A polypropylene capacitor film with low thermal shrinkage rate, its preparation method and application

Abstract

The present invention belongs to the technical field of capacitor films, and relates to a polypropylene capacitor film with a low thermal shrinkage rate, a preparation method thereof, and an application. The raw materials of the polypropylene capacitor film of the present invention are composed of the following components in weight percentage: polypropylene: 94.5-98.9%, polystyrene: 1-5%, styrene-ethylene / butene-styrene copolymer: 0.1-0.5%, and the total weight percentage of polypropylene, polystyrene, and styrene-ethylene / butene-styrene copolymer is 100%; the weight average molecular weight of the polypropylene is 1×10 5 ~5×10 5 g / mol, and the molecular weight distribution is 8-12; the Brookfield viscosity of the styrene-ethylene / butene-styrene copolymer (25 wt% toluene solution, 25 °C) ≥ 50000 mPa·s. This polypropylene capacitor film has a low thermal shrinkage rate at high temperatures and simultaneously has excellent tensile strength and breakdown strength.

Description

Technical Field

[0001] The present invention belongs to the technical field of capacitor films, and relates to a polypropylene capacitor film with a low thermal shrinkage rate, a preparation method thereof, and an application thereof. Background Art

[0002] The increasing use of automobiles has increased energy consumption and environmental pollution. Under the background of China's efforts to achieve the "dual carbon" goals of "carbon peak" and "carbon neutrality", in order to achieve the revolutionary emission reduction goals in the automotive field, new energy vehicle technology is a strategic choice under the "dual carbon" goals and has become the main tool for future transportation. New energy vehicles have always faced problems such as short cruising range, long charging time, and doubts about safety. In order to enhance consumers' trust in new energy vehicles, automobile manufacturers have continuously researched and developed this core system. The role of thin-film capacitors in new energy vehicles is reflected in the performance improvement of the drive inverters in the power supply system. Therefore, thin-film capacitors are replacing traditional aluminum electrolytic capacitors and are being widely used in the pure electric new energy vehicle market. As the core dielectric of thin-film capacitors, the technical level of capacitor films restricts the development of new energy vehicles.

[0003] Existing ordinary biaxially stretched polypropylene films have a relatively large thermal shrinkage rate and poor heat resistance. As the capacitors wound with this film operate for a longer time, the internal temperature rise is relatively fast, resulting in a sharp decline in the stability of the capacitors and even causing the capacitors to fail, posing a serious safety hazard to electrical equipment or the power grid. In addition, the high thermal shrinkage rate of polypropylene films can cause the hardness of the film rolls to be too large and the winding to be too tight after winding, so that the films are easily bonded or broken under high-speed slitting; during metal evaporation, excessive heat energy conversion can cause the films to shrink and cause cracks in the metal layer. Especially when double-sided metallization is performed on polypropylene films, a higher thermal shrinkage rate is more likely to cause uneven evaporation of the metal layer and cause cracks in the metal layer.

[0004] Chinese Patent CN111454517A discloses a polypropylene resin dedicated to high-speed biaxially oriented films, which comprises the following components in parts by mass: 100 parts of polypropylene powder, 0.02 - 0.1 part of primary antioxidant 1010, 0.02 - 0.1 part of auxiliary antioxidant 626, and 0.015 - 0.05 part of acid absorbent hydrotalcite. Through the improvement of the gas-phase polymerization process and formulation optimization, the processing speed of the BOPP film is higher, with high rigidity and strength, and the thermal shrinkage rate decreases. However, the polypropylene film prepared from this polypropylene resin has a relatively large thickness and is difficult to meet the requirements of some specific capacitors. Chinese Patent CN102509610B discloses a preparation method of a 2.5-μm polypropylene capacitor film. By using polypropylene particles with an ash content of 10 - 20 ppm, a melt index of 3.2 - 4.0 g / 10 min, and a melting point of 162 - 170 °C as raw materials, combined with the preparation process, a polypropylene capacitor film with a thickness of 2.5 μm is obtained, and the thermal shrinkage rate is also reduced, but the reduction of the longitudinal thermal shrinkage rate is limited. Summary of the Invention

[0005] The purpose of the present invention is to provide a polypropylene capacitor film with a low thermal shrinkage rate and its preparation method. The polypropylene capacitor film has a low thermal shrinkage rate at high temperatures and simultaneously has excellent tensile strength and breakdown strength.

[0006] One purpose of the present invention is achieved through the following technical solutions:

[0007] A polypropylene capacitor film with a low thermal shrinkage rate, the raw materials of which are composed of the following components in weight percentage:

[0008] Polypropylene: 94.5 - 98.9%,

[0009] Polystyrene: 1 - 5%,

[0010] Styrene-ethylene / butylene-styrene copolymer (SEBS): 0.1 - 0.5%,

[0011] The total weight percentage of polypropylene, polystyrene and styrene-ethylene / butylene-styrene copolymer is 100%;

[0012] The weight-average molecular weight (Mw) of the polypropylene is 1×10 5 - 5×10 5 g / mol, and the molecular weight distribution (Mw / Mn) is 8 - 12; the Brookfield viscosity (25 wt% toluene solution, 25 °C) of the styrene-ethylene / butylene-styrene copolymer (SEBS) ≥ 50000 mPa·s.

[0013] The thermal shrinkage rate of the film made of polymer is related to the flexibility of molecular chains, the intermolecular interaction force, and the degree of stretching orientation. In the present invention, polystyrene and SEBS are added to the polypropylene raw material. The benzene rings in the molecular chains of polystyrene and SEBS limit the movement of the molecular chains; and the presence of SEBS promotes the mutual solubility of polypropylene and polystyrene, improving the intermolecular interaction force between the molecular chains of polypropylene and polystyrene. Therefore, in the present invention, polystyrene and SEBS are simultaneously added to polypropylene as raw materials to prepare polypropylene capacitor film, which is beneficial to reducing the thermal shrinkage rate of the film and improving other properties of the film. However, the addition amounts of polystyrene and SEBS are relatively small and cannot be too much, otherwise it will affect the stretching orientation degree of the film and cause adverse effects on the film properties.

[0014] Further, the weight-average molecular weight of the polypropylene selected in the present invention is 1×10 5 ~5×10 5 g / mol, effectively ensuring that the mechanical properties of the film meet the requirements, and the polypropylene has better stretching orientation degree after stretching. The molecular weight distribution of the polypropylene selected in the present invention is 8 - 12. The distribution of high-molecular-weight and low-molecular-weight polypropylene is appropriate, which can take into account the mechanical properties and improve the processability of polypropylene, and it is not easy to have film breakage phenomenon during the production process. The SEBS selected in the present invention has a relatively large Brookfield viscosity, which to a certain extent represents its relatively high molecular weight, and is beneficial to improving the intermolecular interaction force between the molecular chains of polypropylene and polystyrene. The above Brookfield viscosity is the viscosity value obtained by dissolving SEBS in toluene to form a 25wt% toluene solution and measuring the 25wt% SEBS toluene solution with a Brookfield viscometer at 25°C.

[0015] Preferably, the isotactic index of the polypropylene is ≥97%, the melt index MFR is 2.9 - 4.0 g / 10 min (230 °C, 2.16 kg), and the ash content is ≤20 ppm. Examples include Daelim 5014L HPT-1 or Borealis HC300BF. The ash content of the polypropylene raw material is the mass ratio of the inorganic oxides remaining after high-temperature calcination of the polypropylene at 800 °C to the original material. The ash in the polypropylene participates in the crystallization process to a certain extent and affects the microscopic morphology of the polypropylene film; at the same time, a high ash content exacerbates the ionization of impurities inside the film medium, promotes ion conduction, and increases the conduction loss. Therefore, the ash content of the polypropylene raw material selected in the present invention is controlled to be ≤20 ppm. The higher the isotactic index of the polypropylene raw material, the more uniform the arrangement of each propylene monomer in the polypropylene molecular chain, the relatively better the crystallinity, and the dense and compact microscopic structure formed helps to improve the breakdown field strength of the film and reduce the thermal shrinkage rate. The melt index MFR refers to the mass value of the melt of a thermoplastic resin passing through a standard capillary within 10 minutes under specified temperature and load, expressed in grams per 10 minutes, and is measured using a melt index instrument. The present invention selects a polypropylene raw material with a melt index MFR of 2.9 - 4.0 g / 10 min (measurement conditions: temperature 230 °C, 2160 g load) to effectively control the melt to better adapt to the flow rate of the extrusion system and control the film thickness.

[0016] Preferably, 3000 mPa·s ≤ the Brookfield viscosity of the styrene-ethylene / butene-styrene copolymer (15 wt% toluene solution, 25 °C) ≤ 100000 mPa·s. The above Brookfield viscosity is the viscosity value obtained by dissolving SEBS in toluene to form a 15 wt% toluene solution and measuring the 15 wt% SEBS toluene solution using a Brookfield viscometer at 25 °C. Examples of the above SEBS include Kraton G1633.

[0017] Preferably, the styrene content of the styrene-ethylene / butene-styrene copolymer is 28 - 35%.

[0018] Preferably, the melt index MFR of the polystyrene is 0.5 - 5 g / 10 min (200 °C, 5.0 kg).

[0019] More preferably, the isotactic index of the polypropylene is ≥97%, the melt index MFR is 2.9 - 4.0 g / 10 min (230 °C, 2.16 kg), and the ash content is ≤20 ppm; 3000 mPa·s ≤ the Brookfield viscosity of the styrene-ethylene / butene-styrene copolymer (15 wt% toluene solution, 25 °C) ≤ 100000 mPa·s; the melt index MFR of the polystyrene is 0.5 - 5 g / 10 min (200 °C, 5.0 kg).

[0020] Preferably, the polypropylene capacitor film with low thermal shrinkage rate has a breakdown field strength ≥ 600 kV / mm at 25°C, a longitudinal tensile strength ≥ 190 MPa, a transverse tensile strength ≥ 315 MPa, a transverse thermal shrinkage rate ≤ 0.4% @ 120°C, and a longitudinal thermal shrinkage rate ≤ 2.5% @ 120°C; the film thickness ≤ 3.0 μm.

[0021] Another object of the present invention is achieved by the following technical solution:

[0022] A preparation method of a polypropylene capacitor film with low thermal shrinkage rate includes the following steps:

[0023] After mixing polypropylene, polystyrene, and styrene-ethylene / butylene-styrene copolymer, put them into a screw extruder, melt and extrude to obtain a melt, cool and shape the melt to obtain a cast sheet, and then perform a longitudinal stretching step and a transverse stretching step on the cast sheet in sequence, followed by corona treatment, winding, first aging treatment, slitting, and second aging treatment to obtain the polypropylene capacitor film.

[0024] Preferably, put polypropylene, polystyrene, and styrene-ethylene / butylene-styrene copolymer into a high-speed mixer, stir evenly with the high-speed mixer for mixing, the rotation speed of the high-speed mixer is 500 - 2000 rpm, and the mixing time is 1 - 20 min.

[0025] Preferably, the screw extruder is a twin-screw extruder, and the melt extrusion parameters of the twin-screw extruder are: the temperature of the first zone is 185 - 205°C, the temperature of the second zone: 200 - 235°C, the temperature of the third zone is 235 - 255°C, the temperature of the fourth zone: 240 - 260°C, the temperature of the fifth zone is 240 - 260°C, the temperature of the sixth zone: 230 - 260°C, the die head temperature: 235 - 260°C, and the screw rotation speed is: 60 - 300 rpm.

[0026] Preferably, the melt is cooled and shaped through a chill roll and air shower, the temperature of the chill roll is 85 - 105°C, and the thickness of the obtained cast sheet is 200.0 - 400.0 μm.

[0027] Preferably, the longitudinal stretching step includes: longitudinal preheating, longitudinal stretching, and longitudinal shaping, the temperature of longitudinal preheating is 110 - 130°C, the temperature of longitudinal stretching is 152 - 157°C, and the temperature of longitudinal shaping is 153 - 159°C.

[0028] Preferably, the longitudinal stretching ratio is 5 - 7 times.

[0029] Preferably, the transverse stretching step includes: transverse preheating, transverse stretching, and transverse shaping, the temperature of transverse preheating is 160 - 175°C, the temperature of transverse stretching is 163 - 169°C, and the temperature of transverse shaping is 175 - 183°C.

[0030] Preferably, the transverse stretching ratio is 7 to 10 times.

[0031] In the past, during the stretching process of polypropylene films without adding polystyrene and SEBS, the inventors found that by setting the longitudinal stretching temperature at 140 - 145°C, the longitudinal shaping temperature at 142 - 148°C, the transverse stretching temperature at 155 - 160°C, and the transverse shaping temperature at 165 - 175°C, the film properties can be effectively improved. In the present invention, since a small amount of polystyrene and SEBS are added to the polypropylene raw material, the inventors found that by raising the longitudinal stretching temperature to 152 - 157°C, the longitudinal shaping temperature to 153 - 159°C, simultaneously raising the transverse stretching temperature to 163 - 169°C, and the transverse shaping temperature to 175 - 183°C, the molecular chains inside the film can be greatly stretched, the orientation degree of the molecular chains in the stretching direction increases, the molecular chains are arranged more regularly, the interaction between the molecular chains is stronger, the film exhibits a lower thermal shrinkage rate, and at the same time has excellent tensile strength and breakdown strength.

[0032] Preferably, during corona treatment, the output power is 5 - 18 kW, the output voltage is 5 - 10 kV, and the bombardment intensity of the electrode is 7 - 10 W·min / m 2 。

[0033] Preferably, two electrodes can be used to perform corona treatment on both sides of the film simultaneously, and a polypropylene capacitor film that can be metallized on both sides can be obtained.

[0034] Preferably, the first aging treatment is carried out at a dust-free level of 10,000 or below, and is treated in an environment of 20 - 40°C for 50 - 90 h.

[0035] Preferably, the second aging treatment is carried out at a dust-free level of 10,000 or below, and is treated in an environment of 20 - 40°C for 15 - 40 h.

[0036] The third object of the present invention is achieved by the following technical solutions:

[0037] Application of the above polypropylene capacitor film with low thermal shrinkage rate in electric vehicle capacitors.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) In the present invention, a small amount of polystyrene and SEBS are simultaneously added to the polypropylene raw material to prepare a polypropylene capacitor film, which is beneficial to reducing the thermal shrinkage rate of the film, improving the tensile strength and breakdown field strength;

[0040] (2) The present invention selects polypropylene with a suitable molecular weight, polystyrene with a suitable melt index, and SEBS with a high Brookfield viscosity to enhance the intermolecular interaction force of the polymer molecular chains, thereby improving the tensile strength and breakdown field strength of the film and reducing the thermal shrinkage rate.

[0041] (3) The present invention specifies that the isotactic index of the polypropylene raw material is ≥97%, the melt index MFR is 2.9 - 4.0 g / 10 min (230 °C, 2.16 kg), and the ash content is ≤20 ppm. The high isotactic index and low ash content are beneficial to improving the performance of the polypropylene film.

[0042] (4) The present invention adds a small amount of polystyrene and SEBS to the polypropylene raw material, and at the same time raises the stretching temperature and setting temperature to a suitable range during the stretching process, which can greatly stretch the molecular chains inside the film, increase the orientation degree of the molecular chains in the stretching direction, make the molecular chain arrangement more regular, enhance the intermolecular interaction, and the film shows a lower thermal shrinkage rate, and at the same time has excellent tensile strength and breakdown strength.

[0043] (5) The polypropylene capacitor film of the present invention has a DC breakdown field strength ≥600 kV / mm at 25 °C, a longitudinal tensile strength ≥190 MPa, a transverse tensile strength ≥315 MPa, a transverse thermal shrinkage rate ≤0.4% @120 °C, and a longitudinal thermal shrinkage rate ≤2.5% @120 °C; the film thickness ≤3.0 μm; it can be used as a capacitor film in electric vehicle capacitors to promote the development of new energy vehicles. Detailed Embodiments

[0044] The technical solutions of the present invention will be further described and illustrated below through specific embodiments. It should be understood that the specific embodiments described here are only used to help understand the present invention and are not used for specific limitations of the present invention. If there is no special description, the raw materials used in the embodiments of the present invention are all common raw materials in the art, and the methods used in the embodiments are all conventional methods in the art.

[0045] Polypropylene raw materials:

[0046] 1. Borealis, model HC300BF, its isotactic index is about 98.1%, the melt index MFR is about 3.8 g / 10 min (230 °C, 2.16 kg), the ash content is about 10 ppm, Mw is about 397610, and Mw / Mn is about 9.4.

[0047] 2. Korea Petrochemical, model 5014L HPT-1, its isotactic index is about 98.0%, the melt index MFR is about 3.1 g / 10 min (230 °C, 2.16 kg), the ash content is about 9.9 ppm, Mw is about 375980, and Mw / Mn is about 9.2.

[0048] Polystyrene:

[0049] 1. Trinseo STYRON TM 685F, melt index is 3 g / 10 min (200 °C / 5.0 kg, ASTM D1238).

[0050] 2. Trinseo STYRON TM 685D, melt index is 1.5 g / 10 min (200 °C / 5.0 kg, ASTM D1238).

[0051] 3. Trinseo STYRON TM 656D, melt index is 8.5 g / 10 min (200 °C / 5.0 kg, ASTM D1238).

[0052] Styrene-ethylene / butylene-styrene copolymer:

[0053] 1. Kraton of the United States TM G1633, Brookfield viscosity (25 wt% toluene solution, 25 °C) > 50000 mPa·s, Brookfield viscosity (15 wt% toluene solution, 25 °C) > 30000 mPa·s.

[0054] 2. Kraton of the United States TM G1651, Brookfield viscosity (25 wt% toluene solution, 25 °C) > 50000 mPa·s, Brookfield viscosity (15 wt% toluene solution, 25 °C) < 30000 mPa·s.

[0055] 3. Kraton of the United States TM G1654, Brookfield viscosity (25 wt% toluene solution, 25 °C) > 50000 mPa·s, Brookfield viscosity (15 wt% toluene solution, 25 °C) < 30000 mPa·s.

[0056] 4. Kraton of the United States TM G1650, Brookfield viscosity (25 wt% toluene solution, 25 °C) < 50000 mPa·s.

[0057] 5. Kraton of the United States TM G1652, Brookfield viscosity (25 wt% toluene solution, 25 °C) < 50000 mPa·s.

[0058] Example 1

[0059] The raw materials of the polypropylene capacitor film in this embodiment include 2.5 wt% polystyrene Trinseo STYRON TM 685F, 0.3 wt% SEBS KRATON TM G1633, and the balance is polypropylene HC300BF.

[0060] The preparation method of the polypropylene capacitor film in this embodiment is obtained by the following steps:

[0061] 1) Mix polypropylene, polystyrene, and SEBS in a high-speed mixer at a speed of 800 rpm for 5 minutes, and then put them into a twin-screw extruder to melt and extrude to obtain a melt. The melting and extrusion parameters of the twin-screw extruder are as follows: the temperature of the first zone is 192 °C, the temperature of the second zone is 215 °C, the temperature of the third zone is 250 °C, the temperature of the fourth zone is 250 °C, the temperature of the fifth zone is 250 °C, the temperature of the sixth zone is 250 °C, the die head temperature is 255 °C, and the main screw speed is 150 rpm;

[0062] 2) Chilling roll and air shower cooling and shaping: Cool and shape the melt obtained in step 1) through a chilling roll and an air shower to obtain a cast sheet, and the temperature of the chilling roll is 95 °C;

[0063] 3) Longitudinal stretching step: Sequentially preheat, longitudinally stretch, and longitudinally shape the cast sheet obtained in step 2). There are 8 rolls for longitudinal preheating, the temperature of the 1st - 2nd rolls is set at 112 °C, the temperature of the 3rd - 4th rolls is set at 120 °C, the temperature of the 5th - 6th rolls is set at 125 °C, and the temperature of the 7th - 8th rolls is set at 130 °C; There are 6 rolls for longitudinal stretching, the temperature of the 1st - 2nd rolls is set at 152 °C, the temperature of the 3rd - 4th rolls is set at 155 °C, and the temperature of the 5th - 6th rolls is set at 152 °C; There are 4 rolls for longitudinal shaping, the temperature of the 1st - 2nd rolls is set at 154 °C, and the temperature of the 3rd - 4th rolls is set at 157 °C; The longitudinal stretching ratio is 6 times;

[0064] 4) Transverse stretching step: Sequentially preheat, transversely stretch, and transversely shape the film obtained in step 3). There are 8 sections for transverse preheating, the temperature of the 1st - 3rd sections is set at 165 °C, the temperature of the 4th - 6th sections is set at 171 °C, and the temperature of the 7th - 8th sections is set at 173 °C; There are 6 sections for transverse stretching, the temperature of the 1st - 2nd sections is set at 164 °C, the temperature of the 3rd - 4th sections is set at 165 °C, and the temperature of the 5th - 6th sections is set at 168 °C; There are 5 sections for transverse shaping, the temperature of the 1st - 3rd sections is set at 177 °C, and the temperature of the 4th - 5th sections is set at 180 °C; The transverse stretching ratio is 9 times;

[0065] 5) Thickness measurement, edge cutting and corona treatment: The thickness of the film was measured to be 2.3 μm. The two sides of the film were simultaneously subjected to corona treatment using two electrodes. During corona treatment, the output power of the two electrodes was 8 KW, the output voltage was 8 KV, the pulse frequency was 20 KHz, the discharge gap was 2 mm, and the bombardment intensity of the electrodes was 8 W·min / m 2 , and the discharge state of the electrodes was arc-shaped dense fog-like discharge, and the exhaust was a four-air duct ozone exhaust device;

[0066] 6) Rewinding: The rewinding tension was 25%, and the rewinding pressure was 42%;

[0067] 7) First aging treatment: The wound film was subjected to aging treatment for 85 hours in an environment with a dust-free class of 10,000 and a temperature of 32 °C;

[0068] 8) Slitting: The film after the first aging treatment was slit into small roll films of a certain width according to requirements;

[0069] 9) Second aging treatment: The slit small roll films were aged for 20 hours in an environment with a dust-free class of 10,000 and a temperature of 32 °C to obtain the finished polypropylene capacitor film.

[0070] Example 2

[0071] The raw materials of the polypropylene capacitor film in this example include 2.5 wt% polystyrene Trinseo STYRON TM 685F, 0.3 wt% SEBS KRATON TM G1651, and the balance is polypropylene HC300BF.

[0072] The preparation method of the polypropylene capacitor film in this example is the same as that of Example 1, and the finished polypropylene capacitor film is obtained.

[0073] Example 3

[0074] The raw materials of the polypropylene capacitor film in this example include 2.5 wt% polystyrene Trinseo STYRON TM 685F, 0.3 wt% SEBS KRATON TM G1654, and the balance is polypropylene HC300BF.

[0075] The preparation method of the polypropylene capacitor film in this example is the same as that of Example 1, and the finished polypropylene capacitor film is obtained.

[0076] Example 4

[0077] The raw materials of the polypropylene capacitor film in this example include 2.5 wt% polystyrene Trinseo STYRON TM656D, 0.3 wt% SEBS KRATON TM G1633, with the balance being polypropylene HC300BF.

[0078] The preparation method of the polypropylene capacitor film in this example is the same as that in Example 1, and the finished polypropylene capacitor film is obtained.

[0079] Example 5

[0080] The raw materials of the polypropylene capacitor film in this example include 1 wt% polystyrene Trinseo STYRON TM 685F, 0.1 wt% SEBS KRATON TM G1633, with the balance being polypropylene HC300BF.

[0081] The preparation method of the polypropylene capacitor film in this example is the same as that in Example 1, and the finished polypropylene capacitor film is obtained.

[0082] Example 6

[0083] The raw materials of the polypropylene capacitor film in this example include 5 wt% polystyrene Trinseo STYRON TM 685F, 0.5 wt% SEBS KRATON TM G1633, with the balance being polypropylene HC300BF.

[0084] The preparation method of the polypropylene capacitor film in this example is the same as that in Example 1, and the finished polypropylene capacitor film is obtained.

[0085] Example 7

[0086] The raw materials of the polypropylene capacitor film in this example include 2.5 wt% polystyrene Trinseo STYRON TM 685F, 0.3 wt% SEBS KRATON TM G1633, with the balance being polypropylene HC300BF.

[0087] The preparation method of the polypropylene capacitor film in this example is prepared by the following steps:

[0088] 1) Mix polypropylene, polystyrene, and SEBS in a high-speed mixer at a speed of 800 rpm for 5 minutes, then put them into a twin-screw extruder and melt-extrude to obtain a melt. The melt-extrusion parameters of the twin-screw extruder are as follows: the temperature of the first zone is 192 °C, the temperature of the second zone is 215 °C, the temperature of the third zone is 250 °C, the temperature of the fourth zone is 250 °C, the temperature of the fifth zone is 250 °C, the temperature of the sixth zone is 250 °C, the die head temperature is 255 °C, and the main screw speed is 150 rpm;

[0089] 2) Chilling roll and air shower cooling and shaping: The melt obtained in step 1) is cooled and shaped by a chilling roll and an air shower to obtain a cast film, and the temperature of the chilling roll is 95°C;

[0090] 3) Longitudinal stretching step: The cast film obtained in step 2) is successively subjected to longitudinal preheating, longitudinal stretching, and longitudinal shaping. There are 8 rollers for longitudinal preheating, the temperature of the 1st - 2nd rollers is set at 112°C, the temperature of the 3rd - 4th rollers is set at 120°C, the temperature of the 5th - 6th rollers is set at 125°C, and the temperature of the 7th - 8th rollers is set at 130°C; There are 6 rollers for longitudinal stretching, the temperature of the 1st - 2nd rollers is set at 141°C, the temperature of the 3rd - 4th rollers is set at 144°C, and the temperature of the 5th - 6th rollers is set at 142°C; There are 4 rollers for longitudinal shaping, the temperature of the 1st - 2nd rollers is set at 145°C, and the temperature of the 3rd - 4th rollers is set at 147°C; The longitudinal stretching ratio is 6 times;

[0091] 4) Transverse stretching step: The film obtained in step 3) is successively subjected to transverse preheating, transverse stretching, and transverse shaping. There are 8 sections for transverse preheating, the temperature of the 1st - 3rd sections is set at 165°C, the temperature of the 4th - 6th sections is set at 171°C; the temperature of the 7th - 8th sections is set at 173°C; There are 6 sections for transverse stretching, the temperature of the 1st - 2nd sections is set at 155°C, the temperature of the 3rd - 4th sections is set at 158°C, and the temperature of the 5th - 6th sections is set at 160°C; There are 5 sections for transverse shaping, the temperature of the 1st - 3rd sections is set at 170°C, and the temperature of the 4th - 5th sections is set at 174°C; The transverse stretching ratio is 9 times;

[0092] Steps 5), 6), 7), 8) and 9) are the same as those in Example 1.

[0093] Example 8

[0094] The raw materials of the polypropylene capacitor film in this example include 3wt% polystyrene Trinseo STYRON TM 685D, 0.2wt% SEBS KRATON TM G1633, and the balance is polypropylene 5014L HPT - 1.

[0095] The preparation method of the polypropylene capacitor film in this example is prepared by the following steps:

[0096] 1) Mix polypropylene, polystyrene, and SEBS in a high-speed mixer at a rotational speed of 1000 rpm for 5 minutes, then feed them into a twin-screw extruder and melt-extrude to obtain a melt. The melt-extrusion parameters of the twin-screw extruder are as follows: the temperature of the first zone is 186 °C, the temperature of the second zone is 210 °C, the temperature of the third zone is 245 °C, the temperature of the fourth zone is 250 °C, the temperature of the fifth zone is 250 °C, the temperature of the sixth zone is 250 °C, the die head temperature is 250 °C, and the main screw rotational speed is 150 rpm;

[0097] 2) Chilling roll and air shower cooling and shaping: Cool and shape the melt obtained in step 1) through a chilling roll and an air shower to obtain a cast sheet, and the temperature of the chilling roll is 100 °C;

[0098] 3) Longitudinal stretching step: Sequentially perform longitudinal preheating, longitudinal stretching, and longitudinal shaping on the cast sheet obtained in step 2). There are 8 rolls for longitudinal preheating, and the temperature of the 1st - 2nd rolls is set to 115 °C, the temperature of the 3rd - 4th rolls is set to 123 °C, the temperature of the 5th - 6th rolls is set to 126 °C, and the temperature of the 7th - 8th rolls is set to 130 °C; There are 6 rolls for longitudinal stretching, and the temperature of the 1st - 2nd rolls is set to 153 °C, the temperature of the 3rd - 4th rolls is set to 156 °C, and the temperature of the 5th - 6th rolls is set to 154 °C; There are 4 rolls for longitudinal shaping, and the temperature of the 1st - 2nd rolls is set to 155 °C, the temperature of the 3rd - 4th rolls is set to 158 °C; The longitudinal stretching ratio is 6 times;

[0099] 4) Transverse stretching step: Sequentially perform transverse preheating, transverse stretching, and transverse shaping on the film obtained in step 3). There are 8 sections for transverse preheating, and the temperature of the 1st - 3rd sections is set to 162 °C, the temperature of the 4th - 6th sections is set to 168 °C, and the temperature of the 7th - 8th sections is set to 172 °C; There are 6 sections for transverse stretching, and the temperature of the 1st - 2nd sections is set to 163 °C, the temperature of the 3rd - 4th sections is set to 166 °C, and the temperature of the 5th - 6th sections is set to 169 °C; There are 5 sections for transverse shaping, and the temperature of the 1st - 3rd sections is set to 176 °C, the temperature of the 4th - 5th sections is set to 181 °C; The transverse stretching ratio is 9 times;

[0100] 5) Thickness measurement, edge trimming, and corona treatment: Measure the thickness of the film to be 2.3 μm, and simultaneously perform corona treatment on both sides of the film using two electrodes. When performing corona treatment, the output power of the two electrodes is 10 KW, the output voltage is 7 KV, the pulse frequency is 22 KHz, the discharge gap is 2 mm, and the bombardment intensity of the electrode is 7 W·min / m 2 , and the discharge state of the electrode is arc-shaped dense fog diffusion discharge, and the exhaust is a four-duct ozone exhaust device;

[0101] 6) Rewinding: The rewinding tension is 20%, and the rewinding pressure is 40%;

[0102] 7) First aging treatment: The wound film is subjected to aging treatment for 80 hours in an environment with a dust-free class of 10,000 and a temperature of 30°C;

[0103] 8) Slitting: The film after the first aging treatment is slit into small rolls of film with a certain width according to requirements;

[0104] 9) Second aging treatment: The small rolls of slit film are subjected to aging treatment for 25 hours in an environment with a dust-free class of 10,000 and a temperature of 30°C to obtain the finished polypropylene capacitor film.

[0105] Example 9

[0106] The raw materials of the polypropylene capacitor film in this example include 3.5 wt% polystyrene Trinseo STYRON TM 685F, 0.35 wt% SEBS KRATON TM G1633, and the balance is polypropylene HC300BF.

[0107] The preparation method of the polypropylene capacitor film in this example is prepared by the following steps:

[0108] 1) Polypropylene, polystyrene and SEBS are stirred and mixed in a high-speed mixer at a speed of 900 rpm for 7 min, and then put into a twin-screw extruder for melt extrusion to obtain a melt. The melt extrusion parameters of the twin-screw extruder are: the temperature of the first zone is 195°C, the temperature of the second zone: 220°C, the temperature of the third zone is 245°C, the temperature of the fourth zone: 250°C, the temperature of the fifth zone is 255°C, the temperature of the sixth zone: 255°C, the die head temperature: 255°C, and the main screw speed is: 150 rpm;

[0109] 2) Chilling roll and air shower cooling and shaping: The melt obtained in step 1) is cooled and shaped by a chilling roll and an air shower to obtain a cast sheet, and the temperature of the chilling roll is 90°C;

[0110] 3) Longitudinal stretching step: The cast sheet obtained in step 2) is successively subjected to longitudinal preheating, longitudinal stretching and longitudinal shaping. There are 8 rolls for longitudinal preheating, the temperature of the 1st - 2nd rolls is set at 113°C, the temperature of the 3rd - 4th rolls is set at 122°C, the temperature of the 5th - 6th rolls is set at 127°C, the temperature of the 7th - 8th rolls is set at 130°C; There are 6 rolls for longitudinal stretching, the temperature of the 1st - 2nd rolls is set at 154°C, the temperature of the 3rd - 4th rolls is set at 157°C, the temperature of the 5th - 6th rolls is set at 153°C; There are 4 rolls for longitudinal shaping, the temperature of the 1st - 2nd rolls is set at 156°C, the temperature of the 3rd - 4th rolls is set at 158°C; The longitudinal stretching ratio is 6 times;

[0111] 4) Transverse stretching step: The film obtained in step 3) is successively subjected to transverse preheating, transverse stretching, and transverse shaping. There are 8 sections for transverse preheating, the temperature of the 1st - 3rd sections is set at 164°C, the temperature of the 4th - 6th sections is set at 170°C; the temperature of the 7th - 8th sections is set at 174°C; there are 6 sections for transverse stretching, the temperature of the 1st - 2nd sections is set at 164°C, the temperature of the 3rd - 4th sections is set at 166°C, the temperature of the 5th - 6th sections is set at 168°C; there are 5 sections for transverse shaping, the temperature of the 1st - 3rd sections is set at 175°C, the temperature of the 4th - 5th sections is set at 182°C; the transverse stretching ratio is 9 times;

[0112] 5) Thickness measurement, edge trimming and corona treatment: The thickness of the film is measured to be 2.3 μm. The two sides of the film are simultaneously subjected to corona treatment using two electrodes. When corona treatment is carried out, the output power of the two electrodes is 9 KW, the output voltage is 9 KV, the pulse frequency is 24 KHz, the discharge gap is 2 mm, and the bombardment intensity of the electrode is 9 W·min / m 2 , the discharge state of the electrode is arc - shaped dense fog - like discharge, and the exhaust is a four - duct ozone exhaust device;

[0113] 6) Rewinding: The rewinding tension is 22%, and the rewinding pressure is 42%;

[0114] 7) First aging treatment: The rewound film is subjected to aging treatment for 72 hours in an environment with a dust - free level of 10,000 and a temperature of 35°C;

[0115] 8) Slitting: The film after the first aging treatment is slit into small - roll films of a certain width according to requirements;

[0116] 9) Second aging treatment: The small - roll films after slitting are subjected to aging treatment for 18 hours in an environment with a dust - free level of 10,000 and a temperature of 35°C to obtain the finished polypropylene capacitor film.

[0117] Comparative Example 1

[0118] The difference between the raw materials of the polypropylene capacitor film in Comparative Example 1 and those in Example 1 is that the raw materials of Comparative Example 1 do not include polystyrene. The specific raw materials of Comparative Example 1 are: 0.3 wt% SEBS KRATON TM G1633 and the balance is polypropylene HC300BF.

[0119] The preparation method of the polypropylene capacitor film in Comparative Example 1 is the same as that in Example 1, and the finished polypropylene capacitor film is obtained.

[0120] Comparative Example 2

[0121] The raw materials of the polypropylene capacitor film of Comparative Example 2 are different from those of Example 1 in that the raw materials of Comparative Example 2 do not include SEBS. The specific raw materials of Comparative Example 1 are: 2.5 wt% polystyrene Trinseo STYRON TM 685F, and the balance is polypropylene HC300BF.

[0122] The preparation method of the polypropylene capacitor film of Comparative Example 2 is the same as that of Example 1, and the finished polypropylene capacitor film is obtained.

[0123] Comparative Example 3

[0124] The raw material of the polypropylene capacitor film of Comparative Example 3 is polypropylene HC300BF.

[0125] The preparation method of the polypropylene capacitor film of Comparative Example 3 is prepared by the following steps:

[0126] 1) Stir and mix polypropylene in a high-speed mixer at a speed of 800 rpm for 5 min, and then put it into a twin-screw extruder to melt and extrude to obtain a melt. The melt extrusion parameters of the twin-screw extruder are: the temperature of the first zone is 192 °C, the temperature of the second zone: 215 °C, the temperature of the third zone is 250 °C, the temperature of the fourth zone: 250 °C, the temperature of the fifth zone is 250 °C, the temperature of the sixth zone: 250 °C, the die head temperature: 255 °C, and the main screw speed is: 150 rpm;

[0127] 2) Quenching roll and air shower cooling and shaping: Cool and shape the melt obtained in step 1) through a quenching roll and an air shower to obtain a cast sheet, and the temperature of the quenching roll is 95 °C;

[0128] 3) Longitudinal stretching step: The cast sheet obtained in step 2) is successively subjected to longitudinal preheating, longitudinal stretching, and longitudinal shaping. There are 8 rollers for longitudinal preheating, and the temperature of the 1st - 2nd rollers is set to 112 °C, the temperature of the 3rd - 4th rollers is set to 120 °C, the temperature of the 5th - 6th rollers is set to 125 °C, and the temperature of the 7th - 8th rollers is set to 130 °C; There are 6 rollers for longitudinal stretching, and the temperature of the 1st - 2nd rollers is set to 141 °C, the temperature of the 3rd - 4th rollers is set to 144 °C, and the temperature of the 5th - 6th rollers is set to 142 °C; There are 4 rollers for longitudinal shaping, and the temperature of the 1st - 2nd rollers is set to 145 °C, the temperature of the 3rd - 4th rollers is set to 147 °C; The longitudinal stretching ratio is 6 times;

[0129] 4) Transverse stretching step: The film obtained in step 3) is sequentially subjected to transverse preheating, transverse stretching, and transverse shaping. There are 8 segments for transverse preheating, with the temperature of the 1st - 3rd segments set at 165°C, the temperature of the 4th - 6th segments set at 171°C, and the temperature of the 7th - 8th segments set at 173°C; there are 6 segments for transverse stretching, with the temperature of the 1st - 2nd segments set at 155°C, the temperature of the 3rd - 4th segments set at 158°C, and the temperature of the 5th - 6th segments set at 160°C; there are 5 segments for transverse shaping, with the temperature of the 1st - 3rd segments set at 170°C and the temperature of the 4th - 5th segments set at 174°C; the transverse stretching ratio is 9 times.

[0130] Steps 5), 6), 7), 8) and 9) are the same as those in Example 1.

[0131] Comparative Example 4

[0132] The raw material of the polypropylene capacitor film in Comparative Example 4 is polypropylene HC300BF.

[0133] The preparation method of the polypropylene capacitor film in Comparative Example 4 is the same as that in Example 1, and the polypropylene capacitor film finished product is obtained.

[0134] Comparative Example 5

[0135] The raw material of the polypropylene capacitor film in Comparative Example 5 includes 8wt% polystyrene Trinseo STYRON TM 685F, 0.3wt% SEBS KRATON TM G1633, and the balance is polypropylene HC300BF.

[0136] The preparation method of the polypropylene capacitor film in Comparative Example 5 is the same as that in Example 1, and the polypropylene capacitor film finished product is obtained.

[0137] Comparative Example 6

[0138] The raw material of the polypropylene capacitor film in Comparative Example 6 includes 10wt% polystyrene Trinseo STYRON TM 685F, 0.3wt% SEBS KRATON TM G1633, and the balance is polypropylene HC300BF.

[0139] The preparation method of the polypropylene capacitor film in Comparative Example 6 is the same as that in Example 1, and the polypropylene capacitor film finished product is obtained.

[0140] Comparative Example 7

[0141] The raw material of the polypropylene capacitor film in Comparative Example 7 includes 2.5wt% polystyrene Trinseo STYRON TM 685F, 1wt% SEBS KRATONTM G1633 and the balance is polypropylene HC300BF.

[0142] The preparation method of the polypropylene capacitor film of Comparative Example 7 is the same as that of Example 1, and the polypropylene capacitor film finished product is obtained.

[0143] Comparative Example 8

[0144] The raw materials of the polypropylene capacitor film of Comparative Example 8 include 2.5 wt% polystyrene Trinseo STYRON TM 685F, 1.5 wt% SEBS KRATON TM G1633 and the balance is polypropylene HC300BF.

[0145] The preparation method of the polypropylene capacitor film of Comparative Example 8 is the same as that of Example 1, and the polypropylene capacitor film finished product is obtained.

[0146] Comparative Example 9

[0147] The raw materials of the polypropylene capacitor film of Comparative Example 9 include 2.5 wt% polystyrene Trinseo STYRON TM 685F, 0.3 wt% SEBS KRATON TM G1650 and the balance is polypropylene HC300BF.

[0148] The preparation method of the polypropylene capacitor film of Comparative Example 9 is the same as that of Example 1, and the polypropylene capacitor film finished product is obtained.

[0149] Comparative Example 10

[0150] The raw materials of the polypropylene capacitor film of Comparative Example 10 include 2.5 wt% polystyrene Trinseo STYRON TM 685F, 0.3 wt% SEBS KRATON TM G1652 and the balance is polypropylene HC300BF.

[0151] The preparation method of the polypropylene capacitor film of Comparative Example 10 is the same as that of Example 1, and the polypropylene capacitor film finished product is obtained.

[0152] Experimental results

[0153] The tensile strength (including longitudinal tensile strength and transverse tensile strength), breakdown field strength and thermal shrinkage rate of the polypropylene films of Examples 1-19 and Comparative Examples 1-10 were tested, and the results are shown in Table 1 below.

[0154] I. The test method of tensile strength is as follows:

[0155] 1. Test equipment

[0156] ZWICK / 2010 tensile tester

[0157] Edge cutting machine

[0158] Sampling plate: 300mm×170mm

[0159] 2. Specimen

[0160] Specimen width: (15±1)mm, tensile speed: (100±10)mm / min, with the gripping distance as the reference, the gripping distance is (100±0.1)mm

[0161] 3. Test method

[0162] Use the sampling plate to take 1 piece of film each along the transverse and longitudinal directions of the test film, put it into the edge cutting machine and cut it into the standard width. Take a specimen film and put it into the tensile tester, turn on the computer, click the tensile test software, input the specimen thickness in the interface, and then click the experiment start button. After the test is completed, directly read the data on the computer. Test three times each for the transverse and longitudinal directions, and take the average value as the test result

[0163] II. The test method for the thermal shrinkage rate is as follows

[0164] 1. Test equipment

[0165] Oven: Temperature control accuracy ±2℃

[0166] Timer

[0167] Steel ruler: Least count 0.5mm

[0168] 2. Specimen

[0169] Uniformly take 5 specimens of 100mm×100mm in the film roll direction and make longitudinal and transverse marks. The surface of the specimen should be smooth, the edges should be straight, and there should be no mechanical damage

[0170] 3. Test method

[0171] Measure the longitudinal and transverse dimensions of each specimen respectively, accurate to 0.5mm. Then put the specimen into the oven, control the oven temperature at 120±2℃ or 140±2℃, support the specimen with A4 paper and bake it in the oven for 15min. After baking, take out the specimen, cool it to room temperature, and re-measure the longitudinal and transverse dimensions of the specimen

[0172] 4. Result

[0173] The shrinkage rate is calculated according to the following formula

[0174]

[0175] Where

[0176] X1——Shrinkage rate %;

[0177] L0——Longitudinal or transverse dimension of the specimen before baking, mm;

[0178] L1——Longitudinal or transverse dimension of the specimen after baking, mm.

[0179] The average value of 5 measurement values for both the longitudinal and transverse directions is taken as the test result in that direction.

[0180] III. The test method for breakdown field strength is as follows:

[0181] 1. Test equipment

[0182] Use a CDI-20 withstand voltage tester, DC electrical strength (50-point electrode method).

[0183] 2. Specimen

[0184] Take three rectangular specimens not less than 450mm×650mm. The specimens should be kept clean, flat, without wrinkles or damage.

[0185] 3. Electrode

[0186] The upper electrode is a brass cylindrical electrode with a diameter of 25mm, a chamfer radius of 2.5mm, and a height of 120mm. The surface roughness Ra of the working surface is less than 1.25μm.

[0187] Lay a rubber sheet about 3mm thick on the platform, with a Shore A hardness of (60 - 70) HA. Lay an annealed aluminum foil on the rubber sheet as the lower electrode.

[0188] Ensure that the working surface of the electrode is flat, smooth, and without scars.

[0189] 4. Breakdown device

[0190] The capacity of the high-voltage test transformer should ensure that the secondary rated current is not less than 0.1A. The voltage ripple coefficient of the DC power supply should not exceed 5%. The protection resistance is (0.2 - 0.5)Ω / V. The voltage regulator should be able to adjust the voltage evenly. The overcurrent relay should have sufficient sensitivity to cut off the power supply within 0.1s when the specimen breaks down. The operating current should be selected appropriately to avoid non-operation after breakdown or false operation without breakdown. The voltage measurement error does not exceed 4%.

[0191] 5. Test procedure

[0192] a) Place the specimen on the lower electrode at 25°C and use the continuous voltage increase method.

[0193] b) Set the voltage increase rate to 500V / s.

[0194] c) Measure 50 points evenly and at equal distances on the specimen.

[0195] d) Thickness test: Use a thickness gauge to evenly test 40 values at equal intervals on the test sample, and take the arithmetic mean as the thickness of the specimen.

[0196] 6. Results

[0197] Among the breakdown measurement values at 50 points, remove 5 maximum values and 5 minimum values respectively, and calculate the arithmetic mean of the remaining 40 points. Divide the average breakdown voltage by the thickness of the specimen, which is the average electrical strength of this batch of films.

[0198] Table 1 Performance data of polypropylene films in Examples 1-9 and Comparative Examples 1-10

[0199]

[0200] Comparative Example 3 is a conventional polypropylene film obtained without adding SEBS and polystyrene and by stretching at conventional stretching and setting temperatures. By comparing the data of Comparative Example 3 and Example 7, it can be seen that adding a small amount of polystyrene and SEBS to the polypropylene film helps to improve the film performance; and when further increasing the stretching temperature and setting temperature during the stretching process, the performance of the polypropylene film is better (see Example 1 and Example 7). Comparative Example 4 is a polypropylene film obtained without adding SEBS and polystyrene but by stretching at the stretching and setting temperatures of the present invention. From Comparative Example 3 and Comparative Example 4, it can be seen that when SEBS and polystyrene are not added, stretching and setting at a slightly higher temperature will have an adverse effect on the film performance. Therefore, only when SEBS and polystyrene are added to polypropylene and then the stretching and setting temperatures are increased can more excellent effects be obtained.

[0201] By comparing the data of Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4, it can be seen that adding only SEBS or only polystyrene to the polypropylene raw material has little improvement on the performance of the capacitor film.

[0202] The polystyrene content in Comparative Example 5 and Comparative Example 6 is too high, and the SEBS content in Comparative Example 7 and Comparative Example 8 is too high. Excessive polystyrene content or SEBS content will significantly reduce the breakdown field strength.

[0203] By comparing Example 1, Example 2, Example 3 and Comparative Examples 9 and 10, it can be seen that the Brookfield viscosity value of the selected SEBS has a great influence on the film performance. Generally speaking, the greater the Brookfield viscosity value of SEBS, the better the film performance.

[0204] Aspects, embodiments, and features of the present invention should be considered illustrative in all respects and not limiting of the present invention, the scope of which is defined only by the claims. Without departing from the spirit and scope of the claimed invention, those skilled in the art will appreciate other embodiments, modifications, and uses.

[0205] In the preparation method of the present invention, the order of each step is not limited to the recited order. For those of ordinary skill in the art, without creative efforts, changes in the sequence of each step are also within the protection scope of the present invention. In addition, two or more steps or actions can be carried out simultaneously.

[0206] Finally, it should be noted that the specific embodiments described herein are merely illustrative of the present invention and do not limit the implementation of the present invention. Those skilled in the technical field to which the present invention pertains can make various modifications or supplements to the described specific embodiments or use similar methods for substitution. It is not necessary and impossible to list all implementation manners here. And these obvious changes or variations derived from the essential spirit of the present invention still fall within the protection scope of the present invention. Interpreting them as any additional restrictions is contrary to the spirit of the present invention.

Claims

1. A polypropylene capacitor film with a low thermal shrinkage rate, characterized in that, The raw materials are composed of the following components by weight percentage as follows: Polypropylene: 94.5 - 98.9%, Polystyrene: 1 - 5%, Styrene - ethylene / butene - styrene copolymer: 0.1 - 0.5%, The total weight percentage of polypropylene, polystyrene and styrene - ethylene / butene - styrene copolymer is 100%; The weight-average molecular weight of the polypropylene is 1×10 5 ~5×10 5 g / mol, and the molecular weight distribution is 8 to 12; the Brookfield viscosity of the styrene-ethylene / butene-styrene copolymer is ≥50000 mPa‧s, and the Brookfield viscosity is measured at 25 °C for a 25 wt% toluene solution of the styrene-ethylene / butene-styrene copolymer.

2. The polypropylene capacitor film with a low thermal shrinkage rate according to claim 1, wherein The isotactic index of the polypropylene ≥ 97%, the melt index MFR is measured at 230°C and 2.16 kg to be 2.9 - 4.0 g / 10 min, and the ash content ≤ 20 ppm.

3. A polypropylene capacitor film with a low thermal shrinkage rate according to claim 1, characterized in that, 3000 mPa·s ≤ the Brookfield viscosity of the styrene - ethylene / butene - styrene copolymer ≤ 100000 mPa·s, and the Brookfield viscosity is measured from a 15 wt% toluene solution of the styrene - ethylene / butene - styrene copolymer at 25°C.

4. A polypropylene capacitor film with a low thermal shrinkage rate according to claim 1, characterized in that, The styrene content of the styrene - ethylene / butene - styrene copolymer is 28 - 35%.

5. A polypropylene capacitor film with a low thermal shrinkage rate according to claim 1, characterized in that, The melt index MFR of the polystyrene is measured at 200°C and 5.0 kg to be 0.5 - 5 g / 10 min.

6. The polypropylene capacitor film with a low thermal shrinkage rate according to claim 1, characterized in that The isotactic index of the polypropylene ≥ 97%, the melt index MFR is measured at 230°C and 2.16 kg to be 2.9 - 4.0 g / 10 min, and the ash content ≤ 20 ppm; 3000 mPa·s ≤ the Brookfield viscosity of the styrene - ethylene / butene - styrene copolymer ≤ 100000 mPa·s, and the Brookfield viscosity is measured from a 15 wt% toluene solution of the styrene - ethylene / butene - styrene copolymer at 25°C; the melt index MFR of the polystyrene is measured at 200°C and 5.0 kg to be 0.5 - 5 g / 10 min.

7. A polypropylene capacitor film with a low thermal shrinkage rate according to claim 1, characterized in that, The breakdown field strength of the polypropylene capacitor film with low thermal shrinkage rate ≥ 600 kV / mm at 25°C, the longitudinal tensile strength ≥ 190 MPa, the transverse tensile strength ≥ 315 MPa, the transverse thermal shrinkage rate ≤ 0.4% @ 120°C, the longitudinal thermal shrinkage rate ≤ 2.5% @ 120°C; the film thickness ≤ 3.0 μm.

8. A method for preparing a polypropylene capacitor film with a low thermal shrinkage rate as described in claim 1, characterized in that, It includes the following steps: After mixing polypropylene, polystyrene and styrene - ethylene / butene - styrene copolymer, put them into a twin - screw extruder, melt - extrude to obtain a melt, cool and shape the melt to get a cast sheet, and then perform longitudinal stretching step and transverse stretching step on the cast sheet in sequence, and then through corona treatment, winding, first aging treatment, slitting, second aging treatment to obtain the polypropylene capacitor film.

9. The preparation method according to claim 8, characterized in that, The twin - screw extruder is a double - screw extruder, and the melt - extrusion parameters of the double - screw extruder are: the temperature of zone 1 is 185 - 205°C, the temperature of zone 2: 200 - 235°C, the temperature of zone 3 is 235 - 255°C, the temperature of zone 4: 240 - 260°C, the temperature of zone 5 is 240 - 260°C, the temperature of zone 6: 230 - 260°C, the die head temperature: 235 - 260°C, and the screw speed is: 60 - 300 rpm.

10. The preparation method according to claim 8, characterized in that, The melt is cooled and shaped through a chill roll and air shower, the temperature of the chill roll is 85 - 105°C, and the thickness of the obtained cast sheet is 200.0 - 400.0 μm.

11. The preparation method according to claim 8, characterized in that, The longitudinal stretching step includes: longitudinal preheating, longitudinal stretching, and longitudinal shaping. The temperature for longitudinal preheating is 110 - 130 °C, the temperature for longitudinal stretching is 152 - 157 °C, and the temperature for longitudinal shaping is 153 - 159 °C.

12. The preparation method according to claim 8 or 11, characterized in that, The longitudinal stretching ratio is 5 - 7 times.

13. The preparation method according to claim 8 or 11, characterized in that, The transverse stretching step includes: transverse preheating, transverse stretching, and transverse shaping. The temperature for transverse preheating is 160 - 175 °C, the temperature for transverse stretching is 163 - 169 °C, and the temperature for transverse shaping is 175 - 183 °C.

14. The preparation method according to claim 8, characterized in that, The transverse stretching ratio is 7 - 10 times.

15. The preparation method according to claim 8, characterized in that, During corona treatment, the output power is 5 - 18 kW, the output voltage is 5 - 10 kV, and the bombardment intensity of the electrode is 7 - 10 W·min / m 2 ; And / or, the first aging treatment is carried out under a dust-free class of 10,000 or below, and is treated in an environment of 20 - 40 °C for 50 - 90 h; And / or, the second aging treatment is carried out under a dust-free class of 10,000 or below, and is treated in an environment of 20 - 40 °C for 15 - 40 h.

16. Application of a polypropylene capacitor film with a low thermal shrinkage rate in an electric vehicle capacitor as described in claim 1.

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