An insulating coating for zinc oxide resistors, its preparation method and applications

Abstract

This invention belongs to the technical field of electrical equipment insulation materials, specifically relating to an insulating coating for zinc oxide resistor sheets, its preparation method, and its applications. The insulating coating for zinc oxide resistor sheets of this invention is mainly prepared by magnetic field-assisted curing of a water-based insulating varnish composition. The water-based insulating varnish composition comprises the following components in parts by weight: 85-95 parts water-based epoxy resin emulsion, 25-35 parts water-based polyamine curing agent, 4-6 parts magnetic responsive filler, and 0.5-2 parts wetting and dispersing agent. The magnetic responsive filler is silane-modified iron(III) oxide nanoparticles. Magnetic field-assisted curing involves heating and curing under the assistance of an external magnetic field. This invention, through magnetic field-assisted curing, enables the filler to form an ordered, reinforced structure, significantly improving the thermal anisotropy, adhesion, density, and resistance to high-current surges of the insulating coating. Furthermore, the use of a water-based system is highly environmentally friendly and suitable for the large-scale production of high-performance surge arresters.

Description

Technical Field

[0001] This invention belongs to the technical field of electrical equipment insulation materials, specifically relating to an insulating coating for zinc oxide resistor sheets, its preparation method, and its applications. Background Technology

[0002] Metal oxide surge arresters (MOAs) are critical protective devices in power systems used to limit lightning overvoltages and switching overvoltages. Their core performance depends on the electrical characteristics of the nonlinear resistive element, namely the zinc oxide resistive element. To ensure the long-term operational stability of the resistive element and prevent lateral flashover, an insulating varnish layer is typically coated on the cylindrical surface of the resistive element to provide electrical insulation, mechanical protection, and environmental isolation.

[0003] However, the insulating coating of existing zinc oxide resistor elements used in surge arresters still faces a series of severe challenges in actual operation, directly affecting the reliability and lifespan of the surge arresters. The specific problems mainly manifest in the following three aspects: First, the insulating coating is susceptible to moisture degradation, leading to insulation failure. During operation, condensation may occur inside the sealed structure of the resistor element due to temperature differences and changes in ambient humidity. Traditional insulating varnishes, especially some solvent-based or insufficiently dense water-based varnishes, may contain microscopic pores or defects within the varnish film. Moisture intrusion not only reduces the volume resistivity and surface resistivity of the coating but may also cause electrochemical corrosion of the resistor element substrate or create leakage current paths under the influence of an electric field. Long-term effects will ultimately lead to insulation breakdown, causing the surge arrester to lose its protective function. Although the industry has addressed this by improving the sealing structure or using hydrophobic coatings, the long-term impermeability of the coating itself and its insulation stability in humid and hot environments remain technical bottlenecks.

[0004] Secondly, insufficient mechanical and thermodynamic properties of the coating lead to coating damage and peeling. During operation, surge arresters undergo cyclic heating (such as absorbing overvoltage energy) and cooling, and are also affected by stresses such as mechanical vibration. Due to the difference in thermal expansion coefficients between the insulating coating material and the ceramic substrate and metal electrodes of the resistor sheet, micro-cracks easily form within the coating under thermal cycling and mechanical stress. The interfacial bonding gradually deteriorates, manifesting as coating powdering, cracking, or even localized peeling from the substrate. Once the physical structure of the coating is damaged, its insulation and moisture-proof functions are immediately lost, and the damaged area easily becomes a point of electric field distortion, triggering partial discharge and accelerating equipment aging.

[0005] Third, the resistance of the coating to transient high-current surges urgently needs improvement. When a surge arrester is struck by lightning or subjected to switching overvoltage, it needs to discharge a huge surge current (typically thousands to tens of thousands of amperes) for a duration of microseconds to milliseconds. During this process, a huge amount of energy is instantaneously converted into heat energy in the resistive element and surface coating. The thermal conductivity of traditional insulating varnishes is usually limited, and under rapid thermo-mechanical coupling shocks, the interface between the coating and the substrate may debond due to excessive instantaneous stress, and the coating itself may also undergo thermal decomposition or electrical breakdown. Therefore, developing insulating coatings that can withstand extreme transient electrothermal shocks is key to improving the operational reliability and repeatability of surge arresters.

[0006] To improve the performance of insulating coatings, existing technologies mainly focus on modifying the resin matrix, adding functional fillers (such as thermally conductive fillers and reinforcing fillers), and optimizing the curing process. For example, using silicone resins or epoxy resins with better heat resistance, or adding alumina or boron nitride to improve thermal conductivity. However, these methods often improve one property (such as thermal conductivity) while potentially damaging other properties (such as adhesion and processability), or making it difficult to achieve ideal dispersion and arrangement of fillers in the coating, resulting in bottlenecks in performance improvement. In particular, water-based insulating varnishes are gradually replacing solvent-based varnishes due to their environmental advantages, but the density, adhesion, and water resistance of their cured coatings usually face greater challenges, and the above three technical problems are more prominent in water-based systems.

[0007] Therefore, there is an urgent need in this field to develop novel insulating coating solutions that can synergistically improve the coating's adhesion, density, thermal conductivity, and electrical shock resistance, while also taking into account the requirements of environmentally friendly water-based systems. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide an insulating coating for zinc oxide resistor sheets, its preparation method, and its applications. The insulating coating provided by this invention not only possesses excellent electrical insulation properties, mechanical adhesion, and resistance to environmental aging, but more importantly, it significantly improves the reliability of resisting instantaneous high-current surges. Furthermore, it is suitable for environmentally friendly water-based systems, providing crucial material support for the manufacture of high-performance surge arresters.

[0009] To achieve the above objectives, the first aspect of the present invention adopts the following technical solution: An insulating coating for zinc oxide resistor sheets, the insulating coating being mainly prepared by magnetic field-assisted curing of an aqueous insulating varnish composition; The water-based insulating varnish composition comprises the following components in parts by weight: 85-95 parts of water-based epoxy resin emulsion, 25-35 parts of water-based polyamine curing agent, 4-6 parts of magnetic responsive filler, and 0.5-2 parts of wetting and dispersing agent; the magnetic responsive filler is silane-modified iron tetroxide nanoparticles. The magnetic field-assisted curing is a heating and curing process performed under the assistance of an external magnetic field; the external magnetic field can be any one of the following: a constant static magnetic field parallel to the cylindrical axis of the resistor sheet, a constant static magnetic field perpendicular to the cylindrical axis of the resistor sheet, a dynamically changing magnetic field of the rotating resistor sheet platform, an alternating magnetic field parallel to the cylindrical axis of the resistor sheet, or a gradually changing magnetic field parallel to the cylindrical axis of the resistor sheet with varying intensity along the axis.

[0010] Preferably, the average particle size of the magnetically responsive filler is 40~60nm.

[0011] Preferably, the preparation method of the silane-modified iron oxide nanoparticles includes: grafting hydrolyzed γ-aminopropyltriethoxysilane with iron oxide nanoparticles, followed by washing, drying, and grinding to obtain the silane-modified iron oxide nanoparticles.

[0012] More preferably, the hydrolysis is performed by hydrolyzing γ-aminopropyltriethoxysilane under acidic conditions with a pH of 2 to 4; the grafting reaction is carried out at a temperature of 50 to 90°C for a time of 3 to 8 hours.

[0013] To improve the solidification effect of the magnetic field, preferably, the magnetic field strength of the external magnetic field is 0.1~1.0T, more preferably 0.2~0.5T.

[0014] To further improve the electrical insulation performance, mechanical adhesion, environmental aging resistance, and reliability of the insulating coating in withstanding instantaneous high current surges, it is further preferred that the external magnetic field is a constant static magnetic field parallel to the cylindrical axis of the resistor sheet.

[0015] Preferably, the heating and curing temperature is 80~120℃ and the time is 60~180min.

[0016] The second aspect of this invention is the following technical solution: A method for preparing the insulating coating for zinc oxide resistor sheets as described above includes the following steps: S1. Preparation of water-based insulating varnish composition: Mix water-based epoxy resin emulsion with wetting and dispersing agent, then add magnetic responsive filler and disperse evenly, and finally add water-based polyamine curing agent, stir to defoam, and obtain water-based insulating varnish composition. S2. Coating treatment: The surface of the zinc oxide resistor sheet is cleaned, and a water-based insulating varnish composition is coated on the surface of the zinc oxide resistor sheet. Then, a leveling treatment is performed to obtain a zinc oxide resistor sheet coated with a wet film. S3. Magnetic field-assisted curing: The zinc oxide resistive sheet coated with a wet film is heated and cured under the condition of applying an external magnetic field, so that the magnetic response filler is arranged in an orderly manner along the magnetic field direction under the action of the magnetic field. After cooling, the insulating coating is obtained.

[0017] Preferably, in step S2, the coating is performed by dip coating, spray coating or flow coating; the dip coating temperature is 20~30℃, the dip coating time is 20~80s; the leveling treatment time is 5~12min; and the thickness of the wet film is 100~150μm.

[0018] Preferably, in step S3, the external magnetic field is a constant static magnetic field parallel to the cylindrical axis of the resistor sheet; the magnetic field strength of the external magnetic field is 0.2~0.5T.

[0019] The third aspect of this invention is the following technical solution: The use of the above-mentioned insulating coating or the insulating coating prepared by the above-mentioned method in the preparation of zinc oxide surge arresters.

[0020] The technical solution of the present invention has the following advantages and beneficial effects: I. Constructing an ordered microstructure for synergistic performance improvement. This invention introduces a magnetic field-assisted curing process, causing the magnetically responsive fillers in the insulating coating to align in an ordered manner along a predetermined direction. This ordered structure is not a simple physical addition, but rather an anisotropic reinforcing framework formed under the induction of a physical field. Parallel-aligned fillers effectively bridge defects in the resin matrix, forming continuous pathways for mechanical reinforcement and efficient thermal conductivity; vertical alignment significantly extends the leakage current path under an electric field. This transformation from disordered to ordered microstructure is key to solving the problem of insufficient overall coating performance.

[0021] II. Significantly Improved Resistance to High Current Surges in the Insulating Coating. The orderly arrangement of the filler, especially along the axis of the resistor element (i.e., the main heat flow direction), greatly improves the thermal anisotropy of the coating. This allows the heat generated inside the resistor element during a lightning strike to dissipate more quickly through the coating, reducing the risk of decomposition or debonding due to instantaneous high temperatures. Simultaneously, the reinforcing structure formed by the arranged filler also enhances the structural integrity of the coating under transient electro-thermal-mechanical coupling impacts, thereby significantly improving the reliability of the surge arrester when withstanding multiple high current surges.

[0022] Third, it greatly enhances the adhesion, density, and environmental aging resistance of the coating. Under the influence of a magnetic field, the magnetic filler migrates and aligns directionally into the coating and the interface region, which not only improves the physical and chemical bonding strength between the coating and the ceramic substrate of the resistor, but also effectively fills the micropores that may be generated during resin curing. This makes the cured coating more dense, greatly hindering the penetration paths of media such as moisture and corrosive ions, fundamentally alleviating the problem of insulation performance degradation caused by moisture absorption, and extending the service life of the surge arrester in harsh environments such as humid heat and salt spray.

[0023] Fourth, it has good process compatibility and conforms to the trend of environmental protection. The insulating coating of this invention is based on a water-based insulating varnish system, which avoids the VOC emission problem of traditional solvent-based varnishes. At the same time, magnetic field-assisted curing, as a non-contact, energy-controllable physical field process, is easy to integrate with existing coating production lines and heating curing equipment. It can be achieved by adding a permanent magnet array or electromagnetic coil. The process is simple, suitable for large-scale production, and has significant industrial application value. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the magnetic field-assisted curing operation in Embodiment 1 of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described clearly and completely below in conjunction with specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Without departing from the scope or spirit of this invention, those skilled in the art can make various improvements and changes to the specific embodiments described in this specification, which will be obvious to those skilled in the art.

[0026] Specific experimental steps or conditions are not specified in the examples; they were performed according to conventional experimental procedures and conditions in the art. Unless otherwise specified, all raw materials or instruments used are commercially available products.

[0027] In the following embodiments, the waterborne epoxy resin emulsion used has a solid content of 59-61%, is from the DIESON brand, and its model is EPICLON EXA-8420-60W. The waterborne polyamine curing agent is from the DIESON brand, and its model is LUCKAMIDEWN-720Z. The wetting and dispersing agent is from BYK Chemical AG (Germany), and its model is BYK-346, which belongs to the polyether-modified siloxane solution category.

[0028] Example 1

[0029] This embodiment provides an insulating coating for zinc oxide resistor sheets, which is prepared by curing with a magnetic field parallel to the cylindrical axis of the zinc oxide resistor sheet. The specific preparation method includes the following steps: Step S1, Ingredients Weigh the following raw materials by weight: 90 parts of waterborne epoxy resin emulsion, 30 parts of waterborne polyamine curing agent, 5 parts of magnetic responsive filler, and 1 part of wetting and dispersing agent.

[0030] The waterborne epoxy resin emulsion is model EPICLON EXA-8420-60W. The waterborne polyamine curing agent is model LUCKAMIDE WN-720Z. The wetting and dispersing agent is model BYK-346. The magnetically responsive filler is γ-aminopropyltriethoxysilane-modified iron(III) oxide nanoparticles.

[0031] The specific preparation process of the above-mentioned magnetically responsive filler is as follows: 1) Mix γ-aminopropyltriethoxysilane (KH-550), anhydrous ethanol, and deionized water at a volume ratio of 1:3.6:0.4 (specifically, take 5 mL of KH-550, 18 mL of anhydrous ethanol, and 2 mL of deionized water). Adjust the pH to 3.0 with 1.0 mol / L hydrochloric acid and stir at room temperature for 30 min to hydrolyze the solution, obtaining a hydrolyzed KH-550 solution. Separately, disperse 10 g of iron(III) oxide nanoparticles in 200 mL of anhydrous ethanol and sonicate for 30 min to obtain a Fe3O4 dispersion.

[0032] 2) Under continuous stirring, the KH-550 solution hydrolyzed in step 1) was added dropwise to the above Fe3O4 dispersion. After the addition was complete, the temperature was raised to 70℃ and the reaction was continued to be stirred for 5h. After the reaction was completed, the product was collected by magnetic separation, washed with anhydrous ethanol and deionized water respectively, and centrifuged 3 times (centrifugation speed 1200r / min, 10min each time). Finally, it was vacuum dried at 80℃ for 18h. After grinding, γ-aminopropyltriethoxysilane modified iron tetroxide nanoparticles were obtained with an average particle size of 40~60nm.

[0033] Step S2: Preparation of water-based insulating varnish composition 90 parts of waterborne epoxy resin emulsion and 1 part of wetting and dispersing agent were added to a container and mechanically stirred at 500 rpm until homogeneous. Then, 5 parts of magnetic responsive filler were added. After the addition was complete, the stirring speed was increased to 1500 rpm, and stirring was continued for 60 minutes until a uniform and stable black slurry was obtained. Finally, 30 parts of waterborne polyamine curing agent were added under low-speed stirring at 300 rpm, mixed evenly, and allowed to stand to defoam, thus obtaining the waterborne insulating varnish composition.

[0034] Step S3, Coating After sandblasting the surface of a standard circular zinc oxide resistor sheet (40 mm in diameter and 20 mm in thickness), the zinc oxide resistor sheet is immersed in the water-based insulating varnish composition obtained in step S2. The insulating varnish composition is then applied to the surface of the zinc oxide resistor sheet by dip coating at a temperature of 25°C for 45 seconds. After dip coating, the resistor sheet is lifted vertically at a uniform lifting speed of 8 cm / min. After lifting, the sheet is leveled at room temperature (25°C) for 8 minutes, and the wet film thickness is controlled to be 120 ± 5 μm, thus obtaining a zinc oxide resistor sheet coated with a wet film.

[0035] Step S4: Magnetic field-assisted curing A schematic diagram of magnetic field-assisted curing operation is shown below. Figure 1 As shown, the zinc oxide resistance sheet processed in step S3 is fixed to the center of the uniform magnetic field area inside the oven using an Al2O3 ceramic support. The zinc oxide resistance sheet is placed horizontally (i.e., the axis of the cylinder is parallel to the horizontal plane), and the oven wall is made of 316L stainless steel (non-magnetic). Samarium cobalt (SmCo) permanent magnets are symmetrically arranged on both sides of the oven exterior to form N and S poles. The two cylindrical surfaces of the zinc oxide resistance sheet are parallel to the end faces of the S and N poles. The N and S poles generate a uniform static magnetic field with a direction parallel to the axis of the cylindrical surface of the zinc oxide resistance sheet, and the magnetic field strength is 0.3T. While the magnetic field is applied, the zinc oxide resistance sheet is cured at 100°C for 120 minutes. The magnetic field is continuously applied throughout the curing process. After curing and cooling, the insulating coating for the zinc oxide resistance sheet of this embodiment is obtained.

[0036] Example 2

[0037] This embodiment provides an insulating coating for zinc oxide resistor sheets, which is prepared by curing with a magnetic field perpendicular to the cylindrical axis of the zinc oxide resistor sheet. The specific preparation method includes the following steps: Steps S1 to S3 are exactly the same as steps S1 to S3 in Example 1.

[0038] Step S4: This step is basically the same as step S4 in Example 1, except that the position of the samarium cobalt permanent magnet is adjusted so that the N and S poles generate a uniform static magnetic field with a direction perpendicular to the axis of the cylindrical surface of the zinc oxide resistor sheet, and the magnetic field strength is 0.3T. While the magnetic field is applied, the zinc oxide resistor sheet is cured at 100°C for 120 minutes, and all other conditions remain unchanged.

[0039] Example 3

[0040] This embodiment provides an insulating coating for zinc oxide resistor sheets, which uses a dynamically oriented magnetic field on a rotating resistor sheet platform to construct a gradient orientation structure within a single coating. The specific preparation method includes the following steps: Steps S1 to S3 are exactly the same as steps S1 to S3 in Example 1.

[0041] Step S4: Fix the zinc oxide resistor sheet processed in Step S3 onto a rotating platform, with the zinc oxide resistor sheet placed vertically (i.e., the axis of the cylinder is perpendicular to the horizontal plane). The rotating platform rotates at 10 rpm. Simultaneously, samarium cobalt permanent magnets are symmetrically arranged on both sides of the zinc oxide resistor sheet to form N and S poles. The N and S poles generate a uniform static magnetic field parallel to the axis of the cylindrical surface of the zinc oxide resistor sheet, with a magnetic field strength of 0.3T. While the magnetic field is applied, the resistor sheet continues to rotate, causing the relative magnetic field direction to change continuously. Simultaneously, the zinc oxide resistor sheet is cured at 100°C for 120 minutes. Throughout the curing process, the magnetic field is continuously applied. After curing and cooling, the insulating coating for the surge arrester resistor sheet of this embodiment is obtained.

[0042] Example 4

[0043] This embodiment provides an insulating coating for zinc oxide resistor sheets, which is cured using an alternating magnetic field. The specific preparation method includes the following steps: Steps S1 to S3 are exactly the same as steps S1 to S3 in Example 1.

[0044] Step S4: The zinc oxide resistor sheet processed in Step S3 is fixed to the center of the uniform magnetic field area inside the oven using an Al2O3 ceramic support. The zinc oxide resistor sheet is placed horizontally (i.e., the axis of the cylinder is parallel to the horizontal plane), and the oven wall is made of 316L stainless steel (non-magnetic). An electromagnet is placed outside the oven and a 50Hz alternating current is applied, generating an alternating magnetic field with a frequency of 50Hz, parallel to the cylindrical axis of the zinc oxide resistor sheet, around the zinc oxide resistor sheet, with a peak value of 0.3T. While the magnetic field is applied, the zinc oxide resistor sheet is cured at 100℃ for 120 minutes. The magnetic field is continuously applied throughout the curing process. After curing and cooling, the insulating coating for the surge arrester resistor sheet of this embodiment is obtained.

[0045] Example 5

[0046] This embodiment provides an insulating coating for zinc oxide resistor sheets, which is prepared using a gradient magnetic field driven method. The specific preparation method includes the following steps: Steps S1 to S3 are exactly the same as steps S1 to S3 in Example 1.

[0047] Step S4: This step is essentially the same as step S4 in Example 1, except that an asymmetric permanent magnet is used to generate a magnetic field whose direction is parallel to the axis of the cylindrical surface of the zinc oxide resistive sheet and whose intensity gradually changes along the axis. A large magnet is used on the N pole side, and a small magnet is used on the S pole side. The N pole is 40 mm from the oven wall, and the S pole is 60 mm from the oven wall, forming a magnetic field distribution in the resistive sheet area that gradually weakens from the N pole to the S pole, with the magnetic field intensity varying from 0.5T to 0.1T. While the magnetic field is applied, the zinc oxide resistive sheet is cured at 100°C for 120 minutes, with all other conditions remaining unchanged.

[0048] Comparative Example 1 This comparative example provides an insulating coating for zinc oxide resistor sheets. The preparation method is basically the same as that of Example 1, except that in step S4, during the curing stage, no magnetic field is applied, and the zinc oxide resistor sheet treated in step S3 is directly placed in an oven and cured at 100°C for 120 minutes. All other conditions are exactly the same as those in Example 1.

[0049] Comparative Example 2 This comparative example provides an insulating coating for zinc oxide resistor sheets, the preparation method of which is basically the same as that of Example 1, except that: in the curing stage of step S4, an electromagnet is used instead of a samarium cobalt permanent magnet, and the timing controller of the electromagnet realizes the intermittent application of a static magnetic field with a direction parallel to the axis of the cylindrical surface of the zinc oxide resistor sheet, with an on / off ratio of 1:1, that is, a cycle of 30 seconds on and 30 seconds off, and a magnetic field strength of 0.3T. All other conditions are exactly the same as those in Example 1.

[0050] Comparative Example 3 This comparative example provides an insulating coating for zinc oxide resistor sheets, the preparation method of which is basically the same as that of Example 1, except that: in the curing stage of step S4, a small permanent magnet is used instead of a samarium cobalt permanent magnet. The small permanent magnet generates a uniform weak static magnetic field with a direction parallel to the axis of the cylindrical surface of the zinc oxide resistor sheet, and the magnetic field strength is 0.05T. All other conditions are exactly the same as those in Example 1.

[0051] Performance testing The insulating coating samples prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to the following performance tests, mainly including coating adhesion, electrical strength, thermal conductivity, resistance to high current surge, volume resistivity, thermal cycling adhesion, and water vapor transmission rate. Five coating samples were tested for each performance index, and the results were averaged.

[0052] The coating adhesion test was conducted according to GB / T 9286-2021 "Cross-cut Test for Paints and Varnishes", using a 1mm spacing cross-cut tester, with an evaluation grade of 0-5 (0 being the best and 5 the worst). The electrical strength test was conducted according to GB / T 1408.1-2016 "Test Method for Electrical Strength of Insulating Materials", measuring the short-time electrical strength of the coating in the vertical direction (i.e., the thickness direction). The thermal conductivity was measured using the laser flash method (referencing ASTM E1461), measuring the thermal conductivity of the coating along the axial and radial directions of the resistor after curing. The high-current impact resistance test was conducted using a simulated lightning impact method, applying an 8 / 20μs waveform, 65kA peak current to the coated resistor once, followed by a 1-hour rest period, and observing for any visible damage such as cracking, ablation, or peeling on the coating surface. Volume resistivity was tested according to GB / T 1410-2019 "Test Methods for Volume Resistivity and Surface Resistivity of Solid Insulating Materials", with a test voltage of 500V and a temperature of 25℃, measuring the volume resistivity of the resistive element. Thermal cycling adhesion testing involved cycling the resistive element sample 100 times between -40℃ (30min) and 85℃ (30min), followed by a cross-cut adhesion test. Water vapor transmission rate was tested according to GB / T1037-2021 "Determination of Water Vapor Transmission Performance of Plastic Films and Sheets - Cup Method for Weight Gain and Loss", using the cup method.

[0053] The performance test results of the zinc oxide resistor insulating coating samples prepared in Examples 1-5 and Comparative Examples 1-3 are shown in Table 1.

[0054] Table 1. Performance test results of zinc oxide resistor sheet insulation coating samples

[0055] As shown in Table 1, the insulating coating sample of Example 1 exhibits the best overall performance, with an axial thermal conductivity of 0.85 W / m·K, which is 183% higher than that of Comparative Example 1 without an applied magnetic field. Furthermore, it showed no damage during high-current impact testing. A comparison between Example 1 and Example 2 shows that the axial magnetic field significantly improves axial thermal conductivity, while the radial magnetic field primarily improves radial thermal conductivity with limited improvement in axial heat dissipation. Examples 3-5 demonstrate that rotating, alternating, and gradient magnetic fields can all achieve orderly arrangement of the filler, but their orientation stability is not as good as the constant axial magnetic field of Example 1. Comparative Example 3 shows that a weak magnetic field of 0.05T has almost no effect, indicating that a certain magnetic field strength (e.g., 0.3T) is required to achieve effective orientation. Comparative Example 2 shows that intermittent magnetic fields lead to discontinuous orientation, resulting in limited performance improvement.

[0056] In summary, this invention enables the filler to form an ordered and reinforced structure through magnetic field-assisted curing, which significantly improves the thermal anisotropy, adhesion, density, and resistance to high current surges of the insulating coating. At the same time, the use of a water-based system is very environmentally friendly and suitable for the large-scale production of high-performance surge arresters.

[0057] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An insulating coating for zinc oxide resistor sheets, characterized in that, The insulating coating is mainly prepared by magnetic field-assisted curing of a water-based insulating paint composition; The water-based insulating varnish composition comprises the following components in parts by weight: 85-95 parts of water-based epoxy resin emulsion, 25-35 parts of water-based polyamine curing agent, 4-6 parts of magnetic responsive filler, and 0.5-2 parts of wetting and dispersing agent; the magnetic responsive filler is silane-modified iron tetroxide nanoparticles. The magnetic field-assisted curing is a heating and curing process performed under the assistance of an external magnetic field; the external magnetic field can be any one of the following: a constant static magnetic field parallel to the cylindrical axis of the resistor sheet, a constant static magnetic field perpendicular to the cylindrical axis of the resistor sheet, a dynamically changing magnetic field of the rotating resistor sheet platform, an alternating magnetic field parallel to the cylindrical axis of the resistor sheet, or a gradually changing magnetic field parallel to the cylindrical axis of the resistor sheet with varying intensity along the axis.

2. The insulating coating for zinc oxide resistor sheets according to claim 1, characterized in that, The average particle size of the magnetically responsive filler is 40~60nm.

3. The insulating coating for zinc oxide resistor sheets according to claim 1, characterized in that, The preparation method of the silane-modified iron oxide nanoparticles includes: grafting hydrolyzed γ-aminopropyltriethoxysilane with iron oxide nanoparticles, followed by washing, drying, and grinding to obtain the silane-modified iron oxide nanoparticles.

4. The insulating coating for zinc oxide resistor sheets according to claim 3, characterized in that, The hydrolysis involves hydrolyzing γ-aminopropyltriethoxysilane under acidic conditions with a pH of 2 to 4; the grafting reaction is carried out at a temperature of 50 to 90°C for 3 to 8 hours.

5. The insulating coating for zinc oxide resistor sheets according to claim 1, characterized in that, The magnetic field strength of the external magnetic field is 0.1~1.0T.

6. The insulating coating for zinc oxide resistor sheets according to claim 1, characterized in that, The heating and curing temperature is 80~120℃, and the time is 60~180min.

7. A method for preparing an insulating coating for zinc oxide resistor sheets as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Mix the waterborne epoxy resin emulsion with the wetting and dispersing agent, then add the magnetic responsive filler and disperse it evenly. Finally, add the waterborne polyamine curing agent, stir to defoam, and obtain the waterborne insulating paint composition. S2. Coat the surface of the zinc oxide resistor sheet to be treated with the water-based insulating varnish composition, and then perform a leveling treatment to obtain a zinc oxide resistor sheet coated with a wet film. S3. The zinc oxide resistor sheet coated with a wet film is heated and cured under the condition of applying an external magnetic field, and after cooling, the insulating coating for the zinc oxide resistor sheet is obtained.

8. The method for preparing the insulating coating for zinc oxide resistor sheets according to claim 7, characterized in that, In step S2, the coating is performed by dip coating, spray coating or flow coating; the dip coating temperature is 20~30℃, the dip coating time is 20~80s; the leveling treatment time is 5~12min; the thickness of the wet film is 100~150μm.

9. The method for preparing an insulating coating for zinc oxide resistor sheets according to claim 7, characterized in that, In step S3, the external magnetic field is a constant static magnetic field parallel to the cylindrical axis of the resistor sheet; the magnetic field strength of the external magnetic field is 0.2~0.5T.

10. Use of an insulating coating as described in any one of claims 1 to 6 or an insulating coating prepared by any one of claims 7 to 9 in the preparation of zinc oxide surge arresters.

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