Bidirectional asynchronous stretching high-temperature-resistant polypropylene film for capacitor with three-layer structure and preparation method of polypropylene film

Abstract

The invention discloses a polypropylene film for a bidirectional asynchronous stretching high-temperature-resistant three-layer structure capacitor and a preparation method of the polypropylene film. The film comprises a first surface layer, a second surface layer and a middle layer clamped between the two surface layers, the middle layer is made of polypropylene, the first surface layer is made of a cycloolefin copolymer (COC), and the second surface layer is made of polypropylene; the mass of the middle layer accounts for 85wt%-88wt% of the total mass, the mass of the first surface layer accounts for 5wt%-7wt% of the total mass, and the mass of the second surface layer accounts for 7wt%-8wt% of the total mass. The temperature resistance can be effectively improved by utilizing the characteristics of the cycloolefin copolymer, the film-forming property is improved by controlling the proportion content of the cycloolefin copolymer, the temperature resistance of the film can be improved to 125 DEG C or above, and the temperature resistance of the film is greatly improved. When the COC content is controlled to be 5%-7%, the film-forming property and the temperature resistance can be both considered, and the temperature reaches 125 DEG C.

Description

Technical Field

[0001] The invention relates to a polypropylene film for capacitors, and in particular to a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors and a preparation method thereof. Background Art

[0002] As the product usage scenarios are further reduced, users have put forward higher requirements for the temperature resistance of the film in terms of product volume, cost saving and safety. At different temperatures, the insulation performance of the PP material itself will change. As the temperature rises, the field strength of the material will decrease, which will eventually affect the safety of the product.

[0003] At present, various manufacturers have increased a lot of research and development funds on the heat resistance of films, hoping to improve the heat resistance of films to the maximum extent through improvements in raw material selection and process optimization.

[0004] With the development and progress of the industry, the traditional production process and raw material matching have reached a bottleneck. The traditional temperature resistance of 115°C can no longer meet the market demand. The current demand is to reach a temperature resistance of 125°C, or even 145°C. Therefore, from the perspective of process optimization, it has reached the limit. Only by adding new elements can the existing balance be broken. In the existing technology, the problem of film temperature resistance can be improved by modifying COC. Because it is a cyclic polyolefin, it has extremely high thermal stability. The temperature resistance in the laboratory has reached 145°C, but industrial production still has certain difficulties. Because cyclic polyolefins are highly stable, they show insufficient fluidity and are not conducive to processing during the production process. Therefore, improvements in the production process and adjustments in the production formula will be made to improve its processability while retaining its heat resistance characteristics. This will become the content of our next study. Summary of the invention

[0005] Based on the above description, the present invention provides a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors and a preparation method thereof. By optimizing the raw materials and the ratio, the temperature resistance of the film is improved to above 125°C, so as to solve the problem that the traditional temperature resistance of 115°C cannot meet the market requirements.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: A bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, the film comprising a first surface layer, a second surface layer and an intermediate layer sandwiched between the two surface layers; The material of the middle layer is polypropylene, the material of the first surface layer is cyclic olefin copolymer (COC), and the material of the second surface layer is polypropylene; The mass of the middle layer accounts for 85wt-88wt% of the total mass, the mass of the first surface layer accounts for 5-7wt% of the total mass, and the mass of the second surface layer accounts for 7-8wt% of the total mass.

[0007] Based on the above technical solution, the present invention can also be improved as follows.

[0008] Furthermore, the dielectric strength of the above-mentioned bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors has a dielectric breakdown field strength of 500-550 V / μm at 115° C.; and / or The film has a heat shrinkage rate of 2.0-2.5% in the MD direction (machine direction) and 0.05-0.1% in the TD direction (transverse direction) at 120°C for 15 minutes; and / or Surface roughness, Ra (arithmetic average roughness): 0.05-0.08μm, Rmax (maximum roughness height): 0.1-0.15μm.

[0009] The present invention also provides a method for preparing a bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, comprising the steps of batching, melt extrusion and compounding, sheet casting, and asynchronous stretching; The ingredients are as follows: polypropylene with an isotacticity greater than 98%, a melting point of 120°C, a melt flow rate of 2.8-3.2g / 10min, and an ash content of less than 30ppm is used as the raw material for the middle layer, accounting for 85wt%-88wt%; cyclic olefin copolymer (COC) with a melting point of 91°C, a melt flow rate of 26-30g / min, and an ash content of less than 100ppm is used as the first surface layer, accounting for 5-7wt% of the total mass; the material of the second surface layer is the same as that of the middle layer, accounting for 7-8wt% of the total mass.

[0010] Based on the above technical solution, the present invention can also be improved as follows.

[0011] Furthermore, the above-mentioned method for preparing a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors, the melt extrusion and compounding: the middle layer ingredients are added to the main extruder, the first surface layer ingredients are added to the first auxiliary extruder, and the second surface layer ingredients are added to the second auxiliary extruder, all of which are melted, plasticized, and stably extruded by a metering pump, and then passed through a filter, and the three layers of ingredients are compounded at the die head to flow out a sheet of melt; During compounding, the extrusion temperature of the first surface layer is 55-65°C lower than that of the second surface layer, and the first surface layer is located on the air knife surface; the second surface layer is located on the cold roller surface.

[0012] Furthermore, in the above-mentioned method for preparing a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors, the cast sheet: the composite sheet melt is cooled and shaped under the action of a cold roller and an air knife to obtain a uniform cast sheet.

[0013] The temperature of the cold roller can effectively control the crystallinity of the second surface layer PP (polypropylene) material. The main function of the air knife is to make the casting stick to the roller better. Generally, temperature control is not required, but the air volume must be sufficient to obtain a good casting effect.

[0014] Furthermore, the above-mentioned method for preparing a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors also includes subsequent steps: pulling and winding to finally obtain the capacitor film, wherein thickness measurement, edge trimming and corona treatment are required during pulling.

[0015] Furthermore, the above-mentioned method for preparing a biaxially asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors, the asynchronous stretching: the shaped cast sheet is longitudinally and transversely stretched in sequence, and a film is obtained after biaxial stretching, and both the longitudinal stretching and the transverse stretching have three functional zones, namely a preheating zone, a stretching zone, and a shaping zone; the stretching ratio of the longitudinal stretching is controlled between 4.5-5.3 times, and the stretching ratio of the transverse stretching is controlled between 8-10 times, so that a higher quality film can be obtained.

[0016] Furthermore, in the above-mentioned method for preparing a biaxially asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors, the melt extrusion and compounding steps: the temperature of the cold roller is set between 90-105°C; the material used for the air knife surface is the non-crystalline material COC, and the air volume of the air knife is not less than 120-150mbar.

[0017] The temperature of the cold roller is set between 90-105℃, which can effectively control the crystallinity of the second surface layer PP; because the material used for the air knife surface is non-crystalline, the air knife does not need temperature control, but the air volume of the air knife must be no less than 120-150mbar to ensure the roller-sticking effect of the melt.

[0018] Furthermore, in the above-mentioned method for preparing a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors, the extrusion temperatures of the middle layer PP material and the second surface layer PP material are both between 220-230°C, and the design temperature of the first surface layer is between 150-170°C.

[0019] The extrusion temperatures of the middle layer PP material and the second surface layer PP material are both between 220-230°C, which can effectively ensure the acquisition of a uniformly plasticized PP melt; however, the first surface layer COC has a low melting point and good fluidity, so the design temperature of the extruder can be overall lower than that of PP, so the design temperature is between 150-170°C to obtain a uniformly plasticized COC melt, in which case the melt composite performance is the best. Because the volume of the cyclic polyolefin in COC is relatively large, the molecules cannot form a crystalline structure and have non-crystalline characteristics. In order to better adapt to the characteristics of our biaxial stretching, the COC used in the present invention is a small molecular weight COC, which can meet our film drawing needs, wherein the sum of the mass of the middle layer PP and the second surface layer PP accounts for approximately 93-95%, and COC accounts for approximately 5-7%.

[0020] Furthermore, in the above-mentioned method for preparing a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors, the asynchronous stretching step is bidirectional asynchronous stretching, first longitudinal stretching is performed, and then transverse stretching is performed, the preheating temperature of the longitudinal stretching preheating zone is between 110-130°C, the temperature of the stretching zone is between 140-145°C, and the temperature of the shaping zone is between 145-150°C; the longitudinal stretching ratio is between 4.5-5.3, and the setting varies according to different specifications of films; the preheating temperature of the transverse stretching preheating zone is between 165-170°C, the temperature of the stretching zone is between 155-160°C, the temperature of the shaping zone is between 165-170°C, the transverse stretching ratio is controlled at 8-10 times, and the film after transverse stretching and heat shaping is sent to the traction station through the cooling zone.

[0021] Furthermore, in the preparation method of the above-mentioned bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors, the thickness is first measured by an infrared thickness gauge during traction, and then the edge is trimmed to ensure the effective width of the film that is conducive to winding, and then the film is subjected to corona treatment. At this time, the power of the corona treatment is controlled between 16-22kW. Because the PP material is a non-polar material, after the corona treatment, it has polarity, which is convenient for the next process of metallization vacuum plating. After winding, a 2.0-9.8μm capacitor film can be obtained, and the scraps generated during edge trimming are granulated and recycled. After winding, the original film is inspected, aged, slit and formed, the finished product is inspected, and packaged before being put into storage and out of the warehouse.

[0022] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects: COC has low dielectric loss and high dielectric constant, good thermal stability, and can work at higher temperatures without degradation, which is particularly important for capacitors used in high temperature environments. COC film has high mechanical strength and toughness, and can provide good protection and stability in various applications. COC can be processed by a variety of methods (such as extrusion, injection molding, coating, etc.), and can be easily prepared into films of different shapes and thicknesses to meet different capacitor design requirements. Its low water absorption does not significantly affect its electrical performance when used in a humid environment, which is crucial for the long-term stability of the capacitor.

[0023] In order to better adapt to the characteristics of our biaxial stretching, the COC used in the present invention is a small molecular weight COC, which can meet our film drawing needs. By optimizing the raw materials and utilizing the inherent characteristics of cyclic polyolefins, the temperature resistance can be effectively improved, and then by controlling the proportion of cyclic polyolefins, the film forming property can be improved, and the temperature resistance of the film can be increased to above 125°C, greatly improving the temperature resistance of the film. After several samples with different process parameter ratios, it was found that when the COC content was controlled at 5%-7%, the film forming property and temperature resistance could be taken into account and reached 125°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention provides a process flow chart of a method for preparing a bidirectional asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors. DETAILED DESCRIPTION

[0025] In order to facilitate understanding of the present application, the present application will be described more comprehensively below. The present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0027] The COC is purchased from Mitsui Chemicals, Inc. Polymer Materials Research Institute, model number APEL-APL6509T. PP can be HC300, Borealis, or 5014LHPT-1 from Korea Petrochemical.

[0028] The breakdown field strength described below is tested using a computer-controlled withstand voltage strength tester (model: HT-20) from Guilin Liyuan Electronic Instrument Co., Ltd., using the 50-point method, which is to collect 50 data points, remove the 5 highest and lowest values, and calculate the average and minimum values ​​of the remaining data to improve the accuracy of the measurement. The withstand voltage strength tester comes with an oven with an adjustable oven temperature to test the temperature resistance of the film.

[0029] The heat shrinkage rate described below was tested using a hot air circulation drying oven (model: HF-881-1) produced by Wujiang Huafei Electric Heating Equipment Co., Ltd., and the temperature was adjustable.

[0030] The present invention relates to a biaxially asynchronously stretched high-temperature resistant three-layer structure polypropylene film for capacitors. The film prepared by the method is a BOPP (biaxially oriented polypropylene) film for three-layer composite structure capacitors. The temperature resistance of the film is improved by adjusting the proportion of the three layers and optimizing the surface raw materials. The processing adaptability and temperature resistance of the film are controlled by melt extrusion casting and biaxial asynchronous stretching processes. The obtained film can withstand a temperature of up to 125°C, which exceeds the melting point of PP materials, thereby making the application scenarios of capacitors wider.

[0031] A method for preparing a bidirectional asynchronously stretched high-temperature resistant three-layer capacitor polypropylene film comprises the steps of batching, melt extrusion and compounding, sheet casting, asynchronous stretching and subsequent steps, wherein the subsequent steps include traction and winding to finally obtain a capacitor film, wherein thickness measurement, edge trimming and corona treatment are performed during traction, and after winding, the original film is inspected, aging treated, slit and formed, finished product inspected, packaged and then put into storage and out of storage, and the scraps generated during edge trimming are granulated and recycled.

[0032] The ingredients: polypropylene with an isotacticity greater than 98%, a melting point of 120°C, a melt flow rate of 2.8-3.2g / 10min, and an ash content less than 30ppm is used as the raw material for the middle layer, accounting for about 85%; Cyclic olefin copolymer (COC) with a melting point of 91°C, a melt flow rate of 26-30g / min, and an ash content of less than 100ppm is used as one of the surface layer materials, namely the first surface layer, accounting for about 5-7%. The other surface layer, namely the second surface layer, still uses the same raw material as the middle layer, that is, PP material, accounting for about 7-8%.

[0033] The melt extrusion and compounding: add the middle layer ingredients to the main extruder, add the first surface layer ingredients to the first auxiliary extruder, and add the second surface layer ingredients to the second auxiliary extruder, all of which are melted, plasticized, and stably extruded through a metering pump, and then passed through a filter. After the three layers of ingredients are compounded at the die head, a sheet of melt flows out. It should be noted that because the melting point of the first surface layer COC material is relatively low and the fluidity is relatively good, in order to better integrate with the PP material, when designing the extrusion temperature, the temperature of the PP material should be about 60°C lower than that of the middle layer and the second surface layer, depending on the actual situation on site, and the first surface layer must be placed on the air knife surface, because other anti-adhesion additives cannot be added to the capacitor film. The surface of the film is roughened by using the crystal form of the PP (polypropylene) material itself to meet the winding performance. Therefore, the other surface layer, that is, the second surface layer, must be a normal PP material and on the cold roller surface in order to better control the roughening structure.

[0034] The casting sheet: the composite sheet melt is cooled and shaped under the action of cold rollers and air knives to obtain uniform casting sheets. The temperature of the cold roller can effectively control the crystallinity of the PP material. The main function of the air knife is to make the casting sheet stick to the roller better. Generally, temperature control is not required, but the air volume must be sufficient to obtain a good casting sheet effect.

[0035] The asynchronous stretching: the shaped cast sheet is sequentially stretched longitudinally and transversely, and a film is obtained after biaxial stretching. Both longitudinal and transverse stretching have three functional zones, namely, preheating zone, stretching zone, and shaping zone; and the stretching ratio of longitudinal stretching is controlled between 4.5-5.3 times, and the stretching ratio of transverse stretching is controlled between 8-10 times, so that a higher quality film can be obtained.

[0036] The film is then pulled, thickness measured, trimmed, corona treated and rolled up to finally obtain a capacitor film.

[0037] The mixing step: The reason why the middle layer uses PP material, the first surface layer uses COC material, and the second surface layer uses PP material is because different materials have different functions at different levels, and the structural design directly determines the final quality of the film.

[0038] The melt extrusion and compounding step: the extrusion temperature of the middle layer PP material and the second surface layer PP material are both between 220-230°C, which can effectively ensure the acquisition of a uniformly plasticized PP melt. Because the first surface layer COC has a low melting point and good fluidity, the temperature of the extruder can be designed to be lower than that of PP. Therefore, the design temperature is between 150-170°C to obtain a uniformly plasticized COC melt, and the melt compounding performance in this state is also the best. Because the volume of the cyclic polyolefin in COC is relatively large, the molecules cannot form a crystalline structure and have non-crystalline characteristics. In order to better adapt to the characteristics of our biaxial stretching, the COC selected in the present invention is a small molecular weight COC, which can meet our film drawing needs. The sum of the mass of the middle layer PP and the second surface layer PP accounts for about 93-95%, and COC accounts for about 5-7%.

[0039] Furthermore, in the casting step, the temperature of the cold roller can effectively control the crystallinity of PP, and the temperature can be set between 90-105°C. Because the material used for the air knife surface is non-crystalline, the air knife does not need temperature control, but the air volume of the air knife is guaranteed to be no less than 120-150mbar to ensure the roller-sticking effect of the melt.

[0040] Furthermore, the asynchronous stretching is bidirectional asynchronous stretching, first longitudinal stretching, then transverse stretching, the preheating temperature of the longitudinal stretching preheating zone is between 110-130°C, the temperature of the stretching zone is between 140-145°C, and the temperature of the shaping zone is between 145-150°C; the longitudinal stretching ratio is between 4.5-5.3, which varies according to the settings of different specifications of the film; the preheating temperature of the transverse stretching preheating zone is between 165-170°C, the temperature of the stretching zone is between 155-160°C, the temperature of the shaping zone is between 165-170°C, and the transverse stretching ratio is controlled at 8-10 times. The film after transverse stretching heat setting is sent to the traction station through the cooling zone.

[0041] Furthermore, during traction, the film is first measured for thickness by an infrared thickness gauge, and then trimmed to ensure an effective width of the film that is conducive to winding. It is then subjected to corona treatment, and the power of the corona treatment is controlled between 16-22kW. Because PP material is a non-polar material, after corona treatment, it becomes polar, which is convenient for the next process of metallization vacuum plating. After winding, a 2.0-9.8μm capacitor film can be obtained.

[0042] The capacitor film is finally obtained according to the above method. The obtained capacitor film has the characteristic that the dielectric breakdown field strength of the dielectric strength at 115° C. is between 500-550 V / μm.

[0043] The thermal shrinkage of the film at 140°C for 15 minutes is 2.0-2.5% in the MD direction and 0.05-0.1% in the TD direction. The surface roughness is Ra (arithmetic mean roughness): 0.05-0.08μm, and Rmax (maximum roughness height): 0.1-0.15μm.

[0044] Example 1 A bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, the film comprising a first surface layer, a second surface layer and an intermediate layer sandwiched between the two surface layers; The material of the middle layer is polypropylene, the material of the first surface layer is cyclic olefin copolymer (COC), and the material of the second surface layer is polypropylene; The mass of the middle layer accounts for 85wt% of the total mass, the mass of the first surface layer accounts for 7wt% of the total mass, and the mass of the second surface layer accounts for 8wt% of the total mass.

[0045] A method for preparing a bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, comprising the steps of batching, melt extrusion and compounding, sheet casting, and asynchronous stretching; The ingredients are as follows: polypropylene with an isotacticity greater than 98%, a melting point of 120°C, a melt flow rate of 2.8-3.2 g / 10 min, and an ash content less than 30 ppm is used as the raw material for the middle layer; cycloolefin copolymer (COC) with a melting point of 91°C, a melt flow rate of 26-30 g / min, and an ash content less than 100 ppm is used as the first surface layer, and the material of the second surface layer is the same as that of the middle layer.

[0046] The melt extrusion and compounding: add the middle layer ingredients to the main extruder, add the first surface layer ingredients to the first auxiliary extruder, and add the second surface layer ingredients to the second auxiliary extruder, all of which are melted, plasticized, and stably extruded by a metering pump, and then pass through a filter. After the three layers of ingredients are compounded at the die head, a sheet-like melt flows out; during compounding, the extrusion temperature of the first surface layer is 55°C lower than that of the second surface layer, and the surface layer is located on the air knife surface; the second surface layer is located on the cold roller surface. The temperature of the cold roller is set to 90°C; the material used for the air knife surface is the non-crystalline material COC, and the air volume of the air knife is 120mbar. The extrusion temperatures of the middle layer PP material and the second surface layer PP material are both 220°C, and the design temperature of the first surface layer is 150°C.

[0047] The asynchronous stretching: the shaped cast sheet is longitudinally stretched and transversely stretched in sequence to obtain a film after biaxial stretching. Both the longitudinal stretching and the transverse stretching have three functional zones, namely, a preheating zone, a stretching zone, and a shaping zone; the stretching ratio of the longitudinal stretching is controlled to be 4.5 times, and the stretching ratio of the transverse stretching is controlled to be 8 times; the preheating temperature of the longitudinal stretching preheating zone is 110°C, the temperature of the stretching zone is 140°C, and the temperature of the shaping zone is 145°C; the preheating temperature of the transverse stretching preheating zone is 165°C, the temperature of the stretching zone is 155°C, and the temperature of the shaping zone is 165°C. The film after heat shaping by transverse stretching is sent to the traction station through the cooling zone.

[0048] During traction, the film is first measured by an infrared thickness gauge, and then trimmed to ensure an effective width of the film that is conducive to winding. It is then corona treated, and the power of the corona treatment is 16kW. After winding, a 4.5μm capacitor film can be obtained.

[0049] The capacitor film obtained by the above method can withstand a high temperature of 125°C, and the dielectric breakdown field strength of the dielectric strength at 115°C is 500-550V / μm. The thermal shrinkage rate of the film at 140°C for 15 minutes is 2-2.5% in the MD direction and 0.05-0.1% in the TD direction. The surface roughness of the film is Ra: 0.05-0.08μm, Rmax: 0.1-0.15μm.

[0050] Comparative Example 1 A bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, the film comprising a first surface layer, a second surface layer and an intermediate layer sandwiched between the two surface layers; The material of the middle layer is polypropylene, the material of the first surface layer is cyclic olefin copolymer (COC), and the material of the second surface layer is polypropylene; The mass of the middle layer accounts for 87wt% of the total mass, the mass of the first surface layer accounts for 5wt% of the total mass, and the mass of the second surface layer accounts for 8wt% of the total mass.

[0051] A method for preparing a bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, comprising the steps of batching, melt extrusion and compounding, sheet casting, and asynchronous stretching; The ingredients are as follows: polypropylene with an isotacticity greater than 98%, a melting point of 120°C, a melt flow rate of 2.8-3.2 g / 10 min, and an ash content less than 30 ppm is used as the raw material for the middle layer; cycloolefin copolymer (COC) with a melting point of 91°C, a melt flow rate of 26-30 g / min, and an ash content less than 100 ppm is used as the first surface layer, and the material of the second surface layer is the same as that of the middle layer.

[0052] The melt extrusion and compounding: add the middle layer ingredients to the main extruder, add the first surface layer ingredients to the first auxiliary extruder, and add the second surface layer ingredients to the second auxiliary extruder, all of which are melted, plasticized, and stably extruded by a metering pump, and then pass through a filter. After the three layers of ingredients are compounded at the die head, a sheet-like melt flows out; during compounding, the extrusion temperature of the first surface layer is 60°C lower than that of the second surface layer, and the surface layer is located on the air knife surface; the second surface layer is located on the cold roller surface. The temperature of the cold roller is set to 100°C; the material used for the air knife surface is non-crystalline material COC, and the air volume of the air knife is 130mbar. The extrusion temperatures of the middle layer PP material and the second surface layer PP material are both 225°C, and the design temperature of the first surface layer is 160°C.

[0053] The asynchronous stretching: the shaped cast sheet is longitudinally stretched and transversely stretched in sequence to obtain a film after biaxial stretching. Both the longitudinal stretching and the transverse stretching have three functional zones, namely, a preheating zone, a stretching zone, and a shaping zone; the stretching ratio of the longitudinal stretching is controlled to be 5.0 times, and the stretching ratio of the transverse stretching is controlled to be 9 times; the preheating temperature of the longitudinal stretching preheating zone is 120°C, the temperature of the stretching zone is 143°C, and the temperature of the shaping zone is 148°C; the preheating temperature of the transverse stretching preheating zone is 167°C, the temperature of the stretching zone is 158°C, and the temperature of the shaping zone is 168°C. The film after heat shaping by transverse stretching is sent to the traction station through the cooling zone.

[0054] During traction, the film is first measured by an infrared thickness gauge, and then trimmed to ensure an effective width of the film that is conducive to winding. It is then corona treated, and the power of the corona treatment is controlled at 20kW. After winding, a 4.5μm capacitor film can be obtained.

[0055] The capacitor film obtained by the above method can withstand a high temperature of 120°C, and the dielectric breakdown field strength of the dielectric strength at 115°C is 480-530V / μm. The thermal shrinkage rate of the film at 120°C for 15 minutes is 2.2-2.8% in the MD direction and 0.1-0.15% in the TD direction. The surface roughness of the film is Ra: 0.05-0.08μm, Rmax: 0.1-0.15μm.

[0056] Comparative Example 2 A bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, the film comprising a first surface layer, a second surface layer and an intermediate layer sandwiched between the two surface layers; The material of the middle layer is polypropylene, the material of the first surface layer is cyclic olefin copolymer (COC), and the material of the second surface layer is polypropylene; The mass of the middle layer accounts for 90wt% of the total mass, the mass of the first surface layer accounts for 2wt% of the total mass, and the mass of the second surface layer accounts for 8wt% of the total mass.

[0057] A method for preparing a bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, comprising the steps of batching, melt extrusion and compounding, sheet casting, and asynchronous stretching; The ingredients are as follows: polypropylene with an isotacticity greater than 98%, a melting point of 120°C, a melt flow rate of 2.8-3.2 g / 10 min, and an ash content less than 30 ppm is used as the raw material for the middle layer; cycloolefin copolymer (COC) with a melting point of 91°C, a melt flow rate of 26-30 g / min, and an ash content less than 100 ppm is used as the first surface layer, and the material of the second surface layer is the same as that of the middle layer.

[0058] The melt extrusion and compounding: add the middle layer ingredients to the main extruder, add the first surface layer ingredients to the first auxiliary extruder, and add the second surface layer ingredients to the second auxiliary extruder, all of which are melted, plasticized, and stably extruded by a metering pump, and then pass through a filter. After the three layers of ingredients are compounded at the die head, a sheet of melt flows out; during compounding, the extrusion temperature of the first surface layer is 65°C lower than that of the second surface layer, and the surface layer is located on the air knife surface; the second surface layer is located on the cold roller surface. The temperature of the cold roller is set to 105°C; the material used for the air knife surface is a non-crystalline material COC, and the air volume of the air knife is 150mbar. The extrusion temperatures of the middle layer PP material and the second surface layer PP material are both 230°C, and the design temperature of the first surface layer is 170°C.

[0059] The asynchronous stretching: the shaped cast sheet is longitudinally stretched and transversely stretched in sequence to obtain a film after biaxial stretching. Both the longitudinal stretching and the transverse stretching have three functional zones, namely, a preheating zone, a stretching zone, and a shaping zone; the stretching ratio of the longitudinal stretching is controlled to be 5.3 times, and the stretching ratio of the transverse stretching is controlled to be 10 times; the preheating temperature of the longitudinal stretching preheating zone is 130°C, the temperature of the stretching zone is 145°C, and the temperature of the shaping zone is 150°C; the preheating temperature of the transverse stretching preheating zone is 170°C, the temperature of the stretching zone is 160°C, and the temperature of the shaping zone is 170°C. The film after heat shaping by transverse stretching is sent to the traction station through the cooling zone.

[0060] During traction, the film is first measured by an infrared thickness gauge, and then trimmed to ensure an effective width of the film that is conducive to winding. It is then corona treated, and the power of the corona treatment is controlled at 22kW. After winding, a 4.5μm capacitor film can be obtained.

[0061] The capacitor film obtained by the above method can withstand a high temperature of 115°C, and the dielectric strength at 115°C has a dielectric breakdown field strength of 460-500V / μm. The thermal shrinkage of the film at 140°C for 15 minutes is 2.5-3% in the MD direction and 0.2-0.25% in the TD direction. The surface roughness of the film is Ra: 0.05-0.08μm, Rmax: 0.15-0.2μm.

[0062] Comparative Example 3 This comparative example is basically the same as Example 1, except that the mass of the first surface layer, i.e., the COC layer, accounts for 10wt% of the total mass, the mass of the intermediate layer accounts for 85wt% of the total mass, and the mass of the second surface layer accounts for 5wt% of the total mass. The obtained film was subjected to film drawing test and temperature resistance test.

[0063] The results show that the capacitor film finally obtained according to the above method can withstand high temperature of 129°C, and its dielectric strength at 115°C has a dielectric breakdown field strength of 480-520V / μm. The thermal shrinkage of the film at 140°C for 15 minutes is 2.2-2.5% in the MD direction and 0.02-0.08% in the TD direction. The surface roughness of the film is Ra: 0.05-0.08μm, Rmax: 0.15-0.25μm. However, the film-forming property is poor, and uneven stretching occurs on the surface, which may be caused by the excessive thickness of the COC surface layer.

[0064] Comparative Example 4 The difference between this comparative example and Example 1 is that the COC material of the first surface layer is replaced by a material in which COC and PP are mixed in proportion (10%, 20%, 50%). The obtained film was subjected to film drawing test and temperature resistance test.

[0065] The results show that the capacitor film finally obtained according to the above method can withstand a high temperature of 112°C, and its dielectric strength at 115°C has a dielectric breakdown field strength of 430-460V / μm. The thermal shrinkage of the film at 120°C for 15 minutes is 2.8-3.2% in the MD direction and 0.2-0.3% in the TD direction. The surface roughness of the film is Ra: 0.05-0.08μm, Rmax: 0.2-0.3μm. The test results show that there is no problem with film drawing, but the temperature resistance has not been significantly improved, and on the contrary, the surface layer is torn.

[0066] The present invention can effectively improve the temperature resistance by optimizing the raw materials and utilizing the inherent characteristics of cyclic polyolefins, and then improve the film-forming property by controlling the proportion of cyclic polyolefins, so that the temperature resistance of the film can be increased to above 125°C, greatly improving the temperature resistance of the film. After several samples with different process parameter ratios, it was found that when the COC content was controlled at 5%-7%, the film-forming property and the temperature resistance could be taken into account and reached 125°C.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors, characterized in that: The film comprises a first surface layer, a second surface layer and an intermediate layer sandwiched between the two surface layers; The material of the middle layer is polypropylene, the material of the first surface layer is cyclic olefin copolymer COC, and the material of the second surface layer is polypropylene; The mass of the middle layer accounts for 85wt-88wt% of the total mass, the mass of the first surface layer accounts for 5-7wt% of the total mass, and the mass of the second surface layer accounts for 7-8wt% of the total mass.

2. The bidirectional asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 1, characterized in that: The dielectric strength of the film has a dielectric breakdown field strength of 500-550 V / μm at 115°C; and / or The film has a heat shrinkage rate of 2.0-2.5% in the MD direction and 0.05-0.1% in the TD direction at 120°C for 15 minutes; and / or Surface roughness Ra: 0.05-0.08μm, Rmax: 0.1-0.15μm.

3. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 1 or 2, characterized in that: The method comprises the following steps: batching, melt extrusion and compounding, sheet casting and asynchronous stretching; The ingredients are as follows: polypropylene with an isotacticity greater than 98%, a melting point of 120°C, a melt flow rate of 2.8-3.2g / 10min, and an ash content of less than 30ppm is used as the raw material for the middle layer, accounting for 85wt%-88wt%; cycloolefin copolymer COC with a melting point of 91°C, a melt flow rate of 26-30g / min, and an ash content of less than 100ppm is used as the first surface layer, accounting for 5-7wt% in the total mass; the material of the second surface layer is the same as that of the middle layer, accounting for 7-8wt% in the total mass.

4. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 3, characterized in that: The melt extrusion and compounding: the middle layer ingredients are added to the main extruder, the first surface layer ingredients are added to the first auxiliary extruder, and the second surface layer ingredients are added to the second auxiliary extruder, all of which are melted, plasticized, and stably extruded through a metering pump, and then passed through a filter, and the three layers of ingredients are compounded at the die head and flow out as a sheet melt; During compounding, the extrusion temperature of the first surface layer is 55-65°C lower than that of the second surface layer, and the first surface layer is located on the air knife surface; the second surface layer is located on the cold roller surface.

5. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 4, characterized in that: The casting sheet: the composite sheet melt is cooled and shaped under the action of cold rollers and air knives to obtain a uniform casting sheet.

6. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 3, characterized in that: The subsequent steps include pulling and winding to finally obtain a capacitor film, wherein thickness measurement, edge trimming and corona treatment are required during pulling.

7. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 3, characterized in that: The asynchronous stretching: the shaped cast sheet is sequentially longitudinally stretched and transversely stretched to obtain a film after biaxial stretching, and the longitudinal stretching and transverse stretching both have three functional zones, namely, a preheating zone, a stretching zone, and a shaping zone; the stretching ratio of the longitudinal stretching is controlled between 4.5-5.3 times, and the stretching ratio of the transverse stretching is controlled between 8-10 times; The melt extrusion and compounding step: the temperature of the cold roller is set between 90-105° C.; the material used for the air knife surface is the non-crystalline material COC, and the air volume of the air knife is not less than 120-150 mbar.

8. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 4, characterized in that: The extrusion temperatures of the middle layer PP material and the second surface layer PP material are both between 220-230°C, and the design temperature of the first surface layer is between 150-170°C.

9. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 7, characterized in that: The asynchronous stretching step is bidirectional asynchronous stretching, which is performed longitudinally first and then transversely. The preheating temperature of the longitudinal stretching preheating zone is between 110-130°C, the temperature of the stretching zone is between 140-145°C, and the temperature of the shaping zone is between 145-150°C; the preheating temperature of the transverse stretching preheating zone is between 165-170°C, the temperature of the stretching zone is between 155-160°C, and the temperature of the shaping zone is between 165-170°C. The film after transverse stretching and heat setting is sent to the traction station through the cooling zone.

10. The method for preparing a biaxially asynchronously stretched high temperature resistant three-layer structure polypropylene film for capacitors according to claim 6, characterized in that: During traction, the film is first measured by an infrared thickness gauge, then trimmed, and then corona treated. At this time, the power of the corona treatment is controlled between 16-22kW; after winding, a capacitor film with a thickness of 2.0-9.8μm can be obtained.

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