Nitrogen-rich DOPO-based flame retardant and synthesis and application thereof

Abstract

The invention relates to the technical field of flame retardants and flame-retardant composite materials, in particular to a nitrogen-rich DOPO-based flame retardant and synthesis and application thereof. The polarity matching degree of a conventional phosphorus-based flame retardant and epoxy resin is poor, phase separation or migration precipitation is prone to occurring, the flame retardance of flame-retardant epoxy resin is unstable, and the flame-retardant effect needs to be further improved. Aiming at the technical problems, the invention provides the nitrogen-rich DOPO-based flame retardant which belongs to a reaction type phosphorus-nitrogen flame retardant, a secondary amine group in a molecular structure of the nitrogen-rich DOPO-based flame retardant can generate a chemical bond with an epoxy group in an epoxy resin structure, and compared with a conventional phosphorus-based flame retardant, the nitrogen-rich DOPO-based flame retardant has better compatibility with epoxy resin, is not easy to fall off, and can be used for preparing the flame retardant. The influence on the mechanical property and transparency of the epoxy resin is relatively small, and the application prospect is relatively good.

Description

Technical Field

[0001] The invention relates to the technical field of flame retardants, and in particular to a nitrogen-rich DOPO-based flame retardant and synthesis and application thereof. Background Art

[0002] As an important type of thermosetting polymer material, epoxy resin is widely used in the fields of electronics, home appliance manufacturing, aerospace, etc. due to its excellent chemical corrosion resistance, insulation performance and thermal stability. However, the limiting oxygen index (LOI) of general-purpose epoxy resin is usually lower than 24%, showing significant flammability. During the combustion process, it not only releases a large amount of heat, but also is accompanied by molten droplets, which can easily cause secondary fire risks. Therefore, improving the intrinsic flame retardant properties of epoxy resin has become a key issue that needs to be solved in this field.

[0003] In traditional flame retardant technology, halogen flame retardants (such as brominated epoxy resin) have been used for a long time due to their high flame retardant effect. However, such flame retardants will produce highly toxic gases such as hydrogen halides and dioxins and corrosive smoke when thermally decomposed or burned, posing a serious threat to the ecological environment and human health. With the increasingly stringent global green chemistry and environmental regulations, the development of efficient halogen-free flame retardant systems has become an industry consensus.

[0004] In the halogen-free flame retardant system, phosphorus-based flame retardants have shown significant advantages due to their low toxicity, high flame retardant efficiency and multiple flame retardant mechanisms (gas phase quenching, condensed phase carbonization, etc.). However, the application of existing phosphorus-based flame retardants in epoxy resin matrix still faces many technical bottlenecks:

[0005] Compatibility defects: Most phosphorus-based flame retardants have a low polarity match with epoxy resins, which are prone to phase separation or migration and precipitation, resulting in reduced long-term stability of the material;

[0006] Performance tradeoff: To achieve UL-94V-0 flame retardant effect, it is usually necessary to add more than 15wt.% of flame retardant. Excessive introduction will significantly deteriorate the mechanical strength (such as tensile modulus reduction > 20%) and optical transparency (transmittance loss > 30%) of the composite material.

[0007] Reaction inertness: Some flame retardants lack the ability to chemically bond with epoxy groups. It is difficult to form stable interfacial interactions only through physical blending and dispersion, which further aggravates the compatibility problem.

[0008] Taking the prior art CN 102428091A as an example, it discloses a flame retardant based on DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) derivatives, which improves the flame retardant efficiency by modifying the halogen end group. However, this technology has the following limitations:

[0009] The flame retardant molecule is not designed with reactive functional groups (such as amino groups, hydroxyl groups) that can react with epoxy groups, resulting in only physical adsorption between it and the resin matrix, and the interfacial bonding force is weak;

[0010] Although a high halogen content improves flame retardancy, it exacerbates combustion toxicity, which is contrary to the trend of halogen-free;

[0011] The addition amount needs to reach more than 10 wt.% to meet the V-0 level requirement, and the flexural strength of the composite material decreases by about 18%.

[0012] Based on the above technical pain points, it is urgent to develop a new type of phosphorus-nitrogen synergistic flame retardant system, which needs to have the following characteristics:

[0013] By introducing reactive groups through molecular structure design, chemical bonding with epoxy resin is achieved, and compatibility and dispersion stability are improved;

[0014] A high flame retardancy level (LOI ≥ 30%, UL-94 V-0 level) is achieved with a low addition amount (≤ 5 wt.%), and the mechanical and optical properties of the matrix are retained to the greatest extent;

[0015] Avoid halogen elements and meet the requirements of environmental protection.

[0016] In view of the above needs, the present invention proposes a nitrogen-rich DOPO-based flame retardant and its composite material. By precisely regulating the molecular structure and reaction path of the flame retardant, the core defects of the existing technology are effectively overcome, providing an innovative solution for the development of high-performance flame retardant epoxy resins. Summary of the Invention

[0017] The problems existing in the prior art are as follows: The polarity matching degree between conventional phosphorus-based flame retardants and epoxy resins is poor, and phase separation or migration and precipitation are likely to occur, resulting in unstable flame retardancy of flame retardant epoxy resins, and the flame retardant effect needs to be further improved. In view of the above technical problems, the present invention provides a nitrogen-rich DOPO-based flame retardant, and its chemical structural formula is as follows:

[0018]

[0019] The R group in the above chemical structure includes at least one of a hydrogen atom, C 1 -C 3 a straight-chain alkyl group, a hydroxyl group, a hydroxymethyl group, a nitro group, a dimethylamino group, a boric acid group, an acetoxy group, and a methoxy group.

[0020] The preparation method of the nitrogen-rich DOPO-based flame retardant includes the following steps:

[0021] (1) 2-Thiophenecarboxaldehyde or a 2-thiophenecarboxaldehyde derivative reacts with 5-aminotetrazole to generate a Schiff base intermediate, and the structural formula of the Schiff base intermediate is as follows:

[0022]

[0023] In the above chemical structural formula, the R group includes at least one of a hydrogen atom, a C1-C3 straight-chain alkyl group, a hydroxyl group, a hydroxymethyl group, a nitro group, a dimethylamino group, a boronic acid group, an acetoxy group, and a methoxy group;

[0024] (2) The Schiff base intermediate undergoes a phosphine-hydrogen addition reaction with DOPO to obtain a nitrogen-rich DOPO-based flame retardant.

[0025] Preferably, the 2-thiophenecarboxaldehyde derivative includes at least one of 5-hydroxy-2-thiophenecarboxaldehyde, 5-nitro-2-thiophenecarboxaldehyde, 5-methyl-2-thiophenecarboxaldehyde, 5-ethyl-2-thiophenecarboxaldehyde, 5-propyl-2-thiophenecarboxaldehyde, 5-hydroxymethyl-2-thiophenecarboxaldehyde, 5-dimethylamino-2-thiophenecarboxaldehyde, formylthiophene-2-boronic acid, 5-acetylmethyl-2-thiophenecarboxaldehyde, and 5-methoxy-2-thiophenecarboxaldehyde.

[0026] Preferably, the solvent used in the reaction in step (1) includes one or a combination of two or more of absolute ethanol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, dichloromethane, dichloroethane, chloroform, ethyl acetate, benzene, toluene, and 1,4-dioxane.

[0027] Preferably, the temperature of the reaction in step (1) is 40-100 °C, and the reaction time is 8-12 h.

[0028] Preferably, the molar ratio of 2-thiophenecarboxaldehyde or 2-thiophenecarboxaldehyde derivative to 5-aminotetrazole in step (1) is 1-1.1:1.

[0029] Preferably, the molar ratio of the Schiff base intermediate to DOPO in step (2) is 1:1-1.1.

[0030] Preferably, the temperature of the reaction in step (2) is 40-100 °C, and the reaction time is 12-24 h.

[0031] A method for preparing a flame-retardant epoxy resin includes the following steps:

[0032] (1) Heat the epoxy resin to 140-160 °C, add the above nitrogen-rich DOPO-based flame retardant, stir until the nitrogen-rich DOPO-based flame retardant is completely dissolved in the epoxy resin, and keep warm for 8-20 min to obtain a mixed solution;

[0033] (2) After cooling the mixed solution obtained in step (1) to 90-100 °C, add an epoxy curing agent to the reaction system, stir at a constant temperature until the epoxy curing agent is completely dissolved, quickly pour the reaction system into a mold preheated to the curing temperature, and perform thermal curing to obtain the flame-retardant epoxy resin.

[0034] Preferably, the epoxy curing agent is selected from one or more of amine curing agents, amide curing agents, and imidazole curing agents.

[0035] More preferably, the selected amine curing agent can be one or more of m-phenylenediamine, m-xylylenediamine, diaminodiphenylmethane, diaminodiphenylsulfone, m-aminomethylamine, diaminotoluene, and methylene bisphenylenediamine.

[0036] More preferably, the amide curing agent can be one or both of polyamide 650 and polyamide 651.

[0037] More preferably, the imidazole curing agent can be one or more of imidazole, 2-methylimidazole, 2-ethylimidazole, and 2-ethyl-4-methylimidazole.

[0038] The present invention has the following beneficial effects:

[0039] (1) The present invention obtains a nitrogen-rich DOPO-based flame retardant, which belongs to a reactive phosphorus-nitrogen flame retardant. The secondary amino group in its molecular structure can form a chemical bond with the epoxy group in the epoxy resin structure. Compared with conventional phosphorus-based flame retardants, it has better compatibility with epoxy resin, is not easy to fall off, and has less influence on the mechanical properties and transparency of epoxy resin.

[0040] (2) The nitrogen-rich DOPO-based flame retardant obtained in the present invention is obtained by a simple two-step one-pot synthesis reaction of 2-thiophenecarboxaldehyde or 2-thiophenecarboxaldehyde derivatives with 5-5-aminotetrazole and DOPO. The raw materials used are easily available, the reaction conditions are mild, the purification is simple, the reaction solvent can be recycled, the molecular structure of the product is simple, the steric hindrance is small, and at the same time, the purity and yield of the product are relatively high.

[0041] (3) The present invention uses a nitrogen-rich DOPO-based flame retardant as a flame retardant additive in the preparation of flame-retardant epoxy resin. The obtained flame-retardant epoxy resin has good flame retardant properties. The addition amount of the nitrogen-rich DOPO-based flame retardant is small (not higher than 4 wt%), but the flame retardancy of the obtained flame-retardant epoxy resin can reach V-0 level (UL-94), has better market prospects, and more considerable economic benefits.

[0042] (4) The 2-thiophenecarboxaldehyde derivatives selected in the present invention are all 2-thiophenecarboxaldehyde derivatives with good compatibility and small steric hindrance with epoxy resin. The obtained flame-retardant epoxy resin has better transparency and flame retardancy. Description of the Drawings

[0043] Figure 1 : It is the infrared spectrum diagram of THAMP obtained in Example 1 of the present invention.

[0044] Figure 2: These are the char residue morphologies and SEM images of char residues of EP, EP / THAMP (2 wt.%), and EP / THAMP (4 wt.%) after cone calorimeter tests in Example 1 of the present invention.

[0045] Figure 3 : These are the transmissive digital pictures of 3-mm-thick EP, THAMP (2 wt.%), and THAMP (4 wt.%) in Example 1 of the present invention.

[0046] Figure 4 : These are the transmittance test charts of EP, EP / THAMP (2 wt.%), and EP / THAMP (4 wt.%) in Example 1 of the present invention. Detailed implementation mode

[0047] The present invention will be described in detail below with reference to the embodiments. It should be understood that the following embodiments are only illustrative examples of the implementation modes of the present invention, rather than limitations on the scope of the present invention.

[0048] Example 1

[0049] A method for synthesizing a nitrogen-rich DOPO-based flame retardant is as follows:

[0050] Add 23.53 g of 2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole into a 500-mL four-necked flask, then add 200 mL of absolute ethanol as a solvent into the four-necked flask, stir and heat to 80 °C, keep stirring and reacting for 8 h, take samples for on-line analysis. After 2-thiophenecarboxaldehyde is completely converted, add 45 g of DOPO into the system, keep stirring and reacting for 12 h, then cool down to make the solid product in the reaction system completely precipitate, filter it, and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP, and the yield is 86.20%.

[0051] As shown in the Figure 1 specification appendix, this is the infrared spectrum of the flame retardant THAMP. The spectral analysis results are as follows: -NH- (3321 cm -1 ); P=O (1239 cm -1 ); P-O-Ar (1211 cm -1 ); P-C (758 cm -1 ).

[0052] Specific applications:

[0053] Heat the epoxy resin (E51, epoxy equivalent 0.51) to 160 °C, then add the THAMP obtained in Example 1, and stir until the THAMP is completely dissolved in the epoxy resin. Then, stir at a constant temperature for 8 min. After that, cool down to 90 °C, and add the curing agent 4,4'-diaminodiphenylmethane while stirring. Stir until the 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system. Then, quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0054] When the mass percentage content of THAMP in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the epoxy resin reaction system is 19.57 wt.%, the obtained flame-retardant epoxy resin is denoted as EP / THAMP(2 wt.%). The flame-retardant properties of the obtained transparent flame-retardant epoxy resin are shown in Table 1, and the mechanical properties are shown in Table 2.

[0055] When the mass percentage content of THAMP in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the epoxy resin reaction system is 18.97 wt.%, the obtained flame-retardant epoxy resin is denoted as EP / THAMP(4 wt.%). The flame-retardant properties of the obtained transparent flame-retardant epoxy resin are shown in Table 1, and the mechanical properties are shown in Table 2.

[0056] The flame-retardant properties of pure epoxy resin are also shown in Table 1, and the mechanical properties are shown in Table 2.

[0057] Table 1

[0058]

[0059] None in Table 1 means the specimen burns to the fixture and does not meet the UL-94 grade standard; the flame-retardant grade of NO means there is no dripping phenomenon during the combustion process, and Yes means there is a dripping phenomenon during the combustion process.

[0060] As shown in Table 1, pure epoxy resin shows dripping, while it is found that when the flame retardant THAMP is added, the dripping disappears. After adding 2 wt% THAMP to the epoxy resin, the flame-retardant grade reaches V-1, and after adding 4 wt% THAMP to the epoxy resin, the standard test reaches V-0. This shows that THAMP can achieve high flame retardancy of epoxy resin at extremely low addition amounts.

[0061] In Table 1, the limiting oxygen index standard: ASTM D2863, and the specimen size is: 130×6.5×3 mm 3 The vertical burning test standard: ASTM D3801, and the specimen size is: 130×13×3 mm 3 .

[0062] The mechanical property test results of EP, EP / THAMP (2 wt.%), and EP / THAMP (4 wt.%) are shown in Table 2.

[0063] Mechanical properties: The test standards are ASTM D638 (tensile properties) and ASTM D790 (flexural stiffness and strength properties of materials).

[0064] Table 2

[0065]

[0066] It can be analyzed from Table 2 that adding 2 wt% or 4 wt% of THAMP to epoxy resin has a minimal impact on the mechanical properties of epoxy resin.

[0067] Attached to the specification Figure 2 are the combustion char images and char SEM images tested by a cone calorimeter (CCT). Among them, a 1 ~a 3 、b 1 ~b 3 、c 1 ~c 3 belong to the test images of pure EP, EP / THAMP (2 wt.%), and EP / THAMP (4 wt.%) respectively. The images show that the char morphology and carbon layer expansion height of EP / THAMP (2 wt.%) and EP / THAMP (4 wt.%) are better than those of pure EP, and with the increase of the THAMP addition amount in epoxy resin, the carbon layer integrity and expansion height of the obtained flame-retardant epoxy resin are better. The SEM images further confirm that with the introduction of THAMP, the compactness and continuity of the char layer after combustion of the obtained flame-retardant epoxy resin are significantly improved. Therefore, after adding THAMP, the obtained flame-retardant epoxy resin can form a more stable carbon layer after combustion, which can effectively block oxygen and prevent the combustible gas generated by pyrolysis from escaping to the combustion zone, thus achieving a better flame-retardant effect.

[0068] Attached to the specification Figure 3 It can be intuitively seen that the transparency of EP / THAMP (2 wt.%) and EP / THAMP (4 wt.%) is not much different from that of pure EP. From Figure 4 the ultraviolet-visible transmission spectrum, it can be seen that when measured in the range of 300 - 800 nm, the transmittance of EP / THAMP (2 wt.%) and EP / THAMP (4 wt.%) decreases slightly compared with that of pure EP, but still remains at a relatively high transmittance. This shows that adding THAMP to epoxy resin has little impact on its transparency.

[0069] Example 2

[0070] A method for synthesizing a nitrogen-rich DOPO-based flame retardant is as follows:

[0071] Add 160.45 g of 5-hydroxy-2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole into a 500 mL four-necked flask. Then add 200 mL of absolute ethanol as a solvent into the four-necked flask. Stir and heat up to 80 °C, and keep the temperature constant for reaction for 8 h. Take a sample for in-process control analysis. After the complete conversion of 5-hydroxy-2-thiophenecarboxaldehyde, add 45 g of DOPO into the system, and keep stirring at a constant temperature for 12 h. Then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-1, and the yield is 84.78%.

[0072] Specific application

[0073] Heat the epoxy resin to 160 °C, then add THAMP-1 obtained in Example 2, and stir until THAMP-1 is completely dissolved in the epoxy resin. Then keep stirring at a constant temperature for 8 min. Then cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, and stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system. Then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0074] When the mass percentage content of THAMP-1 in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.58 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-1(2 wt.%). The flame-retardant properties of the obtained transparent flame-retardant epoxy resin are shown in Table 3, and the mechanical properties are shown in Table 4.

[0075] When the mass percentage content of THAMP-1 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 18.99 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-1(4 wt.%). The flame-retardant properties of the obtained transparent flame-retardant epoxy resin are shown in Table 3, and the mechanical properties are shown in Table 4.

[0076] Table 3

[0077]

[0078] In Table 3, the limiting oxygen index standard: ASTM D2863, and the sample bar size is: 130×6.5×3 mm 3Vertical burning test standard: ASTM D3801, spline size: 130×13×3 mm 3 。

[0079] The mechanical property test results of EP, EP / THAMP-1 (2 wt.%), and EP / THAMP-1 (4 wt.%) are shown in Table 2.

[0080] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0081] Table 4

[0082]

[0083] Example 3

[0084] A synthesis method of nitrogen-rich DOPO-based flame retardant is as follows:

[0085] Add 16.09 g of 5-nitro-2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole into a 500 mL four-necked flask, then add 200 mL of absolute ethanol as a solvent into the four-necked flask, stir and heat up to 80 °C, keep the temperature constant and react for 8 h, take samples for in-process control analysis. After the complete conversion of 5-nitro-2-thiophenecarboxaldehyde, add 45 g of DOPO into the reaction system, keep the temperature constant and stir for 12 h, then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-2, and the yield is 85.23%.

[0086] Specific application

[0087] Heat the epoxy resin to 160 °C, then add THAMP-2 obtained in Example 3, stir until THAMP-2 is completely dissolved in the above epoxy resin, then keep the temperature constant and stir for 8 min. Then, cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system, then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0088] When the mass percentage content of THAMP-2 in the above epoxy resin reaction system is 2 wt.% and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.59 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-2 (2 wt.%). The flame retardancy of the obtained transparent flame-retardant epoxy resin is shown in Table 5, and the mechanical properties are shown in Table 6.

[0089] When the mass percentage content of THAMP-2 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.01 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-2(4 wt.%). The flame-retardant properties of the obtained transparent flame-retardant epoxy resin are shown in Table 5, and the mechanical properties are shown in Table 6.

[0090] Table 5

[0091]

[0092] In Table 5, the limiting oxygen index standard: ASTM D2863, and the specimen size is: 130×6.5×3 mm 3 The vertical burning test standard: ASTM D3801, and the specimen size is: 130×13×3 mm 3 .

[0093] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0094] Table 6

[0095]

[0096] Example 4

[0097] A synthesis method of a nitrogen-rich DOPO-based flame retardant is as follows:

[0098] Add 16.09 g of 5-methyl-2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole into a 500 mL four-necked flask, then add 200 mL of absolute ethanol as a solvent into the four-necked flask, stir and heat up to 80 °C, keep the temperature constant and react for 8 h. Take samples for on-line analysis. After 5-methyl-2-thiophenecarboxaldehyde is completely converted, add 45 g of DOPO into the system; keep stirring and reacting at a constant temperature for 12 h, then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-3, and the yield is 82.23%.

[0099] Specific application

[0100] Heat the epoxy resin to 160 °C, then add THAMP-3 obtained in Example 4, stir until THAMP-3 is completely dissolved in the epoxy resin, then stir at a constant temperature for 8 min. After that, cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system, then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0101] When the mass percentage content of THAMP-3 in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.58 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-3(2 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 7, and the mechanical properties are shown in Table 8.

[0102] When the mass percentage content of THAMP-3 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 18.98 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-3(4 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 7, and the mechanical properties are shown in Table 8.

[0103] Table 7

[0104]

[0105] In Table 7, the limiting oxygen index standard: ASTM D2863, the specimen size is: 130×6.5×3 mm 3 。

[0106] Vertical burning test standard: ASTM D3801, the specimen size is: 130×13×3 mm 3 。

[0107] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0108] Table 8

[0109]

[0110] Example 5

[0111] A method for synthesizing a nitrogen-rich DOPO-based flame retardant is as follows:

[0112] Add 20.72 g of 5-dimethylamino-2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole into a 500 mL four-necked flask. Then add 200 mL of absolute ethanol as a solvent into the four-necked flask. Stir and heat up to 80 °C, and keep the temperature constant for reaction for 8 h. Take samples for in-process control analysis. After the complete conversion of 5-dimethylamino-2-thiophenecarboxaldehyde, add 45 g of DOPO into the system, and keep stirring at a constant temperature for 12 h. Then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-4, and the yield is 80.58%.

[0113] Specific application

[0114] Heat the epoxy resin to 160 °C, then add THAMP-4 obtained in Example 5, and stir until THAMP-4 is completely dissolved in the epoxy resin. Then keep stirring at a constant temperature for 8 min. After that, cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, and stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system. Then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0115] When the mass percentage content of THAMP-4 in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.59 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-4(2 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 9, and the mechanical properties are shown in Table 10.

[0116] When the mass percentage content of THAMP-4 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.01 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-4(4 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 9, and the mechanical properties are shown in Table 10.

[0117] Table 9

[0118]

[0119] In Table 9, the limiting oxygen index standard: ASTM D2863, the spline size is: 130×6.5×3 mm 3 。Vertical burning test standard: ASTM D3801, the spline size is: 130×13×3 mm 3 。

[0120] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0121] Table 10

[0122]

[0123] Example 6

[0124] A method for synthesizing a nitrogen-rich DOPO-based flame retardant is as follows:

[0125] Add 20.39 g of 5-acetoxy-2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole to a 500 mL four-necked flask, then add 200 mL of absolute ethanol as a solvent to the four-necked flask, stir and heat to 80 °C, keep the temperature constant for 8 h, take samples for analysis during the process. After the complete conversion of 5-dimethylamino-2-thiophenecarboxaldehyde, add 45 g of DOPO to the system, stir at a constant temperature for 12 h, then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-5, and the yield is 80.58%.

[0126] Specific application

[0127] Heat the epoxy resin to 160 °C, then add THAMP-5 obtained in Example 5, stir until THAMP-5 is completely dissolved in the epoxy resin, then stir at a constant temperature for 8 min. After that, cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system, then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C for 2 h in a forced-air oven, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0128] When the mass percentage content of THAMP-5 in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.41 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-5(2 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 9, and the mechanical properties are shown in Table 10.

[0129] When the mass percentage content of THAMP-5 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.00 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-4(4 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 11, and the mechanical properties are shown in Table 12.

[0130] Table 11

[0131]

[0132] In Table 11, the limiting oxygen index standard: ASTM D2863, the size of the spline is: 130×6.5×3 mm 3 。The vertical burning test standard: ASTM D3801, the size of the spline is: 130×13×3 mm 3 。

[0133] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0134] Table 12

[0135]

[0136] Example 7

[0137] A synthesis method of a nitrogen-rich DOPO-based flame retardant is as follows:

[0138] Add 18.64 g of 5-hydroxymethyl-2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole into a 500 mL four-necked flask, then add 200 mL of absolute ethanol as a solvent into the four-necked flask, stir and heat up to 80 °C, keep the temperature constant and react for 8 h, take samples for in-process control analysis. After the complete conversion of 5-dimethylamino-2-thiophenecarboxaldehyde, add 45 g of DOPO into the system, keep the temperature constant and stir and react for 12 h. Then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-6, and the yield is 84.5%.

[0139] Specific application

[0140] Heat the epoxy resin to 160 °C, then add THAMP-6 obtained in Example 5, stir until THAMP-6 is completely dissolved in the epoxy resin, then keep the temperature constant and stir for 8 min. Then cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system, then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0141] When the mass percentage content of THAMP-6 in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.59 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-6(2 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 9, and the mechanical properties are shown in Table 10.

[0142] When the mass percentage content of THAMP-5 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.00 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-6(4 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 13, and the mechanical properties are shown in Table 14.

[0143] Table 13

[0144]

[0145] In Table 13, the limiting oxygen index standard: ASTM D2863, the spline size is: 130×6.5×3 mm 3 . The vertical burning test standard: ASTM D3801, the spline size is: 130×13×3 mm 3 .

[0146] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0147] Table 14

[0148]

[0149] Example 8

[0150] A method for synthesizing a nitrogen-rich DOPO-based flame retardant is as follows:

[0151] Add 24.82 g of 5-formyl-2-thiopheneboronic acid and 17.85 g of 5-aminotetrazole to a 500 mL four-necked flask, then add 200 mL of absolute ethanol as a solvent to the four-necked flask, stir and heat up to 80 °C, and react at a constant temperature for 8 h. Take samples for on-site control and analysis. After the complete conversion of 5-dimethylamino-2-thiophenecarboxaldehyde, add 45 g of DOPO to the system, stir and react at a constant temperature for 12 h. Then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it 3 times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-7, with a yield of 84.5%.

[0152] Specific application

[0153] Heat the epoxy resin to 160 °C, then add THAMP-7 obtained in Example 5, stir until THAMP-7 is completely dissolved in the epoxy resin, then stir at a constant temperature for 8 min. After that, cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system, then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0154] When the mass percentage content of THAMP-7 in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.26 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-7(2 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 9, and the mechanical properties are shown in Table 10.

[0155] When the mass percentage content of THAMP-6 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 18.35 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-7(4 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 15, and the mechanical properties are shown in Table 16.

[0156] Table 15

[0157]

[0158] In Table 15, the limiting oxygen index standard: ASTM D2863, the specimen size is: 130×6.5×3 mm 3 . Vertical burning test standard: ASTM D3801, the specimen size is: 130×13×3 mm 3 .

[0159] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0160] Table 16

[0161]

[0162] Example 9

[0163] A method for synthesizing a nitrogen-rich DOPO-based flame retardant is as follows:

[0164] Add 18.48 g of 5-methoxy-2-thiophenecarboxaldehyde and 17.85 g of 5-aminotetrazole into a 500 mL four-necked flask. Then add 200 mL of absolute ethanol as the solvent into the four-necked flask. Stir and heat up to 80 °C, and keep the temperature constant for reaction for 8 h. Take samples for in-process control analysis. After the complete conversion of 5-dimethylamino-2-thiophenecarboxaldehyde, add 45 g of DOPO into the system, and keep stirring at a constant temperature for 12 h. Then cool down. After the solid product in the reaction solution is completely precipitated, filter it and wash it three times with absolute ethanol. The obtained solid product is vacuum dried at 80 °C for 24 h to obtain the nitrogen-rich DOPO-based flame retardant, denoted as THAMP-8, and the yield is 84.5%.

[0165] Specific application

[0166] Heat the epoxy resin to 160 °C, then add THAMP-8 obtained in Example 5, and stir until THAMP-8 is completely dissolved in the epoxy resin. Then keep stirring at a constant temperature for 8 min. After that, cool down to 90 °C, add 4,4'-diaminodiphenylmethane while stirring, and stir until 4,4'-diaminodiphenylmethane is completely dissolved in the reaction system. Then quickly transfer it to a mold preheated to 100 °C and cure it at 100 °C in a forced-air oven for 2 h, and then cure it at 150 °C for 3 h to obtain the flame-retardant epoxy resin.

[0167] When the mass percentage content of THAMP-8 in the above epoxy resin reaction system is 2 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 19.59 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-8(2 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 9, and the mechanical properties are shown in Table 10.

[0168] When the mass percentage content of THAMP-8 in the above epoxy resin reaction system is 4 wt.%, and the mass percentage content of 4,4'-diaminodiphenylmethane in the above epoxy resin reaction system is 18.99 wt.%, this flame-retardant epoxy resin is denoted as EP / THAMP-8(4 wt.%). The flame-retardant properties of the obtained flame-retardant epoxy resin are shown in Table 17, and the mechanical properties are shown in Table 18.

[0169] Table 17

[0170]

[0171] In Table 17, the limiting oxygen index standard: ASTM D2863, and the sample bar size is: 130×6.5×3 mm 3 。The vertical burning test standard: ASTM D3801, and the sample bar size is: 130×13×3 mm 3 。

[0172] Mechanical properties: The test standards are ASTM D638 and ASTM D790.

[0173] Table 18

[0174]

[0175]

[0176] Comparative Example 1 is the same as Example 1, except that in Comparative Example 1, 2-thiophenecarboxaldehyde was replaced with an equimolar amount of 5-(4-(diphenylamino)phenyl)thiophene-2-carboxaldehyde. The reaction solvent was chloroform, and the obtained flame retardant was denoted as THAMP-9. Although 5-(4-(diphenylamino)phenyl)thiophene-2-carboxaldehyde is also a thiophenecarboxaldehyde derivative, THAMP-9 obtained in Comparative Example 1 could not be completely dissolved in epoxy resin, and the processed EP / THAMP-9 flame-retardant epoxy resin showed an opaque state. Compared with pure EP, both the flame retardancy and mechanical properties decreased significantly. The possible reason is that the molecular structure of the flame retardant is large, with a large steric hindrance and poor compatibility with epoxy resin, affecting many of its properties. The flame retardancy is shown in Table 19, and the mechanical properties are shown in Table 20. The specific test results are shown in Tables 19 and 20 as follows:

[0177] Table 19

[0178]

[0179] Table 20

[0180]

[0181] Comparative Example 2 is the same as Example 1, except that in Comparative Example 2, 2-thiophenecarboxaldehyde was replaced with an equimolar amount of 5-[bis(4-methylphenyl)amino]-2-thiophenecarboxaldehyde. The reaction solvent used was chloroform, and the obtained flame retardant was denoted as THAMP-10. THAMP-10 could not be completely dissolved in epoxy resin, and the EP / THAMP-10 flame-retardant epoxy resin showed an opaque state. Compared with pure EP, both the flame retardancy and mechanical properties decreased significantly. The flame retardancy is shown in Table 21, and the mechanical properties are shown in Table 22. The specific test results are as follows:

[0182] Table 21

[0183]

[0184]

[0185] Table 22

[0186]

[0187] Inspired by the above-described ideal embodiments of the present invention, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of the present invention. The technical scope of the present invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A nitrogen-rich DOPO-based flame retardant, characterized in that: Its chemical structure is as follows: The R group in the above chemical structure includes at least one of a hydrogen atom, a C1-C3 straight-chain alkyl group, a hydroxyl group, a hydroxymethyl group, a nitro group, a dimethylamino group, a boric acid group, an acetoxy group, and a methoxy group.

2. A nitrogen-rich DOPO-based flame retardant according to claim 1, characterized in that: The preparation method comprises the following steps: (1) 2-Thiophenecarboxaldehyde or a 2-thiophenecarboxaldehyde derivative reacts with 5-aminotetrazolyl to generate a Schiff base intermediate, the structural formula of the Schiff base intermediate is as follows: In the above chemical structure, the R group includes at least one of a hydrogen atom, a C1-C3 straight-chain alkyl group, a hydroxyl group, a hydroxymethyl group, a nitro group, a dimethylamino group, a boric acid group, an acetoxy group, and a methoxy group; (2) The Schiff base intermediate reacts with DOPO to undergo a phosphine-hydrogen addition reaction to obtain a nitrogen-rich DOPO-based flame retardant.

3. A nitrogen-rich DOPO-based flame retardant according to claim 2, characterized in that: The 2-thiophenecarboxaldehyde derivative includes at least one of 5-hydroxy-2-thiophenecarboxaldehyde, 5-nitro-2-thiophenecarboxaldehyde, 5-methyl-2-thiophenecarboxaldehyde, 5-ethyl-2-thiophenecarboxaldehyde, 5-propyl-2-thiophenecarboxaldehyde, 5-hydroxymethyl-2-thiophenecarboxaldehyde, 5-dimethylamino-2-thiophenecarboxaldehyde, formaldehyde thiophene-2-boric acid, and 5-acetylmethyl-2-thiophenecarboxaldehyde.

4. A nitrogen-rich DOPO-based flame retardant according to claim 2, characterized in that: The solvent used in the reaction in step (1) includes one or a combination of two or more of anhydrous ethanol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, dichloromethane, dichloroethane, chloroform, ethyl acetate, benzene, toluene, and 1,4-dioxane.

5. A nitrogen-rich DOPO-based flame retardant according to claim 2, characterized in that: The reaction temperature in step (1) is 40-100° C. and the reaction time is 8-12 h.

6. A nitrogen-rich DOPO-based flame retardant according to claim 2, characterized in that: In step (1), the molar ratio of 2-thiophenecarboxaldehyde or 2-thiophenecarboxaldehyde derivative to 5-aminotetrazolyl is 1-1.1:

1.

7. A nitrogen-rich DOPO-based flame retardant according to claim 2, characterized in that: In step (2), the molar ratio of the Schiff base intermediate to DOPO is 1:1-1.

1.

8. A nitrogen-rich DOPO-based flame retardant according to claim 2, characterized in that: The reaction temperature in step (2) is 40-100° C. and the reaction time is 12-24 h.

9. A flame retardant epoxy resin, characterized in that: The nitrogen-rich DOPO-based flame retardant according to any one of claims 1 to 8 is used as a flame retardant additive.

10. A flame retardant epoxy resin according to claim 9, characterized in that: The preparation method comprises the following steps: (1) heating the epoxy resin to 140-160° C., adding the nitrogen-rich DOPO-based flame retardant, stirring until the nitrogen-rich DOPO-based flame retardant is completely dissolved in the epoxy resin, and keeping the temperature for 8-20 minutes to obtain a mixed solution; (2) After cooling the mixed solution obtained in step (1) to 90-100° C., an epoxy curing agent is added to the reaction system, and the mixture is stirred at a constant temperature until the epoxy curing agent is completely dissolved. The reaction system is quickly poured into a mold preheated to the curing temperature and thermally cured to obtain a flame retardant epoxy resin.

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