A flame-retardant cyclotriphosphazene epoxy compound, a preparation method and application thereof

Abstract

The application relates to the technical field of thermosetting resin materials, and discloses a flame-retardant cyclotriphosphazene epoxy compound, a preparation method and application thereof, and a structural formula as shown in formula I. The epoxy resin provided by the application is a halogen-free flame-retardant epoxy system, the epoxy curing crosslinking network contains rich cyclotriphosphazene ring structures, DOPO and Schiff bases can improve the flame-retardant performance of the epoxy resin, meanwhile, due to the existence of a large number of rigid benzene ring structures and hexafunctional epoxy structures, the crosslinking density is high, and the epoxy resin has excellent flame-retardant performance and relatively high mechanical strength.

Description

Technical Field

[0001] This invention relates to the field of thermosetting resin materials technology, specifically to a flame-retardant cyclotriphosphazene epoxy compound, its preparation method, and its applications. Background Technology

[0002] Epoxy resin possesses excellent mechanical properties, bonding properties, chemical corrosion resistance, and electrical insulation, making it widely used in composite materials, adhesives, coatings, and electronic packaging. However, epoxy resin is a flammable material with a limiting oxygen index of approximately 19.8%, and its combustion is accompanied by molten dripping, posing a serious fire hazard and limiting its application. Flame retardants are commonly used to modify epoxy resin for flame retardancy; two common types are additive and reactive. Additive flame retardants require a larger dosage. Although organic flame retardants have good compatibility with epoxy resin and can be uniformly dispersed in the resin matrix, their small molecular weight leads to a decrease in the mechanical properties of the resin matrix. Reactive flame retardants can chemically react with the epoxy resin matrix or curing agent, introducing flame-retardant structural units into the cross-linked network structure of the epoxy resin through chemical bonding, thereby improving its flame retardancy while enhancing or maintaining high mechanical properties. Therefore, improving or maintaining high mechanical properties while modifying epoxy resin for flame retardancy has become a research hotspot for flame-retardant epoxy resins.

[0003] Phosphazene and phenanthrene compounds are widely used in the field of epoxy resin flame retardancy due to their high flame retardant efficiency and halogen-free and low-toxicity properties. Their flame retardant mechanism involves the combined action of condensed-phase and gas-phase flame retardancy. In the condensed phase, phosphorus-containing structural units degrade upon heating to generate phosphoric acid and metaphosphoric acid, promoting the formation of a char layer in the resin matrix, thus providing oxygen and heat insulation. In the gas phase, the flame-retardant structural units decompose upon heating to produce non-flammable gases such as nitrogen and ammonia, diluting the concentration of flammable gases like oxygen, while simultaneously releasing phosphorus-oxygen free radicals to quench active flammable free radicals and terminate the combustion chain reaction. Schiff base structures possess high-temperature self-crosslinking properties, forming highly thermally stable nitrogen heterocyclic structures at high temperatures that strengthen the char layer and further enhance the flame-retardant effect of the condensed phase. Therefore, Schiff bases, phosphazene, and phenanthrene groups are highly promising synergistic flame-retardant structural units.

[0004] Patent application CN118344705A discloses a halogen-free flame-retardant epoxy resin and its preparation method. First, an aldehyde-containing cyclotriphosphazene intermediate is synthesized from hydroxy aldehydes and hexachlorocyclotriphosphazene as raw materials. Then, DOPO is reacted with the aldehyde group to obtain an additive halogen-free flame retardant based on phosphazene and phosphaphenanthrene groups, which can be used for flame retardancy of epoxy resins. However, this flame retardant does not form a chemical bond with the epoxy resin matrix, which reduces the mechanical properties of the flame-retardant epoxy resin.

[0005] Patent application CN117820605A discloses a method for preparing flame-retardant epoxy resin, using hydroxy aldehydes and hexachlorocyclotriphosphazene as raw materials to prepare the flame retardant. A diamine curing agent is used, and the aldehyde group of the flame retardant reacts with the amino group of the curing agent to form a Schiff base reaction, thereby embedding the cyclotriphosphazene and Schiff base flame-retardant structural units into the polymer long chain to achieve flame retardancy of the epoxy resin. However, the Schiff base reaction between the aldehyde group of the flame retardant and the amino group of the curing agent releases small molecule water, requiring dehydration treatment during preparation. This can easily lead to the formation of pores in the resin, thus affecting the mechanical properties of the resin.

[0006] Therefore, developing highly efficient flame-retardant intrinsically flame-retardant epoxy resins or reactive flame retardants, and preparing halogen-free flame-retardant epoxy resins that not only have high flame retardancy but also improve or maintain high mechanical properties, is of great significance for expanding their application fields. Summary of the Invention

[0007] This invention addresses the flammability problem of epoxy resins by providing a novel flame-retardant cyclotriphosphazene epoxy compound. This compound can be used alone as an intrinsically flame-retardant epoxy monomer, or as a reactive epoxy flame retardant, and can be cured with a curing agent to prepare flame-retardant epoxy resins, while also possessing excellent flame-retardant properties and mechanical strength.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A flame-retardant cyclotriphosphazene epoxy compound having the general formula shown in Formula I: (I) R1 to R6 are independently selected from any one of the structures shown in ① to ④: In equations ① to ④, R is H, -CH3, or -OCH3.

[0009] This invention also provides a method for preparing the flame-retardant cyclotriphosphazene epoxy compound, wherein when all R1 to R6 are selected from ① or ③, the compound contains only cyclotriphosphazene and Schiff base structures, and the preparation process includes the following steps: Step 1: Hexa(4-aminophenoxy)cyclotriphosphazene is mixed with a hydroxy aldehyde compound as shown in Formula I-1 in a solvent and reacted to obtain hexahydroxycyclotriphosphazene with a Schiff base structure as shown in Formula I-2. Step 2: The compound shown in Formula I-2 is mixed with epichlorohydrin and reacted. After the reaction system is homogeneous, a phase transfer catalyst is added to continue the reaction. Then, an inorganic base is added dropwise to continue the reaction. After purification, the flame-retardant cyclotriphosphazene epoxy compound is obtained. When at least one of R1 to R6 is selected from ② or ④, the compound simultaneously contains a cyclotriphosphazene, a Schiff base, and a DOPO structure, and the preparation process includes the following steps: Step 1: Hexa(4-aminophenoxy)cyclotriphosphazene is mixed and reacted with a hydroxy aldehyde compound as shown in Formula I-1 in a solvent, and then treated to obtain hexahydroxycyclotriphosphazene with a Schiff base structure as shown in Formula I-2. Steps 1-2: The compound shown in Formula I-2 is mixed with DOPO and refluxed to react, and the product is purified to obtain the compound shown in Formula I-3; Step 2: Mix the compound shown in Formula I-3 with epichlorohydrin and react. After the reaction system is homogeneous, add a phase transfer catalyst to continue the reaction. Then add an inorganic base dropwise to continue the reaction. After purification, the flame-retardant cyclotriphosphazene epoxy compound is obtained. R1'~R6' are selected from any one of the structures shown in ⑤~⑧, and at least one of them is ⑥ or ⑧: Wherein, R is as described above.

[0010] The reaction in step 1 is carried out at 50~90℃ for 4~15 hours; In step 2, after adding epichlorohydrin, the reaction is carried out at 60-110℃ until the system is a homogeneous phase; after adding a phase transfer catalyst, the reaction continues for 6-15 hours; after adding an inorganic base, the reaction is carried out at 50-90℃ for 2-10 hours. In step 2, the molar ratio of the compound shown in formula I-2 or I-3 to epichlorohydrin is 1:75~120. In step 1-2, the molar ratio of the compound shown in Formula I-2 to DOPO is 1:1-6. During this process, some Schiff bases are grafted with DOPO reactive groups onto carbon. When the molar ratio of the compound shown in Formula I-2 to DOPO is 1:6, all Schiff base structures are grafted with DOPO groups, and the compound exhibits higher flame retardant properties, but the mechanical toughness and flexural strength are somewhat affected, while the overall performance remains excellent.

[0011] The reaction temperature in steps 1-2 is 100~115℃, and the reaction time is 8-36h.

[0012] The hydroxy aldehyde compound shown in Formula I-1 is one of p-hydroxybenzaldehyde, m-hydroxybenzaldehyde, vanillin, syringin, and 2-methyl-4-hydroxybenzaldehyde; The solvent used in the reaction process is at least one of 1,4-dioxane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, and methylpyrrolidone; The phase transfer catalyst is tetrabutylammonium bromide, benzyltriethylammonium chloride, hexadecyltrimethylammonium bromide, or triphenylphosphine.

[0013] This invention also provides an intrinsically flame-retardant epoxy resin, composed of the aforementioned flame-retardant cyclotriphosphazene epoxy compound and a curing agent. The flame-retardant cyclotriphosphazene epoxy compound is an epoxy compound containing a Schiff base and cyclotriphosphazene groups, or an epoxy compound containing a Schiff base, cyclotriphosphazene, and DOPO groups, or an epoxy compound containing cyclotriphosphazene and DOPO groups, all exhibiting excellent intrinsic flame retardancy and can be directly used as a cured epoxy resin.

[0014] The present invention also provides a flame-retardant epoxy resin, comprising an epoxy resin, the flame-retardant cyclotriphosphazene epoxy compound, and a curing agent; wherein in the flame-retardant cyclotriphosphazene epoxy compound, at least one of R1 to R6 is selected from the structure shown in ② or ④.

[0015] Ungrafted epoxy compounds contain only Schiff bases and cyclotriphosphazene groups. When cured as intrinsic epoxy resins alone, they exhibit excellent properties. However, when used as flame retardant additives mixed with conventional non-flame-retardant epoxy resin matrices, the flame retardant performance is generally poor at concentrations below 15%. In contrast, DOPO-grafted epoxy compounds, at the same concentration, produce resin materials that combine both flame retardant and mechanical properties.

[0016] In terms of the flame-retardant mechanism, during combustion, the epoxy compound of this invention releases phosphorus-containing free radicals from the phosphazene and phosphaphenanthrene groups in the gas phase. These free radicals combine with active flammable free radicals in the flame, terminating the combustion chain reaction. Simultaneously, non-flammable gases such as ammonia and nitrogen are generated during pyrolysis, diluting the concentration of oxygen and flammable gases. In the condensed phase, the phosphoric acid and metaphosphoric acid generated by the pyrolysis of the phosphazene and phosphaphenanthrene structures catalyze the carbonization of the epoxy resin matrix, forming a char layer. Furthermore, the Schiff base structure self-crosslinks at high temperatures to form a highly thermally stable nitrogen heterocyclic structure, which further strengthens the char layer, isolates oxygen and heat transfer, and inhibits the volatilization of pyrolysis products, thereby improving the flame-retardant efficiency.

[0017] The epoxy resin includes epoxy resins commonly found in the prior art, such as bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, hydrogenated bisphenol A type epoxy resin, phenolic epoxy resin, etc.

[0018] The curing agent includes one or more of the following: 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenylmethane, bis(nitrile)amine, maleic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, and methylhexahydrophthalic anhydride. The curing agent is added according to a molar ratio of 1:0.5-1.5 between the epoxy groups in the epoxy compound and the active hydrogen in the curing agent.

[0019] The flame-retardant cyclotriphosphazene epoxy compound accounts for 5-30% of the total raw materials.

[0020] The present invention also provides the intrinsic flame-retardant epoxy resin or a method for preparing the flame-retardant epoxy resin, comprising the steps of: dissolving the flame-retardant cyclotriphosphazene epoxy compound, epoxy resin and curing agent in a solvent, removing the solvent by vacuum distillation, and obtaining the flame-retardant epoxy resin by degassing and curing.

[0021] Preferably, the curing temperature is 120-200℃.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) The epoxy resins provided by the present invention are all halogen-free flame-retardant epoxy systems. The epoxy curing network contains abundant phosphazene ring structures, DOPO and Schiff bases, which can improve the flame retardant properties of epoxy resins. At the same time, due to the presence of a large number of rigid benzene ring structures and hexafunctional epoxy structures, the crosslinking density is high, resulting in excellent flame retardant properties and high mechanical strength.

[0023] (3) The epoxy resin crosslinking network provided by the present invention contains abundant phosphorus and nitrogen elements. The synergistic flame retardant effect of phosphorus and nitrogen gives the epoxy curing system excellent flame retardant properties, including excellent flame retardant effects in oxygen index, vertical burning and anti-dripping at multiple angles. Moreover, the method for preparing cyclotriphosphazene epoxy compounds is simple to operate, easy to control and has a high yield. Attached Figure Description

[0024] Figure 1 The 1H NMR spectrum of CH3O-HCCP-6CN-EP, a cyclotriphosphazene epoxide compound containing a Schiff base, prepared in Example 1.

[0025] Figure 2 The phosphorus NMR spectrum of CH3O-HCCP-6CN-EP, a cyclotriphosphazene epoxide compound containing a Schiff base, prepared in Example 1. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Modifications or equivalent substitutions made by those skilled in the art based on their understanding of the technical solutions of this invention, without departing from the spirit and scope of the invention, should be covered within the protection scope of this invention.

[0027] The raw materials used in the following specific embodiments were all purchased from the market: bisphenol A type epoxy resin, brand name EP CYD-128, epoxy value 0.51 mol / 100 g, China Petroleum & Chemical Corporation; 4,4'-diaminodiphenyl sulfone (DDS), analytical grade, purity 99.0%, Sinopharm Chemical Reagent Co., Ltd.

[0028] Example 1 Preparation of cyclotriphosphazene epoxy compound (CH3O-HCCP-6CN-EP) and flame-retardant epoxy resin (CH3O-HCCP-6CN-EP / DDS and DGEBA / CH3O-HCCP-6CN-EP (15%) / DDS) containing Schiff base structure.

[0029] Step 1: Hexa(4-aminophenoxy)cyclotriphosphazene (51 g, 0.065 mol) and vanillin (59.28 g, 0.39 mol) were mixed and dissolved in 500 mL of 1,4-dioxane. The mixture was stirred and reacted at 90 °C for 10 h. After the reaction was complete, the product was precipitated with deionized water, then vacuum filtered, washed, and dried to obtain 94.34 g of hexahydroxycyclotriphosphazene containing a Schiff base structure, denoted as CH3O-HCCP-6CN-OH, with a yield of 91.4%.

[0030] Step 2: CH3O-HCCP-6CN-OH (60.34 g, 0.038 mol) was dissolved in 266 mL of epichlorohydrin. The mixture was heated at 90 °C with stirring until a homogeneous solution was formed. Then, 3 g of tetrabutylammonium bromide was added and the reaction was allowed to proceed for 10 h. Subsequently, the temperature was lowered to room temperature, and 40 wt% sodium hydroxide aqueous solution (27 g, 0.675 mol) was added dropwise. After the addition was complete, the reaction was carried out at 60 °C for 6 h. After the reaction was completed, the reaction solution was washed several times with deionized water until neutral. Then, residual epichlorohydrin was removed by rotary evaporation to obtain 62 g of a cyclotriphosphazene epoxy compound containing a Schiff base structure, denoted as CH3O-HCCP-6CN-EP, with a yield of 85%.

[0031] Step 3: Dissolve 27 g of CH3O-HCCP-6CN-EP in 25 mL of 1,4-dioxane, add 5.22 g of curing agent 4,4'-diaminodiphenyl sulfone (DDS), mix and heat and stir until a homogeneous solution is formed, then remove the solvent by vacuum distillation, pour the resulting mixture into a mold for vacuum degassing, and mold according to the curing regime of 180℃ / 2h, 200℃ / 2h to obtain flame-retardant epoxy resin, denoted as CH3O-HCCP-6CN-EP / DDS.

[0032] Step 4: Dissolve 15 g of CH3O-HCCP-6CN-EP in 20 mL of 1,4-dioxane, add 64.52 g of bisphenol A epoxy resin (DGEBA) and 20.48 g of 4,4'-diaminodiphenyl sulfone (DDS), mix and heat and stir until a homogeneous solution is formed, then remove the solvent by vacuum distillation, pour the resulting mixture into a mold for vacuum degassing, and mold according to the curing regime of 180℃ / 2h, 200℃ / 2h, to obtain a flame-retardant epoxy resin with CH3O-HCCP-6CN-EP added at 15 wt%, denoted as DGEBA / CH3O-HCCP-6CN-EP (15%) / DDS.

[0033] Example 2 Prepare cyclotriphosphazene epoxy compounds (CH3O-HCCP-6DPCN-EP) and flame-retardant epoxy resins (CH3O-HCCP-6DPCN-EP / DDS, DGEBA / CH3O-HCCP-6DPCN-EP(5%) / DDS, DGEBA / CH3O-HCCP-6DPCN-EP(10%) / DDS and DGEBA / CH3O-HCCP-6DPCN-EP(15%) / DDS) containing DOPO structure.

[0034] Step 1: Hexa(4-aminophenoxy)cyclotriphosphazene (51 g, 0.065 mol) and vanillin (59.28 g, 0.39 mol) were mixed and dissolved in 500 mL of 1,4-dioxane. The mixture was stirred and reacted at 90 °C for 10 h. After the reaction was complete, the product was precipitated with deionized water, then vacuum filtered, washed, and dried to obtain 94.34 g of hexahydroxycyclotriphosphazene containing a Schiff base structure, denoted as CH3O-HCCP-6CN-OH, with a yield of 91.4%.

[0035] Step 2: The reactants were added at a molar ratio of CH3O-HCCP-6CN-OH to DOPO of 1:6. CH3O-HCCP-6CN-OH (31.76 g, 0.02 mol) and DOPO (25.92 g, 0.12 mol) were mixed and dissolved in 500 mL of 1,4-dioxane. The mixture was refluxed at 110 °C for 30 h. After the reaction was complete, the product was precipitated with ethanol, then vacuum filtered, washed, and dried to obtain 54.30 g of a hexahydroxycyclotriphosphazene containing the DOPO structure, designated CH3O-HCCP-6DPCN-OH, with a yield of 94.1%.

[0036] Step 3: CH3O-HCCP-6DPCN-OH (57.7 g, 0.02 mol) was dissolved in 180 mL of epichlorohydrin. The mixture was heated at 90 °C with stirring until a homogeneous solution was formed. Then, 3 g of tetrabutylammonium bromide was added and the reaction proceeded for 15 h. The temperature was then lowered to room temperature, and 40 wt% sodium hydroxide aqueous solution was added dropwise. After the addition was complete, the reaction proceeded at 60 °C for 10 h. After the reaction was complete, the reaction solution was washed several times with deionized water until neutral. Residual epichlorohydrin was then removed by rotary evaporation, yielding 52 g of a cyclotriphosphazene epoxy compound containing a DOPO structure, designated CH3O-HCCP-6DPCN-EP, with a yield of 80.7%.

[0037] Step 4: Dissolve 30 g of CH3O-HCCP-6DPCN-EP in 25 mL of 1,4-dioxane, add 3.5 g of 4,4'-diaminodiphenyl sulfone, mix and heat and stir until a homogeneous solution is formed, then remove the solvent by vacuum distillation, pour the resulting mixture into a mold for vacuum degassing, and mold according to the curing regime of 180℃ / 2h, 200℃ / 2h to obtain flame-retardant epoxy resin, denoted as CH3O-HCCP-6DPCN-EP / DDS.

[0038] Step 5: Dissolve 5 g of CH3O-HCCP-6DPCN-EP in 10 mL of 1,4-dioxane, add 72.12 g of DGEBA and 22.88 g of 4,4'-diaminodiphenyl sulfone, mix and heat and stir until a homogeneous solution is formed, then remove the solvent by vacuum distillation, pour the resulting mixture into a mold for vacuum degassing, and mold according to the curing regime of 180℃ / 2h, 200℃ / 2h, to obtain a flame-retardant epoxy resin with CH3O-HCCP-6DPCN-EP added at 5 wt%, denoted as DGEBA / CH3O-HCCP-6DPCN-EP (5%) / DDS. Flame-retardant epoxy resins with CH3O-HCCP-6DPCN-EP addition amounts of 10 wt% and 15 wt% were prepared according to this method, and were denoted as DGEBA / CH3O-HCCP-6DPCN-EP(10%) / DDS and DGEBA / CH3O-HCCP-6DPCN-EP(15%) / DDS.

[0039] Example 3 Prepare a cyclotriphosphazene epoxy compound (CH3O-HCCP-5CN-1DPCN-EP) containing Schiff base and DOPO structure in a molar ratio of 5:1 and a flame-retardant epoxy resin (DGEBA / CH3O-HCCP-5CN-1DPCN-EP (15%) / DDS).

[0040] Step 1: Feed the materials at a molar ratio of CH3O-HCCP-6CN-OH to DOPO of 1:1. The rest of the process is the same as step 2 in Example 2. The resulting hexahydroxycyclotriphosphazene containing Schiff base and DOPO structure is denoted as CH3O-HCCP-5CN-1DPCN-OH, with a yield of 95.4%.

[0041] Step 2: React CH3O-HCCP-5CN-1DPCN-OH with epichlorohydrin, following the same preparation process as step 3 in Example 2, to obtain a cyclotriphosphazene epoxy compound containing a Schiff base and DOPO structure, denoted as CH3O-HCCP-5CN-1DPCN-EP, with a yield of 84.5%.

[0042] Step 3: Same as step 5 in Example 2, prepare a flame-retardant epoxy resin with CH3O-HCCP-5CN-1DPCN-EP added at 15 wt%, denoted as DGEBA / CH3O-HCCP-5CN-1DPCN-EP(15%) / DDS.

[0043] Example 4 Prepare a cyclotriphosphazene epoxy compound (CH3O-HCCP-4CN-2DPCN-EP) with a Schiff base and DOPO structure molar ratio of 4:2 and a flame-retardant epoxy resin (DGEBA / CH3O-HCCP-4CN-2DPCN-EP (15%) / DDS).

[0044] Step 1: Feed the materials at a molar ratio of CH3O-HCCP-6CN-OH to DOPO of 1:2. The rest of the process is the same as step 2 in Example 2. The resulting hexahydroxycyclotriphosphazene containing Schiff base and DOPO structure is designated as CH3O-HCCP-4CN-2DPCN-OH, with a yield of 95.1%.

[0045] Step 2: React CH3O-HCCP-4CN-2DPCN-OH with epichlorohydrin, and prepare the product using the same procedure as in Step 3 of Example 2. The resulting cyclotriphosphazene epoxide containing Schiff base and DOPO structure is designated CH3O-HCCP-4CN-2DPCN-EP, with a yield of 83.7%.

[0046] Step 3: Same as step 5 in Example 2, prepare a flame-retardant epoxy resin with CH3O-HCCP-4CN-2DPCN-EP addition of 15 wt%, denoted as DGEBA / CH3O-HCCP-4CN-2DPCN-EP(15%) / DDS.

[0047] Example 5 Prepare a cyclotriphosphazene epoxy compound (CH3O-HCCP-3CN-3DPCN-EP) with a Schiff base and DOPO structure molar ratio of 3:3 and a flame-retardant epoxy resin (DGEBA / CH3O-HCCP-3CN-3DPCN-EP (15%) / DDS).

[0048] Step 1: The feed was prepared with CH3O-HCCP-6CN-OH and DOPO in a molar ratio of 1:3. Other steps were the same as in Step 2 of Example 2. The resulting hexahydroxycyclotriphosphazene containing Schiff base and DOPO structure was designated CH3O-HCCP-3CN-3DPCN-OH with a yield of 94.6%.

[0049] Step 2: React CH3O-HCCP-3CN-3DPCN-OH with epichlorohydrin. The preparation process is the same as step 3 in Example 2. The resulting cyclotriphosphazene epoxy compound containing Schiff base and DOPO structure is designated CH3O-HCCP-3CN-3DPCN-EP, with a yield of 82.7%.

[0050] Step 3: Same as step 5 in Example 2, prepare a flame-retardant epoxy resin with CH3O-HCCP-3CN-3DPCN-EP addition of 15 wt%, denoted as DGEBA / CH3O-HCCP-3CN-3DPCN-EP(15%) / DDS.

[0051] Example 6 Prepare a cyclotriphosphazene epoxy compound (CH3O-HCCP-2CN-4DPCN-EP) with a Schiff base and DOPO structure molar ratio of 2:4 and a flame-retardant epoxy resin (DGEBA / CH3O-HCCP-2CN-4DPCN-EP (15%) / DDS).

[0052] Step 1: The feed was prepared with CH3O-HCCP-6CN-OH and DOPO in a molar ratio of 1:4. Other steps were the same as in Step 2 of Example 2. The resulting hexahydroxycyclotriphosphazene containing Schiff base and DOPO structure was designated CH3O-HCCP-2CN-4DPCN-OH with a yield of 93.7%.

[0053] Step 2: React CH3O-HCCP-2CN-4DPCN-OH with epichlorohydrin. The preparation process is the same as step 3 in Example 2. The resulting cyclotriphosphazene epoxy compound containing Schiff base and DOPO structure is designated CH3O-HCCP-2CN-4DPCN-EP, with a yield of 81.4%.

[0054] Step 3: Same as step 5 in Example 2, prepare a flame-retardant epoxy resin with CH3O-HCCP-2CN-4DPCN-EP added at 15 wt%, denoted as DGEBA / CH3O-HCCP-2CN-4DPCN-EP(15%) / DDS.

[0055] Example 7 Prepare a cyclotriphosphazene epoxy compound (CH3O-HCCP-1CN-5DPCN-EP) with a Schiff base and DOPO structure molar ratio of 1:5 and a flame-retardant epoxy resin (DGEBA / CH3O-HCCP-1CN-5DPCN-EP (15%) / DDS).

[0056] Step 1: The feed was prepared with CH3O-HCCP-6CN-OH and DOPO in a molar ratio of 1:5. Other steps were the same as in Step 2 of Example 2. The resulting hexahydroxycyclotriphosphazene containing Schiff base and DOPO structure was designated CH3O-HCCP-1CN-5DPCN-OH with a yield of 92.1%.

[0057] Step 2: React CH3O-HCCP-1CN-5DPCN-OH with epichlorohydrin, following the same preparation process as step 3 in Example 2, to obtain a cyclotriphosphazene epoxy compound containing Schiff base and DOPO structure, denoted as CH3O-HCCP-1CN-5DPCN-EP, with a yield of 81.2%.

[0058] Step 3: Same as step 5 in Example 2, prepare a flame-retardant epoxy resin with CH3O-HCCP-1CN-5DPCN-EP added at 15 wt%, denoted as DGEBA / CH3O-HCCP-1CN-5DPCN-EP (15%) / DDS.

[0059] Comparative Example 1 25 g of bisphenol A epoxy resin and 7.9 g of 4,4'-diaminodiphenyl sulfone curing agent were heated to melt and stirred evenly. The mixture was then placed in an oven and vacuumed to remove air bubbles. The mixture was then poured into a mold and cured at 180℃ for 2 hours and 200℃ for 2 hours. The cured resin was denoted as DGEBA / DDS.

[0060] The epoxy resin systems prepared in the examples and comparative examples were subjected to flexural performance, limiting oxygen index, and vertical burning tests. The test standards were as follows: oxygen index was tested according to GB / T 2406.2; horizontal and vertical burning was tested according to GB / T 2408; flexural strength and flexural modulus of elasticity were tested according to GB / T 9341. The test results are shown in Tables 1 and 2.

[0061] Table 1. Flame retardant and mechanical properties of the epoxy resins prepared in Examples 1-2 and Comparative Example 1. As shown in Table 1, the Schiff base-containing cyclotriphosphazene epoxy compound CH3O-HCCP-6CN-EP in Example 1 exhibits excellent flame retardant and mechanical properties after curing with DDS. However, when added as a reactive flame retardant at 15 wt% to DGEBA / DDS, it did not achieve the required flame retardant rating; only the limiting oxygen index was slightly improved, and the phenomenon of dripping melt after ignition was overcome. Therefore, it is evident that direct cross-linking of cyclotriphosphazene epoxy compounds containing Schiff bases with curing agents can yield epoxy resins with excellent flame retardant and mechanical properties.

[0062] The DOPO-containing cyclotriphosphazene epoxy compound CH3O-HCCP-6DPCN-EP in Example 2 is based on CH3O-HCCP-6CN-EP in Example 1, with the introduction of six DOPO groups. After direct crosslinking of CH3O-HCCP-6DPCN-EP as an epoxy monomer with DDS, the limiting oxygen index reached a maximum of 37.5%. Because the epoxy value of CH3O-HCCP-6DPCN-EP is relatively low and the steric hindrance of DOPO significantly reduces the crosslinking density of the epoxy network, the mechanical properties of CH3O-HCCP-6DPCN-EP / DDS are slightly lower than those of CH3O-HCCP-6CN-EP / DDS, but still slightly higher than those of DGEBA / DDS. Adding CH3O-HCCP-6DPCN-EP as a reactive flame retardant to DGEBA / DDS increases both its flame retardant and mechanical properties with increasing addition amount. Therefore, cyclotriphosphazene epoxy compounds containing the DOPO structure are more suitable as reactive flame retardants to be compounded with epoxy resins to prepare flame-retardant epoxy resins with excellent comprehensive performance.

[0063] Table 2 Flame retardant and mechanical properties of the cyclotriphosphazene flame retardant epoxy resin systems prepared in Examples 1-7 As shown in Table 2, the epoxy system prepared by compounding epoxy compounds containing DOPO and Schiff base structures with epoxy resin as reactive flame retardants exhibits excellent flame retardant and mechanical properties at an addition amount of 15 wt%. When the DOPO content in the flame retardant increases, the limiting oxygen index of the flame-retardant resin increases, but the flexural strength decreases. This is mainly because the steric hindrance of the DOPO structural units is relatively large, reducing the crosslinking density of the crosslinking network.

[0064] To illustrate the relevant properties of the flame-retardant epoxy resin provided by this invention, the accompanying drawings are provided.

[0065] Figure 1This is the 1H NMR spectrum of the cyclotriphosphazene epoxy compound CH3O-HCCP-6CN-EP containing a Schiff base, provided in Example 1. The methoxy proton absorption peak is at 3.75 ppm, the proton absorption peak is at 8.36 ppm, the aromatic hydrogen absorption peaks are at 6.79–7.48 ppm on the two benzene rings connected by the imine bond, methylene proton absorption peaks are at 3.85 ppm and 4.34 ppm, and proton absorption peaks are at 2.73 ppm, 2.88 ppm, and 3.37 ppm on the epoxy group.

[0066] Figure 2 The phosphorus NMR spectrum of the Schiff base-containing cyclotriphosphazene epoxide CH3O-HCCP-6CN-EP provided in Example 1 is shown. A single peak at a chemical shift of 9.52 ppm indicates that there is only one type of P atom in the chemical environment. Both the proton and phosphorus NMR spectra demonstrate the successful synthesis of the Schiff base-containing cyclotriphosphazene epoxide CH3O-HCCP-6CN-EP.

[0067] The above description is merely a preferred embodiment of the present invention and is 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 within the scope of protection of the present invention.

Claims

1. A flame-retardant epoxy resin, characterized in that, It includes epoxy resin, flame-retardant cyclotriphosphazene epoxy compound, and curing agent; in the flame-retardant cyclotriphosphazene epoxy compound, at least one of R1 to R6 is selected from the structure shown in ② or ④; the flame-retardant cyclotriphosphazene epoxy compound accounts for 15-30% of the total raw materials by mass; The flame-retardant cyclotriphosphazene epoxy compound has the general formula shown in Formula I: （I） R1 to R6 are independently selected from any one of the structures shown in ① to ④, and at least one of R1 to R6 is selected from ② or ④: In equations ① to ④, R stands for -OCH3.

2. The flame-retardant epoxy resin according to claim 1, characterized in that, The curing agent is one or more of 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenylmethane, bis(nitrile)amine, maleic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, and methylhexahydrophthalic anhydride.

3. The flame-retardant epoxy resin according to claim 1, characterized in that, The preparation method of the flame-retardant cyclotriphosphazene epoxy compound includes the following steps: Step 1: Hexa(4-aminophenoxy)cyclotriphosphazene is mixed and reacted with a hydroxy aldehyde compound as shown in Formula I-1 in a solvent, and then treated to obtain hexahydroxycyclotriphosphazene with a Schiff base structure as shown in Formula I-2. Steps 1-2: The compound shown in Formula I-2 is mixed with DOPO and refluxed to react, and the product is purified to obtain the compound shown in Formula I-3; Step 2: Mix the compound shown in Formula I-3 with epichlorohydrin and react. After the reaction system is homogeneous, add a phase transfer catalyst to continue the reaction. Then add an inorganic base dropwise to continue the reaction. After purification, the flame-retardant cyclotriphosphazene epoxy compound is obtained. R1'~R6' are selected from any one of the structures shown in ⑤~⑧, and at least one of them is ⑥ or ⑧: Wherein, R is as described in claim 1.

4. The flame-retardant epoxy resin according to claim 3, characterized in that, The reaction in step 1 is carried out at 50~90℃ for 4~15 hours; In step 2, after adding epichlorohydrin, the reaction is carried out at 60-110℃ until the system is a homogeneous phase; after adding a phase transfer catalyst, the reaction continues for 6-15 hours; after adding an inorganic base, the reaction is carried out at 50-90℃ for 2-10 hours. In step 1-2, the molar ratio of the compound shown in formula I-2 to DOPO is 1:1-6; The reaction temperature in steps 1-2 is 100~115℃, and the reaction time is 8-36h.

5. The flame-retardant epoxy resin according to claim 3, characterized in that, The hydroxy aldehyde compound shown in Formula I-1 is vanillin; The solvent used in the reaction process is at least one of 1,4-dioxane, dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, and methylpyrrolidone; The phase transfer catalyst is tetrabutylammonium bromide, benzyltriethylammonium chloride, hexadecyltrimethylammonium bromide, or triphenylphosphine.

6. The flame-retardant epoxy resin according to claim 1, wherein the amount of curing agent is added according to a molar ratio of epoxy groups in the epoxy compound to active hydrogen in the curing agent of 1:0.5-1.

5.

7. The method for preparing the flame-retardant epoxy resin according to claim 1, characterized in that, The process includes the following steps: dissolving the flame-retardant cyclotriphosphazene epoxy compound, epoxy resin, and curing agent in a solvent, removing the solvent by vacuum distillation, and then curing by degassing to obtain the flame-retardant epoxy resin.

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