Flame-retardant epoxy resin as well as preparation method and application thereof

Abstract

The invention discloses flame-retardant epoxy resin as well as a preparation method and application thereof. The flame-retardant epoxy resin provided by the invention is co-crosslinked flame-retardant epoxy resin containing cyclotriphosphazene and nitrogen heteroaromatic ring, and the nitrogen heteroaromatic ring comprises at least one of an isoindole ring, a triazine ring and a phthalocyanine ring. The flame-retardant epoxy resin is realized by introducing a branched nitrile resin containing cyclotriphosphazene and a para-ammonia structure into epoxy resin. The flame retardant property of the flame-retardant epoxy resin is effectively improved through the synergistic flame retardant effect formed by the cyclotriphosphazene and the nitrogen-containing heteroaromatic ring structure.

Description

Technical Field

[0001] The present invention relates to the technical field of flame-retardant materials, and in particular, to a flame-retardant epoxy resin, a preparation method thereof, and uses thereof. Background Art

[0002] Epoxy resin (EP) is a type of thermosetting resin containing two or more epoxy groups. Its epoxy functional groups can react with functional groups containing active H (hydroxyl, primary amine, secondary amine, mercapto, etc.) at relatively low temperatures (<200 °C) to form a three-dimensional crosslinked network, endowing the epoxy resin with good mechanical properties, heat resistance, adhesiveness, and electrical insulation properties. Combining with the excellent processing and forming properties of epoxy resin, epoxy resin is widely used in the fields of adhesion, coating, electronic information, aerospace, automotive machinery, and entertainment and leisure.

[0003] Due to its structure containing a large number of aliphatic chains and carbon-hydrogen bonds, epoxy resin has the characteristic of being flammable. According to data, the limiting oxygen index (LOI) of bisphenol A epoxy resin is only 20%, and at the same time, epoxy resin will not self-extinguish when on fire, and is accompanied by thick smoke and obvious dripping. Therefore, unmodified epoxy resin is difficult to meet the industry's requirements for the flame-retardant performance of materials.

[0004] There are mainly two existing methods for modifying the flame-retardant performance of epoxy resin. One is to introduce a flame retardant into the epoxy resin, and the other is to introduce a flame-retardant structure into the epoxy resin structure to prepare an intrinsically flame-retardant epoxy resin. Flame retardants can be divided into inorganic flame retardants and organic halogen-free flame retardants. Inorganic flame retardants mainly include magnesium hydroxide, aluminum hydroxide, boric acid (salt), red phosphorus, ammonium polyphosphate, and hypophosphite. Such flame-retardant particles require a very high addition amount to achieve the flame-retardant effect, and there are problems of difficult dispersion and low decomposition temperature. Organic flame retardants are mainly organic compounds containing nitrogen, boron, and phosphorus. Organic flame retardants containing nitrogen mainly include guanidine, ammonium salts, dicyandiamide, melamine, triazine compounds, etc.; organic flame retardants containing boron are mainly derivatives prepared based on boric acid. However, organic flame retardants containing a single nitrogen or boron usually have poor flame-retardant effects and often need to be used in combination to achieve better flame-retardant effects. Organic phosphorus flame retardants have good flame-retardant effects and are a type of organic halogen-free flame retardants that are widely studied and used. Organic phosphorus flame retardants are mainly derivatives of phosphorus-containing phenanthrene compounds 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and cyclotriphosphazene (CTP) structures. The structural formulas of DOPO and CTP are as follows: Among them, the flame retardant containing the DOPO structure has a relatively low decomposition temperature, resulting in a problem of mismatch with the ambient temperature of the material. Intrinsically flame-retardant epoxy resins mainly introduce the DOPO and CTP structures into the main chain of the epoxy resin molecule through molecular design. Although this method overcomes the defects of externally added flame retardants, there is a problem of complex preparation process for preparing intrinsically flame-retardant epoxy resins by this method.

[0005] Synthesis of phosphonitrilederivative-modified halloysite flameretardantsand their simultaneous enhancement of epoxy resins flame retardancyandmechanical properties,Applied Clay Science,2025(263):107629, recorded that: First, an amino-functionalized cyclotriphosphazene intermediate was prepared, and then the cyclotriphosphazene-modified halloysite flame retardant (HCCP-APTES@Hal) was prepared by hydrolysis condensation with the surface hydroxyl groups of halloysite. HCCP-APTES@Hal was introduced into E44 epoxy resin, and its limiting oxygen index reached up to 27.2%. The prepared flame-retardant epoxy resin only contains cyclotriphosphazene, and the preparation process is complex.

[0006] Preparation and thermal properties of fl ame retardantphthalonitrile / epoxy blends,Sci Eng Compos Mater 19(2012):89–92, recorded that: Bisphenol A bis(phthalonitrile) was introduced into E44 epoxy resin (4,4'-diaminodiphenylsulfone as the curing agent) to improve the flame retardancy of the epoxy resin. When the mass ratio of bisphenol A bis(phthalonitrile) to epoxy resin was 1:1 and the curing temperature reached 270 °C, its limiting oxygen index was 34.5%. The prepared flame-retardant epoxy resin only contains a single nitrogen heteroaromatic ring, and the curing temperature is relatively high.

[0007] "Improving the Heat Resistance and Flame Retardancy of Epoxy Resin Composites by Novel Multifunctional Cyclophosphazene Derivatives", Science and Engineering of Composite Materials, Polymers, 2023, 15, 59, records that: A polycyclic molecule HCCP-SA containing P and N was prepared by the nucleophilic substitution reaction of salicylamide (SA) and hexachlorocyclotriphosphazene. HCCP-SA (with a mass of 20% of E51 type epoxy resin) was used to improve the flame retardancy of epoxy resin, and its limiting oxygen index was 27.1%. In the above flame retardant materials, the dosage of the polycyclic molecule HCCP-SA introduced into the epoxy resin is relatively large. Summary of the Invention

[0008] The purpose of the present invention is to provide a flame retardant epoxy resin, its preparation method and uses to overcome the defects existing in the above-mentioned prior art.

[0009] The present invention solves its technical problems by adopting the following technical solutions.

[0010] The present invention provides a flame retardant epoxy resin, which is a co-crosslinked flame retardant epoxy resin containing cyclotriphosphazene and nitrogen-containing heterocycles, wherein the nitrogen-containing heterocycles include at least one of isoindole ring, triazine ring and phthalocyanine ring.

[0011] The present invention also provides a preparation method of the above-mentioned flame retardant epoxy resin, including: reacting epoxy resin, a branched nitrile resin containing cyclotriphosphazene and secondary amine structure, and a curing agent to obtain the flame retardant epoxy resin.

[0012] The present invention also provides uses of the above-mentioned flame retardant epoxy resin in the fields of electronic information, new energy vehicles and wind power generation.

[0013] The present invention has the following beneficial effects:

[0014] The present invention provides a flame-retardant epoxy resin, its preparation method and uses. The flame-retardant epoxy resin is a co-crosslinked flame-retardant epoxy resin containing cyclotriphosphazene and nitrogen-containing aromatic heterocycles. Among them, the nitrogen-containing aromatic heterocycles include at least one of isoindole ring, triazine ring and phthalocyanine ring. It is obtained by introducing a branched nitrile resin (CTP-PN) containing cyclotriphosphazene and secondary amine structure into the epoxy resin. On the one hand, a crosslinking reaction occurs between the secondary amine structure in CTP-PN and the epoxy groups in the epoxy resin (EP) to form an EP / CTP-PN co-crosslinked resin; on the other hand, a nitrogen-containing aromatic heterocyclic structure is formed through the crosslinking reaction of the cyano groups in CTP-PN. The synergistic flame-retardant effect formed by cyclotriphosphazene and the nitrogen-containing aromatic heterocyclic structure effectively improves the flame-retardant performance of the epoxy resin. In addition, it also has the advantage of a small amount of flame retardant used. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0016] Figure 1 is the synthesis route diagram of the flame-retardant epoxy resin;

[0017] Figure 2 is the DSC curve of the epoxy resin prepolymers of Examples 1-4;

[0018] Figure 3 is the FT-IR diagram of the epoxy resin prepolymers of Examples 1-4;

[0019] Figure 4 is the FT-IR diagram of the cured epoxy resins of Examples 1-4;

[0020] Figure 5 is the TGA curve of the cured epoxy resins of Examples 1-4. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0021] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0022] The following specifically describes a flame-retardant epoxy resin, its preparation method and uses provided by the embodiments of the present invention.

[0023] In a first aspect, an embodiment of the present invention provides a flame-retardant epoxy resin, which is a co-crosslinked flame-retardant epoxy resin containing cyclotriphosphazene and nitrogen-containing aromatic heterocycles, wherein the nitrogen-containing aromatic heterocycles include at least one of isoindole ring, triazine ring and phthalocyanine ring.

[0024] In some optional embodiments, it is obtained by introducing a branched nitrile resin containing cyclotriphosphazene and secondary amine structure into an epoxy resin, wherein: the co-crosslinked structure formed with the epoxy resin is provided by the crosslinking reaction of the secondary amine structure introduced into the branched nitrile resin with the epoxy group;

[0025] Preferably, the epoxy resin is selected from one or more of E51 epoxy resin, AG80 epoxy resin, TDE85 epoxy resin, E44 epoxy resin, E68 epoxy resin, E20 epoxy resin, E44F epoxy resin, E51F epoxy resin and bisphenol AF type epoxy resin;

[0026] Preferably, the structural formula of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is as follows: wherein: R3 is selected from one of the following groups:

[0027]

[0028] The flame-retardant epoxy resin provided in the present invention is obtained by introducing a branched nitrile resin containing cyclotriphosphazene and secondary amine structure into an epoxy resin, wherein: the structure of the epoxy resin is shown as follows:

[0029]

[0030] The structural formula of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure (CTP-PN) is shown as follows:

[0031]

[0032] The flame-retardant resin provided in the present invention is an EP / CTP-PN co-crosslinked resin formed by crosslinking epoxy resin (EP) and a branched nitrile resin (CTP-PN) containing a cyclotriphosphazene and secondary amine structure. The co-crosslinked structure formed with the epoxy resin is provided by the crosslinking reaction between the secondary amine structure introduced into the nitrile resin and the epoxy group. The flame-retardant epoxy resin obtained from the crosslinking reaction is a co-crosslinked resin containing cyclotriphosphazene and nitrogen-containing aromatic heterocycles. Among them, the nitrogen-containing aromatic heterocycles are provided by the crosslinking reaction of phthalonitrile groups or monocyano groups to generate isoindole rings, triazine rings, phthalocyanine rings, etc. These structures are crosslinked with each other to form a complex aromatic heterocyclic network of the cured product, and at the same time, they synergistically flame-retard with cyclotriphosphazene (CTP) containing two flame-retardant elements P and N, so that the co-crosslinked resin formed by cyclotriphosphazene and nitrogen-containing aromatic heterocyclic structures has excellent flame-retardant properties. Compared with the flame-retardant epoxy resin containing only cyclotriphosphazene or the crosslinked resin formed by crosslinking a nitrile resin with an epoxy group using a phthalonitrile structure as the end-capping group, its flame-retardant performance is significantly improved. This is because cyclotriphosphazene and nitrogen-containing aromatic heterocycles form a synergistic flame-retardant effect, jointly improving the flame-retardant performance of the epoxy resin. While synergistically flame-retarding and jointly improving the flame-retardant performance of the epoxy resin, it also has the advantage of a small amount of flame retardant used.

[0033] In some alternative embodiments, the amount of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is 3wt% - 40wt% of the mass of the epoxy resin. When the amount of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is low, the effect of improving the flame-retardant performance of the epoxy resin cannot be achieved, and when the amount is large, the application cost will increase.

[0034] In some alternative embodiments, the oxygen index of the flame-retardant epoxy resin is 23.6% - 41.3%, and the UL-94 vertical burning rating is V-0.

[0035] In a second aspect, an embodiment of the present invention provides a method for preparing the above-mentioned flame-retardant epoxy resin, including: reacting an epoxy resin, a branched nitrile resin containing cyclotriphosphazene and secondary amine structure, and a curing agent to prepare the flame-retardant epoxy resin.

[0036] In some alternative embodiments, it includes the following steps: after uniformly mixing a solution of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure with the epoxy resin, adding a curing agent, performing vacuum drying treatment to obtain an epoxy resin prepolymer, and then taking out the epoxy resin prepolymer, and performing crushing and programmed temperature curing treatment to obtain a cured resin sample.

[0037] In some alternative embodiments, the solution of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is obtained by dissolving the branched nitrile resin containing cyclotriphosphazene and secondary amine structure in a solvent;

[0038] Preferably, the solvent is N,N-dimethylformamide (DMF) or acetonitrile (ACN);

[0039] Preferably, the mass-to-liquid ratio of the branched nitrile resin containing a cyclotriphosphazene and a secondary amine structure to the solvent is 1 g:(0.5 - 2.0) mL.

[0040] In some alternative embodiments, the curing agent is a diamine curing agent, selected from 4,4'-diaminodiphenylmethane (DDM) or 4,4'-diaminodiphenylsulfone (DDS);

[0041] Preferably, the dosage of the diamine curing agent is calculated according to formula (1), where w is the mass of the diamine curing agent required for every 100 parts of epoxy resin; M r is the relative molecular mass of the diamine curing agent; [H] is the number of hydrogen atoms in the diamine curing agent, which is 4 in the diamine curing agent; M is the mass of 100 parts by mass of epoxy resin; m is the epoxy resin equivalent.

[0042]

[0043] In some alternative embodiments, the conditions for the vacuum drying treatment are: the vacuum degree is set to -0.06 MPa to -0.08 MPa, the temperature is 80 - 120 °C, and the drying time is 4 - 6 h. Performing the vacuum drying treatment under the above conditions can reduce the crosslinking degree of the epoxy resin while removing the solvent.

[0044] In some alternative embodiments, the conditions for the programmed temperature curing treatment are: first keep the temperature at 100 - 120 °C for 1 - 2 h, then keep the temperature at 150 - 160 °C for 2 - 3 h, and then keep the temperature at 190 - 210 °C for 2 - 3 h. Performing the programmed temperature curing treatment under the above conditions can control the staged curing of the epoxy group and the phthalonitrile group during the heating process to obtain a more perfect crosslinked network.

[0045] In a third aspect, an embodiment of the present invention provides a use of the above flame-retardant epoxy resin in the fields of electronic information, new energy vehicles, wind power generation, etc.

[0046] The present invention will be further described below in conjunction with embodiments.

[0047] A preparation method of a flame-retardant epoxy resin, the specific steps are as follows:

[0048] (1) Weigh a certain amount of epoxy resin (EP) and place it in a beaker;

[0049] (2) Dissolve the measured branched nitrile resin containing a cyclotriphosphazene and a secondary amine structure (CTP-PN) in a solvent at room temperature to form a homogeneous solution;

[0050] (3) Add the resin solution obtained in step (2) to the epoxy resin in step (1), and stir with a glass rod to form a homogeneous solution;

[0051] (4) Weigh a measured amount of diamine curing agent, add the diamine curing agent to the solution obtained in step (3), and stir with a glass rod to form a homogeneous solution;

[0052] (5) Place the solution obtained in step (4) under vacuum conditions for drying to obtain an epoxy resin prepolymer;

[0053] (6) Take out the epoxy resin prepolymer obtained in step (5), and obtain epoxy resin prepolymer powder after mechanical crushing;

[0054] (7) Place the epoxy resin prepolymer powder obtained in step (6) in a mold, and cure it by programmed temperature rise in a forced air oven to obtain a cured resin spline.

[0055] Among them, the preparation method of CTP-PN resin can be referred to ZL 202310052357.3.

[0056] The dosage of the CTP-PN resin is 3wt%-40wt% of the mass of the epoxy resin.

[0057] The solvent is N,N-dimethylformamide (DMF) or acetonitrile (ACN); the mass-to-liquid ratio of the CTP-PN resin to the solvent is 1g:(0.5-2.0)mL.

[0058] The diamine curing agent is 4,4'-diaminodiphenylmethane (DDM) or 4,4'-diaminodiphenyl sulfone (DDS). The dosage of the diamine curing agent is calculated according to formula 1, where w is the mass of the diamine curing agent required for every 100 parts of epoxy resin; M r is the relative molecular mass of the diamine curing agent; [H] is the number of hydrogen atoms in the diamine curing agent, which is 4 in the diamine curing agent; M is the mass of 100 mass parts of epoxy resin; m is the epoxy resin equivalent.

[0059]

[0060] The conditions for drying in the vacuum oven are: the vacuum degree is -0.06MPa to -0.08MPa, the temperature is 80-120°C, and the drying time is 4-6h.

[0061] The conditions for programmed temperature rise curing are: first keep the temperature at 120°C for 1-2h, then keep the temperature at 150°C for 2-3h, and then keep the temperature at 200°C for 2-3h.

[0062] Example 1:

[0063] Preparation method of flame-retardant epoxy resin, comprising the following steps:

[0064] (1) Weigh 10 g of E51 epoxy resin and place it in a beaker;

[0065] (2) Dissolve 0.5 g of hexakis(4-(3,4-dicyanophenoxy)phenylamino)-cyclotriphosphazene (CTP-PN) in 1 mL of DMF solvent at room temperature to form a homogeneous solution;

[0066] (3) Add the CTP-PN / DMF solution obtained in step (2) to the epoxy resin in step (1), and stir with a glass rod to form a homogeneous solution;

[0067] (4) Weigh 0.3 g of DDS diamine curing agent, add it to the solution obtained in step (3), and stir with a glass rod to form a homogeneous solution;

[0068] (5) Place the solution obtained in step (4) in a vacuum oven at 120 °C to dry and remove the DMF solvent to obtain an epoxy resin prepolymer;

[0069] (6) Take out the epoxy resin prepolymer obtained in step (5), and obtain epoxy resin prepolymer powder after mechanical crushing;

[0070] (7) Place the epoxy resin prepolymer powder obtained in step (6) in a mold, and cure it in a forced-air oven at 120 °C / 1 h - 150 °C / 2 h - 200 °C / 2 h to obtain a cured epoxy resin spline.

[0071] The synthesis route of the above flame-retardant resin is shown in Figure 1 , cyclotriphosphazene (CTP) is an organic-inorganic hybrid material composed of alternating phosphorus and nitrogen atoms, with the properties of both organic and inorganic materials, having good structural stability and high-temperature resistance. At the same time, CTP contains two flame-retardant elements, P and N, and is a flame-retardant unit with excellent flame-retardant performance. Introducing a branched phthalonitrile resin containing cyclotriphosphazene and secondary amine structure (hexakis(4-(3,4-dicyanophenoxy)phenylamino)-cyclotriphosphazene) into epoxy resin, on the one hand, through the secondary amine structure in CTP to cross-link with epoxy groups to form an EP / CTP-PN co-cross-linked resin; on the other hand, through a cyano cross-linking reaction to form a nitrogen-containing aromatic heterocyclic structure.

[0072] Example 2:

[0073] A preparation method of a cyclotriphosphazene and nitrogen-containing aromatic heterocyclic co-cross-linked flame-retardant epoxy resin, the difference from the method steps disclosed in Example 1 is only that the mass of hexakis(4-(3,4-dicyanophenoxy)phenylamino)-cyclotriphosphazene (CTP-PN) is 1 g and the DMF solvent is 2 mL.

[0074] Example 3:

[0075] A preparation method of a crosslinked flame-retardant epoxy resin containing cyclotriphosphazene and nitrogen-containing aromatic heterocycles is different from the method steps disclosed in Example 1 only in that the mass of hexakis(4-(3,4-dicyanophenoxy)phenylamino)-cyclotriphosphazene (CTP-PN) is 2 g and the DMF solvent is 3 mL.

[0076] Example 4:

[0077] A preparation method of a crosslinked flame-retardant epoxy resin containing cyclotriphosphazene and nitrogen-containing aromatic heterocycles is different from the method steps disclosed in Example 1 only in that the mass of hexakis(4-(3,4-dicyanophenoxy)phenylamino)-cyclotriphosphazene (CTP-PN) is 3 g and the DMF solvent is 4 mL.

[0078] Test results:

[0079] The differential scanning calorimetry curves (DSC) and infrared spectra (FT-IR) of the epoxy resin prepolymers of Examples 1-4 are as Figure 2 and Figure 3 shown. It can be seen from Figure 2 that when CTP-PN is 5%, a broad exothermic peak appears at 220 °C corresponding to the curing crosslinking reaction of the epoxy resin. As the content of CTP-PN increases, the exothermic peak of the epoxy resin gradually becomes smaller and at the same time shifts towards a higher temperature. This is because with the increase of the secondary amine structure in the CTP-PN structure, the reaction between the epoxy group and the CTP-PN resin is promoted, forming a co-crosslinked structure. When the CTP-PN content ≥ 10%, an obvious exothermic peak appears at 290 °C, which corresponds to the crosslinking reaction of the cyano group, forming nitrogen-containing aromatic heterocycles such as isoindole rings, triazine rings and phthalocyanine rings. In Figure 3 it can be observed that obvious characteristic absorption peaks of hydroxyl groups, cyano groups and P-NH-Ph appear. The characteristic absorption peak of C-H on the aromatic ring and aliphatic chain can be observed at about 3000 cm -1 -1.

[0080] Figure 4 is the FT-IR diagram of the cured epoxy resins of Examples 1-4. After the resin is cured, the intensity of the hydroxyl absorption peak increases, indicating that the epoxy resin further undergoes a crosslinking reaction. The intensity of the characteristic absorption peak of P-NH-Ph decreases, indicating that the secondary amine structure in CTP-PN undergoes a co-crosslinking reaction with the epoxy resin. The intensity of the cyano absorption peak decreases, and at the same time, the characteristic absorption peaks of isoindole rings and phthalocyanine rings appear, indicating that a nitrogen-containing aromatic heterocyclic structure is formed in the cured epoxy resin.

[0081] Figure 5It is the thermogravimetric curve (TGA) of the cured epoxy resin in Examples 1-4 under a nitrogen atmosphere. It can be seen from the figure that under the synergistic effect of cyclotriphosphazene and nitrogen-containing aromatic heterocycles, the char residue rate of the epoxy resin is effectively increased, and the flame retardancy is improved.

[0082] The flame retardancy of the cured epoxy resin in Examples 1-4 was characterized by the limiting oxygen index (LOI). The LOIs of the cured resins were 23.6%, 29.3%, 35.8% and 41.3% respectively as the content of CTP-PN increased. This is because as the content of CTP-PN increases, the synergistic flame retardant effect formed by cyclotriphosphazene and the nitrogen-containing aromatic heterocyclic structure effectively improves the flame retardancy of the epoxy resin.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A flame-retardant epoxy resin, characterized in that, The flame-retardant epoxy resin is a co-crosslinked flame-retardant epoxy resin containing cyclotriphosphazene and nitrogen-containing aromatic heterocycles, wherein the nitrogen-containing aromatic heterocycles include at least one of isoindole ring, triazine ring and phthalocyanine ring.

2. The flame-retardant epoxy resin according to claim 1, wherein It is obtained by introducing a branched nitrile resin containing cyclotriphosphazene and secondary amine structure into the epoxy resin, wherein: the co-crosslinked structure formed with the epoxy resin is provided by the crosslinking reaction of the secondary amine structure introduced into the branched nitrile resin with the epoxy group; Preferably, the epoxy resin is selected from one or more of: E51 epoxy resin, AG80 epoxy resin, TDE85 epoxy resin, E44 epoxy resin, E68 epoxy resin, E20 epoxy resin, E44F epoxy resin, E51F epoxy resin and bisphenol AF type epoxy resin; Preferably, the structural formula of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is as follows: Wherein: R3 is selected from one of the following groups: Preferably, the amount of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is 3wt% - 40wt% of the mass of the epoxy resin.

3. The flame-retardant epoxy resin according to claim 1 or 2, characterized in that, The oxygen index of the flame-retardant epoxy resin is 23.6% - 41.3%, and the UL-94 vertical burning level is V-0.

4. A method for preparing a flame-retardant epoxy resin according to any one of claims 1-3, characterized in that, It includes: reacting an epoxy resin, a branched nitrile resin containing cyclotriphosphazene and secondary amine structure, and a curing agent to obtain the flame-retardant epoxy resin.

5. The preparation method according to claim 4, characterized in that, It includes the following steps: uniformly mixing a solution of a branched nitrile resin containing cyclotriphosphazene and secondary amine structure with an epoxy resin, adding a curing agent, subjecting to vacuum drying treatment to obtain an epoxy resin prepolymer, then taking out the epoxy resin prepolymer, and subjecting to crushing and programmed temperature curing treatment to obtain a cured resin sample.

6. The preparation method according to claim 5, wherein The amount of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is 3wt% - 40wt% of the mass of the epoxy resin.

7. The preparation method according to claim 5, characterized in that, The solution of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure is obtained by dissolving the branched nitrile resin containing cyclotriphosphazene and secondary amine structure in a solvent; Preferably, the solvent is N,N-dimethylformamide (DMF) or acetonitrile (ACN); Preferably, the mass-to-liquid ratio of the branched nitrile resin containing cyclotriphosphazene and secondary amine structure to the solvent is 1g:(0.5 - 2.0)mL.

8. The preparation method according to claim 5, characterized in that, The curing agent is a diamine curing agent, selected from 4,4'-diaminodiphenylmethane (DDM) or 4,4'-diaminodiphenyl sulfone (DDS); Preferably, the dosage of the diamine curing agent is calculated according to the following formula (1), where w is the mass of the diamine curing agent required for every 100 parts of epoxy resin; M r is the relative molecular mass of the diamine curing agent; [H] is the number of hydrogen atoms in the diamine curing agent, which is 4 in the diamine curing agent; M is the mass of 100 parts by mass of epoxy resin; m is the epoxy resin equivalent; 9. The preparation method according to claim 5, characterized in that, The conditions of the vacuum drying treatment are: the vacuum degree is -0.06MPa to -0.08MPa, the temperature is 80 - 120°C, and the drying time is 4 - 6h. The conditions of the programmed temperature curing treatment are: first keep warm at 100 - 120°C for 1 - 2h, then keep warm at 150 - 160°C for 2 - 3h, and then keep warm at 190 - 210°C for 2 - 3h.

10. Use of the flame-retardant epoxy resin according to any one of claims 1 - 3 in the fields of electronic information, new energy vehicles and wind power generation.

Images