Difunctional modified environment-friendly ester-based insulating oil based on nano aluminum oxide as well as preparation method and application of difunctional modified environment-friendly ester-based insulating oil

Abstract

The invention provides difunctional modified environment-friendly ester-based insulating oil based on nano aluminum oxide as well as a preparation method and application of the difunctional modified environment-friendly ester-based insulating oil, and belongs to the technical field of electrical insulating materials. On the basis of a difunctional surface modification technology of nano aluminum oxide, the nano aluminum oxide is subjected to first surface functionalization modification in a PEI solution, and the PEI modified nano aluminum oxide is subjected to second surface functionalization modification by using a silane coupling agent; and dispersing the difunctional modified nano aluminum oxide particles in the ester-based insulating oil to obtain the difunctional modified environment-friendly ester-based insulating oil. According to the environment-friendly ester-based insulating oil, the power frequency breakdown voltage and the heat conductivity coefficient of the environment-friendly ester-based insulating oil can be remarkably improved and the acid value can be remarkably reduced by adjusting the ratio and the content of the difunctional modified nano aluminum oxide and combining the synergistic effect of the PEI and the silane coupling agent, and the blank in the prior art is filled.

Description

Technical Field

[0001] The present invention relates to the technical field of electrical insulation materials, and in particular to a nano-alumina-based dual-functional modified environmentally friendly ester-based insulating oil, and a preparation method and application thereof. Background Art

[0002] Environmentally friendly ester-based insulating oils include natural ester insulating oils and synthetic ester insulating oils. As a new type of environmentally friendly insulating medium, it has received widespread attention in high-voltage equipment such as power transformers in recent years due to its advantages such as high biodegradability, high flash point, and strong flame resistance. However, environmentally friendly ester-based insulating oils still face some technical difficulties in practical applications: environmentally friendly ester oils are easy to absorb moisture in a humid environment, resulting in a significant drop in their breakdown voltage, affecting the operating safety of high-voltage equipment, and their electrical insulation strength under high electric fields is still slightly inferior to that of optimized mineral oils; the unsaturated fatty acid chains of environmentally friendly ester oils are prone to oxidative decomposition, producing acidic substances and sediments, resulting in a decline in the quality of the insulating oil and a shortened operating life of the equipment; at the same time, it is easy to generate trace amounts of water and acidic products during operation, which not only accelerate the aging of the oil, but may also cause corrosion to the metal parts of the equipment, further affecting the insulation performance and operating stability.

[0003] In order to improve the dielectric properties of environmentally friendly ester oil in long-term operation and extend its service life, researchers have proposed some technical methods for modifying environmentally friendly ester oil, including chemical modification, adding functional additives and introducing nanomaterials. Among them, nanomaterials are an effective method to improve the performance of environmentally friendly ester-based insulating oil due to their unique physical and chemical properties, such as high specific surface area, high dielectric constant and excellent thermal conductivity. Among them, the surface modification method can modify the surface of nanomaterials, such as introducing coupling agents or functional polymers to improve their compatibility with environmentally friendly ester oil. This method can significantly improve the dispersibility and stability of nanoparticles.

[0004] However, although the environmentally friendly ester oil modification methods in the prior art have made some progress, there are still some problems, such as insufficient dispersibility of nanomaterials, unstable modification effects, and high process costs. These problems have limited the large-scale application of environmentally friendly ester-based insulating oils in power equipment to a certain extent. Therefore, it is particularly important to develop a simple, efficient and economically feasible modification method to significantly improve the electrical properties, thermal conductivity and anti-aging ability of environmentally friendly ester-based insulating oils. Summary of the invention

[0005] The purpose of the present invention is to provide a nano-alumina-based dual-functional modified environmentally friendly ester-based insulating oil and a preparation method and application thereof, which can significantly improve the electrical properties, thermal conductivity and anti-aging ability of the environmentally friendly ester-based insulating oil, and the method is simple, efficient and economically feasible.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a nano-alumina-based dual-functional modified environmentally friendly ester-based insulating oil, comprising the following steps:

[0008] The nano-alumina and the polyethyleneimine solution are first ultrasonically mixed and first modified to obtain PEI-modified nano-alumina;

[0009] The PEI-modified nano-alumina and the silane coupling agent solution are subjected to a second ultrasonic mixing to perform a second modification to obtain a double-modified nano-alumina;

[0010] The double-modified nano-alumina is mixed with ester-based insulating oil to obtain a double-functional modified environmentally friendly ester-based insulating oil based on nano-alumina.

[0011] Preferably, before mixing the nano-alumina with the polyethyleneimine solution, the nano-alumina is ultrasonically dispersed in an alcohol solvent, and then vacuum filtered and dried in sequence.

[0012] Preferably, the mass ratio of the nano-alumina to the alcohol solvent is 1:16-20; the power of the ultrasonic dispersion is 600-800W, and the time is 25-35min;

[0013] The vacuum filtration conditions include: vacuum degree of 0.08-0.1Mpa, filter membrane pore size of 0.2-0.45μm, and filtration time of 15-35min;

[0014] The drying conditions include: vacuum drying, a temperature of 50 to 90° C., and a drying time of 3 to 5 hours.

[0015] Preferably, the mass concentration of the polyethyleneimine solution is 5-8%, and the solvent is anhydrous ethanol;

[0016] The mass ratio of the nano-alumina to the polyethyleneimine solution is 1:15-25;

[0017] The ultrasonic power of the first ultrasonic mixing is 800-1400W, and the time is 30-60min;

[0018] The stirring speed of the first modification is 140-200 rpm, and the time is 2-4 hours.

[0019] Preferably, the components of the silane coupling agent solution are silane coupling agent, anhydrous ethanol and water, and the volume ratio of KH-550, anhydrous ethanol and water is 2-5:80:1-2; the silane coupling agent includes KH550, KH560, KH570 or KH602.

[0020] Preferably, the mass ratio of the PEI-modified nano-alumina to the silane coupling agent solution is 1:15-25;

[0021] The power of the second ultrasonic mixing is 800-1400W, and the time is 30-60min;

[0022] The stirring speed of the second modification is 140-200 rpm, and the time is 2-4 hours.

[0023] Preferably, the ester-based insulating oil includes rapeseed oil, soybean oil, trimethylolpropane ester oil or pentaerythritol-based synthetic ester oil; the mass of the double-modified nano-alumina is 0.01-0.16wt% of the total mass of the ester-based insulating oil.

[0024] Preferably, the step of mixing the double-modified nano-alumina with the ester-based insulating oil comprises:

[0025] The ester-based insulating oil is heated to 60°C, modified nano-alumina is added, ultrasonic dispersion is performed, the obtained mixture is subjected to high shear stirring, and allowed to stand for degassing;

[0026] The power of the ultrasonic dispersion is 800-1400W, and the time is 30-60min;

[0027] The high shear stirring speed is 2000-2500 rpm, and the stirring time is 1-1.5 hours; the static degassing time is 12-15 hours.

[0028] The present invention provides a nano-alumina-based bifunctional modified environmentally friendly ester-based insulating oil prepared by the preparation method described in the above technical solution.

[0029] The present invention provides the use of the bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina described in the above technical solution in high-voltage equipment.

[0030] The present invention provides a preparation method of a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina. Based on the bifunctional surface modification technology of nano-alumina, the nano-alumina is subjected to a first surface functional modification in a PEI solution, and a silane coupling agent is used to perform a second surface functional modification on the PEI-modified nano-alumina; the bifunctionally modified nano-alumina particles are dispersed in the ester-based insulating oil to obtain the bifunctionally modified environmentally friendly ester-based insulating oil. The bifunctionally modified nano-alumina of the present invention has a high dielectric constant and a high thermal conductivity, so that it has a high efficiency in dielectric enhancement and slows down the degradation of oil at high temperature in the environmentally friendly ester-based insulating oil; the surface of the nano-alumina modified by PEI can capture and neutralize the free radicals generated by oxidation, and can effectively improve the antioxidant capacity of the environmentally friendly ester oil; the coupling group of the silane coupling agent can form a more stable chemical environment with the environmentally friendly ester oil molecules, reducing the influence of acidic byproducts on the oil performance; the bifunctional modification layer also improves the uniform distribution of nanoparticles in the oil, avoiding the local electric field concentration phenomenon caused by particle agglomeration in the electric field. The present invention can significantly improve the power frequency breakdown voltage (electrical performance), thermal conductivity (thermal conductivity) and reduce the acid value (anti-aging ability) of environmentally friendly ester-based insulating oil by adjusting the ratio and content of dual-functional modified nano-alumina and combining the synergistic effect of PEI and silane coupling agent, thus filling the gap in the prior art.

[0031] The method of the invention has simple process, low cost and high material stability and is suitable for large-scale industrial application. DETAILED DESCRIPTION

[0032] In the present invention, unless otherwise specified, the raw materials or reagents used are commercially available products well known in the art.

[0033] The present invention provides a method for preparing a nano-alumina-based dual-functional modified environmentally friendly ester-based insulating oil, comprising the following steps:

[0034] The nano-alumina and the polyethyleneimine solution are first ultrasonically mixed and first modified to obtain PEI-modified nano-alumina;

[0035] The PEI-modified nano-alumina and the silane coupling agent solution are subjected to a second ultrasonic mixing to perform a second modification to obtain a double-modified nano-alumina;

[0036] The double-modified nano-alumina is mixed with ester-based insulating oil to obtain a double-functional modified environmentally friendly ester-based insulating oil based on nano-alumina.

[0037] The invention performs a first ultrasonic mixing of nano-alumina and polyethyleneimine solution, performs a first modification, and obtains PEI-modified nano-alumina.

[0038] In the present invention, before mixing the nano-alumina with the polyethyleneimine solution, the nano-alumina is preferably dispersed in an alcohol solvent by ultrasonication, and then vacuum filtered and dried in sequence.

[0039] In the present invention, the alcohol solvent is preferably anhydrous ethanol; the mass ratio of the nano-alumina to the alcohol solvent is preferably 1:16-20, more preferably 1:16-18; the power of the ultrasonic dispersion is preferably 600-800W, more preferably 600-650W, and the time is preferably 25-35min, more preferably 30min;

[0040] The conditions for the vacuum filtration preferably include: a vacuum degree of 0.08 to 0.1 MPa, more preferably 0.1 MPa, a filter membrane pore size of 0.2 to 0.45 μm, further preferably 0.3 to 0.4 μm, and a filtration time of 15 to 35 min, further preferably 25 to 30 min; the conditions for the drying preferably include: a drying method of vacuum drying, a temperature of 50 to 90°C, further preferably 60 to 80°C, more preferably 65 to 70°C, and a time of preferably 3 to 5 h, more preferably 4 h.

[0041] The invention disperses nano-alumina in an alcohol solvent by ultrasonic, and then performs vacuum filtration and drying in sequence, thereby removing physically adsorbed moisture, impurities or other pollutants that may exist on the surface of the nano-particles, dissociating the agglomerates between the particles into dispersed particles, and improving the modification effect.

[0042] The present invention adds the dried nano aluminum oxide into a polyethyleneimine (PEI) solution, performs ultrasonic mixing, and then performs a first modification under magnetic stirring conditions.

[0043] In the present invention, the mass concentration of the polyethyleneimine solution is preferably 5-8%, more preferably 5-6%, and the solvent is preferably anhydrous ethanol; the mass ratio of the nano-alumina to the polyethyleneimine solution is preferably 1:15-25, more preferably 1:18-21, and more preferably 1:19-20. The present invention has no special restrictions on the preparation process of the polyethyleneimine solution, and the materials can be mixed uniformly according to the process well known in the art.

[0044] In the present invention, the ultrasonic power of the first ultrasonic mixing is preferably 800-1400 W, more preferably 1000-1200 W, and the time is preferably 30-60 min, more preferably 40-50 min; the stirring speed of the first modification is preferably 140-200 rpm, more preferably 170-180 rpm, and the time is preferably 2-4 h, more preferably 2.5-3 h.

[0045] After completing the first modification, the present invention preferably centrifuges and vacuum-dries the obtained product in sequence to obtain PEI-modified nano-alumina. The centrifugal speed is preferably 5000-8000 rpm, more preferably 6500-7000 rpm; the time is preferably 15-30 min, more preferably 22-25 min; the vacuum drying temperature is preferably 60-80° C., more preferably 70° C., and the time is preferably 4-6 h, more preferably 5 h.

[0046] In the present invention, in order to make the environmentally friendly ester-based insulating oil after adding nano-alumina material have better node performance and antioxidant ability, PEI solution is used to modify the surface of nano-alumina; the surface of nano-alumina modified with PEI can capture and neutralize free radicals generated by oxidation, slow down the oxidative decomposition reaction of environmentally friendly ester oil, enhance the lipophilicity of nano-alumina, reduce the van der Waals force between particles, and thus effectively prevent particle agglomeration.

[0047] After obtaining the PEI-modified nano-alumina, the present invention performs a second ultrasonic mixing on the PEI-modified nano-alumina and the silane coupling agent solution, performs a second modification, and obtains the double-modified nano-alumina.

[0048] In the present invention, the components of the silane coupling agent solution are preferably silane coupling agent, anhydrous ethanol and water, and the volume ratio of KH-550, anhydrous ethanol and water is preferably 2-5:80:1-2, and more preferably 3-4:80:1.2-1.6.

[0049] In the present invention, the silane coupling agent preferably includes KH550, KH560, KH570 or KH602, and more preferably KH550.

[0050] In the present invention, the mass ratio of the PEI-modified nano-alumina to the silane coupling agent solution is preferably 1:15-25, more preferably 1:18-21, and more preferably 1:19-20.

[0051] In the present invention, the PEI-modified nano-alumina is preferably added to the silane coupling agent solution, and after the second ultrasonic mixing, the second modification is performed under magnetic stirring conditions.

[0052] In the present invention, the power of the second ultrasonic mixing is preferably 800-1400W, more preferably 1000-1200W; the time is preferably 30-60min, more preferably 40-50min; the stirring speed of the second modification is preferably 140-200rpm, more preferably 160-170rpm; the time is preferably 2-4h, more preferably 2.5-3h.

[0053] After completing the second modification, the present invention preferably centrifuges the obtained product and vacuum dries it to obtain double modified nano alumina. The centrifugal speed is preferably 5000-8000 rpm, more preferably 6500-7000 rpm; the time is preferably 15-30 min, more preferably 22-25 min; the vacuum drying temperature is preferably 60-80°C, more preferably 70°C, and the time is preferably 4-6 h, more preferably 5 h.

[0054] In the present invention, in order to make the environmentally friendly ester-based insulating oil after adding nano-alumina material have better ability to slow down the growth rate of acid value and stability, a silane coupling agent is used to modify the surface of the PEI-modified nano-alumina; the coupling group of the silane coupling agent can form a more stable chemical environment with the environmentally friendly ester oil molecules, reduce the influence of acidic by-products on the oil performance, and further improve the interface compatibility between particles and oil, and significantly enhance the dispersion stability of particles in the oil.

[0055] After obtaining the double-modified nano-alumina, the present invention mixes the double-modified nano-alumina with ester-based insulating oil to obtain a double-functional modified environmentally friendly ester-based insulating oil based on nano-alumina.

[0056] In the present invention, the ester-based insulating oil preferably includes rapeseed oil, soybean oil, trimethylolpropane ester oil or pentaerythritol-based synthetic ester oil; the trimethylolpropane ester oil is preferably trimethylolpropane trioctanoate; the pentaerythritol-based synthetic ester oil is preferably pentaerythritol monooleate.

[0057] The ester-based insulating oil of the present invention is an environmentally friendly ester-based insulating oil. The present invention has no special limitation on the source of the ester-based insulating oil, and any commercially available product known in the art may be used.

[0058] In the present invention, the mass of the double-modified nano-alumina is preferably 0.01-0.16wt% of the total mass of the ester-based insulating oil, more preferably 0.03-0.12wt%, and more preferably 0.05-0.09wt%.

[0059] In the present invention, the step of mixing the double-modified nano-alumina with the ester-based insulating oil preferably comprises:

[0060] The ester-based insulating oil is heated to 60°C, modified nano-alumina is added, ultrasonic dispersion is performed, the obtained mixture is subjected to high shear stirring, and allowed to stand for degassing;

[0061] The power of the ultrasonic dispersion is preferably 800-1400W, more preferably 1000-1200W; the time is preferably 30-60min, more preferably 40-50min;

[0062] The high shear stirring speed is preferably 2000-2500 rpm, more preferably 2200-23000 rpm; the stirring time is preferably 1-1.5 h, more preferably 1.2-1.5 h; the static degassing time is preferably 12-15 h, more preferably 12 h.

[0063] The present invention provides a nano-alumina-based bifunctional modified environmentally friendly ester-based insulating oil prepared by the preparation method described in the above technical solution.

[0064] The present invention provides the application of the nano-alumina-based dual-functional modified environmentally friendly ester-based insulating oil in high-voltage equipment. The present invention has no special limitation on the application method, and the application can be carried out according to methods well known in the art.

[0065] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0066] The reagents or instruments used in the examples of the present application without indicating the manufacturer are all conventional reagent products that can be obtained through commercial purchase.

[0067] Example 1

[0068] (1) Nano-alumina and anhydrous ethanol were mixed in a mass ratio of 1:16, and ultrasonically dispersed at room temperature for 30 min at a power of 800 W, followed by vacuum filtration with a vacuum degree of 0.1 MPa, a filter membrane pore size of 0.3 μm, and a filtration time of 25 min; the filtered nano-alumina was vacuum dried at a temperature of 70° C. for 4 h and set aside;

[0069] (2) mixing the dried nano-alumina in step (1) with a PEI solution (solvent: anhydrous ethanol) having a mass concentration of 5% at a mass ratio of 1:20, and performing ultrasonic dispersion at room temperature at a power of 1000 W for 40 min; then performing magnetic stirring at a stirring speed of 170 rpm for 3 h; then performing centrifugation at a centrifugal speed of 6500 rpm for 22 min; vacuum drying the nano-alumina after centrifugation at a temperature of 70° C. for 5 h to obtain PEI-modified nano-alumina;

[0070] (3) KH-550, anhydrous ethanol and deionized water were mixed in a volume ratio of 4:80:1.6, and the obtained KH-550 solution was mixed with the PEI-modified nano-alumina after drying in step (2) in a mass ratio of 1:20 of PEI-modified nano-alumina to KH-550 solution, and ultrasonically dispersed at room temperature for 40 minutes at a power of 1000 W; then magnetic stirring was performed at a stirring speed of 170 rpm for 3 hours; then centrifugation was performed at a centrifugal speed of 6500 rpm for 22 minutes; the nano-alumina after centrifugation was vacuum dried at a temperature of 70° C. for 5 hours to obtain double-modified nano-alumina;

[0071] (4) The obtained dual-modified nano-alumina was added to FR3 refined soybean oil (Cargill, made in China) heated to 60°C, wherein the mass of the nano-alumina accounted for 0.05wt% of the FR3 refined soybean oil, and ultrasonic dispersion was performed at a power of 1000 W for 40 min, and high-speed shear stirring was performed at a speed of 2200 rpm for 1.5 h, and then the mixture was allowed to stand for degassing for 12 h to obtain a dual-functional modified environmentally friendly ester-based insulating oil.

[0072] Example 2

[0073] The method of Example 1 was used for preparation, and the only difference from Example 1 was that the mass of the double-modified nano-alumina was 0.07% of the total mass of the environmentally friendly ester-based insulating oil.

[0074] Example 3

[0075] The method of Example 1 was used for preparation, and the only difference from Example 1 was that the mass of the double-modified nano-alumina was 0.09% of the total mass of the environmentally friendly ester-based insulating oil.

[0076] Example 4

[0077] The method of Example 1 was used for preparation, and the only difference from Example 1 was that the mass of the double-modified nano-alumina was 0.11% of the total mass of the environmentally friendly ester-based insulating oil.

[0078] Example 5

[0079] The method of Example 1 was used for preparation, and the only difference from Example 1 was that the mass of the double-modified nano-alumina was 0.12% of the total mass of the environmentally friendly ester-based insulating oil.

[0080] Example 6

[0081] The method of Example 1 is used for preparation, and the only difference from Example 1 is that the mass of the double-modified nano-alumina is 0.09% of the total mass of the environmentally friendly ester-based insulating oil; and the mixing mass ratio of the dried nano-alumina and the PEI solution in step (2) is 1:19.

[0082] Example 7

[0083] The method of Example 1 is used for preparation, and the only difference from Example 1 is that the mass of the double-modified nano-alumina is 0.09% of the total mass of the environmentally friendly ester-based insulating oil; and the mixing mass ratio of the dried nano-alumina and the PEI solution in step (2) is 1:21.

[0084] Example 8

[0085] The method of Example 1 is used for preparation, and the only difference from Example 1 is that the mass of the double-modified nano-alumina is 0.09% of the total mass of the environmentally friendly ester-based insulating oil; and the mixing mass ratio of the PEI-modified nano-alumina and the KH-550 solution in step (3) is 1:19.

[0086] Example 9

[0087] The method of Example 1 was used for preparation, and the only difference from Example 1 was that the mass of the double-modified nano-alumina was 0.09% of the total mass of the environmentally friendly ester-based insulating oil; and the mixing mass ratio of the PEI-modified nano-alumina and the KH-550 solution in step (3) was 1:21.

[0088] Comparative Example 1

[0089] FR3 refined soybean oil without nano-alumina (source is the same as in Example 1).

[0090] Comparative Example 2

[0091] Karamay #25 mineral oil without nano-alumina.

[0092] Comparative Example 3

[0093] The method of Example 1 was used for preparation, and the only difference from Example 1 was that FR3 refined soybean oil was replaced with Karamay #25 mineral oil.

[0094] Test Case

[0095] The power frequency breakdown voltage, 25°C thermal conductivity and acid value of the insulating oils prepared in Examples 1 to 9 and Comparative Examples 1 to 3 were tested by the methods described in national standards GB / T 507-2002, GB / T 29313-2012 and GB / T 28552-2012. The performance test data are shown in Table 1 below:

[0096] Table 1 Insulating oil performance test data in Examples 1 to 9 and Comparative Examples 1 to 3

[0097]

[0098]

[0099] As shown in Table 1, after the bifunctionally modified nano-alumina material is added to the environmentally friendly ester-based insulating oil, it can significantly improve the power frequency breakdown voltage, increase the thermal conductivity and reduce the acid value of the environmentally friendly ester-based insulating oil; it has a good effect on improving the insulation performance of the environmentally friendly ester-based insulating oil; when the content of the bifunctionally modified nano-alumina is 0.12wt% of the total content of the environmentally friendly ester-based insulating oil, the nanoparticles are prone to agglomeration and sedimentation, and are prone to form conductivity channels in the oil, which leads to a weakening of the improvement effect of the power frequency breakdown voltage and acid value. Compared with the environmentally friendly ester-based insulating oil, the bifunctionally modified nano-alumina material does not have a very obvious effect on the improvement of mineral oil. This is because there are few polar molecules in the mineral oil, the interface effect between the particles is weak, and the ability of the nano-alumina particles to capture free radicals is limited by the properties of the oil.

[0100] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a bifunctional modified environmentally friendly ester-based insulating oil based on nano-alumina, characterized in that: The following steps are involved: The nano-alumina and the polyethyleneimine solution are first ultrasonically mixed and first modified to obtain PEI-modified nano-alumina; The PEI-modified nano-alumina and the silane coupling agent solution are subjected to a second ultrasonic mixing to perform a second modification to obtain a double-modified nano-alumina; The double-modified nano-alumina is mixed with ester-based insulating oil to obtain a double-functional modified environmentally friendly ester-based insulating oil based on nano-alumina.

2. The preparation method according to claim 1, characterized in that: Before mixing the nano-alumina with the polyethyleneimine solution, the nano-alumina is ultrasonically dispersed in an alcohol solvent, and then vacuum filtered and dried in sequence.

3. The preparation method according to claim 2, characterized in that: The mass ratio of the nano-alumina to the alcohol solvent is 1:16-20; the power of the ultrasonic dispersion is 600-800W, and the time is 25-35min; The vacuum filtration conditions include: vacuum degree of 0.08-0.1Mpa, filter membrane pore size of 0.2-0.45μm, and filtration time of 15-35min; The drying conditions include: vacuum drying, a temperature of 50 to 90° C., and a drying time of 3 to 5 hours.

4. The preparation method according to claim 1, characterized in that: The mass concentration of the polyethyleneimine solution is 5-8%, and the solvent is anhydrous ethanol; The mass ratio of the nano-alumina to the polyethyleneimine solution is 1:15-25; The ultrasonic power of the first ultrasonic mixing is 800-1400W, and the time is 30-60min; The stirring speed of the first modification is 140-200 rpm, and the time is 2-4 hours.

5. The preparation method according to claim 1, characterized in that: The components of the silane coupling agent solution are silane coupling agent, anhydrous ethanol and water, and the volume ratio of KH-550, anhydrous ethanol and water is 2-5:80:1-2; the silane coupling agent includes KH550, KH560, KH570 or KH602.

6. The preparation method according to claim 1 or 5, characterized in that: The mass ratio of the PEI-modified nano-alumina to the silane coupling agent solution is 1:15-25; The power of the second ultrasonic mixing is 800-1400W, and the time is 30-60min; The stirring speed of the second modification is 140-200 rpm, and the time is 2-4 hours.

7. The preparation method according to claim 1, characterized in that: The ester-based insulating oil includes rapeseed oil, soybean oil, trimethylolpropane ester oil or pentaerythritol-based synthetic ester oil; the mass of the double-modified nano-alumina is 0.01-0.16wt% of the total mass of the ester-based insulating oil.

8. The preparation method according to claim 1, characterized in that: The step of mixing the double modified nano-alumina with the ester-based insulating oil comprises: The ester-based insulating oil is heated to 60°C, modified nano-alumina is added, ultrasonic dispersion is performed, the obtained mixture is subjected to high shear stirring, and allowed to stand for degassing; The power of the ultrasonic dispersion is 800-1400W, and the time is 30-60min; The high shear stirring speed is 2000-2500 rpm, and the stirring time is 1-1.5 hours; the static degassing time is 12-15 hours.

9. The nano-alumina-based bifunctional modified environmentally friendly ester-based insulating oil prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the nano-alumina-based bifunctional modified environmentally friendly ester-based insulating oil according to claim 9 in high-voltage equipment.

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