A high-temperature resistant polypropylene membrane and its preparation method, a metallized membrane, and a membrane capacitor.

Abstract

This invention relates to the field of capacitor film technology, specifically to a high-temperature resistant polypropylene film and its preparation method, a metallized film, and a film capacitor. The preparation method of the high-temperature resistant polypropylene film includes melt extrusion, casting, MD stretching, TD stretching, and shaping. The key features of this invention are as follows: the number of MD stretching segments is ≥3 to enhance the real-time release of internal stress during MD stretching and reduce shrinkage; appropriate rate control and annealing can effectively optimize film crystallization and improve film performance. Following the above technical route, the BOPP film of this invention has a shrinkage rate SMD ≤ 4.2% at 120℃ for 15 min; the absolute value of the difference in the coefficient of thermal expansion between 100℃ and 75℃ at a TMA heating rate of 5℃ / min is |CTE. 100‑ CTE 75 | < 150 μm / m·℃; breakdown strength E120 at 120℃ > 400 V / μm. This film exhibits low shrinkage and high temperature resistance, making it particularly suitable as a base film for metallized films in capacitors.

Description

Technical Field

[0001] This invention relates to the field of capacitor film technology, and in particular to a high-temperature resistant polypropylene film and its preparation method, a metallized film, and a film capacitor. Background Technology

[0002] Low-ash, narrow-distribution, and high-isotactic polypropylene resin, through processes such as melt extrusion, casting, biaxial stretching, and post-treatment, yields thin-film dielectrics with thicknesses as low as 2 μm and room-temperature breakdown strength exceeding 600 V / μm, making it an ideal dielectric material. The polypropylene film produced by biaxial stretching is then metallized by Zn and Al vapor deposition, and finally wound to obtain a thin-film capacitor. Biaxially oriented polypropylene (BOPP) metallized films have wide applications in miniaturization, high-voltage applications, and pulse capacitors.

[0003] Microscopically, molecular chain orientation is the foundation of the excellent electrical properties of BOPP films, but it is also the cause of film shrinkage. The stretched and oriented long-chain structure stores internal stress. When the external force disappears, this internal stress is released over time and with changes in the environment such as temperature—a process known as chain "disorientation"—resulting in macroscopic film size shrinkage. Existing commercially available BOPP capacitor films with a thickness below 5μm, according to GB / T13542.2-2021, under heat treatment at 120℃ for 15 minutes, generally exhibit a MD (medium-density) heat shrinkage rate of 3.0%~7.0%, while the TD (dielectric-density) heat shrinkage rate remains at 0%~0.3%. This difference between the two directions is because MD stretching at lower temperatures (below 150℃) and magnification results in a less complete oriented crystal structure and insufficient structural stability; while at higher temperatures, the pre-formed film is stretched to a greater magnification in the TD direction, resulting in better structural stability. Compared to the TD direction, MD is relatively "cold-stretched," resulting in higher internal stress. Furthermore, TD heat setting is performed at high temperatures (≥165℃), resulting in extremely high chain mobility. The mechanical position of the fixture shrinks inward by 0%~10%, and the internal stress is completely released. However, due to the limitation of mechanical stroke, the mechanical stroke in the MD direction cannot be retracted, and the MD tension traction is maintained throughout the entire processing. Therefore, the "cold drawing" property of MD and the high-speed mechanical traction work together to result in a high shrinkage rate.

[0004] The macroscopic behavior of base film shrinkage plays a crucial role in subsequent capacitor core manufacturing processes. Before and after metallization, the base film undergoes varying degrees of thermal stress release processes to shrink appropriately, preventing coating peeling caused by subsequent dimensional changes. More importantly, after the metallized film is wound (MD direction), the core needs further heat treatment at temperatures near or above 100°C to release internal stress as completely as possible. During core heat treatment, shrinkage in the MD direction forces air out of the gaps between capacitor layers, resulting in close contact between the metallized electrode and the non-evaporated film surface, thus achieving good performance. This is because air has a dielectric constant of only 1, and its presence significantly degrades the electrical performance of the core. However, if the film shrinkage rate is too large during this process, the internal stress cannot be fully released, easily leading to dimensional instability due to thermal changes during subsequent use, directly causing capacitor failure. This is one of the important causes of capacitor failure.

[0005] Because ultrathin film (below 4.0 μm) capacitor cores have more base film winding layers and relatively lower heat conduction efficiency during heat treatment, the manufacturing process of ultrathin film capacitor cores is particularly sensitive to the shrinkage rate in the winding direction (film MD direction). Excessive shrinkage rate can easily cause the outer layer of the core to shrink more than the inner layer, resulting in a "tight outer layer and loose inner layer" phenomenon, which can easily lead to "fine lines" and cause electric field distortion inside the capacitor, making it a defective product.

[0006] It is generally believed that a higher stretching ratio in biaxial stretching implies a more refined orientation structure, which is beneficial for improving electrical and mechanical properties. However, as mentioned earlier, an increased stretching ratio inevitably leads to a decrease in chain conformation entropy, resulting in a greater tendency for disorientation and relaxation, and more pronounced thermal shrinkage. High thermal shrinkage negatively impacts the performance of capacitor films, especially capacitor cores wound with films smaller than 4.0 μm. Ensuring a high MD stretching ratio while minimizing macroscopic shrinkage of the film is currently a relatively difficult task.

[0007] Therefore, reducing shrinkage in the MD direction is a crucial issue in the field of polypropylene capacitor films, but it cannot be achieved simply by reducing the stretch ratio. Ensuring sufficient MD stretch ratio while simultaneously reducing film shrinkage remains a technical challenge in the industry. Numerous patented technologies disclose techniques to achieve lower MD shrinkage. Existing technical literature includes:

[0008] Patent documents 1-3: CN117209899A, CN117558558A, CN117866255A, control film shrinkage rate with additives and fillers.

[0009] Patent document 4: CN112638645A, achieves low shrinkage rate of base film through strict raw material selection.

[0010] Patent document 5: CN119348120A, optimizing the shrinkage rate of film using stretching process.

[0011] On the other hand, the reduction of heat shrinkage rate is closely related to the dimensional stability and reliable performance of the film at high temperatures, as illustrated in Patent Document 6 (CN111051400A) and Patent Document 7 (CN 115135703 A). Improving the operational reliability of polypropylene film capacitors at high temperatures is an important direction for enhancing film performance. Summary of the Invention

[0012] To address the shortcomings of the prior art, this invention provides a high-temperature resistant polypropylene film, its preparation method, a metallized film, and a film capacitor.

[0013] To address the aforementioned technical problems, one of the technical solutions provided by this invention is as follows:

[0014] A method for preparing a high-temperature resistant polypropylene film includes the following steps:

[0015] Polypropylene is melt-extruded, cast, and cooled to obtain a polypropylene film;

[0016] The diaphragm is asynchronously biaxially stretched to obtain a high-temperature resistant polypropylene film;

[0017] The asynchronous bidirectional stretching includes longitudinal stretching and transverse stretching;

[0018] The longitudinal stretching is achieved using n stretching rollers to perform n-1 stretching segments. The stretching rollers are numbered sequentially from 1 to n, and the speed of the i-th roller is denoted as v. i ;

[0019] Wherein, the roller speed v1 of the first roller is the initial roller speed, and the roller speed v of the i-th roller (i≥2) is... i The roller speed v of the (i-1)th roller i-1 Satisfy v i =k i-1 ·v i-1 k i-1 The speed ratio of adjacent rollers; the speed ratios of each roller k1, k2, ..., k n-1 All values ​​are greater than or equal to 1, and the product of all roller speed ratios satisfies 6.0 ≤ k1·k2·...·k n-1 ≤8.0.

[0020] In one embodiment of the present invention, n≥4, v1=30~40 m / min, v n =180~230 m / min.

[0021] In one embodiment of the present invention, the temperature of the longitudinal stretching is 135~150°C.

[0022] In one embodiment of the present invention, the temperature of the transverse stretching is 155~167°C, and the transverse stretching ratio is 8.0~9.9 times.

[0023] In one embodiment of the present invention, the asynchronous bidirectional stretching further includes heat setting after bidirectional stretching;

[0024] The asynchronous stretching further includes heat setting and annealing after biaxial stretching; the heat setting temperature is 140~155℃; the annealing temperature is 130~150℃.

[0025] In one embodiment of the present invention, it further includes longitudinal stretching preheating before the longitudinal stretching and transverse stretching preheating before the transverse stretching;

[0026] The longitudinal stretching preheating temperature is 120~135℃; the transverse stretching preheating temperature is 150~163℃.

[0027] In one embodiment of the present invention, the isotactic index of the polypropylene is above 98.5%; the rheological polydispersity index (PI) is satisfied at 200°C and φ25mm parallel plate rheological test, with 2.5≤PI≤4.5; and the melt index is 2.5~4.0 g / 10min at 230°C and 2.16kg conditions.

[0028] The second technical solution provided by this invention is as follows:

[0029] A high-temperature resistant polypropylene film is prepared by the high-temperature resistant polypropylene film preparation method described above;

[0030] The thickness of the high-temperature resistant polypropylene film is 2.0~4.0μm; under conditions of 120℃ and 15min, the MD heat shrinkage rate (SMD) is ≤4.2%; with TMA heating at 5℃ / min, the absolute value of the difference in the coefficient of thermal expansion between 100℃ and 75℃ in the MD direction is |CTE 100 -CTE 75 |<150 μm / m·℃; The high-temperature resistant polypropylene film has a breakdown strength of >400V / μm at 120℃.

[0031] The third technical solution provided by this invention is as follows:

[0032] A metallized film is formed by providing a metal film on at least one side of a high-temperature resistant polypropylene film as described above.

[0033] The fourth technical solution provided by this invention is as follows:

[0034] A film capacitor is formed using a metallized film as described above.

[0035] Based on the above, compared with the prior art, this invention, starting from the perspective of multi-segment stretching of MD, achieves a solution that stretches MD to 6.0 times or more while maintaining a low heat shrinkage rate. This effectively ensures the improvement of film performance brought about by high stretching ratio, while minimizing the adverse effects of heat shrinkage, thus achieving reduced film shrinkage and improved high-temperature resistance.

[0036] Other features and beneficial effects of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other beneficial effects of the invention can be realized and obtained by means of the structures particularly pointed out in the description, claims and drawings. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Unless otherwise specified, the positional relationships shown in the drawings in the following description are based on the direction in which the components are drawn in the figure.

[0038] Figure 1 A process flow diagram of the preparation method of high-temperature resistant polypropylene film provided in the embodiments of the present invention;

[0039] Figure 2 This is a top view schematic diagram illustrating the basic overview of step D of the present invention;

[0040] Figure 3 This is a cross-sectional schematic diagram of the hot roll annealing step D of the present invention;

[0041] Figure 4 The expansion coefficient diagrams of Examples 1-2 and Comparative Examples 1 and 3 of the present invention were obtained by TMA measurement at 40°C to 120°C.

[0042] Figure 5 The graph shows the changes in the infrared peak intensity of the regular chain segments during the dynamic heating process of Embodiment 1 and Comparative Example 1 of the present invention.

[0043] Figure 6 The diagram shows the breakdown strength variation of some embodiments and comparative examples of the present invention at different temperatures. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The technical features designed in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0045] In the description of this invention, it should be noted that all terms used in this invention (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and should not be construed as limiting the invention; it should be further understood that the terms used in this invention should be understood to have the same meaning as those in the context of this specification and in the relevant field, and should not be understood in an idealized or overly formal sense, except as expressly defined in this invention.

[0046] Terminology Explanation:

[0047] MD (Machine Direction): This refers to the longitudinal direction of the film as it moves along the production line (machine running direction) in the production equipment. For example, in roller stretching, the direction in which the film is stretched along the rotation direction of the rollers is MD; the stretching is mainly achieved by adjusting the speed difference between the front and rear roller groups, which can improve the longitudinal mechanical properties of the film.

[0048] TD (Transverse Direction): This refers to the horizontal direction perpendicular to the film production direction (perpendicular to the machine's running direction). For example, in tenter frame stretching, the direction in which the film is stretched by widening the gap between the clamp chains on both sides is TD; this stretching can balance the transverse properties of the film and improve the difference between longitudinal and transverse properties.

[0049] In the field of polypropylene capacitor film, the process stability of technologies that reduce MD shrinkage by adding additives and fillers is difficult to assess, and it is impossible to conduct large-scale, comprehensive verification of whether there are any problems in different capacitor applications. From a purely process engineering perspective, the raw material structure—the stretching process (especially MD stretching)—is the key factor affecting MD shrinkage. Intuitively, balancing a high MD stretching ratio with a low MD shrinkage rate based on MD stretching is a technical challenge in engineering.

[0050] Regardless of the method used, fundamentally, any process that reduces MD shrinkage involves increasing the relaxation of molecular chains in that direction. This inevitably weakens the orientation in that direction to some extent, potentially leading to film performance degradation. This is further supported by data in subsequent comparative examples (Comparative Examples 3 & 6). The essence of this weakening of orientation stems from the fact that, under relatively high chain mobility (high temperature / slow speed), oriented molecular chains relax more than unoriented ones. However, highly crystalline films with well-developed crystal structures can reduce shrinkage and improve high-temperature resistance, as illustrated in Patent Documents 6 & 7. This can be understood as the highly crystalline regions restricting the movement of amorphous regions, hindering chain deorientation. Research has found that after biaxial stretching, maintaining tension and annealing at temperatures below the film's melting point maintains relatively moderate molecular chain mobility, making the chains more prone to orientation and crystallization rather than deorientation (relaxation), thus achieving crystallization optimization and ultimately improving film dimensional stability and high-temperature breakdown strength. Compensating for the negative impact of "relaxation" through crystallization optimization has been found to be feasible in this invention.

[0051] For this purpose, please refer to Figure 1 An embodiment of the present invention provides a method for preparing a high-temperature resistant polypropylene film, comprising the following steps:

[0052] Step 1: Provide polypropylene resin;

[0053] In specific implementation, the polypropylene used in this embodiment is electrical grade polypropylene resin, and the isotactic index of the polypropylene resin is above 98.5%, preferably above 99%. The polypropylene resin undergoes parallel plate rheological testing at 200℃, φ25mm, and δ1mm, and its rheological polydispersity (PI) satisfies 2.5 ≤ PI ≤ 4.5, for example, 2.5, 2.8, 3.0, 3.2, 3.5, 3.8, 4.0, 4.2, 4.5, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. More preferably, the rheological polydispersity (PI) of the polypropylene resin is 2.5 ≤ PI ≤ 3.5. The polypropylene resin, under conditions of 230℃ and 2.16kg, has a melt flow index of 2.5~4.0 g / 10min, for example, 2.5 g / 10min, 2.8 g / 10min, 3.0 g / 10min, 3.2 g / 10min. The melt flow index of the polypropylene is 2.5~3.5 g / 10min, 3.8 g / 10min, 4.0 g / 10min, or any value between them; more preferably, the melt flow index of the polypropylene is 2.5~3.5 g / 10min; and the weight-average molecular weight of the polypropylene resin is 3.5×10⁻⁶ g / 10min. 5 g / mol ~ 5.5 × 10 5g / mol, for example 3.5 × 10 5 g / mol, 4.0×10 5 g / mol, 4.5×10 5 g / mol, 5.0×10 5 g / mol, 5.5×10 5 g / mol or any value therebetween; the ash content of the polypropylene resin is less than 20 ppm, for example, less than 15 ppm or less than 10 ppm.

[0054] It should be noted that, at the polypropylene raw material end, the raw materials are screened from the perspectives of isotacticity, molecular weight polydispersity, and flowability (chain mobility). This is to ensure a higher molecular weight and narrower polydispersity, so that the raw material structure is conducive to the crystallization homogenization of the film during the stretching process, which in turn helps to fully restrict the amorphous regions by the crystalline regions, reduce the shrinkage rate, and improve the structural stability.

[0055] It is worth noting that the polypropylene resin may also contain other additives or copolymers or blends that do not affect the present invention, such as antioxidants, calcium stearate, nucleating agents, cyclic olefin copolymers, quasi-cyclic block polymers, and other saturated or unsaturated olefin polymers.

[0056] Step 2: Melt-extrude polypropylene, cast and cool it to obtain a polypropylene film;

[0057] In specific implementation, step 2 of this embodiment includes two steps: melt extrusion and casting cooling;

[0058] The melt extrusion process includes melting the raw material inside an extruder, which is a single-screw extruder equipped with a filter screen and a metering pump. The average temperature of the extruder is controlled between 250°C and 260°C, for example, 250°C, 251°C, 252°C, 253°C, 254°C, 255°C, 256°C, 257°C, 258°C, 259°C, 260°C, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The die temperature of the extruder is controlled between 235°C and 245°C, for example, 235°C, 236°C, 237°C, 238°C, 239°C, 240°C, 241°C, 242°C, 243°C, 244°C, 245°C, or any value between them.

[0059] The casting cooling process includes cooling and shaping the melt obtained from the extruder into a sheet using a cold roller. The temperature of the cold roller is controlled between 82°C and 95°C, for example, 82°C, 83°C, 84°C, 85°C, 86°C, 87°C, 88°C, 89°C, 90°C, 91°C, 92°C, 93°C, 94°C, 95°C, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. Preferably, the temperature of the cold roller is between 85°C and 90°C.

[0060] Step 3: The membrane is asynchronously biaxially stretched to obtain a high-temperature resistant polypropylene membrane;

[0061] In practice, the asynchronous bidirectional stretching includes longitudinal stretching and transverse stretching.

[0062] In this embodiment, the longitudinal stretching process may include three parts: longitudinal stretching preheating, longitudinal stretching, and longitudinal stretching shaping, with each part distinguished by roller speed;

[0063] In this embodiment, the roller speed for longitudinal preheating, measured in linear velocity, is 30-40 m / min, such as 30 m / min, 31 m / min, 32 m / min, 33 m / min, 34 m / min, 35 m / min, 36 m / min, 37 m / min, 38 m / min, 39 m / min, 40 m / min, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The number of preheating rollers used for longitudinal preheating can be multiple, but the roller speeds for each longitudinal preheating are preferably constant relative to each other.

[0064] The longitudinal preheating temperature is 120~135℃, for example, 120℃, 121℃, 122℃, 123℃, 124℃, 125℃, 126℃, 127℃, 128℃, 129℃, 130℃, 131℃, 132℃, 133℃, 134℃, 135℃, or any value between them. The number of preheating rollers used for longitudinal preheating can be multiple, but the temperature of each longitudinal preheating roller can be constant relative to the others, preferably increasing sequentially from roller to roller.

[0065] In this embodiment, the longitudinal stretching is achieved through multiple stretching rollers with progressively increasing speeds, resulting in multi-segment longitudinal stretching. Specifically, the longitudinal stretching uses n stretching rollers to achieve n-1 segments of stretching, with each stretching roller numbered sequentially from 1 to n, and the speed of the i-th roller denoted as v. i ;

[0066] Wherein, the roller speed v1 of the first roller is the initial roller speed, and the roller speed v of the i-th roller (i≥2) is... i The roller speed v of the (i-1)th roller i-1 Satisfy v i =ki-1 ·v i-1 k i-1 The speed ratio of adjacent rollers; the speed ratios of each roller k1, k2, ..., k n-1 All values ​​are greater than or equal to 1, and the product of all roller speed ratios satisfies 6.0 ≤ k1·k2·...·k n-1 ≤8.0.

[0067] This invention incorporates multiple stretching rollers in the longitudinal stretching process, with the roller speeds increasing monotonically one by one. This improves upon the fundamental process point of low shrinkage in microstructure (MD), namely, the excessive "orientation" and insufficient "relaxation" in the MD direction during processing. Relaxation refers to the release of residual internal stress during processing.

[0068] In existing biaxially oriented film processes, high stretching ratios and high temperatures in the TD direction lead to recrystallization into perfectly oriented TD crystals with a high degree of amorphous chain binding, making them less prone to shrinkage. Furthermore, after TD stretching, internal stress can be effectively released through higher-temperature heat setting and mechanical stroke retraction. In contrast, MD stretching, with its lower-temperature "cold drawing," inherently stores large amounts of internal stress and maintains tension without retraction. This invention adjusts the existing MD stretching process, which involves stretching in one section between two rollers, to three or more sections, extending the relaxation time of each section without reducing the MD stretching ratio. This alleviates the problem of insufficient relaxation at the MD stretching process point.

[0069] The high shrinkage of membrane membranes (MD) is caused by the high-speed mechanical traction of the production line. Research has found that controlling the production speed to around 200 m / min ensures both production efficiency and stability while further extending the relaxation time. Too low a speed results in an excessively long relaxation time, potentially degrading membrane performance; more seriously, a too low speed significantly hinders efficiency. Too high a speed leaves a large amount of residual internal stress, making it difficult to achieve the low-shrinkage effect described in this invention.

[0070] In practice, research has shown that excessive segmented MD stretching and / or excessively slow production rates are not entirely beneficial to the mechanical and electrical properties of the film. This may be due to excessive "relaxation." To address this, this application proposes a strategy to optimize crystallization through post-crystallization processing, namely, releasing internal stress near 150°C and performing thermal annealing near 140°C. This can be understood as reducing melting (disorientation) and promoting recrystallization (crystallization orientation), thus compensating for and optimizing the performance degradation that may result from relaxation, thereby improving the film's performance.

[0071] It should be noted that for BOPP films formed by single-stage MD stretching, direct processing in the aforementioned post-crystallization strategy can cause tension film breakage due to rapid cooling. Therefore, multi-roller, multi-stage MD stretching is necessary. The internal stress released during the multi-stage MD stretching process matches the stress during subsequent low-temperature setting and annealing, which improves production stability and is a crucial factor in the excellent performance and stable production of the products of this invention.

[0072] In a preferred embodiment of the present invention, the MD stretching is a multi-roller contact stretching; the MD stretching uses ≥4 stretching rollers to achieve 3-stage stretching, thereby enhancing the real-time release of internal stress in MD stretching, extending chain relaxation and reducing shrinkage rate; the production line speed is controlled at 180m / min~230m / min to ensure a low shrinkage rate under a large stretching ratio; the production line speed is, for example, 180 m / min, 190 m / min, 200 m / min, 210 m / min, 220 m / min, 230 m / min, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable;

[0073] More preferably, the MD stretching uses 4 to 6 stretching rollers;

[0074] For example, the longitudinal stretching uses four stretching rollers to achieve three-stage stretching. The initial roller speeds are v1, v2 = k1·v1, v3 = k2·v2, and v4 = k3·v3 = V. d The condition 6.0 ≤ k1·k2·k3 ≤ 8.0, and k1, k2, and k3 are all greater than 1, is given by V. d The speed of the longitudinal stretching and shaping rollers.

[0075] Taking the longitudinal stretching using 5 stretching rollers to achieve 4-segment stretching as an example, the initial roller speeds are: v1, v2 = k1, v3 = k2 * v2, v4 = k3 * v3, and v5 = k4 * v4 = V. d The condition 6.0 ≤ k1·k2·k3·k4 ≤ 8.0, and k1, k2, k3, and k4 are all greater than 1, is given by V. d The speed of the longitudinal stretching and shaping rollers.

[0076] Taking the longitudinal stretching using 6 stretching rollers to achieve 5-segment stretching as an example, the initial roller speeds are: v1, v2 = k1, v3 = k2 = k2, v4 = k3 = k3, v5 = k4 = k4, and v6 = k5 = k5. d The condition V satisfies 6.0 ≤ k1·k2·k3·k4·k5 ≤ 8.0, and k1, k2, k3, k4, and k5 are all greater than 1. d The speed of the longitudinal stretching and shaping rollers.

[0077] It should be noted that non-speed differential rollers may also exist in the MD stretching rollers. For example, taking the aforementioned six stretching rollers as an example, k2 and k4 = 1, meaning that the speeds of the third and fifth stretching rollers located in the middle are the same as those of the preceding stretching rollers. Here, for the sake of simplicity, a concise example is used.

[0078] It is worth noting that although only 3 to 5 segments of MD stretching are shown in the above example, based on the basic principles provided by this invention, it should be understood that as the number of segments continues to increase, the same effect as described in the high-temperature resistant polypropylene film of this invention can be achieved.

[0079] In a preferred embodiment of the present invention, the initial roller speed v1 is 30~40 m / min, for example 30 m / min, 31 m / min, 32 m / min, 33 m / min, 34 m / min, 35 m / min, 36 m / min, 37 m / min, 38 m / min, 39 m / min, 40 m / min, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. n The speed of the longitudinally stretched end stretching roller, v n =180~230 m / min, for example 180 m / min, 190 m / min, 200 m / min, 210 m / min, 220 m / min, 230 m / min, etc., but not limited to the listed values; other unlisted values ​​within this range also apply. It should be noted that the v... n Can be used with V d They can be consistent, or they can be inconsistent; when they are inconsistent, the difference does not exceed V. d 2%.

[0080] The longitudinal stretching temperature is 135~150℃, such as 135℃, 136℃, 137℃, 138℃, 139℃, 140℃, 141℃, 142℃, 143℃, 144℃, 145℃, 146℃, 147℃, 148℃, 149℃, 150℃, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0081] In this embodiment, the longitudinal stretching and shaping roller speed is 180~230 m / min, such as 180 m / min, 190 m / min, 200 m / min, 210 m / min, 220 m / min, 230 m / min, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. The number of shaping rollers used for longitudinal stretching and shaping can be multiple, but the speed of each longitudinal stretching and shaping roller is preferably constant among themselves.

[0082] The longitudinal stretching and shaping temperature is 148~155℃, for example 148℃, 149℃, 150℃, 151℃, 152℃, 153℃, 154℃, 155℃ or any value between them.

[0083] In this embodiment, the transverse stretching process may include transverse preheating, transverse stretching, heat setting, and annealing.

[0084] In this embodiment, the temperature for the transverse stretching preheating is 150~163℃, such as 150℃, 151℃, 152℃, 153℃, 154℃, 155℃, 156℃, 157℃, 158℃, 159℃, 160℃, 161℃, 162℃, 163℃, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0085] The lateral stretching is performed by a mechanical clamp, which provides lateral tension and assists in forward movement. The temperature of the lateral stretching is 155~167℃, and the lateral stretching ratio is 8.0~9.9 times, followed by heat setting.

[0086] The lateral stretching temperature is, for example, 155℃, 156℃, 157℃, 158℃, 159℃, 160℃, 161℃, 162℃, 163℃, 164℃, 165℃, 166℃, 167℃, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The lateral stretching ratio is, for example, 8.0 times, 8.2 times, 8.5 times, 8.8 times, 9.0 times, 9.2 times, 9.5 times, 9.8 times, 9.9 times, or any value between them.

[0087] In this embodiment, please refer to Figure 2 and Figure 3The asynchronous stretching further includes heat setting and annealing after biaxial stretching; wherein, the heat setting is preferably air-heat setting, during which the clamp tension is maintained at all times, the stroke in the TD direction retraction is within 2%, preferably less than 1.5%, and the heat setting temperature is 140~155℃, thus completing the setting; the heat setting temperature is, for example, 140℃, 141℃, 142℃, 143℃, 144℃, 145℃, 146℃, 147℃, 148℃, 149℃, 150℃, 151℃, 152℃, 153℃, 154℃, 155℃, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable;

[0088] After heat setting, the TD fixture is disengaged, and the subsequent traction rollers are circulated with oil heat. Multiple heated rollers (≥2 rollers) can be used for traction. The annealing temperature is 130~150℃, the wrap angle of the film on each roller is greater than 45° and less than 180°, and the film moves in an S-shape between the rollers. The annealing temperature is, for example, 130℃, 131℃, 132℃, 133℃, 134℃, 135℃, 136℃, 137℃, 138℃, 139℃, 140℃, 141℃, 142℃, 143℃, 144℃, 145℃, 146℃, 147℃, 148℃, 149℃, 150℃, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. Preferably, the annealing temperature is 140~150℃.

[0089] It should be noted that the asynchronous bidirectional stretching includes stretching laterally first and then stretching longitudinally.

[0090] This invention targets polypropylene resin materials and polypropylene films. Specifically, compared to the linear flexible chain polypropylene material of this invention, film materials such as polyester have relatively poor crystallinity and weaker chain mobility due to the inherent rigidity and steric hindrance of their chain structure. They also have relatively higher application temperatures. Polyester films (taking PET as an example) typically exhibit a heat shrinkage rate of <2% at 150℃ / 30min and <1% at 120℃; while BOPP films typically show a heat shrinkage rate of 5%~10% at 120℃. In other words, PET films generally maintain excellent performance at temperatures between 100~150℃, while PP films experience dimensional instability below 100℃. This is the temperature resistance problem related to dimensional instability in PP films that this invention aims to solve.

[0091] However, due to the inherent differences in their chain structures, the effect of multi-segment stretching of MD on reducing shrinkage in polyester and other films is weak, as their shrinkage is already sufficiently low. This application specifically targets polypropylene capacitor films, combining the MD stretching process with an annealing and setting process based on the film's linear flexible chain structure to achieve the effects described in this application.

[0092] Step 4: Post-process the obtained high-temperature resistant polypropylene film;

[0093] In practice, the post-processing includes corona treatment, aging treatment, and winding treatment.

[0094] The corona treatment generates polar groups on the surface of the polypropylene film through high-frequency high-voltage discharge, increasing surface tension, improving the wettability and adhesion of the film, facilitating subsequent coating with metal layers or other functional coatings, ensuring a firm bond between the metal layer and the film, preventing detachment, and improving the stability of the capacitor.

[0095] The aging treatment involves placing the film in a specific environment (such as constant temperature and humidity) for a period of time to release internal residual stress, further stabilize the molecular chain structure, reduce dimensional changes in the film during subsequent processing or use, and ensure the geometric accuracy and performance consistency of the capacitor.

[0096] The winding process allows the treated film to be neatly wound into a roll according to the specified tension and diameter, facilitating storage, transportation, and subsequent cutting. At the same time, controlling the winding tension can prevent the film from wrinkling or stretching, ensuring that the film can be smoothly unfolded and used in subsequent processes.

[0097] This invention provides a high-temperature resistant polypropylene film, which is prepared by the high-temperature resistant polypropylene film preparation method described above;

[0098] In a preferred embodiment of the present invention, the thickness of the high-temperature resistant polypropylene film is 2.0~4.0μm, for example 2.0μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm, 2.6μm, 2.7μm, 2.8μm, 2.9μm, 3.0μm, 3.1μm, 3.2μm, 3.3μm, 3.4μm, 3.5μm, 3.6μm, 3.7μm, 3.8μm, 3.9μm, 4.0μm, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 2.9~3.4μm; under 120℃&15min conditions, the MD heat shrinkage rate SMD≤4.2%; when the TMA temperature rises at 5℃ / min, the absolute value of the difference in the coefficient of thermal expansion between 100℃ and 75℃ in the MD direction |CTE 100 -CTE 75 |<150 μm / m·℃; The high-temperature resistant polypropylene film has a breakdown strength of >400V / μm at 120℃.

[0099] The metallized film of the present invention is formed by providing a metal layer on at least one side of the above-mentioned high-temperature resistant polypropylene film.

[0100] The metal layer functions as an electrode. The metals used in the metal layer can be, for example, elemental metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, as well as various mixtures and alloys thereof. Among these, zinc and aluminum are preferred due to their environmental impact, economic efficiency, and excellent capacitor performance.

[0101] There are no particular limitations on the method of laminating a metal layer on at least one side (one side or two sides) of a polypropylene film, and examples include vacuum evaporation and sputtering. From the viewpoint of superior productivity and economy, vacuum evaporation is preferred. Examples of vacuum evaporation methods include crucible deposition and wire deposition, and the optimal method can be selected appropriately.

[0102] There are no particular limitations on the edge pattern when stacking metal layers by vapor deposition. From the viewpoint of further improving the safety of the capacitor, preventing capacitor damage, and further suppressing short circuits, it is preferable to apply patterns containing so-called special edges, such as fishing net patterns or T-shaped edge patterns, to a single surface of the biaxially stretched polypropylene film.

[0103] There are no particular limitations on the method for forming the edge; it can be formed using known methods such as the strip method or the oil method.

[0104] The thickness of the metallized film in this invention is not particularly limited.

[0105] The membrane capacitor of the present invention is formed using the above-described metallized film. The metallized film of this disclosure can be laminated or wound using conventionally known methods to form the membrane capacitor.

[0106] The aforementioned film capacitor can have a structure with multiple stacked metallized films, or it can have a wound metallized film. Such film capacitors are suitable for use as capacitors in inverter power supply equipment for controlling the drive engines of electric vehicles, hybrid vehicles, etc. Furthermore, they are also suitable for applications in railway vehicles, wind power generation, solar power generation, and general household appliances.

[0107] The experimental results demonstrating the technical advantages of the present invention will be described below using examples and comparative examples.

[0108] Example 1

[0109] This embodiment provides a method for preparing polypropylene film for capacitors, the steps of which are melt extrusion, casting and cooling, MD preheating-stretching-setting, TD preheating-stretching, setting zone, and post-treatment (corona treatment, aging, slitting, etc.).

[0110] (A) In the melt extrusion step, electrical-grade polypropylene resin is fed into the extruder, specifically HC300BF from Borealis, with an isotacticity of 98.9% and a weight-average molecular weight of 4.2 × 10⁻⁶. 5The extruder exhibits a polydispersity index (PI) of 3.4, a melt index of 3.1 g / 10 min at a test temperature of 230℃ and a load of 2.16 kg, and an ash content of 18 ppm. The average temperature inside the extruder is 255℃, and the die temperature is 238℃.

[0111] (B) In the rapid cooling of the thick sheet step, the melt is cast onto a single roll for cooling and forming of the thick sheet. The temperature of the rapid cooling roll is 90°C, the temperature of the air knife is 45°C, and the pressure of the air knife is 140mba, to ensure that the melt adheres to the surface of the rapid cooling roll.

[0112] (C) In the MD preheating-stretching-setting step, the total number of rollers is 16. There are 6 preheating rollers with a speed of v1 = 30 m / min and roller temperatures from front to back along the MD direction of 122±1℃, 124±1℃, 128±1℃, 130±1℃, 134±1℃, and 135±1℃, respectively. There are 6 stretching rollers with speeds from front to back along the MD direction of 30 m / min, 40 m / min, 100 m / min, 100 m / min, 195 m / min, and 195 m / min, respectively, with a total ratio of 6.5 times, and temperatures of 138±1℃, 140±1℃, 142±1℃, 142±1℃, 145±1℃, and 145±1℃, respectively. There are 4 setting rollers with a speed of V... d =195m / min, temperature is 146±1℃.

[0113] (D) In ​​the preheating-stretching-heat setting-annealing steps of TD, air heating is adopted, with a preheating temperature of 162±2℃; a stretching temperature of 164±2℃ and a ratio of 9.2 times; a heat setting temperature of 149±1℃; and a roll annealing temperature of 140±1℃.

[0114] Then, after corona treatment and winding, a high-temperature resistant polypropylene film is obtained.

[0115] Example 2

[0116] This embodiment provides a method for preparing polypropylene film for capacitors, the steps of which are (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching and setting zone, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0117] Steps (A) and (B) are consistent with those in Example 1.

[0118] (C) In the MD preheating-stretching-setting step, the total number of rollers is 16. There are 6 preheating rollers with a speed of v1 = 30 m / min and roller temperatures from front to back along the MD direction of 122±1℃, 124±1℃, 128±1℃, 130±1℃, 134±1℃, and 135±1℃, respectively. There are 6 stretching rollers with speeds from front to back along the MD direction of 30 m / min, 60 m / min, 120 m / min, 180 m / min, 180 m / min, and 210 m / min, respectively, with a total ratio of 7.0, and temperatures of 140±1℃, 140±1℃, 142±1℃, 142±1℃, 145±1℃, and 147±1℃, respectively. There are 4 setting rollers with a speed of V... d =210m / min, temperature is 148±1℃.

[0119] (D) In ​​the preheating-stretching-heat setting-annealing steps of TD, air heating is adopted, with a preheating temperature of 162±2℃; a stretching temperature of 164±2℃ and a ratio of 9.2 times; a heat setting temperature of 149±1℃; and a roll annealing temperature of 140±1℃.

[0120] Example 3

[0121] This embodiment provides a method for preparing polypropylene film for capacitors, the steps of which are (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching and setting zone, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0122] Steps (A) and (B) are consistent with those in Example 1.

[0123] In step (C), a total of 16 rollers are used: 6 preheating rollers with a speed of v1=35 m / min; 6 stretching rollers with speeds of 32 m / min, 45 m / min, 60 m / min, 80 m / min, 105 m / min, and 208 m / min from front to back in the MD direction, respectively, with a total magnification of 6.5 times; and temperatures of 140±1℃, 142±1℃, 144±1℃, 146±1℃, 148±1℃, and 149±1℃, respectively; and 4 setting rollers with a temperature of 149±1℃ and a speed of V. d =208 m / min, temperature is 146±1℃.

[0124] In step (D), the preheating temperature is 163±2℃; the stretching temperature is 165±2℃, the ratio is 9.2 times; the heat setting temperature is 142±1℃; and the roll annealing temperature is 135±1℃.

[0125] Then, after corona treatment and winding, a high-temperature resistant polypropylene film is obtained.

[0126] Example 4

[0127] This embodiment provides a method for preparing polypropylene film for capacitors, with the following steps in sequence: (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching-heat setting-annealing, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0128] The only difference between this embodiment and Embodiment 1 is the speed of the stretching rollers in step (C). There are six stretching rollers with speeds of 30 m / min, 30 m / min, 56 m / min, 104 m / min, 195 m / min, and 195 m / min respectively from front to back in the MD direction, with a total speed ratio of 6.5. The remaining steps (A) to (D) are consistent with Embodiment 1.

[0129] After biaxial stretching, shaping, and annealing, the polypropylene film is obtained through corona treatment and winding.

[0130] Example 5

[0131] This embodiment provides a method for preparing polypropylene film for capacitors, with the following steps in sequence: (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching-heat setting-annealing, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0132] The only difference between this embodiment and Embodiment 2 is that in step (C), there are six stretching rollers with speeds of 30 m / min, 35 m / min, 35 m / min, 45 m / min, 180 m / min, and 180 m / min respectively from front to back in the MD direction, resulting in a total multiplier of 6.0. d =180m / min. The remaining steps (A) to (D) are consistent with those in Example 1.

[0133] After biaxial stretching, shaping, and annealing, the polypropylene film is obtained through corona treatment and winding.

[0134] Example 6

[0135] This embodiment provides a method for preparing polypropylene film for capacitors, with the following steps in sequence: (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching-heat setting-annealing, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0136] (A) In the melt extrusion step, electrical-grade polypropylene resin, specifically HTP-5014L from Daehan Oil Chemical Co., Ltd., was fed into the extruder. The resin had an isotacticity of 99.1%, a weight-average molecular weight of 4.5 × 10⁵ g / mol, a rheological polydispersity index (PI) of 4.1, a melt index of 3.5 g / 10 min at a test temperature of 230℃ and a load of 2.16 kg, and an ash content of 17 ppm. The average temperature inside the extruder was 260℃, and the die temperature was 240℃.

[0137] The subsequent steps (B), (C), and (D) are consistent with those in Example 1.

[0138] After biaxial stretching, shaping, and annealing, the polypropylene film is obtained through corona treatment and winding.

[0139] Comparative Example 1

[0140] This comparative example provides a method for preparing polypropylene film for capacitors, with the following steps in sequence: (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching and setting zone, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0141] Steps (A) and (B) are consistent with those in Example 1.

[0142] In step (C), a total of 16 rollers are used: 6 preheating rollers with a speed of v1=35 m / min; 6 stretching rollers with speeds of 35 m / min, 35 m / min, 35 m / min, 35 m / min, 210 m / min, and 220 m / min from front to back in the MD direction, respectively, with a total scaling factor of 6.3 times; and temperatures of 140±1℃, 142±1℃, 144±1℃, 146±1℃, 148±1℃, and 149±1℃, respectively; and 4 setting rollers with a speed of V... d =220m / min, temperature is 146±1℃.

[0143] In step (D), the preheating temperature is 163±2℃; the stretching temperature is 165±2℃, and the stretching ratio is 9.2 times; heat setting is carried out only under the tension of the clamps at a temperature of 167~171℃ to fully release internal stress. The hot roller thermal circulation in the setting zone of this scheme is shut off, no annealing treatment is performed, and the temperature is 25~35℃, with direct cooling at room temperature.

[0144] Then, after corona treatment and winding, a high-temperature resistant polypropylene film is obtained.

[0145] Comparative Example 2

[0146] The process details and parameters of Comparative Example 2 are completely identical to those of Comparative Example 1. The only difference is that in step (C), in Comparative Example 2, v1 = 20 m / min, V d =130m / min.

[0147] Comparative Example 3

[0148] The process details and parameters of Comparative Example 3 are completely identical to those of Comparative Example 1. The only difference is that in step (C), in Comparative Example 3, v1 = 44 m / min, V d =290m / min.

[0149] Comparative Example 4

[0150] This comparative example provides a method for preparing polypropylene film for capacitors, with the following steps in sequence: (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching-heat setting-annealing, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0151] Steps (A) & (B) are consistent with (C) and Comparative Example 1, except for step (D);

[0152] In step (D), the preheating temperature is 163±2℃; the stretching temperature is 165±2℃, and the stretching ratio is 9.2 times; the setting process is carried out only under the tension of the clamps, at a temperature of 167~171℃, to fully release internal stress. The temperature of the hot roller in the setting zone of the aforementioned scheme is similar to that of the TD setting temperature, but higher, at 160℃.

[0153] Then, after corona treatment and winding, a high-temperature resistant polypropylene film is obtained.

[0154] In this comparative example, the production stability was poor during the experiment, and the film breakage mostly occurred directly during the hot roller contact stage of the shaping process; the time interval between multiple film breakages was <30 min.

[0155] Comparative Example 5

[0156] This comparative example provides a method for preparing polypropylene film for capacitors, with the following steps in sequence: (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching-heat setting-annealing, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0157] The difference between this comparative example and Comparative Example 1 lies only in step (C). In step (C), there are six MD stretching rollers, with speeds from front to back along the MD direction of 32 m / min, 32 m / min, 32 m / min, 32 m / min, 220 m / min, and 220 m / min, respectively, resulting in a total stretching ratio of 6.9 times. This is achieved through a single MD stretching operation. All other parameters are consistent with those of Example 2. In particular, in step (D), this comparative example also undergoes heat setting with a clamp at 149±1℃, followed by hot roller annealing at 140±1℃.

[0158] Then, after corona treatment and winding, a high-temperature resistant polypropylene film is obtained.

[0159] In this comparative example, the production stability was poor during the experiment, and the film breakage mostly occurred during the heat setting of the fixture.

[0160] Comparative Example 6

[0161] This comparative example provides a method for preparing polypropylene film for capacitors, with the following steps in sequence: (A) melt extrusion, (B) casting and cooling, (C) MD preheating-stretching-setting, (D) TD preheating-stretching-heat setting-annealing, and finally post-treatment (corona treatment, aging, slitting, etc.) to obtain a high-temperature resistant polypropylene film.

[0162] Steps (A) and (B) are consistent with those in Example 1.

[0163] In step (C), there are a total of 16 rollers: 6 preheating rollers with a speed of v1=35m / min; 6 stretching rollers with speeds of 35 m / min, 45 m / min, 60 m / min, 80 m / min, 105 m / min, and 200 m / min from front to back in the MD direction, respectively, with a total magnification of 5.7 times, and temperatures of 140±1℃, 142±1℃, 144±1℃, 146±1℃, 148±1℃, and 149±1℃, respectively; and 4 setting rollers with a temperature of 149±1℃.

[0164] In step (D), the preheating temperature is 163±2℃; the stretching temperature is 165±2℃, and the ratio is 9.2 times; the temperature of the air-heat setting zone is 165℃, and the hot roller annealing zone is closed, which is equivalent to the cold roller cooling and setting.

[0165] Then, after corona treatment and winding, a high-temperature resistant polypropylene film is obtained.

[0166] Test case

[0167] The high-temperature resistant polypropylene films prepared in the above examples and comparative examples were tested using the following methods:

[0168] Rheological polydispersity testing was performed based on the principles described in Phys. Fluids, 35, 063108 (2023). A DHR-20 rotational rheometer (TA, America) was used. A 25 mm parallel plate with a 1 mm gap was selected for the test. The test mode was oscillation mode, the temperature was 200 °C, the pulse velocity was 500~0.1 rad / s, and the strain was controlled at 1%.

[0169] Thermomechanical analysis (TMA) was performed using a TMA-Q450 (TA, America) with a fiber / film fixture, a sample length of 16 mm, and a width of 4 mm. The test temperature range was 40~150℃, and the heating rate was 5℃ / min. The coefficient of thermal expansion (CTE) was calculated based on the dimensional changes.

[0170] Crystallinity and melting peak temperature were measured by differential thermal analysis, with the standard enthalpy of crystallization for polypropylene taken as 207 J / g. A DSC8500 (PerkinElmer, America) was used to heat the polypropylene from 30℃ to 220℃ at a rate of 10℃ / min, recording the heat flow-temperature curves, melting peak temperature, and calculating the crystallinity.

[0171] Dynamic-heated infrared detection, using Nicolet-FTIR, was conducted to test the chain motion of the film under heated conditions of 120~200℃, in order to explore the structural reasons for the improved high-temperature performance of the film.

[0172] The physicochemical properties described in this invention include breakdown strength (at 25°C, 85°C, 100°C, and 120°C), tensile strength, and heat shrinkage rate (at 120°C for 15 min), and are tested according to the method in GB / T13542.2-2021. In the electrode method breakdown strength test, the electrode is cylindrical with a diameter of 25 mm.

[0173] Table 1. Properties of polypropylene capacitor films in the examples and comparative examples.

[0174]

[0175] In Table 1, the unit for thickness is "μm", the unit for MD shrinkage rate is "%", the unit for breakdown strength is "V / μm", the absolute value of the difference in thermal expansion coefficient is the absolute value of the difference in thermal expansion coefficient at 100℃ and 75℃, and its unit is "μm / m·℃", the unit for MD tensile strength is "MPa", the unit for TD tensile strength is "MPa", and the unit for crystallinity is "%".

[0176] It should be noted that you should refer to [link / reference]. Figure 5 The present invention conducted dynamic temperature-increasing FT-IR tests on Example 1 and Comparative Example 1 to provide feedback on the structural reasons for the improved high-temperature stability of the thin film in the present invention. 998cm-1 and 841cm -1 The symbols represent the relevant information of regular sequence bands with a minimum number of units (n) of 10 and 12, respectively. As the test temperature increased from 125°C to 200°C, the absorption intensity of the two regular sequence signal peaks in the PP film gradually decreased, indicating that the high temperature was disrupting the film's orderliness. However, in Example 1 of this invention, the decrease was more slow, indicating a more complete ordered molecular chain structure, which contributes to improved high-temperature electrical performance.

[0177] It should be noted that this invention conducted systematic breakdown strength tests at different temperatures on typical comparative examples and embodiments. As mentioned earlier, the study found that while simple low-rate and multi-segment stretching can effectively reduce MD shrinkage, its performance is also affected to some extent due to increased MD relaxation. For example, Comparative Examples 3 and 6, although they can achieve very low shrinkage, exhibit significantly poor electrical and mechanical properties at high temperatures. Furthermore, when the hot roll annealing temperature approaches the melting point, it easily causes damage to the film, as seen in Comparative Example 4 (e.g., Figure 6 (Right) This poor electrical performance may be due to the excessively high temperature of the hot roller directly melting the film.

[0178] The foregoing has shown and described the basic principles, main features and advantages of this invention. Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are only illustrative of the principles of this invention. Various changes and modifications can be made to this invention without departing from the spirit and scope of this invention. All such changes and modifications fall within the scope of this invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

[0179] Furthermore, those skilled in the art should understand that although many problems exist in the prior art, each embodiment or technical solution of the present invention can be improved in only one or a few aspects, without necessarily solving all the technical problems listed in the prior art or the background art simultaneously. Those skilled in the art should understand that any content not mentioned in a claim should not be construed as a limitation on that claim.

[0180] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a high-temperature resistant polypropylene film, characterized in that, Includes the following steps: Polypropylene is melt-extruded, cast, and cooled to obtain a polypropylene film; The diaphragm is asynchronously biaxially stretched to obtain a high-temperature resistant polypropylene film; The asynchronous bidirectional stretching includes longitudinal stretching and transverse stretching; The longitudinal stretching is achieved using n stretching rollers to perform n-1 stretching segments. The stretching rollers are numbered sequentially from 1 to n, and the speed of the i-th roller is denoted as v. i ; Wherein, the roller speed v1 of the first roller is the initial roller speed, and the roller speed v of the i-th roller (i≥2) is... i The roller speed v of the (i-1)th roller i-1 Satisfy v i =k i-1 ·v i-1 k i-1 The speed ratio of adjacent rollers; the speed ratios of each roller k1, k2, ..., k n-1 All values ​​are greater than or equal to 1, and the product of all roller speed ratios satisfies 6.0 ≤ k1·k2·...·k n-1 ≤8.0; The longitudinal stretching temperature is 135~150℃; the transverse stretching temperature is 155~167℃. The asynchronous biaxial stretching also includes heat setting and annealing after biaxial stretching; the heat setting temperature is 140~155℃; the annealing temperature is 130~150℃. It also includes longitudinal preheating before the longitudinal stretching and transverse preheating before the transverse stretching; The longitudinal preheating temperature is 120~135℃; the transverse preheating temperature is 150~163℃.

2. The method for preparing the high-temperature resistant polypropylene film according to claim 1, characterized in that, Where n≥4, v1=30~40 m / min, v n =180~230 m / min.

3. The method for preparing the high-temperature resistant polypropylene film according to claim 1, characterized in that, The lateral stretching ratio is 8.0 to 9.9 times.

4. The method for preparing the high-temperature resistant polypropylene film according to claim 1, characterized in that, The isotactic index of the polypropylene is above 98.5%; the rheological polydispersity index (PI) meets the requirements of 2.5 ≤ PI ≤ 4.5 in the rheological test of parallel plates at 200℃ and φ25mm, and the melt index is 2.5~4.0 g / 10min under the conditions of 230℃ and 2.16kg.

5. A high-temperature resistant polypropylene film, prepared by the method for preparing the high-temperature resistant polypropylene film according to any one of claims 1 to 4; The thickness of the high-temperature resistant polypropylene film is 2.0~4.0μm; under conditions of 120℃ and 15min, the MD heat shrinkage rate (SMD) is ≤4.2%; with TMA heating at 5℃ / min, the absolute value of the difference in the coefficient of thermal expansion between 100℃ and 75℃ in the MD direction is |CTE 100 -CTE 75 | <150 μm / m·℃; the breakdown strength of the high-temperature resistant polypropylene film at 120℃ is >400V / μm.

6. A metallized film is formed by providing a metal film on at least one side of the high-temperature resistant polypropylene film according to claim 5.

7. A film capacitor formed using the metallized film of claim 6.

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