Multi-layer co-extrusion polypropylene composite film for capacitor and preparation method of multi-layer co-extrusion polypropylene composite film

Abstract

The invention relates to the technical field of capacitor films, in particular to a multilayer co-extrusion polypropylene composite film for a capacitor and a preparation method of the multilayer co-extrusion polypropylene composite film. The layer A contains a modified flame retardant, and the layer B contains the following raw materials: BN coated SiO2-NH2 and MAH-g-PP; the modified flame retardant is obtained by carrying out polymerization reaction on BN coated SiO2-NH2, glycidyl methacrylate and an unsaturated phosphorus-nitrogen flame retardant; the BN (at) SiO2-NH2 is prepared by the following steps: hydrolyzing mechanically stripped BN and tetraethoxysilane to prepare BN (at) SiO2, and finally, carrying out reflux grafting on the BN (at) SiO2 and a silane coupling agent to prepare the BN (at) SiO2-NH2. BN coated SiO2-NH2 with high breakdown strength is prepared by modifying a BN material, the modified BN coated SiO2-NH2 is placed in the A layer and the B layer respectively, the ABA three-layer composite film is prepared through a co-extrusion die head, mutual interference of functional filler is avoided, performance of each layer is independently optimized, and synergistic improvement of flame retardance, dielectricity and breakdown strength is achieved.

Description

Technical Field

[0001] This invention relates to the field of capacitor film technology, specifically to a multilayer co-extruded polypropylene composite film for capacitors and its preparation method. Background Technology

[0002] Polypropylene (PP) film is widely used in food packaging, medical packaging, production and transportation pipelines, textiles, electronic components and other fields due to its advantages such as low density, good chemical corrosion resistance and low cost.

[0003] Capacitors are one of the three major passive components, found in various electronic products. Film capacitors, in particular, are widely used in new energy fields such as new energy vehicles, photovoltaics, and wind power generation due to their high voltage resistance, temperature resistance, and stability. With continuous technological advancements and rapid development, the performance requirements for film capacitors are becoming increasingly stringent. As the core material of film capacitors, improving the performance of the capacitor film material to optimize capacitor performance is currently a major focus in this field. Biaxially oriented polypropylene (BOPP) film is currently the most commercially successful polymer dielectric material, possessing high breakdown strength, excellent temperature stability, relatively low cost, and processability suitable for large-scale automated production. Despite the significant advantages of BOPP film, its physical properties limit its application in some high-end scenarios: while BOPP performs well at low temperatures, its breakdown electric field strength decreases rapidly and dielectric loss increases significantly at high temperatures (>70℃). Improving the dielectric constant is difficult, often sacrificing insulation strength and temperature resistance. Furthermore, polypropylene has poor flame retardancy, with a limiting oxygen index generally only around 17-18, posing a safety hazard. To improve its flame retardancy, flame retardants are usually added to PP. However, the application of inorganic flame retardants in polymer systems may lead to local stress concentration due to agglomeration, which in turn increases the brittleness of the film and reduces its mechanical strength.

[0004] To address the aforementioned issues, existing technologies mostly employ single modification strategies, such as adding a single functional filler or simple blending, which struggle to simultaneously meet multiple requirements including flame retardancy, high dielectric constant, and high breakdown strength. Therefore, developing a polypropylene capacitor film with superior overall performance has become a pressing technical challenge in this field. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a heat-resistant, flame-retardant, and electrically breakdown-resistant multilayer co-extruded polypropylene film that meets the performance requirements of capacitors.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A multilayer co-extruded polypropylene composite film for capacitors, wherein the composite film has an ABA three-layer co-extruded structure; wherein, the A layer raw materials, by weight, include: 100 parts polypropylene resin, 2-8 parts modified flame retardant, 0.1-0.5 parts nucleating agent, and 0.1-0.5 parts antioxidant; the B layer raw materials, by weight, include: 100 parts polypropylene resin, 0.5-2 parts alumina, 2-5 parts BN@SiO2-NH2, and 8-10 parts MAH-g-PP; wherein, The modified flame retardant is obtained by polymerizing BN@SiO2-NH2 with glycidyl methacrylate and an unsaturated phosphorus-nitrogen flame retardant. The preparation process of BN@SiO2-NH2 is as follows: Boron nitride and urea were mixed and placed in a ball mill. After ball milling for 10-20 h, mechanical exfoliation was performed. Then, the mixture was ultrasonically dispersed in an ethanol aqueous solution, and tetraethyl orthosilicate and ammonia were added. The mixture was stirred and hydrolyzed to obtain BN@SiO2. Finally, it was refluxed and grafted with γ-aminopropyltriethoxysilane to obtain BN@SiO2-NH2. The structural formula of the unsaturated phosphorus-nitrogen flame retardant is: .

[0007] Furthermore, in the B layer raw material, MAH-g-PP and BN@SiO2-NH2 are first mixed and fed into a twin-screw extruder, and melt-extruded and granulated at 180~220℃ to obtain premixed granules, which are then blended with the remaining raw materials.

[0008] Furthermore, the grafting rate of the MAH-g-PP is 1.2%~1.5%.

[0009] Furthermore, the preparation process of the BN@SiO2-NH2 is as follows: 1) Mix boron nitride and urea at a mass ratio of 1:20~40 and place them in a ball mill. The ball-to-material ratio is 80:1~120:1. The speed is 400~600 r / min. The ball milling is carried out for 15~20 h for mechanical stripping to obtain stripped boron nitride, which is denoted as BN. 2) BN was ultrasonically dispersed in an ethanol aqueous solution, and the pH was adjusted to 8-9 with dilute ammonia. Tetraethyl orthosilicate was slowly added dropwise to the BN dispersion. The mixture was stirred at 40°C for 6 hours, centrifuged and washed to obtain BN@SiO2. 3) BN@SiO2 is dispersed in ethanol, and 30-50% by weight of γ-aminopropyltriethoxysilane is added for reflux grafting to obtain BN@SiO2-NH.

[0010] Furthermore, the preparation process of the modified flame retardant is as follows: Add BN@SiO2-NH2 and initiator AIBN to the reaction flask, sonicate to disperse evenly, and purge with nitrogen for 30 min; dissolve glycidyl methacrylate and unsaturated phosphorus nitrogen flame retardant in tetrahydrofuran to form a mixed monomer solution, add the mixed monomer solution to the reaction flask under nitrogen protection, and heat to 70℃ for 4-8 h. After the reaction is complete, cool to room temperature, add tetrahydrofuran to dilute the reaction system, then precipitate in anhydrous methanol, filter, wash, and dry to obtain the final product.

[0011] Furthermore, the molar ratio of glycidyl methacrylate to unsaturated phosphorus-nitrogen flame retardant is 3~6:4~7, and the mass ratio of total monomer mass to BN@SiO2-NH2 mass is 0.5~2:1.

[0012] Furthermore, the nucleating agent is talc powder with a particle size of 100-200 nm; the antioxidant is antioxidant 1010 or antioxidant 168.

[0013] Furthermore, the thickness ratio of the ABA three-layer structure of the composite membrane is 1:1 to 3:1.

[0014] A second objective of this invention is to provide a method for preparing the multilayer co-extruded polypropylene composite film for capacitors as described above, comprising the following steps: S1. Polypropylene resin, modified flame retardant, nucleating agent and antioxidant are mixed in a high-speed mixer to obtain layer A raw material; MAH-g-PP and BN@SiO2-NH2 are mixed and fed into a twin-screw extruder, melt extruded and granulated at 180~220℃ to obtain premixed granules, and then mixed with polypropylene resin and alumina in a high-speed mixer to obtain layer B raw material. S2. The raw materials of layer A and layer B are melted at 200~240℃ through independent screw extruders, and then formed into ABA three-layer composite castings through co-extrusion die head; S3. The casting sheet is subjected to biaxial stretching. First, it is preheated at 50~70℃, and then longitudinally stretched at 110~130℃ with a stretching ratio of 3-3.6. Then, it is preheated at 80~120℃ and transversely stretched at 110~130℃ with a stretching ratio of 2.6~3.2. S4. The film is heat-set after biaxial stretching, then cooled at 50°C, corona treated, drawn, and wound to obtain the final product.

[0015] Boron nitride (BN), as a two-dimensional layered material, possesses high thermal conductivity, excellent insulation properties, and low dielectric loss, making it an ideal filler for polymer modification. However, BN has a highly inert surface and poor interfacial compatibility with the polypropylene matrix. Direct blending can easily lead to agglomeration, affecting the overall performance of the composite material.

[0016] In this application, boron nitride is first mechanically exfoliated. Urea exfoliation of boron nitride has been shown to significantly reduce the average flake size, effectively inhibit inter-flake aggregation, and introduce active amino groups. Building upon this, the exfoliated boron nitride is impregnated in an aqueous ethanol solution, followed by the addition of tetraethyl orthosilicate for hydrolysis. This hydrolysis partially allows the growth of silica particles between the boron nitride layers. Furthermore, by adding γ-aminopropyltriethoxysilane to form an organic bridge, stable BN@SiO2-NH2 composite particles are obtained. The introduction of active amino groups serves two purposes. First, the active amino groups undergo nucleophilic ring-opening with glycidyl methacrylate, followed by free radical polymerization of glycidyl methacrylate with reactive phosphorus-nitrogen flame retardants under the initiation of AIBN to form flame-retardant polymer chains. This modified flame retardant is added to the surface structure of the film, i.e., layer A. The polymer chain structure of the flame retardant does not agglomerate in the PP matrix and also assists in the dispersion of BN@SiO2-NH2, avoiding stress concentration. The grafted phosphorus-nitrogen flame-retardant segments form a dense char layer during combustion, synergistically improving the limiting oxygen index in conjunction with the physical barrier effect of BN. Second, in layer B, it is pre-melted and blended with MAH-g-PP. The surface amino groups undergo amidation reactions with the anhydride groups of MAH-g-PP to form chemically bonded premixed granules, preventing the agglomeration of BN@SiO2-NH2 and assisting in the dispersion of alumina, further improving thermal conductivity and dielectric properties. The core layer B serves as a support layer. Its BN@SiO2-NH2 contains a two-dimensional layered boron nitride structure and silicon dioxide particles grown on it, which can enhance the interface and improve the breakdown strength.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: By modifying the BN material, a BN@SiO2-NH2 with high breakdown strength is obtained. This material is reacted and mixed with a reactive flame retardant in the surface raw material, and then pre-melted and blended with MAH-g-PP to promote its own dispersion and that of alumina in the B layer raw material. An ABA three-layer composite film is obtained by co-extrusion die, avoiding mutual interference of functional fillers, and optimizing the performance of each layer independently, thereby achieving a synergistic improvement in flame retardancy, dielectric properties, and breakdown strength. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0020] Example: A multilayer co-extruded polypropylene composite film for capacitors, wherein the composite film has an ABA three-layer co-extruded structure, and the preparation process is as follows: S1. Mix 100 parts of polypropylene resin (Suzhou Yitianli Plastics Co., Ltd., brand name R370Y), 2-8 parts of modified flame retardant, 0.1-0.5 parts of nucleating agent talc powder (particle size 100-200 nm), and 0.1-0.5 parts of antioxidant 1010 in a high-speed mixer to obtain layer A raw material. 2-5 parts of BN@SiO2-NH2 and 8-10 parts of MAH-g-PP (Jiangsu Runfeng Synthetic Technology Co., Ltd., grafting rate 1.2%-1.5%) are mixed and fed into a twin-screw extruder, and melt-extruded and granulated at 180-220℃ to obtain premixed granules. Then, it is mixed with 100 parts of polypropylene resin (Suzhou Yitianli Plastics Co., Ltd., grade R370Y) and 0.5-2 parts of alumina (50-100 nm) in a high-speed mixer to obtain the B layer raw material. S2. The raw materials of layer A and layer B are melted at 200~240℃ through independent screw extruders, and then formed into ABA three-layer composite castings through co-extrusion die head; S3. The casting sheet is subjected to biaxial stretching. It is first preheated at 70°C, and then longitudinally stretched at 120°C with a stretching ratio of 3. Then, it is preheated at 120°C and transversely stretched at 130°C with a stretching ratio of 3.2. S4. The film is heat-set after biaxial stretching, then cooled at 50°C, corona treated, drawn, and wound to obtain the final product.

[0021] Most of the raw materials used in this application are commodities with numerous commercially available channels, and will not be elaborated upon here. Some raw materials require in-house preparation; the preparation process of the BN@SiO2-NH2 is as follows: 1) Take 10g of hexagonal boron nitride powder (Suzhou Sailon Nano New Material Industry Co., Ltd., BN-2) and mix it with 300g of urea. Add the mixture to a ball mill jar with a ball-to-material ratio of 100:1 and mill at 500r / min for 20 hours. Take out the product, wash it three times with deionized water, and vacuum dry it at 60℃ for 12 hours to obtain exfoliated boron nitride BN.

[0022] 2) Take 5 g of BN and disperse it in 100 mL of ethanol-water mixed solvent (volume ratio 4:1). Adjust the pH to 8-9 with dilute ammonia water, slowly add 10 g of tetraethyl orthosilicate, stir at 40 °C for 6 h, centrifuge the product, wash it 3 times with ethanol, and dry it at 60 °C to obtain BN@SiO2.

[0023] 3) Disperse 3g of BN@SiO2 in 100mL of anhydrous ethanol, add 1.5g of 3-aminopropyltriethoxysilane, and reflux at 70℃ for 8 hours. Centrifuge the product, wash it three times with ethanol, and dry it under vacuum at 60℃ for 12 hours to obtain BN@SiO2-NH2.

[0024] The structural formula of the unsaturated phosphorus-nitrogen flame retardant is: The reactive phosphorus-nitrogen flame retardant was prepared according to patent CN110483578B, which describes a method for preparing the same.

[0025] The preparation process of the modified flame retardant is as follows: Add 2g of BN@SiO2-NH2 and an initiating amount of AIBN to the reaction flask, sonicate to disperse evenly, and purge with nitrogen for 30 min; dissolve 0.86g of glycidyl methacrylate and 2.14g of unsaturated phosphorus nitrogen flame retardant (molar ratio 1:1) in tetrahydrofuran to form a mixed monomer solution. Under nitrogen protection, add the mixed monomer solution to the reaction flask and heat to 70℃ for 6 h. After the reaction is complete, cool to room temperature, add tetrahydrofuran to dilute the reaction system, then precipitate in anhydrous methanol, filter, wash, and dry to obtain the final product.

[0026] Example 1: A composite membrane was prepared according to the above process, wherein the raw materials of layer A by weight are: 100 parts polypropylene resin, 2 parts modified flame retardant, 0.1 parts nucleating agent, and 0.1 parts antioxidant; the raw materials of layer B by weight are: 100 parts polypropylene resin, 0.5 parts alumina, 2 parts BN@SiO2-NH2, and 8 parts MAH-g-PP.

[0027] The film thickness ratio is 1:2:1.

[0028] Example 2: A composite membrane was prepared according to the above process, wherein the raw materials of layer A by weight are: 100 parts polypropylene resin, 5 parts modified flame retardant, 0.3 parts nucleating agent, and 0.3 parts antioxidant; the raw materials of layer B by weight are: 100 parts polypropylene resin, 1 part alumina, 3 parts BN@SiO2-NH2, and 9 parts MAH-g-PP.

[0029] The film thickness ratio is 1:2:1.

[0030] Example 3: A composite membrane was prepared according to the above process, wherein the raw materials of layer A by weight are: 100 parts polypropylene resin, 8 parts modified flame retardant, 0.5 parts nucleating agent, and 0.5 parts antioxidant; the raw materials of layer B by weight are: 100 parts polypropylene resin, 1 part alumina, 4 parts BN@SiO2-NH2, and 10 parts MAH-g-PP.

[0031] The film thickness ratio is 1:2:1.

[0032] Comparative Example 1: Basically the same as Example 3, except that no modified flame retardant is added to layer A.

[0033] Comparative Example 2: Basically the same as Example 3, except that BN@SiO2-NH2 is not added to the B layer.

[0034] Comparative Example 3: Basically the same as Example 3, except that BN@SiO2-NH2 and MAH-g-PP in layer B are not premixed, but directly blended with polypropylene resin and alumina.

[0035] Performance Testing The composite membranes prepared in the above embodiments and comparative examples were subjected to the following performance tests: 1. Dielectric constant and dielectric loss: Tested according to ASTM D150-22, with a test frequency of 1kHz, a temperature of 23±2℃, and a humidity of 50±5%RH.

[0036] 2. Breakdown strength: Tested according to ASTM D149-19, at standard atmospheric pressure, 50±5%RH, and 25℃.

[0037] 3. Heat shrinkage rate: According to GB / T 13541-2020, the longitudinal heat shrinkage rate was measured after treatment at 120℃ for 30 minutes.

[0038] 4. Limiting oxygen index: conducted according to GB / T 2406.2-2009, with a test sample size of 100mm × 10mm × thickness.

[0039] 5. Interfacial bonding strength: The interlayer peel test is used to peel the composite film along the interlayer and measure the peel force.

[0040] Table 1 As shown in Table 1, Examples 1-3 exhibited excellent performance across all parameters. Example 2 demonstrated the best overall performance with a dielectric constant of 3.24, a breakdown strength of 572 kV / mm, an LOI of 28.5%, a heat shrinkage rate of only 1.21%, and a peel strength of 2.28 N / cm. Comparative Example 1, lacking flame retardant, had a limiting oxygen index close to pure PP, but its breakdown strength and peel strength were slightly lower. Comparative Example 2, lacking BN@SiO2-NH2 in its B layer, showed a significant decrease in breakdown strength. Comparative Example 3, without premixing BN@SiO2-NH2 and MAH-g-PP in its B layer, resulted in weakened interfacial bonding and a decrease in peel strength.

[0041] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details.

Claims

1. A multilayer co-extruded polypropylene composite film for capacitors, characterized in that, The composite membrane has an ABA three-layer co-extruded structure; wherein, the A layer raw materials, by weight, include: 100 parts polypropylene resin, 2-8 parts modified flame retardant, 0.1-0.5 parts nucleating agent, and 0.1-0.5 parts antioxidant; the B layer raw materials, by weight, include: 100 parts polypropylene resin, 0.5-2 parts alumina, 2-5 parts BN@SiO2-NH2, and 8-10 parts MAH-g-PP; wherein, The modified flame retardant is obtained by polymerizing BN@SiO2-NH2 with glycidyl methacrylate and an unsaturated phosphorus-nitrogen flame retardant. The preparation process of BN@SiO2-NH2 is as follows: Boron nitride and urea were mixed and placed in a ball mill. After ball milling for 10-20 h, mechanical exfoliation was performed. Then, the mixture was ultrasonically dispersed in an ethanol aqueous solution, and tetraethyl orthosilicate and ammonia were added. The mixture was stirred and hydrolyzed to obtain BN@SiO2. Finally, it was refluxed and grafted with γ-aminopropyltriethoxysilane to obtain BN@SiO2-NH2. The structural formula of the unsaturated phosphorus-nitrogen flame retardant is: .

2. The multilayer co-extruded polypropylene composite film for capacitors according to claim 1, characterized in that, In the B layer raw material, MAH-g-PP and BN@SiO2-NH2 are first mixed and fed into a twin-screw extruder, and melt-extruded and granulated at 180~220℃ to obtain premixed granules, which are then blended with the remaining raw materials.

3. The multilayer co-extruded polypropylene composite film for capacitors according to claim 1, characterized in that, The grafting rate of the MAH-g-PP is 1.2%~1.5%.

4. The multilayer co-extruded polypropylene composite film for capacitors according to claim 1, characterized in that, The preparation process of BN@SiO2-NH2 is as follows: 1) Mix boron nitride and urea at a mass ratio of 1:20~40 and place them in a ball mill. The ball-to-material ratio is 80:1~120:

1. The speed is 400~600 r / min. The ball milling is carried out for 15~20 h for mechanical stripping to obtain stripped boron nitride, which is denoted as BN. 2) BN was ultrasonically dispersed in an ethanol aqueous solution, and the pH was adjusted to 8-9 with dilute ammonia. Tetraethyl orthosilicate was slowly added dropwise to the BN dispersion. The mixture was stirred at 40°C for 6 hours, centrifuged and washed to obtain BN@SiO2. 3) BN@SiO2 is dispersed in ethanol, and 30-50% by weight of γ-aminopropyltriethoxysilane is added for reflux grafting to obtain BN@SiO2-NH2.

5. The multilayer co-extruded polypropylene composite film for capacitors according to claim 1, characterized in that, The preparation process of the modified flame retardant is as follows: Add BN@SiO2-NH2 and initiator AIBN to the reaction flask, sonicate to disperse evenly, and purge with nitrogen for 30 min; dissolve glycidyl methacrylate and unsaturated phosphorus nitrogen flame retardant in tetrahydrofuran to form a mixed monomer solution, add the mixed monomer solution to the reaction flask under nitrogen protection, and heat to 70℃ for 4-8 h. After the reaction is complete, cool to room temperature, add tetrahydrofuran to dilute the reaction system, then precipitate in anhydrous methanol, filter, wash, and dry to obtain the final product.

6. The multilayer co-extruded polypropylene composite film for capacitors according to claim 1, characterized in that, The molar ratio of glycidyl methacrylate to unsaturated phosphorus-nitrogen flame retardant is 3~6:4~7, and the mass ratio of total monomer mass to BN@SiO2-NH2 mass is 0.5~2:

1.

7. The multilayer co-extruded polypropylene composite film for capacitors according to claim 1, characterized in that, The nucleating agent is talc powder with a particle size of 100-200 nm; the antioxidant is antioxidant 1010 or antioxidant 168.

8. The multilayer co-extruded polypropylene composite film for capacitors according to claim 1, characterized in that, The composite membrane has an ABA three-layer structure with a thickness ratio of 1:1 to 3:

1.

9. The method for preparing a multilayer co-extruded polypropylene composite film for capacitors according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Polypropylene resin, modified flame retardant, nucleating agent and antioxidant are mixed in a high-speed mixer to obtain layer A raw material; MAH-g-PP and BN@SiO2-NH2 are mixed and fed into a twin-screw extruder, melt extruded and granulated at 180~220℃ to obtain premixed granules, and then mixed with polypropylene resin and alumina in a high-speed mixer to obtain layer B raw material. S2. The raw materials of layer A and layer B are melted at 200~240℃ through independent screw extruders, and then formed into ABA three-layer composite castings through co-extrusion die head; S3. The casting sheet is subjected to biaxial stretching. First, it is preheated at 50~70℃, and then longitudinally stretched at 110~130℃ with a stretching ratio of 3-3.

6. Then, it is preheated at 80~120℃ and transversely stretched at 110~130℃ with a stretching ratio of 2.6~3.

2. S4. The film is heat-set after biaxial stretching, then cooled at 50°C, corona treated, drawn, and wound to obtain the final product.

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