DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound as well as preparation method and application thereof

Abstract

The invention relates to the technical field of preparation and application of multi-element synergistic flame retardants, in particular to a DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound as well as a preparation method and application thereof. The preparation method of the DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound comprises the following steps: (1) reacting 2-amino-1, 3, 4-thiadiazole with p-methylsulfonylbenzaldehyde to synthesize a Schiff base intermediate, and (2) adding DOPO to perform a phosphorus-hydrogen addition reaction to synthesize the DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound. The DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound is synthesized by adopting a one-pot method, and the reaction method is simple and efficient; besides, the DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound has a multi-element (P, S and N) synergistic flame-retardant effect, the flame retardance of the epoxy resin can be remarkably improved, meanwhile, high transparency and stable mechanical properties of the epoxy resin can be kept, and the compound can be expanded to application in optical electronic products.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation and application of multi-element synergistic flame retardants, and specifically relates to a preparation and application method of a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound. Background Art

[0002] Epoxy resin is a thermosetting resin with excellent performance. It has excellent solvent resistance, corrosion resistance, high temperature resistance, electrical insulation, high transparency and other advantages. It is widely used in electronics, coatings, construction, optics, insulation and other industries. However, epoxy resin is easy to burn and produces a lot of smoke and toxic gas during combustion, which greatly limits its application in high-end fields. Therefore, the development of high-efficiency flame-retardant epoxy resin is of great significance in the application expansion of epoxy resin.

[0003] Halogen flame retardants are currently the most popular in the market, but with in-depth research, it is found that the gases produced by the combustion of halogen flame retardants are harmful to the human body and pollute the environment. The most widely studied are phosphorus-containing flame retardants. In addition, nitrogen, sulfur, silicon, boron and other atoms are added to synergize with phosphorus for flame retardancy. Among them, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) is an important intermediate of organic phosphorus-containing flame retardants. This flame retardant intermediate has high stability, and its phosphorus-hydrogen bond can react with double bonds, carbonyl groups, epoxy groups, amino groups and other groups, and has high reactivity.

[0004] However, many flame retardants nowadays improve flame retardancy while often sacrificing the light transmittance of epoxy resin. Chinese patent CN108864193A discloses a phosphaphenanthrene compound and its preparation method and use, which first uses a benzaldehyde derivative with a 5-dimethyl-1,3,2-dioxaphosphinane-2-oxide group in the para position and p-phenylenediamine or a similar compound with a diamine structure for dehydration condensation to obtain a Schiff base intermediate, and then reacts with DOPO for addition reaction to prepare a phosphaphenanthrene flame retardant compound. Although this method has a reactive group (-NH) with an epoxy group, it is not easy to react with the epoxy group due to its large structure and large steric hindrance, resulting in part of the flame retardant being free in the resin solution, poor light transmittance, and opaque resin after curing, resulting in a difficult balance between flame retardancy and transparency and mechanical properties.

[0005] The disclosure of the background technology information is only used for understanding the overall background technology of the present invention, and should not be regarded as admitting or suggesting in any form that the information constitutes the prior art known to those skilled in the art. Summary of the invention

[0006] One of the purposes of the present invention is to provide a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound to solve the problem that it is difficult to achieve transparency, mechanical properties and flame retardancy of epoxy resin.

[0007] The above objectives are achieved through the following technical solutions:

[0008] A DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound is prepared by synthesizing a Schiff base intermediate (MBAT) using 2-amino-1,3,4-thiadiazole and p-methylsulfonylbenzaldehyde, and then adding MBAT to DOPO to obtain a DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound (MBATP). Since the Schiff base intermediate MBAT has low structural rigidity, the influence on the mechanical properties of epoxy resin can be reduced. Meanwhile, the synthesized MBATP has a small molecular weight, a simple structure, low rigidity, high compatibility with epoxy resin, and little influence on the transparency of flame-retardant epoxy resin, so as to solve the problem of difficult balance between mechanical properties and transparency of flame-retardant epoxy resin.

[0009] The chemical structure of the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound is as follows:

[0010]

[0011] The preparation method of the above DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound adopts a one-pot method to simplify the synthesis process. The two-step synthesis reaction conditions are unchanged, which is easy to control and facilitates the industrialization of the product. The specific steps are as follows:

[0012] S1: Add 2-amino-1,3,4-thiadiazole and solvent into a four-necked round-bottom flask with a condenser and a thermometer, heat and stir until all dissolved, then add p-methylsulfonylbenzaldehyde into the flask to carry out condensation reaction to synthesize Schiff base intermediate MBAT;

[0013] The preparation process of Schiff base intermediate MBAT is as follows:

[0014]

[0015] S2: directly adding DOPO to the Schiff base intermediate MBAT solution in step S1 to carry out a phosphorus-hydrogen addition reaction, cooling the temperature to room temperature after the reaction is completed, and performing post-treatment to obtain a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound.

[0016] The preparation process of DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound (MBATP) is as follows:

[0017]

[0018] Preferably, the molar ratio of 2-amino-1,3,4-thiadiazole to p-methylsulfonylbenzaldehyde DOPO is 1:1-1.5:1-1.3. A slight excess of the latter two can ensure that 2-amino-1,3,4-thiadiazole and Schiff base intermediates can be completely converted, ensure that the reaction proceeds in the forward direction, and improve the conversion rate of the raw materials.

[0019] Preferably, the molar ratio of the 2-amino-1,3,4-thiadiazole to p-methylsulfonylbenzaldehyde DOPO is 1:1:1.

[0020] Preferably, the volume ratio of 2-amino-1,3,4-thiadiazole to solvent is 1 mol:6000-8000 mL, ensuring that 2-amino-1,3,4-thiadiazole can be fully dissolved at the reaction temperature to avoid waste of raw materials and increased difficulty in post-processing.

[0021] Preferably, the condensation reaction conditions are: stirring at 40 to 80° C. for 6 to 12 hours. When the temperature is lower than 40° C., 2-amino-1,3,4-thiadiazole cannot be completely dissolved and the reaction rate will also be reduced. When the temperature is higher than 80° C., the solvent will boil, thereby affecting the reaction.

[0022] Preferably, the conditions for the phosphorus-hydrogen addition reaction are: 40-80° C. for 8-24 hours. By precisely controlling the reaction temperature and reaction time, the forward reaction can be well ensured to avoid side reactions and reduce the product yield.

[0023] Preferably, the solvent in step S1 is selected from one or more of anhydrous ethanol, methanol, dioxane, dichloromethane, chloroform, tetrahydrofuran, benzene, toluene, and acetonitrile. The reaction raw materials have high solubility in the above solvents, will not react with reactants and intermediates, and have stable chemical and physical properties.

[0024] Preferably, the post-treatment method in step S2 is: obtain a crude product by suction filtration or rotary evaporation, rinse with anhydrous ethanol for multiple times to remove unreacted aldehyde, Schiff base intermediate and DOPO, and then dry at 80-100° C. under vacuum conditions for 12-24 hours.

[0025] The flame-retardant epoxy resin prepared by the above-mentioned DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound, -NH- in the DOPO-based phosphorus-nitrogen-sulfur synergistic flame-retardant compound can react with the epoxy group in the epoxy resin, and the good compatibility ensures high transparency, solving the problem of balancing the transparency, mechanical properties and flame retardant properties of the epoxy resin.

[0026] Preferably, the concentration of the DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound in the flame retardant epoxy resin is 1 to 7%.

[0027] Preferably, the concentration of the DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound in the flame retardant epoxy resin is 2-4%.

[0028] The method for preparing the flame retardant epoxy resin comprises the following steps:

[0029] V1: Weigh a certain amount of epoxy resin and heat it to 160℃. Add DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compounds and stir until a transparent uniform solution is obtained. Keep the mixture warm for a while. -NH- in the flame retardant can react with the epoxy group in the epoxy resin. Good compatibility ensures high transparency.

[0030] V2: After cooling the solution described in V1 to 80-100°C, add the curing agent and stir at a reduced speed until all the solution is dissolved. The purpose of lowering the temperature of the epoxy resin and adding the curing agent is to prevent the epoxy resin from curing too quickly and to increase the molding preparation time.

[0031] V3: The solution in step V2 is quickly poured into a mold preheated at 100° C. and cured under certain conditions in an oven to obtain a flame retardant epoxy resin.

[0032] Preferably, the curing agent includes 4,4-diaminodiphenylmethane (DDM);.

[0033] Preferably, the curing conditions are: curing at 100° C. for 2 to 3 hours and curing at 150° C. for 3 to 4 hours. Curing at 100° C. for 2 to 3 hours can ensure that the epoxy resin and the curing agent can fully react, avoiding problems such as bubbles or defects caused by too fast a curing speed, and curing at 150° C. for 3 to 4 hours can ensure that the epoxy resin is fully cured.

[0034] The present invention has the following beneficial effects:

[0035] The invention uses 2-amino-1,3,4-thiadiazole, p-methylsulfonylbenzaldehyde and DOPO to synthesize a DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound, which can be effectively applied to epoxy resin. The synthesized Schiff base intermediate MBAT has a small molecular weight and low rigidity, thereby ensuring the mechanical properties of the epoxy resin. Meanwhile, the DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound has a simple structure, small steric hindrance, high compatibility with the epoxy resin, and can evenly disperse MBATP in the epoxy resin, with stable flame retardant performance and little influence on the transparency of the flame retardant epoxy resin. The flame retardant epoxy resin is expected to be applied to optical materials and microelectronic materials.

[0036] The DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound provided by the present invention utilizes three elements (phosphorus, nitrogen, and sulfur) for synergistic flame retardancy, and can reach V-0 level (UL-94) at a flame retardant addition amount of 4wt%. Not only is the addition amount small, the impact on the performance of epoxy resin is greatly reduced, and a high flame retardant efficiency can be achieved. Compared with traditional halogen flame retardants, it is more environmentally friendly and safer. In addition, the synthesis process is simple, the purification method is easy to operate, the reaction is easy to control, the side reaction is small, and the reaction waste liquid is easy to recycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1This is an infrared spectrum of the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound in Example 1 of the present invention;

[0038] Figure 2 1H-NMR spectrum of the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound in Example 1 of the present invention;

[0039] Figure 3 is the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound in Example 1 of the present invention 31 P-NMR nuclear magnetic spectrum;

[0040] Figure 4 This is a graph showing the transmittance test results of 1EP / MBATP-2, 1EP / MBATP-4 and epoxy resin prepared in Example 1 of the present invention.

[0041] Figure 5 SEM images of carbon residues of 1EP / MBATP-2 (b), 1EP / MBATP-4 (c) and epoxy resin (a) prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0042] In order to clearly and completely describe the technical solution of the present invention, the specific description of the reaction process will be carried out in conjunction with the drawings in the embodiments of the present invention. In addition, the embodiments described in the present invention are only part of the present invention, not all, and other embodiments not described in the present invention belong to the protection scope of the present invention. In addition, the drugs and reagents used in the embodiments can be obtained from commercial channels.

[0043] Example 1

[0044] A method for preparing a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound is as follows:

[0045] Weigh 2-amino-1,3,4-thiadiazole (0.02 mol) and 150 mL of anhydrous ethanol and add them to a 250 mL four-necked round-bottom flask with a condenser and a thermometer, heat to 80 ° C, stir until all the solids are dissolved, add p-methylsulfonylbenzaldehyde (0.02 mol) to the flask, react for 12 hours, add DOPO (0.02 mol), continue to react for 12 hours, then stop the reaction, cool naturally to room temperature, filter and wash with a total of 150 mL of anhydrous ethanol for three times, and then vacuum dry at 80 ° C for 24 hours to obtain the product DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound, recorded as MBATP, with a yield of 76.5%.

[0046] A method for applying a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound specifically comprises the following steps:

[0047] Heat the epoxy resin (EP, Hefei Jiangfeng Chemical Co., Ltd., epoxy value 44%) to 160°C, add MBATP, stir until a transparent uniform solution, keep warm for 10 minutes, slowly cool to 90°C, add the curing agent 4,4'-diaminodiphenylmethane (DDM), slow the stirring until it is completely dissolved, and quickly pour it into a mold preheated at 100°C, cure it in an oven at 100°C for 2 hours and at 150°C for 3 hours to obtain a flame retardant epoxy resin.

[0048] In this embodiment, the concentration of MBATP in the flame retardant epoxy resin is 2 wt % and 4 wt %, and the concentration of 4,4'-diaminodiphenylmethane (DDM) is 19.58 wt %, which are respectively denoted as 1EP / MBATP-2 and 1EP / MBATP-4.

[0049] The flame retardant compound prepared in Example 1 was tested and characterized as follows:

[0050] Figure 1 The results of mid-infrared spectroscopy characterization (KBr tablet method) are as follows: -NH-(3434cm -1 ); C=N(1634cm -1 ); P = O (1230cm -1 ); P-0-Ar(1205cm -1 , 974cm -1 ); PC(753cm -1 ).

[0051] Figure 2 H NMR analysis results: 1H NMR (DMS0-d6, 500 MHz), δ (ppm): 7.00-8.32 (Ar-H); 5.35-5.45 (NH); 6.53-6.73 (C*-H).

[0052] Figure 3 Nuclear magnetic resonance phosphorus spectrum analysis31P NMR (DMSO-d6, 400MHz), (ppm): 27.01, 28.19 (PC).

[0053] The flame retardant performance test results of the flame retardant epoxy resin of Example 1 are shown in Table 1 below.

[0054] Table 1 Flame retardant properties test results of the flame retardant epoxy resin synthesized in Example 1 of the present invention

[0055]

[0056] a. The limiting oxygen index test standard is: ASTM D2863, and the sample size is: 130×6.5×3mm.

[0057] b. The vertical burning test standard is: ASTM D3801, the sample size is: 130×13×3mm.

[0058] As shown in Table 1, the LOI value of pure epoxy resin is only 25%, and it does not meet the UL-94 flame retardant grade requirement. With the addition of MBATP, the flame retardant grade and LOI value of flame retardant EP are gradually improved. When 2wt% MBATP is added, the LOI value of flame retardant EP reaches 31.5%, and the flame retardant grade reaches V-1. When 4wt% MBATP is added, the LOI value of flame retardant EP reaches 33.5%, and the flame retardant grade reaches V-0. This shows that the flame retardant properties of epoxy resin are greatly improved with the addition of MBATP.

[0059] The test results of the mechanical properties of the flame retardant epoxy resin of Example 1 are shown in Table 2 below.

[0060] Table 2 Mechanical properties test results of the flame retardant epoxy resin synthesized in Example 1 of the present invention

[0061]

[0062] Mechanical properties test standards are: According to GB / T1040-2006 standard, the test is carried out on a tensile testing machine. According to GB / T9341-2008 standard, the test is carried out on a three-point bending machine. All tests take the average value of five test samples.

[0063] It can be seen from Table 2 that the mechanical properties of flame-retardant EP are not much different from those of pure EP, among which the tensile strength, tensile modulus and flexural modulus are slightly improved, which shows that the addition of MBATP does not affect the mechanical properties of epoxy resin.

[0064] Figure 4 The transmittance test results of 1EP / MBATP-2, 1EP / MBATP-4 and epoxy resin (EP) are shown. The transmittance test method is: at room temperature, measure the UV-visible transmission spectra of EP and MBATP samples (3mm thickness) in the range of 300-800nm. Figure 4 It can be seen that the incorporation of MBATP has little effect on the light transmittance of epoxy resin.

[0065] Example 2

[0066] A method for preparing a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound is as follows: the difference from Example 1 is that "2-amino-1,3,4-thiadiazole (0.02 mol) and 150 mL of anhydrous ethanol" are changed to "2-amino-1,3,4-thiadiazole (0.02 mol) and 150 mL of dioxane", and the rest is the same as Example 1; the product 2MBATP is obtained with a yield of 75.3%.

[0067] A method for applying a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound specifically comprises the following steps:

[0068] In this embodiment, the flame retardant epoxy resins are respectively recorded as 2EP / MBATP-2 and 2EP / MBATP-4.

[0069] The flame retardant performance test results of the flame retardant epoxy resin of Example 2 are shown in Table 3 below.

[0070] Table 3 Flame retardant properties test results of the flame retardant epoxy resin synthesized in Example 2 of the present invention

[0071]

[0072] The test results of the mechanical properties of the flame retardant epoxy resin of Example 2 are shown in Table 4 below.

[0073] Table 4 Mechanical properties of the flame retardant epoxy resin synthesized in Example 2 of the present invention

[0074]

[0075] Example 3

[0076] A method for preparing a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound is as follows: the difference from Example 1 is that the temperature is raised to 60° C. instead of 80° C., and anhydrous ethanol is replaced by anhydrous chloroform, and the rest is the same as Example 1; the product 3MBATP is obtained with a yield of 75.3%.

[0077] A method for applying a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound specifically includes the following steps: the same as Example 1.

[0078] In this embodiment, the flame retardant epoxy resins are respectively recorded as 3EP / MBATP-2 and 3EP / MBATP-4.

[0079] The flame retardant performance test results of the flame retardant epoxy resin of Example 3 are shown in Table 5 below:

[0080] Table 5 Flame retardant performance test parameters of the flame retardant epoxy resin synthesized in Example 3 of the present invention.

[0081]

[0082] Table 6 Mechanical properties of the flame retardant epoxy resin synthesized in Example 3 of the present invention

[0083]

[0084] Example 4

[0085] A preparation method of a DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound is as follows: the difference from Example 3 is that anhydrous chloroform is replaced by anhydrous benzene, and the rest is the same as Example 3; the product 4MBATP is obtained with a yield of 75.3%. A method for applying a DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound specifically includes the following steps: the same as Example 3.

[0086] In this embodiment, the flame retardant epoxy resins are respectively recorded as 4EP / MBATP-2 and 4EP / MBATP-4.

[0087] The flame retardant performance test results of the flame retardant epoxy resin of Example 4 are shown in Table 7 below.

[0088] Table 7 Flame retardant properties test results of the flame retardant epoxy resin synthesized in Example 4 of the present invention

[0089]

[0090] The test results of the mechanical properties of the flame retardant epoxy resin of Example 4 are shown in Table 8 below.

[0091] Table 8 Mechanical properties of the flame retardant epoxy resin synthesized in Example 1 of the present invention

[0092]

[0093] Example 5

[0094] A method for preparing a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound is as follows: the difference from Example 1 is that anhydrous ethanol is replaced with anhydrous toluene, and the rest is the same as Example 3, to obtain the product 5MBATP; the yield is 75.3%.

[0095] A method for applying a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound specifically includes the following steps: the same as Example 1.

[0096] In this embodiment, the flame retardant epoxy resins are respectively recorded as 5EP / MBATP-2 and 5EP / MBATP-4.

[0097] The flame retardant performance test results of the flame retardant epoxy resin of Example 5 are shown in Table 9 below.

[0098] Table 9 Flame retardant properties test results of the flame retardant epoxy resin synthesized in Example 5 of the present invention

[0099]

[0100] The test results of the mechanical properties of the flame retardant epoxy resin of Example 5 are shown in Table 10 below.

[0101] Table 10 Mechanical properties of the flame retardant epoxy resin synthesized in Example 1 of the present invention

[0102]

[0103] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, from any starting point, the examples should be regarded as illustrative rather than restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all changes within the meaning and range of equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be regarded as referring to the claim.

[0104] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound, characterized in that: The DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound has the following structure:

2. The method for preparing the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound according to claim 1, characterized in that: The specific steps are as follows: S1: Add 2-amino-1,3,4-thiadiazole and solvent into a four-necked round-bottom flask with a condenser and a thermometer, heat and stir until all dissolved, then add p-methylsulfonylbenzaldehyde into the flask to carry out condensation reaction to synthesize Schiff base intermediate MBAT; S2: directly adding DOPO to the Schiff base intermediate MBAT solution in step S1 to carry out a phosphorus-hydrogen addition reaction, cooling the temperature to room temperature after the reaction is completed, and performing post-treatment to obtain a DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound.

3. The method for preparing the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound according to claim 2, characterized in that: The molar ratio of the 2-amino-1,3,4-thiadiazole, p-methylsulfonylbenzaldehyde and DOPO is 1:1-1.5:1-1.

3.

4. The method for preparing the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound according to claim 2, characterized in that: The volume ratio of 2-amino-1,3,4-thiadiazole substance to solvent is 1 mol: 6000-8000 mL.

5. The method for preparing the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound according to claim 2, characterized in that: The condensation reaction conditions are: stirring at 40-80° C. for 6-12 hours.

6. The method for preparing the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound according to claim 2, characterized in that: The conditions of the phosphorus-hydrogen addition reaction are: reaction at 40-80° C. for 8-24 hours.

7. The method for preparing the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound according to claim 2, characterized in that: The solvent includes one or more of anhydrous ethanol, methanol, dioxane, dichloromethane, chloroform, tetrahydrofuran, benzene, toluene and acetonitrile.

8. The method for preparing the DOPO-based phosphorus-nitrogen-sulfur synergistic flame retardant compound according to claim 2, characterized in that: The post-treatment method is: sequentially filtering or rotary evaporating, washing, and drying.

9. A flame retardant epoxy resin, characterized in that: The concentration of the DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound in the flame retardant epoxy resin is 1 to 7 wt %.

10. The flame retardant epoxy resin according to claim 9, characterized in that: The concentration of the DOPO-based phosphorus, nitrogen and sulfur synergistic flame retardant compound in the flame retardant epoxy resin is 2-4 wt %.

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