Polypropylene-based composite film as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of polymer-based composite dielectric materials, and particularly relates to a polypropylene-based composite film and a preparation method and application thereof. According to the composite film obtained by the preparation method provided by the invention, polypropylene is taken as a matrix, the polyvinylidene fluoride-hexafluoropropylene copolymer is taken as a dispersion phase, and the composite film forms a directionally arranged fiber embedded structure by optimizing uniaxial drawing process parameters. The prepared composite film has the advantages of high dielectric constant, low dielectric loss, high energy storage density and the like, and the preparation process is simple, easy to operate, low in preparation cost and suitable for preparing high-performance film capacitors and high-energy-storage equipment.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polymer-based composite dielectric materials, and particularly relates to a polypropylene-based composite film and a preparation method and application thereof. BACKGROUND

[0002] Thin film capacitors are widely used in key fields such as power electronics, frequency converters, new energy vehicles and rail transit due to their excellent insulation performance, non-polar characteristics and high reliability. Especially in direct current support, alternating current-direct current conversion and filtering applications, high-performance capacitors are crucial to the stable operation of the system. In recent years, with the rapid development of high-end equipment, new energy systems and smart grid technologies, energy storage capacitors have higher requirements for dielectric materials in terms of energy density, insulation strength and long-term stability.

[0003] In the existing dielectric material system, polypropylene (PP) film is widely used in commercial thin film capacitors due to its low loss, high breakdown field strength and good processability. However, its intrinsic dielectric constant is relatively low (about 2.2), which limits the further improvement of its energy density. Among many polar polymers, polyvinylidene fluoride-hexafluoropropylene copolymer [P(VDF-HFP)] is of interest due to its high dielectric constant and good electrical insulation. Therefore, composite materials become a solution. Combining high-polarity high-energy storage organic materials with high-insulation easy-to-process polypropylene materials can improve the overall electrical performance and breakdown strength of the material while maintaining good processability. However, traditional composite film preparation methods usually involve multiple steps such as solution mixing and blending calendering. These methods are very complex, and different materials have large polarity differences and poor compatibility, which can easily lead to phase separation or interface defects, thereby affecting the macroscopic electrical performance of the composite material. Therefore, developing a polypropylene-based composite dielectric film preparation method with uniform structure, simple process and excellent performance is an important direction in the current research of dielectric film materials. In particular, how to further improve the dielectric constant and energy density while maintaining high breakdown strength has become a key technical problem that needs to be solved. SUMMARY

[0004] The purpose of the present application is to provide a polypropylene-based composite film and a preparation method and application thereof. The method provided by the present application can realize one-step film formation, avoid complex multi-step operations, realize an efficient and simple preparation process, reduce production cost and time, and improve industrial production efficiency.

[0005] To achieve the above-mentioned purpose, the present application provides the following technical solutions: The present application provides a preparation method of a polypropylene-based composite film, comprising the following steps: Melt and extrude polypropylene and polyvinylidene fluoride-hexafluoropropylene copolymer to obtain a composite medium casting sheet; Uniaxially stretch the composite medium casting sheet to obtain the polypropylene-based composite film. The conditions of the uniaxial stretching include that the speed of the differential roller is 500-700 mm / min, the torque of the traction roller is 40-60 N·m, and the stretching ratio is 30-60.

[0006] Preferably, the particle size of the polyvinylidene fluoride-hexafluoropropylene copolymer is 1-3 μm.

[0007] Preferably, the mass ratio of difluoroethylene to hexafluoropropylene in the polyvinylidene fluoride-hexafluoropropylene copolymer is 85:15-90:10.

[0008] Preferably, the mass percentage content of the polyvinylidene fluoride-hexafluoropropylene copolymer in the polypropylene-based composite film is 10-30%.

[0009] Preferably, the conditions of the melting include that the melting is performed by using a double-screw extruder, the melting temperature is 200-220℃, the melting time is 10-15 min, the stirring speed of the double-screw extruder is 10-20 rpm, and the maximum torque is 10-12 N·m.

[0010] Preferably, the conditions of the extrusion include that the extrusion is performed by using a double-screw extruder, the extrusion speed is 100-120 rpm, the extrusion port temperature is 200-220℃, and the maximum torque is 10-12 N·m.

[0011] Preferably, after the extrusion, the obtained sheet is further cooled.

[0012] The application further provides a polypropylene-based composite film prepared by the preparation method.

[0013] Preferably, the thickness of the polypropylene-based composite film is 10-20 μm, and the deviation of the thickness is within ±200 nm.

[0014] The application further provides an application of the polypropylene-based composite film in preparing a film capacitor.

[0015] The application provides a preparation method of a polypropylene-based composite film, comprising the following steps: melting and extruding polypropylene and polyvinylidene fluoride-hexafluoropropylene copolymer to obtain a composite medium casting piece; uniaxially stretching the composite medium casting piece to obtain the polypropylene-based composite film; and the uniaxial stretching conditions comprise that the speed of a differential roller is 500-700 mm / min, and the torque of a traction roller is 40-60 N·m; and the stretching ratio is 30-60.

[0016] Compared with the prior art, the application has the following beneficial effects: (1) The application significantly improves the dielectric constant and breakdown strength of the polypropylene-based composite film by optimizing the formula and preparation process technology; the uniaxial stretching process ensures that the polar filler forms a fiber type embedded structure in the polypropylene matrix and is uniformly distributed. While maintaining high insulation and low loss, the energy storage density of the composite film is effectively improved.

[0017] (2) The application effectively simplifies the preparation process of the composite medium film by using the one-step film forming process of melting and extrusion combined with uniaxial stretching, overcomes the problems of complicated steps and low efficiency of the traditional method, significantly improves the preparation efficiency and reduces the overall cost. The process is simple to operate, has strong parameter adjustability and good repeatability, and is suitable for high-performance thin film capacitors such as filter, power electronics and new energy equipment to support the application requirements of lightweight, miniaturization and high energy storage of capacitors. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 A polypropylene-based composite film cross-section morphology diagram prepared for Example 3; Figure 2 A dielectric constant and dielectric loss curve diagram of the polypropylene-based composite film obtained in the examples and comparative examples at a frequency of 10-10 6 Hz; Figure 3 A breakdown strength Weibull distribution diagram of the polypropylene-based composite film obtained in the examples and comparative examples; Figure 4 A discharge energy density and charge-discharge efficiency diagram of the polypropylene-based composite film obtained in the examples and comparative examples. DETAILED DESCRIPTION

[0019] The application provides a preparation method of a polypropylene-based composite film, comprising the following steps: melting and extruding polypropylene and polyvinylidene fluoride-hexafluoropropylene copolymer to obtain a composite medium casting piece; uniaxially stretching the composite medium casting piece to obtain the polypropylene-based composite film; The single-axis stretching condition comprises that the speed of the differential roller is 500-700 mm / min, the torque of the traction roller is 40-60 N*m, and the stretching ratio is 30-60.

[0020] The polypropylene and the polyvinylidene-hexafluoropropylene copolymer are melted and extruded to obtain a composite medium casting sheet.

[0021] In the application, the polypropylene is preferably a domestic electrical-grade ultra-clean polypropylene pellet produced by PetroChina Lanzhou Petrochemical.

[0022] In the application, the mass percentage content of the polyvinylidene-hexafluoropropylene copolymer in the polypropylene-based composite film is preferably 10-30%, and can be 10%, 15%, 20%, 25%, or 30%.

[0023] In the application, the melting condition preferably comprises that the melting is performed by using a double-screw extruder, the melting temperature is 200-220 DEG C, and can be 200 DEG C, 210 DEG C, or 220 DEG C; the melting time is 10-15 min, and can be 10 min, 11 min, 12 min, 13 min, 14 min, or 15 min; the stirring speed of the double-screw extruder is 10-20 rpm, and can be 10 rpm, 12 rpm, 14 rpm, 16 rpm, 18 rpm, or 20 rpm; and the maximum torque is 10-12 N*m, and can be 10 N*m, 11 N*m, or 12 N*m.

[0024] In the application, the extrusion condition preferably comprises that the extrusion is performed by using a double-screw extruder, the extrusion speed is 100-120 rpm, and can be 100 rpm, 110 rpm, or 120 rpm; the extrusion temperature is 200-220 DEG C, and can be 200 DEG C, 210 DEG C, or 220 DEG C; and the maximum torque is 10-12 N*m, and can be 10 N*m, 11 N*m, or 12 N*m.

[0025] In the application, the double-screw extruder is preferably a Dutch Xplore double-screw extruder.

[0026] In the application, after the extrusion, the application further preferably comprises cooling the extruded sheet, and the cooling mode is preferably air knife cooling.

[0027] After obtaining the composite medium cast sheet, the composite medium cast sheet is uniaxially stretched to obtain the polypropylene-based composite film.

[0028] In the present application, the conditions of the uniaxial stretching include that the speed of the differential roller is 500-700 mm / min, and specifically can be 500 mm / min, 600 mm / min or 700 mm / min; the torque of the traction roller is 40-60 N·m, and specifically can be 40 N·m, 50 N·m or 60 N·m; and the stretching ratio is 30-60, and specifically can be 30, 40, 50 or 60.

[0029] In the present application, after the uniaxial stretching, the obtained film is preferably wound up.

[0030] In the present application, the uniaxial stretching preferably adopts the uniaxial stretching equipment of the CPFL model of the Netherlands Xplore.

[0031] The present application also provides a polypropylene-based composite film prepared by the preparation method, which comprises a polypropylene matrix and a polyvinylidene-hexafluoropropylene copolymer filled in the polypropylene matrix. In the present application, the polyvinylidene-hexafluoropropylene copolymer preferably forms a fiber type embedded structure in the polypropylene-based composite film.

[0032] In the present application, the thickness of the polypropylene-based composite film is preferably 10-20 μm, and the deviation of the thickness is preferably within ±200 nm.

[0033] The present application also provides the application of the polypropylene-based composite film in the preparation of a film capacitor.

[0034] Unless otherwise specified, the materials and equipment used in the present application are commercially available in the art.

[0035] The technical solutions in the present application will be clearly and completely described below by combining with the embodiments in the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0036] In the following embodiments, the polypropylene is a domestic electrical grade ultra-clean polypropylene pellet produced by PetroChina Lanzhou Petrochemical; The particle size of the polyvinylidene-hexafluoropropylene copolymer is 1-3 μm; the mass ratio of difluoroethylene to hexafluoropropylene in the polyvinylidene-hexafluoropropylene copolymer is 85:15; and the polyvinylidene-hexafluoropropylene copolymer is a pellet of the American Poly-K company; The twin-screw extruder is a Xplore twin-screw extruder from the Netherlands. The uniaxial stretching uses a uniaxial stretching device from the Netherlands with the model CPFL.

[0037] Example 1 9 g of polypropylene and 1 g of polyvinylidene fluoride-hexafluoropropylene copolymer are simultaneously added to the twin-screw extruder for melt blending, preheating and melting at 200℃ for 10 min, and in the process, the twin-screw is stirred at a speed of 10 rpm, a maximum torque of 10 N·m; After the melting is completed, the extrusion port of the twin-screw extruder is opened, and the molten composite material is extruded, the speed of the twin-screw is 100 rpm, the temperature of the extrusion port is 210℃, the maximum torque is 10 N·m, and the molten composite material continuously extrudes from the mold at the extrusion port, and after being cooled by the air knife at the extrusion port, a composite medium casting piece with a thickness of about 600 μm is obtained. The continuously extruded composite medium casting piece is guided to the uniaxial stretching device for uniaxial stretching and winding, the composite medium casting piece is pulled to the differential speed roller for longitudinal stretching, the speed of the differential speed roller is 500 mm / min, and then it is wound on the traction roller with a torque of 50 N·m, and the stretching ratio is controlled to be 60, to obtain a polypropylene-based composite film with a thickness of 10 μm±200 nm.

[0038] Example 2 The polypropylene-based composite film is prepared in the manner of Example 1, except that 8.5 g of polypropylene and 1.5 g of polyvinylidene fluoride-hexafluoropropylene copolymer are added, and the mass ratio of polyvinylidene fluoride-hexafluoropropylene is adjusted to 15%.

[0039] Example 3 The polypropylene-based composite film is prepared in the manner of Example 1, except that 8 g of polypropylene and 2 g of polyvinylidene fluoride-hexafluoropropylene copolymer are added, and the mass ratio of polyvinylidene fluoride-hexafluoropropylene is adjusted to 20%.

[0040] Example 4 The polypropylene-based composite film is prepared in the manner of Example 1, except that 7.5 g of polypropylene and 2.5 g of polyvinylidene fluoride-hexafluoropropylene copolymer are added, and the mass ratio of polyvinylidene fluoride-hexafluoropropylene is adjusted to 25%.

[0041] Example 5 The polypropylene-based composite film is prepared in the manner of Example 1, except that 7 g of polypropylene and 3 g of polyvinylidene fluoride-hexafluoropropylene copolymer are added, and the mass ratio of polyvinylidene fluoride-hexafluoropropylene is adjusted to 30%.

[0042] Comparative Example 1 A polypropylene-based composite film was prepared according to the method in Example 1, except that 10g of polypropylene was added and the mass ratio of polyvinylidene fluoride-hexafluoropropylene was adjusted to 0%, that is, no polyvinylidene fluoride-hexafluoropropylene copolymer was added.

[0043] Performance testing Test Example 1 The cross-sectional morphology of the polypropylene-based composite film prepared in Example 3 was tested using a scanning electron microscope at room temperature, with the transverse direction representing the tensile direction. The cross-sectional morphology of the material is shown below. Figure 1 As shown.

[0044] Depend on Figure 1 It can be seen that the stretched composite dielectric film forms fibers in the stretching direction and exhibits a multilayer structure. This multilayer structure is due to the high orientation under the stretching field. Specifically, the polyvinylidene fluoride-hexafluoropropylene filler is stretched into fibers and uniformly embedded in the polypropylene matrix to form a fiber-embedded structure. The filler diameter is 1~3μm and the distribution is uniform.

[0045] Test Example 2 The dielectric properties of the polypropylene-based composite films prepared in Examples 1-5 and Comparative Example 1 were tested after gold deposition using a small ion sputtering apparatus. The dielectric constant and dielectric loss of the materials at different frequencies were measured using a broadband dielectric spectrometer from Novocontrol (Germany) at room temperature.

[0046] The dielectric constant and dielectric loss of the polypropylene-based composite films prepared in Examples 1-5 and Comparative Example 1 at 1 kHz are shown in Table 1. As can be seen from Table 1, with the increase of the mass fraction of polyvinylidene fluoride-hexafluoropropylene copolymer, the dielectric constant of the composite dielectric film gradually increases, while the dielectric loss factor remains at a low level, exhibiting good dielectric properties. Compared with Comparative Example 1, the uniaxially stretched composite dielectric film prepared by this invention has stable dielectric properties and maintains a consistently low dielectric loss factor, demonstrating excellent performance.

[0047] Examples 1-5 and Comparative Example 1 were in the range of 10-10 6 The dielectric constant and dielectric loss at Hz frequency are shown in the test results. Figure 2 As shown in Table 1. According to Figure 2 It can be seen that the dielectric constant of the polypropylene-based composite film prepared in Example 4 is between 10 and 10. 6 It maintains a stable dielectric strength of 2.99 Hz and has a dielectric loss of less than 0.02 Hz, demonstrating excellent dielectric performance.

[0048] Test Example 3 The polypropylene-based composite films prepared in Examples 1-5 and Comparative Example 1 were subjected to breakdown strength tests after gold plating using a small ion sputtering apparatus. At room temperature, the samples were placed between ball-and-plate electrodes, and a high-voltage withstand tester was used to apply DC voltage to the electrodes. To avoid surface flashover, the samples were placed in silicone oil during the test. The DC voltage ramp rate was 500 V / s, and breakdown was determined when the leakage current reached 50 mA. The results are expressed using a Weibull distribution, as shown in Table 1.

[0049] As shown in Table 1, the breakdown strength of Examples 1-3 is higher than 600 MV / m, demonstrating excellent insulation performance, indicating that the insulation strength of the polypropylene-based composite film prepared in the embodiments of the present invention meets the requirements for use.

[0050] The breakdown strength Weibull distributions of the uniaxially stretched composite dielectric films of Examples 1-5 and Comparative Example 1 are as follows: Figure 3 As shown. From Figure 3 It can be seen that the breakdown performance of Examples 1 to 3 is not significantly different from that of Comparative Example 1, indicating that the uniaxial tensile composite dielectric film prepared by the present invention has good insulation strength.

[0051] Test Example 4 The discharge energy density and charge / discharge efficiency of the polypropylene-based composite films of Examples 1-5 and Comparative Example 1 were tested using a Radiant ferroelectric testing instrument. The samples were placed in silicone oil for testing, and the test results are shown in Table 1. The polypropylene-based composite films prepared in Examples 1-5 showed relatively stable energy storage performance, with charge / discharge efficiencies all greater than 90% at 500 kV / mm. Compared with Comparative Example 1, the discharge energy density of the polypropylene-based composite film gradually increased with the increase of the mass fraction of polyvinylidene fluoride-hexafluoropropylene copolymer, indicating that the polypropylene-based composite film prepared by this invention has good discharge energy density and high charge / discharge efficiency.

[0052] The discharge energy density and charge / discharge efficiency of the polypropylene-based composite films in Examples 1-5 and Comparative Example 1 are as follows: Figure 4 As shown in the figure. The test results show that, compared with the sample of Comparative Example 1, the composite films prepared in Examples 1-5 exhibit higher discharge energy density and charge / discharge efficiency that is always greater than 90%, meeting the requirements of high energy storage applications.

[0053] Table 1 shows the properties of the polypropylene-based composite films obtained in the examples.

[0054] The test results above show that the polypropylene-based composite film provided by the present invention maintains high efficiency and breakdown strength while improving energy density and dielectric constant.

[0055] Although the above embodiments have been described in detail, they are only some embodiments of the present application, not all embodiments. Other embodiments can be obtained under the premise of not being creative according to the above embodiments, and these embodiments all belong to the protection scope of the present application.

Claims

1. A method for producing a polypropylene-based composite film, characterized by, The method comprises the following steps: melting and extruding polypropylene and polyvinylidene fluoride-hexafluoropropylene copolymer to obtain a composite medium casting sheet; uniaxially stretching the composite medium casting sheet to obtain the polypropylene-based composite film; the uniaxial stretching condition comprises that the speed of the differential roller is 500-700 mm / min, and the torque of the traction roller is 40-60 N·m; the stretching ratio is 30-60.

2. The production method according to claim 1, characterized by, The particle size of the polyvinylidene fluoride-hexafluoropropylene copolymer is 1-3 μm.

3. The preparation method according to claim 1, characterized in that, The mass ratio of difluoroethylene to hexafluoropropylene in the polyvinylidene fluoride-hexafluoropropylene copolymer is 85:15-90:

10.

4. The method of claim 1, wherein, The mass percentage content of the polyvinylidene fluoride-hexafluoropropylene copolymer in the polypropylene-based composite film is 10-30%.

5. The preparation method according to claim 1, characterized in that, The melting condition comprises that the melting is performed by using a double-screw extruder, the melting temperature is 200-220 ℃, the melting time is 10-15 min, the stirring speed of the double-screw extruder is 10-20 rpm, and the maximum torque is 10-12 N·m.

6. The method of claim 1, wherein, The extrusion condition comprises that the extrusion is performed by using a double-screw extruder, the extrusion speed is 100-120 rpm, the extrusion port temperature is 200-220 ℃, and the maximum torque is 10-12 N·m.

7. The preparation method according to claim 1, characterized in that, After the extrusion, the method further comprises cooling the extruded sheet.

8. The polypropylene-based composite film prepared by the method according to any one of claims 1 to 7, characterized in that, The polypropylene-based composite film comprises a polypropylene matrix and polyvinylidene fluoride-hexafluoropropylene copolymer filled in the polypropylene matrix.

9. The polypropylene-based composite film according to claim 8, wherein, The thickness of the polypropylene-based composite film is 10-20 μm, and the deviation of the thickness is within ±200 nm.

10. Application of the polypropylene-based composite film in claim 8 or 9 in the preparation of a thin-film capacitor.

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