Base film for metallized capacitor with uniform double-sided structure and preparation method thereof

Abstract

The present invention relates to the technical field of capacitor films, and particularly relates to a base film for a metallized capacitor with a uniform double-sided structure and a preparation method thereof. The thick sheet moves backward through the front roller, the transition roller and the rear roller in sequence, and the contact surfaces of the three rollers with the thick sheet are the same. The temperatures of the front roller and the rear roller are both 105°C to 140°C, and the temperature of the transition roller is 20°C to 50°C. The significant advantage of the present invention is reflected in the transformation of the peeling roller group. The quenching roller and the front roller rotate in the same direction, and the contact surfaces are both the CR surface of the thick sheet. The front roller and the rear roller with temperatures higher than that of the quenching roller transfer heat to the CR surface of the thick sheet in a contact manner, annealing the spherulites on the CR surface of the thick sheet, making the crystallization on both sides of the thick sheet uniform and close to large-sized spherulites of the AK surface; the thickness of the film is 2 to 5 μm, the roughened structures on both sides are uniform and consistent, and the double-sided R z >1300 nm, double-sided SSK≥0.5, the effective withstand voltage thickness is improved, and the film has both excellent roughening performance and high withstand voltage performance, and is suitable for most metallized film capacitors.

Description

Technical Field

[0001] The invention relates to the technical field of capacitor films, and in particular to a base film for metallized capacitors with uniform double-sided structure and a preparation method thereof. Background Art

[0002] Film capacitors use polymer film as dielectric material. Biaxially oriented polypropylene (BOPP) 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 existing technology, low-ash electrical grade polypropylene is put into the extruder, and the high-temperature melt is plasticized and sheared before being extruded through the die head to the chilled roller. The air knife load makes the melt adhere tightly to the surface of the chilled roller and cools it into a sheet; then it is subjected to biaxial stretching, heat setting and post-processing processes to obtain a base film; one side of the base film is evaporated with metals such as Al and Zn to complete metallization, and is wound into a metalized film capacitor, which is then applied to power grid circuits, new energy, electronic appliances and other fields.

[0003] Tight winding between film layers and metal adhesion on the surface require BOPP to have appropriate surface roughening ability. It is generally believed that the increase in film surface roughness will cause a decrease in film breakdown strength. Higher roughness is crucial to metal coating adhesion, friction tension between wound film layers, and component performance improvement of metallized film capacitors. This requires the base film to be as rough as possible without degrading the breakdown strength. Excellent surface roughening ability and breakdown strength are both required in practical applications.

[0004] The cold roller temperature is the key to ensure the surface roughening of BOPP capacitor base film, and it is also the process feature that distinguishes it from packaging film and battery separator. Different degrees of surface roughening are important indicators to distinguish the three types of films. The principle is that the structural origin of BOPP surface roughening is the polycrystalline state (α / β crystal, spherulite size) of the cast thick film, which not only involves the lamellar exfoliation cavitation induced by the β→α crystal transformation during the stretching process ( Zhang C. B, Polymer, 312, 2024, 127651 ), and also involves shear / cavitation competition caused by the size difference of α spherulites ( Wang Y. X, Polymer, 273, 2023, 126150 ), after a series of crystalline structure evolutions induced by stretching, a coarsened ring structure is finally presented on the film surface. The high gloss and low roughening of the surface required for packaging films match the low cold roller temperature to construct small-sized spherulites and grains in the thick sheet; battery separators often need to have through holes and high roughening to match the large-sized spherulites and grains constructed by high-temperature rollers; and for capacitor films, the surface roughness often needs to be maintained within an appropriate range and cannot be too high or too low. From the perspective of mature industrial processes, the β-crystal content and moderate spherulite size in the thick sheet of the melt quenched at the above cold roller temperature match the roughness required for capacitor films.

[0005] Despite this, the surface roughening structure of BOPP capacitor film is still unsatisfactory. The main reason is that the surface roughening structure uniformity between the two sides of the film is poor. The fundamental reason is that the melt is cooled and crystallized under the combined action of the chilled roller and the air knife, but the cooling rate on the two surfaces of the thick film is not consistent. The contact heat transfer of the CR surface (cold roller surface) is always faster than the non-contact heat transfer of the AK surface (air knife surface), which leads to a long crystal growth period on the AK surface, and the spherulite and grain size are always larger than the CR surface ( J. Appl. Polym. Sci, 130(5), 3555~3564, 201 3). Even if the appropriate casting process makes the spherulite size and β-crystal content of the entire thick film match the roughness required by the capacitor film. However, based on the large-ratio stretching, the crystallization difference on both sides of the thick film will be amplified by large-scale processing, resulting in obvious differences in the coarsening structure on both sides of the film. The number and size of the roughening rings on the AK surface are always greater than those on the CR surface. On the one hand, the inconsistent double-sided structure manifests as a difference in double-sided coarsening, which is not conducive to the base film metallization and device winding. On the other hand, the essence reveals that the overall structural uniformity of the film needs to be improved. The structure in the thickness direction from AK to CR is uneven, and there is a lot of room for performance optimization.

[0006] In the prior art, the surface roughness can be regulated in the subsequent biaxial stretching process control, but when the roughness of one side is controlled to achieve the ideal effect through process control, the roughness of the other side is too high or too low. This difference between the AK side and the CR side is particularly obvious under the non-contact wind-heat stretching system with synchronous stretching. It can be seen that, on the surface, the uniform and consistent double-sided roughening structure is conducive to the metallization of the base film and the interlayer winding, and improves the stability of the device performance after winding; in essence, the uniform and consistent double-sided surface structure is associated with the uniformity of the overall structure, which is an important means to achieve high performance of BOPP capacitor film.

[0007] Regarding the uniform roughening of both sides of the film, the Chinese patent document (CN101863120B) discloses the use of air knife gas for cooling. When the melt is cooled, the cooling temperature of both sides of the thick sheet is consistent, so that the difference in crystallization rate, crystal morphology and spherulite growth size on both sides of the formed thick sheet is reduced, and a film with consistent roughening, uniform roughening and uniform structure is prepared by synchronous biaxial stretching; another Chinese patent document (CN103434151B) also discloses that by controlling the cooling temperature of the chilled roller and the high-pressure air knife to 95℃~105℃, it is ensured that both sides of the film are uniformly cooled, the difference in crystallization rate, crystal morphology and spherulite growth size on both sides is reduced, and the effect of uniform roughening degree on both sides of the film is achieved after crystal transformation. The technical principle of these two patents is to make the cooling rate of the AK surface close to the CR surface, so that the crystal structures of the two sides are close, and they are both in the crystallization state of the CR surface. This technical principle can undoubtedly make the double-sided roughening structure uniform, reflecting the uniformity of the overall structure of the film, which is conducive to high performance. But it is only based on a simple average roughness R aIt is a standard and cannot reflect more specific surface structures, let alone involve the adjustment and optimization of surface structures.

[0008] The surface structure of the film is a very complex topic, involving the complex microscopic geometric shapes of the film surface, which always coexist with convexities and concavities. Studies have shown that the correlation between the surface roughening structure and its breakdown strength is highly correlated with the surface average concave-convex deviation SSK. SSK>0 means that the overall surface morphology is high in convex morphology, which helps to increase the effective pressure-resistant thickness of the film, thereby improving the breakdown strength (see Figure 3 ).and R z It represents the sum of the average value of the maximum protrusion height and the average value of the maximum depression depth in the plane. R z With SSK, we can perfectly reflect the convexity and concavity of the film surface. When SSK>0, the larger R z This means that the higher the proportion of protrusions, the more obvious the effective pressure-resistant thickness can be. On the contrary, if SSK is less than 0, the greater R z This means that the proportion of depressions increases and the effective pressure-resistant thickness decreases. IEEE T DIELECT EL IN, 30(3),1188~1196, 2023 Under the action of electric field, charges always gather at surface defects (depressions), resulting in uneven electric field distribution and local charge accumulation on the film surface, thus stimulating flashover and breakdown. Therefore, at the same thickness, how to reduce the number and depth of depressions, improve SSK, and increase R z , the average film coarsening structure is the key to improving the pressure resistance of the film through a fine surface structure.

[0009] From the above, it can be seen that, first, the excellent surface roughening of BOPP film and high breakdown strength need to be balanced; second, the uniform and consistent structure of both sides of the film is conducive to achieving high performance of the film. The existing technical principle of double-sided uniform roughening is to accelerate the cooling of the AK surface and make its crystal structure close to the CR surface. R a The surface structure and effective pressure-resistant thickness are not taken into consideration. From the perspective of effective pressure-resistant thickness, the uniform roughening of both sides is achieved while SSK>0, which improves R z , which can effectively increase the proportion of surface protrusions and help improve the breakdown strength. Summary of the invention

[0010] The object of the present invention is to provide a base film for metallized capacitors with uniform double-sided structure and a preparation method thereof. The base film has uniform roughening structure on both sides and has appropriate surface roughness, which is conducive to surface metallization and tight adhesion between winding layers; double-sided SSK>0, R z Improved, effective withstand voltage thickness increased, film breakdown strength improved. This film structure is uniform, resulting in a double-sided uniform surface structure (small difference between the two sides), achieving both excellent roughening ability and withstand voltage performance, and is very suitable for use in most cases of metallized film capacitors, especially in the field of miniaturized high-performance capacitors.

[0011] To achieve the above object, the technical solution adopted by the present invention is:

[0012] The method for preparing a base film for a metallized capacitor with uniform double-sided structure specifically comprises the following steps:

[0013] S1. Melt extrusion: Use electrical grade special polypropylene resin material to melt and extrude in the extruder;

[0014] S2, rapid cooling of thick slices: the melt is cast onto a single chilling roller to cool the thick slice into shape. The chilling roller temperature is 90℃~99℃, the air knife temperature is 90℃~105℃, and a front roller, a transition roller and a rear roller are arranged on one side of the chilling roller. The key point is that the front roller has an annealing effect on the CR surface crystals of the thick slice, which is beneficial to the increase of the CR surface crystal size and the approach to the AK surface crystal state. The contact surfaces of the front roller, the rear roller and the chilling roller with the thick slice are all CR surfaces; the thick slice moves backwards through the front roller, the transition roller and the rear roller in turn. The temperatures of the front roller and the rear roller are both 105℃~140℃, and the temperature of the transition roller is 20℃~50℃;

[0015] S3, bidirectional synchronous stretching: longitudinal stretching ratio is 4.5~6 times, transverse stretching ratio is 7~9 times, preheating temperature is 125℃~145℃, stretching temperature is 154℃~165℃, heat setting temperature is 140℃~155℃, and heat setting return is 2%~10%.

[0016] 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 2.16kg and is 2.0g / 10min~4.0g / 10min. The ash content is ≤20 ppm.

[0017] Preferably, the transition roller is arranged between the front roller and the rear roller, the front roller has the same rotation direction as the chill roller and the rear roller, and the linear speed difference between the front roller and the chill roller is 2% to 5%, so as to ensure the peeling of the thick slice; the rotation direction of the transition roller is opposite to that of the front roller and the rear roller; the length L of the thick slice on the peripheral surface of the chill roller is 1 The length L of the thick sheet on the circumference of the front roller and the circumference of the rear roller 2 Satisfies the following relationship: 5≤L 1 / L 2 ≤15. Except for the chilled rollers, which are cooled by water circulation, the rest of the rollers are heated by hot oil circulation. 1 Ensure that the melt is sufficiently cooled and hardened, and the appropriate ratio ensures that the small spherulites initially formed by the CR surface after cooling by the chilled roller have a suitable annealing heat treatment time. On the one hand, appropriate annealing promotes the growth of small-sized CR spherulites; on the other hand, the annealing time and temperature should not be too long, causing excessive growth of the crystals and seriously reducing the stability of subsequent biaxial stretching.

[0018] Preferably, in step S2, the wrap angle of the thick slice on the chilled roller is 170°~200°, the diameter of the chilled roller is 1800mm~2200mm, and the diameters of the front roller, transition roller and rear roller are all smaller than the chilled roller; the temperature of the front roller is preferably 120~140℃.

[0019] Preferably, the stretching temperature in step S3 is divided into three stages, and the temperature first rises and then falls, which are 154°C~156°C, 158°C~165°C and 154°C~158°C 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 to weaken the crystal melting effect during the high-temperature stretching process; the setting is divided into three stages, and the temperature is successively reduced to 148°C~155°C, 143°C~147°C and 140°C~142°C, so as to weaken the deorientation of the film under high-temperature heat setting.

[0020] Preferably, the production line speed is 150 m / min~240 m / min.

[0021] Preferably, after step S3, post-processing and cutting are further performed to obtain a base film, wherein the base film has a thickness of 2 μm to 5 μm, a double-sided roughened structure, and consistent structures on both sides.

[0022] The present invention also provides a base film for metallized capacitors with uniform double-sided structure, which is prepared by the preparation method.

[0023] Preferably, the roughness difference between the AK surface and the CR surface of the base film satisfies: 0.05≤| R a-CR - R a-AK | / R a-AK ≤0.2 or 0.05≤|R z-CR - R z-AK | / R z-AK ≤0.2; SSK≥0.5 and Rz>1300nm of the CR surface and AK surface of the base film, the effective withstand voltage thickness is improved, and excellent surface roughening and high breakdown strength are taken into account.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention adopts a synchronous stretching method with a higher degree of uniformity in film structure. The more common asynchronous stretching method has a better ability to uniformize film structure and improve performance, and can greatly reduce the film thickness to less than 5 μm.

[0026] The present invention abandons the scheme of accelerating the cooling rate on the AK side to make the thick slice crystallize on both sides uniformly close to the small spherulite size of the CR surface, but grows crystals on the CR surface from the opposite angle to make the thick slice crystallize on both sides uniformly close to the large spherulite size and morphology of the AK surface; the significant advantage of the present invention is reflected in the transformation of the stripping roller group, the stripping roller group is composed of a front roller, a transition roller and a rear roller, the quenching roller and the front roller maintain the same rotation direction, and the contact surface is the CR surface of the thick slice. The temperature of the front roller and the rear roller, which are higher than the temperature of the quenching roller, are in contact with the CR surface of the thick slice for heat transfer, and annealing treatment is applied to the spherulites on the CR surface of the thick slice to increase its size so that its crystal morphology and spherulite size are close to those of the AK surface; the present invention controls the temperature of the stripping roller group and the contact length relationship of the thick slice on the peripheral surface of the quenching roller and the peripheral surface of the stripping roller group to ensure the time and temperature control of the sheet forming and the annealing process of the thick slice CR surface.

[0027] The present invention reduces the wrap angle of the chilling roller and increases the diameter of the chilling roller, ensuring that the cooling and hardening of the melt can still be achieved when the wrap angle is less than 200 degrees, and the front roller and the chilling roller maintain the same contact surface; there is a slight linear speed difference between the front roller and the chilling roller, and the thick sheet is peeled off from the chilling roller through the action of tension.

[0028] Based on the above roller group modification, the difference in crystal morphology and spherulite size between the AK surface and the CR surface is reduced, so that the spherulite size of the CR surface is close to that of the AK surface, and then the roughening rings with uniform roughening and high density on both sides are obtained during the synchronous stretching process. The uniform roughening rings are conducive to the uniform distribution of the electric field, and the higher roughness is conducive to the adhesion of the metal coating. What is more noteworthy is that the uniform roughening rings with good consistency on both sides, and the SSK on both sides of the film are ≥0.5, which greatly increases the effective pressure resistance thickness of the film, and the two sides are R z Above 1300 nm, the roughening is excellent and the breakdown strength is improved, which is very suitable for use in metallized film capacitors in most cases, especially in the field of miniaturized high-performance capacitors. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a diagram showing the movement of the thick sheet of the present invention on the chilled roller and the stripping roller group.

[0030] Figure 2 This is a diagram showing the trend of the thick sheet of comparative example 1 of the conventional process on the chilled roller and the peeling roller.

[0031] Figure 3 This is a relationship diagram between breakdown strength and film surface unevenness deviation SSK.

[0032] Figure 4 These are microscope photos of the basement membranes prepared in Example 1 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0033] Example 1

[0034] This embodiment provides a method for preparing a base film for a metallized capacitor with a uniform double-sided structure, and the steps are as follows:

[0035] S1. Melt extrusion: Use electrical grade polypropylene resin, 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 melting 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 melt is extruded in an extruder, and the temperatures of the melting section, plasticizing section and extrusion section are 230℃, 250℃ and 240℃ respectively.

[0036] The electrical grade special polypropylene resin can be replaced by HC310BF, HC330BF commonly found on the market, FS3030, FS3031 of Singapore TPC, and HTP-5014L of Korea Petrochemical.

[0037] S2, rapid cooling thick slice: the melt is cooled by a single chilling roller to form a thick slice. The chilling roller temperature is 95℃, the air knife temperature is 95℃, and the air knife pressure is 140mba to ensure that the melt adheres to the surface of the chilling roller. A front roller, a transition roller and a rear roller are arranged on one side of the chilling roller in sequence. The roller surface of the front roller and the chilling roller are the same as the contact surface of the thick slice. The thick slice moves backward through the front roller, the transition roller and the rear roller in sequence. The wrap angle of the thick slice on the chilling roller is 180°. The diameter of the chilling roller is 2000mm. The diameters of the front roller, the transition roller and the rear roller are all smaller than the chilling roller. The temperatures of the front roller and the rear roller are both 125℃, and the temperature of the transition roller is 40℃. The thickness of the thick slice is controlled to be 130±2μm by the production line thick slice thickness measurement and the die head.

[0038] The transition roller is set between the front roller and the rear roller. The front roller has the same rotation direction as the chill roller and the rear roller, and the contact surface with the thick sheet is the thick sheet CR surface. The linear speed difference between the front roller and the chill roller is 3% to ensure the peeling of the thick sheet; the rotation direction of the transition roller is opposite to that of the front roller and the rear roller; the length L of the thick sheet on the peripheral surface of the chill roller 1 The length L of the thick sheet on the circumference of the front roller and the circumference of the rear roller 2 Satisfies the following relationship: L 1 / L 2 = 10. Longer L 1 Ensure that the melt is sufficiently cooled and hardened, and the appropriate ratio is used to ensure that the annealing time of the CR surface small spherulites is appropriate.

[0039] S3, bidirectional synchronous stretching: the longitudinal stretching ratio is 5 times, the transverse stretching ratio is 8 times, and the production line speed is 210m / min; the preheating adopts wind-heating uniform heating, which is divided into three sections, at 125℃, 135℃ and 145℃ respectively, to ensure that the thick sheet is heated evenly; the synchronous stretching heating adopts wind-heating uniform heating, which is divided into three sections and the temperature rises first and then drops, specifically 155℃, 160℃ and 155℃; the setting is divided into three sections, and the temperature decreases successively, at 150℃, 145℃ and 140℃ respectively; the heat setting return stroke is 4% to prevent the film from breaking due to tension caused by lowering the temperature.

[0040] S4, after post-processing and cutting, a polypropylene base film for metallized capacitors is obtained.

[0041] Example 2

[0042] This embodiment provides a method for preparing a base film for a metallized capacitor with a uniform double-sided structure, and the steps are as follows:

[0043] S1. Melt extrusion: same as in Example 1.

[0044] S2, Rapidly cooled thick slices: The melt is cooled by a single chilling roller to form thick slices. The chilling roller temperature is 95℃, the air knife temperature is 95℃, and the air knife pressure is 120mba to ensure that the melt adheres to the surface of the chilling roller. A front roller, a transition roller and a rear roller are arranged on one side of the chilling roller in sequence. The roller surfaces of the front roller and the chilling roller are the same as the contact surface of the thick slice. The thick slice moves backward through the front roller, the transition roller and the rear roller in sequence. The wrap angle of the thick slice on the chilling roller is 170°. The diameter of the chilling roller is 2000mm. The diameters of the front roller, the transition roller and the rear roller are all smaller than the chilling roller. The temperatures of the front roller and the rear roller are both 135℃, and the temperature of the transition roller is 40℃. The thickness of the thick slice is controlled to be 130±2μm.

[0045] The transition roller is arranged between the front roller and the rear roller. The front roller has the same rotation direction as the chill roller and the rear roller. The linear speed difference between the front roller and the chill roller is 2.2%, which ensures that the thick slice is peeled off without slipping. The rotation direction of the transition roller is opposite to that of the front roller and the rear roller. The length L of the thick slice on the circumference of the chill roller is1 The length L of the thick sheet on the circumference of the front roller and the circumference of the rear roller 2 Satisfies the following relationship: L 1 / L 2 =7.5. The smaller circumferential length ratio than that in Example 1 reduces the hardening degree of the thick sheet when it reaches the front roller, and the actual temperature of the CR surface of the thick sheet is higher than that in Example 1, which is conducive to the growth of crystals on the CR surface of the thick sheet.

[0046] S3, bidirectional synchronous stretching, and S4 post-processing and slitting processes are consistent with those in Example 1.

[0047] Example 3

[0048] This embodiment provides a method for preparing a base film for a metallized capacitor with a uniform double-sided structure, and the steps are as follows:

[0049] S1. Melt extrusion: The same as in Example 1. The difference is that the temperature of the extruder is lowered, and the temperatures of the three stages are 230° C., 255° C., and 245° C. respectively.

[0050] S2, rapid cooling thick sheet: the melt is cooled by a single chilling roller to form a thick sheet. The temperature of the chilling roller is 90℃, the temperature of the air knife is 90℃, and the pressure of the air knife is 140mba to ensure that the melt adheres to the surface of the chilling roller. A front roller, a transition roller and a rear roller are arranged on one side of the chilling roller in sequence. The roller surface of the front roller and the chilling roller are the same as the contact surface of the thick sheet. The thick sheet moves backward through the front roller, the transition roller and the rear roller in sequence. The wrap angle of the thick sheet on the chilling roller is 200°, the diameter of the chilling roller is 2000mm, the diameters of the front roller, the transition roller and the rear roller are all smaller than the chilling roller, the temperature of the front roller and the rear roller are both 115℃, and the temperature of the transition roller is 40℃. The thickness of the thick sheet is controlled to be 182±2μm by the production line thick sheet thickness measurement and the die head.

[0051] The transition roller is arranged between the front roller and the rear roller. The rotation direction of the front roller is the same as that of the chill roller and the rear roller. The linear speed difference between the front roller and the chill roller is 2% to ensure the peeling of the thick slice. The rotation direction of the transition roller is opposite to that of the front roller and the rear roller. The length L of the thick slice on the peripheral surface of the chill roller is 1 The length L of the thick sheet on the circumference of the front roller and the circumference of the rear roller 2 Satisfies the following relationship: L 1 / L 2 = 13. Longer L 1 To ensure that the melt is sufficiently cooled and hardened, a larger ratio than that in Example 1 reduces the actual temperature of the slab when it reaches the front roller, thereby shortening the annealing time.

[0052] S3, bidirectional synchronous stretching: the longitudinal stretching ratio is 5 times, the transverse stretching ratio is 7.5 times, and the production line speed is 190 m / min; the preheating, stretching and shaping stages are consistent with Example 1.

[0053] S4, after post-processing and cutting, a polypropylene base film for metallized capacitors is obtained.

[0054] Comparative Example 1

[0055] This comparative example adopts the conventional peeling roller process. Figure 2 , the difference from the above-mentioned embodiment 1 is only in step S2. Specifically, in step S2, the melt is subjected to single chilled roller cooling to form a thick sheet. The chilled 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 chilled roller in sequence. The wrap angle of the thick sheet on the chilled roller is 235°, the diameter of the chilled roller is 2000mm, the rotation direction of the stripping single roller is opposite to that of the chilled roller, the contact surface of the stripping single roller is the AK surface, the temperature of the stripping single roller is 40°C, the ratio of the length of the thick sheet on the circumferential surface of the chilled roller to the length of the thick sheet on the circumferential surface of the stripping single roller exceeds 15, and the CR surface of the thick sheet is in contact with the chilled roller for a long time.

[0056] Comparative Example 2

[0057] This comparative example relates to the design of the stripping roller set of the present invention, but eliminates its annealing effect. It is consistent with the steps S1 and S3 of the above embodiment 1, except that the temperature of the front roller, the rear roller and the transition roller in step S2 are all set to 40°C.

[0058] The capacitive films of the above-mentioned embodiments 1-3 and comparative examples 1-2 were respectively tested for physical properties. The test results are shown in Table 1.

[0059] Roughness and surface structure test: Optical detection instruments are used instead of contact roughness testers. Three-dimensional optical profiler (ComtourX~200, Bruker, Germany), each surface is tested 15 times at different points, and the results are recorded. R a , R z and SSK, and take the average value. This method is a non-contact optical detection method, which can more accurately reflect the difference in double-sided structures compared with traditional contact detection.

[0060] Surface morphology: Observed and recorded using an optical microscope. Microscopic photographs of Example 1 and Comparative Example 1 are as follows: Figure 4 .

[0061] 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.

[0062] Table 1: Physical properties of polypropylene base films for metallized capacitors of Examples 1 to 3 of the present invention and Comparative Examples 1 to 3

[0063]

[0064] In summary, the thickness of the polypropylene base film for metallized capacitors of the present invention is 2 μm to 5 μm, and the roughness difference between the AK surface and the CR surface of the polypropylene base film for metallized capacitors satisfies: 0.05≤| R a-CR - R a-AK | / R a-AK ≤0.2 or 0.05≤| R z-CR - R z-AK | / R z-AK ≤0.2; Annealing of thick CR surface crystal can improve the double-sided R z Compared with SSK, the CR surface is significantly improved. The average degree of concavity and convexity of the CR surface and the AK surface of the polypropylene base film embodiment of the metallized capacitor satisfies SSK, 1≥SSK≥0.5, and the double-sided R z It is stable above 1300nm, and the surface structure on both sides is more dominated by protruding structures, indicating that its effective breakdown resistance thickness is significantly improved compared with the comparative sample.

[0065] For the process described in the present invention, higher front and rear roller temperatures and shorter L 1 / L 2 The ratio is more conducive to the annealing growth of thick CR surface crystals. R a and R z Higher, as in Example 2 above.

[0066] The contact surface of the high-temperature peeling roller is the thick sheet AK surface, which will aggravate the double-sided difference, as shown in the above-mentioned comparative example 1.

[0067] Therefore, the film of the embodiment of the present invention can achieve uniform double-sided structure, has excellent surface roughening ability, and has an increased effective pressure-resistant thickness, and the breakdown strength of the film of the embodiment is improved.

[0068] 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 base film for a metallized capacitor with uniform double-sided structure, 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 thick sheet: the melt is cast onto a single chilling roller to cool the thick sheet into shape, the chilling roller temperature is 90℃~99℃, the air knife temperature is 90℃~105℃, and a front roller, a transition roller and a rear roller are arranged on one side of the chilling roller, and the contact surfaces of the front roller, the rear roller and the chilling roller with the thick sheet are all CR surfaces. The thick sheet moves backward through the front roller, the transition roller and the rear roller in sequence, the temperatures of the front roller and the rear roller are both 105℃~140℃, and the temperature of the transition roller is 20℃~50℃; S3, bidirectional synchronous stretching: longitudinal stretching ratio is 4.5~6 times, transverse stretching ratio is 7~9 times, preheating temperature is 125℃~145℃, stretching temperature is 154℃~165℃, heat setting temperature is 140℃~155℃, and heat setting return is 2%~10%.

2. The method for preparing a base film for a metallized capacitor with uniform double-sided structure 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 , 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 base film for a metallized capacitor with uniform double-sided structure according to claim 1, characterized in that: In step S2, the transition roller is arranged between the front roller and the rear roller, the front roller has the same rotation direction as the chill roller and the rear roller, and the linear speed difference between the front roller and the chill roller is 2% to 5%; the rotation direction of the transition roller is opposite to that of the front roller and the rear roller; the length L1 of the thick slice on the peripheral surface of the chill roller and the length L2 of the thick slice on the peripheral surface of the front roller and the peripheral surface of the rear roller satisfy the following relationship: .

4. The method for preparing a base film for a metallized capacitor with uniform double-sided structure according to claim 1, characterized in that: In step S2, the wrap angle of the thick slice on the chilled roller is 170°~200°, the diameter of the chilled roller is 1800mm~2200mm, the diameters of the front roller, transition roller and rear roller are all smaller than the chilled roller, and the temperature of the front roller and rear roller is 120℃~140℃.

5. The method for preparing a base film for a metallized capacitor with uniform double-sided structure according to claim 1, characterized in that: The stretching temperature in step S3 is divided into three stages, the temperature first rises and then falls, which are 154°C-156°C, 158°C-165°C and 154°C-158°C respectively.

6. The method for preparing a base film for a metallized capacitor with uniform double-sided structure according to claim 1, characterized in that: The shaping in step S3 is divided into three stages and the temperature is successively reduced to 148° C.-155° C., 143° C.-147° C. and 140° C.-142° C.

7. The method for preparing a base film for a metallized capacitor with uniform double-sided structure according to claim 1, characterized in that: The production line speed in step S3 is 150 m / min~240 m / min.

8. The method for preparing a base film for a metallized capacitor with uniform double-sided structure according to any one of claims 1 to 7, characterized in that: After step S3, the base film is obtained through post-processing and slitting, and the thickness is 2 μm to 5 μm, the double-sided roughening structure is uniform, and the structures on both sides are consistent.

9. A base film for a metallized capacitor having a uniform double-sided structure, prepared by the preparation method according to any one of claims 1 to 8.

10. The double-sided uniform structure metallized capacitor base film according to claim 9, characterized in that: The roughness difference between the AK surface and the CR surface of the base film satisfies: The double-sided base film .

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