A DOPO-based cyclotriphosphazene halogen-free flame retardant and its preparation method and application

Abstract

The present invention belongs to the technical field of flame retardant materials, and specifically relates to a DOPO-based cyclotriphosphazene halogen-free flame retardant, a preparation method thereof, and its application in epoxy resin. The present invention is based on hexachlorocyclotriphosphazene, and obtains a halogen-free flame retardant containing phosphorus and nitrogen elements through a nucleophilic substitution reaction between hexachlorocyclotriphosphazene and a phenolic compound containing an aldehyde group and an esterification reaction between hexachlorocyclotriphosphazene and a DOPO-based acid anhydride compound. The epoxy resin modified by the flame retardant of the present invention can significantly improve the flame retardant properties and mechanical properties of the epoxy resin. The flame retardant epoxy resin prepared based on the flame retardant; in terms of the thermal properties of the epoxy resin cured product (thermal decomposition temperature, T g ) can significantly improve its smoke suppression and charring performance and mechanical properties with little influence, and at the same time has excellent flame retardant efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of flame retardant materials, and in particular relates to a DOPO-based cyclotriphosphazene halogen-free flame retardant, a preparation method thereof, and application thereof in epoxy resin. Background Art

[0002] Traditional halogenated flame retardants are widely used in polymer flame retardant applications due to their high flame retardant efficiency, low dosage, and cost-effectiveness. However, since halogenated flame retardants can release toxic gases and smoke during combustion, posing serious risks to human health and the environment, there is a growing desire to develop safe and environmentally friendly halogen-free flame retardants. Among halogen-free flame retardants for epoxy resin (EP), phosphorus-based flame retardants offer advantages such as low toxicity, long-lasting flame retardancy, low ignition rates during degradation, and excellent re-ignition resistance, making them widely used in epoxy resins. EP phosphorus-based flame retardants include red phosphorus, hexachlorocyclotriphosphazene (HCCP), and DOPO. Hexachlorocyclotriphosphazene, with its six active chlorine atoms, can undergo substitution reactions with various nucleophiles to produce a variety of derivatives with diverse properties and functions. These compounds, containing the highly effective flame-retardant elements phosphorus and nitrogen and exhibiting a synergistic flame retardant effect, have attracted widespread attention as flame retardants.

[0003] When heated, cyclotriphosphazene derivatives decompose to produce non-flammable gases such as NH3 and N2. These gases can dilute the concentration of combustible and combustion-supporting gases near the surface of the material and absorb a large amount of energy required for the reaction, thereby reducing the surface temperature of the material. At the same time, during the decomposition process, cyclotriphosphazene derivatives also produce compounds containing structural units such as PO bonds or POC bonds. These compounds can catalyze further degradation of the matrix and promote the formation of a dense and stable carbonized layer on the surface of the material. At the same time, they isolate the interior of the matrix from the invasion of external air and flames and inhibit dehydration, cracking, and oxidation reactions within the matrix. However, cyclotriphosphazene flame retardants also have the problem of poor compatibility with the resin matrix, which leads to a serious reduction in the mechanical properties of the composite material.

[0004] Another common, highly effective phosphorus-based flame retardant material is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), one of the most important phosphorus-based flame retardants. DOPO is widely used as a flame retardant intermediate because the PH bond in DOPO can react with various functional groups, such as double bonds, amino groups, epoxy groups, carbonyl groups, and halogens, converting into different types of bonds, such as PC, PO, and PN. These different chemical bonds have varying thermal and flame retardant properties, leading to its use as a raw material for epoxy resin flame retardants. DOPO has high flame retardant efficiency and is environmentally friendly, but DOPO-based flame retardants exhibit low thermal stability, which affects their flame retardant effectiveness. Furthermore, their compatibility with the resin matrix is ​​less than ideal, adversely affecting the mechanical properties of the composite material.

[0005] Therefore, it is necessary to develop a flame retardant with excellent flame retardancy, good thermal stability, little effect on the mechanical properties of materials, and at the same time non-toxic and low smoke. Summary of the Invention

[0006] To address the problems of existing halogen-containing flame retardants producing toxic gases and large amounts of smoke, as well as the poor flame retardant effect of DOPO-based flame retardants and their significant impact on mechanical properties, a novel and highly efficient DOPO-based cyclotriphosphazene halogen-free flame retardant preparation method and application are provided. This DOPO-based cyclotriphosphazene halogen-free flame retardant combines the structures of DOPO and cyclotriphosphazene, exhibiting excellent thermal stability, a high char formation rate, excellent flame retardant effectiveness, and significantly improved mechanical properties.

[0007] In order to solve the problems of the prior art, the present invention provides the following technical solutions:

[0008] The first aspect of the present invention is to provide a DOPO-based cyclotriphosphazene halogen-free flame retardant, the molecular structure of which is shown in formula (I):

[0009]

[0010] X1 and X2 are independently selected from H or methoxy; X3 is selected from H or methyl.

[0011] Under different raw material ratios, different numbers of X1, X2, and X3 in R1-R5 are substituted, and correspondingly different DOPO-based cyclotriphosphazene halogen-free silicon flame retardants are obtained. In one example, all X1 is substituted with H, and the molecular structure of the DOPO-based cyclotriphosphazene halogen-free silicon flame retardant is shown in Formula (I-1):

[0012]

[0013] wherein X2 is selected from H or methoxy; and X3 is independently selected from H or methyl.

[0014] The second aspect of the present invention is to provide a method for preparing a DOPO-based cyclotriphosphazene halogen-free flame retardant, comprising the following steps:

[0015] S1: In an inert atmosphere, hexachlorocyclotriphosphazene and an aldehyde-containing phenolic compound undergo nucleophilic substitution to obtain an aldehyde-containing cyclotriphosphazene compound, which is then reduced to obtain an intermediate;

[0016] S2: generating a DOPO-based anhydride compound through an addition reaction between DOPO and an anhydride compound in an inert atmosphere;

[0017] S3: In an inert atmosphere, the intermediate prepared in step S1 and the DOPO-based anhydride compound prepared in step S2 undergo an esterification reaction to obtain a DOPO-based cyclotriphosphazene halogen-free flame retardant.

[0018] Furthermore, in step S1, the aldehyde-containing phenolic compound is selected from one or a combination of p-hydroxybenzaldehyde, vanillin or syringaldehyde;

[0019] Furthermore, in step S1, the molar ratio of hexachlorocyclotriphosphazene to the aldehyde-containing phenolic compound is (4-6):33;

[0020] In one embodiment, in step S1, the nucleophilic substitution reaction uses tetrahydrofuran as a solvent and an acid binding agent is added to react to obtain an aldehyde cyclotriphosphazene compound;

[0021] In one embodiment, in step S1, the reduction reaction is carried out using tetrahydrofuran-methanol as a solvent, and a reducing agent is added to react to obtain an intermediate;

[0022] Furthermore, step S1 is carried out by reacting at 30° C. to 70° C. for 14 to 24 hours to obtain the product.

[0023] In one embodiment, in step S1, the nucleophilic substitution is reacted at 70° C. for 24 hours;

[0024] In one embodiment, in step S1, the reduction reaction is carried out at 30° C. for 14 hours.

[0025] Further, the reducing agent is selected from one or a combination of lithium aluminum hydride, sodium borohydride, diborane, platinum or nickel;

[0026] Furthermore, the acid binding agent is selected from one or a combination of sodium hydroxide, potassium carbonate, sodium carbonate, triethylamine, and pyridine.

[0027] In a preferred embodiment, the acid binding agent is potassium carbonate.

[0028] In a preferred embodiment, in step S1, the molar ratio of the acid-binding agent to the aldehyde-containing phenolic compound is 1: 1. The acid-binding agent reacts with the chlorine atoms on the hexachlorocyclophosphazene to form potassium chloride precipitate.

[0029] In a preferred embodiment, the reducing agent is sodium borohydride.

[0030] In some preferred embodiments, in step S1, the amount of the organic solvent is 1 g of raw material: 5 to 20 mL of solvent. Specifically, the amount of the organic solvent is 1 g of hexachlorocyclotriphosphazene or aldehyde-containing phenolic compound or aldehyde-cyclotriphosphazene compound dissolved in 5 to 20 mL of solvent; preferably, the amount of the organic solvent is 1 g of raw material: 10 mL of solvent.

[0031] In a preferred embodiment, in step S1, the molar ratio of the reducing agent to the aldehyde-containing cyclotriphosphazene compound is 10:1.

[0032] Furthermore, in step S2, the acid anhydride compound is selected from one or a combination of itaconic anhydride and ethylene succinic anhydride.

[0033] Furthermore, in step S2, the molar ratio of DOPO to the acid anhydride compound is 1:(1-2);

[0034] Furthermore, in step S2, the reaction temperature is 50-70° C., and the reaction time is 6-10 hours.

[0035] In some preferred embodiments, in step S2, the addition reaction uses tetrahydrofuran as a solvent, and the amount of the organic solvent used is 1 g of DOPO or anhydride compound dissolved in 6 to 15 mL of solvent. Specifically, the amount of the organic solvent used is 1 g of DOPO or anhydride compound dissolved in 6 to 15 mL of solvent; preferably, the amount of the organic solvent used is 1 g of raw material: 10 mL of solvent.

[0036] Furthermore, in step S3, the molar ratio of the intermediate to the DOPO-based anhydride compound is (1-2):6; different molar ratios of the intermediate to the DOPO-based anhydride compound can obtain compounds with different numbers of hydroxyl groups substituted, and correspondingly obtain different DOPO-based cyclotriphosphazene halogen-free flame retardants;

[0037] In one example, when the molar ratio of the intermediate to the DOPO-based anhydride compound is 1:1, only one hydroxyl group is substituted.

[0038] Similarly, in another example, when the molar ratio of the intermediate to the DOPO-based anhydride compound is 2:1, two hydroxyl groups are substituted. When the molar ratio of the intermediate to the DOPO-based anhydride compound is 3:1, 4:1, 5:1, 6:1, and so on, the same can be deduced.

[0039] In another example, an excess of DOPO-based anhydride compound is used to react with the intermediate, that is, the molar ratio of the two is higher than 6:1, such as a molar ratio of 6.5:1. At this time, the reaction is fully completed, all hydroxyl groups are replaced, and the molecular structure of the obtained DOPO-based cyclotriphosphazene halogen-free flame retardant is shown in the above formula (I-1).

[0040] Furthermore, in step S3, the reaction temperature is 80-120° C., and the reaction time is 18-24 hours.

[0041] In some preferred embodiments, in step S3, the esterification reaction uses dioxane as the solvent, and the amount of the organic solvent used is 1 g of the intermediate or DOPO-based anhydride compound dissolved in 15 to 30 mL of the solvent; specifically, the amount of the organic solvent used is 1 g of the intermediate or DOPO-based anhydride compound dissolved in 15 to 30 mL of the solvent; preferably, the amount of the organic solvent used is 1 g of the raw material: 10 mL of the solvent.

[0042] The third aspect of the present invention is to provide a flame-retardant epoxy resin comprising an epoxy resin prepolymer, a curing agent, and the DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 1, or the DOPO-based cyclotriphosphazene halogen-free flame retardant prepared by the preparation method described above;

[0043] Furthermore, in the flame-retardant epoxy resin, the mass ratio of the epoxy resin prepolymer, the curing agent and the DOPO-based cyclotriphosphazene halogen-free flame retardant is 100:(20-26):(10-18).

[0044] Furthermore, the epoxy resin prepolymer is selected from one or a combination of glycidyl ether epoxy resin, glycidyl ester epoxy resin, glycidyl amine epoxy resin or alicyclic epoxy resin;

[0045] Furthermore, the curing agent is one or a combination of acid anhydride, polyamine, dicyandiamide or phenolic resin.

[0046] A fourth aspect of the present invention is to provide a method for preparing the flame-retardant epoxy resin described above, comprising the steps of: stirring a DOPO-based cyclotriphosphazene halogen-free flame retardant and an epoxy resin prepolymer to form a uniform liquid; then adding a curing agent and stirring until dissolved, followed by curing treatment, to obtain a flame-retardant epoxy resin cured product.

[0047] Furthermore, the cyclotriphosphazene-based silicon-containing flame retardant and the epoxy resin prepolymer are stirred at 110 to 140° C. for 15 to 30 minutes to form a uniform liquid.

[0048] Furthermore, the curing treatment includes adding a curing agent to a uniform liquid and pouring it into a mold, placing the mold in a drying oven, and curing it at 100° C., 120° C., 140° C., 160° C., and 180° C. for 1 to 2 hours respectively.

[0049] In some embodiments, the prepared DOPO-based cyclotriphosphazene halogen-free flame retardant and epoxy resin prepolymer are stirred at 120° C. for 30 minutes to form a uniform liquid, and then a curing agent is added and stirred until dissolved, and then quickly poured into a preheated stainless steel mold. The mold is then placed in a blast drying oven and cured at 100° C., 120° C., 140° C., 160° C., and 180° C. for 1 to 2 hours respectively. The mass ratio of the epoxy resin prepolymer, the curing agent, and the DOPO-based cyclotriphosphazene halogen-free flame retardant is 100:(20 to 26):(10 to 18), and a flame-retardant epoxy resin is obtained after cooling.

[0050] A fifth aspect of the present invention provides the use of the DOPO-based cyclotriphosphazene halogen-free flame retardant in flame-retardant materials. The present invention also provides uses of the flame-retardant epoxy resin in the field of flame-retardant materials, including using the flame-retardant epoxy resin as a flame-retardant material or using it to prepare other flame-retardant epoxy resin compositions.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] 1. The DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention can exert the synergistic flame retardant effect of phosphorus and nitrogen elements. The epoxy resin cured product prepared by applying it to epoxy resin has good flame retardant effect, and has good smoke suppression and charring properties.

[0053] 2. The flame retardant of the present invention has good compatibility with epoxy resin, is easy to disperse in the resin matrix, improves the stability of epoxy resin cured product during processing, and can improve the mechanical properties of epoxy cured product with little effect on the thermal properties (thermal decomposition temperature, Tg) of epoxy cured product.

[0054] 3. The preparation method of the DOPO-based cyclotriphosphazene halogen-free flame retardant provided by the present invention has simple steps and a high yield of up to 85%;

[0055] 4. The flame-retardant epoxy resin provided by the present invention has good flame retardancy while maintaining high thermal stability; the limiting oxygen index can reach 31%; the vertical burning grade is above V-0; the vertical burning test time is less than 8+5s; the initial decomposition temperature is higher than 354°C; and the residual carbon content of the flame-retardant epoxy resin can reach more than 24%. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The preferred embodiments of the present invention will be described in further detail below with reference to the accompanying drawings;

[0057] Figure 1 This is the hydrogen nuclear magnetic resonance spectrum of the intermediate prepared in Example 1 of the present invention;

[0058] Figure 2 This is a hydrogen nuclear magnetic resonance spectrum of the DOPO-based cyclotriphosphazene halogen-free flame retardant prepared in Example 3 of the present invention;

[0059] Figure 3 The thermogravimetric curves of the flame retardant epoxy resin cured products prepared in Example 9, Example 10, Example 11, and Comparative Example 1 of the present invention are shown;

[0060] Figure 4 Graphs showing the flexural strength and impact strength of cured epoxy resins obtained in Examples 9, 10, 11, and 1 of the present invention (A: flexural strength and flexural modulus; B: impact strength). DETAILED DESCRIPTION

[0061] The present invention is further described below with reference to the embodiments and drawings, but the present invention is not limited in any way. Any changes or substitutions made based on the present invention fall within the scope of protection of the present invention.

[0062] It should be noted that the embodiments of the present application use an excess of a DOPO-based anhydride compound to fully react with an intermediate to obtain a DOPO-based cyclotriphosphazene halogen-free flame retardant in which all hydroxyl groups are esterified. However, those skilled in the art will appreciate that when the molar ratio of the DOPO-based anhydride compound to the intermediate is changed, different amounts of esterified products will be obtained, and this application will not elaborate on these details.

[0063] Example 1

[0064] The preparation method of one of the intermediates of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention specifically comprises the following steps:

[0065] Step 1, first 40.37g (0.33mol) p-hydroxybenzaldehyde is added to a three-necked flask equipped with mechanical stirring and nitrogen, 300mL tetrahydrofuran is added to stir and dissolve, then 45.5g (0.33mol) potassium carbonate is added and stirred at room temperature for 30 minutes. Then 17.36g (0.05mol) hexachlorocyclotriphosphazene is dissolved in 50mL tetrahydrofuran, and the solution is added dropwise to the reaction system, stirred at room temperature for 1 hour, then warmed to 70°C and stirred for 24 hours. After the reaction is completed, the solution is cooled to room temperature and filtered to remove the solid generated, then the filtrate is concentrated by rotary evaporation, the concentrated solution is added to water to precipitate solid, and the crude product is recrystallized from ethyl acetate to obtain a white solid product with a yield of 78.3%, which is an aldehyde cyclotriphosphazene compound. The molecular structure of this aldehyde cyclotriphosphazene compound is as follows, named as N3P3-CHO.

[0066] The reaction equation is as follows:

[0067]

[0068] Step 2: Weigh 23.33 mmol of the prepared aldehyde cyclotriphosphazene compound into a three-necked flask equipped with a magnetic stirrer and nitrogen, add 300 mL of tetrahydrofuran-methanol and stir to dissolve. Then add 8.81 g (0.233 mol) of sodium borohydride into the reaction system 7 times with an interval of 10 minutes, and stir at room temperature for 14 hours. After the reaction is completed, the solution is concentrated by rotary evaporation, and the concentrate is added to water to precipitate the solid. The crude product is recrystallized from ethanol to obtain a white solid product with a yield of 92.5%, which is the intermediate. Its nuclear magnetic resonance hydrogen spectrum is shown as follows: Figure 1 The molecular structure of the intermediate is shown below and is named N3P3-OH.

[0069] The reaction equation is as follows:

[0070]

[0071] Example 2

[0072] The method for preparing one of the DOPO-based anhydride compounds of the present invention specifically comprises the following steps:

[0073] 45.36 g (0.21 mol) of DOPO was weighed and added to a three-necked flask equipped with a mechanical stirrer and nitrogen, and 300 mL of tetrahydrofuran was added and stirred to dissolve. 22.44 g (0.20 mol) of itaconic anhydride was then dissolved in 50 mL of tetrahydrofuran, and the solution was added dropwise to the reaction system. The temperature was raised to 50°C and stirred for 6 hours. After the reaction, the solution was cooled to room temperature, and the filtrate was evaporated to remove the solvent. The crude product was washed with ethanol to obtain a white solid product with a yield of 76.6%, which was a DOPO-based anhydride compound. Its H NMR spectrum is shown below. Figure 2 The molecular structure of the DOPO-based anhydride compound is shown below and is named DI.

[0074] The reaction equation is as follows:

[0075]

[0076] Example 3

[0077] The preparation method of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention specifically comprises the following steps:

[0078] 10 mmol of the DOPO-based anhydride compound prepared in Example 2 was weighed and added to a three-necked flask equipped with a mechanical stirrer and nitrogen. 300 mL of dioxane was added and stirred to dissolve. 61 mmol of the intermediate prepared in Example 1 was then dissolved in 50 mL of dioxane. The solution was added dropwise to the reaction system, heated to 110°C, and stirred for 20 hours. After the reaction, the solution was cooled to room temperature, and the filtrate was evaporated to remove the solvent. The crude product was washed with dichloromethane and vacuum-dried to constant weight to obtain a white solid product with a yield of 78.4%. This is a DOPO-based cyclotriphosphazene halogen-free flame retardant. The molecular structure of this DOPO-based cyclotriphosphazene halogen-free flame retardant is shown below and is designated N3P3-DI.

[0079] The reaction equation is as follows:

[0080]

[0081] Example 4

[0082] The preparation method of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention specifically comprises the following steps:

[0083] Step 1: Weigh 0.21 mol of DOPO into a three-necked flask equipped with a mechanical stirrer and nitrogen, then add 300 mL of tetrahydrofuran and stir to dissolve. Then, dissolve 0.20 mol of ethylene succinic anhydride in 50 mL of tetrahydrofuran and add the resulting solution dropwise to the reaction system. Heat the mixture to 60°C and stir for 8 hours. After the reaction, cool the solution to room temperature, then remove the solvent by rotary evaporation. The crude product is washed with ethanol to obtain a white solid with a yield of 78.8%, which is the DOPO-based anhydride compound.

[0084] In step 2, 10 mmol of the DOPO-based anhydride compound prepared in this example was weighed and added to a three-necked flask equipped with a mechanical stirrer and nitrogen. 300 mL of dioxane was added and stirred to dissolve. Then, 61 mmol of the intermediate prepared in Example 1 was dissolved in 50 mL of dioxane. The solution was added dropwise to the reaction system, heated to 100°C, and stirred for 24 hours. After the reaction, the solution was cooled to room temperature, and the filtrate was evaporated to remove the solvent. The crude product was washed with dichloromethane and vacuum-dried to constant weight to obtain a white solid product with a yield of 81.2%.

[0085] Example 5

[0086] The preparation method of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention specifically comprises the following steps:

[0087] Step 1: First, 0.33 mol of vanillin is added to a three-necked flask equipped with mechanical stirring and nitrogen, 300 mL of tetrahydrofuran is added and stirred to dissolve, and then 0.33 mol of potassium carbonate is added and stirred at room temperature for 30 minutes. Then 0.05 mol of hexachlorocyclotriphosphazene is dissolved in 50 mL of tetrahydrofuran, and the solution is added dropwise to the reaction system, stirred at room temperature for 1 hour, and then heated to 75 ° C and stirred for 20 hours. After the reaction is completed, the solution is cooled to room temperature and filtered to remove the generated solids, and the filtrate is then concentrated by rotary evaporation. The concentrated solution is added to water to precipitate solids, and the crude product is recrystallized from ethyl acetate to obtain a white solid product with a yield of 82.6%, which is an aldehyde cyclotriphosphazene compound.

[0088] In step 2, 23.33 mmol of the aldehyde cyclotriphosphazene compound prepared in this example was weighed and added to a three-necked flask equipped with a magnetic stirrer and nitrogen atmosphere. 300 mL of tetrahydrofuran-methanol was added and stirred to dissolve. 8.81 g (0.233 mol) of sodium borohydride was then added to the reaction system in seven portions at 10-minute intervals, and stirred at room temperature for 12 hours. After completion of the reaction, the solution was concentrated by rotary evaporation, and the concentrate was added to water to precipitate a solid. The crude product was recrystallized from ethanol to obtain a white solid product with a yield of 90.6%, which was the intermediate.

[0089] Step 3: 10 mmol of the DOPO-based anhydride compound prepared in Example 2 was weighed and added to a three-necked flask equipped with a mechanical stirrer and nitrogen. 300 mL of dioxane was added and stirred to dissolve. Then, 61 mmol of the intermediate prepared in this example was dissolved in 50 mL of dioxane. The solution was added dropwise to the reaction system, heated to 110°C, and stirred for 22 hours. After the reaction, the solution was cooled to room temperature, and the filtrate was evaporated to remove the solvent. The crude product was washed with dichloromethane and vacuum-dried to constant weight to obtain a white solid product with a yield of 84.5%.

[0090] Example 6

[0091] The preparation method of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention specifically comprises the following steps:

[0092] 10 mmol of the DOPO-based anhydride compound prepared in Example 5 was weighed and added to a three-necked flask equipped with a mechanical stirrer and nitrogen. 300 mL of dioxane was added and stirred to dissolve. Then, 61 mmol of the intermediate prepared in Example 4 was dissolved in 50 mL of dioxane. The solution was added dropwise to the reaction system, heated to 120°C, and stirred for 20 hours. After the reaction, the solution was cooled to room temperature, and the filtrate was evaporated to remove the solvent. The crude product was washed with dichloromethane and dried under vacuum to constant weight to obtain a white solid product with a yield of 78.8%.

[0093] Example 7

[0094] The preparation method of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention specifically comprises the following steps:

[0095] Step 1: First, 0.33 mol of syringaldehyde is added to a three-necked flask equipped with mechanical stirring and nitrogen, 300 mL of tetrahydrofuran is added and stirred to dissolve, and then 0.33 mol of potassium carbonate is added and stirred at room temperature for 30 minutes. Then 0.05 mol of hexachlorocyclotriphosphazene is dissolved in 50 mL of tetrahydrofuran, and the solution is added dropwise to the reaction system, stirred at room temperature for 1 hour, and then warmed to 75 ° C and stirred for 24 hours. After the reaction is completed, the solution is cooled to room temperature and filtered to remove the generated solid, and the filtrate is then concentrated by rotary evaporation. The concentrated solution is added to water to precipitate solids, and the crude product is recrystallized from ethyl acetate to obtain a white solid powder with a yield of 80.8%, which is an aldehyde cyclotriphosphazene compound.

[0096] In step 2, 23.33 mmol of the aldehyde cyclotriphosphazene compound prepared in this example was weighed and added to a three-necked flask equipped with a magnetic stirrer and nitrogen atmosphere. 300 mL of tetrahydrofuran-methanol was added and stirred to dissolve. 8.81 g (0.233 mol) of sodium borohydride was then added to the reaction system in seven portions at 10-minute intervals, and stirred at room temperature for 15 hours. After completion of the reaction, the solution was concentrated by rotary evaporation, and the concentrate was added to water to precipitate a solid. The crude product was recrystallized from ethanol to obtain a white solid product with a yield of 92.5%, which was the intermediate.

[0097] Step 3: 10 mmol of the DOPO-based anhydride compound prepared in Example 2 was weighed and added to a three-necked flask equipped with a mechanical stirrer and nitrogen. 300 mL of dioxane was added and stirred to dissolve. Then, 61 mmol of the intermediate prepared in this example was dissolved in 50 mL of dioxane, and the solution was added dropwise to the reaction system. The temperature was raised to 110°C and stirred for 20 hours. After the reaction, the solution was cooled to room temperature, and the filtrate was evaporated to remove the solvent. The crude product was washed with dichloromethane and vacuum dried to constant weight to obtain a white solid product with a yield of 80.5%.

[0098] Example 8

[0099] The preparation method of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention specifically comprises the following steps:

[0100] 10 mmol of the DOPO-based anhydride compound prepared in Example 7 was weighed and added to a three-necked flask equipped with a mechanical stirrer and nitrogen. 300 mL of dioxane was added and stirred to dissolve. Then, 61 mmol of the intermediate prepared in Example 4 was dissolved in 50 mL of dioxane and added dropwise to the reaction system. The temperature was raised to 110°C and stirred for 24 hours. After the reaction, the solution was cooled to room temperature, and the filtrate was evaporated to remove the solvent. The crude product was washed with dichloromethane and dried under vacuum to constant weight to obtain a white solid product with a yield of 82.4%.

[0101] Example 9

[0102] The method for preparing the flame retardant epoxy resin of the present invention comprises the following steps:

[0103] 10.01 g of the DOPO-based cyclotriphosphazene halogen-free flame retardant prepared in Example 3 and 100 g of the epoxy resin prepolymer were stirred at 120° C. for 30 minutes to form a uniform liquid, and then 24.21 g of a curing agent (4,4'-diaminodiphenylmethane) was added and stirred until dissolved. The mixture was quickly poured into a preheated stainless steel mold, and the mold was placed in a blast drying oven and cured at 100° C., 120° C., 140° C., 160° C., and 180° C. for 2 hours each. After cooling, a flame-retardant epoxy resin sample was obtained. Its thermogravimetric analysis was as follows: Figure 3 shown.

[0104] Example 10

[0105] The method for preparing the flame retardant epoxy resin of the present invention comprises the following steps:

[0106] 13.67 g of the DOPO-based cyclotriphosphazene halogen-free flame retardant prepared in Example 3 and 100 g of the epoxy resin prepolymer were stirred at 120° C. for 30 minutes to form a uniform liquid, and then 23.81 g of a curing agent (4,4'-diaminodiphenylmethane) was added and stirred until dissolved. The mixture was quickly poured into a preheated stainless steel mold, and the mold was placed in a blast drying oven and cured at 100° C., 120° C., 140° C., 160° C., and 180° C. for 2 hours each. After cooling, a flame retardant epoxy resin sample was obtained. Its thermogravimetric analysis was as follows: Figure 3 shown.

[0107] Example 11

[0108] The method for preparing the flame retardant epoxy resin of the present invention comprises the following steps:

[0109] 17.51 ​​g of the DOPO-based cyclotriphosphazene halogen-free flame retardant prepared in Example 3 and 100 g of the epoxy resin prepolymer were stirred at 120° C. for 30 minutes to form a uniform liquid, and then 23.40 g of a curing agent (4,4'-diaminodiphenylmethane) was added and stirred until dissolved. The mixture was quickly poured into a preheated stainless steel mold, and the mold was placed in a blast drying oven and cured at 100° C., 120° C., 140° C., 160° C., and 180° C. for 2 hours each. After cooling, a flame retardant epoxy resin sample was obtained. Its thermogravimetric analysis was as follows: Figure 3 shown.

[0110] Comparative Example 1: No flame retardant

[0111] 100g of epoxy resin prepolymer was mixed with 25.28g of curing agent (4,4'-diaminodiphenylmethane) at 90℃ to form a uniform liquid, which was quickly poured into a preheated stainless steel mold. The mold was then placed in a blast drying oven and cured at 100℃, 120℃, 140℃, 160℃, and 180℃ for 2 hours each. After cooling, epoxy resin samples were obtained for further comparative tests. The thermogravimetric analysis results are as follows: Figure 3 shown.

[0112] Test Case

[0113] The flame retardant performance of the flame retardant epoxy resins prepared by adding DOPO-based cyclotriphosphazene halogen-free flame retardant in Examples 9-11 and the flame retardant epoxy resin of the comparative example was tested. The performance test data are shown in Table 1:

[0114] Table 1 Flame retardant performance test results of flame retardant epoxy resin

[0115]

[0116] like Figure 3 As shown, the initial thermal decomposition temperature of the epoxy resin cured product (Examples 9-11) to which the flame retardant prepared according to the present invention is added is slightly reduced, but still maintains good thermal stability.

[0117] As can be seen from the comparison of Examples 9-11 with Comparative Example 1 (shown in Table 1), the addition of the DOPO-based cyclotriphosphazene halogen-free flame retardant of the present invention (Example 3) can significantly improve the limiting oxygen index and vertical burning rating of the epoxy resin sample, and its carbon residue is also increased. Specifically, the limiting oxygen index of the flame-retardant epoxy resin is greater than 27%; the vertical burning rating is above V-0, and the vertical burning test time is less than 8+5 seconds; the initial decomposition temperature is higher than 354°C; the carbon residue is higher than 15%, even higher than 20%, and as high as 24%; the total smoke production is reduced by more than 27.0%, with the highest reduction being 32.9%.

[0118] like Figure 4-A, the flexural strength and flexural modulus of the epoxy resin cured products prepared in Examples 9-11 are significantly improved compared with those in Comparative Example 1. Figure 4 As shown in Figure 1-B, the impact strength of the cured epoxy resins produced in Examples 9-11 was significantly improved. The flexural strength increased by more than 50.5%, with a maximum increase of 76.1%. The impact strength also increased by more than 28.2%, with a maximum increase of 46.2%. This demonstrates that the flame retardant-modified epoxy resins of the present invention can significantly enhance their flame retardancy and mechanical properties.

[0119] In summary, the present invention provides a DOPO-based cyclotriphosphazene halogen-free flame retardant, its preparation method, and application. Based on hexachlorocyclotriphosphazene, a halogen-free flame retardant containing phosphorus and nitrogen is obtained through a nucleophilic substitution reaction with an aldehyde-containing phenolic compound and an esterification reaction with a DOPO-based anhydride compound. The halogen-free flame retardant is then used to modify epoxy resin to prepare a flame-retardant epoxy resin; the thermal properties of the epoxy resin cured product (thermal decomposition temperature, T g ) can significantly improve its smoke suppression and charring performance and mechanical properties with little influence, and has excellent flame retardant efficiency.

[0120] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made based on the essence of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A DOPO-based cyclotriphosphazene halogen-free flame retardant, characterized in that: The molecular structure is shown in formula (I): X1 and X2 are independently selected from H or methoxy; X3 is selected from H or methyl.

2. The method for preparing the DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 1, characterized in that: The steps include: S1: In an inert atmosphere, hexachlorocyclotriphosphazene and an aldehyde-containing phenolic compound undergo nucleophilic substitution to obtain an aldehyde-containing cyclotriphosphazene compound, which is then reduced to obtain an intermediate; S2: generating a DOPO-based anhydride compound through an addition reaction between DOPO and an anhydride compound in an inert atmosphere; S3: In an inert atmosphere, the intermediate prepared in step S1 and the DOPO-based anhydride compound prepared in step S2 undergo an esterification reaction to obtain a DOPO-based cyclotriphosphazene halogen-free flame retardant.

3. The method for preparing the DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 2, wherein: In step S1, the aldehyde-containing phenolic compound is selected from one or a combination of p-hydroxybenzaldehyde, vanillin or syringaldehyde; In step S2, the acid anhydride compound is selected from one or a combination of itaconic anhydride and ethylene succinic anhydride.

4. The method for preparing a DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 2, wherein: In step S1, the molar ratio of hexachlorocyclotriphosphazene to the aldehyde-containing phenolic compound is (4-6):33; The nucleophilic substitution reaction uses tetrahydrofuran as solvent and an acid-binding agent to obtain an aldehyde cyclotriphosphazene compound. The reduction reaction uses tetrahydrofuran-methanol as solvent, adds a reducing agent, and reacts to obtain an intermediate; The reducing agent is selected from one or a combination of lithium aluminum hydride, sodium borohydride, diborane, platinum or nickel; The acid binding agent is selected from one or a combination of sodium hydroxide, potassium carbonate, sodium carbonate, triethylamine and pyridine.

5. The method for preparing the DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 2, wherein: In step S2, the molar ratio of DOPO to the acid anhydride compound is 1:(1-2); The reaction temperature is 50-70°C, and the reaction time is 6-10 hours.

6. The method for preparing the DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 2, characterized in that: In step S3, the molar ratio of the intermediate to the DOPO-based anhydride compound is (1-2):6; In step S3, the reaction temperature is 80-120° C., and the reaction time is 18-24 hours.

7. A flame retardant epoxy resin comprising an epoxy resin prepolymer and a curing agent, characterized in that: It also includes the DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 1, or the DOPO-based cyclotriphosphazene halogen-free flame retardant prepared by the preparation method according to claims 2-6.

8. The flame retardant epoxy resin according to claim 7, characterized in that The epoxy resin prepolymer is selected from one or a combination of glycidyl ether epoxy resin, glycidyl ester epoxy resin, glycidyl amine epoxy resin or alicyclic epoxy resin; The curing agent is selected from one or a combination of acid anhydride, polyamine, dicyandiamide or phenolic resin; In the flame-retardant epoxy resin, the mass ratio of the epoxy resin prepolymer, the curing agent and the DOPO-based cyclotriphosphazene halogen-free flame retardant is 100:(20-26):(10-18).

9. The method for preparing the flame retardant epoxy resin according to claim 7 or 8, characterized in that: The steps include: The DOPO-based cyclotriphosphazene halogen-free flame retardant and the epoxy resin prepolymer are stirred to form a uniform liquid, and then a curing agent is added and stirred until dissolved and then cured. After curing, a flame-retardant epoxy resin cured product can be obtained.

10. Use of the DOPO-based cyclotriphosphazene halogen-free flame retardant according to claim 1 or the DOPO-based cyclotriphosphazene halogen-free flame retardant prepared by the preparation method according to claims 2 to 6 in epoxy resin.

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