Environment-friendly flame-retardant insulating oil and application thereof

Abstract

The invention discloses environment-friendly flame-retardant insulating oil and application thereof, and belongs to the technical field of insulating materials. The environment-friendly flame-retardant insulating oil is used for solving the technical problems that in the prior art, insulating oil needs to have a high flash point and good environment-friendly performance and flame retardant performance, and the environment-friendly flame-retardant insulating oil is obtained by mixing pretreated epoxidized soybean oil and an additive, heating, reacting, adding silicon dioxide gel after the reaction is finished, standing and filtering. Wherein in an inert gas atmosphere, maleic anhydride grafted lignin, DOPO and N, N-dimethylformamide are mixed, stirred, heated and subjected to a reflux reaction, and modified lignin is obtained; the preparation method comprises the following steps: adding hydroxyapatite and lysine into N, N-dimethylformamide, adding a catalyst, and heating for reaction to obtain modified hydroxyapatite; adding the modified lignin, the modified hydroxyapatite and triethylamine into N, N-dimethylformamide, and carrying out heating reaction to obtain an additive; the environment-friendly flame-retardant insulating oil obtained by the invention has relatively high flash point and ignition point and good biodegradability.

Description

Technical Field

[0001] The present invention relates to the technical field of insulating materials, and particularly relates to an environmentally friendly flame-retardant insulating oil and its application. Background Art

[0002] Insulating oil is a liquid insulating medium and is widely used in oil-immersed high-voltage equipment such as transformers, circuit breakers, and voltage transformers. Insulating oils are mainly divided into three categories: silicone oil, mineral oil, and ester oil. The biodegradation rate of mineral insulating oil is only about 30%, and the remaining mineral insulating oil will cause changes in soil structure. A synergistic reaction with the residual heavy metal copper in the soil may also cause the possibility of combined pollution, affecting plant growth and endangering human health. In view of the non-renewable nature of petroleum resources, people will eventually face problems such as shortages and depletion. At present, people's awareness of environmental protection is gradually increasing, and more requirements are put forward for insulating oil.

[0003] Chinese Patent CN115141671B discloses an alkyl-modified silicone oil insulating grease. In the present invention, the silicone oil is modified by introducing CF3 groups on the side chains of the silicone oil molecules and phosphate ester groups at the molecular chain ends. The CF3 groups can endow the silicone oil with excellent anti-friction and anti-wear properties; the introduced phosphate ester groups belong to effective flame-retardant components and can significantly improve the flame resistance of the silicone oil. However, silicone oil itself has poor biodegradability and may cause pollution if it exists in the environment for a long time. Moreover, silicone oil may decompose under high temperature or arc action, generating harmful substances such as formaldehyde, posing a potential threat to the environment and human health. Chinese Patent CN102682869B discloses a preparation method of plant insulating oil. The plant insulating oil uses refined vegetable oil as a raw material and undergoes a transesterification reaction with low-molecular-weight alcohol. After the reaction stops, it is refined, washed, distilled under reduced pressure to remove water and alcohol, decolorized, deacidified, filtered, and then deeply dehydrated. Finally, an antioxidant and a pour point depressant are added. The flash point of the plant insulating oil prepared by this invention is optimally 270°C, and its flash point needs to be improved to expand the scope of application. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide an environmentally friendly flame-retardant insulating oil and its application, so that the insulating oil has a high flash point, good environmental protection and flame retardancy, and expands the scope of application.

[0005] To achieve the above purpose, the present invention provides a preparation method of an environmentally friendly flame-retardant insulating oil, including the following steps:

[0006] Mix the pretreated epoxy soybean oil and additives, heat, react, and after the reaction ends, add silica gel, let it stand, and filter to obtain the environmentally friendly flame-retardant insulating oil;

[0007] Among them, the preparation method of the additive includes the following steps:

[0008] Step (1) Under an inert gas atmosphere, maleic anhydride-grafted lignin, DOPO, and N,N-dimethylformamide are mixed, stirred, heated under reflux, and reacted. After the reaction is completed, vacuum distillation is carried out, followed by filtration, washing, and drying to obtain modified lignin;

[0009] Step (2) Hydroxyapatite and lysine are added to N,N-dimethylformamide, a catalyst is added, and the mixture is heated for reaction. After the reaction is completed, filtration, washing, and drying are carried out to obtain modified hydroxyapatite;

[0010] Step (3) Modified lignin, modified hydroxyapatite, and triethylamine are added to N,N-dimethylformamide, and the mixture is heated for reaction. After the reaction is completed, filtration, washing, and drying are carried out to obtain the additive.

[0011] Preferably, the preparation method of the pretreated epoxy soybean oil includes the following steps:

[0012] Epoxy soybean oil and silica gel are mixed and dried for dehydration in a vacuum drying oven at -0.1 MPa and 80 °C for 48 h, and then cooled to room temperature; silica gel is added, stirred, allowed to stand, and filtered, and this is repeated 3 times to obtain the pretreated epoxy soybean oil.

[0013] Preferably, the preparation method of maleic anhydride-grafted lignin includes the following steps:

[0014] Lignin is repeatedly washed 3 times with an aqueous hydrochloric acid solution and then dried; maleic anhydride, 4-dimethylaminopyridine, and N,N-dimethylformamide are added and mixed, heated, and reacted. After the reaction is completed, deionized water is added, followed by filtration, washing, and drying to obtain maleic anhydride-grafted lignin;

[0015] Among them, the mass ratio of lignin, maleic anhydride, 4-dimethylaminopyridine, and N,N-dimethylformamide is (8 - 10):(10 - 14):(0.1 - 0.14):(50 - 70);

[0016] Among them, the reaction conditions are reaction at 80 - 100 °C for 8 - 12 h.

[0017] Preferably, the mass ratio of the pretreated epoxy soybean oil to the additive is (8 - 10):(1 - 1.4).

[0018] Preferably, the reaction conditions for the pretreated epoxy soybean oil and the additive are: reaction at 65 - 75 °C for 11 - 12 h.

[0019] Preferably, in step (1), the mass ratio of maleic anhydride-grafted lignin, DOPO, and N,N-dimethylformamide is (150 - 210):(50 - 70):(1500 - 3500).

[0020] Preferably, in step (1), the reaction conditions are reacting at 110 - 130 °C for 22 - 26 h.

[0021] Preferably, in step (2), the mass ratio of hydroxyapatite, lysine, N,N - dimethylformamide, and the catalyst is (520 - 600):(120 - 140):(2000 - 2800):(8 - 12).

[0022] Preferably, the catalyst is any one of concentrated sulfuric acid and p - toluenesulfonic acid.

[0023] Preferably, in step (2), the reaction temperature is 60 - 80 °C, and the reaction time is 8 - 10 h.

[0024] Preferably, in step (3), the mass ratio of modified lignin, modified hydroxyapatite, triethylamine, and N,N - dimethylformamide is (20 - 40):(26 - 50):(1.4 - 2.2):(1200 - 2400); the reaction temperature is 75 - 95 °C, and the reaction time is 7 - 9 h.

[0025] Preferably, the environmentally friendly flame - retardant insulating oil prepared by the preparation method of the environmentally friendly flame - retardant insulating oil.

[0026] Preferably, the application of the environmentally friendly flame - retardant insulating oil in a transformer.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] 1. The epoxy soybean oil used in the present invention is a vegetable oil, which has certain biodegradability, belongs to renewable resources, and has fewer unsaturated carbon - carbon bonds compared with soybean oil, having better stability and antioxidant properties. Lignin is rich in sources, has a three - dimensional network structure, has the ability to form carbon during combustion, and can generate a large amount of carbon through dehydration and polycondensation carbonization during the heating process, forming a relatively dense carbon layer, which is beneficial for flame retardancy. Maleic anhydride - grafted lignin introduces carboxylic acid groups, which react with the P - H bond of DOPO to form a phosphate ester structure, obtaining modified lignin. Hydroxyapatite is a high - temperature - resistant non - combustible substance with a very high phosphorus content in its structure and good biodegradability. The modified hydroxyapatite reduces aggregation and is beneficial for its dispersion in epoxy soybean oil. During the combustion process, the added DOPO can generate acid substances such as phosphoric acid and phosphorous acid, promoting the dehydration and carbonization of the matrix during combustion. Grafted onto the surface of hydroxyapatite, it can promote the formation of the carbon layer faster. At the same time, the generated PO· free radicals can quench the combustion chain reaction and prevent the combustion reaction from proceeding. The nitrogen element in lysine will also generate non - combustible gases during combustion, diluting the oxygen concentration in the combustion system. The four work together to further improve the flame - retardant performance.

[0029] 2. In the present invention, an esterification reaction occurs between the hydroxyl groups in hydroxyapatite and the carboxyl groups in lysine to obtain modified hydroxyapatite; a reaction occurs between the amino groups in the modified hydroxyapatite and the carboxyl groups in the modified lignin to obtain an additive; and under the action of a catalyst, the primary amino groups in the additive undergo a ring-opening reaction with the epoxy groups in epoxidized soybean oil to obtain an environmentally friendly flame-retardant insulating oil. At this time, the additive has a relatively large molecular size and a complex spatial structure. After grafting with epoxidized soybean oil, a certain steric hindrance is formed, which hinders the regular arrangement and aggregation of epoxidized soybean oil molecules, thereby reducing the occurrence of crystallization and solidification, improving the performance of not being easily solidified at low temperatures, reducing the pour point; at the same time, reducing the flow resistance, improving the fluidity at low temperatures, and lowering the cloud point.

[0030] 3. The present invention uses epoxidized soybean oil and performs pretreatment on it to remove impurities, improving the stability of epoxidized soybean oil. At the same time, hydroxyapatite and maleic anhydride-grafted lignin both have certain biodegradability, which is beneficial to environmental protection. Moreover, the preparation method of the environmentally friendly flame-retardant insulating oil provided by the present invention is efficient, improving the flash point and fire point in vegetable oil insulating oil. The stability and antioxidant property of the environmentally friendly flame-retardant insulating oil are improved, reducing the probability of increased viscosity caused by high-temperature degradation, maintaining the long-term heat dissipation efficiency, and being beneficial for application in transformers. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is the process flow chart of the preparation of the environmentally friendly flame-retardant insulating oil in the present invention;

[0032] Figure 2 is the process flow chart of the preparation of the additive in the present invention;

[0033] Figure 3 is the bar chart of the biodegradability test results of the environmentally friendly flame-retardant insulating oil in Examples 1-5 and Comparative Examples 1-3 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] Example 1

[0037] This example provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0038] The pretreated epoxy soybean oil and the additive are mixed at a mass ratio of 8:1, reacted at 65 °C for 12 h. After the reaction is completed, silica gel is added, allowed to stand for 60 min, filtered, and an environmentally friendly flame-retardant insulating oil is obtained;

[0039] Among them, the silica gel is 5% of the pretreated epoxy soybean oil;

[0040] Among them, the preparation method of the additive includes the following steps:

[0041] Step (1) Under an Ar atmosphere, maleic anhydride-grafted lignin, DOPO and N,N-dimethylformamide are mixed in a mass ratio of 150:50:1500, heated under reflux, and reacted at 110 °C for 26 h. After the reaction is completed, vacuum distillation is carried out, filtered, washed with ethyl acetate 3 times, and dried at 60 °C for 8 h to obtain modified lignin;

[0042] Step (2) Hydroxyapatite, lysine, N,N-dimethylformamide and p-toluenesulfonic acid are mixed in a mass ratio of 520:120:2000:8, reacted at 60 °C for 10 h. After the reaction is completed, filtered, washed with deionized water, and dried at 80 °C for 7 h to obtain modified hydroxyapatite;

[0043] Step (3) Modified lignin, modified hydroxyapatite, triethylamine and N,N-dimethylformamide are mixed in a mass ratio of 20:26:1.4:1200, reacted at 75 °C for 9 h. After the reaction is completed, filtered, washed with absolute ethanol, and dried at 100 °C for 9 h to obtain the additive.

[0044] Example 2

[0045] This example provides a preparation method of an environmentally friendly flame-retardant insulating oil, including the following steps:

[0046] The pretreated epoxy soybean oil and the additive are mixed at a mass ratio of 8.5:1.1, reacted at 67 °C for 11.8 h. After the reaction is completed, silica gel is added, allowed to stand for 55 min, filtered, and an environmentally friendly flame-retardant insulating oil is obtained;

[0047] Among them, the silica gel is 5% of the pretreated epoxy soybean oil;

[0048] Among them, the preparation method of the additive includes the following steps:

[0049] Step (1) Under an Ar atmosphere, maleic anhydride-grafted lignin, DOPO and N,N-dimethylformamide are mixed in a mass ratio of 165:55:2000, heated under reflux, and reacted at 115 °C for 25 h. After the reaction is completed, vacuum distillation is carried out, filtered, washed with ethyl acetate 3 times, and dried at 65 °C for 7.5 h to obtain modified lignin;

[0050] Step (2): Mix hydroxyapatite, lysine, N,N-dimethylformamide, and p-toluenesulfonic acid in a mass ratio of 540:125:2200:9, react at 65 °C for 9.5 h. After the reaction, filter, wash with deionized water, and dry at 85 °C for 6.5 h to obtain modified hydroxyapatite;

[0051] Step (3): Mix modified lignin, modified hydroxyapatite, triethylamine, and N,N-dimethylformamide in a mass ratio of 25:32:1.6:1500, react at 80 °C for 8.5 h. After the reaction, filter, wash with absolute ethanol, and dry at 102 °C for 8.5 h to obtain the additive.

[0052] Example 3

[0053] This example provides a preparation method of an environmentally friendly flame-retardant insulating oil, which includes the following steps:

[0054] Mix the pretreated epoxy soybean oil and the additive in a mass ratio of 9:1.2, react at 70 °C for 11.5 h. After the reaction, add silica gel, let stand for 50 min, filter to obtain the environmentally friendly flame-retardant insulating oil;

[0055] Among them, the silica gel is 5% of the pretreated epoxy soybean oil;

[0056] Among them, the preparation method of the additive includes the following steps:

[0057] Step (1): Under an Ar atmosphere, mix maleic anhydride-grafted lignin, DOPO, and N,N-dimethylformamide in a mass of 180:60:2500, heat under reflux, and react at 120 °C for 24 h. After the reaction, perform vacuum distillation, filter, wash with ethyl acetate 3 times, and dry at 70 °C for 7 h to obtain modified lignin;

[0058] Step (2): Mix hydroxyapatite, lysine, N,N-dimethylformamide, and p-toluenesulfonic acid in a mass ratio of 560:130:2400:10, react at 70 °C for 9 h. After the reaction, filter, wash with deionized water, and dry at 90 °C for 6 h to obtain modified hydroxyapatite;

[0059] Step (3): Mix modified lignin, modified hydroxyapatite, triethylamine, and N,N-dimethylformamide in a mass ratio of 30:38:1.8:1800, react at 85 °C for 8 h. After the reaction, filter, wash with absolute ethanol, and dry at 105 °C for 8 h to obtain the additive.

[0060] Example 4

[0061] This embodiment provides a preparation method of an environmentally friendly flame-retardant insulating oil, which includes the following steps:

[0062] Mix the pretreated epoxy soybean oil and the additive in a mass ratio of 9.5:1.3, react at 72 °C for 11.3 h. After the reaction ends, add silica gel, let it stand for 45 min, filter, and obtain the environmentally friendly flame-retardant insulating oil;

[0063] Among them, the silica gel is 5% of the pretreated epoxy soybean oil;

[0064] Among them, the preparation method of the additive includes the following steps:

[0065] Step (1) Under an Ar atmosphere, mix maleic anhydride-grafted lignin, DOPO, and N,N-dimethylformamide in a mass ratio of 195:65:3000, heat under reflux, react at 125 °C for 23 h. After the reaction ends, perform vacuum distillation, filter, wash with ethyl acetate 3 times, and dry at 75 °C for 6.5 h to obtain modified lignin;

[0066] Step (2) Mix hydroxyapatite, lysine, N,N-dimethylformamide, and p-toluenesulfonic acid in a mass ratio of 580:135:2600:11, react at 75 °C for 8.5 h. After the reaction ends, filter, wash with deionized water, and dry at 95 °C for 5.5 h to obtain modified hydroxyapatite;

[0067] Step (3) Mix modified lignin, modified hydroxyapatite, triethylamine, and N,N-dimethylformamide in a mass ratio of 35:44:2:2100, react at 90 °C for 7.5 h. After the reaction ends, filter, wash with absolute ethanol, and dry at 107 °C for 7.5 h to obtain the additive.

[0068] Example 5

[0069] This embodiment provides a preparation method of an environmentally friendly flame-retardant insulating oil, which includes the following steps:

[0070] Mix the pretreated epoxy soybean oil and the additive in a mass ratio of 10:1.4, react at 75 °C for 11 h. After the reaction ends, add silica gel, let it stand for 40 min, filter, and obtain the environmentally friendly flame-retardant insulating oil;

[0071] Among them, the silica gel is 5% of the pretreated epoxy soybean oil;

[0072] Among them, the preparation method of the additive includes the following steps:

[0073] Step (1) Under an Ar atmosphere, maleic anhydride grafted lignin, DOPO, and N,N-dimethylformamide were mixed in a mass ratio of 210:70:3500, heated under reflux, and reacted at 130 °C for 22 h. After the reaction, it was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 80 °C for 6 h to obtain modified lignin;

[0074] Step (2) Hydroxyapatite, lysine, N,N-dimethylformamide, and p-toluenesulfonic acid were mixed in a mass ratio of 600:140:2800:12, reacted at 80 °C for 8 h. After the reaction, it was filtered, washed with deionized water, and dried at 100 °C for 5 h to obtain modified hydroxyapatite;

[0075] Step (3) Modified lignin, modified hydroxyapatite, triethylamine, and N,N-dimethylformamide were mixed in a mass ratio of 40:50:2.2:2400, reacted at 95 °C for 7 h. After the reaction, it was filtered, washed with absolute ethanol, and dried at 110 °C for 7 h to obtain the additive.

[0076] Example 6

[0077] This example provides a preparation method of pretreated epoxy soybean oil, including the following steps:

[0078] Epoxy soybean oil and silica gel were mixed in a mass ratio of 1:3, dried and dehydrated in a vacuum drying oven at -0.1 MPa and 80 °C for 48 h, and cooled to room temperature to obtain Intermediate I;

[0079] Intermediate I and silica gel were mixed in a mass ratio of 22:1, stirred at 400 r / min for 5 min, stirred at 200 r / min for 2 h, allowed to stand for 80 min, filtered, and repeated 3 times to obtain pretreated epoxy soybean oil.

[0080] Example 7

[0081] This example provides a preparation method of maleic anhydride grafted lignin, including the following steps:

[0082] Lignin was repeatedly washed 3 times with 2 mol / L hydrochloric acid aqueous solution and dried; maleic anhydride, 4-dimethylaminopyridine, and N,N-dimethylformamide were added and mixed, heated, and reacted.

[0083] Lignin, maleic anhydride, 4-dimethylaminopyridine, and N,N-dimethylformamide were mixed in a mass ratio of 8:10:0.1:50, reacted at 80 °C for 12 h. After the reaction, 600 ml of deionized water was added, filtered, washed with deionized water, and vacuum dried at 50 °C for 6 h to obtain maleic anhydride grafted lignin.

[0084] Comparative Example 1

[0085] This comparative example provides a preparation method of an environmentally friendly flame-retardant insulating oil, including the following steps:

[0086] Mix the pretreated epoxy soybean oil, modified lignin, and modified hydroxyapatite in a mass ratio of 8:0.43:0.57, react at 65 °C for 12 h. After the reaction ends, add silica gel, let it stand for 60 min, filter, and obtain the environmentally friendly flame-retardant insulating oil;

[0087] Among them, the silica gel is 5% of the pretreated epoxy soybean oil;

[0088] Among them, the preparation method of the modified lignin includes the following steps:

[0089] Under an Ar atmosphere, mix maleic anhydride-grafted lignin, DOPO, and N,N-dimethylformamide in a mass ratio of 150:50:1500, heat under reflux, react at 110 °C for 26 h. After the reaction ends, perform vacuum distillation, filter, wash with ethyl acetate 3 times, and dry at 60 °C for 8 h to obtain the modified lignin;

[0090] Among them, the preparation method of the modified hydroxyapatite includes the following steps:

[0091] Mix hydroxyapatite, lysine, N,N-dimethylformamide, and p-toluenesulfonic acid in a mass ratio of 520:120:2000:8, react at 60 °C for 10 h. After the reaction ends, filter, wash with deionized water, and dry at 80 °C for 7 h to obtain the modified hydroxyapatite.

[0092] Comparative Example 2

[0093] This comparative example provides a preparation method of an environmentally friendly flame-retardant insulating oil, including the following steps:

[0094] Mix the pretreated epoxy soybean oil and modified lignin in a mass ratio of 8.57:0.43, react at 65 °C for 12 h. After the reaction ends, add silica gel, let it stand for 60 min, filter, and obtain the environmentally friendly flame-retardant insulating oil;

[0095] Among them, the silica gel is 5% of the pretreated epoxy soybean oil;

[0096] Among them, the preparation method of the modified lignin includes the following steps:

[0097] In an Ar atmosphere, maleic anhydride-grafted lignin, DOPO, and N,N-dimethylformamide were mixed in a mass ratio of 150:50:1500, heated under reflux, and reacted at 110 °C for 26 h. After the reaction, distillation under reduced pressure was carried out, followed by filtration. The product was washed three times with ethyl acetate and dried at 60 °C for 8 h to obtain modified lignin.

[0098] Comparative Example 3

[0099] This comparative example provides a method for preparing an environmentally friendly flame-retardant insulating oil, which includes the following steps:

[0100] The pretreated epoxy soybean oil and maleic anhydride-grafted lignin were mixed in a mass ratio of 8.57:0.43 and reacted at 65 °C for 12 h. After the reaction, silica gel was added, and the mixture was allowed to stand for 60 min and then filtered to obtain the environmentally friendly flame-retardant insulating oil;

[0101] Among them, the silica gel was 5% of the pretreated epoxy soybean oil.

[0102] The pretreated epoxy soybean oil in the examples and comparative examples of the present invention was all the pretreated epoxy soybean oil prepared in Example 6.

[0103] The maleic anhydride-grafted lignin in the examples and comparative examples of the present invention was all the maleic anhydride-grafted lignin prepared in Example 7.

[0104] In the present invention, the epoxy soybean oil is from Shandong Baijiahe Chemical Technology Co., Ltd., with a single product item number 41455-4, a flash point ≥ 280 °C, and a content of 99.9%; the lignin is from Tokyo Chemical Industry Co., Ltd.; the lysine is from Qingdao Haozeyang Biotechnology Co., Ltd., with a CAS number: 56-78-1; the hydroxyapatite is from Xi'an Luteng Biotechnology Co., Ltd., with a particle size of 4.5 μm.

[0105] The environmentally friendly flame-retardant insulating oils prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to corresponding tests. Among them, the flash point and fire point were determined according to the method described in GB / T 3536-2008 "Determination of Flash Point and Fire Point of Petroleum Products - Cleveland Open Cup Method"; the pour point was determined according to GB / T 3535-2006; the flame-retardant performance test was determined according to GB / T 16581-1996 "Test Method for Combustion Performance of Insulating Liquids - Oxygen Index Method"; the biodegradability was determined by the soil experiment method. Taking 28 days as a cycle, soil samples without pollution records were taken. The soil sampling was carried out using a core sampler (a stainless steel pipe with a diameter of about 5 cm); when in use, the sampler was inserted into the soil near a big tree about 20 cm deep to take out the soil sample; the soil sample was stored refrigerated before the test; 100 g of soil sample was placed in a wide-mouth glass bottle, and the addition amount of the environmentally friendly flame-retardant insulating oils prepared in Examples 1-5 and Comparative Examples 1-3 was 5 g / 100 g of soil; the test process was kept at 22 °C, and the unsealed method was adopted. The content of hydrocarbons in the sample before and after the test was measured by an ultraviolet spectrophotometer to represent the residual amount of the environmentally friendly flame-retardant insulating oil in the soil. The biodegradation rate of the environmentally friendly flame-retardant insulating oil was determined by the absorption spectra before and after the experiment. Among them, the test instrument can be selected as the UV-1601 Shimadzu multi-functional spectrometer, the wavelength range is 200-800 nm, and anhydrous ethanol was selected as the reference solution for the test; the specific test results are shown in Table 1;

[0106] Table 1

[0107]

[0108] Data analysis:

[0109] Comparing and analyzing the data in the above table, the flash point of the environmentally friendly flame-retardant insulating oil prepared by the present invention is up to 336°C at the highest, and the pour point is optimal at -26°C. Moreover, the average limiting oxygen index of the environmentally friendly flame-retardant insulating oil prepared in Examples 1-5 is 28.8%, which can be used as the insulating oil of a transformer, all having good biodegradability and meeting the environmental protection requirements. During combustion, hydroxyapatite can form a relatively stable heat-insulating layer on the material surface, hindering the transfer of heat from the flame to the interior of the material, reducing the heating rate of the material, and thus delaying the combustion process. However, the absence or uneven dispersion of hydroxyapatite results in an incomplete heat conduction network, and heat accumulation leads to a decrease in the ignition point. In Example 1, the carboxyl group in the modified lignin and the amino group in the modified hydroxyapatite reacted under the action of a catalyst to generate an additive containing an amide bond, increasing the compatibility with the pretreated epoxidized soybean oil, facilitating dispersion in the pretreated epoxidized soybean oil, and being beneficial to the flame-retardant performance and stability. Therefore, the flame-retardant performance of Comparative Example 1 is lower than that of Example 1. Comparative Example 2 lacks the modified hydroxyapatite compared to Comparative Example 1. Therefore, both the flash point and the ignition point are lower than those of Comparative Example 1, and the flame-retardant performance is lower than that of Comparative Example 1. In Comparative Example 3 compared to Comparative Example 2, the introduction of DOPO is missing. The phosphorus element contained therein can generate acid substances such as phosphoric acid and phosphorous acid, promoting the dehydration and carbonization of the matrix during combustion. The generated PO· free radicals can quench the combustion chain reaction and prevent the combustion reaction from proceeding, being beneficial to flame retardancy. Once missing, the flame-retardant performance will naturally decrease.

[0110] The above content is only an example and illustration of the structure of the present invention. Those skilled in the art of this technology can make various modifications or supplements to the specific embodiments described or use similar methods for substitution, as long as they do not deviate from the structure of the invention or exceed the scope defined by this claims, they should fall within the protection scope of the present invention.

[0111] In the description of this specification, the descriptions referring to terms such as "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0112] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments. Obviously, according to the content of this specification, many modifications and changes can be made. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, so that those skilled in the art can well understand and utilize the present invention. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A method for preparing an environmentally friendly flame-retardant insulating oil, characterized in that: The following steps are involved: The pretreated epoxidized soybean oil and the additive are mixed, heated, reacted, and after the reaction is completed, silica gel is added, allowed to stand, and filtered to obtain an environmentally friendly flame-retardant insulating oil; The preparation method of the additive comprises the following steps: Step (1) in an inert gas atmosphere, maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide are mixed, stirred, heated and refluxed to react, and after the reaction is completed, vacuum distillation, filtration, washing and drying are performed to obtain modified lignin; Step (2) adding hydroxyapatite and lysine to N,N-dimethylformamide, adding a catalyst, heating for reaction, filtering, washing, and drying after the reaction is completed to obtain modified hydroxyapatite; Step (3) adding modified lignin, modified hydroxyapatite and triethylamine to N,N-dimethylformamide, heating for reaction, filtering, washing and drying after the reaction is completed to obtain an additive.

2. The method for preparing an environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that: The preparation method of maleic anhydride grafted lignin comprises the following steps: The lignin was repeatedly washed with hydrochloric acid aqueous solution for 3 times and dried; maleic anhydride, 4-dimethylaminopyridine and N,N-dimethylformamide were added, mixed, heated, reacted, and after the reaction was completed, deionized water was added, filtered, washed, and dried to obtain maleic anhydride grafted lignin.

3. The method for preparing an environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that: The mass ratio of the pretreated epoxidized soybean oil to the additive is (8-10):(1-1.4).

4. The method for preparing an environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that: In step (1), the mass ratio of maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide is (150-210):(50-70):(1500-3500).

5. The method for preparing an environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that: In step (1), the reaction conditions are 110-130° C. for 22-26 hours.

6. The method for preparing an environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that: In step (2), the mass ratio of hydroxyapatite, lysine, N,N-dimethylformamide and catalyst is (520-600):(120-140):(2000-2800):(8-12).

7. The method for preparing an environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that: In step (2), the reaction temperature is 60-80°C and the reaction time is 8-10h.

8. The method for preparing an environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that: In step (3), the mass ratio of modified lignin, modified hydroxyapatite, triethylamine and N,N-dimethylformamide is (20-40):(26-50):(1.4-2.2):(1200-2400); the reaction temperature is 75-95°C, and the reaction time is 7-9h.

9. An environmentally friendly flame-retardant insulating oil prepared by the method for preparing the environmentally friendly flame-retardant insulating oil according to any one of claims 1 to 8.

10. Use of the environmentally friendly flame-retardant insulating oil according to claim 9 in a transformer.

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