Preparation method of multi-layer co-extrusion composite insulating material

Abstract

The invention provides a preparation method of a multi-layer co-extrusion composite insulating material, and relates to the technical field of battery material manufacturing. The preparation method comprises the following steps: respectively melting and mixing an upper layer component, a middle layer component and a lower layer component which are respectively prepared, co-extruding, casting and forming to obtain a multi-layer membrane; performing two-way stretching on the multiple layers of membranes to obtain a composite insulating material; wherein raw materials adopted by the middle layer component comprise isotactic polypropylene and graphene in-situ polymerized polystyrene, raw materials adopted by the upper layer component comprise isotactic polypropylene and norbornene dianhydride, and raw materials adopted by the lower layer component comprise isotactic polypropylene and norbornene dianhydride. According to the obtained composite insulating material, good bonding strength exists between the layers and between the composite insulating material and the metal layer, and the composite insulating material is especially suitable for being used as a base film of 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 used to collect current in a battery. In lithium-ion batteries, it usually refers to metal current collectors such as copper foil, aluminum foil, or composite current collectors composed of metal and polymer. The core function of the current collector is to carry active substances and collect and output the current generated by electrochemical reactions, or input current into active substances to achieve the conversion of chemical energy into electrical energy. Among them, the composite current collector adopts a sandwich structure of "metal-polymer material-metal", which has shown 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 polymer material such as polyethylene terephthalate (PET), polypropylene (PP) or polyimide (PI) as a base film through processes such as vacuum coating.

[0004] Among various base film materials, the advantages of polypropylene film are excellent processing adaptability, stable physical and chemical properties, and low cost. Polypropylene film is made of polypropylene resin as the main raw material through processes such as casting, blow molding or biaxial stretching. In addition to being used as a base film for composite current collectors, it is also widely used in packaging, agriculture, medical and other fields.

[0005] In terms of the relatively mature biaxial stretching process in the prior art, the basic steps for preparing the polypropylene base film include: melt mixing, sheet casting, longitudinal stretching and transverse stretching. After obtaining the polypropylene base film, a metal layer of copper or aluminum can be deposited on both sides thereof by magnetron sputtering or evaporation process to form a composite material. A series of prior applications of applicants including publication numbers or authorization announcement numbers CN118700669B, CN119735842A, and CN118342812A have all studied polypropylene base film materials.

[0006] At present, the deficiency in the prior art is that, since polypropylene is a non-polar material, the wetting tension of its film surface is small, 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, when the polypropylene base film, especially the isotactic polypropylene base film containing non-polar small molecule additives, is compounded with the metal layer, its interlayer peeling strength is insufficient and easy to delaminate.

[0007] How to improve the adhesion of polypropylene-based film materials and enhance their interfacial bonding strength with 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 interfacial bonding strength with metals.

[0009] To solve at least one of the above problems, the present invention provides a method for preparing a multi-layer co-extruded composite insulating material, and the preparation method includes: Melting and kneading the separately prepared upper layer component, middle layer component, and lower layer component respectively, co-extruding, and casting into sheets to obtain a multi-layer film sheet; Biaxially stretching and heat setting the multi-layer film sheet to obtain a composite insulating material; Among them, the raw materials used in the middle layer component include isotactic polypropylene and polystyrene in-situ polymerized with graphene, the raw materials used in the upper layer component include isotactic polypropylene and norbornene dianhydride, and the raw materials used in the lower layer component include isotactic polypropylene and norbornene dianhydride.

[0010] 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 middle layer thickness is 2.5 to 5 microns.

[0011] In the above technical solution, the temperature conditions for melting and kneading the upper layer component and the lower layer component are 170°C to 220°C respectively; and / or the temperature conditions for melting and kneading the middle layer component are 180°C to 240°C; and / or the temperature conditions for casting into sheets are 80°C to 110°C.

[0012] In the above technical solution, before each longitudinal stretching or each transverse stretching during biaxial stretching, the multi-layer film sheet is preheated to 80°C to 100°C; and / or the stretching temperature for longitudinal stretching is 130°C to 150°C, and the stretching ratio is 3 to 5 times; and / or the stretching temperature for transverse stretching is 130°C to 150°C, and the stretching ratio is 4 to 6 times; and / or the temperature conditions for heat setting are 120°C to 140°C.

[0013] In the above technical solution, based on the total amount of the raw materials in the middle layer being 100 parts by mass, the raw materials used in the middle layer component include: isotactic polypropylene, 80 to 84 parts by mass; polystyrene in-situ polymerized with graphene, 16 to 18 parts by mass; antioxidant, 0.5 to 1 part by mass; slip agent, 1 to 2 parts by mass.

[0014] In the above technical solution, the preparation method of polystyrene in-situ polymerized with graphene includes: S110. Intercalate graphene with phthalic anhydride to obtain modified graphene; S120. Prepare a monomer dispersion using raw materials including a dispersant, modified graphene, and styrene monomer; S130. Add an initiator to the monomer dispersion, carry out a polymerization reaction under heating and stirring conditions, and after the reaction ends, perform centrifugal separation, washing, and drying to obtain a solid; S140. Under a protective atmosphere, treat the solid with an organic solvent containing a Lewis acid and nitrobenzene, remove the organic solvent after the treatment, wash, and dry to obtain polystyrene in-situ polymerized on graphene.

[0015] In the above technical solution, in S120, the dispersant includes sodium dodecyl sulfate; and / or in S130, the initiator includes potassium persulfate; and / or in S140, the Lewis acid includes aluminum trichloride.

[0016] In the above technical solution, the preparation method of polystyrene in-situ polymerized on graphene includes: S111. Place graphene in an aqueous solution of dilute hydrochloric acid, heat to 50 °C to 60 °C, stir for 2 h to 4 h, filter, wash, and dry to obtain graphene powder; S112. Mix phthalic anhydride evenly in acetone, then add graphene powder and water, heat to 60 °C to 70 °C, perform ultrasonic dispersion for 0.5 h to 1 h, filter, wash, and dry to obtain modified graphene; S120. Add sodium dodecyl sulfate, modified graphene, and styrene monomer to water and mix, adjust the pH value to 7 to 8, and prepare to obtain a monomer dispersion; S131. After preparing potassium persulfate into an initiator aqueous solution, under a protective atmosphere, drop the initiator aqueous solution into the monomer dispersion heated to 75 °C to 95 °C and stir synchronously. After the dropping is completed, keep the temperature for reaction for 4 h to 12 h; S132. Add a terminator to stop the reaction, cool to room temperature, then adjust the pH value of the system to 5 to 6, let it stand for precipitation, perform centrifugal separation, washing, and drying to obtain a solid; S140. Under a protective atmosphere, mix nitrobenzene evenly in dichloromethane, add the solid and disperse it evenly, then add aluminum trichloride, stir for 6 h to 8 h. After the stirring ends, evaporate dichloromethane, remove aluminum trichloride by pickling, wash, and dry to obtain polystyrene in-situ polymerized on graphene.

[0017] In the above technical solution, in S111, by mass ratio, graphene: aqueous hydrochloric acid solution = (1 - 10): 100; and / or in S111, the volume concentration of the aqueous hydrochloric acid solution is 4% to 8%; and / or in S112, by mass ratio, phthalic anhydride: graphene powder: acetone: water = (2 - 4): (6 - 12): (30 - 40): 100; and / or in S120, by mass ratio, sodium dodecyl sulfate: modified graphene: styrene monomer: water = (0.5 - 1.5): (2 - 6): (40 - 50): 100; and / or in S131, by mass ratio, potassium persulfate: styrene monomer = (0.05 - 0.1): (40 - 50); and / or in S131, in the initiator aqueous solution, the concentration of potassium persulfate is 3 wt% to 6 wt%; and / or in S132, the terminator includes hydroquinone; and / or in S140, by mass ratio, aluminum trichloride: nitrobenzene: solid matter: dichloromethane = (4 - 8): (4 - 8): (10 - 30): 100.

[0018] In the above technical solution, based on the total amount of the raw materials of the upper layer component and the lower layer component being 100 parts by mass respectively, the raw materials used for the upper layer component and the lower layer component respectively include: isotactic polypropylene, 92 to 96 parts by mass; norbornene dianhydride, 3.9 to 5 parts by mass; initiator, 0.1 to 1 part by mass; antioxidant, 0.5 to 1 part by mass; slip agent, 2 to 3 parts by mass.

[0019] Beneficial effects The preparation method of the present invention prepares a composite insulating material through a multi - layer co - extrusion process. The main material used for the composite insulating material is polypropylene. The composite insulating material includes an upper layer, a middle layer, and a lower layer. Among them, graphene is added to the middle layer to improve the thermal stability, chemical stability, and mechanical properties of polypropylene. The upper layer and the lower layer are graft - modified to polypropylene through a norbornene structure to utilize the norbornene structure to limit the migration of graphene into the upper and lower surfaces of the composite insulating material.

[0020] The specific beneficial effects of the present invention are as follows: (1) The upper and lower layers of the composite insulating material are both used to cover metal films by means such as vacuum evaporation or magnetron sputtering. The materials used for the upper and lower layers of the composite insulating material are polypropylene graft-modified with norbornene. The way to achieve the modification is to melt and knead norbornene dianhydride and isotactic polypropylene under the action of an initiator at high temperature conditions, and carry out graft modification in a melt grafting manner. The norbornene structure forms a rigid bicyclic structure by sharing two carbon atoms between two cyclopentane rings. It has a large steric hindrance, has polar groups, and can be filled into the crystalline region of polypropylene as an amorphous unit. The graft modification of norbornene dianhydride not only improves the surface energy of polypropylene, increases the polarity of polypropylene, but also uses its large steric hindrance to limit the migration of graphene in the middle layer into the surface layer, thereby avoiding the contact between graphene small molecules and the metal layer. Since graphene small molecules are restricted from entering the upper and lower surface layers, the problem of poor adhesion caused by the formation of a graphene weak boundary layer on the upper and lower surface layers can be avoided.

[0021] (2) In order to limit the migration and aggregation of graphene in the middle layer in polypropylene, the present invention cures graphite in polystyrene by means of in-situ polymerization of graphene, and then adds this polystyrene material to polypropylene. The achieved effect is that polystyrene can physically fix graphite to limit its migration and aggregation. In addition, the addition of polystyrene is also beneficial to improving the processability and dimensional stability of polypropylene.

[0022] (3) Another characteristic of the graphene used in this composite insulating material is that it is intercalated with phthalic anhydride. After the graphene is mixed with styrene monomer, a polystyrene material uniformly dispersed with graphene can be obtained by in-situ polymerization. After treating this polystyrene material with an organic solution containing Lewis acid and nitrobenzene, the phthalic anhydride between the graphene layers can react with polystyrene in a Friedel-Crafts reaction, which not only improves the bonding strength between graphene and polystyrene, but also imparts polarity to polystyrene to improve the bonding strength between the middle layer and the upper and lower surface layers. Description of the Drawings

[0023] Figure 1 It is the FTRI spectrum of polystyrene sample 2. Detailed Embodiments

[0024] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following will make a detailed description in combination with specific embodiments of the present invention.

[0025] Unless otherwise specified, the reagents and raw materials used in the present invention can be purchased through commercial channels. The experimental methods without specific conditions noted in the following examples are carried out according to conventional methods and conditions, or selected according to the product instructions.

[0026] The present invention provides a method for preparing a multi-layer coextruded composite insulating material. The composite insulating material obtained by this preparation method can be used as the 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 coextrusion preparation process.

[0027] Multi-layer Coextrusion is a molding process in which different molten resins are simultaneously extruded through multiple extruders and then layered and fused through a composite die head (such as a coextrusion die head or a laminate die head) to finally form a multi-layer composite film or sheet. The present invention is dedicated to preparing a multi-layer polypropylene-based film material including at least an upper layer, a middle layer, and a lower layer through multi-layer coextrusion. Among them, each layer uses polypropylene as the main raw material. The middle layer contains graphene for promoting CH-π interaction and polystyrene for improving the processing performance of polypropylene. The graphene and styrene monomer are in-situ polymerized to obtain a graphene in-situ polymerized polystyrene material with uniformly dispersed graphene inside. This process can use polystyrene to lock graphene to limit and slow down its migration and agglomeration. The upper and lower surface layers of the composite insulating material contain polypropylene grafted with norbornene. The norbornene structure can limit the migration of graphite into the upper and lower surface layers of the composite insulating material, and the polypropylene modified by grafting the norbornene structure has more polar groups than conventional polypropylene materials, so the connection strength with the metal layer will be improved.

[0028] The method for preparing the multi-layer coextruded composite insulating material of the present invention includes the following steps: A. Melting, mixing, and coextruding the separately prepared upper layer component, middle layer component, and lower layer component respectively, and then casting and forming to obtain a multi-layer film sheet; B. Biaxially stretching and thermally setting the multi-layer film sheet to obtain the composite insulating material; Among them, the raw materials used for the middle layer component include isotactic polypropylene and polystyrene in-situ polymerized with graphene, the raw materials used for the upper layer component include isotactic polypropylene and norbornene dianhydride, and the raw materials used for the lower layer component include isotactic polypropylene and norbornene dianhydride.

[0029] Preferably, the present invention uses isotactic polypropylene with a molecular weight of 300,000 Mw to 500,000 Mw and a melt index of 1.5 g / 10 min to 2.5 g / 10 min. It can be understood that isotactic polypropylene (iPP) is a stereoisomer of polypropylene with a highly regular molecular chain structure, characterized in that the methyl (CH3) side groups are located on the same side of the polymer main chain. This regularity of the same spatial configuration endows isotactic polypropylene with good crystallization ability. The regular arrangement of methyl groups enables the molecular chains of isotactic polypropylene to pack closely, and makes isotactic polypropylene have better mechanical properties, thermal stability and chemical stability compared with syndiotactic polypropylene or atactic polypropylene.

[0030] In order to further improve the crystallinity of isotactic polypropylene, the prior art usually adds small-molecule inorganic or polymer additives (such as: phenolic resin, polystyrene, fullerene, carbon nanotubes or graphene, etc.) to isotactic polypropylene. These small-molecule additives are used to guide the segments of isotactic polypropylene to pack more closely through CH-π interaction, thereby reducing the defects in the amorphous region.

[0031] Specifically, the CH-π interaction can guide the methyl groups in the molecular chains of isotactic polypropylene to be oriented with the aromatic rings of the π-electron system, enhance the order between segments, and thus improve the crystallinity. Higher crystallinity helps to further improve various properties such as the mechanical strength, thermal stability and chemical stability of the polypropylene-based film. In addition, the highly ordered chain arrangement can also reduce the dipole loss and improve the insulation performance of the polypropylene-based film. Therefore, it is very necessary to add small-molecule additives that can achieve or promote the CH-π interaction to isotactic polypropylene.

[0032] Among many small-molecule additives, graphene is particularly suitable as an additive for polypropylene-based films 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 ordered arrangement of polypropylene segments, improve the crystallinity, and can also inhibit the excessive growth of polypropylene spherulites, thereby forming smaller and more uniform crystals, better enhancing the mechanical strength and thermal stability of the base film, and ensuring the service life of the current collector.

[0033] However, there are also some problems with the above graphene additives in practical applications. First, the surface energy of graphene is relatively high, while that of polypropylene is relatively low. This surface energy difference causes graphene to tend to migrate to the surface of polypropylene during processing and use to reduce the system energy. The migration of graphene leads to the formation of a weak boundary layer (WBL). The weak boundary layer not only has poor adhesion but also causes deterioration of the mechanical properties of the base film. Moreover, the graphene migrating to the surface will prevent direct contact between the metal and polypropylene, and this phenomenon will also lead to a decrease in the adhesion between the polypropylene base film and the metal layer, and even peeling. Therefore, for the polypropylene base film added with graphene, restricting the migration of graphene and avoiding its contact with the metal on the surface layer are the keys to improving the performance of the polypropylene base film, especially the adhesion between the polypropylene base film and the metal layer. Second, polypropylene is composed of non-polar hydrocarbon chains, while the surface of graphene is composed of sp² hybridized carbon atoms, which also shows non-polarity. Therefore, the addition of graphene will further reduce the surface energy of polypropylene, and this problem will also have an adverse effect on its adhesion to the metal layer. Finally, non-polar graphene and polypropylene are mainly combined by weak van der Waals forces, and there is a lack of strong chemical bonds or polar interactions between them, resulting in weak interfacial bonding force between the two. Graphene is difficult to be uniformly dispersed in polypropylene and is prone to agglomeration. The agglomeration problem will have an adverse effect on the mechanical properties of the base film.

[0034] To solve at least one of the above problems, the present invention provides a preparation method of a multilayer co-extruded composite insulating material, which is used to prepare the polypropylene base film for the composite current collector. The main raw material of the polypropylene base film is isotactic polypropylene, and graphene for promoting CH-π interaction is added thereto. To improve the bonding ability between the polypropylene base film and the metal layer, the present invention uses polystyrene to fix graphene by in-situ polymerization and adopts a multilayer co-extrusion process to perform norbornene graft modification on the upper and lower surface layers, so as to utilize the steric hindrance of the norbornene structure to restrict the migration of graphene to the upper and lower surface layers and avoid contact between graphene and the metal layer. Specifically, the norbornene structure contained in the upper and lower surface layers is a cycloolefin with a bicyclic structure, and its structure is formed by two cyclopentane rings sharing two carbon atoms to form a rigid bicyclic structure and containing a carbon-carbon double bond. It has a large steric hindrance and can restrict the migration of graphene in the middle layer into the surface layer, thereby avoiding contact between graphene small molecules and the metal layer. Norbornene can also introduce polar groups into polypropylene, enhance the adhesion or bonding force between polypropylene and metals (such as aluminum, copper), and make metal atoms more easily anchored to the base film. In addition, the structure of norbornene belongs to a non-crystalline unit and can also be used as a heterogeneous nucleating agent to promote polypropylene to form finer and more uniform crystals, improving the mechanical strength and thermal stability of the product.

[0035] To prepare polystyrene in-situ polymerized with graphene, the present invention intercalates graphene with phthalic anhydride, mixes the intercalated graphene with styrene monomer, and prepares polystyrene through a polymerization reaction under the action of an initiator. The polystyrene is uniformly dispersed with graphene intercalated with phthalic anhydride. By soaking the polystyrene material with an organic solvent containing Lewis acid and nitrobenzene, the phthalic anhydride between the graphene layers can react with the polystyrene through a Friedel-Crafts reaction, improving 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 Lewis acid, its carbonyl group forms an acyl carbocation with the Lewis acid, and then undergoes an electrophilic substitution reaction with the benzene ring of the polystyrene through a Friedel-Crafts acylation reaction. The hydrogen atom on the benzene ring is substituted to generate polystyrene substituted with phthaloyl groups, thereby uniformly fixing the graphene in the polystyrene. In addition, the acyl groups generated by the above Friedel-Crafts acylation reaction will ultimately be hydrolyzed to carboxyl groups under the catalytic action of Lewis acid. The carboxyl groups can impart polarity to the polystyrene to increase the polarity of the intermediate layer and its bonding strength with the upper and lower surface layers.

[0036] Example 1 A series of polystyrene samples 1 to 4 in-situ polymerized with graphene were prepared in this example. The raw materials and their ratios (mass ratio) used are shown in Table 1.

[0037] Table 1 The preparation methods of the polystyrene samples 1 to 4 in-situ polymerized with graphene used in this example are as follows: S1. Pretreatment of graphene powder Add 400 mL of an aqueous solution of dilute hydrochloric acid with a volume concentration of 5% (prepared from commercially available 37% concentrated hydrochloric acid and deionized water) to a 1000 mL beaker. According to the mass ratio in Table 1, add commercially available graphene powder (purity ≥ 99.8%, sheet thickness 5 - 10 nm). Place the beaker in a magnetic stirring water bath, heat it to 55°C ± 2°C, and stir at a stirring rate of 200 rpm for 2 hours. After the stirring is completed, filter it hot with a medium-speed quantitative filter paper, wash the filter cake with deionized water until the filtrate is neutral, transfer the solid to a vacuum drying oven, and dry it at 60°C to constant weight to obtain graphene powder with an acid-activated surface.

[0038] S2. Preparation of intercalated and modified graphene According to the mass ratio in Table 1, add phthalic anhydride and acetone into a 500 mL beaker, and stir magnetically at a stirring rate of 100 rpm until the phthalic anhydride is completely dissolved (about 20 min). Add the graphene powder prepared in S1, then add deionized water (the added amount of deionized water is 100 mL), transfer it to an ultrasonic device (power 400 W, frequency 80 kHz), and ultrasonically disperse it at 65 °C ± 2 °C for 30 min to form a uniform black suspension. Filter it with a 0.45 μm polyvinylidene fluoride membrane, wash the filter cake with acetone three times to remove the phthalic anhydride that has not entered the interlayer, and then dry it in a vacuum drying oven at 50 °C to constant weight to obtain intercalation-modified graphene.

[0039] S3. Preparation of monomer dispersion According to the mass ratio in Table 1, add 300 mL of deionized water into a 1000 mL beaker, and then successively add sodium dodecyl sulfate, the intercalation-modified graphene prepared in S2, and styrene monomer (the inhibitor is removed by vacuum distillation before use). Place the beaker in a magnetic stirring water bath, and stir at a stirring rate of 60 rpm at room temperature for 1 h. After stirring, adjust the pH of the system to 8 with 10% aqueous sodium bicarbonate solution to form a monomer dispersion.

[0040] S4. In-situ polymerization reaction Prepare an aqueous initiator solution of 4 wt% potassium persulfate and place it in a constant pressure dropping funnel. Add the monomer dispersion into a three-necked flask, and introduce nitrogen gas (flow rate 50 mL / min) into the three-necked flask for 30 minutes to remove air and maintain a nitrogen atmosphere. Place the three-necked flask in a magnetic stirring water bath, heat it up to 80 °C ± 2 °C, and stir at a stirring rate of 60 rpm. According to the mass ratio in Table 1, slowly drop the initiator solution (the dropping rate is about 1 drop / second). After dropping, keep stirring at a constant temperature of 80 °C ± 2 °C for 8 h, and the system gradually becomes a viscous milky white suspension.

[0041] S5. Post-treatment of polymerization product After the reaction is completed, add 2 mL of 5% hydroquinone ethanol solution (terminator), and stir for 10 minutes to terminate the reaction. Naturally cool to room temperature, adjust the pH to 6 with 8% hydrochloric acid aqueous solution by volume, and let it stand for 1 h to precipitate the product. Transfer it to a centrifuge, centrifuge at 4000 rpm for 15 minutes, collect the precipitate, and wash it three times with deionized water. The product is dried in a vacuum drying oven at 50 °C to constant weight to obtain a solid.

[0042] S6. Friedel-Crafts reaction According to the mass ratio in Table 1, add 80 mL of dichloromethane and nitrobenzene into a 250 mL three-necked flask, and stir magnetically until evenly mixed. Add the solid obtained in S5, and disperse it by ultrasonic wave for 15 minutes (power 400 W, frequency 80 kHz) to form a uniform suspension. Slowly add anhydrous aluminum trichloride, and stir magnetically at room temperature for 6 h (rotation speed 100 rpm). After the reaction is completed, remove dichloromethane by a rotary evaporator (water bath temperature 40 °C, vacuum degree ≤ 0.06 MPa). Wash the residue three times with a hydrochloric acid solution with a volume concentration of 5% to remove aluminum trichloride, and then wash it successively with deionized water, tetrahydrofuran, and methanol. The product is dried in a vacuum drying oven at 50 °C to constant weight to obtain polystyrene in-situ polymerized on graphene.

[0043] Performance Test 1 See Figure 1 As shown, in the FTIR spectrum of the polystyrene sample 2 in-situ polymerized on graphene, characteristic absorption peaks of the benzene ring of polystyrene appear at 1450 to 1600 cm -1 and a characteristic peak of the C=O stretching vibration of the carboxylic acid group appears at 1700 cm -1 This indicates that polystyrene has been successfully in-situ polymerized on graphene.

[0044] Example Two In this example, 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 example are as follows: S1. Prepare the upper layer component and the lower layer component Based on the total amount of raw materials of the upper layer component and the lower layer component being 100 parts by mass respectively, the raw materials of the upper layer component and the lower layer component are as follows: isotactic polypropylene, 93 parts by mass; norbornene dicarboxylic anhydride, 4 parts by mass; initiator (azobisisobutyronitrile), 0.2 parts by mass; antioxidant (2,6-di-tert-butyl-p-cresol), 0.5 part by mass; slip agent (calcium stearate), 2.3 parts by mass.

[0045] S2. Prepare the middle layer component Based on the total amount of raw materials of the middle layer being 100 parts by mass, the raw materials used in the middle layer component include: isotactic polypropylene (isotactic structure content 95%), 81.5 parts by mass; polystyrene in-situ polymerized on graphene, 16 parts by mass; antioxidant (2,6-di-tert-butyl-p-cresol), 0.5 part by mass; slip agent (calcium stearate), 2 parts by mass. Among them, the polystyrene in-situ polymerized on graphene used in the preparation of the composite insulation material samples 1 to 4 corresponds to the raw material ratios and process parameters of the polystyrene samples 1 to 4 in Example One in sequence.

[0046] S3. Preparation of multi-layer film The raw materials of the upper layer, the lower layer, and the middle layer are respectively added to a high-speed mixer and mixed at 120°C for 10 minutes, and then extruded and melted through a twin-screw extruder (screw speed: 120 rpm). The temperature conditions for melting and kneading the upper layer component and the lower layer component are 170°C (feeding section) - 200°C (homogenizing section) - 220°C (die head) respectively. The temperature conditions for melting and kneading the middle layer component are 180°C (feeding section) - 220°C (homogenizing section) - 240°C (die head). The melted upper and lower surface layers and the middle layer materials are extruded into a sheet-like fluid through the die head of the twin-screw extruder (die lip gap: 1.2 mm), and the sheet-like fluid is formed into a cast film through a quenching roller at 90°C and a high-pressure air knife (traction speed: 8 m / min) to obtain a three-layer film (thickness ratio of the upper layer: the middle layer: the lower layer = 0.4:1:0.4, and the thickness of the middle layer is about 4 microns).

[0047] S4. Biaxial stretching and heat setting The film is preheated to 90°C (preheating time: 30 s), and then longitudinally stretched at 140°C with a stretching ratio of 4 times (stretching rate: 300% per second) using a biaxial stretching machine. After cooling to room temperature, the film is preheated to 90°C again and then transversely stretched at 145°C with a stretching ratio of 5 times (stretching rate: 300% per second). It is heat set at 130°C for 20 s, slowly cooled to 80°C, and then naturally cooled to room temperature, and the composite insulation material samples 1 to 4 are wound up.

[0048] Performance test 2 A metal aluminum layer is vapor-deposited on the surfaces of the composite insulation material samples 1 to 4 to obtain metallized film samples 1 to 4. The breakdown strengths of the metallized film samples 1 to 4 are tested. Among them, the conditions for vacuum evaporation include: the vacuum degree does not exceed 2×10 -5 Pa, the evaporation rate is 0.07 Å / s, the evaporation time is 240 s, the substrate temperature is 145°C, the wire feeding speed is 840 mm / min, and the evaporation boat temperature is 660°C. The 100°C breakdown strengths of the metallized film samples 1 to 4 are 654 V / μm, 668 V / μm, 663 V / μm, and 659 V / μm in sequence. After the metallized film samples 1 to 4 are placed in an environment with a temperature of 65°C and a relative humidity of 80% for 15 days, their breakdown strengths after the aging test are tested. Among them, the 100°C breakdown strengths of the metallized film samples 1 to 4 after the aging test are 602 V / μm, 634 V / μm, 627 V / μm, and 611 V / μm in sequence.

[0049] 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 protection scope of the present invention should be subject to the scope defined by the claims.

Claims

1. A preparation method of a multi-layer co-extruded composite insulating material, characterized in that, The preparation method includes: Melting and kneading the separately prepared upper layer component, middle layer component, and lower layer component respectively, co-extruding, and casting into a sheet to obtain a multi-layer film; Biaxially stretching and heat setting the multi-layer film to obtain a composite insulating material; Among them, the raw materials used for the middle layer component include isotactic polypropylene and polystyrene in-situ polymerized with graphene, the raw materials used for the upper layer component include isotactic polypropylene and norbornene anhydride, and the raw materials used for the lower layer component include isotactic polypropylene and norbornene anhydride.

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

3. The preparation method according to claim 1, characterized in that The temperature conditions for melting and kneading the upper layer component and the lower layer component are 170°C to 220°C respectively; and / or The temperature conditions for melting and kneading the middle layer component are 180°C to 240°C; and / or The temperature conditions for casting into a sheet are 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, preheat the multi-layer film 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 temperature conditions for heat setting are 120°C to 140°C.

5. The preparation method according to claim 1, wherein, Based on the total amount of the raw materials of the middle layer being 100 parts by mass, the raw materials used for the middle layer component include: The isotactic polypropylene, 80 to 84 parts by mass; The polystyrene in-situ polymerized with graphene, 16 to 18 parts by mass; Antioxidant, 0.5 to 1 part by mass; Slip agent, 1 to 2 parts by mass.

6. The preparation method according to any one of claims 1 to 5, characterized in that, The preparation method of the polystyrene in-situ polymerized with graphene includes: S110. Intercalate graphene with phthalic anhydride to obtain modified graphene; S120. Prepare a monomer dispersion liquid using raw materials including a dispersant, the modified graphene, and styrene monomer; S130. Add an initiator to the monomer dispersion liquid, carry out a polymerization reaction under heating and stirring conditions, and after the reaction, carry out centrifugal separation, washing, and drying to obtain a solid; S140. Under a protective atmosphere, treat the solid with an organic solvent containing a Lewis acid and nitrobenzene, remove the organic solvent after the treatment, wash, and dry to obtain the polystyrene in-situ polymerized with graphene.

7. The preparation method according to claim 6, characterized in that In S120, the dispersant includes sodium dodecyl sulfate; and / or In S130, the initiator includes potassium persulfate; and / or In S140, the Lewis acid includes aluminum trichloride.

8. The preparation method according to any one of claims 1 to 5, characterized in that, The preparation method of the polystyrene in-situ polymerized with graphene includes: S111. Place graphene in an aqueous solution of dilute hydrochloric acid, heat it to 50°C to 60°C, stir for 2 h to 4 h, filter, wash, and dry to obtain graphene powder; S112. Mix phthalic anhydride evenly in acetone, then add the graphene powder and water, heat it to 60°C to 70°C, perform ultrasonic dispersion for 0.5 h to 1 h, filter, wash, and dry to obtain modified graphene; S120. Add sodium dodecyl sulfate, the modified graphene, and styrene monomer to water and mix them, adjust the pH value to 7 to 8 to prepare the monomer dispersion liquid; S131. After preparing an aqueous solution of potassium persulfate as an initiator, under a protective atmosphere, drop the aqueous solution of the initiator into the monomer dispersion liquid heated to 75°C to 95°C, and stir synchronously. After the dropping is completed, keep the temperature for reaction for 4 h to 12 h; S132. Add a terminator to stop the reaction, cool to room temperature, then adjust the pH value of the system to 5 to 6, let it stand for precipitation, perform centrifugal separation, wash, and dry to obtain a solid; S140. Under a protective atmosphere, mix nitrobenzene evenly in dichloromethane, add the solid and disperse it evenly, then add aluminum trichloride, stir for 6 h to 8 h. After the stirring ends, distill off the dichloromethane, remove the aluminum trichloride by pickling, wash, and dry to obtain the polystyrene in-situ polymerized by graphene.

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

100.

10. The preparation method according to any one of claims 1 to 5, characterized in that, Based on the total amount of the raw materials of the upper layer component and the lower layer component being 100 parts by mass respectively, the raw materials used for the upper layer component and the lower layer component respectively include: Isotactic polypropylene, 92 parts by mass to 96 parts by mass; Norbornene dianhydride, 3.9 parts by mass to 5 parts by mass; Initiator, 0.1 part by mass to 1 part by mass; Antioxidant, 0.5 part by mass to 1 part by mass; Slip agent, 2 parts by mass to 3 parts by mass.

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