Polypropylene base film for capacitor and preparation method thereof

Abstract

The present invention relates to the technical field of capacitor films, and specifically to a polypropylene base film for capacitors and a preparation method thereof. A front roller, a middle roller and a rear roller are provided on one side of a chilled roller, and the cast sheet moves backward through the front roller, the middle roller and the rear roller in sequence, and the contact length ratio of the CR surface and the AK surface of the cast sheet on the peripheral surfaces of the chilled roller, the front roller, the middle roller and the rear roller is controlled at 5-20:1. The innovation of the present invention lies in annealing the precursor cast sheet, specifically, the chilled roller and the front roller group before stretching are transformed and the temperature of the front roller group is controlled as well as the contact length relationship between the AK surface and the CR surface of the cast sheet on the peripheral surfaces of each roller. After experimental verification, under the coordinated cooperation of asynchronous stretching process innovation, it is helpful to achieve the improvement of the crystallinity and melting point of the cast sheet, and then improve the electrical properties of the film, and achieve the withstand voltage level of the 3.8μm thick film with a thickness of 3.2μm, which is suitable for the field of miniaturized and high-voltage resistant capacitors, such as the high-voltage charging platform of electric vehicles.

Description

Technical Field

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

[0002] Film capacitors use polymer film as dielectric material. Biaxially oriented polypropylene film is a non-polar material with extremely low dielectric loss and high breakdown voltage. It has been widely used in high-power power electronic capacitors and other fields. In the prior art, low-ash electrical grade polypropylene is put into an extruder with an extruder temperature of 230℃-250℃. After plasticization and shearing, the high-temperature melt is extruded through a die head to a chilled roller. The air knife load makes the melt adhere tightly to the surface of the chilled roller and cools it into a sheet. After stretching, heat setting and post-processing, a base film is obtained. One side of the base film is evaporated with metals such as Al and Zn to complete metallization. After winding, it is formed into a metalized film capacitor, which is then applied to power grid circuits, new energy, electronic appliances and other fields.

[0003] In order to solve the range anxiety of new energy vehicles, the charging platform technology is being transformed from 400V level to 800V or even 900V high voltage level to achieve the purpose of fast charging. For the charging platform DC-Link capacitor (polypropylene film capacitor), the most direct way to improve the device's withstand voltage level is to increase the film thickness, but in new energy vehicle capacitors, the increase in thickness is more disadvantageous than beneficial. How to achieve a higher level of withstand voltage with a low film thickness is a key problem in the field of vehicle capacitors.

[0004] For vehicle-mounted capacitors, the film thickness needs to be as low as possible to meet the needs of miniaturization of capacitors, reduce the volume and increase the energy density of capacitors. However, the actual voltage that the film can withstand is also reduced as the thickness decreases. It is widely believed that films with high crystallinity and high melting point often have a higher degree of withstand voltage. The current technical means for high crystallinity and high melting point films focus on increasing the stretching ratio of the film and improving the annealing effect on the improvement of the crystal structure of the film. Although these methods are effective, they are all for improving the film structure during the stretching process and the post-stretching treatment stage. For biaxially oriented polypropylene film, crystallization is always related to each other. Not only does the stretching process and post-treatment process have a significant effect on the improvement of the film crystal structure, but the crystal state of the cast sheet before stretching also has a regulating effect on the crystal state of the film. A typical example is the control of the cast sheet β crystal. The transformation of the β crystal to the α crystal induced by stretching directly affects the "crater" structure on the surface of the film. Similarly, the improvement of the crystal structure of the cast sheet (the improvement of crystallinity) is also positively correlated with the crystal perfection of the film. The prior art does not reflect a method for promoting the crystal perfection of the film from the perspective of the perfection of the crystal of the stretching precursor-the cast sheet. Summary of the invention

[0005] The object of the present invention is to provide a polypropylene base film for capacitors and a preparation method thereof, so as to improve the degree of crystal perfection of the cast sheet and thus have a positive effect on the crystallinity and melting point of the subsequent film.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for preparing a polypropylene base film for a capacitor comprises the following steps:

[0008] S1. Melt extrusion: Use electrical grade special polypropylene resin material to melt and extrude in the extruder;

[0009] S2, rapid cooling of the casting sheet: the melt is cast onto a single chilling roller to cool the casting sheet into shape, a front roller, a middle roller and a rear roller are arranged on one side of the chilling roller, and the casting sheet moves backward through the front roller, the middle roller and the rear roller in sequence, and the contact length ratio of the CR surface (roller contact surface) and the AK surface (air knife surface) of the casting sheet on the peripheral surfaces of the chilling roller, the front roller, the middle roller and the rear roller is controlled at 5-20:1; and among the front roller, the middle roller and the rear roller, the temperature of the roller in contact with the CR surface of the casting sheet is controlled at 105℃-140℃ and the temperature of the roller in contact with the AK surface of the casting sheet is controlled at 40℃-90℃;

[0010] S3. Bidirectional asynchronous stretching: first longitudinal stretching, then transverse stretching and finally heat setting. The longitudinal stretching ratio is 4.5-6 times, the transverse stretching ratio is 7-9 times, the transverse stretching temperature rises first and then decreases in the temperature range of 158℃-165℃, the heat setting temperature gradually decreases in the temperature range of 140℃-155℃, and the heat setting return is 2%-10%.

[0011] Preferably, the electrical grade special polypropylene resin material is isotactic polypropylene resin with an isotacticity of ≥97% and a weight average molecular weight of 2.5×10 5 g / mol~4.5×10 5 g / mol, polydispersity index P d The melt index is 4.0~7.0 at a test temperature of 230℃ and a load of 2.16 kg and is 2.0g / 10min~5.0g / 10min. The ash content is ≤20 ppm.

[0012] Preferably, the longitudinal stretching area of ​​step S3 is divided into a first half and a second half according to the number of rollers. The temperature of the longitudinal stretching roller in the first half is set to 125°C~140°C, and the temperature of the longitudinal stretching roller in the second half is set to 135°C-150°C. The longitudinal stretching position is only between two longitudinal stretching rollers with the same roller temperature in the second half, and the speed of the longitudinal stretching roller after the longitudinal stretching position is increased by 250%-400% compared with the speed of the longitudinal stretching roller before the longitudinal stretching position. The stretching only occurs between the two rollers with a speed difference and not between the two rollers with a temperature difference. The MD stretching ratio is set at one time, and the stability is high.

[0013] Preferably, the longitudinal stretching rollers before the longitudinal stretching position have the same roller speed and are set to 40 m / min to 75 m / min, and the longitudinal stretching rollers after the longitudinal stretching position have the same roller speed and are set to 150 m / min to 300 m / min.

[0014] Preferably, the transverse stretching in step S3 is divided into three stages and the temperature is first increased and then decreased, which are 158°C-162°C, 162°C-165°C and 158°C-162°C respectively.

[0015] Preferably, the shaping in step S3 is divided into three stages, namely 148°C-155°C, 143°C-147°C and 140°C-142°C.

[0016] Preferably, in step S2, the rotation direction of the front roller and the rear roller are the same as that of the chill roller, the rotation direction of the middle roller is opposite to that of the chill roller, the front roller and the rear roller are both in contact with the CR surface of the casting and the middle roller is in contact with the AK surface of the casting, and the ratio of the contact length of the CR surface of the casting on the peripheral surfaces of the chill roller, the front roller and the rear roller to the contact length of the AK surface of the casting on the peripheral surface of the middle roller is 5-20:1.

[0017] Preferably, in step S2, the rotation direction of the front roller and the rear roller are opposite to that of the chill roller, the rotation direction of the middle roller is the same as that of the chill roller, the front roller and the rear roller are both in contact with the AK surface of the casting and the middle roller is in contact with the CR surface of the casting, and the ratio of the contact length of the CR surface of the casting on the peripheral surfaces of the chill roller and the middle roller to the contact length of the AK surface of the casting on the peripheral surfaces of the front roller and the rear roller is 5-20:1.

[0018] Preferably, the linear speed difference between the front roller and the chill roller is 2%-5%.

[0019] The present invention also provides a polypropylene base film for capacitors prepared by the preparation method.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention innovatively proposes to anneal the precursor casting, specifically to transform the chilled roller and the stripping roller group before stretching and control the temperature of the stripping roller group and the contact length relationship of the AK surface and CR surface of the casting on the circumferential surface of each roller. The stripping roller group consists of a front roller, a middle roller and a rear roller. It has been verified through experiments that, with the coordinated cooperation of the asynchronous stretching process innovation, it helps to achieve the improvement of the crystallinity and melting point of the film, thereby improving the electrical properties of the film, and achieving the voltage resistance level of a 3.8μm thick film with a thickness of 3.2μm.

[0022] The longitudinal stretching position of the present invention is after half of the rollers in the longitudinal stretching area (i.e., the second half), and the rollers before the longitudinal stretching position play a more uniform preheating and annealing role on the cast sheet. At the same time, the longitudinal stretching position is controlled between two longitudinal stretching rollers with the same roller temperature to avoid longitudinal stretching during the temperature change process, which is more conducive to the improvement of the crystal structure of the cast sheet before stretching; the transverse stretching temperature of the present invention first rises and then falls within the temperature range of 158°C to 165°C, which is 158°C-162°C, 162°C-165°C and 158°C-162°C respectively, firstly, to prevent the semi-finished film with large ratio and low thickness from melting and breaking due to the temperature increase; secondly, to fix the structure in a partially molten state at a lower temperature, reduce the deorientation of the molecular chain and promote its crystallization to the fiber crystal, and stabilize the fiber network of biaxial stretching. The heat setting temperature adopted by the present invention gradually decreases within the temperature range of 140°C to 155°C, which weakens the deorientation of heat setting at high temperature. In short, a more stable self-reinforced fiber network is formed inside the film, and high-melting-point crystals with better thermal stability are constructed, which plays a positive role in improving the breakdown field strength of the film, and the melting point of the crystal reaches above 170°C.

[0023] It should be noted that in the film asynchronous stretching process, the longitudinal stretching method is multi-roller contact stretching, and the transverse stretching method is air-heat non-contact stretching. The cast sheet is first longitudinally stretched, then transversely stretched at a higher temperature, and finally heat-set, post-processed, and rolled. In the longitudinal stretching area, although there are many rollers in the longitudinal stretching area, the longitudinal stretching is only between the two rollers with speed difference and not between the two rollers with temperature difference, realizing contact stretching. There is no speed difference in other areas, and the MD stretching ratio is set at one time, with high stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a diagram showing the trend of the cast pieces of Examples 1 and 2 of the present invention in the melt extrusion step.

[0025] Figure 2 This is a diagram showing the direction of the cast sheet in Example 3 of the present invention during the melt extrusion step.

[0026] Figure 3 This is a diagram showing the trend of the cast sheet of Comparative Example 1 of the present invention during the melt extrusion step.

[0027] Figure 4 The DSC melting curves of the embodiments and comparative examples are shown in FIG.

[0028] Figure 5 The one-dimensional X-ray diffraction spectra of each embodiment and comparative example (a) and the Gaussian fitting example diagram of embodiment 2 (b) are shown. DETAILED DESCRIPTION

[0029] Example 1

[0030] This embodiment provides a method for preparing a polypropylene base film for capacitors, the steps of which are melt extrusion, rapid cooling casting, bidirectional asynchronous stretching, cooling and shaping, trimming, and thickness measurement.

[0031] In the melt extrusion step, electrical grade polypropylene resin was used, specifically HC300BF from Borealis, with an isotacticity of 98.5% and a weight average molecular weight of 3.5×10 5 g / mol, polydispersity index P d The melting index is 5.5, the melt index is 3.4 g / 10min at a test temperature of 230℃ and a load of 2.16kg, and the ash content is 18 ppm. The extruder is melted and extruded, and the temperatures of the melting section, plasticizing section and extrusion section are 230~240℃, 250~260℃ and 230~240℃ respectively.

[0032] In other examples, the electrical grade special polypropylene resin can also be replaced by HC310BF commonly found on the market, FS3030 and FS3031 of Singapore TPC, and HTP-5014L of Korea Petrochemical.

[0033] In the step of rapidly cooling the cast sheet, the melt is cast onto a single chilling roller to cool the cast sheet, the chilling roller temperature is 90°C-99°C, the air knife temperature is 90°C-105°C, specifically in this embodiment, the chilling roller temperature is 95°C, the air knife temperature is 95°C, and the air knife pressure is 140mba, to ensure that the melt is attached to the surface of the chilling roller. A front roller, a middle roller and a rear roller are provided on one side of the chilling roller, and the cast sheet moves backward through the front roller, the middle roller and the rear roller in sequence, the rotation direction of the front roller and the rear roller are the same as the chilling roller, the rotation direction of the middle roller is opposite to that of the chilling roller, the linear speed difference between the front roller and the chilling roller is 3%, the front roller and the rear roller are both in contact with the CR surface of the cast sheet, and the middle roller is in contact with the AK surface of the cast sheet, and the ratio of the contact length of the CR surface of the cast sheet on the peripheral surface of the chilling roller, the front roller and the rear roller to the contact length of the AK surface of the cast sheet on the peripheral surface of the middle roller is 20:1. The wrap angle of the casting on the chilled roller is 180°, the diameter of the chilled roller is 2000mm, the diameters of the front roller, middle roller and rear roller are all smaller than the chilled roller, the temperatures of the front roller and the rear roller are both 125°C, the temperature of the middle roller is 40°C, and the casting thickness is controlled to be 130±2μm by the production line casting thickness measurement and the die head.

[0034] In the bidirectional asynchronous stretching step, longitudinal stretching is first performed, then transverse stretching, and finally heat setting. The temperature of the preheating roller before longitudinal stretching is 125°C. Eight longitudinal stretching rollers are set in the longitudinal stretching area. There are four longitudinal stretching rollers in the first half and the roller temperature is set to 133°C. There are four longitudinal stretching rollers in the second half and the roller temperature is set to 142°C. Contact roller speed difference stretching is adopted. The longitudinal stretching occurs between the 5th and 6th rollers. The roller speeds of the 1st, 2nd, 3rd, 4th, and 5th longitudinal stretching rollers are all set to 53m / min and the roller speed difference is zero. The roller speeds of the 6th, 7th, and 8th longitudinal stretching rollers are all set to 210m / min and the roller speed difference is zero. The MD stretching ratio is set once, and the longitudinal stretching ratio is 5 times. The preheating temperature before transverse stretching is set to 160℃, and wind-heated uniform heating is adopted in the transverse stretching area. It is divided into three stages and the temperature rises first and then drops, which are 160℃, 165℃ and 160℃ respectively. The first two stages of heating make the film sufficiently reduce its thickness, and the latter stage of cooling appropriately reduces the stretching ratio and weakens the crystal melting effect during high-temperature stretching. The transverse stretching ratio is 8 times; the setting is divided into three stages, and the temperature decreases successively, which are 150℃, 145℃ and 140℃ respectively; the heat setting return is 4% to prevent the film from breaking due to tension caused by lowering the temperature.

[0035] Example 2

[0036] This embodiment provides a method for preparing a polypropylene base film for capacitors, the steps of which are melt extrusion, rapid cooling casting, bidirectional asynchronous stretching, cooling and shaping, trimming, and thickness measurement.

[0037] In the melt extrusion step, electrical grade polypropylene resin was used, specifically HC300BF from Borealis, with an isotacticity of 98.5% and a weight average molecular weight of 3.5×10 5 g / mol, polydispersity index P d The melting index is 5.5, the melt index is 3.4 g / 10min at a test temperature of 230℃ and a load of 2.16kg, and the ash content is 18 ppm. The extruder is melted and extruded, and the temperatures of the melting section, plasticizing section and extrusion section are 230~240℃, 250~260℃ and 230~240℃ respectively.

[0038] In the step of rapidly cooling the casting, the melt is cast onto a single chilling roller to cool the casting into shape. The chilling roller temperature is 95°C, the air knife temperature is 95°C, and the air knife pressure is 120mba to ensure that the melt adheres to the surface of the chilling roller. A front roller, a middle roller, and a rear roller are provided on one side of the chilling roller. The casting moves backward through the front roller, the middle roller, and the rear roller in sequence. The rotation direction of the front roller and the rear roller is the same as that of the chilling roller, and the rotation direction of the middle roller is opposite to that of the chilling roller. The linear speed difference between the front roller and the chilling roller is 2.2%. The front roller and the rear roller are in contact with the CR surface of the casting, while the middle roller is in contact with the AK surface of the casting. The ratio of the contact length of the CR surface of the casting on the peripheral surface of the chilling roller, the front roller, and the rear roller to the contact length of the AK surface of the casting on the peripheral surface of the middle roller is 17:1. The wrap angle of the casting on the chilled roller is 170°, the diameter of the chilled roller is 2000mm, the diameters of the front roller, middle roller and rear roller are all smaller than the chilled roller, the temperatures of the front roller and the rear roller are both 120℃, the temperature of the middle roller is 90℃, and the casting thickness is controlled to be 130±2μm by the production line casting thickness measurement and the die head.

[0039] In the bidirectional asynchronous stretching step, longitudinal stretching is performed first, then transverse stretching, and finally heat setting. The temperature of the preheating roller before longitudinal stretching is 125°C. Eight longitudinal stretching rollers are set in the longitudinal stretching area, with four longitudinal stretching rollers in the first half and the roller temperature set to 130°C, and four longitudinal stretching rollers in the second half and the roller temperature set to 140°C. Contact roller speed difference stretching is adopted, and longitudinal stretching occurs between the 5th and 6th rollers. The roller speeds of the 1st, 2nd, 3rd, 4th, and 5th longitudinal stretching rollers are all set to 50m / min and the roller speed difference is zero. The roller speeds of the 6th, 7th, and 8th longitudinal stretching rollers are all set to 200m / min and the roller speed difference is zero. The MD stretching ratio is set once, and the longitudinal stretching ratio is 5 times. The preheating temperature before transverse stretching is set to 160℃, and wind-heated uniform heating is adopted in the transverse stretching area. It is divided into three stages and the temperature rises first and then drops, which are 160℃, 165℃ and 160℃ respectively. The first two stages of heating make the film sufficiently reduce its thickness, and the latter stage of cooling appropriately reduces the stretching ratio and weakens the crystal melting effect during high-temperature stretching. The transverse stretching ratio is 8 times; the setting is divided into three stages, and the temperature decreases successively, which are 150℃, 145℃ and 140℃ respectively; the heat setting return is 5% to prevent the film from breaking due to tension caused by lowering the temperature.

[0040] Example 3

[0041] This embodiment provides a method for preparing a polypropylene base film for capacitors, the steps of which are melt extrusion, rapid cooling casting, bidirectional asynchronous stretching, cooling and shaping, trimming, and thickness measurement.

[0042] In the melt extrusion step, electrical grade polypropylene resin was used, specifically HC300BF from Borealis, with an isotacticity of 98.5% and a weight average molecular weight of 3.5×10 5 g / mol, polydispersity index P dThe melting index is 5.5, the melt index is 3.4 g / 10min at a test temperature of 230℃ and a load of 2.16kg, and the ash content is 18 ppm. The extruder is melted and extruded, and the temperatures of the melting section, plasticizing section and extrusion section are 230~240℃, 250~260℃ and 230~240℃ respectively.

[0043] In the quenching casting step, the melt is cast onto a single chilling roller to cool the casting into a sheet. The chilling roller temperature is 95°C, the air knife temperature is 95°C, and the air knife pressure is 140mba to ensure that the melt adheres to the surface of the chilling roller. A front roller, a middle roller, and a rear roller are provided on one side of the chilling roller. The casting moves backward through the front roller, the middle roller, and the rear roller in sequence. Figure 2 . The rotation direction of the front roller and the rear roller is opposite to that of the chill roller, and the rotation direction of the middle roller is the same as that of the chill roller. The front roller and the rear roller are in contact with the AK surface of the casting, and the middle roller is in contact with the CR surface of the casting. The ratio of the contact length of the CR surface of the casting on the peripheral surface of the chill roller and the middle roller to the contact length of the AK surface of the casting on the peripheral surface of the front roller and the rear roller is 20:2. The wrap angle of the casting on the chill roller is 235°, the diameter of the chill roller is 2000mm, the diameters of the front roller, the middle roller and the rear roller are all smaller than the chill roller, the temperature of the front roller and the rear roller is 90℃, the temperature of the middle roller is 110℃, and the thickness of the casting is controlled to be 146±2μm by the casting thickness measurement of the production line and the die head.

[0044] In the bidirectional asynchronous stretching step, longitudinal stretching is performed first, then transverse stretching, and finally heat setting. The temperature of the preheating roller before longitudinal stretching is 130°C. Eight longitudinal stretching rollers are set in the longitudinal stretching area. There are four longitudinal stretching rollers in the first half and the roller temperature is set to 135°C. There are four longitudinal stretching rollers in the second half and the roller temperature is set to 145°C. Contact roller speed difference stretching is adopted. The longitudinal stretching occurs between the 5th and 6th rollers. The roller speeds of the 1st, 2nd, 3rd, 4th, and 5th longitudinal stretching rollers are all set to 45m / min and the roller speed difference is zero. The roller speeds of the 6th, 7th, and 8th longitudinal stretching rollers are all set to 180m / min and the roller speed difference is zero. The MD stretching ratio is set once, and the longitudinal stretching ratio is 5 times. The preheating temperature before transverse stretching is set to 160℃, and wind-heated uniform heating is adopted in the transverse stretching area. It is divided into three stages and the temperature rises first and then drops, which are 160℃, 165℃ and 160℃ respectively. The first two stages of heating make the film sufficiently reduce its thickness, and the latter stage of cooling appropriately reduces the stretching ratio and weakens the crystal melting effect during high-temperature stretching. The transverse stretching ratio is 9 times; the setting is divided into three stages, and the temperature decreases successively, which are 155℃, 152℃ and 147℃ respectively; the heat setting return is 6% to prevent the film from breaking due to tension caused by lowering the temperature.

[0045] Comparative Example 1

[0046] This comparative example adopts the conventional peeling roller process. Figure 3, the only difference from the above-mentioned Example 1 is the rapid cooling step of the cast sheet, specifically: the melt is subjected to single chilling roller cooling to form the cast sheet, the chilling roller temperature is 95°C, the air knife temperature is 95°C, the air knife pressure is 140mba, and a stripping single roller is arranged on one side of the chilling roller, the wrap angle of the cast sheet on the chilling roller is 235°, the diameter of the chilling roller is 2000mm, the rotation direction of the stripping single roller is opposite to that of the chilling roller, the contact surface of the stripping single roller is the AK surface, the temperature of the stripping single roller is 40°C, and the ratio of the contact length of the CR surface of the cast sheet on the peripheral surface of the chilling roller to the contact length of the AK surface of the cast sheet on the peripheral surface of the stripping single roller is 35:1.

[0047] Comparative Example 2

[0048] This is a conventional film production process. The difference between this comparative example and the comparative example 1 is that in step S2, the front roller, rear roller and middle roller are turned off for heating and kept at room temperature, and the thickness of the cast sheet is measured to be 155±3μm. In step S3, in the biaxial asynchronous stretching step, the longitudinal stretching ratio is 5 times and the transverse stretching ratio is 8 times; the temperature of the preheating roller before longitudinal stretching is 125°C, and 8 longitudinal stretching rollers are set in the longitudinal stretching area, and the contact roller speed difference stretching is adopted, wherein the longitudinal stretching roller temperature in the first half is set to 133°C, the roller speed is 53m / min, and the speed difference between the rollers is zero; the longitudinal stretching roller temperature in the second half is set to 142°C, the roller speed is increased to 210m / min, and the stretching occurs between the 4th and 5th rollers. The preheating temperature before transverse stretching is set to 160°C, and the transverse stretching heating adopts wind-heated uniform heating, which is divided into three stages and the temperature gradually increases, namely 160°C, 163°C and 166°C; the setting temperature is constant at 163±1°C; the heat setting return is 12.5%.

[0049] The films prepared in Examples 1-3 and Comparative Examples 1-2 were tested for physical properties. The test results are shown in Table 1. Melting point and crystallinity test: A differential scanning calorimeter (DSC) instrument (Q250, TA, USA) was used to prepare a 4-10 mg sample aluminum crucible and put it into the instrument. The temperature was raised to 200°C at 10°C / min under a nitrogen atmosphere. The DSC melting curve was recorded at Figure 4 The crystallinity measured by DSC is calculated as the ratio of the thermal enthalpy value, that is, the ratio of the melting peak area (thermal melting enthalpy value) to the enthalpy value of the thermal melting when 100% crystallization is achieved (taken as 207 J / g).

[0050] Crystallinity test: To prevent the influence of sample cold crystallization during DSC test, the crystallinity of the film sample was determined by X-ray diffraction (XRD). The crystallinity calculation method of X-ray diffractometer (Ultima IV, Rigaku, Japan) is as follows: 110 , α 040 , α 130 ) is the ratio of the sum of the diffraction peak areas to the total diffraction area. That is: ,like Figure 5 , amorphous refers to the amorphous region.

[0051] It should be noted that the two methods use different principles to test the crystallinity of samples, and the values ​​are not consistent. It is generally believed that for thin film samples, the crystallinity measured by optical methods (X-rays) is more accurate.

[0052] Electrode method voltage withstand test: refer to the provisions of GB / T 13542.2~2021 on the electrode method breakdown strength test. The electrode is cylindrical with a diameter of 25mm.

[0053] Table 1: Physical properties of polypropylene-based films of Examples 1 to 3 of the present invention and Comparative Examples 1 to 2

[0054]

[0055] Capacitor performance: After multiple confirmations by multiple capacitor manufacturers, the films described in Examples 1, 2, and 3 above all effectively improve the withstand voltage level of capacitors of the same size. Among them, the capacitor devices of the 3.2μm film of Examples 1 and 2 can achieve a level and service life similar to that of conventional 3.8μm film capacitors, and are expected to achieve a comprehensive replacement of the film used in capacitors for high-voltage charging platforms of new energy vehicles.

[0056] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for illustrating the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which shall fall within the scope of the present invention to be protected. The scope of protection of the present invention shall be defined by the attached claims and their equivalents.

Claims

1. A method for preparing a polypropylene base film for capacitors, characterized in that: The specific steps include: S1. Melt extrusion: Use electrical grade special polypropylene resin material to melt and extrude in the extruder; S2, rapid cooling of the casting sheet: the melt is cast onto a single chilling roller to cool the casting sheet into shape, a front roller, a middle roller and a rear roller are arranged on one side of the chilling roller, the casting sheet moves backward through the front roller, the middle roller and the rear roller in sequence, the contact length ratio between the CR surface and the AK surface of the casting sheet on the peripheral surfaces of the chilling roller, the front roller, the middle roller and the rear roller is controlled at 5-20:1; and among the front roller, the middle roller and the rear roller, the temperature of the roller in contact with the CR surface of the casting sheet is controlled at 105°C-140°C and the temperature of the roller in contact with the AK surface of the casting sheet is controlled at 40°C-90°C; S3. Bidirectional asynchronous stretching: first longitudinal stretching, then transverse stretching and finally heat setting. The longitudinal stretching ratio is 4.5-6 times, the transverse stretching ratio is 7-9 times, the transverse stretching temperature rises first and then decreases in the temperature range of 158℃-165℃, the heat setting temperature gradually decreases in the temperature range of 140℃-155℃, and the heat setting return is 2%-10%.

2. The method for preparing a polypropylene base film for capacitors according to claim 1, characterized in that: The electrical grade special polypropylene resin material is isotactic polypropylene resin with an isotacticity of ≥97% and a weight average molecular weight of 2.5×10 5 g / mol~4.5×10 5 g / mol, polydispersity index P d The melt index is 4.0~7.0 at a test temperature of 230℃ and a load of 2.16 kg and is 2.0g / 10min~5.0g / 10min. The ash content is ≤20 ppm.

3. The method for preparing a polypropylene base film for capacitors according to claim 1, characterized in that: The longitudinal stretching area of ​​step S3 is divided into a first half and a second half according to the number of rollers. The temperature of the longitudinal stretching roller in the first half is set to 125°C~140°C, and the temperature of the longitudinal stretching roller in the second half is set to 135°C-150°C. The longitudinal stretching position is only between two longitudinal stretching rollers with the same roller temperature in the second half, and the speed of the longitudinal stretching roller after the longitudinal stretching position is increased by 250%-400% compared with the speed of the longitudinal stretching roller before the longitudinal stretching position.

4. The method for preparing a polypropylene base film for capacitors according to claim 3, characterized in that: The longitudinal roller speeds before the longitudinal stretching position are the same and are set to 40 m / min to 75 m / min, and the longitudinal roller speeds after the longitudinal stretching position are the same and are set to 150 m / min to 300 m / min.

5. The method for preparing a polypropylene base film for capacitors according to claim 1, characterized in that: The transverse stretching in step S3 is divided into three stages and the temperature is first increased and then decreased, which are 158°C-162°C, 162°C-165°C and 158°C-162°C respectively.

6. The method for preparing a polypropylene base film for capacitors according to claim 1, characterized in that: The setting in step S3 is divided into three stages, namely 148°C-155°C, 143°C-147°C and 140°C-142°C.

7. The method for preparing a polypropylene base film for capacitors according to claim 1, characterized in that: In step S2, the rotation direction of the front roller and the rear roller are the same as that of the chill roller, the rotation direction of the middle roller is opposite to that of the chill roller, the front roller and the rear roller are both in contact with the CR surface of the casting, and the middle roller is in contact with the AK surface of the casting, and the ratio of the contact length of the CR surface of the casting on the peripheral surfaces of the chill roller, the front roller and the rear roller to the contact length of the AK surface of the casting on the peripheral surface of the middle roller is 5-20:

1.

8. The method for preparing a polypropylene base film for capacitors according to claim 1, characterized in that: In step S2, the rotation direction of the front roller and the rear roller is opposite to that of the chill roller, the rotation direction of the middle roller is the same as that of the chill roller, the front roller and the rear roller are both in contact with the AK surface of the casting, and the middle roller is in contact with the CR surface of the casting, and the ratio of the contact length of the CR surface of the casting on the peripheral surfaces of the chill roller and the middle roller to the contact length of the AK surface of the casting on the peripheral surfaces of the front roller and the rear roller is 5-20:

1.

9. The method for preparing a polypropylene base film for capacitors according to claim 7 or 8, characterized in that: The linear speed difference between the front roller and the chill roller is 2%-5%.

10. A polypropylene-based film for capacitors prepared by the preparation method according to any one of claims 1 to 9.

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