A graphene composite enameled wire and its preparation method

Abstract

This invention discloses a graphene composite enameled wire and its preparation method, belonging to the technical field of electromagnetic wire insulation materials. The enameled wire comprises a metal conductor and a double-layer insulating coating covering its surface. The double-layer insulating coating includes an inner dense insulating layer and an outer thermally conductive and wear-resistant layer. A biphase graphene intermediate modified by sequential silane coupling is used, combined with sheet-like hexagonal boron nitride, nano-alumina, and a resin system to construct a thermally conductive and insulating composite structure. Through multi-coating and segmented imidization curing, the thermal conductivity, wear resistance, breakdown voltage, and thermal stability of the enameled wire are improved, while reducing heat accumulation during operation.

Description

Technical Field

[0001] This invention relates to the technical field of electromagnetic wire insulation materials, and in particular to a graphene composite enameled wire and its preparation method. Background Technology

[0002] Enamelled wire is a commonly used winding conductor in motors, transformers, compressors, reactors, and various electromagnetic devices. It typically consists of a metallic conductor and an insulating coating covering the surface of the conductor. Besides meeting basic insulation requirements, the insulating coating must maintain good adhesion, heat resistance, mechanical integrity, and dielectric stability during winding, embedding, impregnation, thermal cycling, and long-term energized operation. Therefore, the stability of the enamelled wire insulation structure has a significant impact on the service life and reliability of electrical equipment.

[0003] As motors and related electromagnetic devices develop towards higher power density, higher frequency, and smaller size, the electrical and thermal loads borne by the windings per unit volume are constantly increasing, making the service environment of the enameled wire insulation layer more demanding. Under continuous energization or high-load operation, the heat generated by the conductor during operation is continuously transferred to the insulation layer; if the insulation layer's ability to adapt to heat dissipation and heat accumulation is insufficient, it can easily lead to an increase in local temperature rise. Prolonged exposure to high temperatures or temperature fluctuations may cause performance degradation in the insulation layer, thus affecting the overall stability of the enameled wire.

[0004] Existing enameled wire insulation systems are typically designed around heat resistance, insulation strength, and basic mechanical properties, and have a certain foundation in meeting conventional usage requirements. However, under higher heat load conditions, the insulation layer often needs to simultaneously consider electrical insulation stability, structural integrity, and heat transfer performance. These properties are often difficult to maintain a good balance in practical material systems over long periods. When heat becomes relatively concentrated inside the insulation layer or near the conductor-insulation interface, it can easily accelerate the aging of the insulation material, increase local performance fluctuations, and ultimately adversely affect the reliability of the enameled wire under complex operating conditions.

[0005] Therefore, how to reduce the adverse effects of heat accumulation on the insulation structure during operation while ensuring the basic insulation performance and structural stability of the enameled wire insulation layer remains a technical problem to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and provide a graphene composite enameled wire and its preparation method, so as to reduce the adverse effects of heat accumulation on the insulation structure during the operation of the enameled wire while ensuring the electrical insulation performance and structural stability of the insulation layer.

[0007] To achieve the above objectives, the first aspect of the present invention provides a graphene composite enameled wire, comprising a metal conductor and an insulating coating covering the surface of the metal conductor, wherein the insulating coating comprises an inner dense insulating layer and an outer thermally conductive and wear-resistant layer arranged sequentially.

[0008] The inner dense insulating layer is formed of a first insulating varnish, which comprises, by weight of solid components: 55-72 parts of polyimide precursor, 18-30 parts of polyamide-imide resin, 5-12 parts of fluorinated polyimide resin, 2-6 parts of benzoxazine resin, 1.2-2.5 parts of pre-coupled biphase graphene intermediate, and 0.5-1.5 parts of nano-alumina;

[0009] The outer thermally conductive and wear-resistant layer is formed of a second insulating varnish, which comprises, by weight of solid components: 38-55 parts of polyimide precursor, 20-32 parts of polyamide-imide resin, 8-15 parts of fluorinated polyimide resin, 3-8 parts of benzoxazine resin, 3-8 parts of pre-coupled biphase graphene intermediate, 4-12 parts of sheet-like hexagonal boron nitride, 1-4 parts of nano-alumina, and 0.15-0.80 parts of hydrophobic fumed silica.

[0010] The pre-coupled biphase graphene intermediate is formed by fluorinated graphene and partially reduced graphene oxide, and is obtained by sequential coupling modification with epoxy silane coupling agent and amino silane coupling agent.

[0011] The insulating coating is formed by multiple coating layers and segmental imidization and curing at 300-500°C.

[0012] The graphene composite enameled wire of the present invention adopts a double-layer insulation structure consisting of an inner dense insulating layer and an outer thermally conductive and wear-resistant layer. The inner layer is composed of an insulating skeleton consisting of a polyimide precursor, a polyamide-imide resin, a fluorinated polyimide resin, and a benzoxazine resin. The polyimide precursor is gradually imidized in subsequent heat treatment to form a host phase with high heat resistance. The polyamide-imide resin is used to improve the adhesion stability and thermomechanical strength between the enamel film and the metal conductor. The fluorinated polyimide resin can reduce local interfacial polarity fluctuations and suppress dielectric loss at high temperatures. The benzoxazine resin further participates in cross-linking under thermal action, so that the inner layer forms an insulating layer with both density and thermal stability.

[0013] The amounts of pre-coupled biphase graphene intermediates and nano-alumina added to the inner layer are controlled to a low range. Furthermore, the graphene sheets are modified by sequential coupling of epoxy silanes and amino silanes before entering the resin system, forming an organic interface layer of a certain thickness on the graphene surface. This interface layer can form a stable interfacial bond between the graphene sheets and the polyimide precursor, ensuring that the graphene sheets are uniformly coated and maintained in a dispersed state during resin curing. Due to the resin phase isolation between graphene sheets and the spatial barrier created by the coupling layer, direct face-to-face stacking between sheets is difficult, resulting in graphene exhibiting discrete thermally conductive nodes within the coating film rather than forming a continuous conductive network. Simultaneously, the low surface energy of the fluorinated graphene component further weakens the tendency for direct contact between sheets, making it easier for different graphene sheets to be separated by the resin phase. On the other hand, the nano-alumina particles in the system do not merely exist as auxiliary fillers. Their surface polar sites can generate interfacial adsorption with the modified graphene sheets, allowing some nano-alumina to be distributed around the graphene sheets, forming a particle-spaced structure. This structure is equivalent to introducing microscale insulating gaps between the graphene sheets, dividing the originally high in-plane conductivity graphene sheets into multiple insulating particles, further blocking the possibility of forming continuous conductive paths between graphene sheets from a microstructural perspective. Therefore, the system forms a discrete thermally conductive network composed of graphene sheets, nano-alumina particles, and the resin matrix. Heat can rapidly diffuse along the high thermal conductivity path of the graphene sheets over short distances and continue to diffuse outwards through interfacial transfer between adjacent fillers. Meanwhile, charge is unlikely to form continuous migration paths between different sheets, thus avoiding the formation of obvious conductive channels. This allows heat to be transferred along short-range paths between local sheets and particles without compromising the inner layer's requirements for breakdown strength and film integrity.

[0014] Secondly, the outer layer, while maintaining the continuous phase of polyimide resin, increases the amount of pre-coupled biphase graphene intermediates, sheet-like hexagonal boron nitride, and nano-alumina, and introduces hydrophobic fumed silica to regulate the rheology and curing morphology of the outer layer, enabling it to primarily perform thermal conductivity, scratch resistance, and surface damage resistance. The pre-coupled biphase graphene intermediates in the outer layer simultaneously act as an interfacial transition and sheet framework. The partially reduced graphene oxide retains defect sites and residual oxygen-containing groups, allowing it to form strong interfacial bonds with the aforementioned resin system; while fluorinated graphene reduces the surface energy of the sheets, mitigates direct agglomeration between sheets, and improves the slip characteristics of the paint film surface. Therefore, during the curing process of the outer layer, the pre-coupled biphase graphene intermediates promote the formation of interlaced thermally conductive pathways of hexagonal boron nitride along the in-plane direction of the paint film, and inhibit the hard aggregation of inorganic fillers in local areas, reducing stress concentration and electric field distortion caused by filler agglomeration. This allows the outer layer to possess both rapid heat dissipation and surface bearing capacity.

[0015] In summary, the inner layer of the insulating coating of this invention first ensures the adhesion stability and basic insulation integrity between the inner layer and the metal conductor, allowing localized heat from the conductor surface to be orderly conducted to the outer layer under high-temperature operation. The outer layer establishes a heat diffusion network with a higher proportion of lamellar and particulate fillers, rapidly dispersing the heat transferred from the inner layer along the axial and radial directions of the coating film, thereby reducing localized heat accumulation in the enameled wire under continuous load. Since heat is no longer concentrated in the interface region near the conductor, the tendency of the inner layer resin to undergo thermo-oxidative aging, softening, and microcrack initiation at high temperatures is suppressed. Therefore, the final result is a simultaneous improvement in softening breakdown temperature, breakdown voltage, scratch resistance, and winding crack resistance.

[0016] As a further improvement of the present invention, the pre-coupled biphase graphene intermediate contains 55-80 wt% partially reduced graphene oxide and 20-45 wt% fluorinated graphene.

[0017] Partially reduced graphene oxide retains a certain number of oxygen-containing functional groups and defect sites during the reduction process. These structures can chemically bond with epoxy silanes and amino silanes, forming strong interfacial interactions when subsequently contacting polyimide precursors or polyamide-imide resins. This allows graphene sheets to be firmly embedded in the resin matrix, thus establishing stable thermal conductivity nodes. Meanwhile, fluorinated graphene has low surface energy and high chemical stability. Its presence in the composite system weakens the π–π stacking tendency between graphene sheets, making it easier for the sheets to remain separated during dispersion. When these two structures coexist in the same sheet system, on the one hand, the partially reduced graphene oxide provides interfacial bonding points, preventing graphene sheets from precipitating out of the system or re-aggregating during resin curing; on the other hand, the fluorinated graphene reduces the mutual adsorption forces between sheets, making the dispersed sheet structure more stable.

[0018] When the proportion of partially reduced graphene oxide is below this range, the number of oxygen-containing sites in the graphene system decreases significantly, weakening the interfacial bonding ability with coupling agents and resins. Graphene sheets are more prone to recombination during curing, leading to thicker aggregates in localized areas. This structure not only weakens the uniformity of the heat conduction path but may also create localized areas of concentrated electric field under the influence of an electric field. When the proportion of partially reduced graphene oxide is too high, the interaction between graphene sheets intensifies, the regulating effect of fluorinated graphene on the sheet surface energy is insufficient, and the dispersion stability of the sheets in high-solids paint decreases. They are prone to recombination after shear dispersion, resulting in an uneven heat conduction network. Conversely, when the proportion of fluorinated graphene is controlled within the above range, its low surface energy structure can effectively cover part of the graphene sheet edges, maintaining an appropriate spatial separation distance between the sheets and thus preserving a stable sheet dispersion structure.

[0019] The graphene composite system formed under this ratio exhibits a relatively stable layer distribution structure during the coating film curing process. Partially reduced graphene oxide is interfacially bonded to the resin framework, allowing the layers to maintain relatively stable positions during curing. Fluorinated graphene, by reducing interfacial interactions and inhibiting recombination, presents graphene as numerous dispersed thermally conductive nodes within the coating film. Heat is rapidly diffused along individual graphene layers during conduction, then transferred to adjacent layers or inorganic fillers through the resin interface, gradually forming a continuous heat diffusion path. However, because resin or insulating particles remain between the layers, charge cannot form continuous migration pathways between different layers. Through this structural regulation, while maintaining the electrical insulation stability of the coating film, the thermally conductive paths can be more uniformly distributed within the film, thereby improving overall heat dissipation efficiency and reducing localized heat accumulation.

[0020] As a further improvement of the present invention, the amount of epoxy silane coupling agent added to the pre-coupled biphase graphene intermediate is 6-10% of the total mass of graphene, and the amount of amino silane coupling agent added is 2-5% of the total mass of graphene.

[0021] One end of the epoxy silane coupling agent molecule contains a hydrolyzable siloxane group, which can form siloxane bonds near defect sites or residual oxygen-containing groups on the graphene sheet surface during the reaction, thereby stably anchoring the silane molecule to the graphene sheet surface. The epoxy group at the other end can undergo a ring-opening reaction with the active groups in the resin system during subsequent paint mixing and curing, enabling the graphene sheet to form chemical or strong interfacial bonds with the polyimide precursor or polyamide-imide resin. This structure means that the graphene sheet is no longer in a simple physical filling state during paint curing, but is partially fixed by the resin skeleton, thus reducing the possibility of the sheet re-aggregating due to surface energy during high-temperature curing.

[0022] After epoxy silane forms the first interfacial structure, introducing an aminosilane coupling agent can further construct a second interfacial-regulated structure on the graphene surface. The amino groups in the aminosilane molecule have strong polarity, enabling them to generate strong interfacial interactions with carbonyl and amide groups in polyimide precursors or polyamide-imide resins. This creates locally enhanced polarity regions around the graphene sheets, thereby improving the dispersion stability of graphene in highly polar resin systems. Simultaneously, aminosilane molecules can also participate in partial interfacial crosslinking during curing, resulting in a more stable interfacial connection structure between the graphene sheets and the resin.

[0023] When the amount of epoxy silane is below the above range, the number of anchoring points on the graphene sheet surface is insufficient, and some graphene sheets still maintain a strong tendency to self-aggregate. During the dispersion and curing of the paint, local sheet stacking structures are easily formed, resulting in uneven spatial distribution of heat conduction paths. When the amount of epoxy silane is too high, an excessively thick organic coating layer will form on the graphene surface, isolating the thermal contact interface between graphene sheets by a large amount of organic layer, thereby reducing the effective heat conduction efficiency between graphene sheets. Conversely, when its addition is controlled within the above range, it can ensure that the graphene sheet surface has sufficient interface anchoring points without forming an excessively thick interface layer, thus maintaining the necessary thermal contact between graphene sheets.

[0024] If the amount of aminosilane coupling agent added is below the above range, its effect on regulating the dispersion stability of graphene is not significant, and graphene sheets may still locally aggregate in high-solids paint systems. If the amount added is too high, too many polar groups will be formed on the graphene surface, causing new mutual adsorption tendencies between sheets through hydrogen bonds or polar interactions, thereby increasing the probability of aggregation again. When the amount added is within the above range, the polar interface layer formed by aminosilane can form a stable solvation environment in the resin system, allowing the graphene sheets to maintain a stable spacing during dispersion, while not destroying the first interface structure formed by epoxy silane.

[0025] By controlling the dosage and sequence of the two types of silane coupling agents mentioned above, an interface structure that gradually transitions from the inside to the outside is formed on the surface of the graphene sheets. The region near the graphene surface is dominated by a chemically anchored structure formed by epoxy silanes, enabling the graphene to be stably embedded in the resin framework; the outer side is dominated by a polar interface layer formed by amino silanes, improving the dispersion stability of the graphene sheets in the paint. This interface structure allows the graphene sheets to maintain a relatively uniform spatial distribution within the paint film while also forming a stable bond with the resin, thus creating uniformly distributed thermally conductive nodes after curing. Heat can diffuse and transfer along these nodes to adjacent filler or resin regions step by step. Because the sheets are always separated by the resin or insulating filler, it is difficult for charges to form continuous migration paths between different graphene sheets, thereby improving thermal diffusion while maintaining the stability of the insulating structure.

[0026] As a further improvement of the present invention, the pre-coupled biphase graphene intermediate is further grafted with polyamic acid oligomers, wherein the amount of polyamic acid oligomers added is 8 to 16% of the total mass of graphene.

[0027] The pre-coupled biphase graphene intermediate is further grafted with polyamic acid oligomers, enabling the graphene sheet surface to form a polymeric interface structure compatible with the polyimide system beyond the silane coupling layer. This significantly improves the dispersion stability and interfacial bonding strength of the graphene sheets in polyimide resins. The polyamic acid oligomer itself is a structural form of polyimide precursor, containing both amide and carboxyl structures in its molecular chain. During curing, it can gradually undergo imidization and transform into a polyimide structure. Therefore, when this oligomer is grafted onto the graphene sheet surface, its molecular chain can undergo structural transformation and segment fusion with the surrounding polyimide precursor during subsequent curing, transforming the graphene sheet surface from a simple filler interface into part of the resin skeleton.

[0028] This structural change transforms the interface between the graphene sheets and the resin matrix from simple physical adsorption or weak interfacial bonding into an interfacial region with a certain degree of chemical and structural continuity. During the curing process, the polyamic acid oligomers grafted onto the surface of the graphene sheets gradually undergo imidization as the system temperature rises, forming a continuous polyimide structure with the surrounding resin molecules, thus creating a transition layer between the graphene sheets and the resin. This transition layer not only alleviates the interfacial stress caused by the modulus difference between the graphene sheets and the resin but also forms a more stable interfacial bond at the microstructure level, making it less likely for the graphene sheets to detach or migrate from the resin during curing and subsequent thermal cycling.

[0029] Furthermore, the grafted polyamic acid oligomers gradually transform into polyimide structures during curing, creating an interface region around the graphene sheets with properties similar to the host resin. This interface region can reduce the difference in thermal expansion coefficients between different materials at the microscale, thereby reducing the probability of microcracks forming in the coating film during high-temperature operation. Because a structurally continuous transition interface is formed between the graphene sheets and the resin, the thermally conductive sheets can maintain a more stable dispersion state and form uniformly distributed thermally conductive nodes in the cured coating film, allowing heat to gradually diffuse along the sheet structure into the surrounding material. Simultaneously, since the graphene sheets are always encapsulated and isolated by the polyimide structure, direct contact between different sheets is difficult, further reducing the possibility of forming continuous conductive paths. This allows the material to maintain high electrical insulation stability while improving thermal conductivity.

[0030] As a further improvement of the present invention, the average particle size of the plate-like hexagonal boron nitride is 2-15 μm and the thickness is 50-200 nm.

[0031] The control of the average particle size of the lamellar hexagonal boron nitride is to enable the material to form a lamellar structure with obvious in-plane orientation characteristics in the coating film. Hexagonal boron nitride itself has high in-plane thermal conductivity and high electrical insulation properties. When it is dispersed in a lamellar form in a polyimide resin system, it easily achieves a certain degree of orientational alignment along the surface direction of the coating film during coating and curing, thereby forming a heat diffusion path mainly in the in-plane direction within the coating film. If the particle size is too small, the lamellar structure is difficult to form an effective thermally conductive surface, and the efficiency of heat transfer across the lamellars decreases; if the particle size is too large, sedimentation or local accumulation is likely to occur during the coating film dispersion process, resulting in structural inhomogeneity in the thickness direction of the coating film. When the thickness is controlled within the above range, the lamellars have sufficient lateral dimensions to form a stable heat diffusion path and can maintain a good dispersion state in the resin system. This structure allows heat to diffuse rapidly within the coating film surface and be gradually transferred through the interface between adjacent lamellars, thereby improving the overall heat dissipation efficiency of the enameled wire insulation layer. Meanwhile, since hexagonal boron nitride is an electrically insulating material, its layered structure not only does not form a conductive path inside the coating film, but also forms an electrically insulating barrier between the graphene layers, so that the heat conduction path and the electrical conduction path are spatially separated, thereby improving heat dissipation capacity while maintaining insulation stability.

[0032] As a further improvement of the present invention, the average particle size of the nano-alumina is 20-80 nm.

[0033] The average particle size of nano-alumina is controlled to create a spacer-like particle structure between graphene and hexagonal boron nitride (BN) sheets. The nano-alumina particles have a strong polar surface, enabling them to form interfacial adsorption with the modified graphene and BN sheets in the resin system, thus distributing them microscopically around the sheet structure. When the particle size is smaller than this range, the specific surface area increases significantly, making them prone to aggregation in the paint and reducing particle dispersion stability. When the particle size is too large, their filling effect within the paint film weakens, making it difficult to form an effective particle spacer structure. By controlling the particle size within the aforementioned range, nano-alumina can form microscale insulating spacers between graphene sheets, maintaining a certain distance between different sheets and reducing the possibility of direct contact between them. Simultaneously, nano-alumina exhibits high thermal stability; its particle distribution within the paint film maintains structural stability under high temperatures, mitigating localized stress concentration caused by differences in thermal expansion and ensuring good structural integrity of the paint film under long-term thermal cycling.

[0034] As a further improvement of the present invention, the hydrophobic fumed silica has an average particle size of 10–40 nm and a specific surface area of ​​150–300 m². 2 / g.

[0035] The control of the average particle size and specific surface area of ​​hydrophobic fumed silica is aimed at enabling it to form a stable three-dimensional microstructure network in the paint system. This material possesses a high specific surface area and surface activity, allowing it to form a spatial structure in the paint through weak interactions between particles, thereby improving the thixotropic properties of the paint and inhibiting filler sedimentation. When the particle size of fumed silica is too small and the specific surface area is too high, its structural network in the system becomes overly dense, easily leading to a significant increase in paint viscosity and affecting the flowability of the coating process. When the particle size is too large or the specific surface area is too low, the resulting structural network is insufficient to support the high-density filler system, and the filler is prone to sedimentation during storage or coating. When the particle size and specific surface area are controlled within the aforementioned range, fumed silica can form a moderately stable microstructure network in the paint, maintaining a uniform dispersion of graphene, hexagonal boron nitride, and nano-alumina in the system, thus preventing the thermally conductive filler from agglomerating in localized areas. The uniformly dispersed filler structure not only helps to form a stable heat-conducting network, but also reduces the electric field concentration caused by localized filler accumulation, thereby maintaining the overall electrical insulation stability of the coating film.

[0036] As a further improvement of the present invention, the total thickness of the insulating coating is 20-60 μm, wherein the thickness of the inner dense insulating layer is 8-20 μm and the thickness of the outer thermally conductive and wear-resistant layer is 12-40 μm.

[0037] The total thickness of the insulating coating is controlled between 20 and 60 micrometers, with the inner dense insulating layer having a thickness of 8 to 20 micrometers and the outer thermally conductive and wear-resistant layer having a thickness of 12 to 40 micrometers. This creates clear functional zones in the coating structure along the thickness direction. The thinner, denser inner insulating layer ensures stable adhesion to the metal conductor while reducing the thermal resistance path for heat transfer from the conductor surface to the outer layer, allowing heat generated by the conductor to be transferred to the outer region more quickly. With a relatively increased outer layer thickness, the thermal diffusion network formed by the thermally conductive filler has a larger spatial distribution range in this region, enabling rapid diffusion of heat from the inner layer to the surrounding environment along the axial and radial directions of the coating. When the inner layer is too thick, heat accumulates significantly before being transferred to the outer layer through the inner insulating material, hindering the effectiveness of the outer thermally conductive structure. When the outer layer is too thin, the spatial distribution of the thermally conductive filler is restricted, making it difficult to form a continuous and stable thermal diffusion network.

[0038] A second aspect of the present invention provides a method for preparing the graphene composite enameled wire as described above, comprising the following steps:

[0039] S1 Preparation of pre-coupled biphase graphene intermediate: Partially reduced graphene oxide and fluorinated graphene are dispersed in an organic solvent to form a graphene dispersion system. An epoxy silane coupling agent is added to the dispersion system, and the reaction is carried out at 40-65°C for 1.5-3 hours. Then, an amino silane coupling agent is added and the reaction is continued for 1-2 hours. Subsequently, a polyamic acid oligomer is added for grafting reaction to obtain the pre-coupled biphase graphene intermediate.

[0040] S2 Preparation of the first insulating varnish: Polyimide precursor, polyamide-imide resin, fluorinated polyimide resin and benzoxazine resin are added to a mixed solvent and stirred to dissolve. Then, the pre-coupled biphase graphene intermediate and nano-alumina obtained in step S1 are added. After dispersion and filtration, the first insulating varnish is obtained.

[0041] S3 Preparation of the second insulating varnish: Polyimide precursor, polyamide-imide resin, fluorinated polyimide resin and benzoxazine resin are added to a mixed solvent and mixed and dissolved. Then, the pre-coupled biphase graphene intermediate obtained in step S1, sheet-like hexagonal boron nitride, nano-alumina and hydrophobic fumed silica are added. After dispersion, degassing and filtration, the second insulating varnish is obtained.

[0042] S4 Inner Insulation Layer Coating: After the metal conductor is cleaned and preheated, it is coated with multiple layers of first insulating varnish, and after each layer is coated, it is heated in multiple temperature ranges from low to high to form a dense inner insulation layer.

[0043] S5 Coating of the outer insulating layer: Multiple layers of a second insulating varnish are applied to the outer side of the inner dense insulating layer, and after each layer, the coating is sequentially subjected to segmented imidization curing in multiple temperature ranges from low to high, thereby forming an outer thermally conductive and wear-resistant layer and obtaining a graphene composite enameled wire.

[0044] In step S4, four coats of the first insulating varnish are applied, and the temperature ranges and heating times for each coat are as follows: the first temperature is 140–180℃, and the time is controlled at 5–10 s; the second temperature is 220–260℃, and the time is controlled at 5–10 s; the third temperature is 300–360℃, and the time is controlled at 6–12 s; the fourth temperature is 380–430℃, and the time is controlled at 4–8 s.

[0045] As a further improvement of the present invention, in step S5, a second insulating varnish is applied in three coats, and the temperature ranges and heating times experienced sequentially after the first coat are as follows: the first temperature is 160-220°C, and the time is controlled at 5-8 s; the second temperature is 240-320°C, and the time is controlled at 5-10 s; the third temperature is 360-420°C, and the time is controlled at 5-10 s; the temperature ranges and heating times experienced sequentially after the second and third coats are as follows: the first temperature is 280-320°C, and the time is controlled at 6-10 s; the second temperature is 380-420°C, and the time is controlled at 8-12 s; the third temperature is 440-480°C, and the time is controlled at 3-8 s.

[0046] By setting up preparation steps including the preparation of pre-coupled biphase graphene intermediates, the formulation of two insulating varnishes, and multi-stage coating and curing, graphene sheets, resin matrix, and inorganic fillers sequentially complete interface construction, stable dispersion, and thermosetting structural rearrangement during the structure formation process, thereby forming a uniform and stable thermally conductive and insulating composite structure within the varnish film. Firstly, during the preparation of the pre-coupled biphase graphene intermediates, the graphene is interface-modified to form an interface layer compatible with the polyimide system on the surface of the graphene sheets. This interface layer can form a stable interface with the polyimide precursor and polyamide-imide resin during subsequent resin mixing, ensuring the graphene sheets remain stably dispersed in the varnish system and reducing the tendency for direct stacking between sheets. Thus, during subsequent dispersion, graphene can gradually form a composite filler system with a certain spacing structure with hexagonal boron nitride sheets and nano-alumina particles, providing a uniform structural basis for the formation of the thermally conductive network.

[0047] In the preparation of the first and second insulating varnishes, the pre-coupled biphase graphene intermediate is dispersed separately with the resin system and inorganic fillers, allowing the graphene sheets to first establish a stable spatial distribution structure in the varnish. Since the graphene sheet surface already possesses an interface structure compatible with the resin, it can form a stable interfacial bond with the polyimide precursor and polyamide-imide resin during dispersion, thereby reducing the re-agglomeration of graphene sheets in high-solids systems. Simultaneously, sheet-like hexagonal boron nitride gradually forms an interlaced arrangement around the graphene sheets during dispersion, while nano-alumina particles are distributed between the sheets to form a microscale spacing structure, maintaining an appropriate distance between different thermally conductive fillers. This structure allows the thermally conductive sheets to maintain good thermal coupling without forming a direct, through-contact structure, thus maintaining a balance between thermal conductivity and insulation stability.

[0048] During the formation of the inner insulating layer, four consecutive coatings are applied, followed by staged heating at 140–180℃, 220–260℃, 300–360℃, and 380–430℃ respectively. This allows the resin system to complete solvent evaporation and initial imidization reactions under gradually increasing temperatures. The lower temperature range is mainly used to allow the solvent to evaporate slowly and stabilize the film structure, thus avoiding filler migration or local aggregation caused by rapid solvent evaporation. The medium temperature range allows the polyimide precursor to gradually undergo imidization, enabling the resin system to begin forming a continuous network structure. The higher temperature range further densifies the resin structure, stabilizing the graphene sheets and nano-alumina particles within the resin framework. Because the curing process uses a stepwise heating method, the position of the filler in the film remains stable before the structure is formed, thus allowing the graphene sheets to form uniformly distributed thermally conductive nodes in the inner insulating structure.

[0049] During the formation of the outer thermally conductive and wear-resistant layer, three coats are applied and cured in stages using different temperature ranges, allowing the filler structure to gradually rearrange during curing. The temperature ranges of 160–220℃, 240–320℃, and 360–420℃ after the first coat are primarily used to allow the solvent in the outer coating to evaporate and begin forming a preliminary structure, gradually stabilizing the graphene and hexagonal boron nitride sheets within the resin system. Subsequently, the temperature ranges of 280–320℃, 380–420℃, and 440–480℃ after the second and third coats further promote complete imidization and cross-linking reactions in the resin system, ensuring the complete formation of the polyimide structure and cross-linking of the benzoxazine resin, thereby constructing a more stable high-temperature resin network. Within these temperature ranges, the resin system gradually shrinks and densifies, resulting in a more stable spatial arrangement between the graphene and hexagonal boron nitride sheets.

[0050] As the curing process progresses, the distance between the sheet fillers gradually decreases, allowing heat to be transferred gradually through the interface between the graphene sheets and the hexagonal boron nitride sheets, forming a continuous thermal diffusion path. Simultaneously, because the graphene sheets are always spatially isolated by the resin phase and nano-alumina particles, it is difficult for them to form a continuous structure with direct contact, thus avoiding the formation of a through-conducting conductive path. This staged curing method allows the three processes of graphene interface modification, filler dispersion stabilization, and resin structure curing to be completed sequentially in time, preventing graphene agglomeration or recombination during high-temperature curing and ensuring that the inorganic fillers form a uniform and stable thermally conductive structure within the coating film. The resulting insulation layer not only has good thermal diffusion capabilities but also maintains a stable electrical insulation structure, effectively reducing localized temperature accumulation and slowing down the thermal aging process of the insulation material under high-power operating conditions, thereby improving overall operational stability.

[0051] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0052] (1) This invention constructs a double-layer insulation structure on the surface of a metal conductor, consisting of an inner dense insulation layer and an outer thermally conductive and wear-resistant layer, thereby creating clearly defined functional zones in the thickness direction of the insulation layer. The inner layer is composed of a dense insulating skeleton made of polyimide precursor, polyamide-imide resin, fluorinated polyimide resin, and benzoxazine resin, ensuring adhesion stability and electrical insulation integrity with the metal conductor. The outer layer constructs a heat diffusion network by increasing the content of graphene and inorganic thermally conductive fillers, enabling the heat generated from the conductor interface to diffuse rapidly outward. Through this layered structural design, the insulation layer maintains both high breakdown strength and thermal stability, while significantly improving the heat dissipation capacity of the enameled wire, thereby reducing the risk of insulation aging caused by heat accumulation during operation.

[0053] (2) This invention employs a biphase graphene system composed of partially reduced graphene oxide and fluorinated graphene, and modifies it by sequential coupling with epoxy silane and amino silane to form an interface layer with a gradient structure on the surface of the graphene sheets. This structure provides interfacial binding sites through partially reduced graphene oxide, enabling graphene to be stably embedded in the resin framework; on the other hand, fluorinated graphene reduces the surface energy of the sheets and inhibits π–π stacking, thus keeping the graphene dispersed within the coating film. Through this interface regulation, the graphene exists in the system as discrete thermally conductive nodes, which can both exert its high thermal conductivity and avoid the formation of a continuous conductive network, thereby improving thermal conductivity while maintaining insulation stability.

[0054] (3) This invention introduces graphene sheets, sheet-like hexagonal boron nitride, and nano-alumina into the outer insulating system to form a multi-scale composite filler system. The graphene sheets provide high thermal conductivity nodes, the hexagonal boron nitride sheets form in-plane thermal conductivity pathways during coating, and the nano-alumina particles form insulating spacer structures between the sheets. This structure allows heat to diffuse rapidly along the sheet structure and be transferred step-by-step through the interfaces between the fillers, thus forming a continuous thermal diffusion network. Simultaneously, the spacer effect of the nano-alumina effectively blocks direct contact between graphene sheets, reduces the possibility of forming local conductive channels, and reduces electric field concentration caused by filler aggregation, thereby improving the overall electric field stability of the insulating layer.

[0055] (4) Polyamic acid oligomers are further grafted onto the surface of the graphene sheets to form a polymeric interface structure compatible with the polyimide system. During subsequent curing, the oligomers can undergo imidization with the polyimide precursor to form a continuous polyimide structure, creating a structurally continuous interfacial transition layer between the graphene sheets and the resin matrix. This transition layer can alleviate the modulus difference and thermal expansion difference between the filler and the resin, reduce the interfacial stress generated during high-temperature operation and thermal cycling, thereby reducing the probability of microcracks or interfacial detachment inside the coating film and improving the structural stability of the insulation layer during long-term operation.

[0056] (5) This invention utilizes a multi-coating process and a segmented imidization curing process from low to high temperatures to ensure that solvent evaporation, filler stable distribution, and resin imidization reaction are completed sequentially over time. The lower temperature stage allows the solvent to evaporate slowly and stabilize the film structure; the medium temperature stage promotes the gradual imidization of the polyimide precursor; and the high temperature stage further densifies the resin structure and fixes the spatial position of the filler. Through this gradient curing process, graphene, hexagonal boron nitride, and nano-alumina can form a uniform and stable spatial distribution structure within the film, thereby preventing filler migration or re-agglomeration during high-temperature curing and ultimately forming a composite insulating layer with thermal conductivity, mechanical strength, and electrical insulation stability. Detailed Implementation

[0057] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0058] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0059] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0060] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0061] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0062] Example 1

[0063] This embodiment discloses a graphene composite enameled wire, which uses a copper round conductor as the substrate, and forms an inner dense insulating layer and an outer thermally conductive and wear-resistant layer on its surface in sequence, finally obtaining a graphene composite enameled wire with a double-layer insulation structure.

[0064] In this embodiment, TU1 oxygen-free copper round wire with a diameter of 0.80 mm is selected as the metal conductor.

[0065] The main raw materials used in the pre-coupled biphase graphene intermediate are as follows:

[0066] The partially reduced graphene oxide was prepared using graphene oxide products from Changzhou Sixth Element Materials Technology Co., Ltd. as a precursor, specifically using SE2430W graphene oxide filter cake, which was reduced before use.

[0067] Fluorinated graphene, in this embodiment, is prepared in laboratory-prepared sheet-like fluorinated graphene powder;

[0068] The epoxy silane coupling agent selected is γ-glycidoxypropyltrimethoxysilane, brand name KH-560.

[0069] The aminosilane coupling agent selected is γ-aminopropyltriethoxysilane, brand name KH-550;

[0070] The polyamic acid oligomer was added in the form of a 15 wt% N-methylpyrrolidone solution, which was pre-prepared in this embodiment according to the method described later.

[0071] The main raw materials for the first and second insulating varnishes are as follows:

[0072] The polyimide precursor is polyamic acid varnish, which is a polyamic acid solution obtained by polycondensation of aromatic dianhydride and aromatic diamine, with a solid content of 20%; the viscosity measured by a Brookfield viscometer at 25℃ is 5200 mPa·s; and the acid value is 105 mg KOH / g.

[0073] The polyamide-imide resin used is Torlon AI-10 LM manufactured by Synesqo, which is a solvent-based polyamide-imide resin soluble in N-methylpyrrolidone and N,N-dimethylacetamide. The viscosity of the 25% NMP solution is approximately 850 cP, and the acid value is approximately 82 mg KOH / g.

[0074] The preferred benzoxazine resin is JBZ-OP100I from JFE Chemical Corporation, with a softening point of 60-90℃, a viscosity of 200-700 mPa·s at 150℃, a Tg of 200℃, and a Td of 5% of 409℃.

[0075] Fluorinated polyimide resin is a fluorinated aromatic polyimide resin whose repeating structural units contain hexafluoroisopropyl or fluoroaryl structures; its fluorine content is 8.5%; and its number average molecular weight is 1.0 × 10⁻⁶. 4 ~3.0×10 4 In this embodiment, the preferred size is approximately 1.8 × 10⁻⁶. 4 Its glass transition temperature is 268℃; and its viscosity when prepared as a 15 wt% solution in N-methylpyrrolidone at 25℃ is 1600 mPa·s.

[0076] The preparation method of graphene composite enameled wire is as follows:

[0077] S1. Preparation of pre-coupled biphase graphene intermediates

[0078] First, 70 parts by weight of partially reduced graphene oxide and 30 parts by weight of fluorinated graphene were added to 600 parts by weight of a mixed organic solvent, which was a mixture of N-methylpyrrolidone and N,N-dimethylformamide in a mass ratio of 3:1. The mixture was then dispersed at 3000 rpm for 20 min using a high-speed disperser, followed by ultrasonic dispersion in an ice-water bath for 45 min to form a uniform graphene dispersion system.

[0079] Then, a hydrolysate of the epoxy silane coupling agent was prepared: 8 parts by weight of γ-glycidoxypropyltrimethoxysilane were added to a mixture of 40 parts by weight of anhydrous ethanol, 4 parts by weight of deionized water, and 0.6 parts by weight of glacial acetic acid, and pre-hydrolyzed at room temperature for 20 min. The pre-hydrolyzed epoxy silane coupling agent was then slowly added dropwise to the above graphene dispersion system, with the addition time controlled at 15 min. After the addition was completed, the temperature was raised to 55 °C, and the reaction was maintained at this temperature for 2 h.

[0080] After the above reaction was completed, 3 g of γ-aminopropyltriethoxysilane was added to the system, and the reaction was continued at 50 °C for 1.5 h. Then, a polyamic acid oligomer solution with a solid content of 15% was added to the system, and the amount added was 12 parts by weight based on solids. The reaction was continued at 45 °C for 1 h, so that the polyamic acid oligomer was grafted onto the surface of the sequentially coupled modified biphase graphene.

[0081] After the reaction was completed, the resulting system was centrifuged at 8000 rpm for 10 min, the supernatant was discarded, the precipitate was washed once with N-methylpyrrolidone, and then vacuum dried at 60℃ and -0.08 MPa for 12 h. The precipitate was then slightly pulverized and passed through a 100-mesh sieve to obtain the pre-coupled biphase graphene intermediate.

[0082] S2. Preparation of the first insulating varnish

[0083] In this step, the first insulating varnish comprises, by weight of solid components: 64 parts of polyimide precursor, 24 parts of polyamide-imide resin, 8 parts of fluorinated polyimide resin, 4 parts of benzoxazine resin, 1.8 parts of pre-coupled biphase graphene intermediate, and 1.0 part of nano-alumina; the average particle size of the nano-alumina is 20 nm.

[0084] In the specific preparation process, 310 parts by weight of a mixed solvent, consisting of N-methylpyrrolidone, N,N-dimethylacetamide, and xylene in a mass ratio of 70:20:10, is first added to a reactor equipped with a mechanical stirrer and a jacketed temperature control device. After heating the system to 55°C, polyimide precursor, polyamide-imide resin, fluorinated polyimide resin, and benzoxazine resin are added sequentially at a stirring speed of 250 rpm. The mixture is stirred continuously for 3.5 h until the resin is completely dissolved, resulting in a homogeneous resin base solution.

[0085] Subsequently, 1.8 parts of pre-coupled biphase graphene intermediate and 1.0 parts of nano-alumina were pre-dispersed in a small amount of the above-mentioned mixed solvent to form a filler slurry. This filler slurry was then slowly added to the resin base liquid and dispersed at 4000 rpm for 25 min using a high-speed disperser. After dispersion, the mixture was further milled for 40 min to improve the uniformity of filler dispersion in the resin system. Finally, the resulting system was degassed at -0.095 MPa for 30 min and filtered through a 5 μm filter to obtain the first insulating varnish.

[0086] The solid content of the first insulating varnish is controlled at 24.5%, and the viscosity at 25℃ is controlled at 920 mPa·s.

[0087] S3. Preparation of the second insulating varnish

[0088] In this step, the second insulating varnish comprises, by weight of solid components: 46 parts of polyimide precursor, 25 parts of polyamide-imide resin, 11 parts of fluorinated polyimide resin, 5 parts of benzoxazine resin, 5 parts of pre-coupled biphase graphene intermediate, 8 parts of sheet-like hexagonal boron nitride, 2 parts of nano-alumina, and 0.40 parts of hydrophobic fumed silica.

[0089] Among them, the average particle size of the plate-like hexagonal boron nitride is 8.6 μm and the thickness is 120 nm; the average particle size of the nano-alumina is 20 nm; and the average particle size of the hydrophobic fumed silica is approximately 20 nm with a specific surface area of ​​180 m². 2 / g.

[0090] In the specific preparation process, 285 parts by weight of a mixed solvent, consisting of N-methylpyrrolidone, N,N-dimethylacetamide, and xylene in a mass ratio of 68:20:12, is first added to a reaction vessel. After heating to 55°C, the polyimide precursor, polyamide-imide resin, fluorinated polyimide resin, and benzoxazine resin are added sequentially under stirring, and stirring is continued for 3 hours until completely dissolved.

[0091] Then, pre-coupled biphase graphene intermediates, sheet-like hexagonal boron nitride, nano-alumina, and hydrophobic fumed silica were added sequentially, with the hydrophobic fumed silica added last. After addition, the mixture was premixed at 2500 rpm for 15 min, then dispersed at 4500 rpm for 30 min; subsequently, it was milled in a sand mill for 50 min to further improve the dispersion of the multi-scale fillers. After milling, the system was degassed at -0.095 MPa for 40 min and filtered through a 5 μm filter to obtain the second insulating varnish.

[0092] The solid content of the second insulating varnish is controlled at 26.0%, and the viscosity at 25℃ is controlled at 1380 mPa·s.

[0093] S4, Coating the inner insulating layer

[0094] In this step, the TU1 oxygen-free copper round wire is first cleaned and preheated, and then coated with four layers of first insulating varnish. After each coating, it is heated in multiple temperature ranges from low to high to form a dense inner insulating layer.

[0095] Specifically, the copper wire is first cleaned with acetone and then anhydrous ethanol to remove surface oil and impurities, and then dried in hot air at 80°C for 5 minutes. After drying, the copper wire enters the preheating zone and is preheated at 110°C for 60 seconds.

[0096] The preheated copper round wire enters the enameling coating device at a linear velocity of 8.0 m / min, and undergoes four consecutive coats of the first insulating varnish. After each coat, the wire passes through four heating zones with the following temperatures and heating times: first zone 160℃, 7 s; second zone 240℃, 7 s; third zone 330℃, 8 s; fourth zone 410℃, 6 s. After each coat and heating cycle, the wire is cooled for 8 s in a cold air section before proceeding to the next coat.

[0097] S5, Coating the outer insulating layer

[0098] In this step, a second insulating varnish is applied to the outer side of the inner dense insulating layer in three coats. After each coat, the coating is sequentially subjected to segmented imidization curing in multiple temperature ranges from low to high, thereby forming an outer thermally conductive and wear-resistant layer and obtaining a graphene composite enameled wire.

[0099] Specifically, the surface of the semi-finished wire obtained after step S4 is further coated with three consecutive layers of a second insulating varnish. After the first coating, the wire is heated in three temperature ranges in sequence: the first range is 190°C for 6 seconds; the second range is 280°C for 7 seconds; and the third range is 390°C for 7 seconds, so that the outer varnish film can complete the initial evaporation and initial shaping.

[0100] After the second and third coats, the polyimide precursors were sequentially subjected to segmented imidization curing at three different temperature ranges: the first range was 300°C for 8 seconds; the second range was 400°C for 10 seconds; and the third range was 460°C for 5 seconds. After the third coat, a final curing zone of 470°C for 20 seconds was added to further imidize the polyimide precursor and to ensure more complete crosslinking of the benzoxazine resin, thereby improving the structural stability and surface density of the outer thermally conductive and wear-resistant layer.

[0101] After the above three coating processes and segmented curing, a graphene composite enameled wire is formed.

[0102] Example 2

[0103] This embodiment discloses a graphene composite enameled wire, whose structure is basically the same as that of Embodiment 1, including a copper round conductor and an inner dense insulating layer and an outer thermally conductive and wear-resistant layer sequentially coated on the surface of the copper round conductor. In this embodiment, TU1 oxygen-free copper round wire with a diameter of 0.80 mm is selected as the metal conductor.

[0104] The main difference between this embodiment and Embodiment 1 is that the mass ratio of partially reduced graphene oxide to fluorinated graphene in the pre-coupled biphase graphene intermediate is adjusted to 60:40, specifically 60 parts by weight of partially reduced graphene oxide and 40 parts by weight of fluorinated graphene.

[0105] Except for the change in proportion, the source, type and parameters of the other raw materials are the same as in Example 1.

[0106] Example 3

[0107] This embodiment discloses a graphene composite enameled wire, whose structure is basically the same as that of Embodiment 1, including a copper round conductor and an inner dense insulating layer and an outer thermally conductive and wear-resistant layer sequentially coated on the surface of the copper round conductor.

[0108] The main difference between this embodiment and Embodiment 1 is that the mass ratio of partially reduced graphene oxide to fluorinated graphene in the pre-coupled biphase graphene intermediate is adjusted to 80:20, specifically 80 parts by weight of partially reduced graphene oxide and 20 parts by weight of fluorinated graphene, and the amount of flake hexagonal boron nitride in the second insulating varnish is changed to 10 parts.

[0109] Comparative Example 1

[0110] The main difference between this comparative example and Example 1 is that no pre-coupled biphase graphene intermediate is added to the insulation system, and only sheet-like hexagonal boron nitride and nano-alumina are retained as inorganic fillers.

[0111] Comparative Example 2

[0112] The difference between this comparative example and Example 1 is that graphene filler is added, but the sequential coupling modification of epoxy silane and amino silane is not performed; the graphene is added to the system only through physical dispersion.

[0113] Step S1 is as follows: 70 parts by weight of partially reduced graphene oxide and 30 parts by weight of fluorinated graphene are added to 600 parts by weight of N-methylpyrrolidone / DMF mixed solvent, dispersed at 3000 rpm for 20 min using a high-speed disperser, and ultrasonically treated for 45 min to form a graphene dispersion.

[0114] Comparative Example 3

[0115] The difference between this comparative example and Example 1 is that the graphene is only modified by coupling of epoxy silane and amino silane, but not by grafting polyamic acid oligomers.

[0116] Comparative Example 4

[0117] The difference between this comparative example and Example 1 is that no fluorinated graphene is added to the biphase graphene system, and only 100 parts by weight of partially reduced graphene oxide is used.

[0118] Comparative Example 5

[0119] The difference between this comparative example and Example 1 is that the insulating coating does not use a segmented imidization curing process during the curing process, but instead uses a single high-temperature range for continuous curing.

[0120] Step S4 differs from Example 1 in that after each coating, the material is directly placed in a single high-temperature curing zone for curing. The curing temperature is set to 400°C and the dwell time is 20 seconds.

[0121] Step S5 differs from Example 1 in that after each coating, the material is directly placed in a single high-temperature curing zone for curing. The curing temperature is set at 400°C, and the dwell time is 20 seconds. After each coating, a single-stage high-temperature curing process is also used, with the curing temperature set at 440°C and the dwell time at 25 seconds.

[0122] Performance testing

[0123] To verify the thermal conductivity, wear resistance and insulation stability of the graphene composite enameled wire of the present invention, performance tests were conducted on the enameled wires prepared in Examples 1-3 and Comparative Examples 1-5.

[0124] 1. Insulation Layer Thickness Measurement: The insulation layer thickness was measured using a cross-sectional microscopic measurement method. A sample segment approximately 10 mm in length was cut from the enameled wire and cold-mounted and cured with epoxy resin, ensuring the wire cross-section was completely encapsulated in the resin. After curing, sections were cut perpendicular to the wire axis and progressively sanded with 800-grit, 1200-grit, and 2000-grit sandpaper, followed by polishing with a polishing cloth until the cross-section was clear.

[0125] The cross-sectional structure was then observed under an optical microscope, and the outer diameter of the copper conductor, the thickness of the inner dense insulating layer, and the thickness of the outer thermally conductive and wear-resistant layer were measured. Five measurement points were selected at different locations for each sample, and the average value was calculated as the insulation layer thickness of the sample. The results are shown in Table 1.

[0126] Table 1. Test results of insulation layer thickness for each sample.

[0127] 2. Thermal conductivity test: Thermal conductivity was measured using a steady-state heat flow test device. Before testing, enameled wire was cut into short segments of approximately 50 mm in length and arranged parallel to each other in a metal mold in the same direction. The segments were then pressed under a pressure of 5 MPa for 5 minutes to form a dense sample sheet. The resulting sample sheet was approximately 2 mm thick and 25 mm in diameter.

[0128] During testing, the sample was placed between two constant-temperature metal hot plates, with the temperature difference between the upper and lower plates controlled at 10 °C. Insulation material was used around the sample to reduce heat loss. After the system reached a stable heat flux state, the thermal conductivity was calculated by measuring the heat flux density and temperature gradient. Each sample was tested three times, and the average value was taken as the final result.

[0129] 3. Reciprocating Scratch Test: The scratch resistance performance was tested using a reciprocating scratch test apparatus. During the test, the enameled wire was fixed on the test platform, and a round-headed carbide steel needle was used as the scratch needle, with a needle head radius of 0.25 mm.

[0130] The scraping needle was applied perpendicularly to the surface of the conductor, with a loading force of 7 N, a reciprocating stroke of 10 mm, and a reciprocating frequency of 60 times / min. During the test, the number of reciprocations was recorded when the insulation layer was scraped through and the metal conductor was exposed. Each sample was tested 5 times, and the average value was taken as the result.

[0131] 4. Breakdown Voltage Test: The breakdown voltage is tested using an AC withstand voltage tester. The enameled wire sample is wound into a single-layer coil with a diameter of 20 mm and fixed on an insulating support. During the test, two metal electrodes are brought into contact with both ends of the wire, and the test is conducted in ambient air at room temperature.

[0132] During the test, the AC voltage was gradually increased at a rate of approximately 500 V / s. When electrical breakdown of the insulation layer occurred and arc discharge was observed, the corresponding voltage value was recorded. Each sample was tested five times, and the average value was taken as the breakdown voltage result.

[0133] 5. Softening Breakdown Temperature Test: The softening breakdown temperature is used to evaluate the ability of the insulation layer to maintain electrical insulation stability under elevated temperature conditions. During the test, the enameled wire sample is fixed in a constant tension device to keep the conductor straight, and a constant test voltage is applied to both ends of the conductor.

[0134] The samples were then placed in a controlled-heating furnace and heated continuously at a rate of approximately 2 °C / min. The temperature at which electrical breakdown occurred due to softening of the insulation layer caused by heat was recorded. Each sample was tested five times, and the average value was taken as the softening breakdown temperature result.

[0135] 6. Thermal shock test: Thermal shock performance is evaluated by observing changes in the appearance of the insulation layer after heating at high temperature. During the test, the sample is wound on a round steel rod with a diameter 5 times that of the wire, and then placed in a forced-air drying oven preheated to 300 ℃ for 30 min.

[0136] After removing the sample, allow it to cool naturally to room temperature. Then, observe the surface of the insulation layer under a magnifying glass to see if cracks, blistering, or peeling occur, and record the observation results.

[0137] 7. Winding Crack Resistance Test: The winding crack resistance performance is evaluated through a room temperature winding test. During the test, the sample is wound on a cylindrical mandrel with a diameter 5 times that of the wire at room temperature, and held for 24 hours after one turn.

[0138] Then observe whether cracks, local peeling or detachment appear on the surface of the insulation layer, and record the test results.

[0139] The results of tests 2-6 above are shown in Table 2 below.

[0140] Table 2 Performance test results of each sample

[0141] As can be seen from Table 2, the overall performance of Examples 1 to 3 is significantly better than that of the comparative examples, indicating that the pre-coupled biphase graphene intermediate, polyamic acid oligomer grafting, and segmented imidization curing process used in this invention have a significant effect on the formation and performance improvement of the insulating layer structure.

[0142] Among them, Example 1 exhibits the best balance between thermal conductivity, number of reciprocating scratches, breakdown voltage, softening breakdown temperature, and thermal shock performance. Its thermal conductivity reaches 1.21 W·m. -1 ·K -1The results were significantly higher than those of Comparative Example 1, which did not contain graphene intermediates; the number of reciprocating scratches reached 214, the breakdown voltage reached 8.7 kV, and the softening breakdown temperature reached 432℃, indicating that the double-layer insulation structure and the two-phase graphene interface modification system can effectively improve the thermal diffusion capacity and surface wear resistance while maintaining high insulation stability.

[0143] In Example 2, due to the increased proportion of fluorinated graphene in the biphase graphene, the surface energy of the graphene sheets was further reduced, and the slippage characteristics between the sheets were more obvious. Therefore, the number of reciprocating scratches increased to 236, which was the highest among all samples, indicating that the scratch resistance and wear resistance of the outer surface were further enhanced. However, due to the relatively reduced proportion of partially reduced graphene oxide, the interfacial bonding ability and the continuity of thermally conductive nodes were slightly weakened, so its thermal conductivity was slightly lower than that of Example 1.

[0144] In Example 3, by reducing the proportion of fluorinated graphene and increasing the amount of sheet-like hexagonal boron nitride, the number of in-plane thermal conduction pathways in the outer layer was further increased, thus increasing the thermal conductivity to 1.33 W·m. -1 ·K -1 The coefficient of thermal conductivity was the highest among all samples; however, due to the decrease in the proportion of fluorinated graphene, the surface slippage of the graphene sheets was weakened, and the number of repeated scratches was lower than that in Example 1, indicating that while the thermal conductivity was improved, the improvement in surface scratch resistance was not as significant as that in Example 1.

[0145] In Comparative Example 1, since no pre-coupled biphase graphene intermediate was added to the insulation system, and only hexagonal boron nitride and nano-alumina fillers were retained, it was difficult to establish an efficient composite thermally conductive network with graphene thermally conductive nodes in the resin system, resulting in a significant decrease in thermal conductivity. At the same time, the surface wear resistance and thermal stability also decreased significantly, indicating that the graphene intermediate not only provides a thermally conductive path in the system of this invention, but also plays an important role in improving the overall structural stability of the coating film.

[0146] In Comparative Example 2, although graphene was added, the dispersion stability of graphene in the high polarity resin was poor due to the lack of sequential coupling modification of epoxy silane and amino silane. It was more prone to local agglomeration during the curing process. Therefore, its thermal conductivity, breakdown voltage and thermal shock performance were lower than those of Example 1, indicating that sequential coupling modification plays an important role in constructing a uniform and stable filler dispersion structure.

[0147] In Comparative Example 3, the graphene was coupled but not grafted with polyamic acid oligomers. Although it had a better dispersion state than Comparative Example 2, it still lacked a more continuous interfacial transition layer with the polyimide system. Therefore, its softening breakdown temperature, thermal shock resistance and winding crack resistance were still lower than those of Example 1. This indicates that grafting with polyamic acid oligomers can further improve the interfacial compatibility and thermal cycling stability between graphene and the resin matrix.

[0148] In Comparative Example 4, only partially reduced graphene oxide was used without the introduction of fluorinated graphene, resulting in insufficient surface energy adjustment between graphene sheets. The sheets were more likely to recombine during dispersion and curing, which reduced the uniformity of the thermally conductive network and the surface scratch resistance. This indicates that fluorinated graphene in the biphase graphene system has a significant effect on inhibiting agglomeration, improving sheet distribution, and enhancing overall performance.

[0149] In Comparative Example 5, a single-stage high-temperature curing method was used instead of a segmented imidization curing process. This lack of a gradual process for solvent evaporation, resin imidization, and filler fixation resulted in a greater likelihood of micropore formation, localized blistering, and filler migration within the coating film. Consequently, its breakdown voltage, softening breakdown temperature, thermal shock resistance, and winding crack resistance all decreased significantly. These results demonstrate that segmented imidization curing on a multi-coat base not only improves the film density but also helps maintain the uniform distribution of graphene and inorganic fillers in the insulation layer, thereby enhancing the overall insulation and thermal management performance of the enameled wire.

[0150] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A graphene composite enameled wire, characterized by, It includes a metal conductor and an insulating coating covering the surface of the metal conductor, wherein the insulating coating comprises an inner dense insulating layer and an outer thermally conductive and wear-resistant layer arranged sequentially. The inner dense insulating layer is formed of a first insulating varnish, which comprises, by weight of solid components: 55-72 parts of polyimide precursor, 18-30 parts of polyamide-imide resin, 5-12 parts of fluorinated polyimide resin, 2-6 parts of benzoxazine resin, 1.2-2.5 parts of pre-coupled biphase graphene intermediate, and 0.5-1.5 parts of nano-alumina; The outer thermally conductive and wear-resistant layer is formed of a second insulating varnish, which comprises, by weight of solid components: 38-55 parts of polyimide precursor, 20-32 parts of polyamide-imide resin, 8-15 parts of fluorinated polyimide resin, 3-8 parts of benzoxazine resin, 3-8 parts of pre-coupled biphase graphene intermediate, 4-12 parts of sheet-like hexagonal boron nitride, 1-4 parts of nano-alumina, and 0.15-0.80 parts of hydrophobic fumed silica. The pre-coupled biphase graphene intermediate is formed by fluorinated graphene and partially reduced graphene oxide, and is obtained by sequential coupling modification with epoxy silane coupling agent and amino silane coupling agent. The insulating coating is formed by multiple coating layers and segmental imidization and curing at 300-500°C.

2. The graphene composite enameled wire according to claim 1, characterized in that, The pre-coupled biphase graphene intermediate contains 55-80 wt% partially reduced graphene oxide and 20-45 wt% fluorinated graphene.

3. The graphene composite enameled wire according to claim 1, characterized in that, The amount of epoxy silane coupling agent added to the pre-coupled biphase graphene intermediate is 6-10% of the total mass of graphene, and the amount of amino silane coupling agent added is 2-5% of the total mass of graphene.

4. The graphene composite enameled wire according to claim 1, wherein the graphene is coated on the surface of the core wire. The pre-coupled biphase graphene intermediate is further grafted with polyamic acid oligomers, wherein the amount of polyamic acid oligomers added is 8 to 16% of the total mass of graphene.

5. The graphene composite enameled wire according to claim 1, wherein the graphene is coated on the surface of the core wire. The average particle size of the plate-like hexagonal boron nitride is 2–15 μm, and the thickness is 50–200 nm.

6. The graphene composite enameled wire according to claim 1, wherein the graphene is coated on the surface of the core wire. The average particle size of the nano-alumina is 20–80 nm.

7. The graphene composite enameled wire according to claim 1, wherein the graphene is coated on the surface of the core wire. The hydrophobic fumed silica has an average particle size of 10 to 40 nm and a specific surface area of 150 to 300 m 2 / g.

8. The graphene composite enameled wire according to claim 1, wherein the graphene is coated on the surface of the core wire. The total thickness of the insulating coating is 20–60 μm, of which the thickness of the inner dense insulating layer is 8–20 μm and the thickness of the outer thermally conductive and wear-resistant layer is 12–40 μm.

9. A method of manufacturing a graphene composite enameled wire according to any one of claims 1 to 8, characterized by, Includes the following steps: S1 Preparation of pre-coupled biphase graphene intermediate: Partially reduced graphene oxide and fluorinated graphene are dispersed in an organic solvent to form a graphene dispersion system. An epoxy silane coupling agent is added to the dispersion system, and the reaction is carried out at 40-65°C for 1.5-3 hours. Then, an amino silane coupling agent is added and the reaction is continued for 1-2 hours. Subsequently, a polyamic acid oligomer is added for grafting reaction to obtain the pre-coupled biphase graphene intermediate. S2 Preparation of the first insulating varnish: Polyimide precursor, polyamide-imide resin, fluorinated polyimide resin and benzoxazine resin are added to a mixed solvent and stirred to dissolve. Then, the pre-coupled biphase graphene intermediate and nano-alumina obtained in step S1 are added. After dispersion and filtration, the first insulating varnish is obtained. S3 Preparation of the second insulating varnish: Polyimide precursor, polyamide-imide resin, fluorinated polyimide resin and benzoxazine resin are added to a mixed solvent and mixed and dissolved. Then, the pre-coupled biphase graphene intermediate obtained in step S1, sheet-like hexagonal boron nitride, nano-alumina and hydrophobic fumed silica are added. After dispersion, degassing and filtration, the second insulating varnish is obtained. S4 Inner Insulation Layer Coating: After the metal conductor is cleaned and preheated, it is coated with multiple layers of first insulating varnish, and after each layer is coated, it is heated in multiple temperature ranges from low to high to form a dense inner insulation layer. S5 Coating of the outer insulating layer: Multiple layers of a second insulating varnish are applied to the outer side of the inner dense insulating layer, and after each layer, the coating is sequentially subjected to segmented imidization curing in multiple temperature ranges from low to high, thereby forming an outer thermally conductive and wear-resistant layer and obtaining a graphene composite enameled wire.

10. The method for preparing a graphene composite enameled wire according to claim 9, characterized in that, In step S4, four coats of the first insulating varnish are applied, and the temperature ranges and heating times for each coat are as follows: the first temperature is 140–180℃, and the time is controlled at 5–10 s; the second temperature is 220–260℃, and the time is controlled at 5–10 s; the third temperature is 300–360℃, and the time is controlled at 6–12 s; the fourth temperature is 380–430℃, and the time is controlled at 4–8 s. In step S5, three coats of the second insulating varnish are applied, and the temperature ranges and heating times after the first coat are as follows: the first temperature is 160–220℃, and the time is controlled at 5–8 s; the second temperature is 240–320℃, and the time is controlled at 5–10 s; the third temperature is 360–420℃, and the time is controlled at 5–10 s. The temperature ranges and heating times after the second and third coats are as follows: the first temperature is 280–320℃, and the time is controlled at 6–10 s; the second temperature is 380–420℃, and the time is controlled at 8–12 s; the third temperature is 440–480℃, and the time is controlled at 3–8 s.

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