Preparation method of a flame-retardant epoxy resin

Abstract

The present invention provides a preparation method of a flame-retardant epoxy resin, belonging to the technical field of polymer materials. The present invention uses bio-based raw materials to prepare a flame retardant by reacting with hexachlorocyclotriphosphazene; the flame retardant is uniformly mixed with an epoxy resin monomer, and a diamine compound is used as a curing agent to prepare a flame-retardant epoxy resin. During the curing process, an imine bond is formed by the reaction of an aldehyde group and an amino group in the flame retardant, thereby introducing a flame-retardant structure into the polymer long chain and endowing the epoxy resin with stable and lasting flame-retardant performance. The present invention can regulate the mechanical properties of the epoxy resin material by changing the carbon chain length and molecular flexibility of the diamine; the present invention has the advantages of mild reaction conditions, simple operation, high flame-retardant efficiency, etc., and has broad application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and particularly relates to a preparation method of a flame-retardant epoxy resin. Background Art

[0002] As an important thermosetting resin, bisphenol A epoxy resin has the advantages of excellent mechanical properties, high chemical stability, good electrical properties and processing properties, and has been widely used in the fields of adhesives, coatings, electronic components, aerospace, etc. The bisphenol A epoxy resin has excellent mechanical properties, but due to its high crosslinking density, the epoxy resin has large brittleness, resulting in poor tensile and impact properties. In addition, like most general-purpose plastics, epoxy resin is an organic polymer material composed of carbon, hydrogen and oxygen elements, resulting in its inherently flammable defect, and the limiting oxygen index (LOI) is only about 20%, which greatly limits its application in some special fields (especially electronic appliances). Therefore, the flame-retardant modification of epoxy resin has always been a research hotspot.

[0003] In the field of polymer flame retardancy, with the increasingly strict restrictions on the use of some halogen-containing flame retardants by regulations such as RoHS, WEEE and Reach, the trend of halogen-free flame retardants is obvious. P-N type flame retardants have the advantages of both phosphorus-based flame retardants and nitrogen-based flame retardants, and can play a synergistic role to achieve efficient flame retardancy. Under heating conditions, when the P-N type flame retardant decomposes by heat, strong acids are generated, which can catalyze the carbonization of the polymer; the generated phosphoric acid, polyphosphoric acid and other substances are viscous and can adhere to the surface of the polymer, playing a covering effect. In the gas phase: the gas-phase decomposition products of the phosphorus-based flame retardant contain PO free radicals, which can react with free radicals such as H and OH to reduce the free radical concentration in the combustion zone; at the same time, CO2, NH3, N2, NO x , H2O and other non-combustible gases are released by decomposition. These gases can dilute the concentration of combustible gases and oxygen, play the effect of delaying the combustion of materials, and form an expanded carbon layer, so as to better isolate the transmission of oxygen, heat and combustible gases and play a flame-retardant role.

[0004] The methods for preparing flame-retardant materials are divided into physical blending method and chemical method. The physical blending method has the advantages of simple preparation conditions, low raw material budget, simple preparation operation, diverse structures and wide application, and has been widely used in early research; however, the flame retardant is easy to migrate and precipitate, resulting in a decrease in flame retardancy efficiency, and its application has great limitations. In order to solve the problem that the physical blending method is prone to flame retardant migration and precipitation, resulting in performance degradation, flame retardants with reactive functional groups are usually synthesized and combined with the matrix material by chemical bonding to endow the material with long-term flame retardancy ability. Summary of the Invention

[0005] To solve the existing problems involved in the technical background, the purpose of the present invention is to overcome the deficiencies of the existing technologies and provide a method for preparing a flame-retardant epoxy resin. The present invention uses the reaction of hydroxy-formaldehyde compounds with hexachlorocyclotriphosphazene to prepare a flame retardant, and uses a diamine compound as a curing agent. By utilizing the Schiff base reaction between amino groups and aldehyde groups to generate imine bonds, the flame-retardant components are embedded into the polymer long chain, which not only achieves long-term flame retardancy but also improves the stability of the carbon layer formed during combustion, further enhancing the flame-retardant performance. The starting point of the present invention is to produce a polymer material with inherent flame-retardant properties. Inherent flame retardancy means that the flame retardant is chemically bonded to the polymer material, featuring high efficiency and durability of flame retardancy, and fundamentally solving the defects of traditional additive flame-retardant methods.

[0006] To achieve the above object, the present invention specifically adopts the following technical solutions:

[0007] The present invention provides a method for preparing a flame-retardant epoxy resin, comprising the following steps:

[0008] Step 1: Add a hydroxy-formaldehyde compound, an acid-binding agent, and a solvent to a container, stir evenly, and then slowly drop hexachlorocyclotriphosphazene into the container under solvent conditions. Heat the system to 65 - 75 °C and react for 20 - 30 h. After the reaction is completed, cool and filter to obtain a flame retardant.

[0009] Step 2: Take an epoxy resin monomer, a diamine compound, and the flame retardant obtained in Step 1, heat to 60 - 80 °C in a solvent and continuously stir and react for 10 - 12 h. After completion, cure the obtained reaction product to obtain a flame-retardant epoxy resin.

[0010] Preferably, the hydroxy-formaldehyde compound in Step 1 is at least one of vanillin, p-hydroxybenzaldehyde, o-hydroxybenzaldehyde, and m-hydroxybenzaldehyde.

[0011] Preferably, the acid-binding agent in Step 1 is at least one of anhydrous sodium carbonate, anhydrous potassium carbonate, triethylamine, and pyridine; the solvent in Step 1 is all tetrahydrofuran.

[0012] Preferably, the molar ratio of the hydroxy-formaldehyde compound to hexachlorocyclotriphosphazene in Step 1 is 6:1.

[0013] Preferably, after cooling and filtering in Step 1, the filter cake needs to be washed, recrystallized, rinsed, and dried in sequence; the solvent for recrystallization is at least one of acetone, methanol, and ethanol.

[0014] Based on the previously prepared flame retardant, the present invention selects a diamine compound as the curing agent. The amino groups at both ends of the diamine can react with both epoxy functional groups and the prepared flame retardant containing aldehyde groups to form imine bonds, thereby introducing the flame retardant component into the long chain of the cured epoxy polymer. The flame-retardant epoxy resin prepared by the present invention has excellent flame retardancy. At the same time, the imine bonds formed in the flame-retardant epoxy resin can form a C-N six-membered ring at high temperature, which can improve the stability of the carbon layer formed by combustion, thereby further improving the flame retardancy.

[0015] Preferably, the epoxy resin monomer in step two is bisphenol A diglycidyl ether.

[0016] Preferably, the diamine compound in step two is at least one of ethylenediamine, hexamethylenediamine, decamethylenediamine, polyetheramine D-230, polyetheramine D-400, and polyetheramine D-2000.

[0017] Preferably, the addition amount of the flame retardant in step two is 1%-15% of the mass of the epoxy resin monomer.

[0018] Preferably, the molar ratio of the epoxy functional groups in the epoxy resin monomer to the amino groups in the diamine compound in step two is 2:(1.0-1.2).

[0019] Preferably, the solvent in step two is dimethyl sulfoxide, and its addition amount is 50%-200% of the total mass of the epoxy resin monomer, the diamine compound, and the flame retardant.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. The present invention prepares a P-N type flame retardant based on bio-based raw materials and introduces it into the polymer long chain by a bonding method to prepare a flame-retardant bisphenol A epoxy resin (the reaction process is as Figure 1 shown): The process reaction conditions are mild, the operation is simple, the prepared product has high and lasting flame retardancy efficiency, and solves the problems of low efficiency, migration and precipitation, mechanical property loss, and poor dispersibility existing in traditional additive flame retardants from the source.

[0022] 2. The flame retardant prepared by the present invention can react with a diamine curing agent to form imine bonds. At the same time, the imine bonds can crosslink under high-temperature conditions to form a C-N six-membered ring with excellent thermal stability (as Figure 2 shown); the imine bonds of the flame-retardant epoxy resin prepared by the present invention can further improve the carbon layer stability under combustion conditions and play a stronger flame retardant role.

[0023] 3. The synthesis process of the present invention does not affect the physical properties of bisphenol A epoxy resin itself.

[0024] 4. The present invention can control the crosslinking degree and chain segment length of bisphenol A epoxy resin by regulating the structure of diamine compounds, and obtain bisphenol A epoxy resin materials with different mechanical properties.

[0025] 5. The present invention can realize the regulation of the flame retardant performance of bisphenol A epoxy resin by adjusting the addition amount of flame retardant. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the preparation reaction process of the flame retardant epoxy resin of the present invention.

[0027] Figure 2 is a schematic diagram of the formation of a C-N six-membered ring by imine bonds at high temperature.

[0028] Figure 3 is the infrared spectrum of the flame retardant prepared in Example 1 of the present invention.

[0029] Figure 4 is the nuclear magnetic resonance hydrogen spectrum of the flame retardant prepared in Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified in the manufacturer are all conventional products that can be obtained through commercial purchase.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field of the present invention. The terms used in the description of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0032] Example 1

[0033] A flame retardant bisphenol A epoxy resin, comprising:

[0034] 1. Add 15.76 g of vanillin (0.104 mol), 11.98 g of anhydrous potassium carbonate, and 216.0 g of tetrahydrofuran to a 500 mL three-necked flask, and stir at room temperature for one hour; dissolve 6.0 g of hexachlorocyclotriphosphazene (0.017 mol) in 42.6 mL of tetrahydrofuran, and slowly add it dropwise to the three-necked flask within 60 minutes at room temperature; then raise the temperature to 70 °C and react for 24 hours. After the reaction is completed, cool to room temperature, filter to obtain a filter cake, wash it three times with dichloromethane, combine the filtrates, rotary evaporate the solvent, then add 20 mL of ethanol, stir at 0 - 5 °C for 3 hours, filter the solid, wash it with ice-cold ethanol at 0 - 5 °C, and finally dry it in an oven at 60 °C for 24 hours to obtain the flame retardant.

[0035] 2. Take 100 parts by weight of bisphenol A diglycidyl ether, 17.5 parts of hexamethylenediamine, and 1 part of the above-obtained flame retardant and add them to a 500 mL three-necked flask equipped with a stirrer, a condenser, and a thermometer, and then add 50 parts of dimethyl sulfoxide as a solvent. Raise the temperature to 70 °C and continuously stir and react for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and vacuum dry to remove the solvent to obtain the flame-retardant bisphenol A epoxy resin.

[0036] Example 2

[0037] A flame-retardant bisphenol A epoxy resin, comprising:

[0038] 1. Add 15.76 g of vanillin (0.104 mol), 11.98 g of anhydrous potassium carbonate, and 216.0 g of tetrahydrofuran to a 500 mL three-necked flask, and stir at room temperature for one hour; dissolve 6.0 g of hexachlorocyclotriphosphazene (0.017 mol) in 42.6 mL of tetrahydrofuran, and slowly add it dropwise to the three-necked flask within 60 minutes at room temperature; then raise the temperature to 70 °C and react for 24 hours. After the reaction is completed, cool to room temperature, filter to obtain a filter cake, wash it three times with dichloromethane, combine the filtrates, rotary evaporate the solvent, then add 20 mL of ethanol, stir at 0 - 5 °C for 3 hours, filter the solid, wash it with ice-cold ethanol at 0 - 5 °C, and finally dry it in an oven at 60 °C for 24 hours to obtain the flame retardant.

[0039] 2. Take 100 parts by weight of bisphenol A diglycidyl ether, 17.5 parts of hexamethylenediamine, and 3 parts of the above-obtained flame retardant and add them to a 500 mL three-necked flask equipped with a stirrer, a condenser, and a thermometer, and then add 50 parts of dimethyl sulfoxide as a solvent. Raise the temperature to 70 °C and continuously stir and react for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and vacuum dry to remove the solvent to obtain the flame-retardant bisphenol A epoxy resin.

[0040] Example 3

[0041] A flame-retardant bisphenol A epoxy resin, comprising:

[0042] 1. Add 15.76 g of vanillin (0.104 mol), 11.98 g of anhydrous potassium carbonate, and 216.0 g of tetrahydrofuran to a 500 mL three-necked flask, and stir at room temperature for one hour. Dissolve 6.0 g of hexachlorocyclotriphosphazene (0.017 mol) in 42.6 mL of tetrahydrofuran, and slowly add it dropwise to the three-necked flask within 60 minutes at room temperature. Then raise the temperature to 70 °C and react for 24 hours. After the reaction is completed, cool to room temperature, filter to obtain a filter cake, wash it three times with dichloromethane, combine the filtrates, rotary evaporate the solvent, then add 20 mL of ethanol, stir at 0 - 5 °C for 3 hours, filter the solid, wash it with ice-cold ethanol at 0 - 5 °C, and finally dry it in an oven at 60 °C for 24 hours to obtain the flame retardant.

[0043] 2. Take 100 parts by weight of bisphenol A diglycidyl ether, 18 parts of hexamethylenediamine, and 5 parts of the above-obtained flame retardant and add them to a three-necked flask equipped with a stirrer, a condenser, and a thermometer. Then add 50 parts of dimethyl sulfoxide as a solvent. Raise the temperature to 70 °C and continuously stir and react for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and vacuum dry to remove the solvent to obtain the flame-retardant bisphenol A epoxy resin.

[0044] Example 4

[0045] A flame-retardant bisphenol A epoxy resin, comprising:

[0046] 1. Add 15.76 g of vanillin (0.104 mol), 11.98 g of anhydrous potassium carbonate, and 216.0 g of tetrahydrofuran to a 500 mL three-necked flask, and stir at room temperature for one hour. Dissolve 6.0 g of hexachlorocyclotriphosphazene (0.017 mol) in 42.6 mL of tetrahydrofuran, and slowly add it dropwise to the three-necked flask within 60 minutes at room temperature. Then raise the temperature to 70 °C and react for 24 hours. After the reaction is completed, cool to room temperature, filter to obtain a filter cake, wash it three times with dichloromethane, combine the filtrates, rotary evaporate the solvent, then add 20 mL of ethanol, stir at 0 - 5 °C for 3 hours, filter the solid, wash it with ice-cold ethanol at 0 - 5 °C, and finally dry it in an oven at 60 °C for 24 hours to obtain the flame retardant.

[0047] 2. Take 100 parts by weight of bisphenol A diglycidyl ether, 18.5 parts of hexamethylenediamine, and 7 parts of the above-obtained flame retardant and add them to a three-necked flask equipped with a stirrer, a condenser, and a thermometer. Then add 50 parts of dimethyl sulfoxide as a solvent. Raise the temperature to 70 °C and continuously stir and react for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and vacuum dry to remove the solvent to obtain the flame-retardant bisphenol A epoxy resin.

[0048] Example 5

[0049] A flame-retardant bisphenol A epoxy resin, comprising:

[0050] 1. Add 15.76 g of vanillin (0.104 mol), 11.98 g of anhydrous potassium carbonate and 216.0 g of tetrahydrofuran to a 500 mL three-necked flask, and stir at room temperature for one hour; dissolve 6.0 g of hexachlorocyclotriphosphazene (0.017 mol) in 42.6 mL of tetrahydrofuran, and slowly add it dropwise to the three-necked flask within 60 minutes at room temperature; then raise the temperature to 70 °C and react for 24 hours. After the reaction is completed, cool to room temperature, filter to obtain a filter cake, wash it three times with dichloromethane, combine the filtrates, spin-dry the solvent, then add 20 mL of ethanol, stir at 0 - 5 °C for 3 hours, filter the solid, rinse it with ice-cold ethanol at 0 - 5 °C, and finally dry it in an oven at 60 °C for 24 hours to obtain the flame retardant.

[0051] 2. Take 100 parts by weight of bisphenol A diglycidyl ether, 19 parts of hexamethylenediamine and 10 parts of the above-obtained flame retardant and add them to a three-necked flask equipped with a stirrer, a condenser and a thermometer, and then add 50 parts of dimethyl sulfoxide as a solvent. Raise the temperature to 70 °C and continuously stir and react for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and vacuum dry to remove the solvent to obtain the flame-retardant bisphenol A epoxy resin.

[0052] Example 6

[0053] A flame-retardant bisphenol A epoxy resin, comprising:

[0054] 1. Add 15.76 g of vanillin (0.104 mol), 11.98 g of anhydrous potassium carbonate and 216.0 g of tetrahydrofuran to a 500 mL three-necked flask, and stir at room temperature for one hour; dissolve 6.0 g of hexachlorocyclotriphosphazene (0.017 mol) in 42.6 mL of tetrahydrofuran, and slowly add it dropwise to the three-necked flask within 60 minutes at room temperature; then raise the temperature to 70 °C and react for 24 hours. After the reaction is completed, cool to room temperature, filter to obtain a filter cake, wash it three times with dichloromethane, combine the filtrates, spin-dry the solvent, then add 20 mL of ethanol, stir at 0 - 5 °C for 3 hours, filter the solid, rinse it with ice-cold ethanol at 0 - 5 °C, and finally dry it in an oven at 60 °C for 24 hours to obtain the flame retardant.

[0055] 2. Take 100 parts by weight of bisphenol A diglycidyl ether, 20 parts of hexamethylenediamine and 15 parts of the above-obtained flame retardant and add them to a three-necked flask equipped with a stirrer, a condenser and a thermometer, and then add 50 parts of dimethyl sulfoxide as a solvent. Raise the temperature to 70 °C and continuously stir and react for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and vacuum dry to remove the solvent to obtain the flame-retardant bisphenol A epoxy resin.

[0056] Comparative Example 1

[0057] Same as Example 4, but without adding the flame retardant:

[0058] Take 100 parts by weight of bisphenol A diglycidyl ether and 18.5 parts of hexamethylenediamine and add them to a three-necked flask equipped with a stirrer, a condenser and a thermometer. Then add 50 parts of dimethyl sulfoxide as a solvent. Heat up to 70 °C and stir continuously for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and remove the solvent by vacuum drying to obtain an epoxy resin.

[0059] Comparative Example 2

[0060] Same as Example 4, but replace the flame retardant with a commercially available additive flame retardant (ammonium polyphosphate) of the same weight:

[0061] Take 100 parts by weight of bisphenol A diglycidyl ether, 18.5 parts of hexamethylenediamine and 7 parts of ammonium polyphosphate and add them to a three-necked flask equipped with a stirrer, a condenser and a thermometer. Then add 50 parts of dimethyl sulfoxide as a solvent. Heat up to 70 °C and stir continuously for 12 hours. After the reaction is completed, pour the product into a mold, place the mold in an oven for curing for 24 hours, and remove the solvent by vacuum drying to obtain a flame-retardant epoxy resin.

[0062] Experimental test analysis

[0063] Figure 3 and Figure 4 are the infrared spectrum and nuclear magnetic resonance hydrogen spectrum of the flame retardant respectively.

[0064] Figure 3 In, the absorption peak at 1698 cm -1 is the stretching vibration absorption peak of C=O on -CHO, and the absorption peaks at 1276 cm -1 and 874 cm -1 are the stretching vibration absorption peaks of P=N and P-N on the cyclophosphazene functional group respectively; at the same time, the -OH infrared absorption peak at the wavelength of 3215 cm -1 and the P-Cl absorption peaks at 609 cm -1 , 523 cm -1 disappear, proving the successful preparation of the flame retardant of the present invention.

[0065] Figure 4 In, the chemical shift at 3.70 ppm belongs to the H atom H5 of the methyl group on vanillin, the chemical shifts at 7.16 - 7.27 ppm belong to the H atoms H2 - H4 on the benzene ring of vanillin, and the chemical shift at 9.78 ppm corresponds to the H atom H1 on the aldehyde group.

[0066] Perform performance tests on the samples prepared in the above examples and comparative examples, and the results are as follows. (The detection standards are: oxygen index: ASTM D2863 - 97, vertical burning: ASTM D3801)

[0067] Table 1 Initial Flame Retardancy Performance Test Results

[0068]

[0069]

[0070] Table 2 Flame Retardancy Performance Test Results after One Year

[0071]

[0072] As can be seen from the test results in Table 1 and Table 2, pure epoxy resin does not have flame retardancy. As the addition amount of the flame retardant increases, the flame retardancy of the material gradually improves. When the addition amount of the flame retardant is 7%, the flame retardancy of the epoxy resin material reaches UL-94 V-0 level, and its LOI value rises from 22.1 to 28.0 (by adjusting the addition amount of the flame retardant, it can be better applied to scenarios with different flame retardancy requirements). Through the test on the durability of its flame retardancy performance, it is found that the flame retardancy performance of the sample in the example does not decrease significantly, but the flame retardancy performance of the sample in Comparative Example 2 decreases from UL-94 V-0 level to UL-94 V-2 level.

[0073] In summary, the flame retardant prepared by the present invention has excellent flame retardancy performance in epoxy resin, and has more excellent flame retardancy durability than conventional commercially available additive flame retardants, and will not cause a decrease in flame retardancy performance with the increase of service time. The present invention prepares the flame retardant with bio-based raw materials, uses a diamine compound as a curing agent, introduces a polymer long chain through reaction with amino groups to play a flame retardant role, and can adjust the macroscopic properties of the epoxy resin material by adjusting the carbon chain length and molecular flexibility of the diamine. The present invention has the advantages of mild reaction conditions, simple operation, high flame retardancy efficiency, etc., and has broad application prospects.

[0074] The above-described embodiments only represent several preferred embodiments of the present invention. The description is relatively specific and detailed, but it does not limit the present invention. It should be noted that for those skilled in the art, the present invention can also have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the concept and principle of the present invention should be included within the protection scope of the present invention.

Claims

1. A preparation method of a flame-retardant epoxy resin, characterized in that, It includes the following steps: Step 1: Add a hydroxyaldehyde compound, an acid-binding agent and a solvent into a container, mix them evenly, and then slowly drop hexachlorocyclotriphosphazene into the container under solvent conditions. Heat the system to 65 - 75 °C and react for 20 - 30 h. After the reaction is completed, cool and filter to obtain a flame retardant; Step 2: Take an epoxy resin monomer, a diamine compound and the flame retardant obtained in Step 1, heat them to 60 - 80 °C in a solvent and continuously stir and react for 10 - 12 h. After completion, cure the obtained reaction product to obtain a flame-retardant epoxy resin; Among them, the epoxy resin monomer is bisphenol A diglycidyl ether, the diamine compound is hexamethylenediamine, the molar ratio of the epoxy functional group in the epoxy resin monomer to the amino group in the diamine compound is 2:(1.0 - 1.2), and the addition amount of the flame retardant is 1% - 15% of the mass of the epoxy resin monomer.

2. The preparation method of the flame-retardant epoxy resin according to claim 1, characterized in that, The hydroxyaldehyde compound in Step 1 is at least one of vanillin, p-hydroxybenzaldehyde, o-hydroxybenzaldehyde, and m-hydroxybenzaldehyde.

3. The preparation method of the flame-retardant epoxy resin according to claim 1, characterized in that, The acid-binding agent in Step 1 is at least one of anhydrous sodium carbonate, anhydrous potassium carbonate, triethylamine, and pyridine; the solvent in Step 1 is all tetrahydrofuran.

4. The preparation method of the flame-retardant epoxy resin according to claim 1, wherein, The molar ratio of the hydroxyaldehyde compound and hexachlorocyclotriphosphazene in Step 1 is 6:

1.

5. The preparation method of the flame-retardant epoxy resin according to claim 1, characterized in that, After cooling and filtering in Step 1, the filter cake needs to be washed, recrystallized, rinsed, and dried in sequence; the solvent for recrystallization is at least one of acetone, methanol, or ethanol.

6. The preparation method of the flame-retardant epoxy resin according to claim 1, characterized in that, The solvent in Step 2 is dimethyl sulfoxide, and its addition amount is 50% - 200% of the total mass of the epoxy resin monomer, the diamine compound, and the flame retardant.

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