Preparation method of flame-retardant curing agent and epoxy resin containing curing agent

Abstract

The invention relates to the technical field of epoxy resin modification and preparation, in particular to a preparation method of a flame-retardant curing agent and epoxy resin containing the curing agent. Comprising the following steps: preparation of a phosphorus-containing intermediate, preparation of a phosphorus-nitrogen intermediate and preparation of the flame-retardant curing agent. According to the invention, phosphorus, nitrogen and sulfur elements are integrated into the structure of the latent flame-retardant curing agent, and the synergistic flame-retardant effect of phosphorus, nitrogen and sulfur is utilized, so that the dual mechanisms of the gas-phase flame-retardant effect of phosphorus free radical capture and nitrogen dilution and the condensed-phase flame-retardant effect of promoting char formation by phosphoric acid and sulfuric acid are exerted, and the flame-retardant efficiency of the latent flame-retardant curing agent is remarkably improved. When 8.7 wt% of the flame retardant is added into epoxy resin, a compact carbon layer is formed after ignition, so that exchange of oxygen and heat is isolated, molten drops are avoided, meanwhile, smoke release is reduced, the UL-94 V0 level is reached, and the LOI value is 32.8%. Under the same addition amount, the LOI value is increased by 24.7% compared with that of a traditional phosphorus flame retardant such as triphenyl phosphate, and the flame retardant efficiency is excellent.

Description

Technical Field

[0001] This invention relates to the field of epoxy resin modification and preparation technology, specifically to a method for preparing a flame-retardant curing agent and an epoxy resin containing the curing agent. Background Technology

[0002] As an indispensable thermosetting polymer in daily life, epoxy resin has broad application prospects in construction, medical, electronics, and aerospace fields due to its excellent bonding properties, good chemical resistance, and outstanding electrical insulation properties. However, epoxy resin is a flammable material with a low limiting oxygen index, and it exhibits dripping during combustion, making it highly susceptible to causing serious fire hazards. Furthermore, for commonly used two-component epoxy resins, the epoxy resin monomers and curing agents must be stored separately before use, and then weighed and mixed before use, which is complex and carries a certain risk of operational errors. Moreover, the mixed epoxy resin has a very short shelf life and must be prepared and used immediately, otherwise it is easily spoiled. Therefore, developing flame-retardant one-component epoxy resins with good flame retardant properties and a long shelf life is beneficial for expanding the practical application areas of epoxy resin.

[0003] Flame retardants are typically added to improve the flame retardant properties of epoxy resins. Due to increasingly stringent environmental regulations, phosphorus-based flame retardants are gradually replacing traditional halogen-based flame retardants. Phosphorus-based flame retardants have advantages such as low toxicity, multiple flame retardant mechanisms, and designable molecular structures, and are widely used in the field of epoxy resin flame retardancy. Furthermore, to extend the shelf life of epoxy resins, a common method is to modify the curing agent. For example, this involves using microencapsulation to encapsulate the curing agent, forming complexes between metal ions and the curing agent, and introducing inert groups to weaken the reactivity of the curing agent.

[0004] Patent CN115785871A discloses a heat-curing flame-retardant epoxy resin one-component adhesive and its preparation method. This system uses flame-retardant fillers, latent curing agents, and accelerators as additives. However, this method has some drawbacks: the flame-retardant filler has low flame-retardant efficiency, and excessive addition leads to a significant decrease in the mechanical properties of the matrix material; the preparation process is cumbersome and complex; and the additives are difficult to disperse in the epoxy resin, which adversely affects the curing of the epoxy resin.

[0005] In summary, existing flame-retardant single-component epoxy resins typically suffer from cumbersome operating processes, complex preparation procedures, low flame-retardant efficiency of latent flame-retardant curing agents, poor compatibility with epoxy resins, easy deactivation during storage, and inability to guarantee mechanical strength and toughness, causing numerous problems for practical application. Therefore, developing flame-retardant single-component epoxy resins that simultaneously achieve excellent flame retardancy, storage stability, and mechanical properties, demonstrating superior overall performance, is of great significance. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a method for preparing a flame retardant curing agent and an epoxy resin containing the curing agent.

[0007] A method for preparing a flame-retardant curing agent includes the following steps: S1: Preparation of phosphorus-containing intermediates 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 4-aminoacetophenone, and aniline were mixed, then a protective gas was introduced and an acidic catalyst was added. The mixture was reacted at 120℃~150℃ for 20h~36h, solvent I was added, and stirring was continued for 2h~4h. The mixture was filtered, the precipitate was collected, washed with solvent I, and dried under vacuum to obtain a phosphorus-containing intermediate. S2: Preparation of phosphorus and nitrogen intermediates Maleic anhydride and a phosphorus-containing intermediate were mixed and dissolved in solvent II. A protective gas was introduced and the mixture was stirred in an ice bath. Then, a dehydrating agent was added to the solution, and the mixture was heated to reflux. The reaction was continued for 6 to 8 hours. The mixture was filtered, the precipitate was collected, washed with solvent I, and dried under vacuum to obtain the phosphorus-nitrogen intermediate. S3: Preparation of flame retardant curing agent Under a protective atmosphere, the phosphorus-nitrogen intermediate and dimercaptodiphenyl sulfide are dissolved in solvent III and stirred for 1 to 4 hours. Then, an alkaline catalyst is added to the solution and stirring is continued for 4 to 8 hours. The mixture is poured into solvent IV, filtered, the precipitate is collected, and repeatedly washed with solvent IV. After vacuum drying, the flame-retardant curing agent is obtained.

[0008] Further, the preparation of the phosphorus-containing intermediate in step S1 is specifically as follows: 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 4-aminoacetophenone, and aniline were mixed in a molar ratio of 1:(1~1.2):(5~6). A protective gas was then introduced and 0.5wt%~0.8wt% of an acidic catalyst (one of p-toluenesulfonic acid, benzenesulfonic acid, or concentrated sulfuric acid) was added. The mixture was reacted at 120℃~150℃ for 20h~36h. Solvent I, with a mass of 1~2 times that of aniline, was added at 60℃~80℃. The mixture was stirred for 2h~4h, filtered, and the precipitate was collected and washed 3~5 times with solvent I. The mixture was then vacuum dried at 100℃~130℃ for 20h~30h to obtain a phosphorus-containing intermediate.

[0009] Furthermore, the preparation of the phosphorus-nitrogen intermediate in step S2 is specifically as follows: Maleic anhydride and a phosphorus-containing intermediate were mixed and dissolved in solvent II at a molar ratio of 1:(2~2.2) in 5~6 times the mass of the phosphorus-containing intermediate. A protective gas was introduced and the mixture was stirred in an ice bath for 3~6 hours. Then, 39.0wt%~42.5wt% of a dehydrating agent, equal in mass of the total reactants, was added to the solution in three batches. The dehydrating agent was one of acetic anhydride and sodium acetate or acetic anhydride and triethylamine. The mixture was then heated to reflux and the reaction was continued for 6~8 hours. The mixture was filtered, the precipitate was collected, washed with solvent I, and dried under vacuum to obtain the phosphorus-nitrogen intermediate.

[0010] Further, the preparation of the flame retardant curing agent in step S3 is as follows: Under a protective atmosphere, a phosphorus-nitrogen intermediate and a dimercaptodiphenyl sulfide in a molar ratio of 1:(1.8~2.2) are dissolved in solvent III, which is 2~3 times the mass of the phosphorus-nitrogen intermediate. Solvent III is one of m-cresol, dimethylformamide, and dimethyl sulfoxide. The mixture is stirred at 30°C for 1h~4h. Then, an alkaline catalyst is added to the solution, and stirring is continued for 4h~8h. The mixture is then poured into solvent IV, which is 10~20 times the mass of the phosphorus-nitrogen intermediate. Solvent IV is one of acetic acid-acidified methanol, acetic acid-acidified ethanol, and acetic acid-acidified ethyl acetate. The mixture is then filtered, the precipitate is collected, and the mixture is washed repeatedly with solvent IV 3~5 times. The mixture is then vacuum dried at 100°C~130°C for 18h~24h to obtain a flame-retardant curing agent.

[0011] Furthermore, the protective gas in steps S1 to S3 is one of nitrogen, argon, and helium.

[0012] An epoxy resin containing a flame retardant curing agent, wherein the flame retardant curing agent is prepared by the above-mentioned flame retardant curing agent preparation process, and the components are as follows by mass fraction: epoxy resin monomer 56.3wt%~60.2wt%, thiol curing agent 32.4wt%~36.8wt%, and flame retardant curing agent 3.0wt%~11.3wt%.

[0013] Furthermore, the specific preparation process of epoxy resin containing flame-retardant curing agent is as follows: S1: Epoxy resin monomer, thiol curing agent and flame retardant curing agent are heated and stirred at 60℃~80℃ for 1h~2h in proportion to obtain a homogeneous mixture; S2: The mixture is bubbled at 60℃~80℃ for 1h~3h, then poured into a mold and placed in a forced-air drying oven for staged heating and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3: After cooling to 30℃, the epoxy resin containing flame retardant curing agent is obtained after demolding.

[0014] The present invention has the following advantages: 1. The flame-retardant curing agent of the present invention has a simple synthesis step, mild reaction conditions, convenient post-processing, and a yield of over 90%. In addition, the phosphorus-containing polysulfide imide has good compatibility with epoxy resin, and a homogeneous epoxy resin containing the flame-retardant curing agent can be obtained by simple heating and mixing without the need for external solvents.

[0015] 2. This invention integrates phosphorus, nitrogen, and sulfur elements into the structure of a latent flame-retardant curing agent. Utilizing the synergistic flame-retardant effects of phosphorus, nitrogen, and sulfur, it leverages the dual mechanisms of phosphorus free radical capture, nitrogen dilution (gas-phase flame retardancy), and phosphoric acid and sulfuric acid promoting char formation (condensed-phase flame retardancy), significantly improving the flame-retardant efficiency of the latent flame-retardant curing agent. When added to epoxy resin at 8.7 wt%, a dense char layer forms after ignition, isolating oxygen and heat exchange, preventing dripping, and reducing smoke release, achieving a UL-94 V0 rating with an LOI value of 32.8%. At the same addition amount, the LOI value is 24.7% higher than traditional phosphorus-based flame retardants such as triphenyl phosphate, demonstrating excellent flame-retardant efficiency.

[0016] 3. The flame-retardant curing agent of this invention is composed of rigid imide rings and aromatic rings, as well as flexible thioether bonds, possessing a "rigid-flexible integrated" structure. The rigid structure has a reinforcing effect, increasing the rigidity of the epoxy crosslinking network and improving the tensile and flexural strength of the epoxy resin. The flexible structure has a toughening effect; under external force, it causes uniform shear deformation of the crosslinking network and dissipates more fracture energy through intermolecular motion, improving the impact strength of the epoxy resin. When 8.7 wt% is added to the epoxy resin, the tensile strength and impact strength increase to 59.1 MPa and 39.0 kJ / m, respectively. 2 .

[0017] 4. The flame-retardant curing agent of this invention contains a thioetherimide structure, which readily undergoes a thiols exchange reaction with thiols to produce highly nucleophilic thiophenols. Unlike traditional thiol-based curing agents, the resulting thiophenols are inert at room temperature, while their reaction temperature with epoxy resins is moderate, thus achieving good storage stability. When added to epoxy resin at 8.7 wt%, the shelf life extends to 25 days. Furthermore, this latent flame-retardant curing agent is an addition-type curing agent with stable chemical properties, unaffected by air, moisture, and impurities during long-term storage, and is relatively difficult to deactivate. Attached Figure Description

[0018] Figure 1 This is a flowchart of the preparation method of the flame retardant curing agent of the present invention.

[0019] Figure 2 This is a schematic diagram of the structure of the phosphorus-containing intermediate of the present invention.

[0020] Figure 3 This is a schematic diagram of the structure of the phosphorus-nitrogen intermediate of the present invention.

[0021] Figure 4 This is a schematic diagram of the structure of the flame-retardant curing agent of the present invention.

[0022] Figure 5 This is the NMR spectrum of the phosphorus-containing intermediate in Example 1 of the present invention.

[0023] Figure 6 The NMR spectra of the phosphorus-nitrogen intermediate and the flame-retardant curing agent in Example 1 of this invention are shown.

[0024] Figure 7 The infrared spectrum of the flame-retardant curing agent in Example 1 of this invention is shown.

[0025] Figure 8 This is the chemical structural formula of the phosphorus-containing polysulfide curing agent in Comparative Example 1 of the present invention.

[0026] Figure 9 This is the chemical structural formula of the phosphorus-containing polyetherimide flame retardant curing agent in Comparative Example 2 of the present invention. Detailed Implementation

[0027] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of this invention. Example 1

[0028] A method for preparing a flame-retardant curing agent, such as Figure 1 As shown, it includes the following steps: S1, Preparation of the phosphorus-containing intermediate: In a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and a nitrogen inlet, 100 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 100 mmol of 4-aminoacetophenone, and 500 mmol of aniline were added and stirred until homogeneous. Then, 2.5 mmol of p-toluenesulfonic acid was slowly added dropwise to the mixture, and the reaction was carried out at 130 °C for 24 h. After cooling to 60 °C, 50 mL of ethanol was added, and the mixture was stirred for another 3 h. The precipitate was collected by filtration and washed three times with ethanol. The precipitate was then dried under vacuum at 100 °C for 20 h to obtain a pale yellow phosphorus-containing intermediate with a yield of 92.3%. Its structural formula is as follows: Figure 2 As shown, the 1H NMR spectrum of this sample is as follows: Figure 5As shown, the values ​​of δ (ppm) are assigned as follows: 8.10 (dd, J = 8.2, 4.6 Hz, 1H), 8.02 (dd, J = 8.1, 1.6 Hz, 1H), 7.66 (t, J = 7.7 Hz, 1H), 7.40-7.34 (m, 1H), 7.32 (td, J = 7.5, 2.6 Hz, 1H), 7.19 (t, J = 7.6 Hz, 1H), 7.14 (d, J = 8.1 Hz, 1H), 7.13-7.04 (m, 3H), 6.95 (dd, J = 8.6, 2.0 Hz, 2H), 6.41 (dd, J = 20.8, 8.3 Hz, 4H), 5.03 (s, 4H), 1.50 (d, J = 17.7 Hz, 3H). Consistent with theoretical values, indicating that the phosphorus-containing intermediate was successfully synthesized.

[0029] S2, Preparation of the phosphorus-nitrogen intermediate: In a three-necked flask equipped with a nitrogen inlet, a reflux condenser, and a magnetic stirrer, 50 mmol of the phosphorus-containing intermediate and 100 mmol of maleic anhydride were dissolved in 250 mL of acetone. The mixture was stirred in an ice bath for 5 h. Then, 120 mmol of acetic anhydride and 1.2 mmol of sodium acetate were added to the solution in three portions. The mixture was heated to reflux and reacted for another 6 h. After cooling to 30 °C, a large amount of precipitate appeared. The precipitate was washed three times with ethanol and dried under vacuum at 100 °C for 24 h to obtain a yellow phosphorus-nitrogen powder with a yield of 95.4%. Its structural formula is as follows: Figure 3 As shown, the 1H NMR spectrum of this sample is as follows: Figure 6 As shown, the δ (ppm) values ​​were assigned as follows: 8.15 (dd, J = 8.3, 4.9 Hz, 1H), 8.03 (dd, J = 8.2, 1.6 Hz, 1H), 7.77–7.69 (m, 1H), 7.56–7.44 (ddd, J = 47.9, 8.7, 1.6 Hz, 4H), 7.43–7.31 (m, 4H), 7.31–7.15 (m, 8H), 7.07–7.00 (m, 1H), and 1.71 (d, J = 17.2 Hz, 3H). These results confirm the successful synthesis of the phosphorus-nitrogen intermediate.

[0030] S3, Preparation of the flame-retardant curing agent: 100 mmol of phosphorus-nitrogen intermediate, 200 mmol of dimercaptodiphenyl sulfide, and 250 mL of m-cresol were added sequentially to a three-necked round-bottom flask. The mixture was stirred at 30°C under nitrogen for 1 h to obtain a homogeneous solution. Then, 5 mmol of triethylamine was added dropwise to the solution, and stirring continued for 5 h. During this time, the solution gradually thickened. The mixture was poured into 600 mL of acetic acid-acidified methanol, filtered, and the precipitate was collected. The precipitate was washed three times with acetic acid-acidified methanol and dried under vacuum at 130°C for 24 h to obtain a white powdery flame-retardant curing agent with a yield of 94.8%. Its structural diagram is shown below. Figure 4 As shown, the 1H NMR spectrum of this sample is as follows: Figure 6 As shown, the -CH=CH- ions disappeared at δ = 7.56 ppm - 7.44 ppm, while proton peaks of -CH-SC-, -SH, and -CH2- appeared at 4.56 ppm, 3.39 ppm, and 2.79 ppm, indicating that the phosphorus-nitrogen intermediate reacted completely with the excess dimercaptodiphenyl sulfide. The infrared spectrum of this sample is attached. Figure 7 The spectrum showed characteristic peaks of the imide structure at 1780 cm⁻¹, 1720 cm⁻¹, and 1375 cm⁻¹, and absorption peaks belonging to P=O and PO bonds at 1280 cm⁻¹ and 1045 cm⁻¹, respectively. In addition, there was no absorption peak of -CH=CH- near 3100 cm⁻¹, while characteristic peaks of -CH-SC- and -SH were shown at 1080 cm⁻¹ and 2560 cm⁻¹. The relative molecular weight (Mn) of the sample was 2.3 × 10³ g / mol, confirming that it is a low molecular weight oligomer.

[0031] Preparation of epoxy resin containing flame retardant curing agent: S1, without a catalyst, 57.7 wt% bisphenol A epoxy resin, 33.6 wt% pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 8.7 wt% flame retardant curing agent were placed in a beaker and heated and stirred at 80°C for 1 h to obtain a homogeneous epoxy resin mixture. S2, Place the epoxy resin mixture in a vacuum oven and bubble it at 80℃ for 3 hours. Then pour it into a mold preheated to 80℃ while it is still hot, and place it in a forced-air drying oven for staged heating and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3, after cooling to 30℃, is used to obtain an epoxy resin containing a flame-retardant curing agent after demolding. Its infrared spectrum is shown below. Figure 7 As shown. Example 2

[0032] A method for preparing a flame-retardant curing agent, such as Figure 1 As shown, it includes the following steps: S1, Preparation of phosphorus-containing intermediate: In a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and a nitrogen inlet, 100 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 100 mmol of 4-aminoacetophenone, and 600 mmol of aniline were added and stirred until homogeneous. Then, 3.2 mmol of p-toluenesulfonic acid was slowly added dropwise to the mixture. The reaction was carried out at 150 °C for 20 h, cooled to 60 °C, and 50 mL of methanol was added. The mixture was stirred for another 2 h, and the precipitate was collected by filtration. The precipitate was washed repeatedly with ethanol four times and dried under vacuum at 110 °C for 22 h to obtain a pale yellow phosphorus-containing intermediate with a yield of 90.6%.

[0033] S2, Preparation of phosphorus-nitrogen intermediate: In a three-necked flask equipped with a helium inlet, a reflux condenser, and a magnetic stirrer, 50 mmol of phosphorus-containing intermediate and 110 mmol of maleic anhydride were dissolved in 300 mL of tetrahydrofuran. The mixture was stirred in an ice bath for 3 h. Then, 130 mmol of acetic anhydride and 1.3 mmol of sodium acetate were added to the solution in three portions. The mixture was heated to reflux and the reaction was continued for 6 h. After cooling to 30 °C, a large amount of precipitate appeared. The precipitate was washed four times with ethyl acetate and dried under vacuum at 100 °C for 22 h to obtain a yellow phosphorus-nitrogen intermediate with a yield of 91.2%.

[0034] S3, Preparation of flame retardant curing agent: 100 mmol of phosphorus-nitrogen intermediate, 200 mmol of dimercaptodiphenyl sulfide and 250 mL of dimethylformamide were added sequentially to a three-necked round-bottom flask. The mixture was stirred at 30 °C under helium for 1 h to obtain a homogeneous solution. Then, 4 mmol of triethylamine was added dropwise to the solution and stirring was continued for 4 h. During this period, the solution gradually became viscous. The mixture was poured into 500 mL of acetic acid-acidified ethanol, filtered, and the precipitate was collected. The precipitate was washed four times with acetic acid-acidified ethanol and dried under vacuum at 120 °C for 18 h to obtain a white powdery flame retardant curing agent with a yield of 94.3%.

[0035] Preparation of epoxy resin containing flame retardant curing agent: S1, without a catalyst, 58.9 wt% bisphenol A epoxy resin, 35.2 wt% pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 5.9 wt% flame retardant curing agent were placed in a beaker and heated and stirred at 70°C for 2 hours to obtain a homogeneous epoxy resin mixture. S2, Place the epoxy resin mixture in a vacuum oven and bubble it at 70℃ for 3 hours. Then pour it into a mold preheated at 70℃ while it is still hot, and place it in a forced-air drying oven for staged temperature rise and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3, after cooling to 30℃, is used to obtain an epoxy resin containing a flame-retardant curing agent after demolding. Example 3

[0036] A method for preparing a flame-retardant curing agent, such as Figure 1 As shown, it includes the following steps: S1, Preparation of phosphorus-containing intermediate: In a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and an argon inlet, 100 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 100 mmol of 4-aminoacetophenone, and 550 mmol of aniline were added and stirred until homogeneous. Then, 3.2 mmol of benzenesulfonic acid was slowly added dropwise to the mixture, and the reaction was carried out at 130 °C for 30 h. The temperature was lowered to 70 °C and 50 mL of toluene was added. The mixture was stirred for another 4 h, and the precipitate was collected by filtration. The precipitate was washed three times with toluene and dried under vacuum at 120 °C for 26 h to obtain a pale yellow phosphorus-containing intermediate with a yield of 92.2%.

[0037] S2, Preparation of phosphorus-nitrogen intermediate: In a three-necked flask equipped with an argon inlet, a reflux condenser, and a magnetic stirrer, 50 mmol of the phosphorus-containing intermediate and 110 mmol of maleic anhydride were dissolved in 320 mL of dichloromethane. The mixture was stirred in an ice bath for 6 h. Then, 116 mmol of acetic anhydride and 1.2 mmol of triethylamine were added to the solution in three portions. The mixture was heated to reflux and the reaction was continued for 8 h. After cooling to 30 °C, a large amount of precipitate appeared. The precipitate was washed five times with ethyl acetate and dried under vacuum at 90 °C for 24 h to obtain a yellow phosphorus-nitrogen intermediate with a yield of 93.1%.

[0038] S3, Preparation of flame retardant curing agent: 110 mmol of phosphorus-nitrogen intermediate, 200 mmol of dimercaptodiphenyl sulfide and 300 mL of dimethyl sulfoxide were added sequentially to a three-necked round-bottom flask. The mixture was stirred at 30 °C under argon atmosphere for 2 h to obtain a homogeneous solution. Then, 7.3 mmol of imidazole was added dropwise to the solution and stirring was continued for 6 h. During this period, the solution gradually became viscous. The mixture was poured into 550 mL of acetic acid-acidified ethyl acetate, filtered, and the precipitate was collected. The precipitate was washed five times with acetic acid-acidified ethyl acetate and dried under vacuum at 110 °C for 20 h to obtain a white powdery flame retardant curing agent with a yield of 92.7%.

[0039] Preparation of epoxy resin containing flame retardant curing agent: S1, without a catalyst, 57.4 wt% bisphenol A epoxy resin, 36.2 wt% pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 6.4 wt% flame retardant curing agent were placed in a beaker and heated and stirred at 70°C for 2 hours to obtain a homogeneous epoxy resin mixture. S2, Place the epoxy resin mixture in a vacuum oven and bubble it at 70℃ for 2 hours. Then pour it into a mold preheated at 70℃ while it is still hot, and place it in a forced-air drying oven for staged heating and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3, after cooling to 30℃, is used to obtain an epoxy resin containing a flame-retardant curing agent after demolding. Example 4

[0040] A method for preparing a flame-retardant curing agent includes the following steps: S1, Preparation of phosphorus-containing intermediate: In a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and a nitrogen inlet, 100 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 100 mmol of 4-aminoacetophenone, and 500 mmol of aniline were added and stirred until homogeneous. Then, 5.6 mmol of concentrated sulfuric acid was slowly added dropwise to the mixture, and the reaction was carried out at 120 °C for 36 h. The temperature was lowered to 80 °C and 60 mL of ethanol was added. The mixture was stirred for another 2 h, and the precipitate was collected by filtration. The precipitate was washed three times with ethanol and dried under vacuum at 110 °C for 20 h to obtain a pale yellow phosphorus-containing intermediate with a yield of 92.2%.

[0041] S2, Preparation of phosphorus-nitrogen intermediate: In a three-necked flask equipped with an argon inlet, a reflux condenser, and a magnetic stirrer, 50 mmol of the phosphorus-containing intermediate and 100 mmol of maleic anhydride were dissolved in 250 mL of acetonitrile. The mixture was stirred in an ice bath for 3 h. Then, 120 mmol of acetic anhydride (12.26 g) and 1.2 mmol of triethylamine were added to the solution in three portions. The mixture was heated to reflux and the reaction was continued for 7 h. After cooling to 30 °C, a large amount of precipitate appeared. The precipitate was washed three times with ethanol and dried under vacuum at 100 °C for 24 h to obtain a yellow phosphorus-nitrogen intermediate with a yield of 91.8%.

[0042] S3, Preparation of flame retardant curing agent: 100 mmol of phosphorus-nitrogen intermediate, 220 mmol of dimercaptodiphenyl sulfide and 300 mL of m-cresol were added sequentially to a three-necked round-bottom flask. The mixture was stirred at 30 °C under nitrogen for 4 h to obtain a homogeneous solution. Then, 5 mmol of triethylamine was added dropwise to the solution and stirring was continued for 8 h. During this period, the solution gradually became viscous. The mixture was poured into 700 mL of acetic acid-acidified methanol, filtered, and the precipitate was collected. The precipitate was washed four times with acetic acid-acidified methanol and dried under vacuum at 120 °C for 24 h to obtain a white powdery flame retardant curing agent with a yield of 94.2%.

[0043] Preparation of epoxy resin containing flame retardant curing agent: S1, without a catalyst, 60.2 wt% bisphenol A epoxy resin, 36.8 wt% pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 3.0 wt% flame retardant curing agent were placed in a beaker and heated and stirred at 60°C for 2 hours to obtain a homogeneous epoxy resin mixture. S2, Place the epoxy resin mixture in a vacuum oven and bubble it at 60℃ for 2 hours. Then pour it into a mold preheated to 60℃ while it is still hot, and place it in a forced-air drying oven for staged heating and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3, after cooling to 30℃, is used to obtain an epoxy resin containing a flame-retardant curing agent after demolding. Example 5

[0044] A method for preparing a flame-retardant curing agent includes the following steps: S1, Preparation of phosphorus-containing intermediate: In a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and a helium inlet, 120 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 120 mmol of 4-aminoacetophenone, and 500 mmol of aniline were added and stirred until homogeneous. Then, 3.5 mmol of benzenesulfonic acid was slowly added dropwise to the mixture, and the reaction was carried out at 150 °C for 30 h. The temperature was lowered to 60 °C and 80 mL of ethyl acetate was added. The mixture was stirred for another 3 h, and the precipitate was collected by filtration. The precipitate was washed repeatedly with ethyl acetate four times and dried under vacuum at 130 °C for 24 h to obtain a pale yellow phosphorus-containing intermediate with a yield of 93.2%.

[0045] S2, Preparation of phosphorus-nitrogen intermediate: In a three-necked flask equipped with a helium inlet, a reflux condenser, and a magnetic stirrer, 50 mmol of phosphorus-containing intermediate and 110 mmol of maleic anhydride were dissolved in 330 mL of dichloromethane. The mixture was stirred in an ice bath for 6 h. Then, 120 mmol of acetic anhydride and 1.3 mmol of sodium acetate were added to the solution in three batches. The mixture was heated to reflux and the reaction was continued for 6 h. After cooling to 30 °C, a large amount of precipitate appeared. The precipitate was washed four times with ethanol and dried under vacuum at 80 °C for 21 h to obtain a yellow phosphorus-nitrogen intermediate with a yield of 93.2%.

[0046] S3, Preparation of flame retardant curing agent: 100 mmol of phosphorus-nitrogen intermediate, 200 mmol of dimercaptodiphenyl sulfide and 300 mL of m-cresol were added sequentially to a three-necked round-bottom flask. The mixture was stirred at 30 °C under helium for 2 h to obtain a homogeneous solution. Then, 3 mmol of pyridine was added dropwise to the solution and stirring was continued for 5 h. During this period, the solution gradually became viscous. The mixture was poured into 600 mL of acetic acid-acidified ethyl acetate, filtered, and the precipitate was collected. The precipitate was washed five times with acetic acid-acidified ethyl acetate and dried under vacuum at 120 °C for 20 h to obtain a white powdery flame retardant curing agent with a yield of 93.2%.

[0047] Preparation of epoxy resin containing flame retardant curing agent: S1, without a catalyst, 56.3 wt% bisphenol A epoxy resin, 32.4 wt% pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 11.3 wt% flame retardant curing agent were placed in a beaker and heated and stirred at 70°C for 2 hours to obtain a homogeneous epoxy resin mixture. S2, Place the epoxy resin mixture in a vacuum oven and bubble it at 70℃ for 1 hour. Then pour it into a mold preheated at 70℃ while it is still hot, and place it in a forced-air drying oven for staged heating and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3, after cooling to 30℃, is used to obtain an epoxy resin containing a flame-retardant curing agent after demolding. Example 6

[0048] A method for preparing a flame-retardant curing agent includes the following steps: S1, Preparation of phosphorus-containing intermediate: In a three-necked flask equipped with a magnetic stirrer, a reflux condenser, and an argon inlet, 100 mmol of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 100 mmol of 4-aminoacetophenone, and 600 mmol of aniline were added and stirred until homogeneous. Then, 2.6 mmol of p-toluenesulfonic acid was slowly added dropwise to the mixture. The reaction was carried out at 140 °C for 20 h, cooled to 80 °C, and 80 mL of ethanol was added. The mixture was stirred for another 2 h, and the precipitate was collected by filtration. The precipitate was washed repeatedly with ethanol 5 times and dried under vacuum at 130 °C for 26 h to obtain a pale yellow phosphorus-containing intermediate with a yield of 91.9%.

[0049] S2, Preparation of phosphorus-nitrogen intermediate: In a three-necked flask equipped with an argon inlet, a reflux condenser, and a magnetic stirrer, 60 mmol of the phosphorus-containing intermediate and 130 mmol of maleic anhydride were dissolved in 280 mL of acetone and stirred in an ice bath for 4 h. Then, 120 mmol of acetic anhydride and 1.2 mmol of sodium acetate were added to the solution in three batches, and the mixture was heated to reflux and reacted for another 8 h. After cooling to 30 °C, a large amount of precipitate appeared. The precipitate was washed four times with toluene and dried under vacuum at 90 °C for 24 h to obtain a yellow phosphorus-nitrogen intermediate with a yield of 92.9%.

[0050] S3, Preparation of flame retardant curing agent: 100 mmol of phosphorus-nitrogen intermediate, 190 mmol of dimercaptodiphenyl sulfide and 300 mL of m-cresol were added sequentially to a three-necked round-bottom flask. The mixture was stirred at 30 °C under argon atmosphere for 1 h to obtain a homogeneous solution. Then, 4.1 mmol of triethylamine was added dropwise to the solution and stirring was continued for 5 h. During this period, the solution gradually became viscous. The mixture was poured into 700 mL of acetic acid-acidified methanol, filtered, and the precipitate was collected. The precipitate was washed three times with acetic acid-acidified methanol and dried under vacuum at 100 °C for 24 h to obtain a white powdery flame retardant curing agent with a yield of 94.6%.

[0051] Preparation of epoxy resin containing flame retardant curing agent: S1, without a catalyst, 59.0 wt% bisphenol A epoxy resin, 33.8 wt% pentaerythritol tetrakis(3-mercaptopropionic acid) ester and 7.2 wt% flame retardant curing agent were placed in a beaker and heated and stirred at 80°C for 2 hours to obtain a homogeneous epoxy resin mixture. S2, Place the epoxy resin mixture in a vacuum oven and bubble it at 80℃ for 1 hour. Then pour it into a mold preheated to 80℃ while it is still hot, and place it in a forced-air drying oven for staged heating and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3, after cooling to 30℃, is used to obtain an epoxy resin containing a flame-retardant curing agent after demolding.

[0052] The bisphenol A epoxy resin used in this invention is a product of China Petroleum & Chemical Corporation Baling Branch, with the brand name CYD-128; the pentaerythritol tetrakis(3-mercaptopropionic acid) ester used in this invention is a product of Yifeng New Materials Co., Ltd., with the brand name polythiol 405.

[0053] Comparative Example 1: Preparation of phosphorus-containing polysulfide curing agent: In a three-necked round-bottom flask, 100 mmol of 2-[(6-oxo-6H-dibenzo[c,e][1,2]oxophosphoroxane-6-yl)methyl]succinic acid, 200 mmol of dimercaptodiphenyl sulfide, and 300 mL of toluene were added sequentially. The mixture was stirred at 30 °C under nitrogen for 1 h. Subsequently, 8.7 mmol of p-toluenesulfonic acid was added dropwise to the solution, and the mixture was refluxed and stirred at 120 °C for 10 h. The mixture was poured into methanol, filtered, and the precipitate was collected and washed four times with methanol. The precipitate was then dried under vacuum at 130 °C for 24 h to obtain a light white powdery phosphorus-containing polysulfide curing agent with a yield of 82.7%. The structure is shown in the attached figure. Figure 8 As shown.

[0054] Preparation of epoxy resin containing flame retardant curing agent: Comparative Example 1 formulation consists of: replacing the flame retardant curing agent of Example 1 with an equal amount of phosphorus-containing polysulfide curing agent, while keeping other components unchanged. The chemical structural formula of the phosphorus-containing polysulfide curing agent is as follows: Figure 8 As shown.

[0055] The difference from Example 1 is that, compared to the flame-retardant curing agent, the phosphorus-containing polysulfide curing agent in this comparative example does not contain an imide ring.

[0056] Comparative Example 2: Preparation of phosphorus-containing polyetherimide flame retardant curing agent In a three-necked round-bottom flask, 100 mmol of phosphorus-nitrogen intermediate, 200 mmol of dihydroxydiphenyl ether, and 350 mL of m-cresol were added sequentially. The mixture was stirred at 30 °C under nitrogen for 1 h. Then, 5 mmol of triethylamine was added dropwise, and stirring continued for another 5 h. During this time, the solution gradually thickened. The mixture was poured into 600 mL of acetic acid-acidified methanol, filtered, and the precipitate was collected. The precipitate was washed three times with acetic acid-acidified methanol and dried under vacuum at 120 °C for 24 h to obtain a light white powdery phosphorus-containing polyetherimide flame retardant curing agent with a yield of 84.7%. The structure is as follows: Figure 9 As shown.

[0057] Preparation of epoxy resin containing flame retardant curing agent: The formulation of Comparative Example 2 is as follows: the same amount of phosphorus-containing polyetherimide flame retardant curing agent is used to replace the flame retardant curing agent of Example 1, while other components remain unchanged.

[0058] The difference from Example 1 is that, compared to the flame retardant curing agent, the phosphorus-containing polyetherimide flame retardant curing agent in this comparative example does not contain a sulfide structure.

[0059] Comparative Example 3: The flame-retardant curing agent in Example 1 was replaced with DOPO-AI prepared by the method described in existing technical literature I (Huo, S.; Yang, S.; Wang, J.; Cheng, J.; Zhang, Q.; Hu, Y.; Ding, G.; Zhang, Q.; Song, P.; Wang, H. A Liquid Phosphaphenanthrene-Derived Imidazole for Improved Flame Retardancy and Smoke Suppression of Epoxy Resin. ACS Appl. Polym. Mater. 2020, 2 (8), 3566-3575. DOI: 10.1021 / acsapm.0c00577), while all other aspects remained unchanged, to obtain an epoxy resin containing a flame-retardant curing agent.

[0060] Comparative Example 4: The flame-retardant curing agent in Example 1 was replaced with VAIP synthesized by the method described in existing technical literature II (Yang, B.; Wei, Y.; Qiu, Y.; Ramakrishna, S.; Liu, YA novel bio-based, flame retardant and latent imidazole compound-Itssynthesis and uses as curing agent for epoxy resins. J. Appl. Polym. Sci.2022, 139 (44), e53079. DOI: 10.1002 / app.53079), while all other aspects remained unchanged, to obtain an epoxy resin containing a flame-retardant curing agent.

[0061] Comparative Example 5: An epoxy resin containing a flame retardant curing agent was prepared by replacing the flame retardant curing agent in Example 1 with MPMZ synthesized by the method of Chinese invention patent CN114181495A, while keeping other aspects unchanged.

[0062] The performance of Example 1 was tested six times, as shown in Table 1.

[0063] Table 1 Limiting oxygen index (LOI) of Examples 1-6 and Comparative Examples 1-5 was tested according to ASTM D2863-97 using a limiting oxygen index tester, model FTA-SC48 (FTT Company, UK). Vertical burning tests (UL-94) were performed according to ASTM D3801-10 using a vertical burning tester, model UL94-SC50 (FTT Company, UK).

[0064] The formulations and flame retardant properties of Examples 1-6 and Comparative Examples 1-5 are shown in Table 2.

[0065] Table 2 The data in Table 2, comparing Examples 1, 2, 3, and 4, show that as the amount of flame-retardant curing agent increased from 3.0 wt% to 8.7 wt%, the UL-94 flame retardancy rating of the resulting epoxy resin containing the flame-retardant curing agent improved from V-2 to the highest rating, V-0. Simultaneously, the LOI value increased from 28.2% to 32.8%, demonstrating a significant improvement in flame-retardant performance. This is attributed to the highly efficient flame-retardant groups (phosphine, imide rings, and thioether bonds) of the flame-retardant curing agent, which can exert a dual mechanism of gas-phase flame retardancy (phosphorus radical capture and nitrogen dilution) and condensed-phase flame retardancy (phosphoric acid and sulfuric acid promote char formation). Furthermore, they participate in the construction of the epoxy resin crosslinking network, with phosphorus, nitrogen, and sulfur—the three flame-retardant elements—covalently embedded in the epoxy resin molecular structure, further enhancing the flame-retardant effect.

[0066] Comparing Example 1 and Comparative Example 1, it is evident that using phosphorus and nitrogen-containing flame retardants alone, at low addition levels (≤8.7wt%), results in limited flame retardant effects; specifically, Comparative Example 1 achieves a UL-94 flame retardant rating of V-2 and a low LOI value (27.3%). However, Example 1 demonstrates that introducing imide rings into a latent flame-retardant curing agent to form a phosphorus / nitrogen / sulfur ternary flame retardant significantly improves the flame retardant performance of the resulting epoxy resin containing the flame-retardant curing agent, achieving a UL-94 V-0 rating and an LOI value of 32.8%. Therefore, imide rings play a crucial role in enhancing flame retardant performance. This is primarily due to the high-temperature carbonization effect of the imide rings. At high temperatures, the imide rings undergo cracking, rearrangement, and cyclization reactions, producing nitrogen-containing heterocyclic compounds. These compounds not only promote the formation of a char layer but also remain in the condensed phase to improve the quality of the char layer. This dense char layer acts as a physical barrier, hindering the transfer of oxygen and heat, thereby achieving highly efficient flame retardancy.

[0067] Comparing Example 1 with Comparative Examples 3, 4, and 5, it can be found that, at the same addition amount (8.7 wt%), the flame-retardant curing agent of the present invention exhibits higher flame-retardant efficiency. Specifically, the LOI value of Example 1 is 32.8%, higher than the corresponding values ​​of Comparative Examples 3, 4, and 5 (30.2%, 29.8%, and 31.0%, respectively). Compared to reported latent flame-retardant curing agents, the molecular structure of the latent flame-retardant curing agent of the present invention comprises phenanthrene, an imide ring, and a thioether bond. These components exert a synergistic flame-retardant effect in both the gas and condensed phases, not only releasing free radical scavengers and non-flammable gases to interrupt the gas-phase chain reaction but also promoting cross-linking to char to cut off flame propagation and spread. Therefore, phenanthrene, an imide ring, and thioether bonds synergistically enhance the flame-retardant properties of epoxy resins containing the flame-retardant curing agent.

[0068] Tensile properties of Examples 1-6 and Comparative Examples 1-5 were tested according to GB / T 1040.2-2006 standard using a tensile testing machine, model BTI-FR010TH (Zwick Roell, Germany). Bending properties were tested according to GB / T 9341-2008 standard using a universal testing machine, model AGS-10KNI (Shimadzu Corporation, Japan). Impact properties were tested according to GB / T 1843-2008 standard using a cantilever beam impact testing machine, model PTM 2302 (Shenzhen Sansi Zongheng Technology Co., Ltd.). Storage stability was tested according to GB / T 7123.2-2002 standard, with the storage period defined as the time required for the normalized viscosity to double. The results are shown in Table 3.

[0069] Table 3 The data in Table 3 show that introducing a flame-retardant curing agent can improve the mechanical properties of epoxy resins containing the flame-retardant curing agent. For example, when the addition amount is 8.7%, the tensile strength, flexural strength, and impact strength of the epoxy resin containing the flame-retardant curing agent reach as high as 59.1 MPa, 78.6 MPa, and 39.0 kJ / m, respectively. 2 This indicates that it has a reinforcing and toughening effect on epoxy resin. However, comparing Example 1 with Comparative Examples 3, 4 and 5, it can be seen that although the introduction of the reported latent flame retardant curing agents can improve the tensile strength and flexural strength of epoxy resin containing flame retardant curing agents, it will sacrifice impact strength.

[0070] The epoxy resin containing a flame-retardant curing agent prepared by this invention exhibits excellent mechanical properties. This is because the flame-retardant curing agent is composed of rigid imide and aromatic rings, as well as flexible thioether bonds, possessing a "rigid-flexible integrated" structure. On one hand, the rigid structure has a reinforcing effect, increasing the rigidity of the epoxy crosslinking network and improving the tensile and flexural strength of the epoxy resin. On the other hand, the flexible structure has a toughening effect; under external force, it causes uniform shear deformation of the crosslinking network and dissipates more fracture energy through intermolecular motion, thereby improving the impact strength of the epoxy resin.

[0071] Furthermore, as shown in Table 3, the epoxy resin containing the flame-retardant curing agent obtained in this invention exhibits good storage stability, with a shelf life of up to 42 days. In contrast, reported epoxy resins containing flame-retardant curing agents, such as Comparative Example 4, have a shelf life of less than 14 days, indicating that the flame-retardant curing agent possesses good thermal latency. Comparing Example 1 with Comparative Examples 1 and 2, it can be found that Example 1 has a longer shelf life at the same addition amount (8.7 wt%). Unlike Comparative Examples 1 and 2, the flame-retardant curing agent in Example 1 contains a thioetherimide structure, which readily undergoes a thiols exchange reaction with thiols to produce highly nucleophilic thiophenols. Compared to traditional thiol-based curing agents, the resulting thiophenols are inert at room temperature, while their reaction temperature with epoxy resin is moderate (approximately 130 °C), thus achieving good storage stability. Moreover, this latent flame-retardant curing agent is an addition-type curing agent with stable chemical properties, unaffected by air, moisture, and impurities during long-term storage, and is less prone to deactivation.

[0072] Therefore, this invention successfully develops a flame-retardant curing agent, and the epoxy resin containing the flame-retardant curing agent prepared using it possesses excellent flame-retardant properties, superior mechanical strength and impact toughness, as well as good storage stability. This overcomes the shortcomings of existing latent flame-retardant curing agents that cannot simultaneously achieve good flame-retardant properties, mechanical properties, and storage stability when modifying epoxy resins. This invention provides an effective solution to meet the demands of modern technological development for multifunctional and integrated materials, and it does not require changes to existing industrial production processes; therefore, this method has broad application value.

[0073] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims. Parts not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A method for preparing a flame-retardant curing agent, characterized in that, Includes the following steps: S1: Preparation of phosphorus-containing intermediates 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 4-aminoacetophenone, and aniline were mixed, then a protective gas was introduced and an acidic catalyst was added. The mixture was reacted at 120℃~150℃ for 20h~36h, solvent I was added, and stirring was continued for 2h~4h. The mixture was filtered, the precipitate was collected, washed with solvent I, and dried under vacuum to obtain a phosphorus-containing intermediate. S2: Preparation of phosphorus and nitrogen intermediates Maleic anhydride and a phosphorus-containing intermediate were mixed and dissolved in solvent II. A protective gas was introduced and the mixture was stirred in an ice bath. Then, a dehydrating agent was added to the solution, and the mixture was heated to reflux. The reaction was continued for 6 to 8 hours. The mixture was filtered, the precipitate was collected, washed with solvent I, and dried under vacuum to obtain the phosphorus-nitrogen intermediate. S3: Preparation of flame retardant curing agent Under a protective atmosphere, the phosphorus-nitrogen intermediate and dimercaptodiphenyl sulfide are dissolved in solvent III and stirred for 1 to 4 hours. Then, an alkaline catalyst is added to the solution and stirring is continued for 4 to 8 hours. The mixture is poured into solvent IV, filtered, the precipitate is collected, and repeatedly washed with solvent IV. After vacuum drying, the flame-retardant curing agent is obtained.

2. The method for preparing a flame-retardant curing agent according to claim 1, characterized in that, Step S1, the preparation of the phosphorus-containing intermediate, specifically involves: 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 4-aminoacetophenone, and aniline were mixed in a molar ratio of 1:(1~1.2):(5~6). A protective gas was then introduced and 0.5wt%~0.8wt% of an acidic catalyst (one of p-toluenesulfonic acid, benzenesulfonic acid, or concentrated sulfuric acid) was added. The mixture was reacted at 120℃~150℃ for 20h~36h. Solvent I, with a mass of 1~2 times that of aniline, was added at 60℃~80℃. The mixture was stirred for 2h~4h, filtered, and the precipitate was collected and washed 3~5 times with solvent I. The mixture was then vacuum dried at 100℃~130℃ for 20h~30h to obtain a phosphorus-containing intermediate.

3. The method for preparing a flame-retardant curing agent according to claim 2, characterized in that, The preparation of the phosphorus-nitrogen intermediate in step S2 is as follows: Maleic anhydride and a phosphorus-containing intermediate were mixed and dissolved in solvent II at a molar ratio of 1:(2~2.2) in 5~6 times the mass of the phosphorus-containing intermediate. A protective gas was introduced and the mixture was stirred in an ice bath for 3~6 hours. Then, 39.0wt%~42.5wt% of a dehydrating agent, equal in mass of the total reactants, was added to the solution in three batches. The dehydrating agent was one of acetic anhydride and sodium acetate or acetic anhydride and triethylamine. The mixture was then heated to reflux and the reaction was continued for 6~8 hours. The mixture was filtered, the precipitate was collected, washed with solvent I, and dried under vacuum to obtain the phosphorus-nitrogen intermediate.

4. The method for preparing a flame-retardant curing agent according to claim 3, characterized in that, Step S3, the preparation of the flame retardant curing agent, is as follows: Under a protective atmosphere, a phosphorus-nitrogen intermediate and a dimercaptodiphenyl sulfide in a molar ratio of 1:(1.8~2.2) are dissolved in solvent III, which is 2~3 times the mass of the phosphorus-nitrogen intermediate. Solvent III is one of m-cresol, dimethylformamide, and dimethyl sulfoxide. The mixture is stirred at 30°C for 1h~4h. Then, an alkaline catalyst is added to the solution, and stirring is continued for 4h~8h. The mixture is then poured into solvent IV, which is 10~20 times the mass of the phosphorus-nitrogen intermediate. Solvent IV is one of acetic acid-acidified methanol, acetic acid-acidified ethanol, and acetic acid-acidified ethyl acetate. The mixture is then filtered, the precipitate is collected, and the mixture is washed repeatedly with solvent IV 3~5 times. The mixture is then vacuum dried at 100°C~130°C for 18h~24h to obtain a flame-retardant curing agent.

5. The method for preparing a flame-retardant curing agent according to claim 4, characterized in that, The protective gas in steps S1 to S3 is one of nitrogen, argon, and helium.

6. An epoxy resin containing a flame-retardant curing agent, characterized in that, The flame retardant curing agent is prepared by the flame retardant curing agent preparation process of any one of claims 1-5, and the components are as follows by mass fraction: epoxy resin monomer 56.3wt%~60.2wt%, thiol curing agent 32.4wt%~36.8wt%, and flame retardant curing agent 3.0wt%~11.3wt%.

7. The epoxy resin containing a flame-retardant curing agent according to claim 6, characterized in that, Its preparation process is as follows: S1: Epoxy resin monomer, thiol curing agent and flame retardant curing agent are heated and stirred at 60℃~80℃ for 1h~2h in proportion to obtain a homogeneous mixture; S2: The mixture is bubbled at 60℃~80℃ for 1h~3h, then poured into a mold and placed in a forced-air drying oven for staged heating and curing. The curing process is 120℃×1h, 150℃×2h, and 160℃×2h. S3: After cooling to 30℃, the epoxy resin containing flame retardant curing agent is obtained after demolding.

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