Capacitor Thin Films and Their Preparation Methods

Abstract

This invention relates to the field of capacitor film technology, and discloses a capacitor film and its preparation method. The capacitor film comprises a polypropylene film, a first poly(4-methyl-1-pentene) film disposed on the upper surface of the polypropylene film, and a second poly(4-methyl-1-pentene) film disposed on the lower surface of the polypropylene film. The polypropylene film comprises polypropylene resin, and the poly(4-methyl-1-pentene) film comprises an antioxidant and poly(4-methyl-1-pentene) resin. Based on the total amount of the capacitor film, the content of the polypropylene resin is 50-70 wt%, and the total amount of antioxidant and poly(4-methyl-1-pentene) resin in the first and second poly(4-methyl-1-pentene) films are each independently 15-25 wt%. This invention employs a biaxial stretching process, and the capacitor film obtained has excellent temperature resistance and compressive strength, uniform thickness, and good continuous film formation properties.

Description

Technical Field

[0001] This invention relates to the field of capacitor film technology, and more specifically to a capacitor film and its preparation method. Background Technology

[0002] Currently, polymer films and their capacitors are widely used in various electronic fields, but the application of capacitors made from them is limited due to the properties of the polymer films themselves.

[0003] Polypropylene capacitor film, also known as biaxially oriented polypropylene film (BOPP) for capacitors, has become the main dielectric material for power capacitors due to its low specific gravity, good heat resistance, and chemical stability. However, the dielectric loss factor of polypropylene film is only 0.0002, the dielectric heating is not high, and the maximum operating temperature can only reach 105℃, which limits the application of polypropylene film capacitors in high-temperature environments. Furthermore, existing polypropylene film capacitors generally use single-layer polypropylene capacitor films, whose surface and interior are composed of the same raw material, lacking secondary structure (layer structure). This results in relatively concentrated tensile stress on the product, making it extremely prone to breakage and causing the film to be scrapped during subsequent processing.

[0004] Compared with polyethylene and polypropylene resins, poly(4-methyl-1-pentene) resin has superior heat resistance and is widely used in industrial production and food packaging. However, the molecular side chains of this material are very large, which restricts molecular movement. In the molten state, the viscosity decreases sharply and it is easy to decompose. It is also easy to break during stretching, and the thickness uniformity is difficult to control, making film formation difficult.

[0005] Therefore, developing a capacitor film with high temperature resistance and high toughness is of great value. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of low temperature resistance and easy breakage of capacitor films in the prior art, and to provide a capacitor film and its preparation method. The capacitor film includes a polypropylene film and a poly4-methyl-1-pentene film, which improves the temperature resistance and pressure resistance of the capacitor film.

[0007] To achieve the above objectives, a first aspect of the present invention provides a capacitor film, wherein the capacitor film comprises a polypropylene film, a first poly(4-methyl-1-pentene) film disposed on the upper surface of the polypropylene film, and a second poly(4-methyl-1-pentene) film disposed on the lower surface of the polypropylene film.

[0008] Wherein, the polypropylene film includes polypropylene resin, and the poly4-methyl-1-pentene film includes an antioxidant and poly4-methyl-1-pentene resin;

[0009] Based on the total amount of the capacitor film, the content of the polypropylene resin is 50-70 wt%, and the total amount of antioxidant and poly4-methyl-1-pentene in the poly4-methyl-1-pentene film is 15-25 wt% each.

[0010] In the first poly-4-methyl-1-pentene film and the second poly-4-methyl-1-pentene film, based on the total amount of the poly-4-methyl-1-pentene film, the content of antioxidant in each poly-4-methyl-1-pentene film is independently 0.1-0.49 wt%, and the content of poly-4-methyl-1-pentene resin is independently 99.51-99.9 wt%.

[0011] A second aspect of the present invention provides a method for preparing the capacitor film described in the first aspect above, wherein the method includes the following steps:

[0012] (1) Polypropylene resin, a mixture containing poly(4-methyl-1-pentene) resin and antioxidant are extruded separately to obtain sheet fluid;

[0013] (2) The sheet fluid is sequentially shaped, longitudinally stretched and transversely stretched to obtain a basic film;

[0014] (3) The base film is processed and wound up, and then subjected to a first aging treatment and a second aging treatment to obtain the capacitor film.

[0015] Through the above technical solution, the present invention can achieve the following technical effects:

[0016] (1) The capacitor film provided by the present invention uses poly(4-methyl-1-pentene) film as the upper and lower surface layers of the capacitor film and polypropylene film as the middle layer. It is composed of three layers of film stacked together. Compared with a single-layer BOPP capacitor film, the capacitor film is beneficial to weaken the electrical weaknesses caused by the metal ash and other impurities in the raw materials and improve the voltage resistance of the film.

[0017] (2) The capacitor film provided by the present invention, in a preferred case, is a poly4-methyl-1-pentene film obtained by using a specific poly4-methyl-1-pentene resin. Compared with the traditional poly4-methyl-1-pentene resin, it can further increase the plasticity and tensile properties, and the capacitor film has good thickness uniformity, smooth surface and good film-forming properties.

[0018] (3) The method for preparing capacitor film provided by the present invention adopts a suitable biaxial stretching production process. In the preferred case, the shaping temperature is higher than the stretching temperature, which reduces the thermal shrinkage rate of capacitor film and improves the temperature resistance of capacitor film. Detailed Implementation

[0019] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0020] The first aspect of the present invention provides a capacitor film, wherein the capacitor film comprises a polypropylene film, a first poly(4-methyl-1-pentene) film disposed on the upper surface of the polypropylene film, and a second poly(4-methyl-1-pentene) film disposed on the lower surface of the polypropylene film;

[0021] Wherein, the polypropylene film includes polypropylene resin, and the poly4-methyl-1-pentene film includes an antioxidant and poly4-methyl-1-pentene resin;

[0022] Based on the total amount of the capacitor film, the content of the polypropylene resin is 50-70 wt%, and the total amount of antioxidant and poly4-methyl-1-pentene resin in the first poly4-methyl-1-pentene film and the second poly4-methyl-1-pentene film is independently 15-25 wt%.

[0023] In the first poly-4-methyl-1-pentene film and the second poly-4-methyl-1-pentene film, based on the total amount of the poly-4-methyl-1-pentene film, the content of antioxidant is independently 0.1-0.49 wt%, and the content of poly-4-methyl-1-pentene resin is independently 99.51-99.9 wt%.

[0024] In this invention, the capacitor film uses a polypropylene film as the intermediate layer, and a poly4-methyl-1-pentene film is disposed on the upper and lower surfaces of the polypropylene film. The poly4-methyl-1-pentene film is made from poly4-methyl-1-pentene resin and antioxidants, resulting in a poly4-methyl-1-pentene film that is easier to process. The capacitor film made by combining it with a polypropylene film solves the problems of easy breakage during stretching, difficulty in controlling thickness uniformity, and difficulty in film formation of traditional single-layer poly4-methyl-1-pentene films. The capacitor film has excellent temperature resistance and pressure resistance.

[0025] In this invention, the poly(4-methyl-1-pentene) film comprises a first poly(4-methyl-1-pentene) film and a second poly(4-methyl-1-pentene) film. The polypropylene resin content is controlled at 50-70 wt%, and the total amount of antioxidant and poly(4-methyl-1-pentene) in the first and second poly(4-methyl-1-pentene) films is controlled at 15-25 wt%, which is beneficial to give the obtained capacitor film excellent temperature resistance and voltage resistance.

[0026] In this invention, if the polypropylene resin content is less than 50 wt%, the polypropylene film in the capacitor film is too thin and the poly4-methyl-1-pentene film is too thick. The capacitor film has good temperature resistance and pressure resistance, but the continuous film-forming property is reduced, the yield is low, and the raw material cost is high. If the polypropylene resin content is greater than 70 wt%, the polypropylene film in the capacitor film is too thick and the poly4-methyl-1-pentene film is too thin. The improvement in temperature resistance and pressure resistance of the capacitor film is limited.

[0027] In some embodiments of the present invention, preferably, based on the total amount of the capacitor film, the content of the polypropylene resin is 55-65 wt%, and the total amount of antioxidant and poly4-methyl-1-pentene in the first poly4-methyl-1-pentene film and the second poly4-methyl-1-pentene film is independently 17.5-22.5 wt%.

[0028] In this invention, the total amount of poly4-methyl-1-pentene and antioxidant in the first poly4-methyl-1-pentene film and the second poly4-methyl-1-pentene film can be the same or different. Preferably, the total amount of poly4-methyl-1-pentene and antioxidant in the first poly4-methyl-1-pentene film and the second poly4-methyl-1-pentene film is the same, which is more conducive to making the obtained capacitor film have excellent temperature resistance and voltage resistance.

[0029] In some embodiments of the present invention, preferably, in the first poly-4-methyl-1-pentene film and the second poly-4-methyl-1-pentene film, based on the total amount of the poly-4-methyl-1-pentene film, the content of antioxidant in each poly-4-methyl-1-pentene film is independently 0.2-0.4 wt%, and the content of poly-4-methyl-1-pentene resin is independently 99.6-99.8 wt%.

[0030] In this invention, the contents of poly4-methyl-1-pentene resin and antioxidant in the first poly4-methyl-1-pentene film and the second poly4-methyl-1-pentene film can be the same or different. Preferably, the contents of poly4-methyl-1-pentene resin and antioxidant in the first poly4-methyl-1-pentene film and the second poly4-methyl-1-pentene film are the same. The resulting capacitor film has excellent temperature resistance and voltage resistance, which is beneficial to improving the continuous film formation of the capacitor film.

[0031] In this invention, controlling the content of poly4-methyl-1-pentene resin and antioxidant in the first and second poly4-methyl-1-pentene films within the above-mentioned preferred range is more conducive to improving the continuous film-forming properties of the capacitor film, resulting in a capacitor film with superior temperature resistance and voltage resistance.

[0032] In some embodiments of the present invention, preferably, the polypropylene resin has a weight-average molecular weight of 200,000-500,000 g / mol, a melt index of 0.5-3 g / 10 min at 230°C and a load of 2.16 kg, a molecular weight distribution index of 4-8, an isotacticity of ≥97.5%, and an ash content of <20 ppm.

[0033] In this invention, controlling the weight-average molecular weight, melt index, molecular weight distribution, isotacticity, and ash content of the polypropylene resin within the above-mentioned range is more conducive to improving the continuous production performance of the polypropylene resin. The capacitor film obtained by stacking the poly(4-methyl-1-pentene) film has better thickness uniformity, tensile properties, and film-forming properties.

[0034] In some embodiments of the present invention, preferably, the antioxidant is selected from hindered phenolic antioxidants and / or phosphite antioxidants, and more preferably from pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], N,N'-bis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hydrazine, 2,2'-thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2 At least one of 4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-p-cresol, tris[2,4-di-tert-butylphenyl]phosphite, pentaerythritol diphosphite of bis(2,4-di-tert-butylphenol), tris(4-nonylphenyl)phosphite, distearate of thiodipropionate, dilaurate of thiodipropionate, and 4,4'-thiobis(6-tert-butyl-3-methylphenol), more preferably 2,6-di-tert-butyl-p-cresol.

[0035] In this invention, the antioxidant is combined with poly4-methyl-1-pentene resin to obtain a poly4-methyl-1-pentene film, which is then placed on the upper and lower surfaces of a polypropylene film. This process is more conducive to inhibiting polymer oxidation and delaying the aging of poly4-methyl-1-pentene resin during use.

[0036] In some embodiments of the present invention, preferably, the poly4-methyl-1-pentene resin comprises poly4-methyl-1-pentene homopolymer and / or 4-methyl-1-pentene copolymer.

[0037] In some embodiments of the present invention, preferably, the poly(4-methyl-1-pentene) homopolymer has a weight-average molecular weight of 200,000-600,000 g / mol, a melt index of 10-50 g / 10 min at 260°C and a load of 5 kg, a molecular weight distribution index of 4-10, isotacticity ≥95%, ash content <30 ppm, and a melting point of 235-245°C.

[0038] In this invention, controlling the weight-average molecular weight, melt index, molecular weight distribution, isotacticity, ash content, and melting point of the poly(4-methyl-1-pentene) homopolymer within the above-mentioned ranges is more conducive to improving the mechanical properties, temperature resistance, and film-forming processability of the poly(4-methyl-1-pentene) resin.

[0039] In some embodiments of the present invention, preferably, the poly(4-methyl-1-pentene) copolymer has a weight-average molecular weight of 200,000-500,000 g / mol, a melt index of 5-30 g / 10 min at 260°C and a load of 5 kg, a molecular weight distribution index of 5-10, isotacticity ≥65%, ash content <30 ppm, and a melting point of 220-240°C.

[0040] In this invention, controlling the weight-average molecular weight, melt index, molecular weight distribution, isotacticity, ash content and melting point of the poly(4-methyl-1-pentene) copolymer within the above-mentioned range is more conducive to improving the plasticity and tensile properties of the poly(4-methyl-1-pentene) resin, making the poly(4-methyl-1-pentene) film easier to process during longitudinal and transverse stretching.

[0041] In some embodiments of the present invention, preferably, the poly-4-methyl-1-pentene copolymer is obtained by copolymerization of 4-methyl-1-pentene and α-olefin.

[0042] In this invention, the poly(4-methyl-1-pentene) copolymer is prepared by copolymerizing 4-methyl-1-pentene and α-olefin in the presence of a catalyst. The catalyst is a conventional catalyst in the art and may be at least one selected from Ziegler-Natta catalysts, metallocene catalysts and transition metal catalysts.

[0043] In some embodiments of the present invention, preferably, the content of 4-methyl-1-pentene structural units in the poly-4-methyl-1-pentene copolymer is 50-95 wt%.

[0044] In some embodiments of the present invention, preferably, the α-olefin is selected from α-olefins having 2-18 carbon atoms, more preferably selected from at least one of ethylene, propylene, 1-butene, 1-hexene, 1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, and 1-octadecene, more preferably selected from at least one of 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, and 1-octadecene, and even more preferably selected from at least one of ethylene, 1-hexene, and 1-octene.

[0045] In some embodiments of the present invention, preferably, the thickness of the capacitor film is 4-12 μm, more preferably 5.3-10.8 μm.

[0046] In this invention, the total thickness of the capacitor film is 4-12 μm, the thickness of the polypropylene film is 2-10 μm, and the thickness of the poly4-methyl-1-pentene film is 1-5 μm. These thicknesses are beneficial for the resulting capacitor film to have excellent overall performance. Preferably, the total thickness of the capacitor film is 5.3-10.8 μm, the thickness of the polypropylene film is 2.3-8 μm, and the thickness of the poly4-methyl-1-pentene film is 1.4-4.3 μm. This preferred thickness range allows for better continuous film formation, uniform thickness, and superior temperature resistance and pressure resistance.

[0047] In this invention, the thickness of the capacitor film is tested by a thickness gauge, wherein the thickness gauge can be a 1202HR+P2004MA thickness gauge manufactured by Marl Thin Film Thickness Gauge GmbH in Germany.

[0048] In some embodiments of the present invention, preferably, the capacitor film has a heat shrinkage rate of 0.3-1.2% at 100°C and a DC voltage breakdown strength of 622-645V / μm; the capacitor film has a heat shrinkage rate of 1.3-2% at 120°C and a DC voltage breakdown strength of 561-596V / μm.

[0049] In this invention, the capacitor film uses a polypropylene film as the intermediate layer, with poly(4-methyl-1-pentene) films disposed on the upper and lower surfaces of the polypropylene film. It is composed of three layers stacked together. Compared to a single-layer BOPP capacitor film, this capacitor film effectively mitigates electrical weaknesses caused by metallic ash and other impurities in the raw materials, exhibits higher withstand voltage, a heat shrinkage rate of less than 2% at 120°C, and a DC voltage breakdown strength of not less than 561V / μm, demonstrating excellent temperature resistance and withstand voltage. In this invention, the DC voltage breakdown strength is tested using a voltage breakdown strength tester.

[0050] A second aspect of the present invention provides a method for preparing the capacitor film described in the first aspect above, wherein the method includes the following steps:

[0051] (1) Polypropylene resin, a mixture containing poly(4-methyl-1-pentene) resin and antioxidant are extruded separately to obtain sheet fluid;

[0052] (2) The sheet fluid is sequentially shaped, longitudinally stretched and transversely stretched to obtain a basic film;

[0053] (3) The base film is processed and wound up, and then subjected to a first aging treatment and a second aging treatment to obtain the capacitor film.

[0054] In some embodiments of the present invention, preferably, in step (1), the extrusion temperature of the polypropylene film raw material is 230-255°C, and the extrusion temperature of the poly4-methyl-1-pentene film raw material is 250-300°C.

[0055] In this invention, in step (1), polypropylene resin and a mixture containing poly(4-methyl-1-pentene) resin and antioxidant are melted and plasticized respectively, and then extruded through a three-channel composite die to obtain the sheet fluid. The middle layer of the sheet fluid is a polypropylene resin layer, and the upper and lower surfaces of the polypropylene resin layer are both mixture layers containing poly(4-methyl-1-pentene) resin and antioxidant.

[0056] In this invention, the mixture containing poly(4-methyl-1-pentene) resin and antioxidant is a raw material for both the first and second poly(4-methyl-1-pentene) films. The total amounts of poly(4-methyl-1-pentene) resin and antioxidant in the first and second poly(4-methyl-1-pentene) films can be the same or different. Preferably, the total amounts of poly(4-methyl-1-pentene) resin and antioxidant in the first and second poly(4-methyl-1-pentene) films are the same. The individual contents of poly(4-methyl-1-pentene) resin and antioxidant in the first and second poly(4-methyl-1-pentene) films can also be the same or different. Preferably, the individual contents of poly(4-methyl-1-pentene) resin and antioxidant in the first and second poly(4-methyl-1-pentene) films are the same, which is beneficial for the obtained capacitor film to have excellent temperature resistance and voltage resistance.

[0057] In this invention, in step (2), the molding is preferably performed by casting the sheet fluid obtained in step (1) into a thick film through a chilling roller and a high-pressure air knife, wherein the temperature of the chilling roller is 95-110°C and the gas temperature of the high-pressure air knife is 95-110°C.

[0058] In some embodiments of the present invention, preferably, the longitudinal stretching includes longitudinal preheating, stretching, and shaping; the conditions for the longitudinal stretching include: a preheating temperature of 125-150°C, a stretching temperature of 145-155°C, a shaping temperature of 150-158°C, and a stretching ratio of 3-8 times.

[0059] In some embodiments of the present invention, preferably, the transverse stretching includes stretching and shaping, and the conditions for the transverse stretching include: a stretching temperature of 145-155°C, a shaping temperature of 150-158°C, and a stretching ratio of 5-8 times.

[0060] In this invention, the setting temperature is higher than the stretching temperature. Traditional poly(4-methyl-1-pentene) films exhibit reduced elongation at break at high temperatures, making it difficult to maintain stretching continuity and resulting in film breakage during stretching. Polypropylene films, on the other hand, have better continuous production performance and tensile properties. This invention involves separately applying poly(4-methyl-1-pentene) films to the upper and lower surfaces of a polypropylene film, and independently setting the longitudinal and transverse stretching temperatures to 145-155°C, and independently setting the longitudinal and transverse setting temperatures to 150-158°C. This significantly reduces the thermal shrinkage rate of the film and improves the voltage withstand strength of the capacitor film.

[0061] In some embodiments of the present invention, preferably, in step (3), the process includes sequentially cooling, measuring thickness and trimming the base film; the cooling temperature is 25-30°C.

[0062] In this invention, in step (3), the base film obtained after transverse stretching is sequentially cooled, measured in thickness and trimmed. The thickness of the capacitor film is tested by the aforementioned thickness gauge.

[0063] In some embodiments of the present invention, preferably, the film after edge trimming is wound up, wherein the winding tension is 20-30 N / m and the winding pressure is 150-200 N / m.

[0064] In some embodiments of the present invention, preferably, the conditions for the first aging treatment include: purification level ≤ 10,000, temperature 25-30℃, humidity < 60%, and aging time 24-72h.

[0065] In some embodiments of the present invention, preferably, the conditions for the second aging treatment include: purification level ≤ 10,000, temperature 25-30℃, humidity < 60%, and aging time 12-24h.

[0066] The present invention will be described in detail below through embodiments.

[0067] Melt flow index test: The test was conducted in accordance with ISO 1133 "Determination of melt mass flow rate and melt volume flow rate of thermoplastics" and the test conditions were 230℃ / 2.16kg·10min.

[0068] Weight-average molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC).

[0069] Isotacticity was tested using the n-heptane extraction method;

[0070] Ash content was tested using a combustion method;

[0071] The thickness of the capacitor film was tested using a thickness gauge.

[0072] The breakdown strength of the capacitor film is tested using a voltage breakdown strength tester;

[0073] Heat shrinkage test of capacitor film: After heat treatment in an oven at 100℃ or 120℃ for 15 minutes, the change in the transverse dimension of the film is tested.

[0074] Example 1

[0075] (1) Polypropylene resin: weight average molecular weight 480,000 g / mol, melt index 2.9 g / 10 min (2.16 kg, 230 ℃), molecular weight distribution index 5.8, isotacticity 97.5%, ash content 15 ppm;

[0076] The poly(4-methyl-1-pentene) resin is a homopolymer of 4-methyl-1-pentene with a weight-average molecular weight of 400,000 g / mol, a melt index of 20 g / 10 min (5 kg, 260 °C), a molecular weight distribution index of 7.1, an isotacticity of 95.5%, an ash content of 20 ppm, and a melting point of 240 °C. The poly(4-methyl-1-pentene) resin and 2,6-di-tert-butyl-p-cresol antioxidant are uniformly mixed to obtain a mixture containing poly(4-methyl-1-pentene) and the antioxidant.

[0077] Based on the total amount of capacitor film, the mass of polypropylene resin accounts for 63.6%, the mass of the mixture containing poly(4-methyl-1-pentene) and antioxidant accounts for 36.4%, the content of 2,6-di-tert-butyl-p-cresol antioxidant in the mixture containing poly(4-methyl-1-pentene) and antioxidant is 0.3 wt%, and the raw material composition of the first poly(4-methyl-1-pentene) film and the second poly(4-methyl-1-pentene) film is the same;

[0078] (2) Extrusion: Polypropylene resin and a mixture containing poly4-methyl-1-pentene and antioxidant are melted and plasticized respectively, and then extruded through a three-channel composite die. The extrusion temperature of polypropylene resin is 255°C, and the extrusion temperature of the mixture is 280°C.

[0079] (3) Cooling roller and high-pressure air knife: The sheet fluid obtained by extrusion in step (2) is cast into a thick sheet by passing it through a cooling roller and a high-pressure air knife. The temperature of the cooling roller is 100°C and the gas temperature of the high-pressure air knife is 100°C.

[0080] (4) Longitudinal stretching: The thick sheet is sequentially preheated, stretched and shaped in the longitudinal direction to obtain a thick film. The preheating temperature is 145°C, the stretching temperature is 155°C, the shaping temperature is 158°C, and the stretching ratio is 5.5 times.

[0081] (5) Lateral stretching: The thick film is sequentially stretched and shaped to obtain a thin film. The stretching temperature is 150°C, the sizing temperature is 155°C, and the stretching ratio is 7.5 times.

[0082] (6) Cool the film, measure its thickness and trim its edges. The cooling temperature is 30°C and the thickness is measured after trimming. Then, the film after trimming is wound up with a winding tension of 30 N / m and a winding pressure of 200 N / m.

[0083] (7) Aging treatment: The wound film is subjected to a first aging treatment at 30°C, humidity less than 60%, and cleanliness level 10,000 for 72 hours; then the film after the first aging treatment is subjected to a second aging treatment at 30°C, humidity less than 60%, and cleanliness level 10,000 for 12 hours to obtain the capacitor film.

[0084] The thickness of the upper and lower surface layers (poly4-methyl-1-pentene film) and the intermediate layer (polypropylene film) of the capacitor film, the DC voltage breakdown strength at 100°C and 120°C, and the thermal shrinkage rate are shown in Table 1.

[0085] Example 2

[0086] The method described in Example 1 is different in that, in step (1), based on the total amount of the capacitor film, the mass of the polypropylene resin accounts for 69.2%, and the mass of the mixture containing poly(4-methyl-1-pentene) and antioxidant accounts for 30.8%, wherein the antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris[2,4-di-tert-butylphenyl] phosphite; in step (4), the longitudinal stretching ratio is 6 times. The capacitor film is obtained.

[0087] The thickness of the upper and lower surface layers (poly4-methyl-1-pentene film) and the intermediate layer (polypropylene film) of the capacitor film, the DC voltage breakdown strength at 100°C and 120°C, and the thermal shrinkage rate are shown in Table 1.

[0088] Example 3

[0089] The method described in Example 1 is different in that, in step (1), the mass of the polypropylene resin accounts for 53.8%, the mass of the mixture containing poly(4-methyl-1-pentene) and an antioxidant accounts for 46.2%, and the antioxidant is dilauryl thiodipropionate; in step (5), the transverse stretching ratio is 8 times. The capacitor film is obtained.

[0090] The thickness of the upper and lower surface layers (poly4-methyl-1-pentene film) and the intermediate layer (polypropylene film) of the capacitor film, the DC voltage breakdown strength at 100°C and 120°C, and the thermal shrinkage rate are shown in Table 1.

[0091] Example 4

[0092] The method described in Example 1 is different in that, in step (1), the poly-4-methyl-1-pentene resin is a copolymer of ethylene and 4-methyl-1-pentene with a weight-average molecular weight of 350,000 g / mol, a melt index of 22 g / 10 min (5 kg, 260 °C), a molecular weight distribution index of 6.8, an isotacticity of 75%, an ash content of 25 ppm, and a melting point of 233 °C; in step (5), the transverse stretching temperature is 150 °C, and the setting temperature is 155 °C; the capacitor film is obtained.

[0093] The thickness of the upper and lower surface layers (poly4-methyl-1-pentene film) and the intermediate layer (polypropylene film) of the capacitor film, the DC voltage breakdown strength at 100°C and 120°C, and the thermal shrinkage rate are shown in Table 1.

[0094] Example 5

[0095] The method described in Example 1 is different in that, in step (1), the poly-4-methyl-1-pentene resin is a copolymer of 1-octadecene and 4-methyl-1-pentene, with a weight-average molecular weight of 280,000 g / mol, a melt index of 28 g / 10 min (5 kg, 260 °C), a molecular weight distribution index of 8.3, an isotacticity of 68%, an ash content of 25 ppm, and a melting point of 221 °C; in step (4), the longitudinal stretching preheating temperature is 135 °C, the stretching temperature is 150 °C, the setting temperature is 155 °C, and the stretching ratio is 4.5 times; thus, the capacitor film is obtained.

[0096] The thickness of the upper and lower surface layers (poly4-methyl-1-pentene film) and the intermediate layer (polypropylene film) of the capacitor film, the DC voltage breakdown strength at 100°C and 120°C, and the thermal shrinkage rate are shown in Table 1.

[0097] Example 6

[0098] The method described in Example 1 is different in that, in step (1), the 4-methyl-1-pentene homopolymer has a weight-average molecular weight of 200,000 g / mol, a melt index of 35 g / 10 min (2.16 kg, 260 °C), a molecular weight distribution index of 6.9, an isotacticity of 97%, an ash content of 15 ppm, and a melting point of 235 °C; in step (4), the longitudinal stretching preheating temperature is 135 °C, the stretching temperature is 145 °C, the setting temperature is 150 °C, and the stretching ratio is 4 times; in step (5), the transverse stretching stretching temperature is 145 °C, the setting temperature is 150 °C, and the stretching ratio is 6.5 times; and the capacitor film is obtained.

[0099] The thickness of the upper and lower surface layers (poly4-methyl-1-pentene film) and the intermediate layer (polypropylene film) of the capacitor film, the DC voltage breakdown strength at 100°C and 120°C, and the thermal shrinkage rate are shown in Table 1.

[0100] Comparative Example 1

[0101] The method is the same as in Example 1, except that in step (1), based on the total amount of the capacitor film, the mass of the polypropylene resin accounts for 19%, and the mass of the mixture containing poly(4-methyl-1-pentene) and antioxidant accounts for 81%, thus obtaining the capacitor film.

[0102] The thickness of the upper and lower surface layers (poly4-methyl-1-pentene film) and the intermediate layer (polypropylene film) of the capacitor film, the DC voltage breakdown strength at 100°C and 120°C, and the thermal shrinkage rate are shown in Table 1.

[0103] Comparative Example 2

[0104] (1) Polypropylene resin: weight average molecular weight 480,000 g / mol, melt index 2.9 g / 10 min (2.16 kg, 260 ℃), molecular weight distribution index 5.8, isotacticity 97.5%, ash content 15 ppm;

[0105] (2) Extrusion: The polypropylene resin is melted, plasticized and extruded at an extrusion temperature of 255°C;

[0106] (3) Cooling roller and high-pressure air knife: The sheet fluid obtained by extrusion in step (2) is cast into a thick sheet by passing it through a cooling roller and a high-pressure air knife. The temperature of the cooling roller is 95°C and the gas temperature of the high-pressure air knife is 95°C.

[0107] (4) Longitudinal stretching: The thick sheet is sequentially preheated, stretched and shaped in the longitudinal direction to obtain a thick film. The preheating temperature is 130°C, the stretching temperature is 140°C, the shaping temperature is 145°C, and the stretching ratio is 4.2 times.

[0108] (5) Lateral stretching: The thick film is sequentially stretched and shaped to obtain a thin film. The stretching temperature is 140°C, the shaping temperature is 145°C, and the stretching ratio is 5 times.

[0109] (6) Cool the film, measure its thickness and trim its edges. The cooling temperature is 30°C and the thickness is measured after trimming. Then, the film after trimming is wound up with a winding tension of 25 N / m and a winding pressure of 150 N / m.

[0110] (7) Aging treatment: The wound film is subjected to a first aging treatment at 30°C, humidity less than 60%, and cleanliness level 10,000 for 72 hours; then the film after the first aging treatment is subjected to a second aging treatment at 30°C, humidity less than 60%, and cleanliness level 10,000 for 12 hours to obtain the capacitor film.

[0111] The thickness of the capacitor film (polypropylene film), its DC voltage breakdown strength at 100°C and 120°C, and its thermal shrinkage rate are shown in Table 1.

[0112] Table 1

[0113]

[0114] As can be seen from the results in Table 1, the capacitor film prepared by the method of the present invention has excellent heat resistance and voltage resistance. From Examples 1-6 in Table 1, it can be seen that the capacitor film provided by the present invention has a heat shrinkage rate of 0.3-1.2% at 100℃ and a breakdown strength of 622-645V / μm, and a heat shrinkage rate of 1.3-2% at 120℃ and a breakdown strength of 561-596V / μm. It also exhibits uniform thickness, good film-forming properties, and fully meets the performance requirements of capacitors. In Comparative Example 1, the polypropylene resin content is outside the scope of the claims of the present invention. Under the same processing technology, the obtained capacitor film has poor continuous film-forming properties, insufficient breakdown strength, and a large shrinkage rate. In Comparative Example 2, no poly(4-methyl-1-pentene) resin was added; only a polypropylene film was used. The obtained polypropylene capacitor film had good continuous film-forming properties, but poor heat shrinkage rates at 100℃ and 120℃, and significantly insufficient breakdown strength at 100℃, making it unusable at the high temperature of 120℃. Therefore, the capacitor film provided by the present invention has excellent temperature resistance and voltage resistance, good thickness uniformity, and good continuous film formation in production.

[0115] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A capacitor film for improving breakdown strength while maintaining film-forming properties, characterized in that, The capacitor film includes a polypropylene film, a first poly(4-methyl-1-pentene) film disposed on the upper surface of the polypropylene film, and a second poly(4-methyl-1-pentene) film disposed on the lower surface of the polypropylene film; The polypropylene film comprises polypropylene resin, and the poly(4-methyl-1-pentene) film comprises an antioxidant and poly(4-methyl-1-pentene) resin; the poly(4-methyl-1-pentene) resin is a poly(4-methyl-1-pentene) homopolymer or a poly(4-methyl-1-pentene) copolymer; the poly(4-methyl-1-pentene) copolymer has a melting point of 220-240℃ and a molecular weight distribution index of 5-10; the poly(4-methyl-1-pentene) homopolymer has a melting point of 235-245℃. Based on the total amount of the capacitor film, the polypropylene resin content is 55-65 wt%, and the total amount of antioxidant and poly4-methyl-1-pentene resin in the first and second poly4-methyl-1-pentene films are each independently 17.5-22.5 wt%. In the first poly-4-methyl-1-pentene film and the second poly-4-methyl-1-pentene film, based on the total amount of the poly-4-methyl-1-pentene film, the content of antioxidant is independently 0.1-0.49 wt%, and the content of poly-4-methyl-1-pentene resin is independently 99.51-99.9 wt%.

2. The capacitor film according to claim 1, wherein, In the first poly-4-methyl-1-pentene film and the second poly-4-methyl-1-pentene film, based on the total amount of the poly-4-methyl-1-pentene film, the content of antioxidant is independently 0.2-0.4 wt%, and the content of poly-4-methyl-1-pentene resin is independently 99.6-99.8 wt%.

3. The capacitor film according to claim 1 or 2, wherein, The polypropylene resin has a weight-average molecular weight of 200,000-500,000 g / mol, a melt index of 0.5-3 g / 10 min at 230℃ and a load of 2.16 kg, a molecular weight distribution index of 4-8, isotacticity ≥97.5%, and ash content <20 ppm.

4. The capacitor film according to claim 1, wherein, The antioxidant is selected from hindered phenolic antioxidants and / or phosphite antioxidants.

5. The capacitor film according to claim 4, wherein, The antioxidant is selected from at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], N,N'-bis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl]hydrazine, 2,2'-thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-p-cresol, tris[2,4-di-tert-butylphenyl]phosphite, bis(2,4-di-tert-butylphenol) pentaerythritol diphosphite, tris(4-nonylphenyl)phosphite, distearate thiodipropionate, dilaurate thiodipropionate, and 4,4'-thiobis(6-tert-butyl-3-methylphenol).

6. The capacitor film according to claim 5, wherein, The antioxidant is 2,6-di-tert-butyl-p-cresol.

7. The capacitor film according to claim 1, wherein, The weight-average molecular weight of the poly(4-methyl-1-pentene) homopolymer is 200,000-600,000 g / mol, the melt index at 260℃ and 5 kg load is 10-50 g / 10 min, the molecular weight distribution index is 4-10, the isotacticity is ≥95%, and the ash content is <30 ppm.

8. The capacitor film according to claim 1, wherein, The poly(4-methyl-1-pentene) copolymer has a weight-average molecular weight of 200,000-500,000 g / mol, a melt index of 5-30 g / 10 min at 260 °C and a load of 5 kg, an isotacticity of ≥65%, and an ash content of <30 ppm.

9. The capacitor film according to claim 1, wherein, The poly(4-methyl-1-pentene) copolymer is prepared by copolymerization of 4-methyl-1-pentene and α-olefin.

10. The capacitor film according to claim 9, wherein, The content of 4-methyl-1-pentene structural units in the poly-4-methyl-1-pentene copolymer is 50-95 wt%.

11. The capacitor film according to claim 9, wherein, The α-olefin is selected from α-olefins with 2-18 carbon atoms.

12. The capacitor film according to claim 11, wherein, The α-olefin is selected from at least one of ethylene, propylene, 1-butene, 1-hexene, 1-pentene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, and 1-octadecene.

13. The capacitor film according to claim 12, wherein, The α-olefin is selected from at least one of 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-heptadecene, and 1-octadecene.

14. The capacitor film according to claim 13, wherein, The α-olefin is selected from at least one of ethylene, 1-hexene, and 1-octene.

15. The capacitor film according to claim 1, wherein, The thickness of the capacitor film is 4-12 μm.

16. The capacitor film according to claim 15, wherein, The thickness of the capacitor film is 5.3-10.8 μm.

17. The capacitor film according to claim 1, wherein, The capacitor film has a thermal shrinkage rate of 0.3-1.2% at 100℃ and a DC voltage breakdown strength of 622-645V / μm; the capacitor film has a thermal shrinkage rate of 1.3-2% at 120℃ and a DC voltage breakdown strength of 561-596V / μm.

18. A method for preparing a capacitor film according to any one of claims 1-17, characterized in that, The method includes the following steps: (1) Polypropylene resin, a mixture containing poly(4-methyl-1-pentene) resin and antioxidant are extruded separately to obtain sheet fluid; (2) The sheet fluid is sequentially shaped, longitudinally stretched and transversely stretched to obtain a basic film; The longitudinal stretching includes longitudinal preheating, stretching, and shaping; the transverse stretching includes stretching and shaping; the shaping temperature is higher than the stretching temperature. (3) The base film is processed and wound up, and then subjected to a first aging treatment and a second aging treatment to obtain the capacitor film.

19. The method according to claim 18, wherein, In step (1), the extrusion temperature of the polypropylene resin is 230-255℃, and the extrusion temperature of the mixture containing poly(4-methyl-1-pentene) resin and antioxidant is 250-300℃.

20. The method according to claim 18, wherein, The longitudinal stretching includes longitudinal preheating, stretching, and setting; the conditions for longitudinal stretching include: preheating temperature of 125-150℃, stretching temperature of 145-155℃, setting temperature of 150-158℃, and stretching ratio of 3-8 times.

21. The method according to claim 18, wherein, The transverse stretching includes stretching and shaping. The conditions for transverse stretching include: stretching temperature of 145-155℃, shaping temperature of 150-158℃, and stretching ratio of 5-8 times.

22. The method according to claim 18, wherein, In step (3), the process includes cooling, measuring thickness and trimming the base film in sequence; the cooling temperature is 25-30℃.

23. The method according to claim 22, wherein, The film after edge trimming is wound up, with a winding tension of 20-30 N / m and a winding pressure of 150-200 N / m.

24. The method according to claim 18, wherein, The conditions for the first aging treatment include: purification level ≤ 10,000, temperature 25-30℃, humidity < 60%, and aging time 24-72h.

25. The method according to claim 18, wherein, The conditions for the second aging treatment include: purification level ≤ 10,000, temperature 25-30℃, humidity < 60%, and aging time 12-24h.

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