High energy density bopp composite film and preparation method thereof

Abstract

The present application relates to a kind of high energy density BOPP composite film and its preparation method, belong to dielectric material technical field, solve the low energy density of BOPP film in prior art.The BOPP composite film includes two BOPP film layers and the PVDF layer being arranged between two BOPP film layers, grafting CPP layer is arranged between BOPP film layer and PVDF layer, the grafting monomer of the grafting CPP is at least one of methyl methacrylate, methyl methacrylate trifluoroethyl ester, methyl methacrylate hexafluorobutyl ester, methyl methacrylate twelve fluorine heptyl ester and methyl methacrylate thirteen fluorine octyl ester.The BOPP composite film of the present application has higher energy density.

Description

Technical Field

[0001] This invention relates to the field of dielectric materials technology, and in particular to a high energy density composite thin film and its preparation method. Background Technology

[0002] For dielectric materials, energy density is one of the most critical parameters, and it is positively correlated with dielectric constant and breakdown strength. BOPP (biaxially oriented polypropylene) capacitor film is one of the most widely used dielectric materials on the market, with a breakdown strength as high as 640 MV / m. However, its dielectric constant is only around 2.2, resulting in a relatively low energy density of only 1-1.2 J / cm². 3 The dielectric properties of dielectric capacitors are usually improved by increasing their size and weight, but this greatly limits their industrial production and application. Therefore, improving the energy storage density of dielectric materials to obtain small and lightweight capacitor films is particularly important in industrial production. Summary of the Invention

[0003] Based on the above analysis, the present invention aims to provide a high energy density BOPP composite film and its preparation method to solve the problem of low energy density of existing BOPP films.

[0004] On one hand, the present invention provides a high energy storage density BOPP composite film, the composite film comprising two BOPP film layers and a PVDF layer disposed between the two BOPP film layers, wherein a grafted CPP layer is disposed between the BOPP film layer and the PVDF layer, and the grafting monomer of the grafted CPP is at least one selected from methyl methacrylate, trifluoroethyl methacrylate, hexafluorobutyl methacrylate, dodecafluoroheptyl methacrylate and tridecafluorooctyl methacrylate.

[0005] Preferably, the grafting rate of the grafted CPP is 1-10%, and the chlorination degree of the grafted CPP is 20-40%.

[0006] Preferably, the thickness of the BOPP film layer is 3-20 μm.

[0007] Preferably, the thickness of the PVDF layer is 4-20 μm.

[0008] Preferably, the thickness of each grafted CPP layer is 3-10 μm.

[0009] Preferably, the thickness of the BOPP composite film is 15-50 μm.

[0010] Preferably, the PVDF layer and the grafted CPP layer contain dielectric fillers.

[0011] Preferably, the volume fraction of the dielectric filler in the PVDF layer is 0-80%; the volume fraction of the dielectric filler in each grafted CPP layer is 0-80%.

[0012] Preferably, the dielectric filler is at least one of surface-modified barium titanate, magnesium titanate, bismuth ferrite, lead zirconium titanate, potassium sodium niobate, sodium bismuth titanate and barium strontium titanate.

[0013] In another aspect, the present application provides a preparation method of the high energy storage density BOPP composite film of the present application, which comprises:

[0014] (1) CPP grafting: mixing CPP, solvent 1, grafting monomer and initiator for grafting reaction to obtain grafted CPP;

[0015] (2) Surface modification of dielectric filler: mixing surface modifier, solvent 2 and dielectric filler for surface modification;

[0016] (3) Casting solution preparation: mixing grafted CPP, solvent 1 and dielectric filler to obtain casting solution A; mixing PVDF, solvent DMF and dielectric filler to obtain casting solution B;

[0017] (4) Preparation of BOPP composite film: coating casting solution A and casting solution B on one side of a BOPP film in turn, coating casting solution A on one side of another BOPP film, and then opposing the coated sides of the two BOPP films and hot pressing after the solvent is volatilized to obtain a high energy storage density BOPP composite film.

[0018] Compared with the prior art, the present application can at least achieve one of the following beneficial effects:

[0019] 1. The BOPP composite film of the present application is a BOPP-grafted CPP-PVDF-grafted CPP-BOPP five-layer structure, PVDF is arranged between two layers of BOPP, and grafted CPP is arranged between BOPP and PVDF. On the one hand, grafted CPP is tightly connected with BOPP, and grafted CPP has good compatibility with PVDF, so that the grafted CPP layer and the PVDF layer are tightly connected, the layers are tightly connected without defects, thereby ensuring high dielectric constant and breakdown strength. On the other hand, grafted CPP improves polarity through grafting groups, so that the grafted CPP layer has a high dielectric constant, and the overall BOPP composite film has a high dielectric constant. At the same time, the BOPP composite film has a high breakdown strength through the multi-layer film composite structure. High dielectric constant and breakdown strength make the BOPP composite film have high energy storage density.

[0020] 2. The dielectric constant of the PVDF layer and the grafted CPP layer is further improved by adding dielectric fillers in the PVDF layer and the grafted CPP layer.

[0021] 3. The high energy storage density BOPP composite film also has high tensile strength, elongation at break and charge-discharge efficiency.

[0022] In the present application, the above technical solutions can be combined with each other to realize more preferred combination solutions. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification, or will be understood by implementing the present application. The purposes and other advantages of the present application can be realized and obtained from the contents specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0023] The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated in and constitute a part of this application, illustrate embodiments of the application and are used to explain the principles of the application, but are not intended to limit the scope of the application.

[0024] Figure 1 The high energy storage density BOPP composite film of embodiment 1 of the present application is a cross-section scanning electron microscope image;

[0025] Figure 2 The charge-discharge test diagram of the high energy storage density BOPP composite film product of embodiment 1 of the present application is shown in the figure;

[0026] Figure 3 The energy storage density and charge-discharge efficiency diagram of the high energy storage density BOPP composite film product of embodiment 1 of the present application is shown in the figure. DETAILED DESCRIPTION

[0027] The preferred embodiments of the present application will be described in detail below with reference to the accompanying drawings, which constitute a part of this application, and are used to explain the principles of the embodiments of the present application, but are not intended to limit the scope of the present application.

[0028] The present application provides a high energy storage density BOPP composite film, which comprises two BOPP film layers and a PVDF layer arranged between the two BOPP film layers, and a grafted CPP layer arranged between the BOPP film layer and the PVDF layer, wherein the grafting monomer of the grafted CPP is at least one of methyl methacrylate, trifluoroethyl methacrylate, hexafluorobutyl methacrylate, dodecafluoroheptyl methacrylate and tridecafluorooctyl methacrylate.

[0029] It should be noted that the "high energy storage density" in the present application is relative to the energy storage density of a single layer of BOPP film. The high energy storage density BOPP composite film of the present application is a BOPP-grafted CPP-PVDF-grafted CPP-BOPP five-layer structure, and the energy storage density is higher than that of a single layer of BOPP film.

[0030] The energy storage density formula is W = 1 / 2ε0εE 2 Wherein, ε0 is the vacuum dielectric constant, ε is the dielectric constant, E is the breakdown strength, it can be seen that increasing the dielectric constant or the breakdown strength can increase the energy storage density.

[0031] The BOPP composite film of the present application is a BOPP-grafted CPP-PVDF-grafted CPP-BOPP five-layer structure. On the one hand, the high polarization polymer PVDF is combined with the BOPP film to enhance the polarization intensity of the composite film, thereby increasing the dielectric constant. On the other hand, the grafting CPP has adhesion, the grafting CPP and the BOPP are tightly connected, the grafting CPP and the PVDF have good compatibility after grafting, the grafting CPP layer and the PVDF layer are tightly connected, thereby making each layer tightly connected without defects (if there are defects, they are easily broken down under the condition of an applied electric field, and the air at the defect will cause the dielectric constant of the composite film to decrease), thereby ensuring a high dielectric constant. In addition, the grafting CPP improves the polarity through the grafting group, thereby making the grafting CPP itself have a high dielectric constant, and further making the overall BOPP composite film have a high dielectric constant. Moreover, the BOPP composite film of the present application also has a high breakdown strength through the multi-layer film composite structure. The high dielectric constant and the breakdown strength make the BOPP composite film have a high energy storage density. At the same time, the high energy storage density BOPP composite film of the present application also has a high tensile strength, elongation at break and charge-discharge efficiency.

[0032] In the present application, at least one of methyl methacrylate, trifluoroethyl methacrylate, hexafluorobutyl methacrylate, dodecafluoroheptyl methacrylate and tridecafluorooctyl methacrylate is selected as the grafting monomer. On the one hand, these monomer polymers have stronger polarization than BOPP, which can increase the dielectric constant of CPP itself. On the other hand, these monomer polymers have good compatibility with PVDF, thereby ensuring that the grafting CPP layer and the PVDF layer are tightly combined.

[0033] In the present application, in order to further improve the dielectric constant of the composite film, preferably, the grafting rate of the grafted CPP is 1-10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc. If the grafting rate is too low, the effect of improving the dielectric constant of the BOPP composite film cannot be achieved, and the homopolymerization of monomers in the solution during preparation makes it difficult to achieve a higher grafting rate; the chlorination degree of the grafted CPP is 20-40%, for example, 20%, 22%, 24.5%, 26%, 29.5%, 30%, 32.5%, 40%, etc. The chlorination degree refers to the mass fraction of chlorine element in the grafted CPP. In the present application, the grafted CPP with a chlorination degree of 20-40% can have good binding properties with the BOPP layer and the PVDF layer, avoiding defects on the binding surface; when the chlorination degree is less than 20%, it is difficult to dissolve in the solvent; when the chlorination degree is higher than 40%, the binding properties of the grafted CPP with the BOPP layer and the PVDF layer decrease.

[0034] In the present application, in order to further improve the dielectric constant of the composite film, preferably, the grafting rate of the grafted CPP is 1-10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc. If the grafting rate is too low, the effect of improving the dielectric constant of the BOPP composite film cannot be achieved, and the homopolymerization of monomers in the solution during preparation makes it difficult to achieve a higher grafting rate; the chlorination degree of the grafted CPP is 20-40%, for example, 20%, 22%, 24.5%, 26%, 29.5%, 30%, 32.5%, 40%, etc. The chlorination degree refers to the mass fraction of chlorine element in the grafted CPP. In the present application, the grafted CPP with a chlorination degree of 20-40% can have good binding properties with the BOPP layer and the PVDF layer, avoiding defects on the binding surface; when the chlorination degree is less than 20%, it is difficult to dissolve in the solvent; when the chlorination degree is higher than 40%, the binding properties of the grafted CPP with the BOPP layer and the PVDF layer decrease.

[0035] In the present application, in order to further improve the dielectric constant of the composite film, preferably, the grafting rate of the grafted CPP is 1-10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc. If the grafting rate is too low, the effect of improving the dielectric constant of the BOPP composite film cannot be achieved, and the homopolymerization of monomers in the solution during preparation makes it difficult to achieve a higher grafting rate; the chlorination degree of the grafted CPP is 20-40%, for example, 20%, 22%, 24.5%, 26%, 29.5%, 30%, 32.5%, 40%, etc. The chlorination degree refers to the mass fraction of chlorine element in the grafted CPP. In the present application, the grafted CPP with a chlorination degree of 20-40% can have good binding properties with the BOPP layer and the PVDF layer, avoiding defects on the binding surface; when the chlorination degree is less than 20%, it is difficult to dissolve in the solvent; when the chlorination degree is higher than 40%, the binding properties of the grafted CPP with the BOPP layer and the PVDF layer decrease.

[0036] In the present application, in order to further improve the dielectric constant of the composite film, preferably, the grafting rate of the grafted CPP is 1-10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc. If the grafting rate is too low, the effect of improving the dielectric constant of the BOPP composite film cannot be achieved, and the homopolymerization of monomers in the solution during preparation makes it difficult to achieve a higher grafting rate; the chlorination degree of the grafted CPP is 20-40%, for example, 20%, 22%, 24.5%, 26%, 29.5%, 30%, 32.5%, 40%, etc. The chlorination degree refers to the mass fraction of chlorine element in the grafted CPP. In the present application, the grafted CPP with a chlorination degree of 20-40% can have good binding properties with the BOPP layer and the PVDF layer, avoiding defects on the binding surface; when the chlorination degree is less than 20%, it is difficult to dissolve in the solvent; when the chlorination degree is higher than 40%, the binding properties of the grafted CPP with the BOPP layer and the PVDF layer decrease.

[0037] In the present application, specifically, the thickness of the BOPP composite film is 15-50 μm. For example, the thickness of the BOPP composite film is 15 μm, 20 μm, 25 μm, 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm, 36 μm, 37 μm, 38 μm, 39 μm, 40 μm, 45 μm, 50 μm, and the like, and further preferably, the thickness of the BOPP composite film is 20-30 μm.

[0038] In the present application, further preferably, the thickness ratio of the BOPP film layer, the single grafted CPP layer, and the PVDF layer in the BOPP composite film is 1:0.2-1:0.8-1.5, for example, 1:0.2:1, 1:0.5:1.2, 1:0.8:1.3, 1:1:1.5, 1:1:1, and the like. By controlling the thickness ratio of the BOPP film layer, the single grafted CPP layer, and the PVDF layer, the BOPP composite film has a higher energy storage density.

[0039] In the present application, in order to further improve the energy storage density of the BOPP composite film, preferably, the PVDF layer and the grafted CPP layer contain dielectric fillers. The dielectric fillers can also greatly improve the dielectric constant of the PVDF layer and the grafted CPP layer, thereby further ensuring that the composite film of the present application has a high energy storage density.

[0040] Specifically, in the PVDF layer, the volume fraction of the dielectric filler is 0-80%, for example, 2%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and the like, and if higher than 80%, the percolation threshold of the material will be reached, resulting in breakdown; further preferably, in the PVDF layer, the volume fraction of the dielectric filler is 20-30%; in each grafted CPP layer, the volume fraction of the dielectric filler is 0-80%, for example, 2%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and the like, and if higher than 80%, the breakdown strength will be severely reduced, and further preferably, in each grafted CPP layer, the volume fraction of the dielectric filler is 5-25%.

[0041] In the present application, the dielectric filler is nanoscale or micrometer scale.

[0042] In the present application, in order to improve the compatibility of the interface between the dielectric filler and the polymer matrix (PVDF or grafted CPP), preferably, the dielectric filler is a surface-modified dielectric filler, in particular, the dielectric filler is at least one of surface-modified barium titanate, magnesium titanate, bismuth ferrite, lead zirconate titanate (PZT), potassium sodium niobate (KNN), sodium bismuth titanate (BNT) and barium strontium titanate (BST). Further preferably, the dielectric filler is surface-modified barium titanate and / or barium strontium titanate. Surface modification of the dielectric filler can reduce the surface energy difference between the dielectric filler and the polymer matrix, prevent the dielectric filler from agglomerating in the polymer matrix, and improve the compatibility of the interface between the dielectric filler and the polymer matrix. In the present application, the surface modifier of the surface modification is preferably a long-chain silane coupling agent; in particular, the long-chain silane coupling agent is at least one of n-octyltrimethoxysilane, n-octyltriethoxysilane and gamma-chloropropyltriethoxysilane.

[0043] In the present application, the compatibility of the interface between the dielectric filler and the polymer matrix can also be improved by coating the dielectric filler with an oxide. The oxide is at least one of silicon dioxide, aluminum oxide and magnesium oxide.

[0044] On the other hand, the present application also provides a preparation method of the high-energy-density BOPP composite film of the present application, which comprises:

[0045] (1) CPP grafting: mixing CPP, solvent 1, grafting monomer and initiator for grafting reaction to obtain grafted CPP;

[0046] (2) Surface modification of dielectric filler: mixing surface modifier, solvent 2 and dielectric filler for surface modification to obtain surface-modified dielectric filler;

[0047] (3) Casting solution preparation: mixing grafted CPP, solvent 1 and surface-modified dielectric filler to obtain casting solution A; mixing PVDF, solvent DMF and surface-modified dielectric filler to obtain casting solution B;

[0048] (4) BOPP composite film preparation: coating casting solution A and casting solution B on one side of one BOPP film in turn, coating casting solution A on one side of another BOPP film, and then opposing the coated sides of the two BOPP films and hot pressing to obtain a high-energy-density BOPP composite film.

[0049] In the present application, the CPP can be a benzene-soluble CPP or an ester-soluble CPP. If the CPP is a benzene-soluble CPP, then in steps (1) and (3), the solvent 1 is toluene or xylene; if the CPP is an ester-soluble CPP, then in steps (1) and (3), the solvent 1 is ethyl acetate or butyl acetate, etc.

[0050] In step (1), the initiator can be BPO to achieve as much monomer grafting on the CPP surface as possible instead of homopolymerization in solution. The specific method of CPP grafting is: dissolving CPP in toluene at 75℃, the concentration of CPP is 20-30wt%, adding monomer (the amount of monomer is 10-100wt% of CPP) and initiator BPO (the amount of initiator is 1-5wt% of monomer) under nitrogen atmosphere, sealing and reacting at 80-90℃ for 10-15h, precipitating, washing and drying to obtain grafted CPP.

[0051] In step (2), the solvent 2 is xylene, and the specific method of surface modification can be: adding the modifier into solvent 2 to prepare a 7-8wt% solution, adding the dielectric filler particles into the solution, ultrasonic dispersion for 3-40min, replacing the air in the container with N2, sealing and reacting at 70-90℃ for 12-14h, and then drying in a vacuum oven after centrifugation and washing. The mass ratio of the amount of the modifier to the dielectric filler is 1:10-1:1.

[0052] In the present application, the dielectric filler can also be surface modified by coating with an oxide. The method of coating with an oxide can be a conventional method in the art. For example, taking the coating with silicon dioxide as an example, the specific method can be: weighing a certain amount of ethyl silicate and ethanol solution (TSOS: ethanol = 1:2) and mixing thoroughly to obtain solution C; weighing a certain amount of ammonia water and pouring into the stirring dielectric filler solution to form an alkaline environment, and stirring in a water bath at 60℃ for 12h. Then, solution C is added dropwise at a speed of 1 drop / min while stirring; centrifuging and collecting the product to obtain the dielectric filler coated with silicon dioxide. The amount ratio of the oxide to the dielectric filler is 1:1-1:10.

[0053] In step (3), in the casting solution A, if the content of grafted CPP is too high, the viscosity of the subsequently prepared casting solution will be too high, and it will be difficult to coat the film; if the content of grafted CPP is too low, the viscosity of the subsequently prepared casting solution will be too low, and the film surface will be uneven during the solvent evaporation process. Therefore, preferably, in the mixed solution of grafted CPP and solvent 1, the content of grafted CPP is 10-50wt%. Specifically, first mix grafted CPP with solvent 1 to obtain a grafted CPP solution, then mix the grafted CPP solution with the dielectric filler at room temperature, stir for 20-26h, ultrasonic for 20-40min, and disperse with a homogenizer at 1800-2200rpm for 5-10min to obtain the casting solution A.

[0054] In step (3), the content of PVDF in the casting solution B, preferably a mixed solution of PVDF and solvent DMF, is 20-30 wt%. Specifically, PVDF is first mixed with solvent DMF to obtain a PVDF solution, and then the PVDF solution is mixed with the dielectric filler at room temperature for 20-25 h to obtain the casting solution B.

[0055] In step (4), the coating method can be a solution casting method. Specifically, one BOPP film is vacuum adsorbed on a coating machine with a bottom plate temperature of 30-45℃, and a doctor blade is used to uniformly coat the casting solution A on one side of the BOPP film at a speed of 1-2 mm / s. After the solvent in the casting solution A is completely volatilized, a doctor blade is used to uniformly coat the casting solution B on the coated side of the BOPP film at a speed of 1-2 mm / s. Another BOPP film is vacuum adsorbed on a coating machine with a bottom plate temperature of 30-45℃, and a doctor blade is used to uniformly coat the casting solution A on one side of the BOPP film at a speed of 1-2 mm / s. After the solvents in the coated sides of the two BOPP films are completely volatilized, the coated sides of the BOPP films are oppositely superimposed, and hot pressing is performed at 60-110℃ and a pressure of 1-5 MPa for 5-30 min to obtain a high energy storage density BOPP composite film. The coating thickness of each layer is such that the thickness of each layer after hot pressing meets the requirements of the present application.

[0056] The BOPP composite film prepared by the present application has good dielectric properties. Compared with a commercial BOPP capacitor film, the dielectric constant and breakdown strength of the BOPP composite film are greatly improved, thereby having a higher energy storage density. The tensile strength and elongation at break of the BOPP composite film of the present application are significantly increased, and the BOPP composite film can maintain a high charge and discharge efficiency under high voltage.

[0057] The following specific examples further illustrate the high energy storage density BOPP composite film and the preparation method thereof of the present application.

[0058] In the following examples, the CPP is a benzene solution, and the BOPP is commercially available with product number MP-A(H).

[0059] The performance tests of the film products prepared in the following examples and comparative examples are carried out by the following methods:

[0060] (1) Test of dielectric constant: Before testing the dielectric constant, electrodes with a diameter of 1 cm are plated on both sides of the sample. During testing, the sample is clamped in the center of the electrode plate, and the instrument is Agilent 4980A.

[0061] (2) Test of breakdown strength: The instrument for testing the breakdown strength is a voltage resistance tester CS2673X. The sample is immersed in silicone oil, and the instrument voltage is gradually increased from 0 to 20 kV until the sample is broken down.

[0062] (3) Energy storage density and charge-discharge efficiency test: Copper electrode (2mm in diameter) was plated on the surface of the thin film using high vacuum resistance evaporation coating machine. Monopolar hysteresis loop was tested at 25°C and 10Hz. Energy storage density and charge-discharge efficiency of the material were calculated through the hysteresis loop.

[0063] (4) Mechanical property test: Instron5567 tensile machine was used to test tensile strength and elongation at break at a speed of 20mm / min.

[0064] Example 1

[0065] (1) CPP grafting: CPP (chlorination degree of 29.5%) was dissolved in toluene at 75°C, the concentration of CPP was 25wt%, methyl methacrylate (the amount was 80wt% of CPP) and initiator BPO (the amount was 3wt% of monomer) were added under nitrogen atmosphere, and the reaction was carried out at 85°C for 12h. The reacted solution was added dropwise into methanol under stirring to obtain solid product. The product was dissolved in toluene, and then added dropwise into acetone under stirring to obtain solid product. The solid product was broken into small pieces and dried to constant weight in a vacuum oven at 40°C to obtain grafted CPP (grafting rate of 10%).

[0066] (2) Surface modification of dielectric filler: n-octyltriethoxysilane (OTS) was added to xylene to prepare a 7wt% solution. 20g of barium titanate particles were added to the solution and ultrasonically dispersed for 30min. The container was replaced with N2 to replace the air therein. The reaction was carried out at 80°C for 12h. After centrifugation and washing, the product was dried in a vacuum oven.

[0067] (3) Preparation of casting solution: grafted CPP was mixed with toluene to prepare a grafted CPP solution with a concentration of 30wt%. The grafted CPP solution was mixed with surface-modified barium titanate at room temperature. The amount of surface-modified barium titanate and grafted CPP was such that the volume fraction of dielectric filler in the grafted CPP layer after subsequent hot pressing was 10%. The mixture was stirred for 20h, ultrasonically dispersed for 20min, and dispersed with a homogenizer at 2000rpm for 5min to obtain casting solution A.

[0068] PVDF was mixed with solvent DMF to prepare a PVDF solution with a concentration of 20%. The PVDF solution was mixed with surface-modified barium titanate at room temperature. The amount of surface-modified barium titanate and PVDF was such that the volume fraction of dielectric filler in the grafted PVDF layer after subsequent hot pressing was 30%. The mixture was stirred at room temperature for 24h and ultrasonically dispersed for 30min to obtain casting solution B.

[0069] (4) Preparation of the high energy storage density BOPP composite film: A purchased commercial BOPP film with a thickness of 5 pm was vacuum adsorbed on a coating machine with a bottom plate temperature of 40 °C, and a doctor blade was used to uniformly coat casting solution A on one side of the BOPP film at a speed of 1 mm / s. After the solvent in the casting solution A was completely volatilized, a doctor blade was used to uniformly coat casting solution B on the coated side of the BOPP film at a speed of 1 mm / s. Another BOPP film was vacuum adsorbed on a coating machine with a bottom plate temperature of 40 °C, and a doctor blade was used to uniformly coat casting solution A on one side of the BOPP film at a speed of 1 mm / s. After the solvents on the coated sides of the two BOPP films were completely volatilized, the coated sides of the BOPP films were oppositely superimposed, and hot pressing was performed at 100 °C and a pressure of 3 MPa for 15 min to obtain a high energy storage density BOPP composite film with a total thickness of 20 pm, wherein the thickness of the BOPP film layer was 5 pm, the thickness of the grafted CPP layer was 3 pm, and the thickness of the PVDF layer was 4 pm.

[0070] The cross-sectional scanning electron microscope image of the high energy storage density BOPP composite film of Example 1 is shown in Figure 1 , from which it can be seen that the connection between the two adjacent layers is tight; the charge and discharge test graph of the high energy storage density BOPP composite film product is shown in Figure 2 , and the energy storage density and the charge and discharge efficiency were calculated by integrating the Figure 2 curve, as shown in Figure 3 .

[0071] Example 2

[0072] The high energy storage density BOPP composite film was prepared according to the method of Example 1, except that the thickness of the PVDF layer was 2 pm.

[0073] Example 3

[0074] The high energy storage density BOPP composite film was prepared according to the method of Example 1, except that the thickness of the grafted CPP layer was 2 pm.

[0075] Example 4

[0076] The high energy storage density BOPP composite film was prepared according to the method of Example 1, except that the grafting rate of the grafted CPP was 0.2%.

[0077] Example 5

[0078] The high energy storage density BOPP composite film was prepared according to the method of Example 1, except that the chlorination degree of the grafted CPP was 45%.

[0079] Example 6

[0080] The high energy storage density BOPP composite film was prepared according to the method of Example 1, except that the volume fraction of the surface-modified barium titanate in the PVDF layer was 60%.

[0081] Example 7

[0082] The high energy storage density BOPP composite film was prepared according to the method of Example 1, except that the volume fraction of the surface-modified barium titanate in the grafted CPP layer was 30%.

[0083] Example 8

[0084] The high energy storage density BOPP composite film was prepared according to the method of Example 1, except that magnesium titanate was used instead of barium titanate.

[0085] Comparative Example 1

[0086] The BOPP film was not treated, i.e. a single layer of BOPP film.

[0087] The dielectric constant, breakdown strength, energy storage density, charge and discharge efficiency, tensile strength and elongation at break of the high energy storage density BOPP composite films of Examples 1-11 and Comparative Example 1 were tested, and the results are shown in Table 1.

[0088] Table 1

[0089]

[0090]

[0091] As can be seen from Table 1, the BOPP-grafted CPP-PVDF-grafted CPP-BOPP five-layer structure composite film of the present application has high dielectric constant, breakdown strength, energy storage density, charge and discharge efficiency, tensile strength and elongation at break.

[0092] The above description is merely preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any changes or substitutions easily thought of by those skilled in the art within the technical scope disclosed by the present application should be encompassed within the protection scope of the present application.

Claims

1. A high energy density BOPP composite film, characterized by, The BOPP composite film comprises two BOPP film layers and a PVDF layer arranged between the two BOPP film layers, a grafted CPP layer is arranged between the BOPP film layer and the PVDF layer, the PVDF layer and the grafted CPP layer contain dielectric fillers, and the grafting monomer of the grafted CPP is at least one of methyl methacrylate, trifluoroethyl methacrylate, hexafluorobutyl methacrylate, dodecafluoroheptyl methacrylate and tridecafluorooctyl methacrylate.

2. The BOPP composite film according to claim 1, characterized in that, The grafting rate of the grafted CPP is 1-10%, and the chlorination degree of the grafted CPP is 20-40%.

3. The BOPP composite film according to claim 1, characterized in that, The thickness of the BOPP film layer is 3-20 μm.

4. The BOPP composite film according to claim 1, characterized in that, The thickness of the PVDF layer is 4-20 μm.

5. The BOPP composite film according to claim 1, wherein, The thickness of each grafted CPP layer is 3-10 μm.

6. The BOPP composite film according to claim 1, characterized in that, The thickness of the BOPP composite film is 15-50 μm.

7. The BOPP composite film according to claim 6, characterized in that, In the PVDF layer, the volume fraction of the dielectric filler is 0-80%; in each grafted CPP layer, the volume fraction of the dielectric filler is 0-80%.

8. The BOPP composite film according to claim 1 or 7, characterized in that, The dielectric filler is at least one of surface-modified barium titanate, magnesium titanate, bismuth ferrite, lead zirconium titanate, sodium potassium niobate, bismuth sodium titanate and barium strontium titanate.

9. The process for the production of high energy density BOPP composite film as claimed in any one of claims 1 to 8, wherein, The preparation method comprises: (1) CPP grafting: mixing CPP, solvent 1, grafting monomer and initiator for grafting reaction to obtain grafted CPP; (2) Surface modification of dielectric filler: mixing surface modifier, solvent 2 and dielectric filler for surface modification to obtain surface-modified dielectric filler; (3) Preparation of casting solution: mixing grafted CPP, solvent 1 and surface-modified dielectric filler to obtain casting solution A; mixing PVDF, solvent DMF and surface-modified dielectric filler to obtain casting solution B; (4) Preparation of BOPP composite film: coating casting solution A and casting solution B on one side of one BOPP film in sequence, coating casting solution A on one side of another BOPP film, opposing the coated sides of the two BOPP films after the solvent is volatilized, and hot pressing to obtain a high energy storage density BOPP composite film.

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