Imidazole-based phosphorus-containing flame retardant curing agent and its preparation method and application

Abstract

The present invention discloses an imidazole-based phosphorus-containing flame retardant curing agent and its preparation method and application, which belong to the field of flame retardancy. The imidazole-based phosphorus-containing flame retardant curing agent provided by the present invention can not only play a condensed phase flame retardant mechanism to promote the carbonization performance of the polymer during the combustion process, but also play a gas phase flame retardant mechanism to capture active free radicals during the combustion process; and the chemical structure contains an imidazole structure and can be used as an epoxy curing accelerator. The flame retardant provided by the present invention significantly improves the limiting oxygen index of epoxy resin, and the vertical combustion grade reaches V-0 level. It has the advantages of good carbonization, high flame retardant efficiency, high thermal stability, no migration and precipitation, and antibacterial.

Description

Technical Field

[0001] The present invention belongs to the field of flame retardancy, and in particular relates to an imidazole-based phosphorus-containing flame retardant curing agent, a preparation method thereof, and an application thereof. Background Art

[0002] Epoxy resin, a widely used thermosetting resin material, offers advantages such as low production cost, electrical insulation, chemical resistance, and high thermal stability. Consequently, it is widely used in fields such as construction, aerospace, and electronics. However, as an organic polymer, epoxy resin contains significant amounts of carbon, hydrogen, and oxygen in its structure, making it flammable and releasing large amounts of smoke during combustion, severely limiting its application. In recent years, the flame-retardant modification of epoxy resin using phosphorus-based flame retardants has become a research hotspot in the flame retardant field. Flame-retardant epoxy resins are generally categorized as additive-based and reactive-based. Additive-based epoxy resins are generally simple to manufacture and have a wider range of raw materials, making them the most widely used type worldwide. However, the addition of additives also presents dispersibility and compatibility issues, and flame retardant precipitation can also occur during long-term use. Reactive-based flame-retardant curing accelerators, on the other hand, can directly incorporate flame-retardant elements into the epoxy resin chain. When used as a curing agent, they impart long-term flame retardancy while minimizing the impact on the mechanical properties of the epoxy system. Therefore, the research and development of phosphorus- and nitrogen-containing epoxy resins, as well as silicone-based epoxy resins, has attracted significant attention. A Chinese patent application (CN118772202B) discloses an imidazole-based phosphorus-containing latent flame retardant, but its structure is fixed and cannot be adjusted. A Chinese patent application (CN114539316A) discloses a phosphorus-containing imidazole compound, but this compound suffers from the disadvantages of high addition levels and low flame retardancy. This invention designs a structurally controllable imidazole-based phosphorus-containing flame retardant curing agent for use in the preparation of intrinsically flame-retardant epoxy resin materials. Summary of the Invention

[0003] The present invention aims to provide an imidazole-based phosphorus-containing flame retardant curing agent and a preparation method thereof. The imidazole-based phosphorus-containing flame retardant curing agent has structural characteristics characterized by adjustable phosphorus oxidation state and number of aromatic rings. It can function as an epoxy curing agent while also exerting a phosphorus-nitrogen synergistic flame retardant effect. It exhibits excellent charring properties, high flame retardant efficiency, high thermal stability, no migration or precipitation, and antibacterial properties.

[0004] The imidazole-based phosphorus-containing flame retardant curing agent of the present invention has a chemical structure as shown in the following formula (I) or (II):

[0005] .

[0006] in:

[0007] R1 is selected from any one of the following structures:

[0008] .

[0009] R2 is selected from any one of the following structures:

[0010] .

[0011] *Indicates the connection location.

[0012] The preparation method of the imidazole-based phosphorus-containing flame retardant curing agent of the present invention comprises the following steps:

[0013] ① Dissolve the diamine and 4-imidazolecarboxaldehyde or 2-imidazolecarboxaldehyde in ethanol and heat under the protection of inert gas to react to obtain an imidazolyl Schiff base intermediate;

[0014] ② The imidazolyl Schiff base intermediate and the phosphorus-containing compound obtained in step ① are dissolved in ethanol, and heated to react under the protection of an inert gas to obtain a flame retardant curing agent.

[0015] In step ①, the diamine is selected from one of 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl sulfide, and 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane.

[0016] In step ①, the molar ratio of diamine to 4-imidazolecarboxaldehyde or 2-imidazolecarboxaldehyde is 1:(2.0~2.2).

[0017] In step ①, the inert gas is any one of nitrogen, argon or helium.

[0018] In step ①, the reaction temperature is 20°C to 80°C, and the reaction time is 3 to 24 hours.

[0019] In step ②, the phosphorus-containing compound is selected from any one of diphenylphosphine, diphenyl phosphite, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, dibenzyl phosphite, dimethyl phosphite, diethyl phosphite, dibutyl phosphite, and diisobutyl phosphite.

[0020] In step ②, the molar ratio of the imidazolyl Schiff base intermediate to the phosphorus-containing compound is 1:(2.0-2.2).

[0021] In step ②, the inert gas is any one of nitrogen, argon or helium.

[0022] In step ②, the reaction temperature is 40-80° C., and the reaction time is 3-24 hours.

[0023] The invention discloses an application of the imidazole-based phosphorus-containing flame retardant curing agent in the preparation of a flame retardant and antibacterial epoxy resin functional material.

[0024] Furthermore, the imidazole-based phosphorus-containing flame retardant curing agent is added as an auxiliary agent to the epoxy resin system to improve the flame retardant and antibacterial properties of the epoxy resin.

[0025] Furthermore, the addition amount of the imidazole-based phosphorus-containing flame retardant curing agent is 2.5-7.5% of the total mass of the epoxy resin components, and more preferably 5%.

[0026] In specific applications, the ratio of the imidazole-based phosphorus-containing flame-retardant curing agent to the original curing agent needs to be controlled. In principle, the molar ratio of the imidazole-based phosphorus-containing flame-retardant curing agent plus the active hydrogen of the original curing agent to the epoxy groups in the epoxy resin is fixed at 1:1. Therefore, as the amount of flame-retardant curing agent increases, the amount of original curing agent used decreases accordingly.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. The imidazole structure in the chemical structure of the imidazole-based phosphorus-containing flame retardant curing agent provided by the present invention can undergo a ring-opening reaction with the epoxy group, thereby exerting the advantages of flame retardant elements being difficult to migrate and precipitate, having good compatibility with the matrix, and being antibacterial.

[0029] 2. The phosphorus oxidation state in the chemical structure of the imidazole-based phosphorus-containing flame retardant curing agent provided by the present invention is controllable. It can not only exert a condensed phase flame retardant mechanism to promote the carbonization of the polymer during combustion, but also exert a gas phase flame retardant mechanism to capture active free radicals during combustion, and has the advantages of synergistic flame retardancy and high flame retardant efficiency.

[0030] 3. The imidazole-based phosphorus-containing flame retardant curing agent provided by the present invention has a rich aromatic ring structure, does not deteriorate the glass transition temperature of the epoxy resin matrix, and has the advantage of high thermal stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0032] Figure 1 3 is the thermogravimetric analysis curve of the epoxy resin containing different amounts of flame retardant a in Example 7 of the present invention.

[0033] Figure 2 1 is the heat release rate curve of the epoxy resin containing different amounts of flame retardant a in Example 7 of the present invention.

[0034] Figure 3 This is the total smoke release curve of the epoxy resin containing different amounts of flame retardant a in Example 7 of the present invention.

[0035] Figure 4 This is a differential scanning calorimeter test curve of the epoxy resin containing different amounts of flame retardant a in Example 7 of the present invention.

[0036] Figure 5 These are the antibacterial test results of the epoxy resin, flame retardant epoxy resin-2, flame retardant epoxy resin-9 and flame retardant epoxy resin-12 in the present invention against Aspergillus niger, Fusarium and Penicillium. DETAILED DESCRIPTION

[0037] To further illustrate the technical solution of the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and are not intended to limit the claims of the present invention. All other embodiments obtained by persons of ordinary skill in the art based on the examples of the present invention without creative effort are intended to fall within the scope of protection of the present invention.

[0038] Example 1:

[0039] ① 4, 4'-diaminodiphenylmethane and 4-imidazolecarboxaldehyde were heated to 80°C in an ethanol solvent under nitrogen protection at a molar ratio of 1:2 for 8 hours. The solvent was removed by filtration to obtain an imidazolyl Schiff base intermediate a, the chemical structure of which is shown below:

[0040] .

[0041] ② The above-mentioned imidazolyl Schiff base intermediate a and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were heated to 70° C. in an ethanol solvent at a reaction molar ratio of 1:2, reacted for 24 hours, and the solvent was removed by rotary evaporation to obtain flame retardant a, whose chemical structure is shown below:

[0042] .

[0043] Flame retardant a was analyzed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy ( 1 H-NMR) characterization confirmed its chemical structure as follows: FT-IR (KBr, cm -1 ): 754, 923 (PO-Ph), 1226-1201 (P=O), 1612 (C=N of the imidazole ring). 1H-NMR (400 MHz, DMSO-d6, ppm): 5.05-5.25 (m, 2H, -CH-P=O), 5.5-5.7 (m,2H, NH), 3.5 (m, 2H, -CH2-), 6.5-8.2 (m, 28H, Ar-H).

[0044] Example 2:

[0045] ① 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane and 4-imidazolecarboxaldehyde were heated to 80°C in an ethanol solvent under nitrogen protection for 6 hours. The solvent was removed by rotary evaporation to obtain imidazolyl Schiff base intermediate b, whose chemical structure is shown below:

[0046] .

[0047] ② The above-mentioned imidazolyl Schiff base intermediate b and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were heated to 50° C. in an ethanol solvent at a reaction molar ratio of 1:2.2, reacted for 24 hours, and the solvent was removed by rotary evaporation to obtain flame retardant b, whose chemical structure is shown below:

[0048] .

[0049] Flame retardant b was analyzed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy ( 1 H-NMR) characterization confirmed its chemical structure as follows: FT-IR (KBr, cm -1 ): 932 (POC), 1192 (P=O), 1589 (P-Ar), 1050 (Si-O-Si). 1 H-NMR (400 MHz, DMSO-d6, ppm): 0 (m, 12H, Si-CH3), 0.3 (m, 4H, Si-CH2-), 1.2 (m, 4H, -CH2-), 2.4 (m, 4H, -CH2-N-), 4.2 (m, 2H, -CH-P=O), 4.4 (m,2H, NH), 8.05-8.34 (m, 20H, Ar-H).

[0050] Example 3:

[0051] ① 4, 4'-Diaminodiphenyl sulfone and 4-imidazole carboxaldehyde were heated to 60°C in an ethanol solvent under nitrogen protection at a molar ratio of 1:2.1 for 8 hours. The solvent was removed by rotary evaporation to obtain an imidazolyl Schiff base intermediate c, the chemical structure of which is shown below:

[0052] .

[0053] ② The above-mentioned imidazolyl Schiff base intermediate c and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were heated to 60° C. in an ethanol solvent at a reaction molar ratio of 1:2.1, reacted for 12 hours, and the solvent was removed by rotary evaporation to obtain flame retardant c, whose chemical structure is shown below:

[0054] .

[0055] Flame retardant c was analyzed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy ( 1 H-NMR) characterization confirmed its chemical structure as follows: FT-IR (KBr, cm -1 ): 932 (POC), 1231 (P=O), 1592 (P-Ar). 1 H-NMR (400MHz, DMSO-d6, ppm): 4.05-4.28 (m, 2H, -CH-P=O), 5.5-5.7 (m, 2H, NH), 6.93-8.04 (m, 28H, Ar-H).

[0056] Example 4:

[0057] ① 4, 4'-diaminodiphenylmethane and 2-imidazolecarboxaldehyde were heated to 80°C in an ethanol solvent under nitrogen protection at a molar ratio of 1:2 for 5 hours. The solvent was removed by rotary evaporation to obtain the imidazolyl Schiff base intermediate d, whose chemical structure is shown below:

[0058] .

[0059] ② The above-mentioned imidazolyl Schiff base intermediate d and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide were heated to 80° C. in an ethanol solvent at a reaction molar ratio of 1:2.1, reacted for 6 hours, and the solvent was removed by rotary evaporation to obtain flame retardant d, whose chemical structure is shown below:

[0060] .

[0061] The flame retardant d was analyzed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy ( 1 H-NMR) characterization confirmed its chemical structure as follows: FT-IR (KBr, cm -1 ): 932 (POC), 1202 (P=O), 1589 (P-Ph), 3240 (NH). 1H-NMR (400 MHz, DMSO-d6, ppm): 3.6 (m, 2H, -CH2-), 5.0-5.2 (m, 2H, -CH-P=O), 5.5-5.7 (m, 2H, NH), 6.5-8.1 (m, 28H, Ar-H).

[0062] Example 5:

[0063] ① 4, 4'-diaminodiphenylmethane and 4-imidazolecarboxaldehyde were heated to 80°C in an ethanol solvent under nitrogen protection at a molar ratio of 1:2.1 for 4 hours. The solvent was removed by rotary evaporation to obtain the imidazolyl Schiff base intermediate a.

[0064] ② The above-mentioned imidazolyl Schiff base intermediate a and diphenylphosphine were heated to 80° C. in an ethanol solvent at a reaction molar ratio of 1:2.1, reacted for 12 hours, and the solvent was removed by rotary evaporation to obtain flame retardant e, whose chemical structure is shown below:

[0065] .

[0066] The flame retardant e was analyzed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy ( 1 H-NMR) characterization confirmed its chemical structure as follows: FT-IR (KBr, cm -1 ): 931 (POC), 940 (PO-Ph), 1204 (P=O), 3241 (NH). 1 H-NMR (400 MHz, DMSO-d6, ppm): 3.8 (m, 2H, -CH2-), 4.2 (m, 2H, -CH-P=O), 5.5-5.7 (m, 2H, NH), 6.5-8.5 (m, 32H, Ar-H).

[0067] Example 6:

[0068] ① 4, 4'-diaminodiphenylmethane and 4-imidazolecarboxaldehyde were heated to 40°C in an ethanol solvent under nitrogen protection at a molar ratio of 1:2.1 for 24 hours. The solvent was removed by rotary evaporation to obtain the imidazolyl Schiff base intermediate a.

[0069] ② The above-mentioned imidazolyl Schiff base intermediate a and dimethyl phosphite were heated to 80° C. in an ethanol solvent at a reaction molar ratio of 1:2.1, reacted for 12 hours, and the solvent was removed by rotary evaporation to obtain a flame retardant f, the chemical structure of which is shown below:

[0070] .

[0071] The flame retardant f was analyzed by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy ( 1 H-NMR) characterization confirmed its chemical structure as follows: FT-IR (KBr, cm -1 ): 732 (PC), 935 (POC), 1206 (P=O), 2965 (-CH3), 3238 (NH). 1 H-NMR (400 MHz, DMSO-d6, ppm): 3.5-3.7 (d, 12H, -CH3), 3.9 (s, 2H,-CH2-), 4.0 (m, 2H, -CH-P=O), 5.5-5.7 (m, 2H, NH), 6.5-7.1 (m, 12H, Ar-H).

[0072] Example 7: Preparation of flame-retardant epoxy resin and its oxygen index and vertical combustion test results

[0073] In this embodiment, a flame retardant epoxy resin was prepared according to the formula in Table 1 below.

[0074]

[0075] Note: The epoxy resin is bisphenol A diglycidyl ether epoxy resin (brand: E-44, epoxy value: 0.44 mol / 100 g), and the curing agent is 4, 4'-diaminodiphenylmethane.

[0076] Accurately weigh the epoxy resin, curing agent, and flame retardant. Mix the epoxy resin and flame retardant at 80°C for 1 hour, then add the curing agent and continue mixing for 30 minutes. Immediately pour the mixture into a mold. Curing conditions are 100°C / 2 hours + 150°C / 2 hours. Cool naturally to room temperature to obtain a flame-retardant epoxy resin sample. The results of the oxygen index and vertical combustion tests are shown in Table 2:

[0077]

[0078] Comparative Example 1: Preparation of flame-retardant epoxy resin and its oxygen index and vertical combustion test results

[0079] In this comparative example, a flame retardant epoxy resin was prepared according to the formulation in Table 3 below.

[0080]

[0081] Note: The epoxy resin is bisphenol A diglycidyl ether, the curing agent is 4, 4'-diaminodiphenylmethane, the phosphorus-containing flame retardant is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, the imidazole-containing flame retardant 1 is dibenzo[c,e][1,2]oxaphosphonic acid imidazole complex, the imidazole-containing flame retardant 2 is phenylphosphonic acid diimidazole complex, and the imidazole-containing flame retardant 3 is N-[3-(1H-imidazol-1-yl)propyl]-P,P-diphenylphosphinamide.

[0082] Accurately weigh the epoxy resin, curing agent, and flame retardant (or intermediate). Mix the epoxy resin and flame retardant (or intermediate) at 80°C for 1 hour. Then add the curing agent and continue mixing for 30 minutes. Immediately pour the mixture into a mold. Curing conditions are 100°C / 2 hours + 150°C / 2 hours. Cool naturally to room temperature to obtain a flame-retardant epoxy resin sample. Test the oxygen index and vertical combustion test. The results are shown in Table 4:

[0083]

[0084] The flame retardant test results of each sample in Table 2 and Table 4 show that the oxygen index of the untreated epoxy resin is 25.5%, and the UL-94 vertical burning test has no grade. The oxygen index of the epoxy resin to which 2.5 wt% flame retardant a is added is increased to 31.0%, but the UL-94 vertical burning test only reaches V-2 level. The oxygen index of the epoxy resin to which 5.0 wt% and 7.5 wt% flame retardant a are added respectively is further increased to 33.0% and 33.5%, and both can pass the UL-94 vertical burning test V-0 level. In addition, the oxygen index of the epoxy resin to which 5.0 wt% flame retardant b, flame retardant c and flame retardant d are added respectively reaches more than 32.0%, and both can pass the UL-94 vertical burning test V-0 level, indicating that the imidazole-based phosphorus-containing flame retardant curing agent provided by the present invention has excellent flame retardant efficiency. In contrast, the oxygen index of the epoxy resins added with 5.0 wt% of intermediate a and intermediate b respectively failed to exceed 29.0%, and the UL-94 vertical burning test had no grade; the oxygen index of the epoxy resin added with 5.0 wt% of phosphorus-containing flame retardant reached 30.5%, but only reached the UL-94 V-1 grade; the oxygen index of epoxy resins 10 and 11 added with two imidazole flame retardants were 27.5% and 27.0%, respectively, and the UL-94 vertical burning test showed no grade, which is because the flame retardant efficiency of the ionic bond formed between the imidazole and the phosphonic acid group is not as good as the covalent bond; the oxygen index of the epoxy resin added with imidazole flame retardant 3 was 30.0%, and the UL-94 vertical burning test only reached the V-2 grade, indicating that the flame retardant efficiency of a single imidazole group is not as good as that of a double imidazole group.

[0085] Example 8: Antibacterial test of flame retardant epoxy resin

[0086] The fungal resistance of flame retardant epoxy resin was determined according to the national standard GB / T1741-2020. Flame retardant epoxy resin-2 was used as the test sample, and epoxy resin, flame retardant epoxy resin-9 and flame retardant epoxy resin-12 were used as control samples. The antifungal test results of the above samples against Aspergillus niger, Fusarium spp. and Penicillium spp. are shown in the table below. Figure 5 shown.

[0087] Figure 1 The thermogravimetric analysis curves of epoxy resin containing different amounts of flame retardant a (nitrogen atmosphere, heating rate of 20 ℃ / min). The addition of flame retardant a improves the charring performance of epoxy resin, and the charring rate increases with the increase of the addition amount. o The charring rate at C was 11.8%, while the charring rate of flame retardant epoxy resin was above 14.5%, indicating that flame retardant a had good catalytic charring ability.

[0088] Figure 2 The heat release rate curve of epoxy resin containing different amounts of flame retardant a (cone calorimeter test, 35kW / m 2 The peak heat release rate of untreated epoxy resin is 1167 kW / m 2 The peak heat release rate of epoxy resin with 2.5 wt%, 5 wt% and 7.5 wt% flame retardant a was reduced to 1015, 960 and 884 kW / m, respectively. 2 , indicating that flame retardant a has good flame retardant properties.

[0089] Figure 3 The total smoke release curve of epoxy resin containing different amounts of flame retardant a (cone calorimeter test, 35kW / m 2 The total smoke emission of untreated epoxy resin is 20.9 m 2 The total smoke emission of epoxy resin with 2.5 wt%, 5 wt% and 7.5 wt% of flame retardant a was reduced to 17.2, 17.7 and 17.4 m 2 , indicating that flame retardant a has good smoke suppression performance.

[0090] Figure 4 The following is the differential scanning calorimeter test curve of epoxy resin containing different amounts of flame retardant a (heating rate is 10 ℃ / min). The glass transition temperature (T g ) Take the temperature of the intersection of the midline of the extrapolated baseline on the low temperature side and the high temperature side of the DSC curve and the curve. T g The T of epoxy resin was 151.6℃, and the addition of 2.5 wt%, 5 wt% and 7.5 wt% flame retardant a increased the T of epoxy resin. g When 5 wt% flame retardant a is added, the Tg The increase is close to 10 °C, indicating that flame retardant a can effectively improve the thermal stability of epoxy resin.

[0091] Figure 5 The following table shows the antifungal test results (48 hours) for the epoxy resin, flame-retardant epoxy resin-2, flame-retardant epoxy resin-9, and flame-retardant epoxy resin-12 against Aspergillus niger, Fusarium spp., and Penicillium spp. No significant fungal growth was observed in the culture medium of the three fungi grown on the surface of the flame-retardant epoxy resin-2 film, demonstrating that the flame-retardant epoxy resin-2 exhibited excellent inhibitory effects against the growth of all three fungi. In contrast, the epoxy resin, flame-retardant epoxy resin-9, and flame-retardant epoxy resin-12 exhibited no effective inhibitory effect against any of the three fungi.

[0092] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An imidazole-based phosphorus-containing flame retardant curing agent, characterized in that Its chemical structure is shown below: 。 2. Use of the imidazole-based phosphorus-containing flame retardant curing agent according to claim 1 in the preparation of flame retardant and antibacterial epoxy resin functional materials.

3. The use according to claim 2, characterized in that: The imidazole-based phosphorus-containing flame retardant curing agent is added as an auxiliary agent to the epoxy resin system to improve the flame retardant and antibacterial properties of the epoxy resin; The antibacterial property refers to the property of inhibiting molds, and the molds are Aspergillus niger, Fusarium spp. and Penicillium spp.

4. The use according to claim 3, characterized in that: The added amount of the imidazole-based phosphorus-containing flame retardant curing agent is 2.5-7.5% of the total mass of the epoxy resin components.

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