A polypropylene dielectric film based on hierarchical biaxial stretching, a preparation method and an application

Abstract

The invention relates to the field of material technology, and in particular to a polypropylene dielectric film based on graded biaxial stretching, a preparation method and an application thereof, comprising the following steps: step 1: mixing polypropylene and a crystal regulator to obtain a mixture, and subjecting the mixture to melt blending and hot pressing to obtain a sheet to be stretched; wherein, by weight, the polypropylene is 90 to 99.5 parts and the crystal regulator is 0.5 to 10 parts; step 2: subjecting the sheet to be stretched obtained in step 1 to double-stage stretching to obtain a desired dielectric film; the invention utilizes polymer biaxial stretching technology to prepare a polypropylene film material having high breakdown strength and low leakage current density characteristics and suitable for being used as a polymer-based dielectric film without a high dielectric constant filler.

Description

Technical Field

[0001] The present invention relates to the technical field of polypropylene-based thin film materials, and particularly relates to a polypropylene dielectric film based on hierarchical biaxial stretching, a preparation method and an application thereof. Background Art

[0002] Polymer film capacitors have outstanding advantages such as high reliability, high power density and low cost, and are widely used in hybrid electric vehicles, wind power converters in wind power generation, photovoltaic inverters in photovoltaic power generation, new energy grid connection and advanced electronic devices. However, their energy density output is relatively low. Taking the commercial biaxially stretched polypropylene BOPP as an example, its discharge energy density is 3-4 J / cm 3 . Such film capacitors usually need to occupy more than 30% of the overall volume and weight, which is inevitably limited in the miniaturization trend of modern electronic and electrical systems.

[0003] Generally, the discharge energy density of the dielectric material in a polymer film capacitor is related to the relative dielectric constant and the breakdown strength, and the relationship is as follows.

[0004]

[0005] In the formula, U e is the discharge energy density, is the vacuum permittivity, is the relative dielectric constant, E b is the breakdown strength.

[0006] The method of enhancing the polymer dielectric constant is generally to introduce nano-fillers with high dielectric constants such as barium titanate and bismuth titanate. Although this method can increase the dielectric constant, there is still electric field distortion when it is used as a polymer-based dielectric film. At the same time, the introduction of high dielectric constant fillers, such as inorganic ceramic nano-fillers, will cause interface defects and pores, resulting in a significant reduction in the breakdown strength of the material.

[0007] In the prior art, the publication number is CN118206787A, a high-performance biaxially oriented polypropylene-based capacitor film and its preparation method, which discloses that the main raw materials including polypropylene, wide-bandgap two-dimensional nanosheets and narrow-bandgap fillers are mixed as a mixture, and then successively through melt kneading and hot pressing to obtain a sheet to be stretched for film products. Finally, the obtained sheet to be stretched is subjected to biaxial solid-phase stretching treatment to prepare a high-performance biaxially oriented polypropylene-based capacitor film. This preparation method simultaneously introduces wide-bandgap two-dimensional nanosheets and narrow-bandgap fillers. Although the breakdown strength and dielectric properties of the prepared film material are improved to a certain extent through the synergistic effect of the two. However, in this method, under the condition of a thickness of 10 μm, the highest breakdown strength is only 572 MV / m, which cannot meet the current harsh usage conditions.

[0008] This is because the surface energy of such fillers is relatively high and the dispersibility is poor, resulting in disadvantages such as easy agglomeration. Secondly, the interfacial interaction between the filler and the non-polar polymer is usually weak, which will generate various defects and pores. After biaxial stretching, these defects will be further amplified, resulting in a lower breakdown strength. Therefore, compared with increasing the dielectric constant, enhancing the breakdown strength is more effective for improving the energy density output of polymer film capacitors. There are still some intractable problems in introducing two-dimensional inorganic nanosheets as fillers into the polymer matrix: the exfoliation of two-dimensional inorganic nanosheets usually has problems such as cumbersome process and low yield. For example, only gram-scale nanosheets can be obtained through ball milling for 8 h under laboratory conditions, making this technology usually unable to meet industrial applications; secondly, increasing the breakdown strength by adding fillers usually requires the introduction of a high content of fillers, which is not suitable for the preparation of biaxially stretched films because there is a serious modulus mismatch problem between the inorganic filler and the polymer matrix. During the biaxial stretching process, there is a serious modulus mismatch problem between the high-modulus inorganic filler and the low-modulus polymer, resulting in significant interfacial stress and deformation, causing voids, and ultimately leading to the rupture of the film under the condition of rapid biaxial stretching. Summary of the Invention

[0009] The present invention provides a polypropylene dielectric film based on hierarchical biaxial stretching, a preparation method and an application in view of the problems existing in the prior art.

[0010] The technical solution adopted by the present invention is: a preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching, comprising the following steps:

[0011] Step 1: Mix polypropylene and a crystal regulator to obtain a mixture, and the mixture is melt-blended and hot-pressed to obtain a sheet to be stretched; wherein, by mass, polypropylene is 90-99.5 parts and the crystal regulator is 0.5-10 parts;

[0012] Step 2: The sheet to be stretched obtained in Step 1 is subjected to double-stage stretching to obtain the required dielectric film;

[0013] The double-stage stretching process is as follows:

[0014] First-stage synchronous biaxial stretching: preheat for 30 - 90 s at a temperature of 158 - 163 °C, and stretch to 3×3 - 4.5×4.5 times at a stretching rate of 20 - 40% / s;

[0015] Second-stage synchronous biaxial stretching: preheat for 30 - 90 s at a temperature of 151 - 156 °C, and stretch to 5.5×5.5 - 6.5×6.5 times at a stretching rate of 0.7 - 5% / s.

[0016] Furthermore, the average particle size of polypropylene in the sheet to be stretched is ≤6.4 μm.

[0017] Furthermore, the crystal regulator in Step 1 includes organic nucleating agents, nanocellulose, and polystyrene microspheres.

[0018] Furthermore, the organic nucleating agent is one of organic small molecule nucleating agents, organic phosphate nucleating agents, and organic polyamine nucleating agents.

[0019] Furthermore, the organic small molecule nucleating agent is one of N,N'-dicyclohexyl terephthalamide and bis(3,4-dimethyl dibenzylidene) sorbitol;

[0020] The organic phosphate nucleating agent is sodium 2,2'-methylene-bis(4,6-di-tert-butylphenoxy) phosphate;

[0021] The organic polyamine nucleating agent is one of bis(cyclohexylcarbonyl) terephthalamide and sebacamide.

[0022] Furthermore, the melt blending in Step 1 is carried out by melt blending using a twin-screw extruder, with a melting temperature of 180 - 200 °C and a screw speed of 20 - 80 rpm.

[0023] Furthermore, the hot pressing in Step 1 is carried out using a flat vulcanizing machine.

[0024] Furthermore, during the hot pressing process, the hot pressing pressure is 8 - 10 MPa, the cold pressing pressure is 2 - 3 MPa, the upper and lower plate temperatures are 185 - 195 °C respectively. After preheating for 3 - 5 min, hot press for 4 - 5 min and cold press for 3 - 5 min.

[0025] A polypropylene dielectric film based on graded biaxial stretching, wherein the dielectric film has a thickness of 9 μm, a breakdown strength of 821 MV / m to 901 MV / m, and a leakage current density of 1.1×10 -8 A / cm -2 ~1.5×10 -8 A / cm -2 .

[0026] An application of a graded biaxially stretched polypropylene dielectric film, wherein the film is used in a capacitor.

[0027] The beneficial effects of the present invention are:

[0028] (1) The present invention introduces a crystallization regulator to induce an increase in crystal nucleation sites and a decrease in grain size. The crystal sites are evenly stretched by graded stretching, so that the grains become independent individuals and the grain interfaces increase, which greatly alleviates the grain fragmentation phenomenon and inhibits the generation of defects in the biaxially stretched film. Without sacrificing its dielectric constant, the breakdown strength and insulation capacity are greatly improved;

[0029] (2) The dielectric film obtained by the present invention greatly improves the comprehensive dielectric properties of capacitors prepared from polypropylene-based films, and its energy density is increased from 3.2 J / cm2 of untreated biaxially stretched polypropylene film capacitors to 1.5 J / cm2 of untreated biaxially stretched polypropylene film capacitors. 3 Increased to 10.2J / cm 3 ;

[0030] (3) The preparation method of the present invention is simple in process, easy to operate, and easy to realize industrial production, thus having obvious advantages in commercial promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of polarizing microscope of the sheets to be stretched obtained in Examples 1 and 2 of the present invention and Comparative Examples 1 and 6.

[0032] Figure 2 Schematic diagram of the grain size curve of the sheet to be stretched obtained in Examples 1 and 2 of the present invention and Comparative Examples 1 and 6.

[0033] Figure 3 This is a schematic diagram of the cross-sectional thickness of the film obtained in Example 1 of the present invention.

[0034] Figure 4 This is a physical picture of the film obtained in Example 1 of the present invention.

[0035] Figure 5 It is a schematic diagram of the breakdown strength curve of the film obtained in Example 1 of the present invention and Comparative Examples 1 to 3.

[0036] Figure 6This is the leakage current density curve graph of the samples obtained in Example 1 and Comparative Examples 1 and 6 of the present invention.

[0037] Figure 7 This is the atomic force microscope image of the thin films obtained in Example 1, Example 2 and Comparative Examples 1 and 6 of the present invention.

[0038] Figure 8 This is a schematic diagram of the energy density curve of the thin films obtained in Example 1 and Comparative Example 1 of the present invention. Detailed implementation manners

[0039] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation manners.

[0040] A preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching includes the following steps:

[0041] Step 1: Mix polypropylene and a crystal regulator to obtain a mixed material, and subject the mixed material to melt blending and hot pressing to obtain a sheet to be stretched; wherein, by mass, polypropylene is 90 - 99.5 parts and the crystal regulator is 0.5 - 10 parts; the average particle size of polypropylene in the sheet to be stretched is ≤ 6.4 μm.

[0042] The crystal regulator includes an organic nucleating agent, nano-cellulose, and polystyrene microspheres. The organic nucleating agent is one of an organic small molecule nucleating agent, an organic phosphate nucleating agent, and an organic polyamine nucleating agent. The organic small molecule nucleating agent is one of N,N'-dicyclohexyl terephthalamide and bis(3,4-dimethyl dibenzylidene) sorbitol; the organic phosphate nucleating agent is sodium 2,2'-methylenebis(4,6-di-tert-butylphenoxy) phosphate; the organic polyamine nucleating agent is one of bis(cyclohexylcarbonyl) terephthalamide and sebacamide.

[0043] Among them, conventional homopolypropylene industrial raw materials can be used for polypropylene. The crystal regulator is a substance that can effectively reduce the crystal grain size of polypropylene, and the crystal regulator needs to have good compatibility with polypropylene. In the molten processing temperature range (above 180 °C), the crystal regulator can serve as a crystal nucleation site, improve the crystallization rate of polypropylene, and thus effectively inhibit the growth of crystals. Although inorganic nucleating agents such as talc powder, silica, and mica also have the effect of promoting crystallization and reducing the crystal grain size, they have poor compatibility with polypropylene and are prone to agglomeration, and film rupture is extremely likely to occur during biaxial stretching due to modulus mismatch. The specific selection and addition amount of the crystal regulator should be based on the average crystal grain size of the polypropylene material obtained by melt blending and hot pressing not being greater than 6.4 μm. Before or after mixing and preparing the materials, the residual moisture and ash in the raw materials can be removed as much as possible by heating, drying, and heat preservation treatment to avoid adverse effects.

[0044] Melt blending is a conventional process for polypropylene-based composite materials with polypropylene as the matrix raw material, such as twin-screw blending extrusion or melt kneading blending. In the present invention, twin-screw extrusion is used for melt blending extrusion, with a melting temperature of 180 - 200 °C and a screw rotation speed of 20 - 80 rpm.

[0045] Hot pressing forming is carried out using a flat vulcanizing machine. During the hot pressing forming process, the hot pressing pressure is 8 - 10 MPa, the cold pressing pressure is 2 - 3 MPa, the temperatures of the upper and lower plates are 185 - 195 °C respectively. After preheating for 3 - 5 min, hot pressing is carried out for 4 - 5 min, and cold pressing is carried out for 3 - 5 min.

[0046] Step 2: The sheet to be stretched obtained in Step 1 is subjected to two-stage stretching to obtain the required dielectric film.

[0047] The two-stage stretching process is as follows:

[0048] First-stage synchronous biaxial stretching: Preheat at a temperature of 158 - 163 °C for 30 - 90 s, and stretch to 3×3 - 4.5×4.5 times at a stretching rate of 20 - 40% / s.

[0049] During the first stretching process, when the temperature is too high, it will cause the polypropylene to enter the melting temperature range, and the crystalline region will melt and become in a molten state, making it impossible to form a film. At a lower stretching temperature, the molecular chains and the crystalline region cannot respond to the rapid stretching rate, which exacerbates local stress concentration and structural damage, and its film-forming property is greatly reduced. Any temperature within the above range is acceptable, such as 158 °C, 159 °C, 160 °C, 161 °C, 162 °C, 163 °C, or even any point value. However, if it exceeds this range, the effect of the present invention cannot be achieved.

[0050] The high stretching rate in the first stretching can ensure the maximum extension and dispersion of the crystal regions and improve the orientation degree of the film. However, if the stretching rate exceeds the response limit of the polypropylene molecular chains, film rupture will also occur. Any stretching rate within the above range is acceptable. For example, 20 % / s, 21 % / s, 22 % / s, 23 % / s, 24 % / s, 25 % / s, 26% / s, 27 % / s, 28 % / s, 29 % / s, 30 % / s, 31 % / s, 32 % / s, 33 % / s, 34 % / s, 35 % / s, 36 % / s, 37% / s, 38 % / s, 39 % / s, 40 % / s or any point value is acceptable. However, if it exceeds this range, the effect of the present invention cannot be achieved.

[0051] Second-stage synchronous biaxial stretching: Preheat at a temperature of 151 - 156 °C for 30 - 90 s, and stretch to 5.5×5.5 - 6.5×6.5 times at a stretching rate of 0.7 - 5% / s.

[0052] For the second stretching, the low stretching temperature during low-temperature stretching reduces the activation energy of molecular chains, inhibits the flow of molecular chains, and ensures that a high degree of orientation can still be achieved at a low stretching rate. However, if the stretching temperature is too low, the degree of softening of molecular chains will be low, and the film is extremely likely to rupture under the stretching stress. For example, 151 °C, 152 °C, 153 °C, 154 °C, 155 °C, 156 °C or any point value can be selected, but if it exceeds this range, the effects of the present invention cannot be achieved.

[0053] For the second stretching, the low stretching rate allows sufficient time for molecular chains and crystal regions to orient and rearrange, avoiding chain breakage or non-uniform orientation caused by rapid deformation. The slow stretching process is the key to avoiding crystal fragmentation. The preferred stretching rate range is 0.7-5% / s. For example, 0.7 % / s, 1 % / s, 1.5 % / s, 2 % / s, 2.5 % / s, 3 % / s, 3.5 % / s, 4 % / s, 4.5 % / s, 5 % / s or any point value can be selected, but if it exceeds this range, the effects of the present invention cannot be achieved.

[0054] The thickness of the polymer films obtained in all the following examples is 9 μm, and the thickness of the film can be adjusted by adjusting the thickness of the sheet to be stretched and the stretching ratio.

[0055] The raw material polypropylene (HC300BF) used in the following examples is a commercial product purchased from Borealis, Austria. The sorbitol nucleating agent is purchased from Qianhai Jishengya Technology Co., Ltd., China. The nanocellulose is purchased from Aladdin Biochemical Technology Co., Ltd.

[0056] The breakdown strength test method is as follows:

[0057] The breakdown strength of the sample is characterized by a high-voltage tester (19,057-20) with a DC voltage; the voltage increase rate is 500 V / s. At least 10 measurements were made on each sample for Weibull statistical analysis.

[0058] The leakage current test method is as follows:

[0059] The leakage current test was carried out on a Premier II ferroelectric test system. A hole with a diameter of 3 mm was used to sputter gold on the surface of the sample to form an electrode, and then the test was carried out at a frequency of 10 Hz.

[0060] Example 1

[0061] A preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching includes the following steps:

[0062] Step 1: Weigh 99 parts of polypropylene and 1 part of di(3,4-dimethyl dibenzylidene) sorbitol by weight, mix the above materials for preparation, and use them as the mixed material.

[0063] The mixed material is melt-blended and extruded by a twin-screw extruder. The temperatures of each zone are set at 140 °C, 180 °C, 190 °C, 190 °C, 190 °C, and 170 °C respectively, and the screw speed is 50 rpm. Then, hot pressing is carried out by a flat vulcanizing machine. Among them, the hot pressing pressure is 10 MPa, the cold pressing pressure is 3 MPa, the temperatures of the upper and lower plates are 190 °C respectively. After preheating for 5 min, hot pressing is carried out for 5 min, and cold pressing is carried out for 3 min.

[0064] Step 2: The sheet to be stretched obtained in Step 1 is subjected to two-stage stretching to obtain the required dielectric film.

[0065] The two-stage stretching process is as follows:

[0066] First-stage synchronous biaxial stretching: Preheat at 160 °C for 60 s, and stretch to 4.5×4.5 times at a stretching rate of 40 % / s to obtain the polypropylene film after the first biaxial stretching.

[0067] Second-stage synchronous biaxial stretching: Preheat at 155 °C for 60 s, and stretch to 6.0×6.0 times at a stretching rate of 2 % / s. After cooling, a polypropylene dielectric film is obtained, and its thickness is about 9 μm. The SEM diagram of its cross-section is as Figure 3 shown, and the physical diagram is as Figure 4 shown.

[0068] Example 2

[0069] Other steps of this example are the same as those of Example 1, except that in Step 1, 0.5 part of di(3,4-dimethyl dibenzylidene) sorbitol is used.

[0070] Example 3

[0071] Other steps of this example are the same as those of Example 1, except that in Step 1, 5 parts of nanocellulose are used to replace di(3,4-dimethyl dibenzylidene) sorbitol.

[0072] Example 4

[0073] Other steps of this example are the same as those of Example 1, except that in Step 1, 10 parts of polystyrene microspheres are used to replace di(3,4-dimethyl dibenzylidene) sorbitol.

[0074] Example 5

[0075] The other steps of this example are the same as those of Example 1, except that in step 1, 1.5 parts of sodium 2,2'-methylenebis(4,6-di-tert-butylphenoxy)phosphate are used instead of di(3,4-dimethyl dibenzylidene)sorbitol.

[0076] Example 6

[0077] The other steps of this example are the same as those of Example 1, except that in step 1, 1 part of sebacamide is used instead of di(3,4-dimethyl dibenzylidene)sorbitol.

[0078] Example 7

[0079] The other steps of this example are the same as those of Example 1, except that the primary synchronous stretching conditions in step 2 are as follows:

[0080] Preheat at 158 °C for 90 s, and stretch to 4.0×4.0 times at a stretching rate of 20% / s.

[0081] Example 8

[0082] The other steps of this example are the same as those of Example 1, except that the primary synchronous stretching conditions in step 2 are as follows:

[0083] Preheat at 165 °C for 30 s, and stretch to 3.0×3.0 times at a stretching rate of 30% / s.

[0084] Example 9

[0085] The other steps of this example are the same as those of Example 1, except that the secondary synchronous stretching conditions in step 2 are as follows:

[0086] Preheat at 151 °C for 90 s, and stretch to 5.5×5.5 times at a stretching rate of 0.7% / s.

[0087] Example 10

[0088] The other steps of this example are the same as those of Example 1, except that the secondary synchronous stretching conditions in step 2 are as follows:

[0089] Preheat at 156 °C for 30 s, and stretch to 6.5×6.5 times at a stretching rate of 5% / s.

[0090] Comparative Example 1

[0091] The other steps of this comparative example are the same as those of Example 1, except that no nucleating agent is used.

[0092] Comparative Example 2

[0093] In Comparative Example 1, other steps were the same as those in Example 1, except that for the primary synchronous biaxial stretching and the secondary synchronous biaxial stretching, preheating and stretching were both carried out at 155 °C.

[0094] Comparative Example 3

[0095] In Comparative Example 1, other steps were the same as those in Example 1, except that for the primary synchronous biaxial stretching and the secondary synchronous biaxial stretching, preheating and stretching were both carried out at 160 °C.

[0096] Comparative Example 4

[0097] In Comparative Example 1, other steps were the same as those in Example 1, except that for the primary synchronous biaxial stretching and the secondary synchronous biaxial stretching, stretching was carried out at a stretching rate of 2% / s.

[0098] Comparative Example 5

[0099] In Comparative Example 1, other steps were the same as those in Example 1, except that for the primary synchronous biaxial stretching and the secondary synchronous biaxial stretching, stretching was carried out at a stretching rate of 40% / s.

[0100] Comparative Example 6

[0101] In Comparative Example 1, other steps were the same as those in Example 1, except that the amount of 1,3:2,4 - bis - (3,4 - dimethylbenzylidene)sorbitol in Step 1 was 0.1 part.

[0102] The films obtained in Examples 1 - 4 and Comparative Examples 1 - 6 were tested, and the results are shown in Table 1.

[0103] Table 1. Properties of the films obtained in Examples 1 - 4 and Comparative Examples 1 - 6

[0104]

[0105] As can be seen from Table 1, the breakdown strength of the films obtained in Examples 1 - 4 was significantly higher than that of the comparative examples, and the leakage current density was significantly lower than that of the comparative examples. This shows that the performance of the films obtained in the examples of the present invention has been significantly improved.

[0106] Figure 1 Polarizing microscope images of the sheet materials to be stretched obtained in Examples 1 and 2 and Comparative Examples 1 and 6 are shown. As can be seen from the figures, the particle size of polypropylene in the sheet materials to be stretched obtained in Examples 1 and 2 is significantly smaller than that in Comparative Examples 1 and 6. Among them, 0%-160-155 is the result of Comparative Example 1, 0.1%-160-155 is the result of Comparative Example 6, 0.5%-160-155 is the result of Example 2, and 1%-160-155 is the result of Example 1.

[0107] Figure 2Schematic diagram of the comparison of the grain sizes of the to-be-stretched sheets obtained in Example 1, Example 2, Comparative Example 1, and Comparative Example 6. It can be seen from the figure that Figure 2 the result of Figure 1 is the same as

[0108] Figure 5 Breakdown strength curve diagram of the films obtained in Example 1 and Comparative Examples 1, 2, and 3. It can be seen from the figure that the trend of the results is the same as that in Table 1. Among them, 0%-160-155 is the result of Comparative Example 1, 0.1%-160-155 is the result of Comparative Example 6, 0.5%-160-155 is the result of Example 2, and 1%-160-155 is the result of Example 1.

[0109] Figure 6 Comparison curve diagram of the leakage current density of the films obtained in Example 1, Example 2, Comparative Example 1, and Comparative Example 6. It can be seen from the figure that the results are consistent with the test results in Table 1. Among them, 0%-160-155 is the result of Comparative Example 1, 0.1%-160-155 is the result of Comparative Example 6, 0.5%-160-155 is the result of Example 2, and 1%-160-155 is the result of Example 1.

[0110] Figure 7 Atomic force microscope topography map of the films obtained in Example 1, Example 2, Comparative Example 1, and Comparative Example 6. It can be seen from the figure that after stretching, the large grains in the films obtained in Comparative Example 1 and Comparative Example 6 are broken into small grains, while the grains in the films obtained in Example 1 and Example 2 are relatively complete. Among them, 0%-160-155 is the result of Comparative Example 1, 0.1%-160-155 is the result of Comparative Example 6, 0.5%-160-155 is the result of Example 2, and 1%-160-155 is the result of Example 1.

[0111] Figure 8 Schematic diagram of the energy density of the films obtained in Example 1 and Comparative Example 1. It can be seen from the figure that the energy density of the film obtained in Example 1 is significantly higher than that of the film obtained in Comparative Example 1. Among them, PP is the result of Comparative Example 1, and PP-1% is the result of Example 1.

[0112] The present invention introduces a crystallization regulator, which plays a crucial inhibitory role in crystal fragmentation, the resulting increase in defects, and the increase in free volume during the biaxial stretching process. The original polypropylene is characterized by large α-spherulites composed of closely crossed and frequently branched lamellae. The short crossed lamellae at the branch nodes will generate local stress concentration, forming a relatively large free volume. Under biaxial stress, these weak points will break into individual small lamellae, thus destroying the local potential field and increasing the free volume. Under the action of a high electric field, electrons will accelerate within the free volume and collide with the molecular chains at the edge of the free volume, thereby triggering local dielectric breakdown. After introducing the crystallization regulator, the increase in nucleation density and the acceleration of crystallization kinetics lead to rapid crystal nucleation, greatly reducing the initial grain size and making the α-spherulites more compact, thereby inhibiting crystal fragmentation during the biaxial stretching process and reducing the formation of a relatively large free volume. The small free volume regions between adjacent lamellae act as shallow electron traps, restricting the transport of carriers, thereby significantly improving the dielectric stability of the film.

[0113] It can be seen from the comparison of the above Comparative Examples 1, 6 and Examples that as the content of the crystallization regulator increases, the number of nucleation sites increases, the nucleation density increases, and the crystal growth is accelerated. Through observation with a polarized light microscope and corresponding crystal particle size statistics, the results show that the grain size decreases with the content of the crystallization regulator ( Figure 1 and Figure 2 ). Further summarization reveals that there is a threshold effect on the influence of grain size on the properties of polypropylene dielectric films. When the grain size is smaller than a certain specific value, crystal fragmentation is effectively inhibited, and the breakdown strength and insulation ability are greatly improved without sacrificing its dielectric constant.

[0114] During the preparation process of dielectric films, stretching is a crucial step. In traditional stretching processes, when the grain size of polypropylene does not reach the special threshold range (≤6.4 μm), under the action of high stretching speed and stretching force, the large grains in its matrix often deform and break into small grains, as Figure 7As shown. Once the grain size before stretching exceeds 6.4 μm, the larger initial lamellae after stretching will promote obvious lamella fragmentation and rearrangement, resulting in obvious stacking of short lamellae, along with an increase in defects and free volume. This provides sufficient acceleration space for electrons to collide with molecular chains, thus triggering local dielectric failure. The present invention adopts differential-temperature hierarchical biaxial stretching at different speeds. The polymer crystals soften when approaching the melting temperature, thus greatly reducing the modulus. The first biaxial stretching at a high temperature can smoothly complete the extension and dispersion of the crystal regions in a low-modulus state, making the grains become independent individuals and increasing the grain boundaries. In addition, the stretching speed in this stage is extremely fast, thus ensuring a high degree of orientation of the biaxially stretched film. At the same time, based on the greatly reduced grain size, it is ensured that the α-spherulites have few branches and small radii before stretching, greatly reducing the possibility of grain fragmentation at a high stretching rate. In the low-speed stretching stage of the second biaxial stretching, the lower stretching rate can effectively reduce the internal stress accumulation in the uniformly dispersed crystalline regions, contribute to the formation of a regular and complete crystal structure, further alleviate the grain fragmentation phenomenon, and inhibit the generation of defects in the biaxially stretched film. The decrease in the stretching temperature reduces the activation energy, making the molecular chains easier to orient during the stretching process to ensure the orientation of the biaxially stretched film.

[0115] The dielectric film obtained by the method of the present invention has a breakdown strength and energy density much higher than those of the dielectric film parts of traditional polymer film capacitors. Under the condition of a thickness of 9 μm, the breakdown strength reaches 958 MV / m, and the leakage current density at an electric field strength of 200 MV / m is at most 1.5×10 -8 A / cm -2 . The present invention uses the polymer biaxial stretching technology to prepare a polypropylene film material with high breakdown strength, low leakage current density characteristics and suitable as a polymer-based dielectric film without high-dielectric-constant fillers.

Claims

1. A preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching, characterized in that, It includes the following steps: Step 1: Mix polypropylene and a crystal regulator to obtain a mixture, and the mixture is melt-blended and hot-pressed to obtain a sheet to be stretched; by mass fraction, polypropylene is 90-99.5 parts, and the crystal regulator is 0.5-10 parts; the crystal regulator is one of N,N'-dicyclohexyl terephthalamide, bis(3,4-dimethyl dibenzylidene)sorbitol, sodium 2,2'-methylenebis(4,6-di-tert-butylphenoxy)phosphate, bis(cyclohexylcarbonyl)terephthalamide, sebacamide, nanocellulose, and polystyrene microspheres; Step 2: The sheet to be stretched obtained in Step 1 is subjected to two-stage stretching to obtain the required dielectric film; The two-stage stretching process is as follows: First-stage synchronous biaxial stretching, preheating at a temperature of 158-163 °C for 30-90 s, and stretching at a stretching rate of 20-40 % / s to 3×3-4.5×4.5 times; Second-stage synchronous biaxial stretching, preheating at a temperature of 151-156 °C for 30-90 s, and stretching at a stretching rate of 0.7-5 % / s to 5.5×5.5-6.5×6.5 times.

2. The preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching according to claim 1, characterized in that, The average particle size of polypropylene in the sheet to be stretched is ≤6.4 μm.

3. The preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching according to claim 1, wherein, In Step 1, the melt-blending is carried out by a twin-screw extruder, the melting temperature is 180-200 °C, and the screw speed is 20-80 rpm.

4. The preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching according to claim 1, wherein, In Step 1, the hot-pressing is carried out using a flat vulcanizing machine.

5. The preparation method of a polypropylene dielectric film based on hierarchical biaxial stretching according to claim 4, wherein During the hot-pressing process, the hot-pressing pressure is 8-10 MPa, the cold-pressing pressure is 2-3 MPa, the upper and lower plate temperatures are 185-195 °C respectively. After preheating for 3-5 min, hot-press for 4-5 min, and cold-press for 3-5 min.

6. The polypropylene dielectric film based on hierarchical biaxial stretching obtained by the preparation method according to any one of claims 1 to 5, characterized in that, When the thickness of the dielectric film is 9 μm, the breakdown strength is 821 MV / m to 901 MV / m, and the leakage current density at an electric field strength of 200 MV / m is 1.1×10 -8 A / cm -2 ~1.5×10 -8 A / cm -2 .

7. The application of a polypropylene dielectric film based on hierarchical biaxial stretching according to claim 6, characterized in that, The film is used in a capacitor.

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