Sulfonamide flame-retardant toughening curing agent for epoxy resin and preparation method of sulfonamide flame-retardant toughening curing agent

Abstract

The invention relates to a sulfanilamide flame-retardant toughening curing agent for epoxy resin and a preparation method thereof, and belongs to the technical field of curing agents. The sulfonamide flame-retardant toughening curing agent for epoxy resin is oily liquid, the structural formula is shown in the specification, the initial decomposition temperature is higher than 260 DEG C, the carbon residue amount at 800 DEG C is 1.9-5.2%, and an expanded carbon layer is formed in the combustion process. The sulfonamide flame-retardant toughening curing agent disclosed by the invention is used as a curing agent of epoxy resin, and can be used as a flame retardant of the epoxy resin after being added into the epoxy resin together with a phosphorus-containing flame retardant, and the flame retardance of the epoxy resin is improved through the mutual synergistic effect of multiple elements such as phosphorus (P), nitrogen (N), sulfur (S) and silicon (Si).

Description

Technical Field

[0001] The invention belongs to the technical field of curing agents, and particularly relates to a sulfonamide flame retardant toughening curing agent for epoxy resin and a preparation method thereof. Background Art

[0002] Epoxy resin, due to its excellent adhesion, chemical resistance, and outstanding electrical insulation properties, has become a promising thermosetting material and has been widely used in a variety of fields, including coatings, electronic devices, and engineering plastics. Despite this, epoxy resin still faces two major challenges in its practical application: First, the curing agent is a core substance that realizes the value of epoxy resin. The properties of the cured product depend on the performance of the curing agent. Traditional epoxy resins have poor performance after curing, especially low toughness and brittleness, which greatly affects their use. Therefore, improving the performance of epoxy resins requires improving toughness. Second, the flammability of the epoxy resin network system can cause serious fires, posing a major threat to people's lives and property safety.

[0003] Adding flame retardants is a cost-effective and simple method for flame-retardant epoxy resins. Phosphorus-based flame retardants are the most widely used after halogen-based flame retardants. Phosphorus-containing functional groups offer structural diversity, ease of preparation, low toxicity, and minimal secondary pollution. However, single phosphorus-containing flame retardants require large amounts to ensure adequate flame retardancy. The synergistic effect of phosphorus and nitrogen is particularly significant, enhancing the flame retardancy of composite materials in both the gas and condensed phases. This overall flame retardancy achieves a remarkable '1+1>2' effect, while also offering low toxicity, low smoke, halogen-free, and high efficiency. Common aliphatic, alicyclic, and aromatic amine curing agents are the most widely used and widely used, but they contain low nitrogen. Therefore, even when combined with phosphorus-containing flame retardants, effective flame retardancy is difficult to achieve after curing epoxy resin.

[0004] At present, some progress has been made in improving the flame retardancy or toughness of epoxy resins, but these methods generally have limitations such as complex preparation process, insufficient mechanical properties of materials, and high production costs. It is worth noting that the preparation methods of epoxy resin composites that achieve synergistic modification of flame retardancy and toughening are still rarely reported in existing literature. Due to the increasing demand for optical devices such as reflectors and solar panels, improving the transparency of EP products is also considered important. Therefore, the development of epoxy resin curing agents with both flame retardant and toughening functions, and the preparation of high-performance flame retardant, toughened, and high-transparency epoxy resin composites based on them, have become key scientific issues that need to be broken through in this field. Summary of the Invention

[0005] In order to improve the toughness and flame retardancy of epoxy resin while maintaining high transparency of epoxy resin composite materials, the present invention provides a sulfonamide flame retardant toughening curing agent for epoxy resin, and at the same time, provides a preparation method of the sulfonamide flame retardant toughening curing agent for epoxy resin.

[0006] The structural formula of a sulfonamide flame retardant toughening curing agent for epoxy resin is as follows:

[0007]

[0008] The sulfonamide flame retardant toughening curing agent is an oily liquid with an initial decomposition temperature greater than 260° C., a residual carbon content of 1.9-5.2% at 800° C., and an expanded carbon layer is formed during the combustion process.

[0009] The preparation steps of sulfonamide flame retardant toughening curing agent for epoxy resin are as follows:

[0010] (1) Adding epoxy-terminated silicone oil, sulfaguanidine and a catalyst to a solvent, stirring and dissolving, and heating to no more than 145°C, and reacting under nitrogen protection to obtain a reactant;

[0011] The mass ratio of the epoxy-terminated silicone oil, sulfaguanidine and catalyst is 30:40:0.05;

[0012] The epoxy value of the epoxy-terminated silicone oil is 0.240-0.530 mol / 100 g;

[0013] The catalyst is 2, 4, 6-tris(dimethylaminomethyl)phenol;

[0014] The solvent is N, N-dimethylformamide, and the amount of the solvent is 2 times the total amount of the epoxy-terminated silicone oil and sulfaguanidine in milliliters;

[0015] (2) The reactants were washed three times with deionized water at 90°C to remove sulfaguanidine and 2, 4, 6-tris(dimethylaminomethyl)phenol from the reactants; and vacuum dried to obtain a sulfonamide flame retardant toughening curing agent.

[0016] Further preparation technology scheme is as follows:

[0017] In step (1), the heat preservation reaction time is 2 to 8 hours.

[0018] In step (2), the vacuum drying conditions are: temperature is 110°C, and the drying time is more than 12 hours.

[0019] The beneficial technical effects of the present invention are embodied in the following aspects:

[0020] (1) The sulfonamide flame retardant toughening curing agent prepared by the present invention has a thermal weight loss lower than that of the end-epoxy silicone oil at a temperature of 250°C to 800°C, and its initial decomposition temperature is above 260°C, which is much higher than the 197°C of the end-epoxy silicone oil. The thermal stability is significantly improved, and the residual carbon content of the sulfonamide flame retardant toughening curing agent is 5.2% at 800°C, while the residual carbon content of the unmodified end-epoxy silicone oil is only 1.8%.

[0021] (2) When the unmodified epoxy-terminated silicone oil is ignited, it burns violently and produces a large amount of black smoke. The flame height of the sulfonamide flame retardant toughening curing agent prepared by the present invention is significantly lower after ignition, and only a small amount of white smoke is released. Figure 5 , the unmodified epoxy-terminated silicone oil has almost no carbon residue after cone calorimetry test; see Figure 6 The same weight of sulfonamide flame retardant toughening curing agent formed an expanded carbon layer after cone calorimetry test, and the thickness of the carbon layer reached 8cm.

[0022] (3) The sulfonamide flame retardant toughening curing agent prepared by the present invention grafts sulfaguanidine to both ends of the end-epoxy silicone oil. When the sulfonamide flame retardant toughening curing agent burns, the sulfaguanidine group decomposes in the gas phase to produce gaseous substances such as ammonia (NH3) and sulfur oxygen free radicals (SO·, SO2·). The sulfur-containing free radicals (SO· and SO2·) can capture the active free radicals (H·, O· or HO·) formed during combustion, terminate the chain combustion reaction, and thus play a flame retardant role in the gas phase; among them, ammonia (NH3) can react with oxygen (O2) in the air to produce nitrogen dioxide (NO2) and water (H2O). These substances dilute the oxygen, resulting in self-extinguishing behavior during the initial and subsequent ignition process. The sulfur-containing compounds released during the combustion process can react with water to form strong acids, such as sulfuric acid. This process promotes the dehydration of the matrix into the carbon layer, thereby increasing the carbon residue and improving the quality of the carbon layer, playing a role in blocking the release of smoke and the transfer of heat.

[0023] (4) The sulfonamide flame retardant toughening curing agent prepared by the present invention is used in the field of epoxy resin. As a curing agent for epoxy resin, it can be added to epoxy resin together with a phosphorus-containing flame retardant to serve as a flame retardant for epoxy resin. The flame retardancy of epoxy resin is improved through the synergistic effect of multiple elements such as phosphorus (P), nitrogen (N), sulfur (S), and silicon (Si). A single phosphorus-containing flame retardant requires an extremely high addition amount to ensure good flame retardancy. The synergistic effect of phosphorus and nitrogen elements is particularly significant. It can simultaneously improve the flame retardancy of composite materials in the gas phase and the condensed phase, and the overall flame retardant effect achieves the excellent effect of "1+1>2". Sulfur-containing compounds have always been used as flame retardants or adjuvants to promote the activity of other flame retardant elements, especially phosphorus, which can release sulfur-containing non-combustible gases (such as SO2) or promote the cross-linking reaction of the matrix to reflect flame retardant activity.

[0024] (5) The sulfonamide flame retardant toughening curing agent prepared by the present invention has Si-O bonds. Due to the special properties of the Si-O bonds, the molecular chain is easy to rotate and has good flexibility. It reacts with the epoxy group and introduces flexible chain segments to dilute the crosslinking density of the epoxy resin, which greatly improves the toughness of the prepared epoxy resin composite material. The impact strength of the epoxy resin cured without the addition of the sulfonamide flame retardant toughening curing agent is only 2.57±0.06kJ / m 2 The impact strength of epoxy resin cured by adding 5% by weight of sulfonamide flame retardant toughening curing agent is 4.95±0.13kJ / m 2 The impact strength of epoxy resin cured with 10% by weight of sulfonamide flame retardant toughening curing agent is 7.73±0.24kJ / m 2 The impact strength of epoxy resin cured with 15% by weight of sulfonamide flame retardant toughening curing agent is 2.57±0.06kJ / m 2 . It solves the brittleness problem caused by the high cross-linking density of epoxy resin.

[0025] (6) The epoxy resin composite material cured with the sulfonamide flame retardant toughening curing agent maintains high transparency, meets the requirements of optical devices such as reflectors and solar panels, has a wide range of applications, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the infrared spectrum of the sulfonamide flame retardant toughening curing agent and the raw material end epoxy silicone oil of the present invention;

[0027] Figure 2 The thermogravimetric curves of Examples 1 to 4 of the sulfonamide flame retardant toughening curing agent and the epoxy-terminated silicone oil of the present invention are shown;

[0028] Figure 3 Schematic diagram of combustion of terminated epoxy silicone oil;

[0029] Figure 4 This is a combustion diagram of Example 4 of the sulfonamide flame retardant toughening curing agent of the present invention;

[0030] Figure 5 These are the top and front views of the carbon residue after the cone calorimetry test of epoxy-terminated silicone oil;

[0031] Figure 6 The top view and front view of the carbon residue photograph after the cone calorimetry test of Example 4 of the sulfonamide flame retardant toughening curing agent of the present invention;

[0032] Figure 7 Schematic diagram of a vertical combustion experiment of a comparative example of the epoxy resin composite material of the present invention;

[0033] Figure 8Schematic diagram of a vertical combustion experiment of Application Example 2 of the epoxy resin composite material of the present invention;

[0034] Figure 9 This is a bar graph showing the impact strength of the epoxy resin composite material comparative example and application examples 1 to 3 of the epoxy resin composite material of the present invention;

[0035] Figure 10 1 is the tensile stress-strain curve of the epoxy resin composite material comparative example and application examples 1 to 3 of the epoxy resin composite material of the present invention;

[0036] Figure 11 This is a digital photo of Application Example 2 of the epoxy resin composite material of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described below with reference to the accompanying drawings through embodiments.

[0038] Example 1

[0039] The preparation steps of a sulfonamide flame retardant toughening curing agent are as follows:

[0040] (1) Take 30g of epoxy-terminated silicone oil, 40g of sulfaguanidine and 0.05g of 2,4,6-tris(dimethylaminomethyl)phenol, add them to 140mL of N,N-dimethylformamide, stir and dissolve, and heat the mixed solution to 135℃. Keep the mixture warm under nitrogen protection for 2 hours to obtain the reactant.

[0041] The epoxy value of the epoxy-terminated silicone oil is 0.240 mol / 100 g;

[0042] The solvent is N, N-dimethylformamide, and the amount of the solvent used is 2 times in milliliters the total grams of the terminal epoxy silicone oil and sulfaguanidine.

[0043] (2) The reactants were washed three times with deionized water at 90°C to remove sulfaguanidine and 2, 4, 6-tris(dimethylaminomethyl)phenol; and vacuum dried in a vacuum drying oven at 110°C for 12 hours to obtain a sulfonamide flame retardant toughening curing agent.

[0044] The structural formula of the sulfonamide flame retardant toughening curing agent prepared in Example 1 is as follows:

[0045]

[0046] Sulfonamide flame retardant toughening curing agent is an oily liquid with an initial decomposition temperature of 269°C and a residual carbon content of 1.9% at 800°C. An expanding carbon layer is formed during the combustion process.

[0047] Example 2

[0048] The preparation steps of a sulfonamide flame retardant toughening curing agent are as follows:

[0049] (1) Take 30g of epoxy-terminated silicone oil, 40g of sulfaguanidine and 0.05g of 2,4,6-tris(dimethylaminomethyl)phenol, add them to 140mL of N,N-dimethylformamide, stir and dissolve, and heat the mixed solution to 125℃. Keep the mixture warm under nitrogen protection for 4 hours to obtain the reactant.

[0050] (2) The reactants were washed three times with deionized water at 90°C to remove sulfaguanidine and 2, 4, 6-tris(dimethylaminomethyl)phenol; and vacuum dried in a vacuum drying oven at 110°C for 12 hours to obtain a sulfonamide flame retardant toughening curing agent.

[0051] The structural formula of the sulfonamide flame retardant toughening curing agent prepared in Example 2 is as follows:

[0052]

[0053] Sulfonamide flame retardant toughening curing agent is an oily liquid with an initial decomposition temperature of 261°C and a residual carbon content of 3.4% at 800°C. An expanding carbon layer is formed during the combustion process.

[0054] Example 3

[0055] The preparation steps of a sulfonamide flame retardant toughening curing agent are as follows:

[0056] (1) Take 30g of epoxy-terminated silicone oil, 40g of sulfaguanidine and 0.05g of 2,4,6-tris(dimethylaminomethyl)phenol, add them to 140mL of N,N-dimethylformamide, stir and dissolve, and heat the mixed solution to 130℃. Keep the mixture warm under nitrogen protection for 6 hours to obtain the reactant.

[0057] (2) The reactants were washed three times with deionized water at 90°C to remove sulfaguanidine and 2, 4, 6-tris(dimethylaminomethyl)phenol; and vacuum dried in a vacuum drying oven at 110°C for 12 hours to obtain a sulfonamide flame retardant toughening curing agent.

[0058] The structural formula of the sulfonamide flame retardant toughening curing agent prepared in Example 3 is as follows:

[0059]

[0060] Sulfonamide flame retardant toughening curing agent is an oily liquid with an initial decomposition temperature of 275°C and a residual carbon content of 3.7% at 800°C. An expanding carbon layer is formed during the combustion process.

[0061] Example 4

[0062] The preparation steps of a sulfonamide flame retardant toughening curing agent are as follows:

[0063] (1) Take 30g of epoxy-terminated silicone oil, 40g of sulfaguanidine and 0.05g of 2,4,6-tris(dimethylaminomethyl)phenol, add them to 140mL of N,N-dimethylformamide, stir and dissolve, and heat the mixed solution to 140℃. Keep the mixture warm under nitrogen protection for 7 hours to obtain the reactant.

[0064] (2) The reactants were washed three times with deionized water at 90°C to remove sulfaguanidine and 2, 4, 6-tris(dimethylaminomethyl)phenol; and vacuum dried in a vacuum drying oven at 110°C for 12 hours to obtain a sulfonamide flame retardant toughening curing agent.

[0065] The structural formula of the sulfonamide flame retardant toughening curing agent prepared in Example 4 is as follows:

[0066]

[0067] Sulfonamide flame retardant toughening curing agent is an oily liquid with an initial decomposition temperature of 270°C and a residual carbon content of 5.2% at 800°C. An expanding carbon layer is formed during the combustion process.

[0068] Application Example 1

[0069] The preparation steps of a flame retardant toughened epoxy resin composite material are as follows:

[0070] Mix 5 parts of the sulfonamide flame retardant toughening curing agent prepared in Example 4, 5 parts of 9,10-di-9-oxa-10-phosphaphenanthrene-10-oxide and 70 parts of epoxy resin at 140°C, stir evenly and cool to 90°C, add 20 parts of common amine curing agent diaminodiphenylmethane (DDM), mix evenly and evacuate. When there are no bubbles in the mixture in the vacuum, pour the sample into a preheated mold for curing. The mold is preheated at 100°C for 30 minutes, cured at 120°C for 3 hours, and then heated to 160°C and cured for another 3 hours to obtain a flame retardant toughened epoxy resin composite material. The flame retardant toughened epoxy resin composite material prepared in this application example 1 passed the UL-94 test with a grade of V-1, and no dripping occurred during the combustion process; the impact strength was 4.95±0.13kJ / m 2 .

[0071] Application Example 2

[0072] The preparation steps of a flame retardant toughened epoxy resin composite material are as follows:

[0073] 10 parts of the sulfonamide flame retardant toughening curing agent prepared in Example 4, 5 parts of 9,10-di-9-oxa-10-phosphaphenanthrene-10-oxide and 70 parts of epoxy resin were mixed at 140°C, stirred evenly and cooled to 90°C. 15 parts of the common amine curing agent diaminodiphenylmethane (DDM) were added and mixed evenly and then vacuumed. When there were no bubbles in the mixture in the vacuum, the sample was poured into a preheated mold for curing. The mold was preheated at 100°C for 30 minutes, cured at 120°C for 3 hours, and then heated to 160°C and cured for another 3 hours to obtain a flame retardant toughened epoxy resin composite material. The flame retardant toughened epoxy resin composite material prepared in this application example 2 passed the UL-94 test with a grade of V-0, and no dripping occurred during the combustion process; the impact strength was 7.73±0.24kJ / m 2 .

[0074] Application Example 3

[0075] The preparation steps of a flame retardant toughened epoxy resin composite material are as follows:

[0076] 15 parts of the sulfonamide flame retardant toughening curing agent prepared in Example 4, 5 parts of 9,10-di-9-oxa-10-phosphaphenanthrene-10-oxide and 100 parts of epoxy resin were mixed at 140°C, stirred evenly and cooled to 90°C, 10 parts of curing agent diaminodiphenylmethane (DDM) were added and mixed evenly and then vacuumed. When there were no bubbles in the mixture in the vacuum, the sample was poured into a preheated mold for curing. The mold was preheated at 100°C for 30 minutes, cured at 120°C for 3 hours, and then heated to 160°C and cured for another 3 hours to obtain a flame retardant toughened epoxy resin composite material. The flame retardant toughened epoxy resin composite material prepared in this application example 3 passed the UL-94 test with a grade of V-1, and no dripping occurred during the combustion process; the impact strength was 11.79±0.35kJ / m 2 .

[0077] Comparative Example

[0078] 5 parts of 9,10-di-9-oxa-10-phosphaphenanthrene-10-oxide were mixed with 70 parts of epoxy resin at 140°C, stirred evenly, and then cooled to 90°C. 25 parts of curing agent diaminodiphenylmethane (DDM) were added, mixed evenly, and then vacuumed. When there were no bubbles in the mixture in the vacuum, the sample was poured into a preheated mold for curing. The mold was preheated at 100°C for 30 minutes, cured at 120°C for 3 hours, and then heated to 160°C and cured for another 3 hours to obtain an epoxy resin composite material. The epoxy resin composite material prepared in this comparative example did not pass the UL-94 test, and no dripping occurred during the combustion process; the impact strength was only 2.57±0.06kJ / m 2 .

[0079] Table 1 UL-94 data of flame retardant toughened epoxy resin application examples 1-3 and comparative examples

[0080]

[0081] Table 1 shows the data of the flame-retardant toughened epoxy resins of Application Examples 1-3 and the comparative example passing the UL-94 test. The addition of a sulfonamide flame-retardant toughening curing agent can enable the prepared epoxy resin composite material to pass the UL-94 test. In particular, the flame retardant effect is best when a sulfonamide flame-retardant toughening curing agent with a mass fraction of 10% is added, and the UL-94 grade is V-0.

[0082] Depend on Figure 1 It can be seen that the 910 cm -1 The absorption peak at 3550~3200cm is the absorption peak of epoxy group. The absorption peak at this position disappears in the curing agent, and some new absorption peaks appear in the curing agent, 3550~3200cm -1 The broad absorption peaks at 1540 cm correspond to NH, -NH2 and -OH groups. -1 The absorption peak at 1304 cm belongs to the bending vibration of NH absorption peak. -1 and 1180cm -1 The characteristic absorption peaks belong to S=O symmetric and asymmetric stretching vibrations, 1598cm -1 The characteristic absorption peak is attributed to the C=N stretching vibration of the benzene ring. -1 The disappearance of the epoxy group absorption peak at 3550~3200cm -1 The appearance of NH, -NH2 and -OH absorption peaks at 400 nm proved that the amino group in sulfaguanidine reacted with the epoxy group of the epoxy-terminated silicone oil.

[0083] Depend on Figure 2 It can be seen that the epoxy-terminated silicone oil has completely burned at a temperature of 600°C, leaving essentially no residue, with a carbon residue of only 1.8%. The sulfonamide flame-retardant and toughening curing agents prepared in Examples 1 to 4 of the present invention have a lower thermal weight loss than the epoxy-terminated silicone oil between 250°C and 800°C, and their initial decomposition temperatures are all above 260°C, significantly improving their thermal stability. Furthermore, a certain amount of carbon residue is still present at temperatures above 800°C. The sulfonamide flame-retardant and toughening curing agent prepared in Example 4 has a carbon residue of 5.2% at 800°C. The thermal weight loss of the sulfonamide flame-retardant and toughening curing agent of the present invention is divided into two stages: the first stage is the decomposition of the sulfaguanidine group, and the second stage is the decomposition of the siloxane backbone.

[0084] Depend on Figure 3 It can be seen that the combustion of the epoxy-terminated silicone oil is very intense, with the flame height exceeding 20 cm and accompanied by a large amount of black smoke.

[0085] Depend on Figure 4 It can be seen that the flame of the sulfonamide flame retardant toughening curing agent prepared in Example 4 is about 2 cm, and only a small amount of white smoke is generated, indicating that the sulfonamide flame retardant toughening curing agent of the present invention has a better flame retardant and smoke suppression effect than the end-epoxy silicone oil.

[0086] Figure 5 、 Figure 6 They are respectively the top view and the front view of the carbon residue photos after the cone calorimetry test of the end-epoxy silicone oil and the sulfonamide flame retardant and toughening curing agent Example 4 of the present invention; when the unmodified end-epoxy silicone oil is subjected to the cone calorimetry test, there is almost no carbon residue, while the same weight of the sulfonamide flame retardant and toughening curing agent forms an expanded carbon layer after the cone calorimetry test, and the thickness of the carbon layer reaches 8 cm.

[0087] See also Figure 7 The epoxy resin composite material in the comparative example is very easy to ignite in the air and burns violently, and the combustion is accompanied by the generation of a large amount of black smoke and molten droplets, and cannot pass the UL-94 test.

[0088] See also Figure 8 When 10% by weight of sulfonamide flame retardant toughening curing agent is added, there is no dripping phenomenon, and the ignition is quickly extinguished. The UL-94 grade is V-0, which has good flame retardant effect.

[0089] See also Figure 9 It can be seen that with the increase in the amount of sulfonamide flame retardant toughening curing agent, the impact strength of the epoxy resin composite material is significantly increased. This is because the sulfonamide flame retardant toughening curing agent prepared by the present invention has a longer molecular chain and contains Si-O bonds. Due to the special properties of the Si-O bond, the molecular chain is easier to rotate and has good flexibility. It reacts with the epoxy group and introduces flexible chain segments to dilute the cross-linking density of the epoxy resin, thereby greatly improving the toughness of the epoxy resin composite material.

[0090] See also Figure 10 , are the tensile stress-strain curves of the epoxy resin composite material comparison example and the epoxy resin composite material application examples 1 to 3. With the addition of sulfonamide flame retardant toughening curing agent, the tensile strength of the epoxy resin composite material decreases, but the elongation at break increases a lot.

[0091] See also Figure 11 , which is a digital photo of epoxy resin composite material application example 2. It can be seen that the background font and logo under the epoxy resin composite material remain clearly visible.

[0092] It is easy for those skilled in the art to understand that the above embodiments 1-4 are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A sulfonamide flame retardant toughening curing agent for epoxy resin, characterized in that: The structural formula of the sulfonamide flame retardant toughening curing agent is as follows:

2. The sulfonamide flame retardant toughening curing agent is an oily liquid with an initial decomposition temperature greater than 260°C and a residual carbon content of 1.9-5.2% at 800°C. An expanded carbon layer is formed during the combustion process.

3. The method for preparing the sulfonamide flame retardant toughening curing agent for epoxy resin according to claim 1, characterized in that: The steps are as follows: (1) Adding epoxy-terminated silicone oil, sulfaguanidine and a catalyst to a solvent, stirring and dissolving, and heating to no more than 145°C, and reacting under nitrogen protection to obtain a reactant; The mass ratio of the epoxy-terminated silicone oil, sulfaguanidine and catalyst is 30:40:0.05; The epoxy value of the epoxy-terminated silicone oil is 0.240-0.530 mol / 100 g; The catalyst is 2, 4, 6-tris(dimethylaminomethyl)phenol; The solvent is N, N-dimethylformamide, and the amount of the solvent is 2 times the total amount of the epoxy-terminated silicone oil and sulfaguanidine in milliliters; (2) The reactants were washed three times with deionized water at 90°C to remove sulfaguanidine and 2, 4, 6-tris(dimethylaminomethyl)phenol from the reactants; and vacuum dried to obtain a sulfonamide flame retardant toughening curing agent.

4. The preparation method according to claim 2, wherein: In step (1), the heat preservation reaction time is 2 to 8 hours.

5. The preparation method according to claim 2, wherein: In step (2), the vacuum drying conditions are: temperature is 110°C, and the drying time is more than 12 hours.

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