An environmentally friendly flame-retardant insulating oil and its application

Abstract

The application discloses an environment-friendly flame-retardant insulating oil and application thereof, and belongs to the technical field of insulating materials. The application is used for solving the technical problem that the insulating oil needs to have a high flash point, good environmental protection and flame-retardant performance in the prior art, and comprises the following steps: mixing pretreated epoxy soybean oil and an additive, heating, reacting, adding silica gel after the reaction is completed, standing, filtering, and obtaining the environment-friendly flame-retardant insulating oil; wherein, under an inert gas atmosphere, maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide are mixed, stirred, heated and refluxed to react, and modified lignin is obtained; hydroxyapatite and lysine are added into N,N-dimethylformamide, a catalyst is added, heating is conducted, and reaction is performed to obtain modified hydroxyapatite; the modified lignin, the modified hydroxyapatite and triethylamine are added into N,N-dimethylformamide, heating is conducted, and reaction is performed to obtain the additive; the environment-friendly flame-retardant insulating oil obtained by the application has a high flash point, ignition point and good biodegradability.

Description

Technical Field

[0001] This invention relates to the field of insulating materials technology, specifically to an environmentally friendly flame-retardant insulating oil and its applications. Background Technology

[0002] Insulating oil is a liquid insulating medium widely used in oil-immersed insulated high-voltage equipment such as transformers, circuit breakers, and voltage transformers. There are three main types of insulating oil: silicone oil, mineral oil, and ester oil. Mineral insulating oil has a biodegradability rate of only about 30%, and residual mineral insulating oil can cause changes in soil structure. It can also react synergistically with residual heavy metals like copper in the soil, potentially causing compound pollution, affecting plant growth, and harming human health. Given the non-renewable nature of petroleum resources, we will eventually face shortages and depletion. Currently, people's environmental awareness is gradually increasing, leading to higher demands on insulating oil.

[0003] Chinese patent CN115141671B discloses an alkyl-modified silicone oil insulating grease. This invention modifies silicone oil by introducing CF3 groups on the side chains of the silicone oil molecules and phosphate ester groups at the ends of the molecular chains. The CF3 groups can endow silicone oil with excellent friction reduction and anti-wear properties. The introduced phosphate ester groups are effective flame retardant components, which can significantly improve the flame resistance of 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 electric arc, producing harmful substances such as formaldehyde, which poses a potential threat to the environment and human health. Chinese patent CN102682869B discloses a method for preparing plant-based insulating oil. The plant-based insulating oil is made from refined plant oil as raw material, which undergoes an ester exchange reaction with low molecular weight alcohol. After the reaction is stopped, the oil is refined and washed, then subjected to vacuum distillation to remove water and alcohol, decolorized, deacidified, filtered, and then subjected to deep dehydration. Finally, antioxidants and pour point depressants are added. The plant-based insulating oil prepared by this invention has an optimal flash point of 270°C, but its flash point needs to be improved to expand its application range. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings 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 application range.

[0005] To achieve the above objectives, the present invention provides a method for preparing an environmentally friendly flame-retardant insulating oil, comprising the following steps:

[0006] Pretreated epoxidized soybean oil and additives are mixed, heated, and reacted. After the reaction is complete, silica gel is added, the mixture is allowed to stand, and then filtered to obtain an environmentally friendly flame-retardant insulating oil.

[0007] 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 and heated under reflux to react. After the reaction is completed, the mixture is distilled under reduced pressure, filtered, washed and dried to obtain modified lignin.

[0009] Step (2) Hydroxyapatite and lysine are added to N,N-dimethylformamide, a catalyst is added, the reaction is heated, the reaction is completed, filtered, washed, and dried to obtain modified hydroxyapatite;

[0010] Step (3) Modified lignin, modified hydroxyapatite, and triethylamine are added to N,N-dimethylformamide, heated to react, and after the reaction is completed, filtered, washed, and dried to obtain the additive.

[0011] Preferably, the method for preparing pretreated epoxidized soybean oil includes the following steps:

[0012] Epoxidized soybean oil was mixed with silica gel and dried in a vacuum drying oven at -0.1 MPa and 80°C for 48 hours. The mixture was then cooled to room temperature. Silica gel was added, stirred, allowed to stand, filtered, and the process was repeated three times to obtain pretreated epoxidized soybean oil.

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

[0014] The lignin was washed three times with hydrochloric acid aqueous solution and dried. Maleic anhydride, 4-dimethylaminopyridine and N,N-dimethylformamide were added and mixed. The mixture was heated and reacted. After the reaction was completed, deionized water was added, filtered, washed and dried to obtain maleic anhydride-grafted lignin.

[0015] 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] The reaction conditions are 80-100℃ for 8-12 hours.

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

[0018] Preferably, the reaction conditions for the pretreated epoxidized soybean oil and additives are: 65-75℃ for 11-12 hours.

[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 110-130℃ for 22-26 hours.

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

[0022] Preferably, the catalyst is either concentrated sulfuric acid or p-toluenesulfonic acid.

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

[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℃ and the reaction time is 7-9h.

[0025] Preferably, the environmentally friendly flame-retardant insulating oil is prepared using the aforementioned method.

[0026] Preferably, the aforementioned environmentally friendly flame-retardant insulating oil is used in transformers.

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

[0028] 1. The epoxidized soybean oil used in this invention is a vegetable oil with a certain degree of biodegradability, belonging to renewable resources. Compared with soybean oil, it contains fewer unsaturated carbon-carbon bonds, resulting in better stability and antioxidant properties. Lignin is abundant, possesses a three-dimensional network structure, and has the ability to form char during combustion. During heating, it can generate a large amount of char through dehydration and condensation carbonization, forming a relatively dense char layer, which is beneficial for flame retardancy. Maleic anhydride grafted onto lignin introduces carboxylic acid groups, which undergo an addition reaction with the PH bond of DOPO to form a phosphate ester structure, yielding modified lignin. Hydroxyapatite is a high-temperature resistant non-combustible material with a high phosphorus content and good biodegradability. The modified hydroxyapatite reduces agglomeration and facilitates its dispersibility in epoxidized soybean oil. During combustion, the added DOPO can produce acidic substances such as phosphoric acid and phosphorous acid, which promote the dehydration and carbonization of the matrix during combustion and graft onto the surface of hydroxyapatite, thus accelerating the formation of the char layer. 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 also produces non-combustible gas during combustion, diluting the oxygen concentration in the combustion system. The four elements work together to further improve the flame retardant performance.

[0029] 2. In this invention, the hydroxyl groups in hydroxyapatite and the carboxyl groups in lysine undergo an esterification reaction to obtain modified hydroxyapatite; the amino groups in the modified hydroxyapatite react with the carboxyl groups in modified lignin to obtain an additive; and the primary amino groups in the additive undergo a ring-opening reaction with the epoxy groups in epoxidized soybean oil under the action of a catalyst to obtain an environmentally friendly flame-retardant insulating oil. At this point, the additive has a large molecular size and complex spatial structure. After grafting with epoxidized soybean oil, it forms a certain steric hindrance, hindering the regular arrangement and aggregation of epoxidized soybean oil molecules, thereby reducing crystallization and solidification, improving its resistance to solidification at low temperatures, and lowering the pour point; simultaneously, it reduces flow resistance, improves fluidity at low temperatures, and lowers the pour point.

[0030] 3. This invention uses epoxidized soybean oil, which has undergone pretreatment to remove impurities, thus improving its stability. Furthermore, hydroxyapatite and maleic anhydride-grafted lignin both possess certain biodegradability, which is beneficial to environmental protection. The method for preparing the environmentally friendly flame-retardant insulating oil provided by this invention is highly efficient, improving the flash point and ignition point of vegetable oil insulating oils. The stability and antioxidant properties of the environmentally friendly flame-retardant insulating oil are improved, reducing the probability of viscosity increase due to high-temperature degradation, maintaining long-term heat dissipation efficiency, and facilitating its application in transformers. Attached Figure Description

[0031] Figure 1 This is a process flow diagram for preparing the environmentally friendly flame-retardant insulating oil in this invention;

[0032] Figure 2 This is a flowchart illustrating the preparation process of the additives in this invention.

[0033] Figure 3 The bar chart shows the biodegradability test results of the environmentally friendly flame-retardant insulating oils in Examples 1-5 and Comparative Examples 1-3 of this invention. Detailed Implementation

[0034] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1

[0036] This embodiment provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0037] Pretreated epoxidized soybean oil and additives were mixed at a mass ratio of 8:1 and reacted at 65℃ for 12 hours. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 60 minutes, and then filtered to obtain environmentally friendly flame-retardant insulating oil.

[0038] The silica gel is 5% of the pretreated epoxidized soybean oil;

[0039] The preparation method of the additive includes the following steps:

[0040] Step (1) Under an Ar atmosphere, maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide were mixed in a mass ratio of 150:50:1500, heated to reflux, and reacted at 110°C for 26 h. After the reaction was completed, the mixture was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 60°C for 8 h to obtain modified lignin.

[0041] Step (2) Hydroxyapatite, lysine, N,N-dimethylformamide and p-toluenesulfonic acid are mixed in a mass ratio of 520:120:2000:8 and reacted at 60°C for 10 h. After the reaction is completed, the mixture is filtered, washed with deionized water, and dried at 80°C for 7 h to obtain modified hydroxyapatite.

[0042] Step (3) Modified lignin, modified hydroxyapatite, triethylamine and N,N-dimethylformamide are mixed in a mass ratio of 20:26:1.4:1200 and reacted at 75°C for 9 hours. After the reaction is completed, the mixture is filtered, washed with anhydrous ethanol, and dried at 100°C for 9 hours to obtain the additive.

[0043] Example 2

[0044] This embodiment provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0045] Pretreated epoxidized soybean oil and additives were mixed at a mass ratio of 8.5:1.1 and reacted at 67℃ for 11.8h. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 55min, and then filtered to obtain environmentally friendly flame-retardant insulating oil.

[0046] The silica gel is 5% of the pretreated epoxidized soybean oil;

[0047] The preparation method of the additive includes the following steps:

[0048] Step (1) Under an Ar atmosphere, maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide were mixed in a mass ratio of 165:55:2000, heated to reflux, and reacted at 115°C for 25 h. After the reaction was completed, the mixture was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 65°C for 7.5 h to obtain modified lignin.

[0049] Step (2) Hydroxyapatite, lysine, N,N-dimethylformamide and p-toluenesulfonic acid are mixed in a mass ratio of 540:125:2200:9 and reacted at 65°C for 9.5 h. After the reaction is completed, the mixture is filtered, washed with deionized water, and dried at 85°C for 6.5 h to obtain modified hydroxyapatite.

[0050] Step (3) Modified lignin, modified hydroxyapatite, triethylamine and N,N-dimethylformamide are mixed in a mass ratio of 25:32:1.6:1500 and reacted at 80°C for 8.5 h. After the reaction is completed, the mixture is filtered, washed with anhydrous ethanol, and dried at 102°C for 8.5 h to obtain the additive.

[0051] Example 3

[0052] This embodiment provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0053] Pretreated epoxidized soybean oil and additives were mixed at a mass ratio of 9:1.2 and reacted at 70℃ for 11.5 hours. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 50 minutes, and then filtered to obtain environmentally friendly flame-retardant insulating oil.

[0054] The silica gel is 5% of the pretreated epoxidized soybean oil;

[0055] The preparation method of the additive includes the following steps:

[0056] Step (1) Under an Ar atmosphere, maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide were mixed in a mass ratio of 180:60:2500, heated to reflux, and reacted at 120°C for 24 h. After the reaction was completed, the mixture was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 70°C for 7 h to obtain modified lignin.

[0057] Step (2) Hydroxyapatite, lysine, N,N-dimethylformamide and p-toluenesulfonic acid are mixed in a mass ratio of 560:130:2400:10 and reacted at 70°C for 9 hours. After the reaction is completed, the mixture is filtered, washed with deionized water, and dried at 90°C for 6 hours to obtain modified hydroxyapatite.

[0058] Step (3) Modified lignin, modified hydroxyapatite, triethylamine and N,N-dimethylformamide are mixed in a mass ratio of 30:38:1.8:1800 and reacted at 85°C for 8 hours. After the reaction is completed, the mixture is filtered, washed with anhydrous ethanol, and dried at 105°C for 8 hours to obtain the additive.

[0059] Example 4

[0060] This embodiment provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0061] Pretreated epoxidized soybean oil and additives were mixed at a mass ratio of 9.5:1.3 and reacted at 72℃ for 11.3 hours. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 45 minutes, and then filtered to obtain an environmentally friendly flame-retardant insulating oil.

[0062] The silica gel is 5% of the pretreated epoxidized soybean oil;

[0063] The preparation method of the additive includes the following steps:

[0064] Step (1) Under an Ar atmosphere, maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide were mixed in a mass ratio of 195:65:3000, heated to reflux, and reacted at 125°C for 23 h. After the reaction was completed, the mixture was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 75°C for 6.5 h to obtain modified lignin.

[0065] Step (2) Hydroxyapatite, lysine, N,N-dimethylformamide and p-toluenesulfonic acid are mixed in a mass ratio of 580:135:2600:11 and reacted at 75°C for 8.5 h. After the reaction is completed, the mixture is filtered, washed with deionized water, and dried at 95°C for 5.5 h to obtain modified hydroxyapatite.

[0066] Step (3) Modified lignin, modified hydroxyapatite, triethylamine and N,N-dimethylformamide are mixed in a mass ratio of 35:44:2:2100 and reacted at 90℃ for 7.5h. After the reaction is completed, the mixture is filtered, washed with anhydrous ethanol, and dried at 107℃ for 7.5h to obtain the additive.

[0067] Example 5

[0068] This embodiment provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0069] Pretreated epoxidized soybean oil and additives were mixed at a mass ratio of 10:1.4 and reacted at 75°C for 11 hours. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 40 minutes, and then filtered to obtain environmentally friendly flame-retardant insulating oil.

[0070] The silica gel is 5% of the pretreated epoxidized soybean oil;

[0071] The preparation method of the additive includes the following steps:

[0072] 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 to reflux, and reacted at 130°C for 22 h. After the reaction was completed, the mixture was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 80°C for 6 h to obtain modified lignin.

[0073] Step (2) Hydroxyapatite, lysine, N,N-dimethylformamide and p-toluenesulfonic acid are mixed in a mass ratio of 600:140:2800:12 and reacted at 80°C for 8 hours. After the reaction is completed, the mixture is filtered, washed with deionized water, and dried at 100°C for 5 hours to obtain modified hydroxyapatite.

[0074] Step (3) Modified lignin, modified hydroxyapatite, triethylamine and N,N-dimethylformamide are mixed in a mass ratio of 40:50:2.2:2400 and reacted at 95°C for 7 hours. After the reaction is completed, the mixture is filtered, washed with anhydrous ethanol, and dried at 110°C for 7 hours to obtain the additive.

[0075] Example 6

[0076] This embodiment provides a method for preparing pretreated epoxidized soybean oil, including the following steps:

[0077] Epoxidized soybean oil and silica gel were mixed at a mass ratio of 1:3 and dried and dehydrated in a vacuum drying oven at -0.1 MPa and 80°C for 48 hours. After cooling to room temperature, intermediate I was obtained.

[0078] Intermediate I and silica gel were mixed at 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 the mixture was repeated 3 times to obtain pretreated epoxidized soybean oil.

[0079] Example 7

[0080] This embodiment provides a method for preparing maleic anhydride-grafted lignin, comprising the following steps:

[0081] Lignin was washed three times with 2 mol / L hydrochloric acid aqueous solution and dried; then maleic anhydride, 4-dimethylaminopyridine, and N,N-dimethylformamide were added and the mixture was heated to react.

[0082] Lignin, maleic anhydride, 4-dimethylaminopyridine and N,N-dimethylformamide were mixed in a mass ratio of 8:10:0.1:50 and reacted at 80℃ for 12 h. After the reaction was completed, 600 ml of deionized water was added, filtered, washed with deionized water, and dried under vacuum at 50℃ for 6 h to obtain maleic anhydride-grafted lignin.

[0083] Comparative Example 1

[0084] This comparative example provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0085] Pretreated epoxidized soybean oil, modified lignin and modified hydroxyapatite were mixed in a mass ratio of 8:0.43:0.57 and reacted at 65℃ for 12 hours. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 60 minutes, and then filtered to obtain an environmentally friendly flame-retardant insulating oil.

[0086] The silica gel is 5% of the pretreated epoxidized soybean oil;

[0087] The preparation method of modified lignin includes the following steps:

[0088] Under an Ar atmosphere, maleic anhydride-grafted lignin, DOPO and N,N-dimethylformamide were mixed in a mass ratio of 150:50:1500, heated to reflux, and reacted at 110°C for 26 h. After the reaction was completed, the mixture was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 60°C for 8 h to obtain modified lignin.

[0089] The preparation method of modified hydroxyapatite includes the following steps:

[0090] Hydroxyapatite, lysine, N,N-dimethylformamide, and p-toluenesulfonic acid were mixed in a mass ratio of 520:120:2000:8 and reacted at 60°C for 10 hours. After the reaction was completed, the mixture was filtered, washed with deionized water, and dried at 80°C for 7 hours to obtain modified hydroxyapatite.

[0091] Comparative Example 2

[0092] This comparative example provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0093] Pretreated epoxidized soybean oil and modified lignin were mixed at a mass ratio of 8.57:0.43 and reacted at 65℃ for 12 hours. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 60 minutes, and then filtered to obtain an environmentally friendly flame-retardant insulating oil.

[0094] The silica gel is 5% of the pretreated epoxidized soybean oil;

[0095] The preparation method of modified lignin includes the following steps:

[0096] Under an Ar atmosphere, maleic anhydride-grafted lignin, DOPO, and N,N-dimethylformamide were mixed in a mass ratio of 150:50:1500, heated to reflux, and reacted at 110°C for 26 h. After the reaction was completed, the mixture was distilled under reduced pressure, filtered, washed three times with ethyl acetate, and dried at 60°C for 8 h to obtain modified lignin.

[0097] Comparative Example 3

[0098] This comparative example provides a method for preparing an environmentally friendly flame-retardant insulating oil, including the following steps:

[0099] Pretreated epoxidized soybean oil and maleic anhydride grafted lignin were mixed at a mass ratio of 8.57:0.43 and reacted at 65℃ for 12 hours. After the reaction was completed, silica gel was added, the mixture was allowed to stand for 60 minutes, and then filtered to obtain an environmentally friendly flame-retardant insulating oil.

[0100] The silica gel is 5% of the pretreated epoxidized soybean oil.

[0101] The pretreated epoxidized soybean oil used in the embodiments and comparative examples of this invention is the pretreated epoxidized soybean oil prepared in Example 6.

[0102] The maleic anhydride grafted lignin in the embodiments and comparative examples of this invention are all the maleic anhydride grafted lignin prepared in Example 7.

[0103] In this invention, the epoxidized soybean oil is from Shandong Baijiahe Chemical Technology Co., Ltd., product number 41455-4, flash point ≥280℃, content: 99.9%; lignin is from Tokyo Chemical Industry Co., Ltd.; lysine is from Qingdao Haozeyang Biotechnology Co., Ltd., CAS number: 56-78-1; hydroxyapatite is from Xi'an Lvteng Biotechnology Co., Ltd., particle size 4.5μm.

[0104] The environmentally friendly flame-retardant insulating oils prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to corresponding tests. Flash point and ignition point were determined according to the method described in GB / T 3536-2008 "Determination of Flash Point and Ignition Point of Petroleum Products - Cleveland Open Cup Method"; pour point was determined according to GB / T3535-2006; flame retardant performance was tested according to GB / T 16581-1996 "Test Method for Combustion Performance of Insulating Liquids - Oxygen Index Method"; biodegradability was tested using the soil test method, with a 28-day cycle. Soil samples without prior pollution records were taken, and a core sampler (a stainless steel tube with a diameter of approximately 5 cm) was used for soil sampling. During use, the sampler was inserted approximately 20 cm into the soil near a large tree to extract the soil sample. The soil samples were refrigerated before the test. 100 g of soil sample was placed in a wide-mouthed glass bottle. The environmentally friendly flame-retardant insulating oils prepared in Examples 1-5 and Comparative Examples 1-3 were then used to extract the oil. The amount of insulating oil added was 5g / 100g of soil. The experiment was conducted at a constant temperature of 22℃ using an unsealed method. The hydrocarbon content in the samples before and after the experiment was measured using a UV spectrophotometer to represent the residual amount of 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. The testing instrument could be a UV-1601 Shimadzu multi-functional spectrometer with a wavelength range of 200-800nm. Anhydrous ethanol was used as the reference solution in the experiment. The specific test results are shown in Table 1.

[0105] Table 1

[0106]

[0107] Data Analysis:

[0108] Comparative analysis of the data in the table above shows that the environmentally friendly flame-retardant insulating oil prepared by this invention has a flash point of up to 336℃ and a pour point of -26℃. Furthermore, the limiting oxygen index of the environmentally friendly flame-retardant insulating oils prepared in Examples 1-5 is an average of 28.8%, making them suitable for use as insulating oil in transformers. All exhibit good biodegradability, meeting environmental protection requirements. During combustion, hydroxyapatite forms a relatively stable heat-insulating layer on the material surface, hindering heat transfer from the flame to the material's interior, reducing the material's heating rate, and thus slowing the combustion process. However, the absence or uneven dispersion of hydroxyapatite results in an incomplete thermal conductivity network, leading to heat accumulation and a lower ignition point. In Example 1, the carboxyl groups in the modified lignin and the amino groups in the modified hydroxyapatite, under the action of a catalyst, generate an additive containing amide bonds, increasing compatibility with pretreated epoxidized soybean oil and facilitating dispersion in the pretreated epoxidized soybean oil. This is beneficial for 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 modified hydroxyapatite compared to Comparative Example 1, therefore its flash point and ignition point are both lower than Comparative Example 1, and its flame retardant performance is also lower than Comparative Example 1. Comparative Example 3, compared to Comparative Example 2, lacks the introduction of DOPO. The phosphorus element it contains can produce acidic substances such as phosphoric acid and phosphorous acid, promoting dehydration and carbonization of the matrix during combustion. The generated PO· free radicals can quench the combustion chain reaction, preventing the combustion reaction from proceeding and thus contributing to flame retardancy. Without DOPO, the flame retardant performance naturally decreases.

[0109] The above description is merely an example and illustration of the structure of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

[0110] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0111] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing an environmentally friendly flame-retardant insulating oil, characterized by, The method comprises the following steps: The pretreated epoxy soybean oil and the additive are mixed according to a mass ratio of (8-10):(1-1.4), heated, and reacted at 65-75 DEG C for 11-12 h, then silica gel is added, and the mixture is left to stand, filtered, and an environmentally-friendly flame-retardant insulating oil is obtained; The preparation method of the additive comprises the following steps: In step (1), the maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide are mixed, stirred, heated and refluxed in an inert gas atmosphere, reacted, distilled under reduced pressure, filtered, washed and dried to obtain the modified lignin; In step (2), the hydroxyapatite and lysine are added to N,N-dimethylformamide, a catalyst is added, heated and reacted, then filtered, washed and dried to obtain the modified hydroxyapatite; In step (3), the modified lignin, the modified hydroxyapatite and triethylamine are added to N,N-dimethylformamide, heated and reacted, then filtered, washed and dried to obtain the additive. In step (1), the mass ratio of the maleic anhydride grafted lignin, DOPO and N,N-dimethylformamide is (150-210):(50-70):(1500-3500).

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

3. The preparation method of the environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that, In step (1), the reaction condition is 110-130 DEG C for 22-26 h.

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

5. The preparation method of the environmentally friendly flame-retardant insulating oil according to claim 1, characterized in that, In step (2), the reaction temperature is 60-80 DEG C and the reaction time is 8-10 h.

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

7. An environmentally-friendly flame-retardant insulating oil prepared by the method according to any one of claims 1-6.

8. The environmentally-friendly flame-retardant insulating oil according to claim 7 is used in a transformer.

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