Flame-retardant epoxy resin adhesive based on DOPO and containing boron-nitrogen proton transfer reaction as well as preparation method and application of flame-retardant epoxy resin adhesive

Abstract

The invention discloses a DOPO (9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-based boron-nitrogen proton transfer reaction-containing flame-retardant epoxy resin adhesive as well as a preparation method and application thereof, and relates to a preparation method of a flame-retardant epoxy resin composite material. The flame retardant property of the epoxy resin is improved. Meanwhile, nitrogen atoms on an imidazole ring in the product DTA-B, hydroxyl on boric acid and an epoxy resin composite system form a hydrogen bond and a boric acid ester structure, and hydroxyl and B-O in a polymer system form a hydrogen bond and a dynamic boric acid ester bond on the surface of a base material, and the structural characteristics also improve the mechanical and adhesive properties of the epoxy resin at the same time; the problems of mechanical and adhesive performance degradation and the like caused by addition of a flame retardant into an epoxy resin adhesive are solved. The invention provides a thought for the development of the DOPO-based flame retardant.

Description

Technical Field

[0001] The present invention belongs to the technical field of flame-retardant adhesive preparation and polymer material modification, and specifically relates to a boron-nitrogen proton transfer reaction. In the synthesized flame-retardant adhesive structure, hydrogen bonds and borate structures are formed between the nitrogen atom on the imidazole ring and the hydroxyl group and the epoxy resin composite material, as well as hydrogen bonds and dynamic borate bonds formed between the polymer system and the substrate surface, thereby improving the flame retardancy, mechanical and bonding properties of the epoxy resin adhesive. Background Art

[0002] Epoxy resins have been widely used in the automotive, aerospace, and specialty adhesives sectors due to their exceptional bonding properties, excellent chemical resistance, and good electrical insulation. However, the flammability of epoxy resins limits their application in a wider range of fields. During combustion, epoxy resin adhesives can produce large amounts of heat, smoke, toxic gases, and molten drippings, all of which pose a serious threat to personal safety and property protection. Especially in high-risk applications such as electronic components and automotive parts, the use of epoxy resin adhesives can have catastrophic consequences, including casualties and property losses. Currently, epoxy resin adhesives on the market have not yet achieved a comprehensive improvement in comprehensive performance, such as mechanical strength, high adhesion, flame retardancy, and sustainability, to meet the stringent requirements of industrial applications. Therefore, improving the flame retardant properties of epoxy resin adhesives and expanding their safe application range are important challenges in current research and development.

[0003] Phosphorus-containing flame retardants have attracted widespread attention due to their structural diversity and environmental friendliness. These compounds can contribute to the flame retardancy of epoxy resin adhesives in both the gas phase and the condensed phase. Among the many phosphorus-containing flame retardants, 9,10-dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO) occupies a prominent position due to its wide application. Despite this, DOPO still has some limitations in practical applications. First, in order to achieve the desired flame retardant effect, a high concentration of DOPO is usually required in the epoxy resin formulation. Second, when used as a single flame retardant, DOPO has limited efficiency in suppressing smoke generation.

[0004] In order to overcome the shortcomings of DOPO as a single flame retardant in epoxy resin adhesives, this study adopted an innovative strategy to improve the flame retardant properties by introducing the synergistic effects of other elements. The integration of elements such as nitrogen, boron, sulfur and silicon into DOPO-based systems has been shown to significantly improve their flame retardant properties. In particular, nitrogen-containing compounds promote the flame retardant process by releasing non-flammable gases. These gases can dilute the flammable pyrolysis products and oxygen concentration in the gas phase, thereby reducing the combustion intensity. At the same time, boron-containing compounds are highly favored for their low smoke emissions and non-toxic properties. They can form a protective glass layer on the polymer surface during combustion, effectively delaying the release of smoke and heat. Therefore, the integration of nitrogen and boron into DOPO-based systems can enhance the flame retardant mechanisms in the condensed phase and gas phase, providing a comprehensive approach to solving the limitations of DOPO in epoxy resins.

[0005] In this study, we directly combined elemental boron and nitrogen via proton transfer, using only ethanol as the chemical reaction solvent. This proton transfer reaction between the basic amino group of imidazole and boronic acid allows for high levels of boronic acid doping while ensuring stability, providing insights into the development of DOPO-based flame retardants.

[0006] Based on this research, the present inventors propose to incorporate boron into the existing phosphorus / nitrogen flame retardant DTA via a proton transfer reaction between boric acid and 2-aminobenzimidazole. This effort will ultimately lead to the synthesis of a new organophosphorus flame retardant, designated DTA-B. The anticipated epoxy resin / DTA-B system is expected to significantly reduce fire hazards while improving bond strength through the synergistic flame retardant effects of phosphorus, nitrogen, and boron. Notably, the process for integrating boron into the phosphorus / nitrogen system is designed to be simple and environmentally friendly, contributing to a more sustainable manufacturing approach. Summary of the Invention

[0007] The present invention aims to provide a preparation method for synthesizing a phosphorus / nitrogen / boron flame retardant by a proton transfer reaction. The method can achieve a high level of boric acid doping while ensuring stability through a proton transfer reaction between the basic amino group of imidazole and boric acid. The nitrogen atom and hydroxyl group on the imidazole ring of the product DTA-B form hydrogen bonds and borate structures with the epoxy resin composite system, and the hydroxyl group in the polymer system forms hydrogen bonds and dynamic borate bonds with BO on the surface of the substrate. Therefore, the addition of DTA-B simultaneously enhances the flame retardancy, mechanical and adhesive properties of the epoxy resin adhesive.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a flame-retardant epoxy resin adhesive based on a boron-nitrogen proton transfer reaction of DOPO, the method comprising the following steps:

[0009] (1) Preparation of DTA-B precursor DTA

[0010] There are related literatures on the synthesis of DTA:

[0011] a In a 100 mL three-necked round-bottom flask, 2-aminobenzimidazole (3.99 g, 0.03 mol) and terephthalaldehyde (2.01 g, 0.015 mol) were dissolved in 30 mL of dimethylformamide at room temperature. The solution was gradually heated to 80 °C and vigorously stirred for 6 h to promote the reaction;

[0012] b Subsequently, DOPO (6.48 g, 0.03 mol) was dissolved in an additional 15 mL of dimethylformamide and added to the flask. The temperature was raised to 120 °C and stirred for 12 h to ensure complete reaction.

[0013] c. After that, the excess dimethylformamide was removed by distillation to obtain a crude product, which was then washed repeatedly with ethanol and deionized water and then dried under vacuum at 60 °C to obtain DTA in a purified form;

[0014] (2) Preparation of flame-retardant epoxy resin adhesive DTA-B

[0015] a DTA (7.96 g, 0.01 mol) was finely ground and dispersed in 200 mL of ethanol, and vortexed and ultrasonicated to improve homogeneity;

[0016] b Boric acid (1.85 g, 0.03 mol) was then added to the solution, and the mixture was transferred to a 500 mL three-necked round-bottom flask equipped with a condenser reflux and a magnetic stirrer. The reaction mixture was heated to 70 °C and stirred continuously for 48 h to promote the proton transfer reaction between DTA and boronic acid;

[0017] c After the reaction was completed, the mixture was separated by filtration, and the excess boric acid was removed with ethanol. The crude product was then dried under vacuum at 60°C for 24 h to obtain white powder DTA-B with a yield of 78%;

[0018] (3) Construction and testing of single lap shear test

[0019] Single lap shear tests were conducted on steel and wood strips using DTA-B epoxy adhesive. The bonding process was as follows:

[0020] aFirst, uncured DTA-B epoxy adhesive was applied to the steel or wood strip substrates, and then the substrates were overlapped (12mm x 12mm) to form a specific configuration;

[0021] b Then, they were cured at 70 °C, 100 °C, and 140 °C for 2 h, respectively, and then cooled to room temperature;

[0022] cUse a universal testing machine to perform viscosity performance test.

[0023] In the method, the molar ratio of 2-aminobenzimidazole, terephthalaldehyde, DOPO and boric acid is 2:1:2:2.

[0024] In the method, DTA dispersed in ethanol is treated with vortexing and ultrasonication for 45 to 60 minutes to improve homogeneity.

[0025] In the method, the bonding area is 12 mm×12 mm and the thickness is 0.1 mm. The bonding performance is tested five times for each addition amount at a speed of 5 mm / min using a 20 kN load cell.

[0026] Due to the implementation of the above technical solution, the advantages and effects of the present invention are:

[0027] 1. The present invention adopts proton transfer reaction and uses ethanol as solvent to introduce the flame retardant element boron into the phosphorus / nitrogen flame retardant system with a yield of 78%.

[0028] 2. In the product DTA-B synthesized by the present invention, the nitrogen atom and hydroxyl group on the imidazole ring form hydrogen bonds and borate structures with the epoxy resin composite system, and the hydroxyl group in the polymer system forms hydrogen bonds and dynamic borate bonds with BO on the substrate surface. As a result, the addition of DTA-B simultaneously enhances the flame retardant, mechanical and adhesive properties of the epoxy resin adhesive.

[0029] 3. The present invention provides a new approach for developing novel DOPO-based flame retardants and overcoming the flammability problem of epoxy resin adhesives. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The synthetic route of DTA-B is shown in FIG.

[0031] Figure 2 The following are the Fourier transform infrared (FT-IR) spectra of DTA-B and its raw materials. DETAILED DESCRIPTION

[0032] The present invention will be described in detail below with reference to specific embodiments, but the present invention is not limited to these embodiments.

[0033] Example 1

[0034] The preparation of flame retardant DTA-B specifically includes the following steps:

[0035] a DTA (7.96 g, 0.01 mol) was finely ground and dispersed in 200 mL of ethanol, and vortexed and ultrasonicated to improve homogeneity;

[0036] b Boric acid (1.85 g, 0.03 mol) was then added to the solution, and the mixture was transferred to a 500 mL three-necked round-bottom flask equipped with a condenser reflux and a magnetic stirrer. The reaction mixture was heated to 70 °C and stirred continuously for 48 h to promote the proton transfer reaction between DTA and boronic acid;

[0037] After the reaction was completed, the mixture was separated by filtration, and the excess boric acid was removed with ethanol. The crude product was then dried under vacuum at 60° C. for 24 h to obtain white powder DTA-B with a yield of 78%.

[0038] Figure 2 This is the infrared spectrum of 2-aminobenzimidazole, terephthalaldehyde, DOPO, DTA, boric acid, and DTA-B.

[0039] In the Fourier transform infrared spectroscopy (FTIR), it can be clearly observed that the absorption peak of pH in DTA is at 2435 cm -1 The peaks at 1232 cm-1 and 1247 cm-2 disappeared, which confirmed the complete reaction of DOPO. In addition, the characteristic peaks at 1232 cm-1 and 1247 cm-2 continued to exist in both DTA and DTA-B. -1 and 903cm -1 The peaks of the benzimidazole group are observed in 2-aminobenzimidazole, DTA and DTA-B, namely the C=N stretching vibration peak at 1577 cm -1 The CN bond peak is at 1463 cm -1 In contrast, the -NH2 peak in 2-aminobenzimidazole is at 3385 cm -1 The -CHO peak in terephthalaldehyde is at 2868 cm -1 The disappearance of the BO bond peak at 1378 cm in the DTA-B spectrum -1 The appearance of , corresponds to the BO bond in boric acid, providing conclusive evidence for the successful transfer of boron element into the DTA-B structure.

[0040] Example 2

[0041] The preparation of epoxy resin and its composite material specifically includes the following steps:

[0042] (1) Preparation of epoxy resin

[0043] a. Heat the epoxy resin and mold in a forced air oven at 70°C for 30 minutes;

[0044] b. Combine a certain amount of epoxy resin and 4,4'-diaminodiphenylmethane in a beaker and stir in a 70°C water bath for 10 minutes to promote mixing. Epoxy resin: 4,4'-diaminodiphenylmethane = 4:1 (mass ratio);

[0045] c. After uniform mixing, the mixture was carefully transferred to a vacuum oven and vacuum was applied to effectively remove any remaining bubbles;

[0046] The degassed mixture was then poured into a mold, and the mold was transferred to a vacuum oven for curing at 120°C for 4 h. After high-temperature curing, it was cooled to room temperature to obtain epoxy resin test specimens.

[0047] (2) Preparation of DTA epoxy resin composites

[0048] a. Heat the epoxy resin and mold in a blast oven at 70°C for 30 min;

[0049] b. Mix a certain amount of epoxy resin and the prepared DTA in a beaker and stir in a 70 °C water bath for 10 min to promote initial mixing;

[0050] c. Add a certain amount of 4,4'-diaminodiphenylmethane to the stirred mixture and stir for another 10 minutes to ensure uniform mixing;

[0051] d. After uniform mixing, the mixture was carefully transferred to a vacuum oven and vacuum was applied to effectively remove any remaining bubbles;

[0052] The degassed mixture was then poured into a mold, which was transferred to a vacuum oven for curing at 120°C for 4 h. After high-temperature curing, it was cooled to room temperature to obtain DTA epoxy resin composite test specimens.

[0053] (3) Preparation of DTA-B epoxy resin composite materials

[0054] a. Heat the epoxy resin and mold in a forced air oven at 70°C for 30 minutes;

[0055] b. Mix a certain amount of epoxy resin and the prepared DTA-B in a beaker and stir in a 70 °C water bath for 10 min to promote preliminary mixing;

[0056] c. Add a certain amount of 4,4'-diaminodiphenylmethane to the stirred mixture and stir for another 10 minutes to ensure uniform mixing;

[0057] d. After uniform mixing, the mixture was carefully transferred to a vacuum oven and vacuum was applied to effectively remove any remaining bubbles;

[0058] The degassed mixture was then poured into a mold, which was transferred to a vacuum oven for curing at 120°C for 4 h. After high-temperature curing, it was cooled to room temperature to obtain DTA-B epoxy resin composite test specimens.

[0059] The epoxy resin and composite material prepared in Example 2 were tested for limiting oxygen index according to GB / T 2406.2-2009, for vertical burning rating according to GB / T 2408-2008, and for mechanical and adhesive properties using a WDW-20 universal testing machine. The test results are shown in Table 1.

[0060] Table 1 Test results of combustion performance, mechanical properties and viscosity properties of epoxy resin and its composite materials

[0061]

[0062] It can be seen from Table 1 that the limiting oxygen index of pure epoxy resin is 25.8%, the vertical burning grade result is NR grade, the flexural and tensile strengths are 101.4 MPa and 56.7 MPa respectively, and the shear strengths of the bonded steel strips and wood strips are 7.23 MPa and 5.63 MPa respectively; when the prepared DTA addition amount is 5%, the prepared DTA epoxy resin composite material has a limiting oxygen index of 32.0%, the vertical burning grade result is V-1 grade, and the flexural and tensile strengths are 82.8 MPa and 52.3 MPa respectively; when the prepared DTA-B addition amount is 5%, the prepared DTA-B epoxy resin composite material has a limiting oxygen index of 33.2%, the vertical burning grade result is V-0 grade, the flexural and tensile strengths are 120.4 MPa and 62.3 MPa respectively, and the shear strengths of the bonded steel strips and wood strips are 14.38 MPa and 9.29 MPa respectively.

[0063] The above detailed description of the present invention is intended to enable persons familiar with the art to understand the contents of the present invention and implement them. It does not limit the scope of protection of the present invention. Any equivalent changes or modifications made based on the spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A flame retardant epoxy resin adhesive based on DOPO containing boron-nitrogen proton transfer reaction and its preparation method and application, characterized in that The flame retardant, mechanical and adhesive properties of the epoxy resin are enhanced, and the preparation method thereof comprises the following steps: (1) Preparation of DTA-B precursor DTA There are related literatures on the synthesis of DTA: a In a 100 mL three-necked round-bottom flask, 2-aminobenzimidazole (3.99 g, 0.03 mol) and terephthalaldehyde (2.01 g, 0.015 mol) were dissolved in 30 mL of dimethylformamide at room temperature. The solution was gradually heated to 80 °C and vigorously stirred for 6 h to promote the reaction; b Subsequently, DOPO (6.48 g, 0.03 mol) was dissolved in an additional 15 mL of dimethylformamide and added to the flask. The temperature was raised to 120 °C and stirred for 12 h to ensure complete reaction. c. After that, the excess dimethylformamide was removed by distillation to obtain a crude product, which was then washed repeatedly with ethanol and deionized water and then dried under vacuum at 60 °C to obtain DTA in a purified form; (2) Preparation of flame-retardant epoxy resin adhesive DTA-B a DTA (7.96 g, 0.01 mol) was finely ground and dispersed in 200 mL of ethanol, and vortexed and ultrasonicated to improve homogeneity; b Boric acid (1.85 g, 0.03 mol) was then added to the solution, and the mixture was transferred to a 500 mL three-necked round-bottom flask equipped with a condenser reflux and a magnetic stirrer. The reaction mixture was heated to 70 °C and stirred continuously for 48 h to promote the proton transfer reaction between DTA and boronic acid; c After the reaction was completed, the mixture was separated by filtration, and the excess boric acid was removed with ethanol. The crude product was then dried under vacuum at 60°C for 24 h to obtain white powder DTA-B with a yield of 78%; (3) Construction and testing of single lap shear test Single lap shear tests were conducted on steel and wood strips using DTA-B epoxy adhesive. The bonding process was as follows: aFirst, uncured DTA-B epoxy adhesive was applied to the steel or wood strip substrates, and then the substrates were overlapped (12mm x 12mm) to form a specific configuration; b Then, they were cured at 70 °C, 100 °C, and 140 °C for 2 h, respectively, and then cooled to room temperature; cUse a universal testing machine to perform viscosity performance test.

2. The method according to claim 1, wherein: In the method, the molar ratio of 2-aminobenzimidazole, terephthalaldehyde, DOPO and boric acid is 2:1:2:

2.

3. The method according to claim 1, wherein: In the method, DTA dispersed in ethanol is treated with vortexing and ultrasonication for 45 to 60 minutes to improve homogeneity.

4. The method according to claim 1, wherein: In this method, the bonding area is 12 mm × 12 mm and the thickness is 0.1 mm. The bonding performance is tested five times for each addition amount at a speed of 5 mm / min under a 20 kN load cell.

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