Phosphorus / nitrogen / boron epoxy resin flame retardant with hydrogen bond covalent organic framework constructed through ball milling method and preparation method of phosphorus / nitrogen / boron epoxy resin flame retardant

Abstract

The invention discloses a phosphorus / nitrogen / boron epoxy resin flame retardant with a hydrogen bond covalent organic framework constructed through a ball milling method and a preparation method of the phosphorus / nitrogen / boron epoxy resin flame retardant, and relates to a preparation method of a flame-retardant epoxy resin composite material. A hydrogen bond covalent organic framework is formed between nitrogen atoms on hydroxyl and a pyridine ring through the physical action of ball milling, and a boron element is introduced into a phosphorus / nitrogen flame-retardant system, so that the flame-retardant property of the epoxy resin is remarkably improved. The dry ball milling method provided by the invention has the advantages of strong chemical reaction, environmental protection, no solvent, low cost, simple operation and the like, and effectively avoids the problems of solvent consumption and environmental health caused by the traditional solvent method for synthesizing the flame retardant. The flame retardant property of the final flame retardant PDN-B synthesized by the method is superior to that of a PDN flame retardant, and a thought is provided for development of DOPO-based flame retardants and construction of hydrogen bonds.

Description

Technical Field

[0001] The present invention belongs to the technical field of flame retardant preparation and polymer materials, and particularly relates to a phosphorus / nitrogen / boron epoxy resin flame retardant with a hydrogen bond covalent organic framework constructed by a ball milling method and a preparation method thereof. Background Art

[0002] Epoxy resins, with their excellent mechanical properties, electrical insulation, and chemical resistance, have found widespread application in key areas such as construction, electronics, and coatings. However, their significant flammability significantly limits further expansion of their applications. In particular, when burned, epoxy resins release significant heat, producing dense smoke and molten drippings. These characteristics make the development of highly effective flame retardants a pressing challenge.

[0003] 9,10-Dihydro-9-oxo-10-phosphaphenanthrene-10-oxide (DOPO), a common phosphorus-containing flame retardant, exhibits a moderate flame retardant effect in both the gas and condensed phases. However, in practical applications, the sole use of DOPO presents significant limitations. On the one hand, achieving the desired flame retardant effect typically requires the addition of high concentrations of DOPO to epoxy resin formulations. On the other hand, when used alone as a flame retardant, DOPO's ability to suppress smoke generation is relatively limited.

[0004] In order to overcome the shortcomings of DOPO when used alone as a flame retardant, this study innovatively proposed a strategy to improve flame retardant properties through the synergistic effect of elements. Specifically, the introduction of elements such as nitrogen, boron, sulfur, and silicon into the DOPO system has been proven to effectively enhance its flame retardant ability. Nitrogen-containing compounds can release non-flammable gases during combustion, thereby diluting the concentration of combustible gases and oxygen in the gas phase, achieving the effect of slowing down the fire; boron-containing compounds have attracted much attention due to their low smoke release and non-toxicity during combustion. They can form a layer of protective glassy material on the surface of the material, effectively delaying the release of smoke and heat energy. By strategically introducing nitrogen and boron into the DOPO system, the flame retardant effects of the condensed phase and the gas phase are significantly enhanced, providing a comprehensive and effective solution for improving the flame retardant properties of epoxy resins.

[0005] The history of mechanochemistry dates back to the Stone Age, when simple mechanical processes were used to process materials. However, the last century has seen a surge in systematic and in-depth research into mechanochemistry. Unlike chemical reactions triggered by heat and temperature, mechanical processes such as grinding or sliding can promote specific chemical reactions through mechanochemical effects. The development of mechanochemistry is closely linked to advances in grinding technology, often achieved using high-energy ball mills. During ball milling, the violent collision between stainless steel balls and the milling jar creates a transient environment of high temperature and pressure, unique conditions that strongly promote chemical reactions.

[0006] In this study, a phosphorus-nitrogen flame retardant (PDN) was synthesized by a one-pot reaction of 4-aminopyridine, 4-pyridinecarboxaldehyde, and DOPO. Boric acid was then added to introduce boron. The hydroxyl groups in the boric acid can form a large number of hydrogen-bonded covalent organic frameworks with the nitrogen atoms on the pyridine ring of PDN, successfully introducing boron into the phosphorus / nitrogen flame retardant system and synthesizing the PDN-B flame retardant. During the synthesis of PDN-B, this study employed dry ball milling, a mechanochemical method, to construct the hydrogen-bonded covalent organic framework. This method is simple to operate, environmentally friendly, low-cost, and solvent-free, providing a new approach and path for the development of DOPO-based phosphorus / nitrogen / boron flame retardants. Summary of the Invention

[0007] The present invention aims to provide a mechanochemical method for synthesizing a phosphorus / nitrogen / boron flame retardant and constructing a hydrogen-bonded covalent organic framework using dry ball milling. This method involves placing a synthesized phosphorus / nitrogen / boron flame retardant (PDN), boric acid, and grinding balls into a ball milling jar and subjecting the resulting mechanochemical method to dry ball milling. This mechanical milling operation under certain conditions results in the formation of a hydrogen-bonded covalent organic framework between the PDN and the boric acid, ultimately yielding the phosphorus / nitrogen / boron flame retardant (PDN-B) with a yield of 79.28%. This method is simple to operate, environmentally friendly, low-cost, solvent-free, and produces a product with high flame retardant efficiency.

[0008] The object of the present invention is achieved by the following technical solution: a phosphorus / nitrogen / boron epoxy resin flame retardant that constructs a hydrogen bond covalent organic framework by ball milling and a preparation method thereof, the method comprising the following steps:

[0009] (1) Preparation of flame retardant PDN-B precursor PDN

[0010] a 4-Aminopyridine (0.94 g, 0.01 mol) and 4-pyridinecarboxaldehyde (1.07 g, 0.01 mol) were dissolved in 250 mL of ethanol and transferred to a three-necked round-bottom flask equipped with a condenser reflux and a magnetic stirrer. The solution was then reacted at 85 °C in a water bath for 6 h.

[0011] b Subsequently, DOPO (2.16 g, 0.01 mol) was added to the above solution and stirring was continued for 4 h;

[0012] After the reaction is complete, the mixture is separated by filtration and washed with ethanol. The crude product is then dried under vacuum at 60°C for 24 hours to obtain a white powdery product, PDN.

[0013] (2) Preparation of PDN-B flame retardant by dry ball milling

[0014] a. Place dry PDN and boric acid in a ball mill at a molar mass ratio of 3:2, a ball-to-material ratio of 40:1, and a large ball (10 mm): small ball (5 mm) ratio of 1:1. Ball milling was performed at 800 r / min intermittently (every 5 min) for 2 h to obtain flame retardant PDN-B.

[0015] In the method, the molar ratio of DOPO, 4-aminopyridine and 4-pyridinecarboxaldehyde is 1:1:1.

[0016] In the dry ball milling method, dry PDN and boric acid are placed in a ball milling jar at a molar mass ratio of 3:2, and grinding balls are added to the jar with a ball-to-material ratio of 40:1 and a large ball (10 mm): small ball (5 mm) ratio of 1:1. The flame retardant PDN-B is prepared by intermittent ball milling at 800 r / min (every 5 minutes) for 2 hours.

[0017] Due to the implementation of the above technical solution, the advantages and effects of the present invention are:

[0018] 1. The present invention adopts dry ball milling technology to construct a hydrogen-bonded covalent organic framework and introduces boron into the phosphorus / nitrogen flame retardant system with a yield of 79.28%. This method has the advantages of being solvent-free, green and environmentally friendly, low cost and simple.

[0019] 2. The final product PDN-B synthesized by the present invention has improved flame retardant properties compared to the product PDN.

[0020] 3. This invention provides ideas for the development of new DOPO-based flame retardants and the construction of hydrogen-bonded covalent organic frameworks. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The synthetic route of PDN and PDN-B is shown in FIG.

[0022] Figure 2 Fourier transform infrared (FT-IR) spectra of PDN, PDN-B and their raw materials. DETAILED DESCRIPTION

[0023] The present invention will be described in detail below with reference to specific embodiments, but the present invention is not limited to these embodiments.

[0024] Example 1

[0025] The preparation of flame retardant PDN-B specifically includes the following steps:

[0026] (1) Preparation of flame retardant PDN-B precursor PDN

[0027] a 4-Aminopyridine (0.94 g, 0.01 mol) and 4-pyridinecarboxaldehyde (1.07 g, 0.01 mol) were dissolved in 250 mL of ethanol and transferred to a three-necked round-bottom flask equipped with a condenser reflux and a magnetic stirrer. The solution was then reacted at 85 °C in a water bath for 6 h.

[0028] b Subsequently, DOPO (2.16 g, 0.01 mol) was added to the above solution and stirring was continued for 4 h;

[0029] After the reaction is complete, the mixture is separated by filtration and washed with ethanol. The crude product is then dried under vacuum at 60°C for 24 hours to obtain a white powdery product, PDN.

[0030] (2) Preparation of PDN-B flame retardant by dry ball milling

[0031] a. Place dry PDN and boric acid in a ball mill at a molar mass ratio of 3:2, a ball-to-material ratio of 40:1, and a large ball (10 mm): small ball (5 mm) ratio of 1:1. Ball milling was performed at 800 r / min intermittently (every 5 min) for 2 h to obtain flame retardant PDN-B.

[0032] Figure 2 It is the infrared spectrum of 4-aminopyridine, 4-pyridinecarboxaldehyde, DOPO, PDN, PDN-B, and boric acid.

[0033] In Fourier transform infrared spectroscopy (FTIR), 4-aminopyridine is detected at 3430 cm -1 The characteristic peak at 1710 cm-1 in 4-pyridinecarboxaldehyde disappears. -1 The characteristic peak at 2435cm in PDN disappears, indicating the formation of Schiff base intermediate. -1 The characteristic peaks at 1232 cm-1 and 1247 cm-2 at PDN and PDN-B respectively disappear. -1 and 903cm -1 The 1400 cm-1 region in boric acid corresponds to the P=O and PO bonds, indicating that the phosphorus-containing groups of DOPO are retained. -1 The BO peak at 1378 cm in PDN-B -1 The reason for this is that when boric acid forms a hydrogen bond (OH…N) with the nitrogen atom (N) of the pyridine ring, the -1The wavenumber of the BOH bending vibration peak at [amount missing in original text] does shift, typically toward lower wavenumbers (red-shift). Comparing the FTIR spectra of PDN-B with those of PDN reveals no new peaks other than the BO peak in PDN-B, indicating that no chemical changes occurred during the ball-milling process. This demonstrates the formation of hydrogen bonds between the boric acid and PDN, indicating the successful synthesis of PDN-B.

[0034] Example 2

[0035] The preparation of epoxy resin and its composite material specifically includes the following steps:

[0036] (1) Preparation of epoxy resin

[0037] a. Heat the epoxy resin and mold in a forced air oven at 70°C for 30 minutes;

[0038] 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);

[0039] c. After uniform mixing, the mixture was carefully transferred to a vacuum oven and vacuum was applied to effectively remove any remaining bubbles;

[0040] 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.

[0041] (2) Preparation of PDN epoxy resin composite materials

[0042] a. Heat the epoxy resin and mold in a forced air oven at 70°C for 30 minutes;

[0043] b. Mix a certain amount of epoxy resin and the prepared PDN in a beaker and stir in a 70 °C water bath for 10 min to promote initial mixing;

[0044] c. Add a certain amount of 4,4'-diaminodiphenylmethane to the stirred mixture and stir for another 10 minutes to ensure uniform mixing;

[0045] d. 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 a PDN epoxy resin composite test specimen.

[0047] (3) Preparation of PDN-B epoxy resin composite materials

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

[0049] b. Mix a certain amount of epoxy resin and the prepared PDN-B in a beaker and stir in a 70 °C water bath for 10 min to promote preliminary 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 PDN-B epoxy resin composite material test specimens.

[0053] The epoxy resin and composite material strips prepared in Example 2 were tested for limiting oxygen index according to GB / T 2406.2-2009, and for vertical burning rating according to GB / T 2408-2008. The test results are shown in Table 1.

[0054] Table 1 Combustion performance test results of epoxy resin and its composite materials

[0055]

[0056] It can be seen from Table 1 that the limiting oxygen index of pure epoxy resin is 26.3%, and the vertical burning grade result is NR grade; when the prepared PDN addition amount is 5%, the prepared PDN epoxy resin composite material has a limiting oxygen index of 33.4%, and the vertical burning grade result is V-1 grade; when the prepared PDN-B addition amount is 5%, the prepared PDN-B epoxy resin composite material has a limiting oxygen index of 34.3%, and the vertical burning grade result is V-0 grade.

[0057] 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 phosphorus / nitrogen / boron epoxy resin flame retardant constructed by ball milling to form a hydrogen bond covalent organic framework and a preparation method thereof, characterized in that The hydrogen-bonded covalent organic framework of the prepared phosphorus / nitrogen / boron flame retardant (PDN-B) was constructed by a mechanochemical method called dry ball milling, which improved the flame retardancy of the epoxy resin. The synthesis of the flame retardant PDN-B includes the following steps: (1) Preparation of flame retardant PDN-B precursor PDN a 4-Aminopyridine (0.94 g, 0.01 mol) and 4-pyridinecarboxaldehyde (1.07 g, 0.01 mol) were dissolved in 250 mL of ethanol and transferred to a three-necked round-bottom flask equipped with a condenser reflux and a magnetic stirrer. The solution was then reacted at 85 °C in a water bath for 6 h. b Subsequently, DOPO (2.16 g, 0.01 mol) was added to the above solution and stirring was continued for 4 h; After the reaction is complete, the mixture is separated by filtration and washed with ethanol. The crude product is then dried under vacuum at 60°C for 24 hours to obtain a white powdery product, PDN. (2) Preparation of PDN-B flame retardant by dry ball milling a. Place dry PDN and boric acid in a ball mill at a molar mass ratio of 3:2, a ball-to-material ratio of 40:1, and a large ball (10 mm): small ball (5 mm) ratio of 1:

1. Ball milling was performed at 800 r / min intermittently (every 5 min) for 2 h to obtain flame retardant PDN-B.

2. The method according to claim 1, wherein: In the method, the molar ratio of DOPO, 4-aminopyridine and 4-pyridinecarboxaldehyde is 1:1:

1.

3. The method according to claim 1, wherein: In the dry ball milling method, dry PDN and boric acid are placed in a ball milling jar at a molar mass ratio of 3:2, and grinding balls are added to the jar with a ball-to-material ratio of 40:1 and a large ball (10 mm): small ball (5 mm) ratio of 1:

1. The flame retardant PDN-B is prepared by intermittent ball milling at 800 r / min (every 5 minutes) for 2 hours.

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