Preparation method and application of nitrogen / phosphorus-containing covalent organic framework flame retardant

Abstract

The invention discloses a preparation method and application of a nitrogen / phosphorus-containing covalent organic framework flame retardant, and relates to the technical field of polymer flame-retardant materials. The flame retardant is of a porous network crystal structure constructed by a phosphorus-containing monomer and a nitrogen-containing monomer through a covalent bond, and has a P = O bond and a nitrogen-containing heterocyclic ring structure. And introducing the flame retardant into an epoxy resin system, dispersing, mixing and curing to obtain the flame-retardant composite material. Results show that the flame retardancy and thermal stability of the epoxy resin can be remarkably improved and the heat release rate and the total smoke yield can be reduced under the condition of relatively low addition amount of the material. The flame retardant mechanism is mainly attributed to a synergistic effect of a gas phase free radical quenching effect and a condensed phase charring effect, and a phosphorus-containing structure promotes formation of a compact carbon layer, so that heat and combustible gas transfer is effectively blocked. The material disclosed by the invention has good flame retardance, smoke suppression property and thermal stability, and has a relatively wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of polymer flame retardant materials technology, specifically relating to an epoxy resin flame retardant composite system based on nitrogen / phosphorus covalent organic framework materials and its preparation method, which is applicable to the field of high-performance fireproof materials. Background Technology

[0002] Epoxy resin (EP) is an important class of thermosetting polymer materials, widely used in electronic packaging, aerospace, composite materials, and coatings due to its excellent mechanical properties, adhesive properties, electrical insulation properties, and chemical corrosion resistance. However, the epoxy resin molecular structure contains a large number of flammable organic groups, which easily undergo thermal decomposition under heat or flame, releasing a large amount of flammable volatiles. This is accompanied by a high heat release rate and smoke generation, posing a significant fire safety hazard and severely restricting its further application in fields with high fire safety requirements.

[0003] To improve the flame retardant properties of epoxy resins, existing technologies typically employ additive or reactive flame retardants for modification. While halogenated flame retardants offer high flame retardant efficiency, they release toxic and corrosive gases during combustion, adversely affecting the environment and human health, and have thus been gradually restricted. Halogen-free flame retardant systems, such as those containing phosphorus, nitrogen, and inorganic fillers, have attracted widespread attention due to their environmental friendliness. However, these systems generally suffer from problems such as high addition amounts, poor dispersibility, and adverse effects on the mechanical properties of the material.

[0004] In recent years, covalent organic frameworks (COFs), as a novel type of porous crystalline material, have been widely studied in fields such as adsorption, catalysis, and energy due to their advantages of strong structural designability, large specific surface area, and good thermal stability. Some studies have attempted to introduce COFs into polymer materials to improve their flame retardant properties, utilizing their porous structure and functionalized groups to achieve synergistic flame retardancy in both the gas and condensed phases. However, the application of existing COFs in epoxy resin systems is still in the exploratory stage, mainly due to the following shortcomings:

[0005] (1) The structure is simple and lacks an effective combination of synergistic flame retardant elements, making it difficult to achieve high-efficiency flame retardancy;

[0006] (2) Poor interfacial compatibility leads to uneven dispersion and affects the flame retardant effect;

[0007] (3) Limited ability to suppress flue gas and control heat release;

[0008] (4) It is difficult to balance flame retardant properties and mechanical properties under low addition conditions.

[0009] Therefore, developing a covalent organic framework material that combines good structural stability, phosphorus-nitrogen synergy, and excellent dispersion performance, and applying it to the flame retardant modification of epoxy resin, is of great significance for achieving efficient flame retardancy, smoke suppression, and synergistic performance improvement. Summary of the Invention

[0010] The purpose of this invention is to overcome the above-mentioned shortcomings of the prior art and to provide a method for preparing nitrogen / phosphorus covalent organic framework flame retardants and their applications.

[0011] To achieve the above-mentioned objectives, in a first aspect, the present invention provides a method for preparing a nitrogen / phosphorus covalent organic framework flame retardant, comprising:

[0012] Preparation of nitrogen / phosphorus covalent organic frameworks;

[0013] In one embodiment, a method for preparing a nitrogen / phosphorus covalent organic framework is as follows:

[0014] First, cyanuric chloride and triethylamine were dissolved in N,N-dimethylformamide and stirred in an ice bath for 30 minutes to obtain a mixture. Then, phenylphosphonic acid was dissolved in N,N-dimethylformamide and slowly added dropwise to the mixture over 60 minutes. The mixture was reacted in an ice bath for 2 hours, then stirred at room temperature for 12 hours, and finally stirred at 120°C for 48 hours. After the reaction cooled to room temperature, the mixture was filtered, the filter cake was washed several times with ethanol and acetone, and the product was obtained by rotary evaporation. The product was dried under vacuum for 12 hours to finally obtain a nitrogen / phosphorus covalent organic framework flame retardant.

[0015] The nitrogen / phosphorus covalent organic framework flame retardant is a porous network crystal structure formed by phosphorus-containing monomers and nitrogen-containing monomers connected by covalent bonds, and its structure contains both P=O bonds and nitrogen-containing heterocyclic structures.

[0016] Secondly, the present invention provides the application of the nitrogen / phosphorus covalent organic framework flame retardant prepared by the above-mentioned method.

[0017] In one embodiment, it is used to prepare a flame-retardant epoxy resin.

[0018] In one embodiment, the obtained nitrogen / phosphorus covalent organic framework flame retardant is dispersed in E51 type epoxy resin by mechanical stirring to obtain a uniform mixture. Then, the mixture is reacted at 100°C for a period of time, and 4,4'-diaminodiphenylmethane curing agent is added and stirred until a uniform epoxy resin mixture solution is obtained. Finally, the epoxy resin mixture solution is vacuumed to remove excess air bubbles, and then poured into a preheated mold for thermosetting. After curing, it is allowed to cool to room temperature. Finally, a flame-retardant epoxy resin composite material is prepared.

[0019] In one embodiment, the mass ratio of the nitrogen / phosphorus covalent organic framework flame retardant to E51 type epoxy resin is 3 to 12:100.

[0020] In one embodiment, the reaction temperature of the nitrogen / phosphorus covalent organic framework flame retardant and the E51 type epoxy resin is 90-100°C, and the reaction time is 30-60 minutes.

[0021] Furthermore, the application is characterized in that the peak heat release rate (pHRR) of the flame-retardant epoxy resin composite material is reduced by 66.5% or more.

[0022] Furthermore, the application is characterized in that the peak smoke generation rate of the flame-retardant epoxy resin composite material is reduced by 31.7% or more.

[0023] The advantages and beneficial effects of this invention compared to the prior art are as follows:

[0024] (1) The nitrogen / phosphorus covalent organic framework flame retardant provided by the present invention is simple to prepare and can significantly improve the flame retardant performance of epoxy resin under low addition conditions.

[0025] (2) The nitrogen / phosphorus covalent organic framework flame retardant provided by the present invention is a flame retardant epoxy resin. The covalent organic framework material has good structural tunability and can achieve performance optimization by controlling the monomer structure. It is suitable for flame retardant modification of various polymer systems.

[0026] (3) The nitrogen / phosphorus covalent organic framework flame retardant provided by the present invention retards epoxy resin. During the combustion process, the flame retardant inhibits the reaction of combustible free radicals in the gas phase and promotes the dehydration and carbonization of the matrix in the condensed phase to form a stable and dense carbon layer, thereby achieving multi-mechanism synergistic flame retardancy.

[0027] (4) The nitrogen / phosphorus covalent organic framework flame retardant provided by the present invention has good compatibility with epoxy resin matrix, has little impact on the mechanical properties of the material, and has good application prospects.

[0028] (5) This invention discloses the preparation of nitrogen / phosphorus covalent organic framework composite materials and their application in the field of flame-retardant epoxy resins. A green and environmentally friendly flame-retardant epoxy resin is prepared by using a nitrogen / phosphorus covalent organic framework as a filler and curing it. Due to the catalytic effect of the phosphorus-containing structure, it can promote the formation of a continuous, dense carbon layer with a high degree of graphitization, significantly enhancing its ability to block heat and combustible gases. During pyrolysis or combustion, it can significantly reduce the release of carbon monoxide and carbon dioxide, which is beneficial to air pollution control and sustainable development, and greatly reduces the harm caused by fires. Attached Figure Description

[0029] Figure 1 The infrared spectra of the nitrogen / phosphorus covalent organic framework material, monomer cyanuric chloride, and phenylphosphonic acid obtained in Example 1 are shown.

[0030] Figure 2 The image shows the XRD pattern of the nitrogen / phosphorus covalent organic framework material obtained in Example 1.

[0031] Figure 3 The graph shows the heat release rate of the flame-retardant epoxy resin containing nitrogen / phosphorus covalent organic framework obtained in Experiment Example 1.

[0032] Figure 4 The graph shows the smoke production rate of the flame-retardant epoxy resin containing nitrogen / phosphorus covalent organic framework obtained in Experiment Example 2.

[0033] Figure 5 The graph shows the carbon monoxide formation rate of the flame-retardant epoxy resin obtained from the nitrogen / phosphorus covalent organic framework flame retardant in Experiment Example 3.

[0034] Figure 6 The graph shows the carbon dioxide generation rate of the flame-retardant epoxy resin obtained from the nitrogen / phosphorus covalent organic framework flame retardant in Experiment Example 4. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments in this application without creative effort are within the scope of protection of this application.

[0036] To achieve the aforementioned objectives of the invention, in the first aspect,

[0037] This invention provides a method for preparing a nitrogen / phosphorus covalent organic framework flame retardant, comprising the following steps:

[0038] Preparation of nitrogen / phosphorus covalent organic frameworks;

[0039] In one embodiment, a method for preparing a nitrogen / phosphorus covalent organic framework is as follows:

[0040] First, cyanuric chloride and triethylamine were dissolved in N,N-dimethylformamide and stirred for 30 minutes under ice bath conditions to obtain a mixture. Then, phenylphosphonic acid was dissolved in N,N-dimethylformamide and slowly added dropwise to the above mixture over 60 minutes. The mixture was reacted in an ice bath for 2 hours, then stirred at room temperature for 12 hours, and finally stirred at 120°C for 48 hours. After the reaction cooled to room temperature, the mixture was filtered, the filter cake was washed several times with ethanol and acetone, and rotary evaporation was performed to obtain the product. The obtained product was dried under vacuum for 12 hours to finally obtain a nitrogen / phosphorus covalent organic framework flame retardant.

[0041] Secondly, the present invention provides the application of the nitrogen / phosphorus covalent organic framework flame retardant prepared by the above-mentioned method.

[0042] Furthermore, it is used to prepare flame-retardant epoxy resins.

[0043] Furthermore, the obtained nitrogen / phosphorus covalent organic framework flame retardant was dispersed in E51 epoxy resin by mechanical stirring to obtain a uniform mixture. Then, the mixture was reacted at 100°C for a period of time, and 4,4'-diaminodiphenylmethane curing agent was added. The mixture was stirred until a uniform epoxy resin mixture solution was obtained. Finally, the epoxy resin mixture solution was vacuumed to remove excess air bubbles, and then poured into a preheated mold for thermosetting. After curing, it was allowed to cool to room temperature. Finally, a flame-retardant epoxy resin composite material was prepared.

[0044] Furthermore, the mass ratio of the nitrogen / phosphorus covalent organic framework flame retardant to E51 type epoxy resin is 3 to 12:100, for example, 3:100, 4:100, 6:100, 8:100, 10:100, 12:100, etc. Within this range, the mass ratio of the nitrogen / phosphorus covalent organic framework flame retardant to E51 type epoxy resin can not only effectively improve the flame retardant properties of epoxy resin composites, but also avoid the reduction of mechanical properties of epoxy resin composites due to excessive flame retardant dosage.

[0045] Furthermore, the reaction temperature between the nitrogen / phosphorus covalent organic framework flame retardant and E51 type epoxy resin is 90-100℃, for example, 90℃, 95℃, 100℃, etc.; the reaction time is 30-60 minutes, for example, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 60 minutes, etc.; the optimal reaction conditions are selected according to the ratio of nitrogen / phosphorus covalent organic framework flame retardant and E51 type epoxy resin, and the reaction temperature and reaction time are reasonably controlled to obtain a hot epoxy resin solution that is uniformly mixed with the flame retardant.

[0046] According to experiments, the optimal conditions are a mass ratio of 6:74:20 for nitrogen / phosphorus covalent organic framework flame retardant, E51 type epoxy resin and 4,4'-diaminodiphenylmethane curing agent, and uniform mixing at 100℃ for 40 minutes, which can yield epoxy resin composite material with the highest flame retardant efficiency and the best mechanical properties.

[0047] This invention has undergone numerous experiments, and some of the experimental results are presented here for reference to further describe the invention in detail. The following is a detailed description in conjunction with specific embodiments.

[0048] Example 1

[0049] The preparation of a nitrogen / phosphorus covalent organic framework flame retardant includes the following steps:

[0050] First, 1.84 g of cyanuric chloride and 5 mL of triethylamine were dissolved in 100 mL of N,N-dimethylformamide and stirred in an ice bath for 30 minutes to obtain a mixture. Then, 1.58 g of phenylphosphonic acid was dissolved in 100 mL of N,N-dimethylformamide and slowly added dropwise to the above mixture over 60 minutes. The mixture was reacted in an ice bath for 2 hours, then stirred at room temperature for 12 hours, and finally stirred at 120°C for 48 hours. After the reaction cooled to room temperature, the mixture was filtered, the filter cake was washed several times with ethanol and acetone, and rotary evaporation was performed to obtain the product. The obtained product was dried under vacuum at 80°C for 12 hours to finally obtain a nitrogen / phosphorus covalent organic framework flame retardant.

[0051] Experimental Example 1

[0052] Six parts by weight of the nitrogen / phosphorus covalent organic framework flame retardant prepared in Example 1 were dispersed in 74 parts by weight of E51 type epoxy resin by mechanical stirring. The mixture was then heated to 100°C and reacted for 60 minutes. Subsequently, 20 parts by weight of 4,4'-diaminodiphenylmethane curing agent were added, and the mixture was stirred until a stable and homogeneous epoxy resin solution was obtained. Finally, the epoxy resin solution was vacuum-sealed to remove excess air bubbles, then poured into a preheated mold, and cured at 100°C for half an hour, 120°C for 2 hours, and 150°C for 2 hours. After curing, the mixture was allowed to cool naturally to room temperature and demolded to obtain the flame-retardant epoxy resin composite material.

[0053] The prepared flame-retardant epoxy resin specimens were subjected to cone calorimetry testing. The cone calorimetry specimens were 100mm × 100mm × 3mm in size. The heat release rate of the flame-retardant epoxy resin obtained in Experimental Example 1 is as follows: Figure 1 As shown in the figure. The results indicate that the addition of a nitrogen / phosphorus covalent organic framework flame retardant effectively reduced the heat release rate of the epoxy resin composite. Compared to pure epoxy resin, the peak heat release rate of the epoxy resin composite decreased from 1140.5 kW / m². 2It dropped to 381.4 kW / m 2 The decline rate was 66.5%.

[0054] Experiment Example 2

[0055] Four parts by weight of the nitrogen / phosphorus covalent organic framework flame retardant prepared in Example 1 were dispersed in 76 parts by weight of E51 type epoxy resin by mechanical stirring. The mixture was then heated to 100°C and reacted for 60 minutes. Subsequently, 20 parts by weight of 4,4'-diaminodiphenylmethane curing agent were added, and the mixture was stirred until a stable and homogeneous epoxy resin solution was obtained. Finally, the epoxy resin solution was vacuum-sealed to remove excess air bubbles, then poured into a preheated mold, and cured at 100°C for half an hour, 120°C for 2 hours, and 150°C for 2 hours. After curing, the mixture was allowed to cool naturally to room temperature and demolded to obtain the flame-retardant epoxy resin composite material.

[0056] The prepared flame-retardant epoxy resin specimens were subjected to cone calorimetry testing. The cone calorimetry specimens were 100mm × 100mm × 3mm in size. The smoke generation rate of the flame-retardant epoxy resin obtained in Experiment Example 2 is as follows: Figure 2 As shown in the figure. The results indicate that the addition of a nitrogen / phosphorus covalent organic framework flame retardant effectively reduced the smoke generation rate of the epoxy resin composite. Compared to pure epoxy resin, the peak smoke generation rate of the epoxy resin composite decreased from 0.320 m³ / s. 2 / s decreased to 0.210 m 2 / s, a decrease of 31.7%.

[0057] Experimental Example 3

[0058] Nine parts by weight of the nitrogen / phosphorus covalent organic framework flame retardant prepared in Example 1 were dispersed in 71 parts by weight of E51 type epoxy resin by mechanical stirring. The mixture was then heated to 100°C and reacted for 60 minutes. Subsequently, 20 parts by weight of 4,4'-diaminodiphenylmethane curing agent were added, and the mixture was stirred until a stable and homogeneous epoxy resin solution was obtained. Finally, the epoxy resin solution was vacuum-sealed to remove excess air bubbles, then poured into a preheated mold, and cured at 100°C for half an hour, 120°C for 2 hours, and 150°C for 2 hours. After curing, the mixture was allowed to cool naturally to room temperature and demolded to obtain the flame-retardant epoxy resin composite material.

[0059] The prepared flame-retardant epoxy resin specimens were subjected to cone calorimetry testing. The cone calorimetry specimens were 100 mm × 100 mm × 3 mm in size. The carbon monoxide generation rate of the flame-retardant epoxy resin obtained in Experimental Example 3 is as follows: Figure 3As shown in the figure. The results indicate that the addition of a nitrogen / phosphorus covalent organic framework flame retardant effectively reduced the carbon monoxide formation rate of the epoxy resin composite. Compared with the epoxy resin without flame retardant, the peak carbon monoxide formation rate of the epoxy resin composite decreased from 0.038 g / s to 0.014 g / s, a decrease of 61.4%.

[0060] Experiment Example 4

[0061] Twelve parts by weight of the nitrogen / phosphorus covalent organic framework flame retardant prepared in Example 1 were dispersed in 68 parts by weight of E51 epoxy resin by mechanical stirring. The mixture was then heated to 100°C and reacted for 60 minutes. Subsequently, 20 parts by weight of 4,4'-diaminodiphenylmethane curing agent were added, and the mixture was stirred until a stable and homogeneous epoxy resin solution was obtained. Finally, the epoxy resin solution was vacuum-sealed to remove excess air bubbles, then poured into a preheated mold, and cured at 100°C for half an hour, 120°C for 2 hours, and 150°C for 2 hours. After curing, the mixture was allowed to cool naturally to room temperature and demolded to obtain the flame-retardant epoxy resin composite material.

[0062] The prepared flame-retardant epoxy resin specimens were subjected to cone calorimetry testing. The cone calorimetry specimens were 100mm × 100mm × 3mm in size. The carbon dioxide production rate of the flame-retardant epoxy resin obtained in Experiment Example 4 is as follows: Figure 4 As shown in the figure. The results indicate that the addition of a nitrogen / phosphorus covalent organic framework flame retardant effectively reduced the carbon dioxide generation rate of the epoxy resin composite. Compared to pure epoxy resin, the peak carbon dioxide generation rate of the epoxy resin composite decreased from 0.45 g / s to 0.17 g / s, a reduction of 61.1%.

[0063] Compare with Example 1

[0064] 80 parts by weight of E51 type epoxy resin and 20 parts by weight of 4,4'-diaminodiphenylmethane curing agent were mechanically stirred at 100°C until homogeneous to obtain an epoxy resin solution. Finally, the epoxy resin solution was vacuum-sealed to remove excess air bubbles, then poured into a preheated mold, and cured at 100°C for half an hour, 120°C for 2 hours, and 150°C for 2 hours. After curing, it was allowed to cool naturally to room temperature and demolded to obtain the reference epoxy resin.

[0065] The prepared epoxy resin specimens were subjected to cone calorimetry testing. The cone calorimetry specimens were 100mm × 100mm × 3mm in size.

[0066] As can be seen from the above embodiments or experimental examples, the flame-retardant properties of the flame-retardant epoxy resin composite materials prepared by the present invention are significantly improved. With the introduction of nitrogen / phosphorus covalent organic framework flame retardants into the epoxy resin, the heat release rate, smoke production rate, carbon monoxide generation rate, and carbon dioxide generation rate during the epoxy resin combustion process can be effectively reduced, and the quality of the char layer is significantly improved. This indicates that nitrogen / phosphorus covalent organic framework flame retardants can effectively inhibit the generation of heat, smoke, and harmful gases during the epoxy resin combustion process, and can form a stable heat insulation layer, better preventing external heat from entering the epoxy resin matrix. This suggests that nitrogen / phosphorus covalent organic framework materials are promising candidates for epoxy resin flame retardants.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a nitrogen / phosphorus covalent organic framework flame retardant, characterized in that, Includes the following steps: First, cyanuric chloride and triethylamine were dissolved in N,N-dimethylformamide and stirred in an ice bath for 30 minutes to obtain a mixture. Then, phenylphosphonic acid was dissolved in N,N-dimethylformamide and slowly added dropwise to the mixture over 60 minutes. The mixture was reacted in an ice bath for 2 hours, then stirred at room temperature for 12 hours, and finally stirred at 120°C for 48 hours. After the reaction cooled to room temperature, the mixture was filtered, the filter cake was washed several times with ethanol and acetone, and the product was obtained by rotary evaporation. The product was dried under vacuum for 12 hours to finally obtain a nitrogen / phosphorus covalent organic framework flame retardant. The nitrogen / phosphorus covalent organic framework flame retardant is a porous network crystal structure formed by phosphorus-containing monomers and nitrogen-containing monomers connected by covalent bonds, and its structure contains both P=O bonds and nitrogen-containing heterocyclic structures.

2. The application of the nitrogen / phosphorus covalent organic framework flame retardant prepared by the method of claim 1.

3. The application according to claim 2, characterized in that, Used to prepare flame-retardant epoxy resins.

4. The application according to claim 3, characterized in that, The obtained nitrogen / phosphorus covalent organic framework flame retardant was dispersed in E51 epoxy resin by mechanical stirring to obtain a uniform mixture. Then, the mixture was reacted at 100°C for a period of time, and 4,4'-diaminodiphenylmethane curing agent was added. The mixture was stirred until a uniform epoxy resin mixture solution was obtained. Finally, the epoxy resin mixture solution was vacuumed to remove excess air bubbles, and then poured into a preheated mold for thermosetting. After curing, it was allowed to cool to room temperature. Finally, the flame-retardant epoxy resin composite material was prepared.

5. The application according to claim 4, characterized in that, The mass ratio of the nitrogen- / phosphorus covalent organic framework flame retardant to E51 type epoxy resin is 3-12:

100.

6. The application according to claim 4, characterized in that, The reaction temperature of the nitrogen / phosphorus covalent organic framework flame retardant and the E51 type epoxy resin is 90-100℃, and the reaction time is 30-60 minutes.

7. The application according to any one of claims 4-7, characterized in that, The peak heat release rate (pHRR) of the flame-retardant epoxy resin composite material is reduced by 66.5% or more.

8. The application according to any one of claims 4-7, characterized in that, The peak smoke generation rate of the flame-retardant epoxy resin composite material is reduced by 31.7% or more.

Images