Halogen-free flame retardant containing Schiff base structure, preparation method and application thereof

Abstract

The present application belongs to the technical field of flame-retardant materials, and specifically relates to a halogen-free flame retardant containing a Schiff base structure, a preparation method thereof, and application of the halogen-free flame retardant in epoxy resin. The halogen-free flame retardant containing phosphorus, nitrogen elements and a Schiff base structure is obtained through a Schiff base reaction between an intermediate and 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, wherein the intermediate is prepared through a nucleophilic substitution reaction between an aldehyde group-containing phenolic compound and an acyl chloride compound. The flame retardant has good thermal stability, significantly improved char yield, excellent flame-retardant effect, and significantly improved mechanical properties due to the combination of the Schiff base structure and phosphorus and silicon elements. The halogen-free flame retardant is further used to modify epoxy resin to prepare a flame-retardant epoxy resin, which can significantly improve the char performance and mechanical properties of the epoxy resin with little influence on the thermal performance of the epoxy resin cured product, and has excellent flame-retardant efficiency.

Description

Technical Field

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

[0002] Epoxy resins, with their excellent chemical stability, mechanical properties, dimensional stability, and electrical insulation, are widely used in adhesives, coatings, and construction applications. However, current epoxy resin products are prone to burning, difficult to self-extinguish, and generate large amounts of smoke, toxic gases, and molten drippings during combustion. These issues pose significant fire safety risks, limiting their wider application, particularly in areas such as aerospace, electronics, and automotive, where flame retardancy is paramount. Therefore, the development of highly effective epoxy resin flame retardants has become a key research focus for researchers both domestically and internationally. Traditional epoxy resin flame retardants often use halogenated flame retardants, which offer advantages such as high flame retardancy, low dosage, and a high cost-effectiveness. However, halogenated flame retardants generate toxic substances during their production process, polluting the environment and adversely affecting human health. Furthermore, during combustion, they release large amounts of toxic gases and smoke, increasing fire safety risks. Therefore, the development of new, low-toxic, environmentally friendly halogen-free flame retardants is highly desirable.

[0003] Schiff base compounds are an important class of chemical substances that are widely used in the fields of organic chemistry, catalysis, biomedicine, etc. Schiff base compounds not only contain the flame retardant element N, but the C=N double bond in their structure can undergo a cross-linking reaction at high temperatures to form a stable polybenzoxazine and a triazine ring structure with alternating bonds between C and N elements, forming a stable carbon layer to protect the polymer. The Schiff base reaction is simple and has a high yield. Through reasonable molecular structure design, it can generate a variety of derivatives with different properties and functions to meet the flame retardant needs of different polymer materials, and is very consistent with the current halogen-free and environmentally friendly flame retardant requirements. However, because the Schiff base C=N double bond structure only contains one flame retardant element, N, a higher addition amount is required when used alone to make the epoxy resin have good flame retardant properties.

[0004] Phosphorus-containing and silicon-containing flame retardants are currently the primary alternatives to halogen-based flame retardants. During combustion and degradation, phosphorus-containing flame retardants promote the dehydration of epoxy resins, forming a stable char layer. Furthermore, they generate reactive groups that release phosphorus-containing free radicals, quenching oxygen free radicals and enhancing the flame retardancy of epoxy resins. Silicon-containing flame retardants, due to their low surface tension, readily migrate to the material surface at high temperatures and oxidize to SiO2, enhancing the stability of the char layer. Furthermore, they can form a ceramic-like structure that protects the char layer from further decomposition.

[0005] At present, there is still a lack of an efficient halogen-free flame retardant containing a Schiff base structure. Summary of the Invention

[0006] To address the environmental hazards of existing halogen-containing flame retardants, which release toxic gases and large amounts of smoke during combustion, and the low flame retardant efficiency of Schiff-base flame retardants, the present invention bonds phosphorus-containing groups to terminal amino-containing siloxane molecules through a Schiff-base reaction. The resulting flame retardant achieves excellent flame retardancy through a multi-element synergistic effect. The halogen-free flame retardant containing a Schiff-base structure provided by the present invention combines the Schiff-base structure with phosphorus and silicon, exhibiting excellent thermal stability, significantly improved char formation rate, excellent flame retardancy, and significantly enhanced mechanical properties.

[0007] The present invention provides a preparation method and application of a novel and highly efficient halogen-free flame retardant containing a Schiff base structure. The present invention provides the following technical solutions:

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

[0009]

[0010] Wherein R1 and R2 are independently selected from H or methoxy; R3 is selected from phenyl or phenoxy.

[0011] The second aspect of the present invention is to provide a method for preparing a halogen-free flame retardant containing a Schiff base structure, comprising the following steps:

[0012] S1: In an inert atmosphere, an intermediate is obtained by nucleophilic substitution between an aldehyde-containing phenolic compound and a phosphonyl chloride compound. The reaction formula is as follows:

[0013]

[0014] S2: In an inert atmosphere, a Schiff base reaction between the intermediate and 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane is performed to obtain a halogen-free flame retardant containing a Schiff base structure. The reaction formula is as follows:

[0015]

[0016] Furthermore, in step S1, the aldehyde-containing phenolic compound is selected from one or a combination of p-hydroxybenzaldehyde, vanillin or syringaldehyde;

[0017] Furthermore, in step S1, the acyl chloride compound is selected from one or a combination of diphenylphosphonyl chloride and diphenylphosphinyl chloride.

[0018] Furthermore, in step S1, the aldehyde-containing phenolic compound is dissolved in an organic solvent, an acid-binding agent is added, and after mixing, a phosphonyl chloride compound is added to react to obtain an intermediate.

[0019] Furthermore, in step S1, the organic solvent is selected from a combination of one or more of acetone, tetrahydrofuran, dichloromethane, chloroform, dioxane, toluene, and xylene;

[0020] In a preferred embodiment, in step S1, the organic solvent is tetrahydrofuran.

[0021] In some embodiments of the present invention, in step S1, the acid binding agent is selected from one or a combination of sodium hydroxide, potassium carbonate, sodium carbonate, triethylamine, and pyridine.

[0022] Furthermore, step S1 is reacted at 50° C. to 70° C. for 4 to 12 hours; in some embodiments of the present invention, the reaction time is 6 to 10 hours.

[0023] In a preferred embodiment, in step S1, nucleophilic substitution is carried out at 60° C. for 8 hours to obtain an intermediate.

[0024] In some embodiments of the present invention, in step S1, the molar ratio of the acid binding agent to the aldehyde-containing phenolic compound is 1:1.

[0025] In some embodiments of the present invention, in step S1, the molar ratio of the aldehyde-containing phenol to the acyl chloride compound is 1:1.

[0026] In some preferred embodiments, in step S1, the amount of the organic solvent used is such that 1 g of the aldehyde-containing phenolic compound or acyl chloride compound is dissolved in 5 to 20 mL of the solvent.

[0027] In some embodiments, the organic solvent for the Schiff base reaction is selected from a combination of one or more of ethanol, methanol, dimethylformamide, tetrahydrofuran, dioxane, toluene, and xylene;

[0028] In some preferred embodiments, the Schiff base reaction uses ethanol as a solvent to obtain a halogen-free flame retardant containing a Schiff base structure.

[0029] Furthermore, in step S2, the reaction temperature is 50-70° C., and the reaction time is 4-12 hours; in some embodiments of the present invention, the reaction time is 6-10 hours.

[0030] In some embodiments of the present invention, in step S2, the molar ratio of the intermediate to 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane is 2:1.

[0031] In some preferred embodiments, in step S2, the amount of the organic solvent used is such that 1 g of the intermediate or 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane is dissolved in 5 to 20 mL of the solvent.

[0032] 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 aforementioned halogen-free flame retardant containing a Schiff base structure.

[0033] Furthermore, the mass ratio of the epoxy resin prepolymer, the curing agent and the halogen-free flame retardant containing a Schiff base structure in the flame retardant epoxy resin is 100:(20-90):(10-50).

[0034] In some embodiments of the present invention, 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;

[0035] In some embodiments of the present invention, the curing agent is selected from one or a combination of acid anhydride, polyamine, dicyandiamide or phenolic resin.

[0036] The fourth aspect of the present invention is to provide a method for preparing a flame-retardant epoxy resin cured product, comprising the following steps: stirring a halogen-free flame retardant containing a Schiff base structure with an epoxy resin prepolymer to form a uniform liquid, then adding a curing agent and stirring until dissolved, and then performing a curing treatment. After curing, a flame-retardant epoxy resin cured product can be obtained.

[0037] In some embodiments of the present invention, the Schiff base structure-containing halogen-free flame retardant and the epoxy resin prepolymer are stirred at 40 to 80° C. for 15 to 30 minutes to form a uniform liquid.

[0038] In some embodiments of the present invention, the curing treatment step includes adding a curing agent to a uniform liquid and pouring it into a mold, placing the mold in a drying oven, and curing at 90°C, 110°C, 130°C, and 150°C for 1 to 3 hours respectively.

[0039] In some embodiments, the prepared halogen-free flame retardant containing a Schiff base structure and the epoxy resin prepolymer are stirred at 65°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, and then the mold is placed in a blast drying oven and cured at 90°C, 110°C, 130°C, and 150°C for 1 to 3 hours respectively, and a flame retardant epoxy resin cured product is obtained after cooling.

[0040] The fifth aspect of the present invention is to provide the use of the halogen-free flame retardant containing a Schiff base structure for preparing a flame retardant material.

[0041] Furthermore, the flame retardant material includes a flame retardant resin, which includes but is not limited to a flame retardant epoxy resin, a bismaleimide resin, a benzoxazine resin, and an unsaturated polyester resin.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] (1) The halogen-free flame retardant containing a Schiff base structure prepared by the present invention can exert the synergistic flame retardant effect of phosphorus-nitrogen-silicon elements. The epoxy resin cured product prepared by applying it to epoxy resin has good flame retardant effect and good charring property.

[0044] (2) The preparation method of the halogen-free flame retardant containing a Schiff base structure provided by the present invention has simple steps and a high yield of up to 95.1%;

[0045] (3) The flame-retardant epoxy resin provided by the present invention can maintain high thermal stability while having good flame retardancy; the limiting oxygen index can reach 35.2%; the vertical burning grade can reach above V-0; the vertical burning test time is as low as 1.7+2.5s; the initial decomposition temperature is higher than 300°C; and the residual carbon content of the flame-retardant epoxy resin can reach above 11.3%.

[0046] (4) 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 of epoxy cured product. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The accompanying drawings are used to provide a further understanding of the present application and constitute a part of the specification. Together with the embodiments of the present application, they are used to explain the present application and do not constitute a limitation of the present application. The preferred embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

[0048] Figure 1 This is the hydrogen nuclear magnetic resonance spectrum of the intermediate prepared in Example 4 of the present invention;

[0049] Figure 2 This is a hydrogen nuclear magnetic resonance spectrum of the halogen-free flame retardant containing a Schiff base structure prepared in Example 4 of the present invention;

[0050] 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;

[0051] Figure 4 This is a graph showing the bending strength of epoxy resin cured products obtained in Example 9, Example 10, Example 11, and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0052] To make the above-mentioned objects, features and advantages of the present invention more clearly understood, the following detailed description of the specific embodiments of the present invention is given in conjunction with the examples in the specification. These examples are intended to illustrate the present invention only and are not intended to limit the scope of the present invention. The experimental methods in the following examples, where specific conditions are not specified, are generally based on conventional conditions or conditions recommended by the manufacturer.

[0053] The following description sets forth many specific details to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the scope of the present invention. The present invention is not limited to the specific embodiments disclosed below.

[0054] Example 1 Preparation of a halogen-free flame retardant containing a Schiff base structure

[0055] The steps include:

[0056] Step 1: At 0°C, 12.21g (0.10mol) of p-hydroxybenzaldehyde was added to a three-necked flask equipped with mechanical stirring and nitrogen, 200mL of tetrahydrofuran was added and stirred to dissolve, and then 13.82g (0.10mol) of potassium carbonate was added and stirred for 30 minutes. Then 23.66g (0.10mol) of diphenylphosphinyl chloride was dissolved in 150mL of tetrahydrofuran, and the solution was added dropwise to the reaction system. After the dropwise addition was completed, the reaction was moved into a 50°C oil bath and the reaction was continued for 6h. After the reaction was completed, the solution was cooled to room temperature and then filtered and precipitated. After the solvent was removed by rotary evaporation, it was dissolved in dichloromethane and washed five times with water, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a white solid, i.e., the intermediate, with a yield of 88.2%.

[0057] Step 2: At 0°C, 0.10 mol of the intermediate prepared in this example was added to a three-necked flask equipped with magnetic stirring and nitrogen, and 300 mL of ethanol was added and stirred to dissolve. Then 12.42 g (0.05 mol) of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane was dissolved in 100 mL of ethanol, and the solution was added dropwise to the reaction system. After the addition was completed, the reaction was moved into a 50°C oil bath and the reaction was continued for 6 hours. After the reaction was completed, the solution was cooled to room temperature, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a yellow liquid product with a yield of 90.5%.

[0058] Example 2 Preparation of a halogen-free flame retardant containing a Schiff base structure

[0059] The difference between Example 2 and Example 1 is that: Step 1 is different;

[0060] In step 1, 12.21 g (0.10 mol) of p-hydroxybenzaldehyde was added to a three-necked flask equipped with mechanical stirring and nitrogen at 0°C. 200 mL of tetrahydrofuran was added and stirred to dissolve, followed by the addition of 10.12 g (0.10 mol) of triethylamine and stirring for 30 minutes. 23.66 g (0.10 mol) of diphenylphosphinyl chloride was then dissolved in 150 mL of tetrahydrofuran and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 60°C oil bath and the reaction continued for 8 hours. After the reaction was completed, the solution was cooled to room temperature and the precipitate was filtered. The solvent was removed by rotary evaporation, then dissolved in dichloromethane and washed five times with water. The solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a white solid, one of the intermediates, with a yield of 91.8%.

[0061] Step 2 is the same as in Example 1.

[0062] Example 3 Preparation of a halogen-free flame retardant containing a Schiff base structure

[0063] The steps include:

[0064] In step 1, 15.21 g (0.10 mol) of vanillin was added to a three-necked flask equipped with a mechanical stirrer and nitrogen at 0°C. 200 mL of tetrahydrofuran was added and stirred to dissolve, followed by the addition of 13.82 g (0.10 mol) of potassium carbonate and stirring for 30 minutes. 23.66 g (0.10 mol) of diphenylphosphinyl chloride was then dissolved in 150 mL of tetrahydrofuran and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 60°C oil bath and the reaction continued for 6 hours. After the reaction was completed, the solution was cooled to room temperature and the precipitate was filtered. The solvent was removed by rotary evaporation, then dissolved in dichloromethane and washed five times with water. The solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a white solid, one of the intermediates, with a yield of 87.3%.

[0065] In step 2, 0.10 mol of the intermediate prepared in this example was added to a three-necked flask equipped with magnetic stirring and nitrogen at 0°C, and 400 mL of ethanol was added and stirred to dissolve. Then, 12.42 g (0.05 mol) of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane was dissolved in 100 mL of ethanol, and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 50°C oil bath and the reaction was continued for 6 hours. After the reaction was completed, the solution was cooled to room temperature, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a yellow liquid product with a yield of 89.6%.

[0066] Example 4 Preparation of a halogen-free flame retardant containing a Schiff base structure

[0067] The steps include:

[0068] In step 1, 15.21 g (0.10 mol) of vanillin was added to a three-necked flask equipped with a mechanical stirrer and nitrogen at 0°C. 200 mL of tetrahydrofuran was added and stirred to dissolve, followed by the addition of 10.12 g (0.10 mol) of triethylamine and stirring for 30 minutes. 23.66 g (0.10 mol) of diphenylphosphinyl chloride was then dissolved in 150 mL of tetrahydrofuran and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 70°C oil bath and the reaction continued for 8 hours. After the reaction was completed, the solution was cooled to room temperature and the precipitate was filtered. The solvent was removed by rotary evaporation, then dissolved in dichloromethane and washed five times with water. The solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a white solid, one of the intermediates, with a yield of 93.4%.

[0069] The reaction equation is as follows:

[0070]

[0071] In step 2, 0.10 mol of the intermediate prepared in this example was added to a three-necked flask equipped with magnetic stirring and nitrogen at 0°C, and 400 mL of ethanol was added and stirred to dissolve. Then, 12.42 g (0.05 mol) of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane was dissolved in 100 mL of ethanol, and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 60°C oil bath and the reaction was continued for 8 hours. After the reaction was completed, the solution was cooled to room temperature, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a yellow liquid product with a yield of 95.1%.

[0072] The reaction equation is as follows:

[0073]

[0074] The H NMR spectrum of the intermediate is shown in Figure 1 As shown, the peak at 3.89 ppm (marked as 1) is attributed to the chemical shift of the methoxy group, the peak at 7.14 ppm to 8.00 ppm (marked as Ph-H) is attributed to the aromatic protons on the benzene ring, and the peak at 9.86 ppm (marked as 2) is attributed to the chemical shift of the aldehyde group. The integrated areas of the different proton peaks in the spectrum also correspond to the theoretical values, proving that the intermediate was successfully prepared.

[0075] The nuclear magnetic resonance hydrogen spectrum of flame retardants is as follows Figure 2As shown, the peak at 0.00ppm~0.14ppm (marked as 1) is attributed to the chemical shift of methyl, the peaks at 0.50ppm~1.58ppm and 3.47ppm (marked as 2, 3 and 4) are attributed to the chemical shift of methylene, the peak at 3.89ppm (marked as 5) is attributed to the chemical shift of methoxy, the peak at 7.14ppm~7.89ppm (marked as Ph-H) is attributed to the aromatic proton on the benzene ring, and the peak at 8.17ppm (marked as 6) is attributed to the chemical shift of Schiff base structure, and the integrated areas of different proton peaks in the spectrum also correspond to the theoretical values, proving that the halogen-free flame retardant containing Schiff base structure is successfully prepared.

[0076] Example 5 Preparation of a halogen-free flame retardant containing a Schiff base structure

[0077] The steps include:

[0078] In step 1, 12.21 g (0.10 mol) of p-hydroxybenzaldehyde was added to a three-necked flask equipped with mechanical stirring and nitrogen at 0°C, 200 mL of tetrahydrofuran was added and stirred to dissolve, and then 7.91 g (0.10 mol) of pyridine was added and stirred for 30 minutes. 26.86 g (0.10 mol) of diphenylphosphonyl chloride was then dissolved in 150 mL of tetrahydrofuran, and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved into a 60°C oil bath and the reaction was continued for 8 hours. After the reaction was completed, the solution was cooled to room temperature and the precipitate was filtered. The solvent was removed by rotary evaporation and then dissolved in dichloromethane, washed five times with water, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a white solid, one of the intermediates, with a yield of 84.6%.

[0079] In step 2, 0.10 mol of the intermediate prepared in this example was added to a three-necked flask equipped with magnetic stirring and nitrogen at 0°C, and 300 mL of ethanol was added and stirred to dissolve. Then, 12.42 g (0.05 mol) of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane was dissolved in 100 mL of ethanol and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 60°C oil bath and the reaction was continued for 8 hours. After the reaction was completed, the solution was cooled to room temperature, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a yellow liquid product with a yield of 87.4%.

[0080] Example 6 Preparation of a halogen-free flame retardant containing a Schiff base structure,

[0081] The steps include:

[0082] In step 1, 12.21 g (0.10 mol) of p-hydroxybenzaldehyde was added to a three-necked flask equipped with mechanical stirring and nitrogen at 0°C. 200 mL of tetrahydrofuran was added and stirred to dissolve, followed by the addition of 10.12 g (0.10 mol) of triethylamine and stirring for 30 minutes. 26.86 g (0.10 mol) of diphenylphosphonyl chloride was then dissolved in 150 mL of tetrahydrofuran and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 70°C oil bath and the reaction continued for 10 hours. After the reaction was completed, the solution was cooled to room temperature and the precipitate was filtered. The solvent was removed by rotary evaporation, then dissolved in dichloromethane and washed five times with water. The solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a white solid, one of the intermediates, with a yield of 89.0%.

[0083] In step 2, 0.10 mol of the intermediate prepared in this example was added to a three-necked flask equipped with magnetic stirring and nitrogen at 0°C, and 300 mL of ethanol was added and stirred to dissolve. Then, 12.42 g (0.05 mol) of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane was dissolved in 100 mL of ethanol, and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 70°C oil bath and the reaction was continued for 10 hours. After the reaction was completed, the solution was cooled to room temperature, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a yellow liquid product with a yield of 93.9%.

[0084] Example 7 Preparation of a halogen-free flame retardant containing a Schiff base structure,

[0085] The steps include:

[0086] In step 1, 15.21 g (0.10 mol) of vanillin was added to a three-necked flask equipped with a mechanical stirrer and nitrogen at 0°C, 200 mL of tetrahydrofuran was added and stirred to dissolve, and then 7.91 g (0.10 mol) of pyridine was added and stirred for 30 minutes. 26.86 g (0.10 mol) of diphenylphosphonyl chloride was then dissolved in 150 mL of tetrahydrofuran, and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 60°C oil bath and the reaction was continued for 8 hours. After the reaction was completed, the solution was cooled to room temperature and the precipitate was filtered. The solvent was removed by rotary evaporation and then dissolved in dichloromethane, washed five times with water, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a white solid, which was one of the intermediates, with a yield of 88.1%.

[0087] In step 2, 0.10 mol of the intermediate prepared in this example was added to a three-necked flask equipped with magnetic stirring and nitrogen at 0°C, and 300 mL of ethanol was added and stirred to dissolve. Then, 12.42 g (0.05 mol) of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane was dissolved in 100 mL of ethanol, and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 60°C oil bath and the reaction was continued for 8 hours. After the reaction was completed, the solution was cooled to room temperature, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a yellow liquid product with a yield of 86.2%.

[0088] Example 8 Preparation of a halogen-free flame retardant containing a Schiff base structure

[0089] The steps include:

[0090] In step 1, 15.21 g (0.10 mol) of vanillin was added to a three-necked flask equipped with a mechanical stirrer and nitrogen at 0°C. 200 mL of tetrahydrofuran was added and stirred to dissolve, followed by the addition of 10.12 g (0.10 mol) of triethylamine and stirring for 30 minutes. 26.86 g (0.10 mol) of diphenylphosphonyl chloride was then dissolved in 150 mL of tetrahydrofuran and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 70°C oil bath and the reaction continued for 10 hours. After the reaction was completed, the solution was cooled to room temperature and the precipitate was filtered. The solvent was removed by rotary evaporation, then dissolved in dichloromethane, washed five times with water, and the solvent was removed by rotary evaporation. The product was then dried under vacuum to a constant weight to obtain a white solid, one of the intermediates, with a yield of 90.5%.

[0091] In step 2, 0.10 mol of the intermediate prepared in this example was added to a three-necked flask equipped with magnetic stirring and nitrogen at 0°C, and 300 mL of ethanol was added and stirred to dissolve. Then, 12.42 g (0.05 mol) of 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane was dissolved in 100 mL of ethanol, and the solution was added dropwise to the reaction system. After the addition was complete, the reaction was moved to a 70°C oil bath and the reaction was continued for 10 hours. After the reaction was completed, the solution was cooled to room temperature, the solvent was removed by rotary evaporation, and vacuum dried to constant weight to obtain a yellow liquid product with a yield of 89.7%.

[0092] Example 9 Preparation of flame retardant epoxy resin

[0093] The steps include:

[0094] 13.81 g of the halogen-free flame retardant containing a Schiff base structure prepared in Example 4, 100 g of an epoxy resin prepolymer, and 85.83 g of a curing agent (methylhexahydrophthalic anhydride) were stirred at 60° C. for 30 minutes to form a uniform liquid, and then quickly poured into a preheated stainless steel mold. The mold was then placed in a blast drying oven, and an epoxy resin cured product was prepared according to the curing process of 90° C. / 3 h, 110° C. / 2 h, 130° C. / 1 h, and 150° C. / 3 h. After cooling, a flame retardant epoxy resin sample was obtained, and its thermogravimetric analysis was as follows: Figure 3 shown.

[0095] Example 10 Preparation of flame retardant epoxy resin

[0096] The steps include:

[0097] 32.42 g of the halogen-free flame retardant containing a Schiff base structure prepared in Example 4, 100 g of an epoxy resin prepolymer, and 85.83 g of a curing agent (methylhexahydrophthalic anhydride) were stirred at 60° C. for 30 minutes to form a uniform liquid, and then quickly poured into a preheated stainless steel mold. The mold was then placed in a blast drying oven, and an epoxy resin cured product was prepared according to the curing process of 90° C. / 3 h, 110° C. / 2 h, 130° C. / 1 h, and 150° C. / 3 h. After cooling, a flame retardant epoxy resin sample was obtained, and its thermogravimetric analysis was as follows: Figure 3 shown.

[0098] Example 11 Preparation of flame retardant epoxy resin

[0099] The steps include:

[0100] 53.26 g of the halogen-free flame retardant containing a Schiff base structure prepared in Example 4, 100 g of an epoxy resin prepolymer, and 85.83 g of a curing agent (methylhexahydrophthalic anhydride) were stirred at 60° C. for 30 minutes to form a uniform liquid, and then quickly poured into a preheated stainless steel mold. The mold was then placed in a blast drying oven, and an epoxy resin cured product was prepared according to the curing process of 90° C. / 3 h, 110° C. / 2 h, 130° C. / 1 h, and 150° C. / 3 h. After cooling, a flame retardant epoxy resin sample was obtained, and its thermogravimetric analysis was as follows: Figure 3 shown.

[0101] Comparative Example 1: No flame retardant

[0102] Take 100g of epoxy resin prepolymer and 85.83g of curing agent (methylhexahydrophthalic anhydride) and stir at 60℃ for 30 minutes to form a uniform liquid. Then pour it into a preheated stainless steel mold, and then put the mold into a blast drying oven. Prepare epoxy resin cured product according to the curing process of 90℃ / 3h, 110℃ / 2h, 130℃ / 1h, and 150℃ / 3h. After cooling, obtain flame retardant epoxy resin sample for further comparative test. Its thermogravimetric analysis is as follows: Figure 3shown.

[0103] Test Case

[0104] The flame retardant epoxy resins prepared by adding the Schiff base structure-containing halogen-free flame retardant in Examples 9-11 and the flame retardant epoxy resin of the comparative example were subjected to performance tests. The performance test data are shown in Table 1:

[0105] Table 1 Flame retardant performance test results of flame retardant epoxy resin

[0106]

[0107]

[0108] like Figure 3 As shown in Table 1: the epoxy resin cured product with the flame retardant prepared by the present invention still maintains good thermal stability, and the initial thermal decomposition temperature is slightly reduced. Adding the halogen-free flame retardant containing the Schiff base structure of the present invention (Example 4) can significantly improve the limiting oxygen index and vertical combustion grade of the epoxy resin sample, and its residual carbon content has also been increased. The limiting oxygen index of the flame retardant epoxy resin is greater than 28.5%, and can reach 35.2%; the initial decomposition temperature is higher than 303.8°C. Among them, the residual carbon content of Example 11 can reach 11.3%; the vertical combustion grade is V-0 or above. As shown in Table 1 Figure 4 As shown, the flexural strength and flexural modulus of the epoxy resin cured products prepared in Examples 9-10 were improved compared to those in Comparative Example 1, with the flexural strength increasing by up to 24.7%. This indicates that the flame retardant of the present invention, when applied to epoxy resin, can not only improve the flame retardancy of the cured resin, but also effectively enhance the mechanical properties of the cured resin.

[0109] In summary, the present invention provides a halogen-free flame retardant containing a Schiff base structure, its preparation method, and application. Through a nucleophilic substitution reaction between an aldehyde-containing phenolic compound and an acyl chloride compound, followed by a Schiff base reaction with 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, a halogen-free flame retardant containing phosphorus, nitrogen, and a Schiff base structure is obtained. This halogen-free flame retardant is then used to modify epoxy resin to produce a flame-retardant epoxy resin. This flame-retardant epoxy resin significantly improves the charring and mechanical properties of the cured epoxy resin while minimizing the thermal properties, resulting in excellent flame retardancy.

[0110] 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 halogen-free flame retardant containing a Schiff base structure, characterized in that: The molecular structure is shown in formula (I): （I） Wherein R1 and R2 are independently selected from H or methoxy; R3 is selected from phenyl or phenoxy.

2. The method for preparing the halogen-free flame retardant containing a Schiff base structure according to claim 1, characterized in that: The steps include: S1: In an inert atmosphere, an intermediate is obtained by nucleophilic substitution between an aldehyde-containing phenolic compound and a phosphonyl chloride compound. The reaction formula is as follows: ； S2: In an inert atmosphere, a Schiff base reaction between the intermediate and 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane is performed to obtain a halogen-free flame retardant containing a Schiff base structure. The reaction formula is as follows: 。 3. The method for preparing a halogen-free flame retardant containing a Schiff base structure 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; And / or, in step S1, the acyl chloride compound is selected from diphenylphosphonyl chloride or diphenylphosphinyl chloride.

4. The method for preparing a halogen-free flame retardant containing a Schiff base structure according to claim 2, wherein: Step S1, dissolving an aldehyde-containing phenolic compound in an organic solvent, adding an acid-binding agent, mixing well, and then adding a phosphonyl chloride compound to react to obtain an intermediate.

5. The method for preparing a halogen-free flame retardant containing a Schiff base structure according to claim 4, wherein: In step S1, the organic solvent is selected from one or more combinations of acetone, tetrahydrofuran, dichloromethane, chloroform, dioxane, toluene, and xylene; And / or, in step S1, the acid binding agent is selected from one or a combination of sodium hydroxide, potassium carbonate, sodium carbonate, triethylamine, and pyridine.

6. The method for preparing a halogen-free flame retardant containing a Schiff base structure according to claim 2, wherein: In step S1, the reaction temperature is 50-70°C and the reaction time is 6-10 hours; And / or, in step S2, the organic solvent for the Schiff base reaction is selected from a combination of one or more of ethanol, methanol, dimethylformamide, tetrahydrofuran, dioxane, toluene, and xylene; And / or, in step S2, the reaction temperature is 50-70° C., and the reaction time is 6-10 hours.

7. A flame retardant epoxy resin comprising an epoxy resin prepolymer and a curing agent, characterized in that: It also includes the halogen-free flame retardant containing a Schiff base structure according to claim 1, or the halogen-free flame retardant containing a Schiff base structure obtained by the preparation method according to any one of claims 2 to 6.

8. The flame retardant epoxy resin according to claim 7, characterized in that The mass ratio of the epoxy resin prepolymer, the curing agent and the halogen-free flame retardant containing a Schiff base structure in the flame-retardant epoxy resin is 100: (20-90): (10-50).

9. 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.

10. Use of the halogen-free flame retardant containing a Schiff base structure according to claim 1, or use of the halogen-free flame retardant containing a Schiff base structure obtained by the preparation method according to any one of claims 2 to 6, characterized in that: Used in the preparation of flame retardant materials; The flame retardant material includes a flame retardant resin, which includes but is not limited to epoxy resin, bismaleimide resin, benzoxazine resin, and unsaturated polyester resin.

Images