A method for preparing multi-layer co-extruded composite insulation material

Abstract

The present invention provides a method for preparing a multi-layer co-extruded composite insulating material, which relates to the technical field of battery material manufacturing. The preparation method comprises: melt-mixing, co-extruding, and casting the separately prepared upper layer components, middle layer components, and lower layer components to obtain a multi-layer membrane; biaxially stretching the multi-layer membrane to obtain a composite insulating material; wherein the raw materials used for the middle layer component include isotactic polypropylene and polystyrene obtained by in-situ polymerization of graphene, the raw materials used for the upper layer component include isotactic polypropylene and nadic anhydride, and the raw materials used for the lower layer component include isotactic polypropylene and nadic anhydride. The composite insulating material obtained by the present invention has good bonding strength between each layer and between the composite insulating material and the metal layer, and the composite insulating material is particularly suitable as a base film for a composite current collector.

Description

Technical Field

[0001] The present invention relates to the technical field of battery material manufacturing, and in particular to a method for preparing a multi-layer co-extruded composite insulating material. Background Art

[0002] The current collector is a key structure or component in a battery used to collect current. In lithium-ion batteries, it usually refers to a metal current collector such as copper foil or aluminum foil, or a composite current collector composed of a metal and a polymer. The core function of the current collector is to carry active materials and collect and output the current generated by the electrochemical reaction, or to input current into the active material to achieve the conversion of chemical energy into electrical energy. Among them, the composite current collector adopts a "metal-polymer material-metal" sandwich structure, which has demonstrated significant cost and lightweight advantages in the field of lithium batteries and is an important development direction of current battery technology.

[0003] The composite current collector is a composite material formed by depositing metal layers on both sides of a base film made of a polymer material such as polyethylene terephthalate (PET), polypropylene (PP) or polyimide (PI) through processes such as vacuum coating.

[0004] Among various base film materials, polypropylene film boasts excellent processing adaptability, stable physical and chemical properties, and low cost. Made primarily from polypropylene resin through processes such as cast film, blow molding, or biaxial stretching, polypropylene film serves as a base film for composite current collectors and is widely used in packaging, agriculture, healthcare, and other fields.

[0005] The basic steps for preparing polypropylene-based film using the relatively mature biaxial stretching process in the prior art include melt mixing, sheet casting, longitudinal stretching, and transverse stretching. After obtaining the polypropylene-based film, copper or aluminum metal layers can be deposited on both sides via magnetron sputtering or evaporation to form a composite material. Prior applications by a series of applicants, including those with publication or authorization numbers CN118700669B, CN119735842A, and CN118342812A, have all investigated polypropylene-based film materials.

[0006] At present, the deficiency in the prior art is that, because polypropylene is a non-polar material, the wetting tension of its film surface is less, which causes its adhesion to be less than ideal. In addition, in order to ensure that the polypropylene film material has good mechanical properties, chemical stability and thermal stability, the use of additives such as graphene, fullerene, carbon nanotubes, etc. is very necessary. However, the above additives are usually small-molecule non-polar additives, and their use can further reduce the surface energy of polypropylene. Therefore, the polypropylene base film, especially the isotactic polypropylene base film containing non-polar small-molecule additives, when compounded with the metal layer, has insufficient interlayer peeling strength and is easily delaminated.

[0007] How to improve the adhesion of polypropylene-based film materials and enhance the interlayer bonding strength between them and metals is a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0008] One of the problems solved by the present invention is how to provide a polypropylene-based film material with better adhesion and better interlayer bonding strength with metal.

[0009] In order to solve at least one of the above problems, the present invention provides a method for preparing a multi-layer co-extruded composite insulation material, the preparation method comprising:

[0010] The upper layer component, the middle layer component and the lower layer component are melt-mixed, co-extruded and cast into sheets to obtain a multilayer membrane;

[0011] The multilayer film is biaxially stretched and heat-set to obtain a composite insulation material;

[0012] Among them, the raw materials used for the middle layer component include isotactic polypropylene and graphene in situ polymerized polystyrene, the raw materials used for the upper layer component include isotactic polypropylene and norbornene dic acid anhydride, and the raw materials used for the lower layer component include isotactic polypropylene and norbornene dic acid anhydride.

[0013] In the above technical solution, the thickness ratio of each layer in the composite insulating material is upper layer thickness: middle layer thickness: lower layer thickness = (0.3-0.6): 1: (0.3-0.6), and the thickness of the middle layer is 2.5 microns to 5 microns.

[0014] In the above technical solution, the temperature conditions for melt mixing the upper layer components and the lower layer components are 170°C to 220°C respectively; and / or the temperature conditions for melt mixing the middle layer components are 180°C to 240°C; and / or the temperature conditions for casting are 80°C to 110°C.

[0015] In the above technical solution, before each longitudinal stretching or each transverse stretching of biaxial stretching, the multilayer film is preheated to 80°C to 100°C; and / or the stretching temperature of the longitudinal stretching is 130°C to 150°C, and the stretching ratio is 3 times to 5 times; and / or the stretching temperature of the transverse stretching is 130°C to 150°C, and the stretching ratio is 4 times to 6 times; and / or the temperature condition for heat setting is 120°C to 140°C.

[0016] In the above technical solution, based on the total amount of raw materials of the intermediate layer being 100 parts by mass, the raw materials used for the intermediate layer components include: isotactic polypropylene, 80 to 84 parts by mass; graphene in situ polymerized polystyrene, 16 to 18 parts by mass; antioxidant, 0.5 to 1 part by mass; and lubricant, 1 to 2 parts by mass.

[0017] In the above technical solution, the method for preparing polystyrene by in-situ polymerization of graphene includes:

[0018] S110, performing intercalation treatment on graphene using phthalic anhydride to obtain modified graphene;

[0019] S120, preparing a monomer dispersion using raw materials including a dispersant, modified graphene, and styrene monomer;

[0020] S130, adding an initiator to the monomer dispersion, carrying out a polymerization reaction under heating and stirring conditions, and centrifuging, washing, and drying after the reaction to obtain a solid;

[0021] S140. Under a protective atmosphere, the solid is treated with an organic solvent containing Lewis acid and nitrobenzene. After the treatment, the organic solvent is removed, and the solid is washed and dried to obtain graphene in situ polymerized polystyrene.

[0022] In the above technical solution, in S120, the dispersant includes sodium lauryl sulfate; and / or in S130, the initiator includes potassium persulfate; and / or in S140, the Lewis acid includes aluminum trichloride.

[0023] In the above technical solution, the method for preparing polystyrene by in-situ polymerization of graphene includes:

[0024] S111, placing graphene in a dilute hydrochloric acid aqueous solution, heating to 50° C. to 60° C., stirring for 2 h to 4 h, filtering, washing, and drying to obtain graphene powder;

[0025] S112, mixing phthalic anhydride in acetone uniformly, adding graphene powder and water, heating to 60° C. to 70° C., ultrasonically dispersing for 0.5 h to 1 h, filtering, washing, and drying to obtain modified graphene;

[0026] S120, adding sodium lauryl sulfate, modified graphene, and styrene monomer into water, mixing, adjusting the pH value to 7 to 8, and preparing a monomer dispersion;

[0027] S131, after potassium persulfate is prepared into an initiator aqueous solution, under a protective atmosphere, the initiator aqueous solution is added dropwise into the monomer dispersion heated to 75° C. to 95° C., while stirring simultaneously, and after the addition is complete, the reaction is kept warm for 4 to 12 hours;

[0028] S132, adding a terminator to stop the reaction, cooling to room temperature, adjusting the pH of the system to 5 to 6, allowing the system to settle, centrifuging, washing, and drying to obtain a solid;

[0029] S140. Under a protective atmosphere, nitrobenzene is mixed evenly with dichloromethane, solids are added and dispersed evenly, aluminum chloride is added, and the mixture is stirred for 6 to 8 hours. After the stirring is completed, the dichloromethane is evaporated off, the aluminum chloride is removed by acid washing, and the mixture is washed and dried to obtain graphene in situ polymerized polystyrene.

[0030] In the above technical solution, in S111, the mass ratio of graphene: dilute hydrochloric acid aqueous solution is (1-10):100; and / or in S111, the volume concentration of the dilute hydrochloric acid aqueous solution is 4% to 8%; and / or in S112, the mass ratio of phthalic anhydride: graphene powder: acetone: water is (2-4): (6-12): (30-40):100; and / or in S120, the mass ratio of sodium lauryl sulfate: modified graphene: styrene monomer: water is (0.5-1.5): (2-6): (40-50):100; and / or in S131, the mass ratio of potassium persulfate: styrene monomer is (0.05-0.1): (40-50); and / or in S131, the concentration of potassium persulfate in the initiator aqueous solution is 3wt% to 6wt%. wt%; and / or in S132, the terminator includes hydroquinone; and / or in S140, the mass ratio is aluminum chloride:nitrobenzene:solid matter:dichloromethane = (4-8): (4-8): (10-30): 100.

[0031] In the above technical solution, the total amount of raw materials of the upper layer component and the lower layer component is 100 parts by mass respectively, and the raw materials used for the upper layer component and the lower layer component respectively include: isotactic polypropylene, 92 to 96 parts by mass; nadic anhydride, 3.9 to 5 parts by mass; initiator, 0.1 to 1 part by mass; antioxidant, 0.5 to 1 part by mass; and lubricant, 2 to 3 parts by mass.

[0032] Beneficial effects

[0033] The preparation method of the present invention produces a composite insulating material through a multi-layer co-extrusion process. The composite insulating material uses polypropylene as its main component. The composite insulating material includes an upper layer, an intermediate layer, and a lower layer. The intermediate layer contains graphene to improve the thermal stability, chemical stability, and mechanical properties of the polypropylene. The upper and lower layers are grafted onto polypropylene using a norbornene structure to utilize the norbornene structure to restrict graphene migration into the upper and lower surfaces of the composite insulating material.

[0034] The specific beneficial effects of the present invention are as follows:

[0035] (1) The upper and lower layers of the composite insulating material are used to cover the metal film by vacuum evaporation or magnetron sputtering. The material used for the upper and lower layers of the composite insulating material is polypropylene grafted with norbornene. The modification method is to melt-mix norbornene diacid anhydride and isotactic polypropylene under the action of an initiator at a high temperature and graft-modify them by melt grafting. The norbornene structure is a rigid bicyclic structure formed by two cyclopentane rings sharing two carbon atoms. It has a large steric hindrance, a polar group, and can be filled into the crystalline region of polypropylene as a non-crystalline unit. The grafting modification of norbornene diacid anhydride not only increases the surface energy of polypropylene and increases the polarity of polypropylene, but also can use its large steric hindrance to limit the migration of graphene in the middle layer into the surface layer, thereby preventing the small graphene molecules from contacting the metal layer. Since the small graphene molecules are restricted from entering the upper and lower surface layers, the problem of poor adhesion caused by the formation of a weak boundary layer of graphene between the upper and lower surface layers can be avoided.

[0036] (2) To limit the migration and aggregation of graphene in the middle layer in polypropylene, the present invention solidifies graphite in polystyrene through in-situ polymerization of graphene, and then adds the polystyrene material to the polypropylene. The effect achieved thereby is that the graphite can be physically fixed by polystyrene, limiting its migration and aggregation. In addition, the addition of polystyrene also helps to improve the processability and dimensional stability of polypropylene.

[0037] (3) Another feature of the graphene used in the composite insulating material is that it has been intercalated with phthalic anhydride. After the graphene is mixed with styrene monomer, a polystyrene material uniformly dispersed with graphene can be obtained through in-situ polymerization. After the polystyrene material is treated with an organic solution containing Lewis acid and nitrobenzene, the phthalic anhydride between the graphene layers and the polystyrene can undergo a Fourier reaction, which not only improves the bonding strength between the graphene and the polystyrene, but also gives the polystyrene polarity, thereby improving the bonding strength between the middle layer and the upper and lower surface layers. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is the FTRI spectrum of polystyrene sample 2. DETAILED DESCRIPTION

[0039] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, a detailed description is given below in conjunction with specific embodiments of the present invention.

[0040] Unless otherwise specified, the reagents and raw materials used in the present invention can be purchased from commercial sources. Experimental methods in the following examples where specific conditions are not specified were performed according to conventional methods and conditions, or selected according to the product specifications.

[0041] The present invention provides a method for preparing a multi-layer co-extruded composite insulating material. The composite insulating material obtained by the preparation method can be used as a base film of a composite current collector. The main raw material of the composite insulating material is polypropylene, which is obtained through a multi-layer co-extrusion preparation process.

[0042] Multi-layer coextrusion is a molding process in which different molten resins are extruded simultaneously through multiple extruders, then fused in layers through a composite die (such as a coextrusion die or a stacking die) to ultimately form a multilayer composite film or sheet. The present invention is directed to producing a multilayer polypropylene-based film material comprising at least an upper layer, a middle layer, and a lower layer through multi-layer coextrusion. Each layer is primarily made of polypropylene. The middle layer contains graphene, which promotes CH-π interactions, and polystyrene, which improves the processing properties of polypropylene. The graphene and styrene monomers are polymerized in situ to produce a graphene-in-situ polymerized polystyrene material with graphene uniformly dispersed throughout. This process utilizes polystyrene to lock the graphene, limiting and slowing its migration and aggregation. The upper and lower surface layers of the composite insulating material contain polypropylene grafted with norbornene. The norbornene structure restricts graphite migration into the upper and lower surface layers of the composite insulating material. Furthermore, the norbornene-grafted polypropylene has more polar groups than conventional polypropylene, thereby improving the bond strength with the metal layer.

[0043] The preparation method of the multi-layer co-extruded composite insulation material of the present invention comprises the following steps:

[0044] A. Melting and mixing the upper layer component, the middle layer component and the lower layer component respectively prepared, co-extruding, and casting to form a multilayer film;

[0045] B. biaxially stretching and heat-setting the multilayer film to obtain a composite insulating material;

[0046] Among them, the raw materials used for the middle layer component include isotactic polypropylene and graphene in situ polymerized polystyrene, the raw materials used for the upper layer component include isotactic polypropylene and norbornene dic acid anhydride, and the raw materials used for the lower layer component include isotactic polypropylene and norbornene dic acid anhydride.

[0047] Preferably, the present invention utilizes isotactic polypropylene with a molecular weight of 300,000 to 500,000 Mw and a melt index of 1.5 to 2.5 g / 10 min. It is understood that isotactic polypropylene (iPP) is a polypropylene stereoisomer with a highly regular molecular chain structure, characterized by methyl (CH3) side groups located on the same side of the polymer backbone. This consistent spatial regularity imparts excellent crystallization to isotactic polypropylene. The regular arrangement of the methyl groups enables densely packed molecular chains in isotactic polypropylene, resulting in isotactic polypropylene exhibiting superior mechanical properties, thermal stability, and chemical stability compared to atactic or syndiotactic polypropylene.

[0048] To further enhance the crystallinity of isotactic polypropylene, existing technologies typically add small-molecule inorganic or polymer additives (such as phenolic resins, polystyrene, fullerenes, carbon nanotubes, or graphene) to isotactic polypropylene. These small-molecule additives are used to guide the chain segments of isotactic polypropylene to a closer alignment through the CH-π interaction, thereby reducing defects in the amorphous region.

[0049] Specifically, CH-π interactions can guide the directional alignment of methyl groups and the aromatic rings of the π-electron system in isotactic polypropylene molecular chains, enhancing the order between chain segments and thus improving crystallinity. Higher crystallinity further improves the mechanical strength, thermal stability, and chemical stability of polypropylene-based films. Furthermore, highly ordered chain alignment can reduce dipole losses and enhance the insulating properties of polypropylene-based films. Therefore, it is essential to add small molecule additives to isotactic polypropylene that can achieve or promote CH-π interactions.

[0050] Among many small molecule additives, graphene is particularly suitable as an additive for polypropylene-based membranes used as current collectors. This is because graphene has a two-dimensional honeycomb single-layer planar structure, which can provide nucleation sites for polypropylene, reduce the energy barrier required for the orderly arrangement of polypropylene chain segments, improve crystallinity, and inhibit the excessive growth of polypropylene spherulites, thereby forming smaller and more uniform crystals. This better improves the mechanical strength and thermal stability of the base membrane, thereby ensuring the service life of the current collector.

[0051] However, these graphene additives also present some challenges in practical applications. First, graphene has a higher surface energy, while polypropylene has a lower surface energy. This surface energy difference causes graphene to migrate to the polypropylene surface during processing and use, thereby reducing the system energy. This migration of graphene leads to the formation of a weak boundary layer (WBL). This WBL not only impairs adhesion but also degrades the mechanical properties of the base film. Furthermore, surface-migrating graphene hinders direct contact between the metal and the polypropylene, which in turn reduces adhesion between the polypropylene base film and the metal layer, and may even cause delamination. Therefore, for polypropylene-based films with graphene added, limiting graphene migration and preventing surface contact with metal is key to improving the performance of the polypropylene base film, especially its adhesion to the metal layer. Second, polypropylene is composed of non-polar hydrocarbon chains, while the graphene surface is composed of sp²-hybridized carbon atoms, which is also non-polar. Therefore, the addition of graphene further reduces the surface energy of polypropylene, which also negatively impacts its adhesion to the metal layer. Finally, the non-polar graphene and polypropylene are primarily bound by weak van der Waals forces, lacking strong chemical bonds or polar interactions. This results in a weak interfacial bond between the two. This makes it difficult for graphene to disperse evenly in polypropylene and prone to agglomeration. This agglomeration negatively impacts the mechanical properties of the base film.

[0052] To address at least one of the aforementioned issues, the present invention provides a method for preparing a multilayer co-extruded composite insulating material, which is used to prepare a polypropylene-based film for use in composite current collectors. The primary raw material of the polypropylene-based film is isotactic polypropylene, to which graphene is added to promote CH-π interactions. To enhance the bonding ability of the polypropylene-based film with the metal layer, the present invention employs polystyrene to immobilize the graphene via in-situ polymerization, and employs a multilayer co-extrusion process to graft norbornene onto the upper and lower surface layers. This utilizes the steric hindrance of the norbornene structure to restrict the migration of graphene to the upper and lower surface layers, thereby preventing contact between the graphene and the metal layer. Specifically, the norbornene structure contained in the upper and lower surface layers is a cycloolefin with a bicyclic structure, composed of two cyclopentane rings sharing two carbon atoms to form a rigid bicyclic structure, and contains a carbon-carbon double bond. Its high steric hindrance can restrict the migration of graphene from the intermediate layer into the surface layer, thereby preventing small graphene molecules from contacting the metal layer. Norbornenes can also introduce polar groups into polypropylene, enhancing the adhesion or bonding between polypropylene and metals (such as aluminum and copper), making it easier for metal atoms to anchor to the base film. Furthermore, norbornene, as a non-crystalline unit, can act as a heterogeneous nucleating agent, promoting the formation of finer, more uniform crystals in polypropylene, thereby improving the mechanical strength and thermal stability of the product.

[0053] To prepare polystyrene from graphene in situ polymerization, the present invention uses phthalic anhydride to intercalate graphene, mixes the intercalated graphene with styrene monomer, and prepares polystyrene through polymerization under the action of an initiator. Graphene intercalated with phthalic anhydride is uniformly dispersed in the polystyrene. By soaking the polystyrene material in an organic solvent containing Lewis acid and nitrobenzene, the phthalic anhydride between the graphene layers and the polystyrene undergo a Friedel-Strauss reaction, thereby increasing the bonding strength between the graphene and the polystyrene. Specifically, the phthalic anhydride between the graphene layers acts as an acylating agent. Under the catalytic action of the Lewis acid, its carbonyl group forms an acyl carbonium ion with the Lewis acid, which then undergoes electrophilic substitution with the benzene ring of the polystyrene through a Friedel-Strauss acylation reaction. The hydrogen atoms on the benzene ring are replaced, generating phthaloyl-substituted polystyrene, thereby uniformly fixing the graphene in the polystyrene. In addition, the acyl groups produced by the Friedel acylation reaction will eventually be hydrolyzed into carboxyl groups under the catalysis of Lewis acid. The carboxyl groups can impart polarity to polystyrene to increase the polarity of the middle layer and the bonding strength between the middle layer and the upper and lower surface layers.

[0054] Example 1 In this example, a series of graphene in-situ polymerized polystyrene samples 1 to 4 were prepared. The raw materials and their proportions (mass ratios) used are shown in Table 1.

[0055] Table 1

[0056]

[0057] The preparation method of polystyrene samples 1 to 4 of graphene in situ polymerization used in this embodiment is as follows:

[0058] S1. Pretreatment of graphene powder

[0059] To a 1000mL beaker, add 400mL of a 5% (volume) dilute hydrochloric acid solution (prepared by mixing commercially available 37% concentrated hydrochloric acid with deionized water). Add commercially available graphene powder (purity ≥99.8%, 5-10nm flake thickness) according to the mass ratios listed in Table 1. Place the beaker in a magnetically stirred waterbath, heat to 55°C ± 2°C, and stir at 200rpm for 2 hours. After stirring, filter the mixture while hot using medium-speed quantitative filter paper. Wash the filter cake with deionized water until the filtrate is neutral. Transfer the solids to a vacuum drying oven and dry at 60°C to constant weight to obtain surface-acid-activated graphene powder.

[0060] S2. Preparation of intercalated modified graphene

[0061] Phthalic anhydride and acetone were added to a 500 mL beaker in the mass ratios listed in Table 1. Stir magnetically at 100 rpm until the phthalic anhydride was completely dissolved (approximately 20 minutes). The graphene powder prepared in S1 was added, followed by 100 mL of deionized water. The mixture was transferred to an ultrasonic device (400 W power, 80 kHz frequency) and ultrasonically dispersed at 65°C ± 2°C for 30 minutes to form a uniform black suspension. The mixture was filtered through a 0.45 μm polyvinylidene fluoride membrane. The filter cake was washed three times with acetone to remove any phthalic anhydride that had not entered the interlayers. The mixture was then dried in a vacuum oven at 50°C to constant weight to obtain intercalated graphene.

[0062] S3. Preparation of monomer dispersion

[0063] According to the mass ratios in Table 1, add 300 mL of deionized water to a 1000 mL beaker. Then, add sodium lauryl sulfate, the intercalated graphene prepared from S2, and styrene monomer (remove the polymerization inhibitor by vacuum distillation before use). Place the beaker in a magnetically stirred water bath and stir at room temperature at 60 rpm for 1 hour. After stirring, adjust the pH to 8 with a 10% aqueous sodium bicarbonate solution to form a monomer dispersion.

[0064] S4. In situ polymerization

[0065] Prepare a 4wt% aqueous solution of potassium persulfate as an initiator and place it in a constant-pressure dropping funnel. Add the monomer dispersion to a three-necked flask and introduce nitrogen gas (50mL / min) into the flask for 30 minutes to expel air and maintain a nitrogen atmosphere. Place the flask in a magnetically stirred water bath, raise the temperature to 80°C ± 2°C, and slowly add the initiator solution dropwise (at a rate of approximately 1 drop / second) at a stirring rate of 60 rpm, using the mass ratios listed in Table 1. After the addition is complete, maintain a constant temperature of 80°C ± 2°C with stirring for 8 hours, until the system gradually transforms into a viscous, milky white suspension.

[0066] S5. Post-treatment of polymerization products

[0067] After the reaction is complete, add 2 mL of a 5% (by volume) hydroquinone ethanol solution (terminator) and stir for 10 minutes to terminate the reaction. Cool naturally to room temperature, adjust the pH to 6 with 8% (by volume) hydrochloric acid, and allow to stand for 1 hour to allow the product to precipitate. Transfer the mixture to a centrifuge and centrifuge at 4000 rpm for 15 minutes. Collect the precipitate and wash three times with deionized water. Dry the product in a vacuum oven at 50°C to constant weight to obtain a solid.

[0068] S6, Friedel-Merck reaction

[0069] In a 250 mL three-necked flask, add 80 mL of dichloromethane and nitrobenzene according to the mass ratios in Table 1 and stir magnetically until uniformly mixed. Add the solid obtained in S5 and disperse ultrasonically for 15 minutes (power 400 W, frequency 80 kHz) to form a uniform suspension. Slowly add anhydrous aluminum chloride and stir magnetically at room temperature for 6 hours (speed 100 rpm). After the reaction, remove the dichloromethane using a rotary evaporator (water bath temperature 40°C, vacuum ≤ 0.06 MPa). The residue is washed three times with 5% (volume concentration) hydrochloric acid to remove the aluminum chloride, followed by washing with deionized water, tetrahydrofuran, and methanol. The product is dried in a vacuum oven at 50°C to constant weight to obtain graphene-in-situ polymerized polystyrene.

[0070] Performance Test 1

[0071] See also Figure 1 As shown in the FTIR spectrum of the graphene in situ polymerized polystyrene sample 2, the wavelengths of the FTIR spectrum are between 1450 and 1600 cm -1 The characteristic absorption peak of polystyrene benzene ring appeared at 1700 cm -1 The characteristic peak of C=O stretching vibration of carboxylic acid group appeared, which indicated that polystyrene was successfully in situ polymerized on graphene.

[0072] Example 2

[0073] In this embodiment, a series of composite insulation material samples 1 to 4 were prepared. The preparation methods of the composite insulation material samples 1 to 4 in this embodiment are as follows:

[0074] S1. Preparation of upper and lower components

[0075] Based on 100 parts by mass of the total amount of raw materials for the upper layer component and the lower layer component, the raw materials for each of the upper layer component and the lower layer component are as follows: isotactic polypropylene, 93 parts by mass; nadic anhydride, 4 parts by mass; initiator (azobisisobutyronitrile), 0.2 parts by mass; antioxidant (2,6-di-tert-butyl-p-cresol), 0.5 parts by mass; and lubricant (calcium stearate), 2.3 parts by mass.

[0076] S2. Prepare the middle layer components

[0077] Based on 100 parts by mass of the total raw materials for the intermediate layer, the intermediate layer components include: 81.5 parts by mass of isotactic polypropylene (95% isotactic structure content); 16 parts by mass of graphene in-situ polymerized polystyrene; 0.5 parts by mass of an antioxidant (2,6-di-tert-butyl-p-cresol); and 2 parts by mass of a lubricant (calcium stearate). The graphene in-situ polymerized polystyrene used in preparing composite insulation material samples 1 to 4 was prepared using the same raw material ratios and process parameters as used for polystyrene samples 1 to 4 in Example 1.

[0078] S3. Preparation of multilayer membrane

[0079] The raw materials for the upper, lower, and middle layers were separately added to a high-speed mixer and mixed at 120°C for 10 minutes. The materials were then extruded into a melt through a twin-screw extruder (screw speed 120 rpm). The melt mixing temperatures for the upper and lower layer components were 170°C (feed section) - 200°C (homogenization section) - 220°C (die head). The melt mixing temperatures for the middle layer components were 180°C (feed section) - 220°C (homogenization section) - 240°C (die head). The melted upper and lower surface layer and middle layer materials were extruded through the die of the twin-screw extruder (die lip gap 1.2 mm) into a sheet. The sheet was then passed through a chilled roll at 90°C and a high-pressure air knife for sheet casting (pulling speed 8 m / min), resulting in a three-layer membrane (thickness ratio of upper layer:middle layer:lower layer = 0.4:1:0.4, with the middle layer thickness approximately 4 microns).

[0080] S4, biaxial stretching and heat setting

[0081] The film was preheated to 90°C (preheating time 30 seconds) and stretched longitudinally at 140°C at a 4x stretch ratio (stretching rate 300% per second) using a biaxial stretching machine. After cooling to room temperature, the film was again preheated to 90°C and stretched transversely at 145°C at a 5x stretch ratio (stretching rate 300% per second). Heat-set at 130°C for 20 seconds, slowly cooled to 80°C, and then naturally cooled to room temperature. The film was then rolled to obtain composite insulation material samples 1 to 4.

[0082] Performance Test 2

[0083] A metal aluminum layer was deposited on the surface of composite insulating material samples 1 to 4 to obtain metallized film samples 1 to 4. The breakdown strength of metallized film samples 1 to 4 was tested. The vacuum deposition conditions include: the vacuum degree does not exceed 2×10 -5 The conditions were: a 100°C puncture strength test of 654 V / μm, a 0.07 A / s evaporation rate, a 240 s evaporation time, a substrate temperature of 145°C, a wire feed speed of 840 mm / min, and an evaporation boat temperature of 660°C. The breakdown strengths of metallized film samples 1 to 4 at 100°C were, respectively, 654 V / μm, 668 V / μm, 663 V / μm, and 659 V / μm. Metallized film samples 1 to 4 were aged for 15 days at a temperature of 65°C and a relative humidity of 80%, and their breakdown strengths were then tested. After the aging test, the breakdown strengths of metallized film samples 1 to 4 at 100°C were, respectively, 602 V / μm, 634 V / μm, 627 V / μm, and 611 V / μm.

[0084] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A method for preparing a multi-layer co-extruded composite insulation material, characterized in that: The preparation method comprises: The upper layer component, the middle layer component and the lower layer component are melt-mixed, co-extruded and cast into sheets to obtain a multilayer membrane; Biaxially stretching and heat-setting the multilayer film to obtain a composite insulating material; Based on 100 parts by mass of the total amount of raw materials of the intermediate layer, the raw materials used for the intermediate layer components include: Isotactic polypropylene, 80 to 84 parts by mass; 16 to 18 parts by mass of graphene in situ polymerized polystyrene; Antioxidant, 0.5 to 1 parts by mass; Slip agent, 1 to 2 parts by mass; Based on the total amount of raw materials of the upper layer component and the lower layer component being 100 parts by mass, the raw materials used in the upper layer component and the lower layer component respectively include: Isotactic polypropylene, 92 to 96 parts by mass; Nadic anhydride, 3.9 to 5 parts by mass; Initiator, 0.1 to 1 parts by mass; Antioxidant, 0.5 to 1 parts by mass; Slip agent, 2 to 3 parts by mass; The preparation method of polystyrene by graphene in situ polymerization comprises: S111, placing graphene in a dilute hydrochloric acid aqueous solution, heating to 50° C. to 60° C., stirring for 2 h to 4 h, filtering, washing, and drying to obtain graphene powder; S112, mixing phthalic anhydride in acetone uniformly, then adding the graphene powder and water, heating to 60° C. to 70° C., ultrasonically dispersing for 0.5 h to 1 h, filtering, washing, and drying to obtain modified graphene; S120, adding sodium lauryl sulfate, the modified graphene, and styrene monomer into water, mixing, and adjusting the pH value to 7 to 8 to prepare the monomer dispersion; S131, after preparing potassium persulfate into an initiator aqueous solution, under a protective atmosphere, dropwise add the initiator aqueous solution into the monomer dispersion heated to 75° C. to 95° C., while stirring simultaneously, and after the dropwise addition is completed, keep the temperature to react for 4 to 12 hours; S132, adding a terminator to stop the reaction, cooling to room temperature, adjusting the pH of the system to 5 to 6, allowing the system to settle, centrifuging, washing, and drying to obtain a solid; S140. Under a protective atmosphere, uniformly mix nitrobenzene in dichloromethane, add solids and disperse them evenly, then add aluminum chloride, and stir for 6 to 8 hours. After stirring, evaporate the dichloromethane, remove the aluminum chloride by acid washing, wash, and dry to obtain the graphene in situ polymerized polystyrene.

2. The preparation method according to claim 1, characterized in that The thickness ratio of each layer in the composite insulating material is upper layer thickness: middle layer thickness: lower layer thickness = (0.3-0.6): 1: (0.3-0.6), and the thickness of the middle layer is 2.5 microns to 5 microns.

3. The preparation method according to claim 1, characterized in that The upper layer component and the lower layer component are melt-mixed at temperatures of 170° C. to 220° C., respectively; and / or The temperature condition for melt-kneading the intermediate layer components is 180° C. to 240° C.; and / or The temperature condition for the casting sheet molding is 80°C to 110°C.

4. The preparation method according to claim 1, characterized in that Before each longitudinal stretching or each transverse stretching of the biaxial stretching, the multilayer film is preheated to 80° C. to 100° C.; and / or The stretching temperature of the longitudinal stretching is 130° C. to 150° C., and the stretching ratio is 3 to 5 times; and / or The stretching temperature of the transverse stretching is 130° C. to 150° C., and the stretching ratio is 4 to 6 times; and / or The heat setting temperature condition is 120°C to 140°C.

5. The preparation method according to claim 1, characterized in that In S111, the mass ratio of graphene to dilute hydrochloric acid solution is (1-10):100; and / or In S111, the volume concentration of the dilute hydrochloric acid aqueous solution is 4% to 8%; and / or In S112, the mass ratio of phthalic anhydride: graphene powder: acetone: water is (2-4): (6-12): (30-40): 100; and / or In S120, the mass ratio of sodium lauryl sulfate: modified graphene: styrene monomer: water is (0.5-1.5): (2-6): (40-50): 100; and / or In S131, the mass ratio of potassium persulfate to styrene monomer is (0.05-0.1): (40-50); and / or In S131, the concentration of potassium persulfate in the initiator aqueous solution is 3 wt % to 6 wt %; and / or In S132, the terminator includes hydroquinone; and / or in S140, the mass ratio is aluminum chloride:nitrobenzene:solid matter:dichloromethane = (4-8): (4-8): (10-30): 100.

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