Polypropylene film for film capacitor and preparation method thereof

Abstract

The invention discloses a polypropylene film for a film capacitor, the polypropylene film is composed of polypropylene and a cyclic olefin polymer, a plurality of gaps are distributed in the film, the gaps are approximately annular on the cross section of the film, and the inner diameter of the approximately annular gaps is 20-120nm. The invention also provides a preparation method of the polypropylene film, which comprises the following specific steps: putting the polypropylene and the cyclic olefin polymer into an extruder, heating and melting; the melt is extruded through a T-shaped die head and attached to a piece casting roller through an air knife for piece casting, and the temperature of the piece casting roller is 85-95 DEG C; the method comprises the following steps: preheating a cast sheet, longitudinally stretching at the stretching rate of 4-7 times and at the temperature of 135-150 DEG C, preheating the longitudinally stretched sheet at the temperature of 145-160 DEG C, transversely stretching at the stretching rate of 7-10 times and at the stretching temperature of 165-180 DEG C, and then entering a shaping area at the temperature of 168 DEG C; and carrying out corona treatment, rolling, curing and slitting on the heat-set film to obtain the product. The polypropylene film has good high temperature resistance and high pressure resistance, and is suitable for the field of capacitors.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer materials, and particularly relates to a polypropylene film for film capacitors and a preparation method thereof. Background Art

[0002] The polypropylene film used in film capacitors is made through a biaxial stretching process, which makes full use of the low dielectric constant and low dielectric loss of polypropylene and uses it as an excellent electrical insulating medium. Due to the excellent processing characteristics of polypropylene itself, the biaxial stretching process gives the film higher mechanical properties, temperature resistance and electrical breakdown properties, making film capacitors widely used in new energy vehicles, photovoltaics, wind power generation, energy storage and other fields.

[0003] With the development and application of new energy technologies around the world, the use of film capacitors in inverters faces more challenges of high capacity and miniaturization, especially with the application of new SiC power semiconductors, which enable power devices to have higher electron mobility, higher breakdown electric field strength and lower conduction loss. Therefore, film capacitors are required to be able to serve under higher voltage conditions and higher temperature field conditions.

[0004] The patent application number US20230391966A1 discloses a capacitor with a biaxially oriented film as a dielectric, the biaxially oriented film contains a mixture of polypropylene and cyclic olefin polymers, the proportion of cyclic olefin polymers in the mixture is 3%-18% by weight, and these capacitors have the characteristics of high temperature resistance and high dielectric strength at room temperature. The application uses a scanning electron microscope to observe the cross-section and surface of the film to observe the miscibility effect between polypropylene and cyclic olefins, and the high temperature resistance and voltage resistance of the film can be further improved. Summary of the invention

[0005] In order to solve the above problems, the present invention provides a high temperature and high voltage resistant polypropylene film for film capacitors and a preparation method thereof.

[0006] The technical solution of the present invention is as follows:

[0007] One of the purposes of the present invention is to provide a polypropylene film for film capacitors, wherein the film is composed of polypropylene and cyclic olefin polymers, and a plurality of voids are distributed inside the film, wherein the voids are approximately annular in shape on the cross section of the film, and the inner diameter of the approximately annular voids is 20-120 nm, wherein on the cross section of the film, within each square micron, the number of voids with an inner diameter of less than 100 nm accounts for more than 90%.

[0008] Furthermore, the number of voids in the cross section of the film is ≥50 / um 2 . .

[0009] Furthermore, the film is composed of the following raw materials in percentage by mass: 85-100% polypropylene and 0-15% cyclic olefin polymer.

[0010] Furthermore, the cyclic olefin polymer is a polymer whose main chain or branched chain contains a cyclic structure.

[0011] Furthermore, the cyclic structure of the cyclic olefin polymer is a five-membered ring or a six-membered ring.

[0012] Furthermore, the thickness of the film is 1.5-9.0 um.

[0013] Furthermore, the haze value of the film is 0-15%.

[0014] Furthermore, the longitudinal tensile strength of the film is 100-180 MPa, and the transverse tensile strength of the film is 120-250 MPa.

[0015] Furthermore, the electrical strength of the film at 115° C. is at least 450 V / um.

[0016] A second object of the present invention is to provide a method for preparing a polypropylene film for a film capacitor, comprising the following steps:

[0017] S1: Put polypropylene and cyclic olefin polymer into an extruder together and heat them to melt at 240-260°C;

[0018] S2: extruding the melt obtained in step S1 through a T-die and applying it to a casting roller through an air knife for casting, wherein the casting roller temperature is 85-95°C;

[0019] S3: The cast sheet obtained in step S2 is longitudinally preheated at 105-130°C and then longitudinally stretched, the longitudinal stretching rate is 4-7 times, and the longitudinal stretching temperature is 135-150°C. Then, the longitudinally stretched sheet is transversely preheated at 145-160°C and then transversely stretched, the transverse stretching rate is 7-10 times, and the transverse stretching temperature is 165-180°C, and then enters the shaping zone at 168°C;

[0020] S4: The film that has been heat-set in step S3 is subjected to corona treatment, winding, aging, and slitting to obtain the polypropylene film for film capacitors.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention provides a polypropylene film for film capacitors, wherein a plurality of voids are distributed inside the film, and the voids are approximately ring-shaped on the cross section of the film. The presence of these voids helps to reduce the dielectric constant of the film, reduce its dielectric loss, and improve the withstand voltage characteristics of the film. In addition, the present invention controls the number of voids to ≥50 / um. 2 Within the range of , the film can further exhibit good high temperature resistance and high voltage resistance and can be used in the capacitor field.

[0023] 2. In the present invention, the cyclic structure of the cyclic olefin polymer is a five-membered ring or a six-membered ring. The five-membered ring and the six-membered ring of the olefin have good chemical stability. The cyclic structure itself has high rigidity, which can improve the heat resistance and aging resistance of the material, and improve the high-temperature withstand voltage characteristics of the product when used as a capacitor. After the introduction of the second phase material, it promotes the formation of voids. It is easier to generate voids at the interface during the stretching process, and the generation of voids promotes the improvement of withstand voltage performance.

[0024] 3. The present invention provides a method for preparing a polypropylene film for film capacitors. In the biaxial stretching process, the film undergoes a process of transformation from β crystals to α crystals. Since the density of α crystals is greater than that of β crystals, the volume of α crystals shrinks after they are formed, and gaps are formed in the outer ring of the spherical α crystals. As the film is stretched transversely and longitudinally, the gaps are further enlarged. Therefore, when making the film cross section, the outer ring of the α crystals is more likely to be destroyed first, forming an appearance of approximately annular gaps on the cross section of the film. The present invention controls the number and size of the initial β crystals by controlling the temperature of the casting roller; controls the inner diameter of the film gaps to 20-120nm by controlling the stretching ratio and the stretching temperature, and makes the number of gaps with an inner diameter of less than 100nm account for more than 90% / um. 2 , so that its high temperature and high pressure resistance performance is optimal. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a SEM image of the cross section of a polypropylene film for film capacitors provided by the present invention. DETAILED DESCRIPTION

[0026] The following is in conjunction with the preferred embodiment and with reference to the attached Figure 1, the present invention is further described. The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values; for numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein; the materials, reagents, etc. used in the following examples, unless otherwise specified, can be obtained from commercial channels; the experimental methods in the following examples, unless otherwise specified, are all conventional methods.

[0027] Example 1

[0028] This embodiment provides a method for preparing a polypropylene film for a film capacitor, comprising the following steps:

[0029] S1: Put polypropylene and cyclic olefin polymer into an extruder together and heat them to melt at 260°C;

[0030] In this embodiment, the polypropylene is HC300BF, a raw material of Borealis, and the cyclic olefin polymer is TOPAS cyclic olefin polymer 6013F-04. In the film, the mass percentage of HC300BF is 95%, and the mass percentage of 6013F-04 is 5%.

[0031] S2: The melt obtained in step S1 is extruded through a T-die and applied to a casting roller through an air knife for casting. The casting roller temperature is 90° C.

[0032] S3: The cast sheet obtained in step S2 is longitudinally preheated at 120°C and then longitudinally stretched, the longitudinal stretching rate is 5 times, and the longitudinal stretching temperature is 147°C. Then, the longitudinally stretched sheet is transversely preheated at 155°C and then transversely stretched, the transverse stretching rate is 8 times, and the transverse stretching temperature is 165°C, and then enters the shaping zone at 168°C;

[0033] S4: The film that has been heat-set in step S3 is subjected to corona treatment, winding, aging, and slitting to obtain the polypropylene film for film capacitors.

[0034] In this embodiment, the thickness of the prepared polypropylene film is 3.8 μm.

[0035] Example 2

[0036] This embodiment provides a method for preparing a polypropylene film for a film capacitor, comprising the following steps:

[0037] S1: Put polypropylene and cyclic olefin polymer into an extruder together and heat them to melt at 260°C;

[0038] In this embodiment, the polypropylene is HC300BF, a raw material of Borealis, and the cyclic olefin polymer is USI cyclic olefin polymer Vivion-0645. In the film, the mass percentage of HC300BF is 88%, and the mass percentage of Vivion-0645 is 12%.

[0039] S2: The melt obtained in step S1 is extruded through a T-die and applied to a casting roller through an air knife for casting. The casting roller temperature is 95° C.

[0040] S3: The cast sheet obtained in step S2 is longitudinally preheated at 130°C and then longitudinally stretched, the longitudinal stretching rate is 4.5 times, and the longitudinal stretching temperature is 150°C. Then, the longitudinally stretched sheet is transversely preheated at 160°C and then transversely stretched, the transverse stretching rate is 7.5 times, and the transverse stretching temperature is 170°C, and then enters the shaping zone at 168°C;

[0041] S4: subjecting the film heat-set in step S3 to corona treatment, winding, aging, and slitting to obtain the polypropylene film for film capacitors.

[0042] In this embodiment, the thickness of the prepared polypropylene film is 4.8 μm.

[0043] Example 3

[0044] The method is basically the same as Example 1, except that the raw material of the polypropylene film in this example is 100% HC300BF from Borealis, and the heating and melting temperature in step S1 is 250°C.

[0045] Example 4

[0046] This embodiment provides a method for preparing a polypropylene film for a film capacitor, comprising the following steps:

[0047] S1: Put polypropylene and cyclic olefin polymer into an extruder together and heat them to melt at 240°C;

[0048] In this embodiment, the polypropylene is HC300BF, a raw material of Borealis, and the cyclic olefin polymer is USI cyclic olefin polymer Vivion-0645. In the film, the mass percentage of HC300BF is 85%, and the mass percentage of Vivion-0645 is 15%.

[0049] S2: The melt obtained in step S1 is extruded through a T-die and applied to a casting roller through an air knife for casting. The casting roller temperature is 85° C.

[0050] S3: The cast sheet obtained in step S2 is longitudinally preheated at 105°C and then longitudinally stretched, the longitudinal stretching rate is 4 times, and the longitudinal stretching temperature is 150°C. Then, the longitudinally stretched sheet is transversely preheated at 145°C and then transversely stretched, the transverse stretching rate is 8 times, and the transverse stretching temperature is 172°C, and then enters the shaping zone at 168°C;

[0051] S4: subjecting the film heat-set in step S3 to corona treatment, winding, aging, and slitting to obtain the polypropylene film for film capacitors.

[0052] In this embodiment, the thickness of the prepared polypropylene film is 3.8 μm.

[0053] In this embodiment, the cyclic olefin polymer can be self-made or selected from Polyplastics Company. COC series resin, APEL of Mitsui Chemicals TM Series resin, Ruiong Company COP series resins, USI's Vivion TM CBC series resins, JSR's ARTON series resins.

[0054] Comparative Example 1

[0055] The difference from Example 1 is that in step S3 of the preparation method provided in this comparative example, the longitudinal stretching ratio is 3.7 times, and the thickness of the prepared film is 4.3 μm.

[0056] Comparative Example 2

[0057] The difference from Example 1 is that in step S3 of the preparation method provided in this comparative example, the longitudinal stretching temperature is 137°C.

[0058] Comparative Example 3

[0059] The difference from Example 2 is that the raw materials of the film of this comparative example are HC300BF and USI cyclic olefin polymer Vivion-0645 from Borealis, wherein the mass percentage of HC300BF is 82% and the mass percentage of Vivion-0645 is 18%.

[0060] Implementation effect evaluation

[0061] The following is a specific test of the polypropylene films obtained in Examples 1-4 and Comparative Examples 1-3 to further illustrate the excellent effects achieved by the present invention:

[0062] The following table is a statistical table of experimental data of Examples 1-4 and Comparative Examples 1-3:

[0063] Table 1 Experimental data statistics

[0064]

[0065]

[0066] Tensile strength, thermal shrinkage, electrical strength, haze, and roughness Ra are measured according to the relevant standards of GB / T 13542.2-2021;

[0067] Thin film cross-section test: The thin film cross-section was processed at -60°C using low-temperature ion grinding equipment, and the cross-section was observed using a Zeiss EVO10 electron scanning microscope with a magnification of 20,000 times. Statistical counting software was used to count the size and number of the observed voids, and the number of voids per square micron, the number of voids with an inner diameter less than 100 nm, and the number of all voids were obtained.

[0068] The following table is a statistical table of experimental results of Examples 1-4 and Comparative Examples 1-3:

[0069] Table 2 Experimental results statistics

[0070]

[0071]

[0072] It can be seen from the table above that the proportion of pores with inner diameters <100 nm in the cross-sections of the films prepared in Examples 1-4 is >90% / um 2 When used in combination with polypropylene film, it can show good high temperature resistance and voltage resistance.

[0073] It can be seen from the experimental data of Comparative Examples 1-2 and Example 1 that low longitudinal stretching ratio and low longitudinal stretching temperature are not conducive to the generation of voids in the cross section of the film. These two process conditions are not conducive to the crystal transformation in the film, resulting in a decrease in the number of voids in the cross section of the film.

[0074] It can be seen from the experimental data of Example 3 and Example 2 that when the cyclic olefin content is greater than 15%, this is because the proportion of cyclic olefin components increases, which impairs the ability of the polypropylene material to transform from β crystals to α crystals, which is not conducive to the control of void size and quantity. At the same time, the difference in solubility parameters of the two resins in the film material makes it easier to form an interface. In the presence of a high proportion of the second phase resin, it is more difficult to maintain an independent interface structure, resulting in a decrease in the number of voids.

[0075] The film of the present invention undergoes a process of transformation from β crystals to α crystals during the biaxial stretching process, and the density of α crystals is greater than that of β crystals. Therefore, after the formation of α crystals, the volume is reduced to form voids, and as the film is stretched transversely and longitudinally, the voids are further enlarged. The present invention controls the number and size of initial β crystals by controlling the temperature of the casting roll; controls the inner diameter of the voids in the film to 20-120nm by controlling the stretching ratio and the stretching temperature, and makes the number of voids with an inner diameter of less than 100nm account for more than 90% / um. 2 , so that its high temperature and high pressure resistance performance is optimal.

[0076] The above is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A polypropylene film for film capacitors, characterized in that: The film is composed of polypropylene and cyclic olefin polymer, and has a plurality of voids distributed inside the film. The voids are approximately annular in the cross section of the film, and the inner diameter of the approximately annular voids is 20-120nm. In the cross section of the film, the number of voids with an inner diameter of less than 100nm accounts for more than 90% per square micron.

2. The polypropylene film for film capacitors according to claim 1, characterized in that: The number of voids in the cross section of the film is ≥50 / um 2 .

3. The polypropylene film for film capacitors according to claim 1, characterized in that: The film is composed of the following raw materials in percentage by mass: 85-100% of polypropylene and 0-15% of cyclic olefin polymer.

4. The polypropylene film for film capacitors according to claim 3, characterized in that: The cyclic olefin polymer is a polymer whose main chain or branched chain contains a cyclic structure.

5. The polypropylene film for film capacitors according to claim 3, characterized in that: The cyclic structure of the cyclic olefin polymer is a five-membered ring or a six-membered ring.

6. A polypropylene film for film capacitors according to any one of claims 1 to 5, characterized in that: The thickness of the film is 1.5-9.0 um.

7. A polypropylene film for film capacitors according to any one of claims 1 to 5, characterized in that: The haze value of the film is 0-15%.

8. A polypropylene film for film capacitors according to any one of claims 1 to 5, characterized in that: The longitudinal tensile strength of the film is 100-180 MPa, and the transverse tensile strength of the film is 120-250 MPa.

9. A polypropylene film for film capacitors according to any one of claims 1 to 5, characterized in that: The film has an electrical strength of at least 450 V / um at 115°C.

10. The method for preparing a polypropylene film for a film capacitor according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1: Put polypropylene and cyclic olefin polymer into an extruder together and heat them to melt at 240-260°C; S2: extruding the melt obtained in step S1 through a T-die and applying it to a casting roller through an air knife for casting, wherein the casting roller temperature is 85-95°C; S3: The cast sheet obtained in step S2 is longitudinally preheated at 105-130°C and then longitudinally stretched, the longitudinal stretching rate is 4-7 times, and the longitudinal stretching temperature is 135-150°C. Then, the longitudinally stretched sheet is transversely preheated at 145-160°C and then transversely stretched, the transverse stretching rate is 7-10 times, and the transverse stretching temperature is 165-180°C, and then enters the shaping zone at 168°C; S4: The film that has been heat-set in step S3 is subjected to corona treatment, winding, aging, and slitting to obtain the polypropylene film for film capacitors.

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