Phosphorus-nitrogen flame retardant modified epoxy resin and preparation method thereof

Abstract

The application discloses a phosphorus-nitrogen flame retardant modified epoxy resin and a preparation method thereof, and belongs to the field of flame-retardant materials. The purpose of the application is to obtain a N and P synergistic composite flame retardant by optimizing the structure of the flame retardant, so as to solve the problem that the mechanical properties and the flame-retardant properties of the flame-retardant epoxy resin cannot be considered simultaneously. The phosphorus-nitrogen flame retardant is obtained through two-step reactions of dehydration condensation and addition by using p-xylylene formaldehyde, 4,4-diaminodiphenyl methane and DOPO as raw materials; and the modified epoxy resin is obtained by adding different proportions of the phosphorus-nitrogen flame retardant to m-phenylenediamine as a curing agent. The phosphorus-nitrogen flame retardant modified epoxy resin reaches the standard of the flame-retardant material, the highest limiting oxygen index is up to 36.1%, and the UL-94 test level is V-0. The application has better environmental friendliness and does not release harmful substances. The flame retardant of the application overcomes the above technical defects and realizes high flame-retardant efficiency.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant materials, specifically, it relates to a phosphorus-nitrogen flame retardant modified epoxy resin and its preparation method. Background Technology

[0002] Epoxy resins, with their excellent mechanical properties, chemical corrosion resistance, and superior electrical insulation, have shown broad application prospects in many high-tech fields such as electronic and electrical packaging, aerospace structural components, high-performance coatings, and building composite materials. However, the inherent flammability of this material is significant. The harmful gases and dense smoke released during combustion not only severely restrict its promotion in high-temperature operating environments or scenarios with stringent fire protection requirements, but also pose a potential threat to personnel safety. It is worth noting that, as a core material for load-bearing structures, epoxy resins also face challenges in their service performance under complex stress conditions. It is necessary to simultaneously construct an efficient flame-retardant protection system to cope with sudden fires while maintaining the mechanical load-bearing capacity of the matrix. Therefore, developing modified epoxy resins that combine excellent mechanical properties with efficient flame-retardant functions has become a key technical issue in overcoming the current bottlenecks in epoxy resin applications.

[0003] Currently, the main way to improve the flame retardancy of epoxy resins using traditional processes is by incorporating various flame retardant additives, with phosphorus-based and nitrogen-based compounds being the mainstream. However, while phosphorus-based flame retardants can both retard and plasticize, they are volatile and have poor thermal stability; nitrogen-based flame retardants have high decomposition temperatures and good compatibility with epoxy resins, but their effectiveness is poor when used alone. They are usually incorporated into the epoxy resin matrix through physical blending, but uneven doping leads to unstable flame retardant effects. Furthermore, insufficient interfacial compatibility between the flame retardant and the epoxy matrix often results in agglomeration, which not only worsens processing flowability but also easily forms defective areas within the product, reducing overall reliability. Therefore, establishing a dynamic balance mechanism between flame retardant performance and mechanical properties has become a core research direction for epoxy resin modification.

[0004] To overcome the aforementioned technical bottlenecks, researchers have proposed several innovative approaches: First, refining flame-retardant particles to the nanoscale or performing ultra-micronization treatments to improve their dispersion uniformity and interfacial bonding efficiency in the matrix. However, practice has shown that simply controlling particle size has a threshold effect on improving the flame-retardant performance of the system; once the filler mass fraction exceeds a critical value, further increasing the dosage leads to stagnation in improving the overall performance. Second, introducing reinforcing fibers to construct multiphase composite systems can significantly improve mechanical properties such as flexural strength, but some fibers lack intrinsic flame retardancy and may trigger a "core material ignition effect" at high temperatures, thus accelerating flame spread. Therefore, developing novel functionalized flame retardants with synergistic flame-retardant mechanisms, achieving high-efficiency flame retardancy while maintaining low addition levels, ensuring minimal impact on mechanical properties, and meeting environmental protection requirements are of significant practical importance. Summary of the Invention

[0005] The purpose of this invention is to provide a phosphorus-nitrogen flame retardant and a method for preparing the flame-retardant epoxy resin, thereby solving the bottleneck problem of the inability to simultaneously achieve flame retardant and mechanical properties in existing modified epoxy resins. This invention achieves the synergistic effect of flame-retardant elements through molecular structure design, significantly improving the flame retardant rating while maintaining the excellent mechanical properties of the epoxy resin itself. This provides a new technical direction for solving the problem of simultaneously achieving flame retardant and mechanical properties in the field of high-performance composite materials. This invention proposes the design of a flame retardant containing both nitrogen and phosphorus flame-retardant elements, exhibiting high stability and high flexibility. By combining the active groups in this flame retardant with the epoxy groups of the epoxy resin, the molecular structure of the epoxy resin is modified, thereby optimizing the flame retardant and mechanical properties of the modified epoxy resin.

[0006] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a phosphorus-nitrogen flame retardant modified epoxy resin. The structural formula of the TPN-DOPO phosphorus-nitrogen flame retardant is as follows:

[0007] This invention also provides a method for preparing phosphorus-nitrogen flame retardant modified epoxy resin, comprising the following steps: Step 1: Using anhydrous ethanol as a solvent, terephthalaldehyde and 4,4-diaminodiphenylmethane undergo a dehydration condensation reaction to obtain intermediate TPN; that is, intermediate TPN is prepared by dehydration condensation of terephthalaldehyde and 4,4-diaminodiphenylmethane in anhydrous ethanol.

[0008] Step 2: The intermediate TPN obtained in Step 1 and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) undergo an addition reaction in 1,4-dioxane to obtain a phosphorus-nitrogen flame retardant. Step 3: Heat the epoxy resin to 100~120 ℃, add the phosphorus and nitrogen flame retardant obtained in Step 2, stir until uniform, reduce the temperature of the epoxy resin to 60~80 ℃, add m-phenylenediamine, mix evenly, degas under vacuum, pour into a preheated mold, cure in two stages, and demold after natural cooling to obtain phosphorus and nitrogen flame retardant modified epoxy resin. The amount of phosphorus-nitrogen flame retardant added accounts for 5 to 20% of the mass of the phosphorus-nitrogen flame retardant modified epoxy resin.

[0009] Further specifying, in step 1, the molar ratio of terephthalaldehyde to 4,4-diaminodiphenylmethane is 1:2 to 1:2.5.

[0010] Further specifying, in step 1, the molar ratio of terephthalaldehyde to the volume of anhydrous ethanol is (0.1~0.2) mol: 100 mL.

[0011] Further specifying, in step 1, the molar ratio of 4,4-diaminodiphenylmethane to the volume of anhydrous ethanol is (0.1~0.2) mol: 150 mL.

[0012] Further specifying, in step 2, the addition reaction is carried out at 101~110 °C.

[0013] Further specifying, in step 2, the molar ratio of intermediate TPN to DOPO is 1:2 to 1.1:2.2.

[0014] Further specifying, in step 2, the molar ratio of intermediate TPN to the volume of 1,4-dioxane is (0.1~0.2) mol: 150 mL.

[0015] Further specifying, in step 3, the mass fraction of phosphorus-nitrogen flame retardant used is 5%-20%.

[0016] Further specifying, the curing process involves first curing at 60~80 ℃ for 1.5~2 hours, and then raising the temperature to 130~150 ℃ for another 1.5~2 hours.

[0017] Compared with the prior art, the present invention has the following beneficial effects: The phosphorus-nitrogen flame retardant prepared in this invention, as a polymer compound, contains various types of active hydrogen in its structure, providing more active sites for the ring-opening reaction of epoxy groups in epoxy resin. This results in intertwined molecular chains, increased cross-linking density of the epoxy resin, and a more tightly bonded system with superior mechanical properties. Phosphorus and nitrogen elements provide the core expansion-to-char flame retardant mechanism; the phosphate polyphosphate generated by the combustion decomposition of the phosphorus-nitrogen flame retardant catalyzes the carbonization of the epoxy resin, inhibiting molten dripping during combustion; and polymerization ensures the durability and stability of the flame retardant effect. This invention successfully solves the problem of incompatibility between the flame retardant and mechanical properties of existing modified epoxy resins, providing a standardized and accurate compatibility treatment method.

[0018] The phosphorus and nitrogen-containing flame retardant described in this invention has better environmental friendliness than traditional halogen flame retardants, with no release of harmful substances. Traditional additive flame retardants suffer from precipitation, leading to reduced flame retardant efficiency. The flame retardant of this invention overcomes these technical defects and achieves higher flame retardant efficiency.

[0019] The flame-retardant epoxy resin described in this invention exhibits improved mechanical and adhesive properties compared to unmodified resin. When the flame retardant content is 5% and 10%, the tensile strength increases by 4.11 MPa and 7.56 MPa, respectively, while elongation and plasticity also increase; the peel strength reaches 2.61 kN / m and 2.82 kN / m, respectively, representing increases of 0.14 kN / m and 0.35 kN / m compared to pure epoxy resin. This indicates that adding appropriate amounts of phosphorus-nitrogen flame retardants can effectively improve the adhesive properties of epoxy resin.

[0020] The phosphorus-nitrogen flame retardant-modified epoxy resins prepared by this invention all meet the testing standards for flame-retardant materials. For example, when the flame retardant content is 10%, the limiting oxygen index (LOI) increases from 21% before modification to 36.1%, and the UL-94 test achieves a V-0 rating, demonstrating excellent flame-retardant performance. Furthermore, the amount of flame retardant added can be adjusted according to actual flame-retardant requirements to prepare different grades of flame-retardant epoxy resins, thereby effectively reducing production costs.

[0021] The phosphorus-nitrogen flame retardant used in this invention does not contain halogens or other elements harmful to the environment and human body. It does not produce toxic or corrosive byproducts during combustion, thus meeting increasingly stringent environmental and health standards. Furthermore, the raw materials and solvents used in the preparation process are mostly common chemical substances with low environmental impact, resulting in a minimal overall environmental impact on the manufacturing process.

[0022] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0023] Figure 1 shows the infrared spectrum of the phosphorus-nitrogen flame retardant prepared in Example 5; Figure 2 shows the 1H NMR spectrum of the phosphorus-nitrogen flame retardant prepared in Example 5; Figure 3 shows the NMR phosphorus spectrum of the phosphorus-nitrogen flame retardant prepared in Example 5; Figure 4 is a screenshot from a video of the vertical combustion experiment of phosphorus-nitrogen modified epoxy resin in Example 6; Figure 5 shows the stress-strain curve of the phosphorus-nitrogen modified epoxy resin in Example 6; Figure 6 is a bar chart of the peel strength of the phosphorus-nitrogen modified epoxy resin in Example 6. Detailed Implementation

[0024] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention. Example

[0025] The preparation method of phosphorus-nitrogen flame retardant modified epoxy resin in this embodiment is carried out according to the following steps: Step 1: Dissolve 0.1 mol of terephthalaldehyde in 100 mL of anhydrous ethanol, and dissolve 0.15 mol of 4,4-diaminodiphenylmethane in 100 mL of anhydrous ethanol. Mix the two and carry out a dehydration condensation reaction to obtain the intermediate TPN containing a Schiff base structure. Step 2: Add 0.02 mol of the intermediate TPN obtained in Step 1 and 0.08 mol of DOPO to 250 mL of 1,4-dioxane and carry out an addition reaction at 110 °C for 1 h to obtain a phosphorus-nitrogen flame retardant; Step 3: Heat E-51 epoxy resin to 120℃, add phosphorus and nitrogen flame retardant, mix evenly, then cool to 80℃, add curing agent, mix evenly, vacuum degas, pour into a preheated mold, cure at 80℃ for 2 hours and 150℃ for 2 hours in a vacuum drying oven, cool to room temperature to obtain phosphorus and nitrogen flame retardant modified epoxy resin, wherein the mass fraction of phosphorus and nitrogen flame retardant added to the modified epoxy resin is 10%. Example

[0026] This embodiment is basically the same as Example 1, except that the dehydration condensation reaction described in this embodiment is as follows: terephthalaldehyde and 4,4-diaminodiphenylmethane are added to anhydrous ethanol solvent, wherein the molar ratio of terephthalaldehyde to 4,4-diaminodiphenylmethane is 1:2. 0.2 mol of terephthalaldehyde requires 100 mL of anhydrous ethanol solvent to dissolve, and the mixture is stirred continuously for 2 hours; 0.2 mol of 4,4-diaminodiphenylmethane requires 150 mL of anhydrous ethanol to dissolve, and the mixture is stirred continuously for 2 hours. After complete dissolution, the 4,4-diaminodiphenylmethane solution was slowly added dropwise to the above terephthalic acid solution at a molar ratio of 1:2 between terephthalic acid and 4,4-diaminodiphenylmethane. The mixture was heated in an oil bath with constant stirring and reacted at 60 °C for 6 h. After the reaction was completed and cooled to room temperature, the reaction solution was poured into a large amount of water, the precipitate was filtered, and dried under vacuum at 60 °C for 24 h to obtain the intermediate (TPN), which was a pale yellow solid. Example

[0027] This embodiment is basically the same as Example 2, except that the addition reaction described in this embodiment is as follows: TPN, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and 1,4-dioxane are added to a three-necked flask, wherein the molar ratio of TPN to DOPO is 1:2, and 1,4-dioxane is used as the solvent. 0.03 mol of TPN requires 250 mL of 1,4-dioxane for dissolution, and 0.06 mol of DOPO requires 100 mL of 1,4-dioxane for dissolution. Under nitrogen protection, the mixture is heated in an oil bath with continuous stirring at 110 °C for 12 h. After the reaction is complete, it is cooled to room temperature, and the reaction mixture is transferred to a 1000 mL beaker containing 500 mL of deionized water. After separation by filtration, the precipitate was washed four times each with deionized water and anhydrous ethanol, and finally dried in a vacuum environment at 60 °C for 24 h to obtain the flame retardant (TPN-DOPO), which is a yellow solid.

[0028] Example 4 Weigh the epoxy resin and heat it to 110 ℃ using a constant-temperature heating stirrer. Add 5% phosphorus-nitrogen flame retardant to the epoxy resin. The 5% phosphorus-nitrogen flame retardant is a percentage of the epoxy resin mass added. Stir until homogeneous. Adjust the heater temperature to 80 ℃. Once the epoxy resin has cooled to the appropriate temperature, add the pre-calculated and weighed curing agent to the epoxy resin and stir until homogeneous. Vacuum for 5 minutes, then pour the mixture into a preheated silicone mold coated with release agent. Cure in a constant-temperature vacuum drying oven at 80 ℃ and 150 ℃ for 2 hours respectively. After the resin material has cooled naturally, remove it to obtain the final sample strip, labeled EP2. The reference sample is epoxy resin without added flame retardant, labeled EP1. In the above process, the curing agent is m-phenylenediamine, and the release agent is dimethyl silicone oil.

[0029] Example 5 The difference between this embodiment and Embodiment 4 is that the amount of phosphorus-nitrogen flame retardant added to the modified epoxy resin is 10%, the sample is marked as EP3, and the other steps and parameters are the same as in Embodiment 4.

[0030] Example 6 The difference between this embodiment and Embodiment 4 is that the amount of phosphorus-nitrogen flame retardant added to the modified epoxy resin is 15%. The sample was marked EP4, and the other steps and parameters were the same as in Example 4. Example 7

[0031] The difference between this embodiment and Embodiment 4 is that the amount of TPN-DOPO flame retardant added to the modified epoxy resin is 20%, the sample is marked as EP5, and the other steps and parameters are the same as in Embodiment 4.

[0032] The TPN-DOPO flame retardants prepared in Examples 1-3 were characterized by infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, and phosphorus nuclear magnetic resonance spectroscopy. The test spectra are shown below. Figures 1-3 As shown in the figure. The flame retardant and mechanical properties of the phosphorus-nitrogen flame retardant modified epoxy resin were characterized by vertical burning (UL-94), limiting oxygen index (LOI), tensile property tests, and adhesive T-peel strength tests on the samples prepared in Example 4. The relevant experimental data are recorded in Table 1, and video screenshots of the modified epoxy resin during the vertical burning test are shown in the figure. Figure 4 As shown, the corresponding stress-strain curves and peel strength test results are as follows: Figure 5 and Figure 6 As shown. The determinations of vertical burning, limiting oxygen index, tensile properties, and peel strength were performed according to the following standards: Vertical burning (UL-94) test: GB / T 2408-2021; Limiting Oxygen Index (LOI) Test: GB / T 2406.2-2009; Tensile properties test: GB / T 1040.2-2006; Peel strength test: GB / T 2791-1995.

[0033] Table 1 Flame retardant properties, tensile strength, and adhesive properties of samples from each embodiment.

[0034] In the table above, t1 / t2 represents the first afterflame time / second afterflame time of the sample; – / – indicates that the sample continues to burn after the flame is applied and cannot self-extinguish.

[0035] Pure epoxy resin (EP1) has an LOI value of 21.0%, classifying it as a flammable material, and failed to achieve a rating in the UL-94 test. In contrast, the addition of the flame retardant TPN-DOPO significantly improved the flame retardancy and self-extinguishing properties of the epoxy resin. As the TPN-DOPO content increased, the LOI value of the modified epoxy resin initially increased and then decreased, ultimately reaching a flame-retardant level. At a TPN-DOPO content of 10%, the corresponding modified epoxy resin (EP7) achieved a V-0 rating in the UL-94 test, with an LOI value of 36.1%. The UL-94 test results also indicated that the modified epoxy resin did not exhibit dripping and self-extinguished within a certain timeframe, demonstrating excellent flame retardancy.

[0036] Compare Table 1 with Figure 5 and Figure 6 In summary, compared with the phosphorus-nitrogen flame retardant modified epoxy resin obtained in Example 5, the tensile and adhesive properties of the phosphorus-nitrogen flame retardant modified epoxy resin are improved. That is, at the corresponding addition amount in this example, the phosphorus-nitrogen flame retardant modified epoxy resin has excellent mechanical properties.

[0037] This invention prepares a TPN-DOPO phosphorus-nitrogen synergistic flame retardant with a Schiff base structure by condensation and addition of terephthalaldehyde, 4,4-diaminodiphenylmethane, and DOPO. The modified epoxy resin is then prepared by curing with m-phenylenediamine. Unlike the conventional approach of adding small-molecule flame retardants through physical blending, the core innovation lies in designing the flame retardant molecule as a polymeric structure containing active hydrogen. This is achieved through chemical bonding between active groups and epoxy groups, rather than simple physical doping, thus improving interfacial compatibility and dispersion uniformity, avoiding agglomeration and precipitation. Simultaneously, the synergistic effect of phosphorus and nitrogen expansion into char achieves high-efficiency flame retardancy. Appropriate addition can increase crosslinking density and simultaneously enhance mechanical and adhesive properties, overcoming the bottleneck of traditional flame retardant modification where flame retardancy and mechanical properties are difficult to balance. The use of a halogen-free system is more environmentally friendly. The curing process employs a two-stage stepwise process, achieving UL-94 V-0 and a high limiting oxygen index even with low addition amounts. Overall, this invention achieves integrated modification with reactive flame retardancy, synergistic molecular structure, and balanced performance improvement.

[0038] This invention breaks through the traditional approach of conventional physical blending, small molecule addition, and single-element flame retardancy. It utilizes a customized molecular structure, reactive bonding, and in-situ synergistic char formation of phosphorus and nitrogen as the basis. A Schiff base-containing intermediate, TPN, is constructed through the condensation of terephthalaldehyde and 4,4-diaminodiphenylmethane, followed by addition with DOPO to obtain a high-molecular-weight reactive phosphorus-nitrogen flame retardant. This molecule simultaneously carries multiple active sites, including P=O, C=N, and active NH, enabling ring-opening chemical bonding with epoxy groups, rather than passive filling, thus eliminating phase separation at the interface level. In addition to addressing migration and precipitation issues, phosphorus and nitrogen elements are uniformly distributed at the molecular level. During combustion, they simultaneously exert a synergistic effect of phosphorus catalytic char formation, nitrogen release of inert gas dilution, and expansion insulation, forming a dense and continuous char layer. Furthermore, the active hydrogen inherent in the flame retardant can increase the crosslinking density of the system. With a low addition amount of 5-10%, it not only does not sacrifice mechanical properties but also improves fracture strength and peel strength, achieving simultaneous enhancement of flame retardancy and mechanical properties. Combined with two-stage step-curing, it further optimizes the uniformity of crosslinking. Ultimately, under the premise of halogen-free, low-addition, and environmentally friendly conditions, it achieves an LOI of 36.1% and UL-94 V-0, completely overturning the conventional technical limitations that flame retardancy must sacrifice mechanical properties and that high addition is necessary for high efficiency.

[0039] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A phosphorus-nitrogen flame retardant modified epoxy resin, characterized in that, Its structural formula is 。 2. The preparation method of the phosphorus-nitrogen flame retardant modified epoxy resin as described in claim 1, characterized in that, Includes the following steps: Step 1: Using anhydrous ethanol as a solvent, terephthalaldehyde and 4,4-diaminodiphenylmethane undergo a dehydration condensation reaction to obtain intermediate TPN. Step 2: The intermediates TPN, DOPO and 1,4-dioxane obtained in Step 1 undergo an addition reaction to obtain a phosphorus-nitrogen flame retardant; Step 3: Heat the epoxy resin to 100~120 ℃, add the phosphorus and nitrogen flame retardant obtained in Step 2, stir until uniform, reduce the temperature of the epoxy resin to 60~80 ℃, add m-phenylenediamine, mix evenly, degas under vacuum, pour into a preheated mold, cure, and demold after cooling to obtain phosphorus and nitrogen flame retardant modified epoxy resin. The amount of phosphorus-nitrogen flame retardant added accounts for 5 to 20% of the mass of the phosphorus-nitrogen flame retardant modified epoxy resin.

3. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 2, characterized in that, In step 1, the molar ratio of terephthalaldehyde to 4,4-diaminodiphenylmethane is 1:2 to 1:2.

5.

4. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 2, characterized in that, In step 1, the molar ratio of terephthalaldehyde to the volume of anhydrous ethanol is (0.1~0.2) mol: 100 mL.

5. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 2, characterized in that, In step 1, the molar ratio of 4,4-diaminodiphenylmethane to the volume of anhydrous ethanol is (0.1~0.2) mol: 150 mL.

6. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 2, characterized in that, In step 2, the addition reaction takes place at 101~110 °C.

7. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 2, characterized in that, In step 2, the molar ratio of intermediate TPN to DOPO is 1:2 to 1.1:2.

2.

8. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 2, characterized in that, In step 2, the molar ratio of intermediate TPN to volume of 1,4-dioxane is (0.1~0.2) mol: 150 mL.

9. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 3, characterized in that, In step 3, the mass fraction of phosphorus-nitrogen flame retardant is 5%-20%.

10. The method for preparing the phosphorus-nitrogen flame retardant modified epoxy resin according to claim 3, characterized in that, In step 3, the curing process involves first curing at 60~80 ℃ for 1.5~2 hours, and then raising the temperature to 130~150 ℃ for another 1.5~2 hours.

Images