Polypropylene film and preparation method thereof, metallized film and film capacitor

Abstract

The present invention relates to the technical field of polypropylene films, and in particular to a polypropylene film and a preparation method thereof, a metallized film and a film capacitor. The method comprises the following steps: melt-extrude polypropylene, cast and cool to obtain a polypropylene film sheet; biaxially stretch the polypropylene film sheet to obtain; the extruder die temperature of the melt-extrusion is T1, the cold roller temperature of the cast and cool is T2, and the difference between the die temperature and the cold roller temperature is supercooling ΔT; the ratio of the supercooling ΔT to the cold roller temperature T2 satisfies 1.9≤ΔT / T2≤2.25; wherein ΔT=T1‑T2. The polypropylene film prepared by the preparation method of the present invention has a low degree of corona surface fluctuation, and the corona surface topology satisfies 300nm≤S z ≤700nm, ‑0.3≤SSK≤0. The polypropylene film is coated and wound to obtain a capacitor with low ESR and low loss, and is particularly suitable for high-frequency pulse capacitors and related fields.

Description

Technical Field

[0001] The present invention relates to the technical field of polypropylene films, in particular to a polypropylene film and a preparation method thereof, a metallized film and a film capacitor. Background Art

[0002] Isotactic polypropylene, with its highly ordered isotactic optical structure and non-polar chemical structure, possesses exceptionally strong crystallization ability. This ordered chain structure and crystallization behavior give polypropylene film the characteristics of high breakdown field strength, low dielectric loss, and high charge and discharge efficiency. Pulse capacitors, with their high electromagnetic pulse withstand capability, low inductance, and rapid charge and discharge, are widely used in pulse power technology, lasers, radar, and other fields, capable of withstanding high charge and discharge frequencies and short-term high voltages. Polypropylene film is currently the most commonly used base dielectric material in pulse capacitors.

[0003] In existing technologies, the polypropylene raw material undergoes melt extrusion, cast cooling thick sheets, and biaxial stretching to complete the base film preparation process. The base film undergoes a complex crystalline structure evolution within, possessing a uniform fiber crystal network structure, enabling it to withstand sufficiently high voltages. Meanwhile, a large number of roughened rings (topological structures) are generated on the surface of the base film, giving the film a complex surface undulation and a certain degree of roughness. The base film then undergoes a continuous post-processing process, where one of the two surfaces is corona treated to introduce polar groups to enhance its adhesion. This surface is referred to as the corona surface. Finally, the aged base film is cut into rolls of varying sizes and shapes for subsequent metallization.

[0004] Metallization involves passing the base film through a coating machine to complete the evaporation treatment on the corona surface. After metal is vapor-deposited, a metal coating is deposited on the corona surface. The resulting composite film, consisting of the coating and the base film, is called a metallized film. After a specific design, the metallized film is repeatedly and alternately stacked and wound to complete the assembly of a capacitor.

[0005] The design of the metal coating is crucial in the design of high-frequency pulse capacitors. The extremely high pulse counts and charge-discharge frequencies require capacitor designs with low energy loss to ensure stable operation over long periods of time. Increased losses lead to comprehensive degradation in capacitor device performance, including not only a decrease in capacitance but also a reduction in withstand voltage due to factors such as heat generation. This poses numerous risks for pulse capacitors used in extreme environments. To mitigate the effects of excessive losses, the base film must first be low-loss, making polypropylene the preferred material. Secondly, the metal coating must also be low-loss, which requires a vapor-deposited coating with low sheet resistance (Ω / □). To achieve this, existing inventions and solutions primarily focus on increasing coating thickness, as exemplified by CN108497865A and CN103077821A. Some capacitor design principles, such as CN114360904A, CN112271082A, and CN104078233A, also incorporate considerations of increased coating thickness. It can be understood that the voltage applied to a capacitor in the short term can be considered constant, so the actual work done by the current loss is closely related to the physical shape of the coating. According to the resistance law R = ρ(L / S), where R is the sheet resistance of the coating, ρ is the density of the coating material, L is the total length of the coating (i.e., the total circumference of the winding), and S is the cross-sectional area of ​​the coating. As the coating thickness increases, the actual S increases, and the overall sheet resistance of the coating decreases.

[0006] However, this method has exposed many problems in practical applications. First, increasing the thickness of the coating does not improve the capacitance value, but rather increases the volume and mass of the capacitor because it reduces the specific energy density. Second, it is generally believed in the industry that the thinner the coating, the better the voltage resistance of the capacitor device, that is, the higher the "self-healing" efficiency. In high-frequency pulse capacitors, when high-energy and high-frequency currents pass through the capacitor, the short-term voltage resistance time and the thickening of the coating are in conflict. The coating generally reduces the thickness of the evaporated coating as much as possible to prioritize "self-healing" and voltage resistance stability. In addition, thickening the coating is also accompanied by higher inductance, which also needs to be avoided.

[0007] From the above, how to achieve low charge and discharge energy loss on the basis of the thinnest possible coating thickness is an important technical challenge faced by high-frequency pulse capacitors.

[0008] Currently, the technical means in this area focus on the consideration of coating, and no one has yet provided a solution to reduce losses from the structural perspective of the polypropylene base film. This is because it is indeed very difficult to reduce losses simply from the intrinsic crystal structure of the base film. The existing polypropylene base film used for pulse capacitors has a dielectric loss factor that can be maintained at 10 -4 ~10 -3 Order of magnitude, this is extremely low.

[0009] However, the base film and the coating of the metallized film are a whole. The physical structure of the junction between the coating and the film surface is actually determined by the topological structure of the film surface. Due to the existence of the roughening ring, the surface of the base film is always undulating. On these undulating topological structures, the metal coating attached to it also has corresponding shape changes in the microscopic cross section. The cross-sectional thickness of the coating is not consistent, and it is inevitable that a relatively thin coating area will be produced. This part forms a high steric resistance area with a narrow current path, such as Figure 1 The effect of a single or several high-square-resistance regions may be weak, but when magnified to the entire capacitor, these high-square-resistance regions are ubiquitous and numerous, resulting in a significant impact on capacitor loss.

[0010] Therefore, by controlling the surface topology of the base film on the thin-film metallization surface and reducing its undulations, the high-sheet resistance areas of the subsequent metallization film are reduced, promoting uniform thickness across the entire metal coating. This allows for the thinnest coating possible, reducing energy loss during repeated charge and discharge cycles in pulse capacitors. Summary of the Invention

[0011] In order to solve the problem of excessive fluctuations in the surface topological structure of the polypropylene film base film in the above-mentioned prior art, the present invention provides a polypropylene film and a preparation method thereof, a metallized film and a film capacitor.

[0012] In order to solve the above technical problems, one of the technical solutions provided by the present invention is as follows:

[0013] A method for preparing a polypropylene film comprises the following steps:

[0014] The polypropylene is melt-extruded, cast and cooled to obtain a polypropylene film;

[0015] The polypropylene film is biaxially stretched to obtain a polypropylene film;

[0016] The die head temperature of the melt extrusion extruder is T1, the cold roller temperature of the cast cooling is T2, and the difference between the die head temperature and the cold roller temperature is the supercooling degree ΔT; the ratio of the supercooling degree ΔT to the cold roller temperature T2 satisfies 1.9≤ΔT / T2≤2.25; wherein, ΔT=T1-T2.

[0017] In one embodiment, the isotactic index of the polypropylene is above 98.5%; and / or

[0018] The molecular weight distribution index of the polypropylene is 5.0 to 6.5; and / or

[0019] The melt flow index of the polypropylene is 3.0 to 3.5 g / 10 min; and / or

[0020] The ash content of the polypropylene is below 20 ppm.

[0021] In one embodiment, the method for preparing the polypropylene film further comprises pre-treating the polypropylene before the melt extrusion;

[0022] The pretreatment includes performing a heat drying treatment on the polypropylene and / or adding a nucleating agent capable of reducing the β crystal form to the polypropylene;

[0023] Preferably, the temperature of the heat drying treatment is 30°C to 45°C, and the time of the heat drying treatment is 48 to 72 hours; preferably, the nucleating agent is an α-nucleating agent, and the mass percentage of the nucleating agent in the polypropylene film is 0.05% to 0.9%.

[0024] In one embodiment, the extruder die head temperature is 235-250°C, and the cooling roller temperature is 75-85°C.

[0025] In one embodiment, the biaxial stretching is simultaneous biaxial stretching.

[0026] In one embodiment, the heating method of the stretching section of the simultaneous biaxial stretching is air-heat cycle heating, including preheating, stretching and heat setting, wherein the preheating temperature is 140~160°C, the stretching temperature is 157~166°C, the heat setting temperature is 163~170°C, the longitudinal stretching ratio is 5.9~6.9 times, and the transverse stretching ratio is 8.2~9.5 times.

[0027] In one embodiment, the method for preparing the polypropylene film further comprises post-processing the obtained polypropylene film after the biaxial stretching;

[0028] The post-treatment includes corona treatment and / or aging treatment;

[0029] Preferably, the corona treatment is a single-sided treatment of the roller surface, and the intensity of the corona treatment is 10-30 W·min / m 2 ;

[0030] Preferably, a preheating roller is provided before the corona treatment, and the temperature of the preheating roller is above 50°C and below 100°C.

[0031] The second technical solution provided by the present invention is as follows:

[0032] A polypropylene film is prepared by the above-mentioned method for preparing a polypropylene film;

[0033] Preferably, the thickness of the polypropylene film is 2.0-6.0 μm, and the vertical distance S between the highest peak and the lowest valley of the topological structure of the polypropylene film roller surface is z Satisfy 300nm≤S z≤700nm, the average concave-convex deviation SSK satisfies -0.3≤SSK≤0.

[0034] The third technical solution provided by the present invention is as follows:

[0035] A metallized film is formed by disposing a metal film on at least one side of the polypropylene film.

[0036] The fourth technical solution provided by the present invention is as follows:

[0037] A film capacitor is formed using the metallized film described above.

[0038] Based on the above, compared with the prior art, the present invention coordinates the relationship between the melt supercooling ΔT and the cold roller temperature T2, so that the CR surface of the thick sheet is quickly crystallized into small-sized spherulites, so that the surface undulation of the CR surface is reduced during the subsequent stretching process and is suitable for the field of capacitor films; if the ratio of the two is too low, since the melt cooling rate is proportional to the supercooling ΔT, and the crystallization rate is inversely proportional to the cold roller temperature T2, the cooling is slow, the crystallization is slow, the spherulite size is too large, and the surface undulation cannot be reduced; if the ratio of the two is too high, the cooling is fast, the crystallization is fast, the crystal size is too small, too smooth and the crystallinity is not high, then the base film is too smooth, which seriously affects the metallization.

[0039] Other features and beneficial effects of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The objectives and other beneficial effects of the present invention can be achieved and obtained by the structures particularly pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work. The positional relationships shown in the drawings in the following description are based on the directions of the components drawn in the drawings, unless otherwise specified.

[0041] Figure 1 Shown is a schematic diagram of the topological structure of the base film corona surface and the high square resistance area of ​​the coating;

[0042] Figure 2 Shown is a process flow chart of a method for preparing a polypropylene film provided by an embodiment of the present invention;

[0043] Figure 3 Shown are surface profiles of the corona surfaces of the base films of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments; the technical features designed in different implementation modes of the present invention described below can be combined with each other as long as they do not conflict with each other; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0045] In the description of the present invention, it should be noted that all terms used in the present invention (including technical terms and scientific terms) have the same meanings as those generally understood by ordinary technicians in the field to which the present invention belongs, and should not be understood as limiting the present invention; it should be further understood that the terms used in the present invention should be understood to have the same meanings as these terms in the context of this specification and the relevant field, and should not be understood in an idealized or overly formal sense, unless explicitly defined as such in the present invention.

[0046] One embodiment of the present invention aims to provide a process for producing biaxially oriented polypropylene base film for capacitors. The film's roll-applying surface (corona surface) exhibits minimal undulation, which helps reduce high-resistance regions in the metal coating. This allows for the evaporation of low-thickness coatings while reducing energy loss during capacitor operation, making it suitable for high-frequency pulse capacitors.

[0047] See also Figure 2 One embodiment of the present invention provides a method for preparing a polypropylene film, comprising the following steps:

[0048] Step 1, pre-treating polypropylene;

[0049] In a specific implementation, the polypropylene used in this embodiment is a commercial electrical-grade polypropylene resin. For example, the polypropylene has an isotactic index of 98.5% or higher; and / or the polypropylene has a molecular weight distribution index of 5.0 to 6.5, such as 5.6, 5.7, 5.8, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, etc., but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable. More preferably, the polypropylene has a molecular weight distribution index of 5.6 to 6.5; and / or the polypropylene has a melt flow index of 3.0 to 3.5 g / 10 min, such as 3.0 g / 10 min, 3.1 g / 10 min, 3.2 g / 10 min, 3.3 g / 10 min, 3.4 g / 10 min, 3.5 g / 10 min, or any value therebetween; and / or the polypropylene has an ash content of 20 ppm or less, such as 15 ppm or less, or 10 ppm or less.

[0050] The pretreatment of this embodiment includes heat drying the polypropylene and / or adding a nucleating agent capable of reducing the β crystal form to the polypropylene;

[0051] In a preferred embodiment of the present invention, the temperature of the thermal drying treatment is 30°C to 45°C, for example, 30°C, 31°C, 32°C, 33°C, 34°C, 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the time of the thermal drying treatment is 48 to 72 h, for example, 48 h, 50 h, 52 h, 54 h, 56 h, 58 h, 60 h, 62 h, 64 h, 66 h, 68 h, 70 h, 72 h or any value therebetween;

[0052] In a preferred embodiment of the present invention, the nucleating agent is an α-nucleating agent, and the mass percentage of the nucleating agent in the polypropylene film is 0.05% to 0.9%, for example, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.80%, 0.85%, 0.9%, etc., but is not limited to the values ​​listed above, and other values ​​not listed within this numerical range are also applicable. The addition of a trace amount of α-nucleating agent in the present invention does not significantly affect the ash content of the raw material. It can also effectively inhibit the formation of β crystals, thereby suppressing the increase in surface undulation caused by the transformation of β crystal form to α crystal form. Increasing nucleation sites, supplemented by supercooling and cold roller temperature control, reduces crystal size.

[0053] Specifically, in this embodiment, a trace amount of heterogeneous α-nucleating agent is added to the polypropylene, which has a minimal effect on ash content and effectively reduces the β-crystal content. Furthermore, the ratio of supercooling ΔT to the cold roll temperature T2 is controlled to satisfy 1.9≤ΔT / T2≤2.25, allowing the melt to rapidly cool and crystallize into small crystals. These two factors synergistically minimize cavitation during the stretching process, achieving a low-contour cold roll surface. Subsequently, the cold roll surface can optionally be subjected to corona treatment to ensure that the coating is located on this low-contour side.

[0054] Furthermore, in one embodiment of the present invention, the pretreatment includes sequentially performing a heat drying treatment on the polypropylene; in order to achieve a more significant effect, a nucleating agent capable of reducing the β crystal form is added to the polypropylene that has completed the heat drying treatment.

[0055] Step 2: Melting and extruding polypropylene, casting and cooling the polypropylene to obtain a polypropylene film;

[0056] In specific implementation, step 2 of this embodiment includes two steps: melt extrusion and casting cooling;

[0057] wherein the melt extrusion comprises feeding a raw material into an extruder for melting, the raw material comprising polypropylene mixed with a nucleating agent, or polypropylene alone; the extruder is a single-screw extruder; the extrusion temperature is controlled at 230°C to 265°C, for example, 230°C, 235°C, 240°C, 245°C, 250°C, 255°C, 260°C, 265°C, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the die head temperature of the extruder is 235 to 250°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, 246°C, 247°C, 248°C, 249°C, 250°C or any value therebetween;

[0058] The casting cooling is a single-roll air knife cooling, which includes melting the polypropylene melt obtained by the extruder and then cooling it through an air knife and a single-roll cooling cast sheet; the cooling roller adopts water circulation cooling, and the cooling roller temperature is 75-85°C. The cooling temperature can be 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, etc., but is not limited to the listed values. Other values ​​not listed in this numerical range are also applicable;

[0059] The die head temperature of the melt extrusion extruder is T1, the temperature of the cold roller of the cast cooling is T2, the difference between the die head temperature and the cold roller temperature is the supercooling ΔT, that is, ΔT=T1-T2; the ratio of the supercooling ΔT to the cold roller temperature T2 satisfies 1.9≤ΔT / T2≤2.25, and ΔT / T2 can be 1.9, 1.95, 2.0, 2.05, 2.1, 2.15, 2.2, 2.25 or any value therebetween. The relationship between the temperature T2 and the temperature T2 makes the CR surface of the thick sheet roller quickly crystallize into small-sized spherulites, so that the surface undulation of the CR surface can be reduced in the subsequent stretching process and is suitable for the field of capacitor films; if the ratio of the two is too low, since the melt cooling rate is proportional to the supercooling ΔT, and the crystallization rate is inversely proportional to the cold roller temperature T2, the cooling and crystallization are slow, the spherulites are large, and the surface undulation cannot be reduced; if the ratio of the two is too high, the cooling and crystallization are fast, the crystal size is too small, too smooth and the crystallinity is not high, then the base film is too smooth, which seriously affects the metallization.

[0060] Step 3, biaxially stretching the polypropylene film to obtain a polypropylene film;

[0061] In specific implementation, the biaxial stretching in this embodiment is simultaneous biaxial stretching;

[0062] In a preferred embodiment of the present invention, the stretching section of the simultaneous biaxial stretching process uses air-heat circulation heating. This utilizes a non-contact, simultaneous biaxial stretching process, eliminating contact between the stretching rollers and allowing for air-heat circulation throughout the entire stretching process. This results in significantly lower SSK compared to conventional contact-based asynchronous stretching.

[0063] The simultaneous biaxial stretching includes preheating, stretching and heat setting, wherein the preheating temperature is 140-160°C, for example, 140°C, 141°C, 142°C, 143°C, 144°C, 145°C, 146°C, 147°C, 148°C, 149°C, 150°C, 151°C, 152°C, 153°C, 154°C, 155°C, 156°C, 157°C, 158°C, 159°C, 160°C, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the stretching temperature is 157-166°C, for example, 157°C, 158°C, 159°C, 160°C, 161°C, 162°C, 163°C, 164°C, 165°C , 166°C or any value therebetween; the heat setting temperature is 163~170°C, for example, 163°C, 164°C, 165°C, 166°C, 167°C, 168°C, 169°C, 170°C or any value therebetween; the longitudinal stretching ratio is 5.9~6.9 times, for example, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9 times or any value therebetween; the transverse stretching ratio is 8.2~9.5 times, for example, 8.2, 8.3, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9,2, 9.3, 9.4, 9.5 times or any value therebetween.

[0064] It's important to note that asynchronous stretching in the MD region involves heated roller contact. Roller contact significantly impacts the film's surface structure, easily causing increased surface undulation. This is influenced by the smoothness of the roller surface. Furthermore, under the same conditions, contact-type asynchronous stretching produces greater surface undulation than non-contact synchronous stretching.

[0065] Step 4, post-processing the obtained polypropylene film;

[0066] In a specific implementation, the post-treatment includes corona treatment and / or aging treatment;

[0067] In a preferred embodiment of the present invention, the corona treatment is a single-sided treatment of the roller surface, and the intensity of the corona treatment is 10-30 W·min / m 2 , for example 10W·min / m 2 、11W·min / m 2 、12W·min / m 2 、13W·min / m 2 、14W·min / m 2 、15W·min / m 2 、16W·min / m 2 、17W·min / m 2 、18W·min / m2 、19W·min / m 2 、20W·min / m 2 , 21W·min / m 2 、22W·min / m 2 、23W·min / m 2 、24W·min / m 2 、25W·min / m 2 、26W·min / m 2 、27W·min / m 2 、28W·min / m 2 、29W·min / m 2 、30W·min / m 2 The values ​​listed are not limited to these values; other values ​​not listed within this range also apply. The corona treatment in this embodiment occurs on the roller surface (CR surface), a low-contour surface. This allows the subsequent coating to adhere to the low-contour corona surface, reducing the presence of high-resistance areas. Preheating before corona treatment enhances the efficiency of the corona treatment. Although the physical roughening of the corona surface is reduced, its polarity is enhanced, and the metallization capability is not diminished.

[0068] Preferably, a preheating roller is provided before the corona treatment, and the temperature of the preheating roller is above 50°C and less than 100°C to increase the grafting efficiency of the polar groups during the corona treatment, for example, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0069] The present invention, based on the low-loss design of high-frequency pulse capacitors and the principle of reducing high-square-resistance areas in the coating, provides a base film process technology solution based on the design of polypropylene film structures. This approach, rather than relying on traditional coating processes, provides a new approach to high-frequency pulse capacitor design based on the base film.

[0070] In the specific implementation process of the present invention, the supercooling degree ΔT is proportional to the melt cooling rate, that is, the greater the supercooling degree, the faster the cooling rate, which follows the basic laws of thermodynamics; and T2 is inversely proportional to the crystallization rate. This is because, despite the problem of heat transfer efficiency, it is still believed that T2 is positively correlated with the actual crystallization temperature. The crystallization rate of the polymer presents a typical peak function curve with temperature, that is, as the temperature increases, the crystallization rate increases to a maximum value and then decreases. The temperature involved in the embodiment of the present invention is the crystallization nucleation control zone. As the temperature increases, the crystallization rate decreases, that is, T2 is inversely proportional to the crystallization rate. Therefore, the larger ΔT / T2 is, the faster the cooling rate and crystallization rate are, and the easier it is for the crystal growth to approach the corresponding state under T2, that is, small-sized crystals at a low cold roller temperature. However, it cannot be too large. If it is too large, it will be similar to a packaging film and the surface will be too smooth. This ratio cannot be too small. If it is too small, it will easily cause sufficient crystallization of the thick sheet, large crystal size, and unstable film pulling.

[0071] The transformation of β-crystals to α-crystals and the cavitation effect of spherulites are the two main causes of the topological structure of the film surface. Adding a trace amount of α-nucleating agent to the polypropylene raw material inhibits the formation of metastable β-crystals, thereby reducing the increase in topological fluctuations caused by the crystal transformation during stretching.

[0072] The crystal size of the thick sheet affects the competition between shear and cavitation during the stretching process. The relationship between this size and the surface roughening (topology) of the base film is as follows: larger sizes lead to more pronounced cavitation and more prone to surface roughening, even leading to the formation of through-holes due to excessive cavitation. Therefore, small crystals on the thick sheet's roller surface can achieve a low-volume state in the surface topology.

[0073] In summary, within the context of the present invention, a larger ΔT / T2 ratio results in faster cooling and crystallization rates, smaller crystal size on the thick film roller surface, and a greater ease in achieving low-roughness on the film roller surface. However, ΔT / T2 should not be too large or too small. If it is too small, the roller surface becomes rougher, contradicting the principles of the present invention and preventing low-roughness. If it is too large, the surface becomes too smooth, hindering the metallization process, affecting capacitor lifespan, and even directly affecting film winding.

[0074] Furthermore, for base films that include corona treatment, corona treatment is actually the key to enhancing the adhesion of the coating, in order to enhance its surface polarity and adhesion. The corona surface is a metallized surface. It should be understood that the corona surface must have a certain degree of roughening to increase the surface energy. With respect to the embodiments of the present invention, the smoother the corona surface, the better. For non-corona surfaces, surface roughening is equally important, and sufficient winding tension is required to ensure efficient molding and winding, and to ensure the dimensional stability of the capacitor device.

[0075] An embodiment of the present invention provides a polypropylene film, which is prepared by the above-mentioned method for preparing a polypropylene film;

[0076] In a preferred embodiment of the present invention, the thickness of the polypropylene film is 2.0-6.0 μm, for example, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3.0 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, 4.0 μm, 4.1 μm, 4.2 μm, 4.3μm, 4.4μm, 4.5μm, 4.6μm, 4.7μm, 4.8μm, 4.9μm, 5.0μm, 5.1μm, 5.2μm, 5.3μm, 5.4μm, 5.5μm, 5.6μm, 5.7μm, 5.8μm, 5.9μm, 6.0μm, etc., but are not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; the vertical distance S between the highest peak and the lowest valley of the topological structure of the polypropylene film roller surface z Satisfy 300nm≤S z ≤700nm, the average concave-convex deviation SSK satisfies -0.3≤SSK≤0.

[0077] The metallized film of the present invention is formed by providing a metal layer on at least one surface of the polypropylene film.

[0078] The metal layer functions as an electrode. Examples of metals used in the metal layer include single metals such as zinc, lead, silver, chromium, aluminum, copper, and nickel, mixtures thereof, and alloys thereof. Among these, zinc and aluminum are preferred due to their environmental impact, economic efficiency, and excellent capacitor performance.

[0079] The method for laminating the metal layer on at least one side (one side or both sides) of the polypropylene film is not particularly limited, and examples thereof include vacuum evaporation and sputtering. From the viewpoint of excellent productivity and economic efficiency, vacuum evaporation is preferred. Examples of vacuum evaporation include crucible method and wire method, and the best one can be selected as appropriate.

[0080] The edge pattern when laminating the metal layer by vapor deposition is not particularly limited. From the perspective of further improving the safety of the capacitor and further suppressing capacitor failure and short circuits, it is preferred to apply a pattern containing so-called special edges such as a fishnet pattern or a T-shaped edge pattern to a single surface of the biaxially oriented polypropylene film.

[0081] The method for forming the edge is not particularly limited, and the edge may be formed by a known method such as a tape method or an oil method.

[0082] The thickness of the metallized film of the present invention is not particularly limited, but is preferably 2.0 μm or more and 6.0 μm or less, and more preferably 4.0 μm or more and 5.8 μm or less.

[0083] The film capacitor of the present invention is formed using the metallized film described above. The metallized film disclosed herein can be stacked or wound using conventionally known methods to form a film capacitor.

[0084] The film capacitors may have a structure comprising multiple stacked metallized films or a wound metallized film. Such film capacitors are suitable for use as capacitors for inverter power supply equipment that control the drive motors of electric vehicles, hybrid vehicles, etc. They are also suitable for use in applications such as railway vehicles, wind power generation, solar power generation, and general household appliances.

[0085] Experimental results showing the technical advantages of the present invention will be described below using Examples and Comparative Examples. Example 1

[0086] This embodiment provides a method for preparing a polypropylene film for a pulse capacitor, the steps of which are as follows:

[0087] (a) Raw material processing: The raw material was commercial electrical-grade polypropylene, specifically Borealis HC300BF, with an isotacticity of 98.5%, a distribution index of 6.0, a melt index of 3.4 g / 10 min, and an ash content of ≤20 ppm. The product was heat-dried in a constant temperature room at 40°C for 48 hours.

[0088] (b) Melt Extrusion: The extruder is a single-screw extruder equipped with a high-precision loss-in-weight scale. Sorbitol α-nucleating agent is added to the raw materials at a mass ratio of 0.1%. The raw materials are melted in the extruder. The average temperature of each extrusion section is controlled at 250°C, and the die temperature is 243°C.

[0089] (c) Cast thick sheet: The cast cooling is single-roll air knife cooling, and the melt is single-roll cooled to cast sheet. The cooling roller temperature is 75°C, and the relationship between supercooling and cooling roller temperature is ΔT / T2=2.24.

[0090] (d) Biaxial stretching: A non-contact synchronous stretching process is used, with air-heated circulation heating. This process involves preheating, stretching, and heat setting. The preheating temperature is increased in stages from 140°C to 160°C. The thick sheet is then drawn into the stretching section, where the temperatures are 159°C, 163°C, and 165°C, respectively. The longitudinal stretching ratio is 6x, and the transverse stretching ratio is 8.5x. Heat setting is performed while the stretching tension is maintained, with the temperatures in the heat setting sections at 165°C, 167°C, and 169°C, respectively. The production line speed is 210 m / min.

[0091] (e) Post-treatment: The post-treatment section is divided into corona treatment and aging treatment. The corona treatment surface is the roller surface (CR surface), the roller temperature before corona treatment is 80℃, and the corona intensity is 15W·min / m 2, and then reel; aging treatment, the reeled large coil is left to stand at room temperature for 72 hours, and then sampled and tested. Example 2

[0092] This embodiment provides a method for preparing a polypropylene film for a pulse capacitor, the steps of which are as follows:

[0093] (a) is consistent with Example 1.

[0094] (b) Melt Extrusion: The extruder is a single-screw extruder equipped with a high-precision loss-in-weight scale. Sorbitol α-nucleating agent is added to the raw materials at a mass ratio of 0.5%. The raw materials are melted in the extruder. The average temperature of each extrusion section is controlled at 250°C, and the die temperature is maintained at 247°C.

[0095] (c) Cast thick sheet: The cast cooling is single-roll air knife cooling, and the melt is single-roll cooled to cast sheet. The cooling roller temperature is 85°C, and the relationship between supercooling and cooling roller temperature is ΔT / T2=1.91.

[0096] (d) Biaxial stretching: the same as in Example 1.

[0097] (e) Post-treatment: The post-treatment section includes corona treatment and aging treatment. Heat setting and corona treatment are performed while maintaining the tensile tension. The heat setting temperature is 165°C. The corona treatment surface is the roller surface (CR surface). The roller temperature before corona treatment is 80°C and the corona intensity is 18 W·min / m 2 , and then reel; aging treatment, the reeled large coil is left to stand at room temperature for 72 hours, and then sampled and tested. Example 3

[0098] This embodiment provides a method for preparing a polypropylene film for a pulse capacitor, the steps of which are as follows:

[0099] (a) is consistent with Example 1.

[0100] (b) Melt Extrusion: The extruder is a single-screw extruder equipped with a high-precision loss-in-weight scale. Sorbitol α-nucleating agent is added to the raw materials at a mass ratio of 0.5%. The raw materials are fed into the extruder for melting. The average temperature of each extrusion section is controlled at 250°C, and the die temperature is maintained at 240°C.

[0101] (c) Cast thick sheet: The cast cooling is single-roll air knife cooling, and the melt is single-roll cooled to cast sheet. The cooling roller temperature is 79°C, that is, the relationship between supercooling and cooling roller temperature is ΔT / T2= 2.03.

[0102] (d) Biaxial stretching: the same as in Example 1.

[0103] (e) Post-treatment: The post-treatment section includes heat setting, corona treatment, and aging treatment. Heat setting and corona treatment are performed while maintaining the tensile strength. The heat setting temperature is 165°C. The corona treatment surface is the roller surface (CR surface). The corona preheating roller temperature is 65°C and the corona intensity is 14 W·min / m 2 , and then reel; aging treatment, the reeled large coil is left to stand at room temperature for 72 hours, and then sampled and tested. Example 4

[0104] This embodiment provides a method for preparing a polypropylene film for a pulse capacitor, the steps of which are as follows:

[0105] (a) Raw material processing: The raw material was commercial electrical-grade polypropylene, specifically Borealis HC300BF, with an isotacticity of 98.5%, a distribution index of 6.0, a melt index of 3.4 g / 10 min, and an ash content of ≤20 ppm. The product was heat-dried in a constant temperature room at 40°C for 48 hours.

[0106] (b) Melt extrusion: The extruder is a single-screw extruder. The raw materials are directly melted in the extruder without adding a nucleating agent. The average temperature of each extrusion section is controlled at 250°C, and the die temperature is 243°C.

[0107] (c) Cast thick sheet: The cast cooling is single-roll air knife cooling, and the melt is single-roll cooled to cast sheet. The cooling roller temperature is 75°C, and the relationship between supercooling and cooling roller temperature is ΔT / T2=2.24.

[0108] (d) (e) Biaxial stretching and post-treatment: the same as in Example 1. Comparative Example 1

[0109] This comparative example provides a method for preparing a polypropylene film for capacitors, the steps of which are as follows:

[0110] (a) The raw materials are put into storage and heat-dried in a constant temperature room at 40℃ for 48 hours. The raw materials are HC300BF.

[0111] (b) Melt extrusion: The extruder is a single-screw extruder. Commercial raw materials are directly put into the extruder for melting. The average temperature of each extrusion section is controlled at 245°C, and the die head temperature is 230°C.

[0112] (c) Cast thick sheet: The cast cooling is single-roll air knife cooling, and the melt is single-roll cooled to form a sheet. The cooling roller temperature is 95°C, that is, the relationship between the supercooling degree and the cooling roller temperature is ΔT / T2=1.52;

[0113] (d) Biaxial stretching: the same as in Example 1.

[0114] (e) Post-processing: same as in Example 1. Comparative Example 2

[0115] This comparative example provides a method for preparing a polypropylene film for capacitors, the steps of which are as follows:

[0116] (a) The raw materials are put into storage and heat-dried in a constant temperature room at 40℃ for 48 hours. The raw materials are HC300BF.

[0117] (b) Melt extrusion: The extruder is a single-screw extruder. Commercial raw materials are directly put into the extruder for melting. The average temperature of each extrusion section is controlled at 252°C, and the die head temperature is 245°C.

[0118] (c) Cast thick sheet: The cast cooling is single-roll air knife cooling, and the melt is single-roll cooled to form a sheet. The cooling roller temperature is 90 °C, that is, the relationship between the supercooling degree and the cooling roller temperature is ΔT / T2= 1.72;

[0119] (d) Biaxial stretching: the same as in Example 1.

[0120] (e) Post-processing: same as in Example 1. Comparative Example 3

[0121] This comparative example provides a method for preparing a polypropylene film for capacitors, the steps of which are as follows:

[0122] (a) The raw materials are put into storage and heat-dried in a constant temperature room at 40℃ for 48 hours. The raw materials are HC300BF.

[0123] The processes (b), (c), (d), etc. are consistent with those in Example 2.

[0124] (e) Post-treatment: The only difference from Example 2 is the corona treatment; the corona treated surface is the air knife surface (AK surface), the corona preheating roller is 50 ° C, and the corona intensity is 14 W·min / m 2 . Comparative Example 4

[0125] This comparative example provides a method for preparing a polypropylene film for capacitors, the steps of which are as follows:

[0126] Stages (a), (b), and (c): all remain consistent with Example 1.

[0127] (d) Biaxial Stretching: Utilizing an asynchronous biaxial stretching process, the cast sheet passes through transition rollers and enters the longitudinal stretching zone (MD). This zone consists of 8 to 12 rollers, with the sheet surface in direct contact with the rollers. This zone comprises three components: preheating, stretching, and shaping. The heated rollers utilize an oil-heated circulation system for temperature control. The average temperature of the multi-roller MD stretching zone ranges from 125 to 155°C, and the MD stretching ratio is 6x. The sheet then enters the transverse stretching zone (TD stretching). This zone utilizes an air-heated circulation system for temperature control and is divided into three sections: preheating, stretching, and shaping. The average temperature range is 150 to 170°C, and the TD stretching ratio is 8.5x. The production line speed is 210 m / min.

[0128] (e) The post-treatment process remains the same as in Example 1. Comparative Example 5

[0129] This comparative example provides a method for preparing a polypropylene film for capacitors, the steps of which are as follows:

[0130] The overall process remained consistent with Example 1, with the addition of a trace amount of nucleating agent and the use of a simultaneous biaxial stretching process. The only difference was that in (b) and (c), the die temperature was 255°C and the chill roll temperature was 70°C. The relationship between supercooling and chill roll temperature was ΔT / T² = 2.64.

[0131] The film post-processing and winding stages experienced severe slippage, preventing production tests from running normally and stably.

[0132] Test Case

[0133] (1) 3D optical profiler

[0134] The topology of the base film corona surface was measured using a ComtourX-200 (Bruker, Germany) optical profilometer. The vertical distance Sz between the highest peak and the lowest valley of the base film corona surface and the average deviation of the concavity and convexity SSK were recorded.

[0135] (2) Broadband dielectric tester

[0136] The dielectric loss tangent of the base film was measured using a Concept-50 (Novocontrol, Germany) broadband dielectric spectrometer. The electrode diameter was 25 mm. The base film was double-sided gold-sprayed before testing. The test frequency was 1000 Hz.

[0137] (3) Capacitor performance test

[0138] The base films are cut separately and drawn into the vacuum evaporation step, where a layer of aluminum or zinc is deposited on the surface of the base film to form a metallized polypropylene film. The metallized polypropylene film is then wound or laminated and assembled with leads and insulating packaging materials. Parameters such as the number of film layers and the tightness of the winding are precisely controlled. After welding and encapsulation, a capacitor with a rated capacitance of 35μF is finally produced. The capacitor's equivalent series resistance (ESR) is measured using an EA980A (Agilent, USA) LCR meter at 25°C and 10kHz.

[0139] The physical properties of the films prepared in Examples 1-4 and Comparative Examples 1-4 were tested respectively. The test results are shown in Table 1.

[0140] Table 1 Test results of polypropylene films of Examples 1-4 and Comparative Examples 1-4

[0141]

[0142] See also Figure 3 It can be found from Example 1, Comparative Example 1 and Comparative Example 3 that the undulation degree of the polypropylene film roller surface prepared in Example 1 is significantly lower than that in Comparative Example 1.

[0143] Finally, it should be noted that the polypropylene film provided by the present invention and the capacitor using the same have wide applications in important high-frequency pulse capacitor fields such as military and industrial high-frequency and high-voltage pulse capacitors and aircraft carrier electromagnetic guns.

[0144] In addition, those skilled in the art should understand that, although there are many problems in the prior art, each embodiment or technical solution of the present invention may be improved in only one or several aspects, without having to simultaneously solve all the technical problems listed in the prior art or background art. Those skilled in the art should understand that any content not mentioned in a claim should not be construed as limiting the claim.

[0145] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 polypropylene film, characterized in that: The following steps are involved: The polypropylene is melt-extruded, cast and cooled to obtain a polypropylene film; The polypropylene film is biaxially stretched to obtain a polypropylene film; The die head temperature of the melt extrusion extruder is T1, the cold roller temperature of the cast cooling is T2, and the difference between the die head temperature and the cold roller temperature is the supercooling degree ΔT; the ratio of the supercooling degree ΔT to the cold roller temperature T2 satisfies 1.9≤ΔT / T2≤2.25; wherein, ΔT=T1-T2.

2. The method for preparing a polypropylene film according to claim 1, wherein The isotactic index of the polypropylene is above 98.5%; and / or The molecular weight distribution index of the polypropylene is 5.0 to 6.5; and / or The melt flow index of the polypropylene is 3.0 to 3.5 g / 10 min; and / or The ash content of the polypropylene is below 20 ppm.

3. The method for preparing a polypropylene film according to claim 1, wherein: The method for preparing the polypropylene film further comprises pre-treating the polypropylene before the melt extrusion; The pretreatment includes performing a heat drying treatment on the polypropylene and / or adding a nucleating agent capable of reducing the β crystal form to the polypropylene; The temperature of the heat drying treatment is 30° C. to 45° C., and the time of the heat drying treatment is 48 to 72 h. The nucleating agent is an α-nucleating agent, and the mass percentage of the nucleating agent in the polypropylene film is 0.05% to 0.9%.

4. The method for preparing a polypropylene film according to claim 1, wherein The extruder die head temperature is 235-250°C, and the cooling roller temperature is 75-85°C.

5. The method for preparing a polypropylene film according to claim 1, wherein: The biaxial stretching is simultaneous biaxial stretching.

6. The method for preparing a polypropylene film according to claim 5, wherein: The heating method of the stretching section of the synchronous biaxial stretching is air-heat cycle heating, including preheating, stretching and heat setting, wherein the preheating temperature is 140~160℃, the stretching temperature is 157~166℃, the heat setting temperature is 163~170℃, the longitudinal stretching ratio is 5.9~6.9 times, and the transverse stretching ratio is 8.2~9.5 times.

7. The method for preparing a polypropylene film according to claim 1, wherein: The method for preparing the polypropylene film further comprises post-processing the obtained polypropylene film after the biaxial stretching; The post-treatment includes corona treatment and / or aging treatment; The corona treatment is a single-sided treatment of the roller surface, and the intensity of the corona treatment is 10-30 W·min / m 2 ; A preheating roller is provided before the corona treatment, and the temperature of the preheating roller is above 50°C and below 100°C.

8. A polypropylene film prepared by the method for preparing a polypropylene film according to any one of claims 1 to 7; The thickness of the polypropylene film is 2.0-6.0 μm, and the vertical distance S between the highest peak and the lowest valley of the topological structure of the polypropylene film roller surface is z Satisfy 300nm≤S z ≤700nm, the average concave-convex deviation SSK satisfies -0.3≤SSK≤0.

9. A metallized film comprising the polypropylene film according to claim 8 and a metal film provided on at least one surface thereof. 10 . A film capacitor formed using the metallized film according to claim 9 .

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