Environment-friendly low-toxicity nitrogen-containing boric acid ester flame retardant as well as preparation method and application thereof

Abstract

The invention relates to an environment-friendly low-toxicity nitrogen-containing boric acid ester flame retardant which is of a structure containing a nitrogen coordinated boric acid ester ring and is prepared from organic boric acid, nitrogen-containing polyhydric alcohol and acid anhydride. The nitrogen-containing boric acid ester flame retardant can be used for preparing high-strength and high-toughness transparent flame-retardant epoxy resin, the flame retardant property can reach UL-94V0 grade, and compared with epoxy resin before modification, the mechanical strength of the nitrogen-containing boric acid ester flame retardant is improved by 30% or above, and the toughness is improved by 50% or above.

Description

Technical Field

[0001] The present invention relates to the technical field of epoxy resin flame retardants, and particularly relates to an environmentally friendly and low-toxic nitrogen-containing borate ester flame retardant, a preparation method thereof and an application thereof. Background Art

[0002] Epoxy resin (EP), as a commonly used thermosetting plastic, has been widely used in the fields of electronics and electrics, aerospace, solar cell encapsulation, etc. due to its good transparency, excellent mechanical properties and excellent dimensional stability. Although epoxy resin has the above excellent properties, it is highly flammable, with a limiting oxygen index of only about 22, and it undergoes melt dripping and releases toxic and harmful gases during combustion, seriously endangering personal safety; at the same time, its toughness is poor, and the elongation at break usually does not exceed 10%. These problems limit its application in fields where flame retardancy and toughness are required.

[0003] To address its flammability problem, early studies often used halogen flame retardants to improve its flame retardant performance. However, halogen flame retardants release a large amount of toxic and harmful gases during combustion, and currently the European Union, the United States and China have restricted and gradually prohibited their use. Currently, the commonly used and highly efficient epoxy resin flame retardants are phosphorus-based flame retardants, but phosphorus-based flame retardants have serious negative impacts on the toughness and thermal stability of epoxy resin. The problem of the decrease in the thermal stability of phosphorus-based flame retardants is mainly caused by the relatively low bond energies of the P-C bond and the P-O bond in the phosphorus-containing flame retardant; while its poor compatibility with the resin and weak interfacial interaction are the main reasons for its poor mechanical properties.

[0004] Due to the problems of the above-mentioned halogen-based flame retardants and phosphorus-based flame retardants, the development trend of flame retardants in recent years has gradually shifted towards aspects such as low toxicity, environmental friendliness, and good compatibility. Boron-based flame retardants meet this trend due to their low toxicity and low smoke release. The literature "Synthesis of a novel type of high-efficiency boron-containing Schiff base flame retardant for epoxy resin" (Fire and Materials 47 (2023) 341-351) prepared a borate ester flame retardant B-HIBD and applied it to epoxy resin, improving the thermal stability of the resin. However, the efficiency of the boron-based flame retardant prepared is relatively low, and the resulting borate ester flame-retarded epoxy resin fails to achieve the UL94 V0 rating. At the same time, the preparation of the borate ester flame retardant seriously affects the mechanical properties of the resin. When only 5% by mass of B-HIBD is added, the tensile strength of the epoxy resin drops significantly by 43%, and the elongation at break drops by 30%. Patent CN114456547A synthesized and prepared a polyborate ester with a hyperbranched structure and used it to modify epoxy resin. This type of hyperbranched polyborate ester (HBPB) contains a large number of active hydroxyl groups at its ends, aggregates in the resin matrix, and participates in curing crosslinking to form a dynamic supramolecular polymer network. The resulting HBPB epoxy resin has a relatively high limiting oxygen index, a low heat release rate during combustion, and a low smoke release amount. However, the toughness of the HBPB-modified epoxy resin prepared decreases significantly with the increase in the addition amount of HBPB. The impact strength of the epoxy resin with 3% HBPB added is 40 kJ / m 2 , and when the addition amount of HBPB is increased to 12%, the impact strength of the resin rapidly drops to 25 kJ / m 2 . At the same time, its flame retardant efficiency is relatively low. Even when 12% by mass of HBPB is added, the modified epoxy resin prepared does not reach the UL-94 V0 rating. In addition, past research often only focuses on the flame retardant properties of epoxy resin and does not pay attention to the mechanical properties of the char formed after the combustion of epoxy resin. The mechanical properties and denseness of the char are crucial for maintaining the strength of the structure to reduce the hazards caused by fires. For example, the World Trade Center had been equipped with flame retardant materials before the "911" incident, but after the accident, the flame retardant materials were difficult to isolate heat and maintain mechanical strength, resulting in a chain failure of the building structure and ultimately the collapse of the building.

[0005] Therefore, aiming at the above problems, researching an epoxy resin with low toxicity, environmental friendliness, high efficiency, high strength, high toughness, and flame retardancy and low fire hazards has important practical application value and good application prospects. Summary of the Invention

[0006] In view of the deficiencies in the prior art, the present invention provides an environmentally friendly, low-toxic, colorless nitrogen-containing borate flame retardant and a method for preparing flame-retarded epoxy resin therefrom. The nitrogen-containing borate flame retardant has high compatibility with epoxy resin and good flame retardancy. The prepared flame-retarded epoxy resin has good flame retardancy and better mechanical properties than unmodified epoxy resin, which has important practical application value.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows: In the first aspect, an environmentally friendly and low-toxic nitrogen-containing borate flame retardant is provided. The environmentally friendly and low-toxic nitrogen-containing borate flame retardant does not contain phosphorus and halogen elements; the nitrogen-containing borate flame retardant has a structure of a borate ring coordinated with nitrogen, and the structural formula is , where R1 and R2 are each one of carboxyacryloyloxy or carboxypropionyloxy.

[0008] In the second aspect, the present invention also provides a method for preparing the above nitrogen-containing borate flame retardant, specifically, an organic boronic acid, a nitrogen-containing polyol, and an acid anhydride are reacted in an organic solvent to obtain a nitrogen-containing borate flame retardant having the structure of formula (I); Preferably, the organic boronic acid is p-phenylenediboronic acid; Preferably, the nitrogen-containing polyol is triethanolamine; Preferably, the acid anhydride is selected from one of maleic anhydride and succinic anhydride; Preferably, the organic solvent is one of N,N'-dimethylformamide and tetrahydrofuran; More preferably, the amount of the organic solvent used is 10 to 30 times the mass of the nitrogen-containing polyol.

[0009] Preferably, in the method for preparing the nitrogen-containing borate flame retardant having the structure of formula (I) as described above, the molar ratio of the organic boronic acid, the nitrogen-containing polyol, and the acid anhydride is (1:1:1) to (1:6:3); The reaction temperature of the organic boronic acid, the nitrogen-containing polyol, and the acid anhydride is 50°C to 80°C, the reaction time is 2 h to 8 h, and the stirring rate is 200 r / min to 1400 r / min.

[0010] Preferably, after all the above reactions are completed, the synthesized flame retardant is purified by recrystallization.

[0011] In the third aspect, the present invention also provides the application of the above nitrogen-containing borate flame retardant in the preparation of epoxy resin, especially in the preparation of high-strength, high-toughness, and transparent flame-retarded epoxy resin.

[0012] Fourthly, the present invention also provides a high-strength, high-toughness, transparent and flame-retardant epoxy resin, which comprises the following components: an epoxy prepolymer, an epoxy curing agent and the aforementioned nitrogen-containing borate flame retardant.

[0013] Preferably, the epoxy prepolymer is one of E-44 epoxy resin and E-51 epoxy resin; Preferably, the epoxy curing agent is one of 4,4'-diaminodiphenyl sulfone and 4,4'-diaminodiphenyl methane; Preferably, the dosage of the nitrogen-containing borate flame retardant is 5% - 10% of the total mass of the three components of the nitrogen-containing borate flame retardant, the epoxy prepolymer and the epoxy curing agent; the equivalent ratio of the active hydrogen of the epoxy curing agent to the epoxy group of the epoxy prepolymer is controlled to be (0.8 - 1.0):1.0.

[0014] Fifthly, the present invention also provides a method for preparing a high-strength, high-toughness, transparent and flame-retardant epoxy resin by using the nitrogen-containing borate flame retardant as described above. Specifically, the nitrogen-containing borate flame retardant is dissolved in a low-boiling organic solvent, and then mixed with the epoxy prepolymer and the epoxy curing agent. After degassing under vacuum, it is cured and formed to obtain an epoxy cured product.

[0015] Preferably, the low-boiling organic solvent is one of acetone, methanol and ethanol.

[0016] The method for preparing the flame-retardant epoxy resin as described above comprises the following steps: (1) Dissolve the nitrogen-containing borate flame retardant, the epoxy prepolymer and the epoxy curing agent in a low-boiling organic solvent, and after mixing evenly, obtain a solution with an epoxy prepolymer concentration of 100 g / L - 400 g / L; (2) Pour the mixed solution into a mold, and evacuate for 1 h - 2 h at 40°C - 60°C to remove bubbles; (3) Heat and cure the defoamed mixture, and the curing conditions used are: the curing temperature is gradually increased from 50±5°C to 200±5°C, the curing time is 1 h - 8 h, and after curing, a high-strength, high-toughness, transparent nitrogen-containing borate flame-retardant epoxy resin product is obtained; Preferably, the dosage of the nitrogen-containing borate flame retardant is 5% - 10% of the total mass of the three components of the nitrogen-containing borate flame retardant, the epoxy prepolymer and the epoxy curing agent; the equivalent ratio of the active hydrogen of the epoxy curing agent to the epoxy group of the epoxy prepolymer is controlled to be 0.8:1.0 - 1.0:1.0.

[0017] The beneficial effects of the present invention are as follows: (1) The nitrogen-containing borate flame retardant obtained by the present invention does not contain halogen and phosphorus elements, is low-toxic, environmentally friendly and easy to synthesize, and only requires one-step reaction; (2)The nitrogen-containing borate ester flame-retardant modified epoxy resin obtained in the present invention not only has a flame retardancy that can reach the UL-94 V0 grade, but also has a mechanical strength increased by more than 30% and a toughness increased by more than 50% compared with the epoxy resin before modification, solving the negative impact of traditional flame retardants on mechanical properties and having broad application prospects. Description of the Drawings

[0018] Figure 1 It is a schematic diagram of the synthesis preparation route of Example 1; Figure 2 It is an infrared spectrum diagram of PdBECOOH prepared in Example 1; Figure 3 It is a nuclear magnetic resonance hydrogen spectrum diagram of PdBECOOH prepared in Example 1; Figure 4 It is a digital image of the char residue of EP@PdBECOOH-0.10 prepared in Application Example 3 and a digital image of the char residue of EP@APP-0.10 prepared in Application Comparative Example 2; Figure 5 It is a scanning electron microscope image of the char residue of EP@PdBECOOH-0.10 prepared in Application Example 3 and EP prepared in Application Comparative Example 1.

[0019] Figure 6 It is a scanning electron microscope image of the impact fracture surface of EP@PdBECOOH-0.10 prepared in Application Example 3 and EP prepared in Application Comparative Example 1. Detailed Embodiments

[0020] To better understand the present invention, the present invention will be further described below in conjunction with the drawings, examples, comparative examples, application examples and application comparative examples. However, the embodiments of the present invention are not limited to this. Any changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be regarded as equivalent replacement methods and are all included in the protection scope of the present invention.

[0021] The organic borate ester, nitrogen-containing polyol and succinic anhydride used in each example, comparative example, application example and application comparative example of the present invention are all provided by Beijing InnoChem Technology Co., Ltd.; The test methods for each application example and application comparative example in the present invention are specifically as follows: The tensile property test of the cured product is determined in accordance with GB / T 1040.1-2018 "Determination of Tensile Properties of Plastics"; The impact property test of the cured product is determined in accordance with GB / T 1843-2008 "Determination of Izod Impact Strength of Plastics"; The limiting oxygen index of the cured product was measured in accordance with the Limited Oxygen Index (LOI) method as specified in GB / T 2406.2-2009 "Plastics - Determination of burning behaviour by oxygen index method". The vertical burning test of the cured product was carried out in accordance with GB / T 2408-2021 "Plastics - Determination of burning behaviour - Horizontal and vertical methods".

[0022] Figure 1 is a schematic diagram of the synthesis of the nitrogen-containing borate ester flame retardant in Example 1 and Example 2. Example 1

[0023] 6.6 g of triethanolamine (44 mmol), 4.5 g of succinic anhydride (44 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 4 h. Then, 3.0 g of terephthalic acid diborate (18 mmol) was added and mixed evenly, followed by reaction for 8 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain PdBECOOH. Its infrared spectrum is shown in Figure 2, and the stretching vibration peak of its carboxyl group is shown at 2600 cm -1 The stretching vibration peak of its carboxyl group and carbon-oxygen double bond is shown at 1750 cm -1 The stretching vibration peak of its boron-oxygen bond is shown at 1350 cm -1 The nuclear magnetic resonance hydrogen spectrum is shown in Figure 3.

[0024] The chemical structural formula of PdBECOOH is: , Example 2

[0025] 16.2 g of triethanolamine (108 mmol), 5.5 g of succinic anhydride (54 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 4 h. Then, 3.0 g of terephthalic acid diborate (18 mmol) was added and mixed evenly, followed by reaction for 8 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain the product PdBE2. Example 3

[0026] 2.7 g of triethanolamine (18 mmol), 1.8 g of succinic anhydride (18 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 4 h. Then, 3.0 g of terephthalic acid diborate (18 mmol) was added and mixed evenly, followed by reaction for 8 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain the product PdBE3. Example 4

[0027] 6.6 g of triethanolamine (44 mmol), 3.6 g of succinic anhydride (36 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 4 h. Subsequently, 3.0 g of terephthalic acid diboronic acid (18 mmol) was added and mixed evenly, followed by reaction for 8 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain the product PdBE4. Example 5

[0028] 6.6 g of triethanolamine (44 mmol), 5.3 g of maleic anhydride (54 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 4 h. Subsequently, 3.0 g of terephthalic acid diboronic acid (18 mmol) was added and mixed evenly, followed by reaction for 8 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain the product PdBE5. Example 6

[0029] 6.6 g of triethanolamine (44 mmol), 4.3 g of maleic anhydride (44 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 4 h. Subsequently, 3.0 g of terephthalic acid diboronic acid (18 mmol) was added and mixed evenly, followed by reaction for 8 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain the product PdBE6.

[0030] Comparative Example 1 1.5 g of triethanolamine (10 mmol), 1.0 g of succinic anhydride (10 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 4 h. Subsequently, 3.0 g of terephthalic acid diboronic acid (18 mmol) was added and mixed evenly, followed by reaction for 8 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain the product PdBE7.

[0031] Comparative Example 2 6.6 g of triethanolamine (44 mmol), 3.0 g of terephthalic acid diboronic acid (18 mmol) and 200 g of N,N'-dimethylformamide were mixed evenly and reacted at 50 °C for 12 h. Finally, it was distilled under reduced pressure at 80 °C and recrystallized in methanol to obtain the product PdBE8.

[0032] Application Example 1 Take 32.0 g of E51 epoxy prepolymer and 2.0 g of PdBECOOH, add them to 48 g of methanol, heat and stir at 50 °C for 15 min. Add 6.6 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 60 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, a nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBECOOH-0.05 is obtained.

[0033] Application Example 2 Take 22.0 g of E51 epoxy prepolymer and 3.0 g of PdBECOOH, add them to 60 g of ethanol, heat and stir at 60 °C for 15 min. Add 4.6 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 60 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, a nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBECOOH-0.10 is obtained.

[0034] Application Example 3 Take 30.0 g of E44 epoxy prepolymer and 3.9 g of PdBECOOH, add them to 48 g of acetone, heat and stir at 50 °C for 15 min. Add 5.2 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 60 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, a nitrogen-containing borate ester flame-retardant epoxy resin EP1@PdBECOOH-0.10 is obtained.

[0035] Application Example 4 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBECOOH, add them to 160 g of acetone, heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, a nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBECOOH-0.15 is obtained.

[0036] Application Example 5 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBE2 and add them to 160 g of acetone. Heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBE2-0.15 is obtained.

[0037] Application Example 6 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBE3 and add them to 160 g of acetone. Heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBE3-0.15 is obtained.

[0038] Application Example 7 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBE4 and add them to 160 g of acetone. Heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBE4-0.15 is obtained.

[0039] Application Example 8 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBE5 and add them to 160 g of acetone. Heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBE5-0.15 is obtained.

[0040] Application Example 9 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBE6, add them to 160 g of acetone, heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBE6-0.15 is obtained.

[0041] Application Comparative Example 1 Take 22.0 g of E51 epoxy prepolymer, add it to 60 g of methanol, heat and stir at 60 °C for 15 min. Add 5.6 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 60 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the unmodified epoxy resin EP is obtained.

[0042] Application Comparative Example 2 Take 22.0 g of E51 epoxy prepolymer and 3.0 g of ammonium polyphosphate (APP), add them to 48 g of methanol, heat and stir at 50 °C for 15 min. Add 4.6 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate under vacuum at 60 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@APP-0.10 is obtained.

[0043] Application Comparative Example 3 Prepare the boron-based flame retardant 12% HBPB / EP modified epoxy resin by the method described in Patent CN114456547A.

[0044] Application Comparative Example 4 Prepare the boron-based flame retardant 5% B-HIBD / EP modified epoxy resin by the method described in the literature Synthesis of a novel type of high-efficiency boron-containing Schiff base flame retardant for epoxy resin (Fire and Materials 47 (2023) 341-351).

[0045] Application Comparative Example 5 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBE7 and add them to 160 g of acetone. Heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBE7-0.15 is obtained.

[0046] Application Comparative Example 6 Take 25.0 g of E51 epoxy prepolymer and 5.1 g of PdBE8 and add them to 160 g of acetone. Heat and stir at 60 °C for 15 min. Add 4.4 g of 4,4'-diaminodiphenylmethane to the system, stir for 5 min, then evacuate at 40 °C for 2 h. Subsequently, place it in an oven and cure it in the order of heating at 100 °C for 2 h, 120 °C for 2 h, and 160 °C for 3 h. After cooling to room temperature, the nitrogen-containing borate ester flame-retardant epoxy resin EP@PdBE8-0.15 is obtained.

[0047] After testing, the flame retardant properties of Application Examples 1-9 and Application Comparative Examples 1-6 are shown in Table 1, and the mechanical properties are shown in Table 2.

[0048] Table 1 Flame Retardant Properties of Application Examples 1-9 and Application Comparative Examples 1-6

[0049] Table 2 Mechanical Properties of Application Examples 1-9 and Application Comparative Examples 1-6

[0050] As can be seen from Table 1, compared with Comparative Application Example 1 without modification, which has no UL-94 rating and a limiting oxygen index of 22.1, the flame retardancy of Application Examples 1 to 9 has been significantly improved. Among them, Application Examples 2 to 9 all reached the UL-94 V-0 level, having similar performance to Comparative Application Example 2 with a commercial phosphorus-containing flame retardant added, and the effect exceeded that of Comparative Application Example 3 reported in the patent and Comparative Application Example 4 with a boron-containing flame retardant reported in the literature. At the same time, the flame retardancy of Comparative Application Example 5 lacking nitrogen element and Comparative Application Example 6 lacking carboxyl group were both not rated (NR). The former is because there is less nitrogen element in PdBE7 prepared in Comparative Application Example 1, resulting in less nitrogen-containing gas released, and thus a poorer dilution effect on oxygen; while the latter is because PdBE8 synthesized in Comparative Application Example 2 has no carboxyl group, and it is difficult for the flame retardant to react with epoxy resin, leading to difficult dispersion of the flame retardant in epoxy resin, so the flame retardant efficiency is not good. The above comparative samples confirm the importance of nitrogen-containing groups and carboxyl groups for flame retardant efficiency. The excellent flame retardancy of the application examples is mainly because the nitrogen-containing borate ester catalyzes the formation of a carbon layer during combustion and releases nitrogen-containing gas to dilute the oxygen concentration. From the comparison of the char digital images of EP@PdBECOOH-0.10 prepared in Application Example 3 in Figure 4 and EP@APP-0.10 prepared in Comparative Application Example 2, it can be seen that the char of EP@PdBECOOH-0.10 is extremely tough and does not deform under the compression and bending of a 200 g weight, while the char of EP@APP-0.10 with a commercial flame retardant added is very fragile, and its shape changes significantly after being compressed by a 200 g weight, and its size is difficult to maintain stable. Such a strong carbon layer can effectively maintain the strength of the structure, block the transfer of heat and support the structure from collapsing during a fire, thereby reducing the harm caused by the fire. From the scanning electron microscope image of the char in Figure 5, it can be seen that the char of EP@PdBECOOH-0.10 prepared in Application Example 2 is dense and flat. Even when the magnification is up to 10 µm scale, the char is still dense and flat. Such a char can effectively isolate heat and oxygen and thus prevent the spread of combustion; while the char of EP prepared in Comparative Application Example 1 is broken and porous and has no protective effect on the inner layer material, which further illustrates that the strong char-forming catalytic effect of the nitrogen-containing borate ester flame retardant of the present invention is the reason for its good flame retardancy.

[0051] While achieving good flame retardancy performance, Application Examples 1-9 incorporating the nitrogen-containing borate flame retardant of the present invention also obtained excellent mechanical properties. As can be seen from Table 2, the tensile strength, Young's modulus, and elongation at break of Application Examples 1-9 were significantly improved compared to unmodified Application Comparative Example 1. Among them, Application Examples 2 and 4 with a relatively large addition amount of the nitrogen-containing borate flame retardant had particularly obvious improvements. While the strength increased by 30%, the elongation at break increased by more than 100%. This is mainly because the carboxyl groups and borate groups carried by the nitrogen-containing borate flame retardants added in Application Examples 2 and 4 brought a large number of dynamic bonds to the epoxy resin, which could undergo reversible fracture upon impact to absorb energy and improve toughness; when subjected to tensile forces, these dynamic non-covalent bonds could provide additional crosslinking density to increase strength. At the same time, the carboxyl groups carried by them could participate in the curing process of the epoxy resin, which improved the compatibility between the flame retardant and the resin matrix, and thus improved the mechanical properties of the resin; the scanning electron microscope images of the impact fracture surfaces of Application Example 2 and Application Comparative Example 1 in Figure 6 also confirmed the above properties: the impact fracture surface of Application Example 2 had dense cracks and a rough surface, indicating that it absorbed more energy during fracture; while the impact fracture surface of Application Comparative Example had fewer cracks and a smooth layered surface, indicating that it absorbed less energy during the fracture process. However, the mechanical properties of Application Comparative Example 2 incorporating a commercial phosphorus-containing flame retardant and Application Comparative Example 4 incorporating a boron-containing flame retardant reported in the literature were severely degraded compared to unmodified Application Comparative Example 1. This is mainly because these flame retardants were simply physically blended with the epoxy resin, without strong interactions, resulting in a large number of interfaces. These interfaces were more likely to fracture as defects upon external impact, thus significantly degrading the mechanical properties of the cured product.

[0052] The above results indicate that the nitrogen-containing borate flame retardant of the present invention and its modified flame-retardant epoxy resin have excellent flame retardancy performance, super-strong carbon layers, and good mechanical properties. Their comprehensive properties can prevent the spread of combustion and maintain the strength of the structure in a fire. Their good flame retardancy performance and super-strong carbon layers mainly stem from: (1) the formation of a carbon layer catalyzed by the borate during combustion, blocking the transfer of heat and oxygen; (2) releasing nitrogen-containing gases to dilute the oxygen concentration. Their excellent mechanical properties are due to: (1) carrying a large number of dynamic non-covalent bonds, which can undergo reversible fracture upon impact to absorb energy and provide additional crosslinking density. (2) having good compatibility with the epoxy resin, and groups such as carboxyl groups, tertiary amino groups, and hydroxyl groups carried by them can participate in the epoxy curing reaction, providing chemical bonding with the epoxy resin. In addition, the nitrogen-containing borate flame retardant of the present invention also has advantages such as simple synthesis, low toxicity, and good repeatability.

[0053] The above are only the preferred embodiments of the present invention and are not used to limit the scope of the present invention. It should be noted that without departing from the technical principle of the present invention, several improvements, combinations and deformations can be made to the present invention by those of ordinary skill in the art, and these improvements, combinations and deformations should also be regarded as within the protection scope of the present invention.

Claims

1. An environmentally friendly and low-toxic nitrogen-containing borate ester flame retardant, characterized in that, The environmentally friendly low-toxic nitrogen-containing borate flame retardant does not contain phosphorus and halogen elements; the nitrogen-containing borate flame retardant contains a structure of a borate ring coordinated with nitrogen, and the structural formula is: Wherein R1 and R2 are each one of carboxyacryloyloxy or carboxypropionyloxy.

2. A method for preparing the environmentally friendly and low-toxic nitrogen-containing borate ester flame retardant according to claim 1, characterized in that, The nitrogen-containing borate flame retardant with the structure of formula (Ⅰ) is prepared by reacting organic boric acid, nitrogen-containing polyol, and acid anhydride in an organic solvent.

3. The preparation method according to claim 2, wherein, The organic boric acid is p-phenylenediboronic acid; The nitrogen-containing polyol is triethanolamine; The acid anhydride is selected from one of maleic anhydride and succinic anhydride; The organic solvent is selected from one of N,N'-dimethylformamide and tetrahydrofuran; The molar ratio of the organic boric acid, nitrogen-containing polyol, and acid anhydride is (1:1:1) to (1:6:3); the amount of the organic solvent used is 10 to 30 times the mass of the nitrogen-containing polyol.

4. The method according to any one of claims 2-3, characterized in that The reaction temperature of the reaction is 50°C to 80°C, the time is 2h to 8h, and the stirring rate is 200r / min to 1400r / min.

5. The method according to any one of claims 2 to 4, characterized in that, After the reaction, the synthesized flame retardant is purified by recrystallization.

6. Application of the nitrogen-containing borate flame retardant described in claim 1 or the nitrogen-containing borate flame retardant prepared according to any one of claims 2-5 in the preparation of a high-strength, high-toughness transparent flame-retardant epoxy resin.

7. A high-strength, high-toughness, transparent and flame-retardant epoxy resin, characterized in that, It includes the following components: epoxy prepolymer, epoxy curing agent, and the nitrogen-containing borate flame retardant described in claim 1 or the nitrogen-containing borate flame retardant prepared according to any one of claims 2-5.

8. The flame-retardant epoxy resin according to claim 7, wherein The epoxy prepolymer is selected from one of E-44 epoxy resin and E-51 epoxy resin; The epoxy curing agent is selected from one of 4,4'-diaminodiphenyl sulfone and 4,4'-diaminodiphenyl methane; The amount of the nitrogen-containing borate flame retardant used is 5% to 10% of the total mass of the three components of the nitrogen-containing borate flame retardant, epoxy prepolymer, and epoxy curing agent; the equivalent ratio of the active hydrogen of the epoxy curing agent to the epoxy group of the epoxy prepolymer is (0.8 to 1.0):1.

0.

9. A method for preparing the high-strength, high-toughness, transparent and flame-retardant epoxy resin according to claim 7 or 8, characterized in that, After dissolving the nitrogen-containing borate flame retardant in a low-boiling organic solvent, it is mixed with the epoxy prepolymer and the epoxy curing agent, and then cured and molded after vacuum defoaming to obtain a flame-retardant epoxy resin.

10. The preparation method according to claim 9, characterized in that, The low-boiling organic solvent is selected from one of acetone, methanol, and ethanol.

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