown, the casting machine includes a feeding zone 100, a feeding cooling zone 200, a barrel heating zone 300, a mold heating zone 400, a casting roller 500, a traction roller 600, and a take-up roller 700.

[0060] Feeding zone 100 is used to add blended granules A. Feeding zone 100 includes a feeding tank. The feeding tank can be funnel-shaped. Feeding cooling zone 200 is connected to feeding zone 100 and is located below the feeding tank, used to receive blended granules A and to stir blended granules A.

[0061] The barrel heating zone 300 is horizontally positioned, with one end connected to the feeding cooling zone 200. It heats the blended granules A and pushes the heated melt C of the blended granules A into the mold heating zone 400. Specifically, the barrel heating zone 300 includes a heating cylinder 301 and a screw 302. The heating cylinder 301 has a cylindrical structure, and the screw 302 is horizontally positioned inside the heating cylinder 301. The screw 302, when rotating, pushes the heated melt C of the blended granules A into the mold heating zone 400.

[0062] One end of the mold heating zone 400 is connected to the other end of the barrel heating zone 300, and has an outlet for spreading the melt C of the blended granules A into a film shape. This outlet is used to output the melt C spread into a film shape, forming an initial polypropylene film. The mold heating zone 400 is horizontally arranged. The mold heating zone 400 includes a connecting portion 401 and an outlet portion 402. The cross-section of the connecting portion 401 is approximately triangular. The outlet portion 402 is flat and similar in shape to the film.

[0063] The temperature of the feeding cooling zone 200 is 148℃. The temperature of the barrel heating zone 300 is 200℃. The temperature of the mold heating zone 400 is 200℃.

[0064] The casting roller 500 is positioned close to the mold heating zone 400. The casting roller 500 includes an upper casting roller 501 and a lower casting roller 502 arranged symmetrically. The upper casting roller 501 and the lower casting roller 502 stretch the initial polypropylene film by rotating.

[0065] The traction roller 600 includes an upper traction roller 601 and a lower traction roller 602 symmetrically arranged vertically. The upper traction roller 601 and lower traction roller 602 further stretch the initial polypropylene film by rotation, obtaining a polypropylene film B, and then convey the polypropylene film B towards the take-up roller 700. The take-up roller 700 winds up the obtained polypropylene film B into a roll. The resulting polypropylene film is designated PP-90.

[0066] Example 2

[0067] The only difference from Example 1 is the content of the cyclic olefin copolymer. In Example 2, the cyclic olefin copolymer accounted for 5% of the blended granules, and the resulting polypropylene film was designated PP-95.

[0068] Example 3

[0069] The only difference from Example 1 is the content of the cyclic olefin copolymer. In Example 3, the content of the cyclic olefin copolymer was 0, and the resulting polypropylene film was designated as PP.

[0070] Comparative Example 1

[0071] The only difference from Example 1 is the production method. The obtained blended granules were hot-pressed using the hot press described in Example 1 of CN116515201A to obtain a polypropylene film.

[0072] Comparative Example 2

[0073] The only difference from Example 2 is the production method. The production method used is the same as that of Comparative Example 1.

[0074] Comparative Example 3

[0075] The only difference from Example 3 is the production method. The production method used is the same as that of Comparative Example 1.

[0076] The polypropylene films obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to double-sided gold sputtering using an ion sputtering apparatus. The breakdown field strength (denoted as Eb, unit: MV / m) of the metallized films was tested at different temperatures using an oil bath. The leakage current change was observed to determine whether breakdown had occurred. The results are shown in Tables 1 and 2.

[0077] Table 1

[0078]

[0079] Table 2

[0080]

[0081] From Table 1 and Figure 3 As shown in Table 2, the polypropylene film of the present invention exhibits good breakdown field strength at high temperatures. When the temperature is 125°C, the breakdown field strength of the polypropylene film in Example 1 reaches 456.79 MV / m, which is significantly improved compared with Comparative Examples 1, 2, and 3. This blended cast film is expected to be applied in the field of high-temperature capacitors.

[0082] This invention is not limited to the above-described embodiments. Any modifications, improvements, or substitutions that can be conceived by those skilled in the art without departing from the essential content of this invention fall within the scope of this invention.

Claims

1. An industrial production method for polypropylene film, characterized in that, Includes the following steps: 1) Provides a blend of polypropylene and cyclic olefin copolymer granules; wherein the cyclic olefin copolymer accounts for 0-15% of the mass percentage of the blend granules; 2) The blended granules are melt-cast using a casting machine to obtain a polypropylene film; wherein, the temperature of the mold heating zone in the casting machine where the melt of the blended granules is spread to form a film shape is above 195°C.

2. The industrial production method according to claim 1, characterized in that, The cyclic olefin copolymer accounts for 5-12% of the mass percentage of the blended granules.

3. The industrial production method according to claim 1, characterized in that, The casting machine further includes a feeding zone, a feeding cooling zone, and a barrel heating zone; wherein... The feeding zone is used to add the blended granules; The feeding cooling zone is connected to the feeding zone and is used to receive the blended granules and stir the blended granules. The barrel heating zone is horizontally arranged, with one end connected to the feeding cooling zone, and is used to heat the blended granules and push the melt of the heated blended granules into the mold heating zone; The mold heating zone is connected to the other end of the barrel heating zone, and has an outlet for spreading the melt into a film shape. This outlet is used to output the melt spread into a film shape, which forms an initial polypropylene film after being output.

4. The industrial production method according to claim 3, characterized in that, The feeding cooling zone is located below the feeding zone and its temperature is set below 150°C; the temperature of the barrel heating zone is set above 190°C; a screw is provided in the barrel heating zone, and the screw is configured to push the melt of the heated blended granules into the mold heating zone when rotating.

5. The industrial production method according to claim 4, characterized in that, The temperature of the feeding cooling zone is set to 90-150℃; the temperature of the barrel heating zone is set to 190-240℃; and the temperature of the mold heating zone is set to 195-240℃.

6. The industrial production method according to claim 3, characterized in that, The casting machine further includes casting rollers, traction rollers, and take-up rollers; the casting rollers are positioned close to the mold heating zone, and include an upper casting roller and a lower casting roller symmetrically arranged vertically; the upper casting roller and the lower casting roller are configured to stretch the initial polypropylene film by rotation; the traction rollers include an upper traction roller and a lower traction roller symmetrically arranged vertically, and are configured to further stretch the initial polypropylene film by rotation to obtain a polypropylene film, and convey the polypropylene film towards the take-up roller; the take-up roller is used to collect the obtained polypropylene film into a roll.

7. The industrial production method according to claim 1, characterized in that, In step 1), a co-rotating twin-screw extruder is used to prepare blended granules, and the temperatures of the feeding section, compression section and metering section of the twin-screw extruder are set to 160-170℃, 175-185℃ and 175-185℃, respectively.

8. The industrial production method according to claim 7, characterized in that, In step 1), a co-rotating twin-screw extruder is used to prepare blended granules, and the temperatures of the feeding section, compression section and metering section of the twin-screw extruder are set to 160-165℃, 180-185℃ and 180-185℃, respectively.

9. The industrial production method according to claim 1, characterized in that, The cyclic olefin copolymer is formed from bicyclo[2,2,1]-2-heptene and ethylene; the weight ratio of ethylene to the sum of the weights of bicyclo[2,2,1]-2-heptene and ethylene is 30-40:100; the cyclic olefin copolymer is an amorphous copolymer; the glass transition temperature of the cyclic olefin copolymer is 132-136°C; and the melting point of the polypropylene is 160-168°C.

10. A polypropylene film produced by an industrial production method according to any one of claims 1 to 9, characterized in that, The breakdown field strength of the polypropylene film at a temperature of 125°C is above 456 MV / m.

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